CN115181897B

CN115181897B - 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel and rapid heat treatment manufacturing method

Info

Publication number: CN115181897B
Application number: CN202110360536.4A
Authority: CN
Inventors: 王健; 李俊; 杜小峰; 张宝平
Original assignee: Baoshan Iron and Steel Co Ltd
Current assignee: Baoshan Iron and Steel Co Ltd
Priority date: 2021-04-02
Filing date: 2021-04-02
Publication date: 2023-07-11
Anticipated expiration: 2041-04-02
Also published as: CN115181897A

Abstract

1280MPa low-carbon low-alloy ultra-high strength dual-phase steel and a rapid heat treatment manufacturing method, wherein the steel comprises the following chemical components in percentage by mass: 0.10 to 0.17 percent of C, 0.2 to 0.7 percent of Si, 1.8 to 2.8 percent of Mn, 0.3 to 0.9 percent of Cr, 0.02 to 0.07 percent of Nb, 0.02 to 0.07 percent of Ti, 0.002 to 0.005 percent of B, 0.02 to 0.05 percent of Al, less than or equal to 0.02 percent of P, less than or equal to 0.005 percent of S, one or two of Mo and V, less than or equal to 1.1 percent of Cr+Mo+Ti+Nb+V, and the balance of Fe and other unavoidable impurities. The manufacturing method comprises the following steps: smelting, casting, hot rolling, cold rolling and rapid heat treatment; the whole process of the rapid heat treatment is 41-297 s. According to the invention, by controlling the rapid heating, short-time and rapid cooling processes in the rapid heat treatment process, the recovery, recrystallization and austenite transformation processes of a deformed structure are changed, the nucleation rate (including the recrystallization nucleation rate and the austenite transformation nucleation rate) is increased, the grain growth time is shortened, the grains are refined, the strength of the material is improved, and the material performance interval range is expanded.

Description

1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel and rapid heat treatment manufacturing method

Technical Field

The invention belongs to the technical field of rapid heat treatment of materials, and particularly relates to 1280 MPa-level low-carbon low-alloy ultra-high-strength dual-phase steel and a rapid heat treatment manufacturing method.

Background

Along with the gradual improvement of people on energy conservation and material service safety consciousness, a plurality of automobile manufacturers select high-strength steel as automobile materials, and the automobile industry adopts the high-strength steel plate to reduce the thickness of the steel plate and simultaneously can improve the dent resistance, the durable strength, the large deformation impact toughness and the collision safety of the automobile, so that the automobile steel plate is bound to develop towards the directions of high strength, high toughness and easy molding.

Among high-strength steels for automobiles, dual-phase steels are most widely used and have the best application prospect. The low-carbon low-alloy dual-phase steel has the characteristics of small yield ratio, high initial work hardening rate, good strength and plasticity matching property and the like, and becomes the automobile structure stamping steel with high strength and good formability which is widely used at present.

The traditional dual-phase steel is obtained by rapid cooling treatment or hot rolling controlled cooling after soaking and annealing cold-rolled low-carbon steel or low-alloy high-strength steel in critical areas, and the microstructure of the dual-phase steel mainly comprises ferrite and martensite. The dual phase steel utilizes the principle of "composite materials" to allow the advantages of the phases (ferrite and martensite) in the steel to be exploited as much as possible while alleviating or eliminating the disadvantages or drawbacks of one phase due to the presence of the other phases.

The mechanical properties of dual phase steels are mainly dependent on the following three aspects:

1. grain size of the matrix phase and alloy element distribution;

2. the size, shape, distribution, and volume fraction of the second phase;

3. the combination of the matrix and the second phase.

Therefore, how to obtain a low-cost high-performance dual-phase steel product with good strong plastic matching becomes a target pursued by various large steel enterprises, and the dual-phase steel product is widely focused by steel enterprises and automobile users.

The cold-rolled dual-phase steel is obtained by a rapid cooling treatment process after soaking at critical area temperature, and the process mainly comprises the following three steps:

the first step: heating the strip steel to the temperature of a ferrite and austenite two-phase critical region for soaking and heat preservation;

and a second step of: cooling the sample to M at a cooling rate higher than the critical cooling rate _s ～M _f A certain temperature in the two-phase structure of martensite and ferrite is obtained;

and a third step of: heat-insulating or heating the strip steel to not higher than M _s And (3) carrying out tempering treatment at the temperature of (2) to obtain good structure matching of hard phase martensite and soft phase ferrite, and finally obtaining a dual-phase structure of the martensite and the ferrite.

At present, 1280 MPa-grade cold-rolled dual-phase steel produced by a traditional continuous annealing mode has relatively long heating time and soaking time due to slow heating rate, and the whole continuous annealing period needs 5-8min; the recovery, recrystallization and transformation processes in the heating process are respectively and sequentially carried out, and the overlapping phenomenon is generally avoided, so that ferrite recrystallized grains and austenite grains are respectively nucleated and fully grown, and finally the obtained ferrite and martensite two-phase grain structure size is relatively large, and is generally about 3-10 mu m.

The main regulation means for the dual-phase steel in the prior art is to change the phase structure proportion and distribution of the dual-phase steel by adding alloy elements and adjusting the temperature and time of quenching and tempering processes in an annealing process, so as to obtain relatively optimized product performance.

Chinese patent CN102021483B discloses a "cold rolled dual-phase steel sheet with 1200MPa level tensile strength and preparation method", the high strength dual-phase steel composition of the invention comprises the following components in weight percentage: c:0.19 to 0.21 percent, si:0.7 to 0.9 percent, mn:1.9 to 2.1 percent, cr:0.01 to 0.02 percent, nb: 0.02-0.04%, P is less than or equal to 0.005%, S is less than or equal to 0.003%, and the balance is Fe and other unavoidable impurity elements. The patent is based on a conventional continuous annealing process: the heating rate is 10 ℃/s, the annealing temperature is 820-860 ℃, the heat preservation time is 100-200 s, the slow cooling rate is 5 ℃/s, the rapid cooling starting temperature is 710 ℃, the quenching speed is 45-65 ℃/s, the overaging temperature is 240-320 ℃, and the overaging time is 200-260 s; the obtained yield strength ranges from 580 to 660MPa, the tensile strength ranges from 1200 to 1205MPa, and the elongation is 8 to 10 percent. The welding strength is mainly characterized in that the strength is improved by adopting higher C content, which is unfavorable for the subsequent welding use of users, and meanwhile, the tensile strength can only reach about 1200MPa, and the welding strength does not meet the requirement of 1280MPa products.

Chinese patent CN102471821B discloses "a method for manufacturing a cold-rolled steel sheet excellent in press formability and a cold-rolled steel sheet", the inventive steel comprises the following chemical components in weight percent: c:0.1 to 0.3 percent, mn is more than or equal to 0.2 percent, ni is more than or equal to 0.01 percent, mn+Ni:0.5 to 2.5 percent, cr:1.2 to 9 percent of Fe and other unavoidable impurities. The patent is mainly characterized in that the high-C and high-Cr alloy is adopted based on the traditional continuous annealing process so as to improve the strength and the hardenability, and the excessively high alloy element can lead to the reduction of the welding performance, so that the risk is brought to the subsequent use, and meanwhile, the difficulty is increased for each manufacturing process of steelmaking, hot rolling and cold rolling.

Chinese patent CN105543674B discloses a method for manufacturing cold-rolled ultra-high strength dual-phase steel with high local formability, and the high strength dual-phase steel of the invention comprises the following components in percentage by weight: c:0.08 to 0.12 percent of Si:0.1 to 0.5 percent of Mn:1.5 to 2.5 percent of Al:0.015 to 0.05 percent, and the balance of Fe and other unavoidable impurities. Selecting raw materials for the chemical components, and smelting the raw materials into casting blanks; heating the casting blank at 1150-1250 ℃ for 1.5-2 hours, and then carrying out hot rolling, wherein the hot rolling start temperature is 1080-1150 ℃, and the final rolling temperature is 880-930 ℃; cooling to 450-620 ℃ at a cooling speed of 50-200 ℃/s after rolling, and coiling to obtain a hot rolled steel plate taking bainite as a main structure type; and (3) cold rolling the hot-rolled steel plate, heating to 740-820 ℃ at a speed of 50-300 ℃/s for annealing, keeping the temperature for 30-3 min, cooling to 620-680 ℃ at a cooling speed of 2-6 ℃/s, and then performing overaging treatment for 3-5 min at a cooling speed of 30-100 ℃/s to 250-350 ℃ to obtain the ferrite-martensite dual-phase structure ultra-high strength dual-phase steel. The yield strength of the ultra-high strength dual-phase steel is 650-680 MPa, the tensile strength is 1023-1100 MPa, the elongation is 12.3-13%, and the steel is not cracked when being bent at 180 degrees along the rolling direction.

The main characteristic of the patent is that the control of cooling condition after hot rolling is combined with the rapid heating in the continuous annealing process, namely, the strip-shaped structure is eliminated by controlling the cooling process after hot rolling, so as to realize the homogenization of the structure; and rapid heating is adopted in the subsequent continuous annealing process, so that the tissue refinement is realized on the basis of ensuring the tissue uniformity. The patent technology can be seen to adopt rapid heating annealing, and the premise is that hot rolling is carried out to obtain a hot rolling raw material taking bainite as a main structure, so that the aim is to ensure the uniformity of the structure and avoid uneven local deformation caused by the occurrence of a strip-shaped structure.

