CN115181893A

CN115181893A - 1180 MPa-grade low-carbon low-alloy hot-galvanized TRIP steel and rapid heat treatment hot-galvanized manufacturing method

Info

Publication number: CN115181893A
Application number: CN202110360524.1A
Authority: CN
Inventors: 李俊; 王健; 陈培林; 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: CN115181893B

Abstract

1180 MPa-grade low-carbon low-alloy hot-dip galvanized TRIP steel and a rapid heat treatment hot-dip galvanizing manufacturing method, wherein the steel comprises the following components in percentage by mass: 0.19 to 0.25 percent of C, 1.3 to 2.0 percent of Si, 1.8 to 2.4 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.002 percent of S, 0.02 to 0.05 percent of Al, and one or two of Cr, mo, ti, nb and V, less than or equal to 0.5 percent of Cr + Mo + Ti + Nb + V, and the balance of Fe and other inevitable impurities. The rapid hot galvanizing step comprises: rapid heating, short-time heat preservation, rapid cooling, bainite isothermal treatment, reheating, hot galvanizing and rapid cooling (hot-dip galvanizing of a pure zinc GI product); or after hot galvanizing, the hot galvanized steel is heated, alloyed and rapidly cooled (alloyed hot galvanized GA product). By controlling the recrystallization and phase change process in the heating process and the phase change process in the cooling process, the strength of the material is obviously improved, and meanwhile, good plasticity and toughness are obtained; the rapid heat treatment improves the mechanical property of the material while improving the heat treatment efficiency, and expands the range of the material property interval.

Description

1180 MPa-grade low-carbon low-alloy hot-galvanized TRIP steel and rapid heat treatment hot-galvanized manufacturing method

Technical Field

The invention belongs to the technical field of rapid heat treatment of materials, and particularly relates to 1180 MPa-grade low-carbon low-alloy hot-dip galvanized TRIP steel (comprising hot-dip galvanized GI products and hot-dip galvanized GA products) and a rapid heat treatment hot-dip galvanized manufacturing method.

Background

With the gradual improvement of awareness of energy conservation and material safety service, the use of high-strength steel, especially advanced high-strength steel, is increasing, so that the development of the advanced high-strength steel by iron and steel enterprises and scientific research institutes is more and more important.

In order to further increase the product of strength and elongation of steel products, in particular, the development of advanced high-strength steels represented by TRIP (transformation induced plasticity) steels has been increasingly emphasized. The cold-rolled transformation induced plasticity TRIP steel structure is composed of bainite and residual austenite contained in a ferrite matrix, and metastable residual austenite is transformed into martensite at the time of plastic deformation, thereby having excellent elongation and formability at the time of working high strength. The forming performance of the steel is improved through the deformation induced transformation effect of the metastable retained austenite in a large strain range, the total elongation can reach 20-40%, and the tensile strength of the steel is generally not more than 980MPa due to the existence of a softer polygonal ferrite structure. The mechanical properties of the TRIP steel are determined by the volume fractions and strengths of ferrite, bainite and austenite, and the structure morphology and distribution of each phase, especially the stability of the retained austenite for inhibiting the transformation of strain-induced martensite. The heat treatment process of the TRIP steel mainly comprises two main stages, namely an austenitizing annealing process and a bainite isothermal treatment process.

1. Heating and austenitizing process

In the continuous heating annealing process, the deformed matrix structure recovers and recrystallizes first, and in the temperature range, cementite in the matrix begins to dissolve in ferrite. When the heating temperature exceeds A _C1 Thereafter, if the temperature is high and the time is sufficient, cementite can be completely dissolved in austenite, completing the austenitizing process. By critical annealing, the carbon in the austenite phase is enriched to A _c3 Wire, thereafter, if the proeutectoid cementite is suppressed by the alloying elements Si, al, etc., the carbon concentration reaches T during austempering ₀ Or T' ₀ 。

2. Rapid cooling and bainite isothermal process

The material is rapidly cooled after austenitizing, heat preservation (isothermal) treatment is carried out when the material is cooled to a bainite transformation temperature, bainite transformation starts to occur at a supercooled austenite crystal boundary, residual carbon diffuses into austenite which does not undergo phase transformation reaction to form carbon-rich austenite after the carbon content in the bainite is lower than that in the austenite, the carbon-rich austenite stops phase transformation when the carbon content in the carbon-rich austenite reaches a certain critical value, and residual austenite is formed after the material is cooled to room temperature.

At present, aiming at the development of a hot galvanizing TRIP process, the structure performance of hot galvanizing TRIP steel is mainly changed by adding alloy elements and adjusting the temperature and time of quenching and distribution processes in the hot galvanizing TRIP process.

Japanese patent JP2010-053020 discloses "a high-strength hot-dip galvanized steel sheet with excellent workability and a manufacturing method thereof", which has a composition structure in mass percent (%): c:0.04 to 0.15%, mn:0.8 to 2.2%, si:0.7 to 2.3 percent of Fe, less than or equal to 0.1 percent of P, less than or equal to 0.01 percent of S, less than or equal to 0.1 percent of Al, less than or equal to 0.008 percent of N, and the balance of Fe and other inevitable impurity elements. The metallographic structure is composed of more than 70% of ferrite phase, 2-10% of bainite phase, 0-12% of pearlite phase and 1-8% of residual austenite phase. The ferrite has an average grain size of 18 μm or less, and the retained austenite has an average grain size of 2 μm or less. The steel of the present invention has a tensile strength of 590MPa or more and excellent workability (ductility and hole expansibility). The invention is a steel with tensile strength of 600-700MPa, which can not meet the strength requirement of ultra-high strength steel.

Chinese patent CN103805840B discloses a high-formability hot-dip galvanized ultrahigh-strength steel plate and a manufacturing method thereof, wherein the steel comprises the following chemical components in percentage by weight: c:0.15 to 0.25%, mn:1.5 to 3.0%, si:1.0 to 2.0 percent of N, less than or equal to 0.008 percent of P, less than or equal to 0.015 percent of S, and the balance of Fe and other inevitable impurities. The steel has a room temperature structure comprising 10-30% of ferrite, 60-80% of martensite and 5-15% of residual austenite; the yield strength is 600-900MPa, the tensile strength is 980-1200MPa, and the elongation is 15-20%. The steel is mainly characterized in that through proper component design and adjustment, the traditional continuous annealing production process is adopted, the contents of Si and Mn are improved, and the annealing and furnace atmosphere control process are combined, so that the material has high strength and better plasticity. The method of the invention has high continuous annealing temperature and rapid cooling initial temperature, which puts higher requirements on production organization and plate shape control, and the invention adopts high Si and high Mn content to improve the strength, which is easy to increase the difficulty for the production and manufacturing process, and simultaneously, the high Si and high Mn alloy content can bring corresponding difficulty for downstream users, the platability of the hot-dip process is also very difficult, and the surface quality and the corrosion resistance of the product can be influenced.

