A heat treatment process for improving the low-temperature toughness of low-carbon high-strength martensitic stainless steel

Abstract

The present invention belongs to the technical field of steel materials, and in particular relates to a heat treatment process for improving the low-temperature toughness of low-carbon high-strength martensitic stainless steel, which solves the problem of temper brittleness that is prone to occur during low-temperature tempering of low-carbon high-strength martensitic stainless steel, and is mainly used for manufacturing high-strength corrosion-resistant castings or forgings. First, measure the phase transformation points of relevant materials; then, heat the relevant castings or forgings to complete austenitization, hold them according to the conventional process, and after the holding is completed, quench them by air cooling, oil cooling or water cooling; after quenching, first perform an intermediate low-temperature tempering treatment on the castings or forgings at 150-250 °C to reduce the quenching stress and at the same time form certain carbon and nitrogen segregation zones in the material; finally, perform a tempering treatment on the castings or forgings at 380-480 °C, which can effectively eliminate the problem of temper brittleness and obtain high-strength and high-toughness martensitic stainless steel castings or forgings.

Description

Technical Field:

[0001] The present invention belongs to the technical field of steel materials, and particularly relates to a heat treatment process for improving the low-temperature toughness of low-carbon high-strength martensitic stainless steel, which is mainly used for manufacturing high-strength corrosion-resistant castings or forgings. Background Art:

[0002] High-strength and high-toughness martensitic stainless steel has extensive application requirements in major engineering fields. Currently, the most common high-strength martensitic stainless steel is precipitation-hardened martensitic stainless steel represented by PH17-4, PH15-5, etc. This type of martensitic stainless steel generally needs to be solution-treated at a temperature above 480°C after quenching to precipitate intermetallic compound strengthening phases to improve the yield strength of the material. In addition, Patent ZL200310104844.2 (Publication No. CN1528937A) and the applicant's patent (Application No. 202211524123.6, Title: A Low-Carbon Nitrogen-Containing High-Strength and Tough Martensitic Stainless Steel and Its Heat Treatment Method) disclose high yield-strength ratio high-strength and tough martensitic stainless steel and its production method obtained by optimizing the composition and heat treatment process based on 0Cr13Ni4Mo series materials.

[0003] In order to achieve the purpose of high yield-strength ratio and high strength and toughness, Patent ZL200310104844.2 (Publication No. CN1528937A) requires that the tempering temperature of the material ≥ 450°C; the applicant's patent (Application No. 202211524123.6, Title: A Low-Carbon Nitrogen-Containing High-Strength and Tough Martensitic Stainless Steel and Its Heat Treatment Method) proposes to control the tempering heat treatment temperature ≤ 450°C, and by controlling the content of interstitial atoms and low-temperature tempering treatment, while maintaining the high dislocation density generated by martensitic quenching to improve the strength of the material, good toughness is retained.

[0004] While maintaining the high strength of martensitic steel, how to further improve the toughness of the material to increase the service safety of related components is of great significance. As is well known, although low-temperature tempering of martensitic stainless steel is beneficial to retaining the high strength after quenching, temper embrittlement is likely to occur, making it difficult to achieve a reasonable match between strength and toughness. Summary of the Invention:

[0005] The purpose of the present invention is to provide a heat treatment process for improving the low-temperature toughness of low-carbon high-strength martensitic stainless steel. Without changing the composition of the workpiece, without increasing the equipment cost, and without significantly changing the original heat treatment process, by adjusting the initial microstructure state before tempering, the precipitation phase that induces temper embrittlement is avoided, so that the material maintains good low-temperature toughness while maintaining high strength.

[0006] Technical Solution of the Present Invention:

[0007] A heat treatment process for improving the low-temperature toughness of low-carbon high-strength martensitic stainless steel. The heat treatment process flow includes quenching → intermediate low-temperature tempering → tempering, where:

[0008] (1) Quenching: Heat the casting or forging to complete austenitization, hold it according to the conventional quenching process of martensitic stainless steel, and after the holding is completed, quench it by air cooling, oil cooling or water cooling to obtain a martensite structure;

[0009] (2) Intermediate low-temperature tempering: Perform a tempering treatment on the casting or forging at 150 - 250 °C to reduce the quenching stress, and at the same time form carbon and nitrogen segregation zones in the casting or forging;

[0010] (3) Tempering: Perform a tempering treatment on the casting or forging at 380 - 480 °C.

