Thick steel plate and method for manufacturing the same

Abstract

To provide a thick steel plate excellent in tensile property in a thickness direction and having a plate thickness of more than 130 mm and a producing method thereof.SOLUTION: A plate thickness is more than 130 mm, and the chemical components are C, Si, Mn, P, S, Al, Cr, Mo, Nb, N, in a predetermined range and a metallographic structure is a single phase structure of any of bainite, tempered bainite, and tempered martensite, or a mixed structure made of two or more of the bainite, tempered bainite, and tempered martensite. A major axis of each of MnS inclusions and NbCN inclusions is 10 μm or less, and at a position of 1 / 2 thickness±10 mm of the plate thickness, the number of porosities of 0.1-1.0 mm φ diameter in the total width is 10 / 100 mm or less, the aperture of the plate thickness directional tensile test is 35% or more on average when three plates are tested, and a lowest value of the aperture of the three plate thickness directional tensile test is 25% or more.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a thick steel plate applicable to a boiler, a turbine chamber, etc. used in thermal power generation, and a method for manufacturing the same.

Background Art

[0002] Generally, thick steel plates are manufactured using continuous casting slabs or ingots obtained by block rolling of mold-cast ingots as raw materials. Comparing the two, the former is more attractive in terms of manufacturing cost. However, in the case of continuous casting slabs, central segregation often occurs, resulting in deterioration of the tensile performance in the plate thickness direction. When the tensile performance in the plate thickness direction deteriorates, welding cracks occurring in the vertical direction in the plate thickness direction may become a problem in members that require the production of cruciform welded joints. In particular, when high-Cr thick steel plates are manufactured using continuous casting slabs, the tensile performance in the plate thickness direction is generally inferior to that of ordinary low-carbon low-alloy steels.

[0003] Therefore, when manufacturing high-Cr thick steel plates that require tensile performance in the plate thickness direction, conventionally, ingot slabs have been used. However, in the manufacturing process, trimming of unsteady parts such as the hot pressing part and the precipitation crystal part and block rolling are required, leading to an increase in cost and a decrease in productivity. Therefore, in recent years, there has been a demand for establishing a technology for thick steel plates obtained using continuous casting slabs so as not to cause an increase in cost and a decrease in productivity.

[0004] Several proposals have been made in the past regarding the improvement of the internal quality of continuous casting slabs.

[0005] Patent Documents 1 and 2 disclose technologies for eliminating the center porosity of continuous casting slabs by applying pressure reduction using rolls or surface pressing devices on the outlet side of a continuous casting machine.

[0006] Patent Document 3 discloses a technique for manufacturing extra-thick steel plates of 125 mm or more in thickness under processing conditions where the total reduction ratio is 20-60%, in which, prior to thick plate rolling, the ends of the continuously cast slab are forged and reduced from the width direction, reducing the width of the original slab by 150 mm or more and increasing its thickness, and then forging and reducing in the slab thickness direction to eliminate center porosity.

[0007] Patent documents 4 and 5 disclose a technique for manufacturing extra-thick stainless steel plates with a thickness of 80 mm or more, in which, prior to thick plate rolling, the edges of a continuously cast slab are forged and rolled down from the width direction, reducing the width of the original slab by 300 mm or more and increasing its thickness, and then forging and rolling down in the slab thickness direction to eliminate center porosity.

[0008] Furthermore, Non-Patent Document 1 discloses various common methods for improving the tensile performance of low-carbon, low-alloy steel sheets in the thickness direction, such as reducing the amount of sulfur and adding calcium. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 55-114404 [Patent Document 2] Japanese Patent Application Publication No. 61-273201 [Patent Document 3] Patent No. 3528504 [Patent Document 4] Japanese Patent Publication No. 2014-188579 [Patent Document 5] Japanese Patent Publication No. 2014-189886 [Non-patent literature]

[0010] [Non-Patent Document 1] Bulletin of the Japan Institute of Metals, Vol. 18, No. 5, pp. 368-376 [Overview of the project] [Problems that the invention aims to solve]

[0011] However, Patent Documents 1 to 5 aim to improve internal properties and do not directly disclose a method for improving tensile performance in the thickness direction of the plate.

[0012] Furthermore, while Non-Patent Document 1 directly describes a method for improving tensile performance in the thickness direction of the plate, this knowledge pertains to low-carbon, low-alloy steel, and it is not necessarily applicable to the high-chromium, thick steel plates that are the subject of this study.

