[0001]The present invention relates to a process for the production of austenitic stainless steel and austenitic stainless steel.
Background technique
[0002]
　Recently, as energy alternative to fossil fuels, practical studies have been conducted transport equipment utilizing hydrogen as energy. In its practical application, storing hydrogen under high pressure, transport can use environment (hereinafter, also referred to as a hydrogen equipment.) Requires maintenance is together a. Hydrogen facility for example, a high-pressure hydrogen gas and the equipment for liquid hydrogen for equipment. The materials used in hydrogen facilities, hydrogen embrittlement resistance are required.
[0003]
　WO 2004/083476, WO 2004/083477, WO 2004/110695, International Publication No. 2012/132992, the austenitic stainless steel of high strength is disclosed. In these documents, increase the solubility of N by high Mn of, and by the addition of V and Nb, the precipitation strengthening by solid solution strengthening and nitride by N, more grain refinement due to the pinning effect leverage, high strength have been attempted.
[0004]
　When using austenitic stainless steel as a construction, it is determined from the cost can be assembled by welding. JP-5-192785, JP-2010-227949 and JP WO 2004/110695, supra, Al, actively utilizing Ti and Nb, the 800MPa by performing heat treatment after welding welded joint having a tensile strength greater than is disclosed.
[0005]
　WO 2013/005570, the N content of the welding material, by managing shielding gas during welding, and the weld pool area, by increasing the N content in the weld metal, without post-weld heat treatment welded joints were also as high strength is obtained is disclosed.
Disclosure of the Invention
[0006]
　The material used as the structure, in addition to hydrogen embrittlement resistance and strength, various characteristics are required. For example, when using these materials as a piping, depending on the structure thereof, which may or performed or carried out bending in cold welding under various conditions. Therefore, in order to obtain a sound structure is required to achieve both of sufficient ductility and excellent weldability.
[0007]
　By utilizing the techniques described in the above patent document, the base material or welded joint strength with excellent resistance to hydrogen embrittlement is obtained. In particular, WO 2004/083476 Patent and WO 2004/083477, austenitic stainless steel high strength with ductility greater than 30% elongation at break is disclosed. However, in order to obtain a positively the addition effect of Nb as an alloying element is contained at a high concentration, the welding property is lowered, there is a case where cracking in the weld heat affected zone during welding results.
[0008]
　WO 2012/132992, was subjected to cold working after solution heat treatment, it is by performing heat treatment again, disclosed to obtain austenitic stainless steel having a strength of at least 800 MPa. However, when used as construction, all of the members performing the cold working is difficult, in a state which has been subjected to solution heat treatment after hot working, and more in material strength, etc. required to obtain preferable.
[0009]
　An object of the present invention is to provide strength, ductility, and weldability superior austenitic stainless steels.
[0010]
　Austenitic stainless steel according to an embodiment of the present invention, the chemical composition, in mass%, C: 0.005 ~ 0.07%, Si: 0.1 ~ 1.2%, Mn: 3.2 ~ 6 .5%, Ni: 9 ~ 14%, at least one of the sum of Cu and Co: less than 0.005% or more and 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 %, the balance is Fe and impurities, Nb amount to be analyzed as an electrolytic extraction residue is from 0.01 to 0.3 mass%.
[0011]
　According to the present invention, strength, ductility, and weldability superior austenitic stainless steel can be obtained.
DESCRIPTION OF THE INVENTION
[0012]
　The present inventors have, by mass%, C: 0.005 ~ 0.07%, Si: 0.1 ~ 1.2%, Mn: 3.2 ~ 6.5%, Ni: 9 ~ 14%, at least one of the sum of Cu and Co: less than 0.005% or more and 3%, Cr: 19 ~ 24%, Mo: 1 ~ 4%, Nb: 0.05 ~ 0.4%, N: 0.15 ~ 0.50%, Al: against material containing 0.05% or less such as, hot working and solution treatment investigated strength and ductility of austenitic stainless steel produced by implementation, the following findings Obtained.
[0013]
　Strength and ductility of austenitic stainless steel, Nb amount to be analyzed as an electrolytic extraction residue, i.e., related to the amount of the precipitates containing Nb. Precipitates containing Nb is Nb carbonitride was generated during the production of austenitic stainless steel or Nb nitride.
