Cast slab

Abstract

To provide a cast slab of steel including at least one selected from Nb, Ti, and V, the cast slab exhibiting superior resistance to crack sensitivity.SOLUTION: A cast slab that satisfies a condition of 0.00005≤([Ti]+0.5[Nb]+[V])×[N]≤0.00250 in terms of contents of Ti, Nb, V, and N, and in which, at a position 5 mm in a depth direction from a surface of the cast slab, a total number ratio of nitrides, carbonitrides, and carbides having a circle-equivalent diameter of 200 nm to 5000 nm and containing Zr in an amount of 10 mass% or more among the nitrides, carbonitrides, and carbides is 50% or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a cast slab having excellent cracking susceptibility. [Background technology]

[0002] In recent years, many high-strength steel alloys containing elements such as Nb, Ti, and V have been produced to improve mechanical properties. However, the addition of these elements makes surface cracks more likely to occur in the surface layer of the cast slab during continuous casting, posing problems for both operation and product quality.

[0003] Surface cracks that occur after the secondary cooling zone of continuous casting are known to occur along prior austenite grain boundaries on the surface of the slab. These surface cracks are caused by stress concentration on austenite grain boundaries embrittled by the precipitation of AlN, NbC, etc., or on film-like ferrite formed along prior austenite grain boundaries. The morphology of surface cracks varies depending on the direction of the applied stress: transverse cracks are caused by tensile stress in the casting direction, while longitudinal cracks are caused by tensile stress in the width direction of the slab. These cracks are particularly likely to occur in the temperature range near the austenite-to-ferrite phase transformation region. Therefore, continuous casting methods are typically used to prevent crack occurrence by avoiding the surface temperature in bending and straightening zones, where mechanical stress is applied to the slab surface, above the temperature range where ductility decreases (the embrittlement temperature range).

[0004] On the other hand, in recent years, as the number of steel types to which various elements are added to improve mechanical properties has increased, the number of steel types with high cracking susceptibility has also increased, and the occurrence of cracking cannot necessarily be prevented by bending or straightening the steel while avoiding the brittle temperature range. Therefore, as a technology for preventing cracking during continuous casting, Patent Document 1 discloses a method of suppressing cracking by specifying the amounts of Nb and Ti relative to the amount of N. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-166038 [Non-patent literature]

[0006] [Non-Patent Document 1] Haiwen LUO, L. Pentti KARJALAINEN, David A. PORTER, Heidi-Marja LIIMATAINEN and Yan ZHANG: "The Influence of Ti on the Hot Ductility of Nb-bearing Steels in Simulated Continuous Casting Process", ISIJ International, Vol.42(2002), No.3, pp.273-282 Summary of the Invention [Problem to be solved by the invention]

[0007] In steels containing elements such as Nb, Ti, and V, the embrittlement temperature range is significantly expanded, making bending and straightening in the embrittlement temperature range unavoidable in operation. It is known that the expansion of the embrittlement temperature range is caused by the formation of carbonitrides of Nb, Ti, and V, either singly or in combination, which are densely present on grain boundaries (see Non-Patent Document 1). The method described in Patent Document 1 still results in the dense presence of carbonitrides of Nb and other elements on grain boundaries, so that it may not be possible to sufficiently suppress the occurrence of cracks in the temperature range near the phase transformation region.

[0008] In view of the above-mentioned problems, an object of the present invention is to provide a cast slab of steel containing at least one of Nb, Ti and V, which has excellent cracking susceptibility. [Means for solving the problem]

[0009] The present inventors focused on the fact that carbonitrides containing Nb, Ti, and V are responsible for the high-temperature embrittlement of steels containing Nb, Ti, and V, and investigated the control of carbonitride precipitation. Specifically, they focused on Zr, which has a higher nitride and carbide formation ability than these elements, and investigated the high-temperature ductility of steels with Zr addition. They found that ZrN first forms within grains immediately after solidification, and then transforms into large complex carbonitrides containing Ti, Nb, and V as the temperature decreases. In other words, they found that by increasing the formation of nitrides, carbonitrides, and carbides containing Zr within grains, the formation of carbonitrides containing Ti, Nb, and V at grain boundaries can be suppressed, thereby drastically improving high-temperature embrittlement.

