A heat treatment method for improving the content of residual austenite of high-strength steel to improve low-temperature impact toughness

Abstract

The application discloses a heat treatment method for improving the content of residual austenite of high-strength steel and improving low-temperature impact toughness, the quenching temperature is 830-870 DEG C, and the holding time is 10-30 min; the quenching furnace is set as follows: the steel plate heat transmittance of the heating section is 1.4-1.8 min / mm, the heating time is 1.4*h min, wherein h is the thickness of the steel plate, the target temperature of each section is lower than the quenching temperature by 70 DEG C, 50 DEG C, 20 DEG C and 0 DEG C; the quenching cooling process is as follows: after the austenitized steel plate is taken out from the quenching furnace, the steel plate is sent into a quenching machine to perform isothermal quenching, the quenching medium temperature is a constant temperature between 20 DEG C and 200 DEG C, the water amount ratio of each region of the quenching machine is set according to the following table parameters, the quenching machine roller speed is controlled to be 15-25 m / min, and the quenching end temperature is controlled to be 400-500 DEG C; and after quenching, the steel plate is transported to a cooling bed to be air-cooled to room temperature. The application solves the problems of high manufacturing cost and long delivery cycle of high-strength steel plates.

Description

Technical Field

[0001] This invention relates to the field of metal heat treatment, and more particularly to a heat treatment method for improving the low-temperature impact toughness by increasing the residual austenite content in high-strength steel. Background Technology

[0002] Low-alloy high-strength steel is produced by alloying elements such as Cr, Ni, and Mo. It is generally produced by heat treatment process of "quenching + tempering" or "direct quenching + tempering" after rolling to obtain tempered martensite structure.

[0003] The heat treatment process employing quenching and tempering involves first achieving complete austenitization, followed by quenching to room temperature, and then low-temperature tempering to obtain a high-hardness, high-strength tempered martensite structure. However, this structure results in poor plasticity and impact toughness. Furthermore, while quenching to room temperature ensures the formation of martensite, it also retains high quenching stress (structural stress + thermal stress), which is difficult to completely eliminate during tempering, posing a risk of stress cracking during subsequent processing and application. Moreover, during tempering, supersaturated carbon atoms continuously decompose along the martensite lath interfaces, releasing cementite and reducing the fracture strength of grain boundaries, making them pathways for crack propagation and increasing the risk of brittle fracture. Therefore, high tempering temperatures can effectively eliminate quenching stress, but strength and hardness are reduced; low tempering temperatures are difficult to completely eliminate stress, resulting in temper brittleness. Additionally, end-users have increasingly stringent requirements for the plate shape of high-strength steel, with a flatness requirement of <3mm per meter, necessitating the production line to be equipped with a high-power straightening machine.

[0004] Heat treatment requires two processes: quenching and tempering. This not only places strict requirements on equipment and processes, but also restricts the development of manufacturing, fails to meet market demands, affects the effective connection between upstream and downstream processes, and increases manufacturing and time costs.

[0005] Patent documents

[0006] Patent Document 1: Chinese Invention Patent Application Publication No.: CN 107354382 A. Water quenching to a temperature below 250°C without matching a suitable amount of quenching water makes it difficult to guarantee the shape of the plate after quenching. Moreover, by adding precious metals such as W, Nd, Mg, and rare earth elements, this invention can achieve a Brinell hardness of 361 or higher without adding the above alloys.

[0007] Patent Document 2: Chinese Invention Patent Application Publication No.: CN 113025894 A High-strength coil with a yield strength of 960MPa can be obtained through ultra-fast cooling + self-tempering process. However, the maximum thickness that can be produced is limited by the winding capacity, and the cooling rate is >40℃ / s, which makes it difficult to compensate for the plate shape problem caused by rapid cooling.

[0008] Patent Document 3: Chinese Invention Patent Application Publication No.: CN 113430460 A uses controlled rolling and enhanced cooling processes to obtain a Charpy impact energy of ≥47J at -20°C. Although the amount of alloy added is small, it cannot be used in extremely cold environments of -40°C. It is difficult to guarantee the stringent plate shape requirements when the rolling state reaches a cooling rate of 30°C / S. Summary of the Invention

[0009] The purpose of this invention is to provide a heat treatment method that increases the residual austenite content in high-strength steel to improve its low-temperature impact toughness, thus solving the problems of high manufacturing cost and long delivery cycle of high-strength steel plates.