[0014]
　After hot working, in order to obtain the required strength in a state where the solution heat treated, it is necessary that the Nb content to be analyzed as an electrolytic extraction residue than 0.005 mass%. On the other hand, when the Nb amount to be analyzed as an electrolytic extraction residue of more than 0.3 mass%, ductility is lowered.
[0015]
　Nb amount to be analyzed as an electrolytic extraction residue is not less than 0.005 wt%, even steel having the necessary strength, welded When used, liquation cracking occurred in the weld heat affected zone close to the fusion line there is a case in which another problem that may occur. This is because, when a small amount of Nb to be analyzed as an electrolytic extraction residue, the effect of pinning by carbonitride or nitride of Nb is not sufficiently obtained, the crystal grains in the weld is considered to be due to coarsening . To suppress the liquation cracking during welding, it is necessary that the Nb content to be analyzed as an electrolytic extraction residue than 0.01 mass%.
[0016]
　Accordingly, the austenitic stainless steel having the above chemical composition, superior strength, in order to obtain ductility and weldability, them to the amount of Nb to be analyzed as an electrolytic extraction residue from 0.01 to 0.3 mass% Bayoi.
[0017]
　We further conditions solution heat treatment be carried out after hot working, by adjusting in accordance with the Nb content of the material, further excellent ductility and weldability was found that the resulting. Specifically, in solution heat treatment temperature of 950 ~ 1300 ° C., if solution treatment under conditions satisfying the formula (1) below, could be obtained an excellent ductility and weldability.
　40 × [% Nb] + 100 ≦ T × log (1.2 + t / 60) ≦ -200 × [% Nb] + 700 ... (1)
　In formula (1), the Nb content of the material in [% Nb] is substituted by mass%, the T is substituted by the solution treatment temperature is ° C., solution heat treatment time is substituted by min to t.
[0018]
　Based on the above findings, the present invention has been completed. It described in detail below austenitic stainless steel and a manufacturing method thereof according to an embodiment of the present invention.
[0019]
　[Chemical composition]
　Austenitic stainless steels according to the present embodiment has a chemical composition described below. In the following description, "%" of the content of the element means mass%.
[0020]
　C: 0.005 ~ 0.07%
　carbon (C) is an effective element to stabilize the austenite. Further, to produce a carbide of Nb, it contributes to securing strength. In order to obtain this effect sufficiently, it is necessary to set the C content to 0.005% or more. However, when the C content is too high, Nb carbides are excessively generated, the ductility of the steel is lowered. Therefore, C content is from 0.005 to 0.07%. The lower limit of the C content is preferably 0.01%, further preferably 0.02%. The upper limit of the C content is preferably 0.06%, more preferably 0.05%.
[0021]
　Si: 0.1 ~ 1.2% or less
　silicon (Si), together with an effective element as a deoxidizing agent, is an element effective in improving the corrosion resistance. In order to obtain this effect sufficiently, it is necessary that the Si content more than 0.1%. However, when the Si content is too high, with reducing the stability of the austenitic structure, the ductility of the steel is lowered. Therefore, Si content is 0.1 to 1.2%. The lower limit of the Si content is preferably 0.15%, more preferably 0.2%. The upper limit of the Si content is preferably 1.1%, more preferably 1.0%.
[0022]
　Mn: 3.2 ~ 6.5%
　manganese (Mn), as well as contributes to deoxidation at the time of manufacture, an effective element to stabilize the austenite. Mn further increase the solubility of N, indirectly contribute to increase the strength. In order to obtain this effect sufficiently, it is necessary to set the Mn content to more than 3.2%. On the other hand, when the Mn content is too high, not only these effects are saturated, it becomes fumes adhering to the weld during welding, reduces the corrosion resistance. Therefore, Mn content is from 3.2 to 6.5%. The lower limit of the Mn content is preferably 3.4%, more preferably from 3.5%. The upper limit of the Mn content is preferably 6.3%, more preferably from 6.0%.
[0023]
　Ni: 9 ~ 14%
　nickel (Ni) is an essential element for obtaining a stable austenite, increase the stacking fault energy, reducing the embrittlement susceptibility in a hydrogen environment. In order to obtain this effect sufficiently, it is necessary to set the Ni content above 9%. However, Ni is because an expensive element, a large amount of content leads to increase in cost. Therefore, Ni content is 9 to 14%. The lower limit of Ni content is preferably 9.5%, more preferably 10%. The upper limit of the N content is preferably 13.5%, more preferably 13%.
