0cr15ni5cu4nb stainless steel and a method for manufacturing a rod thereof

Abstract

The present application relates to the technical field of stainless steel, in particular to a 0Cr15Ni5Cu4Nb stainless steel and a preparation method of a rod thereof; the chemical components include: C: 0.03%-0.05%, Cr: 14.0-15.5%, Ni: 3.5-5.5%, Cu: 2.5-4.5%, Nb: 5 times of the content of C-0.45%, Si≤1.00%, Mn≤1.00%, S≤0.005%, P≤0.010%, O≤0.002%, N≤0.001%, and the balance of Fe. The preparation method is realized based on the above chemical components. The present application cooperatively optimizes the deformation amount and the terminal temperature in the last forging process, so that the precipitation strengthening phase (such as copper-rich phase) is more fine and dispersedly distributed, the precipitation strengthening effect is improved, and the more fine grains are obtained, which is helpful to improve the strength and toughness of the material.

Description

Technical Field

[0001] This invention relates to the field of stainless steel technology, specifically to a method for preparing 0Cr15Ni5Cu4Nb stainless steel and its bars. Background Technology

[0002] With the rapid development of industrial technology, the demand for lightweighting has become increasingly prominent, and the application rate of traditional steel has gradually decreased. High-strength steel has become a key material for achieving weight reduction in structural components. Against this backdrop, (ultra)high-strength steels have experienced unprecedented rapid development in recent years, with the research and application of precipitation-hardening stainless steel being particularly prominent.

[0003] 0Cr15Ni5Cu4Nb is a martensitic precipitation hardening stainless steel with excellent machinability and dimensional stability. This steel not only possesses high strength and good transverse toughness, but also features simple heat treatment processes, minimal heat treatment deformation, and a combination of performance characteristics and processing properties. Furthermore, it exhibits excellent mechanical properties and a certain degree of corrosion resistance, and is currently widely used.

[0004] However, the tensile strength and yield strength of existing 0Cr15Ni5Cu4Nb stainless steel cannot meet the requirements of some special application scenarios. Summary of the Invention

[0005] The purpose of this invention is to solve the technical problem that the tensile strength and yield strength of existing 0Cr15Ni5Cu4Nb stainless steel cannot meet the requirements of some special application scenarios, and to provide a method for preparing 0Cr15Ni5Cu4Nb stainless steel and its bars.

[0006] To solve the above-mentioned technical problems, the technical solution provided by the present invention is as follows: A 0Cr15Ni5Cu4Nb stainless steel, by mass percentage, comprises: C: 0.03%-0.05%, Cr: 14.0-15.5%, Ni: 3.5-5.5%, Cu: 2.5-4.5%, Nb: 5 times the C content - 0.45%, Si≤1.00%, Mn≤1.00%, S≤0.005%, P≤0.010%, O≤0.002%, N≤0.001%, and the balance Fe.

[0007] Furthermore, the 0Cr15Ni5Cu4Nb stainless steel has a room temperature tensile strength of 1350-1353 MPa, a yield strength of 1231-1236 MPa, and a grain size grade of 7.

[0008] A method for preparing 0Cr15Ni5Cu4Nb stainless steel bars includes the following steps: S1. Prepare furnace charge, melt the furnace charge, and obtain steel billet with chemical composition after cooling; S2. Grind the surface of the steel billet to remove defects. S3. Preheat the steel billet after grinding at 600-700℃ for 60 minutes; After preheating, heat the billet to 800-850℃ at a heating rate of 50-80℃ / min and perform an initial heat preservation for 1 hour. After the initial heat preservation is completed, the steel billet is heated to 1140-1160℃ at a heating rate of 50-80℃ / min and then subjected to a second heat preservation for 6 hours. S4. After the heat preservation is completed, the steel billet is forged in multiple heats, and after each heat preservation is completed, it is returned to the furnace for heat preservation. In the final forging process, the steel billet is forged with a deformation of 55%-65%, an initial forging temperature of ≥1050℃, and a final forging temperature of 870℃-880℃ for 8-10 minutes; after forging, a forged billet is obtained. S5. Heat the forging billet to 610-630℃ and hold for 4-6 hours. After air cooling, the forging billet is annealed to complete the preparation of 0Cr15Ni5Cu4Nb stainless steel bar.

