Low-temperature high-toughness hot continuous rolling thin steel plate with yield strength of 960 MPa or above and production method

Abstract

The invention relates to a low-temperature high-toughness hot continuous rolling thin steel plate with yield strength of more than 960MPa and a production method thereof. The steel plate comprises the following chemical components: 0.12%-0.15% of C, 0.20%-0.40% of Si, 1.50%-1.60% of Mn, less than or equal to 0.015% of P, less than or equal to 0.003% of S, 0.015%-0.045% of Al, 0.7%-0.9% of Mo + Cr, 0.3%-0.5% of Cu, 0.02%-0.04% of Nb, 0.2%-0.4% of Ni and the balance of Fe and impurities. The component design that a small number of alloy elements are added into low-carbon steel is adopted, the strength is improved through the quenched-tempered heat treatment process after smelting rolling, the smelting difficulty is low, rolling is more facilitated, and the thickness specification of a hot-rolled product is smaller than or equal to 4 mm; meanwhile, a continuous heat treatment transverse cutting process is applied, so that rapid and efficient production is realized; the finished steel plate has high low-temperature impact toughness and high wear resistance, and can meet the manufacturing requirements of engineering machinery, mining machinery, polar machinery and other equipment with different purposes.

Description

Technical Field

[0001] The present invention relates to the technical field of hot-rolled steel plate production, and in particular to a low-temperature high-toughness hot-rolled thin steel plate with a yield strength of more than 960 MPa and a production method thereof. Background Art

[0002] As the equipment manufacturing industry develops towards high performance, large-scale and reduced volume, the market demand for high-strength and ultra-high-strength steel for engineering machinery is increasing, and the performance requirements are increasing. In order to achieve the purpose of weight reduction, it has gradually become a trend to use thin-gauge high-strength steel to replace low-strength thick-gauge low-alloy steel plates. At present, most of the steel for engineering machinery with a strength specification of more than 960MPa has a thickness of more than 15mm. Although a new high-strength steel plate with a thickness of 12mm has been developed, the effect of weight reduction is not obvious and cannot be applied to equipment such as muck trucks and mining machinery. Therefore, there is an urgent need for a thin-gauge steel plate with a higher strength level and good bending performance and high and low temperature toughness.

[0003] The Chinese patent application with publication number CN 110423946A discloses "a method for producing ultra-thick plates of ultra-high-strength steel Q960E with low compression ratio". The chemical composition of the steel is C=0.15~0.20, Si=0.15~0.35, Mn=1.0~1.50, P≤0.012, S≤0.003, Nb=0.020~0.035, V=0.030~0.060, Ti=0.010~0.025, Als=0.020~0.045, Cr=0.50~0.80, Mo=0.40~0.60, Ni=1.00~1.40, B=0.0008~0.0025, CEV≤0.68, and the balance is Fe and other trace elements. The process steps include hot metal pretreatment → converter smelting → LF refining → VD vacuum treatment → continuous casting → heating → rolling → straightening → quenching → tempering → finishing → performance inspection → flaw detection. It breaks through the limitation of the compression ratio of traditional slab production of ultra-high strength steel, and achieves a rolling compression ratio of 2.2 to 3.0 to produce thick ultra-high strength steel. The performance difference in the thickness direction of the steel plate is small, and the mechanical properties are poor: yield strength ≤30MPa, tensile strength ≤25MPa, elongation ≤2%, -40℃ longitudinal impact energy ≤20J, transverse impact energy ≤15J. The Chinese patent application with publication number CN 109023114A discloses "an ultra-high strength steel Q960E thick plate", whose chemical composition and mass percentage are as follows: C: 0.15% to 0.18%, Si: 0.20% to 0.50%, Mn: 0.80% to 1.30%, P≤0.010%, S≤0.003%, Cr: 0.30% to 0.50%, Mo: 0.40% to 0.60%, Ni: 0.80% to 1.0%, Ti: 0.008% to 0.030%, Nb: 0.015% to 0.050%, B: 0.0008% to 0.0025%, and the rest is Fe and unavoidable impurities. It adopts medium-high carbon and alloying composition design, and auxiliary control rolling and offline heat treatment methods to provide a 90mm ultra-high strength steel thick plate. Although both have achieved the production of ultra-high-strength steel plates, neither can achieve the weight reduction goal, and there is no clear explanation of the tempering process.

