JP2023128426A - Thick steel plate and method for manufacturing the same - Google Patents

Abstract

To provide a thick steel plate which suppresses or prevents occurrence of burning, and accordingly is useful for being used in 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 in an interface vicinity region of 10 μm×100 μm of an interface length in a plate thickness direction with the position of the interface between the steel plate and the scale as a center, 50 μm2 or more of a metal-concentrated phase in which the total of Cu, Ni, Mo, Sn and Sb concentrations (mass%) is 3.0 times or more of the total of Cu, Ni, Mo, Sn and Sb contents (mass%) of the steel plate is contained. A method for manufacturing a thick steel plate includes the steps of: heating a slab, including holding the slab at a surface temperature of the slab of 1,100-1,300°C for 30-120 minutes; holding the slab in the atmosphere for 50-120 seconds; and hot-rolling the slab, including subjecting the slab to descaling by final high-pressure water at a temperature of 1,050°C or higher.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 useful for use in laser cutting and a method for manufacturing the same.

A large amount of thick steel plates are used in steel structures such as shipbuilding, architecture, industrial machinery, and bridges. When working on these steel structures, welding and cutting account for much of the construction cost and man-hours. In addition to conventional gas cutting, plasma cutting, laser cutting, and the like are known as methods for cutting steel plates.

Compared to conventional gas cutting, laser cutting has become popular mainly in the thin plate processing industry because it has superior cut surface accuracy, a smaller heat-affected zone, and can be automated. However, in recent years, with the commercialization of high-power laser cutting machines, laser cutting machines have come to be used even for cutting thick steel plates.

The production of steel plates such as thick steel plates generally involves the process of hot rolling slabs. On the other hand, it is known that hot rolled steel plates are oxidized in the atmosphere and scale (oxides) is formed on their surfaces. ing. When laser cutting thick steel plates, if this scale peels off on the surface of the steel plate or is peeled off by the laser during cutting, the thick steel plate may not be cut properly or abnormal cuts such as gouges may occur on the cut surface. In some cases, stable cutting may not be possible.

In connection with this, for example, in Patent Document 1, a scale layer on the steel surface has fine metal particles made of an alloy mainly composed of Fe, Cu, and Ni dispersed in the scale near the interface with the steel base material. A steel plate is described which is characterized in that it has a structured scale/metal mixed layer with a thickness of 5 to 30 μm. In addition, in Patent Document 1, the scale/metal mixed layer greatly contributes to improving the adhesion of the scale, and more specifically, the scale/metal mixed layer in which fine metal particles are dispersed in the scale is made from metal oxides. Because it has a larger heat capacity than scale alone, 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. has been done.

In Patent Document 2, in weight %, C: 0.03 to 0.25%, Si: 0.10 to 0.50%, Mn: 0.05 to 1.60%, Mo: 0.005 to 0 A steel material for laser gas cutting is described that has a scale layer with a thickness of 10 to 60 μm, with the balance consisting of Fe and unavoidable impurities. Further, Patent Document 2 states that the steel material improves the adhesion of the scale layer, which is an important controlling factor for laser gas cutting performance, and significantly expands the stable cutting range in laser gas cutting.

Japanese Patent Application Publication No. 2006-219712 Japanese Patent Application Publication No. 9-279305

In order to achieve stable cutting in laser cutting, in addition to improving the adhesion of the scale as taught in Patent Documents 1 and 2, it is important to prevent the occurrence of cutting defects called burning. More specifically, in laser cutting of a steel plate, the steel plate is heated by laser irradiation, the heated steel plate is reacted with oxygen blown into the surrounding area, and the generated heat is used to melt the steel plate. For example, in laser cutting, because the cutting width is narrow, the molten material may become clogged within the cutting width, and this clogging causes excessive melting in the surrounding area or the molten material blowing up, a phenomenon called burning. Poor cutting may occur. Therefore, there is a need in the art to suppress or prevent the occurrence of such cutting defects.

Although Patent Documents 1 and 2 discuss improving the adhesion of scale in order to achieve stable laser cutting, sufficient consideration is not necessarily made from the viewpoint of preventing the occurrence of burning. Therefore, the steel materials described in these patent documents still have room for improvement in cutting performance.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a thick steel plate that suppresses or prevents the occurrence of burning and is therefore useful for laser cutting, and a method for manufacturing the same.

In order to achieve the above object, the present inventors conducted a study considering that it is important not to excessively melt Fe on the surface of the steel plate, which is the starting point of cutting. As a result, the present inventors were able to suppress excessive melting of Fe by concentrating specific elements in the region near the interface between the steel plate and the scale, and in connection with this, the occurrence of burning when cutting thick steel plates. The present invention was completed based on the discovery that this can be suppressed or prevented.

The present invention that achieves the above object is as follows.
(1) In a region near the interface that includes a steel plate and a scale formed on the surface of the steel plate and has an area of 10 μm in the plate thickness direction and an interface length of 100 μm centered on the position of the interface between the steel plate and the scale, Cu, A metal-enriched phase in which the total Ni, Mo, Sn, and Sb concentration (mass %) is 3.0 times or more the total Cu, Ni, Mo, Sn, and Sb content (mass %) of the steel sheet is 50 μm ² Thick steel plate containing the above.
(2) The steel plate is mass%,
C: 0.01-0.30%,
Si: 0.01-0.60%,
Mn: 0.01 to 2.00%,
P: 0.001-0.050%,
S: 0.001-0.050%,
Al: 0.001-0.100%,
Cr: 0.01-0.50%,
Cu: 0.010 to 0.50%,
Ni: 0.010 to 0.50%,
Mo: 0.010-0.500%,
Sn: 0-0.500%,
Sb: 0 to 0.500%,
Nb: 0 to 0.500%,
V: 0 to 0.500%,
Ti: 0 to 0.500%,
B: 0 to 0.0100%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
REM: 0 to 0.0100%, and the balance: consisting of Fe and impurities,
The thick steel plate according to (1) above, having a chemical composition satisfying 0.060≦[Cu]+[Ni]+[Mo]+[Sn]+[Sb]≦1.000.
Here, [Cu], [Ni], [Mo], [Sn], and [Sb] are the contents (mass%) of each element in the steel plate, and are 0 when no element is contained.
(3) the chemical composition is in mass%;
Sn: 0.001 to 0.500%,
Sb: 0.001 to 0.500%,
Nb: 0.001-0.500%,
V: 0.001-0.500%,
Ti: 0.001 to 0.500%,
B: 0.0001-0.0100%
Ca: 0.0001-0.0100%,
Mg: 0.0001 to 0.0100%, and REM: 0.0001 to 0.0100%
The thick steel plate according to (2) above, comprising one or more selected from the group consisting of:
(4) The thick steel plate according to any one of (1) to (3) above, wherein the scale has a thickness of 5 to 50 μm.
(5) The thick steel plate according to any one of (1) to (4) above, having a thickness of 6 to 40 mm.
(6) The thick steel plate according to any one of (1) to (5) above, which is used for laser cutting.
(7) a step of heating the slab in a heating furnace, the step comprising maintaining the surface temperature of the slab in a range of 1100 to 1300° C. for 30 to 120 minutes;
taking out the slab from the heating furnace and then holding it in the atmosphere for 50 to 120 seconds, and hot rolling the slab, the final high-pressure water descaling being carried out at a temperature of 1050° C. or higher. The method for producing a thick steel plate according to any one of (1) to (6) above, including the step of:

