Austenitic stainless steel material

Abstract

This austenitic stainless steel material has a chemical composition in which the Md30 value is 15.0-70.0, the recrystallization rate is 60% or less, and the phase fraction of processing-induced martensite is 5% or less (including 0).

Description

Austenitic stainless steel material

[0001] The present disclosure relates to an austenitic stainless steel material.

[0002] Conventionally, techniques for improving material properties such as strength (mechanical properties) in metastable austenitic stainless steel materials have been developed (see Patent Documents 1 to 3).

[0003] Japanese National Publication of Translated Version 2024-529999, Japanese Patent No. 6623761, Japanese Patent No. 5347600

[0004] In general, metastable austenitic stainless steel materials are put into practical use after adjusting mechanical properties such as strength by performing temper rolling to generate deformation-induced martensite. In order to control mechanical properties by temper rolling, it is necessary to finely control the temper rolling conditions. From the viewpoint of productivity and manufacturing cost, there is a demand for producing an austenitic stainless steel material having high mechanical properties without performing temper rolling.

[0005] For example, Patent Document 1 describes a technique for improving strength and ductility without performing temper rolling by refining austenite grains (γ grains) while limiting the steel to a specific chemical composition. In this type of technique, there is a limit to the amount of strength improvement only by strengthening through grain refinement.

[0006] In the technique described in Patent Document 2, the density of recrystallized grain boundaries is increased by refining recrystallized grains and reducing the amount of unrecrystallized regions. Thereby, the tensile strength and ductility of the austenitic stainless steel material are improved without performing temper rolling. However, in the technique described in Patent Document 2, final annealing is performed under conditions where unrecrystallized regions remain in order to refine recrystallized grains, and a relatively large amount of deformation-induced martensite may remain as part of the unrecrystallized regions in the metal structure after final annealing. In this case, ductility may be reduced. Further, even if high tensile strength is obtained by work hardening through increasing the number of recrystallized grains, yield stress may become low due to a decrease in dislocation density.

[0007] The technology described in Patent Document 3 improves the strength and formability of austenitic stainless steel by adjusting the chemical composition and refining the crystal grains. However, because the austenite phase has a relatively stable chemical composition, the amount of work-induced martensite generated by cold rolling may be insufficient. In this case, the yield stress after the final heat treatment may be low. Furthermore, Patent Document 3 only discloses the mechanical properties of austenitic stainless steel that has undergone temper rolling, and there is room for further investigation into technologies for producing austenitic stainless steel with high mechanical properties without temper rolling.

[0008] One aspect of this disclosure aims to provide an austenitic stainless steel material that can be manufactured without temper rolling and has an excellent balance of yield strength, tensile strength, and ductility.

[0009] To solve the above problems, an austenitic stainless steel material in one aspect of the present disclosure has a chemical composition in which the Md30 value defined by the following formula (1) is 15.0 or more and 70.0 or less, the recrystallization rate is 60% or less, and the phase fraction of work-induced martensite is 5% or less (including 0): Md30 value [°C] = 551 - 462 (C + N) - 9.2Si - 8.1Mn - 13.7Cr - 29 (Ni + Cu) - 18.5Mo - 68Nb ... (1) The elemental symbols in the above formula (1) are replaced with the content (mass%) of each element contained in the austenitic stainless steel material, and 0 is replaced for elements that are not added.

[0010] According to one aspect of this disclosure, it is possible to provide an austenitic stainless steel material that can be manufactured without temper rolling and has an excellent balance of tensile strength, yield stress, and ductility.

[0011] An embodiment of this disclosure is described below. In this specification, "%" in relation to the content of each element in the chemical composition of austenitic stainless steel means "mass %". The term "steel sheet" is used to include steel strip unless otherwise specified. For numerical values ​​X1 and X2 (where X1 < X2), "X1 to X2" means "X1 or greater and X2 or less".

[0012] <Definitions of Terms, etc.> In this specification, the term "austenitic stainless steel" is used to describe the properties of the material itself and refers to austenitic stainless steel material that is not particularly limited to a specific shape. Examples of steel materials include steel strips, steel plates, steel bars, steel wires, steel pipes, and steel bars. Furthermore, since "steel plate" can be considered a part of "steel strip," "austenitic stainless steel sheet" includes "austenitic stainless steel strip."

[0013] The austenitic stainless steel material in one embodiment of this disclosure is a so-called metastable austenitic stainless steel material having a chemical composition that undergoes work-induced martensitic transformation upon processing. Examples of metastable austenitic stainless steel include SUS301 series steel grades.

[0014] In this specification, an austenitic stainless steel sheet means a cold-rolled and annealed sheet that has undergone cold rolling and final annealing. Cold-rolled and annealed sheets include those that have undergone various post-treatments as appropriate after final annealing, as specified in JIS 4305:2021, for example. An austenitic stainless steel sheet in one embodiment of this disclosure does not need to be temper-rolled, as will be described in detail later. However, if an increase in manufacturing costs is acceptable, the mechanical properties may be adjusted by temper rolling, and therefore, the scope of this disclosure also includes cold-rolled and annealed sheets that have undergone temper rolling.

[0015] <Regarding General Methods> To facilitate understanding of this disclosure, a general example of a typical manufacturing process for metastable austenitic stainless steel sheets is outlined below.

[0016] First, a slab having the chemical composition of metastable austenitic stainless steel is prepared. The slab is hot-rolled to obtain a hot-rolled sheet. Next, the hot-rolled sheet is hot-rolled and annealed to obtain a hot-rolled and annealed sheet. The microstructure of this hot-rolled and annealed sheet is composed of a recrystallized austenite phase. Hereinafter, the recrystallized austenite phase will be referred to as the recrystallized γ phase.

[0017] Cold-rolled steel sheets are obtained by pickling hot-rolled and annealed sheets and then cold-rolling them. During cold rolling (processing), a portion of the recrystallized γ phase transforms into work-induced martensite. The portion of the recrystallized γ phase that was affected by cold rolling but did not transform into work-induced martensite is called the cold-rolled γ phase. The cold-rolled γ phase contains a lot of strain introduced by cold rolling.

