Steel plate and method for manufacturing the same

Abstract

To provide a steel plate and a method for manufacturing the steel plate.SOLUTION: A steel plate of the present invention contains C:0.01 to 0.15%, Si:0.01 to 0.50%, Mn:0.05 to 1.20%, Ni:6.0 to 7.5%, Cr:0.01 to 1.00%, Mo:0.05 to 0.50%, Al:0.008 to 0.10%, P:0.03% or less, S:0.005% or less and N:0.0010 to 0.0080% in mass%, and the balance has a component composition consisting of Fe and inevitable impurities. In a micro structure at a (1 / 4)t position of a plate thickness, the number density of a local Ni concentrated range having an average Ni concentration of 10 mass% or more and an equivalent circle diameter of 500 nm or less is 5×105 pcs. / mm2 or more, and the number density of a local Ni concentrated range having an average Ni concentration of 8 to 12 mass% and an equivalent circle diameter of more than 500 nm is 5×104 pcs. / mm2 or less, the plate contains austenite by 1 to 15% in volume ratio, and has the tensile strength of 690 MPa or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to steel sheets and methods for manufacturing the same, and more particularly to cryogenic thick steel sheets that have consistently excellent cryogenic toughness over a wide range of sheet thicknesses, and to methods for manufacturing the same. [Background technology]

[0002] Steel plates used in structures such as liquefied gas storage tanks are required to have not only excellent strength but also excellent toughness at cryogenic temperatures (hereinafter sometimes referred to as "cryogenic toughness"), due to the extremely low operating environment. For example, when hot-rolled steel plates are used in liquefied natural gas storage tanks, it is necessary to ensure excellent toughness under cryogenic conditions below -164°C, the boiling point of liquefied natural gas. If the cryogenic toughness of the steel material is poor, the safety of the structure as a cryogenic storage facility may not be maintained, so there is a high demand for improved cryogenic toughness in the steel plates used.

[0003] For marine fuel tanks, where the tank volume is relatively small, relatively thin steel plates are required. On the other hand, for land-based applications, where the tank volume is relatively large, thicker steel plates are required. To meet these requirements, the application of 7% nickel steel plates is progressing.

[0004] As an example of a 7% Ni steel sheet, the steel sheets described in Patent Documents 1 and 2 have been proposed.

[0005] Patent Document 1 discloses a cryogenic thick steel sheet containing Ni: greater than 5.0% to less than 10.0% by mass, and predetermined amounts of C, Si, Mn, and Al. The thick steel sheet of Patent Document 1 has a plate thickness of 6 to 50 mm, and the average value of the V-notch Charpy absorption energy vE-196 per unit area is 1.25 J / mm². 2 That's all.

[0006] In addition, Patent Document 2 discloses a low-temperature Ni-containing steel containing Ni: 7.0 to 10.5% by mass and a predetermined amount of C, Si, Mn, and Al. The steel in Patent Document 2 has an average absorbed energy vE-196°C in the Charpy impact test of 150 J or more over a plate thickness of 30 to 60 mm.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] As a result of the inventors' intensive investigation of thick steel plates of 7% Ni steel, it has newly been found that even when the absorbed energy of the steel plate is high, minute brittle fracture may occur. And it has been found that this cause is due to the formation state of a local Ni-concentrated region, which is partly affected by two-phase region heating conditions such as the heating rate immediately before holding at soaking temperature. It is presumed that this is because γ (austenite) with low stability is generated if an appropriate local Ni-concentrated region is not formed, and as a result, minute brittle fracture occurs.

[0009] In the present invention, γ being stable means that austenite has a tendency to be difficult to transform into a martensite structure at -196°C. On the contrary, γ being unstable means that austenite has a tendency to be easily transformed into a martensite structure at -196°C.

[0010] However, in the above-mentioned Patent Documents 1 and 2, only the average value of the absorbed energy is considered for extremely low-temperature toughness, and no consideration is given to the above problems, specifically, the occurrence of minute brittle fracture.

[0011] This invention has been made in view of the above problems, and aims to provide a steel sheet that ensures high strength while exhibiting excellent cryogenic toughness regardless of the thickness of the steel sheet, that is, a steel sheet that has high absorbed energy within the steel sheet and does not undergo minute brittle fracture, and a method for manufacturing the same. [Means for solving the problem]

[0012] To solve the above problems, the inventors diligently researched the component composition, microstructure, and manufacturing conditions of 7% Ni steel sheets and obtained the following findings.

[0013] 1) To increase the absorbed energy within the steel plate, the number density of localized Ni-enriched regions, where the average Ni concentration is 10% by mass or more and the equivalent circle diameter is 500 nm or less, should be 5 × 10⁻⁶. 5 pieces / mm 2 It is important to ensure the above conditions are met. Furthermore, in order to obtain this microstructure, it is important that the Ni content is 6.0% or more, that in the two-phase heating process the average heating rate in the temperature range between Ac1 point and 650°C is 1.0°C / second or less, that the heating temperature (hereinafter also referred to as "heating holding temperature") is in the two-phase temperature range between 650°C and below Ac3 point and that the holding time in the said two-phase temperature range is 15 minutes or more, that in the second accelerated cooling process the average cooling rate in the temperature range between 600°C and 300°C is 1.0°C / second or more and that the cooling stop temperature is 300°C or less, and that in the tempering process the tempering temperature is 550°C or higher and the holding time is 10 minutes or more.

[0014] 2) To suppress the occurrence of minute brittle fractures, the number density of localized Ni-enriched regions, where the average Ni concentration is between 8% and 12% by mass and the equivalent circle diameter is greater than 500 nm, should be 5 × 10⁻⁶. 4 pieces / mm 2It is important to satisfy the following conditions. To obtain this microstructure, the C content should be 0.15% or less, the average cooling rate in the first accelerated cooling process should be 1.0 °C / sec or more and the cooling stop temperature should be 300 °C or less in the temperature range of 550 °C or less and 300 °C or more, the heating and holding temperature in the two-phase region heating process should be 650 °C or more, the average cooling rate in the second accelerated cooling process should be 1.0 °C / sec or more and the cooling stop temperature should be 300 °C or less in the temperature range of 600 °C or less and 300 °C or more, and it is important to control the tempering temperature to (Ac1 point + 30 °C) or less in the tempering process.

[0015] 3) In order to increase the absorbed energy in the steel sheet, it is important to contain austenite in a volume ratio of 1 to 15%, and this microstructure can be obtained by controlling the tempering temperature in the tempering process to 550 °C or more.

[0016] The present invention has been completed based on the above findings, and the gist thereof is as follows. [1] In mass%, C: 0.01 to 0.15%, Si: 0.01 to 0.50%, Mn: 0.05 to 1.20%, Ni: 6.0 to ...... Cr: 0.01 to 1.00%, Mo: 0.05 to 0.50%, Al: 0.008 to 0.10%, P: 0.03% or less, S: 0.005% or less, and N: 0.0010 to 0.0080%, and has a component composition in which the balance consists of Fe and unavoidable impurities, The microstructure at a position 1 / 4 of the plate thickness from the surface of the steel sheet has a number density of local Ni enrichment regions with an average Ni concentration of 10 mass% or more and a circle equivalent diameter of 500 nm or less of 5 × 10 or more, and a number density of local Ni enrichment regions with an average Ni concentration of 8 mass% or more and 12 mass% or less and a circle equivalent diameter exceeding 500 nm of 5 × 10 5 pieces / mm 2 or more, and 4 pieces / mm 2The following: It contains 1-15% austenite by volume, A steel plate with a tensile strength of 690 MPa or higher. [2] The above component composition is further, in mass%, Cu: 0.40% or less, Nb: 0.05% or less, V: 0.05% or less, Ti: 0.03% or less, B: 0.0030% or less, Ca: 0.007% or less, REM: 0.010% or less, and Mg: 0.007% or less The steel plate according to [1] above, comprising one or more selected from the group consisting of the above. [3] A method for manufacturing steel plates as described in [1] or [2] above, A steel material having the above-mentioned component composition is heated to a temperature of 900°C to 1200°C and hot-rolled to produce a hot-rolled steel sheet. Next, a first accelerated cooling process is performed at a temperature at a depth of 1 / 4 of the thickness direction from the surface of the steel plate, under cooling conditions where the average cooling rate in the temperature range of 300°C or higher and the cooling stop temperature is 1.0°C / second or higher, and the cooling stop temperature is 300°C or lower. Next, two-phase heating is performed under the following conditions: at a temperature at a depth of 1 / 4 of the thickness direction from the surface of the steel plate, the average heating rate in the temperature range between Ac1 point and 650°C is 1.0°C / second or less, the heating and holding temperature is in the two-phase temperature range between 650°C and Ac3 point, and the holding time in the two-phase temperature range is 15 minutes or more. Next, a second accelerated cooling process is performed at a temperature at a depth of 1 / 4 of the plate thickness from the surface of the steel plate, under cooling conditions where the average cooling rate in the temperature range of 600°C to 300°C is 1.0°C / second or higher, and the cooling stop temperature is 300°C or lower. Next, a method for manufacturing a steel sheet, comprising tempering the steel sheet at a temperature at a depth of 1 / 4 of the thickness direction from the surface of the steel sheet, under the conditions that the tempering temperature is 550°C or higher (Ac1 point + 30°C) or lower, and the holding time is 10 minutes or more. [4] The method for manufacturing a steel sheet according to [3] above, wherein the two-phase heating further comprises an average heating rate of 0.10°C / second or less in the temperature range of 650°C or higher and below the heating and holding temperature. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a steel sheet that ensures high strength while exhibiting excellent cryogenic toughness regardless of the thickness of the steel sheet, and a method for manufacturing the same. The steel sheet of the present invention has high energy absorption within the sheet and does not undergo minute brittle fracture. Therefore, when used in steel structures used in cryogenic environments, such as liquefied gas storage tanks, for example, the safety of the steel structure can be improved, resulting in significant industrial benefits. [Modes for carrying out the invention]

[0018] The embodiments of the present invention will be described in detail below. The following description illustrates preferred embodiments of the present invention, and the invention is not limited thereto.

[0019] <Steel plate> First, the steel plate of the present invention will be described.

[0020] [Component composition] The steel sheet of the present invention has a predetermined component composition. Furthermore, the steel material used in the manufacture of the steel sheet of the present invention, as described later, also has the same predetermined component composition. The elements included in this component composition will be described below. Unless otherwise specified, in this invention, "%" used as the unit for the content of each element means "mass%".

[0021] C: 0.01~0.15% Carbon (C) is an element that has the effect of improving the strength of steel sheets. To obtain this effect, the C content should be 0.01% or more. Preferably, the C content should be 0.03% or more. On the other hand, if the C content exceeds 0.15%, the number density of localized Ni-enriched regions, where the average Ni concentration is 8% to 12% by mass and the equivalent circle diameter exceeds 500 nm, increases, leading to a larger amount of unstable γ and a decrease in the cryogenic toughness of the steel sheet. Therefore, the C content should be 0.15% or less. Preferably, the C content should be 0.12% or less.

[0022] Si: 0.01~0.50% Si is an element that contributes to improving the strength of steel sheets and also acts as a deoxidizing agent. To achieve these effects, the Si content should be 0.01% or more. Preferably, the Si content should be 0.02% or more. On the other hand, if the Si content is excessively high, the cryogenic toughness decreases. Therefore, the Si content should be 0.50% or less. Preferably, the Si content should be 0.30% or less.

[0023] Mn: 0.05~1.20% Mn is an element that enhances the hardenability of steel and is effective in increasing the strength of steel sheets. To obtain this effect, the Mn content should be 0.05% or more. Preferably, the Mn content should be 0.10% or more. On the other hand, if the Mn content exceeds 1.20%, the susceptibility to tempering embrittlement increases and the cryogenic toughness decreases, so the Mn content should be limited to 1.20% or less. Preferably, the Mn content should be less than 1.00%, and more preferably 0.60% or less.

[0024] Ni: 6.0~7.5% Ni is an extremely effective element for improving the cryogenic toughness of steel sheets. Specifically, if the Ni content is less than 6.0%, the number density of localized Ni-enriched regions, where the average Ni concentration is 10 mass% or more and the equivalent circle diameter is 500 nm or less, which is necessary to increase the absorbed energy within the steel sheet, is 5 × 10⁻⁶. 5 pieces / mm 2If the Ni content is not higher than this, the cryogenic toughness decreases. Therefore, the Ni content is set to 6.0% or higher. On the other hand, since Ni is an expensive element, the cost of the steel sheet increases as its content increases. Therefore, in this invention, the Ni content is set to 7.5% or lower. The Ni content is preferably 6.5% or higher, and preferably 7.4% or lower.

[0025] Cr: 0.01~1.00% Cr is an element that can improve the strength of steel sheets without significantly impairing their cryogenic toughness. To obtain the above effect, the Cr content should be 0.01% or more. Preferably, the Cr content should be 0.30% or more. However, if the Cr content exceeds 1.00%, the cryogenic toughness of the steel sheet decreases. Therefore, the Cr content should be 1.00% or less. Preferably, the Cr content should be 0.80% or less.

[0026] Mo: 0.05~0.50% Mo, like Cr, is an element that can improve the strength of steel sheets without significantly impairing cryogenic toughness. If the Mo content is less than 0.05%, the desired strength cannot be obtained. Therefore, the Mo content should be 0.05% or more. Preferably, the Mo content should be more than 0.10%. On the other hand, if the Mo content exceeds 0.50%, cryogenic toughness actually decreases. Therefore, the Mo content should be 0.50% or less. Preferably, the Mo content should be 0.30% or less, and more preferably 0.25% or less.

[0027] Al: 0.008~0.10% Al is an element contained in deoxidizing agents. If the Al content is less than 0.008%, the deoxidizing effect is poor. Therefore, the Al content should be 0.008% or more. Preferably, the Al content should be 0.010% or more. On the other hand, if the Al content exceeds 0.10%, the cleanliness of the steel is impaired. Therefore, the Al content should be 0.10% or less. Preferably, the Al content should be 0.05% or less.

[0028] P:0.03% or less P is an unavoidable impurity and a harmful element that adversely affects the cryogenic toughness of steel plates. For example, when steel plates are welded to form a welded structure, it is preferable to reduce the P content as much as possible in order to obtain a sound base material and welded joint. Therefore, the P content should be suppressed to 0.03% or less. Preferably, the P content should be 0.025% or less. More preferably, it should be 0.02% or less. Also, from the viewpoint of cryogenic toughness, the lower the P content, the better, so the lower limit is not particularly limited and may be 0%, although its inclusion as an unavoidable impurity is acceptable. On the other hand, excessive reduction will cause increased costs, so from the viewpoint of cost, it is preferable that the lower limit of the P content be 0.001%. Therefore, the P content is preferably 0.001% or more.

[0029] S: 0.005% or less Since sulfur (S) forms MnS in steel, significantly degrading its cryogenic toughness, it is desirable to limit the S content to 0.005% and reduce it as much as possible. Therefore, the S content should be 0.005% or less. Preferably, the S content should be 0.002% or less. On the other hand, since a lower S content is better, the lower limit of the S content is not particularly limited and may be 0%, although its inclusion as an unavoidable impurity is acceptable. However, excessive reduction will increase costs, so from a cost perspective, it is preferable to have an S content of 0.0003% or more.

[0030] N: 0.0010~0.0080% N forms precipitates in steel, and if the N content exceeds 0.0080%, it causes a decrease in the cryogenic toughness of the base material. However, N is also an element that contributes to the fine-graining of the base material by forming AlN, and this effect can be obtained by setting the N content to 0.0010% or more. Therefore, the N content should be between 0.0010% and 0.0080%. Preferably, the N content should be 0.0020% or more, and preferably 0.0060% or less.

[0031] The basic component composition of the steel sheet of the present invention shall consist of the above-mentioned predetermined amounts of elements, with the remainder being Fe and unavoidable impurities.

[0032] With this basic component composition, the steel sheet of the present invention can obtain the desired properties. In the present invention, in order to further improve the properties, in addition to this basic component composition, one or more elements selected from the group consisting of Cu, Nb, V, Ti, B, Ca, REM, and Mg may be optionally included. Note that each of the components Cu, Nb, V, Ti, B, Ca, REM, and Mg can be included as needed, and these components may be present at 0%.

[0033] Cu: 0.40% or less Cu is an element that enhances the strength of steel sheets by improving hardenability. However, if the Cu content exceeds 0.40%, the cryogenic toughness of the steel sheet decreases, and the surface properties of the steel (slab) after casting deteriorate. Therefore, when Cu is included, the Cu content should be 0.40% or less. Preferably, the Cu content should be 0.30% or less. On the other hand, there is no particular lower limit to the Cu content. When Cu is included to obtain the above effects, it is preferable that the Cu content be 0.10% or more.

[0034] Nb: 0.05% or less Nb is an effective element for increasing the strength of steel sheets through precipitation strengthening. However, if the Nb content becomes excessively high, the cryogenic toughness of the steel sheet decreases. Therefore, when Nb is included, the Nb content should be 0.05% or less. Preferably, the Nb content should be 0.03% or less. On the other hand, there is no particular lower limit to the Nb content. When Nb is included to obtain the above effects, it is preferable that the Nb content be 0.010% or more.

[0035] V: 0.05% or less Like Nb, nitrate (V) is an effective element for increasing the strength of steel sheets through precipitation strengthening. However, if the V content becomes excessively high, the cryogenic toughness of the steel sheet decreases. Therefore, when V is included, the V content should be 0.05% or less. Preferably, the V content should be 0.04% or less. On the other hand, there is no particular lower limit to the V content. When V is included to obtain the above effects, it is preferable that the V content be 0.010% or more.

[0036] Ti: 0.03% or less Ti is an element that enhances the cryogenic toughness of the welded joint without degrading the mechanical properties of the base material when welding steel plates to form welded structures. Therefore, it is possible to optionally include Ti in a range of 0.03% or less. For this reason, when Ti is included, the Ti content should be 0.03% or less. Preferably, the Ti content should be 0.02% or less. Furthermore, preferably, the Ti content should be 0.01% or more.

[0037] B: 0.0030% or less B is an element that enhances the hardenability of steel sheets when added in small amounts. To effectively achieve this effect, B can be optionally included in a range of 0.0030% or less. On the other hand, if the B content exceeds 0.0030%, the cryogenic toughness deteriorates. Therefore, when B is included, the B content should be 0.0030% or less. Preferably, the B content should be 0.0020% or less. Furthermore, preferably, the B content should be 0.0003% or more.

[0038] Ca: 0.007% or less Ca is an element that improves the cryogenic toughness of steel sheets by controlling the morphology of inclusions in the steel. However, if the Ca content is excessively high, it impairs the cleanliness of the steel. Therefore, when Ca is included, the Ca content should be 0.007% or less. Preferably, the Ca content should be 0.004% or less. On the other hand, there is no particular lower limit to the Ca content. When Ca is included to obtain the above effect, it is preferable that the Ca content be 0.001% or more.

[0039] REM: 0.010% or less Rare earth metals (REMs), like calcium (Ca), are elements that improve the cryogenic toughness of steel sheets by controlling the morphology of inclusions in the steel. However, excessively high REM content impairs the cleanliness of the steel. Therefore, when REM is included, the REM content should be 0.010% or less. Preferably, the REM content should be 0.008% or less. On the other hand, there is no particular lower limit to the REM content. When REM is included to obtain the above effects, it is preferable that the REM content be 0.001% or more.

[0040] Here, REM refers to the collective term for 17 elements, which are the 15 lanthanides plus Y and Sc. These elements can be included individually or in combination. Therefore, the REM content represents the total content of these elements.

[0041] Mg: 0.007% or less Like Ca and REM, Mg is an element that improves the cryogenic toughness of steel sheets by controlling the morphology of inclusions in the steel. However, if the Mg content becomes excessively high, it impairs the cleanliness of the steel. Therefore, when Mg is included, the Mg content should be 0.007% or less. Preferably, the Mg content should be 0.004% or less. On the other hand, there is no particular lower limit to the Mg content. When Mg is included to obtain the above effects, it is preferable that the Mg content be 0.001% or more.

[0042] [Microorganisms] The steel sheet of the present invention has a microstructure at a depth of 1 / 4 of the thickness direction from the surface of the steel sheet, in which the number density of localized Ni-enriched regions, where the average Ni concentration is 10% by mass or more and the equivalent circle diameter is 500 nm or less, is 5 × 10⁻¹⁰ 5 pieces / mm 2 Furthermore, the number density of localized Ni-enriched regions where the average Ni concentration is between 8% and 12% by mass and the equivalent circle diameter exceeds 500 nm is 5 × 10⁻¹⁰ 4 pieces / mm 2 The following conditions apply, and the austenite content is assumed to be between 1% and 15% by volume.

[0043] Number density of localized Ni-enriched regions where the average Ni concentration is 10% by mass or more and the equivalent circle diameter is 500 nm or less: 5 × 10 5 pieces / mm 2 That's all. The "localized Ni-enriched region where the average Ni concentration is 10% by mass or more and the equivalent circle diameter is 500 nm or less" refers to the localized Ni-enriched region extracted under the measurement conditions described in the examples below. Specifically, a test piece is taken so that the observation position is 1 / 4 of the thickness of the steel plate from the surface (hereinafter sometimes referred to as the "position of (1 / 4)t of the plate thickness," where "t" is the plate thickness), polished to remove polishing distortion, and prepared as a microstructure test piece. A surface analysis is performed on this microstructure observation test piece using energy-dispersive X-ray spectroscopy with a scanning electron microscope to create a Ni concentration distribution, and the Ni-enriched region and average Ni concentration are extracted from this Ni concentration distribution using the image processing software described below. Subsequently, the area of ​​each Ni-enriched region where the average Ni concentration is 10% by mass or more is converted to an equivalent circle diameter, and the locations where the equivalent circle diameter is 500 nm or less are the localized Ni-enriched regions described above. Furthermore, by dividing the number of locations where the equivalent circle diameter obtained is 500 nm or less by the measurement area, the number density of the localized Ni-enriched regions can be determined.

[0044] The number density of localized Ni-enriched regions, where the average Ni concentration is 10% by mass or more and the equivalent circle diameter is 500 nm or less, is 5 × 10⁻⁶. 5 pieces / mm 2 By doing so, a predetermined amount of stable γ is present in the steel plate, and good absorption energy can be obtained. The number density of this local Ni-enriched region is preferably 6 × 10⁻⁶ 5 pieces / mm 2 This concludes the explanation. There is no particular upper limit to the number density of this localized Ni-enriched region. From the viewpoint of manufacturing costs, the number density of this localized Ni-enriched region is preferably 8 × 10⁻⁶. 6 pieces / mm 2 The following is more preferable: 6 × 10 6 pieces / mm 2 The following applies. Furthermore, the equivalent diameter of the circle is preferably 40 nm or larger.

[0045] Number density of localized Ni-enriched regions where the average Ni concentration is between 8% and 12% by mass and the equivalent circle diameter exceeds 500 nm: 5 × 10 4 pieces / mm 2 below The "localized Ni-enriched regions where the average Ni concentration is 8% by mass or more and 12% by mass or less, and the equivalent circle diameter exceeds 500 nm" refers to the localized Ni-enriched regions extracted under the measurement conditions described in the examples below. Specifically, as described above, the Ni-enriched regions and average Ni concentration are extracted from the Ni concentration distribution of a microstructure test specimen prepared so that the observation position is at (1 / 4)t of the plate thickness. Then, the area of ​​each Ni-enriched region where the average Ni concentration is 8-12% by mass is converted to an equivalent circle diameter, and the locations where the equivalent circle diameter exceeds 500 nm are the localized Ni-enriched regions described above. The number density of the localized Ni-enriched regions can be determined by dividing the number of locations where the equivalent circle diameter exceeds 500 nm by the measurement area.

[0046] The number density of localized Ni-enriched regions where the average Ni concentration is between 8% and 12% by mass and the equivalent circle diameter is greater than 500 nm is 5 × 10⁻¹⁰ 4 pieces / mm 2 By doing the following, the amount of unstable γ that causes the occurrence of micro-brittle fractures is sufficiently reduced, and as a result, the occurrence of micro-brittle fractures can be suppressed. The number density of this localized Ni-enriched region is preferably 2 × 10⁻⁶ 4 pieces / mm 2 The following applies. Since it is desirable to have fewer localized Ni-enriched areas, the number density of these localized Ni-enriched areas should be 0 particles / mm³. 2 That is also acceptable.

[0047] Austenite volume fraction: 1-15% If the volume fraction of austenite is less than 1%, the toughness improvement effect due to stable austenite cannot be obtained. On the other hand, if the volume fraction of austenite exceeds 15%, it becomes difficult to obtain stable austenite, and as a result, minute brittle fracture occurs. Therefore, the volume fraction of austenite should be between 1% and 15%. Preferably, the volume fraction of austenite should be 2% or more, and preferably 10% or less.

[0048] The steel sheet of the present invention can be made to have the above-described microstructure, thereby obtaining the mechanical properties described later.

[0049] Furthermore, for the purpose of further improving the mechanical properties, the microstructure of the steel sheet of the present invention preferably has an austenite volume fraction of 1 to 15% at a position (1 / 4)t of the sheet thickness, with the remainder being bainite and martensite.

[0050] Total area ratio of bainite and martensite: 85% or more (favorable conditions) A microstructure primarily composed of bainite and martensite allows for excellent cryogenic toughness through the effects of fine-grained structure, self-tempering, and tempering, while also easily achieving sufficient strength. The above-mentioned "microstructure primarily composed of bainite and martensite" refers to a microstructure where the total area ratio of bainite and martensite at (1 / 4)t of the plate thickness is 85% or more of the entire microstructure. The ratio of bainite to martensite is not particularly important as long as it falls within the above total area ratio range.

[0051] The total area ratio of bainite and martensite is preferably 99% or less. More preferably, the total area ratio of bainite and martensite is 90% or more, and more preferably 98% or less.

[0052] Each of the above microstructures can be obtained by controlling the conditions of the first accelerated cooling, two-phase heating, second accelerated cooling, and tempering described later. Furthermore, the area percentages of the above-mentioned bainite, martensite, and austenite microstructures can be measured by the method described in the examples below.

[0053] [plate thickness] The thickness of the steel plate of the present invention is not particularly limited and can be any thickness. From the viewpoint of applying the steel plate of the present invention to the steel material of the above-mentioned steel structure, it is preferable that the plate thickness be 6 mm or more and 50 mm or less. More preferably, the plate thickness is 10 mm or more. In this invention, "thick steel plate" refers to a steel plate with a plate thickness of 6 mm or more.

[0054] [Mechanical properties] The steel sheet of the present invention having the component composition and microstructure described above has the following mechanical properties.

[0055] (Tensile strength) The tensile strength (TS) of the steel plate shall be 690 MPa or higher. Preferably, the tensile strength shall be 720 MPa or higher. This is because the plate thickness can be reduced when the steel plate is applied to the tank, thereby reducing the weight of the steel material. On the other hand, the upper limit of the tensile strength is not particularly limited and can be any value, but it is preferably 930 MPa or lower, and more preferably 900 MPa or lower. In this invention, "high strength" refers to a steel plate with a TS of 690 MPa or higher.

[0056] The tensile strength can be measured by the method described in the examples below.

[0057] (Cryogenic toughness) The toughness of a steel plate is determined by the Charpy absorption energy (vE) at -196°C. -196℃ ) preferably has a full-size Charpy impact force of 200 J or more. -196℃ The temperature is more preferably 205J or higher, and even more preferably 240J or higher. -196℃ It may be 350J or less, or 280J or less. In addition, in the half-size Charpy impact test, vE -196℃ It is preferable that it is 100J or more. -196℃ The temperature is more preferably 102J or higher, and even more preferably 120J or higher. -196℃ This may be less than 200J, or 150J or less.

[0058] Furthermore, it is preferable that an instrumented Charpy impact test is conducted and that the resulting load-displacement curve does not show a sudden drop in load, which would indicate the initiation of unstable cracks.

[0059] In other words, "excellent cryogenic toughness" in this invention means that in an instrumented Charpy impact test, the Charpy absorbed energy (vE) at full size is excellent. -196℃ This refers to a force of 200 J or more (100 J or more for half-size tests) and the absence of unstable crack formation. The instrumented Charpy impact test described above can be performed using the method described in the examples below.

[0060] <Method of manufacturing steel plates> Next, an embodiment of the steel sheet manufacturing method of the present invention will be described.

[0061] In the following description of the manufacturing method, unless otherwise specified, temperature refers to the temperature at a point (1 / 4)t of the plate thickness in the steel material and steel plate. The temperature at a point (1 / 4)t of the plate thickness can be determined, for example, by heat transfer calculations from the surface temperature of the steel plate measured with a radiation thermometer.

[0062] The steel sheet of the present invention can be manufactured by sequentially performing a heating step, a hot rolling step, a first accelerated cooling step, a two-phase heating step, a second accelerated cooling step, and a tempering step under the specific conditions described below. A cooling step may also be provided after the tempering step.

[0063] [Heating process for steel materials] First, in this heating step, the steel material having the above-mentioned component composition is heated to a heating temperature of 900°C to 1200°C. The method of manufacturing the steel material is not particularly limited. For example, the steel material can be manufactured by melting molten steel having the above-mentioned component composition and then casting it. Melting can be done by any method, such as a converter, electric furnace, or induction furnace. Furthermore, casting is preferably done by continuous casting from the viewpoint of productivity, but it can also be done by ingot-breakdown rolling. As the steel material, for example, a steel slab can be used.

[0064] Furthermore, the heating of the steel material may be carried out after the steel material obtained by methods such as casting has been cooled, or the obtained steel material may be subjected to heating directly without cooling.

[0065] If the heating temperature of the steel material is below 900°C, the deformation resistance of the steel material is high, which increases the load on the rolling mill during subsequent hot rolling, making hot rolling difficult. Therefore, the heating temperature of the steel material should be 900°C or higher. Preferably, the heating temperature should be 1000°C or higher. On the other hand, if the heating temperature of the steel material is higher than 1200°C, oxidation of the steel becomes significant, and the loss due to the removal of the oxide film due to oxidation increases, resulting in a decrease in yield. Therefore, the heating temperature of the steel material should be 1200°C or lower.

[0066] [Hot rolling process] After the above heating process is completed, a hot rolling process is carried out. In the hot rolling process, the heated steel material is hot-rolled to form a hot-rolled steel sheet. The final thickness of the hot-rolled steel sheet is not particularly limited, but as described above, it is preferably 6 mm to 50 mm. The hot rolling conditions should be set appropriately so that the final thickness falls within this range.

[0067] [First accelerated cooling process] After the completion of the hot rolling process described above, a first accelerated cooling process is performed. In the first accelerated cooling process, the hot-rolled steel sheet is accelerated cooling (hereinafter referred to as "first accelerated cooling"). Specifically, the average cooling rate in the temperature range of 300°C or below 550°C at a temperature at (1 / 4)t of the sheet thickness is set to 1.0°C / second or higher, and the cooling stop temperature is set to 300°C or lower at a temperature at a temperature at (1 / 4)t of the sheet thickness. By performing the first accelerated cooling under these conditions, no unstable γ remains after cooling, and a localized Ni-enriched region of 5 × 10⁻¹⁶ is formed where the average Ni concentration is between 8% by mass and 12% by mass and the equivalent circle diameter exceeds 500 nm. 4 pieces / mm 2 The desired organization can be obtained as follows.

[0068] Average cooling rate in the temperature range of 300°C to 550°C: 1.0°C / second or higher In the first accelerated cooling process, if the average cooling rate in the above temperature range is less than 1.0°C / second at the temperature at (1 / 4)t of the plate thickness, unstable austenite is formed at this point. This makes it easier for localized Ni-enriched regions to form where the average Ni concentration is between 8% and 12% by mass and the equivalent circle diameter exceeds 500 nm. As a result, unstable γ is more likely to remain in the steel, and consequently, the cryogenic toughness tends to decrease. The average cooling rate is preferably 5.0°C / second or higher.

[0069] The upper limit of the average cooling rate is not particularly limited. If the average cooling rate is higher than 200°C / second, temperature control at each position within the steel sheet becomes difficult, and variations in material properties tend to occur in the width direction and rolling direction. As a result, variations in material properties such as tensile strength and cryogenic toughness tend to occur. For this reason, it is preferable that the average cooling rate be 200°C / second or less. More preferably, the average cooling rate is 100°C / second or less, and even more preferably 60°C / second or less.

[0070] Cooling stop temperature: 300℃ or less In the first accelerated cooling process, if the cooling stop temperature is higher than 300°C at a point (1 / 4)t of the plate thickness, the phase transformation will not be fully completed during accelerated cooling. As a result, γ is easily generated even in an unstable state, and the localized Ni-enriched region where the average Ni concentration is between 8% and 12% by mass and the equivalent circle diameter exceeds 500 nm is 5 × 10⁻⁶. 4 pieces / mm 2 This is likely to exceed the limit. As a result, minute brittle fractures are more likely to occur. The cooling stop temperature is preferably 250°C or lower, and more preferably 200°C or lower. There is no specific lower limit for the cooling stop temperature, but it is preferably 50°C or higher.

[0071] The first accelerated cooling method is not particularly limited and can be performed by any method. For example, air cooling and / or water cooling can be used. As for water cooling, any cooling method using water can be used, such as spray cooling, mist cooling, laminar cooling, etc.

[0072] [Two-phase region heating process] After the completion of the first accelerated cooling process described above, a two-phase heating process is performed. In the two-phase heating process, the hot-rolled steel sheet that has undergone the first accelerated cooling after hot rolling is subjected to two-phase heating. Specifically, the hot-rolled steel sheet that has undergone the first accelerated cooling is heated under the following conditions: the average heating rate in the temperature range between Ac1 point and 650°C at a temperature at (1 / 4)t of the sheet thickness is 1.0°C / second or less; the heating temperature (hereinafter referred to as the "heating and holding temperature") is in the two-phase temperature range between 650°C and below Ac3 point; and the holding time in the said two-phase temperature range is 15 minutes or more. By performing this two-phase heating, a part of the structure of the hot-rolled steel sheet undergoes a reverse transformation from bainite and / or martensite to austenite, forming a Ni-enriched region. Subsequently, it is cooled back to martensite. After this, the desired structure is obtained by performing the tempering treatment described later. Furthermore, in the two-phase heating process, in order to effectively obtain the desired effects, in addition to the above conditions, the average heating rate in the temperature range above 650°C and below the heating and holding temperature may be controlled.

[0073] Average heating rate in the temperature range between Ac1 point and 650°C: 1.0°C / second or less If the average heating rate in the temperature range from Ac1 point to 650°C at a temperature at (1 / 4)t of the plate thickness exceeds 1.0°C / second, Ni cannot diffuse sufficiently, and the number density of localized Ni-enriched regions where the average Ni concentration is 10% by mass or more and the equivalent circle diameter is 500 nm or less is 5 × 10⁻¹⁰ 5 pieces / mm 2 It is not possible to exceed the above. As a result, it is not possible to secure a sufficient number density of fine stable γ, making it difficult to secure excellent cryogenic toughness. The average heating rate is preferably 0.8°C / second or less. There is no particular lower limit for the average heating rate in the above temperature range. From the viewpoint of manufacturing efficiency, the average heating rate is preferably 0.2°C / second or more.

[0074] Average heating rate in the temperature range above 650°C and below the heating and holding temperature: 0.10°C / second or less (preferred conditions) If the average heating rate in the temperature range above 650°C and below the heating and holding temperature at a position (1 / 4)t of the plate thickness exceeds 0.10°C / second, Ni will diffuse, and there will not be enough time for Ni-enriched regions to form. Ultimately, the number density of local Ni-enriched regions, where the average Ni concentration is 10% by mass or more and the equivalent circle diameter is 500 nm or less, will be 5 × 10⁻⁶. 5 pieces / mm 2 In some cases, it may not be possible to achieve the above. As a result, it may not be possible to secure a sufficient number density of fine, stable γ, making it difficult to ensure excellent cryogenic toughness. For this reason, the average heating rate is preferably 0.10°C / second or less, and more preferably 0.08°C / second or less. There is no particular lower limit for the average heating rate in the above temperature range. From the viewpoint of manufacturing efficiency, the average heating rate is preferably 0.01°C / second or more.

[0075] Heating holding temperature: 650℃ or more, less than Ac 3 points If the heating and holding temperature at a point (1 / 4)t of the plate thickness is less than 650°C, the Ni-enriched region will remain as austenite after the subsequent second accelerated cooling process, and further localized Ni enrichment cannot be expected in the subsequent tempering process. As a result, a large localized Ni-enriched region is formed in the final steel plate, which makes it prone to fine brittle fracture and makes it difficult to obtain the desired cryogenic toughness. For these reasons, the heating and holding temperature should be 650°C or higher. Preferably, the heating and holding temperature should be 660°C or higher.

[0076] On the other hand, if the heating and holding temperature is above the Ac3 point at a temperature at (1 / 4)t of the plate thickness, almost all of the bainite and martensite undergo reverse transformation, resulting in localized Ni-enriched regions with an average Ni concentration of 10% by mass or more and an equivalent circle diameter of 500 nm or less, with a number density of 5 × 10⁻¹⁶ 5 pieces / mm 2 As a result, it is difficult to form the above. Consequently, it is difficult to secure stable residual γ, and consequently, it is difficult to secure excellent cryogenic toughness. For these reasons, the heating and holding temperature should be below the Ac3 point. Preferably, the heating and holding temperature should be (Ac3 point - 20°C) or lower.

[0077] The above two-phase heating process can use any heating method as long as the heating and holding temperature can be controlled as described above. One example of a heating method is furnace heating. This furnace heating is not particularly limited, and a general heat treatment furnace can be used.

[0078] The Ac1 point (Ac1 transformation point) and the Ac3 point (Ac3 transformation point) can be determined by the following equations (1) and (2), respectively. Ac1 point (°C) = 750.8 - 26.6 × C + 17.6 × Si - 11.6 × Mn - 22.9 × Cu - 23 × Ni + 24.1 × Cr + 22.5 × Mo - 39.7 × V - 5.7 × Ti + 232.4 × Nb - 169.4 × Al …(1) Ac3(℃)=937.2 - 436.5×C + 56×Si - 19.7×Mn - 16.3×Cu - 26.6×Ni - 4.9×Cr + 38.1×Mo+ 124.8×V + 136.3×Ti - 19.1×Nb + 198.4×Al …(2) The element symbols in equations (1) and (2) represent the mass percentage of each element contained in the steel. If an element is not present, its content in the steel is set to 0.

[0079] Holding time: 15 minutes or more After reaching the heating and holding temperature within the two-phase temperature range described above, the material is held at that temperature for 15 minutes or more. If the holding time at that heating and holding temperature (sensing time) is less than 15 minutes, Ni diffusion will be insufficient, resulting in localized Ni-enriched regions with an average Ni concentration of 10% by mass or more and an equivalent circle diameter of 500 nm or less, with a number density of 5 × 10⁶ 5 pieces / mm 2As described above, it is difficult to form. In particular, there is a risk that the Ni concentration in the γ structure in the steel sheet cannot be sufficiently increased. As a result, it is difficult to secure a sufficient number density of fine, stable γ particles, and it is difficult to secure excellent cryogenic toughness. The holding time at the heating and holding temperature is preferably 20 minutes or more. There is no particular upper limit specified for the holding time at the heating and holding temperature, but from the viewpoint of manufacturing costs, it is preferable to set the holding time at the heating and holding temperature to 120 minutes or less, and more preferably to 60 minutes or less.

[0080] [Second accelerated cooling process] After the completion of the above two-phase heating process, a second accelerated cooling process is performed. In the second accelerated cooling process, the hot-rolled steel sheet, which has been heated and held in the two-phase region, is accelerated cooling (hereinafter referred to as "second accelerated cooling"). Specifically, the second accelerated cooling is performed under the conditions that the average cooling rate in the temperature range of 300°C or less and 600°C or less at the temperature at the (1 / 4)t position of the sheet thickness is 1.0°C / second or more, and the cooling stop temperature is 300°C or less at the temperature at the (1 / 4)t position of the sheet thickness. By performing the second accelerated cooling under these conditions, no unstable γ remains after cooling, and a localized Ni-enriched region of 5 × 10⁻¹⁶ is formed where the average Ni concentration is 8% by mass or more and 12% by mass or less and the equivalent circle diameter exceeds 500 nm. 4 A desired organization with fewer than or equal to 1 can be obtained.

[0081] Average cooling rate in the temperature range of 300°C to 600°C: 1.0°C / second or higher In the second accelerated cooling process, if the average cooling rate in the above temperature range is less than 1.0°C / second at the temperature at (1 / 4)t of the plate thickness, unstable austenite is formed at this point, making it easy for Ni-enriched regions to remain in the steel as γ. As a result, further Ni enrichment cannot be expected during tempering in the subsequent process. Consequently, localized Ni-enriched regions with an average Ni concentration of 8% to 12% and an equivalent circle diameter exceeding 500 nm are likely to form, reducing the cryogenic toughness of the steel plate. Therefore, the average cooling rate should be 1.0°C / second or higher. Preferably, the average cooling rate should be 5.0°C / second or higher. There is no particular upper limit to the average cooling rate. If the average cooling rate is higher than 200°C / second, temperature control at each position within the steel plate becomes difficult, making stable manufacturing difficult. Therefore, the average cooling rate is preferably 200°C / second or lower, and more preferably 100°C / second or lower.

[0082] Here, the average cooling rate of the second accelerated cooling described above refers to the average rate at which the temperature decreases per unit time in the temperature range of 300°C or below 600°C.

[0083] Cooling stop temperature: 300℃ or less In the second accelerated cooling process, if the cooling stop temperature exceeds 300°C at the temperature at (1 / 4)t of the plate thickness, unstable γ tends to remain, making it difficult to reduce the rate of decrease in the amount of residual γ. As a result, cryogenic toughness tends to decrease. Therefore, the cooling stop temperature should be 300°C or lower. Preferably, the cooling stop temperature should be 250°C or lower. There is no specific lower limit for the cooling stop temperature, but it is preferable to set it at 50°C or higher.

[0084] The cooling method for the second accelerated cooling is not particularly limited and can be any cooling method, as described above for the first accelerated cooling.

[0085] [Tempering process] After the completion of the second accelerated cooling process described above, a tempering process is performed. In the tempering process, the hot-rolled steel sheet that has undergone the second accelerated cooling after heating in the two-phase region is tempered. Specifically, the tempering is performed under conditions where the tempering temperature is 550°C or higher (Ac1 point + 30°C) or lower, and the holding time is 10 minutes or longer.

[0086] Tempering temperature: 550℃ or more (Ac1 point + 30℃) or less If the tempering temperature is below 550°C at the (1 / 4)t position of the plate thickness, reverse transformation to γ ​​does not occur during tempering, and Ni enrichment is also unlikely to occur. As a result, cryogenic toughness tends to decrease. On the other hand, if the tempering temperature exceeds (Ac1 point + 30°C), the strength decreases. Also, the retention of unstable γ tends to reduce cryogenic toughness. For these reasons, the tempering temperature should be between 550°C and (Ac1 point + 30°C). Preferably, the tempering temperature should be 570°C or higher, and preferably below Ac1 point.

[0087] For heating the hot-rolled steel sheet in this tempering process, any heating method can be used as long as the heating temperature can be controlled to the tempering temperature. As an example of a heating method, furnace heating can be used, similar to the method described in the two-phase heating process above.

[0088] Holding time: 10 minutes or more After reaching the tempering temperature, the material is held at this temperature for at least 10 minutes. The holding time is preferably 20 minutes or more, and preferably 60 minutes or less.

[0089] [Cooling process] (Optional process) As described above, a cooling step may be provided after the tempering process. If this cooling step is provided, the hot-rolled steel sheet after tempering can be cooled under any conditions. The cooling method is not particularly limited, but from the viewpoint of ease of work during manufacturing and cost, air cooling is preferred.

[0090] Through the above process, the steel sheet of the present invention can be obtained. The steel sheet of the present invention, having the above-described microstructure and properties, can stably ensure excellent cryogenic toughness over a wide range of plate thicknesses. Therefore, the steel sheet of the present invention can be suitably used as a steel material for structures used in cryogenic environments, such as liquefied gas storage tanks for ships and on land. [Examples]

[0091] The present invention will be described in detail below based on examples. However, the present invention is not limited to these embodiments.

[0092] In this example, steel plates were manufactured according to the procedure described below, and the properties of the obtained steel plates were evaluated.

[0093] First, molten steel having the component composition shown in Table 1 was melted in a converter, and steel slabs (thickness: 200 mm), which are the steel material, were manufactured by continuous casting. Note that "0%" in Table 1 indicates that elements were not intentionally added, and includes not only cases where elements are not present, but also cases where elements are inevitably present. In addition, the Ac1 point (°C) obtained using the above formula (1) and the Ac3 point (°C) obtained using the above formula (2) are also shown in Table 1.

[0094] [Table 1]

[0095] Next, the obtained steel slabs were heated and hot-rolled according to the manufacturing conditions for each step shown in Table 2 to obtain hot-rolled steel sheets of the thicknesses shown in Table 2. Subsequently, the hot-rolled steel sheets were subjected to heat treatment including first accelerated cooling, two-phase heating, second accelerated cooling, and tempering. The thicknesses in Table 2 are the final thicknesses. In all examples, air cooling was performed after tempering to obtain steel sheets with various thicknesses from 10 mm to 50 mm. A heat treatment furnace was used for heating in each step. Furthermore, "-" in Table 2 indicates that the procedure was not performed. Also, for No. 7 in Table 2, "Average heating rate in the temperature range of Ac1 point to 650°C at (1 / 4)t" in the two-phase heating process refers to "Average heating rate in the temperature range of Ac1 point to 640°C," and for No. 41, "Average heating rate in the temperature range of Ac1 point to 650°C at (1 / 4)t" in the two-phase heating process refers to "Average heating rate in the temperature range of Ac1 point to 630°C."

[0096] [Table 2]

[0097] For each of the obtained steel sheets, (1) microstructure, (2) tensile strength (TS), and (3) cryogenic toughness were evaluated according to the following methods. The evaluation results are shown in Table 3.

[0098] (1) Microorganisms A specimen for microstructural observation was taken from each steel plate at a position 1 / 4 of the thickness direction from the surface (i.e., at the position of (1 / 4)t of the plate thickness). This specimen was embedded in resin so that the cross-section perpendicular to the rolling direction became the observation surface, and then mirror-polished. Next, nital etching was performed, and then the specimen was observed with a scanning electron microscope (SEM) at magnifications of 2000x and 10000x to capture images of the microstructure of the steel plate. The microstructure was identified from the obtained images. In this steel, the microstructure of the region excluding the island-like distribution of tissue is defined as the remaining tissue described above (specifically, bainite and martensite), and this includes carbides.

[0099] Furthermore, the number density of each of the local Ni-enriched regions described above was measured as follows. [Measurement of local Ni concentration size (i.e., equivalent circle diameter), average Ni concentration, and number density] From each steel plate, a specimen for microstructural observation was taken so that the observation position was at the plate thickness (1 / 4)t. Next, the observation surface of the specimen was roughly polished using wet polishing paper with waterproof abrasive paper, and then the observation surface was polished to a mirror finish using diamond paste. Then, to remove the polishing distortion introduced by the above polishing, a final finish was performed using colloidal silica. Surface analysis of the obtained specimens was performed using energy dispersive X-ray spectroscopy (EDS) with a scanning electron microscope (SEM) to measure the Ni distribution. The observation magnification was 10,000x, and the SEM acceleration voltage was 5kV. Measurements were taken at five locations. From the obtained EDS surface analysis results, a Ni concentration distribution was created, and then the Ni-enriched region, average Ni concentration, and equivalent circle diameter were determined using image processing software (Avizo ver2023.1, Thermo Fisher Scientific). Next, from the above process, the number of localized Ni-enriched regions where the average Ni concentration is 10% by mass or more and the equivalent circle diameter is 500 nm or less, and the number of localized Ni-enriched regions where the average Ni concentration is 8% by mass or more and 12% by mass or less and the equivalent circle diameter is greater than 500 nm were determined, and the number density of each was calculated by dividing these numbers by the measurement area. Here, the average value of the number densities from five locations was used.

[0100] Furthermore, the amount of gamma was measured as follows. [γ amount] To determine the γ content of each steel plate, five X-ray diffraction specimens were taken, with a size of 1 mm (thickness direction) × 20 mm (rolling direction) × 20 mm (width direction), so that the observation position (i.e., measurement surface) was at the position of the plate thickness (1 / 4)t of each steel plate. Next, each specimen was ground and chemically polished and subjected to X-ray diffraction. The diffraction intensities of the (200) and (211) planes of α-Fe and the (200), (220), and (311) planes of γ-Fe appearing in the symmetric reflection X-ray diffraction pattern were determined, the volume fraction of γ-Fe was calculated, and the average value of the five specimens was taken as the γ content (unit: volume fraction).

[0101] (2) Tensile strength A JIS No. 4 tensile test specimen was taken from the (1 / 4) t position of the steel plate thickness, such that the longitudinal direction of the test specimen coincided with the width direction of the plate. Using this tensile test specimen, a tensile test was performed in accordance with the provisions of JIS Z2241 (2022) to evaluate the tensile strength (TS) of the steel plate.

[0102] (3) Cryogenic toughness In the evaluation of toughness at extremely low temperatures in this invention, the Charpy absorption energy (vE) at -196°C is used. -196℃ The load-displacement curve was used. A V-notch test specimen was taken from the position of the steel plate thickness (1 / 4)t in accordance with the provisions of JIS Z 2242 (2018) so that the longitudinal direction of the test specimen coincided with the width direction of the plate. Using this V-notch test specimen, an instrumented Charpy impact test was performed in accordance with the provisions of JIS B 7755 (2011), and the Charpy absorbed energy (vE) at -196℃ was determined. -196℃ The load-displacement curve was determined.

[0103] The Charpy absorption energy was evaluated by taking three samples from three test pieces and performing a total of three measurements, using the average value. The obtained average value is shown in Table 3 as "vE -196℃ This was noted in the document. For full-size cameras, an average value of 200J or higher was considered "pass," and for half-size cameras, an average value of 100J or higher was considered "pass."

[0104] Furthermore, the load-displacement curve was used to evaluate whether or not a sudden load drop occurred in each measurement. If no sudden load drop occurred in any of the measurements (three in this case), the symbol "○" was written in the "Presence or Absence of Sudden Load Drop" column in Table 3. Otherwise, the symbol "×" was written in the same column in Table 3. Here, "Pass" was defined as the absence of a sudden drop in all three measurements that would indicate the onset of unstable crack propagation.

[0105] In this embodiment, for Nos. 2 and 38, which have smaller plate thicknesses, half-size Charpy impact tests were performed using half-size specimens (i.e., sub-size specimens), while for the other examples, full-size Charpy impact tests were performed using full-size specimens.

[0106] [Table 3]

[0107] As shown in Tables 1 to 3, it was confirmed that the steel plates of the inventive examples according to the present invention all have a structure containing 1 to 15% austenite by volume.

[0108] Furthermore, it was confirmed that the steel plates in the examples of the present invention all exhibited high strength, excellent cryogenic toughness, and suppressed the abrupt load drop in the load-displacement curve that would otherwise indicate the initiation of minute brittle cracks. This effect was achieved even with relatively thin steel plates, for example, with a thickness of 6 to 25 mm.

[0109] On the other hand, in comparative examples of steel sheets that fall outside the scope of the present invention, the full-size Charpy absorption energy was lower than 200 J, and / or a sharp drop in load occurred in the load-displacement curve. In other words, in the comparative examples, the cryogenic toughness was reduced due to the occurrence of minute brittle cracks, and the above-mentioned target performance could not be satisfied. [Industrial applicability]

[0110] According to the present invention, steel plates of various thicknesses can exhibit excellent cryogenic toughness without the occurrence of fine brittle cracks, while ensuring high strength.

Claims

1. In mass percent, C: 0.01-0.15%, Si: 0.01 to 0.50%, Mn: 0.05-1.20%, Ni: 6.0-7.5%, Cr: 0.01-1.00%, Mo: 0.05-0.50%, Al: 0.008-0.10%, P: 0.03% or less, S: 0.005% or less, and N: Contains 0.0010 to 0.0080%, The composition consists of Fe and unavoidable impurities. The microstructure at a depth of 1 / 4 of the thickness direction from the surface of the steel plate is: The number density of localized Ni-enriched regions with an average Ni concentration of 10% by mass or more and an equivalent circle diameter of 500 nm or less is 5 × 10⁻¹⁴ ５ pieces / mm ２ Furthermore, the number density of localized Ni-enriched regions with an average Ni concentration of 8% to 12% by mass and an equivalent circle diameter exceeding 500 nm is 5 × 10⁻¹⁴. ４ pieces / mm ２ The following: It contains 1-15% austenite by volume, A steel plate with a tensile strength of 690 MPa or more.

2. The aforementioned component composition is further expressed in mass%, Cu: 0.40% or less, Nb: 0.05% or less, V: 0.05% or less, Ti: 0.03% or less, B: 0.0030% or less, Ca: 0.007% or less, REM: 0.010% or less, Mg: 0.007% or less The steel plate according to claim 1, comprising one or more selected from the group consisting of the following.

3. A method for manufacturing a steel sheet according to claim 1 or 2, A steel material having the above-mentioned component composition is heated to a temperature of 900°C to 1200°C and hot-rolled to produce a hot-rolled steel sheet. Next, a first accelerated cooling is performed at a temperature at a depth of 1 / 4 of the thickness direction from the surface of the steel plate, under cooling conditions where the average cooling rate in the temperature range of 300°C or higher and the cooling stop temperature is 1.0°C / second or higher, and the cooling stop temperature is 300°C or lower. Next, the temperature at a position 1 / 4 of the way down the thickness of the steel plate from the surface is measured by Ac １ The average heating rate in the temperature range between 1.5°C and 650°C is 1.0°C / second or less, and the heating and holding temperature is 650°C or higher. ３ Two-phase heating is performed under the conditions that the temperature is in the two-phase region below 15°C and the holding time in the two-phase region is 15 minutes or more. Next, a second accelerated cooling process is performed at a temperature at a depth of 1 / 4 of the thickness direction from the surface of the steel plate, under cooling conditions where the average cooling rate in the temperature range of 600°C to 300°C is 1.0°C / second or higher, and the cooling stop temperature is 300°C or lower. Next, the temperature at a position 1 / 4 of the way down the thickness direction from the surface of the steel plate, where the tempering temperature is 550°C or higher (Ac １ A method for manufacturing steel plates, comprising tempering at a temperature of +30°C or lower and holding time of 10 minutes or more.

4. The method for manufacturing a steel sheet according to claim 3, wherein the two-phase heating further comprises an average heating rate of 0.10°C / second or less in the temperature range of 650°C or higher and below the heating and holding temperature.

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