Chinese patent CN105274301B discloses a 'production method of an iron-zinc alloy coated steel plate with yield strength more than or equal to 220 MPa', which comprises the steps of molten iron desulphurization, converter smelting and continuous casting to form a blank; carrying out hot rolling: the rough rolling temperature is 1045 ℃, and the finish rolling temperature is 880 ℃; the coiling temperature is 675 ℃; cold rolling to the required thickness; continuously hot galvanizing, wherein the speed of a machine set is 100-130m/min, and the temperature of zinc liquid is 460 ℃; rapidly cooling at a cooling speed of 43 ℃/s; and cooling by adopting gas mist after the zinc and iron are alloyed at the cooling speed of 38 ℃/s. The invention can ensure that the yield strength is 220-260MPa, the tensile strength is 300-380MPa, and the elongation is more than or equal to 43 percent, the surface crystal grains of the zinc-iron alloy coating are fine and uniform in size distribution, the area ratio of the surface cavity of the coating is less than or equal to 5 percent, the surface has no microcracks, the coating is not easy to have the phenomena of pulverization and shedding during stamping forming, namely, the 90-degree V-bend test rating reaches 2 grades.

The invention is mainly characterized in that on the premise of ensuring the mechanical property, the zinc-iron alloy coating is rapidly cooled to obtain the coating property that the surface crystal grains are fine, the size distribution is uniform, the surface cavities of the coating are few, micro cracks do not exist, and the coating is not easy to be pulverized and fall off during stamping. The method obtains better zinc-iron alloy coating performance only by rapid cooling after plating or alloying; the substrate structure and properties cannot be adjusted by the process adjustment of the whole heat treatment and hot-dip process, and thus the strength of the resulting substrate is not high.

Chinese patent 201711385129.9 discloses 780 MPa-grade low-carbon low-alloy hot-galvanized TRIP steel and a rapid heat treatment method thereof, 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 impurity elements, 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 two-phase region heating target temperature interval is 60-100s; 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 200-300s; the strip steel is heated to 460-470 ℃ from 410-430 ℃ and then is immersed in a zinc pot for heat preservation. After the strip steel is galvanized, rapidly cooling the strip steel from 460 to 470 ℃ (the cooling rate is 50 to 150 ℃/s) to room temperature to obtain a hot-dip pure zinc (GI) product; after hot galvanizing of the strip steel, the strip steel can also be heated (the reheating rate is 10-300 ℃/s) to 480-550 ℃ for alloying treatment for 5-20 s, and the alloyed hot Galvanizing (GA) product is obtained after the alloying treatment and rapid cooling (the cooling rate is 10-250 ℃/s) to the room temperature. 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-level low-carbon low-alloy hot-galvanized 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 that of the 780 MPa-level product, the use effect of a user is not good, and the tensile strength of the TRIP steel product is too low as that of the 980 MPa-level 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 60-100s, which is almost 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, the patent must perform a bainite isothermal treatment time of 200-300s, which is actually too long for rapid heat treatment of the product, and has limited and unnecessary effects. And 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 also not beneficial to the high-speed stable operation of the strip steel in the furnace.

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 speed of 2-6 ℃/s, and then cooling to 250-350 ℃ at the cooling speed of 30-100 ℃/s, and carrying out overaging treatment for 3-5min to obtain the ferrite and martensite dual-phase structure ultrahigh-strength dual-phase steel. The yield strength of the ultra-high strength dual-phase steel is 650-680MPa, the tensile strength is 1023-1100MPa, and the elongation is 12.3-13%. Bending 180 degrees along the rolling direction does not crack.

The most important characteristics of the patent are: 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, so that the tissue thinning 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 the defect of local 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 to be obtained, the hot rolling raw material has high strength and large subsequent cold rolling deformation resistance, and brings 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, and the material is heated 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 increase of the soaking time inevitably partially weakens the grain refining effect generated by rapid heating and is not beneficial to improving the strength and the toughness of the material;

fourth, the patent 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 CN108774681A discloses a rapid heat treatment method for high-strength steel, which adopts a ceramic chip electric heating device, can obtain a maximum heating rate of 400 ℃/s, and rapidly cools to room temperature at a cooling rate of nearly 3000 ℃/s after heating to 1000-1200 ℃. The high-strength steel with beneficial effects contains 0.16-0.55% of carbon, and simultaneously contains: alloying elements such as Si, mn, cr, mo, etc.; the method disclosed by the method is mainly suitable for steel wires, wire rods or steel belts with the thickness of less than 5 mm. The patent describes a rapid heat treatment method by means of ceramic wafer electrical heating, which has to be in surface contact with the product in order to obtain a high heating rate, resulting in problems such as the surface quality of the product being not guaranteed; the invention mainly aims to solve the problems of low heat treatment efficiency of high-strength steel, energy waste and environmental pollution; 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 steel grades, causes bad plate shapes due to overhigh cooling speed, and is not suitable for the industrialized continuous heat treatment production of the wide and thin steel plates.

Chinese patent CN107794357B and U.S. patent US2019/0153558A1 discloses a method for producing an ultrahigh-strength martensite cold-rolled steel plate by an ultrahigh-speed heating process, wherein the chemical components of the high-strength dual-phase steel are as follows by weight percent: 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 percent, less than or equal to 0.02 percent of P, less than or equal to 0.02 percent of S, 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 =1800-2300MPa, the maximum elongation of 12.3% and the uniform elongation of 5.5-6%. 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 not more than 5 seconds to obtain the ultrahigh-strength cold-rolled steel plate.

The disadvantages of the process described in this patent include:

firstly, the inventive steel contains more alloy elements (such as Mo, ti, cr, B and the like), and the yield strength and the tensile strength of the material exceed 1000MPa, which brings great difficulty to the manufacturing of the process and the procedures before heat treatment and the subsequent use of users;

secondly, the ultra-fast heating annealing method of the invention adopts the holding time not exceeding 5s, which can cause uneven distribution of alloy elements in the final product and cause uneven and unstable product structure performance;

thirdly, the final quick cooling adopts water quenching to cool to room temperature without necessary tempering treatment, so that the structural performance of the obtained final product and alloy elements in the final structural structure are not uniformly distributed, the obdurability is not well matched, the final product has excessive strength, and the plasticity and the toughness are insufficient;

fourthly, the method of the invention can cause the problems of poor plate shape, surface oxidation and the like of the steel plate due to the overhigh water quenching speed, so the technology of the patent has no practical application value or has small practical application value, and particularly, the patent only relates to a cold rolled product and does not relate to a hot galvanized product.

Currently, limited by the capacity of the traditional continuous hot galvanizing production line equipment, hot galvanizing TRIP steel products and relevant researches of an annealing process are based on the heating rate (5-20 ℃/s) of the existing industrial equipment to slowly heat strip steel so as to complete recrystallization and austenitizing phase transformation in turn, so that the heating time is long; meanwhile, the soaking time of the traditional continuous hot galvanizing production line is required to be 1-3min generally, the soaking time of the strip steel in a high-temperature furnace section is long, and the number of rollers in the high-temperature section is large (the number of rollers in the high-temperature furnace section of the traditional production line with the unit speed of about 180 m/min is different from 20-40). This is disadvantageous to the energy consumption index and correspondingly increases the requirements for equipment, and the equipment investment is inevitably increased. The difficulty of controlling the surface quality of the strip steel is increased.

