Material for stainless steel molding, stainless steel molding material, and method for producing stainless steel molding material

Abstract

Provided is a molding material excellent in balance between strength and toughness. The present invention relates to a stainless steel molding material: that has a composition containing, in terms of mass%, 0.070% or less of C, 0.30% or less of Si, 1.0% or less of Mn, 0.040% or less of P, 0.030% or less of S, 14.0-17.5% of Cr, 3.0-6.0% of Ni, 0.01-0.10% of V, 0.100% or less of N, 0.03-0.10% of Mo, 2.5-5.0% of Cu, and 0.15-0.45% of Nb, with the remainder consisting of inevitable impurities and Fe; and that satisfies formula (1). (1): Nieq ≥ 0.923Creq − 8.537

Description

Stainless steel molding material, stainless steel molding material, and method for manufacturing stainless steel molding material

[0001] The present invention relates to a stainless steel molding material used in molding, a stainless steel molding material using said material, and a method for manufacturing said stainless steel molding material.

[0002] In recent years, the application of metal additive manufacturing (hereinafter referred to as AM) technology using 17Cr martensitic stainless steel powder as a raw material has been progressing. The application of AM technology enables the production of parts with more complex shapes than conventional technologies, and is expected to provide added value such as shorter lead times and reduced costs in addition to higher functionality due to increased design freedom. 17Cr martensitic stainless steel is primarily used in the aerospace and industrial machinery fields, where high strength and toughness are required. For example, Patent Document 1 describes a metal additive manufacturing material (AM material) made of 17Cr martensitic stainless steel.

[0003] Patent Document 1 describes a stainless steel additive manufacturing material having a predetermined composition and a structure containing 45% by volume or more of a martensite phase and 25% by volume or less of an austenite phase.

[0004] Japanese Patent Application Publication No. 2022-6584

[0005] S. Sabooni, A. Chabok, SC Feng, H. Blaauw, TC Pijper, HJ Yang and YT Pei: Addit. Manuf., 46 (2021) 102176.,p.5

[0006] However, the AM material of Patent Document 1 does not achieve sufficient yield strength while maintaining good toughness. Furthermore, the hardenability is poor, and the presence of 10% or more of residual ferrite phase deteriorates mechanical properties, resulting in a lack of a sufficient balance between strength and toughness. The present invention was made against the background of the above circumstances, and aims to obtain a shaped material with excellent toughness and strength.

[0007] A stainless steel molding material according to one embodiment of the present disclosure has specific components and a composition in which Ni (Ni equivalent) and Cr (Cr equivalent) satisfy Ni≧0.923Cr − 8.537. The stainless steel molding material according to one embodiment has a martensitic structure as molded. A method for producing a stainless steel molding material according to one embodiment involves creating a molding material having a martensitic structure using a molding material having the composition, and then performing a solution heat treatment and an aging treatment to produce a stainless steel molding material having a martensitic structure.

[0008] According to the present invention, a shaped material having a martensite structure can be obtained even in the as-shaped state, and excellent properties in toughness and strength can be obtained.

[0009] FIG. 1 is a diagram showing the structure morphology according to the relationship between Ni and Cr in a molding material of the present disclosure and a comparative molding material. FIG. 2A is a diagram showing one condition of a solution heat treatment in an example. FIG. 2B is a diagram showing one condition of a solution heat treatment in an example. FIG. 2C is a diagram showing one condition of an aging heat treatment in an example. FIG. 3 is a photograph in lieu of a drawing showing the structure of a test material in an example. FIG. 4 is a graph showing the relationship between strength and toughness of a test material in an example.

[0010] A stainless steel forming material according to an embodiment of the present disclosure contains, in mass%, C: 0.070% or less, Si: 0.30% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Cr: 14.0% or more and 17.5% or less, Ni: 3.0% or more and 6.0% or less, V: 0.01% or more and 0.10% or less, N: 0.100% or less, Mo: 0.03% or more and 0.10% or less, Cu: 2.5% or more and 5.0% or less, and Nb: 0.15% or more and 0.45% or less, with the balance being unavoidable impurities and Fe, and has a composition that satisfies the following formula (1): Ni≧0.923Cr−8.537 (1), where Ni and Cr are calculated by formulas (2) and (3). Nieq=(mass%Ni)+30(mass%C)+0.5(mass%Mn)+0.3(mass%Cu)+25(mass%N)...(2) Creq = (mass % Cr) + 2 (mass % Si) + 1.5 (mass % Mo) + 5 (mass % V) + 1.75 (mass % Nb) + 1.5 (mass % Ti) + 5.5 (mass % Al) (3).

