Heavy steel plate and its manufacturing method

Abstract

To provide a thick steel plate in which occurrence of burning in laser cutting is suppressed or prevented, and therefore, which is useful for application of laser cutting, and a method for manufacture of the same.SOLUTION: Provided is a thick steel plate which contains a steel plate and scale, and in which the scale has, in order from a surface thereof, a first layer comprising hematite, a second layer comprising magnetite, and a third layer comprising wustite or comprising wustite and an eutectoid structure of ferrite and magnetite. An average thickness of the first layer is 0.5 to 5.0 μm, a surface coverage of the steel plate with the first layer is 50% or more, an average thickness of the second layer is 10 to 40%, an average thickness of the third layer is 50% or more of a scale thickness, a deviation of third layer thickness in a plate width direction is 0.40 or less, and an average particle diameter of wustite in the third layer is 15 μm or less. Further, a method for manufacture of the thick steel plate is provided.SELECTED DRAWING: None

Description

[Technical Field]

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

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

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

[0004] The production of steel plates, such as thick steel plates, typically involves hot-rolling slabs. Hot-rolled steel plates are known to oxidize in the atmosphere, forming scale (iron oxide) on their surfaces. Laser cutting of steel plates involves applying heat to the plate by irradiating it with a laser, melting it, and cutting it. This process significantly alters the melting behavior depending on the heat absorption properties of the area irradiated by the laser. Because the heat absorption properties of steel plates vary significantly depending on the condition of the surface scale, this can lead to instability in cutting or the formation of irregular cuts on the cut surface, resulting in increased labor hours and reduced cutting accuracy. In particular, when the 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] Patent Document 4 proposes a technology in which, when a hot-rolled slab is cooled, the coiling temperature is controlled to a range of 525 to 325°C, thereby generating magnetite in the scale at the interface with the base steel, improving the adhesion of the scale and enhancing laser cuttability. Furthermore, Patent Document 5 proposes a technology in which a hot-rolled steel sheet is soaked in an atmosphere with a surface oxygen concentration of less than 20% at a temperature range of 250 to 400°C for 5 to 240 minutes with a temperature change of 0.400°C / min or less, thereby giving the scale on the steel sheet surface a structure consisting of a layer made of a magnetite phase and a layer made of a wüstite phase and a granular magnetite phase, thereby improving the adhesion of the scale and enhancing laser cuttability.

[0008] Patent Document 6 proposes a technique for improving the adhesion of scale and laser cuttability by forming a concentrated layer of alloying elements at the interface between the scale and the base steel of a thick steel plate. Furthermore, Patent Document 7 proposes a technique for improving the adhesion of scale and laser cuttability by setting the fraction of magnetite phase in the scale to 50% or more. [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. 10-158734 [Patent Document 5] Japanese Patent Application Publication No. 2020-114938 [Patent Document 6] Japanese Patent Application Laid-Open No. 2002-332541 [Patent Document 7] Japanese Patent Application Laid-Open No. 2008-095155 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. Meanwhile, steel plates with even better laser cuttability than before are needed to cope with the increasing thickness of the cutting target due to the larger size of structures and the increased cutting speeds achieved by high-power fiber lasers. Specifically, to improve laser cuttability, steel plates that do not burn during cutting, even when irradiated with higher-power lasers than before, are needed. In laser cutting, the narrow cutting width can sometimes cause the molten material to clog within the cutting width. This can lead to excessive melting of the surrounding area or the molten material blowing upward, resulting in a cutting defect known as burning. Burning can be a major issue, especially with high-power laser irradiation.

[0011] Although Patent Documents 1 to 7 propose improving laser cuttability by increasing the laser absorption rate or by increasing the adhesion of the scale, they do not necessarily provide sufficient consideration from the perspective of suppressing or preventing the occurrence of burning. Therefore, the steel materials described in these Patent Documents still have room for improvement in terms of improving laser cuttability.

[0012] Therefore, an object of the present invention is to provide a thick steel plate in which the occurrence of burning during laser cutting is suppressed or prevented, and therefore useful for applying laser cutting, and a method for manufacturing the same. [Means for solving the problem]

[0013] In order to achieve the above object, the inventors have investigated the surface layer of a steel plate necessary for stable continuous cutting in laser cutting. As a result, they have found that by optimizing the scale phases and the thickness of each phase in the surface layer of a steel plate that is continuously irradiated with a laser, and by further reducing the crystal grain size of the wüstite phase, which is the main phase that forms the scale, the occurrence of burning can be significantly suppressed or prevented, thereby improving the laser cuttability of thick steel plate.

[0014] Although the mechanism by which the above-mentioned scale characteristics affect laser cuttability is unknown, it is presumed that optimizing the scale phases and their respective thicknesses stabilizes the amount of heat absorbed by the melting of the surface scale during laser irradiation. Furthermore, it is presumed that fixing the main phase present on the outermost surface of the scale and randomizing the crystal orientation of the wüstite phase relative to the laser incident direction as a result of refinement stabilizes the heat absorption of the heat input due to laser irradiation of the scale. It is presumed that these effects enable the steel plate according to the present invention to suppress the occurrence of burning when attempting to cut with a high-power laser.

