CN115181897A

CN115181897A - 1280 MPa-level low-carbon low-alloy ultrahigh-strength dual-phase steel and rapid thermal treatment manufacturing method

Info

Publication number: CN115181897A
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: 2022-10-14
Anticipated expiration: 2041-04-02
Also published as: CN115181897B

Abstract

The 1280 MPa-level low-carbon low-alloy ultrahigh-strength dual-phase 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 can be contained, cr + Mo + Ti + Nb + V is less than or equal to 1.1 percent, and the balance of Fe and other inevitable impurities. The manufacturing method comprises the following steps: smelting, casting, hot rolling, cold rolling and rapid heat treatment; the whole process of the rapid thermal treatment takes 41 to 297s. The invention changes the recovery, recrystallization and austenite phase transformation processes of the deformed structure, increases the nucleation rate (including the recrystallization nucleation rate and the austenite phase transformation nucleation rate), shortens the grain growth time, refines the grains, improves the strength of the material and expands the range of the material performance interval by controlling the rapid heating, short-time heat preservation and rapid cooling processes in the rapid heat treatment process.

Description

1280 MPa-level low-carbon low-alloy ultrahigh-strength dual-phase steel and rapid thermal treatment manufacturing method

Technical Field

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

Background

With the gradual improvement of people's awareness of energy conservation and material service safety, many automobile manufacturers select high-strength steel as automobile materials, and the automobile industry can improve the dent resistance, the endurance strength, the large deformation impact toughness and the collision safety of automobiles while adopting high-strength steel plates to reduce the thickness of the steel plates, so the automobile steel plates are bound to develop towards the directions of high strength, high toughness and easy forming.

In the high-strength steel for automobiles, the dual-phase steel is most widely applied and has 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 steel which is widely used at present and has high strength and good formability and is used for stamping automobile structures.

The traditional dual-phase steel is obtained by carrying out soaking annealing on cold-rolled low-carbon steel or low-alloy high-strength steel in a critical zone and then carrying out rapid cooling treatment or hot rolling, controlled rolling and controlled cooling, and the microstructure of the traditional dual-phase steel mainly comprises ferrite and martensite. Dual phase steels utilize the principle of "composite materials" to maximize the benefits of the phases (ferrite and martensite) in the steel, while mitigating or eliminating the disadvantages or drawbacks of one phase due to the presence of the other phase.

The mechanical properties of dual-phase steels are mainly determined by three aspects:

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

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 plasticity matching becomes a target pursued by various large iron and steel enterprises and is widely concerned by steel enterprises and automobile users.

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

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

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

the third step: keeping the temperature of the strip steel or heating the strip steel to be not higher than M _s The temperature is kept, tempering treatment is carried out, good structure matching of hard phase martensite and soft phase ferrite is obtained, and finally a dual-phase structure of the martensite and the ferrite is obtained.

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 low heating rate, and the whole continuous annealing period needs 5-8min; the recovery, recrystallization and phase transformation processes in the heating process are respectively carried out in sequence, and generally no overlapping phenomenon occurs, so ferrite recrystallization grains and austenite grains are respectively nucleated and fully grown, and finally the obtained ferrite and martensite two-phase grain structure has relatively large size, generally about 3-10 μm.

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

Chinese patent CN102021483B discloses a cold-rolled dual-phase steel plate with 1200 MPa-level tensile strength and a preparation method thereof, and the chemical components of the high-strength dual-phase steel of the invention are as follows by weight percent: c:0.19 to 0.21%, si:0.7 to 0.9%, mn:1.9 to 2.1%, cr:0.01 to 0.02%, 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 of Fe and other unavoidable impurity elements. This 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 fast 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 high-strength steel wire is mainly characterized in that the strength is improved by adopting higher C content, which is unfavorable for subsequent welding of a user, and meanwhile, the tensile strength of the high-strength steel wire can only reach about 1200MPa, and the high-strength steel wire is not in line with the requirements of 1280 MPa-level products.

Chinese patent CN102471821B discloses a method for manufacturing a cold-rolled steel sheet with excellent press formability and a cold-rolled steel sheet, wherein the steel comprises the following chemical components in percentage by weight: c: 0.1-0.3%, mn is more than or equal to 0.2%, ni is more than or equal to 0.01%, mn + Ni:0.5 to 2.5%, cr: 1.2-9%, and the balance of Fe and other inevitable impurities. The method is mainly characterized in that high C and high Cr are adopted based on the traditional continuous annealing process to improve the strength and the hardenability, and excessively high alloy elements can cause the reduction of welding performance, bring risks to subsequent use and increase the difficulty of manufacturing procedures of steel making, hot rolling and cold rolling.

Chinese patent CN105543674B discloses a method for manufacturing cold-rolled ultrahigh-strength dual-phase steel with high local forming performance, and the chemical components of the high-strength dual-phase steel comprise the following components in percentage by weight: c:0.08 to 0.12%, si:0.1 to 0.5%, mn:1.5 to 2.5%, al:0.015 to 0.05 percent, and the balance of Fe and other inevitable impurities. Selecting and matching raw materials for the chemical components, and smelting into a casting blank; heating the casting blank at 1150-1250 ℃ for 1.5-2 hours, and then carrying out hot rolling, wherein the initial rolling temperature of the hot rolling 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 with bainite as a main structure type; and (2) cold rolling the hot rolled steel plate, heating to 740-820 ℃ at the speed of 50-300 ℃/s, annealing, keeping the temperature for 30s-3min, cooling to 620-680 ℃ at the cooling rate of 2-6 ℃/s, and then cooling to 250-350 ℃ at the cooling rate of 30-100 ℃/s for overaging treatment for 3-5 min to obtain the ferrite-martensite dual-phase structure ultrahigh-strength dual-phase steel. The yield strength of the ultrahigh-strength dual-phase steel is 650-680 MPa, the tensile strength is 1023-1100 MPa, the elongation is 12.3-13%, and the ultrahigh-strength dual-phase steel is not cracked when bent at 180 degrees along the rolling direction.

The patent is mainly characterized in that the control of cooling conditions after hot rolling is combined with the rapid heating in the continuous annealing process, namely, the cooling process after hot rolling is controlled to eliminate banded structures and realize the homogenization of the structures; and rapid heating is adopted in the subsequent continuous annealing process, and the tissue refinement is realized on the basis of ensuring the tissue uniformity. Therefore, the patent technology adopts rapid heating annealing, and the premise is that the hot rolling raw material with bainite as a main structure is obtained after hot rolling, and the purpose is mainly to ensure the uniformity of the structure and avoid local uneven deformation caused by the occurrence of banded structures.