In recent years, the development of rapid heating technologies such as transverse magnetic induction heating and novel direct-fired heating has led to the industrial application of rapid thermal treatment processes. The cold-rolled strip steel finishes the austenitizing process within tens of seconds or even seconds from room temperature, greatly shortens the length of a heating section, and is convenient for improving the speed and the production efficiency of a machine set. Meanwhile, the austenitizing process completed in a very short time can provide a more flexible and flexible structure design and production line design method, so that the performance of the TRIP steel material is improved on the premise of not changing alloy components and a rolling process.

The advanced high-strength steel represented by transformation induced plasticity TRIP steel has wide application prospect, the rapid heat treatment technology has great development value, and the combination of the two technologies can provide larger space for the development and production of the TRIP steel.

Disclosure of Invention

The invention aims to provide 1180 MPa-grade low-carbon low-alloy hot-galvanized TRIP steel and a quick heat treatment hot-galvanized manufacturing method, wherein the recovery, recrystallization and phase change processes of a deformed structure are changed through quick heat treatment, the grain growth time is shortened, grains are refined, and bainite in the metallographic structure of the TRIP steel is obtained and is in submicron-grade particles; austenite is equiaxed grains distributed in an island shape; bainite and austenite are uniformly distributed on a ferrite matrix, the yield strength is 771 to 821MPa, and the tensile strength is increased to 1182 to 1284MPa; the elongation is 18 to 22.2 percent; the product of strength and elongation is 22.6-26.4 GPa%; the strength of the material is obviously improved, and simultaneously good plasticity and toughness are obtained; meanwhile, the rapid heat treatment process is adopted, so that the production efficiency is improved, the production cost and the energy consumption are reduced, the number of furnace rollers is obviously reduced, and the surface quality of the steel strip is improved.

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

1180 MPa-grade low-carbon low-alloy hot-dip galvanized TRIP steel comprises the following chemical components in percentage by mass: c:0.19 to 0.25%, si:1.3 to 2.0%, mn: 1.8-2.4%, P is less than or equal to 0.015%, S is less than or equal to 0.002%, al: 0.02-0.05%, and one or two of Cr, mo, ti, nb and V, wherein Cr + Mo + Ti + Nb + V is less than or equal to 0.5%, and the balance of Fe and other inevitable impurities, and 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 temperature of the hot rolling is more than or equal to A _r3 The coiling temperature is 550-680 ℃;

3) Cold rolling of steel

The cold rolling reduction rate is 40-80%;

4) Rapid heat treatment and hot galvanizing

Rapidly heating the cold-rolled steel plate to 770-860 ℃, 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-stage rapid heating is adopted, the first stage is heated from room temperature to 550-625 ℃ at the heating rate of 15-500 ℃/s, and the second stage is heated from 550-625 ℃ to 770-860 ℃ at the heating rate of 50-500 ℃/s; then, soaking, wherein the soaking temperature is as follows: 770-860 ℃, soaking time: 30-120 s;

after the heat equalization is finished, slowly cooling to 670-770 ℃ at a cooling rate of 5-15 ℃/s, and then rapidly cooling to 410-430 ℃ at a cooling rate of 40-100 ℃/s; carrying out isothermal treatment in the temperature range for 60-150 s; after the isothermal treatment is finished, heating to 460-470 ℃ at the heating rate of 10-30 ℃/s, and then soaking in a zinc pot for hot galvanizing;

after hot galvanizing, rapidly cooling to room temperature at a cooling rate of 30-150 ℃/s to obtain a hot-dip pure zinc GI product; alternatively, the first and second electrodes may be,

after hot galvanizing, heating to 480-550 ℃ at the heating rate of 30-300 ℃/s for alloying treatment, wherein the alloying treatment time is 5-20 s; after alloying treatment, the alloy is rapidly cooled to room temperature at a cooling rate of 30-250 ℃/s, and an alloying hot galvanizing GA product is obtained.

Preferably, the content of C is 0.21 to 0.23%.

Preferably, the Si content is 1.5 to 1.9%.

Preferably, the Mn content is 2.0 to 2.2%.

Preferably, the time for the whole process of the rapid thermal treatment and the hot galvanizing is 118 to 238s.

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 is performed in two stages, wherein the first stage is heated from room temperature to 550-625 ℃ at a heating rate of 15-300 ℃/s, and the second stage is heated from 550-625 ℃ to 770-860 ℃ 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 550-625 ℃ at the heating rate of 30-300 ℃/s, and the second section is heated from 550-625 ℃ to 770-860 ℃ at the heating rate of 80-300 ℃/s.

The metallographic structure of the hot galvanizing TRIP steel is a three-phase structure with 40-80% of bainite, 10-50% of ferrite and 7-18% of austenite, and the average grain size is 1-3 mu m; bainite is submicron-sized particles; austenite is equiaxed grains distributed in an island shape; bainite and austenite are distributed uniformly on the ferrite matrix.

The yield strength of the hot-galvanized TRIP steel is 771-821 MPa, and the tensile strength is improved 1182-1284 MPa; the elongation is 18 to 22.2 percent; the product of strength and elongation is 22.6-26.4 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 forming steel, low yield strength, high uniform elongation and total elongation are required, so the carbon content is not too high. Carbon phases in steel generally exist in two ways: ferrite and cementite, the carbon content has a great influence on the mechanical properties of steel, the quantity of reinforcing phases such as bainite, pearlite and martensite is increased 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 are obviously reduced.

For transformation induced plasticity TRIP steel, carbon element is dissolved in austenite in a solid solution mode, an austenite phase region can be enlarged, the number of residual austenite is increased, the stability of the transformation induced plasticity TRIP steel is improved, a C curve of ferrite and bainite transformation is shifted to the right, the transformation of ferrite and bainite is delayed, and the Ms point temperature is reduced. The content of carbon in austenite determines the amount and stability of retained austenite, the higher the carbon content of retained austenite, the better the stability of retained austenite, and as the carbon content increases, the content of retained austenite also increases. However, too high a carbon content may lower the formability and weldability of the steel; too low a carbon content results in a considerable reduction in the stability of the retained austenite, even without the TRIP effect. Therefore, the present invention limits the carbon content to the range of 0.19 to 0.25%.

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 of the steel 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 the pearlite is indirectly enhanced. Manganese also strongly enhances the hardenability of the steel, further improving its strength.

For transformation induced plasticity TRIP steel, manganese plays a role in solid solution strengthening and Ms point reduction in the steel, so that the stability of residual austenite is improved. However, when the manganese content is high, on one hand, the structure is banded, on the other hand, the retained austenite is excessively stable, which is not beneficial to the occurrence of phase transformation, and on the other hand, the grains in the steel are coarsened, the overheating sensitivity of the steel is increased, and when the steel is cooled improperly after smelting, pouring and forging, white spots are easily generated in the carbon steel. In addition, increasing the Mn content increases the alloy cost, the production cost of the pre-heat treatment process, and the production difficulty. In consideration of the above factors, the manganese content is designed to be within the range of 1.8-2.4%.

Si: silicon forms a solid solution in ferrite or austenite, thereby enhancing the yield strength and tensile strength of steel, and silicon can increase 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 the 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.

