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: There are provided a thick steel plate that includes a steel plate, and a scale formed on the surface of the steel plate, wherein a maximum length in a plate thickness direction of cracks existing in the scale is 50% or less average thickness of the scale, the scale includes a laminar fayalite formed on the interface between the scale and the steel plate, an average thickness of the laminar fayalite is 0.3-2.0 μm, a covering ratio of the interface by the laminar fayalite is 50% or more, the scale further includes a laminar magnetite, an average thickness of the laminar magnetite is 2.0 μm or more, and a thickness deviation of the scale is 0.25 or less; and a method for manufacturing the same.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, 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 in which a dry coating film containing titania powder, zinc powder, aluminum powder, and a coloring pigment consisting of one or more of titania powder, zinc powder, aluminum powder, and black iron oxide pigment and black calcined pigment is applied to the surface of the steel sheet. Patent Document 1 teaches that laser cuttability can be improved by adding titania powder, which has high laser absorption, to the coating film of the coated steel material to increase the laser absorption rate. Patent Document 2 also proposes a steel material in which 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 is applied to the surface.

[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 Documents 4 and 5 propose improving laser cuttability by reducing peeling at the interface between the scale and base steel that exists in thick steel plates before laser cutting and by reducing voids that exist in the scale.

[0008] Patent Document 6 describes that stable cuttability can be ensured by the presence of a layer of Si-enriched regions with an Si content of 0.4% or more in the scale and a layer of Al-enriched regions with an Al / Si ratio of 0.3 or more on the surface side of the Si-enriched regions. Patent Document 7 proposes forming an internal oxide layer enriched in alloying elements on the steel substrate side of the interface between the scale and the steel substrate of a steel plate, thereby improving the spalling resistance of the scale. Furthermore, Patent Document 8 proposes improving laser cuttability by forming a layer of enriched alloying elements at the interface between the scale and the steel substrate of a thick steel plate and reducing voids in the scale. [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 Laid-Open No. 2002-332540 [Patent Document 5] Japanese Patent Application Laid-Open No. 2005-271074 [Patent Document 6] International Publication No. 2012 / 014851 [Patent Document 7] Japanese Patent Application Publication No. 09-078180 [Patent Document 8] Japanese Patent Application Laid-Open No. 2002-332541 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 are 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. Also, Patent Document 7 considers improving scale peeling resistance, but there is still room for improvement in terms of increasing 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] To achieve the above-mentioned object, the inventors investigated the surface layer of steel sheet necessary for stable laser cutting. As a result, they found that if cracks extending in the thickness direction exist in the scale, the scale easily cracks when irradiated with a high-power laser, resulting in early spalling, in which the scale peels off irregularly. This is presumably due to thermal stress caused by laser irradiation, but this phenomenon occurs even if the amount of voids (air gaps) in the scale is reduced, if large cracks occur.

[0014] The inventors conducted research to suppress the coarsening of these cracks and obtain a steel sheet with improved laser cuttability, and discovered that laser cuttability can be further improved by covering the scale / steel sheet (base steel) interface with fayalite (Fe2SiO4) during the hot rolling process and precisely controlling the rolling conditions to promote the formation of homogeneous scale with fewer cracks and improve scale adhesion, and by having magnetite, which has excellent heat absorption properties for laser heat input and also has excellent adhesion, be present homogeneously within the scale as layered magnetite.

