JP7780082B2 - Heavy steel plate and its manufacturing method - Google Patents

Description

The present invention relates to thick steel plates and their manufacturing methods, and more specifically to thick steel plates that are laser cut and used to form structures, and their manufacturing methods.

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

In addition to conventional gas cutting, laser cutting and plasma cutting are known methods for cutting steel plates. Compared to conventional gas cutting, laser cutting has superior cut surface accuracy, produces a smaller heat-affected zone, and allows for automation to reduce labor costs, making it popular 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 large steel structures mentioned above.

The manufacturing of steel plates, such as thick steel plates, generally involves hot-rolling slabs. It is known that hot-rolled steel plates oxidize in the atmosphere, forming scale (iron oxide) on their surfaces. Laser cutting of steel plates involves irradiating the plate with a laser, which heats it and melts it to cut it. Therefore, the melting conditions vary significantly depending on the heat absorption properties of the area irradiated by the laser. Because the heat absorption properties of steel plates vary greatly depending on the condition of the surface scale, depending on the condition, the steel plate may not be cut reliably or may have abnormal cuts with gouges on the cut surface, 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 causing molten material to erupt from the cut area, a phenomenon known as burning.

As a method for improving laser cuttability, for example, Patent Document 1 proposes a steel material in which the surface of the steel sheet is coated with a dry coating 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. Patent Document 1 also teaches that laser cuttability can be improved by adding highly laser-absorbent titania powder to the coating of the coated steel material to increase the laser absorption rate. Patent Document 2 also proposes a steel material in which the surface is coated with a coating composition containing an alkoxysilane having two or more alkoxy groups and/or its hydrolysate or condensate (e.g., tetraalkoxysilane), zinc powder, and aluminum phosphate (preferably aluminum tripolyphosphate) powder or a mixed powder of aluminum phosphate and zinc phosphate.

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.

Patent Document 4 proposes a technology in which, when cooling a hot-rolled slab after hot rolling, 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 scale adhesion and 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 forming a structure in which the scale on the steel sheet surface consists of a layer made of a magnetite phase and a layer made of a wüstite phase and a granular magnetite phase, thereby improving scale adhesion and laser cuttability.

Patent Document 6 proposes a technology that improves scale adhesion and laser cuttability by forming a concentrated layer of alloying elements at the interface between the scale and base steel of a thick steel plate. Furthermore, Patent Document 7 proposes a technology that improves scale adhesion and laser cuttability by setting the magnetite phase fraction in the scale to 50% or more.

International Publication No. 2013/065349 Japanese Patent Application Laid-Open No. 2008-156377 Japanese Patent Application Laid-Open No. 2003-221640 Japanese Patent Application Publication No. 10-158734 Japanese Patent Application Laid-Open No. 2020-114938 Japanese Patent Application Laid-Open No. 2002-332541 Japanese Patent Application Laid-Open No. 2008-095155

In order to reduce costs in a society with a declining population, there is an increasing demand for automation in laser cutting of steel plates. At the same time, steel plates with even better laser cuttability than before are needed to cope with the increasing thickness of the objects to be cut due to the larger size of structures and the faster 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. With laser cutting, the narrow cutting width can sometimes cause the molten material to become clogged within the cutting width. This can cause excessive melting of the surrounding area or the molten material to spray upward, resulting in a cutting defect known as burning. Burning can be a major issue, especially with high-power laser irradiation.

Patent Documents 1 to 7 propose improving laser cuttability by increasing the laser absorption rate or by improving the adhesion of the scale, but do not necessarily provide sufficient consideration from the perspective of suppressing or preventing the occurrence of burning. Therefore, there is still room for improvement in the laser cuttability of the steel materials described in these patent documents.

The present invention therefore aims to provide a thick steel plate and a manufacturing method thereof that suppress or prevent the occurrence of burning during laser cutting and are therefore useful for applying laser cutting.

To achieve the above objective, the inventors conducted research into the steel plate surface layer necessary for stable, continuous laser cutting. As a result, they discovered that by optimizing the scale phases and thickness of each phase in the steel plate surface layer 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, it is possible to significantly suppress or prevent the occurrence of burning, thereby improving the laser cuttability of thick steel plate.

The mechanism by which the above-mentioned scale characteristics affect laser cuttability is unknown, but it is presumed that optimizing the scale phases and their respective thicknesses stabilizes the amount of heat absorbed when the surface scale melts 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 the phase becomes finer stabilizes the heat absorption of the heat input due to laser irradiation by 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.

