A method for producing a deep-drawing cold-rolled steel strip having high corrosion resistance

Abstract

The application discloses a production method of a deep-drawing cold-rolled steel strip with high corrosion resistance, comprising: a steelmaking process: molten iron, converter steelmaking, RH refining, slab continuous casting; a hot rolling process; and a cold rolling process; and the chemical composition in percentage by mass is as follows: C 0.0010-0.0035%, Si <=0.030%, Mn <=0.20%, P <=0.015%, Ti 0.040-0.070%, Al 0.020-0.060%, S <=0.010%, N <=0.0040%, Ce 0.0005-0.0025%, and the balance is Fe and inevitable impurities. The application aims to ensure that the cold-rolled steel strip has excellent deep-drawing forming performance, significantly improve the corrosion resistance of the cold-rolled steel strip, enable the cold-rolled steel strip to pass the neutral salt spray test certification specified by automobile manufacturers, and not sacrifice the mechanical properties and process feasibility.

Description

Technical Field

[0001] This invention belongs to the field of material forming technology, and in particular relates to a method for producing cold-rolled steel strip for deep drawing with high corrosion resistance. Background Technology

[0002] With the automotive industry's ever-increasing demands for lightweighting, safety, and durability, high-strength cold-rolled steel strip with high corrosion resistance has become the preferred material for key components such as automotive exterior panels and structural parts. To ensure the long-term service life and appearance quality of vehicles in harsh environments (such as de-icing agents and marine atmospheres), automakers generally use cyclic corrosion testing or neutral salt spray testing as the core certification standard for evaluating the corrosion resistance of steel. The core challenge of existing technologies lies in the difficulty of simultaneously achieving the high level of corrosion resistance required by automakers while ensuring the excellent deep-drawing performance (high r-value, high n-value) of the steel. Conventional single-technology approaches often compromise on one aspect while neglecting the other, failing to achieve an ideal balance between the contradictory attributes of "high formability" and "high corrosion resistance."

[0003] Rare earth microalloying treatment of automotive steel sheets allows rare earth elements to "purify" the steel, "modify" inclusions, and "optimize the passivation film," thereby improving its corrosion resistance. Lowering the hot-rolling coiling temperature inhibits excessive growth of iron oxide scale and refines grains, reducing surface defects and corrosion initiation points to some extent. During continuous annealing, increasing the annealing rate promotes microstructure refinement and achieves fine-grain strengthening. The smaller grain structure increases grain boundary area, making the corrosion path more tortuous and thus slowing down the corrosion rate. Simultaneously, the dew point temperature in the furnace area of ​​the continuous annealing process must be -30℃ to -50℃ to prevent surface oxidation of the steel strip, resulting in a cleaner, brighter, and higher-performance surface, further enhancing corrosion resistance. Summary of the Invention

[0004] The purpose of this invention is to provide a method for producing cold-rolled steel strip for deep drawing with high corrosion resistance. While ensuring excellent deep-drawing forming performance, the method significantly improves the corrosion resistance of the cold-rolled steel strip, enabling it to pass the neutral salt spray test certification required by automobile manufacturers, without sacrificing its mechanical properties and process feasibility. To solve this technical problem, this invention combines rare earth microalloying treatment with optimized hot rolling coiling and annealing processes to form a synergistic technical system, giving the product both good stamping forming performance and high corrosion resistance. The resulting product exhibits a yield strength of 140~180MPa, tensile strength of 270~330MPa, elongation after fracture of 41.0~48.0% (transverse, tensile test: L0=80mm, b=20mm), n value of 0.21~0.24, r value of 2.50~3.35, and surface roughness range of 0.7~1.3μm. The steel strip is designed to have both excellent stamping formability and high corrosion resistance, meeting both the "high formability" requirements of automotive OEMs and the neutral salt spray corrosion resistance certification test.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] This invention discloses a method for producing cold-rolled steel strip for deep drawing with high corrosion resistance, comprising:

