Preparation method of low-cost 220MPa-grade IF high-strength steel for automobiles

Abstract

The invention discloses a preparation method of low-cost 220MPa-grade IF high-strength steel for an automobile, and belongs to the technical field of metallurgy and metal materials. The steel rail comprises the following chemical components in percentage by weight: less than or equal to 0.0030% of C; 0.14% or less of Si; mn < = 0.75%; less than or equal to 0.070% of P; less than or equal to 0.015% of S; alt is greater than or equal to 0.015%; ti is greater than or equal to 0.040%; n is less than or equal to 0.0040%, and the balance is Fe and inevitable impurities. The invention also discloses main optimized related process parameters. According to the preparation method of the low-cost 220MPa-grade IF high-strength steel for the automobile, the strength and the forming performance of a steel belt are guaranteed, and meanwhile the production and manufacturing cost of the steel belt is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of metallurgy and metal materials, and in particular relates to a method for preparing low-cost 220MPa grade IF high-strength steel for automobiles. Background Art

[0002] With the increasing demand for lightweighting and greening in the global automotive industry, the proportion of high-strength steel used in body structural parts continues to increase. Among them, IF (interstitial-free) high-strength steel with a yield strength of 220MPa has become an ideal choice for complex formed parts such as door inner panels and fenders due to its excellent deep drawing performance, high r value (plastic strain ratio) and n value (work hardening index). However, traditional IF steel relies on the addition of microalloying elements such as niobium (Nb) and titanium (Ti) to achieve high strength, which increases the cost of the alloy. For example, patent CN114231845A adopts a Nb-Ti composite microalloying design. Although the yield strength reaches 220MPa level strength, the material cost is relatively high. Patent CN119144887A achieves fine grain strengthening through low-temperature rolling coupled with controlled cooling, but the target is 355 / 390MPa grade steel, and the carbon content (0.13-0.18%) is too high and is not suitable for IF steel. In summary, driven by environmental protection policies and the cost reduction needs of automobile companies, it is of great significance to develop a new type of low-cost 220MPa grade IF high-strength steel for automobiles.

[0003] The present invention optimizes the composition design and reasonably adjusts the preparation process, thereby reducing the alloy cost while ensuring the mechanical properties and forming properties of 220Mpa grade IF high-strength steel, thereby achieving low cost of the IF high-strength steel. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for preparing low-cost 220MPa grade IF high-strength steel for automobiles, which reduces its production cost while ensuring the strength and formability of the steel strip.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] The present invention provides a method for preparing low-cost 220MPa grade IF high-strength steel for automobiles, comprising: a steelmaking process, a hot rolling process, a cold rolling process, and a continuous annealing process; wherein the steelmaking process comprises: converter top and bottom blowing smelting, RH, and slab continuous casting; the hot rolling process comprises: slab heating, high-pressure water descaling, width-fixing press, E1 and R1 roughing mill rolling, E2 and R2 roughing mill rolling, insulation cover, flying shear, high-pressure water descaling, F1-F7 finishing mill rolling, dense laminar cooling, and coiling; the cold rolling process comprises: pickling uncoiling, welding, tension leveling, pickling, rinsing, drying, trimming, continuous rolling by a continuous rolling mill, and coiling; and the continuous annealing process comprises: continuous annealing uncoiling, welding, cleaning, entry looper, annealing furnace, exit looper, leveling, trimming, surface inspection, oiling, sampling, coiling, weighing, and packaging; the method is characterized in that:

[0007] 1) Converter production

[0008] After desulfurization pretreatment, the molten iron is smelted in a converter. The sulfur content of the molten iron entering the converter must be less than 0.005%, and the slag removal area of the molten iron must be greater than 95%. The converter tapping temperature must be ≥1670°C, and a turnover bag must be used for tapping. The tapping temperature drop must be less than 70°C. The RH molten steel supplied by the converter must contain [C]≤0.04%, [Mn]≤0.70%, and [O]: 0.040-0.07%.

