A method for producing a 540mpa grade hot-rolled dual phase steel

Abstract

This invention discloses a method for producing 540MPa grade hot-rolled duplex steel, mainly including: 1) steel smelting; 2) continuous casting; 3) heating and rolling of the continuously cast billet; 4) cooling, with laminar flow cooling after rolling for a three-stage cooling process. The chemical composition of the duplex steel, by weight percentage, includes: C: 0.05-0.07%, Si: 0.20-0.3%, Mn: 0.85-0.95%, P: ≤0.015%, S≤0.005%, Cr: 0.71~0.81%, Als: 0.022-0.030%, Ce: 0.0006~0.0012%, with the balance being Fe and unavoidable impurities. This invention, through reasonable composition design combined with appropriate rolling and cooling processes, obtains duplex steel plates with low yield strength ratio and high strength and toughness.

Description

Technical Field

[0001] This invention belongs to the field of hot-rolled steel plate technology, and particularly relates to a production method of 540MPa grade hot-rolled duplex steel. Background Technology

[0002] Dual-phase steel utilizes composite material concepts in alloy design. Its basic principle is to use a composite material that maximizes the advantages of each phase while reducing or eliminating their disadvantages due to the presence of other phases. During dynamic deformation, dual-phase steel exhibits higher energy absorption compared to other steels with the same static yield strength. This property allows for its application in automotive components. For example, its widespread use in the manufacture of stamped components for car bodies and chassis can significantly improve component strength and reduce component weight.

[0003] The invention disclosed in authorization announcement number CN102732786B is an economical 540MPa grade ferritic bainitic hot-rolled duplex steel and its production method. By mass percentage, it contains C: ≤0.20%, Si: ≤1.80%, Mn: ≤2.00%, P: ≤0.050%, S: ≤0.015%, Nb: ≤0.10%, and the balance Fe. The production process involves the following steps: hot metal pretreatment; converter smelting; alloy fine-tuning station; LF refining; continuous casting; and hot continuous rolling. The difference between this invention and the aforementioned invention lies primarily in the significantly different composition design. This invention's composition design does not contain precious metals such as Nb, resulting in lower alloy costs and a clear cost advantage. The manufacturing process of this invention also differs significantly from the aforementioned inventions.

[0004] The invention disclosed in application publication number CN108411206A is a thin-gauge hot-rolled duplex steel with a tensile strength of 540MPa and its manufacturing method, belonging to the field of metallurgical technology; the chemical composition of the duplex steel by mass percentage is C: 0.04-0.065%, Si: 0.05-0.14%, Mn: 0.40-0.56%, Cr: 0.20-0.30%, S: ≤0.014%, P: ≤0.018%, Als: 0.02-0.04%, with the balance being Fe and unavoidable impurities. The manufacturing method of duplex steel is as follows: 1) molten steel is poured into ingots; 2) the ingots are directly rolled; 3) the strip is cooled in three stages: water-cooling-air-water-cooling. This invention employs a direct rolling process for the cast billet, reducing the pre-rolling heating step, fully utilizing the grain-refining effect of large deformation, reducing the use of manganese, chromium, and silicon, and eliminating the need for other expensive microalloying elements. This significantly reduces production costs, improves production efficiency, and results in a steel plate with uniform microstructure and good surface quality, achieving heat-based production of duplex steel. The difference between this invention and the aforementioned inventions lies primarily in the significantly different composition design. The manufacturing processes of this invention are also significantly different from those of the aforementioned inventions.

[0005] The invention disclosed in application publication number CN108315663 B is a 540MPa grade Ti microalloyed hot-rolled duplex steel plate and its preparation method, belonging to the field of metallurgical technology. The chemical composition of the duplex steel plate, by mass percentage, is: C: 0.04–0.08%, Si: 0.05–0.15%, Mn: 0.40–0.60%, S: ≤0.015%, P: ≤0.018%, Als: 0.02–0.05%, Ti: 0.03–0.05%, with the balance being Fe and unavoidable impurities. The preparation method of the hot-rolled duplex steel plate is described below. Method: 1) Heat the steel billet to 1200–1240℃ and hold for 1.5–2.5 hours; 2) Rough roll the heated steel billet; 3) Finish roll the intermediate billet; 4) Perform a three-stage cooling process of water-air-water cooling on the strip. This invention uses inexpensive microalloyed titanium to replace expensive alloys chromium and niobium, reducing the use of manganese and silicon, lowering the mill load, and producing steel plates with uniform microstructure and good surface quality. This achieves low-cost, easy-to-roll, and high-efficiency production of hot-rolled duplex steel plates with a tensile strength of 540 MPa. The difference between this invention and the aforementioned inventions lies primarily in the significantly different composition design. The manufacturing process of this invention also differs significantly from the aforementioned inventions. Summary of the Invention