[0015] The present invention, which has achieved the above object, is as follows. (1) A thick steel plate comprising a steel plate and scale formed on the surface of the steel plate, the scale having, in order from the surface, a first layer consisting of hematite, a second layer consisting of magnetite, and a third layer consisting of wustite or a eutectoid structure of wustite, ferrite, and magnetite, wherein the average thickness of the first layer is 0.5 to 5.0 μm, the surface coverage of the steel plate by the first layer is 50% or more, the average thickness of the second layer is 10 to 40% of the average thickness of the scale, the average thickness of the third layer is 50% or more of the average thickness of the scale, the deviation of the thickness of the third layer in the plate width direction is 0.40 or less, and the average grain size of wustite in the third layer is 15 μm 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: 0.01 to 1.00%, Mn: 0.10~2.50% P: 0.001 to 0.050%, S: 0.0001 to 0.0100%, Al: 0.001 to 0.200%, N: 0.0150% or less, O: 0.0050% or less, Cu: 0-1.00% Ni: 0-2.00% Cr: 0~1.00%, Mo: 0-1.00%, W: 0~0.50%, Nb: 0 to 0.500%, Ti: 0 to 0.500% V: 0 to 1.000%, B: 0~0.0100%, Sn: 0 to 0.500% Sb: 0 to 0.500% Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Hf: 0 to 0.0100%, Te: 0~0.0100%, Sr: 0 to 0.0100%, REM: 0 to 0.0100%, and The steel plate according to (1) or (2) above, having a chemical composition consisting of the balance: Fe and impurities. (4) The chemical composition is in mass%: Cu: 0.01 to 1.00%, Ni: 0.01 to 2.00% Cr: 0.01 to 1.00%, Mo: 0.01 to 1.00%, W: 0.003 to 0.50%, Nb: 0.003 to 0.500%, Ti: 0.003 to 0.500%, V: 0.003 to 1.000%, B: 0.0003~0.0100%, Sn: 0.003 to 0.500%, Sb: 0.003 to 0.500%, Ca: 0.0003 to 0.0100%, Mg: 0.0003 to 0.0100%, Hf: 0.0003 to 0.0100%, Te: 0.0003 to 0.0100%, Sr: 0.0003 to 0.0100%, and REM: 0.0003 to 0.0100% The steel plate according to (3) above, comprising one or more selected from the group consisting of: (5) The steel plate according to any one of the above (1) to (4), wherein the proportion of the eutectoid structure in the third layer is 20 to 80%. (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 is 1050 to 1300°C, and the elapsed time from when the temperature exceeds 1050°C to when the heating step is completed is controlled so as to satisfy the following formula (1): a hot rolling step of hot rolling the slab, the hot rolling step including rolling the slab at a cumulative reduction of 15 to 30% in a temperature range of 1000 to 1100°C such that the surface temperature of the slab is in the range of 1000 to 1100°C, followed by high-pressure water descaling in a temperature range of 1000 to 1100°C such that the surface temperature of the obtained rolled material is in the range of 1000 to 1100°C, and further rolling in a temperature range of 1000°C or less to a cumulative reduction of 30% or more compared to the plate thickness at the time when the temperature reaches 1000°C, the hot rolling step including two or more rolling passes, the reduction of each rolling pass being less than 30%, the reduction of at least one rolling pass being 5% or more, and the rolling temperature of the final rolling pass being 800 to 950°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 (2), and the water cooling stop temperature is set to 500 to 600 ° C. The method for producing a steel plate according to any one of the above (1) to (5), comprising: 1.0≦x 10 ≦10.0...Equation (1) x1=D1·D2·(T1 3 +D3T1 2 +D4T1+D5)·(T1-D6) 0.5 ·{1-exp(D7T1+D8)} 0.5 Δt 0.5 t n =x n 2 D1 -2 D2 -2 ·(T n+1 3 +D3T n+1 2 +D4T n+1 +D5) -2 ·(T n+1-875) -1 ·{1-exp(D6T n+1 +D7)} -1 x n =D1·D2·(T n 3 +D3T n 2 +D4T n +D5)·(T n -D6) 0.5 ·{1-exp(D7T n +D8)} 0.5 (t n-1 +Δt ) 0.5 D1=(1-0.850[C]-0.052[Si]-0.026[Mn]-0.065[Al]) 0.5 x n is an index that represents the degree of scale growth after the time elapsed from when the surface temperature of the slab exceeds 1050°C during the heating process until the heating process is completed, and n indicates that the calculation corresponds to the nth of the 10 equal divisions. D1 takes into account the influence of the chemical composition of the slab, and [C], [Si], [Mn], and [Al] in the above formula are the contents [mass%] of each element in the slab. D2, D3, D4, D5, D6, D7 and D8 are constants, each equal to 8.66 × 10 -10 , -3.99×10 3 , 5.36 x 10 6 , -2.32 × 10 9 , 8.75 x 10 2 , -3.50×10 -3 and 3.06 × 10 0 and T n is the average slab temperature [℃] in the nth region of the 10 equally divided sections, Δt is 1 / 10 of the elapsed time [seconds], x 10 is obtained by calculating x1, x2, x3, etc. in order using the above formula, 1.0≦y 10 ≦10.0...Equation (2) y1=E1·(1+E2·Mn)·exp{E3J1+E4 / (J1+E5)}·Δk 0.5 k n =y n 2 E1 -2 (1+E2 Mn) -2 ·exp{-2·E3J n+1 -2·E4 / (J n+1 +E5)} y n =E1·(1+E2·Mn)·exp{E3J n +E4 / (J n +E5)}·(k n-1 +Δk) 0.5 y n is an index that represents the degree of scale growth after the elapsed time from the completion of the hot rolling process to the start of water cooling is divided into 10 equal sections in the cooling process, and n indicates that the calculation corresponds to the nth section out of 10 equal sections. E1, E2, E3, E4, and E5 are constants, each equal to 5.00 × 10 7 , 1.24×10 -1 , -9.56×10 -3 , -1.05×10 4 and 2.73 × 10 2 and J n is the average steel plate temperature [°C] in the nth region of the 10 equally divided sections, Δk is one-tenth of the elapsed time [seconds], y 10 can be obtained by calculating y1, y2, y3, etc. in order using the above formula. (7) The method for producing a steel plate according to (6) above, wherein after water cooling is stopped, the average cooling rate in the temperature range of 400 to 500°C is 0.10 to 10.0°C / min. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a thick steel plate that suppresses or prevents the occurrence of burning during laser cutting and is therefore useful for applying laser cutting, and a method for manufacturing the same. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a schematic diagram showing a scale structure of 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] An example of identifying wüstite grains using crystal orientation analysis using the EBSD method is shown below. 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 chemical composition of the steel sheet is not particularly limited and may be appropriately determined within a range useful for laser cutting. As described above, the present invention aims to provide a steel sheet that suppresses or prevents burning during laser cutting and is therefore useful for laser cutting. The objective is achieved by providing a steel sheet having a scale consisting of, in order from the surface, a first layer composed of hematite, a second layer composed of magnetite, and a third layer composed of wüstite or a eutectoid structure of wüstite, ferrite, and magnetite, the first layer having an average thickness of 0.5 to 5.0 μm, a surface coverage of the steel sheet by the first layer of 50% or more, an average thickness of the second layer being 10 to 40% of the average thickness of the scale, an average thickness of the third layer being 50% or more of the average thickness of the scale, a deviation in the thickness of the third layer in the sheet width direction of 0.40 or less, and an average grain size of wüstite in the third layer being 15 μm or less. Therefore, it is clear that the chemical composition of the steel sheet itself is not an essential technical feature for achieving the objective of the present invention. Hereinafter, preferred chemical compositions of steel sheets according to embodiments of the present invention will be described, but these descriptions are intended to be merely illustrative and are not intended to limit the present invention to steel sheets having such specific chemical compositions. Furthermore, in the following description, "%," which is the unit of content of each element, means "mass %" unless otherwise specified.

[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] [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. 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.

[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 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.

[0039] [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.

