Low-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 low-yield-ratio complex-phase steel and a preparation method and application thereof. A steel billet with the same chemical component is adopted, the steel billet comprises, by weight, 0.09%-0.12% of C, 0.5%-0.7% of Si, 2.2%-2.4% of Mn, 0.7%-0.9% of Alt, 0.3%-0.4% of Cr, 0.0018%-0.0030% of B, Nb + Ti larger than or equal to 0.04% and smaller than or equal to 0.06%, P smaller than or equal to 0.005%, S smaller than or equal to 0.005%, N smaller than or equal to 0.0035%, O smaller than or equal to 0.005% and the balance Fe and impurities, and hot-rolled low-yield-ratio complex-phase steel, cold-rolled low-yield-ratio complex-phase steel and galvanized low-yield-ratio complex-phase steel are prepared. The problems of high alloy cost, multiple application categories, multiple mixed casting batches, multiple product specifications and the like of the low-yield-ratio complex-phase steel 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 low-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 low-yield ratio complex phase steels at the 1.0GPa level and above bring many inconveniences to steel enterprises in actual production organization, mainly manifested in the high rate of mixed casting downgrading and re-judgment, and the frequent transition of different steel grades and specifications, resulting in poor product quality stability. Therefore, exploring the preparation method of low-yield ratio complex phase steels with low cost, economy, diversified user needs and stable quality can solve many problems in the actual industrial production of low-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 low 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 low 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 low yield 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 low yield 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 low 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 low 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 low yield ratio complex phase steel and a preparation method and application thereof, which solves the many problems in the actual production of low 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 low yield ratio complex phase steel, comprising the following chemical components in weight percentage: C 0.09%~0.12%, Si 0.5%~0.7%, Mn 2.2%~2.4%, Alt 0.7%~0.9%, Cr 0.3%~0.4%, B 0.0018%~0.0030%, 0.04%≤Nb+Ti≤0.06%, 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 <0.60 and a tensile strength of >1.0 GPa; the complex phase steel is a hot-rolled low yield ratio complex phase steel, a cold-rolled low yield ratio complex phase steel or a galvanized low yield ratio complex phase steel.

[0010] Furthermore, the microstructure of the hot-rolled low yield ratio complex phase steel is 25%~45% ferrite + 45%~65% martensite / bainite + ≥5% retained austenite; the elongation after fracture of the hot-rolled low yield ratio complex phase steel is ≥17%.

[0011] Furthermore, the microstructure of the cold-rolled low yield ratio complex phase steel is 30% to 50% ferrite + 40% to 60% martensite / bainite + ≥8% retained austenite; the elongation after fracture of the cold-rolled low yield ratio complex phase steel is ≥20%.

[0012] Furthermore, the microstructure of the galvanized low yield ratio complex phase steel is 35% to 45% ferrite + 40% to 60% martensite / bainite + ≥6% retained austenite; the elongation after fracture of the galvanized low yield ratio complex phase steel is ≥18%.

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

[0014] Furthermore, the method for preparing hot-rolled low yield ratio complex phase steel comprises: (1) Heating process: heat the steel billet, the soaking temperature is 1200~1240℃, and the soaking time is 30~40min; (2) Rolling process: rolling includes rough rolling and finishing rolling. The starting temperature of rough rolling is 1170~1190℃, the temperature of finishing rolling is 1050~1080℃, and the final rolling temperature of finishing rolling is 865~890℃. When the thickness of the finished product is ≤4.0mm, the edge heater is used. (3) Cooling process: After finishing rolling, a three-stage cooling process is adopted. The first stage adopts a pre-ultra-fast cooling process, and the steel is rapidly cooled to 700-730°C at a cooling rate of >50°C / s. The second stage adopts an air natural cooling process to cool the steel to 660-690°C. The third stage adopts a laminar cooling mode to rapidly cool the steel to 320-360°C at a cooling rate of >30°C / s. (4) Coiling process: the cooled steel strip is coiled into a steel coil and placed in an insulation pit to cool to room temperature. The slow cooling time is 72 hours. (5) Flattening process: the flattening elongation is set to 0.9%~1.3% to produce hot-rolled low yield strength ratio complex phase steel.

