Method for producing high-strength steel from low-grade raw materials

Abstract

The invention belongs to the technical field of metal material processing, and particularly relates to a method for producing high-strength steel from low-grade raw materials, which comprises the following steps: smelting the low-grade raw materials mainly comprising scrap steel after pretreatment to obtain initial molten steel; deep purification and component fine adjustment are carried out through LF furnace refining and RH vacuum degassing; a high-quality casting blank is obtained through continuous casting under dynamic soft reduction; and finally, two-stage controlled rolling is combined with a multi-stage flexible cooling process to complete preparation. According to the method, residual elements are converted into a strengthening phase by optimizing the Ni / Cu proportion, phase change strengthening and multi-scale precipitation strengthening are synchronously achieved through the quenching-partitioning-tempering (Q-P-T) technology, the defects of unstable performance and hot brittleness caused by raw material impurities in a traditional technology are effectively overcome, the purpose that high-strength and high-toughness steel is produced through low-cost raw materials is achieved, and the method is suitable for large-scale production. The defects of unstable product performance and low percent of pass caused by many raw material impurities and large component fluctuation in the traditional process are overcome from the source.

Description

Technical Field

[0001] This invention relates to the field of metal material processing technology, specifically to a method for producing high-strength steel from low-grade raw materials. Background Technology

[0002] In the steel metallurgy industry, with the deepening of green manufacturing and sustainable development concepts, improving the utilization rate of low-grade or low-quality steel raw materials, mainly scrap steel, has become a core demand of the industry. Existing processes for producing high-strength steel using such raw materials generally follow the conventional technical route of "raw material pretreatment—smelting—refining—continuous casting—thermomechanical rolling (TMCP)". The general idea is to reduce impurities through physical sorting, perform desulfurization and deoxidation during the smelting process, and rely on the addition of microalloying elements (such as Nb, V, and Ti) combined with controlled rolling and cooling to improve the strength and toughness of the final product.

[0003] Existing technologies have the following shortcomings: conventional processes have significant and urgent defects that need to be addressed. First, the contradiction between poor raw material adaptability and cost control is prominent. The residual elements such as copper (Cu) and tin (Sn) abundant in low-grade scrap steel are difficult to remove completely and economically effectively under existing refining technologies. When the content of these elements exceeds a critical value (e.g., Cu > 0.15%), they are prone to segregation at grain boundaries during subsequent hot processing, leading to "hot brittle" surface cracks and a significant decrease in product yield. Using large amounts of high-purity molten iron or undergoing deep purification to avoid this risk would drastically increase production costs, contradicting the initial intention of using low-cost raw materials. Second, process stability and performance uniformity face challenges. Due to the large fluctuations in the composition of low-grade raw materials, even small changes in composition can lead to drastic fluctuations in the final microstructure and properties, making the production of high-strength steel difficult to maintain stability and control.

[0004] Therefore, the reason for improving existing technologies is urgent and clear: it is imperative to develop an innovative method that can systematically tolerate fluctuations in raw material composition and transform traditionally harmful residual elements into beneficial ones. This would allow for the stable and efficient production of high-strength, high-toughness steel from low-grade raw materials without significantly increasing equipment investment and production costs. This not only has enormous economic value but is also a key technological path for driving the steel industry towards resource conservation and environmental friendliness. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for producing high-strength steel from low-grade raw materials, thus solving the problem of poor raw material adaptability.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A method for producing high-strength steel from low-grade raw materials, comprising the following steps: S1: Raw material pretreatment and smelting: Low-grade steel raw materials, mainly scrap steel, are sorted, crushed, and cleaned before being smelted in an electric arc furnace or converter to obtain initial molten steel; S2: Ladle refining and composition fine-tuning: The initial molten steel is transferred to an LF refining furnace for deep desulfurization, deoxidation, and inclusion modification treatment, and the composition of the molten steel is adjusted to the target range by adding alloys; S3: Vacuum circulation degassing: The refined molten steel is subjected to RH or VD vacuum circulation treatment for a treatment time of not less than 12 minutes; S4: Continuous casting and dynamic light reduction: Continuous casting is performed using dynamic light reduction technology to obtain a billet; S5: Thermomechanical rolling and flexible cooling: The billet is subjected to two-stage controlled rolling, and immediately after rolling, multi-stage accelerated cooling is performed, followed by final cooling and air cooling to room temperature.