[0015] 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 maximum length in the plate thickness direction of cracks present in the scale is 50% or less of the average thickness of the scale, the scale contains stratified fayalite formed at the interface between the scale and the steel plate, the average thickness of the stratified fayalite is 0.3 to 2.0 μm, the coverage of the interface with the stratified fayalite is 50% or more, the scale further contains stratified magnetite, the average thickness of the stratified magnetite is 2.0 μm or more, and the thickness deviation of the scale is 0.25 or less. (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: 0.10 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 the above (1) to (4), wherein the layered fayalite accounts for 50% or more of all fayalite in the scale. (6) 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 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 to a rolling completion temperature of 1050°C to (T0-30)°C and a cumulative reduction of 15 to 30%, followed by high-pressure water descaling in a temperature range of 1000°C to (T0-30)°C, and then rolled two or more times with a reduction in each pass of 30% or less compared to the plate thickness before rolling, controlling the interpass time of the rolling after the high-pressure water descaling to satisfy the following formula (3), and the rolling completion temperature in the final rolling pass is 850°C or higher; 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 to satisfy the following formula (4), and the water cooling stop temperature is set to 550 to 650 ° C. The method for producing a steel plate according to any one of the above (1) to (5), comprising: T0 is the temperature calculated by the following formula (1). T0=1175-8.4[Mn]-135[P] 0.5 -52[Al]-24[Cr]...Formula (1) [Mn], [P], [Al], and [Cr] are the contents [mass%] of each element in the slab. 1.0≦x 10 ≦10.0...Equation (2) x1=D1·(T1 3 +D2·T1 2 +D3·T1+D4)· [ 1-exp{D5·(T1-T0) }] 0.5 Δt 0.5 t n =x n 2 D1 ー2 ·(T n+1 3 +D2·T n+1 2 +D3·T n+1 +D4) -2 · [ 1-exp{D5·(T n+1 -T0) }] -1 x n =D1·(T n 3+D2·T n 2 +D3·T n +D4)· [ 1-exp{D5·(T n -T0) }] 0.5 (t n-1 +Δt) 0.5 x n is an index that represents the degree of fayalite settling after the time elapsed from when the surface temperature of the slab exceeds T0℃ during the heating process until the completion of the heating process is divided into 10 equal parts, and n indicates that the calculation corresponds to the nth of the 10 equal parts. D1, D2, D3, D4, and D5 are constants, each equal to 4.00 × 10 -9 , -4.22×10 3 , 5.93×10 6 , -2.74×10 9 and -1.85 × 10 -2 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. y n ≦1.00...Equation (3) y m =y m-1 ·exp[E1·k m-1 ·exp{E2·(J m-1 +J m )}]+E3·R m ·exp(E4·J m ) y0=0.00 y m is an index that represents the degree of cracking in the scale that occurs in the mth rolling out of a total of n rollings that are performed from the end of high-pressure water descaling to the completion of rolling, E1, E2, E3, and E4 are constants, each equal to -2.57 × 10 -10 , -1.02 × 10 -2 , 2.70×101 and -8.33 × 10 -3 and k m is the elapsed time [seconds] from the mth rolling to the (m+1)th rolling, J m is the rolling material temperature [℃] at the mth rolling, R m is the reduction ratio [%] of the thickness before rolling in the mth rolling pass, y n is obtained by calculating y1, y2, y3, etc. in order using the above formula. 1.0≦z 10 ≦10.0...Equation (4) z1=F1·exp{F2 / (H1-F3)}·(-H1 2 +F4H1+F5) 0.5 Δp 0.5 p n =z n 2 Formula 1 -2 ·exp{2·F2 / (H1-F3)}·(-H n+1 2 +F4H n+1 +F5) -1 z n =F1·exp{F2 / (H n -F3)}·(-H n 2 +F4H n +F5) 0.5 ·(p n-1 +Δp) 0.5 z 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. F1, F2, F3, F4, and F5 are constants, each equal to 1.63 × 10 -6 , -2.50×10 2 , 3.25×10 2 , 2.94 × 10 2 and 1.36 × 10 6 and Hn is the average steel plate temperature [°C] in the nth region of the 10 equally divided sections, Δp is one-tenth of the elapsed time [seconds], z 10 is obtained by calculating z1, z2, z3, etc. in order using the above formula. (7) The method for producing a steel plate according to (6) above, wherein in the rolling after the high-pressure water descaling, high-pressure water descaling is further carried out once or twice or more times before the hot rolling step is completed. [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. 2 is a schematic diagram of scale on a thick steel plate according to an embodiment of the present invention. [Figure 2] The distribution of hematite, magnetite, and wüstite within the scale is shown by crystal orientation analysis using the EBSD method. [Figure 3] The Si concentration map obtained by EPMA is shown. [Figure 4] The oxygen concentration map obtained by EPMA is shown. 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 embodiments of the present invention, the chemical composition of the steel plate is not particularly limited and may be appropriately determined within a range useful for application in laser cutting. As described above, the present invention aims to provide a thick steel plate that improves scale adhesion, suppresses or prevents scale spalling during laser irradiation, and is therefore useful for application in laser cutting. This objective is achieved by: (a) limiting the maximum length of cracks in the thickness direction in the scale to 50% or less of the average thickness of the scale; (b) forming layered fayalite with an average thickness of 0.3 to 2.0 μm at the interface between the scale and the steel plate and achieving an interface coverage of 50% or more; (c) further including layered magnetite with an average thickness of 2.0 μm or more; and (d) limiting the thickness deviation of the scale to 0.25 or less. Therefore, it is clear that the chemical composition of the steel plate itself is not an essential technical feature for achieving the objectives of the present invention. Below, preferred chemical compositions of steel plates according to embodiments of the present invention are described, but these descriptions are intended merely as examples and are not intended to limit the present invention to steel plates having such specific chemical compositions. In the following description, 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: 0.10-1.00%] Si is an element that forms fayalite (Fe2SiO4) in the scale, and by including an appropriate amount of Si in the steel sheet, it is possible to form a scale having the characteristics of the thick steel plate according to the embodiment of the present invention. To sufficiently form fayalite at the scale / steel sheet interface, the Si content is preferably 0.10% or more. To improve scale adhesion, the Si content is preferably 0.12% or more, and more preferably 0.15% or more. On the other hand, if the Si content is high, excessive fayalite forms at the scale / steel sheet interface, which causes the scale formation behavior on the steel sheet surface to become non-uniform and deteriorates laser cuttability. Therefore, the Si content is preferably 1.00% or less. Furthermore, Si is an element that impairs the toughness of the steel sheet. 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 in an excessive amount, the formation of fayalite is excessively promoted, which causes the behavior of scale formation on the surface of the steel sheet to become non-uniform and deteriorates laser cuttability, so the Mn content is preferably limited to 2.50% or less. 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 processes, 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 P content is preferably 0.001% or more. Furthermore, P is an element that promotes the formation of fayalite, and from this perspective, the P content is preferably 0.005% or more, and more preferably 0.008% or more. On the other hand, excessive P content excessively promotes the formation of fayalite, resulting in non-uniform scale formation behavior on the surface of the steel sheet and degraded laser cuttability. From this perspective, the P content is preferably limited to 0.050% or less, more preferably 0.035% or less, and even more preferably 0.025% 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 in order to obtain this effect, the Al content is preferably 0.001% or more. In order to fully exert the deoxidizing effect, the Al content is preferably 0.005% or more. Furthermore, Al is an element that promotes the formation of fayalite, and from this perspective, the Al content is preferably 0.012% or more. On the other hand, if an excessive Al content is contained, the formation of fayalite is excessively promoted and laser cuttability is impaired, so the Al content is preferably limited to 0.200% or less, more preferably 0.120% or less, and even more preferably 0.080% or less.

[0026] [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.

[0027] [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.

[0028] 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.

[0029] [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.

[0030] [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.

[0031] [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. Cr is also an element that promotes the formation of fayalite. From this perspective, the Cr content is preferably 0.05% or more, and more preferably 0.15% or more. On the other hand, excessive Cr content may generate coarse Cr carbonitrides, which may significantly deteriorate the toughness of the steel plate. From this perspective, it is preferable to limit the Cr content to 1.00% or less. Cr is also an element that impairs weldability and the toughness of welds. From this perspective, the Cr content is preferably 0.60% or less.

[0032] [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.

[0033] [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.

[0034] [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.

[0035] [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.

[0036] [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.

[0037] [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.

[0038] [Sn: 0~0.500%] Sn concentrates at the interface between the steel sheet and scale, promoting the formation of fayalite. The Sn content may be 0%, but in order to enhance the adhesion of the scale and improve laser cuttability, 0.500% or less of Sn may be contained. To fully obtain this effect, the Sn content is preferably 0.001% or more or 0.005% or more, and more preferably 0.025% or more. On the other hand, since a large amount of Sn content impairs the weldability and 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.