[0015] Furthermore, the preparation method of cold-rolled low yield ratio complex phase steel comprises: (1) Heating process: heat the steel billet, the soaking temperature is 1200~1240℃, and the soaking time is 35min; (2) Rolling process: rolling includes rough rolling and finishing rolling. The starting temperature of rough rolling is 1170~1190℃, the temperature of finishing rolling is 1070~1100℃, and the final temperature of finishing rolling is 890~910℃. When the thickness of the finished product is ≤4.0mm, the edge heater is used. (3) Cooling process: After finishing rolling, the steel is cooled to 620-660°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 680°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 >55%, 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 a preheating temperature of 430-460°C, a two-phase annealing temperature of 810-830°C, and an annealing time of 60-90s; then, it is slowly cooled to 680-700°C at a cooling rate of 5°C / s, and then rapidly flash-cooled to 330-350°C at a cooling rate of >35°C / s by high hydrogen cooling, and isothermally treated at an aging temperature of 320-340°C for 250-350s. Finally, the strip is cooled to room temperature at a cooling rate of 18°C / s to obtain a cold-rolled low yield ratio complex phase steel.

[0016] Furthermore, the preparation method of galvanized low yield ratio complex phase steel includes: (1) Heating process: heat the steel billet, the soaking temperature is 1200~1240℃, and the soaking time is 35min; (2) Rolling process: rolling includes rough rolling and finishing rolling. The starting temperature of rough rolling is 1170~1190℃, the temperature of finishing rolling is 1070~1100℃, and the final temperature of finishing rolling is 890~910℃. When the thickness of the finished product is ≤4.0mm, the edge heater is used. (3) Cooling process: After finishing rolling, the steel is cooled to 620-660°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 680°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 >55%, to obtain a cold-hardened steel strip with a thickness of 0.8 to 2.5 mm; (6) Annealing and galvanizing process: the cold hardened strip is heated to an annealing temperature of 850~870℃ for 30~60s; then slowly cooled to 700~720℃ at a cooling rate of 5℃ / s; then high hydrogen cooling is used to quickly cool to a zinc pot temperature of 465±5℃ at a cooling rate of >35℃ / s. After exiting the zinc pot, a mobile bellows is used to cool the galvanized steel strip to ≤170℃ to obtain galvanized low yield ratio complex phase steel. Furthermore, in order to ensure the adhesion and surface quality of the zinc layer of the strip, the galvanizing process is equipped with a pre-oxidation-reduction device.

[0017] In a third aspect, the present invention provides an application of a low yield ratio complex phase steel. When the complex phase steel is a hot-rolled low 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 low yield ratio complex phase steel, it is used to produce thin-gauge cold-formed automobile parts with a thickness specification of 0.8 to 2.5mm and high drawing requirements; when the complex phase steel is a galvanized low yield ratio complex phase steel, it is used to produce thin-gauge cold-formed automobile parts with corrosion resistance and high drawing requirements.

[0018] The beneficial effects of the present invention are as follows: the present invention realizes the preparation of three types of low yield ratio complex phase steels by using steel billets designed with the same chemical composition, including hot-rolled low yield ratio complex phase steel, cold-rolled low yield ratio complex phase steel and galvanized low yield ratio complex phase steel, thereby solving many problems in actual industrial production of low 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 low yield ratio complex phase steel prepared in Example 2-I.