[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] [1st layer: hematite (Fe2O3), 2nd layer: magnetite (Fe3O4), 3rd layer: wustite (FeO) or a wustite phase and a eutectoid structure of ferrite and magnetite] A steel plate according to an embodiment of the present invention has a scale having three layers on the surface. FIG. 1 shows a schematic diagram of the scale structure of a steel plate according to an embodiment of the present invention. Referring to FIG. 1, a steel plate 10 includes a steel plate (base steel) 11 and a scale 12 formed on the surface of the steel plate 11, the scale 12 including three layers. A first layer 13 of the scale 12, located on the outermost surface of the steel plate 10, is composed of hematite, a subsequent second layer 14 is composed of magnetite, and a third layer 15, which is closest to the scale / steel plate (base steel) interface, is composed of wüstite or a eutectoid structure of wüstite, ferrite, and magnetite. The surface scale is formed by a reaction between iron (Fe) diffusing from the base steel and oxygen (O) in the atmosphere. Therefore, lower-order iron oxides such as wüstite are formed closer to the base steel, while higher-order iron oxides such as hematite are formed closer to the atmosphere. In the steel plate according to the embodiment of the present invention, the thicknesses of the above three layers are appropriate, and the grain size of wüstite, the main phase that constitutes the scale, is reduced to improve laser cuttability. Each layer will be described in more detail below.

[0045] [Average thickness of the first layer: 0.5 to 5.0 μm] The first layer of scale in the steel plate according to the embodiment of the present invention is the layer that is primarily exposed to laser irradiation. By being present at a sufficient thickness, the first layer can absorb the heat input from the laser depending on the physical properties of the phase that forms the first layer. If the average thickness of the first layer is less than 0.5 μm, a wide area of ​​the first layer will not function as a laser-irradiated layer due to evaporation caused by laser irradiation or wear during handling of the steel plate, resulting in significant fluctuations in the heat absorption properties of the scale, and thus impairing laser cuttability. On the other hand, if the average thickness of the first layer is more than 5.0 μm, excessive oxygen supply will cause numerous cracks and voids to form in the scale, making the scale more susceptible to spalling. Furthermore, the thermal stress associated with laser irradiation will cause the scale to spall, significantly fluctuating the heat absorption properties of the steel plate, thereby impairing laser cuttability. From these perspectives, the average thickness of the first layer of scale in the steel plate according to the embodiment of the present invention is set to 0.5 to 5.0 μm, preferably 0.8 to 4.0 μm. When the first layer has such an average thickness, it can stably absorb the irradiated laser, i.e., the heat absorption of the heat input due to the laser irradiation is stabilized, and the occurrence of burning and the like can be sufficiently suppressed or prevented, thereby improving the laser cutting properties.

[0046] [Surface coverage of the first layer: 50% or more] To stabilize the heat absorption of scale due to laser irradiation, the surface coverage of the first layer on the scale surface of the steel sheet must be 50% or more. If the surface coverage is less than 50%, the influence of the second layer on the heat absorption of the scale becomes greater, resulting in greater variations in heat absorption depending on the location and impairing laser cutting properties. Since a higher surface coverage is preferable, it is preferably 65% ​​or more, more preferably 80% or more, and most preferably 100%.

[0047] [Average thickness of the second layer: 10-40% of the average thickness of the scale] The average thickness of the second layer of scale in the steel plate according to the embodiment of the present invention is limited to 10 to 40% of the average scale thickness. If the thickness of the second layer is excessively thin, there will be areas where the second layer is absent, causing the heat absorption amount of the scale to vary significantly depending on the location, impairing laser cuttability. Therefore, the average thickness of the second layer is set to 10% or more of the average scale thickness. The average thickness of the second layer may be 15% or more or 20% or more of the average scale thickness. On the other hand, if the thickness of the second layer is excessively thick, it will be difficult to obtain the first layer and / or third layer having the specified thickness, impairing laser cuttability. Therefore, the average thickness of the second layer is set to 40% or less of the average scale thickness. The average thickness of the second layer may be 30% or less or 35% or less of the average scale thickness.

[0048] [Average thickness of the third layer: 50% or more of the average thickness of the scale] The third layer of scale in the steel plate according to the embodiment of the present invention is the main layer constituting the scale of the steel plate according to the embodiment of the present invention, and by increasing the proportion of the third layer in the scale, the amount of heat absorbed during melting of the scale can be stabilized. From this perspective, the average thickness of the third layer is set to 50% or more, and preferably 60% or more, of the average thickness of the scale. On the other hand, if the proportion of the third layer in the scale is excessively large, the first and / or second layers cannot be sufficiently present, and laser cuttability deteriorates. Therefore, the average thickness of the third layer is preferably set to 85% or less, and more preferably 75% or less, of the average thickness of the scale.

[0049] [Average grain size of wustite in the third layer: 15 μm or less] The heat absorption properties of the third layer, like those of the first layer, significantly affect the heat absorption properties of the scale. The heat absorption properties of scale depend on its crystal orientation, and any deviation in the crystal orientation results in deviations in the heat absorption properties, impairing laser cutting properties. To stabilize the heat absorption properties of the third layer, the wüstite grain size in the third layer is made fine, resulting in a scale with a more diverse crystal orientation than in the case of coarse crystal grains. Furthermore, the third layer also contains a eutectoid structure of ferrite and magnetite. Since this eutectoid structure is formed using wüstite as the parent phase, making the wüstite grain size fine also results in a scale with a more diverse crystal orientation. From these perspectives, the average grain size of wüstite in the third layer is set to 15 μm or less, preferably 12 μm or less or 10 μm or less, and may be set to 8 μm or less. Although there is no particular lower limit set for the average grain size of wüstite, there is a concern that cracks may occur in the scale during the process of refining the wüstite, which may impair the adhesion of the scale and deteriorate the laser cuttability, so it is preferable to keep the average grain size of wüstite at 1 μm or more or 3 μm or more.

[0050] [Three layer thickness deviation in the plate width direction: 0.40 or less] Since the thickness of the third layer significantly affects the amount of heat absorbed in the melting of scale, it is preferable that the thickness be uniform throughout the steel plate. The deviation in the thickness of the third layer in the plate width direction is preferably 0.40 or less, more preferably 0.25 or less, and even more preferably 0.15 or less. The lower limit is not particularly limited, but for example, the deviation in the thickness of the third layer in the plate width direction may be 0.01 or more or 0.02 or more.

[0051] [Proportion of eutectoid structure of ferrite and magnetite in the third layer: 20-80%] The third layer is composed of wüstite or a layer composed of a eutectoid structure of wüstite, ferrite, and magnetite. Preferably, by setting the proportion of the eutectoid structure to 20% or more, it is possible to significantly suppress large variations in the proportion of the eutectoid structure depending on the location, thereby making it possible to further stabilize the endothermic properties of the third layer and the amount of heat absorbed during melting. More preferably, the proportion of the eutectoid structure of ferrite and magnetite in the third layer is 30% or more. On the other hand, even if the proportion of the eutectoid structure exceeds 80%, the effect of suppressing variations in the proportion of the eutectoid structure saturates, and achieving that proportion requires long periods of slow cooling in the rolling process. Therefore, from an economical point of view, it is preferable that the proportion of the eutectoid structure of ferrite and magnetite in the third layer be 80% or less, and more preferably 70% or less.

