A cold-deformable, heat-treatable hardenable high-strength stainless steel without Ni and a method for producing the same

Abstract

The application belongs to the technical field of stainless steel preparation, and discloses a cold-deformable and heat-treatable hardening Ni-free high-strength stainless steel and a preparation method thereof. The stainless steel comprises the following components in percentage by mass: Cr: 13-15%, C: 0.25-0.4%, Mn: 0.5-2%, Cu: 0.1-3%, Mo: 2-3%, Si: 0-0.5%, and the balance of Fe and inevitable impurities. The stainless steel does not contain Ni element, has a low work hardening rate (strain hardening index n≤0.185) and a fracture elongation rate of more than 20% in a soft state (annealed state), is cold-deformable, and has low raw material and processing costs; after quenching and aging heat treatment, the stainless steel not only has an optimal combination of strength and toughness (yield strength and tensile strength are more than 1300 MPa and 1600 MPa respectively, and elongation is greater than 10%), but also has a high yield strength ratio.

Description

Technical Field

[0001] This invention relates to the field of stainless steel preparation technology, and in particular to a Ni-free high-strength stainless steel that can be cold-deformed and heat-hardened, and its preparation method. Background Technology

[0002] High-strength stainless steel possesses both excellent comprehensive mechanical properties and good corrosion resistance, making it widely used in high-end equipment fields such as aerospace, marine engineering, petrochemicals, and nuclear industry. Currently, ultra-high-strength steels with strengths exceeding 1.3 GPa are mostly high-carbon martensitic steels, with carbon contents reaching up to ~1.1 wt.%. These materials form supersaturated martensite after quenching, and carbide precipitation during tempering achieves strengthening. Generally, higher carbon content results in higher strength, but significantly reduced toughness, and elongation typically does not exceed 5%. Furthermore, the rapid quenching process easily generates significant residual stress, leading to deformation, warping, and even cracking of components. Subsequent correction and precision forming are difficult, hindering the mass production of precision structural parts.

[0003] Precision forming and efficient mass production of components rely on the material's good cold working ability. Good cold deformation performance enables alloys to achieve near-net-shape forming and efficient industrial production, effectively reducing machining allowances, waste loss and energy consumption. While improving the dimensional accuracy of components, it further controls the overall manufacturing cost. Therefore, high-strength stainless steel must have excellent cold working adaptability to meet the needs of low-cost, large-scale production of precision components.

[0004] Precipitation-hardening martensitic stainless steel can effectively improve the above-mentioned problems. This type of material uses low-carbon martensite as a matrix, and its strength is enhanced through the precipitation of nano-strengthening phases via low-temperature aging. It possesses both high strength and good toughness, with an elongation exceeding 10%. In its solution-treated (soft) state, the material has low strength and excellent machinability, requiring no quenching or rapid cooling. It exhibits minimal deformation and high dimensional accuracy during heat treatment, making it suitable for forming precision and complex components. After aging treatment, intermetallic compounds such as Ni3Ti and NiAl precipitate in the matrix, significantly increasing strength. However, the formation of these strengthening phases requires the addition of large amounts of elements such as nickel and cobalt. For example, PH13-8Mo contains 8 wt.% Ni, Custom465 contains 11 wt.% Ni, and Ferrium S53Cr contains 5.5 wt.% Ni and 14 wt.% Co. The high cost of nickel and cobalt raw materials increases the alloy production cost, limiting the large-scale application of the material. Therefore, the industry urgently needs to develop precipitation-hardening high-strength stainless steel that can be cold-deformed, heat-treatable, and free of high-cost alloying elements such as nickel. Summary of the Invention

[0005] The purpose of this invention is to provide a cold-deformable and heat-hardenable Ni-free high-strength stainless steel and its preparation method, thereby solving the above-mentioned problems existing in the prior art.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a cold-deformable and heat-treatable Ni-free high-strength stainless steel, comprising the following components by mass percentage: Cr: 13~15%, C: 0.25~0.4%, Mn: 0.5~2%, Cu: 0.1~3%, Mo: 2~3%, Si: 0~0.5%, balance Fe and unavoidable impurities.

[0007] This invention also provides a method for preparing Ni-free high-strength stainless steel that can be cold-deformed and heat-treatable, comprising the following steps: 1) Smelting and casting The billet is obtained by melting and casting according to the chemical composition and content of the cold-deformable and heat-treatable Ni-free high-strength stainless steel described above. 2) Heat treatment and annealing After hot working of the billet in step 1), air cool it to room temperature, then anneal it and furnace cool it to room temperature. 3) Cold working After the annealing process in step 2), cold working is performed; 4) Quenching treatment After the cold working in step 3), quenching treatment is performed; 5) Timeliness processing After the quenching treatment in step 4), an aging treatment is performed to obtain the Ni-free high-strength stainless steel that can be cold-deformed and heat-hardened.

