High-strength high-toughness maraging stainless steel and method for manufacturing the same

Abstract

The application relates to the technical field of materials, in particular to a kind of ultra-low carbon high-strength high-toughness maraging stainless steel and a preparation method thereof.The chemical composition of the stainless steel is as follows: Cr: 11.0%-14.0%, Ni: 8.5%-11.5%, Mo: 0.5%-1.5%, Al: 0.1%-1.5%, Ti: 0.5%-2.0%, Mn: <=0.20%, P: <=0.01%, S: <=0.01%, Si: <=0.20%, O: <=0.01%, C: <=0.03%, and the balance is Fe.The preparation method comprises vacuum induction melting, homogenization treatment, forging, hot rolling and heat treatment processes.By introducing hot rolling to refine the structure and regulating the number and size of precipitated phases, the maraging stainless steel prepared finally has a tensile strength of more than 2000 MPa, a macro hardness of more than 51 HRC, an elongation of more than 15%, a reduction of area of more than 50%, a U-type impact energy of greater than 50 J, and a fracture toughness of more than 100 MPa*m 1 / 2 The maraging stainless steel has the advantages of low preparation cost, high strength, high yield ratio, good toughness, high corrosion resistance and the like.

Description

Technical Field

[0001] This invention belongs to the field of materials technology, and in particular relates to a high-strength, high-toughness martensitic age-hardening stainless steel and its preparation method. Background Technology

[0002] Martensitic age-hardening stainless steel, strengthened by martensitic phase transformation and synergistic strengthening by intermetallic compounds, possesses high strength, hardness, toughness, and corrosion resistance, making it suitable for aircraft landing gear, high-temperature corrosion-resistant bearing steel, gear steel, and other components. Facing the demands for high-speed and high-precision components under corrosion conditions, stainless steel requires higher strength and hardness while also exhibiting higher ductility and toughness. Custom465 steel, by forming Ni3Ti precipitates as a strengthening phase, not only improves strength and hardness but also significantly enhances toughness and corrosion resistance, achieving a strength of 1700 MPa, an elongation of 10%, and a fracture toughness of 80 MPa·m. 1 / 2 Custom475, by adding a higher amount of Co, can increase its strength by over 2000 MPa, but its elongation is less than 5%. Therefore, simultaneously improving the strength and toughness of martensitic aging stainless steel can significantly enhance its application prospects.

[0003] Martensitic aging stainless steel utilizes ultra-low carbon and high Ni to ensure the ductility of martensite, while adding elements such as Co, Mo, and Ti to form intermetallic compounds for age-hardening to improve its strength and hardness. However, high Co content typically leads to increased costs and insufficient stiffness and toughness. Increasing the retained austenite content through alloying or heat treatment can effectively improve ductility and toughness, but this often results in reduced or coarsened precipitates, leading to decreased strength and hardness. Therefore, it is not easy to achieve good ductility and toughness in Co-free martensitic aging stainless steel while maintaining high strength and hardness (≥2000MPa, 51HRC) simply by controlling the alloying and heat treatment process. Summary of the Invention

[0004] This invention addresses the problem that existing Co-free martensitic stainless steels cannot simultaneously possess high strength, high hardness, and high ductility and toughness. It provides a martensitic age-hardening stainless steel with high strength, high hardness, and good toughness, along with its preparation method. The stainless steel prepared using the method provided by this invention exhibits a tensile strength exceeding 2000 MPa, a macroscopic hardness exceeding 51 HRC, an elongation exceeding 15%, a reduction of area exceeding 50%, a U-impact energy greater than 50 J, and a fracture toughness exceeding 100 MPa·m. 1 / 2 .

[0005] The objective of this invention is achieved through the following technical solution:

[0006] The high-strength and high-toughness martensitic age-hardening stainless steel of the present invention has the following chemical composition by mass percentage: Cr: 11.0%~14.0%, Ni: 8.5%~11.5%, Mo: 0.5%~1.5%, Al: 0.1%~1.5%, Ti: 0.5%~2.0%, Mn: ≤0.20%, P: ≤0.01%, S: ≤0.01%, Si: ≤0.20%, O: ≤0.01%, C: ≤0.03%, with the balance being Fe.

[0007] The microstructure of the high-strength and high-toughness martensitic age-hardening stainless steel is composed of a matrix consisting of fine lath martensite, retained austenite (5%-8%) with high dislocation density, and thin-film inverted austenite, with high-density nano-sized Ni3Ti, Ni3Al, Ni3(Ti,Al) precipitates and Mo clusters dispersed on the matrix.

