Production method of St14 cold-rolled steel strip for rare earth microalloyed vehicle door inner plate

Abstract

The invention discloses a production method of an St14 cold-rolled steel strip for a rare earth microalloyed vehicle door inner plate. The production method is characterized by comprising the following steps: a steelmaking process; a hot rolling process; cold rolling process and annealing process parameters; key parameters of each process are determined; the steel comprises the following chemical components in percentage by mass: less than or equal to 0.0025% of C, less than or equal to 0.020% of S i, 0.05-0.15% of Mn, less than or equal to 0.015% of P, 0.060-0.080% of Ti, 0.025-0.050% of Al, less than or equal to 0.010% of S, less than or equal to 0.0030% of N, 0.0006-0.0020% of Ce and the balance of Fe and inevitable impurities. The rare earth Ce is added for microalloying, and the optimized process is coordinated, so that the r value of the product is increased, the steel strip has more excellent stamping formability, the forming requirement of the inner plate of the automobile door can be met, and cost reduction and efficiency improvement of an automobile main engine plant are facilitated.

Description

Technical Field

[0001] This invention belongs to the field of materials forming technology, and in particular relates to a production method of St14 cold-rolled steel strip for rare earth microalloyed car door inner panels. Background Technology

[0002] Cold-rolled deep-drawing steel possesses extremely high tensile strength (high r-value), excellent work hardening ability (high n-value), and uniform plastic deformation capacity, making it widely used in the stamping of complex parts in the automotive industry. Door inner panels are among the most complex formed automotive interior panels, typically using high-grade deep-drawing steels such as DC06 (St16) and above. In recent years, with increasingly fierce competition in the automotive industry, OEMs have demanded cost reduction and efficiency improvement. Conventional St14 cold-rolled steel strip employs an ultra-low carbon design and fixes interstitial atoms by adding microalloying elements such as niobium (Nb) and titanium (Ti). The production process typically includes converter smelting, ladle refining, continuous casting, hot rolling, pickling and cold rolling, continuous annealing, and leveling. Summary of the Invention

[0003] The purpose of this invention is to provide a production method for St14 cold-rolled steel strip for rare earth microalloyed automotive door inner panels. By adding rare earth Ce for microalloying and simultaneously optimizing the process, the product's r-value is improved, giving the steel strip better stamping formability, which can meet the forming requirements of automotive door inner panels and is beneficial for automotive OEMs to reduce costs and increase efficiency.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0005] This invention discloses a method for producing St14 cold-rolled steel strip for rare-earth microalloyed automotive door inner panels, comprising:

[0006] Steelmaking process: molten iron → converter steelmaking → RH refining → slab continuous casting; molten iron is pre-treated by desulfurization and then smelted in a converter. The sulfur content of the molten iron entering the converter must be less than 0.002%, and the slag removal area must be greater than 95%; the tapping temperature of the converter smelting must be ≥1670℃, and a turnover ladle must be used for tapping. The temperature drop at tapping must 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 must be ensured for more than 6 minutes before temperature measurement and sampling. After the addition of ferrotitanium, the pure degassing time must be ensured for more than 8 minutes. In the later stage of vacuum treatment in the RH furnace, 30-50 kg of rare earth cerium ferroalloy is added, and the superheat of the molten steel is 20-40℃.

[0007] 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; slab heating temperature is 1170~1200℃; heating time is 180~260min; roughing mode adopts 3+3; finishing rolling temperature is ≥900℃, coiling temperature is 720~740℃;

[0008] 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 820~830℃, 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 0.6~1.0%;

[0009] Its chemical composition by mass percentage is as follows: C≤0.0025%, Si≤0.020%, Mn: 0.05~0.15%, P≤0.015%, Ti 0.060~0.080%, Al 0.025~0.050%, S≤0.010%, N≤0.0030%, Ce 0.0006~0.0020%, with the balance being Fe and unavoidable impurities.

[0010] Furthermore, its chemical composition by mass percentage is as follows: C 0.0020%, Si 0.005%, Mn 0.14%, P 0.009%, Ti 0.065%, Al 0.036%, S 0.007%, N 0.0016%, Ce 0.0007%, with the balance being Fe and unavoidable impurities.

