Stainless steel sheet and method for manufacturing same

Abstract

Provided is a stainless steel sheet that contains martensite and an austenite phase and that is excellent in strength-ductility balance and toughness. The stainless steel sheet has a chemical composition including, in mass %, C: 0.005-0.200%, Si: 0.05-2.50%, Mn: 0.1-8.0%, P: 0.0800% or less, S: 0.030% or less, Cr: 12.0-20.0%, Ni: 0.30% or more and less than 8.00%, Mo: 0.01-1.00%, Al: 0.500% or less, N: 0.005-0.100%, optional elements, and the balance made up of Fe and impurities, wherein: the Cr equivalent is 10.0-20.0%; the Ni equivalent is 5.0-15.0%; the Ms point is 50°C or higher; the metal structure includes martensite and an austenite phase; the area ratio of the austenite phase is 30% or more and less than 100%; and the average crystal grain size of the austenite phase is 2.0 μm or less.

Description

Stainless steel sheet and its manufacturing method

[0001] The present invention relates to a stainless steel sheet and a method for manufacturing the same.

[0002] Martensitic stainless steel sheets have excellent strength and are used for automobile parts, etc. However, it is known that martensitic stainless steel sheets have low ductility and toughness in proportion to their high strength. For this reason, as shown in Patent Document 1, for example, stainless steel materials have been developed that are excellent in both strength and ductility by controlling the structure of the austenite phase contained in addition to martensite to improve the balance between strength and ductility.

[0003] JP 2015-86405 A

[0004] However, the stainless steel sheet disclosed in Patent Document 1 has room for further improvement from the viewpoint of improving toughness while enhancing the balance between strength and ductility (hereinafter also referred to as "strength-ductility balance"). Therefore, an object of the present invention is to provide a stainless steel sheet that contains martensite and austenite phases and has an excellent strength-ductility balance and toughness.

[0005] The present invention has been made to solve the above-mentioned problems, and is summarized as follows: a stainless steel sheet and a method for manufacturing the same.

[0006] (1) Chemical composition, in mass%, is: C: 0.005 to 0.200%, Si: 0.05 to 2.50%, Mn: 0.1 to 8.0%, P: 0.0800% or less, S: 0.030% or less, Cr: 12.0 to 20.0%, Ni: 0.30% or more but less than 8.00%, Mo: 0.01 to 1.00%, Al: 0.500% or less, N: 0.005 to 0.100%, Ca: 0 to 0.0100%, Cu: 0 to 7.0%, Ti: 0 to 0.70%, Nb: 0 to 1.00%, B: 0 to 0.0050%, V: 0 to 0.50%, Sn: 0 to 0.10%, A stainless steel sheet comprising: Co: 0 to 0.30%, and the balance: Fe and impurities; a Cr equivalent calculated by the following formula (i) of 10.0 to 20.0%; a Ni equivalent calculated by the following formula (ii) of 5.0 to 15.0%; an Ms point calculated by the following formula (iii) of 50°C or higher; a metal structure containing martensite and an austenite phase; an area fraction of the austenite phase being 30% or higher but less than 100%; and an average crystal grain size of the austenite phase being 2.0 μm or less. Cr equivalent (%) = Cr + Mo + 1.5Si + (0.5Nb) ... (i) Ni equivalent (%) = Ni + 30(C + N) + 0.5Mn + (0.3Cu) ... (ii) Ms point (°C) = 545 - 330(C + N) + Al + 7Co-14Cr-13C-23Mn-5Mo-4Nb-13Ni-7Si + 3Ti + 4V ... (iii) However, each element symbol in the above formula represents the content (mass%) of each element contained in the stainless steel plate, and if the element is not contained, it is set to zero.

[0007] (2) The stainless steel sheet according to (1) above, wherein the chemical composition contains, in mass%, one or more elements selected from Ca: 0.0001 to 0.0100%, Cu: 0.5 to 7.0%, Ti: 0.05 to 0.70%, Nb: 0.01 to 1.00%, B: 0.0005 to 0.0050%, V: 0.01 to 0.50%, Sn: 0.02 to 0.10%, and Co: 0.01 to 0.30%.

[0008] (3) The stainless steel sheet according to (1) or (2) above, wherein the area ratio of the austenite phase is 85% or more.

[0009] (4) A method for producing the stainless steel sheet described in (1) or (2) above, comprising: (a) a step of heating a slab having the chemical composition described in (1) or (2) above in the range of 1100 to 1300°C, and hot-rolling it to obtain a hot-rolled steel sheet; (b) a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet by holding it at an annealing temperature of not less than the Af point (°C) calculated by the following formula (I) and not more than 1200°C for 20 to 3000 seconds, and cooling it to not more than 100°C, and pickling it after the annealing to obtain a pickled steel sheet; and (c) a cold-rolling step of cold-rolling the pickled steel sheet at a reduction rate of not less than 50% to obtain a cold-rolled steel sheet. (d) an annealing step of heating the cold-rolled steel sheet at a rate of 20°C / s or more, holding the temperature at a temperature in the range of from the As point (°C) to 900°C (calculated by the following formula (II)) for 0 to 30 seconds, and then cooling at a cooling rate of 20°C / s or more. Af point (°C) = As point + 100 (°C) ... (I) The As point in the above formula (I) can be calculated by the following formula (II): As point (°C) = 937.2 - 436.5C + 56Si - 19.7Mn - 16.3Cu - 26.6Ni - 4.9Cr + 38.1Mo + 124.8V + 136.3Ti - 19.1Nb + 198.4Al + 3315B ... (II)

