High-strength and high-toughness martensitic precipitation hardening stainless steel and method for manufacturing the same

Abstract

The application belongs to the technical field of materials, and relates to a high-strength and high-toughness martensite precipitation hardening stainless steel and a preparation method thereof. The chemical composition of the stainless steel is as follows in mass percent: C: 0.15-0.20, Cr: 13.0-17.5, Co: 11.0-14.0, Mo: 3.0-5.0, V: 0.2-0.6, N: 0.02-0.10, Nb: 0.2-0.70, Mn: <=0.30, P: <=0.02, S: <=0.01, Si: <=0.20, O <=0.01, and the balance is Fe. The application introduces a hot rolling process to refine the structure, increase the number of precipitated phases, and form high-stability residual austenite with dispersed distribution of precipitated phases. Finally, the martensite precipitation hardening stainless steel prepared by the preparation method has a tensile strength and hardness of more than 2000 MPa and 54 HRC respectively, and the elongation, reduction of area and U-type impact energy are significantly improved, and are more than 17%, 40% and 60 J respectively. The martensite precipitation hardening stainless steel has excellent comprehensive mechanical properties and is suitable for industrial production.

Description

Technical Field

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

[0002] Martensitic precipitation-hardening stainless steel utilizes a combination of martensitic phase transformation strengthening and secondary precipitation hardening to achieve high strength, hardness, toughness, and corrosion resistance. It is widely used in aerospace, nuclear power, and marine industries, and has become a key component in load-bearing, corrosion-resistant (or high-temperature) parts of civil aircraft, fighter jets, carrier-based aircraft landing gear, and submarine power plants. It is also used in high-temperature corrosion-resistant bearing steels, gear steels, and ball screw assemblies. Facing the demands for high-speed and high-precision components under corrosion conditions, stainless steel requires even higher strength and hardness while simultaneously possessing higher ductility and toughness. Therefore, improving the strength, hardness, and toughness of martensitic precipitation-hardening stainless steel while simultaneously enhancing its ductility and toughness has significant theoretical and practical value.

[0003] Patent CN106119736A discloses a method for preparing martensitic aging stainless steel 1Cr14Co13Mo5, achieving a tensile strength of 1900 MPa and a hardness of HRC50. Patent CN110358983A discloses a method for preparing martensitic aging stainless steel, achieving a tensile strength of 1800 MPa, a hardness of 50 HRC, an elongation exceeding 12%, and an impact energy exceeding 60 J.

[0004] Currently, martensitic precipitation-hardening stainless steel primarily relies on martensite, solid solution strengthening through interstitial atoms such as C / N, and second-phase strengthening through precipitates like carbides to improve strength and hardness. While the formation of more carbides effectively increases strength and hardness, it significantly reduces ductility and toughness. The ductility and toughness of martensitic precipitation-hardening stainless steel are mainly based on the TRIP effect induced by retained austenite and inverted austenite. While alloying or reducing the cooling rate to increase the retained austenite content, or increasing the tempering temperature to increase the inverted austenite content, can improve ductility and toughness, they often significantly reduce the strength and hardness of the stainless steel. Therefore, under conditions where tensile strength exceeds 2000 MPa and hardness exceeds 50 HRC, it is difficult to achieve higher elongation and impact toughness solely through heat treatment. Summary of the Invention

[0005] In view of the above problems, this invention discloses a high-strength, high-toughness martensitic precipitation hardening stainless steel and its preparation method. The stainless steel prepared using the method provided by this invention has a tensile strength exceeding 2000 MPa, a macroscopic hardness exceeding 54 HRC, an elongation exceeding 17%, a reduction of area exceeding 40%, and a U-shaped impact energy greater than 60 J.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A high-strength, high-toughness martensitic precipitation-hardening stainless steel, wherein the stainless steel has the following chemical composition by mass percentage: C: 0.15–0.20, Cr: 13.0–17.5, Co: 11.0–14.0, Mo: 3.0–5.0, V: 0.2–0.6, N: 0.02–0.10, Nb: 0.2–0.70, Mn: ≤0.30, P: ≤0.02, S: ≤0.01, Si: ≤0.20, O ≤0.01, with the balance being Fe.

[0008] The preparation method of the high-strength and high-toughness martensitic precipitation hardening stainless steel specifically includes the following steps:

[0009] Step 1: Load scrap steel, ferrochrome, ferrocobalt and pure iron into the smelting furnace, heat until melted, and then add a second batch of materials, including carbon raisers, high melting point carbides and intermediate alloy powder. After purging with argon gas to remove impurities, deoxidize and then cast.

