High-yield-ratio complex-phase steel as well as preparation method and application thereof

Abstract

The invention relates to the technical field of high-strength steel materials, in particular to high-yield-ratio complex-phase steel and a preparation method and application thereof. A steel billet with the same chemical component is adopted, and the steel billet comprises, by weight, 0.11%-0.14% of C, 0.55%-0.75% of Si, 2.1%-2.3% of Mn, 0.6%-0.8% of Alt, 0.3%-0.4% of Cr, 0.04%-0.07% of Nb, smaller than or equal to 0.005% of P, smaller than or equal to 0.005% of S, smaller than or equal to 0.0035% of N, smaller than or equal to 0.005% of O and the balance Fe and inevitable impurities. The hot-rolled high-yield-ratio complex-phase steel, the cold-rolled high-yield-ratio complex-phase steel and the galvanized high-yield-ratio complex-phase steel are prepared, and the problems that the high-yield-ratio complex-phase steel is high in alloy cost, multiple in application category, multiple in mixed casting batch, multiple in product specification and the like in actual industrial production are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of high-strength steel materials, and in particular to a high-yield ratio complex phase steel and a preparation method and application thereof. Background Art

[0002] The booming new energy vehicle industry has led the direction of automobile development and provided strong support for the low-carbon transformation of the automobile industry. Compared with traditional fuel vehicles, new energy vehicles pay more attention to cruising range, safety and service life, and due to the increase in battery packs, they are usually heavier than fuel vehicles of the same level. Taking a compact SUV as an example, a fuel vehicle weighs about 1.4 tons, while a new energy electric vehicle weighs more than 1.7 tons; the weight of the best-selling electric vehicles on the market is mainly concentrated in 2.2~2.4 tons, which is 300~500 kg heavier than the fuel vehicles of the same specifications, so the demand for lightweight is stronger. Compared with aluminum alloys, carbon fiber and other materials, high-strength steel materials have the characteristics of high strength, light weight, manufacturing process, high cost performance and good processability, as well as taking into account safety and low-carbon environmental protection. It is an important material used in the manufacture of new energy vehicles.

[0003] Steel manufacturing companies are also actively promoting low-carbon transformation and are committed to reducing energy consumption and carbon emissions in the material manufacturing process to improve the environmental friendliness of materials. In recent years, some researchers have proposed the concept of material elementalization, the manufacturing concept of one steel for multiple uses, and the design and manufacturing ideas of low-entropy advanced high-strength steel products, all of which are aimed at achieving the goals of green, environmental protection, and energy saving in material manufacturing.

[0004] However, the multi-purpose, multi-specification and small-batch ordering characteristics of high-yield ratio complex phase steels at the 1.0GPa level and above have brought many inconveniences to steel companies in actual production organization, mainly manifested in the high downgrading and re-judgment rate of mixed casting billets and the frequent transition of different steel grades and specifications, resulting in poor product quality stability. Therefore, exploring the preparation method of high-yield ratio complex phase steels with low cost and economy, diversified user needs and stable quality can solve many problems in the actual industrial production of high-yield ratio complex phase steels, such as high alloy cost, multiple categories of uses, multiple mixed casting batches, and multiple product specifications, and has become a technical problem that technicians in this field need to solve urgently.

[0005] CN118291856A provides a 980MPa grade hot-dip galvanized high yield ratio complex phase steel with high hole expansion and a preparation method, wherein the yield ratio is greater than 0.80, the tensile strength is greater than or equal to 980MPa, the elongation is greater than or equal to 10%, and the hole expansion ratio is greater than or equal to 50%; CN116949261A provides a method for preparing a 980MPa grade BQ&P steel of Bayer / Massachusetts complex phase based on a thin slab continuous casting and rolling process, with a yield strength ratio of >0.595, a tensile strength of ≥1000MPa, and an elongation of ≥22%; CN116043120A provides a 1000MPa grade cold-rolled high yield ratio complex phase steel with excellent forming performance and a preparation method thereof, wherein alloy elements such as Mo, Cu, and Ni are added, the tensile strength is ≥1000MPa, the yield strength is 820-950MPa, the elongation is ≥11%, and the hole expansion rate is ≥70%; CN116043118A provides a 980MPa grade hot-dip galvanized high yield strength ratio complex phase steel with ultra-high hole expansion performance and a preparation method, wherein the tensile strength is above 980MPa, the yield strength is 700-850MPa, the elongation is ≥12%, and the hole expansion rate is ≥60%; CN116043133A provides an ultra-high forming 980MPa grade hot-dip galvanized high yield strength ratio complex phase steel and its preparation method, which adds a large amount of alloy elements such as Mo and Cr, has a tensile strength of more than 980MPa, a yield strength of 700-850MPa, an elongation of ≥20%, and a hole expansion rate of ≥60%.