[0008] A method for preparing high-strength, high-toughness martensitic age-hardening stainless steel includes the following steps:

[0009] Step 1: Select pure Fe, pure Ni, ferrochrome, ferromolybdenum, and titanium-aluminum alloy, mix them, and fill them into the crucible of a vacuum induction furnace. Heat them until they are completely melted and cast. Then, the ingot is remelted by vacuum consumable melting or vacuum electroslag remelting before casting.

[0010] Step 2: After homogenizing the ingot, cool it to the austenitic region for forging, and then cool it after forging;

[0011] Step 3: After heat treatment, the forging is cooled and hot rolled, and then cooled after rolling;

[0012] Step 4: Heat-treat the hot-rolled part to obtain high-strength and high-toughness martensitic age-hardening stainless steel.

[0013] The homogenization process in step 2 involves holding the material at 1050℃~1200℃ for 5-10 hours; the forging ratio during forging is 5-8, and the final forging temperature is 850℃~950℃.

[0014] The post-forging cooling process in step 2 involves holding the forging at 850℃~950℃ for 2 hours, then furnace cooling to 500℃ before removing it from the furnace, followed by asbestos covering or sand burying for slow cooling.

[0015] In step 3, the forging is heated to 1050℃~1150℃ and held for 1h-3h, then cooled to 900℃~1050℃ and held for another 1h~2h before hot rolling. The final rolling temperature is 750℃~900℃, the total hot rolling reduction rate is 40%~80%, and the single-pass reduction rate is 10%~20%.

[0016] The cooling process after hot rolling in step 3 involves cooling the hot-rolled part to 600℃~700℃ at a rate of 2℃ / s~10℃ / s, followed by oil quenching to room temperature.

[0017] The hot-rolled part in step 4 is subjected to deep cryogenic treatment at -80℃ to -196℃, held for 1h to 4h and then restored to room temperature. Finally, it is subjected to aging treatment at 350℃ to 500℃ and then air-cooled 2 to 3 times, with each holding time being 4h to 6h.

[0018] Advantages of this invention:

[0019] (1) The high-strength and high-toughness martensitic age-hardening stainless steel and its preparation method of the present invention form a composite strengthening system composed of Ni3Ti, Ni3Al, Ni3(Ti,Al) precipitates and nano-clusters Mo by alloying elements such as Ti, Al, and Mo.

[0020] (2) The high-strength and high-toughness martensitic age-hardening stainless steel and its preparation method of the present invention introduce a hot rolling and cooling process with a large reduction rate. On the one hand, it can significantly refine the grains and introduce high-density dislocations in the austenite matrix, and promote the precipitation of a large number of nano-sized intermetallic compounds and nano-clusters in the austenite, so as to strengthen the strength of the residual austenite matrix and thus significantly improve the strength and toughness of stainless steel. On the other hand, by controlling the cooling process after hot rolling, the distribution of Ni, Ti, Al and Mo elements is achieved after hot rolling, thereby promoting the transformation of residual austenite and controlling the composition and characteristics of the nano-precipitated phase, providing a basis for heat treatment to strengthen the austenite and martensitic matrix.

[0021] (3) The high-strength and high-toughness martensitic age-hardening stainless steel and its preparation method of the present invention further propose to match the processes of microalloying elements, hot rolling and deep cryogenic treatment and low-temperature long-term aging treatment, and finally obtain a matrix composed of fine lath martensite, residual austenite with high dislocation density and thin film inverted austenite, and disperse high density Ni3Ti, Ni3Al and Ni3(Ti,Al) precipitates and nanoclusters Mo with size of 0.02~0.1μm on the matrix, thereby obtaining high strength and high hardness while significantly improving the plasticity and toughness of stainless steel, and achieving a comprehensive improvement in strength and toughness.

[0022] (4) The high-strength and high-toughness martensitic age-hardening stainless steel and its preparation method of the present invention do not contain expensive metals such as Co, and have low Mo and Ti content, which reduces costs while maintaining high strength, high toughness, high hardness and high corrosion resistance; the high-strength and high-toughness martensitic age-hardening stainless steel provided by the preparation method of the present invention has a tensile strength of over 2000 MPa, a Rockwell hardness of over 51 HRC, an elongation of over 15%, a reduction of area of ​​over 40%, and an impact toughness of over 50 J / cm 2 Fracture toughness exceeding 100 MPa·m1 / 2 . Attached Figure Description

[0023] Figure 1 The metallographic morphology of component A after heat treatment in Embodiment 1 of the present invention is shown.

