JP7798124B2 - Heavy steel plate and its manufacturing method - Google Patents

Description

The present invention relates to thick steel plates and manufacturing methods thereof, and in particular to thick steel plates with excellent laser cuttability and machinability, and manufacturing methods thereof.

When thick hot-rolled steel plate is used for components such as molds and machine parts, it is typically cut or machined to the desired shape before assembly. Cutting and machining work accounts for a high proportion of the manufacturing process for steel structures, and there is a strong demand for high efficiency and low cost operations. Furthermore, as the shapes of steel structures become more complex due to design considerations and other factors, cut surfaces are becoming more complex due to the omission of processes after cutting, requiring high precision for the cut surfaces. Furthermore, in the machining process, it is necessary to machine parts with the specified shape, precision, and roughness from the specified workpiece material as cheaply as possible, taking into account factors such as the replacement life of cutting tools and labor costs.

Traditionally, gas cutting and plasma cutting have been used as methods for cutting thick steel plates. Gas cutting is the most widely used method because it requires the lowest equipment installation costs and can cut even very thick steel plates. However, it is difficult to automate the control and monitoring of the gas flame, and the cutting speed is slow, making it less user-friendly.

Plasma cutting is capable of high-speed cutting up to thicknesses of approximately 50 mm, but the torch has a lifespan of only a few hours, and frequent replacement work makes it less efficient and difficult to automate.

On the other hand, as laser oscillators become more powerful and cheaper, laser cutting is now also being used to cut thick steel plates. The advantages of laser cutting are that it can be easily fully automated, there is little heat input during cutting and the thermal impact is minimal, and the cut surface is of good quality.

However, with current laser output, cutting stability drops sharply when the plate thickness exceeds 20 mm. While efforts are being made to improve cutting performance by further increasing the output of laser cutting machines, research has also been conducted into improving the laser cutting performance of the steel plate itself, i.e., how to prevent cutting defects.

Patent Document 1 describes a steel sheet with excellent laser cuttability that has a specific chemical composition, an average thickness of a scale layer of 10 μm or less, and contains 60 mass% or more of magnetite.

Patent Document 2 describes a thick steel plate with excellent laser cuttability, in which the amounts of alloy elements such as Al, Cu, Ni, and Cr are controlled and the surface scale layer has an Al ₂ O ₃ -containing layer at the interface with the base steel.

Patent Document 3 describes a method for manufacturing thick steel plate with excellent laser cuttability, in which scale is removed by descaling at the start of rolling and the rolling end temperature is controlled to keep the sum of the porosity in the scale and the peeling rate at the interface between the scale and the base steel to 15% or less.

Patent Document 4 describes a thick steel plate that contains a specified amount of Sb, has a surface oxide layer with a thickness of 15 μm to 50 μm, and has excellent laser cuttability.

Another example of a steel material with excellent machinability is MnS free-cutting steel (Non-Patent Document 1).

JP 2012-87339 A Japanese Patent Application Publication No. 11-323478 Japanese Patent Application Laid-Open No. 2004-169093 Japanese Patent Application Laid-Open No. 2022-054486

Chubu Kohan Co., Ltd. website, "Improved Machinability Steel Plates," searched February 1, 2023, Internet, <URL: https://www.chubukohan.co.jp/product/standard/info/>

In Patent Document 1, strict temperature control during rolling is required to obtain thin scale containing 60% or more by mass of magnetite, resulting in a lack of stable production. In Patent Document 2, adjustment of the amount of alloying elements is required to improve laser cuttability. In Patent Document 3, problems such as dross adhesion occur when laser cutting plates 25 mm or thicker. Furthermore, Patent Documents 1 to 3 do not take machinability into consideration. Similarly, Patent Document 4 only discusses improvements to laser cuttability, not machinability itself. Furthermore, Non-Patent Document 1 discusses improved machinability, but lacks laser cuttability.

The present invention was made in consideration of these circumstances, and aims to provide a thick steel plate that has excellent laser cuttability and machinability, and a method for manufacturing the same.

In order to achieve the above objectives and ensure excellent laser cutting properties for thick steel plates, the inventors conducted extensive research into the effects on laser cutting of the steel plate's chemical composition, manufacturing method, and the thickness and adhesion of the surface oxide layer (hereinafter sometimes simply referred to as "scale"), and have arrived at the following findings.

(1) When high-density energy is applied to thick steel plates by laser irradiation, thermal stress is generated, destroying the surface scale and making cutting defects more likely. To prevent this, it is necessary to improve the adhesion of the scale.