[0009] Furthermore, in step S1, the furnace charge consists of metallic chromium with a particle size of ≤20mm, electrolytic nickel, oxygen-free copper rods, metallic manganese, pure iron, niobium bars, crystalline silicon, and graphite carbon.

[0010] Furthermore, in step S1, the O content in the steel billet is ≤0.002%, and the N content is ≤0.001%.

[0011] Furthermore, in step S4, the multi-fire forging is replaced by a two-fire forging; The steel billet is placed in the forging mill for one-time forging, and the steel billet is forged and drawn into an initial forging billet with an octagonal cross-section; the initial forging temperature is greater than or equal to 1070℃, and the final forging temperature is greater than or equal to 905℃. After the first forging is completed, the initial forging billet is reheated to 1064-1065℃ and held for 78-82 minutes. After heat preservation, the initial forging billet is forged twice with a deformation amount of 55%-65%, an initial forging temperature of ≥1050℃, and a final forging temperature of 870℃-880℃ to obtain the forging billet.

[0012] Furthermore, step S4 specifically includes: The steel billet is placed in a forging mill for multiple forging cycles. The initial forging temperature in each cycle is greater than or equal to 1070℃, and the final forging temperature is greater than or equal to 905℃. After each forging cycle, the billet is reheated to 1064-1065℃ and held for 78-82 minutes. The steel billet is then forged and drawn into an initial forging billet with an octagonal cross-section. In the final forging process, the forging billet is forged with a deformation of 55%-65%, an initial forging temperature of ≥1050℃, and a final forging temperature of 870℃-880℃ to obtain the forging billet.

[0013] Further, in step S5, after the forging billet is annealed, the annealed forging billet is turned to round it, so as to obtain a 0Cr15Ni5Cu4Nb stainless steel bar with a diameter of 150-205mm.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: The preparation method of 0Cr15Ni5Cu4Nb stainless steel and its bars provided by this invention, compared with the traditional forging process, in order to take into account the hot working performance of 0Cr15Ni5Cu4Nb stainless steel and avoid forging cracks, usually controls the deformation amount of the last forging at about 50% and sets the final forging temperature at about 900℃. Although this conservative process can ensure smooth production, it also has the following obvious shortcomings: the higher final forging temperature is prone to recrystallization and growth of grains after forging, making it difficult to obtain a fine grain structure; while the lower deformation amount cannot fully break the as-cast structure, making it difficult to introduce sufficient dislocations and deformation energy, thereby limiting the formation efficiency of precipitation strengthening phases in subsequent heat treatment, ultimately resulting in the material's tensile strength and yield strength not reaching the optimal level.

[0015] This invention optimizes the deformation amount and final temperature during the final forging process, increasing the deformation amount to 55%-65% and reducing the final forging temperature to 870℃. Increasing the deformation amount increases the strain energy during forging, introducing higher density dislocations and deformation bands. This provides more nucleation sites for the precipitation of elements such as Cr and Cu during subsequent heat treatment, resulting in finer and more dispersed precipitation-strengthening phases (such as copper-rich phases), thus significantly improving precipitation strengthening. Lowering the final forging temperature effectively suppresses grain growth within the forged billet. Furthermore, the lower final forging temperature limits dynamic and static recrystallization processes, helping to preserve and "freeze" the fine substructures and deformed microstructures generated by the large deformation, thereby obtaining finer grains and improving the material's strength and toughness. This enables 0Cr15Ni5Cu4Nb stainless steel to meet the requirements of certain special applications.

[0016] Compared with 0Cr15Ni5Cu4Nb stainless steel bars produced by the traditional process (50% deformation, final forging temperature 900℃), the room temperature tensile strength (Rm) of 0Cr15Ni5Cu4Nb stainless steel bars produced by the process of this invention can be increased by 30-70MPa and the yield strength (Rp0.2) can be increased by 30-50MPa after the same solution + aging (H900) heat treatment, while the plasticity and toughness indicators remain at an excellent level. Attached Figure Description

[0017] Figure 1 Metallographic image of the 0Cr15Ni5Cu4Nb stainless steel bar prepared in Example 1 of this invention; Figure 2 Metallographic image of the 0Cr15Ni5Cu4Nb stainless steel bar prepared in Example 2 of this invention; Figure 3 The image shows the metallographic structure of the 0Cr15Ni5Cu4Nb stainless steel bar prepared in Comparative Example 1 of this invention. Figure 4 The image shows the metallographic structure of the 0Cr15Ni5Cu4Nb stainless steel bar prepared in Comparative Example 2 of this invention. Detailed Implementation

[0018] The technical solutions of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. 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.

