A flash heat treatment process for ultra-high strength steel plates

Abstract

A flash heat treatment process for ultra-high strength steel plates belongs to the field of metallic materials. The chemical composition of the ultra-high strength steel plate is: C 0.25-0.35%, Si 0.3-1.5%, Mn 2.0-2.5%, Ni 0.4-0.6%, Cr 0.3-0.8%, Mo 0.1-0.5%, Al 0.01-0.04%, B 0.0015-0.0020%, Nb 0.02-0.05%, Ti 0.005-0.020%, N≤0.003%, S≤0.010%, P≤0.010%, with the balance being Fe. During preparation, the rolled plate is held at 600–650℃ for 20–40 min, heated to 900–950℃ at 100–150℃ / s, held for 5–15 s, cooled to room temperature at 50–100℃ / s, and finally held at 150–250℃ for 60–120 min to obtain the yield strength (R) of the ultra-high strength steel plate. p0.2 ≥1100MPa, tensile strength (R) m ≥1200MPa, elongation (A)≥12%, impact toughness at -40℃≥70J.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, specifically relating to a flash heat treatment process for ultra-high strength steel plates. Background Technology

[0002] Currently, there are few domestic enterprises capable of producing ultra-high strength steel with a yield strength of 1100MPa. Ultra-high strength steel plates have broad market prospects. They are generally produced using an offline / online quenching and tempering (QT / DQ-T) heat treatment process. During low-temperature tempering, the steel plate has sufficiently high strength and a certain degree of plasticity, but its low-temperature impact toughness is insufficient, making it unsuitable for special applications such as extreme cold. While high-temperature tempering improves toughness, it significantly reduces strength, and the energy consumption from prolonged high-temperature tempering is high. Enabling steel to possess good low-temperature impact toughness while ensuring strength and plasticity, without significantly increasing costs, is a pressing issue that needs to be addressed.

[0003] Invention patent CN108315671A proposes a method for preparing ultra-high strength steel with a yield strength of 1000MPa and a low yield ratio. This invention patent uses an offline quenching + low temperature tempering process to prepare Q1000 ultra-high strength steel. However, the strength is relatively low and only the impact toughness value at -20℃ is considered. At the same time, the process flow is long and the production efficiency is reduced.

[0004] Invention patent CN100372962C proposes an ultra-high strength steel plate with a yield strength of over 1100MPa and its manufacturing method. This invention uses a heat treatment process of TMCP + online quenching + tempering at 350~750℃ to manufacture Q1100 ultra-high strength steel plate with a thickness of 16~25mm and an impact toughness value of ≤40J at -40℃.

[0005] Invention patent CN114196879B proposes a structural steel plate with a yield strength of 1000MPa and its manufacturing method. This invention uses a quenching and tempering process to manufacture ultra-high strength plates with a thickness of 15-40mm and a yield strength ≥1065MPa, tensile strength ≥1166MPa, elongation after fracture ≥18.5%, and impact energy at -85℃ ≥106J. However, it adds a large number of expensive alloying elements, such as 10.0%-15.0% Ni and 0.80%-1.50% Mo, which significantly increases the cost and is not suitable for industrial production.

[0006] Invention patent CN115537508A proposes a flash heat treatment method for high-strength steel plates. This invention replaces the traditional long-time tempering process with flash heat treatment technology while ensuring mechanical properties, thereby improving production efficiency and achieving energy conservation and emission reduction. However, the thickness of the steel plate in the example is only 5mm and the impact of flash heat treatment on low-temperature impact toughness is not considered. Summary of the Invention

[0007] This invention addresses the problems of traditional QT and DQ-T heat treatment processes, which cannot balance strength and low-temperature toughness during low-temperature tempering, and the decrease in strength and increase in energy consumption during high-temperature tempering. It proposes a flash heat treatment process for ultra-high strength steel, which can replace the traditional quenching and tempering process. It improves low-temperature toughness while ensuring strength and plasticity, and reduces energy consumption, thereby achieving energy conservation, emission reduction and improved production efficiency.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] This invention discloses a flash heat treatment process for ultra-high strength steel plates. The ultra-high strength steel is smelted and forged to obtain a forged billet, which is then hot-rolled and rapidly cooled to room temperature. Following this, a heat treatment process is performed: the billet is first held at 600–650°C for 20–40 minutes; then, it is heated to 900–950°C at a heating rate of 100–150°C / s, held for 5–15 seconds, and then cooled to room temperature at a rate of 50–100°C / s; finally, it is held at 150–250°C for 60–120 minutes to obtain an ultra-high strength steel plate with a good strength-toughness ratio.

