A method for manufacturing a 780mpa grade reinforced formability cold rolled dh steel sheet for automotive structures

Abstract

The application discloses a preparation method of a 780MPa-grade reinforced formability cold-rolled DH steel plate for automobile structure, which comprises the following steps: smelting-continuous casting, wherein the chemical composition of the molten steel for the caster is as follows: C: 0.12-0.18%, Si: 0.30-0.60%, Mn: 2.00-2.50%, P≤0.020%, S≤0.010%, Alt: 0.60-1.00%, Cr: 0.10-0.28%, Ti: 0.005-0.020%, Ca: 0.0008-0.0025%, N: 0.0030-0.0070%, and the rest is Fe and impurities; hot rolling; pickling and cold rolling; continuous annealing and skin passing. The mechanical properties of the steel plate meet the following requirements: yield strength R p0.2 : 440-550MPa, tensile strength R m ≥780MPa, elongation A 50mm ≥19%, and strain hardening index n0≥0.13.

Description

Technical Field

[0001] This invention relates to the field of metallurgical sheet technology, and in particular to a method for preparing 780MPa grade reinforced formable cold-rolled DH steel sheet for automotive structures. Background Technology

[0002] As the global automotive industry rapidly moves towards lightweighting, energy conservation, emission reduction, and enhanced safety, automotive structural components are facing increasingly stringent requirements for the strength and formability of steel. On the one hand, to reduce overall vehicle energy consumption, high-strength steel is needed to reduce the weight of structural components. Advanced high-strength steel of 780MPa grade has become the mainstream material choice for core structural components such as automotive anti-collision beams, B-pillar reinforcement plates, and floor beams because it can reduce weight by 20%-30% while maintaining structural load-bearing capacity. On the other hand, the stamping process of complex structural components (such as flanging, bending, and hole expansion) places extremely high demands on the local formability of steel, requiring a balance between low yield strength ratio, high work hardening index, and excellent deformation coordination to avoid defects such as cracking and wrinkling during the forming process.

[0003] In terms of manufacturing processes, existing technologies still face numerous bottlenecks: some solutions improve hardenability by adding large amounts of alloying elements such as Cr, Mo, and Nb, but this not only increases production costs but also deteriorates the weldability of the material; another process obtains bainitic structure through hot rolling followed by cooling to ensure uniformity, but the high hardness of the bainitic raw material increases the deformation resistance of subsequent cold rolling, increasing processing difficulty and cost. Furthermore, existing 780MPa grade steel plates generally suffer from insufficient strength-ductility product and low forming qualification rate, making it difficult to meet the comprehensive requirements of automotive structural components for "high strength-high formability-low cost".

[0004] Therefore, developing a cold-rolled DH steel sheet and its preparation method that can significantly improve local formability while ensuring a strength of 780MPa and reducing costs through precise organization and process optimization is key to solving the current technical pain points of steel used in automotive structural components and promoting the coordinated development of automotive lightweighting and safety, and has important industrial application value. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing 780MPa grade reinforced formable cold-rolled DH steel sheet for automotive structures. This invention provides a 780MPa grade reinforced formable cold-rolled DH steel sheet for automotive structures that exhibits excellent strength and plasticity matching, high elongation, and good formability. It can meet users' high-end material selection needs, enabling applications such as lightweighting, high strength, and improved weldability in vehicle body structures. The mechanical properties meet the following requirements: yield strength R... p0.2 440~550MPa, tensile strength R m ≥780MPa, elongation A 50mm ≥19%, strain hardening index n0≥0.15.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] This invention discloses a method for preparing 780MPa grade reinforced formable cold-rolled DH steel sheet for automotive structures, comprising:

[0008] The smelting-continuous casting production process focuses on controlling the purity of molten steel, precise alloying, and ensuring the quality of cast billets. It employs a process flow of "hot metal pretreatment—converter—LF refining—RH vacuum treatment—slab continuous casting." In the converter top and bottom blowing smelting stage, oxygen blowing is used for decarburization and temperature rise. During the converter tapping process, aluminum ferromanganese, ferrosilicon, ferrochrome, and titanium ferroalloys are added in proportion for deoxidation and alloying, controlling P and S content to prevent over-oxidation of the molten steel. The tapping temperature is stabilized at 1620~1660℃. In the LF refining stage, sulfur and oxygen impurities in the molten steel are further removed by creating reducing slag, and aluminum ferromanganese, ferrosilicon, and ferrosilicon alloys are added to adjust to target values. Subsequently, RH vacuum treatment is performed, with a vacuum degree requirement of ≤2.6 mbar and a vacuum time of ≥15 min, achieving dehydrogenation, denitrification, and inclusion flotation removal, thus improving the purity of the molten steel. During RH vacuum treatment, titanium ferroalloys are added according to the steel composition to adjust the composition, ensuring a circulating pure degassing time of ≥ 7 min; After RH vacuum treatment, calcium treatment is performed, and the soft blowing time is guaranteed to be ≥8 min after calcium treatment; Adjust the molten steel composition to the target composition according to the steel composition. The chemical composition of the molten steel supplied to the casting machine by mass percentage is C: 0.12~0.18%, Si: 0.30~0.60%, Mn: 2.00~2.50%, P≤0.020%, S≤0.010%, Alt: 0.60~1.00%, Cr: 0.10~0.28%, Ti: 0.005~0.020%, Ca: 0.0008~0.0025%, N: 0.0030~0.0070%, with the remainder being Fe and impurities; The superheat of the molten steel for slab continuous casting is controlled at 15~40℃, and the slab casting speed is controlled at 0.8~1.6m / min; With the help of electromagnetic stirring technology, the grain structure of the slab can be refined, reducing central segregation and porosity defects. The obtained continuous casting slab is used for subsequent hot rolling processes.

[0009] (2) Hot rolling production process: slab heating—high pressure water descaling—fixed width press → E1R1 roughing mill rolling → E2R2 roughing mill rolling → heat insulation cover → flying shear head and tail → high pressure water descaling → F1~F7 finishing mill rolling → dense laminar flow cooling → coiling → pallet transport system → weighing → warehousing; slab time in the furnace is 160~260min, furnace exit temperature is 1210~1290℃, roughing is carried out using a 3+3 mode 2-stand mill. The finishing rolling process uses a 7-stand mill with an initial rolling temperature of 950~1030℃ and a final rolling temperature of 870~910℃. The hot-rolled steel strip thickness is 2.30~4.00mm. The cooling process uses an advanced, high-efficiency, dense cooling device with a maximum cooling rate of 60℃ / second. This process reduces the temperature to 540~600℃ at a cooling rate of 30~50℃ / s before coiling, which inhibits the precipitation of hard and brittle phases such as pearlite and cementite and induces a uniform transformation of ferrite and austenite.

[0010] (3) Pickling and cold rolling process flow: pickling and uncoiling → welding → tension leveling → pickling → rinsing → drying → edge trimming → rolling → coiling → weighing → marking → bundling → warehousing. The process section adopts a new and advanced shallow tank turbulent flow pickling process; a total of three pickling tanks and five rinsing tanks are set up to realize the circulation and heating of acid liquid; a shutdown flushing system is provided to prevent residual acid liquid from corroding the strip steel and forming shutdown spots when the machine stops; a two-bend one-straightening tension leveling and descaling machine is set at the inlet to improve the plate shape and iron oxide scale on the surface of the broken strip steel through bending and stretching, accelerate the chemical reaction, and improve the pickling speed and acid content. To improve washing quality and shorten pickling time, hot-rolled steel strip is pickled to remove surface iron oxide scale. The elongation rate is ≤3.0% during tension leveling. The acid solution temperature is 75~90℃, the concentration of free acid #1 is 30~90g / L, the concentration of free acid #2 is 60~100g / L, and the concentration of free acid #3 is 90~150g / L. The rinsing water temperature is 50~80℃, the drying temperature is 100~120℃, and the pickling section speed is ≥30m / min. Afterward, it is cold rolled on a 5-stand cold rolling mill with a cold rolling reduction rate of 50~65% and a thickness of 0.80~2.00mm.

[0011] (4) Continuous annealing and leveling process flow: continuous annealing uncoiling → welding → cleaning → annealing furnace → leveling → edge trimming → surface inspection → oiling → sampling and inspection → coiling → weighing → marking → bundling → packaging → warehousing; after uncoiling the cold-rolled steel strip, continuous welding production is carried out; the continuous annealing cleaning process includes hot alkaline washing, alkaline brushing, electrolytic cleaning, desalinated water brushing, hot water rinsing and strip drying; mechanical brushing and electrolytic cleaning methods are used to remove residual rolling oil and other surface stains on the surface of cold-rolled strip steel, and alkaline solution is used to react with the rolling oil to generate water-soluble soap or glycerin, and hydrogen and oxygen bubbles are generated on the surface of the strip steel by electrolysis to destroy the oil film and remove the grease; the alkaline solution temperature is 55~85℃, the conductivity of 1# and 2# alkaline washing is 20~45ms, the conductivity of electrolytic cleaning is 45~85ms, and the drying temperature is 100~135℃; the annealing process is as follows: The furnace consists of an inlet seal, preheating section, heating section, soaking section, slow cooling section, rapid cooling section, over-aging section, secondary cooling, final cooling section, outlet seal, and water quenching. The furnace is filled with a nitrogen-hydrogen mixed protective gas to prevent oxidation of the strip surface during heating, and the hydrogen also reduces residual oxides on the strip surface. The continuous annealing and leveling process uses a six-roll CVC + wet leveling machine. By appropriately controlling the flatness and rolling force, the yield strength and surface hardness of the strip can be controlled to a certain extent. The furnace speed is 60~150 m / min, the soaking temperature is 800~830℃, slow cooling to 660~700℃, then further cooling to 310~350℃. A rapid cooling box with 15~30% high hydrogen content is used to cool the strip. The rapid cooling rate is 25℃ / s, the over-aging temperature is 310~350℃, and the flattening elongation is 0.20%~0.40%.