[0010] (5) A method for producing the stainless steel sheet described in (3) above, comprising: (a) a step of heating a slab having the chemical composition described in (1) or (2) above in the range of 1100 to 1300°C, and hot rolling it to obtain a hot-rolled steel sheet; (b) a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet by holding it at an annealing temperature of not less than the Af point (°C) calculated by the following formula (I) and not more than 1200°C for 20 to 3000 seconds, and cooling it to not more than 100°C, and pickling it after the annealing to obtain a pickled steel sheet; and (c) a cold rolling step of cold-rolling the pickled steel sheet at a reduction rate of not less than 50% to obtain a cold-rolled steel sheet. and (d) an annealing step of raising the temperature of the cold-rolled steel sheet at a rate of 20°C / s or more, holding the temperature in the range of the As point (°C) or higher to 900°C or lower for 0 to 30 seconds as calculated by the following formula (II), and then cooling at a cooling rate of 20°C / s or higher, wherein in the hot rolling in (a), the finish rolling start temperature is 1000 to 1100°C, the finish rolling end temperature is 950 to 1000°C, and the coiling temperature is 600°C or lower, in the hot-rolled sheet annealing step in (b), the annealing temperature is 1050°C or lower, and in the cold rolling step in (c), the rolling reduction is 80% or higher. Af point (°C) = As point + 100 (°C) (I) Note that the As point in the above formula (I) can be calculated from the following formula (II): As point (℃)=937.2-436.5C+56Si-19.7Mn-16.3Cu-26.6Ni-4.9Cr+38.1Mo+124.8V+136.3Ti-19.1Nb+198.4Al+3315B...(II)

[0011] According to the present invention, it is possible to obtain a stainless steel sheet that contains martensite and austenite phases and has an excellent balance of strength and ductility and toughness.

[0012] Fig. 1 is a graph showing the relationship between the area fraction of the austenite phase and the strength-ductility balance (the product of tensile strength and total elongation). Fig. 2 is a graph showing the relationship between the average grain size of the austenite phase and the strength-ductility balance (the product of tensile strength and total elongation).

[0013] The present inventors have conducted various studies and have obtained the following findings.

[0014] (a) Martensite has high strength but low ductility. Therefore, by including a certain amount of austenite phase, ductility can be improved. The inventors have clarified that when the ratio of the austenite phase is 30% or more in terms of area ratio, the strength-ductility balance and toughness are improved. Furthermore, they have clarified that not only including a certain amount of austenite phase but also making the austenite phase fine, specifically, setting the average crystal grain size to 2.0 μm or less, is effective in improving the strength-ductility balance and toughness.

[0015] (b) Typically, martensitic stainless steel grades are softened by decomposing the martensite structure into a ferrite phase and carbides through hot-rolled sheet annealing or the like, followed by cold rolling and annealing at a temperature below the As point so as to prevent the formation of martensite during cooling. This is a method for obtaining a soft structure that is mainly composed of ferrite phase and has excellent workability, but it is difficult to obtain a certain amount of the desired fine austenite phase with this method.

[0016] Therefore, it is desirable to perform cold rolling while maintaining a full martensite structure, i.e., a metal structure mainly composed of martensite, before cold rolling, and then annealing by raising the temperature to a temperature range where an austenite phase is generated by diffusionless reverse transformation. Furthermore, it is desirable to set the annealing temperature at this time to a temperature relatively lower than usual. This allows a desired amount of fine austenite phase to be obtained.

[0017] (c) The reason for using such manufacturing conditions is due to the following mechanism. By heating a cold-rolled steel sheet mainly composed of martensite at a predetermined temperature, a diffusionless reverse transformation from martensite to the austenite phase occurs. Normally, the transformation from martensite to the austenite phase is a diffusional transformation accompanied by atomic movement. On the other hand, in the steel sheet of this embodiment, the phase transformation occurs by a diffusionless transformation not accompanied by atomic movement, thereby suppressing grain growth of the austenite phase and making the crystal grains finer. In addition, by cold-rolling a full martensite structure, a large amount of strain is introduced, promoting nucleation, and also contributing to making the austenite phase finer.

[0018] In addition, the fine austenite phase has excellent thermal stability, so that the transformation from austenite to martensite during the cooling process during annealing is suppressed, and the desired amount of austenite phase can be secured.

[0019] An embodiment of the present invention has been made based on the above findings. Each requirement of the stainless steel sheet of this embodiment will be described in detail below.

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

[0021] C: 0.005 to 0.200% C (carbon) has the effect of improving strength. For this reason, the C content is 0.005% or more. The C content is preferably 0.010% or more, and more preferably 0.020% or more. However, excessive C content reduces ductility and toughness. For this reason, the C content is 0.200% or less. The C content is preferably 0.150% or less, and more preferably 0.100% or less.

[0022] Si: 0.05 to 2.50% Si (silicon) is an element that has a deoxidizing effect. Therefore, the Si content is 0.05% or more. The Si content is preferably 0.10% or more, and more preferably 0.20% or more. However, excessive Si content narrows the austenite single phase temperature range, reduces the stability of the austenite phase, and reduces the area fraction of the austenite phase. As a result, the strength-ductility balance deteriorates. Therefore, the Si content is 2.50% or less. The Si content is preferably 1.80% or less, and more preferably 1.50% or less.