[0013] The present invention was made to solve the above problems, and aims to provide a thick steel plate with a thickness exceeding 130 mm that has excellent tensile performance in the thickness direction, and a method for manufacturing the same. [Means for solving the problem]

[0014] The inventors conducted various studies to achieve the above objective and obtained the following findings.

[0015] First, to identify the cause of the deterioration in the thickness-direction tensile performance of high-chromium thick steel sheets manufactured from continuously cast slabs, fracture surface observations were performed on test specimens with low values ​​for thickness-direction tensile performance (reduction of area). This revealed the presence of coarse MnS-based inclusions, which have been known to exist even in low-carbon, low-alloy steels, as well as the presence of coarse NbCN-based inclusions. This means that the high-chromium thick steel sheets targeted by this invention require high Nb and N content to ensure heat resistance, which causes coarse NbCN-based inclusions to crystallize in the central segregation of the continuous casting slab at the end of solidification, and these remain in the final product, degrading the tensile performance in the thickness direction. Therefore, it has become clear that the knowledge gained so far regarding low-carbon low-alloy steels cannot be directly applied to high-chromium thick steel sheets. Furthermore, porosity was observed in some products, and it was found that in order to stably ensure tensile performance in the thickness direction of the sheet, it is also necessary to suppress the occurrence of coarse center porosity.

[0016] From the above findings, it has been found that thick steel plates such as high-Cr thick steel plates manufactured from continuously cast slabs need to dissolve or crush MnS-based inclusions and NbCN-based inclusions during the period from casting to heat treatment. The inventors of the present invention have clarified that as a method for dissolving and crushing these, normal hot rolling is insufficient, and it is effective to perform hot forging that applies a large reduction in thickness to the center in the thickness direction before hot rolling. Further, it has been found that it is possible to crimp center porosity by ensuring a sufficient reduction ratio by this hot forging and hot rolling.

[0017] The present invention has been made by further examining based on the obtained findings. That is, the gist configuration of the present invention is as follows. [1] The plate thickness is more than 130 mm, The chemical composition is in mass%, C: 0.02 to 0.13%, Si: 0.02 to 0.50%, Mn: 0.30 to 0.70%, P: 0.030% or less, S: 0.006% or less, Al: 0.060% or less, Cr: 8.0 to 14.0%, Mo: 0.03 to 2.20%, Nb: 从0.01到0.10%, N: 0.003到0.080%, contains, and the balance is Fe and inevitable impurities, The metal structure is any single-phase structure of bainite, tempered bainite, and tempered martensite, or a mixed structure composed of any two or more of bainite, tempered bainite, and tempered martensite, The major diameters of each of the MnS-based inclusions and NbCN-based inclusions are 10 μm or less, At the position of 1 / 2 thickness of the plate thickness ± 10 mm, the number of porosities with a diameter of 0.1 to 1.0 mmφ in the entire width is 10 or less per 100 mm, A thick steel plate having an average reduction in the plate thickness direction tensile test of 35% or more for 3 tests and a minimum value of the reduction in the 3 plate thickness direction tensile tests of 25% or more. [2] The above chemical components are further, in mass%, Cu: 0~0.50%, Ni: 0~0.40%, V: 0~0.40%, Ti: 0~0.15%, B: 0~0.006%, Ca: 0~0.0050%, Mg: 0~0.0050%, W: 0~2.0%, The thick steel plate according to [1] above, comprising one or more selected from the above. [3] A continuous cast slab with a thickness of 195 mm or more having the chemical components described in [1] or [2] above is heated to 1050 to 1300°C. After heating, the ends of the slab in the width direction are forged and rolled down in the width direction, reducing the slab width by 300 mm or more. Hot forging is performed at a temperature of 1000°C or higher, with the contact area between the second surface facing the first surface of the slab being three times or more than the contact area between the first surface of the slab and the second surface facing the first surface, and the slab is reduced in the thickness direction by a reduction ratio of 10% or more using a forging die. Cooling is performed, and after cooling, it is reheated and hot-rolled to a plate thickness of more than 130 mm. Cooling is performed, and after cooling, the product is baked at a temperature of 900-1100°C. A method for manufacturing thick steel plates, wherein the sum of the reduction ratio in hot forging and the reduction ratio in hot rolling is 30% or more. [4] A method for manufacturing a thick steel plate according to [3], wherein after normalizing, cooling is performed, and after cooling, tempering is performed at a temperature of 700 to 850°C. [Effects of the Invention]

