Metal injection molding 440C stainless steel and preparation method thereof

Abstract

The invention relates to the technical field of stainless steel materials, in particular to metal injection molding 440C stainless steel and a preparation method of the metal injection molding 440C stainless steel. The content of Cr is 16-18%; the content of Si is 0.6 to 0.8 percent; the content of Mn is 0.6 to 0.8 percent; the content of Mo is 0.45 to 0.55 percent; the content of Nb is 2.98%-3.02%; the O content is less than or equal to 0.25%; the P content is less than or equal to 0.02%; the S content is less than or equal to 0.01%; the single content of other impurity elements is less than or equal to 0.03%; the total content of other impurity elements is less than or equal to 0.15%; the stainless steel adopts a specific preparation process, and has high hardness and high wear resistance.

Description

Technical Field

[0001] The present invention relates to the technical field of stainless steel materials, and in particular to metal injection molded 440C stainless steel and a preparation method thereof. Background Art

[0002] 440C is a high-carbon, high-chromium martensitic stainless steel. After quenching and tempering, its hardness can reach 58HRC (654HV). It has good hardness and wear resistance. 440C stainless steel also has good corrosion resistance and is one of the hardest stainless steels. It is widely used in corrosive environments and non-lubricated conditions, such as bearing steel in the aerospace field. It is also often used to make high-quality knives such as scissors and scalpels.

[0003] Metal injection molding (MIM) is an emerging powder metallurgy process. It is a near-net-shape technology that integrates modern plastic injection molding concepts into traditional powder metallurgy. It is used to manufacture parts with complex geometries and high dimensional accuracy. While inheriting the material savings, uniform crystallization, and cutting-free nature of traditional powder metallurgy, it also combines the high precision and complexity of plastic injection molded products. The heat treatment process for MIM-molded 440C stainless steel generally follows that of as-cast 440C stainless steel. However, the microstructure of MIM-molded 440C before heat treatment (after sintering) differs significantly from that of as-cast material. Ring-shaped chromium carbides are prevalent in the MIM-molded 440C microstructure, and these carbides retain a significant amount of carbon. Conventional heat treatment processes prevent carbon from fully dissolving into the austenite matrix, significantly limiting the formation of high-carbon martensite and significantly reducing the hardness and wear resistance of MIM-molded 440C stainless steel.

[0004] Therefore, there is an urgent need to provide a metal injection molded 440C stainless steel with high hardness and high wear resistance and a preparation method thereof. Summary of the Invention

[0005] The present invention aims to solve the technical problem of how to provide a 440C stainless steel with high hardness and high wear resistance and a preparation method thereof.

[0006] In order to achieve the above object, the first aspect of the present invention provides a metal injection molded 440C stainless steel, wherein the components and their weight percentages in the stainless steel are as follows: C content is 1.15-1.25%; Cr content is 16-18%; Si content is 0.6-0.8%; Mn content is 0.6-0.8%; Mo content is 0.45-0.55%; Nb content is 2.98-3.02%; O content ≤ 0.25%; P content ≤ 0.02%; S content ≤ 0.01%; The content of other impurity elements is ≤0.03%; The total content of other impurity elements is ≤0.15%; The balance is Fe; The hardness of the metal injection molded 440C stainless steel is 720-830 HV.

[0007] The HV is Vickers hardness.

[0008] A second aspect of the present invention provides a method for preparing the above-mentioned metal injection molded 440C stainless steel, wherein the method comprises: S1. Inject, degrease and sinter 440C stainless steel powder to obtain sintered samples; S2. The sintered sample is first heated to 800-900°C, kept at this temperature for 1-48 hours, and then cooled. From the first heating temperature to 200°C, the relationship between the cooling time and the temperature is: , where t is the cooling time in min, T(t) is the temperature at the corresponding moment in °C, To keep the temperature warm, cool from 200℃ to 20-30℃ and use furnace cooling; S3, heating for the second time to an austenitic state, cooling to restore the temperature to 20-30° C., heating for the third time, and cooling to obtain the stainless steel.

