A hot-rolled high-strength high-plasticity steel and a manufacturing method thereof

Abstract

A hot-rolled high-strength high-plasticity steel and a manufacturing method thereof, using a relatively high carbon content and silicon content, and controlling the contents of C, Mn, Si, and Al, while meeting the following requirements: 1.0≤C+Mn≤3.0, 0.8≤Si+Al≤3.0, to obtain a steel plate with a good match of high tensile strength and ultra-high plasticity; and using relatively cheap elements such as C, Si, Mn, Al, and not containing precious metal elements, greatly reducing the alloy cost. On the basis of the above-mentioned component design, the present invention adopts a hot rolling process for the first time, that is, a brand-new preparation process of hot rolling + medium-temperature coiling + annealing or tempering, to achieve a good match of high tensile strength and ultra-high plasticity of the steel plate, and also has a lower yield strength, which can avoid cracking during the stamping process of parts and improve the forming performance of steel. It is particularly suitable for the manufacture of various complex parts of passenger cars or commercial vehicles, and has good application prospects.

Description

Technical Field

[0001] The invention relates to the field of hot-rolled high-strength steel, and in particular to a hot-rolled high-strength high-plasticity steel and a manufacturing method thereof. Background Art

[0002] Automobiles occupy a very important position in the development of the national economy. As we all know, the lightweighting of passenger cars has always been the trend of the industry. This is mainly because the passenger car industry has strict laws and regulations, which makes passenger car manufacturers constantly pursue the lightweighting of the whole vehicle, and carry out high-strength weight reduction from multiple aspects such as chassis, body, seats, etc., and even adopt other new materials such as aluminum alloy and carbon fiber. Compared with passenger cars, the lightweighting process of commercial vehicles is much slower. This is not only because the laws and regulations on commercial vehicles are not perfect, and the policy implementation is not strict enough. For example, the overloading and over-limit of commercial vehicles have not been completely eradicated for many years. This makes the design units of commercial vehicles take into account that commercial vehicles may be overloaded in actual use during actual design, so that sufficient margin is given during structural design, which affects the lightweighting process of commercial vehicles. However, with the continuous strengthening of the implementation of commercial vehicle policies and regulations, there is huge potential for lightweighting in the field of commercial vehicles in the future.

[0003] Commercial vehicle chassis parts such as fuel tank brackets, battery brackets, gas cylinder brackets, front lower protection, etc. are currently produced with low-strength thick-gauge ordinary steel such as Q235 or Q345, and the process is relatively complicated, some require bolt connection, and some require welding. With the development of lightweight commercial vehicles, many users have hoped to form these parts of commercial vehicles in one piece by cold stamping, which not only reduces the process but also achieves lightweight. This puts higher requirements on the performance of hot-rolled high-strength steel, with higher elongation and better formability while having high strength. For example, when parts such as fuel tank brackets are stamped with traditional high-strength steel, cracks occur at the large arc of the parts, and smooth stamping cannot be achieved. This requires the development of new high-strength steel with high tensile strength and high formability; considering the user's mold life, the yield strength of the new high-strength steel cannot be too high, otherwise not only will the parts rebound severely during actual stamping, but also the forming difficulty will be great.

[0004] Existing high-strength and high-plasticity steels are mainly concentrated in cold-rolled high-strength steels for automobiles, such as cold-rolled TRIP steel, QP steel, DH steel, etc. These steels all contain a certain amount of retained austenite. The cold rolling and continuous annealing process can achieve precise control of the structure, especially the stability of the retained austenite, thereby obtaining good comprehensive performance.

[0005] For example, US Patent No. 7468109B2 discloses "a cold-rolled high-strength and high-plasticity steel containing retained austenite", which uses a cold rolling process to obtain a steel plate structure containing up to 15% polygonal or quasi-polygonal ferrite. In addition, the patent involves a continuous annealing process for high-strength and high-plasticity steel.

[0006] European patent EP3225708A1 discloses "a cold-rolled high-strength and high-plasticity steel containing retained austenite", which adopts cold rolling and continuous annealing, and the obtained steel plate structure type is relatively complex. In addition to more than 8% of retained austenite, there are also bainite, martensite, tempered bainite, tempered martensite, etc. From the perspective of its mechanical properties, when the strength reaches more than 980MPa, its elongation is low.

