Method for increasing the yield strength ratio of vanadium-containing martensitic precipitation hardening stainless steels

Abstract

The application discloses a method for improving yield strength ratio of vanadium-containing martensite precipitation hardening stainless steel, and belongs to the field of metal material hot working process. The method is characterized in that: the vanadium-containing martensite precipitation hardening stainless steel after smelting is subjected to hot deformation treatment, so that the stainless steel with a grain size of 9 levels is obtained; and the stainless steel is subjected to heat treatment process, so that the vanadium-containing martensite precipitation hardening stainless steel with a Rp0.02 / Rp0.2 yield strength ratio of greater than or equal to 0.80 is obtained. The method can effectively solve the problem that the Rp0.02 / Rp0.2 yield strength ratio of the existing hardening stainless steel is low and cannot meet the application requirements.

Description

Methods to improve the yield strength ratio of vanadium-containing martensitic precipitation hardening stainless steel Technical Field

[0001] This invention belongs to the field of hot working technology of metal materials, and relates to a method for manufacturing vanadium-containing martensitic precipitation hardening stainless steel, specifically a method for improving the yield strength ratio of vanadium-containing martensitic precipitation hardening stainless steel Rp0.02 / Rp0.2. Background Technology

[0002] Martensitic precipitation-hardening stainless steels, represented by 0Cr17Ni4Cu4Nb and 0Cr13Ni8Mo2Al, have been widely used in power, aerospace, nuclear reactors, and petrochemical industries. Examples include low-pressure turbine blades, compressor blades for gas turbines and aero-engines, cold-heading and machined fasteners, nuclear reactor components, and key components for high-end equipment such as petrochemical equipment. With technological upgrades, higher requirements are being placed on the materials used in these key components. Specifically, while ensuring material strength, it is necessary to improve ductility and toughness, and simultaneously increase the Rp0.02 / Rp0.2 yield strength ratio. Due to the high alloy content and large deformation resistance of these materials, improper hot deformation processes can easily lead to billet cracking, significantly reducing the production yield of this steel. At the same time, improper heat treatment processes also make it difficult to improve the Rp0.02 / Rp0.2 yield strength ratio, thus affecting the material's production cost, economic benefits, and application scenarios. Summary of the Invention

[0003] The technical problem to be solved by the present invention is that the yield strength ratio of existing hardened stainless steel is low (Rp0.02 / Rp0.2), which cannot meet the application requirements.

[0004] The technical solution adopted by this invention to solve its technical problem is: a method to improve the yield strength ratio of vanadium-containing martensitic precipitation hardening stainless steel: by hot deformation treatment of smelted vanadium-containing martensitic precipitation hardening stainless steel to obtain stainless steel with a grain size of grade 9; and then by heat treatment process to obtain vanadium-containing martensitic precipitation hardening stainless steel with a yield strength ratio of Rp0.02 / Rp0.2 ≥ 0.80.

[0005] The mechanical properties of the above-mentioned vanadium-containing martensitic precipitation hardening stainless steel are Rm≥1200MPa, Rp0.2≥1050MPa, A≥18%, Z≥60%, and KV2≥50J.

[0006] The aforementioned vanadium-containing martensitic precipitation-hardening stainless steel has the following chemical composition by weight percentage: C 0.05-0.10, Cr 14.0-15.0, Ni 3.0-5.5, Cu 3.0-5.0, Mo 0.50-2.0, W 0-1.0, V 0.10-0.60, Mn 0.30-0.70, Si 0.2-0.6, Nb 0.2-0.4, Ta 0-0.02, Al 0.01-0.03, with the balance being Fe and unavoidable impurities. The unavoidable impurities include P and S, with P ≤ 0.002% and S ≤ 0.003% by weight.

