Stainless steel material and method for manufacturing the same

Abstract

To provide a stainless steel material that is both high-strength and has excellent hot-workability. [Solution] By mass, the material contains C: 0.001~0.050%, Si: 0.01~0.50%, Mn: 1.0~3.5%, P: 0.050% or less, S: 0.0300% or less, Ni: 1.5~2.5%, Cr: 19.6~22.5%, Mo: 0.01~1.00%, Cu: 0.01~0.50%, N: 0.010~0.090%, with the remainder being Fe and impurities. Formula (1): DF=7.2(Cr+0.88Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9(1) The DF shown is 65.0-80.0, The ferrite phase accounts for 65-80% by volume, the martensite phase for 5-30% by volume, and the austenite phase for 5-20% by volume. The stainless steel material has a micro-Vickers hardness of 300 HV or higher in the ferrite phase.

Description

Technical Field

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

Background Art

[0002] Quasi-stable austenitic stainless steel materials such as SUS301 and SUS304 can be strengthened by various treatments such as temper rolling, and thus are used for high-strength members and spring materials. However, since quasi-stable austenitic stainless steel materials contain a large amount of Ni, they are costly. Therefore, there is a need for a duplex stainless steel material containing a ferrite phase and an austenite phase that can be used as an alternative to quasi-stable austenitic stainless steel materials.

[0003] As a method for strengthening duplex stainless steel materials, Patent Document 1 discloses that after performing a first heat treatment in a temperature range of 950 to 1150°C, the duplex stainless steel is deformed at a rolling reduction rate of at least 10% to obtain a high-level tensile strength of at least 1000 MPa while maintaining formability, and after the deformation, a second heat treatment is performed within a temperature range of 100°C to 420°C for 1 second to 20 minutes to further improve the yield strength. A method for producing a duplex stainless steel material has been proposed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The technology described in Patent Document 1 uses duplex stainless steel with a nitrogen content of 0.16-0.26% and a relatively high proportion of the austenite phase as the material, and induces work-induced martensitic transformation (TRIP effect) through a first heat treatment (temper rolling). Generally, the hardness of the martensite phase depends on the carbon and nitrogen content, so high strength is achieved by martensizing the austenite phase, which has a high nitrogen content. Furthermore, the remaining ferrite phase is also strengthened through a second heat treatment (aging treatment). However, when using duplex stainless steel with a high nitrogen content as the material, the increased proportion of the austenite phase reduces hot workability, and productivity tends to decrease due to issues such as edge breakage during hot rolling.

[0006] This invention was made to solve the above-mentioned problems and aims to provide a stainless steel material that is high in strength and has excellent hot workability, as well as a method for manufacturing the same. [Means for solving the problem]

[0007] The hot workability of stainless steel can be ensured by reducing the nitrogen content while increasing the ferrite phase density (DF). On the other hand, the increase in strength due to work-induced martensitic transformation (TRIP effect) by temper rolling is insufficient due to the decrease in nitrogen content and the amount of austenite phase. Therefore, we attempted to achieve high strength of the ferrite phase through aging treatment. As a result of diligent research under the above-mentioned background, the inventors have discovered that the above-mentioned problems can be solved by controlling the composition of the stainless steel material (including the values ​​of DF and Md), the proportions of the ferrite phase, martensite phase, and austenite phase, and the micro-Vickers hardness of the ferrite phase to an appropriate range, and have completed the present invention.

[0008] In other words, the present invention contains, by mass, C: 0.001~0.050%, Si: 0.01~0.50%, Mn: 1.0~3.5%, P: 0.050% or less, S: 0.0300% or less, Ni: 1.5~2.5%, Cr: 19.6~22.5%, Mo: 0.01~1.00%, Cu: 0.01~0.50%, N: 0.010~0.090%, with the remainder being Fe and impurities. Formula (1): DF=7.2(Cr+0.88Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9 (1) The DF value shown in the formula (where element symbols represent the content (mass%) of each element) is between 65.0 and 80.0. The following equation (2): Md=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo... (2) The value of Md shown in the formula (where the element symbol represents the content (mass %) of each element) is between 100.0 and 150.0°C. The ferrite phase accounts for 65-80% by volume, the martensite phase for 5-30% by volume, and the austenite phase for 5-20% by volume. This invention relates to a stainless steel material in which the ferrite phase has a micro-Vickers hardness of 300 HV or higher.

