CN116497266A

CN116497266A - Hot-rolled high-strength high-plasticity steel and manufacturing method thereof

Info

Publication number: CN116497266A
Application number: CN202210058426.7A
Authority: CN
Inventors: 王焕荣; 张晨; 杨阿娜
Original assignee: Baoshan Iron and Steel Co Ltd
Current assignee: Baoshan Iron and Steel Co Ltd
Priority date: 2022-01-19
Filing date: 2022-01-19
Publication date: 2023-07-28

Abstract

A hot rolled high strength high plasticity steel and its manufacturing method, which adopts higher carbon content and silicon content, and controls C, mn, si, al content, and simultaneously satisfies the following requirements: C+Mn is more than or equal to 1.0 and less than or equal to 3.0,0.8, si+Al is more than or equal to 3.0, and a steel plate with high tensile strength and ultrahigh plasticity and good matching is obtained; and C, si, mn, al and other relatively cheap elements are adopted, so that the alloy does not contain noble metal elements, and the alloy cost is greatly reduced. On the basis of the component design, the invention adopts a hot rolling process for the first time, namely a brand-new preparation process of hot rolling, medium temperature coiling and cover annealing or tempering, realizes good matching of high tensile strength and ultrahigh plasticity of the steel plate, has lower yield strength, can avoid cracking in the part stamping process, improves the forming performance of steel, is particularly suitable for manufacturing various complex parts of passenger vehicles or commercial vehicles, and has good application prospect.

Description

Hot-rolled high-strength high-plasticity steel and manufacturing method thereof

Technical Field

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

Background

Automobiles occupy a very important position in national economy development. As is well known, the light weight of passenger cars is always a trend of industry development, mainly because the passenger car industry has strict legal and regulatory requirements, so that passenger car manufacturers continuously pursue the light weight of the whole car, and high-strength weight reduction is performed from various aspects of chassis, car body, seat and the like, and even other new materials such as aluminum alloy, carbon fiber and the like are adopted. Compared with a passenger car, the light weight process of the commercial car is much slower, not only is the policy execution strength not strict enough because the laws and regulations of the commercial car are not perfect, but also the problems of overload and overrun of the commercial car are not thoroughly radically solved for many years, so that the design unit of the commercial car needs to consider that the commercial car can be overloaded in actual use in actual design, and therefore enough surplus is given in structural design, and the light weight process of the commercial car is affected. However, with the continuous enhancement of the implementation force of the policy and regulation of the commercial vehicle, the future commercial vehicle field has huge light weight potential.

The chassis parts of the commercial vehicle such as an oil tank bracket, a battery bracket, a gas cylinder bracket, front lower protection and the like are produced by adopting low-strength-level thick-specification common steel such as Q235 or Q345 at present, the process is complex, some of the chassis parts need to be connected by bolts, and other chassis parts need to be welded. With the development of light weight of commercial vehicles, many users wish to integrally form the parts of the commercial vehicle by adopting a cold stamping mode, so that the procedures are reduced, and the light weight is realized. This puts higher demands on the performance of hot rolled high strength steel, and has higher elongation and better formability while having high strength. When parts such as an oil tank bracket and the like are stamped by adopting traditional high-strength steel, cracking occurs at the large arc part of the parts, and smooth stamping cannot be realized. There is a need to develop new high strength steels with high tensile strength and high formability; considering the service life of a die of a user, the yield strength of the novel high-strength steel cannot be too high, otherwise, the rebound of the part is serious during actual stamping, and the forming difficulty is high.

The existing high-strength high-plasticity steel is mainly concentrated on cold-rolled high-strength steel for automobiles, such as cold-rolled TRIP steel, QP steel, DH steel and the like, and the steel types all contain a certain amount of retained austenite, and the structure, particularly the stability of the retained austenite, can be accurately regulated and controlled through a cold-rolling continuous annealing process, so that good comprehensive performance is obtained.

As disclosed in US patent 7468109B2, "a cold-rolled high strength high plasticity steel containing retained austenite", a cold-rolling process is employed to obtain a steel sheet structure containing up to 15% polygonal or quasi-polygonal ferrite. In addition, the continuous annealing process of the high-strength and high-plastic steel is disclosed in the patent.