[0006] CN117418165A provides a 1000MPa grade Zr-containing complex phase steel and a preparation method thereof, wherein the tensile strength is above 1000MPa and the thickness of the steel plate is 2.0 to 10.0mm; CN116219303A provides a 980MPa grade hot-dip galvanized high yield ratio complex phase steel and its preparation method, with a yield strength of 800-860MPa, a tensile strength of 1000-1100MPa, an elongation value of 13.5-18.0%, and a hole expansion rate of 65-85%.

[0007] Judging from the above-mentioned related patents, the above technical problems have not been well solved. The existing disclosed technologies only explain the production method, processing method or manufacturing method for a specific purpose of 1.0GPa level and above complex phase steel, and there are still many problems in actual industrial production, such as high alloy cost, multiple application categories, multiple mixed casting batches, and multiple product specifications. Summary of the invention

[0008] In view of the technical problems in actual production of high yield ratio complex phase steel, such as high alloy cost, multiple application categories, multiple mixed casting batches, and multiple product specifications, the present invention provides a high yield ratio complex phase steel and a preparation method and application thereof, which solves the many difficult problems in the actual production of high yield ratio complex phase steel, such as high alloy cost, multiple application categories, multiple mixed casting batches, and multiple product specifications.

[0009] In a first aspect, the present invention provides a high yield ratio complex phase steel, wherein the steel billet comprises the following chemical components in weight percentage: C 0.11%-0.14%, Si 0.55%-0.75%, Mn 2.1%-2.3%, Alt 0.6%-0.8%, Cr 0.3%-0.4%, Nb 0.04%-0.07%, P≤0.005%, S≤0.005%, N≤0.0035%, O≤0.005%, and the balance is Fe and unavoidable impurities; the complex phase steel has a yield ratio of >0.70, a tensile strength of >1.0 GPa, and a hole expansion rate of >35%; and the high yield ratio complex phase steel is hot-rolled high yield ratio complex phase steel, cold-rolled high yield ratio complex phase steel, or galvanized high yield ratio complex phase steel.

[0010] Furthermore, the microstructure of the hot-rolled high yield ratio complex phase steel is 15% to 25% ferrite + 60% to 75% martensite / bainite + ≥8% retained austenite; the elongation after fracture of the hot-rolled high yield ratio complex phase steel is ≥16%.

[0011] Furthermore, the microstructure of the cold-rolled high yield ratio complex phase steel is 18% to 28% ferrite + 60% to 70% martensite / bainite + ≥10% retained austenite; the elongation after fracture of the cold-rolled high yield ratio complex phase steel is ≥20%.

[0012] Furthermore, the microstructure of the galvanized high yield ratio complex phase steel is 15% to 25% ferrite + 65% to 75% martensite / bainite + ≥8% retained austenite; the elongation after fracture of the galvanized high yield ratio complex phase steel is ≥18%.

[0013] In a second aspect, the present invention provides a method for preparing high yield ratio complex phase steel, comprising using a steel billet designed with the same chemical composition to prepare hot-rolled high yield ratio complex phase steel, cold-rolled high yield ratio complex phase steel or galvanized high yield ratio complex phase steel.

[0014] Furthermore, the preparation method of hot-rolled high yield ratio complex phase steel includes: (1) Heating process: heat the steel billet, the soaking temperature is 1195~1225℃, and the soaking time is 35min; (2) Rolling process: rolling includes rough rolling and finishing rolling. The starting temperature of rough rolling is 1150~1180℃, and the cumulative reduction rate of rough rolling is more than 55%; the temperature of finishing rolling is 1050~1080℃, the final rolling temperature of finishing rolling is 865~890℃, and the cumulative reduction rate of finishing rolling is more than 65%; when the thickness of the finished product is ≤3.5mm, the edge heater is used; (3) Cooling process: After finishing and final rolling, the steel strip adopts a post-rolling pre-ultra-fast cooling process, and is rapidly cooled to 460~500℃ at a cooling rate of >50℃ / s; (4) Coiling process: coiling the steel strip cooled in step (3) into a steel coil and placing it in a heat preservation pit to cool it to room temperature. The cooling time is 72 hours. (5) Flattening process: the flattening elongation is set to 1.2%~1.5% to produce hot-rolled high yield-to-strength ratio complex phase steel.

