Steel sheet and method for manufacturing the same

Abstract

To provide a low-cost steel sheet having high strength and high toughness, and a method for manufacturing the same.SOLUTION: A steel plate has a chemical composition, by mass %, C:0.080 to 0.160%, Si:0.08 to 0.25%, Mn:0.90 to 1.50%, P:0.020% or less, S:0.010% or less, Cr:0.50 to 1.50%, Mo:0.25 to 0.45%, V:0.020 to 0.060%, Nb:0.005 to 0.020%, Al:0.010 to 0.050%, N:0.0010 to 0.0070%, B:0.0006 to 0.0015% in mass%, and the balance comprised of Fe and impurities, the steel plate has such a metallographic structure that the total area ratio of tempered bainite and tempered martensite is 95.0% or more, the average crystal grain diameter of prior γ grains is 35.0 μm or less, the steel plate has a tensile strength of 710 to 930 MPa, and the yield stress of 630 MPa or more, the absorption energy of Charpy impact test at -20°C is 100 J or more, and the thickness of the steel plate is more than 60 mm and 150 mm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel sheet and a method for manufacturing the same. [Background technology]

[0002] In recent years, there has been a demand for higher strength steel plates for use in construction machinery, industrial machinery, pressure vessels, etc., and the use of steel plates with a tensile strength of 570 MPa or more is increasing. At the same time, there has also been an increasing demand for thicker steel plates, leading to the production of steel plates with a thickness of 50 mm or more.

[0003] However, in general, the thicker the steel, the more difficult it becomes to achieve both strength and toughness, so methods are used to adjust the alloying elements contained or to devise manufacturing conditions such as hot rolling and cooling.

[0004] For example, Patent Documents 1, 2, 3 and 4 all disclose high-tensile steel plates that are excellent in toughness and have a tensile strength of 570 MPa or more. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 06-025739 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-229441 [Patent Document 3] Japanese Patent Application Publication No. 04-002715 [Patent Document 4] Japanese Patent Application Laid-Open No. 2012-92377 Summary of the Invention [Problem to be solved by the invention]

[0006] The steel plate described in Patent Document 1 generally contains a positive amount of Ni, which is advantageous for improving strength and toughness. However, since Ni is expensive, there is still room for improvement in the invention described in Patent Document 1 from the viewpoint of manufacturing costs.

[0007] The steel plate described in Patent Document 2 is manufactured by hot rolling at a controlled low rolling temperature, followed by direct quenching and then tempering in order to achieve both high strength and high toughness. However, with this manufacturing method, it is difficult for the controlled rolling effect to be fully extended to the interior of the steel plate, far from the surface, in an extremely thick steel plate having a thickness of more than 60 mm, and it is therefore not possible to achieve both strength and toughness at the same time.

[0008] The steel plate described in Patent Document 3 is produced by hot rolling, cooling once, and then reheating and quenching in order to achieve both high strength and high toughness. However, in order to stably produce steel plates with a thickness of over 60 mm that achieve both high strength and high toughness, there remains room for improvement in the adjustment of rolling conditions, the selection of alloying elements, and the content of alloying elements.

[0009] The steel plate described in Patent Document 4 is produced by reheating, quenching, and tempering to achieve both high strength and high toughness. However, in order to produce a steel plate having a thickness of more than 60 mm that achieves both high strength and high toughness, there is still room for improvement in controlling the reduction ratio in hot rolling.

[0010] The present invention aims to solve the above problems and to provide a low-cost steel plate having high strength and high toughness, and a method for manufacturing the same. [Means for solving the problem]

[0011] The present invention has been made to solve the above-mentioned problems, and is summarized as follows: a steel sheet and a method for manufacturing the same.

[0012] (1) The chemical composition of the steel sheet is, in mass%, C: 0.080~0.160%, Si: 0.08 to 0.25%, Mn: 0.90 to 1.50% P: 0.020% or less, S: 0.010% or less, Cr: 0.50~1.50%, Mo: 0.25 to 0.45% V: 0.020~0.060%, Nb: 0.005 to 0.020%, Al: 0.010 to 0.050%, N: 0.0010~0.0070%, B: 0.0006~0.0015%, The balance is Fe and impurities. The metal structure has a total area ratio of tempered bainite and tempered martensite of 95.0% or more, The average grain size of prior austenite grains is 35.0 μm or less, The tensile strength is 710 to 930 MPa, The yield stress is 630 MPa or more, The absorbed energy of the Charpy impact test at -20°C is 100J or more, The thickness of the steel plate is more than 60 mm and 150 mm or less, steel plate.

[0013] (2) The chemical composition of the steel sheet is, in mass%, replacing a part of the Fe, Ti: 0.025% or less, Cu: 0.50% or less, Ni: 0.30% or less, W: 0.20% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and REM: 0.0050% or less, It contains one or more selected from The steel sheet according to (1) above.

