JP5167628B2 - Steel slab with fine solidification structure - Google Patents

Description

  The present invention relates to a steel slab having a fine solidification structure, and in particular, an oxide that can advantageously refine the solidification structure by promoting the formation of equiaxed crystals when the steel solidifies δ. It relates to a steel slab containing a fine solidified structure. The steel slab with a fine solidification structure of the present invention is mainly applied to the steelmaking process of the steel industry, and the steel slab to which the present invention is applied includes various steel plates, steel pipes, shaped steel, bar steel, wire rods and the like. Used as a material for steel products.

  Steel products such as steel plates, steel pipes, shaped steels, steel bars, and steel wires are required to have various mechanical properties such as tensile properties and fracture properties, and the level of demand from users is increasing year by year. Furthermore, due to intensifying competition accompanying the globalization of the steel products market, not only quality such as mechanical properties but also shortening of delivery time and lowering prices are becoming important indicators of competitiveness. .

  The mechanical properties, production time, and production cost that affect the quality, delivery date, and price of steel products are greatly affected by the quality of steel slabs such as slabs, blooms, and billets. In steel slabs, there are compositionally inhomogeneous parts and voids such as solidification segregation and porosity. In order to reduce the negative effects of these casting defects on the quality of steel products, low-temperature casting processing, electromagnetic stirring processing, light reduction processing in continuous casting, homogenization heat treatment of steel slabs prior to hot working, and hot working There is a problem that a large manufacturing load such as high-temperature high-strength processing occurs, the manufacturing time becomes long, and the manufacturing cost increases. Furthermore, in response to demands for higher functionality of steel products, there are many cases where the casting defects of the material hinder the improvement of the mechanical properties, and in order to solve the above problems, the casting defects are drastically improved. Technology is desired.

  If the solidification segregation and porosity of steel slabs can be refined and made harmless than before, the manufacturing load mentioned above will be reduced, and the obstacles to high functionality will be removed, so the quality, delivery date, and price competitiveness of steel products May improve. As a means of detoxifying solidification segregation and porosity, there is a method of promoting the generation of solidified equiaxed crystals by inoculation technology and making the particle size finer, and patents that explain the effectiveness of such methods are described below Known by the literature.

As an inoculation nucleus when the solidification initial phase is δ-Fe, such as low carbon steel and ferritic stainless steel, in the invention described in Patent Document 1, the effectiveness of REM oxides, sulfides and oxysulfides However, in the invention described in Patent Document 2, the effectiveness of Ba-based oxides is described, and in the invention described in Patent Document 3, effectiveness of TiN compounded with a single TiN or Mg-based oxide is described in Patent Documents 4-6. In this invention, the effectiveness of Mg-based oxides is disclosed.
On the other hand, ZrO ₂ , MgS, CaO, REM nitride, Ti ₂ O _3, etc. are effective as inoculation nuclei when the solidification initial phase is γ-Fe, such as high carbon steel and austenitic stainless steel. It has been found.

As described above, the types of effective seeding nuclei are different between the cases where the crystal structure of the solidification initial phase is δ-Fe and γ-Fe. This is because, as described in Non-Patent Document 1, a substance having a smaller degree of lattice mismatch with the solidification initial phase has a higher ability as a solidification nucleus and is considered to promote the formation of solidified equiaxed crystals. It can be determined that the effectiveness of the seeded nuclei varies depending on the crystal structure of the initial solidification phase.
JP 2005-177848 A JP 2004-276041 A JP 2002-030395 A JP 2001-192882 A JP 2000-192200 A Japanese Patent Laid-Open No. 11-254107 Tetsuro Ohashi et al., “Effect of Oxides on Heterogeneous Nucleation of Iron”, Iron and Steel, 62nd (1976), No. 6, p. 614 to 623

  However, in the steel whose solidification initial phase is δ-Fe, when the above-mentioned known inoculation nuclei are generated using the deoxidation reaction, desulfurization reaction, and denitrification reaction of molten steel, there are the following problems, It became clear that there was a limit to the practical application. That is, as in the invention described in Patent Document 1, when REM is added in order to produce REM oxides and sulfides, and further composite precipitates thereof in molten steel, coarse REM inclusions are produced. In addition, the continuous casting nozzle may be blocked and the quality of steel products may be deteriorated.