EP3225708A1 discloses "a cold-rolled high-strength high-plastic steel containing residual austenite", which adopts cold rolling and continuous annealing, and the obtained steel sheet has a relatively complex structure type, and besides more than 8% of residual austenite, bainite, martensite, tempered bainite, tempered martensite and the like. From the mechanical property, when the strength reaches over 980MPa, the elongation is lower.

For cold-rolled high-strength steel, the hot-rolled coil structure before cold-rolling continuous annealing is ferrite and pearlite coiled at high temperature, and the main purpose is to provide a substrate with uniform thickness and performance for subsequent cold-rolling continuous annealing, and high-strength and ultrahigh-plasticity steel is not obtained through a hot-rolling process at present.

Disclosure of Invention

The invention aims to provide hot-rolled high-strength high-plasticity steel and a manufacturing method thereof, and the steel plate with high tensile strength and ultrahigh plasticity and good matching performance is obtained, so that the steel plate can be widely applied to parts with complex shape requirements such as commercial vehicles or passenger vehicles or other parts needing high strength thinning; meanwhile, a brand new preparation process of hot rolling, medium temperature coiling, cover annealing or tempering is adopted for the first time, and the high-strength high-plasticity steel plate can be obtained.

In order to achieve the above purpose, the technical scheme of the invention is as follows:

the hot-rolled high-strength high-plasticity steel comprises the following chemical components in percentage by mass: c:0.10 to 0.20 percent, si:0.8 to 2.0 percent, mn:1.0 to 2.5 percent, P is less than or equal to 0.02 percent, S is less than or equal to 0.003 percent, al:0.01 to 1.5 percent, N is less than or equal to 0.004 percent, and the balance is Fe and other unavoidable impurities, and the requirements are simultaneously satisfied: C+Mn is more than or equal to 1.0 and less than or equal to 3.0,0.8, si+Al is more than or equal to 3.0;

the microstructure of the steel is bainite and residual austenite, wherein the content of bainite is 85-90%, and the content of residual austenite is 10-15%; the yield strength of the steel is more than or equal to 300MPa, the tensile strength is more than or equal to 590MPa, and the elongation is more than or equal to 35%.

The hot rolled high-strength high-plasticity steel comprises the following chemical components in percentage by mass: c:0.15 to 0.25 percent, si:0.8 to 2.0 percent, mn:1.0 to 2.5 percent, P is less than or equal to 0.02 percent, S is less than or equal to 0.003 percent, al:0.01 to 1.5 percent, N is less than or equal to 0.004 percent, and the balance is Fe and other unavoidable impurities, and the requirements are simultaneously satisfied: C+Mn is more than or equal to 1.0 and less than or equal to 3.0,0.8, si+Al is more than or equal to 3.0;

the microstructure of the steel is bainite and residual austenite, wherein the content of bainite is 80-85%, and the content of residual austenite is 15-20%; the yield strength of the steel is more than or equal to 400MPa, the tensile strength is more than or equal to 780MPa, and the elongation is more than or equal to 30%.

Furthermore, the hot rolled high-strength high-plasticity steel comprises the following chemical components in percentage by mass: c:0.20 to 0.30 percent, si:0.8 to 2.0 percent, mn:1.0 to 2.5 percent, P is less than or equal to 0.02 percent, S is less than or equal to 0.003 percent, al:0.01 to 1.5 percent, N is less than or equal to 0.004 percent, and the balance is Fe and other unavoidable impurities, and the requirements are simultaneously satisfied: C+Mn is more than or equal to 1.0 and less than or equal to 3.0,0.8, si+Al is more than or equal to 3.0;

the microstructure of the steel is bainite and residual austenite, wherein the content of bainite is 75-80%, and the content of residual austenite is 20-25%; the yield strength of the steel is more than or equal to 500MPa, the tensile strength is more than or equal to 980MPa, and the elongation is more than or equal to 25%.

Also, a hot rolled high strength high plasticity steel comprising the chemical components by mass percent: c:0.25 to 0.35 percent, si:0.8 to 2.0 percent, mn:1.0 to 2.5 percent, P is less than or equal to 0.02 percent, S is less than or equal to 0.003 percent, al:0.01 to 1.5 percent, N is less than or equal to 0.004 percent, and the balance is Fe and other unavoidable impurities, and the requirements are simultaneously satisfied: C+Mn is more than or equal to 1.0 and less than or equal to 3.0,0.8, si+Al is more than or equal to 3.0;

the microstructure of the steel is bainite and residual austenite, wherein the content of bainite is 70-75%, and the content of residual austenite is 25-30%; the yield strength of the steel is more than or equal to 600MPa, the tensile strength is more than or equal to 1180MPa, and the elongation is more than or equal to 20%.

