A high-plating-property and high strength-ductility product TRIP steel and its manufacturing method

Abstract

The present invention belongs to the field of manufacturing high-strength steel for automobiles, and particularly relates to a high-plating-property and high-strength-ductility-product TRIP steel and a manufacturing method thereof. The chemical composition of the TRIP steel includes, by mass percentage, C: 0.16 - 0.24%, Mn: 1.5 - 2.4%, Si: 0.5 - 1.2%, Al: 0.6 - 1.5%, Ti: 0.008 - 0.08%, Cu: 0.05 - 0.3%, Bi: 0.008 - 0.05%, B: 0.001 - 0.003%, P: below 0.015%, S: below 0.002%, and the balance is Fe. The heating temperature of the slab is 1100 - 1250°C, the finish rolling temperature is 20 - 50°C below the A3 line, the coiling temperature is 500 - 600°C, the dew point of the annealing atmosphere is above -30°C, the critical annealing temperature is 780 - 900°C, the critical annealing holding time is 90 - 600 s, the cooling rate is 10 - 50°C / s, the bainite isothermal annealing temperature is 350 - 450°C, and the isothermal annealing holding time is 40 - 600 s. The present invention can greatly improve the surface oxidation problem of the TRIP steel during annealing on the premise of considering mechanical properties, improve the plating property, and improve the process stability.

Description

Technical Field

[0001] The present invention belongs to the field of production and manufacturing of high-strength steel for automobiles, and particularly relates to a high-plating-property, high-strength and high-ductility product TRIP steel and a manufacturing method thereof. Background Art

[0002] With the rapid development of automotive lightweighting and the increasingly stringent requirements for automotive collision safety, advanced high-strength steels represented by transformation-induced plasticity (TRIP) steel are increasingly used as automotive sheet materials. The microstructure of TRIP steel contains soft-phase ferrite, hard-phase bainite, and retained austenite. When the steel deforms under load, the retained austenite in the steel undergoes stress-strain-induced martensitic transformation, introducing transformation strengthening and plasticity growth mechanisms, making the steel sheet have good strength and plasticity. To meet the corrosion resistance requirements of automotive steel sheets, hot-dip galvanizing treatment is generally required for TRIP steel sheets. However, different from traditional steels, the galvanizing defect problem of TRIP steel is very prominent, and the surface quality problem of galvanized steel has been difficult to effectively solve for a long time. The main reason is that active alloying elements such as Mn and Si in the steel undergo surface selective oxidation during the continuous annealing process before hot-dip galvanizing, and the presence of external oxides significantly deteriorates the plating property of the steel, leading to subsequent galvanizing quality defects. In response to the poor plating property of TRIP steel, the industry and academia have found that Si is the element that has the greatest impact on the plating property, and reducing the Si content can significantly improve the plating property of the steel. However, Si is an important alloying element in TRIP steel, which has the effect of inhibiting the precipitation of cementite and thus retaining retained austenite; in addition, Si can further improve the strength of the steel through solid solution strengthening. Therefore, for high-strength and high-ductility product TRIP steel in particular, Si is irreplaceable. Therefore, how to make TRIP steel have both high strength and high ductility and high plating property has become an international bottleneck problem.

[0003] Chinese patents (publication numbers CN 115181899A, CN 115181892A, CN 111996467A) have successively disclosed a series of composition systems and preparation methods of high-strength (980 MPa, 1180 MPa) grade TRIP steels; however, the Mn and Si contents in the steel are relatively high, and there are no special composition regulation and process improvement measures to inhibit external oxidation during annealing. After subsequent hot-dip galvanizing treatment, the surface quality problem is worrying.

