Austenitic stainless steel hot rolled sheet and method for producing the same

Abstract

To provide an austenitic stainless steel which is low cost and combines high low-temperature strength with sufficient corrosion resistance. The austenitic stainless steel disclosed herein has a component composition, by mass%, of C: 0.050% or more and 0.100% or less, Si: 0.05% or more and 1.00% or less, Mn: 5.0% or more and 20.0% or less, P: 0.030% or less, S: 0.0050% or less, Al: 0.070% or less, Cu: 1.5% or more and 3.5% or less, Ni: 5.0% or more and 10.0% or less, Cr: 11.0% or more and 20.0% or less, N: 0.05% or more and 0.20% or less, W: 0.05% or more and 0.50% or less, O: 0.0050% or less, Ti: 0.005% or less, and Nb: 0.005% or less, with the balance being Fe and unavoidable impurities, and an average crystal grain size of less than 30 μm and a dislocation density of 1.0 × 10 14 m -2 or more, and has a microstructure in which the amount of precipitated Cr is less than 1.0 mass %, and mechanical properties in which the yield stress in a tensile test at -269°C is 1000 MPa or more.

Description

[Technical field]

[0001] The present disclosure relates to austenitic stainless steels and methods for producing same. [Background technology]

[0002] To achieve carbon neutrality, it is desirable to utilize hydrogen as an energy source. To achieve a stable supply of hydrogen, hydrogen storage tanks are necessary. To achieve a large-scale supply of hydrogen, large storage tanks are necessary. To realize large storage tanks, it is required to improve the strength of the storage tanks. Austenitic stainless steels are sometimes used as materials for storage tank structures for liquid hydrogen, liquid helium, liquefied gas, etc. However, austenitic stainless steels generally undergo solution heat treatment to restore corrosion resistance, and therefore have the problem of low strength. Therefore, high-strength austenitic stainless steels are desirable.

[0003] Patent Document 1 discloses an austenitic stainless steel and a method for manufacturing the same. The austenitic stainless steel has a chemical composition, in mass%, of C: 0.005-0.07%, Si: 0.1-1.2%, Mn: 3.2-6.5%, Ni: 9-14%, the total of at least one of Cu and Co: 0.005% or more and less than 3%, Cr: 19-24%, Mo: 1-4%, Nb: 0.05-0.4%, N: 0.15-0.50%, Al: 0.05% or less, P: 0.03% or less, S: 0.002% or less, O: 0.02% or less, V: 0-0.5%, Ti: 0-0.5%, B: 0-0.01%, Ca: 0-0.05%, Mg: 0-0.05%, REM: 0-0.5%, balance: Fe and impurities, and the amount of Nb analyzed as the electrolytic extraction residue is 0.01-0.3 mass%. Patent Document 1 discloses that this austenitic stainless steel is excellent in strength, ductility, and weldability, and for example, discloses a case where the tensile strength is 690 MPa or more at room temperature. Patent Document 1 discloses that the strength and ductility of austenitic stainless steel are related to the amount of Nb analyzed as the electrolytic extraction residue, that is, the amount of precipitates containing Nb. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2017 / 056619 Summary of the Invention [Problem to be solved by the invention]

[0005] The austenitic stainless steel described in Patent Document 1 requires a solution heat treatment process in addition to a hot working process as part of its manufacturing process. Therefore, although the stainless steel has good corrosion resistance, it does not have sufficient strength in low-temperature environments (hereinafter referred to as low-temperature strength), which results in high costs.

[0006] The present disclosure has been made in consideration of the above-mentioned circumstances, and its object is to provide an austenitic stainless steel that combines high low-temperature strength and sufficient corrosion resistance at low cost, and a method for producing the same. [Means for solving the problem]

[0007] In order to achieve the above object, the austenitic stainless steel and the method for producing the same according to the present disclosure are as follows.