[0011] Furthermore, its chemical composition by mass percentage is as follows: C 0.0018%, Si 0.002%, Mn 0.10%, P 0.014%, Ti 0.072%, Al 0.037%, S 0.004%, N 0.0025%, Ce 0.0011%, with the balance being Fe and unavoidable impurities.

[0012] Furthermore, the billet heating temperature is 1182-1195℃, and the heating time is 186-245min.

[0013] Furthermore, the finishing rolling temperature is 915-934℃.

[0014] Furthermore, the winding temperature is 720–736°C.

[0015] Further, the annealing process parameters are as follows: slow cooling section outlet temperature 620-654℃, rapid cooling section outlet temperature 340-370℃, over-aging section temperature 334-360℃, and final cooling section outlet temperature 143-147℃.

[0016] Furthermore, the finished product thickness is 0.7-1.2mm, and the finished product mechanical properties are: 142-147MPa, tensile strength 284-295MPa; elongation after fracture A 80 42.8-45.6%; r 90 The range is 2.92-3.19, n 90 It is 0.23-0.24.

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

[0018] This invention adds a certain amount of alloying element Ti to ultra-low carbon steel to fix interstitial C and N atoms, resulting in interstitial atom-free IF steel. Simultaneously, it narrows the process window and improves the steel's purity. Further addition of rare earth element Ce for microalloying strongly promotes the formation of favorable {111} textures, thereby increasing the plastic strain ratio (r-value) of the steel strip. This gives it a deeper drawing capability exceeding that of conventional St14 steel strip, meeting the stringent stamping requirements of complex components such as the inner panel of heavy-duty truck front doors. The finished product achieves a yield strength of 135–150 MPa, tensile strength of 280–310 MPa, elongation after fracture of 42.5–50.0% (transverse, tensile test: L0 = 80 mm, b = 20 mm), n-value of 0.23–0.24, r-value of 2.80–3.28, and surface roughness of 0.6–1.9 μm. The high elongation after fracture and r-value ensure excellent stamping performance. Attached Figure Description

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

[0020] Figure 1 The metallographic structure of the finished product in Example 1;

[0021] Figure 2 A horizontal comparison of the r-values ​​of St14 products with and without rare earth elements. Detailed Implementation

[0022] A method for producing St14 cold-rolled steel strip for rare-earth microalloyed car door inner panels includes:

[0023] 1. Smelting process

[0024] 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 ​​the hot metal slag removed should be greater than 95% to prevent high sulfur slag from being added to the converter and causing sulfur reversion.

[0025] 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.

[0026] 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, 30-50 kg of rare earth cerium ferroalloy is added to improve the cleanliness of the molten steel.

[0027] 1.4 Continuous Casting: Continuous casting is a protective process to prevent secondary oxidation of the molten steel and to avoid secondary oxidation of the steel quality. A constant casting speed is adopted, controlled between 1.0 and 1.8 m / min, and the superheat is controlled appropriately during casting, maintaining a superheat of approximately 20–40°C to promote the flotation of inclusions and enable multi-furnace casting.

[0028] 2. Hot rolling process

[0029] The billet is heated by a walking beam furnace. The roughing rolling adopts a double-stand R1 and R2 reciprocating rolling process with a 3+3 roughing mode. The descaling water pressure is not less than 200 bar. The finishing rolling adopts the F1 to F7 continuous rolling process with a finishing rolling temperature ≥900℃ to ensure rolling in the single-phase austenite region. The coiling temperature is 710 to 760℃ to ensure normal precipitation of carbonitrides and improve deep drawing performance. The specific hot rolling heating regime and rolling process are shown in Table 1.

[0030] Table 1. Hot rolling heating regime and rolling process

[0031]

[0032] 3 Annealing process

[0033] 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 820–830℃ to ensure complete recrystallization of the grains. 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.

[0034] Table 2 Annealing process

[0035]

[0036] Table 3 Elongation of the leveling machine

[0037]

[0038] 4. Example Analysis

[0039] 4.1 Steelmaking Composition

[0040] Based on the above steelmaking process requirements, the actual chemical composition (mass percentage) of the slab is shown in Table 4 below.