[0018] Generally, highly hardened (very low ductility) cold-rolled steel sheets containing cold-rolled γ phase and work-induced martensite are subjected to annealing (e.g., continuous annealing) to soften them. For example, the cold-rolled steel sheet is heated in a burner-heated furnace to a temperature range above the recrystallization initiation temperature (e.g., around 1000°C). Heating causes austenite phases with less strain to nucleate from the work-induced martensite. These nucleated austenite phases can also be considered as recrystallized grains.

[0019] Then, the nucleated austenite phase grains grow by encroaching on the surrounding work-induced martensite, causing a reverse transformation from work-induced martensite to the austenite phase. Hereafter, this reverse transformation will be referred to as the diffusion-type reverse transformation, and the austenite phase produced by the diffusion-type reverse transformation will be referred to as the diffusion-type reverse transformation γ phase. For the sake of explanation, the above annealing treatment for cold-rolled steel sheets will be referred to as "annealing treatment by conventional heating."

[0020] The diffusive reverse transformation γ phase produced by conventional annealing is less strained, and the crystal grains of the diffusive reverse transformation γ phase can be considered as recrystallized grains. In conventional annealing, it can be said that recrystallization progresses along with the progression of the diffusive reverse transformation. Furthermore, if the diffusive reverse transformation is partially incomplete in conventional annealing, the microstructure of the cold-rolled annealed sheet after annealing will contain residual work-induced martensite. This residual work-induced martensite can be considered the unrecrystallized portion.

[0021] Metastable austenitic stainless steel sheets obtained by conventional annealing require temper rolling to adjust the balance between strength and ductility in order to achieve the desired mechanical properties.

[0022] <Summary of the findings of this disclosure> The inventors have conceived the technology of this disclosure by obtaining novel findings that improve the mechanical properties of metastable austenitic stainless steel materials by controlling the metal structure generated by annealing. As an example of the application of the technology of this disclosure, steel sheets will be used as an example and explained below. For the sake of explanation, the annealing treatment of cold-rolled steel sheets based on the findings of this disclosure will be referred to as the "annealing treatment of this disclosure".

[0023] Similar to the general manufacturing method described above, a cold-rolled steel sheet containing cold-rolled γ phase and work-induced martensite is prepared. Then, the cold-rolled steel sheet is subjected to the annealing treatment according to this disclosure. In the annealing treatment according to this disclosure, the heating rate is relatively fast compared to annealing treatment by conventional heating. In addition, in the annealing treatment according to this disclosure, the holding time (soaking time) at the annealing temperature is set to 0 seconds or a short time. The specific conditions will be described in detail later, but in general terms, the cold-rolled steel sheet is heated to the annealing temperature at a heating rate of 100°C / s or more. The soaking time is set to, for example, less than 10 seconds. In the annealing treatment according to this disclosure, cooling may be started immediately after reaching the maximum temperature, which is the annealing temperature, in which case the soaking time is 0 seconds. For example, it is difficult to achieve a heating rate of 100°C / s or more and to significantly shorten the soaking time in a burner-heated type heating furnace, but the annealing treatment according to this disclosure can be carried out using a known heating means selected as appropriate. For example, an induction heating device may be used.

[0024] According to the annealing treatment of this disclosure, heating at a heating rate of 100°C / s or higher causes a rapid reverse transformation from work-induced martensite to the austenite phase. In this case, the work-induced martensite undergoes a reverse transformation to the austenite phase not through a diffusion transformation involving atomic diffusion, but through a so-called diffusionless transformation mechanism. In other words, the work-induced martensite undergoes a shear-type reverse transformation to the austenite phase without nucleation, taking over the strain of the work-induced martensite. For the sake of explanation, the austenite phase thus produced is referred to as the shear-type reverse transformation γ phase.

[0025] The shear-type reverse-transformed γ phase has an unrecrystallized structure and inherits the strain from cold rolling. Although the shear-type reverse-transformed γ phase is an austenite phase, it maintains a high level of strain and has a very fine substructure. In the annealing treatment of this disclosure, the shear-type reverse-transformed γ phase may be generated from the work-induced martensite, and some recrystallized γ phase may also be generated.

[0026] In the annealing process described herein, unlike the conventional heating annealing process described above, the progression of reverse transformation and the progression of recrystallization are not correlated, and the reverse transformation (shear-type reverse transformation) proceeds faster than the recrystallization. Furthermore, the short soaking time can delay the progression of recrystallization. As a result, a metal structure is obtained in which the entire structure is composed of the austenite phase, with a large amount of unrecrystallized material. In addition, by heating at a heating rate of 100°C / s or higher and a short soaking time, the heating time in the annealing temperature range is shortened, which can refine the metal structure.

[0027] An austenitic stainless steel sheet obtained by annealing according to the present disclosure has a microstructure comprising a cold-rolled γ phase and a shear-type reverse-transformed γ phase. The microstructure of the austenitic stainless steel sheet in one embodiment of the present disclosure may consist of a cold-rolled γ phase and a shear-type reverse-transformed γ phase. Furthermore, the microstructure of the austenitic stainless steel sheet in one embodiment of the present disclosure may consist of a cold-rolled γ phase, a shear-type reverse-transformed γ phase and a recrystallized γ phase. Of course, the microstructure of the austenitic stainless steel sheet in one embodiment of the present disclosure may also include an unavoidable formation phase in addition to the cold-rolled γ phase, the shear-type reverse-transformed γ phase and the recrystallized γ phase. Furthermore, a trace amount of work-induced martensite may remain in the microstructure of the austenitic stainless steel sheet in one embodiment of the present disclosure. In this specification, the expression "consisting of" with respect to the microstructure does not exclude embodiments that include a trace amount of unavoidable formation phase or work-induced martensite. Inevitable formation phases include various precipitates and inclusions.

[0028] Generally, in metastable austenitic stainless steels such as SUS301, there is a trade-off relationship between strength and ductility, and it is necessary to adjust the balance of strength and ductility by temper rolling after annealing of cold-rolled sheets.