According to the present invention, it is possible to provide a thick steel plate in which the occurrence of burning is suppressed or prevented, and therefore useful for use in laser cutting, and a method for manufacturing the same.

Hereinafter, the present invention will be explained in more detail regarding a thick steel plate used for laser cutting, but the thick steel plate of the present invention is not limited to such a specific use, and can be used for any purpose where burning may occur. It goes without saying that the present invention can also be applied to cutting operations such as gas cutting and plasma cutting.

<Thick steel plate>
The thick steel plate according to the embodiment of the present invention includes a steel plate and a scale formed on the surface of the steel plate, and has a size of 10 μm x interface length of 100 μm in the plate thickness direction centered on the position of the interface between the steel plate and the scale. In the region near the interface, the total concentration of Cu, Ni, Mo, Sn and Sb (mass%) is 3.0 times or more the total of the Cu, Ni, Mo, Sn and Sb content (mass%) of the steel sheet. It is characterized by containing a metal-enriched phase of 50 μm ² or more.

As mentioned above, in laser cutting a steel plate, the steel plate is heated by laser irradiation, the heated steel plate is reacted with oxygen blown into the surrounding area, and the heat generated is used to melt the steel plate. Cutting is then performed by removing the molten oxide or metal by blowing oxygen. In the case of laser cutting, since the width to be melted and removed is relatively narrow, advantages include small size changes due to cutting and the possibility of microfabrication. However, on the other hand, if the viscosity of the molten material is high or there is a large amount of molten material due to the narrow cutting width, the discharge property (also called "flowability") of the molten material will be poor, and the molten material will not flow within the cutting width. It may get clogged. If the molten material becomes clogged, the clogging may cause excessive melting of the surrounding area or the molten material may blow up, resulting in cutting defects called burning. When burning occurs, it becomes necessary to stop the automatic operation of laser cutting, for example, and furthermore, the scattering of molten material may cause equipment trouble. Therefore, in steel plates such as thick steel plates that are subjected to laser cutting, it is required to be able to suppress or prevent the occurrence of burning under typical laser cutting conditions.

In order to suppress or prevent the occurrence of burning, it is important to maintain the flowability of the melt at a high level during cutting and to prevent clogging of the melt. In connection with this, the present inventors believe that it is important not to excessively melt Fe on the surface of the steel sheet, which is the starting point of cutting, in order to improve the fluidity of the metal, and therefore, we have developed methods to suppress such excessive melting of Fe. We investigated the structure of the steel plate that can be used. As a result, the present inventors found that certain elements, more specifically the elements Cu, Ni, Mo, Sn and Sb, were concentrated and present on the steel plate surface, more specifically at the interface between the steel plate and the scale. It has been found that by doing so, it is possible to suppress excessive melting of Fe on the surface of the steel sheet during cutting with a laser or the like. Based on this knowledge, the present inventors conducted a more detailed study and found that in a region near the interface of 10 μm in the plate thickness direction and 100 μm in interface length centered on the position of the interface between the steel plate and the scale, Cu, Ni, Mo, The sum of the Sn and Sb concentrations (mass%) (Ms) is 3.0 times or more the sum of the Cu, Ni, Mo, Sn, and Sb contents (Mm) of the entire steel sheet, that is, the Ms/Mm ratio is 3.0 or more. By containing 50 μm ² or more of the metal-enriched phase, it is possible to sufficiently suppress excessive melting of Fe on the surface of the steel sheet during cutting with a laser, etc., and improve the flowability of the melt. It has been found that the occurrence of burning can be prevented or significantly suppressed.

Although not intending to be bound by any particular theory, since Cu, Ni, Mo, Sn, and Sb are more noble elements than Fe, these elements form oxides at the interface between the steel plate and the scale. It is thought that it exists as a metal rather than as a metal. These elements, which are relatively difficult to oxidize, are concentrated in a relatively wide range near the interface between the steel plate and the scale (i.e., the surface of the base steel plate), which reduces the amount of Fe on the surface of the steel plate during cutting. It is considered that oxidation and heat generation accompanying the oxidation can be suppressed. As a result, excessive melting of Fe on the surface of the steel sheet during cutting can be significantly suppressed, so gas cutting and plasma cutting, which have wider cutting widths than laser cutting, are of course suitable for gas cutting and plasma cutting. Even with a cutting method in which the cutting width is relatively narrow and clogging of the molten material is likely to occur, it is believed that it is possible to reliably improve the flowability of the molten material. Therefore, according to the thick steel plate according to the embodiment of the present invention, Cu, Ni, Mo, Sn and By containing 50 μm ^or more of a metal-enriched phase whose total Sb concentration (mass%) is 3.0 times or more the total content of Cu, Ni, Mo, Sn, and Sb in the entire steel sheet, cutting such as burning can be prevented. It becomes possible to reliably suppress or prevent defects. The fact that by concentrating these specific metal elements that are less oxidizable than Fe at the interface between the steel plate and the scale, excessive melting of Fe can be suppressed and cutting defects such as burning can be reliably suppressed or prevented. has not been previously known, and has now been revealed for the first time by the present inventors.

