A super deep draw dual phase steel treated by twice cold rolling and a method for manufacturing the same

Abstract

The application discloses a kind of super deep-drawing dual-phase steel treated by twice cold rolling and a preparation method thereof, and belongs to the technical field of deep-drawing steel preparation.The chemical composition and mass content of the super deep-drawing dual-phase steel are as follows: C: 0.02-0.03%, Mn: 1.70-2.10%, Si: 0.10-0.20%, Ti: 0.010-0.018%, Nb: 0.025-0.045%, Mo: 0.10-0.20%, N≤0.003%, S≤0.008%, P≤0.015%, and the rest is Fe and inevitable impurities.The preparation method comprises the following steps: smelting, casting, hot rolling, first cold rolling, recrystallization annealing, second cold rolling, and dual-phase annealing process.The r value of the super deep-drawing dual-phase steel produced by the application reaches 2.0-2.8, which can be applied to automobile panels and other complex shaped parts, significantly reduces the weight of vehicle body, achieves the purpose of energy saving and consumption reduction, and improves safety.

Description

Technical Field

[0001] This invention belongs to the field of deep-drawing steel preparation technology, and relates to an ultra-deep-drawing dual-phase steel that has undergone two cold rolling processes and its preparation method. Background Technology

[0002] In recent years, the automotive industry has developed rapidly, and the number of cars on the road has increased daily. However, the accompanying problems of global warming, energy shortages, and environmental pollution have become increasingly serious. Reducing vehicle weight and saving energy consumption while ensuring safety and comfort is a new direction for the automotive industry. Lightweight automotive design, which reduces vehicle weight while ensuring safety, is an effective way to address these issues. Using high-strength steel instead of lower-strength steel to reduce steel usage is the leading direction for automotive lightweighting. Automotive exterior body panels are large in size and account for a high percentage of weight, exceeding 20% ​​of the body-in-white's mass. Therefore, developing higher-strength steel for automotive exterior body panels, ensuring component safety while reducing steel usage, is an effective way to achieve automotive lightweighting.

[0003] Automotive exterior panels have complex shapes and are mostly made from cold-rolled automotive steel sheets with a thickness of 0.6~0.8mm through stamping. Therefore, automotive steel sheets and strips are required to have high strength, excellent stamping formability, weldability, dent resistance, stability, and coating properties, while the surface morphology and roughness of the steel strips are strictly limited. Currently, the steel used in automotive exterior panels is mainly interstitial atomic (IF) steel. IF steel with a single-phase structure generally has an r-value (planar anisotropic plastic strain ratio) of 1.5 or higher. The r-value is related to the {111} texture of the steel sheet. A higher proportion of {111} texture results in a larger r-value and better deep-drawing performance of IF steel, which can meet the requirements of complex stamping. However, due to its lower yield strength and poor dent resistance, it is difficult to achieve the goal of thinning and weight reduction.

[0004] Duplex (DP) series high-strength steels, primarily composed of martensite and ferrite, feature no yield elongation, no room temperature aging, low yield strength ratio, and a high work hardening index. They are currently the preferred steel for automotive structural components, mainly used in automotive parts requiring high strength and high impact energy absorption, and are among the most widely used high-strength steels in automobiles. However, the excessively high content of interstitial carbon atoms and hard martensite in DP steel produced by steel companies results in poor deep-drawing performance, with r-values ​​generally below 1.0 and a low work hardening index. This significantly limits the use of traditional DP steel in automotive panels and body panels with high stamping performance requirements.

[0005] The development trends in the automotive industry indicate an urgent need for high-strength steel sheets with higher strength and better deep-drawing performance. Developing high-formability dual-phase (DP) steel with a high r-value can replace some IF steel in automotive body panels and other complexly formed components, significantly reducing vehicle weight, achieving energy conservation, emission reduction, and improved safety. Therefore, there is an urgent need to develop dual-phase steel for automotive outer panels with excellent deep-drawing performance, along with related production processes and control methods. Summary of the Invention

[0006] To address the technical problems of low r-value and poor deep-drawing performance of duplex steel produced by existing technologies, this invention provides an ultra-deep-drawing duplex steel subjected to two cold rolling processes and its preparation method.

