Patent Publication Number: US-2012043304-A1

Title: Melt type high basicity flux for submerged arc welding use

Description:
TECHNICAL FIELD  
     The present invention relates to melt flux for submerged arc welding use which is used for welding when making UO pipe for pipeline use from which strength and low temperature toughness for transporting petroleum, natural gas, etc. are demanded and which has a base material of a tensile strength of 800 MPa to 1200 MPa. 
     BACKGROUND ART   
     As steel pipes for use in pipelines transporting oil, natural gas, etc., welded steel pipes obtained by welding steel pipes together in their longitudinal directions may be mentioned. Among these, UO pipes are produced by using a C-press, U-press, and O-press to shape steel plate into a tube, then welding together the abutting parts of the plate. For the welding method used to join the parts, considering quality and productivity, submerged arc welding is often used. 
     “Submerged arc welding” is a method which uses flux and weld wire and causes an arc in the flux to perform welding. The weld zone of UO pipe obtained by submerged arc welding usually is comprised of a total of two welds: one layer from the inner surface of the steel pipe and one layer from the outer surface of the steel pipe. The weld metal fabricated from the flux and weld wire is required to have a strength and toughness commensurate with the base material. Further, at the same time, a good weld bead shape and defect-free weld metal are being demanded. 
     For control for such quality, the composition of the flux and the ingredients of the weld wire are important. In the past, various art has been developed. 
     For example, in the invention described in PLT 1, to obtain a weld metal with a high toughness, the ranges of ingredients and the ranges of ratios of several ingredients have been prescribed so as to thereby reduce the oxygen content in the weld metal and achieve specific objectives. Specifically, CaF 2  is added in a suitable quantity to decrease the amount of oxygen in the weld metal. Further, to deal with the instability in the arc, which becomes a problem along with addition of CaF 2 , SiO 2  and Al 2 O 3  are added to vitrify the slag and, furthermore, SIC 2 , MgO, and CaO are added in a suitable ratio so as to stabilize the arc. Furthermore, by vitrification of the slag by addition of suitable quantities of Al 2 O 3  and SiO 2 , the effect of improvement of the work efficiency is also obtained. As a result, a base material containing C: 0.09%, Si: 0.29%, Mn: 1.32%, P: 0.019%, S: 0.007%, and Al: 0.035% is being used to improve the 0° C. toughness. 
     In the invention described in PLT 2, the chemical compositions of the base material and weld metal of large sized pipes for applications requiring resistance to sour environments are defined. Furthermore, in the weld metal, the ratios of the ingredients of the weld metal are defined to try to obtain seam weld metal for UO pipe with excellent low temperature toughness. In this case, a low oxygen type flux is used so that the amount of oxygen of the weld metal in the flux used becomes 0.015% to 0.025%. The flux used here is the low oxygen type flux which is disclosed in PLT 3. To decrease the amount of oxygen in the weld metal, for example, the SiO 2  is decreased to a low level of 5.4% so as to lower the oxygen content. 
     The invention described in PLT 4 limits the ingredients of the weld wire used and the basicity of the flux used or the chemical ingredients of the base material so as to secure low temperature toughness of the weld metal. The flux used here is CaO—CaF 2 -AiO 2 -based flux. To make the amount of oxygen of the weld metal the 0.035% or less required for securing toughness, the basicity is made 0.4 or more. The main composition of the flux used in the examples is SiO 2 : 25% to 29%, MnO: 2% to 5%, TiO 2 : 5% to 7%, Al 2 O 3 : 5%, CaO: 18% to 33%, MgO: 4% to 9%, and CaF 2 : 17% to 26%. By using such flux and welding X65 steel pipe, weld metal of a high toughness of an absorption energy at −30° C. of about 20 kgfm (about 200 J) is obtained. 
     The invention described in PLT 5 is applied to welding of particularly thick-walled large-sized steel pipe, so is predicated on multilayer welding and defines the ingredients of the weld-use flux so as to try to obtain toughness. Specifically, to control the hardenability of the weld metal and secure low temperature toughness of the weld metal, the ingredients of the flux are optimized for the first layer and second layer and for the third layer and on. 
     The invention described in PLT 6 discloses flux for submerged arc welding use applied to weld metal predicated on heat treatment after welding. In PLT 6, the ingredients of the flux are defined so as to secure low temperature toughness of the weld metal after heat treatment after welding. In particular, the lower limits of the Si and Mn in the flux are set from the viewpoint of securing deoxidation, while the upper limits are set from the viewpoint of securing low temperature toughness. A suitable quantity of CaF 2  is added to decrease the amount of oxygen of the weld metal. 
     In the above way, the ingredients of the flux have been studied for the purpose of securing low temperature toughness of the weld metal. 
     CITATION LIST  
     Patent Literature 
     PLT 1: Japanese Patent Publication (A) No. 58-055197 
     PLT 2: Japanese Patent Publication (A) No. 03-285770 
     PLT 3: Japanese Patent Publication (A) No. 60-191691 
     PLT 4: Japanese Patent Publication (A) No. 05-000375 
     PLT 5: Japanese Patent Publication (A) No. 06-155076 
     PLT 6: Japanese Patent Publication (A) No. 08-257789 
     PLT 7: Japanese Patent Publication (A) No. 2006-305604 
     PLT 8: Japanese Patent Publication (A) No. 2007-90399 
     SUMMARY OF INVENTION  
     Technical Problem 
     In recent years, for the purpose of improving the transport efficiency and reducing costs, demand for high strength UO pipe with a tensile strength of the base material of 800 MPa or more, higher than the strength of conventional UO pipe, has been rising. Naturally, the weld metal of the welds of such UO pipe also is required to have a strength of a tensile strength of 800 MPa or more and a low temperature toughness commensurate with the base material. 
     However, in the findings of studies by the inventors, when welding UO pipe for pipeline use with a base material tensile strength of 800 MPa to 1200 MPa by submerged arc welding, the phenomenon such as shown in  FIG. 1  of slag being entrained inside of the weld metal near the top of the bead (below, this phenomenon and the parts where this phenomenon occurs will be called “top slag-in ST”) is sometimes seen and there was a tendency for low temperature cracking to easily occur after welding. 
     For example, with the flux of the composition disclosed in the examples of PLT 1, application to high strength weld metal with a high low temperature cracking sensitivity is not possible. 
     With the flux of the invention described in PLT 3 used in the invention described in PLT 2, the problem of low temperature cracking after welding arises. 
     With welding using the flux disclosed in PLT 4 to obtain high strength weld metal, there is the problem of occurrence of top slag-in. 
     With the flux of the composition disclosed in the examples of PLT 5, application to high strength weld metal with a high low temperature cracking sensitivity is not possible. 
     With the invention described in PLT 6, securing toughness is easy by the effect of reheating by subsequent welding, but application is not possible with single layer welding at the inside and outside surfaces where the reheating effect cannot be expected. 
     Furthermore, in the invention described in this literature, low temperature cracking easily occurs after welding. This is therefore not suitable for high strength weld metal. 
     The invention described in PLT 7 proposes melt flux for submerged arc welding use which enables horizontal position welding by a large current and can be applied to any posture of a horizontal, horizontal fillet, and downward posture, but does not focus on the amount of oxygen in the weld metal to prevent a drop in toughness when preparing high strength weld metal. 
     The invention described in PLT 8 proposes melt flux for submerged arc welding use, used when preparing high strength weld metal with a strength corresponding to 800 MPa to 1200 MPa class high strength steel plate, which improves the low temperature toughness, but application of UO pipe etc. in more severe environments is being demanded. 
     Therefore, the present invention, in view of this background, has as its task the provision of high basicity melt flux for submerged arc welding use where even when preparing high strength weld metal, sound, high toughness weld metal resistant to low temperature cracking after welding and free of top slag-in can be obtained. 
     Solution to Problem 
     The inventors studied the above such melt flux for submerged arc welding use which can give high strength weld metal which is resistant to low temperature cracking after welding and is free of top slag-in and excellent in low temperature toughness. As a result, they discovered that by setting suitable ranges for CaO, MgO, Al 2 O 3 , CaF 2 , and SiO 2  and optimizing the amount of addition of MnO, even when preparing high strength weld metal, as shown below, it is possible to optimize the amount of oxygen of the weld metal so as to obtain flux which enables the preparation of a weld bead which secures a high low temperature toughness while giving a good shape free of top slag-in and is resistant to low temperature cracking after welding. 
     The strength of the weld metal can be improved by using, as the weld wire used for the submerged arc welding, one in which Ni, Cr, Mo, or other alloy elements have been added. According to the results of studies of the inventors, in such weld metal increased in strength and having a tensile strength of 800 MPa to 1200 MPa, as shown in  FIG. 2 , the amount of oxygen of the weld metal is important. As shown by the broken line in  FIG. 2 , when the amount of oxygen in the weld metal is about 0.018% to 0.035% in range, a good toughness where the −30° C. impact absorption energy becomes 100J or more can be obtained. 
     With weld metal with a tensile strength of 800 MPa to 1200 MPa, the microstructure becomes an acicular ferrite microstructure to bainite microstructure. With these microstructures, the amount of oxygen in the weld metal plays an important part in the toughness of the weld metal. On the other hand, with weld metal of over 1200 MPa, the microstructure becomes a martensite microstructure. With just control of the oxygen amount, securing the toughness becomes difficult. 
     Further, according to the results of studies by the inventors, top slag-in is believed to occur in the following way. That is, to obtain the strength of the weld metal, the above-mentioned alloy elements are added to the weld wire used for the submerged arc welding, but the above alloy elements cause the viscosity of the weld metal melted at the time of welding and the interfacial tension between the molten weld metal and the melt flux to change. For this reason, it is believed, it becomes difficult for the slag melted at the time of welding to float up and separate and slag-in ends up remaining inside of the weld metal at the final solidified part, that is, the top of the bead. 
     Furthermore, the inventors analyzed the trends in occurrence of this top slag-in by types of tensile strength of the weld metal using the basicity B of the flux calculated by formula (1) shown below. As a result, as shown in  FIG. 3 , they learned that in weld metal of 800 MPa or higher strength, when the basicity B is 1.2 or more, top slag-in does not occur: 
       B=6.05N[CaO]+4.0N[MgO]+5.1N[CaF 2 ]+4.8N[MnO]−0.2N[Al 2 O 3 ]−6.31N[SiO 2 ]  (1)
 
     wherein, N[k] indicates the molar percentage of the ingredient “k”. 
     In this regard, there is a correlation between the basicity B of flux and the amount of oxygen in the weld metal. In surveys conducted by the inventors on various fluxes as well, a relationship such as in  FIG. 4  was observed. That is, even when the basicity B expressed by formula (1) is 1.2 or more and top slag-in does not occur, sometimes the amount of oxygen in the weld metal becomes less than 0.018%. In that case, as shown in  FIG. 1 , the toughness at a low temperature is expected to become lower. From this, it is necessary that the basicity B be 1.2 or more and that the amount of oxygen in the weld metal be stable at 0.018% or more. 
     Furthermore, the inventors investigated in detail the effects of the individual ingredients contained in flux on the occurrence of top slag-in. As a result, as shown in  FIG. 5 , they learned that if the tensile strength of the weld metal is 800 MPa or more, the amount of SiO 2  of the flux becomes over 20% and top slag-in occurs. Furthermore, they learned that the amount of CaF 2  also has an effect on the trend in occurrence of top slag-in and, as shown in  FIG. 6 , that in high strength weld metal, if the amount of CaF 2  in the flux is less than 30%, top slag-in occurs. 
     In this way, in high strength weld metal, to avoid top slag-in, it is necessary to adjust the basicity B obtained by formula (1) and the amount of SiO 2  and amount of CaF 2  in the flux. Furthermore, the basicity B and the amount of SiO 2  or the amount of CaF 2  in the flux have to be satisfied in a range where top slag-in can be avoided while controlling the amount of oxygen in the weld metal to 0.018% to 0.035%. 
     MnO, by being added to the flux in a suitable quantity, enables the amount of oxygen of the weld metal to be increased without inviting the occurrence of top slag-in. By adding MnO in a suitable quantity, it is possible to obtain a basicity B of 1.2 or more and an amount of oxygen of a stable 0.018% or more. 
     Further, in welding of high strength weld metal, if the flux is crystalline, a trend is seen where the surface area of the flux increases, adsorption of moisture becomes greater, and low temperature cracking easily occurs after welding.  FIG. 7 ,  FIG. 8 , and  FIG. 9  show the results when holding flux in an environment of a temperature of 25° C. and a humidity of 60% for 48 hours, then drying it at 500° C. for 24 hours and measuring the loss of mass. Note that, in  FIG. 7 ,  FIG. 8 , and  FIG. 9 , the reduced mass is attributed to the moisture which had been adsorbed at the flux, the ratio of the reduced mass to the original mass is obtained, and that ratio is used as the amount of moisture adsorption.  FIG. 7  shows the relationship between the amount of Al 2 O 3  in the flux and the amount of moisture adsorption.  FIG. 8  shows the relationship between the amount of SiO 2  in the flux and the amount of moisture adsorption.  FIG. 9  shows the relationship between the basicity B of flux and the amount of moisture adsorption. The basicity B shown in  FIG. 9  is the value calculated by the above formula (1). 
     As shown in  FIG. 7 , when the Al 2 O 3  in the flux is 15% or less, the amount of moisture adsorption rapidly increases. Further, as shown in  FIG. 8 , when the amount of SiO 2  in the flux is less than 10%, the amount of moisture adsorption similarly rapidly increases. Furthermore, as shown in  FIG. 9 , even if the basicity B determined by the composition of the flux is over 3.2, the amount of moisture adsorption rapidly increases. 
     This is because if the Al 2 O 3  in the flux is 15% or less or the SiO 2  is less than 10% or the basicity B is over 3.2, the surface of the flux becomes crystalline and moisture is more easily adsorbed. In welding of high strength steel, the moisture in the flux causes the diffusive hydrogen of the weld metal to increase and makes low temperature cracking after welding easier to occur. Therefore, in flux used for welding of high strength steel, the Al 2 O 3  in the flux has to be over 15%, the amount of SiO 2  has to be 10% or more, and the basicity B has to be 3.2 or less. 
     That is, the gist of the present invention is as follows:
     (1) Melt type high basicity flux for submerged arc welding use which is used when welding the seams of UO pipe for pipeline use having a tensile strength of a base material of 800 MPa to 1200 MPa, said melt type high basicity flux for submerged arc welding use characterized by containing, by mass%, CaO: 5.0% to 25.0%, MgO: 1.0% to 5.0%, Al 2 O 3 : over 15.0% to 30.0%, CaF 2 : 30.0% to 50.0%, SiO 2 : 10.0% to 20.0%, and MnO: 0.5% to 15.0%, having a balance of unavoidable impurities, and having a basicity B obtained by formula (1) of 1.2 to 3.2:   

