1.4 GPa-grade low-yield-ratio hot-dip galvanized TRIP steel and preparation method thereof

Abstract

The invention relates to the technical field of ultrahigh-strength steel materials, in particular to 1.4 GPa-grade low-yield-ratio hot-dip galvanized TRIP steel and a preparation method thereof.The 1.4 GPa-grade low-yield-ratio hot-dip galvanized TRIP steel is prepared from, by mass, 0.25%-0.28% of C, 0.8%-1.2% of Si, 2.9%-3.2% of Mn, 0.055%-0.075% of Nb, 0.035%-0.055% of Ti, 0.55%-0.75% of Cr, 1.0%-1.3% of Alt, smaller than or equal to 0.005% of P, smaller than or equal to 0.003% of S, smaller than or equal to 0.0035% of N, smaller than or equal to 0.005% of O and the balance Fe and inevitable impurities. The obtained 1.4 GPa-grade low-yield-ratio hot-dip galvanized TRIP steel can meet the comprehensive requirements of modern new energy automobiles for high strength, high plasticity and high formability of materials.

Description

Technical Field

[0001] The invention relates to the technical field of ultra-high strength steel materials, and in particular to a 1.4 GPa-grade low yield ratio hot-dip galvanized TRIP steel and a preparation method thereof. Background Art

[0002] ‌‌TRIP steel, the full name of which is transformation induced plasticity steel, is a type of ultra-high strength steel. Through the transformation induced plasticity effect, the retained austenite in the steel plate induces martensite nucleation under the action of plastic deformation, thereby introducing phase transformation strengthening and plastic growth mechanisms to improve the strength and toughness of the steel. Compared with aluminum alloys, carbon fiber and other materials, ultra-high strength steel has the characteristics of high strength, light weight, high cost performance and good machinability, and is both safe and low-carbon and environmentally friendly. It is an important material used in the manufacture of new energy vehicles. ‌‌

[0003] However, ultra-high strength steels such as TRIP steel have been found to have the following problems in current research and applications: (1) High manufacturing cost: The unreasonable addition ratio of alloying elements such as Ni, Mo, and V increases the preparation cost and difficulty of ultra-high strength steel. (2) Unstable comprehensive mechanical properties: After the tensile strength of ultra-high strength steel is increased, especially when the tensile strength is increased to 1.4GPa level and above, its plasticity index is significantly reduced, and the forming performance and ductility properties are sharply reduced. The mechanical properties have shortcomings and cannot meet the comprehensive requirements of modern new energy vehicles for high strength, high plasticity and high formability of materials. (3) The heating and cooling rates of the existing hot-dip galvanizing process are not high: The heating and cooling rates affect the microstructure of TRIP steel, and thus affect its comprehensive mechanical properties.

[0004] CN 114574760A provides a 1500MPa grade high-formability alloyed hot-dip galvanized dual-phase steel and a preparation method thereof, which adopts a high Cr and high Ti design, and also adds alloys such as Mo and B, which has high cost and increased preparation difficulty; the existing hot-dip galvanizing process is adopted during preparation, and the heating and cooling rates are not high, which affects its comprehensive mechanical properties; although its tensile strength is high, reaching 1500MPa level, its elongation after fracture is ≤10.3%, and its strength-plasticity product is 12.0~16.3GPa·%, and its plasticity is not high, and its comprehensive mechanical properties have shortcomings, so its practical application is limited. Summary of the invention

[0005] In view of the technical problems in the prior art that when the tensile strength of TRIP steel is increased to 1.4 GPa, there are high manufacturing costs and unstable comprehensive mechanical properties such as strength and plasticity. The present invention provides a 1.4 GPa-level low yield ratio hot-dip galvanized TRIP steel and a preparation method thereof. The chemical component ratio is reasonable, the tensile strength is greater than 1.4 GPa, the yield ratio is less than 0.55, the strength-plasticity product is greater than 20 GPa·%, and the hole expansion rate is greater than 30%, which can meet the comprehensive requirements of modern new energy vehicles for high strength, high plasticity and high formability of materials.

[0006] In a first aspect, the present invention provides a 1.4 GPa grade low yield ratio hot-dip galvanized TRIP steel, the chemical composition and its mass percentage are: C 0.25%~0.28%, Si 0.8%~1.2%, Mn 2.9%~3.2%, Nb 0.055%~0.075%, Ti0.035%~0.055%, Cr 0.55%~0.75%, Alt 1.0%~1.3%, P≤0.005%, S≤0.003%, N≤0.0035%, O≤0.005%, and the balance is Fe and unavoidable impurities; At the same time, 1.8%≤Alt+Si≤2.5% and 1.00≤Alt / Si≤1.25, 3.45%≤Mn+Cr≤3.85% and 0.18≤Cr / Mn ≤0.24, 0.09%<Nb+Ti<0.12%; The microstructure is ferrite + bainite + retained austenite; Yield-to-strength ratio <0.55, tensile strength >1.4GPa, elongation after break ≥13.4%, hole expansion rate >30%, strength-to-ductility product >20GPa·%.

