Heavy steel plate and its manufacturing method

Abstract

To provide a thick steel plate which enhances adhesion of a scale, suppresses or prevents scale peeling at the time of laser irradiation, and accordingly is useful for applying laser cutting, and a method for manufacturing the same.SOLUTION: A thick steel plate includes a steel plate, and a scale formed on the surface of the steel plate, wherein a density of a CuNi-concentrated part existing in an island state on the interface between the steel plate and the scale is 1.0×107 pieces / m2 or more, and thickness deviation of the scale is 10.0% or less of an average thickness of the scale. A method for manufacturing a thick steel plate includes the steps of: applying bending work with the amount of strain on the surface of a slab of 20-40% at a surface temperature of the slab in continuous casting of 1,000-1,200°C; heating the slab at a predetermined highest heating temperature in predetermined time; rolling the slab at a cumulative draft of 15% or more, then subjecting the slab to descaling by high-pressure water, setting temperature deviation in a width direction of a rolled material at 25°C or lower, and then applying additional hot-rolling; and cooling the steel plate under a predetermined condition.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 the cutting time and perform cutting with high precision to reduce the 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, generally involves the hot-rolling of slabs, and it is known that hot-rolled steel plates oxidize in the atmosphere, forming scale (iron oxide) on their surfaces. When laser cutting thick steel plates, this scale peels off from the steel plate surface before and / or during cutting, which can result in the steel plate being unable to be cut or in abnormal cuts with gouges on the cut surface, making stable cutting impossible and, instead, increasing the number of steps and deteriorating cutting accuracy.

[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, Patent Document 4 describes a high-tensile steel sheet containing appropriate amounts of C, Si, Mn, etc. as its chemical composition, based on the view that controlling the heat of oxidation reaction of steel within an appropriate range is important for laser cuttability, and teaches that the rolling end temperature of hot rolling should be set to 700 to 850°C in order to obtain an appropriate scale thickness and thereby suppress the oxidation reaction.

[0008] Furthermore, Patent Document 5 describes how appropriate amounts of Cu and Ni are added to the chemical composition of a steel sheet, and the steel sheet is heated in the rolling process at a temperature equal to or higher than the melting point of the Cu-Ni alloy, followed by a descaling treatment at a temperature equal to or lower than the melting point of the Cu-Ni alloy, thereby forming a scale / metal mixed layer containing metal particles made of an alloy whose main components are Fe, Cu, and Ni in the scale.It also teaches that because this scale / metal mixed layer has a larger heat capacity than the scale itself, it acts as a buffer when hit by laser light, absorbing the impact of the laser light and preventing the scale layer from peeling off from the steel surface. [Prior art documents] [Patent documents]

[0009] [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. 09-194988 [Patent Document 5] Japanese Patent Application Laid-Open No. 2006-219712 Summary of the Invention [Problem to be solved by the invention]

[0010] To reduce costs in a society with a declining population, there is an increasing demand for automation of laser cutting of steel plates. On the other hand, to cope with the increasing thickness of the objects to be cut due to the larger size of structures and the increased cutting speeds using high-power fiber lasers, steel plates with even better laser cuttability than conventional ones are required. Specifically, to improve laser cuttability, there is a demand for steel plates with a scale that has excellent adhesion and does not peel off irregularly when irradiated with a higher-power laser than conventional ones.

[0011] In the steel sheet described in Patent Document 3, measures have been taken to prevent irregular peeling of scale, but descaling with high-pressure water is required in every rolling pass, and there is still room for improvement in terms of addressing the fundamental issue of cost reduction.In the steel sheet described in Patent Document 5, laser cuttability is improved by increasing the adhesion of the scale, but there is still room for improvement in terms of increasing the laser output.

[0012] Therefore, an object of the present invention is to provide a thick steel plate that improves the adhesion of scale, suppresses or prevents scale peeling during laser irradiation, and is therefore useful for applying laser cutting, and a method for manufacturing the same. [Means for solving the problem]

[0013] In order to achieve the above object, the inventors investigated the scale spallation resistance required for stable continuous cutting in laser cutting. As a result, they found that in a steel sheet heated by continuous laser irradiation, preventing scale spalling at the laser irradiated locations is effective for stable continuous cutting. As a result of further investigation, they found that, as a means for obtaining such scale spallation resistance, the scale spallation resistance can be improved by increasing the number of Cu and Ni enriched regions (hereinafter referred to as CuNi enriched regions) at the scale / steel sheet (base steel) interface, particularly on the steel sheet-side surface of the scale / steel sheet interface, and further by making the scale thickness on the steel sheet uniform.

[0014] Although the mechanism by which the CuNi-enriched areas affect the scale spalling resistance is unknown, it is presumed that the CuNi-enriched areas act as scale formation sites after descaling, increasing their density and promoting the uniform formation of scale. Furthermore, because there is extremely good adhesion at the interface between the CuNi-enriched areas and the scale that forms therein, the connection of microscopic spalls between the steel plate and the scale that occurs due to thermal stress during laser irradiation is suppressed, preventing the progression of large-scale spalling, in which the scale peels off from the steel plate.

