Development of high hole-expanding ratio hot-rolled complex phase steel for automotive wheels

A micro-alloyed hot-rolled complex phase steel with high porosity for automobile wheels was designed and industrially trial-produced. The microstructure and mechanical properties of hot rolled multiphase steel with high hole expansion rate were studied by means of metallographic examination and mechanical properties test. The results show that the microstructure of the trial product was mainly composed of bainite and ferrite, and a small amount of martensite, retained austenite and dispersed second phase particles were also distributed. The yield strength, tensile strength and elongation of the product exceed 520, 610 MPa and 24%, respectively. The yield ratio was 0.85~0.87, the hole expansion rate was more than 83%, and the hole expansion rate along the width of the plate fluctuated within 10%. Increasing the content of Si element could significantly improve the hole expansion performance and affect the surface quality of the product. It was more appropriate to control the mass fraction of Si element at about 0.20%. The trial product had excellent local formability, good fatigue resistance and welding performance. Under the condition of fatigue test bearing capacity of 1 700 N·m, the bending fatigue life of the wheel reached more than 600 000 times.