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 made of hematite, a second layer made of magnetite, and a third layer made of wustite or a eutectoid structure of wustite 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 of the thickness of the third layer in the plate width direction being 0.40 or less, and the average grain size of wustite in the third layer being 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 contains, 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%,
The steel plate according to (1) or (2) above, having a chemical composition consisting of: REM: 0 to 0.0100%, and the balance: 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 to 0.0100%
The steel plate according to (3) above, comprising one or more selected from the group consisting of:
(5) The steel plate according to any one of (1) to (4) above, wherein the 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;
The method for producing a thick steel plate according to any one of (1) to (5), comprising a step of cooling the obtained steel plate, 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.
1.0≦ _x10 ≦10.0...Formula (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 elapsed time from when the surface temperature of the slab exceeds 1050°C to when the heating process is completed, which is divided into 10 equal sections, and n indicates that the calculation corresponds to the nth section of the 10 equal sections.
_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,
D ₂ , D ₃ , D ₄ , D ₅ , D ₆ , D ₇ and D ₈ are constants, which are 8.66×10 ⁻¹⁰ , −3.99×10 ³ , 5.36×10 ⁶ , −2.32×10 ⁹ , 8.75×10 ² , −3.50×10 ⁻³ and 3.06×10 ⁰ , respectively;
_Tn 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],
_x10 is obtained by calculating _x1 , _x2 , _x3 , etc. in order using the above formula,
1.0≦ _y10 ≦10.0...Formula (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・exp{−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 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 of the 10 equal sections,
E ₁ , E ₂ , E ₃ , E ₄ and E ₅ are constants, which are 5.00×10 ⁷ , 1.24×10 ⁻¹ , −9.56×10 ⁻³ , −1.05×10 ⁴ and 2.73×10 ² , respectively;
_Jn is the average steel sheet temperature [°C] in the nth region of the 10 equally divided sections,
Δk is 1/10 of the elapsed time [seconds],
_y10 can be obtained by calculating _y1 , _y2 , _y3 , and so on 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.

The present invention provides a thick steel plate and a manufacturing method thereof that suppress or prevent the occurrence of burning during laser cutting and are therefore useful for applying laser cutting.

1 is a schematic diagram showing a scale structure of a thick steel plate according to an embodiment of the present invention. The distribution of hematite, magnetite, and wustite within the scale is shown by crystal orientation analysis using the EBSD method. An example of determining wustite grains by crystal orientation analysis using the EBSD method is shown.

Heavy steel plates and manufacturing methods according to embodiments of the present invention will be described in more detail below. However, these descriptions are intended to exemplify preferred forms of the present invention and are not intended to limit the present invention to specific embodiments.

[Preferred Chemical Composition]
In an embodiment 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 laser cutting. As described above, the present invention aims to provide a thick steel plate in which burning during laser cutting is suppressed or prevented and therefore useful for laser cutting. The object is achieved by providing a steel plate having, from the surface, a scale comprising, in order from the surface, a first layer consisting of hematite, a second layer consisting of magnetite, and a third layer consisting 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 plate by the first layer of 50% or more, the second layer having an average thickness of 10 to 40% of the average thickness of the scale, the third layer having an average thickness of 50% or more of the average thickness of the scale, a thickness deviation of the third layer in the plate width direction of 0.40 or less, and an average grain size of wüstite in the third layer of 15 μm or less. Therefore, it is clear that the chemical composition of the steel plate itself is not an essential technical feature for achieving the object 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.

[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.

[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.

[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, and there is a concern that the toughness of the steel plate will be significantly deteriorated. 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 a weld, and from this viewpoint, the Mn content is preferably 2.00% or less, and more preferably 1.80% or less.

[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 large burden on the smelting process and is economically undesirable. From this perspective, the P content is preferably 0.001% or more. On the other hand, P is an element that impairs toughness, and from this perspective, the P content is preferably limited to 0.050% or less. Furthermore, P is an element that impairs weldability and the toughness of welds, and from this perspective, the P content is preferably 0.030% or less, and more preferably 0.020% or less.

[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 large 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.

[Al: 0.001-0.200%]
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. On the other hand, Al is an element that impairs toughness, and the Al content is preferably limited to 0.200% or less. Furthermore, since Al also impairs weldability and the toughness of the weld, the Al content is preferably 0.120% or less, and more preferably 0.080% or less.

