Green low-carbon production method for replacing hardening and tempering with direct quenching and tempering of HG785 steel plate

Abstract

The invention discloses a green low-carbon production method of an HG785 steel plate by replacing hardening and tempering with direct quenching and tempering, the thickness of the steel plate is 30-50mm, and the production method comprises the following steps: smelting and continuously casting various components according to a reasonable proportion; heating and rolling; directly quenching; two-phase region coupling tempering; and after discharging, performing air cooling or water mist cooling to room temperature to obtain a finished steel plate. The production method provided by the invention is high in universality, low in modification requirement on an existing medium-thickness plate production line (with accelerated cooling and tempering capabilities), easy to realize industrial large-scale production and remarkable in economic benefit and social benefit; through the collaborative design of'component fine adjustment and gradient quenching + two-phase region coupling tempering ', the mechanical property of the steel plate comprehensively reaches the standard (the yield strength is larger than or equal to 785 MPa, and the impact energy stability at the temperature of-40 DEG C is larger than or equal to 80 J) while greenness and low carbon are achieved, it is proved that'green production and product performance improvement can be synchronously achieved', and the practicability and popularization value of the green low-carbon technology are further enhanced.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for low-alloy high-strength steel plates, specifically involving a green and low-carbon production method for HG785 steel plates that uses direct quenching and tempering instead of heat treatment. It adopts direct quenching (DQ) combined with a specific tempering process to replace the traditional offline reheating quenching and tempering (heat treatment, Q+T) heat treatment production method. Background Technology

[0002] HG785 is a high-strength structural steel with a yield strength of not less than 785MPa, widely used in critical load-bearing structures such as engineering machinery, mining equipment, and large bridges. The mainstream traditional process for producing this type of thick high-strength steel plate is quenching and tempering: the rolled steel plate is cooled to room temperature, then reheated in a roller hearth furnace to the complete austenitizing temperature (usually 30-50℃ above Ac3), held at that temperature, quenched, and then tempered at high temperature to obtain a tempered sorbite structure, thereby achieving a balance between high strength and good toughness.

[0003] However, this traditional process has significant drawbacks: (1) huge energy consumption: the residual heat after rolling is wasted, and reheating consumes a lot of energy; (2) long production cycle: additional heating, heat preservation and multiple transfer times are required; (3) serious iron oxide scale: reheating produces a large amount of iron oxide scale, resulting in loss of yield and surface quality problems; (4) large equipment investment and land occupation: a high-power roller hearth heat treatment furnace and supporting equipment are required.

[0004] To save energy and reduce consumption, direct quenching (DQ) technology has been explored. Its principle is to use the high-temperature residual heat after rolling to directly perform online quenching, eliminating the need for reheating. However, when applying DQ to high hardenability steels such as 30-50mm thick HG785, it faces recognized technical bottlenecks: (1) Difficulty in controlling the uniformity of cooling in the thickness direction: The cooling rate of the core of the thick plate is much lower than that of the surface, which easily leads to the generation of non-target medium-temperature or high-temperature transformation products (such as bainite and pearlite) in the core, resulting in uneven structure and insufficient core strength and toughness; (2) Extremely high quenching stress: The thermal stress and structural stress generated by direct quenching of thick plates are easily superimposed, leading to severe warping or even cracking of the plate shape; (3) Narrow tempering parameter window: The microstructure after direct quenching (often a mixture of martensite and a small amount of bainite) is different from the single tempered martensite / sorbite after traditional quenching and tempering. It is difficult to ensure strength and toughness at the same time using traditional tempering temperature and time, especially the low-temperature impact toughness fluctuates greatly.

