Cold-rolled DH980 steel with excellent rebound resilience resistance and preparation method of cold-rolled DH980 steel

Abstract

The invention discloses cold-rolled DH980 steel with excellent rebound resilience resistance and a preparation method of the cold-rolled DH980 steel. The cold-rolled DH980 steel comprises 0.05%-0.15% of C, 0.5%-1.5% of Mn, 0.1%-1.0% of Si, 0.02%-6.0% of Al, 0.4%-1.0% of Cr, 0.2%-1.0% of Mo, 0.5%-1.20% of Ni, less than or equal to 0.01% of P, less than or equal to 0.01% of S, less than or equal to 0.005% of N, 0.005%-0.50% of Mg, 0.01%-0.50% of Nb, 0.01%-0.50% of V, more than or equal to 1.0% and less than or equal to 6.0% of (Al + Si) and the balance Fe and inevitable impurities. Through component design and cooperation with electric furnace smelting, continuous casting and rolling and continuous annealing, the prepared steel material gives consideration to resilience resistance, high plasticity and high formability, and meets the requirements of resilience resistance, high strength, high plasticity and excellent formability of automobiles.

Description

Technical Field

[0001] The present invention relates to the technical fields of materials and metallurgy, and more specifically, to a cold-rolled DH980 steel with excellent anti-springback performance and a preparation method thereof. Background Art

[0002] In recent years, with the continuous improvement of the requirements for the formability of materials in the automotive industry, traditional dual-phase steel (DP steel) has been difficult to meet the needs of complex stamped parts for high drawability. Although transformation-induced plasticity steel (TRIP steel) with high alloy content has excellent properties, its high production cost limits its wide application. In this context, a new type of dual-phase steel with enhanced formability (DH steel) has emerged. By introducing a certain amount of retained austenite into traditional dual-phase steel and utilizing the transformation-induced plasticity (TRIP) effect, DH steel significantly improves the formability of materials, effectively overcomes the deficiencies of DP steel and TRIP steel in practical applications, and has become one of the research hotspots in the field of automotive steel development. However, during the stamping process of ultra-high-strength steel parts, the springback phenomenon is relatively serious, which seriously affects the normal use of the parts and has also attracted wide attention from manufacturers and users. Chinese patent application with publication number CN112048680A discloses an alloyed hot-dip galvanized DH980 steel and a preparation method thereof, and its main chemical components are: C: 0.16% - 0.23%, Mn: 1.5% - 2.5%, Si: 0.2% - 0.9%, Al: 0.02% - 0.9%, Cr: 0.02% - 0.70%, P≤0.01%, S≤0.01%, Nb≤0.05%, V≤0.05%, Ti≤0.05%, and the rest is Fe and inevitable impurities. This invention does not consider the springback problem of ultra-high-strength steel, and the preparation process does not consider the electric furnace + scrap steel smelting process. Chinese patent application with publication number CN113403551A discloses a cold-rolled DH980 steel plate with a high yield ratio and hydrogen embrittlement resistance and a preparation method thereof, and its main chemical components are: C: 0.16% - 0.23%, Mn: 1.8% - 2.5%, Si: 0.4% - 1.2%, Al: 0.30% - 0.90%, Cr: 0.10 - 0.50%, Mo: 0.10 - 0.60%, P≤0.01%, S≤0.01%, N≤0.005%, Nb: 0.01% - 0.10%, Ti: 0.01% - 0.10%, and the balance is Fe and other inevitable impurities. The ductility of this product is relatively poor, it is difficult to balance high plasticity and anti-springback problems, and the preparation process does not consider the electric furnace + scrap steel smelting process.

[0003] At the same time, driven by the "dual carbon" goals, the steel industry is accelerating its transformation towards energy conservation, environmental protection and greening. Dual-phase steel products with enhanced formability for automobiles face special demands such as diversified parts and components, diversified user needs, numerous product specifications and small batch orders, which bring many challenges to the actual production organization of steel companies. The main manifestations are the high downgrading and re-judgment rate of mixed billets and the frequent transitions between different steel grades and specifications, resulting in poor quality stability of DH steel products. Therefore, the development of green and low-carbon DH steel products for automobiles, that is, the realization of alloy design with multiple uses of one steel and short-process and low-cost preparation technology, can not only meet the special requirements of the automotive industry for multiple parts, diversified needs, multiple specifications and small batch orders, but also effectively enhance the market competitiveness of steel companies, and has become the research focus of major steel suppliers.