[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 plate, so the N content is preferably limited to 0.0150% or less. Furthermore, N is an element that impairs the toughness of a weld. From this viewpoint, the N content is preferably limited to 0.0100% or less, and more preferably to 0.0060% or less. There is no particular lower limit for the N content, but limiting it to less than 0.0003% imposes a large load on the smelting process and is not economically preferable, so it is preferably 0.0003% or more.

[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 a weld. From this viewpoint, 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% imposes a large load on the smelting process and is not economically preferable, so it is preferably 0.0002% or more.

The basic chemical composition of the steel sheet according to an 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 are described in detail below.

[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.

[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 may 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.

[Cr: 0-1.00%]
Cr is an element that contributes to improving strength. The Cr 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 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.

[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 excessively, there is a concern that the weldability and the toughness of the welded portion may be impaired. From this viewpoint, the Mo content is preferably limited to 1.00% or less, and more preferably 0.30% or less.

[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 excessively, there is a concern that the weldability and the toughness of the welded portion may be impaired. From this viewpoint, the W content is preferably limited to 0.50% or less, and more preferably 0.30% or less.

[Nb: 0 to 0.500%]
Nb is an element that contributes to improving strength. The Nb 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 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.

[Ti: 0 to 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.

[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 excessively, 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.

[B: 0 to 0.0100%]
B is an element that contributes to improving strength. The B content may be 0%, but when 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 excessively, there is a concern that the weldability and the toughness of the welded portion may be impaired. From this viewpoint, the B content is preferably limited to 0.0100% or less, and more preferably 0.0035% or less.

[Sn: 0 to 0.500%]
Sn is an element that concentrates at the interface between the steel sheet and the 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. The Sn content may be 0%, but in order to enhance the adhesion of the scale and improve laser cuttability, the Sn content is preferably 0.001% or more or 0.003% or more. To fully achieve this effect, the Sn content is preferably 0.005% or more, and more preferably 0.025% or more. On the other hand, 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 viewpoint, the Sn content is preferably 0.300% or less, and more preferably 0.200% or less.

[Sb: 0 to 0.500%]
Sb is an element that concentrates at the interface between the steel sheet and the scale and promotes the concentration of other metal elements near the interface. It promotes the formation of CuNi-enriched portions and has the effect of improving scale adhesion. 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 achieve 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 viewpoint, the Sb content is preferably 0.300% or less, and more preferably 0.200% or less.

[Ca: 0-0.0100%]
[Mg: 0 to 0.0100%]
[Hf: 0 to 0.0100%]
[Te: 0 to 0.0100%]
[Sr: 0 to 0.0100%]
[REM: 0 to 0.0100%]
Ca, Mg, Hf, Te, Sr, and REM are elements that refine sulfides and improve the toughness of steel sheets. The contents of Ca, Mg, Hf, Te, Sr, and REM may be 0%, but 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. On the other hand, 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.

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

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

Next, we will describe the scale characteristics of the steel plate according to an embodiment of the present invention.

[First layer: hematite (Fe ₂ O ₃ ), second layer: magnetite (Fe ₃ O ₄ ), third 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 , the 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 side and oxygen ( _O2 ) 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 set to be appropriate, while the grain size of wüstite, which is the main phase constituting the scale, is reduced, thereby improving the laser cuttability. Each layer will be described in more detail below.

[Average thickness of 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 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, thereby impairing laser cuttability. On the other hand, if the average thickness of the first layer is greater than 5.0 μm, excessive oxygen supply will cause numerous cracks and voids 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 receive 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.

[Surface coverage of 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 larger surface coverage is preferable, it is preferably 65% or more, more preferably 80% or more, and most preferably 100%.

[Average thickness of second layer: 10 to 40% of average thickness of scale]
In the steel plate according to the embodiment of the present invention, the average thickness of the second layer of scale 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 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 the 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 35% or less or 30% or less of the average scale thickness.

[Average thickness of the third layer: 50% or more of the average thickness of the scale]
The third layer of the scale in the steel plate according to the embodiment of the present invention is a major 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 layer and/or the second layer 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.

[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 a deviation in the heat absorption properties, impairing laser cutting properties. To stabilize the heat absorption properties of the third layer, the wüstite crystal grain size in the third layer is made fine, thereby enabling the scale to have 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 finer eutectoid structure, resulting in a scale with a 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 8 μm or less. Although no particular lower limit is 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, impairing the adhesion of the scale and deteriorating the laser cuttability, and therefore it is preferable to keep the average grain size of wüstite at 1 μm or more or 3 μm or more.