[0052] In the steel plate according to the embodiment of the present invention, the surface scale has the three layers described above, but other layers or components may be included in the scale as long as the characteristics of each layer are satisfied. For example, the scale of the steel plate according to the embodiment of the present invention may include a thin alloy element-enriched layer composed of fayalite (Fe2SiO4) or the like between the third layer and the steel plate. Furthermore, each layer in the scale may contain fine particles composed of a phase different from the phase that constitutes the respective layer, such as metallic Cu or fayalite, or wüstite in the second layer.

[0053] [Average scale thickness: 6-60μm] The average thickness of the scale in the steel plate according to the embodiment of the present invention is preferably 6 to 60 μm. By making the average thickness of the scale 6 μm or more, it is possible to ensure the thickness of each layer, and in particular to ensure a sufficient thickness of the first layer, making it relatively easy to satisfy the above-mentioned characteristics of the scale. The average thickness of the scale may be 10 μm or more, 15 μm or more, or 20 μm or more. On the other hand, from the viewpoint of ensuring sufficient adhesion of the scale and further improving laser cuttability, the average thickness of the scale is preferably a moderate thickness, for example, preferably 60 μm or less, and may be 50 μm or less, 40 μm or less, or 30 μm or less.

[0054] The scale characteristics described above were evaluated by crystal orientation analysis and microstructural observation of the scale cross section. Specifically, small pieces were cut from each of the 1 / 4, 1 / 2, and 3 / 4 widths of the steel plate. The cross sections parallel to the rolling direction and perpendicular to the plate surface were used as the observation surfaces. These were then wet-polished and polished with colloidal silica to a mirror finish. The section from the outermost surface of the steel plate to the scale / steel plate interface was then observed using a field emission scanning electron microscope (FE-SEM) and analyzed using a crystal orientation analyzer equipped with the FE-SEM, using electron backscattering diffraction (EBSD). Crystal orientation analysis using EBSD revealed the distribution of hematite, magnetite, and wüstite within the scale, as shown in Figure 2. Furthermore, wüstite grains could be identified, as shown in Figure 3. Furthermore, FE-SEM observation revealed the distribution of the eutectoid structure of ferrite and magnetite.

[0055] For each observation sample, the surface layer on the observation surface was evaluated within a 100 μm long range parallel to the rolling direction. The thickness of the scale and each layer within the scale was determined by drawing five lines perpendicular to the plate surface of each observation sample, evaluating the length of each line segment corresponding to each layer on each line, and calculating the simple average of the values ​​along the five lines to determine the thickness of each layer on the scale and each layer within the scale for that sample. The average thickness of the scale and each layer within the scale for each steel plate was calculated by calculating the simple average of the thicknesses of three samples cut from the 1 / 4, 1 / 2, and 3 / 4 widths. The deviation of the third layer thickness in the plate width direction was calculated by dividing the difference between the maximum and minimum third layer thickness values ​​for the three samples by the average third layer thickness for the three samples.

[0056] The surface coverage of the first layer is obtained by reading the length of the outermost surface of the scale and the total length of the area where the hematite phase exists on the outermost surface of the scale from the phase distribution map obtained by EBSD, and dividing the latter by the former. The average grain size of the wüstite phase in the third layer is determined by taking the simple average of the circle-equivalent diameters of any 10 wüstite crystal grains read from the phase distribution map obtained by EBSD, and taking the simple average of the wüstite grain sizes in the three samples as the average wüstite grain size.

[0057] The proportion of the eutectoid structure of ferrite and magnetite in the third layer was determined by observing a scale of 100 μm in length parallel to the rolling direction using an FE-SEM, drawing five arbitrary lines parallel to the plate thickness direction on the photograph, evaluating the length of the line segment occupied by the eutectoid structure on each line to determine the thickness of the eutectoid structure, averaging the thicknesses of the eutectoid structure in the three samples to determine the average thickness of the eutectoid structure, and dividing this by the average thickness of the third layer.

[0058] 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.

[0059] 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.

[0060] A method for manufacturing a steel plate according to an embodiment of the present invention includes: A step of heating a slab, in which the slab is heated to a maximum heating temperature at which the surface temperature of the slab is 1050 to 1300°C, and the elapsed time from when the temperature exceeds 1050°C until the completion of the heating step is controlled so as to satisfy the following formula (1): a hot rolling step of hot rolling the slab, the hot rolling step including rolling the slab at a cumulative reduction of 15 to 30% in a temperature range of 1000 to 1100°C such that the surface temperature of the slab is in the range of 1000 to 1100°C, followed by high-pressure water descaling in a temperature range of 1000 to 1100°C such that the surface temperature of the obtained rolled material is in the range of 1000 to 1100°C, and further rolling in a temperature range of 1000°C or less to a cumulative reduction of 30% or more compared to the plate thickness at the time when the temperature reaches 1000°C, the hot rolling step including two or more rolling passes, the reduction of each rolling pass being less than 30%, the reduction of at least one rolling pass being 5% or more, and the rolling temperature of the final rolling pass being 800 to 950°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 (2), and the water cooling stop temperature is set to 500 to 600 ° C. The present invention is characterized in that it includes: 1.0≦x 10 ≦10.0...Equation (1) x1=D1·D2·(T1 3 +D3T1 2+D4T1+D5)·(T1-D6) 0.5 ·{1-exp(D7T1+D8)} 0.5 Δt 0.5 t n =x n 2 D1 -2 D2 -2 ·(T n+1 3 +D3T n+1 2 +D4T n+1 +D5) -2 ·(T n+1 -875) -1 ·{1-exp(D6T n+1 +D7)} -1 x n =D1·D2·(T n 3 +D3T n 2 +D4T n +D5)·(T n -D6) 0.5 ·{1-exp(D7T n +D8)} 0.5 (t n-1 +Δt ) 0.5 D1=(1-0.850[C]-0.052[Si]-0.026[Mn]-0.065[Al]) 0.5 x n is an index that represents the degree of scale growth after the time elapsed from when the surface temperature of the slab exceeds 1050°C during the heating process until the heating process is completed, and n indicates that the calculation corresponds to the nth of the 10 equal divisions. D1 takes into account the influence of the chemical composition of the slab, and [C], [Si], [Mn], and [Al] in the above formula are the contents [mass%] of each element in the slab. D2, D3, D4, D5, D6, D7 and D8 are constants, each equal to 8.66 × 10 -10 , -3.99×10 3 , 5.36 x 10 6 , -2.32 × 10 9, 8.75 x 10 2 , -3.50×10 -3 and 3.06 × 10 0 and T n is the average slab temperature [℃] in the nth region of the 10 equally divided sections, Δt is 1 / 10 of the elapsed time [seconds], x 10 is obtained by calculating x1, x2, x3, etc. in order using the above formula, 1.0≦y 10 ≦10.0...Equation (2) y1=E1·(1+E2·Mn)·exp{E3J1+E4 / (J1+E5)}·Δk 0.5 k n =y n 2 E1 -2 (1+E2 Mn) -2 ·exp{-2·E3J n+1 -2·E4 / (J n+1 +E5)} y n =E1·(1+E2·Mn)·exp{E3J n +E4 / (J n +E5)}·(k n-1 +Δk) 0.5 y n is an index that represents the degree of scale growth after the elapsed time from the completion of the hot rolling process to the start of water cooling is divided into 10 equal sections in the cooling process, and n indicates that the calculation corresponds to the nth section out of 10 equal sections. E1, E2, E3, E4, and E5 are constants, each equal to 5.00 × 10 7 , 1.24×10 -1 , -9.56×10 -3 , -1.05×10 4 and 2.73 × 10 2 and J n is the average steel plate temperature [°C] in the nth region of the 10 equally divided sections, Δk is one-tenth of the elapsed time [seconds], y 10can be obtained by calculating y1, y2, y3, etc. in order using the above formula.