[0029] In contrast, as described above, by applying the annealing treatment of this disclosure, an austenitic stainless steel sheet with a controlled microstructure can be obtained. The annealing treatment of this disclosure is performed as a so-called final annealing treatment. The properties and manufacturing method of the austenitic stainless steel material in one embodiment of this disclosure will be described in detail later, but the advantages over the prior art are described in general terms as follows. In the austenitic stainless steel sheet in one embodiment of this disclosure, the yield stress and tensile strength can be effectively increased by dislocation strengthening due to the microstructure containing a cold-rolled γ phase, which is an unrecrystallized austenite phase, and a shear-type reverse-transformed γ phase with high strain. In the austenitic stainless steel sheet in one embodiment of this disclosure, the yield stress and tensile strength can also be effectively increased by grain refinement strengthening due to the microstructure. Furthermore, since the microstructure contains multiple states of austenite phase (γ phase), transformation-induced plasticity (TRIP) can occur at different timings in each of the multiple states of austenite phase. As a result, ductility can be effectively increased. Therefore, the austenitic stainless steel sheet in one embodiment of the present disclosure has a controlled metallic structure, resulting in an excellent balance of yield strength, tensile strength, and ductility even without temper rolling.

[0030] As can be seen from the above, by performing the annealing treatment of the present disclosure on a semi-finished product having a metal structure including a strained austenite phase (e.g., cold-rolled γ phase) and work-induced martensite, an austenitic stainless steel material according to one embodiment of the present disclosure can be obtained. Therefore, the specific form of the austenitic stainless steel material according to one embodiment of the present disclosure is not necessarily limited to steel sheets.

[0031] <Chemical Composition> The chemical composition of the austenitic stainless steel material in one embodiment of this disclosure is described below. For convenience of explanation, the austenitic stainless steel material in one embodiment of this disclosure may be abbreviated as "this austenitic stainless steel material" below. Also, the austenitic stainless steel sheet in one embodiment of this disclosure may be abbreviated as "this austenitic stainless steel sheet".

[0032] This austenitic stainless steel material may have a chemical composition in which the Md30 value defined by the following formula (1) is between 15.0 and 70.0: Md30 value [°C] = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 13.7 Cr - 29 (Ni + Cu) - 18.5 Mo - 68 Nb ... (1) The elemental symbols in formula (1) above are replaced with the content (mass %) of each element contained in the austenitic stainless steel material, and 0 is replaced for elements that are not added.

[0033] A Md30 value of 15.0 or higher allows for a larger amount of work-induced martensite generated, for example, by cold rolling. Therefore, it becomes easier to generate a shear-type reverse transformation γ phase with high strain after final annealing. As a result, the strength can be increased.

[0034] If too much shear-type reverse transformation γ phase is formed after the final annealing, ductility may decrease. By setting the Md30 value to 70.0 or less, the possibility of excessive shear-type reverse transformation γ phase formation after the final annealing can be reduced. Therefore, it becomes easier to increase ductility.

[0035] This austenitic stainless steel material satisfies the above-mentioned requirements for the Md30 value and may have a chemical composition in mass percent containing C: 0.01-0.20%, Si: 0.1-1.0%, Mn: 0.1-3.0%, Cr: 15.0-20.0%, Ni: 5.0-10.0%, N: 0.01-0.20%, and P: 0.005-0.050%, with an S content of 0.01% or less. This austenitic stainless steel material may have a chemical composition in which the remainder is Fe (iron) and impurities. Each of the above-mentioned elements is described below.

[0036] (C: Carbon) C is an austenite-forming element that facilitates the formation of the austenite phase, possesses a high solid solution strengthening effect, and is also an effective element for obtaining strength. This austenitic stainless steel material may contain 0.01% by mass or more and 0.20% by mass or less of C. A C content of 0.01% by mass or more provides sufficient solid solution strengthening. Excessive addition of C can cause undissolved C to precipitate as carbides, which can reduce corrosion resistance and workability, so the C content may be limited to 0.20% by mass or less. This austenitic stainless steel material may contain 0.02% by mass or more and 0.15% by mass or less of C.

[0037] (Si: Silicon) Si is an effective deoxidizing agent and also has a solid solution strengthening effect. This austenitic stainless steel material may contain 0.1% by mass or more and 1.0% by mass or less of Si. When the Si content is 0.1% by mass or more, the deoxidizing effect and solid solution strengthening effect are effectively exhibited in austenitic stainless steel. In addition, Si is a ferrite-forming element that facilitates the formation of the ferrite phase, and if added in excess, a δ-ferrite phase may be formed. Since the δ-ferrite phase can cause edge breakage or splitting in two layers during hot rolling, the Si content may be limited to 1.0% by mass or less. This austenitic stainless steel material may contain 0.2% by mass or more and 0.8% by mass or less of Si.

[0038] (Mn: Manganese) Mn is an austenite-forming element and is also an effective element for maintaining the austenite phase. On the other hand, excessive addition of Mn can lead to a decrease in the hot workability of austenitic stainless steel. This austenitic stainless steel material may contain 0.1% by mass or more and 3.0% by mass or less of Mn. This austenitic stainless steel material may contain 0.2% by mass or more and 2.0% by mass or less of Mn.

[0039] (Cr: Chromium) Cr is an element effective for securing the corrosion resistance of austenitic stainless steel. On the other hand, like Si, Cr is also a ferrite-forming element, so if excessive Cr is added, an excessive δ-ferrite phase may be formed. The present austenitic stainless steel material may contain Cr in an amount of 15.0 mass% or more and 20.0 mass% or less. The present austenitic stainless steel material may contain Cr in an amount of 15.0 mass% or more and 19.0 mass% or less.

[0040] (Ni: Nickel) Ni is an austenite-forming element, and is an element effective for stabilizing the austenite phase. On the other hand, Ni is an expensive element, and excessive addition thereof causes an increase in cost. The present austenitic stainless steel material may contain Ni in an amount of 5.0 mass% or more and 10.0 mass% or less. The present austenitic stainless steel material may contain Ni in an amount of 5.0 mass% or more and 9.5 mass% or less.

