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

Abstract

The present disclosure provides a low-cost austenitic stainless steel having excellent low-temperature toughness in the heat-affected zone of welding. The austenitic stainless steel of the present disclosure has, by mass%, 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: 0.070% to 0.050%, Cu: 1.5% to 3.5%, Ni: 5.0% to 10.0%, Cr: 11.0% to 20.0%, N: 0.05% to 0.20%, and W: 0.05% to 0.50%. The steel has a composition containing 0.0050% or less O, 0.005% or less Ti, and 0.005% or less Nb, with the balance being Fe and unavoidable impurities; a microstructure in which the number ratio of crystal grains having an aspect ratio (long side / short side) of 3.5 or less is 85% or more and the amount of precipitated Cr having a diameter of more than 100 nm is 0.2 mass % or less; and a characteristic in which the absorbed energy in a Charpy impact test at -269°C in the coarse grain region of the weld heat affected zone is 55 J or more.

Description

[Technical field]

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

[0002] In order to use hot-rolled steel sheets as materials for storage tank structures for liquefied gases such as liquid hydrogen and liquid helium, the hot-rolled steel sheets are required to have excellent toughness at low temperatures. This is because the materials for such storage tank structures are used in extremely low temperatures. For example, when using hot-rolled steel sheets for storage tanks for liquid helium, it is desirable to ensure excellent toughness at temperatures below -269°C, which is the boiling point of helium. In general, the weld heat-affected zone in the welded portion of a steel sheet has inferior low-temperature toughness compared to the base material. Therefore, it is desirable to improve the low-temperature toughness of the weld heat-affected zone of the steel sheet.

[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: 0.10% or less, Si: 1.00% or less, Mn: 6.0-12.0%, P: 0.050% or less, S: 0.0500% or less, Ni: 15.0-17.0%, Cr: 15.0-30.0%, V: 0.01-0.30%, Nb: 0.01-0.30%, N: 0.40-0.60%, Mo: 0-5.0%, and the balance being Fe and impurities, which satisfies a predetermined relational formula, and the undissolved V content is 0.20% or less by mass and the undissolved Nb content is 0.20% or less by mass. Patent Document 1 discloses that the austenitic stainless steel has high strength and excellent low-temperature toughness of the welded portion.

[0004] Patent Document 2 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 2 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 2 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]

[0005] [Patent Document 1] JP 2020-132979 A [Patent Document 2] International Publication No. 2017 / 056619 Summary of the Invention [Problem to be solved by the invention]

[0006] The austenitic stainless steels described in Patent Documents 1 and 2 have a problem in that their manufacturing process requires a solution heat treatment in addition to a hot working process, resulting in high costs. Therefore, there is a demand for a method for manufacturing austenitic stainless steels that have excellent low-temperature toughness in the weld heat-affected zone without carrying out solution heat treatment.

[0007] The present disclosure has been made in consideration of the above circumstances, and an object of the present disclosure is to provide a low-cost austenitic stainless steel having excellent low-temperature toughness in a weld heat-affected zone, and a method for producing the same. [Means for solving the problem]

[0008] 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.

[0009] [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; A microstructure in which the number ratio of crystal grains having an aspect ratio (long side / short side) of 3.5 or less is 85% or more and the amount of precipitated Cr having a diameter of more than 100 nm is 0.2 mass% or less; The characteristic that the absorbed energy in the Charpy impact test at -269°C in the coarse grain region of the welded heat affected zone is 55J or more; An austenitic stainless steel having a

[0010] [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:

[0011] [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 1350°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 with an average pass reduction of 5% or more; and (ii) a second rolling process which is carried out after the first rolling process and hot-rolls the steel material at less than 950°C for 5 passes or less, and the finish rolling end temperature is 900°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, which is in a temperature range of 100°C lower than the finish rolling end temperature, to a cooling stop temperature, which is in a temperature range of 600°C or lower.

[0012] [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

[0013] According to the present disclosure, it is possible to provide a low-cost austenitic stainless steel having excellent low-temperature toughness in a weld heat-affected zone, and a method for producing the same. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] An austenitic stainless steel and a method for producing the same according to an embodiment of the present disclosure will be described.