[0007] The above-mentioned heat deformation treatment includes the following steps:

[0008] A. Heating of electroslag ingots: Heat to 850±10℃ at a heating rate of ≤50℃ / h, hold for 1-3h, then heat to 1180℃±10℃ at a heating rate of ≤150℃ / h, hold for more than 12h.

[0009] B. Forging blanking: The steel ingot processed in step A is forged once, and after completion, it is heated in the furnace to 1180±10℃ and held for ≥1h. After being taken out of the furnace, it is forged again and drawn once. After completion, it is heated in the furnace to 1180±10℃ and held for ≥1h. After being taken out of the furnace, it is drawn again.

[0010] C. Precision forging: Heat the steel billet processed in step B to 1180±10℃ and hold for ≥2h, then precision forge it.

[0011] D. Annealing heat treatment: After the steel billet is processed in step C, it is loaded into the annealing furnace and heated to 650±10℃ at a heating rate of ≤80℃ / h and held at that temperature. Then, it is slowly cooled in the furnace at a cooling rate of ≤50℃ / h until the billet temperature reaches 350℃ before being removed from the furnace and air-cooled.

[0012] Furthermore, in step B above, at least one of the following conditions must be met:

[0013] The forging process involves upsetting a steel ingot radially and axially into a square billet, such that the length of the billet is no more than 60% of the original length.

[0014] The secondary forging and primary drawing process involves upsetting the steel billet after secondary heating along the diagonal edge to a square billet with a length not exceeding 40% of the original length, and then drawing the steel billet axially to more than 60% of the original length.

[0015] The secondary drawing process involves drawing the steel billet to more than 100% of its original length.

[0016] The initial forging temperature should be controlled to be no less than 1100±10℃, and the final forging temperature should be no less than 1010℃.

[0017] Furthermore, in step C above, the initial forging temperature for precision forging is not lower than 1100±10℃, and the final forging temperature is not lower than 1010℃.

[0018] Furthermore, in step D above, the annealing furnace temperature is controlled at 400-500℃, and the holding time is calculated as t(h)=1.5+0.025*(bar diameter mm-100).

[0019] The above heat treatment process includes the following steps:

[0020] a. Solution heat treatment: Heat the heat-deformed material to 1040±10℃ at a heating rate of 20-50℃ / min and hold for 1 hour;

[0021] b. First aging treatment: Place the material treated in step a into an aging furnace at 480±10℃ and keep it at that temperature for 3-5 hours. Then, remove the material from the furnace and air cool it to room temperature.

[0022] c. Secondary aging heat treatment: Place the material treated in step b into an aging furnace at 450-480±10℃ and keep it at that temperature for 3-5 hours. Then, remove the material from the furnace and air cool it to room temperature.

[0023] The beneficial effects of this invention are as follows: This invention utilizes controlled heating temperature, heating rate, hot deformation variables, and subsequent heat treatment processes to produce a vanadium-containing vanadium precipitation-hardening stainless steel material after smelting. This results in a grain size control of approximately grade 9 after hot deformation. The resulting steel exhibits excellent comprehensive mechanical properties after heat treatment, namely Rm≥1200MPa, Rp0.2≥1050MPa, Rp0.02 / Rp0.2≥0.80, A≥18%, Z≥60%, and KV2≥50J. This overcomes the production limitations of this steel and completely solves the cracking problem caused by improper hot deformation processes. The steel ingots produced by this invention exhibit stable mechanical properties after tempering heat treatment, and the Rp0.02 / Rp0.2 ratio increases from approximately 0.70 to over 0.80, meeting the user's requirements for high strength and high precision under medium-temperature conditions. Detailed Implementation

[0024] The technical solution of the present invention can be implemented in the following manner.

