Metal injection molded 440c stainless steel and method of making

Abstract

The present application relates to the technical field of stainless steel material, in particular to a kind of metal injection molding 440C stainless steel and preparation method thereof, the weight percentage of each component in the stainless steel is 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 is ≤0.25%; P content is ≤0.02%; S content is ≤0.01%; the single content of other impurity elements is ≤0.03%; the total content of other impurity elements is ≤0.15%; the balance is Fe, the stainless steel uses specific preparation process, has high hardness and high wear resistance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of stainless steel materials, in particular to a metal injection molding 440C stainless steel and a preparation method thereof. BACKGROUND

[0002] 440C is a high-carbon high-chromium martensitic stainless steel, and after quenching and tempering, its hardness can reach 58HRC (654HV), and it has good hardness and wear resistance. 440C stainless steel also has good corrosion resistance, and is one of the hardest stainless steels, which is widely used in corrosive environments and under no lubrication conditions, such as bearings in the aerospace field, and is often used to make high-quality knives, such as scissors, surgical knives, etc.

[0003] Metal injection molding is a new powder metallurgy process, which is a near-net shaping technology formed by introducing the modern plastic injection molding idea into the traditional powder metallurgy field, and is used to manufacture parts with complex geometry and high dimensional accuracy, which inherits the characteristics of traditional powder metallurgy such as material saving, uniform crystallization, and no cutting, and also has high precision and high complexity of plastic injection molding products. The heat treatment process of metal injection molding 440C stainless steel generally follows the heat treatment process of as-cast 440C stainless steel, but there is a significant difference in microstructure between metal injection molding 440C and as-cast material before heat treatment (after sintering). There are ring-shaped chromium carbides in the microstructure of metal injection molding 440C, which fix a large amount of carbon elements. Under the conventional heat treatment process, carbon elements are difficult to fully dissolve into the austenite matrix, which greatly limits the formation of high-carbon martensite and significantly reduces the hardness and wear resistance of metal injection molding 440C stainless steel. Therefore, it is urgent to provide a metal injection molding 440C stainless steel with high hardness and high wear resistance and a preparation method thereof. SUMMARY

[0004] The present application 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.

[0005] To achieve the above-mentioned purpose, the first aspect of the present application provides a metal injection molding 440C stainless steel, wherein the components and their weight percentages in the stainless steel are as follows:

[0006] The content of C is 1.15-1.25%;

[0007] The content of Cr is 16-18%;

[0008] The content of Si is 0.6-0.8%;

[0009] The content of Mn is 0.6-0.8%;

[0010] The content of Mo is 0.45-0.55%;

[0011] Nb content is 2.98-3.02%;

[0012] O content is ≤0.25%;

[0013] P content is ≤0.02%;

[0014] S content is ≤0.01%;

[0015] Other impurity elements are ≤0.03% individually;

[0016] Other impurity elements are ≤0.15% in total;

[0017] The balance is Fe;

[0018] The hardness of the metal injection molding 440C stainless steel is 720-830HV.

[0019] The HV is Vickers hardness.

[0020] The second aspect of the application provides a preparation method of the above-mentioned metal injection molding 440C stainless steel, wherein the method comprises:

[0021] S1, injecting, defatting and sintering 440C stainless steel powder to obtain a sintered sample;

[0022] S2, first heating the sintered sample to 800-900℃, holding for 1-48h, and cooling after holding, wherein the first heating temperature is within 200℃, and the relationship between the cooling time and the temperature is wherein t is the cooling time, the unit is min, T(t) is the temperature at the corresponding time, the unit is ℃, the holding temperature is 200℃, and the cooling from 200℃ to 20-30℃ is furnace cooling;

[0023] S3, second heating to austenite state, cooling treatment, restoring to 20-30℃, third heating, and cooling to obtain the stainless steel.

[0024] The preparation method of the metal injection molding 440C stainless steel provided by the application has the heat treatment principle that:

[0025] The Ac1 temperature of the 440C stainless steel is about 815℃, the sintered 440C is treated by S2, and the specific holding temperature selected in S2 can make the carbon element fully diffuse to the matrix, thereby providing an element basis for subsequent formation of high-hardness cryptocrystalline martensite. Subsequently, furnace cooling is adopted to make the material mainly undergo diffusion type phase change (such as formation of bainite and pearlite), thereby providing a basic structure for the martensite transformation in the following S3 step.