[0004] Based on the above situation, developing a new type of automotive steel that has anti-rebound, high plasticity and high formability has become an important issue in the industry, in order to solve the rebound problem of automotive high-strength steel and at the same time adapt to the needs of green and low-carbon product design of automotive high-strength steel under the background of "dual carbon". Summary of the invention

[0005] The purpose of the present invention is to overcome the above-mentioned defects of the prior art, provide a cold-rolled DH980 steel with excellent anti-rebound performance and a preparation method thereof, and the present invention realizes the low-carbon, green and lightweight development of automotive high-strength steel through a short process and low-cost process path, combined with an extremely cost-reducing alloy design. At the same time, it can meet the personalized needs of ultra-high-strength steel in terms of anti-rebound resilience, high plasticity and high formability, and provide an efficient and reliable technical solution for automobile manufacturers and steel companies, promoting the sustainable development of automotive steel.

[0006] To achieve the above object, the technical solution of the present invention is as follows:

[0007] A cold-rolled DH980 steel with excellent anti-rebound performance comprises the following components in percentage by mass: C: 0.05%-0.15%, Mn: 0.5%-1.5%, Si: 0.1%-1.0%, Al: 0.02%-6.0%, Cr: 0.4%-1.0%, Mo: 0.2%-1.0%, Ni: 0.5%-1.20%, P≤0.01%, S≤0.01%, N≤0.005%, Mg: 0.005%-0.50%, Nb: 0.01%-0.50%, V: 0.01%-0.50%, 1.0%≤(Al+Si)≤6.0%, and the balance is Fe and unavoidable impurities.

[0008] The present invention also discloses a method for preparing the cold-rolled DH980 steel with excellent anti-rebound performance as described above, comprising the following steps: electric furnace smelting, medium-thin slab continuous casting and rolling, pickling cold rolling, continuous annealing and skin-passing;

[0009] In the continuous annealing process, the pickled and cold-rolled steel sheet is subjected to continuous annealing. The strip speed is 60 m / min to 180 m / min, the furnace temperature in the soaking section is 760 °C to 880 °C, the soaking time is 10 s to 600 s, the slow cooling outlet temperature is 700 °C to 760 °C, the rapid cooling rate > 25 °C / s, the rapid cooling temperature is 300 °C to 450 °C, the aging temperature is 350 °C to 550 °C, and the aging time is 60 s to 1000 s.

[0010] Implementing the embodiments of the present invention will have the following beneficial effects:

[0011] (1) The chemical composition of the steel in the present invention mainly consists of C, Mn, Al, and Si, with a relatively low original cost. Thus, a significant reduction in cost is achieved at the raw material stage, laying an economic foundation for the subsequent production process.

[0012] (2) The present invention adopts a new short-process production process of "100% scrap steel + electric furnace smelting + medium and thin slab continuous casting and rolling". This process route not only significantly reduces carbon emissions but also remarkably saves energy consumption, conforms to the trend of green and low-carbon development in the steel industry, and further reduces production costs.

[0013] (3) Based on traditional cold-rolled dual-phase steel, the present invention increases a certain proportion of retained austenite and utilizes the transformation-induced plasticity (TRIP) effect, enabling the steel to have high plasticity and excellent formability while maintaining high strength. This unique microstructure design effectively solves the springback problem that easily occurs during the forming process of traditional high-strength steel.

[0014] (4) By adding a large amount of aluminum element, the present invention realizes the low density of high-strength steel, meeting the requirements of the automotive industry for lightweight design.

[0015] (5) The cold-rolled DH980 steel produced by the present invention has excellent mechanical properties: yield strength ≥ 700 MPa, tensile strength 980 - 1100 MPa, elongation after fracture of A80 ≥ 15.0%, hole expansion rate ≥ 60%, and density is 6.5 g / cm 3 ~7.5 g / cm 3 ; the springback angle ≤ 2°, and the anti-springback performance is excellent.

