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    • 专利摘要:
    It is a high-resistance austenitic stainless steel that has resistance to hydrogen embrittlement and satisfactory hydrogen fatigue resistance. Austenitic stainless steel has a chemical composition that includes, in % by mass, C: up to 0.10%; Si: up to 1.0%; Mn: not less than 3.0% and less than 7.0%; Cr: 15 to 30%; Ni: not less than 12.0% and less than 17.0%; Al: up to 0.10%; N: 0.10 to 0.50%; P: up to 0.050%; S: up to 0.050%; at least one of V: 0.01 to 1.0% and Nb: 0.01 to 0.50%; and other elements, where the balance is Fe and impurities, where the ratio of the minor axis to the major axis of the austenite crystal grains is greater than 0.1, the grain crystal grain size number of austenite crystal is not less than 8.0 and the tensile strength is not less than 1000 MPa.
    公开号:BR112017000121B1
    申请号:R112017000121-7
    申请日:2015-10-22
    公开日:2021-06-08
    发明作者:Jun Nakamura；Tomohiko Omura；Hiroyuki Hirata；Kana JOTOKU；Takahiro Osuki
    申请人:Nippon Steel Corporation；
    IPC主号:
    专利说明:

    FIELD OF TECHNIQUE
    [0001] The present invention relates to an austenitic stainless steel and a method for manufacturing such stainless steel, and more particularly to an austenitic stainless steel that has a high resistibility and a satisfactory resistance to hydrogen embrittlement and a resistance to hydrogen fatigue required of a member such as a valve or gasket exposed to high pressure hydrogen gas and a method of fabricating such stainless steel. BACKGROUND OF THE INVENTION
    [0002] Research is underway to develop a fuel cell vehicle that uses hydrogen as a fuel for travel and to deploy hydrogen stations that fuel such a fuel cell vehicle with hydrogen. Stainless steel is one of the candidate materials that can be used for such applications. However, in a high-pressure hydrogen gas environment, even stainless steel can be embrittled by hydrogen (hydrogen embrittlement). Automotive pressurized hydrogen container standards specified by the High-Pressure Gas Safety Law allow the use of SUS316L as a stainless steel that does not suffer from embrittlement in a hydrogen environment.
    [0003] However, in order to achieve light fuel cell vehicle and compact hydrogen stations and to address the need for high pressure operation of hydrogen stations, it is desired that a stainless steel for use in a container or gasket or piping does not suffer from embrittlement in an environment with hydrogen in an environment with hydrogen gas and has a high resistivity not inferior to SUS316L, as is conventional. In recent years, high resistivity steels have been proposed which have a high N content and which use the resulting solute reinforcement and fine particle nitrides as disclosed in WO 2004/111285, WO 2004/083477, WO 2004/083476 and in Patent No. JP 5131794. DESCRIPTION OF THE INVENTION
    [0004] Materials with even higher resistibilities than the high resistibility steels described in the patent documents are desired. Cold working is known as a means of increasing the resistibility of austenitic stainless steel. However, cold-worked austenitic stainless steel has significantly decreased resistance to hydrogen embrittlement. Especially, in austenitic stainless steels with high N contents that have low stacking sequence failure energy, stresses during deformation can be localized, which results in an even more significant decrease in resistance to hydrogen embrittlement. Consequently, it is believed that cold working to increase resistibility cannot be applied to a material that is intended for use in a high pressure hydrogen environment.
    [0005] In addition, a member that is exposed to high pressure hydrogen gas, such as a tube or valve at a hydrogen station, is used in an environment in which the pressure of hydrogen gas varies. Consequently, a certain fatigue strength that can be caused by varying the pressure of hydrogen gas (hereafter referred to as "hydrogen fatigue strength") is desirable, however, the patent documents listed above do not consider hydrogen fatigue strength. That is, there is no material that has satisfactory resistibility, satisfactory hydrogen embrittlement resistance, and satisfactory hydrogen fatigue resistance.
    [0006] The present invention was developed in view of the current circumstances described above. An object of the present invention is to provide an austenitic stainless steel of high strength which has satisfactory hydrogen embrittlement resistance and hydrogen fatigue resistance.
    [0007] An austenitic stainless steel according to the present invention has a chemical composition consisting, in % by mass, of C: up to 0.10%; Si: up to 1.0%; Mn: not less than 3.0% and less than 7.0%; Cr: 15 to 30%; Ni: not less than 12.0% and less than 17.0%; Al: up to 0.10%; N: 0.10 to 0.50%; P: up to 0.050%; S: up to 0.050%; at least one of V: 0.01 to 1.0% and Nb: 0.01 to 0.50%; Mo: 0 to 3.0%; W: 0 to 6.0%, Ti: 0 to 0.5%, Zr: 0 to 0.5%; Hf: 0 to 0.3%; Ta: 0 to 0.6%; B: 0 to 0.020%; Cu: 0 to 5.0%; Co: 0 to 10.0%, Mg: 0 to 0.0050%, Ca: 0 to 0.0050%; La: 0 to 0.20%; Ce: 0 to 0.20%; Y: 0 to 0.40%; Sm: 0 to 0.40%; Pr: 0 to 0.40%; Nd: 0 to 0.50%; where the balance is Fe and impurities, the steel has an austenite crystal grain with a ratio of a smaller axis to a larger axis that is greater than 0.1, the austenite crystal grain has a size number of crystal grain that is not less than 8.0, the steel has a tensile strength that is not less than 1,000 MPa.