[0008] Preferably, the melting atmosphere in step 1) is a vacuum or an inert atmosphere; the inert atmosphere includes, but is not limited to, argon.

[0009] Preferably, the hot working in step 2) includes, but is not limited to, one or more of hot rolling, hot forging, hot extrusion, and hot drawing; the temperature of the hot working is 850~1250℃; and the temperature of the annealing treatment is 700~1000℃, and the time is ≥5min.

[0010] Preferably, the cold working described in step 3) includes, but is not limited to, one or more of the following: cold rolling, cold drawing, cold stamping.

[0011] Preferably, the austenitizing temperature of the quenching treatment in step 4) is 900~1100℃, and the time is ≥5min; the quenching treatment method is water cooling or oil cooling.

[0012] Preferably, the aging treatment in step 5) is performed at a temperature of 400~550℃ for a time of ≥10 min.

[0013] As can be seen from the above technical solution, compared with the prior art, the present invention has the following beneficial effects: (1) The cold-deformable and heat-treatable Ni-free high-strength stainless steel described in this invention, in its annealed (soft) state, is a single-phase ferrite with excellent cold workability. It has low strength (yield strength ≤ 500 MPa, tensile strength ≤ 850 MPa), low hardness (≤ 230 HB), low work hardening rate (strain hardening index n ≤ 0.185), and excellent ductility (elongation ≥ 20%, reduction of area ≥ 65%). In this state, it has low deformation resistance, making it suitable for cold deformation processing and avoiding processing problems such as localized cracking during stamping and cold heading.

[0014] (2) The high-strength stainless steel described in this invention can be strengthened by aging treatment at a relatively low temperature (only 400~550℃). After aging, the alloy exhibits excellent comprehensive mechanical properties, high strength (yield strength ≥1300 MPa, tensile strength ≥1600 MPa), and good plasticity (elongation ≥10%), achieving an excellent balance between strength and plasticity, while maintaining a yield strength ratio ≥80%. Furthermore, since no strong MC carbide-forming elements such as V, Ti, Nb, and Ta are added, the alloy of this invention is essentially free of carbides (carbides are incoherent with the matrix and are prone to becoming crack initiation sites under fatigue service conditions, damaging the fatigue life of the alloy and components). The high strength, high yield strength ratio, and absence of carbides make the high-strength stainless steel of this invention highly suitable for manufacturing structural components that need to withstand high loads, resist fatigue, and ensure dimensional stability during service.

[0015] (3) By rationally adjusting the content and ratio of alloying elements, this invention can form a large amount of copper-rich nanophase after aging without adding high-valence metal elements such as Ni, Co, and V, but only by adding a small amount of elements such as C and Cu. Due to the presence of a certain amount of carbon, the martensitic matrix itself also has high strength, thereby significantly improving the overall strength of the alloy. Therefore, this invention achieves excellent comprehensive mechanical properties with lower raw material and processing costs. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0017] Figure 1 The diagram shows the EBSD phase distribution of stainless steel in the annealed and aged states of Example 1. Figure 2 The images show the TEM and EDS analysis results of the stainless steel in the aged state of Example 1. Detailed Implementation

[0018] This invention provides a cold-deformable and heat-treatable Ni-free high-strength stainless steel, comprising the following components by mass percentage: Cr: 13~15%, C: 0.25~0.4%, Mn: 0.5~2%, Cu: 0.1~3%, Mo: 2~3%, Si: 0~0.5%, balance Fe and unavoidable impurities.

[0019] In this invention: Chromium: Chromium is a key element in ensuring the corrosion resistance of stainless steel. When the chromium content is below 13%, the corrosion resistance of the steel deteriorates sharply. However, when the chromium content is too high, Cr is easily precipitated. 23 Chromium (C6) consumes carbon in the alloy, reducing its strength, and a high chromium content also increases the price of raw materials. Therefore, the chromium content should be controlled between 13% and 15%.

