TRIP steel and its manufacturing method, cold-rolled steel sheet and hot-dip galvanized steel sheet

Abstract

The present invention discloses a TRIP steel, a method for producing the same, a cold-rolled steel sheet, and a hot-dip galvanized steel sheet, the TRIP steel containing the following chemical components in mass percent: C: 0.15-0.3%, Si: 0.6-1.0%, Mn: 1.7-2.5%, Al: 0.5-0.9%, P≦0.01%, S≦0.01%, N≦0.007%, Fe≧90%. The TRIP steel of the present invention is based on a carbon-silicon-manganese steel, and by optimizing the ratios of carbon, silicon, manganese, and aluminum, Si in conventional TRIP steels is partially replaced by Al, thereby suppressing carbide precipitation and stabilizing retained austenite, ensuring the mechanical properties of the TRIP steel, and improving the surface quality and galvanizability of the steel sheet.

Description

Technical Field

[0001] The present invention relates to the field of metallurgical technology, and particularly to TRIP steel and its manufacturing method, cold-rolled steel sheets, and hot-dip galvanized steel sheets.

Background Art

[0002] In recent years, advanced high-strength steels for automobiles have been widely used for purposes such as achieving vehicle body weight reduction, energy conservation, improving impact safety, and reducing manufacturing costs. Advanced high-strength steels have a reduced steel sheet thickness due to the improvement of the steel sheet strength, and at the same time, excellent formability is maintained. Therefore, they are currently the vehicle body weight reduction materials with the highest overall competitiveness.

[0003] In the case of advanced high-strength steels based on the transformation-induced plasticity (TRIP) effect, it is necessary to maintain high strength and at the same time have good ductility. From the perspective of the microstructure, TRIP steel consists of ferrite, bainite, and retained austenite, and is mainly used in the manufacture of automobile door protection rods, bumpers, and floor structural parts.

[0004] In TRIP steel, the volume fraction and stability of retained austenite are the main factors influencing its mechanical properties. In TRIP steel, stabilizing retained austenite and suppressing carbide precipitation is crucial. Conventional TRIP steel suppresses carbide precipitation primarily by high Si content; however, high Si content leads to the formation of a Si oxide layer on the steel surface, significantly affecting the material's plating properties. Simultaneously, even the TRIP steel with the best strength and ductility among first-generation advanced high-tensile steels generally exhibits strength and ductility below 20 GPa.%. To further improve strength and ductility, the design principles for third-generation advanced high-tensile steels for automobiles are primarily as follows: (1) Adding a large amount of Mn achieves an austenite reverse transformation process, resulting in austenite with excellent formability, but this increases material costs, makes manufacturing difficult, and excessively high Mn levels negatively impact weldability; (2) Quenching distribution (QP process) yields a martensitic steel structure containing a certain amount of retained austenite, but this presents significant challenges to the annealing process and equipment.

[0005] For example, Chinese invention patent CN201610867413.9 describes an economical high-strength cold-rolled TRIP steel with strength and ductility exceeding 20 GPa.% and a method for producing the same. The main chemical composition of the TRIP steel is, in weight percent, the following components: C: 0.15~0.25%, Si: 1.3~1.7%, Mn: 1.5~2.5%, P: ≤0.030%, S: ≤0.020%, Al: 0.02~0.06%, with the remainder being Fe and unavoidable impurities. The manufacturing method includes smelting, hot rolling, pickling and cold rolling, and continuous annealing. The material obtained by this method has excellent strength and ductility, but due to its high Si content, the surface quality of the material is poor, and color unevenness is likely to occur on the surface of the product after hot-dip galvanizing.

[0006] Chinese invention patent CN201310520998.3 describes a heat treatment process for 980 MPa class high-strength ductile automotive steel sheets. After quenching the treated TRIP780 cold-rolled steel sheet, it is heated to a constant temperature to distribute the strength, resulting in a 980 MPa class high-strength ductile automotive steel sheet with a strength ductility of 26.3-26.8 GPa.% and an elongation of 23% or more. The drawback is that the rapid quenching and redistribution process requires high cooling and heating performance of the equipment, which current annealing equipment cannot fully meet.

[0007] Chinese invention patent CN200810119818.X describes a high-strength cold-rolled hot-dip galvanized TRIP steel sheet and its manufacturing method. Its advantages include a low Si component design and good galvanizing properties. However, it has disadvantages such as a high Al content making continuous casting difficult and stable batch production challenging, as well as increased production costs due to the addition of alloying elements such as Nb, Ti, Cu, and Ni in the composition.