[0011] A stainless steel forming material according to another embodiment of the present disclosure contains, in mass%, C: 0.070% or less, Si: 0.30% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Cr: 14.0% or more and 17.5% or less, Ni: 3.0% or more and 6.0% or less, V: 0.01% or more and 0.10% or less, N: 0.100% or less, Mo: 0.03% or more and 0.10% or less, Cu: 2.5% or more and 5.0% or less, and Nb: 0.15% or more and 0.45% or less, with the balance consisting of inevitable impurities and Fe, and has a composition that satisfies the following formula (1), and has an as-formed structure of a martensitic stainless steel forming material: Nieq≧0.923Creq − 8.537 … (1) where Nieq and Creq are calculated by the formulas (2) and (3): Nieq = (mass% Ni) + 30 (mass% C) + 0.5 (mass% Mn) + 0.3 (mass% Cu) + 25 (mass% N) ... (2) Creq = (mass% Cr) + 2 (mass% Si) + 1.5 (mass% Mo) + 5 (mass% V) + 1.75 (mass% Nb) + 1.5 (mass% Ti) + 5.5 (mass% Al) ... (3).

[0012] A stainless steel forming material according to one embodiment of the present disclosure is a heat-treated stainless steel forming material, containing, in mass%, C: 0.070% or less, Si: 0.30% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Cr: 14.0% or more and 17.5% or less, Ni: 3.0% or more and 6.0% or less, V: 0.01% or more and 0.10% or less, N: 0.100% or less, Mo: 0.03% or more and 0.10% or less, Cu: 2.5% or more and 5.0% or less, and Nb: 0.15% or more and 0.45% or less, with the balance consisting of inevitable impurities and Fe, and having a composition that satisfies the following formula (1): Ni≧0.923Cr−8.537 (1), where Ni and Cr are calculated by formulas (2) and (3). Nieq = (mass% Ni) + 30 (mass% C) + 0.5 (mass% Mn) + 0.3 (mass% Cu) + 25 (mass% N) ... (2) Creq = (mass% Cr) + 2 (mass% Si) + 1.5 (mass% Mo) + 5 (mass% V) + 1.75 (mass% Nb) + 1.5 (mass% Ti) + 5.5 (mass% Al) ... (3) The 0.2% proof stress (0.2% Y.S.; MPa) at room temperature in accordance with JIS Z 2241:2022 and the absorbed energy E (J / cm) in a Charpy impact test at 20°C in accordance with JIS Z2242:2022 are ２ ) satisfies the following formula (5) in the range of 500≦0.2%Y.S.≦1200 MPa: E≧−0.167(0.2%Y.S.)+260...(5).

[0013] A method for manufacturing a stainless steel shaped material according to one embodiment of the present disclosure includes, in mass %, adding C: 0.070% or less, Si: 0.30% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Cr: 14.0% or more and 17.5% or less, Ni: 3.0% or more and 6.0% or less, V: 0.01% or more and 0.10% or less, N: 0.100% or less, Mo: 0.03% or more and 0.10% or less, Cu: 2.5% or more and 5.0% or less, Nb: 0.15% or more and 0.10% or less. The shaping material contains 45% or less of Fe, with the remainder consisting of unavoidable impurities and Fe, and has a composition that satisfies the following formula (1): shaping is performed to have a martensite structure with an average crystal grain size of less than 10 μm; the shaping material is further subjected to solution heat treatment and aging treatment to obtain a martensite structure with an average crystal grain size of less than 15 μm and a martensite volume fraction of 85% or more, with an oxide area fraction of less than 0.2% and an average diameter of 80 μm; and the 0.2% proof stress (Y.S.; MPa) at room temperature in accordance with JIS Z 2241:2022 and absorbed energy E (J / cm) in a Charpy impact test at 20°C in accordance with JIS Z2242:2022 are obtained. ２ ) satisfies the following formula (5) in the range of 500≦0.2%Y.S.≦1200 MPa. Nieq≧0.923Creq−8.537 (1) where Nieq and Creq are calculated by formulas (2) and (3): Nieq=(mass%Ni)+30(mass%C)+0.5(mass%Mn)+0.3(mass%Cu)+25(mass%N) (2) Creq=(mass%Cr)+2(mass%Si)+1.5(mass%Mo)+5(mass%V)+1.75(mass%Nb)+1.5(mass%Ti)+5.5(mass%Al) (3) E≧−0.167(0.2%Y.S.)+260 (5).