[0010] Using a lower alloy ratio and excluding precious alloys such as Ni or Mo, the steel plate is heated to above the Ac3 line under existing equipment conditions, and then quenched in a medium at a certain temperature. By controlling the quenching cooling rate and the water flow rate of the quenching water beam in the cross-section of the steel plate, the reheat temperature and plate shape after quenching are controlled. Self-tempering after quenching allows the high-strength steel plate to achieve its performance requirements. The high-strength steel product has a thickness of 50-100mm, a tensile strength range of 500MPa-1450MPa, a Brinell hardness (HBW) of up to 450, a retained austenite content of up to 6.58%, and a residual stress intensity that is more than 10% lower than that of the quenching + tempering process. The impact energy (KV2) at -60℃ can reach over 33J, and the unevenness per meter can be reduced to <3mm. This approach reduces production costs, controls the plate shape effectively, and shortens delivery time, meeting the market's stringent requirements for the performance and shape of high-strength steel plates.

[0011] Ingredient design:

[0012] Carbon (C) is a crucial element affecting the strength, hardness, toughness, and hardenability of high-strength steel. Increasing the C content will improve the product strength, but it will also impair impact toughness, elongation, and weldability. Therefore, the C content should be controlled between 0.09% and 0.21% while ensuring strength.

[0013] Si is dissolved in the steel matrix, increasing the matrix strength. Si can reduce the diffusion rate of carbon in ferrite, increasing the tempering stability of steel. However, excessive content will increase the brittleness of steel; its content should be controlled between 0.2% and 0.6%.

[0014] Mn can expand the austenite region, and increasing the Mn content can lower the austenite-ferrite phase transformation temperature, improve the stability of austenite, and refine the grains. However, excessive Mn content will increase the carbon equivalent, which is detrimental to the weldability of high-strength steel. Therefore, its content should be controlled between 1.0% and 1.6%.

[0015] The main role of Cr in high-strength steel is to improve the hardenability of the steel, strengthen the matrix through solid solution, refine the microstructure, and significantly improve the steel's oxidation and corrosion resistance. However, it also tends to increase temper brittleness. Therefore, its content should be controlled between 0.4% and 1.3%.

[0016] Adding trace amounts of Ti and Nb to high-strength steel can improve its strength and toughness. During the solidification process of steel, Ti and Nb can combine with N and C to form stable TiN, TiC, and Nb(C,N). After rolling, they precipitate from the matrix and strongly hinder the migration of austenite grain boundaries, thereby refining the austenite grains and playing a precipitation strengthening role. The amount of Ti and Nb added should be 3 to 4 times the sum of N and S in the steel, with Nb controlled between 0.01% and 0.025% and Ti content controlled between 0.008% and 0.02%.

[0017] Boron (B) is a key element for improving the hardenability of high-strength steel. Adding even a very small amount of B (0.0005–0.0030%) can significantly improve hardenability. Therefore, adding trace amounts of B to high-strength steel can effectively replace some expensive alloying elements such as Ni and Mo. B atoms dissolved in austenite tend to segregate and adsorb at austenite grain boundaries, reducing the free energy of these boundaries and hindering carbon atom diffusion, thus making it difficult for new phases to nucleate. The B content in high-strength steel should not exceed 0.004%, otherwise, non-solid-dissolved borides are easily formed, causing boron embrittlement. Its content should be controlled between 0.001% and 0.002%.

[0018] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0019] This invention discloses a heat treatment method for improving the low-temperature impact toughness of high-strength steel by increasing the residual austenite content. The heat treatment process includes:

[0020] The quenching temperature is 830-870℃, and the holding time is 10-30 min. In order to ensure uniform heating of the steel plate before quenching, the quenching furnace is set as follows: the heat penetration rate of the steel plate in the heating section is 1.4-1.8 min / mm, and the heating time is 1.4*h minutes, where h is the thickness of the steel plate. The heating temperature is increased segment by segment, and the target temperature of each segment is 70℃, 50℃, 20℃ and 0℃ lower than the quenching temperature, respectively. The heating temperature is consistent with the quenching temperature.