       B=6.05N [CaO]+4.0N[MgO]+5.1N [CaF 2 ]+4.8N [MnO]−0.2N [Al 2 O 3 ]−6.31N [SiO 2 ]  (1)
 
     where, N[k] indicates the molar percentage of the ingredient “k”.
     (2) Melt type high basicity flux for submerged arc welding use as set forth in (1) characterized by further containing, by mass%, one or both of Li 2 O and K 2 O in a total of 0.2% to 5.0%.   

     According to the melt type high basicity flux for submerged arc welding use of the present invention, it is possible to obtain high strength weld metal resistant to low temperature cracking after welding, free of top slag-in, and excellent in low temperature toughness, so this can be used when welding the seams of UO pipe for pipeline use having a tensile strength of the base material of 800 MPa to 1200 MPa. 
     In this regard, the invention described in PLT 1, as shown in the examples, does not contain Ni, Mo, Cr, etc., so, for example, covers low strength steel with a tensile strength of the base material of the 500 MPa to 600 MPa class. Therefore, in PLT 1, the problem of the present invention of the occurrence of top slag-in does not become an issue. Further, with the flux of the composition disclosed in the examples of PLT 1, there are the problems that the amount of Al 2 O 3  is small, the flux becomes crystalline, and adsorption of moisture is great. In particular, this cannot be applied to high strength weld metal with a high low temperature cracking sensitivity. 
     The invention described in PLT 2, as shown in the examples, makes the weld metal a weld metal of a low C with little content of Mo, Ni, or Cr and a strength of the 700 MPa class. Problems arise if applying this as is to weld metal with a further higher strength. For example, with the flux of the invention described in PLT 3 used for the invention described in PLT 2, the problems arise that the amount of SiO 2  is small, the flux becomes crystalline, and adsorption of moisture is great. 
     Further, the flux described in PLT 3, as described in Table 3 of the examples of PLT 3, does not contain Ni, Cr, or Mo, for example, is flux for application to low strength steel with a tensile strength of the base material of the 500 MPa to 600 MPa class. Therefore, the flux described in PLT 3 does not give high strength weld metal and is not predicated on application to high strength steel with a tensile strength of the base material of the 800 MPa class to 1200 MPa class. 
     The invention described in PLT 4 covers the X65 class in its application as shown in the examples. Low strength weld metal of a weld metal strength of for example 600 MPa to 700 MPa or so is covered. Therefore, with welding using the flux disclosed in PLT 4 and for obtaining a high strength weld metal, since the amount of SiO 2  is large and the amount of CaF 2  is small, there is the problem of the occurrence of top slag-in. 
     The invention described in PLT 5, like the invention described in PLT 1, covers low strength steel pipe. With the flux of the composition disclosed in the examples, there are the problems of the flux becoming crystalline and adsorption of moisture being great, so this cannot be applied to high strength weld metal particularly high in low temperature cracking sensitivity. 
     The flux disclosed in PLT 6 is applied to welding predicated on heat treatment after welding. It assumes multilayer welding as disclosed in the examples. Therefore, the invention described in this literature enables toughness to be easily secured by the reheating effect due to later welding. This cannot be applied in single-layer inner circumference and outer circumference welding, upon which the present invention is predicated, where the reheating effect cannot be expected. Furthermore, the invention described in that literature has a small amount of SiO 2 , has advanced crystallization of the flux, and has larger adsorption of moisture as well, so is unsuitable for high strength weld metal. Further, the flux described in PLT 6 is a bond type flux. This is a separate type of flux differing in manufacturing process from the melt flux of the present invention. 
     Advantageous Effects of Invention 
     According to the melt type high basicity flux for submerged arc welding use of the present invention, it is possible to obtain high strength weld metal resistant to low temperature cracking after welding, free of top slag-in, and excellent in low temperature toughness. More particularly, when welding the seams of UO pipe for pipeline use having a tensile strength of the base material of 800 MPa to 1200 MPa, by using the melt flux for submerged arc welding use of the present invention, it is possible to obtain a weld bead with a good bead shape and free of the “top slag-in” defect inside the weld bead. Furthermore, due to the system of ingredients of the present invention, even with a composition of flux in which top slag-in does not occur, the amount of oxygen of the weld metal can be optimized and weld metal having excellent low temperature toughness can be obtained. Due to this, it becomes possible to easily produce sound, high strength weld metal. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view of top slag-in. 
         FIG. 2  is a view showing a relationship between an amount of oxygen of weld metal and toughness. 
         FIG. 3  is a view showing a relationship between a basicity B of flux, a tensile strength of weld metal, and trend in occurrence of top slag-in. 
         FIG. 4  is a view showing a relationship between a basicity B of flux and an amount of oxygen in weld metal. 
         FIG. 5  is a view showing a relationship between an amount of SiO 2  in flux, a tensile strength of weld metal, and trend in occurrence of top slag-in. 
         FIG. 6  is a view showing a relationship between an amount of CaF 2  in flux, a tensile strength of weld metal, and trend in occurrence of top slag-in. 
         FIG. 7  is a view showing a relationship between an amount of Al 2 O 3  in flux and moisture adsorption in flux. 
         FIG. 8  is a view showing a relationship between an amount of SiO 2  in flux and moisture adsorption in flux. 
         FIG. 9  is a view showing a relationship between a basicity B of flux and moisture adsorption in flux. 
         FIG. 10  is a schematic view of bottom slag-in. 
         FIG. 11  is a view showing a relationship between a basicity B of flux containing MnO and an amount of oxygen in weld metal. 
         FIG. 12  is a view showing a groove shape used in the examples and comparative examples. 
         FIG. 13  is a view showing the procedure for obtaining a mechanical strength test piece. 
     
    
    
     DESCRIPTION OF EMBODIMENTS  
     Below, the reasons for limitation of the ingredients of the present invention will be explained. 
     [CaO: 5.0% to 25.0%] 
     CaO has an effect on the shape of the weld bead of the weld metal. If less than 5.0%, the softening and melting temperature becomes higher and pock marks are caused due to the blocked dispersion of the molten metal gas and otherwise poor appearance of the weld bead surface is caused. On the other hand, if excessive, the viscosity becomes high and the excessive weld metal becomes higher. Further, the strippability of the slag also falls. Therefore, the upper limit was made 25% or less. 
     Further, CaO is an ingredient having an effect on the basicity B. If less than 5.0%, the basicity B becomes smaller and occurrence of top slag-in is aggravated. If over 25.0%, the basicity B becomes high, the flux becomes crystalline, and the hydrogen cracking sensitivity rises. 
     [MgO: 1.0% to 5.0%] 
     MgO has an effect on the viscosity of the molten slag. If less than 1.0%, the viscosity of the slag becomes too low and pock mark shaped defects are formed on the surface. On the other hand, if over 5.0%, the viscosity of the slag becomes higher and undercut occurs. 
     Al 2 O 3 : over 15.0% to 30.0%] 
     Al 2 O 3  is one of the important ingredients in the ingredients forming the slag, so as added as an ingredient forming the flux. However, if 15.0% or less, as shown in  FIG. 7 , the flux strongly tends to become crystalline and easily adsorbs moisture. As a result, the amount of hydrogen in the weld metal increases and the low temperature cracking sensitivity rises. On the other hand, addition of Al 2 O 3  has the action of lowering the basicity B, so excessive addition aggravates top slag-in. Further, Al 2 O 3  has an effect on the bead shape. If excessively added, undercut occurs or a sharply upward curved projection forms at the top of the weld bead. From these viewpoints, the upper limit was made 30.0% or less. 
     [CaF 2 : 30.0% to 50.0%] 
     CaF 2  is an ingredient which has an effect on the trend in occurrence of top slag-in separate from the basicity B. If CaF 2  is less than 30.0%, top slag-in occurs, so the lower limit was made 30.0% or more. On the other hand, if excessively added, the arc becomes unstable and pock mark shaped patterns are formed on the bead surface. Therefore, the upper limit was made 50.0%. Further, CaF 2  lowers the viscosity and softening and melting temperature of the flux, so prevents the weld bead from becoming an excessively bulging shape and makes the weld bead surface smooth. To obtain this effect, 30.0% or more, preferably over 35%, more preferably over 37%, has to be added. 
     [SiO 2 : 10.0% to 20.0%] 
     If excessively adding SiO 2  over 20.0%, top slag-in occurs. Therefore, the upper limit was made 20.0% or less. On the other hand, as shown in  FIG. 8 , if less than 10.0%, the flux becomes crystalline and easily adsorbs moisture. Therefore, the amount of hydrogen in the weld metal is increased and as a result the low temperature cracking sensitivity of the weld metal becomes higher. Therefore, the lower limit was made 10.0% or more. 
     At the same time, SiO 2  improves the contact angle of the weld toe with the base material, reduces sticking of slag at the weld toe, etc. to improve the affinity with the base material at the weld toe. Furthermore, it has the effect of smoothing the weld bead surface and preventing extreme bulging of the bead so as to improve the bead shape. To obtain this effect, 10.0% or more is required. 
     [MnO: 0.5% to 15.0%] 
     MnO, by addition of a suitable quantity to the flux, enables an increase in the amount of oxygen in the weld metal without inviting the occurrence of top slag-in.  FIG. 11  shows the relationship between the basicity B of flux to which MnO has been added and the amount of oxygen in the weld metal. By adding MnO in a suitable quantity as illustrated, the basicity B becomes the 1.2 or more shown in  FIG. 3  and an amount of oxygen of 0.018% or more can be stably obtained. To obtain this effect, 0.5% or more has to be added. However, if adding over 15.0%, bottom slag-in SL easily occurs near the weld line or on the weld line at the bottom of the weld bead as shown in  FIG. 10 . For this reason, the upper limit was made 15.0% or less. Further, if adding MnO over 15.0%, when the basicity is low, the amount of oxygen in the weld metal becomes excessive and the toughness may fall. Therefore, the upper limit was made 15.0% or less. 
     [Basicity B obtained by formula (1): 1.2 to 3.2] 
       B=6.05N[CaO]+4.0N[MgO]+5.1N[CaF 2 ]+4.8N[MnO]−0.2N[Al 2 O 3 ]−6.31N[SiO 2 ]  (1)
 
