A bake-hardened steel sheet for electrogalvanizing and a production method thereof

Abstract

The invention discloses a bake-hardened steel plate for electro-galvanizing and a production method thereof, and belongs to the field of electro-galvanizing automobile steel production. The steel plate comprises the following components by weight percentage: C: 0.0010-0.0015%, Mn: 0.10-0.70%, Si≤0.02%, Al: 0.020-0.050%, P≤0.015%, S≤0.015%, N≤0.0030%, O≤0.0030%, and the rest is Fe and inevitable impurities. The preparation method comprises the following steps: continuous casting, hot rolling, coiling, pickling and cold rolling, continuous annealing, cooling, leveling, alkali washing, brushing, electrolytic degreasing and rinsing, and electro-galvanizing. The steel plate is manufactured into bake-hardened high-strength steel that meets the requirements of automobile parts by controlling the reasonable mass percentage of carbon and manganese elements and the matching relationship of nitrogen elements, supplemented by appropriate process flow, without adding strong carbon and nitrogen combining elements such as Nb, V, and Ti and precious elements such as Mo and Cr.

Description

Technical Field

[0001] The invention discloses a bake-hardened steel plate for electro-galvanizing and a production method thereof, and belongs to the field of production of electro-galvanized automobile steel. Background Art

[0002] Since the automobile manufacturing industry has very high requirements for exterior panels, steel plates are required to have ultra-high stamping performance and excellent surface quality. At present, most domestic and foreign automobile manufacturers use hot-dip galvanized deep-drawn plates, that is, the steel plates after continuous annealing enter the zinc pot, and the excess zinc liquid on the surface is blown away by an air knife. Its surface quality is difficult to control stably. When used by automobile factories, the protruding zinc slag on the surface of the steel plate is easy to form nodules in the stamping die, affecting subsequent stamping; in the painting process, the defects on the surface of the steel plate are easily magnified and affect the appearance of the car body. Compared with hot-dip galvanized automobile plates, the electro-galvanized zinc layer is uniform, the roughness is stable, and it has better surface quality and roughness controllability.

[0003] Metals such as Koch's gas, iron, etc. have carbon and nitrogen atoms in solid solution. These atoms cause slight lattice distortion through diffusion, and accordingly produce a residual stress concentration area around these impurity atoms. The diffusion of these impurities forms certain dislocations, which produce aging and tensile strain marks of steel.

[0004] Bake hardening steel is a high-quality automotive steel sheet that has a low yield strength before stamping. After stamping and aging treatment at baking paint temperature, the yield strength can be improved to a certain extent. It is mainly used for processing automotive outer panels. Its characteristics are the organic combination of product strength, deep drawing performance and part anti-sag performance. In bake hardening steel, the desired bake hardening value is mainly achieved by the interstitial solid solution C content, and it is generally believed that the appropriate range is 10-15ppm. In order to control the interstitial atoms in the steel, under-alloying (adding less Nb, Ti and other alloys than IF steel) is used. The ratio of the amount of Nb added to the C content satisfies 10≤Ceff≤15, where Ceff=C-Nb / 7.75.

[0005] Patent CN 105970094 B discloses a production process of electro-galvanized bake-hardened steel plate, wherein the chemical composition mass percentage of the hardened steel plate is: C: 0.0016-0.0035%, Si≤0.030%, Mn: 0.4-0.7%, P: 0.035-0.060%, S≤0.015%, Nb: 0.0006-0.0012%, Als: 0.02-0.055%, N≤0.004%, and the balance is iron and unavoidable impurities; the production method comprises the following preparation steps: 1) selecting the composition of the electro-galvanized bake-hardened steel plate; 2) smelting and casting into a slab; 3) hot rolling after conventional heating: The final rolling temperature is controlled at 900-960°C; 4) conventional water cooling to the coiling temperature and coiling: the coiling temperature is controlled at 680-740°C; 5) cold rolling: the total cold rolling reduction rate is controlled to be no more than 85%; 6) continuous annealing: the annealing soaking temperature is controlled at 780-840°C, the strip running speed is controlled between 160-280m / min, and the strip is rapidly cooled to 370-430°C; 7) leveling: the leveling elongation is ensured to be 1.1-1.8%; 8) conventional alkali washing, brushing, electrolytic degreasing and rinsing are performed; it adopts Nb under-alloying treatment, a wider production process and a higher leveling elongation.