[0015] Furthermore, the preparation method of cold-rolled high yield ratio complex phase steel includes: (1) Heating process: heat the steel billet, the soaking temperature is 1220~1250℃, and the soaking time is 35min; (2) Rolling process: rolling includes rough rolling and finishing rolling. The starting temperature of rough rolling is 1150~1180℃, the temperature of finishing rolling is 1055~1085℃, and the final rolling temperature of finishing rolling is 880~900℃. When the thickness of the finished product is ≤3.5mm, the edge heater is used. (3) Cooling process: After finishing rolling, the steel is cooled to 580-620°C at a cooling rate of >30°C / s in a laminar cooling mode. The steel is then coiled and placed in a heat preservation pit to cool to room temperature. The cooling time is 72 hours. (4) a single annealing process, wherein the steel coil obtained in step (3) is placed in a 670°C annealing furnace for 6 to 8 hours and then cooled in the furnace; (5) Acid rolling process, wherein the steel coil obtained in step (4) is pickled and then cold rolled, with a cold rolling reduction ratio of more than 60%, to obtain a cold-hardened steel strip with a thickness of 0.8 to 2.5 mm; (6) The cold-hardened strip is subjected to a continuous annealing process, with the annealing temperature in the two-phase region being 800-820°C and the annealing time being 30-60s; then, it is slowly cooled to 690-710°C at a cooling rate of 5°C / s, and then rapidly flash-cooled to 360-390°C at a cooling rate of >35°C / s by high hydrogen cooling, and isothermally treated at an aging temperature of 350-380°C for 250-350s. Finally, the strip is cooled to room temperature at a cooling rate of 20°C / s to obtain a cold-rolled high yield strength ratio complex phase steel.

[0016] Furthermore, the preparation method of galvanized high yield ratio complex phase steel includes: (1) Heating process: heat the steel billet, the soaking temperature is 1220~1250℃, and the soaking time is 35min; (2) Rolling process: rolling includes rough rolling and finishing rolling. The starting temperature of rough rolling is 1150~1180℃, the temperature of finishing rolling is 1055~1085℃, and the final rolling temperature of finishing rolling is 880~900℃. When the thickness of the finished product is ≤3.5mm, the edge heater is used. (3) Cooling process: After finishing rolling, the steel is cooled to 580-620°C at a cooling rate of >30°C / s in a laminar cooling mode. The steel is then coiled and placed in a heat preservation pit to cool to room temperature. The cooling time is 72 hours. (4) a single annealing process, wherein the steel coil obtained in step (3) is placed in a 670°C annealing furnace for 6 to 8 hours and then cooled in the furnace; (5) Acid rolling process, wherein the steel coil obtained in step (4) is pickled and then cold rolled, with a cold rolling reduction ratio of more than 60%, to obtain a cold-hardened steel strip with a thickness of 0.8 to 2.5 mm; (6) Annealing and galvanizing process, heating the cold hardened steel strip obtained in step (5) to an annealing temperature of 840-860°C for 60-90s; then cooling it to 690-710°C at a cooling rate of 5°C / s; then adopting high hydrogen cooling method, rapidly cooling it to a zinc pot temperature of 485±5°C at a cooling rate of >35°C / s, and then entering the zinc pot. After the strip is galvanized and exits the zinc pot, a mobile bellows is used to cool the galvanized steel strip to ≤160°C, thereby obtaining a galvanized high yield strength ratio complex phase steel.