[0008] [1] In mass percent, C: 0.050% or more and 0.100% or less, Si: 0.05% or more and 1.00% or less, Mn: 5.0% or more and 20.0% or less, P: 0.030% or less, S: 0.0050% or less, Al: 0.070% or less, Cu: 1.5% or more and 3.5% or less, Ni: 5.0% or more and 10.0% or less, Cr: 11.0% or more and 20.0% or less, N: 0.05% or more and 0.20% or less, W: 0.05% or more and 0.50% or less, O: 0.0050% or less, Ti: 0.005% or less, and Nb: 0.005% or less and the balance being Fe and unavoidable impurities; The average grain size is less than 30 μm and the dislocation density is less than 1.0 × 10 14 m -2 or more, and the amount of precipitated Cr is less than 1.0 mass %; Mechanical properties with a yield stress of 1000 MPa or more in a tensile test at -269°C; An austenitic stainless steel having a

[0009] [2] The composition further comprises, in mass%, Mo: 2.0% or less, V: 0.5% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0200% or less The austenitic stainless steel according to the above [1], which contains one or more selected from the group consisting of:

[0010] [3] In mass percent, C: 0.050% or more and 0.100% or less, Si: 0.05% or more and 1.00% or less, Mn: 5.0% or more and 20.0% or less, P: 0.030% or less, S: 0.0050% or less, Al: 0.070% or less, Cu: 1.5% or more and 3.5% or less, Ni: 5.0% or more and 10.0% or less, Cr: 11.0% or more and 20.0% or less, N: 0.05% or more and 0.20% or less, W: 0.05% or more and 0.50% or less, O: 0.0050% or less, Ti: 0.005% or less, and Nb: 0.005% or less A heating step of heating a steel material having a composition containing Fe and unavoidable impurities to a temperature range of 1000°C to 1300°C; a hot rolling process for hot rolling the steel material after the heating process; a cooling step of cooling the steel material after the hot rolling; Including, The hot rolling process includes: (i) a first rolling process in which the steel material is hot-rolled at 950°C or higher to a total rolling reduction of 52% or more; and (ii) a second rolling process that is carried out after the first rolling process and hot-rolls the steel material at less than 900°C for one or more passes, and the finish rolling end temperature is 800°C or higher; A method for producing an austenitic stainless steel, wherein the cooling step comprises water cooling at an average water cooling rate of 10°C / s or more from a cooling start temperature in a temperature range of 100°C lower than the finish rolling end temperature to a cooling stop temperature in a temperature range of 600°C or lower.

[0011] [4] The composition further comprises, in mass%, Mo: 2.0% or less, V: 0.5% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0200% or less The method for producing an austenitic stainless steel according to the above [3], which contains one or more selected from the group consisting of: Effect of the Invention

[0012] According to the present disclosure, it is possible to provide an austenitic stainless steel that combines high low-temperature strength and sufficient corrosion resistance at low cost, and a method for producing the same. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] An austenitic stainless steel and a method for producing the same according to an embodiment of the present disclosure will be described.

[0014] The austenitic stainless steel according to the present embodiment has a composition, in mass%, of C: 0.050% to 0.100%, Si: 0.05% to 1.00%, Mn: 5.0% to 20.0%, P: 0.030% to 0.0050%, S: 0.0050%, Al: 0.070%, Cu: 1.5% to 3.5%, Ni: 5.0% to 10.0%, Cr: 11.0% to 20.0%, N: 0.05% to 0.20%, W: 0.05% to 0.50%, O: 0.0050%, Ti: 0.005%, and Nb: 0.005%, with the balance being Fe and unavoidable impurities. The austenitic stainless steel according to the present embodiment has an average grain size of less than 30 μm and a dislocation density of 1.0 × 10 14 m-2 The austenitic stainless steel according to this embodiment has a microstructure in which the amount of precipitated Cr is less than 1.0 mass %. The austenitic stainless steel according to this embodiment has mechanical properties in which the yield stress in a tensile test at -269°C is 1000 MPa or more.

[0015] The method for producing austenitic stainless steel according to the present embodiment includes a heating step of heating a steel material having the composition described in the previous paragraph to a temperature range of 1000°C to 1300°C, a hot rolling step of hot rolling the steel material after the heating step, and a cooling step of cooling the steel material after the hot rolling. The hot rolling step includes (i) a first rolling step of hot rolling the steel material at 950°C or higher to a total rolling reduction of 52% or more, and (ii) a second rolling step performed after the first rolling step of hot rolling the steel material at less than 900°C for one or more passes, and the finish rolling end temperature is 800°C or higher. In the cooling step, water cooling is performed at an average water cooling rate of 10°C / s or more from a cooling start temperature in a temperature range of 100°C lower than the finish rolling end temperature to a cooling stop temperature in a temperature range of 600°C or lower.

[0016] The austenitic stainless steel according to this embodiment can be manufactured at low cost because the manufacturing process does not require a solution heat treatment. In addition, in this embodiment, a structure having a predetermined average crystal grain size and dislocation density can be obtained by optimizing the manufacturing conditions, such as adopting a predetermined component composition, not performing solution heat treatment, and adopting a predetermined slab heating temperature and hot rolling conditions, thereby realizing high low-temperature strength. In this embodiment, sufficient corrosion resistance can be ensured without performing solution heat treatment by optimizing the manufacturing conditions, such as adopting a predetermined C content and Cr content, and adopting a predetermined finish rolling end temperature and water cooling conditions.

