Austenitic stainless steel material

Abstract

To provide an austenitic stainless steel material capable of suppressing separation and exhibiting high hardness under low-temperature conditions.SOLUTION: An austenitic stainless steel material comprises, in mass%, C: 0.080% or less, Si: 1.20% or less, Mn: 3.50% or less, P: 0.050% or less, S: 0.050% or less, Ni: 6.00 to 15.00%, Cr: 16.00 to 21.00%, Mo: 5.00% or less, Ti: 0.30% or less, Nb: 0.10% or less, N: 0.010 to 0.30%, optional elements, and the balance: Fe and impurities, wherein the Brinell hardness is 187 HBW or more and the maximum crack length is 1 mm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an austenitic stainless steel material. [Background technology]

[0002] Due to their properties, austenitic stainless steel materials are used in a variety of fields, including fuel tanks and water gates. For example, for low-temperature applications such as fuel tanks, the development of heat-treated materials that have undergone solution heat treatment is underway. Therefore, as shown in Patent Document 1, austenitic stainless steel materials that have undergone solution heat treatment are being developed. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-133001 Summary of the Invention [Problem to be solved by the invention]

[0004] On the other hand, when the above-mentioned steel material (hereinafter simply referred to as "heat-treated material") is subjected to solution heat treatment, the hardness of the material decreases. Therefore, it is not suitable for use when high hardness is desired. Furthermore, from the viewpoint of reducing environmental load and manufacturing load, it is preferable not to perform solution heat treatment.

[0005] However, when an impact test was conducted on a non-heat-treated material that had not undergone solution heat treatment, assuming low-temperature applications, the inventors discovered a phenomenon in which cracks of several millimeters in size (hereinafter also referred to as "separation") occurred on the fracture surface of the impact-tested material. Note that the non-heat-treated material of the present invention refers to a hot-rolled steel material, but will hereinafter be referred to as the non-heat-treated material.

[0006] Although non-heat-treated materials have excellent low-temperature impact properties like heat-treated materials, the morphology of the low-temperature impact fracture surface is accompanied by separation in addition to the fracture surface morphology that has been conventionally recognized, and the challenge is to suppress the occurrence of separation without performing heat treatment.

[0007] An object of the present invention is to provide a non-heat-treated austenitic stainless steel material that has high hardness at room temperature and suppresses separation in a low-temperature environment. [Means for solving the problem]

[0008] The present invention has been made to solve the above-mentioned problems, and the gist of the present invention is the following austenitic stainless steel material.

[0009] (1) Chemical composition, in mass%, C: 0.080% or less, Si: 1.20% or less, Mn: 3.50% or less, P: 0.050% or less, S: 0.050% or less, Ni: 6.00~15.00%, Cr: 16.00~21.00%, Mo: 5.00% or less Ti: 0.30% or less, Nb: 0.10% or less, N: 0.010~0.10%, Al: 0 to 0.10% Cu: 0-2.0% V: 0 to 1.0%, Co: 0-3.0% Ca: 0 to 0.0100%, Mg: 0 to 0.0050%, Sb: 0 to 0.2% Sn: 0 to 0.10% Se: 0~0.080%, W: 0-1.0%, Ta: 0 to 0.20%, Hf: 0 to 0.10% Zr: 0 to 0.05%, Te: 0~0.080%, B: 0~0.01%, Bi: 0 to 0.300% Pb: 0~0.100%, REM: 0~0.10%, The balance is Fe and impurities. Brinell hardness is 187HBW or more, Austenitic stainless steel material with a maximum crack length of 1 mm or less.

[0010] (2) Chemical composition, in mass %, C: 0.080% or less, Si: 1.20% or less, Mn: 3.50% or less, P: 0.050% or less, S: 0.050% or less, Ni: 6.00~15.00%, Cr: 16.00~21.00%, Mo: 5.00% or less Ti: 0.30% or less, Nb: 0.10% or less, N: more than 0.10% and less than 0.30%, Al: 0 to 0.10% Cu: 0-2.0% V: 0 to 1.0%, Co: 0-3.0% Ca: 0 to 0.0100%, Mg: 0 to 0.0050%, Sb: 0 to 0.2% Sn: 0 to 0.10% Se: 0~0.080%, W: 0-1.0%, Ta: 0 to 0.20%, Hf: 0 to 0.10% Zr: 0 to 0.05%, Te: 0~0.080%, B: 0~0.01%, Bi: 0 to 0.300% Pb: 0~0.100%, REM: 0~0.10%, The balance is Fe and impurities. Brinell hardness is 217HBW or more, Austenitic stainless steel material with a maximum crack length of 1 mm or less.

