Preparation method of low-cost 180-grade IF high-strength steel for automobiles

Abstract

The invention discloses a preparation method of a heat-treated steel rail with good comprehensive mechanical properties. The preparation method comprises a steelmaking process, a hot rolling process, a cold rolling process and a continuous annealing process, 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; al t is greater than or equal to 0.015%; ti is greater than or equal to 0.035%; n is less than or equal to 0.0040%, and the balance is Fe and inevitable impurities. According to the preparation method of the low-cost 180-grade IF high-strength steel for the automobile, the strength and forming performance of the yield strength 180-grade IF high-strength steel for the automobile are guaranteed, and meanwhile the production and manufacturing cost of the 180-grade IF high-strength steel for the automobile is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of metallurgy and metal materials, and particularly relates to a preparation method of a low-cost 180-grade IF high-strength steel for automobiles. Background Art

[0002] The greening and cost reduction of automotive steels have become the focus of common concern in the steel and automotive industries. The 180-grade IF steel with a yield strength is commonly used in multiple components such as the front and rear outer panels of automobiles, the left and right front fenders, the upper and lower outer panels of the trunk lid, and the outer panel of the engine lid, with a large consumption and high demand. Since traditional IF steels rely on adding microalloying elements such as niobium (Nb) and titanium (Ti) to achieve high strength, resulting in a relatively high alloy cost, it is of great significance to develop a low-cost 180 MPa-grade IF high-strength steel for automobiles. Characteristics and applications of IF steel: IF steel (Interstitial Free steel) is a kind of ultra-low carbon steel with excellent deep drawing performance and non-aging property, and is widely used in fields such as automotive sheets and household appliance sheets. Its main principle is that on the premise of very low carbon and nitrogen contents, by adding microalloying elements such as Nb and Ti to combine with carbon and nitrogen to form carbonitride precipitates, thereby making the steel in an interstitial free atom state. This steel grade needs to have good formability, corrosion resistance and surface quality while ensuring high strength to meet the manufacturing requirements of automotive outer panels and complex structural parts. The present invention optimizes the composition design and preparation process to reduce the alloy cost, so as to achieve the low-cost of 180-grade IF high-strength steel for automobiles. Summary of the Invention

[0003] The purpose of the present invention is to provide a preparation method of a low-cost 180-grade IF high-strength steel for automobiles, which reduces the production and manufacturing cost while ensuring the strength and formability of the 180-grade automotive steel with a yield strength.

[0004] To solve the above technical problems, the present invention adopts the following technical solutions:

[0005] A preparation method of a low-cost 180-grade IF high-strength steel for automobiles according to the present invention includes:

[0006] 1) Converter production

[0007] The hot metal is subjected to desulfurization pretreatment and then smelted in a converter. It is required that the sulfur content of the hot metal entering the converter is less than 0.005%, and the slag cleaning area of the hot metal is greater than 95%; the tapping temperature of the converter is ≥1670 °C, and it is necessary to use a turnover ladle for tapping, and the tapping temperature drop is required to be less than 70 °C. In the RH steel supplied by the converter, [C] ≤ 0.04%, [Mn] ≤ 0.60%, and [O]: 0.040 - 0.07%;

[0008] 2) RH furnace production

[0009] The RH furnace is subjected to deep decarburization treatment. After decarburization is completed, aluminum pellets are added for deoxidation according to the oxygen determination situation. After circulating for more than 4 minutes, ferro-manganese, ferro-titanium and other alloys are added to adjust the composition. After the composition adjustment is completed, ensure that the vacuum circulation is ≥6 minutes;

[0010] 3) Continuous casting production

[0011] Slab continuous casting, the superheat control range of the tundish during the casting process of the casting machine is 30-45°C, and the production casting speed is 1.0-1.8 m / min;

[0012] 4) During the heating of the casting blank, the heating temperature is 1170-1230°C, the heating time is 150-250 min, the soaking temperature is 1180-1230, the soaking time is 25-60 min, and the tapping temperature is 1180-1230°C;

[0013] 5) The finishing rolling temperature of the finish rolling is 880-950°C, and the coiling temperature is 580-650°C;

[0014] 6) In the continuous annealing process, the annealing heating and soaking section temperatures are 770-830°C, the outlet temperature of the slow cooling section is ≤700°C, the outlet temperature of the rapid cooling section is ≤450°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 0.5-1.0%, and the speed of the process section is 50-200 m / min;

[0015] The weight percentages of the chemical components of the rail include: C≤0.0030%; Si≤0.14%; Mn≤0.75%; P≤0.070%; S≤0.015%; Alt≥0.015%; Ti≥0.035%; N≤0.0040%, and the balance is Fe and unavoidable impurities.