The disadvantages of this patent are mainly:

firstly, to obtain a hot rolled raw material with a bainite structure, the hot rolled raw material has high strength and large deformation resistance, and great difficulty is brought to subsequent pickling and cold rolling production;

secondly, the understanding of the rapid heating is limited to shortening the heating time and refining the grain level, the heating rate is not divided according to the material organization structure change of different temperature sections, and the rapid heating is carried out at the speed of 50-300 ℃/s, so that the rapid heating production cost is increased;

thirdly, the soaking time is 30s-3min, and the effect of refining grains generated by rapid heating is necessarily partially weakened by increasing the soaking time, so that the strength and toughness of the material are improved disadvantageously;

Fourth, the process must be over-aged for 3-5 minutes, which is in fact too long for rapid heat treatment of DP steel, and is not necessary. The increase of soaking time and overaging time is not beneficial to saving energy, reducing the investment of unit equipment and the occupied area of the unit, and is not beneficial to the high-speed stable operation of strip steel in a furnace, and obviously, the method is not a rapid heat treatment process in a strict sense.

Chinese patent 201711385126.5 discloses 780 MPa-level low-carbon low-alloy TRIP steel, which comprises the following chemical components in percentage by mass: c:0.16-0.22%, si:1.2-1.6%, mn:1.6-2.2%, the balance being Fe and other unavoidable impurities, obtained by a rapid thermal treatment process comprising: the strip steel is rapidly heated to an austenite and ferrite two-phase region at the temperature of 790-830 ℃ from room temperature, and the heating rate is 40-300 ℃/s; the residence time of the heating target temperature interval in the two-phase region is 60 to 100s; the strip steel is rapidly cooled to 410-430 ℃ from the temperature of the two-phase region, the cooling speed is 40-100 ℃/s, and the strip steel stays for 200-300 s in the temperature region; the strip steel is rapidly cooled from 410 to 430 ℃ to room temperature. The method is characterized in that: the TRIP steel metallographic structure is a bainitic, ferritic and austenitic three-phase structure; the average grain size of the TRIP steel is obviously refined; tensile strength is 950-1050 MPa; the elongation rate is 21-24%; the maximum product of strong plastic can reach 24GPa percent.

The deficiencies of this patent are mainly the following:

firstly, the patent discloses a 780 MPa-level low-carbon low-alloy TRIP steel product and a process technology thereof, but the tensile strength of the TRIP steel product is 950-1050 MPa, the tensile strength of the TRIP steel product is 780 MPa-level, the use effect of a user cannot be good, the tensile strength of the TRIP steel product is 980 MPa-level, and the strength requirement of the user cannot be well met;

secondly, the patent adopts one-section rapid heating, the same rapid heating rate is adopted in the whole heating temperature interval, and the rapid heating is carried out at a speed of 40-300 ℃/s without distinguishing treatment according to the material tissue structure change of different temperature sections, so that the production cost of the rapid heating process is necessarily increased;

thirdly, the soaking time of the patent is set to be 60-100 s, which is similar to that of the traditional continuous annealing, and the increase of the soaking time necessarily partially weakens the grain refining effect generated by rapid heating, thus being very unfavorable for improving the strength and toughness of the material;

fourth, the patent must be subjected to a bainite isothermal treatment time of 200 to 300 seconds, which is practically too long for a rapid thermal treatment product to function as intended, and is unnecessary. The increase of soaking time and isothermal treatment time is not beneficial to saving energy, reducing the investment of unit equipment and the occupied area of the unit, and is not beneficial to the high-speed stable operation of strip steel in a furnace, and obviously, the method is not a rapid heat treatment process in a strict sense.

Chinese patent CN108774681a discloses a "rapid heat treatment method for high-strength steel", which adopts a ceramic electric heating device, and can obtain a heating rate up to 400 ℃/s, and after heating to 1000-1200 ℃, adopts a blower fan for blowing and cooling, and the cooling speed is nearly 3000 ℃/s. In the method, a ceramic plate electric heating heat treatment device is adopted, and the treatment speed is 50cm/min. The inventive steel is characterized in that the carbon content is up to 0.16-0.55%, and simultaneously contains: si, mn, cr, mo and other alloying elements; the method is mainly suitable for steel wires, wire rods or steel belts with the diameter below 5 mm.

This patent describes a rapid thermal process by electrical heating of ceramic sheets; the invention mainly aims to solve the problems of low heat treatment efficiency, energy waste and environmental pollution of products such as high-strength steel wires, wire rods and the like; the effect and effect of rapid heating on the material tissue properties are not mentioned; the invention does not combine the components and the tissue characteristics of the grade of steel, adopts a blower blowing cooling mode, and the fastest cooling speed approaches 3000 ℃/s and is the instantaneous cooling speed of the high-temperature section, and the average cooling speed is less than 3000 ℃/s; meanwhile, the high-temperature section adopts too high cooling speed to produce the wide and thin strip steel, so that the problems of too high internal stress, poor plate shape of the steel plate and the like are caused, and the method is not suitable for large-scale industrialized continuous heat treatment production of the wide and thin steel plate.

Chinese patent CN106811698B discloses "a high-strength steel sheet based on fine control of structure and a manufacturing method thereof", the high-strength dual-phase steel composition comprises the following components in weight percentage: c:0.08 to 0.40 percent of Si:0.35 to 3.5 percent of Mn:1.5 to 7.0 percent, P is less than or equal to 0.02 percent, S is less than or equal to 0.02 percent, al:0.02 to 3.0 percent, and also contains Cr:0.50 to 1.5 percent of Mo:0.25 to 0.60 percent of Ni:0.5 to 2.5 percent of Cu:0.20 to 0.50 percent, B:0.001 to 0.005 percent, V:0.10 to 0.5 percent of Ti:0.02 to 0.20 percent of Nb:0.02 to 0.20 percent of at least one of Fe and other unavoidable impurities. The mechanical properties are as follows: tensile strength R _m Elongation A of more than 1000MPa _50mm Greater than 28%. The components C, si and Mn of the invention are relatively high, soaking-free annealing is carried out on the traditional continuous annealing production line, and recrystallization annealing is carried out on the steel strips with different components in a mode of removing soaking heat preservation sections. The specific annealing parameter ranges are as follows: rapidly heating to 800-930 ℃ at a speed of more than 20 ℃/s, and immediately cooling at a cooling speed of more than 40 ℃/sTo M _s -M _f Spot and then reheat to M _f ～M _f The temperature of +100 ℃ is kept for 30s to 30min, and finally cooled to room temperature.

The invention is mainly characterized in that a fine martensite structure with fine needle shapes and short rod shapes is obtained by controlling the shape and the structure of martensite Gao Jiangxiang, C atoms are diffused into residual austenite by reheating, and finally, more stable residual austenite is obtained, so that the residual austenite has certain deformability, and the plasticity and the toughness of high-strength steel are improved.

The rapid heating of the invention has lower heating rate, the heating rate is 20-60 ℃/s, the rapid heating belongs to medium heating rate, and the cooling rate is 40-100 ℃/s. The consideration of rapid heating, rapid cooling and omission of a soaking section is to shorten the residence time of high-strength steel in a high-temperature section, ensure that crystal grains of the steel are tiny in an austenitizing process and the structure and chemical components are not completely homogenized, further ensure that a large amount of large-size lath-shaped martensite is not generated after cooling, and simultaneously obtain a certain amount of membranous residual austenite structure. However, this necessarily results in a heating temperature that is difficult to control and increases in fluctuations in the structure and performance.

The method is still based on the heating technology and the cooling technology of the traditional continuous annealing unit, and the high-strength steel product with certain toughness matching is finally obtained by omitting a soaking section (shortening soaking time to 0), increasing alloy content and quenching and tempering treatment, and the invention does not carry out specific refinement research and development on the grade of steel of each strength grade. And the heating rate belongs to a medium heating rate, does not belong to rapid heating and does not have soaking time, so that a rapid heat treatment method and a complete annealing cycle in a real sense cannot be embodied, and the method has no prospect of commercial application.

Chinese patent CN107794357B and US patent US2019/0153558A1 disclose "a method for producing an ultra-high strength martensitic cold-rolled steel sheet by an ultra-rapid heating process", wherein the high strength dual-phase steel composition comprises, in weight percent: c:0.10 to 0.30 percent of Mn:0.5 to 2.5 percent of Si:0.05 to 0.3 percent of Mo:0.05 to 0.3 percent of Ti:0.01 to 0.04 percent of Cr:0.10 to 0.3% and B:0.001 to 0.004 percent, P: less than or equal to 0.02 percent, S: less than or equal to 0.02 percent, and the balance of Fe and other unavoidable impurities. Mechanical properties of the dual-phase steel: yield strength Rp _0.2 More than 1100MPa, tensile strength R _m =1800-2300 MPa, elongation is maximum 12.3%, uniform elongation is 5.5-6%. The invention provides an ultra-fast heating production process of an ultra-high strength martensitic cold-rolled steel plate, which is characterized in that firstly, the cold-rolled steel plate is heated to 300-500 ℃ at a speed of 1-10 ℃/s, and then is reheated to a single-phase austenite region of 850-950 ℃ at a heating speed of 100-500 ℃/s; and then, immediately cooling the steel plate to room temperature after heat preservation is not more than 5 seconds, and obtaining the ultra-high strength cold-rolled steel plate.