[0014]

[0023] An embodiment of the present disclosure will be described below. (Production of a stainless steel forming material) A stainless steel forming material containing, by mass%, C: 0.070% or less, Si: 0.30% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Cr: 14.0% to 17.5%, Ni: 3.0% to 6.0%, V: 0.01% to 0.10%, N: 0.100% or less, Mo: 0.03% to 0.10%, Cu: 2.5% to 5.0%, and Nb: 0.15% to 0.45%, with the balance consisting of inevitable impurities and Fe, is prepared to have a composition that satisfies the following formula (1): Nieq≧0.923Creq − 8.537 (1) where Nieq and Creq are calculated by the formulas (2) and (3). Nieq=(mass% Ni)+30(mass% C)+0.5(mass% Mn)+0.3(mass% Cu)+25(mass% N) (2) Creq=(mass% Cr)+2(mass% Si)+1.5(mass% Mo)+5(mass% V)+1.75(mass% Nb)+1.5(mass% Ti)+5.5(mass% Al) (3)

[0015] Below, we will explain the component composition of the stainless steel molding material (hereinafter sometimes referred to as the "molding material" or "molding material") disclosed herein and the reasons for its limitations.

[0016] C: 0.070% or less C improves the hardness of the as-formed AM material and promotes cracking during forming, so it needs to be kept low. For this reason, the content is set to 0.070% or less in mass% (hereinafter simply referred to as "%." Unless otherwise specified in this specification, "%" represents mass%). More preferably, it is 0.050% or less. Even more preferably, it is 0.025% or less.

[0017] Si: 0.30% or less Si combines with O in the AM material to form Si-based oxides. These oxides become the starting point of fracture during tensile tests and stress loading, significantly reducing toughness, so the content must be kept low. For this reason, the content is set to 0.30% or less. More preferably, it is 0.25% or less. Even more preferably, it is 0.21% or less.

[0018] Mn: 1.0% or less Mn is an element that combines with S to form MnS, which reduces mechanical properties and corrosion resistance. Therefore, the Mn content is set to 1.0% or less, more preferably 0.8% or less, and even more preferably 0.6% or less.

[0019] P: 0.040% or less P is an element that easily segregates at grain boundaries and reduces mechanical properties. Therefore, the content is set to 0.040% or less, more preferably 0.030% or less, and even more preferably 0.025% or less.

[0020] S: 0.030% or less S is an element that forms MnS and reduces mechanical properties. Therefore, the content is set to 0.030% or less, more preferably 0.005% or less, and even more preferably 0.002% or less.

[0021] Cr: 14.0% or more and 17.5% or less. Cr is an element that forms a passive film on the surface of the AM material, contributing to improved corrosion resistance. If the Cr content is less than 14.0%, the desired corrosion resistance cannot be ensured. On the other hand, a large amount of Cr increases Cr, changing the as-formed structure into a coarse ferrite structure, or leaving ferrite after solution treatment, preventing the formation of a uniform martensite structure. For this reason, the content is set to 14.0% or more and 17.5% or less. For the same reason, the lower limit is preferably 15.0% and the upper limit is preferably 16.0%. More preferably, the upper limit is 15.3%.

[0022] Ni: 3.0% or more and 6.0% or less Ni is an element that increases Nieq and transforms the as-formed structure into fine martensite. It also has the effect of increasing the adhesion of the oxide film of Cr oxide, improving corrosion resistance. On the other hand, a large amount of Ni increases the retained austenite fraction of the AM material and reduces the martensite fraction, thereby impairing strength. For this reason, the Ni content is set to 3.0% or more and 6.0% or less. For the same reason, the lower limit is preferably 4.5% and the upper limit is preferably 5.5%. More preferably, the lower limit is 4.8% and the upper limit is 5.2%.

[0023] V: 0.01% or more and 0.10% or less V contributes to improving strength as a solid solution element. It also bonds with C and N to form fine V carbonitrides, preventing grain coarsening during heat treatment and contributing to the refinement of the martensite structure. To achieve this effect, a V content of 0.01% or more is required. On the other hand, a large amount of V increases Cr and changes the as-formed structure to a ferrite structure, or forms coarse V carbonitrides, degrading mechanical properties. Therefore, the V content is set to 0.01% or more and 0.10% or less. For the same reason, the lower limit is preferably 0.03% and the upper limit is preferably 0.08%. More preferably, the lower limit is 0.04% and the upper limit is 0.06%.

[0024] N: 0.100% or less If N is contained in a large amount, it forms NbCrN (Z phase) and nitrides, which reduces toughness. Therefore, the N content is set to 0.100% or less, preferably 0.040% or less, and more preferably 0.025% or less.

[0025] Mo: 0.03% or more and 0.10% or less Mo has the effect of improving the toughness and hardenability of AM materials. To obtain these effects, a Mo content of 0.03% or more is required. On the other hand, a large amount of Mo increases Cr, changing the as-formed structure to ferrite. For this reason, the Mo content is set to 0.03% or more and 0.10% or less. For the same reason, the lower limit is preferably 0.05% and the upper limit is preferably 0.08%. More preferably, the lower limit is 0.06%.

