Thick steel plate and producing method thereof

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 boilers, turbine rooms, etc. used in thermal power generation, and a manufacturing method thereof. [Background technology]

[0002] Generally, thick steel plates are manufactured from continuous cast slabs or from ingot slabs that are made by rolling mold-cast ingots into blooms. Comparing the two, the former is more attractive in terms of manufacturing costs, but in the case of continuous cast slabs, center segregation often occurs, which deteriorates the tensile properties in the thickness direction. When the tensile properties in the thickness direction deteriorate, weld cracks that occur perpendicular to the thickness direction can become a problem in components that require the creation of cross welded joints. In particular, when high Cr thick steel plate is generally produced using a continuous cast slab, the tensile properties through the plate thickness are inferior to those of ordinary low carbon low alloy steel.

[0003] Therefore, in the case of manufacturing high Cr thick steel plate that requires tensile performance in the plate thickness direction, ingot casting slabs have been used in the past, but the manufacturing process requires cutting off the non-steady parts of the riser and precipitated crystal parts and blooming, which increases costs and reduces productivity. Therefore, in recent years, there has been a demand for the establishment of technology for producing thick steel plate using continuous casting slabs that does not result in increases in costs or decreases in productivity.

[0004] Several proposals have been made in the past to improve the internal quality of continuously cast slabs.

[0005] Patent Documents 1 and 2 disclose techniques for eliminating center porosity in a continuously cast slab by applying pressure reduction using rolls or a planar pressure device at the outlet side of a continuous casting machine.

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

[0007] Patent Documents 4 and 5 disclose a technology for producing extra-thick stainless steel plate having a thickness of 80 mm or more, in which, prior to plate rolling, the ends of a continuously cast slab are forged and reduced in the width direction, reducing the width of the original slab by 300 mm or more and thickening it, and then forging and reducing the slab in the thickness direction, thereby eliminating center porosity.

[0008] Furthermore, as a method for improving the through-thickness tensile properties of low-carbon low-alloy steel sheets, Non-Patent Document 1 discloses various general methods such as reducing the amount of S and adding Ca. [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] JP 2014-188579 A [Patent Document 5] JP 2014-189886 A [Non-patent literature]

[0010] [Non-Patent Document 1] Journal of the Japan Institute of Metals, Vol. 18, No. 5, pp. 368-376 Summary of the Invention [Problem to be solved by the invention]

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

[0012] In addition, Non-Patent Document 1 directly shows a method for improving the tensile performance through the plate thickness, but the knowledge is related to low carbon low alloy steel, and the knowledge may not be applicable to the high Cr thick steel plate which is the subject of this study.

[0013] The present invention has been made to solve the above-mentioned problems, and has an object to provide a thick steel plate having a thickness of more than 130 mm and excellent tensile performance in the plate thickness direction, and a manufacturing method thereof. [Means for solving the problem]

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

[0015] First, to identify the cause of the deterioration of the through-thickness tensile performance of high-Cr steel plates manufactured from continuous cast slabs, we performed fracture surface observation of test pieces with low through-thickness tensile performance (reduction of area). As a result, in addition to the presence of coarse MnS-based inclusions, which have been known to exist even in low-carbon low-alloy steels, the presence of coarse NbCN-based inclusions was confirmed. This means that the high Cr thick steel plate targeted in this invention needs to have a high Nb and N content to ensure heat resistance, and this causes coarse NbCN inclusions to crystallize in the central segregation portion of the continuously cast slab at the end of solidification and remain in the final product, thereby degrading the tensile performance in the plate thickness direction. It was therefore found that the knowledge gained so far about low carbon, low alloy steels cannot be directly applied to high Cr thick steel plate. Furthermore, porosity was observed in some products, and it was found that in order to stably ensure tensile performance in the plate thickness direction, it is also necessary to suppress the occurrence of large center porosity.

[0016] From the above findings, it was found that thick steel plates such as high Cr thick steel plates manufactured from continuous casting slabs need to dissolve or crush MnS-based inclusions and NbCN-based inclusions between casting and heat treatment. The inventors have clarified that normal hot rolling is insufficient as a method for dissolving or crushing the inclusions, and that hot forging, which applies a large reduction to the center in the plate thickness direction before hot rolling, is effective. It was also found that center porosity can be bonded by ensuring a sufficient reduction rate in this hot forging and hot rolling.