[0004] Chinese Patent (Publication No. CN 111020119 A) discloses "A 700 MPa Grade Cold-Rolled TRIP Steel Sheet and Its Production Method". The Si content in this steel sheet is very low (≤0.2 wt.%), and Al element is used to replace Si element to improve the surface quality of the steel sheet. However, although replacing Si with Al can improve the plating property to a certain extent, the solution strengthening effect of Al is far less than that of Si. Reducing the Si content in the steel to such a low level sacrifices a large amount of the strength of the steel. In addition, although the problem of surface selective oxidation of Al element during annealing is not as prominent as that of Si, its impact on the plating property still cannot be ignored.

[0005] Patent CN 113604728 A discloses "A Hot-Dip Galvanized High-Strength Steel with High Surface Quality and Its Manufacturing Method", attempting to improve the surface quality of hot-dip galvanized high-strength steel from two aspects of alloy composition and preparation process. From the perspective of composition, this patent attempts to significantly reduce complex oxides of Fe, Si, Mn, and Cr through appropriate Si, Mn, and Cr alloy contents to ensure good surface quality. However, the Si, Mn, and Cr involved in this patent are still at relatively high contents, which are still sufficient to cause serious surface oxidation during annealing, thereby triggering surface quality defects. From the perspective of process, this patent focuses on the improvement of pre-processes such as hot rolling, while the improvement of the annealing treatment process, which is a key process stage for the plating property, is limited.

[0006] In order to improve the surface quality problem of galvanized TRIP steel, POSCO in South Korea proposed to add a small amount of Bi element to the steel. Through its surface segregation and enrichment, it inhibits the selective oxidation of alloy elements on the surface during annealing, thereby improving the plating property (J. OH, et al., Metallurgical and Materials Transactions A, 47A (2016) 5474). Patents CN 113061807 A, CN 113046644 A, and CN 113061806 A successively disclose a 780 MPa, 980 MPa, and 1180 MPa grade lightweight high-strength steel, adding 0.05 - 0.2 wt.%, 0.06 - 0.18 wt.%, and 0.07 - 0.19 wt.% Bi element to the steel respectively, hoping to "inhibit the diffusion of C and O elements at grain boundaries through the distribution of Bi at grains and grain boundaries, playing a role in reducing decarburization and oxidation inhibition, reducing steel plate rolling cracking, and improving the surface quality and mechanical properties of the steel plate" through the distribution of Bi at grains and grain boundaries. However, Bi is one of the five harmful elements in steel, and its grain boundary segregation is extremely likely to make the steel brittle. It is generally believed that the critical content of Bi in steel should not be higher than 0.05% (or even lower). Therefore, using the surface segregation characteristics of Bi can improve the problem of surface selective oxidation and improve the galvanizing quality, but the method of using the grain boundary segregation of Bi to improve properties such as oxidation resistance is not suitable, and the negative impact of Bi on mechanical properties needs to be paid special attention.

[0007] In addition to adjusting the alloy composition, the continuous development and optimization of the production process are also crucial for improving the microstructure and properties of TRIP steel. In the "Low-carbon low-alloy TRIP steel or hot-dip galvanized TRIP steel with a tensile strength ≥ 980 MPa and its manufacturing method" disclosed in international patent WO 2022 / 206912A1, the finish rolling temperature is set at ≥ A3, that is, rolling is carried out in the austenite phase region. This is also the widely adopted finish rolling temperature. However, in order to obtain an appropriate proportion of ferrite and retained austenite with an appropriate C content after hot rolling and cooling, it is advisable to first rapidly cool to the temperature range where a large amount of ferrite transforms, then reduce the cooling rate in this temperature range, and then rapidly cool to the coiling temperature for coiling. This poses a great challenge to the cooling control ability of the equipment. Therefore, how to make up for the deficiency of equipment capacity through process adjustment is another difficult problem in improving the microstructure and properties of TRIP steel and stabilizing the production capacity. Summary of the Invention

[0008] The purpose of the present invention is to provide a high-plating-property and high-strength-ductility-product TRIP steel and its manufacturing method to solve the difficult problem of hard to balance the mechanical properties and plating property of TRIP steel.