[0018] According to the present invention, a thick steel plate with excellent tensile performance in the thickness direction and a method for manufacturing the same are provided. [Modes for carrying out the invention]

[0019] The thick steel plate of the present invention has a thickness of more than 130 mm, and its chemical composition, in mass%, is as follows: C: 0.02~0.13%, Si: 0.02~0.50%, Mn: 0.30~0.70%, P: 0.030% or less, S: 0.006% or less, Al: 0.060% or less, Cr: 8.0~14.0%, Mo: 0.03~2.20%, Nb: 0.01~0.10%, N: 0.003~0.080%, with the remainder being Fe and unavoidable impurities, and its microstructure is bainite, tempered bainite, and tempered martensite. The material has one of the following single-phase structures, or a mixed structure consisting of two or more of bainite, tempered bainite, and tempered martensite, with the major axis of each of the MnS-based inclusions and NbCN-based inclusions being 10 μm or less, the number of porosity with a diameter of 0.1 to 1.0 mm in the entire width at a position of 1 / 2 thickness ± 10 mm, the reduction of area in the thickness direction tensile test being 35% or more on average over three tests, and the lowest reduction of area among the three thickness direction tensile tests being 25% or more.

[0020] Next, the chemical composition of the thick steel plate (heat-resistant steel) of the present invention will be described. Note that the "%" for the content of each element means "mass%".

[0021] C: 0.02~0.13% Carbon (C) enhances hardenability and ensures strength at a low cost. To ensure the creep rupture strength required for heat-resistant steel, it contains 0.02% or more carbon. On the other hand, if the carbon content exceeds 0.13%, the weldability deteriorates significantly, so the carbon content should be 0.13% or less.

[0022] Si: 0.02~0.50% Si should be included at a concentration of 0.02% or more as a deoxidizing agent. However, since including more than 0.50% Si leads to a decrease in creep ductility and toughness, the upper limit of Si content should be 0.50%. Preferably, the Si content is 0.10 to 0.50%.

[0023] Mn: 0.30~0.70% Similar to Si, Mn should be included at a concentration of 0.30% or more as a deoxidizing agent. However, if the Mn content exceeds 0.70%, it will lead to creep embrittlement and a decrease in toughness, and will also form MnS-based inclusions, degrading the tensile performance in the thickness direction of the plate. Therefore, the upper limit for Mn content is set at 0.70%.

[0024] P:0.030% or less If the phosphorus (P) content exceeds 0.030%, along with sulfur (S) and boron (B), it segregates at grain boundaries in the coarse-grained heat-affected zone (HAZ), lowering the melting point and causing liquefaction cracking. To prevent this, the phosphorus content should be 0.030% or less. The lower limit is not particularly limited and may be 0%.

[0025] S: 0.006% or less If the sulfur content exceeds 0.006%, segregation occurs at grain boundaries in the coarse-grained HAZ (heat-affected zone of welding), lowering the melting point and leading to liquefaction cracking. Furthermore, MnS-based inclusions are formed, degrading the tensile performance in the thickness direction of the plate. Therefore, the sulfur content should be 0.006% or less. The lower limit is not particularly limited and may be 0%.

[0026] Al: 0.060% or less Since an Al content exceeding 0.060% leads to a decrease in creep ductility and toughness, the Al content should be limited to an upper limit of 0.060%. The Al content should be 0% or more, preferably more than 0%, preferably 0.005% or more, and more preferably 0.010% or more.

[0027] Cr: 8.0~14.0% Cr is an essential element in heat-resistant steel for ensuring oxidation resistance and high-temperature corrosion resistance, as well as for stably obtaining a martensitic structure in the matrix. To achieve this effect, the steel should contain 8.0% or more Cr. On the other hand, if the Cr content exceeds 14.0%, the formation of a large amount of Cr carbides reduces the stability of the carbides, leading to a decrease in creep strength and deterioration of toughness. Therefore, the Cr content should be 14.0% or less.

[0028] Mo: 0.03~2.20% Mo is an element that strengthens the matrix through solid solution, contributing to improved creep strength. To achieve this effect, the Mo content should be 0.03% or higher. On the other hand, if the Mo content exceeds 2.20%, it generates coarse intermetallic compounds, leading to an extreme decrease in toughness. Therefore, the Mo content should be 2.20% or less.

