400 MPa-grade high-strength cold-rolled steel for automobile inner and outer plates based on ultra-fast heating and continuous annealing technology and preparation method of 400 MPa-grade high-strength cold-rolled steel

Abstract

The invention belongs to the field of automobile steel manufacturing, and particularly relates to 400 MPa-grade high-strength cold-rolled automobile inner and outer plate steel based on an ultra-fast heating and continuous annealing technology and a preparation method of the 400 MPa-grade high-strength cold-rolled automobile inner and outer plate steel. The steel comprises the following chemical components in percentage by mass: 0.003%-0.008% of C, 0.3%-0.7% of Mn, 0.01%-0.08% of Si, 0.02%-0.06% of Ti, less than or equal to 0.03% of P, less than or equal to 0.002% of N, less than or equal to 0.005% of S and the balance of Fe and inevitable impurities. The yield strength of the steel plate is 240-320 MPa, the tensile strength is 400 MPa or more, the percentage elongation after fracture A80 is 36% or more, the plastic strain ratio r is 2.0 or more, and the strain hardening index n is 0.2 or more.

Description

Technical Field

[0001] This invention belongs to the field of automotive steel manufacturing, and particularly relates to a 400MPa grade high-strength cold-rolled steel for automotive inner and outer panels based on ultra-fast heating and continuous annealing technology and its preparation method. Background Technology

[0002] With the rapid development of the automotive industry, environmental pollution has become increasingly serious, making the development of environmentally friendly vehicles an inevitable trend for the future automotive industry. Therefore, the automotive industry has proposed lightweighting measures. It is well known that the amount of steel used in a car body is one of the key factors determining a car's weight. Therefore, reducing the amount of steel used in the car body and lowering the vehicle's weight has become one of the important ways to achieve the development of environmentally friendly vehicles. However, the safety performance of a vehicle is paramount and cannot be ignored. The application of high-strength steel plates provides an effective solution to this problem. By using high-strength steel plates, while reducing the amount of steel used in the car body, it is possible to still ensure good safety performance of the vehicle, and even further improve the vehicle's impact resistance, thereby better ensuring the safety and service life of the vehicle.

[0003] Currently, steels used for stamping automotive body panels typically employ interstitial steels and bake-hardening steels. These products generally utilize solid solution strengthening (such as adding elements like P and Mn) and precipitation strengthening (containing precipitates like Nb, V, and Ti) to improve steel sheet strength; however, the strength grades of commonly used products still do not exceed 340 MPa. Therefore, developing high-strength steels for automotive inner and outer body panels with excellent formability will contribute to advancing the process of automotive lightweighting.

[0004] Chinese invention patent CN200810119821.1 discloses a 440MPa niobium-containing high-strength IF steel and its preparation method. The steel's chemical composition by weight percentage is: C: 0.005%–0.007%, Si: 0.02%–0.03%, Mn: 1.2%–2.1%, P < 0.08%, S < 0.006%, N ≤ 0.003%, Nb: 0.05%–0.11%, B: 0.0005%–0.002%, Cr: 0.2%–0.5%, Ti: 0.005%–0.01%, Al: 0.01%–0.04%, with the remainder being Fe and unavoidable impurities. The production process is: smelting → hot rolling → cold rolling → continuous annealing or hot-dip galvanizing. The controlled process parameters are as follows: hot-rolled slab heating temperature 1200–1250°C, holding time 0.5–1.5 hours, final rolling temperature 910–920°C, coiling temperature 640–680°C, cold rolling reduction 80%, annealing temperature 840–860°C, and annealing holding time 100–120 seconds. The resulting steel plate exhibits a strength ≥440MPa and an r-value ≥1.5, demonstrating excellent deep-drawing properties and high-strength IF steel. This invention employs a traditional IF steel production method, which is entirely different from this invention. Furthermore, this invention, based on solid solution strengthening with Mn (1.2–2.1%) and Si, adds a significant amount of Nb to achieve grain refinement and precipitation strengthening, ultimately resulting in high-strength steel. However, excessive precipitates in the steel plate negatively impact formability, and the addition of the precious metal Nb increases costs, which differs from the composition and process design philosophy of this invention.