[0006] The purpose of this invention is to provide a production method for 540MPa grade hot-rolled duplex steel, which obtains duplex steel plates with low yield strength ratio and high strength and toughness by combining reasonable composition design with appropriate rolling and cooling processes.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] This invention discloses a method for producing 540MPa grade hot-rolled duplex steel, the main process parameters of which include:

[0009] 1) Steel smelting

[0010] The steelmaking process uses desulfurized pretreated molten iron for smelting. A top-and-bottom blown converter is used to decarburize and dephosphorize the molten iron. The tapping temperature of the converter is 1650℃. The molten steel is then transferred to an LF furnace for refining to complete the alloying treatment of the molten steel, so that the molten steel meets the design composition requirements and thus obtains molten steel with qualified composition.

[0011] 2) Continuous casting

[0012] The molten steel obtained in the previous step is transferred to a continuous casting machine and continuously cast into slabs. During the continuous casting process, a stable casting speed must be maintained, and electromagnetic stirring and gentle reduction techniques for the slabs are employed to improve slab quality. The slabs are then inspected, cleaned, and stacked for slow cooling.

[0013] 3) Heating and rolling of continuously cast billets

[0014] The continuously cast billet is heated in the heating furnace before rolling at a temperature of 1230~1250℃. After the billet is taken out of the furnace, it undergoes thorough surface descaling and then enters the rolling process. The initial rolling temperature for finishing rolling is 1010~1030℃, and the final rolling temperature for finishing rolling is 835~845℃.

[0015] 4) Cooling

[0016] After rolling, the steel plate undergoes a three-stage laminar flow cooling process: the first stage is water cooling at a rate of 34-36℃ / s, with a final cooling temperature of 630~640℃; the second stage is air cooling of the steel plate for 4~6s; then it enters the third stage of water cooling at a rate of ≥95℃ / s, with a final cooling temperature of 130~150℃, and is then rolled into a steel coil, followed by air cooling to room temperature;

[0017] The chemical composition of the duplex steel by weight percentage includes: C: 0.05-0.07%, Si: 0.20-0.3%, Mn: 0.85-0.95%, P: ≤0.015%, S≤0.005%, Cr: 0.71~0.81%, Als: 0.022-0.030%, Ce: 0.0006~0.0012%, with the balance being Fe and unavoidable impurities.

[0018] Furthermore, the first stage of water cooling has a cooling rate of 35℃ / s.

[0019] Furthermore, the chemical composition of the duplex steel by weight percentage includes: C: 0.05%, Si: 0.20%, Mn: 0.95%, P: 0.012%, S: 0.004%, Cr: 0.71%, Als: 0.022%, Ce: 0.0006%, with the balance being Fe and unavoidable impurities.

[0020] Furthermore, the chemical composition of the duplex steel by weight percentage includes: C: 0.06%, Si: 0.24%, Mn: 0.91%, P: 0.012%, S: 0.004%, Cr: 0.77%, Als: 0.023%, Ce: 0.0010%, with the balance being Fe and unavoidable impurities.

[0021] Furthermore, the chemical composition of the duplex steel by weight percentage includes: C: 0.07%, Si: 0.30%, Mn: 0.85%, P: 0.012%, S: 0.003%, Cr: 0.81%, Als: 0.028%, Ce: 0.0011%, with the balance being Fe and unavoidable impurities.

[0022] Furthermore, the continuously cast billet is heated in a heating furnace before rolling at a temperature of 1250°C. After the billet exits the furnace, it undergoes thorough surface descaling before entering the rolling process. The initial rolling temperature for finishing is 1030°C, and the final rolling temperature for finishing is 845°C.