[0026] If S1+S3 process path is adopted, needle-like martensite is formed finally due to low carbon content of the matrix. If S1+S2+S3 process path is adopted, cryptocrystalline martensite is obtained after S3 step due to good structure foundation provided by S2 step. Compared with needle-like martensite obtained by traditional process, the cryptocrystalline martensite prepared by the process has higher hardness. Compared with prior art, the application solves the problem of hardness gap between metal injection molding 440C stainless steel and as-cast 440C stainless steel sample. The metal injection molding 440C stainless steel treated by the process has certain performance improvement compared with the sample obtained by the as-cast 440C stainless steel heat treatment process, and can reach 720-830HV. The hardness of the sample prepared by the original heat treatment process is 672-693HV. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The heat treatment process flow chart of the metal injection molding 440C stainless steel of the application;

[0028] Figure 2 The microstructure morphology of S1;

[0029] Figure 3 The microstructure morphology after S2 of example 1;

[0030] Figure 4 The microstructure morphology after the final heat treatment of example 1;

[0031] Figure 5 The microstructure morphology after the final heat treatment of example 8;

[0032] Figure 6 The microstructure morphology after the final heat treatment of example 10;

[0033] Figure 7 The microstructure morphology after the final heat treatment of example 12;

[0034] Figure 8 The microstructure morphology after the final heat treatment of comparative example 1;

[0035] Figure 9 The microstructure morphology after the final heat treatment of comparative example 4;

[0036] Figure 10 The microstructure morphology after the final heat treatment of comparative example 5; Figure 11 The microstructure morphology after the final heat treatment of comparative example 6. DETAILED DESCRIPTION

[0037] The endpoints of the ranges and any values claimed herein are not to be understood as being limited to the exact values recited as implicitly split into a narrower range between each pair of endpoints. The use of "about" in conjunction with a reference to a value is intended to mean that the value is approximate, and, indeed, may allow for a reasonable amount of deviation from the stated value. For numerical ranges, the end points are included in the range. The use of "at least" indicates open-ended ranges. For numerical ranges, the end points are included in the range. The use of "at least" indicates open-ended ranges. For ranges given as A to B, A to C, D to B or D to C, this is intended to mean A to C inclusive of B, and D to C inclusive of B, unless the context clearly indicates otherwise.

[0038] The hardness and wear resistance of the 440C stainless steel made by metal injection molding cannot meet the requirements. In the prior art, the method for improving the hardness and wear resistance of the 440C stainless steel made by metal injection molding is to add a large amount of carbon elements in the raw material preparation or to change the sintering temperature. However, increasing carbon will affect the stability in the subsequent sintering process, making it difficult to produce, and changing the sintering temperature will cause deformation. Moreover, the hardness and wear resistance of the metal injection molding 440C stainless steel made by the method of the prior art are not significantly improved.

[0039] To improve the hardness and wear resistance of the metal injection molding 440C stainless steel, the inventors found that the microstructure of the metal injection molding 440C stainless steel is greatly different from that of the traditional cast 440C stainless steel before heat treatment. The metal injection molding 440C stainless steel cannot release the carbon elements fixed by the ring-shaped distributed chromium carbide generated during sintering using the existing heat treatment process, which limits the formation of high-carbon martensite, and reduces the hardness and wear resistance of the metal injection molding 440C stainless steel.

[0040] The inventors found that prolonging the second heating time during quenching (the second heating of step S3) can only form individual carbides during the diffusion of carbon, and cannot form high-hardness cryptocrystalline martensite.

[0041] Therefore, the inventors conceived that the microstructure of the metal injection molding 440C stainless steel can be adjusted by process treatment to improve the hardness and wear resistance of the metal injection molding 440C stainless steel. To this end, the inventors found that a heat treatment at a specific temperature and time after sintering can improve the hardness and wear resistance of the metal injection molding 440C stainless steel.

[0042] The inventors found that the ring-shaped chromium carbide formed during sintering of the metal injection molding 440C stainless steel is easily destroyed at a holding temperature of 800-900℃. If the holding temperature is too low, such as 700℃, it is not easy to destroy the ring-shaped chromium carbide; if the holding temperature is too high, such as 1000℃, the released carbon elements will form new carbides, both of which will result in carbon-poor martensite and make it difficult to obtain high-hardness cryptocrystalline martensite.