[0029] Nb: 0.01~0.10% Nb is an element that forms stable, fine carbonitrides within the grain even at high temperatures, and significantly contributes to improving creep strength. To obtain this effect, Nb should be included in a concentration of at least 0.01%. On the other hand, if the Nb content exceeds 0.10%, it leads to an increase in the growth rate of carbonitrides, causing the dispersion strengthening effect to disappear prematurely and resulting in a decrease in toughness. Therefore, the Nb content should be 0.10% or less.

[0030] N: 0.003~0.080% N forms fine carbonitrides containing V and Nb, making it an effective element for ensuring creep strength. To obtain this effect, it should be included in a concentration of 0.003% or more. On the other hand, including more than 0.080% of nitrogen leads to an increase in the amount of carbonitride precipitate, causing embrittlement. Therefore, the upper limit for the nitrogen content is set at 0.080%.

[0031] The above constitutes the basic components of the present invention, with the remainder being Fe and unavoidable impurities. In the present invention, Cu, Ni, V, Ti, B, Ca, Mg, and W may be further included as needed, within the ranges shown below.

[0032] Cu: 0~0.50% Although Cu is an effective element for stabilizing austenite, when manufactured by normalizing and tempering as in the present invention, it precipitates alone in the steel as ε-Cu (metallic Cu). When heated to 1100°C or higher during hot working, iron is selectively oxidized, and if Cu accumulates at the grain boundaries, localized low-melting-point metal accumulation zones are formed, which can induce grain boundary delamination cracking (red-hot brittleness). Thus, in the present invention, although Cu contributes to austenite stabilization, it has a significant impact on hot workability, so if Cu is included, the Cu content is limited to 0.50% or less.

[0033] Ni: 0~0.40% Ni is an effective element for improving toughness and stabilizing austenite, but it increases dislocation mobility and significantly reduces creep rupture strength. Therefore, in this invention, its content is limited. In this invention, in order to suppress the decrease in creep rupture strength over a long period of time, if Ni is included, the Ni content is limited to 0.40% or less.

[0034] V: 0~0.40% V is an element that forms fine carbonitrides within the grains, significantly contributing to improved creep strength. However, if the V content exceeds 0.40%, it leads to an increase in the carbonitride growth rate, causing its dispersion strengthening effect to disappear prematurely and resulting in a decrease in toughness. Therefore, when V is included, the V content should be 0.40% or less.

[0035] Ti: 0~0.15% In this invention, Ti can be added as needed to precipitate TiN, a nitride, as a dislocation movement obstruction to improve creep strength, both within grains and at grain boundaries. When precipitated at grain boundaries, it contributes to improving grain boundary coverage and exhibits a creep strength improvement effect. When precipitated within grains, it acts directly as resistance to dislocation movement, making it possible to reduce the apparent mobility of dislocations. However, if the Ti content exceeds 0.15%, the high deoxidizing power may lead to the formation of oxide clusters and a decrease in toughness. Therefore, when Ti is included, the upper limit of the Ti content is restricted to 0.15%.

[0036] B: 0~0.006% B segregates at grain boundaries in the heat-affected zone (HAZ), lowering the grain boundary energy, thereby delaying austenite phase nucleation and suppressing grain refinement. However, in coarse-grained HAZ, segregated B promotes a decrease in the melting point at grain boundaries, and in conjunction with segregation of S and P, causes liquefaction cracking. To prevent this, if B is included, the B content should be kept below 0.006%.

[0037] Ca: 0~0.0050% Like magnesium (Mg), calcium (Ca) is an element that improves the hot workability of steel. When it is necessary to improve hot workability, Ca can be included together with Mg or alone. However, if the content exceeds 0.0050%, it can lead to the coarsening of inclusions, conversely impairing workability and toughness. Therefore, when Ca is included, the upper limit of the Ca content should be 0.0050%.

[0038] Mg: 0~0.0050% Like calcium (Ca), magnesium (Mg) is an element that improves the hot workability of steel. When it is necessary to improve hot workability, Mg can be included together with Ca or alone. However, if the Mg content exceeds 0.0050%, it can lead to the coarsening of inclusions, which conversely impairs workability and toughness. Therefore, when Mg is included, the upper limit of the Mg content should be 0.0050%.