[0017] The present invention has been made based on the obtained findings and further studies, and the gist and configuration of the present invention are as follows. [1] The plate thickness is more than 130 mm, Chemical composition, by mass%, C: 0.02 to 0.13%, Si: 0.02 to 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: 0.003~0.080%, The balance is Fe and unavoidable impurities. The metal structure is a single-phase structure of any one of bainite, tempered bainite, and tempered martensite, or a mixed structure consisting of two or more types of bainite, tempered bainite, and tempered martensite, The major axis of each of the MnS-based inclusions and the NbCN-based inclusions is 10 μm or less, At a position of 1 / 2 thickness ±10 mm, the number of porosity with a diameter of 0.1 to 1.0 mm is 10 pieces / 100 mm or less across the entire width, A thick steel plate having an average reduction in area in a thickness direction tensile test of 35% or more, and the minimum reduction in area in the three thickness direction tensile tests is 25% or more. [2] The chemical composition further comprises, in mass %, Cu: 0-0.50%, Ni: 0-0.40%, V: 0 to 0.40%, Ti: 0 to 0.15%, B: 0 to 0.006%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, W: 0-2.0%, The steel plate according to [1] above, comprising one or more selected from the following: [3] A continuous casting slab having a thickness of 195 mm or more and having the chemical composition described in [1] or [2] above is heated to 1050 to 1300 ° C., After heating, both ends of the slab are forged 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 more, with a contact area with a second surface opposed to the first surface being three times or more larger than a contact area with the first surface of the slab, and the slab is reduced in a thickness direction by a forging die at a reduction rate of 10% or more; Cooling is performed, and after cooling, reheating is performed and hot rolling is performed to a plate thickness of more than 130 mm. Cooling is performed, and after cooling, normalizing is performed at a temperature of 900 to 1100 ° C. A method for producing a thick steel plate, wherein a total reduction rate of the hot forging and the hot rolling is 30% or more. [4] The method for producing a steel plate according to [3] above, wherein after the normalizing, the steel plate is cooled, and after the cooling, the steel plate is tempered at a temperature of 700 to 850°C. Effect of the Invention

[0018] According to the present invention, there are provided a thick steel plate having excellent through-thickness tensile performance and a manufacturing method thereof. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The steel plate of the present invention has a plate thickness of more than 130 mm and contains, 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: 0.01 to 0.10%, N: 0.003 to 0.080%, with the balance being Fe and unavoidable impurities, and has a metal structure of bainite, tempered bainite and tempered martensite. The steel has a single phase structure of any one of these, 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, the number of porosity with a diameter of 0.1 to 1.0 mmφ over the entire width at a position 1 / 2 thickness ±10 mm is 10 pieces / 100 mm or less, the reduction in area in the thickness direction tensile test is 35% or more on average out of three test pieces, and the minimum reduction in area in the thickness direction tensile test is 25% or more.

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

[0021] C: 0.02 to 0.13% C improves hardenability and ensures strength at low cost. In order to ensure the creep rupture strength required for heat-resistant steel, C is contained at 0.02% or more. On the other hand, if the C content exceeds 0.13%, weldability is significantly deteriorated, so the C content is set to 0.13% or less.

[0022] Silicon: 0.02 to 0.50% Si is contained in an amount of 0.02% or more as a deoxidizer. On the other hand, if the Si content exceeds 0.50%, creep ductility and toughness are deteriorated, so the upper limit of the Si content is set to 0.50%. The Si content is preferably 0.10 to 0.50%.

[0023] Mn: 0.30-0.70% Mn, like Si, is contained as a deoxidizer in an amount of 0.30% or more. However, if the Mn content exceeds 0.70%, creep embrittlement and a decrease in toughness are caused, and MnS-based inclusions are formed, deteriorating the tensile properties through the plate thickness. Therefore, the upper limit of the Mn content is set at 0.70%.