    [0008] A method for manufacturing an austenitic stainless steel, according to the present invention, includes the steps of: preparing a steel material that has a chemical composition consisting, in % by mass, of C: up to 0.10% ; Si: up to 1.0%; Mn: not less than 3.0% and less than 7.0%; Cr: 15 to 30%; Ni: not less than 12.0% and less than 17.0%; Al: up to 0.10%; N: 0.10 to 0.50%; P: up to 0.050%; S: up to 0.050%; at least one of V: 0.01 to 1.0% and Nb: 0.01 to 0.50%; Mo: 0 to 3.0%; W: 0 to 6.0%, Ti: 0 to 0.5%, Zr: 0 to 0.5%; Hf: 0 to 0.3%; Ta: 0 to 0.6%; B: 0 to 0.020%; Cu: 0 to 5.0%; Co: 0 to 10.0%, Mg: 0 to 0.0050%, Ca: 0 to 0.0050%; La: 0 to 0.20%; Ce: 0 to 0.20%; Y: 0 to 0.40%; Sm: 0 to 0.40%; Pr: 0 to 0.40%; Nd: 0 to 0.50%; where the balance is Fe and impurities; perform a solution treatment on the steel material at a solution treatment temperature of 1000 to 1200 °C; cold working the steel material that has been subjected to solution treatment with a reduction in area of not less than 20%; perform a heat treatment on the steel material that has been cold worked at a temperature not below 900 °C and below the solution treatment temperature; and cold working the steel material that has undergone heat treatment with a reduction in area of not less than 10% and less than 65%.
    [0009] The present invention provides a high resistibility austenitic stainless steel with satisfactory hydrogen embrittlement resistance and hydrogen fatigue resistance. BRIEF DESCRIPTION OF THE DRAWINGS
    [0010] [Figure 1] Figure 1 is a flowchart of a method for fabricating an austenitic stainless steel, according to an embodiment of the present invention.
    [0011] [Figure 2] Figure 2 is a scatter diagram showing the relationship between reduction in area in secondary cold work and relative elongation at break.
    [0012] [Figure 3] Figure 3 is a scatter diagram showing the relationship between Ni content and relative elongation at break.
    [0013] [Figure 4] Figure 4 is a scatter diagram showing the relationship between Ni content and fatigue duration in hydrogen. MODALITIES FOR CARRYING OUT THE INVENTION
    [0014] The present inventors have tried to find a way to increase the resistibility of austenitic stainless steel while maintaining the resistance to hydrogen embrittlement and the resistance to hydrogen fatigue. The following findings were obtained, (a) and (b).
    [0015] (a) Those of the austenitic stainless steels described in Patent No. 5131794 that have a Ni content of 12.0% or greater are suitable as the steel base material.
    [0016] (b) These austenitic stainless steels must be further cold worked with a reduction in area that is less than 10% and less than 65%. This will provide an austenitic stainless steel that has a high resistibility of 1000 MPa or greater and that has satisfactory hydrogen embrittlement resistance and hydrogen fatigue strength without excessive anisotropy in the cold worked crystal grains.
    [0017] Traditionally, it has been believed that cold working an austenitic stainless steel can cause stress-induced transformation or deformation of the crystal grains, which will prevent the resistance to hydrogen embrittlement and the resistance to hydrogen fatigue from being maintained. However, the investigation of the present inventors has shown that, in a steel with fine carbonitride precipitations, the anchoring effect prevents the crystal grains from being deformed. Furthermore, it has been shown that if the Ni content is 12.0% or greater, then satisfactory hydrogen embrittlement strength and hydrogen fatigue strength can be maintained even if the steel is cold worked with a reduction in area that is not less than 10% and less than 65%.
    [0018] The austenitic stainless steel of the present invention was produced based on the findings discussed above. Austenitic stainless steel in accordance with an embodiment of the present invention will now be described in detail. CHEMICAL STEEL COMPOSITION
    [0019] Austenitic stainless steel according to the present modality has the chemical composition described below. In the description below, "%" for the content of an element means % by mass.
    [0020] C: up to 0.10%
    [0021] Carbon (C) is not an element that is intentionally added in accordance with the present modality. If the C content exceeds 0.10%, the carbides precipitate at grain boundaries, which in turn can affect tenacity and other properties. With this in mind, the C content should not be greater than 0.10%. The C content is preferably not greater than 0.04% and more preferably not greater than 0.02%. The lower the C content, the better it will be; however, reducing the C content involves excessively increasing refining costs, so for practical reasons it is preferred that the C content be not less than 0.001%.
    [0022] Si: up to 1.0%
    [0023] Silicon (Si) deoxidizes steel. However, if a large amount of Si is contained, it can, together with Ni, Cr and/or other elements, form intermetallic compounds or it can facilitate the formation of intermetallic compounds, such as o-phase, which can significantly decrease the hot workability. In view of this, the Si content should not be greater than 1.0%. The Si content is preferably not greater than 0.5%. The lower the Si content, the better; however, from the perspective of refining costs, it is preferred that Si content is not less than 0.01%.