  Moreover, when adding Ba in order to produce | generate Ba type | system | group oxide in molten steel like the invention of the said patent document 2, or in the molten steel like the invention of the said patent documents 3-6, When adding Mg to produce Mg-based oxides, these elements have large evaporation losses and low yields, so restrictions on the addition method, increase in alloy costs, and unstable steel content There is a possibility that the problem of becoming.

Moreover, it is difficult to use a method for adding a large amount of N to low Cr steel in order to generate TiN in molten steel, such as the invention described in Patent Document 3, and a large amount of Ti is added. When the method is used, the base material of the welded structural steel or the weld heat affected zone (HAZ) may be excessively cured to impair mechanical properties.
Due to the problems described above, in the production of steel slabs whose solidification initial phase is δ-Fe, no harmful coarse inclusions are generated, element addition to molten steel is easy, and a large amount of Ti or N is required. No new inoculation technology was required.

  The present invention has been made in view of the above-mentioned problems, and it is possible to make the segregation and porosity of steel slabs whose solidification initial phase is δ-Fe finer, and to make them harmless, and to make them harmless. The purpose is to provide a piece.

As a result of intensive studies on the components contained in the steel slab, the present inventors have promoted the formation of solidified equiaxed crystals by using the oxide produced by the deoxidation reaction of molten steel as the solidification nuclei, and refined the particle size. As a result, it was found that the segregation and porosity of the steel slab whose solidification initial phase is δ-Fe can be refined and made harmless advantageously, and a steel slab having a fine solidification structure can be obtained.
The present invention is configured based on the above-described knowledge, and the main points thereof are as follows.

(1) The composition of the steel slab is mass%, C: 0.5% or less, S: 0.01% or less, N: 0.05% or less, O: 0.001 to 0.01%, Al: 0.001 to 0.1%, Ca: 0.0020 to 0.005%, Zr: 0.0005 to 0.05%, Hf: 0.001 to 0.1% Is a steel slab containing iron and unavoidable impurities, and the solidification initial phase is δ-Fe, and among the oxides in the steel slab, Ca, One or two of Zr and Hf are included at the same time, and all or a part of the surface thereof is present in direct contact with the ground iron without passing through a third substance, and a maximum of 0.1 to 10 μm CaX-based oxide having a diameter of, alone particles or composite particles, there are one or more in an area 1 mm ² per in any cross-section It characterized the door, equiaxed grain structures steel slab. However, X of the CaX-based oxide is one or two of Zr and Hf.

(2) While containing 1 type or 2 or more types in Si, Mn , Nb, V, Cu, Ni, Cr, Mo , B which is a component whose deoxidizing power in molten steel is smaller than Al, The content of these components is such that the Cr equivalent calculated by the following formula (1) is 30% by mass or less and the Ni equivalent calculated by the following formula (2) is 15% by mass or less. The steel slab having a fine solidification structure according to claim 1, wherein
Cr equivalent = Cr + Mo + 1.5Si + 0.5Nb + 21.4Al + 40B + 8.29V (1)
Ni equivalent = Ni + 30C + 30N + 0.5Mn + 0.16Cu (2)

  According to the steel slab with a fine solidification structure of the present invention, segregation and porosity are refined through refinement of the solidification structure, and the effect of making it harmless than before, and eventually, the casting defect It is possible to reduce the load process performed for the purpose of reducing the adverse effect, and it is possible to remove the obstacle caused by the slab with respect to the demand for higher functionality. By using a steel slab having a fine solidification structure of the present invention, it is possible to advantageously provide steel products with high competitiveness in terms of quality, delivery time, price, etc. unfathomable.

Hereinafter, embodiments of a steel slab having a fine solidified structure according to the present invention will be described.
In addition, since this embodiment is described in detail for better understanding of the gist of the invention, the present invention is not limited unless otherwise specified.

The steel slab having a fine solidification structure of the present invention (hereinafter sometimes abbreviated as steel slab) is a steel having a C content of 0.5 mass% or less and a solidification initial phase of δ-Fe. It is a slab, Ca is essential among the oxides in the steel slab, and one or two of Zr and Hf are the main components, and all or part of the surface is third. One or more oxides having a maximum diameter of 0.1 to 10 μm exist as single particles or composite particles per area of 1 mm ² in an arbitrary cross-section as they are in direct contact with the base iron without any substance. And it is schematically configured.