[0008] Chinese invention patent CN201510596778.8 describes a treatment method for improving the surface quality of TRIP steel. This method also has the advantage of a wider annealing temperature range than conventional TRIP steel due to its low Si (≤0.5%) and high Al (1-5%) composition. However, due to the high Al composition, stable continuous casting production is difficult, and the resulting product has low tensile strength (445-520 MPa) and a strength ductility of only 16-18 GPa.%.

[0009] Chinese invention patent CN201510112679.8 describes a method for manufacturing high-Al medium-manganese steel having strength and ductility exceeding 30 GPa.%. The advantages of this TRIP steel are its high strength, high elongation, and low density, which can meet the material selection requirements for vehicle body weight reduction. However, it has the disadvantage of being difficult to manufacture and costly due to its excessively high Mn and Al content.

[0010] Chinese invention patent CN201110280804.8 describes cold-rolled TRIP steel of 1000 MPa or higher and its manufacturing method, with a high Si and high Al composition design combined with Nb and V microalloys, and the material has an elongation A when Rm is 1000 MPa or higher. 80 This exceeds 18%. However, due to the high Si, Al, and Mn content, as well as the Nb and V microalloys, continuous casting production is difficult, and the production cost of the materials also increases. [Overview of the project] [Problems that the invention aims to solve]

[0011] To address the problems of conventional TRIP steel, such as the deterioration of surface quality due to high Si content, the difficulty of continuous casting production processes due to high Mn and Al content, and the increase in production costs due to the use of alloying elements such as Nb, Ti, Cu, Ni, and V, the present invention partially replaces Si with Al in conventional TRIP steel and optimizes the ratios of C, Si, Mn, and Al, thereby suppressing carbide precipitation, stabilizing retained austenite, ensuring the mechanical properties of TRIP steel, while simultaneously improving the surface quality and plating properties of the steel sheet, and reducing processing difficulties and production costs.

[0012] One of the objectives of the present invention is to provide TRIP steel. The TRIP steel obtained by the technical proposal of the present invention has good plated surface quality and high strength ductility. Specifically, its yield strength is 420 MPa or higher, for example 420 to 600 MPa; its tensile strength is 800 MPa or higher, for example 800 to 950 MPa; its elongation at break is 25% or higher, for example 25% to 35%; and its strength ductility exceeds 23 GPa.%. [Means for solving the problem]

[0013] The TRIP steel of the present invention contains, by mass percent, the following chemical components: C: 0.15-0.3%, Si: 0.6-1.0%, Mn: 1.7-2.5%, Al: 0.5-0.9%, P ≤ 0.01%, S ≤ 0.01%, N ≤ 0.007%, Fe ≥ 90%.

[0014] Furthermore, the TRIP steel of the present invention contains, by mass percent, the following chemical components and their percentage content: C: 0.15~0.3%, Si: 0.6~1.0%, Mn: 1.7~2.5%, Al: 0.5~0.9%, P ≤ 0.01%, S ≤ 0.01%, N ≤ 0.007%, with the remainder being Fe and other unavoidable impurities.

[0015] In one embodiment, the uniform elongation ratio (UEL) of the TRIP steel of the present invention is 18.5% or more, for example, 18.5 to 26.0%.

[0016] In this invention, the design principles for each element are as follows: Carbon (C): Carbon directly affects the strength, weldability, and formability of steel sheets / strips. A higher carbon content is advantageous for improving the strength of the steel sheet. If the carbon content is less than 0.15%, the strength of the steel sheet / strip will not meet the target, but if the carbon content exceeds 0.30%, the carbon equivalent is too high, and the weldability of the steel sheet tends to deteriorate. At the same time, C is also an austenite-stabilizing element, and austenite containing a certain amount of C can exist stably even at room temperature, and through the TRIP effect, it can meet the requirement for high-strength ductility of the material. Therefore, in this invention, the mass percentage of carbon is controlled to 0.15-0.30%, preferably the C content to 0.17-0.23%.