[0024] Figure 2 The image shows the morphology of EBSD after heat treatment of component B in Embodiment 1 of the present invention.

[0025] Figure 3 The tensile fracture morphology of component C after heat treatment in Embodiment 1 of the present invention is shown. Detailed Implementation

[0026] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0027] The present invention discloses a high-strength and high-toughness martensitic age-hardening stainless steel. The chemical composition of the high-strength and high-toughness martensitic age-hardening stainless steel is as follows (by mass percentage): Cr: 11.0%~14.0%, Ni: 8.5%~11.5%, Mo: 0.5%~1.5%, Al: 0.1%~1.5%, Ti: 0.5%~2.0%, Mn: ≤0.20%, P: ≤0.01%, S: ≤0.01%, Si: ≤0.20%, O: ≤0.01%, C: ≤0.03%, with the balance being Fe.

[0028] The microstructure of the high-strength, high-toughness martensitic age-hardening stainless steel consists of a matrix composed of fine lath martensite, retained austenite with high dislocation density (5%-8%), and thin-film inverted austenite. High-density nano-sized Ni3Ti, Ni3Al, Ni3(Ti,Al) precipitates and Mo clusters are dispersed on the matrix. Ni3(Ti,Al) represents an atomic ratio of Ni to Ti+Al of 3:1.

[0029] A method for preparing high-strength, high-toughness martensitic age-hardening stainless steel includes the following steps:

[0030] Step 1: Select pure Fe, pure Ni, ferrochrome, ferromolybdenum, and titanium-aluminum alloy, mix them, and fill them into the crucible of a vacuum induction furnace. Heat them until they are completely melted and cast. Then, the ingot is remelted by vacuum consumable melting or vacuum electroslag remelting before casting.

[0031] Step 2: After homogenizing the ingot, cool it to the austenitic region for forging, and then cool it after forging;

[0032] Step 3: After heat treatment, the forging is cooled and hot rolled, and then cooled after rolling;

[0033] Step 4: Heat-treat the hot-rolled part to obtain high-strength and high-toughness martensitic age-hardening stainless steel.

[0034] The homogenization process in step 2 involves holding the material at 1050℃~1200℃ for 5 h - 10 h; the forging ratio during forging is 5-8, and the final forging temperature is 850℃~950℃.

[0035] The post-forging cooling process in step 2 involves holding the forging at 850℃~950℃ for 2 hours, then furnace cooling to 500℃ before removing it from the furnace, followed by asbestos covering or sand burying for slow cooling.

[0036] In step 3, the forging is heated to 1050℃~1150℃ and held for 1h-3h, then cooled to 900℃~1050℃ and held for another 1h~2h before hot rolling. The final rolling temperature is 750℃~900℃, the total hot rolling reduction rate is 40%~80%, and the single-pass reduction rate is 10%~20%.

[0037] The cooling process after hot rolling in step 3 involves cooling the hot-rolled part to 600℃~700℃ at a rate of 2℃ / s~10℃ / s, followed by oil quenching to room temperature.

[0038] The hot-rolled part in step 4 is subjected to deep cryogenic treatment at -80℃ to -196℃, held for 1h to 4h and then restored to room temperature. Finally, it is subjected to aging treatment at 350℃ to 500℃ and then air-cooled 2 to 3 times, with each holding time being 4h to 6h.

[0039] Metallographic samples from various heat treatment regimes of this invention were etched using a mixed solution of picric acid, hydrochloric acid, and alcohol, and the grain structure was observed using a metallographic microscope. Grain size and martensite lath size were determined using EBSD. The tensile fracture morphology was observed using a scanning electron microscope.

[0040] In the mechanical property tests, the tensile specimens were standard specimens according to GB / T228-2002, with a gauge length of 25 mm and a diameter of 5 mm. The hardness of the heat-treated stainless steel was tested using a Rockwell hardness tester, with at least three measurements taken for each specimen, and the average value was recorded. Impact specimens were U-shaped specimens according to GB / T229-1994, with dimensions of 10 mm × 10 mm × 55 mm, and the average of three tests was taken as the average impact toughness. Plane strain fracture toughness specimens were tested according to the GB4161-2007 standard, with dimensions of 30 mm × 37.5 mm × 15 mm, pre-cracked at 3 mm, using an Instron 8801 fatigue testing machine with a beam movement speed of 0.5 mm / min. The average of three tests was taken as the average fracture toughness. Example 1