In the transformation induced plasticity TRIP steel, the Si element is a ferrite forming element, and can improve the stability of the retained austenite and also play a role in solid solution strengthening, thereby improving the strength of the TRIP steel. Meanwhile, the silicon element has the functions of reducing an austenite phase region and improving the activity of the C element in ferrite. Higher silicon content is advantageous for obtaining more retained austenite, but too high silicon content may cause the steel to have, for example, a hard oxide layer, poor surface properties, reduced wettability of hot-rolled steel sheet, surface quality problems, etc. An excessively low content of silicon does not bring about a stable and satisfactory TRIP effect, so that the content of silicon must be controlled within a certain range. The main effect of silicon is to reduce the austenite volume fraction at final equilibrium for a given annealing temperature time. Silicon has no obvious influence on the growth rate of austenite, but has obvious influence on the formation form and distribution of the austenite. By combining the factors, the silicon content is determined to be within the range of 1.3-2.1%.

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 can form an intermetallic compound sigma phase (FeCr) with iron, and 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 brittleness tendency of the steel. The chromium element can improve the strength and the hardness of the steel, and has more obvious effect when being used with other alloy elements. 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 suitable to be too much in consideration of factors such as cost increase and the like.

Mo: molybdenum can inhibit the self-diffusion of iron and the diffusion speed 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 effect of Mo in steel also depends on 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, but the effect is less pronounced than C and Cr. Mo inhibits the transformation of pearlite areas and accelerates the transformation of medium-temperature areas, so that Mo-containing steel can form a certain amount of bainite strengthening phases under the condition of higher 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.

Microalloying elements Ti, nb and V: the addition of trace microalloy elements Nb, V and Ti in the steel can ensure that the steel has good weldability and usability by the dispersion precipitation of carbon and nitride particles (the size is less than 5 nm) and the solid solution of Nb, V and Ti to refine grains under the condition of low carbon equivalent. Nb, V, and Ti are carbide-forming elements and nitride-forming elements that satisfy such requirements at relatively low concentrations, and are strong carbide-forming elements, and most of Nb, V, and Ti exist as carbides, nitrides, and carbonitrides in steel at normal temperature, and a small portion of Nb, V, and Ti are solid-dissolved in ferrite.

For TRIP steels, the addition of microalloying elements can strengthen the ferritic matrix by grain refinement and precipitation, and can also delay bainite formation. The reason why the bainite formation is delayed is to strengthen the formation of ferrite upon cooling, which is a result of grain refinement of the microstructure. The formation of ferrite causes a carbon enrichment of the residual austenite, delaying the transformation of austenite to bainite, while the finely dispersed carbonitride inhibits the nucleation of bainite, thereby also delaying the kinetics of bainite formation. The addition of Nb, V and Ti can prevent austenite grains from growing and raise the coarsening temperature of steel, and the dispersed small grains of carbon and nitride can fix austenite grain boundary, block the migration of austenite grain boundary, raise austenite recrystallization temperature and enlarge unrecrystallized area.

The effect of adding a small amount of Nb, V and Ti in the steel is as follows:

firstly, the strength can be improved while the carbon equivalent content is reduced, and the welding performance of the steel is improved;

secondly, fixing impurities such as oxygen, nitrogen, sulfur, etc., thereby improving weldability of steel;

thirdly, due to the effect of microscopic particles, such as insolubility of TiN at high temperature, coarsening of grains in the heat affected zone is prevented, and the material toughness of the heat affected zone is improved, thereby improving the weldability of 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 hot galvanizing 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 or grow inconspicuously. 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. And then the crystal grain growth time in the soaking process is shortened by short-time heat preservation and quick cooling, and the fine and uniform distribution of the crystal grain structure is ensured.

The whole heat treatment process is comprehensively controlled: 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 multiphase structure of ferrite, austenite and bainite obtained by the rapid heat treatment method is 1-3 mu m, 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 structures of the ferrite, the bainite and the residual austenite obtained by the method are mainly in various shapes such as blocks, granules and the like, and are distributed more uniformly, so that better strong plasticity can be obtained in a deformation stage.

The invention relates to a rapid heat treatment hot galvanizing manufacturing method of 1180 MPa-grade low-carbon low-alloy hot galvanizing TRIP steel, which 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 temperature of the hot rolling is more than or equal to A _r3 Then cooling to 550-680 ℃ for coiling;

3) Cold rolling of steel

The cold rolling reduction rate is 40-80%, 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 a two-phase region of austenite and ferrite at 770-860 ℃, wherein the rapid heating adopts a one-stage type or a 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 550-625 ℃ at the heating rate of 15-500 ℃/s, and the second section is heated from 550-625 ℃ to 770-860 ℃ at the heating rate of 50-500 ℃/s;

b) Soaking heat

Soaking at the target temperature of 770-860 ℃ in an austenite and ferrite two-phase region for 30-120 s;

c) Cooling

Slowly cooling the band steel or the steel plate to 670-770 ℃ at a cooling rate of 5-15 ℃/s after the heat equalization; then rapidly cooling to 410-430 ℃ at a cooling rate of 40-100 ℃/s;

d) Bainite isothermal treatment

Bainite isothermal treatment is carried out on the strip steel or the steel plate at the temperature of 410-430 ℃, and the isothermal treatment time is 60-150 s;

e) Reheating

After the isothermal treatment is finished, heating to 460-470 ℃ at a heating rate of 10-30 ℃/s;

f) Hot dip galvanizing

Then, soaking the strip steel or the steel plate into a zinc pot for hot galvanizing;

g) After hot galvanizing of strip steel or steel plate, rapidly cooling to room temperature at a cooling rate of 30-150 ℃/s to obtain a hot-dip pure zinc GI product; alternatively, the first and second electrodes may be,

after hot galvanizing strip steel or steel plate, heating to 480-550 ℃ at the heating rate of 30-300 ℃/s for alloying treatment, wherein the alloying treatment time is 5-20 s; after alloying treatment, rapidly cooling to room temperature at a cooling rate of 30-250 ℃/s to obtain an alloying hot galvanizing GA product.

Preferably, the time for the whole process of the rapid thermal treatment and the hot galvanizing is 118 to 328 seconds.

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 two stages, wherein the first stage is heated from room temperature to 550-625 ℃ at a heating rate of 30-300 ℃/s, and the second stage is heated from 550-625 ℃ to 770-860 ℃ at a heating rate of 80-300 ℃/s.

Preferably, in the step 4), the rapid heating final temperature is 790 to 860 ℃.

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 860 ℃, and the temperature after the temperature reduction is not lower than 770 ℃.

Preferably, in the step 4), after hot galvanizing the strip steel or the steel plate, heating the strip steel or the steel plate to 480-550 ℃ at a heating rate of 30-200 ℃/s for alloying treatment, wherein the alloying treatment time is 5-20 s; after alloying treatment, rapidly cooling to room temperature at a cooling rate of 30-200 ℃/s to obtain an alloying hot galvanizing GA product.

The invention relates to a rapid heat treatment hot galvanizing manufacturing method of 1180 MPa-grade low-carbon low-alloy hot galvanizing TRIP steel, which comprises the following steps:

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 _star Is the recrystallization onset temperature; β is the heating rate; b (T) is obtained by the following formula:

b＝b ₀ exp(-Q/RT)

it can be derived from the above formula and the related experimental data that the recrystallization onset temperature (T) increases with the rate of heating _star ) And end temperature (T) _fin ) All rise; when the heating rate is more than 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.

