Gigapascal grade cold rolled high strength steel for seat frames and method of production thereof

Abstract

A kind of seat framework uses giga-level cold-rolled high-strength steel, chemical composition (wt%): C 0.08~0.12, Si 0.4~0.8, Mn 2.0~2.5, Als 0.03~0.06, Nb 0.02~0.04, Ti 0.02~0.04, Cr 0.1~0.3, La 0.02~0.05, P≤0.01, S≤0.01, N≤0.005, P+S+N≤0.022, the rest is Fe and inevitable impurities;Production method is annealed for continuous annealing, with 2~3 ℃ / s to 800~850 ℃ Steel plate is heated, keeps warm 110~150 s, with 3~6 ℃ / s slow cooling to 600~650 ℃, with 20~40 ℃ / s fast cooling to 300~360 ℃, overaging 200~310 s, flatness elongation 0.2~0.5%.Steel plate has the characteristics of low carbon equivalent, high elongation, high bending performance.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical rolling technology, specifically relating to a gigapascal-grade cold-rolled high-strength steel for seat frames and its production method. Background Technology

[0002] Low energy consumption and high safety are the trends in the automotive industry. Reducing the weight of a car is the most direct and effective way to reduce fuel consumption. A large amount of experimental data has proven that if the weight of a car is reduced by 50%, fuel consumption will be reduced by nearly half.

[0003] The seat frame is the core structural component of a seat, providing support during daily driving and crucial protection for occupants in collisions. With the rapid development of automotive lightweighting technology, gigapascal high-strength steel is increasingly used in seat frames. However, seat frames often feature complex curved hollow structures and complex flanged structures, requiring materials with high elongation (A≥14%) and high bending performance (180° cold bending radius r≤1t, where t is the steel plate thickness). Currently used seat frame steels cannot meet these requirements. Therefore, it is necessary to develop a gigapascal cold-rolled high-strength steel suitable for automotive seat frames that combines high elongation and high cold bending performance. Summary of the Invention

[0004] This invention provides a gigapascal-grade cold-rolled high-strength steel for seat frames. The steel plate has the characteristics of low carbon equivalent, high elongation, high bending performance, and high strength, and has excellent welding and forming performance. This invention also provides a method for producing gigapascal-grade cold-rolled high-strength steel for seat frames.

[0005] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0006] A type of cold-rolled high-strength steel sheet with a gigapascal grade for seat frames has the following chemical composition by mass percentage: C: 0.08%–0.12%, Si: 0.4%–0.8%, Mn: 2.0%–2.5%, Als: 0.03%–0.06%, Nb: 0.02%–0.04%, Ti: 0.02%–0.04%, Cr: 0.1%–0.3%, La: 0.02%–0.05%, P≤0.01%, S≤0.01%, N≤0.005%, P+S+N≤0.022%, with the remainder being Fe and unavoidable impurities.

[0007] The rationale for the alloy design of this invention is as follows:

[0008] C: Carbon enhances the strength of steel through solid solution strengthening and helps stabilize austenite, providing the necessary conditions for the presence of a small amount of stable retained austenite after annealing, thereby improving the plasticity of the steel. Too low a C content will not yield the mechanical properties of the steel described in this invention; too high a content will cause the steel to become brittle, posing a risk of delayed fracture. Therefore, the C content in this invention is controlled at 0.08%–0.12%.

[0009] Si: Silicon is a ferrite-forming element, whose main functions are solid solution strengthening and deoxidation of molten steel. Si is a non-carbide-forming element, which can expand the α+γ region in the Fe-C phase diagram and increase the austenite-to-ferrite transformation temperature, thereby promoting ferrite precipitation. However, excessive Si content will reduce the weldability of steel plates and easily produce red rust, leading to surface quality problems. In this invention, the Si content is controlled at 0.4% to 0.8%.

[0010] Mn: Manganese can play a role in solid solution strengthening and grain refinement. If the Mn content is too low, the supercooled austenite is not stable enough, reducing the processing properties of the steel plate, such as plasticity and toughness; if the Mn content is too high, it easily forms banded structures, and MnS readily becomes a hydrogen accumulation site, worsening the material's resistance to delayed cracking. Therefore, this invention controls the Mn content to 2.0%–2.5%.

[0011] P: Phosphorus is an inclusion element in steel, which can cause segregation in the center of the steel. The lower its content, the better. Considering cost, this invention controls the content of P element to ≤0.01%.