[0043] Finally, it is found that, before quenching in S3, the sintered sample of metal injection molding 440C stainless steel is kept at 800-900℃ to destroy the distributed chromium carbide and release carbon elements. And after keeping, the cooling speed is controlled, so that the released carbon elements are not formed into separate carbides, but are melted in the matrix. These carbons will promote the formation of aphanitic martensite during subsequent quenching, so that the metal injection molding 440C stainless steel with high hardness is obtained.

[0044] Therefore, the present application provides a preparation method of metal injection molding 440C stainless steel, and the prepared metal injection molding 440C stainless steel has excellent hardness and wear resistance.

[0045] Specifically, the first aspect of the present application provides a metal injection molding 440C stainless steel, wherein the components and their weight percentages in the stainless steel are as follows:

[0046] The content of C is 1.15-1.25%;

[0047] The content of Cr is 16-18%;

[0048] The content of Si is 0.6-0.8%;

[0049] The content of Mn is 0.6-0.8%;

[0050] The content of Mo is 0.45-0.55%;

[0051] The content of Nb is 2.98-3.02%;

[0052] The content of O is ≤0.25%;

[0053] The content of P is ≤0.02%;

[0054] The content of S is ≤0.01%;

[0055] The balance is Fe;

[0056] The hardness of the metal injection molding 440C stainless steel is 720-830HV.

[0057] S1, injecting 440C stainless steel powder, defatting, sintering to obtain a sintered sample;

[0058] S2, first heating the sintered sample to 800-900℃, keeping for 1-48h, and cooling after keeping. The relationship between cooling time and temperature in the temperature from the first heating temperature to 200℃ is as follows: Wherein, t is the cooling time, unit is min, T(t) is the temperature at the corresponding time, unit is ℃, The keeping temperature is 800-900℃, and the cooling from 200℃ to 20-30℃ is furnace cooling;

[0059] S3, second heating to austenite state, cooling treatment, recovery to 20-30 DEG C, third heating, cooling to obtain the stainless steel.

[0060] S2 and S3 are the heat treatment processes herein, the description is attached Figure 1 for the heat treatment process flow chart.

[0061] In the present application, the sintered state after S1 treatment generally exists annular distribution of chromium carbide, which fixes a large amount of carbon element, the description is attached Figure 2 for its microstructure. After S2 treatment, the hardness of the sample is 550-580HV, at this time, carbon diffuses fully, and some diffusion type conversion products (bainite, pearlite) are obtained in the process of S2, which provides the organizational basis for the subsequent step, the description is attached Figure 3 for the microstructure after S2.

[0062] Step S2 is cooled after a specific first heating temperature and holding time, and the cooling has a specific relationship with the holding temperature. If the cooling rate is too fast, the released carbon element will form a separate carbide; if the cooling rate is too slow, the performance will not be improved infinitely with the cooling rate. The cooling provided by the present application has a specific relationship between the cooling time and the holding temperature, which can make the carbon element melt in the matrix. These carbon elements will promote the formation of hidden martensite during subsequent quenching, thereby obtaining high hardness of metal injection molding 440C stainless steel.

[0063] In the present application, in step S1, the injection, debinding and sintering can use conventional parameters in the art, as long as the sintered sample can be obtained.

[0064] Specifically, the injection conditions include: mixing 440C stainless steel powder with thermoplastic polymer binder, and then injecting into the mold.

[0065] The debinding conditions include: solvent debinding and thermal debinding. Solvent debinding is to put the injection sample into dichloromethane solvent, and thermal debinding is to put the sample into a tube furnace with argon, hydrogen and other protective atmosphere.

[0066] The sintering conditions include: heating the debinding sample to 1200-1250 DEG C under vacuum or argon.

[0067] According to the present application, the second heating conditions include: heating temperature is 1030-1050 DEG C, and holding time is 10-30 min.

[0068] According to the present application, the cooling treatment is any one of liquid nitrogen cooling, water cooling, oil cooling or air cooling.

[0069] According to the application, the liquid nitrogen cooling condition comprises: the liquid nitrogen cooling holding time is 2-6h, and the liquid nitrogen is cooled to-196℃.

[0070] According to the application, the water cooling condition comprises: the water cooling holding time is 2-6h, and the water is cooled to 20-30℃.

[0071] According to the application, the oil cooling condition comprises: the oil cooling holding time is 2-6h, and the oil is cooled to 20-30℃.

[0072] According to the application, the air cooling condition comprises: the air cooling holding time is 2-6h, and the air is cooled to 20-30℃.