Preferably, the chemical composition of the steel also contains one or more of Mo, nb, V, ti, cu, ni, cr, B elements, wherein Mo is less than or equal to 0.5%, nb is less than or equal to 0.06%, V is less than or equal to 0.10%, ti is less than or equal to 0.15%, cu is less than or equal to 0.5%, ni is less than or equal to 0.5%, cr is less than or equal to 0.5% and B is less than or equal to 0.001%.

In the composition design of the hot rolled high strength high plasticity steel of the invention:

carbon (C): carbon is a basic element in the steel, and is also one of important elements in the steel of the invention, and carbon enlarges an austenite phase region and stabilizes austenite. Carbon plays a very important role in improving the strength of steel as a interstitial atom in steel, and has the greatest influence on the yield strength and tensile strength of steel. At the same time, carbon is the most effective element for stabilizing the retained austenite, and its content is usually at a higher level. In the present invention, in order to obtain advanced high-strength steels of different strength levels, it is required to control the carbon content within a reasonable range, respectively. Meanwhile, in order to obtain a large content of stable retained austenite, the content of carbon must be ensured to be 0.10% or more. According to different intensity levels, the carbon content should be controlled in different ranges. Specifically, the carbon content of 590 MPa-level high-strength steel with tensile strength is controlled to be 0.10-0.20%, the carbon content of 780 MPa-level high-strength steel with tensile strength is controlled to be 0.15-0.25%, the carbon content of 980 MPa-level high-strength steel with tensile strength is controlled to be 0.20-0.30%, and the carbon content of 1180 MPa-level high-strength steel with tensile strength is controlled to be 0.25-0.35%.

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 silicon content is increased, the solid solution strengthening effect is improved, more importantly, the effect of greatly reducing the unrecrystallized temperature of the steel is achieved, and the steel can be dynamically recrystallized in a wide temperature range. In the actual rolling process, the final rolling temperature can be controlled in a wider temperature range, so that the difference of transverse and longitudinal tissues is reduced, the uniformity of the tissues is improved, and the strength and the plasticity are improved; another function is to suppress cementite precipitation. In the present invention, in order to obtain a structure mainly composed of carbide-free bainite and to obtain more stable retained austenite, it is necessary to add higher silicon. This carbide formation inhibiting effect of silicon must be evident at levels above 0.8%; however, the silicon content should not be too high, otherwise the rolling force load in the actual rolling process is too large, which is unfavorable for stable production. Therefore, the Si content is controlled to be 0.8-2.0%.

Manganese (Mn): manganese is the most basic element in the steel and is also one of the most important elements in the steel according to the invention. Mn enlarges the austenite phase region, reduces the critical quenching speed of steel, stabilizes austenite, refines grains, and delays the transformation from austenite to pearlite. Meanwhile, manganese can be distributed in the heat treatment process, and the manganese diffuses from bainite to residual austenite to further stabilize the residual austenite and obtain more residual austenite. The above effect can be achieved only when the manganese content is at least 1.0%, but the manganese content is not too high, the manganese content exceeds 2.5%, the continuous casting billet is easy to segregate, and more MnS inclusions are formed. Therefore, the Mn content is controlled to be 1.0-2.5%.

Phosphorus (P): phosphorus is an impurity element in steel. P is easily biased to grain boundary, and Fe is formed when the content of P in steel is higher (more than or equal to 0.1 percent) ₂ P is precipitated around the crystal grains, and the plasticity and toughness of the steel are reduced, so that the lower the content is, the better the content is, and the content of P is generally controlled to be less than or equal to 0.02 percent on the premise of not increasing the steelmaking cost, so that the content of P is controlled to be less than or equal to 0.02 percent in the invention.

Sulfur (S): sulfur is an impurity element in steel. S in steel is usually combined with Mn to form MnS inclusion, more MnS is formed in the steel especially when the contents of S and Mn are high, the MnS has certain plasticity, and the MnS deforms along the rolling direction in the subsequent rolling process, so that the transverse plasticity of the steel is reduced, the tissue anisotropy is increased, and the reaming performance is unfavorable. Therefore, the lower the S content in the steel, the better, and in order to reduce the MnS content, the S content needs to be strictly controlled, so that the S content is controlled to be less than or equal to 0.003 percent in the invention.

