JP2023128419A - Thick steel plate and method for manufacturing the same - Google Patents

Abstract

To provide a thick steel plate which enhances adhesion of a scale, suppresses or prevents scale peeling at the time of laser irradiation, and accordingly is useful for applying laser cutting, and a method for manufacturing the same.SOLUTION: A thick steel plate includes a steel plate, and a scale formed on the surface of the steel plate, wherein a density of a CuNi-concentrated part existing in an island state on the interface between the steel plate and the scale is 1.0×107 pieces/m2 or more, and thickness deviation of the scale is 10.0% or less of an average thickness of the scale. A method for manufacturing a thick steel plate includes the steps of: applying bending work with the amount of strain on the surface of a slab of 20-40% at a surface temperature of the slab in continuous casting of 1,000-1,200°C; heating the slab at a predetermined highest heating temperature in predetermined time; rolling the slab at a cumulative draft of 15% or more, then subjecting the slab to descaling by high-pressure water, setting temperature deviation in a width direction of a rolled material at 25°C or lower, and then applying additional hot-rolling; and cooling the steel plate under a predetermined condition.SELECTED DRAWING: None

Description

The present invention relates to a thick steel plate and a method for manufacturing the same, and more particularly to a thick steel plate that is subjected to laser cutting for use in forming a structure, and a method for manufacturing the same.

Large quantities of thick steel plates are used in large steel structures such as ships, building materials, industrial machinery, and bridges. When constructing these steel structures, cutting and welding account for the majority of the construction work. Therefore, it is required to cut with high precision in order to reduce the cutting man-hours and to reduce the welding man-hours.

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 become popular mainly for cutting thin steel plates because it has superior cut surface accuracy, has a smaller heat-affected zone, and can reduce man-hours through automation. . In recent years, with the practical use of high-power laser cutting machines, laser cutting is sometimes performed even when cutting thick steel plates used for the above-mentioned large steel structures.

The production of steel plates such as thick steel plates generally involves the process of hot rolling slabs, and it is known that hot rolled steel plates oxidize in the atmosphere and form scale (iron oxide) on their surfaces. . When laser cutting thick steel plates, this scale peels off on the surface of the steel plate before and/or during cutting, resulting in the thick steel plate not being able to be cut, or creating abnormal cuts with gouges on the cut surface, resulting in unstable cuts. This results in an increase in man-hours and a deterioration in cutting accuracy.

As a method for improving laser cutting properties, for example, Patent Document 1 discloses a drying method in which the surface contains a colored pigment consisting of one or more of titania powder, zinc powder, aluminum powder, black iron oxide pigment, and black fired pigment. Steel materials with coatings have been proposed. Further, in Patent Document 2, alkoxysilane having two or more alkoxy groups on the surface and/or its hydrolyzate or condensate (e.g., tetraalkoxysilane), zinc powder, and aluminum phosphate (preferably aluminum tripolyphosphate) are used. A steel material coated with a coating composition containing a powder of aluminum phosphate or a mixed powder of aluminum phosphate and zinc phosphate has been proposed.

On the other hand, in Patent Document 3, in order to improve laser cutting properties, the proportion of magnetite phase (Fe ₃ O ₄ ) in the scale on the steel sheet surface is increased to 85% or more to increase adhesion, and the thickness of the scale is A steel plate limited to 6 μm or less has been proposed.

In addition, in Patent Document 4, from the viewpoint that it is important to control the heat of oxidation reaction of steel within an appropriate range for laser cuttability, it is proposed that the chemical composition of the steel sheet is high tensile strength with the addition of appropriate amounts of C, Si, Mn, etc. A steel plate is described, and it is taught that the end-of-rolling temperature of hot rolling should be between 700 and 850° C. in order to obtain a suitable scale thickness and thereby suppress oxidation reactions.

Furthermore, in Patent Document 5, appropriate amounts of Cu and Ni are added to the chemical composition of the steel sheet, and after being heated at a temperature higher than the melting point of the Cu-Ni alloy in the rolling process, descaling is performed at a temperature lower than the melting point of the Cu-Ni alloy. It is described that by applying the treatment, a scale/metal mixed layer containing fine metal particles made of an alloy mainly composed of Fe, Cu, and Ni is formed in the scale, and the scale/metal mixed layer is formed in the scale alone. It is taught that because it has a relatively large heat capacity, it acts as a buffer when laser light hits it, absorbing the impact of the laser light, and preventing the scale layer from peeling off from the steel surface.

International Publication No. 2013/065349 Japanese Patent Application Publication No. 2008-156377 Japanese Patent Application Publication No. 2003-221640 Japanese Patent Application Publication No. 09-194988 Japanese Patent Application Publication No. 2006-219712

In order to reduce costs in a society with a declining population, there is an increasing demand for automation when cutting steel plates using lasers. On the other hand, in order to cope with the increase in the thickness of the objects to be cut due to larger structures and the faster cutting speed using high-power fiber lasers, there is a need for steel plates with even better laser cutting properties than conventional ones. Specifically, in order to improve laser cuttability, there is a need for a steel plate that does not peel irregularly when irradiated with a laser of higher power than conventional steel sheets and has a scale that has excellent adhesion.

In the steel plate described in Patent Document 3, measures are taken to prevent irregular peeling of scale, but descaling using high-pressure water must be performed during all rolling passes, making it difficult to address the fundamental issue of cost reduction. However, there is still room for improvement. In the steel plate described in Patent Document 5, the laser cuttability is improved by increasing the adhesion of the scale, but there is still room for improvement with respect to the increase in laser output.

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

In order to achieve the above object, the present inventors investigated the scale peeling resistance necessary for stable and continuous cutting in laser cutting. As a result, it was found that in a steel plate heated by continuous laser irradiation, it is effective for stable continuous cutting to prevent scale peeling from occurring at the laser irradiated area. Further studies were carried out, and as a means to obtain such anti-scale peeling characteristics, we found that at the scale/steel plate (substrate) interface, particularly at the steel plate side surface of the scale/steel plate interface, areas with concentrated Cu and Ni (hereinafter referred to as , CuNi enriched portions) and by making the scale thickness on the steel sheet uniform, the scale peeling resistance was improved.

The mechanism by which the CuNi-enriched area affects the anti-scale peeling properties is unknown, but the CuNi-enriched area acts as a scale generation site after descaling, increasing its density and promoting the homogeneous formation of scale. Furthermore, since the adhesion between the CuNi-enriched area and the scale generated therefrom is extremely good, the connection of the microscopic peeling between the steel plate and the scale caused by thermal stress during laser irradiation is suppressed, and the separation from the steel plate is suppressed. It is presumed that large-scale flaking of the scale will not develop.