[0005] Chinese invention patent CN201010291524.2 discloses a high-strength ultrafine-grained cold-rolled IF steel with a tensile strength ≥390MPa and its production method. This product has good surface quality, formability, and resistance to secondary processing brittleness, and can be used as a substrate for galvanized sheets. The chemical composition of the steel (by weight %) is: C: 0.006~0.01, Si: 0.004~0.009, Mn: 1.1~1.6, P: 0.06~0.1, S: ≤0.01, Als: 0.05~0.06, N: ≤0.003, Nb: 0.1~0.15, B: 0.0005~0.0015, with the balance being Fe and unavoidable impurities. This invention utilizes "grain refinement and precipitation strengthening combined with existing solid solution strengthening" as a strengthening mechanism, and "adds a certain amount of Nb to form more fine niobium carbonitride compounds Nb(C,N) and disperse them. These fine precipitates help to obtain fine ferrite grains, and high strength is obtained through grain refinement strengthening and the dispersed precipitation of Nb(C,N)." This is significantly different from the present invention. Summary of the Invention

[0006] This invention provides a 400MPa grade high-strength cold-rolled steel for automotive inner and outer panels based on ultra-rapid heating and continuous annealing technology and its preparation method. By adopting rapid heating and continuous annealing technology and reasonable composition design, a high-strength and highly formable cold-rolled steel for automotive inner and outer panels is prepared, with a yield strength of 240-320MPa, a tensile strength of over 400MPa, an elongation after fracture (A80) of over 34%, a plastic strain ratio r of over 2.0, and a strain hardening index n of over 0.2.

[0007] To achieve the above objectives, the present invention employs the following technical solution:

[0008] The 400MPa grade high-strength cold-rolled steel for automotive inner and outer panels based on ultra-fast heating and continuous annealing technology is characterized by the following chemical composition by mass percentage: C: 0.003%~0.008%, Mn: 0.3%~0.7%, Si: 0.01%~0.08%, Ti: 0.02%~0.06%, P≤0.03%, N≤0.002%, S≤0.005%, with the balance being Fe and unavoidable impurities.

[0009] The microstructure of the steel plate includes ferrite and Ti(N,C) precipitates with a size of 30–70 nm and a grain size of 3–12 μm.

[0010] The steel plate has a yield strength of 240-320 MPa, a tensile strength of 400 MPa or more, an elongation after fracture (A80) of 36% or more, a plastic strain ratio (r) of 2.0 or more, and a strain hardening index (n) of 0.2 or more.

[0011] The rationale for the alloy design of this invention is as follows:

[0012] Carbon (C) is an important element in steel, playing a crucial role in interstitial solid solution strengthening. It also forms fine carbides with alloying elements in steel, contributing to precipitation strengthening. However, excessive C content reduces the plasticity of the steel sheet and decreases the {111} texture density, leading to a decrease in the r-value and n-value, thus affecting the formability of the steel sheet. Therefore, this invention requires a C content of 0.003% to 0.008%.

[0013] Mn: Mn is the most important alloying element in this invention and a major strengthening element that contributes to increased strength. However, excessive Mn content can lead to inclusions in the steel, reducing the r-value of the steel plate. Furthermore, it is prone to oxidation in subsequent processes, causing surface quality problems. Therefore, this invention requires an Mn content of 0.3% to 0.7%.

[0014] Si: Si exists in steel as substitutional atoms, effectively strengthening the ferrite matrix, with a strengthening effect better than that of Mn. However, increasing the silicon content in steel reduces its deep-drawing properties. Therefore, this invention requires a Si content of 0.01% to 0.08%.

[0015] Ti: Ti is an important additive element in steel. It readily reacts with nitrogen (N) to form TiN, thus eliminating harmful N and improving the formability of the steel sheet. Simultaneously, Ti forms carbides with carbon (C) during hot rolling. Therefore, Ti-containing precipitates hinder grain boundary migration during dynamic recrystallization in hot rolling, which is beneficial for refining the initial microstructure of the steel sheet and ultimately achieving a fine microstructure. However, excessive Ti can cause the precipitation of FeTiP phase in the steel, which is detrimental to the formation of a favorable γ-texture ({111} plane texture) during recrystallization, severely reducing the γ-value of the steel sheet. Therefore, this invention requires a Ti content of 0.02% to 0.06%.

[0016] P: P is a harmful element in steel, easily causing grain boundary segregation and reducing the performance of the steel plate. Therefore, this invention requires P content to be ≤0.030%.

[0017] N: N is an impurity element in steel, and the lower its content, the better. Therefore, this invention requires that the N content be ≤0.002%.

[0018] S: S is an impurity element in steel, which readily reacts with Mn to form MnS, deteriorating the performance of the steel plate. Therefore, its content should be as low as possible. Thus, this invention requires an S content ≤ 0.005%.