[0017] In the third aspect, the present invention provides an application of high yield ratio complex phase steel, including: when the complex phase steel is hot-rolled high yield ratio complex phase steel, it is applied to the production of automobile chassis structural parts, reinforcement parts or safety parts with a thickness specification of ≥2.0mm; when the complex phase steel is cold-rolled high yield ratio complex phase steel or the complex phase steel is galvanized high yield ratio complex phase steel, it is applied to the production of cold-formed automobile parts with a thickness specification of 0.8 to 2.5mm. Furthermore, cold-rolled high yield ratio complex phase steel is applied to thin-gauge cold-formed automobile parts with high drawing and hole expansion requirements, and galvanized high yield ratio complex phase steel is applied to thin-gauge cold-formed automobile parts with corrosion resistance, high drawing and hole expansion requirements.

[0018] The beneficial effect of the present invention is that the present invention realizes the preparation of high yield ratio complex phase steel for three purposes by using steel billets designed with the same chemical composition, including hot-rolled high yield ratio complex phase steel, cold-rolled high yield ratio complex phase steel and galvanized high yield ratio complex phase steel, thereby solving many problems in actual industrial production of high yield ratio complex phase steel, such as high alloy cost, multiple application categories, multiple mixed casting batches, and multiple product specifications. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0020] Figure 1 This is a typical microscopic scanning structure diagram of the hot-rolled high yield ratio complex phase steel prepared in Example 1-I.

[0021] Figure 2 This is a typical microscopic scanning structure diagram of the cold-rolled high yield ratio complex phase steel prepared in Example 1-II.

[0022] Figure 3 This is a typical microscopic scanning structure diagram of the galvanized high yield ratio complex phase steel prepared in Example 1-Ⅲ. DETAILED DESCRIPTION

[0023] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0024] The present invention adopts steel billets designed with the same chemical composition to prepare high yield ratio complex phase steels for three purposes, including hot-rolled high yield ratio complex phase steel, cold-rolled high yield ratio complex phase steel, and galvanized high yield ratio complex phase steel.

[0025] The steel billets in Examples 1 to 3 of the present invention include the following chemical compositions in weight percentage: C 0.11% to 0.14%, Si 0.55% to 0.75%, Mn 2.1% to 2.3%, Alt 0.6% to 0.8%, Cr 0.3% to 0.4%, Nb 0.04% to 0.07%, P≤0.005%, S≤0.005%, N≤0.0035%, O≤0.005%, and the balance is Fe and unavoidable impurities.

[0026] The chemical composition of the molten steel from the steel billets in Examples 1 to 3 is shown in Table 1.

[0027] Table 1 Actual smelting composition of the embodiment (mass percentage, %)

[0028] The preparation method of the hot-rolled high yield ratio complex phase steel in Examples 1 to 3 includes: (1) Heating process: Heat the steel billet to the austenitizing temperature (i.e., soaking temperature) of 1195~1225℃, and the soaking time is about 35min; (2) Rolling process: the starting temperature of rough rolling is 1150~1180℃, the cumulative reduction rate of rough rolling is more than 55%; the temperature of finishing rolling is 1050~1080℃, the final rolling temperature of finishing rolling is 865~890℃, and the cumulative reduction rate of finishing rolling is more than 65%; when the thickness of the finished product is ≤3.5mm, the edge heater is used; (3) Cooling process: After finishing rolling, the steel is cooled to 460-500°C at a cooling rate of >50°C / s by adopting a post-rolling pre-ultra-fast cooling process. (4) Coiling process: The cooled steel strip is coiled into a steel coil and placed in a heat preservation pit, and slowly cooled to room temperature for 72 hours; (5) Flattening process: The flattening elongation is set to 1.2%~1.5% to produce hot-rolled high yield-to-strength ratio complex phase steel.

[0029] The main process control parameters of the hot-rolled high yield ratio complex phase steel in Examples 1 to 3 (1-I in the table represents the hot-rolled high yield ratio complex phase steel in Example 1) are shown in Table 2.