[0061] [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.

[0062] [Heating process] The produced slab is subjected to a heat treatment with a maximum heating temperature in the range of 1050 to 1300°C for hot rolling. This heat treatment promotes scale growth on the slab surface, generating voids and cracks within the scale, making it more likely to peel off. This results in a slab with scale that peels off uniformly in the descaling process described below. As a result, subsequent scale formation is made uniform, and the desired scale structure is ultimately achieved, stabilizing the heat absorption of heat input by laser irradiation and making it possible to sufficiently suppress or prevent the occurrence of burning and other problems. If the heating temperature is below 1050°C, scale growth is insufficient, resulting in a slab with scale that is difficult to peel off. Excessive scale remains after descaling, resulting in inhomogeneous subsequent scale formation and increased thickness deviation of the third layer. As a result, heat absorption during laser irradiation becomes unstable, making it impossible to sufficiently suppress or prevent the occurrence of burning and other problems, resulting in impaired laser cuttability. On the other hand, if the heating temperature exceeds 1300°C, the unevenness of the scale / steel sheet interface in the slab becomes excessively large, the degree of scale remaining after descaling becomes non-uniform, the subsequent scale formation becomes non-uniform, and the laser cuttability is impaired. From the above viewpoints, the maximum heating temperature of the slab is set to 1050 to 1300°C, and preferably 1080 to 1250°C.

[0063] The growth of scale on the slab surface during the heating process is greatly affected not only by the heating temperature but also by the heating time. Therefore, heating is performed so that the elapsed time from when the temperature exceeds 1050°C until the end of the heating process satisfies the following formula (1). Here, x in formula (1) 10 is an index that represents the degree of scale growth on the slab surface, taking into account the change in the influence of the temperature on scale growth over time during the heating process, which is calculated using the calculations described below. If the elapsed time is too short, x 10 If x is less than 1.0, the growth of scale is insufficient, resulting in a slab with scale that is difficult to peel off, and excessive scale remains after descaling, which causes subsequent scale formation to be non-uniform and impairs laser cutting properties. 10 x is set to 1.0 or more. In order to reduce the adhesion of scale to the slab and make it easier to remove the scale by descaling, 10 is preferably 1.5 or more, and more preferably 2.0 or more. On the other hand, if the elapsed time is too long, the scale growth causes excessively large irregularities at the scale / steel sheet interface in the slab, and the degree of remaining scale after descaling becomes non-uniform, which causes subsequent scale formation to become non-uniform and impairs laser cuttability. For this reason, x 10 is limited to 10.0 or less, preferably 9.0 or less, and more preferably 8.0 or less. 1.0≦x 10 ≦10.0...Equation (1) x1=D1·D2·(T1 3 +D3T1 2 +D4T1+D5)·(T1-D6) 0.5 ·{1-exp(D7T1+D8)} 0.5 Δt 0.5 t n =x n 2 D1 -2 D2 -2 ·(T n+1 3 +D3T n+1 2 +D4T n+1 +D5) -2 ·(T n+1-875) -1 ·{1-exp(D6T n+1 +D7)} -1 x n =D1·D2·(T n 3 +D3T n 2 +D4T n +D5)·(T n -D6) 0.5 ·{1-exp(D7T n +D8)} 0.5 (t n-1 +Δt ) 0.5 D1=(1-0.850[C]-0.052[Si]-0.026[Mn]-0.065[Al]) 0.5

[0064] These calculations were carried out by dividing the time elapsed from when the surface temperature of the slab (also called the slab temperature) exceeded 1050°C during the heating process until the end of the heating process into 10 equal parts, and calculating the degree of scale growth (x n ), where the subscript n indicates that the calculation is for the nth interval of 10 equal divisions. D1 is a term that takes into account the influence of the chemical composition of the slab, and is the value calculated using the above formula, where [C], [Si], [Mn], and [Al] are the contents [mass%] of each element in the slab. D2, D3, D4, D5, D6, D7, and D8 are constants, each of which is 8.66×10 -10 , -3.99×10 3 , 5.36 x 10 6 , -2.32 × 10 9 , 8.75 x 10 2 , -3.50×10 -3 and 3.06 × 10 0 T n is the average slab temperature [°C] in the nth region of the 10-divided section, that is, the arithmetic mean of the temperature measurements taken every predetermined time, for example, every 10 seconds, in the entire nth region. Δt is one-tenth of the elapsed time [seconds]. x in Equation (1) 10can be obtained by calculating x1, x2, x3, etc. in order using the above formula.

[0065] [Hot rolling process] The surface of the heat-treated slab has heterogeneous scale that is prone to peeling. Therefore, by performing hot rolling and high-pressure water descaling to remove most of the heterogeneous scale and making the amount of remaining scale uniform across the entire surface of the steel plate, the subsequent scale formation behavior can be homogenized, and then appropriate rolling and cooling treatments can be performed to obtain the scale structure of the steel plate according to the embodiment of the present invention.

[0066] To thoroughly remove scale by descaling, hot rolling is performed prior to descaling to crush the scale. The rolling prior to descaling is performed in one or more stages at a surface temperature of 1000 to 1100°C, with a cumulative reduction of 15 to 30% relative to the slab thickness. That is, when rolling prior to descaling is performed in multiple stages, the cumulative reduction calculated based on the plate thickness after the multiple stages is 15 to 30% relative to the slab thickness. If the slab temperature is below 1000°C, the scale is excessively crushed, resulting in non-uniform scale remaining after descaling, which leads to non-uniform subsequent scale formation behavior and degraded laser cuttability. On the other hand, if the slab temperature exceeds 1100°C, the scale is not crushed sufficiently, resulting in non-uniform scale remaining after descaling, which leads to non-uniform subsequent scale formation behavior and degraded laser cuttability.

