Steel plate and method for manufacturing the same

Abstract

The present invention provides a steel plate having high strength and excellent toughness, and a method for manufacturing the same. [Solution] The chemical composition is, in mass%, C: 0.080%~0.180%, Si: 0.08%~0.30%, Mn: 0.56%~1.50%, P: 0.020% or less, S: 0.010% or less, Cr: 0.50~1.50%, Mo: 0.30~1.06%, Al: 0.010~0.080%, N: 0.0010~0.0070%, B: 0.0006~0.0015%, remainder: Fe and fus. A pure steel sheet having a total area ratio of tempered bainite and tempered martensite of 95% or more, an average grain size of prior γ grains of 35.0 μm or less, an average grain size of grains observed by electron backscatter diffraction of 22.0 μm or less, a tensile strength of 760 to 1150 MPa, a yield stress of 665 MPa or more, and an absorbed energy of 100 J or more at -20°C.

Description

[Technical Field]

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

[0002] In recent years, higher strength has been required for steel sheets used in construction machinery, industrial machinery, pressure vessels and the like, and steel sheets having a tensile strength of 570 MPa or more, further 780 MPa, have been increasingly used. In addition, demand for thicker steel sheets is also increasing.

[0003] However, generally, as a steel sheet becomes thicker, it tends to be more difficult to achieve both strength and toughness. Therefore, methods of adjusting the content of alloy elements added or devising production conditions such as hot rolling and cooling are used.

[0004] For example, Patent Document 1, Patent Document 2, and Patent Document 3 all disclose high-tensile-thick steel sheets in which the tensile strength is 570 MPa or more and both strength and toughness are considered. [Prior Art Documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2010-229441 [Patent Document 2] Japanese Unexamined Patent Application Publication No. 04-002715 (Heisei 4) [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2012-92377 [Summary of the Invention] [Problem to be Solved by the Invention]

[0006] In the thick steel plate described in Patent Document 1, in order to achieve both high strength and toughness, the manufacturing process involves hot rolling with a low controlled rolling temperature, direct quenching, and then tempering. However, with this manufacturing method, it is difficult to sufficiently extend the effect of controlled rolling to the interior of the steel plate, away from the surface, when producing extremely thick steel plates with a thickness exceeding 30 mm and a tensile strength of 780 MPa or more, making it insufficient to achieve both strength and toughness.

[0007] Furthermore, the thick steel plate described in Patent Document 2 is manufactured by hot rolling, followed by cooling and then reheating and quenching, in order to achieve both high strength and excellent toughness. However, from the viewpoint of stably achieving both a tensile strength of 780 MPa or more and toughness at -20°C or below in thick steel plates, there is still room for improvement regarding the rolling temperature conditions and the selection and content of alloying elements.

[0008] Furthermore, the thick steel plate described in Patent Document 3 is manufactured by reheating, quenching, and tempering in order to achieve both high strength and excellent toughness. However, from the viewpoint of manufacturing thick steel plates with a thickness exceeding 60 mm, there is still room for improvement in controlling the Mo content and the reduction ratio in hot rolling.

[0009] The present invention aims to solve the above problems and provide a steel plate having high strength and excellent toughness. [Means for solving the problem]

[0010] This invention was made to solve the above-mentioned problems, and its gist is as follows: steel plate and method for manufacturing the same.

[0011] (1) The chemical composition is expressed in mass%, C: 0.080%~0.180%, Si: 0.08%~0.30% Mn: 0.56%~1.50% P: 0.020% or less, S: 0.010% or less, Cr: 0.50~1.50%, Mo: 0.30~1.06%, Al: 0.010~0.080%, N: 0.0010~0.0070%, B: 0.0006~0.0015%, the balance being Fe and impurities, in the metal structure, the total area fraction of tempered bainite and tempered martensite is 95% or more, the average grain size of prior austenite grains is 35.0μm or less, the average grain size of crystal grains observed by electron backscatter diffraction is 22.0μm or less, the tensile strength is 760~1150MPa, the yield stress is 665MPa or more, the absorbed energy in a Charpy impact test at -20°C is 100J or more, a steel plate.

[0012] (2) the chemical composition, instead of part of said Fe, contains, by mass%, V: 0.060% or less, Nb: 0.021% or less, Ti: 0.025% or less, Cu: 0.50% or less, Ni: 0.30% or less, W: 0.20% or less, Sn: 0.20% or less, Sb: 0.05% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and REM: 0.0050% or less, containing one or more selected from the above, the steel plate according to (1) above.

[0013] (3) A method for producing the steel plate according to (1) or (2) above, comprising: a soaking step of heating a steel slab having the chemical composition according to (1) or (2) above to a temperature range of 1050~1250°C and performing soaking, a hot rolling step of subjecting the steel slab after the soaking step to hot rolling at a reduction ratio of 30% or more in a temperature range of 730 to 920°C to obtain a steel sheet; a cooling step of cooling the steel sheet until the surface temperature of the steel sheet becomes 300°C or lower; a quenching step of heating and soaking the steel sheet after the cooling step in a temperature range of 860 to 950°C, and then performing accelerated cooling such that the reheated surface temperature of the steel sheet becomes 300°C or lower; a tempering step of tempering the steel sheet after the quenching step in a temperature range of 450 to 680°C, a method for producing a steel sheet.