[0030] Table 2 Main process control parameters of hot-rolled high yield ratio composite steel in Examples 1 to 3

[0031] The preparation method of the cold-rolled high yield ratio complex phase steel of embodiments 1 to 3 comprises: (1) Hot rolling process: The steel billet is heated to the austenitizing temperature (i.e., the soaking temperature) of 1220~1250℃, and the soaking time is about 35min; the starting temperature of the rough rolling is 1150~1180℃; the finishing rolling temperature is 1055~1085℃, and the finishing rolling temperature is 870~900℃; when the thickness of the finished product is ≤3.5mm, the edge heater is used; after the finishing rolling, the laminar cooling mode is adopted, and the steel billet is rapidly cooled to 580~620℃ at a cooling rate of >30℃ / s, and then the steel billet is coiled and placed in a heat preservation pit, and slowly cooled to room temperature, and the slow cooling time is 72h; (2) One-step annealing process: the steel coil is placed in a 670°C annealing furnace for 6-8 hours and cooled with the furnace; (3) Pickling process: After pickling the steel coil, cold rolling is performed with a cold rolling reduction ratio of >60% to obtain a cold-hardened steel strip with a thickness of 0.8 to 2.5 mm. (4) Secondary continuous annealing process: The cold-hardened strip after pickling is subjected to a continuous annealing process, wherein the annealing temperature in the (γ+α) two-phase region is 800~820℃ and the annealing time is 30~60s; then the strip is slowly cooled to 690~710℃ at a rate of about 5℃ / s; then the strip is rapidly flash-cooled to 360~390℃ at a cooling rate of >35℃ / s by high hydrogen cooling, and is isothermally aged at a temperature of 350~380℃ for 250~350s; finally, the strip is cooled to room temperature at a cooling rate of about 20℃ / s to obtain a cold-rolled high yield ratio complex phase steel.

[0032] The main process control parameters of the cold-rolled high yield ratio complex phase steel in Examples 1 to 3 (1-II in the table represents the cold-rolled high yield ratio complex phase steel in Example 1) are shown in Table 3.

[0033] Table 3 Main process control parameters of cold-rolled high yield ratio complex phase steel in Examples 1 to 3

[0034] The preparation method of the galvanized high yield ratio complex phase steel of embodiments 1 to 3 comprises: (1) Hot rolling process: The steel billet is heated to the austenitizing temperature (i.e., the soaking temperature) of 1220~1250℃, and the soaking time is about 35min; the starting temperature of the rough rolling is 1150~1180℃; the finishing rolling temperature is 1055~1085℃, and the finishing rolling temperature is 870~900℃; when the thickness of the finished product is ≤3.5mm, the edge heater is used; after the finishing rolling, the laminar cooling mode is adopted, and the steel billet is rapidly cooled to 580~620℃ at a cooling rate of >30℃ / s, and then the steel billet is coiled and placed in a heat preservation pit, and slowly cooled to room temperature, and the slow cooling time is 72h; (2) One-step annealing process: the steel coil is placed in a 670°C annealing furnace for 6-8 hours and cooled with the furnace; (3) Pickling process: After pickling the steel coil, cold rolling is performed with a cold rolling reduction ratio of >60% to obtain a cold-hardened steel strip with a thickness of 0.8 to 2.5 mm. (4) Annealing and galvanizing process: The cold hardened steel strip after acid rolling is heated to the (γ+α) two-phase region annealing temperature of 840~860℃, and the annealing time is 60~90s; then slowly cooled to 690~710℃ at a rate of about 5℃ / s; then high hydrogen cooling is adopted to rapidly flash cool to 485±5℃ at a cooling rate of >35℃ / s, and then enters the zinc pot. After the strip is galvanized and exits the zinc pot, a mobile bellows is used to cool the galvanized steel strip to ≤160℃, thereby obtaining the galvanized high yield strength ratio complex phase steel. In addition, in order to ensure the adhesion and surface quality of the zinc layer of the steel strip, the galvanizing process is equipped with a pre-oxidation-reduction device.

[0035] The main process control parameters of the galvanized high yield ratio complex phase steel in Examples 1 to 3 (1-III in the table represent the galvanized high yield ratio complex phase steel in Example 1) are shown in Table 4.

[0036] Table 4 Main process control parameters of galvanized high yield ratio complex phase steel in Examples 1 to 3

[0037] The high yield strength ratio complex phase steel prepared in Examples 1-3 was sampled for microstructure analysis and mechanical property testing. The test results are shown in Table 5.