[0006] Patent CN 109161814 B discloses an ultra-low carbon bake hardening steel plate and a production method thereof, wherein the chemical composition and mass percentage of the steel plate are: C: 0.0020-0.0030%, Si≤0.03%, Mn: 0.38-0.45%, P: 0.045-0.060%, S≤0.008%, Als: 0.025-0.055%, Nb: 0.008-0.015%, B: 0.0005-0.0015% , N≤0.0040%, solid solution C: 0.0008~0.0015%, the rest are Fe and inevitable impurity elements, the steel plate thickness is 0.6~1.5mm; tensile strength is 320~400MPa, yield strength is 220~270MPa, elongation A80≥35%, n90 value ≥0.18, r90 value ≥2.0, bake hardening value BH2: 35~50MPa; Nb is used for under-alloying treatment.

[0007] Patent CN 109321839 B discloses a method for manufacturing 240MPa grade bake hardening steel. The chemical components and mass percentages of the 240MPa grade bake hardening steel are: C: 0.0015-0.0030%; Si≤0.03%; Mn: 0.40-0.6%; P: 0.052-0.065%; S: ≤0.01%; Alt: 0.02-0.06%; Nb: 0.006-0.014%; B: 0.0003-0.0008%; N: ≤0.004%. It adopts Nb under-alloying treatment, adds B to solidify N, and adopts high temperature heating and low temperature curling in the process.

[0008] In the existing bake hardening steel, Nb is used to control the solid solution C. However, it is difficult to accurately control the Nb and C contents and Nb / C, and it is difficult to control the interstitial solid solution C content. There is no control over the solid solution N and solid solution AlN, which leads to a long yield platform, which makes it difficult to eliminate the subsequent flattening process, narrow process window requirements, large strength fluctuations, and BH 2 The value is unstable and tensile strain scar defects are easily generated during the stamping process, which affects the surface quality of the outer plate. Therefore, there is an urgent need for a production method of bake-hardened steel for electrogalvanizing that is suitable for industrial production, easy to control the process, and can reduce costs and increase efficiency for enterprises. Summary of the invention

[0009] The object of the present invention is to provide a production process preparation method of bake-hardened steel for electrogalvanizing. The method adopts (steelmaking process) molten iron pretreatment → converter smelting → (LF+RH) refining → continuous casting → (hot rolling process) billet heating → rough rolling → finishing rolling → controlled cooling → hot rolling coiling → (cold rolling process) hot rolled coil uncoiling → pickling, cold rolling → coiling → chilled coil uncoiling → continuous annealing → flattening → electroplating → finishing → surface and function inspection to produce bake-hardened steel for electrogalvanizing, which not only ensures the mechanical properties of bake-hardened steel for electrogalvanizing, but also ensures that the product has good application performance and is easy for large-scale industrial production.

[0010] In order to achieve the above-mentioned invention object, the present invention provides a composition design and process design of bake-hardened steel for electrogalvanizing, and the specific steps are as follows: Raw materials: (steelmaking process) molten iron pretreatment → converter smelting → (LF+RH) refining → continuous casting → (hot rolling process) heating of ingot → rough rolling → finishing rolling → controlled cooling → hot rolling coiling → (cold rolling process) hot rolled coil uncoiling → pickling, cold rolling → coiling → chilled coil uncoiling → continuous annealing → leveling → electroplating → finishing → surface and functional inspection. The quality requirements of bake-hardened steel for electrogalvanizing are met, which can produce bake-hardened high-strength steel with high strength, low cost, high surface quality and processing performance that meets the requirements of automotive parts by controlling the reasonable mass percentage of carbon and manganese elements and the adaptation relationship of nitrogen elements without adding precious metals such as Mo, Cr, Ti, and Nb, and supplemented by appropriate process flow. At the same time, it solves the problems of long yield platform, narrow process window, difficult to control solid solution C content, large strength fluctuation, unstable BH value and easy tensile strain scar defects during stamping and forming, which improves the low stamping pass rate of advanced outer panels.

[0011] In order to solve the above problems, the present invention provides a bake-hardening steel sheet for electrogalvanizing, which comprises the following components by weight percentage: C: 0.0010-0.0015%, Mn: 0.10-0.70%, Si≤0.02%, Al: 0.020-0.050%, P: not more than 0.015%, S: not more than 0.015%, N not more than 0.0030%, O not more than 0.0030%, no strong carbon and nitrogen combining elements such as Nb, V, Ti and precious elements such as Mo and Cr are added, and the rest are Fe and unavoidable impurities.