(2) When a specified amount of Sb is added to improve scale adhesion, Sb concentrates at the interface between the scale and the base steel. While the addition of Si, Cu, and Ni is considered effective in improving scale adhesion, these elements are undesirable for scale uniformity. Because Si, Cu, and Ni concentrate locally at specific locations such as grain boundaries, causing differences in the scale growth rate, excessive inclusion of Si, Cu, and Ni results in uneven scale. In the present invention, by keeping the Si, Cu, and Ni contents below a specified amount and adding a specified amount of Sb, Sb concentrates at the interface between the scale and the base steel, allowing for the formation of scale with excellent adhesion and uniformity.

(3) By setting the scale thickness to 15 μm or more and 60 μm or less, cutting defects during laser cutting can be suppressed, the cut surface can be made smooth, and dross adhesion can also be suppressed.

Furthermore, in order to ensure excellent machinability for thick steel plates, the inventors conducted extensive research into the effects of the steel plate's chemical composition and manufacturing method on machinability, and obtained the following findings. Note that machinability is determined by a comprehensive evaluation of tool wear, chip disposal, cutting resistance, cutting temperature, and adhesion, but if each individual evaluation item is excellent, it can naturally be said that machinability is good.

(4) To reduce friction between the tool and the chip, it would be possible to supply lubricant to the contact surface between the tool and the chip, but this is not possible because the contact surface is subject to high temperature and pressure. Adding a combination of B and N, or Ca and N, to the steel plate acts as a solid film that prevents direct contact between the tool and the backside of the chip, even under high temperature and pressure conditions, thereby reducing tool wear and improving machinability.

The present invention was made based on the above findings and further investigation, and its gist is as follows:

[1] In mass%,
C: more than 0.03% and less than 0.20%,
Sb: 0.003 to 0.050%,
Si: 0.60% or less,
Mn: 0.10 to 2.50%,
P: 0.030% or less,
S: 0.030% or less,
Al: 0.150% or less,
N: 0.02% or less, B: more than 0.0030% and 0.0100% or less, and the balance being Fe and unavoidable impurities;
A thick steel plate having a surface oxide layer having a thickness of 15 μm or more and 60 μm or less.
[2] The component composition further includes, in mass%,
Cu: 1.00% or less,
Ni: 1.00% or less,
Cr: 0.01-1.00%,
Mo: 0.01-1.00%,
W: 0.01-1.00%,
V: 0.003-0.100%,
Nb: 0.003 to 0.030%,
Ti: 0.003 to 0.050%,
REM: 0.0001-0.0030%,
Ca: 0.0001-0.0030%,
Mg: 0.0001 to 0.0030%, and Sn: 0.001 to 0.030%
The steel plate according to [1], containing one or more selected from the following:
[3] In mass%,
C: more than 0.03% and less than 0.20%,
Sb: 0.003 to 0.050%,
Si: 0.60% or less,
Mn: 0.10 to 2.50%,
P: 0.030% or less,
S: 0.030% or less,
Al: 0.150% or less,
N: 0.02% or less, Ca: more than 0.0030% and 0.0100% or less, and the balance being Fe and unavoidable impurities;
A thick steel plate having a surface oxide layer having a thickness of 15 μm or more and 60 μm or less.
[4] The component composition further includes, in mass%,
Cu: 1.00% or less,
Ni: 1.00% or less,
Cr: 0.01-1.00%,
Mo: 0.01-1.00%,
W: 0.01-1.00%,
V: 0.003-0.100%,
Nb: 0.003 to 0.030%,
Ti: 0.003 to 0.050%,
REM: 0.0001-0.0030%,
B: 0.0001 to 0.0030%,
Mg: 0.0001 to 0.0030%, and Sn: 0.001 to 0.030%
The steel plate according to [3], containing one or more selected from the following:
[5] The steel plate according to any one of [1] to [4], wherein the standard deviation of the thickness of the surface oxide layer is 5.0 μm or less.
[6] A method for producing a thick steel plate, in which a steel material having the component composition described in any one of [1] to [4] is heated to 1000 to 1200°C, and then hot rolling is performed at a rolling end temperature of 700 to 1000°C, and descaling is performed five or more times by spraying water onto at least one surface of the steel plate during a rolling pass in a temperature range from (rolling end temperature + 100°C) to the rolling end temperature.

The present invention provides thick steel plates that have excellent laser cuttability and machinability.

The steel plate of the present invention provides excellent workability and cutting quality when laser cut, and also has excellent machinability and tool life when cut.
Specifically, it has excellent cutting properties, such as no cutting defects such as notches on the cross section of the steel plate after laser cutting and no dross adhesion on the back surface of the steel plate, and excellent machinability, such that the workpiece (thick steel plate) is easily cut during cutting, cutting tools are less likely to wear, and cutting chips are easy to dispose of.
Use of the steel plate of the present invention contributes greatly to improving the manufacturing efficiency when manufacturing steel structures, and provides significant industrial benefits.

The present invention will now be described in detail. Note that the following description is merely an example of a preferred embodiment of the present invention, and the present invention is not limited thereto.