[0020] Carbon: Higher carbon content results in higher martensitic strength but lower toughness. Too low a carbon content leads to lower alloy strength. Too high a carbon content results in an excessively high volume fraction of martensite in the aged alloy, and since martensite itself has poor toughness, it impairs the alloy's toughness. Carbon also readily combines with chromium to form Cr. 23 C6 depletes chromium in the matrix, weakening corrosion resistance; the carbides are incompatible with the matrix, potentially reducing the alloy's fatigue life. Therefore, the carbon content should be controlled at 0.25~0.4%.

[0021] Manganese: Manganese is an effective desulfurizer, readily forming manganese sulfide inclusions with sulfur in steel, reducing sulfur-induced hot brittleness. However, when the manganese content is too low, the desulfurization effect is significantly weakened. Simultaneously, manganese is also an austenite-forming element; excessive manganese content leads to the formation of austenite in the steel, significantly reducing the alloy's yield strength and yield-to-tensile ratio. Therefore, the manganese content should be controlled between 0.5% and 2%.

[0022] Copper: Copper is an austenite-forming element with limited solid solubility in iron and does not form carbides with carbon. Adding an appropriate amount of copper, after low-temperature aging heat treatment, can precipitate fine, dispersed copper-rich phases in the martensitic and ferrite matrix, simultaneously improving the strength and hardness of the steel. However, excessive copper content may introduce austenite, reducing the alloy's yield strength and yield ratio, and also increasing raw material costs. Therefore, the copper content should be controlled between 0.1% and 3%.

[0023] Molybdenum: Molybdenum is an effective solid solution strengthening element and can also improve the alloy's resistance to pitting corrosion. Adding an appropriate amount of molybdenum can improve the alloy's strength, hardness, and corrosion resistance. However, molybdenum raw materials are expensive, and excessive content will significantly increase the production cost of raw materials. Therefore, the molybdenum content should be controlled at 2-3%.

[0024] Silicon: Silicon is a ferrite-forming element and an important deoxidizing element in steelmaking. However, excessive silicon content will increase the ferrite content in aged alloys, reducing their strength. Therefore, the silicon content should be controlled at 0-0.5%.

[0025] This invention also provides a method for preparing Ni-free high-strength stainless steel that can be cold-deformed and heat-treatable, comprising the following steps: 1) Smelting and casting The billet is obtained by melting and casting according to the chemical composition and content of the cold-deformable and heat-treatable Ni-free high-strength stainless steel described above. 2) Heat treatment and annealing After hot working of the billet in step 1), air cool it to room temperature, then anneal it and furnace cool it to room temperature. 3) Cold working After the annealing process in step 2), cold working is performed; 4) Quenching treatment After the cold working in step 3), quenching treatment is performed; 5) Timeliness processing After the quenching treatment in step 4), an aging treatment is performed to obtain the Ni-free high-strength stainless steel that can be cold-deformed and heat-hardened.

[0026] In this invention, the melting atmosphere in step 1) is preferably a vacuum or an inert atmosphere; the inert atmosphere preferably includes argon.

[0027] In this invention, the smelting furnace described in step 1) preferably includes an induction furnace or an electric arc furnace.

[0028] In this invention, the hot working in step 2) preferably includes one or more of hot rolling, hot forging, hot extrusion, and hot drawing; the temperature of the hot working is preferably 850~1250℃, more preferably 950~1150℃, and even more preferably 1050℃.

[0029] In this invention, the annealing temperature in step 2) is preferably 700~1000℃, more preferably 750~900℃, and even more preferably 880℃, and the time is preferably ≥5 min, more preferably 90~240 min, and even more preferably 120 min.

[0030] In this invention, the cold working described in step 3) preferably includes one or more of cold rolling, cold drawing, cold rolling, and cold stamping.

[0031] In this invention, the austenitizing temperature of the quenching treatment in step 4) is preferably 900~1100℃, more preferably 950~1050℃, and even more preferably 1038℃, and the time is preferably ≥5 min, more preferably 15~30 min, and even more preferably 15~30 min; the quenching treatment method is preferably water cooling or oil cooling, and even more preferably water cooling.

[0032] In this invention, the temperature of the aging treatment in step 5) is preferably 400~550℃, more preferably 450~520℃, and even more preferably 475℃, and the time is preferably ≥10 min, more preferably 2~4 h, and even more preferably 2 h.

[0033] In this invention, the annealed (soft) stainless steel has a yield strength ≤ 500 MPa, tensile strength ≤ 850 MPa, elongation ≥ 20%, reduction of area ≥ 65%, strain hardening index n ≤ 0.185, and hardness ≤ 230 HB; the aged (hardened) stainless steel has a yield strength ≥ 1300 MPa, tensile strength ≥ 1600 MPa, elongation ≥ 10%, and yield strength ratio ≥ 80%. The resulting cold-deformable and heat-hardenable Ni-free high-strength stainless steel (hardened state) has a microstructure consisting of two phases (lamellar martensite and equiaxed ferrite), with a high-density Cu-rich nanophase dispersed throughout both phases.