Advantageous Effects of Invention

[0014] According to the present invention, a steel sheet having high strength and excellent toughness and a method for producing the same can be provided.

Brief Description of Drawings

[0015] [Figure 1] FIG. 1 is a diagram for explaining the sampling position of a test piece used in a Charpy impact test.

Mode for Carrying Out the Invention

[0016] The present inventors have conducted detailed investigations on the strength and toughness of thick steel sheets in order to solve the above-described problems, and as a result have obtained the following findings.

[0017] To achieve both strength and toughness in thick steel plates, it is crucial to manage the thermal history. We have concluded that it is necessary to appropriately control the microstructure by properly managing the conditions of rolling, reheating and quenching, and subsequent tempering. Specifically, in the microstructure of a steel plate where the total area ratio of tempered bainite and tempered martensite is 95% or more, the average grain size of prior austenite grains (hereinafter also referred to as "prior γ grains") is controlled to 35.0 μm or less. In addition, the average grain size of grains observed by electron backscatter diffraction (EBSD) (hereinafter simply referred to as "EBSD grains") is controlled to 22.0 μm or less. By controlling in this way, excellent toughness can be imparted to thick steel plates.

[0018] Furthermore, martensite and bainite have a hierarchical structure, such as packets and blocks, within the prior γ grains. As will be explained in detail in a later paragraph, EBSD grains are observed as regions surrounded by grain boundaries with a crystal orientation difference of 15° or more, and are considered to be regions surrounded by boundaries such as packets and blocks.

[0019] To stably reduce the size of the original gamma grain, the rolling conditions prior to reheating and quenching are crucial, rather than being determined solely by the reheating and quenching temperature. The ideal method is to reduce the grain size by 30% or more at a rolling temperature of 900°C or lower, set the finish rolling temperature to 730°C or higher, and then complete the rolling process at the lowest possible temperature. This type of rolling imparts significant strain to the gamma grains, increasing the nucleation sites for austenite transformation during reheating, thereby reducing the gamma grain size.

[0020] On the other hand, if the steel structure after reheating and quenching consists mainly of ferrite and pearlite, the strain imparted by rolling prevents the full utilization of the gamma grain refining effect during reheating and quenching. To prevent this, it is necessary to generate bainite and / or martensite in the steel structure after cooling following quenching. Specifically, by incorporating Mo into the steel and using Cr, B, etc. as essential additives to improve quenching, it becomes easier to generate bainite and / or martensite in the steel structure after cooling.

[0021] Then, the steel is reheated, hardened, and tempered to adjust the hardness of bainite and martensite, further enhancing the toughness of the steel. By controlling the metal structure in this way, both strength and toughness can be achieved.

[0022] This invention is based on the above findings. The requirements of this invention will be described in detail below.

[0023] (A) Chemical composition The reasons for the limitations on each element are as follows. Note that in the following explanation, "%" for content refers to "mass%".

[0024] (C: 0.080~0.180%) Since carbon (C) contributes to improved strength, the C content should be 0.080% or higher. Preferably, the C content should be 0.100% or higher. On the other hand, since a C content exceeding 0.180% leads to a decrease in toughness, the C content should be 0.180% or lower. To improve toughness, the C content should preferably be 0.160% or lower.

[0025] (Si: 0.08~0.30%) Since Si is a deoxidizing element and also contributes to improving strength, the Si content should be 0.08% or more. Preferably, the Si content should be 0.10% or more. On the other hand, excessive Si content leads to a deterioration of toughness, so the Si content should be 0.30% or less. To ensure toughness, the Si content should preferably be 0.25% or less, and more preferably 0.20% or less.

[0026] (Mn: 0.56~1.50%) Since manganese (Mn) contributes to increased hardenability and improved strength, the Mn content should be 0.56% or more. To further increase strength, the Mn content is preferably 0.90% or more, and more preferably 1.10% or more. On the other hand, since a Mn content exceeding 1.50% impairs toughness, the Mn content should be 1.50% or less. Preferably, the Mn content is 1.30% or less.

[0027] (P: 0.020% or less, S: 0.010% or less) P and S are impurities present in steel and can cause welding cracks due to solidification segregation and a decrease in toughness; therefore, it is preferable to reduce them as much as possible. For this reason, the P content should be 0.020% or less and the S content 0.010% or less. A P content of 0.010% or less is preferable, and 0.008% or less is more preferable. Similarly, a S content of 0.008% or less is preferable, and 0.006% or less is more preferable.

[0028] (Cr: 0.50~1.50%) Cr contributes to improved tensile strength by increasing hardenability, so the Cr content should be 0.50% or more. Preferably, the Cr content should be 0.80% or more to increase strength. On the other hand, toughness decreases when the Cr content exceeds 1.50%, so the Cr content should be 1.50% or less. Preferably, the Cr content should be 1.30% or less, and more preferably 1.10% or less.