    [0024] Mn: not less than 3.0% and less than 7.0%
    [0025] Manganese (Mn) is an inexpensive austenite stabilizing element. According to the present modality, Mn is appropriately combined with Cr, Ni, N and/or with other elements to contribute to the increase in resistibility and to the improvement of ductility and toughness. Furthermore, in accordance with the present embodiment, fine particle precipitation of carbonitrides produces fine crystal grains; however, if the amount of dissolved N is small, carbonitrides with a sufficient density cannot be precipitated even after the process comprising a solution treatment, cold working and secondary heat treatment described further below. Mn has the effect of increasing the solubility of N; in view of this, the Mn content should not be less than 3.0%. On the other hand, if the Mn content is not less than 7.0%, the technique described in the document no. WO 2004/083477 can be applied; in view of this, according to the present modality, the Mn content must be less than 7.0%. Thus, the Mn content is not less than 3.0% and is less than 7.0%. The lower limit for the Mn content is preferably 4%. The upper limit for the Mn content is preferably 6.5%, more preferably 6.2.
    [0026] Cr: 15 to 30%
    [0027] Chromium (Cr) is an element that provides sufficient corrosion resistance to produce a stainless steel and is therefore an essential component. On the other hand, excessive Cr content facilitates the production of large amounts of coarse carbide particles, such as M23C6, which can decrease ductility and toughness. With this in mind, the Cr content should be in the range of 15 to 30%. The lower limit for the Cr content is preferably 18%, more preferably 20%. The upper limit for the Cr content is preferably 24%, more preferably 23.5%.
    [0028] Ni: not less than 12.0% and less than 17.0%
    [0029] Nickel (Ni) is added as an austenite stabilizing element. According to the present modality, Ni is appropriately combined with Cr, Mn, N and/or with other elements to contribute to the increase in resistibility and to the improvement of ductility and toughness. If the Ni content is less than 12.0%, cold working can cause the stability of the austenite to decrease. On the other hand, if the Ni content is not less than 17.0%, the steel is saturated in relation to the Ni effects described above, which means increases in material costs. In view of this, the Ni content should not be less than 12.0% and less than 17.0%. The lower limit of Ti content is preferably 13%, more preferably 13.5%. The upper limit of the Ti content is preferably 15%, more preferably 14.5%.
    [0030] Al: up to 0.10%
    [0031] Aluminum (Al) deoxidizes steel. On the other hand, the excessive Al content facilitates the production of intermetallic compounds, such as o-phase. In view of this, the Al content should not be greater than 0.10%. In order to ensure that the steel is deoxidized, the Al content is preferably not less than 0.001%. The upper limit for the Al content is preferably 0.05% and more preferably 0.03%. Al, as used herein, means the so-called "Al sol. (Acid-soluble Al)".
    [0032] N: 0.10 to 0.50%
    [0033] Nitrogen (N) is the most important solute reinforcement element and, at the same time, according to the present modality, it produces fine crystal grains forming fine particles of binding carbonitrides, thus contributing to the increase in resistibility. On the other hand, excessive N content can result in coarse nitride particles, which decrease toughness and other mechanical properties. With this in mind, N content should be in the range of 0.10 to 0.50%. The lower limit of the N content is preferably 0.20%, more preferably 0.30%.
    [0034] V: 0.01 to 1.0% and/or Nb: 0.01 to 0.50%
    [0035] Vanadium (V) and niobium (Nb) promote the production of binding carbonitrides and contribute to thinner crystal grains; in view of this, the two are contained, or one of them. On the other hand, if excessive amounts of these elements are contained, the steel will be saturated with respect to its effects, which means increases in material costs. With this in mind, the V content should be in the range of 0.01 to 1.0%, and the Nb content in the range of 0.01 to 0.50%. The lower limit for the V content is preferably 0.10%. The upper limit for the V content is preferably 0.30%. The lower limit for the Nb content is preferably 0.15%. The upper limit for the Nb content is preferably 0.28%. This is most effective if both V and Nb are contained.
    [0036] P: up to 0.050%
    [0037] Phosphorus (P) is an impurity and can adversely affect the toughness and other properties of steel. The P content should not be greater than 0.050%, the lower the P content the better. The P content is preferably not greater than 0.025% and more preferably not greater than 0.018%.
    [0038] S: up to 0.050%
    [0039] Sulfur (S) is an impurity and can adversely affect the toughness and other properties of steel. The S content should not be greater than 0.050%, where the lower the S content, the better. The S content is preferably not greater than 0.010% and more preferably not greater than 0.005%.
    [0040] The balance of the chemical composition of austenitic stainless steel according to the present modality is Fe and impurities. Impurity, as used herein, means an element that originates from ore or scrap used as a raw material of a steel that is manufactured on an industrial basis or an element that has been inserted from the environment or the like during the process. of manufacturing.
    [0041] Austenitic stainless steel according to the present embodiment may have a chemical composition that includes, instead of part of the Fe described above, one or more elements selected from the first to fourth groups given below. All elements belonging to the first to fourth groups below are optional elements. That is, the elements belonging to the first to the fourth groups given below do not need to be contained in the austenitic stainless steel according to the present modality. Only one or a few of these elements can be contained.