[0009] The preparation method of metal injection molded 440C stainless steel provided by the present invention has the following heat treatment principles: The Ac1 temperature of 440C stainless steel is approximately 815°C. The sintered 440C is then subjected to step S2. The specific holding temperature selected for S2 allows carbon to fully diffuse into the matrix, providing the elemental foundation for the subsequent formation of high-hardness cryptocrystalline martensite. Furnace cooling is then used to induce a primarily diffusion-type phase transformation (such as bainite and pearlite formation) in the material, which serves as the foundation for the martensite transformation in step S3.

[0010] If the S1+S3 process is used, acicular martensite is ultimately formed due to the low carbon content of the matrix. If the S1+S2+S3 process is used, the S2 step provides a good microstructural foundation, resulting in cryptocrystalline martensite after the S3 step. Compared to the acicular martensite obtained by traditional processes, the cryptocrystalline martensite produced by this process has higher hardness.

[0011] Compared with existing technologies, this invention addresses the problem of a hardness gap between metal injection molded 440C stainless steel and as-cast 440C stainless steel samples. The metal injection molded 440C stainless steel treated using this process exhibits improved performance compared to samples obtained using the original as-cast 440C stainless steel heat treatment process, reaching a hardness of 720-830 HV. Samples produced using the original heat treatment process have a hardness of 672-693 HV. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 This is a process flow chart of heat treatment of 440C stainless steel by metal injection molding according to the present invention; Figure 2 The microstructure of S1; Figure 3 This is the microstructure morphology after Example 1S2; Figure 4 This is the microstructure morphology after the final heat treatment of Example 1; Figure 5 This is the microstructure morphology after the final heat treatment of Example 8; Figure 6 This is the microstructure morphology after the final heat treatment of Example 10; Figure 7 This is the microstructure morphology after the final heat treatment of Example 12; Figure 8 This is the microstructure morphology of Comparative Example 1 after final heat treatment; Figure 9 This is the microstructure morphology of Comparative Example 4 after final heat treatment; Figure 10 This is the microstructure morphology of comparative example 5 after final heat treatment; Figure 11 This is the microstructure morphology of comparative example 6 after final heat treatment. DETAILED DESCRIPTION

[0013] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0014] The hardness and wear resistance of 440C stainless steel produced by metal injection molding cannot meet the requirements. In the existing technology, the method of improving the hardness and wear resistance of 440C stainless steel produced by metal injection molding is to add a large amount of carbon element in the raw material preparation or change the sintering temperature. However, adding carbon will affect the stability during the subsequent sintering process, making it difficult to produce. Changing the sintering temperature will cause deformation. Moreover, the hardness and wear resistance of 440C stainless steel produced by metal injection molding using the existing technology are not significantly improved.

[0015] To improve the hardness and wear resistance of metal injection molded 440C stainless steel, the inventors discovered that the microstructure of 440C stainless steel produced by metal injection molding (MIM) differs significantly from that of conventional cast 440C stainless steel before heat treatment. Existing heat treatment processes for 440C stainless steel produced by MIM cannot release the carbon fixed by the ring-shaped chromium carbides generated during sintering, limiting the formation of high-carbon martensite and reducing the hardness and wear resistance of the 440C stainless steel.

[0016] The inventors found that during quenching (the second heating in step S3 ), extending the second heating time can only allow carbon to form separate carbides during diffusion, and cannot form cryptocrystalline martensite with high hardness.

[0017] Therefore, the inventors envisioned adjusting the microstructure of 440C stainless steel produced by metal injection molding through process treatment, thereby improving the hardness and wear resistance of the 440C stainless steel produced by metal injection molding. To this end, the inventors discovered that if a heat treatment at a specific temperature and time is performed after the sintering state, the hardness and wear resistance of the 440C stainless steel produced by metal injection molding can be improved.

[0018] The inventors discovered that the ring-shaped chromium carbides formed during the sintering of metal injection molded 440C stainless steel are easily destroyed by holding at 800-900°C. Holding at temperatures too low, such as 700°C, makes the ring-shaped chromium carbides difficult to destroy; while holding at temperatures too high, such as 1000°C, causes the released carbon to form new carbides. Both of these factors deplete the final martensite carbon content, making it difficult to obtain high-hardness cryptocrystalline martensite.