The steel plate (base steel of the steel plate) of the present invention has the chemical composition of the first or second embodiment shown below. The reasons for limiting each chemical composition are explained below. Note that "%" in the chemical composition means "mass %" unless otherwise specified.

[Component composition of the first embodiment]
The steel plate according to the first embodiment of the present invention has a component composition containing, in mass%, C: more than 0.03% and not more than 0.20%, Sb: 0.003 to 0.050%, Si: 0.60% or less, Mn: 0.10 to 2.50%, P: 0.030% or less, S: 0.030% or less, Al: 0.150% or less, N: 0.02% or less, and B: more than 0.0030% and not more than 0.0100%, with the balance being Fe and unavoidable impurities.

The above-mentioned composition may further contain, in mass percent, one or more of the following: Cu: 1.00% or less, Ni: 1.00% or less, Cr: 0.01-1.00%, Mo: 0.01-1.00%, W: 0.01-1.00%, V: 0.003-0.100%, Nb: 0.003-0.030%, Ti: 0.003-0.050%, REM: 0.0001-0.0030%, Ca: 0.0001-0.0030%, Mg: 0.0001-0.0030%, and Sn: 0.001-0.030%.

C: more than 0.03% and not more than 0.20% C is an element necessary for increasing the strength of steel and ensuring the strength required for thick steel plates. To achieve this effect, it is necessary to contain more than 0.03% C. The C content is preferably 0.04% or more, and more preferably 0.10% or more. On the other hand, if the C content exceeds 0.20%, the toughness deteriorates and the weldability decreases. Therefore, the C content is set to 0.20% or less. The C content is preferably 0.18% or less.

Sb: 0.003-0.050%
Sb is an element necessary for improving scale adhesion. Sb diffuses to the surface during steel material heating and hot rolling, and uniformly concentrates at the interface between the scale (surface oxide layer) and the base steel. This concentrated layer makes the scale less likely to peel off. Furthermore, uniform concentration prevents local differences in the scale growth rate and does not impair scale uniformity. As a result, desired laser cuttability is obtained. To achieve this effect, an Sb content of 0.003% or more is required. The Sb content is preferably 0.005% or more. On the other hand, if the Sb content exceeds 0.050%, scratches are more likely to occur on the steel plate surface, deteriorating laser cuttability. Therefore, the Sb content is set to 0.050% or less. The Sb content is preferably less than 0.030%, and more preferably 0.020% or less.

Si: 0.60% or less Si is an element that acts as a deoxidizer. However, if the Si content exceeds 0.60%, the toughness of the base material and the cold cracking resistance of the weld may be significantly deteriorated. For this reason, the Si content is set to 0.60% or less. The Si content is preferably set to 0.40% or less. There is no particular lower limit for the Si content, but the Si content is preferably 0.01% or more.

Mn: 0.10-2.50%
Mn has the effect of increasing the hardenability of steel, and is contained at 0.10% or more to ensure the strength of the base metal. The Mn content is preferably 0.20% or more, and more preferably 1.30% or more. On the other hand, if the Mn content exceeds 2.50%, the toughness, ductility, and weldability of the base metal may be significantly deteriorated. Therefore, the Mn content is set to 2.50% or less. The Mn content is preferably set to 2.00% or less.

P: 0.030% or less P exists in steel as an impurity, and by segregating at grain boundaries, it may have adverse effects such as deteriorating the low-temperature toughness and ductility of the base material. For this reason, it is desirable to keep the P content as low as possible, but 0.030% or less is acceptable. Therefore, the P content is set to 0.030% or less. There is no particular lower limit for the P content, but reducing it to less than 0.001% is difficult in industrial-scale production. Therefore, from the viewpoint of productivity, it is preferable to set the P content to 0.001% or more.

S: 0.030% or less. S, present in steel as sulfide-based inclusions such as MnS, may have adverse effects, such as degrading the low-temperature toughness and ductility of the base material. Therefore, it is desirable to keep the S content as low as possible. On the other hand, when a certain amount of S is contained, the MnS present in the steel is softer than the base material at high temperatures. It is stretched during deformation in the shear region and acts as a stress concentration point, reducing the deformation resistance at the tool cutting edge and the shear deformation region of the workpiece, thereby improving machinability. However, C-direction toughness decreases with increasing S content. In consideration of the above, the S content is set to 0.030% or less. While there is no particular lower limit for the S content, reducing it to less than 0.001% is difficult in industrial-scale production. Therefore, from the viewpoint of productivity, it is preferable to set the S content to 0.001% or more.

Al: 0.150% or less. Al acts as a deoxidizer and fixes N in steel as AlN, contributing to improving the toughness of the base material and welds. In cutting, Al prevents diffusion and adhesion between the tool and the backside of the chips in the workpiece, while also adhering firmly to the tool surface as a film during cutting. However, if the Al content exceeds 0.150%, the toughness of the base material may decrease. Therefore, the Al content is set to 0.150% or less. The Al content is preferably set to 0.110% or less. While there is no particular lower limit for the Al content, an Al content of 0.015% or more is preferred.