[0039] W: 0~2.0% W is an element that strengthens the matrix through solid solution, contributing to improved creep strength. However, if W is included in excess beyond 2.0%, it generates coarse intermetallic compounds, leading to an extreme decrease in toughness. Therefore, when W is included, the upper limit of the W content should be 2.0%.

[0040] Next, the metal structure of the thick steel plate of the present invention, and the inclusions remaining in the central segregation area, etc., will be specified.

[0041] Metallic structure: A single-phase structure consisting of bainite, tempered bainite, or tempered martensite, or a mixed structure consisting of two or more of bainite, tempered bainite, and tempered martensite. In the chemical composition of the present invention, the metallic structure of the thick steel sheet inevitably consists mainly of bainite and martensite. However, in the case of martensite, the desired performance cannot be achieved in tensile tests in the thickness direction unless the structure is tempered. Therefore, the metallic structure of the thick steel sheet of the present invention is a single-phase structure of bainite, tempered bainite, and tempered martensite, or a mixed structure containing two or more types selected from bainite, tempered bainite, and tempered martensite.

[0042] Longest diameter of MnS-type inclusions and NbCN-type inclusions: 10 μm or less. In thick steel plates exceeding 130 mm in thickness manufactured using continuous casting slabs, the causes of low tensile performance in the thickness direction are MnS-based inclusions, NbCN-based inclusions, and porosity as described below. In central segregation areas, etc., if the major axis of these inclusions exceeds 10 μm, the desired tensile performance in the thickness direction cannot be secured. Therefore, the major axis of each of the MnS-based and NbCN-based inclusions should be limited to 10 μm.

[0043] At a position ±10mm from half the plate thickness, the number of porosity particles with a diameter of 0.1 to 1.0mm across the entire width should be 10 or less per 100mm. In thick steel plates exceeding 130 mm in thickness manufactured using continuous casting slabs, one of the causes of low tensile strength in the thickness direction is porosity. The porosity size that affects tensile strength in the thickness direction is minute, with a diameter of 0.1 to 1.0 mm. When the number of porosity particles exceeds 10 per 100 mm, the tensile strength in the thickness direction becomes low, so the upper limit is set at 10 particles per 100 mm. Since the porosity targeted in this invention occurs most frequently at a position of 1 / 2 thickness ± 10 mm, the number of porosity at this position is limited to 10 per 100 mm or less across the entire width (overall width direction).

[0044] Next, the performance of thick steel plates will be specified.

[0045] The reduction of area in the thickness direction tensile test is 35% or more on average across three tests, and the lowest reduction of area among the three thickness direction tensile tests is 25% or more. The reason for specifying a range for the reduction of area as the desired tensile performance in the thickness direction is that the steel sheet of this invention is required to prevent cracking in the direction perpendicular to the thickness direction when a cross joint is manufactured. Therefore, the same value as Z35 in JIS G 3199 (2021), which is applied to lamellar tear steel sheets, was adopted. In other words, in the thick steel plate of the present invention, the reduction of area in the thickness direction tensile test is 35% or more on average over three tests, and the lowest reduction of area among the three thickness direction tensile tests is 25% or more.

[0046] Plate thickness: over 130mm For plate thicknesses of 130mm or less, center porosity is not a problem even without using the special hot forging method applied in this study. Therefore, the plate thickness is limited to over 130mm. While there is no particular upper limit, the thickness of the steel plate of the present invention is preferably 300 mm or less, and more preferably 280 mm or less.

[0047] Furthermore, the width of the thick steel plate of the present invention is not particularly limited, but is preferably 1000 to 5000 mm, and more preferably 1000 to 4400 mm.

[0048] Next, the method for manufacturing thick steel plates according to the present invention will be specified. The thick steel plate of the present invention is manufactured from a continuously cast slab having the chemical composition described above. A continuously cast slab with a thickness of 195 mm or more is heated to 1050 to 1300°C. After heating, both ends of the slab in the width direction are forged and reduced in the width direction to reduce the slab width by 300 mm or more. Hot forging is performed at a temperature of 1000°C or higher, with a forging die, reducing the contact area with the second surface opposite the first surface of the slab by 3 times or more compared to the contact area with the first surface of the slab, with a reduction ratio of 10% or more in the thickness direction of the slab. After cooling, it is reheated and hot-rolled to a plate thickness of more than 130 mm. After cooling, it is normalized at a temperature of 900 to 1100°C, and the total reduction ratio of the reduction ratio in hot forging and the reduction ratio in hot rolling is 30% or more. Regarding cooling after hot forging, the cooling stop temperature can be set to room temperature (-20 to 50°C). Furthermore, regarding cooling after hot rolling, the cooling stop temperature can be set to room temperature (-20 to 50°C). Furthermore, each temperature specified in the manufacturing method of the thick steel plate of the present invention refers to the temperature of the surface of the slab or steel plate.