[0021] Quenching and cooling process: After the austenitic steel plate comes out of the quenching furnace, it enters the quenching machine for isothermal quenching. The quenching medium temperature is a constant temperature between 20-200℃. The water volume ratio of each area of ​​the quenching machine is set according to the parameters in Table 1 below. The roller speed of the quenching machine is controlled at 15-25m / min. The final quenching temperature is controlled at 400-500℃. After quenching, the steel plate is transported to the cooling bed for air cooling to room temperature.

[0022] Table 1

[0023]

[0024] The unit for water allocation in the table above is m³ / h (Note: The target water volume for the two lines on the left and right sides of the steel plate is the same).

[0025] Furthermore, in order not to affect hardenability, the rust removal grade of the steel plate reaches level 2.5 or above, and the oxygen content in the furnace is controlled below 800 ppm.

[0026] Furthermore, oil is used as the quenching medium for temperatures above 100°C.

[0027] Furthermore, the composition of the high-strength steel by mass percentage is designed as follows:

[0028] C 0.09%-0.21%; Si 0.2%-0.6%; Mn 1.0%-1.6%; Cr 0.4%-1.3%; the amount of Ti and Nb added is 3-4 times the sum of N and S in the steel, Nb 0.01%-0.025%, Ti 0.008%-0.02%; B 0.001%-0.002%, the remainder being Fe and unavoidable impurities.

[0029] Furthermore, the overall production process is as follows: desulfurization → converter smelting → LF refining → RH refining → continuous casting → heating → rolling → cooling → hot straightening → shearing → heat treatment → sampling and quality inspection.

[0030] Furthermore, the thickness of the high-strength steel products is 50-100mm, and the performance meets the following requirements: tensile strength range of 500MPa-1450MPa, Brinell hardness HBW reaches 450, retained austenite content reaches 6.58%, residual stress intensity is reduced by more than 10% compared with quenching + tempering process, and impact energy KV2 at -60℃ reaches more than 33J.

[0031] Furthermore, the unevenness per meter reaches a level of <3mm.

[0032] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0033] The closer the quenching temperature is to Mf, the lower the quenching internal stress. Therefore, isothermal quenching at 20℃~200℃ can significantly reduce the stress in the quenched structure. Simultaneously, the residual heat cooling process allows for self-tempering, eliminating thermal stress and reducing carbide precipitation, thus lowering the risk of stress and brittle cracking. Furthermore, the main alloying element in high-strength steel is Mn, a stable austenite element. Due to the compositional inhomogeneity, isothermal quenching retains more residual austenite than quenching + tempering. XRD results show that the residual austenite content is 6.58% after sub-temperature quenching, compared to 4.09% after quenching + tempering, representing a 2.5% increase in residual austenite content. The residual stress intensity is reduced by more than 10% compared to quenching + tempering. During deformation, the retained austenite undergoes a transformation-induced plasticity effect (TRIP effect) under external force, gradually transforming into high-strength martensite accompanied by volume expansion. This suppresses the instability of plastic deformation, increases the range of uniform elongation, and thus simultaneously improves the material's strength and plasticity. Tensile strength can reach 1450 MPa, surface hardness (HBW) of the steel plate can reach over 450, core hardness (HBW) can reach over 410, and impact energy (KV2) at -60℃ can reach over 33 J. Therefore, the sub-temperature quenching process without tempering is more conducive to obtaining a comprehensive performance ratio of strength, hardness, and plasticity.

[0034] Compared with existing technologies, the present invention has the following significant advantages and effects:

[0035] Compared with the traditional quenching and tempering process, this invention achieves the product microstructure, properties and plate shape requirements of the traditional quenching and tempering process in just one quenching step, under the same composition and without performance reduction. It improves the residual austenite content and low-temperature impact toughness, reduces residual stress intensity, and saves one tempering step, which not only saves production costs but also shortens the contract delivery time.

[0036] The process parameters of this invention are stable, and the process control is simple and stable. With a fixed water (oil) volume in the quenching machine, the steel plate is cooled evenly, ensuring that the steel plate's structure and properties are uniform and stable. At the same time, optimizing the flow rate in different areas of the quenching machine can guarantee the stringent flatness requirements of the steel plate.