[0011] In the heat treatment process for improving the low-temperature toughness of low-carbon high-strength martensitic stainless steel, in step (1), after the quenching heat treatment holding is completed, quench it by air cooling, oil cooling or water cooling to obtain a martensite structure in the casting or forging, ensuring that the material has a relatively high strength in the quenched state.

[0012] In the heat treatment process for improving the low-temperature toughness of low-carbon high-strength martensitic stainless steel, in step (2), add an intermediate low-temperature tempering treatment at 150 - 250 °C between the quenching and tempering heat treatments. On the one hand, it reduces the quenching stress, and on the other hand, through the diffusion of C and N atoms, C and N element segregation zones or precipitation phases are formed at the dislocation and lath interstitial defect positions.

[0013] In the heat treatment process for improving the low-temperature toughness of low-carbon high-strength martensitic stainless steel, in step (3), the tempering temperature is 380 - 480 °C, and the cooling method after tempering is air cooling, so that the martensitic stainless steel maintains a relatively high dislocation density and at the same time generates partial precipitation phase strengthening.

[0014] In the heat treatment process for improving the low-temperature toughness of low-carbon high-strength martensitic stainless steel, by mass percentage, the composition and content of the low-carbon high-strength martensitic stainless steel are as follows: C ≤ 0.1%; Cr 12 - 16.5%; Ni 3.5 - 6.0%; Si ≤ 1.0%; Mn ≤ 1.0%; N ≤ 0.08%; Mo ≤ 1.0%; Fe: the balance.

[0015] The design idea and principle of the present invention are as follows:

[0016] Low-carbon high-strength martensitic stainless steel (with the following contents by mass percentage: C ≤ 0.1%; Cr 12 - 16.5%; Ni 3.5 - 6.0%; Si ≤ 1.0%; Mn ≤ 1.0%; N ≤ 0.08%; Mo ≤ 1.0%; Fe: the balance) usually has high strength and good toughness in the quenched state. At the same time, due to the relatively high content of alloying elements, the temper resistance is relatively good. After low-temperature tempering treatment at ≤ 480 °C, it can retain the high-density dislocations in the quenched state, thus maintaining relatively high strength. However, there is also a decrease in toughness, mainly low-temperature toughness, due to temper brittleness.

[0017] The present invention proposes to add a low-temperature tempering treatment between the quenching treatment and the tempering treatment to avoid the precipitation of harmful precipitates by changing the initial element distribution state of the precipitates (or element segregation) that induce temper brittleness, so that the high-strength martensitic stainless steel can still maintain good toughness after low-temperature tempering at ≤ 480 °C.

[0018] The advantages and beneficial effects of the present invention are as follows:

[0019] 1. The present invention adds an intermediate low-temperature tempering treatment at 150 - 250 °C between the quenching and tempering heat treatments. On the one hand, it reduces the quenching stress. On the other hand, through the diffusion of atoms such as C and N, C and N element segregation zones or precipitates are formed at defect positions such as dislocations and lath interfaces, thereby changing the precipitation behavior of the precipitates after the final tempering and avoiding the occurrence of traditional temper brittleness phenomena.

[0020] 2. The tempering temperature of the present invention is 380 - 480 °C, and the cooling method after tempering is air cooling, so that the martensitic stainless steel maintains a relatively high dislocation density, and at the same time, partial precipitates may be formed for strengthening and high strength is maintained.

[0021] 3. Using the heat treatment process of the present invention to improve the low-temperature toughness of low-carbon high-strength martensitic stainless steel, its technical indicators are as follows: yield strength 900 - 1100 MPa, tensile strength 1100 - 1300 MPa, elongation 15 - 20%, room temperature Akv: 150 - 270 J; -28 °C Akv: 120 - 260 J; -40 °C Akv: 100 - 200 J. Description of the drawings:

[0022] Figure 1 : Carbon atom distribution map measured by three-dimensional atom probe for the casting material after heat treatment in Example 1.