  Further, in the steel slab having a fine solidification structure according to this embodiment, the component composition of the steel slab is mass%, C: 0.5% or less, S: 0.01% or less, N: 0.05%. Hereinafter, O: 0.001 to 0.01%, Al: 0.001 to 0.1%, Ca: 0.0005 to 0.005%, Zr: 0.0005 to 0.05% , Hf: One or two of 0.001 to 0.1% are contained, the balance being iron and inevitable impurities.

In the production of a slab in which the solidification initial phase is δ-Fe, the present inventors do not generate harmful coarse inclusions, it is easy to add elements to the molten steel, and a large amount of Ti or N is not required. The new inoculation technique was studied earnestly. As a result, a new type of inoculum nucleus not known in the past has been found and the present invention has been completed. This new inoculum nucleus is an oxide (hereinafter referred to as CaX-based oxide) mainly composed of Ca and X (X: one or two of Zr and Hf, hereinafter may be simply referred to as X). May be abbreviated as ")".
That is, when the oxide is produced by the deoxidation reaction of the molten steel, the area ratio of equiaxed crystals increases in the δ solidified structure of the slab, and the area ratio of columnar crystals decreases. As a result of analyzing oxides in the slab where the inoculation effect such as miniaturization was recognized, or in the arbitrary cross section of the processed product, the oxide composition measured by wavelength dispersive X-ray analysis (WDS) is The characteristics of containing Ca and X at the same time were confirmed. Further, the crystal structure of the oxide identified by electron beam diffraction was confirmed to have a characteristic that all or part of each oxide particle has a perovskite structure (CaTiO ₃ structure). Further, when X constituting the oxide is mainly Zr, it is identified as CaZrO ₃ having an orthorhombic perovskite structure (lattice constants: a = 0.576 nm, b = 0.802 nm, c = 0.559 nm). Many oxides were confirmed.

Here, assuming that the {100} plane of δ-Fe nucleates on the (010) plane of CaZrO _3, the lattice mismatch degree defined in Non-Patent Document 1 can be estimated to be 3.1%. it can. Conventionally, the lattice mismatch degree of TiN, which is known to have a high δ-Fe inoculation ability, is 3.9%, and that of Ce ₂ O ₃ is 5.0%. Therefore, the oxide CaZrO ₃ is known in the art. It is considered that the degree of lattice mismatch with respect to δ-Fe is smaller than that of inoculated nuclei and the inoculation ability is high. In the case where X-based nitride or Ca-based sulfide is partially complex-precipitated on the surface of the oxide CaZrO ₃ (in the present invention, in the case of such a case with respect to a single particle of CaX-based oxide) However, the surface of the oxide CaZrO ₃ is not entirely covered with these nitrides or sulfides, and all or part of the oxide surface is X-based nitride. It was observed in a form that was in direct contact with the steel without passing through a third substance such as Ca or Ca-based sulfide.

On the other hand, the inoculation effect was small in the steel added with a large amount of S and N and further added with a large amount of Ca and X. Such a slab with a small inoculation effect has the same CaX-based oxide as described above, but the surface of the CaX-based oxide is almost entirely covered with X-based nitride and Ca-based sulfide. There was no portion where the surface of the system oxide was in direct contact with the steel. Coarse nitrides and sulfides that cover the entire surface of the CaX-based oxide are mainly generated in the molten steel. In such cases, the CaX-based oxide is not in direct contact with the molten steel. It is presumed that the inoculation effect expected from this point of view is not achieved.
From the above, the characteristics of the composition and form of the inoculum nucleus newly found by the present inventors are oxides mainly composed of Ca and X, and the entire surface of the oxide with respect to the base iron (molten steel). Alternatively, a part is in direct contact without a third substance.

The higher the number of inoculum nuclei and the greater the number, the greater the inoculation effect on the coagulated tissue. However, in addition to the composition and form of the inoculum nuclei described above, the size of the inoculum nuclei has an effect. To do. Furthermore, the larger the size of the seeded nucleus, the higher the capacity of the seeded nucleus. On the other hand, there is a disadvantage that the number of seeded cores decreases. Therefore, in order to stably develop the inoculation effect, it is important to appropriately control the size and number in addition to the composition and form of the inoculum nucleus.
As a result of examining the appropriate particle diameter and number of CaO-based oxide inoculation nuclei newly found in the present invention, the maximum diameter of the oxide is 0.1 to 10 μm, and the area of an arbitrary cross section is 1 mm ² . It was revealed that the inoculation effect was stably expressed when one or more were present. When the maximum diameter of oxide is less than 0.1 μm, the inoculation capacity is low, and when the maximum diameter of oxide exceeds 10 μm, the number is less than 1 per 1 mm ² and the inoculation capacity decreases. The above particle diameter and number are necessary.

