The yield point of a steel structural member determines the stress that the structure can withstand without permanent deformation. The typical yield point of a typical carbon structural steel is 235 MPa. The typical low alloy high strength steel has a minimum yield point of 345 MPa. Therefore, depending on the proportional relationship of the yield points, the use of low-alloy high-strength steel allows the stress to be 1.4 times higher than that of carbon structural steel. Compared with carbon structural steel, the use of low alloy high strength steel can reduce the size of the structural parts and reduce the weight. It must be noted that for components that may be bent, the allowable stress must be corrected to ensure structural robustness. Sometimes the low-alloy high-strength steel is substituted for the carbon structural steel without changing the cross-sectional size, and its sole purpose is to obtain a stronger and more durable structure without increasing the weight. Saving weight is of the utmost importance to the structure of the transport vehicle so that it can transport heavier weight and reduce energy consumption.
The strength of the steel is often increased by adding a small amount of niobium or vanadium, or titanium. These elements are economically enhanced by precipitation hardening. Other alloying elements added for various purposes can also achieve the purpose of strengthening. For hot rolled sheets, the hot rolling and coiling processes are strictly controlled to achieve the required uniform strength. For cold-rolled sheets, special annealing and flattening processes are used to achieve higher strength while maintaining good formability.
The latest development is the use of low-alloy high-strength steels that obtain martensitic and ferrite two-phase microstructures (or two-phase microstructures) by critical annealing and rapid cooling. This steel sheet product has excellent forming properties, and the yield point is generally 310 to 345 MPa. The strain point can be increased to 550 MPa or higher by strain generated by auto-forming of automobile parts.