[0029] (Mo: 0.30~1.06%) Mo is an element that, during quenching, dissolves in the steel to improve hardenability and contribute to increased tensile strength. During tempering, it precipitates as carbonitrides, suppressing the decrease in tensile strength. Furthermore, it also contributes to improved toughness. For this reason, the Mo content should be 0.30% or more. Preferably, the Mo content should be 0.35% or more. On the other hand, if the Mo content exceeds 1.06%, it may lead to a decrease in toughness, so the Mo content should be 1.06% or less. Preferably, the Mo content should be 0.50% or less.

[0030] (Al: 0.010~0.080%) Since Al is a deoxidizing element, the Al content should be 0.010% or more. Preferably, the Al content should be 0.020% or more. On the other hand, if Al is included in excess, the oxide becomes coarser, which becomes the starting point for brittle fracture and reduces toughness. For this reason, the Al content should be 0.080% or less. Preferably, the Al content should be 0.070% or less, and more preferably, 0.040% or less.

[0031] (N: 0.0010~0.0070%) N is an element that forms precipitates with Mo, Nb, and Ti, contributing to the refinement of the microstructure and precipitation strengthening. For this reason, the N content should be 0.0010% or more. Preferably, the N content should be 0.0020% or less. On the other hand, if the N content is excessive, the toughness of the base material will decrease, leading to surface cracking during casting and material defects due to strain aging of the manufactured steel. Therefore, the N content should be 0.0070% or less. Preferably, the N content should be 0.0050% or less.

[0032] (B: 0.0006~0.0015%) Since B has the effect of improving hardenability and thus strength, the B content should be 0.0006% or more. Preferably, the B content should be 0.0007% or more. On the other hand, if B is included in excess, toughness may be impaired, so the B content should be 0.0015% or less. Preferably, the B content should be 0.0012% or less.

[0033] (Remainder: Fe and impurities) In the chemical composition of the steel material of the present invention, the remainder is Fe and impurities. Here, impurities refer to components that are mixed in during the industrial production of steel material due to raw materials such as ore and scrap, or other factors, and are acceptable within a range that does not adversely affect the properties of the steel material of the present invention. Specifically, it may contain As, Co, Hf, Te, Sr, Bi, Ta, Zn, Pb, and Se. In this case, the total content of these elements is 0.20% or less.

[0034] In the chemical composition of the steel sheet of the present invention, one or more elements selected from V, Nb, Ti, Cu, Ni, W, Ca, Mg, and REM may be further included within the ranges shown below in order to improve strength and / or toughness. Since these elements are not necessarily essential in the steel sheet, the lower limit of their content is 0%. The reasons for limiting each element are explained below.

[0035] (V:0.060% or less) V contributes to precipitation strengthening by forming VC or VN. Furthermore, V carbonitrides precipitated within austenite grains act as ferrite transformation nuclei, and also have the effect of refining the effective crystal grains of ferrite. For this reason, V may be included as needed. However, excessive V content may impair toughness due to the coarsening of precipitates. Therefore, the V content should be 0.060% or less. Preferably, the V content should be 0.050% or less. If the above effects are to be obtained more reliably, the V content should preferably be 0.001% or more, and more preferably 0.020% or more.

[0036] (Nb:0.021% or less) Nb contributes to the refinement of ferrite grains in bainite after quenching by suppressing austenite recrystallization during hot rolling and accumulating processing strain in the steel, and also increases the strain accumulated in the bainite. As a result, it contributes to the refinement of prior austenite grains during reheating quenching. For this reason, Nb may be included as needed. However, excessive Nb content can lead to a significant decrease in toughness. Therefore, the Nb content should be 0.021% or less. Preferably, the Nb content should be 0.015% or less. If the above effects are to be obtained more reliably, the Nb content should preferably be 0.001% or more, and more preferably 0.010% or more.

[0037] (Ti:0.025% or less) Ti forms TiN, which has the effect of refining austenite through a pinning effect. For this reason, Ti may be included as needed. However, if Ti is included in excess, coarse TiN will be generated, impairing toughness. Therefore, the Ti content should be 0.025% or less. Preferably, the Ti content should be 0.020% or less. If the above effect is to be obtained more reliably, the Ti content should preferably be 0.001% or more, and more preferably 0.005% or more.

[0038] (Cu:0.50% or less) Cu improves hardenability and contributes to increased tensile strength. For this reason, Cu may be included as needed. However, excessive Cu content may reduce toughness. Therefore, the Cu content should be 0.50% or less. Preferably, the Cu content should be 0.30% or less. If the above effects are to be obtained more reliably, the Cu content should preferably be 0.01% or more, and more preferably 0.10% or more.

[0039] (Ni:0.30% or less) Ni is an element that dissolves in steel to enhance hardenability and contributes to improved tensile strength. For this reason, Ni may be included as needed. However, excessive Ni content leads to an excessive improvement in hardenability and a decrease in toughness, and also increases manufacturing costs because Ni is expensive. Therefore, the Ni content should be 0.30% or less. Preferably, the Ni content should be 0.20% or less. If the above effects are to be obtained more reliably, the Ni content should preferably be 0.01% or more, and more preferably 0.10% or more.