[0011] (3) The chemical composition is in mass%: Al: 0.003 to 0.10%, Cu: 0.01 to 2.0% V: 0.01 to 1.0%, Co: 0.01 to 3.0%, Ca: 0.0010 to 0.0100%, Mg: 0.0001 to 0.0050%, Sb: 0.02 to 0.2% Sn: 0.001 to 0.10% Se: 0.005 to 0.080%, W: 0.01 to 1.0%, Ta: 0.005 to 0.20%, Hf: 0.01 to 0.10%, Zr: 0.001 to 0.05%, Te: 0.0050~0.080%, B: 0.0002~0.01%, Bi: 0.03 to 0.300%, Pb: 0.010 to 0.100%, and REM: 0.005~0.10%, The austenitic stainless steel material according to (1) or (2) above, containing one or more selected from the following:

[0012] (4) The chemical composition is in mass%: Al: 0.003 to 0.10%, Cu: 0.01 to 2.0% V: 0.01 to 1.0%, Co: 0.01 to 3.0%, Ca: 0.0010 to 0.0100%, Mg: 0.0001 to 0.0050%, Sb: 0.02 to 0.2% Sn: 0.001 to 0.10% Se: 0.005 to 0.080%, W: 0.01 to 1.0%, Ta: 0.005 to 0.20%, Hf: 0.01 to 0.10%, Zr: 0.001 to 0.05%, Te: 0.0050~0.080%, B: 0.0002~0.01%, Bi: 0.03 to 0.300%, Pb: 0.010 to 0.100%, and REM: 0.005~0.10%, The austenitic stainless steel material according to (2) above, containing one or more selected from the following:

[0013] (5) The austenitic stainless steel material according to any one of (1) to (4) above, which is used for a tank. [Effects of the Invention]

[0014] According to the present invention, it is possible to obtain a non-heat-treated austenitic stainless steel material that has high hardness at room temperature and suppresses separation in a low-temperature environment. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present inventors have investigated the occurrence of separation and have obtained the following findings.

[0016] (a) In the case of non-heat-treated materials, since no solution heat treatment is performed, there may be areas where the metal structure is not uniform compared to heat-treated materials. As a result, the fracture surface of the impact test conducted at -196°C is a ductile fracture surface, but separation occurs. The separation is a crack of approximately several millimeters in size, and the occurrence of such separation is unique for a fracture surface morphology of ductile fracture.

[0017] (b) The inventors have come up with the idea of ​​suppressing separation by controlling the chemical composition and manufacturing method. Regarding the chemical composition, it is preferable to add a small amount of Ti. This is because the addition of a small amount of Ti results in a finer metal structure. It is also effective to perform two-heat rolling, in which the slab is heated twice at a temperature of 1000 to 1300°C and hot-rolled. It is also effective to control the rolling start temperature of the final three passes in hot rolling depending on the C, N, and Mn contents.

[0018] (c) By controlling the chemical composition and manufacturing method described above, it is possible to reduce the non-uniformity of the metal structure of the non-heat-treated material of the present invention, and to suppress separation.

[0019] An embodiment of the present invention has been made based on the above findings. Hereinafter, each requirement of the austenitic stainless steel material of this embodiment will be described in detail.

[0020] 1.Chemical composition The reasons for limiting the content of each element are as follows: In the following description, "%" in the content means "% by mass."

[0021] C: 0.080% or less C (carbon) has the effect of improving strength, but if it is contained in excess, sensitization tends to occur. Therefore, the C content is 0.080% or less. The C content is preferably 0.060% or less, and more preferably 0.030% or less. On the other hand, in order to obtain the above effect, the C content is preferably 0.008% or more.

[0022] Si: 1.20% or less Silicon (Si) is added for deoxidation. However, excessive Si content may reduce toughness. Therefore, the Si content is 1.20% or less. The Si content is preferably 1.00% or less, and more preferably 0.70% or less. On the other hand, to obtain the above effects, the Si content is preferably 0.15% or more.