[0017] The austenitic stainless steel and the manufacturing method thereof according to this embodiment will be described in detail below.

[0018] The austenitic stainless steel according to this embodiment is particularly suitable as a structural steel for use in extremely low temperature environments, such as tanks for storing liquid hydrogen, liquid helium, liquefied gas, and the like.

[0019] The composition of the austenitic stainless steel according to this embodiment will be described. The austenitic stainless steel according to this embodiment has the above-mentioned composition. As described above, the steel material used to manufacture the austenitic stainless steel according to this embodiment also has the same composition as the austenitic stainless steel according to this embodiment.

[0020] In the following description, the indication of "%" regarding the composition of a component means "mass%" unless otherwise specified. In the following, austenitic stainless steels and steel materials used in the production of the same will be collectively referred to simply as steel materials.

[0021] The C content in the steel material is 0.050% or more and 0.100% or less. C is an inexpensive austenite stabilizing element, and is an important element for obtaining austenite, and also has the effect of dissolving in steel and strengthening the steel material by solid solution strengthening. In order to fully obtain such an effect, the steel material contains 0.050% or more of C. If the steel material contains more than 0.100% of C, excessive Cr precipitates are formed, and the corrosion resistance of the steel material decreases. The C content in the steel material is preferably 0.060% or more and preferably 0.090% or less.

[0022] The Si content in the steel material is 0.05% or more and 1.00% or less. Si acts as a deoxidizer and is necessary for steelmaking, but also has the effect of dissolving in steel and strengthening the steel material through solid solution strengthening. To obtain such an effect, the steel material contains 0.05% or more of Si. If the steel material contains more than 1.00% of Si, the non-thermal stress increases excessively, and the low-temperature toughness of the steel material deteriorates. The Si content in the steel material is preferably 0.07% or more and preferably 0.80% or less.

[0023] The Mn content in the steel material is 5.0% or more and 20.0% or less. Mn is an inexpensive austenite stabilizing element, and is an important element for obtaining austenite, and also has the effect of dissolving in steel and strengthening the steel material through solid solution strengthening. In order to fully obtain such an effect, the steel material contains 5.0% or more of Mn. If the steel material contains more than 20.0% of Mn, the manufacturability and cuttability may deteriorate. The Mn content in the steel material is preferably 6.0% or more and preferably 19.0% or less.

[0024] The P content in the steel material is 0.030% or less. If the steel material contains more than 0.030% P, the hot ductility of the steel material may decrease, which may result in increased intergranular cracking. For this reason, it is desirable to reduce the P content in the steel material as much as possible. The P content in the steel material is preferably 0.028% or less. However, excessive reduction in P content increases refining costs and is economically disadvantageous, so the P content in the steel material is preferably 0.002% or more.

[0025] The S content in steel is 0.0050% or less. If the S content in steel exceeds 0.0050%, the hot ductility of the steel may decrease, which may result in increased intergranular cracking. For this reason, it is desirable to reduce the S content in steel as much as possible. The S content in steel is preferably 0.0045% or less. However, excessive reduction in S content increases refining costs and is economically disadvantageous, so the S content in steel is preferably 0.0010% or more.

[0026] The Al content in the steel material is 0.070% or less. Al acts as a deoxidizer and is most commonly used in the molten steel deoxidization process. In order to obtain such an effect, the steel material preferably contains 0.010% or more Al. If the steel material contains more than 0.070% Al, the hot ductility of the steel material may decrease, which may increase the occurrence of grain boundary cracks. The Al content in the steel material is preferably 0.060% or less.

[0027] The Cu content in the steel material is 1.5% or more and 3.5% or less, and the Ni content is 5.0% or more and 10.0% or less. Cu and Ni are austenite stabilizing elements, and are important elements for obtaining austenite, and also increase the strength of the steel material by solid solution strengthening. In order to fully obtain such effects, the steel material contains 1.5% or more of Cu and 5.0% or more of Ni. If the steel material contains these elements excessively, the surface properties may deteriorate during rolling of the steel material, and the manufacturing cost may be increased. For this reason, the Cu content in the steel material is 3.5% or less, and the Ni content is 10.0% or less. Preferably, the Cu content in the steel material is 2.0% or more and 3.0% or less. Also, preferably, the Ni content in the steel material is 6.0% or more and 9.0% or less.