     where, N[k] indicates the molar percentage of the ingredient “k” 
     If analyzing the relationship between the basicity B and the occurrence of top slag-in, as shown in  FIG. 3 , if the basicity B is 1.2 or more, top slag-in no longer occurs. On the other hand, from the viewpoint of top slag-in, there is no upper limit to the basicity B, but as shown in  FIG. 9 , if the basicity B is over 3.2, the flux strongly tends to become crystalline and therefore easily adsorbs moisture. As a result, the amount of hydrogen in the weld metal increases and the low temperature cracking sensitivity rises. From such a viewpoint, the upper limit of the basicity B was made 3.2 or less. 
     Further, if the basicity B is less than 1.2, due to the synergistic effect with the addition of MnO, the oxygen becomes excessive and the possibility arises of the toughness of the weld metal falling, so the lower limit was made 1.2 or more. 
     Tensile strength of base material: 800 MPa to 1200 MPa] 
     In the present invention, the tensile strength of the base material is made 800 MPa to 1200 MPa, preferably is over 980 MPa to 1200 MPa. 
     If the tensile strength of the base material is less than 800 MPa, the strength is low for the field in which the present invention is applied, that is, UO pipe, so the problem of top slag-in does not arise. Further, if the tensile strength of the base material exceeds 1200 MPa, sometimes a sufficient toughness cannot be secured. 
     The chemical composition of the base material used, from the viewpoints of strength and toughness, is preferably, by mass%, C: 0.03% to 0.12%, Si: 0.5% or less, Mn: 1.2% to 2.5%, Ni: 2.0% or less, Mo: 0.6% or less, Ti: 0.030% or less, Al: 0.07% or less, P: 0.015% or less, S: 0.003% or less, Nb: 0.0015% to 0.1%, and N: 0.008% or less. 
     Further, the chemical composition of the weld wire used, from the viewpoints of the strength and toughness of the weld metal and the manufacturability of the wire, is preferably, by mass%, C: 0.02% to 0.15%, Si: 0.5% or less, Mn: 1.0% to 2.5%, Ni: 10.0% or less, Mo: 4.0% or less, Ti: 0.030% or less, 0: 0.008% or less, Al: 0.05% or less, P: 0.015% or less, S: 0.01% or less, and N: 0.008% or less. 
     One of both of Li 2 O and K 2 O in total: 0.2% to 5.0%] 
     The total amount of one or both of Li 2 O and K 2 O is preferably 0.2% to 5.0%, more preferably over 2% to 5.0%. 
     MnO increases the reignition voltage of the arc. Therefore, the voltage raised at the time when the arc is reignited during welding causes the surface of the molten pool to vibrate and as a result tends to make the wave pattern on the surface of the weld bead, that is, the ripple lines, uneven. This is not a problem in terms of mechanical properties and is not recognized as a defect, but is not preferable in terms of appearance and, when coating the surface after welding, becomes a cause of lowered work efficiency. As opposed to this, Li 2 O and K 2 O have the effect of calming this and eliminating unevenness of ripple lines. To obtain this effect, the total amount of one or both of Li 2 O and K 2 O has to be 0.2% or more. On the other hand, if adding over 5.0%, the slag strippability falls and sticking of slag occurs. Therefore, the upper limit was made 5.0% or less. 
     Further, in flux for submerged arc welding use, the amount of nitrogen in the weld metal is decreased by adding CaCO 3  to the flux in an amount of 0.05% to 2.0% or the strength of the weld metal is adjusted by adding B 2 O 3  in an amount of 0.05% to 2.0%, but these have no effect at all on the action targeted by the present invention of the effect of avoiding top slag-in. These ingredients may be added in accordance with need. 
     EXAMPLES  
     Next, examples and comparative examples will be used to specifically explain the effects of the present invention. 
     In the examples and comparative examples, the inventors used three-electrode submerged arc welding under the welding conditions shown in Table 1, combined wires of different compositions and strength levels shown in Table 2, formed grooves of cross-sectional shapes shown in  FIG. 12  in base materials having the chemical compositions and tensile strengths shown in Table 3 and plate thicknesses of 20 mm and lengths of 1500 mm, and performed welding using the fluxes of the ingredients shown in Table 5 or Table 6. The combinations of wires used in the examples and comparative examples are shown in Table 4. 
     Note that the groove shown in  FIG. 12  is a cross-sectional V-shaped groove provided between base materials for welding in the examples and comparative examples. As shown in  FIG. 12 , the depth of the groove is 9 mm, and the angle formed by the V-shaped groove is 80°. Further, in the examples and comparative examples, the flux used was kept in 20° C., 60% humidity environment for 24 hours, then, before use, dried in a 150° C. oven for 2 hours. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 1st 
                 2nd 
                 3rd 
                 Welding 
                 Input 
               
               
                 No. of 
                   
                 elec- 
                 elec- 
                 elec- 
                 speed 
                 heat 
               
               
                 electrodes 
                   
                 trode 
                 trode 
                 trode 
                 (m/min) 
                 (kJ/mm) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 3-electrode 
                 Current (A) 
                 1000 
                 800 
                 600 
                 1.6 
                 3.0 
               
               
                 welding 
                 Voltage (V) 
                 32 
                 32 
                 36 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Wire 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 diameter 
                   
                 Strength 
               
               
                 (mm) 
                 Wire 
                 level 
                 C 
                 Si 
                 Mn 
                 Ni 
                 Cr 
                 Mo 
                 Ti 
                 O 
                 N 
                 Al 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 4.0 
                 A 
                  800 MPa class 
                 0.11 
                 0.21 
                 2.3 
                 0.28 
                 0.00 
                 0.00 
                 0.010 
                 0.005 
                 0.006 
                 0.005 
               
               
                   
                 B 
                  900 MPa class 
                 0.12 
                 0.15 
                 1.9 
                 1.30 
                 0.80 
                 0.90 
                 0.010 
                 0.005 
                 0.005 
                 0.018 
               
               
                   
                 C 
                 1000 MPa class 
                 0.07 
                 0.16 
                 1.9 
                 4.20 
                 1.50 
                 1.70 
                 0.025 
                 0.005 
                 0.004 
                 0.004 
               
               
                   
                 D 
                 1200 MPa class 
                 0.06 
                 0.25 
                 1.7 
                 5.20 
                 2.10 
                 2.21 
                 0.022 
                 0.005 
                 0.002 
                 0.002 
               
               
                   
                 E 
                 1400 MPa class 
                 0.04 
                 0.38 
                 1.7 
                 7.60 
                 2.69 
                 3.79 
                 0.005 
                 0.005 
                 0.002 
                 0.002 
               
               
                   
                 F 
                  600 MPa class 
                 0.07 
                 0.21 
                 1.8 
                 0.00 
                 0.00 
                 0.00 
                 0.005 
                 0.005 
                 0.007 
                 0.004 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                   
                 Tensile 
                   
               
               
                 Base 
                 strength 
                 Chemical composition (mass %) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 material 
                 (MPs) 
                 C 
                 Si 
                 Mn 
                 Ni 
                 Cr 
                 Mo 
                 Nb 
                 Ti 
                 N 
                 Al 
               
               
                   
               
               
                 i 
                 965 
                 0.05 
                 0.21 
                 1.9 
                 1.0 
                 0.6 
                 0.3 
                 0.050 
                 0.013 
                 0.004 
                 0.018 
               
               
                 ii 
                 750 
                 0.08 
                 0.20 
                 1.7 
                 0.0 
                 0.6 
                 0.0 
                 0.002 
                 0.020 
                 0.006 
                 0.010 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                   
                 TABLE 4 
               
             
            
               
                   
                   
               
               
                   
                 Wire 
                 Example of weld 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Base 
                 1st 
                 2nd 
                 3rd 
                 metal tensile 
               
               
                 Symbol 
                 material 
                 electrode 
                 electrode 
                 electrode 
                 strength (MPa) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 a 
                 i 
                 B 
                 A 
                 C 
                 828 
               
               
                 b 
                 i 
                 C 
                 B 
                 B 
                 892 
               
               
                 c 
                 i 
                 C 
                 C 
                 B 
                 958 
               
               
                 d 
                 i 
                 C 
                 C 
                 C 
                 1007 
               
               
                 e 
                 i 
                 D 
                 E 
                 D 
                 1150 
               
               
                 f 
                 i 
                 E 
                 E 
                 E 
                 1246 
               
               
                 g 
                 ii 
                 B 
                 F 
                 F 
                 732 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
             
            
               
                   
                   
                   
                   
                   
                 Amount of 
                 Strength 
               
               
                   
                   
                 Combination 
                   
                   
                 oxygen in 
                 of weld 
               
               
                   
                 Base 
                 of weld 
                 Ingredients of flux 
                   
                 weld metal 
                 metal 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Symbol 
                 material 
                 materials 
                 CaO 
                 MgO 
                 Al 2 O 3   
                 CaF 2   
                 SiO 2   
                 MnO 
                 Li 2 O 
                 K 2 O 
                 Others 
                 Basicity B 
                 (%) 
                 (MPa) 
               
               
                   
               
               
                 Ex. 
                 1 
                 i 
                 a 
                 5.5 
                 4.2 
                 29.5 
                 48.5 
                 11.2 
                 0.6 
                 0.0 
                 0.0 
                 0.5 
                 2.3 
                 0.0205 
                 825 
               
               
                 Ex. 
                 2 
                 i 
                 b 
                 12.5 
                 4.5 
                 18.0 
                 41.4 
                 18.0 
                 5.0 
                 0.0 
                 0.0 
                 0.6 
                 2.1 
                 0.0220 
                 892 
               
               
                 Ex. 
                 3 
                 i 
                 b 
                 15.1 
                 2.6 
                 19.5 
                 46.9 
                 12.7 
                 2.5 
                 0.0 
                 0.0 
                 0.7 
                 2.7 
                 0.0195 
                 901 
               
               
                 Ex. 
                 4 
                 i 
                 a 
                 5.2 
                 3.7 
                 27.9 
                 42.1 
                 19.6 
                 0.6 
                 0.0 
                 0.0 
                 0.9 
                 1.2 
                 0.0222 
                 823 
               
               
                 Ex. 
                 5 
                 i 
                 c 
                 5.3 
                 2.1 
                 28.3 
                 35.2 
                 19.5 
                 9.2 
                 0.0 
                 0.0 
                 0.4 
                 1.2 
                 0.0325 
                 967 
               
               
                 Ex. 
                 6 
                 i 
                 c 
                 5.5 
                 3.8 
                 28.5 
                 40.5 
                 19.1 
                 2.2 
                 0.0 
                 0.0 
                 0.4 
                 1.3 
                 0.0252 
                 972 
               
               
                 Ex. 
                 7 
                 i 
                 c 
                 11.8 
                 3.2 
                 20.4 
                 43.8 
                 18.2 
                 2.1 
                 0.0 
                 0.0 
                 0.5 
                 1.9 
                 0.0222 
                 968 
               
               
                 Ex. 
                 8 
                 i 
                 c 
                 20.5 
                 2.9 
                 20.4 
                 39.0 
                 10.2 
                 6.1 
                 0.0 
                 0.0 
                 0.9 
                 3.1 
                 0.0185 
                 971 
               
               
                 Ex. 
                 9 
                 i 
                 d 
                 10.5 
                 3.2 
                 26.1 
                 33.5 
                 17.1 
                 9.2 
                 0.0 
                 0.0 
                 0.4 
                 1.8 
                 0.0258 
                 1002 
               
               
                 Ex. 
                 10 
                 i 
                 a 
                 17.7 
                 1.2 
                 16.2 
                 31.5 
                 18.2 
                 14.1 
                 0.0 
                 0.0 
                 1.1 
                 2.2 
                 0.0242 
                 831 
               
               
                 Ex. 
                 11 
                 i 
                 d 
                 11.8 
                 4.0 
                 22.2 
                 48.4 
                 12.5 
                 0.6 
                 0.0 
                 0.0 
                 0.5 
                 2.6 
                 0.0201 
                 998 
               
               
                 Ex. 
                 12 
                 i 
                 c 
                 16.1 
                 4.2 
                 20.1 
                 40.8 
                 16.2 
                 2.1 
                 0.0 
                 0.0 
                 0.5 
                 2.3 
                 0.0195 
                 974 
               
               
                 Ex. 
                 13 
                 i 
                 d 
                 10.7 
                 4.1 
                 22.9 
                 42.1 
                 18.0 
                 0.8 
                 0.0 
                 0.0 
                 1.4 
                 1.8 
                 0.0201 
                 1008 
               
               
                 Ex. 
                 14 
                 i 
                 b 
                 14.7 
                 3.2 
                 21.5 
                 43.3 
                 14.8 
                 2.1 
                 0.0 
                 0.0 
                 0.4 
                 2.4 
                 0.0235 
                 902 
               
               
                 Ex. 
                 15 
                 i 
                 e 
                 24.3 
                 2.2 
                 20.5 
                 31.7 
                 15.3 
                 5.5 
                 0.0 
                 0.0 
                 0.5 
                 2.6 
                 0.0222 
                 1162 
               
               
                 Ex. 
                 16 
                 i 
                 d 
                 12.8 
                 1.2 
                 24.5 
                 37.5 
                 15.2 
                 8.2 
                 0.0 
                 0.0 
                 0.6 
                 2.1 
                 0.0228 
                 1005 
               
               
                 Ex. 
                 17 
                 i 
                 e 
                 11.1 
                 3.2 
                 20.1 
                 43.1 
                 12.1 
                 9.8 
                 0.0 
                 0.0 
                 0.6 
                 2.7 
                 0.0242 
                 1150 
               