Aluminum (Al): the role of aluminum in steel is mainly deoxidation and nitrogen fixation. In the presence of strong carbide forming elements such as Ti and the like, the primary role of aluminum is to deoxidize and refine the grains. In the invention, aluminum can be used as a common additive element for deoxidizing and fixing nitrogen, and can be increased to a certain content to promote the diffusion of carbon element into the residual austenite. As the elements of common deoxidizing and refining crystal grains, the content is controlled to be 0.01 to 0.08 percent; as an element for promoting diffusion of carbon into austenite, the aluminum content is lower than 0.1%, and the effect of promoting refinement of carbon atoms into austenite is achieved; also, at an aluminum content higher than 1.5%, the effect of promoting carbon diffusion and enrichment is saturated. Therefore, the aluminum content is controlled to be 0.01 to 1.5% in the present invention.

Nitrogen (N): n is an impurity element in the present invention, and the lower the content thereof, the better. Nitrogen is an inevitable element in the steelmaking process. Although the content thereof is small, the formed TiN particles have an adverse effect on the properties of the steel in combination with a strong carbide forming element such as Ti or the like. Therefore, the nitrogen content is controlled to be less than or equal to 0.004 percent in the invention.

Titanium (Ti): titanium is one of the elements that can be added to the steel of the present invention. Since the steel contains more retained austenite, the retained austenite belongs to a soft phase with lower strength, and thus the yield strength is lower. In order to increase the yield strength of steel, microalloying elements such as titanium and the like may be added under certain conditions, and the yield strength is increased by precipitation strengthening of titanium in bainite and retained austenite. With the increase of the titanium content, the precipitation strengthening effect is gradually enhanced. When the content of titanium increases to 0.15%, the precipitation strengthening effect of titanium is saturated. Therefore, the addition amount of titanium can be adjusted according to actual needs, and the content of titanium in steel is controlled within 0.15 percent.

Molybdenum (Mo): molybdenum is one of the elements that can be added in the present invention. The addition of molybdenum to steel can greatly delay ferrite and pearlite transformation, which is beneficial to obtaining bainite structure. In addition, molybdenum has strong weld softening resistance. Since the main purpose of the invention is to obtain a structure with bainite and retained austenite as main components, and the bainite is easy to soften after welding, the addition of a certain amount of molybdenum can effectively reduce the welding softening degree. In addition, the molybdenum has strong high-temperature tempering softening resistance, and can ensure that the strength of the strip steel is not reduced too much in the long-time covering and annealing process. Therefore, the content of molybdenum should be controlled within 0.5%.

Copper (Cu): copper is one of the additive elements in the present invention. Copper is added into the steel to improve the corrosion resistance of the steel, and when the copper and the P element are added together, the corrosion resistance effect is better; when the Cu addition amount exceeds 1%, an epsilon-Cu precipitated phase can be formed under certain conditions, and a stronger precipitation strengthening effect is achieved. However, cu is easy to form a "Cu embrittlement" phenomenon during rolling, and in order to fully utilize the corrosion resistance improving effect of Cu in some applications, the Cu content is generally controlled to be within 0.5% without causing a significant "Cu embrittlement" phenomenon.

Nickel (Ni): nickel is one of the additizable elements of the present invention. The nickel added into the steel has certain corrosion resistance, but the corrosion resistance effect is weaker than that of copper, and the nickel added into the steel has little influence on the tensile property of the steel, but can refine the structure and the precipitated phase of the steel, so that the low-temperature toughness of the steel is greatly improved; meanwhile, in the steel added with copper element, the occurrence of Cu embrittlement can be restrained by adding a small amount of nickel. The addition of higher nickel has no significant adverse effect on the properties of the steel itself. If copper and nickel are added at the same time, not only the corrosion resistance can be improved, but also the structure and the precipitated phase of the steel are refined, and the low-temperature toughness is greatly improved. But since copper and nickel are both relatively noble alloying elements. Therefore, in order to reduce the cost of alloy design as much as possible, the addition amount of nickel is usually less than or equal to 0.5%.