[0007] Steelmaking process: Molten iron → Converter steelmaking → RH refining → Slab continuous casting; Molten iron is pre-treated by desulfurization and then smelted in the converter. The sulfur content of the molten iron entering the converter is required to be less than 0.002%, and the slag removal area is greater than 95%; The tapping temperature of the converter smelting is required to be ≥1670℃, and a turnover ladle must be used for tapping. The temperature drop at tapping is required to be less than 60℃; The RH process requires deep decarburization treatment according to the composition and temperature of the RH steel supplied to the converter. After decarburization, aluminum particles are added for deoxidation according to the oxygen content. After circulation for more than 6 minutes, alloys such as ferrotitanium are added to adjust the composition; After the composition adjustment is completed, vacuum circulation is ensured for more than 6 minutes before temperature measurement and sampling. After the addition of ferrotitanium, the pure degassing time is guaranteed to be more than 8 minutes. In the later stage of vacuum treatment in the RH furnace, 10~50kg of rare earth cerium ferroalloy is added, and the superheat of the molten steel is 20~60℃.

[0008] Hot rolling process: slab heating → high-pressure water descaling → fixed-width press → E1R1 roughing mill rolling → E2R2 roughing mill rolling → flying shear → high-pressure water descaling → F1~F7 finishing mill rolling → dense laminar flow cooling → coiling → pallet transport system → sampling and inspection; the slab heating temperature is 1160~1220℃; the heating time is 160~300min; the roughing mode adopts 3+3; the finishing rolling temperature is ≥900℃, and the coiling temperature is 670~700℃;

[0009] Cold rolling process: Pickling and uncoiling → Welding → Tension leveling → Pickling → Rinsing → Drying → Trimming → Continuous rolling mill cold rolling → Slitting → Coiling → Offline inspection → Weighing → Marking → Bundling → Packaging → Warehousing → Continuous annealing and uncoiling → Welding → Cleaning → Inlet looper → Annealing furnace → Outlet looper → Leveling → Inspection looper → Trimming → Surface inspection → Oiling → Coiling → Weighing → Sampling and inspection; Annealing process parameters: Heating and soaking zone outlet temperature 810~850℃, slow cooling zone outlet temperature 600~700℃, rapid cooling zone outlet temperature ≤400℃, over-aging zone temperature ≤400℃, final cooling zone outlet temperature ≤150℃, leveling mill elongation 0.7~1.1%, annealing speed 180~230 m / min, furnace area dew point temperature -30℃~-50℃;

[0010] Its chemical composition by mass percentage is as follows: C 0.0010~0.0035%, Si ≤0.030%, Mn≤0.20%, P ≤0.015%, Ti 0.040~0.070%, Al 0.020~0.060%, S ≤0.010%, N ≤0.0040%, Ce 0.0005~0.0025%, with the balance being Fe and unavoidable impurities.

[0011] Furthermore, its chemical composition by mass percentage is as follows: C 0.0022%, Si 0.005%, Mn 0.15%, P 0.007%, Ti 0.063%, Al 0.046%, S 0.008%, N 0.0016%, Ce 0.0007%, with the balance being Fe and unavoidable impurities.

[0012] Furthermore, its chemical composition by mass percentage is as follows: C 0.0016%, Si 0.002%, Mn 0.20%, P 0.013%, Ti 0.052%, Al 0.055%, S 0.005%, N 0.0028%, Ce 0.0011%, with the balance being Fe and unavoidable impurities.

[0013] Furthermore, its chemical composition by mass percentage is as follows: C 0.0031%, Si 0.003%, Mn 0.12%, P 0.010%, Ti 0.070%, Al 0.037%, S 0.004%, N 0.0020%, Ce 0.0018%, with the balance being Fe and unavoidable impurities.

[0014] Furthermore, the heat spread temperature is 1192-1215℃, and the heat spread time is 28-32min.

[0015] Furthermore, the finishing rolling temperature is 915-934℃.

[0016] Furthermore, the winding temperature is 680-693℃.

[0017] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0018] This invention provides a method for producing highly corrosion-resistant cold-rolled steel strip for automobiles. The method involves rare-earth microalloying of the automotive steel sheet to reduce the "starting point" and "rapid pathway" of corrosion, altering the morphology and properties of inclusions to transform them from "harmful" to "relatively harmless." Rare-earth elements promote the formation of a dense passivation film on the steel strip surface. Simultaneously, combined with optimized hot-rolling coiling and annealing processes, the method achieves grain refinement and optimizes the surface quality of the steel strip. While ensuring excellent deep-drawing performance, the method significantly improves the corrosion resistance of the cold-rolled steel strip, enabling it to pass the neutral salt spray test certification specified by automobile manufacturers without sacrificing its mechanical properties and process feasibility. This forms a synergistic technical system of multiple optimization measures, resulting in a product that combines good stamping performance with high surface corrosion resistance. Attached Figure Description

[0019] The present invention will be further described below with reference to the accompanying drawings.