[0016] In summary, the cold-rolled DH980 steel of the present invention not only performs excellently in terms of mechanical properties and formability but also has significant advantages in cost control and green manufacturing. It can provide an efficient, economical, and environmentally friendly technical solution for automotive manufacturers and steel enterprises, promoting the upgrading and sustainable development of automotive steels. Description of the Drawings

[0017] Figure 1This is the typical microstructure of Example 1 of the present invention. Detailed implementation manners

[0018] The present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited in any way.

[0019] I. Chemical composition and mechanical properties

[0020] The present invention discloses a cold-rolled DH980 steel with excellent springback resistance, which comprises the following components by mass percentage: C: 0.05% - 0.15%, Mn: 0.5% - 1.5%, Si: 0.1% - 1.0%, Al: 0.02% - 6.0%, Cr: 0.4% - 1.0%, Mo: 0.2% - 1.0%, Ni: 0.5% - 1.20%, P≤0.01%, S≤0.01%, N≤0.005%, Mg: 0.005% - 0.50%, Nb: 0.01% - 0.50%, V: 0.01% - 0.50%, and 1.0% ≤ (Al + Si) ≤ 6.0%, and the balance is Fe and inevitable impurities.

[0021] The reasons for the alloy design of the present invention are as follows:

[0022] C: The carbon element ensures the strength requirement of the steel through solid solution strengthening. A sufficient amount of carbon element helps to stabilize austenite, thereby improving the formability of the steel. If the content of the C element is too low, the mechanical properties of the steel in the present invention cannot be obtained; if the content is too high, the steel will be embrittled, and there are risks of hydrogen embrittlement and springback. Therefore, in the present invention, the content of the C element is controlled to be 0.05% - 0.15%.

[0023] Mn: Manganese is an austenite stabilizing element in steel, which can expand the austenite phase region and reduce the critical quenching rate of the steel. At the same time, it can also refine the grains and contribute to solid solution strengthening to improve the strength. If the content of the Mn element is too low, the supercooled austenite is not stable enough, reducing the processing properties such as plasticity and toughness of the steel plate; if the content of the Mn element is too high, it will lead to poor welding performance of the steel plate and an increase in production cost, which is not conducive to industrial production. Therefore, in the present invention, the content of the Mn element is controlled to be 0.5% - 1.5%.

[0024] Si: Silicon has a certain solid solution strengthening effect in ferrite, ensuring that the steel has sufficient strength. At the same time, Si can also inhibit the decomposition of retained austenite and the precipitation of carbides, reducing inclusions in the steel. If the content of the Si element is too low, it cannot play a strengthening role; if the content of the Si element is too high, it will reduce the surface quality and welding performance of the steel plate. Therefore, in the present invention, the content of the Si element is controlled to be 0.1% - 1.0%.

[0025] Al: The density of aluminum element is much lower than that of iron element. Adding an appropriate amount of aluminum element to steel can significantly reduce the density of steel, which helps the development of lightweight steel. At the same time, aluminum element has an antioxidant effect, and adding it in combination with silicon element can effectively improve the surface quality of steel, which is beneficial to the design of multi-purpose steel products. In addition, aluminum element can also inhibit the decomposition of retained austenite and the precipitation of carbides, and accelerate the bainite transformation to improve the coordinated deformation ability. Moreover, aluminum element can inhibit the decomposition of retained austenite and the precipitation of carbides, and accelerate the bainite transformation to improve the coordinated deformation ability. If the content of aluminum element is too high, it will not only increase the production cost, but also cause difficulties in continuous casting production, etc. When the aluminum content is too low, the low-density design of the material cannot be achieved. Therefore, in the present invention, the content range of aluminum element is controlled at 0.02% - 6.0%. In the present invention, silicon element and aluminum element are used in combination, and the design concept of replacing silicon with aluminum can significantly improve the surface quality of steel. Therefore, the present invention further sets 1.0% ≤ (Al + Si) ≤ 6.0%.

[0026] Cr: Chromium element can increase the hardenability of steel to ensure the strength of steel, and can stabilize retained austenite. If the content of Cr is too low, it will affect the hardenability of steel, and if the content is too high, it will increase the production cost. Therefore, in the present invention, the content range of chromium element is controlled at 0.4% - 1.0%.