The defects of the patent mainly lie in that:

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

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

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

fourth, the process must be overaged for 3-5 minutes, which is actually too long for rapid heat treating DP steels and is not necessary. And 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 also not beneficial to the high-speed stable operation of the strip steel in the furnace, obviously, the rapid heat treatment process is not strictly defined.

Chinese patent 201711385126.5 discloses 'a 780 MPa-grade 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 processing process comprising: rapidly heating the strip steel from room temperature to a temperature of 790-830 ℃ in an austenite and ferrite two-phase region, wherein the heating rate is 40-300 ℃/s; the retention time of the heating target temperature interval in the two-phase region is 60 to 100s; rapidly cooling the strip steel from the temperature of the two-phase region to 410-430 ℃, wherein the cooling speed is 40-100 ℃/s, and the strip steel stays in the temperature region for 200-300 s; the strip steel is rapidly cooled to the room temperature from the temperature of 410-430 ℃. The method is characterized in that: the TRIP steel metallographic structure is a bainite, ferrite and austenite three-phase structure; the average grain size of the TRIP steel is obviously refined; the tensile strength is 950-1050 MPa; the elongation is 21-24%; the maximum product of strength and elongation can reach 24GPa%.

The defects of the patent mainly comprise the following aspects:

firstly, the patent discloses a 780 MPa-grade 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 too high as the tensile strength of the 780 MPa-grade product, the use effect of a user is not good, and the tensile strength of the TRIP steel product is too low as the tensile strength of the 980 MPa-grade product, so that the strength requirement of the user can not be well met;

secondly, the patent adopts one-stage rapid heating, the same rapid heating rate is adopted in the whole heating temperature interval, the materials are not processed differently according to the change of the tissue structures of the materials in different temperature sections, and the materials are all rapidly heated at the speed of 40-300 ℃/s, so that the production cost in the rapid heating process is inevitably increased;

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

fourth, this patent requires 200 to 300 seconds of bainite isothermal treatment, which is actually too long for rapid heat treatment of the product to be effective and is not necessary. And the increase of soaking time and isothermal treatment time is not beneficial to saving energy, reducing unit equipment investment and unit occupied area, and is also not beneficial to the high-speed stable operation of the strip steel in the furnace, obviously, the rapid heat treatment process is not in strict meaning.

Chinese patent CN108774681A discloses a rapid heat treatment method for high-strength steel, which adopts a ceramic chip electric heating device, can obtain a heating rate with the maximum value of 400 ℃/s, and adopts a fan to blow and cool after heating to 1000-1200 ℃, and the fastest cooling rate is about 3000 ℃/s to cool to room temperature. The processing speed of the heat treatment device adopting the ceramic plate for electric heating in the method is 50cm/min. The steel of the invention is characterized in that the carbon content is up to 0.16-0.55%, and the steel simultaneously contains: alloying elements such as Si, mn, cr, mo and the like; the method is mainly suitable for steel wires, wire rods or steel belts with the thickness of less than 5 mm.

This patent describes a rapid thermal treatment method by electrical heating of ceramic plates; 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 influence and effect of rapid heating on the texture properties of the material are not mentioned; the invention does not combine the components and the structure characteristics of the grade of the steel, adopts a fan blowing cooling mode, the fastest cooling speed is close to the instant cooling speed of the high-temperature section which is supposed to be indicated by 3000 ℃/s, and the average cooling speed is less than 3000 ℃/s; meanwhile, the high-temperature section adopts overhigh cooling speed to produce the wide thin strip steel, which can cause the problems of overlarge internal stress, poor steel plate profile and the like, and is not suitable for large-scale industrial continuous heat treatment production of the wide thin strip steel.

Chinese patent CN106811698B discloses a high-strength steel plate based on fine tissue control and a manufacturing method thereof, and the chemical components of the high-strength dual-phase steel comprise the following components in percentage by weight: c:0.08 to 0.40%, si:0.35 to 3.5%, mn: 1.5-7.0%, P is less than or equal to 0.02%, S is less than or equal to 0.02%, al:0.02 to 3.0%, further comprising Cr:0.50 to 1.5%, mo:0.25 to 0.60%, ni:0.5 to 2.5%, cu:0.20 to 0.50%, B:0.001 to 0.005%, V:0.10 to 0.5%, ti:0.02 to 0.20%, nb: 0.02-0.20 percent of at least one of the components, and the balance of Fe and other inevitable impurities. The mechanical properties are as follows: tensile strength R _m Greater than 1000MPa, elongation A _50mm Greater than 28%. The steel strip has high contents of C, si and Mn, and is subjected to non-soaking annealing on a traditional continuous annealing production line and recrystallization annealing on steel strips with different components by adopting a mode of removing a soaking section. The specific annealing parameter ranges are as follows: after the mixture is rapidly heated to 800-930 ℃ at a temperature of more than 20 ℃/s, the mixture is immediately cooled to M at a cooling rate of more than 40 ℃/s _s -M _f Point and then reheated to M _f ～M _f Keeping the temperature at +100 ℃ for 30s to 30min, and finally cooling to room temperature.

The invention is mainly characterized in that a fine acicular and short rod-shaped fine martensite structure is obtained by controlling the form and structure of a martensite high-strength phase, C atoms are diffused into residual austenite by reheating, and stable residual austenite is finally obtained, so that the high-strength steel has certain deformability, and the plasticity and toughness of the high-strength steel are improved.

The rapid heating of the invention has the advantages of low actual heating rate of 20-60 ℃/s, medium heating rate and cooling rate of 40-100 ℃/s. The consideration of the rapid heating, the rapid cooling and the omission of the soaking section is to shorten the retention time of the high-strength steel in the high-temperature section and ensure that the steel has fine grains and the structure and chemical components are not completely homogenized in the austenitizing process, thereby ensuring that a large amount of large-size lath-shaped martensite is not generated after the cooling and simultaneously obtaining a certain amount of film-shaped residual austenite structures. However, this entails that the heating temperature is difficult to control and the structural and performance fluctuations are increased.

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

Chinese patent CN107794357B and US patent US2019/0153558A1 disclose 'a method for producing a super-strength martensite cold-rolled steel sheet by a super-rapid heating process', and the chemical components of the high-strength dual-phase steel are as follows according to weight percentage: c:0.10 to 0.30%, mn:0.5 to 2.5%, si:0.05 to 0.3%, mo:0.05 to 0.3%, ti:0.01 to 0.04%, cr:0.10 to 0.3%, B:0.001 to 0.004%, 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 inevitable impurities. The mechanical properties of the dual-phase steel are as follows: yield strength Rp _0.2 Greater than 1100MPa, tensile strength R _m The elongation is maximally 12.3 percent and the uniform elongation is 5.5 to 6 percent, wherein the elongation is 1800 to 2300 MPa. The invention provides a super-fast heating production process of an ultrahigh strength martensite cold-rolled steel plate, which is characterized in that the cold-rolled steel plate is heated to 300-500 ℃ at the speed of 1-10 ℃/s, and then is heated to a single-phase austenite region 850-950 ℃ at the heating speed of 100-500 ℃/s; and then, immediately cooling the steel plate to room temperature after keeping the temperature for no more than 5s to obtain the ultrahigh-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 enters 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 both exceed 1000MPa, so that the steel brings great difficulty to the manufacturing of the heat treatment process and the working procedures before the heat treatment and the use of subsequent users.