[0023] Mn: 0.1 to 8.0% Mn (manganese) has the effect of improving strength. It also expands the austenite single-phase region and stabilizes the austenite phase. For this reason, the Mn content is 0.1% or more. The Mn content is preferably 0.3% or more, and more preferably 0.5% or more. However, if Mn is added in excess, the hardenability improves too much, resulting in excessive martensite formation and a decrease in the amount of austenite phase. As a result, the strength-ductility balance deteriorates. For this reason, the Mn content is 8.0% or less. The Mn content is preferably 6.0% or less, and more preferably 5.0% or less.

[0024] P: 0.0800% or less P (phosphorus) is an element contained as an impurity in stainless steel and reduces strength and ductility. For this reason, the P content is 0.0800% or less. It is preferable to reduce the P content as much as possible, but excessive reduction leads to increased refining costs, so the P content is preferably 0.0085% or more.

[0025] S: 0.030% or less S (sulfur) is an element contained as an impurity in stainless steel, and forms sulfide-based inclusions, reducing corrosion resistance. S also reduces hot workability and increases the edge cracking susceptibility of hot-rolled steel sheets. For this reason, the S content is 0.030% or less. It is preferable to reduce the P content as much as possible, but excessive reduction leads to increased refining costs, so the S content is preferably 0.001% or more.

[0026] Cr: 12.0 to 20.0% Cr (chromium) has the effect of improving the corrosion resistance of stainless steel materials. It also has the effect of increasing hardenability. For this reason, the Cr content is 12.0% or more. The Cr content is preferably 12.5% ​​or more, and more preferably 13.0% or more. However, excessive Cr content actually reduces the stability of the austenite phase, and the amount of austenite phase decreases. As a result, the strength-ductility balance deteriorates. For this reason, the Cr content is 20.0% or less. The Cr content is preferably 18.0% or less, and more preferably 17.0% or less.

[0027] Ni: 0.30% or more and less than 8.00% Ni (nickel), like Mn, is an element that stabilizes the austenite phase. It is also an element that improves hardenability. For this reason, the Ni content is 0.30% or more. The Ni content is preferably 0.50% or more, and more preferably 0.70% or more. However, excessive Ni content actually reduces the amount of austenite phase. As a result, the strength-ductility balance is degraded. For this reason, the Ni content is less than 8.00%. The Ni content is preferably 7.50% or less, and more preferably 7.00% or less.

[0028] Mo: 0.01 to 1.00% Mo (molybdenum) is an element effective in improving the corrosion resistance of martensitic structures. Therefore, the Mo content is 0.01% or more. The Mo content is preferably 0.05% or more, and more preferably 0.10% or more. However, Mo is an element that stabilizes the ferrite phase, and an excessive Mo content narrows the austenite single-phase temperature range, thereby reducing the amount of austenite phase. As a result, the strength-ductility balance deteriorates. For this reason, the Mo content is 1.00% or less. The Mo content is preferably 0.90% or less, and more preferably 0.80% or less.

[0029] Al: 0.500% or less Al (aluminum) has a deoxidizing effect. It also has the effect of improving oxidation resistance. However, excessive Al content increases the amount of inclusions, reducing strength and ductility. For this reason, the Al content is 0.500% or less. The Al content is preferably 0.450% or less, more preferably 0.010% or less, and even more preferably 0.005% or less. On the other hand, in order to obtain the above effects, the Al content is preferably 0.001% or more.

[0030] N: 0.005 to 0.100% N (nitrogen) has the effect of improving strength. It also has the effect of improving corrosion resistance. For this reason, the N content is 0.005% or more. The N content is preferably 0.010% or more, and more preferably 0.020% or more. However, if the N content is excessive, the corrosion resistance will decrease. The austenite amount will also decrease. For this reason, the N content is 0.100% or less. The N content is preferably 0.090% or less, and more preferably 0.080% or less.

[0031] In addition to the above elements, one or more elements selected from Ca, Cu, Ti, Nb, B, V, Sn, and Co may be contained within the ranges shown below. In other words, the lower limit of the above elements is 0%. The reasons for limiting the content of each element will be explained below.

[0032] Ca: 0 to 0.0100% Ca (calcium) has a deoxidizing effect. Therefore, it may be contained as needed. However, if an excessive amount of Ca is contained, a large amount of inclusions are formed, which tends to reduce manufacturability. Therefore, the Ca content is 0.0100% or less. The Ca content is preferably 0.0090% or less, and more preferably 0.0080% or less. On the other hand, in order to obtain the above effect, the Ca content is preferably 0.0001% or more, more preferably 0.0002% or more, and even more preferably 0.0003% or more.

[0033] Cu: 0 to 7.0% Cu (copper) is an element that improves strength. It also has the effect of stabilizing the austenite phase. Therefore, it may be contained as needed. However, excessive Cu content reduces hot workability. For this reason, the Cu content is 7.0% or less. The Cu content is preferably 6.5% or less, and more preferably 6.0% or less. On the other hand, in order to obtain the above effects, the Cu content is preferably 0.5% or more.

[0034] Ti: 0 to 0.70% Ti (titanium) has the effect of forming carbonitrides and improving corrosion resistance. Therefore, it may be contained as needed. However, if Ti is contained in excess, coarse TiN is formed, making hot rolling defects more likely to occur. Furthermore, toughness is more likely to decrease. Therefore, the Ti content is 0.70% or less. The Ti content is preferably 0.60% or less, and more preferably 0.50% or less. On the other hand, in order to obtain the above effects, the Ti content is preferably 0.05% or more.