[0023] Mn:3.50% or less Manganese (Mn) has the effect of improving low-temperature impact properties. However, excessive Mn content reduces corrosion resistance. For this reason, the Mn content is 3.50% or less. The Mn content is preferably 3.00% or less, and more preferably 2.50% or less. On the other hand, to obtain the above effect, the Mn content is preferably 0.20% or more.

[0024] P:0.050% or less P (phosphorus) is an impurity element contained in steel, which reduces toughness and hot workability. For this reason, the P content is 0.050% or less. It is preferable to reduce the P content as much as possible, but excessive reduction in the P content increases manufacturing costs. For this reason, the P content is preferably 0.0001% or more.

[0025] S: 0.050% or less S (sulfur) is an impurity element contained in steel, which reduces toughness and hot workability. For this reason, the S content is 0.050% or less. It is preferable to reduce the S content as much as possible, but excessive reduction of the S content increases manufacturing costs. For this reason, the S content is preferably 0.0001% or more.

[0026] Ni: 6.00~15.00% Ni (nickel) not only improves toughness but also has the effect of stabilizing the γ phase. In particular, it suppresses the formation of δ phase in the cast slab, which leads to a decrease in hot workability. For this reason, the Ni content is 6.00% or more. The Ni content is preferably 6.50% or more, and more preferably 7.00% or more. However, excessive Ni content increases the manufacturing cost. For this reason, the Ni content is 15.00% or less. The Ni content is preferably 10.50% or less.

[0027] Cr: 16.00~21.00% Cr (chromium) has the effect of improving corrosion resistance. Therefore, the Cr content is 16.00% or more. The Cr content is preferably 17.00% or more, and more preferably 18.00% or more. However, if Cr is contained in excess, the δ phase tends to form. Therefore, the Cr content is 21.00% or less. The Cr content is preferably 20.00% or less.

[0028] Mo: 5.00% or less Mo (molybdenum) has the effect of improving corrosion resistance. However, since Mo is an expensive element, excessive Mo content increases production costs. It also makes the δ phase more likely to form. For this reason, the Mo content is 5.00% or less. The Mo content is preferably 4.00% or less, and more preferably 3.00% or less. On the other hand, in order to obtain the above effects, the Mo content is preferably 0.10% or more.

[0029] Ti: 0.30% or less Titanium (Ti) has a very strong affinity with nitrogen (N) and forms Ti nitrides in steel. By utilizing the fine dispersion of Ti nitrides (TiN), grain refinement is achieved through their action during slab heating and in the high-temperature region of hot rolling, suppressing separation. However, excessive Ti content can result in mixed grains, leading to a non-uniform structure and promoting separation. For this reason, the Ti content is 0.30% or less. The Ti content is preferably less than 0.10%, more preferably 0.05% or less. Since even a small amount of Ti is effective in the present invention, the Ti content may be less than 0.020%, or even less than 0.010%. On the other hand, to achieve the above effects, the Ti content is preferably 0.0005% or more, more preferably 0.0008% or more, and even more preferably 0.0010% or more.

[0030] Nb: 0.10% or less Nb (niobium) has the effect of improving strength. However, excessive Nb content causes mixed grains, making the structure non-uniform and promoting the occurrence of separation. For this reason, the Nb content is 0.10% or less. The Nb content is preferably 0.075% or less, and more preferably 0.05% or less. On the other hand, in order to obtain the above effect, the Nb content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.

[0031] N: 0.010 to 0.30% N (nitrogen) has the effect of improving strength. Therefore, the N content is 0.010% or more. The N content is preferably 0.030% or more, more preferably 0.050% or more, more preferably 0.060% or more, even more preferably 0.10% or more, and even more preferably more than 0.10%. However, if N is contained in excess, a decrease in toughness occurs due to the formation of bubbles and nitride precipitation. Therefore, the N content is 0.30% or less. The N content is preferably 0.25% or less, more preferably 0.22% or less.