[0025] This invention provides a method for improving the yield strength ratio of vanadium-containing martensitic precipitation-hardening stainless steel, wherein the chemical composition of the vanadium-containing martensitic precipitation-hardening stainless steel is as follows (by weight percentage):

[0026] C: 0.05-0.10, Cr: 14.0-15.0, Ni: 3.0-5.5, Cu: 3.0-5.0, Mo: 0.50-2.0, W: 0-1.0, V: 0.10-0.60, Mn: 0.30-0.70, Si: 0.2-0.6, Nb: 0.2-0.4, Ta: 0-0.02, Al: 0.01-0.03, with the balance being Fe and unavoidable impurities. The unavoidable impurities include P and S, with P ≤ 0.002% and S ≤ 0.003% by weight.

[0027] The hot deformation method for vanadium-containing martensitic precipitation-hardening stainless steel includes the following steps:

[0028] ① Electroslag ingot heating

[0029] The electroslag ingot is placed in a heating furnace and heated to 850±10℃ at a heating rate of ≤50℃ / h, held for 1-3 hours, and then heated to 1180℃±10℃ at a heating rate of ≤150℃ / h, held for more than 12 hours. Because this steel has a high alloy element content, cracking often occurs during the initial heating process due to excessive thermal stress; therefore, the heating rate must be controlled.

[0030] ② Forging blanking

[0031] Heated steel ingots are removed from the heating furnace for forging. The forging process employs a two-stage upsetting and two-stage drawing process: First, the steel ingot is upset radially and axially to form a square billet, with the billet length not exceeding 60% of its original length. The billet from the first upsetting is returned to the heating furnace for reheating at 1180±10℃. After holding at this temperature for at least 1 hour, it is removed from the furnace for a second upsetting and a first drawing: the reheated billet is upset diagonally to form a square billet with a length not exceeding 40% of its original length, and then drawn axially to more than 60% of its original length. The billet from the second upsetting and first drawing process is returned to the heating furnace for reheating at 1180±10℃. After holding at this temperature for at least 1 hour, it is removed from the furnace for a second drawing: the billet from the third heating process is drawn to form a round billet exceeding 100% of its original length. To ensure billet quality, the initial forging temperature is typically no lower than 1100±10℃, and the final forging temperature is no lower than 1010℃. Through a two-stage upsetting and two-stage drawing process, the uniformity of the billet's microstructure can be improved, the grain size refined, and the mechanical properties of the material enhanced.

[0032] ③ Precision forging

[0033] The steel billet after step ② is precision forged. The precision forging process involves placing the steel billet after forging in a heating furnace and heating it at a temperature of 1180±10℃ for ≥2 hours. The heated steel billet is then precision forged into finished product specifications using a precision forging machine. The precision forging temperature in step ③ is not lower than 1100±10℃, and the final forging temperature is not lower than 1010℃.

[0034] ④ Annealing heat treatment

[0035] The steel billet after precision forging in step ③ is loaded into an annealing furnace for waiting. The temperature of the annealing furnace is controlled between 400-500℃. The billet is heated to 650±10℃ at a heating rate of ≤80℃ / h, and then held at that temperature for a time calculated as t(h)=1.5+0.025*(bar diameter mm-100). It is then slowly cooled in the furnace to a temperature of 350℃ before being removed and air-cooled. The slow cooling rate in the furnace must be ≤50℃ / h. Because the steel bar experiences significant thermal stress due to the large temperature difference between its inner and outer surfaces after hot rolling or forging, to prevent deformation and cracking of high-alloy vanadium-containing martensitic precipitation-hardening stainless steel after forging, the material should be placed in an annealing furnace immediately after forging for stress-relieving annealing.

[0036] The heat treatment process for the vanadium-containing martensitic precipitation-hardening stainless steel includes the following steps:

[0037] ① Solution heat treatment

[0038] The precision-forged material is placed in a resistance furnace and heated to 1040±10℃ at a heating rate of 20-50℃ / min, and held at that temperature for 1 hour (the time to reach the desired temperature is calculated as effective thickness × 0.3 min / mm). The purpose of this process is to remelt carbides, copper-rich phases, and other precipitates in the material, thereby fully austenitizing the matrix and eliminating compositional inhomogeneities. This facilitates the formation of uniform supersaturated martensite after cooling, which is beneficial for subsequent aging treatment.