[0007] Furthermore, the microstructure of low yield ratio TRIP steel is 8%~15% ferrite, 65%~80% bainite and 12%~20% retained austenite.

[0008] Furthermore, the ferrite grain size is ≤1.8 μm.

[0009] In a second aspect, the present invention provides a method for preparing a 1.4 GPa grade low yield ratio hot-dip galvanized TRIP steel, comprising the following steps: (1) Smelting and forging: Smelting produces a steel billet, which is then slowly cooled and then heated and forged to produce a forged billet; (2) Hot rolling and ultra-fast cooling: The forging billet is hot rolled, the austenitizing soaking temperature is 1230~1260℃, and the soaking period is 45~60min; the starting rolling temperature is 1140~1170℃, and the final rolling temperature is 845~875℃; after rolling, ultra-fast water cooling mode is used to cool the steel sheet to obtain a 5~9mm hot-rolled steel plate; (3) Warm rolling and pickling: The hot-rolled steel plate is heated to 700±20℃ and kept warm for 50±10min; then it is taken out of the furnace for rolling, with a cumulative reduction of 70%~85% and a final rolling temperature of 540~590℃; after rolling, it is slowly cooled to room temperature and pickled to remove surface oxides to obtain a strip; (4) Continuous annealing and hot-dip galvanizing: The strip is rapidly heated to 860-890°C at a heating rate of 80-150°C / s and kept at this temperature for 40-90s; then it is rapidly cooled to the bainite region at a cooling rate of >60°C / s, and the temperature after cooling is 360-390°C, and the temperature is kept at this temperature for 120-180s; then induction heating is used to rapidly increase the temperature at >30°C / s for hot-dip galvanizing, and the galvanized steel strip is cooled to ≤155°C after exiting the zinc pot; (5) Skin finishing and coiling: The galvanized steel strip is skin-finished, and the skin finishing elongation is controlled at 1.0% to 1.5%, and then coiled.

[0010] Furthermore, in step (1), the steel billet is slowly cooled and then heated to 1235-1255°C, and kept at this temperature for 90±20 min; the start forging temperature of forging is 1150-1210°C, and the final forging temperature is 950-1100°C.

[0011] Furthermore, in step (2), the ultra-fast water cooling mode is to rapidly cool to <160°C at a cooling rate of >100°C / s.

[0012] Furthermore, the reduction ratios of the last two passes of warm rolling in step (3) are set to 15% to 20%.

[0013] Furthermore, in step (4), the galvanizing temperature is 450-475°C, and the zinc liquid temperature is 465±5°C.

[0014] The beneficial effects of the present invention are: The present invention provides a 1.4GPa-level low yield ratio hot-dip galvanized TRIP steel, which has a reasonable chemical composition ratio, a microstructure of ferrite+bainite+retained austenite, a yield ratio of less than 0.55, a tensile strength of more than 1.4GPa, an elongation after fracture of ≥13.4%, a hole expansion rate of more than 30%, and a strength-plasticity product of more than 20GPa·%, and its comprehensive mechanical properties can meet the comprehensive requirements of modern new energy vehicles for high strength, high plasticity and high formability of materials. In addition, by adopting the preparation process of the present invention, the heating and cooling rates of the hot-dip galvanizing process are appropriately improved, and a 1.4GPa-level low yield ratio hot-dip galvanized TRIP steel with good comprehensive mechanical properties can be prepared. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] 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.

[0016] Figure 1 This is a typical microstructure diagram of the 1.4 GPa grade low yield ratio hot-dip galvanized TRIP steel prepared in Example 1-I.

[0017] Figure 2 This is a typical engineering stress-strain curve of the 1.4 GPa grade low yield ratio hot-dip galvanized TRIP steel prepared in Example 1-I. DETAILED DESCRIPTION

[0018] 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.

