An economical micro-titanium high-hole-expanding 440Mpa grade pickling automobile steel production method

Abstract

This invention discloses an economical method for producing 440MPa grade pickled automotive steel with low Si, low Ti, and high porosity. It primarily addresses the problems of high cost, surface quality, and porosity not meeting the requirements of automotive users for existing 440MPa automotive steel with thicknesses of 2.5mm to 5mm. The pickled steel sheet provided by this invention has the following chemical composition by weight percentage: C: 0.060%-0.075%; Mn: 0.55%-0.65%; Ti: 0.05%-0.06%; Alt: 0.01%-0.05%; P: ≤0.018%; S: ≤0.006%, with the remainder being Fe and unavoidable impurities. Its longitudinal yield strength Rp0.2 ≥ 305MPa, tensile strength Rm ≥ 440MPa, and elongation after fracture A... 50 ≥32%, hole expansion rate ≥90%. The pickled steel sheet of this invention is mainly used to manufacture pickled automotive chassis structural parts.

Description

Technical fields:

[0001] This application relates to an economical method for producing 440MPa grade pickled automotive steel with high micro-Ti and high hole expansion performance, belonging to the technical field of hot-rolled pickled steel sheet production and processing. Background technology:

[0002] High-grade pickled automotive structural steel (440 MPa and above) has a wide range of applications. User-end processing methods primarily involve stamping, flanging, and reaming. For general stamping applications, high elongation performance is sufficient. However, for processes like curved stretching, flanging, and reaming, the reaming performance requirements are much higher. The microstructure of conventional 440 MPa automotive structural steel is ferrite + pearlite. The significant hardness difference between these two phases leads to uneven hardness distribution, generating substantial internal stress, which is detrimental to improving the reaming performance. Currently, domestic steel mills generally use a ferrite + bainite microstructure to achieve high reaming performance. Existing reaming steel processes often contain silicon, which easily leads to defects such as red iron scale on the pickled surface. This is unacceptable to users and can easily cause quality disputes. Laminar flow cooling processes often employ complex segmented cooling techniques, resulting in imprecise temperature control and significant challenges in production organization. This can easily lead to poor coiling or scrap steel. Due to cost considerations, this method is generally only used in high-end vehicles or for parts with extremely high requirements for hole expansion ratio. However, for most parts, users order through trading companies, and the end-user's intended use is often unknown. The superior hole expansion performance of conventional grade materials can avoid many unforeseen forming problems such as hole expansion cracking and edge cracking, thus reducing the risk of quality disputes.

[0003] Automakers require materials to meet the performance requirements for stamping, flanging, and hole expansion while minimizing costs. To meet these demands, it is necessary to develop steel grades that balance cost and hole expansion performance.

[0004] Chinese patent application CN 109487153A discloses a high-expansion hot-rolled pickled steel sheet with a tensile strength of 440 MPa. Its chemical composition by weight percentage is: C: 0.035%–0.055%, Si: 0.40%–0.60%, Mn: 0.95%–1.05%, P≤0.015%, S≤0.005%, Al: 0.020%–0.060%, with the balance being iron and unavoidable inclusions. The microstructure of the hot-rolled pickled steel sheet is ferrite with a small amount of bainite, where the volume content of bainite is 10%–15%, and the grain size of ferrite is 11–12.5. This design contains high Mn and actively added Si, but does not contain Ti alloying elements, and uses a three-stage laminar cooling method. Although the expansion rate is high, it is detrimental to the hot-rolled sheet shape and the pass rate of the mill.

[0005] Chinese patent application CN200710093966.4 discloses a high-strength, high-toughness steel plate with a tensile strength of 440 MPa and its manufacturing method, 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. This invention also provides a method for manufacturing this hot-rolled high-perforation steel plate, which eliminates the need for heat treatment, reducing costs, and eliminates the need for complex controlled cooling technology after hot rolling, making it easy to produce. The steel plate provided by this invention has excellent perforation performance, formability, and cold working performance, and is mainly used for forming complex-shaped automotive chassis parts. This solution contains Nb and actively adds Si.

