Heavy steel plate and its manufacturing method

Abstract

To provide a thick steel plate with superior laser cutting properties and superior machinability, and a method for producing the same.SOLUTION: A thick steel plate comprises, in mass%, C: more than 0.03% to 0.20% or less, Sb: 0.003-0.050%, Si: 0.60% or less, Mn: 0.10-2.50%, 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% to 0.0100% or less or Ca: more than 0.0030% to 0.0100% or less, with the balance being Fe and inevitable impurities. The thick steel plate has a surface oxide layer with a thickness of 15 μm or more and 60 μm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a thick steel plate and a manufacturing method thereof, and more particularly to a thick steel plate having excellent laser cuttability and machinability, and a manufacturing method thereof. [Background technology]

[0002] When thick hot-rolled steel plates are used for components such as molds and machine parts, they are typically cut or milled to the desired shape before assembly. Cutting and milling operations account for a large proportion of the steel structure manufacturing process, creating a demand for high efficiency and low cost. Furthermore, as the shapes of steel structures become more complex due to design considerations and the elimination of processes after cutting, the cut surfaces also become more complex, requiring high precision. Furthermore, in the milling 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.

[0003] Gas cutting and plasma cutting have traditionally been used to cut 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.

[0004] Plasma cutting is capable of high-speed cutting up to a maximum thickness of about 50 mmt, but the torch has a lifespan of only a few hours, and frequent replacement work makes it inefficient to use, making automation difficult.

[0005] On the other hand, laser cutting is now being applied to cutting thick steel plates as laser oscillators become more powerful and cheaper. The advantages of laser cutting are that it is easy to fully automate, there is little heat input and thermal impact during cutting, and the quality of the cut surface is good.

[0006] 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., preventing cutting defects.

[0007] Patent Document 1 describes a steel sheet with excellent laser cuttability, which has a specific chemical composition, a scale layer with an average thickness of 10 μm or less, and a magnetite content of 60 mass % or more in the scale layer.

[0008] Patent Document 2 describes a thick steel plate with excellent laser cuttability, in which the amounts of alloying elements such as Al, Cu, Ni, and Cr are controlled and the surface scale layer has an Al2O3-containing layer at the interface with the base steel.

[0009] 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.

[0010] Patent Document 4 describes a thick steel plate that contains a predetermined amount of Sb, has a surface oxide layer with a thickness of 15 μm or more and 50 μm or less, and has excellent laser cuttability.

[0011] Furthermore, MnS free-cutting steel is known as an example of a steel material with excellent machinability (Non-Patent Document 1). [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-87339 [Patent Document 2] Japanese Patent Application Publication No. 11-323478 [Patent Document 3] Japanese Patent Application Laid-Open No. 2004-169093 [Patent Document 4] Japanese Patent Publication No. 2022-054486 [Non-patent literature]

[0013] [Non-Patent Document 1] Chubu Kohan Co., Ltd. website, "Improved machinability steel plate," searched February 1, 2023, Internet,<URL:https: / / www.chubukohan.co.jp / product / standard / info / > Summary of the Invention [Problem to be solved by the invention]

[0014] In Patent Document 1, strict temperature control during rolling is required to obtain a thin scale containing 60 mass% or more of magnetite, which results in a lack of stable production. Patent Document 2 states that the amount of alloying elements needs to be adjusted to improve laser cuttability. Patent Document 3 states that problems such as dross adhesion occur when laser cutting plates with a thickness of 25 mm or more. Furthermore, Patent Documents 1 to 3 do not take machinability into consideration. Furthermore, Patent Document 4 also describes only improvements in laser cuttability, and does not consider machinability at all. Furthermore, Non-Patent Document 1 describes improved machinability, but lacks laser cuttability.

[0015] The present invention has been made in view of the above circumstances, and has as its object to provide a thick steel plate that has excellent laser cuttability and machinability, and a method for manufacturing the same. [Means for solving the problem]

[0016] In order to achieve the above object, the present inventors have conducted extensive research into the influence 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") on laser cutting in order to ensure excellent laser cuttability of thick steel plate, and have obtained the following findings.