[Deviation of the thickness of the third layer in the sheet 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.

[Proportion of eutectoid structure of ferrite and magnetite in the third layer: 20 to 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, and it is possible to further stabilize the endothermic property 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.

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 contents 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 phases constituting the respective layers, such as metallic Cu or fayalite, or wustite in the second layer.

[Average scale thickness: 6 to 60 μm]
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, the thickness of each layer can be ensured, and in particular, the thickness of the first layer can be sufficiently ensured, 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.

The scale characteristics described above are evaluated through crystal orientation analysis and microstructural observation of the cross section of the scale. Specifically, small pieces are cut from each of the 1/4, 1/2, and 3/4 width locations of the thick 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 thick steel plate to the scale/steel plate interface is 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) method. Crystal orientation analysis using the EBSD method reveals the distribution of hematite, magnetite, and wüstite within the scale, as shown in Figure 2, and furthermore, makes it possible to identify wüstite crystal grains, as shown in Figure 3. Furthermore, observation using an FE-SEM reveals the distribution of the eutectoid structure of ferrite and magnetite.

For each observation sample, the surface layer on the observation surface is evaluated within a 100 μm long range parallel to the rolling direction. Five lines are drawn randomly in a direction perpendicular to the plate surface of each observation sample, and the length of the line segments corresponding to each layer on each line is evaluated. The simple average of the values on the five lines is used to determine the thickness of the scale and each layer on that sample. The average thickness of the scale and each layer on that scale for each steel plate is calculated as the simple average of the thicknesses of three samples cut from each of the 1/4, 1/2, and 3/4 width locations. Furthermore, the deviation in the thickness of the third layer in the plate width direction is calculated by dividing the difference between the maximum and minimum thicknesses of the third layer for the three samples by the average thickness of the third layer for the three samples.

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 the EBSD method, and dividing the latter by the former. Furthermore, 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 the EBSD method as the grain size of the wüstite in that sample, and then taking the simple average of the grain sizes of the wüstite in the three samples as the average grain size of the wüstite.

The proportion of the eutectoid structure of ferrite and magnetite in the third layer is determined by observing a scale of 100 μm in length parallel to the rolling direction using an FE-SEM, drawing five random 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, and determining the thickness of the eutectoid structure. The average thickness of the eutectoid structure for the three samples is then determined as the average thickness of the eutectoid structure, and this is then divided by the average thickness of the third layer.

The thick steel plate according to an embodiment of the present invention may have any thickness that allows for laser cutting, and is not particularly limited, but may have a thickness of, for example, 6 to 40 mm. 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.

Next, a preferred manufacturing method for the 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 the steel plate according to an embodiment of the present invention, and is not intended to limit the steel plate to those manufactured by the manufacturing method described below.

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. 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;
The method is characterized by including a cooling step in which the obtained steel sheet is cooled, 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.
1.0≦ _x10 ≦10.0...Formula (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 elapsed time from when the surface temperature of the slab exceeds 1050°C to when the heating process is completed, which is divided into 10 equal sections, and n indicates that the calculation corresponds to the nth section of the 10 equal sections.
_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,
D ₂ , D ₃ , D ₄ , D ₅ , D ₆ , D ₇ and D ₈ are constants, which are 8.66×10 ⁻¹⁰ , −3.99×10 ³ , 5.36×10 ⁶ , −2.32×10 ⁹ , 8.75×10 ² , −3.50×10 ⁻³ and 3.06×10 ⁰ , respectively;
_Tn 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],
_x10 is obtained by calculating _x1 , _x2 , _x3 , etc. in order using the above formula,
1.0≦ _y10 ≦10.0...Formula (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・exp{−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 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 of the 10 equal sections,
E ₁ , E ₂ , E ₃ , E ₄ and E ₅ are constants, which are 5.00×10 ⁷ , 1.24×10 ⁻¹ , −9.56×10 ⁻³ , −1.05×10 ⁴ and 2.73×10 ² , respectively;
_Jn is the average steel sheet temperature [°C] in the nth region of the 10 equally divided sections,
Δk is 1/10 of the elapsed time [seconds],
_y10 can be obtained by calculating _y1 , _y2 , _y3 , and so on in order using the above formula.