Under the traditional low-speed heating condition, the deformation matrix is recovered, recrystallized and grown up, then phase transformation from ferrite to austenite is generated, phase transformation nucleation points are mainly concentrated at ferrite grain boundaries with grown up grains, the nucleation rate is low, and the finally obtained grain structure is relatively coarse.

Under the rapid heating condition, the deformed matrix begins to recrystallize without fully recovering, the phase transformation from ferrite to austenite begins to occur when the recrystallization is not completed or the grain growth is not started, and the nucleation rate is obviously improved and the grains are obviously refined because the grains are fine and the grain boundary area is large when the recrystallization is just completed. Particularly, after the ferrite recrystallization 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, so that a large number of nucleation points are provided for austenite, the austenite presents explosive nucleation, and austenite grains are further refined. The retained high-density dislocation line defects also become channels for high-speed diffusion of carbon atoms, so that each austenite grain can be rapidly generated, and the volume fraction of austenite is increased.

By finely controlling the structural evolution and the distribution of alloy elements and phase components in the rapid heating process, 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 the heating rate is set to be 50-500 ℃/s when one-stage rapid heating is adopted, and the heating rate is set to be 15-500 ℃/s when two-stage rapid heating is adopted.

In different temperature interval ranges, rapid heating has different influences on the structure evolution processes of recovery, recrystallization, grain growth and the like of the material, and in order to obtain optimal structure control, the optimal heating rates of different heating temperature intervals are different: the heating rate has the greatest influence on the recovery process from 20 ℃ to 550-625 ℃, and the heating rate is preferably controlled to be 15-300 ℃/s, and is further preferably controlled to be 30-300 ℃/s; the heating temperature is from 550-625 ℃ to the austenitizing temperature of 770-860 ℃, the heating rate has the greatest influence on the grain growth process, and the heating rate is preferably controlled to be 50-300 ℃/s; more preferably 80 to 300 ℃/s.

2. Soaking temperature control

The soaking temperature is selected by combining the material structure evolution process control of 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 content of C in steel, and the soaking temperature is generally set at A in the traditional heat treatment process _C1 To A _C3 Or A is _C3 Above 30-50 ℃. The invention utilizes the rapid heating technology to retain a large amount of dislocation in the ferrite which is not fully recrystallized and provides nucleation work for austenite transformation, so that the soaking temperature is only required to be heated to A _C1 To A _C3 More austenite can be obtained, and the C content of the TRIP steel is as follows: 0.19-0.25%, A _C1 And A _C3 Respectively at about 730 ℃ and 870 ℃.

A large amount of undissolved carbides which are fine and evenly distributed exist in TRIP steel, and can play a role in mechanical obstruction to the growth of austenite grains in the heating process, so that the grain size of alloy steel can be refined, but if the heating temperature is too high, the number of the undissolved carbides is greatly reduced, the obstruction effect is weakened, the growth tendency of the grains is enhanced, and the strength of the steel is further reduced. 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 alloy steel, so that not only can the abnormal growth of austenite grains be prevented, but also 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 is also selected with the aim of obtaining fine and uniform austenite grains, so as to achieve the aim of obtaining fine and uniform ferrite, bainite and retained austenite after cooling. The austenite grains are coarse due to the overhigh soaking temperature, workpieces are easy to crack in the quenching process, and the obtained structure after quenching is coarse, so that the mechanical property of the steel is poor; too low soaking temperature can cause insufficient carbon and alloy element contents dissolved in austenite, so that the austenite carbon concentration is not uniformly distributed, the hardenability of the steel is greatly reduced, and the mechanical property of the alloy steel is adversely affected. The soaking temperature of the hypoeutectoid steel should be Ac ₃ + 30-50 ℃. In the case of ultra-high strength steels, the presence of carbide-forming elements hinders the transformation of carbides, so the soaking temperature can be suitably increased. In combination with the above factors, 770-860 ℃ is selected as the soaking temperature in the invention, so as to obtain reasonable quenching process and ideal structure performance.

3. Soaking time control

Because the invention adopts rapid heating, the material in the two-phase region contains a large amount of dislocation, which provides a large amount of nucleation points for the formation of austenite and provides a rapid diffusion channel for carbon atoms, the austenite can be formed very rapidly; the shorter the soaking and heat preservation time is, the shorter the carbon atom diffusion distance is, the larger the carbon concentration gradient in the austenite is, and the more the residual austenite carbon content is finally reserved; 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 soaking time is also related to the content of carbon and alloy elements in the steel, when the content of the carbon and the alloy elements in the steel is increased, the thermal conductivity of the steel is reduced, and the alloy elements obviously delay the structure transformation of the steel because the diffusion speed of the alloy elements is slower than that of the carbon elements, so the soaking time is properly prolonged. 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 30-120 s.

4. Fast cooling rate control

In order to obtain bainite, the cooling speed of a sample during rapid cooling is required to be greater than the critical cooling speed to obtain a bainite structure, the critical cooling speed mainly depends on material components, the content of Si in the TRIP steel is 1.3-2.0%, the content of Mn in the TRIP steel is 1.8-2.4%, and the content of Mn is relatively high, so that the hardenability of the TRIP steel is greatly enhanced by the Si and Mn, and the critical cooling speed is reduced.

The cooling rate also needs to comprehensively consider the structure evolution and alloy diffusion distribution results of the early heating process and the soaking process so as to finally obtain reasonable structure distribution and alloy element distribution of each phase. The bainite structure cannot be obtained due to too low cooling rate, so that the mechanical property cannot meet the requirement; too high cooling rate can generate larger quenching stress (namely, structural stress and thermal stress) to cause serious poor plate shape, and even easily cause serious deformation and cracking of the test sample. Therefore, the rapid cooling speed is set to be 40-100 ℃/s.

5. Bainite isothermal treatment temperature

The bainite isothermal temperature of TRIP steels is generally chosen to be at a temperature (T) where the free energies of bainite, ferrite and austenite are equal ₀ ) In this case, the bainite free energy is smaller than the austenite free energyThe reduction in free energy provides a driving force for transformation of bainite. Because the chemical components of steel materials are different, the isothermal treatment temperature of the bainite is different, the isothermal temperature of the bainite is generally selected to be 350-550 ℃, when the isothermal treatment temperature is higher, the atomic diffusion capability is strong, the austenite is partially converted into granular bainite, carbides are precipitated, the stability of the undercooled austenite is reduced, and the volume fraction of the residual austenite is lower. During isothermal treatment at a lower temperature, bainite transformation requiring atomic diffusion is difficult to perform, martensite phase transformation without atomic diffusion possibly occurs, martensite is a supersaturated structure of C, C diffuses too slowly in the isothermal process and is difficult to enrich in supercooled austenite, and the volume fraction of residual austenite is reduced, so that the isothermal temperature of bainite is selected in a temperature range of 410-430 ℃.