[0019] Example 1 A 0Cr15Ni5Cu4Nb stainless steel, by mass percentage, comprises: C: 0.03%-0.05%, Cr: 14.0-15.5%, Ni: 3.5-5.5%, Cu: 2.5-4.5%, Nb: 5 times the C content - 0.45%, Si≤1.00%, Mn≤1.00%, S≤0.005%, P≤0.010%, O≤0.002%, N≤0.001%, and the balance Fe.

[0020] Among them, C is used for deoxidation, increasing the strength and hardness of the material, but high C content will affect the corrosion resistance of stainless steel. Therefore, the C content is 0.03%-0.05%.

[0021] Cr is used to improve the corrosion resistance and strength of materials. The added Cr element will form a Cr2O3 film on the surface of the steel ingot, which will protect the internal material. The corrosion resistance will be significantly enhanced with the increase of Cr element content. However, if the Cr content is too high, it will promote the precipitation of harmful phases and affect the mechanical property stability of 0Cr15Ni5Cu4Nb stainless steel. Therefore, the Cr content is 14.0-15.5%.

[0022] Mn is used to improve the malleability (i.e., plasticity) of stainless steel. However, excessive Mn combines with S to form MnS-type inclusions at grain boundaries, which reduces the stainless steel's resistance to pitting corrosion and intergranular corrosion. This solution reduces the adverse effects of Mn by lowering the S content, so the Mn content is less than 1.00%.

[0023] Ni is a key element in the formation of austenite and is used to stabilize the austenite structure. By adding Ni, the crystal structure of ferrite can be transformed from body-centered cubic (BCC) to face-centered cubic (FCC) structure, thereby improving the plasticity, weldability, toughness and other properties of stainless steel. Therefore, the Ni content is 3.5-5.5%.

[0024] Cu helps to enhance the mechanical properties of 0Cr15Ni5Cu4Nb steel through aging strengthening and copper phase precipitation, and also improves corrosion resistance; therefore, the Cu content is 2.5-4.5%.

[0025] The specific preparation method of the above-mentioned 0Cr15Ni5Cu4Nb stainless steel bar is as follows: 1) Prepare the furnace charge, which consists of metallic chromium with a particle size of ≤20mm, electrolytic nickel, oxygen-free copper rods, metallic manganese, pure iron, niobium bars, crystalline silicon, and graphite carbon. The furnace charge is smelted and cooled to obtain a steel billet with the correct chemical composition. 2) Grind the surface of the steel billet to remove defects; 3) Preheat the ground steel billet at 700℃ for 60 minutes; After preheating, the billet is heated to 840℃ at a heating rate of 69℃ / min and then held for the first time for 1 hour. After the initial heat preservation is completed, the steel billet is heated to 1150℃ at a heating rate of 62℃ / min and then subjected to a second heat preservation for 6 hours. 4) After the heat preservation is completed, the steel billet is placed in the forging machine for one-time forging. The steel billet is forged and drawn into an initial forging billet with an octagonal cross section and a length of 320mm. The initial forging temperature is greater than or equal to 1070℃ and the final forging temperature is greater than or equal to 905℃. After the first forging is completed, the initial forging billet is reheated to 1064℃ and held for 78 minutes; After heat preservation, the initial forging billet is subjected to two-stage forging with a deformation of 55%-65%, an initial forging temperature of ≥1050℃, and a final forging temperature of 870℃-880℃. The forging time of the two-stage forging is 8 minutes and 20 seconds. After forging, the forging billet is obtained. During the entire forging process, the initial single-pass deformation is 25-35%, the cumulative deformation is 40-60%, and the deformation rate is 0.1-0.5. The purpose is to break up the grains, allow for full dynamic recrystallization, and ensure a uniform microstructure. The deformation per pass in the later stages is 8-15%, the cumulative deformation is 10-25%, and the deformation rate is 0.01-0.05. The purpose is to control the shape and size, avoid grain growth, and ensure surface quality. The total deformation amount (final heat) is 55-65%.

[0026] 5) Heat the forging billet to 610-630℃ and hold for 4-6 hours. After air cooling, the forging billet is annealed to complete the preparation of 0Cr15Ni5Cu4Nb stainless steel bar. The annealed forging billet was machined to achieve roundness, resulting in a 0Cr15Ni5Cu4Nb stainless steel bar with a diameter of 200mm.