The present invention that achieves the above object is as follows.
(1) The density of a Cu and/or Ni enriched area (CuNi enriched area) that includes a steel plate and a scale formed on the surface of the steel plate and exists in an island shape at the interface between the steel plate and the scale. A thick steel plate, wherein the number of scales is 1.0×10 ⁷ pieces/m ² or more, and the thickness deviation of the scales is 10.0% or less of the average thickness of the scales.
(2) The thick steel plate according to (1) above, wherein the scale has an average thickness of 6 to 60 μm.
(3) The steel plate is in mass%,
C: 0.001-0.300%,
Si: 0.01-1.00%,
Mn: 0.10 to 2.50%,
P: 0.001-0.050%,
S: 0.0001-0.0100%,
Al: 0.001-0.200%,
Cu: 0.01 to 1.00%,
Ni: 0.01-2.00%,
N: 0.0150% or less,
O: 0.0050% or less,
Cr: 0-1.00%,
Mo: 0-1.00%,
W: 0-0.50%,
Nb: 0 to 0.500%,
Ti: 0 to 0.500%,
V: 0-1.000%,
B: 0 to 0.0100%,
Sn: 0-0.500%,
Sb: 0 to 0.500%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Hf: 0-0.0100%,
Te: 0 to 0.0100%,
Sr: 0 to 0.0100%,
REM: 0 to 0.0100%, and the remainder: Fe and impurities,
The thick steel plate described in (1) or (2) above, having a chemical composition that satisfies the following formula (1).
0.05≦[Cu]+0.5[Ni]≦1.00...Formula (1)
[Cu] and [Ni] represent the content [mass%] of each element in the steel plate.
(4) the chemical composition is in mass%;
Cr: 0.01-1.00%,
Mo: 0.01-1.00%,
W: 0.003-0.50%,
Nb: 0.003 to 0.500%,
Ti: 0.003 to 0.500%,
V: 0.003-1.000%,
B: 0.0003 to 0.0100%,
Sn: 0.003 to 0.500%,
Sb: 0.003 to 0.500%,
Ca: 0.0003-0.0100%,
Mg: 0.0003 to 0.0100%,
Hf: 0.0003-0.0100%,
Te: 0.0003 to 0.0100%,
Sr: 0.0003 to 0.0100%, and REM: 0.0003 to 0.0100%
The thick steel plate according to (3) above, comprising one or more selected from the group consisting of:
(5) A process of manufacturing a slab by a continuous casting method, in which the surface temperature of the slab during continuous casting is in the range of 1000 to 1200°C, and the amount of strain on the surface of the slab is 20 to 40%. Casting process that involves bending,
The step of heating the slab, in which the surface temperature of the slab is heated to a maximum heating temperature of (T ₀ +20) °C to 1300 °C, and the elapsed time from exceeding T ₀ °C to completion of the heating process is as follows. a heating step controlled to satisfy formula (2);
The step of hot rolling the slab, in which the surface temperature of the slab is rolled at a temperature range of 1080°C to T ₀ and a cumulative reduction rate of 15% or more, the surface temperature of the obtained rolled material is High-pressure water descaling is performed in a temperature range of 1080 to T ₀ , the temperature deviation in the width direction of the rolled material after high-pressure water descaling is 25°C or less, and then additional hot rolling is carried out until the rolling completion temperature is 900 to 900°C. The hot rolling process is carried out to achieve a temperature of 1050°C, and the process of cooling the obtained steel plate, in which the elapsed time from the completion of the hot rolling process to the start of water cooling is controlled so that the following formula (3) is satisfied. The method for producing a thick steel plate according to any one of (1) to (4) above, including a cooling step.
1.00≦ _x10 ≦10.00...Formula (2)
x ₁ =D ₁・{(T ₁ −T ₀ ) ² +D ₂ T ₁ +D ₃ }・Δt ^0.5・exp(−D ₄ /T ₁ )
t _n =x _n ²・D ₁ ^-2・{(T _n+1 −T ₀ ) ² +D ₂ T _n+1 +D ₃ } ^-2・exp(D ₄ /T _n+1 )
x _n =D ₁・{(T _n −T ₀ ) ² +D ₂ T _n +D ₃ }・(t _n-1 +Δt) ^0.5・exp(−D ₄ /T _n )
x _n is an index representing the degree of formation of the CuNi-enriched portion after each section is completed by dividing the elapsed time from the time when the surface temperature of the slab exceeds T ₀ °C to the completion of the heating process into 10 equal parts. Yes, n indicates that the calculation corresponds to the nth section of the interval divided into 10 equal parts,
D ₁ , D ₂ , D ₃ and D ₄ are constants and are 8.92×10 ⁻⁴ , 2.47×10 ⁰ , −1.09×10 ³ and 5.89×10 ³ , respectively;
T _n is the average slab temperature [°C] in the nth region of the 10 equally divided sections,
Δt is one tenth of the elapsed time [seconds],
x ₁₀ is obtained by sequentially calculating x ₁ to x ₂ , x ₃ . . . using the above calculation formula.
1.00≦y ₁₀ ≦10.00...Formula (3)
y ₁ =E ₁・(J ₁ ² +E ₂ J ₁ +E ₃ )・Δk ^0.5・exp(−E ₄ /J ₁ )
k _n =y _n ²・E ₁ ^-2・(J _n+1 ² +E ₂ J _n+1 +E ₃ ) ^-2・exp(2・E ₄ /J _n+1 )
y _n =E ₁・(J _n ² +E ₂ J _n +E ₃ )・(k _n-1 +Δk) ^0.5・exp(−E ₄ /J _n )
y _n is an index representing the degree of scale growth after each section is completed by dividing the elapsed time from the completion of the hot rolling process to the start of water cooling into 10 equal parts in the cooling process, and n is the index representing the degree of scale growth after each section is completed. Indicates that it is the nth calculation,
E ₁ , E ₂ , E ₃ and E ₄ are constants and are 9.14×10 ⁻⁴ , 2.47×10 ⁰ , −1.09×10 ³ and 5.89×10 ³ , respectively;
J _n is the average steel plate temperature [°C] in the nth region of the 10 equally divided sections,
Δk is one-tenth of the elapsed time [seconds],
y ₁₀ is obtained by sequentially calculating from y ₁ to y ₂ , y ₃ , etc. using the above calculation formula.
T ₀ is the temperature determined by the following equation (4).
T ₀ = K ₁ + K ₂ [Cu] [Ni] + K ₃ [Cu] [Ni] ^0.5 (K ₄ - [Cu]) + K ₅ (K ₆ + log ₁₀ [Si]) ² + K ₇ Al + K ₈ Mn... Formula (4)
K ₁ =1.158×10 ³ , K ₂ =6.288×10 ⁰ , K ₃ =3.337×10 ¹ , K ₄ =1.783×10 ⁰ , K ₅ =1.400×10 ⁰ , K ₆ =1.000×10 ⁰ , K ₇ =−3.500×10 ¹ , and K ₈ =−1.560×10 ¹ ,
[Cu] and [Ni] represent the content [mass%] of each element in the slab.

According to the present invention, peeling of the scale during laser irradiation is suppressed or prevented by increasing the adhesion of the scale, and therefore it is possible to provide a thick steel plate useful for applying laser cutting and a method for manufacturing the same. can.

FIG. 2 is a schematic diagram of an elemental map obtained by EPMA analysis for measuring the density of a CuNi enriched area.

Hereinafter, thick steel plates and methods for manufacturing the same according to embodiments of the present invention will be explained in more detail, but these explanations are intended to be exemplifications of preferred embodiments of the present invention, and the present invention is not limited to specific embodiments. It is not intended to.

[Preferred chemical composition]
In the embodiment of the present invention, the steel plate is any material that satisfies the requirement that the density of CuNi enriched areas existing in island form at the interface between the steel plate and the scale is 1.0×10 ⁷ pieces/m ² or more. It's good. Therefore, the chemical composition of the steel plate is not particularly limited, and may be appropriately determined within a range that satisfies the requirements. More specifically, as described above, the present invention aims to improve scale adhesion, suppress or prevent scale peeling during laser irradiation, and therefore provide a thick steel plate useful for laser cutting. The density of the CuNi enriched areas existing in the form of islands at the interface between the steel plate and the scale shall be 1.0×10 ⁷ pieces/m ² or more, and the thickness deviation of the scale shall be the average thickness of the scale. This objective is achieved by reducing the amount of carbon dioxide to 10.0% or less. Therefore, it is clear that the chemical composition of the entire steel sheet is not an essential technical feature to achieve the object of the present invention. The preferred chemical composition of the steel plate according to the embodiment of the present invention will be explained below, but these explanations are based on the assumption that the density of the CuNi enriched parts existing in the form of islands at the interface between the steel plate and the scale is 1.0 × 10 ⁷ / This is intended to be merely an example of a preferable chemical composition in a steel plate to meet the requirement that the chemical composition be ² m2 or more, and the present invention is not intended to be limited to steel plates having such a specific chemical composition. . Furthermore, in the following description, "%", which is the unit of content of each element, means "% by mass" unless otherwise specified.

[C: 0.001-0.300%]
C is an element that is unavoidably included in general iron manufacturing methods, and limiting it to less than 0.001% places a large burden on the smelting process, which is economically undesirable. From this point of view, the content of C is preferably 0.001% or more. C is an element that greatly increases strength, and in order to increase strength, it is preferably contained in an amount of 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 will be significantly degraded, so the C content is preferably 0.300% or less. Further, C is an element that impairs weldability and the toughness of the weld zone, and from this point of view, 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. In order to fully obtain these effects, the Si content is preferably 0.01% or more. In particular, in order to increase the strength, the Si content is preferably 0.05% or more, more preferably 0.10% or more. On the other hand, if the Si content is high, the scale formation behavior on the surface of the steel plate may become non-uniform, so the Si content is preferably 1.00% or less. Further, Si is an element that impairs the toughness of a steel sheet, and from this point of view, 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, more preferably 0.50% or more. On the other hand, if Mn is contained excessively, coarse MnS will be generated, and there is a concern that the toughness of the steel sheet will be significantly deteriorated. From this point of view, it is preferable to limit the Mn content to 2.50% or less. Further, Mn is an element that impairs weldability and the toughness of the weld zone, and from this point of view, 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 unavoidably included in general iron manufacturing methods, and limiting it to less than 0.001% imposes a large burden on the smelting process, which is economically undesirable. From this viewpoint, the P content is preferably 0.001% or more. On the other hand, P is an element that impairs toughness, and from this point of view, it is preferable to limit the P content to 0.050% or less. Furthermore, P is an element that impairs weldability and the toughness of the weld zone, and from this point of view, 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 unavoidably included in general iron manufacturing methods, and limiting it to less than 0.0001% imposes a large burden on the smelting process, which is economically undesirable. From this point of view, the S content is preferably 0.0001% or more. On the other hand, S is an element that forms coarse sulfides and impairs toughness, and from this point of view, it is preferable to limit the S content to 0.0100% or less. Further, S is an element that impairs weldability and the toughness of the welded part, and from this point of view, the S content is preferably 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 content of Al is preferably 0.001% or more. In order to fully exhibit 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 content of Al is preferably limited to 0.200% or less. Furthermore, since Al also impairs weldability and the toughness of the welded part, the Al content is preferably 0.120% or less, more preferably 0.080% or less.