[0010] Based on the above, the present invention is as follows. [1] In mass%, C: 0.02%~0.50%, Si: 0.05% to 3.00%, Mn: 0.50%~4.00%, Al: 0.001%~0.200%, N: 0.0100% or less, Zr: 0.0003%~0.0500%, Ti: 0% to 0.100%, P: 0%~0.100%, S: 0%~0.0100%, Nb: 0% to 0.100%, V: 0%~0.100%, B: 0%~0.0050%, Cr: 0%~0.10%, Mo: 0% to 0.10%, Sn: 0% to 0.50%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, REM: 0%~0.0100%, Ni: 0% to 0.50%, and Cu: 0% to 0.50%, A cast slab comprising the remainder Fe and impurities, The contents of Ti, Nb, V and N satisfy the following formula (1), A cast slab characterized in that the total number ratio of nitrides, carbonitrides and carbides containing 10 mass% or more of Zr among nitrides, carbonitrides and carbides having an equivalent circle diameter of 200 nm to 5000 nm at a position 5 mm deep from the surface of the cast slab is 50% or more. 0.00005≦([Ti]+0.5[Nb]+[V])×[N]≦0.00250 ···(1) Here, [Ti], [Nb], [V], and [N] each represent the content (mass%) in the cast slab. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a cast slab of steel containing at least one of Nb, Ti, and V, which has excellent cracking susceptibility. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 2 is a diagram showing the relationship between the contents of Ti, Nb, V, and N and the content of Zr. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention will be described below with reference to the drawings. First, the chemical composition of a cast slab according to an embodiment of the present invention will be described. In the following description, "%", which is the unit of content of each element contained in the cast slab, means "mass %" unless otherwise specified. Furthermore, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. Furthermore, cast slabs containing the following components include steel slabs such as slabs and blooms.

[0014] <C:0.02%~0.50%> C is an element that improves the strength of steel. If the C content is less than 0.02%, it cannot meet the requirements for use as a high-strength steel plate. Also, if the C content exceeds 0.50%, the hardness becomes too high and the necessary bendability cannot be ensured. Therefore, the C content is set to 0.02% - 0.50%. The lower limit of the C content is preferably 0.05% or more, and the upper limit is preferably 0.30% or less.

[0015] <Si: 0.05% - 3.00%> Si is an element that improves the strength of steel. If the Si content is less than 0.05%, it cannot meet the requirements for use as a high-strength steel plate. Also, if the Si content exceeds 3.00%, it has an adverse effect on weldability. Therefore, the Si content is set to 0.05% - 3.00%. The lower limit of the Si content is preferably 0.10% or more, and the upper limit is preferably 2.0% or less.

[0016] <Mn: 0.50% - 4.00%> Mn is an element that improves the strength of steel. If the Mn content is less than 0.50%, it cannot meet the requirements for use as a high-strength steel plate. Also, if the Mn content exceeds 4.00%, since Mn is a segregation element, it may cause uneven strength in the slab or steel plate. Therefore, the Mn content is set to 0.50% - 4.00%. The lower limit of the Mn content is preferably 0.80% or more, and the upper limit is preferably 2.50% or less.

[0017] <Al: 0.001% - 0.200%> Al is the element most widely used for deoxidation purposes. Also, Al has the effect of suppressing the coarsening of crystal grains by generating AlN. To obtain these effects, the lower limit of the Al content is 0.001% or more, preferably 0.005% or more. On the other hand, if there is too much Al, there may be problems such as nozzle clogging during casting due to the aggregation of Al2O3, or the performance may be deteriorated due to the remaining Al2O3 in the steel. Therefore, the upper limit of the Al content is 0.200% or less, preferably 0.1% or less.