[0039] [Sb: 0~0.500%] Sb concentrates at the interface between the steel sheet and scale, promoting the formation of fayalite. The Sb content may be 0%, but in order to improve scale adhesion and laser cuttability, the Sb content is preferably 0.001% or more or 0.003% or more. To fully obtain this effect, the Sb content is preferably 0.005% or more, and more preferably 0.025% or more. On the other hand, since a large amount of Sb impairs weldability and weld toughness, 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.

[0040] [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.

[0041] 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.

[0042] 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.

[0043] Next, the characteristics of the steel plate according to the embodiment of the present invention in terms of its scale will be described.

[0044] A schematic diagram of scale in a steel plate according to an embodiment of the present invention is shown in Figure 1. Referring to Figure 1, a steel plate 10 according to an embodiment of the present invention includes a steel plate (base steel) 11 and a scale 12 formed on the surface of the steel plate, and the scale 12 has layered fayalite 14 formed at an interface 13 between the scale 12 and the steel plate 11, and further has layered magnetite 15, and has cracks 16 therein.

[0045] [Maximum crack length in the thickness direction: 50% or less of the average thickness of the scale] In a steel plate according to an embodiment of the present invention, the thickness-wise size of cracks present within the surface scale is limited to a certain range, thereby suppressing scale spalling and improving laser cuttability. Here, a crack refers to a space, including voids (air gaps), where no scale or metal is present, regardless of its shape. The length of the longest crack in the thickness direction among multiple cracks present in the scale is limited to 50% or less of the average scale thickness. Since the shorter the maximum thickness of a crack, the better the laser cuttability. Therefore, the length is preferably 43% or less, and more preferably 35% or less, of the average scale thickness. While there is no specific lower limit for the maximum thickness-wise length of a crack, maintaining it below 15% of the average scale thickness is difficult in actual production processes and requires specialized equipment to control the oxidation behavior of the surface layer, making it economically undesirable. Therefore, the maximum thickness-wise length of a crack may be 15% or more of the average scale thickness.

[0046] [Average thickness of layered fayalite: 0.3-2.0 μm] [Coverage rate of scale / steel sheet interface by layered fayalite: 50% or more] The steel plate according to the embodiment of the present invention has lamellar fayalite, and by disposing the lamellar fayalite at the scale / steel plate interface, the scale growing therefrom is made uniform, and further, the adhesion of the scale is increased, thereby improving the laser cuttability. If the average thickness of this lamellar fayalite is excessively thin, the effect of increasing the adhesion of the scale is insufficient, and the laser cuttability is not sufficiently improved. On the other hand, if the average thickness of this lamellar fayalite is excessively thick, cracks are likely to occur in the fayalite and its vicinity due to thermal stress caused by laser irradiation, which in turn deteriorates the adhesion of the scale and the laser cuttability. From the above viewpoints, the average thickness of the lamellar fayalite is set to 0.3 to 2.0 μm or less, and preferably 0.5 to 1.7 μm or less.

[0047] For stratified fayalite to exert the above-mentioned effects, the stratified fayalite must be located at the scale / steel sheet interface, and the interface must be sufficiently covered by the stratified fayalite. If the coverage of the interface by the stratified fayalite is insufficient, scale peeling cannot be suppressed at locations where the stratified fayalite is not present at the interface, reducing the adhesion of the scale. Furthermore, the growth behavior of the scale differs between locations where the stratified fayalite is present and locations where it is not present at the interface, resulting in a deviation in scale thickness and a deterioration in laser cuttability. From the above perspectives, in the thick steel plate according to an embodiment of the present invention, the coverage of the scale / steel sheet interface by the stratified fayalite is set to 50% or more. The higher the coverage of the scale / steel sheet interface by the stratified fayalite, the better, preferably 65% ​​or more, more preferably 80% or more, and most preferably 100%.

[0048] [Proportion of stratified fayalite in total fayalite: 50% or more] In addition to the layered fayalite in the scale, fayalite also exists fragmentarily within other phases (wüstite, magnetite, and hematite) in the scale. Because such fragmented fayalite is not present at the scale / steel sheet interface, it does not enhance the scale adhesion and uniformity of scale growth. Instead, it acts as a foreign substance within the scale, which actually degrades the scale adhesion and impairs laser cuttability. Therefore, it is preferable to reduce the proportion of such fragmented fayalite. Specifically, the proportion of layered fayalite among all fayalite in the scale is preferably 50% or more, more preferably 65% ​​or more, and even more preferably 80% or more. While a 100% proportion of layered fayalite is acceptable, the effects saturate at 80% or more. Therefore, the properties of the steel plate according to the present invention are not impaired even if the proportion does not reach 100%.

[0049] [Average thickness of layered magnetite: 2.0 μm or more] The scale in the steel plate according to the embodiment of the present invention further contains lamellar magnetite. Magnetite has excellent heat absorption properties for heat input by a laser and also has excellent adhesion, so when it is uniformly present in the scale as lamellar magnetite, it can improve laser cuttability. To fully obtain this effect, the average thickness of the lamellar magnetite needs to be 2.0 μm or more, and preferably 3.5 μm or more. There is no particular upper limit set for the average thickness of the lamellar magnetite, but a thickness of 30.0 μm or more requires the supply of a large amount of oxygen, which may cause cracks in the scale to coarsen. Therefore, it is preferably 30.0 μm or less, and more preferably 20.0 μm or less.

[0050] The scale in the steel plate according to the embodiment of the present invention may further contain hematite (Fe2O3), wustite (FeO), alloy oxides, ferrite, and metal particles, as long as the above characteristics are satisfied.