Chromium (Cr): chromium is one of the additizable elements in the present invention. Chromium is added into steel to improve the strength of the steel mainly through solid solution strengthening, tissue refining and other modes. Because the structure is fine bainitic ferrite plus nano precipitated carbide and the movable dislocation in the structure is reduced after the high-temperature covering and annealing process is carried out, the ratio of the yield strength to the tensile strength of the steel, namely the yield ratio, is higher and is generally more than 0.90. The addition of a small amount of chromium element can properly reduce the yield strength of steel, thereby reducing the yield ratio. In addition, the addition of a small amount of chromium can also play a role in improving corrosion resistance, and the addition amount of chromium is generally less than or equal to 0.5%.

Niobium (Nb): niobium is one of the additizable elements of the present invention. Niobium is similar to titanium and is a strong carbide element in steel, the unrecrystallized temperature of the steel can be greatly increased by adding the niobium into the steel, deformed austenite with higher dislocation density can be obtained in the finish 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 large, but on the one hand, the amount of niobium added exceeds 0.06%, and relatively coarse niobium carbonitrides are easily formed in the structure, which is disadvantageous in low-temperature impact toughness of the steel. Meanwhile, the niobium content is high, and the anisotropy of the hot rolled austenitic structure is easy to cause. Therefore, the niobium content in the steel is usually controlled to be 0.06% or less.

Vanadium (V): vanadium is one of the additizable elements in the present invention. Vanadium, like titanium, niobium, is a strong carbide forming element. However, the vanadium carbide has a low solution or precipitation temperature, and is usually entirely dissolved in austenite in the finish rolling stage. Vanadium starts to form in ferrite only when the temperature decrease starts to change phase. Since the solid solubility of vanadium carbide in ferrite is greater than that of niobium and titanium, the size of vanadium carbide formed in ferrite is large and is easily formed on grain boundaries, which is disadvantageous to toughness of steel. Therefore, the addition amount of vanadium in steel is usually not more than 0.10%.

Vanadium (B): boron is one of the additizable elements in the present invention. Boron can greatly improve the hardenability of steel and is beneficial to obtaining a martensitic structure. Considering that the microstructure expected to be obtained in the hot rolling stage of the present invention is bainite, it is necessary to strictly control the content of boron element in steel to prevent the formation of martensite due to excessive addition of boron element, so that the addition amount of boron in steel is usually controlled to be 0.001% or less.

In addition, the content of C, mn, si, al also needs to meet the following requirements:

1.0≤C+Mn≤3.0，0.8≤Si+Al≤3.0。

carbon and manganese are elements for stabilizing austenite, and the carbon content is one of important elements for determining the strength of steel and the final residual austenite content and stability in component design; manganese is also an austenite stabilizing element, and there is also distribution of manganese element during slow cooling after hot rolling and coiling and during the annealing. Carbon and manganese together control the strength and retained austenite content and stability of the steel. When the sum of the contents of carbon and manganese is less than 1.0, the strength and the retained austenite stability of the steel are insufficient, resulting in low elongation; when the sum of the contents of carbon and manganese is more than 3.0, the weldability of the steel becomes poor, and weld cold cracking easily occurs in the actual welding process. Therefore, the sum of the carbon and manganese contents should be controlled to be 1.0-3.0.

Also, silicon is the most important element for inhibiting cementite formation, and aluminum also has a certain effect of inhibiting cementite formation, and when the sum of silicon and aluminum content is less than 0.8, it cannot sufficiently inhibit cementite formation; when the sum of the silicon and aluminum contents exceeds 3.0, not only is the cost increased, but also the excessive aluminum content can cause the blockage of a water gap in the steelmaking process and greatly increase the rolling load, so that the manufacturability of the steel is reduced; meanwhile, the contents of silicon and aluminum are too high, and hot cracks are easy to occur in the welding process. Therefore, the sum of the silicon and aluminum contents should be controlled to be 0.8-3.0.

In the component design of the steel, the high carbon content and the high silicon content are adopted, the high carbon content is favorable for obtaining high strength, and more available carbon atoms are diffused into the residual austenite, so that the residual austenite is stabilized. The main purpose of adding higher silicon content is to inhibit carbide formation and obtain carbide-free bainite; and the content of C+Mn is more than or equal to 1.0 and less than or equal to 3.0,0.8, the content of Si+Al is more than or equal to 3.0, and the manganese can further improve the stability of the residual austenite and refine the structure; aluminum can promote the diffusion of carbon atoms into residual austenite, so that the steel plate can obtain good matching of high tensile strength and ultrahigh plasticity, and meanwhile, the generation of cracks at high temperature or low temperature can be prevented.