[0015] The present invention, which has achieved the above object, is as follows. (1) A steel sheet comprising a steel sheet and a scale formed on the surface of the steel sheet, wherein the density of Cu and / or Ni enriched portions (CuNi enriched portions) present in an island shape at the interface between the steel sheet and the scale is 1.0 × 10 7 pieces / m 2 or more, and the thickness deviation of the scale is 10.0% or less of the average thickness of the scale. (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%, Cu: 0.01 to 1.00%, Ni: 0.01 to 2.00% N: 0.0150% or less, O: 0.0050% or less, 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 balance is composed of Fe and impurities. The steel plate according to (1) or (2) above, having a chemical composition that satisfies the following formula (1): 0.05≦[Cu]+0.5[Ni]≦1.00...Equation (1) [Cu] and [Ni] represent the content [mass %] of each element in the steel sheet. (4) The chemical composition is in mass%: 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) A step of producing a slab by a continuous casting method, in which a bending process is performed so that the strain amount on the surface of the slab is 20 to 40% while the surface temperature of the slab during continuous casting is in the range of 1000 to 1200°C; a step of heating the slab, in which the slab is heated to a maximum heating temperature at which the surface temperature of the slab becomes (T0+20)°C to 1300°C, and the elapsed time from exceeding T0°C to the completion of the heating step is controlled so as to satisfy the following formula (2); a hot rolling step of hot rolling the slab, in which the slab is rolled at a cumulative reduction rate of 15% or more in a temperature range of 1080°C to T0 such that the surface temperature of the slab is in the temperature range of 1080 to T0, and then the obtained rolled material is subjected to high-pressure water descaling in a temperature range of 1080 to T0 such that the temperature deviation in the width direction of the rolled material after the high-pressure water descaling is 25°C or less, and then additional hot rolling is performed so that the rolling completion temperature is 900 to 1050°C; 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 (3): The method for producing a steel plate according to any one of the above (1) to (4), comprising: 1.00≦x 10 ≦10.00...Equation (2) x1=D1·{(T1-T0) 2 +D2T1+D3}·Δt 0.5 exp(-D4 / T1) t n =x n2 D1 -2 ·{(T n+1 -T0) 2 +D2T n+1 +D3} -2 exp(D4 / T n+1 ) x n =D1·{(T n -T0) 2 +D2T n +D3}·(t n-1 +Δt) 0.5 exp(-D4 / T n ) x n is an index that represents the degree of formation of CuNi enriched areas after the time elapsed from when the surface temperature of the slab exceeds T0°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 10 equal sections. D1, D2, D3, and D4 are constants, each equal to 8.92 × 10 -4 , 2.47 × 10 0 , -1.09×10 3 and 5.89 × 10 3 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 obtained by calculating x1, x2, x3, etc. in order using the above formula. 1.00≦y 10 ≦10.00...Equation (3) y1=E1·(J1 2 +E2J1+E3) Δk 0.5 exp(-E4 / J1) k n =y n 2 E1 -2 ·(J n+1 2 +E2J n+1 +E3) -2 ·exp(2·E4 / J n+1 ) y n =E1·(Jn 2 +E2J n +E3)·(k n-1 +Δk) 0.5 ·exp(-E4 / J n ) 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, and E4 are constants, each equal to 9.14 × 10 -4 , 2.47 × 10 0 , -1.09×10 3 and 5.89 × 10 3 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. T0 is the temperature calculated by the following formula (4). T0 = ​​K1 + K2 [Cu][Ni] + K3 [Cu][Ni] 0.5 (K4-[Cu])+K5(K6+log 10 [Si]) 2 +K7 [ Al ] +K8 [ Mn ] ...Equation (4) K1=1.158×10 3 , K2=6.288×10 0 , K3=3.337×10 1 , K4=1.783×10 0 , K5=1.400×10 0 , K6=1.000×10 0 , K7=-3.500×10 1 , and K8=-1.560×10 1 and [Cu] 、 [Ni] , [Al] and [Mn]represents the content [mass %] of each element in the slab. [Effects of the Invention]

[0016] According to the present invention, by increasing the adhesion of the scale, peeling of the scale during laser irradiation is suppressed or prevented, and therefore it is possible to provide a thick steel plate and a manufacturing method thereof that are useful for applying laser cutting. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic diagram of an element map obtained by EPMA analysis for measuring the density of CuNi enriched parts. DETAILED DESCRIPTION OF THE INVENTION

[0018] 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.

[0019] [Preferred chemical composition] In an embodiment of the present invention, the steel sheet has a density of CuNi enriched parts existing in an island shape at the interface between the steel sheet and the scale of 1.0 × 10 7 pieces / m 2 The steel plate may be made of any material that satisfies the requirement that the CuNi content be 1.0×10 or more. Therefore, the chemical composition of the steel plate is not particularly limited, and may be appropriately determined within a range that satisfies the requirement. More specifically, as described above, the present invention aims to provide a thick steel plate that improves the adhesion of scale, suppresses or prevents scale spalling during laser irradiation, and is therefore useful for applying laser cutting, and ... aims to provide a thick steel plate that improves the adhesion of scale and suppresses or prevents scale spalling during laser irradiation, and 7 pieces / m 2This object is achieved by limiting the thickness deviation of the scale to 10.0% or less of the average thickness of the scale. Therefore, it is clear that the chemical composition of the entire steel sheet is not an essential technical feature for achieving the object of the present invention. Below, a preferred chemical composition of the steel sheet according to the embodiment of the present invention will be explained. However, this explanation is based on the assumption that the density of CuNi enriched parts existing in an island shape at the interface between the steel sheet and the scale is 1.0 × 10 7 pieces / m 2 These are merely examples of preferred chemical compositions for steel sheets that satisfy the above requirements, 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.

[0020] [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.

[0021] [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.

[0022] [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.

[0023] [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.

[0024] [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.

[0025] [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.

[0026] [Cu: 0.01 to 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. From this perspective, the Cu content is preferably 0.01% or more. To improve scale adhesion, the Cu content is preferably 0.03% or more, and more preferably 0.08% or more. The Cu content may be 0.10% or more, 0.12% or more, 0.15% or more, 0.20% or more, or 0.30% or more. On the other hand, if the Cu content is excessive, there is a concern that defects may 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 preferably 0.70% or less, and more preferably 0.50% or less.

[0027] [Ni: 0.01 to 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. From this perspective, the Ni content is preferably 0.01% or more. To improve scale adhesion, the Ni content is preferably 0.03% or more, and more preferably 0.05% or more. The Ni content may be 0.10% or more, 0.12% or more, 0.15% or more, 0.20% or more, 0.30% or more, 0.40% or more, or 0.50% or more. On the other hand, an excessive Ni content may cause defects on the surface of the cast slab, which may interfere with rolling, so the Ni content is preferably limited to 2.00% or less. Furthermore, because Ni deteriorates weldability, the Ni content is preferably 1.50% or less, more preferably 1.00% or less, and even more preferably 0.70% or less.