[0019] Finally, it was discovered that holding the sintered sample of MIM 440C stainless steel at 800-900°C before S3 quenching can destroy the distributed chromium carbides and release carbon. Furthermore, by controlling the cooling rate after holding, the released carbon does not form separate carbides, but is instead dissolved into the matrix. This carbon promotes the formation of cryptocrystalline martensite during subsequent quenching, resulting in high-hardness MIM 440C stainless steel.

[0020] Therefore, the present invention provides a method for preparing metal injection molded 440C stainless steel, and the prepared metal injection molded 440C stainless steel has excellent hardness and wear resistance.

[0021] Specifically, the first aspect of the present invention provides a metal injection molded 440C stainless steel, wherein the components and their weight percentages in the stainless steel are: C content is 1.15-1.25%; Cr content is 16-18%; Si content is 0.6-0.8%; Mn content is 0.6-0.8%; Mo content is 0.45-0.55%; Nb content is 2.98-3.02%; O content ≤ 0.25%; P content ≤ 0.02%; S content ≤ 0.01%; The balance is Fe; The hardness of the metal injection molded 440C stainless steel is 720-830 HV.

[0022] S1. Inject, degrease and sinter 440C stainless steel powder to obtain sintered samples; S2. The sintered sample is first heated to 800-900°C, kept at this temperature for 1-48 hours, and then cooled. From the first heating temperature to 200°C, the relationship between the cooling time and the temperature is: , where t is the cooling time in min, T(t) is the temperature at the corresponding moment in °C, To keep the temperature warm, cool from 200℃ to 20-30℃ and use furnace cooling; S3, heating for the second time to an austenitic state, cooling to restore the temperature to 20-30° C., heating for the third time, and cooling to obtain the stainless steel.

[0023] S2 and S3 are the heat treatment processes in this article. Figure 1 This is the heat treatment process flow chart.

[0024] In the present invention, ring-shaped chromium carbides are commonly found in the sintered state after S1 treatment. These carbides fix a large amount of carbon elements. Figure 2 After S2 treatment, the hardness of the sample in this state is 550-580HV. At this time, carbon is fully diffused, and some diffusion-type transformation products (bainite, pearlite) are obtained in the S2 process, which provides the organizational basis for the subsequent steps. Figure 3 This is the microstructure diagram after S2.

[0025] In step S2, after a specific first heating temperature and holding time, cooling is performed with a specific relationship between the cooling time and the holding temperature. If the cooling rate is too fast, the released carbon element will eventually form a separate carbide; if the cooling rate is too slow, the performance will not be infinitely improved with the cooling rate. The cooling with a specific relationship between the limited cooling time and the holding temperature provided by the present invention can allow the carbon element to be melted into the matrix. These carbon elements will promote the formation of cryptocrystalline martensite during subsequent quenching, thereby obtaining high-hardness metal injection molded 440C stainless steel.

[0026] In the present invention, in step S1, the injection, degreasing and sintering may adopt conventional parameters in the art as long as a sintered sample can be obtained.

[0027] Specifically, the injection conditions include: mixing 440C stainless steel powder with a thermoplastic polymer binder and then injecting it into the mold.

[0028] Degreasing conditions include solvent degreasing and thermal degreasing. Solvent degreasing involves placing the injection sample in a solvent such as dichloromethane, while thermal degreasing involves placing the sample in a tube furnace with a protective atmosphere such as argon or hydrogen.

[0029] The sintering conditions include heating the degreased sample to 1200-1250°C under vacuum or argon and keeping the temperature.

[0030] According to the present invention, the second heating conditions include: a heating temperature of 1030-1050° C. and a holding time of 10-30 minutes.

[0031] According to the present invention, the cooling treatment is any one of liquid nitrogen cooling, water cooling, oil cooling or air cooling.

[0032] According to the present invention, the liquid nitrogen cooling conditions include: the liquid nitrogen cooling and holding time is 2-6 hours, and the liquid nitrogen is cooled to -196°C.

[0033] According to the present invention, the water cooling conditions include: water cooling insulation time is 2-6 hours, and water cooling to 20-30°C.

[0034] According to the present invention, the oil cooling conditions include: the oil cooling insulation time is 2-6 hours, and the oil is cooled to 20-30°C.

[0035] According to the present invention, the air cooling conditions include: air cooling and heat preservation time is 2-6 hours, and air cooling to 20-30°C.

