Microstructure,Mechanical Properties,and Strengthening Mechanism of Cr-Mo Microalloy Cold Heading Steel

Lightweight and safety of automobiles is the development trend of high-strength automobile fasteners, which is conducive to saving resources and protecting the environment. High-strength fasteners connect parts of engine, and their strength affects the overall life of the engine, thereby affecting the safety performance of the vehicle. Presently, high-strength fastener steels for cars are mainly 35CrMo, and the fastener strength reaches 10.9/12.9 level, which must fulfill sufficient delayed fracture and fatigue properties. Therefore, it is important to develop cold heading steel with high fatigue and strong plasticity matching. In this study, the microstructure, mechanical properties, and strengthening mechanism of Cr-Mo microalloyed cold heading steel, under different thermomechanical control processes (TMCPs), were investigated using OM, SEM, and TEM. The results show that the TMCP parameters affect the structure and mechanical properties of the experimental steel. With an increase in the finish rolling temperature and acceleration of the cooling rate, the ferrite-pearlite composite structure in the steel gradually changes to bainitic, dislocation density gradually increases, tensile strength monotonously increases, and elongation fluctuates. When the finish rolling temperature is 935 degrees C, the microstructure is mainly uniformly distributed bainite phase, which is in the form of short rod and granular, and there is dislocation entanglement. The experimental results show that this process has the best strength and toughness matching. Its tensile strength and elongation reach 925 MPa and 20%, respectively, and the hardness at 7 mm from the quenched end (J7) is 53.1 HRC. When the finish rolling temperature is 900 degrees C, grain refinement strengthening is the main strengthening mechanism, accounting for 31%-36% of the yield strength; when the finish rolling temperature is more than 935 degrees C, dislocation strengthening is the main strengthening mechanism, accounting for the total strength of 35%-38%. The hardenability results show that the hardenability of the experimental steel is unaffected by the microstructure and mechanical properties, and it maintains high-quality hardenability. In addition, a model of the end quenching curve of the Cr-Mo microalloyed steel is established to predict hardenability.