[0024]
　At least one of the sum of Cu and Co: less than 0.005% or more and 3%
　Cu (copper) and Co (cobalt), like Ni, is an element effective for obtaining a stable austenitic structure. In order to obtain this effect sufficiently, it is necessary that the total content of Cu and Co in 0.005% or more. Cu and Co may be the only one is contained, both may be contained. However, since Cu and Co is an expensive element, a large amount of content leads to increase in cost. If the content of Cu and Co is excessive, the ductility of the steel is lowered. Accordingly, the total content of Cu and Co are less than 0.005% to 3%. Lower limit of the total content of Cu and Co is preferably 0.01%, more preferably 0.02%. Total upper limit of Cu and Co content is preferably 2%, more preferably 1%.
[0025]
　Cr: 19 ~ 24%
　chromium (Cr) is an essential element for ensuring the corrosion resistance in the environment of use. Cr is further to increase the solubility of N during preform manufacturing, indirectly contribute to increase the strength. In order to obtain this effect sufficiently, it is necessary that the Cr content more than 19%. However, when the Cr content is too high, the austenite structure becomes unstable. Therefore, Cr content is 19-24%. The lower limit of the Cr content is preferably 19.5%, more preferably 20%. The upper limit of the Cr content is preferably 23.5%, more preferably 23%.
[0026]
　Mo: 1 ~ 4%
　molybdenum (Mo), the improvement of corrosion resistance in the environment of use, and is an effective element to increase the strength. In order to obtain this effect sufficiently, it is necessary that the Mo content to 1% or more. However, Mo is because an expensive element, a large amount of content leads to increase in cost. Further, the austenite structure becomes unstable when Mo content is too high. Therefore, Mo content is 1-4%. The lower limit of the Mo content is preferably 1.2%, more preferably from 1.5%. The upper limit of the Mo content is preferably 3.8%, more preferably from 3.5%.
[0027]
　Nb: 0.05 ~ 0.4%
　niobium (Nb) is precipitated as fine carbonitride or nitride in the matrix, it is an element effective for improving the strength. Further, carbonitrides or nitrides finely precipitated inhibits grain coarsening of the heat affected zone during welding, it reduces the liquation cracking susceptibility. To obtain the effect sufficiently, it is necessary that the Nb content to 0.05% or more. However, when the Nb content is too high, with increases crack susceptibility of the weld heat affected zone, carbonitride or nitride by a large amount of precipitation, the ductility of the material is reduced. Therefore, Nb content is 0.05 to 0.4%. The lower limit of the Nb content is preferably 0.12%, more preferably 0.15%. The upper limit of the Nb content is preferably 0.38%, more preferably 0.35%.
[0028]
　The Nb content herein, means the total amount of Nb contained in the austenitic stainless steel. That means the sum of the amount of Nb present as the amount of Nb that dissolves in the matrix, deposit. In the present embodiment, in addition to the Nb content, the amount of Nb present as precipitates, i.e. the amount of Nb to be analyzed as an electrolytic extraction residue must also be in the specified range.
[0029]
　N: 0.15 ~ 0.50%
　nitrogen (N), together with a solid solution in the matrix to form a Nb or the like and fine carbonitrides and nitrides and contributes to higher strength. N are further effective element to stabilize the austenite structure. In order to obtain these effects sufficiently, it is necessary that the N content to 0.15% or more. However, when the N content is too high, the thermal workability during manufacturing is reduced, it precipitates Nb is excessively precipitated, reduce the ductility of the steel. Therefore, N content is 0.15 to 0.50%. The lower limit of the N content is preferably 0.22%, further preferably 0.25%. The upper limit of the N content is preferably 0.48%, more preferably 0.45%.
[0030]
　Al: 0.05% or less
　Al (aluminum), like Si, is contained as a deoxidizing agent. However, when the Al content is too high, hot workability is deteriorated cleanliness of steel is deteriorated. Therefore, Al content is 0.05% or less. Al content is preferably not more than 0.04%, more preferably not more than 0.03%. Although not required, the lower limit is particularly provided the Al content, an extreme reduction causes an increase in steelmaking cost. Therefore, the lower limit of the Al content is preferably 0.0005%, more preferably 0.001%.
[0031]
　The remainder of the chemical composition of the austenitic stainless steel according to the present embodiment is Fe and impurities. The impurity as used herein means an element that tramp elements mixed from ore and scrap to be used as a raw material of steel or from the environment or the like of the manufacturing process.