[0040] (W: 0.20% or less) W is an element that dissolves in steel to enhance hardenability and contributes to improved tensile strength. Therefore, W may be included as needed. However, excessive W content can lead to a decrease in toughness. For this reason, the W content should be 0.20% or less, preferably 0.15% or less. If the above effects are to be obtained more reliably, the W content should preferably be 0.01% or more, and more preferably 0.05% or more.

[0041] (Sn:0.20% or less) Sn contributes to improving the corrosion resistance of steel when incorporated into it. Therefore, it may be included as needed. However, excessive Sn content can lead to a decrease in toughness. For this reason, the Sn content should be 0.20% or less, preferably 0.15% or less. If the above effect is to be obtained more reliably, the Sn content should preferably be 0.001% or more, and more preferably 0.05% or more.

[0042] (Sb:0.05% or less) Sb (Sulfur) contributes to improving the corrosion resistance of steel when incorporated into it. Therefore, Sb may be included as needed. However, excessive Sb content can lead to a decrease in toughness. For this reason, the Sb content should be 0.05% or less, preferably 0.04% or less. If the above effects are to be obtained more reliably, the Sb content should preferably be 0.001% or more, and more preferably 0.02% or more.

[0043] (Ca:0.0050% or less) Ca is an effective element for controlling the morphology of sulfides, suppressing the formation of coarse MnS and contributing to improved toughness. For this reason, Ca may be included as needed. However, excessive Ca content may reduce toughness. Therefore, the Ca content should be 0.0050% or less. Preferably, the Ca content should be 0.0030% or less. If the above effects are to be obtained more reliably, the Ca content should preferably be 0.0005% or more, and more preferably 0.0010% or more.

[0044] (Mg:0.0050% or less) Mg is effective as a deoxidizing element in removing oxygen from steel, and also has the effect of refining austenite grains through the formation of sulfides. For this reason, Mg may be included as needed. However, excessive Mg content may reduce toughness. Therefore, the Mg content should be 0.0050% or less, preferably 0.0040% or less. If the above effects are to be obtained more reliably, the Mg content should preferably be 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 through the removal of dissolved oxygen and sulfur from the steel. For this reason, REM may be included as needed. However, excessive REM content will not only reduce toughness but also lead to an excessive increase in cost. Therefore, the REM content should be 0.0050% or less, preferably 0.0030% or less. If the above effects are to be obtained more reliably, the REM content should preferably be 0.0005% or more, more preferably 0.0010% or more.

[0046] Here, REM is a collective term for 17 elements including Sc, Y, and lanthanides, and the REM content represents the total amount of these elements. Note that lanthanides are added industrially in the form of mischmetal.

[0047] (B) Metal structure The metallographic 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, an average grain size of prior γ grains of 35.0 μm or less, and an average grain size of EBSD grains of 22.0 μm or less. The reasons for each limitation will be explained below.

[0048] (Total area ratio of tempered bainite and tempered martensite: 95.0% or more) Strength cannot be ensured if the total area ratio of tempered bainite and tempered martensite is less than 95.0%. Therefore, the total area ratio of tempered bainite and tempered martensite should be 95.0% or more. Preferably, the total area ratio of tempered bainite and tempered martensite is 99.0% or more, and more preferably 100%. The area ratio of tempered bainite may be 100%, and the area ratio of tempered martensite may be 100%.

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

[0050] The area ratio of the metal structure is measured by the following method: When the thickness of the steel plate is t, a test piece is taken so that the observation surface includes a position at a depth of 1 / 4t from the surface of the steel plate, and when the width of the plate is W, it includes a position at 1 / 4W, and the surface includes the thickness direction and the rolling direction of the steel plate. The observation surface is mechanically polished with diamond powder with a particle size of 1.0 μm, and then etched with Nital etching solution. A microscopic photograph of the structure is then taken at a magnification of 200x, and 1000 measurement points are placed in a grid pattern at 20 μm intervals within an area of ​​780 μm in the rolling direction × 480 μm in the width direction, centered at the position at a depth of 1 / 4t and the position at 1 / 4W.

[0051] Subsequently, at each measurement point, it is determined whether the material is tempered bainite, tempered martensite, ferrite, pearlite, or tempered MA, and the number of measurement points is counted for each microstructure. For each microstructure, the ratio of the number of counted measurement points out of 1000 measurement points is calculated to determine the total area ratio of tempered bainite and tempered martensite. In this invention, in order to evaluate the average strength in the thickness direction, the area ratio of the microstructure at a depth of 1 / 4t is determined.

[0052] (Average crystal grain size of the old γ grains: 35.0 μm or less) If the average grain size of the prior γ grains exceeds 35.0 μm, stable toughness at -20°C cannot be ensured. Therefore, the average grain size of the prior γ grains should be 35.0 μm or less. Preferably, the average grain size of the prior γ grains is 30.0 μm or less. There is no particular lower limit to the average grain size of the prior γ grains. However, in the manufacturing method described later, the effective lower limit of the average grain size of the prior γ grains is 1.0 μm. Furthermore, the effective lower limit of the average grain size of the prior γ grains is 1.0 μm, which is observable with an optical microscope.