               
                 Comp. ex. 
                 1 
                 i 
                 a 
                 11.4 
                 2.8 
                 19.1 
                 51.1 
                 14.9 
                 0.2 
                 0.0 
                 0.0 
                 0.5 
                 2.4 
                 0.0175 
                 826 
               
               
                 Comp. ex. 
                 2 
                 i 
                 c 
                 19.2 
                 2.9 
                 20.5 
                 45.5 
                 11.1 
                 0.3 
                 0.0 
                 0.0 
                 0.5 
                 3.0 
                 0.0165 
                 965 
               
               
                 Comp. ex. 
                 3 
                 i 
                 b 
                 18.1 
                 1.3 
                 15.2 
                 31.5 
                 17.0 
                 16.5 
                 0.0 
                 0.0 
                 0.4 
                 2.4 
                 0.0275 
                 893 
               
               
                 Comp. ex. 
                 4 
                 i 
                 d 
                 5.2 
                 1.3 
                 25.5 
                 30.6 
                 19.5 
                 17.5 
                 0.0 
                 0.0 
                 0.4 
                 1.3 
                 0.0365 
                 1007 
               
               
                 Comp. ex. 
                 5 
                 i 
                 b 
                 7.8 
                 4.8 
                 18.3 
                 27.6 
                 28.5 
                 12.5 
                 0.0 
                 0.0 
                 0.5 
                 0.7 
                 0.0385 
                 898 
               
               
                 Comp. ex. 
                 6 
                 i 
                 d 
                 14.2 
                 2.6 
                 15.5 
                 48.5 
                 9.2 
                 9.5 
                 0.0 
                 0.0 
                 0.5 
                 3.4 
                 0.0215 
                 989 
               
               
                 Comp. ex. 
                 7 
                 i 
                 c 
                 3.9 
                 4.2 
                 28.5 
                 49.1 
                 8.5 
                 5.2 
                 0.0 
                 0.0 
                 0.6 
                 2.7 
                 0.0224 
                 962 
               
               
                 Comp. ex. 
                 8 
                 i 
                 b 
                 8.4 
                 3.0 
                 18.3 
                 52.5 
                 12.1 
                 5.2 
                 0.0 
                 0.0 
                 0.5 
                 2.7 
                 0.0212 
                 901 
               
               
                 Comp. ex. 
                 9 
                 i 
                 d 
                 13.7 
                 4.1 
                 15.3 
                 52.5 
                 8.5 
                 5.3 
                 0.0 
                 0.0 
                 0.6 
                 3.4 
                 0.0195 
                 1001 
               
               
                 Comp. ex. 
                 10 
                 i 
                 c 
                 27.5 
                 3.2 
                 17.2 
                 35.0 
                 11.2 
                 5.5 
                 0.0 
                 0.0 
                 0.4 
                 3.3 
                 0.0201 
                 964 
               
               
                 Comp. ex. 
                 11 
                 i 
                 b 
                 5.0 
                 4.8 
                 35.0 
                 31.1 
                 19.5 
                 4.1 
                 0.0 
                 0.0 
                 0.5 
                 0.9 
                 0.0324 
                 904 
               
               
                 Comp. ex. 
                 12 
                 i 
                 d 
                 10.2 
                 3.2 
                 20.1 
                 38.2 
                 22.5 
                 5.3 
                 0.0 
                 0.0 
                 0.5 
                 1.4 
                 0.0284 
                 1008 
               
               
                 Comp. ex. 
                 13 
                 i 
                 f 
                 20.3 
                 3.6 
                 23.2 
                 28.5 
                 18.5 
                 5.3 
                 0.0 
                 0.0 
                 0.6 
                 2.0 
                 0.0265 
                 1245 
               
               
                 Comp. ex. 
                 14 
                 i 
                 c 
                 17.2 
                 6.4 
                 14.2 
                 40.2 
                 16.2 
                 5.3 
                 0.0 
                 0.0 
                 0.5 
                 2.6 
                 0.0254 
                 952 
               
               
                 Comp. ex. 
                 15 
                 i 
                 a 
                 11.0 
                 0.5 
                 23.2 
                 49.5 
                 11.0 
                 5.3 
                 0.0 
                 0.0 
                 0.5 
                 2.7 
                 0.0242 
                 825 
               
               
                 Comp. ex. 
                 16 
                 ii 
                 g 
                 10.2 
                 3.2 
                 20.1 
                 38.2 
                 22.5 
                 5.3 
                 0.0 
                 0.0 
                 0.5 
                 1.4 
                 0.0290 
                 735 
               
               
                 Comp. ex. 
                 17 
                 ii 
                 g 
                 20.3 
                 3.6 
                 23.1 
                 28.4 
                 18.8 
                 5.3 
                 0.0 
                 0.0 
                 0.5 
                 1.9 
                 0.0260 
                 740 
               
               
                 Comp. ex. 
                 18 
                 ii 
                 g 
                 5.0 
                 4.8 
                 35.0 
                 31.1 
                 19.5 
                 4.1 
                 0.0 
                 0.0 
                 0.5 
                 0.9 
                 0.0320 
                 738 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 −30° C. weld 
                 No. of 
               
               
                   
                 metal 
                 top 
               
               
                   
                 toughness (J) 
                 slag-ins/ 
                   
                 Bead surface 
                 Pock 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Symbol 
                 Average 
                 Lowest 
                 1 m 
                 Bead shape 
                 Undercut 
                 condition 
                 marks 
                 Others 
               
               
                   
                   
               
               
                   
                 Ex. 
                 1 
                 184 
                 178 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
               
               
                   
                 Ex. 
                 2 
                 176 
                 166 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
               
               
                   
                 Ex. 
                 3 
                 174 
                 168 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
               
               
                   
                 Ex. 
                 4 
                 188 
                 176 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 B 2 O 3 : 0.2% added 
               
               
                   
                 Ex. 
                 5 
                 158 
                 142 
                 0 
                 Good 
                 None 
                 Minor relief shapes 
                 None 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 not defects 
               
               
                   
                 Ex. 
                 6 
                 158 
                 143 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
               
               
                   
                 Ex. 
                 7 
                 161 
                 149 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
               
               
                   
                 Ex. 
                 8 
                 145 
                 111 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 CaCO 3 : 0.5% added 
               
               
                   
                 Ex. 
                 9 
                 148 
                 139 
                 0 
                 Good 
                 None 
                 Relief shapes not 
                 None 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 defects 
               
               
                   
                 Ex. 
                 10 
                 189 
                 175 
                 0 
                 Good 
                 None 
                 Relief shapes not 
                 None 
                 B 2 O 3 : 0.7% added 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 defects. 
               
               
                   
                 Ex. 
                 11 
                 158 
                 146 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
               
               
                   
                 Ex. 
                 12 
                 148 
                 129 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
               
               
                   
                 Ex. 
                 13 
                 155 
                 143 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 CaCO 3 : 1.0% added 
               
               
                   
                 Ex. 
                 14 
                 176 
                 164 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
               
               
                   
                 Ex. 
                 15 
                 128 
                 111 
                 0 
                 Good 
                 None 
                 Relief shapes not 
                 None 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 defects 
               
               
                   
                 Ex. 
                 16 
                 145 
                 139 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
               
               
                   
                 Ex. 
                 17 
                 124 
                 116 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
               
               
                   
                 Comp. ex. 
                 1 
                 128 
                 91 
                 0 
                 Good 
                 None 
                 Good 
                 Yes 
               
               
                   
                 Comp. ex. 
                 2 
                 115 
                 82 
                 0 
                 Good 
                 None 
                 Good 
                 None 
               
               
                   
                 Comp. ex. 
                 3 
                 152 
                 135 
                 0 
                 Good 
                 None 
                 Relief shapes not 
                 None 
                 Bottom slag-in occurs 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 defects 
               
               
                   
                 Comp. ex. 
                 4 
                 112 
                 84 
                 0 
                 Good 
                 None 
                 Relief shapes not 
                 None 
                 Bottom slag-in occurs 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 defects 
               
               
                   
                 Comp. ex. 
                 5 
                 109 
                 79 
                 86 
                 Good 
                 None 
                 Relief shapes on 
                 None 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 bead surface 
               
               
                   
                 Comp. ex. 
                 6 
                 135 
                 125 
                 0 
                 Good 
                 None 
                 Good 
                 None 
                 Low temperature 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 cracking after 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 welding 
               
               
                   
                 Comp. ex. 
                 7 
                 154 
                 138 
                 0 
                 Good 
                 None 
                 Many pock mark like 
                 None 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 relief shapes 
               
               
                   
                 Comp. ex. 
                 8 
                 185 
                 168 
                 0 
                 Good 
                 None 
                   
                 Yes 
               
               
                   
                 Comp. ex. 
                 9 
                 132 
                 118 
                 0 
                 Good 
                 None 
                 Good 
                 Yes 
                 Low temperature 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 cracking after 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 welding 
               
               
                   
                 Comp. ex. 
                 10 
                 162 
                 136 
                 0 
                 Upward 
                 None 
                 Good 
                 None 
                 Low temperature 
               
               
                   
                   
                   
                   
                   
                   
                 curved bead 
                   
                   
                   
                 cracking after 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 welding 
               
               
                   
                 Comp. ex. 
                 11 
                 168 
                 143 
                 52 
                 Sharp upward 
                 None 
                 Relief shapes not 
                 None 
               
               
                   
                   
                   
                   
                   
                   
                 curved bead 
                   
                 defects 
               
               
                   
                 Comp. ex. 
                 12 
                 148 
                 125 
                 18 
                 Good 
                 None 
                 Good 
                 None 
               
               
                   
                 Comp. ex. 
                 13 
                 108 
                 81 
                 54 
                 Good 
                 None 
                 Relief shapes on 
                 None 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 bead surface 
               
               
                   
                 Comp. ex. 
                 14 
                 154 
                 135 
                 0 
                 Good 
                 Yes 
                 Good 
                 None 
                 Low temperature 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 cracking after 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 welding 
               
               
                   
                 Comp. ex. 
                 15 
                 185 
                 152 
                 0 
                 Good 
                 None 
                 Many pock mark like 
                 None 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 relief shapes 
               
               
                   
                 Comp. ex. 
                 16 
                 200 
                 185 
                 0 
                 Good 
                 None 
                 Good 
                 None 
               
               
                   
                 Comp. ex. 
                 17 
                 212 
                 192 
                 0 
                 Good 
                 None 
                 Relief shapes on 
                 None 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 bead surface 
               
               
                   
                 Comp. ex. 
                 18 
                 220 
                 182 
                 0 
                 Sharp upward 
                 None 
                 Relief shapes not 
                 None 
               
               
                   
                   
                   
                   
                   
                   
                 curved bead 
                   
                 defects 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
             
            
               
                   
                   
                   
                   
                   
                 Amount of 
                   
               
               
                   
                   
                   
                   
                   
                 oxygen 
                 Strength 
               
               
                   
                   
                 Combination 
                   
                   
                 in weld 
                 of weld 
               
               
                   
                 Base 
                 of weld 
                 Ingredients of flux 
                   
                 metal 
                 metal 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Symbol 
                 material 
                 materials 
                 CaO 
                 MgO 
                 Al 2 O 3   
                 CaF 2   
                 SiO 2   
                 MnO 
                 Li 2 O 
                 K 2 O 
                 Others 
                 Basicity B 
                 (%) 
                 (MPa) 
               
               
                   
               
               
                 Ex. 
                 18 
                 i 
                 c 
                 13.2 
                 3.3 
                 21.5 
                 44.2 
                 13.7 
                 2.1 
                 0.6 
                 1.0 
                 0.4 
                 2.5 
                 0.0233 
                 958 
               
               
                 Ex. 
                 19 
                 i 
                 c 
                 13.5 
                 3.2 
                 21.4 
                 44.4 
                 13.4 
                 2.1 
                 0.3 
                 1.1 
                 0.6 
                 2.5 
                 0.0232 
                 962 
               
               
                 Ex. 
                 20 
                 i 
                 c 
                 13.2 
                 3.3 
                 21.3 
                 43.1 
                 13.7 
                 2.2 
                 1.2 
                 1.1 
                 0.9 
                 2.4 
                 0.0225 
                 955 
               
               
                 Ex. 
                 21 
                 i 
                 c 
                 13.0 
                 3.1 
                 20.9 
                 42.2 
                 13.2 
                 2.1 
                 2.2 
                 2.1 
                 1.2 
                 2.5 
                 0.0222 
                 961 
               
               
                 Ex. 
                 22 
                 i 
                 c 
                 13.8 
                 3.2 
                 21.5 
                 42.3 
                 13.1 
                 2.2 
                 3.5 
                 0.0 
                 0.4 
                 2.5 
                 0.0212 
                 952 
               