[Cu: 0.01 to 1.00%]
Cu is an element that, together with Ni, increases the adhesion between the steel plate and the scale, and improves the laser cuttability of the steel plate. From this viewpoint, the content of Cu is preferably 0.01% or more. In order to improve scale adhesion, the Cu content is preferably 0.03% or more, more preferably 0.08% or more. The content of Cu may be 0.10% or more, 0.12% or more, 0.15% or more, 0.20% or more, or 0.30% or more. On the other hand, if the Cu content is excessive, there is a concern that flaws will occur on the surface of the slab and that rolling will be hindered, so the Cu content is preferably limited to 1.00% or less. Further, since Cu deteriorates weldability, the content of Cu is preferably 0.70% or less, and more preferably 0.50% or less.

[Ni:0.01-2.00%]
Ni, together with Cu, is an element that increases the adhesion between the steel plate and the scale, and improves the laser cuttability of the steel plate. From this point of view, the Ni content is preferably 0.01% or more. In order to improve scale adhesion, the Ni content is preferably 0.03% or more, more preferably 0.05% or more. Even if the Ni content is 0.10% or more, 0.12% or more, 0.15% or more, 0.20% or more, 0.30% or more, 0.40% or more, or 0.50% or more good. On the other hand, if the Ni content is excessive, there is a concern that flaws will occur on the surface of the slab and that rolling will be hindered, so the Ni content is preferably limited to 2.00% or less. Further, since Ni deteriorates weldability, the Ni content is preferably 1.50% or less, more preferably 1.00% or less, and even more preferably 0.70% or less. .

[Cu+0.5Ni: 0.05-1.00%]
The scale with excellent adhesion in the thick steel plate according to the embodiment of the present invention is formed by Cu and Ni forming a CuNi-enriched area at the interface between the scale on the steel plate surface and the steel plate, particularly on the steel plate side surface of the interface. is obtained. In order to obtain the scale, it is preferable that the content of Cu and Ni in the steel plate satisfies the following formula (1) in addition to the above content range for each individual element.
0.05≦[Cu]+0.5[Ni]≦1.00...Formula (1)
Here, [Cu] and [Ni] represent the content [mass%] of each element in the steel plate.
When the content of Cu and Ni is low and does not satisfy formula (1), the surface of the steel sheet has CuNi enriched areas with a sufficient density, that is, island-like areas with a density of 1.0 × 10 ⁷ pieces/m ² or more. A CuNi-enriched area may not be formed, and in such a case, a scale with poor adhesion is formed. On the other hand, if the content of Cu and Ni is excessive and does not satisfy formula (1), CuNi-enriched areas are unevenly distributed, some of the CuNi-enriched areas become coarse, and on the contrary, island-like CuNi-enriched areas become Density may be reduced. The coarse CuNi-enriched areas formed by unevenly distributed CuNi-enriched areas have excessively high adhesion with the scale formed thereon, and the defects caused by the heating of the slab in the hot rolling process described below. During descaling to remove homogeneous scale, some scale may remain partially. In such a case, a non-uniform scale is formed in the subsequent cooling step, and the adhesion of the scale is rather poor. In order to increase the density of the CuNi-enriched area and more reliably improve scale adhesion, the value of formula (1) is preferably 0.08% or more and 0.70% or less, and 0.10% Above, it is more preferable that it is 0.50% or less.

[N: 0.0150% or less]
N is an element that is inevitably included in general iron manufacturing methods. If a large amount of N is included, coarse nitrides will be formed and the toughness of the steel sheet will be impaired, so it is preferable to limit the N content to 0.0150% or less. Further, N is an element that impairs the toughness of the weld zone, and from this point of view, the N content is preferably 0.0100% or less, and more preferably 0.0060% or less. Although there is no particular lower limit to the N content, limiting it to less than 0.0003% places a heavy burden on the smelting process and is economically unfavorable, so it is preferably 0.0003% or more.

[O: 0.0050% or less]
O is an element that is inevitably included in general iron manufacturing methods. If a large amount of O is included, coarse oxides are formed and the toughness of the steel sheet is impaired, so it is preferable to limit the O content to 0.0050% or less. Furthermore, O is an element that impairs the toughness of the weld zone, and from this point of view, the content of O is preferably 0.0035% or less, more preferably 0.0025% or less. There is no particular lower limit for the content of O, but limiting it to less than 0.0002% puts a heavy burden on the smelting process and is economically unfavorable, so it is preferably 0.0002% or more.

The basic chemical composition of the steel plate according to the embodiment of the present invention is as described above. Furthermore, the steel plate may contain one or more of the following optional elements as necessary. The steel plate may contain one or more selected from the group consisting of Cr: 0 to 1.00%, Mo: 0 to 1.00%, and W: 0 to 0.50%. Further, the steel plate may contain one or more selected from the group consisting of Nb: 0 to 0.500%, Ti: 0 to 0.500%, and V: 0 to 1.000%. . Further, the steel plate may contain B: 0 to 0.0100%. Further, the steel plate may contain one or two selected from the group consisting of Sn: 0 to 0.500% and Sb: 0 to 0.500%. In addition, the steel plate contains Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Hf: 0 to 0.0100%, Te: 0 to 0.0100%, Sr: 0 to 0.0100% and REM: May contain one or more selected from the group consisting of 0 to 0.0100%. These optional elements will be explained in detail below.

[Cr: 0-1.00%]
Cr is an element that contributes to improving strength. The content of Cr may be 0%, but when included, 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, more preferably 0.15% or more. On the other hand, if Cr is contained excessively, coarse Cr carbonitrides will be generated, and there is a concern that the toughness of the steel sheet will be significantly deteriorated. From this point of view, it is preferable to limit the Cr content to 1.00% or less. Further, Cr is an element that impairs weldability and the toughness of the weld zone, and from this point of view, the Cr content is preferably 0.60% or less.

[Mo: 0-1.00%]
Mo is an element that contributes to improving strength. The content of Mo may be 0%, but when it is included, it may be 0.001% or more or 0.01% or more. From the viewpoint of increasing strength, the content of Mo is preferably 0.02% or more, 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 part will be impaired. From this point of view, the Mo content is preferably limited to 1.00% or less, more preferably 0.60% or less.

[W: 0-0.50%]
W is an element that contributes to improving strength. The content of W may be 0%, but when included, 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 part will be impaired. From this point of view, the content of W is preferably limited to 0.50% or less, more preferably 0.30% or less.

[Nb: 0 to 0.500%]
Nb is an element that contributes to improving strength. The content of Nb may be 0%, but when included, 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 will be generated, and there is a concern that the toughness of the steel plate and welded portion will be significantly deteriorated. 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 content of Ti may be 0%, but when included, 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 will be generated, and there is a concern that the toughness of the steel plate and the welded portion will be significantly deteriorated. 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 content of V may be 0%, but when it is included, it may be 0.001% or more or 0.003% or more. From the viewpoint of increasing strength, the content of V is preferably 0.030% or more, 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 part may be impaired. From this point of view, the V content is preferably limited to 1.000% or less, more preferably 0.600% or less.

[B: 0 to 0.0100%]
B is an element that contributes to improving strength. The content of B may be 0%, but when included, it may be 0.0001% or more. From the viewpoint of increasing strength, the content of B is preferably 0.0003% or more, 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 part may be impaired. From this point of view, the content of B is preferably limited to 0.0100% or less, more preferably 0.0035% or less.

[Sn: 0 to 0.500%]
Sn is an element that is concentrated at the interface between the steel plate and the scale and promotes the concentration of other metal elements near the interface, and has the effect of promoting the formation of a CuNi-enriched area and increasing the adhesion of the scale. The Sn content may be 0%, but in order to improve scale adhesion and laser cutting properties, the Sn content is preferably 0.001% or more or 0.003% or more. . In order to fully obtain this effect, the Sn content is preferably 0.005% or more, more preferably 0.025% or more. On the other hand, if a large amount of Sn is contained, the weldability and the toughness of the welded part will be impaired, so it is preferable to limit the Sn content to 0.500% or less. From this viewpoint, the content of Sn is preferably 0.300% or less, more preferably 0.200% or less.

[Sb: 0 to 0.500%]
Sb is an element that concentrates at the interface between the steel plate and the scale and promotes the concentration of other metal elements near the interface, and has the effect of promoting the formation of a CuNi-enriched area and increasing the adhesion of the scale. The Sb content may be 0%, but in order to improve scale adhesion and laser cutting properties, the Sb content is preferably 0.001% or more or 0.003% or more. . In order to fully obtain this effect, the content of Sb is preferably 0.005% or more, and more preferably 0.025% or more. On the other hand, if a large amount of Sb is contained, the weldability and the toughness of the welded part will be impaired, so it is preferable to limit the Sb content to 0.500% or less. From this point of view, the Sb content is preferably 0.300% or less, 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 the steel sheet. The contents of Ca, Mg, Hf, Te, Sr, and REM may be 0%, but in order to obtain this effect, the contents of Ca, Mg, Hf, Te, Sr, and REM should be 0%, respectively. It is preferably at least 0.0001% or at least 0.0003%. On the other hand, if these elements are contained in excess, the effects will be saturated, and therefore, containing Ca, Mg, Hf, Te, Sr, and REM in a steel sheet in excess of necessary will lead to an increase in manufacturing costs. Therefore, the content of Ca, Mg, Hf, Te, Sr and REM is preferably limited to 0.0100% or less, more preferably 0.0040% or less. Here, REM is a general term for Sc, Y, and lanthanoids (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), and these 17 Let the total content of elements be the content of REM.