[0049] Continuous casting slab thickness: 195 mm or more The thicker the continuously cast slab, the greater the reduction during subsequent hot forging and hot rolling, which is advantageous from the viewpoint of crushing inclusions. If the continuously forged slab thickness is less than 195 mm, the desired tensile performance in the thickness direction cannot be obtained even if a thick steel plate is manufactured under the specified hot forging and hot rolling conditions, so the lower limit is set at 195 mm. Preferably it is 210 mm or more, more preferably 215 mm or more. While there is no particular upper limit, the continuous casting slab thickness is preferably 500 mm or less, and more preferably 450 mm or less, in order to ensure internal quality during casting. Furthermore, while the width of the continuous casting slab is not particularly limited, it is preferably 1000 to 2500 mm, and more preferably 1500 to 2500 mm.

[0050] Hot forging Hot forging heating temperature: 1050~1300℃ In this invention, in hot forging, first, the continuous cast slab is heated to 1050 to 1300°C. If the heating temperature in hot forging (hot forging heating temperature) is less than 1050°C, sufficient deformation due to forging cannot be obtained. On the other hand, if the heating temperature in hot forging exceeds 1300°C, the hot ductility decreases and cracks occur during forging. Therefore, the heating temperature in hot forging should be 1050 to 1300°C. Preferably, the heating temperature in hot forging is 1100°C or higher, and more preferably 1150°C or higher. Also, preferably, the heating temperature in hot forging is 1250°C or lower.

[0051] Widthwise reduction (forging reduction) After heating as described above, the ends of the continuously cast slab (hereinafter simply referred to as "slab") are forged and rolled down from the width direction, reducing the slab width by 300 mm or more, thereby thickening both ends of the slab. The reduction in slab width (r) due to widthwise reduction should be 300 mm or more. By reducing the slab width (r) by 300 mm or more and then performing FM forging, the reduction effect of thick plate rolling is also taken into account, making it possible to reduce the three-sided cooling area at both ends of the slab (the width ends of the slab cooled from above, below, and the side), and especially the center porosity near the area equivalent to the slab thickness from the ends and in the center of the slab width.

[0052] FM forging: Slab thickness direction reduction After the widthwise reduction described above, FM forging is performed at a temperature of 1000°C or higher, in which the contact area with the second surface opposite the first surface of the slab is made three times or more than the contact area with the first surface of the slab, and the slab is reduced in the thickness direction by a reduction ratio of 10% or more. Specifically, FM forging involves using a forging die (asymmetrical type) with an upper and lower anvil at a temperature of 1000°C or higher, setting the contact length (contact area) of the upper anvil with the slab to 1 and the contact length (contact area) of the lower anvil with the slab to 3 or more, and reducing the slab thickness in the slab thickness direction by a reduction ratio of 10% or more. If the reduction ratio is less than 10%, the reduction near the center of the slab thickness will be insufficient; therefore, the reduction ratio should be 10% or more. There is no particular upper limit to the reduction ratio, but in order to suppress the occurrence of cracks due to forging reduction, the reduction ratio is preferably 40% or less, and more preferably 30% or less.

[0053] In FM forging (reduction in the slab thickness direction), if the reduction temperature (slab temperature) is below 1000°C, the forging effect cannot be obtained sufficiently; therefore, it should be 1000°C or higher. While there is no particular upper limit, the reduction temperature is preferably 1200°C or lower, and more preferably 1100°C or lower, in order to suppress the occurrence of scale defects. When the contact area between the upper anvil of the forging die and the first surface of the slab (e.g., the top side) is set to 1, if the contact area between the lower anvil and the second surface of the slab (e.g., the bottom side) is less than 3, the effect of FM forging, where the tensile stress at the center of the slab thickness shifts from the center in the direction of the slab thickness, and the hydrostatic pressure at the center increases, cannot be sufficiently obtained. Therefore, the contact area between the second surface opposite the first surface of the slab should be at least 3 times the contact area with the first surface (contact area with the second surface / contact area with the first surface ≥ 3). The ratio of the contact area with the second surface to the contact area with the first surface is preferably 10 times or less, and preferably 8 times or less.