[0032] In addition to the above elements, one or more elements selected from Al, Cu, V, Co, Ca, Mg, Sb, Sn, Se, W, Ta, Hf, Zr, Te, B, Bi, Pb, and REM may be contained within the ranges shown below. In other words, the lower limit of the above elements is 0%. The reasons for limiting each element will be explained below.

[0033] Al: 0 to 0.10% Al (aluminum) has the effect of deoxidizing steel. Therefore, it is contained as needed. However, if Al is contained in excess, Al nitrides are formed, which reduces toughness. Therefore, the Al content is 0.10% or less. The Al content is preferably 0.08% or less. On the other hand, in order to obtain the above effect, the Al content is preferably 0.003% or more.

[0034] Cu: 0 to 2.0% Copper (Cu) is an element that additionally enhances the corrosion resistance of stainless steel to acids and also has the effect of improving toughness, and is contained as needed. Although the effects of Cu can be obtained even with trace amounts, if Cu is contained, it is preferable that the Cu content be 0.01% or more. In consideration of hot workability, the Cu content is preferably 2.00% or less, and preferably 1.9%.

[0035] V: 0 to 1.0% Vanadium (V) has an affinity with N and C, forms nitrides and carbides, and is an element that contributes to grain refinement. Therefore, it is contained as needed. However, if V is contained in excess, a large amount of V carbonitrides precipitates, reducing toughness. For this reason, the V content is 1.0% or less. The V content is preferably 0.8% or less. On the other hand, to obtain the above effects, the V content is preferably 0.01% or more.

[0036] Co: 0-3.0% Co (cobalt) has the effect of improving the toughness and corrosion resistance of steel. Therefore, Co may be contained as necessary. However, Co is an expensive element, and if Co is contained in excess, the manufacturing cost increases. For this reason, the Co content is 3.0% or less. The Co content is preferably 2.8% or less. On the other hand, in order to obtain the above effect, the Co content is preferably 0.01% or more.

[0037] Ca: 0 to 0.0100% Ca (calcium) is an element that improves the hot workability of steel and is contained as needed. Although the effect of Ca can be obtained even with a small amount, if Ca is contained, the content is preferably 0.0010% or more. These elements form relatively large oxides in the steel, which reduces the toughness of the steel. For this reason, the Ca content is 0.0100% or less. The Ca content is preferably 0.0050% or less.

[0038] Mg: 0 to 0.0050% Mg (magnesium) is an element that improves the hot workability of steel and is contained as needed. Although the effects of Mg can be obtained even with trace amounts, if Mg is contained, the content is preferably 0.0010% or more. These elements form relatively large oxides in the steel, reducing the toughness of the steel. For this reason, the Mg content is 0.0050% or less. The Mg content is preferably 0.0040% or less. On the other hand, to obtain the above effects, the Mg content is preferably 0.0001% or more.

[0039] Sb: 0 to 0.2% Sb (antimony) has the effect of improving corrosion resistance. Therefore, Sb may be contained as necessary. However, if Sb is contained in excess, weldability and toughness decrease. Therefore, the Sb content is 0.2% or less. The Sb content is preferably 0.15% or less, and more preferably 0.1% or less. On the other hand, in order to obtain the above effect, the Sb content is preferably 0.02% or more.

[0040] Sn: 0 to 0.10% Sn (tin) has the effect of improving corrosion resistance against acids. Therefore, it is contained as needed. However, if Sn is contained in excess, hot workability deteriorates. Therefore, the Sn content is 0.10% or less. The Sn content is preferably 0.08% or less. On the other hand, in order to obtain the above effect, the Sn content is preferably 0.001% or more.

[0041] Se: 0 to 0.080% Se (selenium) has the effect of improving machinability. Therefore, it is contained as needed. However, if Se is contained in excess, hot workability and corrosion resistance decrease. Therefore, the Se content is 0.080% or less. On the other hand, to obtain the above effect, the Se content is preferably 0.005% or more.

[0042] W: 0 to 1.0% W (tungsten), like Mo, is an element that improves the corrosion resistance of stainless steel and is contained as needed. The effect of W can be obtained even with a small amount, but if W is contained, the W content is preferably 0.01% or more. In consideration of cost, the W content is preferably 1.00% or less. The W content is preferably 0.70% or less.