The disadvantages of the process described in this patent include:

firstly, the annealing temperature of the steel of the invention is in the ultra-high temperature range of an austenite single-phase region, and the steel also contains more alloy elements, and the yield strength and the tensile strength are both more than 1000MPa, so that great difficulties are brought to the heat treatment of the process, the manufacture of the working procedure before the heat treatment and the subsequent use of users.

Secondly, the ultra-rapid heating and annealing method disclosed by the invention adopts the heat preservation time of not more than 5 seconds, so that not only is the controllability of the heating temperature poor, but also the uneven distribution of alloy elements in a final product is caused, and the uneven and unstable structure performance of the product is caused;

thirdly, the final quick cooling adopts water quenching to cool to room temperature, and necessary tempering treatment is not carried out, so that the obtained final product has the structure property and the alloy element distribution condition in the final structure, the product cannot obtain the optimal toughness, the final product has excessive strength and insufficient plasticity and toughness;

fourth, the method of the invention causes problems of poor plate shape and surface oxidation of the steel plate due to the excessively high water quenching speed, so that the technology of the patent has no or little practical application value.

The prior researches on cold-rolled dual-phase steel products and annealing processes are limited by the equipment capacity of the traditional continuous annealing furnace production line, and the related researches are based on the heating rate (5-20 ℃/s) of the prior industrial equipment to slowly heat the strip steel, so that the strip steel is sequentially subjected to reversion, recrystallization and austenitizing phase transformation, the heating and soaking time is relatively long, the energy consumption is high, and meanwhile, the traditional continuous annealing production line also has the problems of long stay time of the strip steel in a high-temperature furnace section, more number of passing rollers and the like. The conventional continuous annealing unit generally has soaking time of 1-3 min according to the product outline and the productivity requirement, and the number of rollers in a high-temperature furnace section of the conventional production line with the unit speed of about 180 meters/min is generally 20-40 different, so that the difficulty in controlling the surface quality of strip steel is increased.

Disclosure of Invention

The invention aims to provide 1280 MPa-level low-carbon low-alloy ultrahigh-strength dual-phase steel and a rapid heat treatment manufacturing method, which change the recovery of deformed structures, recrystallization grain growth, austenite phase transformation process, grain growth and other processes through rapid heat treatment, increase nucleation rate (comprising recrystallization nucleation rate and austenite phase transformation nucleation rate), shorten grain growth time, refine grains, and then form ferrite matrix phase in the cooling process, and generate various strengthening phases and structures and component gradient distribution in the phases, so that the yield strength of the dual-phase steel is 902-1114 MPa, the tensile strength is 1264-1443 MPa, the elongation is 7-9.8%, and the strength-plastic product is 9.5-12.1 GPa; the dual-phase steel obtained by the method has lower alloy content in the steel with the same level, and good plasticity and toughness are obtained while the material strength is improved; meanwhile, the rapid heat treatment process is adopted to improve the production efficiency, reduce the production cost and the energy consumption, obviously reduce the number of furnace rollers and improve the surface quality of the steel plate.

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

1280MPa grade low-carbon low-alloy ultra-high strength dual-phase steel comprises the following chemical components in percentage by mass: c:0.10 to 0.17 percent, si:0.2 to 0.7 percent, mn:1.8 to 2.8 percent, cr:0.3 to 0.9 percent, nb:0.02 to 0.07 percent, ti:0.02 to 0.07 percent, B: 0.002-0.005%, P is less than or equal to 0.02%, S is less than or equal to 0.005%, al:0.02 to 0.05 percent; one or two of Mo and V, cr+Mo+Ti+Nb+V less than or equal to 1.1 percent, and the balance of Fe and other unavoidable impurities, and is obtained by the following process:

1) Smelting and casting

Smelting and casting the steel into a slab according to the chemical components;

2) Hot rolling and coiling

The coiling temperature is 550-680 ℃;

3) Cold rolling

The cold rolling reduction rate is 40-85%;

4) Rapid thermal treatment

The cold rolled steel plate is quickly heated to 750-845 ℃, and the quick heating adopts one-section or two-section; when the one-stage rapid heating is adopted, the heating rate is 50-500 ℃/s; when two-section rapid heating is adopted, the first section is heated to 550-650 ℃ from room temperature at a heating rate of 15-500 ℃/s, and the second section is heated to 750-845 ℃ from 550-650 ℃ at a heating rate of 50-500 ℃/s; soaking at 750-845 deg.c for 10-60 s;

slowly cooling to 670-770 ℃ at a cooling rate of 5-15 ℃/s after soaking, and then rapidly cooling to room temperature from 670-770 ℃ at a cooling rate of 50-200 ℃/s;

or rapidly cooling from 670-770 ℃ to 230-280 ℃ at a cooling rate of 50-200 ℃/s, and performing overaging treatment in the temperature range, wherein the overaging treatment time is as follows: and (5) cooling to room temperature after 200s or less.

Preferably, the C content is 0.055 to 0.110%.

Preferably, the Si content is 0.15 to 0.45%.

Preferably, the Mn content is 1.6 to 2.0%.

Preferably, the Cr content is 0.5 to 0.7%.

Preferably, the Ti content is 0.02 to 0.05%.

Preferably, the Nb content is 0.02 to 0.05%.

Preferably, the whole rapid thermal processing process is 41-297 s.

Preferably, in step 2), the hot rolling temperature is not less than A _r3 。

Preferably, in the step 2), the winding temperature is 580 to 650 ℃.

Preferably, in step 3), the cold rolling reduction is 60 to 80%.

Preferably, in the step 4), the heating rate is 50-300 ℃/s when the rapid heating is performed by adopting one-stage heating.

Preferably, in step 4), the rapid heating is performed by two-stage heating: the first section is heated from room temperature to 550-650 ℃ at a heating rate of 15-300 ℃/s; the second section is heated from 550 to 650 ℃ to 750 to 845 ℃ at a heating rate of 50 to 300 ℃/s.

Preferably, in step 4), the rapid heating is performed by two-stage heating: the first section is heated from room temperature to 550-650 ℃ at a heating rate of 50-300 ℃/s; the second section is heated from 550 to 650 ℃ to 750 to 845 ℃ at a heating rate of 80 to 300 ℃/s.

Preferably, in step 4), the soaking time is 10 to 40 seconds.

Preferably, in step 4), the rapid cooling rate is 50 to 150 ℃/s.

The microstructure of the dual-phase steel is a ferrite and martensite dual-phase structure with evenly distributed average grain size of 1-3 mu m.

The yield strength of the dual-phase steel is 902-1114 MPa, the tensile strength is 1264-1443 MPa, the elongation is 7-9.8%, and the product of strength and elongation is 9.5-12.1 GPa%.

In the composition and process design of the steel of the invention:

c: carbon is the most common strengthening element in steel, and increases the strength and decreases the plasticity of steel, but for cold-stamped steel sheets, low yield strength, high uniform elongation and high total elongation are required, so the carbon content is not necessarily too high. There are generally two ways in which carbon phases in steel exist: ferrite and cementite. The carbon content has great influence on the mechanical properties of the steel, the number of reinforcing phases such as pearlite and the like can be increased along with the increase of the carbon content, so that the strength and the hardness of the steel are greatly improved, but the plasticity and the toughness of the steel can be obviously reduced, if the carbon content is too high, obvious netlike carbide can appear in the steel, the existence of the netlike carbide can obviously reduce the strength, the plasticity and the toughness of the steel, the reinforcing effect generated by the increase of the carbon content in the steel can be obviously weakened, the technological properties of the steel are worsened, and the carbon content is reduced as much as possible on the premise of ensuring the strength.

For dual phase steels, the carbon element mainly affects the volume fraction of austenite formed during annealing, and during the formation of austenite, the diffusion process of the carbon element in the austenite or ferrite actually plays a role in controlling the growth of austenite grains. As the carbon content or the heating temperature of the critical region increases, the volume fraction of austenite increases, and the martensite phase structure formed after cooling increases, and the strength of the material increases, but at the same time the plasticity decreases. The increase of the carbon content increases the manufacturing difficulty of the working procedure before heat treatment, so the invention limits the carbon content within the range of 0.10-0.17 percent by comprehensively considering the toughness matching of the material, the rapid heat treatment characteristic and the structural property change rule of carbon on the final product.

Mn: manganese can form solid solution with iron, so that the strength and hardness of ferrite and austenite in carbon steel are improved, finer pearlite with higher strength is obtained in the cooling process of steel after hot rolling, and the content of pearlite is increased along with the increase of the content of Mn. Manganese is also a carbide forming element, and the carbide of manganese can be dissolved into the cementite, thereby indirectly enhancing the strength of pearlite. Manganese can also strongly enhance the hardenability of the steel, further increasing its strength.