[0016] Furthermore, the hot rolling thickness is: 3.0 mm - 5.5 mm.

[0017] Furthermore, the cold rolling thickness is 0.60 - 1.80 mm.

[0018] Furthermore, the microstructure of the rail is ferrite.

[0019] Furthermore, the grain size of the microstructure is 8.0 - 10.0 grades.

[0020] Furthermore, the mechanical property uniformity of products with each thickness group distance is achieved, the surface quality of the steel strip is FD, and the surface roughness range is 0.6 - 1.9 μm.

[0021] Furthermore, the mechanical properties of the rail meet: yield strength 180 - 240 MPa, tensile strength ≥340 MPa. Elongation after fracture A 80mm ≥34%, r 90 ≥1.7, n 90≥0.19.

[0022] Functions and mechanisms of each alloying element in the present invention:

[0023] The main functions of C in steel are to increase strength and hardness, promote pearlite transformation, and form carbides. With the increase of C content, the strength and hardness of steel increase, while plasticity and toughness decrease. The main forms of C in steel are dissolved in steel as interstitial atoms or forming alloy cementite with Fe or other metal elements. However, interstitial atom C will cause lattice distortion of the metal and reduce formability (such as deep drawing performance). Therefore, the negative impact on formability is eliminated through ultra-low C design, so the C content range is set to ≤0.003%. In addition, similar to C, N atoms will also occupy lattice interstices, and its content needs to be strictly limited to improve the ductility of steel, and the N content is set to ≤0.0040%.

[0024] The main function of Si in steel is to inhibit the formation of cementite and, as a solid solution strengthening element, improve the strength and hardness of the ferrite matrix. In automotive steels, its main function is to assist in deoxidation. Excessive content will deteriorate the surface quality. Therefore, the Si content range is set to ≤0.14%.

[0025] As a solid solution strengthening element and carbide forming element in steel, Mn can strengthen both ferrite and cementite, improving the strength and hardness of steel. In addition, Mn element in steel can also replace Fe to form MnS with S element, avoiding the formation of FeS (FeS and Fe are easy to form low melting point compounds) which causes hot brittleness of steel, and at the same time can also play a role in deoxidation. Mn can also lower the pearlite transformation temperature, and then reduce the pearlite lamellar spacing. However, the increase of Mn content will significantly reduce the welding performance of steel, increase the grain size, and increase the sensitivity to the formation of white spots and slab segregation. There is no cementite in the microstructure of automotive steels, so its strengthening effect on carbides is considered; when the Mn content > 0.80%, excessive Mn element will cause the grain size to increase, affecting the microstructure change of steel during the production process and reducing the plasticity and toughness of steel. Considering comprehensively, the Mn content range is set to ≤0.75%.

[0026] Nb, V, and Ti mainly exist in the form of carbides, nitrides, or oxides in steel, playing the role of precipitation strengthening and grain refinement. In automotive steels, in addition to the above functions, Nb, V, and Ti also mainly play the role of combining with residual C and N to form compounds such as TiC, TiN, NbC, and VC, completely fixing interstitial atoms and avoiding their interference with the lattice, thus significantly improving the deep drawing performance of steel. However, due to the high prices of Nb and V, and only Ti element can play the role of completely fixing interstitial atoms, so Nb and V elements are not designed to be added to reduce costs. At the same time, according to the calculation formula, the addition amount of Ti ≥ 1.1 - 1.3×(3.4×N content + 4×C content), and the Ti content range is set to ≥0.035%.

[0027] P and S are both impurity elements that cannot be completely removed in ordinary steel. They will significantly increase the crack sensitivity of the steel, and at the same time increase the low-temperature brittle transition temperature of the steel, reducing the low-temperature impact performance of the steel. Therefore, on the premise of not affecting the performance of the steel, it is generally required that the contents of P and S be as low as possible. However, a certain amount of P content needs to be added or retained in automotive steel to improve the strength of automotive steel through its solid solution strengthening effect. Therefore, it is set that P ≤ 0.070%, while avoiding the cold brittleness effect brought by it while increasing the strength. S is still an impurity element in automotive steel, so its content needs to be controlled below 0.015%.

