Heavy steel plate and its manufacturing method

Abstract

To provide a thick steel plate which suppresses or prevents occurrence of burning upon laser cutting, and therefore, is useful for application of laser cutting, and to provide a manufacturing method of the thick steel plate.SOLUTION: A thick steel plate includes a steel plate and a scale. Therein, two-dimensional arithmetic average roughness of scale surface and scale / steel plate interface are each 5.0 μm or less, and kurtosis of ruggedness of the interface is 2.50 or more. A manufacturing method of the thick steel plate includes a process in which a slab is heated up to 1050 to 1300°C and a time up to heating process completion after exceeding 1050°C satisfies predetermined conditions, a process in which rolling is performed at a cumulation of 15% or more in a predetermined temperature range, high-pressure water descaling is performed in a predetermined temperature range, rolling is performed at a rolling reduction rate of 5% or more by using a rolling roll of arithmetic average roughness of 1.00 μm or less within 30 s and, further, rolling is performed at a rolling reduction rate of 3 to 15% by using the rolling roll of arithmetic average roughness of 1.00 μm or less, and a cooling process in predetermined conditions.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a thick steel plate and a manufacturing method thereof, and more particularly to a thick steel plate that is laser cut and used in forming a structure, and a manufacturing method thereof. [Background technology]

[0002] Large steel structures such as ships, building components, industrial machinery, and bridges use a large amount of thick steel plates. In the construction of these steel structures, cutting and welding account for a large portion of the construction time. Therefore, there is a need to reduce cutting time and perform cutting with high precision to reduce welding time.

[0003] In addition to conventional gas cutting, laser cutting and plasma cutting are known as methods for cutting steel plates. Compared to conventional gas cutting, laser cutting has superior cut surface accuracy, a smaller heat-affected zone due to cutting, and the possibility of reducing labor hours through automation, so it has become popular mainly for cutting thin steel plates. In recent years, with the practical application of high-power laser cutting machines, laser cutting is also sometimes used to cut thick steel plates used in the above-mentioned large steel structures.

[0004] The production of steel plates, such as thick steel plates, typically involves hot-rolling slabs. Hot-rolled steel plates are known to oxidize in the atmosphere, forming scale (iron oxide) on their surfaces. Laser cutting of steel plates involves applying heat to the plate by irradiating it with a laser, melting it, and then cutting it. This process significantly alters the melting behavior depending on the heat absorption properties of the area irradiated by the laser. The heat absorption properties of steel plates vary significantly depending on the condition of the surface scale and the interface between the scale and the base steel. Depending on these conditions, the steel plate may not be cut reliably or may develop irregular cuts, resulting in increased labor hours and reduced cutting accuracy. In particular, when surface scale peels off irregularly due to laser irradiation, the heat absorption properties also fluctuate irregularly, making laser cutting particularly unstable and resulting in the ejection of molten material from the cut area, a phenomenon known as burning.

[0005] As a method for improving laser cuttability, for example, Patent Document 1 proposes a steel material having a surface coated with a dry coating film containing a coloring pigment consisting of titania powder, zinc powder, aluminum powder, and one or more of a black iron oxide pigment and a black calcined pigment. Patent Document 2 also proposes a steel material having a surface coated with a coating composition containing an alkoxysilane having two or more alkoxy groups and / or its hydrolysate or condensate (e.g., tetraalkoxysilane), zinc powder, and aluminum phosphate (preferably aluminum tripolyphosphate) powder or a mixed powder of aluminum phosphate and zinc phosphate.

[0006] On the other hand, Patent Document 3 presents a steel sheet in which the proportion of magnetite phase (Fe3O4) in the scale on the steel sheet surface is set to 85% or more to increase adhesion and the thickness of the scale is limited to 6 μm or less in order to improve laser cuttability.

[0007] Furthermore, techniques have been proposed for improving laser cuttability by controlling the surface properties of the scale that is directly irradiated with a laser. For example, Patent Documents 4 to 6 teach that laser cuttability can be improved by controlling the roughness of the scale surface of the steel sheet, reducing diffuse reflection of the laser, and increasing the laser absorption rate. Furthermore, Patent Documents 7 and 8 propose increasing the roughness of the interface between the scale on the steel sheet surface and the base steel to improve adhesion of the scale, thereby improving laser cuttability. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2013 / 065349 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-156377 [Patent Document 3] Japanese Patent Application Laid-Open No. 2003-221640 [Patent Document 4] Japanese Patent Application Publication No. 07-155975 [Patent Document 5] Japanese Patent Application Publication No. 08-003692 [Patent Document 6] Japanese Patent Application Publication No. 11-131183 [Patent Document 7] Japanese Patent Application Publication No. 11-343541 [Patent Document 8] Japanese Patent Application Laid-Open No. 2014-005504 Summary of the Invention [Problem to be solved by the invention]

[0009] To reduce costs in a society with a declining population, there is an increasing demand for automation of laser cutting of steel plates. Meanwhile, steel plates with even better laser cuttability than before are needed to cope with the increasing thickness of the cutting target due to the larger size of structures and the increased cutting speeds achieved by high-power fiber lasers. Specifically, to improve laser cuttability, steel plates that do not burn during cutting, even when irradiated with higher-power lasers than before, are needed. In laser cutting, the narrow cutting width can sometimes cause the molten material to clog within the cutting width. This can lead to excessive melting of the surrounding area or the molten material blowing upward, resulting in a cutting defect known as burning. Burning can be a major issue, especially with high-power laser irradiation.

[0010] Although Patent Documents 1 to 8 propose improving laser cuttability by increasing scale adhesion, reducing diffused reflection of the laser, increasing laser absorption, etc., they do not necessarily provide sufficient consideration from the perspective of suppressing or preventing the occurrence of burning. Therefore, the steel materials described in these Patent Documents still have room for improvement in terms of improving laser cuttability.

[0011] Therefore, an object of the present invention is to provide a thick steel plate in which the occurrence of burning during laser cutting is suppressed or prevented, and therefore useful for applying laser cutting, and a method for manufacturing the same. [Means for solving the problem]

[0012] In order to achieve the above object, the inventors have investigated the surface layer of a steel sheet necessary for stable continuous cutting in laser cutting. As a result, they have found that, in the surface layer of a steel sheet that is continuously irradiated with a laser, reducing the irregularities of the scale surface that receives the laser and, at the same time, reducing the irregularities of the interface between the scale and the steel sheet (base steel) stabilizes the heat absorption during laser irradiation and significantly suppresses or prevents the occurrence of burning during cutting with a high-power laser.

[0013] In particular, to stabilize the heat absorption in response to the heat input from the laser, it was extremely important to simultaneously suppress both the short-period irregularities that occur on the micrometer scale and the long-period irregularities that occur on the submillimeter scale when reducing the unevenness at the interface between the scale and steel plate. Simultaneous suppression of both types of irregularities was achieved by evaluating the arithmetic mean roughness when evaluating the unevenness at the interface between the scale and steel plate, as well as the kurtosis, which represents the kurtosis of the unevenness distribution, and controlling both within a certain range.