[0051] [Scale thickness deviation: 0.25 or less] The steel plate according to the embodiment of the present invention has a uniform scale thickness, which suppresses the generation of thermal stress, improves scale adhesion, and exhibits excellent laser cuttability. Large variations in scale thickness result in irregular changes in the amount of heat consumed by melting the scale upon laser irradiation, irregularly generating thermal stress and degrading scale adhesion. From this perspective, the steel plate according to the embodiment of the present invention has a scale thickness deviation, calculated by dividing the difference between the maximum and minimum measured scale thicknesses by the average thickness, limited to 0.25 or less. The smaller the scale thickness deviation, the better the scale adhesion. Therefore, the scale thickness deviation is preferably 0.20 or less, more preferably 0.15 or less, and even more preferably 0.10 or less. While there is no particular lower limit for the scale thickness deviation, the effect of reducing the scale thickness deviation saturates when the deviation is less than 0.03. Therefore, from an economical perspective, it is preferable to limit the scale thickness deviation to 0.03 or more.

[0052] The scale characteristics described above are evaluated by crystal orientation analysis and microstructural observation of the cross section of the scale. Specifically, a small piece is cut from halfway across the width of the steel plate, and the cross section parallel to the rolling direction and perpendicular to the plate surface is used as the observation surface. The observation surface is wet polished and polished with colloidal silica to a mirror finish, and the section from the outermost surface of the steel plate to the scale / steel plate interface is observed using a field emission scanning electron microscope (FE-SEM), analyzed using a crystal orientation analyzer that uses electron backscattering diffraction (EBSD) mounted on the FE-SEM, and elemental mapping is performed using an electron probe microanalyzer (EPMA).

[0053] Crystal orientation analysis using EBSD revealed the distribution of magnetite within the scale, as shown in Figure 2. EBSD also detected the presence of fayalite, but because the crystalline structure of fayalite is complex and the detection pattern obtained by EBSD is weak, the thickness of fayalite cannot be determined from the map shown in Figure 2. Therefore, when elemental mapping was performed using EPMA at locations where fayalite was detected by EBSD, Si-enriched areas were observed, as shown in Figure 3. Measurement points where the detected Si concentration was 5.0 mass% or greater were determined to be fayalite. A region where the magnetite and fayalite phases are adjacent to each other and connected by 20 μm or more along the rolling direction was determined to be a layered region. Furthermore, as shown in Figure 4, if fayalite is present within 2.0 μm in the thickness direction from the scale / steel sheet interface as determined from the oxygen concentration map obtained by elemental mapping, that portion of the scale was determined to be covered by fayalite. Furthermore, from the element concentration map of iron, oxygen, and substitutional elements (Si, Mn, Ni, Cu, and Cr), measurement points where the total amount of these elements is less than 50 at% are determined to be voids, and locations where these exist continuously in the scale are determined to be cracks in the scale.

[0054] For each observation sample, evaluation of the surface layer was performed within a 100 μm long range parallel to the rolling direction. EBSD crystal orientation analysis and EPMA element mapping were performed in 0.3 μm increments. The thickness of the scale and each layer within the scale was determined by drawing five lines perpendicular to the sheet surface of each observation sample. The length of each line segment corresponding to the scale and each layer within the scale was evaluated, and the simple average of the values ​​along the five lines was used to determine the thickness of the layered magnetite and layered fayalite within the scale for that sample. Similarly, the difference between the maximum and minimum scale thickness values ​​measured at five locations was calculated, and this difference was divided by the average of the five scale thickness values ​​to determine the scale thickness deviation. The coverage rate of the scale / steel sheet interface by layered fayalite (as determined from the Si concentration map shown in Figure 3) was calculated as the percentage of the total length of the scale / steel sheet interface determined from the oxygen concentration map shown in Figure 4.

[0055] 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.

[0056] 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.

[0057] 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 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 to a rolling completion temperature of 1050°C to (T0-30)°C and a cumulative reduction of 15 to 30%, followed by high-pressure water descaling in a temperature range of 1000°C to (T0-30)°C, and then rolled two or more times with a reduction in each pass of 30% or less compared to the plate thickness before rolling, controlling the interpass time of the rolling after the high-pressure water descaling to satisfy the following formula (3), and the rolling completion temperature in the final rolling pass is 850°C or higher; 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 to satisfy the following formula (4), and the water cooling stop temperature is set to 550 to 650 ° C. The present invention is characterized in that it includes: T0 is the temperature calculated by the following formula (1). T0=1175-8.4[Mn]-135[P] 0.5 -52[Al]-24[Cr]...Formula (1) [Mn], [P], [Al], and [Cr] are the contents [mass%] of each element in the slab. 1.0≦x 10 ≦10.0...Equation (2) x1=D1·(T1 3 +D2·T1 2 +D3·T1+D4)· [ 1-exp{D5·(T1-T0) }] 0.5 Δt 0.5 t n =x n 2 D1 ー2 ·(T n+1 3 +D2·T n+1 2 +D3·T n+1 +D4) -2 · [ 1-exp{D5·(Tn+1 -T0) }] -1 x n =D1·(T n 3 +D2·T n 2 +D3·T n +D4)· [ 1-exp{D5·(T n -T0) }] 0.5 (t n-1 +Δt) 0.5 x n is an index that represents the degree of fayalite settling after the time elapsed from when the surface temperature of the slab exceeds T0℃ during the heating process until the completion of the heating process is divided into 10 equal parts, and n indicates that the calculation corresponds to the nth of the 10 equal parts. D1, D2, D3, D4, and D5 are constants, each equal to 4.00 × 10 -9 , -4.22×10 3 , 5.93×10 6 , -2.74×10 9 and -1.85 × 10 -2 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. y n ≦1.00...Equation (3) y m =y m-1 ·exp[E1·k m-1 ·exp{E2·(J m-1 +J m )}]+E3·R m ·exp(E4·J m ) y0=0.00 y mis an index that represents the degree of cracking in the scale that occurs in the mth rolling out of a total of n rollings that are performed from the end of high-pressure water descaling to the completion of rolling, E1, E2, E3, and E4 are constants, each equal to -2.57 × 10 -10 , -1.02 × 10 -2 , 2.70×10 1 and -8.33 × 10 -3 and k m is the elapsed time [seconds] from the mth rolling to the (m+1)th rolling, J m is the rolling material temperature [℃] at the mth rolling, R m is the reduction ratio [%] of the thickness before rolling in the mth rolling pass, y n is obtained by calculating y1, y2, y3, etc. in order using the above formula. 1.0≦z 10 ≦10.0...Equation (4) z1=F1·exp{F2 / (H1-F3)}·(-H1 2 +F4H1+F5) 0.5 Δp 0.5 p n =z n 2 Formula 1 -2 ·exp{2·F2 / (H1-F3)}·(-H n+1 2 +F4H n+1 +F5) -1 z n =F1·exp{F2 / (H n -F3)}·(-H n 2 +F4H n +F5) 0.5 ·(p n-1 +Δp) 0.5 z 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. F1, F2, F3, F4, and F5 are constants, each equal to 1.63 × 10 -6 , -2.50×10 2 , 3.25×10 2 , 2.94 × 10 2 and 1.36 × 10 6 and H n is the average steel plate temperature [°C] in the nth region of the 10 equally divided sections, Δp is one-tenth of the elapsed time [seconds], z 10 is obtained by calculating z1, z2, z3, etc. in order using the above formula.