The steel of the invention mainly adopts C, si, mn, al which are relatively cheap elements and does not contain noble metal elements, thereby greatly reducing the alloy cost.

The manufacturing method of the hot rolled high-strength high-plasticity steel comprises the following steps:

1) Smelting and casting

Smelting in a converter or an electric furnace according to the components, secondarily refining in a vacuum furnace, and casting into casting blanks or cast ingots;

2) Reheat of

The reheating temperature of the casting blank or the cast ingot is more than or equal to 1100 ℃, and the heat preservation time is as follows: 1 to 2 hours;

3) Hot rolling

Start rolling temperature: 1050-1150 ℃, 3-5 times of reduction at the temperature of over 1050 ℃ and accumulated deformation of more than or equal to 50%; then the intermediate blank is heated to 950-1000 ℃, and then is rolled for 3-5 passes and the accumulated deformation is more than or equal to 70%; the finishing temperature is between 800 and 950 ℃;

4) Heat treatment of

After finishing the finish rolling, cooling the steel plate to 400-550 ℃ at a cooling rate of more than or equal to 30 ℃/s, and then coiling at 400-550 ℃ and cooling to room temperature; the obtained steel coil is covered and retreated at 100-400 ℃ for 4-48 h; or alternatively, the first and second heat exchangers may be,

after finishing the finish rolling, the steel plate is cooled to 400-550 ℃ at a cooling rate of more than or equal to 30 ℃/s, and then tempered at 100-400 ℃ for 1-240 min.

The steel provided by the invention adopts higher carbon content and silicon content in component design, combines with other alloy elements, and is matched with hot rolling coiling, covering annealing or tempering process to obtain bainite and stable residual austenite microstructure with higher content.

On the basis of component design, the invention adopts a hot rolling process, controls the reheating temperature of casting blank or cast ingot to be more than or equal to 1100 ℃, and maintains the temperature for a period of time: 1-2 hours, the initial rolling temperature: 1050-1150 ℃, 3-5 times of high pressure above 1050 ℃ and accumulated deformation not less than 50%, and the main purpose is to refine austenite grains; then the intermediate blank is heated to 950-1000 ℃, and then is rolled for 3-7 passes and the accumulated deformation is more than or equal to 70%; after finishing the finish rolling at 800-950 ℃, water-cooling the steel plate to 400-550 ℃ at a cooling rate of more than or equal to 30 ℃/s, and cooling to room temperature after coiling to obtain bainite and residual austenite structures, wherein the residual austenite is not stable enough, so that the stability of the residual austenite needs to be improved, the content of the residual austenite is stabilized, and the stability of the residual austenite is further improved by utilizing a subsequent cover annealing or tempering process.

In the annealing or tempering process, carbon atoms are further diffused from the bainite into the residual austenite, meanwhile, silicon can inhibit carbon atoms from forming cementite in the process, a certain amount of aluminum element can promote carbon diffusion into the residual austenite, all components and processes are matched, and finally, the lean-carbon bainite and a more stable rich-carbon residual austenite structure are obtained. Thus, the steel grades obtained have a low yield strength, high tensile strength and ultra-high elongation match.

When the annealing temperature is lower than 100 ℃, the diffusion of carbon atoms from bainite to residual austenite can be completed in a long time, and the production period is too long; when the shield annealing temperature is higher than 400 ℃, more residual austenite is decomposed after long-time shield annealing, and the stability of the residual austenite is reduced. Therefore, the cover annealing temperature is controlled between 100 ℃ and 400 ℃, and the cover annealing time is adjusted from 4 hours to 48 hours according to the actual conditions according to the difference of the cover annealing temperature and the difference of the number of the steel coils subjected to the cover annealing treatment.

When the obtained steel is a steel plate, tempering heat treatment is adopted, the tempering temperature is 100-400 ℃, the tempering time can be greatly shortened, and the heat preservation time is adjusted from 1-240 min according to different heat treatment temperatures.

The invention obtains the steel with high tensile strength and ultra-high elongation through the precise matching of the components and the process, and can be widely applied to parts with complex shape requirements such as commercial vehicles or passenger vehicles or other parts needing high strength thinning.

