Thin gauge high-precision 440mpa grade high-bore pickling automobile steel and production method

Abstract

The application discloses a kind of high-precision 440MPa level high-bore pickling automobile steel of thin specification, its main chemical components include Fe, C, Si, Mn and the like element, the rest is residual element, by mass percentage: C 0.04-0.065%, Si 0.05~0.2%, Mn 0.5~0.9%, Ti 0.02~0.05%, P≤0.015%, S≤0.005%, N≤0.005%, the rest is Fe and inevitable impurities.The tensile strength of product ≥440MPa, yield strength ≥305MPa, product elongation ≥35%, bore expansion rate ≥125%, thickness fluctuation ≤±15 μm, can solve the existing 440MPa level automobile pickling steel bore, flanging and the like forming performance insufficient, ≤1.6mm thin specification high-precision product production limited key technical bottleneck, realize "acid instead of cold", not only reduce the procurement cost for user, also provide solution for the environmental protection upgrade of automobile industry chain.

Description

Technical Field

[0001] This invention belongs to the field of strip steel production technology, and particularly relates to a thin-gauge, high-precision 440MPa grade high-expansion pickling automotive steel and its production method. Background Technology

[0002] 440MPa grade high-expansion automotive structural steel is widely used in automotive chassis and body parts. During the forming process, the parts need to be flanged and expanded, requiring a certain expansion rate. At the same time, the parts need to withstand a certain load during service, requiring a tensile strength ≥440MPa. The higher the expansion rate of the material, the lower the risk of cracking during flanging and expansion forming.

[0003] Pickled steel is a type of product between hot-rolled and cold-rolled products. Its microstructure and dimensional accuracy are controlled through hot rolling, followed by pickling to remove surface iron oxide scale, achieving surface quality and mechanical properties similar to cold-rolled products. Compared to cold-rolled products, pickled steel avoids the cold rolling and annealing processes required for cold rolling, resulting in a shorter production process, lower costs, and effectively reducing procurement costs for users. However, because the pickling process cannot improve product thickness and thickness accuracy, and due to limitations in the rolling capacity of hot rolling mills, pickled materials can currently only provide products with a thickness >1.6mm and less stringent thickness accuracy requirements. High-precision, ≤1.6mm thin-gauge steel with high-expansion holes still requires cold rolling and annealing processes, leading to higher energy consumption and manufacturing costs.

[0004] CN200710093966.4 discloses a hot-rolled high-expansion steel plate with a tensile strength of 440 MPa, comprising the following chemical composition (wt%): C: 0.01–0.06%, Si: 0–0.6%, Mn: 0.8–1.3%, P≤0.035%, S≤0.010%, Al: 0.025–0.060%, N≤0.0060%, Nb: 0–0.25%, with the remainder being Fe and unavoidable impurities. Its typical production process is as follows: coiling temperature 560–660℃, heating temperature 1150–1250℃, hot rolling deformation greater than 80%, finishing rolling temperature 830–900℃, and laminar cooling rate 20℃ / s. The 440 MPa grade hot-rolled high-expansion steel plate produced by this process has an elongation of 28–34% and an expansion rate of 103–113%. However, the elongation and expansion rate of this hot-rolled high-expansion steel plate are both low, posing a risk of forming cracks, and the thickness accuracy of the product is unclear.

[0005] CN201310628175.2) discloses a hot-rolled high-expansion steel plate with a tensile strength of 440MPa and its manufacturing method. The chemical composition by weight percentage is: C: 0.05~0.1%, Si: 0.1~0.6%, Mn: 0.9~1.8%, P≤0.02%, S≤0.005%, Al: 0.015~0.060%, Ca<0.0050%, with the remainder being Fe and unavoidable impurities; wherein, ([C]×[Mn])≤0.1, ([P]+10[S])≤0.04, the thickness of the obtained steel plate is 1.6~6.0mm, and the product of the steel plate strength and the expansion rate TSxλ≥44000MPa%, which can meet the steel requirements for chassis and complex stamping parts in the automotive industry. The expansion rate of the product is 100~126%. However, the expansion rate of this hot-rolled high-expansion steel plate is low, which poses a risk of forming cracks, and the thickness accuracy of the product is unclear.