[0012] Furthermore, in the above technical solution, the metallographic structure of the steel plate is ferrite, the grain size is 9 to 9.5, and the inclusions are only D-type 1.0; the elongation of the steel plate A80 is ≥42%, n 90 Value ≥ 0.22, r90 value ≥ 2.1, bake hardening value BH 2 ≥37MPa.

[0013] The present invention further provides a method for preparing a bake-hardened steel sheet for electrogalvanizing, which is characterized by comprising the following specific steps:

[0014] (1) Continuous casting;

[0015] (2) Hot rolling: low temperature heating ≤ 1200°C, preheating section 860-900°C, heating section 1080-1120°C, soaking section 1140-1180°C, heating time 2.5-3h; final rolling temperature 890-930°C;

[0016] (3) Coiling;

[0017] (4) pickling and cold rolling;

[0018] (5) Continuous annealing: annealing temperature is 830-850°C, annealing speed is 120-180 m / min;

[0019] (6) Cooling: first slowly cool to 670-690°C at a cooling rate of 9-11°C / s; then quickly cool to 440-470°C at a cooling rate of 37-50°C / s; perform aging treatment at 400-440°C for 400-1000s; finally cool to 160-180°C;

[0020] (7) Leveling and electrogalvanizing pre-treatment;

[0021] (8)Electrogalvanizing.

[0022] Furthermore, in the above technical solution, in the step (1), the platform temperature is 1555-1565°C, the tundish temperature is 1540-1550°C, the superheat is less than 25°C, and the pulling speed is 0.8-1.0 m / min.

[0023] Furthermore, in the above technical solution, in step (2), a reduction ratio distribution system is adopted in which the middle rolling mill has a large reduction ratio and the two end rolling mills have a small reduction ratio; and the finishing mills F1 to F7 are provided with an automatic thickness control device AGC.

[0024] Furthermore, in the above technical scheme, in step (2), in the above technical scheme, in step (2), in the pickling process in step (4), the acid solution temperature is controlled at 65-75°C, the free acid concentration in the 1# acid tank is controlled at 50-80g / L, the free acid concentration in the 2# acid tank is controlled at 80-120g / L, and the free acid concentration in the 3# acid tank is controlled at 120-160g / L; the pickling speed is 80-220m / min.

[0025] Furthermore, in the above technical solution, in step (2) and step (4), the total cold rolling reduction rate is ≥75%; the cold rolling adopts five-stand full six-roller cold rolling, and the reduction amount is distributed according to 28-32%, 20-24%, 16-20%, 14-18%, and 10-14%.

[0026] Furthermore, in the above technical scheme, in the step (2) and in the step (7), a flattening machine is used for flattening, and the flattening elongation is 1.1% to 1.3%; the electroplating pretreatment is conventional alkaline washing, brushing, electrolytic degreasing and rinsing; the temperature of the degreasing solution in the electrolytic degreasing is controlled at 45 to 75°C, and the pH of the degreasing solution is ≥9.5.

[0027] Furthermore, in the above technical solution, in step (2) and step (3), the coiling is water-cooled to the coiling temperature and slowly cooled in the warehouse; the coiling temperature is 710-750°C.

[0028] Furthermore, in the above technical solution, in step (2) and step (8), gravity electrogalvanizing is performed, the electroplating solution is zinc sulfate solution, the zinc sulfate concentration in the electroplating solution is 100-120 g / L, the pH value is 1.3-1.8, the electroplating solution temperature is 50-60°C, the running speed of the steel plate in the electrogalvanizing tank is 80-140 m / min; the zinc layer thickness is 10-90 g / m 2 .

[0029] Design of specific parameters for each production process of bake-hardened steel for electrogalvanizing:

[0030] 1. Continuous casting: platform temperature 1560±5℃, tundish temperature 1540℃~1550℃, ensure that the overheating is controlled less than 25℃ during the production process, casting speed 0.8~1.0m / min, reduce liquid level fluctuation and control vibration mark depth. The output is a rectangular continuous casting billet.

[0031] 2. Hot rolling: low temperature heating ≤ 1200℃, preheating section 880±20℃, heating section 1100±20℃, soaking section 1160±20℃, heating time 2.5~3h, slab is heated evenly, temperature control precision is high, and the overall temperature difference of slab can be controlled within ±30℃. In order to ensure the surface quality of steel, the atmosphere in the furnace (weak oxidizing atmosphere) is strictly controlled during heating to control the final rolling temperature at 890~930℃; the reduction ratio distribution system is adopted with large reduction ratio of the middle rolling mill and small reduction ratio of the rolling mills at both ends. Finishing mills F1~F7 are equipped with thickness automatic control device AGC to effectively control the thickness precision of strip steel.