N: 0.02% or less N is an element present in steel as an impurity. If the N content exceeds 0.02%, the toughness of the base material may be significantly reduced. On the other hand, N in steel is fixed as AlN by Al and contributes to improving machinability, so it is preferable to include an appropriate amount. For this reason, the N content is set to 0.02% or less. Furthermore, the N content is preferably 0.001% or more.

B: More than 0.0030% and less than 0.0100%. At high temperatures, such as during cutting, Al and BN in the steel form AlN, and the presence of an AlN film on the tool wear surface suppresses the diffusion reaction between the tool and the workpiece, thereby reducing tool wear. This effect is greater as the B content increases. Furthermore, B is an element that increases the strength of steel by improving hardenability. To achieve these effects, the B content is set to more than 0.0030%. Preferably, the B content is set to more than 0.0050%, and more preferably, to 0.0055% or more. On the other hand, if the B content exceeds 0.0100%, the hardenability is significantly increased and the toughness and ductility of the base material are deteriorated. Furthermore, if the B content exceeds 0.0100% and the ratio of the N content to the B content (N/B ratio) becomes small, free B present in the steel remains, resulting in deterioration of machinability. For this reason, the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less.

The steel plate according to the first embodiment of the present invention may have the above-mentioned components as its basic components. The balance may be Fe and unavoidable impurities.

In addition to the above-mentioned composition, the steel plate according to the first embodiment of the present invention may further contain one or more of Cu, Ni, Cr, Mo, W, V, Nb, Ti, REM, Ca, Mg, and Sn, each in the content ranges below.

Cu: 1.00% or less Cu is an element that contributes to improving the strength of thick steel plates. To achieve this effect, when Cu is contained, the Cu content is preferably 0.01% or more. On the other hand, when the Cu content exceeds 1.00%, hot embrittlement may occur, resulting in significant deterioration of the surface properties of the steel plate. Therefore, when Cu is contained, the Cu content is preferably 1.00% or less.

Ni: 1.00% or less Ni is an element that contributes to improving the strength of thick steel plates. To achieve this effect, when Ni is contained, the Ni content is preferably 0.01% or more. On the other hand, when the Ni content exceeds 1.00%, the scale thickness becomes significantly non-uniform, and the laser cuttability deteriorates. Therefore, when Ni is contained, the Ni content is preferably 1.00% or less.

Cr:0.01~1.00%
Cr is an element that contributes to improving the strength of thick steel plates. To achieve this effect, when Cr is contained, the Cr content is preferably 0.01% or more. On the other hand, when Cr is contained in an amount exceeding 1.00%, the toughness, ductility, and weldability of the base material may be significantly deteriorated. Therefore, when Cr is contained, the Cr content is preferably 1.00% or less.

Mo: 0.01~1.00%
Mo is an element that contributes to improving the strength of thick steel plates. To achieve this effect, when Mo is contained, the Mo content is preferably 0.01% or more. On the other hand, when the Mo content exceeds 1.00%, it may have an adverse effect on the base material toughness, ductility, and weld cracking resistance. Therefore, when Mo is contained, the Mo content is preferably 1.00% or less.

W: 0.01~1.00%
W is an element that contributes to improving the strength of thick steel plates. To achieve this effect, when W is contained, the W content is preferably 0.01% or more. On the other hand, when the W content exceeds 1.00%, it may have an adverse effect on the base material toughness, ductility, and weld crack resistance. Therefore, when W is contained, the W content is preferably 1.00% or less.

V:0.003~0.100%
V is an element that significantly contributes to improving the strength of thick steel plates. To achieve this effect, when V is contained, the V content is preferably 0.003% or more. On the other hand, if the V content exceeds 0.100%, the toughness and ductility of the base material may be deteriorated. Therefore, when V is contained, the V content is preferably 0.100% or less.

Nb: 0.003-0.030%
Nb is an element that greatly contributes to improving the strength of thick steel plates. To achieve this effect, when Nb is contained, the Nb content is preferably 0.003% or more. The Nb content is preferably 0.005% or more. On the other hand, when the Nb content exceeds 0.030%, the toughness and ductility of the base material deteriorate. Therefore, when Nb is contained, the Nb content is preferably 0.030% or less.

Ti: 0.003~0.050%
Ti is an element that has a strong affinity with N and precipitates as TiN during solidification, suppressing the coarsening of austenite grains in the weld heat-affected zone and contributing to increased toughness. To achieve this effect, when Ti is contained, the Ti content is preferably 0.003% or more. On the other hand, if the Ti content exceeds 0.050%, the TiN particles may coarsen, deteriorating the toughness of the base material and the weld. Therefore, when Ti is contained, the Ti content is preferably 0.050% or less.