[0007] In some embodiments, in step S1, the low-grade raw material includes at least one of Q235 series scrap steel and scrapped automobile steel sheet, and its residual elements are controlled to satisfy Cu+Sn+Cr+Ni+Mo≤0.45%.

[0008] In some embodiments, the steel composition within the target range in step S2, by mass percentage, is: C: 0.05%–0.12%, Si: 0.15%–0.40%, Mn: 1.50%–2.20%, P≤0.018%, S≤0.006%, with the balance being Fe and unavoidable impurities.

[0009] In some embodiments, the molten steel composition further includes microalloying elements Nb, V, and Ti, with contents of: Nb: 0.020%–0.055%, V: 0.035%–0.085%, Ti: 0.008%–0.030%, and Ti: 0.015%–0.055%.

[0010] In some embodiments, the mass percentages of the microalloying elements Nb, V, and Ti satisfy the relationship: (Nb+V) / Ti = 3.0~8.0.

[0011] In some embodiments, the two-stage controlled rolling in step S4 includes: S4.1: rough rolling in the recrystallization zone: the billet is heated to 1150℃~1250℃, held for 1.5~2.5 hours, and then rolled in multiple passes in the temperature range of 1100℃~1000℃, with a cumulative deformation of ≥55%; S4.2: finish rolling in the non-recrystallization zone: multiple passes are rolled in the temperature range of 950℃ to the Ar3 phase transformation point, with a cumulative deformation of ≥45%.

[0012] In some embodiments, the multi-stage accelerated cooling in step S4 includes: S4.3: First stage ultra-fast cooling: cooling the rolled piece to 600℃~670℃ at a cooling rate of 35℃ / s~60℃ / s; S4.4: Second stage slow cooling: cooling the rolled piece to 450℃~520℃ at a cooling rate of 5℃ / s~20℃ / s; S4.5: Third stage air cooling: subsequently air cooling to room temperature.

[0013] In some embodiments, the multi-stage accelerated cooling is achieved by setting different water pressures and nozzle openings in the cooling system.

[0014] In some embodiments, the dynamic light pressing in step S3 has a total pressing amount of 3 to 8 mm and is applied to the solidification end range where the solid fraction fs of the billet is 0.3 to 0.7.

[0015] Compared with the prior art, the present invention provides a method for producing high-strength steel from low-grade raw materials, which has the following beneficial effects: A method for producing high-strength steel from low-grade raw materials, through the optimized "raw material pretreatment-LF furnace refining-RH vacuum degassing" synergistic purification process in steps S1 to S3, can effectively control the sulfur, phosphorus and gas content in molten steel. At the same time, calcium treatment is used to achieve harmless modification of inclusions, which significantly improves the applicability of raw materials and the purity of molten steel. This method solves the defects of traditional processes, such as unstable product performance and low pass rate caused by many impurities and large fluctuations in composition of raw materials, from the source. The method for producing high-strength steel using low-grade raw materials employs a "two-stage controlled rolling process (rough rolling in the recrystallization zone and fine rolling in the non-recrystallization zone)" in step S4. This process can fully refine the original austenite grains and introduce a large number of defects into the grains, providing a large number of nucleation sites for subsequent phase transformations and laying the foundation for microstructure refinement. This innovative "multi-stage flexible cooling" process, which organically combines "first-stage ultra-fast cooling, second-stage slow cooling, and third-stage air cooling," successfully achieves precise control over the phase transformation process and stably obtains a multiphase structure of ultrafine ferrite and bainite. This structural feature ensures that the final steel achieves high strength while maintaining excellent plasticity, toughness, and formability, and its comprehensive mechanical properties are significantly better than those of conventional processes using a single cooling path. Detailed Implementation