[0007] For cold-rolled high-strength steel, the hot-rolled coil structure before cold-rolled continuous annealing is ferrite and pearlite coiled at high temperature. The main purpose is to provide a substrate with uniform thickness and performance for subsequent cold-rolled continuous annealing. At present, high-strength and ultra-high plasticity steel has not been obtained through hot rolling process. Summary of the invention

[0008] The purpose of the present invention is to provide a hot-rolled high-strength and high-plasticity steel and a method for manufacturing the same, so as to obtain a steel plate with a good match of high tensile strength and ultra-high plasticity, which can be widely used in parts with complex shape requirements such as commercial vehicles or passenger vehicles, or other parts requiring high strength and thinning; at the same time, a new preparation process of hot rolling + medium-temperature coiling + annealing or tempering is used for the first time to obtain a high-strength and high-plasticity steel plate.

[0009] To achieve the above object, the technical solution of the present invention is:

[0010] A hot-rolled high-strength high-plasticity steel, the chemical composition mass percentage of which is: C: 0.10-0.20%, Si: 0.8-2.0%, Mn: 1.0-2.5%, P≤0.02%, S≤0.003%, Al: 0.01-1.5%, N≤0.004%, the rest is Fe and other inevitable impurities, and it also needs to meet the following conditions at the same time: 1.0≤C+Mn≤3.0, 0.8≤Si+Al≤3.0;

[0011] The microstructure of the steel is bainite+residual austenite, wherein the bainite content is 85-90% and the residual austenite content is 10-15%; the yield strength of the steel is ≥300MPa, the tensile strength is ≥590MPa, and the elongation is ≥35%.

[0012] In addition, a hot-rolled high-strength high-plasticity steel, the chemical composition mass percentage of which is: C: 0.15-0.25%, Si: 0.8-2.0%, Mn: 1.0-2.5%, P≤0.02%, S≤0.003%, Al: 0.01-1.5%, N≤0.004%, the rest is Fe and other inevitable impurities, and it is also necessary to simultaneously meet the following conditions: 1.0≤C+Mn≤3.0, 0.8≤Si+Al≤3.0;

[0013] The microstructure of the steel is bainite+residual austenite, wherein the bainite content is 80-85% and the residual austenite content is 15-20%; the yield strength of the steel is ≥400MPa, the tensile strength is ≥780MPa, and the elongation is ≥30%.

[0014] Furthermore, a hot-rolled high-strength and high-plasticity steel, whose chemical composition by mass percentage is: C: 0.20-0.30%, Si: 0.8-2.0%, Mn: 1.0-2.5%, P≤0.02%, S≤0.003%, Al: 0.01-1.5%, N≤0.004%, and the rest is Fe and other inevitable impurities, and it is also necessary to simultaneously meet the following conditions: 1.0≤C+Mn≤3.0, 0.8≤Si+Al≤3.0;

[0015] The microstructure of the steel is bainite+residual austenite, wherein the bainite content is 75-80% and the residual austenite content is 20-25%; the yield strength of the steel is ≥500MPa, the tensile strength is ≥980MPa, and the elongation is ≥25%.

[0016] Also, a hot-rolled high-strength high-plasticity steel, whose chemical composition by mass percentage is: C: 0.25-0.35%, Si: 0.8-2.0%, Mn: 1.0-2.5%, P≤0.02%, S≤0.003%, Al: 0.01-1.5%, N≤0.004%, and the rest is Fe and other inevitable impurities, and it is also necessary to simultaneously meet the following conditions: 1.0≤C+Mn≤3.0, 0.8≤Si+Al≤3.0;

[0017] The microstructure of the steel is bainite+residual austenite, wherein the bainite content is 70-75% and the residual austenite content is 25-30%; the yield strength of the steel is ≥600MPa, the tensile strength is ≥1180MPa, and the elongation is ≥20%.

[0018] Preferably, the chemical composition of the steel also contains one or more of the elements Mo, Nb, V, Ti, Cu, Ni, Cr, and B, and Mo≤0.5%, Nb≤0.06%, V≤0.10%, Ti≤0.15%, Cu≤0.5%, Ni≤0.5%, Cr≤0.5%, and B≤0.001%.