[0041] According to the composition set in Table 1, 20 kg steel ingots were melted in a vacuum induction furnace. Pure Fe, pure Ni, ferrochrome, ferromolybdenum, and titanium-aluminum alloys were mixed and placed in a crucible, heated until completely melted, and then cast. The ingots were then remelted under vacuum electroslag remelting and cast again. The billets were homogenized by holding at 1050℃ for 10 hours. Forging was then performed with a forging ratio of 5 and a final forging temperature of 850℃. The forgings were held at 850℃ for 2 hours, then furnace-cooled to 500℃ and slowly cooled while covered with asbestos. The forgings were reheated to 1050℃ for solution treatment for 3 hours, then furnace-cooled to 900℃ and held for 2 hours before hot rolling at a final rolling temperature of 750℃. Four hot rolling passes were performed, with a total hot rolling reduction of 40%. After hot rolling, the forgings were cooled to 600℃ at a rate of 2℃ / s and then oil-quenched to room temperature. After pickling, the hot-rolled plate was kept in a liquid nitrogen + alcohol mixture (‒196℃) for 1 hour, and then kept at 500℃ for 4 hours, repeated twice. The final microstructure and properties are shown in Table 2. Figure 1 The image shows the metallographic morphology of component A after heat treatment. Figure 2 The image shows the EBSD morphology of component B after heat treatment. Figure 3 The fracture morphology of the tensile sample after heat treatment of component C is shown.

[0042] Table 1. List of chemical compositions of stainless steel in embodiments of the present invention.

[0043] Element C Cr Ni Mo Ti Al Fe A 0.010 11.9 11.50 0.58 0.91 0.50 margin B 0.026 11.0 9.85 1.50 1.52 0.97 margin C 0.015 12.5 10.14 0.98 0.50 1.50 margin D 0.021 14.0 8.50 1.28 2.00 0.10 margin E 0.030 13.2 11.14 0.50 1.78 0.89 margin

[0044] Table 2. Tensile properties, hardness, impact toughness and fracture toughness of Example 1.

[0045] Element Yield strength Rp0.2 (MPa) Tensile strength Rm (MPa) Elongation A% Reduction of area Z% Hardness HRC <![CDATA[Impact toughness ak (J / cm 2 )]]> <![CDATA[Fracture toughness KIC (MPa·m 1 / 2 )]]> A 1951 2010 17.9 42.5 51.0 59 112 B 1924 2051 19.5 48.7 51.5 55 118 C 1999 2104 15.2 40.9 52.0 56 102 D 1948 2018 20.4 41.5 51.0 62 115 E 1985 2024 17.8 42.1 51.5 59 117 Example 2

[0046] 25kg steel ingots with the composition of steel B (Table 1) were melted in a vacuum induction furnace. Pure Fe, pure Ni, ferrochrome, ferromolybdenum, and titanium-aluminum alloy were mixed and placed in a crucible, heated to complete melting, and then cast. The ingots were then remelted under vacuum electroslag remelting before casting. The billets were homogenized by holding at 1100℃ for 8 hours. Forging was then performed with a forging ratio of 7 and a final forging temperature of 900℃. The forgings were held at 900℃ for 2 hours, then furnace-cooled to 500℃ and slowly cooled with sand. The forgings were reheated to 1100℃ for solution treatment for 2 hours, then furnace-cooled to 1000℃ and held for 1.5 hours before hot rolling. The final rolling temperature was 850℃, and the process involved four hot rolling passes with a total hot rolling reduction of 60%. After hot rolling, the forgings were cooled to 650℃ at a rate of 2℃ / s to 10℃ / s and then oil-quenched to room temperature. After pickling, the hot-rolled plate was kept in a liquid nitrogen + alcohol mixture (‒130℃) for 2 hours, and then kept at 450℃ for 6 hours, repeated 3 times. The final microstructure and properties are shown in Table 3.

[0047] Table 3. Tensile properties, hardness, impact toughness and fracture toughness of Example 2.

[0048] Cooling rate after rolling (°C / s) Yield strength Rp0.2 (MPa) Tensile strength Rm (MPa) Elongation A% Reduction of area Z% Hardness HRC <![CDATA[Impact toughness ak (J / cm 2 )]]> <![CDATA[Fracture toughness KIC (MPa·m 1 / 2 )]]> 2 1901 2023 21.1 51.4 51.0 59 127 4 1910 2044 18.5 47.9 51.5 57 120 6 1905 2071 16.3 45.2 51.0 55 121 8 1914 2085 15.9 43.8 51.5 55 111 10 1921 2087 15.7 42.1 52.0 52 107 Example 3