[0014] (3) A method for producing a steel sheet according to (1) above, A soaking process in which the billet is heated to a temperature range of 1050 to 1200°C and soaked in water. a hot rolling step in which the steel slab after the soaking step is hot rolled in a temperature range of 730 to 920°C at a rolling reduction of 30% or more to form a steel plate; a cooling step of cooling the steel plate so that the surface temperature thereof is 300°C or less; a quenching process in which the steel plate after the cooling process is heated to a temperature range of 860 to 950 ° C. and soaked, and then subjected to accelerated cooling so that the reheating temperature is 300 ° C. or less at the surface temperature of the steel plate; A tempering process in which the steel plate after the quenching process is tempered in a temperature range of 500 to 680 ° C., The chemical composition of the steel slab is, in mass%, C: 0.080~0.160%, Si: 0.08 to 0.25%, Mn: 0.90 to 1.50% P: 0.020% or less, S: 0.010% or less, Cr: 0.50~1.50%, Mo: 0.25 to 0.45% V: 0.020~0.060%, Nb: 0.005 to 0.020%, Al: 0.010 to 0.050%, N: 0.0010~0.0070%, B: 0.0006~0.0015%, The balance is Fe and impurities. Steel plate manufacturing method.

[0015] (4) A method for producing a steel sheet according to (2) above, The chemical composition of the steel slab comprises, in mass %, replacing a part of the Fe, Ti: 0.025% or less, Cu: 0.50% or less, Ni: 0.30% or less, W: 0.20% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and REM: 0.0050% or less, It contains one or more selected from The method for manufacturing a steel sheet according to (3) above. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a steel plate having high strength and high toughness at low cost, and a method for manufacturing the same. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a diagram for explaining the positions at which test specimens used in a Charpy impact test are taken. DETAILED DESCRIPTION OF THE INVENTION

[0018] In order to solve the above-mentioned problems, the present inventors have conducted detailed investigations into the strength and toughness of thick steel plates, and have come to the following findings.

[0019] We have come to the realization that in order to achieve both strength and toughness at low cost in thick steel plates over 60 mm thick, it is important to adjust the type and content of alloying elements to be contained within an appropriate range, and then to manage the thermal history in the hot rolling, cooling, quenching, and tempering processes.

[0020] Specifically, the average grain size of prior austenite grains (hereinafter referred to as "prior γ grains") in the metallographic structure of the steel sheet is controlled to 35.0 μm or less. As a result, the toughness of the steel sheet can be improved. To reduce the prior γ grain size of the steel sheet, Nb, which contributes to grain refinement, is added to the steel. Furthermore, lowering the heating temperature during the heating stage before quenching, where austenite grains (hereinafter referred to as "γ grains") tend to coarsen, is effective. However, it has been found that in addition to the heating temperature before quenching, it is also important to perform a hot rolling process prior to quenching, performing a reduction of 30% or more at 920°C or less, setting the finish rolling temperature to 730°C or higher, and completing the hot rolling at the lowest possible temperature. By performing such rolling, large strain is imparted to the γ grains, which increases the number of nucleation sites for austenite transformation during heating before quenching. As a result, the prior γ grain size can be reduced.

[0021] Furthermore, if the metal structure after quenching is primarily ferrite and / or pearlite, the grain refinement effect achieved by the strain introduced by hot rolling cannot be fully utilized. To prevent this, the metal structure after quenching must be primarily bainite and / or martensite. Specifically, it is effective to add Mo, Cr, etc. to the steel to improve its hardenability, and then perform accelerated cooling during the quenching process so that the reheating temperature is 300°C or less.

[0022] After the quenching process, a tempering process is carried out to adjust the hardness of the bainite and martensite, thereby further increasing the toughness of the steel.

[0023] By controlling the metal structure of the steel plate in this way, it is possible to achieve both strength and toughness without adding Ni, which is generally known to contribute to improving strength and toughness, to the steel.

[0024] The present invention was made based on the above findings. Each of the features of the present invention will be described in detail below.

[0025] (A) Chemical composition The reasons for limiting the content of each element are as follows: In the following description, "%" in the content means "% by mass."

[0026] (C: 0.080 to 0.160%) C is an element effective in improving the strength of steel. Therefore, the C content is set to 0.080% or more. Preferably, the C content is set to 0.100% or more. On the other hand, if the C content exceeds 0.160%, it will result in a decrease in toughness. Therefore, the C content is set to 0.160% or less. To improve toughness, the C content is preferably set to 0.140% or less.

[0027] (Si: 0.08 to 0.25%) Si is a deoxidizing element and also contributes to improving the strength of steel. Therefore, the Si content is set to 0.08% or more. Preferably, the Si content is set to 0.10% or more. On the other hand, excessive Si content causes a deterioration in toughness. Therefore, the Si content is set to 0.25% or less. To ensure toughness, the Si content is preferably set to 0.22% or less, more preferably 0.20% or less.

[0028] (Mn: 0.90 to 1.50%) Mn improves the hardenability of steel and contributes to improving strength. Therefore, the Mn content is set to 0.90% or more. To increase strength, the Mn content is preferably set to 1.00% or more, more preferably 1.10% or more. On the other hand, if the Mn content exceeds 1.50%, toughness is impaired. Therefore, the Mn content is set to 1.50% or less. Preferably, the Mn content is set to 1.30% or less.