[0032]
　Of impurities, limiting P, S, and O content, respectively, in the ranges set forth below.
[0033]
　P: 0.03% or less
　Phosphorus (P) is included in steel as an impurity. When the P content is too high, the thermal workability during manufacturing is reduced, liquation cracking susceptibility of the weld heat affected zone is increased at the time of welding. P content is preferably as low, an extreme reduction causes an increase in manufacturing cost. Therefore, P content is 0.03% or less. P content is preferably not more than 0.025%, more preferably not more than 0.02%.
[0034]
　S: 0.002% or less
　Sulfur (S) is included in steel as an impurity. When S content is too high, the thermal workability during manufacturing is reduced, the ductility of the steel is lowered. Further, when the S content is too high, liquation cracking susceptibility of the weld heat affected zone is increased at the time of welding. S content is preferably as low, an extreme reduction causes an increase in manufacturing cost. Therefore, S content is 0.002% or less. S content is preferably not more than 0.0018%, further preferably 0.0015% or less.
[0035]
　O: 0.02% or less
　oxygen (O) is included as an impurity in steel. When O content is too high, the thermal workability during manufacturing is reduced, the ductility is reduced cleanliness of steel is deteriorated. Therefore, O content is 0.02% or less. O content is preferably 0.015% or less, still more preferably 0.01% or less. The lower limit of O content is not necessary to particularly provide, but extreme reduction causes an increase in steelmaking cost. Therefore, the lower limit of the O content is preferably 0.001%, more preferably 0.002%.
[0036]
　The chemical composition of the austenitic stainless steel according to the present embodiment, instead of a part of the above Fe, V, Ti, B, Ca, may contain one or more elements selected Mg, and from REM . V, Ti, B, Ca, Mg, and REM are both optional elements. That is, the chemical composition of the austenitic stainless steel according to the present embodiment, V, Ti, B, Ca, of Mg, and REM, may not contain some or all.
[0037]
　V: 0 ~ 0.5%
　vanadium (V), like Nb, precipitated as carbonitrides and improves the strength of steel. If V is long as it is contained even a little, this effect can be obtained. However, when the V content is too high, carbonitrides are excessively precipitated, ductility of the steel is lowered. Therefore, V content is 0 to 0.5%. The lower limit of the V content is preferably 0.001%, still more preferably 0.005%, still more preferably 0.01%. The upper limit of the V content is preferably 0.45%, more preferably 0.40%.
[0038]
　Ti: 0 ~ 0.5%
　of titanium (Ti) is, V, as with Nb, precipitated as carbonitrides and improves the strength of steel. If Ti is long is contained even a little, the effect is obtained. However, if the Ti content is too high, carbonitrides are excessively precipitated, ductility of the steel is lowered. Therefore, Ti content is 0 to 0.5%. The lower limit of the Ti content is preferably 0.001%, still more preferably 0.003%, still more preferably 0.005%. The upper limit of the Ti content is preferably 0.45%, more preferably 0.40%.
[0039]
　B: 0 ~ 0.01%
　boron (B) enhances the grain boundary bonding strength segregates to grain boundaries, it contributes to improvement of strength, to improve ductility. B also inhibits embrittlement in a hydrogen environment. If B is long is contained even a little, the effect is obtained. However, when the B content is too high, it increases the liquation cracking susceptibility of the weld heat affected zone. Therefore, B content is 0 to 0.01%. The lower limit of the B content is preferably 0.0001%, more preferably 0.0002%, more preferably 0.0005%. The upper limit of the B content is preferably 0.008%, and more preferably 0.005%.
[0040]
　Ca: 0 ~ 0.05%
　of calcium (Ca) improves the hot workability of the steel. If Ca is long is contained even a little, the effect is obtained. However, when the Ca content is too high, Ca combines with O, hot workability is deteriorated cleanliness of steel is deteriorated. Therefore, Ca content is from 0 to 0.05%. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0005%, more preferably from 0.001%. The upper limit of the Ca content is preferably 0.03%, more preferably 0.01%.
[0041]
　Mg: 0 ~ 0.05%
　magnesium (Mg), like Ca, improves the hot workability of the steel. If Mg is long as it is contained even a little, this effect can be obtained. However, if the Mg content is too high, Mg combines with O, hot workability is deteriorated cleanliness of steel is deteriorated. Therefore, Mg content is 0 to 0.05%. The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0005%, more preferably from 0.001%. The upper limit of the Mg content is preferably 0.03%, more preferably 0.01%.
