Low-yield-ratio hot-rolled quenched and tempered steel plate with yield strength of 1100 MPa or above and production method

Abstract

The invention relates to a low-yield-ratio hot-rolled quenched and tempered steel plate with yield strength of more than 1100MPa and a production method thereof. The steel plate comprises the following components: 0.14%-0.17% of C, 0.15%-0.25% of Si, 1.15%-1.25% of Mn, less than or equal to 0.015% of P, less than or equal to 0.003% of S, 0.001%-0.01% of Al, 0.45%-0.55% of Mo, 0.3%-0.4% of Cr, 0.3%-0.5% of Ni and the balance of Fe and impurities. The component design that Cr, Mo and Ni are added into low-carbon steel is adopted, the hot-rolled steel plate is produced through smelting, continuous casting, rolling and hardening and tempering, the yield strength of the finished steel plate is larger than or equal to 1100 MPa, the tensile strength ranges from 1120 MPa to 1300 MPa, the hardness is larger than or equal to 350 HBW, the ductility is larger than or equal to 8%, and the structure is tempered martensite and bainite; a continuous heat treatment transverse cutting process is applied to production, and the steel plate with the thickness smaller than or equal to 6 mm, the maximum width of 1500 mm, high low-temperature impact toughness, low yield ratio and high wear resistance is rapidly and efficiently produced.

Description

Technical Field

[0001] The invention relates to the technical field of hot-rolled steel plate production, and in particular to a hot-rolled quenched and tempered steel plate with a yield strength of more than 1100 MPa and a low yield ratio and a production method thereof. Background Art

[0002] As the machinery industry continues to develop in the direction of high strengthening and lightweight, high-strength steel and ultra-high-strength steel have been used more and more widely, mainly to meet the higher requirements of mechanical properties for steel used in engineering machinery. Ultra-high-strength steel with a yield strength of 1100MPa is mainly used in the construction machinery industry for the production of key structural parts such as telescopic booms, crawler crane bases and pull plates, and large-tonnage hydraulic supports in heavy equipment such as concrete pump trucks and truck cranes. This grade of steel has excellent formability, good weldability, and good low-temperature impact toughness.

[0003] In recent years, the market demand for ultra-high strength, thin-gauge (5-10mm thick) steel plates has increased. Although the "quenching + tempering" heat treatment process can make the yield strength reach 1100MPa level, most of them require a high-temperature tempering process.

[0004] The Chinese patent application with application number CN202310567689.5 discloses "a low yield ratio 1100MPa grade high strength Q&P steel and its production method". The chemical composition of the steel is as follows by mass percentage: C 0.15-0.17%, Si 0.4-0.6%, Mn 1.4-2.6%, Cr 0.4-0.8%, Ni 0.7-1.6%, Mo 0.2-0.6%, Nb 0.02-0.08%, V0.02-0.08%, Ti 0.01-0.03%, Al 0.01-0.05%, Cu 0.6-1.5%, and the balance is Fe and unavoidable impurities. The low yield ratio 1100MPa grade high strength steel with a thickness of 15mm was prepared by controlled rolling and controlled cooling and offline salt bath Q&P heat treatment process. The heat treatment adopts the salt bath two-step Q&P process, with a complete austenitization temperature of 900-920°C, a salt bath quenching temperature of 200-250°C, a salt bath partitioning temperature of 350-450°C, and finally water cooling to room temperature. The yield strength of the low yield ratio 1100MPa grade high-strength steel plate is ≥1100MPa, the yield ratio is ≤0.88, the elongation is ≥14%, and the impact energy at -40°C is ≥30J.

[0005] The Chinese patent application with application number CN202210653524.5 discloses "a 1100MPa grade cold-rolled martensitic steel and its manufacturing method". The chemical composition of the steel is C: 0.13-0.17%, Si: 1.05-1.15%, Mn: 1.70-1.80%, P: ≤0.020%, S: ≤0.010%, Cr: 0.25-0.35%, Alt: 0.030-0.050%, Ti: 0.020-0.030%, Ce: 10-20ppm, and the rest is Fe and unavoidable impurities. The manufacturing method includes a continuous annealing process, in which the heating and soaking temperatures are both 790-800°C.