[0054] Hot rolling to a specified plate thickness exceeding 130 mm. After the hot forging described above, the slab is cooled to the cooling stop temperature, then reheated, and hot-rolled until the resulting steel plate thickness exceeds a predetermined value of 130 mm. The cooling stop temperature after hot forging can be room temperature. By continuing this hot rolling process until the plate thickness exceeds a predetermined value of 130 mm, the final thickness of the resulting thick steel plate can be made to exceed 130 mm. The thickness of the resulting steel sheet can be adjusted by the number of passes, reduction ratio, and finish rolling temperature during hot rolling. Furthermore, cooling is performed after hot rolling, and the cooling stop temperature can be set to room temperature.

[0055] The sum of the reduction ratios in hot forging and hot rolling (total reduction ratio): 30% or more Ensuring a sufficient reduction ratio in hot forging and hot rolling is necessary for the crushing of inclusions and the compression of porosity. In order to crush inclusions that occur in the center of the slab thickness and to compress porosity, it is necessary to reduce the center of the slab thickness. The larger the total reduction ratio (the sum of the reduction ratios in hot forging and hot rolling), the greater the crushing effect on inclusions and the compression effect on porosity. To sufficiently crush inclusions and compress porosity, the total reduction ratio needs to be 30% or more. Therefore, the lower limit of the total reduction ratio is set at 30%.

[0056] Tempura temperature: 900~1100℃ After hot rolling the steel sheet, it is cooled, and then normalized at a temperature of 900-1100°C. If the normalizing temperature is set higher than 1100°C, the prior γ grain size increases, and toughness deteriorates. On the other hand, if the normalizing temperature is set lower than 900°C, the decomposition and solid solution of various elements are insufficient, and the prior γ grain size decreases, resulting in reduced hardenability and insufficient strength. Therefore, in this invention, the normalizing temperature is limited to 900 to 1100°C.

[0057] Tempering temperature: 700~850℃ After normalizing the steel plate, it is cooled and then tempered at a temperature of 700-850°C. The cooling stop temperature after normalizing can be set to room temperature (-20-50°C). If the tempering temperature is lower than 700°C, the creep properties may deteriorate. On the other hand, if the tempering temperature is higher than 850°C, the precipitates on the grain boundaries may coarseen, reducing the strengthening of precipitates in a creep environment and potentially degrading the creep properties. Therefore, in this invention, the tempering temperature is set to 700-850°C.

[0058] The thick steel plate of the present invention has been described above. According to the present invention, a thick steel plate with excellent tensile performance in the thickness direction, which was conventionally manufactured using block slabs as a material, can be manufactured from a continuously cast slab using a forging press, making it extremely useful industrially. [Examples]

[0059] Next, embodiments of the present invention will be described. The conditions in the embodiments are just one example of conditions adopted to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to this one example of conditions. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention.

[0060] Steel with the chemical composition shown in Table 1 was melted and slabs were manufactured by continuous casting. Hot forging, hot rolling, and heat treatment (normalizing and tempering) were then performed under the conditions shown in Table 2. The thickness of the resulting thick steel plates is the thickness after hot rolling (rolled finish thickness) shown in Table 2. The width of the slabs used was 2100 mm. In hot forging, the ends of the slab in the width direction were forged and reduced from the width direction after heating, thereby thickening the ends of the slab by 300 mm or more. A forging die (asymmetrical type) having an upper and lower anvil was used, with the contact length (contact area) of the upper anvil with the slab set to 1 and the contact length (contact area) of the lower anvil with the slab set to 4, and the slab was reduced in the thickness direction at the reduction temperature and reduction ratio shown in Table 2. The room temperature used for cooling to room temperature during hot rolling, normalizing, and tempering was 25°C.