[0005] Therefore, existing publicly available technologies generally believe that for HG785 grade steel plates with a thickness of 30-50mm, it is difficult and risky to completely replace the quenching and tempering process with direct quenching. Either the performance (especially the core toughness and plate shape) will not meet the stringent standards, or an extremely complex and expensive ultra-strong jet cooling system will be required, which limits industrial application. Summary of the Invention

[0006] To address the aforementioned technical deficiencies, the present invention aims to provide a green and low-carbon production method for HG785 steel plates that replaces quenching and tempering with direct quenching and tempering. This method overcomes the biases and obstacles of existing technologies and, without relying on ultra-strong cooling equipment, successfully replaces traditional quenching and tempering through a unique composition fine-tuning and precisely matched "gradient quenching + two-phase region coupled tempering" process. While ensuring high strength, high toughness, and excellent plate shape across the entire thickness range of the steel plate (especially the core), it achieves significant energy saving, consumption reduction, and efficiency improvement.

[0007] To achieve the above objectives, the technical solution adopted by this invention is as follows: A green and low-carbon production method for HG785 steel plates, wherein direct quenching and tempering replaces heat treatment, the steel plate thickness is 30-50mm, and the production method includes the following steps: 1) Smelting and Continuous Casting: Molten steel is smelted by weight percentage, and its chemical composition meets the following requirements: C: 0.06-0.10%, Si: 0.15-0.35%, Mn: 1.40-1.70%, P≤0.015%, S≤0.005%, Nb: 0.025-0.045%, Ti: 0.008-0.020%, Mo: 0.20-0.40%, Cr: 0.30-0.60%, Ni: 0.30-0.60%, V: 0.03-0.06%, B: 0.0008-0.0020%, Alt: 0.020-0.050%, N≤0.0040%, and satisfies that (Nb+Ti) / (C+N) is between 3.5 and 5.5, Ti / N > 3.5, with the balance being Fe and unavoidable impurities; it is then continuously cast into billets. 2) Heating and rolling: The continuously cast billet is heated to 1180-1220℃ and held to allow the microalloying elements to be fully dissolved; rough rolling is performed first to complete most of the deformation in the recrystallization zone; then finish rolling is performed, with the final rolling temperature controlled at 830-870℃ (non-recrystallization zone), and the cumulative reduction rate ≥50% to obtain a severely deformed non-recrystallized austenitic structure; 3) Direct quenching: The rolled steel plate immediately enters the accelerated cooling device and adopts the "gradient quenching" mode: In the first stage, the surface of the steel plate is cooled to below 450℃ at a cooling rate of ≥20℃ / s, and the core temperature is reduced to about 600-650℃ (between Bs and Bf); In the second stage, the water cooling intensity is adjusted, and the entire steel plate (core) is cooled to below 150℃ at a cooling rate of 5-15℃ / s before leaving the quenching zone; The final cooling temperature is controlled at 80-150℃. The gradient quenching design described above ensures that the surface forms martensite in the first stage of strong cooling, while the core begins to transform near the Bs point. The second stage of slow cooling aims to allow the core to slowly complete the martensitic transformation in the temperature range between the Ms and Mf points, while allowing a small amount of bainite to form on the already formed (Nb,Ti) (C,N) and deformation bands, thus synergistically dissipating stress and avoiding the huge internal stress caused by rapid cooling to room temperature.

[0008] 4) Two-phase coupled tempering: The quenched steel plate is placed in the tempering furnace and tempered according to the following procedure: First stage: Heating to 740-760℃ at a heating rate of ≤5℃ / min and holding for 10-25 minutes; Second stage: Then furnace cooling to 620-660℃ at a free cooling rate or a controlled slow cooling rate of ≤3℃ / min and holding for 40-80 minutes; The total tempering time (including heating, holding and second stage slow cooling) is determined by 1.5-2.5 minutes per millimeter of steel plate thickness. The aforementioned two-phase coupled tempering process involves a first-stage (α+γ region) short-time tempering, which rapidly softens the martensite while simultaneously precipitating fine, Mo and V-rich alloy cementite "seeds" from the retained austenite. Subsequent cooling and a second-stage tempering allow these "seeds" to grow further or act as heterogeneous nucleation sites, promoting the precipitation of more stable and finer Ti-Mo-V composite carbides. This achieves "hierarchical strengthening" and refinement of the intragranular structure, which is crucial for obtaining high core toughness.

[0009] 5) After exiting the furnace, air-cool or water-mist-cool to room temperature to obtain the finished steel plate.