[0032] Low temperature heating and high pressure dephosphorization water ensure less primary iron oxide scale generation and improve product surface quality.

[0033] 3. High temperature coiling: Conventional water cooling to the coiling temperature and coiling, slow cooling storage in the warehouse, control the coiling temperature at 710 ~ 750 ℃;

[0034] High temperature curling and slow cooling storage in the warehouse can make AlN precipitate in the grain boundary in the form of particles instead of being dissolved inside the grain boundary. AlN precipitation at the grain boundary can ensure stable bake hardenability.

[0035] 4. Carry out pickling and cold rolling: the acid solution temperature is controlled at 65-75°C, the free acid concentration of 1# acid tank is controlled at 50-80g / L, the free acid concentration of 2# acid tank is controlled at 80-120g / L, and the free acid concentration of 3# acid tank is controlled at 120-160g / L; the pickling speed is 80-220m / min; the total cold rolling reduction rate is controlled to be ≥75%; the five-stand full six-roller cold rolling, the reduction amount is distributed as 28-32%, 20-24%, 16-20%, 14-18%, and 10-14%.

[0036] 5. Carry out continuous annealing: control the annealing temperature at 830-850°C and the annealing speed at 120-180 m / min;

[0037] 6. Cooling: The method is to first slowly cool to 680°C at a cooling rate of 9-11°C / s; then quickly cool to 440-470°C at a cooling rate of 37-50°C / s, perform aging treatment at 400-440°C for 400-1000s; finally cool to 160-180°C;

[0038] 7. Leveling on the leveling machine: Control the leveling elongation to be: 1.2±0.1%;

[0039] 8. Pre-treatment for electroplating: conventional alkaline washing, brushing, electrolytic degreasing and rinsing, the degreasing liquid temperature is controlled at 45-75°C, and the degreasing liquid pH is ≥9.5;

[0040] 9. Use gravity method for electrogalvanizing: the electroplating solution is zinc sulfate solution, the zinc sulfate concentration in the electroplating solution is 100-120g / L, the pH value is 1.3-1.8, the electroplating solution temperature is 50-60℃, and the running speed of the steel plate in the electrogalvanizing tank is 80-140m / min; the target thickness of the zinc layer can be controlled according to user requirements, ranging from 10-90g / m 2 .

[0041] The zinc coating amount of each electroplating tank is evenly distributed according to the thickness to ensure uniform zinc layer and reduce energy consumption.

[0042] Beneficial effects of the present invention

[0043] (1) The chemical composition design adopts low carbon (aluminum killed steel). By controlling the C content and AlN precipitates to generate a stable content of interstitial carbon and nitrogen atoms, the material has good and stable baking hardening characteristics, stable aging, and easy to control. The product has stable performance, high r value, high elongation, and no tensile strain marks, which improves the problem of low stamping pass rate of high-grade outer panels. (2) Reasonable composition design and production process ensure product performance stability.

[0044] (3) The finished steel has a uniform ferrite structure and contains a small amount of 0.0008-0.0015% interstitial solid solution C; N atoms fully combine with AlS (excessive) to form AlN, which precipitates as large-sized aluminum nitride particles at the grain boundaries under slow cooling during hot rolling and high-temperature curling, inhibiting the formation of solid solution AlN, thereby ensuring the plasticity of the product and inhibiting the formation of solid solution N atoms. Stable solid solution C can ensure a stable "Coriolis gas mass", which has stable bake hardening properties, ensures stable aging, a shorter yield platform, and inhibits the existence of tensile strain marks; non-metallic inclusions and gas content meet the surface quality requirements of high-end automotive products. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 A schematic diagram of the cooling process in a bake-hardened steel plate for electrogalvanizing provided by the present invention.

[0046] Figure 2 The metallographic microstructure of Example 1 DETAILED DESCRIPTION

[0047] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution 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 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 are within the scope of protection of the present invention.

[0048] Best practice:

[0049] The thickness of the bake-hardened steel for automobile outer panels in all embodiments and comparative examples is 0.80 mm. Its chemical composition and mass percentage are shown in Table 1, and the rest are Fe and unavoidable impurities.