[0035] Nb: 0 to 1.00% Nb (niobium) forms carbonitrides and refines the metal structure. As a result, it has the effect of improving strength. Therefore, it may be contained as needed. However, if Nb is contained in excess, ductility tends to decrease. For this reason, the Nb content is 1.00% or less. The Nb content is preferably 0.90% or less, and more preferably 0.80% or less. On the other hand, in order to obtain the above effect, the Nb content is preferably 0.01% or more.

[0036] B: 0 to 0.0050% B (boron) has the effect of improving hot workability. Therefore, it may be added as needed. However, excessive B content may reduce hardenability. Therefore, the B content is 0.0050% or less. The B content is preferably 0.0040% or less, and more preferably 0.0030% or less. On the other hand, in order to obtain the above effect, the B content is preferably 0.0005% or more.

[0037] V: 0 to 0.50% V (vanadium) forms fine carbonitrides and has the effect of improving corrosion resistance. Therefore, it may be contained as needed. However, excessive V content tends to decrease ductility and toughness. For this reason, the V content is 0.50% or less. The V content is preferably 0.45% or less, and more preferably 0.40% or less. On the other hand, in order to obtain the above effect, the V content is preferably 0.01% or more.

[0038] Sn: 0 to 0.10% Sn (tin) has the effect of improving corrosion resistance. Therefore, it may be contained as needed. However, excessive Sn content promotes edge cracking during hot rolling. Therefore, the Sn content is 0.10% or less. The Sn content is preferably 0.09% or less, and more preferably 0.08% or less. On the other hand, in order to obtain the above effect, the Sn content is preferably 0.02% or more.

[0039] Co: 0 to 0.30% Co (cobalt) is an element that improves heat resistance. Therefore, it may be contained as needed. However, because Co is expensive, an excessive Co content leads to an increase in manufacturing costs. For this reason, the Co content is 0.30% or less. The Co content is preferably 0.10% or less, and more preferably 0.05% or less. On the other hand, in order to obtain the above effects, the Co content is preferably 0.01% or more.

[0040] The structure of a stainless steel sheet is affected by its chemical composition. While the chemical composition is not particularly limited as long as it satisfies the above requirements, the following chemical composition is preferred, for example, to obtain the metal structure described later in this application. In particular, to obtain a full martensite structure before cold rolling and annealing, the Cr equivalent is 10.0 to 20.0%, and the Ni equivalent is in the range of 5.0 to 15.0%. Furthermore, the Ms point is 50°C or higher.

[0041] The above-mentioned Cr equivalent can be calculated from the following formula (i): The above-mentioned Ni equivalent can be calculated from the following formula (ii): Cr equivalent (%) = Cr + Mo + 1.5Si + (0.5Nb) ... (i) Ni equivalent (%) = Ni + 30(C + N) + 0.5Mn + (0.3Cu) ... (ii) Note that the element symbols in the above formulas represent the content (mass%) of each element contained in the stainless steel sheet, and if an element is not contained, it is set to zero.

[0042] The above-mentioned Ms point can be calculated from the following formula (iii): Ms point (°C) = 545 - 330 (C + N) + Al + 7Co-14Cr-13C-23Mn-5Mo-4Nb-13Ni-7Si + 3Ti + 4V ... (iii) Note that each element symbol in the above formula represents the content (mass%) of that element contained in the stainless steel sheet, and if the element is not contained, it is set to zero.

[0043] The Ms point is the temperature at which martensitic transformation begins during cooling, and the Ms point at which full martensite is achieved by the completion of cooling is 50° C. or higher, preferably 100° C. or higher, and more preferably 200° C. or higher.

[0044] It is important to control the manufacturing conditions within a predetermined range in order to achieve both strength and ductility while keeping the area ratio of the austenite phase, which will be described later, within a desired range. As long as the above-mentioned chemical composition is satisfied, the chemical composition is not particularly limited. For example, it is preferable that Md30, which is an index showing the mechanical stability of the austenite phase, be in the range of 23 to 250°C.

[0045] When a steel sheet's austenite phase is subjected to processing strain, it must undergo processing-induced transformation to martensite. The standard Md30 shown in the following formula (iv) is used as an index of processing-induced transformation. Md30 indicates the temperature at which 50% of the γ phase transforms into processing-induced martensite when a true strain of 0.3 is applied. The lower the Md30, the higher the mechanical stability and the less likely the processing-induced martensite transformation is. Setting the Md30 at room temperature (23°C) or higher allows processing-induced transformation to occur, making it easier to improve strength and ductility. While Md30 is determined by the chemical composition, if it is too high, the mechanical stability of austenite may be reduced, making it difficult for austenite to form. Therefore, Md30 is preferably 250°C or lower.

[0046] The Md30 (°C) can be calculated using the following formula (iv): Md30 (°C) = 551 - 462 (C + N) - 9.2Si - 8.1Mn - 13.7Cr - 29 (Ni + Cu) - 18.5Mo - 68Nb ... (iv) The element symbols in the formula above represent the content (mass%) of each element contained in the stainless steel sheet, and if an element is not contained, it is set to zero.

[0047] In the chemical composition of this embodiment, the balance is Fe and impurities. Here, "impurities" refer to components that are mixed in during the industrial production of stainless steel sheets due to various factors in raw materials such as ores and scraps, and in the manufacturing process, and are acceptable within a range that does not adversely affect this embodiment.

[0048] 2. Metallographic Structure In the stainless steel sheet of this embodiment, the metallographic structure includes martensite and austenite phases. Note that the metallographic structure of the stainless steel sheet of this embodiment is basically composed of martensite and austenite phases, but may also include, for example, δ-ferrite, inclusions, and precipitates such as carbides and intermetallic compounds in addition to the austenite phase and martensite. Note that δ-ferrite, inclusions, carbides, intermetallic compounds, and other precipitates are collectively referred to as unavoidably formed structures.