[0004] The Chinese invention patent with announcement number CN 116377324 B discloses "a 960MPa grade ultra-high strength and high toughness seamless steel pipe for crane boom and its manufacturing method", and its chemical composition by weight percentage is: C: 0.13% to 0.18%, Si: 0.30% to 0.50%, Mn: 0.8% to 1.00%, P≤0.020%, S≤0.010%, Cr: 0.20% to 0.80%, Mo: 0.30% to 0.60%, Ni: 0.50% to 1.40%, V: 0.030% to 0.070%, Al: 0.015% to 0.050%, Nb: 0.020% to 0.060%, Ti: 0.010% to 0.030%, Cu: 0.30% to 0.50%, N<0.0060%, and the balance is Fe and unavoidable impurities. It adopts normalizing + quenching + high temperature tempering heat treatment process. The microstructure of the steel pipe is tempered bainite + ≤10% tempered bainite, and the austenite grain size is ≥10, with high strength and low temperature toughness. However, the normalizing + quenching and tempering process adopted in its production process cannot be achieved for thin hot-rolled strip steel.

[0005] The Chinese invention patent with the announcement number CN 114941068 B discloses "a method for preparing a rare earth microalloyed high-toughness 960MPa grade ultra-high strength steel", which uses wide and thick plate continuous casting billets as hot rolling raw materials, and conducts heating, controlled rolling and controlled cooling, and heat treatment (quenching + high temperature tempering), and finally obtains a steel plate with good low-temperature impact toughness; through impact test detection, the steel plate has a low-temperature impact of -60℃ ≥60J. However, rare earth elements such as RE (Ce) are added to the steel for strengthening, which can obtain good low-temperature impact toughness, but increases the manufacturing cost. Summary of the invention

[0006] The present invention provides a low-temperature high-toughness hot-rolled thin steel plate with a yield strength of more than 960 MPa and a production method. The composition design adopts a small amount of alloy elements added to low-carbon steel, and after smelting and rolling, a tempering heat treatment process is used to improve the strength. Not only is the smelting difficulty lower, but it is also more conducive to rolling. The thickness specification of the hot-rolled product is ≤4mm. At the same time, a continuous heat treatment cross-cutting process is applied to achieve fast and efficient production. The finished steel plate has high low-temperature impact toughness and high wear resistance, which can meet the manufacturing needs of equipment with different uses such as engineering machinery, mining machinery, and polar machinery.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A low-temperature high-toughness hot-rolled thin steel plate with a yield strength of more than 960 MPa. The chemical composition of the steel plate is as follows by weight: C: 0.12%-0.15%, Si: 0.20%-0.40%, Mn: 1.50%-1.60%, P≤0.015%, S≤0.003%, Al: 0.015%-0.045%, Mo+Cr: 0.7%-0.9%, Cu: 0.3%-0.5%, Nb: 0.02%-0.04%, Ni: 0.2%-0.4%, and the rest is Fe and unavoidable impurities.

[0009] The thickness of the finished steel plate is ≤4mm.

[0010] The microstructure of the finished steel plate is tempered martensite + bainite, and in terms of volume percentage, the tempered martensite accounts for 83% to 88%, and the bainite accounts for 12% to 17%.

[0011] The yield strength of the finished steel plate is ≥960MPa, the tensile strength is 980-1050MPa, the elongation is ≥10%, the hardness is 300±10HBW, and the impact energy at -40℃ is >30J.