[0007] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows:

[0008] A type of ultra-deep drawing dual-phase steel subjected to two cold rolling processes has the following chemical composition and mass percentage: C: 0.02-0.03%, Mn: 1.70-2.10%, Si: 0.10-0.20%, Ti: 0.010-0.018%, Nb: 0.025-0.045%, Mo: 0.10-0.20%, N≤0.003%, S≤0.008%, P≤0.015%, with the remainder being Fe and unavoidable impurities.

[0009] Furthermore, the thickness of the ultra-deep drawing dual-phase steel is 0.5–3.5 mm, and the microstructure at room temperature consists of ferrite and martensite, wherein the area ratio of martensite is <8%.

[0010] Furthermore, the ultra-deep drawing dual-phase steel has a tensile strength ≥500MPa, a yield strength ≥345MPa, an elongation ≥30%, and an r value of 2.0~2.8.

[0011] The roles of the main elements in the chemical composition of the above-mentioned ultra-deep drawing dual-phase steel are as follows:

[0012] C: A key element ensuring the formation of a two-phase microstructure in duplex steel. It increases both the strength of the steel and the hardenability of austenite. When the C content is <0.01% (wt.%), martensite is difficult to form. However, when the C content is too high (>0.03%), the volume fraction of martensite formed after annealing in the two-phase region will increase significantly, and the solid solution C content in ferrite will also increase significantly, thus adversely affecting the texture of DP steel. Therefore, considering the content of alloying elements and the precipitation of second-phase particles in this invention, the C content is controlled within the range of 0.02% to 0.03%.

[0013] Mn: An element that expands the austenite phase region, it can improve the hardenability of austenite and reduce the critical cooling rate for martensite formation. Simultaneously, a suitable Mn to C ratio can reduce the solid solution of C in ferrite, making the ferrite purer and beneficial for improving recrystallization texture. However, excessively high Mn content in steel can easily lead to an excessively high proportion of martensite in the microstructure. Considering all factors, the Mn content should be controlled within the range of 1.70% to 2.10%.

[0014] Si: The main element for solid solution strengthening, especially under conditions of low C content. Si can improve the activity of C and inhibit the formation of carbides, thus giving duplex steel a good balance of strength and plasticity. However, high silicon content can easily deteriorate the plating properties of the material. Considering all factors, the Si content should be controlled within the range of 0.10-0.20%.

[0015] Nb and Ti: For dual-phase steels, to achieve interstitial atom-free structure while simultaneously satisfying the requirements of a ferrite and martensite dual-phase structure, it is necessary to add a certain amount of C, along with a certain amount of Nb and Ti. This fixes some C atoms while dissolving others, ensuring that the remaining C atoms do not hinder the development of recrystallization texture, and also ensuring the formation of a small amount of hard martensite during annealing in the two-phase region. Considering all factors, the Ti content is controlled within the range of 0.01–0.018%, and the Nb content within the range of 0.025–0.045%.

[0016] Mo (Mo) possesses the characteristic of precipitating at low temperatures and dissolving at high temperatures, so Mo primarily controls the state of carbon (C) during heat treatment. On one hand, before the dual-phase steel is heated to its recrystallization temperature, Mo precipitates as MoC at the ferrite grain boundaries, hindering ferrite recrystallization and promoting the development of the {111} texture, thereby improving the deep-drawing performance of the dual-phase steel. When held at high temperatures, MoC decomposes, causing some C to diffuse into the austenite, increasing its stability. Ultimately, annealing in the ferrite and austenite dual-phase regions yields a ferrite-martensite microstructure. Considering the addition of Nb and Ti and cost factors, the Mo content is controlled within the range of 0.10–0.20%.

[0017] The above-mentioned method for preparing ultra-deep drawing dual-phase steel after two cold rolling processes includes smelting, casting, hot rolling, primary cold rolling, recrystallization annealing, secondary cold rolling, and dual-phase annealing processes.

[0018] The cold rolling process in this first cold rolling step has a cold rolling reduction rate of 50-70%;

[0019] The secondary cold rolling process has a cold rolling reduction rate of 60-75%.

[0020] Furthermore, in the hot rolling process, the billet is heated to 1180-1250℃, held for 2-3 hours, and then rolled; the initial rolling temperature is 1100-1150℃, the final rolling temperature is 860-900℃, and the coiling temperature is 640-700℃.

