Abstract:
A mold powder for use in continuous casting, containing specified amounts of CaO, Al 2  O 3 , TiO 2 , Li 2  O, BaO, MgO, F and Na 2  O, and further containing at least one of BN and C as aggregate. The mold powder, when used in continuous casting of a steel rich in strong reducing metal elements such as Al, Ti, REM and so forth, remarkably suppresses generation of deterioration in the surface quality of the slab and break out of the slab which are attributable inferior lubrication.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention broadly relates to continuous casting of a steel which contains slag reducing metal elements such as Al, Ti, REM (Rare Earth Metal) or the like and, more particularly, to a mold powder which can effectively be used for improving lubrication between the steel and the mold in order to avoid problems such as deterioration of the surface quality of the slab and break out of the slab which otherwise may occur due to inferior lubrication. 
     2. Description of the Related Art 
     Hitherto, in continuous casting of a steel containing 0.010 wt % or more Al, reducing reactions take place in accordance with the following formulae (1) and (2) between Al component in the molten steel and low-grade oxides such as SiO 2 , MnO and so forth contained in the powder which is used for providing lubrication between the mold wall and the slab and for covering the surface of the molten steel. At the same time, Al 2  O 3  in the molten steel, which is a deoxidation product or a product of secondary oxidation of the molten steel, floats in the molten steel in the mold so as to be trapped by the powder. ##STR1## 
     Consequently, the molten powder composition is changed to enhance the tendency of generation of 2CaO.Al 2  O 3 .SiO 2  (gehlenite), which undesirably promotes generation of slag and impairs lubrication. 
     Under this circumstance, methods are being developed which can avoid the above-mentioned change in the molten powder composition despite the increase in the Al 2  O 3  content in the powder, thereby ensuring stable casting while preventing deterioration of the surface quality of the product slab. 
     For instance, Japanese Patent Laid-Open No. 61-186155  discloses a powder which contains BaO and Li 2  O, with the composition adjusted to meet the conditions of BaO+CaO+SiO 2  =60 to 80 wt % and CaO/SiO 2  =0.30 to 0.60 wt %. 
     Japanese Patent Laid-Open No. 63-10052 discloses a powder having a composition meeting the condition of CaO/SiO 2  =0.6 to 0.8, melting point of 800° to 1000° C. and viscosity of 1300° C.&lt;1.5 poise. Japanese Patent Laid-Open No. 3-77753 proposes a powder containing 10 to 20 wt % of SiO 2  and not more than 15 wt % of Al 2  O 3 , the SiO 2  and Al 2  O 3  contents being determined to meet a specific condition. 
     All these proposed powders, however, have SiO 2  content ranging from 10 to 40 wt % and, therefore, cannot completely eliminate the above-described problems. Thus, when these known powders are used, thermit reactions take place in accordance with the aforesaid formulae (1) and (2) to generate heat, thus posing the following problems. 
     (i) Generation of bleed (small scale of breakage of solidification shell) along oscillation mark due to delay of solidification at the meniscus. 
     (ii) Reduced Si is mixed in the molten steel to make the Si content of the molten steel fall out of the target range. At the same time, Al in the molten steel is changed to Al 2  O 3  to make the Al content fall out of the target range. Consequently, the desired composition of molten steel cannot be obtained. 
     (iii) Flame is formed as a result of the thermit reaction, which causes danger in the casting operation, making it difficult to safely continue the casting. 
     Meanwhile, Japanese Patent Publication No. 63-56019 proposes a powder in which SiO 2  content is diminished as much as possible so as to avoid the reaction in accordance with the aforesaid formula (1). 
     This powder has an increased CaO content of 40 to 60 wt % at the cost of the reduced SiO 2  content. Therefore, the powder exhibits inferior fluidity, which impairs lubrication of the steel resulting in deterioration of the surface quality of the product slab. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a powder which can suppress, in continuous casting of a steel containing slag reducing metal elements such as Al, Ti, REM and so forth, any tendency of deterioration of the slab surface quality or break out of the slab attributable to inferior lubrication effect. 