6. Bainite isothermal treatment time

If the bainite isothermal time is short, bainite transformation cannot be fully performed, the enrichment degree of C elements to austenite is low, the content of austenite C is low, the stability is poor, and in the subsequent cooling process, supercooled austenite is converted into a large amount of martensite. The martensite structure has the characteristics of high strength and low elongation, so that the martensite structure is not favorable for improving the strong plasticity. As the isothermal time is prolonged, the bainite is fully transformed, and the volume fraction of the bainite in the TRIP steel is increased. The isothermal time is prolonged, the change of an SEM microstructure is not obvious, the volume fraction and the morphology of bainite are not changed greatly, the process of enriching C elements into residual austenite is mainly adopted, the residual austenite content and the carbon content are increased along with the extension of the heat preservation time, the stability is increased, the residual austenite continuously generates martensite phase transformation along with the generation of strain in the using process of the TRIP steel material, the material strength of the phase transformation area is enhanced, so that the strain can be transferred to other areas of the material, and the elongation of the material can be obviously improved. Therefore, the bainite isothermal time is set to 60 to 150 seconds in the present invention.

7. Hot dip galvanizing and alloying process control

For high-strength hot-dip galvanized products, the rapid heat treatment process reduces the retention time of strip steel in a high-temperature furnace, so that the enrichment amount of alloy elements on the surface of the high-strength strip steel in the heat treatment process is obviously reduced, the platability of the high-strength hot-dip galvanized products is improved, the surface plating leakage defect is reduced, the corrosion resistance is improved, and the yield can be improved.

By the method, the alloy content in the steel of the same grade can be reduced, the crystal grains are refined, and good soft and hard phase structure composition and matching of strength and toughness are obtained; meanwhile, the traditional continuous hot galvanizing unit is improved by a rapid heating and rapid cooling process, so that the rapid heat treatment process is realized, the length of a heating and soaking section of the annealing furnace can be greatly shortened (at least one third of the length of the traditional continuous annealing furnace), the production efficiency of the traditional continuous hot galvanizing unit is improved, the production cost and the energy consumption are reduced, the number of furnace rollers of the continuous annealing furnace is obviously reduced, particularly the number of furnace rollers of a high-temperature furnace section, and the energy consumption and the investment on equipment can be reduced.

By establishing the novel continuous hot galvanizing unit adopting the rapid heat treatment process technology, the purposes of short and stout unit, flexible material transition, strong regulation and control capability and the like can be realized; 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 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 deformed structure and the ferrite recrystallization process in the heat treatment process through rapid heat treatment, 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 grain growth time is shortened. Compared with the transformation induced plasticity TRIP steel of the same grade obtained by the traditional heat treatment mode, the transformation induced plasticity TRIP steel obtained by the invention has the advantages that the alloy components can be greatly reduced, the average grain size is 1-3 mu m, the metallographic structure of the steel after the rapid heat treatment is a three-phase structure of bainite (40-80%), ferrite (10-50%) and austenite (8-18%), and the bainite is submicron-grade particles; austenite is equiaxed grains distributed in an island shape; bainite and austenite are uniformly distributed on the ferrite matrix; and austenite with different orientations and different forms can continuously generate TRIP effect under different strain conditions, so that the plasticity is obviously improved, and the strength of the material can be improved through grain refinement, thereby simultaneously obtaining good strong plasticity and toughness and improving the service performance of the material.

(2) Compared with the transformation induced plasticity TRIP steel obtained by the traditional heat treatment mode, the TRIP steel obtained by the invention is a multi-phase structure with fine crystal grains, the average crystal grain size is 1-3 mu m, and the average crystal grain size is reduced by 30-50%; the toughness of the material can be obviously improved, the yield strength is 771-821 MPa, and the tensile strength is increased to 1182-1284 MPa; the elongation is 18 to 22.2 percent; the product of strength and elongation is 22.6-26.4 GPa%.

(3) According to the heat treatment process of the hot-dip galvanized TRIP steel, the shortest time of the whole heat treatment process can be shortened to 118s, the time of the whole rapid heat treatment process is greatly reduced (the hot-dip time of the TRIP heat treatment of the traditional continuous annealing process is usually 9-11 min), and particularly the retention time of the strip steel at a high temperature of more than 600 ℃ is shortened, so that the production efficiency is improved, the energy consumption is reduced, and the production cost is reduced.

(4) Compared with the traditional transformation induced plasticity TRIP steel and the heat treatment process thereof, the rapid heat treatment method of the TRIP steel shortens the length and time of the heating section and the soaking section of the continuous hot galvanizing annealing furnace (the length of the heating section and the soaking section can be shortened by 60-80 percent compared with the traditional continuous hot galvanizing annealing furnace) and the whole heat treatment process time, can save energy, reduce emission and consumption, remarkably reduces one-time investment of furnace equipment, and remarkably reduces the production running cost and the equipment maintenance cost; the alloy content of the product produced by the process is lower, the production cost of the heat treatment and the previous working procedure can be reduced, and the manufacturing difficulty of each working procedure before the heat treatment can be reduced.

(5) Compared with the traditional transformation induced plasticity TRIP steel and the heat treatment process thereof, the rapid heat treatment process technology is adopted, so that the time of a heating process and a soaking process can be reduced, the length of a furnace is shortened, the number of furnace rollers is obviously reduced, the probability of generating surface defects in the furnace is reduced, and the surface quality of a product is obviously improved.

For high-strength hot-dip galvanized products, the rapid heat treatment process reduces the retention time of strip steel in a high-temperature furnace, so that the enrichment amount of alloy elements on the surface of the high-strength strip steel in the heat treatment process is obviously reduced, the improvement of the platability of the high-strength hot-dip galvanized products is facilitated, the surface plating leakage defect is reduced, the corrosion resistance is improved, and the yield can be improved.

In addition, due to the refinement of product grains and the reduction of material alloy content, the processing forming performance such as hole expanding performance, bending performance and the like and the user service performance such as welding performance and the like of the TRIP steel product obtained by adopting the technology are also improved.

The 1180 MPa-grade low-carbon low-alloy hot-dip galvanized TRIP 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 hot-dip galvanized TRIP steel (GI) produced in example 1, which is test steel A of the present invention.

FIG. 2 is a photograph of the microstructure of hot-dip galvanized TRIP steel (GI) produced according to conventional Process 1, which is test steel A of the present invention.

FIG. 3 is a photograph showing the microstructure of a galvannealed dual phase steel (GA) produced in example 17, which is test steel I of the present invention.

FIG. 4 is a photograph of the microstructure of hot-dip galvanized dual-phase steel (GI) produced in example 22, which is test steel D of the present invention.

FIG. 5 is a photograph showing the microstructure of a galvannealed dual phase steel (GA) produced in example 34, which is test steel I of the present invention.

Detailed Description

The present invention is further illustrated by the following examples and the accompanying drawings, wherein the examples are implemented on the premise of the technical solution of the present invention, and detailed embodiments and specific operation procedures are provided, but the scope of the present invention is not limited to the following examples.