[0012] Furthermore, the final cooling temperature of the final cooling section is ≤150℃.

[0013] Furthermore, the water quenching temperature is 60℃.

[0014] Furthermore, the chemical composition of the steel plate by weight percentage is C: 0.14~0.16%, Si: 0.40%, Mn: 2.07-2.20%, P: 0.010%, S: 0.001%, Alt: 0.75~0.85%, Cr: 0.15~0.20%, Ti: 0.018%, Ca: 0.0009~0.0012%, N: 0.0035~0.0038%, with the remainder being Fe and impurities.

[0015] Furthermore, the chemical composition of the steel plate by weight percentage is C: 0.14%, Si: 0.40%, Mn: 2.07%, P: 0.010%, S: 0.001%, Alt: 0.75%, Cr: 0.15%, Ti: 0.018%, Ca: 0.0009%, N: 0.0035%, with the remainder being Fe and impurities.

[0016] Furthermore, the chemical composition of the steel plate by weight percentage is C: 0.16%, Si: 0.40%, Mn: 2.20%, P: 0.010%, S: 0.001%, Alt: 0.85%, Cr: 0.20%, Ti: 0.018%, Ca: 0.0012%, N: 0.0038%, with the remainder being Fe and impurities.

[0017] Furthermore, the mechanical properties of the steel plate satisfy the following: yield strength R p0.2 442~480MPa, tensile strength R m 790~842MPa, elongation A 50mm : 19.36~24.55%.

[0018] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0019] This invention utilizes 780MPa grade reinforced formable cold-rolled DH steel sheet for automotive structures, exhibiting excellent strength and plasticity matching, high elongation, and superior formability. This enables lightweight, high-strength vehicle body structures with improved weldability. The mechanical properties meet the following requirements: yield strength R... p0.2 440~550MPa, tensile strength R m ≥780MPa, elongation A 50mm ≥19%, strain hardening index n0≥0.13. Attached Figure Description

[0020] The present invention will be further described below with reference to the accompanying drawings.

[0021] Figure 1 This is the main process flow of the continuous annealing operation;

[0022] Figure 2 Heating curves for 780MPa grade reinforced formability cold-rolled DH steel;

[0023] Figure 3 This is a microstructure diagram of an embodiment of the present invention. Detailed Implementation

[0024] The present invention will be described in detail below through specific embodiments. These embodiments are only intended to help understand the present invention and do not limit the scope of the present invention.

[0025] Example

[0026] According to the above steelmaking process requirements, the actual chemical composition of the slab is shown in Table 1.

[0027] Table 1 Chemical composition (wt%) of embodiments of the present invention

[0028]

[0029] According to the above hot rolling process requirements, the hot rolling thickness is 2.30~4.00mm, and the actual process is shown in Table 2.

[0030] Table 2 Hot rolling process of embodiments of the present invention

[0031]

[0032] According to the above requirements for cold rolling annealing and leveling processes, the cold rolling thickness is 0.80~2.00mm. The actual cold rolling annealing and leveling processes are shown in Table 3.

[0033] Continuous annealing and leveling process: After uncoiling the chilled hardened steel strip, continuous welding production is carried out. The furnace speed is 60~150m / min, the soaking temperature is 800~830℃, the slow cooling is 660~700℃, and then the cooling is 310~350℃. The over-aging temperature is 310~350℃, and the leveling elongation is 0.20%~0.40%.

[0034] Table 3. Continuous shearing and leveling processes in embodiments of the present invention.

[0035]

[0036] The mechanical properties of the steel strip in this embodiment of the invention were tested, and the test results are shown in Table 4.