[0024] P:0.030% or less If the P content exceeds 0.030%, it will segregate to the grain boundaries in the coarse-grained HAZ (weld heat affected zone) together with S and B, lowering the melting point and inducing liquation cracking. To prevent this, the P content is set to 0.030% or less. There is no particular lower limit, and it may be set to 0%.

[0025] S: 0.006% or less If the S content exceeds 0.006%, it will segregate at grain boundaries in the coarse-grained HAZ (weld heat affected zone), lowering the melting point and inducing liquation cracking, and furthermore, MnS-based inclusions will be generated, deteriorating the tensile performance through the plate thickness, so the S content is set to 0.006% or less. There is no particular lower limit, and it may be set to 0%.

[0026] Al: 0.060% or less If the Al content exceeds 0.060%, creep ductility and toughness are reduced, so the upper limit of the Al content is set to 0.060%. The Al content is set to 0% or more, preferably exceeds 0%, preferably is 0.005% or more, and more preferably is 0.010% or more.

[0027] Cr: 8.0~14.0% Cr is an essential element for ensuring oxidation resistance and high-temperature corrosion resistance in heat-resistant steel and for stably obtaining a martensite structure in the matrix. To obtain these effects, Cr is contained in an amount of 8.0% or more. On the other hand, if the Cr content exceeds 14.0%, a large amount of Cr carbide is generated, which reduces the stability of the carbide, leading to a decrease in creep strength and a deterioration in toughness. Therefore, the Cr content is set to 14.0% or less.

[0028] Mo: 0.03 to 2.20% Mo is an element that strengthens the matrix through solid solution and contributes to improving creep strength. To obtain this effect, Mo is contained in an amount of 0.03% or more. On the other hand, if Mo is contained in excess of 2.20%, coarse intermetallic compounds are formed, resulting in a significant decrease in toughness, so the Mo content is set to 2.20% or less.

[0029] Nb: 0.01 to 0.10% Nb is an element that forms fine carbonitrides within grains that are stable up to high temperatures and contributes greatly to improving creep strength. To obtain this effect, the Nb content is set to at least 0.01%. On the other hand, if the Nb content exceeds 0.10%, the growth rate of carbonitrides increases, the dispersion strengthening effect disappears early, and toughness decreases, so the Nb content is set to 0.10% or less.

[0030] N: 0.003~0.080% N forms fine carbonitrides containing V and Nb, and is an effective element for ensuring creep strength. To obtain this effect, the content is set to 0.003% or more. On the other hand, if the N content exceeds 0.080%, the amount of carbonitrides precipitated increases, causing embrittlement, so the upper limit of the N content is set at 0.080%.

[0031] The above are the basic components in the present invention, with the balance being Fe and unavoidable impurities. In the present invention, Cu, Ni, V, Ti, B, Ca, Mg, and W may further be contained in the ranges shown below, if necessary.

[0032] Cu: 0-0.50% Cu is an element effective in stabilizing austenite, but when manufacturing by normalizing (normalizing)-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 when Cu gathers at grain boundaries, a local low melting point metal accumulation zone is formed, which may induce grain boundary peeling cracks (red brittleness). Thus, in the present invention, Cu contributes to the stabilization of austenite, but has a large effect on hot workability, so when Cu is contained, the Cu content is limited to 0.50% or less.

[0033] Ni: 0 to 0.40% Ni is an element effective in improving toughness and stabilizing austenite, but since it increases dislocation mobility and significantly reduces creep rupture strength, the present invention restricts its content to 0.40% or less in order to suppress the decrease in long-term creep rupture strength.

[0034] V: 0~0.40% V is an element that forms fine carbonitrides within grains and contributes greatly to improving creep strength. However, if V is contained in an amount exceeding 0.40%, the growth rate of the carbonitrides increases, the dispersion strengthening effect disappears early, and toughness decreases. Therefore, if V is contained, the V content is set to 0.40% or less.

[0035] Ti: 0 to 0.15% In the present invention, Ti can be added as necessary for the purpose of precipitating TiN, a nitride, as a dislocation movement obstacle in grains and grain boundaries to improve creep strength. In the case of grain boundary precipitation, it contributes to improving the grain boundary coverage rate and exerts an effect of improving creep strength, and in the case of precipitation in grains, it acts directly as a dislocation movement resistance and can reduce the apparent dislocation mobility. However, if Ti is contained in an amount exceeding 0.15%, the deoxidizing power is high, and therefore oxide clusters may be formed, which may reduce toughness. Therefore, when Ti is contained, the upper limit of the Ti content is limited to 0.15%.