In the steel plate according to the 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 industrial manufacturing of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap.

The chemical composition of the steel plate may be measured by a general analytical method. For example, the chemical composition of a steel plate may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). For C and S, combustion-infrared absorption method is used.

Next, the scale, the scale/steel plate interface, and the characteristics in the vicinity of the interface will be described regarding the thick steel plate according to the embodiment of the present invention.

[Density of concentrated areas of Cu and/or Ni existing in the form of islands at the interface between the steel plate and the scale ≧1.0×10 ⁷ pieces/m ² ]
The thick steel plate according to the embodiment of the present invention has island-shaped Cu and/or Ni enriched areas (CuNi enriched areas) at the interface between the steel plate (substrate) and the scale, particularly on the steel plate side surface of the interface. characterized by its existence. The greater the density of the island-shaped CuNi-enriched portions at the interface, the higher the scale adhesion and the better the laser cuttability. In order to fully obtain this effect, in the thick steel plate according to the embodiment of the present invention, the density of the CuNi enriched portions at the interface is set to 1.0×10 ⁷ pieces/m ² or more.

The island-like CuNi enriched areas present at the interface function as scale formation sites after descaling in the hot rolling process, so their density increases, which increases the homogeneity of the scale and reduces the scale during laser irradiation. It is thought that the heat absorption and expansion behavior of the steel become constant, and as a result, the peeling of the scale is suppressed, thereby improving the laser cuttability. In addition, since it also functions as a scale formation site, the adhesion between the CuNi enriched area and the scale layer, and ultimately the adhesion between the steel plate and the scale layer, increases. It is thought that the occurrence of macroscopic peeling phenomenon due to the connection of microscopic peeled parts between the scale and the steel plate caused by thermal expansion of the steel plate due to laser irradiation is suppressed. Therefore, if a CuNi enriched area is formed over the entire area of the interface, scale formation will be promoted over the entire area of the steel sheet, the homogeneity of the scale will increase, and the adhesion of the scale will improve, leading to laser cutting. It is also considered that the performance is more markedly improved. However, in order to do this, it is necessary to contain a large amount of Cu and/or Ni, and it is also necessary to increase the strain applied in the casting process described below, but both deteriorate the surface properties of the slab and This is not necessarily preferable because it becomes difficult to subject the slab to a hot rolling process. Therefore, in order to further improve scale adhesion while avoiding such manufacturing disadvantages, the density is preferably 1.5 x 10 ⁷ pieces/m ² or more, and 2.0 x 10 ⁷ pieces/m 2 or more. /m ² or more is more preferable. Although no upper limit is set for the density, if an attempt is made to increase the density of the island-shaped CuNi-enriched parts too much, the CuNi-enriched parts will connect with each other, and the density will actually decrease. In this case, as the density decreases, coarse CuNi enriched areas will be formed, and scale formation will be abnormally promoted only in the area where coarse CuNi enriched areas exist. It is thought that the homogeneity is impaired, and the heat absorption and expansion behavior of the scale during laser irradiation become non-uniform, promoting peeling of the scale and deteriorating the laser cuttability. From the above viewpoint, it is preferable that the density of the CuNi enriched portions at the interface be kept at 1.0×10 ⁸ pieces/m ² or less.

The density of the island-shaped CuNi enriched portions present at the interface, particularly on the steel plate side, is measured by the following procedure. Grind one side of a thick steel plate containing a scale layer to a thickness of 2.0 mm, bend the plate at 90 degrees with a bending radius of 5.0 mm with the surface where the scale remains outside the bending process, and then Perform an unbending process to flatten the bent part, then apply cellophane tape to the scale where the unbending process was applied and peel it off to peel off the scale layer and expose the steel plate side at the interface between the scale and the steel plate. . If the scale layer cannot be peeled off sufficiently, the part where the same bending and unbending process has been applied is used as the inside of the bending process, and the bending process is performed again and the scale is removed using cellophane tape. The bending and unbending process is repeated while alternating between outside bending and inside bending until the scale layer is sufficiently peeled off.

Subsequently, elemental distribution analysis is performed using an electron probe microanalyzer (EPMA). Analysis by EPMA is performed with a step size of 0.5 μm for measurement at locations where scale appears to have peeled off visually. A schematic diagram of an elemental map obtained by EPMA analysis is shown in FIG. When obtaining the density of the CuNi enriched portion, even in locations where scale appears to have peeled off visually, there are locations where fine scale remains and the steel plate surface is coated. Therefore, in order to obtain the density of the CuNi-enriched area on the steel plate surface, the area of the scale remaining area covered with the steel plate surface is determined, and the area of the analyzed steel plate surface is obtained by subtracting it from the area where the EPMA analysis was performed. There is a need. As the elemental amount analyzed by EPMA, locations where the concentration of O (oxygen) is 5.0% or more in mass % are determined to be locations where scale remains, and the area where scale remains is determined from the number of measurement points. Find S. The area obtained by subtracting S from the area S ₀ where EPMA analysis was performed is the area (S ₀ - S) where the steel plate surface was analyzed to obtain the density of the CuNi enriched part, and in order to obtain the average density of the CuNi enriched part, EPMA analysis is performed so that this area is 1.0×10 ⁻⁸ m ² or more.

Measurements in which the concentration of Cu and Ni satisfies the following formula (5) and there are 12 or more consecutive points in the region of the elemental map obtained by EPMA analysis where it is determined that the steel plate is observed from the above analysis. The point group is determined to be an island-like or island-like CuNi enriched area. The reason why the island-like CuNi enriched areas are determined in this way is that when the concentration of Cu and Ni is low, the ability to promote scale generation is weak, and when the enriched areas are minute, they promote scale generation. This is because it is thought that the ability to do so will be weakened. The thick steel plate according to the embodiment of the present invention is characterized by counting the number N of island-shaped CuNi enriched areas obtained in this way and dividing the number by the analyzed area (S ₀ - S) of the steel plate surface. The density of island-like CuNi enriched areas at the scale/steel plate interface can be obtained.
[Cu]+0.5[Ni]≧5.0% by mass...Formula (5)
[Cu] and [Ni] represent the content [mass%] of each element in the steel plate.

[Scale thickness deviation is 10.0% or less of the average scale thickness]
The thick steel plate according to the embodiment of the present invention is characterized in that the scale thickness is uniform. The homogeneity of scale thickness is evaluated using the following procedure. A cross section parallel to the rolling direction and parallel to the plate surface direction was cut out from a thick steel plate, and the surface including scale was observed using a scanning electron microscope (SEM), and a 1 mm area parallel to the rolling direction was cut out. Measure the distance from the interface between the steel plate and the scale to the scale surface at five arbitrary points from the scale, and use the average value as the scale thickness (h ₁ ) at that point. Using the above method, scale thickness was measured at two locations: at the center in the width direction of a thick steel plate and at a location 100 mm or more apart from each other in the width direction, and the scale thickness at a total of three locations (h ₁ , h ₂ , h ₃ ) is taken as the average scale thickness (h) for the same thickness steel plate. Further, among the three scale thicknesses h ₁ , h ₂ , and h ₃ , the difference between the maximum and minimum scale thicknesses is defined as the scale thickness deviation (Δh) in steel plates of the same thickness.

The larger the thickness deviation of the scale compared to the average thickness of the scale, the more uneven the heat input to the steel plate due to laser irradiation becomes, the larger the local thermal stress is applied to the scale, and the more peeling of the scale is promoted. As a result, laser cuttability deteriorates. Therefore, the thickness deviation of the scale in the thick steel plate according to the embodiment of the present invention is limited to 10.0% or less of the average thickness of the scale (that is, Δh/h×100≦10.0%). The smaller the scale thickness deviation, the more the scale peeling during laser irradiation is suppressed, so the scale thickness deviation is preferably 8.0% or less of the average thickness of the scale, and 6.0% or less. It is more preferable that There is no particular lower limit set for the scale thickness deviation, and the smaller the thickness deviation, the more suppressed the peeling of the scale during laser irradiation. Although not particularly limited, for example, the thickness deviation of the scale may be 1.0% or more or 2.0% or more of the average thickness of the scale.