[0017] Silicon (Si): Silicon is mainly present in TRIP steel as a solid solution and suppresses cementite formation during bainite transformation. At the same time, Si is distributed in ferrite, increasing the chemical potential of C and promoting the diffusion of C into the austenite in ferrite. This is because the formation of retained austenite with a high carbon content is a fundamental condition for the TRIP effect to occur. To ensure the excellent plasticity of the material, the Si content must be at least 0.6%. However, Si is also an element that deteriorates the surface quality of steel sheets, and if the Si content exceeds 1.0%, a severe hot-rolled iron oxide film is likely to form on the surface of the hot-rolled steel sheet / strip, which deteriorates the surface quality of the steel sheet / strip, not only negatively affecting the production of hot-dip galvanized steel sheet / strip but also worsening the plating properties of the steel sheet / strip. Therefore, in this invention, the mass percentage of silicon is limited to 0.6-1.0%, and preferably, the Si content is limited to 0.7-0.9%.

[0018] Manganese (Mn): In the TRIP steel of the present invention, Mn expands the austenite phase region, reduces Ms and Mf, improves austenite stability and hardenability of the steel, lowers the critical transformation rate, and is advantageous for storing retained austenite down to room temperature. At the same time, it is also an important solid solution strengthening element. However, it is important to note that if the Mn content in the steel is too high, corrosion resistance and weldability deteriorate, the tendency for grain coarsening worsens, and the plasticity and toughness of the steel decreases. For this reason, in the present invention, the mass percentage of Mn is controlled to 1.70 to 2.50%, preferably to 1.8 to 2.0%.

[0019] Aluminum (Al): Aluminum is similar to silicon and can stabilize retained austenite by suppressing carbide formation during the aging process, although its effect is weaker than that of Si. Unlike Si, Al does not adversely affect the surface quality of steel materials and is advantageous for improving the surface quality of hot-dip galvanized products. Since Al also has the effect of suppressing carbide formation, it may be added to partially replace Si to improve the plating properties of TRIP steel. In addition, Al can significantly raise the Ac3 point, and if the Al content is too high, both the hot rolling completion temperature and the finish rolling temperature must be increased, and at the same time, the annealing temperature in the two-phase region must also be increased accordingly. If the mass percentage content of Al exceeds 0.9%, it is very disadvantageous for the steelmaking and continuous casting processes, and problems such as nodules in the continuous casting nozzle are likely to occur. Therefore, in order to obtain TRIP steel that achieves both excellent surface quality and good strength and ductility, the present invention achieves the above objective by adding a composite of Si and Al, replacing Si with Al. However, if the Al content is less than 0.5%, more Si (>1.0%) must be added to achieve the required strength and ductility, which degrades the surface quality of the product and results in defects such as uneven coloring and red iron scales. Therefore, in this invention, the mass percentage of Al is controlled to 0.5-0.9%, preferably to 0.6-0.8%.

[0020] Phosphorus (P), sulfur (S), and nitrogen (N) are impurity elements in steel, and the lower their content, the higher the purity and performance of the steel. Of these, P plays a certain role in solid solution strengthening, is beneficial in suppressing carbide formation and improving the stability of retained austenite, but if the mass percentage of P is too high, the grain boundaries weaken, brittleness increases, and weldability deteriorates, so the mass percentage of P needs to be controlled to P ≤ 0.01%. For N, if the mass percentage of N is too high, steelmaking and continuous casting become difficult, and it is also unfavorable in limiting inclusions, so in this invention, the mass percentage of N is controlled to N ≤ 0.007%. Also, if the mass percentage of element S in steel is too high, the plasticity of the material deteriorates significantly, so in this invention, the mass percentage of S is controlled to S ≤ 0.01%.

[0021] Furthermore, the metallographic structure of the TRIP steel of the present invention contains, in terms of volume fraction, 38 - 58% ferrite, 30 - 50% bainite, 10 - 12% retained austenite, and ≤2% martensite.

[0022] When the bainite content exceeds 50%, the material strength is high but the plasticity is insufficient. When the bainite content is less than 30%, the strength is insufficient and the plasticity is excessive. Therefore, in the present invention, the volume fraction of bainite is controlled to be 30 - 50%. The retained austenite content is important for the excellent strength and ductility of the material. By controlling the volume fraction of retained austenite to be 10 - 12%, a certain TRIP effect is exerted to improve the strength and ductility of the steel. By controlling the volume fraction of martensite to be low, the excellent strength and ductility of the material can be ensured.

[0023] Furthermore, among the ferrite of the TRIP steel of the present invention, ferrite crystal grains of 10 μm or less account for 80% or more, and among them, ferrite crystal grains of 5 μm or less account for 60% or more. The average crystal grain size of the retained austenite is 2 μm or less, and the average C content of the retained austenite crystal grains is 1 wt% or more.