[0020] Figure 1 Comparison of the metallographic structures of steel strips in Example 1 and Comparative Example 1; wherein Figure 1 a represents the metallographic structure of the steel strip in the example. Figure 1 b represents the metallographic structure of the steel strip in the comparative example.

[0021] Figure 2 This is a comparison of the morphology of the phosphate phase on the surface of the steel strips in Example 1 and Comparative Example 1; wherein Figure 2 a represents the morphology of the phosphate phase on the surface of the steel strip in the example; Figure 2 b shows the morphology of the phosphate phase on the surface of the comparative steel strip;

[0022] Figure 3 This is a comparison of the width of scratch corrosion on the steel strip surface in Example 1 and Comparative Example 1; wherein Figure 3 a represents the width of the scratch corrosion on the steel strip surface in the example; Figure 3 b represents the width of the surface scratch corrosion on the comparative steel strip. Detailed Implementation

[0023] A method for producing cold-rolled steel strip for deep drawing with high corrosion resistance:

[0024] 1. Smelting process

[0025] 1.1 Desulfurization Pretreatment: KR hot metal desulfurization technology is adopted, requiring the final sulfur content after deep desulfurization to be within 0.002%. KR desulfurization is carried out by mechanical stirring. The stirring head is inserted into the hot metal and rotated to create a vortex. Then, the desulfurizing agent is added into the hot metal vortex, so that the desulfurizing agent and the sulfur in the hot metal undergo a desulfurization reaction during continuous stirring. After desulfurization, the desulfurization slag is completely removed. The area of ​​slag removal should be greater than 95% to prevent high sulfur slag from being added to the converter and causing sulfur reversion.

[0026] 1.2 Converter Smelting: After desulfurization pretreatment, molten iron is smelted in a converter. The converter dephosphorization process mainly relies on slag with high basicity, high oxidizing properties, and a large slag volume for dephosphorization. However, to achieve deep dephosphorization, conditions must be created to allow a large amount of phosphorus in the molten iron to be oxidized and enter the slag. The phosphorus content of the molten iron entering the converter is ≤0.015%. Oxygen blowing is used for decarburization and temperature rise. P and S compositions are controlled to prevent over-oxidation of the molten steel. The final converter temperature is controlled above 1670℃. Steel must be tapped using a reusable ladle, and the temperature drop at tapping must be less than 60℃. Ferromanganese is added during the converter tapping process for alloying, and quicklime and modifiers are added for top slag modification.

[0027] 1.3 Refining: The RH refining furnace performs decarburization treatment based on the composition and temperature of the molten steel. RH decarburization is the most important decarburization step in the production of ultra-low carbon steel. Due to the high temperature and intense stirring during vacuum treatment, all factors that are conducive to improving the decarburization rate can accelerate the decarburization reaction. During the rapid decrease in pressure in the vacuum chamber, the increase in the lifting gas flow rate, circulation flow rate, and volume coefficient increases, thereby improving the decarburization rate and accelerating the decarburization reaction. After decarburization, deoxidizer and aluminum are added according to the oxygen content. After circulation for more than 6 minutes, alloys such as ferrotitanium, ferroniobium, metallic manganese, micro-carbon ferrosilicon, and ferrophosphorus are added to adjust the composition. After adjusting the composition, vacuum circulation is ensured for 6 minutes before temperature measurement and sampling. After the addition of ferrotitanium, the pure degassing time is guaranteed to be more than 8 minutes to ensure the uniformity of the temperature and composition of the molten steel, and at the same time, it is conducive to the flotation of inclusions in the molten steel. In the later stage of vacuum treatment, 10-50 kg of rare earth cerium ferroalloy is added to improve the cleanliness of the molten steel.