REM: 0.0001-0.0030%, Mg: 0.0001-0.0030%
Both REM and Mg combine with S to control the morphology of sulfides, thereby contributing to improving the toughness of steel. To achieve this effect, when these elements are contained, it is preferable that each content be 0.0001% or more. On the other hand, even if each of these elements is contained in an amount exceeding 0.0030%, the effect saturates. Therefore, when these elements are contained, it is preferable that each content be 0.0030% or less. Here, REM (rare earth metal) is a collective term for 17 elements, including Sc, Y, and lanthanoid elements. One or more of these 17 elements can be contained in steel, and the REM content refers to the total content of these elements.

Ca: 0.0001-0.0030%
Ca contributes to improving the toughness of steel by bonding with S and controlling the morphology of sulfides. In order to obtain this effect, when Ca is contained, the Ca content is preferably 0.0001% or more. On the other hand, even if the Ca content exceeds 0.0030%, the effect saturates. Therefore, when Ca is contained, the Ca content is preferably 0.0030% or less.

Sn: 0.001-0.030%
Sn contributes to improving corrosion resistance. To achieve this effect, when Sn is contained, the Sn content is preferably 0.001% or more. On the other hand, if the Sn content exceeds 0.030%, there is a risk of deteriorating toughness. Therefore, when Sn is contained, the Sn content is preferably 0.030% or less.

Note that when the content of Cu, Ni, Cr, Mo, W, V, Nb, Ti, REM, Ca, Mg, or Sn described above as optional components is less than the lower limit (however, for Cu and Ni, when the content is less than 0.01%), that component is considered to be included as an unavoidable impurity.

[Component composition of second embodiment]
The steel plate according to the second embodiment of the present invention has a component composition containing, in mass%, C: more than 0.03% and not more than 0.20%, Sb: 0.003 to 0.050%, Si: 0.60% or less, Mn: 0.10 to 2.50%, P: 0.030% or less, S: 0.030% or less, Al: 0.150% or less, N: 0.02% or less, and Ca: more than 0.0030% and not more than 0.0100%, with the balance being Fe and unavoidable impurities.

The above-mentioned composition may further contain, in mass percent, one or more of the following: Cu: 1.00% or less, Ni: 1.00% or less, Cr: 0.01-1.00%, Mo: 0.01-1.00%, W: 0.01-1.00%, V: 0.003-0.100%, Nb: 0.003-0.030%, Ti: 0.003-0.050%, REM: 0.0001-0.0030%, B: 0.0001-0.0030%, Mg: 0.0001-0.0030%, and Sn: 0.001-0.030%.

The reasons for limiting the chemical composition of the steel plate according to the second embodiment of the present invention to C: more than 0.03% and not more than 0.20%, Sb: 0.003 to 0.050%, Si: 0.60% or less, Mn: 0.10 to 2.50%, P: 0.030% or less, S: 0.030% or less, Al: 0.150% or less, and N: 0.02% or less are the same as those for the first embodiment described above.

Ca: More than 0.0030% and Less than 0.0100% Ca combines with S to control the morphology of sulfides, thereby improving the toughness of the steel. It also acts as a deoxidizer, and when the Ca content is 0.0020% or more and the Al content is 0.150% or less, a protective film (Belaag) forms on the tool surface, reducing tool wear. On the other hand, even if the Ca content exceeds 0.0100%, the effect of reducing tool wear tends to saturate, so the upper limit of the Ca content is set to 0.0100%. Furthermore, if the Ca content exceeds 0.0100%, the proportion of CaS, which has a higher melting point than MnS, increases, resulting in reduced machinability. Therefore, the Ca content is set to 0.0100% or less. Furthermore, if the Ca content is too low, the effect of reducing tool wear is low, so the Ca content is set to more than 0.0030%.

The steel plate according to the second embodiment of the present invention may have the above-mentioned components as its basic components. The balance may be Fe and unavoidable impurities.

In addition to the above-mentioned composition, the steel plate according to the second embodiment of the present invention may further contain one or more of Cu, Ni, Cr, Mo, W, V, Nb, Ti, REM, B, Mg, and Sn, each in the content ranges below.

When the chemical composition of the steel plate according to the second embodiment of the present invention contains one or more selected from Cu: 1.00% or less, Ni: 1.00% or less, Cr: 0.01 to 1.00%, Mo: 0.01 to 1.00%, W: 0.01 to 1.00%, V: 0.003 to 0.100%, Nb: 0.003 to 0.030%, Ti: 0.003 to 0.050%, REM: 0.0001 to 0.0030%, Mg: 0.0001 to 0.0030%, and Sn: 0.001 to 0.030%, the reasons for limiting the content of each of these elements are the same as those for the first embodiment described above.