[0039] ② One-time processing

[0040] The material after solution heat treatment is placed in an aging furnace at 480±10℃ and held for 3-5 hours. After being removed from the furnace, it is air-cooled to room temperature.

[0041] ③ Secondary aging heat treatment

[0042] After the material has undergone one aging treatment, it is placed in an aging furnace at 450-480±10℃ and kept at that temperature for 3-5 hours. After being removed from the furnace, it is air-cooled to room temperature.

[0043] The technical solution and effects of the present invention will be further explained below through practical examples.

[0044] Example

[0045] Example 1

[0046] This embodiment provides a method for improving the yield strength ratio of vanadium-containing martensitic precipitation-hardening stainless steel. Its chemical composition, by weight percentage, is: C: 0.079, Cr: 14.48, Ni: 5.10, Cu: 3.00, Mo: 1.00, W: 0.602, V: 0.599, Mn: 0.480, Si: 0.412, Nb: 0.277, Ta: 0.008, Al: 0.017, with the balance being Fe and unavoidable impurities. The unavoidable impurities in the novel vanadium-containing martensitic precipitation-hardening stainless steel include P and S, with P ≤ 0.002% and S ≤ 0.003% by weight. Specifically, taking a Φ180mm steel billet as an example, its hot deformation process is as follows:

[0047] (1) Electroslag ingot heating

[0048] Place the Φ550mm×1500mm electroslag ingot in a heating furnace, heat it to 850℃ at a heating rate of 40℃ / h, hold it for 2.5 hours, then heat it to 1180℃ at a heating rate of 120℃ / h, and hold it for 12 hours.

[0049] (2) Electroslag ingot forging and billet preparation

[0050] The heated steel ingot is then removed from the heating furnace for forging. This rapid forging process employs a two-stage upsetting and two-stage drawing process: First, the steel ingot is upset radially and axially to form a square billet with a length of 880mm. The billet is then returned to the heating furnace for reheating at 1180±10℃. After holding at this temperature for one hour, it undergoes a second upsetting and a first drawing: the reheated billet is upset to a square billet with a length of 600mm, and then drawn axially by 950mm. The billet, after the second upsetting and first drawing, is returned to the heating furnace for reheating at 1180±10℃. After holding at this temperature for one hour, it undergoes a second drawing: the billet, after three heating stages, is drawn to a round billet of 1600mm. To ensure billet quality, the initial forging temperature is typically not lower than 1100±10℃, and the final forging temperature is not lower than 1010℃.

[0051] (3) Precision forging

[0052] The rapidly forged steel ingot is then precision forged. This involves placing the forged steel billet in a heating furnace and heating it to 1180℃±10℃ for 12 hours. The heated billet is then precision forged into a finished product with a diameter of Φ180mm using a precision forging machine. The initial forging temperature is typically not lower than 1100±10℃, and the final forging temperature is not lower than 1010℃.

[0053] (1) Solution heat treatment

[0054] Cut samples from the precision forged material and place them in a resistance furnace. Heat the samples to 1040±10℃ at a heating rate of 25℃ / min and hold for 1 hour. Remove the samples from the furnace and cool them to room temperature.

[0055] (2) One-time time-sensitive processing

[0056] After solution treatment, the sample was placed in an aging furnace at 480±10℃ and kept at that temperature for 4 hours. After being removed from the furnace, it was air-cooled to room temperature.

[0057] (3) Secondary aging heat treatment

[0058] The material after one aging treatment is placed in an aging furnace at 450℃±10℃ and 480℃±10℃, held for 4 hours, and then air-cooled to room temperature.

[0059] According to the product technical requirements in this implementation, as shown in Table 1, the mechanical properties of the obtained product after quenching and tempering heat treatment are shown in Table 2.