[0023] Figure 2 : Scanning electron microscope image of the microstructure of the bar after heat treatment in Example 2.

[0024] Figure 3 : Scanning electron microscope image of the microstructure of the bar after heat treatment in Example 3.

[0025] Figure 4 : SEM micrograph of the microstructure of the bar after heat treatment in Example 4. Specific implementation method:

[0026] In the specific implementation process, the present invention proposes a heat treatment process for improving the low-temperature toughness of low-carbon high-strength martensitic stainless steel: First, measure the phase transformation points of the relevant materials; then, heat the relevant castings or forgings to complete austenitization, hold them according to the conventional process, and after the holding is completed, quench them by air cooling, oil cooling or water cooling; after quenching, first perform an intermediate low-temperature tempering treatment on the castings or forgings at 150-250 °C to reduce the quenching stress and at the same time form certain carbon and nitrogen segregation zones in the material; finally, perform a tempering treatment on the castings or forgings at 380-480 °C to effectively eliminate the temper brittleness problem and obtain high-strength and high-toughness martensitic stainless steel castings or forgings.

[0027] Preferably, the quenching temperature is 950-1050 °C, and the holding time is 5-10 hours; the temperature of the intermediate low-temperature tempering treatment is 160-200 °C, and the holding time is 8-12 hours; the tempering treatment temperature is 400-480 °C, and the holding time is 8-12 hours.

[0028] Next, the present invention will be further elaborated in detail through examples.

[0029] Example 1

[0030] In this example, the chemical composition of the low-carbon high-strength martensitic stainless steel is (mass fraction): C: 0.09%; Cr: 12.0%; Ni: 3.55%; Si: 0.5%; Mn: 0.5%; N: 0.01%; Mo: 0.4%; Fe: the balance.

[0031] After the above-mentioned martensitic stainless steel material is prepared into a casting through vacuum smelting and pouring, it is first austenitized at 1000 °C, air-cooled and quenched to room temperature after holding for 5 hours to obtain 100% martensite structure; then, according to the present invention, it is first tempered at 250 °C for 10 hours, then cooled to room temperature, and then tempered at 450 °C for 10 hours and then air-cooled to room temperature. The mechanical properties of the material are measured as follows: yield strength 1040 MPa, tensile strength 1250 MPa, elongation 17.5%, room temperature Akv: 182 J; -28 °C Akv: 170 J; -40 °C Akv: 124 J. For comparison, after the relevant casting is heat-treated by austenitizing at 1000 °C, holding for 5 hours, air-cooled and quenched to room temperature, it is directly heat-treated by tempering at 450 °C for 10 hours. The mechanical properties of the material are measured as follows: yield strength 1025 MPa, tensile strength 1240 MPa, elongation 15.5%, room temperature Akv: 128 J; -28 °C Akv: 86 J; -40 °C Akv: 24 J. It can be seen that after using the heat treatment process proposed by the present invention, the strength of the casting basically remains unchanged, but the low-temperature impact toughness is significantly improved.

[0032] As Figure 1 shown, it can be seen from the carbon atom distribution map measured by three-dimensional atom probe after heat treatment of the casting material in Example 1 that adding low-temperature tempering at 250 °C can form a C element segregation zone at defect positions such as dislocations and lath gaps, thereby avoiding the occurrence of temper brittleness phenomenon during subsequent tempering at 450 °C.

[0033] Example 2

[0034] In this example, the chemical composition of the low-carbon high-strength martensitic stainless steel is (mass fraction): C: 0.03%; Cr: 16.5%; Ni: 4.5%; Si: 0.2%; Mn: 0.9%; N: 0.04%; Mo: 0.8%; Fe: balance.