[0010] Furthermore, the chemical composition of the ultra-high strength steel plate, by mass percentage of element content, is as follows: C: 0.25–0.35%, Si: 0.3–1.5%, Mn: 2.0–2.5%, Ni: 0.4–0.6%, Cr: 0.3–0.8%, Mo: 0.1–0.5%, Al: 0.01–0.04%, B: 0.0015–0.0020%, Nb: 0.02–0.05%, Ti: 0.005–0.020%, N≤0.003%, S≤0.010%, P≤0.010%; the balance being Fe and other unavoidable impurities.

[0011] Furthermore, the ultra-high strength steel smelting and forging process involves smelting the raw materials using a vacuum induction furnace, strictly controlling the content of O, N, S, and P elements, casting the billet after smelting, removing the riser from the billet, and forging it into an 80mm*80mm square billet. The initial forging temperature is ≤1150℃ and the final forging temperature is ≥900℃ to obtain the forged billet.

[0012] Furthermore, the rapid cooling to room temperature after hot rolling involves holding the forging billet at 1150–1200℃ for 60–120 minutes for homogenization; after exiting the furnace, removing the surface iron oxide scale, and performing two-stage rolling: the rough rolling start temperature is 1100–1150℃, the waiting temperature is ≥960℃, the finish rolling start temperature is 920–880℃, and the final rolling temperature is ≥840℃; and then the billet is rapidly cooled to room temperature.

[0013] Furthermore, the ultra-high strength steel plate has an initial microstructure of martensite after hot rolling and water cooling, a microstructure of tempered sorbite after high-temperature tempering, and a martensite microstructure after flash heating with a grain size of 11-12, exhibiting significant fine-grain strengthening effect. The final microstructure after low-temperature tempering consists of tempered martensite + nano-sized precipitated carbides + thin-film retained austenite. The precipitated carbides have a size of 20-100 nm, and the thin-film retained austenite is located between the martensite laths with a width of 5-40 nm, serving to coordinate toughness.

[0014] Furthermore, the yield strength (R) of the ultra-high strength steel plate p0.2 ≥1100MPa, tensile strength (R) m ≥1200MPa, elongation (A)≥12%, impact toughness at -40℃≥70J.

[0015] Furthermore, the thickness of the ultra-high strength steel plate is 10-30 mm.

[0016] Compared with the prior art, the main advantages of the present invention are:

[0017] (1) The present invention utilizes the production process of online quenching after rolling to avoid reheating and quenching, thereby shortening the process flow, improving production efficiency and saving energy.

[0018] (2) The key point of this invention is the flash heat treatment process, which precipitates cementite by holding at 600-650℃ for 20-40 min, providing a large number of nucleation sites for subsequent austenite inversion transformation; the slab is heated to 900-950℃ at a heating rate of 100-150℃ / s and held for 5-15s. The rapid heating and short holding time allow the austenite to nucleate explosively and not have time to grow, resulting in a grain size of 11-12 for the ultra-high strength steel, with a significant fine grain strengthening effect; when cooled to room temperature at a rate of 50-100℃ / s, martensite is obtained. The phase transformation strengthening and dislocation strengthening provide high strength for the ultra-high strength steel, while the thin film-like residual austenite between the martensite laths coordinates the toughness.

[0019] (3) This invention uses a flash heat treatment process to replace the traditional quenching and tempering process, which not only improves the low-temperature toughness while ensuring strong plasticity, but also shortens the heat treatment time of ultra-high strength steel due to the short heating, holding and cooling time of flash heating, thereby improving production efficiency and reducing energy consumption.

[0020] (4) The thickness of the ultra-high strength steel described in this invention is 10-30mm. At present, flash heat treatment process is mostly used for thin plates, which solves the technical problem of flash heat treatment process for medium and thick plates. Attached Figure Description

[0021] Figure 1SEM image of ultra-high strength steel prepared by traditional quenching and tempering heat treatment process;

[0022] Figure 2 SEM image of the ultra-high strength steel prepared by flash heat treatment process in Example 1;

[0023] Figure 3 SEM image of the ultra-high strength steel prepared by flash heat treatment process in Example 2;

[0024] Figure 4 SEM image of the ultra-high strength steel prepared by flash heat treatment process in Example 3;

[0025] Figure 5 SEM image of the ultra-high strength steel prepared by flash heat treatment process in Example 4; Detailed Implementation

[0026] The heat treatment process of the present invention will be described below through specific embodiments, including but not limited to the following embodiments.