[0024] Furthermore, the TRIP steel of the present invention has a yield strength of 420 MPa or more, for example, 420 - 600 MPa, a tensile strength of 800 MPa or more, for example, 800 - 950 MPa, an elongation at break of 25% or more, for example, 25% - 35%, and a strength-ductility product exceeding 23 GPa·%.

[0025] Another object of the present invention is to provide a method for producing the above TRIP steel, including the following steps: Smelting and casting; Hot rolling and coiling; Cold rolling; Continuous annealing.

[0026] Including the above steps to provide a method for producing the above TRIP steel Furthermore, in the above smelting and casting process, the superheat degree of continuous casting is controlled to be 15 - 30°C.

[0027] In a continuous casting process, a certain degree of superheating is necessary for the molten steel to flow smoothly in a continuous manner. If the superheating is below 15°C, the molten steel cannot be poured continuously and stably, and if the superheating exceeds 30°C, the internal quality of the slab's center deteriorates, making it prone to defects such as segregation and shrinkage cavities.

[0028] Furthermore, in the hot rolling process described above, the heating temperature of the slab is controlled to 1150-1250°C, and the finishing rolling completion temperature is controlled to 850-950°C.

[0029] In the hot rolling process, the cast slab is first heated at a high temperature in the complete austenite zone for a certain period of time to soften the material and uniformly diffuse its components. After rough rolling to the specified thickness of the intermediate slab, the coarse structure generated in the heating furnace is broken down and refined, and the slab is then rolled to the required thickness by finish rolling, forming a uniform and dense recrystallized material structure. If the finish rolling end temperature is below 850°C, ferrite will precipitate before finish rolling, reducing the bainite content in the final structure, resulting in insufficient hot rolling strength and affecting subsequent cold rolling annealing properties. Considering the upper limit of the slab heating temperature and the temperature drop during the rolling process, the finish rolling end temperature usually does not exceed 950°C.

[0030] Furthermore, in the winding process described above, the winding temperature should be 520-600°C. Furthermore, in the cold rolling process described above, the cold rolling reduction amount is set to 50-70%.

[0031] The hot-rolling coil temperature is one of the most important processes that affects the characteristics of hot rolling. If the coiling temperature exceeds 600°C, internal oxidation of Si and Mn is more likely to occur on the steel sheet surface, and a surface fracture layer is formed during pickling, affecting the surface quality of the final product. If the coiling temperature is below 520°C, the hot-rolling strength is too high, which is unfavorable for subsequent cold rolling and results in excessive deformation resistance. After pickling to remove the iron oxide film on the surface of the hot-rolled coil, the steel sheet is reduced to the target thickness by a cold-rolling reduction of 50-70%, accumulating a certain amount of deformation energy, which is beneficial for subsequent annealing and recrystallization.

[0032] Furthermore, in the continuous annealing process described above, the steel plate / strip is first kept warm at an annealing temperature of 800-860°C for 60-200 seconds, then cooled to a rapid cooling start temperature of 690-730°C at a first cooling rate of 3-10°C / s, then cooled to a rapid cooling end temperature of 350-450°C at a second cooling rate of 30-60°C / s, followed by an aging treatment where it is kept warm for 200-400 seconds, and finally cooled to below 150°C at a third cooling rate of 2-10°C / s.

[0033] The deformed structure after cold rolling is annealed in the two-phase region at an annealing holding temperature of 800-860°C for 60-200 seconds. The main purpose of this process is to recrystallize the structure of the steel sheet after cold rolling. However, if the temperature is too low, the recrystallization of the steel sheet structure will be incomplete, and if the temperature is too high, the steel sheet structure will coarseen, precipitates will decompose, and the strength of the steel sheet will decrease. Therefore, in this invention, the annealing holding temperature is controlled to 800-860°C. Time control of this process is also important. If the time is too short, the recrystallization of the steel sheet structure will be incomplete, and the diffusion of austenite-stabilizing elements (C, Mn) will be insufficient. If the time is too long, the crystal grains of the steel sheet structure will become abnormally coarse, and abnormal conditions such as precipitate decomposition are likely to occur. Therefore, the annealing holding time is controlled to 60-200 seconds. In this process, a portion of the steel sheet structure is austenitized, and austenite-stabilizing elements such as C and Mn in the ferrite migrate to austenite.