[0017] (1) When high-density energy is applied to thick steel plates by laser irradiation, thermal stress is generated, which destroys the surface scale and makes cutting defects more likely. To prevent this, it is necessary to improve the adhesion of the scale.

[0018] (2) When a predetermined amount of Sb is added to improve scale adhesion, Sb is concentrated at the interface between the scale and the base steel. Although 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 locally concentrate at specific locations, such as grain boundaries, causing differences in the scale growth rate, excessive inclusion of Si, Cu, and Ni results in non-uniform scale. In the present invention, by setting the contents of Si, Cu, and Ni to a predetermined amount or less and adding a predetermined amount of Sb, Sb is concentrated at the interface between the scale and the base steel, allowing the formation of scale with excellent adhesion and uniformity.

[0019] (3) By setting the thickness of the scale 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 the adhesion of dross can also be suppressed.

[0020] Furthermore, in order to ensure excellent machinability for thick steel plates, the inventors have conducted extensive research into the effects of the component composition and manufacturing method of steel plates on machinability, and have obtained the following findings: Note that machinability is obtained by comprehensively evaluating tool wear, chip disposal, cutting resistance, cutting temperature, and adhesion, but when each individual evaluation item is excellent, it can naturally be said that machinability is good.

[0021] (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. By adding a combination of B and N, or Ca and N, to the steel plate, which 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, tool wear can be reduced and machinability can be improved.

[0022] The present invention was made based on the above findings and further investigations, and the gist of the present invention is as follows.

[0023] [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: 0.10~2.50% 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 less than 0.0100% and the balance being Fe and unavoidable impurities, A thick steel plate with a surface oxide layer that is 15 μm or more and 60 μm or less in thickness. [2] The component composition further includes, in mass%, 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: 0.0001~0.0030%, Ca: 0.0001 to 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: 0.10~2.50% 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 less than 0.0100% and the balance being Fe and unavoidable impurities, A thick steel plate with a surface oxide layer that is 15 μm or more and 60 μm or less in thickness. [4] The component composition further includes, in mass%, 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: 0.0001~0.0030%, B: 0.0001~0.0030%, 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 manufacturing thick steel plate, comprising heating a steel material having a chemical composition according to any one of [1] to [4] above to 1000 to 1200°C, and then hot rolling the material at a rolling end temperature of 700 to 1000°C, and then descaling the material by spraying water onto at least one side of the steel plate five or more times during a rolling pass in a temperature range of (rolling end temperature + 100°C) to the rolling end temperature. [Effects of the Invention]

[0024] According to the present invention, a thick steel plate having excellent laser cuttability and machinability can be provided.

[0025] 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. DETAILED DESCRIPTION OF THE INVENTION

[0026] The present invention will be specifically described below. Note that the following description shows examples of preferred embodiments of the present invention, and the present invention is not limited thereto.

[0027] The steel plate (base steel of the steel plate) of the present invention has the chemical composition of the first embodiment or the second embodiment shown below. The reasons for limiting each chemical composition will be explained below. Note that "%" regarding the chemical composition means "mass %" unless otherwise specified.

[0028] [Component Composition of First Embodiment] The steel plate according to the first embodiment of the present invention has a chemical 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.

[0029] The composition may further contain, in mass %, 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%, Ca: 0.0001 to 0.0030%, Mg: 0.0001 to 0.0030%, and Sn: 0.001 to 0.030%.

[0030] C: More than 0.03% and less 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, the C content must exceed 0.03%. 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. For this reason, the C content is set to 0.20% or less. The C content is preferably 0.18% or less.

[0031] Sb: 0.003 to 0.050% Sb is an element necessary for improving scale adhesion. Sb diffuses to the surface during the steel material heating and hot rolling process, 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 the uniformity of the scale. 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 likely to occur on the surface of the steel plate, 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.

[0032] 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.

[0033] Mn: 0.10 to 2.50% Mn has the effect of increasing the hardenability of steel, and is contained in an amount of 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. For this reason, the Mn content is set to 2.50% or less. The Mn content is preferably set to 2.00% or less.

