Patent Publication Number: US-2016244865-A1

Title: Steel plate with excellent sour resistance, haz toughness and haz hardness, and steel pipe for line pipe

Description:
TECHNICAL FIELD 
     The present invention relates to a steel plate with excellent sour resistance, HAZ toughness and HAZ hardness which is suitable as a structural component for energy field such as a line pipe and a marine structure, and a steel pipe for a line pipe manufactured using the steel plate. 
     BACKGROUND ART 
     In recent years, accompanying increase of the global energy demand, development and practical application of various energy including renewable energy have been in progress. On the other hand, oil, natural gas and coal which are fossil fuel occupy a major part of the energy resources, how to produce, transport and store the fossil energy safely and efficiently is also an important issue in securing energy, and particularly, a highly functional steel material for energy field becomes indispensable in production, transportation and the like of the fossil energy. 
     With respect to this steel material for energy field, when the function thereof cannot be exerted and an accident occurs once, the damage becomes enormous, and therefore high safety is required. 
     Steel for a line pipe is one of the steel material for energy field and is used for transportation of oil and natural gas (LNG), and not only the mechanical properties (strength, toughness) as a structural component but also corrosion resistance to oil and natural gas passing through the pipe is required for the steel. In recent years, in oil fields and gas fields of oil and natural gas, the quality of oil and gas produced has deteriorated and a large amount of H 2 S has been mixed, and sour resistance represented by hydrogen-induced cracking resistance (HIC resistance) has been strongly required in addition to the specification of the past. 
     Also, the steel plate of the steel for a line pipe and the like is used as a welded structural component. In general, the weakest part in terms of the material of the welded structure is the heat-affected zone (HAZ) in the vicinity of the weld metal, it is required to secure the toughness of the portion, and, on the other hand, from the viewpoint of the welding efficiency, there is a constant demand of improving weldability. Namely, both of securing the HAZ toughness and weldability have been required. 
     In order to improve the weldability, heat input for welding should be increased for example, however, in the welding condition with a large heat input, the HAZ toughness extremely deteriorates which has become a problem. 
     As a related art that achieved these sour resistance and securing HAZ toughness, Patent Literature 1 and the like can be cited. 
     Namely, in Patent Literature 1, by the balance of the main composition (Ceq value) and control of the precipitates of 20 nm or less mainly of Ti, both of the sour resistance without the HIC cracking in the base plate of X65-X 80 strength class and the HAZ toughness of vTrs of approximately −10° level after the thermal cycle of 1,400° C.×1 s, Tc=50 s have been achieved. However, to reduce and smooth the slope of the HAZ hardness from coarse grains to fine grains (reducing the dispersion of the HAZ hardness) has not been particularly mentioned. 
     On the other hand, as one for improving the average value and minimum value of the HAZ toughness even in a large heat input welding condition by introducing inclusions and controlling the composition of inclusions, Patent Literature 2 and the like can be cited. 
     Namely, in Patent Literature 2, thick steel plates for bridges and ship building are the objects, and the HAZ toughness of −40° level vTrs is achieved in a condition equivalent to large heat input welding of 1,400° C.×60 s, Tc=400 s (equivalent to heat input of 50 kJ/mm) by controlling the composition of oxide containing Ti, Ca, Al and the like and the amount of the trace amount thereof and increasing the intragranular a forming rate. Therefore, the present literature does not intend improvement of the sour resistance required as the line pipe use and focuses solely on the control of the oxides described above, and specific desulphurization is not executed in order to suppress coarse sulfide that deteriorates the sour resistance. Further, although to reduce the dispersion of the HAZ toughness is emphasized, there is no concrete description on the dispersion of the HAZ hardness from coarse grains to fine grains. 
     According to all of these related arts, three of the sour resistance, the HAZ toughness and reduction of the dispersion of the HAZ hardness are not achieved simultaneously. 
     Also, line pipes are manufactured by bending a thick steel plate for a line pipe into a tubular shape and welding both edges. The line pipes thus manufactured are joined by welding the pipes each other, and are used as an actual oil transportation line. 
     Because the T-cross weld joint that receives two kinds of thermal histories of seam welding in working a thick steel plate into a pipe and girth welding in joining pipes with each other is subjected to complicated thermal histories such as rapid heating and rapid cooling, the strength (hardness) increases and a cracking called sulfide stress corrosion cracking (SSCC) is liable to occur in the HAZ. Therefore, in the steel for a line pipe, it is required that the SSCC resistance of the T-cross weld joint is also secured in addition to the HIC resistance (sour resistance) of the base plate described above. 
     As related arts taking improvement of the SSCC resistance into consideration, technologies described in Patent Literature 3 and Patent Literature 4 can be cited. The technology described in Patent Literature 3 is a technology of utilizing precipitation strengthening by fine Nb and V carbonitride and achieving high strength of 56 kgf/mm 2  or more of the tensile strength. However, the HIC resistance of the base plate is not taken into consideration, and only the HAZ of seam welding is taken into consideration with respect to the SSCC resistance. Also, the immersion time into a solution that simulates the sour environment is 21 days which is not a sufficiently severe test condition. Further, in Patent Literature 4, such a composition as suppressing increase of the hardness which is deemed to deteriorate the SSCC resistance of the T-cross weld joint is shown. However, the SSCC resistance itself is not evaluated, and the HIC resistance of the base plate also is not taken into consideration. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2009-52137 
     Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2010-168644 
     Patent Literature 3: Japanese Unexamined Patent Application Publication No. Hei 1-96329 
     Patent Literature 4: Japanese Unexamined Patent Application Publication No. 2005-186162 
     SUMMARY OF INVENTION 
     Technical Problem 
     Therefore, the object (assignment) of the present invention is to provide a steel plate satisfying three of the sour resistance, HAZ toughness and reduction of the dispersion of the HAZ hardness simultaneously, and suitable to a steel material for energy field such as a line pipe use having high yield strength and tensile strength, and a steel plate for a line pipe manufactured using the steel plate. 
     Solution to Problem 
     The present invention provides a steel plate with excellent sour resistance, HAZ toughness and HAZ hardness including, in terms of mass %: 
     C: 0.02-0.20%; 
     Si: 0.02-0.50%; 
     Mn: 0.6-2.0%; 
     P: over 0% and 0.030% or less; 
     S: over 0% and 0.004% or less; 
     Al: 0.010-0.08%; 
     N: 0.001-0.01%; 
     Nb: 0.002% or more and less than 0.05%; 
     O: over 0% and 0.0040% or less; 
     REM: 0.0002-0.05%; and 
     Zr: 0.0003-0.020% 
     with the remainder consisting of iron and inevitable impurities; in which 
     an expression 10,000×[Nb]+31×Di−82≧0 
     (where 
         Di =([C]/10) 0.5 ×(1+0.7×[Si])×(1+3.33×[Mn])×(1+0.35×[Cu])×(1+0.36×[Ni])×(1+2.16×[Cr])×(1+3×[Mo])×(1+1.75×[V])×1.115)
 
     is satisfied, and 
     in the composition of inclusions with 1 μm or more width contained in steel, 
     Zr amount is 1-40%, 
     REM amount is 5-50%, 
     Al amount is 3-30%, and 
     S amount is over 0% and less than 20%. 
     The steel plate may further include at least one element selected from a group consisting of: 
     Ca: 0.0003-0.0060%, and 
     Mg: 0.0003-0.005%. 
     In the steel plate, it may be arranged that, in the composition of the inclusions with 1 μm or more width contained in steel, 
     Zr amount is 1-40%, 
     REM amount is 5-50%, 
     Al amount is 3-30%, 
     Ca amount is 5-60%, and 
     S amount is over 0% and less than 20%. 
     Also, the steel plate may further include at least one element selected from a group consisting of: 
     Ti: 0.003-0.03%, 
     B: 0.0002-0.005%, 
     V: 0.003-0.1%, 
     Cu: 0.01-1.5%, 
     Ni: 0.01-3.5%, 
     Cr: 0.01-1.5%, and 
     Mo: 0.01-1.5%. 
     The steel plate may be for use of a line pipe. 
     Also, the present invention provides a steel pipe for a line pipe manufactured using the steel plate described above. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide a steel plate that is excellent in the sour resistance such as the hydrogen-induced cracking resistance, has the excellent HAZ toughness and HAZ hardness even in a large heat input welding condition, simultaneously satisfies three of the sour resistance, HAZ toughness and reduction of the dispersion of the HAZ hardness, and can be adaptable advantageously as a steel material for energy field such as a line pipe use and a marine structure use having a high functional property of high yield strength and tensile strength, and a steel pipe for a line pipe manufactured using the steel plate. 
    