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

[0022] Figure 3 This is a typical microscopic scanning structure diagram of the galvanized low yield ratio complex phase steel prepared in Example 2-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 three types of low yield ratio complex phase steels, including hot-rolled low yield ratio complex phase steel, cold-rolled low yield ratio complex phase steel, and galvanized low 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: 0.09-0.12%, Si: 0.5-0.7%, Mn: 2.2-2.4%, Alt: 0.7-0.9%, Cr: 0.3-0.4%, 0.04%≤Nb+Ti≤0.06%, B: 0.0018-0.0030%, and P is limited to 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 method for preparing hot-rolled low yield ratio complex phase steel in Examples 1 to 3 comprises: (1) Heating process: Heat the steel billet to the austenitizing temperature (soaking temperature) of 1200~1240℃, and the soaking time is 30~40min; (2) Rolling process: Rough rolling starting temperature 1170~1190℃; Finishing rolling temperature 1050~1080℃, Finishing rolling final temperature 865~890℃; When the finished product thickness is ≤4.0mm, the edge heater is used; (3) Cooling process: After finishing rolling, a three-stage cooling process is adopted. The first stage of cooling is to control the microstructure before the γ phase transformation. The ultra-fast cooling process after rolling is adopted to rapidly cool to 700-730°C at a cooling rate of >50°C / s. The second stage of cooling is to carry out the ferrite phase transformation and fully diffuse the C element into the untransformed γ. The temperature holding mode of natural slow cooling in air is adopted to cool to 660-690°C. The third stage of cooling is to promote the martensite / bainite phase transformation. The laminar cooling mode of concentrated cooling in the front stage is adopted to rapidly cool to 320-360°C at a cooling rate of >30°C / s. (4) Coiling process: The steel strip is coiled into a steel coil and placed in an insulation pit, and slowly cooled to room temperature for 72 hours.

[0029] (5) Flattening process: The flattening elongation is set to 0.9~1.3% to obtain hot-rolled low yield ratio complex phase steel.

[0030] The main process control parameters of the hot-rolled low yield ratio complex phase steel in Examples 1 to 3 (1-I in the table represents the hot-rolled low yield ratio complex phase steel in Example 1) are shown in Table 2.

[0031] Table 2 Main process control parameters of hot-rolled low yield ratio composite steel in Examples 1 to 3

[0032] The preparation method of the cold-rolled low yield ratio complex phase steel of embodiments 1 to 3 comprises: (1) Hot rolling process: The steel billet is heated to an austenitizing temperature (soaking temperature) of 1200-1240°C, and the soaking time is 35 min; the starting temperature of the rough rolling is 1170-1190°C; the finishing rolling temperature is 1070-1100°C, and the finishing rolling temperature is 890-910°C; when the thickness of the finished product is ≤4.0 mm, the edge heater is used; after the finishing rolling, the laminar cooling mode is adopted, and the steel billet is rapidly cooled to 620-660°C at a cooling rate of >30°C / 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 72 hours; (2) One-step annealing process: the steel coil obtained in step (1) is placed in a 680°C annealing furnace for 6-8 hours and cooled in the furnace; (3) Pickling process: The steel coil obtained in step (2) is pickled and then cold rolled with a cold rolling reduction ratio of >55% 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 obtained in step (3) is subjected to a continuous annealing process, wherein the preheating temperature is 430-460°C; the (γ+α) two-phase region annealing temperature is 810-830°C, and the annealing time is 60-90s; then the strip is slowly cooled to 680-700°C at a rate of 5°C / s; then the strip is rapidly flash cooled to 330-350°C at a cooling rate of >35°C / s by high hydrogen cooling, and isothermally treated at an aging temperature of 320-340°C for 250-350s; finally, the strip is cooled to room temperature at a cooling rate of 18°C / s to obtain a cold-rolled low yield ratio complex phase steel.

[0033] The main process control parameters of the cold-rolled low yield ratio complex phase steel in Examples 1 to 3 (1-II in the table represents the cold-rolled low yield ratio complex phase steel in Example 1) are shown in Table 3.

[0034] Table 3 Main process control parameters of cold-rolled low yield ratio complex phase steel in Examples 1 to 3