     Briefly, the powder in accordance with the present invention contains, as the major components, CaO, Al 2  O 3 , TiO 2 , Li 2  O, BaO, MgO and Na 2  O but does not substantially contain SiO 2 . 
     According to one aspect of the present invention, there is provided a mold powder for continuous casting, having a composition which contains: from 10 to 35 wt % of CaO, from 10 to 35 wt % of Al 2  O 3 , from 3 to 15 wt % of TiO 2 , from 3 to 20 wt % of Li 2  O, from 5 to 40 wt % of BaO, not more than 15 wt % of F, not more than 20 wt % of Na 2  O, at least one of from 0.5 to 4.0 wt % of BN and from 0.5 to 4.0 wt % of C as aggregate, and the balance inevitable impurities. 
     The above and other objects, features and advantages of the present invention will become clear from the following detailed description of the invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Throughout intense study and experiments, the present inventors have found that the above-described problems of the prior art is overcome by using a powder having a CaO-Al 2  O 3  type basic composition avoiding as much as possible inclusion of SiO 2  and with addition of suitable amounts of TiO 2  and Li 2  O which have a large effect on lowering the melting point. 
     More specifically, the powder of the present invention has a composition which contains: from 10 to 35 wt % of CaO, from 10 to 35 wt % of Al 2  O 3 , from 3 to 15 wt % of Ti 2  O, from 3 to 20 wt % of Li 2  O, from 5 to 40 wt % of BaO, not more than 15 wt % of F, not more than 20 wt % of Na 2  O, from 0.5 to 4.0 wt % of BN and from 0.5 to 4.0 wt % of C as aggregate, and the balance inevitable impurities. 
     A description will now be given of the reasons of limiting the contents of the respective elements. TiO 2  : from 3 to 15 wt % 
     Known mold powders rich in SiO 2  are expected to lower the melting point while promoting vitrification. When Al 2  O 3  content of the powder is increased as a result of the reaction of the aforesaid formula (1), the large SiO 2  content of the powder leads to generation of 2CaO.Al 2  O 3 .SiO 2  (gehlenite) which exhibits high melting point. 
     In contrast, the mold powder in accordance with the present invention, TiO 2  is added in an amount of 3 to 15 wt % while SiO 2  is minimized. TiO 2  exhibits vitrification promoting effect as is the case of SiO 2  and exhibits a reducing rate which is about half that of SiO 2  in the molten steel. Furthermore, in the case of a CaO-Al 2  O 3  type slag (powder), addition of 3 to 15 wt % of TiO 2  causes about 200° to 300° C. reduction in the melting point of the powder as compared with the case where no TiO 2  is added. The effect to lower the melting point, however, is not appreciable when the TiO 2  content is below 3 wt % and when the same exceeds 15 wt %. For these reasons, the TiO 2  content in the powder is determined to be from 3 to 15 wt %. Li 2  O: from 3 to 20  wt % 
     Addition of 3 to 20 wt % of Li 2  O to a CaO--Al 2  O 3  --TiO 2  slag offers the same effect as that produced by the addition of TiO 2 . Addition of Li 2  O also produces an effect to energize activity of Al 2  O 3  in the molten slag so as to suppress oxidation of Al in the molten steel. These effects, however, are not noticeable when the content of Li 2  O is below 3 wt %. Addition of Li 2  O in excess of 20 wt % is not preferred because it enhances the tendency of the reduction of Li 2  O in accordance with the following formula (3) so as to promote generation of Al 2  O 3 , resulting in elevation of the melting point. For these reasons, the content of Li 2  O 3  is determined to range from 3 to 20 wt %. 