[0050] The production method (steelmaking process) of bake-hardened steel for automobile outer panels in this embodiment and comparative example is as follows: molten iron pretreatment → converter smelting → (LF+RH) refining → continuous casting → (hot rolling process) heating of ingot → rough rolling → finishing rolling → controlled cooling → hot rolling coiling → (cold rolling process) hot rolled coil uncoiling → pickling, cold rolling → coiling → chilled coil uncoiling → continuous annealing → leveling → electroplating → finishing → surface and function inspection. The specific process steps are as follows:

[0051] Chemical composition: C: 0.0010-0.0015%, Mn: 0.10-0.70%, Si≤0.02%, Al: 0.020-0.050%, P: ≤0.015%, S: not more than 0.015%, N not more than 0.0030%, O not more than 0.0030%, no addition of strong carbon and nitrogen combining elements such as Nb, V, Ti and precious elements such as Mo and Cr, the rest are Fe and unavoidable impurities.

[0052] Continuous casting: platform temperature 1560±5℃, tundish temperature 1540℃~1550℃, ensure that the overheating is controlled less than 25℃ during the production process, casting speed 0.8~1.0 / min, reduce liquid level fluctuation and control vibration mark depth. The output is a rectangular continuous casting billet.

[0053] Hot rolling process: low temperature heating ≤ 1200℃, preheating section 880±20℃, heating section 1100±20℃, soaking section 1160±20℃, heating time 2.5 hours, slabs are heated evenly, temperature control accuracy is high, and the overall temperature difference of the slabs can be controlled within ±30℃. In order to ensure the surface quality of the steel, the atmosphere in the furnace (weakly oxidizing atmosphere) is strictly controlled during heating to control the final rolling temperature at 890~930℃; a reduction ratio distribution system is adopted with a large reduction ratio for the middle mill and a small reduction ratio for the two end mills. Finishing mills F1~F7 are equipped with an automatic thickness control device AGC to effectively control the thickness accuracy of the strip.

[0054] High temperature coiling: conventional water cooling to coiling temperature and coiling, slow cooling storage in the warehouse, control the coiling temperature at 710 ~ 750 ℃;

[0055] Carry out pickling and cold rolling: the acid solution temperature is controlled at 70℃, the free acid concentration of 1# acid tank is controlled at 70g / L, the free acid concentration of 2# acid tank is controlled at 100g / L, and the free acid concentration of 3# acid tank is controlled at 140g / L; the pickling speed is 180m / min; the total cold rolling reduction rate is controlled to be ≥75%; five-stand full six-roller cold rolling, the reduction amount is distributed as 30%, 23%, 19%, 16%, and 12%.

[0056] Perform continuous annealing: control the annealing temperature at 830-850°C and the annealing speed at 120-180 m / min;

[0057] Cooling: the method is to first slowly cool to 680°C at a cooling rate of 9-11°C / s; then quickly cool to 440-470°C at a cooling rate of 37-50°C / s, perform aging treatment at 400-440°C for 400-1000s; finally cool to 160-180°C;

[0058] Leveling is performed on a leveling machine, and the leveling elongation is controlled to be: 1.2±0.1%;

[0059] Carry out conventional alkaline washing, brushing, electrolytic degreasing and rinsing, the degreasing liquid temperature is controlled at 65℃, and the degreasing liquid pH is 10.0;

[0060] Gravity electrogalvanizing is adopted: the electroplating solution is zinc sulfate solution, the zinc sulfate concentration in the electroplating solution is 100-120g / L, the pH value is 1.4, the electroplating solution temperature is 58℃, and the running speed of the steel plate in the electrogalvanizing tank is 120m / min; the target thickness of the zinc layer can be controlled according to user requirements, ranging from 10-90g / m 2 .

[0061] The chemical composition and mass percentage of the bake-hardened steel sheets for electrogalvanizing prepared in Examples 1 to 3 of the present invention and Comparative Examples 1 and 2 are shown in Table 1, and the remainder is Fe and unavoidable impurities.

[0062] Table 1 Mass percentage of chemical components of the embodiments and comparative examples / wt%

[0063]

[0064] The bake-hardened steel sheets for electro-galvanizing obtained by the above-mentioned preparation method in Examples 1 to 3 of the present invention and Comparative Examples 1 to 2 differ in parameters. The specific parameters are shown in Table 2.