<Chemical composition of steel slab>
In order to produce CaX-based oxides having the composition, form, dimensions, and number as described above in the steel, it is preferable to appropriately adjust the chemical components of the steel in which the solidification initial phase is δ-Fe.
Below, the reason for limitation of the chemical component composition prescribed | regulated by this invention is demonstrated.

[C (carbon): 0.5% or less]
The C content needs to be 0.5% or less. When C exceeds 0.5%, the solidification initial phase changes from δ-Fe to γ-Fe, so that the inoculation effect expected for the degree of lattice mismatch of the CaX-based oxide cannot be obtained.

[S (sulfur): 0.01% or less]
The S content is preferably 0.01% or less. If S exceeds 0.01%, coarse CaS is produced in the molten steel, and there is a possibility that Ca for deoxidation is insufficient and CaX oxide cannot be produced stably. Moreover, when S is added excessively when there is a large amount of Ca, coarse CaS generated in the molten steel may cover the surface of the CaX-based oxide and reduce the ability of the seeding nucleus.

[N (nitrogen): 0.05% or less]
The N content is preferably 0.05% or less. If N exceeds 0.05%, coarse XN is generated in the molten steel, and there is a possibility that X for deoxidation is insufficient and CaX oxide cannot be stably generated. In addition, if N is excessively added when X is large, coarse XN generated in the molten steel may cover the surface of the CaX-based oxide and reduce the ability of the seeding nucleus.

[O (oxygen): 0.001 to 0.01%]
The O content is preferably 0.001 to 0.01%. If O is less than 0.001%, the particle diameter and number of CaX-based oxides may be insufficient. Further, if O exceeds 0.01%, the particle diameter of the CaX-based oxide becomes coarse, and the number thereof is insufficient, and at the same time, there is a possibility that the quality of the steel product may be harmful.

[Al (aluminum): 0.001 to 0.1%]
The Al content is preferably 0.001 to 0.1%. If Al is less than 0.001%, the amount of O may not be stably reduced to 0.01% or less. On the other hand, if Al exceeds 0.1%, the Al content in the oxide may increase and the CaX-based oxide may not be stably generated. Furthermore, when Al is excessively added when O is large, coarse alumina clusters are generated, and there is a possibility that the casting nozzle is blocked and the quality of the steel product is deteriorated.

[Ca (calcium): 0.0005 to 0.005%]
The Ca content is preferably 0.0005 to 0.005%. If Ca is less than 0.0005%, the Ca content in the oxide may be reduced, and the CaX-based oxide may not be stably generated. On the other hand, if Ca exceeds 0.005%, the Ca content in the oxide may increase, and it may not be possible to stably generate the CaX-based oxide. Further, when Ca is excessively added when there is a large amount of S, coarse CaS is generated in the molten steel, covering the surface of the CaX-based oxide, which may reduce the ability of the seeding nucleus.

[Zr (zirconium): 0.0005 to 0.05%]
The Zr content is preferably 0.0005 to 0.05%. If Zr is less than 0.0005%, the Zr content in the oxide may be reduced, and the CaX-based oxide may not be stably generated. On the other hand, if Zr exceeds 0.05%, the Zr content in the oxide may increase, and the CaX-based oxide may not be stably generated. Furthermore, when Zr is excessively added when N is large, coarse ZrN is generated in the molten steel, covering the surface of the CaX-based oxide, which may reduce the ability of the seeding nucleus. Furthermore, excessive Zr addition is not preferable because it increases the alloy cost.

[Hf (hafnium): 0.001 to 0.1%]
The Hf content is preferably 0.001 to 0.1%. If Hf is less than 0.001%, the Hf content in the oxide may decrease, and the CaX-based oxide may not be stably generated. On the other hand, if Hf exceeds 0.1%, the Hf content in the oxide may increase, and the CaX-based oxide may not be stably generated. Further, when Hf is excessively added when N is large, coarse HfN is generated in the molten steel, covering the surface of the CaX-based oxide, which may reduce the ability of the seeding nucleus. Furthermore, excessive Hf addition is undesirable because it increases the alloy cost.