[0014] The present invention, which has achieved the above object, is as follows. (1) A thick steel plate comprising a steel plate and a scale formed on the surface of the steel plate, wherein the two-dimensional arithmetic mean roughness of the surface of the scale is 5.0 μm or less, the two-dimensional arithmetic mean roughness of the interface between the scale and the steel plate is 5.0 μm or less, and the kurtosis calculated from the unevenness of the interface is 2.50 or more. (2) The steel plate according to (1) above, wherein the average thickness of the scale is 6 to 60 μm. (3) The steel plate is, in mass%, C: 0.001 to 0.300%, Si: 0.01 to 1.00%, Mn: 0.10~2.50% P: 0.001 to 0.050%, S: 0.0001 to 0.0100%, Al: 0.001 to 0.200%, N: 0.0150% or less, O: 0.0050% or less, Cu: 0-1.00% Ni: 0-2.00% Cr: 0~1.00%, Mo: 0-1.00%, W: 0~0.50%, Nb: 0 to 0.500%, Ti: 0 to 0.500% V: 0 to 1.000%, B: 0~0.0100%, Sn: 0 to 0.500% Sb: 0 to 0.500% Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Hf: 0 to 0.0100%, Te: 0~0.0100%, Sr: 0 to 0.0100%, REM: 0 to 0.0100%, and The steel plate according to (1) or (2) above, having a chemical composition consisting of the balance: Fe and impurities. (4) The chemical composition is in mass%: Cu: 0.01 to 1.00%, Ni: 0.01 to 2.00% Cr: 0.01 to 1.00%, Mo: 0.01 to 1.00%, W: 0.003 to 0.50%, Nb: 0.003 to 0.500%, Ti: 0.003 to 0.500%, V: 0.003 to 1.000%, B: 0.0003~0.0100%, Sn: 0.003 to 0.500%, Sb: 0.003 to 0.500%, Ca: 0.0003 to 0.0100%, Mg: 0.0003 to 0.0100%, Hf: 0.0003 to 0.0100%, Te: 0.0003 to 0.0100%, Sr: 0.0003 to 0.0100%, and REM: 0.0003 to 0.0100% The steel plate according to (3) above, comprising one or more selected from the group consisting of: (5) The steel plate according to any one of (1) to (4) above, wherein the skewness calculated from the unevenness of the interface is 0.00 or less. (6) The steel plate according to any one of the above (1) to (5), wherein the deviation of the scale thickness is 10% or less of the average thickness of the scale. (7) A step of heating a slab, in which the slab is heated to a maximum heating temperature at which the surface temperature of the slab is 1050 to 1300°C, and the elapsed time from when the temperature exceeds 1050°C to when the heating step is completed is controlled so as to satisfy the following formula (1): a hot rolling step of hot rolling the slab, the hot rolling step being such that the slab is rolled at a cumulative reduction rate of 15% or more at a surface temperature in the range of 1050 to 1250°C, the resulting rolled material is then subjected to high-pressure water descaling at a surface temperature in the range of 1050 to 1250°C, and within 30 seconds after the high-pressure water descaling, the slab is rolled at a reduction rate of 5% or more using rolls having an arithmetic mean roughness of 1.00 μm or less, and the final rolling pass of the successive rolling steps is rolled at a temperature in the range of 850 to 1000°C using rolls having an arithmetic mean roughness of 1.00 μm or less at a reduction rate of 3 to 15%; A cooling step of cooling the obtained steel sheet, in which the elapsed time from the completion of the hot rolling step to the start of water cooling is controlled so as to satisfy the following formula (2): The method for producing a steel plate according to any one of the above (1) to (6), comprising: 1.0≦x 10 ≦10.0...Equation (1) x1=D1·D2·(T1 3 +D3T1 2 +D4T1+D5)·(T1-D6) 0.5 ·{1-exp(D7T1+D8)} 0.5 Δt 0.5 t n =x n 2D1 -2 D2 -2 ·(T n+1 3 +D3T n+1 2 +D4T n+1 +D5) -2 ·(T n+1 -875) -1 ·{1-exp(D6T n+1 +D7)} -1 x n =D1·D2·(T n 3 +D3T n 2 +D4T n +D5)·(T n -D6) 0.5 ·{1-exp(D7T n +D8)} 0.5 (t n-1 +Δt ) 0.5 D1=(1+1.160[C]-0.850[Si]+0.135[Mn]-5.300[P]-0.300[Al]-0.355[Ni] -0.147[Cu]-0.200[Cr]) (1-0.850[C]-0.052[Si]-0.026[Mn]-0.065[Al]) 0.5 x n is an index that represents the degree of scale growth after the time elapsed from when the surface temperature of the slab exceeds 1050°C during the heating process until the heating process is completed, and n indicates that the calculation corresponds to the nth of the 10 equal divisions. D1 takes into consideration the influence of the chemical composition of the steel sheet, and [C], [Si], [Mn], [P], [Al], [Ni], [Cu], and [Cr] in the above are the contents [mass%] of each element in the steel sheet. D2, D3, D4, D5, D6, D7 and D8 are constants, each equal to 5.77 × 10 -10 , -3.99×10 3 , 5.36 x 10 6 , -2.32 × 10 9 , 8.75 x 10 2 , -3.50×10-3 and 3.06 × 10 0 and T n is the average slab temperature [℃] in the nth region of the 10 equally divided sections, Δt is 1 / 10 of the elapsed time [seconds], x 10 is obtained by calculating x1, x2, x3, etc. in order using the above formula. 1.0≦y 10 ≦10.0...Equation (2) y1=E1·(J1 2 +E2J1+E3) exp{E4 / (J1+E5)} Δk 0.5 k n =y n 2 E1 -2 ·(J n+1 2 +E2J n+1 +E3) -2 ·exp{-2E4 / (J n+1 +E5)} y n =E1·(J n 2 +E2J n +E3)·exp{E4 / (J n +E5)}·(k n-1 +Δk) 0.5 y n is an index that represents the degree of scale growth after the elapsed time from the completion of the hot rolling process to the start of water cooling is divided into 10 equal sections in the cooling process, and n indicates that the calculation corresponds to the nth section out of 10 equal sections. E1, E2, E3, E4, and E5 are constants, each of which is −2.72 × 10 -2 , -2.15×10 3 , 7.89 x 10 5 , -1.05×10 4 and 2.73 × 10 2 and J n is the average steel plate temperature [°C] in the nth region of the 10 equally divided sections, Δk is one-tenth of the elapsed time [seconds], y 10 is obtained by calculating y1, y2, y3, etc. in order using the above formula. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide a thick steel plate that suppresses or prevents the occurrence of burning during laser cutting and is therefore useful for applying laser cutting, and a method for manufacturing the same. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a diagram for explaining the appearance of irregularities at the scale / steel plate interface in a thick steel plate, and corresponds to a thick steel plate of the prior art. [Figure 2] FIG. 1 is a diagram for explaining the appearance of irregularities at the scale / steel plate interface in a thick steel plate, and corresponds to a thick steel plate according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, a steel plate according to an embodiment of the present invention and a method for manufacturing the same will be described in more detail. However, these descriptions are intended to exemplify preferred embodiments of the present invention and are not intended to limit the present invention to specific embodiments.

[0018] [Preferred chemical composition] In an embodiment of the present invention, the steel sheet may be made of any material that satisfies the requirements that the two-dimensional arithmetic mean roughness of the interface between the scale and the steel sheet is 5.0 μm or less and the kurtosis calculated from the unevenness of the interface is 2.50 or more. Therefore, the chemical composition of the steel sheet is not particularly limited and may be appropriately determined within a range that satisfies these requirements. More specifically, as described above, the present invention aims to provide a thick steel sheet that suppresses or prevents the occurrence of burning during laser cutting and is therefore useful for laser cutting. This objective is achieved by achieving a two-dimensional arithmetic mean roughness of the scale surface of 5.0 μm or less, a two-dimensional arithmetic mean roughness of the interface between the scale and the steel sheet of 5.0 μm or less, and a kurtosis calculated from the unevenness of the interface between the scale and the steel sheet of 2.50 or more. Therefore, it is clear that the chemical composition of the steel sheet itself is not an essential technical feature for achieving the objective of the present invention. Hereinafter, preferred chemical compositions of steel sheets according to embodiments of the present invention will be described, but these descriptions are intended to be merely illustrative and are not intended to limit the present invention to steel sheets having such specific chemical compositions. Furthermore, in the following description, "%," which is the unit of content of each element, means "mass %" unless otherwise specified.

[0019] [C:0.001~0.300%] C is an element that is inevitably contained in general steelmaking methods, and limiting it to less than 0.001% places a heavy burden on the smelting process and is therefore economically undesirable. From this perspective, the C content is preferably 0.001% or more. C is an element that significantly increases strength, and in order to increase strength, it is preferable to contain 0.030% or more, and more preferably 0.050% or more. On the other hand, if C exceeds 0.300%, the toughness of the steel plate is significantly deteriorated, so the C content is preferably 0.300% or less. Furthermore, C is an element that impairs weldability and the toughness of welds, and from this perspective, the C content is preferably 0.230% or less, and more preferably 0.200% or less.

[0020] [Si: 0.01 to 1.00%] Si is a deoxidizing element and also contributes to improving strength. To fully obtain these effects, the Si content is preferably 0.01% or more. In particular, to increase strength, the Si content is preferably 0.05% or more, and more preferably 0.10% or more. On the other hand, if the Si content is high, the behavior of scale formation on the surface of the steel sheet may become non-uniform, so the Si content is preferably 1.00% or less. Furthermore, Si is an element that impairs the toughness of the steel sheet, and from this perspective, the Si content is preferably 0.70% or less, and more preferably 0.50% or less.

[0021] [Mn: 0.10~2.50%] Mn is an element that contributes to improving strength, and in order to fully obtain this effect, the Mn content is preferably 0.10% or more. From the viewpoint of increasing strength, the Mn content is preferably 0.30% or more, and more preferably 0.50% or more. On the other hand, if Mn is contained excessively, coarse MnS is generated, which may significantly deteriorate the toughness of the steel plate. From this viewpoint, the Mn content is preferably limited to 2.50% or less. Furthermore, Mn is an element that impairs weldability and the toughness of welds, and from this viewpoint, the Mn content is preferably 2.00% or less, and more preferably 1.80% or less.

[0022] [P: 0.001~0.050%] P is an element that is inevitably contained in general steelmaking methods, and limiting it to less than 0.001% places a heavy burden on the smelting process and is therefore economically undesirable. From this perspective, it is preferable that the P content be 0.001% or more. On the other hand, P is an element that impairs toughness, and from this perspective, it is preferable that the P content be limited to 0.050% or less. Furthermore, P is an element that impairs weldability and the toughness of welds, and from this perspective, it is preferable that the P content be 0.030% or less, and more preferably 0.020% or less.

[0023] [S:0.0001~0.0100%] S is an element that is inevitably contained in general steelmaking methods, and limiting the S content to less than 0.0001% places a heavy burden on the smelting process and is therefore economically undesirable. From this perspective, it is preferable that the S content be 0.0001% or more. On the other hand, S is an element that forms coarse sulfides and impairs toughness, and from this perspective, it is preferable that the S content be limited to 0.0100% or less. Furthermore, S is an element that impairs weldability and the toughness of welds, and from this perspective, it is preferable that the S content be 0.0060% or less, and more preferably 0.0040% or less.

[0024] [Al: 0.001 to 0.200%] Al is a deoxidizing element, and to obtain this effect, the Al content is preferably 0.001% or more. To fully exert the deoxidizing effect, the Al content is preferably 0.005% or more. On the other hand, Al is an element that impairs toughness, so the Al content is preferably limited to 0.200% or less. Furthermore, since Al also impairs weldability and the toughness of welds, the Al content is preferably 0.120% or less, and more preferably 0.080% or less.

[0025] [N:0.0150% or less] N is an element that is inevitably contained in general steelmaking methods. If a large amount of N is contained, coarse nitrides are formed, impairing the toughness of the steel sheet, so the N content is preferably limited to 0.0150% or less. Furthermore, N is an element that impairs the toughness of welds. From this perspective, the N content is preferably limited to 0.0100% or less, and more preferably 0.0060% or less. There is no particular lower limit for the N content, but limiting it to less than 0.0003% places a heavy load on the smelting process and is not economically preferable, so it is preferably 0.0003% or more.