[0035] After the above-mentioned martensitic stainless steel material is prepared into an ingot through vacuum smelting and casting, it is then forged into a φ200mm bar according to the conventional process. The bar is first austenitized at 1000°C, oil quenched and cooled to room temperature after holding for 4 hours to obtain a 100% martensite structure. Then, according to the present invention, it is first tempered at 180°C for 8 hours, then cooled to room temperature, and then tempered at 480°C for 8 hours and then air cooled to room temperature. The mechanical properties of the material are measured as follows: yield strength 980 MPa, tensile strength 1170 MPa, elongation 18%, room temperature Akv: 226 J; -28°C Akv: 215 J; -40°C Akv: 184 J. For comparison, after the relevant bar is heat treated by austenitizing at 1000°C, holding for 4 hours, oil quenching and cooling to room temperature, it is directly tempered at 480°C for 8 hours. The mechanical properties of the material are measured as follows: yield strength 965 MPa, tensile strength 1150 MPa, elongation 18%, room temperature Akv: 135 J; -28°C Akv: 85 J; -40°C Akv: 33 J. It can be seen that after using the heat treatment process proposed by the present invention, the strength of the bar basically remains unchanged, but the low-temperature impact toughness is significantly improved.

[0036] As Figure 2 shown, it can be seen from the scanning electron microscope image of the microstructure of the bar after heat treatment in Example 2 that after the intermediate low-temperature tempering treatment at 180°C + tempering treatment at 480°C, fine precipitates are precipitated within the martensite laths (rather than only precipitating between the laths during tempering at 480°C). These precipitates are due to the segregation of carbon and nitrogen atoms near the dislocation lines within the laths during the low-temperature tempering at 180°C, providing a basis for the precipitation of precipitates within the subsequent laths. These precipitates within the laths not only slightly increase the strength of the material but also do not damage the toughness of the material.

[0037] Example 3

[0038] In this example, the chemical composition of the low-carbon high-strength martensitic stainless steel is (mass fraction): C: 0.08%; Cr: 13.5%; Ni: 5.8%; Si: 0.9%; Mn: 0.9%; N: 0.07%; Mo: 0.5%; Fe: balance.

[0039] After the above-mentioned martensitic stainless steel material is prepared into an ingot through vacuum smelting and casting, it is then forged into a φ200mm bar according to the conventional process. The bar is first austenitized at 980°C, oil quenched and cooled to room temperature after holding for 4 hours to obtain 100% martensite structure. Then, according to the present invention, it is first tempered at 200°C for 8 hours, then cooled to room temperature, and then tempered at 380°C for 8 hours and then air cooled to room temperature. The mechanical properties of the material are measured as follows: yield strength 1090MPa, tensile strength 1280MPa, elongation 16%, room temperature Akv: 153J; -28°C Akv: 122J; -40°C Akv: 108J. For comparison, after the relevant bar is heat treated by austenitizing at 980°C, holding for 4 hours, oil quenching and cooling to room temperature, it is directly tempered at 350°C for 8 hours. The mechanical properties of the material are measured as follows: yield strength 1060MPa, tensile strength 1270MPa, elongation 16%, room temperature Akv: 114J; -28°C Akv: 67J; -40°C Akv: 25J. It can be seen that after using the heat treatment process proposed by the present invention, the strength of the bar basically remains unchanged, but the low-temperature impact toughness is significantly improved.

[0040] As Figure 3 shown, it can be seen from the scanning electron microscope image of the microstructure after heat treatment of the bar in Example 3 that for the quenched material, after first performing an intermediate low-temperature tempering treatment at 200°C, and then performing a tempering at 380°C, a large number of fine needle-shaped precipitates appear inside the laths of the microstructure. The precipitation of these precipitates is because the prior tempering treatment at 200°C causes the carbon and nitrogen elements to segregate at the defects within the martensite laths, providing preferential nucleation sites for the precipitation of the precipitates within the laths during the subsequent tempering at 380°C, avoiding the precipitation of the precipitates at the lath boundaries and grain boundaries, thereby significantly improving the toughness of the material compared with traditional direct tempering.