[0029] (P: 0.020% or less, S: 0.010% or less) P and S are impurities that can cause weld cracking and reduced toughness due to solidification segregation, so it is preferable to reduce their contents as much as possible. Therefore, the P content is set to 0.020% or less, and the S content is set to 0.010% or less. The P content is preferably set to 0.010% or less, and more preferably to 0.008% or less. The S content is preferably set to 0.008% or less, and more preferably to 0.006% or less.

[0030] (Cr: 0.50 to 1.50%) Cr improves hardenability and contributes to improving tensile strength. Therefore, the Cr content is set to 0.50% or more. To increase strength, the Cr content is preferably set to 0.80% or more. On the other hand, if the Cr content exceeds 1.50%, toughness decreases. Therefore, the Cr content is set to 1.50% or less. Preferably, the Cr content is set to 1.30% or less, and more preferably, the Cr content is set to 1.10% or less.

[0031] (Mo: 0.25 to 0.45%) Mo is an element that dissolves in steel during quenching, improving hardenability and contributing to improved tensile strength. It also precipitates as carbonitrides during tempering, preventing a decrease in tensile strength. When high toughness is required, the γ grain size must be small, but the plate thickness is very large, exceeding 60 mm, which reduces hardenability and easily reduces strength. To compensate for this decrease in strength, the Mo content is set to 0.25% or more. Preferably, the Mo content is set to 0.30% or more. On the other hand, a Mo content exceeding 0.45% can result in a decrease in toughness. Therefore, the Mo content is set to 0.45% or less. Preferably, the Mo content is set to 0.40% or less.

[0032] (V:0.020~0.060%) V contributes to precipitation strengthening by forming VC or VN. Furthermore, V carbonitrides precipitated within γ grains act as ferrite transformation nuclei and also have the effect of refining the effective ferrite grain size. For this reason, the V content is set to 0.020% or more, and preferably to 0.030% or more. On the other hand, excessive V content may cause coarsening of precipitates, resulting in a loss of toughness. For this reason, the V content is set to 0.060% or less, and preferably to 0.050% or less.

[0033] (Nb: 0.005 to 0.020%) Nb suppresses the recrystallization of γ grains during hot rolling and accumulates processing strain in the steel, thereby contributing to the refinement of ferrite grains in bainite after quenching. It also increases the strain accumulated in bainite, contributing to the refinement of prior γ grains during reheating and quenching. Therefore, the Nb content is set to 0.005% or more. Preferably, the Nb content is set to 0.010% or more. On the other hand, excessive Nb content can significantly reduce toughness. Therefore, the Nb content is set to 0.020% or less. Preferably, the Nb content is set to 0.015% or less.

[0034] (Al: 0.010 to 0.050%) Al is a deoxidizing element. Therefore, the Al content is set to 0.010% or more. Preferably, the Al content is set to 0.020% or more. On the other hand, if Al is included in excess, oxides become coarse and act as starting points for brittle fracture, reducing toughness. Therefore, the Al content is set to 0.050% or less. Preferably, the Al content is set to 0.040% or less.

[0035] (N: 0.0010 to 0.0070%) N is an element that forms precipitates with Mo, Nb, and Ti, and contributes to refining the grain structure and precipitation strengthening. Therefore, the N content is set to 0.0010% or more. Preferably, the N content is set to 0.0020% or more. On the other hand, excessive N content reduces the toughness of the base material, causing surface cracks during casting and poor material properties due to strain aging of the manufactured steel. Therefore, the N content is set to 0.0070% or less. Preferably, the N content is set to 0.0050% or less.

[0036] (B: 0.0006 to 0.0015%) B has the effect of increasing hardenability and improving strength. Therefore, the B content is set to 0.0006% or more, and preferably 0.0008% or more. On the other hand, if B is added in excess, toughness may be impaired. Therefore, the B content is set to 0.0015% or less, and preferably 0.0012% or less.

[0037] The balance of the chemical composition of the steel sheet of the present invention is Fe and impurities, which refer to components that are mixed in from raw materials such as ores and scraps or other factors during industrial production of the steel sheet, and are acceptable within a range that does not adversely affect the steel sheet of the present invention.

[0038] In order to enhance strength and toughness, the chemical composition of the steel of the present invention may further contain one or more elements selected from Ti, Cu, Ni, W, Ca, Mg, and REM within the ranges shown below. Note that these elements are not necessarily essential for the steel material, and therefore the lower limit of their content is 0%. The reasons for limiting each element will be explained below.

[0039] (Ti:0.025% or less) Ti may be added as needed because it forms TiN and has the effect of refining γ grains through a pinning effect. However, excessive Ti content generates coarse TiN, impairing toughness. Therefore, the Ti content is set to 0.025% or less. The Ti content is preferably set to 0.020% or less. To more reliably obtain the above effects, the Ti content is preferably set to 0.005% or more, and more preferably set to 0.010% or more.