    
     DESCRIPTION OF EMBODIMENTS 
     In order to achieve the object of the present invention, the present inventors repeated intensive researches and studies from the viewpoint of controlling the inclusions in steel in addition to the componential composition of the steel that became the basis in exerting the property of a steel plate. As a result, it was found out that an excellent steel plate was obtained which simultaneously satisfied all properties of the sour resistance, HAZ toughness and HAZ hardness described above by holding the coarse inclusions with 1 μm or more width in a predetermined componential composition, and the present invention has been completed based on the knowledge. 
     As a result of the study from the viewpoint of the sour resistance, it is assumed that, when hydrogen intrudes into steel in a sour environment, inclusions such as MnS which are coarse and have a larger coefficient of thermal expansion than that of steel form coarse voids around them, therefore the hydrogen having intruded is accumulated concentrically to these voids, and the cracking namely the hydrogen-induced cracking is generated and propagates in steel by the pressure generated by vaporization of the hydrogen. Therefore, it was considered that the sour resistance of steel could be improved and secured by convertingly preparing the coarse inclusions of 1 μm or more which became a cause of this hydrogen-induced cracking from inclusions having a larger coefficient of thermal expansion than that of steel into inclusions having a smaller coefficient of thermal expansion than that of steel. Also, as the inclusions having a smaller coefficient of thermal expansion than that of steel, in concrete terms, oxides of Zr, Al, REM and the like are effective. 
     On the other hand, as a result of the study from the viewpoint of improving the HAZ toughness and reducing the hardness slope (reducing the dispersion of the hardness) from coarse grains to fine grains, miniaturization of the microstructure which was one of the methods for improving these properties was focused on. In general, when the weld heat input is added, the microstructure is coarsened, and the HAZ toughness deteriorates. Also, because the vicinity of the weld metal is exposed to various thermal histories according to the distance from the weld metal, the hardness becomes distributional. Particularly, the hardness difference increases in a position where transformation shifts from ferrite to bainite. With this regard, it is possible to improve the HAZ toughness and the dispersion of the HAZ hardness by introducing inclusions that can promote transformation within the austenite grain and can form fine microstructure. 
     Also, at the same time, by positively introducing intragranular acicular α whose transformation point is in a middle degree between ferrite whose transformation point is high and bainite whose transformation point is low, the hardness slope can be reduced. In order to make inclusions exert such effect, there are methods such as lowering the melting point and increasing lattice matching with the matrix a phase of steel, and in concrete terms, Zr, REM, Al, TiN and Ti—Ca-based oxide of a small amount are supposed to be effective. 
     However, when such viewpoints of the sour resistance, HAZ toughness and reduction of the slope of the HAZ hardness as described above are considered together, although effective inclusions are common in terms of the composition, they do not necessarily agree with each other in terms of the concrete composition (composition rate). Therefore, the present inventors proceeded with further studies and experiments focusing to properly balance these inclusion compositions along with the composition and microstructure of steel, and succeeded in finding out an optimum range in which three properties described above could be achieved simultaneously. 
     Further, it was also found out that, by making the Ca concentration in the inclusions an amount within a predetermined range, the intragranular acicular α originated from the inclusions came to grow actively even in the T-cross weld joint, and the SSCC resistance of the T-cross weld joint was also improved by the formation of the fine microstructure. 
     Below, the composition of steel, the microstructure and the composition of inclusions of the steel plate of the present invention will be explained in detail including the reason of the determination. Also, all of % which is an expression unit of the composition means mass %. Here, in the present description, the percentage (mass %) based on the mass is same with the percentage (wt %) based on the weight. Also, with respect to the content of each chemical composition, the fact “X % or less (not inclusive of 0%)” may be expressed as “over 0% and X % or less”. 
     (Steel Composition) 
     [C: 0.02-0.20%] 
     C is an indispensable element for securing the quenchability of the HAZ part, and should be contained by 0.02% or more. C amount is preferably 0.03% or more, and more preferably 0.05% or more. When C amount is excessive, martensite (inclusive of MA=island martensite) is liable to be formed, and the HAZ toughness deteriorates. Therefore, C amount should be 0.20% or less. C amount is preferably 0.15% or less, and more preferably 0.12% or less. 
     [Si: 0.02-0.50%] 
     Si is effective in deoxidation. In order to secure such an effect, Si amount is made 0.02% or more. Si amount is preferably 0.05% or more, and more preferably 0.15% or more. However, when Si amount is excessive, island martensite is liable to be formed, and the HAZ toughness deteriorates. Therefore, Si amount should be suppressed to 0.50% or less. Si amount is preferably 0.45% or less, and more preferably 0.35% or less. 
     [Mn: 0.6-2.0%] 
     Mn is an element effective in securing the quenchability of the HAZ part, and is contained by 0.6% or more in the present invention. Mn amount is preferably 0.8% or more, and more preferably 1.0% or more. However, when Mn amount is excessive, MnS is formed, not only the hydrogen-induced cracking resistance deteriorates but also the HAZ toughness deteriorates. Therefore, the upper limit of Mn amount is made 2.0%. Mn amount is preferably 1.8% or less, and more preferably 1.6% or less. 
     [P: 0.030% or Less (not Inclusive of 0%)] 
     P is an element inevitably included in steel material. When P amount exceeds 0.030%, deterioration of the HAZ is extreme, and the hydrogen-induced cracking resistance also deteriorates. Therefore, in the present invention, P amount is suppressed to 0.030% or less. P amount is preferably 0.020% or less, and more preferably 0.010% or less. 
     [S: 0.004% or Less (not Inclusive of 0%)] 
     When S is excessive, a large amount of MnS is formed and the hydrogen-induced cracking resistance is deteriorated extremely, and therefore the upper limit of S amount is made 0.004% in the present invention. S amount is preferably 0.003% or less, more preferably 0.0025% or less, and still more preferably 0.0020% or less. Thus, from the viewpoint of improving the hydrogen-induced cracking resistance, S amount is preferable to be as little as possible. However, because it is hard to make S amount less than 0.0001% industrially, the lower limit of S amount is approximately 0.0001%. 
     [Al: 0.010-0.08%] 
     Al is an element effective in reducing the voids against the matrix phase of steel by reducing the coefficient of thermal expansion of inclusions, and securing the sour resistance. Also, Al is effective in lowering the melting point of the inclusions to increase the forming rate of the intragranular acicular α, securing the HAZ toughness, and reducing the hardness slope from coarse grains to fine grains. In order to exert the effect, Al should be made 0.010% or more. Al amount is preferably 0.020% or more, and more preferably 0.030% or more. 
     On the other hand, when Al is added prior to Zr and Al content is excessive, the oxide of Al is formed prior to the oxide of Zr, Zr concentration in inclusions lowers, and the particles of oxide of Al agglomerate and become the origin of the hydrogen-induced cracking. Therefore, Al amount should be 0.08% or less. Al amount is preferably 0.06% or less, and more preferably 0.05% or less. 
     [N: 0.001-0.01%] 
     N is an element precipitating as TiN in the steel microstructure, suppressing coarsening of the austenitic grain of the HAZ part, promoting the ferritic transformation, and improving the toughness of the HAZ part. In order to secure these effects, N should be contained by 0.001% or more. N amount is preferably 0.003% or more, and more preferably 0.0040% or more. However, when N amount is excessive, the HAZ toughness deteriorates adversely because of presence of solute N, and therefore N amount should be 0.01% or less. N amount is preferably 0.008% or less, and more preferably 0.0060% or less. 
     [Nb: 0.002-0.05% (not Inclusive of 0.05%)] 
     Nb is an element effective in increasing the strength without deteriorating the weldability. In order to secure this effect, Nb amount should be 0.002% or more. Nb amount is preferably 0.010% or more, and more preferably 0.020% or more. However, when Nb amount becomes 0.05% or more, the toughness of HAZ deteriorates. Therefore, in the present invention, Nb amount is made less than 0.05%. Nb amount is preferably 0.040% or less, and more preferably 0.030% or less. 
     [O: 0.0040% or Less (not Inclusive of 0%)] 
     O (oxygen) is preferable to be less from the viewpoint of improving the cleanliness. When a large amount of O is contained, in addition to that the toughness deteriorates, the HIC is generated from the origin of the oxide, and the hydrogen-induced cracking resistance deteriorates. From this viewpoint, O amount should be 0.0040% or less, is preferably 0.0030% or less, and more preferably 0.0020% or less. 
     [REM: 0.0002-0.05%] 
     REM (Rare Earth Metal) is effective in reducing the voids against the matrix phase of steel by reducing the coefficient of thermal expansion of inclusions, and securing the sour resistance. Also, REM is effective in lowering the melting point of the inclusions to increase the forming rate of the intragranular acicular α, securing the HAZ toughness, and reducing the hardness slope from coarse grains to fine grains. In order to exert such an effect, REM should be contained by 0.0002% or more. REM amount is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, even when a large amount of REM is contained, the effects saturate. Therefore, the upper limit of REM amount is made 0.05%. From the viewpoint of suppressing blockage of the immersion nozzle in casting and improving the productivity, REM amount is preferably 0.03% or less, more preferably 0.010% or less, and still more preferably 0.0050% or less. 
     In the present invention, the REM means the lanthanoid elements (15 elements from La to Lu), Sc (scandium), and Y (yttrium). 
     [Zr: 0.0003-0.020%] 
     Zr is effective in reducing the voids against the matrix phase of steel by reducing the coefficient of thermal expansion of inclusions, and securing the sour resistance. Also, Zr is effective in lowering the melting point of the inclusions to increase the forming rate of the intragranular acicular α, securing the HAZ toughness, and reducing the hardness slope from coarse grains to fine grains. In order to make Zr concentration in inclusions 5% or more for significantly improving the hydrogen-induced cracking resistance, Zr amount should be 0.0003% or more. Zr amount is preferably 0.0005% or more, more preferably 0.0010% or more, and still more preferably 0.0015% or more. On the other hand, when Zr is added excessively, solute Zr in the molten steel is increased and is crystallized so as to surround oxides and sulfides in casting, and the HAZ toughness and the hydrogen-induced cracking resistance are deteriorated. Therefore, Zr amount should be 0.020% or less. Zr amount is preferably 0.010% or less, more preferably 0.0070% or less, and still more preferably 0.0050% or less. 
     The componential composition of the steel of the steel plate of the present invention is as described above, and the remainder is iron and inevitable impurities. Also, by further containing at least one element selected from a group consisting of Ca, Mg, Ti, B, V, Cu, Ni, Cr, and Mo of the amount described below in addition to the elements described above, the HAZ toughness can be improved, the strength can be improved, and so on. Below, these elements will be explained. 
     [Ca: 0.0003-0.0060%] 
     Ca has an action of forming CaS and finely dispersing sulfides. In order to secure this effect, Ca amount should be 0.0003% or more. Ca amount is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, when Ca amount exceeds 0.0060%, CaS is formed excessively, is agglomerated, and affects the HAZ toughness and the HIC property adversely. Therefore, in the present invention, the upper limit of Ca amount is made 0.0060%. Ca amount is preferably 0.0050% or less, and more preferably 0.0040% or less. 
     [Mg: 0.0003-0.005%] 
     Mg has an action of forming MgS and finely dispersing sulfides. In order to secure this effect, it is preferable to contain Mg by 0.0003% or more. Mg amount is more preferably 0.001% or more. On the other hand, even when Mg is contained so as to exceed 0.005%, the effect saturates, and therefore the upper limit of Mg amount is preferably 0.005%. Mg amount is more preferably 0.0030% or less. 
     [Ti: 0.003-0.03%] 
     Ti is an element required for improving the toughness of the HAZ part because Ti prevents coarsening of the austenitic grains and promotes ferritic transformation in the HAZ part at the time of welding by precipitating as TiN in steel. In order to secure such effects, it is preferable to contain Ti by 0.003% or more. Ti amount is more preferably 0.005% or more, and still more preferably 0.010% or more. On the other hand, when Ti content becomes excessive, the amount of solute Ti and the number of TiC precipitates increase, the HAZ toughness deteriorates, and therefore 0.03% or less is preferable. Ti amount is more preferably 0.02% or less. 
     [B: 0.0002-0.005%] 
     B enhances the quenchability, and therefore improves the HAZ toughness. In order to secure this effect, it is preferable to contain B by 0.0002% or more. B amount is more preferably 0.0005% or more, and still more preferably 0.0010% or more. However, when B content becomes excessive, the HAZ toughness deteriorates and deterioration of the weldability is caused, and therefore B content is preferably 0.005% or less. B amount is more preferably 0.004% or less, and still more preferably 0.003% or less. 
     [V: 0.003-0.1%] 
     V is an element effective in improving the strength, and, in order to secure this effect, it is preferable to contain V by 0.003% or more. V amount is more preferably 0.010% or more. On the other hand, when V content exceeds 0.1%, the weldability deteriorates. Therefore, V amount is preferably 0.1% or less, and more preferably 0.08% or less. 
     [Cu: 0.01-1.5%] 
     Cu is an element effective in improving the quenchability and increasing the strength. In order to secure these effects, it is preferable to contain Cu by 0.01% or more. Cu amount is more preferably 0.05% or more, and still more preferably 0.10% or more. However, because the strength is increased excessively and the toughness deteriorates when Cu content exceeds 1.5%, 1.5% or less is preferable. Cu amount is more preferably 1.0% or less, and still more preferably 0.50% or less. 
     [Ni: 0.01-3.5%] 
     Ni is an element effective in improving the strength of the base plate and the HAZ toughness. In order to secure the effect, it is preferable to make Ni amount 0.01% or more. Ni amount is more preferably 0.05% or more, and still more preferably 0.10% or more. However, when Ni is contained excessively, the cost increases extremely as a structural steel material, and therefore it is preferable to make Ni amount 1.5% or less from the economical viewpoint. Ni amount is more preferably 1.0% or less, and still more preferably 0.50% or less. 
     [Cr: 0.01-1.5%] 
     Cr is an element effective in improving the strength, and, in order to secure this effect, it is preferable to contain Cr by 0.01% or more. Cr amount is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, when Cr amount exceeds 1.5%, the HAZ toughness deteriorates. Therefore it is preferable to make Cr amount 1.5% or less. Cr amount is more preferably 1.0% or less, and still more preferably 0.50% or less. 
     [Mo: 0.01-1.5%] 
     Mo is an element effective in improving the strength of the base plate. In order to secure the effect, it is preferable to make Mo amount 0.01% or more. Mo amount is more preferably 0.05% or more, and still more preferably 0.10% or more. However, when Mo amount exceeds 1.5%, the HAZ toughness and weldability deteriorate. Therefore, Mo amount is preferably 1.5% or less, more preferably 1.0% or less, and still more preferably 0.50% or less. 
       [10,000×[Nb]+31×Di−82≧0  Expression 1]
 