Hereinafter, thick steel plates according to embodiments of the present invention will be explained in more detail, but these explanations are intended to be merely illustrative of preferred embodiments of the present invention, and the present invention is not limited to such specific embodiments. It is not intended to be limited to.

[Metal enriched phase with Ms/Mm ratio of 3.0 or more in the region near the interface: 50 μm ² or more]
In the thick steel plate according to the embodiment of the present invention, Cu, Ni, Mo, Sn, and Sb concentrations (mass %) is 3.0 times or more the total content (Mm) of Cu, Ni, Mo, Sn, and Sb in the entire steel sheet, that is, Ms/Mm≧3.0. It is necessary to contain ² or more. As mentioned above, Cu, Ni, Mo, Sn, and Sb are elements nobler than Fe, and therefore are less likely to be oxidized than Fe. Therefore, by allowing a metal-enriched phase in which these elements are concentrated so that Ms/Mm≧3.0 to exist in a relatively wide area of 50 μm ² or more in the region near the interface, the steel plate surface during cutting can be improved. The oxidation of Fe and the heat generation associated with the oxidation can be suppressed, and as a result, the flowability of the melt can be significantly improved. In the thick steel plate according to the embodiment of the present invention, the total concentration of Cu, Ni, Mo, Sn, and Sb in each metal-enriched phase in the region near the interface meets the above requirements with respect to the total content of these elements in the entire steel plate. The metal-enriched phase or steel plate does not necessarily need to contain all of these elements. That is, the metal concentrated phase or the steel plate does not need to contain any one or more of Cu, Ni, Mo, Sn, and Sb.

From the viewpoint of suppressing or preventing cutting defects such as burning, it is preferable that the area of the metal-enriched phase in the region near the interface is large. Specifically, the area is preferably 52 μm ² or more, more preferably 55 μm ² or more, even more preferably 60 μm ² or more, and most preferably 65 μm ² or more. Although the upper limit of the area of the metal-enriched phase in the region near the interface is not particularly limited, the area may be, for example, 200 μm ² or less, 150 μm ² or less, or 120 μm ^{2 or} less.

The metal-enriched phase exists in a layered or granular form in the region near the interface, and the existing form and mass of the metal-enriched phase can be quantitatively evaluated by EPMA (electron beam microanalyzer) observation. be. Therefore, the area of the metal-enriched phase in the region near the interface according to the present invention is determined using EPMA as follows. Specifically, a quantitative measurement using EPMA is performed at a magnification of 1500 times on a cross section of an arbitrary location parallel to the thickness direction of the steel plate. First, in the EPMA measurement results of O (oxygen), the average value of the O concentration in the area of one pixel in the thickness direction of the scale and all pixels in the direction perpendicular to it is calculated. Next, this average value is calculated in order from the scale side to the substrate direction, and the point where this average value falls below 10% for the first time is determined as the interface between the steel plate and the scale, and the scale in the plate thickness direction is calculated from the determined interface position. A region of 5 μm on each of the side and the base metal side, that is, a region of 10 μm in the plate thickness direction centering on the position of the interface, is determined as the region near the interface. Next, the elemental distribution of Cu, Ni, Mo, Sn, and Sb in the region near this interface is observed. In the image of elemental distribution obtained by EPMA measurement, a quantitative measurement value is assigned to each pixel. Therefore, by adding these values for Cu, Ni, Mo, Sn, and Sb, it is possible to obtain the total distribution of Cu, Ni, Mo, Sn, and Sb concentrations (mass %) in the region near the interface. In the present invention, a region near the interface in a straight line length (interface length) of 100 μm in the direction perpendicular to the plate thickness direction, that is, a region near the interface of 100 μm x 10 μm = 1000 μm ² is specified based on the position of the interface determined previously. Then, the total Ms (mass %) of the concentrations of Cu, Ni, Mo, Sn, and Sb in each metal phase in the specified region near the interface is calculated. By dividing the calculated Ms by the total Mm (mass%) of the Cu, Ni, Mo, Sn, and Sb contents of the entire steel plate, the enrichment ratio of Cu, Ni, Mo, Sn, and Sb (Ms/Mm) and its distribution can be obtained. Finally, by multiplying the number of pixels for which Ms/Mm is 3.0 or more in the EPMA measurement image in the 1000 μm ² near-interface region by the actual area per pixel, we can calculate the amount of Cu, Ni, Mo, Sn and The area of the metal-enriched phase where the total Sb concentration (mass %) is 3.0 times or more the sum of the Cu, Ni, Mo, Sn, and Sb contents (mass %) of the steel sheet is determined.

The content of Cu, Ni, Mo, Sn, and Sb in the steel plate may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES).

[Preferred chemical composition of steel plate]
In the embodiment ^of the present invention ^, the steel plate has Ms. The material may be any material that satisfies the requirement of containing 50 μm ² or more of a metal-enriched phase where /Mm≧3.0. 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 provide a thick steel plate in which the occurrence of burning is suppressed or prevented, and is therefore useful for use in laser cutting. This objective is achieved by containing 50 μm ² or ^more of a metal-enriched phase in which Ms/Mm≧3.0 per 1000 μm 2 . Therefore, it is clear that the chemical composition of the steel sheet itself is not an essential technical feature to achieve the object of the present invention. The preferred chemical composition of the steel sheet according to the embodiment of the present invention will be explained in detail below, but these explanations are based on a steel plate containing 50 μm ² or ^more of a metal-enriched phase with Ms/Mm≧3.0 per 1000 μm 2 near the interface. This is intended to be merely an example of a preferred chemical composition of a steel plate to meet the requirements of the above, and is not intended to limit the present invention to steel plates having such specific chemical compositions. In addition, in the following description, "%", which is the unit of content of each element, means "mass %" unless otherwise specified. Further, in this specification, the term "~" indicating a numerical range is used to include the numerical values listed before and after the range as the lower limit and upper limit, unless otherwise specified.

[C:0.01-0.30%]
C is an element necessary for stabilizing hardness and/or ensuring strength. In order to fully obtain these effects, the C content is preferably 0.01% or more. The C content may be 0.03% or more, 0.05% or more, or 0.10% or more. On the other hand, if C is contained excessively, toughness, bendability and/or weldability may deteriorate. Therefore, the C content is preferably 0.30% or less. The C content may be 0.28% or less, 0.26% or less, or 0.24% or less.