[0038] Table 5 Microstructure volume fraction and mechanical properties of high yield ratio composite steel prepared in Examples 1 to 3

[0039] From the results analysis in Table 5, it can be seen that the high yield ratio complex phase steel prepared in Examples 1-3 of the present invention has a tensile strength of >1.0GPa, a hole expansion rate of >35%, and a yield ratio of >0.70. Among them, the hot-rolled high yield ratio complex phase steel has an elongation after fracture of ≥16%, which is particularly suitable for manufacturing automobile chassis structural parts, reinforcement parts, safety parts, etc. with a thickness specification of ≥2.0mm and requirements for hole expansion and flanging; the cold-rolled high yield ratio complex phase steel has an elongation after fracture of ≥20%, which is particularly suitable for manufacturing thin-gauge cold-formed automobile parts with a thickness specification of 0.8-2.5mm and high drawing and hole expansion requirements; the galvanized high yield ratio complex phase steel has an elongation after fracture of ≥18%, which is particularly suitable for manufacturing thin-gauge cold-formed automobile parts with a thickness specification of 0.8-2.5mm and requirements for corrosion resistance, high drawing and hole expansion.

[0040] Although the present invention has been described in detail with reference to the accompanying drawings and in combination with the preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and essence of the present invention, a person of ordinary skill in the art may make various equivalent modifications or substitutions to the embodiments of the present invention, and these modifications or substitutions shall be within the scope of the present invention. Any person of ordinary skill in the art may easily think of changes or substitutions within the technical scope disclosed by the present invention, and these shall be within the scope of protection of the present invention.

Claims

1. A high yield-to-strength ratio complex phase steel, characterized in that: The steel billet used includes the following chemical compositions in weight percentage: C 0.11%~0.14%, Si 0.55%~0.75%, Mn 2.1%~2.3%, Alt 0.6%~0.8%, Cr 0.3%~0.4%, Nb 0.04%~0.07%, P≤0.005%, S≤0.005%, N≤0.0035%, O≤0.005%, the balance is Fe and unavoidable impurities; the yield ratio of the complex phase steel is greater than 0.70, the tensile strength is greater than 1.0 GPa, and the hole expansion rate is greater than 35%; the high yield ratio complex phase steel is hot-rolled high yield ratio complex phase steel, cold-rolled high yield ratio complex phase steel or galvanized high yield ratio complex phase steel.

2. A high yield ratio complex phase steel as claimed in claim 1, characterized in that: The microstructure of the hot-rolled high yield ratio complex phase steel is 15% to 25% ferrite + 60% to 75% martensite / bainite + ≥8% retained austenite; the elongation after fracture of the hot-rolled high yield ratio complex phase steel is ≥16%.

3. A high yield ratio complex phase steel as claimed in claim 1, characterized in that: The microstructure of the cold-rolled high yield ratio complex phase steel is 18% to 28% ferrite + 60% to 70% martensite / bainite + ≥10% retained austenite; the elongation after fracture of the cold-rolled high yield ratio complex phase steel is ≥20%.

4. The high yield ratio complex phase steel according to claim 1, characterized in that: The microstructure of the galvanized high yield ratio complex phase steel is 15% to 25% ferrite + 65% to 75% martensite / bainite + ≥8% retained austenite; the elongation after fracture of the galvanized high yield ratio complex phase steel is ≥18%.

5. A method for preparing high yield ratio complex phase steel as claimed in claim 1, characterized in that: The steel billet designed with the same chemical composition is used to prepare hot-rolled high yield ratio complex phase steel, cold-rolled high yield ratio complex phase steel or galvanized high yield ratio complex phase steel.

6. The method for preparing high yield ratio complex phase steel according to claim 5, characterized in that: The preparation method of hot-rolled high yield ratio complex phase steel comprises: (1) Heating process: heat the steel billet, the soaking temperature is 1195~1225℃, and the soaking time is 35min; (2) Rolling process: rolling includes rough rolling and finishing rolling. The starting temperature of rough rolling is 1150~1180℃, and the cumulative reduction rate of rough rolling is more than 55%; the temperature of finishing rolling is 1050~1080℃, the final rolling temperature of finishing rolling is 865~890℃, and the cumulative reduction rate of finishing rolling is more than 65%; when the thickness of the finished product is ≤3.5mm, the edge heater is used; (3) Cooling process: After finishing and final rolling, the steel strip adopts a post-rolling pre-ultra-fast cooling process, and is rapidly cooled to 460~500℃ at a cooling rate of >50℃ / s; (4) Coiling process: coiling the steel strip cooled in step (3) into a steel coil and placing it in a heat preservation pit to cool it to room temperature. The cooling time is 72 hours. (5) Flattening process: the flattening elongation is set to 1.2%~1.5% to produce hot-rolled high yield-to-strength ratio complex phase steel.