[0012] S: Sulfur is an inclusion element in steel. It forms MnS nonmetallic inclusions with Mn, which reduces the ductility of steel. In this invention, the content of S element is controlled to ≤0.01%.

[0013] Als: Acid-soluble aluminum plays a role in deoxidation and grain refinement in steel. Excessive Al content not only increases production costs but also leads to difficulties in continuous casting. This invention controls the Al content to 0.03%–0.06%.

[0014] Niobium (Nb) enhances the strength of steel through grain refinement and precipitation strengthening. It exists in steel as Nb (C, N), inhibiting austenite grain growth and reducing ferrite grain size, thus refining the microstructure. In this invention, the Nb content is selected to be 0.02%–0.04%.

[0015] Ti: Titanium is a strengthening nitrogen and oxygen-removing element. At high temperatures, the slab forms TiN, which hinders the growth of austenite grains in the cast slab and refines the grain size. Simultaneously, at low temperatures, Ti (C, N) precipitates, strengthening the material and improving its yield strength and toughness. This invention selects a Ti content of 0.02%–0.04%.

[0016] Cr: Chromium is an effective element for improving hardenability, which helps to refine grains during hot rolling and improves the strength and plasticity of materials. This invention selects a Cr content of 0.1% to 0.3%.

[0017] La (La): The rare earth element lanthanum reacts with O and S to form high-melting-point spherical inclusions, and can also modify Al2O3 and MnS inclusions into smaller spherical rare earth inclusions. These inclusions are uniformly distributed, avoiding or reducing cracks and defects caused by elongated MnS, thus improving the material's strength, isotropy, and plasticity. In this invention, the La content is selected to be 0.02%~0.05%.

[0018] N: Nitrogen is an impurity element in steel, and the content of nitrogen in this invention is required to be controlled at ≤0.005%.

[0019] The aforementioned seat frame uses a gigapascal-grade cold-rolled high-strength steel plate with a carbon equivalent (CE(IIW)) ≤ 0.60%; samples were taken from the steel plate along the rolling direction, and the yield strength... R p0.2 Tensile strength is 700-800 MPa. R m The elongation after fracture is 1000–1100 MPa. A 80 Rolling direction ≥15%, 180° cold bending radius r≤1t; t represents the sample thickness; the microstructure of the steel plate is ferrite, martensite, bainite and retained austenite, of which the bainite content is 30%~40% and the retained austenite content is 10%~20%, which meets the requirements of high strength, high elongation and high bending performance for seat frame steel.

[0020] A method for producing gigapascal-grade cold-rolled high-strength steel sheet for seat frames includes converter smelting, hot rolling, cold rolling, and annealing processes; the chemical composition of the smelting billet by mass percentage is as follows: C: 0.08%–0.12%, Si: 0.4%–0.8%, Mn: 2.0%–2.5%, Als: 0.03%–0.06%, Nb: 0.02%–0.04%, Ti: 0.02%–0.04%, Cr: 0.1%–0.3%, La: 0.02%–0.05%, P≤0.01%, S≤0.01%, N≤0.005%, P+S+N≤0.022%, with the remainder being Fe and unavoidable impurities.

[0021] The above-mentioned method for producing a gigapascal-grade cold-rolled high-strength steel sheet for a seat frame includes a continuous annealing process. The steel sheet is heated to 800–850°C at a heating rate of 2–3°C / s, held at that temperature for 110–150s, then slowly cooled to 600–650°C at a cooling rate of 3–6°C / s, followed by rapid cooling to 300–360°C at a cooling rate of 20–40°C / s, and then aged for 200–310s, resulting in a flattening elongation of 0.2–0.5%.

[0022] The above-mentioned method for producing a gigapascal-grade cold-rolled high-strength steel plate for a seat frame includes the following steps: in the hot rolling process, the temperature of the slab exiting the heating furnace is controlled at 1240~1280℃, the final rolling temperature is controlled at 880~930℃, and after rolling, it is cooled to 540~600℃ by a laminar flow cooling system before being coiled; in the cold rolling process, the cold rolling reduction rate is controlled at 45~55%.

[0023] This invention controls the continuous annealing temperature at 800–850℃ in the α+γ two-phase region. The microstructure consists of ferrite, martensite, bainite, and a small amount of retained austenite. The retained austenite is mainly distributed at the ferrite grain boundaries and between the ferrite and bainite phase interfaces. During heating in the two-phase region, austenite-stabilizing elements such as C and Mn in the ferrite diffuse into the austenite, increasing the stability of the retained austenite. In this invention, the Mn content is 2.0%–2.5%. Mn readily accumulates at the austenite phase boundaries, making the austenite in the enriched region more stable. After annealing, 10%–15% of retained austenite is retained. The TRIP effect of the retained austenite ensures the product's plasticity and bending performance.