[0028] The Cr content in the steel material is 11.0% or more and 20.0% or less. Cr is an element necessary for ensuring the corrosion resistance of stainless steel. In order to obtain such an effect, the steel material contains 11.0% or more of Cr. If the steel material contains more than 20.0% of Cr, excessive Cr precipitates are formed, and the corrosion resistance of the steel material decreases. The Cr content in the steel material is preferably 12.0% or more and preferably 19.0% or less.

[0029] The N content in the steel material is 0.05% or more and 0.20% or less. N is an inexpensive austenite stabilizing element. N is an important element for obtaining austenite, and has the effect of dissolving in steel and strengthening the steel material by solid solution strengthening. In order to obtain such an effect, the steel material contains 0.05% or more of N. If the steel material contains more than 0.20% of N, the nitrides or carbonitrides in the steel material may become coarse, and the low-temperature toughness of the steel material may decrease. The N content in the steel material is preferably 0.06% or more and preferably 0.19% or less.

[0030] The W content in the steel material is 0.05% or more and 0.50% or less. W contributes to improving the strength of the steel material and also contributes to improving the corrosion resistance of the steel material. To obtain such effects, the steel material contains 0.05% or more of W. If the steel material contains W in excess of 0.50%, the manufacturing cost of the steel material will be increased. The W content in the steel material is preferably 0.10% or more and preferably 0.40% or less.

[0031] The O content in the steel product is 0.0050% or less. O forms oxides and deteriorates the low-temperature toughness of the steel product. The O content in the steel product is preferably 0.0045% or less. However, excessive reduction of O increases the refining cost and is economically disadvantageous, so the O content in the steel product is preferably 0.0010% or more.

[0032] The Ti content in the steel material is 0.005% or less, and the Nb content is 0.005% or less. Ti and Nb form carbonitrides with high melting points in the steel material, and may reduce the low-temperature toughness of the steel material. Ti and Nb are components mixed in from raw materials, etc. In general, the Ti content is mixed in the steel material in the range of more than 0.005% to 0.010% or less, and the Nb content is mixed in the range of more than 0.005% to 0.010% or less. Therefore, it is necessary to avoid the mixing of Ti and Nb in the steel material and suppress the Ti and Nb contents to 0.005% or less, respectively, according to the melting method described later. By suppressing the Ti and Nb contents in the steel material to 0.005% or less, respectively, the adverse effects of the carbonitrides described above can be eliminated, and excellent low-temperature toughness and ductility can be ensured in the steel material. The Ti content in the steel material is preferably 0.003% or less. In addition, the Nb content in the steel material is preferably 0.003% or less. Of course, the Ti and Nb contents in the steel material may be 0.000% or more, or may be 0.001% or more.

[0033] The steel material in this embodiment is composed of iron (Fe) and inevitable impurities other than the above-mentioned components. Examples of inevitable impurities include H and B, and the total of these elements is acceptable if it is 0.01% or less. In this embodiment, inevitable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, manufacturing equipment, etc., and are acceptable to be contained within a range that does not impair the object of the present invention. Examples of raw materials include iron ore, reduced iron, scrap, etc.

[0034] The steel material in this embodiment may contain the elements described below as optional components as necessary.

[0035] The Mo content in the steel material may be 2.0% or less. The V content in the steel material may be 0.5% or less. Mo and V contribute to stabilizing austenite and improving the strength of the steel material. In order to obtain such effects, the steel material preferably contains Mo and V in an amount of 0.1% or more each. If the steel material contains Mo and V in excess of 2.0% and 0.5%, respectively, the manufacturing cost may be increased.

[0036] The Ca content in the steel material may be 0.0100% or less. The Mg content in the steel material may be 0.0100% or less. The REM (rare earth metal) content in the steel material may be 0.0200% or less. Ca, Mg and REM are elements useful for controlling the shape of inclusions. The control of the shape of inclusions means making expanded sulfide-based inclusions into granular inclusions. Through this control of the shape of inclusions, the ductility, low-temperature toughness and resistance to sulfide stress corrosion cracking in the steel material are improved. In order to obtain such effects, the steel material preferably contains 0.0005% or more of Ca and Mg and 0.0010% or more of REM. If the steel material contains these elements excessively, the amount of nonmetallic inclusions may increase, and the ductility, low-temperature toughness or resistance to sulfide stress corrosion cracking of the steel material may decrease. In addition, it may be economically disadvantageous. It is preferable that the Ca content be 0.0080% or less, the Mg content be 0.0080% or less, and the REM content be 0.0150% or less.