[0036] B: 0 to 0.006% B segregates to grain boundaries in the HAZ and lowers the grain boundary energy, thereby delaying the nucleation of the austenite phase and suppressing grain refinement. However, in coarse-grained HAZ, the B segregated at the grain boundaries promotes a decrease in the melting point of the grain boundaries, which, combined with the segregation of S and P, causes liquation cracking. To prevent this, if B is included, the B content should be kept to 0.006% or less.

[0037] Ca: 0 to 0.0050% Like Mg, Ca is an element that improves the hot workability of steel, and when it is necessary to improve the hot workability, Ca can be contained together with Mg or alone. However, if the Ca content exceeds 0.0050%, it causes coarsening of inclusions, which in turn impairs workability and toughness. Therefore, when Ca is contained, the upper limit of the Ca content is set to 0.0050%.

[0038] Magnesium: 0 to 0.0050% Like Ca, Mg is an element that improves the hot workability of steel, and when it is necessary to improve the hot workability, Mg can be added alone or together with Ca. However, if the Mg content exceeds 0.0050%, it leads to coarsening of inclusions, which in turn impairs workability and toughness. Therefore, when Mg is added, the upper limit of the Mg content is set to 0.0050%.

[0039] W: 0~2.0% W is an element that strengthens the matrix through solid solution and contributes to improving creep strength. However, if W is contained in excess of 2.0%, coarse intermetallic compounds are formed, resulting in a significant decrease in toughness. Therefore, if W is contained, the upper limit of the W content is set to 2.0%.

[0040] Next, the metal structure of the steel plate of the present invention and the inclusions remaining in the center segregation portion and the like will be specified.

[0041] Metal structure: 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. With the chemical composition of the present invention, the metal structure of the steel plate is inevitably a structure mainly composed of bainite or martensite, but in the case of martensite, the desired performance cannot be achieved in a tensile test in the plate thickness direction unless the structure is tempered. Therefore, the metal structure of the steel plate of the present invention is a single-phase structure of any one 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] The major axis of each of the MnS inclusions and NbCN inclusions: 10 μm or less In thick steel plates with a thickness of over 130 mm manufactured using continuous casting slabs, the causes of low tensile performance through the thickness are MnS-based inclusions and NbCN-based inclusions, and the porosity described below. If the major axis of these inclusions exceeds 10 μm in the central segregation area, etc., the desired tensile performance through the thickness cannot be ensured. Therefore, the upper limit of the major axis of each of the MnS-based inclusions and NbCN-based inclusions is set to 10 μm.

[0043] At a position 1 / 2 thickness ±10 mm from the plate thickness, the number of porosity with a diameter of 0.1 to 1.0 mm is 10 pieces / 100 mm or less across the entire width In thick steel plates over 130 mm thick manufactured using continuous casting slabs, one of the causes of low tensile performance through the plate thickness is porosity. The porosity size that affects the tensile performance through the plate thickness is very small, with a diameter of 0.1 to 1.0 mm, and when the number of porosities exceeds 10 / 100 mm, the tensile performance through the plate thickness becomes low, so the upper limit is set at 10 / 100 mm. The porosity targeted in this invention occurs most frequently at 1 / 2 thickness ±10 mm positions of the plate thickness, so the limit is 10 porosity / 100 mm or less across the entire width (overall width direction) at 1 / 2 thickness ±10 mm positions of the plate thickness.

[0044] Next, the performance of the steel plate is specified.

[0045] The reduction in the thickness direction tensile test is 35% or more on average for three tests, and the minimum reduction in the thickness direction tensile test is 25% or more The reason why the reduction in area is set to a specific range as the desired tensile performance in the thickness direction is that the steel plate of the present invention is required to prevent cracks from occurring in the direction perpendicular to the thickness direction when a cross joint is made. Therefore, the same value as Z35 of JIS G 3199 (2021) applied to lamellar tear steel plates was adopted. That is, in the thick steel plate of the present invention, the average reduction in area in the thickness direction tensile test of three specimens is 35% or more, and the minimum reduction in area in the three thickness direction tensile tests is 25% or more.