[0034] P:0.030% or less P exists in steel as an impurity and may have adverse effects such as degrading the low-temperature toughness and ductility of the base material by segregating at grain boundaries. 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. For this reason, from the viewpoint of productivity, it is preferable to set the P content to 0.001% or more.

[0035] S: 0.030% or less S, present in steel as sulfide inclusions such as MnS, can 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 added, 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 in the shear deformation region of the tool cutting edge and the workpiece material, thereby improving machinability. However, the toughness in the C direction decreases with increasing S content. Considering these points, 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 perspective of productivity, it is preferable to set the S content to 0.001% or more.

[0036] 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, it also prevents diffusion and adhesion between the tool and the backside of the chip 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.

[0037] 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.

[0038] B: More than 0.0030% and less than 0.0100% At high temperatures, such as during cutting, Al and BN in steel form AlN. The presence of an AlN film on the tool wear surface suppresses the diffusion reaction between the tool and the workpiece, 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 increases significantly, deteriorating the toughness and ductility of the base material. Furthermore, if the B content exceeds 0.0100% and the ratio of the N content to the B content (N / B ratio) decreases, free B remains in the steel, deteriorating machinability. Therefore, the B content is set to 0.0100% or less. The B content is preferably set to 0.0080% or less.

[0039] The steel plate according to the first embodiment of the present invention may have the above-mentioned components as the basic components, with the balance being Fe and unavoidable impurities.

[0040] In addition to the above-described 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 following content ranges.

[0041] 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, causing significant deterioration in the surface properties of the steel plate. Therefore, when Cu is contained, the Cu content is preferably 1.00% or less.

[0042] 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.

[0043] Cr: 0.01 to 1.00% Cr is an element that contributes to improving the strength of steel plates. To achieve this effect, when Cr is contained, the Cr content is preferably 0.01% or more. On the other hand, if the Cr content exceeds 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.

[0044] Mo: 0.01 to 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, if 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.

[0045] W: 0.01 to 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, if 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.

[0046] V: 0.003 to 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 deteriorate. Therefore, when V is contained, the V content is preferably 0.100% or less.

[0047] Nb: 0.003 to 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.

[0048] Ti: 0.003 to 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%, 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.

[0049] 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 a total of 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.

[0050] Ca: 0.0001 to 0.0030% Ca bonds with S to control the morphology of sulfides, thereby contributing to improving the toughness of steel. To achieve 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.

[0051] Sn: 0.001 to 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.

[0052] In addition, 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, in the case of Cu and Ni, when the content is less than 0.01%), the component is considered to be contained as an unavoidable impurity.

[0053] [Component Composition of Second Embodiment] The steel plate according to the second embodiment of the present invention has a chemical 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.

[0054] The composition may further contain, in mass%, 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%, B: 0.0001 to 0.0030%, Mg: 0.0001 to 0.0030%, and Sn: 0.001 to 0.030%.

[0055] In the chemical composition of the steel plate according to the second embodiment of the present invention, the reasons for limiting the contents of C to more than 0.03% and not more than 0.20%, Sb to 0.003 to 0.050%, Si to 0.60% or less, Mn to 0.10 to 2.50%, P to 0.030% or less, S to 0.030% or less, Al to 0.150% or less, and N to 0.02% or less are the same as those for the first embodiment described above.

[0056] 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 steel. It also acts as a deoxidizer, forming a protective film (Belaag) on ​​the tool surface when the Ca content is 0.0020% or more and the Al content is 0.150% or less, thereby reducing tool wear. However, 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 limited, so the Ca content is set to more than 0.0030%.

[0057] The steel plate according to the second embodiment of the present invention may have the above-mentioned components as the basic components, with the balance being Fe and unavoidable impurities.

[0058] In addition to the above-described 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 following content ranges.

[0059] In the chemical composition of the steel plate according to the second embodiment of the present invention, when one or more elements 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% are contained, the reasons for limiting the content of each of the elements are the same as those for the first embodiment described above.