[0049] 2-1. Austenite Phase Area Fraction Figure 1 is a graph showing the relationship between the austenite phase area fraction and the strength-ductility balance (the product of tensile strength and total elongation). As is clear from Figure 1, when the austenite phase area fraction is 30% or more, the strength-ductility balance and toughness are improved. Furthermore, when the austenite phase area fraction is 85% or more, the strength-ductility balance is significantly improved and toughness is further improved.

[0050] On the other hand, in the stainless steel sheet of this embodiment, the area ratio of the austenite phase is less than 100% because martensite is included. Therefore, in the stainless steel sheet of this embodiment, the area ratio of the austenite phase is 30% or more and less than 100%.

[0051] From the viewpoint of achieving a good balance between strength and ductility, the area fraction of the austenite phase is preferably 95% or less, and more preferably 90% or less. The remainder other than the austenite phase is preferably martensite, but the above-mentioned unavoidably formed structure is permissible if its area fraction is 5% or less because it does not adversely affect the properties of the stainless steel sheet of this embodiment.

[0052] 2-2. Average grain size of the austenite phase Figure 2 is a graph showing the relationship between the average grain size of the austenite phase and the strength-ductility balance (the product of tensile strength and total elongation). As is clear from Figure 2, when the average grain size of the austenite phase is 2.0 μm or less, a desired strength-ductility balance is obtained. On the other hand, when the average grain size of the austenite phase exceeds 2.0 μm, toughness decreases. For this reason, in the stainless steel sheet of this embodiment, the average grain size of the austenite phase is 2.0 μm or less.

[0053] The average grain size of the austenite phase is preferably 1.5 μm or less, more preferably 1.0 μm or less, 0.8 μm or less, or 0.5 μm or less. Furthermore, when it is particularly desired to improve toughness, the average grain size of the austenite phase is preferably 1.5 μm or less. The lower limit of the average grain size of the austenite phase is not particularly limited, but is 0.1 μm, taking into account the manufacturing conditions described below.

[0054] Observation and measurement of the metal structure may be performed according to the following procedure. A sample is prepared so that the C-section of the steel sheet (a cross section parallel to the sheet thickness direction and perpendicular to the rolling direction) serves as the observation surface, and measurements are performed using an SEM and an EBSD attached to the SEM. First, an optical microscope or SEM is used to search for characteristic structures such as lath structures and confirm the presence or absence of martensite. Next, an EBSD attached to the SEM is used to identify austenite phase crystal grains having an FCC structure. The obtained measurement data is analyzed using an OIM4.0-CCD / ADV crystal orientation analysis system (manufactured by TSL Solutions), and the value (%) obtained by dividing the area of ​​the austenite phase crystal grains by the measurement range is used as the austenite phase area ratio. In addition, the average crystal grain size is calculated from the crystal grains identified as austenite phase. The SEM-EBSD measurement conditions are 5000x magnification, a step of 0.07 μm, and a measurement range of 18 × 52 μm.

[0055] 3. Target Properties The stainless steel sheet of this embodiment is judged to have a good balance of strength and ductility if the product of tensile strength (MPa) and total elongation (%) is 12,000 or more. The product of tensile strength (MPa) and total elongation (%) is preferably 14,000 or more.

[0056] The measurement of tensile strength and total elongation may be carried out by the following procedure. A tensile test piece according to JIS No. 13B is prepared so that the rolling direction is the longitudinal direction, and a tensile test is carried out to measure the tensile strength and total elongation. The rate of stress rise is 10 N / (mm ２ The strain rate was set to 25 mm / min after the yield point. The yield point was determined as the point at which a strain of 0.2% occurred. Other test conditions may be in accordance with JIS Z 2253:2020.

[0057] The stainless steel plate of this embodiment has a Charpy impact value of 130 J / cm ２ If the Charpy impact value is 150 J / cm or more, it is judged to have good toughness. ２ More preferably, it is equal to or greater than this.

[0058] The Charpy impact value can be measured by the following procedure: A notched Charpy test piece (3 mm thick) is taken from a direction parallel to the rolling direction. The test is performed at 25°C (room temperature) in accordance with JIS Z 2242:2023, and the Charpy impact value is measured.

[0059] 4. Manufacturing Method The stainless steel sheet of this embodiment can be stably manufactured by the following manufacturing method, which includes, for example, a hot rolling process, a hot-rolled sheet annealing process, a cold rolling process, and a cold-rolled sheet annealing process. In order to increase the area ratio of the austenite phase, particularly to exceed 85%, (1) the metallographic structure is refined before the cold rolling process, and (2) the reduction ratio in the cold rolling process is increased. Specifically, (1) the metallographic structure is refined before the cold rolling process by lowering the finish rolling temperature and coiling temperature in the hot rolling and lowering the annealing temperature of the hot-rolled sheet, as described below. This allows a large amount of strain to be introduced during hot rolling. Annealing this highly strained hot-rolled structure under the conditions described below promotes austenite nucleation during annealing of the hot-rolled sheet while suppressing grain growth, thereby refining the structure before cold rolling. (2) Cold-rolling this refined structure introduces a higher-than-normal cold-rolling strain. Furthermore, by performing cold rolling at a sufficiently high reduction rate, as described below, it is possible to introduce a large amount of strain. Furthermore, by performing cold-rolled sheet annealing on the highly strained metal structure obtained through the above (1) and (2), austenite transformation is promoted, the austenite grains are sufficiently refined, and the area ratio of the austenite phase can be made to exceed 85%.