[0006] CN201610556820.8 discloses a high-surface-quality hot-rolled pickled steel for automobiles with a tensile strength of 440 MPa. Its composition and wt% are: C: 0.090–0.120%, Si ≤ 0.080%, Mn: 1.100–1.500%, P ≤ 0.025%, S ≤ 0.006%, Als: 0.015–0.040%. The production method is as follows: conventional smelting and continuous casting into billets; heating the billets; hot rolling; coiling; laminar flow cooling; and pickling. This invention utilizes the surplus capacity of a cold-rolled pickling production line to produce hot-rolled pickled steel for automobiles. The surface roughness of the pickled steel is controlled using the last stand of a five-stand mill, resulting in a surface roughness of 0.4–1.0 μm. The invention has an unclear porosity and a high carbon content in its composition design, which is detrimental to both phosphating and porosity expansion performance. Furthermore, the invention uses a five-stand continuous rolling mill to control surface roughness, resulting in a long production process and high costs. Summary of the Invention

[0007] The technical problem to be solved by this invention is to provide a thin-gauge, high-precision 440MPa grade high-hole-expansion pickled automotive steel and its production method, addressing the shortcomings of the existing technology. The product has a tensile strength ≥440MPa, yield strength ≥305MPa, elongation ≥35%, hole expansion rate ≥125%, and thickness fluctuation ≤±15μm. This invention solves the key technical bottlenecks of insufficient hole expansion and flanging forming performance of existing 440MPa grade automotive pickled steel, and limited production of thin-gauge, high-precision products ≤1.6mm. It realizes "acid instead of cold", which not only reduces procurement costs for users, but also provides a solution for the environmental upgrading of the automotive industry chain.

[0008] The technical solution adopted by the present invention to solve the above-mentioned problems is as follows:

[0009] A thin-gauge, high-precision 440MPa grade high-pore-expanding pickled automotive steel, whose main chemical composition includes Fe, C, Si, Mn and other elements, with the remainder being residual elements, in the following mass percentages: C 0.04~0.065%, Si 0.05~0.2%, Mn 0.5~0.9%, Ti 0.02~0.05%, P≤0.015%, S≤0.005%, N≤0.005%, with the remainder being Fe and unavoidable impurities.

[0010] Preferably, the thin-gauge, high-precision 440MPa grade high-pore-expansion pickled automotive steel, by mass percentage, comprises: C 0.04–0.055%, Si 0.08–0.13%, Mn 0.5–0.7%, P ≤0.012%, S ≤0.003%, Ti 0.03–0.04%, N ≤0.004%, with the remainder being Fe and unavoidable impurities. Within this preferred chemical composition range, the product exhibits a tensile strength of 455–480MPa, a yield strength of 325–350MPa, an elongation ≥37%, and a pore-expansion rate ≥135%.

[0011] Preferably, the thickness of the thin-gauge, high-precision 440MPa grade high-pore-expansion pickled automotive steel is 0.8–1.6 mm, with a thickness fluctuation of ≤ ±15 μm.

[0012] This invention also provides a production method for the aforementioned thin-gauge, high-precision 440MPa grade high-permeability pickled automotive steel, mainly including steps such as billet casting, rough rolling, induction heating, finish rolling, laminar flow cooling, coiling, and pickling. The characteristics of this production method are:

[0013] (8) Casting of billet: The billet casting speed is 5.5~7m / min, and the billet thickness is 90~130mm;

[0014] (9) Rough rolling: The billet is continuously rough rolled to the intermediate billet, the thickness of the intermediate billet is 9-20 mm, and the single-pass reduction rate of rough rolling is 50-65%.

[0015] (10) Induction heating: Induction heating is used to raise the temperature of the intermediate billet to 1160-1220℃;

[0016] (11) Finishing rolling: The F1-F5 finishing rolling mill is used to roll the intermediate billet to the required product thickness in a fully continuous manner. The product thickness is 0.8-1.6 mm, the rolling passes are 5, the reduction rate of F1-F2 is 40-60%, the reduction rate of F3 is 25-40%, the reduction rate of the remaining passes (i.e. F4-F5) is 10%-35%, the rolling speed of F5 is 6-14 m / s, and the final rolling temperature is 840-880℃.