[Si:0.01-0.60%]
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. The Si content may be 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, if Si is contained excessively, toughness may decrease or surface quality defects called scale defects may occur. Therefore, the Si content is preferably 0.60% or less. The Si content may be 0.50% or less, 0.45% or less, or 0.40% or less.

[Mn: 0.01-2.00%]
Mn is an effective element for improving hardenability and/or strength, and is also an effective austenite stabilizing element. In order to fully obtain these effects, the Mn content is preferably 0.01% or more. The Mn content may be 0.10% or more, 0.30% or more, or 0.50% or more. On the other hand, if Mn is contained excessively, MnS, which is harmful to toughness, may be generated. Therefore, the Mn content is preferably 2.00% or less. The Mn content may be 1.60% or less, 1.40% or less, or 1.20% or less.

[P:0.001-0.050%]
P is an element mixed in during the manufacturing process. Although it is preferable to have less P, reducing the P content to less than 0.001% requires time for refining, which may lead to a decrease in productivity. Therefore, the P content is preferably 0.001% or more. The P content may be 0.005% or more, 0.007% or more, or 0.010% or more. On the other hand, if too much P is contained, workability and/or toughness may decrease. Therefore, the P content is preferably 0.050% or less. The P content may be 0.040% or less, 0.030% or less, or 0.020% or less.

[S: 0.001-0.050%]
S is an element mixed in during the manufacturing process. Although it is preferable to have less S, reducing the S content to less than 0.001% requires time for refining, which may lead to a decrease in productivity. Therefore, the S content is preferably 0.001% or more. The S content may be 0.002% or more, 0.004% or more, or 0.006% or more. On the other hand, if S is contained excessively, toughness may decrease. Therefore, the S content is preferably 0.050% or less. The S content may be 0.035% or less, 0.020% or less, or 0.015% or less.

[Al: 0.001-0.100%]
Al is a deoxidizing element and is also an effective element for improving corrosion resistance and/or heat resistance. In order to fully obtain these effects, the Al content is preferably 0.001% or more. The Al content may be 0.005% or more, 0.010% or more, or 0.020% or more. On the other hand, if Al is contained excessively, coarse inclusions may be formed and the toughness may be reduced. Therefore, the Al content is preferably 0.100% or less. The Al content may be 0.080% or less, 0.060% or less, or 0.040% or less.

[Cr:0.01~0.50%]
Cr is an element that contributes to improving strength and/or corrosion resistance. In order to fully obtain these effects, the Cr content is preferably 0.01% or more. The Cr content may be 0.03% or more, 0.05% or more, or 0.10% or more. On the other hand, if Cr is contained excessively, the toughness may decrease in addition to an increase in alloy cost. Therefore, the Cr content is preferably 0.50% or less. The Cr content may be 0.40% or less, 0.30% or less, or 0.20% or less.

[Cu:0.010-0.50%]
Cu is an element that is effective in concentrating at the interface between the steel plate and the scale and suppressing the oxidation of Fe on the surface of the steel plate during cutting by laser cutting or the like and the heat generated thereby. In addition, Cu melts at high temperatures during the manufacturing process of steel sheets, incorporates other metal elements such as Ni, Mo, Sn, and/or Sb into liquid Cu, and causes the concentration of these elements at the interface between the steel sheet and the scale. It also has the function of promoting. In order to fully obtain these effects, the Cu content is preferably 0.010% or more. The Cu content may be 0.03% or more, 0.05% or more, 0.10% or more, 0.20% or more, or 0.30% or more. On the other hand, if Cu is contained excessively, the effect may be saturated and the toughness and weldability may deteriorate. Therefore, the Cu content is preferably 0.50% or less. The Cu content may be 0.45% or less, 0.40% or less, or 0.35% or less.

[Ni:0.010-0.50%]
Ni is an element that is effective in concentrating at the interface between the steel plate and the scale and suppressing the oxidation of Fe on the surface of the steel plate during cutting by laser cutting or the like and the heat generated thereby. In order to sufficiently obtain such effects, the Ni content is preferably 0.010% or more. The Ni content may be 0.03% or more, 0.05% or more, 0.10% or more, 0.20% or more, or 0.30% or more. On the other hand, if Ni is contained excessively, the effect will be saturated and the alloy cost will increase. Therefore, the Ni content is preferably 0.50% or less. The Ni content may be 0.45% or less, 0.40% or less, or 0.35% or less.

[Mo: 0.010-0.500%]
Mo is an element that is effective in concentrating at the interface between the steel plate and the scale and suppressing the oxidation of Fe on the surface of the steel plate during cutting by laser cutting or the like and the heat generated thereby. In order to sufficiently obtain such effects, the Mo content is preferably 0.010% or more. The Mo content may be 0.015% or more, 0.030% or more, 0.100% or more, or 0.300% or more. On the other hand, if Mo is contained excessively, deformation resistance during hot working may increase, and equipment load may increase. Therefore, the Mo content is preferably 0.500% or less. The Mo content may be 0.450% or less, 0.400% or less, or 0.350% or less.

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. For example, 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%. Further, the steel plate may contain one or more selected from the group consisting of Nb: 0 to 0.50%, V: 0 to 0.50%, and Ti: 0 to 0.50%. . Further, the steel plate may contain B: 0 to 0.0100%. Further, the steel plate may contain one or more selected from the group consisting of Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, and REM: 0 to 0.0100%. . These optional elements will be explained in detail below.

[Sn: 0 to 0.500%]
Sn is an element that is effective in concentrating at the interface between the steel plate and the scale and suppressing the oxidation of Fe on the surface of the steel plate and the heat generation associated with it during cutting by laser cutting or the like. Furthermore, as in the case of Cu, Sn melts at high temperatures during the manufacturing process of steel sheets, incorporates other metal elements into liquid phase Sn, and has the function of promoting the concentration of these elements at the interface between the steel sheet and the scale. It also has Although the Sn content may be 0%, in order to obtain these effects, the Sn content is preferably 0.001% or more. The Sn content may be 0.003% or more, 0.005% or more, 0.010% or more, 0.030% or more, or 0.100% or more. On the other hand, if Sn is contained excessively, the effect may be saturated and the toughness, particularly low-temperature toughness, may deteriorate. Therefore, the Sn content is preferably 0.500% or less. The Sn content may be 0.400% or less, 0.300% or less, or 0.200% or less.