[0012] A method for producing a low-temperature high-toughness hot-rolled thin steel plate with a yield strength of more than 960 MPa, comprising smelting, casting, heating, rolling, cooling, tempering, leveling, straightening and cross-cutting processes; specifically controlling the following processes:

[0013] 1) Heating: cold or hot charging, heating temperature is 1245-1255℃, total heat preservation time for cold charging is 150-170min, total heat preservation time for hot charging is 140-160min; heat preservation time in the soaking section is 50-70min;

[0014] 2) Rolling: The rough rolling adopts a multi-pass rolling process, and the rough rolling final rolling temperature is 1120-1150°C; the insulation cover of the intermediate roller between the rough rolling and the finishing rolling is put into use; the finishing rolling inlet temperature is 1000-1100°C, and the finishing rolling adopts multi-stand continuous rolling, large reduction rolling, the thickness ratio of the intermediate billet to the finished steel plate is ≥5, and the finishing rolling final rolling temperature is 850-950°C; in the finishing rolling stage, the PC angle of the F2 frame, the F3 frame and the F4 frame is adjusted, and the bending roll force of the F5 frame to the F7 frame is adjusted to control the crown value C40 to 90-98μm;

[0015] 3) Cooling: The laminar rapid cooling process is adopted. The strip is cooled immediately after leaving the finishing mill, and is cooled to 700-750°C at a cooling rate of 20-50°C / s for coiling. After coiling, the strip is stacked and slowly cooled to below 100°C.

[0016] 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 900-910°C, and the holding time is 20-25min; 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 550-600°C, and the holding time is 1-7min, and a thin steel strip with a thickness of ≤4mm is obtained;

[0017] 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.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] 1) The composition design adopts a small amount of alloy elements (Cr, Mo, Ni) added to low carbon steel, and the strength is improved by quenching and tempering heat treatment process after smelting and rolling. The yield strength of the finished steel plate is ≥960MPa, the tensile strength is 980~1050MPa, the hardness is 300±10HBW, the elongation is ≥10%, and the structure is lamellar tempered martensite + bainite.

[0020] 2) In the finishing rolling stage, the PC+bending roll coupling effect is used to control the strip shape. After rolling, the continuous heat treatment cross-cutting process is applied to quickly and efficiently produce finished products with a thickness of ≤4mm and a maximum width range of 1500mm. The steel plates have high low-temperature impact toughness and high wear resistance, meeting the needs of manufacturing equipment for different purposes such as engineering machinery, mining machinery, and polar machinery;

[0021] 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 ≤4 mm, which is significantly lower than the conventional product specifications of the same strength level;

[0022] 4) Compared with the traditional thick Q960 series steel plates, 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

[0023] The invention discloses a low-temperature high-toughness hot-rolled thin steel plate with a yield strength of more than 960 MPa, wherein the chemical composition is, by weight percentage, C: 0.12%-0.15%, Si: 0.20%-0.40%, Mn: 1.50%-1.60%, P≤0.015%, S≤0.003%, Al: 0.015%-0.045%, Mo+Cr: 0.7%-0.9%, Cu: 0.3%-0.5%, Nb: 0.02%-0.04%, Ni: 0.2%-0.4%, and the rest is Fe and unavoidable impurities.

[0024] The chemical composition design principles of the low temperature and high toughness hot-rolled thin steel plate of the present invention are as follows:

[0025] C (0.12% to 0.15%): As an important element of carbon steel, its content can play a decisive role in material properties. The hot-rolled thin steel plate of the present invention adopts an offline 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 relatively large, while the toughness of low carbon steels is relatively good. The present invention adopts a low-carbon design to enhance the toughness of the material.

[0026] Si (0.20% to 0.40%): 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.

[0027] Mn (1.50% to 1.60%): Since the hot-rolled thin 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 infinitely solid-dissolve with iron, which has little effect on plasticity while improving the strength of steel. The hot-rolled thin steel plate described in the present invention adopts a post-rolling offline 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.

[0028] 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.

[0029] Al (0.015% to 0.045%): 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 thin 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.