[0027] Example 2 The difference between Example 2 and Example 1 is as follows: The steel billet is placed in a forging machine for multiple forging cycles, and the steel billet is forged and drawn into an initial forging billet with an octagonal cross-section and a length of 320mm. The initial forging temperature in each forging process is greater than or equal to 1070℃, and the final forging temperature is greater than or equal to 905℃. After each forging cycle, the billet is reheated to 1064-1065℃ and held for 78-82 minutes. In the final forging process, the billet is forged with a deformation of 55%, an initial forging temperature of ≥1050℃, and a final forging temperature of 877℃. The forging time for the final forging process is 8 minutes and 20 seconds. After forging, the billet is obtained.

[0028] Comparative Example 1 Compared with Example 1, Comparative Example 1 differs in that the deformation amount in the last forging pass is 49%, and the final forging temperature is 905°C; a stainless steel bar with a diameter of 150 mm is obtained.

[0029] Comparative Example 2 Compared with Example 2, Comparative Example 2 differs in that the deformation amount in the last forging is 50%, and the final forging temperature is 910℃; a stainless steel bar with a diameter of 200mm is obtained.

[0030] The room temperature tensile strength, room temperature yield strength, room temperature elongation, and hardness of the 0Cr15Ni5Cu4Nb stainless steel bars prepared in Examples 1 and 2, as well as Comparative Examples 1 and 2, were tested (2-3 data points were tested), and the test results are shown in Table 1.

[0031] Table 1: Performance Test Table As shown in Table 1, the 0Cr15Ni5Cu4Nb stainless steel bar prepared by this invention has a tensile strength ≥1310MPa, a yield strength ≥1172MPa, an elongation ≥6.0%, and a reduction of area ≥20%. Compared with the comparative example, the tensile strength increased by 5.6%, the yield strength increased by 5.3%, and the elongation and reduction of area were not affected.

[0032] Meanwhile, the 0Cr15Ni5Cu4Nb stainless steel bars prepared in Examples 1 and 2, as well as Comparative Examples 1 and 2, were observed using a metallographic microscope, and the grain size grade of each stainless steel bar was evaluated; specific results are as follows. Figures 1 to 4 As shown.

[0033] The grain size of Examples 1 and 2 is grade 7, and the grain size of Comparative Examples 1 and 2 is grade 5.5. According to GB / T 6394 standard, the larger the grain size grade number, the finer the grains.

[0034] Combination Figures 1 to 4 It can be seen that the grains in Examples 1 and 2 are significantly finer and more uniform, while the grains in Comparative Examples 1 and 2 are coarser and less uniform. This directly verifies that the process measures of reducing the final forging temperature and increasing the deformation amount in the final heat have a significant effect on refining the grains of 0Cr15Ni5Cu4Nb stainless steel.

[0035] For 200mm round bars: the grain size of Example 1 (grade 7) is much smaller than that of Comparative Example 1 (grade 5.5), and for 150mm round bars: the grain size of Example 2 (grade 7) is also much smaller than that of Comparative Example 2 (grade 5.5); this shows that regardless of the size of the forged bar, the optimized final forging process can effectively refine the grains.

[0036] The grain size of both Example 1 (200 mm) and Example 2 (150 mm) is grade 7, and the grain size and uniformity are very similar; this shows that under the optimized process, even if the size of the forging bar is increased from 150 mm to 200 mm, a stable and fine equiaxed grain structure can still be obtained.

[0037] Comparative Example 1 (200 mm) and Comparative Example 2 (150 mm) both had a grain size of 5.5, with relatively coarse grains and obvious mixed grain phenomenon. This indicates that under the original process, the increase in the size of the forging bar has a more significant impact on grain coarsening, and large-size forgings are more prone to uneven microstructure.

[0038] As is well known, the higher the grain size grade, the finer the grains. Therefore, the grains of the 0Cr15Ni5Cu4Nb stainless steel bars prepared by the process of this invention are significantly finer than those prepared by the existing process, which helps to improve the strength and toughness of the material.

[0039] In summary, under the same heat treatment regime, the 0Cr15Ni5Cu4Nb forgings produced using the process of this invention exhibit significantly improved tensile strength and yield strength, fully demonstrating the effectiveness and advancement of this invention.