[0018] <N: 0.0100% or less> N is an element that is present in steel through normal refining and continuous casting processes. Its unavoidable presence in steel produces nitrides that adversely affect high-temperature ductility, so the lower the N content, the better the ductility tends to be. Therefore, the N content is set to 0.0100% or less. While the lower the N content, the better, taking into account the cost of the refining process, the N content is preferably set to 0.0010% or more. The N content may be 0.0015% or more, or 0.0080% or less.

[0019] <Zr:0.0003%~0.0500%> Zr is an important element because it forms nitrides within crystal grains immediately after solidification and then incorporates elements such as Ti, Nb, and V, which significantly expand the embrittlement temperature range, to form large carbonitrides. Therefore, the Zr content is set to 0.0003% or more. On the other hand, because Zr is a strong deoxidizing element and is an expensive element, the Zr content is set to 0.0500% or less. The lower limit of the Zr content is preferably 0.0005% or more, and the upper limit is preferably 0.0300% or less.

[0020] <Ti:0%~0.100%> Ti reacts with nitrogen to form complex nitrides together with Zr and other elements, suppressing the formation of AlN and improving high-temperature ductility. However, if Ti is included in excess, the effect saturates, resulting in unnecessary costs. Therefore, the Ti content is set to 0.100% or less. The lower limit of the Ti content is preferably 0.001% or more, and the upper limit of the Ti content is preferably 0.080% or less.

[0021] <P:0%~0.100%> P is an impurity element contained in iron ore and scrap, and excessive P can deteriorate the quality of steel. Therefore, the P content is set to 0.100% or less, and preferably 0.060% or less.

[0022] <S:0%~0.010%> S is an impurity element contained in iron ore and scrap, and excessive S content can deteriorate the quality of steel. Therefore, the S content is set to 0.010% or less. The S content is preferably 0.006% or less.

[0023] <Nb:0%~0.100%> Nb is an element that reacts with nitrogen to form nitrides and also contributes to improving strength. However, if Nb is included in excess, the effect saturates, resulting in unnecessary costs. Furthermore, the nitrides that are formed expand the embrittlement temperature range, just like AlN. Therefore, the Nb content is set to 0.100% or less. The lower limit of the Nb content is preferably 0.001% or more, and the upper limit of the Nb content is preferably 0.080% or less.

[0024] <V:0%~0.100%> V is an element that reacts with nitrogen to form nitrides and also contributes to improving strength. However, if excessive V is included, the effect saturates, resulting in unnecessary costs. Furthermore, the nitrides that are formed expand the embrittlement temperature range, similar to AlN. Therefore, the V content is set to 0.100% or less. The lower limit of the V content is preferably 0.001% or more, and the upper limit of the V content is preferably 0.080% or less.

[0025] <B:0%~0.0050%> B segregates to the austenite grain boundaries in steel, strengthening the grain boundaries and improving the high-temperature ductility of high-Al steel. However, if B is included in excess, the effect saturates and unnecessary costs are incurred. Therefore, the B content is set to 0.0050% or less. The B content is preferably 0.0030% or less.

[0026] <Cr:0%~0.10%> Cr is an element that reacts with nitrogen to form nitrides and also contributes to improving strength. However, if Cr is included in excess, the effect saturates, resulting in unnecessary costs. Furthermore, the nitrides that are formed expand the embrittlement temperature range, just like AlN. Therefore, the Cr content is set to 0.10% or less. The Cr content is preferably 0.08% or less.

[0027] <Mo:0%~0.10%> Mo contributes to improving strength by dissolving in steel. However, if Mo is included in excess, the effect saturates and extra costs are incurred. Therefore, the Mo content is set to 0.10% or less. The Mo content is preferably 0.08% or less.

[0028] <Sn:0%~0.50%> Sn contributes to improving strength and corrosion resistance by dissolving in steel. However, excessive Sn content can deteriorate the quality of steel. Therefore, the Sn content is set to 0.50% or less. The Sn content is preferably 0.3% or less.