[0021] Furthermore, in the recrystallization annealing process, the annealing temperature is 790-810℃, the holding time is 1-2 hours, and the furnace is cooled to room temperature.

[0022] Furthermore, in the two-phase annealing process, the annealing temperature is 800-880℃, the heating rate is 12-15℃ / s, and the holding time is 100-200s; then it is rapidly cooled to 250℃ at a cooling rate of 55-65℃ / s, then slowly cooled to 200℃ at a cooling rate of 0.3-0.5℃ / s, and finally cooled to room temperature at a cooling rate of 10-15℃ / s.

[0023] The dual-phase steel produced by this invention has a high r-value and excellent deep-drawing performance, mainly due to the strong {111} in its microstructure. <110> This invention employs a double cold rolling process to avoid the high reduction rate (single cold rolling) that is detrimental to γ-texture formation. This allows the grains to gradually transform towards γ orientation under optimal reduction conditions, ultimately resulting in an extremely strong γ-texture. When the total reduction rate is high and both cold rolling reduction rates are favorable for γ-texture formation, the double cold rolling process allows the texture to gradually transform into a γ-texture beneficial for deep drawing performance, ultimately achieving a higher strength γ-texture than that obtained with single cold rolling under the same total reduction rate. By employing the double cold rolling and double annealing processes of this invention, the γ-texture is strengthened each time, resulting in a stronger recrystallized texture than that obtained with single cold rolling.

[0024] This invention involves the composite addition of alloying elements Nb, Ti, and Mo to dual-phase steel. The aim is to induce rapid precipitation of these elements as carbides and carbonitrides at low temperatures, hindering ferrite recrystallization and simultaneously purifying the ferrite matrix, thus promoting the development of the {111} texture. During high-temperature holding in the austenite and ferrite regions, the carbides and carbonitrides decompose, allowing some carbon to diffuse into the austenite, increasing its stability. Subsequent cooling yields a suitable ratio of ferrite and martensite. Ultimately, the final ratio of ferrite to martensite, grain size and distribution, and the strength of the {111} texture depend not only on the chemical composition but also on the rolling and annealing processes, especially the cold rolling process.

[0025] In summary, this invention, through rational composition design and comprehensive control of process parameters in each step of the preparation method, yields a ferrite structure with a small amount of granular martensite of suitable size. The average ferrite grain size is 12–16 μm; the granular martensite is dispersed and accounts for less than 8% of the total martensite content; the r-value is significantly improved compared to existing deep-drawing dual-phase steels, reaching 2.0–2.8; the mechanical properties include tensile strength ≥500 MPa, yield strength ≥345 MPa, and elongation ≥30%. The ultra-deep-drawing dual-phase steel produced using this invention can be applied to automotive panels and other complexly formed components, significantly reducing vehicle weight, achieving energy conservation, emission reduction, and improved safety. Attached Figure Description

[0026] Figure 1 This is a microstructure diagram of the ultra-deep drawing dual-phase steel of this invention. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to specific embodiments.

[0028] The chemical composition is prepared, smelted, and cast into a billet according to the following embodiments, and then the billet is subjected to subsequent processes. Example 1

[0029] The chemical composition of the ultra-deep drawing dual-phase steel in this embodiment is as follows by mass percentage: C: 0.02%, Mn: 2.10%, Si: 0.10%, Ti: 0.010%, Nb: 0.025%, Mo: 0.2%, N: 0.003%, S: 0.007%, P: 0.015%, with the remainder being Fe and unavoidable impurities.

[0030] The preparation method of the ultra-deep drawing dual-phase steel includes the following steps:

[0031] Hot rolling: The billet is heated to 1180℃, held for 3 hours and then hot rolled. The initial rolling temperature is 1100℃, the final rolling temperature is 860℃, and the coiling temperature is 640℃.

[0032] First cold rolling: The hot-rolled plate is subjected to a first cold rolling, with a cold rolling reduction rate of 50%;

[0033] Recrystallization annealing: Place the above hot-rolled plate in an annealing furnace, heat it to 790℃ and hold it for 2 hours, then cool it to room temperature with the furnace;

[0034] Secondary cold rolling: The annealed sheet is subjected to a second cold rolling, with a cold rolling reduction rate of 75%;

[0035] Two-phase annealing: The second cold-rolled plate is placed in an annealing furnace and heated to 800°C at a heating rate of 12°C / s, held for 200s, then cooled to 250°C at a cooling rate of 55°C / s, then slowly cooled to 200°C at a cooling rate of 0.3°C / s, and finally rapidly cooled to room temperature at a cooling rate of 10°C / s.