[0026] Cu: 2.5% or more and 5.0% or less Cu forms fine precipitates by aging heat treatment, improving strength. However, if the content is less than 2.5%, this effect is not fully obtained, and the 0.2% yield strength (0.2% Y.S.; MPa) is insufficient. However, if the content exceeds 5.0%, Cu does not fully dissolve in the solid solution treatment, and coarse Cu precipitates, degrading mechanical properties. For this reason, the content is set to 2.5% or more and 5.0% or less. Preferably, it is 3.0% or more. More preferably, it is 3.6% or more.

[0027] Nb: 0.15% or more and 0.45% or less Nb is an element that combines with C and N to precipitate as fine carbonitrides, preventing grain coarsening during heat treatment and contributing to the refinement of the martensite structure. To achieve this effect, a Nb content of 0.15% or more is required. On the other hand, a large amount of Nb changes the as-formed structure to a ferrite structure, and the coarse carbonitrides further degrade mechanical properties. Therefore, the Nb content is set to 0.15% or more and 0.45% or less. For the same reason, the lower limit is preferably 0.20% and the upper limit is preferably 0.35%. More preferably, the upper limit is 0.33%.

[0028] (Inevitable Impurities) In the alloy composition of the present disclosure, the balance consists of inevitable impurities and Fe. The inevitable impurities may include Al, Ti, O, etc., and the contents of Al, Ti, and O are exemplified below. Note that the inevitable impurities are not limited to the following components.

[0029] Al: 0.100% or less If Al is contained in a large amount, coarse oxides are formed, which reduces toughness. Therefore, the Al content is preferably 0.100% or less, more preferably 0.010% or less, and even more preferably 0.005% or less.

[0030] Ti: 0.05% or less If Ti is contained in large amounts, it forms coarse oxides and reduces toughness. Therefore, the Ti content is preferably 0.05% or less, more preferably 0.03% or less, and even more preferably 0.01% or less.

[0031] O: 0.100% or less O combines with other elements in the AM material to form coarse oxides, which reduces toughness. Therefore, the O content is preferably 0.100% or less, more preferably 0.050% or less, and even more preferably 0.030% or less.

[0032] Nieq≧0.923Creq−8.537 (1) where Nieq and Creq are calculated by equations (2) and (3). Nieq=(mass%Ni)+30(mass%C)+0.5(mass%Mn)+0.3(mass%Cu)+25(mass%N) (2) Creq=(mass%Cr)+2(mass%Si)+1.5(mass%Mo)+5(mass%V)+1.75(mass%Nb)+1.5(mass%Ti)+5.5(mass%Al) (3) where equations (2) and (3) are referenced from Non-Patent Document 1.

[0033] AM materials that satisfy formula (1) have a fine martensitic structure as-formed, and exhibit excellent toughness. It is desirable for the martensitic structure to have an average grain size of less than 10 μm. If the average grain size of the as-formed structure is 10 μm or more, it is not possible to obtain a martensitic structure with an average grain size of less than 15 μm after solution heat treatment, making it difficult to achieve a sufficient balance between strength and toughness.

[0034] The material can be melted by a conventional method, and can be prepared in the form of a block or powder, and is used as a molding material for molding.

[0035] (Production of molding material) The molding material can be used for metal additive manufacturing, metal powder injection molding, thermal spray surface treatment, and the like.

[0036] For materials having the amounts of components disclosed herein (those that satisfy formula (1) and those that do not), the as-formed structural morphology is shown in FIG. 1 in relation to the relationship between Cr and Ni, which is related to hardenability. As is clear from FIG. 1, in the relationship between Ni and Cr, in the region where Ni is equal to or greater than (0.923Cr - 8.537), the structure is a martensite structure, and in the region where Ni is smaller than (0.923Cr - 8.537), the structure is a mixed phase of martensite and ferrite or a ferrite phase, and the significance of the above formula is recognized. Note that a martensite structure is considered to be a martensite structure when the area fraction of the martensite structure is 85% or more.

[0037] In metal additive manufacturing, for example, a block or powdered stainless steel molding material is prepared as the raw material. The preparation of the raw material is not particularly limited and can be carried out by any appropriate method. The raw material is melted and additively manufactured. The molding can be carried out without employing any special method, and methods such as discharging the molten raw material through a nozzle or melting the material using a laser or the like to create the shape can be employed. The molding material can be left to cool as is, or, if necessary, can be rapidly cooled by a cooling means. Regardless of the cooling method, the resulting molding material has a martensitic structure and a fine grain structure with an average diameter of less than 10 μm. This result is due to the composition of the molding material.