[0037] Next, the microstructure of the austenitic stainless steel according to this embodiment will be described.

[0038] The austenitic stainless steel according to this embodiment has an average crystal grain size of less than 30 μm and a dislocation density of 1.0×10 14 m -2 The average grain size is less than 30 μm and the dislocation density is less than 1.0×10 14 m -2 Achieving this or higher is a necessary condition for achieving excellent low-temperature strength with a yield stress of 1000 MPa or more in a tensile test at -269°C. A precipitated Cr amount of less than 1.0 mass% is a necessary condition for ensuring sufficient corrosion resistance.

[0039] The average grain size of the austenitic stainless steel according to this embodiment is preferably 27 μm or less, and more preferably 25 μm or less. There is no particular lower limit for the average grain size of the austenitic stainless steel according to this embodiment. In order to improve the toughness of the austenitic stainless steel according to this embodiment, the average grain size is preferably 18 μm or more, and more preferably 20 μm or more. The average grain size of the austenitic stainless steel according to this embodiment is a value measured by the method described in the examples below.

[0040] The dislocation density of the austenitic stainless steel according to this embodiment is preferably 2.0×10 14 m -2 More preferably, it is 3.0×10 14 m -2 The upper limit of the dislocation density of the austenitic stainless steel according to this embodiment is not particularly specified. In order to improve the toughness of the austenitic stainless steel according to this embodiment, the dislocation density is 9.0×10 14 m -2 Less than 8.0×10 is preferable. 14 m -2The following is more preferable. The dislocation density of the austenitic stainless steel according to this embodiment is a value measured by the method described in the examples described later.

[0041] The amount of precipitated Cr in the austenitic stainless steel according to this embodiment is preferably 0.8 mass% or less, and more preferably 0.5 mass% or less. The lower limit of the amount of precipitated Cr in the austenitic stainless steel according to this embodiment is not particularly specified, and the lower the better, and it may be 0.0 mass%. The amount of precipitated Cr in the austenitic stainless steel according to this embodiment is a value measured by the method described in the examples below.

[0042] In this embodiment, the average grain size and dislocation density of the austenitic stainless steel according to this embodiment can be controlled within the above-mentioned numerical ranges by heating and hot rolling the steel material according to the conditions described below and not carrying out a solution heat treatment, and as a result, the above-mentioned low temperature strength can be achieved in the austenitic stainless steel according to this embodiment.

[0043] In this embodiment, the austenitic stainless steel may be a steel plate having a thickness of 10 mm or more. When the austenitic stainless steel according to this embodiment is a steel plate, the thickness of the steel plate is preferably 12 mm or more from the viewpoint of suitable use as a structural steel material used in extremely low temperature environments. The upper limit of the thickness of the steel plate is not particularly limited and may be any thickness, but is preferably 30 mm or less.

[0044] Next, an example of a method for producing the austenitic stainless steel according to this embodiment will be described.

[0045] First, molten steel having the above-mentioned composition is obtained by melting using a known melting method such as a converter or an electric furnace. Secondary refining may also be performed in a vacuum degassing furnace.

[0046] When obtaining the austenitic stainless steel according to the present embodiment, in order to limit the contents of Ti and Nb, which hinder the structure control, to the above-mentioned numerical range, it is necessary to take measures to prevent Ti and Nb from being mixed in from raw materials as much as possible and to reduce the contents of these elements. For example, by lowering the basicity of the slag in the refining stage, these alloys may be concentrated in the slag and discharged, thereby reducing the concentrations of Ti and Nb in the slab product. Alternatively, oxygen may be blown into the molten steel to oxidize it, and the Ti and Nb alloys may be floated and separated during reflux.

[0047] Thereafter, it is preferable to form the slab or other steel material of a predetermined size by a known casting method such as a continuous casting method or an ingot casting-blooming rolling method.

[0048] The method for producing an austenitic stainless steel according to this embodiment will be described in detail below.

[0049] In the method for producing an austenitic stainless steel according to this embodiment, as described above, the heating step, the hot rolling step, and the cooling step are performed.

[0050] In the following description, unless otherwise specified, the temperature indicated in "°C" refers to the surface temperature of the steel material or steel plate (austenitic stainless steel). In each process described below, the temperature is controlled based on the surface temperature of the steel material or steel plate. The surface temperature can be measured, for example, by a radiation thermometer.