    [0042] More specifically, for example, only one among the first to fourth groups can be selected and one or more elements can be selected from that group. In this case, not all elements belonging to the selected group need to be selected. Alternatively, a plurality of groups can be selected from the first through the fourth groups and one or more elements can be selected from each of these groups. Again, not all elements that belong to the selected groups need to be selected. FIRST GROUP
    [0043] Mo: 0 to 3.0%
    [0044] W: 0 to 6.0%
    [0045] The elements that belong to the first group are molybdenum (Mo) and tungsten (W). These elements have the common effects of promoting the production and stabilization of carbonitrides and contributing to solute reinforcement. On the other hand, if excessive amounts of them are contained, the steel is saturated in relation to their effects. With this in mind, the upper bound for Mo should be 3.0% and that for W should be 6.0%. The preferred lower limit for these elements is 0.3%. SECOND GROUP
    [0046] Ti: 0 to 0.5%
    [0047] Zr: 0 to 0.5%
    [0048] Hf: 0 to 0.3%
    [0049] Ta: 0 to 0.6%
    [0050] The elements that belong to the second group are titanium (Ti), zirconium (Zr), hafnium (Hf) and tantalum (Ta). These elements have the common effects of promoting the production of carbonitrides and producing fine crystal grains. On the other hand, if excessive amounts of them are contained, the steel is saturated in relation to their effects. With this in mind, the upper limit for Ti and Zr is 0.5%, that for Hf is 0.3% and that for Ta is 0.6%. The upper limit for Ti and Zr is preferably 0.1% and more preferably 0.03%. The upper limit for Hf is preferably 0.08% and more preferably 0.02%. The upper limit for Ta is preferably 0.4% and more preferably 0.3%. The preferred lower limit for these elements is 0.001%. THIRD GROUP
    [0051] B: 0 to 0.020%
    [0052] Cu: 0 to 5.0%
    [0053] Co: 0 to 10.0%
    [0054] The elements that belong to the third group are boron (B), copper (Cu) and cobalt (Co). These elements have the common effect of contributing to an increase in the resistibility of steel. B increases the resistibility of the steel producing fine precipitates and therefore fine crystal grains. On the other hand, if the B is contained in excess, it can cause compounds with low melting points to be formed, which decreases the hot workability. With this in mind, the upper limit for the B content is 0.020%. Cu and Co are austenite stabilizing elements and increase the resistibility of steel through solute reinforcement. On the other hand, if excessive amounts of them are contained, the steel is saturated in relation to their effects. With this in mind, the upper limit for Cu is 5.0% and that for Co is 10.0%. The preferred lower limit for B is 0.0001% and the preferred lower limit for Cu and Co is 0.3%. FOURTH GROUP
    [0055] Mg: 0 to 0.0050%
    [0056] Ca: 0 to 0.0050%
    [0057] La: 0 to 0.20%
    [0058] Ce: 0 to 0.20%
    [0059] Y: 0 to 0.40%
    [0060] Sm: 0 to 0.40%
    [0061] Pr: 0 to 0.40%
    [0062] Nd: 0 to 0.50%
    [0063] The elements that belong to the fourth group are magnesium (Mg), calcium (Ca), lanthanum (La), cero (Ce), yttrium (Y), samarium (Sm), praseodymium (Pr) and neodymium (Nd) . These elements have the common effect of preventing cracking by solidification during steel melting. On the other hand, their excessive contents decrease the hot workability. With this in mind, the upper bound for Mg and Ca is 0.0050%, that for La and Ce is 0.20%, that for Y, Sm and Pr is 0.40% and that for Nd is 0.350%. The preferred lower limit for these elements is 0.0001%. INTERNAL STEEL MICROSTRUCTURE
    [0064] Although nitrogen is effective in solute reinforcement, it decreases the failure energy of the stacking sequence to locate stresses during deformation, which can decrease durability against embrittlement in a hydrogen environment. In addition, as discussed further below, although the present modality attempts to reinforce steel through cold working, cold working can increase displacement density and increase the amount of trapped hydrogen, which can decrease durability against embrittlement in an environment with hydrogen.
    [0065] According to the present modality, the microstructure present after the cold work carried out after the secondary heat treatment further described below (hereinafter referred to as secondary cold work) is adjusted to increase the resistibility up to 1,500 MPa and at the same time , prevent embrittlement in an environment with hydrogen. More specifically, the ratio of minor axis (B) to major axis (A) of austenite crystal grains, B/A, becomes greater than 0.1 to provide satisfactory resistance to hydrogen embrittlement in a Cold worked microstructure.
    [0066] In order to make the ratio of minor axis to major axis of austenite crystal grains after secondary cold working greater than 0.1, the microstructure before secondary cold working needs to be controlled; for this to be accomplished, anchoring using bonding carbonitrides is effective. In order to obtain this effect, it is preferable to have 0.4/μm2 or more particles (in an observed cross section) of binding carbonitrides with a size of 50 to 1000 nm to be precipitated. These bonding carbonitrides contain Cr, V, Nb, Mo, W, Ta, etc. as main components and have a Z-phase crystal microstructure, that is, the type of Cr (Nb, V) (C, N) and MX (M: Cr, V, Nb, Mo, W, Ta etc., X: C,N). The bonding carbonitrides in this embodiment contain almost no Fe, the amount of Fe, if any, at most 1% by atom. Carbonitrides, according to the present embodiment, can have an extremely low C (carbon) content, i.e. they can be nitrides.