[0009] Furthermore, the present invention contains, by mass, C: 0.001~0.050%, Si: 0.01~0.50%, Mn: 1.0~3.5%, P: 0.050% or less, S: 0.0300% or less, Ni: 1.5~2.5%, Cr: 19.6~22.5%, Mo: 0.01~1.00%, Cu: 0.01~0.50%, N: 0.010~0.090%, with the remainder being Fe and impurities. Formula (1): DF=7.2(Cr+0.88Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9 (1) The DF value shown in the formula (where element symbols represent the content (mass%) of each element) is between 65.0 and 80.0. The following equation (2): Md=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo... (2) This invention relates to a method for manufacturing stainless steel, which involves temper-rolling a rolled material having an Md value of 100.0 to 150.0°C (wherein the formula, the elemental symbols represent the content (mass%) of each element) at a rolling rate of 40 to 80%, and then performing an aging treatment by heating under conditions in which the LMP represented by the following formula (3) becomes 12000 to 17000. LMP = (t + 273) × (C + log tr) (3) In the formula, t is the heating temperature (°C), C is the material constant, and tr is the holding time at the heating temperature (h). [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a stainless steel material that is highly strong and has excellent hot workability, as well as a method for manufacturing the same. [Modes for carrying out the invention]

[0011] The embodiments of the present invention will be described in detail below. The present invention is not limited to the embodiments described below, and it should be understood that modifications, improvements, etc., made to the embodiments described below, based on the ordinary knowledge of those skilled in the art, without departing from the spirit of the invention, also fall within the scope of the present invention. In this specification, unless otherwise specified, any "%" indication for ingredients refers to "mass%".

[0012] The stainless steel material according to the embodiment of the present invention has a composition comprising C: 0.001-0.050%, Si: 0.01-0.50%, Mn: 1.0-3.5%, P: 0.050% or less, S: 0.0300% or less, Ni: 1.5-2.5%, Cr: 19.6-22.5%, Mo: 0.01-1.00%, Cu: 0.01-0.50%, N: 0.010-0.090%, with the remainder being Fe and impurities.

[0013] In this specification, "stainless steel material" means a material formed from stainless steel, and its shape is not particularly limited. Examples of shapes include plates (including strips), rods, and tubes. Furthermore, the cross-sectional shape may be various types of steel, such as T-shaped or I-shaped. Among these, the stainless steel material according to the embodiment of the present invention is preferably plate-shaped (including strips). Furthermore, in this specification, "impurities" refers to components that are mixed in during the industrial production of stainless steel materials due to various factors in the raw materials such as ore and scrap, and the manufacturing process, and which are acceptable as long as they do not adversely affect the present invention. For example, impurities include unavoidable impurities. An example of an impurity is oxygen (O). The oxygen content is, for example, 0.0001 to 0.0070%. Regarding the content of each element, "containing xx% or less" means that it contains xx% or less, but also more than 0% (especially above the impurity level).

[0014] The stainless steel material according to the embodiment of the present invention may further include, as necessary, one or more elements selected from Nb: 0.010-0.500%, Ti: 0.01-0.50%, V: 0.01-0.50%, W: 0.05-0.50%, Co: 0.01-0.30%, B: 0.0002-0.0050%, Sn: 0.010-0.500%, Al: 0.010-0.050%, Mg: 0.0002-0.0100%, Ca: 0.0002-0.0100%, Ta: 0.050% or less, Ga: 0.050% or less, Zr: 0.01-0.50%, and REM: 0.0002-0.0100%. The following provides a detailed explanation of each component.

[0015] <C:0.001~0.050%> C is an element that has a great influence on the stability of the austenite phase. If the C content is too high, the ductility (workability) may decrease, or the precipitation of Cr carbides may be promoted, leading to intergranular corrosion. Therefore, the C content is set to 0.050% or less, preferably 0.045% or less, more preferably 0.040% or less. Also, from the perspective of corrosion resistance, a lower C content is better, but reducing the C content too much will lead to an increase in cost. Therefore, the C content is set to 0.001% or more, preferably 0.002% or more, more preferably 0.005% or more.

[0016] <Si: 0.01 - 0.50%> Si is added as a deoxidizing element and is also useful for improving oxidation resistance. However, if the Si content is too high, it will harden and the ductility will decrease. Therefore, the Si content is set to 0.50% or less, preferably 0.45% or less, more preferably 0.40% or less. Also, excessively reducing the Si content will increase the cost during steelmaking. Therefore, the Si content is set to 0.01% or more, preferably 0.02% or more, more preferably 0.05% or more.

[0017] <Mn: 1.0 - 3.5%> Mn is an element that plays an important role in concentrating in the austenite phase and stabilizing the austenite phase. However, if the Mn content is too high, in addition to ductility, the corrosion resistance and hot workability will also decrease. Therefore, the Mn content is set to 3.5% or less, preferably 3.3% or less, more preferably 3.0% or less. Also, excessively reducing the Mn content will increase the cost during steelmaking. Therefore, the Mn content is set to 1.0% or more, preferably 1.1% or more, more preferably 1.2% or more.

[0018] <P: 0.050% or less> P is an element contained in raw materials such as Cr. When the content of P is high, the formability decreases. Therefore, the content of P is set to 0.050% or less, preferably 0.045% or less, more preferably 0.040% or less. On the other hand, it is preferable that the content of P is lower, but there is a limit to reducing the content of P. The lower limit value of the content of P is generally 0.001%, preferably 0.002%, more preferably 0.003%.

[0019] <S: 0.0300% or less> S is an element contained in various raw materials. Since S combines with Mn to form inclusions and may become the starting point of rusting, the lower the content of S, the better the corrosion resistance. Therefore, the content of S is set to 0.0300% or less, preferably 0.0250% or less, more preferably 0.0200% or less. On the other hand, there is a limit to reducing the content of S. The lower limit value of the content of S is generally 0.0001%, preferably 0.0005%.