[0037] Table 4 Mechanical properties of steel strips in embodiments of the present invention

[0038]

[0039] As shown in Table 4, the mechanical properties of the 780MPa grade reinforced formable cold-rolled DH steel sheet for automotive structures and its preparation method of the present invention are as follows: Yield strength R p0.2 442~480MPa, tensile strength R m 790~842MPa, elongation A 50mm The yield is 19.36%~24.55%. Based on the above, the steel strip products obtained by the process have been used by users and have suitable performance, so they can be promoted for widespread use.

[0040] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for preparing 780MPa grade reinforced formable cold-rolled DH steel sheet for automotive structures, characterized in that: include: The smelting-continuous casting production process focuses on controlling the purity of molten steel, precise alloying, and ensuring the quality of cast billets. It employs a process flow of "hot metal pretreatment—converter—LF refining—RH vacuum treatment—slab continuous casting." In the converter top and bottom blowing smelting stage, oxygen blowing is used for decarburization and temperature rise. During the converter tapping process, aluminum ferromanganese, ferrosilicon, ferrochrome, and titanium ferroalloys are added in proportion for deoxidation and alloying, controlling P and S content to prevent over-oxidation of the molten steel. The tapping temperature is stabilized at 1620~1660℃. In the LF refining stage, sulfur and oxygen impurities in the molten steel are further removed by creating reducing slag, and aluminum ferromanganese, ferrosilicon, and ferrosilicon alloys are added to adjust to target values. Subsequently, RH vacuum treatment is performed, with a vacuum degree requirement of ≤2.6 mbar and a vacuum time of ≥15 min, achieving dehydrogenation, denitrification, and inclusion flotation removal, thus improving the purity of the molten steel. During RH vacuum treatment, titanium ferroalloys are added according to the steel composition to adjust the composition, ensuring a circulating pure degassing time of ≥ 7 min; After RH vacuum treatment, calcium treatment is performed, and the soft blowing time is guaranteed to be ≥8 min after calcium treatment; Adjust the molten steel composition to the target composition according to the steel composition. The chemical composition of the molten steel supplied to the casting machine by mass percentage is C: 0.12~0.18%, Si: 0.30~0.60%, Mn: 2.00~2.50%, P≤0.020%, S≤0.010%, Alt: 0.60~1.00%, Cr: 0.10~0.28%, Ti: 0.005~0.020%, Ca: 0.0008~0.0025%, N: 0.0030~0.0070%, with the remainder being Fe and impurities; The superheat of the molten steel for slab continuous casting is controlled at 15~40℃, and the slab casting speed is controlled at 0.8~1.6m / min; With the help of electromagnetic stirring technology, the grain structure of the slab can be refined, reducing central segregation and porosity defects. The obtained continuous casting slab is used for subsequent hot rolling processes. (2) Hot rolling production process: slab heating—high pressure water descaling—fixed width press → E1R1 roughing mill rolling → E2R2 roughing mill rolling → heat insulation cover → flying shear head and tail → high pressure water descaling → F1~F7 finishing mill rolling → dense laminar flow cooling → coiling → pallet transport system → weighing → warehousing; slab time in the furnace is 160~260min, furnace exit temperature is 1210~1290℃, roughing is carried out using a 3+3 mode 2-stand mill. The finishing rolling process uses a 7-stand mill with an initial rolling temperature of 950~1030℃ and a final rolling temperature of 870~910℃. The hot-rolled steel strip thickness is 2.30~4.00mm. The cooling process uses an advanced, high-efficiency, dense cooling device with a maximum cooling rate of 60℃ / second. This process reduces the temperature to 540~600℃ at a cooling rate of 30~50℃ / s before coiling, which inhibits the precipitation of hard and brittle phases such as pearlite and cementite and induces a uniform transformation of ferrite and austenite. (3) Pickling and cold rolling process flow: pickling and uncoiling → welding → tension leveling → pickling → rinsing → drying → edge trimming → rolling → coiling → weighing → marking → bundling → warehousing. The process section adopts a new and advanced shallow tank turbulent flow pickling process; a total of three pickling tanks and five rinsing tanks are set up to realize the circulation and heating of acid liquid; a shutdown flushing system is provided to prevent residual acid liquid from corroding the strip steel and forming shutdown spots when the machine stops; a two-bend one-straightening tension leveling and descaling machine is set at the inlet to improve the plate shape and iron oxide scale on the surface of the broken strip steel through bending and stretching, accelerate the chemical reaction, and improve the pickling speed and acid content. To improve washing quality and shorten pickling time, hot-rolled steel strip is pickled to remove surface iron oxide scale. The elongation rate is ≤3.0% during tension leveling. The acid solution temperature is 75~90℃, the concentration of free acid #1 is 30~90g / L, the concentration of free acid #2 is 60~100g / L, and the concentration of free acid #3 is 90~150g / L. The rinsing water temperature is 50~80℃, the drying temperature is 100~120℃, and the pickling section speed is ≥30m / min. Afterward, it is cold rolled on a 5-stand cold rolling mill with a cold rolling reduction rate of 50~65% and a thickness of 0.80~2.00mm. (4) Continuous annealing and leveling process flow: continuous annealing uncoiling → welding → cleaning → annealing furnace → leveling → edge trimming → surface inspection → oiling → sampling and inspection → coiling → weighing → marking → bundling → packaging → warehousing; after uncoiling the cold-rolled steel strip, continuous welding production is carried out; the continuous annealing cleaning process includes hot alkaline washing, alkaline brushing, electrolytic cleaning, desalinated water brushing, hot water rinsing and strip drying; mechanical brushing and electrolytic cleaning methods are used to remove residual rolling oil and other surface stains on the surface of cold-rolled strip steel, and alkaline solution is used to react with the rolling oil to generate water-soluble soap or glycerin, and hydrogen and oxygen bubbles are generated on the surface of the strip steel by electrolysis to destroy the oil film and remove the grease; the alkaline solution temperature is 55~85℃, the conductivity of 1# and 2# alkaline washing is 20~45ms, the conductivity of electrolytic cleaning is 45~85ms, and the drying temperature is 100~135℃; the annealing process is as follows: The furnace consists of an inlet seal, preheating section, heating section, soaking section, slow cooling section, rapid cooling section, over-aging section, secondary cooling, final cooling section, outlet seal, and water quenching. The furnace is filled with a nitrogen-hydrogen mixed protective gas to prevent oxidation of the strip surface during heating, and the hydrogen also reduces residual oxides on the strip surface. The continuous annealing and leveling process uses a six-roll CVC + wet leveling machine. By appropriately controlling the flatness and rolling force, the yield strength and surface hardness of the strip can be controlled to a certain extent. The furnace speed is 60~150 m / min, the soaking temperature is 800~830℃, slow cooling to 660~700℃, then further cooling to 310~350℃. A rapid cooling box with 15~30% high hydrogen content is used to cool the strip. The rapid cooling rate is 25℃ / s, the over-aging temperature is 310~350℃, and the flattening elongation is 0.20%~0.40%.