       (where 
         Di =[C]/10) 0.5 ×(1+0.7×[Si])×(1+3.33×[Mn])×(1+0.35×[Cu])×(1+0.36×[Ni])×(1+2.16×[Cr])×(1+3×[Mo])×(1+1.75×[V])×1.115  Expression 2)
 
     This item is to determine the Nb-Di balance namely the relation between the Nb amount and the Di value (quenchability: Expression 2), and above Expression 1 should be satisfied. All of the compositions within [ ] are in mass %. Also, above Expression 2 on the quenchability Di value is one that is described as the Grossmann Expression (Trans. Metall. Soc. AIME, 150 (1942), p. 227). 
     Because the Nb-Di balance controls the steel composition so as to satisfy above Expression 1, higher strength (yield strength, tensile strength) of the base plate can be secured by accelerated cooling, and a steel plate having smaller slope of the HAZ hardness and excellent in the hydrogen-induced cracking resistance can be obtained. 
     Further, although the Nb-Di balance does not particularly become an issue with respect to the properties specified in the present invention, from the viewpoint of the tolerance in controlling the property of the base plate, preferable range of the Nb-Di balance is [200≦10,000×[Nb]+31×Di≦300]. 
     (Composition of Inclusions) 
     [Composition of Inclusions with 1 μm or More Width Contained in Steel] 
     [Zr Amount is 1-40%] 
     Because Zr oxide has smaller coefficient of thermal expansion than that of the steel, when Zr amount in inclusions is secured, the voids against the surrounding matrix phase of steel can be reduced, which can function effectively in securing the sour resistance. Also, Zr oxide is effective in lowering the melting point of inclusions, increasing the intragranular forming rate, improving the HAZ toughness, and reducing the dispersion of the HAZ hardness. In order to exert such effects, Zr amount in inclusions is made 1-40%. When Zr amount is less than 1%, the sour resistance and/or HAZ toughness and reduction of the dispersion of the HAZ hardness become insufficient. On the other hand, when Zr amount exceeds 40%, because the lattice matching between the inclusions and the matrix phase of steel deteriorates, the intragranular a forming rate lowers, and the HAZ toughness and reduction of the dispersion of the HAZ hardness deteriorate. 
     [REM Amount is 5-50%] 
     Because REM oxide has smaller coefficient of thermal expansion than that of the steel, when REM amount in inclusions is secured, the voids against the surrounding matrix phase of steel can be reduced and S can be fixed and finely dispersed, which can function effectively in securing the sour resistance. Also, REM oxide is effective in lowering the melting point of inclusions, increasing the intragranular forming rate, improving the HAZ toughness, and reducing the dispersion of the HAZ hardness. In order to exert such effects, REM amount in inclusions is made 5-50%. When REM amount is less than 5%, the sour resistance and/or HAZ toughness and reduction of the dispersion of the HAZ hardness become insufficient. On the other hand, when REM amount exceeds 50%, because the lattice matching between the inclusions and the matrix phase of steel deteriorates, the intragranular a forming rate lowers, the HAZ toughness lowers, and the dispersion of the HAZ hardness increases. 
     [Al Amount is 3-30%] 
     Because Al oxide has smaller coefficient of thermal expansion than that of the steel, when Al amount in inclusions is secured, the voids against the surrounding matrix phase of steel can be reduced, which can function effectively in securing the sour resistance. Also, Al oxide is effective in lowering the melting point of inclusions, increasing the intragranular forming rate, improving the HAZ toughness, and reducing the dispersion of the HAZ hardness. In order to exert such effects, Al amount in inclusions is made 3-30%. When Al amount is less than 3%, the sour resistance and/or HAZ toughness and reduction of the dispersion of the HAZ hardness become insufficient. On the other hand, when Al amount exceeds 30%, because the lattice matching between the inclusions and the matrix phase of steel deteriorates, the intragranular a forming rate lowers, the HAZ toughness lowers, and the dispersion of the HAZ hardness increases. 
     [S Amount is Over 0% and Less than 20%] 
     Because coarse sulfide deteriorates the sour resistance, it should be reduced. In order to reduce the adverse effect of the coarse sulfide on the sour resistance, it is effective to reduce the S amount in steel by desulfurization and finely dispersing S. By adding REM, S can be fixed and finely dispersed. This effect can be grasped indirectly by measuring the S amount in the inclusions, and the adverse effect of coarse sulfides on the sour resistance can be suppressed when the S amount in the inclusions is over 0% and less than 20%. The case the S amount is 0% shows that S has not been fixed, and the sour resistance deteriorates. On the other hand, when the S amount is 20% or more, although S has been fixed, the coarse sulfide is liable to be formed, and the sour resistance deteriorates. 
     [Ca Amount is 5-60%] 
     When the steel plate of the present invention contains Ca, by making the Ca amount in inclusions a predetermined range, intragranular acicular α originated from inclusions comes to be formed vigorously even in the T-cross weld joint, and the SSCC resistance of the T-cross weld joint is improved by the formation of the fine microstructure. In order to exert such an effect, Ca amount in inclusions is made 5-60%. When Ca amount is less than 5% or exceeds 60%, the SSCC resistance of the T-cross weld joint cannot be improved. 
     (Manufacturing Method) 
     [Molten Steel Processing Step] 
     Next, a method for manufacturing the steel plate of the present invention will be explained in detail below. 
     In order to obtain the steel plate of the present invention having the microstructure described above, it is necessary in the molten steel processing step that 
     (A) desulfurizing step of making S 0.004% or less using the slag that satisfies Fe: 0.1-10%,
 
(B) deoxidizing step of making the dissolved oxygen concentration Of of the molten steel 10 or less in terms of the ratio relative to the S concentration of the molten steel (Of/S), and
 
(C) step for adding Zr, REM and Ca either in the order of Zr, REM, Ca, or in the order of adding Zr and REM simultaneously and then adding Ca (where the time after adding REM until adding Ca is to be made 4 min or more) are included in this order, the time after adding Ca until completion of solidification is made 200 min or less, and the cooling time at t/4 position (t: plate thickness) of the slab of 1,300° C.-1,200° C. in casting is made 460 s or less. Also, it is preferable to make the cooling time at t/4 position (t: plate thickness) of the slab of 1,500-1,450° C. in casting 300 s or less because the SSCC resistance can be improved.
 