[0028] [Cu+0.5Ni:0.05~1.00%] Cu and Ni form a CuNi-enriched portion at the interface between the scale on the steel sheet surface and the steel sheet, particularly on the steel sheet-side surface of the interface, thereby obtaining a scale with excellent adhesion in the thick steel plate according to an embodiment of the present invention. In order to obtain such a scale, it is preferable that the Cu and Ni contents of the steel sheet satisfy the following formula (1) in addition to the above-mentioned ranges of the contents of each individual element. 0.05≦[Cu]+0.5[Ni]≦1.00...Equation (1) Here, [Cu] and [Ni] represent the content [mass %] of each element in the steel sheet. When the Cu and Ni contents are low and formula (1) is not satisfied, the CuNi enriched area on the surface of the steel sheet has a sufficient density, i.e., 1.0 × 10 7 pieces / m 2In some cases, island-shaped CuNi-enriched regions having a density equal to or greater than the above value may not be formed, resulting in the formation of scale with poor adhesion. On the other hand, when the Cu and Ni contents are excessive and formula (1) is not satisfied, the CuNi-enriched regions may be unevenly distributed, resulting in some of the CuNi-enriched regions becoming coarse, which may in turn reduce the density of the island-shaped CuNi-enriched regions. The coarse CuNi-enriched regions formed by unevenly distributed CuNi-enriched regions have excessively high adhesion to the scale formed thereon. This may result in partial residual scale during descaling, which is performed to remove inhomogeneous scale formed by heating the slab in the hot rolling process described below. In such cases, inhomogeneous scale is formed in the subsequent cooling process, resulting in poor scale adhesion. To increase the density of the CuNi-enriched regions and more reliably improve scale adhesion, the value of formula (1) is preferably 0.08% or more and 0.70% or less, and more preferably 0.10% or more and 0.50% or less.

[0029] [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.

[0030] [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.

[0031] 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 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.

[0032] [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.

[0033] [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 0.60% or less.

[0034] [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.

[0035] [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.

[0036] [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.

[0037] [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.

[0038] [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.

[0039] [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.

[0040] [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.

[0041] [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 contents of Ca, Mg, Hf, Te, Sr, and REM may be 0%, to achieve this effect, the contents of Ca, Mg, Hf, Te, Sr, and REM 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 contents of Ca, Mg, Hf, Te, Sr, and REM 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] [The density of Cu and / or Ni enriched islands at the interface between the steel sheet and the scale is ≥ 1.0 × 10 7 pieces / m 2 ] The steel plate according to the embodiment of the present invention is characterized in that Cu and / or Ni enriched regions (CuNi enriched regions) are present in an island-like pattern at the interface between the steel plate (base steel) and the scale, particularly on the surface of the steel plate side of the interface. The greater the density of the island-like CuNi enriched regions at the interface, the greater the adhesion of the scale and the improved laser cuttability. To fully obtain this effect, in the steel plate according to the embodiment of the present invention, the density of the CuNi enriched regions at the interface is set to 1.0 × 10 7 pieces / m 2 That's all.

[0046] The island-like CuNi-enriched regions present at the interface function as scale formation sites after descaling in the hot rolling process. Their increased density enhances the uniformity of the scale, resulting in consistent heat absorption and expansion behavior of the scale during laser irradiation. As a result, scale spalling is suppressed, improving laser cuttability. Furthermore, because they also function as scale formation sites, adhesion between the CuNi-enriched regions and the scale layer, and ultimately adhesion between the steel sheet and the scale layer, is enhanced. This enhanced adhesion is believed to suppress the occurrence of macroscopic spalling due to the connection of microscopic spalls between the scale and the steel sheet caused by thermal expansion of the steel sheet during laser irradiation. Therefore, if CuNi-enriched regions are formed throughout the interface, scale formation is promoted throughout the steel sheet, increasing the uniformity of the scale and improving scale adhesion, resulting in a more significant improvement in laser cuttability. However, this requires the inclusion of a large amount of Cu and / or Ni, and also requires an increase in the strain imparted in the casting process described below. However, both of these are not necessarily desirable because they deteriorate the surface quality of the slab, making it difficult to subject the slab to the hot rolling process. Therefore, in order to further increase the scale adhesion while avoiding such manufacturing disadvantages, the density is set to 1.5 × 10 7 pieces / m 2 It is preferable that the value is 2.0×10 or more. 7 pieces / m 2 It is more preferable that the density is 1.0 × 10 or more. Although no upper limit is set for the density, if the density of the island-shaped CuNi-enriched portions is excessively increased, the CuNi-enriched portions will connect to each other, and the density will decrease. In this case, as the density decreases, coarse CuNi-enriched portions will be formed, and scale formation will be abnormally promoted only in the regions where the coarse CuNi-enriched portions exist, which will actually impair the homogeneity of the scale, causing the heat absorption and expansion behavior of the scale during laser irradiation to become non-uniform, promoting scale peeling and deteriorating laser cuttability. From the above viewpoints, it is considered that the density of the CuNi-enriched portion at the interface should be 1.0 × 10 8 pieces / m 2 It is preferable to keep it below this.

[0047] The density of the island-like CuNi enriched regions present at the interface, particularly on the steel plate side, is measured using the following procedure. One side of a steel plate containing a scale layer is ground to a thickness of 2.0 mm, and the steel plate is bent 90 degrees with a bending radius of 5.0 mm, with the surface with the remaining scale positioned on the outside of the bend. The bent portion is then bent back to flatten it. Cellophane tape is applied to the scale at the bent-back portion and then peeled off to remove the scale layer, exposing the steel plate side of the interface between the scale and the steel plate. If the scale layer is not sufficiently removed, the bent-back portion is positioned on the inside of the bend, and the bent-back process is repeated, alternating between the outside and inside of the bend, until the scale layer is sufficiently removed.