[0046] Plate thickness: over 130mm For plate thicknesses of 130 mm or less, center porosity is not an issue even without the special hot forging method applied this time, so plate thickness is limited to over 130 mm. Although 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 producing the steel plate of the present invention will be defined. The thick steel plate of the present invention is manufactured from a continuous cast slab, has the above-mentioned chemical composition, and is heated to 1050 to 1300 ° C., and both ends of the heated slab in the width direction are forged and pressed in the width direction to reduce the slab width by 300 mm or more, and at a temperature of 1000 ° C. or more, the contact area with the second surface facing the first surface is made three times or more larger than the contact area with the first surface of the slab, and hot forging is performed in the slab thickness direction at a reduction rate of 10% or more using a forging die, and then cooled. After cooling, the plate is reheated and hot rolled to a plate thickness of more than 130 mm, cooled, and normalized at a temperature of 900 to 1100 ° C., and the total reduction rate of the hot forging reduction rate and the hot rolling reduction rate is set to 30% or more. Regarding cooling after hot forging, the cooling stop temperature can be room temperature (-20 to 50°C). In addition, with regard to cooling after hot rolling, the cooling stop temperature can be room temperature (-20 to 50°C). Moreover, each temperature specified in the method for producing a steel plate of the present invention refers to the temperature of the surface of the slab or steel plate.

[0049] Continuous casting slab thickness: 195mm or more The thicker the continuous cast slab is, the larger the reduction can be in the subsequent hot forging and hot rolling, which is advantageous from the viewpoint of crushing inclusions. If the continuous 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 to 195 mm. It is preferably 210 mm or more, more preferably 215 mm or more. Although there is no particular upper limit, in order to ensure the internal quality during casting, the thickness of the continuously cast slab is preferably 500 mm or less, and more preferably 450 mm or less. Furthermore, the width of the continuously cast slab is not particularly limited, but is preferably 1000 to 2500 mm, and more preferably 1500 to 2500 mm.

[0050] Hot Forging Hot forging temperature: 1050~1300℃ In the present invention, in the hot forging, first, the above-mentioned continuously 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 by 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 is set to 1050 to 1300°C. The heating temperature in hot forging is preferably 1100°C or higher, more preferably 1150°C or higher. In addition, the heating temperature in hot forging is preferably 1250°C or lower.

[0051] Width direction reduction (forging reduction) After the above heating, both ends in the width direction of the continuous cast slab (hereinafter also simply referred to as slab) are forged and pressed down from the width direction to reduce the slab width by 300 mm or more, thereby thickening both ends of the slab. The reduction in the slab width (r) due to width reduction should be 300 mm or more. If the slab width reduction (r) is 300 mm or more and then FM forging is performed, it is possible to reduce center porosity in the three-sided cooling areas at both width ends (the width ends of the slab that are cooled from above, below, and the sides), especially in the vicinity of the area equivalent to the slab thickness inside from the ends, and in the center of the slab width, with the reduction effect of plate rolling also being taken into account.

[0052] FM forging: Slab thickness reduction After the above-mentioned widthwise reduction, FM forging is performed at a temperature of 1000°C or higher, with the contact area with the second surface opposing the first surface being three times or more larger than the contact area with the first surface of the slab, and the slab is reduced in the thickness direction at a reduction rate of 10% or more. Specifically, for FM forging, a forging die (asymmetric top and bottom) having an upper and lower die is used at a temperature of 1000°C or higher, the contact length (contact area) of the upper die with the slab is set to 1, and the contact length (contact area) of the lower die with the slab is set to 3 or more, and the slab is reduced in the thickness direction at a reduction rate of 10% or more. If the reduction rate is less than 10%, the reduction near the center of the slab thickness will be insufficient, so the reduction rate is set to 10% or more. The upper limit of the reduction rate is not particularly limited, but in order to suppress the occurrence of cracks due to the forging reduction, the reduction rate is preferably 40% or less, and more preferably 30% or less.