The effect of reducing the ratio between the scale thickness deviation and the scale average thickness described above is exerted regardless of the scale average thickness, but the smaller the scale average thickness, the smaller the scale thickness deviation. This increases the operational burden. From this viewpoint, the average thickness of the scale is preferably 6 μm or more, more preferably 10 μm or more, and even more preferably 16 μm or more. The average thickness of the scale may be 17 μm or more, 18 μm or more, 20 μm or more, 22 μm or more, or 25 μm or more. On the other hand, if the average thickness of the scale becomes too large, the scale becomes brittle and may partially peel off due to causes other than laser irradiation, such as impact during transportation, and as a result, there is a concern that laser cuttability may deteriorate. From this viewpoint, the average thickness of the scale is preferably 60 μm or less, more preferably 50 μm or less, and even more preferably 40 μm or less.

The thick steel plate according to the embodiment of the present invention can have any thickness to which laser cutting can be applied, and may have a thickness of, for example, 6 to 40 mm, although it is not particularly limited. The plate thickness may be, for example, 8 mm or more, 10 mm or more, 15 mm or more, or 20 mm or more. Similarly, the plate thickness may be, for example, 35 mm or less, 30 mm or less, or 25 mm or less.

Next, a preferred method for manufacturing the thick steel plate according to the embodiment of the present invention will be described. The following explanation is an illustration of a characteristic method for manufacturing a thick steel plate according to an embodiment of the present invention, and is not intended to limit the thick steel plate to that manufactured by the manufacturing method described below. do not have.

The method for manufacturing a thick steel plate according to an embodiment of the present invention includes:
A process of manufacturing slabs by continuous casting, in which the surface temperature of the slab during continuous casting is in the range of 1000 to 1200°C, and the bending process is performed such that the amount of strain on the surface of the slab is 20 to 40%. casting process,
The step of heating the slab, in which the surface temperature of the slab is heated to a maximum heating temperature of (T ₀ +20) °C to 1300 °C, and the elapsed time from exceeding T ₀ °C to completion of the heating process is as follows. a heating step controlled to satisfy formula (2);
The step of hot rolling the slab, in which the surface temperature of the slab is rolled at a temperature range of 1080°C to T ₀ and a cumulative reduction rate of 15% or more, the surface temperature of the obtained rolled material is High-pressure water descaling is performed in a temperature range of 1080 to T ₀ , the temperature deviation in the width direction of the rolled material after high-pressure water descaling is 25°C or less, and then additional hot rolling is carried out until the rolling completion temperature is 900 to 900°C. The hot rolling process is carried out to achieve a temperature of 1050°C, and the process of cooling the obtained steel plate, in which the elapsed time from the completion of the hot rolling process to the start of water cooling is controlled so that the following formula (3) is satisfied. It is characterized by including a cooling process.
1.00≦ _x10 ≦10.00...Formula (2)
x ₁ =D ₁・{(T ₁ −T ₀ ) ² +D ₂ T ₁ +D ₃ }・Δt ^0.5・exp(−D ₄ /T ₁ )
t _n =x _n ²・D ₁ ^-2・{(T _n+1 −T ₀ ) ² +D ₂ T _n+1 +D ₃ } ^-2・exp(D ₄ /T _n+1 )
x _n =D ₁・{(T _n −T ₀ ) ² +D ₂ T _n +D ₃ }・(t _n-1 +Δt) ^0.5・exp(−D ₄ /T _n )
x _n is an index representing the degree of formation of the CuNi-enriched portion after each section is completed by dividing the elapsed time from the time when the surface temperature of the slab exceeds T ₀ °C to the completion of the heating process into 10 equal parts. Yes, n indicates that the calculation corresponds to the nth section of the interval divided into 10 equal parts,
D ₁ , D ₂ , D ₃ and D ₄ are constants and are 8.92×10 ⁻⁴ , 2.47×10 ⁰ , −1.09×10 ³ and 5.89×10 ³ , respectively;
T _n is the average slab temperature [°C] in the nth region of the 10 equally divided sections,
Δt is one tenth of the elapsed time [seconds],
x ₁₀ is obtained by sequentially calculating x ₁ to x ₂ , x ₃ . . . using the above calculation formula.
1.00≦y ₁₀ ≦10.00...Formula (3)
y ₁ =E ₁・(J ₁ ² +E ₂ J ₁ +E ₃ )・Δk ^0.5・exp(−E ₄ /J ₁ )
k _n =y _n ²・E ₁ ^-2・(J _n+1 ² +E ₂ J _n+1 +E ₃ ) ^-2・exp(2・E ₄ /J _n+1 )
y _n =E ₁・(J _n ² +E ₂ J _n +E ₃ )・(k _n-1 +Δk) ^0.5・exp(−E ₄ /J _n )
y _n is an index representing the degree of scale growth after each section is completed by dividing the elapsed time from the completion of the hot rolling process to the start of water cooling into 10 equal parts in the cooling process, and n is the index representing the degree of scale growth after each section is completed. Indicates that it is the nth calculation,
E ₁ , E ₂ , E ₃ and E ₄ are constants and are 9.14×10 ⁻⁴ , 2.47×10 ⁰ , −1.09×10 ³ and 5.89×10 ³ , respectively;
J _n is the average steel plate temperature [°C] in the nth region of the 10 equally divided sections,
Δk is one-tenth of the elapsed time [seconds],
y ₁₀ is obtained by sequentially calculating from y ₁ to y ₂ , y ₃ , etc. using the above calculation formula.
T ₀ is the temperature determined by the following equation (4).
T ₀ = K ₁ + K ₂ [Cu] [Ni] + K ₃ [Cu] [Ni] ^0.5 (K ₄ - [Cu]) + K ₅ (K ₆ + log ₁₀ [Si]) ² + K ₇ Al + K ₈ Mn... Formula (4)
K ₁ =1.158×10 ³ , K ₂ =6.288×10 ⁰ , K ₃ =3.337×10 ¹ , K ₄ =1.783×10 ⁰ , K ₅ =1.400×10 ⁰ , K ₆ =1.000×10 ⁰ , K ₇ =−3.500×10 ¹ , and K ₈ =−1.560×10 ¹ ,
[Cu] and [Ni] represent the content [mass%] of each element in the slab.

[Casting process]
Molten steel adjusted to a predetermined chemical composition is cast by a continuous casting method to produce a slab for hot rolling. The manufacturing method of molten steel is not specified. During continuous casting, bending is performed so that the surface temperature of the slab being formed is in the range of 1000 to 1200°C, and the amount of strain on the surface of the slab is 20 to 40%. Through this treatment, the Cu and Ni in the scale on the slab surface are melted at high temperatures, and the molten Cu and Ni are further concentrated at the interface between the scale and the steel by introducing strain, which is then used in the subsequent hot rolling process. Promotes the formation of CuNi-enriched areas. If the bending temperature is too high or the amount of strain applied during bending is too large, Cu and Ni will excessively penetrate into the steel from the scale-steel interface, causing flaws on the slab surface and causing hot-bending. The appearance of the steel plate after rolling may be impaired, or cracks may occur during hot rolling. On the other hand, if the bending temperature is too low or the amount of strain applied during bending is too small, the concentration of Cu and Ni at the interface between the scale and the steel will be insufficient, resulting in becomes insufficiently concentrated. The amount of strain during bending can be controlled within a desired range, for example, by appropriately selecting the radius of curvature of the curved portion in the continuous casting machine.

Since the cast slab is subjected to the hot rolling process while leaving an interface between the scale and the steel that is moderately enriched with Cu and Ni, the surface of the slab must not be ground.

[Heating process]
The manufactured slab is heated for hot rolling. This heating is performed such that the maximum heating temperature is in the range of (T ₀ +20)° C. to 1300° C., and the elapsed time from exceeding T ₀ ° C. to completion of the heating step satisfies the following formula (2). Here, T ₀ is a temperature derived from the following formula (4) from the chemical composition constituting the slab. Through this heating, while controlling the austenite to an appropriate size, the Cu and Ni concentrated at the interface between the scale and the slab are liquefied and penetrated into the austenite grain boundaries, thereby forming a CuNi-enriched area in the slab. . Here, x ₁₀ in equation (2) is an index representing the degree of formation of the CuNi-enriched portion, which is determined by calculations described later.
1.00≦ _x10 ≦10.00...Formula (2)
T ₀ = K ₁ + K ₂ [Cu] [Ni] + K ₃ [Cu] [Ni] ^0.5 (K ₄ - [Cu]) + K ₅ (K ₆ + log ₁₀ [Si]) ² + K ₇ Al + K ₈ Mn... Formula (4)
K ₁ =1.158×10 ³ , K ₂ =6.288×10 ⁰ , K ₃ =3.337×10 ¹ , K ₄ =1.783×10 ⁰ , K ₅ =1.400×10 ⁰ , K ₆ =1.000×10 ⁰ , K ₇ =−3.500×10 ¹ , and K ₈ =−1.560×10 ¹ ,
[Cu] and [Ni] represent the content [mass%] of each element in the slab.
In the heating step, if the heating temperature is too high, the austenite grains will grow too much, the density of the CuNi-enriched area will be insufficient, and the adhesion of the scale will be impaired. On the other hand, if the heating temperature is too low, Cu and Ni will not sufficiently penetrate into the slab, the density of the CuNi-enriched area will be insufficient, and scale adhesion will be impaired. In order to obtain a sufficient density of the CuNi-enriched portion, the maximum heating temperature is set in the range of (T ₀ +20)°C to 1300°C. In order to improve the adhesion of the scale, the maximum heating temperature is preferably in the range of (T ₀ +40)°C to 1270°C.