[0073] According to the application, the third heating condition comprises: two stages, the first stage heating temperature is 130-145℃, the first stage holding time is 1.5-2.5h, and the first stage is cooled to 20-30℃, the second stage heating temperature is 130-145℃, the second stage holding time is 1.5-2.5h, and the second stage is cooled to 20-30℃.

[0074] The hardness test method is according to GB / T 4340.1-2009.

[0075] The technical solutions of the application will be further described in detail below in combination with the drawings and examples. Obviously, the examples described here are only some of the examples of the application and are not used to limit the application. Based on the examples in the application, all other examples implemented by those skilled in the art without creative improvement are within the protection scope of the application.

[0076] Example 1

[0077] The chemical composition of the 440C stainless steel is as follows: 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%; and the balance is Fe;

[0078] S1, the 440C powder is injected, defatted, and sintered to obtain a sintered sample;

[0079] S2, the sintered sample is first heated to 875℃, 875℃, the cooling rate after S2 holding is cooling, and after cooling to 200℃, the sample is cooled with the furnace, and the sample is cooled to 20℃;

[0080] S3, second heating 1040 °C, holding for 30 min, liquid nitrogen cooling, liquid nitrogen cooling holding time is 2 h, liquid nitrogen cooling to -196 °C, recovery to 20 °C, third heating first stage heating temperature is 145 °C, first stage holding for 1.5 h, cooling to 20 °C, second stage heating temperature is 145 °C, second stage holding for 1.5 h, cooling to obtain stainless steel.

[0081] 440C stainless steel A1 is prepared, the hardness is 783.3HV, the specification is attached. Figure 3 The microstructure diagram of example 1 after step S2. In the S2 step, bainite, pearlite, the product of these diffusion type phase change is generated. Figure 4 The microstructure diagram of example 1 after step S3, finally high hardness cryptocrystalline martensite is obtained.

[0082] Example 2

[0083] According to the preparation method of example 1, except that in step S2, the first heating is to 800 °C, holding for 1 h, 800 °C, the cooling rate after S2 holding Cooling is carried out, after cooling to 200 °C, it is changed to furnace cooling, and cooling to 20 °C.

[0084] 440C stainless steel A2 is prepared, the hardness is 734.2HV.

[0085] Example 3

[0086] According to the preparation method of example 1, except that in step S2, the first heating is to 900 °C, holding for 48 h, 900 °C, the cooling rate after S2 holding Cooling is carried out, after cooling to 200 °C, it is changed to furnace cooling, and cooling to 20 °C.

[0087] 440C stainless steel A3 is prepared, the hardness is 817.2HV.

[0088] Example 4

[0089] According to the preparation method of example 1, except that in step S3, the second heating is to 1030 °C, holding for 10 min to austenite state.

[0090] 440C stainless steel A4 is prepared, the hardness is 767.9HV.

[0091] Example 5

[0092] According to the preparation method of example 1, except that in step S3, the second heating is to 1050 °C, holding for 30 min to austenite state.

[0093] 440C stainless steel A5 was produced with a hardness of 783.1 HV.

[0094] Example 6

[0095] The production method of Example 1 was followed, except that in step S3, the first stage of the second heating was at a temperature of 130°C, the first stage was held for 1.5 h, cooled to 20°C, and the second stage of the heating was at a temperature of 130°C, and the second stage was held for 1.5 h.

[0096] 440C stainless steel A6 was produced with a hardness of 778.3 HV.

[0097] Example 7

[0098] The production method of Example 1 was followed, except that in step S3, the first stage of the second heating was at a temperature of 145°C, the first stage was held for 2.5 h, cooled to 30°C, and the second stage of the heating was at a temperature of 145°C, and the second stage was held for 2.5 h.

[0099] 440C stainless steel A7 was produced with a hardness of 776.1 HV.

[0100] Example 8

[0101] The production method of Example 1 was followed, except that in step S3, water cooling was used, the cooling and holding time was 2 h, cooled to 20°C, and returned to 20°C.

[0102] 440C stainless steel A8 was produced with a hardness of 744.6 HV.

[0103] Description Figure 5 A microstructure of Example 8 after step S3, using water cooling in S3 also resulted in high hardness of the microcrystalline martensite.

[0104] Example 9

[0105] The production method of Example 1 was followed, except that in step S3, water cooling was used, the cooling and holding time was 2 h, cooled to 20°C, and returned to 30°C.

[0106] 440C stainless steel A9 was produced with a hardness of 742.0 HV.