[0060] B: 0.0001 to 0.0030% B is an element that has the effect of increasing the strength of steel by improving hardenability. To obtain this effect, when B is contained, the B content is preferably 0.0001% or more. On the other hand, even if the B content exceeds 0.0030%, the effect saturates. Therefore, when B is contained, the B content is preferably 0.0030% or less.

[0061] In addition, 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, in the case of Cu and Ni, when the content is less than 0.01%), the component is considered to be contained as an unavoidable impurity.

[0062] [Suitable ranges for 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 to be 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.

[0063] 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 standpoint of deoxidation, a Si content of 0.01% or more is preferable. Furthermore, the melting point of SiO2 (silicon oxide) formed by oxidizing Si (around 1700°C) is higher than the melting point of iron (1536°C), and the generation of SiO2 is thought to reduce the fluidity of molten metal during laser cutting, increasing the roughness of the cut surface and leading to burning. Considering this, a low Si content is preferable.

[0064] 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 to achieve uniform scale. From this perspective, when Cu and Ni are contained, the Cu and Ni contents are each allowable if they are 0.03% or less. From this perspective, the Cu and Ni contents may be 0%.

[0065] [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 will occur at the interface between the surface oxide layer and the steel substrate, and the thickness of the surface oxide layer will also tend 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.

[0066] In order to ensure stable laser cuttability, it is preferable to make the surface oxide layer uniform in thickness, with the standard deviation of the thickness of the surface oxide layer being 5.0 μm or less. To achieve a standard deviation of the surface oxide layer thickness of 5.0 μm or less, the contents of Si, Cu, and Ni are controlled to predetermined values, as described above. Here, ensuring stable laser cuttability means, for example, that there are no notches or dross on the cut surface, and that cutting is possible even at high laser cutting speeds.

[0067] The thickness of the surface oxide layer and its standard deviation can be measured by the method shown in the Examples.

[0068] Plate Thickness The thickness of the steel plate of the present invention is not particularly limited and can be any thickness. As an example, the thickness of the steel plate of the present invention is 4.5 mm or more. As another 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 obtain excellent cutting quality, particularly when laser cutting, so the thickness of the steel plate is preferably 4.5 to 32 mm.

[0069] [Manufacturing method] Next, a method for producing a steel plate according to the present invention will be described. The steel plate according to the present invention can be produced 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 a method for producing a steel plate according to one embodiment of 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, whereby descaling is carried out 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. In the following description, the temperature indicated in "°C" refers to the temperature of the surface layer of the steel plate. The temperature can be measured using a surface thermometer or the like.

[0070] Heating temperature of steel material: 1000~1200℃ 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, deteriorating 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.

[0071] Rolling end temperature: 700~1000℃ If the rolling end temperature of hot rolling exceeds 1000°C, not only will blisters occur in the surface oxide layer, but the surface oxide layer will grow excessively during the cooling process after rolling. As a result, the desired surface oxide layer thickness cannot be obtained. Furthermore, the microstructure will become coarse, resulting in a deterioration in toughness. On the other hand, if the rolling end temperature is lower than 700°C, the deformation resistance will be too high, increasing the rolling load and placing a heavy burden on the rolling mill. Furthermore, to lower the rolling temperature, waiting will be required during rolling, which will significantly hinder productivity. Furthermore, the low steel plate temperature will prevent the surface oxide layer from growing and result in a thin layer, preventing the desired thickness from being obtained. Furthermore, the large strain accumulated in the steel plate will cause deformation of the steel plate during laser cutting, resulting in reduced cutting accuracy and interrupted cutting. For these reasons, the rolling end temperature is set to a range of 700 to 1000°C.

[0072] Descaling during rolling passes in the temperature range from (rolling end temperature + 100°C) to the rolling end temperature: 5 times or more In the present invention, in order to stably form a surface oxide layer of the desired thickness, it is important to strictly control the number of descalings during rolling. In the present invention, the number of descalings is set to 5 or more during a rolling pass in the temperature range from (rolling end temperature + 100°C) to the rolling end temperature. If the number of descalings in this temperature range is less than 5, the surface oxide layer that forms and grows during rolling will not be sufficiently peeled off, and the surface oxide layer will grow excessively. Furthermore, since the surface oxide layer peels off locally, the thickness of the surface oxide layer will vary greatly.