[0028] 1.4 Continuous Casting: Continuous casting protects the pouring process, preventing secondary oxidation of the molten steel and avoiding secondary oxidation of the steel quality. A constant casting speed is adopted, controlled at 1.0~1.8 m / min, and the superheat is controlled appropriately during the pouring process, maintaining a superheat of approximately 20~60℃ to promote the flotation of inclusions and enabling multi-furnace casting.

[0029] 2. Hot rolling process

[0030] The billet is heated in a walking beam furnace. Roughing is performed using a two-stand reciprocating rolling mill (R1 and R2) in a 3+3 pattern. The descaling water pressure is no less than 200 bar. Finishing uses a continuous rolling process (F1~F7) with a finishing temperature ≥900℃ to ensure rolling in the austenitic single-phase region. The coiling temperature is 670~700℃, which ensures normal precipitation of carbonitrides, improving deep-drawing performance, and also refines the grains, reducing oxide formation on the steel strip surface. Specific hot rolling heating regimes and rolling processes are shown in Table 1.

[0031] Table 1 Hot rolling heating regime and rolling process

[0032] Heating temperature ℃ heating time (min) Isotropic temperature (°C) Soaking time (min) Finishing rolling temperature (°C) Winding temperature (°C) 1160~1220 160~300 1160~1220 20~60 ≥900 670~700

[0033] 3 Annealing process

[0034] Annealing was performed using a vertical continuous annealing furnace, with a reducing atmosphere and a nitrogen-hydrogen mixed protective atmosphere used for cooling. The temperature range of the heating section and the soaking section was 810~850℃ to ensure complete recrystallization of the grains, increase the annealing speed and refine the microstructure. The outlet temperature control of each section of the heating furnace is shown in Table 2, and the thickness control of different elongation rates of the leveling machine is shown in Table 3.

[0035] Table 2 Annealing process

[0036] Finished product thickness (H) in mm Outlet temperature of heating section and soaking section (°C) Slow cooling section outlet temperature (°C) Rapid cooling section outlet temperature (°C) Exit temperature of the aging stage (°C) Final cooling section outlet temperature (°C) Process section speed (m / min) Dew point temperature (°C) 0.6≤H≤1.0 810~850 600~700 ≤400 ≤400 ≤150 180~230 -30~-50

[0037] Table 3 Elongation of the leveling machine

[0038] Finished product thickness (H) in mm Elongation of leveling machine % 0.6≤H≤1.0 0.7~1.1

[0039] 4. Example Analysis

[0040] 4.1 Steelmaking composition

[0041] Based on the above steelmaking process requirements, the actual slab chemical composition (mass percentage) is shown in Table 4 below. Except for the rare earth element content, the control range of other element contents is the same between the example and the comparative example.

[0042] Table 4. Example Chemical Composition (wt.%)

[0043] Group C Si Mn P S Al Ti N Ce Example 1 0.0022 0.005 0.15 0.007 0.008 0.046 0.063 0.0016 0.0007 Example 2 0.0016 0.002 0.20 0.013 0.005 0.055 0.052 0.0028 0.0011 Example 3 0.0031 0.003 0.12 0.010 0.004 0.037 0.070 0.0020 0.0018 Comparative Example 1 0.0020 0.004 0.14 0.011 0.007 0.046 0.062 0.0023 0 Comparative Example 2 0.0023 0.005 0.17 0.014 0.006 0.043 0.060 0.0018 0

[0044] 4.2 Hot rolling process

[0045] Based on the above-designed chemical composition and hot rolling process, the hot rolling heating regime and rolling process of Examples 1, 2, and 3 and Comparative Examples 1 and 2 are shown in Table 5. Except for the difference in the coiling temperature, the control range of other process parameters is the same.

[0046] Table 5 Heating regime and rolling process for cast billets

[0047] Group Heating temperature ℃ heating time (min) Isotropic temperature (°C) Soaking time (min) Finishing rolling temperature (°C) Winding temperature (°C) Example 1 1182 186 1192 30 922 686 Example 2 1190 230 1215 32 934 680 Example 3 1195 245 1204 28 915 693 Comparative Example 1 1120 220 1210 43 927 736 Comparative Example 2 1120 220 1201 43 920 724

[0048] 4.3 Cold rolling process

[0049] Based on the above hot rolling performance, cold rolling and continuous annealing were carried out. The acid rolling reduction rate of the same thickness products in Examples 1, 2, and 3 was consistent with that in Comparative Examples 1 and 2, ranging from 75% to 80%. The continuous annealing process was executed as shown in Table 7. Examples 1 to 3 adopted high-temperature annealing with a high annealing speed, while Comparative Examples 1 and 2 adopted low-temperature annealing with a low annealing speed. The control range of other process parameters was the same.