Secondly, the ultra-fast heating annealing method adopts the heat preservation time not more than 5s, so that the controllability of the heating temperature is poor, the distribution of alloy elements in a final product is uneven, and the structure performance of the product is uneven and unstable;

thirdly, the final rapid cooling adopts water quenching to cool to room temperature without necessary tempering treatment, so that the structure property of the obtained final product and the distribution condition of alloy elements in the final structure can not ensure that the product can obtain the optimal obdurability, the final product has excessive strength and insufficient plasticity and toughness;

fourthly, the method of the invention causes the problems of poor plate shape, surface oxidation and the like of the steel plate due to the overhigh water quenching speed, so the patented technology has no or low practical application value.

At present, limited by the capability of the traditional continuous annealing furnace production line equipment, cold-rolled dual-phase steel products and relevant researches of an annealing process are carried out on the strip steel based on the heating rate (5-20 ℃/s) of the existing industrial equipment, so that the strip steel is subjected to slow heating to sequentially complete recovery, recrystallization and austenitizing phase change, the heating and soaking time is long, the energy consumption is high, and meanwhile, the traditional continuous annealing production line also has the problems of long retention time of the strip steel in a high-temperature furnace section, more rollers passing through and the like. The traditional continuous annealing unit generally requires soaking time of 1-3 min according to the requirements of product outline and capacity, and for the traditional production line with the unit speed of about 180 m/min, the number of rollers in a high-temperature furnace section is generally 20-40, so that the difficulty of controlling the surface quality of the 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 processes of recovery, recrystallization grain growth, austenite phase transformation process, grain growth and the like of a deformed structure through rapid heat treatment, increase the nucleation rate (including the recrystallization nucleation rate and the austenite phase deformation nucleation rate), shorten the grain growth time, refine grains, form a ferrite matrix phase in the cooling process, generate various strengthening phases and the gradient distribution of structures and components in the phases, and obtain the dual-phase steel with the yield strength of 902-1114 MPa, the tensile strength of 1264-1443 MPa, the elongation of 7-9.8% and the product of strength of 9.5-12.1 GPa%; the dual-phase steel obtained by the method has lower alloy content in the same-grade steel, and obtains good plasticity and toughness while improving the material strength; meanwhile, the rapid heat treatment process is adopted to improve the production efficiency, reduce the production cost and energy consumption, obviously reduce the number of furnace rollers and improve the surface quality of the steel plate.

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

the low-carbon low-alloy ultrahigh-strength dual-phase steel at 1280MPa level comprises the following chemical components in percentage by mass: c:0.10 to 0.17%, si:0.2 to 0.7%, mn: 1.8-2.8%, cr:0.3 to 0.9%, nb:0.02 to 0.07%, ti:0.02 to 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 to 0.05 percent; one or two of Mo and V can be contained, cr + Mo + Ti + Nb + V is less than or equal to 1.1 percent, and the balance is Fe and other inevitable impurities, and the alloy is obtained by the following process:

1) Smelting and casting

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

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 processing

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

slowly cooling to 670-770 ℃ at a cooling rate of 5-15 ℃/s after the heat equalization, 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 carrying out overaging treatment in the temperature range, wherein the overaging treatment time is as follows: less than or equal to 200s, and finally cooling to room temperature.

Preferably, the content of C is 0.055-0.110%.

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 content of Nb is 0.02 to 0.05%.

Preferably, the time of the whole rapid thermal treatment is 41 to 297s.

Preferably, in the step 2), the hot rolling temperature is more than or equal to A _r3 。

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

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

Preferably, in the step 4), the rapid heating is performed in a one-stage heating mode, and the heating rate is 50-300 ℃/s.

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

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

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

Preferably, in the 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 uniformly 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 reduces the plasticity of steel, but for cold-press formed steel sheets, low yield strength, high uniform elongation and high total elongation are required, so the carbon content should not be too high. The carbon phase in steel generally exists in two ways: 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 net-shaped carbides can appear in the steel, the strength, the plasticity and the toughness of the steel can be obviously reduced due to the existence of the net-shaped carbides, 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 poor, and the carbon content is reduced as much as possible on the premise of ensuring the strength.

For dual phase steels, the carbon element primarily affects the volume fraction of austenite formed during annealing, and the diffusion process of the carbon element in austenite or ferrite during austenite formation actually acts to control austenite grain growth. With the increase of the carbon content or the increase of the heating temperature of the critical area, the volume fraction of austenite is increased, further, the martensite phase structure formed after cooling is increased, the strength of the material is increased, but the plasticity is reduced at the same time. The increase of the carbon content increases the manufacturing difficulty of the working procedure before the heat treatment, so the carbon content is limited within the range of 0.10-0.17% by comprehensively considering the matching of the obdurability of the material, the characteristic of rapid heat treatment and the change rule of the carbon to the structure property of the final product.

Mn: manganese can form a solid solution with iron, so that the strength and hardness of ferrite and austenite in the carbon steel are improved, fine pearlite with high strength is obtained in the cooling process after hot rolling, and the content of the pearlite is increased along with the increase of the content of Mn. Manganese is a forming element of carbide at the same time, and the carbide of manganese can be dissolved into a cementite, so that the strength of pearlite is indirectly enhanced. Manganese can also strongly enhance the hardenability of steel, further improving the strength thereof.

