High-toughness high-surface-quality wide and thick steel plate and manufacturing method thereof

Abstract

The invention belongs to the technical field of low-alloy high-strength wide and thick steel plate manufacturing, and relates to a high-toughness high-surface-quality wide and thick steel plate and a manufacturing method thereof.The steel plate comprises the chemical components of C, Si, Mn, P, S, Nb, Ti, Als, Cr, B and the balance iron and other impurities, and the high-toughness high-surface-quality wide and thick steel plate with the yield strength ReL larger than or equal to 550 MPa, the tensile strength Rm larger than or equal to 700 MPa, the ductility A larger than or equal to 20% and the tensile strength Rm larger than or equal to 300 MPa is obtained through smelting, refining, continuous casting, casting blank heating, descaling, rough rolling, finish rolling and cooling. And the comprehensive strength and ductility product of the wide and thick steel plate is greater than or equal to 15GPa. Low-cost Ti and B alloy elements are used for partially replacing high-cost Nb / V / Cr / Mo and other alloy elements, so that the strength and low-temperature toughness of a steel plate matrix are improved; a heat treatment process (quenching and tempering) is not needed after rolling, so that the comprehensive energy consumption is reduced, the production efficiency is improved, and the production cost is obviously reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of manufacturing low-alloy high-strength wide and thick steel plates, and in particular relates to a wide and thick steel plate with high toughness and high surface quality and a manufacturing method thereof. Background Art

[0002] As downstream users increase their requirements for the quality of wide and thick steel plates, users pay more attention to the appearance quality of steel plates while paying attention to the performance of steel plates. Currently, high-strength wide and thick steel plates with a yield strength of 550MPa have the advantages of high strength, good toughness, excellent processing and welding properties, and are widely used in the manufacture of engineering machinery equipment and parts. The use of steel grades of this strength level can significantly improve the load-bearing capacity of the overall components and reduce their weight. As competition in the engineering machinery industry becomes increasingly fierce, major domestic and foreign engineering machinery parts manufacturers have increasingly higher requirements for the surface quality of steel plates. They not only require steel plates to meet mechanical performance requirements, but also require steel plates to have good surface quality, that is, after shot blasting and painting, the surface is free of defects such as pits, pitting, embossing, patterns and color differences. At the same time, the steel plates are required to have a higher plate shape.

[0003] Patent CN 112725703 A discloses "A low yield ratio Q550D high-strength steel plate and its manufacturing method", whose chemical composition by weight percentage is: C: 0.04-0.10%, Mn: 1.50-2.0%, Si: 0.10-0.40%, S≤0.0050%, P: ≤0.015%, Nb: 0.030-0.070%, Ti: 0.008-0.020%, Alt: 0.020-0.060%, B: 0.0010-0.0025%, and the rest is Fe and unavoidable impurities. Using a controlled rolling and controlled cooling process, followed by low-temperature quenching and tempering, the resulting structure consists of acicular ferrite, massive ferrite, and evenly distributed martensite with grains less than 2μm. The yield strength is 610-700MPa, the tensile strength is 720-810MPa, the yield strength ratio is ≤0.85, and the longitudinal impact energy is above 100J. This method uses a low-C design, making the produced steel plate prone to surface defects. Furthermore, the process requires a subsequent heat treatment process, which places strict demands on the process and results in poor overall economic performance.

[0004] Patent CN 113604737 A discloses a "Q550D high-strength steel plate and its preparation method." Its chemical composition, by mass, includes: C: 0.06-0.18%, Si: 0.1-0.6%, Mn: 1.0-2.0%, P ≤ 0.03%, S ≤ 0.025%, V: 0.09-0.12%, N: 0.01-0.013%, Cr: 0.1-0.6%, CEV ≤ 0.47, Pcm ≤ 0.22. This solution reduces alloy costs by adding VN alloy, introducing relatively inexpensive nitrogen, while also reducing vacuum degassing and refining in steelmaking. However, the use of Si alloy for deoxidation in practice can lead to difficulties in casting and excessive inclusions in the steel, resulting in reduced low-temperature toughness and the development of microcracks on the steel plate surface.

[0005] Patent CN 116043104 A discloses "a TMCP process for producing low-cost Q550D steel and its production method", and its chemical composition by mass percentage (unit, wt%): C: 0.13-0.16, Si: 0.10-0.25, Mn: 1.40-1.60, P ≤ 0.015, S ≤ 0.003, Als: 0.020-0.035, Nb: 0.030-0.060, Cr: 0.40-0.6 0, Mo: 0.10-0.20, Ti≤0.020, and B 0.0010-0.0020, with the balance being Fe and residual elements. The steel plate has a microstructure of 50%-60% bainite, 20%-30% acicular ferrite, and 10%-20% pearlite, with a yield strength ≥550 MPa, a tensile strength ≥670 MPa, an elongation ≥17%, and a longitudinal KV2 impact energy ≥100 J at -20°C. While this production process eliminates heat treatment, reducing manufacturing costs, it has a high alloying element content and does not meet high surface quality requirements.