[0023] After rolling, the steel plate undergoes a three-stage laminar flow cooling process: the first stage is water cooling at a rate of 35℃ / s, with a final cooling temperature of 640℃; the second stage involves air cooling of the steel plate for 4 seconds; then, the third stage is water cooling at a rate of ≥95℃ / s, with a final cooling temperature of 150℃, during which the steel plate is rolled into a coil and then air-cooled to room temperature.

[0024] Furthermore, the continuously cast billet is heated in the heating furnace before rolling at a temperature of 1240°C. After the billet is taken out of the furnace, it undergoes thorough surface descaling before entering the rolling process. The initial rolling temperature for finishing is 1020°C, and the final rolling temperature for finishing is 840°C.

[0025] After rolling, the steel plate undergoes a three-stage laminar flow cooling process: the first stage is water cooling at a rate of 35℃ / s, with a final cooling temperature of 635℃; the second stage involves air cooling of the steel plate for 5 seconds; then, the steel plate enters the third stage of water cooling at a rate of ≥95℃ / s, with a final cooling temperature of 140℃, before being rolled into a coil and then air-cooled to room temperature.

[0026] Furthermore, the continuously cast billet is heated in the heating furnace before rolling at a temperature of 1230°C. After the billet is taken out of the furnace, it undergoes thorough surface descaling before entering the rolling process. The initial rolling temperature for finishing is 1010°C, and the final rolling temperature for finishing is 835°C.

[0027] After rolling, the steel plate undergoes a three-stage laminar flow cooling process: the first stage is water cooling at a rate of 35℃ / s, with a final cooling temperature of 630℃; the second stage involves air cooling of the steel plate for 6 seconds; then, the third stage is water cooling at a rate of ≥95℃ / s, with a final cooling temperature of 130℃, during which the steel plate is rolled into a coil and then air-cooled to room temperature.

[0028] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0029] This invention provides a method for manufacturing duplex steel plates with low yield strength ratio, high strength and toughness, and a thickness of 2-6 mm. By combining reasonable composition design with appropriate rolling and cooling processes, duplex steel plates with low yield strength ratio and high strength and toughness can be obtained. Detailed Implementation

[0030] A method for manufacturing duplex steel plates with low yield strength ratio, high strength and toughness, and a thickness of 2-6 mm is disclosed. The method achieves the duplex steel plate with low yield strength ratio and high strength and toughness by combining reasonable composition design with appropriate rolling and cooling processes.

[0031] The production method includes the following steps:

[0032] 1. Steel smelting

[0033] The steelmaking process utilizes desulfurized pretreated molten iron, which is then decarburized and dephosphorized using a top-and-bottom blown converter. The converter tapping temperature is 1650℃. The molten steel is then transferred to an LF furnace for refining, where it undergoes alloying treatment to achieve the designed composition requirements, thus obtaining steel with qualified composition.

[0034] 2. Continuous casting

[0035] The molten steel obtained in the previous step is transferred to a continuous casting machine and continuously cast into slabs. During the continuous casting process, a stable casting speed must be maintained, and electromagnetic stirring and light reduction technology for the continuously cast slabs should be implemented to improve the quality of the slabs. The continuously cast slabs are then inspected, cleaned, and stacked for slow cooling.

[0036] 3. Heating and rolling of continuously cast billets

[0037] The continuously cast billet is heated in a heating furnace before rolling at a temperature of 1230~1250℃. After the billet is taken out of the furnace, it undergoes thorough surface descaling before entering the rolling process. The initial rolling temperature for finishing rolling is 1010~1030℃, and the final rolling temperature for finishing rolling is 835~845℃.

[0038] 4. Cooling

[0039] After rolling, the steel plate undergoes a three-stage laminar flow cooling process. The first stage involves water cooling at a rate of 35℃ / s, with a final cooling temperature of 630~640℃. The second stage involves air cooling of the steel plate for 4~6 seconds to allow it to reach room temperature. Afterward, the steel plate enters the third stage of water cooling at a rate ≥95℃ / s, with a final cooling temperature of 130~150℃, where it is then rolled into a coil and finally air-cooled to room temperature.

[0040] Example 1

[0041] The chemical composition of Example 1 is shown in Table 1, and the mechanical properties of the steel plate are shown in Table 2.

[0042] The continuously cast billet is heated in a heating furnace before rolling at a temperature of 1250℃. After the billet is taken out of the furnace, it undergoes thorough surface descaling before entering the rolling process. The initial rolling temperature for finishing rolling is 1030℃, and the final rolling temperature for finishing rolling is 845℃.