               
                 Ex. 
                 23 
                 i 
                 c 
                 13.8 
                 3.4 
                 21.4 
                 44.5 
                 13.8 
                 2.2 
                 0.0 
                 0.4 
                 0.5 
                 2.5 
                 0.0222 
                 963 
               
               
                 Ex. 
                 24 
                 i 
                 c 
                 13.5 
                 3.3 
                 21.5 
                 44.9 
                 13.9 
                 2.1 
                 0.3 
                 0.0 
                 0.5 
                 2.5 
                 0.0235 
                 964 
               
               
                 Ex. 
                 25 
                 i 
                 b 
                 13.9 
                 3.9 
                 21.7 
                 35.3 
                 13.4 
                 8.2 
                 1.2 
                 1.8 
                 0.6 
                 2.5 
                 0.0265 
                 901 
               
               
                 Ex. 
                 26 
                 i 
                 b 
                 13.2 
                 3.0 
                 21.3 
                 39.1 
                 13.5 
                 8.1 
                 0.3 
                 0.0 
                 1.5 
                 2.5 
                 0.0254 
                 905 
               
               
                 Ex. 
                 27 
                 i 
                 b 
                 13.5 
                 3.3 
                 21.4 
                 38.6 
                 13.7 
                 8.2 
                 0.8 
                 0.0 
                 0.5 
                 2.5 
                 0.0258 
                 900 
               
               
                 Ex. 
                 28 
                 i 
                 b 
                 13.4 
                 3.4 
                 21.4 
                 38.8 
                 13.2 
                 8.1 
                 1.2 
                 0.0 
                 0.5 
                 2.5 
                 0.0262 
                 892 
               
               
                 Ex. 
                 29 
                 i 
                 b 
                 13.3 
                 3.4 
                 21.6 
                 37.7 
                 13.1 
                 8.5 
                 2.0 
                 0.0 
                 0.4 
                 2.5 
                 0.0266 
                 910 
               
               
                 Ex. 
                 30 
                 i 
                 b 
                 13.5 
                 3.5 
                 21.4 
                 36.9 
                 13.0 
                 8.5 
                 2.7 
                 0.0 
                 0.5 
                 2.5 
                 0.0258 
                 922 
               
               
                 Ex. 
                 31 
                 i 
                 d 
                 13.4 
                 3.3 
                 21.5 
                 32.6 
                 13.7 
                 14.5 
                 0.0 
                 0.4 
                 0.6 
                 2.5 
                 0.0254 
                 1001 
               
               
                 Ex. 
                 32 
                 i 
                 d 
                 13.1 
                 3.2 
                 21.4 
                 33.3 
                 13.3 
                 14.1 
                 0.0 
                 1.1 
                 0.5 
                 2.5 
                 0.0255 
                 1005 
               
               
                 Ex. 
                 33 
                 i 
                 d 
                 13.2 
                 3.5 
                 21.2 
                 32.1 
                 13.1 
                 14.2 
                 0.0 
                 2.1 
                 0.6 
                 2.5 
                 0.0245 
                 999 
               
               
                 Ex. 
                 34 
                 i 
                 d 
                 13.4 
                 3.3 
                 21.4 
                 37.1 
                 11.4 
                 8.5 
                 0.0 
                 4.5 
                 0.4 
                 2.7 
                 0.0259 
                 1003 
               
               
                 Ex. 
                 35 
                 i 
                 d 
                 13.1 
                 3.2 
                 21.5 
                 39.2 
                 18.9 
                 2.1 
                 0.6 
                 1.0 
                 0.4 
                 1.8 
                 0.0245 
                 997 
               
               
                 Ex. 
                 36 
                 i 
                 a 
                 12.9 
                 3.2 
                 28.5 
                 32.5 
                 18.8 
                 2.1 
                 0.3 
                 1.1 
                 0.6 
                 1.5 
                 0.0288 
                 829 
               
               
                 Ex. 
                 37 
                 i 
                 a 
                 6.3 
                 3.4 
                 29.1 
                 31.5 
                 18.8 
                 8.5 
                 2.0 
                 0.0 
                 0.4 
                 1.2 
                 0.0312 
                 831 
               
               
                 Ex. 
                 38 
                 i 
                 e 
                 9.1 
                 3.5 
                 16.5 
                 48.1 
                 11.1 
                 8.5 
                 2.7 
                 0.0 
                 0.5 
                 2.9 
                 0.0235 
                 1091 
               
               
                 Comp. ex. 
                 19 
                 i 
                 c 
                 13.2 
                 3.2 
                 21.6 
                 45.6 
                 13.9 
                 2.1 
                 0.0 
                 0.0 
                 0.4 
                 2.5 
                 0.0233 
                 968 
               
               
                 Comp. ex. 
                 20 
                 i 
                 c 
                 13.2 
                 3.2 
                 21.6 
                 45.6 
                 13.8 
                 2.1 
                 0.1 
                 0.0 
                 0.4 
                 2.5 
                 0.0243 
                 952 
               
               
                 Comp. ex. 
                 21 
                 i 
                 c 
                 13.6 
                 3.4 
                 21.7 
                 44.7 
                 13.7 
                 2.2 
                 0.0 
                 0.1 
                 0.6 
                 2.5 
                 0.0232 
                 955 
               
               
                 Comp. ex. 
                 22 
                 i 
                 c 
                 13.2 
                 3.2 
                 21.6 
                 42.7 
                 13.5 
                 2.1 
                 5.2 
                 0.0 
                 0.5 
                 2.4 
                 0.0243 
                 958 
               
               
                 Comp. ex. 
                 23 
                 i 
                 c 
                 13.8 
                 3.2 
                 21.5 
                 42.2 
                 13.4 
                 2.2 
                 0.0 
                 5.3 
                 0.4 
                 2.5 
                 0.0252 
                 961 
               
               
                 Comp. ex. 
                 24 
                 i 
                 d 
                 13.8 
                 3.2 
                 21.5 
                 42.1 
                 13.4 
                 2.2 
                 1.6 
                 3.8 
                 0.4 
                 2.5 
                 0.0232 
                 1002 
               
               
                 Comp. ex. 
                 25 
                 i 
                 d 
                 13.8 
                 3.4 
                 21.4 
                 41.5 
                 13.8 
                 2.2 
                 3.8 
                 1.8 
                 0.5 
                 2.4 
                 0.0242 
                 997 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 No. of 
                   
                   
                   
                   
               
               
                   
                 −30° C. weld 
                 top 
               
               
                   
                 metal 
                 slag- 
               
               
                   
                 toughness (J) 
                 ins/ 
                 Bead 
                 Bead surface 
                 Pock 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Symbol 
                 Average 
                 Lowest 
                 1 m 
                 shape 
                 Undercut 
                 condition 
                 marks 
                 Others 
               
               
                   
                   
               
               
                   
                 Ex. 
                 18 
                 156 
                 142 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Uniform and good 
               
               
                   
                 Ex. 
                 19 
                 165 
                 145 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Uniform and good 
               
               
                   
                 Ex. 
                 20 
                 168 
                 144 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Uniform and good 
                 B 2 O 3 : 0.2% added 
               
               
                   
                 Ex. 
                 21 
                 153 
                 138 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Uniform and good 
                 CaCO 3 : 0.7% 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 added 
               
               
                   
                 Ex. 
                 22 
                 158 
                 136 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Uniform and good 
               
               
                   
                 Ex. 
                 23 
                 152 
                 131 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Uniform and good 
               
               
                   
                 Ex. 
                 24 
                 168 
                 142 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Uniform and good 
               
               
                   
                 Ex. 
                 25 
                 178 
                 151 
                 0 
                 Good 
                 None 
                 Minor relief shapes 
                 None 
                 Uniform and good 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 not constituting 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 defects 
               
               
                   
                 Ex. 
                 26 
                 171 
                 148 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Uniform and good 
                 B 2 O 3 : 1.0% added 
               
               
                   
                 Ex. 
                 27 
                 182 
                 165 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Uniform and good 
               
               
                   
                 Ex. 
                 28 
                 176 
                 155 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Uniform and good 
               
               
                   
                 Ex. 
                 29 
                 169 
                 151 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Uniform and good 
               
               
                   
                 Ex. 
                 30 
                 165 
                 147 
                 0 
                 Good 
                 None 
                 Minor relief shapes 
                 None 
                 Uniform and good 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 not constituting 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 defects 
               
               
                   
                 Ex. 
                 31 
                 148 
                 125 
                 0 
                 Good 
                 None 
                 Relief shapes not 
                 None 
                 Uniform and good 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 constituting defects 
               
               
                   
                 Ex. 
                 32 
                 135 
                 121 
                 0 
                 Good 
                 None 
                 Relief shapes not 
                 None 
                 Uniform and good 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 constituting defects 
               
               
                   
                 Ex. 
                 33 
                 148 
                 131 
                 0 
                 Good 
                 None 
                 Relief shapes not 
                 None 
                 Uniform and good 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 constituting defects 
               
               
                   
                 Ex. 
                 34 
                 151 
                 132 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Uniform and good 
               
               
                   
                 Ex. 
                 35 
                 161 
                 143 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Uniform and good 
               
               
                   
                 Ex. 
                 36 
                 185 
                 161 
                 0 
                 Good 
                 None 
                 Relief shapes not 
                 None 
                 Uniform and good 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 constituting defects 
               
               
                   
                 Ex. 
                 37 
                 172 
                 155 
                 0 
                 Good 
                 None 
                 Relief shapes not 
                 None 
                 Uniform and good 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 constituting defects 
               
               
                   
                 Ex. 
                 38 
                 135 
                 112 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Uniform and good 
               
               
                   
                 Comp. ex. 
                 19 
                 162 
                 134 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Ripple lines 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 uneven 
               
               
                   
                 Comp. ex. 
                 20 
                 151 
                 128 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Ripple lines 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 uneven 
               
               
                   
                 Comp. ex. 
                 21 
                 168 
                 141 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Ripple lines 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 uneven 
               
               
                   
                 Comp. ex. 
                 22 
                 155 
                 133 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Slag strippability 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 falls 
               
               
                   
                 Comp. ex. 
                 23 
                 164 
                 138 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Slag strippability 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 falls 
               
               
                   
                 Comp. ex. 
                 24 
                 151 
                 134 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Slag strippability 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 falls 
               
               
                   
                 Comp. ex. 
                 25 
                 148 
                 132 
                 0 
                 Good 
                 None 
                 Smooth 
                 None 
                 Slag strippability 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 falls 
               
               
                   
                   
               
            
           
         
       