[0050] Table 7 Annealing and Leveling Machine Elongation Process

[0051] Group Finished product thickness (mm) Heating section / soaking section outlet temperature (°C) Slow cooling section outlet temperature (°C) Rapid cooling section outlet temperature (°C) Exit temperature of the aging stage (°C) Final cooling section outlet temperature (°C) Process section speed (m / min) Elongation of leveling machine % Dew point temperature (°C) Example 1 0.7 820 620 340 334 143 215 0.7 -46 Example 2 0.8 830 637 360 356 146 190 0.9 -45 Example 3 1.0 828 654 370 360 147 186 1.1 -48 Comparative Example 1 1.0 796 640 365 353 143 160 1.1 -45 Comparative Example 2 0.8 785 645 354 340 144 156 0.9 -38

[0052] 4.4 Finished Product Structure and Properties

[0053] Table 8 compares the room temperature tensile mechanical properties of Examples 1, 2, and 3 with those of Comparative Examples 1 and 2. The metallographic structure of all examples is equiaxed ferrite, but the grains in the examples are refined to grade 9.0 (compared to grades 7.5-8.0 in the comparative examples), representing a grain size refinement of approximately 1-1.5 grades. A comparison of the microstructures is shown in [Table 8]. Figure 1 The surface roughness of the steel strip is consistent.

[0054] Table 8 Tensile properties of finished products

[0055] Group Finished product thickness (mm) <![CDATA[Yield strength R p0.2 / MPa]]> <![CDATA[Tensile strength R m / MPa]]> <![CDATA[Elongation A 80 %]]> <![CDATA[r 90 ]]> <![CDATA[n 90 ]]> roughness Example 1 0.7 146 301 45.6 3.19 0.24 0.9 Example 2 0.8 148 298 42.8 2.96 0.24 0.8 Example 3 1.0 153 293 44.0 3.00 0.23 1.1 Comparative Example 1 1.0 147 292 45.4 2.92 0.24 1.0 Comparative Example 2 0.8 150 286 44.5 3.01 0.23 0.9

[0056] 4.5 Neutral Salt Spray Test Certification

[0057] For OEM neutral salt spray test certification, the steel plate needs to undergo surface degreasing, phosphating, and electrophoretic coating. A phosphating film and electrophoretic paint are formed on the surface of the steel strip. Subsequently, surface scratches are performed to observe the coating's ability to inhibit corrosion from spreading from the scratch to the surrounding area after damage. The corrosion width is usually measured to evaluate the surface corrosion resistance.

[0058] The surface treatment method of the steel strip in the examples is the same as that in the comparative examples, and the morphology of the resulting surface phosphating phase is as follows: Figure 2 As shown, the phosphate phase on the surface of the steel strip in the embodiment is denser. After 1000 hours of neutral salt spray, the scratch corrosion width is compared to... Figure 3 As shown, the corrosion width on the surface of the steel strip in the embodiment is 0.2~0.3mm, while that in the comparative example is 0.5~0.6mm, indicating that the corrosion resistance is greatly improved.

[0059] In summary, through rare earth microalloying treatment, combined with optimization of hot rolling and continuous annealing processes, the surface corrosion resistance of this product has been greatly improved. It has passed the neutral salt spray test certification of the OEM, and the relevant products have passed the stamping trial and certification of the OEM, and are ready for mass supply, with an annual supply of approximately 1,000 tons.