[0060] Example 2

[0061] This embodiment provides a method for improving the yield strength ratio of vanadium-containing martensitic precipitation-hardening stainless steel. Its chemical composition, by weight percentage, is: C: 0.075, Cr: 14.54, Ni: 4.93, Cu: 3.28, Mo: 1.81, W: 0.075, V: 0.357, Mn: 0.499, Si: 0.410, Nb: 0.282, Ta: 0.007, Al: 0.015, with the balance being Fe and unavoidable impurities. The unavoidable impurities in the novel vanadium-containing martensitic precipitation-hardening stainless steel include P and S, with P ≤ 0.002% and S ≤ 0.003% by weight. Specifically, taking a Φ350mm steel billet as an example, its hot deformation process is as follows:

[0062] (1) Electroslag ingot heating

[0063] Place the Φ550mm×1500mm electroslag ingot in a heating furnace, heat it to 850℃ at a heating rate of 40℃ / h, hold it for 2.5 hours, then heat it to 1180℃ at a heating rate of 120℃ / h, and hold it for 12 hours.

[0064] (2) Electroslag ingot forging and billet preparation

[0065] The heated steel ingot is then removed from the heating furnace for forging. This rapid forging process employs a two-stage upsetting and two-stage drawing process: First, the steel ingot is upset radially and axially to form a square billet with a length of 880mm. The billet is then returned to the heating furnace for reheating at 1180±10℃. After holding at this temperature for one hour, it undergoes a second upsetting and a first drawing: the reheated billet is upset to a square billet with a length of 600mm, and then drawn axially by 950mm. The billet, after the second upsetting and first drawing, is returned to the heating furnace for reheating at 1180±10℃. After holding at this temperature for one hour, it undergoes a second drawing: the billet, after three heating stages, is drawn to a round billet of 1600mm. To ensure billet quality, the initial forging temperature is typically not lower than 1100±10℃, and the final forging temperature is not lower than 1010℃.

[0066] (4) Precision forging

[0067] The rapidly forged steel ingot is then precision forged. This involves placing the forged steel billet in a heating furnace and heating it to 1180℃ for 12 hours. The heated billet is then precision forged into a finished product with a diameter of Φ350mm using a precision forging machine. The initial forging temperature is typically not lower than 1100±10℃, and the final forging temperature is not lower than 1010℃.

[0068] (4) Solution heat treatment

[0069] Cut samples from the precision forged material and place them in a resistance furnace. Heat the samples to 1040±10℃ at a heating rate of 25℃ / min and hold for 1 hour. Remove the samples from the furnace and cool them to room temperature.

[0070] (5) Time-sensitive processing

[0071] After solution treatment, the sample was placed in an aging furnace at 480±10℃ and kept at that temperature for 4 hours. After being removed from the furnace, it was air-cooled to room temperature.

[0072] (6) Secondary aging heat treatment

[0073] The material after one aging treatment is placed in an aging furnace at 450℃±10℃ and 480℃±10℃, held for 4 hours, and then air-cooled to room temperature.

[0074] According to the product technical requirements in this implementation, as shown in Table 1, the mechanical properties of the obtained product after quenching and tempering heat treatment are shown in Table 2.

[0075] Table 1 Product Technical Requirements

[0076]

[0077] Table 2 Mechanical properties of the samples after heat treatment

[0078]

[0079]