[0041] Example 4

[0042] In this example, the chemical composition of the low-carbon high-strength martensitic stainless steel is (mass fraction): C: 0.05%; Cr: 14.6%; Ni: 5.2%; Si: 0.3%; Mn: 0.2%; N: 0.03%; Mo: 0.2%; Fe: the balance.

[0043] After the above-mentioned martensitic stainless steel material is prepared into an ingot through vacuum smelting and casting, it is then forged into a φ300mm bar according to the conventional process. The bar is first austenitized at 1000°C, air-cooled and quenched to room temperature after holding for 6 hours to obtain 100% martensite structure. Then, according to the present invention, it is first tempered at 160°C for 8 hours, then cooled to room temperature, and then tempered at 400°C for 8 hours and then air-cooled to room temperature. The mechanical properties of the material are measured as follows: yield strength 975 MPa, tensile strength 1170 MPa, elongation 19%, room temperature Akv: 254 J; -28°C Akv: 245 J; -40°C Akv: 194 J. For comparison, after the same bar is heat-treated by austenitizing at 1000°C, holding for 6 hours, air-cooling and quenching to room temperature, it is directly tempered at 400°C for 8 hours. The mechanical properties of the material are measured as follows: yield strength 984 MPa, tensile strength 1185 MPa, elongation 18%, room temperature Akv: 156 J; -28°C Akv: 106 J; -40°C Akv: 36 J. It can be seen that after using the heat treatment process proposed by the present invention, the strength of the bar basically remains unchanged, but the low-temperature impact toughness is significantly improved.

[0044] As Figure 4 shown, it can be seen from the scanning electron microscope image of the microstructure after heat treatment of the bar in Example 4 that a large number of fine needle-shaped precipitates appear inside the laths of the microstructure after the quenched material is first subjected to intermediate low-temperature tempering at 160°C and then tempered at 400°C. The precipitation of these precipitates is because the prior tempering treatment at 160°C causes the carbon and nitrogen elements to segregate at the defects inside the martensite laths, providing preferential nucleation sites for the precipitation of precipitates inside the laths during the subsequent tempering at 400°C, avoiding the precipitation of precipitates at the lath boundaries and grain boundaries, thus significantly improving the toughness of the material compared with traditional direct tempering.

[0045] The implementation results show that the present invention avoids the occurrence of temper brittleness and further improves the low-temperature impact toughness of martensitic stainless steel by adding an intermediate low-temperature tempering treatment to change the initial state of the precipitation of subsequent tempering precipitates, solving the problem of easy occurrence of temper brittleness during low-temperature tempering of low-carbon high-strength martensitic stainless steel.

Claims

1. A heat treatment process for improving the low-temperature toughness of low-carbon high-strength martensitic stainless steel, characterized in that, The heat treatment process flow includes quenching → intermediate low-temperature tempering → tempering, where: (1) Quenching: Heat the casting or forging to complete austenitization, hold it according to the conventional quenching process of martensitic stainless steel, and after the holding is completed, quench it by air cooling, oil cooling or water cooling to obtain a martensitic structure; (2) Intermediate low-temperature tempering: Temper the casting or forging at 150-250 °C to reduce the quenching stress and simultaneously form carbon and nitrogen segregation zones in the casting or forging; (3) Tempering: Temper the casting or forging at 380-480 °C; By mass percentage, the composition and content of the low-carbon high-strength martensitic stainless steel are as follows: 0.03% ≤ C ≤ 0.1%; Cr 12-16.5%; Ni 3.5-6.0%; 0.2% ≤ Si ≤ 1.0%; 0.2% ≤ Mn ≤ 1.0%; 0.01% ≤ N ≤ 0.08%; 0.2% ≤ Mo ≤ 1.0%; the balance is Fe.

2. The heat treatment process for improving the low-temperature toughness of low-carbon high-strength martensitic stainless steel according to claim 1, characterized in that, In step (3), the tempering temperature is 380-480 °C, and the cooling method after tempering is air cooling, so that the martensitic stainless steel maintains a relatively high dislocation density and at the same time generates partial precipitation phase strengthening.

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