Classification Of Steel Plates For Automobiles

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The parts that make up the car body are roughly divided into panel parts, structural parts, running parts and reinforcement parts. These parts correspond to different use requirements and have different performances. For example, panel parts require the plate to have good formability, strength, elongation, dent resistance, corrosion resistance, etc.; structural parts require the plate to have good formability, strength, collision energy absorption capacity, fatigue durability, corrosion resistance, and weldability; running parts require good formability, rigidity, fatigue durability, corrosion resistance, and weldability; and excellent collision energy absorption capacity and weldability are particularly important for reinforcement parts.

Although the market demand for automobile weight reduction is becoming stronger and stronger, due to the strength requirements of the car and the safety considerations for the driver and passengers, the structural parts and some panels of mainstream cars are still using steel plates. Automotive steel plates are divided into hot-rolled steel plates, cold-rolled steel plates and coated ...
... steel plates based on production process characteristics; from the perspective of strength, 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 IF steel, BH steel, phosphorus-containing steel and IS steel, etc.) and advanced high-strength steel plates (AHSS), etc.

1. Ordinary steel plates
Ordinary steel plates refer to those with a carbon content between 0.01-0.1%, whose strength generally meets Rp0.2≤250MPa, Rm between 250-350MPa, and elongation of more than 30% to meet general strength requirements. They are generally used for parts with low strength requirements. 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 the hot rolling process with low temperature and large reduction and accelerated cooling immediately after rolling, fine-grained ferrite is obtained, and large reduction rate cold rolling and high temperature annealing are used to obtain the required texture and high forming performance), so that the steel has high strength while ensuring good plasticity and stamping performance, and can meet the performance requirements of complex-shaped car stamping parts.

3. Low-alloy high-strength steel plate
Low-alloy high-strength steel is a type of engineering structural steel developed by adding a small amount of Mn, Si and trace amounts of Nb, V, Ti, Al and other alloying elements on the basis of carbon structural steel, and its yield strength exceeds 275MPa. The so-called low alloy refers to the total amount of alloy elements in steel not exceeding 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 strength ratio. The strength level 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 automotive structural parts and reinforcements, and is mainly used in European series models, such as: Q345, Q390.

The alloying principle of low alloy high strength steel is mainly to improve the strength of steel by using the solid content strengthening, fine grain strengthening and precipitation strengthening produced by alloying elements, and at the same time, the fine grain strengthening is used to reduce the toughness-brittle transition temperature of steel to offset the adverse effect of carbonitride precipitation strengthening in steel to increase the toughness-brittle transition temperature of steel. This allows the steel to obtain high strength while maintaining good low temperature performance.

4. Bake hardening steel plate (BH steel)
Bake hardening steel has both strength and high formability. The strength of the final part is obtained through work hardening during processing and aging during painting. It includes IF steel bake hardening steel plate and low carbon bake hardening steel plate. It mainly focuses on IF bake hardening plate, and its models are such as H180 and H260. The characteristic is that the steel plate has a lower yield strength before stamping, and the yield strength of the steel plate is increased by the painting and baking process after stamping.

BH steel has good bake hardening properties and must also ensure that it has room temperature non-aging properties within a certain period of time, 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 experience natural aging within 3 months. BH steel can improve the dent resistance of the steel plate without affecting the shape stability of the formed part, so it is very suitable for the production of automotive exterior panels.

5.5 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, it is first heated to the ferrite + austenite two-phase region of 760-830℃, so that its structure is a certain proportion of ferrite and austenite. At this time, the steel is quenched below the martensite point, and the austenite is transformed into martensite, resulting in the so-called "dual-phase structure". The matrix of DP steel is soft ferrite, on which hard martensite is distributed, and 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 strength ratio and can obtain a large extension. DP steel has a large amount of solid solution C, so it is also a bake hardening steel. After baking and painting, the yield strength increases by about 100MPa. Such as representative models DP590 and DP780.