[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 purposes 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.

[Heating process]
The produced slab is subjected to a heat treatment in which the maximum heating temperature is 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, resulting 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, the scale growth is insufficient, resulting in a slab with scale that is difficult to peel off. Excessive scale remains after descaling, resulting in subsequent scale formation being non-uniform and increasing the thickness deviation of the third layer. As a result, the heat absorption during laser irradiation becomes unstable, making it impossible to sufficiently suppress or prevent the occurrence of burning and other problems, and laser cuttability is impaired. On the other hand, if the heating temperature exceeds 1300°C, the 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 formation of scale becomes non-uniform, and 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.

Since the growth of scale on the slab surface during the heating process is greatly affected by not only the heating temperature but also the heating time, heating is performed so that the elapsed time from when the temperature exceeds 1050°C until the completion of the heating process satisfies the following formula (1). Here, _x10 in formula (1) is an index that represents the degree of scale growth on the slab surface, calculated by the calculation described below, taking into account changes in the influence on scale growth over time depending on the temperature at each moment during the heating process. If the elapsed time is too short and _x10 is less than 1.0, the scale growth will be insufficient, resulting in a slab with scale that is difficult to peel off. Excessive scale will remain after descaling, resulting in uneven subsequent scale formation and impaired laser cuttability. For this reason, _x10 is set to 1.0 or greater. To reduce the adhesion of scale to the slab and facilitate scale removal by descaling, _x10 is preferably set to 1.5 or greater, and more preferably 2.0 or greater. On the other hand, if the elapsed time is too long, the growth of scale will cause excessively large irregularities at the scale/steel sheet interface in the slab, resulting in non-uniformity in the degree of scale remaining after descaling, and subsequent scale formation will be non-uniform, impairing laser cuttability. For this reason, _x10 is limited to 10.0 or less, preferably 9.0 or less, and more preferably 8.0 or less.
1.0≦ _x10 ≦10.0...Formula (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

These calculations evaluate the degree of scale growth (xn) after the completion of each interval, dividing the time elapsed from when the surface temperature of the slab (also referred to as the slab _temperature ) exceeds 1050°C during the heating process to the completion of the heating process into 10 equal sections, with the subscript n indicating that the calculation corresponds to the nth of the 10 equal sections. _D1 is a term that takes into account the influence of the chemical composition of the slab and is a 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, which are 8.66× ^10-10 , -3.99× ¹⁰³ , 5.36×106, -2.32× ¹⁰⁹ , 8.75× ¹⁰² , -3.50× ^{10-3 ,} and 3.06× ¹⁰⁰ , respectively. _Tn is the average slab temperature [°C] in the nth region of the 10-divided section, i.e., the arithmetic mean of the temperature measurements taken every predetermined time, for example, every 10 seconds, throughout the entire nth region. Δt is 1/10 of the elapsed time [seconds]. _x10 in formula (1) is obtained by calculating _x1 , _x2 , _x3 , ... in order using the above formula.

[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.

To ensure sufficient scale removal 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. In other words, if rolling prior to descaling is performed in multiple stages, the cumulative reduction, calculated based on the plate thickness after multiple stages of rolling, is 15 to 30% relative to the slab thickness. If the slab temperature falls below 1000°C, the scale is excessively crushed, resulting in a non-uniform scale remaining after descaling, which leads to non-uniform subsequent scale formation and degraded laser cuttability. On the other hand, if the slab temperature exceeds 1100°C, the scale is not crushed sufficiently, resulting in a non-uniform degree of scale remaining after descaling, which leads to non-uniform subsequent scale formation and degraded laser cuttability.

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 inhomogeneous after descaling, resulting in non-uniform scale formation behavior and poor laser cuttability. On the other hand, if the cumulative reduction rate of the hot rolling exceeds 30%, the scale will be crushed excessively, and excessive scale will be removed by descaling, resulting in non-uniform scale formation behavior and poor laser cuttability.

After the hot rolling, the resulting rolled material is descaled using high-pressure water. Descaling can be performed, for example, using high-pressure water with an impact pressure of 10 to 15 MPa. If the descaling temperature (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 descaling temperature exceeds 1100°C, excessive scale will be removed by descaling, resulting in uneven scale formation thereafter. From this perspective, the temperature at which descaling is performed after the hot rolling is limited to the range of 1000 to 1100°C.