[0107] Example 10

[0108] The production method of Example 1 was followed, except that in step S3, oil cooling was used, the cooling and holding time was 2 h, cooled to 20°C, and returned to 20°C.

[0109] 440C stainless steel A10 was produced with a hardness of 725.6 HV.

[0110] DescriptionFigure 6 For the microstructure of Example 10 after step S3, high hardness cryptocrystalline martensite can also be obtained by oil cooling in S3.

[0111] Example 11

[0112] According to the preparation method of Example 1, except that in step S3, oil cooling is used, the cooling holding time is 2h, and the cooling is to 20°C, and the recovery is to 30°C.

[0113] 440C stainless steel A11 is prepared, and the hardness is 725.8HV.

[0114] Example 12

[0115] According to the preparation method of Example 1, except that in step S3, air cooling is used, the cooling holding time is 2h, and the cooling is to 20°C, and the recovery is to 20°C.

[0116] 440C stainless steel A12 is prepared, and the hardness is 738.6HV.

[0117] Description Figure 6 For the microstructure of Example 10 after step S3, high hardness cryptocrystalline martensite can also be obtained by air cooling in S3.

[0118] Example 13

[0119] According to the preparation method of Example 1, except that in step S3, air cooling is used, the cooling holding time is 2h, and the cooling is to 20°C, and the recovery is to 30°C.

[0120] 440C stainless steel A13 is prepared, and the hardness is 735.1HV.

[0121] Example 14

[0122] According to the preparation method of Example 1, except that in step S2, the first heating is to 870°C, to 870°C, the cooling rate after S2 holding cooling is performed, after cooling to 200°C, it is changed to furnace cooling, and the cooling is to 20°C, in step S3, the second heating is to 1050°C, and the holding is for 30min to austenite state.

[0123] 440C stainless steel A14 is prepared, and the hardness is 779.6HV.

[0124] Example 15

[0125] According to the preparation method of Example 1, except that in step S2, the first heating is to 865°C, to 865°C, the cooling rate after S2 holding Cooling was performed, and after cooling to 200°C, furnace cooling was performed, and the temperature was decreased to 20°C. In step S3, second heating was performed at 1040°C for 20 minutes to obtain an austenite structure.

[0126] 440C stainless steel A15 was produced, and the hardness was 777.5 HV.

[0127] Example 16

[0128] The production method of Example 1 was followed, except that in step S2, the first heating was performed at 800°C for 48 hours, 800°C, and the cooling rate after S2 Cooling was performed, and after cooling to 200°C, furnace cooling was performed, and the temperature was decreased to 20°C.

[0129] 440C stainless steel A16 was produced, and the hardness was 830.8 HV.

[0130] Example 17

[0131] The production method of Example 1 was followed, except that in step S2, the first heating was performed at 800°C, 800°C, and the cooling rate after S2 Cooling was performed, and after cooling to 200°C, furnace cooling was performed, and the temperature was decreased to 20°C.

[0132] 440C stainless steel A17 was produced, and the hardness was 755.6 HV.

[0133] Example 18

[0134] The production method of Example 8 was followed, except that in step S2, the first heating was performed at 800°C, 800°C, and the cooling rate after S2 Cooling was performed, and after cooling to 200°C, furnace cooling was performed, and the temperature was decreased to 20°C.

[0135] 440C stainless steel A18 was produced, and the hardness was 742.3 HV.

[0136] Example 19

[0137] The production method of Example 10 was followed, except that in step S2, the first heating was performed at 800°C, 800°C, and the cooling rate after S2 Cooling was performed, and after cooling to 200°C, furnace cooling was performed, and the temperature was decreased to 20°C.

[0138] 440C stainless steel A19 was produced, and the hardness was 725.0 HV.

[0139] Example 20

[0140] According to the preparation method of Example 12, except that in step S2, the first heating is to 800℃, 800℃, the cooling rate after S2 holding cooling is carried out, and after cooling to 200℃, furnace cooling is converted, and cooling to 20℃ is carried out.

[0141] 440C stainless steel A20 is prepared, and the hardness is 728.7HV.

[0142] Example 21

[0143] According to the preparation method of Example 1, except that in step S2, the first heating is to 825℃, 825℃, the cooling rate after S2 holding cooling is carried out, and after cooling to 200℃, furnace cooling is converted, and cooling to 20℃ is carried out.

[0144] 440C stainless steel A21 is prepared, and the hardness is 773.8HV.