[0036] According to the present invention, the conditions for the third heating include: divided into two stages, the heating temperature of the first stage is 130-145°C, the first stage is kept warm for 1.5-2.5 hours, and cooled to 20-30°C; the heating temperature of the second stage is 130-145°C, the second stage is kept warm for 1.5-2.5 hours, and cooled to 20-30°C.

[0037] The hardness test method is in accordance with GB / T 4340.1-2009.

[0038] The technical solution of the present invention is further described in detail below with reference to the accompanying drawings and embodiments. Obviously, the embodiments described herein are only some embodiments of the present invention and are not intended to limit the present invention. Based on the embodiments of the present invention, all other embodiments implemented by those of ordinary skill in the art without making creative improvements are within the scope of protection of the present invention.

[0039] Example 1 The chemical composition of 440C stainless steel is C:1.15-1.25%; Cr:16-18%; Si:0.6-0.8%; Mn:0.6-0.8%; Mo:0.45-0.55%; Nb:2.98-3.02%; O≤0.25%; P≤0.02%; S≤0.01%; the balance is Fe; S1. Inject, degrease and sinter 440C powder to obtain sintered samples; S2, first heating the sintered sample to 875°C, The cooling rate after S2 holding is 875℃ Cooling is carried out, and after cooling to 200°C, it is switched to furnace cooling and cooled to 20°C; S3, the second heating is 1040 ° C, the heat preservation time is 30 minutes, and the liquid nitrogen cooling is carried out. The liquid nitrogen is cooled to -196 ° C and restored to 20 ° C. The heating temperature of the first stage of the third heating is 145 ° C, the first stage is kept warm for 1.5 hours, and cooled to 20 ° C. The heating temperature of the second stage is 145 ° C, the second stage is kept warm for 1.5 hours, and cooled to obtain stainless steel.

[0040] 440C stainless steel A1 was obtained with a hardness of 783.3HV. Figure 3 The microstructure diagram after step S2 of Example 1. In step S2, bainite and pearlite are generated as diffusion-type transformation products. Figure 4 This is the microstructure diagram after step S3 of Example 1, and finally high-hardness cryptocrystalline martensite is obtained.

[0041] Example 2 The preparation method of Example 1 is the same as that of Example 1, except that in step S2, the first step is heating to 800°C and keeping the temperature for 1 hour. Cooling rate after S2 holding at 800 °C Cooling was performed and after cooling to 200 °C the furnace cooling was started and cooling to 20 °C was performed.

[0042] 440C stainless steel A2 was produced with a hardness of 734.2 HV.

[0043] Example 3 The production method according to example 1 was followed, except that in step S2 the first heating was to 900 °C and the holding was for 48 h, Cooling rate after S2 holding at 900 °C Cooling was performed and after cooling to 200 °C the furnace cooling was started and cooling to 20 °C was performed.

[0044] 440C stainless steel A3 was produced with a hardness of 817.2 HV.

[0045] Example 4 The production method according to example 1 was followed, except that in step S3 the second heating was to 1030 °C and the holding was for 10 min to austenitic state.

[0046] 440C stainless steel A4 was produced with a hardness of 767.9 HV.

[0047] Example 5 The production method according to example 1 was followed, except that in step S3 the second heating was to 1050 °C and the holding was for 30 min to austenitic state.

[0048] 440C stainless steel A5 was produced with a hardness of 783.1 HV.

[0049] Example 6 The production method according to example 1 was followed, except that in step S3 the second heating was to 130 °C in a first stage with a holding of 1.5 h and cooling to 20 °C, and in a second stage the heating was to 130 °C with a holding of 1.5 h.

[0050] 440C stainless steel A6 was produced with a hardness of 778.3 HV.

[0051] Example 7 The production method according to example 1 was followed, except that in step S3 the second heating was to 145 °C in a first stage with a holding of 2.5 h and cooling to 30 °C, and in a second stage the heating was to 145 °C with a holding of 2.5 h.

[0052] 440C stainless steel A7 was produced with a hardness of 776.1 HV.

[0053] Example 8 The preparation method of Example 1 is followed, except that in step S3, water cooling is adopted, the cooling and heat preservation time is 2 hours, cooling to 20°C, and then returning to 20°C.