2. The method for preparing 780MPa grade reinforced formable cold-rolled DH steel sheet for automotive structures according to claim 1, characterized in that: The final cooling temperature of the final cooling section is ≤150℃.

3. The method for preparing 780MPa grade reinforced formable cold-rolled DH steel sheet for automotive structures according to claim 1, characterized in that: Water quenching temperature: 60℃.

4. The method for preparing 780MPa grade reinforced formable cold-rolled DH steel sheet for automotive structures according to claim 1, characterized in that: The chemical composition of the steel plate by weight percentage is C: 0.14~0.16%, Si: 0.40%, Mn: 2.07-2.20%, P: 0.010%, S: 0.001%, Alt: 0.75~0.85%, Cr: 0.15~0.20%, Ti: 0.018%, Ca: 0.0009~0.0012%, N: 0.0035~0.0038%, with the remainder being Fe and impurities.

5. The method for preparing 780MPa grade reinforced formable cold-rolled DH steel sheet for automotive structures according to claim 4, characterized in that: The chemical composition of the steel plate by weight percentage is C: 0.14%, Si: 0.40%, Mn: 2.07%, P: 0.010%, S: 0.001%, Alt: 0.75%, Cr: 0.15%, Ti: 0.018%, Ca: 0.0009%, N: 0.0035%, with the remainder being Fe and impurities.

6. The method for preparing 780MPa grade reinforced formable cold-rolled DH steel sheet for automotive structures according to claim 4, characterized in that: The chemical composition of the steel plate by weight percentage is C: 0.16%, Si: 0.40%, Mn: 2.20%, P: 0.010%, S: 0.001%, Alt: 0.85%, Cr: 0.20%, Ti: 0.018%, Ca: 0.0012%, N: 0.0038%, with the remainder being Fe and impurities.

7. The method for preparing 780MPa grade reinforced formability cold-rolled DH steel sheet for automotive structures according to claim 1, characterized in that: The mechanical properties of the steel plate satisfy: yield strength R p0.2 442~480MPa, tensile strength R m 790~842MPa, elongation A 50mm : 19.36~24.55%.

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