     Each step described above will be explained below in order. 
     (A) Desulfurizing Step 
     In order to secure the sour resistance, reduction of coarse sulfides is important, and to control the S amount is important in order to achieve it. In a converter or an electric furnace, with respect to the molten steel molten so that the molten steel temperature becomes 1,550° C. or above, S is made 0.004% or less using the slag that satisfies Fe: 0.1-10%. By increasing the Fe concentration in the slag, REM and Zr added after desulfurization and deoxidizing can form oxides preferentially without being dissolved in the molten steel. In order to secure this effect, the Fe concentration in the slag is made 0.1% or more. The Fe concentration in the slag is preferably 0.5% or more, and more preferably 1.0% or more. On the other hand, when the Fe concentration in the slag exceeds 10%, oxides are formed excessively, the oxides not only become the origin of the hydrogen-induced cracking but also deteriorate the toughness of the base plate and the HAZ. Therefore, the Fe concentration in the slag is made 10% or less. The Fe concentration in the slag is preferably 8% or less, and more preferably 5% or less. Also, when Ca is added, by sufficiently executing desulfurization in the slag and suppressing S to 0.004% or less, CaS can be prevented from being formed excessively when Ca is added after adding REM, the composition of inclusions can be prevented from deviating from a predetermined range, and thereby the HIC resistance and the SSCC resistance can be secured. 
     As a means for making S 0.004% or less as described above, (a) and (b) below can be cited. 
     (a) It can be cited to blow-in an inert gas (Ar and the like) of 5 Nm 3 /h or more of the flow rate (preferably 10 Nm 3 /h or more, the upper limit of the flow rate is approximately 300 Nm 3 /h) using a ladle desulfurizing apparatus (LF and the like) for example, and to execute stirring for 3 min or more (preferably 10 min or more, more preferably 20 min or more, the upper limit of the stirring time is approximately 200 min from the viewpoint of the productivity). 
     (b) Also, when Ca is added, the CaO concentration in the slag described above is made 10% or more. By that CaO in the slag reacts with dissolved S in the molten steel and changes to CaS instead of adding Ca, reduction of S in the molten steel namely desulfurization can be executed sufficiently. Also, at this time, if the CaO concentration in the slag is made 10% or more, S can be made 0.004% or less. The CaO concentration in the slag is preferably 15% or more, and more preferably 20% or more. On the other hand, even when CaO in the slag is excessive, desulfurization becomes difficult, and therefore the upper limit of the CaO concentration in the slag is approximately 80%. 
     (B) Deoxidizing Step 
     In order to improve the HAZ toughness, to control oxides is important, and to control the O amount is vital in order to achieve it. In this step, because the S amount that is influential for the sour resistance slightly increases namely so called S-return takes place, it is important to control the O amount and the S amount simultaneously. In this step, prior to adding of REM as described below, the dissolved oxygen concentration Of of the molten steel is made 10 or less in terms of the ratio relative to the S concentration of the molten steel (Of/S). When REM is added into the molten steel, the REM forms oxides at the same time of forming the sulfides thereof. When above Of/S exceeds 10, the major portion of REM added forms oxides, and the composition of the inclusions deviates from the predetermined range. As a result, the HIC resistance and the SSCC resistance deteriorate. Therefore, in the present invention, Of/S is made 10 or less as described above. Of/S is preferably 5 or less, more preferably 3.5 or less, and still more preferably 2.0 or less. Also, the lower limit value of Of/S is approximately 0.1. To make Of/S 10 or less as described above can be achieved by deoxidation by an RH degassing apparatus and/or deoxidation by feeding deoxidizing elements such as Mn, Si, and Ti. 
     (C) Adding Step of Al, Zr, REM (and Ca) 
     With respect to adding Al, Zr, and REM to the molten steel, Al is added first, and (Zr, REM) are added then. In this regard, when deoxidizing capacity of Al and (Zr, REM) is compared, because the deoxidizing power of (Zr, REM) is stronger than that of Al, if (Zr, REM) is added prior to Al, the Al amount in inclusions cannot be made a desired value. Therefore, the adding order should be made Al→(Zr, REM). 
     In the case Ca is further added in addition to these elements, in considering the desulfurizing power and the deoxidizing power of each adding element described below, either method of adding Al first, then adding Zr, adding REM next, and adding Ca lastly, or adding Al first, then adding Zr and REM at the same time, and adding Ca lastly is to be employed. However, in either case, the time after adding REM until adding Ca is to be made 4 min or more. 
     The reason of the above will be explained. First, when the desulfurizing capacity of REM and Ca is compared, the desulfurizing power of REM is weaker than that of Ca, therefore, if Ca is added before adding REM, a large amount of CaS is formed, the composition of the inclusions deviates from the predetermined range, and thereby the sour resistance is deteriorated. Because REM should be added before adding Ca, the adding order of Al, Zr, REM and Ca should be Al→(Zr, REM)→Ca. Also, in order to control the range of the inclusions to the predetermined range, the time of adding REM and the time of adding Ca should be apart from each other by 4 min or more. Also, the time after adding REM until adding Ca is preferably 5 min or more, and more preferably 8 min or more. Further, from the viewpoint of the productivity, the upper limit of the time after adding REM until adding Ca becomes approximately 60 min. 
     Next, when the deoxidizing capacity of Zr, REM and Ca is compared, it is considered in general that the deoxidizing power of Ca is strongest and the deoxidizing power is in the order of Ca&gt;REM&gt;Zr, and Zr is lowest. Therefore, in order to contain Zr in the inclusions (namely to form ZrO 2  as the oxide-based inclusions), Zr should be added prior to adding Ca and REM whose deoxidizing power is stronger than that of Zr. Therefore, the adding order of Al, Zr, REM and Ca should be Al→Zr→REM→Ca. However, because the deoxidizing capacity of REM is smaller compared to Ca, even if REM is added simultaneously with Zr, Zr can be contained in the inclusions, and therefore the adding order of them may also be Al→(Zr, REM)→Ca. 
     With respect to the adding amount of each element described above, the steel plate having each desired element amount only has to be obtained, and such a method can be cited for example to add Zr so as to become 3-100 ppm in terms of the concentration in the molten steel, to add REM thereafter or simultaneously so as to become 2-500 ppm in terms of the concentration in the molten steel, and, after 4 min or more elapses thereafter, to add Ca so as to become 3-60 ppm in terms of the concentration in the molten steel. 
     [Casting Step] 
     After Ca is added as described above, casting is started quickly (within 80 min for example), and casting is executed so that the time after adding Ca until completion of solidification becomes 200 min or less. The reason of doing so is as follows. Because Ca is an element that is high in both of the desulfurizing capacity and the deoxidizing capacity, the Ca concentration in inclusions is liable to increase, and the composition of the inclusions deviates from the predetermined range. Therefore, in the present invention, the time after adding Ca until completion of solidification is made 200 min or less, preferably 180 min or less, and more preferably 160 min or less. The lower limit of the above time is approximately 4 min from the viewpoint of homogenizing Ca. 
     Further, it is important to make the cooling time of 1,300° C.-1,200° C. at the time of casting 270-460 s. When the cooling time exceeds the upper limit, complex formation of the secondary inclusions mainly of the sulfide-basis over the inclusions is promoted, the composition of the inclusions deviates from the predetermined range, and the difference of the HAZ hardness thereby deviates from a predetermined range. On the other hand, when the cooling time becomes less than the lower limit thereof, the cooling load significantly increases, which is not preferable practically. 
     Also, by making the cooling time of 1,500° C.-1,450° C. at the time of casting 300 s or less, complex formation of the oxide-based secondary inclusions to over the inclusions is promoted, the composition of the inclusions more effective for formation of the acicular α is achieved, and the improving effect is secured even in the SSCC resistance of the T-cross weld joint. 
     [Step of Rolling and Onward] 
     After the solidification described above, hot rolling is executed according to an ordinary method, and the steel plate (thick steel plate) can be manufactured. Also, using the steel plate, the steel pipe for a line pipe can be manufactured by a method generally executed. Although the steps of rolling and onward are not particularly limited, it is preferable for example to heat a casted slab to 1,100° C. or above, to execute hot rolling with the compression reduction of 40% or more at the recrystallization temperature, and to cool it (accelerated cooling) from 780° C. with the cooling rate of 10-20° C./s. The conditioning thereafter is not necessary. 
     Example 
     The molten steel melted by an ordinary method using a 240 t converter was subjected to processing (molten steel processing) such as desulfurizing, deoxidizing, composition regulating, and inclusions controlling using an LF furnace, various kinds of molten steel having the steel composition and the composition of the inclusions in steel shown in Tables 1, 2, 9, 10 (invention examples) and Tables 3, 4, 9, 10 (comparative examples) were made slabs by the continuous casting method, the slabs were subjected to accelerated cooling after hot rolling, and steel plates (thick steel plates) with 40 mm thickness and 3,500 mm width were manufactured. Further, in Tables 2, 10 (invention examples) and Tables 4, 10 (comparative examples), the composition of the coarse inclusions in steel is also shown. Tables 5, 11 (invention examples) and Tables 6, 11 (comparative examples) show the main process conditions in the molten steel processing, continuous casting, and accelerated cooling described above. Tables 7, 12 (invention examples) and Tables 8, 12 (comparative examples) show the various properties of each steel plate thus obtained. 
     The analyzing method for the composition of the inclusions shown in Tables 2, 4, 10, the measuring (testing) method for each property of Tables 7, 8, 12, and the method of evaluation will be explained below. 
     [Analysis of Composition of Inclusions] 
     Analysis of the composition of the inclusions was executed as follows. The cross section in the plate thickness direction of the as-rolled material was observed focusing the plate thickness center part using EPMA-8705 made by Shimadzu Corporation. To be more specific, three cross sections were observed with 400 observation magnification and approximately 50 mm 2  field of view (7 mm in the plate thickness direction and 7 mm in the plate width direction so that the plate thickness center part became the center of the field of view), and the componential composition at the inclusion center part was quantitatively analyzed by wave length dispersion spectrometry of the characteristic X-ray for the inclusions with 1 μm or more width. 
     The elements of the analyzing object were made Al, Mn, Si, Mg, Ca, Ti, Zr, S, REM (La, Ce, Nd, Dy, Y), and Nb. The relation between the X-ray strength and the element concentration of each element was obtained beforehand as the analytical curve using known matters, and the element concentration of the inclusions was determined then from the X-ray strength obtained from the inclusions and the analytical curve described above. 
     Also, the average value of the content of each element descried above of the inclusions with 1 μm or more width in 3 cross sections described above (composition of inclusions) was obtained. 
     [Measurement of Yield Strength YS and Tensile Strength TS of Base Plate and Evaluation] 
     No. 4 specimen of JIS Z 2241 was taken in parallel with C direction from the t/4 position (t: plate thickness) of each steel plate, the tensile test was executed by the method described in JIS Z 2241, and the tensile strength TS and the yield strength YS were measured. In the present example, those with 415 MPa or more of YS and 520 MPa or more of TS were evaluated to be excellent (passed) in the base plate strength, and those with less than 415 MPa of YS and less than 520 MPa of TS were evaluated to be inferior (failed) in the base plate strength. 
     [Test of HIC Resistance of Base Plate and Evaluation] 
     The test and evaluation were executed according to the method defined in NACE Standard TM 0284-2003. In concrete terms, the specimen was immersed for 96 hours in the 25° C. (0.5% NaCl+0.5% acetic acid) aqueous solution saturated with 1 atm hydrogen sulfide. 
     With respect to evaluation of the HIC test, each specimen was cut at 10 mm pitch in the longitudinal direction, the cut surface was polished, all cross sections were thereafter observed with 100 magnifications using an optical microscope, the number of piece of the cracking with 200 μm or more of the cracking length of HIC and the number of piece of the cracking with 1 mm or more were measured respectively. In the present invention, those without the cracking with 1 mm or more of the cracking length of HIC described above were evaluated to be excellent (passed) in the HIC resistance, and the case in which one or more of the cracking with 1 mm or more existed was evaluated to be inferior (failed) in the HIC resistance. 
     [Measurement of SSCC Resistance of T-Cross Weld Joint and Evaluation] 
     In order to simulate the seam welding in working the thick steel plate into a pipe, the rolled plate was worked to have a 75° X bevel, welding was executed by 2-pass submerged arc welding, and the pipe was manufactured. The heat input at the time of welding was made first pass: 3.7 kJ/mm, and second pass: 5.4 kJ/mm. Also, in order to simulate the girth welding in joining the pipes with each other, referring to “Practical application of UOE steel pipe excellent in SSCC resistance, Matsuyama et. al., Yosetsu Gijutsu (Welding Technology), September 1988, p. 58”, 1 pass bead-on-plate welding by gas shield arc welding was executed so as to orthogonally cross the seam weld line. The heat input at the time of welding was made 1.0 kJ/mm. 
     The surface of the weld part of the pipe joint body after welding was subjected to grinding, and the excess metal portion of the bead welding was removed. From right below the bead weld part of this pipe joint body, the specimen of 115L×15W×5t was taken so that the longitudinal direction became parallel with the bead weld line. Using this specimen, the SSCC resistance evaluation test with 4-point bending test piece was executed based on ASTM G39, NACE TM 0177-2005, Method B. The specimen was applied with the deflection equivalent to 332 MPa and 374 MPa of the load stress and was immersed for 720 hours in the NACE solution A (5 mass % NaCl-0.5 mass % CH 3 COOH) saturated with 1 atm hydrogen sulfide, and those in which the cracking did not occur thereafter on the surface of the specimen were evaluated to have passed. 
     [Measurement of HAZ Toughness (Percent Brittle Fracture in C Direction) and Evaluation] 
     The thermal cycle test specimen (12.5t×33L×55W) was taken from the t/4 position (t: plate thickness) of each steel plate in parallel with C direction, and was subjected to the thermal cycle of 1,400° C. (maximum temperature)×5 s (heat retention time), Tc (cooling time of 800-500° C.)=400 s. Thereafter, the Charpy impact test specimen (V-notch specimen of JIS Z 2242) was taken by 2 pieces each from the thermal cycle test specimen, the impact test was executed for 3 pieces for each measuring temperature by the method described in JIS Z 2242, and vTrs was obtained. Those in which vTrs was −10° C. or below were evaluated to be excellent (passed) in the HAZ toughness, and those in which vTrs exceeded −10° C. were evaluated to be inferior (failed) in the HAZ toughness. Further, the thermal cycle test described above corresponds to the large heat input welding condition equivalent to heat input=60 kJ/mm. 
     [Measurement of Hardness Difference Between CG-HAZ and FG-HAZ] 
     The thermal cycle test specimen (12.5t×33L×55W) was taken from the t/4 position (t: plate thickness) of each steel plate in parallel with C direction, and was subjected to the thermal cycle of 1,400° C.×5 s, Tc=40 s (CG-HAZ test), and 1,100° C.×5 s, Tc=40 s (FG-HAZ test). Thereafter, the steel pieces equivalent to the Charpy impact test specimen (V-notch specimen of JIS Z 2242) were taken from the CG-HAZ and FG-HAZ thermal cycle test specimens, the hardness of the cross section in C direction was measured by Vickers 10 kg for N=3 or more, the average value was obtained, and the difference of these CG-HAZ hardness and FG-HAZ hardness was calculated. Those in which this hardness difference ((CG-HAZ hardness)−(FG-HAZ hardness)) was 45 or less were evaluated to be low in the dispersion of the hardness and to be excellent (passed) in the HAZ hardness, and those in which the hardness difference exceeded 45 were evaluated to be inferior (failed) in the HAZ hardness. 
     As is clear from the comparison of respective properties of the invention examples of Tables 7, 12 and the comparative examples of Tables 8, 12 showing the result of these examples, it is known that the steel plates of the invention examples which satisfy the steel composition (including the Nb-Di balance) and the composition of the coarse inclusions in steel with 1 μm or more width determined by the present invention secure the high mechanical strength of 415 MPa or more of the yield strength (YS) and 520 MPa or more of the tensile strength (TS), do not cause the cracking by the HIC test, are excellent in the sour resistance, have vTrs (CG) by the impact test which exceeds −10° C. or below, have excellent HAZ toughness even under a large heat input welding condition, have stable difference of the CG-HAZ hardness and the FG-HAZ hardness which is approximately 30, and have excellent HAZ hardness with less dispersion. Further, in the invention examples 26-32 in which the Ca amount in the coarse inclusions in steel with 1 μm or more width is within the range of 5-60%, the cracking by the SSCC test also does not occur. On the other hand, it is clarified that the steel plates of the comparative examples which do not satisfy the steel composition or the composition of the coarse inclusions determined by the present invention are extremely inferior in the sour resistance and/or HAZ toughness, and the HAZ hardness compared to the invention examples although the majority of them secure the properties generally equivalent to those of the invention examples with respect to the mechanical strength described above. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Steel composition 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Nb- 
               