[0026] [O:0.0050% or less] O is an element that is inevitably contained in general steelmaking methods. If a large amount of O is contained, coarse oxides are formed, impairing the toughness of the steel sheet, so the O content is preferably limited to 0.0050% or less. Furthermore, O is an element that impairs the toughness of welds. From this perspective, the O content is preferably limited to 0.0035% or less, and more preferably to 0.0025% or less. There is no particular lower limit for the O content, but limiting it to less than 0.0002% places a heavy load on the smelting process and is not economically preferable, so it is preferably 0.0002% or more.

[0027] The basic chemical composition of the steel sheet according to the embodiment of the present invention is as described above. Furthermore, the steel sheet may contain one or more of the following optional elements, if necessary. The steel sheet may contain one or more elements selected from the group consisting of Cu: 0-1.00%, Ni: 0-2.00%, Cr: 0-1.00%, Mo: 0-1.00%, and W: 0-0.50%. The steel sheet may also contain one or more elements selected from the group consisting of Nb: 0-0.500%, Ti: 0-0.500%, and V: 0-1.000%. The steel sheet may also contain B: 0-0.0100%. The steel sheet may also contain one or two elements selected from the group consisting of Sn: 0-0.500% and Sb: 0-0.500%. The steel sheet may also contain one or more elements selected from the group consisting of Ca: 0-0.0100%, Mg: 0-0.0100%, Hf: 0-0.0100%, Te: 0-0.0100%, Sr: 0-0.0100%, and REM: 0-0.0100%. These optional elements will be described in detail below.

[0028] [Cu: 0-1.00%] Cu, together with Ni, is an element that improves the adhesion between the steel sheet and scale and enhances the laser cuttability of the steel sheet. The Cu content may be 0%, but if contained, it may be 0.001% or more. To achieve this effect, the Cu content is preferably 0.01% or more, and more preferably 0.04% or more. On the other hand, if the Cu content is excessive, there is a concern that defects will occur on the surface of the cast slab, causing problems in rolling, so the Cu content is preferably limited to 1.00% or less. Furthermore, because Cu deteriorates weldability, the Cu content is more preferably 0.50% or less.

[0029] [Ni: 0-2.00%] Ni, together with Cu, is an element that improves the adhesion between the steel sheet and scale and enhances the laser cuttability of the steel sheet. The Ni content may be 0%, but if contained, it may be 0.001% or more. To achieve this effect, the Ni content is preferably 0.01% or more, and more preferably 0.04% or more. On the other hand, if the Ni content is excessive, there is a concern that defects will occur on the surface of the cast slab, causing problems in rolling, so the Ni content is preferably limited to 2.00% or less. Furthermore, because Ni deteriorates weldability, the Ni content is more preferably 1.00% or less.

[0030] [Cr: 0~1.00%] Cr is an element that contributes to improving strength. The Cr content may be 0%, but if contained, it may be 0.001% or more, or 0.01% or more. From the viewpoint of increasing strength, the Cr content is preferably 0.05% or more, and more preferably 0.15% or more. On the other hand, if Cr is contained excessively, coarse Cr carbonitrides may be formed, which may significantly deteriorate the toughness of the steel plate. From this viewpoint, it is preferable to limit the Cr content to 1.00% or less. Furthermore, Cr is an element that impairs weldability and the toughness of welds, and from this viewpoint, the Cr content is preferably 0.60% or less.

[0031] [Mo: 0-1.00%] Mo is an element that contributes to improving strength. The Mo content may be 0%, but when contained, it may be 0.001% or more, or 0.01% or more. From the viewpoint of increasing strength, the Mo content is preferably 0.02% or more, and more preferably 0.05% or more. On the other hand, if Mo is contained in an excessive amount, there is a concern that the weldability and the toughness of the welded joint may be impaired. From this viewpoint, the Mo content is preferably limited to 1.00% or less, and more preferably 0.30% or less.

[0032] [W:0~0.50%] W is an element that contributes to improving strength. The W content may be 0%, but when contained, it may be 0.001% or more, or 0.003% or more. From the viewpoint of increasing strength, the W content is preferably 0.05% or more, and more preferably 0.15% or more. On the other hand, if W is contained in an excessive amount, there is a concern that the weldability and the toughness of the welded joint may be impaired. From this viewpoint, the W content is preferably limited to 0.50% or less, and more preferably 0.30% or less.

[0033] [Nb: 0~0.500%] Nb is an element that contributes to improving strength. The Nb content may be 0%, but when it is contained, it may be 0.001% or more, or 0.003% or more. From the viewpoint of increasing strength, the Nb content is preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if Nb is contained excessively, coarse Nb carbonitrides are formed, which may significantly deteriorate the toughness of the steel plate and welded joints. From this viewpoint, the Nb content is preferably limited to 0.500% or less, and more preferably 0.100% or less.

[0034] [Ti: 0~0.500%] Ti is an element that contributes to improving strength. The Ti content may be 0%, but when contained, it may be 0.001% or more, or 0.003% or more. From the viewpoint of increasing strength, the Ti content is preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if Ti is contained excessively, coarse Ti carbonitrides are formed, which may significantly deteriorate the toughness of the steel plate and welded joints. From this viewpoint, the Ti content is preferably limited to 0.500% or less, and more preferably 0.200% or less.

[0035] [V:0~1.000%] V is an element that contributes to improving strength. The V content may be 0%, but when contained, it may be 0.001% or more, or 0.003% or more. From the viewpoint of increasing strength, the V content is preferably 0.030% or more, and more preferably 0.080% or more. On the other hand, if V is contained in an excessive amount, there is a concern that the weldability and the toughness of the welded portion may be impaired. From this viewpoint, the V content is preferably limited to 1.000% or less, and more preferably 0.600% or less.

[0036] [B: 0~0.0100%] B is an element that contributes to improving strength. The B content may be 0%, but if it is contained, it may be 0.0001% or more. From the viewpoint of increasing strength, the B content is preferably 0.0003% or more, and more preferably 0.0008% or more. On the other hand, if B is contained in an excessive amount, there is a concern that the weldability and the toughness of the welded joint may be impaired. From this viewpoint, the B content is preferably limited to 0.0100% or less, and more preferably 0.0035% or less.

[0037] [Sn: 0~0.500%] Sn is an element that concentrates at the interface between the steel sheet and scale and promotes the concentration of other metal elements near the interface. This promotes the formation of CuNi-enriched areas, thereby enhancing the adhesion of the scale. While the Sn content may be 0%, the Sn content is preferably 0.001% or more or 0.003% or more to enhance the adhesion of the scale and improve laser cuttability. To fully achieve this effect, the Sn content is preferably 0.005% or more, and more preferably 0.025% or more. However, since a large amount of Sn impairs the weldability and the toughness of the weld, the Sn content is preferably limited to 0.500% or less. From this perspective, the Sn content is preferably 0.300% or less, and more preferably 0.200% or less.

[0038] [Sb: 0~0.500%] Sb is an element that concentrates at the interface between the steel sheet and scale and promotes the concentration of other metal elements near the interface. This promotes the formation of CuNi-enriched areas, thereby enhancing the adhesion of the scale. While the Sb content may be 0%, the Sb content is preferably 0.001% or more or 0.003% or more to enhance the adhesion of the scale and improve laser cuttability. To fully achieve this effect, the Sb content is preferably 0.005% or more, and more preferably 0.025% or more. However, since a large amount of Sb impairs the weldability and toughness of the weld, the Sb content is preferably limited to 0.500% or less. From this perspective, the Sb content is preferably 0.300% or less, and more preferably 0.200% or less.

[0039] [Ca: 0~0.0100%] [Mg: 0~0.0100%] [Hf:0~0.0100%] [Te: 0~0.0100%] [Sr: 0~0.0100%] [REM:0~0.0100%] Ca, Mg, Hf, Te, Sr, and REM are elements that refine sulfides and improve the toughness of steel sheets. While the Ca, Mg, Hf, Te, Sr, and REM contents may be 0%, to achieve this effect, the Ca, Mg, Hf, Te, Sr, and REM contents are preferably 0.0001% or more or 0.0003% or more, respectively. However, if these elements are contained in excess, the effect saturates. Therefore, adding more Ca, Mg, Hf, Te, Sr, and REM to steel sheets than necessary increases manufacturing costs. Therefore, the Ca, Mg, Hf, Te, Sr, and REM contents are preferably limited to 0.0100% or less, and more preferably 0.0040% or less. Here, REM is a general term for Sc, Y and lanthanides (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu), and the total content of these 17 elements is defined as the REM content.

[0040] In the steel sheet according to the embodiment of the present invention, the balance other than the above elements consists of Fe and impurities, which are components that are mixed in during the industrial production of steel sheet due to various factors in the production process, including raw materials such as ore and scrap.

[0041] The chemical composition of steel sheets can be measured using common analytical methods. For example, the chemical composition of steel sheets can be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S are measured using the combustion-infrared absorption method.

[0042] Next, the characteristics of the scale, the scale / steel plate interface, and the vicinity of the interface for the steel plate according to the embodiment of the present invention will be described.

[0043] [Two-dimensional arithmetic mean roughness of scale surface ≦5.0μm] A thick steel plate according to an embodiment of the present invention is characterized in that the irregularities of its scale surface are small over a wide area. The smaller the irregularities of the scale surface, the more stable the irradiated laser beam can be received, i.e., the more stable the heat input due to laser irradiation, and the occurrence of burning and other defects is sufficiently suppressed or prevented, thereby improving laser cuttability. To fully achieve this effect, the thick steel plate according to an embodiment of the present invention limits the two-dimensional arithmetic mean roughness of the scale surface to 5.0 μm or less. From the viewpoint of stabilizing the heat input due to laser irradiation and improving laser cuttability, the two-dimensional arithmetic mean roughness of the scale surface is preferably 3.5 μm or less, and more preferably 2.5 μm or less. While no lower limit is set for the two-dimensional arithmetic mean roughness of the scale surface, extremely small irregularities on the scale surface are economically undesirable due to the large burden on managing the rolling and subsequent cooling processes. Therefore, the two-dimensional arithmetic mean roughness of the scale surface is preferably 0.5 μm or more.