For dual phase steels, manganese is a significant contributor to austenite formation kinetics during intercritical annealingOne of the chemical elements, manganese mainly affects the transformation and growth process of austenite to ferrite after the formation of austenite and the final equilibrium process of austenite and ferrite. Because the diffusion speed of the manganese element in the austenite is far less than that of the manganese element in the ferrite, the austenite controlled by the manganese diffusion takes longer time to grow, and the manganese element can take longer time to be uniformly distributed in the austenite. During rapid heating in a critical zone, if the heat preservation time is short, the manganese element cannot be uniformly distributed in austenite, and then if the cooling rate is insufficient, a more uniform martensite austenite island (mao island for short) structure cannot be obtained. In the dual-phase steel produced by adopting the rapid heating process (such as a rapid induction heating or rapid direct heating and water quenching cooling continuous annealing production line), because the manganese content is generally high and a large amount of pearlite exists in a matrix, the austenite generated firstly locally has high manganese content after being generated, the hardenability of an austenite island is ensured, and the uniform structure and uniform performance of a martensite austenite island (short for Mao island) can be easily obtained after cooling. In addition, manganese expands the gamma phase region and reduces A _c1 And A _c3 The manganese containing steel will therefore get a higher martensite volume fraction than the low carbon steel under the same heat treatment conditions. However, when the manganese content is further increased, there is a tendency that crystal grains in the steel are coarsened and the overheating sensitivity of the steel is increased, and when cooling after melt casting and forge rolling is not proper, 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. Considering the above factors comprehensively, 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 to enhance the yield strength and tensile strength of steel, and silicon increases the cold working deformation hardening rate of steel, and is a beneficial element in alloy steel. In addition, silicon has an obvious enrichment phenomenon on the surface of a fracture along the grain boundary of the silicon-manganese steel, and the segregation of silicon at the position of the grain boundary can slow down the distribution of carbon and phosphorus along the grain boundary, so that the embrittlement state of the grain boundary is improved. Silicon can improve the strength, hardness and wear resistance of steel, and can not obviously reduce the plasticity of the steel within a certain range. Silicon has strong deoxidizing capacity, is a common deoxidizing agent in steel making, and can increase the fluidity of molten steel, so that the general steel contains silicon, but when the content of the silicon in the steel is too high, the plasticity and the toughness of the steel are obviously reduced.

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

Nb: nb element is a carbide and nitride forming element and can satisfy such a requirement at a relatively low concentration. At normal temperature, most of the steel exists in the form of carbide, nitride, or carbonitride, and a small part of the steel is dissolved in ferrite. The addition of Nb can prevent austenite grains from growing and improve the coarsening temperature of steel grains. Nb element and carbon form stable NbC, and the addition of trace amount of Nb element in steel can raise the strength of matrix by means of its precipitation strengthening effect. The Nb element has obvious inhibition effect on the growth of ferrite recrystallization and the growth of austenite grains, can refine the grains and improve the strength and toughness of the steel; the Nb element may affect the mobility of grain boundaries, and also the phase transformation behavior and the formation of carbides. Nb can increase the content of carbon in the residual austenite, hinder the formation of bainite, promote the nucleation of martensite, obtain a dispersed martensite structure, improve the stability of the residual austenite, improve the strength of the dual-phase steel by adding Nb element, can obtain the dual-phase steel with certain strength under the conditions of low content of martensite and low content of C, and improve the toughness of the dual-phase steel; an additional benefit of the simultaneous addition of Nb is that the strength of the steel can be increased over a wider annealing temperature range. In the invention, the Nb element is an essential addition element, and the addition amount is not excessively large in consideration of factors such as cost increase.

Ti: ti is a microalloy element, belongs to a ferrite forming element of a closed gamma region, can improve the critical point of steel, and Ti and C in the steel can form very stable TiC which is extremely difficult to dissolve in the austenitizing temperature range of general heat treatment. Since TiC particles refine austenite grains, the chances of new phase nucleation increase during austenite decomposition transformation, which accelerates austenite transformation. Ti forms TiC and TiN precipitates with C and N, and is more stable than carbonitride of Nb and V, and significantly reduces the diffusion rate of C in austenite to significantly reduce the austenite formation rate, and the formed carbonitride precipitates in the matrix and pins at the austenite grain boundary to inhibit the austenite grain growth. In the cooling process, the precipitated TiC has the precipitation strengthening effect; in the tempering process, ti slows down the diffusion of C in an alpha phase, slows down the precipitation and growth of carbides such as Fe, mn and the like, increases the tempering stability, and can play a role in secondary hardening through TiC precipitation. The high temperature strength of the steel can be improved by microalloying of Ti. By adding a trace amount of Ti into the steel, on one hand, the strength and the welding performance of the steel can be improved while the carbon equivalent content is reduced; on the other hand, impurities such as oxygen, nitrogen, sulfur, etc. are fixed, thereby improving weldability of steel; secondly, due to the effect of microscopic particles, such as the insolubility of TiN at high temperature, coarsening of grains in the heat-affected zone is prevented, and the toughness of the heat-affected zone is improved, thereby improving the weldability of the steel. In the invention, ti is an essential additive element, and the addition amount is not too much in consideration of factors such as cost increase and the like.

B: the content of the B element in the steel is very small, the B element mainly has the effect of improving the hardenability of the steel, the influence effect is much larger than that of Cr, mn and other alloy elements, and a large amount of other rare and precious metals such as nickel, chromium, molybdenum and the like can be saved by applying the trace B element. For this purpose, the content is generally defined 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 replacing molybdenum, it being noted that molybdenum cannot be completely replaced with boron because it prevents or reduces temper embrittlement, while boron has a slight tendency to promote temper embrittlement. Boron, nitrogen and oxygen have strong affinity, and the aging phenomenon of the steel can be eliminated by adding 0.007 percent of boron into the boiling steel. However, since only B element existing in a solid solution state has a beneficial effect on the hardenability of steel and B element existing in a compound state has no effect on the hardenability of steel, fixation of C and N elements should be considered when the hardenability is increased by B element. The element B of the present invention is an essential additive element, but the amount of the element B is not limited to be excessively large.

Cr: the main function of chromium in steel is to improve hardenability, so that the steel has better comprehensive mechanical properties after quenching and tempering. Chromium forms a continuous solid solution with iron, narrowing the austenite phase region, forms multiple carbides with carbon, and has a greater affinity for carbon than the elements iron and manganese. Chromium and iron may form an intermetallic sigma phase (FeCr), chromium reducing 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 brittleness tendency of the steel. The chromium element can improve the strength and the hardness of the steel, and other alloy elements are added, so that the effect is obvious. Since Cr increases the quenching ability of steel during air cooling, it adversely affects the weldability of steel. However, when the chromium content is 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., with V), the adverse effect of Cr on weldability is greatly reduced. If Cr, mo, V, etc. are present in the steel at the same time, the weld properties of the steel are not significantly adversely affected even if the Cr content reaches 1.7%. The chromium element is a beneficial and unnecessary addition element, and the addition amount is not too much in consideration of factors such as cost increase and the like.