[0016] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] In the description of this invention, it should be noted that the terms "upper," "lower," "inner," "outer," "front end," "rear end," "both ends," "one end," and "the other end," etc., indicate the orientation or positional relationship based on the shown orientation or positional relationship, and are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0018] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installed," "equipped with," "connected," etc., should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be a connection within two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0019] Please refer to this implementation plan: A method for producing high-strength steel from low-grade raw materials, comprising the following steps: S1: Raw material pretreatment and smelting: Low-grade steel raw materials, mainly scrap steel, are sorted, crushed, and cleaned before being smelted in an electric arc furnace or converter to obtain initial molten steel; S2: Ladle refining and composition fine-tuning: The initial molten steel is transferred to an LF refining furnace for deep desulfurization, deoxidation, and inclusion modification treatment, and the composition of the molten steel is adjusted to the target range by adding alloys; S3: Vacuum circulation degassing: The refined molten steel is subjected to RH or VD vacuum circulation treatment for a treatment time of not less than 12 minutes; S4: Continuous casting and dynamic light reduction: Continuous casting is performed using dynamic light reduction technology to obtain a billet; S5: Thermomechanical rolling and flexible cooling: The billet is subjected to two-stage controlled rolling, and immediately after rolling, multi-stage accelerated cooling is performed, followed by final cooling and air cooling to room temperature.

[0020] In step S1, the low-grade raw materials include at least one of Q235 series scrap steel and shredded scrapped automobile steel sheets, and their residual elements are controlled to satisfy Cu+Sn+Cr+Ni+Mo≤0.45%. In step S2, the steel composition within the target range, by mass percentage, is: C: 0.05%~0.12%, Si: 0.15%~0.40%, Mn: 1.50%~2.20%, P≤0.018%, S≤0.006%, with the balance being Fe and unavoidable impurities.

[0021] It should be noted that the molten steel also contains microalloying elements Nb, V, and Ti, with the following contents: Nb: 0.020%~0.055%, V: 0.035%~0.085%, Ti: 0.008%~0.030%, and Ti: 0.015%~0.055%.

[0022] Furthermore, the mass percentages of the microalloying elements Nb, V, and Ti satisfy the following relationship: (Nb+V) / Ti = 3.0~8.0.

[0023] Furthermore, the two-stage controlled rolling in step S4 includes: S4.1: rough rolling in the recrystallization zone: the billet is heated to 1150℃~1250℃, held for 1.5~2.5 hours, and then rolled in multiple passes in the temperature range of 1100℃~1000℃, with a cumulative deformation of ≥55%; S4.2: finish rolling in the non-recrystallization zone: multiple passes are rolled in the temperature range of 950℃ to the Ar3 phase transformation point, with a cumulative deformation of ≥45%.

[0024] The multi-stage accelerated cooling in step S4 includes: S4.3: First stage ultra-fast cooling: cooling the rolled piece to 600℃~670℃ at a cooling rate of 35℃ / s ~ 60℃ / s; S4.4: Second stage slow cooling: cooling the rolled piece to 450℃~520℃ at a cooling rate of 5℃ / s ~ 20℃ / s; S4.5: Third stage air cooling: subsequently air cooling to room temperature. Multi-stage accelerated cooling is achieved by setting different water pressures and nozzle opening degrees in the cooling system.

[0025] The dynamic light pressing described in step S3 has a total pressing amount of 3 to 8 mm and is applied to the solidification end range where the solid fraction fs of the billet is 0.3 to 0.7.