[0034] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] Example 1

[0036] In this embodiment, the chemical composition of the cold-deformable and heat-hardenable Ni-free high-strength stainless steel is as follows (mass percentage): Cr: 13%, C: 0.32%, Mn: 1%, Cu: 2.2%, Mo: 2.5%, Si: 0.1%, with the balance being Fe and unavoidable impurities.

[0037] The preparation method of Ni-free high-strength stainless steel that can be cold-deformed and heat-treatable is as follows: 1) The chemical composition and content of the Ni-free high-strength stainless steel that can be cold-deformed and heat-treated are as described above, and the steel is melted in an induction melting furnace in an argon atmosphere. After the melting is completed, the steel is cast into an alloy ingot. 2) The alloy ingot is heated to 1050℃ and held for 30 min, followed by hot rolling. The hot rolling deformation is controlled at 80%. After hot rolling deformation, it is annealed at 880℃ for 120 min, then cooled to room temperature in the furnace to obtain the annealed product. The annealed product is then cold rolled to control the cold rolling deformation at 50%. After cold working, it is heated to 1038℃ and held for 30 min for austenitization, followed by rapid cooling by water quenching. Then, it is aged at 475℃ for 2 h and naturally cooled to room temperature to obtain a Ni-free high-strength stainless steel that can be cold-deformed and heat-treatable.

[0038] Figure 1 This is a phase distribution diagram of the EBSD phase in the annealed and aged states of the Ni-free high-strength stainless steel that can be cold-deformed and heat-treated for hardening in Example 1. Clearly, the stainless steel is a single-phase ferrite in the annealed state, and in the aged state, it consists of very fine lath-like martensite and relatively coarse ferrite (10~15 μm) grains.

[0039] Figure 2 These are TEM and EDS images of the aged (hardened) state of the Ni-free high-strength stainless steel that can be cold-deformed and heat-treated in Example 1. It can be seen that the copper-rich precipitates are spherical and dispersed, with a size of 3-5 nm.

[0040] Example 2

[0041] In this embodiment, the chemical composition of the cold-deformable and heat-hardenable Ni-free high-strength stainless steel is as follows (mass percentage): Cr: 13%, C: 0.32%, Mn: 1.5%, Cu: 2.2%, Mo: 2.5%, Si: 0.1%, with the balance being Fe and unavoidable impurities.

[0042] The preparation method of Ni-free high-strength stainless steel that can be cold-deformed and heat-treatable is as follows: 1) The chemical composition and content of the Ni-free high-strength stainless steel that can be cold-deformed and heat-treated are as described above, and the alloy ingots are cast into alloy ingots in an electric arc melting furnace under an argon atmosphere after the melting is completed. 2) The alloy ingot is heated to 1050℃ and held for 30 min, followed by hot rolling. The hot rolling deformation is controlled at 80%. After hot rolling deformation, it is annealed at 880℃ for 120 min, then cooled to room temperature in the furnace to obtain the annealed product. The annealed product is then cold rolled to control the cold rolling deformation at 50%. After cold working, it is heated to 1038℃ and held for 30 min for austenitization, followed by rapid cooling by water quenching. Then, it is aged at 500℃ for 2 h and naturally cooled to room temperature to obtain Ni-free high-strength stainless steel that can be cold-deformed and heat-treatable.

[0043] Example 3

[0044] In this embodiment, the chemical composition of the cold-deformable and heat-hardenable Ni-free high-strength stainless steel is as follows (mass percentage): Cr: 13.5%, C: 0.33%, Mn: 1%, Cu: 2.2%, Mo: 2.5%, balance Fe and unavoidable impurities.

[0045] The preparation method of Ni-free high-strength stainless steel that can be cold-deformed and heat-treatable is as follows: 1) The chemical composition and content of the Ni-free high-strength stainless steel that can be cold-deformed and heat-treated are as described above, and the alloy ingots are cast into alloy ingots in an electric arc melting furnace under an argon atmosphere after the melting is completed. 2) The alloy ingot is heated to 1050℃ and held for 30 min, followed by hot rolling. The hot rolling deformation is controlled at 80%. After hot rolling deformation, it is annealed at 880℃ for 120 min, then cooled to room temperature in the furnace to obtain the annealed product. The annealed product is then cold rolled to control the cold rolling deformation at 50%. After cold working, it is heated to 1038℃ and held for 15 min for austenitization, followed by rapid cooling by water quenching. Then, it is aged at 475℃ for 2 h and naturally cooled to room temperature to obtain a Ni-free high-strength stainless steel that can be cold-deformed and heat-treatable.