[0020] <Ni: 1.5 - 2.5%> Ni is an austenite-forming element and is an important element for adjusting the stability of the austenite phase. Also, Ni has the effect of suppressing the precipitation of nitrides and improving the corrosion resistance. In order to exert these effects, the content of Ni is set to 1.5% or more, preferably 1.6% or more, more preferably 1.7% or more, and still more preferably 1.8% or more. On the other hand, if the content of Ni is too high, it will not only cause an increase in raw material costs, but there is also a possibility of problems such as stress corrosion cracking due to the high proportion of the austenite phase. Therefore, the content of Ni is set to 2.5% or less, preferably 2.4% or less, more preferably 2.3% or less.

[0021] <Cr: 19.6 - 22.5%> Cr is an element necessary to ensure corrosion resistance. To achieve this effect, the Cr content should be 19.6% or more, preferably 19.8% or more, and more preferably 20.0% or more. On the other hand, if the Cr content is too high, it can lead to hot working cracks and increase the cost of the refining process. Therefore, the Cr content should be 22.5% or less, preferably 22.3% or less, and more preferably 22.0% or less.

[0022] <Mo:0.01~1.00%> Mo is an element that improves corrosion resistance. To achieve this effect, the Mo content should be 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, if the Mo content is too high, the raw material cost will increase. Therefore, the Mo content should be 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.

[0023] <Cu:0.01~0.50%> Cu, like Mn and Ni, is an austenite-forming element and has the effect of suppressing nitride precipitation and improving corrosion resistance. To achieve these effects, the Cu content should be 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more. On the other hand, if the Cu content is too high, it will lead to an increase in raw material costs and a decrease in hot workability. Therefore, the Cu content should be 0.50% or less, preferably 0.45% or less, and more preferably 0.40% or less.

[0024] <N:0.010~0.090%> Nitrogen (N), like carbon (C), is an element that significantly affects the stability of the austenite phase. Furthermore, N also enhances corrosion resistance through solid solution. To achieve these effects, the N content should be 0.010% or higher, preferably 0.020% or higher. On the other hand, if the N content is too high, hot workability decreases, and corrosion resistance also decreases due to the precipitation of Cr nitrides. Therefore, the N content should be 0.090% or lower, preferably 0.085% or lower, and more preferably 0.080% or lower.

[0025] <Nb:0.010~0.500%> Nb forms nitrides (NbN) and carbides (NbC), which improve workability. To achieve this effect, the Nb content should be 0.010% or more, preferably 0.013% or more, and more preferably 0.015% or more. On the other hand, if the Nb content is too high, the ductility will decrease. Therefore, the Nb content should be 0.500% or less, preferably 0.300% or less, and more preferably 0.200% or less.

[0026] <Ti:0.01~0.50%> Like Nb, Ti forms nitrides (TiN) and carbides (TiC), which improve workability. To achieve this effect, the Ti content should be 0.01% or more, preferably 0.02% or more, and more preferably 0.03% or more. On the other hand, if the Ti content is too high, the ductility decreases. Therefore, the Ti content should be 0.50% or less, preferably 0.30% or less, and more preferably 0.20% or less.

[0027] <V:0.01~0.50%> V has the effect of forming nitrides and improving workability. To achieve this effect, the V content should be 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, if the V content is too high, the ductility and hot workability will decrease. Therefore, the V content should be 0.50% or less, preferably 0.45% or less, and more preferably 0.40% or less.

[0028] <W:0.05~0.50%> Water (W) is an effective element for improving corrosion resistance. To achieve this effect, the W content should be 0.05% or more, preferably 0.08% or more, and more preferably 0.10% or more. On the other hand, if the W content is too high, the ductility will decrease. Therefore, the W content should be 0.50% or less, preferably 0.45% or less, and more preferably 0.40% or less.

[0029] <Co:0.01~0.30%> Co is an effective element for increasing high-temperature strength and improving hot workability. To achieve these effects, the Co content should be 0.01% or more, preferably 0.02% or more. On the other hand, if the Co content is too high, toughness will decrease. Therefore, the Co content should be 0.30% or less, preferably 0.25% or less, and more preferably 0.20% or less.

[0030] <B:0.0002~0.0050%> B is an element that segregates at grain boundaries and improves hot workability. To achieve this effect, the B content should be 0.0002% or more, preferably 0.0010% or more, and more preferably 0.0015% or more. On the other hand, if the B content is too high, the corrosion resistance will be significantly reduced. Therefore, the B content should be 0.0050% or less, preferably 0.0045% or less, and more preferably 0.0040% or less.

[0031] <Sn:0.010~0.500%> Sn is an element that improves corrosion resistance. To achieve this effect, the Sn content should be 0.010% or more, preferably 0.020% or more, and more preferably 0.025% or more. On the other hand, if the Sn content is too high, the hot workability will decrease. Therefore, the Sn content should be 0.500% or less, preferably 0.450% or less, and more preferably 0.400% or less.