[0026] Example 1: Production of 700MPa grade high-strength steel plates Step 1: Raw material pretreatment and smelting One hundred tons of mixed raw materials, primarily composed of recycled automotive sheet scrap and Q235 scrap steel (in a 7:3 ratio), were selected, sorted, crushed, and cleaned before being fed into an electric arc furnace with a nominal capacity of 100 tons for smelting. The smelting process employed conventional operations, yielding initial molten steel after the raw materials were completely melted. Preliminary testing showed that the total residual element content (Cu+Sn+Cr+Ni+Mo) of the raw materials in this furnace was 0.38%, meeting the requirement of ≤0.45% as specified in the claims.

[0027] Step Two: Ladle Refining and Composition Fine-tuning All of the initial molten steel was transferred to the LF refining station.

[0028] Deep desulfurization and deoxidation: High-alkalinity white slag is produced and thoroughly stirred with bottom-blown argon gas to reduce the sulfur content in the molten steel from 0.025% to 0.004%, achieving an ultra-low sulfur level; at the same time, deep deoxidation is carried out through aluminum feeding wire.

[0029] Inclusion modification treatment: Subsequently, calcium treatment is carried out to transform high-melting-point Al2O3 inclusions into low-melting-point calcium aluminates (such as 12CaO·7Al2O3) to improve the fluidity of molten steel and prevent nozzle blockage.

[0030] Composition fine-tuning: During the refining process, the composition of the molten steel is precisely adjusted to the target range by adding alloys such as ferromanganese and ferrosilicon: C: 0.08%, Si: 0.28%, Mn: 1.80%, P: 0.015%, S: 0.004%. Simultaneously, microalloying elements are added as claimed: Nb: 0.040%, V: 0.060%, Ti: 0.015%, Alt: 0.030%. Calculations show that (Nb+V) / Ti = (0.040+0.060) / 0.015 ≈ 6.67, satisfying the relationship between 3.0 and 8.0.

[0031] Step 3: Vacuum circulation degassing The refined molten steel is then transferred to an RH vacuum treatment unit for circulating degassing. The pressure inside the vacuum tank is controlled below 1 mbar, and the treatment time is strictly controlled to 15 minutes to ensure a significant reduction in the gas content of the molten steel, with the [H] content dropping to 1.8 ppm.

[0032] Step 4: Continuous casting and dynamic light reduction Continuous casting is carried out using an arc-shaped continuous casting machine.

[0033] Dynamic light reduction: Based on the solidification model of the billet, a dynamic light reduction of 5 mm is applied at the end of the billet solidification (the solid fraction fs is about 0.3~0.7), which effectively improves the center segregation and porosity of the billet.

[0034] Step 5: Thermomechanical Rolling and Flexible Cooling – Billet Heating: The obtained billet is placed in a walking beam furnace and heated to 1200℃, then held for 2 hours to ensure uniform heating. – Two-Stage Controlled Rolling: – Recrystallization Zone Rough Rolling: Six passes are performed in the temperature range of 1100℃~1020℃ to reduce the thickness from 220mm to 40mm, with a cumulative deformation of 81.8%. – Non-Recrystallization Zone Finish Rolling: Finish rolling begins when the steel plate cools to 880℃, with seven passes performed in the temperature range of 880℃~810℃, resulting in a final thickness of 8mm and a cumulative deformation of 80.0%.

[0035] – Multi-stage accelerated cooling (according to claim 8, achieved by adjusting the manifold water pressure and nozzles): – First stage ultra-fast cooling: Immediately after rolling, the steel plate is rapidly cooled to approximately 630°C using high-pressure water at a cooling rate of approximately 45°C / s. – Second stage slow cooling: The water pressure is then reduced, switching to a medium cooling mode, and the steel plate is further cooled to approximately 480°C at a cooling rate of approximately 12°C / s. – Third stage air cooling: The steel plate leaves the cooling zone and cools naturally to room temperature in the air.