[0041] (N: Nitrogen) N is an austenite-forming element, and is an element having a solid solution strengthening effect and a corrosion resistance improving effect. In addition, if excessive N is added, a large amount of nitrides are formed during the production process, which may be a factor that reduces elongation and workability. The present austenitic stainless steel material may contain N in an amount of 0.01 mass% or more and 0.20 mass% or less. The present austenitic stainless steel material may contain N in an amount of 0.02 mass% or more and 0.15 mass% or less.

[0042] (P: Phosphorus) P is an element that is inevitably mixed in. From the viewpoint of productivity, the present austenitic stainless steel material may contain P in an amount of 0.005 mass% or more and 0.050 mass% or less. When the P content is 0.050 mass% or less, adverse effects on material properties such as ductility in austenitic stainless steel can be reduced. The present austenitic stainless steel material may contain P in an amount of 0.005 mass% or more and 0.040 mass% or less.

[0043] (S: Sulfur) S is an element that is inevitably present. From the viewpoint of manufacturability, the S content (content rate) in this austenitic stainless steel material may be 0.01% by mass or less, thereby reducing the adverse effects on material properties such as ductility in austenitic stainless steel.

[0044] (Other components) In addition to the elements mentioned above, this austenitic stainless steel material contains, by mass%, Cu: 0.01-1.00%, Mo: 0.01-1.00%, Nb: 0.001-0.500%, Al: 0.01-0.10%, V: 0.001-0.500%, Ti: 0.001-0.500%, B: 0.0010-0.0025%, Sn: 0.001-0.500%, Co: 0.001-0. The chemical composition may further contain one or more elements selected from the group consisting of 500%, W: 0.001 to 1.000%, Sb: 0.001 to 0.500%, Zr: 0.001 to 0.500%, Y: 0.001 to 0.100%, Mg: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, and rare earth elements: 0.001 to 0.100%.

[0045] (Cu) Cu (copper) is an austenite-forming element and is also an effective element for maintaining the austenite phase. Furthermore, Cu is an element that effectively acts on grain refinement. This is thought to be because when εCu precipitates during the heating process in the final annealing step, this εCu exhibits an inhibitory effect on grain growth. On the other hand, if Cu is added in excess, a CuMn phase may be formed in the center of the slab during the solidification process, which can reduce the hot workability of the slab. When this austenitic stainless steel material contains Cu, the Cu content may be 0.01% by mass or more and 1.00% by mass or less, and may be 0.05% by mass or more and 0.50% by mass or less.

[0046] (Mo, V, W) Mo (molybdenum), V (vanadium) and W (tungsten) are elements effective for improving corrosion resistance, so they may be contained as required. Meanwhile, Mo, V and W are ferrite-forming elements and are also expensive elements. Therefore, when the austenitic stainless steel material contains Mo, the content of Mo may be 0.01 mass% or more and 1.00 mass% or less. Further, when the austenitic stainless steel material contains V, the content of V may be 0.001 mass% or more and 0.500 mass% or less. When the austenitic stainless steel material contains W, the content of W may be 0.001 mass% or more and 1.000 mass% or less.

[0047] (Nb) Nb (niobium) is an element effective for reducing sensitization of austenitic stainless steel. It is also effective for refining and homogenizing the structure, so it may be contained as required. When the austenitic stainless steel material contains Nb, the content of Nb may be 0.001 mass% or more and 0.500 mass% or less.

[0048] (Al, Ti, Mg, Ca) Al (aluminum), Ti (titanium), Mg (magnesium) and Ca (calcium) are all elements having a deoxidation effect. The austenitic stainless steel material may contain, as a deoxidizer, one or more selected from the group consisting of Al in an amount of 0.01 mass% or more and 0.10 mass% or less, Ti in an amount of 0.001 mass% or more and 0.500 mass% or less, Mg in an amount of 0.0001 mass% or more and 0.0050 mass% or less, and Ca in an amount of 0.0001 mass% or more and 0.0050 mass% or less.

[0049] (B) B (boron) is an element that improves hot workability and is effective in reducing the occurrence of edge cracking and double splitting in hot rolling. However, excessive addition of B to austenitic stainless steel containing Cr causes Cr ２ Precipitation of borides can lead to a decrease in corrosion resistance. When the austenitic stainless steel material contains B, the content of B may be 0.0010 mass% or more and 0.0025 mass% or less.

[0050] (Sn) Sn (tin) is an effective element for ensuring the corrosion resistance of austenitic stainless steel. However, excessive addition of Sn can lead to a decrease in the hot workability of austenitic stainless steel. When this austenitic stainless steel material contains Sn, the Sn content may be between 0.001% by mass and 0.500% by mass.

[0051] (Co) Cobalt (Co) is an effective element for ensuring the corrosion resistance of austenitic stainless steel. However, Co is an expensive element. When this austenitic stainless steel material contains Co, the Co content may be 0.001% by mass or more and 0.500% by mass or less.

[0052] (Sb, Zr, Y, REM) Sb (antimony), Zr (zirconium), Y (yttrium), and REM (rare earth metals) are all elements that improve hot workability and oxidation resistance. This austenitic stainless steel material preferably contains one or more elements selected from Sb in an amount of 0.001% to 0.500% by mass, Zr in an amount of 0.001% to 0.500% by mass, Y in an amount of 0.001% to 0.100% by mass, and REM in an amount of 0.001% to 0.100% by mass. REM may be added as a single element or as a mixture of multiple elements. Since REM contains Y, if this austenitic stainless steel material contains Y and REM other than Y, the total amount should be in the range of 0.001% to 0.100% by mass.

[0053] (Impurities) This austenitic stainless steel material may have a chemical composition in which the remainder consists of Fe and impurities, and examples of impurities include elements other than those mentioned above.

[0054] <Material Properties> This austenitic stainless steel material has the chemical composition described above and undergoes final annealing under the annealing conditions described later, resulting in a modified microstructure. This microstructure has the following properties.

[0055] (Recrystallized portion) This austenitic stainless steel has a recrystallized portion (recrystallization rate) of 60% or less in its microstructure. The recrystallized portion may be a recrystallized γ phase with relatively little strain. In this austenitic stainless steel, the recrystallized portion can be said to be the portion of the austenite phase in the microstructure other than the cold-rolled γ phase and the shear-type reverse transformation γ phase.