     
         m(Li.sub.2 O)+nM→2mLi+(MnOM)                        (3) 
    
     wherein M represents a reducing element, and m and n are coefficients. CaO, Al 2  O 3  : from 10 to 35 wt % 
     The contents of CaO and Al 2  O 3  respectively range from 10 to 35 wt %. It is therefore necessary to draw an upper limit of these contents in order that the powder will not lose the lubricating functions due to enhancement of melting point and viscosity. It is also necessary that CaO and Al 2  O 3 , when mixed as externally incurred impurities, do not cause a substantial change in the physical properties. Considering these requirements while taking into account also other components, the upper limit of the CaO content, as well as that of Al 2  O 3  , is set to be 35 wt %. Conversely, when the contents of CaO and Al 2  O 3  are below 10 wt %, contents of other components are increased beyond the limits specified above. The lower limits of the contents of CaO and Al 2  O 3 , therefore, is set to be 10 wt %. F: 15 wt % or less 
     F is necessary for the purpose of adjustment of the melting point and viscosity. An F content exceeding 15 wt % causes gasification of fluorides due to reaction between F and other components of the powder, and can no more function as an element for controlling the physical properties. In addition, such large content of F generates white smoke to impair the work. The F content, therefore, is to be limited to be 15 wt % or less. Na 2  O: 20 wt % or less 
     Na 2  O provides the same advantage as F. The content of Na 2  O is determined in accordance with factors such as the casting rate and powder consumption, and is preferably set to be not more than 20 wt %. Addition of Na 2  O in excess of 20 wt % poses a risk of melt down of the immersed nozzles. BaO: from 5 to 40 wt % 
     BaO is added preferably in an amount ranging from 5 to 40 wt %, in order to reduce viscosity and to prevent crystalline precipitation. Addition of BaO also is advantageous in that this compound does not cause reducing reaction with Al. The effect of prevention of crystalline precipitation is not appreciable when the BaO content is less than 5 wt %, whereas addition of BaO in excess of 40wt % causes a saturation in the effect of reducing viscosity. The content of BaO, therefore, is determined to range from 5 to 40 wt %. MgO: 10 wt % or less 
     MgO is an element which lowers the melting point of the powder as is the case of Li 2  O and TiO 2 , and is added as necessitated. The content of MgO, however, is limited to be 10 wt % or less, since the effect to lower the melting point is appreciable only in this range of MgO content. 
     The powder of the invention also can contain suitable amounts of metal fluorides such as LiF and NaF as solvents in addition to BaO and MgO, as well as suitable amounts of C, BN and so forth as aggregates. C, BN: 0.5 to 4.0 wt % 
     When C and/or BN is used as aggregates, the rate of formation of the molten slag layer is lowered when the content of C and/or BN exceeds 4 wt%, resulting in slag inclusion. When C alone is used, thermit reactions take place in accordance with the aforesaid formulae (1), (2) and (3) particularly when the C content exceeds 4 wt %, with the result that C as aggregates is burned to generate flame, thus affecting safety of the casting operation. Conversely, the effect of C and/or BN as aggregate is not noticeable when the content is below 0.5 wt %. For these reasons, the content of C and/or BN is determined to range from 0.5 to 4.0 wt %. 
    
    
     Examples: 
     Slabs having a cross-section of 200 mm thickness and 1000 mm width were prepared by continuous casting from three types of steels A, B and C having chemical compositions shown in Table 1. 
     Referring to Table 1, steels A and B are rich in reducing metal elements such as Al, Ti, REM and so forth, while the steel C has small contents of Al and Ti and is substantially free of REM. The continuous casting was conducted by using mold powders of the invention having compositions as shown in Table 2. For the purpose of comparison, continuous casting also was conducted by using a CaO-SiO 2  type powder having a composition disclosed in Japanese Patent Laid-Open No. 61-186155. Table 3 shows the results of the continuous casting: specifically, defects caused by inclusion of powder and number of bleed defects at the slab surface, and yield after conditioning of the slab surface. 