[0058] [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.

[0059] [Heating process] The produced slab is subjected to a heat treatment for hot rolling, with the maximum heating temperature being in the range of (T0 + 20)°C to 1300°C. This heat treatment causes fayalite within the scale on the slab surface to precipitate to the scale / steel sheet interface, forming lamellar fayalite at the scale / steel sheet interface. T0 is the temperature at which fayalite begins to precipitate within the scale and is calculated using the following formula (1). If the heating temperature is below (T0 + 20)°C, sufficient lamellar fayalite cannot be obtained at the scale / steel sheet interface. On the other hand, if the heating temperature is excessively high, the scale grows non-uniformly, and it cannot be sufficiently removed during descaling after heating. Therefore, scale remains on the lamellar fayalite, resulting in an inhomogeneous scale thickness and poor laser cuttability. For this reason, the heating temperature is limited to 1300°C or less. From the above viewpoints, the heating temperature is limited to (T0+20)°C to 1300°C, and more preferably to (T0+40)°C to 1270°C. T0=1175-8.4[Mn]-135[P] 0.5 -52[Al]-24[Cr]...Formula (1) Here, [Mn], [P], [Al], and [Cr] are the contents [mass %] of each element in the slab.

[0060] The sedimentation behavior of fayalite within the scale during the heating process is greatly affected by not only the heating temperature but also the heating time. Therefore, heating is performed so that the elapsed time from exceeding T0°C to the completion of the heating process satisfies the following formula (2). Here, x in formula (2) 10 is an index that expresses the degree of fayalite precipitation on the slab surface, taking into account the change in the influence of time-dependent temperature changes on scale growth during the heating process, which is calculated using the calculations described below. If the elapsed time is too short, x 10 If x is less than 1.0, the degree of formation of lamellar fayalite at the scale / steel sheet interface becomes insufficient, impairing laser cutting properties. 10 is set to 1.0 or more. In order to promote the formation of layered fayalite and further improve the laser cutting property, x 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 scale will grow and remain uneven after descaling, and the subsequent scale formation will be uneven, impairing the laser cutting properties. For this reason, x 10 is limited to 10.0 or less, preferably 9.0 or less, and more preferably 8.0 or less. 1.0≦x 10 ≦10.0...Equation (2) x1=D1·(T1 3 +D2·T1 2 +D3·T1+D4)· [ 1-exp{D5·(T1-T0) }] 0.5 Δt 0.5 t n =x n 2 D1 ー2 ·(T n+1 3 +D2·Tn+1 2 +D3·T n+1 +D4) -2 · [ 1-exp{D5·(T n+1 -T0) }] -1 x n =D1·(T n 3 +D2·T n 2 +D3·T n +D4)· [ 1-exp{D5·(T n -T0) }] 0.5 (t n-1 +Δt) 0.5

[0061] 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 until the completion of the heating process into 10 equal parts, and calculating the degree of fayalite settling (x n ) and the subscript n indicates that the calculation is the nth of 10 equal intervals. D1, D2, D3, D4, and D5 are constants, each of which is 4.00×10 -9 , -4.22×10 3 , 5.93×10 6 , -2.74×10 9 and -1.85 × 10 -2 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.

[0062] [Hot rolling process] The surface of the heat-treated slab has heterogeneous scale that is prone to peeling. Therefore, hot rolling and high-pressure water descaling are performed to remove the heterogeneous scale that has grown on the layered fayalite, exposing the layered fayalite on the surface of the steel plate, thereby homogenizing the subsequent scale formation behavior, and then appropriate rolling and cooling treatments are performed to obtain the scale structure of the steel plate according to the embodiment of the present invention.

[0063] In order to sufficiently remove scale by descaling, hot rolling is performed prior to descaling to crush the scale that has grown on the layered fayalite. The rolling prior to descaling is performed in one or more stages at a rolling completion temperature in the range of 1050°C to (T0-30)°C, with a cumulative reduction of 15 to 30% relative to the slab thickness. In other words, when rolling prior to descaling is performed in multiple stages, the cumulative reduction calculated based on the plate thickness after completion of the multiple rolling stages relative to the slab thickness is 15 to 30%. If the rolling completion temperature before descaling is below 1050°C, the layered fayalite will be crushed together with the scale above it, and the layered fayalite will be removed by descaling, resulting in a deterioration in laser cuttability. On the other hand, if the rolling completion temperature before descaling exceeds (T0-30)°C, the adhesion between the layered fayalite and the scale growing on it will be strong, the scale will not be crushed sufficiently, the degree of remaining scale after descaling will be inhomogeneous, the subsequent scale formation behavior will be inhomogeneous, and the laser cuttability will be deteriorated.

[0064] Furthermore, if the cumulative reduction rate of the hot rolling is less than 15%, the scale will not be crushed sufficiently, and the scale will remain non-uniform after descaling, resulting in non-uniform behavior of the subsequent scale formation and degraded laser cuttability. On the other hand, if the cumulative reduction rate of the hot rolling exceeds 30%, the layered fayalite will be crushed and removed by descaling, degrading laser cuttability. In order to suppress the crushing of the layered fayalite and avoid the formation of clumped fayalite inside the scale, the cumulative reduction rate of the hot rolling is preferably 25% or less.