[0034] Next, the steel strip, after being kept warm, is slowly cooled to 690-730°C at a first cooling rate of 3-10°C / s. During this process, some of the austenite in the steel sheet structure is converted to ferrite, and elements such as C and Mn become further concentrated in the austenite.

[0035] Finally, the steel strip, which has been slowly cooled by blowing air, is rapidly cooled to 350-450°C at a second cooling rate of 30-60°C / s, followed by an aging treatment where it is kept warm for 200-400 seconds. In this process, if the cooling rate is too slow, pearlite will form, reducing the performance of the steel sheet, and if the cooling rate is too fast, it will lead to difficulties in manufacturing and increased manufacturing costs, so the second cooling rate is controlled to 30-60°C / s. During the aging process, some of the austenite transforms into bainite, improving the strength of the steel strip. After the phase transformation, the carbon in the bainite migrates to austenite that does not undergo bainite transformation, and the austenite is stably preserved at room temperature. At this stage, the carbon content, volume fraction, and distribution size of the retained austenite are determined, which has a decisive impact on the performance of the final product. After the aging treatment, the steel strip is finally cooled to below 150°C at a third cooling rate of 2-10°C / s. Below this temperature, no further phase transformation of the steel microstructure occurs. In this process, the cooling rate must not be too fast to prevent retained austenite remaining during the aging process from forming new martensite during the final cooling process, which would affect the plasticity of the steel strip.

[0036] Furthermore, the above method for producing TRIP steel includes a hot-dip galvanizing process. Another object of the present invention is to provide a cold-rolled steel sheet made from the above-mentioned TRIP steel.

[0037] For example, a steel sheet / strand may undergo processes such as smelting, casting, hot rolling, coiling, cold rolling, and continuous annealing to obtain a cold-rolled steel sheet.

[0038] Another object of the present invention is to provide the cold-rolled steel sheet and a hot-dip galvanized steel sheet including a hot-dip galvanized layer formed on the surface of the cold-rolled steel sheet.

[0039] For example, a hot-dip galvanized steel sheet is obtained by applying a hot-dip galvanizing treatment to a steel strip that has undergone the aging treatment of the present invention, and then immersing the steel strip, which has completed the bainite transformation, in a zinc oven to perform surface zinc plating.

[0040] In one embodiment, the present invention further provides a method for producing a hot-dip galvanized steel sheet. This method includes the steps for producing TRIP steel and the hot-dip galvanizing process described in any embodiment of the present invention. Furthermore, the hot-dip galvanized steel sheet is produced by immersing the TRIP steel strip that has undergone the aging treatment described herein in a zinc oven to perform surface galvanizing.

[0041] The microstructure of the cold-rolled steel sheet and hot-dip galvanized steel sheet described above both contains ferrite, bainite, retained austenite, and trace amounts of martensite phase. Of these, bainite accounts for 30-50%, retained austenite for 10-12%, and martensite for less than 2%, with the remainder being the ferrite phase. There is no significant difference in the mechanical properties of the cold-rolled steel sheet and the hot-dip galvanized steel sheet.

[0042] The beneficial effects of the present invention are as follows: 1. The TRIP steel described in the present invention is based on carbon-silicon-manganese steel, with optimized ratios of carbon, silicon, manganese, and aluminum. By partially replacing the Si in conventional TRIP steel with Al, carbide precipitation is suppressed, retained austenite is stabilized, the mechanical properties of the TRIP steel are ensured, and the surface quality and plating properties of the steel sheet are improved. The elemental composition of the present invention solves surface defect problems such as color unevenness and plating leaks that are prevalent in hot-dip galvanized TRIP steel products with high Si content.

[0043] 2. In this invention, no other expensive alloying elements such as Ti, Nb, Cr, or Mo are added, other than the most basic elements C, Si, Mn, and Al. This ensures the overall mechanical properties and good surface plating properties of TRIP steel while maximizing the reduction of raw material production costs, increasing the applicability of the steel material, making it suitable for market promotion, and giving it good potential for general use.

[0044] 3. In the present invention, by precisely controlling the finish rolling completion temperature, winding temperature, and temperature and cooling rate during the continuous annealing process, the resulting cold-rolled steel sheet and hot-dip galvanized steel sheet have an appropriate microstructure distribution and grain size, and the influence of phenomena such as mixed crystals and grain coarsening on the mechanical properties of the steel is avoided.