[0036] The ultra-deep drawing dual-phase steel prepared in this embodiment has a thickness of 0.5 mm, and its microstructure at room temperature consists of ferrite and martensite, with martensite accounting for 5.2% of the area; its tensile strength is 505 MPa, its yield strength is 345 MPa, its elongation is 36%, and its r value is 2.8. Example 2

[0037] The chemical composition of the ultra-deep drawing dual-phase steel in this embodiment is as follows by mass percentage: C: 0.03%, Mn: 1.70%, Si: 0.20%, Ti: 0.018%, Nb: 0.045%, Mo: 0.1%, N: 0.002%, S: 0.008%, P: 0.013%, with the remainder being Fe and unavoidable impurities.

[0038] The preparation method of the ultra-deep drawing dual-phase steel includes the following steps:

[0039] Hot rolling: The billet is heated to 1250℃, held for 2 hours and then hot rolled. The initial rolling temperature is 1150℃, the final rolling temperature is 900℃, and the coiling temperature is 700℃.

[0040] First cold rolling: The hot-rolled plate is subjected to a first cold rolling, with a cold rolling reduction rate of 55%;

[0041] Recrystallization annealing: Place the above hot-rolled plate in an annealing furnace, heat it to 810℃ and hold it for 1 hour, then cool it to room temperature with the furnace;

[0042] Secondary cold rolling: The annealed sheet is subjected to a second cold rolling, with a cold rolling reduction rate of 60%;

[0043] Two-phase annealing: The second cold-rolled plate is placed in an annealing furnace and heated to 880°C at a heating rate of 15°C / s, held for 100s, then cooled to 250°C at a cooling rate of 65°C / s, then slowly cooled to 200°C at a cooling rate of 0.5°C / s, and finally rapidly cooled to room temperature at a cooling rate of 15°C / s.

[0044] The ultra-deep drawing dual-phase steel prepared in this embodiment has a thickness of 3.5 mm, and its microstructure at room temperature consists of ferrite and martensite, with martensite accounting for 5.9% of the area; its tensile strength is 515 MPa, its yield strength is 350 MPa, its elongation is 31%, and its r value is 2.0. Example 3

[0045] The chemical composition of the ultra-deep drawing dual-phase steel in this embodiment is as follows by mass percentage: C: 0.025%, Mn: 1.75%, Si: 0.15%, Ti: 0.015%, Nb: 0.040%, Mo: 0.15%, N: 0.0025%, S: 0.006%, P: 0.011%, with the remainder being Fe and unavoidable impurities.

[0046] The preparation method of the ultra-deep drawing dual-phase steel includes the following steps:

[0047] Hot rolling: The billet is heated to 1230℃, held for 2.5h and then hot rolled. The initial rolling temperature is 1120℃, the final rolling temperature is 870℃, and the coiling temperature is 650℃.

[0048] First cold rolling: The hot-rolled plate is subjected to a first cold rolling, with a cold rolling reduction rate of 60%;

[0049] Recrystallization annealing: Place the above hot-rolled plate in an annealing furnace, heat it to 800℃ and hold it for 1.5 hours, then cool it to room temperature with the furnace;

[0050] Secondary cold rolling: The annealed sheet is subjected to a second cold rolling, with a cold rolling reduction rate of 70%;

[0051] Two-phase annealing: The second cold-rolled plate is placed in an annealing furnace and heated to 810°C at a heating rate of 13°C / s, held for 180s, then cooled to 250°C at a cooling rate of 57°C / s, then slowly cooled to 200°C at a cooling rate of 0.35°C / s, and finally rapidly cooled to room temperature at a cooling rate of 11°C / s.

[0052] The ultra-deep drawing dual-phase steel prepared in this embodiment has a thickness of 1.0 mm, and its microstructure at room temperature consists of ferrite and martensite, with martensite accounting for 7.0% of the area; its tensile strength is 530 MPa, its yield strength is 365 MPa, its elongation is 32%, and its r value is 2.3. Example 4

[0053] The chemical composition of the ultra-deep drawing dual-phase steel in this embodiment is as follows by mass percentage: C: 0.022%, Mn: 1.82%, Si: 0.17%, Ti: 0.013%, Nb: 0.035%, Mo: 0.17%, N: 0.0023%, S: 0.005%, P: 0.010%, with the remainder being Fe and unavoidable impurities.