[0040] (Cu:0.50% or less) Cu improves hardenability and contributes to improving tensile strength, so it may be added as needed. However, if excessive Cu is added, toughness may decrease. Therefore, the Cu content is set to 0.50% or less. The Cu content is preferably set to 0.30% or less. To more reliably obtain the above effects, the Cu content is preferably set to 0.01% or more, and more preferably set to 0.10% or more.

[0041] (Ni:0.30% or less) Ni is an element that dissolves in steel to improve hardenability and contributes to improving tensile strength, so it may be added as needed. However, if Ni is added in excess, hardenability improves excessively, reducing toughness, and since Ni is expensive, it increases manufacturing costs. Therefore, the Ni content is set to 0.30% or less. The Ni content is preferably set to 0.20% or less. To more reliably obtain the above effects, the Ni content is preferably set to 0.01% or more, and more preferably 0.10% or more.

[0042] (W: 0.20% or less) W is an element that dissolves in steel to improve hardenability and contributes to improving tensile strength, so it may be added as needed. However, excessive W content can lead to a decrease in toughness. Therefore, the W content is set to 0.20% or less. The W content is preferably set to 0.10% or less. To ensure the above effects, the W content is preferably set to 0.01% or more, and more preferably 0.05% or more.

[0043] (Ca:0.0050% or less) Ca is an element effective in controlling the morphology of sulfides, suppressing the formation of coarse MnS and contributing to improving toughness. Therefore, Ca may be added as needed. However, if Ca is added in excess, toughness may decrease. Therefore, the Ca content is set to 0.0050% or less. The Ca content is preferably less than 0.0030%. To more reliably obtain the above effects, the Ca content is preferably set to 0.0005% or more, and more preferably 0.0010% or more.

[0044] (Mg:0.0050% or less) Mg acts as a deoxidizing element, effectively removing oxygen from steel, and also has the effect of refining γ grains by forming sulfides. Therefore, Mg may be added as needed. However, excessive Mg content may reduce toughness. Therefore, the Mg content is set to 0.0050% or less. The Mg content is preferably set to 0.0040% or less. To ensure the above effects, the Mg content is preferably set to 0.0005% or more, and more preferably 0.0010% or more.

[0045] (REM:0.0050% or less) REM generally contributes to improving toughness by purifying the steel billet by removing dissolved oxygen and sulfur from the steel. Therefore, REM may be added as needed. However, excessive REM content not only reduces toughness but also leads to excessive cost increases. Therefore, the REM content is set to 0.0050% or less. The REM content is preferably set to 0.0030% or less. To more reliably obtain the above effects, the REM content is preferably set to 0.0005% or more, and more preferably to 0.0010% or more.

[0046] Here, REM is a general term for 17 elements in total: Sc, Y, and lanthanides, and the REM content refers to the total amount of the above elements. Note that lanthanides are industrially added in the form of misch metals.

[0047] (B) Metal structure The metal structure of the steel sheet according to the present invention has a total area ratio of tempered bainite and tempered martensite of 95.0% or more, and an average crystal grain size of prior γ grains of 35.0 μm or less. The reasons for each limitation will be explained.

[0048] Total area ratio of tempered bainite and tempered martensite: 95.0% or more If the area percentage of tempered bainite and tempered martensite is less than 95.0%, strength cannot be ensured. Therefore, the total area percentage of tempered bainite and tempered martensite is set to 95.0% or more. The total area percentage of tempered bainite and tempered martensite is preferably 99.0% or more, and more preferably 100%. Note that the area percentage of tempered bainite may be 100%, and the area percentage of tempered martensite may be 100%.

[0049] The remainder other than tempered bainite and tempered martensite is ferrite, pearlite, and / or tempered MA (martensite-austenite mixture). If the total area ratio of these remainders exceeds 5.0%, the strength decreases. Therefore, the total area ratio of the remainders is set to 5.0% or less. The smaller the total area ratio of the remainders, the better, and there is no particular lower limit. For example, the total area ratio of the remainders may be 0%.

[0050] The area fraction of the metallographic structure is measured as follows. A test specimen is taken so that the observation surface is the surface that includes the 1 / 4t depth position from the surface of the steel plate and that includes the thickness and rolling directions of the steel plate, where t is the thickness of the steel plate. The observation surface is mechanically polished using diamond powder with a particle size of 1.0 μm and then etched with a nital etchant. Then, a microstructure photograph is taken using an optical microscope at 200x magnification. With the width of the plate as W, 1,000 measurement points are arranged in a grid pattern every 20 μm within an area of ​​780 μm in the rolling direction and 480 μm in the width direction, centered at the 1 / 4W position. Each measurement point is then identified as tempered bainite, tempered martensite, ferrite, pearlite, or tempered MA, and the number of measurement points is counted for each metallographic structure. The percentage of the 1,000 measurement points counted for each metallographic structure is calculated to determine the total area fraction of tempered bainite and tempered martensite. In the present invention, in order to evaluate the average strength in the plate thickness direction, the area ratio of the metal structure and the strength at the 1 / 4t depth position were measured.

