A low cost method for manufacturing high strength automotive steel using ti microalloy precipitation strengthening without a heat shield treatment

Abstract

The application discloses a low-cost method for manufacturing high-strength automobile steel by using Ti micro-alloying precipitation strengthening without heat insulation cover treatment, adopts a C-Si-Mn-Ti component design idea, and utilizes the synergistic effect of solid solution strengthening elements Mn and Si and fine-grain strengthening and precipitation strengthening element Ti to improve the strength and toughness of the material, so that the high-strength 700MPa-grade automobile steel strip with the yield strength ≥600MPa, the tensile strength ≥700MPa, the elongation A 50 ≥18% and the impact performance at-20 DEG C satisfies AKv2 ≥100J is manufactured in a low-cost mode without heat insulation cover treatment.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical materials, specifically relating to a low-cost method for manufacturing high-strength automotive steel using Ti microalloy precipitation strengthening without the need for heat insulation cover treatment. Background Technology

[0002] In recent years, with the increasing emphasis on energy conservation and emission reduction by the state, end-users in various industries have placed increasingly higher demands on the strength of steel materials to meet the trend of weight reduction. This has led to the expanding development and application of high-strength steel. In 2016, the China Society of Automotive Engineers explicitly proposed a weight reduction target for commercial vehicles, aiming for a 35% reduction by 2035. High-strength steel with a strength of 700MPa and above has gradually become the main material for structural components such as automotive beams, bodies, and axles. How to efficiently and cost-effectively produce high-quality high-strength steel has become a key research focus and hot topic in steel materials. Mature research and application of low-alloy high-strength steel mainly utilizes solid solution strengthening, grain refinement strengthening, and microstructure strengthening as primary methods. The main alloying elements added include Mn, Cr, Nb, Ti, V, Cu, and Mo, primarily in the form of acicular ferrite and low-carbon bainite. However, the addition of expensive alloying elements results in higher product costs, which to some extent affects the mass application and promotion of high-strength steel. The goal of this invention is to develop low-alloy high-strength steel with a tensile strength ≥700MPa, mainly composed of acicular ferrite and polygonal ferrite, by utilizing the relatively inexpensive microalloying element Ti, and by rationally controlling the content of key component elements C, S, and N, as well as the controlled rolling and controlled cooling process parameters, to leverage the precipitation strengthening effect of alloyed Ti.

[0003] Patent document CN1962099A discloses "a method for producing 700MPa high-strength weathering steel using Ti microalloying process based on thin slab continuous casting and rolling process". Its composition design system is C (0.03%~0.07%)-Si (0.30%~0.50%)-Mn (0.60%~1.60%)-P (≤0.040%)-S (≤0.008%)-Cu (0.20%~0.50%)-Cr (0.30%~0.70%)-Ni (0.15%~0.35%)-Ti (0.08%~0.14%)-Al (0.025%~0.040%), N≤0.0080, and the remainder is Fe. The production process employs thin slab continuous casting and rolling technology, with a slab thickness of 60mm, a furnace tapping temperature of 1100–1180℃, FDT: 870–920℃, and CT: 550–650℃. This belongs to the thin slab continuous casting and rolling (CSP) process.

[0004] Patent document CN 110616301 A discloses a "production method for improving the precipitation strengthening effect of Ti microalloyed hot-rolled high-strength steel online". It mainly discloses that the amount of microalloying element Ti added is 0.03 to 0.10 wt%, and the coiling temperature of the steel coil is set at 500-700℃. The method refers to covering each hot-rolled coil with an online heat preservation cover after uncoiling, and keeping the steel coil online for ≥60 minutes to improve the precipitation strengthening effect of TiC.

[0005] The literature “A Brief Analysis of the Slow Cooling Process of 2050 Finished High-Strength Steel” introduces a method of using a slow cooling wall to control the cooling process of high-strength steel coils such as BS600MC and BS700MC in the warehouse, in order to improve the precipitation strengthening effect, internal stress distribution and improve the quality of the plate shape.

[0006] The literature "Research and Implementation of Construction Scheme for 620mm Strip Steel Slow Cooling Pit" proposes using a slow cooling pit to control the temperature of steel coils over a 48-hour slow cooling cycle, thereby achieving uniform overall temperature of the coils. However, in actual production, it has been found that the above-mentioned slow cooling process cannot effectively maintain the temperature of the steel coils in a timely manner, and the heat preservation effect is greatly affected by the environment of the slow cooling zone. In particular, it is difficult to achieve effective heat preservation for Ti microalloyed hot-rolled high-strength steel coils, thus failing to improve the precipitation strengthening effect. Summary of the Invention