     
     The tensile strength and toughness of the weld metal were evaluated by a rod-shaped tensile test piece of weld metal and 2 mm V-notched Charpy impact test.  FIG. 13  is a plan view for explaining the procedure for obtaining a tensile test piece from welded plates, while  FIG. 13(   a ) is a cross-sectional view of the impact test piece obtained from welded plates as seen from the weld line direction.  FIG. 13(   b ) is a plan view showing a tensile test piece obtained from welded plates, while  FIG. 13(   c ) is a cross-sectional view of the tensile test piece obtained from welded plates as seen from the weld line direction. 
     The impact test piece, as shown in  FIG. 13(   a ), was taken from the center part of the weld metal in the plate thickness direction. It was worked so that the notch depth direction became the weld line direction. The impact test was repeated three times at a test temperature of −30° C. The toughness of the weld metal was evaluated using the average value of the results of impact tests repeated three times and the lowest value of the results of the three impact tests. 
     Further, the rod-shaped tensile test piece, as shown in  FIG. 13(   b ) and  FIG. 13(   c ), was taken from the center part of the weld metal in the plate thickness direction. It was worked so that the axial direction of the test piece became the weld line direction. The tensile test was repeated once at room temperature. 
     The state of internal defects in the weld metal bead was investigated by radiographic nondestructive testing over the entire thickness of 20 mm and the entire length of the weld line. After that, the plate thickness was reduced to a thickness of 7 mm from the back surface opposite to the weld surface, then radiography nondestructive testing was again performed to investigate for the presence of any top slag-in. The bead shape was evaluated by the shape of the excessive weld metal of the bead, undercut, relief shapes on the bead surface, pock marks, and the state of waves called “ripples” appearing on the weld bead surface, and cross-sectional macro observation. 
     If the surface is formed with sharp relief shapes, these may show up in radiographic testing and be recognized as defects, so judgment is made including the possibility of these being recognized as defects. 
     The concentration of oxygen in the weld metal is based on the method of assaying oxygen in metal materials described in JIS Z2613. 
     Examples and comparative examples according to claim  1  are shown in Table 5. In the flux ingredients of Table 5, “Others” are B 2 O 3  or CaCO 3  or oxides of alkali metals, P, S, and other impurities unavoidably entering from the raw materials. 
     The fluxes used in Examples 1 to 17 all satisfy the range of ingredients and range of basicity B of claim  1  of the present invention. 
     Therefore, the strength of the weld metal is a high strength of 800 MPa or more. Further, the amount of oxygen in the weld metal is optimized. The −30° C. low temperature toughness (Charpy absorption energy) exceeded 100 J in each case. Further, the basicity B of flux and the amount of SiO 2  and CaF 2  are within the ranges of the present invention, so there is also no top slag-in and the bead shape is good. 
     Further, in Examples 4 and Example 10, the amount of B in the weld metal is adjusted to improve the hardenability of the weld metal by the addition of B 2 O 3 , but this has no effect on the effect of prevention of occurrence of top slag-in in the present invention. 
     Further, in Example 8 and Example 13, the increase in nitrogen in the weld metal is prevented by the addition of CaCO 3 , but this has no effect on the effect of prevention of occurrence of top slag-in in the present invention. 
     Next, comparative examples relating to claim  1  will be explained. 
     Comparative Example 1 and Comparative Example 2 have ingredients other than MnO and a basicity B inside the ranges of the present invention, but have MnO less than the range of the present invention. Therefore, the basicity B is high and top slag-in does not occur, but the amount of oxygen is small, the toughness of the weld metal is low, and the average value of the Charpy absorption energy exceeds 100 J, but in the individual values (lowest values), data of under 100 J is obtained. From Comparative Examples 1 and 2, it is shown that by making the range of addition of MnO 0.5% or more, the amount of oxygen is stably obtained and therefore it is possible to critically prevent a drop in the toughness. 
     Comparative Example 3 and Comparative Example 4 have ingredients other than MnO and a basicity B inside the ranges of the present invention, but have MnO added exceeding the range of the present invention. Therefore, slag-in occurs at the bottom. Further, in Comparative Example 4, the oxygen is also excessive, so the toughness of the weld metal (Charpy absorption energy) falls under 100 J in individual values. From Comparative Examples 3 and 4, it is shown that by making the range of addition of MnO 15.0% or less, it is possible to critically prevent the occurrence of slag-in at the bottom. 
     Comparative Example 5 has CaF 2  in the flux less than the range of the present invention and has SiO 2  exceeding the range of the present invention. Further, the basicity B is also low. Therefore, top slag-in occurs. Further, the oxygen is excessive, so the −30° C. toughness (Charpy absorption energy) is above 100 J in average value, but values under 100 J are obtained in individual values. 
     Comparative Example 6 has an amount of SiO 2  in the flux of less than the range of the present invention and has a basicity B exceeding the range of the present invention. Therefore, moisture ends up being adsorbed during storage. Despite the flux being kept under dry conditions before use, low temperature cracking occurs after welding. To prevent low temperature cracking, further sufficient drying of the flux under a high temperature becomes necessary. Further, the SiO 2  in the flux is small, so slag sticks to the weld toes and the affinity with the base material falls. 
     Comparative Example 7 has a CaO in the flux of less than the range of the present invention. Therefore, pock mark-like relief shapes form on the bead surface and the bead appearance falls. Further, the SiO 2  in the flux is small, so slag sticks to the weld toes and the affinity with the base material falls. 
     Comparative Example 8 has an amount of CaF 2  in the flux exceeding the range of the present invention, so the arc becomes unstable and “pock mark” patterns are formed in the bead surface. 
     Comparative Example 9 also, in the same way as Comparative Example 8, has an amount of CaF 2  in the flux exceeding the range of the present invention, so the arc becomes unstable and “pock marks” patterns are formed in the bead surface. Further, the SiO 2  in the flux is small, so slag sticks to the weld toes and the affinity with the base material falls. Furthermore, in the same way, the amount of SiO 2  in the flux is less than the range of the present invention, so under the drying conditions of the flux of this experiment, the drying is insufficient and low temperature cracking occurs after welding. 
     Comparative Example 10 has a CaO in the flux exceeding the range of the present invention, so the viscosity of the molten slag is high, there is high excessive weld metal, and an upward curved bead results. Further, the strippability of the slag also falls. Further, the basicity B also exceeds the range of the present invention, so under the drying conditions of the flux of this experiment, the drying is insufficient and low temperature cracking occurs after welding. 
     Comparative Example 11 has an Al 2 O 3  in the flux exceeding the range of the present invention. Therefore, the shape of excessive weld metal of the bead becomes a triangular sharp upward curved bead. Furthermore, undercut occurred. Further, the basicity B of the flux is less than the range of the present invention. Therefore, top slag-in occurs. 
     Comparative Example 12 has an amount of SiO 2  in the flux exceeding the range of the present invention. Therefore, top slag-in occurs. 
     Comparative Example 13 has an amount of CaF 2  in the flux of less than the range of the present invention. Therefore, top slag-in occurs. Further, the weld metal has a microstructure of a martensite microstructure, has a strength of 1250 MPa or more, and has low toughness. 
     Comparative Example 14 has an MgO in the flux exceeding the range of the present invention. Therefore, the viscosity of the slag becomes high and undercut occurs. Further, the amount of Al 2 O 3  is less than the range of the present invention, so the flux becomes crystalline and moisture is easily adsorbed. Therefore, under the drying conditions of the flux of this experiment, the drying is insufficient and low temperature cracking occurs after welding. 
     Comparative Example 15 has an MgO in the flux less than the range of the present invention. Therefore, pock mark-like defects are formed at the weld bead surface. 
     Comparative Example 16 to Comparative Example 18 are examples using base materials with low strength. The combinations of weld materials also are low in strength at about 740 MPa. Therefore, Comparative Example 16 has an SiO 2  exceeding the range of the present invention, but top slag-in does not occur. Comparative Example 17 has CaF 2  of less than the range of the present invention, but top slag-in does not occur. Further, Comparative Example 18 has an amount of Al 2 O 3  exceeding the range of the present invention, but the shape of the excessive weld metal of the bead becomes a sharp upward curved bead. 
     Next, examples and comparative examples according to claim  2  are shown in Table 6. In the flux ingredients of Table 6, “Others”, in the same way as in Table 5, are B 2 O 3  or CaCO 3  or oxides of alkali metals, P, S, and other impurities unavoidably entering from the raw materials. 
     Example 18 to Example 38 all are fluxes having ingredients within the range of claim  2  of the present invention, so the −30° C. toughness (Charpy absorption energy) is good and no top slag-in occurs either. Furthermore, Li 2 O and K 2 O are suitably added, so the ripple lines on the bead surfaces are even and a beautiful bead surface is formed. 
     Further, in Example 20 and Example 26, the amount of B in the weld metal is adjusted to improve the hardenability of the weld metal by the addition of B 2 O 3 , but this has no effect on the effect of prevention of occurrence of top slag-in in the present invention. 
     Further, in Example 21, the increase in nitrogen in the weld metal is prevented by addition of CaCO 3 , but this has no effect on the effect of prevention of occurrence of top slag-in in the present invention. 
     Next, comparative examples according to claim  2  of the present invention described in Table 6 will be explained. 
     Comparative Example 19 to Comparative Example 21 have amounts of addition of Li 2 O and K 2 O of less than the range of the present invention, so the ripple lines on the bead surfaces become uneven. 
     Comparative Example 22 and Comparative Example 23 have amounts of addition of Li 2 O or K 2 O which exceed the range of the present invention. Therefore, the slag sticks to the bead surfaces and the strippability of the slag falls. 
     Comparative Example 24 and Comparative Example 25 have total amounts of addition of Li 2 O and K 2 O when adding both which exceed the range of the present invention. Therefore, the slag sticks to the bead surfaces and the strippability of the slag falls. 
     Experiment 1 
     The inventors used three-electrode submerged arc welding under the welding conditions shown in Table 1, combined wires of different compositions and strength levels shown in Table 2, formed grooves of cross-sectional shapes shown in  FIG. 12  in base materials having the chemical compositions and tensile strengths shown in Table 3 and plate thicknesses of 20 mm and lengths of 1500 mm, and performed welding using the fluxes of No. 1 to No. 14 of the ingredients shown in Table 7. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 7 
               
               
                   
               
               
                 Flux 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 no. 
                 CaO 
                 MgO 
                 Al 2 O 3   
                 CaF 2   
                 SiO 2   
                 MnO 
                 Others 
                 Basicity B 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 10.8 
                 7.2 
                 25.4 
                 27.6 
                 28.5 
                 0.0 
                 0.5 
                 0.47 
               
               
                 2 
                 14.2 
                 5.5 
                 31.3 
                 23.2 
                 25.2 
                 0.0 
                 0.6 
                 0.65 
               
               
                 3 
                 7.8 
                 4.8 
                 18.3 
                 27.6 
                 28.5 
                 12.5 
                 0.5 
                 0.68 
               
               
                 4 
                 5.2 
                 1.3 
                 25.5 
                 30.6 
                 19.5 
                 17.5 
                 0.4 
                 1.35 
               
               
                 5 
                 5.0 
                 4.8 
                 35.0 
                 31.1 
                 19.5 
                 4.1 
                 0.5 
                 0.93 
               
               
                 6 
                 10.2 
                 3.2 
                 20.1 
                 38.2 
                 22.5 
                 5.3 
                 0.5 
                 1.36 
               
               
                 7 
                 8.5 
                 8.5 
                 25.4 
                 35.5 
                 21.0 
                 0.0 
                 0.5 
                 1.31 
               
               
                 8 
                 14.7 
                 3.2 
                 21.5 
                 43.3 
                 14.8 
                 2.1 
                 0.4 
                 2.39 
               
               
                 9 
                 10.8 
                 7.2 
                 20.4 
                 33.5 
                 27.5 
                 0.0 
                 0.6 
                 0.81 
               
               
                 10 
                 3.9 
                 4.2 
                 28.5 
                 49.1 
                 8.5 
                 5.2 
                 0.6 
                 2.67 
               
               
                 11 
                 11.8 
                 4.0 
                 22.2 
                 48.4 
                 12.5 
                 0.6 
                 0.5 
                 2.59 
               
               
                 12 
                 20.5 
                 2.9 
                 20.4 
                 39.0 
                 10.2 
                 6.1 
                 0.9 
                 3.14 
               
               
                 13 
                 13.6 
                 5.5 
                 32.5 
                 28.6 
                 19.2 
                 0.0 
                 0.6 
                 1.32 
               
               
                 14 
                 21.5 
                 1.2 
                 21.2 
                 42.1 
                 13.7 
                 0.0 
                 0.3 
                 2.69 
               
               
                   
               
            
           
         
       
     
     The inventors measured the above obtained weld metals for −30° C. toughness (Charpy absorption energy), tensile strength, and amount of oxygen in the same way as the above methods of evaluation and investigated the relationship between the amount of oxygen in the weld metal and the toughness in the case where the tensile strength of the weld metal is 800 MPa to 1200 MPa. The results are shown in  FIG. 2 . 
     As shown in  FIG. 2 , they learned that when the amount of oxygen of the weld metal is 0.018% to 0.035% in range, a good toughness of a −30° C. impact absorption energy of 100 J or more is obtained. 
     Experiment 2 
     The inventors used three-electrode submerged arc welding under the welding conditions shown in Table 1, combined wires of different compositions and strength levels shown in Table 2, formed grooves of cross-sectional shapes shown in  FIG. 12  in base materials having the chemical compositions and tensile strengths shown in Table 3 and plate thicknesses of 20 mm and lengths of 1500 mm, and performed welding using the fluxes of No. 15 to No. 24 of the ingredients shown in Table 8. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 8 
               
               
                   
               
               
                 Flux no. 
                 CaO 
                 MgO 
                 Al 2 O 3   
                 CaF 2   
                 SiO 2   
                 Others 
                 Basicity B 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 15 
                 5.5 
                 1.0 
                 42.0 
                 31.2 
                 19.8 
                 0.5 
                 0.47 
               
               
                 16 
                 8.0 
                 1.2 
                 40.3 
                 31.4 
                 18.5 
                 0.6 
                 0.81 
               
               
                 17 
                 4.8 
                 4.8 
                 31.8 
                 38.6 
                 19.5 
                 0.5 
                 1.07 
               
               
                 18 
                 5.5 
                 4.2 
                 28.5 
                 42.1 
                 19.4 
                 0.3 
                 1.26 
               
               
                 19 
                 12.9 
                 3.2 
                 25.3 
                 38.7 
                 19.5 
                 0.4 
                 1.58 
               
               
                 20 
                 8.2 
                 3.3 
                 20.3 
                 49.2 
                 18.5 
                 0.5 
                 1.81 
               
               
                 21 
                 12.6 
                 4.8 
                 15.5 
                 49.5 
                 17.0 
                 0.6 
                 2.32 
               
               
                 22 
                 15.1 
                 3.3 
                 19.5 
                 48.5 
                 12.7 
                 0.9 
                 2.75 
               
               
                 23 
                 23.2 
                 4.2 
                 19.8 
                 41.5 
                 10.7 
                 0.6 
                 3.16 
               