[0053] The average grain size of the old γ grains is measured by the following method: from the above-mentioned tissue photograph, a 500 μm square area (250,000 μm) 2 A section of the microstructure is cut out, and the number of old gamma grains contained within it is counted. The area of ​​the section is then divided by the number of old gamma grains to calculate the average area per old gamma grain, and this value is converted to the diameter of a circle that has the same area to obtain the average grain size of the old gamma grains. Old gamma grains located at the edges of the microstructure photograph are counted as 0.5, and old gamma grains located at the four corners of the field of view are counted as 0.25. In this invention, in order to evaluate the average toughness in the thickness direction of the plate, the average grain size of the old gamma grains at a depth of 1 / 4t is measured.

[0054] (Average crystal grain size of EBSD grains: 22.0 μm or less) To ensure toughness at -20°C, controlling only the prior γ grain size is insufficient; it is necessary to control the EBSD grain size. If the average grain size of the EBSD grains exceeds 22.0 μm, stable toughness at -20°C cannot be ensured. Therefore, the average grain size of the EBSD grains should be 22.0 μm or less. Preferably, the average grain size of the EBSD grains is 15.0 μm or less. There is no particular lower limit to the average grain size of the EBSD grains. However, in the manufacturing method described later, the practical lower limit of the average grain size of the EBSD grains is 2.0 μm. Note that the average grain size of the EBSD grains can be sufficiently calculated by irradiating them with an electron beam at 0.2 μm intervals.

[0055] The average grain size of EBSD grains is measured by the following method: A test specimen is taken so that the observation surface includes a position at a depth of 1 / 4t and a position at 1 / 4W from the surface of the steel plate, and also includes the thickness direction and the rolling direction of the steel plate. Then, a region of 200 μm in the rolling direction × 200 μm in the thickness direction, centered at a depth of 1 / 4t from the surface of the steel plate and at the 1 / 4W position, is observed with EBSD at 0.2 μm intervals, and the crystal orientation information of this region is obtained.

[0056] Here, tempered martensite, tempered bainite, and pearlite are all mixed structures of ferrite (α phase) and cementite phase. However, the cementite phase is generally fine and has a weak EBSD signal intensity. Therefore, if the crystal structure of the observation surface is considered to be only a body-centered cubic lattice of Fe (α phase) and the observation results are analyzed, the cementite phase is found to be an α phase with the same crystal orientation as the surrounding α phase. Utilizing this, the present invention determines the crystal orientation by considering the crystal structure of the observation surface to be only the α phase. Measurement locations where the crystal orientation difference between adjacent grains is 15° or more are considered grain boundaries, and the region enclosed by these grain boundaries is defined as a crystal grain. The average value of the equivalent circle diameter calculated from the area of ​​each crystal grain is defined as the average crystal grain size of the EBSD grain. The equivalent circle diameter of each crystal grain is calculated as the so-called Area grain size. Furthermore, in the present invention, the EBSD grain size is determined at a position 1 / 4t from the surface in the thickness direction, as the location representing the average toughness in the thickness direction.

[0057] (C) Mechanical properties (Tensile strength: 760~1150MPa) The tensile strength of the steel plate according to the present invention shall be 760 MPa or higher. On the other hand, since excessively high strength may impair toughness, the tensile strength shall be 1150 MPa or lower.

[0058] (Yield stress: 665 MPa or higher) The yield strength of the steel sheet according to the present invention shall be 665 MPa or higher. There is no specific upper limit set for the yield strength, but the practical upper limit for the steel sheet according to the present invention is 950 MPa.

[0059] Tensile strength and yield stress are measured by the following method: A No. 4 test specimen, i.e., a round bar specimen with a diameter of 14 mm, as specified in JIS Z 2241:2022, is taken from the position 1 / 4t of the plate thickness and 1 / 4W of the plate width in the direction of the plate width, and a tensile test is performed at room temperature in accordance with JIS Z 2241:2022. If yielding occurs in the stress-strain curve, the yield stress is determined; if yielding does not occur, the 0.2% proof stress is determined. In this invention, in order to evaluate the average strength in the plate thickness direction, the strength at the 1 / 4t depth position is evaluated.

[0060] 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 shall be 100 J or more. A higher absorbed energy in the Charpy impact test is preferable, and there is no particular upper limit, but the practical upper limit for the steel plate according to the present invention is 310 J.

[0061] The absorbed energy in the Charpy impact test is measured by the following method. Figure 1 is a diagram illustrating the sampling location of the test specimen used in the Charpy impact test. (a) is a plan view of the 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 with a thickness of 10 mm × width of 10 mm × length of 55 mm and a notch width of 8 mm is taken in accordance with JIS Z 2242:2023, centered at the position of 1 / 4t of the plate thickness and 1 / 4W of the plate width, so that the rolling direction of the steel plate coincides with the longitudinal direction of the test specimen. Here, the test specimen is taken so that a V-notch is formed on one side of the specimen in the plate width direction, parallel to the thickness direction. Then, the Charpy impact test is performed three times at -20°C in accordance with JIS Z 2242:2023, and the average value is taken as the absorbed energy of the Charpy impact test. In this invention, the toughness is evaluated at a point 1 / 4th of the way from the surface in the thickness direction, as the location representing the average toughness in the thickness direction.