[0029] <Ca:0%~0.0100%> Ca has the effect of modifying Al2O3 and suppressing the coarsening of oxide-based inclusions. However, if there is too much Ca, coarse oxide-based inclusions composed mainly of CaO-Al2O3 are formed, which may become the starting point for fatigue fracture. Therefore, the Ca content is set to 0.0100% or less. The Ca content is preferably 0.0060% or less.

[0030] <Mg:0%~0.0100%> Like Ca, Mg modifies Al2O3 and has the effect of suppressing the coarsening of oxide-based inclusions. It also acts on sulfide-based inclusions, reducing their aspect ratio. However, if there is too much Mg, coarse cluster-like oxide-based inclusions composed mainly of MgO are formed, which may become the starting point for fatigue fracture. Therefore, the Mg content is set to 0.0100% or less. The Mg content is preferably 0.0060% or less.

[0031] <REM:0%~0.0100%> REM also modifies Al2O3 and has the effect of suppressing the coarsening of oxide inclusions. However, excessive REM may reduce the cleanliness of the steel and degrade the toughness of the base material. Therefore, the REM content is set to 0.0100% or less. The REM content is preferably set to 0.0060% or less. REM is a collective term for Sc, Y, and 17 lanthanides, from La with atomic number 57 to Lu with atomic number 71.

[0032] <Ni:0%~0.50%> Ni is also an element that contributes to improving strength and toughness. However, if Ni is contained in excess, the effect saturates and unnecessary costs are incurred. Therefore, the Ni content is set to 0.50% or less. The Ni content is preferably 0.3% or less.

[0033] <Cu:0%~0.50%> Cu contributes to improving the strength of steel by being present in the form of fine particles. However, excessive Cu content can deteriorate the quality of the steel. Therefore, the Cu content is set to 0.50% or less. The Cu content is preferably 0.3% or less.

[0034] In the cast slab of this embodiment, the remainder other than the above elements consists of Fe and impurities. Here, the impurities refer to components that are mixed in during the industrial production of steel sheet due to various factors in the manufacturing process, including raw materials such as ore and scrap. Examples of impurities include O, H, W, Sb, Na, Cl, Co, Zn, Ga, Ge, As, Se, Y, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir, Pt, Au, Pb, Bi, and Po. The total amount of impurities may be 0.100% or less.

[0035] When analyzing these chemical compositions, the sample is taken from a location that represents the composition of the slab, that is, a quarter-width portion and a quarter-thick portion on the upper surface of a cross section perpendicular to the casting direction, and each element is analyzed by a known method.

[0036] Furthermore, this embodiment is premised on an alloy steel containing elements such as Ti, Nb, and V, which significantly expand the embrittlement temperature range, and is premised on a steel type that satisfies the following formula (1). 0.00005≦([Ti]+0.5[Nb]+[V])×[N]≦0.00250 ···(1) Here, [Ti], [Nb], [V], and [N] each represent the content (mass%) in the cast slab. The left side of the above formula (1) is preferably 0.00010, and the right side is preferably 0.00150. That is, ([Ti] + 0.5[Nb] + [V]) × [N] is 0.00005 or more, and preferably 0.00010 or more. Furthermore, ([Ti] + 0.5[Nb] + [V]) × [N] is 0.00250 or less, and preferably 0.00150.

[0037] Next, we will explain the nitrides, carbonitrides, and carbides in the cast slab. As described above, when Zr is added, ZrN forms within the crystal grains immediately after solidification, and then changes from ZrN to large composite carbonitrides containing Ti, Nb, and V as the temperature decreases. In other words, by forming nitrides, carbonitrides, and carbides containing Zr, the formation of carbonitrides at grain boundaries can be suppressed, thereby drastically improving high-temperature embrittlement. Therefore, in the cast slab according to this embodiment, as a guideline for the depth of surface cracks, the total number ratio of nitrides, carbonitrides, and carbides containing 10 mass% or more of Zr among nitrides, carbonitrides, and carbides with a circle-equivalent diameter of 200 nm to 5000 nm at a depth of 5 mm from the surface is 50% or more. This ratio is preferably 60% or more, and more preferably 75% or more.