[0073] Descaling can be performed by spraying water onto at least one surface of the steel plate. When descaling is performed, descaling is usually performed once before, after, or during one pass of rolling. Descaling is preferably performed at a water spray pressure of 10 MPa or more. [Example]

[0074] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0075] 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 produce 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 a 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 was not contained (0%), but also cases where the element was unavoidably contained.

[0076] Six samples, each measuring 20 mm × 20 mm × thickness, were taken from the resulting steel plates at positions 500 mm from the leading edge, the center, and 500 mm from the tail edge, respectively, at the quarter and half positions in the width direction. Each sample was embedded in carbon resin so that the cross section parallel to the rolling direction served as the observation surface, and then polished to a mirror finish. Five optical microscope photographs were taken at 400x magnification of the area containing the steel substrate 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 of a 20 μm × 30 μm area of ​​the cross-sectional sample, including the interface between the surface oxide layer and the steel substrate, using an electron beam microanalyzer (accelerating voltage 15 kV). Specifically, the presence of Sb at the interface was judged to be an indication of the presence of an Sb-enriched layer.

[0077] Regarding the 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 carried out to determine the yield strength and tensile strength. The tensile strength is preferably 400 MPa or more.

[0078] Regarding toughness, in accordance with JIS Z 2242:2018, three test pieces were taken from the 1 / 4 position of the plate thickness (t) with the notch direction in the L direction (parallel to the rolling direction) (test piece: 2 mm V notch, size: 10 mm x 10 mm), and a Charpy impact test was performed. The absorbed energy vE0°C at a test temperature of 0°C was determined to evaluate the impact properties. vE0°C is preferably 27 J or more. The absorbed energy value was the average value of the three test pieces.

[0079] Laser cuttability was evaluated by cutting a thick steel plate using a 6kW 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 steel plate. The cutting test was carried out under the conditions of an oxygen pressure of 0.3kgf / cm. 2The cutting length was 500 mm, and the cutting speeds were 750 mm / min and 900 mm / min for evaluation. "Cuttable" was determined based on whether the plates could be separated regardless of the state of the cut surface. Specifically, plates that could not be separated due to reasons such as the 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 that had no notches on the cross section of the thick steel plate after cutting and no dross adhesion on the back surface of the thick steel plate were evaluated as having excellent laser cuttability (good) and were deemed to have passed.

[0080] Machinability was evaluated by conducting a cutting (peripheral turning) test. The tool material was a carbide tool P20, and the cutting conditions were a feed of 0.20 mm / rev, a depth of cut of 2.0 mm, a cutting speed of 150 m / min, and no lubrication. Tool life was evaluated as the cutting time at which the flank wear width VB reached 0.2 mm. In this test, a tool life of 20 min or more was evaluated as having excellent (good) machinability and was deemed to have passed. The results are shown in Tables 3 to 5.

[0081] All of the examples of the present invention were excellent in laser cuttability and machinability. Furthermore, all of the examples of the present invention had excellent strength, with a tensile strength of 400 MPa or more, and excellent toughness, with a vE0°C of 27 J or more. Furthermore, those with a standard deviation of scale thickness of 5.0 μm or less were cuttable even at a cutting speed of 900 mm / min, ensuring stable laser cuttability, and 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.

[0082] [Table 1]

[0083] [Table 2]

[0084] [Table 3]

[0085] Table 4

[0086] Table 5

Claims

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, and B: More than 0.0030% and less than 0.0100% 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: 0.001-0.030% The steel plate according to claim 1, comprising 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, and Ca: more than 0.0030% and less than 0.0100% 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: 0.001-0.030% The steel plate according to claim 3, comprising one or more selected from the following:

5. The steel plate according to any one of claims 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 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.

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