               
                   
                 C 
                 Si 
                 Mn 
                 S 
                 P 
                 Al 
                 Nb 
                 O 
                 N 
                 REM 
                 Zr 
                 Ca 
                 Ti 
                 Others 
                 Di balance 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Invention 
                 0.06 
                 0.28 
                 1.03 
                 0.0014 
                 0.007 
                 0.026 
                 0.03 
                 0.0012 
                 0.0063 
                 0.0020 
                 0.0010 
                 0.0015 
                 0.012 
                   
                 232 
               
               
                 example 1 
               
               
                 Invention 
                 0.15 
                 0.30 
                 1.00 
                 0.0012 
                 0.007 
                 0.028 
                 0.032 
                 0.0014 
                 0.0052 
                 0.0018 
                 0.0009 
                 0.0018 
                 0.014 
                   
                 256 
               
               
                 example 2 
               
               
                 Invention 
                 0.04 
                 0.29 
                 1.02 
                 0.0016 
                 0.006 
                 0.024 
                 0.032 
                 0.0009 
                 0.0063 
                 0.0019 
                 0.0010 
                 0.0013 
                 0.011 
                   
                 246 
               
               
                 example 3 
               
               
                 Invention 
                 0.06 
                 0.45 
                 1.05 
                 0.0014 
                 0.007 
                 0.025 
                 0.029 
                 0.0014 
                 0.0063 
                 0.0024 
                 0.0010 
                 0.0016 
                 0.011 
                   
                 221 
               
               
                 example 4 
               
               
                 Invention 
                 0.06 
                 0.05 
                 1.00 
                 0.0012 
                 0.008 
                 0.025 
                 0.028 
                 0.0009 
                 0.0059 
                 0.0022 
                 0.0013 
                 0.0018 
                 0.011 
                   
                 206 
               
               
                 example 5 
               
               
                 Invention 
                 0.06 
                 0.29 
                 1.8 
                 0.0014 
                 0.005 
                 0.026 
                 0.031 
                 0.0012 
                 0.0051 
                 0.0024 
                 0.0008 
                 0.0015 
                 0.014 
                   
                 247 
               
               
                 example 6 
               
               
                 Invention 
                 0.05 
                 0.28 
                 0.65 
                 0.0012 
                 0.005 
                 0.024 
                 0.029 
                 0.0013 
                 0.0069 
                 0.0017 
                 0.0007 
                 0.0016 
                 0.011 
                   
                 219 
               
               
                 example 7 
               
               
                 Invention 
                 0.07 
                 0.28 
                 1.06 
                 0.003 
                 0.006 
                 0.025 
                 0.028 
                 0.0014 
                 0.0044 
                 0.0022 
                 0.0008 
                 0.0018 
                 0.010 
                   
                 214 
               
               
                 example 8 
               
               
                 Invention 
                 0.07 
                 0.28 
                 1.01 
                 0.0001 
                 0.006 
                 0.027 
                 0.032 
                 0.0011 
                 0.0085 
                 0.0020 
                 0.0010 
                 0.0019 
                 0.012 
                   
                 250 
               
               
                 example 9 
               
               
                 Invention 
                 0.06 
                 0.26 
                 1.01 
                 0.0012 
                 0.025 
                 0.027 
                 0.028 
                 0.0010 
                 0.0047 
                 0.0018 
                 0.0013 
                 0.0013 
                 0.010 
                   
                 213 
               
               
                 example 10 
               
               
                 Invention 
                 0.06 
                 0.29 
                 1.07 
                 0.0014 
                 0.009 
                 0.025 
                 0.007 
                 0.0010 
                 0.0049 
                 0.0023 
                 0.0012 
                 0.0015 
                 0.013 
                   
                 3 
               
               
                 example 11 
               
               
                 Invention 
                 0.07 
                 0.29 
                 0.98 
                 0.0016 
                 0.007 
                 0.028 
                 0.048 
                 0.0009 
                 0.0045 
                 0.0023 
                 0.0012 
                 0.0013 
                 0.011 
                   
                 413 
               
               
                 example 12 
               
               
                 Invention 
                 0.07 
                 0.29 
                 0.99 
                 0.0012 
                 0.008 
                 0.025 
                 0.028 
                 0.0009 
                 0.0049 
                 0.0016 
                 0.0008 
                 0.0015 
                 0.012 
                   
                 214 
               
               
                 example 13 
               
               
                 Invention 
                 0.06 
                 0.26 
                 1.01 
                 0.0012 
                 0.009 
                 0.024 
                 0.032 
                 0.0032 
                 0.0035 
                 0.0017 
                 0.0008 
                 0.0016 
                 0.013 
                   
                 256 
               
               
                 example 14 
               
               
                 Invention 
                 0.06 
                 0.27 
                 0.98 
                 0.0015 
                 0.006 
                 0.025 
                 0.032 
                 0.0010 
                 0.0051 
                 0.0080 
                 0.0010 
                 0.0013 
                 0.010 
                   
                 247 
               
               
                 example 15 
               
               
                 Invention 
                 0.06 
                 0.29 
                 1.02 
                 0.0013 
                 0.007 
                 0.027 
                 0.032 
                 0.0009 
                 0.0066 
                 0.0024 
                 0.0070 
                 0.0014 
                 0.011 
                   
                 249 
               
               
                 example 16 
               
               
                 Invention 
                 0.07 
                 0.29 
                 1.04 
                 0.0012 
                 0.008 
                 0.026 
                 0.028 
                 0.0014 
                 0.0044 
                 0.0019 
                 0.0011 
                 0.0018 
                 0.013 
                 Mg: 0.0012 
                 213 
               
               
                 example 17 
               
               
                 Invention 
                 0.06 
                 0.29 
                 1.05 
                 0.0012 
                 0.006 
                 0.025 
                 0.028 
                 0.0012 
                 0.0052 
                 0.0020 
                 0.0010 
                 0.0012 
                 0.011 
                 B: 0.0015 
                 214 
               
               
                 example 18 
               
               
                 Invention 
                 0.05 
                 0.26 
                 1.04 
                 0.0012 
                 0.008 
                 0.027 
                 0.030 
                 0.0012 
                 0.0046 
                 0.0021 
                 0.0008 
                 0.0012 
                 0.013 
                 V: 0.05 
                 232 
               
               
                 example 19 
               
               
                 Invention 
                 0.06 
                 0.26 
                 1.04 
                 0.0014 
                 0.006 
                 0.028 
                 0.032 
                 0.0010 
                 0.0043 
                 0.0025 
                 0.0010 
                 0.0014 
                 0.012 
                 Cu: 0.2 
                 251 
               
               
                 example 20 
               
               
                 Invention 
                 0.07 
                 0.29 
                 1.08 
                 0.0011 
                 0.008 
                 0.026 
                 0.032 
                 0.0012 
                 0.0057 
                 0.0024 
                 0.0008 
                 0.0013 
                 0.013 
                 Ni: 0.2 
                 258 
               
               
                 example 21 
               
               
                 Invention 
                 0.07 
                 0.27 
                 1.04 
                 0.0012 
                 0.006 
                 0.026 
                 0.028 
                 0.0016 
                 0.0049 
                 0.0015 
                 0.0013 
                 0.0017 
                 0.012 
                 Cr: 0.15 
                 219 
               
               
                 example 22 
               
               
                 Invention 
                 0.06 
                 0.26 
                 1.03 
                 0.0012 
                 0.005 
                 0.024 
                 0.028 
                 0.0016 
                 0.0052 
                 0.0023 
                 0.0010 
                 0.0017 
                 0.011 
                 Mo: 0.05 
                 211 
               
               
                 example 23 
               
               
                 Invention 
                 0.06 
                 0.27 
                 0.99 
                 0.0014 
                 0.006 
                 0.026 
                 0.030 
                 0.0014 
                 0.0038 
                 0.0020 
                 0.0015 
                 0.0017 
                 — 
                 Ti-free 
                 232 
               
               
                 example 24 
               
               
                 Invention 
                 0.06 
                 0.26 
                 1.03 
                 0.0009 
                 0.005 
                 0.030 
                 0.031 
                 0.0015 
                 0.0051 
                 0.0020 
                 0.0012 
                 — 
                 0.011 
                 Ca-free 
                 242 
               
               
                 example 25 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
                 Composition of coarse inclusions in steel  
               
               
                   
                 (mass %) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Zr  
                 REM  
                 Al  
                 S  
               
               
                   
                 amount 
                 amount 
                 amount 
                 amount 
               
               
                   
               
               
                 Invention example 1 
                 16% 
                 28% 
                 13% 
                 14% 
               
               
                 Invention example 2 
                 15% 
                 33% 
                 15% 
                 14% 
               
               
                 Invention example 3 
                 17% 
                 32% 
                 12% 
                 15% 
               
               
                 Invention example 4 
                 15% 
                 24% 
                 13% 
                 13% 
               
               
                 Invention example 5 
                 18% 
                 28% 
                 11% 
                 16% 
               
               
                 Invention example 6 
                 14% 
                 31% 
                 12% 
                 12% 
               
               
                 Invention example 7 
                 13% 
                 28% 
                 15% 
                 12% 
               
               
                 Invention example 8 
                 12% 
                 24% 
                 17% 
                 11% 
               
               
                 Invention example 9 
                 17% 
                 32% 
                 10% 
                 16% 
               
               
                 Invention example 10 
                 20% 
                 24% 
                 11% 
                 18% 
               
               
                 Invention example 11 
                 17% 
                 34% 
                 11% 
                 15% 
               
               
                 Invention example 12 
                 17% 
                 30% 
                 11% 
                 15% 
               
               
                 Invention example 13 
                 15% 
                 32% 
                 14% 
                 14% 
               