               
                 24 
                 24.9 
                 4.2 
                 10.0 
                 49.8 
                 10.3 
                 0.8 
                 3.62 
               
               
                   
               
            
           
         
       
     
     The inventors measured the above obtained weld metals for tensile strength in the same way as the above-mentioned method of evaluation and investigated the relationship between the tensile strength of the weld metal, the basicity B of flux, and top slag-in. The results are shown in  FIG. 3 . 
     As shown in  FIG. 3 , they learned that in weld metal with a tensile strength of 800 MPa or more, if the basicity B is 1.2 or more, top slag-in does not occur. 
     Experiment 3 
     The inventors used three-electrode submerged arc welding under the welding conditions shown in Table 1, combined wires of different compositions and strength levels shown in Table 2, formed grooves of cross-sectional shapes shown in  FIG. 12  in base materials having the chemical compositions and tensile strengths shown in Table 3 and plate thicknesses of 20 mm and lengths of 1500 mm, and performed welding using the fluxes of No. 25 to No. 79 of the ingredients shown in Table 9. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 9 
               
               
                   
               
               
                 Flux 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 no. 
                 CaO 
                 MgO 
                 Al 2 O 3   
                 CaF 2   
                 SiO 2   
                 MnO 
                 Others 
                 Basicity B 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 25 
                 5.5 
                 4.8 
                 29.5 
                 48.5 
                 11.2 
                 0.0 
                 0.5 
                 2.30 
               
               
                 26 
                 15.1 
                 3.3 
                 19.5 
                 48.5 
                 12.7 
                 0.0 
                 0.9 
                 2.75 
               
               
                 27 
                 12.6 
                 4.8 
                 15.5 
                 49.5 
                 17.0 
                 0.0 
                 0.6 
                 2.32 
               
               
                 28 
                 12.8 
                 3.2 
                 20.4 
                 44.2 
                 18.2 
                 0.0 
                 1.2 
                 1.94 
               
               
                 29 
                 17.8 
                 2.9 
                 23.4 
                 40.0 
                 15.5 
                 0.0 
                 0.4 
                 2.29 
               
               
                 30 
                 12.9 
                 3.2 
                 25.3 
                 38.7 
                 19.5 
                 0.0 
                 0.4 
                 1.58 
               
               
                 31 
                 24.5 
                 3.0 
                 22.7 
                 31.3 
                 18.0 
                 0.0 
                 0.5 
                 2.15 
               
               
                 32 
                 13.4 
                 3.0 
                 22.7 
                 48.2 
                 12.2 
                 0.0 
                 0.5 
                 2.65 
               
               
                 33 
                 15.1 
                 4.2 
                 22.1 
                 40.9 
                 17.1 
                 0.0 
                 0.6 
                 2.08 
               
               
                 34 
                 5.5 
                 4.2 
                 28.5 
                 42.1 
                 19.4 
                 0.0 
                 0.3 
                 1.26 
               
               
                 35 
                 15.0 
                 3.2 
                 21.2 
                 44.2 
                 15.9 
                 0.0 
                 0.5 
                 2.27 
               
               
                 36 
                 23.2 
                 4.2 
                 19.8 
                 41.5 
                 10.7 
                 0.0 
                 0.6 
                 3.16 
               
               
                 37 
                 21.5 
                 1.2 
                 21.2 
                 42.1 
                 13.7 
                 0.0 
                 0.3 
                 2.69 
               
               
                 38 
                 8.2 
                 3.3 
                 20.3 
                 49.2 
                 18.5 
                 0.0 
                 0.5 
                 1.81 
               
               
                 39 
                 4.5 
                 4.8 
                 23.5 
                 44.2 
                 22.5 
                 0.0 
                 0.5 
                 1.07 
               
               
                 40 
                 13.6 
                 5.5 
                 32.5 
                 28.6 
                 19.2 
                 0.0 
                 0.6 
                 1.32 
               
               
                 41 
                 26.5 
                 0.8 
                 14.2 
                 48.3 
                 9.4 
                 0.0 
                 0.8 
                 3.62 
               
               
                 42 
                 11.8 
                 3.2 
                 20.4 
                 52.5 
                 11.4 
                 0.0 
                 0.7 
                 2.81 
               
               
                 43 
                 25.6 
                 3.0 
                 11.2 
                 42.2 
                 17.4 
                 0.0 
                 0.6 
                 2.72 
               
               
                 44 
                 10.8 
                 7.2 
                 25.4 
                 27.6 
                 28.5 
                 0.0 
                 0.5 
                 0.47 
               
               
                 45 
                 14.2 
                 5.5 
                 31.3 
                 23.2 
                 25.2 
                 0.0 
                 0.6 
                 0.65 
               
               
                 46 
                 11.1 
                 6.3 
                 24.1 
                 25.5 
                 32.5 
                 0.0 
                 0.5 
                 0.05 
               
               
                 47 
                 10.8 
                 7.2 
                 20.4 
                 33.5 
                 27.5 
                 0.0 
                 0.6 
                 0.81 
               
               
                 48 
                 5.5 
                 4.2 
                 29.5 
                 48.5 
                 11.2 
                 0.6 
                 0.5 
                 2.30 
               
               
                 49 
                 12.5 
                 4.5 
                 18.0 
                 41.4 
                 18.0 
                 5.0 
                 0.6 
                 2.09 
               
               
                 50 
                 15.1 
                 2.6 
                 19.5 
                 46.9 
                 12.7 
                 2.5 
                 0.7 
                 2.75 
               
               
                 51 
                 5.2 
                 3.7 
                 27.9 
                 42.1 
                 19.6 
                 0.6 
                 0.9 
                 1.23 
               
               
                 52 
                 5.3 
                 2.1 
                 28.3 
                 35.2 
                 19.5 
                 9.2 
                 0.4 
                 1.22 
               
               
                 53 
                 5.5 
                 3.8 
                 28.5 
                 40.5 
                 19.1 
                 2.2 
                 0.4 
                 1.29 
               
               
                 54 
                 11.8 
                 3.2 
                 20.4 
                 43.8 
                 18.2 
                 2.1 
                 0.5 
                 1.93 
               
               
                 55 
                 20.5 
                 2.9 
                 20.4 
                 39.0 
                 10.2 
                 6.1 
                 0.9 
                 3.14 
               
               
                 56 
                 10.5 
                 3.2 
                 26.1 
                 33.5 
                 17.1 
                 9.2 
                 0.4 
                 1.79 
               
               
                 57 
                 17.7 
                 1.2 
                 16.2 
                 31.5 
                 18.2 
                 14.1 
                 1.1 
                 2.22 
               
               
                 58 
                 11.8 
                 4.0 
                 22.2 
                 48.4 
                 12.5 
                 0.6 
                 0.5 
                 2.59 
               
               
                 59 
                 16.1 
                 4.2 
                 20.1 
                 40.8 
                 16.2 
                 2.1 
                 0.5 
                 2.30 
               
               
                 60 
                 10.7 
                 4.1 
                 22.9 
                 42.1 
                 18.0 
                 0.8 
                 1.4 
                 1.80 
               
               
                 61 
                 14.7 
                 3.2 
                 21.5 
                 43.3 
                 14.8 
                 2.1 
                 0.4 
                 2.39 
               
               
                 62 
                 24.3 
                 2.2 
                 20.5 
                 31.7 
                 15.3 
                 5.5 
                 0.5 
                 2.57 
               
               
                 63 
                 12.8 
                 1.2 
                 24.5 
                 37.5 
                 15.2 
                 8.2 
                 0.6 
                 2.14 
               
               
                 64 
                 11.1 
                 3.2 
                 20.1 
                 43.1 
                 12.1 
                 9.8 
                 0.6 
                 2.71 
               
               
                 65 
                 11.4 
                 2.8 
                 19.1 
                 51.1 
                 14.9 
                 0.2 
                 0.5 
                 2.40 
               
               
                 66 
                 19.2 
                 2.9 
                 20.5 
                 45.5 
                 11.1 
                 0.3 
                 0.5 
                 3.00 
               
               
                 67 
                 18.1 
                 1.3 
                 15.2 
                 31.5 
                 17.0 
                 16.5 
                 0.4 
                 2.44 
               
               
                 68 
                 5.2 
                 1.3 
                 25.5 
                 30.6 
                 19.5 
                 17.5 
                 0.4 
                 1.35 
               
               
                 69 
                 7.8 
                 4.8 
                 18.3 
                 27.6 
                 28.5 
                 12.5 
                 0.5 
                 0.68 
               
               
                 70 
                 14.2 
                 2.6 
                 15.5 
                 48.5 
                 9.2 
                 9.5 
                 0.5 
                 3.35 
               
               
                 71 
                 3.9 
                 4.2 
                 28.5 
                 49.1 
                 8.5 
                 5.2 
                 0.6 
                 2.67 
               
               
                 72 
                 8.4 
                 3.0 
                 18.3 
                 52.5 
                 12.1 
                 5.2 
                 0.5 
                 2.74 
               
               
                 73 
                 13.7 
                 4.1 
                 15.3 
                 52.5 
                 8.5 
                 5.3 
                 0.6 
                 3.44 
               
               
                 74 
                 27.5 
                 3.2 
                 17.2 
                 35.0 
                 11.2 
                 5.5 
                 0.4 
                 3.28 
               
               
                 75 
                 5.0 
                 4.8 
                 35.0 
                 31.1 
                 19.5 
                 4.1 
                 0.5 
                 0.93 
               
               
                 76 
                 10.2 
                 3.2 
                 20.1 
                 38.2 
                 22.5 
                 5.3 
                 0.5 
                 1.36 
               
               
                 77 
                 20.3 
                 3.6 
                 23.2 
                 28.5 
                 18.5 
                 5.3 
                 0.6 
                 1.97 
               
               
                 78 
                 17.2 
                 6.4 
                 14.2 
                 40.2 
                 16.2 
                 5.3 
                 0.5 
                 2.58 
               
               
                 79 
                 11.0 
                 0.5 
                 23.2 
                 49.5 
                 11.0 
                 5.3 
                 0.5 
                 2.75 
               
               
                   
               
            
           
         
       
     
     The inventors measured the thus obtained weld metal for the amount of oxygen in the same way as the above method of evaluation and investigated the relationship between the amount of oxygen of the weld metal and the basicity B of flux. The results are shown in  FIG. 4 . 
     As shown in  FIG. 4 , they learned that there is a correlation between the amount of oxygen of the weld metal and the basicity B of flux. 
     Experiment 4 
     The inventors used three-electrode submerged arc welding under the welding conditions shown in Table 1, combined wires of different compositions and strength levels shown in Table 2, formed grooves of cross-sectional shapes shown in  FIG. 12  in base materials having the chemical compositions and tensile strengths shown in Table 3 and plate thicknesses of 20 mm and lengths of 1500 mm, and performed welding using the fluxes of No. 80 to No. 89 of the ingredients shown in Table 10. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 10 
               
               
                   
               
               
                 Flux no. 
                 CaO 
                 MgO 
                 Al 2 O 3   
                 CaF 2   
                 SiO 2   
                 Others 
                 Basicity B 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 80 
                 24.8 
                 1.2 
                 14.9 
                 30.1 
                 28.5 
                 0.5 
                 1.21 
               
               
                 81 
                 19.4 
                 6.8 
                 15.5 
                 30.2 
                 27.5 
                 0.6 
                 1.23 
               
               
                 82 
                 20.1 
                 4.4 
                 20.4 
                 32.1 
                 22.5 
                 0.5 
                 1.61 
               
               
                 83 
                 8.5 
                 8.5 
                 25.4 
                 35.5 
                 21.0 
                 0.5 
                 1.31 
               
               
                 84 
                 5.5 
                 4.2 
                 28.5 
                 42.1 
                 19.4 
                 0.3 
                 1.26 
               
               
                 85 
                 8.2 
                 3.3 
                 20.3 
                 49.2 
                 18.5 
                 0.5 
                 1.81 
               
               
                 86 
                 12.6 
                 4.8 
                 15.5 
                 49.5 
                 17.0 
                 0.6 
                 2.32 
               
               
                 87 
                 15.1 
                 3.3 
                 19.5 
                 48.5 
                 12.7 
                 0.9 
                 2.75 
               
               
                 88 
                 17.8 
                 2.9 
                 23.4 
                 40.0 
                 15.5 
                 0.4 
                 2.29 
               
               
                 89 
                 23.2 
                 4.2 
                 19.8 
                 41.5 
                 10.7 
                 0.6 
                 3.16 
               
               
                   
               
            
           
         
       
     
