The components that make up the vehicle body are roughly divided into panel components, structural components, running components and reinforcement components. These components correspond to different application requirements and have different performances. For example, panel components require plates to have good formability, strength, extensibility, dent resistance, corrosion resistance, etc. Structural components require plates to have good formability, strength, collision energy absorption capacity, fatigue durability, and corrosion resistance. , Weldability; walking parts require good formability, rigidity, fatigue durability, corrosion resistance, and weldability; and excellent collision energy absorption capabilities and weldability are particularly important for reinforced components.
Although the market demand for weight reduction of automobiles is getting stronger and stronger, due to the strength requirements of automobiles and safety considerations for drivers and passengers, steel plates are still used in the structural parts and some panels of mainstream cars. Steel plates for automobiles are divided into hot-rolled steel plates, cold-rolled steel plates and coated steel plates according to the production process characteristics; from the strength point of view, they can be divided into: ordinary steel plates (mild steel plates), low-alloy high-strength steel plates (HSLA), ordinary high-strength steel plates (high-strength steel plates). Strength IF steel, BH steel, phosphorus-containing steel and IS steel, etc.) and advanced high-strength steel plate (AHSS), etc.
1. Ordinary steel plate
Ordinary steel plates refer to carbon content between 0.01-0.1%, their strength generally meets the requirements of Rp0.2≤250MPa, Rm between 250-350MPa, and the elongation reaches more than 30% to meet the general strength requirements. Generally, the strength requirements are not high. Parts are top quality. Such as St12, St13, St14 and other models.
2. High strength IF steel plate
On the basis of IF steel, different types of strengthening elements (such as solid solution strengthening elements P, Mn, Si) and appropriate rolling process control (through hot rolling at low temperature and large reduction and accelerated cooling immediately after rolling) are added. , to obtain fine-grained ferrite, as well as large reduction rate cold rolling and high-temperature annealing to obtain the required texture and high formability), so that the steel has high strength while ensuring good plasticity and stamping performance. Meet the performance requirements of complex-shaped car stamping parts.
3. Low alloy high strength steel plate
Low-alloy high-strength steel is developed on the basis of carbon structural steel by adding a small amount of Mn, Si and trace amounts of Nb, V, Ti, Al and other alloying elements. Its yield strength exceeds 275MPa, a type of engineering structural steel. The so-called low alloy means that the total amount of alloying elements in steel does not exceed 3%. The principle of developing low-alloy high-strength steel is to use as few alloy elements as possible to obtain the highest possible comprehensive mechanical properties, so as to achieve the purpose of satisfying use and low cost.
Main features: High yield-to-strength ratio. Strength levels can be divided into 260, 300, 340, 380 and 420, 460, 500MPa according to the yield strength. Low-alloy high-strength steel is mainly used for automobile structural parts and reinforcement parts, and is mainly used in European series models, such as Q345 and Q390.
The alloying principle of low-alloy high-strength steel mainly uses the solid volume strengthening, fine-grain strengthening and precipitation strengthening produced by alloying elements to improve the strength of the steel. At the same time, the fine-grain strengthening is used to reduce the ductile-brittle transition temperature of the steel to offset the effect of the steel. The precipitation strengthening of medium-carbon nitrides has the adverse effect of increasing the tough-to-brittle transition temperature of steel, allowing the steel to maintain good low-temperature properties while obtaining high strength.
Performance standards for representative grades of low alloy high strength steel
4. Bake hardened steel plate (BH steel)
Bake-hardened steel is both strong and highly formable. The strength of the final part is obtained through work hardening during machining and aging phenomena during the painting process. Including IF steel bake-hardened steel plate and low-carbon bake-hardened steel plate. It mainly focuses on IF bake-hardened boards, with models such as H180 and H260. The characteristic is that the steel plate has a low yield strength before stamping, and the yield strength of the steel plate is increased through the painting and baking process after stamping.
While BH steel has good bake hardening properties, it must also ensure that it is non-aging at room temperature within a certain period of time. It is usually expressed by the aging index AI. If the AI value is less than 30MPa, it can be considered that the steel plate will not appear within 3 months. Natural aging. BH steel can improve the dent resistance of the steel plate without affecting the shape stability of the formed parts, so it is very suitable for the production of automobile exterior panels.
Performance standards for representative grades of bake hardened steel
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5. Dual-Phase steel (DP for short)
DP steel is a low-cost steel with Si and Mn as the main alloy components. In the continuous annealing process, the ferrite + austenite two-phase zone is first heated to 760-830°C to make the structure a certain proportion of ferrite and austenite. At this time, the steel is quenched below the martensite point, and the austenite transforms into martensite, resulting in the so-called "dual-phase structure". The matrix of DP steel is soft ferrite, with hard martensite distributed on it. The two determine the low yield strength and high tensile strength of the material respectively.
DP steel has a higher initial work hardening rate than traditional high-strength steel, so it has a very low yield-to-strength ratio and can achieve great elongation. DP steel has more C in solid solution, so it is also a bake-hardened steel. After baking and painting, the yield strength increases by about 100MPa. For example, the representative models are DP590 and DP780.
DP steel shows higher strength than ordinary high-strength steel during high-speed deformation in vehicle collisions, so it has greater impact energy absorption capacity, which is beneficial to improving vehicle safety. The main structures are ferrite and martensite, of which the martensite content is 5% to 50%. As the martensite content increases, the strength increases linearly, and the strength range is 500 to 1200MPa.
Dual-phase steel also has the characteristics of low yield ratio, high work hardening index, high bake hardening performance, no yield extension and room temperature aging. Generally used for automotive parts that require high strength, high anti-collision energy absorption and strict forming requirements, such as wheels, bumpers, suspension systems and their reinforcements, etc. With the advancement of steel performance and forming technology, DP steel It has also begun to be used for interior and exterior panel parts of automobiles.
6. Transformation Induced Plastic (TRIP)
TRIP steel is a type of steel that has only been commercially developed in the past 10 years. Its main components are C, Si and Mn, and includes hot-rolled, cold-rolled, electroplated and hot-dip galvanized products. The main structures are ferrite, bainite and retained austenite, of which the content of retained austenite is 5% to 15%, and the strength range is 600 to 800MPa. Representative models such as: TR590, TR780.
The essence of the high elongation of TRIP steel is the strain-induced transformation of retained austenite into martensite. At the same time, the volume expansion caused by the phase transformation is accompanied by an increase in the local work hardening index, making it difficult for deformation to be concentrated in local areas. Compared with DP steel, the initial work hardening index of TRIP steel is smaller than that of DP steel, but the work hardening index of TRIP steel remains high over a long strain range, which is especially suitable for situations requiring high bulging performance.
TRIP steel representative grade performance standards
7. ComplexPhase (CP, Multiphase)
The cooling mode of multiphase steel is similar to that of TRIP steel, but the chemical composition needs to be adjusted to form the precipitation phase of strengthened martensite and bainite, with a strength range of 800 to 1000MPa. Its structural characteristics are fine ferrite and a high proportion of hard phases (martensite, bainite), which are further strengthened by precipitation strengthening. It contains Nb, Ti and other elements, and has high impact energy absorption capacity and Good hole expansion performance. Suitable for safety parts such as door anti-collision bars, bumpers and B-pillars.