Mo: molybdenum inhibits the self-diffusion of iron and the diffusion rate of other elements. The atomic radius of Mo is larger than that of alpha-Fe atoms, so that when Mo is dissolved in the alpha solid solution, the solid solution generates strong lattice distortion, and meanwhile, the crystal lattice atomic bond attraction can be increased by Mo, and the recrystallization temperature of alpha ferrite is increased. The strengthening effect of Mo in pearlite type, ferrite type and martensite type steel is also obvious even in high-alloy austenitic steel. The good function of Mo in steel also needs to be determined by the interaction with other alloying elements in the steel. When strong carbide forming elements V, nb and Ti are added into steel, the solid solution strengthening effect of Mo is more obvious. This is because when a strong carbide-forming element is combined with C into a stable carbide, mo can be promoted to be more efficiently dissolved into solid solution, thereby contributing more to the improvement of the heat strength of the steel. Addition of Mo can also increase the hardenability of the steel. Mo inhibits the transformation of pearlite areas and accelerates the transformation in the intermediate temperature area, so that Mo-containing steel can form a certain amount of bainite under the condition of a high cooling speed, and the formation of ferrite is eliminated, which is one of the reasons why Mo favorably influences the heat strength of low-alloy heat-resistant steel. Mo also significantly reduces the hot embrittlement tendency of the steel and reduces the pearlite nodularisation rate. When the Mo content is 0.15% or less, the weldability of the steel is not adversely affected. The molybdenum element is a beneficial and unnecessary addition element, and the addition amount is not too large in consideration of factors such as cost increase and the like.

V: v is a ferrite stabilizing element and a strong carbide forming element, has strong grain refining effect and can make the steel structure compact. The addition of V to steel can improve the strength, plasticity and toughness of steel at the same time. Vanadium also improves the high temperature strength of structural steels. Vanadium does not improve hardenability. The addition of trace microalloy element V in the steel can ensure that the steel has good weldability and other service performances by the dispersion precipitation of carbon and nitride particles (the size is less than 5 nm) and the solid solution of V to refine grains under the condition of low carbon equivalent. A trace amount of V is added into the steel, so that on one hand, the strength and the welding performance of the steel can be improved while the carbon equivalent content is reduced; on the other hand, impurities such as oxygen, nitrogen, sulfur, etc. are fixed, thereby improving weldability of steel; secondly, due to the effect of microscopic particles, such as the insolubility of V (CN) at high temperature, coarsening of the crystal grains in the heat affected zone is prevented, and the toughness of the heat affected zone is improved, thereby improving the weldability of the steel. The microalloy elements in the invention are beneficial and unnecessary addition elements, and the addition amount is not too much 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 rolled hard strip steel in the heat treatment process by a rapid heat treatment method (comprising the processes of rapid heating, short-time heat preservation and rapid cooling), and finally obtains various fine, uniform and dispersedly distributed tissue structures and good strong plasticity matching.

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

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

In the heat treatment process disclosed in chinese patent CN107794357B and US patent US2019/0153558A1, although the heating process is also treated in stages: heating to 300-500 deg.C at a heating rate of 1-10 deg.C/s, heating to 850-950 deg.C at a heating rate of 100-500 deg.C/s, maintaining for no more than 5s, and water-quenching to room temperature. The treatment method requires that the steel plate must be heated to a high-temperature area of single-phase austenite, which improves the high-temperature resistance requirement of equipment and increases the manufacturing difficulty, and simultaneously, a water-cooling mode is adopted, so that although the cooling speed is extremely high, the growth time of a grain structure in the high-temperature area can be greatly reduced, the distribution of alloy elements in a final product is inevitably uneven, the structure performance of the product is uneven and unstable, and the too high cooling speed can also cause a series of problems of poor plate shape of the steel plate, surface oxidation and the like.

Only by comprehensively controlling the whole heat treatment process: the method comprises the processes of rapid heating (heating speed is controlled by sections), short-time soaking and rapid cooling, so that the finely controlled optimal grain size, alloy elements and phase structures are uniformly distributed, and the optimal toughness matching product is finally 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, and is reduced by about 30 percent 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, and meanwhile, good plasticity and toughness are obtained, and the service performance of the material is improved; the ferrite and martensite tissues obtained by the method have various shapes such as blocks, strips, granules and the like, and the two are more uniformly distributed, so that better strong plasticity can be obtained.

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

1) Smelting and casting

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

2) Hot rolling and coiling

The coiling temperature is 550-680 ℃;

3) Cold rolling of steel

Cold rolling reduction rate is 40-85%, and rolling hard state strip steel or steel plate is obtained;

4) Rapid thermal processing

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 ℃, wherein the rapid heating adopts a one-stage type or two-stage type; when one-stage rapid heating is adopted, the heating rate is 50-500 ℃/s; when two-section type rapid heating is adopted, the first section is heated from room temperature to 550-650 ℃ at the heating rate of 15-500 ℃/s, and the second section is heated from 550-650 ℃ to 750-845 ℃ at the heating rate of 50-500 ℃/s;

b) Soaking heat

Soaking at 750-845 ℃ in an austenite and ferrite two-phase region, wherein the soaking time is 10-60 s;

c) Cooling down

Soaking the strip steel or the steel plate, and then 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 quickly cooling from 670-770 ℃ to 230-280 ℃ at a cooling rate of 50-200 ℃/s, and carrying out overaging treatment, wherein the overaging treatment time is as follows: less than or equal to 200s, and cooling to room temperature at the speed of 30-50 ℃/s after overaging treatment.

Preferably, the time for the whole rapid thermal treatment process is 41 to 297s.

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

Preferably, in the 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 550-650 ℃ to 750-845 ℃ at a heating rate of 50-300 ℃/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 550-650 ℃ to 750-845 ℃ at a heating rate of 80-300 ℃/s.

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

Preferably, in the soaking process in the step 4), the temperature of the strip steel or the steel plate is raised or lowered within a small range within the soaking time period, the temperature after the temperature rise is not more than 845 ℃, and the temperature after the temperature reduction is not less than 750 ℃.

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

In the rapid heat treatment manufacturing method of the 1280 MPa-level low-carbon low-alloy ultrahigh-strength dual-phase steel, the steps are as follows:

1. heating rate control

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

wherein X (T) is ferrite recrystallization volume fraction; n is an Avrami index, is related to a phase change mechanism, depends on the decay period of the recrystallization nucleation rate, and is generally selected within the range of 1-4; t is the heat treatment temperature; t is a unit of _star Is the recrystallization onset temperature; β is the heating rate; b is constant at a given isothermal temperature, varying the isothermal temperature, b varies accordingly, b (T) is obtained by the following formula:

b＝b ₀ exp(-Q/RT)

from the above formula and the associated experimental data, it can be concluded that the recrystallization onset temperature (T) increases with increasing heating rate _star ) And end temperature (T) _fin ) All are increased; when the heating rate is above 50 ℃/s, the austenite transformation and recrystallization processes are overlapped, the recrystallization temperature is raised to the temperature of the two-phase region, and the faster the heating rate, the higher the ferrite recrystallization temperature.

The traditional heat treatment process is influenced by the heating technology and is slow heating, under the condition, the deformation matrix is sequentially subjected to recovery, recrystallization and grain growth, then phase transformation nucleation from ferrite to austenite and austenite grain growth are generated, the phase transformation nucleation point is mainly concentrated at the grown ferrite grain boundary, and the nucleation rate is low. The grain structure of the finally obtained dual-phase steel is therefore relatively coarse.