The invention has the beneficial effects that:

1. the steel of the invention adopts higher carbon content and silicon content in the component design, and the content of C, mn, si, al is controlled to simultaneously meet the following requirements: C+Mn is more than or equal to 1.0 and less than or equal to 3.0,0.8, si+Al is more than or equal to 3.0, carbide formation is inhibited, carbide-free bainite is obtained, meanwhile, the effect of stabilizing residual austenite is also achieved, steel with microstructure of bainite and residual austenite is obtained, the steel plate is enabled to obtain good matching of high tensile strength and ultrahigh plasticity, and generation of cracks at high temperature or low temperature is prevented.

2. The traditional high-strength high-plasticity steel is obtained by adopting a cold rolling and continuous annealing process, and even if the preparation of the substrate involves hot rolling, the subsequent process of cold rolling and continuous annealing is required to be carried out again to obtain the high-strength high-plasticity steel.

Based on the component design, the hot rolling process is adopted for the first time, namely a brand-new hot rolling, medium temperature coiling, cover annealing or tempering preparation process is adopted, the bainite and residual austenite structure is obtained in the hot rolling stage, and the subsequent cover annealing or tempering process is combined to promote the diffusion of carbon atoms into the residual austenite, so that the stability of the residual austenite is improved, the amount of the residual austenite is stabilized, and the hot rolled steel with higher strength and ultrahigh plasticity is obtained.

3. The steel manufactured by the method has higher tensile strength and ultrahigh plasticity, also has lower yield strength, can avoid cracking in the part stamping process, improves the forming performance of the steel, is particularly suitable for manufacturing various complex parts of passenger vehicles or commercial vehicles, and has good application prospect.

4. The alloy composition cost of the high-strength high-plasticity steel is lower, the C, si, mn, al relatively cheap elements are mainly adopted, and the high-strength high-plasticity steel does not contain noble metal elements, so that the alloy cost is greatly reduced.

Drawings

FIG. 1 is a typical SEM photograph of steel obtained in example 1 of the present invention.

FIG. 2 is a typical SEM photograph of steel obtained in example 3 of the present invention.

FIG. 3 is a typical SEM photograph of steel obtained in example 5 of the present invention.

FIG. 4 is a typical SEM photograph of steel obtained in example 9 of the present invention.

FIG. 5 is a typical SEM photograph of steel obtained in example 11 of the present invention.

FIG. 6 is a typical SEM photograph of steel obtained in example 15 of the present invention.

FIG. 7 is a typical SEM photograph of steel obtained in example 18 of the invention.

FIG. 8 is a typical SEM photograph of steel obtained in example 22 of the invention.

FIG. 9 is a typical SEM photograph of steel obtained in example 24 of the invention.

FIG. 10 is a typical SEM photograph of steel obtained in example 25 of the invention.

FIG. 11 is a typical SEM photograph of steel obtained in example 28 of the present invention.

FIG. 12 is a typical SEM photograph of steel obtained in example 30 of the invention.

Detailed Description

The invention will be further explained and illustrated with reference to specific embodiments and drawings, which, however, do not constitute an undue limitation on the technical solutions of the invention.

Table 1 shows the mass percentages of the steel components 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.

FIGS. 1 to 3 show typical SEM pictures of steel sheets of examples 1 and 3 after heat treatment, respectively; FIGS. 4 to 6 show typical SEM pictures of steel sheets of examples 9 and 11 after heat treatment, respectively; FIGS. 7 to 9 show typical SEM pictures of steel sheets of examples 18 and 22 after heat treatment, respectively; fig. 10 to 12 show typical SEM photographs of steel sheets of examples 25 and 28 after heat treatment of the steel sheets of example 30, respectively. As can be seen from the figure, the microstructure of the steel obtained by adopting the composition and the process path designed by the invention is bainite+retained austenite.

As can be seen from Table 3, the hot rolled high strength high plasticity steel obtained by the invention has good strength and plasticity matching, is particularly suitable for preparing parts requiring complex forming such as automobile chassis structures, and has wide application prospect.

It should also be noted that the above-recited embodiments are merely specific examples of the present invention. It is apparent that the present invention is not limited to the above embodiments, and similar changes or modifications will be apparent to those skilled in the art from the present disclosure, and it is intended to be within the scope of the present invention.