[0038] The shaped material can be subjected to heat treatment as desired. Examples of heat treatment include solution heat treatment and aging heat treatment. Conditions for solution heat treatment include holding the material at a temperature of 950°C to 1150°C for 0.5 to 10 hours, followed by cooling to room temperature at a rate of 90°C / h or more. Conditions for aging heat treatment include a heat treatment method that results in a tempering parameter P of 17600 to 19500, where P is calculated using equation (4). T is temperature (K), and t is heat treatment time (h). P = T (log (t) + 20) ... (4) That is, in the method for producing a stainless steel shaped material, the solution treatment is preferably performed under conditions of holding at a temperature of 950°C or higher and 1150°C or lower for 0.5 hours or higher and 10 hours or lower, and then cooling to room temperature at a rate of 90°C / h or higher, and the aging treatment is preferably performed under conditions such that the tempering parameter P after the solution heat treatment is 17600 or higher and 19500 or lower, where P is calculated using equation (4), where T is the temperature (K), and t is the heat treatment time (h): P = T (log (t) + 20) ... (4). The stainless steel shaped material may be subjected to solution heat treatment by holding the material at a temperature of 950°C to 1150°C for 0.5 to 10 hours, followed by cooling to room temperature at a rate of 90°C / h or more, followed by aging heat treatment under conditions such that the tempering parameter P is 17,600 to 19,500. The resulting structure may be a martensitic structure. P is calculated using equation (4), where T is temperature (K) and t is heat treatment time (h): P = T (log(t) + 20) (4). After the solution heat treatment and aging heat treatment, the shaped material has a martensitic structure, preferably with an area fraction of 85% or more, and more preferably with an area fraction of 90%. It is desirable for the stainless steel shaped material to have an average grain size of less than 15 μm after the solution heat treatment and aging heat treatment. If the average crystal grain size in the entire structure is 15 μm or more or the martensite structure fraction is less than 85%, the tensile properties and toughness will decrease.

[0039] The heat-treated molding material has excellent strength and toughness, and the balance between these two satisfies the following formula (5): E (J / cm ２ ) ≧−0.167(0.2%Y.S.)+260 … (5) where 0.2%Y.S. is the yield strength (MPa) at room temperature in accordance with JIS Z 2241:2022, and E is the absorbed energy (J / cm) in a Charpy impact test at 20°C in accordance with JIS Z2242:2022. ２ ) Preferably, E (J / cm ２ ) ≧ −0.167(0.2%Y.S.) + 300. More preferably, the 0.2%Y.S. is in the range of 600 to 1150 MPa. Even more preferably, after solution heat treatment and aging heat treatment, the 0.2% proof stress at room temperature in accordance with JIS Z 2241:2022 is 700 MPa or more, and the absorbed energy in a Charpy impact test at 20°C in accordance with JIS Z2242:2022 is 150 J / cm ２ The stainless steel molding material may be a metal additive manufacturing material, and the molding may be performed by metal additive manufacturing.

[0040] Furthermore, it is desirable that the oxides in the shaped body after the solution heat treatment and aging heat treatment have an area ratio of less than 0.2% and an average diameter of the oxides of less than 80 nm. Since oxides formed by Si and the like reduce toughness, it is desirable to set an upper limit on the area ratio, and the average diameter of the oxides is desirably less than 80 nm. Coarse oxides are prone to cracking. It is also desirable that the martensite fraction of the shaped body after the solution heat treatment and aging heat treatment is 85% by volume or more.

[0041] Hereinafter, a method for manufacturing a martensitic steel shaped material by metal additive manufacturing will be described in detail using examples and comparative examples.

[0042] A molding material with the composition shown in Table 1 was prepared and classified into powder with a particle size distribution of 20 to 53 μm using a known electric sieve. Six types of test materials A to F were molded from the above-mentioned stainless steel metal powder using a commercially available laser additive manufacturing device (powder bed fusion method). The molding parameters were: output: 245 W, scanning speed: 910 mm / sec, layer pitch: 30 μm, and hatch pitch: 100 μm. Test materials A and B have a Si content of 0.30% or less and satisfy formula (1) (invention examples). Test material C has a Si content exceeding 0.30% and does not satisfy formula (1) (comparative example). Test materials D to F have a Si content exceeding 0.30% but satisfy formula (1) (comparative example).

[0043]

[0044] Each test material was subjected to the solution heat treatment (i) shown in Fig. 2A, and then to the aging heat treatment shown in Fig. 2C and Table 2. For test material C only, two types were prepared: one was subjected to the solution heat treatment (i), and the other was subjected to the solution heat treatment (ii) shown in Fig. 2B, which included an additional heat treatment step, and then each was subjected to the aging treatment shown in Fig. 2C.