[0027] Mo: Molybdenum element is a strengthening element in steel, which helps to stabilize retained austenite. At the same time, it has a significant effect on improving the hardenability of steel. When Mo element is used in combination with Ti, a large number of TiMoC precipitates can be formed, which is beneficial to make the diffusible hydrogen in steel be diffusely distributed and reduce the aggregation of diffusible hydrogen. Therefore, it can take into account both high strength and hydrogen embrittlement resistance. In the present invention, the content range of Mo element is controlled at 0.2% - 1.0%.

[0028] Ni: Nickel element is an important element to improve the welding performance of steel. The addition of Ni is beneficial to improving the toughness of the weld, especially the low-temperature impact toughness, and reducing the brittle transition temperature; when reasonably combined with Cr, it can effectively control the phase transformation process of the weld metal and obtain a composite structure of martensite and a certain proportion of retained austenite, so as to take into account both high crack resistance and high strength. In the present invention, the content range of Ni element is controlled at 0.5% - 1.20%.

[0029] P: Phosphorus element is a harmful element in steel. It is extremely easy to segregate to the grain boundary and seriously reduce the plasticity and deformation performance of steel. The lower its content, the better. Considering the cost, in the present invention, the content of P element is controlled at P ≤ 0.01%.

[0030] S: Sulfur element is a harmful element in steel. Sulfur is easy to combine with manganese element to form MnS inclusions. After rolling deformation, the transverse performance of the material will be significantly reduced, seriously affecting the formability of steel. The lower its content, the better. Considering the cost, in the present invention, the content of S element is controlled at S ≤ 0.01%.

[0031] N: N element easily reacts with Ti to precipitate large TiN particles, which act as crack sources during deformation and are detrimental to the anti-hydrogen embrittlement performance. Therefore, the N element content in the steel must be strictly controlled. The present invention controls the N content to N≤0.005%.

[0032] Mg: Magnesium is a good deoxidizer, desulfurizer and spheroidizer in steel. Mg can reduce the number of inclusions in steel, reduce their size, make their distribution uniform and improve their morphology. A small amount of magnesium can improve the carbide size and distribution of DH steel, promote the fine and uniform carbide particles, and also help to achieve low-density material design. In order to control production costs, the Mg content is controlled at 0.005% to 0.50% in the present invention.

[0033] Nb: The microalloying element Nb forms compounds with carbon and nitrogen, which helps to delay the recrystallization of the material during hot rolling, refines the grain size, and significantly improves the strength, toughness and fatigue failure resistance of the material. In the present invention, the Nb element content is controlled at 0.01% to 0.50%.

[0034] V: Microalloying element vanadium mainly exists in the form of VC, which improves the strength and fatigue resistance of the material through fine grain strengthening and dispersion strengthening. In the hot-dip galvanizing annealing heating process, the undissolved VC particles can pin the ferrite grain boundary, thereby playing a role in refining the grains; when the annealing temperature increases to the two-phase region, the VC dissolution temperature is low, so it is fully dissolved in the matrix, and the solid solution C atoms are enriched in the austenite to improve its stability; during the annealing process, the VC in the ferrite will reprecipitate, thereby producing obvious precipitation strengthening. Therefore, in the present invention, the V element content is controlled at 0.01% to 0.50%.

[0035] In a specific embodiment, the yield strength of cold-rolled DH980 steel is ≥700MPa, the tensile strength is 980MPa-1100MPa, the A80 elongation after fracture is ≥15.0%, the hole expansion rate is ≥60%, and the density is 6.5g / cm 3 ~7.5g / cm 3 ; Use 180° U-shaped cold bending with a springback angle of ≤2°.

[0036] In a specific embodiment, the microstructure of cold-rolled DH980 steel includes ferrite, martensite, retained austenite and bainite; the volume percentages are as follows: ferrite: 20% to 45%, martensite: 30% to 60%, retained austenite: 3% to 12%, bainite: 3% to 15%; the retained austenite is in a film-like form with a grain size of 0.05 μm to 0.20 μm, and the film-like retained austenite is mainly distributed between bainite and martensite laths.