[0027] The mass percentage of the chemical composition elements of the ultra-high strength steel plate is as follows: C: 0.25-0.35%, Si: 0.3-1.5%, Mn: 2.0-2.5%, Ni: 0.4-0.6%, Cr: 0.3-0.8%, Mo: 0.1-0.5%, Al: 0.01-0.04%, B: 0.0015-0.0020%, Nb: 0.02-0.05%, Ti: 0.005-0.020%, N≤0.003%, S≤0.010%, P≤0.010%; the balance is Fe and other unavoidable impurities.

[0028] The specific steps for smelting and forging the ultra-high strength steel plate are as follows: the raw material is smelted in a vacuum induction furnace, and the content of O, N, S and P elements is strictly controlled. After smelting, it is cast into a billet. After the riser is removed from the billet, it is forged into a square billet of 80mm*80mm. The initial forging temperature is ≤1150℃ and the final forging temperature is ≥900℃.

[0029] Example 1

[0030] This embodiment describes an ultra-high-strength steel plate obtained through this flash heat treatment process. The finished plate thickness is 10mm. The specific rolling and heat treatment process is as follows: the obtained forging billet is held at 1150℃ for 90 minutes for homogenization treatment; after exiting the furnace, the surface iron oxide scale is removed, and a two-stage rolling process is performed. The rough rolling starts at 1117℃, rolling to 30mm in 4 passes, with a waiting temperature of 961℃. The finish rolling starts at 892℃, rolling to 10mm in 4 passes, with a final rolling temperature of 851℃. After rolling, the sample is cooled to room temperature at a rate of ≥50℃ / s. Then, the sample is held at 600℃ for 20 min, followed by rapid heating of the slab to 900℃ at a heating rate of 100℃ / s, holding for 5 s, and then cooling to room temperature at a rate of 50℃ / s. Finally, it is held at 180℃ for 90 min to obtain an ultra-high-strength steel plate with strength comparable to that obtained using traditional quenching and tempering processes, and good toughness. Its final microstructure consists of tempered martensite + nano-sized precipitated carbides + thin-film retained austenite. See details... Figure 2 Its mechanical properties are shown in Table 2. Figure 1 The image shows an SEM image of ultra-high strength steel prepared using the traditional quenching and tempering heat treatment process (holding at 860℃ for 40 minutes followed by rapid cooling and tempering at 600℃). It shows a tempered sorbite structure. In contrast, the flash heat treatment process can significantly refine the original austenite grains, improve low-temperature toughness while ensuring strength, and shorten the heat treatment time and improve production efficiency.

[0031] Example 2

[0032] This embodiment describes an ultra-high strength steel plate obtained through this flash heat treatment process. The finished plate thickness is 10mm. The specific rolling and heat treatment process is as follows: the obtained forging billet is held at 1150℃ for 90 minutes for homogenization treatment; after exiting the furnace, the surface iron oxide scale is removed, and a two-stage rolling process is performed. The rough rolling starts at 1123℃, rolling to 30mm in 4 passes, with a waiting temperature of 977℃. The finish rolling starts at 889℃, rolling to 10mm in 4 passes, with a final rolling temperature of 843℃. After rolling, the sample is cooled to room temperature at a rate of ≥50℃ / s. Then, the sample is held at 600℃ for 20 min, followed by rapid heating of the slab to 900℃ at a heating rate of 100℃ / s, held for 10 s, and then cooled to room temperature at a rate of 50℃ / s. Finally, it is held at 180℃ for 90 min to obtain an ultra-high-strength steel plate with strength comparable to that obtained using traditional quenching and tempering processes, exhibiting excellent toughness. Its final microstructure consists of tempered martensite + nano-sized precipitated carbides + thin-film retained austenite. See details... Figure 3 Its mechanical properties are shown in Table 2.

[0033] Example 3

[0034] This embodiment describes an ultra-high-strength steel plate obtained through this flash heat treatment process. The finished plate thickness is 30mm. The specific rolling and heat treatment process is as follows: the obtained forging billet is held at 1150℃ for 90 minutes for homogenization treatment; after exiting the furnace, the surface iron oxide scale is removed, and a two-stage rolling process is performed. The rough rolling starts at 1132℃, rolling to 45mm in 3 passes, with a waiting temperature of 1004℃. The finish rolling starts at 907℃, rolling to 30mm in 2 passes, with a final rolling temperature of 877℃. After rolling, the sample is cooled to room temperature at a rate of ≥50℃ / s. Then, the sample is held at 650℃ for 40 min, followed by rapid heating of the slab to 950℃ at a heating rate of 150℃ / s, held for 10 s, and then cooled to room temperature at a rate of 50℃ / s. Finally, it is held at 180℃ for 90 min to obtain an ultra-high-strength steel plate with strength comparable to that obtained using traditional quenching and tempering processes, and good toughness. Its final microstructure consists of tempered martensite + nano-sized precipitated carbides + thin-film retained austenite. See details... Figure 4 Its mechanical properties are shown in Table 2.