[0009] 2) RH furnace production

[0010] The RH furnace performs deep decarburization. After decarburization, aluminum particles are added for deoxidation according to the oxygen concentration. After the cycle lasts for more than 4 minutes, ferromanganese and ferrotitanium are added to adjust the composition. After the composition adjustment is completed, ensure that the vacuum cycle lasts ≥ 6 minutes.

[0011] 3) Continuous casting production

[0012] During slab continuous casting, the superheat of the tundish during casting is controlled within the range of 30-45°C, and the production casting speed is 1.0-1.8 m / min;

[0013] 4) During the heating of the casting, the heating temperature is 1180-1230℃, the heating time is 150-250min, the soaking temperature is 1180-1230℃, the soaking time is 30-60min, and the furnace discharge temperature is 1190-1240℃;

[0014] 5) The final rolling temperature of the finishing rolling is 890-940℃, and the coiling temperature is 580-650℃;

[0015] 6) In the continuous annealing process, the temperature of the annealing heating and soaking sections is 770-810°C, the outlet temperature of the slow cooling section is 620-680°C, the outlet temperature of the rapid cooling section is 380-430°C, the temperature of the over-aging section is ≤400°C, the outlet temperature of the final cooling section is ≤160°C, the elongation of the skin pass mill is 0.5-1.4%, and the process section speed is 70-180 m / min;

[0016] The chemical composition of the rail includes by weight: C≤0.0030%; Si≤0.14%; Mn≤0.75%; P≤0.070%; S≤0.015%; Alt≥0.015%; Ti≥0.040%; N≤0.0040%, and the balance is Fe and unavoidable impurities.

[0017] Furthermore, the hot-rolled thickness is 2.5mm-6.0mm, and the cooling mode adopts front-end centralized cooling.

[0018] Furthermore, the cold-rolled thickness is 0.60-1.80 mm.

[0019] Furthermore, the metallographic structure of the steel coil is ferrite.

[0020] Furthermore, the metallographic structure has a grain size of 8.5-10.0.

[0021] Furthermore, the uniformity of mechanical properties of products in each thickness group is achieved, the surface quality FD of the steel strip is good, and the surface roughness range is 0.6-1.9μm.

[0022] Furthermore, its mechanical properties meet the following requirements: yield strength 220-270MPa, tensile strength 340-420MPa, elongation after fracture A 80mm ≥33%, r 90 ≥1.6,n 90 ≥0.18.

[0023] The role and mechanism of each alloy element in the present invention:

[0024] Carbon (C) primarily exists in low-carbon steel as interstitial atoms. Because interstitial C atoms can cause lattice distortion and reduce deep-drawability, an ultra-low C design and the addition of strong carbide-forming elements mitigate this negative impact on formability. Therefore, the C content is set to ≤0.003%. Similarly to C, N atoms occupy interstitial spaces in the lattice, and their content also needs to be limited and fixed to improve formability. Therefore, the N content is set to ≤0.0040%.

[0025] Si is an important reducing agent and deoxidizer in the steelmaking process. Adding a certain amount of Si during aluminum deoxidation can significantly enhance the deoxidation process. Si also provides solid solution strengthening (second only to P), but excessive Si content can deteriorate surface quality. Therefore, the Si content is limited to 0.14% or less.

[0026] As a solid solution strengthening element in steel, Mn can strengthen ferrite and increase the strength of steel. In addition, Mn can replace Fe and S in steel to form MnS, avoiding the formation of FeS (FeS and Fe easily form low-melting-point compounds) that cause hot brittleness of steel, and also plays a deoxidizing role. However, an increase in Mn content will significantly reduce the weldability of steel, increase grain size, and increase the sensitivity to white spot formation and ingot segregation. When the Mn content is greater than 0.80%, excessive Mn causes the grain size to increase, affecting the structural changes of steel during production and reducing the plasticity and toughness of steel. Therefore, the Mn content range is set to ≤0.75%.