[0037] Traditional quenching and tempering processes for high-strength steel plates result in a sorbite microstructure. Quenching alone, by controlling the red-heat temperature, produces a microstructure dominated by fine pearlite and bainite, resulting in a better balance of strength, hardness, and plasticity. Detailed Implementation Example 1

[0038] This embodiment experimentally produces a 60mm thick high-strength steel plate with a Brinell hardness of HBW330. The chemical composition is formulated according to the proportions in Table 2. The steel plate is obtained through smelting, continuous casting, and rolling processes. The steel plate is then quenched. The heat penetration rate of the steel plate in the heating section is 1.8 min / mm, and the heating time is 84 min. A step-by-step temperature increase method is adopted, with target temperatures of 790℃, 810℃, 840℃, and 860℃ for each section. Before quenching, the steel plate is derusted to grade 3.0, with an oxygen content of 650 ppm in the quenching furnace. After reaching the austenitizing temperature of 860℃, the holding time is 20 min.

[0039] After exiting the quenching furnace, the steel plate enters the quenching machine using the cooling parameters in Table 3. Oil is used as the quenching medium at 200℃, and the roller speed of the quenching machine is 25m / min. The final quenching temperature (i.e., the red-hot temperature) of the steel plate is between 450 and 460℃. The quenched steel plate is then transported from the quenching machine to a cooling bed via roller conveyor, where it is cooled to room temperature in air. Samples of the steel plate are taken and their properties are tested. The tensile strength is 40MPa higher than that of the quenched and tempered steel plate, the -60℃ impact strength is 3.6J higher, the hardness is similar, the retained austenite content is 2.82% higher, the residual stress intensity is 11.8% lower, and the flatness is 6mm / m smaller. A comparison of properties and flatness is shown in Table 4. The steel plate in this example exhibits better impact performance and flatness than the traditional quenching and tempering process, while saving tempering time and costs.

[0040] Table 2. Chemical composition (wt%) of Brinell hardness HBW330

[0041]

[0042] Table 3. Flow parameters of HBW330 steel plate with Brinell hardness in different zones of the quenching machine (unit: m³ / h)

[0043]

[0044] Table 4 Comparison of mechanical properties of steel plates treated with heat treatment processes such as no tempering and quenching + tempering.

[0045] Example 2

[0046] This embodiment experimentally produces a high-strength steel plate with a thickness of 80 mm, a Brinell hardness of HBW370, and chemical composition according to the proportions in Table 5. The steel plate is obtained through smelting, continuous casting, and rolling processes. The steel plate is then quenched. The heat penetration rate of the steel plate in the heating section is 1.6 min / mm, and the heating time is 112 min. A step-by-step temperature increase method is adopted, with target temperatures of 800℃, 820℃, 850℃, and 870℃ for each section. Before quenching, the steel plate is derusted to grade 4.5, with an oxygen content of 400 ppm in the quenching furnace. After reaching the austenitizing temperature of 870℃, the holding time is 30 min.

[0047] After exiting the quenching furnace, the steel plate enters the quenching machine using the cooling parameters in Table 6. Oil is used as the quenching medium at a temperature of 150℃, and the roller speed of the quenching machine is 21 m / min. The final quenching temperature (i.e., the red-hot temperature) of the steel plate is between 470 and 480℃. The quenched steel plate is then transported from the quenching machine to a cooling bed via roller conveyor, where it is cooled to room temperature in air. Samples of the steel plate are taken and their properties are tested. The steel plate of this invention has similar tensile strength, elongation, and hardness to the quenched and tempered steel plate. Its impact strength at -60℃ is 1.9 J higher than that of the quenched and tempered steel plate, its retained austenite content is 2.31% higher, its residual stress intensity is 12.2% lower, and its flatness is 5 mm / m less than that of the quenched and tempered steel plate. A comparison of properties and flatness is shown in Table 7.

[0048] Table 5. Chemical composition (wt%) of Brinell hardness HBW370

[0049]

[0050] Table 6. Flow parameters of HBW370 steel plate in different zones of the quenching machine (unit: m³ / h)

[0051]

[0052] Table 7 Steel plates with heat treatment processes including temper-free and quenching + tempering

[0053] Example 3

[0054] This embodiment experimentally produces a 50mm thick high-strength steel plate with a Brinell hardness of HBW420. The chemical composition is formulated according to the proportions in Table 8. The steel plate is obtained through smelting, continuous casting, and rolling processes. The steel plate is then quenched. The heat penetration rate of the steel plate in the heating section is 1.6 min / mm, and the heating time is 70 min. A step-by-step temperature increase method is adopted, with target temperatures of 800℃, 820℃, 850℃, and 870℃ for each section. Before quenching, the steel plate is derusted to grade 4.0, with an oxygen content of 380 ppm in the quenching furnace. After reaching the austenitizing temperature of 870℃, the holding time is 30 min.