    [0067] Furthermore, the austenite crystal grains of austenitic stainless steel, according to the present embodiment, have a crystal grain size number in accordance with ASTM E 112 which is not less than 8.0. Making the crystal grains finer increases the resistance of a high nitrogen steel to embrittlement in a hydrogen environment.
    [0068] Even if a steel contains the above microstructure, it may have low resistance to embrittlement in a hydrogen environment if it has a low Ni content. Furthermore, even if the microstructure prior to cold working is austenite, which has satisfactory resistance to hydrogen embrittlement, cold working can cause a martensite phase to form, which can deteriorate resistance to hydrogen embrittlement. hydrogen. Ni is contained under the present modality to improve austenite stability: Ni content is 12.0% or greater under the present modality to provide sufficient austenite stability against cold working with a large working ratio .
    [0069] The tensile strength of an austenitic stainless steel according to the present modality is not greater than 1,000 MPa and preferably not less than 1,200 MPa. On the other hand, a tensile strength of 1500 MPa or greater can increase the anisotropy of crystal grains, making it difficult to provide sufficient strength to hydrogen embrittlement. Thus, in order to define an upper limit, the tensile strength is preferably less than 1500 MPa. MANUFACTURING METHOD
    [0070] A method for fabricating the austenitic stainless steel, according to an embodiment, of the present invention will be described.
    [0071] With conventional methods, it is impossible to make the crystal grains finer and to make suitable fine binding carbonitrides with a desired number density to precipitate before secondary cold working; however, this is made possible, for example, by successfully carrying out the solution treatment, cold working, secondary heat treatment described below.
    [0072] Figure 1 is a flowchart of the method for manufacturing the austenitic stainless steel, according to the present modality. The method for making austenitic stainless steel in accordance with the present embodiment includes the step of: preparing a steel material (step S1); perform the solution treatment on the steel material (step S2); cold working the steel material that has been subjected to solution treatment (step 3); perform a secondary heat treatment on the steel material that has been cold worked (step S4); and perform secondary cold work on the steel material that has undergone secondary heat treatment (step S5).
    [0073] A steel that has the chemical composition described above (hereinafter referred to as steel material) is prepared (step S1). More specifically, for example, steel with the chemical composition described above is reductively melted and refined. Furthermore, it is possible that the steel material may be a refined steel that has been subjected to hot working, such as hot forging, hot rolling or hot extrusion.
    [0074] The steel material is subjected to solution treatment (step S2). More specifically, the steel material is held at a temperature of 1000-1200°C (hereafter referred to as the solution treatment temperature) for a predetermined period of time and then cooled. In order to make the binding elements sufficiently dissolve, the solution treatment temperature is not less than 1000 °C and more particularly not less than 1,100 °C. On the other hand, if the solution treatment temperature is higher than 1200 °C, the crystal grains become extremely coarse.
    [0075] In the solution treatment, according to the present modality, it is sufficient if the solution occurs to a degree necessary to cause the carbonitrides to precipitate in the subsequent secondary heat treatment (step S4) and not all carbonitrides forming elements need to be dissolved. It is preferred that the steel material which has undergone the solution treatment is cooled rapidly from the solution treatment temperature, preferably water cooled (bathed or immersed).
    [0076] In addition, the solution treatment step (step S2) does not need to be independent step: similar effects can be obtained by means of rapid cooling after the hot working step, such as hot extrusion. For example, blast chilling can occur after hot extrusion to about 1,150 °C.
    [0077] The steel material that has been subjected to solution treatment is cold worked (step S3). Cold working can be, for example, cold rolling, cold forging or cold drawing. Area reduction for cold working is 20% or greater. This increases the precipitation cores for carbonitrides in steel. There is no specific upper limit for area reduction for cold working; however, considering the area reductions applied to normal parts, a reduction of 90% or less is preferred. As used in this document, the reduction in area (%) is (cross section of steel material before cold working - cross section of steel material after cold working) x 100 / (cross section of steel material before of cold work).
    [0078] The steel material that has been cold worked is subjected to secondary heat treatment (step S4). More specifically, the steel material that has been cold worked is maintained at a temperature that is not less than 900 °C and less than the solution treatment temperature of step S2 (hereinafter referred to as the secondary heat treatment temperature) for a predetermined period of time and then cooled. Secondary heat treatment removes stresses due to cold working and causes fine carbonitride particles to precipitate, resulting in fine crystal grains.
    [0079] As described above, the secondary heat treatment temperature is lower than the solution treatment temperature. In order to obtain even finer crystal grains, the secondary heat treatment temperature is preferably not higher than [solution treatment temperature - 20 °C] and more preferably not higher than [solution treatment temperature solution - 50 °C]. The secondary heat treatment temperature is preferably not greater than 1,150 °C and more preferably not greater than 1,080 °C. On the other hand, if the secondary heat treatment temperature is less than 900 °C, coarse Cr carbide particles are produced, which results in a non-uniform microstructure.