[0015] The austenitic stainless steel of this embodiment has a composition containing, by mass%, 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. The austenitic stainless steel according to this embodiment has a microstructure in which the proportion of crystal grains with an aspect ratio (long side / short side) of 3.5 or less is 85% or more and the amount of precipitated Cr with a diameter of more than 100 nm is 0.2 mass % or less, and the absorbed energy in a Charpy impact test at -269°C in the coarse grain region of the weld heat affected zone is 55 J or more.

[0016] 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 1350°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 with an average pass reduction of 5% or more, and (ii) a second rolling step performed after the first rolling step of hot rolling the steel material at less than 950°C for 5 passes or less, and the finish rolling end temperature is 900°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 100°C or more lower than the finish rolling end temperature to a cooling stop temperature in a temperature range of 600°C or less.

[0017] The austenitic stainless steel according to the present embodiment does not require a solution heat treatment in its manufacturing process, and therefore can be manufactured at low cost. Furthermore, the austenitic stainless steel according to the present embodiment has excellent low-temperature toughness in the weld heat affected zone.

[0018] The austenitic stainless steel and the manufacturing method thereof according to this embodiment will be described in detail below.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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 increasing the strength of the steel material by solid solution strengthening through solid solution dissolution in steel and improving the low-temperature toughness of the welded heat affected zone. In order to fully obtain such effects, the steel material contains 0.050% or more of C. If the steel material contains more than 0.100% of C, coarse precipitated Cr is excessively generated, and the low-temperature toughness of the welded heat affected zone decreases when the steel material is welded. The C content in the steel material is preferably 0.060% or more and preferably 0.090% or less.

[0023] 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, and 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, non-thermal stress increases excessively, and the low-temperature toughness of the weld heat affected zone deteriorates. The Si content in the steel material is preferably 0.07% or more and preferably 0.80% or less.

[0024] 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 has the effect of increasing the strength of the steel material by solid solution strengthening through solid solution dissolution in steel. Mn also has the effect of improving the low temperature toughness of the welded heat affected zone. 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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. Cu and Ni also have the effect of improving the low-temperature toughness of the welded heat affected zone. 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 reduced. 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.

[0029] 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, coarse Cr precipitates are excessively generated, and the low-temperature toughness of the weld heat affected zone decreases. The Cr content in the steel material is preferably 12.0% or more and preferably 19.0% or less.

[0030] 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 increasing the strength of the steel material by solid solution strengthening through solid solution dissolution in steel. N also has the effect of improving the low-temperature toughness of the welded heat affected zone. In order to obtain such an effect, the steel material contains 0.05% or more of N. If the steel material contains N in excess of 0.20%, the nitrides or carbonitrides in the steel material may become coarse, and the low-temperature toughness of the welded heat affected zone may decrease. The N content in the steel material is preferably 0.06% or more and preferably 0.19% or less.

[0031] 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.

[0032] The O content in the steel product is 0.0050% or less. O deteriorates the low-temperature toughness of the weld heat affected zone by forming oxides. The O content in the steel product is preferably 0.0045% or less. Note that 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.

[0033] 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 weld heat-affected zone. 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 mixing 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.

[0034] 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.

[0035] The steel material in this embodiment may contain the elements described below as optional components as necessary.

[0036] 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.

[0037] 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.

[0038] Next, the microstructure of the austenitic stainless steel according to this embodiment will be described.

[0039] In the austenitic stainless steel according to this embodiment, the number ratio of crystal grains having an aspect ratio (long side / short side) of 3.5 or less is 85% or more, and the amount of precipitated Cr with a diameter of more than 100 nm is 0.2 mass% or less. When the aspect ratio of the crystal grains of the austenitic stainless steel is large, the grain boundaries are prone to stress concentration, and the coarse Cr present at the grain boundaries becomes the starting point of fracture, resulting in a decrease in low-temperature toughness. By setting the number ratio of crystal grains having an aspect ratio of 3.5 or less to 85% or more and setting the amount of precipitated Cr with a diameter of more than 100 nm to 0.2 mass% or less, excellent toughness can be achieved in the coarse grain region of the weld heat affected zone when the steel is welded, with an absorbed energy of 55 J or more in a Charpy impact test at -269°C.

[0040] In the austenitic stainless steel according to this embodiment, the number ratio of crystal grains having an aspect ratio of 3.5 or less is preferably 87% or more, more preferably 90% or more. In the austenitic stainless steel according to this embodiment, the upper limit of the number ratio of crystal grains having an aspect ratio of 3.5 or less is not limited. In order to ensure the strength of the austenitic stainless steel according to this embodiment, the number ratio of crystal grains having an aspect ratio of 3.5 or less is preferably 97% or less, more preferably 95% or less. In the austenitic stainless steel according to this embodiment, the above aspect ratio is a value measured by the method described in the examples below. The lower limit of the aspect ratio is 1.0 (when the long side and the short side are equal).