[0030] Cr+Mo (0.7%~0.9%): Appropriate addition of chromium to steel can improve the hardenability of steel and have a secondary hardening effect, thereby increasing the hardness of steel products to achieve the purpose of improving wear resistance. This is because chromium can make 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. Molybdenum also has the ability to improve the hardenability of steel and can reduce tempering brittleness. In quenched and tempered steel, molybdenum can make the steel plate hardenable, improve the tempering resistance or tempering stability of steel, and enable the steel plate to be tempered at a higher temperature, thereby more effectively eliminating or reducing residual stress and improving plasticity. In addition, molybdenum plays a beneficial role in improving the ductility, toughness and wear resistance of steel. Therefore, adding an appropriate amount of Cr and Mo to the hot-rolled thin steel plate described in the present invention can significantly improve the elongation, wear resistance and other indicators of the plate.

[0031] Cu (0.3% to 0.5%): Adding an appropriate amount of copper to steel can significantly improve the strength and hardness of the steel; copper can react with carbon to form a compound in the steel, forming a solid solution or a precipitate phase, thereby improving the strength and hardness of the steel. In addition, copper can hinder the growth of grains in the steel, promote grain refinement, and further improve strength and hardness. At the same time, adding a small amount of copper to steel can also improve the heat treatment performance of the steel.

[0032] Nb (0.02%~0.04%): Adding niobium to steel can combine with carbon and nitrogen in the steel to form carbides, nitrides or carbonitrides. These compounds can be fully dissolved in the steel at high temperatures, and can precipitate at low temperatures to inhibit grain growth and precipitation strengthening, thereby significantly reducing the carbon equivalent and improving the strength and toughness of the steel.

[0033] Ni (0.2% to 0.4%): In order to ensure the toughness of the steel, the present invention adds an appropriate amount of nickel element to the steel. Nickel can make the crystal structure of the steel more compact and improve the stability of the steel during heat treatment. In addition, nickel does not dissolve in carbides, so it can enter the austenite at high temperatures, thereby improving the hardenability of the steel.

[0034] The method for manufacturing a low-temperature high-toughness hot-rolled thin steel plate (thickness ≤ 4 mm) with a yield strength of 960 MPa or more described in the present invention comprises the following steps:

[0035] 1) Smelting and casting;

[0036] Smelting and casting are performed according to the designed chemical composition to obtain slabs;

[0037] 2) Slab reheating;

[0038] The present invention has no clear requirements for the charging temperature, and can be hot or cold. The heating temperature is 1245-1255°C, the insulation time is 150-170 minutes when cold charging is adopted, and the insulation time is 140-160 minutes when hot charging is adopted; the insulation time of the soaking section is 50-70 minutes. In the steel burning model, "cold charging" is set to 0, and "hot charging" is set to 1.

[0039] 3) Slab rolling;

[0040] 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 adopted to ensure stable production, and the rough rolling final rolling temperature is 1120-1150℃. The intermediate roller insulation cover between rough rolling and finishing rolling is put into use. The finishing rolling inlet temperature is 1000-1100℃, 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 ≥5, and the finishing rolling final rolling temperature is controlled at 850-950℃. The finishing rolling adopts the method of increasing speed rolling. Since the thickness of the finished strip steel is ≤4mm, in order to ensure the plate shape after rolling and avoid wave-shaped defects, the bending roll force configuration of each frame is adopted in a ladder-like increasing or decreasing manner during the finishing rolling stage. For the convexity problem, the PC angle of the F2 frame and the F3 frame is adjusted mainly, and the PC angle of the F4 frame is adjusted as a supplement, and the bending roll force of the F5 frame to the F7 frame is used to achieve convexity control, and finally the convexity value C40 is controlled at 90-98μm.

[0041] 4) Steel plate cooling;

[0042] The cooling adopts laminar rapid cooling process. The strip is cooled immediately after leaving the finishing mill, and is cooled to 700-750℃ at a cooling rate of 20-50℃ / s for coiling. After coiling, it is stacked and slowly cooled to below 100℃.

[0043] 5) Quenching and tempering of steel plates;

[0044] The cooled steel coil is subjected to continuous quenching and tempering heat treatment by a continuous heat treatment unit. It is heated to 900-910°C in the heating section of the continuous heat treatment unit and kept warm for 20-25 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 and is tempered at a temperature of 550-600°C and kept warm for 1-7 minutes to obtain a thin steel strip with a thickness of ≤4mm.