[0032] <Al:0.010~0.050%> Al is an effective element for desulfurization and deoxidation. To achieve these effects, the Al content should be 0.010% or more, preferably 0.015% or more, and more preferably 0.020% or more. On the other hand, too much Al content leads to an increase in manufacturing defects and raw material costs. Therefore, the Al content should be 0.050% or less, preferably 0.045% or less, and more preferably 0.040% or less.

[0033] <Mg:0.0002~0.0100%> Mg is an element that not only deoxidizes but also has the effect of refining the solidification structure. In order to exert these effects, the Mg content should be 0.0002% or more, preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, if the Mg content is too high, it will lead to an increase in raw material costs. Therefore, the Mg content is set to 0.0100% or less, preferably 0.0095% or less, and more preferably 0.0090% or less.

[0034] <Ca: 0.0002~0.0100%> Ca is an element effective for desulfurization and deoxidation. To exert these effects, the Ca content is set to 0.0002% or more, preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, if the Ca content is too high, hot working cracks are likely to occur and the corrosion resistance also decreases. Therefore, the Ca content is set to 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0050% or less.

[0035] <Ta: 0.050% or less> Ta is an element that improves corrosion resistance by modifying inclusions. However, if the Ta content is too high, it will cause a decrease in normal temperature ductility and toughness. Therefore, the Ta content is 0.050% or less, preferably 0.045% or less, and more preferably 0.040% or less. On the other hand, the lower limit value of the Ta content is not particularly limited, but in order to exert the effect of Ta, it is preferably 0.001%, and more preferably 0.003%.

[0036] <Ga: 0.050% or less> Ga is an element that contributes to improving corrosion resistance and suppressing hydrogen embrittlement. However, if the Ga content is too high, the workability will decrease. Therefore, the Ga content is 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less. On the other hand, the lower limit value of the Ga content is not particularly limited, but in order to exert the effect of Ga, it is preferably 0.001%, and more preferably 0.003%.

[0037] <Zr: 0.01~0.50%> Zr has similar effects to Nb and Ti, and is an element that improves oxidation resistance. To achieve these effects, the Zr content should be 0.01% or more, preferably 0.02% or more. On the other hand, if the Zr content is too high, it will lead to a decrease in ductility and an increase in raw material costs. Therefore, the Zr content should be 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

[0038] <REM:0.0002~0.0100%> Rare earth elements (REMs) are effective in improving hot workability. To achieve this effect, the REM content should be 0.0002% or more, preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, too much REM content impairs manufacturability and increases costs. Therefore, the REM content should be 0.0100% or less, preferably 0.0095% or less, and more preferably 0.0090% or less. REM is a collective term for 15 elements (lanthanides) from Sc, Y, and La to Lu. These elements can be used individually or in combination of two or more as REM.

[0039] The stainless steel material according to the embodiment of the present invention has a DF value represented by the following formula (1) of 65.0 to 80.0, preferably 66.0 to 78.0, and more preferably 68.0 to 76.0. DF=7.2(Cr+0.88Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9 (1) In formula (1), the element symbols represent the percentage (%) of each element. Here, DF is an index representing the amount of ferrite phase. Therefore, 100-DF represents the total amount of austenite and martensite phases. However, it should be noted that since DF is an index determined based on elemental content, it does not necessarily match the actual measured amounts of austenite and martensite phases. By controlling the value of DF within the above range, it is possible to increase the amount of ferrite phase while decreasing the amount of austenite phase, thereby ensuring the hot workability of stainless steel materials. Furthermore, high strength can be achieved through temper rolling (work-induced martensitic transformation) and aging treatment.

[0040] The stainless steel material according to the embodiment of the present invention has an Md value represented by the following formula (2) of 100.0 to 150.0°C, preferably 105.0 to 140.0°C, more preferably 110.0 to 135.0°C, and even more preferably 115.0 to 130.0°C. Md=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo... (2) In equation (2), the element symbols represent the percentage (%) of each element. Here, Md is an index representing the stability of the austenite phase. A larger Md value (higher temperature) indicates that the austenite phase is less stable. By controlling the Md value within the above range, it is possible to adjust the stability of the austenite phase to achieve high strength even with reduced alloying.

[0041] In the stainless steel material according to the embodiment of the present invention, the ferrite phase is 65 to 80 volume%, the martensite phase is 5 to 30 volume%, and the austenite phase is 5 to 20 volume%. By controlling each phase within these ranges, it is possible to achieve both high strength and improved hot workability. From the viewpoint of stably ensuring this effect, the ferrite phase is preferably 65 to 75 volume%, the martensite phase is preferably 10 to 20 volume%, and the austenite phase is preferably 10 to 20 volume%.