[0019] In the composition design of the hot-rolled high-strength and high-plasticity steel of the present invention:

[0020] Carbon (C): Carbon is a basic element in steel and is also one of the important elements in the steel of the present invention. Carbon expands the austenite phase and stabilizes austenite. Carbon, as an interstitial atom in steel, plays a very important role in improving the strength of steel and has the greatest impact on the yield strength and tensile strength of steel. At the same time, as the most effective element for stabilizing residual austenite, carbon is usually contained at a relatively high level. In the present invention, in order to obtain advanced high-strength steels of different strength levels, the carbon content needs to be controlled within a reasonable range. At the same time, in order to obtain a larger content of stable residual austenite, the carbon content must be guaranteed to be above 0.10%. According to different strength levels, it is more reasonable to control the carbon content within different ranges. Specifically, the carbon content of high-strength steel with a tensile strength of 590MPa is controlled at 0.10-0.20%, the carbon content of high-strength steel with a tensile strength of 780MPa is controlled at 0.15-0.25%, the carbon content of high-strength steel with a tensile strength of 980MPa is controlled at 0.20-0.30%, and the carbon content of high-strength steel with a tensile strength of 1180MPa is controlled at 0.25-0.35%.

[0021] Silicon (Si): Silicon is a basic element in steel and is also one of the important elements in the steel of the present invention. The increase in silicon content not only improves the solid solution strengthening effect, but more importantly, it plays the following two roles. One role is to greatly reduce the non-recrystallization temperature of steel, so that the steel can complete dynamic recrystallization in a wide temperature range. In this way, in the actual rolling process, the final rolling temperature can be controlled in a wider temperature range, so that the difference in transverse and longitudinal structures is reduced, the uniformity of the structure is improved, and it is beneficial to improve the strength and plasticity; the other role is to inhibit the precipitation of cementite. In the present invention, in order to obtain a structure dominated by carbide-free bainite and more stable residual austenite, it is necessary to add a higher amount of silicon. The effect of silicon in inhibiting the formation of carbides must be obvious only when its content reaches 0.8% or more; but the silicon content should not be too high, otherwise the rolling force load will be too large during the actual rolling process, which is not conducive to stable production. Therefore, the present invention controls the Si content to 0.8-2.0%.

[0022] Manganese (Mn): Manganese is the most basic element in steel, and is also one of the most important elements in the steel of the present invention. Mn expands the austenite phase region, reduces the critical quenching speed of steel, stabilizes austenite, refines grains, and delays the transformation of austenite to pearlite. At the same time, manganese can be partitioned during the heat treatment process, and manganese diffuses from bainite to residual austenite, further stabilizing the residual austenite and obtaining more residual austenite. The manganese content must be at least 1.0% to achieve the above-mentioned effect, but the manganese content should not be too high. If the manganese content exceeds 2.5%, the continuous casting billet is prone to segregation, and more MnS inclusions will be formed. Therefore, the present invention controls the Mn content to be 1.0-2.5%.

[0023] Phosphorus (P): Phosphorus is an impurity element in steel. P is very easy to segregate to the grain boundary. When the P content in steel is high (≥0.1%), Fe2P is formed and precipitated around the grains, reducing the plasticity and toughness of the steel. Therefore, the lower the content, the better. Under the premise of not increasing the steelmaking cost, it is generally controlled within 0.02%. Therefore, in the present invention, the P content is controlled to be ≤0.02%.

[0024] Sulfur (S): Sulfur is an impurity element in steel. S in steel usually combines with Mn to form MnS inclusions. Especially when the contents of S and Mn are both high, more MnS will be formed in the steel. MnS itself has certain plasticity. During the subsequent rolling process, MnS deforms along the rolling direction, which not only reduces the transverse plasticity of the steel, but also increases the anisotropy of the organization, which is unfavorable to the hole expansion performance. Therefore, the lower the S content in the steel, the better. In order to reduce the content of MnS, the S content needs to be strictly controlled. Therefore, in the present invention, the S content is controlled to be ≤0.003%.

[0025] Aluminum (Al): The main function of aluminum in steel is deoxidation and nitrogen fixation. Under the premise of the presence of strong carbide-forming elements such as Ti, the main function of aluminum is deoxidation and grain refinement. In the present invention, aluminum can be used as a commonly used additive element for deoxidation and nitrogen fixation, and can also be increased to a certain content to promote the diffusion of carbon into the residual austenite. As a common deoxidation and grain refinement element, its content can be controlled at 0.01-0.08%; as an element that promotes the diffusion of carbon into austenite, the aluminum content is less than 0.1%, which plays a role in promoting carbon atoms to refine the grains in austenite; similarly, when the aluminum content is higher than 1.5%, the effect of promoting carbon diffusion and enrichment reaches saturation. Therefore, in the present invention, the aluminum content is controlled to be 0.01-1.5%.