[0036] Example 2: Production of 800MPa grade high-strength steel plates Technical background and purpose This embodiment aims to specifically illustrate how to use the method described in this invention to stably produce high-performance steel plates with a tensile strength of not less than 800MPa using low-cost industrial scrap steel as the main raw material, so as to meet the steel demand in fields such as engineering machinery.

[0037] Raw material preparation and initial conditions Main raw materials: A mixture mainly composed of A3 steel chips from machining and scrap steel from structural components, with a total input of 120 tons. After pretreatment, the total residual element content (Cu+Sn+Cr+Ni+Mo) is approximately 0.30%, far below the upper limit requirement of 0.45%, demonstrating excellent raw material adaptability. Specific implementation steps

[0038] Step 1: Raw Material Pretreatment and Smelting. The pretreated raw materials are fed into a 100-ton ultra-high power electric arc furnace for smelting. Enhanced oxygen supply and foamy slag operation are employed to ensure rapid melting and efficient dephosphorization. After melting and clearing, initial molten steel is obtained at a temperature of approximately 1620℃.

[0039] Step Two: Ladle Refining and Composition Fine-tuning The initial molten steel is transferred to the LF ladle refining furnace.

[0040] A high-alkalinity refining slag system is used in conjunction with strong stirring to achieve deep desulfurization and control the sulfur content at 0.002%.

[0041] Deoxidation is achieved by feeding aluminum wire, so that the acid-soluble aluminum (Alt) content reaches 0.040%.

[0042] Implementing a calcium treatment process effectively modifies inclusions, causing them to spheroidize and improving the castability of molten steel.

[0043] Alloying fine-tuning is performed to bring the final steel composition to the target value. The core controlled components are as follows: Basic elements: C: 0.09%, Si: 0.35%, Mn: 1.95%.

[0044] Harmful elements: P: 0.012%, S: 0.002%.

[0045] Microalloying elements: Nb: 0.050%, V: 0.075%, Ti: 0.022%, Alt: 0.040%.

[0046] Relationship verification: (Nb+V) / Ti = (0.050+0.075) / 0.022 ≈ 5.68, which meets the technical requirements of 3.0~8.0.

[0047] Step 3: After vacuum circulation degassing and refining, the molten steel enters the RH vacuum degassing unit. It is circulated for 18 minutes at a vacuum level of ≤0.5 mbar. After treatment, the [H] content in the molten steel decreases to 1.2 ppm, and the [N] content decreases to 45 ppm.

[0048] Step 4: Continuous casting and dynamic light pressure were performed using a straight-arc continuous casting machine at a casting speed of 1.2 m / min. A total of 6 mm of dynamic light pressure was applied at the end of the billet solidification (in the range of solid fraction fs approximately 0.4–0.6), effectively improving the center quality of the billet.

[0049] Step 5: Thermomechanical rolling and flexible cooling Billet heating: The billet is heated to 1220℃ in a walking beam furnace and held for 150 minutes to ensure that the alloying elements are fully dissolved and the temperature is uniform.

[0050] Two-stage controlled rolling: Rough rolling in the recrystallization zone: After the billet is descaled after exiting the furnace, it is subjected to 7 passes of reversible rolling in the temperature range of 1120℃–1010℃ to reduce the thickness from 250mm to 45mm, with a cumulative deformation of 82%.

[0051] Finishing in the non-recrystallization zone: Starting when the intermediate billet cools to 870℃, eight rolling passes are performed in the temperature range of 870℃–800℃ to finally obtain a steel plate with a thickness of 12mm and a cumulative deformation of 73.3%.

[0052] Multi-stage accelerated cooling (achieved by adjusting the water pressure and flow rate of the cooling line manifold): The first stage of ultra-fast cooling: The ultra-dense tube laminar flow system is started immediately after rolling, and the surface temperature of the steel plate is rapidly reduced to about 610°C at a high cooling rate of about 50°C / s.

[0053] Second stage slow cooling: Then switch to sparse tube cooling mode to continue cooling the steel plate to about 460°C at a cooling rate of about 8°C / s.