[0028] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0029] A low-cost 180-grade IF high-strength steel for automobiles and its preparation method provided by the present invention can meet the mechanical property requirements of 180-grade IF high-strength steel by only adding Ti element (without adding other micro-alloying elements such as Nb and V), so as to achieve the purpose of reducing costs (the cost per ton of steel is reduced by about 60 yuan). It has been proved by actual use that the performance is suitable, and its mechanical properties meet: yield strength 180 - 240 MPa, tensile strength ≥ 340 MPa. Elongation after fracture A 80mm ≥ 34%, r90 ≥ 1.7, n90 ≥ 0.19. The production method of this automotive steel is simple, highly operable, and easy to promote and apply. Brief Description of the Drawings

[0030] The present invention will be further described below in conjunction with the drawings.

[0031] Figure 1 It is the finished product metallographic structure. Detailed Embodiments

[0032] A low-cost 180-grade IF high-strength steel for automobiles and its preparation method. The production method specifically includes the following processes: steelmaking process - hot rolling process - cold rolling process - continuous annealing process.

[0033] The steelmaking process includes: combined blowing top and bottom in converter - RH - slab continuous casting. After the hot metal is pretreated by desulfurization, it is smelted in the converter. The sulfur content of the hot metal entering the converter is required to be less than 0.005%, and the cleaning area of the hot metal slag is greater than 95%; the tapping temperature of the converter is ≥1670°C, and a turnover ladle must be used for tapping, and the tapping temperature drop is required to be less than 70°C. In the molten steel supplied by the converter to RH, [C] ≤ 0.04%, [Mn] ≤ 0.60%, [O]: 0.040 - 0.07%. Deep decarburization treatment is carried out in the RH furnace. After the decarburization is completed, aluminum pellets are added for deoxidation according to the oxygen determination situation. After circulating for more than 4 minutes, ferro-manganese, ferro-titanium and other alloys are added to adjust the composition. After the composition adjustment is completed, ensure that the vacuum circulation is ≥6 minutes; for slab continuous casting, the superheat control range of the tundish during the casting process of the caster is 30 - 45°C, and the production casting speed is 1.0 - 1.8 m / min. According to the above requirements of the steelmaking process, the actual chemical composition (mass percentage) of the slab is shown in Table 1 below, and the balance is Fe and unavoidable impurities (Note: Since the composition design schemes of the comparative example and the example are different, the production process parameters are also different from those described in the present invention, and only a list comparison is made here).

[0034] Table 1: Chemical composition (%)

[0035]

[0036]

[0037] The hot rolling process includes: billet heating - high-pressure water descaling - width press - rough rolling by E1R1 roughing mill - rough rolling by E2R2 roughing mill - flying shear - high-pressure water descaling - finishing rolling by F1 - F7 finishing mills - intensified laminar cooling - coiling; this process uses a walking beam reheating furnace to heat the billets (the heating process is shown in Table 2), and the rough rolling uses a two-stand R1 and R2 reciprocating rolling, and the finishing rolling uses an F1 - F7 continuous rolling process. The specific hot rolling process is shown in Table 3.

[0038] Table 2 Billet heating system

[0039] Heating temperature / °C Heating time / min Soaking temperature / °C Soaking time / min Temperature at the end of rolling / °C 1170-1230 150-250 1180-1230 25-60 1180-1230

[0040] Table 3 Rolling process

[0041] Rolling thickness / mm Temperature at the end of rough rolling / °C Temperature at the end of finish rolling / °C Coiling temperature / °C 3.5-5.0 1000-1100 880-950 580-650

[0042] The cold rolling process includes: uncoiling in acid rolling - welding - tension leveling - pickling - rinsing - drying - trimming - cold rolling by a continuous rolling mill - coiling;

[0043] The continuous annealing process includes: uncoiling in continuous annealing - welding - cleaning - inlet loop - annealing furnace - outlet loop - skin pass - trimming - surface inspection - oiling - sampling - coiling - weighing - packaging; vertical continuous annealing furnace is used for annealing, and reducing atmosphere and nitrogen-hydrogen mixed protective atmosphere are used for cooling in the furnace. The specific process system of the continuous annealing process is shown in Table 4.