[0065] After the hot rolling, the obtained rolled material is subjected to high-pressure water descaling. Descaling can be performed using high-pressure water with a collision pressure of 10 to 15 MPa, for example. If the temperature at which high-pressure water descaling is performed is below 1000°C, some of the unevenly grown scale will remain, and subsequent scale formation will be uneven. On the other hand, if the temperature at which high-pressure water descaling is performed exceeds (T0-30)°C, some of the layered fayalite will peel off due to the high-pressure water descaling. From the above perspective, the temperature at which high-pressure water descaling is performed after the hot rolling is limited to the range of 1000°C to (T0-30)°C.

[0066] After the descaling, the surface layer of the steel sheet is mainly covered with lamellar fayalite, and a scale mainly composed of wüstite forms thereon. The steel sheet is rolled to a thickness appropriate for its intended use. However, if the reduction rate per rolling pass exceeds 30%, large cracks will form in the formed scale, impairing laser cuttability. Therefore, the reduction rate per pass is limited to 30% or less. The smaller the reduction rate per pass, the smaller the cracks formed in the scale. From this perspective, it is preferable to limit the reduction rate per pass to 25% or less, and more preferably to 20% or less. On the other hand, if the reduction rate is excessively small in all rolling passes, the number of rolling passes required to obtain a plate thickness appropriate for the intended use will increase, the rolling temperature will decrease, and the rolling completion temperature will be excessively low. Therefore, it is preferable to set the average reduction rate in rolling after high-pressure water descaling to 10% or more.

[0067] By setting the reduction rate per rolling pass to 30% or less, the size of cracks that occur in the formed scale can be suppressed, but continuous rolling causes the cracks to grow and impairs laser cuttability. Therefore, when performing continuous rolling, by leaving a sufficient amount of time between rolling passes, the growth of the scale can be promoted and the formed cracks can be filled, thereby suppressing the formation of large cracks and improving laser cuttability. From the above perspective, in the rolling performed after the high-pressure water descaling, rolling is performed so that the interpass time between rolling passes satisfies the following formula (3).

[0068] After the high-pressure water descaling, in order to prevent a portion of the layered fayalite from being crushed by the rolling, being incorporated into the scale, and remaining in the scale as granular fayalite, high-pressure water descaling may be carried out once or twice or more times during the continuous rolling to remove excess scale. y n ≦1.00...Equation (3) y m =y m-1 ·exp[E1·k m-1 ·exp{E2·(J m-1 +J m )}]+E3·R m ·exp(E4·J m ) y0=0.00

[0069] y m is an index that represents the degree of cracking in the scale that occurs in the mth rolling of all n rollings performed after high-pressure water descaling until the completion of rolling. By successively calculating y1, y2, y3, etc. according to the above formula, the degree of cracking at the completion of rolling can be calculated. n You can get y n If y is large, the cracks in the scale will become larger, so as shown in the above equation (3), n Limit the value of y to 1.00 or less. To reduce the crack size in the scale, n The value of k is preferably 0.70 or less, and more preferably 0.50 or less.m is the elapsed time [seconds] from the mth rolling to the (m+1)th rolling, and J m is the temperature of the steel plate at the mth rolling [℃], and R m is the reduction ratio [%] of the thickness before the mth rolling in the mth rolling. E1, E2, E3, and E4 are constants, each of which is -2.57×10 -10 , -1.02 × 10 -2 , 2.70×10 1 and -8.33 × 10 -3 is.

[0070] The rolling completion temperature in the final rolling pass after high-pressure water descaling is 850°C or higher. If rolling is performed at a temperature below 850°C, some of the scale will peel off, and the peeled scale will be pressed in, making the scale thickness non-uniform and impairing laser cuttability. The rolling completion temperature is preferably 875°C or higher, and more preferably 900°C or higher. There is no particular upper limit, but for example, the rolling completion temperature in the final rolling pass may be 1000°C or lower.

[0071] [Cooling process] The time elapsed from the completion of the hot rolling process to the start of water cooling is controlled by Equation (4) to reduce the size of cracks formed inside the scale while obtaining an appropriate amount of magnetite inside the scale. Here, if the time elapsed from the completion of the hot rolling process to the start of water cooling is too short, z 10 If z is less than 1.0, the growth of scale is excessively suppressed, magnetite is not sufficiently formed, and laser cutting properties are impaired. 10 On the other hand, if the time from the end of the hot rolling process to the start of water cooling is too long, z 10 If z exceeds 10.0, excessive oxygen is supplied to the scale, and some cracks become coarser. 10 z should be 10.0 or less. To improve the structure of the scale and laser cutting properties, 10 is preferably 2.0 or more and 8.0 or less, and more preferably 3.0 or more and 7.0 or less. 1.0≦z 10 ≦10.0...Equation (4) where z n is calculated as follows: z1=F1·exp{F2 / (H1-F3)}·(-H1 2 +F4H1+F5) 0.5 Δp 0.5 p n =z n 2 Formula 1 -2 ·exp{2·F2 / (H1-F3)}·(-H n+1 2 +F4H n+1 +F5) -1 z n =F1·exp{F2 / (H n -F3)}·(-H n 2 +F4H n +F5) 0.5 ·(p n-1 +Δp) 0.5

[0072] 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 (z n ), where the subscript n indicates that the calculation is the nth of 10 equal intervals. F1, F2, F3, F4, and F5 are constants, each of which is 1.63 × 10 -6 , -2.50×10 2 , 3.25×10 2 , 2.94 × 10 2 and 1.36 × 10 6 H 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. Δp is one-tenth of the elapsed time [seconds]. z in Equation (4) 10 is obtained by calculating z1, z2, z3, etc. in order using the above formula.

[0073] Water cooling is stopped when the steel plate temperature reaches 550 to 650°C. If this water cooling stop temperature is too high, the scale grows unevenly, resulting in an uneven scale thickness and poor laser cuttability. On the other hand, if the water cooling stop temperature is too low, thermal stress will cause cracks in the scale during water cooling, impairing laser cuttability. For this reason, the water cooling stop temperature is set to 550 to 650°C.