[0053] In FM forging (slab thickness direction reduction), if the reduction temperature (slab temperature) is less than 1000°C, the forging effect cannot be fully obtained, so the reduction temperature is set to 1000°C or higher. Although there is no particular upper limit, in order to suppress the occurrence of scale defects, the rolling temperature is preferably 1200° C. or less, and more preferably 1100° C. or less. When the contact area of ​​the upper die, etc. of the forging die with the first surface (e.g., the top surface) of the slab is taken as 1, if the contact area of ​​the lower die, etc. with the second surface (e.g., the bottom surface) of the slab is less than 3, the tensile stress at the center position of the slab thickness due to FM forging will shift from the center position in the slab thickness direction, and the effect of increasing the hydrostatic pressure at the center position will not be fully obtained. Therefore, the contact area with the second surface opposite the first surface is set to be three times or more the contact area with the first surface of the slab (contact area with the second surface / contact area with the first surface ≧ 3). The contact area with the second surface / contact area with the first surface is preferably 10 times or less, and more preferably 8 times or less.

[0054] Hot rolling to a specified thickness of over 130mm After the above hot forging, the slab is cooled to a cooling stop temperature, then reheated, and hot rolled until the thickness of the resulting steel plate reaches a predetermined value exceeding 130 mm. The cooling stop temperature in the cooling after the hot forging can be room temperature. By carrying out this hot rolling until the plate thickness reaches a predetermined value exceeding 130 mm, the plate thickness of the finally obtained 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, and finish rolling temperature in hot rolling. After hot rolling, the steel sheet is cooled, and the cooling stop temperature can be room temperature.

[0055] The total reduction rate of hot forging and hot rolling (total reduction rate): 30% or more Ensuring the reduction ratio in hot forging and hot rolling is necessary to crush inclusions and press porosity. In order to crush inclusions occurring in the center of the slab thickness and press porosity, it is necessary to roll down the center of the slab thickness. The greater the total reduction ratio (total reduction ratio) of the reduction ratios in hot forging and hot rolling, the greater the effect of crushing inclusions and pressing porosity. In order to sufficiently crush inclusions and press porosity, the total reduction ratio must be 30% or more. For this reason, the lower limit of the total reduction ratio is set to 30%.

[0056] Normalizing temperature: 900~1100℃ The steel sheet is hot-rolled, cooled, and then normalized at a temperature of 900 to 1100°C. If the normalizing temperature is higher than 1100°C, the prior γ grain size becomes large and the toughness deteriorates. On the other hand, if the normalizing temperature is lower than 900°C, the decomposition and solid solution of various elements becomes insufficient and the prior γ grain size becomes small, so that the hardenability decreases and the strength is insufficient. Therefore, in the present invention, the normalizing temperature is limited to 900 to 1100°C.

[0057] Tempering temperature: 700~850℃ After normalizing, the steel sheet is cooled and tempered at a temperature of 700 to 850° C. The cooling stop temperature in the cooling after normalizing can be room temperature (−20 to 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 become coarse, which may reduce the precipitation strengthening in a creep environment and deteriorate the creep properties. Therefore, in the present invention, the tempering temperature is set to 700 to 850°C.

[0058] The thick steel plate of the present invention has been described above. According to the present invention, thick steel plates having excellent tensile properties in the thickness direction, which have been conventionally produced from bloomed slabs, can be produced from continuous cast slabs using a forging press, and is extremely useful in industry. EXAMPLES

[0059] Next, an embodiment of the present invention will be described. The conditions in the embodiment are one example of conditions adopted to confirm the feasibility and effects 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 deviate from the gist of the present invention and achieve the object of the present invention.

[0060] Steel having the chemical composition shown in Table 1 was melted and produced into slabs by continuous casting, and hot forging, hot rolling and heat treatment (normalizing and tempering) were carried out under the conditions shown in Table 2. The thickness of the obtained steel plates was the plate thickness after hot rolling (rolling finish thickness) shown in Table 2. The width of the slabs used was 2100 mm. In hot forging, both ends of the heated slab in the width direction were forged and pressed down from the width direction to reduce the slab width by 300 mm or more, thereby thickening both ends of the slab, and a forging die (asymmetric top and bottom) having an upper and lower die was used to press down the slab in the thickness direction, with the contact length (contact area) of the upper die and the slab being 1, and the contact length (contact area) of the lower die and the slab being 4, at the pressing temperature and reduction rate shown in Table 2. The cooling to room temperature during hot rolling, normalizing and tempering was performed at 25°C.

