1.6 GPa-grade high-yield-ratio TRIP steel and production method thereof

Abstract

The invention relates to the technical field of metallurgy, in particular to 1.6 GPa-grade high-yield-ratio TRIP steel and a production method thereof.The production method comprises the steps of smelting and casting, hot rolling and cooling, warm rolling, acid pickling and cold rolling, continuous annealing and leveling, and a steel billet comprises, by mass, 0.26%-0.3% of C, 1.6%-2.1% of Si, 3.5%-4.2% of Mn, 0.055%-0.075% of Nb, 0.045%-0.055% of Ti, 0.62%-0.75% of Cr, 1.2%-1.5% of Alt, smaller than or equal to 0.003% of P, smaller than or equal to 0.003% of S, smaller than or equal to 0.003% of N and the balance Fe. The alloy comprises the following components in percentage by weight: not more than 0.003% of O, not more than 2.8% and not more than 3.5% of Alt + Si, not less than 0.11% and not more than 0.13% of Nb + Ti, and the balance of Fe and impurities. The prepared steel is good in mechanical property, plasticity and processability and wide in application prospect.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgy, and in particular to a 1.6 GPa high yield ratio TRIP steel and a production method thereof. Background Art

[0002] The booming development of new energy vehicles has provided strong support for the low-carbon transformation of the automotive industry. Compared with traditional fuel vehicles, new energy vehicles have built-in battery packs, which are usually heavier than fuel vehicles of the same level. During use, the higher weight will cause more energy consumption and bring greater burden to the drive system. Therefore, new energy vehicles have a strong demand for lightweight.

[0003] Compared with aluminum alloy, carbon fiber and other materials, high-strength steel has the characteristics of high strength, light weight, high cost performance and good processability, and can take into account both safety and low carbon environmental protection, and is an important material used in the manufacture of new energy vehicles. At the same time, as the new energy vehicle manufacturing industry continues to upgrade its demand for lightweight, the proportion of high-strength grade, thin-gauge high-performance steel in automobile manufacturing will further increase.

[0004] In the production of high-strength steel, the TRIP effect (Transformation Induced Plasticity) can be used to obtain steel with both high strength and high plasticity, namely TRIP steel. Typical TRIP steels include Q&P steel, DH steel, CH steel, etc. TRIP steel has a very broad application prospect, but in the existing technology, the preparation of TRIP steel with a tensile strength of 1.6GPa (i.e. 1600MPa) and above still has the following problems:

[0005] (1) The plasticity index is low. When the tensile strength increases, the plasticity of the steel will deteriorate. Especially when the strength increases to 1.6 GPa or above, the plasticity index decreases significantly, which cannot meet the requirements of modern new energy vehicles for high strength and high plasticity at the same time.

[0006] (2) Poor formability and insufficient ductility. When the tensile strength increases to 1.6 GPa, the formability and ductility of the steel material drop sharply, making it difficult to meet the requirements of new energy vehicle manufacturers for high formability and high plasticity.

[0007] (3) The manufacturing cost is high. The addition of alloy elements such as Ni, Mo, V, etc. increases the cost; at the same time, it increases the difficulty of manufacturing. Summary of the invention

[0008] Aiming at the technical problems of low plasticity index, insufficient forming performance and ductility and high manufacturing cost in the prior art of producing 1.6GPa grade TRIP steel, the present invention provides a 1.6GPa grade high yield ratio TRIP steel and a production method thereof. The steel contains fewer alloy elements, and the material cost and process cost are lower. The prepared 1.6GPa grade high yield ratio TRIP steel has excellent mechanical properties, good plasticity, good processing performance and broad application prospects.

[0009] In a first aspect, the present invention provides a production method of a 1.6 GPa grade high yield ratio TRIP steel, comprising smelting and casting, hot rolling and cooling, warm rolling, pickling and cold rolling, continuous annealing, and leveling steps, wherein the chemical composition mass percentage of the steel billet comprises C: 0.26%-0.30%, Si: 1.6%-2.1%, Mn: 3.5%-4.2%, Nb: 0.055%-0.075%, Ti: 0.045%-0.055%, Cr: 0.62%-0.75%, Alt: 1.2%-1.5%, P≤0.003%, S≤0.003%, N≤0.003%, O≤0.003%, and at the same time satisfies 2.8%≤Alt+Si≤3.5%, 0.11%≤Nb+Ti≤0.13%, and the balance is Fe and unavoidable impurities.