[0010] Step 2: Forge the ingot in the austenitic region and cool it after forging;

[0011] Step 3: Solution treat the forging ingot, then cool it down and hot roll it;

[0012] Step 4: After short-time annealing, the hot-rolled parts are subjected to deep cooling and aging treatment to obtain high-strength and high-toughness martensitic precipitation hardening stainless steel.

[0013] Further, in step 1, the high-melting-point carbide is VC and / or NbC, with an added mass fraction of 0.02%–0.10% and a size of 0.2–2 μm, and is uniformly dispersed by stirring.

[0014] Further, in step 1, the intermediate alloy powder is ferromolybdenum, ferrovanadium, ferroniobium and ferrochromium nitride, wherein the microalloying element content is: 3.0-5.0% Mo, 0.20-0.60% V, and 0.20-0.70% Nb.

[0015] Furthermore, in step 2, the forging process involves holding the temperature at 1050℃~1250℃ for 1~3 hours before forging, and the final forging temperature is 850℃~950℃. After forging, the material is covered with asbestos or buried in sand for slow cooling.

[0016] Furthermore, in step 3, the solution treatment involves heating the forging to 1050℃~1200℃ and holding it for 2-5 hours, followed by air cooling or oil cooling.

[0017] Furthermore, in step 3, the cooled forging is heated to 850℃~1050℃ and held for 1~2 hours 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%~30%.

[0018] Further, in step 4, the hot-rolled part from step 3 is first held at 800~900℃ for 0.5~1h and then quenched to room temperature, repeated 1~2 times. Then it is held at 600~700℃ for 0.5~2h and then quenched to room temperature, repeated 1~3 times. Subsequently, it is subjected to deep cryogenic treatment at -80℃~-190℃, held for 0.5~3h and then restored to room temperature. Finally, it is aged twice at 450℃~550℃, each time for 4~10h.

[0019] The high-strength and high-toughness martensitic precipitation hardening stainless steel prepared by this invention has a microstructure consisting of a matrix composed of fine lath martensite, retained austenite with high dislocation density, and thin-film inverted austenite, with high density and fine-sized carbides (nitrides) dispersed on the matrix. The austenite grain size is 10~20μm, and the average lath size of the martensite is ≤2μm.

[0020] The high-strength and high-toughness martensitic precipitation hardening stainless steel prepared by this invention has a tensile strength exceeding 2000 MPa, a Rockwell hardness exceeding 54 HRC, an elongation exceeding 17%, a reduction of area exceeding 40%, and a U-shaped impact energy greater than 60 J.

[0021] This patent proposes to effectively refine austenite grains by adding high-melting-point carbides such as VC and NbC during smelting, and to increase the content of N element in stainless steel, which has a stronger solid solution strengthening and grain refinement effect, thereby obtaining a fine-grained austenite structure after casting and forging.

[0022] This patent further proposes that after solution treatment, hot rolling with a large reduction rate can significantly refine the grains while introducing high-density dislocations into the austenite matrix and promoting the precipitation of a large number of nano-sized carbides and carbonitrides 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.

[0023] This patent further proposes that after hot rolling, the matrix undergoes multiple low-temperature short-time annealing processes to achieve recovery and partial recrystallization, thereby refining the grains. Multiple quenching processes are used to promote martensitic transformation, resulting in austenite being distributed in a lamellar form. At the same time, the amount of carbide and carbonitride precipitates is increased, and their size and morphology are controlled.

[0024] This patent further proposes to match microalloying elements such as Mo, Cr, and Co with a cryogenic + aging process to ultimately obtain a matrix composed of fine lath martensite, retained austenite with high dislocation density, and thin-film inverted austenite. High-density carbides (nitrides) with a size of 0.05~0.2μm are dispersed on the matrix, wherein the austenite grain size is 10~20μm and the average lath size of martensite is ≤2μm. This results in high-strength and high-hardness stainless steel while significantly improving the plasticity and toughness of stainless steel, achieving a comprehensive improvement in strength and toughness.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows.

[0026] 1. The preparation method provided by the present invention introduces dispersed high-melting-point carbides and higher N content in the later stage of smelting, which can significantly refine austenite grains. Furthermore, the hot rolling process is introduced into the austenite phase region to break up the austenite grains. At the same time, it can increase the austenite dislocation density and promote the formation of high-density precipitates.