[0051] Average grain size of prior γ grains: 35.0 μm or less If the average grain size of the prior γ grains exceeds 35.0 μm, toughness at −20°C cannot be reliably ensured. Therefore, the average grain size of the prior γ grains is set to 35.0 μm or less. The average grain size of the prior γ grains is preferably 30.0 μm or less. There is no particular lower limit for the average grain size of the prior γ grains. However, in the manufacturing method described below, the substantial lower limit for the average grain size of the prior γ grains is 1.0 μm. Furthermore, the substantial lower limit for the average grain size of the prior γ grains is 1.0 μm, which is observable with an optical microscope.

[0052] The average grain size of prior γ grains is measured by the following method. 2) is cut out and the number of prior γ grains contained therein is counted. The area of ​​the portion is then divided by the number of prior γ grains to calculate the average area per prior γ grain. This value is then converted into the diameter of a circle having the same area as this value, and this is taken as the average grain size of the prior γ grains. Prior γ grains located at the edge of the structural photograph are counted as 0.5 grains, and prior γ grains located at the four corners of the field of view are counted as 0.25 grains. In addition, in the present invention, in order to evaluate the average toughness in the plate thickness direction, the average grain size and toughness of prior γ grains at the 1 / 4t depth position are measured.

[0053] (C) Mechanical properties Tensile strength: 710~930MPa The tensile strength of the steel plate according to the present invention is set to 710 MPa or more, but if the strength is too high, toughness may be impaired, so the tensile strength is set to 930 MPa or less.

[0054] Yield stress: 630 MPa or more The yield stress of the steel plate according to the present invention is set to 630 MPa or more. Although there is no particular upper limit for the yield stress, the substantial upper limit for the steel plate according to the present invention is 820 MPa.

[0055] Tensile strength and yield stress are measured using the following method. A No. 4 test piece specified in JIS Z 2241:2022, i.e., a round bar test piece with a diameter of 14 mm, is taken from the position 1 / 4t of the plate thickness and 1 / 4W of the plate width in the plate width direction, and a tensile test is carried out at room temperature in accordance with JIS Z 2241:2022. If a yield phenomenon appears in the stress-strain curve, the yield stress is obtained, and if no yield phenomenon appears, the 0.2% proof stress is obtained.

[0056] Absorbed energy in Charpy impact test at -20°C: 100J or more The absorbed energy of the steel plate according to the present invention in a Charpy impact test at −20° C. is set to be 100 J or more. The higher the absorbed energy in the Charpy impact test, the better, and although there is no particular upper limit, the substantial upper limit for the steel plate according to the present invention is 310 J.

[0057] The absorbed energy in a Charpy impact test is measured using the following method. Figure 1 illustrates the location of specimens used in the Charpy impact test. (a) is a plan view of a steel plate, (b) is a right side view of the steel plate, and (c) is a front view of the steel plate. As shown in Figure 1, a V-notch Charpy test specimen conforming to JIS Z 2242:2023, measuring 10 mm in thickness, 10 mm in width, 55 mm in length, and 8 mm below the notch, is prepared, centered at the 1 / 4t position of the plate thickness and the 1 / 4W position of the plate width, so that the rolling direction of the steel plate coincides with the longitudinal direction of the test specimen. The test specimen is prepared so that a V-notch is formed parallel to the thickness direction on one side of the test specimen in the plate width direction. The Charpy impact test is then performed three times at -20°C in accordance with JIS Z 2242:2023, and the average value is used as the absorbed energy of the Charpy impact test.

[0058] (D) Dimensions It is difficult to achieve both strength and toughness with thick steel plates. On the other hand, thick steel plates are used in various structures such as construction and industrial machinery and pressure vessels. Therefore, the thickness of the steel plate according to the present invention is set to be more than 60 mm and 150 mm or less.

[0059] (E) Manufacturing method A method for manufacturing a steel plate according to the present invention will be described. The steel plate according to the present invention is manufactured by the following manufacturing method. In the method for manufacturing a steel plate according to the present invention, a steel billet is subjected to a soaking step, a hot rolling step, a cooling step, a quenching step, and a tempering step. Each step will be described. The temperatures shown in the following steps are the surface temperatures of the steel billet or steel plate, except for the average cooling rate in the quenching step.

[0060] <Soaking process> Steel plates are produced by processing steel billets, but the manufacturing method is not limited. Molten steel having the above-mentioned chemical composition may be cast to obtain steel billets. In this case, the thickness of the steel billets is preferably 240 mm or more from the viewpoint of productivity, and is preferably 350 mm or less in consideration of reducing segregation and ensuring uniformity of the soaking temperature before hot rolling.

[0061] In the soaking step, the slab is heated to a temperature range of 1050 to 1200°C and soaked. If the soaking temperature is less than 1050°C, deformation resistance during finish rolling increases, so the soaking temperature is set to 1050°C or higher. In order to sufficiently dissolve elements that form carbides and / or nitrides, such as Nb, the soaking temperature is preferably 1080°C or higher. On the other hand, if the soaking temperature exceeds 1200°C, it is necessary to wait for the slab to cool so that it can be properly rolled down in the hot rolling step described below, which significantly reduces productivity. Therefore, the soaking temperature for the slab is set to 1200°C or lower. The soaking temperature for the slab is preferably 1180°C or lower.