[0006] Chinese patent application CN200910053943.X discloses a 590MPa grade hot-rolled pickled high-expansion steel strip and its production method. Its chemical composition by weight percentage is: C 0.02~0.10, Si 0~1.6, Mn 0.8~2.0, P≤0.035, S≤0.010, Al 0.025~0.060, N≤0.0060, Nb 0~0.10, Ti 0~0.04, Ca 0~0.0050, with the remainder being Fe and unavoidable impurities. The hot-rolled high-expansion steel sheet manufacturing process of this invention involves heating the steel billet to 1150–1250°C, rolling it in the austenitic region with a rolling deformation greater than 80%, and a final rolling temperature of 830–900°C. The final-rolled steel sheet is cooled to 600–750°C at a cooling rate of 50–100°C / s, then cooled in air at a cooling rate of 5–15°C / s for 3–10 seconds. The steel sheet is then cooled again to 350–500°C at a cooling rate of 70–150°C / s and coiled, and finally air-cooled to room temperature. The steel sheet of this invention is particularly suitable for manufacturing chassis components for automobiles. The strength level of this solution is above 590 MPa. Furthermore, this process involves alloying with Nb and Ti, and employs a two-stage cooling method. Summary of the Invention:

[0007] This invention relates to an economical method for producing 440MPa grade pickled automotive steel with high porosity, free of Si and micro-Ti, and with high porosity expansion performance. It primarily addresses the technical problems of high manufacturing costs and insufficient porosity expansion performance in existing automotive chassis steel. The technical measures to achieve the above objectives are as follows:

[0008] Its chemical composition by weight percentage is: C: 0.060%-0.075%; Mn: 0.55%-0.65%; Ti: 0.05%-0.06%; Alt: 0.01%-0.05%; P: ≤0.018%; S: ≤0.006%. The reason for limiting the chemical composition of the acid-washed automobile with an economic strength of 440 MPa or higher described in this invention to the above range is as follows:

[0009] Carbon (C) is the main strengthening element in automotive structural steel SAPH440. Increasing the C percentage improves strength but reduces elongation. To avoid the peritectic region (0.08%–0.15%) and reduce corner cracks in continuously cast slabs, the C content in this invention is controlled at 0.060%–0.075%.

[0010] Manganese (Mn) is an important strengthening element in SAPH440 low-alloy high-strength automotive structural steel to improve strength. Increasing Mn content reduces plasticity and promotes the formation of pearlite. Generally, Mn content ≥1.0% is too high, causing severe segregation during steelmaking, which adversely affects stamping and welding. In this invention, Mn content is controlled between 0.55% and 0.65%.

[0011] Polymer (P) and sulfur (S): Polymer (P) in automotive structural steel tends to cause segregation and deteriorate toughness, leading to cold brittleness. Sulfur (S) readily forms MnS inclusions with manganese (Mn), reducing the steel's low-temperature toughness and decreasing the yield rate of wide cold bending; S also causes hot brittleness. Therefore, in the design of SAPH440 for stamping automotive parts, the P and S content should be minimized. In this invention, P is controlled at ≤0.018%, and S at ≤0.006%. Ca wire is also fed to improve the castability of the molten steel and reduce nozzle clogging.

[0012] Titanium (Ti): In low-carbon microalloyed steel, the addition of Ti can refine the grain and promote precipitation strengthening, thereby improving the yield strength and toughness of the steel. This improvement in performance is mainly related to Ti's ability to increase the austenite recrystallization temperature and austenite coarsening temperature, thus improving grain size during continuous casting and heating. Since Ti can form TiN high-temperature refractory particles with N at high temperatures, the addition of Ti can also increase the grain size of the weld heat-affected zone, thereby improving its toughness. This invention employs a strategy of increasing titanium and reducing manganese, which can reduce alloy consumption and lower costs. In this invention, Ti is controlled at 0.05%–0.06%.

[0013] Al: In this invention, aluminum acts as a deoxidizer. Aluminum is a strong oxidizing element and reacts with oxygen in steel to form Al₂O₃, which is removed during steelmaking. Excessive aluminum content will lead to excessive Al₂O₃ inclusions; therefore, this invention limits the Al content to 0.010%–0.050%.