[0027] 2. The preparation method provided by this invention introduces multiple low-temperature short-time annealings after hot rolling. On the one hand, this allows the matrix to recover and partially recrystallize, further refining the grains. On the other hand, multiple quenchings promote martensitic transformation, causing the retained austenite to be distributed in a lamellar shape. The final austenite grain size is 10~20μm, and the average martensite lath size is ≤2μm. Simultaneously, it can increase the amount of carbide and carbonitride precipitates in the martensite and retained austenite, and control their size and morphology.

[0028] 3. The preparation method provided by the present invention, based on the above, combines cryogenic + aging treatment to obtain a matrix composed of fine lath martensite, retained austenite with high dislocation density and thin film-like inverted austenite, and disperses high density and fine size carbides (nitrides) on the three matrices.

[0029] 4. The high-strength, high-toughness martensitic precipitation hardening stainless steel prepared by the method of this invention has a tensile strength exceeding 2000 MPa, a macroscopic hardness exceeding 54 HRC, an elongation exceeding 17%, a reduction of area exceeding 40%, and an impact toughness exceeding 60 J / cm². 2 It exhibits significant high strength and toughness. Attached Figure Description

[0030] Figure 1 The microstructure of component A after heat treatment in Example 1 of this invention is shown.

[0031] Figure 2 The tensile fracture morphology of component B after heat treatment in Example 1 of the present invention is shown. Detailed Implementation

[0032] The present invention will be further described in detail below with reference to specific embodiments and accompanying drawings. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention. The following 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, and should not be used to limit the scope of protection of the present invention. All equivalent changes or modifications made according to the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

[0033] A high-strength, high-toughness martensitic precipitation-hardening stainless steel, wherein the stainless steel has the following chemical composition by mass percentage: C: 0.15–0.20, Cr: 13.0–17.5, Co: 11.0–14.0, Mo: 3.0–5.0, V: 0.2–0.6, N: 0.02–0.10, Nb: 0.2–0.70, Mn: ≤0.30, P: ≤0.02, S: ≤0.01, Si: ≤0.20, O ≤0.01, with the balance being Fe.

[0034] The preparation method of the high-strength and high-toughness martensitic precipitation hardening stainless steel specifically includes the following steps:

[0035] Step 1: Load scrap steel, ferrochrome, ferrocobalt, and pure iron into a smelting furnace. Heat to melt, then add a second batch of materials, including a carbon raiser, high-melting-point carbides, and master alloy powder. After purging with argon gas to remove impurities and deoxidizing, the mixture is cast. The high-melting-point carbides are VC and / or NbC, with a mass fraction of 0.02%–0.10% and a size of 0.2–2 μm. Stirring ensures uniform dispersion. The master alloy powder consists of ferromolybdenum, ferrovanadium, ferroniobium, and ferrochrome nitride, with microalloying element contents of 3.0–5.0% Mo, 0.20–0.60% V, and 0.20–0.70% Nb.

[0036] Step 2: Forge the ingot in the austenitic region and cool it after forging. The forging process is to hold it at 1050℃~1250℃ for 1~3 hours before forging. The final forging temperature is 850℃~950℃. After forging, cover it with asbestos or bury it in sand for slow cooling.

[0037] Step 3: Solution treat the forging ingot, then cool it down before hot rolling; the solution treatment involves heating the forging to 1050℃~1200℃ and holding it for 2-5 hours, then air cooling or oil cooling. After cooling, the forging is heated to 850℃~1050℃ and held for 1-2 hours before hot rolling. The final rolling temperature is 750℃~900℃, the total hot rolling reduction is 40%~80%, and the single-pass reduction is 10%~30%.

[0038] Step 4: First, hold the hot-rolled part from Step 3 at 800~900℃ for 0.5~1h, then quench it to room temperature. Repeat this process 1~2 times. Then, hold the part at 600~700℃ for 0.5~2h, then quench it to room temperature. Repeat this process 1~3 times. Next, perform deep cryogenic treatment at -80℃~-190℃, hold it for 0.5~3h, and then restore it to room temperature. Finally, perform aging treatment twice at 450℃~550℃, holding it for 4~10h each time, to obtain high-strength and high-toughness martensitic precipitation hardening stainless steel.

[0039] Metallographic samples from various heat treatment regimes of this invention were ground and polished, then etched with potassium permanganate-sulfuric acid solution, and the grain size was observed using a ZEISS-40MAT metallographic microscope. A mixed solution of picric acid, hydrochloric acid, and alcohol was used for etching, and the grain structure was observed using a ZEISS-40MAT metallographic microscope. The tensile fracture morphology was observed using a JSM-6301F field emission scanning electron microscope.