[0051] In the heating step, the steel material is heated to 1000°C or higher and 1300°C or lower. If the heating temperature of the steel material is less than 1000°C, it may not be possible to achieve a total rolling reduction of 52% or higher at 950°C or higher in the hot rolling step described below. If the heating temperature of the steel material exceeds 1300°C, the crystal grains may become excessively coarse, and high yield stress may not be obtained after the steel material is made into an austenitic stainless steel. The heating temperature of the steel material is preferably 1130°C or higher and 1270°C or lower.

[0052] The hot rolling process includes a first rolling process and a second rolling process carried out after the first rolling process.

[0053] In the first rolling step, the steel material after the heating step is hot-rolled at 950°C or higher. In the first rolling step, the total reduction is set to 52% or higher. As described above, in the present disclosure, it is important to set the average grain size to less than 30 μm in the microstructure of the austenitic stainless steel. If the structure of the steel material cannot be made equiaxed in the first rolling step, i.e., rolling in the recrystallization region, the grains will remain as coarse grains in the subsequent rolling (second rolling step). Therefore, in the first rolling step, it is effective to ensure a total reduction of 52% or higher in the temperature range of 950°C or higher, which is the recrystallization region. In the first rolling step, the total reduction is preferably 55% or higher. The upper limit of the temperature of the steel material in the first rolling step is not particularly limited, but the temperature may be 1300°C or lower. In the first rolling step, i.e., the upper limit of the total reduction in the recrystallization region, is not limited. However, in order to ensure the strength of the austenitic stainless steel, the total reduction in the first rolling step is preferably 80% or less, and more preferably 70% or less.

[0054] In the second rolling step, the steel material is hot-rolled at less than 900° C. for one or more passes. The finish rolling end temperature is 800° C. or higher. That is, the hot rolling of the steel material in the second rolling step is performed at a temperature of 800° C. or higher and lower than 900° C.

[0055] In the second rolling step, in order to refine the equiaxed grains formed in the steel material in the first rolling step, it is important to perform at least one hot rolling pass at less than 900°C, which is an unrecrystallized region. The number of hot rolling passes in the second rolling step, i.e., at less than 900°C, is preferably two or more. There is no particular upper limit on the number of hot rolling passes at less than 900°C, but from the viewpoint of preventing the finish rolling end temperature from decreasing excessively, the number of passes is preferably five or less.

[0056] If hot rolling is performed at less than 800°C in the second rolling step, precipitated Cr is generated and the corrosion resistance of the austenitic stainless steel is reduced, so the finish rolling end temperature in the second rolling step is set to 800°C or higher. The finish rolling end temperature (the temperature of the steel material in the last rolling pass in the second rolling step) is preferably 820°C or higher. The finish rolling end temperature is less than 900°C.

[0057] The cooling step is performed after the second rolling step. In the cooling step, the steel material is water-cooled. In the cooling step, the steel material is water-cooled at an average water-cooling rate of 10°C / s or more from a cooling start temperature in a temperature range of 100°C lower than the above-mentioned finish rolling end temperature (finish rolling end temperature -100°C) to a cooling stop temperature in a temperature range of 600°C or less. This suppresses the generation of precipitated Cr, and sufficient corrosion resistance can be ensured without carrying out a solution heat treatment. The upper limit of the cooling start temperature is not particularly limited, but the cooling start temperature can be 870°C or less. The lower limit of the cooling stop temperature is not particularly limited, but the cooling stop temperature can be 250°C or more. The upper limit of the average water-cooling rate is not particularly limited, but the average water-cooling rate can be 40°C / s or less.

[0058] In the method for producing austenitic stainless steel according to this embodiment, no solution heat treatment is performed after the cooling step. The solution heat treatment for austenitic stainless steel is a process in which the austenitic stainless steel containing precipitated Cr is heated to 1000 to 1100°C, the temperature is maintained until the structure becomes austenite single phase, and then the stainless steel is quenched in water, so that the precipitated Cr is dissolved again to recover the corrosion resistance.

[0059] In this manner, an austenitic stainless steel and a method for producing the same can be provided. EXAMPLES

[0060] The present disclosure will be described below based on examples. Note that the following examples are merely preferred examples of the present disclosure, and the present disclosure is not limited to these examples.