[0014] Si: Si can also improve the strength of steel, but when the Si content is too high, red silicon iron scale is easily formed at the interface between the matrix and the iron scale, which cannot meet the requirements of high surface quality. Therefore, Si is not actively added in this application.

[0015] A method for producing an economical micro-Ti, Si-free, high-pore-expansion-performance 440MPa grade pickling automotive steel includes the following steps:

[0016] Molten steel is continuously cast to obtain continuously cast slabs, wherein the chemical composition of the molten steel is as follows by weight percentage: C: 0.060%-0.075%; Mn: 0.55%-0.65%; Ti: 0.05%-0.06%; Alt: 0.01%-0.05%; P: ≤0.018%; S: ≤0.006%, with the remainder being Fe and unavoidable impurities.

[0017] Continuously cast slabs with a thickness of 210 or 230 mm are heated for 180 to 240 minutes and then exited the furnace at a temperature of 1170°C to 1230°C for hot rolling. The hot rolling process is a roughing and finishing rolling process. Roughing (R1, three-pass reversible rolling) and R2 (three-pass reversible rolling) are performed above the austenite recrystallization temperature, with a finishing temperature of 950°C to 1100°C. The intermediate slab thickness is 32-40 mm. The finishing rolling is a 7-pass continuous rolling process. Rolling is performed in the non-recrystallization temperature range of the martensite, with the finishing rolling temperature being 840℃~880℃. After finishing rolling, laminar cooling is carried out using a special cooling method with a laminar cooling rate ≥50℃ / s. Hot-rolled steel coils are obtained when the coiling temperature is 430℃~470℃. The steel coils are then air-cooled for 3-5 days until the surface temperature of the steel coils is ≤60℃. The steel coils then enter the pickling unit and undergo uncoiling, welding, tension leveling, shallow trough turbulent hydrochloric acid pickling process, rinsing, drying, oiling, and coiling.

[0018] The rationale for adopting the hot rolling process in this invention is as follows:

[0019] 1. Setting the heating temperature and heating time for continuously cast slabs

[0020] The setting of the continuous casting slab tapping temperature and time is crucial to ensuring the dissolution of coarse Ti microalloyed carbide and nitride particles in the slab. In this invention, Ti microalloyed carbide and nitride particles precipitate during the cooling process of the slab. These precipitated particles are coarse and lack strengthening effect. Therefore, it is necessary to fully dissolve these coarse Ti microalloyed carbide and nitride particles during the slab heating process before hot rolling. This allows the combined Ti element to dissolve into the austenite, forming interphase precipitation during the subsequent hot rolling and cooling phase transformation, thus strengthening the ferrite. This is very important for this invention. If the temperature is too low or the heating time is too short, the original coarse Ti microalloyed carbide and nitride particles in the slab will not dissolve sufficiently. If the temperature is too high or the heating time is too long, severe oxidation and decarburization of the slab surface will occur, which is detrimental to the final performance and surface quality of the steel plate, and also consumes energy. Therefore, this invention sets the continuous casting slab heating temperature to 1170℃~1230℃ and the heating time to 180min~240min.

[0021] 2. Setting the finishing rolling temperature

[0022] The finishing rolling temperature setting of this invention serves two purposes. Firstly, rolling in the non-recrystallized austenite zone yields flattened austenite grains with internal deformation bands, which transform into fine ferrite grains during subsequent laminar cooling, thus achieving grain refinement strengthening. Grain refinement is crucial in this invention, as it allows for high toughness without compromising strength. The finishing rolling temperature cannot be set too high, otherwise surface defects such as temperature-dependent oxide scale and blemishes will appear. Secondly, the finishing rolling temperature cannot be too low. Excessively low temperatures can easily induce the precipitation of Ti microalloyed carbides and nitrides in the austenite state during rolling, resulting in insufficient precipitates during subsequent phase transformations and affecting precipitation strengthening. Too low a finishing rolling temperature can also lead to excessive mill load, causing poor plate or coil shape. In this invention, the Ar3 temperature is designed to be 825℃, therefore the finishing rolling end temperature is set to 830℃~870℃.