[0043] Ta: 0 to 0.20% Ta (tantalum) has the effect of refining crystal grains and is therefore included as necessary. Although the effect of Ta can be obtained even in small amounts, when Ta is included, the content of each element is preferably 0.001% or more, and more preferably 0.005% or more. In consideration of the reduction in toughness due to the precipitation of a large amount of Ta carbonitride, the Ta content is 0.20% or less. The Ta content is preferably 0.18% or less.

[0044] Hf: 0 to 0.10% Hf (hafnium) has the effect of refining crystal grains and is therefore included as needed. The effect of Hf can be obtained even with trace amounts, but when included, it is preferable to set the content of each element at 0.01% or more. Considering the reduction in toughness due to the precipitation of large amounts of Hf carbonitrides, the Hf content is set to 0.10% or less.

[0045] Zr: 0 to 0.05% Zr (zirconium) has the effect of refining crystal grains and is therefore included as necessary. The effect of Zr can be obtained even with a small amount, but when it is included, the content of each element is preferably 0.001% or more. In consideration of the reduction in toughness due to the precipitation of a large amount of Zr carbonitride, the Zr content is 0.05% or less. The Zr content is preferably 0.04% or less.

[0046] Te: 0 to 0.080% Te (tellurium) has the effect of spheroidizing MnS inclusions, improving toughness in the direction perpendicular to the rolling direction, and preventing anisotropy from increasing. Therefore, Te may be contained as needed. However, excessive Te content reduces hot workability. Therefore, the Te content is 0.080% or less. On the other hand, to obtain the above effects, the Te content is preferably 0.0050% or more.

[0047] B: 0 to 0.01% B (boron) is an element that improves hot workability and is contained as needed. Considering the reduction in toughness due to the precipitation of nitrides of B, the B content is set to 0.01% or less. On the other hand, in order to obtain the above effects, the B content is preferably 0.0002% or more.

[0048] Bi: 0 to 0.300% Bi (bismuth) has the effect of improving machinability. Therefore, Bi may be contained as needed. However, if Bi is contained in excess, hot workability deteriorates. Therefore, the Bi content is 0.300% or less. On the other hand, to obtain the above effect, the Bi content is preferably 0.03% or more.

[0049] Pb: 0 to 0.100% Pb (lead) has the effect of improving machinability. Therefore, Pb may be contained as necessary. However, excessive Pb content lowers the melting point of the grain boundary and reduces the bonding strength of the grain boundary, resulting in reduced hot workability, such as liquation cracking due to grain boundary melting. Therefore, the Pb content is 0.100% or less. The Pb content is preferably 0.090% or less. On the other hand, to obtain the above effect, the Pb content is preferably 0.010% or more, and more preferably 0.020% or more.

[0050] REM: 0 to 0.10% REM is an element that improves the hot workability of steel and is added as needed. The effect of REM can be obtained even in small amounts, but if REM is added, the content is preferably 0.005% or more, and more preferably 0.010% or more. These elements form relatively large oxides in the steel, which reduces the toughness of the steel. For this reason, the REM content is 0.10% or less. The REM content is preferably 0.08% or less.

[0051] REM refers to a total of 17 elements, including Sc, Y, and lanthanides, and the REM content above refers to the total content of these elements. In industry, REM is often added in the form of misch metal.

[0052] In the chemical composition of this embodiment, the balance is Fe and impurities. Here, "impurities" refer to components that are mixed in due to various factors in raw materials such as ore and scrap, or in the manufacturing process, during industrial production of austenitic stainless steel material, and are acceptable within a range that does not adversely affect this embodiment.

[0053] 2. Hardness The austenitic stainless steel material of this embodiment is a non-heat treated material, and has a higher hardness than a heat treated material that has undergone solution heat treatment. Therefore, when the N content is 0.010 to 0.10%, the Brinell hardness is 187 HBW or more, preferably 200 HBW or more, and more preferably 217 HBW or more.

[0054] Furthermore, when the N content is 0.10 to 0.30%, the Brinell hardness is 217 HBW or more. On the other hand, even if the chemical composition of the austenitic stainless steel material of this embodiment is satisfied, if it is a heat-treated material, the Brinell hardness will be less than 187 HBW. Note that the upper limit of the Brinell hardness of the austenitic stainless steel material of this embodiment is not particularly limited, but the upper limit of the hardness is usually 350 HBW.