[0007] To address the problems existing in the prior art, this invention provides a low-cost manufacturing method for high-strength automotive structural steel with a thickness of 2-6 mm and a tensile strength ≥700 MPa, based on a 2250 mm hot continuous rolling mill (slab heating-roughing-finishing-layer cooling-coiling-stacking slow cooling). This method leverages the microalloying of Ti during slab solidification, precipitation strengthening during rolling, and sedimentation strengthening during coiling. It also works synergistically with solid solution strengthening elements Mn and Si to improve the material's strength and toughness. The innovations of this method lie in the material composition design that fully utilizes the strengthening effect of low-cost Ti microalloying, the narrow-range control of C, S, and N elements during the smelting process, and the TMCP temperature and deformation control strategy during hot rolling. This results in a low-cost, easily formable, high-rigidity automotive high-strength steel that requires no insulation cover. This material is primarily used in high-strength structural steels for commercial vehicle beams, bodies, and axles.

[0008] The present invention is achieved through the following technical solution.

[0009] One aspect of this invention provides a low-cost method for manufacturing high-strength automotive steel using Ti microalloying precipitation strengthening without the need for a heat insulation cover. The high-strength automotive steel has the following chemical composition by mass percentage: C: 0.04%–0.08%; Si: 0.05%–0.20%; Mn: 1.00%–1.50%; P ≤ 0.015%; S ≤ 0.005%; Al ≤ 0.040%; Ti: 0.070%–0.100%; H ≤ 2.0 ppm; O ≤ 30 ppm; N ≤ 50 ppm; the remainder being Fe and unavoidable inclusions.

[0010] The method includes the following processes: smelting continuous casting process, rolling process, and slow cooling process for steel coil stacking in the warehouse; wherein:

[0011] The smelting and continuous casting process is as follows: molten iron → converter smelting → LF refining → RH degassing → slab continuous casting; converter smelting uses KR pre-desulfurized molten iron, and ferrosilicon, ferromanganese and fertitanium are deoxidized and alloyed, and molten steel is stirred by argon blowing throughout the process; vacuum degree ≤2mbar, deep vacuum time ≥10min, superheat ΔT≤30℃.

[0012] The rolling process is as follows: 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; the slab heating temperature is 1250±20℃; the heating time is 180~240min; the roughing mode adopts 1+5, 3+3 or 3+5; the intermediate slab thickness range is 35~45mm; the finishing mill opening temperature is 970~1100℃; the finishing mill closing temperature is 840~890℃; the coiling temperature is 560~620℃; and the cooling rate is 20~40℃ / s.

[0013] Slow cooling process for steel coil storage area: Use steel coils with the same or similar winding temperature range (≥10 coils) to form a slow cooling atmosphere by stacking them together. The steel coils can be set as single or double layers. The spacing between the steel coils in the transverse direction is controlled at 50-100mm. At the same time, the slow cooling area should be selected far away from the ventilation opening of the steel coil storage area. The slow cooling time should be ≥72 hours and the slow cooling rate should be 5-10℃ / hour.

[0014] In some embodiments, the mechanical properties of the high-strength automotive steel meet the following requirements: yield strength ≥ 600 MPa, tensile strength ≥ 700 MPa, and elongation A. 50 ≥18%; Impact performance at -20℃ meets AKv2≥100J (full size).

[0015] In some embodiments, the mechanical properties of the high-strength automotive steel meet the following requirements: yield strength ≥ 660 MPa, tensile strength ≥ 730 MPa, and elongation A. 50≥19%; Impact performance at -20℃ meets AKv2≥155J (full size), 0.89≤yield ratio≤0.93, grain size≥11 grade.

[0016] In some embodiments, the metallographic structure of the high-strength automotive steel is granular bainite, polygonal ferrite, and pearlite.

[0017] In some embodiments, the chemical composition of the high-strength automotive steel, by mass percentage, is: C: 0.07%; Si: 0.14%; Mn: 1.05%; P: 0.012%; S: 0.003%; Alt: 0.030%; Ti: 0.070%; H ≤ 2.0 ppm; O ≤ 30 ppm; N: 44 ppm; the remainder being Fe and unavoidable inclusions.

[0018] In some embodiments, the chemical composition of the high-strength automotive steel, by mass percentage, is: C: 0.08%; Si: 0.15%; Mn: 1.40%; P: 0.010%; S: 0.003%; Alt: 0.025%; Ti: 0.080%; H ≤ 2.0 ppm; O ≤ 30 ppm; N: 40 ppm; the remainder being Fe and unavoidable inclusions.

[0019] In some embodiments, the chemical composition of the high-strength automotive steel, by mass percentage, is: C: 0.07%; Si: 0.18%; Mn: 1.50%; P: 0.013%; S: 0.002%; Alt: 0.032%; Ti: 0.088%; H ≤ 2.0 ppm; O ≤ 30 ppm; N: 38 ppm; the remainder being Fe and unavoidable inclusions.