DP steel shows higher strength than ordinary high-strength steel in high-speed deformation of vehicle collision, so it has greater impact energy absorption capacity, which is beneficial to improve the safety of vehicles. The main organizations are ferrite and martensite, of which the content of martensite is 5% to 50%. With the increase of martensite content, the strength increases linearly, and the strength range is 500 to 1200MPa.

Dual-phase steel also has the characteristics of low yield strength ratio, high work hardening index, high bake hardening performance, no yield extension and room temperature aging. It is 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. With the advancement of steel performance and forming technology, DP steel has also begun to be used for automotive inner and outer panel parts.

6. Transformation Induced Plastic Steel (TRIP)

TRIP steel is a steel type that has been commercially developed in the past 10 years. Its main components are C, Si and Mn, including 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 include: TR590, TR780.

The essence of TRIP steel's high elongation is that strain induces the transformation of retained austenite into martensite. At the same time, the volume expansion caused by 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 in a long strain range, which is particularly suitable for situations requiring high bulging performance.

7. Complex Phase Steel (CP, Multiphase)
The cooling mode of complex phase steel is similar to that of TRIP steel, but the chemical composition needs to be adjusted to form precipitation phases of strengthening martensite and bainite, with a strength range of 800-1000MPa. Its organizational characteristics are fine ferrite and a high proportion of hard phase (martensite, bainite), and it is further strengthened by precipitation strengthening. It contains elements such as Nb and Ti, 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.

8. Martensitic Steels (MP)
The microstructure of steel is almost entirely martensitic, and martensitic steel has high tensile strength. Martensitic steel usually needs to be tempered to improve its plasticity so that it still has sufficient formability at such a high strength. The strength of the steel is 800-1500Mpa.

9. TWIP steel (TWinningInduced Plasticity, referred to as TWIP)
The so-called TWIP means twinning induced plasticity. TWIP steel is a pure austenitic carbon steel mainly composed of Fe and Mn. The use of Mn can expand the austenite phase region. The addition of C can not only stabilize austenite, but also use C solid solution to strengthen austenite. By controlling the content of Mn (17-24%) and C (0.5-0.7%), the best mechanical properties can be obtained. The biggest feature of nTWIP steel is its high strength-plasticity product, which can reach 50000Mpa%, more than twice that of TRIP steel. TWIP steel can stamp out complex-shaped parts like DDQ-grade stamping plates, but its strength is 2 to 5 times higher; its tensile strength is equivalent to that of heat-treated steel, while its plasticity is more than 10 times higher than that of heat-treated steel.

The main mechanism of plastic deformation of TWIP steel is dislocation slip. During the deformation process, very fine twins occur in the crystal, and both the twin boundary and the grain boundary become barriers to dislocation slip, which will lead to very intense work hardening, and its instantaneous hardening rate can be maintained at a high level (>0.45). This will inhibit the occurrence of necking, resulting in very strong work hardening ability and very large elongation of TWIP steel. This performance is still maintained even at high strain rates, so it can ensure very high safety of automobiles during high strain processes such as impact.

10. Hot-formed (heat-treated) steel plate
With the increase of strength, the overall forming performance of steel plates shows a downward trend, the shape freezing property becomes worse, and the forming difficulty increases. At the same time, the material forming force also increases, resulting in an increase in the forming load, and serious wear and damage to tools and molds. Therefore, in recent years, a new technology for heat treatment of materials after hot stamping has been developed to meet the urgent requirements of users for further strength improvement.

This technology is applied to steel plates that can be heat-treated and strengthened, such as B-containing steel plates, which are heated to about 900℃ for stamping, which greatly reduces the forming resistance and improves the forming ability of the material. After the parts are punched, the residual heat is immediately used for quenching. At present, the tensile strength after treatment can reach about 1500Mpa. The treated parts need to be shot peened to remove the iron oxide scale and improve the surface quality. In recent years, steel plates with Al-Si pre-coating are widely used in production, which can avoid oxidation during the heating process before hot stamping. After hot stamping, a Fe-Al-Si alloying layer is formed on the surface. The parts do not need to be shot peened and can be directly painted, and the shape accuracy of the parts is good. Typical grades are Mazda: SPHT-T; Arcelor: Usibor 1500P, 22MnB5.

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