[0010] Furthermore, the specific production steps of 1.6GPa high yield ratio TRIP steel include:

[0011] (1) Smelting and casting: smelting and casting molten steel to make slabs;

[0012] (2) hot rolling and cooling, heating the slab to 1235-1265°C, keeping the temperature for 45-65 min, and then hot rolling, with the hot rolling start temperature being 1085-1145°C and the hot rolling finish temperature being 835-875°C, and then cooling the steel plate to room temperature to obtain a hot rolled steel plate, with a thickness of 6-13 mm;

[0013] (3) warm rolling, heating the hot-rolled steel plate to 700±20°C, keeping the temperature for 40-60 minutes, and then warm rolling, with a cumulative reduction rate of 65%-75%. After rolling, the steel plate is slowly cooled to room temperature to obtain a warm-rolled steel plate, and the thickness of the warm-rolled steel plate is 2.0-4.5 mm;

[0014] (4) pickling and cold rolling, pickling the warm-rolled steel sheet to remove surface oxides, and then cold rolling, the cumulative cold rolling reduction is 40%-50%, and a cold-hardened steel strip is obtained, and the thickness of the cold-hardened steel strip is 1.0-2.5 mm;

[0015] (5) Continuous annealing, heating the cold hardened strip to 860-900°C, keeping the temperature for 50-90s, and then cooling it to the over-aging temperature of 395±10°C, the over-aging isothermal time is 100-150s, and then cooling the strip to room temperature to obtain the strip to be leveled;

[0016] (6) Flattening: Flatten the steel strip to be flattened to obtain 1.6 GPa high yield strength ratio TRIP steel.

[0017] Furthermore, the specific operation of cooling the steel plate to room temperature in step (2) is: adopting an ultra-fast water cooling mode to cool the steel plate to below 280° C. at a cooling rate of >120° C. / s, and then air cooling to room temperature.

[0018] Furthermore, in step (3), the reduction ratio of the penultimate pass of warm rolling is 22%-25%, and the reduction ratio of the last pass is 17%-20%.

[0019] Furthermore, the final rolling temperature of the warm rolling in step (3) is 545-585°C.

[0020] Furthermore, in step (5), the heating rate of the cold-hardened strip is 150-200°C / s.

[0021] Furthermore, in step (5), the cooling rate of cooling the heated cold-hardened steel strip to the over-aging temperature is >55°C / s; and the cooling rate of cooling the steel strip at the over-aging temperature to room temperature is 25°C / s.

[0022] Furthermore, during the leveling process in step (6), the leveling elongation of the steel strip is controlled within a range of 1.4% to 1.6%.

[0023] In a second aspect, the present invention provides a 1.6 GPa grade high yield ratio TRIP steel prepared using the above production method, wherein the 1.6 GPa grade high yield ratio TRIP steel has a tensile strength of >1600 MPa, a yield ratio of 0.76-0.86, an elongation after fracture of >15%, a hole expansion rate of >25%, and a strength-ductility product of >25 GPa·%.

[0024] Furthermore, in the microstructure of the 1.6 GPa grade high yield ratio TRIP steel, the bainite content is 70%-90%, the ferrite content is 5%-18%, and the retained austenite content is 5%-20%.

[0025] The beneficial effects of the present invention are:

[0026] 1. The 1.6GPa-grade high yield ratio TRIP steel prepared by the technical solution of the present invention has a bainite content of 70%-90%, a ferrite content of 5%-18%, and a retained austenite content of 5%-18% in the microstructure, showing high yield ratio characteristics, a tensile strength of>1600MPa, and a yield ratio of 0.76-0.86; it has excellent plasticity, an elongation after fracture of>15%; good processing performance, a hole expansion rate of>25%, and a strength-plasticity product of>25GPa·%, which can meet the diversified needs of lightweight, ultra-high strength level, and thin specifications, has broad application prospects, and is suitable for industries such as new energy vehicle manufacturing.

[0027] 2. The 1.6 GPa high yield ratio TRIP steel raw material of the present invention has a small amount of alloy elements, low material cost and process cost, and high economic benefit. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] 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.

[0029] Figure 1 This is the microstructure diagram of the 1.6 GPa grade high yield ratio TRIP steel prepared in Example 1.

[0030] Figure 2 This is the engineering stress-strain curve of the 1.6 GPa grade high yield ratio TRIP steel prepared in Example 1. DETAILED DESCRIPTION

[0031] 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.