[0056] The recrystallization rate in the microstructure of this austenitic stainless steel can be calculated, for example, using the electron backscatter diffraction (EBSD) method. Specifically, for example, an EBSD pattern of the cross-section of this austenitic stainless steel is obtained using an EBSD detector mounted on a scanning electron microscope (SEM). Next, using analysis software, the acquired EBSD pattern is analyzed to identify grains, with interfaces where the orientation difference is 5° or more being defined as grain boundaries. Then, the GAM (Grain Average Misorientation) value is calculated. The GAM value is calculated as the average value obtained by averaging the crystal orientation differences between adjacent pixels within each grain, based on the acquired EBSD pattern. Based on the GAM map, grains with a GAM value of 0.6 or less are identified as recrystallized grains (recrystallized parts), and the proportion of the recrystallized part (recrystallization rate) can be calculated.

[0057] In the microstructure of this austenitic stainless steel, a high recrystallization rate means that the proportion of cold-rolled γ phase and shear-type reverse-transformed γ phase is relatively reduced, as is the amount of strain within the microstructure. This austenitic stainless steel has a high yield strength due to its recrystallization rate of 60% or less. This austenitic stainless steel may have a recrystallization rate of 1% or more and 60% or less.

[0058] The austenitic stainless steel material annealed according to the present disclosure may have a microstructure consisting of a cold-rolled γ phase and a shear-type reverse-transformed γ phase, as described above, and in this case, the microstructure may not substantially contain recrystallized grains. The austenitic stainless steel material may have a recrystallization rate of 0%, which is a value calculated by identifying crystal grains using image analysis on data obtained by EBSD measurement.

[0059] (Work-induced martensite) The austenitic stainless steel material has a work-induced martensite proportion (phase fraction) of 5% or less (including 0) in its microstructure. As mentioned above, the work-induced martensite in this austenitic stainless steel material undergoes a non-diffusion reverse transformation of shear-type reverse-transformed γ phase by the annealing treatment of this disclosure, so the microstructure does not need to contain substantially any work-induced martensite. A phase fraction of 5% or less for work-induced martensite can also be said to mean that the amount of work-induced martensite is 5% by volume or less.

[0060] The processing-induced martensite in a metal structure can be calculated, for example, based on measured magnetic properties. Specifically, the saturation magnetization value (B) can be calculated from the B-H loop (magnetic hysteresis curve) obtained by a DC hysteresis curve test. ｍａｘ ) is determined. Let σ be the saturation magnetization value if the austenite phase in the metal structure were to be entirely (100%) transformed into work-induced martensite. σ can be calculated using the following equation (2): σ = 1.83 - 0.03 × (Cr + 1.8Si + Mo + 0.5Ni + 0.9Mn + 3.6(C + N) + 1.85Al) ... (2).

[0061] The phase fraction (α) of work-induced martensite in the metal structure is calculated using the following formula (3): α (%) = (B ｍａｘ / σ)×100 (3).

[0062] In the microstructure of this austenitic stainless steel, if work-induced martensite exceeds 5%, its ductility may decrease. This austenitic stainless steel has high ductility because the phase fraction of work-induced martensite is 5% or less.

[0063] In the microstructure of this austenitic stainless steel, both the cold-rolled γ phase and the shear-induced reverse-transformation γ phase exhibit high strain, making it difficult to distinguish them based on the EBSD pattern. For example, in the IQ (Image Quality) map, both the cold-rolled γ phase and the shear-induced reverse-transformation γ phase are shown as relatively dark images. In other words, it is difficult to calculate the phase fractions of the cold-rolled γ phase and the shear-induced reverse-transformation γ phase. Therefore, as described above, the recrystallization rate and the phase fraction of the work-induced martensite are defined. This indirectly identifies the following:

[0064] Specifically, firstly, in the steel grade of this austenitic stainless steel, work-induced martensite is easily formed by cold rolling. When the annealing treatment of the steel grade of this disclosure is performed on a cold-rolled sheet, during the annealing process, some of the highly strained γ phase generated from the work-induced martensite by shear-type reverse transformation may recrystallize, but the remaining portion remains in the microstructure of the austenitic stainless steel after annealing as shear-type reverse transformation γ. Furthermore, at the annealing temperature of the annealing treatment of this disclosure (described later), recrystallization from the cold-rolled γ phase is unlikely to occur, and the cold-rolled γ phase in the cold-rolled sheet tends to remain as an unrecrystallized portion in the microstructure of the austenitic stainless steel after annealing. By performing the annealing treatment of this disclosure, a microstructure in which the recrystallization rate is 60% or less and the phase fraction of work-induced martensite is 5% or less contains 40% or more of unrecrystallized portion. From the above, it can be indirectly determined that the unrecrystallized portion contains, in addition to the cold-rolled γ phase, shear-type reverse-transformed γ, which is derived from processing-induced martensite.

[0065] Furthermore, when cold-rolled steel sheets of the steel types disclosed herein are subjected to annealing treatment by conventional heating, the following can be said. That is, in annealing treatment by conventional heating, a diffusion-type reverse transformation occurs in the work-induced martensite. For example, if the annealing temperature is relatively low, recrystallization from the cold-rolled γ phase is unlikely to occur, and the reverse transformation rate of the diffusion-type reverse transformation is slow. Therefore, in this case, it is not possible to make the phase fraction of work-induced martensite 5% or less, and even if the unrecrystallized portion is 40% or more, almost all of the unrecrystallized portion will be the cold-rolled γ phase. Alternatively, for example, if the annealing temperature is relatively high, the diffusion-type reverse transformation from work-induced martensite proceeds during the heating process, and the recrystallization of the cold-rolled γ phase also proceeds. Therefore, even if the phase fraction of work-induced martensite is 5% or less, it is not possible to obtain a metal structure in which the recrystallization rate is 60% or less. Thus, with annealing treatment by conventional heating, it is not possible to obtain a metal structure in which the recrystallization rate is 60% or less and the phase fraction of work-induced martensite is 5% or less.