[0023] 3. Setting the laminar flow cooling method and cooling rate after finishing rolling

[0024] The pickled steel sheet of this invention employs a special cooling method that facilitates grain recovery. This special cooling method results in a more uniform microstructure, less anisotropy, and smaller transverse and longitudinal variations in the material. The cooling rate is ≥50℃ / s; if the cooling rate is too slow, bainite cannot be formed.

[0025] 4. Setting the hot rolling coiling temperature

[0026] Hot-rolling coiling temperature primarily affects the microstructure and properties of the material. This invention contains Ti, and based on the optimal precipitation temperature of Ti microalloying elements, the coiling temperature is designed to be 430℃~470℃. If the coiling temperature is below 430℃, inaccurate temperature control can easily lead to poor coil or sheet shape, and may even result in scrap steel. If the coiling temperature is above 470℃, the target microstructure—polygonal ferrite + a small amount of bainite—will transform into ferrite + pearlite, resulting in unsatisfactory hole-expanding performance.

[0027] 5. Design of the straightening rate in the pickling process

[0028] The pickling process aims to maintain the plate shape while keeping the tensile elongation as low as possible; in this invention, it is set at 0.8%.

[0029] A high tension leveling rate setting is beneficial for improving the sheet shape, but an excessively high tension leveling rate will prematurely deplete the material's plasticity, increasing the transverse and longitudinal differences in the pickled sheet. A low tension leveling rate setting has limited ability to improve the sheet shape; in this invention, the tension leveling rate setting is 0.8%.

[0030] Compared with the prior art, the present invention has the following advantages:

[0031] 1. This composition system utilizes a titanium-containing process without adding the precious alloy niobium. The composition design employs a strategy of increasing titanium and reducing manganese, thereby reducing alloy consumption and offering advantages in cost control. By using different rolling and laminar flow cooling processes, the same composition system can yield products with different strength grades and microstructure properties. This patented composition system can also produce enamel steel, thus achieving flexible production. It increases the number of continuous casting heats in the tundish, reduces mixed casting and steel grade transitions, thereby lowering costs and improving product competitiveness.

[0032] 2. By combining multiple measures, including titanium content design, hot-rolled low-temperature coiling, and a special cooling method, a polygonal ferrite structure with a small amount of bainite is obtained. The special cooling method results in a more uniform microstructure with less anisotropy, achieving a hole-expanding performance of over 90%. This performance falls between that of ordinary SAPH440 and hole-expanding steel produced through segmented cooling. Appropriate pickling and straightening ratios minimize transverse and longitudinal variations in the hole-expanding steel while maintaining the desired sheet shape. This composition system does not contain added silicon, thus avoiding surface silicon red scale defects and meeting the high surface quality requirements of the automotive industry.

[0033] 3. This invention, through a suitable inclusion treatment process, alters the morphology of alumina inclusions and manganese sulfide inclusions, making them smaller and more dispersed, thus delaying the formation of cracks during the hole expansion process. Attached Figure Description

[0034] Figure 1 This is a metallographic photograph of the hot-rolled steel plate of Embodiment 1 of the present invention. Detailed Implementation

[0035] The present invention will be further described below with reference to Examples 1 to 3, as shown in Tables 1 to 3; Table 1 shows the chemical composition (by weight percentage) of the steel in the examples of the present invention, with the balance being Fe and unavoidable impurities.

[0036] Table 1 Chemical composition of steel in the embodiments of the present invention, unit: weight percentage.

[0037]

[0038]

[0039] Molten steel that meets the chemical composition requirements is obtained by converter smelting. The molten steel is then treated with Ar blowing in the refining process of the LF ladle refining furnace, and then continuously cast into slabs to obtain continuously cast slabs. The thickness of the continuously cast slabs is 210-230 mm, the width is 900-1600 mm, and the length is 8500-11000 mm.

[0040] The slabs produced in steelmaking are sent to a reheating furnace for reheating, and after descaling, they are sent to a hot continuous rolling mill for rolling. Rolling is controlled by a roughing and finishing continuous rolling mill, followed by laminar flow cooling and coiling using a special cooling method; the thickness of the steel plate is 2.0–5.0 mm. Hot rolling process control parameters are shown in Table 2.