[0009] To achieve the above purpose, the technical solution of the present invention is as follows:

[0010] A high-plating-property and high-strength-ductility-product TRIP steel, by mass percentage, the chemical composition of the TRIP steel contains: C 0.16 - 0.24%, Mn 1.5 - 2.4%, Si 0.5 - 1.2%, Al 0.6 - 1.5%, Ti 0.008 - 0.08%, Cu 0.05 - 0.3%, Bi 0.008 - 0.05%, B 0.001 - 0.003%, P below 0.015%, S below 0.002%, and the balance is Fe.

[0011] The high-plating-property and high-strength-ductility-product TRIP steel, by mass percentage, the chemical composition of the TRIP steel further contains at least one of the following: Ni below 0.3%, Sb below 0.03%, Sn below 0.03%, RE below 0.006%, Nb below 0.12%, Cr below 0.5%, Mo below 0.3%, V below 0.1%.

[0012] In the high-plating-property and high-strength-ductility-product TRIP steel, the rare earth element RE is at least one of the 17 elements including lanthanide elements, scandium, and yttrium.

[0013] The described high-plating-property and high strength-ductility product TRIP steel has a metallographic structure consisting of ferrite, bainite, and retained austenite, with an average grain size of 0.5 - 3 μm; among them, the volume fraction of ferrite is 30 - 70%, the volume fraction of bainite is 30 - 50%, and the volume fraction of retained austenite is 5 - 17%.

[0014] For the described high-plating-property and high strength-ductility product TRIP steel, the yield strength of this TRIP steel is ≥750 MPa, the tensile strength is ≥980 MPa, the elongation is ≥22%, and the strength-ductility product is ≥24 GPa%.

[0015] The manufacturing method of the described high-plating-property and high strength-ductility product TRIP steel includes the following steps:

[0016] 1) After smelting, refining, and casting according to the set composition, a slab is made;

[0017] 2) After heating the slab in a heating furnace at 1100 - 1250 °C for 1 - 3 h, rough rolling and finish rolling are carried out, and the final rolling temperature is 20 - 50 °C below the A3 line. Then, it is rapidly cooled to 500 - 600 °C for coiling;

[0018] 3) After uncoiling, pickling, and cold rolling in sequence, critical annealing and bainite isothermal annealing heat treatment are carried out;

[0019] 4) After the annealing heat treatment, it is heated to 450 - 500 °C at 10 - 40 °C / s, and then enters a zinc pot for hot-dip galvanizing treatment, and the galvanizing time is 5 - 10 s.

[0020] For the manufacturing method of the described high-plating-property and high strength-ductility product TRIP steel, the dew point of the annealing atmosphere is above -30 °C.

[0021] For the manufacturing method of the described high-plating-property and high strength-ductility product TRIP steel, before critical annealing, the heating temperature is 5 - 20 °C / s, and then it is held at 780 - 900 °C in the critical annealing temperature range for 90 - 600 s, and then rapidly cooled to the bainite isothermal annealing temperature range at 10 - 50 °C / s.

[0022] For the manufacturing method of the described high-plating-property and high strength-ductility product TRIP steel, the bainite isothermal annealing temperature range is 350 - 450 °C, and the holding time is 40 - 600 s.

[0023] For the manufacturing method of the described high-plating-property and high strength-ductility product TRIP steel, after hot-dip galvanizing, it is rapidly cooled to room temperature at 20 - 100 °C / s to obtain a hot-dip galvanized product; or, it is rapidly heated to 520 - 580 °C at 10 - 50 °C / s for alloying treatment of the coating, and the treatment time is 10 - 30 s, and then it is rapidly cooled to room temperature at 20 - 100 °C / s to obtain a hot-dip galvanized product.

[0024] In the composition and process design of TRIP steel of the present invention:

[0025] C: C is an effective solid solution strengthening element and is also crucial for ensuring the hard phase content. Increasing the C content helps increase the proportion and stability of retained austenite. However, excessive C content significantly degrades the weldability of the steel. Therefore, the present invention limits the C content to 0.16-0.24%.