[0066] As mentioned above, the phase fraction (α) of work-induced martensite in the microstructure of this austenitic stainless steel is calculated as a calculated value. Therefore, even if the microstructure does not actually contain work-induced martensite, or contains almost none, a certain value can be calculated using equation (3) above. For this reason, the index value of the phase fraction of work-induced martensite calculated based on the saturation magnetization value of this austenitic stainless steel may be 5% or less. More specifically, the index value of the phase fraction of work-induced martensite calculated using equations (2) and (3) above based on the test results of the DC hysteresis curve test may be 5% or less.

[0067] (Average grain size) The average grain size of the recrystallized portion of this austenitic stainless steel may be 5.0 μm or less. The average grain size of the recrystallized portion can be determined, for example, as follows: That is, by using analysis software on the EBSD pattern as described above, the interfaces with an orientation difference of 5° or more are identified as grain boundaries, and the grains are identified. Then, the average grain size can be calculated using the Area Method (Average by Area Fraction Method).

[0068] In the microstructure of this austenitic stainless steel, an average grain size greater than 5.0 μm in the recrystallized region means that the effects of the microstructure are reduced. This austenitic stainless steel exhibits even higher yield strength because the average grain size of the recrystallized region in its microstructure is 5.0 μm or less. Furthermore, this austenitic stainless steel also possesses high cross-sectional hardness.

[0069] <Advantages> For example, as mentioned above, prior art document Patent Document 2 describes a technique for increasing the tensile strength and ductility of austenitic stainless steel without performing temper rolling. However, this type of technique contains many recrystallized grains, making it difficult to increase the yield stress. In contrast, the austenitic stainless steel material has a metal structure composed of less recrystallized γ phase than the austenitic stainless steel material described in Patent Document 2, and an unrecrystallized austenite phase containing cold-rolled γ phase and shear-type reverse-transformed γ phase. As a result, it has high tensile strength and yield stress, as well as high ductility. In other words, this austenitic stainless steel material has an excellent balance of yield stress, tensile strength, and ductility due to the adjusted metal structure described above.

[0070] Specifically, this austenitic stainless steel material may have a 0.2% yield strength of 700 MPa or more, a tensile strength of 850 MPa or more, and a total elongation of 20.0% or more. For example, this austenitic stainless steel sheet can have a 0.2% yield strength of 700 MPa or more, a tensile strength of 850 MPa or more, and a total elongation of 20.0% or more by having the above-described metal structure, even without temper rolling. This austenitic stainless steel sheet can have a 0.2% yield strength of 800 MPa or more, a tensile strength of 1000 MPa or more, and a total elongation of 20.0% or more. This austenitic stainless steel sheet can have a 0.2% yield strength of 900 MPa or more, a tensile strength of 1000 MPa or more, and a total elongation of 20.0% or more.

[0071] Furthermore, this austenitic stainless steel material may have a cross-sectional hardness of 350 HV or higher, 360 HV or higher, or 380 HV or higher. The cross-sectional hardness is measured by a Vickers hardness test (load 1.0 kgf) in accordance with JIS Z 2244:2009. The Vickers hardness test is performed at the center of the plate thickness in a cross section parallel to the rolling direction and perpendicular to the plate surface. This austenitic stainless steel material may have a 0.2% yield strength of 800 MPa or higher, a tensile strength of 1000 MPa or higher, a total elongation of 20.0% or higher, and a cross-sectional hardness of 380 HV or higher.

[0072] <Other Compositions> The austenitic stainless steel sheet in question only needs to have the above-described metal structure, and may be in a temper-rolled state. In other words, an austenitic stainless steel sheet having the above-described metal structure in a temper-rolled state is also included within the scope of this disclosure.

[0073] This austenitic stainless steel sheet may not have undergone temper rolling and may have been pickled after final annealing. The absence of temper rolling in this austenitic stainless steel sheet can be determined, for example, as follows: It can be determined based on the difference between the surface condition of a conventional temper-rolled sheet of SUS301 steel and the surface condition of this austenitic stainless steel sheet that has not undergone temper rolling. Examples of surface condition include gloss, surface roughness, temper rolling roll marks, and residual stress near the surface.

[0074] The thickness of this austenitic stainless steel sheet may be, for example, 0.1 mm or more and 3.0 mm or less, or 0.2 mm or more and 2.0 mm or less.

[0075] <Method for Manufacturing Steel Sheets> An example of a method for manufacturing austenitic stainless steel sheets (hereinafter sometimes abbreviated as "this manufacturing method") includes a step of preparing the material to be processed, a cold rolling step of cold rolling the material to be processed, and a final annealing step of annealing the cold-rolled steel sheet obtained by the cold rolling step.

[0076] The material to be treated has a chemical composition in which the Md30 value defined by the following formula (1) is between 15.0 and 70.0: Md30 value [°C] = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 13.7 Cr - 29 (Ni + Cu) - 18.5 Mo - 68 Nb ... (1) The elemental symbols in the above formula (1) are replaced with the content (mass %) of each element contained in the above austenitic stainless steel material, and 0 is replaced for elements that are not added.

[0077] Furthermore, the chemical composition of the material to be treated may be the same as that of the austenitic stainless steel material described above, and therefore, a repeated explanation will be omitted. The material to be treated may be any material having a metallic structure composed of a recrystallized γ phase. Typically, the material to be treated may be a hot-rolled and annealed sheet.

[0078] (Preparation Process) When the material to be processed is a hot-rolled annealed sheet, it may be manufactured as follows, for example. First, molten steel having the desired composition is poured into a mold and cooled to produce a slab. The slab is cut to the desired length and used in the hot rolling process. The hot rolling process can be carried out using known equipment and methods. In this manufacturing method, general manufacturing conditions can be adopted in the hot rolling process. For example, the heating temperature (rolling temperature) can be 1150 to 1250°C, and the total reduction ratio can be 95 to 99%. The coiling temperature after hot rolling can be 200 to 1000°C.

[0079] Next, a hot-rolled sheet annealing step may be performed to anneal the hot-rolled sheet obtained in the hot-rolling step. In this manufacturing method, the hot-rolled sheet annealing step can be carried out using known equipment and methods. General manufacturing conditions can be adopted in the hot-rolled sheet annealing step. In the hot-rolled sheet annealing step, for example, the hot-rolled sheet is heated to a temperature range of 900 to 1200°C and maintained at a uniform temperature. The soaking temperature in the hot-rolled sheet annealing step is preferably 1000 to 1150°C. The soaking time in the hot-rolled sheet annealing step is also preferably 10 to 120 seconds.