[0006] The Chinese patent application with application number CN202010665479.6 discloses "a method for producing ultra-high-strength steel plates with large thickness, high toughness and yield strength of 1100MPa". The process flow is: converter or electric furnace steelmaking → LF refining → VD or RH vacuum degassing → Ca treatment → continuous casting → slow cooling and diffusion treatment of ingots → heating → rolling → slow cooling and diffusion treatment of steel plates → quenching → tempering. The slow cooling and diffusion treatment is carried out using the residual temperature of the ingots and steel plates off the line. The starting temperature of the slow cooling and diffusion of the ingots is controlled at 600-700℃, the diffusion time is ≥48 hours, and the diffusion termination temperature is ≤300℃; the starting temperature of the steel plate diffusion is ≥300℃, and the diffusion time is ≥24 hours.

[0007] The Chinese patent application with application number CN202210846315.2 discloses "a low yield ratio and high low temperature toughness steel plate and its production method". In the steel plate: C 0.06-0.09%, Si 0.15-0.25%, Mn 1.10-1.20%, Ni 0.15-0.30%, Nb 0.015-0.025%, Ti 0.005-0.015%, Alt 0.045-0.070%, N≤0.005%, O≤0.002%, P≤0.010%, S≤0.003%, and the balance is iron and impurities; in the production method, the final rolling temperature of the rough rolling stage in the recrystallization zone is >{1030-360[Si]}℃, and the final rolling temperature of the finishing rolling stage in the non-recrystallization zone is 805-835℃; the cooling rate in the controlled cooling process is. The obtained steel plate has a low yield strength ratio, good low-temperature toughness and welding performance, and can meet the application requirements of ships and liquefied petroleum gas storage and transportation.

[0008] The Chinese patent application with application number CN202211610421.7 discloses "a low yield ratio ultra-low temperature toughness extra-thick marine engineering steel plate and its manufacturing method". The composition of the steel plate is as follows by weight percentage: C: 0.04% to 0.07%, Si: 0.05% to 0.15%, Mn: 0.3% to 0.5%, P≤0.02%, S≤0.01%, Als: 0.01% to 0.03%, Ni: 1.5% to 3.1%, Cr: 0.02% to 0.15%, Nb: 0.045% to 0.06%, and the rest is Fe and unavoidable impurities. The manufacturing method includes smelting, casting, heating, and rolling; the steel plate produced has a yield strength of ≥550MPa, a tensile strength of 640 to 820MPa, an elongation of ≥18%, a Charpy impact energy of -80℃ ≥80J, and a yield ratio of ≤0.84.

[0009] Compared with the above-mentioned technical solutions disclosed above, the present invention abandons the design idea of ​​using microalloys such as Nb and Ti to strengthen steel, and instead adopts a composition design of adding traditional three alloy elements of Cr, Mo and Ni to low-carbon steel, while using a tempering heat treatment process to improve the strength; from the actual production situation, smelting and rolling are easier, and it is possible to produce steel plates with a thickness specification of ≤6mm (significantly lower than the existing product specifications). Summary of the invention

[0010] The invention provides a low yield ratio hot-rolled quenched and tempered steel plate with a yield strength of more than 1100MPa and a production method thereof. The invention adopts a composition design in which Cr, Mo and Ni are added to low carbon steel, and the hot-rolled steel plate is produced through smelting, continuous casting, rolling and quenching and tempering. The yield strength of the finished steel plate is ≥1100MPa, the tensile strength is 1120-1300MPa, the hardness is ≥350HBW, the elongation is ≥8%, and the structure is tempered martensite+bainite. The continuous heat treatment cross-cutting process is used for production, so that the steel plate with a thickness of ≤6mm and a maximum width of 1500mm, high low-temperature impact toughness, low yield ratio and high wear resistance can be produced quickly and efficiently, and the manufacturing needs of special-purpose equipment such as engineering machinery, mining machinery, polar machinery, etc. can be met.