[0054] 440C stainless steel A8 was obtained with a hardness of 744.6 HV.

[0055] Instructions attached Figure 5 This is the microstructure diagram after step S3 of Example 8. High-hardness cryptocrystalline martensite can also be obtained by using water cooling in S3.

[0056] Example 9 The preparation method of Example 1 is followed, except that in step S3, water cooling is adopted, the cooling and heat preservation time is 2 hours, cooling to 20°C, and then recovering to 30°C.

[0057] 440C stainless steel A9 was obtained with a hardness of 742.0 HV.

[0058] Example 10 The preparation method of Example 1 is followed, except that in step S3, oil cooling is adopted, the cooling and heat preservation time is 2 hours, cooling to 20°C, and then returning to 20°C.

[0059] 440C stainless steel A10 was obtained with a hardness of 725.6 HV.

[0060] Instructions attached Figure 6 This is a microstructure diagram after step S3 of Example 10. High-hardness cryptocrystalline martensite can also be obtained by using oil cooling in S3.

[0061] Example 11 The preparation method of Example 1 is followed, except that in step S3, oil cooling is adopted, the cooling and heat preservation time is 2 hours, cooling to 20°C and then recovering to 30°C.

[0062] 440C stainless steel A11 was produced with a hardness of 725.8 HV.

[0063] Example 12 The preparation method of Example 1 is followed, except that in step S3, air cooling is adopted, the cooling and heat preservation time is 2 hours, cooling to 20°C, and then returning to 20°C.

[0064] 440C stainless steel A12 was obtained with a hardness of 738.6HV.

[0065] Instructions attached Figure 6 This is a microstructure diagram after step S3 of Example 10. High-hardness cryptocrystalline martensite can also be obtained by using air cooling in S3.

[0066] Example 13 The preparation method of Example 1 was followed except that in step S3, air cooling was used, the cooling time was 2 h, and the temperature was cooled to 20 °C and then returned to 30 °C.

[0067] 440C stainless steel A13 was produced having a hardness of 735.1 HV.

[0068] Example 14 The preparation method of Example 1 was followed except that in step S2, the first heating was to 870 °C, 870 °C, and the cooling rate after S2 cooling was performed, and after cooling to 200 °C, furnace cooling was used, and the temperature was cooled to 20 °C. In step S3, the second heating was to 1050 °C for 30 min to the austenitic state.

[0069] 440C stainless steel A14 was produced having a hardness of 779.6 HV.

[0070] Example 15 The preparation method of Example 1 was followed except that in step S2, the first heating was to 865 °C, 865 °C, and the cooling rate after S2 cooling was performed, and after cooling to 200 °C, furnace cooling was used, and the temperature was cooled to 20 °C. In step S3, the second heating was to 1040 °C for 30 min to the austenitic state.

[0071] 440C stainless steel A15 was produced having a hardness of 777.5 HV.

[0072] Example 16 The preparation method of Example 1 was followed except that in step S2, the first heating was to 868 °C for 48 h, 868 °C, and the cooling rate after S2 cooling was performed, and after cooling to 200 °C, furnace cooling was used, and the temperature was cooled to 20 °C. In step S3, the second heating was to 1040 °C for 20 min to the austenitic state.

[0073] 440C stainless steel A16 was produced having a hardness of 830.8 HV.

[0074] Example 17 The preparation method of Example 1 was followed except that in step S2, the first heating was to 800 °C, 800 °C, and the cooling rate after S2 cooling was performed, and after cooling to 200 °C, furnace cooling was used, and the temperature was cooled to 20 °C.

[0075] 440C stainless steel A17 was produced having a hardness of 755.6 HV.

[0076] Example 18 The preparation method of Example 8 is as follows, except that in step S2, the first heating is performed to 800°C. The cooling rate after S2 holding at 800℃ is The mixture was cooled to 200°C and then switched to furnace cooling to 20°C.

[0077] 440C stainless steel A18 was obtained with a hardness of 742.3HV.

[0078] Example 19 The preparation method of Example 10 is as follows, except that in step S2, the first heating is performed to 800°C. The cooling rate after S2 holding at 800℃ is The mixture was cooled to 200°C and then switched to furnace cooling to 20°C.

[0079] 440C stainless steel A19 was produced with a hardness of 725.0 HV.