[0042] In this specification, the proportion of each phase in stainless steel material can be determined by EBSD measurement. Specifically, EBSD measurement is performed on a sample of duplex stainless steel material with a thickness-direction cross-section mirror-polished parallel to the rolling direction to identify the bcc phase (ferrite phase and martensite phase) and the fcc phase (austenite phase). The proportion of the austenite phase can be determined by calculating the area of ​​the fcc phase, dividing it by the total area, and multiplying by 100. Furthermore, the ferrite phase and martensite phase can be identified by performing IQ (Image Quality) imaging using OIM analysis software on the bcc phase data obtained from the EBSD measurement. Note that IQ imaging is an image analysis that represents clarity. The martensite phase has a more complex internal structure than the ferrite phase and has lower clarity, so it appears dark in the IQ image. On the other hand, the ferrite phase has a simpler internal structure than the martensite phase and has higher clarity, so it appears bright in the IQ image. Therefore, by binarizing the IQ image, the areas of the ferrite phase and the martensite phase can be calculated, respectively. Consequently, the ratio of the ferrite phase to the martensite phase can be determined by dividing the calculated areas of the ferrite phase and the martensite phase by the total area and multiplying by 100.

[0043] The stainless steel material according to the embodiment of the present invention has a micro-Vickers hardness of 300 HV or higher, preferably 310 HV or higher, in the ferrite phase. If the micro-Vickers hardness of the ferrite phase is within this range, it can be said that the ferrite phase has been strengthened. The upper limit of the micro-Vickers hardness of the ferrite phase is not particularly limited, but is typically 450 HV. In this specification, the micro-Vickers hardness of the ferrite phase can be determined as follows. First, the surface of the stainless steel material is descaled by wet polishing or the like, and then the ferrite phase is identified by electrolytic etching with a KOH aqueous solution. After that, the micro-Vickers hardness of the identified ferrite phase is measured. The micro-Vickers hardness is measured in accordance with JIS Z2244-1:2020 and performed using a micro-Vickers hardness tester. The micro-Vickers hardness of the ferrite phase is measured on 50 or more ferrite phases, and the average is taken as the measurement result.

[0044] In the embodiment of the present invention, the stainless steel material preferably has a Vickers hardness of 350 to 450 HV, and more preferably 360 to 440 HV. If the Vickers hardness is within this range, the stainless steel material can be said to have high strength. However, if the Vickers hardness of the stainless steel material exceeds 450 HV, the toughness decreases and it becomes brittle, and the manufacturing burden also increases. In this specification, the Vickers hardness of stainless steel is measured at 50 or more arbitrary locations on the surface of the stainless steel, and the average of these measurements is used as the measurement result. Furthermore, the Vickers hardness is measured in accordance with JIS Z2244-1:2020 and performed using a Vickers hardness tester.

[0045] The stainless steel material according to the embodiment of the present invention preferably has a fracture reduction value of 90% or more, more preferably 91% or more, and even more preferably 92% or more. If the fracture reduction value is within this range, it is possible to suppress a decrease in productivity due to edge breakage during hot rolling, and thus the material can be said to have excellent hot workability. In this specification, the reduction of area at break can be measured by the method of the embodiment described later.

[0046] The stainless steel material in the embodiment of the present invention may be hot-rolled or cold-rolled, and the hot-rolled or cold-rolled material may be subjected to annealing or pickling.

[0047] The thickness of the stainless steel material according to the embodiment of the present invention is not particularly limited and can be adjusted as appropriate depending on the application, but is preferably 0.2 to 5.0 mm, more preferably 0.2 to 4.0 mm, and even more preferably 0.2 to 3.0 mm. Note that when the stainless steel material is in the form of a rod, the thickness refers to the equivalent diameter of the cross-section. When the stainless steel material is in the form of a structural steel, the thickness refers to the thickness at any point in the cross-section.

[0048] The method for manufacturing stainless steel material according to the embodiment of the present invention is not particularly limited as long as it is a method capable of manufacturing stainless steel material having the above-described characteristics. An example of a method for manufacturing stainless steel material according to an embodiment of the present invention will be described below. The stainless steel material according to the embodiment of the present invention can be manufactured by temper-rolling a rolled material having the above composition and then performing an aging treatment.

[0049] The rolled material having the above composition is not particularly limited, but it can be produced by melting stainless steel having the above composition (including DF and Md) by vacuum melting to form a steel slab, and then rolling it. Specifically, if the rolled material is a hot-rolled material, it can be produced by hot-rolling the steel slab. The hot-rolled material may be annealed after hot-rolling. If the rolled material is a cold-rolled material, it can be produced by hot-rolling and annealing the steel slab, and then cold-rolling it. The cold-rolled material may be annealed after cold-rolling.

[0050] While there are no particular limitations to the hot rolling process, it is preferable to maintain a temperature of 950°C or higher immediately after the final pass, followed by cooling to 800°C at a cooling rate of 20°C / second or higher. Performing hot rolling under these conditions makes it easier to coarse the crystal grains of the ferrite phase. There are two main reasons why the ferrite phase grains become finer. The first is recrystallization during hot rolling or subsequent annealing due to the accumulation of hot rolling strain. The lower the hot rolling temperature, the more strain accumulates and induces recrystallization, making it easier for the ferrite phase grains to become finer. Therefore, it is necessary to raise the temperature to the point immediately after the final pass where this is less likely to occur. The second reason is the suppression of ferrite phase grain growth by the formation of austenite phase. When the austenite phase precipitates within the grain boundaries of the ferrite phase, the movement of the grain boundaries slows down, suppressing grain growth. The austenite phase decreases as the temperature increases, peaking around 900°C, so ferrite phase grains grow more easily at higher temperatures. On the other hand, to maintain a high hot rolling temperature, it is effective to increase the heating temperature of the slab before hot rolling or to increase the rolling speed to shorten the heat dissipation time, but this increases fuel costs and manufacturing difficulties. Therefore, taking these circumstances into consideration, it is preferable to set the lower limit of the final pass temperature for hot rolling to 950°C.