[0020] In some embodiments, the chemical composition of the high-strength automotive steel, by mass percentage, is: C: 0.075%; Si: 0.15%; Mn: 1.30%; P: 0.012%; S: 0.004%; Alt: 0.037%; Ti: 0.092%; H ≤ 2.0 ppm; O ≤ 30 ppm; N: 45 ppm; the remainder being Fe and unavoidable inclusions.

[0021] Another aspect of the present invention provides a high-strength automotive steel, which is manufactured by the method described above.

[0022] The advantages of this invention are as follows: This invention utilizes a C-Si-Mn-Ti composition design, leveraging the synergistic effects of solid solution strengthening elements Mn and Si, and fine-grain strengthening and precipitation strengthening elements Ti to improve the material's strength and toughness. It employs a controlled cooling process using a 2250mm hot rolling mill (slab heating - rough rolling - finish rolling - layer cooling - coiling - stacking slow cooling), and a control scheme for the slow cooling field of the steel strip in the warehouse is designed. This eliminates the need for specially designed insulation covers or pits, not only reducing product manufacturing costs and improving production efficiency, but also ensuring that the product's mechanical properties meet the requirements of yield strength ≥600MPa, tensile strength ≥700MPa, and elongation A. 50 High-strength 700MPa automotive steel strip with an impact performance of ≥18% and meeting AKv2≥100J (full-size) at -20℃, achieving industrial production on a 2250mm hot-rolling production line, and possessing good promotional value. The product can be applied to automotive beams, bodies, and drive shafts. Attached Figure Description

[0023] Figure 1 Metallographic image of the 700MPa grade high-strength automotive steel manufactured for Example 3.

[0024] Figure 2 Metallographic image of the 700MPa grade high-strength automotive steel manufactured for Example 4. Detailed Implementation

[0025] The present invention will be described in detail below through specific embodiments. These embodiments are intended to help understand the present invention and are not intended to limit the scope of the present invention.

[0026] 1. Smelting of materials

[0027] 1.1 Composition Design: The composition design scheme is based on the thickness of the product, as shown in Table 1.

[0028] Table 1: Ingredient Design (Percentage by Mass)

[0029]

[0030] 1.2 Converter smelting: The pretreated molten iron (S≤0.003%) enters the converter, where oxygen is blown to decarburize and raise the temperature. In the later stage of smelting, ferrosilicon and ferromanganese are added for deoxidation and alloying. The P and S content is controlled to prevent over-oxidation of the molten steel and to control the tapping rate to prevent steel slag from entering the molten steel. The tapping temperature is 1600~1650℃. The [P] of the converter tapped steel is ≤0.015%, and [S] is ≤0.008%.

[0031] 1.3 Ladle refining: The LF+RH full-process argon blowing process is adopted. The refining process maintains a good reducing atmosphere. Al wire is used for deoxidation. In the later stage of LF, manganese iron is added for alloying. In the later stage of RH vacuum treatment (vacuum degree ≤2mbar, deep vacuum time ≥10min), titanium iron is added for alloying. After the process is completed, calcium treatment is performed to ensure that the N in the molten steel is ≤50ppm.

[0032] 1.4 Slab continuous casting: molten steel superheat ΔT ≤ 30℃ (20~30℃), casting speed controlled at 0.90~1.10m / min. Equipped with dynamic light reduction technology, straightening temperature ≥ 920℃.

[0033] 1.5 Full-process nitrogen control: N in the molten steel ladle must be ≤50ppm, scrap steel ratio ≤85%, converter tapping temperature ≥1600℃, and bottom blowing argon stirring in the converter is required; alternating argon and nitrogen bottom blowing stirring is not permitted. The ladle is purged with argon gas. Before continuous casting begins and before heat exchange, the ladle is purged with argon gas for ≥10 minutes to remove air. When 30t of steel is poured from the ladle, the argon blowing pipe is removed to reduce nitrogen absorption by the molten steel. During normal casting, full-process argon blowing protection is used.

[0034] 2. Controlled rolling and controlled cooling process

[0035] The billet is heated by a walking beam furnace (heating process is shown in Table 2). The roughing rolling adopts a single stand R1 and R2 reciprocating rolling process, and the roughing rolling mode is 1+5, 3+3 and 3+5. The finishing rolling adopts the F1 to F7 continuous rolling process. The specific controlled rolling and controlled cooling process is shown in Table 3.

[0036] Table 2: Heating Regime for Cast Billets

[0037]

[0038] Table 3: Rolling Process

[0039]

[0040] 3. Example

[0041] 3.1 Based on the above smelting technical requirements, smelt Ti microalloyed high-strength steel billets with the following composition, as shown in Table 4 below.