[0060] 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 producing cold-rolled steel strip for deep drawing with high corrosion resistance, characterized in that: include: Steelmaking process: Molten iron → Converter steelmaking → RH refining → Slab continuous casting; Molten iron is pre-treated by desulfurization and then smelted in the converter. The sulfur content of the molten iron entering the converter is required to be less than 0.002%, and the slag removal area is greater than 95%; The tapping temperature of the converter smelting is required to be ≥1670℃, and a turnover ladle must be used for tapping. The temperature drop at tapping is required to be less than 60℃; The RH process requires deep decarburization treatment according to the composition and temperature of the RH steel supplied to the converter. After decarburization, aluminum particles are added for deoxidation according to the oxygen content. After circulation for more than 6 minutes, alloys such as ferrotitanium are added to adjust the composition; After the composition adjustment is completed, vacuum circulation is ensured for more than 6 minutes before temperature measurement and sampling. After the addition of ferrotitanium, the pure degassing time is guaranteed to be more than 8 minutes. In the later stage of vacuum treatment in the RH furnace, 10~50kg of rare earth cerium ferroalloy is added, and the superheat of the molten steel is 20~60℃. Hot rolling process: slab heating → high-pressure water descaling → fixed-width press → E1R1 roughing mill rolling → E2R2 roughing mill rolling → flying shear → high-pressure water descaling → F1~F7 finishing mill rolling → dense laminar flow cooling → coiling → pallet transport system → sampling and inspection; the slab heating temperature is 1160~1220℃; the heating time is 160~300min; the roughing mode adopts 3+3; the finishing rolling temperature is ≥900℃, and the coiling temperature is 670~700℃; Cold rolling process: Pickling and uncoiling → Welding → Tension leveling → Pickling → Rinsing → Drying → Trimming → Continuous rolling mill cold rolling → Slitting → Coiling → Offline inspection → Weighing → Marking → Bundling → Packaging → Warehousing → Continuous annealing and uncoiling → Welding → Cleaning → Inlet looper → Annealing furnace → Outlet looper → Leveling → Inspection looper → Trimming → Surface inspection → Oiling → Coiling → Weighing → Sampling and inspection; Annealing process parameters: Heating and soaking zone outlet temperature 810~850℃, slow cooling zone outlet temperature 600~700℃, rapid cooling zone outlet temperature ≤400℃, over-aging zone temperature ≤400℃, final cooling zone outlet temperature ≤150℃, leveling mill elongation 0.7~1.1%, annealing speed 180~230 m / min, furnace area dew point temperature -30℃~-50℃; Its chemical composition by mass percentage is as follows: C 0.0010~0.0035%, Si ≤0.030%, Mn≤0.20%, P ≤0.015%, Ti 0.040~0.070%, Al 0.020~0.060%, S ≤0.010%, N ≤0.0040%, Ce 0.0005~0.0025%, with the balance being Fe and unavoidable impurities.

2. The method for producing high corrosion-resistant cold-rolled steel strip for deep drawing according to claim 1, characterized in that: Its chemical composition by mass percentage is: C 0.0022%, Si 0.005%, Mn 0.15%, P 0.007%, Ti 0.063%, Al 0.046%, S 0.008%, N 0.0016%, Ce 0.0007%, with the balance being Fe and unavoidable impurities.

3. The method for producing cold-rolled steel strip for deep drawing with high corrosion resistance according to claim 1, characterized in that: Its chemical composition by mass percentage is: C 0.0016%, Si 0.002%, Mn 0.20%, P 0.013%, Ti 0.052%, Al 0.055%, S 0.005%, N 0.0028%, Ce 0.0011%, with the balance being Fe and unavoidable impurities.

4. The method for producing cold-rolled steel strip for deep drawing with high corrosion resistance according to claim 1, characterized in that: Its chemical composition by mass percentage is: C 0.0031%, Si 0.003%, Mn 0.12%, P 0.010%, Ti 0.070%, Al 0.037%, S 0.004%, N 0.0020%, Ce 0.0018%, with the balance being Fe and unavoidable impurities.

5. The method for producing cold-rolled steel strip for deep drawing with high corrosion resistance according to claim 1, characterized in that: The heat spread temperature is 1192-1215℃, and the heat spread time is 28-32min.

6. The method for producing cold-rolled steel strip for deep drawing with high corrosion resistance according to claim 1, characterized in that: The finishing rolling temperature is 915-934℃.

7. The method for producing cold-rolled steel strip for deep drawing with high corrosion resistance according to claim 1, characterized in that: Winding temperature: 680-693℃.

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