[0006] At present, the domestic production and manufacturing methods of wide and thick steel plates with a yield strength of 550MPa are relatively mature, but the manufacturing processes and methods of steel plates that require both high toughness and high surface quality are relatively imperfect. The cost of traditional Q550D-grade steel plates has certain advantages, but their performance in low-temperature toughness is average, and the production process cannot meet the high surface quality requirements. Other high-toughness steel plates generally add large amounts of precious metal elements such as Mn, Nb, V, and Mo in their composition design, resulting in higher alloy costs. Therefore, how to meet the low-temperature high toughness performance requirements of steel plates while manufacturing high-strength wide and thick steel plates with high surface quality has become an urgent problem for steel mills to solve. Summary of the Invention

[0007] The purpose of the present invention is to provide a wide and thick steel plate with high toughness and high surface quality and a manufacturing method thereof. It only requires the addition of an appropriate amount of precious alloy, based on the Nb+Ti composite refined matrix structure, to improve the matrix strength to achieve the high toughness requirement. At the same time, combined with the production process of high surface quality control, the structure is further optimized through controlled rolling and controlled cooling, so as to achieve the high surface quality and high toughness requirements of the wide and thick steel plate.

[0008] The technical solution adopted by the present invention to solve its technical problems is: a wide and thick steel plate with high toughness and high surface quality, wherein the chemical composition and weight percentage of the steel plate are: C: 0.08%-0.15%, Si: 0.25%-0.45%, Mn: 1.2%-1.8%, P≤0.015%, S≤0.0040%, Nb: 0.030%-0.060%, Ti: 0.080%-0.20%, Als: 0.020%-0.055%, Cr: 0.17%-0.37%, B: 0.0015-0.0025%, and the balance is iron and other impurities.

[0009] Furthermore, the physical properties of the steel plate are as follows: the steel plate thickness is 6.0-30mm, the yield strength is ≥550MPa, the tensile strength is ≥700MPa, and the elongation is ≥20%; the metallographic structure of the steel plate is fine-grained ferrite + martensite + a small amount of bainite, the surface of the steel plate is smooth, and a stable light blue oxide layer is formed.

[0010] The role and mechanism of the key alloying elements of the present invention:

[0011] Carbon: Carbon has a significant strengthening effect, significantly increasing the strength and hardenability of steel. However, excessive carbon content can significantly reduce the toughness of steel, and increased carbon content can reduce the cold forming and welding properties of steel. The carbon content in this invention is controlled within a range of 0.08% to 0.15%.

[0012] Manganese: Manganese is the most effective element for improving strength and toughness. It stabilizes austenite and acts as a desulfurizer and deoxidizer, improving the hardenability of steel. Manganese expands the austenite phase, delays pearlite transformation, and refines ferrite grains. Excessive manganese content can cause segregation and banding, and in severe cases, can lead to delamination of the steel. In the present invention, the manganese content is controlled at 1.2-1.8%.

[0013] Silicon: Silicon dissolves in ferrite, acting as a solid solution strengthener and significantly increasing the strength and hardness of the steel matrix. However, excessive silicon content can make surface oxide scale difficult to remove. The silicon content in this invention is controlled between 0.25-0.45%, and a suitable heating process is required.

[0014] Niobium: Niobium has a significant refining effect in steel, effectively refining the original austenite grain size and expanding the unrecrystallized zone during rolling. The Nb (NC) formed during rolling provides a favorable location for ferrite nucleation, pinning grain boundaries and preventing them from moving, thereby increasing steel strength. This simultaneously refines grains and improves strength and ductility. The niobium content in this invention is controlled to be between 0.030% and 0.060%.

[0015] Titanium: Titanium is a strong carbide-forming element with a strong affinity for carbon and nitrogen. It refines the structure of steel and improves toughness and strength. Data show that adding trace amounts of Ti to steel, particularly in Nb-containing steels, can reduce the steel's crack sensitivity by properly controlling the Nb / Ti ratio, effectively reducing transverse cracks in continuous castings and improving the steel's overall strength and toughness. The titanium content in this invention is controlled within a range of 0.08% to 0.20%.