[0061] [Table 1]

[0062] The tensile performance in the thickness direction was measured using a method compliant with JIS G 3199 (2021), with three samples being tested. The average and minimum values ​​of the reduction in area were evaluated. The microstructure of the thick steel plate was determined by taking a sample with the cross-section parallel to the rolling direction as the observation surface, mirror polishing it, etching it with 5 vol% nital, and observing it at a position 1 / 4 of the way through the plate thickness using an electron microscope (magnification: 2000x). Whether or not tempering had been performed was determined by checking for the formation of carbides in the microstructure observed with the electron microscope; if no carbides were formed, it was determined to be an untempered microstructure. Regions with a lath-like structure where carbides are formed between the laths were identified as bainite, and regions where carbides are formed within the lath were identified as tempered bainite. Regions with a needle-like structure where carbides are not formed were identified as martensite, and regions where carbides are formed were identified as tempered martensite. The longest diameter of the inclusion was determined by observing the fracture surface of a specimen subjected to a tensile test in the thickness direction and measuring the longest diameter. If Mn was detected in elemental analysis, it was classified as a MnS-type inclusion, and if Nb was detected, it was classified as a NbCN-type inclusion. Since all fracture initiations were in the central segregation region, it was determined that both the MnS-type and NbCN-type inclusions with the largest major axis were located in the central segregation region.

[0063] The porosity survey involved observing porosity particles ranging from 0.1 mmφ to 1.0 mmφ at positions ±10 mm of the full width and half the thickness of the steel sheet in a cross section perpendicular to the rolling direction (a plane perpendicular to the rolling direction and parallel to the width direction) 100 mm from the longitudinal end of the steel sheet using an optical microscope (magnification: 200x). The number of particles was measured, and the number per unit length (particles / 100 mm) was determined.

[0064] The test results are shown in Table 2. In the present invention example, all characteristics are satisfied, whereas in the comparative example... It can be seen that the tensile performance in the thickness direction of the plate has deteriorated.

[0065] [Table 2]

Claims

1. The plate thickness is over 130 mm, The chemical composition is expressed in mass percent. C: 0.02-0.13%, Si: 0.02-0.50%, Mn: 0.30-0.70%, P: 0.030% or less, S: 0.006% or less, Al: 0.060% or less, Cr: 8.0-14.0%, Mo: 0.03-2.20%, Nb: 0.01 to 0.10%, N: 0.003 to 0.080%, It contains, moreover, Cu: 0.50% or less, Ni: 0.40% or less, V: 0.40% or less, Ti: 0.15% or less, B: 0.006% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, W: 2.0% or less, It contains one or more selected from the following: The remainder consists of Fe and unavoidable impurities. The metallic structure is either a single-phase structure of bainite, tempered bainite, or tempered martensite, or a mixed structure consisting of two or more of bainite, tempered bainite, and tempered martensite. The major axis of each of the MnS-based inclusions and NbCN-based inclusions is 10 μm or less. At a position ±10 mm of half the plate thickness, the number of porosity with a diameter of 0.1 to 1.0 mm across the entire width is 10 or less per 100 mm. A thick steel plate in which the reduction of area in a thickness-direction tensile test is 35% or more on average over three tests, and the lowest reduction of area among the three thickness-direction tensile tests is 25% or more.

2. A continuous cast slab with a thickness of 195 mm or more having the chemical composition described in claim 1 is heated to 1050 to 1300°C. After heating, the ends of the slab in the width direction are forged and rolled down in the width direction, reducing the slab width by 300 mm or more. Hot forging is performed at a temperature of 1000°C or higher, with the contact area between the second surface facing the first surface of the slab being three times or more than the contact area between the first surface of the slab and the second surface facing the first surface, and the slab is reduced in the thickness direction by a reduction ratio of 10% or more using a forging die. Cooling is performed, and after cooling, it is reheated and hot-rolled to a plate thickness of more than 130 mm. Cooling is performed, and after cooling, the product is baked at a temperature of 900 to 1100°C. The sum of the reduction ratio in the hot forging process and the reduction ratio in the hot rolling process shall be 30% or more. The metallic structure is either a single-phase structure of bainite, tempered bainite, or tempered martensite, or a mixed structure consisting of two or more of bainite, tempered bainite, and tempered martensite. The major axis of each of the MnS-based inclusions and NbCN-based inclusions is 10 μm or less. At a position ±10 mm of half the plate thickness, the number of porosity with a diameter of 0.1 to 1.0 mm across the entire width is 10 or less per 100 mm. The reduction of area in the thickness direction tensile test is 35% or more on average over three tests, and the lowest reduction of area among the three thickness direction tensile tests is 25% or more. A method for manufacturing thick steel plates with a thickness exceeding 130 mm.

3. A method for manufacturing a thick steel plate according to claim 2, wherein after the normalizing process, cooling is performed, and after the cooling, tempering is performed at a temperature of 700 to 850°C.

Images