[Other chemical components]
In the steel slab having a fine solidification structure according to the present invention, another element having a deoxidizing power in molten steel smaller than that of Al may be added as necessary.
That is, when REM, Ba, Mg, or the like having a stronger deoxidizing power than Al is added, the CaX-based oxide of the present invention cannot be stably generated because it competes with Ca and X in the deoxidation reaction.
On the other hand, one or more elements of Si, Mn, Cu , Ni, Cr, Mo , Nb, V, and B, which are components having a deoxidizing power smaller than that of Al, have a solidification initial phase of δ The effect of the present invention can also be obtained when it is added within the range of -Fe. However, in this case, the Cr equivalent calculated by the following equation (1) needs to be 30% or less. This is because if the Cr equivalent exceeds 30%, the metallurgical effects such as corrosion resistance and heat resistance are saturated and the alloy cost is increased, which is not preferable.
Furthermore, in the steel slab of the present invention, the Ni equivalent calculated by the following equation (2) needs to be 15% or less. This is because if the Ni equivalent exceeds 15%, the solidification initial phase changes from δ-Fe to γ-Fe, so that the inoculation effect expected for the degree of lattice mismatch of the CaX-based oxide cannot be obtained .
C r eq = Cr + Mo + 1.5Si + 0.5Nb + 21.4Al + 40B
+ 8.29V (1)
Ni equivalent = Ni + 30C + 30N + 0.5Mn + 0.16Cu (2)

  As described above, the steel slab having a fine solidification structure according to the present invention has the effect of making the segregation and porosity finer through the refinement of the solidification structure and making it harmless than the conventional structure. As a result, it is possible to reduce the loading process performed for the purpose of reducing the adverse effects of casting defects, and it is possible to remove the obstacle caused by the slab for the demand for higher functionality. In addition, by using a steel slab having a fine solidification structure according to the present invention, it is possible to advantageously provide steel products having high competitiveness in terms of quality, delivery time, price, etc., and its industrial effects are immeasurable. Absent.

  Hereinafter, examples of the steel slab having a fine solidification structure according to the present invention will be given and the present invention will be described more specifically. However, the present invention is not limited to the following examples, and The present invention can be carried out with appropriate modifications within a range that can be adapted to the gist of the present invention, and these are all included in the technical scope of the present invention.

[Sample preparation]
A 10 kg steel slab having the chemical composition shown in Table 1 below was produced using a vacuum melting furnace installed in a laboratory. In Table 1 below, only steel B and steel 28 are γ-Fe solidified, and the other steels are all δ-Fe solidified. During casting, the molten steel temperature is adjusted so that the molten steel superheat degree is 60 to 70 ° C., and a slab is produced using the same mold (mold bottom surface: about 80 mm square, mold height: about 200 mm). did.

[Evaluation test]
The number of CaX-based oxides, the equiaxed crystal ratio, and the equiaxed crystal grain size shown in Table 1 below were measured in a half height section of the slab produced by the above method, and the results are shown in Table 2 below. It was.

(Number of CaX-based oxides)
The number of CaX-based oxides was measured by the following procedure at the center of the slab cross section. First, the oxide composition contains Ca and X at the same time, all or part of the oxide surface is in direct contact with the ground iron, and the maximum diameter of the oxide alone is 0.1 to 10 μm. The number of oxides satisfying the conditions was measured over an area of 4 mm ² and converted to the number per 1 mm ² . At this time, the above measurement was performed using WDS and a scanning electron microscope (SEM) without corroding the cross section of the slab until polishing.

  In the WDS measurement of the oxide composition, the detected Fe is subtracted, and further, when S is detected, an equivalent amount of Ca is subtracted, and when N is detected, the same amount of X is subtracted. The oxide composition was determined as accurately as possible by subtracting the influence of the base iron and the influence of CaS and XN on the oxide. When CaS and XN are combined with the oxide, the presence of oxide, CaS, and XN is separated using SEM images and composition images, and at the same time the contact state between the oxide surface and the ground iron is judged, and the oxide The maximum diameter was measured.