[0045] 4. The proposed method of the present invention yields TRIP steel having good surface quality and overall mechanical properties. The microstructure of the steel contains, by volume fraction, 38-58% ferrite, 30-50% bainite, 10-12% retained austenite, and ≤2% martensite, of which 80% or more are ferrite grains of 10 μm or less, of which 60% or more are ferrite grains of 5 μm or less, the average grain size of retained austenite is 2 μm or less, and the average carbon content of retained austenite grains is 1 wt% or more. The TRIP steel has a yield strength of 420 MPa or more (e.g., 420-600 MPa), a tensile strength of 800 MPa or more (e.g., 800-950 MPa), a fracture elongation of 25% or more (e.g., 25%-35%), and a strength-ductile product exceeding 23 GPa.%.

[0046] The drawings illustrate specific embodiments, but do not limit the present invention. [Brief explanation of the drawing]

[0047] [Figure 1] Figure 1 shows a microstructural diagram of the TRIP steel of Example 7 of the present invention; [Figure 2] Figure 2 shows the EBSD diagram of the metallurgical structure of the TRIP steel of Example 7 of the present invention; [Figure 3] Figure 3 shows a surface morphology of a hot-dip galvanized product of TRIP steel according to Embodiment 7 of the present invention; [Figure 4] Figure 4 shows the surface morphology of the hot-dip galvanized product of TRIP steel according to Comparative Example 2 of the present invention; [Figure 5] Figure 5 shows the surface morphology of the hot-dip galvanized product of TRIP steel according to Comparative Example 1 of the present invention. [Modes for carrying out the invention]

[0048] The embodiments of the present invention will be described below with reference to specific examples, but those skilled in the art will readily understand the other advantages and effects of the present invention from what is disclosed herein. The present invention will be described in relation to preferred embodiments, but this does not mean that the features of the present invention are limited to these embodiments. On the contrary, the purpose of describing the present invention in conjunction with embodiments is to cover other choices and modifications that extend based on the claims of the present invention. In order to provide a deeper view of the present invention, the following description includes numerous specific details. The present invention can be carried out without these details. Also, in order to avoid ambiguity of the emphasis of the present invention, some specific details are omitted from the description. To the extent that they do not contradict each other, embodiments of the present invention and the features in the embodiments can be combined with each other.

[0049] Examples 1-20 and Comparative Examples 1-3 In this invention, the TRIP steels of Examples 1-20 were manufactured using the following process: Process 1, Smelting and Continuous Casting: The molten metal was smelted according to the chemical composition shown in Table 1 and cast into slabs by continuous casting. In continuous casting, the target temperature of the tundish was controlled to be 15-30°C higher than the liquidus temperature, and dynamic light pressure and electromagnetic stirring were performed during the pouring process.

[0050] Process 2, Hot rolling of steel slabs: The heating temperature of the slabs was set to 1150-1250°C. After rough rolling, the slabs were rolled to the thickness required for finish rolling. The finish rolling completion temperature was set to 850-950°C.

[0051] Process 3, Post-rolling Cooling and Winding: After rolling, water cooling was performed, and the winding temperature was set to 520-600°C.

[0052] Step 4: The iron oxide film was removed by pickling. Process 5, Cold Rolling: The hot-rolled slag was cold-rolled to the required thickness with a reduction of 50-70%.

[0053] Process 6, Continuous Annealing and Hot-Dip Galvanizing: The annealing was maintained at an annealing temperature of 800-860°C for 60-200 seconds, and then cooled to a rapid cooling start temperature of 690-730°C at a first cooling rate of 3-10°C / s to obtain a certain proportion of ferrite. Furthermore, it was rapidly cooled to a rapid cooling end temperature of 350-450°C at a second cooling rate of 30-60°C / s. Then, an aging treatment was performed by maintaining the temperature for 200-400 seconds. After the aging treatment, the steel strip / steel sheet was subjected to hot-dip galvanizing. Finally, it was cooled to below 150°C at a third cooling rate of 2-10°C / s to obtain the hot-dip galvanized annealed steel sheet product.

[0054] The TRIP steels of Comparative Examples 1-3 were also produced by smelting, continuous casting, hot rolling, coiling, cold rolling, continuous annealing, and hot-dip galvanizing processes, and the chemical composition of the steel and the parameters of the production process are shown in Table 1-2.

[0055] Table 1 shows the mass percentage of each chemical element in the TRIP steel of the examples and comparative examples.

[0056] [Table 1]

[0057] Tables 2-1 and 2-2 show the specific parameters of the TRIP steel in Examples 1-20 and the comparative steel in Comparative Examples 1-3.