[0026] Mn: Mn is an austenite-stabilizing element, lowering the cementite precipitation temperature and increasing the solubility of carbon in austenite. Furthermore, Mn provides solid solution strengthening. However, excessive Mn content can overly stabilize the retained austenite, hindering deformation-induced phase transformation and leading to banded microstructure. Therefore, the present invention limits the Mn content to 1.5-2.4%.

[0027] Si: Si has a very low solubility in cementite. During isothermal annealing of bainite, it inhibits cementite precipitation, thereby promoting carbon enrichment in austenite and preserving the retained austenite to room temperature. Furthermore, Si also provides solid solution strengthening. The Si content in conventional TRIP steels is generally 1.5-1.9%. However, increased Si content causes film-like oxides to form on the steel surface during annealing, significantly degrading plateability and compromising the quality of subsequent galvanizing. Therefore, the present invention limits the Si content to 0.5-1.2%.

[0028] Al: Al is similar to Si in that both have the effect of hindering the precipitation of cementite. Replacing Si with Al can improve the problem of selective oxidation on the surface of the steel plate during annealing, thereby improving the quality of galvanizing; in addition, it can also improve welding performance. Al is a very strong ferrite stabilizing element, which increases the phase transformation temperature, hinders austenitization, and increases the transformation temperature of martensite. However, the solid solution strengthening effect of Al is far inferior to that of Si. If Al completely replaces Si, it will be difficult to obtain high-strength TRIP steel. In addition, when the Al content is too high, martensite and Fe-Al brittle phases will be formed in the steel. Therefore, the present invention limits the Al content to 0.6-1.5%.

[0029] Ti: A strong carbide-forming element, trace additions of Ti to steel can refine grains through the dispersion and precipitation of TiC, significantly improving the steel's strength and toughness. Furthermore, it can also contribute to precipitation strengthening to a certain extent. Ti influences austenitization, recrystallization, grain growth, and element migration in TRIP steel, as well as various phase transformations during hot rolling, annealing, and deformation. Therefore, the present invention limits the Ti content to 0.008% to 0.08%.

[0030] Cu: Cu can improve hardenability and thus increase the proportion of bainite hard phase. In addition, Cu can achieve secondary phase strengthening by precipitating fine and dispersed ε-Cu. However, when the Cu content is too high, the proportion of the hard phase is too high and the precipitates are coarse, which is instead not conducive to the ductility of the steel plate and the forming dimensional accuracy, etc. Therefore, in the present invention, the Cu content is limited to 0.05 - 0.3%.

[0031] Bi: Bi is an extremely important component of the present invention. Although partially replacing Si with Al can to a certain extent inhibit surface selective oxidation and thus improve the plating property, the galvanizing quality problem of the steel at this time cannot be ignored. The role of Bi in the present invention is to rapidly form a hydrophobic oxygen barrier on the surface, inhibit the chemisorption of oxygen into the matrix lattice during annealing, and thus significantly inhibit surface selective oxidation and improve the plating property. The reason why Bi can "rapidly form a hydrophobic oxygen barrier on the surface" is that it meets three necessary conditions: 1) The affinity with oxygen is much lower than that of the matrix Fe; 2) The diffusion coefficient in the Fe matrix is large; 3) The surface segregation tendency in the Fe matrix is large. In addition, the segregation and enrichment of Bi on the surface can also play a role in inhibiting surface decarburization. However, Bi is also a strong grain boundary segregation element, and its segregation at the grain boundary will weaken the grain boundary bonding force and increase the brittleness of the steel. Therefore, the content should not exceed 0.05% (even lower). Therefore, in the present invention, the Bi content is limited to 0.008 - 0.05%.