[0040] In the mechanical property testing, 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. The specimens were coarsely ground and then finely ground to ensure the upper and lower surfaces were parallel and the roughness was higher than 0.8. At least three points were measured on each specimen, and the average value was taken. The impact specimens were U-shaped specimens according to GB / T229-1994, with dimensions of 10 mm × 10 mm × 55 mm. The average value of three tests was taken as the average impact toughness.

[0041] Example 1.

[0042] A 25kg steel ingot was smelted in a vacuum induction furnace, with the steel composition set according to Table 1. Pure Fe, ferrochrome, ferrocobalt, etc., were mixed and loaded into a crucible, heated until completely melted, and a protective gas was added to maintain the temperature. During the smelting process, appropriate weights of ferromolybdenum, ferrovanadium, ferroniobium, and ferrochrome nitride, as well as 0.02% of 0.5-2μm NbC particles and 0.10% of 0.5-2μm VC particles were added, and mechanical stirring was used to ensure uniform dispersion. A deoxidizer was added before casting. The steel ingot was forged at 1150℃ for 2 hours, with a final forging temperature of 900℃, into a 50mm thick plate. After forging, it was covered with asbestos and cooled slowly. The forging was heated to 1150℃ and held for 3 hours, then air-cooled. After solution treatment, the forging was heated to 1000℃ and held for 2 hours, then hot-rolled to a final rolling temperature of 850℃, undergoing three passes with a total hot-rolling reduction of 80%. After pickling, the hot-rolled plate was first held at 900℃ for 0.5 hours and then quenched to room temperature, repeated once. Then, it was held at 700℃ for 1 hour and quenched to room temperature, repeated once. Subsequently, it was held in a liquid nitrogen + alcohol mixture (‒130℃) for 1 hour, then restored to room temperature, and then held at 450℃ for 10 hours, repeated twice. The final microstructure and properties are shown in Table 2. Figure 1The microstructure of component A after heat treatment shows that a large number of fine carbides are distributed on a fine martensitic lath matrix, and the average size of the original austenite is 18 μm. Figure 2 The fracture morphology of the tensile sample after heat treatment of component B is shown. It can be seen that the tensile fracture has obvious necking and a wide shear lip, exhibiting typical ductile fracture characteristics.

[0043] Table 1. List of chemical compositions of stainless steel in Example 1 of the present invention.

[0044]

[0045] Table 2 Tensile properties, hardness, grain size, fracture toughness and impact toughness of Example 1.

[0046]

[0047] Example 2.

[0048] A 25kg steel ingot with the composition of steel B (Table 1) was smelted in a vacuum induction furnace. Pure Fe, ferrochrome, and ferrocobalt were mixed and placed in a crucible, heated until completely melted, and a protective gas was added to maintain the temperature. During the smelting process, appropriate weights of ferromolybdenum, ferrovanadium, ferroniobium, and ferrochrome nitride, as well as 0.05% of 0.5-2μm NbC particles and 0.50% of 0.5-2μm VC particles were added, and mechanical stirring was used to ensure uniform dispersion. A deoxidizer was added before casting. The steel ingot was forged at 1250℃ for 1 hour, and the final forging temperature was 950℃, resulting in a 50mm thick plate. After forging, it was covered with asbestos and cooled slowly. The forging was heated to 1200℃ and held for 2 hours, then air-cooled. After solution treatment, it was reheated to 850~1050℃ and held for 1 hour before hot rolling at a final rolling temperature of 900℃. It underwent three hot rolling passes with a total hot rolling reduction of 60%. After pickling, the hot-rolled plate was first held at 850℃ for 1 hour and then quenched to room temperature, repeated twice. Then, it was held at 650℃ for 0.5 hours and quenched to room temperature, repeated three times. Subsequently, it was held in a liquid nitrogen + alcohol mixture (‒190℃) for 0.5 hours and then restored to room temperature, and then held at 500℃ for 5 hours, twice consecutively. The final microstructure and properties are shown in Table 3.

[0049] Table 3 Tensile properties, hardness, grain size, fracture toughness and impact toughness of Example 2.

[0050]

[0051] Example 3.