[0017] (12) Laminar flow cooling: Cooling rate is 30-70℃ / s;

[0018] (13) Winding: The winding temperature is controlled at 550-590℃;

[0019] (14) Pickling: Pickling is carried out using conventional pickling process to obtain thin-gauge, high-precision 440MPa grade high-expansion pickled automotive steel.

[0020] Preferably, the billet casting speed is 6-7 m / min and the billet thickness is 90-110 mm;

[0021] Preferably, the thickness of the intermediate billet is 9-15 mm, and the single-pass reduction rate of roughing is 58-65%.

[0022] Preferably, the product thickness is 0.8-1.2 mm, the rolling passes are 5, the reduction rate of F1-F2 is 50-60%, the reduction rate of F3 is 33-40%, the reduction rate of the remaining passes (i.e., F4-F5) is 20-35%, the rolling speed of F5 is 10-14 m / s, and the final rolling temperature is 840-860℃.

[0023] The alloy types and their contents are selected based on the following reasons:

[0024] C: Solid solution C will increase the yield strength and tensile strength of steel, but will reduce the plasticity and pore-expanding properties of steel. At the same time, when the carbon content exceeds 0.065%, peritectic reaction may occur during solidification, which can easily cause longitudinal cracks and steel leakage defects when casting thin slabs. In order to balance mechanical properties and production difficulty, this invention controls the C weight percentage content in steel to 0.04-0.065%.

[0025] Si (Si): Si can also improve the strength of steel, but if the Si content is too high, liquid Fe2SiO4 is easily formed at the interface between the matrix and the iron scale, increasing the adhesion of the iron scale and making descaling difficult during finishing rolling. If the Si content is too low, the iron oxide scale grows too quickly during rolling, resulting in greater oxidation and thermal stress on the iron scale, making it prone to blistering, shattering, and pitting defects caused by pressing into the steel matrix. Considering surface quality, the Si weight percentage should be controlled at 0.05-0.2%.

[0026] Mn: Mn is dissolved in ferrite and austenite, which can improve yield strength and tensile strength. However, Mn is not as effective as carbon in improving strength. To improve the yield and tensile strength of the product, a large amount of manganese needs to be added, which will reduce plasticity and promote the formation of pearlite structure, which is not conducive to the expansion rate. In order to achieve a tensile strength ≥440MPa and obtain a high expansion rate, the weight percentage of Mn is controlled at 0.5% to 0.9%.

[0027] Ti: Titanium precipitates TiC can inhibit the growth of austenite and ferrite grains. By refining the grains, it can improve both strength and porosity. However, excessive Ti content will lead to more Ti precipitates, which will reduce the plasticity of the material. Taking all factors into consideration, the weight percentage of Ti should be controlled at 0.02-0.05%.

[0028] P: P is a harmful impurity element in steel. P in steel is prone to segregation, which reduces the toughness and weldability of steel. Therefore, the lower the P content, the better. Taking all factors into consideration, the P content should be ≤0.015%.

[0029] S: S combines with Ti to form TiS or Ti4C2S2, but due to the high precipitation temperature, the precipitates are coarse and cannot effectively inhibit grain growth. Instead, it consumes the Ti that forms TiC precipitates, resulting in a weakening of the strengthening effect. Therefore, the S content in steel is ≤0.005%.

[0030] N: N has a strong bonding force with Ti and will precipitate coarse TiN in the liquid phase. On the one hand, it will consume the Ti that forms TiC precipitates, thus weakening the strengthening effect. On the other hand, the coarse TiN will cause stress concentration during the pore expansion process, resulting in a decrease in the pore expansion rate. Therefore, the N content in steel is ≤0.005%.

[0031] The production method of thin-gauge, high-precision 440MPa grade high-pore-expansion pickling automotive steel described in this invention has the following technical characteristics:

[0032] The reason why the billet thickness is controlled at 90-130mm and the billet casting speed is 5.5-7m / min is to increase the billet cooling rate, form fine and dispersed inclusions, and suppress austenite grain coarsening.