Under the rapid heating condition, the deformed matrix is just recrystallized or is not recrystallized (even the transformation is not fully performed), the phase transformation from ferrite to austenite begins to occur, and because the grains are fine and the grain boundary area is large when the recrystallization is just completed or is not completed, the phase transformation nucleation rate is obviously improved, and the austenite grains are obviously refined. Particularly, when the ferrite recrystallization process and the austenite phase transformation process are overlapped, a large number of crystal defects such as dislocation and the like are reserved in the ferrite crystal, a large number of nucleation points are provided for austenite, and the nucleation of the austenite presents a burst type nucleation, so that the austenite crystal grains are further refined, and the high-density dislocation line defects also become channels for high-speed diffusion of carbon atoms, so that each austenite crystal grain can be rapidly generated and grown, and the austenite crystal grains are fine and the volume fraction is increased.

The structural evolution and the distribution of alloy elements and phase components are finely controlled in the rapid heating process, and a good foundation is laid for the growth of an austenite structure in the subsequent soaking process, the distribution of the alloy components and the transformation from austenite to martensite in the rapid cooling process. Finally, the final product structure with refined grains, reasonable elements and various phase distributions can be obtained. The invention comprehensively considers the factors of the effect of rapidly heating and thinning crystal grains, the manufacturing cost, the manufacturability and the like, and sets the heating rate to be 50-500 ℃/s when one-stage rapid heating is adopted and 15-500 ℃/s when two-stage rapid heating is adopted.

Because the influence of rapid heating on the structure evolution processes of material such as recovery, recrystallization and grain growth is different in different temperature interval ranges, in order to obtain optimal structure control, the optimal heating rates of different heating temperature intervals are different: the influence of the heating rate on the recovery process is the largest from 20 ℃ to 500-650 ℃, the heating rate is controlled to be 15-300 ℃/s, and the preferred heating rate is 50-300 ℃/s; the heating temperature is from 500 to 650 ℃ to the austenitizing temperature of 750 to 845 ℃, the heating rate has the largest influence on the nucleation rate and the grain growth process, and the heating rate is controlled to be 50 to 300 ℃/s; more preferably 80 to 300 ℃/s.

2. Soaking temperature control

The selection of the soaking temperature needs to be combined with the control of the material structure evolution process in each temperature stage of the heating process, and meanwhile, the evolution and the control of the structure in the subsequent rapid cooling process need to be considered, so that the optimal structure and distribution can be finally obtained.

The soaking temperature generally depends on the C content, the C content in the ultrahigh-strength dual-phase steel is 0.10-0.15%, and the A content in 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 addition, a large amount of dislocation of the material 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 austenite structures.

The invention firstly proposes the soaking temperature to be increased and decreased within a certain range for the control of the soaking temperature: namely, the temperature is increased obliquely and decreased obliquely in the soaking process, but the soaking temperature must be kept within a certain range. The benefits of this are: in the process of rapidly increasing and decreasing the temperature within the temperature range of the two-phase region, the superheat degree and the supercooling degree are further increased to facilitate rapid phase transformation, when the temperature increasing and decreasing amplitude and the temperature decreasing and decreasing rate are both large enough, grains can be further refined through repeated transformation from ferrite to austenite and transformation from austenite to ferrite, meanwhile, certain influence is exerted on formation of carbides and uniform distribution of alloy elements, and finally, finer structures and alloy elements with uniform distribution are formed.

After cold rolling, the dual-phase steel contains a large amount of undissolved fine and uniformly distributed carbides, the carbides can become nucleation points of austenite, and in the heating and soaking processes, the carbides can play a role in mechanical obstruction to the growth of austenite grains, thereby being beneficial to refining the grain size of the alloy steel. However, if the heating temperature is too high, the number of undissolved carbides is greatly reduced and the size thereof is increased, which impairs the effect of inhibition, increases the tendency of crystal grains to grow, and further lowers the strength of the steel. When the amount of undissolved carbides is too large, aggregation may occur, resulting in uneven distribution of local chemical components, and when the carbon content in the aggregated portion is too high, local overheating may also occur. Ideally, a small amount of fine granular undissolved carbides should be uniformly distributed in the steel, so that the abnormal growth of austenite grains can be prevented, the content of each alloy element in a 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 chosen with the aim of obtaining fine and homogeneous austenite grains, in order to achieve a fine martensitic structure after cooling. The overhigh soaking temperature can cause the austenite grains to be coarse, and the martensite structure obtained after quick cooling is also coarse, so that the mechanical property of the steel is poor; but also increases the amount of retained austenite, reduces the amount of martensite, and reduces the hardness and wear resistance of the steel. The excessively low soaking temperature not only reduces the amount 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 causes adverse effects on the mechanical properties of the steel. The soaking temperature of the hypoeutectoid steel should be Ac ₃ + 30-50 ℃. In the case of ultra-high strength steel, the presence of carbide-forming elements hinders the transformation of carbides, so the soaking temperature can be suitably increased. By combining the factors, the invention selects 770-845 ℃ as soaking temperature to obtain more ideal and more reasonable final tissue.

3. Soaking time control

Because the rapid heating is adopted, the material in the two-phase region contains a large amount of residual dislocation, a large amount of nucleation points are provided for the formation of austenite, and a rapid diffusion channel is provided for carbon atoms, so that the austenite can be formed very rapidly, and 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 the more the carbon content of the residual austenite is remained; however, if the heat preservation time is too short, the distribution of alloy elements in the steel is uneven, and the austenitizing is insufficient; too long heat preservation time easily causes coarse austenite grains. The influence factor of the soaking time also depends on the contents of carbon and alloy elements in the steel, when the contents of the carbon and the alloy elements in the steel are increased, the thermal conductivity of the steel is reduced, and because the diffusion speed of the alloy elements is slower than that of the carbon elements, the alloy elements can obviously delay the structure transformation of the steel, and the soaking time is prolonged appropriately. Therefore, the control of the soaking time needs to be made by strictly combining the soaking temperature, the rapid cooling and the rapid heating process, and the ideal tissue and element distribution can be finally obtained. In conclusion, the soaking and heat preservation time is set to be 10-60 s.

4. Fast cooling rate control

In order to obtain a martensite strengthening phase, the cooling speed of the material during rapid cooling must be greater than the critical cooling speed to obtain a martensite structure, the critical cooling speed mainly depends on the material components, 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 the Si and the Mn, and the requirement on the critical cooling speed is reduced.