[0062] (D) Dimension Generally, it is difficult to achieve both strength and toughness in thick steel plates. On the other hand, thick steel plates are used in various structures such as construction machinery, industrial machinery, and pressure vessels. The thickness of the steel plate according to the present invention is not particularly specified. The steel billet can be reduced to a predetermined thickness by controlling the reduction process during the hot rolling process so that the grain size of the steel plate becomes the grain size specified in the present invention. The thickness of the steel plate according to the present invention is preferably 4.5 to 75 mm.

[0063] (E) Structure The steel plate according to the present invention may be used in structures. Examples of structures include construction machinery, industrial machinery, pressure vessels, and bridges.

[0064] (F) Manufacturing method The method for manufacturing steel sheets according to the present invention will now be described. The steel material according to the present invention is manufactured by the following manufacturing method. The method for manufacturing steel sheets according to the present invention involves subjecting the steel billet to a soaking process, a hot rolling process, a cooling process, a quenching process, and a tempering process. Each of these processes will be described below. Note that the temperatures shown in each of the following processes, except for the average cooling rate in the quenching process, are the surface temperatures of the steel billet or steel sheet.

[0065] <Soaking process> Steel plates are manufactured by processing steel billets, but the method of manufacturing the steel billets is not specified. It is sufficient to cast molten steel having the chemical composition described above to obtain a steel billet. In this case, the thickness of the steel billet is preferably 240 mm or more from the viewpoint of productivity, and preferably 350 mm or less when considering the reduction of segregation and the homogeneity of the soaking temperature before hot rolling.

[0066] In the soaking process, the steel billet is heated to a temperature range of 1050 to 1250°C to achieve soaking. The soaking temperature of the steel billet should be 1050°C or higher, as a temperature below 1050°C increases the deformation resistance during finish rolling. A soaking temperature of 1080°C or higher is preferable to allow sufficient solid solution of elements that form carbides and / or nitrides, such as Nb. On the other hand, if the soaking temperature exceeds 1250°C, it becomes necessary to wait for the steel billet's temperature to decrease so that it can be properly reduced in the hot rolling process described later, which significantly reduces productivity. Therefore, the soaking temperature of the steel billet should be 1250°C or lower. A soaking temperature of 1180°C or lower is preferable.

[0067] <Hot rolling process> In the hot rolling process, the steel billet after the soaking process is subjected to hot rolling at a temperature range of 730 to 920°C with a reduction ratio of 30% or more to produce a steel plate. Here, the reduction ratio is defined as follows: reduction ratio [%] = (t2-t1) / 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 at a rolling temperature of 730°C or higher.

[0068] The hot rolling process comprises a rough rolling process and a finish rolling process. In the rough rolling process, the steel billet is roughly rolled using a rough rolling mill. The rough rolling process is an auxiliary rolling process for the finish rolling process, and after the rough rolling process, the finish rolling process is performed using a finish rolling mill to obtain a thick steel plate with the final plate thickness. In the hot rolling process, rolling is performed in a rough rolling mill and / or a finish rolling mill at a temperature range of 730 to 920°C with a reduction ratio of 30% or more.

[0069] Even if the steel is significantly reduced at temperatures above 920°C, sufficient strain cannot be imparted to the gamma grains, causing them to coarseen during reheating and quenching, resulting in insufficient toughness of the steel sheet. Rolling at temperatures below 730°C increases the deformation resistance of the steel billet, placing a load on the rolling mill. For this reason, it is preferable to complete the finish rolling at 730°C or higher.

[0070] If the reduction ratio is less than 30% in the 730-920°C temperature range, sufficient strain cannot be applied to the gamma grains, which can cause the gamma grains to coarseen during reheating and quenching, resulting in insufficient toughness of the steel sheet. A reduction ratio of 40% or more in the 730-920°C temperature range is preferable. There is no particular upper limit to the reduction ratio, but considering the final thickness of the steel sheet, the maximum reduction ratio is approximately 70%.

[0071] <Cooling process> In the cooling process, the steel plate is cooled so that its surface temperature is below 300°C. There are no particular restrictions on the cooling method. It may be allowed to cool naturally, or water cooling may be used to improve productivity.

[0072] If the quenching process is performed on a steel sheet without cooling the surface temperature to below 300°C, the steel sheet will be reheated before the transformation is complete in the center of the sheet thickness, resulting in coarse old gamma grains in the steel sheet after tempering. Therefore, it is necessary to cool the sheet to below 300°C. Preferably, it should be cooled to room temperature.

[0073] When cooling by air-cooling, there is generally no reheating. On the other hand, when cooling by water cooling, reheating may occur after the water cooling stops. If the surface temperature rises due to reheating, cooling should be performed so that the reheating temperature is 300°C or less on the surface of the steel plate.

[0074] <Heat treatment process> In the quenching process, the steel plate after the cooling process is heated to a temperature range of 860-950°C and then subjected to accelerated cooling so that the reheating temperature is 300°C or less at the surface temperature of the steel plate. In other words, the steel plate after the cooling process is reheated and quenched.