[0011] In order to achieve the above object, the present invention adopts the following technical solutions:

[0012] A low yield ratio hot-rolled quenched and tempered steel plate with a yield strength of more than 1100 MPa. The chemical composition of the steel plate is C: 0.14%-0.17%, Si: 0.15%-0.25%, Mn: 1.15%-1.25%, P≤0.015%, S≤0.003%, Al: 0.001%-0.01%, Mo: 0.45%-0.55%, Cr: 0.3%-0.4%, Ni: 0.3%-0.5%, and the rest is Fe and unavoidable impurities.

[0013] The thickness of the finished steel plate is ≤6mm.

[0014] The microstructure of the finished steel plate is tempered martensite + bainite, and in terms of volume percentage, the tempered martensite accounts for 88% to 92%, and the bainite accounts for 8% to 12%.

[0015] The yield strength of the finished steel plate is ≥1100MPa, the tensile strength is 1120-1300MPa, the elongation is ≥8%, the hardness is ≥350HBW, and the impact energy value at -40℃ is >35J.

[0016] A method for producing a hot-rolled quenched and tempered steel plate with a yield strength of more than 1100 MPa and a low yield strength ratio, comprising smelting, casting, heating, rolling, cooling, quenching and tempering, flattening, straightening and cross-cutting processes, wherein the following processes are controlled:

[0017] 1) Heating: cold charging furnace, heating temperature is 1200-1220℃, total holding time is 180-190min, of which the holding time of soaking section is 50-70min;

[0018] 2) Rolling: Rough rolling adopts multi-pass rolling process; the rough rolling final rolling temperature is 1055-1070℃; the finishing rolling inlet temperature is 930-950℃, the finishing rolling adopts multi-stand continuous rolling, large reduction rolling, the thickness ratio of the intermediate billet to the finished steel plate is ≥7, and the finishing rolling final rolling temperature is 880-900℃;

[0019] 3) Cooling: Use laminar rapid cooling process, cool to 680-700℃ at a cooling rate of 15-30℃ / s for coiling, and stack and slowly cool after coiling;

[0020] 4) Quenching and tempering: The cooled steel coil is subjected to continuous quenching and tempering heat treatment by a continuous heat treatment unit, the heating temperature of the heating section of the continuous heat treatment unit is 910-920°C, and the holding time is 30-40min; then quenching is carried out, and the quenching medium is clean circulating water with a temperature of ≤25°C; after quenching, it enters the tempering section of the continuous heat treatment unit, the tempering temperature is 550-575°C, and the holding time is 8-10min, and a thin steel strip with a thickness of ≤6mm is obtained;

[0021] 5) Leveling, straightening and cross-cutting: The thin steel strip is leveled, straightened, surface polished and cross-cut in sequence to obtain hot-rolled thin steel plates.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1) The composition design adopts a small amount of alloy elements (Cr, Mo, Ni) added to low carbon steel, and after smelting and rolling, the strength is improved by quenching and tempering heat treatment process, the yield strength is ≥1100MPa, the tensile strength is 1120-1300MPa, the elongation is ≥8%, the hardness is ≥350HBW, and the impact energy value at -40℃ is >35J; the structure is tempered martensite + bainite;

[0024] 2) After rolling, continuous heat treatment and cross-cutting process is applied to quickly and efficiently produce finished steel plates with a thickness of ≤6mm and a maximum width of 1500mm, which have high low-temperature impact toughness, low yield strength ratio and high wear resistance, meeting the manufacturing needs of special-purpose equipment such as engineering machinery, mining machinery, and polar machinery;

[0025] 3) From the perspective of steelmaking, the present invention has a lower smelting difficulty. From the perspective of rolling technology, the present invention is more conducive to rolling, and the thickness of the finished steel plate after rolling is ≤6mm, which is significantly lower than the conventional product specifications of the same strength level;