[0019] The chemical composition and mass percentage of the 1.4 GPa grade low yield ratio hot-dip galvanized TRIP steel in Examples 1 to 4 of the present invention are: C 0.25% to 0.28%, Si 0.8% to 1.2%, Mn 2.9% to 3.2%, Nb 0.055% to 0.075%, Ti 0.035% to 0.055%, Cr 0.55% to 0.75%, Alt 1.0% to 1.3%, P ≤ 0.005%, S ≤ 0.003%, N ≤ 0.0035%, O ≤ 0.005%, and the balance is Fe and unavoidable impurities; At the same time, 1.8%≤Alt+Si≤2.5% and 1.00≤Alt / Si≤1.25, 3.45%≤Mn+Cr≤3.85% and 0.18≤Cr / Mn ≤0.24, 0.09%<Nb+Ti<0.12%.

[0020] The chemical composition (the balance is Fe and unavoidable impurities) of the 1.4 GPa grade low yield ratio hot-dip galvanized TRIP steel in specific embodiments 1 to 4 is shown in Table 1.

[0021] Table 1 Chemical composition of Examples 1 to 4 (wt%)

[0022] The preparation method of the 1.4 GPa grade low yield ratio hot-dip galvanized TRIP steel described in Examples 1 to 4 comprises the following steps: (1) Smelting and forging: Smelting is performed according to the chemical composition of the 1.4 GPa grade low yield ratio hot-dip galvanized TRIP steel in Table 1 to obtain a steel billet with the corresponding composition; the steel billet after slow cooling is heated to 1235-1255°C and kept warm for 90±20 min; then forging is performed with the start forging temperature being 1150-1210°C and the final forging temperature being 950-1100°C to obtain a forged billet; (2) Hot rolling and ultra-fast cooling: The forging billet is hot rolled, the austenitizing soaking temperature is 1230~1260℃, and the soaking period is 45~60min; the starting rolling temperature is 1140~1170℃, and the final rolling temperature is 845~875℃; after rolling, the ultra-fast water cooling mode is adopted, and the cooling rate is >100℃ / s to quickly cool to <160℃, and a 5~9mm hot-rolled steel plate is obtained; (3) Warm rolling and pickling: The hot-rolled steel plate is heated to 700±20℃ and kept warm for 50±10min. It is then taken out of the furnace for rolling, with a cumulative reduction of 70%~85%, and the reduction rate of the last two passes is set to 15%~20%. The final rolling temperature is 540~590℃. After rolling, it is slowly cooled to room temperature, and the surface oxide is removed by hydrochloric acid pickling to obtain a 1.0~2.5mm strip steel. (4) Continuous annealing and hot-dip galvanizing: The strip is rapidly heated to 860-890°C at a heating rate of 80-150°C / s and kept at this temperature for 40-90s; then it is rapidly cooled to the bainite region at a cooling rate of >60°C / s, and the temperature after cooling is 360-390°C, and the temperature is kept at this temperature for 120-180s; then induction heating is used to rapidly increase the temperature by >30°C / s for hot-dip galvanizing, the galvanizing temperature is 450-475°C, the zinc liquid temperature is 465±5°C, and after exiting the zinc pot, a mobile bellows is used to cool the galvanized steel strip to ≤155°C; (5) Skin finishing and coiling: The galvanized steel strip is skin-finished, and the skin finishing elongation is controlled at 1.0% to 1.5%, and then coiled.

[0023] The process parameters of forging, hot rolling, flattening and primary annealing in Examples 1 to 4 are shown in Table 2, and the process parameters of cold rolling and secondary annealing in Examples 1 to 4 are shown in Table 3.

[0024] Table 2 Specific parameters of forging, hot rolling, ultra-fast cooling and warm rolling in Examples 1 to 4

[0025] Table 3 Specific parameters of continuous annealing, hot dip galvanizing and skin pass in Examples 1 to 4

[0026] The 1.4 GPa-grade low yield ratio hot-dip galvanized TRIP steel prepared in Examples 1 to 4 of the present invention has a microstructure composed of 8% to 15% ferrite, 65% to 80% bainite and 12% to 20% retained austenite, wherein the ferrite grain size is ≤1.8 μm. Figure 1 This is a typical microstructure diagram of the 1.4 GPa grade low yield ratio hot-dip galvanized TRIP steel prepared in Example 1-I.

[0027] The 1.4 GPa grade low yield ratio hot-dip galvanized TRIP steel prepared in Examples 1 to 4 was sampled for mechanical property testing and microstructural analysis. The test and analysis results are specifically shown in Table 4. The 1.4 GPa grade low yield ratio hot-dip galvanized TRIP steel presents low yield ratio characteristics, with a yield ratio of <0.55, high strength, and tensile strength>1.4 GPa. It also has high strength-plasticity matching and high formability, with an elongation after fracture ≥13.4%, a strength-plasticity product>20 GPa·%, and a hole expansion rate>30%. Figure 2 This is a typical engineering stress-strain curve of the 1.4 GPa grade low yield ratio hot-dip galvanized TRIP steel prepared in Example 1-I.