[0060] 4-1. Hot Rolling Process Steel having the above chemical composition is melted and produced into a material (slab, etc.) for hot rolling such as a slab. In the method for producing a steel sheet according to this embodiment, for example, a slab obtained by continuous casting after melting is heated to a temperature range of 1100 to 1300°C and hot rolled to produce a hot-rolled sheet.

[0061] Furthermore, in order to make the area ratio of the austenite phase exceed 85% in the metal structure after cold rolling and annealing, in the hot rolling step, the finish rolling start temperature is set to 1000 to 1100°C, the finish rolling end temperature is set to 950 to 1000°C, and the coiling temperature is set to 600°C or less. The lower limit of the coiling temperature is not particularly limited, but may be set to 500°C or higher.

[0062] 4-2. Hot-rolled sheet annealing process The obtained hot-rolled sheet is annealed. In the steel sheet manufacturing method of this embodiment, hot-rolled sheet annealing is performed before cold rolling to change the metal structure of the steel sheet to a full martensite structure, i.e., a structure in which the area ratio of martensite is 95% or more. Usually, in the manufacture of steel sheets containing martensite, the martensite is decomposed into a ferrite phase and carbides to soften the steel sheet, and then cold-rolled. This is because if cold-rolling is performed while maintaining a hard and brittle metal structure such as martensite, the load during cold rolling is large, and the yield, etc., is likely to decrease.

[0063] On the other hand, in the steel sheet of this embodiment, since it is necessary to obtain a fine austenite phase with a certain amount or more, the steel sheet is made into a full martensite structure before cold rolling. By making the metal structure like this, a large amount of strain can be introduced in the cold rolling, and a fine austenite phase with a certain amount or more can be effectively obtained in the subsequent cold-rolled sheet annealing or the like.

[0064] Therefore, the hot-rolled sheet annealing is a process in which the temperature is held at a temperature of not less than the Af point (°C) and not more than 1200°C for 20 to 3000 seconds, and then cooled to 100°C or less. This is because the above-mentioned full martensite structure can be obtained by setting the annealing temperature to not less than the Af point (°C) and not more than 1200°C and cooling to 100°C or less. Furthermore, if the holding time at the annealing temperature in the hot-rolled sheet annealing is in the range of 20 to 3000 seconds, the above-mentioned full martensite structure can also be obtained after cooling. Furthermore, in order to make the area ratio of the austenite phase in the metal structure after cold-rolled sheet annealing exceed 85%, it is more preferable that the annealing temperature in the hot-rolled sheet annealing process be 1050°C or less.

[0065] After annealing the hot-rolled sheet, pickling, etc. may be carried out as appropriate. The As point (°C) and Af point (°C) may be calculated from the formula described below.

[0066] 4-3. Cold Rolling Process The hot-rolled sheet that has undergone annealing and cooling is cold-rolled to produce a cold-rolled sheet. The reduction ratio in cold rolling is preferably 50% or more, more preferably 70% or more. This is because a sufficient amount of strain can be introduced into the martensite structure by using a reduction ratio of 50% or more in cold rolling. Furthermore, in order to achieve an austenite phase area ratio of more than 85% in the metal structure after annealing the cold-rolled sheet, the reduction ratio in the cold rolling process is more preferably 80% or more. In order to achieve the above reduction range, the thickness of the steel sheet in this embodiment is in the range of 3.0 to 0.1 mm. Other conditions in cold rolling are not particularly limited and may be conventional.

[0067] 4-4. Cold-rolled Sheet Annealing (Final Annealing) The obtained cold-rolled sheet is annealed. This annealing is called cold-rolled sheet annealing or final annealing. The transformation strain of martensite generated during hot-rolled sheet annealing and the large amount of processing strain introduced during cold rolling are used as driving forces to promote austenite nucleation during cold-rolled sheet annealing, resulting in very fine austenite phase grains of 2.0 μm or less. The annealing temperature in cold-rolled sheet annealing is generally set to 1000°C or higher for steel sheets containing martensite. On the other hand, in the steel sheet manufacturing method of this embodiment, a relatively low annealing temperature is desirable in order to suppress grain growth of the austenite phase, and the temperature range is from the As point (°C) to 900°C. Setting the annealing temperature within the above range promotes the reverse transformation from martensite to the austenite phase and suppresses grain growth of the austenite phase.

[0068] To achieve the above-mentioned refinement of the structure, it is necessary to adjust the As point, which is the reverse transformation start temperature, to a range of 650°C to 850°C by adjusting the components. At this time, if the As point is higher than 850°C, the diffusion of iron atoms at high temperatures is promoted, so even if annealing is performed at the above-mentioned temperature, the generated fine austenite grains will easily grow. On the other hand, if the As point is 650°C or lower, austenite nucleation will not occur, and the transformation strain and cold-rolling processing strain will be recovered, which may make it difficult to refine the crystal grains.

[0069] The Af point, which is the reverse transformation finish temperature, can be calculated from the following formula (I): The As point (°C) can be calculated from the following formula (II): Af point (°C) = As point + 100 (°C) ... (I) The As point in the above formula (I) can be found from the following formula (II): As point (°C) = 937.2 - 436.5C + 56Si - 19.7Mn - 16.3Cu - 26.6Ni - 4.9Cr + 38.1Mo + 124.8V + 136.3Ti - 19.1Nb + 198.4Al + 3315B ... (II) Note that the element symbols in the above formulas represent the content (mass%) of each element contained in the stainless steel sheet, and if the element is not contained, it is set to zero.