Furthermore, the degree of penetration of Cu and Ni into the slab changes depending on the elapsed time during heating. Note that since the influence of the passage of time changes depending on the temperature at the time, the elapsed time during heating is managed using equation (2). If the elapsed time is too short and x ₁₀ is less than 1.00, Cu and Ni will not fully penetrate into the slab, resulting in insufficient density in the CuNi-enriched area and areas where scale adhesion is impaired. Therefore, x ₁₀ is set to be 1.00 or more. In order to increase the density of the CuNi-enriched portion and further improve the adhesion, x ₁₀ is preferably set to 1.50 or more, and more preferably 2.00 or more. On the other hand, if the elapsed time is too long, austenite grains will grow and the density of the CuNi enriched area will decrease. Therefore, x ₁₀ is limited to 10.00 or less, preferably 9.00 or less, and more preferably 8.00 or less. Here, equation (2) is as follows, and x ₁₀ therein is obtained by the calculation below.
1.00≦x ₁₀ ≦10.00...Formula (2)
x ₁ =D ₁・{(T ₁ −T ₀ ) ² +D ₂ T ₁ +D ₃ }・Δt ^0.5・exp(−D ₄ /T ₁ )
t _n =x _n ²・D ₁ ^-2・{(T _n+1 −T ₀ ) ² +D ₂ T _n+1 +D ₃ } ^-2・exp(D ₄ /T _n+1 )
x _n =D ₁・{(T _n −T ₀ ) ² +D ₂ T _n +D ₃ }・(t _n-1 +Δt) ^0.5・exp(−D ₄ /T _n )

These calculations are performed by dividing the elapsed time from the time when the surface temperature of the slab (also called slab temperature) exceeds T ₀ °C until the completion of the heating process into 10 equal parts, and calculating the CuNi enriched area after each section is completed. It is used to evaluate the degree of formation (x _n ), and the subscript n indicates that the calculation corresponds to the nth section of the interval divided into 10 equal parts. D ₁ , D ₂ , D ₃ , and D ₄ are constants, and are 8.92×10 ⁻⁴ , 2.47×10 ⁰ , −1.09×10 ³ , and 5.89×10 ³ , respectively. T _n is the average slab temperature [° C.] in the n-th region of the 10-equally divided section, that is, the arithmetic mean of the temperature measurements in the entire n-th region at predetermined intervals, for example, every 10 seconds. Δt is one-tenth of the elapsed time [seconds]. x ₁₀ in formula (2) is obtained by calculating x ₁ to x ₂ , x ₃ , etc. in order using the above calculation formula.

[Hot rolling process]
On the surface of the heated slab, there is a heterogeneous scale formed and grown during the casting and heating processes. Therefore, by hot rolling and descaling using high-pressure water to remove the non-uniform scale and re-form a homogeneous scale on the surface of the steel plate with an appropriate CuNi enriched area, we were able to create a scale with excellent adhesion. A thick steel plate having excellent laser cuttability can be obtained.

In order to sufficiently remove scale by descaling, hot rolling is performed prior to descaling to crush the scale. The rolling prior to descaling is performed once or in multiple times at a slab surface temperature in the temperature range of 1080° C. to T ₀ , and the rolling reduction is cumulatively 15% or more with respect to the slab thickness. That is, when rolling is performed in multiple steps prior to descaling, the cumulative reduction ratio determined by the thickness of the slab after completing the multiple rounds of rolling is set to 15% or more. If the hot rolling temperature exceeds T ₀ , the scale will not be sufficiently crushed, and the scale will not be sufficiently removed by descaling. For this reason, the scale finally obtained becomes non-uniform, the thickness deviation of the scale becomes large, and the adhesion of the scale is impaired. On the other hand, if the temperature at which the hot rolling is performed is lower than 1080°C, the adhesion of some scales during the hot rolling increases, and some scales may remain even after descaling with high-pressure water. Similarly, the final scale obtained becomes non-uniform, the thickness deviation of the scale becomes large, and the adhesion of the scale is impaired. The temperature at which hot rolling is performed prior to descaling is preferably 1100° C. or higher from the viewpoint of sufficiently increasing the temperature of the rolled material to be subjected to descaling. In addition, if the cumulative reduction rate of hot rolling performed in the temperature range is less than 15%, the scale will not be crushed sufficiently, and even if descaling is performed using high-pressure water, some scale will remain, resulting in the final The scale obtained is non-uniform. As a result, the thickness deviation of the scale becomes large and the adhesion of the scale is impaired. There is no particular upper limit to the cumulative reduction rate of the hot rolling, but the cumulative reduction rate may be 30% or less, and if the cumulative reduction rate exceeds 30%, the scale has been sufficiently crushed, and the descaling The effect of helping remove heterogeneous scales is saturated.

After the above-mentioned hot rolling, the obtained rolled material is subjected to descaling using high pressure water. Descaling can be carried out using high pressure water with an impingement pressure of 10 to 15 MPa, for example. If the descaling temperature (surface temperature of the rolled material) falls below 1080°C, some non-uniformly grown scales will remain, and the final scale will be non-uniform, resulting in a lack of scale. Adhesion is impaired. On the other hand, if the temperature at which descaling is performed exceeds T ₀ , the density of the CuNi-enriched portion decreases, and the adhesion of the scale is rather impaired. This is presumed to be because if descaling is performed at an excessively high temperature, part of the CuNi enriched area formed up to the heating step is removed together with the scale. From the above point of view, the temperature at which descaling is performed after the hot rolling is preferably limited to a range of 1080°C to T ₀ , and more preferably within a range of 1100°C to (T ₀ -20)°C.

When performing the above descaling, the temperature of the surface of the rolled material is measured in the width direction of the rolled material before and after descaling, and the amount of water for descaling is adjusted in the width direction according to the temperature distribution before descaling. It is possible to reduce the temperature deviation in the width direction of the rolled material after descaling, and to uniformly proceed with scale formation at various locations of the rolled material, which starts immediately after descaling. The amount of water used for descaling varies greatly depending on the size of the rolled material and the state of the equipment, so it is not specified specifically, but the amount of high-pressure water sprayed during descaling is determined according to the temperature measured before descaling. A homogeneous scale can be obtained by increasing or decreasing the amount of water in the width direction or by keeping the amount of water constant and controlling the temperature deviation in the width direction of the rolled material after descaling to 25° C. or less. In order to improve scale homogeneity, it is preferable to control the temperature deviation in the width direction of the rolled material after descaling to 20° C. or less.

The temperature deviation in the width direction of the rolled material after descaling is measured within 10 seconds after the above descaling is completed, and is measured at the center of the rolled material and at 1/4 width from both ends of the rolled material. Among the temperatures, the absolute value of the difference between the highest temperature and the lowest temperature is taken as the temperature deviation in the width direction of the rolled material.

After the above descaling, additional hot rolling is performed to give the rolled material a thickness suitable for the intended use. The scale on the surface of the rolled material is formed and grows mainly from the CuNi enriched area after the descaling described above, and it grows particularly large after all the rolling is completed. Here, if the temperature at which rolling is completed is too high, scale formation and/or growth will be non-uniform, and the uniformity of scale thickness will be impaired. On the other hand, if the temperature at which rolling is completed is too low, some scales may peel off or the peeled scales may be pushed into other locations by rolling, impairing the uniformity of scale thickness. From the above viewpoint, the completion temperature of rolling is set to be 900°C or higher and 1050°C or lower. In order to further improve scale homogeneity, the rolling completion temperature is preferably 915°C or higher and 1010°C or lower, and more preferably 930°C or higher and 980°C or lower.