[0028] Table 4 Mechanical properties and volume fraction of each phase of hot-dip galvanized TRIP steel prepared in Examples 1 to 4

[0029] 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 1.4GPa grade low yield ratio hot dip galvanized TRIP steel, characterized in that: The chemical composition and mass percentage of 1.4GPa grade low yield ratio hot-dip galvanized TRIP steel are: C 0.25%~0.28%, Si 0.8%~1.2%, Mn 2.9%~3.2%, Nb 0.055%~0.075%, Ti 0.035%~0.055%, Cr 0.55%~0.75%, Alt 1.0%~1.3%, P≤0.005%, S≤0.003%, N≤0.0035%, O≤0.005%, the balance is Fe and unavoidable impurities; At the same time, 1.8%≤Alt+Si≤2.5% and 1.00≤Alt / Si≤1.25, 3.45%≤Mn+Cr≤3.85% and 0.18≤Cr / Mn ≤0.24, 0.09%<Nb+Ti<0.12%; The microstructure is ferrite + bainite + retained austenite; Yield-to-strength ratio <0.55, tensile strength >1.4GPa, elongation after break ≥13.4%, hole expansion rate >30%, strength-to-ductility product >20GPa·%.

2. The 1.4 GPa grade low yield ratio hot dip galvanized TRIP steel according to claim 1, characterized in that: The microstructure of low yield ratio TRIP steel is 8%~15% ferrite, 65%~80% bainite and 12%~20% retained austenite.

3. The 1.4 GPa grade low yield ratio hot dip galvanized TRIP steel according to claim 1, characterized in that: Ferrite grain size ≤1.8μm.

4. A method for preparing 1.4 GPa grade low yield ratio hot dip galvanized TRIP steel as claimed in claim 1, characterized in that: The steps include: (1) Smelting and forging: Smelting produces a steel billet, which is then slowly cooled and then heated and forged to produce a forged billet; (2) Hot rolling and ultra-fast cooling: The forging billet is hot rolled, the austenitizing soaking temperature is 1230~1260℃, and the soaking period is 45~60min; the starting rolling temperature is 1140~1170℃, and the final rolling temperature is 845~875℃; after rolling, ultra-fast water cooling mode is used to cool the steel sheet to obtain a 5~9mm hot-rolled steel plate; (3) Warm rolling and pickling: The hot-rolled steel plate is heated to 700±20℃ and kept warm for 50±10min; then it is taken out of the furnace for rolling, with a cumulative reduction of 70%~85% and a final rolling temperature of 540~590℃; after rolling, it is slowly cooled to room temperature and pickled to remove surface oxides to obtain a strip; (4) Continuous annealing and hot-dip galvanizing: The strip is rapidly heated to 860-890°C at a heating rate of 80-150°C / s and kept at this temperature for 40-90s; then it is rapidly cooled to the bainite region at a cooling rate of >60°C / s, and the temperature after cooling is 360-390°C, and the temperature is kept at this temperature for 120-180s; then induction heating is used to rapidly increase the temperature at >30°C / s for hot-dip galvanizing, and the galvanized steel strip is cooled to ≤155°C after exiting the zinc pot; (5) Skin finishing and coiling: The galvanized steel strip is skin-finished, and the skin finishing elongation is controlled at 1.0% to 1.5%, and then coiled.

5. The method for preparing 1.4 GPa grade low yield ratio hot dip galvanized TRIP steel according to claim 4, characterized in that: In step (1), the steel billet is slowly cooled and then heated to 1235-1255°C and kept at this temperature for 90±20 min; the start forging temperature of forging is 1150-1210°C and the final forging temperature is 950-1100°C.

6. The method for preparing 1.4 GPa grade low yield ratio hot dip galvanized TRIP steel according to claim 4, characterized in that: In step (2), the ultra-fast water cooling mode is to cool rapidly to <160°C at a cooling rate of >100°C / s.

7. The method for preparing 1.4 GPa grade low yield ratio hot dip galvanized TRIP steel according to claim 4, characterized in that: The reduction ratio of the last two passes of warm rolling in step (3) is set to 15%~20%.

8. The method for preparing 1.4 GPa grade low yield ratio hot dip galvanized TRIP steel according to claim 4, characterized in that: In step (4), the galvanizing temperature is 450-475°C, and the zinc liquid temperature is 465±5°C.

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