[0044] [Two-dimensional arithmetic mean roughness of the scale / steel sheet (base steel) interface ≦ 5.0 μm] The scale on the surface of the steel plate melts and / or scatters when irradiated with a laser, and then the scale / steel plate (base steel) interface is irradiated with the laser. By minimizing the irregularities at the scale / steel plate interface, the heat input to the steel plate by laser irradiation is stabilized and laser cuttability is improved. To fully achieve this effect, in the steel plate according to an embodiment of the present invention, the two-dimensional arithmetic mean roughness at the scale / steel plate interface is limited to 5.0 μm or less. From the viewpoint of stabilizing the heat input by laser irradiation and improving laser cuttability, the two-dimensional arithmetic mean roughness at the scale / steel plate interface is preferably 4.0 μm or less, and more preferably 3.0 μm or less. From the viewpoint of laser cuttability, the smaller the two-dimensional arithmetic mean roughness at the scale / steel sheet interface, the better, and no lower limit is set. However, making the irregularities at the scale / steel sheet interface extremely small is economically undesirable because it increases the burden of managing the rolling and subsequent cooling processes. From this viewpoint, it is preferable to keep the two-dimensional arithmetic mean roughness at the scale / steel sheet interface at 0.5 μm or more.

[0045] [Kurtosis calculated from the unevenness of the scale / steel plate interface ≧ 2.50] Minimizing the unevenness at the scale / steel sheet interface is effective for stabilizing the heat input to the steel sheet by the laser and improving its laser cuttability. Here, small micrometer-scale unevenness at the interface (i.e., a two-dimensional arithmetic mean roughness at the scale / steel sheet interface of 5.0 μm or less) stabilizes the heat absorption at a given instant upon laser irradiation. However, as the cutting progresses, the location of laser irradiation moves, and the heat absorption changes as the cutting progresses in response to the submillimeter-scale unevenness. To continuously stabilize the heat absorption in response to the laser heat input as the cutting progresses, it is necessary to not only reduce the two-dimensional arithmetic mean roughness obtained from the unevenness at the interface, but also increase the kurtosis (kurtosis) obtained from the unevenness at the interface. By reducing the two-dimensional arithmetic mean roughness and increasing the kurtosis, the scale / steel sheet interface, i.e., the steel sheet surface that receives the laser, becomes sufficiently flat, resulting in a thick steel sheet with stable heat absorption. As the kurtosis increases, the convex portions become sharper, while the other portions become flatter, resulting in an increase in flatness at submillimeter intervals. From the above perspective, the kurtosis calculated from the irregularities at the scale / steel sheet interface is set to 2.50 or more. The kurtosis is preferably set to 2.80 or 3.00 or more, and more preferably 3.20 or 3.50 or more. From the perspective of laser cuttability, the larger the kurtosis calculated from the irregularities at the scale / steel sheet interface, the better, and no upper limit is set. However, an extremely large kurtosis at the scale / steel sheet interface is economically undesirable because it increases the burden of managing the rolling and subsequent cooling processes. From this perspective, it is preferable to keep the kurtosis calculated from the irregularities at the scale / steel sheet interface to 7.50 or less. As mentioned above, in prior art, it has been proposed to improve laser cutting efficiency by limiting the roughness of the scale surface within a specified range, thereby reducing diffuse reflection of the laser.However, the fact that by keeping the two-dimensional arithmetic mean roughness of the scale surface and the two-dimensional arithmetic mean roughness of the scale / steel sheet interface within specified ranges, and in addition setting the kurtosis calculated from the irregularities at the scale / steel sheet interface to 2.50 or more to increase the flatness of the scale / steel sheet interface at a sub-millimeter period, it is possible to stabilize the heat absorption during laser irradiation and suppress or prevent the occurrence of burning, thereby improving laser cutting properties, was not previously known and has now been revealed for the first time by the present inventors.

[0046] 1 and 2 are diagrams for explaining the appearance of unevenness at the scale / steel plate interface in a thick steel plate, with Fig. 1 corresponding to a thick steel plate of the prior art and Fig. 2 corresponding to a thick steel plate according to an embodiment of the present invention. To make the concept easier to understand, Fig. 1 is a schematic diagram of the results of measuring unevenness in one dimension, but as mentioned above, the unevenness at the scale / steel plate interface in a thick steel plate according to an embodiment of the present invention is measured in two dimensions.

[0047] Line A in Figure 1 is an example of a large arithmetic mean roughness at the scale / steel sheet interface. Line B is an example of a small arithmetic mean roughness at the scale / steel sheet interface, but with small kurtosis, resulting in sub-millimeter irregularities at the scale / steel sheet interface. Line C in Figure 2 has the same arithmetic mean roughness as line B in Figure 1, but is an example of large kurtosis, resulting in pointed convex portions, but the majority of the interface being an extremely flat surface, which results in excellent laser cutting properties.

[0048] [Skewness calculated from the unevenness of the scale / steel sheet interface ≦ 0.00] Furthermore, reducing the skewness, which indicates the unevenness of the distribution of irregularities, further stabilizes the heat absorption of the laser, resulting in a more significant improvement in laser cutting performance. Line D in Figure 2 above represents an example of a scale having the same arithmetic mean thickness and kurtosis as line C in Figure 2, but with a smaller skewness. By reducing the skewness, the number of interfaces extending toward the surface, as seen in line C, decreases, while the number of interfaces extending toward the steel sheet increases. The interfaces extending toward the surface indicate localized protrusions of the steel sheet toward the surface, which are presumed to destabilize the heat absorption upon laser irradiation. Limiting the skewness to a small value is believed to suppress such localized protrusions of the steel sheet, further stabilizing the heat absorption of the laser heat input. From this perspective, the skewness calculated from the irregularities at the scale / steel sheet interface is preferably 0.00 or less, and more preferably -1.00 or less. While not particularly limited, the skewness calculated from the irregularities at the scale / steel sheet interface may be, for example, -3.00 or more or -2.00 or more.

[0049] [Method for measuring two-dimensional arithmetic mean roughness, kurtosis and skewness] The unevenness of the scale surface and the scale / steel plate interface was measured using a white light interferometer with vertical scanning low-coherence interferometry at a resolution of 1.0 × 10 -6 m 2 Measurements are made over the above area. Measurements are taken on the surface at the center of the thick steel plate, and two specimens are taken to observe the scale surface and the scale / steel plate interface. The former specimens are used as is for measurement using a white light interferometry microscope. For the latter specimens, the steel plate is polished and removed, leaving the surface scale and scale / steel plate interface intact, to expose the scale / steel plate interface, and the unevenness of the exposed scale / steel plate interface is measured using a white light interferometry microscope. After the latter specimens are taken, the backside is ground using dry and wet polishing to obtain plates with a thickness of 2.0 mm or less, and then electrolytic polishing is performed, which dissolves the steel plate. The electrolysis is stopped when the scale is exposed on the electropolished surface, and the specimen is then washed with alcohol and dried to obtain a test specimen with the scale / steel plate interface exposed.

[0050] The unevenness of the scale surface and the scale / steel plate interface is measured with the side of the thick steel plate being measured as positive and the direction from the scale toward the steel plate as negative, with the pixel size being 0.50 μm or less, and the two-dimensional arithmetic mean roughness, kurtosis, and skewness are calculated as the surface roughness of the measured locations in accordance with the calculation method described in ISO 25178.

[0051] [Scale thickness deviation is 10% or less of the average scale thickness] In the steel plate according to the embodiment of the present invention, the uniformity of the scale thickness further stabilizes heat absorption during laser cutting and improves laser cuttability. The uniformity of the scale thickness is evaluated using the following procedure: a cross section parallel to the rolling direction and the sheet surface direction is cut from the steel plate, and the surface including the scale is observed using a scanning electron microscope (SEM). The distances from the interface between the steel plate and the scale to the scale surface are measured at five arbitrary points within a 1 mm range parallel to the rolling direction, and the average value is defined as the scale thickness (h1) at those points. Using the above method, the scale thickness is measured at two locations: the center of the steel plate in the width direction and two locations at least 100 mm apart from the center in the width direction. The simple average of the three scale thicknesses (h1, h2, h3) is defined as the average scale thickness (h) for the steel plate. Furthermore, the difference between the maximum and minimum of the three scale thicknesses (h1, h2, h3) is defined as the scale thickness deviation (Δh) for the steel plate.

[0052] The greater the scale thickness deviation relative to the average scale thickness, the more non-uniform the heat input to the steel plate during laser irradiation becomes, resulting in localized large thermal stresses on the scale, accelerating scale spalling and deteriorating laser cuttability. For this reason, it is preferable to limit the scale thickness deviation in the steel plate according to the embodiment of the present invention to 10% or less of the average scale thickness (i.e., Δh / h × 100≦10%). Since the smaller the scale thickness deviation, the more effectively scale spalling during laser irradiation is suppressed, the more preferable the scale thickness deviation is 8% or less of the average scale thickness, and even more preferable is 6% or less. There is no particular lower limit set for the scale thickness deviation, and the smaller the deviation, the more effectively scale spalling during laser irradiation is suppressed. While not particularly limited, the scale thickness deviation may be, for example, 1% or more or 2% or more of the average scale thickness.