The compositions of the test steels of the invention are shown in table 1, and the specific parameters of the examples of the invention and the conventional process are shown in table 2 (one-stage heating) and table 3 (two-stage heating); tables 4 and 5 show the main properties of GI hot-dip galvanized TRIP steels prepared by using the test steel compositions of the invention according to the examples and the conventional process, and Table 4 shows the main properties of GI hot-dip galvanized TRIP steels prepared by using the test steel compositions of the invention according to the examples and the conventional process in tables 2 and 3.

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 TRIP steel obtained by the method can reach 771-821 MPa, and the tensile strength is improved to 1182-1284 MPa; the elongation is 18 to 22.2 percent; the product of strength and elongation is 22.6-26.4 GPa%.

FIGS. 1 and 2 are structural diagrams of a typical composition A steel passing through example 1 and comparative conventional process example 1. From the two figures, the structures after hot galvanizing of the two processes are very different. The structure (figure 1) of the A steel after the rapid heat treatment of the invention mainly comprises fine, uniform and mutually dispersed ferrite, bainite, martensite, residual austenite and other structures. The various tissues are distributed in a very fine and uniform dispersion mode, which is very beneficial to improving the strength and the plasticity of the material.

The A steel structure (figure 2) processed by the traditional process is a typical TRIP steel structure diagram. The tissue characteristics treated by the traditional process are as follows: the tissues are distributed in a certain direction along the rolling direction. The proportion of ferrite structure is slightly larger, the martensite, bainite and residual austenite structures are distributed along the rolling direction, and the structures have certain non-uniformity.

FIG. 3 is a structural diagram of a typical composition I steel obtained in example 17 (GA), and FIG. 4 is a structural diagram of a typical composition D steel obtained in example 22 (GI). FIG. 5 is a structural diagram obtained by subjecting a typical composition I steel to example 34 (GA). Examples 17, 22 and 34 are all processes with a short overall heat treatment period. It can be seen from the figure that by using the rapid heat treatment hot galvanizing method of the invention, after alloying treatment, very uniform, fine and dispersedly distributed phase structures (figure 3) are obtained, and in the steel strip metallographic structure prepared by the traditional process 9, structures such as ferrite, martensite, bainite and the like are relatively coarse and have certain directionality. Meanwhile, the ferrite structure content in the structure is relatively more, and the distribution is not uniform. Therefore, the preparation method of the hot galvanizing TRIP steel can refine crystal grains, and enable each phase structure of the material to be uniformly distributed in a matrix, thereby improving the material structure and improving the material performance.

The invention carries out process transformation on the traditional continuous annealing hot galvanizing unit by adopting the rapid heating and rapid cooling process, realizes the rapid heat treatment hot galvanizing process, can greatly shorten the lengths of a heating section and a soaking section of the traditional continuous annealing hot galvanizing furnace, improves the production efficiency of the traditional continuous annealing hot galvanizing unit, reduces the production cost and the energy consumption, reduces the number of furnace rollers of the continuous annealing hot galvanizing furnace, and obviously reduces surface defects such as roller marks, pockmarks, scratches and the like, thereby improving the control capability of the surface quality of the strip steel and easily obtaining the strip steel product with high surface quality; meanwhile, by establishing a novel continuous annealing unit adopting a rapid heat treatment hot galvanizing process technology, the advantages of short and bold hot galvanizing unit, flexible material transition, strong regulation and control capability and the like can be realized; for the hot-dip base plate material, crystal grains 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 use performance of the material for users such as forming, welding and the like is improved.

In conclusion, the invention adopts the rapid heat treatment hot galvanizing process to greatly promote the technical progress of the continuous annealing hot galvanizing process of the cold-rolled strip steel, the austenitizing process of the cold-rolled strip steel from room temperature to the last can be finished within ten seconds or even several seconds, the heating section length of a continuous annealing hot galvanizing furnace is greatly shortened, the speed and the production efficiency of a continuous annealing hot galvanizing unit are conveniently improved, the number of rollers in the furnace of the continuous annealing hot galvanizing unit is obviously reduced, the number of rollers in the high-temperature furnace section of a rapid heat treatment hot galvanizing production line with the unit speed of about 180 m/min is not more than 10, and the surface quality of the strip steel can be obviously improved. Meanwhile, the rapid heat treatment hot galvanizing process method for the recrystallization and austenitization 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 alloy components, rolling process and other previous process conditions.

The hot-dip galvanized advanced high-strength steel represented by transformation induced plasticity TRIP steel has wide application prospects, the rapid heat treatment technology has great development and application values, and the combination of the two has a great space for development and production of the hot-dip galvanized TRIP steel.

Claims

1.1180 MPa-grade low-carbon low-alloy hot-dip galvanized TRIP steel comprises the following chemical components in percentage by mass: c:0.19 to 0.25%, si:1.3 to 2.0%, mn: 1.8-2.4%, P is less than or equal to 0.015%, S is less than or equal to 0.002%, al: 0.02-0.05%, and one or two of Cr, mo, ti, nb and V, wherein Cr + Mo + Ti + Nb + V is less than or equal to 0.5%, and the balance of Fe and other inevitable impurities, and 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

3) Cold rolling

The cold rolling reduction rate is 40-80%;

4) Rapid heat treatment and hot galvanizing

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 550-625 ℃ at the heating rate of 15-500 ℃/s, and the second section is heated from 550-625 ℃ to 770-860 ℃ at the heating rate of 50-500 ℃/s; then, soaking, wherein the soaking temperature is as follows: 770-860 ℃, soaking time: 30-120 s;

after the heat equalization is finished, slowly cooling to 670-770 ℃ at a cooling rate of 5-15 ℃/s, and then rapidly cooling to 410-430 ℃ at a cooling rate of 40-100 ℃/s; carrying out bainite isothermal treatment in the temperature interval for 60-150 s; after the isothermal treatment is finished, heating to 460-470 ℃ at the heating rate of 10-30 ℃/s, and then soaking in a zinc pot for hot galvanizing;

after hot galvanizing, rapidly cooling to room temperature at the cooling rate of 30-150 ℃/s to obtain a hot-dip pure zinc GI product; alternatively, the first and second electrodes may be,

after hot galvanizing, heating to 480-550 ℃ at the heating rate of 30-300 ℃/s for alloying treatment, wherein the alloying treatment time is 5-20 s; after alloying treatment, rapidly cooling to room temperature at a cooling rate of 30-250 ℃/s to obtain an alloying hot galvanizing GA product.

2. The 1180 MPa-grade low-carbon low-alloy hot-dip galvanized TRIP steel according to claim 1, wherein the C content is 0.21-0.23%.

3. The 1180 MPa-grade low-carbon low-alloy hot-dip galvanized TRIP steel according to claim 1, wherein the Si content is 1.5-1.9%.

4. The 1180MPa grade low carbon, low alloy, hot dip galvanized TRIP steel as claimed in claim 1, wherein said Mn content is 2.0-2.2%.

5. The 1180MPa grade low-carbon low-alloy hot-dip galvanized TRIP steel as claimed in claim 1, wherein the time for the whole process of rapid heat treatment and hot-dip galvanizing is 118-328 seconds.

6. The 1180 MPa-grade low-carbon low-alloy hot-dip galvanized TRIP steel as claimed in claim 1, wherein the hot rolling temperature in the step 2) is not less than A _r3 。

7. The 1180 MPa-grade low-carbon and low-alloy hot-dip galvanized TRIP steel as claimed in claim 1 or 6, wherein the coiling temperature in the step 2) is 580-650 ℃.