[0042]
　REM: 0 ~ 0.5%
　rare earth element (REM) is affinity for S strongly improves the hot workability of the steel. If REM is long as it is contained even a little, this effect can be obtained. However, when the REM content is too high, REM is combined with O, hot workability is deteriorated cleanliness of steel is deteriorated. Therefore, REM content is 0 to 0.5%. The lower limit of the REM content is preferably 0.001%, still more preferably 0.002%, still more preferably 0.005%. The upper limit of the REM content is preferably 0.3%, more preferably 0.1%.
[0043]
　The "REM" Sc, is a generic name for a total of 17 elements Y and lanthanoid, and the content of REM refers to the total content of one or more elements of the REM. In addition, REM is generally contained in the misch metal. Thus for example, the addition of misch metal alloy, the content of REM can be set to be within the above range.
[0044]
　[Electrowinning Nb amount is analyzed as a residue]
　Austenitic stainless steels according to the present embodiment, the amount of Nb to be analyzed as an electrolytic extraction residue is from 0.01 to 0.3 mass%.
[0045]
　Nb contained in the material, in the process of solution heat treatment, precipitates as fine carbonitride or nitride. Carbonitrides and / or nitrides of Nb were finely precipitated, thereby improving the strength of the steel, and contributes to grain coarsening suppressing the weld heat affected zone to reduce the liquation cracking susceptibility during welding. In order to obtain this effect, the amount of Nb is precipitated as carbonitrides and / or nitrides, i.e. the amount of Nb to be analyzed as an electrolytic extraction residue must be at least 0.01 mass%. However, when the Nb amount to be analyzed as an electrolytic extraction residue is excessive, the ductility of the steel is lowered. Therefore, Nb amount to be analyzed as an electrolytic extraction residue is from 0.01 to 0.3 mass%. Nb amount of the lower limit to be analyzed as an electrolytic extraction residue is preferably 0.02 mass%, more preferably from 0.03% by mass. Nb amount upper limit to be analyzed as an electrolytic extraction residue is preferably 0.28 mass%, more preferably from 0.25% by mass.
[0046]
　Nb amount to be analyzed as an electrolytic extraction residue can be adjusted Nb content and N content of the material, as well as the conditions of the solution treatment. Specifically, the higher Nb content and N content of the material, Nb amount to be analyzed as an electrolytic extraction residue is higher. As the temperature of the solution heat treatment is low, Nb quantity and / or retention time is analyzed as longer electrolytic extraction residue is higher. However, low temperature of the solid solution heat treatment, and if the short / or retention time, hot working such as, carbonitrides and / or nitrides of Nb generated in the steps up to the solution treatment is not sufficiently dissolved Therefore, Nb amount is also analyzed as an electrolytic extraction residue in this case is higher. In the cooling and the solution heat treatment, the smaller the cooling rate of the temperature range of 1100 ~ 600 ° C. to carbonitrides and / or nitrides of Nb is precipitated, Nb amount to be analyzed as an electrolytic extraction residue is higher.
[0047]
　Nb amount to be analyzed as an electrolytic extraction residue is determined as follows.
[0048]
　From austenitic stainless steel, and collecting the predetermined size of the test material. By 10 vol% acetylacetone -1 wt% tetramethylammonium chloride methanol solution galvanostatic electrolysis method using as an electrolytic solution, current density 20 ~ 25mA / cm 2 of the test material dissolved anodically, residue carbonitride and nitride It is extracted as. After extracted residue acid decomposing performs ICP (inductively coupled plasma) emission spectrometry to measure the mass of Nb in the residue. By dividing the mass of Nb in the residue in dissolution of the test material, the amount of Nb present as carbonitrides and / or nitrides, i.e. determine the amount of Nb to be analyzed as an electrolytic extraction residue.
[0049]
　[Manufacturing Method]
　Hereinafter, a method for manufacturing austenitic stainless steel according to an embodiment of the present invention. Austenitic stainless steel according to the present embodiment includes a step of preparing a raw material, a step of hot working the material, and a step of solution heat treated hot-worked material.
[0050]
　First, a material of the above-mentioned chemical composition. Specifically, for example, by melting a steel having a chemical composition described above, to refining.
[0051]
　The material to hot working. Hot working example, a hot rolling or hot forging.