     
                       TABLE 1______________________________________TYPEOF      CHEMICAL COMPOSITION (wt %)STEEL   C      Si     Mn   Ni   Cr    Al   Ti   La______________________________________A       0.02   0.15   0.17 1.0  13    1    0.4  --B       0.002  0.1    0.4  --   20    5    0.02 0.10C       0.10   0.30   0.80 0.10 0.15  0.02 0.08 --______________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________POWDER                                   MELTING                                           VISCOSITYSAMPLE CHEMICAL COMPOSITION (wt %)        POINT  (Poise atNO.   C BN CaO         Al.sub.2 O.sub.3             TiO.sub.2                 Li.sub.2 O                    BaO                       MgO                          Na.sub.2 O                              P* SiO.sub.2                                    (°C.)                                           1300° C.)                                                  REMARKS__________________________________________________________________________1     1.0   -- 23.9         19.9             4.0 14.1                    28.7                       Tr.                          4.9 3.5                                 Tr.                                    1120   1.4    Example of the2     0.5   1.0      24.4         28.7             5.0 14.0                    9.3                       Tr.                          9.4 6.7                                 1.0                                    1110   2.0    Invention --   2.0      18.5         20.5             5.0 12.5                    15.0                       7.0                          6.3 10.7                                 2.5                                     980   1.54     1.0   -- 21.1         0.9 7.7 4.0                    9.6                       8.0                          7.7 5.5                                 34.5                                     820   1.1    Comparative                                                  Example5     4.0   -- 18.4         26.2             5.0 14.4                    30 Tr.                          1.4 0.6                                 Tr.                                    1140   1.0    Example of the6     2.0   -- 20.0         16.4             5.0 8.0                    20.6                       5.0                          8.0 14.5                                 0.5                                    1150   0.9    Invention7     4.0   -- 24.6         30.0             3.0 5.4                    13.8                       Tr.                          17.5                              1.2                                 0.5                                    1180   1.3__________________________________________________________________________ *P indicates total content of F in flourides such as LiF, NaF and so forth. 
    
     
                                           TABLE 3__________________________________________________________________________             NO. OF             DEFECTS  YIELD TYPE     NO. OF  CAUSED BY                      AFTERPOWDER OF  BLEEDS  POWDER   CONDI-SAMPLE CAST     GENERATED             INCLUSION                      TIONING                            SMOKE/FLAME GENERATIONNO.   STEEL     (per m.sup.2)             (per m.sup.2)                      (%)   DURING CASTING    REMARKS__________________________________________________________________________1     A   0       0.01     99.8  Smoke/flame generated                                              Example of the                            (Lighter than 4)  Invention2     B   0.15    0.05     99.0  Slight smoke/flame                            genertion B   0.20    0.21     99.2  Almost no smoke/flame4     A   2.4     2.9      95.6  Average level of  Comparative Example                            smoke/flame4     B   1.3     1.0      96.5  More smoke/flame than                            average level4     C   0.05    0.24     99.5  Almost no smoke/flame1     C   0       0        100   Almost no smoke/flame                                              Example of the5     B   0       0        100   Smoke/flame generated                                              Invention                            (Lighter than 4)6     B   0.1     0        98.9  Smoke/flame generated                            (Lighter than 4)7     B   0.15    0        98.0  Smoke/flame generated                            (Lighter than 4)__________________________________________________________________________ 
    
     From Table 3, it will be seen that steels rich in slag reducing metal elements such as Al, Ti and REM can be done satisfactorily when the mold powder used is one of the powder sample Nos. 1 to 7 except for No.4. More specifically, when these mold powders were used, generation of bleeds and defects caused by inclusion of powder are remarkably suppressed. In addition, yield after conditioning of the steel slab surface, in terms of [{(weight before conditioning-weight reduction caused by conditioning)/(weight before conditioning)}×100%] was also improved significantly. In particular, these advantageous effects were remarkable particularly on the steels A and B which are rich in Al, Ti and REM. In contrast, many defects were observed, as well as small yield, when the continuous casting was conducted by using the mold powder sample No. 4 which is a conventional powder. This tendency is noticeable particularly in the cases of steels A and B having large contents of Al, Ti and REM. 
     As will be understood from the foregoing description, according to the present invention, the powder composition is so determined as to minimize the contents of oxides which are liable to react with strong reducing elements in the molten steel such as Al, Ti, REM and so forth, while retaining characteristics required for mold powder. Consequently, problems are eliminated such as generation of defects such as bleeds in the product slab surface, defects caused by inclusion of powder, reduction in the yield due to conditioning, generation of flame during casting, and so forth.