[0032] B: Bi + B is an extremely important component of the present invention. In addition to having a strong surface segregation tendency, Bi also has a strong grain boundary segregation tendency, and the latter is not desired. B is a grain boundary strengthening element. In the present invention, it is used to enrich and occupy positions at the grain boundary to inhibit the grain boundary segregation of Bi, thereby avoiding the weakening effect of Bi on the mechanical properties. In addition, the addition of B can also improve hardenability and inhibit the formation and grain growth of ferrite during annealing and cooling. However, too high B content is prone to "B brittleness", which instead reduces the performance. Therefore, in the present invention, the B content is limited to 0.001 - 0.003%.

[0033] P, S: Both are harmful elements in steel. P is prone to segregation along the grain boundary, which will increase the brittleness of the steel. S often exists in the form of sulfide inclusions in steel, making the steel prone to cracking during hot working, reducing the ductility and toughness of the steel. Therefore, in the present invention, the contents of P and S are respectively limited to below 0.015% and below 0.002%.

[0034] In addition, the present invention can also appropriately add the following beneficial elements:

[0035] Ni: Similar to Cu, Ni increases hardenability and the proportion of hard phases, thereby improving strength. However, excessive Ni content can degrade plasticity and toughness. In this invention, Ni is a beneficial but optional addition, with an addition level of less than 0.3%.

[0036] Sb and Sn: The addition of trace amounts of Sb and Sn can inhibit decarburization and oxidation on the steel surface during annealing. However, excessive amounts can increase the brittleness of the steel and negatively impact mechanical properties. In this invention, Sb and Sn are optional elements, and their addition amounts are limited to less than 0.03%.

[0037] RE: A trace amount of RE can purify the grain boundaries, thereby inhibiting the enrichment of Bi, P, Sb, Sn, etc. at the grain boundaries, thereby avoiding the grain boundary weakening caused by these elements. In the present invention, RE is a beneficial and non-essential additive element, and the addition amount is less than 0.006%.

[0038] Nb and V: Similar to Ti, Nb and V are strong carbide formers, refining grain size and improving ductility. Carbides also act as a secondary phase strengthening agent. In this invention, Nb and V are beneficial but optional additions, with amounts limited to less than 0.12% and 0.1%, respectively.

[0039] Cr: Cr can improve hardenability and help increase the proportion of hard phase. However, if the Cr content is too high, it will be detrimental to welding performance. In the present invention, Cr is a beneficial and non-essential additive element, and the addition amount is less than 0.5%.

[0040] Mo: Mo stabilizes austenitization and has a solid solution strengthening effect. Furthermore, Mo inhibits pearlite formation and accelerates bainite transformation in the intermediate temperature range. In the present invention, Mo is a beneficial but optional addition, with an addition level of less than 0.3%.

[0041] The main design ideas of the present invention are:

[0042] The galvanizing quality problems of conventional TRIP steel are very prominent. The main reason is that active elements such as Mn and Si in the steel undergo surface selective oxidation during annealing, thereby deteriorating the plating property of the steel. Although the traditional method of reducing the Si content can improve the plating property, it damages the mechanical properties. Therefore, in the present invention, Bi element with strong oxygen-phobicity, large surface deflection tendency and large diffusion coefficient is added in trace amounts to the steel. By rapidly forming an oxygen-phobic barrier on the surface layer by Bi (Sb and Sn may also be present), the chemical adsorption of oxygen to the surface layer of the substrate is inhibited, thereby inhibiting surface selective oxidation and improving the plating property. At the same time, grain boundary purification elements B (RE may also be present) are added in trace amounts to the steel to avoid the embrittlement of the steel caused by the segregation of Bi at the grain boundaries, so that the Bi element is segregated and enriched on the surface as much as possible and plays a role. In addition, by partially replacing Si element with Al, the problem of surface selective oxidation is further improved and the galvanizing quality is enhanced. Since the addition amount of Bi in the present invention is very low and B is added to avoid the intergranular embrittlement caused by Bi, the influence of Bi on the mechanical properties can be ignored. In addition, in order to avoid the reduction of the steel strength caused by replacing Si with Al, the present invention adopts the following measures: ① retaining a considerable amount of Si; ② adding Ti (Nb and V may also be present) to form carbides, producing fine grain strengthening and precipitation strengthening effects; ③ adding Cu (Ni and Cr may also be present) to improve the hardenability and increase the proportion of hard phases. In addition, in order to improve the production stability, the finish rolling temperature is reduced from above the conventional A3 line to 20-50 °C below the A3 line, so that the temperature can be reduced to the hot coil temperature in one step and the tissue properties can be ensured, avoiding the challenge of the conventional multi-step temperature reduction to the cooling capacity of the equipment.