[0042] Table 4: Chemical composition of each example (mass percentage: %)

[0043]

[0044] 3.2 Based on the above composition design and hot rolling process, the product performance indicators are shown in Table 5. Test methods refer to GB / T228 and GB / T 229.

[0045] Table 5: Performance of Each Example Product

[0046]

[0047] 3.3 The metallographic structure of the product consists of granular bainite, polygonal ferrite, and pearlite. See the attached image for the microstructure. Figure 1 and Figure 2 As shown, where Figure 1 The image shown is the metallographic structure of the product manufactured in Example 3. Figure 2 The metallographic structure of the product manufactured in Example 4 is shown.

[0048] In summary, this product boasts advantages such as low-cost control and high strength and toughness, making it suitable for mass production and application in the lightweighting of automobiles and construction machinery, thus meeting the demands of downstream markets.

[0049] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A low-cost method for manufacturing high-strength automotive steel using Ti microalloying precipitation strengthening without the need for a heat insulation cover, characterized in that, The chemical composition of the high-strength automotive steel, by mass percentage, is as follows: C: 0.04%–0.08%; Si: 0.05%–0.20%; Mn: 1.00%–1.50%; P≤0.015%; S≤0.005%; Al≤0.040%; Ti: 0.070%–0.100%; H≤2.0ppm; O≤30ppm; N≤50ppm; the remainder is Fe and unavoidable inclusions. The method includes the following processes: continuous casting, rolling, and slow cooling of steel coils in a warehouse; and the method does not include heat preservation treatment of the steel coils using insulation covers or insulation pits; wherein: The smelting and continuous casting process is as follows: molten iron → converter smelting → LF refining → RH degassing → slab continuous casting; converter smelting uses KR pre-desulfurized molten iron, and ferrosilicon, ferromanganese and fertitanium are deoxidized and alloyed, and molten steel is stirred by argon blowing throughout the process; vacuum degree ≤2mbar, deep vacuum time ≥10min, superheat ΔT≤30℃. The rolling process is as follows: 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; the slab heating temperature is 1250±20℃; the heating time is 180~240 min; the roughing mode adopts 1+5, 3+3 or 3+5; the intermediate slab thickness range is 35~45mm; the finishing mill opening temperature is 970~1100℃; the finishing mill closing temperature is 840~890℃; the coiling temperature is 560~620℃; and the cooling rate is 20~40℃ / s. Slow cooling process for steel coil storage area: At least 10 steel coils with the same or similar coiling temperature range are stacked together to form a slow cooling atmosphere. The steel coils are set in single or double layers, and the spacing between the steel coils in the transverse direction is controlled at 50~100mm. At the same time, the slow cooling area should be far away from the ventilation opening of the steel coil storage area, and the slow cooling time should be ≥72 hours, with a slow cooling rate of 5~10℃ / hour. The mechanical properties of the high-strength automotive steel meet the following requirements: yield strength ≥ 660 MPa, tensile strength ≥ 730 MPa, and elongation A. 50 ≥19%; Impact performance at -20℃ meets full-size AKv2 ≥155J, yield strength ratio 0.89 ≤ 0.93, grain size ≥11 grade; The metallographic structure of the high-strength automotive steel consists of granular bainite, polygonal ferrite, and pearlite.

2. The method according to claim 1, characterized in that, The chemical composition of the high-strength automotive steel, by mass percentage, is as follows: C: 0.07%; Si: 0.14%; Mn: 1.05%; P: 0.012%; S: 0.003%; Alt: 0.030%; Ti: 0.070%; H≤2.0ppm; O≤30ppm; N: 44ppm; the remainder is Fe and unavoidable inclusions.

3. The method according to claim 1, characterized in that, The chemical composition of the high-strength automotive steel, by mass percentage, is as follows: C: 0.08%; Si: 0.15%; Mn: 1.40%; P: 0.010%; S: 0.003%; Alt: 0.025%; Ti: 0.080%; H≤2.0ppm; O≤30ppm; N: 40ppm; the remainder is Fe and unavoidable inclusions.

4. The method according to claim 1, characterized in that, The chemical composition of the high-strength automotive steel, by mass percentage, is as follows: C: 0.07%; Si: 0.18%; Mn: 1.50%; P: 0.013%; S: 0.002%; Alt: 0.032%; Ti: 0.088%; H≤2.0ppm; O≤30ppm; N: 38ppm; the remainder is Fe and unavoidable inclusions.

5. The method according to claim 1, characterized in that, The chemical composition of the high-strength automotive steel, by mass percentage, is as follows: C: 0.075%; Si: 0.15%; Mn: 1.30%; P: 0.012%; S: 0.004%; Alt: 0.037%; Ti: 0.092%; H≤2.0ppm; O≤30ppm; N: 45ppm; the remainder is Fe and unavoidable inclusions.

6. A high-strength automotive steel, manufactured by the method of any one of claims 1-5.

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