[0058] [Table 2-1]

[0059] [Table 2-2]

[0060] Table 3 shows the results of the mechanical property measurements of TRIP steel for Examples 1-20 and Comparative Examples 1-3. ISO 6892:1998 (Room temperature tensile testing method for metallic materials), P14(A 50 )Measured using a standard tensile sample.

[0061] [Table 3]

[0062] As can be seen from Table 3, the TRIP steel of this embodiment 1-20 has excellent plasticity while ensuring strength, with a yield strength of 420-600 MPa, a tensile strength of 800-950 MPa, and a fracture elongation of 25-35%. At the same time, the strength ductility of the material exceeds 23 GPa.%, reaching a maximum of 28 GPa.%.

[0063] Table 4 shows the microstructural observation results of the TRIP steels from Examples 1-20.

[0064] [Table 4] As can be seen from Tables 3 and 4, the microstructure of the TRIP steels of Examples 1-20 of the present invention consists of 38-58% ferrite + 30-50% bainite + 10-12% retained austenite + trace amounts of martensite (≤2%) by volume fraction. Of the ferrite, grains smaller than 10 μm account for more than 80%, of which grains smaller than 5 μm account for more than 60%. The average grain size of the retained austenite is 2 μm or less, and the average carbon content of the retained austenite grains is 1 wt% or more. In other words, the TRIP steels of each example of the present invention have a certain amount of fine-grained ferrite and exhibit good microstructural uniformity. At the same time, because the TRIP steels of the present invention contain a high content of retained austenite, the TRIP effect during deformation is ensured, and the TRIP steels of each example can have a tensile strength of 800 MPa or more while simultaneously possessing extremely good plasticity.

[0065] The steel grade component system employs a design concept of replacing Si with Al, thereby reducing the Si content in the steel composition. This significantly improves the surface quality of the steel material and solves the problems of surface defects such as color unevenness and plating leaks that are prevalent in hot-dip galvanized products with high Si content. Figures 3 and 4 show the surface quality of hot-dip galvanized products manufactured in Example 7 and Comparative Example 2, respectively. A comparison reveals that the surface quality of Example 7 is clearly improved, and Comparative Example 2 lacks the typical color unevenness surface defect present in the center of the plate width. Simultaneously, Figure 5 shows the surface quality of the hot-dip galvanized product manufactured in Comparative Example 1. Although its Si content is low, the high winding temperature used in the hot rolling process during manufacturing in Comparative Example 1 resulted in surface color unevenness defects.

[0066] Figure 1 shows the microstructure morphology of the TRIP steel of Example 7. Figure 2 shows the metallurgical structure EBSD diagram of the TRIP steel of Example 7.

[0067] Combining Figures 1 and 2, the microstructure of the TRIP steel of Example 7 is 54.43% ferrite + 33.10% bainite + 10.60% retained austenite + 1.87% martensite, with 93.24% of the ferrite grains being 10 μm or smaller and 62.88% being 5 μm or smaller, and the average grain size of the retained austenite being 1.6 μm; and / or the average carbon content of the retained austenite being 1.18 wt%. The TRIP steel has good overall mechanical properties. Specifically, its yield strength is 495 MPa, its tensile strength is 821 MPa, its uniform elongation is 24.9%, its elongation at break is 33.1%, and its strength-ductility area is 27.18 GPa.%.

[0068] Thus, the TRIP steel described in the present invention is based on carbon-silicon-manganese steel, and in order to improve the surface quality and plating properties of the steel sheet, a certain amount of Al is added to replace Si with Al, while not adding any other expensive alloying elements, thereby significantly reducing the Si content that is commonly used in conventional TRIP steel. By optimizing the ratio of carbon, silicon, manganese, and aluminum, a high-strength, ductile cold-rolled steel sheet with excellent surface quality and its hot-dip galvanized product are obtained. The TRIP steel obtained by the technical proposal of the present invention has a better elongation rate under similar strength conditions, and therefore has good prospects for application in automotive safety structural components. In particular, it is suitable for manufacturing vehicle structural components and safety components that have complex shapes and require high forming performance, such as A / B pillars, door collision prevention bars, vertical beams, and bumpers.

[0069] While the present invention has been illustrated and described with reference to certain preferred embodiments, those skilled in the art will understand that the above description, along with specific embodiments, further elaborates on the present invention and that the implementation of the present invention is not limited thereto. Those skilled in the art may make various modifications in form and detail, including simple assumptions or modifications, without departing from the spirit and scope of the present invention.