[0067] Furthermore, if the cumulative reduction rate of the hot rolling is less than 15%, the scale is not crushed sufficiently, and the scale remains non-uniformly after descaling, resulting in non-uniform behavior of subsequent scale formation and degraded laser cuttability.On the other hand, if the cumulative reduction rate of the hot rolling exceeds 30%, the scale is crushed excessively, and excessive scale is removed by descaling, resulting in non-uniform behavior of subsequent scale formation and degraded laser cuttability.

[0068] After the hot rolling, the obtained rolled material is subjected to descaling using high-pressure water. Descaling can be performed using high-pressure water with a collision pressure of 10 to 15 MPa, for example. If the temperature at which descaling is performed (surface temperature of the rolled material) is below 1000°C, some of the unevenly grown scale will remain, resulting in uneven scale formation thereafter. On the other hand, if the temperature at which descaling is performed exceeds 1100°C, excessive scale will be removed by descaling, resulting in uneven scale formation thereafter. From the above perspective, the temperature at which descaling is performed after the hot rolling is limited to the range of 1000 to 1100°C.

[0069] After the descaling, wüstite forms in the surface layer of the steel sheet. To refine the grain size, rolling is performed in a temperature range of 1000°C or less at a cumulative reduction of 30% or more compared to the sheet thickness at the time when 1000°C is reached. More specifically, rolling is performed so that the cumulative reduction, calculated by the sheet thickness after full rolling in a temperature range of 1000°C or less relative to the sheet thickness at the time when 1000°C is reached, is 30% or more. Here, if rolling is performed in one pass at a reduction of 30% or more, cracks will occur in the scale, oxygen will be supplied to the scale / steel sheet interface through the cracks, and a large amount of magnetite will be generated, causing the second layer to become excessively thick and impairing laser cuttability. Rolling is performed in a multi-stage rolling manner in two or more passes, and preferably in four or more passes.

[0070] On the other hand, if the reduction ratios in all rolling passes are all less than 5% compared to the plate thickness before each rolling pass, sufficient strain will not be imparted to the wüstite, causing the wüstite to coarsen and impairing laser cuttability; therefore, the reduction ratio in at least one rolling pass must be 5% or more, and it is preferable to perform rolling with a reduction ratio of 5% or more four or more times.

[0071] Furthermore, if the cumulative reduction is less than 30% relative to the sheet thickness at the time of reaching 1000°C, sufficient strain is not imparted to the wüstite, causing the wüstite to coarsen and impairing laser cuttability. Therefore, the cumulative reduction is set to 30% or more. To sufficiently refine the wüstite, the cumulative reduction is preferably 40% or more, and more preferably 50% or more. On the other hand, while there is no particular upper limit for the cumulative reduction, if the cumulative reduction exceeds 95%, cracks will form in the scale, oxygen will be supplied to the scale / steel sheet interface through the cracks, a large amount of magnetite will be formed, the second layer will become excessively thick, and laser cuttability may be impaired, which is undesirable. Furthermore, since sufficient strain is not imparted to the wüstite during rolling in a temperature range above 1000°C, this cumulative reduction is not taken into consideration.

[0072] Of the rolling passes performed in multiple passes, the rolling temperature in the final rolling pass is set to 800 to 950°C. If rolling is performed at a temperature below 800°C, cracks will occur in the scale, oxygen will be supplied to the scale / steel sheet interface through the cracks, a large amount of magnetite will be generated, the second layer will become excessively thick, and laser cuttability will be impaired. On the other hand, if rolling is completed in a temperature range above 950°C, wüstite will grow and coarsen, impairing laser cuttability. From the above perspectives, the rolling temperature in the final rolling pass is set to 800 to 950°C, and preferably 830 to 920°C.

[0073] In order to suppress excessive growth of scale and improve the appearance quality, descaling may be performed one or more times between the rolling performed following the descaling and the rolling in the final rolling pass.

[0074] [Cooling process] The time elapsed from the completion of the hot rolling process to the start of water cooling is controlled to form hematite and magnetite on the surface of the scale, thereby forming the scale structure in the steel plate according to the embodiment of the present invention. Since the rate at which these phases form differs depending on the temperature, the time elapsed from the completion of the hot rolling process to the start of water cooling is controlled by Equation (2). Here, if the time elapsed from the completion of the hot rolling process to the start of water cooling is too short, y 10If y is less than 1.0, the growth of the scale is excessively suppressed, the formation of hematite and / or magnetite on the scale surface becomes insufficient, and the first and / or second layers cannot be obtained with sufficient thickness, impairing the laser cutting properties. 10 On the other hand, if the time elapsed from the completion of the hot rolling process to the start of water cooling is too long, y 10 If y exceeds 10.0, excessive oxygen is supplied to the scale, causing excessive growth of the first and / or second layers, impairing laser cutting properties. 10 To improve the laser cutting properties by adjusting the scale structure, y 10 is preferably 2.0 or more and 8.0 or less, and more preferably 3.0 or more and 7.0 or less. 1.00≦y 10 ≦10.00...Equation (2) where y n is calculated as follows: y1=E1·(1+E2·Mn)·exp{E3J1+E4 / (J1+E5)}·Δk 0.5 k n =y n 2 E1 -2 (1+E2 Mn) -2 ·exp{-2·E3J n+1 -2·E4 / (J n+1 +E5)} y n =E1·(1+E2·Mn)·exp{E3J n +E4 / (J n +E5)}·(k n-1 +Δk) 0.5

[0075] These calculations were carried out by dividing the time elapsed from the completion of the hot rolling process to the start of water cooling into 10 equal parts, and calculating the degree of scale growth (y n ), where the subscript n indicates that the calculation is the nth of 10 equal intervals. E1, E2, E3, E4, and E5 are constants, each of which is 5.00×10 7 , 1.24×10 -1, -9.56×10 -3 , -1.05×10 4 and 2.73 × 10 2 J n is the average steel sheet temperature [°C] in the nth region of the 10 equally divided sections, that is, the arithmetic mean of the temperature measurements at each predetermined time in the entire nth region. Δk is one-tenth of the elapsed time [seconds]. y in Equation (3) 10 can be obtained by calculating y1, y2, y3, etc. in order using the above formula.

[0076] The water cooling is stopped when the steel sheet temperature reaches 500 to 600°C. If this water cooling stop temperature is too high, some of the hematite on the scale surface will change to magnetite, reducing the surface coverage of the first layer on the scale surface and impairing laser cuttability. On the other hand, if the water cooling stop temperature is too low, cracks will occur in the scale due to thermal stress during water cooling, and after water cooling is stopped, oxygen will penetrate along the cracks, causing the second layer to grow excessively and impairing laser cuttability. From the above perspectives, the water cooling stop temperature is preferably set to 515 to 585°C.