B: 0.0001-0.0030%
B is an element that has the effect of increasing the strength of steel by improving hardenability. In order to obtain this effect, when B is contained, the B content is preferably 0.0001% or more. On the other hand, even if B is contained in an amount exceeding 0.0030%, the effect saturates. Therefore, when B is contained, the B content is preferably 0.0030% or less.

Note that when the content of Cu, Ni, Cr, Mo, W, V, Nb, Ti, REM, B, Mg, or Sn described above as optional components is less than the lower limit (however, for Cu and Ni, when the content is less than 0.01%), that component is considered to be included as an unavoidable impurity.

[Preferable ranges of Si, Cu, and Ni]
In the steel plate according to the present invention, the Si, Cu, and Ni contents are preferably set to 0.10% or less, 0.03% or less, and 0.03% or less, respectively. In addition to the effect of adding Sb, by strictly controlling the Si, Cu, and Ni contents so that they are equal to or less than predetermined amounts, the standard deviation of the thickness of the surface oxide layer can be set to 5.0 μm or less, as described below. This makes it possible to further uniformize the thickness of the surface oxide layer. As a result, laser cuttability can be further improved.

Si: 0.10% or less (preferable range)
Since Si is an easily oxidized element and is easily incorporated into scale, it is desirable to keep the Si content as low as possible to achieve uniform scale. From this perspective, a Si content of 0.10% or less is acceptable. From the viewpoint of deoxidation, a Si content of 0.01% or more is preferable. Furthermore, the melting point of SiO ₂ (silicon oxide), which is formed by oxidizing Si (around 1700°C), is higher than the melting point of iron (1536°C). The generation of SiO ₂ is thought to reduce the fluidity of molten metal during laser cutting, increase the roughness of the cut surface, and contribute to burning. Considering these points, a low Si content is preferable.

Cu: 0.03% or less (preferable range), Ni: 0.03% or less (preferable range)
As with Si, it is desirable to keep the Cu and Ni contents as low as possible in order to make the scale uniform. From this point of view, when Cu and Ni are contained, the Cu and Ni contents are each permissible if they are 0.03% or less. From this point of view, the Cu and Ni contents may be 0%.

[Surface Oxide Layer]
The surface oxide layer (scale) of the steel plate of the present invention is an oxide layer formed by oxidizing the surface of the steel plate during its manufacture. The surface oxide layer has a higher laser light absorption rate than the Fe of the base material (steel substrate) and also serves as an oxygen source necessary for cutting, making it a necessary layer for laser cutting. In order to increase the laser light absorption energy and improve cutting efficiency, the thickness of the surface oxide layer is set to 15 μm or more. The thicker the surface oxide layer, the higher the laser absorption energy. However, if the surface oxide layer is too thick, peeling occurs easily at the interface between the surface oxide layer and the steel substrate, and the thickness of the surface oxide layer is also likely to become uneven, resulting in poor laser cutting properties. For this reason, the thickness of the surface oxide layer is set to 60 μm or less.

In order to ensure stable laser cutting performance, it is preferable to make the surface oxide layer uniform in thickness, with the standard deviation of the surface oxide layer thickness being 5.0 μm or less. To achieve a standard deviation of the surface oxide layer thickness of 5.0 μm or less, as described above, the Si, Cu, and Ni contents are controlled to predetermined levels. Here, ensuring stable laser cutting performance means, for example, that there are no notches or dross adhesions on the cut surface, and that cutting is possible even at high laser cutting speeds.

The thickness of the surface oxide layer and its standard deviation can be measured using the method shown in the examples.

[Thickness]
The thickness of the steel plate of the present invention is not particularly limited and can be any thickness. For example, the thickness of the steel plate of the present invention is 4.5 mm or more. For example, the thickness of the steel plate of the present invention is 32 mm or less. In the present invention, a steel plate having a thickness of 4.5 to 32 mm can provide excellent cutting quality, particularly when laser cutting, so the thickness of the steel plate is preferably 4.5 to 32 mm.

[Manufacturing method]
Next, a method for manufacturing a steel plate according to the present invention will be described. The steel plate according to the present invention can be manufactured by heating a steel material having the chemical composition according to the first or second embodiment described above, and then descaling the surface oxide layer formed during hot rolling under predetermined conditions. Specifically, in one embodiment of the method for manufacturing a steel plate according to the present invention, a steel material having the chemical composition described above is heated to 1000 to 1200°C, and then hot-rolled at a rolling end temperature of 700 to 1000°C. During the rolling passes in the temperature range from (rolling end temperature + 100°C) to the rolling end temperature, descaling is performed five or more times by spraying water onto at least one surface of the steel plate. Note that the "°C" designation for temperature in the following description refers to the temperature of the surface layer of the steel plate. The temperature can be measured using a surface thermometer or the like.