[0043] After rolling, the steel plate undergoes a three-stage laminar flow cooling process. The first stage involves water cooling at a rate of 35℃ / s, with a final cooling temperature of 640℃. The second stage involves air cooling of the steel plate for 4 seconds. Afterward, the steel plate enters the third stage of water cooling at a rate ≥95℃ / s, with a final cooling temperature of 150℃, where it is then rolled into a coil and finally air-cooled to room temperature.

[0044] Example 2

[0045] The chemical composition of Example 2 is shown in Table 1, and the mechanical properties of the steel plate are shown in Table 2.

[0046] The continuously cast billet is heated in a heating furnace before rolling at a temperature of 1240℃. After the billet is taken out of the furnace, it undergoes thorough surface descaling before entering the rolling process. The initial rolling temperature for finishing is 1020℃, and the final rolling temperature for finishing is 840℃.

[0047] After rolling, the steel plate undergoes a three-stage laminar flow cooling process. The first stage involves water cooling at a rate of 35℃ / s, with a final cooling temperature of 635℃. The second stage involves air cooling of the steel plate for 5 seconds. Afterward, the steel plate enters the third stage of water cooling at a rate ≥95℃ / s, with a final cooling temperature of 140℃, where it is then rolled into a coil and finally air-cooled to room temperature.

[0048] Example 3

[0049] The continuously cast billet is heated in a heating furnace before rolling at a temperature of 1230℃. After the billet is taken out of the furnace, it undergoes thorough surface descaling before entering the rolling process. The initial rolling temperature for finishing rolling is 1010℃, and the final rolling temperature for finishing rolling is 835℃.

[0050] After rolling, the steel plate undergoes a three-stage laminar flow cooling process. The first stage involves water cooling at a rate of 35℃ / s, with a final cooling temperature of 630℃. The second stage involves air cooling of the steel plate for 6 seconds. Afterward, the steel plate enters the third stage of water cooling at a rate ≥95℃ / s, with a final cooling temperature of 130℃, where it is then rolled into a coil and finally air-cooled to room temperature.

[0051] Table 1: Chemical composition (wt%) of steel plates from the examples

[0052] Serial Number thickness C Si Mn P S Cr Als Ce Example 1 2 0.05 0.2 0.95 0.012 0.004 0.71 0.022 0.0006 Example 2 4 0.06 0.24 0.91 0.012 0.004 0.77 0.023 0.001 Example 3 6 0.07 0.3 0.85 0.012 0.003 0.81 0.028 0.0011

[0053] Table 2: Steel Plate Properties of Examples

[0054] Serial Number Specifications / mm Yield strength / MPa Tensile strength / MPa Elongation A / % The ratio of yield strength Bending D=0a, 180° Example 1 2 315 561 38.5 0.56 qualified Example 2 4 307 554 39.0 0.55 qualified Example 3 6 305 557 39.5 0.55 qualified

[0055] 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 method for producing 540MPa grade hot-rolled duplex steel, characterized in that: The main process parameters include: 1) Steel smelting The steelmaking process uses desulfurized pretreated molten iron for smelting. A top-and-bottom blown converter is used to decarburize and dephosphorize the molten iron. The tapping temperature of the converter is 1650℃. The molten steel is then transferred to an LF furnace for refining to complete the alloying treatment of the molten steel, so that the molten steel meets the design composition requirements and thus obtains molten steel with qualified composition. 2) Continuous casting The molten steel obtained in the previous step is transferred to a continuous casting machine and continuously cast into slabs. During the continuous casting process, a stable casting speed must be maintained, and electromagnetic stirring and gentle reduction techniques for the slabs are employed to improve slab quality. The slabs are then inspected, cleaned, and stacked for slow cooling. 3) Heating and rolling of continuously cast billets The continuously cast billet is heated in the heating furnace before rolling at a temperature of 1230~1250℃. After the billet is taken out of the furnace, it undergoes thorough surface descaling and then enters the rolling process. The initial rolling temperature for finishing rolling is 1010~1030℃, and the final rolling temperature for finishing rolling is 835~845℃. 4) Cooling After rolling, the steel plate undergoes a three-stage laminar flow cooling process: the first stage is water cooling at a rate of 34-36℃ / s, with a final cooling temperature of 630~640℃; the second stage is air cooling of the steel plate for 4~6s; then it enters the third stage of water cooling at a rate of ≥95℃ / s, with a final cooling temperature of 130~150℃, and is then rolled into a steel coil, followed by air cooling to room temperature; The chemical composition of the duplex steel by weight percentage includes: C: 0.05-0.07%, Si: 0.20-0.3%, Mn: 0.85-0.95%, P: ≤0.015%, S≤0.005%, Cr: 0.71~0.81%, Als: 0.022-0.030%, Ce: 0.0006~0.0012%, with the balance being Fe and unavoidable impurities.