[0041] The amount of precipitated Cr having a diameter exceeding 100 nm in the austenitic stainless steel according to this embodiment is preferably 0.1 mass% or less. The lower limit of the amount of precipitated Cr having a diameter exceeding 100 nm in the austenitic stainless steel according to this embodiment is not particularly specified, and the lower the better, it may be 0.0 mass%. The amount of precipitated Cr having a diameter exceeding 100 nm 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 ratio of the number of crystal grains having an aspect ratio of 3.5 or less and the amount of precipitated Cr having a diameter of more than 100 nm can be controlled within the above-mentioned numerical ranges by performing hot rolling and water cooling according to the conditions described below. As a result, the low-temperature toughness of the weld heat-affected zone 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 a steel material such as a slab 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 1350°C or lower. By setting the heating temperature of the steel material to 1000°C or higher, segregation can be reduced in the hot rolling step described below, and an austenitic structure can be obtained even in the negative segregation portion, thereby ensuring the low-temperature toughness of the welded heat affected zone. If the heating temperature of the steel material exceeds 1350°C, the steel may start to melt. The heating temperature of the steel material is preferably 1130°C or higher and 1320°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 average pass reduction is set to 5% or higher. As described above, in the present disclosure, it is important to set the ratio of crystal grains having an aspect ratio (long side / short side) of 3.5 or less in the microstructure of the austenitic stainless steel to 85% or more. If the grains become mixed grains in the first rolling step, i.e., rolling in the recrystallization region, the aspect ratio of the crystal grains increases in the subsequent rolling (second rolling step). Therefore, in the first rolling step, it is effective to set the average pass reduction to 5% or more for the purpose of promoting recrystallization in the temperature range of 950°C or higher, which is the recrystallization region. The upper limit of the temperature of the steel material in the first rolling step is not particularly limited, but the temperature can be 1350°C or lower. In the first rolling step, the average pass reduction is preferably 6% or more. In the first rolling step, the higher the average pass reduction, the more preferable; however, from the viewpoint of the equipment capacity of the rolling mill, the average pass reduction may be 10% or less.

[0054] In the second rolling step, the steel material is hot-rolled for 5 passes or less at a temperature of less than 950° C. The finish rolling end temperature is 900° C. or higher. That is, the hot rolling of the steel material in the second rolling step is performed at a temperature of 900° C. or higher and lower than 950° C.

[0055] Since the partial recrystallization region is from less than 950°C to more than 900°C, the aspect ratio of the crystal grains of the steel material increases in the second rolling step. Therefore, it is important to limit the number of hot rolling passes at less than 950°C in the second rolling step to 5 or less. The number of hot rolling passes at less than 950°C in the second rolling step is preferably 3 or less. The lower limit of the number of hot rolling passes at less than 950°C is 0.

[0056] If hot rolling is performed at less than 900°C in the second rolling process, the aspect ratio of the crystal grains increases significantly due to the non-recrystallized region. Therefore, the finish rolling end temperature (the temperature of the steel material at the entry side of the last rolling pass in the second rolling process) is set to 900°C or higher. The finish rolling end temperature is preferably 920°C or higher. The finish rolling end temperature is less than 950°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 can suppress the generation of precipitated Cr and suppress the decrease in low-temperature toughness of the welded heat-affected zone of the austenitic stainless steel. The upper limit of the cooling start temperature is not particularly limited, but the cooling start temperature can be 920°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 29) having the composition shown in Table 1 were produced by the converter-ladle refining (secondary refining)-continuous casting method. The "-" 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 and 23 to 29 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 plate" in Table 2), and then water-cooled to produce austenitic stainless steel plates (Sample Nos. 1 to 36) having a plate thickness of 10 to 30 mm. In addition, rectangular joint test plates (size: 250 mm x 500 mm) were taken from the produced steel plates, and welded joints were produced by MAG welding. The welding conditions were as follows: groove shape: square groove, backing material: ceramics, shielding gas: Ar-30%CO2, torch sweep angle: 5-10°, average welding heat input: 2.0 kJ / mm. In Table 2, steel plates of samples No. 1-10 and 30-36 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 specified in this embodiment.