For dual phase steels, manganese is one of the elements that significantly affects the kinetics of austenite formation upon intercritical annealing, and manganese primarily affects the process of austenite to ferrite transformation and growth after austenite formation, as well as the final equilibrium process of austenite and ferrite. Since the diffusion rate of manganese element in austenite is much smaller than that in ferrite, the time required for the austenite to grow under the control of manganese diffusion is longer, and the time for manganese element to reach uniform distribution in austenite is longer. When the critical zone is heated rapidly, if the heat preservation time is shorter, the manganese element is not distributed uniformly in the austenite, and then when the cooling rate is insufficient, a more uniform martensite austenite island (for short, mao)Islands) of tissue. In dual-phase steel produced by adopting a rapid heating process (such as a continuous annealing production line of rapid induction heating or rapid direct fire heating and water quenching cooling), due to the fact that a large amount of pearlite exists in a matrix with a high manganese content, austenite generated locally at first has high manganese content after being generated, hardenability of austenite islands is guaranteed, and uniform martensite austenite island (Mao island for short) tissues and uniform performance are easy to obtain after cooling. In addition, manganese enlarges the gamma-phase region and reduces A _c1 And A _c3 Temperature, and therefore manganese-containing steels will give a higher martensitic volume fraction than low carbon steels under the same heat treatment conditions. However, when the manganese content is further increased, the crystal grains in the steel tend to be coarsened, and the overheat sensitivity of the steel is increased, and when cooling is improper after the smelting pouring and forging, white spots are easily generated in the carbon steel. The increase in manganese content will increase the difficulty of manufacturing the pre-heat treatment process. By considering the above factors, the manganese content is designed to be within the range of 1.8-2.8%.

Si: silicon forms a solid solution in ferrite or austenite, thereby enhancing the yield strength and tensile strength of the steel, and silicon can increase the cold working deformation hardening rate of the steel, which is a beneficial element in alloy steel. In addition, silicon has obvious enrichment phenomenon on the surface of the crystal boundary of the silicon-manganese steel, and the segregation of silicon at the crystal boundary position can slow down the distribution of carbon and phosphorus along the crystal boundary, thereby improving the embrittlement state of the crystal boundary. Silicon can improve the strength, hardness and wear resistance of steel, and the plasticity of steel can not be obviously reduced in a certain range. The silicon deoxidizer has strong capability, is a deoxidizer commonly used in steelmaking, and can increase the fluidity of molten steel, so that the common steel contains silicon, but when the silicon content in the steel is too high, the plasticity and toughness of the steel can be obviously reduced.

For dual phase steels, silicon has no significant effect on the austenite growth rate, but has a significant effect on the morphology and distribution of austenite formation. The increase of the silicon content increases the manufacturing difficulty of the pre-heat treatment process, and the invention aims to reduce the manufacturing difficulty of the pre-heat treatment process, reduce the cost and improve the welding performance. By combining the factors, the silicon content is determined to be within the range of 0.2-0.7 percent.

Nb: nb element is a carbide and nitride forming element and can meet this requirement at relatively low concentrations. At normal temperature, most of the steel exists in the form of carbide, nitride and carbonitride, and a small part is solid-dissolved in ferrite. The addition of Nb can prevent austenite grains from growing up and increase the coarsening temperature of steel grains. Nb element and carbon form stable NbC, and the addition of trace Nb element into steel can improve the strength of the matrix by utilizing the precipitation strengthening effect. Nb has obvious inhibition effect on the growth of ferrite recrystallization and austenite grain growth, can refine grains and improve the strength and toughness of steel; nb element can affect the mobility of grain boundaries and also has an effect on phase transformation behavior and carbide formation. Nb can raise the content of carbon in the residual austenite, prevent the formation of bainite, promote the nucleation of martensite, obtain a dispersed martensitic structure, improve the stability of the residual austenite, and improve the strength of the dual-phase steel by adding Nb element, so that the dual-phase steel with certain strength can be obtained under the conditions of low content of martensite and low C content, and the toughness of the dual-phase steel is improved; an additional benefit of adding Nb element at the same time is that the strength of the steel can be improved over a wider annealing temperature range. In the invention, nb element is an essential additive element, and the addition amount is not excessive in consideration of factors such as cost increase and the like.

Ti: ti is a microalloy element, belongs to ferrite forming elements of a closed gamma zone, can improve the critical point of steel, and Ti and C in the steel can form very stable TiC, so that the TiC is extremely difficult to dissolve in the austenitizing temperature range of common heat treatment. Since TiC particles refine austenite grains, the chance of nucleation of new phases increases during the decomposition transformation of austenite, which accelerates the transformation of austenite. In addition, ti can form TiC and TiN precipitated phases with C and N, and the TiC and TiN precipitated phases are more stable than carbon nitrides of Nb and V, so that the diffusion speed of C in austenite is obviously reduced, the formation speed of the austenite is greatly reduced, and the formed carbon nitrides precipitate in a matrix and are pinned at the grain boundary of the austenite to prevent the growth of austenite grains. During the cooling process, the precipitated TiC has a precipitation strengthening effect; in the tempering process, ti slows down the diffusion of C in alpha phase, slows down the precipitation and growth of carbide such as Fe, mn and the like, increases tempering stability, and can play a secondary hardening role by precipitating TiC. The high temperature strength of the steel can be improved by micro-alloying of Ti. On one hand, the strength and the welding performance of the steel can be improved while the equivalent carbon content is reduced by adding a trace amount of Ti into the steel; on the other hand, impurities such as oxygen, nitrogen, sulfur, etc. are fixed, thereby improving the weldability of the steel; secondly, due to the action of microscopic particles, such as undissolved TiN at high temperature, coarsening of grains in a heat affected zone can be prevented, and toughness of the heat affected zone can be improved, so that welding performance of steel can be improved. In the invention, ti element is an essential additive element, and the addition amount is not excessive in consideration of factors such as cost increase and the like.

B: the content of B element in steel is very tiny, the main function is to increase the hardenability of the steel, the effect is much larger than that of Cr, mn and other alloy elements, and a large amount of other more rare and noble metals such as nickel, chromium, molybdenum and the like can be saved by applying trace B element. For this purpose, the content thereof is generally specified to be in the range of 0.001% to 0.005%. It can replace 1.6% nickel, 0.3% chromium or 0.2% molybdenum, with boron substituting molybdenum, it being noted that molybdenum cannot be completely replaced by boron because it prevents or reduces temper embrittlement, whereas boron has a slight tendency to promote temper embrittlement. Boron, nitrogen and oxygen have strong affinity, and 0.007% of boron is added into boiling steel, so that the aging phenomenon of the steel can be eliminated. However, only the B element existing in a solid solution state has a beneficial effect on the hardenability of steel, and the B existing in a compound state has no effect on the hardenability of steel, so that the fixation of C, N element should be considered when the hardenability is increased by the B element. The element B is an essential additive element, but the addition amount is not excessive.

Cr: chromium has the main function of improving hardenability in steel, so that the steel has better comprehensive mechanical property after quenching and tempering. Chromium forms a continuous solid solution with iron, reduces the austenite phase region, forms various carbides with carbon, and has a greater affinity with carbon than iron and manganese elements. Chromium and iron can form intermetallic compound sigma phase (FeCr), chromium reduces the concentration of carbon in pearlite and the limiting solubility of carbon in austenite; chromium slows down the decomposition speed of austenite and obviously improves the hardenability of steel. But also increases the temper embrittlement tendency of the steel. The chromium element can improve the strength and hardness of the steel, and has obvious effect when other alloy elements are added. Cr improves the quenching ability of steel in air cooling, and thus has an adverse effect on the welding performance of steel. However, at a chromium content of less than 0.3%, the adverse effect on weldability is negligible; when the content is more than this, defects such as cracks and slag inclusion are likely to occur during welding. When Cr is present with other alloying elements (e.g., coexisting with V), the adverse effect of Cr on weldability is greatly reduced. If elements such as Cr, mo, V and the like are simultaneously present in the steel, even if the Cr content reaches 1.7%, the welding performance of the steel is not significantly adversely affected. The chromium element is beneficial and unnecessary, and the addition amount is not excessive in consideration of factors such as cost increase and the like.

Mo: the molybdenum element can inhibit self-diffusion of iron and diffusion rate of other elements. Mo atoms have larger radius than alpha-Fe atoms, so that when Mo is dissolved in alpha solid solution, the solid solution is subjected to strong lattice distortion, and meanwhile, mo can increase the bond attraction of lattice atoms and improve the recrystallization temperature of alpha ferrite. Mo has remarkable strengthening effect in pearlite, ferrite and martensite steels, and even in high alloy austenitic steels. The good effect of Mo in steel is also dependent on the interactions with other alloying elements in the steel. When the strong carbide forming elements V, nb and Ti are added into the steel, the solid solution strengthening effect of Mo is more remarkable. This is because when a strong carbide-forming element is combined with C to form a stable carbide, mo is promoted to be more effectively dissolved into solid solution, thereby more contributing to the improvement of the heat resistance of steel. The addition of Mo may also increase the hardenability of the steel. Mo suppresses transformation of pearlite area and accelerates transformation of medium temperature area, so that Mo-containing steel can form a certain amount of bainite even in case of a large cooling rate, and eliminates formation of ferrite, which is one of reasons why Mo has an advantageous effect on heat resistance of low alloy heat resistant steel. Mo also significantly reduces the hot embrittlement tendency of the steel and reduces the pearlite spheroidization speed. When the Mo content is 0.15% or less, there is no adverse effect on the weldability of the steel. The molybdenum element is beneficial and unnecessary, and the addition amount is not excessive in consideration of factors such as cost increase and the like.