[0016] Chromium (Cr): Chromium mainly improves the hardenability of steel, strengthens the matrix through solid solution, refines the structure, improves the high-temperature strength, hardness and wear resistance of steel, significantly improves the steel's antioxidant effect, and increases its corrosion resistance. When properly combined with Mn and Si, chromium can greatly improve hardenability. In the present invention, the Cr content is controlled at 0.17%-0.37%.

[0017] Boron (B): Boron is a key element for improving hardenability. Even a minimal addition of B (0.0005%-0.0030%) to steel significantly enhances hardenability. Adding trace amounts to steel effectively replaces some expensive alloying elements. The B content in wear-resistant steel should not exceed 0.004%, as this can easily form insoluble borides and cause boron embrittlement. In the present invention, the B content is controlled to 0.0015-0.0025%.

[0018] The method for manufacturing the above-mentioned wide and thick steel plate with high toughness and high surface quality comprises the following steps:

[0019] (1) Smelting: Slide plates are used for slag blocking and steel tapping, and LF+RH double refining process is adopted. The LF treatment cycle is ≥15min, and the RH soft blowing time is ≥15min;

[0020] (2) Continuous casting: The continuous casting process adopts argon protection casting throughout the process, the superheat is controlled at 15-30℃, and the crystallizer liquid level fluctuation is controlled to be ≤±3mm;

[0021] (3) Heating: The slab temperature out of the furnace is controlled at 1220-1280℃, and the holding time of the heating furnace is optimized;

[0022] (4) Descaling: The high-pressure descaling pressure after the furnace must ensure that the outlet pressure is ≥25MPa, the height of the descaling nozzle and the slab must be ensured to be 130-180mm, and the inclination or deflection angle of the descaling nozzle must be ensured to be between 12°-15° to ensure high-pressure water jet descaling;

[0023] (5) Rough rolling: The descaling water before the stand should be kept open for the first three passes of rough rolling, and the descaling water before the stand must be opened for the last pass of rough rolling;

[0024] (6) Finishing rolling: Keep the descaling water in front of the stand open for the first two passes of finishing rolling, and the descaling water in front of the stand must be opened for the last two passes of finishing rolling; the finishing rolling temperature is 780-820℃;

[0025] (7) Cooling process: After finishing rolling, rapid cooling is adopted, and the surface temperature is 180-240℃, so as to obtain a wide and thick steel plate with high surface and high toughness strength, such as yield strength ReL ≥ 550MPa, tensile strength Rm ≥ 700MPa, elongation A ≥ 20%, and comprehensive strength-ductility product ≥ 15GPa.

[0026] Furthermore, the method for optimizing the holding time of the heating furnace in step (3) is specifically as follows: according to the different thicknesses of the ingots, mandatory regulations are imposed on the holding time of the heating furnace: the holding time of 200mm ingots is required to be ≥30min, the holding time of 250mm ingots is required to be ≥35min, and the holding time of 300mm ingots is required to be ≥40min. The holding time cannot exceed 100min at most. If the specified time is exceeded, the ingots are required to be returned to the furnace and taken offline.

[0027] The present invention has the following beneficial effects:

[0028] 1. The present invention meets the requirements of high toughness and high surface area steel plates in combination with actual production. The design composition uses low-cost Ti and B alloy elements to partially replace the higher-cost alloy elements such as Nb / V / Cr / Mo, thereby improving the strength and toughness while refining the grains. At the same time, an appropriate amount of Si element is added to appropriately increase the ferrite content, thereby improving the strength and low-temperature toughness of the steel plate matrix.

[0029] 2. The present invention cooperates with a reasonable rolling process and adopts a controlled cooling method to achieve the production of wide and thick plates with high toughness and high surface requirements of 550MPa yield strength. After rolling, it does not need to undergo heat treatment process (quenching + tempering), which reduces comprehensive energy consumption, improves production efficiency, and significantly reduces production costs.

[0030] 3. The alloy in the present invention is mainly composed of Nb+Ti, with a small amount of Cr / B and other elements, which reduces the Mn alloy content and does not add V element, thereby reducing the overall cost.

[0031] 4. Compared with existing solutions, the rolling process of the present invention specifies the opening pass for descaling between rolling passes and controls the finishing temperature during finishing. The surface of the product of the present invention produces a smooth, light blue oxide scale at a specific temperature. This scale is firmly bonded to the steel substrate and resists shedding. The surface quality is significantly superior to that of products produced using conventional processes, and is distinctly different from the shedding of black oxide scale or iron powder found on conventional surfaces. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a surface photograph of the steel plate prepared in Example 1 of the present invention.