    [0080] The steel material that has undergone secondary heat treatment is subjected to secondary cold working (step S5). Secondary cold working can be, for example, cold rolling, cold forging or cold drawing. The area reduction for secondary cold working is not less than 10% and less than 65%. If the area reduction for secondary cold work is not less than 65%, the material anisotropy and austenite stability decrease, which decreases the resistance to hydrogen embrittlement and the fatigue life in hydrogen. According to the present modality, the increase in Ni content, which is an element that increases the stability of austenite, and the anchoring effect of carbonitrides provide a desired resistance to hydrogen embrittlement and resistance to hydrogen fatigue even though the reduction in area is relatively high. This will increase resistibility while preventing embrittlement in a hydrogen environment. In order to define a lower limit, the reduction in area for secondary cold working is preferably greater than 30% and more preferably not less than 40%. EXAMPLES
    [0081] The present invention will now be described in more detail by way of examples. The present invention is not limited to these examples.
    [0082] 50 kg of stainless steels having the chemical compositions shown in Table 1 were vacuum cast and hot forged into blocks with a thickness of 40 to 60 mm.


    [0083] The blocks were hot rolled to a predetermined thickness in order to provide steel materials. Each of the steel materials was subjected to solution treatment, cold working, secondary heat treatment and secondary cold working under the conditions shown in Table 2 to provide a plate with a thickness of 8 mm. The retention time for each of the solution treatment and the secondary heat treatment was one hour. Cold rolling was performed as each of cold working and secondary cold working.


    MICROSTRUCTURE OBSERVATION
    [0084] Samples were extracted from the plates obtained to allow the observation of cross-sections parallel to the lamination direction and the direction of thickness and were incorporated into resin and were corroded in a mixed acid (hydrochloric acid to nitric acid = 1:1), before their crystal grain size numbers have been measured in accordance with ASTM E 112. In addition, in each of these samples, the ratio of the minor axis to the major axis of austenite crystal grains ( minor geometric axis/major geometric axis) was determined. After secondary heat treatment, samples were similarly extracted from the plates before secondary cold working and their crystal grain size numbers were measured. RESISTANCE TO TENSION AND RUPTURE Elongation
    [0085] Round bar tensile test specimens extending in the longitudinal direction of the plates and with a parallel portion having a diameter of 3 mm were extracted, and the tensile tests were conducted in the atmosphere at room temperature or in a high pressure hydrogen gas at 85 MPa at room temperature at a strain ratio of 3*10-6/s to measure tensile strength and elongation at break. Since a significant influence of hydrogen is a decrease in toughness, the ratio of the elongation at break in hydrogen to the elongation at break in the atmosphere was treated as a relative elongation at break, and it was considered that a steel with an elongation at break relative of 80% or greater, preferably 90% or greater has a negligible decrease in ductility due to hydrogen and has a resistance to embrittlement in a satisfactory hydrogen environment. DURATION OF FATIGUE
    [0086] Tubular fatigue test specimens extending in the longitudinal direction of the plates and with an outer diameter of 7.5 mm were extracted and fatigue tests were conducted in argon gas at room temperature or in a high pressure hydrogen gas at 85 MPa at room temperature to measure fatigue duration. The number of cycles that occurred when a crack originating from the inner surface of a specimen reached the outer surface was treated as fatigue duration. Since a significant influence of hydrogen is a decrease in fatigue duration, the ratio of fatigue duration in hydrogen to fatigue duration in argon was treated as relative fatigue duration, and it was considered that a steel with a duration of fatigue. Relative fatigue of 70% or greater has a negligible decrease in fatigue duration due to hydrogen and has satisfactory hydrogen fatigue strength. TEST RESULTS
    [0087] The tensile strength values after secondary heat treatment, the tensile strength after secondary cold working, the ratio of the minor axis to the major axis of austenite crystal grain, the size number of austenite crystal grain crystal grain after secondary heat treatment, the relative elongation at break, the relative fatigue duration, the fatigue duration in hydrogen, the fatigue duration in argon and the number of crystal grain size of Austenite crystal grains after secondary cold working are listed in Table 2 given above.
    [0088] In each of Tests Nos. 1 to 15, the ratio between the minor axis and the major axis of austenite crystal grains was greater than 0.1, the number of crystal grain size of crystal grains of austenite after secondary cold work was not less than 8.0, and the tensile strength was not less than 1,000 MPa and, at the same time, the relative elongation at break was not less than 80% and the relative fatigue duration it was not less than 70%, exhibiting sufficient hydrogen embrittlement resistance and hydrogen fatigue resistance.
    [0089] In each of Tests Nos. 16 and 17, the relative elongation at break and the duration of relative fatigue were small. Presumably, this is due to the fact that the ratio of the minor axis to the major axis of austenite crystal grains was not greater than 0.1, that is, due to anisotropy of the crystal grains. Furthermore, the ratio of minor axis to major axis of austenite crystal grains was not greater than 0.1 presumably due to the fact that the reduction in area for secondary cold working was very high.
    [0090] In Test No 18, the relative elongation at break and the relative fatigue duration were small. Presumably this is due to the fact that the crystal grains were thick. The crystal grains were presumably coarse due to the fact that the solution treatment temperature was too high.
    [0091] In Test No. 19, the relative elongation at break and the relative fatigue duration were small. Presumably this is due to the fact that the crystal grains were thick. The crystal grains were presumably coarse due to the fact that the secondary heat treatment temperature was too low, precipitating the Cr2N.
    [0092] In each of Tests Nos. 20 to 23, the relative elongation at break and the relative fatigue duration were small. Presumably this is due to the fact that Ni contents in steel grades L, M, N and O were very low and the stability of austenite after cold working was not guaranteed.