[0019] The method for preparing 400MPa grade high-strength cold-rolled steel for automotive inner and outer panels based on ultra-fast heating and continuous annealing technology includes the following steps: smelting, hot rolling, pickling, cold rolling, continuous annealing and galvanizing, and finishing. Specific steps include:

[0020] 1) Smelting: The alloy composition within the above range is obtained by smelting in a converter or refining furnace, and then cast into a billet.

[0021] 2) Hot rolling: The heating temperature is between 1220 and 1250℃, and the furnace time is ≥120 minutes. This limited temperature and time ensures uniform diffusion of elements, eliminates element segregation present in the cast billet, and makes the steel plate's microstructure uniform. At the same time, this limited temperature reduces the hot rolling deformation resistance of the steel plate and eliminates defects. If the heating temperature is too high, the original austenite grains will grow excessively, causing the steel plate to overheat.

[0022] The roughing temperature is between 1120 and 1165℃, the finishing temperature is between 1055 and 1090℃, and the final rolling temperature is ≥905℃; this ensures sufficient dynamic recrystallization of the steel plate's microstructure during the hot rolling process and refines the grains.

[0023] The coiling temperature is between 675 and 710℃, and after coiling, it is cooled to room temperature at a rate >20℃ / s. A coiling temperature of 675–710℃ facilitates the complete precipitation and dispersion of Ti carbides in the steel sheet, eliminating interstitial solid solution atoms while refining the grains. A cooling rate >20℃ / s after coiling prevents the growth of ferrite grains and precipitated phases, thus ensuring fine and uniform grains, which is beneficial for improving the material's strength and plasticity.

[0024] 3) Pickling: Removes iron oxides from the surface of hot-rolled steel sheets to ensure the surface quality of cold-rolled sheets.

[0025] 4) Cold rolling: The cold rolling reduction rate is controlled at 65%–80%. A larger cold rolling reduction provides sufficient driving force for the recovery, recrystallization, and phase transformation of the cold-rolled fiber structure in subsequent heat treatment stages, ensuring the uniformity of recrystallization and phase transformation of the overall steel sheet structure, and also enabling the steel sheet to obtain a high-density {111} surface texture. However, an excessively high rolling reduction rate will increase the load on the cold rolling mill, and may not guarantee the achievement of the target thickness. Therefore, the cold rolling reduction rate in this invention is controlled at 65%–80%.

[0026] 5) Continuous annealing: The rapid heating rate is 50–150℃ / s, the isothermal annealing temperature is 820–850℃, and the annealing time is 5–20s. The mechanism is that during the heating and annealing stages, the cold-rolled fiber structure mainly undergoes structural recovery, recrystallization, and phase transformation, resulting in a decrease in dislocation density, grain boundary migration, and a transformation towards equiaxed grains. However, rapid heating inhibits recovery, increases the temperature for complete recrystallization, and reduces the time required for complete recrystallization. Therefore, compared to traditional low heating rates, rapid heating provides more nucleation sites for recrystallization (at intragranular defects or grain boundaries), increasing the nucleation rate and resulting in finer austenite grains during the annealing stage. The isothermal temperature of 820–850℃ and the annealing time of 5–20s ensure complete recrystallization of the deformed steel sheet structure, forming a strong recrystallized {111} texture, while simultaneously preventing excessive grain growth.

[0027] The steel sheet is slowly cooled to 665–710℃ at a cooling rate of 10–15℃ / s, then rapidly cooled to room temperature at a rate greater than 25℃ / s. Finally, the steel sheet enters a finishing machine for shape adjustment, with the finishing elongation controlled at 0.4%–1.2%. During the slow cooling stage, the Ti-containing precipitates will grow. The slow cooling temperature of 665–710℃ and the slow cooling rate of 10–15℃ / s can ensure that the size of the precipitates is reasonable (30–70 nm), avoiding an increase in strength and a decrease in plasticity due to excessively fine precipitates.

[0028] The final microstructure of the steel plate of this invention consists of ferrite + Ti precipitates (30-70 nm). Through the aforementioned low-cost composition design, combined with a rapid heating annealing process, the goal of achieving fine grain strength and toughness is achieved. The yield strength is 240-320 MPa, the tensile strength is above 400 MPa, the elongation after fracture (A80) is above 36%, the plastic strain ratio r is above 2.0, and the strain hardening index n is above 0.2.

[0029] Compared with the prior art, the beneficial effects of the present invention are:

[0030] 1. This invention adopts a low-cost composition design, abandoning the traditional method of adding microalloying elements such as Nb and V to achieve fine grains, and adopts a rapid heating continuous annealing process to realize the production of steel for automotive inner and outer panels with high formability.