[0080] Example 20 The preparation method of Example 12 is as follows, except that in step S2, the first heating is performed to 800°C. The cooling rate after S2 holding at 800℃ is The mixture was cooled to 200°C and then switched to furnace cooling to 20°C.

[0081] 440C stainless steel A20 was obtained with a hardness of 728.7 HV.

[0082] Example 21 The preparation method of Example 1 is the same as that of Example 1, except that in step S2, the first heating is performed to 825°C. The cooling rate after S2 holding is 825℃ The mixture was cooled to 200°C and then switched to furnace cooling to 20°C.

[0083] 440C stainless steel A21 was obtained with a hardness of 773.8 HV.

[0084] Example 22 The preparation method of Example 8 is as follows, except that in step S2, the first heating is performed to 825°C. The cooling rate after S2 holding is 825℃ The mixture was cooled to 200°C and then switched to furnace cooling to 20°C.

[0085] 440C stainless steel A22 was obtained with a hardness of 728.4HV.

[0086] Example 23 The preparation method of Example 10 is as follows, except that in step S2, the first heating is performed to 825°C. The cooling rate after S2 holding is 825℃ The mixture was cooled to 200°C and then switched to furnace cooling to 20°C.

[0087] 440C stainless steel A23 was obtained with a hardness of 729.7 HV.

[0088] Example 24 The preparation method of Example 12 is as follows, except that in step S2, the first heating is performed to 825°C. The cooling rate after S2 holding is 825℃ The mixture was cooled to 200°C and then switched to furnace cooling to 20°C.

[0089] 440C stainless steel A24 was obtained with a hardness of 734.2 HV.

[0090] Example 25 The preparation method of Example 1 is the same as that of Example 1, except that in step S2, the first heating is performed to 850°C. The cooling rate after S2 holding at 850℃ is The mixture was cooled to 200°C and then switched to furnace cooling to 20°C.

[0091] 440C stainless steel A25 was obtained with a hardness of 766.9 HV.

[0092] Example 26 The preparation method of Example 8 is as follows, except that in step S2, the first heating is performed to 850°C. The cooling rate after S2 holding at 850℃ is The mixture was cooled to 200°C and then switched to furnace cooling to 20°C.

[0093] 440C stainless steel A26 was obtained with a hardness of 741.9 HV.

[0094] Example 27 The preparation method of Example 10 is as follows, except that in step S2, the first heating is performed to 850°C. The cooling rate after S2 holding at 850℃ is The mixture was cooled to 200°C and then switched to furnace cooling to 20°C.

[0095] 440C stainless steel A27 was obtained with a hardness of 734.9 HV.

[0096] Example 28 The preparation method of Example 12 is as follows, except that in step S2, the first heating is performed to 850°C. The cooling rate after S2 holding at 850℃ is The mixture was cooled to 200°C and then switched to furnace cooling to 20°C.

[0097] 440C stainless steel A28 was obtained with a hardness of 743.8 HV.

[0098] Example 29 The preparation method of Example 8 is as follows, except that in step S2, the first heating is performed to 900°C. The cooling rate after S2 holding at 900℃ is The mixture was cooled to 200°C and then switched to furnace cooling to 20°C.

[0099] 440C stainless steel A29 was obtained with a hardness of 744.6 HV.

[0100] Example 30 The preparation method of Example 10 is as follows, except that in step S2, the first heating is performed to 900°C. The cooling rate after S2 holding at 900℃ is The mixture was cooled to 200°C and then switched to furnace cooling to 20°C.

[0101] 440C stainless steel A30 was obtained with a hardness of 725.6 HV.

[0102] Example 31 The preparation method of Example 12 is as follows, except that in step S2, the first heating is performed to 900°C. The cooling rate after S2 holding at 900℃ is The mixture was cooled to 200°C and then switched to furnace cooling to 20°C.

[0103] 440C stainless steel A31 was obtained with a hardness of 738.6HV.

[0104] Example 32 The preparation method of Example 1 is the same as that of Example 1, except that in step S2, the cooling rate after S2 is kept warm is The mixture was cooled to 200°C and then switched to furnace cooling to 20°C.

[0105] 440C stainless steel A32 was obtained with a hardness of 725.1 HV.