[Sb: 0 to 0.500%]
Sb is an element that is effective in concentrating at the interface between the steel plate and the scale and suppressing the oxidation of Fe on the surface of the steel plate during cutting by laser cutting or the like and the heat generated thereby. In addition, as in the case of Cu and Sn, Sb melts at high temperatures during the manufacturing process of steel sheets, incorporates other metal elements into the liquid phase Sb, and promotes the concentration of these elements at the interface between the steel sheet and the scale. It also has the function of Although the Sb content may be 0%, in order to obtain these effects, the Sb content is preferably 0.001% or more. The Sb content may be 0.003% or more, 0.005% or more, 0.010% or more, 0.030% or more, or 0.100% or more. On the other hand, if Sb is contained excessively, the effect may be saturated and the toughness, particularly low temperature toughness, may be reduced. Therefore, the Sb content is preferably 0.500% or less. The Sb content may be 0.400% or less, 0.300% or less, or 0.200% or less.

[Nb: 0 to 0.500%]
[V: 0-0.500%]
[Ti: 0 to 0.500%]
Nb, V, and Ti are elements that contribute to improving the strength of the steel plate through precipitation strengthening and the like. The Nb, V, and Ti contents may be 0%, but in order to obtain such effects, the Nb, V, and Ti contents are preferably 0.001% or more, and 0.010% or more. % or more, 0.050% or more, or 0.100% or more. On the other hand, if these elements are contained excessively, the effect may be saturated and the workability and/or toughness may be reduced. Therefore, the Nb, V and Ti contents are each preferably 0.500% or less, and may be 0.400% or less, 0.300% or less, or 0.200% or less.

[B: 0 to 0.0100%]
B is an element that contributes to improving strength. Although the B content may be 0%, in order to obtain such an effect, the B content is preferably 0.0001% or more. The B content may be 0.0003% or more, 0.0005% or more, or 0.0007% or more. On the other hand, if B is contained excessively, toughness and/or weldability may deteriorate. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0080% or less, 0.0050% or less, or 0.0030% or less.

[Ca: 0-0.0100%]
[Mg: 0 to 0.0100%]
[REM: 0 to 0.0100%]
Ca, Mg, and REM are elements that can control the morphology of sulfides. The Ca, Mg and REM contents may be 0%, but in order to obtain such effects, it is preferable that the Ca, Mg and REM contents are each 0.0001% or more, and 0.0003% or more. % or more, 0.0005% or more, or 0.0007% or more. On the other hand, when these elements are contained excessively, the effects are saturated, and therefore, containing Ca, Mg, and REM in a steel sheet more than necessary may lead to an increase in manufacturing costs. Therefore, the Ca, Mg, and REM contents are each preferably 0.0100% or less, and may be 0.0080% or less, 0.0050% or less, or 0.0030% or less. In this specification, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids such as lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. is a general term for 17 elements, and the REM content is the total content of these elements.

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.

[0.060≦[Cu]+[Ni]+[Mo]+[Sn]+[Sb]≦1.000]
It is preferable that the chemical composition of the steel plate according to the embodiment of the present invention satisfies the following formula.
0.060≦[Cu]+[Ni]+[Mo]+[Sn]+[Sb]≦1.000
Here, [Cu], [Ni], [Mo], [Sn], and [Sb] are the contents (mass%) of each element in the steel plate, and are 0 when no element is contained. A metal in which the Ms/Mm ratio is 3.0 or more in the area near the interface between the steel plate and the scale per 1000 μm ² by setting the total content of Cu, Ni, Mo, Sn, and Sb in the entire steel plate to 0.060% or more. It can be ensured that the concentrated phase is present in the desired area amount, more specifically in an area amount of 50 μm ² . As a result, it is possible to suppress the oxidation of Fe on the surface of the steel sheet and the associated heat generation during cutting by laser cutting, etc., which in turn suppresses excessive melting of Fe and significantly improves the flowability of the molten material. . From the viewpoint of further increasing the area of the metal-enriched phase in the region near the interface, the total content of Cu, Ni, Mo, Sn, and Sb in the entire steel sheet (i.e., [Cu] + [Ni] + [Mo] + [Sn ]+[Sb] or Mm) is preferably large. Therefore, [Cu] + [Ni] + [Mo] + [Sn] + [Sb] is preferably at least 0.070, more preferably at least 0.100, even more preferably at least 0.200 or 0.300. It is most preferably 0.500 or more. From the viewpoint of further increasing the area of the metal-enriched phase in the region near the interface, the upper limit of [Cu]+[Ni]+[Mo]+[Sn]+[Sb] is not particularly limited. However, even if these elements are contained excessively, the effect may be saturated and the alloy cost may increase. Therefore, from the economic point of view, [Cu] + [Ni] + [Mo] + [Sn] + [Sb] is preferably 1.000 or less, for example 0.900 or 0.800. Good too.

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.

[Scale thickness: 5 to 50 μm]
According to a preferred embodiment of the present invention, the thickness of the scale formed on the surface of the steel plate is 5 to 50 μm. Since the scale has a function of stabilizing the heat input of the laser, in order to realize stable laser cutting, it is preferable that the thick steel plate has a certain scale thickness. By setting the scale thickness to 5 μm or more, such stable laser cutting can be ensured. The scale thickness may be 10 μm or more, 20 μm or more, 30 μm or more, or 40 μm or more. On the other hand, if the scale thickness becomes too thick, the scale may peel off and stable laser cutting may be inhibited. Therefore, from the viewpoint of reliably avoiding such scale peeling, the scale thickness is preferably 50 μm or less, and may be 45 μm or less. As will be explained in detail later in connection with the method for producing thick steel plates, the formation and growth of scale during the production process is an important factor in producing the desired metal-enriched phase. However, the scale thickness in the final product is not necessarily related to the form of the metal-enriched phase, and therefore does not directly affect the occurrence of burning during cutting by laser cutting or the like. Therefore, the scale thickness of the thick steel plate may be appropriately selected from the viewpoint of achieving stable cutting.

Scale thickness is determined using scanning electron microscopy (SEM). More specifically, the scale thickness is determined by taking a sample using a cross section parallel to the thickness direction of a thick steel plate as an observation surface, polishing the observation surface, and observing the observation surface with an SEM at a magnification of 500 times. The distance from the interface between the scale and the base metal to the scale surface is measured at five or more points in any three visual fields, and the distance is determined as the average value of the obtained distances.