[0033] Figure 2 This is a microstructure photograph of the steel plate prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0034] The following are specific embodiments of the present invention to further describe the technical solution of the present invention, but the scope of protection of the present invention is not limited to these embodiments. Any changes or equivalent substitutions that do not deviate from the concept of the present invention are included in the scope of protection of the present invention.

[0035] A wide and thick steel plate with high toughness and high surface quality. The chemical composition and weight percentages of the steel plate are as follows: C: 0.08%-0.15%, Si: 0.25%-0.45%, Mn: 1.2%-1.8%, P ≤ 0.015%, S ≤ 0.0040%, Nb: 0.030%-0.060%, Ti: 0.080%-0.20%, Als: 0.020%-0.055%, Cr: 0.17%-0.37%, B: 0.0015-0.0025%, with the remainder being iron and other impurities. The physical properties of the steel plate are as follows: the steel plate has a thickness of 6.0-30mm, a yield strength ≥ 550MPa, a tensile strength ≥ 700MPa, and an elongation ≥ 20%. The metallographic structure of the steel plate is fine-grained ferrite + martensite + a small amount of bainite. The steel plate has a smooth surface and forms a stable light blue oxide layer.

[0036] The method for manufacturing wide and thick steel plates with high toughness and high surface quality comprises the following steps:

[0037] (1) Smelting: Slide plates are used for slag blocking and steel tapping, and LF+RH double refining process is adopted. The LF treatment cycle is ≥15min, and the RH soft blowing time is ≥15min.

[0038] (2) Continuous casting: The continuous casting process adopts argon protection casting throughout the entire process, the superheat is controlled at 15-30℃, and the crystallizer liquid level fluctuation is controlled to be ≤±3mm, thereby improving the purity of the molten steel and thus improving the quality of the ingot.

[0039] (3) Heating: The temperature of the slab out of the furnace is controlled at 1220-1280℃, and the holding time of the heating furnace is optimized. According to the thickness of the billet, the holding time of the heating furnace is mandatory: the holding time of the 200mm billet is required to be ≥30min, the holding time of the 250mm billet is required to be ≥35min, and the holding time of the 300mm billet is required to be ≥40min. The holding time cannot exceed 100min at most. If the specified time is exceeded, the billet is required to be returned to the furnace and taken off the line.

[0040] (4) Descaling: The high-pressure descaling pressure after the furnace must ensure that the outlet pressure is ≥25MPa, the height of the descaling nozzle and the slab must be ensured to be 130-180mm, and the inclination or deflection angle of the descaling nozzle must be ensured to be between 12°-15° to ensure high-pressure water jet descaling and ensure good descaling effect on the plate surface.

[0041] (5) Rough rolling: In the first three passes of rough rolling, the descaling water in front of the stand must be kept open to quickly remove the secondary iron oxide produced on the surface, and the surface temperature of the intermediate billet must be appropriately reduced again. In addition, the descaling water in front of the stand must be turned on in the last pass of rough rolling.

[0042] (6) Finishing rolling: Keep the descaling water in front of the stand open for the first two passes of finishing rolling, and the descaling water in front of the stand must be opened for the last two passes of finishing rolling; the final rolling temperature of finishing rolling is 780-820℃.

[0043] (7) Cooling process: After finishing rolling, rapid cooling is adopted, and the surface temperature is 180-240℃, so as to obtain a wide and thick steel plate with high surface and high toughness strength, such as yield strength ReL ≥ 550MPa, tensile strength Rm ≥ 700MPa, elongation A ≥ 20%, and comprehensive strength-ductility product ≥ 15GPa.

[0044] Example 1

[0045] Take the steel plant's 4300mm wide and heavy plate production line, 18*2800mm wide and heavy plates, as an example:

[0046] 1. The chemical composition and weight percentage of the smelted steel plate are: C: 0.12%, Si: 0.36%, Mn: 1.54%, P: 0.012%, S: 0.0020%, Nb: 0.048%, Ti: 0.13%, Als: 0.038%, Cr: 0.28%, B: 0.0019%, and the balance is iron and other impurities.

[0047] 2. Using LF+RH double refining process, LF treatment cycle: 18min, RH soft blowing time 19min.

[0048] 3. The entire continuous casting process is protected by argon, the superheat is controlled at 15-30℃, and the crystallizer liquid level fluctuation is controlled to ≤±3mm.

[0049] 4. The slab furnace temperature is controlled at 1220-1280℃, and the holding time of 250mm slab is 45min.

[0050] 5. The outlet pressure of the high-pressure descaling after the furnace is ≥25MPa, the height between the descaling nozzle and the slab is 146mm, and the inclination or deflection angle of the descaling nozzle is 14° to ensure good descaling effect on the plate surface.