[0053] The effect of reducing the ratio of the scale thickness deviation to the average scale thickness is exhibited regardless of the average scale thickness. However, as the average scale thickness decreases, the scale thickness deviation must be reduced even more, resulting in increased operational load. From this perspective, the average scale thickness is preferably 6 μm or more, more preferably 10 μm or more, and even more preferably 16 μm or more. The average scale thickness may be 17 μm or more, 18 μm or more, 20 μm or more, 22 μm or more, or 25 μm or more. On the other hand, if the average scale thickness is excessively large, the scale becomes brittle and may partially peel off due to causes other than laser irradiation, such as impact during transportation, which may result in deterioration of laser cutting properties. From this perspective, the average scale thickness is preferably 60 μm or less, more preferably 50 μm or less, and even more preferably 40 μm or less.

[0054] The steel plate according to the embodiment of the present invention may have any thickness that allows for laser cutting, and may have a thickness of, for example, 6 to 40 mm, without being particularly limited thereto. The thickness may be, for example, 8 mm or more, 10 mm or more, 15 mm or more, or 20 mm or more. Similarly, the thickness may be, for example, 35 mm or less, 30 mm or less, or 25 mm or less.

[0055] Next, a preferred method for manufacturing a steel plate according to an embodiment of the present invention will be described. The following description is an example of a characteristic method for manufacturing a steel plate according to an embodiment of the present invention, and is not intended to limit the steel plate to one manufactured by the manufacturing method described below.

[0056] A method for manufacturing a steel plate according to an embodiment of the present invention includes: A step of heating a slab, in which the slab is heated to a maximum heating temperature at which the surface temperature of the slab is 1050 to 1300°C, and the elapsed time from when the temperature exceeds 1050°C until the completion of the heating step is controlled so as to satisfy the following formula (1): a hot rolling step of hot rolling the slab, the hot rolling step being such that the slab is rolled at a cumulative reduction rate of 15% or more at a surface temperature in the range of 1050 to 1250°C, the resulting rolled material is then subjected to high-pressure water descaling at a surface temperature in the range of 1050 to 1250°C, and within 30 seconds after the high-pressure water descaling, the slab is rolled at a reduction rate of 5% or more using rolls having an arithmetic mean roughness of 1.00 μm or less, and the final rolling pass of the successive rolling steps is rolled at a temperature in the range of 850 to 1000°C using rolls having an arithmetic mean roughness of 1.00 μm or less at a reduction rate of 3 to 15%; A cooling step of cooling the obtained steel sheet, in which the elapsed time from the completion of the hot rolling step to the start of water cooling is controlled so as to satisfy the following formula (2): The present invention is characterized in that it includes: 1.0≦x 10 ≦10.0...Equation (1) x1=D1·D2·(T1 3 +D3T1 2 +D4T1+D5)·(T1-D6)0.5 ·{1-exp(D7T1+D8)} 0.5 Δt 0.5 t n =x n 2 D1 -2 D2 -2 ·(T n+1 3 +D3T n+1 2 +D4T n+1 +D5) -2 ·(T n+1 -875) -1 ·{1-exp(D6T n+1 +D7)} -1 x n =D1·D2·(T n 3 +D3T n 2 +D4T n +D5)·(T n -D6) 0.5 ·{1-exp(D7T n +D8)} 0.5 (t n-1 +Δt ) 0.5 D1=(1+1.160[C]-0.850[Si]+0.135[Mn]-5.300[P]-0.300[Al]-0.355[Ni] -0.147[Cu]-0.200[Cr]) (1-0.850[C]-0.052[Si]-0.026[Mn]-0.065[Al]) 0.5 x n is an index that represents the degree of scale growth after the time elapsed from when the surface temperature of the slab exceeds 1050°C during the heating process until the heating process is completed, and n indicates that the calculation corresponds to the nth of the 10 equal divisions. D1 takes into consideration the influence of the chemical composition of the steel sheet, and [C], [Si], [Mn], [P], [Al], [Ni], [Cu], and [Cr] in the above are the contents [mass%] of each element in the steel sheet. D2, D3, D4, D5, D6, D7 and D8 are constants, each equal to 5.77 × 10-10 , -3.99×10 3 , 5.36 x 10 6 , -2.32 × 10 9 , 8.75 x 10 2 , -3.50×10 -3 and 3.06 × 10 0 and T n is the average slab temperature [℃] in the nth region of the 10 equally divided sections, Δt is 1 / 10 of the elapsed time [seconds], x 10 is obtained by calculating x1, x2, x3, etc. in order using the above formula. 1.0≦y 10 ≦10.0...Equation (2) y1=E1·(J1 2 +E2J1+E3) exp{E4 / (J1+E5)} Δk 0.5 k n =y n 2 E1 -2 ·(J n+1 2 +E2J n+1 +E3) -2 ·exp{-2E4 / (J n+1 +E5)} y n =E1·(J n 2 +E2J n +E3)·exp{E4 / (J n +E5)}·(k n-1 +Δk) 0.5 y n is an index that represents the degree of scale growth after the elapsed time from the completion of the hot rolling process to the start of water cooling is divided into 10 equal sections in the cooling process, and n indicates that the calculation corresponds to the nth section out of 10 equal sections. E1, E2, E3, E4, and E5 are constants, each of which is −2.72 × 10 -2 , -2.15×10 3 , 7.89 x 10 5 , -1.05×10 4 and 2.73 × 102 and J n is the average steel plate temperature [°C] in the nth region of the 10 equally divided sections, Δk is one-tenth of the elapsed time [seconds], y 10 is obtained by calculating y1, y2, y3, etc. in order using the above formula.

[0057] [Casting process] The method for producing the slabs used in the steel plates according to the embodiments of the present invention is not particularly specified, and they can be produced by, for example, a continuous casting method or a blooming method. Furthermore, for the purpose of stabilizing laser cuttability and improving the appearance of the product, the surface of the slab after casting may be ground to remove scale and the scale / steel plate interface.

[0058] [Heating process] The produced slab is subjected to a heat treatment with a maximum heating temperature in the range of 1050 to 1300°C for hot rolling. This heat treatment promotes the growth of scale on the slab surface, generating voids and cracks within the scale, making it more likely to peel off. This results in a slab with scale that peels off uniformly in the descaling process described below. As a result, the flatness of the scale / steel plate interface of the final steel plate is improved, stabilizing the heat input due to laser irradiation and making it possible to sufficiently suppress or prevent the occurrence of burning and other problems. If the heating temperature is below 1050°C, the scale growth is insufficient, resulting in a slab with scale that is difficult to peel off. Fine scale remains after descaling and is pressed into the surface by rolling, increasing the unevenness of the scale / steel plate interface. As a result, the heat absorption during laser irradiation becomes unstable, making it impossible to sufficiently suppress or prevent the occurrence of burning and other problems, and laser cuttability is impaired. On the other hand, if the heating temperature exceeds 1300°C, the growth of scale causes excessively large irregularities at the scale / steel sheet interface in the slab, and the irregularities cannot be sufficiently eliminated even by rolling after descaling, resulting in a small kurtosis calculated from the irregularities at the scale / steel sheet interface and impaired laser cuttability. From these viewpoints, the maximum heating temperature of the slab is set to 1050 to 1300°C, and preferably 1080 to 1250°C.

[0059] The growth of scale on the slab surface during the heating process is greatly affected not only by the heating temperature but also by the heating time. Therefore, heating is performed so that the elapsed time from when the temperature exceeds 1050°C until the end of the heating process satisfies the following formula (1). Here, x in formula (1) 10 is an index that represents the degree of scale growth on the slab surface, taking into account the change in the influence of the temperature on scale growth over time during the heating process, which is calculated using the calculations described below. If the elapsed time is too short, x 10If x is less than 1.0, the growth of scale is insufficient, resulting in a slab with scale that is difficult to peel off, and fine scale remains after descaling and is pressed into the slab by rolling, increasing the unevenness of the scale / steel sheet interface and / or reducing the kurtosis calculated from the unevenness, impairing laser cuttability. 10 x is set to 1.0 or more. In order to reduce the adhesion of scale to the slab and make it easier to remove the scale by descaling, 10 is preferably 1.5 or more, and more preferably 2.0 or more. On the other hand, if the elapsed time is too long, the growth of scale causes excessively large irregularities at the scale / steel sheet interface in the slab, and the irregularities cannot be sufficiently eliminated even by rolling after descaling, so that the kurtosis calculated from the irregularities at the scale / steel sheet interface becomes small, impairing laser cuttability. For this reason, x 10 is limited to 10.0 or less, preferably 9.0 or less, and more preferably 8.0 or less. 1.00≦x 10 ≦10.00...Equation (1) x1=D1·D2·(T1 3 +D3T1 2 +D4T1+D5)·(T1-D6) 0.5 ·{1-exp(D7T1+D8)} 0.5 Δt 0.5 t n =x n 2 D1 -2 D2 -2 ·(T n+1 3 +D3T n+1 2 +D4T n+1 +D5) -2 ·(T n+1 -875) -1 ·{1-exp(D6T n+1 +D7)} -1 x n =D1·D2·(T n 3 +D3T n 2 +D4Tn +D5)·(T n -D6) 0.5 ·{1-exp(D7T n +D8)} 0.5 (t n-1 +Δt ) 0.5 D1=(1+1.160[C]-0.850[Si]+0.135[Mn]-5.300[P]-0.300[Al]-0.355[Ni] -0.147[Cu]-0.200[Cr]) (1-0.850[C]-0.052[Si]-0.026[Mn]-0.065[Al]) 0.5

[0060] These calculations were carried out by dividing the time elapsed from when the surface temperature of the slab (also called the slab temperature) exceeded 1050°C during the heating process until the end of the heating process into 10 equal parts, and calculating the degree of scale growth (x n ), where the subscript n indicates that the calculation corresponds to the nth interval of 10 equal divisions. D1 is a term that takes into account the influence of the chemical composition of the steel sheet, and is the value calculated using the above formula, where [C], [Si], [Mn], [P], [Al], [Ni], [Cu], and [Cr] are the contents [mass%] of each element in the steel sheet. D2, D3, D4, D5, D6, D7, and D8 are constants, each of which is 5.77 x 10 -10 , -3.99×10 3 , 5.36 x 10 6 , -2.32 × 10 9 , 8.75 x 10 2 , -3.50×10 -3 and 3.06 × 10 0 T n is the average slab temperature [°C] in the nth region of the 10-divided section, that is, the arithmetic mean of the temperature measurements taken every predetermined time, for example, every 10 seconds, in the entire nth region. Δt is one-tenth of the elapsed time [seconds]. x in Equation (1) 10 can be obtained by calculating x1, x2, x3, etc. in order using the above formula.