Claims

1. A method for improving the yield strength ratio of vanadium-containing martensitic precipitation-hardening stainless steel, characterized in that: By hot deformation treatment of vanadium-containing martensitic precipitation-hardening stainless steel after smelting, stainless steel with a grain size of grade 9 is obtained; after further heat treatment, vanadium-containing martensitic precipitation-hardening stainless steel with a yield strength ratio of Rp0.02 / Rp0.2 ≥ 0.80 is obtained; the chemical composition of the vanadium-containing martensitic precipitation-hardening stainless steel, by weight percentage, is: C 0.05-0.10, Cr 14.0-15.0, Ni 3.0-5.5, Cu 3.0-5.0, Mo 0.50-2.0, W 0-1.0, V 0.10-0.60, Mn 0.30-0.70, Si 0.2-0.6, Nb 0.2-0.4, Ta 0-0.02, Al 0.01-0.03, balance being Fe and unavoidable impurities; the hot deformation treatment includes the following steps: A. Electroslag ingot heating: heating to 850±10℃ at a heating rate of ≤50℃ / h, holding for 1-3h, then heating to 1180℃±10℃ at a heating rate of ≤150℃ / h, holding for more than 12h; B. Forging blanking: the steel ingot treated in step A is forged once, then reheated in the furnace to 1180±10℃ and held for ≥1h, then taken out of the furnace for a second forging and a first drawing, then reheated in the furnace to 1180±10℃ and held for ≥1h, then taken out of the furnace for a second drawing; C. Precision forging: the steel billet treated in step B is heated to 1180±10℃ and held for ≥2h, then precision forged; D. Unloading Heat treatment: The steel billet treated in step C is loaded into an annealing furnace and heated to 650±10℃ at a heating rate of ≤80℃ / h and held at that temperature. Then, it is slowly cooled in the furnace to 350℃ at a cooling rate of ≤50℃ / h before being removed from the furnace and air-cooled. The heat treatment process includes the following steps: a. Solution heat treatment: The material after heat deformation treatment is heated to 1040±10℃ at a heating rate of 20-50℃ / min and held for 1 hour; b. First aging treatment: The material after step a is placed in an aging furnace at 480±10℃ and held for 3-5 hours, then removed from the furnace and air-cooled to room temperature; c. Second aging heat treatment: The material after step b is placed in an aging furnace at 450-480±10℃ and held for 3-5 hours, then removed from the furnace and air-cooled to room temperature.

2. The method for improving the yield strength ratio of vanadium-containing martensitic precipitation-hardening stainless steel according to claim 1, characterized in that: The mechanical properties of the vanadium-containing martensitic precipitation hardening stainless steel are Rm≥1200MPa, Rp0.2≥1050MPa, A≥18%, Z≥60%, and KV2≥50J.

3. The method for improving the yield strength ratio of vanadium-containing martensitic precipitation-hardening stainless steel according to claim 1, characterized in that: The unavoidable impurities in the chemical composition of the vanadium-containing martensitic precipitation hardening stainless steel include P and S, with P ≤ 0.002% and S ≤ 0.003% by weight.

4. The method for improving the yield strength ratio of vanadium-containing martensitic precipitation-hardening stainless steel according to claim 1, characterized in that: In step B of the hot deformation treatment, at least one of the following conditions must be met: the first forging is to upset the steel ingot radially and axially into a square billet, such that the length of the square billet is no more than 60% of the original length; the second forging and first drawing are to upset the steel ingot after secondary heating along the diagonal edges into a square billet with a length no more than 40% of the original length, and then draw the steel billet axially to more than 60% of the original length; the second drawing is to draw the steel billet to more than 100% of the original length; the initial forging temperature is controlled to be no less than 1100±10℃, and the final forging temperature is controlled to be no less than 1010℃.

5. The method for improving the yield strength ratio of vanadium-containing martensitic precipitation-hardening stainless steel according to claim 1, characterized in that: In step C of the hot deformation treatment, the initial forging temperature is not lower than 1100±10℃ and the final forging temperature is not lower than 1010℃.

6. The method for improving the yield strength ratio of vanadium-containing martensitic precipitation-hardening stainless steel according to claim 1, characterized in that: In step D of the heat deformation treatment, the annealing furnace temperature is controlled at 400-500℃, and the holding time is calculated as t(h)=1.5+0.025×(bar diameter mm-100).

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