[0055] Brinell hardness can be measured using the following procedure. A hard ball with a diameter of 10 mm is used as the test ball, and a load of 3000 kgf is applied to measure using a Brinell hardness tester. The test piece used for the measurement is 30 mm square and 30 mm thick. One surface of the 30 mm x 30 mm size is polished with #400 abrasive paper, and this surface is used as the measurement surface. The other surface is polished with #200 abrasive paper to remove scale. The hardness is measured at one point in the center of the measurement surface polished with #400 abrasive paper, and a similar test piece is prepared and the hardness is measured at a total of five points. The average of these measurements is taken as the Brinell hardness of the steel. The test is conducted at room temperature.

[0056] 3. Maximum crack length The austenitic stainless steel material of this embodiment can suppress separation. Here, separation refers to cracks of approximately several millimeters in size that are observed with the naked eye along with ductile fracture surfaces in impact tests conducted in a low-temperature environment, for example, at −196°C. More than one such crack may be observed. Therefore, the austenitic stainless steel material of this embodiment has a maximum crack length of 1 mm or less. It is preferable that the maximum crack length be as small as possible.

[0057] The maximum crack length refers to the length of the longest crack that occurs on the fracture surface in the impact test. In this application, if the maximum crack length is 1 mm or less, it is determined that separation has been suppressed. The maximum crack length may be measured by the following procedure.

[0058] When the thickness of the steel plate is t, three V-notch Charpy test specimens are taken in the T direction from the t / 2 portion. The size of the taken test specimens is full size, 10mm square x 55mm (W). If it is not possible to take test specimens of this size, it is acceptable to use sub-size test specimens of the largest size possible.

[0059] Using the above test pieces, a Charpy impact test is performed at a temperature of -196°C in accordance with JIS Z 2242:2018, and the fracture surfaces are observed. After the test, the six fracture surfaces of the three test pieces are observed with the naked eye, and the length of the longest crack is measured with a ruler.

[0060] 4. Maximum grain thickness The austenitic stainless steel material of this embodiment preferably has a maximum grain thickness of 200 μm or less, where the maximum grain thickness is the longest value among the lengths of each grain in the sheet thickness direction when the steel material is electrolytically etched with nitric acid and observed at t / 4 part of the L cross section at 25x magnification in five fields of view.

[0061] 5. Manufacturing method The austenitic stainless steel material of this embodiment can be stably produced, for example, by the following production method.

[0062] A slab having the above chemical composition is manufactured. The obtained slab is heated to a temperature of 1000 to 1300°C and subjected to primary hot rolling. If the slab is heated to a temperature below 1000°C, the hot workability decreases and scratches and cracks tend to occur. Therefore, the slab is heated to a temperature of 1000°C or higher. On the other hand, if the slab is heated to a temperature above 1300°C, the heating temperature is too high, which tends to cause rough surfaces due to high-temperature oxidation and reduces yield. Therefore, the slab is heated to a temperature of 1300°C or lower.

[0063] After heating the slab within the above temperature range, primary hot rolling is performed to produce an intermediate material. Here, the reduction rate during the primary hot rolling is 20% or more. If the reduction rate during the primary hot rolling is less than 20%, the structure cannot be sufficiently refined, making separation more likely to occur. The reduction rate during the primary hot rolling is preferably 30% or more, and more preferably 40% or more. The upper limit of the reduction rate during the primary hot rolling is not particularly limited, but is usually 90%. After the primary hot rolling, the material may be cooled to room temperature.

[0064] Next, the intermediate material is heated again at a temperature of 1000 to 1300°C. If the heating temperature of the intermediate material is less than 1000°C, defects and edge cracks are likely to occur due to reduced hot workability. In addition, residual strain increases, the yield ratio becomes too high, and toughness and ductility are likely to decrease. Furthermore, the rolling reduction cannot be sufficiently ensured, making it difficult to widen the steel material. For this reason, the heating temperature of the intermediate material is 1000°C or higher. On the other hand, if the heating temperature of the intermediate material is more than 1300°C, the heating temperature is too high, making it prone to surface roughening due to high-temperature oxidation and reducing yield. For this reason, the heating temperature of the slab is 1300°C or lower.