8. The 1180 MPa-grade low-carbon and low-alloy hot-dip galvanized TRIP steel as claimed in claim 1, wherein in the step 3), the cold rolling reduction is 60-80%.

9. The 1180 MPa-grade low-carbon low-alloy hot-dip galvanized TRIP steel according to claim 1, wherein in the step 4), the rapid heating is performed in a one-stage heating mode, and the heating rate is 50-300 ℃/s.

10. The 1180MPa grade low-carbon low-alloy hot-dip galvanized TRIP steel as claimed in claim 1, wherein in the step 4), the rapid heating adopts two-stage heating: the first section is heated from room temperature to 550-625 ℃ at the heating rate of 15-300 ℃/s, and the second section is heated from 550-625 ℃ to 770-860 ℃ at the heating rate of 50-300 ℃/s.

11. The 1180 MPa-grade low-carbon low-alloy hot-dip galvanized TRIP steel according to claim 1, wherein in the step 4), the rapid heating adopts two-stage heating: the first section is heated from room temperature to 550-625 ℃ at the heating rate of 30-300 ℃/s, and the second section is heated from 550-625 ℃ to 770-860 ℃ at the heating rate of 80-300 ℃/s.

12. The low-carbon low-alloy hot-dip galvanized TRIP steel as claimed in any one of claims 1 to 11, characterized in that the metallographic structure of the hot-dip galvanized TRIP steel is a three-phase structure of 40 to 80% bainite, 10 to 50% ferrite and 7 to 18% austenite, and the average grain size is 1 to 3 μm; bainite is submicron-sized particles; austenite is equiaxed grains distributed in an island shape; bainite and austenite are distributed uniformly on the ferrite matrix.

13. The 1180 MPa-grade low-carbon low-alloy hot-dip galvanized TRIP steel as claimed in any one of claims 1 to 12, wherein the yield strength of the hot-dip galvanized TRIP steel is 771 to 821MPa, the tensile strength is increased to 1182 to 1284MPa, the elongation is 18 to 22.2 percent, and the product of strength and elongation is 22.6 to 26.4GPa%.

14. The method for rapidly heat-treating and hot-galvanizing a TRIP steel with low carbon and low alloy at 1180MPa level according to any one of claims 1 to 13, 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 hot rolling finishing temperature is more than or equal to A _r3 Then cooling to 550-680 ℃ for coiling;

3) Cold rolling

4) Rapid heat treatment of hot dip zinc

a) Rapid heating

when one-stage rapid heating is adopted, the heating rate is 50-500 ℃/s;

when two-stage rapid heating is adopted, the first stage is heated from room temperature to 550-625 ℃ at the heating rate of 15-500 ℃/s, and the second stage is heated from 550-625 ℃ to 770-860 ℃ at the heating rate of 50-500 ℃/s;

b) Soaking heat

c) Cooling

Slowly cooling the band steel or the steel plate to 670-770 ℃ at a cooling rate of 5-15 ℃/s after the heat equalization;

then rapidly cooling to 410-430 ℃ at a cooling rate of 40-100 ℃/s;

d) Bainite isothermal treatment

Carrying out bainite isothermal treatment on the strip steel or the steel plate at the temperature of 410-430 ℃ for 60-150 s;

e) Reheating the material

f) Hot dip galvanizing

g) After hot galvanizing, rapidly cooling to room temperature at a cooling rate of 30-150 ℃/s to obtain a hot-dip pure zinc GI product; alternatively, the first and second electrodes may be,

15. The method for manufacturing the hot-dip galvanized TRIP steel by the rapid heat treatment at 1180MPa grade according to claim 14, wherein the time for the whole process of the rapid heat treatment and the hot-dip galvanized zinc is 118 to 328 seconds.

16. The method for rapidly heat-treating hot-dip galvanized TRIP steel with low carbon and low alloy at 1180MPa level according to claim 14, wherein the hot rolling temperature in step 2) is not lower than a _r3 。

17. The method for manufacturing hot-dip galvanized TRIP steel by rapid heat treatment of low carbon and low alloy hot-dip galvanized TRIP steel according to claim 14 or 16, characterized in that in step 2), the coiling temperature is 580 to 650 ℃.

18. The method for rapid thermal processing hot dip galvanizing manufacturing of 1180MPa grade low-carbon low-alloy hot dip galvanized TRIP steel according to claim 14, wherein the cold rolling reduction in the step 3) is 60 to 80%.

19. The method for rapidly heat-treating hot-dip galvanized TRIP steel with low carbon and low alloy at 1180MPa level according to claim 14, wherein the rapid heating in step 4) is performed in a single stage at a heating rate of 50-300 ℃/s.

20. The method for rapid thermal processing hot dip galvanizing manufacturing of low carbon and low alloy hot dip galvanized TRIP steel of 1180MPa class according to claim 14, characterized in that in the step 4), the rapid heating adopts two-stage heating, the first stage is heated from room temperature to 550-625 ℃ at a heating rate of 15-300 ℃/s, and the second stage is heated from 550-625 ℃ to 770-860 ℃ at a heating rate of 50-300 ℃/s.

21. The method for rapid thermal processing hot dip galvanizing manufacturing of low carbon and low alloy hot dip galvanized TRIP steel of 1180MPa class according to claim 14, characterized in that in the step 4), the rapid heating adopts two-stage heating, the first stage is heated from room temperature to 550-625 ℃ at a heating rate of 30-300 ℃/s, and the second stage is heated from 550-625 ℃ to 770-860 ℃ at a heating rate of 80-300 ℃/s.

22. The method for rapid thermal processing hot dip galvanizing fabrication of low carbon low alloy hot dip galvanized TRIP steel of 1180MPa grade according to claim 14, 20 or 21, characterized in that in the step 4), the rapid heating final temperature is 790 to 860 ℃.

23. The method for rapidly heat-treating hot-dip galvanized TRIP steel with low carbon and low alloy at 1180MPa level according to claim 14, wherein the strip steel or the steel plate is heated to the target temperature of the austenite and ferrite two-phase region in the soaking step of step 4), and then soaked while keeping the temperature constant.

24. The rapid heat treatment hot dip galvanizing manufacturing method of 1180MPa grade low-carbon low-alloy hot dip galvanized TRIP steel as claimed in claim 14, characterized in that in the soaking process of the step 4), the strip steel or the steel plate is heated up or cooled down in a small amplitude within the soaking time period, the temperature after heating up is not more than 860 ℃, and the temperature after cooling down is not less than 770 ℃.

25. The method for rapidly heat-treating hot-dip galvanized steel sheet TRIP steel of 1180MPa grade low-carbon and low-alloy hot-dip galvanized steel sheet according to claim 14, wherein in the step 4), after hot-dip galvanizing the strip steel or the steel sheet, the strip steel or the steel sheet is heated to 480 to 550 ℃ at a heating rate of 30 to 200 ℃/s for alloying treatment for 5 to 20s; after alloying treatment, rapidly cooling to room temperature at a cooling rate of 30-200 ℃/s to obtain an alloying hot galvanizing GA product.