[Cooling process]
If the scale grows excessively after the hot rolling process is completed, the uniformity of the scale thickness will be impaired. On the other hand, if scale growth is excessively suppressed, the scale becomes thinner, the deviation of the scale thickness from the average thickness of the scale increases, and the scale easily peels off during laser irradiation. From the above point of view, the elapsed time from the completion of the hot rolling process until water cooling is applied to stop scale growth is controlled to improve scale adhesion. Note that since the influence of the passage of time changes depending on the temperature at the time, the elapsed time after the completion of the hot rolling process is managed using equation (3). If the elapsed time from the completion of the hot rolling process to the water cooling is too short and _y10 is less than 1.00, scale growth will be excessively suppressed, resulting in scale thickness deviation and scale adhesion. Since there are places where the properties are impaired, _y10 is set to 1.00 or more. On the other hand, if the time from the completion of the hot rolling process to the water cooling is too long and _y10 exceeds 10.00, the scale will grow excessively, causing thickness deviation of the scale and areas where the adhesion of the scale is impaired. occurs, so y ₁₀ is set to 10.00 or less. In order to homogenize the scale and improve the adhesion of the scale, y ₁₀ is preferably set to 2.00 or more and 8.00 or less, and more preferably 3.00 or more and 7.00 or less.
1.00≦y ₁₀ ≦10.00...Formula (3)
Here, y _n is determined by the following calculation.
y ₁ =E ₁・(J ₁ ² +E ₂ J ₁ +E ₃ )・Δk ^0.5・exp(−E ₄ /J ₁ )
k _n =y _n ²・E ₁ ^-2・(J _n+1 ² +E ₂ J _n+1 +E ₃ ) ^-2・exp(2・E ₄ /J _n+1 )
y _n =E ₁・(J _n ² +E ₂ J _n +E ₃ )・(k _n-1 +Δk) ^0.5・exp(−E ₄ /J _n )

In these calculations, the elapsed time from the completion of the hot rolling process to the start of water cooling in the cooling process is divided into 10 equal parts, and the degree of scale growth (y _n ) after the completion of each section is evaluated, and the subscript n indicates that the calculation corresponds to the n-th interval divided into 10 equal parts. E ₁ , E ₂ , E ₃ , and E ₄ are constants, and are 9.14×10 ⁻⁴ , 2.47×10 ⁰ , −1.09×10 ³ , and 5.89×10 ³ , respectively. J _n is the average steel plate temperature [° C.] in the nth region of the 10 equally divided sections, that is, the arithmetic mean of the temperature measurements at predetermined time intervals in the entire nth region. Δk is a time [second] that is one-tenth of the elapsed time. y ₁₀ in equation (3) is obtained by sequentially calculating y ₁ to y ₂ , y ₃ , etc. using the above calculation formula.

Note that, after the descaling and until the hot rolling process is completed, descaling using high-pressure water may be further performed for the purpose of suppressing excessive scale growth and improving the appearance quality. When descaling is additionally applied, it is preferable to spray the water at a constant amount in the width direction so as not to increase the temperature deviation of the rolled material.

In the method for manufacturing a thick steel plate according to an embodiment of the present invention, water cooling conditions are not particularly defined, but if the steel plate temperature at which water cooling is completed exceeds 700°C, scale will grow even after water cooling and scale homogeneity will be impaired. Therefore, it is preferable that the temperature of the steel plate at which water cooling is completed is 700° C. or lower. In addition, if the temperature of the steel plate at which water cooling is completed is below 300°C, the thermal stress generated in the steel plate will increase, and there is a concern that the scale will partially fracture and the homogeneity of the scale will be impaired. The temperature is preferably 300°C or higher.

In the method for manufacturing a thick steel plate according to an embodiment of the present invention, the cooling conditions after water cooling the steel plate are not particularly defined, but there is a concern that the toughness of the steel plate will be impaired if excessive heat retention is performed after water cooling, so air cooling and/or Preferably, it is cooled. Alternatively, the steel plate may be wound into a coil after being water-cooled or water-cooled, and may be further water-cooled, air-cooled, and/or allowed to cool. Further, after the steel plate is completely cooled, a tempering treatment may be performed within a range that does not impair the characteristics of the thick steel plate according to the embodiment of the present invention.

In addition to the above-mentioned casting process, heating process, rolling process, and cooling process, the method for manufacturing a thick steel plate according to the embodiment of the present invention may further include a flattening process using a hot leveler or the like.

The thick steel plate manufactured by the thick steel plate manufacturing method according to the embodiment of the present invention has a density of 1.0×10 ⁷ Cu and/or Ni enriched areas existing in an island shape at the interface between the steel plate and the scale. /m ² or more, and the thickness deviation of the scale is 10.0% or less of the average thickness of the scale, so that peeling of the scale is suppressed when a thick steel plate is irradiated with a laser during laser cutting work. I can do it. Therefore, such thick steel plates can be stably cut into any shape without causing any problems such as burning during laser cutting, and are suitable for construction, construction, industrial machinery, bridges, etc. It can be used for structures such as

Hereinafter, the present invention will be explained in more detail with reference to examples, and the conditions in the examples are examples of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these example conditions. The present invention may adopt various conditions as long as the objectives of the present invention are achieved without departing from the gist 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 degree of occurrence of scale peeling when the obtained thick steel plates were irradiated with a laser was investigated.

First, using molten steel having the chemical composition shown in Table 1, a slab was cast by a continuous casting method while undergoing the bending process shown in Table 2.The obtained slab was subjected to a heating process under the conditions shown in Table 3. By performing the inter-rolling process and the cooling process, thick steel plates as experimental examples including examples and comparative examples were obtained. In addition, Experimental Examples 56 and 69 are thick steel plates obtained by cooling to 330°C and 260°C, respectively, by water cooling after the completion of the hot rolling process, and then cooling them in the atmosphere. 68 is a thick steel plate obtained by cooling to 630° C. and 670° C., respectively, by water cooling after the completion of the hot rolling process, and then allowing it to cool in the atmosphere. Furthermore, other experimental examples are thick steel plates obtained by water cooling after the completion of the hot rolling process to 450 to 600 ° C. and then cooling in the atmosphere, and in particular, experimental examples 13 and 22, These thick steel plates were obtained by water-cooling the steel plates to 460°C and 510°C, respectively, and then winding the steel plates into a coil and leaving them to cool in the atmosphere.

[Evaluation of scale adhesion]
The scale adhesion of the obtained thick steel plate was evaluated by a test simulating the state in which the thick steel plate was irradiated with a laser. First, a thick steel plate containing scale on the surface layer was cut into 100 mm x 100 mm, heated to 200°C with a burner to simulate the state in the middle of laser cutting, which was heated to some extent by the laser, and the following was placed in the center of the thick steel plate surface. A laser beam was irradiated over a length of 10 mm under the following conditions, and a 1 mm long area at the center of the laser irradiation mark on the surface was observed using an optical microscope to evaluate the degree of scale peeling in the same area. If the area ratio of scale peeling off at the relevant location exceeds 30% after one irradiation, it will be marked "x", if it exceeds 5% and 30% or less, it will be marked "○", if it is 5% or less, it will be marked "◎", and "○". A thick steel plate that received a rating of "◎" was considered to be a steel plate that had excellent scale adhesion and suppressed or prevented scale peeling during laser irradiation, and was judged to be acceptable. The results are shown in Table 4.
Laser power: 500W
Pulse frequency: 60kHz
Focusing diameter: 0.70mm
Irradiation speed: 3000m/sec

In the experimental examples listed in Tables 1 to 4, in experimental example 35, the temperature at which strain is applied to the surface of the slab in the casting process is low, and the CuNi enriched part is not sufficiently formed at the interface between the steel plate and the scale, and the scale This is a comparative example in which the adhesion was inferior. On the other hand, in Experimental Example 64, the temperature at which strain is applied to the surface of the slab during the casting process is high, and cracks occurred on the surface of the slab manufactured through the casting process. This is a comparative example in which it was necessary to grind the surface of the slab, and as a result, a CuNi enriched portion was not sufficiently formed at the interface between the steel plate and the scale, resulting in inferior scale adhesion.
Experimental Example 7 is a comparative example in which the amount of strain applied in the predetermined temperature range of the casting process was small, and a CuNi enriched part was not sufficiently formed at the interface between the steel plate and the scale, resulting in inferior scale adhesion. . On the other hand, in Experimental Example 11, the amount of strain applied in the predetermined temperature range of the casting process was large, and cracks occurred on the surface of the slab manufactured through the casting process. This is a comparative example in which it was necessary to grind the surface of the steel plate, and as a result, a CuNi-enriched portion was not sufficiently formed at the interface between the steel plate and the scale, resulting in inferior scale adhesion.
Experimental example 20 is a comparative example in which the maximum heating temperature of the slab in the heating step was low, and the CuNi enriched portion was not sufficiently formed at the interface between the steel plate and the scale, resulting in inferior scale adhesion. On the other hand, Experimental Example 12 is a comparative example in which the maximum heating temperature of the slab in the heating step was high, the density of the CuNi-enriched portion was insufficient, and the adhesion of the scale was inferior.
Experimental Example 8 is a case where the heating time in the heating process was short and formula (2) was not satisfied, and the CuNi enriched area was not sufficiently formed at the interface between the steel plate and the scale, resulting in inferior scale adhesion. This is a comparative example. On the other hand, in Experimental Example 27, the heating time in the heating process was long and equation (2) was not satisfied, and the density of the CuNi enriched area at the interface between the steel plate and the scale was reduced, and the adhesion of the scale was inferior. This is a comparative example.
Experimental Example 3 is a comparative example in which the cumulative reduction ratio of hot rolling performed before descaling was small, the thickness deviation of the scale was large, and the adhesion of the scale was inferior. Further, Experimental Example 16 is a comparative example in which the hot rolling temperature before descaling was low, the scale thickness deviation was large, and the scale adhesion was inferior. On the other hand, Experimental Example 24 is a comparative example in which the hot rolling temperature before descaling was high, the scale thickness deviation was large, and the scale adhesion was inferior.
Experimental example 15 is a comparative example in which the descaling temperature was low, the scale thickness deviation was large, and the scale adhesion was inferior. On the other hand, Experimental Example 32 is a comparative example in which the descaling temperature was high, the density of the CuNi-enriched portion was reduced, and the scale adhesion was inferior.
Experimental Example 19 is a comparative example in which the temperature deviation of the rolled material after descaling was large, the scale thickness deviation was large, and the scale adhesion was inferior.
Experimental example 4 is a comparative example in which the rolling completion temperature was low, the scale thickness deviation was large, and the scale adhesion was inferior. On the other hand, Experimental Example 23 is a comparative example in which the rolling completion temperature was high, the scale thickness deviation was large, and the scale adhesion was inferior.
Experimental example 31 is a case in which the elapsed time from the completion of the hot rolling process to the start of water cooling is short and formula (3) is not satisfied, and the scale thickness deviation is large and the scale adhesion is inferior. This is an example. On the other hand, in Experimental Example 28, the elapsed time from the completion of the hot rolling process to the start of water cooling was long, and the equation (3) was not satisfied, so the thickness deviation of the scale was large and the adhesion of the scale was inferior. This is a comparative example.
Experimental Examples 73 to 77 are comparative examples in which the chemical composition of the thick steel plate was not necessarily appropriate, the desired CuNi enriched area was not formed at the interface between the steel plate and the scale, and the adhesion of the scale was inferior.