[0080] After the hot-rolled sheet annealing process, the hot-rolled annealed sheet obtained in the hot-rolled sheet annealing process may be subjected to an acid pickling process. The acid pickling process is a process of removing scale adhering to the surface of the hot-rolled annealed sheet using an acid pickling solution such as sulfuric acid, hydrochloric acid, or a mixture of nitric acid and hydrofluoric acid.

[0081] (Cold Rolling Process) In this manufacturing method, a cold rolling process is performed on the hot-rolled annealed sheet, which is prepared as described above, as the material to be processed. The cold rolling process is a process in which a cold-rolled steel sheet of a predetermined thickness is obtained by cold-rolling the hot-rolled annealed sheet (for example, at room temperature to 200°C). In the cold rolling process, cold rolling is performed under conditions of a cold rolling ratio of 50% or more. The cold rolling ratio refers to the total rolling ratio in the cold rolling process. The higher the cold rolling ratio, the easier it is to introduce strain into the metal structure of the cold-rolled steel sheet and the easier it is to increase the amount of work-induced martensite. On the other hand, the higher the cold rolling ratio, the more work-induced martensite is produced, and the harder the cold-rolled steel sheet becomes. Achieving a cold rolling ratio of more than 90% requires a very large number of rolling passes, and in this case, the manufacturing cost increases. The cold rolling ratio may be 50% or more and 90% or less, or 50% or more and 85% or less.

[0082] Multiple cold rolling processes may be performed between the hot-rolled sheet annealing process and the final annealing process. For example, the first cold rolling process may be performed on a hot-rolled annealed sheet that has been pickled. After that, intermediate annealing may be performed, followed by pickling and a second cold rolling process. Intermediate annealing can be performed under general conditions, for example, the same conditions as the hot-rolled sheet annealing process described above. When multiple cold rolling processes are performed, a cold rolling ratio may be set for each cold rolling process. In this manufacturing method, the cold rolling process is performed with a cold rolling ratio of 50% or more in the final cold rolling process (in other words, the cold rolling process immediately before the final annealing process). That is, the rolling ratio in the final cold rolling process (final rolling ratio) is set to 50% or more. The rolling ratios of the cold rolling processes prior to the final cold rolling process may be appropriately set based on a predetermined sheet thickness of the cold-rolled steel sheet to be used in the final annealing process.

[0083] (Final Annealing Process) In this manufacturing method, a final annealing process is performed on the cold-rolled steel sheet that has undergone the cold-rolling process. In the final annealing process, the cold-rolled steel sheet is heated to the highest annealing temperature within the temperature range of 700 to 900°C at a heating rate of 100°C / s or more, and after holding the temperature for less than 10 seconds to ensure even heating or after reaching the highest annealing temperature, cooling is started.

[0084] In this manufacturing method, the heating rate is set to 100°C / s or higher, and the soaking time is shortened. This induces shear-type reverse transformation as described above, while preventing excessive recrystallization. In practice, the changes in the metal structure during the final annealing process are influenced by the settings in the final annealing process, as well as various other conditions such as the cold rolling rate and chemical composition. Furthermore, the settings in the final annealing process and various other conditions such as the cold rolling rate and chemical composition are intricately related to each other. Therefore, in the manufacture of this austenitic stainless steel material, it is sufficient that the metal structure is created to have the desired properties, and the specific manufacturing conditions can be adjusted as appropriate by referring to the explanation below.

[0085] By setting the heating rate to 100°C / s or higher, the time elapsed from the point where the recrystallization initiation temperature is exceeded to the point where the maximum annealing temperature is reached can be shortened. This allows for the effective generation of shear-type reverse transformation in the microstructure, rather than the nucleation that leads to the aforementioned diffusion-type reverse transformation γ phase. As a result, the shear-type reverse transformation γ phase is generated from the work-induced martensite. Furthermore, the heating time in the temperature range of 700-900°C, i.e., the recrystallization temperature range, can be reduced, making it easier to retain a large amount of unrecrystallized material and to refine the microstructure. Therefore, an austenitic stainless steel sheet with a microstructure that combines high strength and high ductility can be obtained.

[0086] The heating rate in the final annealing process may be 100 to 2000°C / s or 100 to 1500°C / s. If the maximum annealing temperature is lower than 700°C, the reverse transformation of work-induced martensite is difficult to proceed, and a longer soaking time is required to promote the reverse transformation of work-induced martensite. In this case, recrystallization will proceed, making it difficult to keep the recrystallization rate below 60%. Also, if the maximum annealing temperature is higher than 900°C, even with a short soaking time, excessive recrystallization may occur, potentially reducing the strength. Therefore, the maximum annealing temperature is set in the temperature range of 700 to 900°C.

[0087] In the final annealing process, if the metal is held at the highest annealing temperature within the 700-900°C temperature range for more than 10 seconds, the metal structure may coarseen, potentially reducing its strength. Therefore, if soaking is performed in the final annealing process, the soaking time should be set to less than 10 seconds. If soaking is not performed in the final annealing process, cooling should begin as soon as the highest annealing temperature is reached.

[0088] In the final annealing process, starting cooling from the point where the maximum annealing temperature is reached corresponds to a soaking time of 0 seconds. In the final annealing process, cooling from the maximum annealing temperature may be by air cooling, and may be carried out at a cooling rate faster than that of air cooling.

[0089] After the final annealing process, further processes are carried out as needed to obtain the austenitic stainless steel sheet. A typical example of a further process is pickling. In this manufacturing method, the austenitic stainless steel sheet may be produced without performing temper rolling after the final annealing process. Temper rolling may be performed as the above-mentioned further process.

[0090] [Additional Notes] The present invention is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0091] An embodiment of this disclosure is described below. Note that the method for manufacturing an austenitic stainless steel sheet described in this embodiment is just one example.