After the descaling, wüstite forms on the surface of the steel sheet. To refine its grain size, rolling is performed at a temperature below 1000°C with a cumulative reduction of 30% or more compared to the sheet thickness at the time 1000°C is reached. More specifically, rolling is performed so that the cumulative reduction, calculated by the thickness after full rolling in a temperature range below 1000°C relative to the sheet thickness at the time 1000°C is reached, is 30% or more. Here, if rolling is performed with a reduction of 30% or more in one pass, cracks will form 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 two or more passes, preferably four or more passes.

On the other hand, if the reduction in all rolling passes is 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 in at least one rolling pass must be 5% or more, and it is preferable to perform rolling with a reduction of 5% or more four times.

Furthermore, if the cumulative reduction is less than 30% relative to the plate thickness at the time when 1000°C is reached, sufficient strain is not imparted to the wüstite, causing it 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 through the cracks to the scale/steel plate interface, 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 when rolling at temperatures above 1000°C, this cumulative reduction is not taken into consideration.

Of the multiple rolling passes, the rolling temperature in the final rolling pass should be 800 to 950°C. If rolling is performed below 800°C, 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 produced, causing the second layer to become excessively thick and impairing laser cuttability. On the other hand, if rolling is completed at a temperature above 950°C, wüstite will grow and coarsen, impairing laser cuttability. From these perspectives, the rolling temperature in the final rolling pass should be 800 to 950°C, with 830 to 920°C being preferable.

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

[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 an embodiment of the present invention. Because the formation rates of these phases vary 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, causing _y10 to be less than 1.0, scale growth is excessively suppressed, resulting in insufficient formation of hematite and/or magnetite on the scale surface, making it impossible to obtain a first layer and/or second layer of sufficient thickness, and impairing laser cuttability. Therefore, _y10 is set to be 1.0 or greater. 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, causing _y10 to exceed 10.0, excessive oxygen is supplied to the scale, causing the first layer and/or second layer to grow excessively, impairing laser cuttability. Therefore, _y10 is set to be 10.0 or less. In order to regulate the structure of the scale and improve the laser cutting properties, y ₁₀ is preferably 2.0 or more and 8.0 or less, and more preferably 3.0 or more and 7.0 or less.
1.00≦y ₁₀ ≦10.00...Formula (2)
Here, y _n can be calculated as follows:
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・exp{−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

These calculations evaluate the degree of scale growth (y _n ) after the elapsed time from the completion of the hot rolling process to the start of water cooling is divided into 10 equal sections during the cooling process, with the subscript n indicating that this calculation corresponds to the nth of the 10 equal sections. _E1 , _E2 , _E3 , _E4 , and _E5 are constants, which are 5.00 x 10 ⁷ , 1.24 x 10 ^-1 , -9.56 x 10 ^-3 , -1.05 x 10 ⁴ , and 2.73 x 10 ^{2 ,} respectively. _{J n} is the average steel sheet temperature [°C] in the nth section of the 10 equal sections, i.e., the arithmetic mean of the temperature measurements at each predetermined time throughout the entire nth section. Δk is one-tenth of the elapsed time [seconds]. _y10 in equation (3) is obtained by calculating _y1 , _y2 , _y3 , ... in order using the above formula.

The water cooling is stopped when the steel plate temperature reaches 500 to 600°C. If the water cooling stop temperature is too high, some of the hematite on the scale surface will convert 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 form 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 these perspectives, it is preferable that the water cooling stop temperature be 515 to 585°C.

Furthermore, after water cooling is complete, in order to adequately promote the eutectoid transformation in the third layer, 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. If the average cooling rate is excessively small, magnetite growth 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, resulting in partial formation of eutectoid structures and inhomogeneous scale, impairing laser cuttability.

In the manufacturing method of 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. However, excessive heat retention after water cooling may impair the toughness of the steel plate, so natural cooling or air cooling is preferable. 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 to the extent that the characteristics of the steel plate according to an embodiment of the present invention are not impaired.

The method for manufacturing steel plates according to an embodiment of the present invention may further include a flattening process using a hot leveler or the like in addition to the heating process, hot rolling process, and cooling process described above.