[Thickness of thick steel plate: 6 to 40 mm]
The thick steel plate according to the embodiment of the present invention can have any thickness to which a cutting method such as 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.

<Manufacturing method of steel plate>
Next, a preferred method for manufacturing a thick steel plate according to an embodiment of the present invention will be described. The following description is intended to illustrate a characteristic method for manufacturing a thick steel plate according to an embodiment of the present invention, and the thick steel plate is manufactured by the manufacturing method as described below. It is not intended to be limited to.

The method for manufacturing a steel plate according to an embodiment of the present invention includes:
A step of heating the slab in a heating furnace, the step comprising maintaining the surface temperature of the slab in a range of 1100 to 1300° C. for 30 to 120 minutes (heating step);
A step of taking out the slab from the heating furnace and then holding it in the atmosphere for 50 to 120 seconds (atmosphere holding step); and a step of hot rolling the slab, the final high-pressure water descaling being carried out at a temperature of 1050°C or higher. Processes that involve carrying out at temperatures (hot rolling processes)
It is characterized by including.

[Heating process]
First, prior to the heating process, molten steel is cast to form a slab. The casting method may be a normal casting method, and a continuous casting method, an ingot-forming method, etc. can be adopted, but a continuous casting method is preferable from the viewpoint of productivity. The slab may have, for example, but not limited to, the preferred chemical composition described above in connection with sheet steel.

The resulting slab is then heated in a heating furnace. It is important that the heating of the slab includes maintaining the surface temperature of the slab in a range of 1100 to 1300° C. for 30 to 120 minutes. By holding the film at such a high temperature for a predetermined period of time, scale growth can be promoted. Here, "maintenance" does not necessarily mean that the slab is kept at a constant temperature, but also includes cases where the temperature fluctuates within the range of 1100 to 1300°C. As the scale grows, metal elements such as Cu, Ni, Mo, Sn, and Sb that cannot be dissolved in the scale are discharged from the scale, so these elements are concentrated on the surface of the steel plate, that is, the interface between the steel plate and the scale. be able to. In addition, if the slab contains Cu, Sn and/or Sb, especially Cu, the Cu etc. expelled from the scale will melt under such high temperatures, and other metal elements will be dissolved in the liquid phase such as Cu. It becomes possible to further promote the concentration of these elements at the interface between the steel plate and the scale. As a result, it becomes possible to generate a metal enriched phase having a desired enrichment ratio in a sufficient area amount in the region near the interface of the finally obtained thick steel plate.

If the heating temperature is less than 1100°C or the holding time is less than 30 minutes, sufficient amounts of Cu, Ni, Mo, Sn, and Sb are added to the interface between the steel plate and the scale because the scale does not grow sufficiently. It will no longer be possible to thicken it. Therefore, the heating temperature is 1100°C or higher, preferably 1110°C or higher. Similarly, the holding time is 30 minutes or more. On the other hand, if the heating temperature exceeds 1300°C or the holding time exceeds 120 minutes, the scale will grow too thick and will peel off together with the metal concentrated phase during the subsequent manufacturing process, which will cause the steel sheet to deteriorate as well. It may become impossible to concentrate Cu, Ni, Mo, Sn, and Sb in sufficient amounts at the scale interface. Further, if the scale grows too thick, there is a problem that the yield decreases, and furthermore, the fuel cost increases, which is not preferable. Therefore, the heating temperature is 1300°C or lower, preferably 1280°C or lower. Similarly, the holding time is 120 minutes or less, preferably 100 minutes or less.

[Atmosphere maintenance process]
The heated slab is then removed from the furnace and then held in ambient air for 50-120 seconds. The slab taken out from the heating furnace is subjected to hot rolling with the scale formed on the interface sufficiently removed while maintaining the interface properties in which metal elements such as Cu are enriched in the heating furnace. There is a need. Here, the scale formed on the interface has a three-layer structure composed of wustite (FeO), magnetite (Fe ₃ O ₄ ), and hematite (Fe ₂ O ₃ ) in this order from the base iron (steel plate) side. It is generally known that These iron oxides are produced by the reaction between iron (Fe) diffused from the steel base and oxygen (O ₂ ) in the atmosphere. Therefore, lower-order iron oxide is produced closer to the base metal, and higher-order iron oxide is produced closer to the atmosphere. In order to efficiently remove scale with such a three-layer structure by high-pressure water descaling, etc., it is effective to increase the proportion of hematite, which corresponds to higher-order oxides formed on the outermost layer of the scale. It is. In order to increase the proportion of such higher-order oxides, the slab taken out of the heating furnace is held in the atmosphere for a predetermined period of time to promote the oxidation reaction on the atmospheric side of the scale, and to remove wustite and magnetite. It is effective to modify it to hematite.

From this point of view, in this manufacturing method, the slab taken out from the heating furnace is held in the atmosphere for 50 to 120 seconds, and more specifically, the first high-pressure water descaling is performed after it is taken out from the heating furnace. By holding the slab in the air for 50 to 120 seconds during this period, the proportion of hematite formed in the outermost layer of the scale is increased, thereby improving the descaling properties of the outer layer of the slab. As a result, by high-pressure water descaling after holding the atmosphere and before the hot rolling process, the outer layer of the scale can be sufficiently removed and the metal-enriched phase at the interface can be left behind. By carrying out this process, it becomes possible to achieve the desired interface properties between the steel plate and the scale in the finally obtained thick steel plate. On the other hand, if the retention time in the atmosphere is less than 50 seconds, sufficient hematite will not be generated on the surface layer of the scale, and the scale will not be sufficiently removed even by high-pressure water descaling before the hot rolling process. . As a result, scale encroachment occurs frequently during hot rolling, making it impossible to obtain the desired interface properties between the steel plate and the scale in the finally obtained thick steel plate. On the other hand, if the retention time in the atmosphere exceeds 120 seconds, excessive hematite will be produced and the descaling property will be excessively improved, resulting in the scale being removed together with the metal-enriched phase during high-pressure water descaling. There may be cases where In such a case, it becomes impossible to obtain desired interface properties between the steel plate and the scale in the finally obtained thick steel plate. Therefore, in order to avoid removal of such a metal-enriched phase by high-pressure water descaling before the hot rolling process, the retention time in the atmosphere is 120 seconds or less, preferably 100 seconds or less. Further, high-pressure water descaling after maintaining the atmosphere is not particularly limited, and can be carried out using, for example, high-pressure water with a collision pressure of 10 to 15 MPa.