[0037] 2. Production technology

[0038] The present invention also discloses a preparation method of cold-rolled DH980 steel with excellent anti-rebound performance as in any embodiment of the present invention, including the following steps: electric furnace smelting, medium and thin slab continuous casting and rolling, pickling and cold rolling, continuous annealing, and skin pass rolling.

[0039] S1. In the electric furnace smelting, scrap steel is used as the furnace charge, and the mass percentage of the scrap steel in the furnace charge is 100%; the temperature of the converter molten steel is 1600°C to 1750°C.

[0040] S2. In the medium and thin slab continuous casting and rolling, a special protective slag for high-aluminum steel is used, the casting temperature is 1530°C to 1600°C, the casting machine drawing speed is 1.0 m / min to 5.5 m / min, and the thickness of the continuous casting slab is 60 mm to 115 mm; the starting rolling temperature is 1000°C to 1150°C, the final rolling temperature is ≥900°C, and the coiling temperature is 600°C to 700°C.

[0041] In a specific embodiment, the thickness of the hot-rolled steel plate is 2.0 mm to 4.5 mm, and the microstructure of the hot-rolled steel plate includes ferrite, pearlite, bainite, and a small amount of cementite / impurities; the volume percentages of the respective microstructures are as follows: ferrite: 30% to 50%, pearlite: 10% to 30%, bainite: 20% to 50%, cementite / impurities: 1% to 5%.

[0042] In a specific embodiment, the mass percentage of Li2O in the special protective slag for high-aluminum steel is 0.5% to 10.0%.

[0043] S3. In the pickling and cold rolling, the cold rolling reduction rate is 45% to 70%, and the finished product thickness is 0.8 to 2.3 mm.

[0044] S4. In the continuous annealing, the pickled and cold-rolled steel plate is continuously annealed, the strip speed is 60 m / min to 180 m / min, the furnace temperature in the soaking section is 760°C to 880°C, the soaking time is 10 s to 600 s, the slow cooling outlet temperature is 700°C to 760°C, the rapid cooling rate > 25°C / s, the rapid cooling temperature is 300°C to 450°C, the aging temperature is 350°C to 550°C, and the aging time is 60 s to 1000 s.

[0045] S5. In the skin pass rolling, the skin pass rolling process uses rolling force control, the rolling force is 1000 kN to 3500 kN, and the rolling tension is 500 kN to 2000 kN.

[0046] The following are specific embodiments

[0047] The composition of the steel in the embodiments of the present invention is shown in Table 1. The main process parameters of steel smelting, continuous casting and rolling, and the hot-rolled microstructure in the embodiments of the present invention are shown in Table 2. The main process parameters of continuous annealing of the steel in the embodiments of the present invention are shown in Table 3. The properties of the steel in the embodiments of the present invention are shown in Table 4. The microstructures of the steel in the embodiments of the present invention are shown in Table 5.

[0048] Table 1 Composition of the steel in Examples 1-6 of the present invention (%)

[0049]

[0050] Table 2 Main process parameters of steel smelting, continuous casting and rolling, and hot-rolled microstructure in Examples 1-6 of the present invention

[0051]

[0052] Table 3 Main process parameters of continuous annealing and skin pass of the steel in Examples 1-6 of the present invention

[0053]

[0054] Table 4 Properties of the steel in Examples 1-6 of the present invention

[0055]

[0056] Table 5 Microstructures of the steel in Examples 1-6 of the present invention

[0057] Embodiment Ferrite / % Martensite / % Retained austenite / % Bainite / % 1 35.8 47.7 8.9 7.6 2 30.2 57.4 6.0 6.4 3 36.6 48.6 9.1 5.7 4 43.5 41.2 11.7 3.6 5 41.2 38.6 8.2 12.0 6 42.6 41.8 6.9 8.7

[0058] As can be seen from the above embodiments, by using the composition design of the present invention in combination with the process of "large proportion of scrap steel + electric furnace smelting, continuous casting and rolling, and continuous annealing process", cold-rolled DH980 steel with excellent anti-springback performance is prepared, with a yield strength ≥ 700 MPa, a tensile strength of 980 MPa - 1100 MPa, an elongation after fracture of A80 ≥ 15.0%, an expansion rate ≥ 60%, and a density of 6.5 g / cm 3 ~7.5 g / cm 3 ; The springback angle is ≤ 2° with a 180° U-shaped cold bend (bending radius of 2 mm), meeting the requirements of anti-springback performance, high strength and high plasticity, and excellent formability of automobiles.