V: v is a ferrite stabilizing element and is a strong carbide forming element, has a strong grain refining effect, and can compact the structure of steel. The addition of V to the steel results in the steel having improved strength, plasticity and toughness. Vanadium can also improve the high temperature strength of structural steels. Vanadium does not improve hardenability. The micro-alloying element V is added into the steel, so that the steel can be ensured to have good weldability and other service performances under the condition of lower carbon equivalent by the dispersion precipitation of carbon and nitride particles (the size is smaller than 5 nm) and the solid solution of V, the crystal grains are refined, the strength and toughness of the steel are greatly improved, and particularly the low-temperature toughness is improved. On one hand, the strength and the welding performance of the steel can be improved while the equivalent carbon content is reduced by adding a trace amount of V into the steel; on the other hand, impurities such as oxygen, nitrogen, sulfur, etc. are fixed, thereby improving the weldability of the steel; secondly, due to the effect of microscopic particles, such as the undissolved property of V (CN) at high temperature, the coarsening of grains in a heat affected zone can be prevented, and the toughness of the heat affected zone is improved, so that the welding performance of steel is improved. The microalloy elements are beneficial and unnecessary, and the addition amount is not excessive in consideration of factors such as cost increase and the like.

The invention finely controls the recovery, recrystallization and phase change processes of the deformed structure of the hard rolled strip steel in the heat treatment process by a rapid heat treatment method (comprising rapid heating, short-time temperature and rapid cooling processes), and finally obtains all the structure structures which are fine, uniform and distributed in a dispersing way and good strong plastic matching.

The concrete principle is as follows: different heating rates are adopted at different temperature stages in the heating process, recovery of deformed tissues mainly occurs at a low-temperature stage, and relatively low heating rate can be adopted to reduce energy consumption; the high temperature section mainly causes recrystallization and grain growth of different phase structures, and a relatively high heating rate is required to shorten the residence time of the structures in a high temperature section so as to ensure that the grains cannot grow. The recovery of the deformed structure and the ferrite recrystallization process in the heating process are restrained by controlling the heating rate in the heating process, so that the recrystallization process is overlapped with the austenite transformation process, nucleation points of recrystallized grains and austenite grains are increased, and the grains are refined finally. The time for grain growth in the soaking process is shortened by short-time heat preservation and rapid cooling, and fine and uniform grain structure distribution is ensured.

The heat treatment process disclosed in the Chinese patent CN106811698B does not distinguish the whole heating process, the heating rate adopted in the heating process is 20-60 ℃/s, which belongs to a medium heating rate, is realized based on the heating technology of the traditional continuous annealing unit, and cannot be controlled in a large range according to the requirement of material organization transformation.

In the heat treatment process disclosed in chinese patent CN107794357B and US patent US2019/0153558A1, although the heating process is also staged: heating to 300-500 ℃ at a heating rate of 1-10 ℃/s, heating to 850-950 ℃ in a single-phase austenite region at a heating rate of 100-500 ℃/s, preserving heat for not more than 5s, and quenching to room temperature. The treatment method requires that the steel plate must be heated to a high temperature region of single-phase austenite, which increases the high temperature resistance requirement of equipment and increases the manufacturing difficulty, and meanwhile, the method adopts a water cooling mode, although the cooling speed is extremely high, the growth time of a grain structure in the high temperature region can be greatly reduced, but uneven distribution of alloy elements in a final product is inevitably brought, uneven and unstable product structure performance is caused, and a series of problems such as poor plate shape and surface oxidation of the steel plate are also caused due to the excessively high cooling speed.

Only by comprehensively controlling the whole heat treatment process: the method comprises the steps of rapid heating (heating speed is controlled by sections), short-time soaking and rapid cooling, so that the optimal grain size, the uniform distribution of alloy elements and phase structures can be obtained through fine control, and finally the optimal obdurability matching product is obtained.

The average grain size of the ferrite and martensite dual-phase structure obtained by the rapid heat treatment method is 1-3 mu m, which is reduced by about 30% compared with the grain size (usually 3-10 mu m) of the product produced by the prior art, the strength of the material can be improved by grain refinement, good plasticity and toughness can be obtained, and the service performance of the material can be improved; the ferrite and martensite structure obtained by the method has various forms such as blocks, strips, particles and the like, and the ferrite and martensite structure is more uniformly distributed, so that better strength and plasticity can be obtained.

The invention relates to a rapid heat treatment manufacturing method of 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel, which comprises the following steps:

1) Smelting and casting

2) Hot rolling and coiling

The coiling temperature is 550-680 ℃;

3) Cold rolling

The cold rolling reduction rate is 40-85%, and the rolled hard strip steel or steel plate is obtained;

4) Rapid thermal treatment

a) Rapid heating

Rapidly heating cold-rolled strip steel or a steel plate from room temperature to a target temperature of an austenite and ferrite two-phase region at 750-845 ℃, wherein the rapid heating adopts a one-section type or a two-section type; when the one-stage rapid heating is adopted, the heating rate is 50-500 ℃/s; when two-section rapid heating is adopted, the first section is heated to 550-650 ℃ from room temperature at a heating rate of 15-500 ℃/s, and the second section is heated to 750-845 ℃ from 550-650 ℃ at a heating rate of 50-500 ℃/s;

b) Soaking heat

Soaking at the target temperature of 750-845 ℃ in the austenite and ferrite two-phase region for 10-60 s;

c) Cooling

Soaking the strip steel or the steel plate, and slowly cooling to 670-770 ℃ at a cooling rate of 5-15 ℃/s; then rapidly cooling to room temperature from 670-770 ℃ at a cooling rate of 50-200 ℃/s;

or rapidly cooling from 670-770 ℃ to 230-280 ℃ at a cooling rate of 50-200 ℃/s for overaging treatment, wherein the overaging treatment time is as follows: and (3) cooling to room temperature at 30-50 ℃ per second after over-aging treatment for less than or equal to 200 s.

Preferably, the whole rapid thermal processing process is 41-297 s.

Preferably, in step 2), the hot rolling temperature is not less than A _r3 。

Preferably, in the step 2), the winding temperature is 580 to 650 ℃.

Preferably, in step 3), the cold rolling reduction is 60 to 80%.

Preferably, in the step 4), the rapid heating adopts two-stage heating, and the first stage is heated from room temperature to 550-650 ℃ at a heating rate of 15-300 ℃/s; the second section is heated from 550 to 650 ℃ to 750 to 845 ℃ at a heating rate of 50 to 300 ℃/s.

Preferably, in the step 4), the rapid heating adopts two-stage heating, and the first stage is heated from room temperature to 550-650 ℃ at a heating rate of 50-300 ℃/s; the second section is heated from 550 to 650 ℃ to 750 to 845 ℃ at a heating rate of 80 to 300 ℃/s.

Preferably, in step 4), the rapid cooling rate is 50 to 150 ℃/s. Preferably, in the soaking process of step 4), after the steel strip or the steel plate is heated to the target temperature of the austenite and ferrite two-phase region, the soaking is performed while keeping the temperature unchanged.

Preferably, in the soaking process of the step 4), the strip steel or the steel plate is subjected to small-amplitude heating or small-amplitude cooling in the soaking time period, the temperature after heating is not more than 845 ℃, and the temperature after cooling is not less than 750 ℃.

Preferably, in step 4), the soaking time is 10 to 40 seconds.

The rapid heat treatment manufacturing method of 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel comprises the following steps:

1. heating rate control

The recrystallization kinetics of a continuous heating process can be quantitatively described by a relationship affected by the heating rate, where the ferrite recrystallization volume fraction as a function of temperature T:

wherein X (T) is ferrite recrystallization volume fraction; n is Avrami index, is related to a phase change mechanism, and generally takes a value in a range of 1-4 depending on the decay period of the recrystallization nucleation rate; t is the heat treatment temperature; t (T) _star Is the recrystallization onset temperature; beta is the heating rate; b is constant at a given isothermal temperature, varying the isothermal temperature, with b varying, b (T) being obtained by:

b＝b ₀ exp(-Q/RT)

from the above formula and the related experimental data, it can be seen that the recrystallization onset temperature (T _star ) End temperature (T) _fin ) Are all raised; when the heating rate is above 50 ℃/s, the austenite transformation and recrystallization processes are overlapped, the recrystallization temperature is increased to the temperature of the two-phase region, and the faster the heating rate is, the higher the ferrite recrystallization temperature is.

The influence of the heating technology in the traditional heat treatment process is slow heating, under the condition, the deformed matrix is recovered, recrystallized and grown in turn, and then ferrite-to-austenite transformation nucleation and austenite grain growth are carried out, the transformation nucleation points are mainly concentrated at the grown ferrite grain boundaries, and the nucleation rate is low. The grain structure of the finally obtained dual phase steel is relatively coarse.

Under the condition of rapid heating, the deformed matrix just completes recrystallization or does not complete recrystallization (even does not fully recover yet), and then the phase transformation from ferrite to austenite starts to happen, and the grains are fine and the grain boundary area is large when the recrystallization just completes or does not complete, so the phase transformation nucleation rate is obviously improved, and the austenite grains are obviously refined. Particularly, when the ferrite recrystallization process and the austenite transformation process are overlapped, a large number of crystal defects such as dislocation remain in ferrite crystals, a large number of nucleation points are provided for austenite, so that the nucleation of the austenite presents explosive nucleation, austenite grains are further refined, the dislocation line defects with high density are also taken as channels for high-speed diffusion of carbon atoms, each austenite grain can be quickly generated and grown up, and therefore, the austenite grains are fine and the volume fraction is increased.