[0035] Example 4

[0036] This embodiment describes an ultra-high-strength steel plate obtained through this flash heat treatment process. The finished plate thickness is 30mm. The specific rolling and heat treatment process is as follows: the obtained forging billet is held at 1150℃ for 90 minutes for homogenization treatment; after exiting the furnace, the surface iron oxide scale is removed, and a two-stage rolling process is performed. The rough rolling starts at 1127℃, is rolled to 45mm in 3 passes, and the waiting temperature is 998℃. The finish rolling starts at 902℃, is rolled to 30mm in 2 passes, and the final rolling temperature is 867℃. After rolling, the sample is cooled to room temperature at a rate of ≥50℃ / s. Then, the sample is held at 650℃ for 40 min, followed by rapid heating of the slab to 950℃ at a heating rate of 150℃ / s, held for 15 s, and then cooled to room temperature at a rate of 50℃ / s. Finally, it is held at 180℃ for 90 min to obtain an ultra-high-strength steel plate with strength comparable to that obtained using traditional quenching and tempering processes, and good toughness. Its final microstructure consists of tempered martensite + nano-sized precipitated carbides + thin-film retained austenite. See details... Figure 5 Its mechanical properties are shown in Table 2.

[0037] Table 1. Elemental content (Wt.%) of chemical components in each embodiment.

[0038]

[0039] Table 2 Comparison of mechanical properties of ultra-high strength steel prepared by flash heat treatment process and traditional quenching and tempering heat treatment process

[0040]

Claims

1. A flash heat treatment process for ultra-high strength steel plates, characterized in that, The process involves smelting and forging ultra-high strength steel to obtain a forging billet, then hot rolling it and rapidly cooling it to room temperature. Subsequently, heat treatment is performed. The billet is first held at 600–650°C for 20–40 minutes, then heated to 900–950°C at a heating rate of 100–150°C / s, held for 5–15 seconds, and then cooled to room temperature at a rate of 50–100°C / s. Finally, it is held at 150–250°C for 60–120 minutes to obtain an ultra-high strength steel plate. The chemical composition of the ultra-high strength steel plate, by mass percentage of element content, is as follows: C: 0.25–0.35%, Si: 0.3–1.5%, Mn: 2.0–2.5%, Ni: 0.4–0.6%, Cr: 0.3–0.8%, Mo: 0.1–0.5%, Al: 0.01–0.04%, B: 0.0015–0.0020%, Nb: 0.02–0.05%, Ti: 0.005–0.020%, N≤0.003%, S≤0.010%, P≤0.010%; the balance is Fe and other unavoidable impurities. The final microstructure of the ultra-high strength steel plate is tempered martensite + nano-sized precipitated carbides + thin film retained austenite. The precipitated carbides have a size of 20-100 nm, and the thin film retained austenite is located between the martensite laths with a width of 5-40 nm and a grain size of 11-12.

2. The heat treatment process according to claim 1, characterized in that, The ultra-high strength steel smelting and forging process involves smelting the raw materials using a vacuum induction furnace, casting the smelting process, removing the riser from the cast billet, and forging it into an 80mm*80mm square billet. The initial forging temperature is ≤1150°C and the final forging temperature is ≥900°C to obtain the forged billet.

3. The heat treatment process according to claim 1, characterized in that, The rapid cooling to room temperature after hot rolling involves holding the forged billet at 1150–1200°C for 60–120 minutes for homogenization; after exiting the furnace, removing the surface iron oxide scale, and then performing two-stage rolling: the rough rolling start temperature is 1100–1150°C, the waiting temperature is ≥960°C, the finish rolling start temperature is 920–880°C, and the final rolling temperature is ≥840°C; and then the billet is rapidly cooled to room temperature.

4. The heat treatment process according to claim 1, characterized in that, The yield strength (R) of the ultra-high strength steel plate p0.2 ≥1100MPa, tensile strength (R) m ≥1200MPa, elongation (A)≥12%, impact toughness at -40°C≥70J.

5. The heat treatment process according to claim 1, characterized in that, The thickness of the ultra-high strength steel plate is 10-30mm.

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