[0041] Table 4. Chemical composition (wt.%) of examples

[0042]

[0043] 4.2 Hot-rolled properties

[0044] Based on the above design chemical composition and hot rolling process (hot rolling heating regime and rolling process are shown in Table 5), the room temperature tensile properties of hot-rolled plates are shown in Table 6. The tensile specimens have L0 = 80 mm and b = 20 mm (transverse). The test method is in accordance with GB / T 228.1.

[0045] Table 5. Heating Regime and Rolling Process for Cast Billets

[0046]

[0047]

[0048] Table 6 Hot-rolled tensile properties

[0049]

[0050] 4.3 Finished Product Performance

[0051] Based on the above-mentioned hot-rolled properties, cold rolling and annealing were performed (process execution is shown in Table 7). The room temperature tensile mechanical properties of the finished product are shown in Table 8. The metallographic structure is equiaxed ferrite with a grain size of 7.0–8.5. The microstructure is shown in Table 8. Figure 1 The range of r-values ​​in the steel strip before and after rare earth treatment is as follows: Figure 2 As shown, the overall range has narrowed, while the average value has increased.

[0052] Table 7 Annealing and Leveling Machine Elongation Process

[0053]

[0054] Table 8 Tensile properties of finished products

[0055]

[0056] In summary, this product has passed performance testing and meets the performance range of high-level DC06 products. After user trial molding, it meets the forming requirements of the car door inner panel. The supply of key parts has been achieved, with an annual supply volume of approximately 2,000 tons.

[0057] 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 St14 cold-rolled steel strip for rare earth microalloyed car door inner panels, 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, 30-50 kg of rare earth cerium ferroalloy is added, and the superheat of the molten steel is 20-40℃. Hot rolling process: slab heating temperature is 1170~1200℃; heating time is 180~260min; roughing mode adopts 3+3; finishing rolling temperature is ≥900℃, coiling temperature is 720~740℃; Annealing process parameters for cold rolling: Heating and soaking zone outlet temperature 820~830℃, 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 0.6~1.0%. Its chemical composition by mass percentage is as follows: C≤0.0025%, Si≤0.020%, Mn: 0.05~0.15%, P≤0.015%, Ti 0.060~0.080%, Al 0.025~0.050%, S≤0.010%, N≤0.0030%, Ce 0.0006~0.0020%, with the balance being Fe and unavoidable impurities.

2. The production method of St14 cold-rolled steel strip for rare earth microalloyed car door inner panels according to claim 1, characterized in that: Its chemical composition by mass percentage is: C 0.0020%, Si 0.005%, Mn 0.14%, P 0.009%, Ti 0.065%, Al 0.036%, S 0.007%, N 0.0016%, Ce 0.0007%, with the balance being Fe and unavoidable impurities.

3. The production method of St14 cold-rolled steel strip for rare earth microalloyed car door inner panels according to claim 1, characterized in that: Its chemical composition by mass percentage is: C 0.0018%, Si 0.002%, Mn 0.10%, P 0.014%, Ti 0.072%, Al 0.037%, S 0.004%, N 0.0025%, Ce 0.0011%, with the balance being Fe and unavoidable impurities.

4. The production method of St14 cold-rolled steel strip for rare earth microalloyed car door inner panels according to claim 1 or 2, characterized in that: The billet is heated at 1182-1195℃ for 186-245 minutes.

5. The production method of St14 cold-rolled steel strip for rare earth microalloyed car door inner panels according to claim 4, characterized in that: The finishing rolling temperature is 915-934℃.

6. The production method of St14 cold-rolled steel strip for rare earth microalloyed car door inner panels according to claim 5, characterized in that: The winding temperature is 720–736℃.

7. The production method of St14 cold-rolled steel strip for rare earth microalloyed car door inner panels according to claim 6, characterized in that: Annealing process parameters: slow cooling section outlet temperature 620-654℃, rapid cooling section outlet temperature 340-370℃, over-aging section temperature 334-360℃, final cooling section outlet temperature 143-147℃.

8. The production method of St14 cold-rolled steel strip for rare earth microalloyed car door inner panels according to claim 7, characterized in that: Finished product thickness: 0.7-1.2mm; Finished product mechanical properties: 142-147MPa, tensile strength: 284-295MPa; Elongation after fracture: A 80 42.8-45.6%; r 90 The range is 2.92-3.19, n 90 It is 0.23-0.24.

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