[0043] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0044] 1) The TRIP steel can have both high plating property and high strength-ductility product. In terms of the plating property, in the present invention, by adding Bi element in trace amounts, the surface selective oxidation problem of TRIP steel during annealing can be greatly reduced, thereby significantly improving the plating property, and by replacing Si with Al, the plating property is further improved. In terms of the mechanical properties, the steel with high strength-ductility product can be obtained by the following measures in the present invention: ① the addition amount of Bi is very low and its influence on the mechanical properties is limited; ② adding grain boundary purification elements B (RE may also be present) in trace amounts to further avoid the potential intergranular embrittlement problem of Bi; ③ adding strengthening elements to form a suitable structure and produce solid solution strengthening, fine grain strengthening and precipitation strengthening effects. Thus, the present invention solves the problem that it is difficult to balance the plating property and mechanical properties of conventional TRIP steel.

[0045] 2) It can improve the stable production capacity of TRIP steel. The conventional finish rolling temperature is higher than the A3 line. To obtain an appropriate proportion of ferrite and retained austenite with an appropriate C content after hot rolling and cooling, it is necessary to rapidly cool down to the temperature range where a large amount of ferrite transforms, then reduce the cooling rate, and then rapidly cool to the hot coil temperature, which poses a huge challenge to the cooling capacity of the equipment. In the present invention, the finish rolling temperature is reduced to 20 - 50 °C below the A3 line, so that it can be cooled down to the hot coil temperature by a one-step method while ensuring the tissue properties, avoiding the production stability problems caused by the conventional multi-step cooling method. Description of the Drawings

[0046] Figure 1 It is an ultra-depth-of-field optical microscope image of the surface of the TRIP steel in Example 1 of the present invention after continuous annealing treatment.

[0047] Figure 2 It is an ultra-depth-of-field optical microscope image of the surface of the TRIP steel in Comparative Example 1 of the present invention after continuous annealing treatment.

[0048] Figure 3 It is a scanning electron microscope image of the surface of the TRIP steel in Example 1 of the present invention after continuous annealing treatment.

[0049] Figure 4 It is a scanning electron microscope image of the surface of the TRIP steel in Comparative Example 1 of the present invention after continuous annealing treatment. Detailed Embodiments

[0050] In the specific implementation process, the present invention provides a high-plating-property and high-strength-plasticity-product TRIP steel and its manufacturing method. The chemical composition of the TRIP steel includes, by mass percentage: C: 0.16 - 0.24%, Mn: 1.5 - 2.4%, Si: 0.5 - 1.2%, Al: 0.6 - 1.5%, Ti: 0.008 - 0.08%, Cu: 0.05 - 0.3%, Bi: 0.008 - 0.05%, B: 0.001 - 0.003%, P: below 0.015%, S: below 0.002%, and the balance is Fe. The heating temperature of the continuous casting billet is 1100 - 1250 °C, the finish rolling temperature is 20 - 50 °C below the A3 line, the coiling temperature is 500 - 600 °C, the dew point of the annealing atmosphere is above -30 °C, the critical annealing temperature is 780 - 900 °C, the critical annealing holding time is 90 - 600 s, the cooling rate is 10 - 50 °C / s, the bainite isothermal annealing temperature is 350 - 450 °C, and the isothermal annealing holding time is 40 - 600 s. The present invention can greatly improve the surface oxidation problem of TRIP steel during annealing, improve the plating property, and improve the process stability while taking into account the mechanical properties.