Experimental examples other than the above comparative examples, i.e. Experimental examples 1, 2, 5, 6, 9, 10, 13, 14, 17, 18, 21, 22, 25, 26, 29, 30, 33, 34, 36 -63 and 65-72 are examples of the present invention, and thick steel plates with excellent scale adhesion were obtained.

Claims (5)

A density of a Cu and/or Ni enriched area (CuNi enriched area) that includes a steel plate and a scale formed on the surface of the steel plate and exists in an island shape at the interface between the steel plate and the scale is 1.0. ×10 ⁷ pieces/m ² or more, and the thickness deviation of the scales is 10.0% or less of the average thickness of the scales. The thick steel plate according to claim 1, wherein the scale has an average thickness of 6 to 60 μm. The steel plate is in mass%,
C: 0.001-0.300%,
Si: 0.01-1.00%,
Mn: 0.10 to 2.50%,
P: 0.001-0.050%,
S: 0.0001-0.0100%,
Al: 0.001-0.200%,
Cu: 0.01 to 1.00%,
Ni: 0.01-2.00%,
N: 0.0150% or less,
O: 0.0050% or less,
Cr: 0-1.00%,
Mo: 0-1.00%,
W: 0-0.50%,
Nb: 0 to 0.500%,
Ti: 0 to 0.500%,
V: 0-1.000%,
B: 0 to 0.0100%,
Sn: 0-0.500%,
Sb: 0 to 0.500%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Hf: 0-0.0100%,
Te: 0 to 0.0100%,
Sr: 0 to 0.0100%,
REM: 0 to 0.0100%, and the remainder: Fe and impurities,
The thick steel plate according to claim 1 or 2, having a chemical composition that satisfies the following formula (1).
0.05≦[Cu]+0.5[Ni]≦1.00...Formula (1)
[Cu] and [Ni] represent the content [mass%] of each element in the steel plate. The chemical composition is in mass%,
Cr: 0.01-1.00%,
Mo: 0.01-1.00%,
W: 0.003-0.50%,
Nb: 0.003 to 0.500%,
Ti: 0.003 to 0.500%,
V: 0.003-1.000%,
B: 0.0003 to 0.0100%,
Sn: 0.003 to 0.500%,
Sb: 0.003 to 0.500%,
Ca: 0.0003-0.0100%,
Mg: 0.0003 to 0.0100%,
Hf: 0.0003-0.0100%,
Te: 0.0003 to 0.0100%,
Sr: 0.0003 to 0.0100%, and REM: 0.0003 to 0.0100%
The thick steel plate according to claim 3, comprising one or more selected from the group consisting of: A process of manufacturing slabs by continuous casting, in which the surface temperature of the slab during continuous casting is in the range of 1000 to 1200°C, and the bending process is performed such that the amount of strain on the surface of the slab is 20 to 40%. casting process,
The step of heating the slab, in which the surface temperature of the slab is heated to a maximum heating temperature of (T ₀ +20) °C to 1300 °C, and the elapsed time from exceeding T ₀ °C to completion of the heating process is as follows. a heating step controlled to satisfy formula (2);
The step of hot rolling the slab, in which the surface temperature of the slab is rolled at a temperature range of 1080°C to T ₀ and a cumulative reduction rate of 15% or more, the surface temperature of the obtained rolled material is High-pressure water descaling is performed in a temperature range of 1080 to T ₀ , the temperature deviation in the width direction of the rolled material after high-pressure water descaling is 25°C or less, and then additional hot rolling is carried out until the rolling completion temperature is 900 to 900°C. The hot rolling process is carried out to achieve a temperature of 1050°C, and the process of cooling the obtained steel plate, in which the elapsed time from the completion of the hot rolling process to the start of water cooling is controlled so that the following formula (3) is satisfied. The method for producing a thick steel plate according to any one of claims 1 to 4, comprising a cooling step.
1.00≦ _x10 ≦10.00...Formula (2)
x ₁ =D ₁・{(T ₁ −T ₀ ) ² +D ₂ T ₁ +D ₃ }・Δt ^0.5・exp(−D ₄ /T ₁ )
t _n =x _n ²・D ₁ ^-2・{(T _n+1 −T ₀ ) ² +D ₂ T _n+1 +D ₃ } ^-2・exp(D ₄ /T _n+1 )
x _n =D ₁・{(T _n −T ₀ ) ² +D ₂ T _n +D ₃ }・(t _n-1 +Δt) ^0.5・exp(−D ₄ /T _n )
x _n is an index representing the degree of formation of the CuNi-enriched portion after each section is completed by dividing the elapsed time from the time when the surface temperature of the slab exceeds T ₀ °C to the completion of the heating process into 10 equal parts. Yes, n indicates that the calculation corresponds to the nth section of the interval divided into 10 equal parts,
D ₁ , D ₂ , D ₃ and D ₄ are constants and are 8.92×10 ⁻⁴ , 2.47×10 ⁰ , −1.09×10 ³ and 5.89×10 ³ , respectively;
T _n is the average slab temperature [°C] in the nth region of the 10 equally divided sections,
Δt is one tenth of the elapsed time [seconds],
x ₁₀ is obtained by sequentially calculating x ₁ to x ₂ , x ₃ . . . using the above calculation formula.
1.00≦y ₁₀ ≦10.00...Formula (3)
y ₁ =E ₁・(J ₁ ² +E ₂ J ₁ +E ₃ )・Δk ^0.5・exp(−E ₄ /J ₁ )
k _n =y _n ²・E ₁ ^-2・(J _n+1 ² +E ₂ J _n+1 +E ₃ ) ^-2・exp(2・E ₄ /J _n+1 )
y _n =E ₁・(J _n ² +E ₂ J _n +E ₃ )・(k _n-1 +Δk) ^0.5・exp(−E ₄ /J _n )
y _n is an index representing the degree of scale growth after each section is completed by dividing the elapsed time from the completion of the hot rolling process to the start of water cooling into 10 equal parts in the cooling process, and n is the index representing the degree of scale growth after each section is completed. Indicates that it is the nth calculation,
E ₁ , E ₂ , E ₃ and E ₄ are constants and are 9.14×10 ⁻⁴ , 2.47×10 ⁰ , −1.09×10 ³ and 5.89×10 ³ , respectively;
J _n is the average steel plate temperature [°C] in the nth region of the 10 equally divided sections,
Δk is one-tenth of the elapsed time [seconds],
y ₁₀ is obtained by sequentially calculating from y ₁ to y ₂ , y ₃ , etc. using the above calculation formula.
T ₀ is the temperature determined by the following equation (4).
T ₀ = K ₁ + K ₂ [Cu] [Ni] + K ₃ [Cu] [Ni] ^0.5 (K ₄ - [Cu]) + K ₅ (K ₆ + log ₁₀ [Si]) ² + K ₇ Al + K ₈ Mn... Formula (4)
K ₁ =1.158×10 ³ , K ₂ =6.288×10 ⁰ , K ₃ =3.337×10 ¹ , K ₄ =1.783×10 ⁰ , K ₅ =1.400×10 ⁰ , K ₆ =1.000×10 ⁰ , K ₇ =−3.500×10 ¹ , and K ₈ =−1.560×10 ¹ ,
[Cu] and [Ni] represent the content [mass%] of each element in the slab.