7. The method for preparing high yield ratio complex phase steel according to claim 5, characterized in that: The preparation method of cold-rolled high yield ratio complex phase steel comprises: (1) Heating process: heat the steel billet, the soaking temperature is 1220~1250℃, and the soaking time is 35min; (2) Rolling process: rolling includes rough rolling and finishing rolling. The starting temperature of rough rolling is 1150~1180℃, the temperature of finishing rolling is 1055~1085℃, and the final rolling temperature of finishing rolling is 880~900℃. When the thickness of the finished product is ≤3.5mm, the edge heater is used. (3) Cooling process: After finishing rolling, the steel is cooled to 580-620°C at a cooling rate of >30°C / s in a laminar cooling mode. The steel is then coiled and placed in a heat preservation pit to cool to room temperature. The cooling time is 72 hours. (4) a single annealing process, wherein the steel coil obtained in step (3) is placed in a 670°C annealing furnace for 6 to 8 hours and then cooled in the furnace; (5) Acid rolling process, wherein the steel coil obtained in step (4) is pickled and then cold rolled, with a cold rolling reduction ratio of more than 60%, to obtain a cold-hardened steel strip with a thickness of 0.8 to 2.5 mm; (6) The cold-hardened strip is subjected to a continuous annealing process, with the annealing temperature in the two-phase region being 800-820°C and the annealing time being 30-60s; then, it is slowly cooled to 690-710°C at a cooling rate of 5°C / s, and then rapidly flash-cooled to 360-390°C at a cooling rate of >35°C / s by high hydrogen cooling, and isothermally treated at an aging temperature of 350-380°C for 250-350s. Finally, the strip is cooled to room temperature at a cooling rate of 20°C / s to obtain a cold-rolled high yield strength ratio complex phase steel.

8. The method for preparing high yield ratio complex phase steel according to claim 5, characterized in that: The preparation method of galvanized high yield ratio complex phase steel includes: (1) Heating process: heat the steel billet, the soaking temperature is 1220~1250℃, and the soaking time is 35min; (2) Rolling process: rolling includes rough rolling and finishing rolling. The starting temperature of rough rolling is 1150~1180℃, the temperature of finishing rolling is 1055~1085℃, and the final rolling temperature of finishing rolling is 880~900℃. When the thickness of the finished product is ≤3.5mm, the edge heater is used. (3) Cooling process: After finishing rolling, the steel is cooled to 580-620°C at a cooling rate of >30°C / s in a laminar cooling mode. The steel is then coiled and placed in a heat preservation pit to cool to room temperature. The cooling time is 72 hours. (4) a single annealing process, wherein the steel coil obtained in step (3) is placed in a 670°C annealing furnace for 6 to 8 hours and then cooled in the furnace; (5) Acid rolling process, wherein the steel coil obtained in step (4) is pickled and then cold rolled, with a cold rolling reduction ratio of more than 60%, to obtain a cold-hardened steel strip with a thickness of 0.8 to 2.5 mm; (6) Annealing and galvanizing process, heating the cold hardened steel strip obtained in step (5) to an annealing temperature of 840-860°C for 60-90s; then cooling it to 690-710°C at a cooling rate of 5°C / s; then adopting high hydrogen cooling method, rapidly cooling it to a zinc pot temperature of 485±5°C at a cooling rate of >35°C / s, and then entering the zinc pot. After the strip is galvanized and exits the zinc pot, a mobile bellows is used to cool the galvanized steel strip to ≤160°C, thereby obtaining a galvanized high yield strength ratio complex phase steel.

9. An application of the high yield ratio complex phase steel as claimed in claim 1, characterized in that: When the complex phase steel is a hot-rolled high yield ratio complex phase steel, it is used to produce automobile chassis structural parts, reinforcement parts or safety parts with a thickness specification of ≥2.0mm; when the complex phase steel is a cold-rolled high yield ratio complex phase steel or the complex phase steel is a galvanized high yield ratio complex phase steel, it is used to produce cold-formed automobile parts with a thickness specification of 0.8 to 2.5mm.

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