[0035] The preparation method of the galvanized low yield ratio complex phase steel of embodiments 1 to 3 comprises: (1) Hot rolling process: The steel billet is heated to an austenitizing temperature (soaking temperature) of 1200-1240°C, and the soaking time is 35 min; the starting temperature of the rough rolling is 1170-1190°C; the finishing rolling temperature is 1070-1100°C, and the finishing rolling temperature is 890-910°C; when the thickness of the finished product is ≤4.0 mm, the edge heater is used; after the finishing rolling, the laminar cooling mode is adopted, and the steel billet is rapidly cooled to 620-660°C at a cooling rate of >30°C / 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 72 hours; (2) One-step annealing process: the steel coil obtained in step (1) is placed in a 680°C annealing furnace for 6-8 hours and cooled in the furnace; (3) Pickling process: The steel coil obtained in step (2) is pickled and then cold rolled with a cold rolling reduction ratio of >55% 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 obtained in step (3) is heated to the (γ+α) two-phase region annealing temperature of 850~870℃ for 30~60s; then slowly cooled to 700~720℃ at 5℃ / s; then rapidly cooled to the zinc pot temperature of 465±5℃ at a cooling rate of >35℃ / s by high hydrogen cooling, and the galvanized steel strip is cooled to ≤170℃ by a mobile bellows after exiting the zinc pot, thereby obtaining galvanized low yield ratio complex phase steel. Furthermore, 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.

[0036] The main process control parameters of the galvanized low yield ratio complex phase steel in Examples 1 to 3 (1-III in the table represent the galvanized low yield ratio complex phase steel in Example 1) are shown in Table 4.

[0037] Table 4 Main process control parameters of galvanized low yield ratio complex phase steel in Examples 1 to 3

[0038] The low 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.

[0039] Table 5 Microstructure volume fraction and mechanical properties of low yield ratio composite steel prepared in Examples 1 to 3

[0040] From the results analysis in Table 5, it can be seen that the low yield ratio complex phase steel prepared in Examples 1-3 of the present invention has a tensile strength of >1.0GPa and a yield ratio of <0.60. Among them, the hot-rolled low yield ratio complex phase steel has an elongation after fracture of ≥17%, which is particularly suitable for manufacturing automobile chassis structural parts, reinforcement parts, safety parts, etc. with a thickness specification of ≥2.0mm; the cold-rolled low 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 requirements; the galvanized low 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 corrosion resistance and high drawing requirements.

[0041] Figure 1 A typical microscopic scanning structure diagram of the hot-rolled low yield ratio complex phase steel prepared in Example 2-I; Figure 2 A typical microscopic scanning structure diagram of the cold-rolled low yield ratio complex phase steel prepared in Example 2-II; Figure 3 This is a typical microscopic scanning structure diagram of the galvanized low yield ratio complex phase steel prepared in Example 2-III. The specific microstructure analysis is shown in Table 5.

[0042] 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 low yield strength ratio complex phase steel, characterized in that: The invention comprises the following chemical compositions in weight percentage: C 0.09%~0.12%, Si 0.5%~0.7%, Mn 2.2%~2.4%, Alt 0.7%~0.9%, Cr 0.3%~0.4%, B 0.0018%~0.0030%, 0.04%≤Nb+Ti≤0.06%, P≤0.005%, S≤0.005%, N≤0.0035%, O≤0.005%, and the balance is Fe and unavoidable impurities; the yield ratio of the complex phase steel is less than 0.60, and the tensile strength is greater than 1.0GPa; the low yield ratio complex phase steel is hot-rolled low yield ratio complex phase steel, cold-rolled low yield ratio complex phase steel or galvanized low yield ratio complex phase steel.

2. A low yield ratio complex phase steel as claimed in claim 1, characterized in that: The microstructure of the hot-rolled low yield ratio complex phase steel is 25%~45% ferrite + 45%~65% martensite / bainite + ≥5% retained austenite; the elongation after fracture of the hot-rolled low yield ratio complex phase steel is ≥17%.

3. The low yield ratio complex phase steel according to claim 1, characterized in that: The microstructure of the cold-rolled low yield ratio complex phase steel is 30% to 50% ferrite + 40% to 60% martensite / bainite + ≥8% retained austenite; the elongation after fracture of the cold-rolled low yield ratio complex phase steel is ≥20%.

4. The low yield ratio complex phase steel according to claim 1, characterized in that: The microstructure of the galvanized low yield ratio complex phase steel is 35% to 45% ferrite + 40% to 60% martensite / bainite + ≥6% retained austenite; the elongation after fracture of the galvanized low yield ratio complex phase steel is ≥18%.