[0026] Nitrogen (N): N is an impurity element in the present invention, and the lower its content, the better. However, nitrogen is an inevitable element in the steelmaking process. Although its content is relatively small, it combines with strong carbide-forming elements such as Ti to form TiN particles that have an adverse effect on the properties of steel. Therefore, in the present invention, the nitrogen content is controlled to be ≤0.004%.

[0027] Titanium (Ti): Titanium is one of the elements that can be added to the steel of the present invention. Since the steel contains more residual austenite, which is a soft phase with lower strength, its yield strength is lower. In order to improve the yield strength of the steel, under certain conditions, microalloying elements such as titanium can be added to improve the yield strength through precipitation strengthening of titanium in bainite and residual austenite. As the titanium content increases, the precipitation strengthening effect gradually increases. When the titanium content increases to 0.15%, the precipitation strengthening effect of titanium is saturated. Therefore, the amount of titanium added can be adjusted according to actual needs, and the titanium content in the steel should be controlled within 0.15%.

[0028] Molybdenum (Mo): Molybdenum is one of the elements that can be added in the present invention. Adding molybdenum to steel can greatly delay the phase transformation of ferrite and pearlite, which is conducive to obtaining bainite structure. In addition, molybdenum has a strong resistance to welding softening. Since the main purpose of the present invention is to obtain a structure mainly composed of bainite and residual austenite, and bainite is prone to softening after welding, adding a certain amount of molybdenum can effectively reduce the degree of welding softening. In addition, molybdenum has a strong resistance to high-temperature tempering softening, which can ensure that the strength of the strip does not decrease too much during the long-term tempering process. Therefore, the molybdenum content should be controlled within 0.5%.

[0029] Copper (Cu): Copper is one of the additive elements in the present invention. Copper can be added to steel to improve the corrosion resistance of the steel. When it is added together with the P element, the corrosion resistance effect is better. When the Cu addition exceeds 1%, under certain conditions, an ε-Cu precipitation phase can be formed, which has a strong precipitation strengthening effect. However, the addition of Cu is prone to cause "Cu brittleness" during the rolling process. In order to fully utilize the corrosion resistance improvement effect of Cu in certain applications without causing significant "Cu brittleness" phenomenon, the Cu content is usually controlled within 0.5%.

[0030] Nickel (Ni): Nickel is one of the additive elements in the present invention. Nickel has a certain corrosion resistance when added to steel, but the corrosion resistance effect is weaker than that of copper. Nickel added to steel has little effect on the tensile properties of steel, but it can refine the structure and precipitation phase of steel, greatly improving the low-temperature toughness of steel; at the same time, in steel with copper added, adding a small amount of nickel can inhibit the occurrence of "Cu brittleness". Adding higher nickel has no obvious adverse effect on the performance of the steel itself. If copper and nickel are added at the same time, not only can the corrosion resistance be improved, but the structure and precipitation phase of steel can also be refined, greatly improving the low-temperature toughness. However, since copper and nickel are both relatively expensive alloying elements. Therefore, in order to minimize the cost of alloy design, the amount of nickel added is usually ≤0.5%.

[0031] Chromium (Cr): Chromium is one of the additive elements in the present invention. Chromium is added to steel mainly to improve the strength of the steel by solid solution strengthening or refinement of the structure. Since the structure of the present invention is fine bainitic ferrite plus nano-precipitated carbides, and the movable dislocations in the structure are reduced after the high-temperature annealing process, the ratio of the yield strength to the tensile strength of the steel, i.e., the yield strength ratio, is high, usually reaching more than 0.90. Adding a small amount of chromium can appropriately reduce the yield strength of the steel, thereby reducing the yield strength ratio. In addition, the addition of a small amount of chromium can also improve corrosion resistance. Usually, the amount of chromium added is ≤0.5%.