[0026] 4) Compared with traditional thick steel plates of the same grade, the present invention can produce coils with thinner specifications, larger width range and better plate shape, achieving the dual goals of reducing costs and weight, and can be cut into thin steel plates of different lengths according to subsequent usage. DETAILED DESCRIPTION

[0027] The invention discloses a low yield ratio hot-rolled quenched and tempered steel plate with a yield strength of more than 1100 MPa. The chemical composition of the steel plate is C: 0.14%-0.17%, Si: 0.15%-0.25%, Mn: 1.15%-1.25%, P≤0.015%, S≤0.003%, Al: 0.001%-0.01%, Mo: 0.45%-0.55%, Cr: 0.3%-0.4%, Ni: 0.3%-0.5%, and the rest is Fe and unavoidable impurities.

[0028] The chemical composition design principles of the low temperature and high toughness hot-rolled thin steel plate of the present invention are as follows:

[0029] C (0.14% to 0.17%): As an important element of carbon steel, its content can play a decisive role in material properties. The hot-rolled quenched and tempered steel plate of the present invention adopts an offline quenching and tempering process after rolling. Due to the presence of carbon, the steel can be heat treated to adjust and change its mechanical properties. The carbon content directly affects the toughness of the material. The brittleness and hardness of medium and high carbon steels are greater, and the toughness of low carbon steels is better. The present invention adopts a low-carbon design to enhance the toughness of the material.

[0030] Si (0.15% to 0.25%): Adding silicon to carbon steel can significantly improve the solid solubility of steel. In the steelmaking process, silicon is also used as a reducing agent and deoxidizer, and silicon can improve the yield strength of steel. However, too high a silicon content will increase the oxide adhesion of steel during the production process, thereby reducing the surface quality of the product. Therefore, the present invention adopts a low silicon content design.

[0031] Mn (1.15% to 1.25%): Since the hot-rolled quenched and tempered steel plate described in the present invention is mainly used in construction machinery, mining machinery and other scenes with harsh working conditions, it is necessary to add manganese to the steel to make it have higher hardness, rigidity and wear resistance. Manganese can be infinitely dissolved with iron, which has little effect on plasticity while improving the strength of steel. The hot-rolled quenched and tempered steel plate described in the present invention adopts a post-rolling offline quenching and tempering process, so it is also necessary to add an appropriate amount of manganese to improve the hardenability of the steel and improve the hot working performance of the steel.

[0032] P (≤0.015%) and S (≤0.003%): Sulfur combines with manganese and other elements in steel to form plastic inclusions such as manganese sulfide, which is particularly detrimental to the transverse plasticity and toughness of steel. Therefore, the sulfur content should be as low as possible. Phosphorus is also a harmful element in steel, which seriously damages the plasticity and toughness of the steel plate. For the present invention, sulfur and phosphorus are both unavoidable impurity elements, and the lower the content, the better.

[0033] Al (0.001% to 0.01%): Aluminum is mainly used as a deoxidizer and grain refinement element in steel. As a deoxidizer, it can react with oxygen and nitrogen elements in steel, thereby effectively removing impurities in the steel and improving the purity and quality of the steel. The hot-rolled quenched and tempered steel plate described in the present invention belongs to the category of high-strength steel. Improving strength requires grain refinement, while the ability of grain refinement by simply applying quenching and tempering heat treatment is limited. Therefore, it is necessary to add appropriate aluminum elements to promote grain refinement and improve the strength and toughness of the steel, especially the toughness at low temperatures.

[0034] Mo (0.45% to 0.55%): Molybdenum as an alloying element can improve the hardenability of steel and reduce temper brittleness. In quenched and tempered steel, molybdenum can make the steel plate hardenable, improve the tempering resistance or tempering stability of the steel, and allow the plate to be tempered at a higher temperature, thereby more effectively eliminating or reducing residual stress and improving plasticity. From the perspective of organization, molybdenum can be dissolved in ferrite, austenite and carbide in steel. It is an element that reduces the austenite phase region. In addition, molybdenum plays a beneficial role in improving the ductility, toughness and wear resistance of steel. Therefore, the purpose of appropriately adding Mo to the hot-rolled quenched and tempered steel plate of the present invention is to improve the elongation and wear resistance of the plate.