[0059] The above-described embodiments merely represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the patent application. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all fall within the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A cold-rolled DH980 steel with excellent anti-rebound performance, characterized in that: The following components are included in mass percentage: C: 0.05% ~ 0.15%, Mn: 0.5% ~ 1.5%, Si: 0.1% ~ 1.0%, Al: 0.02% ~ 6.0%, Cr: 0.4% ~ 1.0%, Mo: 0.2% ~ 1.0%, Ni: 0.5% ~ 1.20%, P ≤ 0.01%, S ≤ 0.01%, N ≤ 0.005%, Mg: 0.005% ~ 0.50%, Nb: 0.01% ~ 0.50%, V: 0.01% ~ 0.50%, and 1.0% ≤ (Al + Si) ≤ 6.0%, the balance is Fe and unavoidable impurities.

2. The cold-rolled DH980 steel with excellent anti-rebound performance according to claim 1, characterized in that: The yield strength of the cold-rolled DH980 steel is ≥700MPa, the tensile strength is 980MPa-1100MPa, the A80 elongation after fracture is ≥15.0%, the hole expansion rate is ≥60%, and the density is 6.5g / cm 3 ~7.5g / cm 3 ; Rebound angle ≤2°.

3. The cold-rolled DH980 steel with excellent anti-rebound performance according to claim 1, characterized in that: The microstructure of the cold-rolled DH980 steel includes ferrite, martensite, retained austenite and bainite; the volume percentages are as follows: ferrite: 20% to 45%, martensite: 30% to 60%, retained austenite: 3% to 12%, bainite: 3% to 15%; The residual austenite is in a film-like form, with a grain size of 0.05 μm to 0.20 μm, and the film-like residual austenite is mainly distributed between bainite and martensite laths.

4. A method for preparing cold-rolled DH980 steel with excellent anti-rebound performance as claimed in any one of claims 1 to 3, characterized in that: The following steps are involved: Electric furnace smelting, medium and thin slab continuous casting and rolling, pickling and cold rolling, continuous annealing and skin-passing; In the continuous annealing, the steel plate is continuously annealed after pickling and cold rolling, the belt speed is 60m / min-180m / min, the furnace temperature of the soaking section is 760°C-880°C, the soaking time is 10s-600s, the slow cooling outlet temperature is 700°C-760°C, the rapid cooling rate is greater than 25°C / s, the rapid cooling temperature is 300°C-450°C, the aging temperature is 350°C-550°C, and the aging time is 60s-1000s.

5. The preparation method according to claim 4, characterized in that: In the electric furnace smelting, scrap steel is used as the furnace charge, and the mass percentage of the scrap steel in the furnace charge is 100%; the temperature of the molten steel in the converter is 1600°C to 1750°C.

6. The preparation method according to claim 4, characterized in that: In the continuous casting and rolling of the medium-thin slab, special protective slag for high aluminum steel is used, the casting temperature is 1530℃~1600℃, the casting machine pulling speed is 1.0m / min~5.5m / min, the thickness of the continuous casting slab is 60mm~115mm; the starting rolling temperature is 1000℃~1150℃, the final rolling temperature is ≥900℃, and the coiling temperature is 400℃~600℃.

7. The preparation method according to claim 6, characterized in that: The thickness of the steel plate after hot rolling is 2.0mm~4.5mm, and the microstructure of the hot rolled steel plate includes ferrite, pearlite, bainite, and a small amount of cementite / impurities; the volume percentage of each microstructure is as follows: ferrite: 30%~50%, pearlite: 10%~30%, bainite: 20%~50%, cementite / impurities: 1%~5%.

8. The preparation method according to claim 4, characterized in that: In the pickling cold rolling, the cold rolling reduction rate is 45% to 70%, and the finished product thickness is 0.8 mm to 2.3 mm.

9. The preparation method according to claim 4, characterized in that: In the skin-finishing, the skin-finishing process is controlled by rolling force, the rolling force is 1000kN to 3500kN, and the rolling tension is 500kN to 2000kN.

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