[0106] Example 33 The preparation method of Example 1 is the same as that of Example 1, except that in step S2, the cooling rate after S2 is kept warm is The mixture was cooled to 200°C and then switched to furnace cooling to 20°C.

[0107] 440C stainless steel A33 was obtained with a hardness of 781.3HV.

[0108] Comparative Example 1 The preparation method of Example 1 is followed, except that in step S2, there is no first heating.

[0109] 440C stainless steel DA1 was obtained with a hardness of 693.0 HV.

[0110] Instructions attached Figure 8 This is the microstructure diagram of comparative example 1 after step S3. Compared with example 1, since it does not have steps such as S2 heating, heat preservation and cooling, its matrix structure is acicular martensite with a hardness lower than that of the cryptocrystalline martensite in example 1.

[0111] Comparative Example 2 The preparation method of Example 1 is followed, except that in step S2, the first heating is performed to 700°C.

[0112] 440C stainless steel DA2 was obtained with a hardness of 693 HV.

[0113] Comparative Example 3 The preparation method of Example 1 is followed, except that in step S2, the first heating is performed to 1000°C.

[0114] 440C stainless steel DA3 was obtained with a hardness of 678.3HV.

[0115] Comparative Example 4 The preparation method of Example 8 is followed, except that in step S2, there is no first heating.

[0116] 440C stainless steel DA4 was obtained with a hardness of 677.9 HV.

[0117] Instructions attached Figure 9 This is the microstructure diagram of comparative example 4 after step S3. Compared with example 8, since it does not have steps such as S2 heating, heat preservation and cooling, its matrix structure is acicular martensite with a hardness lower than that of the cryptocrystalline martensite in example 8.

[0118] Comparative Example 5 The preparation method of Example 10 is followed, except that in step S2, there is no first heating.

[0119] 440C stainless steel DA5 was obtained with a hardness of 688.3HV.

[0120] Instructions attached Figure 10 This is the microstructure diagram of comparative example 5 after step S3. Compared with example 10, since it does not have the steps of S2 heating, heat preservation and cooling, its matrix structure is acicular martensite with a hardness lower than that of the cryptocrystalline martensite in example 10.

[0121] Comparative Example 6 The preparation method of Example 12 is followed, except that in step S2, there is no first heating.

[0122] 440C stainless steel DA6 was obtained with a hardness of 672.6HV.

[0123] Comparative Example 7 The preparation method of Example 1 is the same as that of Example 1, except that in step S2, the cooling rate after S2 is kept warm is The mixture was cooled to 200°C and then switched to furnace cooling to 20°C.

[0124] 440C stainless steel DA7 was obtained with a hardness of 687.2HV.

[0125] Instructions attached Figure 11 This is the microstructure diagram of comparative example 6 after step S3. Compared with example 12, since it does not have steps such as S2 heating, heat preservation and cooling, its matrix structure is acicular martensite with a hardness lower than that of the cryptocrystalline martensite in example 12.

[0126] In Comparative Example 5, the martensite hardness obtained by the heat treatment process used for the cast 440C is slightly lower for metal injection molded 440C stainless steel. In order to obtain martensite with higher hardness, Comparative Examples 4 and 6 changed the cooling method after quenching and holding to faster water cooling and slower air cooling. It was found that no matter whether the quenching cooling speed is faster oil cooling or slower air cooling, it is impossible to obtain martensite with higher hardness. The Mf (martensitic transformation end temperature) of 440C stainless steel is below room temperature. Considering that water cooling, oil cooling and air cooling cannot ultimately cool below room temperature. Comparative Example 1 uses liquid nitrogen as the cooling medium to make it reach its Mf point, the hardness of the obtained metal injection molded 440C stainless steel DA1 is not significantly improved. Changing the quenching cooling medium and the final quenching cooling temperature cannot significantly improve the hardness of metal injection molded 440C stainless steel.

[0127] As long as S2 treatment is performed before quenching, the subsequent quenching cooling medium, regardless of the four quenching cooling media, will show a significant increase in hardness compared to those without S2 treatment. In other words, while traditional metal injection molded 440C stainless steel cannot improve its performance through raw material element content and sintering process, its hardness and wear resistance can be reasonably improved through improved heat treatment process.