               
                 Invention example 14 
                 12% 
                 18% 
                 18% 
                 11% 
               
               
                 Invention example 15 
                 1.3%  
                 49% 
                  7% 
                 10% 
               
               
                 Invention example 16 
                 38% 
                 13% 
                  5% 
                 19% 
               
               
                 Invention example 17 
                 16% 
                 33% 
                 14% 
                 15% 
               
               
                 Invention example 18 
                 17% 
                 31% 
                 11% 
                 15% 
               
               
                 Invention example 19 
                 14% 
                 33% 
                 12% 
                 12% 
               
               
                 Invention example 20 
                 16% 
                 27% 
                 11% 
                 14% 
               
               
                 Invention example 21 
                 14% 
                 23% 
                 11% 
                 12% 
               
               
                 Invention example 22 
                 18% 
                 36% 
                 15% 
                 17% 
               
               
                 Invention example 23 
                 14% 
                 31% 
                 13% 
                 13% 
               
               
                 Invention example 24 
                 18% 
                 44% 
                  3% 
                 16% 
               
               
                 Invention example 25 
                 19% 
                 48% 
                  5% 
                 17% 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                 Steel composition 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 C 
                 Si 
                 Mn 
                 S 
                 P 
                 Al 
                 Nb 
                 O 
                 N 
                 REM 
                 Zr 
                 Ca 
                 Ti 
                 Others 
                 Nb-Di balance 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Comparative 
                 0.23 
                 0.26 
                 0.98 
                 0.0011 
                 0.007 
                 0.027 
                 0.030 
                 0.0011 
                 0.0053 
                 0.0015 
                 0.0012 
                 0.0013 
                 0.013 
                   
                 243 
               
               
                 example 1 
               
               
                 Comparative 
                 0.01 
                 0.27 
                 1.00 
                 0.0016 
                 0.009 
                 0.026 
                 0.032 
                 0.0011 
                 0.0052 
                 0.0018 
                 0.0011 
                 0.0018 
                 0.011 
                   
                 246 
               
               
                 example 2 
               
               
                 Comparative 
                 0.06 
                 0.65 
                 1.04 
                 0.0014 
                 0.005 
                 0.028 
                 0.032 
                 0.0009 
                 0.0043 
                 0.0022 
                 0.0007 
                 0.0015 
                 0.013 
                   
                 251 
               
               
                 example 3 
               
               
                 Comparative 
                 0.07 
                 0.30 
                 2.2 
                 0.0012 
                 0.008 
                 0.024 
                 0.029 
                 0.0016 
                 0.0064 
                 0.0016 
                 0.0010 
                 0.0013 
                 0.011 
                   
                 237 
               
               
                 example 4 
               
               
                 Comparative 
                 0.05 
                 0.26 
                 1.06 
                 0.005 
                 0.005 
                 0.027 
                 0.029 
                 0.0010 
                 0.0059 
                 0.0018 
                 0.0013 
                 0.0013 
                 0.011 
                   
                 224 
               
               
                 example 5 
               
               
                 Comparative 
                 0.06 
                 0.29 
                 0.99 
                 0.0016 
                 0.037 
                 0.025 
                 0.030 
                 0.0012 
                 0.0056 
                 0.0018 
                 0.0013 
                 0.0017 
                 0.012 
                   
                 231 
               
               
                 example 6 
               
               
                 Comparative 
                 0.05 
                 0.27 
                 1.07 
                 0.0014 
                 0.007 
                 0.1 
                 0.031 
                 0.0009 
                 0.0057 
                 0.0019 
                 0.0011 
                 0.0014 
                 0.012 
                   
                 237 
               
               
                 example 7 
               
               
                 Comparative 
                 0.05 
                 0.29 
                 1.00 
                 0.0014 
                 0.008 
                 0.005 
                 0.028 
                 0.0011 
                 0.0050 
                 0.0024 
                 0.0012 
                 0.0013 
                 0.013 
                   
                 215 
               
               
                 example 8 
               
               
                 Comparative 
                 0.06 
                 0.27 
                 1.04 
                 0.0012 
                 0.008 
                 0.024 
                 0.072 
                 0.0010 
                 0.0059 
                 0.0019 
                 0.0009 
                 0.0019 
                 0.011 
                   
                 652 
               
               
                 example 9 
               
               
                 Comparative 
                 0.05 
                 0.27 
                 1.03 
                 0.0015 
                 0.008 
                 0.028 
                 0.029 
                 0.0055 
                 0.0044 
                 0.0024 
                 0.0013 
                 0.0016 
                 0.013 
                   
                 225 
               
               
                 example 10 
               
               
                 Comparative 
                 0.06 
                 0.28 
                 1.06 
                 0.0012 
                 0.005 
                 0.027 
                 0.031 
                 0.0012 
                 0.0057 
                 0.0000 
                 0.0013 
                 0.0016 
                 0.012 
                   
                 245 
               
               
                 example 11 
               
               
                 Comparative 
                 0.06 
                 0.27 
                 1.00 
                 0.0015 
                 0.006 
                 0.027 
                 0.031 
                 0.0016 
                 0.0054 
                 0.0023 
                 0.0250 
                 0.0013 
                 0.010 
                   
                 240 
               
               
                 example 12 
               
               
                 Comparative 
                 0.06 
                 0.29 
                 0.99 
                 0.0011 
                 0.007 
                 0.025 
                 0.032 
                 0.0012 
                 0.0045 
                 0.0019 
                 0.0000 
                 0.0016 
                 0.011 
                   
                 255 
               
               
                 example 13 
               
               
                 Comparative 
                 0.05 
                 0.16 
                 1.56 
                 0.0020 
                 0.005 
                 0.015 
                 0.003 
                 0.0030 
                 0.0060 
                 0.0021 
                 0.0017 
                 0.002 
                 0.000 
                   
                 −35 
               
               
                 example 14 
               
               
                 Comparative 
                 0.07 
                 0.28 
                 0.99 
                 0.0015 
                 0.009 
                 0.026 
                 0.030 
                 0.0011 
                 0.0047 
                 0.0021 
                 0.0007 
                 0.0018 
                 0.010 
                   
                 234 
               
               
                 example 15 
               
               
                 Comparative 
                 0.07 
                 0.27 
                 0.99 
                 0.0015 
                 0.008 
                 0.027 
                 0.032 
                 0.0013 
                 0.0045 
                 0.0015 
                 0.0012 
                 0.0014 
                 0.011 
                   
                 251 
               
               
                 example 16 
               
               
                 Comparative 
                 0.06 
                 0.27 
                 1.02 
                 0.0011 
                 0.005 
                 0.024 
                 0.032 
                 0.0016 
                 0.0050 
                 0.0023 
                 0.0009 
                 0.0017 
                 0.011 
                   
                 248 
               
               
                 example 17 
               
               
                 Comparative 
                 0.06 
                 0.30 
                 1.07 
                 0.0015 
                 0.007 
                 0.028 
                 0.028 
                 0.0014 
                 0.0040 
                 0.0021 
                 0.0012 
                 0.0019 
                 0.013 
                   
                 217 
               
               
                 example 18 
               
               
                 Comparative 
                 0.06 
                 0.29 
                 1.03 
                 0.0013 
                 0.008 
                 0.025 
                 0.030 
                 0.0012 
                 0.0051 
                 0.0020 
                 0.0014 
                 0.0016 
                 0.013 
                   
                 236 
               
               
                 example 19 
               
               
                 Comparative 
                 0.06 
                 0.29 
                 1.07 
                 0.0012 
                 0.006 
                 0.025 
                 0.029 
                 0.0010 
                 0.0040 
                 0.0019 
                 0.0012 
                 0.0014 
                 0.011 
                   
                 224 
               
               
                 example 20 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                   
                 Composition of coarse inclusions in steel  
               
               
                   
                 (mass %) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Zr  
                 REM  
                 Al  
                 S  
               
               
                   
                 amount 
                 amount 
                 amount 
                 amount 
               
               
                   
               
               
                 Comparative example 1 
                 19% 
                 29% 
                 13% 
                 17% 
               
               
                 Comparative example 2 
                 16% 
                 25% 
                 14% 
                 14% 
               
               
                 Comparative example 3 
                 13% 
                 29% 
                 13% 
                 12% 
               
               
                 Comparative example 4 
                 16% 
                 33% 
                 15% 
                 14% 
               
               
                 Comparative example 5 
                 15% 
                 14% 
                 18% 
                 13% 
               
               
                 Comparative example 6 
                 18% 
                 29% 
                 14% 
                 16% 
               
               
                 Comparative example 7 
                  4% 
                  4% 
                 32% 
                  4% 
               
               
                 Comparative example 8 
                 17% 
                 29% 
                 11% 
                 16% 
               
               
                 Comparative example 9 
                 15% 
                 33% 
                 13% 
                 14% 
               
               
                 Comparative example 10 
                 13% 
                 16% 
                 18% 
                 12% 
               
               
                 Comparative example 11 
                  6% 
                  0% 
                  7% 
                 14% 
               
               
                 Comparative example 12 
                 42% 
                  7% 
                  2% 
                  3% 
               
               
                 Comparative example 13 
                 0.4%  
                 40% 
                 17% 
                  4% 
               
               
                 Comparative example 14 
                 16% 
                 30% 
                 13% 
                 14% 
               
               
                 Comparative example 15 
                  4% 
                  3% 
                  3% 
                  4% 
               
               
                 Comparative example 16 
                  9% 
                 62% 
                 11% 
                  9% 
               
               
                 Comparative example 17 
                 14% 
                 36% 
                  2% 
                 13% 
               
               
                 Comparative example 18 
                 16% 
                 33% 
                  5% 
                  3% 
               
               
                 Comparative example 19 
                  4% 
                  4% 
                  2% 
                  2% 
               
               
                 Comparative example 20 
                 19% 
                 31% 
                 11% 
                 25% 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                   
                 Time from 
                   
                 Cooling 
               
               
                   
                   
                   
                   
                   
                   
                 adding 
                   
                 rate 
               
               
                   
                   
                   
                   
                   
                 Time from 
                 Ca until 
                   
                 from 
               
               
                   
                 Fe 
                 CaO 
                   
                   
                 adding REM 
                 completion 
                   
                 780° C., 
               
               
                   
                 concentration 
                 concentration 
                 Of/S 
                 Adding order of 
                 until 
                 of 
                 Casting time of 
                 after 
               
               
                   
                 in slag 
                 in slag 
                 ratio 
                 Al, Zr, REM and Ca 
                 adding Ca 
                 solidification 
                 1,300° C.-1,200° C. 
                 rolling 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Invention example 1 
                 1.5% 
                 40% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 140 
                 330 
                 18 
               
               
                 Invention example 2 
                 1.4% 
                 39% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 141 
                 293 
                 18 
               
               
                 Invention example 3 
                 1.4% 
                 37% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 146 
                 353 
                 18 
               
               
                 Invention example 4 
                 1.4% 
                 38% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 9 
                 143 
                 347 
                 18 
               
               
                 Invention example 5 
                 1.5% 
                 39% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 144 
                 349 
                 18 
               
               
                 Invention example 6 
                 1.4% 
                 39% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 142 
                 312 
                 18 
               
               
                 Invention example 7 
                 1.5% 
                 39% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 146 
                 308 
                 18 
               