[0051] The annealing after hot rolling is not particularly limited, but it is preferable to hold the material at a target temperature of 1030-1150°C for 10 seconds or more, and then cool it to 400°C or below at a cooling rate of 20°C / second or more. The reason for performing annealing under these conditions is to sufficiently dissolve the carbides and nitrides precipitated during cooling after hot rolling, and to suppress the precipitation of carbides and nitrides during the cooling process after annealing. In addition, by keeping the proportion of the austenite phase relatively low, the suppression of grain growth in the ferrite phase is reduced, and the grains of the ferrite phase are made to coarseen more easily. In particular, if the target temperature is lower than 1030°C, the solid solution of carbides and nitrides will be insufficient, and the proportion of the austenite phase will be too high. Conversely, if the target temperature is higher than 1150°C, although the carbides and nitrides will be sufficiently dissolved, the proportion of the austenite phase will be too low. Furthermore, a certain amount of carbon and nitrogen dissolves in the ferrite phase, and there is a risk that precipitates will form during cooling in the ferrite phase, which has a small solid solubility limit, potentially degrading its corrosion resistance.

[0052] The conditions for cold rolling are not particularly limited, but it is preferable to have a rolling ratio of 40-90%. A rolling ratio of 40% or more is preferable because crushing or spreading precipitates such as carbides increases the surface area, thereby promoting solid solution during heat treatment. On the other hand, a rolling ratio of 90% or less is preferable to suppress edge breakage due to excessive rolling. It is also preferable to prevent the microstructure of the finished annealed material from becoming too fine due to the accumulation of rolling strain. As mentioned above, excessive strain induces recrystallization, which results in finer grains, so this should be avoided. If cold rolling is performed two or more times, intermediate annealing may be carried out between each cold rolling step. If intermediate annealing is performed, the conditions should be the same as those for annealing after hot rolling.

[0053] The conditions for annealing after cold rolling (finish annealing) are not particularly limited, but it is preferable to hold the material at a target temperature of 1000-1150°C for 5 seconds or more, followed by cooling at a cooling rate of 50°C / second or more. Finish annealing is performed under these conditions to suppress the precipitation of carbides and nitrides during heating, to complete recrystallization, to achieve solid solution of carbides and nitrides, to control the proportion of the austenite phase, to suppress fluctuations in the proportion of the austenite phase during cooling, and to suppress the reprecipitation of carbides and nitrides. Furthermore, due to the microstructure control during cold rolling, the ferrite phase grains become coarser after finish annealing.

[0054] Temper rolling is a rolling process performed to adjust the strength of a material, and is carried out by rolling the material to a rolling ratio of 40-80%. By performing temper rolling, the hardness of the ferrite phase can be increased through work hardening. Furthermore, by controlling the rolling ratio within the above range, the strength of the stainless steel material can be increased through work-induced martensitic transformation. From the viewpoint of stably ensuring this effect, a rolling ratio of 45-75% is preferable.

[0055] Aging treatment is a heat treatment for precipitation strengthening, and is performed by heating under conditions where the LMP (Larson-Miller parameter), represented by the following formula (3), is between 12,000 and 17,000. LMP = (t + 273) × (C + log tr) (3) In the formula, t is the heating temperature (°C), C is the material constant, and tr is the holding time at the heating temperature (h). Note that the material constant C is around 20 for most stainless steel materials. By controlling LMP within the above range, the ferrite phase can be strengthened, thereby increasing the overall strength of the stainless steel material. From the viewpoint of stably ensuring this effect, LMP is preferably 12500 to 16500, and more preferably 13000 to 16200. The heating temperature during the aging treatment is preferably 350 to 500°C. Furthermore, the holding time during the aging treatment is preferably 0.5 to 2 hours. Under these conditions, the above effects are more easily and consistently obtained.

[0056] The stainless steel material according to the embodiment of the present invention has a lower Ni content compared to metastable austenitic stainless steel materials such as SUS301 and SUS304, and is therefore less expensive. Furthermore, the stainless steel material according to the embodiment of the present invention has better hot workability compared to conventional duplex stainless steel materials and can achieve high strength. For this reason, the stainless steel material according to the embodiment of the present invention can be used in various applications (for example, high-strength components and spring materials) that require high strength, low cost, and hot workability. [Examples]