[0077] Furthermore, after water cooling is completed, it is preferable to limit the average cooling rate in the temperature range of 400 to 500 ° C to 0.10 to 10.0 ° C / min in order to moderately promote the eutectoid transformation in the third layer. If the average cooling rate is excessively small, the growth of magnetite proceeds in parallel with the eutectoid transformation, eroding hematite and thinning the first layer, impairing laser cuttability. On the other hand, if the average cooling rate is excessively large, the eutectoid transformation does not proceed sufficiently, a eutectoid structure is partially formed, the scale becomes inhomogeneous, and laser cuttability is impaired.

[0078] In the manufacturing method of a steel plate according to an embodiment of the present invention, the cooling conditions for the steel plate at 400°C or below 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, it may be subjected to a tempering treatment within a range that does not impair the characteristics of the steel plate according to an embodiment of the present invention.

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

[0080] The thick steel plate manufactured by the method for manufacturing a thick steel plate according to an embodiment of the present invention includes a steel plate and scale formed on the surface of the steel plate, the scale having, in order from the surface, a first layer made of hematite, a second layer made of magnetite, and a third layer made of wüstite or a eutectoid structure of wüstite and ferrite and magnetite, the average thickness of the first layer being 0.5 to 5.0 μm, the surface coverage of the steel plate by the first layer being 50% or more, the average thickness of the second layer being 10 to 40% of the average thickness of the scale, the average thickness of the third layer being 50% or more of the average thickness of the scale, the deviation in thickness of the third layer in the plate width direction being 0.40 or less, and the average grain size of the wüstite in the third layer being 15 μm or less, thereby stabilizing the heat absorption of heat input by laser irradiation, thereby suppressing or preventing the occurrence of burning in laser cutting operations and enabling stable cutting into any shape, and the steel plate can be used for structures such as land development, architecture, industrial machinery, and bridges.

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

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

[0083] First, using a slab having the chemical composition shown in Table 1, a heating 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. In particular, experimental examples 2, 5, 13, 19, 31, 32, 37, 42, and 67 are experimental examples in which the rolled material was reheated by induction heating between the completion of heating and the time of descaling, thereby controlling the temperature of the rolled material.

[0084] [Table 1]

[0085] [Table 2-1]

[0086] [Table 2-2]

[0087] [Evaluation by laser cutting] The laser cuttability of the resulting steel plates was evaluated based on the presence or absence of burning during laser cutting tests. First, a 500 mm × 500 mm steel plate was cut into 100 mm × 100 mm pieces using a CO2 laser irradiated under the following conditions. The same cutting test was then performed three times. Those that did not experience burning were marked with a "◎", those that experienced burning once were marked with a "○", those that experienced burning twice were marked with a "△", and those that experienced burning all three times were marked with an "×". Steel plates that achieved a "◎" or "○" were judged to have reduced or prevented burning during laser cutting and therefore had excellent laser cuttability. The thickness of the steel plates used for cutting was 16 mm. In the experimental examples, steel plates with thicknesses exceeding 16 mm were ground on the side opposite the laser irradiated surface to reduce the thickness of the cut portion to 16 mm. In the experimental examples, thick steel plates with a thickness of less than 16 mm were ground on the side opposite to the side to be irradiated with the laser, and then stacked with the same thick steel plate that had also had its surface ground so that the ground surfaces overlapped, and then cut so that the total thickness of the plates was 16 mm. The results are shown in Table 3. Laser power: 3500W Frequency: 500Hz Duty: 75% Assist gas pressure: 15MPa Cutting speed: 750mm / min

[0088] [Table 3-1]

[0089] [Table 3-2]

[0090] Among the experimental examples listed in Tables 1 to 3, experimental example 10 is a comparative example in which the maximum heating temperature of the slab in the heating process was low and the thickness of the third layer of scale was non-uniform, i.e., the deviation in the thickness of the third layer in the plate width direction was large, resulting in poor laser cuttability. On the other hand, experimental example 29 is a comparative example in which the maximum heating temperature of the slab in the heating process was high and the thickness of the third layer of scale was similarly non-uniform, resulting in poor laser cuttability. Experimental Example 23 is a comparative example in which the heating time in the heating step was short, formula (1) was not satisfied, the thickness of the third layer of scale was non-uniform, and the laser cuttability was poor. On the other hand, Experimental Example 50 is a comparative example in which the heating time in the heating step was long, formula (1) was not satisfied, the thickness of the third layer of scale was non-uniform, and the laser cuttability was poor. Experimental Example 13 is a comparative example in which the temperature at which hot rolling was performed before descaling was low, resulting in an inhomogeneous thickness of the third layer of scale, and thus inferior laser cuttability. Experimental Example 56 is a comparative example in which the temperature at which hot rolling was performed before descaling was high, resulting in an inhomogeneous thickness of the third layer of scale, and thus inferior laser cuttability. Experimental Example 28 is a comparative example in which the cumulative reduction ratio of the hot rolling performed before descaling was small, the thickness of the third layer of scale was non-uniform, and the laser cuttability was poor. Experimental Example 22 is a comparative example in which the cumulative reduction ratio of the hot rolling performed before descaling was large, the thickness of the third layer of scale was non-uniform, and the laser cuttability was poor. Experimental Example 57 is a comparative example in which the descaling temperature was low, the thickness of the third layer of scale was non-uniform, and the laser cuttability was poor. On the other hand, Experimental Example 5 is a comparative example in which the descaling temperature was high, the thickness of the third layer of scale was non-uniform, and the laser cuttability was poor. Experimental Example 8 is a comparative example in which the maximum reduction ratio in the rolling performed at 1000°C or less was small, causing the wüstite in the third layer of scale to become coarse, resulting in poor laser cuttability. Experimental Example 16 is a comparative example in which the maximum reduction ratio in the rolling performed at 1000°C or less was large, causing the thicknesses of the second and third layers of scale to fall outside the range of the present invention, resulting in poor laser cuttability. Experimental Example 61 is a comparative example in which the cumulative reduction rate of the rolling performed at 1000° C. or less was small, causing the wustite in the third layer of the scale to become coarse, resulting in poor laser cuttability. Experimental Example 4 is a comparative example in which the rolling temperature in the final rolling pass was low, resulting in a thick second layer of scale and poor laser cuttability, while Experimental Example 60 is a comparative example in which the rolling temperature in the final rolling pass was high, resulting in coarsening of the wüstite in the third layer of scale and poor laser cuttability. Experimental Examples 21 and 74 are comparative examples in which the time elapsed from the completion of the hot rolling process to the start of water cooling was short, so that formula (2) was not satisfied, and the first or second scale layer was not sufficiently formed, resulting in poor laser cuttability. On the other hand, Experimental Examples 3 and 53 are comparative examples in which the time elapsed from the completion of the hot rolling process to the start of water cooling was long, so that formula (2) was not satisfied, resulting in the first and / or second scale layer becoming too thick, resulting in poor laser cuttability. Experimental Example 75 is a comparative example in which the water cooling stop temperature was low, the second layer of scale was excessively thick, and the laser cuttability was poor. Experimental Example 9 is a comparative example in which the water cooling stop temperature was high, the first layer of scale had a small scale surface coverage, and the laser cuttability was poor. Experimental Example 82 is a comparative example in which the Si content of the steel sheet was excessive, the thickness of the third layer of the scale was non-uniform, and the laser cuttability was poor.