[0054] The preparation method of the ultra-deep drawing dual-phase steel includes the following steps:

[0055] Hot rolling: The billet is heated to 1240℃, held for 2.2 hours and then hot rolled. The initial rolling temperature is 1130℃, the final rolling temperature is 880℃, and the coiling temperature is 670℃.

[0056] First cold rolling: The hot-rolled plate is subjected to a first cold rolling, with a cold rolling reduction rate of 70%;

[0057] Recrystallization annealing: Place the above hot-rolled plate in an annealing furnace, heat it to 795℃ and hold it for 1.2 hours, and then cool it to room temperature with the furnace;

[0058] Secondary cold rolling: The annealed sheet is subjected to a second cold rolling, with a cold rolling reduction rate of 75%;

[0059] Two-phase annealing: The second cold-rolled plate is placed in an annealing furnace and heated to 820°C at a heating rate of 14°C / s, held for 160s, then cooled to 250°C at a cooling rate of 60°C / s, then slowly cooled to 200°C at a cooling rate of 0.4°C / s, and finally rapidly cooled to room temperature at a cooling rate of 12°C / s.

[0060] The ultra-deep drawing dual-phase steel prepared in this embodiment has a thickness of 1.5 mm, and its microstructure at room temperature consists of ferrite and martensite, with martensite accounting for 7.8% of the area; its tensile strength is 560 MPa, its yield strength is 385 MPa, its elongation is 37%, and its r value is 2.7. Example 5

[0061] The chemical composition of the ultra-deep drawing dual-phase steel in this embodiment is as follows by mass percentage: C: 0.027%, Mn: 1.91%, Si: 0.14%, Ti: 0.017%, Nb: 0.042%, Mo: 0.13%, N: 0.0017%, S: 0.0075%, P: 0.009%, with the remainder being Fe and unavoidable impurities.

[0062] The preparation method of the ultra-deep drawing dual-phase steel includes the following steps:

[0063] Hot rolling: The billet is heated to 1200℃, held for 2.4h and then hot rolled. The initial rolling temperature is 1140℃, the final rolling temperature is 890℃, and the coiling temperature is 690℃.

[0064] First cold rolling: The hot-rolled plate is subjected to a first cold rolling, with a cold rolling reduction rate of 60%;

[0065] Recrystallization annealing: Place the above hot-rolled plate in an annealing furnace, heat it to 805℃ and hold it for 1.6 hours, and then cool it to room temperature with the furnace;

[0066] Secondary cold rolling: The annealed sheet is subjected to a second cold rolling, with a cold rolling reduction rate of 65%;

[0067] Two-phase annealing: The second cold-rolled plate is placed in an annealing furnace and heated to 830°C at a heating rate of 13.5°C / s, held for 140s, then cooled to 250°C at a cooling rate of 62°C / s, then slowly cooled to 200°C at a cooling rate of 0.45°C / s, and finally rapidly cooled to room temperature at a cooling rate of 13°C / s.

[0068] The ultra-deep drawing dual-phase steel prepared in this embodiment has a thickness of 2.0 mm, and its microstructure at room temperature consists of ferrite and martensite, with martensite accounting for 6.3% of the area; its tensile strength is 520 MPa, its yield strength is 352 MPa, its elongation is 33%, and its r value is 2.4. Example 6

[0069] The chemical composition of the ultra-deep drawing dual-phase steel in this embodiment is as follows by mass percentage: C: 0.021%, Mn: 2.0%, Si: 0.11%, Ti: 0.012%, Nb: 0.039%, Mo: 0.16%, N: 0.0029%, S: 0.0065%, P: 0.014%, with the remainder being Fe and unavoidable impurities.

[0070] The preparation method of the ultra-deep drawing dual-phase steel includes the following steps:

[0071] Hot rolling: The billet is heated to 1190℃, held for 2.6 hours and then hot rolled. The initial rolling temperature is 1110℃, the final rolling temperature is 875℃, and the coiling temperature is 660℃.