[0027] Nb, V, and Ti exist primarily in steel as carbides, nitrides, or oxides, contributing to precipitation strengthening and grain refinement. In automotive steel, in addition to these functions, Nb, V, and Ti also primarily combine with residual C and N to form compounds such as TiC, TiN, NbC, and VC, effectively anchoring interstitial atoms and preventing them from interfering with the crystal lattice, thereby significantly improving the steel's deep-drawing properties. Because Nb and V are relatively expensive, and Ti alone can completely anchor interstitial atoms, the present invention eliminates the addition of Nb and V to reduce costs. Furthermore, based on the calculation formula (Ti addition: ≥1.1-1.3 × (3.4 × N content + 4 × C content)) and actual production conditions, the Ti content is set to ≥0.040%.

[0028] P and S are impurity elements that cannot be completely removed in general steel. They will greatly increase the crack sensitivity of steel, increase the low-temperature brittle transition temperature of steel, and reduce the low-temperature impact performance of steel. Therefore, under the premise of not affecting the performance of steel, it is generally required that the P and S content is as low as possible. However, P is the most powerful and relatively cheap element for strengthening ferrite and is widely used in deep-drawing automotive steel. The present invention sets P≤0.070%, which improves the strength while avoiding the cold brittleness effect it brings. S is still an impurity element in automotive steel, so its content needs to be controlled below 0.015%.

[0029] Al is generally added to steel as a deoxidizer. Its deoxidizing ability is stronger than that of Si and Mn. During the steelmaking process, it preferentially reacts with oxygen to form Al2O3, reducing oxidized inclusions in the steel, improving the purity of the molten steel, and preventing secondary oxidation, thereby improving the ductility and surface quality of the material. In addition, Al can also inhibit the solid solution of the nitrogen element in ferrite, eliminate strain aging, and improve plasticity. However, excessive Al may cause the viscosity of the molten steel to increase, and the risk of nozzle blockage during continuous casting increases. Therefore, according to general design requirements, the present invention sets the acid-soluble aluminum content in the steel to ≥0.015%.

[0030] The reasons for selecting the above process parameters are as follows:

[0031] The steelmaking process strictly controls the oxygen blowing amount to reduce the generation of inclusions; the hot rolling process uses a low heating temperature to effectively control the generation of iron oxide scale and save energy. At the same time, the descaling water pressure is increased to remove the iron oxide scale on the slab surface, improving the surface quality of the hot-rolled steel strip; the continuous annealing process uses high-temperature annealing to achieve rapid recrystallization, and different skin-pass mill elongation rates are used according to thickness group spacing to achieve a stable performance range for products of various thickness specifications. The finished product has a yield strength of 220-270MPa, a tensile strength of 340-420MPa, and an elongation after fracture of A 80mm ≥33% (transverse tensile test: L0=80mm, b0=20mm), n value ≥0.18, r value ≥1.6, surface roughness 0.6-1.9μm, with excellent stamping performance.

[0032] Compared with the prior art, the present invention has the following beneficial technical effects:

[0033] The present invention provides a low-cost 220MPa grade IF high-strength steel for automotive use and its preparation method. By adding only Ti (without other microalloying elements such as Nb and V), the mechanical properties required for 220MPa grade IF high-strength steel are achieved, thereby reducing costs (the cost per ton of steel is reduced by approximately 60 yuan). Actual use has proven that the performance is suitable, and its mechanical properties meet the following requirements: yield strength of 220-270MPa, tensile strength of 340-420MPa. Elongation after fracture A 80mm ≥33%, r90≥1.6, n90≥0.18. The production method of automobile steel is simple, highly operable and easy to promote and apply. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The present invention will be further described below with reference to the accompanying drawings.

[0035] Figure 1 This is the metallographic structure of the finished product. DETAILED DESCRIPTION

[0036] A low-cost 220MPa grade IF high-strength steel for automobiles and a preparation method thereof. The production method specifically comprises the following steps: steelmaking step - hot rolling step - cold rolling step - continuous annealing step.