[0062] <Hot rolling process> In the hot rolling process, the steel slab after the soaking process is hot rolled at a reduction rate of 30% or more in a temperature range of 730 to 920°C to produce a steel plate. Here, the reduction rate is defined as the reduction rate [%] = (t1 - t2) / t1 × 100, where t1 is the plate thickness at the final stage when the rolling temperature is above 920°C, and t2 is the plate thickness after the final rolling when the rolling temperature is 730°C or higher.

[0063] The hot rolling process includes a rough rolling process and a finish rolling process. In the rough rolling process, a rough rolling mill is used to perform rough rolling on the steel slab. The rough rolling process is auxiliary rolling for the finish rolling process, and a finish rolling mill is used after the rough rolling process to obtain a thick steel plate having a final plate thickness. In the hot rolling process, rolling is performed in the rough rolling mill and / or the finish rolling mill at a temperature range of 730 to 920°C with a reduction of 30% or more.

[0064] Even if a large reduction is performed at temperatures above 920°C, sufficient strain cannot be imparted to the gamma grains, which coarsen during reheating and quenching, resulting in insufficient toughness in the steel plate. If rolling is performed at temperatures below 730°C, the deformation resistance of the billet increases, placing a strain on the rolling mill. For this reason, finish rolling is completed at 730°C or above.

[0065] If the reduction rate in the temperature range of 730 to 920°C is less than 30%, sufficient strain cannot be imparted to the gamma grains, which may cause the gamma grains to coarsen during reheating and quenching, resulting in insufficient toughness of the steel plate. There is no particular upper limit to the reduction rate, but considering the final thickness of the steel plate, the maximum reduction rate is 65%.

[0066] <Cooling process> In the cooling step, the steel sheet is cooled so that the surface temperature becomes 300°C or less. There are no particular restrictions on the cooling method. The steel sheet may be left to cool as is, or may be water-cooled if productivity is to be improved.

[0067] If the steel sheet is quenched without being cooled to a surface temperature of 300°C or below, the steel sheet will be reheated before transformation is complete at the center of the sheet thickness, resulting in coarsening of prior γ grains in the steel sheet after tempering. For this reason, the steel sheet is cooled to 300°C or below, preferably to room temperature.

[0068] When cooling by natural cooling, heat recovery does not occur. On the other hand, when cooling by water cooling, heat recovery may occur after water cooling is stopped. If the surface temperature rises due to heat recovery, cooling is performed so that the heat recovery temperature is 300°C or less on the surface temperature of the steel plate.

[0069] <Quenching process> In the quenching process, the steel plate after the cooling process is heated to a temperature range of 860 to 950°C and soaked, and then accelerated cooling is performed so that the reheating temperature is 300°C or less at the surface temperature of the steel plate.

[0070] If the heating temperature before quenching is less than 860°C, the amount of alloying elements dissolved in the solid solution will be small, resulting in poor hardenability during accelerated cooling and insufficient strength. The heating temperature before quenching is preferably 870°C or higher. Furthermore, if the heating temperature before quenching is more than 950°C, the prior γ grain size will become coarse and toughness will decrease. The heating temperature before quenching is preferably 930°C or lower.

[0071] Accelerated cooling is a cooling method that adjusts the steel structure and improves the mechanical strength of steel plates by cooling at a rate not achievable by natural cooling. Industrially, this method is usually performed by spraying water onto the top and / or bottom surfaces of the steel plate using a spray nozzle, but it is not necessarily limited to water cooling. Specifically, it refers to cooling at an average cooling rate of 2.0°C / s or greater from the pre-quenching heating temperature to 300°C. This type of cooling increases the total area fraction of bainite and martensite, thereby consistently improving the strength of the steel plate. The average cooling rate is the average cooling rate at the 1 / 4th thickness position and is estimated by simulation based on the surface temperature of the steel plate and taking into account the plate thickness.

[0072] Accelerated cooling is performed so that the reheating temperature is 300°C or less at the surface of the steel plate. If the reheating temperature exceeds 300°C, the hardenability will be insufficient, resulting in insufficient strength.

[0073] <Tempering process> In the tempering process, the steel plate after the quenching process is tempered in the temperature range of 500 to 680°C. If the tempering temperature is less than 500°C, the tempering effect is insufficient and toughness is insufficient. On the other hand, if the tempering temperature exceeds 680°C, strength decreases. For this reason, the tempering temperature is set to 500 to 680°C. After the steel plate has been soaked, it may be air-cooled as is, but water cooling is also acceptable.

[0074] By going through the above steps, the metal structure described above can be obtained.

[0075] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples. [Example]

[0076] Steel was melted in a converter and subjected to primary deoxidation, after which alloying elements were added to adjust the composition as shown in Table 1, followed by vacuum degassing to produce steel billets with thicknesses of 240 to 300 mm. The resulting steel billets were then heated in a heating furnace and subjected to rough rolling, finish rolling, reheating, quenching, and tempering under the conditions shown in Table 2 to produce steel plates.