[0038] The total number ratio of nitrides, carbonitrides, and carbides containing 10% by mass or more of Zr can be measured by the following method. First, a surface 5 mm from the surface of a slab is observed (for example, a 50 μm × 50 μm field of view) using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyzer (EDS), and nitrides, carbonitrides, and carbides with equivalent circle diameters of 200 nm to 5,000 nm are identified. The observation surface used for this observation is prepared as follows. First, a block of approximately 200 mm × 200 mm × 200 mm, including the slab surface, is cut from the slab by gas cutting. Next, a sample located 5 mm from the surface and having an observation surface measuring 25 mm × 25 mm is mechanically cut from the block. This sample is embedded in resin, and the observation surface is prepared by mirror polishing. Note that this observation is performed within 100 mm of each end of the slab in the width and length directions. Next, the identified nitrides, carbonitrides, and carbides are separated into those containing 10 mass% or more of Zr and those not. The total number of nitrides, carbonitrides, and carbides containing 10 mass% or more of Zr is divided by the total number of nitrides, carbonitrides, and carbides with a circle-equivalent diameter of 200 nm to 5000 nm to calculate the number ratio. 2 If this is the case, it can be considered as the number density at a position 5 mm from the surface of the slab.

[0039] As described above, the formation of Zr-containing nitrides, carbonitrides, and carbides can suppress the formation of carbonitrides at grain boundaries. The reason for determining the number density of nitrides, carbonitrides, and carbides containing 10 mass% or more Zr rather than the average Zr concentration contained in all nitrides, carbonitrides, and carbides is as follows: The number ratio of nitrides, carbonitrides, and carbides containing 10 mass% or more Zr is not necessarily proportional to the average Zr concentration contained in all nitrides, carbonitrides, and carbides. Furthermore, because the amount of Zr is relatively small compared to the amounts of Ti, Nb, and V, the average Zr concentration contained in all nitrides, carbonitrides, and carbides tends to be small. Furthermore, the Zr concentration in complex inclusions tends to be small. Even if the average Zr concentration contained in all nitrides, carbonitrides, and carbides is small, the formation of a certain number of nitrides, carbonitrides, and carbides containing 10 mass% or more Zr can suppress the formation of carbonitrides at grain boundaries. Therefore, the number density of nitrides, carbonitrides, and carbides containing 10 mass % or more of Zr can be determined and regarded as an index for the ability to suppress the formation of carbonitrides at grain boundaries.

[0040] Next, the conditions for generating the above-mentioned Zr-containing nitrides, carbonitrides, and carbides will be described in detail. The order of nitride and carbide generation ability is Zr > Ti > Nb > V, and the higher the generation ability, the higher the temperature at which nitrides and carbides are generated. The inventors focused on the high generation ability of Zr and, as a result of investigating high-temperature ductility, found that there is an appropriate Zr content that significantly improves high-temperature ductility.

[0041] On the premise that the straightening of the slabs would be performed in a general temperature range at the straightening point, the inventors conducted high-temperature tensile tests to confirm the extent to which the addition of Zr to Nb steel and Nb-Ti steel would improve the high-temperature ductility. In the high-temperature tensile tests, a high-frequency induction heating type high-temperature tensile tester equipped with a cold crucible was used, and after melting, the tensile test specimens were cooled to a predetermined tensile temperature at a cooling rate of 1.0°C / s, and then subjected to a strain rate of 3.3 × 10 while maintaining the predetermined tensile temperature. -4The specimen was subjected to tension at a rate of (1 / s) until fracture. The reduction of area (%) was calculated as {(cross-sectional area of ​​the tensile test specimen before the test - cross-sectional area of ​​the tensile test specimen after fracture) / cross-sectional area of ​​the tensile test specimen before the test} x 100. As a result, it was found that the addition of Zr increases the reduction of area, especially in the temperature range of 750 to 1000°C, and improves high-temperature ductility.