[0145] Example 22

[0146] According to the preparation method of Example 8, except that in step S2, the first heating is to 825℃, 825℃, the cooling rate after S2 holding cooling is carried out, and after cooling to 200℃, furnace cooling is converted, and cooling to 20℃ is carried out.

[0147] 440C stainless steel A22 is prepared, and the hardness is 728.4HV.

[0148] Example 23

[0149] According to the preparation method of Example 10, except that in step S2, the first heating is to 825℃, 825℃, the cooling rate after S2 holding cooling is carried out, and after cooling to 200℃, furnace cooling is converted, and cooling to 20℃ is carried out.

[0150] 440C stainless steel A23 is prepared, and the hardness is 729.7HV.

[0151] Example 24

[0152] According to the preparation method of Example 12, except that in step S2, the first heating is to 825℃, 825℃, the cooling rate after S2 holding cooling is carried out, and after cooling to 200℃, furnace cooling is converted, and cooling to 20℃ is carried out.

[0153] A 24 440C stainless steel was produced having a hardness of 734.2 HV.

[0154] Example 25

[0155] The production method of Example 1 was followed except that in step S2 the first heating was to 850°C, 850°C, the cooling rate after S2 holding Cooling was performed, after cooling to 200°C the furnace cooling was changed to air cooling, and cooling to 20°C.

[0156] A 25 440C stainless steel was produced having a hardness of 766.9 HV.

[0157] Example 26

[0158] The production method of Example 8 was followed except that in step S2 the first heating was to 850°C, 850°C, the cooling rate after S2 holding Cooling was performed, after cooling to 200°C the furnace cooling was changed to air cooling, and cooling to 20°C.

[0159] A 26 440C stainless steel was produced having a hardness of 741.9 HV.

[0160] Example 27

[0161] The production method of Example 10 was followed except that in step S2 the first heating was to 850°C,

[0162] 850°C, the cooling rate after S2 holding Cooling was performed, after cooling to 200°C the furnace cooling was changed to air cooling, and cooling to 20°C.

[0163] A 27 440C stainless steel was produced having a hardness of 734.9 HV.

[0164] Example 28

[0165] The production method of Example 12 was followed except that in step S2 the first heating was to 850°C, 850°C, the cooling rate after S2 holding Cooling was performed, after cooling to 200°C the furnace cooling was changed to air cooling, and cooling to 20°C.

[0166] A 28 440C stainless steel was produced having a hardness of 743.8 HV.

[0167] Example 29

[0168] The production method of Example 8 was followed except that in step S2 the first heating was to 900°C, 900 °C, cooling rate after S2 holding

[0169] cooling was performed, and after cooling to 200 °C, furnace cooling was performed to 20 °C.

[0170] 440C stainless steel A29 was produced, and the hardness was 744.6 HV.

[0171] Example 30

[0172] The production method according to Example 10 was followed, except that in Step S2, the first heating was to 900 °C, 900 °C, cooling rate after S2 holding cooling was performed, and after cooling to 200 °C, furnace cooling was performed to 20 °C.

[0173] 440C stainless steel A30 was produced, and the hardness was 725.6 HV.

[0174] Example 31

[0175] The production method according to Example 12 was followed, except that in Step S2, the first heating was to 900 °C, 900 °C, cooling rate after S2 holding cooling was performed, and after cooling to 200 °C, furnace cooling was performed to 20 °C.

[0176] 440C stainless steel A31 was produced, and the hardness was 738.6 HV.

[0177] Example 32

[0178] The production method according to Example 1 was followed, except that in Step S2, the cooling rate after S2 holding cooling was performed, and after cooling to 200 °C, furnace cooling was performed to 20 °C.

[0179] 440C stainless steel A32 was produced, and the hardness was 725.1 HV.

[0180] Example 33

[0181] The production method according to Example 1 was followed, except that in Step S2, the cooling rate after S2 holding

[0182] cooling was performed, and after cooling to 200 °C, furnace cooling was performed to 20 °C.

[0183] 440C stainless steel A33 was produced, and the hardness was 781.3 HV.

[0184] Comparative Example 1

[0185] The preparation method is the same as in Example 1, except that there is no first heating in step S2.

[0186] 440C stainless steel DA1 was obtained with a hardness of 693.0 HV.

[0187] Instruction manual attached Figure 8 The image shows the microstructure of Comparative Example 1 after step S3. Compared to Example 1, because it does not have the heating, heat preservation, and cooling steps of S2, its matrix structure is acicular martensite with a lower hardness than the cryptocrystalline martensite in Example 1.