[0065] After heating the intermediate material within the above temperature range, secondary hot rolling is performed and the material is cooled to room temperature under appropriate conditions to produce an austenitic stainless steel material. The reduction rate during secondary hot rolling is 20% or more. If the reduction rate during secondary hot rolling is less than 20%, the structure cannot be sufficiently refined, making separation more likely to occur. The reduction rate during secondary hot rolling is preferably 30% or more, and more preferably 40% or more. The upper limit of the reduction rate during secondary hot rolling is not particularly limited, but is usually 90%.

[0066] Here, the rolling start temperature T of the final three passes in the secondary hot rolling satisfies the following formula (i): By satisfying the following formula (i), separation can be suppressed and hardness can be increased. T≦-((-314-244(C+N)+20.6Mn) / 0.31 ···(i)

[0067] In the above formula (i), C, N, and Mn represent the contents of the respective elements in the steel material. That is, the contents of C (carbon), N (nitrogen), and Mn (manganese) can be substituted for C, N, and Mn, respectively.

[0068] In the rolling process, a 2Hi roughing mill and a 4Hi finishing mill are installed on the same line. The steel plate is rolled by passing between the rolls of each of these two rolling mills. Here, the process of a steel plate passing between the rolls of one rolling mill is called a pass. The starting temperature of the third pass, counting from the final pass of hot rolling by the finishing mill, is set to satisfy the above formula (i).

[0069] The steel is subjected to primary and secondary hot rolling to a desired thickness. In the case of steel plates, the thickness is preferably 4 to 100 mm. After the secondary hot rolling, the steel is cooled to produce an austenitic stainless steel material. Pickling may be performed as needed, but subsequent solution heat treatment is not performed because it reduces hardness.

[0070] EXAMPLES The austenitic stainless steel material according to the present invention will be described in more detail below with reference to examples, but the present embodiment is not limited to these examples. [Example]

[0071] Slabs having the chemical compositions shown in Tables 1 and 2 were produced. Some of the obtained slabs were heated at the heating temperatures shown in Table 3, subjected to first hot rolling with the reduction shown in Table 3, and then reheated at the temperatures shown in Table 3 and subjected to second hot rolling with the reduction shown in Table 3. The rolling start temperatures for the final three passes were also as shown in Table 3. Note that for some examples (Nos. 32 and 36), slab heating and hot rolling were carried out only once, followed by cooling to produce austenitic stainless steel sheets.

[0072] [Table 1]

[0073] [Table 2]

[0074] [Table 3]

[0075] The hardness test and maximum crack length of the obtained austenitic stainless steel material were measured by the following procedures.

[0076] (Hardness) The test ball was a 10 mm diameter hard ball, and a load of 3000 kgf was used to measure the Brinell hardness using a Brinell hardness tester. The test specimens used for the measurements were 30 mm square and 30 mm thick. One surface of the 30 mm x 30 mm size was polished with #400 abrasive paper, and this surface was used as the measurement surface. The other surface was polished with #200 abrasive paper to remove scale. The hardness was measured at one point in the center of the measurement surface polished with #400 abrasive paper, and similar test specimens were prepared and the hardness was measured at a total of five points. The average value was used as the Brinell hardness of the steel. The test was conducted at room temperature.

[0077] (Maximum crack length) When the thickness of the steel plate is t, three V-notch Charpy test specimens were taken in the T direction from the t / 2 portion. The size of the taken test specimens was full size, 10 mm square x 55 mm (W). Note that when the plate thickness was 10 mm or less, a sub-size test specimen was used, which was as large as possible.

[0078] Using the above test specimens, a Charpy impact test was performed at a temperature of -196°C in accordance with JIS Z 2242:2018, and the fracture surfaces were observed. After the test, the six fracture surfaces of the three test specimens were observed with the naked eye, and the length of the largest crack was measured with a ruler. The results are summarized in Table 4 below. If the crack was more than 1 mm, separation was judged to be present (×); if it was 1 mm or less and it was difficult to discern the crack with the naked eye, separation was judged to be absent (○); and if it was clear that there was no crack with the naked eye, it was judged to be (◎).