[0042] Next, a test was conducted to confirm the amount of Zr that needed to be added to prevent surface cracking. Specifically, a tensile temperature was set to 900°C, and a number of samples with different amounts of Zr, N, Ti, Nb, and V were prepared and subjected to a high-temperature tensile test to determine the area reduction (%) for each. The specific method for the high-temperature tensile test was the same as that described above. As a guideline, it was determined that no surface cracking would occur if the area reduction was 50% or more.

[0043] As a result of the test, it was found that the Zr content is correlated with the Ti, Nb, V, and N contents, and that satisfying the condition of the following formula (2) is the condition for making the number density of nitrides, carbonitrides, and carbides containing Zr 50% or more. [Zr]-6.0×(([Ti]+0.5[Nb]+[V])×[N])≧0···(2) Here, [Zr], [Ti], [Nb], [V], and [N] represent the respective contents (mass%) in the cast slab. There is no particular upper limit for [Zr] - 6.0 × (([Ti] + 0.5[Nb] + [V]) × [N]), but it is preferable to set it to 0.03 or less. [Zr] - 6.0 × (([Ti] + 0.5[Nb] + [V]) × [N]) is more preferably 0.001 or more and 0.01 or less, at which point the number density of Zr-containing nitrides, carbonitrides, and carbides becomes 60% or more.

[0044] On the other hand, Zr also easily bonds with oxygen in steel. It is necessary for Zr to be effectively consumed in the formation of carbonitrides, without being consumed in the formation of oxides. If the Zr content is high relative to the content of Al, a common and inexpensive deoxidizing element, Zr is more likely to form oxides, making it difficult to form nitrides, carbonitrides, and carbides containing Zr. Therefore, if the Al content in the cast slab is [Al] (mass%), the number density of Zr-containing nitrides, carbonitrides, and carbides can be increased to 50% or more by satisfying the condition of the following equation (3) in addition to the above equation (2). [Al] / [Zr]≧2 (3) Although there is no particular restriction on the upper limit of the formula (3), it is preferably set to 50 or less. [Al] / [Zr] is preferably 3 or more.

[0045] FIG. 1 is a diagram showing the relationship between the contents of Ti, Nb, V, and N and the Zr content. In FIG. 1, line 10 represents the boundary line that satisfies formula (2), and the area above the line represents conditions that satisfy formula (2). Circles represent examples in which no surface cracks were observed in the slab, and triangles represent examples in which surface cracks were observed in the slab. As shown in FIG. 1, under the conditions to the left of line 10, no cracks occurred except for plot 11. The example of plot 11 is an example in which formula (2) is satisfied but formula (3) is not satisfied, and surface cracks were observed in the slab.

[0046] Next, the above-mentioned continuous casting method for slabs will be described. A slower cooling rate in the 1450-1000°C temperature range results in the formation of stable ZrN at high temperatures, and Zr-containing carbonitrides grow and become coarse. On the other hand, a faster cooling rate results in less Zr-based carbonitrides being formed, preventing grain growth. To ensure stable formation of Zr-containing nitrides, carbonitrides, and carbides, when cooling a slab having the above-mentioned composition, it is preferable to set the average cooling rate from 1450 to 1000°C at a position 5 mm from the surface of the slab width center in the continuous casting machine to 300°C / min or less. On the other hand, since a too slow average cooling rate unnecessarily lengthens the operating time, it is preferable to set the average cooling rate from 1450 to 1000°C at a position 5 mm from the surface of the slab width center to 5°C / min or more. The average cooling rate is measured by measuring the surface temperature of the slab width center using a thermocouple or the like, and calculating the average cooling rate at a position 5 mm deep using two-dimensional heat transfer calculations.