[0061] [Hot rolling process] The surface of the heat-treated slab has heterogeneous scale that is easily peeled off. Therefore, hot rolling and high-pressure water descaling are performed to remove the heterogeneous scale, thereby exposing the steel sheet surface over the entire surface, and then appropriate rolling can be performed to control the unevenness of the steel sheet surface.

[0062] In order to thoroughly remove scale by descaling, hot rolling is performed prior to descaling to crush the scale. The rolling prior to descaling is performed in one or more stages at a slab surface temperature (also referred to as the slab temperature) in the range of 1050 to 1250°C, with a cumulative reduction of 15% or more relative to the slab thickness. In other words, when rolling prior to descaling is performed in multiple stages, the cumulative reduction calculated from the plate thickness after the completion of the multiple rolling stages relative to the slab thickness is 15% or more. If the slab temperature exceeds 1250°C, the crushed scale grows before descaling, increasing the adhesion of the scale and making it impossible to thoroughly remove the scale by descaling. On the other hand, if the slab temperature is below 1050°C, some of the scale is pressed into the steel plate, and even when descaling is performed using high-pressure water, some of the scale remains. Thus, if the slab temperature deviates from the range of 1050 to 1250°C, scale removal by descaling is not performed sufficiently, and the properties of the scale / steel sheet interface are impaired. The slab temperature is preferably 1080°C or higher. Furthermore, by setting the slab temperature to 1125°C or higher, the convex portions of the scale / steel sheet interface facing the scale side are rolled, and the skewness calculated from the unevenness of the scale / steel sheet interface becomes 0.00 or negative, improving laser cuttability. Therefore, it is even more preferable to set the slab temperature to 1125°C or higher.

[0063] Furthermore, if the cumulative reduction rate of the hot rolling is less than 15%, the scale is not sufficiently crushed, and even after descaling with high-pressure water, some of the scale remains, resulting in an inhomogeneous scale. As a result, the irregularities at the scale / steel sheet interface become large, i.e., the two-dimensional arithmetic mean roughness at the scale / steel sheet interface becomes large, and further, the kurtosis calculated from the irregularities at the interface becomes small, impairing laser cuttability. Although there is no particular upper limit set for the cumulative reduction rate of the hot rolling, the cumulative reduction rate may be 30% or less. If the cumulative reduction rate exceeds 30%, the scale is sufficiently crushed, and the effect of descaling in assisting the removal of inhomogeneous scale becomes saturated.

[0064] After the hot rolling, the obtained rolled material is subjected to descaling using high-pressure water. Descaling can be performed using high-pressure water with an impingement pressure of 10 to 15 MPa, for example. If the descaling temperature (surface temperature of the rolled material) is below 1050°C, some unevenly grown scale will remain, impairing the quality of the scale / steel sheet interface. On the other hand, if the descaling temperature exceeds 1250°C, scale will form immediately after descaling, impairing the quality of the scale / steel sheet interface. From the above viewpoints, the temperature at which descaling is performed after the rolling is limited to the range of 1050 to 1250°C, and preferably, it is performed in the range of 1080 to 1200°C.

[0065] When carrying out the above-mentioned descaling, the temperature of the surface of the rolled material is measured in the width direction of the rolled material before and after descaling, and the amount of water used for descaling is adjusted in the width direction according to the temperature distribution before descaling, thereby reducing the temperature deviation in the width direction of the rolled material after descaling and allowing the formation of scale after descaling to proceed uniformly at all locations on the rolled material. The amount of water used for descaling varies greatly depending on the size of the rolled material and the state of the equipment, so a specific amount is not specified, but in order to achieve a uniform scale thickness, it is preferable to increase or decrease the amount of high-pressure water sprayed in descaling in the width direction in accordance with the temperature measured before descaling, or to keep the amount of water constant, and thereby reduce the temperature deviation in the width direction of the rolled material after descaling to 25°C or less.

[0066] When measuring the temperature deviation in the width direction of the rolled material after descaling, the absolute value of the difference between the highest and lowest temperatures among the three surface temperatures measured at the center and 1 / 4 width from each end of the rolled material within 10 seconds after the completion of the descaling is taken as the temperature deviation in the width direction of the rolled material.

[0067] By performing appropriate rolling on the steel sheet surface exposed by the descaling and then allowing the scale to grow, a steel plate according to an embodiment of the present invention having a desirable scale / steel sheet interface can be obtained. The elapsed time from the descaling to the subsequent rolling is set to 30 seconds or less. This is to prevent scale from being formed non-uniformly on the steel sheet surface exposed by the descaling, which would impair the effect of the rolling in flattening the steel sheet surface. From this perspective, the elapsed time is preferably set to 20 seconds or less. It is preferable not to set a lower limit for the elapsed time and to perform rolling immediately after descaling. However, if the elapsed time is too short, there is a concern that scale scattered by the descaling may be caught in the rolling, impairing the properties of the steel sheet surface. Therefore, the elapsed time is preferably set to 0.1 seconds or more.

[0068] The rolling performed following the descaling is performed using a rolling roll having a surface roughness of 1.00 μm or less in arithmetic mean roughness (Ra). If the surface roughness of the rolling roll exceeds 1.00 μm in arithmetic mean roughness, the irregularities of the rolling roll are transferred to the steel sheet, making it impossible to achieve a sufficiently flat steel sheet surface, and impairing the quality of the scale / steel sheet interface. To achieve a flat steel sheet surface, the surface roughness of the rolling roll is preferably 0.50 μm or less in arithmetic mean roughness, more preferably 0.30 μm or less, and even more preferably 0.15 μm or less. There is no particular lower limit set for the surface roughness of the rolling roll, but even if the roughness is less than 0.03 μm in arithmetic mean roughness, the effect of the rolling on flattening the scale / steel sheet interface is saturated, and therefore, from an economical viewpoint, it is preferable to keep the arithmetic mean roughness at 0.03 μm or more.

[0069] Furthermore, the reduction ratio in the rolling performed following the descaling is set to 5% or more relative to the sheet thickness before rolling. If the reduction ratio is less than 5%, the steel sheet surface cannot be made sufficiently flat by rolling, and the quality of the scale / steel sheet interface is impaired. From this perspective, the reduction ratio is preferably set to 8% or more, and more preferably to 12% or more. There is no particular upper limit to the reduction ratio, but since the effect on the quality of the scale / steel sheet interface saturates when the reduction ratio exceeds 30%, it is preferable from an economical perspective to keep it to 30% or less.

[0070] Further rolling is performed to obtain a thickness appropriate for the intended use. Scale on the surface of the rolled material occurs and grows after the descaling and subsequent rolling. To improve flatness, the final rolling pass is performed using a roll with a surface roughness of 1.00 μm or less in arithmetic mean roughness. If the surface roughness of the roll exceeds 1.00 μm in arithmetic mean roughness, the irregularities of the roll are transferred to the scale surface, resulting in increased irregularities on the scale surface. To reduce the irregularities on the scale surface, the surface roughness of the roll is preferably 0.50 μm or less in arithmetic mean roughness, more preferably 0.30 μm or less, and even more preferably 0.15 μm or less. While there is no particular lower limit for the roughness of the roll, a roughness of less than 0.03 μm saturates the effect of flattening the scale surface, so from an economical standpoint, it is preferable to keep it at 0.03 μm or more.

[0071] The final rolling pass is carried out at a temperature in the range of 850 to 1000°C. If this temperature is too high, the flattened scale will grow unevenly immediately after rolling, resulting in increased irregularities on the scale surface. On the other hand, if this temperature is too low, the scale will be crushed, causing localized oxygen penetration into the steel sheet surface, which will damage the irregularities at the scale / steel sheet interface and further increase the irregularities on the scale surface. From these perspectives, the rolling temperature is preferably set to 875 to 975°C.

[0072] The reduction ratio in the final rolling pass is set to 3 to 15% compared to the thickness immediately before. If the reduction ratio is less than 3%, the effect of flattening the scale surface is not sufficiently achieved. On the other hand, if the reduction ratio exceeds 15%, the scale breaks down, causing localized oxygen penetration into the steel sheet surface, which can impair the irregularities at the scale / steel sheet interface and further increase the irregularities on the scale surface. From these perspectives, the reduction ratio is preferably set to 5 to 12%.

[0073] Between the rolling performed following the descaling and the rolling in the final rolling pass, one or more rolling passes may be performed to obtain a thickness appropriate for the intended use. The temperature and reduction ratio in these rolling passes are not particularly specified, but in order to avoid impairing the laser cuttability of the product, it is preferable to limit the reduction ratio per pass, relative to the thickness immediately before, to 30% or less between the temperature at which the descaling is performed and the temperature at which the final rolling pass is performed. Furthermore, while the steel plate according to the embodiment of the present invention can be obtained without particularly specifying the surface roughness of the mill rolls used in these rolling passes, if the roughness of the mill rolls used in some or all of these rolling passes is 1.00 μm or less in arithmetic mean roughness, unevenness at the scale / steel sheet interface and on the scale surface is reduced, which is preferable from the viewpoint of laser cuttability.