[0070] In cold-rolled sheet annealing, the holding time at the annealing temperature is set to a range of 0 to 20 seconds to suppress grain growth of the austenite phase. If the holding time at the annealing temperature exceeds 20 seconds, the time held at or above the As point becomes longer, which makes grain growth of the austenite phase more likely to occur. Note that a holding time at the annealing temperature of 0 seconds means that cooling begins the moment the annealing temperature is reached.

[0071] Furthermore, when heating to the annealing temperature, heating is performed under the condition that the heating rate is 20°C / s or more. By setting the heating rate within the above range, diffusion transformation is suppressed and diffusionless reverse transformation without atomic movement is likely to occur. At this time, the processing strain introduced by cold rolling is inherited, which also promotes nucleation of austenite recrystallized grains and refines the crystal grains of the austenite phase. Furthermore, the area fraction of the austenite phase can be made 30% or more. The heating rate is preferably 20°C / s or more, and more preferably 100°C / s or more.

[0072] In order to obtain finer austenite grains, the heating rate is more preferably 300°C / s or more, and even more preferably 400°C / s or more. The upper limit of the heating rate is not particularly limited, but is preferably 2000°C / s from the viewpoint of production facilities.

[0073] After holding at the annealing temperature for 0 to 20 seconds, cooling is performed at a cooling rate of 20°C / s or more. By setting the cooling rate within the above range, excessive grain growth can be suppressed. The cooling rate is preferably 40°C / s or more, and more preferably 50°C / s or more. Thereafter, cooling to 400°C or less is preferable. After cooling, pickling or the like may be performed as appropriate.

[0074] In the above-mentioned production method, conditions not specifically mentioned may be carried out in accordance with conventional methods as appropriate.

[0075] EXAMPLES The stainless steel sheet according to the present invention will be described in more detail below with reference to examples, but the present embodiment is not limited to these examples.

[0076] Steel having the chemical composition shown in Table 1 was melted and a material for hot rolling was produced. The obtained material for hot rolling was heated at 1200°C and hot rolled to produce a hot-rolled sheet. The obtained hot-rolled sheet was annealed under the conditions shown in Table 2, cooled to room temperature, and pickled, and then cold-rolled under the conditions shown in Table 2 to produce a cold-rolled sheet. The obtained cold-rolled sheet was annealed under the conditions shown in Table 2, cooled to room temperature, and pickled to obtain a stainless steel sheet with a sheet thickness of 1.0 mm. All examples contained martensite. The cooling rate in the table is the cooling rate from the cold-rolled sheet annealing temperature to 400°C.

[0077]

[0078]

[0079] The area ratio and average grain size of the austenite phase of the obtained stainless steel sheets were measured by the following procedure. Furthermore, the obtained steel sheets were subjected to a tensile test by the following procedure to measure the tensile strength and total elongation (%). Furthermore, a Charpy impact test was performed by the following procedure to measure the Charpy impact value.

[0080] (Austenite Phase Area Fraction and Average Grain Size) Samples were prepared so that the C-section of the steel sheet (a cross section parallel to the sheet thickness direction and perpendicular to the rolling direction) served as the observation surface, and measurements were performed using an SEM and an EBSD attached to the SEM. First, the presence or absence of martensite was confirmed using an SEM to search for characteristic structures such as lath structures. Next, the EBSD attached to the SEM was used to identify austenite phase grains having an FCC structure. The obtained measurement data was analyzed using an OIM4.0-CCD / ADV crystal orientation analysis system (manufactured by TSL Solutions), and the value (%) obtained by dividing the area of ​​the austenite phase grains by the measurement range was used to determine the austenite phase area fraction. In addition, the average grain size was calculated from the grains identified as austenite phase. The SEM-EBSD measurement conditions were 5000x magnification, a step of 0.07 μm, and a measurement range of 18 × 52 μm.

[0081] (Tensile strength and total elongation) A tensile test piece according to JIS No. 13B was prepared from the cold-rolled annealed sheet so that the rolling direction was the longitudinal direction, and a tensile test was carried out to measure the tensile strength and total elongation. ２ The strain rate was set to 25 mm / min after the yield point. The yield point was determined as the point at which a strain of 0.2% occurred. Other test conditions were in accordance with JIS Z 2253:2020.

[0082] (Charpy impact value) V-notch Charpy test pieces (3 mm thick) were taken from the cold-rolled annealed sheets in a direction parallel to the rolling direction. Charpy impact tests were performed on these test pieces at 25°C (room temperature) in accordance with JIS Z 2242:2023 to measure the Charpy impact value.

[0083]

[0084] Inventive Examples a1 to a21, which satisfied the requirements of this embodiment, had a good strength-ductility balance and toughness, while Comparative Examples a1 to a17, which did not satisfy the requirements of this embodiment, had poor strength-ductility balance and / or toughness. In particular, Inventive Examples a20 and a21 had an austenite phase area fraction of more than 85% and an austenite phase average grain size of 1.0 μm or less, and were excellent in both strength-ductility balance and toughness.

[0085] For the steel types of Steel Nos. A17 and A18 in Table 1, a 6.2 mm hot-rolled sheet was subjected to hot-rolled sheet annealing, cold rolling, and cold-rolled annealing under the manufacturing conditions shown in Table 4 to produce a steel sheet having a sheet thickness of 3 mm. Other conditions were the same as in Example 1. The obtained steel sheet was evaluated for the same items (area ratio of the austenite phase, average grain size of the austenite phase) and properties as in Example 1. The measurement methods were the same.