A steel plate manufactured by a steel plate manufacturing method according to an embodiment of the present invention includes a steel plate and scale formed on the surface of the steel plate, the scale comprising, in order from the surface, a first layer of hematite, a second layer of magnetite, and a third layer of wüstite or a eutectoid structure of wüstite, ferrite, and magnetite. The first layer has an average thickness of 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 in the thickness of the third layer in the plate width direction is 0.40 or less, and the average grain size of the wüstite in the third layer is 15 μm or less. This stabilizes the heat absorption of heat input by laser irradiation, thereby suppressing or preventing burning during laser cutting operations and enabling stable cutting into any shape, making the steel plate suitable for use in structures such as construction, architecture, industrial machinery, and bridges.

The present invention will be described in more detail below using examples. The conditions in the examples are merely examples used to confirm the feasibility and effectiveness of the present invention. The present invention is not limited to these example conditions. Various conditions may be used in the present invention as long as they do not deviate from the gist of the present invention and achieve the objectives of the present invention.

In the following examples, thick steel plates according to embodiments of the present invention were manufactured under various conditions, and the occurrence of burning when the resulting thick steel plates were laser cut was investigated.

First, using a slab having the chemical composition shown in Table 1, the heating process, hot rolling process, and 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 rolled material temperature.

[Evaluation by laser cutting]
The laser cuttability of the obtained steel plates was evaluated as follows based on the presence or absence of burning during laser cutting tests. First, a steel plate cut to dimensions of 500 mm x 500 mm was laser-cut into 100 mm x 100 mm pieces using a _CO2 laser irradiated under the following conditions. The same cutting was then performed three times, and those that did not experience burning were marked with an "◎", those that experienced burning once were marked with an "○", those that experienced burning twice were marked with an "△", and those that experienced burning all three times were marked with an "×". Steel plates that received an "◎" 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 surface opposite to the surface irradiated with the laser 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: 15 MPa
Cutting speed: 750mm/min

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 step 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 sheet 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 step 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 a non-uniform thickness of the third layer of scale, and thus resulting in poor 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 a non-uniform thickness of the third layer of scale, and thus resulting in poor 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, and the first and/or second scale layer became excessively thick, resulting in poor laser cuttability.
Experimental Example 75 is a comparative example in which the water-cooling stop temperature was low, the second scale layer 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, and the scale surface coverage of the first scale layer was small, and the laser cuttability was poor.
Experimental Example 82 is a comparative example in which the Si content of the steel plate was excessive, the thickness of the third layer of scale was non-uniform, and the laser cuttability was poor.

The experimental examples excluding the above 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.

10 Thick steel plate 11 Steel plate 12 Scale 13 First layer 14 Second layer 15 Third layer

Claims (7)

A heavy 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; 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 in 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. The steel plate according to claim 1, wherein the average thickness of the scale is 6 to 60 μm. 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%,
The steel plate according to claim 1 or 2, having a chemical composition consisting of: REM: 0 to 0.0100%, and the balance: Fe and impurities. 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 to 0.0100%
The steel plate according to claim 3, comprising one or more selected from the group consisting of: 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%. 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;
The method for producing a thick steel plate according to any one of claims 1 to 5, comprising a step of cooling the obtained steel plate, 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.
1.0≦ _x10 ≦10.0...Formula (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 elapsed time from when the surface temperature of the slab exceeds 1050°C to when the heating process is completed, which is divided into 10 equal sections, and n indicates that the calculation corresponds to the nth section of the 10 equal sections.
_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,
D ₂ , D ₃ , D ₄ , D ₅ , D ₆ , D ₇ and D ₈ are constants, which are 8.66×10 ⁻¹⁰ , −3.99×10 ³ , 5.36×10 ⁶ , −2.32×10 ⁹ , 8.75×10 ² , −3.50×10 ⁻³ and 3.06×10 ⁰ , respectively;
_Tn 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],
_x10 is obtained by calculating _x1 , _x2 , _x3 , etc. in order using the above formula,
1.0≦ _y10 ≦10.0...Formula (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・exp{−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 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 of the 10 equal sections,
E ₁ , E ₂ , E ₃ , E ₄ and E ₅ are constants, which are 5.00×10 ⁷ , 1.24×10 ⁻¹ , −9.56×10 ⁻³ , −1.05×10 ⁴ and 2.73×10 ² , respectively;
_Jn is the average steel sheet temperature [°C] in the nth region of the 10 equally divided sections,
Δk is 1/10 of the elapsed time [seconds],
_y10 can be obtained by calculating _y1 , _y2 , _y3 , and so on in order using the above formula. The method for manufacturing thick steel plates described in 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.