[0074] Furthermore, for the purpose of suppressing excessive growth of scale and improving the appearance quality, descaling may be performed one or more times between the rolling performed following the above descaling and the rolling in the final rolling pass. When additional descaling is performed, it is preferable to perform the descaling so as not to increase the temperature deviation of the rolled material, for example by making the amount of water sprayed constant in the width direction.

[0075] [Cooling process] If the growth of scale proceeds excessively after the completion of the hot rolling process, the irregularities on the scale surface will become large, impairing laser cuttability. On the other hand, if the growth of scale is excessively suppressed, fine cracks inside the scale caused by rolling will remain, and the scale will partially peel off due to thermal stress during water cooling, which will actually increase the irregularities on the scale surface and impair laser cuttability. Therefore, from the above perspective, the elapsed time from the completion of the hot rolling process until water cooling is performed to stop the scale growth is controlled to reduce the irregularities on the scale surface. Note that the effect of the passage of time on the degree of scale growth varies depending on the temperature at the time, so the elapsed time after the completion of the hot rolling process is managed by the following formula (2). Here, if the elapsed time from the completion of the hot rolling process until water cooling is too short, y 10If y is less than 1.0, the growth of scale is excessively suppressed, the unevenness of the scale surface becomes large, and the laser cutting ability is impaired. 10 On the other hand, if the time from the completion of the hot rolling process to water cooling is too long, y 10 If y exceeds 10.0, the scale grows excessively and the unevenness of the scale surface increases. 10 To reduce the unevenness of the scale surface and improve laser cutting properties, y 10 is preferably 2.0 or more and 8.0 or less, and more preferably 3.0 or more and 7.0 or less. 1.00≦y 10 ≦10.00...Equation (2) where y n is calculated as follows: y1=E1·(J1 2 +E2J1+E3) exp{E4 / (J1+E5)} Δk 0.5 k n =y n 2 E1 -2 ·(J n+1 2 +E2J n+1 +E3) -2 ·exp{-2E4 / (J n+1 +E5)} y n =E1·(J n 2 +E2J n +E3)·exp{E4 / (J n +E5)}·(k n-1 +Δk) 0.5

[0076] These calculations were carried out by dividing the time elapsed from the completion of the hot rolling process to the start of water cooling into 10 equal parts, and calculating the degree of scale growth (y n ), where the subscript n indicates that the calculation is the nth of 10 equal intervals. E1, E2, E3, E4, and E5 are constants, each of which is -2.72×10 -2 , -2.15×10 3 , 7.89 x 105 , -1.05×10 4 and 2.73 × 10 2 J n is the average steel sheet temperature [°C] in the nth region of the 10 equally divided sections, that is, the arithmetic mean of the temperature measurements at each predetermined time in the entire nth region. Δk is one-tenth of the elapsed time [seconds]. 10 can be obtained by calculating y1, y2, y3, etc. in order using the above formula.

[0077] In the method for manufacturing a steel plate according to an embodiment of the present invention, the conditions for water cooling are not particularly specified, but if the steel plate temperature at which water cooling is completed exceeds 700°C, there is a concern that scale will continue to grow even after water cooling, impairing the homogeneity of the scale, and therefore the steel plate temperature at which water cooling is completed is preferably 700°C or lower. Furthermore, if the steel plate temperature at which water cooling is completed is lower than 300°C, there is a concern that thermal stress generated in the steel plate will increase, causing partial fracture of the scale and impairing the homogeneity of the scale, and therefore the steel plate temperature at which water cooling is completed is preferably 300°C or higher.

[0078] In the manufacturing method of a steel plate according to an embodiment of the present invention, the cooling conditions after water-cooling of the steel plate are not particularly specified, but excessive heat retention after water-cooling may impair the toughness of the steel plate, so natural cooling or air-cooling is preferred. Alternatively, the steel plate after water-cooling or during water-cooling may be wound into a coil and further water-cooled, air-cooled, and / or natural cooling may be performed. Furthermore, after cooling of the steel plate is complete, it may be subjected to a tempering treatment within a range that does not impair the characteristics of the steel plate according to an embodiment of the present invention.

[0079] The method for producing a steel plate according to an embodiment of the present invention may further include a flattening step using a hot leveler or the like in addition to the heating step, hot rolling step, and cooling step described above.

[0080] The thick steel plate manufactured by the method for manufacturing thick steel plate according to an embodiment of the present invention has a two-dimensional arithmetic mean roughness of 5.0 μm or less on the surface of the scale formed on the surface of the steel plate, a two-dimensional arithmetic mean roughness of 5.0 μm or less at the interface between the scale and the steel plate, and a kurtosis calculated from the unevenness of the interface of 2.50 or more. This stabilizes the heat input due to laser irradiation, thereby suppressing or preventing the occurrence of burning during laser cutting work and enabling stable cutting into any shape, and the plate can be used for structures such as land development, architecture, industrial machinery, and bridges.

[0081] The present invention will be described in more detail below with reference to examples. The conditions in the examples are merely examples employed to confirm the feasibility and effects of the present invention. The present invention is not limited to these example conditions. Various conditions may be employed in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved. [Example]

[0082] In the following examples, thick steel plates according to the embodiments of the present invention were produced under various conditions, and the occurrence of burning when the obtained thick steel plates were cut with a laser was examined.

[0083] First, using a slab having the chemical composition shown in Table 1, a heating step, a hot rolling step, and a cooling step were carried out under the conditions shown in Table 2 to obtain steel plates as experimental examples including examples and comparative examples. Note that experimental examples 46 and 51 are steel plates obtained by winding the water-cooled steel plate into a coil and allowing it to cool in the atmosphere.

[0084] [Table 1]

[0085] [Table 2-1]

[0086] [Table 2-2]

[0087] [Evaluation by laser cutting] The laser cuttability of the resulting steel plates was evaluated based on the presence or absence of burning during laser cutting tests. First, a 500 mm × 500 mm steel plate was cut into 80 mm × 80 mm pieces using a CO2 laser irradiated under the following conditions. The same cutting test was then performed three times. Those that did not experience burning were marked with a "◎", those that experienced burning once were marked with a "○", those that experienced burning twice were marked with a "△", and those that experienced burning all three times were marked with an "×". Steel plates that achieved a "◎" or "○" were judged to have reduced or prevented burning during laser cutting and therefore had excellent laser cuttability. The thickness of the steel plates used for cutting was 20 mm. In the experimental examples, steel plates with thicknesses exceeding 20 mm were ground on the side opposite the laser irradiated surface to reduce the thickness of the cut to 20 mm. In the experimental examples, thick steel plates with a thickness of less than 20 mm were ground on the side opposite to the side to be irradiated with the laser, and then stacked with the same thick steel plate that had also had its surface ground so that the ground surfaces overlapped, and then cut so that the total thickness of the plates was 20 mm. The results are shown in Table 3. Laser power: 4200W Frequency: 500Hz Duty: 75% Assist gas pressure: 30MPa Cutting speed: 650mm / min

[0088] [Table 3-1]

[0089] [Table 3-2]

[0090] Among the experimental examples listed in Tables 1 to 3, experimental example 52 is a comparative example in which the maximum heating temperature of the slab in the heating process was low, the kurtosis calculated from the unevenness at the scale / steel sheet interface was small, and the laser cuttability was inferior. On the other hand, experimental example 73 is a comparative example in which the maximum heating temperature of the slab in the heating process was high, the unevenness at the scale / steel sheet interface was large (i.e., the two-dimensional arithmetic average roughness at the scale / steel sheet interface was large), and the kurtosis calculated from the unevenness was small, and the laser cuttability was inferior. Experimental Example 57 is a comparative example in which the heating time in the heating step was short, formula (1) was not satisfied, the kurtosis calculated from the unevenness at the scale / steel sheet interface was small, and the laser cuttability was inferior. On the other hand, Experimental Example 3 is a comparative example in which the heating time in the heating step was long, formula (1) was not satisfied, the unevenness at the scale / steel sheet interface was large, and furthermore, the kurtosis calculated from the unevenness was small, and the laser cuttability was inferior. Experimental Example 65 is a comparative example in which the temperature at which hot rolling was performed before descaling was low, the kurtosis calculated from the unevenness at the scale / steel sheet interface was small, and the laser cuttability was poor. Experimental Example 76 is a comparative example in which the temperature at which hot rolling was performed before descaling was high, the unevenness at the scale / steel sheet interface was large, and the kurtosis calculated from the unevenness was small, and the laser cuttability was poor. On the other hand, Experimental Example 71 is a comparative example in which the cumulative reduction rate in hot rolling was low, the unevenness at the scale / steel sheet interface was large, and the kurtosis calculated from the unevenness was small, and the laser cuttability was poor. Experimental Example 61 is a comparative example in which the descaling temperature was low, the unevenness at the scale / steel sheet interface was large, and the kurtosis calculated from the unevenness was small, resulting in inferior laser cuttability. On the other hand, Experimental Example 27 is a comparative example in which the descaling temperature was high, the kurtosis calculated from the unevenness at the scale / steel sheet interface was small, resulting in inferior laser cuttability. Experimental Example 6 is a comparative example in which the time elapsed after descaling until the next rolling was performed was long, the kurtosis calculated from the unevenness of the scale / steel sheet interface was small, and the laser cuttability was poor. Experimental Example 17 is a comparative example in which the reduction ratio of the rolling performed after descaling was small, resulting in large unevenness at the scale / steel sheet interface, and furthermore, the kurtosis calculated from the unevenness was small, resulting in poor laser cuttability. Experimental Example 79 is a comparative example in which the surface roughness of the rolling roll used in the rolling performed after descaling was high, resulting in large unevenness at the scale / steel sheet interface, resulting in poor laser cuttability. Experimental Example 37 is a comparative example in which the temperature in the final rolling pass was low, the unevenness at the scale / steel sheet interface was large, the kurtosis calculated from the unevenness was small, and the unevenness on the scale surface of the steel sheet was large, resulting in poor laser cuttability. Experimental Example 45 is a comparative example in which the temperature in the final rolling pass was high, the unevenness on the scale surface was large, resulting in poor laser cuttability. Experimental Example 20 is a comparative example in which the reduction ratio in the final rolling pass was small, resulting in large irregularities on the scale surface and poor laser cuttability. Experimental Example 11 is a comparative example in which the reduction ratio in the final rolling pass was large, resulting in large irregularities at the scale / steel sheet interface and further large irregularities on the scale surface, resulting in poor laser cuttability. On the other hand, Experimental Example 32 is a comparative example in which the surface roughness of the roll used for rolling in the final rolling pass was large, resulting in large irregularities on the scale surface and poor laser cuttability. Experimental Example 8 is a comparative example in which the time elapsed from the completion of the hot rolling process to the start of water cooling was short, so that formula (2) was not satisfied, resulting in large irregularities on the scale surface and poor laser cuttability. On the other hand, Experimental Example 42 is a comparative example in which the time elapsed from the completion of the hot rolling process to the start of water cooling was long, so that formula (2) was not satisfied, resulting in large irregularities on the scale surface and poor laser cuttability. Experimental Example 81 is a comparative example in which the Si content of the steel sheet was excessive, resulting in large irregularities at the scale / steel sheet interface and on the scale surface, and in which the laser cuttability was poor.