[0086]

[0087] When the average grain size of the austenite phase is 1.5 μm or less, the Charpy impact value is further improved, and toughness is improved. In particular, when the area fraction of the austenite phase is 40% or more, or the average grain size of the austenite phase is 1.0 μm or less, the Charpy impact value is 200 J / cm ２ The results were better than those mentioned above, and the toughness was also improved.

Claims

1. Chemical composition, in mass%, is: C: 0.005 to 0.200%, Si: 0.05 to 2.50%, Mn: 0.1 to 8.0%, P: 0.0800% or less, S: 0.030% or less, Cr: 12.0 to 20.0%, Ni: 0.30% or more but less than 8.00%, Mo: 0.01 to 1.00%, Al: 0.500% or less, N: 0.005 to 0.100%, Ca: 0 to 0.0100%, Cu: 0 to 7.0%, Ti: 0 to 0.70%, Nb: 0 to 1.00%, B: 0 to 0.0050%, V: 0 to 0.50%, Sn: 0 to 0.10%, A stainless steel sheet comprising: Co: 0 to 0.30%, and the balance: Fe and impurities; a Cr equivalent calculated by the following formula (i) of 10.0 to 20.0%; a Ni equivalent calculated by the following formula (ii) of 5.0 to 15.0%; an Ms point calculated by the following formula (iii) of 50°C or higher; a metal structure containing martensite and an austenite phase; an area fraction of the austenite phase being 30% or higher but less than 100%; and an average crystal grain size of the austenite phase being 2.0 μm or less. Cr equivalent = Cr + Mo + 1.5Si + (0.5Nb) ... (i) Ni equivalent = Ni + 30(C + N) + 0.5Mn + (0.3Cu) ... (ii) Ms point (°C) = 545 - 330(C + N) + Al + 7Co-14Cr-13C-23Mn-5Mo-4Nb-13Ni-7Si + 3Ti + 4V ... (iii) However, each element symbol in the above formula represents the content (mass%) of each element contained in the stainless steel, and if the element is not contained, it is set to zero.

2. The stainless steel sheet according to claim 1, wherein the chemical composition contains, in mass%, one or more elements selected from Ca: 0.0001 to 0.0100%, Cu: 0.5 to 7.0%, Ti: 0.05 to 0.70%, Nb: 0.01 to 1.00%, B: 0.0005 to 0.0050%, V: 0.01 to 0.50%, Sn: 0.02 to 0.10%, and Co: 0.01 to 0.30%.

3. The stainless steel sheet according to claim 1 or 2, wherein the area ratio of the austenite phase is 85% or more.

4. A method for producing the stainless steel sheet according to claim 1 or 2, comprising: (a) a step of heating a slab having the chemical composition according to claim 1 or 2 in the range of 1100 to 1300°C, and hot rolling it to obtain a hot-rolled steel sheet; (b) a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet by holding it at an annealing temperature of not less than the Af point (°C) calculated by the following formula (I) and not more than 1200°C for 20 to 3000 seconds, and cooling it to not more than 100°C, and pickling it after the annealing to obtain a pickled steel sheet; and (c) a cold rolling step of cold-rolling the pickled steel sheet at a reduction rate of not less than 50% to obtain a cold-rolled steel sheet. (d) an annealing step of heating the cold-rolled steel sheet at a rate of 20°C / s or more, holding the temperature at a temperature in the range of from the As point (°C) to 900°C (calculated by the following formula (II)) for 0 to 30 seconds, and then cooling at a cooling rate of 20°C / s or more. Af point (°C) = As point + 100 (°C) ... (I) The As point in the above formula (I) can be calculated by the following formula (II): As point (°C) = 937.2 - 436.5C + 56Si - 19.7Mn - 16.3Cu - 26.6Ni - 4.9Cr + 38.1Mo + 124.8V + 136.3Ti - 19.1Nb + 198.4Al + 3315B ... (II) 5. A method for producing a stainless steel sheet according to claim 3, comprising: (a) a step of heating a slab having the chemical composition according to (1) or (2) above in the range of 1100 to 1300°C, and hot rolling it to obtain a hot-rolled steel sheet; (b) a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet by holding it at an annealing temperature of not less than the Af point (°C) calculated by the following formula (I) and not more than 1200°C for 20 to 3000 seconds, and cooling it to not more than 100°C, and pickling it after the annealing to obtain a pickled steel sheet; and (c) a cold rolling step of cold-rolling the pickled steel sheet at a reduction rate of 50% or more to obtain a cold-rolled steel sheet. and (d) an annealing step of raising the temperature of the cold-rolled steel sheet at a rate of 20°C / s or more, holding the temperature in the range of the As point (°C) or higher to 900°C or lower for 0 to 30 seconds as calculated by the following formula (II), and then cooling at a cooling rate of 20°C / s or higher, wherein in the hot rolling in (a), the finish rolling start temperature is 1000 to 1100°C, the finish rolling end temperature is 950 to 1000°C, and the coiling temperature is 600°C or lower, in the hot-rolled sheet annealing step in (b), the annealing temperature is 1050°C or lower, and in the cold rolling step in (c), the rolling reduction is 80% or higher. Af point (°C) = As point + 100 (°C) (I) Note that the As point in the above formula (I) can be calculated from the following formula (II): As point (℃)=937.2-436.5C+56Si-19.7Mn-16.3Cu-26.6Ni-4.9Cr+38.1Mo+124.8V+136.3Ti-19.1Nb+198.4Al+3315B...(II)

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