[0128] By comparing the examples with the comparative examples, it can be seen that the stainless steel provided by the present invention has a certain improvement in performance compared to the sample obtained by the original heat treatment process of the cast 440C stainless steel, and can reach 720-830HV. Figure 4 、 Figure 5 、 Figure 6 and Figure 7 microstructure) and Comparative Examples 1, 4, 5 and 6 (the corresponding drawings are Figure 8 、 Figure 9 、 Figure 10 and Figure 11 Observing the microstructure of the sample, it can be seen that regardless of the cooling method used after the first stage of heat preservation in S3 (liquid nitrogen, water, oil, or air), the acicular martensite in the matrix of the comparative example is extremely prominent, and a distinct transition zone is formed between the martensitic matrix and the chromium carbides. In contrast, the example, because the S2 treatment is performed before S3, which improves the microstructure and allows carbon diffusion, ultimately produces cryptocrystalline martensite after S3, and the transition zone between the martensitic matrix and the chromium carbides is smaller than that of the comparative example. The cryptocrystalline martensite in the example is harder than the acicular martensite in the comparative example, ultimately resulting in a significantly greater hardness in the example than in the comparative example.

[0129] The examples herein, after treatment in step S2, diffuse carbon from the grain boundaries into the matrix, subsequently forming cryptocrystalline martensite with a high carbon content in step S3. In contrast, the comparative examples without S2 treatment produced acicular martensite. Cryptocrystalline martensite is harder than acicular martensite, thus demonstrating that this process can improve the hardness of martensite in metal injection molding (MIM) at 440C.

[0130] The above description is only a preferred specific implementation method of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, can make equivalent replacements or changes based on the technical solutions and inventive concepts of the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A metal injection molded 440C stainless steel, characterized in that, The components and their weight percentages in the stainless steel are: C content is 1.15-1.25%; Cr content is 16-18%; Si content is 0.6-0.8%; Mn content is 0.6-0.8%; Mo content is 0.45-0.55%; Nb content is 2.98-3.02%; O content ≤ 0.25%; P content ≤ 0.02%; S content ≤ 0.01%; The individual content of other impurity elements is ≤0.03%; The total content of other impurity elements is ≤0.15%; The balance is Fe; The hardness of the metal injection molded 440C stainless steel is 720-830 HV.

2. A method for preparing the metal injection molded 440C stainless steel according to claim 1, characterized in that: The method comprises: S1. Inject, degrease and sinter 440C stainless steel powder to obtain sintered samples; S2. The sintered sample is first heated to 800-900°C, kept warm for 1-48 hours, and then cooled. The relationship between the cooling time and temperature within the first heating temperature to 200°C is: , where t is the cooling time in min, T(t) is the temperature at the corresponding moment in °C, To keep the temperature, cool from 200℃ to 20-30℃ and use furnace cooling; S3, heating for the second time to an austenitic state, cooling to restore the temperature to 20-30° C., heating for the third time, and cooling to obtain the stainless steel.

3. The method according to claim 2, characterized in that The second heating conditions include: a heating temperature of 1030-1050° C. and a holding time of 10-30 minutes.

4. The method according to claim 2, characterized in that The cooling treatment is any one of liquid nitrogen cooling, water cooling, oil cooling or air cooling.

5. The method according to claim 4, characterized in that The liquid nitrogen cooling conditions include: the liquid nitrogen cooling and holding time is 2-6 hours, and the liquid nitrogen is cooled to -196°C.

6. The method according to claim 4, characterized in that The water cooling conditions include: water cooling insulation time is 2-6 hours, and water cooling to 20-30°C.

7. The method according to claim 4, characterized in that The oil cooling conditions include: the oil cooling insulation time is 2-6 hours, and the oil is cooled to 20-30°C.

8. The method according to claim 4, characterized in that The air cooling conditions include: air cooling and heat preservation time of 2-6 hours, and air cooling to 20-30°C.

9. The method according to claim 3, characterized in that The conditions of the third heating include: divided into two stages, the heating temperature of the first stage is 130-145°C, the first stage is kept warm for 1.5-2.5 hours, and cooled to 20-30°C; the heating temperature of the second stage is 130-145°C, the second stage is kept warm for 1.5-2.5 hours, and cooled to 20-30°C.

Images