[0074] In the manufacturing method of a steel plate according to an embodiment of the present invention, the cooling conditions for the steel plate after water cooling 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 then water cooling, air cooling, and / or natural cooling may be performed after satisfying the characteristics of the manufacturing method described above. Furthermore, after cooling of the steel plate is complete, a tempering treatment may be performed within a range that does not impair the characteristics of the steel plate according to an embodiment of the present invention.

[0075] 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.

[0076] The thick steel plate manufactured by the method for manufacturing a thick steel plate according to an embodiment of the present invention includes a steel plate and a scale formed on the surface of the steel plate, wherein the maximum length in the plate thickness direction of cracks present in the scale is 50% or less of the average thickness of the scale, the scale includes stratified fayalite formed at the interface between the scale and the steel plate, the average thickness of the stratified fayalite is 0.3 to 2.0 μm, the coverage of the interface with the stratified fayalite is 50% or more, the scale further includes stratified magnetite, the average thickness of the stratified magnetite is 2.0 μm or more, and the thickness deviation of the scale is 0.25 or less. Therefore, irregular peeling of the scale during laser irradiation in a laser cutting operation can be suppressed or prevented, and cutting into any shape can be stably carried out, and the steel plate can be used for structures such as construction, architecture, industrial machinery, and bridges.

[0077] 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]

[0078] 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.

[0079] First, using a slab having the chemical composition shown in Table 1, a heating process, a hot rolling process, and a cooling process were carried out under the conditions shown in Table 2 to obtain steel plates as experimental examples, including examples and comparative examples. Experimental Examples 1, 8, 10, 36, 51, and 74 are examples in which reheating by induction heating was carried out between heating and descaling.

[0080] [Table 1]

[0081] [Table 2-1]

[0082] [Table 2-2]

[0083] [Scale adhesion evaluation] The scale adhesion of the resulting steel plates was evaluated using a test simulating the conditions of laser irradiation on steel plates. First, steel plates containing scale on the surface were cut into 100 mm x 100 mm pieces. The center of the steel plate surface, held at room temperature (20-30°C), was irradiated with a laser beam over a 10 mm length under the following conditions. A 1 mm-long area at 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 if the area ratio of scale spalling in that area after a single irradiation exceeded 30%, "○" if it was greater than 5% but not more than 30%, and "◎" if it was 5% or less. Steel plates that achieved a rating of "○" or "◎" were deemed to have excellent scale adhesion and in which scale spalling during laser irradiation was suppressed or prevented, and were therefore deemed to have passed the test. The results are shown in Table 3. Laser power: 500W Pulse frequency: 60kHz Focusing diameter: 0.70 mm Irradiation speed: 3m / sec

[0084] [Table 3-1]

[0085] [Table 3-2]

[0086] Among the experimental examples listed in Tables 1 to 3, experimental example 25 is a comparative example in which the maximum heating temperature of the slab in the heating process was low, and sufficient layered fayalite was not obtained at the scale / steel sheet interface, resulting in poor laser cuttability. On the other hand, experimental example 14 is a comparative example in which the maximum heating temperature of the slab in the heating process was high, and the scale thickness deviation was large, resulting in poor laser cuttability. Experimental Example 17 is a comparative example in which the heating time in the heating step was short, and formula (1) was not satisfied, and sufficient layered fayalite was not obtained at the scale / steel sheet interface, resulting in poor laser cuttability. On the other hand, Experimental Example 33 is a comparative example in which the heating time in the heating step was long, and formula (1) was not satisfied, resulting in large deviation in scale thickness and poor laser cuttability. Experimental Example 36 is a comparative example in which the rolling completion temperature before descaling was low, the coverage of the scale / steel sheet interface with lamellar fayalite was low, and the laser cuttability was poor. Experimental Example 3 is a comparative example in which the rolling completion temperature before descaling was high, the scale thickness deviation was large, and the laser cuttability was poor. Experimental Example 73 is a comparative example in which the cumulative reduction ratio in the rolling prior to descaling was small, the deviation in scale thickness was large, and the laser cuttability was poor. On the other hand, Experimental Example 22 is a comparative example in which the cumulative reduction ratio in the rolling prior to descaling was large, the coverage of the scale / steel sheet interface by lamellar fayalite was low, and the laser cuttability was poor. Experimental Example 44 is a comparative example in which the descaling temperature was low, the scale thickness deviation was large, and the laser cuttability was poor. On the other hand, Experimental Example 74 is a comparative example in which the descaling temperature was high, the coverage of the scale / steel sheet interface with layered fayalite was low, and the laser cuttability was poor. Experimental Example 6 is a comparative example in which the maximum reduction ratio in rolling after descaling was large, causing large cracks in the scale and resulting in poor laser cuttability. Experimental Example 11 is a comparative example in which the time elapsed between rolling passes after descaling was short and formula (3) was not satisfied, resulting in large cracks in the scale and poor laser cuttability. Experimental Examples 52 and 59 are comparative examples in which the rolling completion temperature in the final rolling pass was low, the deviation in scale thickness was large, and the laser cuttability was poor. Experimental Example 47 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 (4) was not satisfied, so lamellar magnetite was not obtained sufficiently, and laser cuttability was poor. On the other hand, Experimental Example 64 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 (4) was not satisfied, so cracks in the scale became large, and laser cuttability was poor. Experimental Example 39 is a comparative example in which the water cooling stop temperature was low, and cracks in the scale became large, resulting in poor laser cuttability. On the other hand, Experimental Example 28 is a comparative example in which the water cooling stop temperature was high, and the deviation in scale thickness was large, resulting in poor laser cuttability. Experimental Example 77 is a comparative example in which the Si content of the steel sheet was low, so that sufficient lamellar fayalite was not obtained at the scale / steel sheet interface, resulting in poor laser cuttability. On the other hand, Experimental Example 78 is a comparative example in which the Si content of the steel sheet was high, so that excessive lamellar fayalite was present at the scale / steel sheet interface, resulting in poor laser cuttability.