               
                 Invention example 8 
                 1.5% 
                 38% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 9 
                 146 
                 348 
                 18 
               
               
                 Invention example 9 
                 1.5% 
                 41% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 11 
                 147 
                 297 
                 18 
               
               
                 Invention example 10 
                 1.4% 
                 42% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 147 
                 289 
                 18 
               
               
                 Invention example 11 
                 1.5% 
                 43% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 144 
                 307 
                 18 
               
               
                 Invention example 12 
                 1.5% 
                 40% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 9 
                 141 
                 353 
                 18 
               
               
                 Invention example 13 
                 1.4% 
                 42% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 148 
                 332 
                 18 
               
               
                 Invention example 14 
                 1.5% 
                 40% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 9 
                 142 
                 302 
                 18 
               
               
                 Invention example 15 
                 1.5% 
                 42% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 146 
                 330 
                 18 
               
               
                 Invention example 16 
                 1.4% 
                 42% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 145 
                 320 
                 18 
               
               
                 Invention example 17 
                 1.5% 
                 38% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 9 
                 149 
                 290 
                 18 
               
               
                 Invention example 18 
                 1.4% 
                 43% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 11 
                 144 
                 328 
                 18 
               
               
                 Invention example 19 
                 1.4% 
                 39% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 9 
                 149 
                 327 
                 18 
               
               
                 Invention example 20 
                 1.4% 
                 38% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 11 
                 146 
                 287 
                 18 
               
               
                 Invention example 21 
                 1.5% 
                 40% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 9 
                 147 
                 307 
                 18 
               
               
                 Invention example 22 
                 1.5% 
                 41% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 9 
                 143 
                 299 
                 18 
               
               
                 Invention example 23 
                 1.5% 
                 40% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 150 
                 336 
                 18 
               
               
                 Invention example 24 
                 1.5% 
                 40% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 145 
                 305 
                 18 
               
               
                 Invention example 25 
                 1.5% 
                 — 
                 0.4 
                 Al→(Zr, REM) 
                 — 
                 — 
                 307 
                 18 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                   
                 Time from 
                   
                 Cooling 
               
               
                   
                   
                   
                   
                   
                 Time from 
                 adding 
                   
                 rate 
               
               
                   
                   
                   
                   
                   
                 adding 
                 Ca until 
                   
                 from 
               
               
                   
                 Fe 
                 CaO 
                   
                   
                 REM 
                 completion 
                   
                 780° C. 
               
               
                   
                 concentration 
                 concentration 
                 Of/ 
                 Adding order of 
                 until 
                 of 
                 Casting time of 
                 after 
               
               
                   
                 in slag 
                 in slag 
                 S ratio 
                 Al, Zr, REM and Ca 
                 adding Ca 
                 solidification 
                 1,300° C.-1,200° C. 
                 rolling 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Compatative example 1 
                 1.4% 
                 41% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 11 
                 146 
                 311 
                 18 
               
               
                 Compatative example 2 
                 1.5% 
                 42% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 9 
                 145 
                 345 
                 18 
               
               
                 Compatative example 3 
                 1.5% 
                 37% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 9 
                 142 
                 330 
                 18 
               
               
                 Compatative example 4 
                 1.5% 
                 39% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 141 
                 290 
                 18 
               
               
                 Compatative example 5 
                 1.4% 
                 41% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 142 
                 353 
                 18 
               
               
                 Compatative example 6 
                 1.5% 
                 41% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 9 
                 144 
                 351 
                 18 
               
               
                 Compatative example 7 
                 1.4% 
                 42% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 150 
                 354 
                 18 
               
               
                 Compatative example 8 
                 1.5% 
                 42% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 9 
                 149 
                 296 
                 18 
               
               
                 Compatative example 9 
                 1.4% 
                 40% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 146 
                 354 
                 18 
               
               
                 Compatative example 10 
                 1.4% 
                 43% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 9 
                 145 
                 302 
                 18 
               
               
                 Compatative example 11 
                 1.5% 
                 38% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 11 
                 142 
                 323 
                 18 
               
               
                 Compatative example 12 
                 1.4% 
                 41% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 140 
                 348 
                 18 
               
               
                 Compatative example 13 
                 1.4% 
                 37% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 142 
                 350 
                 18 
               
               
                 Compatative example 14 
                 1.5% 
                 40% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 140 
                 330 
                 20 
               
               
                 Compatative example 15 
                 0.03%  
                 38% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 9 
                 142 
                 296 
                 18 
               
               
                 Compatative example 16 
                 1.5% 
                 41% 
                 12.0 
                 Al→(Zr, REM)→Ca 
                 9 
                 143 
                 301 
                 18 
               
               
                 Compatative example 17 
                 1.5% 
                 41% 
                 0.4 
                 (Zr, REM)→Al 
                 3 
                 149 
                 310 
                 18 
               
               
                 Compatative example 18 
                 1.5% 
                 41% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 9 
                 148 
                 287 
                 18 
               
               
                 Compatative example 19 
                 1.4% 
                 42% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 260 
                 312 
                 18 
               
               
                 Compatative example 20 
                 1.5% 
                 42% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 142 
                 600 
                 18 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                   
                 TABLE 7 
               
             
            
               
                   
                   
               
               
                   
                 Properties 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 HAZ toughness 
                 CG-HAZ 
                 FG-HAZ 
                 Hardness difference 
                   
               
               
                   
                 YS 
                 TS 
                 vTrs (CG) 
                 hardness 
                 hardness 
                 (CG − FG) 
                 HIC test 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Invention example 1 
                 442 
                 555 
                 −15 
                 212 
                 183 
                 29 
                 Without cracking 
               
               
                 Invention example 2 
                 529 
                 664 
                 −5 
                 254 
                 219 
                 35 
                 Without cracking 
               
               
                 Invention example 3 
                 438 
                 507 
                 −10 
                 193 
                 164 
                 29 
                 Without cracking 
               
               
                 Invention example 4 
                 489 
                 564 
                 −5 
                 218 
                 185 
                 32 
                 Without cracking 
               
               
                 Invention example 5 
                 443 
                 508 
                 −25 
                 194 
                 165 
                 29 
                 Without cracking 
               
               
                 Invention example 6 
                 535 
                 666 
                 −5 
                 253 
                 222 
                 31 
                 Without cracking 
               
               
                 Invention example 7 
                 412 
                 479 
                 −25 
                 186 
                 156 
                 30 
                 Without cracking 
               
               
                 Invention example 8 
                 495 
                 567 
                 −15 
                 217 
                 186 
                 31 
                 Without cracking 
               
               
                 Invention example 9 
                 489 
                 563 
                 −20 
                 215 
                 187 
                 28 
                 Without cracking 
               
               
                 Invention example 10 
                 473 
                 546 
                 −15 
                 207 
                 178 
                 30 
                 Without cracking 
               
               
                 Invention example 11 
                 427 
                 540 
                 −15 
                 208 
                 176 
                 32 
                 Without cracking 
               
               
                 Invention example 12 
                 575 
                 575 
                 −15 
                 221 
                 189 
                 32 
                 Without cracking 
               
               
                 Invention example 13 
                 482 
                 559 
                 −25 
                 215 
                 181 
                 34 
                 Without cracking 
               
               
                 Invention example 14 
                 473 
                 542 
                 −5 
                 207 
                 180 
                 27 
                 Without cracking 
               
               
                 Invention example 15 
                 456 
                 526 
                 −5 
                 200 
                 171 
                 29 
                 Without cracking 
               
               
                 Invention example 16 
                 481 
                 553 
                 −5 
                 210 
                 181 
                 29 
                 Without cracking 
               
               
                 Invention example 17 
                 492 
                 561 
                 −20 
                 213 
                 186 
                 27 
                 Without cracking 
               
               
                 Invention example 18 
                 550 
                 627 
                 −25 
                 240 
                 205 
                 35 
                 Without cracking 
               
               
                 Invention example 19 
                 467 
                 580 
                 −15 
                 221 
                 189 
                 32 
                 Without cracking 
               
               
                 Invention example 20 
                 457 
                 570 
                 −15 
                 217 
                 187 
                 30 
                 Without cracking 
               
               
                 Invention example 21 
                 452 
                 565 
                 −25 
                 215 
                 188 
                 26 
                 Without cracking 
               
               
                 Invention example 22 
                 467 
                 580 
                 −15 
                 220 
                 189 
                 30 
                 Without cracking 
               
               
                 Invention example 23 
                 472 
                 585 
                 −25 
                 226 
                 191 
                 35 
                 Without cracking 
               
               
                 Invention example 24 
                 450 
                 520 
                 −5 
                 201 
                 172 
                 29 
                 Without cracking 
               
               
                 Invention example 25 
                 471 
                 540 
                 −20 
                 204 
                 177 
                 26 
                 Without cracking 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                   
                 TABLE 8 
               
             
            
               
                   
                   
               
               
                   
                 Properties 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 HAZ toughness 
                 CG-HAZ 
                 FG-HAZ 
                 Hardness difference 
                   
               
               
                   
                 YS 
                 TS 
                 vTrs (CG) 
                 hardness 
                 hardness 
                 (CG − FG) 
                 HIC test 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Comparative example 1 
                 582 
                 729 
                 Over 0° C. 
                 279 
                 243 
                 35 
                 Without cracking 
               
               
                 Comparative example 2 
                 364 
                 424 
                 Over 0° C. 
                 197 
                 155 
                 42 
                 Without cracking 
               
               
                 Comparative example 3 
                 523 
                 599 
                 Over 0° C. 
                 231 
                 199 
                 31 
                 Without cracking 
               
               
                 Comparative example 4 
                 600 
                 756 
                 Over 0° C. 
                 290 
                 246 
                 44 
                 With cracking 
               
               
                 Comparative example 5 
                 470 
                 539 
                 Over 0° C. 
                 208 
                 178 
                 30 
                 With cracking 
               
               
                 Comparative example 6 
                 470 
                 542 
                 Over 0° C. 
                 209 
                 180 
                 30 
                 With cracking 
               
               
                 Comparative example 7 
                 477 
                 545 
                 −10 
                 209 
                 155 
                 54 
                 With cracking 
               
               
                 Comparative example 9 
                 469 
                 536 
                 −10 
                 206 
                 151 
                 55 
                 With cracking 
               
               
                 Comparative example 9 
                 560 
                 697 
                 Over 0° C. 
                 265 
                 228 
                 38 
                 Without cracking 
               
               
                 Comparative example 10 
                 463 
                 530 
                 Over 0° C. 
                 200 
                 175 
                 24 
                 With cracking 
               
               
                 Comparative example 11 
                 489 
                 557 
                 −10 
                 213 
                 163 
                 50 
                 Without cracking 
               
               
                 Comparative example 12 
                 461 
                 534 
                 Over 0° C. 
                 206 
                 152 
                 54 
                 With cracking 
               
               
                 Comparative example 13 
                 473 
                 539 
                 −10 
                 208 
                 153 
                 55 
                 Without cracking 
               
               
                 Comparative example 14 
                 417 
                 525 
                 −15 
                 201 
                 150 
                 51 
                 With cracking 
               
               
                 Comparative example 15 
                 476 
                 552 
                 Over 0° C. 
                 213 
                 184 
                 29 
                 With cracking 
               