The cooling rate also needs to comprehensively consider the structure evolution and the alloy diffusion distribution results of the heating process and the soaking process so as to finally obtain reasonable structure distribution and alloy element distribution of each phase. The cooling rate is too low to obtain a martensite structure, so that the strength is reduced, and the mechanical property cannot meet the requirement; too large cooling rate can generate larger quenching stress (namely, structural stress and thermal stress) to cause serious defect of the plate shape, and the defect of the plate shape is particularly serious when the cooling is not uniform, and even the sample is easy to be seriously deformed and cracked. Therefore, the rapid cooling speed is set to be 50-200 ℃/s.

5. Overaging treatment

After traditional heat treatment, overaging is mainly used for tempering quenched martensite to improve the comprehensive performance of the dual-phase steel. Improper setting of the overaging temperature and time causes martensite to decompose, directly deteriorating the mechanical properties of the dual phase steel. The overaging temperature and time are set by comprehensively considering the shape and distribution of martensite structure, the content and distribution of elements and the size and distribution of other structures. Therefore, overaging control needs to be established by integrating various parameters of the previous heating process, soaking process and cooling process. According to the method, the tissue evolution and element distribution conditions in the processes of rapid heating, short-time heat preservation and rapid cooling are combined, and the overaging temperature range is set to be 230-280 ℃; the overaging time is controlled to be less than or equal to 200s.

The invention realizes the rapid heat treatment process by carrying out rapid heating and rapid cooling process transformation on the traditional continuous annealing unit, can greatly shorten the length of a heating and soaking section of the annealing furnace (at least one third shorter than the traditional continuous annealing furnace), improves the production efficiency of the traditional continuous annealing unit, reduces the production cost and energy consumption, obviously reduces the number of furnace rollers of the continuous annealing furnace, particularly the number of furnace rollers of a high-temperature furnace section, can improve the surface quality control capability of strip steel, and obtains the strip steel product with high surface quality. Meanwhile, the novel continuous annealing unit of the rapid heat treatment process technology is established, so that the purposes of short and bold unit, flexible material transition, strong regulation and control capability and the like can be realized; for the product material, the crystal grains of the strip steel can be refined, the strength of the material is further improved, the alloy cost and the manufacturing difficulty of the working procedures before heat treatment are reduced, and the use performances of the material for users such as forming, welding and the like are improved.

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

(1) The invention inhibits the recovery of a deformation structure and a ferrite recrystallization process in a heat treatment process through rapid heat treatment, so that the recrystallization process is overlapped with an austenite phase 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 (usually 3-10 mu m) of the product produced by the prior art is reduced by about 30 percent; the ferrite and martensite tissues obtained by the method have various shapes such as blocks, strips, granules and the like, and the ferrite and the martensite tissues are more uniformly distributed, so that better strong plasticity can be obtained; the strength of the material is improved, and simultaneously, the material has good plasticity and toughness, and the service performance of the material is improved.

(2) Compared with the dual-phase steel obtained by the traditional heat treatment mode, the dual-phase steel obtained by the invention has the advantages that the grain size is reduced by more than 30 percent, the strength and toughness of the material are obviously improved, the yield strength is 902-1114 MPa, the tensile strength is 1264-1443 MPa, the yield strength can be controlled in a smaller interval 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 elongation is 9.5-12.1 GPa%, and the forming performance is excellent.

(3) According to the rapid heat treatment process for the low-carbon low-alloy ultrahigh-strength 1280 MPa-grade dual-phase steel, the time for the whole heat treatment process can be shortened to 41-297 s, the time of the whole heat treatment process is greatly reduced (the time of a traditional continuous annealing process 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 one-time 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 process technology reduces the time of the heating process and the soaking process, shortens the length of the furnace, reduces the number of furnace rollers and reduces the probability of generating surface defects in the furnace, so the surface quality of the product is obviously improved; in addition, due to the refinement of product grains and the reduction of the alloy content of the material, the processing and forming performances such as hole expansion performance, bending performance and the like, and the user service performances such as welding performance and the like of the dual-phase steel obtained by adopting the technology of the invention are also improved.

The low-carbon low-alloy ultra-high-strength 1280MPa grade dual-phase steel obtained by the invention has important values on the development of new-generation light-weight transportation tools such as automobiles, trains, ships, airplanes and the like and the healthy development of corresponding industries and advanced manufacturing industries.

Drawings

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

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

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

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

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

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

Detailed Description

The present invention is further illustrated with reference to the following examples and the accompanying drawings, wherein the examples are carried out on the premise of the technical solution of the present invention, and detailed embodiments and specific operating procedures are given, but the scope of the present invention is not limited to the following examples.

The compositions of the test steels according to the present invention are shown in Table 1, the specific parameters of the examples according to the present invention and the conventional process are shown in tables 2 and 3, and tables 4 and 5 show the main properties of the steels prepared according to 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 steel of the same grade can be reduced, the crystal grains are refined, and the material structure composition and the matching of the strength and the toughness are 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%, and the product of strength and elongation is 9.5-12.1 GPa%, which is higher than that of the dual-phase steel produced by the traditional process.

FIG. 1 is a structural diagram of a typical composition A steel obtained in example 1, and FIG. 2 is a structural diagram of a typical composition A steel obtained in conventional process example 1. From the figure, the tissues treated by different heat treatment modes have great differences. The dual-phase steel structure obtained after the rapid heat treatment process of the embodiment of the invention consists of fine and uniform martensite structures and a small amount of carbides which are dispersedly distributed on a ferrite matrix, and the ferrite, the martensite crystal grain structures and the carbides are very fine and are uniformly distributed in the matrix, which is very favorable for improving the strength and the plasticity of the material. The microstructure diagram of the typical dual-phase steel is obtained by the traditional process treatment, namely a small amount of black martensite structures exist on white ferrite grain boundaries, and due to element segregation and other reasons, the material structure treated by the traditional process presents certain directionality, and the ferrite structures of the material structure are in strip distribution along the rolling direction. The tissue characteristics treated by the traditional process are as follows: the grains are coarse and have a certain banded structure, martensite and carbide are in a net distribution along ferrite grain boundaries, ferrite grains are relatively coarse, and the two-phase structure of ferrite and martensite is not uniformly distributed.

FIG. 3 is a structural diagram of a typical composition F steel obtained in example 6, and FIG. 4 is a structural diagram of a typical composition M steel obtained in example 12. FIG. 5 is a structural diagram obtained by subjecting an S steel, which is an exemplary composition, to example 23, and FIG. 6 is a structural diagram obtained by subjecting an M steel, which is an exemplary composition, to example 24. Examples 6, 12, 23, 24 are all processes with a short overall heat treatment period. It can be seen from the figure that by using the method of the present invention, the removal of the aging treatment section can also obtain a very uniform, fine and dispersedly distributed phase structure. Therefore, the preparation method of the dual-phase steel can refine the crystal 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 transform 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 is 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 exquisite structure, flexible material transition, strong regulation and control capability and the like; for the material, the grain of the strip steel can be refined, the strength of the material is further improved, the alloy cost and the manufacturing difficulty of the working procedure before heat treatment are reduced, and the welding performance of the material and other user service performances are improved.