[0055] After exiting the quenching furnace, the steel plate enters the quenching machine using the water-cooling parameters in Table 9. Water is used as the quenching medium at a temperature of 20℃, and the roller speed of the quenching machine is 16 m / min. The final quenching temperature (i.e., the red-hot temperature) of the steel plate is between 420 and 440℃. The quenched steel plate is then transported from the quenching machine to a cooling bed via roller conveyor, where it is cooled to room temperature in air. Samples of the steel plate are taken and their properties are tested. The steel plate of this invention has similar tensile strength, elongation, and hardness to the quenched and tempered steel plate. Its impact strength at -60℃ is 1.9 J higher than that of the quenched and tempered steel plate, its retained austenite content is 2.31% higher, its residual stress intensity is 13% lower, and its flatness is 5 mm / m less than that of the quenched and tempered steel plate. A comparison of performance and flatness is shown in Table 10.

[0056] Table 8. Chemical composition (wt%) of Brinell hardness HBW420

[0057]

[0058] Table 9. Flow parameters of HBW370 steel plate in different zones of the quenching machine (unit: m³ / h)

[0059]

[0060] Table 10 Steel plates with heat treatment processes including temper-free and quenching + tempering

[0061]

[0062] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A heat treatment method for improving low-temperature impact toughness by increasing the content of residual austenite in high-strength steel, characterized by, The heat treatment process comprises: The quenching temperature is 830-870℃, and the holding time is 10-30min; in order to make the steel plate heated uniformly before quenching, the quenching furnace is set as follows: the steel plate heat penetration rate of the heating section is 1.4-1.8min / mm, the heating time is 1.4*h minutes, wherein h is the thickness of the steel plate, the heating temperature is gradually increased in each section, and the target temperature of each section is 70℃, 50℃, 20℃ and 0℃ lower than the quenching temperature, and the heating temperature is consistent with the quenching temperature; The quenching cooling process is as follows: after the austenitized steel plate is taken out from the quenching furnace, the steel plate is subjected to isothermal quenching in the quenching machine, the quenching medium temperature is a constant temperature between 20-200℃, the water amount ratio of each region of the quenching machine is set according to the following table parameters, the quenching machine roller speed is controlled at 15-25m / min, and the quenching end temperature is controlled at 400-500℃; after quenching, the steel plate is transported to the cooling bed and air-cooled to room temperature; The water amount ratio unit in the above table is m³ / h; The mass percentage of the high-strength steel is designed as follows: C 0.09%-0.21%; Si 0.2%-0.6%; Mn 1.0%-1.6%; Cr 0.4%-1.3%; the addition amount of Ti and Nb is 3-4 times of the sum of N and S in the steel, Nb 0.01%-0.025%, Ti 0.008%-0.02%; B 0.001%-0.002%, and the rest is Fe and inevitable impurities.

2. The heat treatment method for improving low-temperature impact toughness by increasing residual austenite content of high-strength steel according to claim 1, characterized by, In order to not affect the hardenability, the steel plate rust grade is above 2.5, and the oxygen content in the furnace is controlled below 800ppm.

3. The heat treatment method for improving low-temperature impact toughness by increasing residual austenite content of high-strength steel according to claim 1, characterized by, The overall process of the production process is: desulfurization→ converter smelting→ LF refining→ RH refining→ continuous casting→ heating→ rolling→ cooling→ hot straightening→ shearing→ heat treatment→ sampling and quality inspection.

4. The heat treatment method for improving low-temperature impact toughness by increasing residual austenite content of high-strength steel according to claim 1, characterized by, The high-strength steel product thickness is 50-100mm, and the performance meets: the tensile strength is 500MPa-1450MPa, the Brinell hardness HBW reaches 450, the residual austenite content reaches 6.58%, the residual stress strength is reduced by more than 10% compared with the quenching+tempering process, and the impact energy KV2 at-60℃ reaches more than 33J.

5. The heat treatment method for improving low-temperature impact toughness by increasing residual austenite content of high-strength steel according to claim 1, characterized by, The per-meter unevenness reaches the level of <3mm.

Images