[0044] Table 4: Annealing process

[0045]

[0046] Through hot rolling, cold rolling and continuous annealing processes, the room temperature tensile mechanical properties of the finished product are shown in Table 5. The metallographic structure is equiaxed ferrite, the grain size is 8.0 - 10.0 grades, and the microstructure morphology is shown in Figure 1 .

[0047] Table 5: Room temperature tensile properties of the finished product

[0048]

[0049] To sum up, the performance of this product meets the requirements through performance inspection, and meets the forming requirements through trial die use by users, and can be widely promoted and used.

[0050] The above-described embodiments are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A preparation method of low-cost 180-grade IF high-strength steel for automobiles, characterized in that: Including: 1) Converter production After the hot metal is pretreated by desulfurization, converter smelting is carried out. It is required that the sulfur content of the hot metal entering the converter is less than 0.005%, and the slag cleaning area of the hot metal is greater than 95%; the tapping temperature of the converter is ≥1670°C, and the tapping ladle must be used for tapping, and the tapping temperature drop is required to be less than 70°C. The [C] in the RH molten steel supplied by the converter is ≤0.04%, [Mn] is ≤0.60%, and [O] is 0.040 - 0.07%; 2) RH furnace production Deep decarburization treatment is carried out in the RH furnace. After the decarburization is completed, aluminum particles are added for deoxidation according to the oxygen determination situation. After circulating for more than 4 minutes, ferro-manganese, ferro-titanium and other alloys are added to adjust the composition. After the composition adjustment is completed, ensure that the vacuum circulation is ≥6 minutes; 3) Continuous casting production Slab continuous casting, the superheat control range of the tundish during the casting process of the caster is 30 - 45°C, and the production casting speed is 1.0 - 1.8 m / min; 4) During the heating of the cast slab, the heating temperature is 1170 - 1230°C, the heating time is 150 - 250 min, the soaking temperature is 1180 - 1230, the soaking time is 25 - 60 min, and the furnace outlet temperature is 1180 - 1230°C; 5) The finishing rolling temperature of finish rolling is 880 - 950°C, and the coiling temperature is 580 - 650°C; 6) In the continuous annealing process, the annealing heating and soaking section temperatures are 770 - 830°C, the outlet temperature of the slow cooling section is ≤700°C, the outlet temperature of the rapid cooling section is ≤450°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 - 1.0%, and the speed of the process section is 50 - 200 m / min; The weight percentages of the chemical components of the said rail include: C ≤ 0.0030%; Si ≤ 0.14%; Mn ≤ 0.75%; P ≤ 0.070%; S ≤ 0.015%; Alt ≥ 0.015%; Ti ≥ 0.035%; N ≤ 0.0040%, and the balance is Fe and unavoidable impurities.

2. The preparation method of the low-cost 180-grade IF high-strength steel for automobiles according to claim 1, wherein: The hot-rolled thickness is: 3.0 mm - 5.5 mm.

3. The preparation method of the low-cost 180-grade IF high-strength steel for automobiles according to claim 1, wherein: The cold-rolled thickness is 0.60 - 1.80 mm.

4. The preparation method of the low-cost 180-grade IF high-strength steel for automobiles according to claim 1, characterized in that: The metallographic structure of the said rail is ferrite.

5. The preparation method of the low-cost 180-grade IF high-strength steel for automobiles according to claim 4, characterized in that: The grain size of the metallographic structure is 8.0 - 10.0 grades.

6. The preparation method of the low-cost 180-grade IF high-strength steel for automobiles according to claim 1, characterized in that: This method realizes the mechanical property uniformity of products in each thickness group interval, the surface quality of the steel strip is FD, and the surface roughness range is 0.6 - 1.9 μm.

7. The preparation method of the low-cost 180-grade IF high-strength steel for automobiles according to claim 1, characterized in that: The mechanical properties of the rail meet the following requirements: yield strength of 180 - 240 MPa, tensile strength ≥ 340 MPa. Elongation after fracture A 80mm ≥ 34%, r 90 ≥ 1.7, n 90 ≥ 0.19.

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