[0052] A 25kg steel ingot with the composition of C steel as shown in Table 1 was smelted in a vacuum induction furnace. Pure Fe, ferrochrome, ferrocobalt, etc., were mixed and loaded into a crucible, heated until completely melted, and a protective gas was added to maintain the temperature. During the smelting process, appropriate weights of ferromolybdenum, ferrovanadium, ferroniobium, and ferrochrome nitride, as well as 0.10% of 0.5-2μm NbC particles and 0.20% of 0.5-2μm VC particles were added, and mechanical stirring was used to ensure uniform dispersion. A deoxidizer was added before casting. The steel ingot was forged at 1050℃ for 3 hours, with a final forging temperature of 850℃, into a 50mm thick plate. The forging was then slowly cooled by burying it in sand. After forging, the forging was heated to 1050℃ for 5 hours, then oil-cooled, solution-treated, and reheated to 850℃ for 2 hours before hot rolling at a final rolling temperature of 750℃. This involved three passes of hot rolling, with a total hot rolling reduction of 40%-80%. The hot-rolled plate was pickled, then held at 800℃ for 1 hour, quenched to room temperature, and this process was repeated twice. Next, it was held at 600℃ for 2 hours and quenched to room temperature, and this process was repeated twice. Finally, it was held in a liquid nitrogen + alcohol mixture (‒80℃) for 3 hours, then restored to room temperature, and finally held at 550℃ for 4 hours, repeating this process twice. The final microstructure and properties are shown in Table 3.

[0053] Table 4 Tensile properties, hardness, grain size, fracture toughness and impact toughness of Example 3.

[0054]

[0055] 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 preparing high-strength, high-toughness martensitic precipitation-hardening stainless steel, characterized in that, Specifically, the following steps are included: Step 1: Load scrap steel, ferrochrome, ferrocobalt and pure iron into the smelting furnace, heat to melt, add materials a second time, introduce argon gas to remove impurities, deoxidize and then cast. The second material includes carbon raiser, high melting point carbide and intermediate alloy powder. Step 2: Forge the ingot in the austenitic region and cool it after forging; Step 3: Solution treat the forging ingot, then cool it down and hot roll it; the hot rolling is as follows: after cooling, the forging ingot is heated to 850℃~1050℃ and held for 1~2 hours 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%~30%; Step 4: After short-term annealing, the hot-rolled parts are subjected to deep cryogenic and aging treatments: The hot-rolled parts from Step 3 are first held at 800~900℃ for 0.5~1h and then quenched to room temperature, repeated 1~2 times; then held at 600~700℃ for 0.5~2h and quenched to room temperature, repeated 1~3 times; then deep cryogenic treatment is performed at -80℃~-190℃, held for 0.5~3h and then restored to room temperature; finally, aging treatment is performed twice at 450℃~550℃, each time held for 4~10h, to obtain high-strength and high-toughness martensitic precipitation hardening stainless steel; The chemical composition (mass percentage) of the high-strength, high-toughness martensitic precipitation hardening stainless steel is as follows: C: 0.15–0.20, Cr: 13.0–17.5, Co: 11.0–14.0, Mo: 3.0–5.0, V: 0.2–0.6, N: 0.02–0.10, Nb: 0.2–0.70, Mn: ≤0.30, P: ≤0.02, S: ≤0.01, Si: ≤0.20, O ≤0.01, with the balance being Fe.

2. The method for preparing high-strength, high-toughness martensitic precipitation-hardening stainless steel according to claim 1, characterized in that, In step 1, the high-melting-point carbide is VC and / or NbC, with an added mass fraction of 0.02%–0.10% and a size of 0.2–2 μm. It is uniformly dispersed by stirring.

3. The method for preparing high-strength, high-toughness martensitic precipitation-hardening stainless steel according to claim 1, characterized in that, In step 2, the forging process involves holding the temperature at 1050℃~1250℃ for 1~3 hours before forging, and the final forging temperature is 850℃~950℃. After forging, the material is covered with asbestos or buried in sand for slow cooling.

4. The method for preparing high-strength, high-toughness martensitic precipitation-hardening stainless steel according to claim 1, characterized in that, In step 3, the solution treatment involves heating the forging to 1050℃~1200℃ and holding it for 2-5 hours, followed by air cooling or oil cooling.

5. The method for preparing high-strength, high-toughness martensitic precipitation-hardening stainless steel according to claim 1, characterized in that, The microstructure of the high-strength and high-toughness martensitic precipitation hardening stainless steel is composed of a matrix consisting of fine lath martensite, retained austenite with high dislocation density, and thin-film inverted austenite. High-density and fine-sized carbides or carbonitrides are dispersed on the matrix, wherein the austenite grain size is 10~20μm and the average lath size of martensite is ≤2μm.

6. The method for preparing high-strength, high-toughness martensitic precipitation-hardening stainless steel according to claim 1, characterized in that, The prepared high-strength and high-toughness martensitic precipitation hardening stainless steel has a tensile strength exceeding 2000 MPa, a Rockwell hardness exceeding 54 HRC, an elongation exceeding 17%, a reduction of area exceeding 40%, and a U-shaped impact energy greater than 60 J.

Images