5. A method for preparing a low 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 low-yield ratio complex phase steel, cold-rolled low-yield ratio complex phase steel or galvanized low-yield ratio complex phase steel.

6. The method for preparing low yield ratio complex phase steel according to claim 5, characterized in that: The method for preparing hot-rolled low yield ratio complex phase steel comprises: (1) Heating process: heat the steel billet, the soaking temperature is 1200~1240℃, and the soaking time is 30~40min; (2) Rolling process: rolling includes rough rolling and finishing rolling. The starting temperature of rough rolling is 1170~1190℃, the temperature of finishing rolling is 1050~1080℃, and the final rolling temperature of finishing rolling is 865~890℃. When the thickness of the finished product is ≤4.0mm, the edge heater is used. (3) Cooling process: After finishing rolling, a three-stage cooling process is adopted. The first stage adopts a pre-ultra-fast cooling process, and the steel is rapidly cooled to 700-730°C at a cooling rate of >50°C / s. The second stage adopts an air natural cooling process to cool the steel to 660-690°C. The third stage adopts a laminar cooling mode to rapidly cool the steel to 320-360°C at a cooling rate of >30°C / s. (4) Coiling process: the cooled steel strip is coiled into a steel coil and placed in an insulation pit to cool to room temperature. The slow cooling time is 72 hours. (5) Flattening process: the flattening elongation is set to 0.9%~1.3% to produce hot-rolled low yield strength ratio complex phase steel.

7. The method for preparing low yield ratio complex phase steel according to claim 5, characterized in that: The preparation method of cold-rolled low yield ratio complex phase steel comprises: (1) Heating process: heat the steel billet, the soaking temperature is 1200~1240℃, and the soaking time is 35min; (2) Rolling process: rolling includes rough rolling and finishing rolling. The starting temperature of rough rolling is 1170~1190℃, the temperature of finishing rolling is 1070~1100℃, and the final temperature of finishing rolling is 890~910℃. When the thickness of the finished product is ≤4.0mm, the edge heater is used. (3) Cooling process: After finishing rolling, the steel is cooled to 620-660°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 680°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 >55%, 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 a preheating temperature of 430-460°C, a two-phase annealing temperature of 810-830°C, and an annealing time of 60-90s; then, it is slowly cooled to 680-700°C at a cooling rate of 5°C / s, and then rapidly flash-cooled to 330-350°C at a cooling rate of >35°C / s by high hydrogen cooling, and isothermally treated at an aging temperature of 320-340°C for 250-350s. Finally, the strip is cooled to room temperature at a cooling rate of 18°C / s to obtain a cold-rolled low yield ratio complex phase steel.

8. The method for preparing low yield ratio complex phase steel according to claim 5, characterized in that: The preparation method of galvanized low yield ratio complex phase steel includes: (1) Heating process: heat the steel billet, the soaking temperature is 1200~1240℃, and the soaking time is 35min; (2) Rolling process: rolling includes rough rolling and finishing rolling. The starting temperature of rough rolling is 1170~1190℃, the temperature of finishing rolling is 1070~1100℃, and the final temperature of finishing rolling is 890~910℃. When the thickness of the finished product is ≤4.0mm, the edge heater is used. (3) Cooling process: After finishing rolling, the steel is cooled to 620-660°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 680°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 >55%, to obtain a cold-hardened steel strip with a thickness of 0.8 to 2.5 mm; (6) Annealing and galvanizing process: heat the cold hardened strip to an annealing temperature of 850~870℃ for 30~60s; then slowly cool it to 700~720℃ at a cooling rate of 5℃ / s; then use high hydrogen cooling method to quickly cool it to the zinc pot temperature of 465±5℃ at a cooling rate of >35℃ / s. After leaving the zinc pot, use a mobile bellows to cool the galvanized steel strip to ≤170℃ to obtain galvanized low yield strength ratio complex phase steel.

9. An application of the low yield ratio complex phase steel as claimed in claim 1, characterized in that: When the complex phase steel is a hot-rolled low 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 low yield ratio complex phase steel or the complex phase steel is a galvanized low 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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