[0010] The traditional quenching and tempering (Q+T) process for HG785 steel plates requires reheating the steel plate, which has been cooled to room temperature after rolling, to the full austenitizing temperature (30-50°C above Ac3). This results in a waste of residual heat after rolling and consumes a large amount of energy during reheating. The core innovation of this invention is to utilize the residual heat of rolling to achieve direct quenching (DQ), completely eliminating the "reheating and quenching" process. This reduces energy consumption at the source, explicitly stating "energy savings of over 30%", while also reducing carbon dioxide emissions, directly aligning with the core requirements of green and low-carbon development: "reducing energy consumption and greenhouse gas emissions".

[0011] The reheating process in traditional quenching and tempering processes generates a large amount of iron oxide scale, which not only reduces the yield of steel plates (wasting resources) but also causes surface quality problems. Subsequent processing may increase additional energy consumption and pollutant emissions. This invention eliminates the reheating process, avoiding the large-scale generation of iron oxide scale. This improves the yield (reducing resource waste) and reduces the environmental burden caused by iron oxide scale treatment, aligning with the green production concept of "efficient resource utilization and reduced pollutant generation."

[0012] The core innovation of this invention lies in: "the design for phase transformation stress management based on composite precipitates and the precise control technology for coupled tempering microstructure in two-phase regions." Specifically, this invention does not simply optimize the direct quenching cooling rate, but creatively achieves synergistic effects from the following two aspects: Innovations in composition and microstructure design: Based on the HG785 base composition, trace amounts of Nb and Ti are introduced, and the form of B is controlled, with its quantitative relationship with C and N defined. The aim is not only traditional grain refinement and precipitation strengthening, but more importantly, to promote the pre-formation of high-density, nanoscale composite (Nb,Ti) (C,N) and Ti-Mo-C precipitates within the austenite grains and at grain boundaries during rolling in the non-recrystallized zone before direct quenching and subsequent cooling. These precipitates act as "pinning points" and "phase deformation nuclei," actively guiding and refining martensite lath bundles / blocks during subsequent direct quenching, while simultaneously promoting the non-uniform nucleation of a small amount (3-8% by volume) of lath bainite at specific locations (such as near grain boundaries and precipitates). This hybrid microstructure of "martensite matrix + trace amounts of dispersed lath bainite," compared to a single martensite microstructure, effectively absorbs and channels phase transformation stress during quenching, significantly reducing the risk of cracking in thick plates and improving plate shape. The control of (Nb+Ti) / (C+N) and Ti / N ratios ensures the formation of sufficient nanoscale (Nb,Ti)(C,N) precipitates to pin austenite grain boundaries and induce phase transformation. A high Ti / N ratio preferentially forms TiN, fixes N, protects the hardenability effect of B, and provides a Ti source for subsequent Ti-Mo-C precipitation.

[0013] Innovation in tempering process: For the specific direct-quenched microstructure described above, the traditional single-temperature tempering method below Ac1 is abandoned, and a "two-phase region coupled tempering" is adopted. That is, a short-term holding is first performed in a low-temperature two-phase region (α+γ) above Ac1 (e.g., 740-760℃), causing rapid tempering and softening of the quenched martensite, while simultaneously precipitating extremely fine alloy cementite (Fe,Mo)3C from the retained austenite and carbon-rich regions. Subsequently, the temperature is lowered to a conventional high-temperature tempering region below Ac1 (e.g., 620-660℃) for sufficient holding. The fine, dispersed alloy cementite generated in the first stage (α+γ region) tempering acts as a "template" or "catalyst" during the second stage tempering, promoting the epitaxial growth or independent precipitation of stable, fine Ti-Mo-V composite carbides on it. This process achieves "hierarchical precipitation" and "size distribution" of precipitates, ultimately yielding a multiphase microstructure dominated by ultrafine tempered sorbite and containing high-density, thermally stable nanoscale composite carbides. While ensuring strength, this microstructure exhibits exceptionally excellent and stable low-temperature impact toughness (-40℃ KV2) in the core, fully comparable to or even superior to traditional quenched and tempered steel plates.