    [0093] In each of Tests Nos 24 and 25, the tensile strength was less than 1,000 MPa and the relative elongation at break and the relative fatigue duration were small. In type P for Test No 24, the Mn content was very low and, as a result, a sufficient amount of N was not contained. In steel grade Q for Test No 25, the N content was very low. In any case, the solute reinforcement due to N was insufficient, resulting in insufficient tensile strength.
    [0094] In each of Tests Nos. 26 to 28, the relative elongation at break and the duration of relative fatigue were small. Presumably, this is due to the fact that the ratio of the minor axis to the major axis of austenite crystal grains was not greater than 0.1, that is, due to anisotropy of the crystal grains. The ratio of minor axis to major axis of austenite crystal grains was not greater than 0.1 presumably due to the fact that steel grade R for Test Nos. 26 to 28 did not contain Nb or V, therefore , the anchoring effect by carbonitrides was not obtained.
    [0095] Figure 2 is a scatter diagram showing the relationship between reduction in area in secondary cold work and relative elongation at break. Figure 2 was created by extracting, from Table 2, data for the same type of steel (ie, steel type A). Figure 2 shows that if the reduction in area is not greater than 65%, a relative elongation at break of 80% or greater can be stably obtained. Furthermore, it shows that, even if the area reduction is less than 65%, the relative elongation at break is low if the solution treatment temperature is too high (Test No 18) or the secondary heat treatment temperature be too low (Assay No 19).
    [0096] Figure 3 is a scatter diagram showing the relationship between Ni content and relative elongation at break. Figure 3 was created by extracting, from Table 2, data with the same area reduction (60%) in secondary cold work. Figure 3 shows that if the Ni content is not less than 12.0%, the relative elongation at break is significantly higher. Furthermore, it shows that, even if the Ni content is not less than 12.0%, the relative elongation at break is lower if the N content is very low (steel types P and Q). Furthermore, it shows that, even if the Ni content is not lower than 12.0%, the relative breaking elongation is lower if there is no Nb or V content (type of steel R).
    [0097] Figure 4 is a scatter diagram that shows the relationship between Ni content and fatigue duration in hydrogen. Figure 4 was created by extracting, from Table 2, data with the same area reduction (60%) in secondary cold work. Figure 4 shows that if the Ni content is not less than 12.0%, the fatigue life in hydrogen is significantly long. Furthermore, it shows that, even if the Ni content is not less than 12.0%, the fatigue life in hydrogen is short if the N content is very low (P and Q steel types). Furthermore, it shows that, even if the Ni content is not less than 12.0%, the fatigue life in hydrogen is short if there is no Nb or V content (type of steel R). INDUSTRIAL APPLICABILITY
    [0098] The present invention provides a high resistibility austenitic stainless steel with satisfactory hydrogen embrittlement resistance and hydrogen fatigue resistance that are required of a member for use in high pressure hydrogen that is used without welding, for example.
    权利要求:
    Claims (11)
    [0001]
    1. Austenitic stainless steel CHARACTERIZED by the fact that it has a chemical composition consisting, in % by mass, of C: up to 0.10%; Si: up to 1.0%; Mn: not less than 3.0% and less than 7.0%; Cr: 15 to 30%; Ni: not less than 12.0% and less than 17.0%; Al: up to 0.10%; N: 0.10 to 0.50%; P: up to 0.050%; S: up to 0.050%; at least one of V: 0.01 to 1.0% and Nb: 0.01 to 0.50%; Mo: 0 to 3.0%; W: 0 to 6.0%; Ti: 0 to 0.5%; Zr: 0 to 0.5%; Hf: 0 to 0.3%; Ta: 0 to 0.6%; B: 0 to 0.020%; Cu: 0 to 5.0%; Co: 0 to 10.0%; Mg: 0 to 0.0050%; Ca: 0 to 0.0050%; La: 0 to 0.20%; Ce 0 to 0.20%; Y 0 to 0.40%; Sm: 0 to 0.40%; Pr: 0 to 0.40%; Nd: 0 to 0.50%; and where the balance is Fe and impurities, the steel has an austenite crystal grain with a ratio of a minor axis to a major axis that is greater than 0.1, the austenite crystal grain has a number of Crystal grain size conforming to ASTM E 112 standard which is not less than 8.0, the steel has a tensile strength which is not less than 1,000 Mpa and up to 1,500 Mpa.
    [0002]
    2. Austenitic stainless steel, according to claim 1, CHARACTERIZED by the fact that the chemical composition contains one or more elements selected from one or more of the first to fourth groups, the first group consisting of Mo: 0.3 at 3.0% and W: 0.3 to 6.0%; the second group consists of Ti: 0.001 to 0.5%, Zr: 0.001 to 0.5%, Hf: 0.001 to 0.3% and Ta: 0.001 to 0.6%; the third group consists of B: 0.0001 to 0.020%, Cu: 0.3 to 5.0% and Co: 0.3 to 10.0%; the fourth group consists of Mg: 0.0001 to 0.0050%, Ca: 0.0001 to 0.050%, La: 0.0001 to 0.20%, Ce: 0.0001 to 0.20%, Y : 0.0001 to 0.40%, Sm: 0.0001 to 0.40%, Pr: 0.0001 to 0.40% and Nd: 0.0001 to 0.50%.