[0035] Cr (0.3% to 0.4%): Chromium and iron can form a continuous solid solution, reducing the austenite phase area. Proper addition of chromium to steel can improve the hardenability of steel and have a secondary hardening effect, thereby increasing the hardness of the steel and achieving the purpose of improving wear resistance. This is because chromium makes the quenched and tempered steel have better comprehensive mechanical properties after quenching and tempering, forming chromium-containing carbides, thereby improving the wear resistance of the material. Therefore, the purpose of properly adding chromium to the hot-rolled quenched and tempered steel plate of the present invention is to improve the wear resistance of the plate.

[0036] Ni (0.3% to 0.5%): Adding a small amount of nickel to quenched and tempered steel can mainly improve the strength of the steel and maintain good plasticity and toughness. At the same time, it can reduce the quenching temperature during heat treatment and reduce quenching deformation. The presence of nickel in steel can effectively prevent the steel from overheating during the heating process, prevent the growth of grains at high temperatures, and maintain fine-grained structure at high temperatures.

[0037] The method for manufacturing a low yield ratio hot-rolled quenched and tempered steel plate with a yield strength of more than 1100 MPa according to the present invention comprises the following steps:

[0038] 1) Smelting and casting;

[0039] Smelting and casting are performed according to the designed chemical composition to obtain slabs;

[0040] 2) Slab reheating;

[0041] Cold charging furnace. The heating temperature is 1200-1220℃, the total holding time is 180-190min, of which the holding time in the soaking section is 50-70min.

[0042] 3) Slab rolling;

[0043] Rolling is divided into two stages: rough rolling and finishing rolling. In the rough rolling stage, a multi-pass rolling process (preferably 3+3 rolling process) is used to ensure stable production, and the rough rolling final rolling temperature is 1055-1070°C. The finishing rolling inlet temperature is 930-950°C, and the finishing rolling adopts multi-stand continuous rolling (preferably 7-stand continuous rolling), large reduction rolling, the thickness ratio of the intermediate billet to the finished steel plate is ≥7, the finishing rolling final rolling temperature is controlled at 880-900°C, and the finishing rolling adopts speed increase rolling.

[0044] 4) Steel plate cooling;

[0045] The laminar cooling process is applied, and the steel is cooled to 680-700°C at a cooling rate of 15-30°C / s for coiling. After coiling, the steel is stacked and slowly cooled.

[0046] 5) Quenching and tempering of steel plates;

[0047] The cooled steel coil is subjected to continuous quenching and tempering heat treatment by a continuous heat treatment unit. It is heated to 910-920°C in the heating section of the continuous heat treatment unit and kept warm for 30-40 minutes. It is then quenched. The quenching medium is clean circulating water with a temperature of ≤25°C. Finally, it enters the tempering section of the continuous heat treatment unit, is tempered at a temperature of 550-575°C, and kept warm for 10 minutes to obtain a thin steel strip with a thickness of ≤6mm.

[0048] 6) Leveling, straightening and cross-cutting;

[0049] The thin steel strips coming out of the continuous heat treatment unit are subjected to subsequent processing, such as flattening, straightening, surface grinding, cross-cutting and other processes according to the order requirements to obtain hot-rolled thin steel plates.

[0050] The thickness of the finished steel plate is ≤6mm.

[0051] The microstructure of the finished steel plate is tempered martensite + bainite, and in terms of volume percentage, the tempered martensite accounts for 88% to 92%, and the bainite accounts for 8% to 12%.

[0052] The yield strength of the finished steel plate is ≥1100MPa, the tensile strength is 1120-1300MPa, the elongation is ≥8%, the hardness is ≥350HBW, and the impact energy value at -40℃ is >35J.