[0040] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions within the technical scope disclosed in the present invention should be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A 0Cr15Ni5Cu4Nb stainless steel, characterized in that, Its chemical composition, by mass percentage, includes: C: 0.03%-0.05%, Cr: 14.0-15.5%, Ni: 3.5-5.5%, Cu: 2.5-4.5%, Nb: 5 times the C content - 0.45%, Si≤1.00%, Mn≤1.00%, S≤0.005%, P≤0.010%, O≤0.002%, N≤0.001%, and the balance Fe.

2. The 0Cr15Ni5Cu4Nb stainless steel according to claim 1, characterized in that, The 0Cr15Ni5Cu4Nb stainless steel has a room temperature tensile strength of 1350-1353 MPa, a yield strength of 1231-1236 MPa, and a grain size grade of 7.

3. A method for preparing 0Cr15Ni5Cu4Nb stainless steel bars based on the 0Cr15Ni5Cu4Nb stainless steel of claim 1, characterized in that, Includes the following steps: S1. Prepare furnace charge, melt the furnace charge, and obtain steel billet with chemical composition after cooling; S2. Grind the surface of the steel billet to remove defects. S3. Preheat the steel billet after grinding at 600-700℃ for 60 minutes; After preheating, heat the billet to 800-850℃ at a heating rate of 50-80℃ / min and perform an initial heat preservation for 1 hour. After the initial heat preservation is completed, the steel billet is heated to 1140-1160℃ at a heating rate of 50-80℃ / min and then subjected to a second heat preservation for 6 hours. S4. After the heat preservation is completed, the steel billet is forged in multiple heats, and after each heat preservation is completed, it is returned to the furnace for heat preservation. In the final forging process, the steel billet is forged with a deformation of 55%-65%, an initial forging temperature of ≥1050℃, and a final forging temperature of 870℃-880℃ for 8-10 minutes; after forging, a forged billet is obtained. S5. Heat the forging billet to 610-630℃ and hold for 4-6 hours. After air cooling, the forging billet is annealed to complete the preparation of 0Cr15Ni5Cu4Nb stainless steel bar.

4. The method for preparing 0Cr15Ni5Cu4Nb stainless steel bars according to claim 3, characterized in that, In step S1, the furnace charge consists of metallic chromium with a particle size of ≤20mm, electrolytic nickel, oxygen-free copper rods, metallic manganese, pure iron, niobium bars, crystalline silicon, and graphite carbon.

5. The method for preparing 0Cr15Ni5Cu4Nb stainless steel bars according to claim 3, characterized in that, In step S1, the O content in the steel billet is ≤0.002%, and the N content is ≤0.001%.

6. The method for preparing 0Cr15Ni5Cu4Nb stainless steel bars according to claim 3, characterized in that, In step S4, the multi-fire forging is replaced by a two-fire forging. The steel billet is placed in the forging mill for one-time forging, and the steel billet is forged and drawn into an initial forging billet with an octagonal cross-section; the initial forging temperature is greater than or equal to 1070℃, and the final forging temperature is greater than or equal to 905℃. After the first forging is completed, the initial forging billet is reheated to 1064-1065℃ and held for 78-82 minutes. After heat preservation, the initial forging billet is forged twice with a deformation amount of 55%-65%, an initial forging temperature of ≥1050℃, and a final forging temperature of 870℃-880℃ to obtain the forging billet.

7. The method for preparing 0Cr15Ni5Cu4Nb stainless steel bars according to claim 3, characterized in that, Step S4 is as follows: The steel billet is placed in a forging mill for multiple forging cycles. The initial forging temperature in each cycle is greater than or equal to 1070℃, and the final forging temperature is greater than or equal to 905℃. After each forging cycle, the billet is reheated to 1064-1065℃ and held for 78-82 minutes. The steel billet is then forged and drawn into an initial forging billet with an octagonal cross-section. In the final forging process, the forging billet is forged with a deformation of 55%-65%, an initial forging temperature of ≥1050℃, and a final forging temperature of 870℃-880℃ to obtain the forging billet.

8. The method for preparing 0Cr15Ni5Cu4Nb stainless steel bars according to claim 3, characterized in that, In step S5, after the forging billet is annealed, the annealed forging billet is turned to round it, and a 0Cr15Ni5Cu4Nb stainless steel bar with a diameter of 150-205mm is obtained.

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