[0037] The steelmaking process includes top- and bottom-blowing smelting in a converter (BOF), followed by RH (Reverse Oxygen Refractory) and slab continuous casting. After desulfurization pretreatment, the molten iron enters the BOF. The sulfur content of the molten iron entering the BOF must be less than 0.005%, and the slag removal area must be greater than 95%. The BOF tapping temperature must be ≥1670°C, and a tundish must be used for tapping. The tapping temperature drop must be less than 70°C. The BOF molten steel must contain C ≤ 0.04%, Mn ≤ 0.70%, and O ≤ 0.040-0.07%. The RH furnace undergoes deep decarburization. After decarburization, aluminum granules are added to adjust the oxygen content. After a cycle of at least 4 minutes, alloys such as ferromanganese and ferrotitanium are added to adjust the composition. After this adjustment, a vacuum cycle of ≥6 minutes is maintained. For slab continuous casting, the tundish superheat during casting is controlled between 30 and 45°C, and the production casting speed is 1.0 to 1.8 m / min. According to the above steelmaking process requirements, the actual slab chemical composition (mass percentage) is shown in Table 1 below, with the balance being Fe and unavoidable impurities (Note: Since the comparative example and the embodiment have different composition designs, their production process parameters are also different from those described in the present invention and are only listed here for comparison).

[0038] Table 1: Chemical composition (%)

[0039]

[0040]

[0041] The hot rolling process includes: billet heating, high-pressure water descaling, width-setting press, rolling in the E1 R1 roughing mill, rolling in the E2R2 roughing mill, flying shearing, high-pressure water descaling, rolling in the F1-F7 finishing mill, dense laminar cooling, and coiling. This process uses a walking beam furnace to heat the billet (see Table 2 for the heating process), a dual-stand R1 and R2 reciprocating rolling system for roughing, and an F1-F7 continuous rolling process for finishing. Cooling is front-end centralized cooling. The specific hot rolling process is shown in Table 3.

[0042] Table 2 Slab heating system

[0043] Heating temperature / ℃ Heating time / min Soaking temperature / ℃ Soaking time / min Oven temperature / ℃ 1180-1230 150-250 1180-1230 30-60 1190-1240

[0044] Table 3 Rolling process

[0045] Rolling thickness / mm Rough rolling and finishing temperature / ℃ Finish rolling temperature / ℃ Coiling temperature / ℃ 3.8-5.0 1000-1100 890-940 580-650

[0046] The cold rolling process includes: pickling uncoiling - welding - straightening - pickling - rinsing - drying - trimming - continuous rolling mill cold rolling - coiling;

[0047] The continuous annealing process includes: continuous annealing uncoiling, welding, cleaning, entry looper, annealing furnace, exit looper, leveling, edge trimming, surface inspection, oiling, sampling, coiling, weighing, and packaging. Annealing is performed in a vertical continuous annealing furnace, using a reducing atmosphere and a nitrogen-hydrogen mixed protective atmosphere for cooling. The specific process specifications for the continuous annealing process are shown in Table 4.

[0048] Table 4: Annealing process

[0049]

[0050]

[0051] After hot rolling, cold rolling and continuous annealing, the room temperature tensile mechanical properties of the finished product are shown in Table 5. The metallographic structure is equiaxed ferrite with a grain size of 8.5-10.0 and a microstructure of Figure 1 .

[0052] Table 5: Room temperature tensile properties of finished products

[0053]

[0054] In summary, this product has passed the performance test and all performance requirements are met. It has also met the forming requirements after user trial use and can be promoted and used on a large scale.