[0065] Table 2 Process parameters of embodiments and comparative examples

[0066]

[0067] The performance test standard of the bake-hardening steel sheets for electrogalvanizing prepared in Examples 1 to 3 of the present invention and Comparative Examples 1 and 2 refers to GB / T20564.1.

[0068] Table 3 Performance data of embodiments and comparative examples

[0069]

[0070] The microstructure and metal inclusion grade of the bake-hardened steel sheets for electrogalvanizing prepared in Examples 1 to 3 of the present invention and Comparative Examples 1 and 2 are shown in Table 4.

[0071] Table 4 Microstructure and non-metallic inclusions

[0072]

[0073] The above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention is described in detail with reference to the above embodiments, those skilled in the art should understand that the present invention can still be modified or replaced by equivalents. Any modification or partial replacement that does not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A bake-hardened steel sheet for electrogalvanizing, characterized in that: The following components are included by weight percentage: C: 0.0010-0.0015%, Mn: 0.10-0.70%, Si≤0.02%, Al: 0.020-0.050%, P≤0.015%, S≤0.015%, N≤0.0030%, O≤0.0030%, and the rest are Fe and unavoidable impurities; The metallographic structure of the steel plate is ferrite, the grain size is 9 to 9.5, and the inclusions are only D-type 1.0; the elongation of the steel plate A80 is ≥42%, n 90 value ≥ 0.22, r90 value ≥ 2.1, bake hardening value BH2MPa ≥ 37 MPa; The method for preparing the bake-hardened steel sheet for electrogalvanizing comprises the following specific steps: (1) Continuous casting; (2) Hot rolling: low temperature heating ≤ 1200°C, preheating section 860-900°C, heating section 1080-1120°C, soaking section 1140-1180°C, heating time 2.5-3h; final rolling temperature 890-930°C; (3) Coiling; (4) Pickling and cold rolling; (5) Continuous annealing: annealing temperature is 830-850°C, annealing speed is 120-180 m / min; (6) Cooling: first slowly cool to 670-690°C at a cooling rate of 9-11°C / s; then quickly cool to 440-470°C at a cooling rate of 37-50°C / s; perform aging treatment at 400-440°C for 400-1000s; finally cool to 160-180°C; (7) Leveling and electrogalvanizing pre-treatment; (8) Electrogalvanizing; In the step (1), the platform temperature is 1555-1565°C, the tundish temperature is 1540-1550°C, the superheat is less than 25°C, and the pulling speed is 0.8-1.0 m / min; In the step (3), the coiling is water-cooled to the coiling temperature and slowly cooled in the warehouse; the coiling temperature is 710-750°C; In the pickling process in step (4), the acid solution temperature is controlled at 65-75°C, the free acid concentration in the 1# acid tank is controlled at 50-80 g / L, the free acid concentration in the 2# acid tank is controlled at 80-120 g / L, and the free acid concentration in the 3# acid tank is controlled at 120-160 g / L; the pickling speed is 80-220 m / min; In the step (4), the total cold rolling reduction ratio is ≥75%; the cold rolling adopts five-stand full six-roller cold rolling, and the reduction ratio is distributed as 28-32%, 20-24%, 16-20%, 14-18%, and 10-14%.

2. The bake-hardening steel sheet for electrogalvanizing according to claim 1, characterized in that: In the step (2), a rolling reduction rate distribution system is adopted in which the middle rolling mill has a large rolling reduction rate and the rolling mills at both ends have a small rolling reduction rate; the finishing mills F1 to F7 are provided with an automatic thickness control device AGC.

3. The bake-hardened steel sheet for electrogalvanizing according to claim 1, characterized in that: In the step (7), the flattening machine is used for flattening, and the flattening elongation is 1.1% to 1.3%; the electroplating pretreatment is conventional alkali washing, brushing, electrolytic degreasing and rinsing; the temperature of the degreasing solution in the electrolytic degreasing is controlled at 45 to 75°C, and the pH of the degreasing solution is ≥9.

5.

4. The bake-hardening steel sheet for electrogalvanizing according to claim 1, characterized in that: In the step (8), the gravity method is used for electrogalvanizing, the electroplating solution is zinc sulfate solution, the zinc sulfate concentration in the electroplating solution is 100-120 g / L, the pH value is 1.3-1.8, the electroplating solution temperature is 50-60°C, the running speed of the steel plate in the electrogalvanizing tank is 80-140 m / min; the zinc layer thickness is 10-90 g / m 2 .

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