[0033] The reason why the thickness of the intermediate billet is controlled to be 9-20 mm and the single-pass reduction rate is 50-65% is to allow the coarse austenite in the cast state to recrystallize fully through high temperature and high reduction, forming fine austenite grains. At the same time, it can promote the dynamic precipitation of TiC particles during rolling, inhibit the growth of austenite grains, and is beneficial to refine the room temperature structure, improve strength and porosity.

[0034] The reason why this invention uses induction heating to raise the temperature of the intermediate billet to 1160-1220℃ is mainly to reduce the finishing rolling load. If the intermediate billet temperature is too low, the finishing rolling load will be too high, the rolling force will be too large, and it will damage the plate shape. However, if the intermediate billet temperature is too high, it may cause TiC particles to dissolve back, which will weaken the effect of inhibiting austenite grain growth.

[0035] This invention controls the finishing rolling process to five passes, with a reduction rate of 50-60% for F1-F2, 30-45% for F3, and 10%-35% for the remaining passes. In the finishing rolling passes F1-F2, austenite recrystallization and TiC particle precipitation are further promoted, refining the austenite grains. The reduction rates of F3, F4, and F5 are controlled primarily to obtain the desired product thickness, while simultaneously improving thickness accuracy and plate shape quality. During the finishing rolling process, the rolling speed of F5 is controlled at 6-14 m / s. Rolling speeds that are too high or too low cannot achieve fully continuous rolling. Fully continuous rolling eliminates the need for strip threading and steel polishing, resulting in uniform temperature and stable reduction rates, leading to high stability and thickness accuracy when rolling thin-gauge products. In the finishing rolling process, the final rolling temperature is 840-880℃. If the final rolling temperature is higher than 880℃, the number of defects such as deformation bands, dislocations, and vacancies that are formed during the cooling process inside the austenite grains will be less. The fewer ferrite nucleation cores will result in coarse room temperature microstructure, which is not conducive to improving strength and porosity. If the final rolling temperature is lower than 840℃, the rolling load of the tail stand will be high, which is not conducive to thickness accuracy and plate shape quality.

[0036] The reason why the laminar cooling rate is controlled at 30-70℃ / s and the winding temperature at 550-590℃ in this invention is to reduce the ferrite transformation temperature, reduce the formation of pearlite structure, obtain a uniform and fine ferrite structure, and improve the strength and porosity.

[0037] Compared with the prior art, the beneficial effects of the present invention are:

[0038] This invention utilizes fully continuous thin slab rolling, combined with end-to-end load distribution, and employs a low-carbon, low-manganese, and micro-titanium composition design. By leveraging mechanisms such as thin-gauge casting, high-speed casting, complete austenite recrystallization, and TiC inhibition of austenite grain growth, it achieves thin-gauge, high-precision 440MPa-grade high-porosity pickled automotive steel. The product thickness ranges from 0.8 to 1.6 mm, with a thickness fluctuation of ≤ ±15 μm. It boasts a tensile strength ≥ 440 MPa, a yield strength ≥ 305 MPa, an elongation ≥ 35%, and a porosity ≥ 125%. This invention realizes "acid-based cooling" for 440MPa-grade high-porosity pickled automotive steel, reducing the manufacturing process of existing thin-gauge, high-precision products. Attached Figure Description

[0039] Figure 1 This invention describes the microstructure of a thin-gauge, high-precision 440MPa grade high-pore-expansion pickled automotive steel produced using the method of this invention, as well as the production method thereof. Detailed Implementation

[0040] To better understand the present invention, the following embodiments further illustrate the content of the invention, but the present invention is not limited to the following embodiments. This section lists 10 sets of embodiments to illustrate the implementation of the present invention.

[0041] Examples 1-10

[0042] The chemical composition of the thin-gauge, high-precision 440MPa grade high-pore-expansion pickled automotive steel provided in Examples 1-10 is shown in Table 1.