[Hot rolling process]
Next, the slab is subjected to hot rolling. The hot rolling may include, for example, rough rolling for adjusting the plate thickness. The conditions for the rough rolling are not particularly limited as long as the desired plate thickness can be ensured. In the hot rolling process of this manufacturing method, as in the case of normal hot rolling, descaling is appropriately carried out by high-pressure water descaling, and such descaling and rolling are repeated to form a slab to a predetermined thickness. Press down. However, in this hot rolling process, it is extremely important that the final high-pressure water descaling is performed at a temperature of 1050°C or higher, or in other words, that the final high-pressure water descaling is not performed at a temperature of less than 1050°C. . When the final high-pressure water descaling is carried out at a temperature of 1050°C or higher, the surface temperature of the steel plate after the final high-pressure water descaling is relatively high, so that subsequent reoxidation of the steel plate surface is promoted. I can do it. As scale is generated and grows due to such reoxidation, metal elements such as Cu, Ni, Mo, Sn, and Sb that cannot be dissolved in the scale are discharged from the scale. It becomes possible to concentrate the

On the other hand, if the final high-pressure water descaling is carried out at a temperature below 1050°C, the metal-enriched phase in the area near the interface between the steel plate and the scale created in the previous heating process and atmosphere holding process will be excessively removed. There may be cases where In addition, since the surface temperature of the steel plate after the final high-pressure water descaling is relatively low, subsequent reoxidation of the steel plate surface cannot be promoted, resulting in further scale formation and growth, and Cu, It becomes impossible to promote interfacial concentration of Ni, Mo, Sn, and Sb. Therefore, the final high pressure water descaling is carried out at a temperature of 1050°C or higher, preferably 1060°C or higher. Further, the final high-pressure water descaling can be carried out using high-pressure water with a collision pressure of 10 to 15 MPa, for example, as in the case of high-pressure water descaling after holding the atmosphere.

[Other processes]
In addition to the above-mentioned heating step, atmosphere holding step, and hot rolling step, this manufacturing method may further include a flattening step using a hot leveler or the like, a cooling step, and, if necessary, a heat treatment step. These additional steps do not particularly affect the morphology of the metal-enriched phase formed in the region near the interface between the steel sheet and the scale, or the cuttability associated with laser cutting or the like. Therefore, these additional steps are not particularly limited, and may be performed by appropriately selecting any appropriate conditions in consideration of other desired steel sheet properties.

The thick steel plate manufactured by this manufacturing method contains 50 μm ² or more of a metal-enriched phase with Ms/Mm≧3.0 per 1000 μm ² of the area near the interface. It is possible to sufficiently suppress excessive melting and improve the flowability of the melt. Therefore, with such thick steel plates, gas cutting and plasma cutting, which have wider cutting widths than laser cutting, naturally cause clogging with molten material, which has relatively narrow cutting widths like laser cutting. Even with an easy cutting method, the flowability of the melt can be reliably improved, so it is possible to prevent or significantly suppress the occurrence of burning. Therefore, the thick steel plate according to the embodiment of the present invention is a thick steel plate used for steel structures such as shipbuilding, architecture, industrial machinery, and bridges, and is suitable for any cutting operation that may cause burning, especially laser cutting. It is useful as a thick steel plate.

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to these Examples in any way.

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 obtained thick steel plates were laser cut was investigated.

First, a slab having the chemical composition shown in Table 1 was cast by a continuous casting method, and then the cast slab was heated and held in a heating furnace under the conditions shown in Table 2. The slabs were then removed from the furnace and then held in air for the times indicated in Table 2. The slab after being kept in the atmosphere was subjected to high-pressure water descaling, and then the slab was subjected to hot rolling to reduce the thickness to 25 mm. Table 2 shows the temperatures at which the final high-pressure water descaling was performed during hot rolling. High-pressure water descaling after holding in the atmosphere and during hot rolling was performed using high-pressure water with an impingement pressure of 10 MPa. The properties of the obtained thick steel plate were measured and evaluated by the following method.

[Scale thickness]
The scale thickness is determined by taking a sample using a cross section parallel to the thickness direction of a thick steel plate as an observation surface, polishing the observation surface, observing the observation surface with an SEM at a magnification of 500 times, and calculating the relationship between the scale and the base steel. Measurements were performed to determine the distance from the interface to the scale surface at five points in three arbitrary fields of view, and the average value of the obtained distances was determined.

[Area of metal enriched phase (Ms/Mm≧3.0) in the region near the interface]
The area of the metal-enriched phase in the region near the interface was determined using EPMA as follows. Specifically, a quantitative measurement was performed using EPMA at a magnification of 1500 times on a cross section of an arbitrary location parallel to the thickness direction of the steel plate. First, in the EPMA measurement results of O (oxygen), the average value of the O concentration in the area of one pixel in the thickness direction of the scale and all pixels in the direction perpendicular thereto was calculated. Next, this average value is calculated in order from the scale side to the base metal direction, and the point where this average value falls below 10% for the first time is determined as the interface between the steel plate and the scale, and from the determined interface position to the scale side in the plate thickness direction. The area near the interface was determined to be 5 μm on both the base and the base rail side. Next, the element distribution of Cu, Ni, Mo, Sn, and Sb in the region near this interface was observed. Specifically, a region near the interface of 100 μm x 10 μm in a straight line length (interface length) of 100 μm in the direction perpendicular to the plate thickness direction is specified based on the position of the interface determined previously, and the region near the interface is determined. The total Ms (mass %) of Cu, Ni, Mo, Sn, and Sb concentrations in each metal phase within 1000 μm ² was calculated. By dividing the calculated Ms by the total Mm (mass%) of the Cu, Ni, Mo, Sn, and Sb contents of the entire steel plate, the enrichment ratio of Cu, Ni, Mo, Sn, and Sb (Ms/Mm) and its distribution was obtained. Finally, by multiplying the number of pixels for which Ms/Mm is 3.0 or more in the EPMA measurement image in the region near the interface by the actual area per pixel, we can calculate the amount of Cu, Ni, Mo, Sn and The area of the metal-enriched phase where the total Sb concentration (mass %) is 3.0 times or more the sum of the Cu, Ni, Mo, Sn, and Sb contents (mass %) of the steel sheet was determined.