    [0003]
    3. Austenitic stainless steel, according to any one of the preceding claims, CHARACTERIZED by the fact that it contains Ni: not less than 13.0%.
    [0004]
    4. Austenitic stainless steel, according to any one of the preceding claims, CHARACTERIZED by the fact that 0.4 / μm2 or more particles, in an observed cross section, of alloy carbonitrides with a dimension of 50 to 1000 nm are precipitated, wherein the carbonitrides comprise Cr, V, Nb Mo, W and Ta; have a Z-phase crystal microstructure; and Fe, if contained, is at most 1% in atom.
    [0005]
    5. Austenitic stainless steel, according to any one of the preceding claims, CHARACTERIZED by the fact that it has a tensile strength that is not less than 1200 MPa.
    [0006]
    6. Use of austenitic stainless steel, according to any one of the preceding claims, CHARACTERIZED by the fact that it is used for a limb exposed to hydrogen gas at high pressure which is used without welding.
    [0007]
    7. Use according to claim 6, CHARACTERIZED by the fact that the member exposed to hydrogen gas at high pressure is a valve or gasket.
    [0008]
    8. Method for manufacturing austenitic stainless steel, according to any one of claims 1 to 5, CHARACTERIZED by the fact that it comprises the steps of: melting and refining a steel material that has a chemical composition consisting of % by mass , in C: up to 0.10%; Si: up to 1.0%; Mn: not less than 3.0% and less than 7.0%; Cr: 15 to 30%; Ni: not less than 12.0% and not less than 17.0%; Al: up to 0.10%; N: 0.10 to 0.50%; P: up to 0.050%; S: up to 0.050%; at least one of V: 0.01 to 1.0% and Nb: 0.01 to 0.50%; Mo: 0 to 3.0%; W: 0 to 6.0%, Ti: 0 to 0.5%, Zr: 0 to 0.5%; Hf: 0 to 0.3%; Ta: 0 to 0.6%; B: 0 to 0.020%; Cu: 0 to 5.0%; Co: 0 to 10.0%, Mg: 0 to 0.0050%, Ca: 0 to 0.0050%; La: 0 to 0.20%; Ce: 0 to 0.20%; Y: 0 to 0.40%; Sm: 0 to 0.40%; Pr: 0 to 0.40%; Nd: 0 to 0.50%, where the equilibrium is Fe and impurities; one of the following steps a) and b) a) heat working the steel material and carrying out (S2) a solution treatment on the steel material at a solution treatment temperature of 1000 to 1200 °C; and b) hot working the steel material at a solution temperature of 1000 to 1200 °C, followed by rapid cooling, performing (S2) a solution treatment; cold working (S3) the steel material that has been subjected to solution treatment with a reduction in area not less than 20%; perform (S4) a heat treatment on the steel material that has been cold worked at a temperature not lower than 900 °C and lower than the solution treatment temperature; and cold working (S5) the steel material that has undergone heat treatment with a reduction in area of not less than 10% and less than 65%.
    [0009]
    9. Method according to claim 8, CHARACTERIZED by the fact that the solution treatment temperature is not less than 1100 °C.
    [0010]
    10. Method according to any one of claims 8 to 9, CHARACTERIZED by the fact that the treatment of the solution includes a water-cooling step of the steel material.
    [0011]
    11. Method, according to any one of claims 8 to 10, CHARACTERIZED by the fact that the heat treatment on the steel material that has been cold worked is carried out (S4) at a temperature that is not higher than the treatment temperature of the solution -20°C.
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    同族专利:
    公开号 | 公开日
    JP6004140B1|2016-10-05|
    JPWO2016068009A1|2017-04-27|
    EP3214194B1|2019-12-04|
    CN106795606A|2017-05-31|
    AU2015338140B2|2018-05-24|
    EP3214194A4|2018-03-14|
    KR101868761B1|2018-06-18|
    EP3214194A1|2017-09-06|
    CA2963770C|2021-01-12|
    CN106795606B|2018-11-23|
    BR112017000121A2|2018-01-09|
    KR20170029617A|2017-03-15|
    US10662497B2|2020-05-26|
    WO2016068009A1|2016-05-06|
    CA2963770A1|2016-05-06|
    ES2769201T3|2020-06-25|
    US20170314092A1|2017-11-02|
    AU2015338140A1|2017-04-06|
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    法律状态:
    2019-10-08| B25D| Requested change of name of applicant approved|Owner name: NIPPON STEEL CORPORATION (JP) |
    2020-01-28| B06U| Preliminary requirement: requests with searches performed by other patent offices: procedure suspended [chapter 6.21 patent gazette]|
    2021-03-09| B09A| Decision: intention to grant [chapter 9.1 patent gazette]|
    2021-06-08| B16A| Patent or certificate of addition of invention granted|Free format text: PRAZO DE VALIDADE: 20 (VINTE) ANOS CONTADOS A PARTIR DE 22/10/2015, OBSERVADAS AS CONDICOES LEGAIS. |
    优先权:
    申请号 | 申请日 | 专利标题
    JP2014220553|2014-10-29|
    JP2014-220553|2014-10-29|
    PCT/JP2015/079800|WO2016068009A1|2014-10-29|2015-10-22|Austenitic stainless steel and manufacturing method therefor|
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