[0043] Table 1 Chemical composition (wt%) of each example

[0044] C Si Mn P S Ti N Example 1 0.057 0.05 0.79 0.0079 0.0017 0.025 0.0033 Example 2 0.04 0.121 0.61 0.0117 0.0025 0.035 0.0026 Example 3 0.055 0.082 0.50 0.0089 0.0016 0.03 0.0024 Example 4 0.042 0.125 0.54 0.0127 0.0033 0.04 0.0029 Example 5 0.044 0.098 0.71 0.0103 0.0022 0.05 0.0042 Example 6 0.055 0.11 0.66 0.009 0.0016 0.036 0.0036 Example 7 0.065 0.132 0.90 0.0091 0.005 0.042 0.002 Example 8 0.045 0.108 0.57 0.0075 0.0023 0.038 0.004 Example 9 0.048 0.159 0.69 0.0108 0.0027 0.031 0.0023 Example 10 0.064 0.175 0.85 0.0116 0.0023 0.03 0.005

[0045] The production method of thin-gauge, high-precision 440MPa grade high-permeability pickled automotive steel described in Examples 1-10 mainly includes the following steps: billet casting, rough rolling, induction heating, finish rolling, laminar flow cooling, coiling, and pickling. The specific steps are as follows:

[0046] (1) Smelting and casting of billets: Smelting and refining the automotive steel according to the chemical composition and mass percentage in Table 1 using a converter or electric furnace, casting billets with a casting speed of 5.5 to 7 m / min and a billet thickness of 90 to 130 mm.

[0047] (2) Rough rolling: The billet is continuously rough rolled to the intermediate billet, the thickness of which is 9-20 mm, and the single-pass reduction rate of rough rolling is 50-65%.

[0048] (3) Induction heating: Induction heating is used to raise the temperature of the intermediate billet to 1160-1220℃;

[0049] (4) Finishing rolling: The F1-F5 finishing rolling mill is used to roll the intermediate billet to the required product thickness in a fully continuous manner. The product thickness is 0.8-1.6 mm, the rolling passes are 5, the reduction rate of F1-F2 is 40-60%, the reduction rate of F3 is 25-40%, and the reduction rate of the remaining passes (i.e. F4-F5) is 10%-35%. The rolling speed of F5 is 6-14 m / s, and the final rolling temperature is 840-880℃.

[0050] (5) Laminar flow cooling: Cooling rate is 30-70℃ / s;

[0051] (6) Winding: The winding temperature is controlled at 550-590℃;

[0052] (7) Pickling: Pickling is carried out using conventional pickling process to obtain thin-gauge, high-precision 440MPa grade high-expansion pickled automotive steel strip.

[0053] The process parameters and main performance characteristics of each embodiment are shown in Tables 1 to 3.

[0054] Table 2 Heating process parameters for each embodiment

[0055]

[0056] Table 2 (continued) Heating process parameters for each embodiment

[0057]

[0058] Table 3 Typical mechanical properties, porosity, and thickness parameters of Examples 1-10

[0059]

[0060] Depend on Figure 1 It can be seen that the microstructure of the thin-gauge, high-precision 440MPa grade high-hole-expansion pickled automotive steel products produced by this invention is a uniform and fine ferrite structure, which is beneficial to improving strength and hole expansion rate.

[0061] As shown in Table 3, the produced thin-gauge, high-precision 440MPa grade high-hole-expansion pickled automotive steel has a thickness of 0.8–1.6 mm, a tensile strength ≥440 MPa, a yield strength ≥305 MPa, an elongation ≥35%, and a hole expansion rate ≥125%. As shown in Examples 2, 6, and 8, optimized composition and rolling processes are used to produce 0.8–1.2 mm ultra-thin, high-surface-value 400MPa grade high-hole-expansion pickled automotive steel. The product has a tensile strength of 455–480 MPa, a yield strength of 325–350 MPa, an elongation ≥37%, and a hole expansion rate ≥135%. In this invention, yield strength, tensile strength, and elongation are tested according to standard GB / T 228.1, and hole expansion rate is tested according to standard GB / T 24524.