[0045]

[0046] Test specimens were taken from each as-formed specimen, each solution-treated specimen (after solution treatment and before aging treatment), and each solution-treated and aging-treated specimen (after solution treatment and aging treatment), so that the cross section perpendicular to the forming direction served as the observation surface. Each specimen was then polished using diamond paste and a colloidal silica suspension. Microstructure observation was then performed using a scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) (field of view: 400 μm × 600 μm), and the martensite phase fraction and average grain size (area basis) were calculated using EDAX OIM Analysis™ software. When calculating the martensite phase fraction, the SEM acceleration voltage was 15 kV, the EBSD analysis magnification was 3000x, and the step size was 0.07 μm. When calculating the average crystal grain size, the SEM acceleration voltage was 15 kV, the EBSD analysis magnification was 200x, and the step size was 1 μm. From the area A of the crystal grain image defined by the grain boundaries with a misorientation of 15° or more, the crystal grain size r was calculated using equation (6), and the average value of r of the crystal grains in the crystal grain image was taken as the average crystal grain size. r = (4A / π)^(1 / 2) ... (6) Furthermore, the area ratio and average diameter of the oxide were investigated from the SEM backscattered electron image using WinROOF (trademark) image analysis software. The SEM acceleration voltage was 15 kV, and the magnification of the SEM image was 5000x.

[0047] Next, for each test material after solution heat treatment and aging treatment, a tensile test piece with a parallel portion 6 mm in diameter was prepared so that the tensile direction was perpendicular to the building direction, and a tensile test was performed at room temperature in accordance with JIS Z 2241:2022. Two or three Charpy test pieces were taken from each test material, and a Charpy impact test was performed at 20°C in accordance with JIS Z 2242:2022. For the Charpy impact test, a test piece with a length of 55 mm, a square cross section with sides of 10 mm, and a V-notch with a depth of 2 mm in the center was used. The impact direction was parallel to the building direction. After the test, the average absorbed energy was calculated using a 0.8 cm2 cross-sectional area of ​​the test piece. ２ Divide by the impact value (J / cm ２ )

[0048] Grain maps (micrographs) of each test material as-formed and each test material after solution heat treatment (after solution heat treatment, before aging heat treatment) are shown in Figure 3. The average grain size of each test material obtained from Figure 3 is shown in Table 3. The above grain maps are maps obtained from the results of EBSD analysis, and show images of crystal grains defined by grain boundaries with a misorientation of 15° or more.

[0049] In Comparative Examples 1 and 2, coarse ferrite with an average grain size of 10 μm or more was observed in the as-formed samples. On the other hand, fine martensite was observed in the other test materials. After solution treatment, coarse martensite with an average grain size of 15 μm or more, in which the as-formed structure's grain morphology remained, was observed in Comparative Example 1, while fine martensite was observed in all other test materials. From the above, it can be seen that test materials with a composition satisfying formula (1) become fine martensite after solution treatment. Furthermore, a comparison between Comparative Examples 1 and 2 revealed that a fine martensite structure can be obtained by increasing the number of solution treatment steps. The grain map after aging treatment showed almost no change compared to before aging treatment, so it is omitted.

[0050] The 0.2% proof stress and impact value of each test material after the solution treatment and aging treatment are shown in Table 3. The 0.2% proof stress (MPa) and impact value (J / cm) of each test material after the solution treatment and aging treatment are also shown in Table 3. ２ ) is shown in Figure 4. The test material of the present disclosure satisfies formula (5), and it has become clear that formula (5) is significant. Formula (5) shows a straight line indicating the boundary between the region of 0.2% yield strength and impact value exhibited by known materials and the region exceeding this. In the examples, the 0.2% yield strength (0.2% Y.S.; MPa) at room temperature and the absorbed energy E (J / cm) in a Charpy impact test at 20°C are ２) tendency. E≧-0.167(0.2%Y.S.)+260...(5) Furthermore, the examples satisfy the following: E≧-0.167(0.2%Y.S.)+300...(5A). This relationship (5A) shows a tendency closer to the 0.2% yield strength and absorbed energy values ​​of the examples. The relationship between yield strength and impact value can be changed by changing the heat treatment conditions. For example, increasing the solution temperature and increasing the grain size tends to decrease both the 0.2%Y.S. and impact value. Furthermore, the relationship between yield strength and impact value can be changed by changing the component composition, etc., within the composition range. For example, reducing the content of Cu, a major component that affects strength, decreases yield strength. Furthermore, changing the oxide area ratio or the average oxide diameter also affects the relationship between yield strength and impact value. These effects enable the relationship between yield strength and impact value to satisfy formula (5), and further, formula (5A) can be satisfied.

