Automotive Steel

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 high-reduction 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. IF can meet the performance requirements of complex-shaped car stamping parts.Typical grades include B220IF, B340IF, B410IF, B500IF, etc.

Including IF steel bake-hardened steel plate and low-carbon bake-hardened steel plate. Mainly focused on IF bake hardened boards. 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 non-aging properties at room temperature within a certain period of time. 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.Typical grades include CR210BH, CR240BH, CR270BH, CR340BH, etc.

Phosphorus steel plate
The solid solution strengthening effect of phosphorus in steel is used for strengthening. The solid solution strengthening effect of phosphorus is outstanding, equivalent to 10 times that of manganese and 7 times that of silicon. While phosphorus increases strength, the elongation decreases very little, and it also has a high plastic strain ratio r value. Phosphorus-containing steel plates can be used to punch some automotive stamping parts with relatively complex shapes.Typical grades include SPCC-P, SGCC-P, etc.

Isotropic steel is a low carbon microalloy steel. This steel is characterized by very low plane anisotropy, that is, Δr tends to zero (generally between -0.15~+0.15), high strength, high n value and good formability. wait. The strength level can be divided into 220, 250, 260, 280 and 300Mpa according to the yield strength. Isotropic steel is mainly used for automobile exterior panels. It is mainly used on European series models and is rarely used on Japanese cars.Typical grades include DP600IS and DP800IS, etc.

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 to 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 carbon in solid solution, so it is also a bake-hardened steel. After baking and painting, the yield strength increases by about 100Mpa.

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%-50%. As the martensite content increases, the strength increases linearly, with a strength range of 500-1200MPa.

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. Generally used for automotive parts that require high strength, high anti-collision energy absorption and strict molding requirements, such as wheels, bumpers, suspension systems and their reinforcements, etc. With the advancement of steel performance and molding technology, DP steel It has also begun to be used for interior and exterior panel parts of automobiles.Typical grades include DP590, DP780, DP980, DP1180, etc.

TRIP steel is a type of steel that has only been commercially developed in the past 10 years, including hot-rolled, cold-rolled, electroplated and hot-dip galvanized products. The main structures are ferrite, bainite and retained austenite. The content of retained austenite is 5%~15%, and the strength range is 600~800MPa. 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 the 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.Typical transformation-induced plasticity steel (TRIP) grades include TRIP590, TRIP780, TRIP980 and TRIP1180.

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 800~1000MPa. 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 door anti-collision bars, bumpers, B-pillars and other safety parts.Typical grades include CP500, CP600, CP800 and CP1000, etc.

The so-called TWIP means twin-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 area. Adding C can not only stabilize austenite, but also utilize C solid solution to strengthen austenite. By controlling the contents of Mn and C, the best mechanical properties can be obtained.

The biggest feature of TWIP steel is its extremely high strength, which can reach 50,000MPa%, which is 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 heat-treated steel, but its plasticity is more than 10 times higher than heat-treated steel. TWIP can ensure very high safety of the car during high-strain processes such as impact.Typical TWIP steel grades include TWIP800, TWIP1000, TWIP1200, etc.

As the strength increases, the formability of the steel plate generally shows a downward trend, the shape freezeability becomes worse, and the forming difficulty increases. At the same time, the material forming force also increases, resulting in increased molding load and serious wear and damage of tools and molds. Therefore, in recent years, new technologies for heat treatment of materials after hot stamping have been developed to meet users' urgent requirements for further improving strength. This technology is applied to steel plates that can be heat-treated and strengthened, such as B-containing steel plates. It is heated to about 900°C for stamping and forming, which greatly reduces the forming resistance and improves the forming ability of the material. After punching the parts, they are immediately quenched using waste heat. The current tensile strength after treatment can reach about 1500MPa. The processed parts need to be shot blasted to remove iron oxide scale and improve the surface quality. In recent years, steel plates with Al-Si pre-coating are commonly used in production, which can avoid oxidation during the heating process before hot stamping. After hot stamping, an Fe-Al-Si alloying layer is formed on the surface, and the parts do not need shot peening. , can be painted directly, and the shape accuracy of the parts is good.Typical hot-formed steel plate grades include QSTE340TM, QSTE420TM, QSTE500TM, etc.