[0051] Next, the present invention will be further described in conjunction with specific embodiments.

[0052] The chemical compositions of the highly electroplatable and high strength-ductility product TRIP steels in each embodiment are shown in Table 1, the production process parameters are shown in Table 2, and the performance test results are shown in Table 3 Figures 1 to 4 As described below, it is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto.

[0053] Table 1 Chemical compositions of the steels in the examples and comparative examples (unit: mass percentage)

[0054] C Mn Si Al Ti Cu Bi B P S Nb Example 1 0..215 1.852 0.731 1.123 0.042 0.213 0.036 0.0017 0.0125 0.0015 / Example 2 0.199 1.743 0.928 1.061 0.012 0.158 0.032 0.0023 0.0094 0.0012 0.031 Example 3 0.230 2.158 1.137 0.871 0.038 0.121 0.047 0.0025 0.0108 0.0010 / Comparative Example 1 0..221 1.915 0.771 1.102 0.038 0.202 / / 0.0125 0.0015 / Comparative Example 2 0..221 1.915 0.771 1.102 0.038 0.202 0.032 / 0.0130 0.0010 /

[0055] Table 2 Process parameters of the manufacturing methods of the steels in the examples and comparative examples

[0056]

[0057] Table 3 Mechanical properties of the steels in the examples and comparative examples

[0058] Yield strength (MPa) Tensile strength (MPa) Elongation (%) Product of strength and plasticity (GPa%) Example 1 824 1087 23.1 25.1 Example 2 805 1103 21.8 24.0 Example 3 782 1124 20.3 22.8 Comparative Example 1 828 1079 23.4 25.2 Comparative Example 2 757 992 17.6 17.5

[0059] In Example 1 and Comparative Example 1, the metallographic structures are all fine, uniform, and dispersedly distributed ferrite, bainite, and retained austenite three-phase structures, and the average grain size is 0.5 - 3 μm. Among them, the volume ratio of ferrite is 40 - 50%, the volume ratio of bainite is 40 - 50%, and the volume ratio of retained austenite is 8 - 15%.

[0060] After annealing heat treatment, the surface of the steel plate in Example 1 still has a metallic luster, and its super-depth-of-field optical microscope morphology is as Figure 1 shown, while the surface of the steel plate in Comparative Example 1 shows a blackish color due to oxidation ( Figure 2 ). Scanning electron microscope analysis shows that the oxidation degree of the surface of the steel plate in Example 1 ( Figure 3 ) is much lower than that in Comparative Example 1 ( Figure 4 ), indicating the beneficial effect of trace Bi on inhibiting external oxidation during annealing and thus improving electroplating performance.

[0061] As can be seen from the above examples, by using the alloy composition design and manufacturing method of the present invention, TRIP steels with a tensile strength of more than 980 MPa, a yield strength of more than 750 MPa, and an elongation of more than 20% can be obtained. From Example 1 and Comparative Examples 1 and 2, it can be seen that when B is not added, the addition of trace Bi will deteriorate the mechanical properties to a certain extent, but when Bi and B are added simultaneously, the mechanical properties do not change significantly. The present invention can obtain TRIP steels with excellent electroplating performance and high strength-ductility product, solving the problem that it is difficult to balance the two.

Claims

1. A high-plating-property and high strength-ductility product TRIP steel, characterized in that, By mass percentage, the chemical composition of the TRIP steel contains: C 0.16 - 0.24%, Mn 1.5 - 2.4%, Si 0.5 - 1.2%, Al 0.6 - 1.5%, Ti 0.008 - 0.08%, Cu 0.05 - 0.3%, Bi 0.032 - 0.05%, B 0.001 - 0.003%, P below 0.015%, S below 0.002%, and the balance is Fe; The manufacturing method of the high-plating-property and high strength-ductility product TRIP steel includes the following steps: 1) After smelting, refining and casting according to the set composition, a slab is made; 2) After heating the slab in a heating furnace at 1100 - 1250 °C for 1 - 3 h, rough rolling and finish rolling are carried out, and the finish rolling temperature is 20 - 50 °C below the A3 line, and then it is rapidly cooled to 500 - 600 °C for coiling; 3) After uncoiling, pickling and cold rolling in sequence, critical annealing and bainite isothermal annealing heat treatment are carried out; 4) After the annealing heat treatment, it is heated to 450 - 500 °C at 10 - 40 °C / s, and then enters a zinc pot for hot-dip galvanizing treatment, and the galvanizing time is 5 - 10 s.