[0061] Steel slabs (steel Nos. 1 to 22) having the composition shown in Table 1 were produced by the converter-ladle refining (secondary refining)-continuous casting method. The symbol "-" in Table 1 indicates that the component is not added intentionally, and includes not only the case where the component is not contained (0%), but also the case where the component is inevitably contained. In Table 1, steel Nos. 1 to 10, 21, and 22 have the composition according to this embodiment. Next, the above steel slabs were heated and hot-rolled under the conditions shown in Table 2 (see "Method of manufacturing steel sheet" in Table 2), and then water-cooled to produce austenitic stainless steel sheets (sample Nos. 1 to 31) having a thickness of 10 to 30 mm. In No. 29, after water-cooling, a solid solution heat treatment was performed at 1010°C. In Table 2, the steel sheets of sample Nos. 1 to 10, 30, and 31 are austenitic stainless steels according to the invention that satisfy the range of this embodiment. In Tables 1 and 2, the underlined values ​​indicate that the values ​​are outside the range defined in this embodiment.

[0062] [Table 1]

[0063] [Table 2]

[0064] The obtained steel sheets were evaluated as follows.

[0065] (1) The grain size was determined as follows.

[0066] The cross section perpendicular to the width direction of the obtained hot-rolled steel sheets was polished and etched, and the electron backscatter diffraction (EBSD) pattern was measured in a scanning electron microscope (SEM) at the center of the sheet thickness. The measurement area was 1 mm x 1 mm. The measured EBSD data was analyzed using OIM-Analysis to obtain the average grain size (μm). These average grain sizes are also shown in Table 2.

[0067] (2) Dislocation density was calculated as follows.

[0068] For the hot-rolled steel sheets obtained, a sample of 10 mm length × 10 mm width × 1.0 mm thickness was taken from the center of the sheet thickness, mirror polished, electrolytically polished, and then an X-ray diffraction device was used to obtain a diffraction pattern and perform dislocation density analysis. The dislocation density thus obtained is also shown in Table 2.

[0069] (3) The amount of precipitated Cr was determined as follows.

[0070] For the obtained hot-rolled steel sheets, samples for electrolytic extraction were taken from the center of the sheet thickness, and the precipitates extracted by electrolytic extraction using a 10% AA (10% acetylacetone-1% tetramethylammonium chloride-methanol) solution were measured for the Cr content in the precipitates by ICP emission spectrometry. The precipitated Cr content is also shown in Table 2.

[0071] (4) Low temperature strength was evaluated as follows.

[0072] From each of the obtained steel plates, a JIS No. 4 tensile test piece was taken from the steel plates having a thickness exceeding 15 mm (except for sample Nos. 5, 7, 18, and 31), and a tensile test was carried out at -269°C to determine the yield stress.

[0073] For the steel plates of Samples No. 5, 7, 18, and 31 having a plate thickness of 15 mm or less, round bar tensile test pieces having a parallel part diameter of 6 mm and a gauge length of 25 mm were taken and subjected to a tensile test at −269° C. to determine the yield stress. Note that the temperature at which the tensile test was performed in this example (−269° C.) corresponds to the boiling point of liquid helium.

[0074] The yield stress at -269°C obtained as described above is also shown in Table 2. When the yield stress is 1000 MPa or more, it is determined that the target performance is satisfied and the low-temperature strength is excellent. That is, in this embodiment, "excellent low-temperature strength" means that the yield stress is 1000 MPa or more in an environment of -269°C, which is the boiling point of liquid helium.

[0075] (5) Corrosion resistance was evaluated as follows.

[0076] A corrosion resistance test was conducted in accordance with the Slow Strain Rate Test Method (hereinafter referred to as SSRT test) of the NACE Standard TM0111-2011. Specifically, the test specimen was a Type A round bar with a notch, and was immersed in artificial seawater (chloride ion concentration 18,000 ppm) at a temperature of 23°C and strain rate of 4×10 -7 A constant velocity tensile test was carried out at 100 mm / s. A breaking stress of 600 MPa or more was judged to be "good" as it had excellent corrosion resistance and satisfied the target performance, and a breaking stress of less than 600 MPa was judged to be "poor" as it had poor corrosion resistance and did not satisfy the target performance. The results of the corrosion resistance evaluation as described above are shown in Table 2.

[0077] As shown in Table 2, the austenitic stainless steel according to this embodiment (samples Nos. 1 to 10, 30, and 31) has an average crystal grain size in the microstructure of less than 30 μm and a dislocation density of 1.0 × 10 14 m -2 or more, and the amount of precipitated Cr is less than 1.0 mass %.

[0078] In the austenitic stainless steel according to this embodiment, the yield stress in the tensile test at -269°C was 1000 MPa or more, satisfying the target performance.