[0061] [Table 1]

[0062] The tensile performance through the plate thickness was measured using a method conforming to JIS G 3199 (2021) on three pieces each, and the average and minimum reduction in area were evaluated. The metal structure of the thick steel plate was determined by taking a sample with a cross section parallel to the rolling direction as the observation surface, mirror polishing it, corroding it with 5 vol% nital, and observing the 1 / 4 position in the plate thickness direction with an electron microscope (magnification: 2000 times).Whether the structure had been tempered or not was determined based on whether carbides were formed in the metal structure observed with the electron microscope, and if no carbides were formed, it was determined to be an untempered structure. The lath-shaped regions where carbides had formed between the laths were judged to be bainite, and the regions where carbides had formed within the laths were judged to be tempered bainite.The needle-shaped regions where carbides had not formed were judged to be martensite, and the regions where carbides had formed were judged to be tempered martensite. The major axis of the inclusions was determined by observing the maximum major axis of the fracture surface of the test piece subjected to the tensile test in the sheet thickness direction. When Mn was detected as a result of elemental analysis, it was determined to be an MnS-based inclusion, and when Nb was detected as a result of elemental analysis, it was determined to be an NbCN-based inclusion. Because the fracture origins were all in the central segregation region, it was determined that the MnS-based inclusions and NbCN-based inclusions with the largest major diameters were both present in the central segregation region.

[0063] The porosity was investigated by observing porosity of 0.1 mmφ to 1.0 mmφ at a position of 1 / 2 the plate thickness ±10 mm across the entire width of the cross section perpendicular to the rolling direction (a surface perpendicular to the rolling direction and parallel to the width direction) 100 mm from the longitudinal end of the steel plate using an optical microscope (magnification: 200x), counting the number of porosity, and measuring the number per unit length (pieces / 100 mm).

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

[0065] [Table 2]

Claims

1. The plate thickness is more than 130 mm, Chemical composition, by mass%, 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%, The balance is Fe and unavoidable impurities, The metal structure is a single-phase structure of any one of bainite, tempered bainite, and tempered martensite, or a mixed structure consisting of any two or more of bainite, tempered bainite, and tempered martensite, The major axis of each of the MnS-based inclusions and the NbCN-based inclusions is 10 μm or less, At a position of 1 / 2 thickness ±10 mm of the plate thickness, the number of porosity with a diameter of 0.1 to 1.0 mmφ is 10 pieces / 100 mm or less over the entire width, A thick steel plate having an average reduction in area in a tensile test in the plate thickness direction of 35% or more, and the minimum reduction in area in the tensile test in the plate thickness direction of the three steel plates being 25% or more.

2. The chemical composition further comprises, in mass %, Cu: 0 to 0.50%, Ni: 0 to 0.40%, V: 0 to 0.40%, Ti: 0 to 0.15%, B: 0 to 0.006%, Ca: 0-0.0050%, Mg: 0 to 0.0050%, W: 0 to 2.0%, The steel plate according to claim 1, comprising one or more selected from the following:

3. A continuous casting slab having a thickness of 195 mm or more and having the chemical composition according to claim 1 or 2 is heated to 1050 to 1300 ° C., After heating, both ends of the slab in the width direction are forged and pressed down in the width direction to reduce the slab width by 300 mm or more. At a temperature of 1000° C. or more, a contact area with a second surface opposed to the first surface of the slab is set to three times or more as compared with a contact area with the first surface of the slab, and hot forging is performed in which the slab is reduced in a thickness direction by a forging die at a reduction rate of 10% or more; Cooling is performed, and after cooling, reheating is performed and hot rolling is performed to a plate thickness of more than 130 mm. Cooling is performed, and after cooling, normalizing is performed at a temperature of 900 to 1100 ° C. The method for producing a thick steel plate, wherein a total reduction rate of the hot forging and the hot rolling is 30% or more.

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

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