[0072] First cold rolling: The hot-rolled plate is subjected to a first cold rolling, with a cold rolling reduction rate of 65%;

[0073] Recrystallization annealing: The hot-rolled plate was placed in an annealing furnace, heated to 802°C and held for 1.8 hours, and then cooled to room temperature with the furnace.

[0074] Secondary cold rolling: The annealed sheet is subjected to a second cold rolling, with a cold rolling reduction rate of 70%;

[0075] Two-phase annealing: The second cold-rolled plate is placed in an annealing furnace and heated to 850°C at a heating rate of 12.5°C / s, held for 120s, then cooled to 250°C at a cooling rate of 63°C / s, then slowly cooled to 200°C at a cooling rate of 0.42°C / s, and finally rapidly cooled to room temperature at a cooling rate of 14°C / s.

[0076] The ultra-deep drawing dual-phase steel prepared in this embodiment has a thickness of 2.5 mm, and its microstructure at room temperature consists of ferrite and martensite, with martensite accounting for 6.8% of the area; its tensile strength is 540 MPa, its yield strength is 375 MPa, its elongation is 32.5%, and its r value is 2.2.

Claims

1. A type of ultra-deep drawing dual-phase steel subjected to two cold rolling processes, characterized in that, Its chemical composition and mass percentage are as follows: C: 0.02-0.03%, Mn: 1.70-2.10%, Si: 0.10-0.20%, Ti: 0.010-0.018%, Nb: 0.025-0.045%, Mo: 0.10-0.20%, N≤0.003%, S≤0.008%, P≤0.015%, with the remainder being Fe and unavoidable impurities; The ultra-deep drawing dual-phase steel has a tensile strength ≥500MPa, a yield strength ≥345MPa, an elongation ≥30%, and an r value of 2.0~2.

8. The preparation method of the ultra-deep drawing dual-phase steel includes smelting, casting, hot rolling, primary cold rolling, recrystallization annealing, secondary cold rolling, and dual-phase annealing processes. The cold rolling reduction rate in the first cold rolling process is 50-70%; the cold rolling reduction rate in the second cold rolling process is 60-75%.

2. The ultra-deep drawing dual-phase steel subjected to two cold rolling processes according to claim 1, characterized in that, The thickness of the ultra-deep drawing dual-phase steel is 0.5–3.5 mm, and its microstructure at room temperature consists of ferrite and martensite, with the area ratio of martensite being <8%.

3. The method for preparing ultra-deep drawing dual-phase steel with two cold rolling processes as described in claim 1 or 2, characterized in that, This includes smelting, casting, hot rolling, primary cold rolling, recrystallization annealing, secondary cold rolling, and dual-phase annealing processes. The cold rolling process in this first cold rolling step has a cold rolling reduction rate of 50-70%; The secondary cold rolling process has a cold rolling reduction rate of 60-75%.

4. The method for preparing ultra-deep drawing dual-phase steel after two cold rolling processes according to claim 3, characterized in that, In the hot rolling process, the billet is heated to 1180-1250℃, held at that temperature for 2-3 hours, and then rolled.

5. The method for preparing ultra-deep drawing dual-phase steel after two cold rolling processes according to claim 4, characterized in that, The hot rolling process has an initial rolling temperature of 1100–1150°C, a final rolling temperature of 860–900°C, and a coiling temperature of 640–700°C.

6. The method for preparing ultra-deep drawing dual-phase steel after two cold rolling processes according to claim 5, characterized in that, The recrystallization annealing process involves an annealing temperature of 790–810°C, a holding time of 1–2 hours, and subsequent furnace cooling to room temperature.

7. The method for preparing ultra-deep drawing dual-phase steel subjected to two cold rolling processes according to any one of claims 3-6, characterized in that, The two-phase annealing process involves an annealing temperature of 800–880°C, a heating rate of 12–15°C / s, and a holding time of 100–200s; followed by rapid cooling to 250°C, then slow cooling to 200°C, and finally cooling to room temperature.

8. The method for preparing ultra-deep drawing dual-phase steel after two cold rolling processes according to claim 7, characterized in that, The two-phase annealing process involves rapidly cooling to 250°C at a cooling rate of 55–65°C / s, then slowly cooling to 200°C at a cooling rate of 0.3–0.5°C / s, and finally cooling to room temperature at a cooling rate of 10–15°C / s.

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