[0079] Furthermore, the austenitic stainless steel according to this embodiment also had good corrosion resistance and satisfied the target performance.

[0080] In contrast, the steel sheets of Samples 11 to 29, which are outside the scope of the present disclosure, could not satisfy the above target performance, regardless of the values ​​of the average crystal grain size, dislocation density, and precipitated Cr amount.

[0081] As shown in the above examples, the austenitic stainless steel according to this embodiment can be manufactured at low cost because the manufacturing process does not require a solution heat treatment. In addition, the austenitic stainless steel according to this embodiment has high strength in a low-temperature environment and excellent corrosion resistance. Therefore, the austenitic stainless steel according to this embodiment can be suitably used as a material for steel structures (such as tanks for liquefied gas storage tanks) used in low-temperature environments. It can greatly contribute to improving the safety and life of such steel structures. In addition, the manufacturing method for the austenitic stainless steel according to this embodiment is economical without causing a decrease in productivity or an increase in manufacturing costs.

[0082] Note that the embodiments disclosed in this specification are merely examples, and the embodiments of the present disclosure are not limited thereto, and may be modified as appropriate within the scope of the purpose of the present disclosure. [Industrial Applicability]

[0083] The present disclosure is applicable to austenitic stainless steels and methods for making same.

Claims

1. In mass percent, C: 0.050% or more and 0.100% or less, Si: 0.05% or more and 1.00% or less, Mn: 5.0% or more and 20.0% or less, P: 0.030% or less, S: 0.0050% or less, Al: 0.070% or less, Cu: 1.5% or more and 3.5% or less, Ni: 5.0% or more and 10.0% or less, Cr: 11.0% or more and 20.0% or less, N: 0.05% or more and 0.20% or less, W: 0.05% or more and 0.50% or less, O: 0.0050% or less, Ti: 0.005% or less, and Nb: 0.005% or less and the balance being Fe and unavoidable impurities; The average grain size is less than 30 μm, and the dislocation density is less than 1.0×10 １４ m －２ or more, and the amount of precipitated Cr is less than 1.0 mass%; Mechanical properties in which the yield stress in a tensile test at −269° C. is 1000 MPa or more; Austenitic stainless hot-rolled steel sheet having the above properties.

2. The composition further comprises, in mass%, Mo: 2.0% or less, V: 0.5% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0200% or less The austenitic stainless steel hot rolled sheet according to claim 1, comprising at least one selected from the group consisting of:

3. In mass percent, C: 0.050% or more and 0.100% or less, Si: 0.05% or more and 1.00% or less, Mn: 5.0% or more and 20.0% or less, P: 0.030% or less, S: 0.0050% or less, Al: 0.070% or less, Cu: 1.5% or more and 3.5% or less, Ni: 5.0% or more and 10.0% or less, Cr: 11.0% or more and 20.0% or less, N: 0.05% or more and 0.20% or less, W: 0.05% or more and 0.50% or less, O: 0.0050% or less, Ti: 0.005% or less, and Nb: 0.005% or less a heating step of heating a steel material having a composition containing Fe and unavoidable impurities to a temperature range of 1000°C to 1300°C; a hot rolling step of hot rolling the steel material after the heating step to obtain a hot rolled steel sheet; A cooling process of cooling the hot-rolled steel sheet after the hot rolling; Including, The hot rolling process includes: (i) a first rolling process in which the steel material is hot-rolled at 950° C. or higher to a total rolling reduction of 52% or higher; and (ii) a second rolling process that is carried out after the first rolling process and hot-rolls the steel material at less than 900° C. for one or more passes, and the finish rolling end temperature is 800° C. or higher; In the cooling step, water cooling is performed at an average water cooling rate of 10 ° C. / s or more from a cooling start temperature in a temperature range of 100 ° C. lower than the finish rolling end temperature to a cooling stop temperature in a temperature range of 600 ° C. or less, A method for producing an austenitic stainless steel hot-rolled steel sheet having a microstructure with an average crystal grain size of less than 30 μm, a dislocation density of 1.0 x 10 14 m -2 or more, and a precipitated Cr amount of less than 1.0 mass %, and mechanical properties with a yield stress of 1000 MPa or more in a tensile test at -269°C.

4. The composition further comprises, in mass%, Mo: 2.0% or less, V: 0.5% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0200% or less The method for producing austenitic stainless steel hot rolled steel sheet according to claim 3, further comprising at least one selected from the group consisting of:

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