[0024] This invention adds 0.02%~0.05% of the rare earth element La. La reacts with O and S to form spherical inclusions La2O3, La2S3, or La2O2S, reducing the amount of traditional inclusions such as MnS and Al2O3. It also modifies Al2O3 and MnS inclusions into smaller composite phase inclusions such as La-Al-Mn-OS, LOS, and La-O-Al. The rare earth inclusions are ≤3.0μm in size and dispersed throughout the microstructure, refining the grain size. The spherical inclusions are more stable than irregular MnS inclusions, and the fine rare earth inclusions pin the austenite grain boundaries and stabilize the austenite, thus ensuring the high plasticity, toughness, bending performance (180° cold bending radius r≤1t), and elongation (A80≥15%) of the gigapascal-grade cold-rolled high-strength steel sheet.

[0025] The beneficial effects of adopting the above technical solution are as follows:

[0026] 1) The high elongation and high bending strength cold-rolled high-strength steel plate provided by this invention has a yield strength of [missing information]. R p0.2 Tensile strength is 700-800 MPa. Rm The rolling elongation after fracture is 1000–1100 MPa. A 80 With a thickness of ≥15% and a 180° cold bending radius r≤1t (t represents the template thickness), it can be used for complex seat frame components with high strength, high elongation, and high bending performance requirements, meeting the lightweight development needs of the automotive industry.

[0027] 2) The steel plate provided by this invention has low carbon equivalent (CE(IIW)≤0.60%), high bending performance (180° cold bending radius r≤1t) and high elongation (A80≥15%). The high Mn content combined with the grain-refining effect of Nb and Ti elements gives it excellent welding performance and forming performance.

[0028] 3) The present invention controls the continuous annealing temperature at 800-850℃. When heating the α+γ two-phase region, austenite-stabilizing elements such as C and Mn in ferrite diffuse into austenite, and Mn forms a Mn-enriched region at the austenite phase boundary, which improves the content and stability of retained austenite. The content of retained austenite is 10%-20%. The TRIP effect of retained austenite ensures the plasticity and bending performance of the product. Attached Figure Description

[0029] Figure 1 This is a typical metallographic image (SEM) prepared in Example 1 of the present invention. Detailed Implementation

[0030] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without making inventive steps are within the protection scope of the present invention.

[0031] The present invention will be further described below through specific embodiments 1 to 10. Examples 1-10

[0032] This invention discloses a gigapascal-grade cold-rolled high-strength steel for seat frames and its production method. The preparation process is as follows:

[0033] (1) Smelting: 260-ton converter is used for smelting, followed by LF+RH refining and continuous casting to produce billets; the composition of the billets is shown in Table 1.

[0034] (2) Hot rolling: The temperature of the billet exiting the heating furnace is controlled at 1240~1280℃, the final rolling temperature is controlled at 880~930℃, and after rolling, it is cooled to 540~600℃ by a laminar flow cooling system and then coiled.

[0035] (3) Cold rolling: The reduction rate is controlled at 45-55%; the hot rolling and cold rolling process parameters of each embodiment are shown in Table 2.

[0036] (4) Annealing: Continuous annealing is adopted. The steel plate is heated to 800-850℃ at a heating rate of 2-3℃ / s, held for 110-150s, then slowly cooled to 600-650℃ at a cooling rate of 3-6℃ / s, and then rapidly cooled to 300-360℃ at a cooling rate of 20-40℃ / s. After aging treatment for 200-310s, the flattening elongation is 0.2-0.5%. The annealing process parameters of each embodiment are shown in Table 3.

[0037] Table 1 Chemical composition (wt%) of the billets in each embodiment

[0038]

[0039] Note: The balance of components in Table 1 represents Fe and unavoidable impurities.

[0040] Table 2. Hot rolling and cold rolling process parameters for each embodiment.

[0041]

[0042] Table 3 Annealing process parameters for each embodiment

[0043]

[0044] The mechanical properties of the cold-rolled duplex steel sheets in each embodiment are shown in Table 4.