[0032] Examples 1-6

[0033] A production method of a 1.6 GPa grade high yield ratio TRIP steel, wherein the chemical composition mass percentage of the steel billet includes C: 0.26%-0.30%, Si: 1.6%-2.1%, Mn: 3.5%-4.2%, Nb: 0.055%-0.075%, Ti: 0.045%-0.055%, Cr: 0.62%-0.75%, Alt: 1.2%-1.5%, P≤0.003%, S≤0.003%, N≤0.003%, O≤0.003%, and at the same time, 2.8%≤Alt+Si≤3.5%, 0.11%≤Nb+Ti≤0.13% are satisfied, and the balance is Fe and unavoidable impurities.

[0034] The specific chemical composition weight percentages of the steel billets in Examples 1-6 are shown in Table 1, with the remainder being Fe and unavoidable impurities.

[0035] Table 1 Chemical composition of steel billet (mass percentage, %)

[0036] Example C Si Mn Nb Ti Cr Alt P S N O 1 0.26 1.9 4.2 0.063 0.048 0.67 1.3 0.001 0.003 0.002 0.003 2 0.26 1.9 4.2 0.063 0.048 0.67 1.3 0.001 0.003 0.002 0.003 3 0.26 1.9 4.2 0.063 0.048 0.67 1.3 0.001 0.003 0.002 0.003 4 0.26 2.0 3.9 0.072 0.045 0.75 1.5 0.003 0.002 0.003 0.002 5 0.28 1.6 3.5 0.055 0.055 0.62 1.2 0.002 0.003 0.001 0.002 6 0.30 1.8 3.8 0.075 0.051 0.71 1.3 0.002 0.001 0.003 0.003

[0037] The specific production steps of 1.6GPa grade high yield ratio TRIP steel include:

[0038] (1) Smelting and casting: smelting and casting molten steel to make slabs;

[0039] (2) hot rolling and cooling, heating the slab to 1235-1265°C, keeping the temperature for 45-65 min, and then hot rolling, with the hot rolling start temperature being 1085-1145°C and the hot rolling finish temperature being 835-875°C, and then adopting an ultra-fast water cooling mode to cool the steel plate to below 280°C at a cooling rate of >120°C / s, and then air cooling to room temperature to obtain a hot-rolled steel plate, with a thickness of 6-13 mm;

[0040] (3) warm rolling, heating the hot rolled steel plate to 700±20°C, keeping the temperature for 40-60min, and then warm rolling, the cumulative reduction rate of warm rolling is 65%-75%, the reduction rate of the penultimate pass of warm rolling is 22%-25%, and the reduction rate of the last pass is 17%-20%; the final rolling temperature of warm rolling is 545-585°C, and the steel plate is slowly cooled to room temperature after rolling to obtain a warm rolled steel plate, and the thickness of the warm rolled steel plate is 2.0-4.5mm;

[0041] (4) pickling and cold rolling, pickling the warm-rolled steel sheet to remove surface oxides, and then cold rolling, the cumulative cold rolling reduction is 40%-50%, and a cold-hardened steel strip is obtained, and the thickness of the cold-hardened steel strip is 1.0-2.5 mm;

[0042] (5) Continuous annealing, heating the cold hardened strip to 860-900°C at a heating rate of 150-200°C / s, keeping the temperature for 50-90s, and then cooling the strip to an over-aging temperature of 395±10°C at a cooling rate of >55°C / s, the over-aging isothermal time is 100-150s, and then cooling the strip to room temperature at a cooling rate of 25°C / s to obtain a strip to be leveled;

[0043] (6) Flattening: The strip to be flattened is flattened. During the flattening process, the flattening elongation of the strip is controlled at 1.4%-1.6%, and a 1.6 GPa grade high yield strength ratio TRIP steel is obtained.

[0044] The specific process parameters of the hot rolling and cooling steps and the warm rolling step in Examples 1-6 are shown in Table 2; the specific process parameters of the cold rolling step, the continuous annealing step, and the leveling step are shown in Table 3.

[0045] Table 2 Process parameters of hot rolling and cooling steps, warm rolling steps

[0046]

[0047] Table 3 Process parameters of cold rolling step, continuous annealing step and leveling step

[0048]

[0049] The 1.6 GPa grade high yield ratio TRIP steel samples prepared in Examples 1-6 were sampled for mechanical property testing and microstructural analysis. The test and analysis results are shown in Table 4.

[0050] Table 4 Mechanical properties test and microstructure analysis results of samples

[0051]

[0052] The microstructure of the 1.6 GPa high yield ratio TRIP steel prepared in Example 1 is shown in FIG. Figure 1 As shown in FIG. 1 , the engineering stress-strain curve of the 1.6 GPa high yield ratio TRIP steel prepared in Example 1 is as follows: Figure 2 shown.