[0055] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing low-cost 220 MPa grade IF high-strength steel for automobiles, comprising: Steelmaking process - hot rolling process - cold rolling process - continuous annealing process; wherein: the steelmaking process includes: converter top and bottom blowing smelting - RH - slab continuous casting; the hot rolling process includes: slab heating - high-pressure water descaling - width setting press - E1R1 roughing mill rolling - E2R2 roughing mill rolling - insulation cover - flying shear - high-pressure water descaling - F1-F7 finishing mill rolling - dense laminar cooling - coiling; the cold rolling process includes: pickling uncoiling - welding - tension leveling - pickling - rinsing - drying - trimming - continuous rolling mill cold rolling - coiling; the continuous annealing process includes: continuous annealing uncoiling - welding - cleaning - entry looper - annealing furnace - exit looper - leveling - trimming - surface inspection - oiling - sampling - coiling - weighing - packaging; the characteristics are: 1) Converter production After desulfurization pretreatment, the molten iron is smelted in a converter. The sulfur content of the molten iron entering the converter must be less than 0.005%, and the slag removal area of the molten iron must be greater than 95%. The converter tapping temperature must be ≥1670°C, and a turnover bag must be used for tapping. The tapping temperature drop must be less than 70°C. The RH molten steel supplied by the converter must contain [C]≤0.04%, [Mn]≤0.70%, and [O]: 0.040-0.07%. 2) RH furnace production The RH furnace performs deep decarburization. After decarburization, aluminum particles are added for deoxidation according to the oxygen concentration. After the cycle lasts for more than 4 minutes, ferromanganese and ferrotitanium are added to adjust the composition. After the composition adjustment is completed, ensure that the vacuum cycle lasts ≥ 6 minutes. 3) Continuous casting production During slab continuous casting, the superheat of the tundish during casting is controlled within the range of 30-45°C, and the production casting speed is 1.0-1.8 m / min; 4) During the heating of the casting, the heating temperature is 1180-1230℃, the heating time is 150-250min, the soaking temperature is 1180-1230℃, the soaking time is 30-60min, and the furnace discharge temperature is 1190-1240℃; 5) The final rolling temperature of the finishing rolling is 890-940℃, and the coiling temperature is 580-650℃; 6) In the continuous annealing process, the temperature of the annealing heating and soaking sections is 770-810°C, the outlet temperature of the slow cooling section is 620-680°C, the outlet temperature of the rapid cooling section is 380-430°C, the temperature of the over-aging section is ≤400°C, the outlet temperature of the final cooling section is ≤160°C, the elongation of the skin pass mill is 0.5-1.4%, and the process section speed is 70-180 m / min; The chemical composition of the rail includes by weight: C≤0.0030%; Si≤0.14%; Mn≤0.75%; P≤0.070%; S≤0.015%; Alt≥0.015%; Ti≥0.040%; N≤0.0040%, and the balance is Fe and unavoidable impurities.

2. The method for preparing low-cost 220 MPa grade IF high-strength steel for automobiles according to claim 1, characterized in that: The hot-rolled thickness is 2.5mm-6.0mm, and the cooling mode adopts front-end centralized cooling.

3. The method for preparing low-cost 220 MPa grade IF high-strength steel for automobiles according to claim 1, characterized in that: The cold rolled thickness is 0.60-1.80mm.

4. The method for preparing low-cost 220 MPa grade IF high-strength steel for automobiles according to claim 1, characterized in that: The metallographic structure of the steel coil is ferrite.

5. The method for preparing low-cost 220 MPa grade IF high-strength steel for automobiles according to claim 4, characterized in that: The metallographic grain size is 8.5-10.

0.

6. The method for preparing low-cost 220 MPa grade IF high-strength steel for automobiles according to claim 1, characterized in that: Achieve uniformity of mechanical properties of products in each thickness group, steel strip surface quality FD, and surface roughness range of 0.6-1.9μm.

7. The method for preparing low-cost 220 MPa grade IF high-strength steel for automobiles according to claim 1, characterized in that: Its mechanical properties meet the following requirements: yield strength 220-270MPa, tensile strength 340-420MPa, elongation after fracture A 80mm ≥33%, r 90 ≥1.6,n 90 ≥0.18.

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