[0045] 6) Leveling, straightening and cross-cutting;

[0046] 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.

[0047] The thickness of the finished steel plate is ≤4mm.

[0048] The microstructure of the finished steel plate is tempered martensite + bainite, and in terms of volume percentage, the tempered martensite accounts for 83% to 88%, and the bainite accounts for 12% to 17%.

[0049] The yield strength of the finished steel plate is ≥960MPa, the tensile strength is 980-1050MPa, the elongation is ≥10%, the hardness is 300±10HBW, and the impact energy at -40℃ is >30J.

[0050] 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.

[0051] [Example]

[0052] 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.

[0053] Table 1 Chemical composition (wt%) and metallographic structure (V%) of steel

[0054]

[0055] Note: CeV=C+Mn / 6+(Cr+Mo+V) / 5+Cu / 15

[0056] Table 2 Steel rolling and tempering process parameters

[0057]

[0058] Note: In the furnace loading mark, 0 means "cold loading" and 1 means "hot loading".

[0059] Table 3 Main mechanical properties parameters of finished steel plates

[0060]

[0061] 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 low temperature and high toughness hot-rolled thin steel plate with a yield strength of 960 MPa or more, characterized in that: The chemical composition of the steel plate is calculated by weight as follows: C: 0.12% ~ 0.15%, Si: 0.20% ~ 0.40%, Mn: 1.50% ~ 1.60%, P ≤ 0.015%, S ≤ 0.003%, Al: 0.015% ~ 0.045%, Mo + Cr: 0.7% ~ 0.9%, Cu: 0.3% ~ 0.5%, Nb: 0.02% ~ 0.04%, Ni: 0.2% ~ 0.4%, and the rest is Fe and unavoidable impurities.

2. The low temperature and high toughness hot-rolled thin steel plate with a yield strength of 960 MPa or more according to claim 1, characterized in that: The thickness of the finished steel plate is ≤4mm.

3. The low temperature and high toughness hot-rolled thin steel plate with a yield strength of 960 MPa or above 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 83% to 88%, and the bainite accounts for 12% to 17%.

4. The low temperature and high toughness hot-rolled thin steel plate with a yield strength of 960 MPa or above according to claim 1, characterized in that: The yield strength of the finished steel plate is ≥960MPa, the tensile strength is 980-1050MPa, the elongation is ≥10%, the hardness is 300±10HBW, and the impact energy at -40℃ is >30J.

5. A method for producing a low temperature high toughness hot-rolled thin steel plate with a yield strength of 960 MPa or more as claimed in any one of claims 1 to 4, characterized in that: Including smelting, casting, heating, rolling, cooling, tempering, leveling, straightening and cross-cutting processes; specifically controlling the following processes: 1) Heating: cold or hot charging, heating temperature is 1245-1255℃, total heat preservation time for cold charging is 150-170min, total heat preservation time for hot charging is 130-150min; heat preservation time for equalizing section is 50-70min; 2) Rolling: The rough rolling adopts a multi-pass rolling process, and the rough rolling final rolling temperature is 1120-1150°C; the insulation cover of the intermediate roller between the rough rolling and the finishing rolling is put into use; the finishing rolling inlet temperature is 1000-1100°C, and the finishing rolling adopts multi-stand continuous rolling, large reduction rolling, the thickness ratio of the intermediate billet to the finished steel plate is ≥5, and the finishing rolling final rolling temperature is 850-950°C; in the finishing rolling stage, the PC angle of the F2 frame, the F3 frame and the F4 frame is adjusted, and the bending roll force of the F5 frame to the F7 frame is adjusted to control the crown value C40 to 90-98μm; 3) Cooling: The laminar rapid cooling process is adopted. The strip is cooled immediately after leaving the finishing mill, and is cooled to 700-750°C at a cooling rate of 20-50°C / s for coiling. After coiling, the strip is stacked and slowly cooled to below 100°C. 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 900-910°C, and the holding time is 20-25min; 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 550-600°C, and the holding time is 1-7min, and a thin steel strip with a thickness of ≤4mm 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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