[0049] A 50kg steel ingot with the composition of steel C (Table 1) was melted in a vacuum induction furnace. Pure Fe, pure Ni, ferrochrome, ferromolybdenum, and titanium-aluminum alloy were mixed and placed in a crucible, heated to complete melting, and then cast. The ingot was remelted under vacuum and cast again. The billet was homogenized by holding at 1200℃ for 5 hours. It was then forged with a forging ratio of 8 and a final forging temperature of 950℃. The forging was held at 950℃ for 2 hours, then furnace-cooled to 500℃ and slowly cooled with sand. The forging was reheated to 1150℃ for solution treatment for 1 hour, then furnace-cooled to 1050℃ and held for 1 hour before hot rolling at a final rolling temperature of 900℃. Four hot rolling passes were performed, with a total hot rolling reduction of 80%. After hot rolling, the forging was cooled to 700℃ at a rate of 10℃ / s and then oil-quenched to room temperature. After pickling, the hot-rolled plate was kept in a liquid nitrogen + alcohol mixture (-80℃) for 4 hours, and then kept at 350℃~500℃ for 6 hours, repeated 3 times. The final microstructure and properties are shown in Table 4.

[0050] Table 4. Tensile properties, hardness, impact toughness and fracture toughness of Example 2.

[0051] Aging temperature (°C) Yield strength Rp0.2 (MPa) Tensile strength Rm (MPa) Elongation A% Reduction of area Z% Hardness HRC <![CDATA[Impact toughness ak (J / cm 2 )]]> Fracture toughness KIC (MPa·m1 / 2) 350 1999 2120 15.2 41.4 52.5 50 101 400 1978 2104 16.5 42.7 51.5 57 110 450 1925 2057 18.3 44.1 51.5 60 112 500 1901 2028 19.9 43.2 51.0 62 121

[0052] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for producing a high-strength high-toughness maraging stainless steel, characterized by comprising: The high-strength and high-toughness maraging stainless steel has the following chemical composition in percentage by mass: Cr: 11.0%-14.0%, Ni: 8.5%-11.5%, Mo: 0.5%-1.5%, Al: 0.1%-1.5%, Ti: 0.5%-2.0%, Mn: ≤0.20%, P: ≤0.01%, S: ≤0.01%, Si: ≤0.20%, O: ≤0.01%, C: ≤0.03%, and the balance of Fe. ​ The method comprises the following steps: Step 1, pure Fe, pure Ni, chromium iron, molybdenum iron, titanium aluminum alloy are mixed and then filled into a crucible of a vacuum induction furnace, heated to complete melting and cast; then the ingot is remelted by vacuum consumable melting or vacuum electroslag remelting and then cast; Step 2, after the ingot is subjected to homogenization treatment, the temperature is lowered to the austenite region for forging, and the forged product is cooled after forging, the cooling process after forging is that the forged product is furnace cooled to 500℃ after being kept at 850℃-950℃ for 2h and then taken out of the furnace, and the forged product is slowly cooled by asbestos wrapping or sand embedding; Step 3, the forged product is heated for solid solution treatment, the forged product is heated to 1050℃-1150℃ and kept for 1h-3h, then the temperature is lowered to 900℃-1050℃ and kept for 1h-2h, and then the forged product is hot-rolled, the final rolling temperature is 750℃-900℃, the total reduction rate of hot rolling is 40%-80%, the single pass reduction rate is 10%-20%, the hot-rolled product is cooled, and the cooling process is that the hot-rolled product is cooled to 600℃-700℃ at a rate of 2℃ / s-10℃ / s and then oil quenched to room temperature; Step 4, the hot-rolled product is subjected to heat treatment, the hot-rolled product is subjected to cryogenic treatment at-80℃--196℃ and kept for 1h-4h, then the temperature is restored to room temperature, and finally the hot-rolled product is subjected to aging treatment at 350℃-500℃ for 2-3 times, each time for 4h-6h, thereby obtaining the high-strength and high-toughness maraging stainless steel.

2. The method of producing a high-strength high-toughness maraging stainless steel according to claim 1, characterized by, The high-strength and high-toughness maraging stainless steel has a microstructure composed of a matrix of fine lath-shaped martensite, residual austenite containing high dislocation density and thin film-shaped reversed austenite, and a large number of nanometer-sized Ni3Ti, Ni3Al, Ni3(Ti, Al) precipitates and Mo clusters dispersedly distributed on the matrix.

3. The method of producing a high-strength high-toughness maraging stainless steel according to claim 1, characterized by, The homogenization treatment in step 2 is keeping at 1050℃-1200℃ for 5h-10h, and the forging ratio is 5-8 during forging, and the final forging temperature is 850℃-950℃.

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