[0075] If the heating temperature before quenching is below 860°C, the amount of alloying elements dissolved in the solid solution decreases, resulting in reduced hardenability during accelerated cooling and insufficient strength. A heating temperature of 870°C or higher is preferable. Furthermore, if the heating temperature before quenching exceeds 950°C, the prior γ grain size becomes coarser, reducing toughness. A heating temperature of 930°C or lower is preferable.

[0076] Accelerated cooling is a cooling method that improves the hardenability of steel sheets by cooling them at a rate that cannot be achieved by normal air cooling, i.e., by lowering the transformation temperature, thereby improving the strength of the steel sheet. Industrially, it is mostly carried out by spraying water onto the top and / or bottom surface of the steel sheet using a spray nozzle, but it is not necessarily limited to water cooling. Specifically, accelerated cooling refers to cooling so that the average cooling rate from the heating temperature before quenching to 300°C is 5°C / s or more. By performing such cooling, the total area ratio of bainite and martensite can be increased, and the strength of the steel sheet can be stably improved. In addition, the EBSD grain size can be refined and toughness can be ensured. The average cooling rate is the average cooling rate at a position of 1 / 4t of the sheet thickness, and is estimated by simulation considering the sheet thickness from the surface temperature of the steel sheet.

[0077] Accelerated cooling should be performed so that the reheat temperature is 300°C or less at the surface temperature of the steel plate. If the reheat temperature exceeds 300°C, the hardenability will be insufficient and the strength will be insufficient. In addition, insufficient hardenability may cause the EBSD grain size to coarseen, reducing toughness. Therefore, the reheat temperature should be 300°C or less. A reheat temperature of 250°C or less is preferable.

[0078] At the stage after quenching and accelerated cooling but before tempering, the total area ratio of bainite and martensite is preferably 95.0% or more. This ensures that the target strength is obtained in the steel sheet after the tempering process. The total area ratio of bainite and martensite is preferably 99.0% or more, and more preferably 100%.

[0079] <Tempering process> In the tempering process, the steel sheet, after the quenching process, is tempered in the temperature range of 450 to 680°C. If the tempering temperature is below 450°C, the tempering effect is insufficient and the toughness is lacking. Conversely, if the tempering temperature exceeds 680°C, the strength decreases. Therefore, the tempering temperature should be between 450 and 680°C. After the steel sheet has been uniformly heated, it can be air-cooled, but water-cooling is also acceptable.

[0080] By going through the above steps, the above-described metallic structure can be obtained, and the steel sheet according to the present invention can be obtained.

[0081] The method for manufacturing a structure using the steel plates according to the present invention is not particularly limited. For example, the steel plates manufactured as described above may be welded together by a conventional method.

[0082] The present invention will be described more specifically below with reference to examples, but the present invention is not limited to these examples. [Examples]

[0083] Steel was melted in a converter, deoxidized, and then alloying elements were added to adjust the composition as shown in Table 1. Vacuum degassing treatment was then performed to produce steel billets with a thickness of 240-300 mm. Subsequently, under the conditions shown in Table 2, the obtained steel billets were heated in a heating furnace, and rough rolling, finish rolling, reheating and quenching, and tempering were performed to produce steel plates.

[0084] [Table 1]

[0085] [Table 2]

[0086] The manufactured steel plates were examined for their microstructure using the following methods, and tensile and Charpy impact tests were performed.

[0087] <Area ratio of metal structure> The area fraction of the metal structure was measured by the following method. A test piece was collected such that the observation surface includes the 1 / 4t depth position from the surface of the steel sheet, includes the 1 / 4W position, and contains the thickness direction and rolling direction of the steel sheet. After mechanically polishing the observation surface with diamond powder having a particle size of 1.0 µm, it was corroded with nital etchant. Then, a structure photograph was taken with an optical microscope at a magnification of 200 times. 1000 grid-shaped measurement points were arranged at intervals of 20 µm within an area of 780 µm in the rolling direction and 480 µm in the width direction, centered on the 1 / 4t depth position and the 1 / 4W position. Thereafter, at each measurement point, it was determined whether the structure is tempered bainite, tempered martensite, ferrite, pearlite, or tempered MA, and the number of measurement points was counted for each metal structure. For each metal structure, the ratio of the number of counted measurement points among the 1000 measurement points was calculated, and the total area fraction of tempered bainite and tempered martensite was obtained.

[0088] <Average grain size of prior γ grains> The average grain size of prior γ grains was measured by the following method. A 500 µm square region (250000 µm 2 ) was cut out, and the number of prior γ grains contained therein was counted. The area of the region was divided by the number of prior γ grains to calculate the average area per prior γ grain, which was converted into the diameter of a circle having the same area as this value, and this was taken as the average grain size of prior γ grains. Note that 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 visual field were counted as 0.25 grains.