The structure evolution, alloy element and each phase component distribution are finely controlled in the rapid heating process, so that a good foundation is laid for the austenite structure growth in the subsequent soaking process, the distribution of each alloy component and the transformation from austenite to martensite phase in the rapid cooling process. The final product structure with refined grains, reasonable elements and distribution of each phase can be finally obtained. The invention sets the heating rate of one-section type rapid heating to 50-500 ℃/s and adopts two-section type rapid heating to 15-500 ℃/s by comprehensively considering the effects of rapid heating and refining crystal grains, manufacturing cost, manufacturability and other factors.

Because the influence of rapid heating on the material recovery, recrystallization, grain growth and other tissue evolution processes is different in different temperature ranges, the optimal tissue control is obtained, so that the optimal heating rate is also different in different heating temperature ranges: the influence of the heating rate on the recovery process is the largest from 20 ℃ to 500-650 ℃, and the heating rate is controlled to be 15-300 ℃/s, and more preferably 50-300 ℃/s; the heating temperature is from 500-650 ℃ to 750-845 ℃ of austenitizing temperature, the heating rate has the greatest influence on nucleation rate and grain growth process, and the heating rate is controlled to be 50-300 ℃/s; further preferably 80 to 300 ℃/s.

2. Soaking temperature control

The soaking temperature is selected by combining the material tissue evolution process control of each temperature stage in the heating process, and simultaneously considering the tissue evolution and control of the subsequent rapid cooling process, so that the optimal tissue structure and distribution can be finally obtained.

The soaking temperature generally depends on the C content, the C content in the ultra-high strength dual-phase steel is 0.10-0.15 percent, and the A of the steel is _C1 And A _C3 About 730 ℃ and 870 ℃, respectively. The rapid heat treatment process of the invention rapidly heats the strip steel from room temperature to A _C1 To A _C3 In between, a large amount of dislocation is reserved in the ferrite which is not fully recrystallized, and a larger nucleation driving force is provided for austenite transformation, so that compared with the traditional continuous annealing process, the rapid heat treatment method can obtain more and finer OtssBody tissue.

The invention firstly proposes that the soaking temperature is increased and reduced within a certain range for the control of the soaking temperature: i.e. the soaking process is ramped up and ramped down, but the soaking temperature must be kept within a certain range. The advantage of this is that: in fact, the rapid temperature rise and fall process is realized by further increasing the superheat degree and the supercooling degree in the temperature range of the two-phase region, so that the rapid phase transformation is facilitated, when the temperature rise and fall range and the speed are large enough, grains can be further refined through repeated ferrite-to-austenite phase transformation and austenite-to-ferrite phase transformation, meanwhile, the carbide formation and the uniform distribution of alloy elements are influenced to a certain extent, and finally finer tissues and the alloy elements with uniform distribution are formed.

The dual-phase steel after cold rolling has a large number of undissolved tiny evenly distributed carbides which can become nucleation points of austenite, and can play a role in mechanical obstruction on the growth of austenite grains in the heating and soaking processes, thereby being beneficial to refining the grain size of alloy steel. However, if the heating temperature is too high, the number of undissolved carbides is greatly reduced and the size is increased, so that the inhibition is weakened, the growth tendency of grains is enhanced, and the strength of the steel is further reduced. When the amount of undissolved carbide is too large, aggregation may be caused, resulting in uneven distribution of local chemical components, and when the carbon content at the aggregation site is too high, local overheating may be caused. Therefore, in ideal conditions, a small amount of fine granular undissolved carbide should be uniformly distributed in the steel, so that abnormal growth of austenite grains can be prevented, the content of each alloy element in the matrix can be correspondingly increased, and the aim of improving the mechanical properties of the alloy steel such as strength, toughness and the like is fulfilled.

The soaking temperature should also be selected to obtain fine and uniform austenite grains for the purpose of obtaining a fine martensitic structure after cooling. Too high soaking temperature can cause austenite grains to be coarse, and a martensitic structure obtained after quick cooling can be coarse, so that the mechanical property of the steel is poor; the amount of residual austenite is increased, the amount of martensite is reduced, and the hardness and the strength of the steel are reduced Wear resistance. The excessively low soaking temperature not only reduces the quantity of austenite, but also causes insufficient carbon and alloy element content in the austenite, causes uneven concentration distribution of the alloy element in the austenite, greatly reduces the hardenability of the steel, and has adverse effects on the mechanical properties of the steel. Soaking temperature of hypoeutectoid steel should be Ac ₃ +30 to 50 ℃. In the case of ultra-high strength steel, the presence of carbide forming elements prevents transformation of carbides, so that soaking temperature can be properly increased. By combining the factors, 770-845 ℃ is selected as soaking temperature, so that more ideal and reasonable final organization is obtained.

3. Soaking time control

Because the invention adopts rapid heating, the material in the two-phase region contains a large amount of residual dislocation, a large amount of nucleation sites are provided for the formation of austenite, and a rapid diffusion channel is provided for carbon atoms, so that the austenite can be formed extremely rapidly, the shorter the soaking and heat preserving time is, the shorter the diffusion distance of the carbon atoms is, the larger the carbon concentration gradient in the austenite is, and finally the more residual austenite carbon content is kept; however, if the heat preservation time is too short, the alloy elements in the steel are unevenly distributed, so that austenitization is insufficient; the austenite grains are easily coarse due to the long heat preservation time. The influence factor of soaking time also depends on the content of carbon and alloy elements in steel, when the content of carbon and alloy elements in steel is increased, not only the thermal conductivity of steel is reduced, but also the alloy elements obviously delay the structural transformation of steel because the diffusion speed of the alloy elements is slower than that of the carbon elements, and the heat preservation time is properly prolonged. Therefore, the soaking time is controlled by strictly combining soaking temperature, rapid cooling and rapid heating process to be comprehensively considered and formulated, so that ideal organization and element distribution can be finally obtained. In conclusion, the soaking and heat preserving time is set to be 10-60 s.

4. Fast cooling rate control

In order to obtain a martensite reinforced phase, the cooling speed of the material during rapid cooling is required to be larger than the critical cooling speed to obtain a martensite structure, and the critical cooling speed mainly depends on the material components, and the optimized Si content is 0.2-0.7%, the Mn content is 1.8-2.8%, and the content is relatively high, so that the hardenability of the dual-phase steel is greatly enhanced by Si and Mn, and the requirement of the critical cooling speed is reduced.

The cooling rate also needs to comprehensively consider the structure evolution and alloy diffusion distribution results of the heating process and the soaking process so as to finally obtain reasonable structure distribution of each phase and alloy element distribution. The cooling rate is too low to obtain a martensitic structure, so that the strength is reduced, and the mechanical properties cannot meet the requirements; and a too large cooling rate can generate larger quenching stress (namely tissue stress and thermal stress) to cause serious plate shape defects, especially serious plate shape defects when the cooling is uneven, and even serious deformation and cracking of the sample are easily caused. The present invention sets the rapid cooling rate to 50-200 deg.c/s.

5. Overaging treatment

After traditional heat treatment, overaging mainly tempers quenched martensite to improve the comprehensive performance of the dual-phase steel. The improper setting of the overaging temperature and time can lead to the martensitic decomposition and directly deteriorate the mechanical properties of the dual-phase steel. The setting of the overaging temperature and time needs to comprehensively consider the morphology and distribution of the martensitic structure, the content and distribution of elements and the size and distribution of other structures. Therefore, the overaging control needs to be formulated by combining various parameters of the heating process, the soaking process and the cooling process. The invention combines the tissue evolution and element distribution conditions of the rapid heating, short-time temperature and rapid cooling process, and sets the overaging temperature range to 230-280 ℃; the overaging time is controlled to be less than or equal to 200s.

According to the invention, the rapid heating and rapid cooling process is improved on the traditional continuous annealing unit, so that the rapid heat treatment process is realized, the heating and heat equalizing section length of the annealing furnace can be greatly shortened (at least one third shorter than that of the traditional continuous annealing furnace), the production efficiency of the traditional continuous annealing unit is improved, the production cost and the energy consumption are reduced, the number of the continuous annealing furnace rollers, particularly the number of the high-temperature furnace section rollers, is obviously reduced, the surface quality control capability of strip steel is improved, and the strip steel product with high surface quality is obtained. Meanwhile, the purposes of short and precise unit, flexible material transition, strong regulation and control capability and the like can be realized by establishing a novel continuous annealing unit of a rapid heat treatment process technology; the grain of the strip steel can be thinned for the product material, the material strength is further improved, the alloy cost and the manufacturing difficulty of the working procedure before heat treatment are reduced, and the use performance of users such as material molding, welding and the like is improved.

Compared with the prior art, the invention has the advantages that:

(1) The invention suppresses the recovery of deformed structure and ferrite recrystallization process in the heat treatment process through rapid heat treatment, so that the recrystallization process overlaps with the austenite transformation process, the nucleation points of recrystallized grains and austenite grains are increased, the grain growth time is shortened, the grains are refined, the microstructure of the obtained dual-phase steel is a dual-phase structure of ferrite and martensite, the average grain size is 1-3 mu m, and the grain size is reduced by about 30% compared with the grain size (usually 3-10 mu m) of a product produced by the prior art; the ferrite and martensite structure obtained by the method has various forms such as blocks, strips, particles and the like, and the ferrite and martensite structure are more uniformly distributed, so that better strong plasticity can be obtained; the strength of the material is improved, meanwhile, good plasticity and toughness are obtained, and the service performance of the material is improved.