[0054] The third stage of air cooling: The steel plate leaves the cooling zone and is air cooled to room temperature on the roller conveyor.

[0055] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for producing high-strength steel from low-grade raw materials, characterized in that: Includes the following steps: S1: Raw material pretreatment and smelting: Low-grade steel raw materials, mainly scrap steel, are sorted, crushed and cleaned before being smelted in an electric arc furnace or converter to obtain initial molten steel. S2: Ladle refining and composition fine-tuning: The initial molten steel is transferred to an LF refining furnace for deep desulfurization, deoxidation, and inclusion modification treatment, and the composition of the molten steel is adjusted to the target range by adding alloys; S3: Vacuum circulation degassing: The refined molten steel is subjected to RH or VD vacuum circulation treatment for a time of not less than 12 minutes; S4: Continuous casting and dynamic light reduction: Continuous casting is carried out using dynamic light reduction technology to obtain a billet; S5: Thermomechanical rolling and flexible cooling: The billet is subjected to two-stage controlled rolling, and multi-stage accelerated cooling is performed immediately after rolling, followed by final cooling and air cooling to room temperature.

2. The method for producing high-strength steel from low-grade raw materials according to claim 1, characterized in that: In step S1, the low-grade raw materials include at least one of Q235 series scrap steel and scrapped automobile steel sheet, and the residual elements are controlled to satisfy Cu+Sn+Cr+Ni+Mo≤0.45%.

3. The method for producing high-strength steel from low-grade raw materials according to claim 1, characterized in that: The steel composition within the target range in step S2, by mass percentage, is as follows: C: 0.05%–0.12%, Si: 0.15%–0.40%, Mn: 1.50%–2.20%, P≤0.018%, S≤0.006%, with the balance being Fe and unavoidable impurities.

4. The method for producing high-strength steel from low-grade raw materials according to claim 3, characterized in that: The molten steel also contains microalloying elements Nb, V, and Ti, with the following contents: Nb: 0.020%–0.055%, V: 0.035%–0.085%, Ti: 0.008%–0.030%, and Ti: 0.015%–0.055%.

5. The method for producing high-strength steel from low-grade raw materials according to claim 4, characterized in that: The mass percentages of the microalloying elements Nb, V, and Ti satisfy the following relationship: (Nb+V) / Ti = 3.0~8.

0.

6. The method for producing high-strength steel from low-grade raw materials according to claim 1, characterized in that: The two-stage controlled rolling in step S4 includes: S4.1: rough rolling in the recrystallization zone: the billet is heated to 1150℃~1250℃, held for 1.5~2.5 hours, and then rolled in multiple passes in the temperature range of 1100℃~1000℃, with a cumulative deformation of ≥55%; S4.2: finish rolling in the non-recrystallization zone: multiple passes are rolled in the temperature range of 950℃ to the Ar3 phase transformation point, with a cumulative deformation of ≥45%.

7. The method for producing high-strength steel from low-grade raw materials according to claim 1, characterized in that: The multi-stage accelerated cooling in step S4 includes: S4.3: First stage ultra-fast cooling: cooling the rolled piece to 600℃~670℃ at a cooling rate of 35℃ / s ~ 60℃ / s; S4.4: Second stage slow cooling: cooling the rolled piece to 450℃~520℃ at a cooling rate of 5℃ / s ~ 20℃ / s; S4.5: Third stage air cooling: subsequently air cooling to room temperature.

8. The method for producing high-strength steel from low-grade raw materials according to claim 7, characterized in that: The multi-stage accelerated cooling is achieved by setting different water pressures and nozzle openings in the cooling system.

9. The method for producing high-strength steel from low-grade raw materials according to claim 1, characterized in that: The dynamic light pressing in step S3 has a total pressing amount of 3 to 8 mm and is applied to the solidification end range of the billet where the solid fraction fs is 0.3 to 0.7.