[0051] 6. The descaling water in front of the stand should be kept on for the first three passes of rough rolling, and the descaling water in front of the stand should be turned on for the last pass of rough rolling.

[0052] 7. The descaling water in front of the stand must also be kept on for the first two passes of finishing rolling. At the same time, the descaling water in front of the stand must be turned on for the last two passes of finishing rolling; the final rolling temperature of finishing rolling is 810℃.

[0053] 8. The cooling process adopts a rapid cooling method, that is, ultra-fast cooling is used to cool the surface temperature to 210℃.

[0054] 9. Sample inspection yield strength ReL: 595MPa, tensile strength Rm: 878MPa, elongation A50: 22%, -20℃ longitudinal KV2 impact energy ≥110J, comprehensive strength and plasticity product ≥18GPa, high toughness and high surface quality wide and thick steel plates. Steel plate surface such as Figure 1 As shown in the figure, the surface of the steel plate is smooth and a stable light blue oxide layer is formed; the microstructure photo of the steel plate is shown in the figure. Figure 2 shown.

[0055] The present invention is not limited to the above-mentioned embodiments. Anyone should be aware that any structural changes made under the guidance of the present invention, and any technical solutions that are the same or similar to those of the present invention, fall within the scope of protection of the present invention.

[0056] The technology, shape, and structure not described in detail in the present invention are all well-known technologies.

Claims

1. A wide and thick steel plate with high toughness and high surface quality, characterized in that: The chemical composition and weight percentage of the steel plate are: C: 0.08%-0.15%, Si: 0.25%-0.45%, Mn: 1.2%-1.8%, P≤0.015%, S≤0.0040%, Nb: 0.030%-0.060%, Ti: 0.080%-0.20%, Als: 0.020%-0.055%, Cr: 0.17%~0.37%, B: 0.0015-0.0025%, and the balance is iron and other impurities.

2. The wide and thick steel plate with high toughness and high surface quality according to claim 1, characterized in that: The physical properties of the steel plate are as follows: the steel plate thickness is 6.0-30mm, the yield strength is ≥550MPa, the tensile strength is ≥700MPa, and the elongation is ≥20%; the metallographic structure of the steel plate is fine-grained ferrite + martensite + a small amount of bainite, the steel plate surface is smooth, and a stable light blue oxide layer is formed.

3. The method for manufacturing a wide and thick steel plate with high toughness and high surface quality according to claim 1 or 2, characterized in that: The following steps are involved: (1) Smelting: Slide plates are used for slag blocking and steel tapping, and LF+RH double refining process is adopted. The LF treatment cycle is ≥15min, and the RH soft blowing time is ≥15min; (2) Continuous casting: The continuous casting process adopts argon protection casting throughout the process, the superheat is controlled at 15-30℃, and the crystallizer liquid level fluctuation is controlled to be ≤±3mm; (3) Heating: The slab temperature out of the furnace is controlled at 1220-1280℃, and the holding time of the heating furnace is optimized; (4) Descaling: The high-pressure descaling pressure after the furnace must ensure that the outlet pressure is ≥25MPa, the height of the descaling nozzle and the slab must be ensured to be 130-180mm, and the inclination or deflection angle of the descaling nozzle must be ensured to be between 12°-15° to ensure high-pressure water jet descaling; (5) Rough rolling: The descaling water before the stand should be kept open for the first three passes of rough rolling, and the descaling water before the stand must be opened for the last pass of rough rolling; (6) Finishing rolling: Keep the descaling water in front of the stand open for the first two passes of finishing rolling, and the descaling water in front of the stand must be opened for the last two passes of finishing rolling; the finishing rolling temperature is 780-820℃; (7) Cooling process: After finishing rolling, rapid cooling is adopted, and the surface temperature is 180-240℃, so as to obtain a wide and thick steel plate with high surface and high toughness strength, such as yield strength ReL ≥ 550MPa, tensile strength Rm ≥ 700MPa, elongation A ≥ 20%, and comprehensive strength-ductility product ≥ 15GPa.

4. The method for manufacturing a wide and thick steel plate with high toughness and high surface quality according to claim 3, characterized in that: The method for optimizing the holding time of the heating furnace in step (3) is specifically as follows: according to the different thicknesses of the ingots, the holding time of the heating furnace is imposed with mandatory regulations: the holding time of 200mm ingots is required to be ≥30min, the holding time of 250mm ingots is required to be ≥35min, and the holding time of 300mm ingots is required to be ≥40min. The holding time cannot exceed 100min at most. If the specified time is exceeded, the ingots are required to be returned to the furnace and taken offline.

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