               
                 Comparative example 16 
                 470 
                 544 
                  0 
                 207 
                 179 
                 28 
                 With cracking 
               
               
                 Comparative example 17 
                 465 
                 538 
                 Over 0° C. 
                 205 
                 155 
                 50 
                 With cracking 
               
               
                 Comparative example 18 
                 483 
                 554 
                  −5 
                 211 
                 154 
                 57 
                 With cracking 
               
               
                 Comparative example 19 
                 476 
                 542 
                  −5 
                 206 
                 151 
                 55 
                 With cracking 
               
               
                 Comparative example 20 
                 479 
                 547 
                  −5 
                 211 
                 153 
                 58 
                 Without cracking 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                   
                 TABLE 9 
               
             
            
               
                   
                   
               
               
                   
                 Steel composition 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 C 
                 Si 
                 Mn 
                 S 
                 P 
                 Al 
                 Nb 
                 O 
                 N 
                 REM 
                 Zr 
                 Ca 
                 Ti 
                 Others 
                 Nb-Di balance 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Invention 
                 0.06 
                 0.27 
                 1.09 
                 0.0011 
                 0.007 
                 0.025 
                 0.029 
                 0.0013 
                 0.006 
                 0.0019 
                 0.0011 
                 0.0014 
                 0.012 
                   
                 223 
               
               
                 example 26 
               
               
                 Invention 
                 0.14 
                 0.29 
                 0.98 
                 0.0011 
                 0.006 
                 0.025 
                 0.031 
                 0.0015 
                 0.0050 
                 0.0019 
                 0.0010 
                 0.0017 
                 0.014 
                   
                 249 
               
               
                 example 27 
               
               
                 Invention 
                 0.06 
                 0.30 
                 0.96 
                 0.0015 
                 0.007 
                 0.027 
                 0.047 
                 0.0010 
                 0.0048 
                 0.0021 
                 0.0013 
                 0.0015 
                 0.012 
                   
                 402 
               
               
                 example 28 
               
               
                 Invention 
                 0.06 
                 0.28 
                 1.01 
                 0.0015 
                 0.006 
                 0.026 
                 0.031 
                 0.0011 
                 0.0050 
                 0.0077 
                 0.0011 
                 0.0015 
                 0.010 
                   
                 242 
               
               
                 example 29 
               
               
                 Invention 
                 0.08 
                 0.28 
                 1.05 
                 0.0015 
                 0.007 
                 0.025 
                 0.031 
                 0.0010 
                 0.0052 
                 0.0026 
                 0.0010 
                 0.0016 
                 0.012 
                 Ni = 0.19 
                 246 
               
               
                 example 30 
               
               
                 Invention 
                 0.07 
                 0.29 
                 1.04 
                 0.0013 
                 0.006 
                 0.021 
                 0.030 
                 0.0015 
                 0.0051 
                 0.0017 
                 0.0011 
                 0.0018 
                 0.012 
                 Cr = 0.1 
                 237 
               
               
                 example 31 
               
               
                 Invention 
                 0.06 
                 0.25 
                 1.00 
                 0.0011 
                 0.006 
                 0.026 
                 0.031 
                 0.0015 
                 0.0043 
                 0.0026 
                 0.0014 
                 0.0014 
                 0.000 
                   
                 242 
               
               
                 example 32 
               
               
                 Invention 
                 0.06 
                 0.28 
                 1.03 
                 0.0014 
                 0.007 
                 0.026 
                 0.030 
                 0.0012 
                 0.0063 
                 0.0020 
                 0.0010 
                 0.0015 
                 0.012 
                   
                 232 
               
               
                 example 33 
               
               
                 Invention 
                 0.06 
                 0.27 
                 0.98 
                 0.0015 
                 0.006 
                 0.025 
                 0.032 
                 0.0010 
                 0.0051 
                 0.0080 
                 0.0010 
                 0.0013 
                 0.010 
                   
                 247 
               
               
                 example 34 
               
               
                 Comparative 
                 0.06 
                 0.28 
                 1.06 
                 0.0012 
                 0.005 
                 0.027 
                 0.031 
                 0.0012 
                 0.0057 
                 0.0000 
                 0.0013 
                 0.0016 
                 0.012 
                   
                 230 
               
               
                 example 21 
               
               
                 Invention 
                 0.06 
                 0.27 
                 0.99 
                 0.0014 
                 0.006 
                 0.026 
                 0.030 
                 0.0014 
                 0.0038 
                 0.0020 
                 0.0015 
                 0.0017 
                 0.000 
                   
                 232 
               
               
                 example 35 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                 TABLE 10 
               
             
            
               
                   
               
               
                   
                 Composition of coarse inclusions in steel  
               
               
                   
                 (mass %) 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Zr  
                 REM  
                 Al  
                 S  
                 Ca  
               
               
                   
                 amount 
                 amount  
                 amount  
                 amount 
                 amount 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Invention example 26  
                 12% 
                 20% 
                 12% 
                 11% 
                 25% 
               
               
                 Invention example 27  
                 13% 
                 21% 
                 11% 
                 10% 
                 31% 
               
               
                 Invention example 28 
                  4% 
                 15% 
                  8% 
                  8% 
                 55% 
               
               
                 Invention example 29 
                 10% 
                 46% 
                  8% 
                 11% 
                  8% 
               
               
                 Invention example 30 
                 13% 
                 20% 
                 10% 
                 12% 
                 30% 
               
               
                 Invention example 31 
                  8% 
                 21% 
                  8% 
                 13% 
                 34% 
               
               
                 Invention example 32 
                 13% 
                 39% 
                  6% 
                 12% 
                 18% 
               
               
                 Invention example 33 
                 16% 
                 28% 
                 13% 
                 14% 
                  3% 
               
               
                 Invention example 34  
                 1.3%  
                 49% 
                  7% 
                 10% 
                  1% 
               
               
                 Comparative example 21 
                  6% 
                  0% 
                  7% 
                 14% 
                 62% 
               
               
                 Invention example 35  
                 18% 
                 44% 
                  3% 
                 16% 
                  4% 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 11 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                 Time 
                 Time from 
                   
                   
                 Cooling 
               
               
                   
                   
                   
                   
                   
                 from 
                 adding 
                   
                   
                 rate 
               
               
                   
                 Fe 
                   
                   
                   
                 adding 
                 Ca until 
                   
                   
                 from 
               
               
                   
                 concen- 
                 CaO 
                   
                 Adding order 
                 REM 
                 completion 
                   
                   
                 780° C. 
               
               
                   
                 tration 
                 concentration 
                 Of/S 
                 of Al, Zr, REM 
                 until 
                 of 
                 Casting time of 
                 Casting time of 
                 after 
               
               
                   
                 in slag 
                 in slag 
                 ratio 
                 and Ca 
                 adding Ca 
                 solidification 
                 1,500° C.-1,450° C. 
                 1,300° C.-1,200° C. 
                 rolling 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Invention 
                 1.5% 
                 40% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 141 
                 271 
                 316 
                 18 
               
               
                 example 26 
               
               
                 Invention 
                 1.4% 
                 39% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 140 
                 281 
                 300 
                 18 
               
               
                 example 27 
               
               
                 Invention 
                 1.5% 
                 40% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 140 
                 188 
                 341 
                 18 
               
               
                 example 28 
               
               
                 Invention 
                 1.5% 
                 40% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 145 
                 221 
                 321 
                 18 
               
               
                 example 29 
               
               
                 Invention 
                 1.5% 
                 40% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 145 
                 253 
                 310 
                 18 
               
               
                 example 30 
               
               
                 Invention 
                 1.5% 
                 40% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 145 
                 208 
                 310 
                 18 
               
               
                 example 31 
               
               
                 Invention 
                 1.5% 
                 40% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 144 
                 293 
                 306 
                 18 
               
               
                 example 32 
               
               
                 Invention 
                 1.5% 
                 40% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 140 
                 311 
                 330 
                 18 
               
               
                 example 33 
               
               
                 Invention 
                 1.5% 
                 42% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 146 
                 311 
                 330 
                 18 
               
               
                 example 34 
               
               
                 Comparative 
                 1.5% 
                 38% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 11 
                 142 
                 321 
                 323 
                 18 
               
               
                 example 21 
               
               
                 Invention 
                 1.5% 
                 40% 
                 0.4 
                 Al→(Zr, REM)→Ca 
                 10 
                 145 
                 315 
                 305 
                 18 
               
               
                 example 35 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                   
                 TABLE 12 
               
             
            
               
                   
                   
               
               
                   
                 Properties 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 HAZ 
                   
                   
                 Hardness 
                   
                   
                   
               
               
                   
                   
                   
                 toughness 
                 CG-HAZ 
                 FG-HAZ 
                 difference 
                   
                 SSCC test 
                 SSCC test 
               
               
                   
                 YS 
                 TS 
                 vTrs (CG) 
                 hardness 
                 hardness 
                 (CG − FG) 
                 HIC test 
                 (332 MPa) 
                 (374 MPa) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Invention example 26 
                 452 
                 563 
                 −15 
                 215 
                 184 
                 31 
                 Without cracking 
                 Without cracking 
                 Without cracking 
               
               
                 Invention example 27 
                 510 
                 653 
                 −5 
                 250 
                 211 
                 39 
                 Without cracking 
                 Without cracking 
                 Without cracking 
               
               
                 Invention example 28 
                 569 
                 582 
                 −15 
                 218 
                 186 
                 32 
                 Without cracking 
                 Without cracking 
                 Without cracking 
               
               
                 Invention example 29 
                 450 
                 530 
                 −10 
                 205 
                 172 
                 33 
                 Without cracking 
                 Without cracking 
                 Without cracking 
               
               
                 Invention example 30 
                 438 
                 559 
                 −25 
                 210 
                 187 
                 23 
                 Without cracking 
                 Without cracking 
                 Without cracking 
               
               
                 Invention example 31 
                 471 
                 572 
                 −15 
                 223 
                 185 
                 38 
                 Without cracking 
                 Without cracking 
                 Without cracking 
               
               
                 Invention example 32 
                 440 
                 522 
                 −10 
                 199 
                 169 
                 30 
                 Without cracking 
                 Without cracking 
                 Without cracking 
               
               
                 Invention example 33 
                 442 
                 555 
                 −15 
                 212 
                 183 
                 29 
                 Without cracking 
                 Without cracking 
                 With cracking 
               
               
                 Invention example 34 
                 456 
                 526 
                 −5 
                 200 
                 171 
                 29 
                 Without cracking 
                 Without cracking 
                 With cracking 
               
               
                 Comparative example 21 
                 489 
                 557 
                 −10 
                 213 
                 163 
                 50 
                 Without cracking 
                 With cracking 
                 With cracking 
               
               
                 Invention example 35 
                 450 
                 520 
                 −5 
                 201 
                 172 
                 29 
                 Without cracking 
                 Without cracking 
                 With cracking 
               
               
                   
               
            
           
         
       
     
     Although the present invention has been explained in detail referring to specific aspects, it is obvious for a person with an ordinary skill in the art that various alterations and amendments are possible without departing from the sprit and scope of the present invention. 
     Further, the present application is based on Japanese Patent Application (No. 2013-256080) applied on Dec. 11, 2013, and the entirety thereof is incorporated by reference into the present application.