2. The high-plating-property and high strength-ductility product TRIP steel according to claim 1, characterized in that, By mass percentage, the chemical composition of the TRIP steel further contains at least one of the following: Ni below 0.3%, Sb below 0.03%, Sn below 0.03%, RE below 0.006%, Nb below 0.12%, Cr below 0.5%, Mo below 0.3%, V below 0.1%.

3. The high-plating-property and high strength-ductility product TRIP steel according to claim 2, characterized in that, The rare earth element RE is at least one of the 17 elements including lanthanide elements, scandium and yttrium.

4. The highly plating - feasible and high strength - ductility product TRIP steel according to claim 1, characterized in that, The metallographic structure is a three-phase structure of ferrite, bainite and retained austenite, and the average grain size is 0.5 - 3 μm; among them, the volume ratio of ferrite is 30 - 70%, the volume ratio of bainite is 30 - 50%, and the volume ratio of retained austenite is 5 - 17%.

5. The highly plating - applicable and high strength - plasticity product TRIP steel according to claim 1, characterized in that, The yield strength of this TRIP steel is ≥750 MPa, the tensile strength is ≥980 MPa, the elongation is ≥22%, and the strength-ductility product is ≥24 GPa%.

6. A manufacturing method of the high-plating-property and high strength-ductility product TRIP steel according to any one of claims 1 to 5, characterized in that, Including the following steps: 1) After smelting, refining and casting according to the set composition, a slab is made; 2) After heating the slab in a heating furnace at 1100 - 1250 °C for 1 - 3 h, rough rolling and finish rolling are carried out, and the finish rolling temperature is 20 - 50 °C below the A3 line, and then it is rapidly cooled to 500 - ~600 °C for coiling; 3) After uncoiling, pickling and cold rolling in sequence, critical annealing and bainite isothermal annealing heat treatment are carried out; 4) After the annealing heat treatment, it is heated to 450 - 500 °C at 10 - 40 °C / s, and then enters a zinc pot for hot-dip galvanizing treatment, and the galvanizing time is 5 - 10 s.

7. The manufacturing method of the high-plating-property and high strength-ductility product TRIP steel according to claim 6, characterized in that, The dew point of the annealing atmosphere is above -30 °C.

8. The manufacturing method of the high-plating-property and high strength-ductility product TRIP steel according to claim 6, characterized in that, Before critical annealing, the heating temperature is 5 - 20 °C / s, then it is held at 780 - 900 °C in the critical annealing temperature range for 90 - 600 s, and then rapidly cooled to the bainite isothermal annealing temperature range at 10 - 50 °C / s.

9. The manufacturing method of the high-plating-property and high strength-ductility product TRIP steel according to claim 6, characterized in that, The bainite isothermal annealing temperature range is 350 - 450 °C, and the holding time is 40 - 600 s.

10. The manufacturing method of the highly plating-friendly and high strength-ductility product TRIP steel according to claim 6, characterized in that, After hot-dip galvanizing, it is rapidly cooled to room temperature at a rate of 20 - 100 °C / s to obtain a hot-dip galvanized product; or, it is rapidly heated to 520 - 580 °C at a rate of 10 - 50 °C / s for alloying treatment of the coating for 10 - 30 s, and then rapidly cooled to room temperature at a rate of 20 - 100 °C / s to obtain a hot-dip galvanized product.

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