[0092] Hot-rolled and annealed sheets having the chemical composition shown in Table 1 below were prepared. In Table 1, the Md30 value is the value calculated by the formula (1) described above.

[0093]

[0094] Cold-rolled steel sheets with a thickness of 1 mm were produced by cold-rolling hot-rolled annealed sheets of the steel types shown in Table 1 at various cold-rolling rates shown in Table 2. Final annealing was performed on the cold-rolled steel sheets under the conditions shown in Table 2. The results of evaluating various physical properties of each steel sheet obtained by final annealing, without heat rolling, are also shown in Table 2.

[0095] The recrystallization rate was determined as follows: A test specimen was cut from each steel sheet with the rolling direction cross-section (L-section) as the observation surface. The specimen was polished, and EBSD measurement was performed on the observation surface. The data obtained from the EBSD measurement was analyzed using the image analysis software OIM (Orientation Imaging Microscopy) 8. A region enclosed by grain boundaries with an orientation difference of 5 degrees or more was considered as a single crystal grain, and a GAM value of ≤ 0.6 was identified as a recrystallized grain. The recrystallization rate was then calculated.

[0096] Furthermore, the grain size of the identified recrystallized grains was calculated using the Area method. This allowed us to determine the average grain size of the recrystallized portion.

[0097] The phase fraction of processing-induced martensite was calculated as follows: A DC hysteresis test was performed on the collected specimens, and the saturation magnetization value (B) was obtained from the B-H curve when a magnetic field of 10,000 Oe was applied. ｍａｘ The saturation magnetization value was determined using a DC magnetization characteristic tester (manufactured by Metron Giken Co., Ltd.). Using the aforementioned equations (2) and (3), the phase fraction (α) of work-induced martensite in the metal structure was calculated as a calculated value.

[0098] Tensile tests were performed on JIS No. 13B test specimens according to JIS Z 2241, and the 0.2% yield strength, tensile strength, and total elongation were measured.

[0099] Furthermore, Vickers hardness tests were conducted in accordance with JIS Z 2244:2009 to measure the cross-sectional hardness. Each steel plate specimen was embedded in resin so that the rolling direction cross-section (L-section) was exposed. The exposed cross-section was polished and electrolytically etched, and then the cross-sectional hardness was measured at the center of the plate thickness. The load was 1.0 kgf. Five measurements were taken at the center of the plate thickness, and the average value was used as the representative value.

[0100]

[0101] As shown in Table 2, all of the steel sheets of the present invention manufactured by the austenitic stainless steel sheet manufacturing method described above had a 0.2% yield strength of 700 MPa or higher, a tensile strength of 850 MPa or higher, and a total elongation of 20.0% or higher, demonstrating an excellent balance of yield strength, tensile strength, and ductility even without temper rolling. Furthermore, all of the steel sheets of the present invention had a cross-sectional hardness HV of 350 or higher.

[0102] In contrast, the comparative steel sheets exhibited insufficient properties in at least one of the following areas: yield strength, tensile strength, and ductility. Comparative Example No. 2 underwent final annealing at a maximum annealing temperature exceeding 900°C, resulting in a 100% recrystallization rate and consequently low strength. Comparative Examples No. 3 and 17 underwent diffusion-type reverse transformation from work-induced martensite due to a heating rate slower than 100°C / s. Furthermore, a large amount of work-induced martensite remained after final annealing. As a result, while the strength was high, the ductility was very low.

[0103] Comparative Examples No. 5, No. 9, and No. 12 underwent significant recrystallization during the final annealing, with a recrystallization rate exceeding 60%. As a result, while tensile strength and ductility were high, the yield stress decreased. In Comparative Example No. 16, the maximum annealing temperature was low, and the reverse transformation did not proceed sufficiently, resulting in a large amount of work-induced martensite remaining. As a result, while strength was high, ductility was very low.

[0104] In Comparative Example No. 19, the steel grade had a low Md30 value, making it difficult to form work-induced martensite, resulting in a metal structure with high strength. In Comparative Example No. 20, the steel grade had a high Md30 value, and after final annealing, a large amount of shear-type reverse transformation γ phase was formed from work-induced martensite. As a result, while the strength was high, the ductility decreased.

Claims

It has a chemical composition in which the Md30 value defined by the following formula (1) is 15.0 or more and 70.0 or less, The recrystallization rate is 60% or less. Austenitic stainless steel materials in which the phase fraction of processing-induced martensite is 5% or less (including 0): Md30 value [℃] = 551-462 (C + N) - 9.2 Si - 8.1 Mn - 13.7 Cr - 29 (Ni + Cu) - 18.5 Mo - 68 Nb ... (1) 　 In the above formula (1), the elemental symbols are replaced with the content (mass%) of each element contained in the austenitic stainless steel material, and 0 is replaced for elements that are not added. The austenitic stainless steel material according to claim 1, wherein the average grain size of the recrystallized portion is 5.0 μm or less. 　 The austenitic stainless steel material according to claim 1, wherein the chemical composition contains, by mass%, C: 0.01 to 0.20%, Si: 0.1 to 1.0%, Mn: 0.1 to 3.0%, Cr: 15.0 to 20.0%, Ni: 5.0 to 10.0%, N: 0.01 to 0.20%, and P: 0.005 to 0.050%, with an S content of 0.01% or less, and the remainder being Fe and unavoidable impurities. 　 The aforementioned chemical composition is as follows, in mass%,: Cu: 0.01-1.00%, Mo: 0.01-1.00%, Nb: 0.001-0.500%, Al: 0.01-0.10%, V: 0.001-0.500%, Ti: 0.001-0.500%, B: 0.0010-0.0025%, Sn: 0.001-0.500%, Co: 0.001-0.500%, W: 0.001-1.0 The austenitic stainless steel material according to claim 3, further containing one or more elements selected from the group consisting of 00%, Sb: 0.001 to 0.500%, Zr: 0.001 to 0.500%, Y: 0.001 to 0.100%, Mg: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, and rare earth elements: 0.001 to 0.100%. An austenitic stainless steel material according to any one of claims 1 to 4, wherein the 0.2% yield strength is 700 MPa or more, the tensile strength is 850 MPa or more, and the total elongation is 20.0% or more.

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