[0051] In Comparative Example 1, the area ratio and average grain size of the Si-based oxides were high due to the high Si and O contents. In addition, because Ni did not satisfy formula (1), the structure after solution heat treatment and aging heat treatment became coarse martensite with an average crystal grain size of 15 μm or more. Therefore, the strength-toughness balance was the worst. In Comparative Example 2, Ni did not satisfy formula (1), but the structure was able to be refined by increasing the number of heat treatment steps. Therefore, the strength-toughness balance was improved compared to Comparative Example 1. However, because the area ratio and average grain size of the Si-based oxides were high, the strength-toughness balance was poor. In Comparative Examples 3 to 5, Ni satisfied formula (1), so the structure after solution heat treatment and aging heat treatment became fine martensite. However, because the Si and O contents were high, the area ratio and average grain size of the Si-based oxides were high, and the strength-toughness balance was poor. In contrast, Inventive Examples 1 and 2 satisfied the conditions of the present invention and therefore had an excellent balance of strength and toughness on the low strength side (a region in which the 0.2% proof stress is 1200 MPa or less). In these examples, a structure having a martensite structure with an area fraction of 85% is shown as a martensite structure.

[0052]

[0053] As described above, by controlling the component composition and Si content in the present disclosure, a martensite structure can be obtained in the as-formed state, and by increasing the fraction of fine martensite to 85% or more by heat treatment, the martensite is further refined, and the oxides are refined, thereby providing a powder and a shaped body with an excellent balance of strength and toughness.

[0054] According to the present disclosure, a shaped material having a martensitic structure can be obtained even in the as-shaped state, and excellent properties in toughness and strength can be obtained.

[0055] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on a Japanese patent application (Patent Application No. 2024-127651) filed on August 2, 2024, the contents of which are incorporated herein by reference.

Claims

1. A stainless steel forming material containing, by mass%, C: 0.070% or less, Si: 0.30% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Cr: 14.0% or more and 17.5% or less, Ni: 3.0% or more and 6.0% or less, V: 0.01% or more and 0.10% or less, N: 0.100% or less, Mo: 0.03% or more and 0.10% or less, Cu: 2.5% or more and 5.0% or less, Nb: 0.15% or more and 0.45% or less, with the balance consisting of inevitable impurities and Fe, and having a composition that satisfies the following formula (1): Nieq≧0.923Creq − 8.537 ... (1), where Nieq and Creq are calculated using formulas (2) and (3). Nieq=(mass%Ni)+30(mass%C)+0.5(mass%Mn)+0.3(mass%Cu)+25(mass%N)...(2) Creq = (mass % Cr) + 2 (mass % Si) + 1.5 (mass % Mo) + 5 (mass % V) + 1.75 (mass % Nb) + 1.5 (mass % Ti) + 5.5 (mass % Al) (3).

2. A stainless steel forming material containing, by mass%, C: 0.070% or less, Si: 0.30% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Cr: 14.0% or more and 17.5% or less, Ni: 3.0% or more and 6.0% or less, V: 0.01% or more and 0.10% or less, N: 0.100% or less, Mo: 0.03% or more and 0.10% or less, Cu: 2.5% or more and 5.0% or less, Nb: 0.15% or more and 0.45% or less, with the balance consisting of inevitable impurities and Fe, and having a composition that satisfies the following formula (1), and whose structure in the as-formed state is a martensitic structure: Nieq≧0.923Creq − 8.537 ... (1), where Nieq and Creq are calculated by formulas (2) and (3). Nieq=(mass%Ni)+30(mass%C)+0.5(mass%Mn)+0.3(mass%Cu)+25(mass%N)...(2) Creq = (mass % Cr) + 2 (mass % Si) + 1.5 (mass % Mo) + 5 (mass % V) + 1.75 (mass % Nb) + 1.5 (mass % Ti) + 5.5 (mass % Al) (3).

3. The stainless steel shaped material according to claim 2, wherein the average crystal grain size is less than 10 μm.

4. The stainless steel forming material according to claim 2, wherein the material is held at a temperature of 950°C to 1150°C for 0.5 to 10 hours, then subjected to solution heat treatment under conditions of cooling to room temperature at a rate of 90°C / h or more, and then subjected to aging heat treatment under conditions of a tempering parameter P of 17600 to 19500, resulting in a martensitic structure, where P is calculated using equation (4), where T is the temperature (K), and t is the heat treatment time (h): P = T (log (t) + 20) ... (4).

5. The stainless steel shaped material according to claim 4, wherein the average grain size after the solution heat treatment and aging heat treatment is less than 15 μm.