[0053] It can be seen that the 1.6 GPa grade high yield ratio TRIP steel prepared in Examples 1-6 has a tensile strength>1600 MPa, a yield ratio of 0.76-0.86, an elongation after fracture>15%, a hole expansion rate>25%, and a strength-ductility product>25 GPa·%; in the microstructure, the bainite content is 70%-85%, the ferrite content is 5%-18%, and the retained austenite content is 10%-20%.

[0054] 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 method for producing 1.6 GPa high yield ratio TRIP steel, characterized in that: The steel billet comprises the steps of smelting and casting, hot rolling and cooling, warm rolling, pickling and cold rolling, continuous annealing and leveling. The chemical composition percentage by mass of the steel billet includes C: 0.26%-0.30%, Si: 1.6%-2.1%, Mn: 3.5%-4.2%, Nb: 0.055%-0.075%, Ti: 0.045%-0.055%, Cr: 0.62%-0.75%, Alt: 1.2%-1.5%, P≤0.003%, S≤0.003%, N≤0.003%, O≤0.003%, and at the same time satisfies 2.8%≤Alt+Si≤3.5%, 0.11%≤Nb+Ti≤0.13%, and the balance is Fe and unavoidable impurities.

2. The production method according to claim 1, characterized in that The specific production steps of 1.6GPa grade high yield ratio TRIP steel include: (1) Smelting and casting molten steel to make slabs; (2) heating the slab to 1235-1265° C., keeping the temperature for 45-65 min, and then hot rolling, wherein the hot rolling start temperature is 1085-1145° C., the hot rolling finish temperature is 835-875° C., and then cooling the steel plate to room temperature to obtain a hot-rolled steel plate, wherein the thickness of the hot-rolled steel plate is 6-13 mm; (3) heating the hot-rolled steel plate to 700±20°C, keeping the temperature for 40-60 minutes, and then warm rolling the steel plate, wherein the cumulative reduction rate of the warm rolling is 65%-75%, and after rolling, the steel plate is slowly cooled to room temperature to obtain a warm-rolled steel plate, wherein the thickness of the warm-rolled steel plate is 2.0-4.5 mm; (4) pickling the warm-rolled steel sheet to remove surface oxides, and then cold rolling the steel sheet, wherein the cumulative reduction of the cold rolling is 40% to 50%, to obtain a cold-hardened steel strip having a thickness of 1.0 to 2.5 mm; (5) heating the cold hardened steel strip to 860-900°C, keeping the temperature for 50-90s, and then cooling it to the over-aging temperature of 395±10°C, the over-aging isothermal time is 100-150s, and then cooling the steel strip to room temperature to obtain the steel strip to be leveled; (6) The flat strip steel is flattened to obtain 1.6 GPa grade high yield strength ratio TRIP steel.

3. The production method according to claim 2, characterized in that The specific operation of cooling the steel plate to room temperature in step (2) is as follows: adopting an ultra-fast water cooling mode to cool the steel plate to below 280° C. at a cooling rate of >120° C. / s, and then air cooling to room temperature.

4. The production method according to claim 2, characterized in that In step (3), the reduction rate of the penultimate warm rolling pass is 22%-25%, and the reduction rate of the last warm rolling pass is 17%-20%.

5. The production method according to claim 2, characterized in that The final rolling temperature of the warm rolling in step (3) is 545-585°C.

6. The production method according to claim 2, characterized in that In step (5), the heating rate of the cold-hardened strip is 150-200°C / s.

7. The production method according to claim 2, characterized in that In step (5), the cooling rate of cooling the heated cold-hardened steel strip to the over-aging temperature is >55°C / s; and the cooling rate of cooling the steel strip at the over-aging temperature to room temperature is 25°C / s.

8. The production method according to claim 2, characterized in that During the leveling process in step (6), the leveling elongation of the steel strip is controlled at 1.4%-1.6%.

9. A 1.6 GPa high yield ratio TRIP steel prepared by the production method according to any one of claims 1 to 8, characterized in that: The tensile strength of 1.6GPa grade high yield ratio TRIP steel is >1600MPa, the yield ratio is 0.76-0.86, the elongation after fracture is >15%, the hole expansion rate is >25%, and the strength-ductility product is >25GPa·%.

10. The 1.6 GPa high yield ratio TRIP steel according to claim 9, characterized in that: In the microstructure of 1.6GPa grade high yield ratio TRIP steel, the bainite content is 70%-90%, the ferrite content is 5%-18%, and the retained austenite content is 5%-20%.

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