[Evaluation by laser cutting]
The thick steel plate after hot rolling was cut into a size of 1 m x 1 m and subjected to a laser cutting test, and the presence or absence of burning at that time was examined. The laser cutting test was conducted by cutting a small piece of 100 mm x 100 mm using a CO ₂ laser under the following conditions. The results are shown in Table 2.
Laser power: 4500W
Frequency: 500Hz
Duty: 75%
Assist gas pressure: 30MPa
Cutting speed: 570mm/min

Referring to Table 2, in Comparative Example 12, the slab surface temperature in the heating process was low, so the scale did not grow sufficiently in the heating process, and elements such as Cu were not sufficiently concentrated at the interface between the steel plate and the scale. It is thought that it was not possible. As a result, the area of the metal-enriched phase in the region near the interface became smaller, and burning occurred in the laser cutting test. In Comparative Example 13, the holding time at a predetermined slab surface temperature during the heating process was short, so the scale did not grow sufficiently during the heating process, and elements such as Cu were not sufficiently concentrated at the interface between the steel plate and the scale. It is thought that it was not possible to do so. As a result, the area of the metal-enriched phase in the region near the interface became smaller, and burning occurred in the laser cutting test. In Comparative Example 14, the slab surface temperature in the heating process was high, and in Comparative Example 15, the holding time in the heating process was long. In both cases, the scale became too thick in the heating process, and the scale was removed in the subsequent manufacturing process. It is considered that the steel sheet partially peeled off along with the metal-enriched phase, and elements such as Cu could not be sufficiently concentrated at the interface between the steel plate and the scale. As a result, the area of the metal-enriched phase in the region near the interface became smaller, and burning occurred in the laser cutting test.

In Comparative Example 16, because the holding time in the air holding step was short, hematite was not sufficiently generated on the surface layer of the scale, and the descaling properties were reduced, making it difficult to remove the scale sufficiently even with subsequent high-pressure water descaling. It is considered that it could not be done. As a result, scale encroachment occurred frequently during hot rolling, it was not possible to obtain the desired interface properties between the steel plate and the scale, and burning occurred in the laser cutting test. In Comparative Example 17, because the holding time in the air holding step was long, hematite was produced excessively on the surface layer of the scale, resulting in excessively improved descaling properties, and during the subsequent high-pressure water descaling, the scale became a metal-concentrated phase. It is thought that it was removed along with the As a result, it was not possible to obtain the desired interface properties between the steel plate and the scale, and burning occurred in the laser cutting test.

In Comparative Example 18, because the final descaling temperature in the hot rolling process was low, subsequent scale growth was insufficient, and elements such as Cu could not be sufficiently concentrated at the interface between the steel plate and the scale. Conceivable. As a result, the area of the metal-enriched phase in the region near the interface became smaller, and burning occurred in the laser cutting test. In Comparative Examples 19 to 22, since the total content (Mm) of Cu, Ni, Mo, Sn, and Sb in the entire steel sheet was low, the area of the metal-enriched phase in the region near the interface was small, and burning occurred in the laser cutting test. There has occurred.

In contrast, in Invention Examples 1 to 11, the ^total concentration of Cu, Ni, Mo, Sn, and Sb in the area near the interface between the steel plate and the scale is By containing 50 μm ² or more of the metal concentrated phase, which is 3.0 times or more the total amount, laser cutting could be performed satisfactorily without causing burning.

Claims (7)

In a region near the interface, including a steel plate and a scale formed on the surface of the steel plate, and having an area of 10 μm in the plate thickness direction and an interface length of 100 μm centered on the position of the interface between the steel plate and the scale, Cu, Ni, Mo , containing 50 μm ^or more of a metal-enriched phase in which the total Sn and Sb concentration (mass %) is 3.0 times or more the total Cu, Ni, Mo, Sn, and Sb content (mass %) of the steel sheet. , thick steel plate. The steel plate is in mass%,
C: 0.01-0.30%,
Si: 0.01-0.60%,
Mn: 0.01 to 2.00%,
P: 0.001-0.050%,
S: 0.001-0.050%,
Al: 0.001-0.100%,
Cr: 0.01-0.50%,
Cu: 0.010 to 0.50%,
Ni: 0.010 to 0.50%,
Mo: 0.010-0.500%,
Sn: 0-0.500%,
Sb: 0 to 0.500%,
Nb: 0 to 0.500%,
V: 0 to 0.500%,
Ti: 0 to 0.500%,
B: 0 to 0.0100%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
REM: 0 to 0.0100%, and the balance: consisting of Fe and impurities,
The thick steel plate according to claim 1, having a chemical composition satisfying 0.060≦[Cu]+[Ni]+[Mo]+[Sn]+[Sb]≦1.000.
Here, [Cu], [Ni], [Mo], [Sn], and [Sb] are the contents (mass%) of each element in the steel plate, and are 0 when no element is contained. The chemical composition is in mass%,
Sn: 0.001 to 0.500%,
Sb: 0.001 to 0.500%,
Nb: 0.001-0.500%,
V: 0.001-0.500%,
Ti: 0.001 to 0.500%,
B: 0.0001-0.0100%
Ca: 0.0001-0.0100%,
Mg: 0.0001 to 0.0100%, and REM: 0.0001 to 0.0100%
The thick steel plate according to claim 2, comprising one or more selected from the group consisting of: The thick steel plate according to any one of claims 1 to 3, wherein the scale has a thickness of 5 to 50 μm. The thick steel plate according to any one of claims 1 to 4, having a plate thickness of 6 to 40 mm. The thick steel plate according to any one of claims 1 to 5, which is used for laser cutting. A step of heating a slab in a heating furnace, the step comprising holding the surface temperature of the slab in a range of 1100 to 1300° C. for 30 to 120 minutes;
taking out the slab from the heating furnace and then holding it in the atmosphere for 50 to 120 seconds, and hot rolling the slab, the final high-pressure water descaling being carried out at a temperature of 1050° C. or higher. The method for producing a thick steel plate according to any one of claims 1 to 6, comprising the step of: