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 a steel plate and a method for manufacturing the same, and more particularly to a thick steel plate for cryogenic use having stable and excellent cryogenic toughness over a wide plate thickness range and a method for manufacturing the same.

Background Art

[0002] Steel plates applied to structures such as tanks for storing liquefied gas are required to be excellent not only in the strength of the steel plate but also in toughness at cryogenic temperatures (hereinafter sometimes referred to as "cryogenic toughness") because the use environment is cryogenic. For example, when a hot-rolled steel plate is used for a tank for storing liquefied natural gas, it is necessary to ensure excellent toughness under cryogenic conditions of -164°C or lower, which is the boiling point of liquefied natural gas. If the cryogenic toughness of the steel material is poor, there is a risk that the safety of the cryogenic storage structure cannot be maintained, so the demand for improving the cryogenic toughness of the applied steel plate is high.

[0003] In the case of a ship fuel tank application where the volume of the tank is relatively small, a steel material with a relatively small plate thickness among thick steel plates is required. On the other hand, in the case of an onshore application where the volume of the tank is relatively large, a steel material with a larger plate thickness is required. In response to these requirements, the application of 7% Ni steel plates is being promoted.

[0004] As 7% Ni steel plates, for example, the steel plates described in Patent Documents 1 and 2 have been proposed.

[0005] Patent Document 1 discloses a thick steel plate for cryogenic use containing Ni: more than 5.0% to less than 10.0% by mass and a predetermined amount of C, Si, Mn, and Al. The thick steel plate of Patent Document 1 has an average value of the V-notch Charpy absorption energy vE-196 per unit area of 1.25 J / mm 2 or more over a plate thickness of 6 to 50 mm.

[0006] In addition, Patent Document 2 discloses a low-temperature Ni-containing steel containing 7.0 to 10.5% by mass of Ni and a predetermined amount of C, Si, Mn, and Al. The steel of 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 intensive investigations by the present inventors on thick steel plates of 7% Ni steel, it has been newly found that even when the absorbed energy of the steel plate is high, minute brittle fractures 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 fractures occur.

[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. Conversely, 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, regarding extremely low-temperature toughness, only the average value of the absorbed energy is considered, and no consideration is given to the above problems, specifically, the occurrence of minute brittle fractures.

[0011] The present invention has been made in view of such problems, and an object thereof is to provide a steel plate that has excellent toughness at extremely low temperatures regardless of the thickness of the thick steel plate while ensuring high strength, that is, a steel plate having high absorbed energy in the steel plate and not generating minute brittle fractures, and a method for manufacturing the same.

Means for Solving the Problems

[0012] In order to solve the above problems, the present inventors have intensively studied the component composition, microstructure, and manufacturing conditions of 7% Ni steel plates and obtained the following findings.

[0013] 1) In order to increase the absorbed energy in the steel plate, it is important that the number density of local Ni enrichment regions having an average Ni concentration of 10 mass% or more and a circle-equivalent diameter of 500 nm or less is 5×10 5 pieces / mm 2 or more. Further, in order to obtain this microstructure, the Ni content is 6.0% or more, the average heating rate in the temperature range of Ac1 or higher and 650°C or lower in the two-phase region heating process is 1.0°C / second or less, the heating temperature (hereinafter, also referred to as "heating holding temperature") is in the two-phase region temperature range of 650°C or higher and less than Ac3, and the holding time in the two-phase region temperature range is 15 minutes or more. In the second accelerated cooling process, the average cooling rate in the temperature range of 600°C or lower and 300°C or higher is 1.0°C / second or more, and the cooling stop temperature is 300°C or lower. Further, in the tempering process, it is important to control the tempering temperature to 550°C or higher and the holding time to 10 minutes or more.

[0014] 2) In order to suppress the occurrence of minute brittle fractures, the number density of local Ni enrichment regions having an average Ni concentration of 8 mass% or more and 12 mass% or less and a circle-equivalent diameter exceeding 500 nm is 5×10 4 pieces / mm 2It is important to satisfy the following conditions. In order to obtain this microstructure, the C content should be 0.15% or less, the average cooling rate in the temperature range of 300°C to 550°C in the first accelerated cooling process should be 1.0°C / second or more, and the cooling stop temperature should be 300°C or less. In the two-phase region heating process, the holding temperature should be 650°C or more. In the second accelerated cooling process, the average cooling rate in the temperature range of 300°C to 600°C should be 1.0°C / second or more, and the cooling stop temperature should be 300°C or less. Also, 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%. 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] By mass, 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%, having a component composition in which the balance consists of Fe and inevitable impurities, The microstructure at a position 1 / 4 of the plate thickness from the surface of the steel sheet is such that the number density of local Ni enrichment regions having an average Ni concentration of 10% by mass or more and an equivalent circle diameter of 500 nm or less is 5×10 5 pieces / mm 2 or more, and the number density of local Ni enrichment regions having an average Ni concentration of 8% by mass or more and 12% by mass or less and an equivalent circle diameter exceeding 500 nm is 5×10 4 pieces / mm 2as follows, containing 1 to 15% by volume of austenite, a steel plate having a tensile strength of 690 MPa or more. [2] The component composition further includes, 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, containing one or more selected from the group consisting of. [3] A method for manufacturing the steel plate according to [1] or [2] above, heating a steel material having the component composition to a temperature of 900 °C or more and 1200 °C or less, and hot rolling to obtain a hot-rolled steel plate, then, at the temperature at a position 1 / 4 of the plate thickness from the steel plate surface in the thickness direction, performing first accelerated cooling under cooling conditions where the average cooling rate in the temperature range of 550 °C or less and 300 °C or more is 1.0 °C / second or more, and the cooling stop temperature is 300 °C or less, then, at the temperature at a position 1 / 4 of the plate thickness from the steel plate surface in the thickness direction, performing two-phase region heating under conditions where the average heating rate in the temperature range of Ac1 point or more and 650 °C or less is 1.0 °C / second or less, the heating holding temperature is in the two-phase region temperature range of 650 °C or more and less than Ac3 point, and the holding time in the two-phase region temperature range is 15 minutes or more, then, at the temperature at a position 1 / 4 of the plate thickness from the steel plate surface in the thickness direction, performing second accelerated cooling under cooling conditions where the average cooling rate in the temperature range of 600 °C or less and 300 °C or more is 1.0 °C / second or more, and the cooling stop temperature is 300 °C or less, then, at the temperature at a position 1 / 4 of the plate thickness from the steel plate surface in the thickness direction, performing tempering under conditions where the tempering temperature is 550 °C or more and (Ac1 point + 30 °C) or less, and the holding time is 10 minutes or more. A method for manufacturing a steel plate. [4] The two-phase region heating further includes the method for manufacturing a steel plate according to [3] above, wherein the average heating rate in the temperature range of 650°C or higher and lower than the holding temperature is 0.10°C / second or less.

Advantages of the Invention

[0017] According to the present invention, it is possible to provide a steel plate having excellent cryogenic toughness regardless of the plate thickness of a thick steel plate while ensuring high strength, and a method for manufacturing the same. Since the steel plate of the present invention has a high absorbed energy within the steel plate and does not generate minute brittle fractures, by using it for steel structures used in cryogenic environments such as liquefied gas storage tanks, etc., the safety of the steel structures can be improved, bringing about an extremely significant effect in industry.

Embodiments for Carrying Out the Invention

[0018] Hereinafter, embodiments of the present invention will be specifically described. Note that the following description shows preferred embodiments of the present invention, and the present invention is not limited thereto.

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

[0020] [Component Composition] The steel plate of the present invention has a predetermined component composition. Also, the steel material used for manufacturing the steel plate of the present invention described later is also assumed to have the said predetermined component composition. Hereinafter, each element included in this component composition will be described. In the present invention, unless otherwise specified, "%" described as the unit of the content of each element means "mass%".

[0021] C: 0.01 to 0.15% C is an element that has the effect of improving the strength of the steel sheet. To obtain this effect, the C content is set to 0.01% or more. The C content is preferably 0.03% or more. On the other hand, when the C content exceeds 0.15%, the number density of local Ni enrichment 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 increases, the amount of unstable γ increases, and the very low temperature toughness of the steel sheet decreases. Therefore, the C content is set to 0.15% or less. The C content is preferably 0.12% or less.

[0022] Si: 0.01 - 0.50% Si is an element that contributes to the improvement of the strength of the steel sheet and also has the function as a deoxidizer. To exhibit these effects, the Si content is set to 0.01% or more. The Si content is preferably 0.02% or more. On the other hand, when the Si content becomes excessively high, the very low temperature toughness decreases. Therefore, the Si content is set to 0.50% or less. The Si content is preferably 0.30% or less.

[0023] Mn: 0.05 - 1.20% Mn is an element that enhances the hardenability of steel and is effective for increasing the strength of the steel sheet. To obtain this effect, the Mn content is set to 0.05% or more. The Mn content is preferably 0.10% or more. On the other hand, when containing Mn exceeding 1.20%, the temper embrittlement susceptibility increases and the very low temperature toughness decreases, so the Mn content is limited to 1.20% or less. The Mn content is preferably less than 1.00%, more preferably 0.60% or less.

[0024] Ni: 6.0 - 7.5% Ni is an element that is extremely effective for improving the very low temperature toughness of the steel sheet. Specifically, when the Ni content is less than 6.0%, the number density of local Ni enrichment regions where the average Ni concentration is 10% by mass or more and the equivalent circle diameter is 500 nm or less, which is necessary to increase the absorbed energy in the steel sheet, is 5×10 5 pieces / mm 2It cannot be the above, and the toughness at extremely low temperatures decreases. Therefore, the Ni content should be 6.0% or more. On the other hand, since Ni is an expensive element, the cost of the steel sheet increases as its content increases. Therefore, in the present invention, the Ni content is set to 7.5% or less. The Ni content is preferably 6.5% or more and preferably 7.4% or less.

[0025] Cr: 0.01 - 1.00% Cr is an element that can improve the strength of the steel sheet without significantly impairing the toughness at extremely low temperatures. To obtain the above effects, the Cr content should be 0.01% or more. The Cr content is preferably 0.30% or more. However, when the Cr content exceeds 1.00%, the toughness at extremely low temperatures of the steel sheet decreases. Therefore, the Cr content is set to 1.00% or less. The Cr content is preferably 0.80% or less.

[0026] Mo: 0.05 - 0.50% Mo, like Cr, is an element that can improve the strength of the steel sheet without significantly impairing the toughness at extremely low temperatures. When the Mo content is less than 0.05%, the desired strength cannot be obtained. Therefore, the Mo content is 0.05% or more. The Mo content is preferably more than 0.10%. On the other hand, when the Mo content exceeds 0.50%, the toughness at extremely low temperatures rather decreases. Therefore, the Mo content is 0.50% or less. The Mo content is preferably 0.30% or less, and more preferably 0.25% or less.

[0027] Al: 0.008 - 0.10% Al is an element contained in the deoxidizer. When the Al content is less than 0.008%, the effect as a deoxidizer is poor. Therefore, the Al content is 0.008% or more. The Al content is preferably 0.010% or more. On the other hand, when the Al content exceeds 0.10%, the cleanliness of the steel is impaired. Therefore, the Al content is 0.10% or less. The Al content is preferably 0.05% or less.

[0028] P: 0.03% or less P is an inevitable impurity and a harmful element that has an adverse effect on the extremely low temperature toughness of the steel plate. For example, when welding the steel plate to form a welded structure, in order to obtain a sound base material and welded joint, it is preferable to reduce the P content as much as possible. Therefore, the P content is suppressed to 0.03% or less. The P content is preferably 0.025% or less. More preferably, it is 0.02% or less. Also, from the perspective of extremely low temperature toughness, the lower the P content, the better, so the lower limit is not particularly limited and may be 0%, but it is allowed to be contained as an inevitable impurity. On the other hand, excessive reduction causes an increase in cost, so from the perspective of cost, the lower limit of the P content is preferably 0.001%. Therefore, the P content is preferably 0.001% or more.

[0029] S: 0.005% or less S forms MnS in the steel and significantly deteriorates the extremely low temperature toughness. Therefore, the S content is set with 0.005% as the upper limit, and it is desirable to reduce it as much as possible. Thus, the S content is 0.005% or less. The S content is preferably 0.002% or less. On the other hand, the lower the S content, the better, so the lower limit of the S content is not particularly limited and may be 0%, but it is allowed to be contained as an inevitable impurity. On the other hand, excessive reduction causes an increase in cost, so from the perspective of cost, it is preferable to set the S content to 0.0003% or more.

[0030] N: 0.0010 - 0.0080% N forms precipitates in the steel. When the N content exceeds 0.0080%, it causes a decrease in the extremely low temperature toughness of the base material. However, N is also an element that contributes to the grain refinement of the base material by forming AlN, and such an effect can be obtained by setting the N content to 0.0010% or more. Therefore, the N content is 0.0010% or more and 0.0080% or less. The N content is preferably 0.0020% or more and preferably 0.0060% or less.

[0031] The basic component composition of the steel plate of the present invention, in addition to the above-mentioned predetermined amounts of elements, is such that the balance consists of Fe and inevitable impurities.

[0032] With this basic component composition, the steel sheet of the present invention can obtain the desired properties. In the present invention, for the purpose of further improving the properties, in addition to this basic component composition, optionally, one or more selected from the group consisting of Cu, Nb, V, Ti, B, Ca, REM, and Mg can be contained. In addition, since each component of Cu, Nb, V, Ti, B, Ca, REM, and Mg can be contained as necessary, these components may be 0%.

[0033] Cu: 0.40% or less Cu is an element having the effect of increasing the strength of the steel sheet by improving hardenability. However, when the Cu content exceeds 0.40%, in addition to the decrease in the toughness of the steel sheet at extremely low temperatures, the surface properties of the steel (slab) after casting deteriorate. Therefore, when Cu is contained, the Cu content is set to 0.40% or less. The Cu content is preferably 0.30% or less. On the other hand, the lower limit of the Cu content is not particularly limited. When Cu is contained to obtain the above effects, the Cu content is preferably 0.10% or more.

[0034] Nb: 0.05% or less Nb is an effective element for increasing the strength of the steel sheet by precipitation strengthening. However, when the Nb content becomes excessively high, the toughness of the steel sheet at extremely low temperatures decreases. Therefore, when Nb is contained, the Nb content is set to 0.05% or less. The Nb content is preferably 0.03% or less. On the other hand, the lower limit of the Nb content is not particularly limited. When Nb is contained to obtain the above effects, the Nb content is preferably 0.010% or more.

[0035] V: 0.05% or less V is, like Nb, an effective element for increasing the strength of the steel sheet by precipitation strengthening. However, when the V content becomes excessively high, the toughness of the steel sheet at extremely low temperatures decreases. Therefore, when V is contained, the V content is set to 0.05% or less. The V content is preferably 0.04% or less. On the other hand, the lower limit of the V content is not particularly limited. When V is contained to obtain the above effects, the V content is preferably 0.010% or more.

[0036] Ti: Below 0.03% Ti is an element that has the effect of enhancing the extremely low temperature toughness of the welded part without degrading the mechanical properties of the base material when welding a steel plate to form a welded structure. Therefore, optionally, Ti can be contained in the range of 0.03% or less. For this reason, when Ti is contained, the Ti content is set to 0.03% or less. The Ti content is preferably 0.02% or less. Incidentally, the Ti content is preferably 0.01% or more.

[0037] B: 0.0030% or less B is an element that enhances the hardenability of the steel plate with a trace addition. In order to effectively exert this effect, optionally, B can be contained in the range of 0.0030% or less. On the other hand, when the B content exceeds 0.0030%, the extremely low temperature toughness deteriorates. Therefore, when B is contained, the B content is set to 0.0030% or less. The B content is preferably 0.0020% or less. Incidentally, the B content is preferably 0.0003% or more.

[0038] Ca: 0.007% or less Ca is an element that has the effect of improving the extremely low temperature toughness of the steel plate by controlling the form of inclusions in the steel. However, when the Ca content becomes excessively high, the cleanliness of the steel is impaired. Therefore, when Ca is contained, the Ca content is set to 0.007% or less. The Ca content is preferably 0.004% or less. On the other hand, the lower limit of the Ca content is not particularly limited. When Ca is contained to obtain the above effect, it is preferable to set the Ca content to 0.001% or more.

[0039] REM: 0.010% or less REM (rare earth metal) is an element that, like Ca, has the effect of improving the extremely low temperature toughness of a steel sheet by controlling the form of inclusions in the steel. However, if the REM content becomes excessively high, the cleanliness of the steel is impaired. Therefore, when REM is contained, the REM content is set to 0.010% or less. The REM content is preferably 0.008% or less. On the other hand, the lower limit of the REM content is not particularly limited. When REM is contained to obtain the above effects, the REM content is preferably 0.001% or more.

[0040] Here, REM is a general term for 17 elements including Y and Sc in addition to the 15 elements of lanthanoids, and these elements can be contained alone or in combination. Therefore, the REM content means the total content of these elements.

[0041] Mg: 0.007% or less Mg is an element that, like Ca and REM, has the effect of improving the extremely low temperature toughness of a steel sheet by controlling the form of inclusions in the steel. However, if the Mg content becomes excessively high, the cleanliness of the steel is impaired. Therefore, when Mg is contained, the Mg content is set to 0.007% or less. The Mg content is preferably 0.004% or less. On the other hand, the lower limit of the Mg content is not particularly limited. When Mg is contained to obtain the above effects, the Mg content is preferably 0.001% or more.

[0042] [Microstructure] In the steel sheet of the present invention, 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% by mass or more and a circle equivalent diameter of 500 nm or less of 5×10 5 pieces / mm 2 or more, and a number density of local Ni enrichment regions with an average Ni concentration of 8% by mass or more and 12% by mass or less and a circle equivalent diameter exceeding 500 nm of 5×10 4 pieces / mm 2 or less, and contains austenite in a volume ratio of 1 to 15%.

[0043] The number density of local Ni enrichment 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 or more The above-mentioned "local Ni enrichment 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 local Ni enrichment region extracted under the measurement conditions described in the following examples. Specifically, a test piece collected so that the position 1 / 4 in depth in the plate thickness direction from the surface of the steel plate (hereinafter, may also be referred to as the position of "(1 / 4)t of the plate thickness". "t" is the plate thickness.) is the observation position is polished to remove polishing strain to obtain a test piece for microstructural observation. For this test piece for microstructural observation, surface analysis by energy dispersive X-ray spectroscopy using a scanning electron microscope is performed to create a Ni concentration distribution, and a Ni enrichment region and an average Ni concentration are extracted from the Ni concentration distribution using the image processing software described below. Then, the area of each Ni enrichment region where the average Ni concentration is 10% by mass or more is converted into an equivalent circle diameter, and the location where the equivalent circle diameter is 500 nm or less is the above-mentioned local Ni enrichment region. Note that the number density of the above-mentioned local Ni enrichment regions can be obtained by dividing the number of locations where the obtained equivalent circle diameter is 500 nm or less by the measurement area.

[0044] By setting the number density of this local Ni enrichment region where the average Ni concentration is 10% by mass or more and the equivalent circle diameter is 500 nm or less to 5×10 5 pieces / mm 2 or more, a predetermined amount of stable γ will exist in the steel plate, and good absorbed energy can be obtained. The number density of this local Ni enrichment region is preferably 6×10 5 pieces / mm 2 or more. Note that the upper limit of the number density of this local Ni enrichment region is not particularly limited. From the perspective of manufacturing cost, the number density of this local Ni enrichment region is preferably 8×10 6 pieces / mm 2 or less, and more preferably 6×10 6 pieces / mm 2 or less. Also, the above equivalent circle diameter is preferably 40 nm or more.

[0045] The number density of local Ni enrichment 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: 5×10 4 pieces / mm 2 or less The above-mentioned "local Ni enrichment region 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 local Ni enrichment region extracted under the measurement conditions described in the following examples. Specifically, in the same manner as described above, from the Ni concentration distribution of the microstructure test piece prepared so that the position of (1 / 4)t of the plate thickness is the observation position, the Ni enrichment region and the average Ni concentration are extracted. Then, the area of each Ni enrichment region where the average Ni concentration is 8 to 12% by mass is converted into the equivalent circle diameter, and the portion where the equivalent circle diameter exceeds 500 nm is the above-mentioned local Ni enrichment region. Note that the number density of the above-mentioned local Ni enrichment region can be obtained by dividing the number of portions where the obtained equivalent circle diameter exceeds 500 nm by the measurement area.

[0046] By setting the number density of this local Ni enrichment region 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 to 5×10 4 pieces / mm 2 or less, the amount of unstable γ generated, which causes the occurrence of fine brittle fractures, becomes sufficiently small, and as a result, the occurrence of fine brittle fractures can be suppressed. The number density of this local Ni enrichment region is preferably 2×10 4 pieces / mm 2 or less. Note that since it is desirable that the number of these local Ni enrichment regions is small, the number density of the local Ni enrichment region may be 0 pieces / mm 2 as well.

[0047] Volume fraction of austenite: 1 to 15% When the volume fraction of austenite is less than 1%, the toughness improvement effect by stable austenite cannot be obtained. On the other hand, when the volume fraction of austenite exceeds 15%, it becomes difficult to obtain stable austenite, and as a result, minute brittle fractures occur. Therefore, the volume fraction of austenite is 1% or more and 15% or less. The volume fraction of austenite is preferably 2% or more and preferably 10% or less.

[0048] By forming the steel sheet of the present invention into a microstructure having the above configuration, the mechanical properties described below can be obtained.

[0049] In addition, 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 the position of (1 / 4)t of the plate thickness, and the remaining structure is bainite and martensite.

[0050] Total area ratio of bainite and martensite: 85% or more (preferred condition) If the structure is mainly composed of bainite and martensite, excellent cryogenic toughness can be ensured by the effects of the fine grain structure and self-tempering or tempering, and sufficient strength can also be easily obtained. The above-mentioned "structure mainly composed of bainite and martensite" means that the total area ratio of bainite and martensite is 85% or more with respect to the entire structure at the position of (1 / 4)t of the plate thickness. As long as it is within the range of the above total area ratio, the ratio of bainite and martensite is not particularly limited.

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

[0052] Each of the above structures can be obtained by controlling the conditions of the following first accelerated cooling, two-phase region heating, second accelerated cooling, and tempering. In addition, the area ratios of the above bainite, martensite, and austenite structures can be measured by the method described in the following examples.

[0053] [Plate thickness] The plate thickness of the steel sheet of the present invention is not particularly limited and can be any thickness. From the viewpoint of applying the steel sheet of the present invention to the steel material of the above steel structure, the plate thickness is preferably 6 mm or more and 50 mm or less. The plate thickness is more preferably 10 mm or more. In addition, the "thick steel sheet" in the present invention refers to a steel sheet having 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 sheet shall be 690 MPa or more. The tensile strength is preferably 720 MPa or more. This is because when applying the steel sheet to a tank, the plate thickness can be reduced and the steel weight can be reduced. 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 less, and more preferably 900 MPa or less. In the present invention, "high strength" means that the TS of the steel sheet is 690 MPa or more.

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

[0057] (Cryogenic Toughness) The toughness of the steel sheet is preferably such that the Charpy impact energy (vE -196℃ ) at -196 °C is 200 J or more in a full-size Charpy impact test. vE -196℃ is more preferably 205 J or more, and even more preferably 240 J or more. vE -196℃ may be 350 J or less, or may be 280 J or less. In the half-size Charpy impact test, vE -196℃ is preferably 100 J or more. vE -196℃ is more preferably 102 J or more, and even more preferably 120 J or more. vE -196℃ may be less than 200 J, or may be 150 J or less.

[0058] Also, it is preferable to conduct an instrumented Charpy impact test and ensure that there is no sudden drop in load indicating the occurrence of unstable cracks in the obtained load-displacement curve.

[0059] That is, "excellent cryogenic toughness" in the present invention means that in the instrumented Charpy impact test, the Charpy absorbed energy (vE -196℃ ) is 200 J or more in full size (in half size, it is 100 J or more), and no unstable crack occurs. The above-mentioned instrumented Charpy impact test can be carried out by the method described in the examples below.

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

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

[0062] The steel plate of the present invention can be manufactured by sequentially performing a heating process, a hot rolling process, a first accelerated cooling process, a two-phase region heating process, a second accelerated cooling process, and a tempering process on the steel material under the specific conditions described below. Further, a cooling process may be provided after the tempering process.

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

[0064] Note that the heating of the steel material may be performed after once cooling the steel material obtained by a method such as casting, or the obtained steel material may be directly subjected to heating without cooling.

[0065] When the heating temperature of the steel material is less than 900 °C, due to the high deformation resistance of the steel material, the load on the rolling mill in subsequent hot rolling increases, and it may be difficult to perform hot rolling. 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, when the heating temperature of the steel material is higher than 1200 °C, the oxidation of the steel becomes remarkable, and as a result of the increase in the loss due to removing the oxide film formed by oxidation, the yield decreases. Therefore, the heating temperature of the steel material should be 1200 °C or lower.

[0066] [Hot rolling process] After the completion of the above heating process, a hot rolling process is performed. In the hot rolling process, the heated steel material is hot rolled into a hot rolled steel sheet. The final plate thickness of the hot rolled steel sheet is not particularly limited, but as described above for the plate thickness, it is preferably 6 mm or more and 50 mm or less. The hot rolling conditions may be appropriately set so as to be within this range of the final plate thickness.

[0067] [First accelerated cooling process] After the completion of the above hot rolling process, a first accelerated cooling process is performed. In the first accelerated cooling process, the hot rolled steel sheet after hot rolling is accelerated cooled (hereinafter referred to as "the first accelerated cooling"). Specifically, the average cooling rate in the temperature range of 550 °C or lower and 300 °C or higher at the temperature at the position of (1 / 4)t of the plate thickness is set to 1.0 °C / second or more, and the cooling stop temperature is set to 300 °C or lower at the temperature at the position of (1 / 4)t of the plate thickness. By performing the first accelerated cooling under such conditions, unstable γ does not remain after cooling, and a desired structure with an average Ni concentration of 8 mass% or more and 12 mass% or less and a local Ni enrichment region with a circle equivalent diameter exceeding 500 nm is obtained as 5×10 4 pieces / mm 2 or less.

[0068] Average cooling rate in the temperature range of 550 °C or lower and 300 °C or higher: 1.0 °C / second or more In the first accelerated cooling, if the average cooling rate in the above temperature range is less than 1.0 °C / second at the temperature at the position of (1 / 4)t of the plate thickness, unstable austenite is generated at this point. As a result, it is likely to form a local Ni enrichment region with an average Ni concentration of 8 mass% or more and 12 mass% or less and a circle equivalent diameter exceeding 500 nm. Thereby, unstable γ is likely to remain in the steel, and as a result, the very low temperature toughness is likely to decrease. The average cooling rate is preferably 5.0 °C / second or more.

[0069] Note that the upper limit of the average cooling rate is not particularly limited. If the average cooling rate is higher than 200 °C / second, it becomes difficult to control the temperature at each position in the steel plate, and variations in material properties are likely to occur in the plate width direction and the rolling direction. As a result, variations are likely to occur in material properties such as tensile strength and very low temperature toughness. Therefore, the average cooling rate is preferably 200 °C / second or less. The average cooling rate is more preferably 100 °C / second or less, and even more preferably 60 °C / second or less.

[0070] Cooling stop temperature: 300 °C or lower In the first accelerated cooling, if the cooling stop temperature is higher than 300 °C at the temperature at the position of (1 / 4)t of the plate thickness, the phase transformation does not complete sufficiently during the accelerated cooling. Therefore, γ is likely to be generated even in an unstable state, and finally, the local Ni enrichment region with an average Ni concentration of 8 mass% or more and 12 mass% or less and a circle equivalent diameter exceeding 500 nm is likely to exceed 5×10 4 pieces / mm 2 As a result, micro brittle fracture is likely to occur. The cooling stop temperature is preferably 250 °C or lower, and also preferably 200 °C or lower. The lower limit of the cooling stop temperature is not particularly specified, but it is preferably 50 °C or higher.

[0071] The first accelerated cooling is not particularly limited and can be performed by any method. For example, one or both of air cooling and water cooling can be used. Examples of the water cooling include any cooling method using water, such as spray cooling, mist cooling, laminar cooling, etc.

[0072] [Two-phase region heating process] After the completion of the first accelerated cooling process, a two-phase region heating process is performed. In the two-phase region heating process, the hot-rolled steel sheet that has undergone the first accelerated cooling after hot rolling is subjected to two-phase region heating. Specifically, the hot-rolled steel sheet that has undergone the first accelerated cooling is heated under the conditions that the average heating rate in the temperature range of Ac1 point or higher and 650°C or lower at the temperature at the position of (1 / 4)t of the plate thickness is 1.0°C / second or lower, and the heating temperature (hereinafter referred to as the "heating holding temperature") is in the two-phase region temperature range of 650°C or higher and less than Ac3 point, and the holding time in the two-phase temperature range is 15 minutes or longer. By performing this two-phase region heating, a part of the structure of the hot-rolled steel sheet is reversely transformed from bainite and / or martensite to austenite to form a Ni enrichment region. Then, by cooling, it becomes martensite again. After performing this, a desired structure can be obtained by performing the tempering treatment described later. In addition, in the two-phase region heating process, from the viewpoint of effectively obtaining the action effect, in addition to the above conditions, the average heating rate in the temperature range exceeding 650°C and below the heating holding temperature may be controlled.

[0073] Average heating rate in the temperature range of Ac1 point or higher and 650°C or lower: 1.0°C / second or lower When the average heating rate in the temperature range of Ac1 point to 650°C at the temperature at the position of (1 / 4)t of the plate thickness exceeds 1.0°C / second, Ni cannot diffuse sufficiently, and the 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 cannot be 5 pieces / mm 2 5×10 or more. As a result, the number density of fine stable γ cannot be ensured sufficiently, and it is difficult to ensure excellent extremely low temperature toughness. The average heating rate is preferably 0.8°C / second or lower. The lower limit of the average heating rate in the above temperature range is not particularly defined. From the viewpoint of production efficiency, the average heating rate is preferably 0.2°C / second or higher.

[0074] Average heating rate in the temperature range of 650°C or higher and below the heating holding temperature: 0.10°C / second or lower (preferred condition) When the average heating rate in the temperature range of 650°C or higher and lower than the holding temperature exceeds 0.10°C / second at the temperature at the position of (1 / 4)t of the plate thickness, Ni diffuses, and the time for forming the Ni-enriched region cannot be sufficiently ensured. Finally, the number density of local Ni-enriched regions with an average Ni concentration of 10% by mass or more and a circle-equivalent diameter of 500 nm or less is 5×10 5 pieces / mm 2 or more may not be achieved. As a result, the number density of fine stable γ cannot be sufficiently ensured, and it may be difficult to ensure excellent very low temperature toughness. Therefore, the average heating rate is preferably 0.10°C / second or less, more preferably 0.08°C / second or less. Note that the lower limit of the average heating rate in the above temperature range is not particularly specified. From the viewpoint of production efficiency, the average heating rate is preferably 0.01°C / second or more.

[0075] Holding temperature: 650°C or higher and less than Ac3 point When the holding temperature is less than 650°C at the temperature at the position of (1 / 4)t of the plate thickness, after the completion of the subsequent second accelerated cooling process, the Ni-enriched region remains as austenite, and further local Ni enrichment in the subsequent tempering process cannot be expected. As a result, coarse local Ni-enriched regions are formed in the finally obtained steel plate, and due to this, fine brittle fracture is likely to occur, and it is difficult to obtain the desired very low temperature toughness. For these reasons, the holding temperature is 650°C or higher. The holding temperature is preferably 660°C or higher.

[0076] On the other hand, when the holding temperature is at or above the Ac3 point at the temperature at the position of (1 / 4)t of the plate thickness, almost all of the bainite and martensite undergo reverse transformation, and the number density of local Ni-enriched regions with an average Ni concentration of 10% by mass or more and a circle-equivalent diameter of 500 nm or less is 5×10 5 pieces / mm 2 or more is difficult to form. As a result, it is difficult to ensure stable retained γ, and due to this, it is difficult to ensure excellent very low temperature toughness. For these reasons, the holding temperature is less than the Ac3 point. The holding temperature is preferably (Ac3 point - 20°C) or less.

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

[0078] Note that the Ac1 point (Ac1 transformation point) and the Ac3 point (Ac3 transformation point) can be obtained 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 (°C) = 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 content (mass%) of each element in the steel. When the element is not contained, the content in the steel is taken as 0.

[0079] Holding time: 15 minutes or more After reaching the heating and holding temperature that becomes the above two-phase region temperature range, hold at the heating and holding temperature for 15 minutes or more. If the holding time (soaking holding time) at the heating and holding temperature is less than 15 minutes, the diffusion of Ni becomes insufficient. Therefore, the local Ni enrichment region with an average Ni concentration of 10 mass% or more and a circle equivalent diameter of 500 nm or less has a number density of 5×10 5 pieces / mm 2It is difficult to form the above. In particular, there is a risk that the Ni concentration in the γ phase in the steel plate cannot be increased sufficiently. As a result, the number density of fine and stable γ cannot be ensured sufficiently, and it is difficult to ensure excellent cryogenic toughness. The holding time at the heating and holding temperature is preferably 20 minutes or more. Although the upper limit of the holding time at the heating and holding temperature is not particularly specified, from the perspective of manufacturing cost, it is preferable that the holding time at the heating and holding temperature is 120 minutes or less, and more preferably 60 minutes or less.

[0080] [Second Accelerated Cooling Step] After the completion of the above two-phase region heating step, a second accelerated cooling step is performed. In the second accelerated cooling step, the hot-rolled steel plate heated and held in the two-phase region is subjected to accelerated cooling (hereinafter referred to as "second accelerated cooling"). Specifically, the average cooling rate in the temperature range of 600°C or lower and 300°C or higher at the temperature at the position of (1 / 4)t of the plate thickness is set to 1.0°C / second or more, and the cooling stop temperature is set to 300°C or lower at the temperature at the position of (1 / 4)t of the plate thickness. By performing the second accelerated cooling under such conditions, unstable γ does not remain after cooling, and a desired structure with a local Ni enrichment region having an average Ni concentration of 8% by mass or more and 12% by mass or less and a circle equivalent diameter exceeding 500 nm is 5×10 4 pieces or less can be obtained.

[0081] Average cooling rate in the temperature range of 600°C or lower and 300°C or higher: 1.0°C / second or more In the second accelerated cooling, if the average cooling rate in the temperature range is less than 1.0 °C / second at the temperature at the position of (1 / 4)t of the plate thickness, unstable austenite is generated at this time, and the Ni-enriched region tends to remain in the steel as γ. Due to this, further Ni enrichment cannot be expected during tempering in subsequent processes. As a result, a local Ni-enriched region with a final average Ni concentration of 8 mass% or more and 12 mass% or less and a circle-equivalent diameter exceeding 500 nm is likely to be formed, and the extremely low-temperature toughness of the steel plate decreases. Therefore, the average cooling rate should be 1.0 °C / second or more. The average cooling rate is preferably 5.0 °C / second or more. Note that the upper limit of the average cooling rate is not particularly limited. If the average cooling rate is higher than 200 °C / second, it becomes difficult to control the temperature at each position in the steel plate, and stable production is difficult. Therefore, the average cooling rate is preferably 200 °C / second or less, and more preferably 100 °C / second or less.

[0082] Here, the average cooling rate of the second accelerated cooling described above refers to the average rate of temperature decrease per unit time in the temperature range of 600 °C or lower and 300 °C or higher.

[0083] Cooling stop temperature: 300 °C or lower In the second accelerated cooling, if the cooling stop temperature exceeds 300 °C at the temperature at the position of (1 / 4)t of the plate thickness, unstable γ tends to remain, and it is difficult to reduce the reduction rate of the residual γ amount due to this. As a result, the extremely low-temperature toughness is likely to decrease. Therefore, the cooling stop temperature is set to 300 °C or lower. The cooling stop temperature is preferably 250 °C or lower. The lower limit of the cooling stop temperature is not particularly defined, but it is preferably 50 °C or higher.

[0084] The cooling method of the second accelerated cooling is not particularly limited, and it may be carried out by any cooling method in the same manner as described in the first accelerated cooling above.

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

[0086] Tempering temperature: 550°C or higher (Ac1 point + 30°C) and lower If the tempering temperature is less than 550°C at the temperature at the position of (1 / 4)t of the plate thickness, reverse transformation to γ does not occur during tempering, and Ni enrichment is also unlikely to occur. As a result, the very low temperature toughness is likely to decrease. On the other hand, if the tempering temperature exceeds (Ac1 point + 30°C), the strength decreases. Also, due to the remaining unstable γ, the very low temperature toughness is likely to decrease. For these reasons, the tempering temperature is set to 550°C or higher (Ac1 point + 30°C) and lower. The tempering temperature is preferably 570°C or higher and preferably Ac1 point °C or lower.

[0087] For heating the hot-rolled steel sheet in this tempering process, any heating method can be used as long as it can control the heating temperature to the above-mentioned tempering temperature. As an example of the heating method, furnace heating can be used in the same manner as described in the above-mentioned two-phase region heating process.

[0088] Holding time: 10 minutes or longer After reaching the above-mentioned tempering temperature, hold at this tempering temperature for 10 minutes or longer. The holding time is preferably 20 minutes or longer and preferably 60 minutes or shorter.

[0089] [Cooling process] (Optional process) As described above, a cooling process may be provided after the tempering process. When this cooling process 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 viewpoints of workability during manufacturing and cost, it is preferable to perform air cooling.

[0090] Through the above steps, 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 plate thickness range. 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.

Example

[0091] Hereinafter, the present invention will be specifically described based on examples. Note that the present invention is not limited to this embodiment.

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

[0093] First, molten steel having the component composition shown in Table 1 was melted in a converter, and a steel slab (thickness: 200 mm), which is a steel material, was manufactured by continuous casting. Note that the content “0%” in Table 1 represents that the element is not intentionally added, and includes not only the case where the element is not contained but also the case where the element is unavoidably contained. In addition, the Ac1 point (°C) obtained using the above-described formula (1) and the Ac3 point (°C) obtained using the formula (2) are also shown in Table 1.

[0094]

Table 1

[0095] Next, according to the manufacturing conditions of each step shown in Table 2, the obtained steel slab was heated and hot-rolled to obtain hot-rolled steel sheets having the plate thicknesses shown in Table 2. Subsequently, the hot-rolled steel sheets were subjected to heat treatment including first accelerated cooling, two-phase region heating, second accelerated cooling, and tempering. Note that the plate thickness in Table 2 is the final plate thickness. Here, in all examples, air cooling was performed after tempering to obtain steel sheets having various plate thicknesses from 10 mm to 50 mm. Note that a heat treatment furnace was used for heating in each step. In addition, "-" in Table 2 indicates that the operation was not performed. For No. 7 in Table 2, the "average heating rate in the temperature range of 650 °C or lower and above the Ac1 point at (1 / 4)t" in the two-phase region heating process indicates the "average heating rate in the temperature range of 640 °C or lower and above the Ac1 point", and for No. 41, the "average heating rate in the temperature range of 650 °C or lower and above the Ac1 point at (1 / 4)t" in the two-phase region heating process indicates the "average heating rate in the temperature range of 630 °C or lower and above the Ac1 point".

[0096] [Table 2]

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

[0098] (1) Microstructure Test pieces for observing the microstructure were taken so that the position at a depth of 1 / 4 in the plate thickness direction from the surface of each steel plate (i.e., the position of the plate thickness (1 / 4)t) was the observation position. This test piece was embedded in resin so that the cross-section perpendicular to the rolling direction was the observation surface and mirror-polished. Next, after nitriding corrosion was performed, observation was carried out with a scanning electron microscope (SEM) at magnifications of 2000 times and 10000 times, and an image of the microstructure of the steel plate was taken. The microstructure was identified from the obtained image. The structure in the region excluding the structure distributed in an island shape was defined as the remaining structure described above in this steel (specifically, bainite and martensite), and this includes carbides.

[0099] In addition, the number density of each of the above-described local Ni enrichment regions was measured as follows. [Measurement of the size (i.e., equivalent circle diameter) of the local Ni concentration, the average Ni concentration, and the number density] From each steel plate, test pieces for microstructure observation were taken such that the position at a plate thickness of (1 / 4)t was the observation position. Next, after wet polishing the taken test pieces using waterproof abrasive paper to roughly polish the observation surface, the observation surface was finished to a mirror surface using diamond paste. Next, in order to remove the polishing strain introduced by the above polishing, final finishing using colloidal silica was performed. For the obtained test pieces, surface analysis by energy dispersive X-ray spectroscopy (EDS) using a scanning electron microscope (SEM) was carried out to measure the Ni distribution. The observation magnification at this time was set to 10,000 times, and the acceleration voltage of the SEM was set to 5 kV. The number of measurement locations was set to 5. From the obtained EDS surface analysis results, a Ni concentration distribution was created, and then, using image processing software (Avizo ver2023.1 manufactured by Thermo Fisher Scientific), the Ni-enriched region, average Ni concentration, and equivalent circle diameter were determined. Next, from the above treatment, the number of local Ni-enriched regions where the average Ni concentration is 10 mass% or more and the equivalent circle diameter is 500 nm or less, and the number of local Ni-enriched regions where the average Ni concentration is 8 mass% or more and 12 mass% or less and the equivalent circle diameter exceeds 500 nm were respectively determined, and the respective number densities were obtained by dividing the number by the measurement area. Here, the average value of the number densities at 5 locations was used.

[0100] Also, the γ amount was measured as follows. [γ amount] To determine the γ amount of each steel plate, 5 test pieces for X-ray diffraction with a test piece size of 1 mm (in the plate thickness direction) × 20 mm (in the rolling direction) × 20 mm (in the plate width direction) were taken such that the position at a plate thickness of (1 / 4)t of each steel plate was the observation position (i.e., the measurement surface). Next, each taken test piece was ground and chemically polished and subjected to X-ray diffraction. The diffraction intensities of the (200), (211) planes of α-Fe and the (200), (220), (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 5 test pieces was determined as the γ amount (unit: volume fraction).

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

[0102] (3) Cryogenic toughness For the evaluation of toughness at cryogenic temperatures in the present invention, the Charpy absorption energy (vE -196℃ ) and the load-displacement curve at -196 °C were used. A V-notch test specimen was taken from the position of the plate thickness (1 / 4)t of the steel plate in accordance with the provisions of JIS Z 2242 (2018) so that the longitudinal direction of the test specimen was aligned with the plate width direction. Using this V-notch test specimen, an instrumented Charpy impact test was carried out in accordance with the provisions of JIS B 7755 (2011) to obtain the Charpy absorption energy (vE -196℃ ) and the load-displacement curve at -196 °C.

[0103] The Charpy absorption energy was evaluated using the average value obtained by taking three test specimens and performing a total of three measurements. The obtained average value was recorded in "vE -196℃ " in Table 3. In the case of full size, when the average value was 200 J or more, it was regarded as "qualified", and in the case of half size, when the average value was 100 J or more, it was regarded as "qualified".

[0104] Also, for the load-displacement curve, the presence or absence of a sudden load drop in each measurement was evaluated. When there was no sudden load drop in all measurements (here, three times), the symbol "〇" was recorded in the column of "Presence or absence of sudden load drop" in Table 3. Otherwise, the symbol "×" was recorded in the same column of Table 3. Here, when no sudden drop indicating the start of unstable crack propagation occurred in all three measurements, it was regarded as "qualified".

[0105] In this example, for No. 2 and 38 with a small plate thickness, a half-size Charpy impact test using a half-size test specimen (i.e., a sub-size test specimen) was carried out, and for other examples, a full-size Charpy impact test using a full-size test specimen was carried out.

[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 austenite in a volume ratio of 1 to 15%.

[0108] In addition, it was confirmed that the steel plates of the inventive examples according to the present invention are all high-strength, have excellent extremely low temperature toughness, and suppress a sharp load drop in the load-displacement curve indicating the occurrence of minute brittle cracks. This effect was achieved even in relatively thin steel plates with a plate thickness of 6 to 25 mm, for example.

[0109] On the other hand, in the steel plates of the comparative examples outside the scope of the present invention, the full-size Charpy absorbed energy became lower than 200 J and / or a sharp load drop in the load-displacement curve occurred. That is, in the comparative examples, the extremely low temperature toughness decreased due to the occurrence of minute brittle cracks, and the above-described target performance could not be satisfied.

Industrial Applicability

[0110] According to the present invention, for steel plates having various plate thicknesses, it is possible to exhibit excellent extremely low temperature toughness without the occurrence of minute 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 to 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: 0.0010 to 0.0080%; The balance is Fe and unavoidable impurities, The microstructure at a position of 1 / 4 depth from the surface of the steel plate in the plate thickness direction is The number density of localized Ni-enriched regions having an average Ni concentration of 10 mass% or more and an equivalent circle diameter of 500 nm or less is 5 × 10 ５ pieces / mm ２ or more, and the number density of localized Ni-enriched regions having an average Ni concentration of 8 mass% or more and 12 mass% or less and an equivalent circle diameter of more than 500 nm is 5×10 ４ pieces / mm ２ is as follows: Contains 1 to 15% austenite by volume; A steel plate having a tensile strength of 690 MPa or more.

2. The composition further comprises, 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 sheet according to claim 1 , comprising one or more selected from the group consisting of:

3. The method for producing a steel sheet according to claim 1 or 2, A steel material having the above-mentioned composition is heated to a temperature of 900° C. or more and 1200° C. or less, and hot-rolled to obtain a hot-rolled steel sheet; Next, a first accelerated cooling is performed at a temperature at a position of 1 / 4 depth from the surface of the steel sheet in the sheet thickness direction under cooling conditions in which the average cooling rate in a temperature range of 550°C to 300°C is 1.0°C / sec or more and the cooling stop temperature is 300°C or less; Next, the temperature at a position 1 / 4 depth from the surface of the steel plate in the plate thickness direction, Ac １ The average heating rate in the temperature range from the temperature point to 650 ° C. is 1.0 ° C. / sec or less, and the heating retention temperature is 650 ° C. or more Ac ３ The heating is performed in a two-phase region temperature range below the temperature point and the holding time in the two-phase region temperature range is 15 minutes or more, Next, a second accelerated cooling is performed at a temperature at a position of 1 / 4 depth from the surface of the steel sheet in the sheet thickness direction under cooling conditions of an average cooling rate of 1.0 ° C. / sec or more in a temperature range of 600 ° C. or less and 300 ° C. or more, and a cooling stop temperature of 300 ° C. or less; Next, the tempering temperature is 550°C or more (Ac １ The method for manufacturing a steel sheet comprises tempering the steel sheet at a temperature of 0.1 +30°C or less for a holding time of 10 minutes or more.

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

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