[0014] The beneficial effects of this invention are: it completely eliminates the reheating and quenching process, utilizes the residual heat of rolling, saves more than 30% of energy, reduces carbon dioxide emissions, and shortens the production cycle by more than 40%; and successfully solves the problem of controlling the core performance and plate shape of thick plates by direct quenching. By actively controlling the phase transformation path through composition design to generate a stress-dissipating structure, combined with gradient quenching, the unevenness of a 50mm thick plate after quenching is ≤8mm / m, allowing it to proceed to the next process without straightening. The mechanical properties are comprehensive, excellent, and stable. The produced 30-50mm thick HG785 steel plates exhibit a yield strength ≥785MPa, tensile strength 850-950MPa, elongation ≥16%, and a longitudinal impact energy (KV2) of ≥80J at -40℃. The performance uniformity deviation between the core and the 1 / 4 thickness is <10%. The method is highly universal, requiring minimal modification to existing medium-thick plate production lines (with accelerated cooling and tempering capabilities), and is easily scalable for large-scale industrial production, resulting in significant economic and social benefits. Traditional direct quenching technology suffers from unstable performance and poor plate shape, making it difficult to replace tempering processes. This invention, however, utilizes "composition fine-tuning and gradient quenching + two-phase region coupled tempering" to address these issues. The collaborative design ensures that the mechanical properties of the steel plate fully meet the standards (yield strength ≥785MPa, impact energy stable at -40℃ ≥80J) while achieving green and low-carbon production, proving that "green production and product performance improvement can be achieved simultaneously", and further strengthening the practicality and promotion value of green and low-carbon technologies. Attached Figure Description

[0015] Figure 1 This is a schematic diagram comparing the production process of the embodiments of the present invention with that of the comparative example (traditional conditioning process).

[0016] Figure 2 These are transmission electron microscopy (TEM) images of the core tissue of Example 1 (40mm thick steel plate) and Comparative Example 1 (conventional DQ+T, single low-temperature tempering) of the present invention, showing an ultrafine tempered sorbite matrix and nanoscale composite precipitates.

[0017] Figure 3 This is a schematic diagram comparing the impact energy of the core at -40℃ in Embodiment 2 (50mm thick steel plate) and Comparative Example 1 (traditional DQ+T, single low-temperature tempering). Detailed Implementation

[0018] The present invention will be further described below with reference to the embodiments, but the present invention is not limited thereto.

[0019] Example 1: Production of 40mm thick HG785 steel plate, the steps are as follows: 1. Steel composition (wt%): C 0.08, Si 0.25, Mn 1.55, P 0.010, S 0.003, Nb 0.035, Ti 0.015, Mo 0.30, Cr 0.45, Ni 0.40, V 0.045, B 0.0015, Alt 0.035, N 0.0035, (Nb+Ti) / (C+N)=4.2, Ti / N=4.3.

[0020] 2. Heat the billet to 1200℃ and hold for 3 hours. Rough roll to 100mm thickness, finish roll at 850℃, with a cumulative reduction of 60%.

[0021] 3. Direct quenching: The first stage is water cooling at a rate of about 25°C / s, cooling the surface of the steel plate to about 400°C and the core to about 630°C; the second stage is adjusted water cooling at a rate of about 10°C / s, finally cooling to about 120°C.

[0022] 4. Tempering: Heat to 750℃ at 4℃ / min and hold for 15 minutes; then cool in the furnace (about 2℃ / min) to 640℃ and hold for 60 minutes; remove from the furnace and cool with water mist.

[0023] Performance test results: Yield strength 815MPa, tensile strength 890MPa, elongation 18%, impact energy (core) at -40℃ average 95J, and plate shape straight.

[0024] Example 2: Production of 50mm thick HG785 steel plate, the steps are as follows: 1. Composition adjustment (wt%): C 0.07, Mn 1.65, Nb 0.040, Ti 0.018, Mo 0.35, B 0.0018, others are the same as in Example 1, (Nb+Ti) / (C+N)=4.8, Ti / N=5.1.

[0025] 2. The final rolling temperature is controlled at 840℃, and the cumulative reduction rate is 55%.