[0091] The experimental examples excluding the above-mentioned comparative examples, i.e., experimental examples 1, 2, 6, 7, 11, 12, 14, 15, 17-20, 24-27, 30-49, 51, 52, 54, 55, 58, 59, 62-73 and 76-81, are examples of the present invention, in which the occurrence of burning during laser cutting was suppressed or prevented, and thick steel plates with excellent laser cuttability were obtained. [Explanation of symbols]

[0092] 10 thick steel plate 11 Steel plate 12 scale 13 1st layer 14 2nd layer 15. Level 3

Claims

1. a first layer made of hematite, a second layer made of magnetite, and a third layer made of wustite or a eutectoid structure of wustite, ferrite, and magnetite, in that order from the surface; an average thickness of the first layer being 0.5 to 5.0 μm; a surface coverage rate of the steel plate by the first layer being 50% or more; an average thickness of the second layer being 10 to 40% of the average thickness of the scale; an average thickness of the third layer being 50% or more of the average thickness of the scale; a deviation in thickness of the third layer in the plate width direction being 0.40 or less; and an average grain size of wustite in the third layer being 15 μm 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: 0.01-1.00%, Mn: 0.10 to 2.50%, P: 0.001-0.050%, S: 0.0001-0.0100%, Al: 0.001-0.200%, N: 0.0150% or less, O: 0.0050% or less, Cu: 0 to 1.00%, Ni: 0-2.00%, Cr: 0-1.00%, Mo: 0-1.00%, W: 0-0.50%, Nb: 0 to 0.500%, Ti: 0 to 0.500%, V: 0-1.000%, B: 0 to 0.0100%, Sn: 0-0.500%, Sb: 0 to 0.500%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Hf: 0-0.0100%, Te: 0 to 0.0100%, Sr: 0 to 0.0100%, REM: 0 to 0.0100%, and The steel plate according to claim 1 or 2, having a chemical composition consisting of the balance being Fe and impurities.

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

5. The steel plate according to any one of claims 1 to 4, wherein the proportion of the eutectoid structure in the third layer is 20 to 80%.

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 is 1050 to 1300 ° C., and the elapsed time from when the temperature exceeds 1050 ° C. to when the heating step is completed is controlled so as to satisfy the following formula (1): a hot rolling step of hot rolling the slab, the hot rolling step including rolling the slab at a cumulative reduction of 15 to 30% in a temperature range of 1000 to 1100°C such that the surface temperature of the slab is in the temperature range of 1000 to 1100°C, followed by high-pressure water descaling in a temperature range of 1000 to 1100°C such that the surface temperature of the obtained rolled material is in the temperature range of 1000 to 1100°C, and further rolling in a temperature range of 1000°C or less such that the cumulative reduction is 30% or more compared to the plate thickness at the time when 1000°C is reached, the hot rolling step including two or more rolling passes, the reduction of each rolling pass being less than 30%, the reduction of at least one rolling pass being 5% or more, and the rolling temperature of the final rolling pass being 800 to 950°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 to satisfy the following formula (2), and the water cooling stop temperature is set to 500 to 600 ° C. The method for producing a steel plate according to any one of claims 1 to 5, comprising: 1.0≦x 10 ≦10.0 ・・・Form (1) x 1 =D 1 ・D 2 ・(T 1 3 +D 3 T 1 2 +D 4 T 1 +D 5 )・(T 1 -D 6 ) 0.5 ・{1-exp(D 7 T 1 +D 8 )} 0.5 ・Δt 0.5 t n =x n 2 ・D 1 -2 ・D 2 -2 ・(T n+1 3 +D 3 T n+1 2 +D 4 T n+1 +D 5 ) -2 ・(T n+1 -875) -1 ・{1-exp(D 6 T n+1 +D 7 )} -1 x n =D 1 ・D 2 ・(T n 3 +D 3 T n 2 +D 4 T n +D 5 )・(T n -D 6 ) 0.5 ・{1-exp(D 7 T n +D 8 )} 0.5 ・(t n-1 +Δt) 0.5 D 1 =(1-0.850[C]-0.052[Si]-0.026[Mn]-0.065[Al]) 0.5 x n is an index representing the degree of scale growth after the time elapsed from when the surface temperature of the slab exceeds 1050°C during the heating process until the completion of the heating process is divided into 10 equal sections, and n indicates that the calculation corresponds to the nth section out of the 10 equal sections. D 1 is a formula taking into consideration the influence of the chemical composition of the slab, and [C], [Si], [Mn], and [Al] in the above formula are the contents [mass%] of each element in the slab, D 2 , D 3 , D 4 , D 5 , D 6 , D 7 and D 8 are constants, and are 8.66×10 -10 , −3.99×10 3 , 5.36 x 10 6 , −2.32 × 10 9 , 8.75 x 10 2 , −3.50×10 -3 and 3.06 × 10 0 and T n is the average slab temperature [°C] in the nth region of the 10 equally divided sections, Δt is 1 / 10 of the elapsed time [seconds], x 10 is calculated by the above formula: 1 From x 2 , x 3 ...and so on, and 1.0≦y 10 ≦10.0 ・・・Form (2) y 1 =E 1 ・(1+E 2 ・Mn)・exp{E 3 J 1 +E 4 / (J 1 +E 5 )}・Δk 0.5 k n =y n 2 ・E 1 -2 ・(1+E 2 ・Mn) -2 ・ex{-2・E 3 J n+1 -2・E 4 / (J n+1 +E 5 )} y n =E 1 ・(1+E 2 ・Mn)・exp{E 3 J n +E 4 / (J n +E 5 )}・(k n-1 +Δk) 0.5 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 , E 4 and E 5 are constants, each of which is 5.00 × 10 7 , 1.24 × 10 -1 , −9.56×10 -3 , −1.05×10 4 and 2.73 x 10 2 and J n is the average steel sheet temperature [°C] in the n-th region of the 10 equally divided sections, Δk is 1 / 10 of the elapsed time [seconds], y 10 According to the above formula, y 1 From y 2 , y 3 ...and so on, and can be calculated in this order.

7. The method for producing a steel plate according to claim 6, wherein after water cooling is stopped, the average cooling rate in the temperature range of 400 to 500°C is 0.10 to 10.0°C / min.

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