[0077] [Table 1]

[0078] [Table 2]

[0079] The metal structure of the produced steel sheets was observed by the following methods, and a tensile test and a Charpy impact test were carried out.

[0080] <Area ratio of metal structure> The area fraction of the metallographic structure was measured as follows. Test specimens were taken so that the observation surface was a cross section parallel to the steel plate surface, including the 1 / 4th depth position from the surface. The observation surface was mechanically polished using diamond powder with a particle size of 1.0 μm and then etched with a nital etchant. Microstructure photographs were then taken using an optical microscope at 200x magnification. One thousand measurement points were arranged in a grid pattern every 20 μm within an area of ​​780 μm in the rolling direction and 480 μm in the width direction, centered at the 1 / 4W position of the plate width. Each measurement point was then identified as tempered bainite, tempered martensite, ferrite, or pearlite, and the number of measurement points for each metallographic structure was counted. The percentage of the 1,000 measurement points counted for each metallographic structure was calculated to determine the total area fraction of tempered bainite and tempered martensite.

[0081] <Average grain size of prior γ grains> The average grain size of prior γ grains was measured by the following method. 2 ) was cut out and the number of prior γ grains contained therein was counted. The area of ​​the section was then divided by the number of prior γ grains to calculate the average area per prior γ grain. This value was converted into the diameter of a circle with the same area as this value, and this was taken as the average crystal grain size of the prior γ grains. Prior γ grains located at the edge of the structure photograph were counted as 0.5 grains, and prior γ grains located at the four corners of the field of view were counted as 0.25 grains.

[0082] <Tensile strength and yield stress> Tensile strength and yield stress were measured using the following method. A No. 4 test piece, i.e., a round bar test piece with a diameter of 14 mm, as specified in JIS Z 2241:2022 was taken from the position 1 / 4t of the plate thickness and 1 / 4W of the plate width in the plate width direction, and a tensile test was performed at room temperature in accordance with JIS Z 2241:2022. If a yield phenomenon appeared in the stress-strain curve, the yield stress was determined, and if no yield phenomenon appeared, the 0.2% proof stress was determined.

[0083] <Absorbed energy in Charpy impact test> The absorbed energy of the Charpy impact test was measured as follows. As shown in FIG. 1, a V-notch Charpy test specimen was prepared in accordance with JIS Z 2242:2023, measuring 10 mm in thickness, 10 mm in width, 55 mm in length, and 8 mm below the notch, with the center at the 1 / 4t position of the plate thickness and the 1 / 4W position of the plate width, so that the rolling direction of the steel plate coincided with the longitudinal direction of the test specimen. The test specimen was prepared so that a V-notch was formed parallel to the thickness direction on one side of the plate width direction. The Charpy impact test was then performed three times at -20°C in accordance with JIS Z 2242:2023, and the average value was used as the absorbed energy of the Charpy impact test.

[0084] Table 3 shows the total area ratio of tempered martensite and tempered bainite, the average grain size of prior γ grains, the yield stress (YS), the tensile strength (TS), and the absorbed energy (vE -20 ) is shown.

[0085] [Table 3]

[0086] As shown in Table 3, the steel plates manufactured by Production Nos. 1, 2, 4, 5, 7, 8, 9, 12 to 17, and 20 had a YS of 630 MPa or more, which was the target lower limit, and a TS of 710 MPa or more and 930 MPa or less. Furthermore, the Charpy absorbed energy at -20°C was 100 J or more, meeting all targets.

[0087] On the other hand, the cooling temperature after reheating was high for the steel plate of Production No. 3. Specifically, the surface temperature of the steel plate rose due to reheating, which prevented sufficient hardening and resulted in insufficient tempered bainite and martensite, resulting in low strength (TS, YS). The cooling temperature after rolling for the steel plate of Production No. 6 was high, which resulted in the average crystal grain size of the prior γ grains becoming coarse, and therefore insufficient toughness was obtained.

[0088] The steel plate of Production No. 10 had a low reduction rate in the temperature range of 730 to 920°C, and insufficient reduction was performed in the non-recrystallized region, resulting in coarse average crystal grain size of prior γ grains and insufficient toughness. The steel plate of Production No. 11 had a low tempering temperature, which made it impossible to reduce the steel hardness, and therefore insufficient toughness was not obtained. The steel plate of Production No. 18 had a high reheating temperature before quenching, which caused γ grains to grow, and the prior γ grain size after tempering also became large, resulting in insufficient toughness.

[0089] The steel plate of production No. 19 was not reheated to a low temperature before quenching after cooling, and even with accelerated cooling, it was not able to be quenched sufficiently, so it was unable to obtain sufficient tempered bainite and tempered martensite, resulting in low strength (TS, YS).The steel plate of production No. 21 was cooled without accelerated cooling, so it was not able to be quenched, and it was unable to obtain sufficient tempered bainite and tempered martensite, resulting in low strength (TS, YS).