[0026] 3. Direct quenching: First stage cooling rate ≥22℃ / s, core cooled to about 650℃; second stage cooling rate 8-12℃ / s, final cooling to about 150℃.

[0027] 4. Tempering: Heat to 755℃ and hold for 20 minutes, then cool to 630℃ and hold for 75 minutes.

[0028] Performance test results: Yield strength 805MPa, tensile strength 875MPa, elongation 17%, impact energy (core) at -40℃ average 85J, good plate shape.

[0029] Comparative Example 1: 40mm thick HG785 produced using conventional conditioning process (same composition as Example 1) After rolling, the rolls are air-cooled to room temperature, reheated to 920℃ and held at that temperature, then water-quenched, and finally tempered at 650℃ for 120 minutes.

[0030] Results: The performance was comparable to that of Example 1, but the energy consumption was high, the cycle was long, and there was iron oxide scale on the surface.

[0031] Comparative Example 2: Conventional direct quenching + single low-temperature tempering (same composition and rolling process as Example 1) Quench directly to room temperature (forced cooling), then directly temper at 640℃ for 90 minutes.

[0032] Results: The plate warping was severe (unevenness > 15 mm / m), requiring hot straightening; the impact energy at -40℃ fluctuated greatly (40-100 J), and the core toughness was unstable.

[0033] Conclusion: Through innovative composition and process design, this invention successfully replaces the traditional quenching and tempering process with direct quenching and tempering for stabilizing 30-50mm thick HG785 steel plates, demonstrating significant comprehensive advantages.

Claims

1. A green and low-carbon production method for HG785 steel plates, characterized by direct quenching and tempering instead of heat treatment, wherein: The steel plate thickness is 30-50mm, and the production method includes the following steps. 1) Smelting and Continuous Casting: Molten steel is smelted by weight percentage, and its chemical composition meets the following requirements: C: 0.06-0.10%, Si: 0.15-0.35%, Mn: 1.40-1.70%, P≤0.015%, S≤0.005%, Nb: 0.025-0.045%, Ti: 0.008-0.020%, Mo: 0.20-0.40%, Cr: 0.30-0.60%, Ni: 0.30-0.60%, V: 0.03-0.06%, B: 0.0008-0.0020%, Alt: 0.020-0.050%, N≤0.0040%, and satisfies that (Nb+Ti) / (C+N) is between 3.5 and 5.5, Ti / N > 3.5, with the balance being Fe and unavoidable impurities; it is then continuously cast into billets. 2) Heating and rolling: The continuously cast billet is heated to 1180-1220℃ and held to allow the microalloying elements to be fully dissolved; rough rolling is performed first to complete most of the deformation in the recrystallization zone; then finish rolling is performed, with the final rolling temperature controlled at 830-870℃ (non-recrystallization zone), and the cumulative reduction rate ≥50% to obtain a severely deformed non-recrystallized austenitic structure; 3) Direct quenching: The rolled steel plate immediately enters the accelerated cooling device and adopts the "gradient quenching" mode: In the first stage, the surface of the steel plate is cooled to below 450℃ at a cooling rate of ≥20℃ / s, and the core temperature is reduced to about 600-650℃ (between Bs and Bf); In the second stage, the water cooling intensity is adjusted, and the entire steel plate (core) is cooled to below 150℃ at a cooling rate of 5-15℃ / s before leaving the quenching zone; The final cooling temperature is controlled at 80-150℃. 4) Two-phase coupled tempering: The quenched steel plate is placed into the tempering furnace and tempered according to the following procedure: First stage: Heating to 740-760℃ at a heating rate of ≤5℃ / min and holding for 10-25 minutes; Second stage: Then, cool the furnace to 620-660℃ by free cooling or slow cooling at a controlled rate of ≤3℃ / min, and hold for 40-80 minutes; The total tempering time (including heating, holding, and the second stage of slow cooling) is determined based on 1.5-2.5 minutes per millimeter of steel plate thickness; 5) After exiting the furnace, air-cool or water-mist-cool to room temperature to obtain the finished steel plate.

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