[0057] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0058] (Examples 1-6 and Comparative Examples 1-8) Stainless steel having the composition shown in Table 1 (the remainder being Fe and impurities) was melted down by vacuum melting to obtain a steel slab. Next, this steel slab was hot-rolled to obtain a hot-rolled sheet with a thickness of 5 mm. In the hot-rolling process, the temperature immediately after the final pass was set to the temperature shown in Table 2, and it was cooled to 800°C by water cooling (cooling rate of 20°C / sec or more). Next, the hot-rolled sheet was annealed by holding it at approximately 1100°C (annealing temperature) for 30 seconds, and then cooled to 400°C or below by water cooling (cooling rate of 20°C / sec or more) to obtain a hot-rolled annealed sheet. Next, the hot-rolled annealed sheet was cold-rolled at a rolling ratio of 70% to obtain a cold-rolled sheet. Next, the cold-rolled sheet was finished annealed by holding it at 1080°C (annealing temperature) for 30 seconds, and then cooled to 400°C or below by water cooling (cooling rate of 50°C / sec or more) to obtain a cold-rolled annealed sheet. Next, the cold-rolled and annealed sheets were temper-rolled at the rolling ratios shown in Table 2, and then aged at the heating temperatures and holding times shown in Table 2 to obtain stainless steel sheets. In Table 1, the values ​​of DF and Md were calculated using the content of each element and based on the formulas described above. In Table 2, LMP was calculated assuming the material constant C was 20.

[0059] [Table 1] [Table 2]

[0060] The stainless steel plates obtained above were evaluated as follows.

[0061] <Proportion of ferrite, martensite, and austenite phases in stainless steel sheet> After cutting test specimens from stainless steel sheets, the thickness-direction cross-sections parallel to the rolling direction were mirror-polished, and EBSD (backscattered electron diffraction) measurements were performed. EBSD measurements were performed using a scanning electron microscope with the TSL OIM Data Collection 7 software (TSL Solutions Co., Ltd.), measuring a 200 μm square region in the center of the specimen's thickness direction with a step size of 0.3 μm. The proportion of the austenite phase was determined by calculating the area of ​​the fcc phase, dividing it by the total area (sum of the bcc and fcc phases), and multiplying by 100. Furthermore, for the ferrite and martensite phases, the bcc phase data obtained from the EBSD measurement was analyzed using the TSL OIM Analysis 7 software (TSL Solutions Co., Ltd.) to create IQ (Image Quality) images, identifying the ferrite and martensite phases, and calculating their respective areas. The ratio of ferrite phase to martensite phase was determined by dividing the area of ​​the obtained ferrite phase and martensite phase by the total area (sum of bcc phase and fcc phase) and multiplying by 100.

[0062] <Micro-Vickers hardness of the ferrite phase> After descaling the surface of a stainless steel plate by wet polishing, the ferrite phase was identified by electrolytic etching with a KOH aqueous solution. Next, the micro-Vickers hardness of the identified ferrite phase was measured. The micro-Vickers hardness was measured using a micro-Vickers hardness tester in accordance with JIS Z2244-1:2020. The micro-Vickers hardness of the ferrite phase was measured for 50 ferrite phases, and the average was used as the measurement result.

[0063] <Vickers hardness of stainless steel sheet> The Vickers hardness was measured at 50 arbitrary locations on the surface of the stainless steel material, and the average was used as the measurement result. The Vickers hardness was measured using a Vickers hardness tester in accordance with JIS Z2244-1:2020.

[0064] <Breakage Aperture> Round bar specimens with a diameter of 8 mm and a longitudinal length of 110 mm were taken from steel slabs having the composition shown in Table 1. Next, the taken round bar specimens were heated to 900-1000°C in a vacuum atmosphere, and then subjected to a strain rate of 1.3 / s. -1 A tensile test was conducted to fracture the round bar specimen. The reduction of area at fracture was calculated from the diameter of the fracture surface of the fractured round bar specimen and the diameter of the round bar specimen before the test using the following formula. The reduction in size at fracture = (Diameter of the original round bar specimen - Diameter of the fracture surface of the fractured round bar specimen) / Diameter of the original round bar specimen × 100

[0065] The evaluation results are shown in Table 3. In Table 3, the "micro-Vickers hardness" of the ferrite phase is abbreviated as "Vickers hardness".

[0066] [Table 3]

[0067] As shown in Tables 1-3, Examples 1-6 had appropriate compositions of stainless steel sheets (including DF and Md), proportions of ferrite, martensite, and austenite phases, and micro-Vickers hardness of the ferrite phase, resulting in high strength (Vickers hardness of stainless steel sheets of 350-450 HV) and excellent hot workability (reduction of area at fracture of 90% or more). In contrast, Comparative Example 1 had a high content of Cu and N, and a low DF (Defensive Factor), resulting in a high proportion of austenite phase and a low proportion of ferrite phase. As a result, its hot workability was insufficient (fracture reduction value was less than 90%). In Comparative Example 2, the DF was too high, resulting in a high proportion of ferrite phase and a low proportion of martensite and austenite phase. As a result, the strength was insufficient (Vickers hardness of the stainless steel sheet was less than 350 HV). Comparative Example 3 had high Ni and N content and excessively low DF and Md content, resulting in a high proportion of austenite phase and low proportions of ferrite and martensite phase. As a result, its strength and hot workability were insufficient (Vickers hardness of the stainless steel sheet was less than 350 HV, and the reduction of area at fracture was less than 90%). In Comparative Example 4, the rolling ratio during temper rolling was too low, resulting in a high proportion of the austenite phase and a low micro-Vickers hardness of the ferrite phase. Consequently, the strength was insufficient (Vickers hardness of the stainless steel sheet was less than 350 HV). In Comparative Example 5, the rolling ratio during temper rolling was too high, resulting in a high proportion of martensite phase and a low proportion of austenite phase. Furthermore, the LMP (Long Maturation Period) during aging treatment was too low. As a result, the strength became excessively high (the Vickers hardness of the stainless steel sheet exceeded 450 HV). Comparative Example 6 did not undergo aging treatment, resulting in a lower micro-Vickers hardness of the ferrite phase. Consequently, its strength was insufficient (the Vickers hardness of the stainless steel plate was less than 350 HV). In Comparative Example 7, the LMP (Low Molecular Strength) of the ferrite phase was too high due to excessive aging treatment, resulting in a low micro-Vickers hardness. Consequently, the strength was insufficient (Vickers hardness of the stainless steel plate was less than 350 HV). In Comparative Example 8, the LMP (Low Molecular Strength) of the ferrite phase was too low due to excessive aging treatment. As a result, the strength was insufficient (the Vickers hardness of the stainless steel plate was less than 350 HV).