[0090] The steel plate of Production No. 22 contained excessive C and therefore did not have sufficient toughness. The steel plate of Production No. 23 contained insufficient C and therefore did not have sufficient strength (TS, YS). The steel plate of Production No. 24 contained excessive Si and therefore did not have sufficient toughness. The steel plate of Production No. 25 contained insufficient Si and therefore did not have sufficient tensile strength (TS).

[0091] The steel plate of Production No. 26 contained excessive Mn, which prevented it from achieving sufficient toughness. It also had a high tensile strength (TS). The steel plate of Production No. 27 contained insufficient Mn, which prevented it from achieving sufficient tempered bainite and martensite, resulting in low strength (TS, YS). The steel plate of Production No. 28 contained excessive Cr, which prevented it from achieving sufficient toughness. It also had a high tensile strength (TS).

[0092] The steel plate No. 29 had a Cr deficiency, which prevented sufficient tempered bainite and martensite from being obtained, resulting in low strength (TS, YS). The steel plate No. 30 contained an excess of Mo, which prevented sufficient toughness. The steel plate No. 31 had a deficiency of Mo, which prevented sufficient tempered bainite and martensite from being obtained, and the prior γ grain size also became large, resulting in low strength (TS, YS).

[0093] The steel plate of Production No. 32 contained excessive V and therefore did not have sufficient toughness. The steel plate of Production No. 33 contained insufficient V and therefore did not have sufficient strength (TS, YS). The steel plate of Production No. 34 contained excessive Nb and therefore did not have sufficient toughness. The steel plate of Production No. 35 contained insufficient Nb, which caused the γ grains to grow and the prior γ grain size after tempering to also become large, so it did not have sufficient strength (TS, YS) or toughness. The steel plate of Production No. 36 contained excessive Ti and therefore did not have sufficient toughness.

[0094] The steel plate of Production No. 37 contained excessive Al and therefore did not have sufficient toughness. The steel plate of Production No. 38 contained excessive B and therefore did not have sufficient toughness. The steel plate of Production No. 39 contained insufficient B, which prevented the formation of sufficient tempered bainite and tempered martensite, resulting in low strength (TS, YS). [Industrial Applicability]

[0095] The steel sheet of the present invention can be produced without requiring advanced steelmaking technology, and therefore makes a significant contribution to industry by improving the reliability of construction machinery, industrial machinery, pressure vessels, etc., without sacrificing economic efficiency.

Claims

1. The chemical composition of the steel plate is, in mass%, C: 0.080-0.160%, Si: 0.08-0.25%, Mn: 0.90 to 1.50%, P: 0.020% or less, S: 0.010% or less, Cr: 0.50-1.50%, Mo: 0.25-0.45%, V: 0.020-0.060%, Nb: 0.005-0.020%, Al: 0.010-0.050%, N: 0.0010-0.0070%, B: 0.0006 to 0.0015%, The balance is Fe and impurities. The metal structure has a total area ratio of tempered bainite and tempered martensite of 95.0% or more, The average grain size of prior austenite grains is 35.0 μm or less, The tensile strength is 710 to 930 MPa, The yield stress is 630 MPa or more, The absorbed energy in a Charpy impact test at -20°C is 100 J or more, The thickness of the steel plate is more than 60 mm and 150 mm or less, steel plate.

2. The chemical composition of the steel plate is, in mass%, replacing a part of the Fe, Ti: 0.025% or less, Cu: 0.50% or less, Ni: 0.30% or less, W: 0.20% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and REM: 0.0050% or less, It contains one or more selected from The steel sheet according to claim 1.

3. The method for producing a steel sheet according to claim 1, a soaking step in which the steel slab is heated to a temperature range of 1050 to 1200°C and soaked; a hot rolling step in which the steel slab after the soaking step is hot rolled in a temperature range of 730 to 920°C at a rolling reduction of 30% or more to obtain a steel plate; a cooling step of cooling the steel plate so that the surface temperature of the steel plate is 300°C or less; a quenching process in which the steel plate after the cooling process is heated to a temperature range of 860 to 950 ° C. and soaked, and then subjected to accelerated cooling so that the reheating temperature is 300 ° C. or less at the surface temperature of the steel plate; A tempering process in which the steel plate after the quenching process is tempered in a temperature range of 500 to 680 ° C., The chemical composition of the steel slab is, in mass%, C: 0.080-0.160%, Si: 0.08-0.25%, Mn: 0.90 to 1.50%, P: 0.020% or less, S: 0.010% or less, Cr: 0.50-1.50%, Mo: 0.25-0.45%, V: 0.020-0.060%, Nb: 0.005-0.020%, Al: 0.010-0.050%, N: 0.0010-0.0070%, B: 0.0006 to 0.0015%, The balance is Fe and impurities. Steel plate manufacturing method.

4. The method for producing a steel sheet according to claim 2, The chemical composition of the steel slab comprises, in mass %, replacing a portion of the Fe, Ti: 0.025% or less, Cu: 0.50% or less, Ni: 0.30% or less, W: 0.20% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and REM: 0.0050% or less, It contains one or more selected from The method for manufacturing a steel sheet according to claim 3.

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