[0048] Next, elemental distribution analysis was performed using an electron probe microanalyzer (EPMA). EPMA analysis was performed at locations where scale was visible to the naked eye, with a measurement step size of 0.5 μm. Figure 1 shows a schematic diagram of the elemental map obtained by EPMA analysis. When determining the density of CuNi-enriched areas, it was necessary to calculate the area of ​​the remaining scale covering the steel sheet surface, even in locations where scale was visible to the naked eye, and subtract this area from the area analyzed by EPMA to obtain the area of ​​the analyzed steel sheet surface. Locations where the O (oxygen) concentration was 5.0% or higher by mass were determined as locations where scale remained, and the area of ​​remaining scale, S, was calculated from the number of measurement points. The area (S0-S) obtained by subtracting S from the area S0 where EPMA analysis was performed is the area analyzed on the steel sheet surface to obtain the density of the CuNi enriched areas. To obtain the average density of the CuNi enriched areas, this area must be 1.0 × 10 -8 m 2 EPMA analysis is performed to ensure the above.

[0049] In an elemental map obtained by EPMA analysis, in a region where it is determined from the above analysis that a steel sheet is observed, a group of measurement points where the concentrations of Cu and Ni satisfy the following formula (5) and there are 12 or more consecutive points is determined to be an island-like or island-like CuNi-enriched region. The reason for determining the island-like CuNi-enriched region in this manner is that low Cu and Ni concentrations are thought to weaken the ability to promote scale formation, and that small Cu and Ni-enriched regions are thought to weaken the ability to promote scale formation. The number N of island-like CuNi-enriched regions obtained in this manner is counted and divided by the analyzed area (S0-S) of the steel sheet surface, thereby obtaining the density of island-like CuNi-enriched regions at the scale / steel sheet interface, which characterizes the steel sheet according to an embodiment of the present invention. [Cu]+0.5[Ni]≧5.0mass%...Equation (5) [Cu] and [Ni] represent the content [mass %] of each element in the steel sheet.

[0050] [Scale thickness deviation is 10.0% or less of the average scale thickness] Steel plates according to embodiments of the present invention are characterized by having a uniform scale thickness. The uniformity of scale thickness is evaluated using the following procedure: a cross section parallel to the rolling direction and the plate 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 in the width direction of the steel plate and two locations at least 100 mm apart from that 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.

[0051] 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, the scale thickness deviation in the steel plate according to the embodiment of the present invention is limited to 10.0% or less of the average scale thickness (i.e., Δh / h × 100≦10.0%). Since the smaller the scale thickness deviation, the more effectively scale spalling during laser irradiation is suppressed, the scale thickness deviation is preferably 8.0% or less, and more preferably 6.0% or less, of the average scale thickness. 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, 1.0% or more or 2.0% or more of the average scale thickness.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] A method for manufacturing a steel plate according to an embodiment of the present invention includes: a casting step for producing a slab by a continuous casting method, in which the surface temperature of the slab during continuous casting is in the range of 1000 to 1200°C, and the slab is subjected to bending so that the strain amount on the surface of the slab is 20 to 40%; a step of heating the slab, in which the slab is heated to a maximum heating temperature at which the surface temperature of the slab becomes (T0+20)°C to 1300°C, and the elapsed time from exceeding T0°C to the completion of the heating step is controlled so as to satisfy the following formula (2); a hot rolling step of hot rolling the slab, in which the slab is rolled at a cumulative reduction rate of 15% or more in a temperature range of 1080°C to T0 such that the surface temperature of the slab is in the temperature range of 1080 to T0, and then the obtained rolled material is subjected to high-pressure water descaling in a temperature range of 1080 to T0 such that the temperature deviation in the width direction of the rolled material after the high-pressure water descaling is 25°C or less, and then additional hot rolling is performed so that the rolling completion temperature is 900 to 1050°C; 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 (3): The present invention is characterized in that it includes: 1.00≦x 10 ≦10.00...Equation (2) x1=D1·{(T1-T0) 2+D2T1+D3}·Δt 0.5 exp(-D4 / T1) t n =x n 2 D1 -2 ·{(T n+1 -T0) 2 +D2T n+1 +D3} -2 exp(D4 / T n+1 ) x n =D1·{(T n -T0) 2 +D2T n +D3}·(t n-1 +Δt) 0.5 exp(-D4 / T n ) x n is an index that represents the degree of formation of CuNi enriched areas after the time elapsed from when the surface temperature of the slab exceeds T0°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 10 equal sections. D1, D2, D3, and D4 are constants, each equal to 8.92 × 10 -4 , 2.47 × 10 0 , -1.09×10 3 and 5.89 × 10 3 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 obtained by calculating x1, x2, x3, etc. in order using the above formula. 1.00≦y 10 ≦10.00...Equation (3) y1=E1·(J1 2 +E2J1+E3) Δk 0.5 exp(-E4 / J1) k n =y n 2 E1 -2 ·(J n+1 2 +E2J n+1+E3) -2 ·exp(2·E4 / J n+1 ) y n =E1·(J n 2 +E2J n +E3)·(k n-1 +Δk) 0.5 ·exp(-E4 / J n ) 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, and E4 are constants, each equal to 9.14 × 10 -4 , 2.47 × 10 0 , -1.09×10 3 and 5.89 × 10 3 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. T0 is the temperature calculated by the following formula (4). T0 = ​​K1 + K2 [Cu][Ni] + K3 [Cu][Ni] 0.5 (K4-[Cu])+K5(K6+log 10 [Si]) 2 +K7 [ Al ] +K8 [ Mn ] ...Equation (4) K1=1.158×10 3 , K2=6.288×10 0 , K3=3.337×10 1 , K4=1.783×10 0 , K5=1.400×10 0 , K6=1.000×10 0 , K7=-3.500×10 1, and K8=-1.560×10 1 and [Cu] 、 [Ni] , [Al] and [Mn] represents the content [mass %] of each element in the slab.