[0087] The experimental examples excluding the above-mentioned comparative examples, i.e., Experimental Examples 1, 2, 4, 5, 7-10, 12, 13, 15, 16, 18-21, 23, 24, 26, 27, 29-32, 34, 35, 37, 38, 40-43, 45, 46, 48-51, 53-58, 60-63, 65-72, 75, and 76, are examples of the present invention, in which steel sheets were obtained that had excellent scale adhesion, suppressed or prevented scale spalling during laser irradiation, and therefore had excellent laser cuttability. Experimental Examples 12, 16, 23, and 50 are examples in which descaling was performed three more times between the completion of rolling following the first descaling and the final rolling pass. Experimental Examples 57, 60, and 70 are examples in which the steel sheets were wound into a coil after water cooling. [Explanation of symbols]

[0088] 10 thick steel plate 11 Steel plate 12 scale 13 Scale / steel plate interface 14 Layered Fayalite 15 Layered magnetite 16 Crack

Claims

1. A thick steel plate comprising a steel plate and a scale formed on a surface of the steel plate, wherein the maximum length in the plate thickness direction of cracks present in the scale is 50% or less of the average thickness of the scale, the scale includes stratified fayalite formed at the interface between the scale and the steel plate, the average thickness of the stratified fayalite is 0.3 to 2.0 μm, the coverage of the interface with the stratified fayalite is 50% or more, the scale further includes stratified magnetite, the average thickness of the stratified magnetite is 2.0 μm or more, and the thickness deviation of the scale is 0.25 or less.

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: 0.10-1.00%, 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. 5. The steel plate according to claim 1, wherein the layered fayalite accounts for 50% or more of all fayalite in the scale.

6. A step of heating a 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 process of hot rolling the slab, wherein the rolling completion temperature is 1050°C to (T 0 -30) ° C., and a cumulative rolling reduction of 15 to 30% is applied, followed by rolling at 1000 ° C. to (T 0 a hot rolling process in which high-pressure water descaling is performed at a temperature in the range of −30°C, followed by two or more passes of rolling in which the reduction rate in one pass is 30% or less compared to the plate thickness before rolling in each pass, the inter-pass time of the rolling after the high-pressure water descaling is controlled to satisfy the following formula (3), and the rolling completion temperature in the final rolling pass is 850°C or higher; 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 to satisfy the following formula (4), and the water cooling stop temperature is set to 550 to 650 ° C. The method for producing a steel plate according to any one of claims 1 to 5, comprising: T 0 is the temperature calculated by the following formula (1). T 0 = 1175 - 8.4[Mn] - 135[P] 0.5 - 52[Al] - 24[Cr] ··· Formula (1) [Mn], [P], [Al] and [Cr] are the contents [mass %] of each element in the slab. 1.0≦x 10 ≦10.0 ・・・Form (2) x 1 =D 1 ・(T 1 3 +D 2 ・T 1 2 +D 3 ・T 1 +D 4 )・[1-exp{D 5 ・(T 1 -T 0 )}] 0.5 ・Δt 0.5 t n =x n 2 ・D 1 ー2 ・(T n+1 3 +D 2 ・T n+1 2 +D 3 ・T n+1 +D 4 ) -2 ・[1-exp{D 5 ・(T n+1 -T 0 )}] -1 x n =D 1 ・(T n 3 +D 2 ・T n 2 +D 3 ・T n +D 4 )・[1-exp{D 5 ・(T n -T 0 )}] 0.5 ・(t n-1 +Δt) 0.5 x n is the temperature at the surface of the slab during the heating process. 0 The time elapsed from when the temperature exceeds °C until the completion of the heating process is divided into 10 equal parts, and the index represents the degree of fayalite settling after each section is completed, where n indicates that the calculation corresponds to the nth section out of 10 equal parts. D 1 , D 2 , D 3 , D 4 and D 5 are constants, each of which is 4.00 × 10 -9 , −4.22 × 10 3 , 5.93 x 10 6 , −2.74 × 10 9 and −1.85 × 10 -2 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 this order. y n ≦1.00...Formula (3) y m =y m-1 ・exp[E 1 ・k m-1 ・exp{E 2 ・(J m-1 +J m )}]+E 3 ・R m ・exp(E 4 ・J m ) y 0 =0.00 y m is an index representing the degree of cracking in the scale that occurs in the mth rolling out of a total of n rollings that are performed from the end of high-pressure water descaling to the completion of rolling, E 1 , E 2 , E 3 and E 4 are constants, and −2.57×10 -10 , −1.02 × 10 -2 , 2.70 x 10 1 and −8.33 × 10 -3 and k m is the elapsed time [seconds] from the mth rolling to the (m+1)th rolling, J m is the rolling material temperature [°C] at the mth rolling, R m is the reduction ratio [%] of the thickness before rolling in the mth rolling, y n is calculated by the above formula: 1 From y 2 , y 3 ...and so on, and can be calculated in this order. 1.0≦z 10 ≦10.0 ・・・Form (4) z 1 =F 1 ・exp{F 2 / (H 1 -F 3 )}・(-H 1 2 +F 4 H 1 +F 5 ) 0.5 ・Δp 0.5 p n =z n 2 ・F 1 -2 ・exp{2・F 2 / (H 1 -F 3 )}・(-H n+1 2 +F 4 H n+1 +F 5 ) -1 z n =F 1 ・exp{F 2 / (H n -F 3 )}・(-H n 2 +F 4 H n +F 5 ) 0.5 ・(p n-1 +Δp) 0.5 z 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. F 1 , F 2 , F 3 , F 4 and F 5 are constants, and are 1.63 × 10 -6 , −2.50×10 2 , 3.25 x 10 2 , 2.94 x 10 2 and 1.36 x 10 6 and H n is the average steel sheet temperature [°C] in the n-th region of the 10 equally divided sections, Δp is 1 / 10 of the elapsed time [seconds], z 10 is calculated by the above formula 1 From Z 2 , z 3 ...and so on, and can be calculated in this order.

7. The method for producing a steel plate according to claim 6, wherein in the rolling after the high-pressure water descaling, high-pressure water descaling is further carried out one or more times before the hot rolling step is completed.

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