[0047] Furthermore, when adjusting the composition of molten steel, it is preferable to add a predetermined amount of metallic Zr at least 2 minutes after adding metallic Al to the molten steel. Because Al and Zr have equivalent deoxidizing abilities, adding metallic Zr before adding metallic Al may cause a deoxidizing reaction by Zr, resulting in less than expected amounts of nitrides, carbonitrides, and carbonitrides containing Zr. [Example]

[0048] Next, an example of the present invention will be described, but the conditions are merely examples of conditions for confirming the feasibility and effects of the present invention, and the present invention is not limited to the description of this example. The present invention can be implemented in various ways to achieve the object of the present invention without departing from the gist of the present invention.

[0049] Twelve types of molten steel, the compositions of which are shown in Table 1, were prepared and poured into molds, followed by continuous casting using a continuous casting machine. A vertical bending continuous casting machine with a mold size of 240 mm thick x 1200 mm wide was used, and the casting speed was 1.3 m / min. The surface temperature of the slab at the straightening point was 850°C in all cases. The experiments were also conducted with an average cooling rate of 60°C / min.

[0050] Representative samples of approximately 10 m were cut from each of the slabs produced under the above conditions, and the slabs were evaluated for cracking. First, the front and back surfaces of the slabs were ground with a 0.7 mm grinder, and then visually inspected for cracks. Furthermore, to evaluate the high-temperature ductility of the slabs, high-temperature tensile tests were performed using the procedure described above, and the area reduction (%) was also determined. Additionally, for slabs that showed no cracks visually, they were subjected to the next hot rolling process without any further treatment (no grinder check for cracks) and the presence or absence of cracking was also confirmed. Specifically, the slabs were heated to 1220°C, then rough-rolled, and then finish-rolled at 900°C to a thickness of 3 mm. They were then cooled at an average cooling rate of 600°C / min to obtain hot-rolled sheets. The surface of the hot-rolled sheets was then inspected for cracks visually or by camera. When there were no cracks in the slab and no cracks occurred after hot rolling, the slab was evaluated as "◎", when there were no cracks in the slab but cracks occurred after hot rolling, the slab was evaluated as "○". When cracks were confirmed in the slab, the slab was evaluated as "×".

[0051] Furthermore, a surface of each slab was taken out at a depth of 5 mm from the surface and mirror-polished, and a 10 mm × 10 mm area was observed with an SEM. The total number ratio of nitrides, carbonitrides, and carbides containing 10 mass% or more of Zr was calculated using the method described above. The test results are shown in Table 1.

[0052] [Table 1]

[0053] As shown in Table 1, when the total number ratio of nitrides, carbonitrides, and carbides containing 10 mass% or more of Zr among all nitrides, carbonitrides, and carbides was 50% or more, no cracks occurred in the cast slab. [Explanation of symbols]

[0054] 10 straight line 11 Plot

Claims

[Claim 1] In mass%, C: 0.02% to 0.50%, Si: 0.05% to 3.00%, Mn: 0.50% to 4.00%, Al: 0.001% to 0.200%, N: 0.0100% or less, Zr: 0.0003% to 0.0500%, Ti: 0% to 0.100%, P: 0% to 0.100%, S: 0% to 0.0100%, Nb: 0% to 0.100%, V: 0% to 0.100%, B: 0% to 0.0050%, Cr: 0% to 0.10%, Mo: 0% to 0.10%, Sn: 0% to 0.50%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, REM: 0% to 0.0100%, Ni: 0% to 0.50%, and Cu: 0% to 0.50%; A cast slab, the balance of which is Fe and impurities, The contents of Ti, Nb, V, and N satisfy the following formula (1), A cast slab, characterized in that the total number ratio of nitrides, carbonitrides and carbides containing 10 mass% or more of Zr among nitrides, carbonitrides and carbides having an equivalent circle diameter of 200 nm to 5000 nm at a position 5 mm deep from the surface of the cast slab is 50% or more. 0.00005≦([Ti]+0.5[Nb]+[V])×[N]≦0.00250 ・・・（１） Here, [Ti], [Nb], [V], and [N] each represent the content (mass %) in the cast slab.

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