[0032] Niobium (Nb): Niobium is one of the additive elements of the present invention. Niobium is similar to titanium and is a strong carbide element in steel. The addition of niobium to steel can greatly increase the non-recrystallization temperature of the steel, and deformed austenite with a higher dislocation density can be obtained in the finishing rolling stage, and the final phase transformation structure can be refined in the subsequent transformation process. However, the amount of niobium added should not be too much. On the one hand, if the amount of niobium added exceeds 0.06%, it is easy to form relatively coarse niobium carbonitrides in the structure, which is not conducive to the low-temperature impact toughness of the steel. At the same time, a high content of niobium is also likely to cause anisotropy of the hot-rolled austenite structure. Therefore, the niobium content in steel is usually controlled at ≤0.06%.

[0033] Vanadium (V): Vanadium is one of the additive elements in the present invention. Vanadium is a strong carbide-forming element, similar to titanium and niobium. However, the solid solution or precipitation temperature of vanadium carbides is low, and they are usually all solid dissolved in austenite during the finishing rolling stage. Only when the temperature drops and the phase transition begins, vanadium begins to form in ferrite. Since the solid solubility of vanadium carbides in ferrite is greater than that of niobium and titanium, the size of vanadium carbides formed in ferrite is larger and is easily formed on grain boundaries, which is not good for the toughness of steel. Therefore, the amount of vanadium added to steel is usually ≤0.10%.

[0034] Vanadium (B): Boron is one of the additive elements in the present invention. Boron can greatly improve the hardenability of steel and is conducive to obtaining a martensitic structure. Considering that the structure desired to be obtained in the hot rolling stage of the present invention is bainite, the content of boron in the steel needs to be strictly controlled to prevent the formation of martensite due to excessive addition of boron, so the addition amount of boron in the steel is usually controlled to ≤0.001%.

[0035] In addition, the contents of C, Mn, Si and Al must also meet the following requirements:

[0036] 1.0≤C+Mn≤3.0, 0.8≤Si+Al≤3.0.

[0037] Both carbon and manganese are elements that stabilize austenite. Carbon content is one of the important elements in the composition design that determines the strength of steel and the final residual austenite content and stability. Manganese is also an austenite stabilizing element. Manganese is also distributed during the slow cooling process after hot rolling and during the annealing process. Carbon and manganese jointly control the strength, residual austenite content and stability of steel. When the sum of the carbon and manganese contents is less than 1.0, the strength and residual austenite stability of the steel are insufficient, resulting in low elongation; when the sum of the carbon and manganese contents is greater than 3.0, the weldability of the steel deteriorates, and welding cold cracking is prone to occur during actual welding. Therefore, the sum of the carbon and manganese contents should be controlled between 1.0 and 3.0.

[0038] Similarly, silicon is the most important element to inhibit the formation of cementite, and aluminum also has a certain effect of inhibiting the formation of cementite. When the sum of silicon and aluminum content is less than 0.8, it cannot play a sufficient role in inhibiting the formation of cementite; when the sum of silicon and aluminum content exceeds 3.0, not only the cost is increased, but also the excessive aluminum content will cause nozzle blockage in the steelmaking process and greatly increase the rolling load, and the manufacturability of steel will be reduced; at the same time, the silicon and aluminum content is too high, and thermal cracking is prone to occur during welding. Therefore, the sum of silicon and aluminum content should be controlled between 0.8 and 3.0.

[0039] The steel of the present invention adopts a higher carbon content and silicon content in the composition design. The higher carbon content is conducive to obtaining high strength, and more available carbon atoms diffuse into the residual austenite, thereby stabilizing the residual austenite. The main purpose of adding a higher silicon content is to inhibit the formation of carbides and obtain carbide-free bainite; and control 1.0≤C+Mn≤3.0, 0.8≤Si+Al≤3.0, manganese can further improve the stability of the residual austenite and refine the structure; aluminum can promote the diffusion of carbon atoms into the residual austenite, so that the steel plate obtains a good match of high tensile strength and ultra-high plasticity, and can also prevent the generation of cracks at high or low temperatures.

[0040] The steel of the present invention mainly adopts relatively cheap elements such as C, Si, Mn and Al, does not contain precious metal elements, and greatly reduces the alloy cost.