Claims

1. A TRIP steel, The chemical composition and mass percentage content of the aforementioned TRIP steel are: C: 0.15–0.3%, Si: 0.6–1.0%, Mn: 1.7–2.5%, Al: 0.5–0.9%, P ≤ 0.01%, S ≤ 0.01%, N ≤ 0.007%, the remainder being Fe and other unavoidable impurities. The metallurgical structure of the aforementioned TRIP steel comprises, by volume fraction, 38-58% ferrite, 30-50% bainite, 10-12% retained austenite, and ≤2% martensite.

2. The TRIP steel according to claim 1, wherein the carbon content is 0.17 to 0.23%.

3. The TRIP steel according to claim 1, wherein the Si content is 0.7 to 0.9%.

4. The TRIP steel according to claim 1, with a Mn content of 1.8 to 2.0%.

5. The TRIP steel according to claim 1, with an Al content of 0.6 to 0.8%.

6. The TRIP steel according to claim 1, wherein ferrite grains of 10 μm or less account for 80% or more of the ferrite in the TRIP steel, of which ferrite grains of 5 μm or less account for 60% or more, the average grain size of the retained austenite is 2 μm or less, and the average carbon content of the retained austenite grains is 1 wt% or more.

7. The TRIP steel according to claim 1, wherein the TRIP steel has a yield strength of 420 MPa or more, a tensile strength of 800 MPa or more, a fracture elongation of 25% or more, and a strength-ductile product of more than 23 GPa.%.

8. The TRIP steel according to claim 7, wherein the TRIP steel has a yield strength of 420 to 600 MPa, a tensile strength of 800 to 950 MPa, a fracture elongation of 25% to 35%, and a strength-ductile product of more than 23 GPa.%.

9. The following steps: Smelting, casting; Hot rolling, coiling; Cold rolling; Continuous annealing; A method for producing TRIP steel according to any one of claims 1 to 8, including the following:

10. The parameters in the above manufacturing method are as follows: In the aforementioned smelting and casting processes, the superheating temperature for continuous casting shall be 15 to 30°C; In the hot rolling process, the slab heating temperature is set to 1150 to 1250°C, and the finish rolling completion temperature is set to 850 to 950°C; In the aforementioned winding process, the winding temperature shall be 520 to 600°C; In the cold rolling process, the cold rolling reduction amount shall be 50-70%; In the aforementioned continuous annealing process, the material is first kept warm at an annealing temperature of 800-860°C for 60-200 seconds, then cooled to a rapid cooling start temperature of 690-730°C at a first cooling rate of 3-10°C / s, then cooled to a rapid cooling end temperature of 350-450°C at a second cooling rate of 30-60°C / s, followed by an aging treatment with a warming period of 200-400 seconds, and finally cooled to below 150°C at a third cooling rate of 2-10°C / s. A method for producing TRIP steel according to claim 9, controlled to satisfy at least one of the following conditions.

11. A method for producing a hot-dip galvanized TRIP steel sheet, comprising the TRIP steel production step and the hot-dip galvanizing step described in claim 9.

12. Among the processes for manufacturing TRIP steel, In the aforementioned smelting and casting processes, the superheating temperature for continuous casting shall be 15 to 30°C; In the hot rolling process, the slab heating temperature is set to 1150 to 1250°C, and the finish rolling completion temperature is set to 850 to 950°C; In the aforementioned winding process, the winding temperature shall be 520 to 600°C; In the cold rolling process, the cold rolling reduction amount shall be 50-70%; In the aforementioned continuous annealing process, the material is first kept warm at an annealing temperature of 800-860°C for 60-200 seconds, then cooled to a rapid cooling start temperature of 690-730°C at a first cooling rate of 3-10°C / s, then cooled to a rapid cooling end temperature of 350-450°C at a second cooling rate of 30-60°C / s, followed by an aging treatment with a warming period of 200-400 seconds, and finally cooled to below 150°C at a third cooling rate of 2-10°C / s. The method according to claim 11.

13. A cold-rolled steel sheet made from TRIP steel according to any one of claims 1 to 8.

14. A hot-dip galvanized steel sheet comprising a cold-rolled steel sheet, a plate, and a hot-dip galvanized layer formed on the surface of the cold-rolled steel sheet, as described in claim 13.

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