Heating temperature of steel material: 1000-1200°C
If the heating temperature of the steel material is below 1000°C, the deformation resistance during hot rolling is high, making it difficult to achieve a large reduction per pass. As a result, the number of rolling passes increases, resulting in a decrease in rolling efficiency and the inability to press-fit casting defects in the steel material (slab). On the other hand, if the heating temperature exceeds 1200°C, excessive high-temperature scale formed during heating is likely to cause surface defects, increasing the burden of post-rolling maintenance and coarsening the crystal grains, degrading the brittleness and ductility of the base material. For this reason, the heating temperature of the steel material is set to a range of 1000 to 1200°C. Note that the present invention can employ the conventional method of cooling the steel material to room temperature after production and then reheating it. In addition, direct rolling can also be applied, in which the steel material is charged into a heating furnace as a hot slab without being cooled to room temperature, or immediately rolled after a short period of heat retention.

Rolling end temperature: 700 to 1000°C
If the rolling end temperature of hot rolling exceeds 1000°C, not only do blisters occur in the surface oxide layer, but the surface oxide layer grows excessively during the cooling process after rolling. As a result, the desired surface oxide layer thickness cannot be obtained. Furthermore, the structure becomes coarse, resulting in a deterioration in toughness. On the other hand, if the rolling end temperature is lower than 700°C, the deformation resistance becomes too high, increasing the rolling load and placing a heavy burden on the rolling mill. Furthermore, in order to lower the rolling temperature, waiting is required during rolling, which not only significantly impairs productivity, but also causes the surface oxide layer to thin without growing due to the low steel plate temperature, making it impossible to obtain the desired surface oxide layer thickness. Furthermore, the strain accumulated in the thick steel plate increases, causing deformation of the thick steel plate during laser cutting, resulting in problems such as reduced cutting accuracy and interruption of cutting. For this reason, the rolling end temperature is set to the range of 700 to 1000°C.

Descaling during rolling passes in the temperature range from (rolling end temperature + 100°C) to the rolling end temperature: 5 or more times In order to consistently form a surface oxide layer of the desired thickness, it is important in the present invention to strictly control the number of descalings during rolling. In the present invention, the number of descalings during rolling passes in the temperature range from (rolling end temperature + 100°C) to the rolling end temperature is set to 5 or more times. If the number of descalings in this temperature range is less than 5 times, the surface oxide layer that forms and grows during rolling will not be sufficiently peeled off, resulting in excessive growth of the surface oxide layer. Furthermore, the surface oxide layer will peel off locally, resulting in greater variation in thickness of the surface oxide layer.

Descaling can be performed by spraying water onto at least one surface of the steel plate. When descaling is performed, it is usually performed once before, after, or during one rolling pass. Descaling is preferably performed at a water spray pressure of 10 MPa or higher.

The present invention will be described in detail below using examples. However, the present invention is not limited to the following examples.

Steel materials (steel slabs) prepared using the converter-ladle refining-continuous casting process to have the various chemical compositions shown in Tables 1 and 2 were then hot-rolled to 25 mm thick steel plates under the various hot-rolling conditions shown in Tables 3 to 5. Descaling was performed on both sides of the steel plates using water injection pressure of 15 MPa. Note that blank spaces in Tables 1 and 2 indicate that the element was not intentionally added, and include not only cases where the element is not contained (0%), but also cases where the element is unavoidably contained.

Six samples, each measuring 20 mm x 20 mm x thickness, were taken from the resulting steel plates at positions 500 mm from the leading edge, center, and tail edge, respectively, at positions 1/4 and 1/2 of the plate width. Each sample was embedded in carbon resin so that the cross section parallel to the rolling direction served as the observation surface, and polished to a mirror finish. Five optical microscope photographs were taken at 400x magnification of the area containing the base steel and the surface oxide layer. The thickness of the surface oxide layer (scale thickness) was measured at 10 random locations on each photograph using an image analyzer. The average value of the measurements for all six samples was taken as the surface oxide layer thickness. The standard deviation of the measurements for all six samples was also taken as the standard deviation of the surface oxide layer thickness. The presence or absence of an Sb-enriched layer was evaluated by Sb mapping analysis using an electron beam microanalyzer (accelerating voltage 15 kV) on a 20 μm x 30 μm area of the cross-sectional sample, including the interface between the surface oxide layer and the base steel. Specifically, the presence of Sb at the interface was determined to be an indication of an Sb-enriched layer.

For tensile properties, a JIS No. 1A test piece was taken from the C direction (perpendicular to the rolling direction) of the thick steel plate in accordance with JIS Z2241:2022, and a tensile test was conducted to determine the yield strength and tensile strength. A tensile strength of 400 MPa or more is preferred.