[0056] [Casting process] Molten steel adjusted to a specified chemical composition is cast using a continuous casting method to produce slabs for hot rolling. The method for producing molten steel is not particularly specified. During continuous casting, the slab undergoes bending, with the surface temperature of the slab being formed reaching a range of 1000–1200°C, resulting in a strain of 20–40% on the surface. This treatment melts the Cu and Ni in the scale on the slab surface at high temperatures, and the molten Cu and Ni are further concentrated at the interface between the scale and steel by introducing strain, promoting the formation of CuNi-enriched areas in the subsequent hot rolling process. If the bending temperature is too high or the strain applied during bending is too great, Cu and Ni will penetrate excessively into the steel from the interface between the scale and steel, causing defects on the slab surface, impairing the appearance of the steel sheet after hot rolling, or causing cracks during hot rolling. On the other hand, if the bending temperature is too low or the amount of strain applied during bending is too small, the concentration of Cu and Ni at the interface between the scale and the steel will be insufficient, resulting in insufficient concentration of Cu and Ni during the hot rolling process. The amount of strain during bending can be controlled within a desired range, for example, by appropriately selecting the radius of curvature of the curved portion in the continuous casting machine.

[0057] The cast slab must not be subjected to surface grinding in order to provide the hot rolling process with the interface between the steel and the scale, which has a moderate concentration of Cu and Ni.

[0058] [Heating process] The produced slab is heated for hot rolling. This heating is performed so that the maximum heating temperature is in the range of (T0+20)°C to 1300°C, and the elapsed time from exceeding T0°C until the completion of the heating process satisfies the following formula (2). Here, T0 is the temperature derived from the chemical composition of the slab using the following formula (4). This heating controls the austenite to an appropriate size, while liquefying Cu and Ni concentrated at the interface between the scale and the slab, causing them to penetrate into the austenite grain boundaries, thereby forming CuNi-enriched parts in the slab. Here, x in formula (2) 10 is an index that represents the degree of formation of CuNi-enriched regions, which is calculated by the calculation described later. 1.00≦x 10 ≦10.00...Equation (2) T0 = ​​K1 + K2 [Cu][Ni] + K3 [Cu][Ni] 0.5 (K4-[Cu])+K5(K6+log 10 [Si]) 2 +K7 [ Al ] +K8 [ Mn ] ...Equation (4) K1=1.158×10 3 , K2=6.288×10 0 , K3=3.337×10 1 , K4=1.783×10 0 , K5=1.400×10 0 , K6=1.000×10 0 , K7=-3.500×10 1 , and K8=-1.560×10 1 and [Cu] 、 [Ni] , [Al] and [Mn] represents the content [mass %] of each element in the slab. In the heating process, if the heating temperature is too high, austenite grains grow too much, the density of the CuNi-enriched portion becomes insufficient, and scale adhesion is impaired. On the other hand, if the heating temperature is too low, Cu and Ni do not penetrate sufficiently into the slab, the density of the CuNi-enriched portion becomes insufficient, and scale adhesion is impaired. To obtain a sufficient density of the CuNi-enriched portion, the maximum heating temperature is set to a range of (T0 + 20) ° C to 1300 ° C. To improve scale adhesion, the maximum heating temperature is preferably set to a range of (T0 + 40) ° C to 1270 ° C.

[0059] Furthermore, the degree of penetration of Cu and Ni into the slab changes depending on the elapsed time during heating. Since the effect of time varies depending on the temperature at that time, the elapsed time during heating is managed by equation (2). If the elapsed time is too short, x 10 If x is less than 1.00, Cu and Ni cannot penetrate sufficiently into the slab, the density of the CuNi enriched area becomes insufficient, and there are areas where the adhesion of the scale is impaired. 10 x is set to 1.00 or more. In order to increase the density of the CuNi concentrated area and further improve adhesion, 10 is preferably 1.50 or more, and more preferably 2.00 or more. On the other hand, if the elapsed time is too long, austenite grains grow and the density of the CuNi enriched portion decreases. 10 is limited to 10.00 or less, preferably 9.00 or less, and more preferably 8.00 or less. Here, formula (2) is as follows, where x 10 is calculated as follows: 1.00≦x 10 ≦10.00...Equation (2) x1=D1·{(T1-T0) 2 +D2T1+D3}·Δt 0.5 exp(-D4 / T1) t n =x n 2 D1 -2 ·{(T n+1 -T0) 2 +D2T n+1 +D3} -2 exp(D4 / Tn+1 ) x n =D1·{(T n -T0) 2 +D2T n +D3}·(t n-1 +Δt) 0.5 exp(-D4 / T n )

[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 T0°C during the heating process until the end of the heating process into 10 equal parts, and calculating the degree of formation of CuNi-enriched areas (x n ) and the subscript n indicates that it is the nth calculation in the 10-divided interval. D1, D2, D3, and D4 are constants, and each is 8.92×10 -4 , 2.47 × 10 0 , -1.09×10 3 , 5.89 x 10 3 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 (2) 10 can be obtained by calculating x1, x2, x3, etc. in order using the above formula.

[0061] [Hot rolling process] The surface of the heated slab is covered with heterogeneous scale that was formed and grown during the casting and heating processes. Therefore, by performing hot rolling and high-pressure water descaling to remove the heterogeneous scale and then forming a homogeneous scale on the surface of the steel plate with an appropriate CuNi enrichment, a thick steel plate with a highly adhesive scale and excellent laser cuttability can be obtained.

[0062] In order to thoroughly remove the 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 with a surface temperature of the slab in the temperature range of 1080°C to T0, 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 temperature at which hot rolling is performed exceeds T0, the scale is not sufficiently crushed, and it is not possible to thoroughly remove the scale by descaling. As a result, the scale obtained in the end becomes non-uniform, with a large deviation in scale thickness and a loss of scale adhesion. On the other hand, if the temperature at which the hot rolling is performed is below 1080°C, the adhesion of some of the scale increases during the hot rolling, and even after descaling with high-pressure water, some of the scale remains. Similarly, the final scale becomes non-uniform, the scale thickness deviation increases, and the scale adhesion is impaired. The temperature at which the hot rolling is performed prior to descaling is preferably 1100°C or higher, from the viewpoint of sufficiently increasing the temperature of the rolled material to be subjected to descaling. Furthermore, if the cumulative reduction rate of the hot rolling performed within this temperature range is less than 15%, the scale is not sufficiently crushed, and even after descaling with high-pressure water, some of the scale remains, and the final scale becomes non-uniform. As a result, the scale thickness deviation increases and the scale adhesion is impaired. 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 heterogeneous scale becomes saturated.

[0063] After the hot rolling, the resulting rolled material is descaled using high-pressure water. Descaling can be performed using high-pressure water with a collision pressure of 10 to 15 MPa, for example. If the descaling temperature (surface temperature of the rolled material) is below 1080°C, some of the non-uniformly grown scale remains, making the final scale non-uniform and impairing scale adhesion. On the other hand, if the descaling temperature exceeds T0, the density of the CuNi-enriched region decreases, and scale adhesion is actually impaired. This is presumably because if descaling is performed at an excessively high temperature, some of the CuNi-enriched region formed before the heating step is removed together with the scale. From the above perspective, the temperature at which descaling is performed after the hot rolling is limited to the range of 1080°C to T0, and preferably, is performed in the range of 1100°C to (T0-20)°C.

[0064] During the descaling process, the temperature of the surface of the rolled material is measured in the width direction of the rolled material before and after descaling. The amount of water used for descaling is adjusted in the width direction according to the temperature distribution before descaling. This reduces the temperature deviation in the width direction of the rolled material after descaling, allowing the scale formation that begins immediately after descaling to proceed uniformly at each location on the rolled material. The amount of water used for descaling varies significantly depending on the size of the rolled material and the equipment configuration, so no specific amount is specified. However, a uniform scale can be obtained by increasing or decreasing the amount of high-pressure water sprayed in descaling in the width direction, or by keeping the amount of water constant, depending on the temperature measured before descaling, and controlling the temperature deviation in the width direction of the rolled material after descaling to 25°C or less. To improve the uniformity of the scale, it is preferable to control the temperature deviation in the width direction of the rolled material after descaling to 20°C or less.

[0065] The width direction temperature deviation of the rolled material after descaling is 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 plate width of the rolled material, measured within 10 seconds after the completion of the descaling.

[0066] After the descaling, additional hot rolling is performed to adjust the rolled material to a plate thickness appropriate for the intended application. After the descaling, scale on the surface of the rolled material forms and grows primarily from the CuNi-enriched areas, but it grows particularly significantly after all rolling is completed. If the temperature at which rolling is completed is too high, the scale formation and / or growth becomes non-uniform, resulting in a loss of uniformity in the scale thickness. On the other hand, if the temperature at which rolling is completed is too low, some of the scale may peel off, or the peeled scale may be pushed into other areas by rolling, resulting in a loss of uniformity in the scale thickness. From these perspectives, the rolling completion temperature is set to 900°C or higher and 1050°C or lower. To further improve the uniformity of the scale, the rolling completion temperature is preferably set to 915°C or higher and 1010°C or lower, and more preferably 930°C or higher and 980°C or lower.

[0067] [Cooling process] If the growth of scale proceeds excessively after the completion of the hot rolling process, the uniformity of the scale thickness will be impaired. On the other hand, if the growth of scale is excessively suppressed, the scale will become thinner, the deviation of the scale thickness from the average thickness of the scale will increase, and the scale will be more likely to peel off during laser irradiation. From the above perspective, the time elapsed from the completion of the hot rolling process until water cooling to stop the scale growth is controlled to improve the adhesion of the scale. Note that the effect of the passage of time changes depending on the temperature at the time, so the time elapsed after the completion of the hot rolling process is managed by Equation (3). Here, if the time elapsed from the completion of the hot rolling process until water cooling is too short, y 10 If y is less than 1.00, the growth of scale is excessively suppressed, and the thickness of the scale is deviated, resulting in areas where the adhesion of the scale 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 10.00, the scale grows excessively, causing deviations in the scale thickness and creating areas where the scale adhesion is impaired. 10 To homogenize the scale and improve its adhesion, y 10is preferably 2.00 or more and 8.00 or less, and more preferably 3.00 or more and 7.00 or less. 1.00≦y 10 ≦10.00...Equation (3) where y n is calculated as follows: y1=E1·(J1 2 +E2J1+E3) Δk 0.5 exp(-E4 / J1) k n =y n 2 E1 -2 ·(J n+1 2 +E2J n+1 +E3) -2 ·exp(2·E4 / J n+1 ) y n =E1·(J n 2 +E2J n +E3)·(k n-1 +Δk) 0.5 ·exp(-E4 / J n )

[0068] 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 ) and the subscript n indicates that the calculation is the nth of 10 equal intervals. E1, E2, E3, and E4 are constants, and each is 9.14×10 -4 , 2.47 × 10 0 , -1.09×10 3 , 5.89 x 10 3 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]. y in Equation (3) 10 can be obtained by calculating y1, y2, y3, etc. in order using the above formula.

[0069] After the descaling step and before the completion of the hot rolling process, further descaling with high-pressure water may be performed in order to suppress excessive growth of scale and improve the appearance quality. 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.

[0070] 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.

[0071] 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 air-cooling and / or natural cooling is preferred. Alternatively, the steel plate after 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.

[0072] 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 above-mentioned casting step, heating step, rolling step, and cooling step.

[0073] In the steel plate manufactured by the method for manufacturing a steel plate according to the embodiment of the present invention, the density of the concentrated Cu and / or Ni islands existing at the interface between the steel plate and the scale is 1.0 × 10 7 pieces / m 2As described above, since the deviation in the thickness of the scale is 10.0% or less of the average thickness of the scale, peeling of the scale can be suppressed when the steel plate is irradiated with a laser in a laser cutting operation. Therefore, such a steel plate can be stably cut into any shape without causing any problems that would hinder the laser cutting operation, such as burning, and can be used for structures such as land development, architecture, industrial machinery, and bridges.

[0074] 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]

[0075] In the following examples, thick steel plates according to the embodiments of the present invention were produced under various conditions, and the degree of scale spalling that occurred when the obtained thick steel plates were irradiated with a laser was investigated.

[0076] First, molten steel having the chemical composition shown in Table 1 was used, and a slab was cast by continuous casting while being subjected to bending as shown in Table 2. The obtained slab was then subjected to the heating step, hot rolling step, and cooling step under the conditions shown in Table 3, thereby obtaining steel plates as experimental examples including Examples and Comparative Examples. Experimental Examples 56 and 69 were steel plates obtained by cooling to 330°C and 260°C, respectively, by water cooling after the completion of the hot rolling step, and then allowed to cool in the atmosphere. Experimental Examples 57 and 68 were steel plates obtained by cooling to 630°C and 670°C, respectively, by water cooling after the completion of the hot rolling step, and then allowed to cool in the atmosphere. Furthermore, other experimental examples are steel plates obtained by water-cooling after the completion of the hot rolling process to 450 to 600°C and then cooling in the atmosphere. In particular, experimental examples 13 and 22 are steel plates obtained by water-cooling to 460°C and 510°C, respectively, then winding the steel plate into a coil and cooling in the atmosphere.

[0077] [Table 1]

[0078] [Table 2-1]

[0079] [Table 2-2]

[0080] [Table 3-1]

[0081] [Table 3-2]

[0082] [Scale adhesion evaluation] The scale adhesion of the resulting steel plates was evaluated using a test simulating the conditions of laser irradiation. First, a steel plate containing surface scale was cut into 100 mm x 100 mm pieces and heated to 200 °C using a burner to simulate the conditions during laser cutting, where the plate is heated to a certain extent by the laser. A 10 mm long laser beam was then irradiated onto the center of the plate surface under the following conditions. A 1 mm long area in the center of the laser irradiation mark on the surface was observed using an optical microscope to evaluate the degree of scale spalling in that area. A rating of "×" was given for a scale spalling area exceeding 30% after a single irradiation; "○" for a scale spalling area exceeding 5% but not exceeding 30%; and "◎" for a scale spalling area less than 5%. Steel plates that achieved a rating of "○" or "◎" were deemed to have excellent scale adhesion and suppressed or prevented scale spalling during laser irradiation and were therefore deemed to have passed the test. The results are shown in Table 4. Laser power: 500W Pulse frequency: 60kHz Focusing diameter: 0.70 mm Irradiation speed: 3000m / sec

[0083] [Table 4-1]

[0084] [Table 4-2]

[0085] Among the experimental examples listed in Tables 1 to 4, experimental example 35 is a comparative example in which the temperature at which strain is imparted to the surface of the slab in the casting process is low, and a CuNi-enriched portion is not sufficiently formed at the interface between the steel sheet and the scale, resulting in poor scale adhesion. On the other hand, experimental example 64 is a comparative example in which the temperature at which strain is imparted to the surface of the slab in the casting process is high, and cracks occur on the surface of the slab produced through the casting process, making it necessary to grind the surface of the slab before heating for the hot rolling process, and as a result, a CuNi-enriched portion is not sufficiently formed at the interface between the steel sheet and the scale, resulting in poor scale adhesion. Experimental Example 7 is a comparative example in which the amount of strain applied within the specified temperature range in the casting process was small, and a CuNi-enriched portion was not sufficiently formed at the interface between the steel sheet and the scale, resulting in poor scale adhesion. On the other hand, Experimental Example 11 is a comparative example in which the amount of strain applied within the specified temperature range in the casting process was large, and cracks occurred on the surface of the slab produced through the casting process, making it necessary to grind the surface of the slab before heating for the hot rolling process, and as a result, a CuNi-enriched portion was not sufficiently formed at the interface between the steel sheet and the scale, resulting in poor scale adhesion. Experimental Example 20 is a comparative example in which the maximum heating temperature of the slab in the heating process was low, and the CuNi-enriched portion was not sufficiently formed at the interface between the steel sheet and the scale, resulting in poor scale adhesion. On the other hand, Experimental Example 12 is a comparative example in which the maximum heating temperature of the slab in the heating process was high, and the density of the CuNi-enriched portion was insufficient, resulting in poor scale adhesion. Experimental Example 8 is a comparative example in which the heating time in the heating step was short, and formula (2) was not satisfied, and CuNi-enriched portions were not sufficiently formed at the interface between the steel sheet and the scale, resulting in poor scale adhesion. On the other hand, Experimental Example 27 is a comparative example in which the heating time in the heating step was long, and formula (2) was not satisfied, and the density of CuNi-enriched portions at the interface between the steel sheet and the scale was reduced, resulting in poor scale adhesion. Experimental Example 3 is a comparative example in which the cumulative reduction ratio of the hot rolling performed before descaling was small, resulting in large scale thickness deviation and poor scale adhesion. Experimental Example 16 is a comparative example in which the temperature at which hot rolling was performed before descaling was low, resulting in large scale thickness deviation and poor scale adhesion. On the other hand, Experimental Example 24 is a comparative example in which the temperature at which hot rolling was performed before descaling was high, resulting in large scale thickness deviation and poor scale adhesion. Experimental Example 15 is a comparative example in which the descaling temperature was low, resulting in a large deviation in scale thickness and poor scale adhesion, while Experimental Example 32 is a comparative example in which the descaling temperature was high, resulting in a decrease in the density of the CuNi enriched parts and poor scale adhesion. Experimental Example 19 is a comparative example in which the temperature deviation of the rolled material after descaling was large, the deviation in the thickness of the scale was large, and the adhesion of the scale was poor. Experimental Example 4 is a comparative example in which the rolling completion temperature was low, resulting in large scale thickness deviation and poor scale adhesion, while Experimental Example 23 is a comparative example in which the rolling completion temperature was high, resulting in large scale thickness deviation and poor scale adhesion. Experimental Example 31 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, and formula (3) was not satisfied, resulting in large scale thickness deviation and poor scale adhesion. On the other hand, Experimental Example 28 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, and formula (3) was not satisfied, resulting in large scale thickness deviation and poor scale adhesion. Examples 73 to 77 are comparative examples in which the chemical composition of the steel plate was not necessarily appropriate, the desired CuNi enriched portion was not formed at the interface between the steel plate and the scale, and the scale adhesion was poor.