[0052]
　The hot worked material to solution heat treated. Specifically, in a predetermined solution heat treatment temperature material, after holding a predetermined solution heat treatment time, it cooled. Thus, carbonitrides coarse Nb precipitated during hot working, etc., and / or a nitride is dissolved, it is re-precipitated as fine carbonitride in the course of cooling and / or nitride it can. Carbonitrides and / or nitrides of Nb were finely precipitated contributes to the improvement of the strength of the steel and ductility.
[0053]
　Solution heat treatment temperature is preferably 950 ~ 1300 ° C.. Solution heat treatment temperature is less than 950 ° C., carbonitrides of Nb precipitated during hot working and / or nitride is not sufficiently dissolved, 0.3 wt% of Nb amount to be analyzed as an electrolytic extraction residue there is a case that can not be. On the other hand, when the solution treatment temperature exceeds 1300 ° C., with the crystal grains become coarse in some cases the grain boundary of some melting begins.
[0054]
　Cooling in solution treatment, water cooling is preferred. In cooling after solution heat treatment, the smaller the cooling rate of the temperature range of 1100 ~ 600 ° C. to carbonitrides and / or nitrides of Nb precipitates, is possible to increase the amount of Nb to be analyzed as an electrolytic extraction residue it can. The cooling rate of this temperature range is preferably not 0.5 ° C. / sec or more, further preferably 1 ° C. / sec or more.
[0055]
　Solution heat treatment is preferably carried out under conditions satisfying the formula (1) below.
　40 × [% Nb] + 100 ≦ T × log (1.2 + t / 60) ≦ -200 × [% Nb] + 700 ... (1)
　In formula (1), the Nb content of the material in [% Nb] is substituted by mass%, the T is substituted by the solution treatment temperature is ° C., solution heat treatment time is substituted by min to t. In addition, log (x) is the logarithm of x.
[0056]
　When T × log (1.2 + t / 60) is less than 40 × [% Nb] +100, is not sufficiently dissolved carbonitride and / or nitride of coarse Nb, fine the process comprising cooling carbonitride and / or nitride is not sufficiently precipitated. Therefore, the effect of improving the strength and ductility is decreased. This higher Nb content of the material is to require high temperatures and / or longer time in order to form a solid solution carbonitride and / or nitride of coarse Nb. Thus, the higher Nb content of the material, it is preferable to increase the value of T × log (1.2 + t / 60).
[0057]
　On the other hand, when T × log (1.2 + t / 60) is greater than -200 × [% Nb] +700, grain coarsening significantly increases the liquation cracking susceptibility during welding. Since Nb is an element which enhances the liquation cracking susceptibility, the higher Nb content of the material, it is preferable to reduce the value of T × log (1.2 + t / 60).
[0058]
　Method of manufacturing austenitic stainless steel according to the present embodiment, preferably, between the solution treatment and hot working, not performed cold working. This, when carrying out the cold working, precipitates strain induced is generated by the heating process of solution heat treatment, because the high temperature or a long time by solution heat treatment is required.
[0059]
　It has been described an embodiment of the present invention. According to this embodiment, the strength, ductility, and weldability superior austenitic stainless steel can be obtained.
[0060]
　The above-described embodiment is merely an example for implementing the present invention. Accordingly, the present invention is not limited to the embodiment described above, without departing from the spirit and can be implemented by modifying as appropriate to the embodiments described above.
Example
[0061]
　The following examples illustrate the present invention more specifically. The present invention is not limited to these examples.
[0062]
　The material of the steel types A ~ F having the chemical compositions shown in Table 1 from the ingot casted dissolved laboratory, hot forging, the hot rolling to produce a sheet having a thickness of 14 mm. Then, it was carried out solution treatment while varying the temperature and time. Cooling after solution treatment was water-cooled. Were solution heat treated sheet is machined plate thickness 12 mm, and the test material. Incidentally, in Table 1, "-" indicates that the content of the corresponding element is an impurity level.
[0063]
[Table 1]

[0064]
　[The residue Analysis]
　10 mm square from each sample were taken test material length 50 mm, by the method described in the above embodiments, to measure the amount of Nb to be analyzed as an electrolytic extraction residue.
[0065]
　[Tensile test]
　from each sample, parallel portion diameter 8 mm, were taken 14A No. round specimens shown in JIS Z2201 (2013) parallel director 55 mm, and a tensile test at room temperature. Tensile strength, was passed more than 690MPa required for hydrogen equipment. Of these, the tensile strength of not less than 800MPa, tensile strength was evaluated to be particularly excellent. Also, the ductility, elongation at break in a tensile test was passed more than 35%. Among them, those elongation at break of 40% or more was evaluated as ductility are particularly excellent.