[0079] [Table 4]

[0080] Nos. 1 to 28, which satisfied the requirements of this embodiment, showed no cracks (no separation) on the low-temperature impact fracture surface and good hardness. On the other hand, Nos. 29 to 38, which did not satisfy this embodiment, showed cracks (separation) on the low-temperature impact fracture surface, and No. 39, which underwent solution heat treatment, showed no cracks but a decrease in hardness. No. 40, whose rolling start temperature for the final three passes did not satisfy the preferred conditions, showed a decrease in hardness and also showed separation.

Claims

1. The chemical composition, in mass%, is C: 0.080% or less, Si: 1.20% or less, Mn: 3.50% or less, P: 0.050% or less, S: 0.050% or less, Ni: 6.00 to 15.00%, Cr: 16.00-21.00%, Mo: 5.00% or less, Ti: 0.30% or less, Nb: 0.10% or less, N: 0.010-0.10%, Al: 0-0.10%, Cu: 0-2.0%, V: 0 to 1.0%, Co: 0-3.0%, Ca: 0-0.0100%, Mg: 0 to 0.0050%, Sb: 0 to 0.2%, Sn: 0 to 0.10%, Se: 0 to 0.080%, W: 0 to 1.0%, Ta: 0 to 0.20%, Hf: 0-0.10%, Zr: 0 to 0.05%, Te: 0 to 0.080%, B: 0 to 0.01%, Bi: 0-0.300%, Pb: 0 to 0.100%, REM: 0-0.10%, The balance is Fe and impurities. The Brinell hardness is 187HBW or more, An austenitic stainless steel material having a maximum crack length of 1 mm or less.

2. The chemical composition, in mass%, is C: 0.080% or less, Si: 1.20% or less, Mn: 3.50% or less, P: 0.050% or less, S: 0.050% or less, Ni: 6.00 to 15.00%, Cr: 16.00-21.00%, Mo: 5.00% or less, Ti: 0.30% or less, Nb: 0.10% or less, N: more than 0.10% and less than 0.30%, Al: 0-0.10%, Cu: 0-2.0%, V: 0 to 1.0%, Co: 0-3.0%, Ca: 0-0.0100%, Mg: 0 to 0.0050%, Sb: 0 to 0.2%, Sn: 0 to 0.10%, Se: 0 to 0.080%, W: 0 to 1.0%, Ta: 0 to 0.20%, Hf: 0-0.10%, Zr: 0 to 0.05%, Te: 0 to 0.080%, B: 0 to 0.01%, Bi: 0-0.300%, Pb: 0 to 0.100%, REM: 0-0.10%, The balance is Fe and impurities. The Brinell hardness is 217HBW or more, An austenitic stainless steel material having a maximum crack length of 1 mm or less.

3. The chemical composition is, in mass %, Al: 0.003-0.10%, Cu: 0.01-2.0%, V: 0.01-1.0%, Co: 0.01 to 3.0%, Ca: 0.0010-0.0100%, Mg: 0.0001 to 0.0050%, Sb: 0.02 to 0.2%, Sn: 0.001 to 0.10%, Se: 0.005-0.080%, W: 0.01-1.0%, Ta: 0.005-0.20%, Hf: 0.01-0.10%, Zr: 0.001 to 0.05%, Te: 0.0050 to 0.080%, B: 0.0002-0.01%, Bi: 0.03-0.300%, Pb: 0.010 to 0.100%, and REM: 0.005-0.10%, The austenitic stainless steel material according to claim 1, comprising one or more selected from the following:

4. The chemical composition is, in mass %, Al: 0.003-0.10%, Cu: 0.01-2.0%, V: 0.01-1.0%, Co: 0.01 to 3.0%, Ca: 0.0010-0.0100%, Mg: 0.0001 to 0.0050%, Sb: 0.02 to 0.2%, Sn: 0.001 to 0.10%, Se: 0.005-0.080%, W: 0.01-1.0%, Ta: 0.005-0.20%, Hf: 0.01-0.10%, Zr: 0.001 to 0.05%, Te: 0.0050 to 0.080%, B: 0.0002-0.01%, Bi: 0.03-0.300%, Pb: 0.010 to 0.100%, and REM: 0.005-0.10%, The austenitic stainless steel material according to claim 2, comprising one or more selected from the following:

5. The austenitic stainless steel material according to any one of claims 1 to 4, which is used for tanks.

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