[0089] <Average grain size of EBSD grains> The average grain size of EBSD grains was measured using an electron backscatter diffractometer (EBSD; manufactured by TSL Solutions Co., Ltd.) attached to a scanning electron microscope (SEM; model number JSM-7200F) manufactured by JEOL Ltd., using the following method. A test specimen was taken so that the observation surface included a position 1 / 4t deep and 1 / 4W from the surface of the steel plate, and also included the thickness direction and rolling direction of the steel plate. Then, a region of 200 μm in the rolling direction × 200 μm in the thickness direction, centered on the position 1 / 4t deep and 1 / 4W from the surface of the steel plate, was observed with EBSD at 0.2 μm intervals, and crystal orientation information of this region was obtained. Then, assuming that the crystal structure was only a body-centered cubic lattice (α phase) of Fe, the crystal orientation was determined, and measurement locations where the orientation difference between adjacent grains was 15° or more were considered as grain boundaries, and the region surrounded by these grain boundaries was defined as a crystal grain. The average equivalent diameter of each crystal grain, calculated from the area of ​​each grain, was defined as the average grain size of the EBSD grains. The equivalent diameter of each crystal grain is calculated as the so-called Area grain size using software (OIM ver3.0).

[0090] <Tensile strength and yield stress> Tensile strength and yield stress were measured using the following method. A No. 4 test specimen, as specified in JIS Z 2241:2022, i.e., a round bar specimen with a diameter of 14 mm, was taken from the position 1 / 4 t of the plate thickness and 1 / 4 W of the plate width in the direction of the plate width, and a tensile test was performed at room temperature in accordance with JIS Z 2241:2022. If yielding phenomena appeared in the stress-strain curve, the yield stress was determined; if yielding phenomena did not appear, the 0.2% proof stress was determined.

[0091] <Energy absorbed in Charpy impact test> The absorbed energy of the Charpy impact test was measured using the following method. As shown in Figure 1, a V-notch Charpy test specimen was taken in accordance with JIS Z 2242:2023, with a thickness of 10 mm, a width of 10 mm, a length of 55 mm, and a notch width of 8 mm, 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 coincided with the longitudinal direction of the test specimen. Here, the test specimen was taken so that a V-notch was formed on one side of the plate in the plate width direction, parallel to the thickness direction. Then, the Charpy impact test was performed three times at -20°C in accordance with JIS Z 2242:2023, and the average value was taken as the absorbed energy of the Charpy impact test.

[0092] Table 3 shows the total area ratio of tempered martensite and tempered bainite, the average grain size of prior γ grains, EBSD grain size, yield stress (YS), tensile strength (TS), and absorbed energy (vE) of the Charpy test. -20 ) indicates.

[0093] [Table 3]

[0094] As shown in Table 3, production numbers 1, 2, 4-8, 13-18, and 22-25 of the present invention satisfied the target YS of 660 MPa or higher and the target TS of 760-1150 MPa or lower. Furthermore, the absorbed energy at -20°C was 100 J or higher, meeting all the targets.

[0095] On the other hand, for production numbers 3, 9-12, 19-21, and 26-38 in Table 3, one or more of the following are outside the scope of the present invention: chemical composition, rolling conditions, cooling temperature, reheating and quenching temperature, accelerated cooling rate, reheating temperature, tempering temperature, total area fraction of tempered bainite and tempered martensite, average grain size of prior austenite, and EBSD grain size. Therefore, one or more of the following did not meet the above objectives: YS, TS, and absorbed energy at -20°C. [Industrial applicability]

[0096] According to the present invention, a steel sheet with high strength and excellent toughness can be obtained.

Claims

1. The chemical composition is expressed in mass percent. C: 0.080% to 0.180%, Si: 0.08% to 0.30%, Mn: 0.56% to 1.50%, P: 0.020% or less, S: 0.010% or less, Cr: 0.50-1.50%, Mo: 0.30-1.06%, Al: 0.010-0.080%, N: 0.0010-0.0070%, B: 0.0006 to 0.0015%, The remainder consists of Fe and impurities. In the metal structure, the total area ratio of tempered bainite and tempered martensite is 95% or more. The average grain size of the prior austenite grains is 35.0 μm or less. The average grain size of the crystal grains observed by electron beam backscatter diffraction is 22.0 μm or less. The tensile strength is 760 to 1150 MPa. The yield strength is 665 MPa or higher. The absorbed energy in the Charpy impact test at -20°C is 100 J or more. steel plate.

2. The aforementioned chemical composition, in place of a portion of the Fe, is expressed in mass % as follows: V: 0.060% or less, Nb: 0.021% or less, Ti: 0.025% or less, Cu: 0.50% or less, Ni: 0.30% or less, W: 0.20% or less, Sn: 0.20% or less, Sb: 0.05% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, REM: 0.0050% or less, It contains one or more selected from the following: The steel plate according to claim 1.

3. A method for manufacturing a steel sheet according to claim 1 or claim 2, A soaking step of heating a steel billet having the chemical composition described in claim 1 or claim 2 to a temperature range of 1050 to 1250°C and soaking it, A hot rolling process is performed on the steel billet after the soaking process, in a temperature range of 730 to 920°C, with a reduction ratio of 30% or more, to form a steel plate. A cooling step in which the surface temperature of the steel plate is cooled until it reaches 300°C or below, The steel plate after the cooling step is heated to a temperature range of 860 to 950°C and uniformly heated, and then subjected to a quenching step in which accelerated cooling is performed so that the reheating temperature is 300°C or less on the surface of the steel plate. The process includes a tempering step in which the steel plate after the quenching step is tempered at a temperature range of 450 to 680°C. A method for manufacturing steel plates.

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