[0041] The method for manufacturing hot-rolled high-strength and high-plasticity steel of the present invention comprises the following steps:

[0042] 1) Smelting and casting

[0043] According to the above composition, smelting is performed in a converter or electric furnace, secondary refining is performed in a vacuum furnace, and casting is performed into a billet or an ingot;

[0044] 2) Reheating

[0045] The reheating temperature of the billet or ingot is ≥1100℃, and the holding time is 1 to 2 hours;

[0046] 3) Hot rolling

[0047] The starting rolling temperature is 1050-1150℃, and the steel is rolled for 3-5 passes at a temperature above 1050℃ with a cumulative deformation of ≥50%; the intermediate billet is then heated to 950-1000℃, and then the last 3-5 passes are rolled with a cumulative deformation of ≥70%; the final rolling temperature is between 800 and 950℃;

[0048] 4) Heat treatment

[0049] After the final rolling, the steel plate is cooled to 400-550°C at a cooling rate of ≥30°C / s, and then coiled at 400-550°C and cooled to room temperature; the obtained steel coil is annealed at 100-400°C for 4-48h; or,

[0050] After the final rolling, the steel plate is cooled to 400-550°C at a cooling rate of ≥30°C / s, and then tempered at 100-400°C for 1-240 minutes.

[0051] The steel of the present invention adopts a relatively high carbon content and silicon content in the composition design, combined with other alloy elements, and matched with hot rolling coiling, annealing or tempering process to obtain a bainite and a relatively high content of stable residual austenite microstructure.

[0052] The invention adopts a hot rolling process on the basis of component design, controls the reheating temperature of the billet or ingot to be ≥1100 DEG C, the holding time is 1-2 hours, the start rolling temperature is 1050-1150 DEG C, and 3-5 passes of large reduction are performed above 1050 DEG C with a cumulative deformation of ≥50%, the main purpose of which is to refine the austenite grains; then the intermediate billet is heated to 950-1000 DEG C, and then the last 3-7 passes of rolling are performed with a cumulative deformation of ≥70%; after the final rolling is completed between 800-950 DEG C, the steel plate is water-cooled to between 400-550 DEG C at a cooling rate of ≥30 DEG C / s, and is cooled to room temperature after coiling to obtain bainite and residual austenite structures, but the residual austenite at this time is not stable enough, and in order to make the steel obtain high strength and high plasticity, it is necessary to improve the stability of the residual austenite, stabilize the content of the residual austenite, and further improve the stability of the residual austenite by using a subsequent annealing or tempering process.

[0053] During the annealing or tempering process, carbon atoms further diffuse from bainite to retained austenite. At the same time, silicon can inhibit carbon atoms from forming cementite in this process, and a certain amount of aluminum can promote the diffusion of carbon into retained austenite. The combination of all components and processes ultimately obtains carbon-poor bainite and more stable carbon-rich retained austenite. Therefore, the obtained steel has a matching of low yield strength, high tensile strength and ultra-high elongation.

[0054] When the annealing temperature is lower than 100℃, it takes a long time to complete the diffusion of carbon atoms from bainite to retained austenite, and the production cycle is too long; when the annealing temperature is higher than 400℃, long-term annealing will cause more retained austenite to decompose, reducing the stability of the retained austenite. Therefore, the annealing temperature should be controlled between 100 and 400℃. According to the different annealing temperatures and the number of steel coils annealed, the annealing time can be adjusted from 4 hours to 48 hours according to the actual situation.

[0055] When the obtained steel is a steel plate, tempering heat treatment is adopted, and the tempering temperature is 100-400°C. The tempering time can be greatly shortened. According to different heat treatment temperatures, the holding time is adjusted from 1 to 240 minutes.

[0056] The present invention obtains steel with high tensile strength and ultra-high elongation through precise matching of the above-mentioned components and processes, and can be widely used in parts with complex shape requirements such as commercial vehicles or passenger vehicles or other parts requiring high strength and thinning.

[0057] Beneficial effects of the present invention:

[0058] 1. The steel of the present invention adopts a relatively high carbon content and silicon content in the composition design, and controls the contents of C, Mn, Si and Al to meet the following requirements at the same time: 1.0≤C+Mn≤3.0, 0.8≤Si+Al≤3.0, thereby inhibiting the formation of carbides and obtaining carbide-free bainite. At the same time, it also plays a role in stabilizing the retained austenite, obtaining a steel with a microstructure of bainite + retained austenite, so that the steel plate obtains a good match of high tensile strength and ultra-high plasticity, and prevents the generation of cracks at high or low temperatures.