[0066]
　Low strain rate tensile test]
　with respect to the tensile test materials passed the test, to evaluate the resistance to hydrogen embrittlement in a high-pressure hydrogen environment, it was carried out low strain rate tensile testing. Specifically, collect plate low strain rate tensile test specimens from the test material was performed in the atmosphere, and a low strain rate tensile test at a high pressure hydrogen environment of 45 MPa. Strain rate × 10 3 -5 was / sec. The aperture value rupture at high pressure hydrogen environment is as acceptable as a more than 90% of the value of the diaphragm rupture in the atmosphere.
[0067]
　[Welding Test
　against test materials passed the tensile test and the low distortion rate tensile test, the test was carried out to evaluate the weldability. Specifically, to prepare a steel sheet having a width 50 mm, length 100 mm, a cross-section along its longitudinal direction, included angle 30 °, it was processed V groove root thickness 1 mm. The steel sheet, thickness 25 mm, width 200 mm, on SM400B steel sheet specified in JIS G 3106 of length 200 mm (2008), using a covered electrode ENi6625 prescribed in JIS Z 3224 (2010), restraining the four sides welded. Then, using the Tiguwaiya applicable SNi6082 specified in JIS Z 3334 (2011) in the GMA performs lamination welding with heat input 10 ~ 15 kJ / cm in the GMA to prepare a welded joint.
[0068]
　From five points of the welded joint produced, the viewing surface is a sample was taken to be the cross-section of the joint (weld bead section perpendicular). After the collected sample was mirror-polished, corrosion, and microscopic examination by an optical microscope to examine the existence of cracks in the weld heat affected zone. Of the five samples, the number of cracks were observed samples were as acceptable was 1 or less. Among them, those cracks were not observed in all samples, was evaluated as weldability is especially excellent.
[0069]
　Solution heat treatment conditions, and the results of each test are shown in Table 2.
[0070]
[Table 2]

[0071]
　In Table 2, "Nb content (% by mass)" indicates the Nb content of the material. "T (° C.)" and "t (min)", respectively, show the solution treatment temperature and the solution treatment time. fn1, fn2, and fn3 the left side of each formula (1), middle side, and shows the right-hand side. "The residue Nb amount (wt%)" indicates the amount of Nb to be analyzed as an electrolytic extraction residue.
[0072]
　In the column "tensile test" as a result of the tensile test are described. In the column of "Tensile Strength", "excellent" indicates that tensile strength of the test material is not less than 800 MPa, "impossible" denotes that was less than 690 MPa. In the column of "stretch", "excellent" is that elongation at break of the test specimen was 40% or more, "good" is that it was less than 35% 40%, "impossible" 35% show that was less than.
[0073]
　The column of "low strain rate tensile test", describes a low strain rate tensile test results. "Pass" in the same column, in the test material, indicating that the value of an aperture rupture at high pressure hydrogen environment was at least 90% of the value of the diaphragm rupture in the atmosphere.
[0074]
　The column of "welding test" is described the results of welding tests. In this column, "excellent" is that the cracks in any five samples was observed, "good" is that the crack only one sample was observed, "impossible" it is 2 or more cracking of the sample indicate that was observed.
[0075]
　The column of "low strain rate tensile test" and "welding test" "-" indicates that no conduct the test.
[0076]
　As shown in Table 2, test material Duif A1 ~ A19, B1 ~ B6, B8 ~ B13, and C1 ~ C12 are tensile test, low strain rate tensile test, and also passed to any welding test. Among them, test pieces of Duif A1, A3 ~ A8, A11 ~ A16, A18, A19, B1, B3 ~ B5, B8 ~ B11, B13, C1 ~ C6, C8 ~ C11 are the tensile test and welding tests It showed excellent results in both. Specifically, a tensile strength of not less than 800 MPa, elongation at break is 40% or more, cracks in any five samples was observed.
[0077]
　Test material Duif A2, A10, and B2 is less than 40% elongation at break less than 35%, the ductility was slightly poor when compared to Duif A1 like. This is too low in relation to the value Nb content of the material of T × log (1.2 + t / 60), probably because that could not be sufficiently solid solution carbonitrides of coarse Nb and / or nitride It is.