[0041] Table 2 Hot rolling process control parameters of the present invention embodiments

[0042]

[0043] For hot-rolled steel sheets obtained using the above method, see [link to relevant documentation]. Figure 1 The metallographic structure of the hot-rolled steel sheet of this invention is polygonal ferrite with a small amount of bainite, wherein the grain size of the ferrite in the structure is 9.5 to 10.5 grade; the longitudinal yield strength Rp0.2 of the 2.5mm to 5mm hot-rolled steel sheet is ≥305MPa, the tensile strength Rm is ≥440MPa to 450MPa, and the elongation after fracture is A 50 ≥32%, expansion rate ≥90%.

[0044] The hot-rolled steel plate obtained by the present invention was subjected to tensile testing in accordance with GB / T228.1-2010 Metallic Materials - Tensile Testing - Part 1: Test Method at Room Temperature. Its mechanical properties are shown in Table 3.

[0045] Table 3 Mechanical properties of pickled steel plates according to embodiments of the present invention

[0046] Performance indicators Hot-rolled steel plate thickness / mm Lower yield strength ReL / MPa Tensile strength Rm / MPa <![CDATA[Elongation after fracture A 50 / %]]> Hole expansion rate % This invention 2.5～5.0 305～450 450-550 ≥32 ≥90 Example 1 3.04 365 480 34 107 Example 2 3.04 363 484 37 102

[0047] In addition to the embodiments described above, the present invention may have other implementations. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by the present invention.

Claims

1. A method for producing an economical micro-Ti high-pore-expansion-performance 440MPa grade pickled automotive steel, characterized in that, Its chemical composition by weight percentage is: C: 0.060%-0.075%; Mn: 0.55%-0.65%; Ti: 0.05%-0.06%; Alt: 0.01%-0.05%. P ：≤0.018％； S: ≤0.006%, the remainder being Fe and unavoidable impurities.

2. The method for producing economical micro-Ti high-pore-expansion-performance 440MPa grade pickled automotive steel according to claim 1, characterized in that, The metallographic structure of the hot-rolled steel sheet is polygonal ferrite + pearlite + a small amount of bainite, and the grain size of the ferrite in the structure is 9.5-10.

5.

3. The method for producing an economical micro-Ti high-pore-expansion-performance 440MPa grade pickled automotive steel according to claim 1, characterized in that, Its production method includes the following steps: Molten steel is continuously cast to obtain a continuously cast slab, wherein the weight percentage of the chemical composition of the molten steel is C: 0.065%-0.075%; Mn: 0.58%-0.63%; Ti: 0.05%-0.06%; Alt: 0.03%-0.04%; P: ≤0.018%; S: ≤0.006%; the remainder is Fe and unavoidable impurities. The continuously cast slab is heated at 1170℃~1230℃ for 180min~240min and then hot-rolled. The hot rolling is a two-stage rolling process of roughing and finishing. The roughing is a 6-pass continuous rolling process, rolled above the austenite recrystallization temperature, and the roughing end temperature is 950℃~1100℃. The finishing is a 7-pass continuous rolling process, rolled in the austenite non-recrystallization temperature range, and the finishing end temperature is 840℃~880℃. After finishing, laminar cooling is carried out using a special cooling method with a laminar cooling rate ≥50℃ / s. The hot-rolled steel coil is obtained when the coiling temperature is 430℃~470℃.

4. The method for producing an economical micro-Ti high-pore-expansion-performance 440MPa grade pickled automotive steel according to claim 1, characterized in that, After hot rolling and finishing, the thickness of the steel plate is controlled to be 2.5mm to 5.0mm.

5. The method for producing an economical micro-Ti high-hole-expansion-performance 440MPa grade pickled automotive steel according to claim 1, characterized in that, The longitudinal yield strength Rp0.2 of automotive steel is ≥305MPa, the tensile strength Rm is ≥440MPa, and the elongation after fracture is A. 50 ≥32%, expansion rate ≥90%.

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