[0086] The experimental examples excluding the above comparative examples, that is, experimental examples 1, 2, 5, 6, 9, 10, 13, 14, 17, 18, 21, 22, 25, 26, 29, 30, 33, 34, 36 to 63 and 65 to 72 are examples of the present invention, and thick steel plates having excellent scale adhesion were obtained.

Claims

1. A steel sheet and a scale formed on the surface of the steel sheet, wherein a density of Cu and / or Ni enriched portions (CuNi enriched portions) existing in an island shape at the interface between the steel sheet and the scale is 1.0 × 10 7 pieces / m 2 or more, and the thickness deviation of the scale is 10.0% or less of the average thickness of the scale.

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%, Cu: 0.01 to 1.00%, Ni: 0.01-2.00%, N: 0.0150% or less, O: 0.0050% or less, 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 balance is composed of Fe and impurities. The steel plate according to claim 1 or 2, having a chemical composition that satisfies the following formula (1): 0.05≦[Cu]+0.5[Ni]≦1.00...Formula (1) [Cu] and [Ni] represent the content [mass %] of each element in the steel sheet.

4. The chemical composition is, in mass %, 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. A process for producing a slab by a continuous casting method, comprising: a casting process in which a bending process is performed so that the strain amount on the surface of the slab is 20 to 40% while the surface temperature of the slab during continuous casting is in the range of 1000 to 1200°C; A step of heating the slab, wherein the surface temperature of the slab is (T 0 +20)℃ to 1300℃, and then heat to the maximum heating temperature. 0 A heating step in which the elapsed time from when the temperature exceeds 100° C. to when the heating step is completed is controlled so as to satisfy the following formula (2): A step of hot rolling the slab, wherein the surface temperature of the slab is 1080°C to T 0 After rolling with a cumulative reduction rate of 15% or more in the temperature range of 0 a hot rolling process in which high-pressure water descaling is performed in a temperature range of 100°C to 25°C, and the temperature deviation in the width direction of the rolled material after the high-pressure water descaling is set to 25°C or less, and then additional hot rolling is performed so that the rolling completion temperature is 900 to 1050°C; 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 (3): The method for producing a steel plate according to any one of claims 1 to 4, comprising: 1.00≦x 10 ≦10.00 ・・・Form (2) x 1 =D 1 ・{(T 1 -T 0 ) 2 +D 2 T 1 +D 3 }・Δt 0.5 ・exp(-D 4 / T 1 ) t n =x n 2 ・D 1 -2 ・{(T n+1 -T 0 ) 2 +D 2 T n+1 +D 3 } -2 ・exp(D 4 / T n+1 ) x n =D 1 ・{(T n -T 0 ) 2 +D 2 T n +D 3 }・(t n-1 +Δt) 0.5 ・exp(-D 4 / T n ) x n is the temperature at the surface of the slab during the heating process. 0 ° C. to the completion of the heating step, and the elapsed time 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 , D 2 , D 3 and D 4 are constants, and 8.92 × 10 -4 , 2.47 × 10 0 , −1.09 × 10 3 and 5.89 x 10 3 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. 1.00≦y 10 ≦10.00 ・・・Form (3) y 1 =E 1 ・(J 1 2 +E 2 J 1 +E 3 )・Δk 0.5 ・exp(-E 4 / J 1 ) k n =y n 2 ・E 1 -2 ・(J n+1 2 +E 2 J n+1 +E 3 ) -2 ・exp(2・E) 4 / J n+1 ) y n =E 1 ・(J n 2 +E 2 J n +E 3 )・(k n-1 +Δk) 0.5 ・exp(-E 4 / J n ) y n is an index representing the degree of scale growth after each of the ten equal sections of the elapsed time from the completion of the hot rolling process to the start of water cooling in the cooling process is completed, and n indicates that the calculation corresponds to the nth of the ten equal sections. E 1 , E 2 , E 3 and E 4 are constants, and 9.14 × 10 -4 , 2.47 × 10 0 , −1.09 × 10 3 and 5.89 x 10 3 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. T 0 is the temperature calculated by the following formula (4). T 0 = K 1 + K 2 [Cu][Ni] + K 3 [Cu][Ni] 0.5 (K 4 − [Cu]) + K 5 (K 6 + log 10 [Si]) 2 + K 7 [Al] + K 8 [Mn] ··· Equation (4) K 1 = 1.158 x 10 3 , K. 2 = 6.288 x 10 0 , K. 3 = 3.337 x 10 1 , K. 4 = 1.783 x 10 0 , K. 5 = 1.400 x 10 0 , K. 6 = 1.000 x 10 0 , K. 7 = -3.500 x 10 1 , and K 8 = -1.560 x 10 1 and [Cu], [Ni], [Al] and [Mn] represent the content [mass %] of each element in the slab.

Images