[0062] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the inventive concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A thin-gauge, high-precision 440MPa grade high-perforation pickling automotive steel, characterized in that: By mass percentage: C 0.04%–0.065%, Si 0.05%–0.082%, Mn 0.5%–0.54%, Ti 0.031%–0.05%, P≤0.015%, S≤0.005%, N≤0.005%, with the remainder being Fe and unavoidable impurities; The production method of thin-gauge, high-precision 440MPa grade high-permeability pickled automotive steel mainly includes the following steps: billet casting, rough rolling, induction heating, finish rolling, laminar flow cooling, coiling, and pickling. (1) Casting of billet: The billet casting speed is 5.6~7m / min, and the billet thickness is 90~130mm; (2) Rough rolling: The billet is continuously rough rolled to the intermediate billet, the thickness of which is 9-20 mm, and the single-pass reduction rate of rough rolling is 50-65%. (3) Induction heating: Induction heating is used to raise the temperature of the intermediate billet to 1192-1220℃; (4) Finishing: The F1-F5 finishing mills are used to roll the intermediate billet to the required product thickness in a fully continuous manner. The product thickness is 0.8-1.6 mm, the rolling passes are 5, the reduction rate of F1-F2 is 40-60%, the reduction rate of F3 is 25-40%, the reduction rate of F4-F5 is 10%-35%, the rolling speed of F5 is 6-14 m / s, and the final rolling temperature is 840-880℃. (5) Laminar flow cooling: Cooling rate is 30-70℃ / s; (6) Winding: The winding temperature is controlled at 550~574℃; (7) Pickling: Pickling is carried out using conventional pickling process to obtain thin-gauge, high-precision 440MPa grade high-hole-expansion pickled automotive steel. The thin-gauge, high-precision 440MPa grade high-expansion pickled automotive steel produced has a tensile strength ≥440MPa, yield strength ≥305MPa, elongation ≥38.3%, expansion rate ≥129%, thickness ≤1.6mm, and thickness fluctuation ≤±15μm.

2. The thin-gauge, high-precision 440MPa grade high-permeability pickling automotive steel according to claim 1, characterized in that: The thickness is 0.8 to 1.6 mm, and the thickness fluctuation is ≤ ±15 μm.

3. The production method of thin-gauge, high-precision 440MPa grade high-permeability pickled automotive steel as described in claim 1 or 2, mainly includes the steps of billet casting, rough rolling, induction heating, finish rolling, laminar flow cooling, coiling, and pickling, characterized in that: (1) Casting of billet: The billet casting speed is 5.6~7m / min, and the billet thickness is 90~130mm; (2) Rough rolling: The billet is continuously rough rolled to the intermediate billet, the thickness of which is 9-20 mm, and the single-pass reduction rate of rough rolling is 50-65%. (3) Induction heating: Induction heating is used to raise the temperature of the intermediate billet to 1192-1220℃; (4) Finishing: The F1-F5 finishing mills are used to roll the intermediate billet to the required product thickness in a fully continuous manner. The product thickness is 0.8-1.6 mm, the rolling passes are 5, the reduction rate of F1-F2 is 40-60%, the reduction rate of F3 is 25-40%, the reduction rate of F4-F5 is 10%-35%, the rolling speed of F5 is 6-14 m / s, and the final rolling temperature is 840-880℃. (5) Laminar flow cooling: Cooling rate is 30-70℃ / s; (6) Winding: The winding temperature is controlled at 550~574℃; (7) Pickling: Pickling is carried out using conventional pickling process to obtain thin-gauge, high-precision 440MPa grade high-expansion pickled automotive steel.

4. The production method of thin-gauge, high-precision 440MPa grade high-permeability pickling automotive steel according to claim 3, characterized in that: The casting speed is 6-7 m / min, and the casting thickness is 90-110 mm.

5. The production method of thin-gauge, high-precision 440MPa grade high-hole-expansion pickling automotive steel according to claim 3, characterized in that: The thickness of the intermediate billet is 9-15 mm, and the single-pass reduction rate of roughing is 58-65%.

6. The production method of thin-gauge, high-precision 440MPa grade high-hole-expansion pickling automotive steel according to claim 3, characterized in that: The product thickness is 0.8–1.2 mm, the rolling passes are 5, the reduction rate of F1–F2 is 50–60%, the reduction rate of F3 is 33–40%, the reduction rate of F4–F5 is 20–35%, the rolling speed of F5 is 10–14 m / s, and the final rolling temperature is 840–860℃.

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