To evaluate toughness, three test specimens (2 mm V-notch, size: 10 mm x 10 mm) were taken from the 1/4 position of the plate thickness (t) with the notch direction aligned in the L direction (parallel to the rolling direction) in accordance with JIS Z 2242:2018. Charpy impact tests were conducted. The absorbed energy vE0°C at a test temperature of 0°C was determined to evaluate impact properties. A vE0°C of 27 J or more is preferred. The absorbed energy value was calculated as the average of the three test specimens.

Laser cuttability was evaluated by cutting thick steel plates using a 6 kW carbon dioxide laser and visually inspecting the cross section of the cut steel plate for the presence or absence of notches and the presence or absence of dross on the back surface of the plate. The cutting test was performed under conditions of an oxygen pressure of 0.3 kgf/cm ² , a cutting length of 500 mm, and cutting speeds of 750 mm/min and 900 mm/min. "Cuttable" was determined based on whether the plates could be separated regardless of the condition of the cut surface. Specifically, plates that could not be separated due to reasons such as cutting stopping midway were deemed "uncuttable." The results are shown in Tables 3 to 5. In this test, plates that could be cut at a cutting speed of 750 mm/min and had no notches on the cross section and no dross on the back surface of the plate were evaluated as having excellent (good) laser cuttability and were deemed to have passed.

Machinability was evaluated by conducting a cutting (peripheral turning) test. The tool material was carbide tool P20, and the cutting conditions were feed 0.20 mm/rev, depth of cut 2.0 mm, cutting speed 150 m/min, and no lubrication. Tool life was evaluated as the cutting time until flank wear width VB = 0.2 mm. In this test, a tool life of 20 minutes or more was evaluated as having excellent (good) machinability and was deemed to have passed. The results are shown in Tables 3 to 5.

All of the inventive examples exhibited excellent laser cuttability and machinability. Furthermore, all of the inventive examples exhibited excellent strength, with a tensile strength of 400 MPa or more, and excellent toughness, with a vE0°C of 27J or more. Furthermore, those with a standard deviation of scale thickness of 5.0 μm or less were capable of cutting even at a cutting speed of 900 mm/min, ensuring stable laser cuttability. Furthermore, there were no notches on the cross section of the thick steel plate after cutting, and no dross adhesion to the back surface of the thick steel plate, demonstrating particularly excellent laser cuttability.

Claims (6)

In mass%,
C: more than 0.03% and less than 0.20%,
Sb: 0.003 to 0.050%,
Si: 0.60% or less,
Mn: 0.10 to 2.50%,
P: 0.030% or less,
S: 0.030% or less,
Al: 0.150% or less,
N: 0.02% or less, B: more than 0.0030% and 0.0100% or less, and the balance being Fe and unavoidable impurities;
A thick steel plate having a surface oxide layer having a thickness of 15 μm or more and 60 μm or less. The component composition further includes, in mass %,
Cu: 1.00% or less,
Ni: 1.00% or less,
Cr: 0.01-1.00%,
Mo: 0.01-1.00%,
W: 0.01-1.00%,
V: 0.003-0.100%,
Nb: 0.003 to 0.030%,
Ti: 0.003 to 0.050%,
REM: 0.0001-0.0030%,
Ca: 0.0001-0.0030%,
Mg: 0.0001 to 0.0030%, and Sn: 0.001 to 0.030%
The steel plate according to claim 1, comprising one or more selected from the following: In mass%,
C: more than 0.03% and less than 0.20%,
Sb: 0.003 to 0.050%,
Si: 0.60% or less,
Mn: 0.10 to 2.50%,
P: 0.030% or less,
S: 0.030% or less,
Al: 0.150% or less,
N: 0.02% or less, Ca: more than 0.0030% and 0.0100% or less, and the balance being Fe and unavoidable impurities;
A thick steel plate having a surface oxide layer having a thickness of 15 μm or more and 60 μm or less. The component composition further includes, in mass %,
Cu: 1.00% or less,
Ni: 1.00% or less,
Cr: 0.01-1.00%,
Mo: 0.01-1.00%,
W: 0.01-1.00%,
V: 0.003-0.100%,
Nb: 0.003 to 0.030%,
Ti: 0.003 to 0.050%,
REM: 0.0001-0.0030%,
B: 0.0001 to 0.0030%,
Mg: 0.0001 to 0.0030%, and Sn: 0.001 to 0.030%
The steel plate according to claim 3, comprising one or more selected from the following: A steel plate according to any one of claims 1 to 4, in which the standard deviation of the thickness of the surface oxide layer is 5.0 μm or less. A method for producing a thick steel plate having a surface oxide layer having a thickness of 15 μm to 60 μm, wherein a steel material having the component composition according to any one of claims 1 to 4 is heated to 1000 to 1200°C, and then hot-rolled at a rolling end temperature of 700 to 1000°C, and descaling is carried out five or more times by spraying water onto at least one surface of the steel plate during rolling passes in a temperature range of (rolling end temperature + 100°C) to the rolling end temperature .