In addition, when it was difficult to perform the above-described measurement over a wide area of 4 mm ² , the measurement was performed instead of a simple measurement method as described below.
First, 10 or more oxide particles are arbitrarily analyzed with a SEM-WDS apparatus, the oxide composition, the contact state between the oxide surface and the ground iron, the maximum diameter of the oxide are measured, and the CaX defined in the present invention. The ratio of the system oxide is determined. Next, the number of oxides in an area of 4 mm ² is measured by O mapping process using WDS. Then, the oxide quantity per area 1 mm ² combined over existing ratio of CaX based oxide, obtaining the CaX based oxide number per 1 mm ^2. In addition, since the measurement of such oxide dispersion | distribution state is substantially the same in the steel slab and the product which processed it, you may perform in the arbitrary cross sections of a product.

(Equiaxial crystal ratio and equiaxed crystal grain size)
The equiaxed crystal ratio and the equiaxed crystal grain size were measured by the following procedure after observing a half height section of the steel slab by the etch print method.
That is, the equiaxed crystal ratio was measured by measuring the area ratio of the equiaxed crystal portion in the entire cross section of the steel slab.
Further, the equiaxed crystal grain size was averaged by measuring 20 particles in order of increasing maximum diameter, considering the boundary where the dendrite direction of the equiaxed crystal part changes discontinuously as a crystal grain boundary.

  A list of chemical composition of each steel slab of this example is shown in Table 1, and a list of measurement results is shown in Table 2.

[Evaluation results]
Steel A and steel 1-6 shown in Table 1, 9 ～13,15 is the invention steels, since the dispersion state of the steel component and CaX based oxide is proper, as shown in Table 2, the present casting condition It was confirmed that the equiaxed crystal ratio was as high as 70% or more and the equiaxed crystal grain size was as small as 1 to 3 mm.

Steel AA, Steel B and Steels 16 to 32 in Table 1 are comparative steels, and Steel AA, Steel B and Steels 16 to 29 are those in which Ca and X are added. Since the dispersion state of the product is not appropriate, as shown in Table 2, the equiaxed crystal ratio is 40% or less, which is lower than the steel of the present invention, and the equiaxed grain size is 5-8 mm, which is smaller than the steel of the present invention. Was also confirmed to be large.
Steels 29 to 32 are comparative steels in which REM, Ba, and Mg are added without adding Ca and X, and the equiaxed crystal ratio is 50 to 60%, which is lower than that of the steel of the present invention. It was confirmed that the diameter was 4 mm, which was larger than the steel of the present invention.

  In addition, when the slab which has the chemical composition similar to steel 30-32 was continuously cast in the steelmaking factory (mass production factory), problems, such as nozzle obstruction | occlusion and an element low yield, generate | occur | produced. On the other hand, when the steel according to the present invention was continuously cast in a steelmaking factory, nozzle clogging did not occur, the yield of Zr and Hf was higher than that of Ca, and no particular problem was observed.

From the results of the examples described above, the steel slab with a fine solidified structure of the present invention has an effect that the segregation and porosity are refined through refinement of the solidified structure, and is made harmless than before, As a result, it is clear that the loading process performed for the purpose of reducing the adverse effects of casting defects can be reduced, and that the obstacle caused by the slab for the demand for higher functionality can be removed.

Claims (2)

The component composition of the steel slab is mass%,
C: 0.5% or less,
S: 0.01% or less,
N: 0.05% or less,
O: 0.001-0.01%,
Al: 0.001 to 0.1%,
Ca: 0.0020 to 0.005%
In addition,
Zr: 0.0005 to 0.05%,
Hf: 0.001 to 0.1%
A steel slab containing one or two of the above, the balance being iron and inevitable impurities, and the solidification initial phase being δ-Fe,
Of the oxides in the steel slab, it contains Ca and one or two of Zr and Hf at the same time, and all or part of the surface is in direct contact with the iron without passing through a third substance. And at least one CaX-based oxide having a maximum diameter of 0.1 to 10 μm is present as an individual particle or a composite particle per 1 mm ² area in an arbitrary cross section. Steel slab with fine structure.
However, X of the CaX-based oxide is one or two of Zr and Hf. Deoxidation force in the molten steel is smaller components than Al, Si, Mn, N b , V, Cu, Ni, Cr, Mo, as well as containing one or more of B, these components The Cr equivalent calculated by the following formula (1) is 30% by mass or less, and the Ni equivalent calculated by the following formula (2) is 15% by mass or less. A steel slab having a fine solidification structure according to claim 1.
Cr equivalent = Cr + Mo + 1.5Si + 0.5Nb + 21.4Al + 40B + 8.29V (1)
Ni equivalent = Ni + 30C + 30N + 0.5Mn + 0.16Cu (2)