[0053] In order to more intuitively embody the present invention, the embodiments of the present invention are further described in conjunction with examples. The following examples are only preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any technical solution that can be obviously obtained by a person skilled in the art within the technical scope disclosed in the present invention, including simple changes or equivalent replacements, is within the protection scope of the present invention.

[0054] [Example]

[0055] The chemical composition and metallographic structure of the steel in each embodiment are shown in Table 1, the rolling and tempering process parameters of the steel in each embodiment are shown in Table 2, and the main mechanical property parameters of the finished steel plate in each embodiment are shown in Table 3.

[0056] Table 1 Chemical composition (wt%) and metallographic structure (V%) of steel

[0057]

[0058] Note: CeV=C+Mn / 6+(Cr+Mo+V) / 5+Cu / 15

[0059] Table 2 Steel rolling and tempering process parameters

[0060]

[0061] Table 3 Main mechanical properties parameters of finished steel plates

[0062]

[0063] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A hot-rolled quenched and tempered steel plate with a yield strength of 1100 MPa or more and a low yield ratio, characterized in that: The chemical composition of the steel plate is calculated by weight as follows: C: 0.14% ~ 0.17%, Si: 0.15% ~ 0.25%, Mn: 1.15% ~ 1.25%, P ≤ 0.015%, S ≤ 0.003%, Al: 0.001% ~ 0.01%, Mo: 0.45% ~ 0.55%, Cr: 0.3% ~ 0.4%, Ni: 0.3% ~ 0.5%, and the rest is Fe and unavoidable impurities.

2. The low yield ratio hot-rolled quenched and tempered steel plate with a yield strength of more than 1100 MPa according to claim 1, characterized in that: The thickness of the finished steel plate is ≤6mm.

3. The low yield ratio hot-rolled quenched and tempered steel plate with a yield strength of more than 1100 MPa according to claim 1, characterized in that: The microstructure of the finished steel plate is tempered martensite + bainite, and in terms of volume percentage, the tempered martensite accounts for 88% to 92%, and the bainite accounts for 8% to 12%.

4. The low yield ratio hot-rolled quenched and tempered steel plate with a yield strength of more than 1100 MPa according to claim 1, characterized in that: The yield strength of the finished steel plate is ≥1100MPa, the tensile strength is 1120-1300MPa, the elongation is ≥8%, the hardness is ≥350HBW, and the impact energy value at -40℃ is >35J.

5. A method for producing a low yield ratio hot-rolled quenched and tempered steel plate with a yield strength of more than 1100 MPa as claimed in any one of claims 1 to 4, characterized in that: It includes smelting, casting, heating, rolling, cooling, quenching and tempering, leveling, straightening and cross-cutting processes, among which the following processes are controlled: 1) Heating: cold charging furnace, heating temperature is 1200-1220℃, total holding time is 180-190min, of which the holding time of soaking section is 50-70min; 2) Rolling: Rough rolling adopts multi-pass rolling process, the rough rolling final rolling temperature is 1055-1070℃; the finishing rolling inlet temperature is 930-950℃, the finishing rolling adopts multi-stand continuous rolling, large reduction rolling, the thickness ratio of the intermediate billet to the finished steel plate is ≥7, and the finishing rolling final rolling temperature is 880-900℃; 3) Cooling: Use laminar rapid cooling process, cool to 680-700℃ at a cooling rate of 15-30℃ / s for coiling, and stack and slowly cool after coiling; 4) Quenching and tempering: The cooled steel coil is subjected to continuous quenching and tempering heat treatment by a continuous heat treatment unit, the heating temperature of the heating section of the continuous heat treatment unit is 910-920°C, and the holding time is 30-40min; then quenching is carried out, and the quenching medium is clean circulating water with a temperature of ≤25°C; after quenching, it enters the tempering section of the continuous heat treatment unit, the tempering temperature is 550-575°C, and the holding time is 8-10min, and a thin steel strip with a thickness of ≤6mm is obtained; 5) Leveling, straightening and cross-cutting: The thin steel strip is leveled, straightened, surface polished and cross-cut in sequence to obtain hot-rolled thin steel plates.

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