[0059] 2. Traditionally, high-strength and high-plasticity steels are obtained by cold rolling + continuous annealing process. Even if the preparation of the substrate involves hot rolling, cold rolling and continuous annealing are still required to obtain high strength and high plasticity.

[0060] Based on the above-mentioned component design, the present invention adopts a hot rolling process for the first time, that is, a brand-new preparation process of hot rolling + medium-temperature coiling + annealing or tempering is adopted, and a bainite and residual austenite structure is obtained in the hot rolling stage. Combined with the subsequent annealing or tempering process, the diffusion of carbon atoms into the residual austenite is promoted, the stability of the residual austenite is improved, thereby stabilizing the amount of the residual austenite, and obtaining a hot-rolled steel with higher strength and ultra-high plasticity.

[0061] 3. The steel manufactured by the present invention has high tensile strength and ultra-high plasticity, and also has low yield strength, which can avoid cracking during the stamping process of parts and improve the forming performance of steel. It is particularly suitable for the manufacture of various complex parts of passenger cars or commercial vehicles and has good application prospects.

[0062] 4. The alloy component cost of the high-strength and high-plasticity steel of the present invention is relatively low, mainly using relatively cheap elements such as C, Si, Mn, and Al, and does not contain precious metal elements, which greatly reduces the alloy cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Figure 1 This is a typical SEM photo of the steel obtained in Example 1 of the present invention.

[0064] Figure 2 This is a typical SEM photo of the steel obtained in Example 3 of the present invention.

[0065] Figure 3 This is a typical SEM photo of the steel obtained in Example 5 of the present invention.

[0066] Figure 4 This is a typical SEM photo of the steel obtained in Example 9 of the present invention.

[0067] Figure 5 This is a typical SEM photograph of the steel obtained in Example 11 of the present invention.

[0068] Figure 6 This is a typical SEM photograph of the steel obtained in Example 15 of the present invention.

[0069] Figure 7 This is a typical SEM photograph of the steel obtained in Example 18 of the present invention.

[0070] Figure 8 This is a typical SEM photograph of the steel obtained in Example 22 of the present invention.

[0071] Fig. 9 This is a typical SEM photograph of the steel obtained in Example 24 of the present invention.

[0072] Fig.10 This is a typical SEM photograph of the steel obtained in Example 25 of the present invention.

[0073] Fig.11 This is a typical SEM photograph of the steel obtained in Example 28 of the present invention.

[0074] Fig.12 This is a typical SEM photograph of the steel obtained in Example 30 of the present invention. DETAILED DESCRIPTION

[0075] The present invention will be further explained and illustrated below in conjunction with specific embodiments and drawings. However, such explanation and illustration do not constitute an improper limitation on the technical solution of the present invention.

[0076] Table 1 shows the mass percentages of the chemical components of the steel of the present invention, Table 2 shows the process parameters of the steel of the present invention, and Table 3 shows the properties of the steel of the present invention.

[0077] Figure 1 to Figure 3 Typical SEM photos of the steel coils of Examples 1 and 3 after annealing and the steel plate of Example 5 after heat treatment are given respectively; Figures 4 to 6 Typical SEM photos of steel coils of Examples 9 and 11 after annealing and steel plate of Example 15 after heat treatment are given respectively; Figure 7 to Figure 9 Typical SEM photos of steel coils of Examples 18 and 22 after annealing and steel plate of Example 24 after heat treatment are given respectively; Figure 10 to Figure 12 Typical SEM photos of steel coils of Examples 25 and 28 after annealing and steel plate of Example 30 after heat treatment are shown. It can be seen from the figure that the microstructure of the steel obtained by using the composition and process path designed by the present invention is bainite + retained austenite.

[0078] It can be seen from Table 3 that the hot-rolled high-strength and high-plasticity steel obtained by the present invention has good strength and plasticity matching, is particularly suitable for preparing parts that require complex forming, such as automobile chassis structures, and has broad application prospects.

[0079] It should also be noted that the above-listed embodiments are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therewith can be directly derived or easily associated with by those skilled in the art from the contents disclosed in the present invention, and all should fall within the protection scope of the present invention.