6. A stainless steel shaped material according to claim 4 or 5, wherein the oxides in the shaped body after the solution heat treatment and aging heat treatment have an area ratio of less than 0.2% and an average diameter of the oxides of less than 80 nm.

7. The stainless steel shaped material according to claim 4 or 5, wherein the martensite fraction of the shaped body after the solution heat treatment and aging heat treatment is 85% by volume or more.

8. A heat-treated stainless steel forming material containing, by mass%, C: 0.070% or less, Si: 0.30% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Cr: 14.0% to 17.5%, Ni: 3.0% to 6.0%, V: 0.01% to 0.10%, N: 0.100% or less, Mo: 0.03% to 0.10%, Cu: 2.5% to 5.0%, Nb: 0.15% to 0.45%, with the balance consisting of inevitable impurities and Fe, and having a composition that satisfies the following formula (1): Nieq≧0.923Creq − 8.537 … (1), where Nieq and Creq are calculated using formulas (2) and (3). Nieq = (mass% Ni) + 30 (mass% C) + 0.5 (mass% Mn) + 0.3 (mass% Cu) + 25 (mass% N) ... (2) Creq = (mass% Cr) + 2 (mass% Si) + 1.5 (mass% Mo) + 5 (mass% V) + 1.75 (mass% Nb) + 1.5 (mass% Ti) + 5.5 (mass% Al) ... (3) The 0.2% proof stress (0.2% Y.S.; MPa) at room temperature in accordance with JIS Z 2241:2022 and the absorbed energy E (J / cm) in a Charpy impact test at 20°C in accordance with JIS Z2242:2022 are ２ ) satisfies the following formula (5) in the range of 500≦0.2%Y.S.≦1200 MPa: E≧−0.167(0.2%Y.S.)+260...(5).

9. The stainless steel shaped material according to claim 8, wherein the 0.2% yield strength (0.2% Y.S.; MPa) is in the range of 600 MPa or more and 1150 MPa or less.

10. The 0.2% yield strength at room temperature in accordance with JIS Z 2241:2022 is 700 MPa or more, and the absorbed energy in a Charpy impact test at 20°C in accordance with JIS Z2242:2022 is 150 J / cm ２ The stainless steel shaped material according to claim 8, wherein the stainless steel shaped material is a shaped material having the above structure.

11. The stainless steel forming material according to any one of claims 2 to 5 and 8 to 10, which is a metal additive manufacturing material.

12. In mass%, C: 0.070% or less, Si: 0.30% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Cr: 14.0% or more and 17.5% or less, Ni: 3.0% or more and 6.0%. Below, V: 0.01% or more and 0.10% or less, N: 0.100% or less, Mo: 0.03% or more and 0.10% or less, Cu: 2.5% or more and 5.0% or less, Nb: 0.15% or more and 0.45% or less, and the remainder is A shaping material is made of inevitable impurities and Fe and has a composition that satisfies the following formula (1), and is used to shaping a martensite structure with an average crystal grain size of less than 10 μm. The shaping material is then subjected to solution heat treatment and aging treatment to obtain a martensite structure with an average crystal grain size of less than 15 μm and a martensite volume fraction of 85% or more, an oxide area fraction of less than 0.2%, and an average diameter of 80 μm. The 0.2% proof stress (Y.S.; MPa) at room temperature in accordance with JIS Z 2241:2022 and absorbed energy E (J / cm) in a Charpy impact test at 20°C in accordance with JIS Z2242:2022 are obtained. ２ %Y.S.) satisfies the following formula (5) in the range of 500≦0.2%Y.S.≦1200 MPa: Nieq≧0.923Creq − 8.537 … (1) where Nieq and Creq are calculated by formulas (2) and (3): Nieq=(mass%Ni)+30(mass%C)+0.5(mass%Mn)+0.3(mass%Cu)+25(mass%N … (2) Creq=(mass%Cr)+2(mass%Si)+1.5(mass%Mo)+5(mass%V)+1.75(mass%Nb)+1.5(mass%Ti)+5.5(mass%Al) … (3) E≧−0.167(0.2%Y.S.)+260 … (5).

13. A method for producing a stainless steel shaped material as set forth in claim 12, wherein the solution treatment is performed under conditions of holding the material at a temperature of 950°C to 1150°C for 0.5 to 10 hours, followed by cooling to room temperature at a rate of 90°C / h or more, and the aging treatment is performed under conditions such that the tempering parameter P after the solution treatment is 17,600 to 19,500, inclusive, wherein P is calculated using equation (4), where T is the temperature (K) and t is the heat treatment time (h): P = T (log (t) + 20) ... (4).

14. A method for manufacturing a stainless steel shaped material according to claim 12 or 13, wherein the shaping is metal additive manufacturing.

Images