     The inventors measured the above obtained weld metal for tensile strength in the same way as the above-mentioned method of evaluation and investigated the relationship between the composition of the flux, the tensile strength of the weld metal, and top slag-in. The results are shown in  FIG. 5 . 
       FIG. 5  is a graph showing the relationship between the amount of SiO 2  in the flux, the tensile strength of the weld metal, and the trend in occurrence of top slag-in. 
     As shown in  FIG. 5 , when the tensile strength of the weld metal is 800 MPa or more, if the amount of SiO 2  in the flux is over 20%, top slag-in occurs. 
     Experiment 5 
     The inventors used three-electrode submerged arc welding under the welding conditions shown in Table 1, combined wires of different compositions and strength levels shown in Table 2, formed grooves of cross-sectional shapes shown in  FIG. 12  in base materials having the chemical compositions and tensile strengths shown in Table 3 and plate thicknesses of 20 mm and lengths of 1500 mm, and performed welding using the fluxes of No. 90 to No. 97 of the ingredients shown in Table 11. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 11 
               
               
                   
               
               
                 Flux no. 
                 CaO 
                 MgO 
                 Al 2 O 3   
                 CaF 2   
                 SiO 2   
                 Others 
                 Basicity B 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 90 
                 22.8 
                 4.2 
                 28.5 
                 25.5 
                 18.5 
                 0.5 
                 1.81 
               
               
                 91 
                 13.6 
                 5.5 
                 32.5 
                 28.6 
                 19.2 
                 0.6 
                 1.32 
               
               
                 92 
                 5.5 
                 4.2 
                 28.5 
                 42.1 
                 19.4 
                 0.3 
                 1.26 
               
               
                 93 
                 12.9 
                 3.2 
                 25.3 
                 38.7 
                 19.5 
                 0.4 
                 1.58 
               
               
                 94 
                 12.6 
                 4.8 
                 15.5 
                 49.5 
                 17.0 
                 0.6 
                 2.32 
               
               
                 95 
                 15.1 
                 3.3 
                 19.5 
                 48.5 
                 12.7 
                 0.9 
                 2.75 
               
               
                 96 
                 22.8 
                 4.6 
                 31.3 
                 23.2 
                 17.5 
                 0.6 
                 1.81 
               
               
                 97 
                 24.5 
                 3.0 
                 22.7 
                 31.3 
                 18.0 
                 0.5 
                 2.15 
               
               
                   
               
            
           
         
       
     
     The inventors measured the above obtained weld metal for tensile strength in the same way as the above-mentioned method of evaluation and investigated the relationship between the composition of the flux, the tensile strength of the weld metal, and the top slag-in. The results are shown in  FIG. 6 . 
       FIG. 6  is a graph showing the relationship between the amount of CaF 2  in the flux, the tensile strength of the weld metal, and the trend in occurrence of top slag-in. 
     As shown in  FIG. 6 , when the amount of CaF 2  in the flux is less than 30%, top slag-in occurs. 
     Experiment 6 
     The inventors investigated the moisture adsorption in the fluxes of No. 98 to No. 105 of the ingredients shown in Table 12 by the above method. The results are shown in  FIG. 7 . 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 12 
               
               
                   
               
               
                 Flux no. 
                 CaO 
                 MgO 
                 Al 2 O 3   
                 CaF 2   
                 SiO 2   
                 Others 
                 Basicity B 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 98 
                 20.5 
                 4.8 
                 5.5 
                 49.2 
                 19.5 
                 0.5 
                 2.60 
               
               
                 99 
                 20.2 
                 4.8 
                 10.1 
                 45.9 
                 18.5 
                 0.5 
                 2.53 
               
               
                 100 
                 20.2 
                 4.9 
                 12.4 
                 43.5 
                 18.5 
                 0.5 
                 2.43 
               
               
                 101 
                 20.1 
                 4.8 
                 15.0 
                 42.4 
                 17.2 
                 0.5 
                 2.48 
               
               
                 102 
                 20.2 
                 4.6 
                 15.8 
                 42.7 
                 16.2 
                 0.5 
                 2.57 
               
               
                 103 
                 13.6 
                 4.7 
                 17.2 
                 47.9 
                 16.1 
                 0.5 
                 2.39 
               
               
                 104 
                 13.2 
                 4.8 
                 20.1 
                 45.2 
                 16.2 
                 0.5 
                 2.24 
               
               
                 105 
                 13.3 
                 4.2 
                 25.5 
                 40.2 
                 16.3 
                 0.5 
                 1.99 
               
               
                   
               
            
           
         
       
     
       FIG. 7  is a graph showing the relationship between the Al 2 O 3  in the flux and the amount of moisture adsorption, while  FIG. 8  is a graph showing the relationship between the amount of SiO 2  in the flux and the amount of moisture adsorption. 
     As shown in  FIG. 7 , with an Al 2 O 3  in the flux of 15% or less, the amount of adsorbed moisture rapidly increases. 
     Experiment 7 
     The inventors investigated the moisture adsorption in the fluxes of No. 106 to No. 112 of the ingredients shown in Table 13 by the above method. The results are shown in  FIG. 8 . 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 13 
               
               
                   
               
               
                 Flux no. 
                 CaO 
                 MgO 
                 Al 2 O 3   
                 CaF 2   
                 SiO 2   
                 Others 
                 Basicity B 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 106 
                 23.5 
                 4.8 
                 32.5 
                 31.2 
                 7.5 
                 0.5 
                 3.05 
               
               
                 107 
                 20.8 
                 3.3 
                 29.5 
                 36.7 
                 9.2 
                 0.5 
                 2.90 
               
               
                 108 
                 20.2 
                 3.5 
                 25.5 
                 40.3 
                 10.0 
                 0.5 
                 2.96 
               
               
                 109 
                 18.5 
                 3.6 
                 25.3 
                 39.7 
                 12.4 
                 0.5 
                 2.62 
               
               
                 110 
                 18.4 
                 3.3 
                 23.2 
                 39.5 
                 15.1 
                 0.5 
                 2.36 
               
               
                 111 
                 18.3 
                 3.2 
                 17.2 
                 40.6 
                 20.2 
                 0.5 
                 1.97 
               
               
                 112 
                 14.9 
                 3.1 
                 16.5 
                 40.5 
                 24.5 
                 0.5 
                 1.39 
               
               
                   
               
            
           
         
       
     
       FIG. 8  is a graph showing the relationship between the amount of SiO 2  in the flux and the amount of moisture adsorption. As shown in  FIG. 8 , when the amount of SiO 2  in the flux is less than 10%, the amount of adsorbed moisture rapidly increases. 
     Experiment 8 
     The inventors investigated the moisture adsorption in the fluxes of No. 113 to No. 120 of the ingredients shown in Table 14 by the above method. The results are shown in  FIG. 9 . 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 14 
               
               
                   
               
               
                 Flux no. 
                 CaO 
                 MgO 
                 Al 2 O 3   
                 CaF 2   
                 SiO 2   
                 Others 
                 Basicity B 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 113 
                 23.9 
                 0.2 
                 15.2 
                 50.0 
                 10.2 
                 0.5 
                 3.44 
               
               
                 114 
                 22.3 
                 0.6 
                 15.5 
                 49.8 
                 11.3 
                 0.5 
                 3.25 
               
               
                 115 
                 22.5 
                 0.6 
                 15.9 
                 48.2 
                 12.3 
                 0.5 
                 3.11 
               
               
                 116 
                 22.3 
                 0.8 
                 15.9 
                 46.2 
                 14.3 
                 0.5 
                 2.85 
               
               
                 117 
                 20.2 
                 1.0 
                 18.6 
                 44.0 
                 15.7 
                 0.5 
                 2.51 
               
               
                 118 
                 17.8 
                 1.0 
                 21.5 
                 41.0 
                 18.2 
                 0.5 
                 2.01 
               
               
                 119 
                 17.2 
                 1.2 
                 28.1 
                 34.8 
                 18.2 
                 0.5 
                 1.70 
               
               
                 120 
                 14.7 
                 4.8 
                 29.5 
                 30.7 
                 19.8 
                 0.5 
                 1.41 
               
               
                   
               
            
           
         
       
     
       FIG. 9  is a graph showing the relationship between the basicity B of flux and the amount of moisture adsorption. As shown in  FIG. 9 , even when the basicity B is over 3.2, the amount of adsorbed moisture rapidly increases. 
     Experiment 9 
     The inventors used three-electrode submerged arc welding under the welding conditions shown in Table 1, combined wires of different compositions and strength levels shown in Table 2, formed grooves of cross-sectional shapes shown in  FIG. 12  in base materials having the chemical compositions and tensile strengths shown in Table 3 and plate thicknesses of 20 mm and lengths of 1500 mm, and performed welding using the fluxes of No. 121 to No. 139 of the ingredients shown in Table 15. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 15 
               
               
                   
               
               
                 Flux 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 no. 
                 CaO 
                 MgO 
                 Al 2 O 3   
                 CaF 2   
                 SiO 2   
                 MnO 
                 Others 
                 Basicity B 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 121 
                 5.5 
                 4.2 
                 29.5 
                 48.5 
                 11.2 
                 0.6 
                 0.5 
                 2.30 
               
               
                 122 
                 12.5 
                 4.5 
                 18.0 
                 41.4 
                 18.0 
                 5.0 
                 0.6 
                 2.09 
               
               
                 123 
                 15.1 
                 2.6 
                 19.5 
                 46.9 
                 12.7 
                 2.5 
                 0.7 
                 2.75 
               
               
                 124 
                 5.2 
                 3.7 
                 27.9 
                 42.1 
                 19.6 
                 0.6 
                 0.9 
                 1.23 
               
               
                 125 
                 5.3 
                 2.1 
                 28.3 
                 35.2 
                 19.5 
                 9.2 
                 0.4 
                 1.22 
               
               
                 126 
                 5.5 
                 3.8 
                 28.5 
                 40.5 
                 19.1 
                 2.2 
                 0.4 
                 1.29 
               
               
                 127 
                 11.8 
                 3.2 
                 20.4 
                 43.8 
                 18.2 
                 2.1 
                 0.5 
                 1.93 
               
               
                 128 
                 20.5 
                 2.9 
                 20.4 
                 39.0 
                 10.2 
                 6.1 
                 0.9 
                 3.14 
               
               
                 129 
                 10.5 
                 3.2 
                 26.1 
                 33.5 
                 17.1 
                 9.2 
                 0.4 
                 1.79 
               
               
                 130 
                 17.7 
                 1.2 
                 16.2 
                 31.5 
                 18.2 
                 14.1 
                 1.1 
                 2.22 
               
               
                 131 
                 11.8 
                 4.0 
                 22.2 
                 48.4 
                 12.5 
                 0.6 
                 0.5 
                 2.59 
               
               
                 132 
                 16.1 
                 4.2 
                 20.1 
                 40.8 
                 16.2 
                 2.1 
                 0.5 
                 2.30 
               
               
                 133 
                 10.7 
                 4.1 
                 22.9 
                 42.1 
                 18.0 
                 0.8 
                 1.4 
                 1.80 
               
               
                 134 
                 14.7 
                 3.2 
                 21.5 
                 43.3 
                 14.8 
                 2.1 
                 0.4 
                 2.39 
               
               
                 135 
                 24.3 
                 2.2 
                 20.5 
                 31.7 
                 15.3 
                 5.5 
                 0.5 
                 2.57 
               
               
                 136 
                 12.8 
                 1.2 
                 24.5 
                 37.5 
                 15.2 
                 8.2 
                 0.6 
                 2.14 
               
               
                 137 
                 11.1 
                 3.2 
                 20.1 
                 43.1 
                 12.1 
                 9.8 
                 0.6 
                 2.71 
               
               
                 138 
                 11.4 
                 2.8 
                 19.1 
                 51.1 
                 14.9 
                 0.2 
                 0.5 
                 2.40 
               
               
                 139 
                 19.2 
                 2.9 
                 20.5 
                 45.5 
                 11.1 
                 0.3 
                 0.5 
                 3.00 
               
               
                   
               
            
           
         
       
     
     The inventors measured the thus obtained weld metal for the amount of oxygen in the same way as the method of evaluation explained above and investigated the relationship of the amount of oxygen of the weld metal and the basicity B of flux. The results are shown in  FIG. 11 . 
     As shown in  FIG. 11 , by adding MnO in an amount of 0.5% or more, with a basicity B of 1.2 or more, it is possible to stably obtain weld metal with an amount of oxygen of 0.018% or more. 
     INDUSTRIAL APPLICABILITY  
     In the above way, when using the submerged arc welding method to prepare high strength weld metal, by using the melt flux for submerged arc welding use of the present invention, it is possible to obtain a weld bead which is excellent in bead shape and also is free of the defect of the inside of the weld bead called “top slag-in”. Furthermore, due to the system of ingredients of the present invention, even with a composition of flux where top slag-in does not occur, the amount of oxygen of the weld metal can be optimized and weld metal having excellent low temperature toughness can be easily obtained. The contribution to industry is extremely great. 
     REFERENCE SIGNS LIST  
     
         
         ST: Top slag-in 
         SL: Bottom slag-in