(2) Compared with the dual-phase steel obtained by the traditional heat treatment mode, the grain size of the dual-phase steel obtained by the method is reduced by more than 30%, the toughness of the material is obviously improved, the yield strength is 902-1114 MPa, the tensile strength is 1264-1443 MPa, the grain size can be controlled within a smaller range, and the stability of the mechanical property of the product is obviously improved; the elongation is still kept at a higher level of 7-9.8%; the product of strength and plasticity is 9.5-12.1 GPa, and the molding property is excellent.

(3) According to the low-carbon low-alloy ultra-high strength 1280 MPa-level dual-phase steel rapid heat treatment process, the total heat treatment time can be shortened to 41-297 s, the time of the whole heat treatment process (the traditional continuous annealing process time is usually 5-8 min), the production efficiency is improved, the energy consumption is reduced, and the production cost is reduced.

(4) Compared with the traditional dual-phase steel and the heat treatment process thereof, the rapid heat treatment method shortens the time of the heating section and the soaking section by 60-80%, shortens the time of the whole heat treatment process to 41-297 s, can save energy, reduce emission and consumption, obviously reduces the disposable investment of furnace equipment, and obviously reduces the production running cost and the equipment maintenance cost; in addition, the alloy content can be reduced by producing products with the same strength grade through rapid heat treatment, the production cost of the heat treatment and the previous working procedures is reduced, and the manufacturing difficulty of each working procedure before the heat treatment is reduced.

(5) Compared with the dual-phase steel obtained by the traditional continuous annealing treatment, the rapid heat treatment technology reduces the time of a heating process and a soaking process, shortens the length of a furnace and reduces the number of furnace rollers, so that the probability of generating surface defects in the furnace is reduced, and the surface quality of a product is obviously improved; in addition, as the grain refinement of the product and the alloy content of the material are reduced, the processing forming performance such as reaming performance and bending performance of the dual-phase steel obtained by the technology of the invention and the use performance of users such as welding performance are also improved.

The low-carbon low-alloy ultra-high strength 1280 MPa-level dual-phase steel obtained by the method has important value for the development of new-generation light-weight vehicles such as automobiles, trains, ships, airplanes and the like, the corresponding industry and the healthy development of advanced manufacturing industry.

Drawings

FIG. 1 is a photograph of the microstructure of a dual phase steel produced as in example 1 for test steel A according to the present invention.

FIG. 2 is a photograph of the microstructure of a dual phase steel produced by conventional process 1 for test steel A according to the present invention.

FIG. 3 is a photograph of the microstructure of a dual phase steel produced as in example 6 for test steel F according to the present invention.

FIG. 4 is a photograph of the microstructure of a dual phase steel produced as in example 12 for test steel M according to the present invention.

FIG. 5 is a photograph of the microstructure of a dual phase steel produced as in example 23 for test steel S according to the present invention.

FIG. 6 is a photograph of the microstructure of a dual phase steel produced as in example 24 for test steel M according to the present invention.

Detailed Description

The present invention is further described below with reference to examples and drawings, wherein the examples are provided on the premise of the technical proposal of the present invention, and detailed implementation and specific operation procedures are given, but the protection scope of the present invention is not limited to the examples below.

The composition of the test steel according to the present invention is shown in Table 1, the specific parameters of the examples and the conventional process according to the present invention are shown in tables 2 and 3, and tables 4 and 5 are the main properties of the steels prepared by the examples and the conventional process.

As can be seen from tables 1 to 5, by the method of the present invention, the alloy content in the same grade steel can be reduced, the grains can be refined, and the matching of the material structure and the strength and toughness can be obtained. The yield strength of the dual-phase steel obtained by the method is 902-1114 MPa, the tensile strength is 1264-1443 MPa, the elongation is 7-9.8%, the strength-plastic product is 9.5-12.1 GPa%, and the dual-phase steel is higher than the dual-phase steel produced by the traditional process.

Fig. 1 is a structure diagram of a typical composition a steel obtained through example 1, and fig. 2 is a structure diagram of a typical composition a steel obtained through conventional process example 1. From the figure, the tissue treated by different heat treatment modes is very different. The dual-phase steel structure obtained after the rapid heat treatment process treatment of the embodiment of the invention is composed of fine and uniform martensitic structure and a small amount of carbide which are dispersed and distributed on a ferrite matrix, and the ferrite, martensitic grain structure and carbide are very fine and uniformly distributed in the matrix, which is very beneficial to improving the strength and plasticity of the material. The dual-phase steel structure diagram is obtained through the traditional process treatment, namely a small amount of black martensite structure exists on a white ferrite grain boundary, and the material structure after the traditional process treatment shows a certain directionality due to element segregation and other reasons, and the ferrite structure is distributed in a long strip shape along the rolling direction. The tissue treated by the traditional process is characterized in that: the grains are coarse, a certain banded structure exists, martensite and carbide are distributed in a net shape along the ferrite grain boundary, the ferrite grains are relatively coarse, and the ferrite and martensite two-phase structure is unevenly distributed.

Fig. 3 is a structure diagram of a typical composition F steel obtained through example 6, and fig. 4 is a structure diagram of a typical composition M steel obtained through example 12. Fig. 5 is a structure diagram of a typical composition S steel obtained through example 23, and fig. 6 is a structure diagram of a typical composition M steel obtained through example 24. Examples 6, 12, 23, 24 are all processes with a shorter overall heat treatment cycle. It can be seen from the figure that by adopting the method of the invention, each phase structure which is very uniform, fine and dispersed can be obtained by removing the aging treatment section. Therefore, the preparation method of the dual-phase steel can refine grains, so that each phase structure of the material is uniformly distributed in the matrix, thereby improving the material structure and the material performance.

The invention can reform the traditional continuous annealing unit by adopting the rapid heating and rapid cooling process, so that the rapid heat treatment process is realized, the lengths of the heating section and the soaking section of the traditional continuous annealing furnace can be greatly shortened, the production efficiency of the traditional continuous annealing unit is improved, the production cost and the energy consumption are reduced, the number of furnace rollers of the continuous annealing furnace is reduced, the control capability of the surface quality of the strip steel can be improved, and the strip steel product with high surface quality can be obtained; meanwhile, by establishing a novel continuous annealing unit adopting a rapid heat treatment process technology, the continuous heat treatment unit has the advantages of short and precise structure, flexible material transition, strong regulation and control capability and the like; the material can refine the grain of the strip steel, further improve the strength of the material, reduce the alloy cost and the manufacturing difficulty of the working procedure before heat treatment, and improve the use performance of users such as the welding performance of the material.

In summary, the rapid heat treatment process is adopted to greatly promote the technical progress of the continuous annealing process of the cold-rolled strip steel, the austenitizing process of the cold-rolled strip steel from room temperature to the final completion is expected to be completed in tens of seconds, tens of seconds or even a few seconds, the length of a heating section of a continuous annealing furnace is greatly shortened, the speed and the production efficiency of a continuous annealing unit are conveniently improved, the number of rollers in the furnace of the continuous annealing unit is obviously reduced, and the surface quality of the strip steel can be obviously improved for a rapid heat treatment production line with the unit speed of about 180 meters/minute, wherein the number of rollers in a high-temperature furnace section is not more than 10. Meanwhile, the rapid heat treatment process method of the recrystallization and austenitizing process completed in a very short time also provides a more flexible and flexible high-strength steel structure design method, so that the material structure is improved and the material performance is improved on the premise of not changing the alloy components, the rolling process and other pre-process conditions.

The advanced high-strength steel represented by the dual-phase steel has wide application prospect, and the rapid heat treatment technology has huge development and application values, and the combination of the two can provide a larger space for the development and production of the dual-phase steel.

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Claims

1.1280MPa low-carbon low-alloy ultra-high strength dual-phase steel comprises the following chemical components in percentage by mass: c: 0.10-0.17%, si: 0.2-0.7%, mn: 1.8-2.8%, cr: 0.3-0.9%, nb: 0.02-0.07%, ti: 0.02-0.07%, B: 0.002-0.005%, P is less than or equal to 0.02%, S is less than or equal to 0.005%, al: 0.02-0.05%; also contains one or two of Mo and V, cr+Mo+Ti+Nb+V is less than or equal to 1.1 percent, and the balance is Fe and other unavoidable impurities, and is obtained by the following process:

1) Smelting and casting

2) Hot rolling and coiling

The coiling temperature is 550-680 ℃;

3) Cold rolling

The cold rolling reduction rate is 40-85%;

4) Rapid thermal treatment

The cold rolled steel plate is rapidly heated to 750-845 ℃, and the rapid heating is one-section or two-section; the heating rate is 50-500 ℃/s when the one-stage rapid heating is adopted; when two-section rapid heating is adopted, the first section is heated to 550-650 ℃ from room temperature at a heating rate of 15-500 ℃/s, and the second section is heated to 750-845 ℃ from 550-650 ℃ at a heating rate of 50-500 ℃/s; soaking, wherein the soaking temperature is 750-845 ℃, and the soaking time is 10-60 s;

after soaking, slowly cooling to 670-770 ℃ at a cooling rate of 5-15 ℃/s, and then rapidly cooling to room temperature from 670-770 ℃ at a cooling rate of 50-200 ℃/s;

Or rapidly cooling to 230-280 ℃ from 670-770 ℃ at a cooling rate of 50-200 ℃/s, and performing overaging treatment in the temperature range, wherein the overaging treatment time is as follows: and (3) cooling to room temperature at 30-50 ℃ per second for less than or equal to 200 s.