[0046] Comparative Example 1

[0047] The only difference between this comparative example and Example 1 is that it does not contain Cu; other chemical components and their contents, as well as the preparation conditions, remain unchanged.

[0048] Comparative Example 2

[0049] The only difference between this comparative example and Example 1 is that it does not contain element C; other chemical components and their contents, as well as the preparation conditions, remain unchanged.

[0050] Comparative Example 3

[0051] The only difference between this comparative example and Example 1 is that the mass percentage of C is 0.5%; the other chemical components and their contents, as well as the preparation conditions, remain unchanged.

[0052] The properties of the stainless steels prepared in the two states of Examples 1-3 and Comparative Examples 1-3 were tested. Tensile strength was tested according to the testing standard GB / T 228.1-2021, elongation was recorded based on the output of a video extensometer, Brinell hardness was used, and the yield strength ratio was recorded as the quotient of yield strength and tensile strength. The test results for the annealed samples are shown in Table 1, and the test results for the hardened samples are shown in Table 2.

[0053] Table 1 Performance test results of stainless steel in annealed state

[0054] Table 2 Performance test results of stainless steel in hardened state

[0055] As shown in the table above, the hardened state of the embodiments of the present invention exhibits the best combination of strength and toughness (yield strength ≥ 1300 MPa, tensile strength ≥ 1600 MPa, elongation ≥ 10%), and has a high yield strength ratio (yield strength ratio ≥ 80%). The annealed state of the embodiments of the present invention has good cold working performance, strain hardening index n ≤ 0.185, hardness ≤ 230 HB, elongation ≥ 20%, and reduction of area ≥ 65%.

[0056] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A cold-deformable and heat-treatable Ni-free high-strength stainless steel, characterized in that, The components include the following mass percentages: Cr: 13~15%, C: 0.25~0.4%, Mn: 0.5~2%, Cu: 0.1~3%, Mo: 2~3%, Si: 0~0.5%, balance Fe and unavoidable impurities.

2. The method for preparing a cold-deformable and heat-treatable Ni-free high-strength stainless steel according to claim 1, characterized in that, Includes the following steps: 1) Smelting and casting The billet is obtained by melting and casting according to the chemical composition and content of the cold-deformable and heat-treatable Ni-free high-strength stainless steel described above. 2) Heat treatment and annealing After hot working of the billet in step 1), air cool it to room temperature, then anneal it and furnace cool it to room temperature. 3) Cold working After the annealing process in step 2), cold working is performed; 4) Quenching treatment After the cold working in step 3), quenching treatment is performed; 5) Timeliness processing After the quenching treatment in step 4), an aging treatment is performed to obtain the Ni-free high-strength stainless steel that can be cold-deformed and heat-hardened.

3. The method for preparing cold-deformable and heat-treatable Ni-free high-strength stainless steel according to claim 2, characterized in that, The melting atmosphere described in step 1) is a vacuum or an inert atmosphere; the inert atmosphere includes, but is not limited to, argon.

4. The method for preparing cold-deformable and heat-treatable Ni-free high-strength stainless steel according to claim 3, characterized in that, The hot working process described in step 2) includes, but is not limited to, one or more of hot rolling, hot forging, hot extrusion, and hot drawing; the temperature of the hot working process is 850~1250℃; the temperature of the annealing treatment is 700~1000℃, and the time is ≥5 min.

5. The method for preparing cold-deformable and heat-treatable Ni-free high-strength stainless steel according to claim 4, characterized in that, The cold working process mentioned in step 3) includes, but is not limited to, one or more of the following: cold rolling, cold drawing, cold stamping.

6. The method for preparing cold-deformable and heat-treatable Ni-free high-strength stainless steel according to claim 5, characterized in that, The austenitizing temperature of the quenching treatment in step 4) is 900~1100℃, and the time is ≥5 min; the quenching treatment method is water cooling or oil cooling.

7. The method for preparing cold-deformable and heat-treatable Ni-free high-strength stainless steel according to any one of claims 2 to 6, characterized in that, The aging treatment described in step 5) is performed at a temperature of 400~550℃ for a time of ≥10 min.

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