[0091] The experimental examples excluding the above-mentioned comparative examples, i.e., Experimental Examples 1, 2, 4, 5, 7, 9, 10, 12-16, 18, 19, 21-26, 28-31, 33-36, 38-41, 43, 44, 46-51, 53-56, 58-60, 62-64, 66-70, 72, 74, 75, 77, 78, and 80, are examples of the present invention. In these examples, burning during laser cutting was suppressed or prevented, and steel plates with excellent laser cuttability were obtained. In particular, in all experimental examples in which the skewness calculated from the irregularities at the scale / steel plate interface was 0.00 or less, the burning resistance was evaluated as excellent, and therefore high laser cuttability was achieved. In this regard, a skewness of 0.00 or less could be reliably achieved by setting the slab temperature during hot rolling before descaling to 1125°C or higher. In addition, Experimental Examples 62, 77, and 80 are examples in which descaling was performed two more times after the completion of rolling following the first descaling and before the final rolling pass, and Experimental Examples 59 and 74 are examples in which all rolling was performed using rolls whose surface roughness was 1.00 or less in arithmetic mean roughness.

Claims

1. A thick steel plate comprising a steel plate and a scale formed on the surface of the steel plate, wherein the surface of the scale has a two-dimensional arithmetic mean roughness of 5.0 μm or less, the interface between the scale and the steel plate has a two-dimensional arithmetic mean roughness of 5.0 μm or less, and the kurtosis calculated from the unevenness of the interface is 2.50 or more.

2. The steel plate according to claim 1, wherein the average thickness of the scale is 6 to 60 μm.

3. The steel plate comprises, in mass%, C: 0.001-0.300%, Si: 0.01-1.00%, Mn: 0.10 to 2.50%, P: 0.001-0.050%, S: 0.0001-0.0100%, Al: 0.001-0.200%, N: 0.0150% or less, O: 0.0050% or less, Cu: 0 to 1.00%, Ni: 0-2.00%, Cr: 0-1.00%, Mo: 0-1.00%, W: 0-0.50%, Nb: 0 to 0.500%, Ti: 0 to 0.500%, V: 0-1.000%, B: 0 to 0.0100%, Sn: 0-0.500%, Sb: 0 to 0.500%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Hf: 0-0.0100%, Te: 0 to 0.0100%, Sr: 0 to 0.0100%, REM: 0 to 0.0100%, and The steel plate according to claim 1 or 2, having a chemical composition consisting of the balance being Fe and impurities.

4. The chemical composition is, in mass %, Cu: 0.01 to 1.00%, Ni: 0.01-2.00%, Cr: 0.01-1.00%, Mo: 0.01-1.00%, W: 0.003-0.50%, Nb: 0.003 to 0.500%, Ti: 0.003 to 0.500%, V: 0.003-1.000%, B: 0.0003 to 0.0100%, Sn: 0.003 to 0.500%, Sb: 0.003 to 0.500%, Ca: 0.0003-0.0100%, Mg: 0.0003 to 0.0100%, Hf: 0.0003-0.0100%, Te: 0.0003 to 0.0100%, Sr: 0.0003 to 0.0100%, and REM: 0.0003-0.0100% The steel plate according to claim 3, comprising one or more selected from the group consisting of:

5. The steel plate according to any one of claims 1 to 4, wherein the skewness calculated from the unevenness of the interface is 0.00 or less.

6. The steel plate according to any one of claims 1 to 5, wherein the deviation in the scale thickness is 10% or less of the average thickness of the scale.

7. A step of heating a slab, in which the slab is heated to a maximum heating temperature at which the surface temperature of the slab is 1050 to 1300 ° C., and the elapsed time from when the temperature exceeds 1050 ° C. to when the heating step is completed is controlled so as to satisfy the following formula (1): a hot rolling step of hot rolling the slab, in which the slab is rolled at a cumulative reduction rate of 15% or more at a surface temperature in the range of 1050 to 1250°C, and then the obtained rolled material is subjected to high-pressure water descaling at a surface temperature in the range of 1050 to 1250°C, and within 30 seconds after the high-pressure water descaling, the slab is rolled at a reduction rate of 5% or more using rolls having an arithmetic mean roughness of 1.00 μm or less, and the final rolling pass of the successive rolling steps is rolled at a temperature in the range of 850 to 1000°C using rolls having an arithmetic mean roughness of 1.00 μm or less at a reduction rate of 3 to 15%; A step of cooling the obtained steel sheet, in which the elapsed time from the completion of the hot rolling step to the start of water cooling is controlled so as to satisfy the following formula (2): The method for producing a steel plate according to any one of claims 1 to 6, comprising: 1.0≦x 10 ≦10.0 ・・・Form (1) x 1 =D 1 ・D 2 ・(T 1 3 +D 3 T 1 2 +D 4 T 1 +D 5 )・(T 1 -D 6 ) 0.5 ・{1-exp(D 7 T 1 +D 8 )} 0.5 ・Δt 0.5 t n =x n 2 ・D 1 -2 ・D 2 -2 ・(T n+1 3 +D 3 T n+1 2 +D 4 T n+1 +D 5 ) -2 ・(T n+1 -875) -1 ・{1-exp(D 6 T n+1 +D 7 )} -1 x n =D 1 ・D 2 ・(T n 3 +D 3 T n 2 +D 4 T n +D 5 )・(T n -D 6 ) 0.5 ・{1-exp(D 7 T n +D 8 )} 0.5 ・(t n-1 +Δt) 0.5 D 1 - 0.147[Cu]-0.200[Cr])・(1-0.850[C]-0.052[Si]-0.026[Mn]-0.065[-]) 0.5 x n is an index representing the degree of scale growth after the time elapsed from when the surface temperature of the slab exceeds 1050°C during the heating process until the completion of the heating process is divided into 10 equal sections, and n indicates that the calculation corresponds to the nth section out of the 10 equal sections. D 1 is a value that takes into consideration the influence of the chemical composition of the steel sheet, and in the above, [C], [Si], [Mn], [P], [Al], [Ni], [Cu], and [Cr] are the contents [mass%] of each element in the steel sheet, D 2 , D 3 , D 4 , D 5 , D 6 , D 7 and D 8 are constants, and 5.77 × 10 -10 , −3.99×10 3 , 5.36 x 10 6 , −2.32 × 10 9 , 8.75 x 10 2 , −3.50×10 -3 and 3.06 × 10 0 and T n is the average slab temperature [°C] in the nth region of the 10 equally divided sections, Δt is 1 / 10 of the elapsed time [seconds], x 10 is calculated by the above formula: 1 From x 2 , x 3 ...and so on, and can be calculated in order. 1.0≦y 10 ≦10.0 ・・・Form (2) y 1 =E 1 ・(J 1 2 +E 2 J 1 +E 3 )・exp{･ 4 / (J 1 +E 5 )}・Δk 0.5 k n =y n 2 ・E 1 -2 ・(J n+1 2 +E 2 J n+1 +E 3 ) -2 ・exp{-2E 4 / (J n+1 +E 5 )} y n =E 1 ・(J n 2 +E 2 J n +E 3 )・exp{･ 4 / (J n +E 5 )}・(k n-1 +Δk) 0.5 y n is an index representing the degree of scale growth after the elapsed time from the completion of the hot rolling process to the start of water cooling is divided into 10 equal sections in the cooling process, and n indicates that the calculation corresponds to the nth section out of the 10 equal sections, E 1 , E 2 , E 3 , E 4 and E 5 are constants, and −2.72 × 10 -2 , −2.15×10 3 , 7.89 x 10 5 , −1.05×10 4 and 2.73 x 10 2 and J n is the average steel sheet temperature [°C] in the n-th region of the 10 equally divided sections, Δk is 1 / 10 of the elapsed time [seconds], y 10 is calculated by the above formula. 1 From y 2 , y 3 ...and so on, and can be calculated in order.

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