[0045] Table 4 Mechanical properties of biaxial steel plates in various embodiments

[0046]

[0047] As can be seen from Tables 1 and 4, the gigapascal grade cold-rolled high-strength steel provided by this invention exhibits excellent mechanical properties and yield strength. R p0.2 Tensile strength is 700-800 MPa. R m The elongation after fracture is 1000–1100 MPa. A 80 Rolling direction ≥15%, 180° cold bending radius r≤1t (t represents the sample thickness); the steel plate has the characteristics of low carbon equivalent (CE(IIW)≤0.60%), high elongation (A80≥15%), high bending performance (180° cold bending radius r≤1t), and high strength, and is particularly suitable for complex parts of seat frames with high strength, high elongation and high bending performance requirements, meeting the development needs of lightweighting in the automotive industry.

[0048] The metallographic structure of the dual-phase steel plate prepared in Example 1 is as follows: Figure 1As shown, the microstructure consists of ferrite (F), martensite (M), bainite (B), and a small amount of retained austenite (RA). Ferrite, as the matrix microstructure, coordinates the deformation of the surrounding hard phases (M+B) through its own plasticity during deformation, reducing stress concentration and ensuring high strength while maintaining a certain level of toughness. Martensite (M) and bainite (B), as hard phase microstructures, significantly contribute to the improvement of strength during plastic deformation. During deformation, retained austenite transforms into martensite, resulting in a transformation-induced plasticity (TRIP) effect, which effectively inhibits necking and simultaneously improves the material's strength and ductility.

[0049] The above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A gigapascal-grade cold-rolled high-strength steel plate for a seat frame, characterized in that: The chemical composition of the steel plate, by mass percentage, is as follows: C: 0.08%–0.12%, Si: 0.4%–0.8%, Mn: 2.0%–2.5%, Als: 0.03%–0.06%, Nb: 0.02%–0.04%, Ti: 0.02%–0.04%, Cr: 0.1%–0.3%, La: 0.02%–0.05%, P≤0.01%, S≤0.01%, N≤0.005%, P+S+N≤0.022%, with the remainder being Fe. And unavoidable impurities; the carbon equivalent CE(IIW) of the steel plate is ≤0.60%; the steel plate is sampled along the rolling direction, the yield strength Rp0.2 is 700~800MPa, the tensile strength Rm is 1000~1100MPa, the elongation after fracture A80 in the rolling direction is ≥15%, and the 180° cold bending radius r≤1t; t represents the sample thickness; the microstructure of the steel plate is ferrite, martensite, bainite and retained austenite, of which the bainite content is 30%~40% and the retained austenite content is 10%~20%.

2. A method for producing gigapascal-grade cold-rolled high-strength steel sheet for a seat frame, comprising converter smelting, hot rolling, cold rolling, and annealing processes; characterized in that: The chemical composition (mass percentage) of the billet used for converter smelting is as follows: C: 0.08%–0.12%, Si: 0.4%–0.8%, Mn: 2.0%–2.5%, Als: 0.03%–0.06%, Nb: 0.02%–0.04%, Ti: 0.02%–0.04%, Cr: 0.1%–0.3%, La: 0.02%–0.05%, P≤0.01%, S≤0.01%, N≤0.005%, P+S+N≤0.022%, and the remainder is... Fe and unavoidable impurities; the carbon equivalent CE(IIW) of the steel plate is ≤0.60%; the steel plate is sampled along the rolling direction, with a yield strength Rp0.2 of 700-800 MPa, a tensile strength Rm of 1000-1100 MPa, an elongation after fracture A80 in the rolling direction ≥15%, and a 180° cold bending radius r ≤1t; t represents the sample thickness; the microstructure of the steel plate is ferrite, martensite, bainite and retained austenite, of which the bainite content is 30%-40% and the retained austenite content is 10%-20%.

3. The method for producing a gigapascal-grade cold-rolled high-strength steel plate for a seat frame as described in claim 2, characterized in that: The annealing process is a continuous annealing process, in which the steel plate is heated to 800-850°C at a heating rate of 2-3°C / s, held at that temperature for 110-150s, then slowly cooled to 600-650°C at a cooling rate of 3-6°C / s, and then rapidly cooled to 300-360°C at a cooling rate of 20-40°C / s, followed by aging treatment for 200-310s, with a flattening elongation of 0.2-0.5%.

4. The method for producing a gigapascal-grade cold-rolled high-strength steel plate for a seat frame as described in claim 2, characterized in that: In the hot rolling process, the temperature of the slab exiting the heating furnace is controlled at 1240~1280℃, the final rolling temperature is controlled at 880~930℃, and after rolling, it is cooled to 540~600℃ by a laminar flow cooling system before being coiled; in the cold rolling process, the cold rolling reduction rate is controlled at 45~55%.

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