[0188] Comparative Example 2

[0189] The preparation method is the same as in Example 1, except that in step S2, the temperature is first heated to 700°C.

[0190] 440C stainless steel DA2 was obtained with a hardness of 693HV.

[0191] Comparative Example 3

[0192] The preparation method is the same as in Example 1, except that in step S2, the temperature is first heated to 1000°C.

[0193] 440C stainless steel DA3 was obtained with a hardness of 678.3HV.

[0194] Comparative Example 4

[0195] The preparation method is the same as in Example 8, except that there is no first heating in step S2.

[0196] 440C stainless steel DA4 was obtained with a hardness of 677.9HV.

[0197] Instruction manual attached Figure 9 The image shown is a microstructure diagram of Comparative Example 4 after step S3. Compared to Example 8, because it does not have the heating, heat preservation, and cooling steps of S2, its matrix structure is acicular martensite with a lower hardness than the cryptocrystalline martensite in Example 8.

[0198] Comparative Example 5

[0199] The preparation method is the same as in Example 10, except that there is no first heating in step S2.

[0200] 440C stainless steel DA5 was obtained with a hardness of 688.3HV.

[0201] Instruction manual attached Figure 10 The image shown is a microstructure diagram of Comparative Example 5 after step S3. Compared to Example 10, because it does not have steps such as heating, heat preservation, and cooling in S2, its matrix structure is acicular martensite with a hardness lower than that of cryptocrystalline martensite in Example 10.

[0202] Comparative Example 6

[0203] According to the preparation method of Example 12, except that in step S2, no first heating.

[0204] The 440C stainless steel DA6 was prepared, and the hardness was 672.6 HV.

[0205] Comparative Example 7

[0206] According to the preparation method of Example 1, except that in step S2, the cooling rate after S2 holding was Cooling was performed, and after cooling to 200℃, furnace cooling was performed until 20℃.

[0207] The 440C stainless steel DA7 was prepared, and the hardness was 687.2 HV.

[0208] The microstructure diagram of Comparative Example 6 after step S3 is shown in the accompanying drawings of the specification. Figure 11 Comparative Example 6 after step S3 is shown in the accompanying drawings of the specification.

[0209] Comparative Example 5 used the heat treatment process of the above-mentioned as-cast 440C to obtain martensite with slightly lower hardness. In order to obtain martensite with higher hardness, the cooling method after quenching holding was changed to faster water cooling and slower air cooling in Comparative Examples 4 and 6. It was found that no matter whether the quenching cooling speed was faster oil cooling or slower air cooling, martensite with higher hardness could not be obtained. The Mf (martensite transformation end temperature) of 440C stainless steel is below room temperature. Considering that water cooling, oil cooling and air cooling cannot finally cool below room temperature. Comparative Example 1 used liquid nitrogen as a cooling medium to reach its Mf point, and the hardness of the metal injection molding 440C stainless steel DA1 did not significantly improve. Changing the quenching cooling medium and the final quenching cooling temperature cannot significantly improve the hardness of the metal injection molding 440C stainless steel.

[0210] As long as S2 treatment is performed before quenching, the subsequent quenching cooling medium, no matter which of the four quenching cooling media is selected, has obvious hardness improvement compared with no S2 treatment. That is, the traditional metal injection molding 440C stainless steel cannot improve its performance in terms of raw material element content and sintering process. Through the improvement of the heat treatment process, the hardness and wear resistance of the metal injection molding 440C stainless steel are reasonably improved.

[0211] Through the comparison of the examples and comparative examples, it can be seen that the stainless steel provided by the present application has certain performance improvement compared with the sample obtained by following the heat treatment process of as-cast 440C stainless steel, and can reach 720-830 HV. Comparing Example 1, Example 8, Example 10 and Example 12 (the corresponding drawings of the specification areFigure 4 , Figure 5 , Figure 6 and Figure 7 microstructure) and Comparative Example 1, Comparative Example 4, Comparative Example 5 and Comparative Example 6 (corresponding to the microstructure of Figure 8 , Figure 9 , Figure 10 and Figure 11 microstructure), it can be found that no matter what kind of cooling method (liquid nitrogen cooling, water cooling, oil cooling, air cooling) is used after the first stage of S3 heat preservation, the acicular martensite in the matrix of the comparative examples is very obvious, and there is a clear transition zone between the martensite matrix and the chromium carbide. The examples have improved the structure by S2 treatment before S3, so that the carbon can diffuse, and finally the cryptocrystalline martensite is obtained after S3 of the examples, and the transition zone between the martensite matrix and the chromium carbide is smaller than that of the comparative examples. The hardness of the cryptocrystalline martensite in the examples is greater than that of the acicular martensite in the comparative examples, which finally leads to the hardness of the examples being significantly greater than that of the comparative examples.