[0031] 2. This invention adopts a rapid heating continuous annealing production method, which effectively improves production efficiency.

[0032] 3. Steel sheets have excellent formability and high dent resistance, making them particularly suitable for stamping and forming automotive body panels, such as door panels, hoods, and roof panels. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of this invention clearer, the specific implementation methods of this invention will be further described below in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of this invention. These embodiments are only general descriptions of the content of this invention and do not limit the content of this invention.

[0034] Table 1 lists the chemical composition of the steels in the examples, Table 2 lists the hot rolling and cold rolling process parameters of the steels in the examples, Table 3 lists the continuous annealing process parameters of the steels in the examples, and Table 4 gives the mechanical properties of the steels in the examples.

[0035] Table 1 Chemical composition of the steel in the examples, wt%

[0036]

[0037] Table 2 Hot rolling and cold rolling processes for steel in the examples

[0038]

[0039] Table 3. Annealing process for steel in the examples

[0040]

[0041] Table 4 Mechanical properties of the steel and iron content of the coating in the examples

[0042]

[0043] As can be seen from the above embodiments, high-strength and highly formable cold-rolled steel for automotive inner and outer panels was prepared through low-cost composition design and innovative rapid heating and annealing process. Its yield strength is 240-320 MPa, tensile strength is above 400 MPa, elongation after fracture (A80) is above 36%, plastic strain ratio r is above 2.0, and strain hardening index n is above 0.2.

Claims

1. A 400 MPa grade high strength cold rolled steel sheet for automotive inner and outer panel based on ultra fast heating continuous annealing technology, characterized in that, The chemical composition of the steel is as follows in mass percent: C: 0.003% to 0.008%, Mn: 0.3% to 0.7%, Si: 0.01% to 0.08%, Ti: 0.02% to 0.06%, P≤0.03%, N≤0.002%, S≤0.005%, and the balance of Fe and inevitable impurities.

2. The 400 MPa grade high strength cold rolled steel sheet for automotive inner and outer panel based on ultra-fast heating continuous annealing technology according to claim 1, characterized in that, The microstructure of the steel sheet comprises ferrite and Ti(N,C) precipitates, the size of the precipitates is 30 to 70 nm, and the grain size is 3 to 12 μm.

3. The 400 MPa grade high strength cold rolled steel sheet for automotive inner and outer panel based on ultra-fast heating continuous annealing technology according to claim 1, characterized in that, The yield strength of the steel sheet is 240 to 320 MPa, the tensile strength is above 400 MPa, the elongation A80 after fracture is above 36%, the plastic strain ratio r is above 2.0, and the strain hardening index n is above 0.

2.

4. The method for preparing 400 MPa grade high strength cold rolled steel sheet for automotive inner and outer panel based on ultra-fast heating continuous annealing technology according to any one of claims 1-3, comprising the following steps: The steel sheet is smelted, hot-rolled, pickled, cold-rolled, continuously annealed, and finished, characterized in that, the continuous annealing comprises a fast heating rate of 50 to 150 ℃ / s, an annealing isothermal temperature of 820 to 850 ℃, and an annealing time of 5 to 20 s; slow cooling at a slow cooling rate of 10 to 15 ℃ / s to 665 to 710 ℃, and then fast cooling at a fast cooling rate of >25 ℃ / s to room temperature.

5. The method of producing a 400 MPa grade high strength cold rolled steel sheet for automotive inner and outer panel based on ultra-fast heating continuous annealing technology according to claim 4, characterized in that, The hot rolling comprises a heating temperature of 1220 to 1250 ℃, a furnace time of ≥120 min, a rough rolling temperature of 1120 to 1165 ℃, a finish rolling temperature of 1055 to 1090 ℃, a final rolling temperature of ≥905 ℃, and a coiling temperature of 675 to 710 ℃, and then cooling to room temperature at a cooling rate of >20 ℃ / s after coiling.

6. The method of producing a 400 MPa grade high strength cold rolled steel sheet for automotive inner and outer panel based on ultra-fast heating continuous annealing technology according to claim 4, characterized in that, The cold rolling reduction is controlled to be 65% to 80%.

7. The method of producing a 400 MPa grade high strength cold rolled steel sheet for automotive inner and outer panel based on ultra-fast heating continuous annealing technology according to claim 4, characterized in that, The finishing elongation is controlled to be 0.4% to 1.2%.

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