[0062] [Table 1]

[0063] [Table 2]

[0064] The obtained steel plates and welded joints were evaluated as follows.

[0065] (1) The ratio of the number of crystal grains having an aspect ratio of 3.5 or less was determined as follows.

[0066] The cross section perpendicular to the width direction of the obtained steel sheet 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, and the number ratio of crystal grains with an aspect ratio (long side / short side) of 3.5 or less was calculated. These number ratios are also shown in Table 2.

[0067] (2) The amount of precipitated Cr with a diameter of more than 100 nm was determined as follows.

[0068] For the obtained hot-rolled steel plate, samples for electrolytic extraction were taken from the center position of the plate thickness, and the precipitates extracted by electrolytic extraction using a 10% AA (10% acetylacetone-1% tetramethylammonium chloride-methanol) solution were filtered and collected using an alumina filter with a pore size of 100 nm, and the Cr amount in the precipitates was measured by ICP emission spectrometry. The amount of precipitated Cr with a diameter of over 100 nm is also shown in Table 2.

[0069] (3) The low temperature toughness of the weld heat affected zone was evaluated as follows.

[0070] For the welded joints produced from each sample, three Charpy V-notch test pieces were taken from each sample in accordance with the provisions of JIS Z 2242 (2023). Using the three Charpy V-notch test pieces, a Charpy impact test was performed at -269°C. The average value of the absorbed energy of the three test pieces was then calculated. In this example, the temperature at which the Charpy impact test was performed (-269°C) corresponds to the boiling point of liquid helium.

[0071] The average values ​​thus obtained are shown in Table 2 as absorbed energies at -269°C. When the average absorbed energy of the three specimens was 55 J or more, the specimen was judged to have excellent low-temperature toughness as satisfying the target performance. That is, in this embodiment, excellent low-temperature toughness means that the average absorbed energy in a Charpy impact test in an environment of -269°C, which is the boiling point of liquid helium, is 55 J or more.

[0072] As shown in Table 2, the austenitic stainless steel according to this embodiment (samples No. 1 to 10, 30 to 36) has a microstructure in which the number ratio of crystal grains having an aspect ratio (long side / short side) of 3.5 or less is 85% or more, and the amount of precipitated Cr with a diameter exceeding 100 nm is 0.2 mass% or less.

[0073] Furthermore, in the austenitic stainless steel according to this embodiment, the absorbed energy in the coarse-grained region of the weld heat affected zone in the Charpy impact test at -269°C was 55 J or more, satisfying the target performance.

[0074] In contrast, the steel sheets of Samples 11 to 29 outside the scope of the present disclosure could not satisfy the above target performance, regardless of the proportion of crystal grains with an aspect ratio of 3.5 or less and the amount of precipitated Cr with a diameter of more than 100 nm.

[0075] As shown in the above examples, the austenitic stainless steel according to this embodiment can be manufactured at low cost since the manufacturing process does not require a heat treatment step. Furthermore, the austenitic stainless steel according to this embodiment has excellent toughness in a low-temperature environment. 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 lifespan of such steel structures. Furthermore, 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.

[0076] 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]

[0077] 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; A microstructure in which the number ratio of crystal grains having an aspect ratio (long side / short side) of 3.5 or less is 85% or more and the amount of precipitated Cr having a diameter of more than 100 nm is 0.2 mass% or less; An austenitic stainless steel hot-rolled steel sheet having The austenitic stainless hot-rolled steel plate has a welded joint produced under the following welding conditions: groove shape: R-shaped groove, backing material: ceramics, shielding gas: Ar-30% CO2, torch sweep angle: 5 to 10°, average welding heat input: 2.0 kJ / mm, and the absorbed energy in the coarse grain region of the weld heat-affected zone in a Charpy impact test at -269°C is 55 J or more.

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 1350°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 with an average pass reduction of 5% or more; and (ii) a second rolling process which is carried out after the first rolling process and hot-rolls the steel material at less than 950 ° C. for 5 passes or less, and the finish rolling end temperature is 900 ° 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, the method comprising the steps of: producing an austenitic stainless steel hot-rolled steel sheet having a microstructure in which the number ratio of crystal grains having an aspect ratio (long side / short side) of 3.5 or less is 85% or more, and the amount of precipitated Cr having a diameter of more than 100 nm is 0.2 mass% or less.

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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