[0212] The examples in this paper make the carbon element diffuse from the grain boundary to the matrix after the S2 step, and generate cryptocrystalline martensite with high carbon content in the subsequent S3. The comparative examples without S2 treatment generate acicular martensite. The hardness of cryptocrystalline martensite is better than that of acicular martensite, so the examples in this paper prove that this process can improve the hardness of metal injection molding 440C by improving the martensite. The above is only a preferred specific embodiment of the present application, but the protection scope of the present application is not limited to this. Any person skilled in the art within the technical scope disclosed in the present application, according to the technical scheme and the inventive concept of the present application, should be covered within the protection scope of the present application.

Claims

1. A metal injection molded 440C stainless steel, characterized in that, The components and their weight percentages in the stainless steel are as follows: The C content is 1.15-1.25%; The Cr content is 16-18%; The Si content is 0.6-0.8%; The Mn content is 0.6-0.8%; The Mo content is 0.45-0.55%; The 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 720-830HV. The preparation methods for 440C stainless steel by metal injection molding include: S1. 440C stainless steel powder is injected, degreased, and sintered to obtain a sintered sample; S2. The sintered sample is first heated to 800-900℃ and held at that temperature for 1-48 hours. After holding, it is cooled. Within the temperature range from the first heating temperature to 200℃, the relationship between cooling time and temperature is as follows: Where t is the cooling time in minutes, and T(t) is the temperature at the corresponding moment in degrees Celsius (°C). To maintain the temperature, cooling from 200℃ to 20-30℃ is achieved using in-furnace cooling. S3. Second heating to the austenitic state, cooling treatment, restoring to 20-30℃, third heating, cooling to obtain the stainless steel; The conditions for the second heating include: a heating temperature of 1030-1050℃ and a holding time of 10-30 minutes; The conditions for the third heating include: two stages. The heating temperature of the first stage is 130-145℃, the first stage is held at this temperature for 1.5-2.5 hours, and then cooled to 20-30℃. The heating temperature of the second stage is 130-145℃, the second stage is held at this temperature for 1.5-2.5 hours, and then cooled to 20-30℃.

2. A method for preparing 440C stainless steel by metal injection molding as described in claim 1, characterized in that, The method includes: S1. 440C stainless steel powder is injected, degreased, and sintered to obtain a sintered sample; S2. The sintered sample is first heated to 800-900℃ and held at that temperature for 1-48 hours. After holding, it is cooled. Within the temperature range from the first heating temperature to 200℃, the relationship between cooling time and temperature is as follows: Where t is the cooling time in minutes, and T(t) is the temperature at the corresponding moment in degrees Celsius (°C). To maintain the temperature, cooling from 200℃ to 20-30℃ is achieved using in-furnace cooling. S3. Second heating to the austenitic state, cooling treatment, restoring to 20-30℃, third heating, cooling to obtain the stainless steel; The conditions for the second heating include: a heating temperature of 1030-1050℃ and a holding time of 10-30 minutes; The conditions for the third heating include: two stages. The heating temperature of the first stage is 130-145℃, the first stage is held at this temperature for 1.5-2.5 hours, and then cooled to 20-30℃. The heating temperature of the second stage is 130-145℃, the second stage is held at this temperature for 1.5-2.5 hours, and then cooled to 20-30℃.

3. The method according to claim 2, characterized in that, The cooling process can be any one of liquid nitrogen cooling, water cooling, oil cooling, or air cooling.

4. The method according to claim 3, characterized in that, The conditions for liquid nitrogen cooling include: liquid nitrogen cooling and holding time of 2-6 hours, and liquid nitrogen cooling to -196℃.

5. The method according to claim 3, characterized in that, The conditions for water cooling include: water cooling and heat preservation time of 2-6 hours, and water cooling to 20-30℃.

6. The method according to claim 3, characterized in that, The conditions for oil cooling include: oil cooling and heat preservation time of 2-6 hours, and oil cooling to 20-30℃.

7. The method according to claim 3, characterized in that, The conditions for air cooling include: air cooling and heat preservation time of 2-6 hours, and air cooling to 20-30℃.

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