In conclusion, the invention adopts the rapid heat treatment process, so that the technological progress of the continuous annealing process of the cold-rolled strip steel is greatly promoted, the austenitizing process of the cold-rolled strip steel from room temperature to the last is expected to be completed within dozens of seconds, dozens of seconds or even several seconds, the heating section length of the continuous annealing furnace is greatly shortened, the speed and the production efficiency of a continuous annealing unit are convenient to improve, the number of rollers in the furnace of the continuous annealing unit is obviously reduced, for a rapid heat treatment production line with the unit speed of about 180 meters/minute, the number of rollers in the high-temperature furnace section is not more than 10, and the surface quality of the strip steel can be obviously improved. Meanwhile, the rapid heat treatment process method of the recrystallization and austenitizing processes finished 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 alloy components, rolling process and other pre-process conditions.

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

Claims

1.1280 MPa-level low-carbon low-alloy ultrahigh-strength dual-phase steel comprises the following chemical components in percentage by mass: c:0.10 to 0.17%, si:0.2 to 0.7%, mn: 1.8-2.8%, cr:0.3 to 0.9%, nb:0.02 to 0.07%, ti:0.02 to 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 to 0.05 percent; one or two of Mo and V can be contained, cr + Mo + Ti + Nb + V is less than or equal to 1.1 percent, and the balance is Fe and other inevitable impurities, and the alloy is obtained by the following process:

1) Smelting and casting

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

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 processing

or rapidly cooling from 670-770 ℃ to 230-280 ℃ at a cooling rate of 50-200 ℃/s, and carrying out overaging treatment in the temperature range, wherein the overaging treatment time is as follows: less than or equal to 200s, and finally cooling to room temperature at the speed of 30-50 ℃/s.

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

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

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

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

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

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

8. The 1280MPa grade low-carbon low-alloy ultrahigh-strength dual-phase steel according to claim 1, wherein the rapid thermal treatment time is 41-297 s.

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

10. The ultra-high strength dual phase steel with low carbon and low alloy at 1280MPa level according to claim 1 or 9, wherein the coiling temperature in step 2) is 580-650 ℃.

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

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

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

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

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

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

17. The ultra-high strength dual phase steel of 1280MPa grade low carbon low alloy according to any of claims 1-16, wherein the microstructure of said dual phase steel is a ferrite and martensite dual phase structure with a uniformly distributed average grain size of 1-3 μm.

18. The 1280 MPa-grade low-carbon low-alloy ultrahigh-strength dual-phase steel according to any one of claims 1 to 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 product of strength and elongation of 9.5 to 12.1GPa%.

19. The rapid thermal processing manufacturing method of 1280MPa grade low carbon low alloy ultra high strength dual phase steel according to any of claims 1 to 18, comprising the steps of:

1) Smelting and casting

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

2) Hot rolling and coiling

The coiling temperature is 550-680 ℃;

3) Cold rolling

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

4) Rapid thermal processing

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 the temperature of 750-845 ℃; the rapid heating adopts a one-stage or two-stage type, and when the one-stage type rapid heating is adopted, the heating rate is 50-500 ℃/s; when two-section type rapid heating is adopted, the first section is heated from room temperature to 550-650 ℃ at the heating rate of 15-500 ℃/s, and the second section is heated from 550-650 ℃ to 750-845 ℃ at the heating rate of 50-500 ℃/s;

b) Soaking heat

c) Cooling down

Soaking the strip steel or the steel plate, and then 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 quickly 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: less than or equal to 200s, and cooling to room temperature at the speed of 30-50 ℃/s after overaging treatment.

20. The rapid thermal processing method for manufacturing a low carbon and low alloy ultra-high strength dual-phase steel of 1280MPa grade according to claim 19, wherein the time required for the entire rapid thermal processing is 41-297 s.

21. The rapid thermal processing method of 1280 MPa-grade low-carbon low-alloy ultrahigh-strength dual-phase steel according to claim 19, wherein the hot rolling temperature in step 2) is not less than A _r3 。

22. The rapid thermal processing method for manufacturing the low carbon and low alloy ultra-high strength dual-phase steel with 1280MPa level according to claim 19 or 21, wherein the coiling temperature in the step 2) is 580 to 650 ℃.

23. The rapid thermal processing method for manufacturing low carbon low alloy ultra high strength dual phase steel of 1280MPa grade according to claim 19, wherein the cold rolling reduction in step 3) is 60-80%.

24. The rapid thermal processing method for manufacturing the ultra-high strength dual-phase steel with low carbon and low alloy at 1280MPa level according to claim 19, wherein in the step 4), the rapid heating is performed in one stage at a heating rate of 50-300 ℃/s.

25. The rapid thermal processing method for manufacturing a low-carbon low-alloy ultrahigh-strength dual-phase steel of 1280MPa level according to claim 18, wherein in the step 4), the rapid heating is performed in two stages, 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 550-650 ℃ to 750-845 ℃ at a heating rate of 50-300 ℃/s.

26. The rapid thermal processing method for manufacturing low-carbon low-alloy ultrahigh-strength dual-phase steel at 1280MPa according to claim 19, wherein the rapid heating in step 4) is performed in two stages, 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 550-650 ℃ to 750-845 ℃ at a heating rate of 80-300 ℃/s.

27. The rapid thermal processing method for manufacturing a low carbon low alloy ultra-high strength dual-phase steel of 1280MPa class according to claim 19, wherein in the step 4), the rapid cooling rate is 50 to 150 ℃/s.

28. The rapid heat treatment manufacturing method of 1280 MPa-level low-carbon low-alloy ultrahigh-strength dual-phase steel according to claim 19, wherein in the soaking step of 4), the soaking is performed while maintaining the temperature after the strip steel or the steel sheet is heated to the target temperature of the two-phase region of austenite and ferrite.

29. The rapid heat treatment manufacturing method of the 1280 MPa-level low-carbon low-alloy ultrahigh-strength dual-phase steel according to claim 19, wherein in the soaking process of the step 4), the steel strip or the steel plate is subjected to small-amplitude temperature rise or small-amplitude temperature drop in the soaking time period, the temperature after temperature rise does not exceed 845 ℃, and the temperature after temperature drop does not fall below 750 ℃.

30. The rapid thermal processing method for manufacturing ultra-high strength dual-phase steel of 1280MPa grade low carbon low alloy according to claim 19, 28 or 29, wherein the soaking time in step 4) is 10-40 s.