[0068] As can be seen from the above results, the present invention provides a stainless steel material that is high in strength and has excellent hot workability, as well as a method for manufacturing the same.

Claims

1. By mass, it contains C: 0.001-0.050%, Si: 0.01-0.50%, Mn: 1.0-3.5%, P: 0.050% or less, S: 0.0300% or less, Ni: 1.5-2.5%, Cr: 19.6-22.5%, Mo: 0.01-1.00%, Cu: 0.01-0.50%, N: 0.010-0.090%, with the remainder being Fe and impurities. The following formula (1): DF=7.2(Cr+0.88Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9... (1) The DF value shown in the formula (where the element symbol represents the content (mass %) of each element) is between 65.0 and 80.

0. Formula (2) below: Md=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo... (2) The value of Md shown in the formula (where the element symbol represents the content (mass %) of each element) is between 100.0 and 150.0°C. The ferrite phase is 65-80% by volume, the martensite phase is 5-30% by volume, and the austenite phase is 5-20% by volume. A stainless steel material having a micro-Vickers hardness of 300 HV or more in the ferrite phase.

2. The stainless steel material according to claim 1, further comprising one or more elements selected by mass from Nb: 0.010 to 0.500%, Ti: 0.01 to 0.50%, V: 0.01 to 0.50%, W: 0.05 to 0.50%, Co: 0.01 to 0.30%, B: 0.0002 to 0.0050%, Sn: 0.010 to 0.500%, Al: 0.010 to 0.050%, Mg: 0.0002 to 0.0100%, Ca: 0.0002 to 0.0100%, Ta: 0.050% or less, Ga: 0.050% or less, Zr: 0.01 to 0.50%, and REM: 0.0002 to 0.0100%.

3. The stainless steel material according to claim 1 or 2, wherein the Vickers hardness of the stainless steel material is 350 to 450 HV.

4. The stainless steel material according to claim 1 or 2, wherein the reduction of area at break is 90% or more.

5. The stainless steel material according to claim 1 or 2, wherein the thickness of the stainless steel material is 0.2 to 5.0 mm.

6. By mass, it contains C: 0.001-0.050%, Si: 0.01-0.50%, Mn: 1.0-3.5%, P: 0.050% or less, S: 0.0300% or less, Ni: 1.5-2.5%, Cr: 19.6-22.5%, Mo: 0.01-1.00%, Cu: 0.01-0.50%, N: 0.010-0.090%, with the remainder being Fe and impurities. The following formula (1): DF=7.2(Cr+0.88Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9... (1) The DF value shown in the formula (where the element symbol represents the content (mass %) of each element) is between 65.0 and 80.

0. Formula (2) below: Md=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo... (2) A method for manufacturing stainless steel, comprising temper-rolling a rolled material having an Md value of 100.0 to 150.0°C (wherein the formula, the elemental symbols represent the content (mass%) of each element) at a rolling rate of 40 to 80%, and then performing an aging treatment by heating under conditions in which the LMP represented by the following formula (3) becomes 12,000 to 17,000. LMP=(t+273)×(C+log tr) (3) In the formula, t is the heating temperature (°C), C is the material constant, and tr is the holding time at the heating temperature (h).

7. The method for manufacturing stainless steel according to claim 6, wherein the rolled material further comprises one or more selected by mass from Nb: 0.010 to 0.500%, Ti: 0.01 to 0.50%, V: 0.01 to 0.50%, W: 0.05 to 0.50%, Co: 0.01 to 0.30%, B: 0.0002 to 0.0050%, Sn: 0.010 to 0.500%, Al: 0.010 to 0.050%, Mg: 0.0002 to 0.0100%, Ca: 0.0002 to 0.0100%, Ta: 0.050% or less, Ga: 0.050% or less, Zr: 0.01 to 0.50%, and REM: 0.0002 to 0.0100%.

8. The method for manufacturing stainless steel according to claim 6 or 7, wherein the heating temperature in the aging treatment is 350 to 500°C.

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