2. The 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 1 wherein the C content is 0.10-0.15%.

3. The 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 1 wherein the Si content is 0.2-0.5%.

4. The 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 1 wherein the Mn content is 2.0% -2.6%.

5. The 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 1 wherein the Cr content is 0.5-0.7%.

6. The 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 1 wherein the Ti content is 0.02-0.05%.

7. The 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 1 wherein the Nb content is 0.02-0.05%.

8. The 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 1, wherein the rapid thermal treatment time is 41-293 s.

9. The 1280 MPa-grade low-carbon low-alloy ultra-high strength dual-phase steel according to claim 1, wherein in step 2), the hot rolling temperature is not less than a _r3 。

10. The 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 1 wherein in step 2) the coiling temperature is 580-650 ℃.

11. The 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 1 wherein in step 3) the cold rolling reduction is 60-80%.

12. The 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 1 wherein in step 4) the rapid heating is performed at a heating rate of 50-300 ℃/s using one-stage heating.

13. The 1280 MPa-grade low-carbon low-alloy, ultra-high strength dual phase steel according to claim 1 wherein in step 4) the rapid heating is performed in two-stage heating: the first section is heated to 550-650 ℃ from room temperature at a heating rate of 15-300 ℃/s; the second section is heated from 550 ℃ to 650 ℃ to 750 ℃ to 845 ℃ at a heating rate of 50 ℃ to 300 ℃/s.

14. The 1280 MPa-grade low-carbon low-alloy, ultra-high strength dual phase steel according to claim 1 wherein in step 4) the rapid heating is performed in two-stage heating: the first section is heated to 550-650 ℃ from room temperature at a heating rate of 50-300 ℃/s; the second section is heated from 550 ℃ to 650 ℃ to 750 ℃ to 845 ℃ at a heating rate of 80 ℃ to 300 ℃/s.

15. The 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 1 wherein in step 4) the soaking time is 10-40 s.

16. The 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 1 wherein in step 4) the rapid cooling rate is 50-150 ℃/s.

17. The 1280 MPa-level low-carbon low-alloy, ultra-high strength dual phase steel according to any one of claims 1 to 16 wherein the dual phase steel has a microstructure of ferrite and martensite dual phase structures with a uniform distribution of average grain sizes of 1 to 3 μm.

18. The 1280 MPa-grade low-carbon low-alloy ultra-high strength dual-phase steel according to any one of claims 1 to 16 wherein the dual-phase steel has a yield strength of 902 to 1114MPa, a tensile strength of 1264 to 1443MPa, an elongation of 7 to 9.8% and a yield strength of 9.5 to 12.1gpa%.

19. The 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 17 wherein the dual-phase steel has a yield strength of 902 to 1114MPa, a tensile strength of 1264 to 1443MPa, an elongation of 7 to 9.8% and a yield strength of 9.5 to 12.1gpa%.

20. The rapid thermal processing method for producing 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to any one of claims 1 to 16, comprising the steps of:

1) Smelting and casting

Smelting and casting into a plate blank according to the chemical components;

2) Hot rolling and coiling

The coiling temperature is 550-680 ℃;

3) Cold rolling

The cold rolling reduction rate is 40-85%, and the rolled hard strip steel or steel plate is obtained after cold rolling;

4) Rapid thermal treatment

a) Rapid heating

Rapidly heating the cold-rolled strip steel or the steel plate from room temperature to a target temperature of an austenite and ferrite two-phase region at 750-845 ℃; the rapid heating is carried out in one section or two sections, and when the rapid heating is carried out in one section, the heating rate is 50-500 ℃/s; when two-section rapid heating is adopted, the first section is heated to 550-650 ℃ from room temperature at a heating rate of 15-500 ℃/s, and the second section is heated to 750-845 ℃ from 550-650 ℃ at a heating rate of 50-500 ℃/s;

b) Soaking heat

c) Cooling

21. The rapid thermal processing method for producing 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 20, wherein the total time of the rapid thermal processing is 41-293 s.

22. The rapid thermal processing manufacturing method of 1280 MPa-level low-carbon low-alloy ultra-high-strength dual-phase steel according to claim 20Characterized in that in the step 2), the hot rolling temperature is more than or equal to A _r3 。

23. The rapid thermal processing method for producing 1280 MPa-level low-carbon low-alloy ultra-high-strength dual-phase steel according to claim 20 or 22, wherein in step 2), the coiling temperature is 580-650 ℃.

24. The rapid thermal processing method for producing 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 20 wherein in step 3) the cold rolling reduction is 60 to 80%.

25. The method for manufacturing 1280 MPa-level low-carbon low-alloy ultrahigh-strength dual-phase steel according to claim 20 wherein in step 4) the heating rate is 50-300 ℃/s when one-stage heating is used.

26. The rapid thermal processing manufacturing method of 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 20, wherein in step 4), the rapid heating is performed by two-stage heating, the first stage is heated from room temperature to 550-650 ℃ at a heating rate of 15-300 ℃/s; the second section is heated from 550 ℃ to 650 ℃ to 750 ℃ to 845 ℃ at a heating rate of 50 ℃ to 300 ℃/s.

27. The rapid thermal processing manufacturing method of 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 20, wherein in step 4), the rapid heating is performed by two-stage heating, the first stage is heated from room temperature to 550-650 ℃ at a heating rate of 50-300 ℃/s; the second section is heated from 550 ℃ to 650 ℃ to 750 ℃ to 845 ℃ at a heating rate of 80 ℃ to 300 ℃/s.

28. The rapid thermal processing method for producing 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 20 wherein in step 4) the rapid cooling rate is 50-150 ℃/s.

29. The rapid thermal processing method for producing 1280MPa low-carbon low-alloy ultra-high strength dual phase steel according to claim 20 wherein in step 4) soaking, the steel strip or sheet is heated to the target temperature for the austenite and ferrite two-phase region and then kept at the same temperature for soaking.

30. The method for rapid heat treatment manufacturing of 1280 MPa-level low-carbon low-alloy ultra-high-strength dual-phase steel according to claim 20 wherein in the soaking process of step 4), the temperature of the steel strip or sheet is raised or lowered by a small extent in the soaking time period, the temperature after the temperature is raised is no more than 845 ℃, and the temperature after the temperature is lowered is no less than 750 ℃.

31. The rapid thermal processing method for producing 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 20, 29 or 30, wherein in step 4), the soaking time is 10 to 40s.

32. The rapid thermal processing method for producing 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 20, 21 or 22 wherein the dual-phase steel has a microstructure of ferrite and martensite having a uniform distribution and an average grain size of 1 to 3 μm.

33. The rapid thermal processing method for producing 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 23 wherein the dual-phase steel has a microstructure of ferrite and martensite having a uniform distribution and an average grain size of 1 to 3 μm.

34. The rapid thermal processing method for producing a low-carbon low-alloy ultra-high strength dual phase steel having a 1280MPa level according to any one of claims 24 to 30, wherein the dual phase steel has a microstructure of ferrite and martensite having a uniform distribution and an average grain size of 1 to 3 μm.

35. The rapid thermal processing method for producing 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 31 wherein the dual-phase steel has a microstructure of ferrite and martensite having a uniform distribution and an average grain size of 1 to 3 μm.

36. The rapid thermal processing method for producing 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 20, 21 or 22, wherein the dual-phase steel has a yield strength of 902 to 1114MPa, a tensile strength of 1264 to 1443MPa, an elongation of 7 to 9.8% and a strength-to-plastic product of 9.5 to 12.1gpa%.

37. The rapid thermal processing method of 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 23 wherein the dual-phase steel has a yield strength of 902 to 1114MPa, a tensile strength of 1264 to 1443MPa, an elongation of 7 to 9.8% and a strength-plastic product of 9.5 to 12.1gpa%.

38. The rapid thermal processing method for producing a 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to any one of claims 24 to 30, wherein the dual-phase steel has a yield strength of 902 to 1114MPa, a tensile strength of 1264 to 1443MPa, an elongation of 7 to 9.8% and a yield strength of 9.5 to 12.1gpa%.

39. The rapid thermal processing method of 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 31 wherein the dual-phase steel has a yield strength of 902 to 1114MPa, a tensile strength of 1264 to 1443MPa, an elongation of 7 to 9.8% and a strength-plastic product of 9.5 to 12.1gpa%.

40. The rapid thermal processing method of 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 32 wherein the dual-phase steel has a yield strength of 902 to 1114MPa, a tensile strength of 1264 to 1443MPa, an elongation of 7 to 9.8% and a strength-plastic product of 9.5 to 12.1gpa%.

41. The rapid thermal processing method of 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 33 wherein the dual-phase steel has a yield strength of 902 to 1114MPa, a tensile strength of 1264 to 1443MPa, an elongation of 7 to 9.8% and a strength-plastic product of 9.5 to 12.1gpa%.

42. The rapid thermal processing method of 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 34 wherein the dual-phase steel has a yield strength of 902 to 1114MPa, a tensile strength of 1264 to 1443MPa, an elongation of 7 to 9.8% and a strength-plastic product of 9.5 to 12.1gpa%.

43. The rapid thermal processing method for producing 1280 MPa-level low-carbon low-alloy ultra-high strength dual-phase steel according to claim 35 wherein the dual-phase steel has a yield strength of 902 to 1114MPa, a tensile strength of 1264 to 1443MPa, an elongation of 7 to 9.8% and a strength-plastic product of 9.5 to 12.1gpa%.