2. The method for producing 540MPa grade hot-rolled duplex steel according to claim 1, characterized in that: The first stage of water cooling has a cooling rate of 35℃ / s.

3. The method for producing 540MPa grade hot-rolled duplex steel according to claim 1, characterized in that: The chemical composition of the duplex steel by weight percentage includes: C: 0.05%, Si: 0.20%, Mn: 0.95%, P: 0.012%, S: 0.004%, Cr: 0.71%, Als: 0.022%, Ce: 0.0006%, with the balance being Fe and unavoidable impurities.

4. The method for producing 540MPa grade hot-rolled duplex steel according to claim 1, characterized in that: The chemical composition of the duplex steel by weight percentage includes: C: 0.06%, Si: 0.24%, Mn: 0.91%, P: 0.012%, S: 0.004%, Cr: 0.77%, Als: 0.023%, Ce: 0.0010%, with the balance being Fe and unavoidable impurities.

5. The method for producing 540MPa grade hot-rolled duplex steel according to claim 1, characterized in that: The chemical composition of the duplex steel by weight percentage includes: C: 0.07%, Si: 0.30%, Mn: 0.85%, P: 0.012%, S: 0.003%, Cr: 0.81%, Als: 0.028%, Ce: 0.0011%, with the balance being Fe and unavoidable impurities.

6. The method for producing 540MPa grade hot-rolled duplex steel according to claim 3, characterized in that: The continuously cast billet is heated in the heating furnace before rolling at a temperature of 1250℃. After the billet is taken out of the furnace, it undergoes thorough surface descaling before entering the rolling process. The initial rolling temperature for finishing rolling is 1030℃, and the final rolling temperature for finishing rolling is 845℃. After rolling, the steel plate undergoes a three-stage laminar flow cooling process: the first stage is water cooling at a rate of 35℃ / s, with a final cooling temperature of 640℃; the second stage involves air cooling of the steel plate for 4 seconds; then, the third stage is water cooling at a rate of ≥95℃ / s, with a final cooling temperature of 150℃, during which the steel plate is rolled into a coil and then air-cooled to room temperature.

7. The method for producing 540MPa grade hot-rolled duplex steel according to claim 4, characterized in that: The continuously cast billet is heated in the heating furnace before rolling at a temperature of 1240℃. After the billet is taken out of the furnace, it undergoes thorough surface descaling before entering the rolling process. The initial rolling temperature for finishing rolling is 1020℃, and the final rolling temperature for finishing rolling is 840℃. After rolling, the steel plate undergoes a three-stage laminar flow cooling process: the first stage is water cooling at a rate of 35℃ / s, with a final cooling temperature of 635℃; the second stage involves air cooling of the steel plate for 5 seconds; then, the steel plate enters the third stage of water cooling at a rate of ≥95℃ / s, with a final cooling temperature of 140℃, before being rolled into a coil and then air-cooled to room temperature.

8. The method for producing 540MPa grade hot-rolled duplex steel according to claim 5, characterized in that: The continuously cast billet is heated in the heating furnace before rolling at a temperature of 1230℃. After the billet is taken out of the furnace, it undergoes thorough surface descaling before entering the rolling process. The initial rolling temperature for finishing rolling is 1010℃, and the final rolling temperature for finishing rolling is 835℃. After rolling, the steel plate undergoes a three-stage laminar flow cooling process: the first stage is water cooling at a rate of 35℃ / s, with a final cooling temperature of 630℃; the second stage involves air cooling of the steel plate for 6 seconds; then, the third stage is water cooling at a rate of ≥95℃ / s, with a final cooling temperature of 130℃, during which the steel plate is rolled into a coil and then air-cooled to room temperature.