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Claims

1. A hot-rolled high-strength and high-plasticity steel, the chemical composition of which is as follows by mass percentage: C: 0.10-0.20%, Si: 0.8-2.0%, Mn: 1.0-2.5%, P≤0.02%, S≤0.003%, Al: 0.01-1.5%, N≤0.004%, the rest is Fe and other inevitable impurities, and it is also necessary to simultaneously meet the following conditions: 1.0≤C+Mn≤3.0, 0.8≤Si+Al≤3.0; The microstructure of the steel is bainite + retained austenite, wherein: The bainite content is 85~90%, and the retained austenite content is 10~15%; The yield strength of the steel is ≥300MPa, the tensile strength is ≥590MPa, and the elongation is ≥35%.

2. A hot-rolled high-strength and high-plasticity steel, the chemical composition of which is as follows by mass percentage: C: 0.15-0.25%, Si: 0.8-2.0%, Mn: 1.0-2.5%, P≤0.02%, S≤0.003%, Al: 0.01-1.5%, N≤0.004%, the rest is Fe and other inevitable impurities, and it is also necessary to simultaneously meet the following conditions: 1.0≤C+Mn≤3.0, 0.8≤Si+Al≤3.0; The microstructure of the steel is bainite + retained austenite, wherein: The bainite content is 80~85%, and the retained austenite content is 15~20%; The yield strength of the steel is ≥400MPa, the tensile strength is ≥780MPa, and the elongation is ≥30%.

3. A hot-rolled high-strength and high-plasticity steel, the chemical composition of which is as follows by mass percentage: C: 0.20-0.30%, Si: 0.8-2.0%, Mn: 1.0-2.5%, P≤0.02%, S≤0.003%, Al: 0.01-1.5%, N≤0.004%, the rest is Fe and other inevitable impurities, and it is also necessary to simultaneously meet the following conditions: 1.0≤C+Mn≤3.0, 0.8≤Si+Al≤3.0; The microstructure of the steel is bainite + retained austenite, wherein: The bainite content is 75~80%, and the retained austenite content is 20~25%; The yield strength of the steel is ≥500MPa, the tensile strength is ≥980MPa, and the elongation is ≥25%.

4. A hot-rolled high-strength and high-plasticity steel, the chemical composition of which is as follows by mass percentage: C: 0.25-0.35%, Si: 0.8-2.0%, Mn: 1.0-2.5%, P≤0.02%, S≤0.003%, Al: 0.01-1.5%, N≤0.004%, the rest is Fe and other inevitable impurities, and it is also necessary to simultaneously meet the following conditions: 1.0≤C+Mn≤3.0, 0.8≤Si+Al≤3.0; The microstructure of the steel is bainite + retained austenite, wherein: The bainite content is 70~75%, and the retained austenite content is 25~30%; The yield strength of the steel is ≥600MPa, the tensile strength is ≥1180MPa, and the elongation is ≥20%.

5. The hot-rolled high-strength and high-plasticity steel according to any one of claims 1 to 4, characterized in that: The chemical composition of the steel also contains one or more of the elements Mo, Nb, V, Ti, Cu, Ni, Cr, and B, and Mo≤0.5%, Nb≤0.06%, V≤0.10%, Ti≤0.15%, Cu≤0.5%, Ni≤0.5%, Cr≤0.5%, and B≤0.001%.

6. The method for producing hot-rolled high-strength and high-plasticity steel according to any one of claims 1 to 5, characterized in that: The steps include: 1) Smelting and casting According to the chemical composition, smelting is performed in a converter or electric furnace, secondary refining is performed in a vacuum furnace, and casting is performed into a billet or an ingot; 2) Reheating The reheating temperature of the billet or ingot is ≥1100℃, and the holding time is 1~2h; 3) Hot rolling Starting rolling temperature: 1050~1150℃, 3~5 passes of rolling above 1050℃ and cumulative deformation ≥50%; then the intermediate billet is heated to 950~1000℃, and then the last 3~5 passes of rolling are carried out with cumulative deformation ≥70%, and the final rolling temperature is between 800~950℃; 4) Heat treatment After the final rolling, the steel plate is cooled to 400-550°C at a cooling rate of ≥30°C / s, and then coiled at 400-550°C and cooled to room temperature; the obtained steel coil is annealed at 100-400°C for 4-48 hours; or, After the final rolling, the steel plate is cooled to 400~550℃ at a cooling rate of ≥30℃ / s, and then tempered at 100~400℃ for 1~240min.

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