[0078]
　Duif A9, A17, B6, B12, C7, and the test material of C12 is observed cracks only five one sample among the weldability as compared with Duif A like was inferior slightly. This is the value of T × log (1.2 + t / 60) is too high in relation to the Nb content, presumably because of increased liquation cracking susceptibility by the particle size is large.
[0079]
　Duif B7, although tensile strength was sufficient, the elongation at break was less than 35%. This is probably because the amount of Nb to be analyzed as an electrolytic extraction residue was too high. The amount of Nb to be analyzed as an electrolytic extraction residue is too high, by the temperature of the solution treatment was low, the cooling start temperature is also lowered, the cooling rate when passing through the temperature region in which the precipitates generated by small precipitates too is considered to be because that is generated excessively.
[0080]
　Duif C13 is, tensile strength was less than 690MPa. This is probably because the amount of Nb to be analyzed as an electrolytic extraction residue was too low. The amount of Nb to be analyzed as an electrolytic extraction residue is too low, by the temperature of the solution treatment was high, the cooling start temperature is also increased, large cooling rate when passing through the temperature region in which the precipitate to produce product precipitates too is considered because it was suppressed.
[0081]
　Duif D1 and D2, while the tensile strength and ductility was sufficient, cracking into five two or more samples of were observed. This is because the Nb content of the steel species D is too high, presumably because of increased liquation cracking susceptibility.
[0082]
　Duif E1 and E2, although the tensile strength was sufficient, the elongation at break was less than 35%. This is probably because the total content of Cu and Co steels E was too high.
[0083]
　Duif F1 and F2, because it did not contain Nb, tensile strength was less than 690 MPa.
[0084]
　As described above, according to the present invention, it has been found strength, ductility, and that the weldability excellent austenitic stainless steel can be obtained.
Industrial Applicability
[0085]
　According to the present invention, strength, ductility, and weldability superior austenitic stainless steel can be obtained. Accordingly, the present invention can be suitably used for various steel materials such as a high-pressure hydrogen gas and the equipment for liquid hydrogen storage tank.

claims

[Claim 1]Chemical composition, in
　mass%, C:
　0.005
　~ 0.07%, Si: 0.1 ~ 1.2%, Mn:
　3.2 ~ 6.5%, Ni: 9 ~
　14%, Cu and at least one of the sum of Co: less than 0.005% or more%
　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%,
　the balance is Fe and impurities,
　the electrolytic extraction residue Nb amount to be analyzed as is, is 0.01 to 0.3 mass%, an austenitic stainless steel.
[Claim 2]
　A austenitic stainless steel according to claim 1,
　wherein the chemical composition, in
　mass%, V:
　0.001 ~ 0.5%, Ti: 0.001
　~ 0.5%, B: 0.0001
　0.01%
　~,
　Mg: 0.0001 ~ 0.05%, REM: 0.001 ~ 0.5%,
　1 kind selected from a group consisting or 2 containing species or more elements, austenitic stainless steel.
[Claim 3]
　A austenitic stainless steel according to claim 1 or 2,
　wherein the austenitic stainless steel, cold has a tensile strength and elongation at break of 35% or more of the above 690MPa, the austenitic stainless steel.
[Claim 4]
　A austenitic stainless steel according to any one of claims 1 to 3,
　wherein the austenitic stainless steel is used in the high-pressure hydrogen gas equipment or liquid hydrogen equipment, austenitic stainless steel.
[Claim 5]
　A method of austenitic stainless steel produced according to any one of claims 1 to 4,
　chemical composition, in mass%, C: 0.005 ~ 0.07% , Si: 0.1 ~ 1 .2%, Mn: 3.2 ~ 6.5 %, Ni: 9 ~ 14%, at least one of the sum of Cu and Co: less than 0.005% or more and 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 %, the balance: a step of preparing a raw material is Fe and impurities,
　a step of the material to hot working,
　the worked material between the hot, 950 In solution heat treatment temperature of 1300 ° C., and a step of solution heat treated under conditions satisfying the formula (1) below,
　between the solution treatment and the hot working, it is not carried out cold working, austenite method for producing a system of stainless steel.
　40 × [% Nb] + 100 ≦ T × log (1.2 + t / 60) ≦ -200 × [% Nb] + 700 ... (1)
　In formula (1), the Nb content of the material in [% Nb] is substituted by mass%, the T is substituted by the solution treatment temperature is ° C., solution heat treatment time is substituted by min to t.