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Claims

1. The hot-rolled high-strength high-plasticity steel comprises the following chemical components in percentage by mass: c:0.10 to 0.20 percent, si:0.8 to 2.0 percent, mn:1.0 to 2.5 percent, P is less than or equal to 0.02 percent, S is less than or equal to 0.003 percent, al:0.01 to 1.5 percent, N is less than or equal to 0.004 percent, and the balance is Fe and other unavoidable impurities, and the requirements are simultaneously satisfied: C+Mn is more than or equal to 1.0 and less than or equal to 3.0,0.8, si+Al is more than or equal to 3.0;

the microstructure of the steel is bainite and residual austenite, wherein the content of bainite is 85-90%, and the content of residual austenite is 10-15%;

the yield strength of the steel is more than or equal to 300MPa, the tensile strength is more than or equal to 590MPa, and the elongation is more than or equal to 35%.

2. The hot-rolled high-strength high-plasticity steel comprises the following chemical components in percentage by mass: c:0.15 to 0.25 percent, si:0.8 to 2.0 percent, mn:1.0 to 2.5 percent, P is less than or equal to 0.02 percent, S is less than or equal to 0.003 percent, al:0.01 to 1.5 percent, N is less than or equal to 0.004 percent, and the balance is Fe and other unavoidable impurities, and the requirements are simultaneously satisfied: C+Mn is more than or equal to 1.0 and less than or equal to 3.0,0.8, si+Al is more than or equal to 3.0;

the microstructure of the steel is bainite and residual austenite, wherein the content of bainite is 80-85%, and the content of residual austenite is 15-20%;

the yield strength of the steel is more than or equal to 400MPa, the tensile strength is more than or equal to 780MPa, and the elongation is more than or equal to 30%.

3. The hot-rolled high-strength high-plasticity steel comprises the following chemical components in percentage by mass: c:0.20 to 0.30 percent, si:0.8 to 2.0 percent, mn:1.0 to 2.5 percent, P is less than or equal to 0.02 percent, S is less than or equal to 0.003 percent, al:0.01 to 1.5 percent, N is less than or equal to 0.004 percent, and the balance is Fe and other unavoidable impurities, and the requirements are simultaneously satisfied: C+Mn is more than or equal to 1.0 and less than or equal to 3.0,0.8, si+Al is more than or equal to 3.0;

the microstructure of the steel is bainite and residual austenite, wherein the content of bainite is 75-80%, and the content of residual austenite is 20-25%;

the yield strength of the steel is more than or equal to 500MPa, the tensile strength is more than or equal to 980MPa, and the elongation is more than or equal to 25%.

4. The hot-rolled high-strength high-plasticity steel comprises the following chemical components in percentage by mass: c:0.25 to 0.35 percent, si:0.8 to 2.0 percent, mn:1.0 to 2.5 percent, P is less than or equal to 0.02 percent, S is less than or equal to 0.003 percent, al:0.01 to 1.5 percent, N is less than or equal to 0.004 percent, and the balance is Fe and other unavoidable impurities, and the requirements are simultaneously satisfied: C+Mn is more than or equal to 1.0 and less than or equal to 3.0,0.8, si+Al is more than or equal to 3.0;

the microstructure of the steel is bainite and residual austenite, wherein the content of bainite is 70-75%, and the content of residual austenite is 25-30%;

the yield strength of the steel is more than or equal to 600MPa, the tensile strength is more than or equal to 1180MPa, and the elongation is more than or equal to 20%.

5. The hot rolled high strength and high plasticity steel as claimed in any one of claims 1 to 4, wherein the steel further comprises one or more of Mo, nb, V, ti, cu, ni, cr, B elements, and wherein Mo is not more than 0.5%, nb is not more than 0.06%, V is not more than 0.10%, ti is not more than 0.15%, cu is not more than 0.5%, ni is not more than 0.5%, cr is not more than 0.5%, and B is not more than 0.001%.

6. The method for manufacturing a hot rolled high strength high plasticity steel as claimed in claims 1 to 5, comprising the steps of:

1) Smelting and casting

Smelting in a converter or an electric furnace, secondarily refining in a vacuum furnace, and casting into casting blanks or cast ingots according to the components;

2) Reheat of

The reheating temperature of the casting blank or the cast ingot is more than or equal to 1100 ℃, and the heat preservation time is as follows: 1-2 h;

3) Hot rolling

Start rolling temperature: 1050-1150 ℃, 3-5 times of reduction at the temperature of over 1050 ℃ and accumulated deformation of more than or equal to 50%; then the intermediate blank is heated to 950-1000 ℃, then the intermediate blank is rolled for 3-5 last passes, the accumulated deformation is more than or equal to 70%, and the final rolling temperature is 800-950 ℃;

4) Heat treatment of