Method Discussion And Theory Analysis For Deeply Exploring The Potential Properties Of Q345Steel

Continuous annealing process was adopted for heat treating cold-rolled Q345steel and the microstructure and mechanical properties of treated material was studied, and a new kind of dual-phase (DP) steel with ferrite and martensite was then produced. The formability and bake hardening behavior of the DP steel were also analyzed. Moreover, hot rolled and cold-drawn Q345seamless steel tubes taken as raw materials were heat treated by the self-designed intermediate-frequency induction quenching device, and the dual-phase steel tube with the matrix of ferrite and martensite was developed. By intermediate-frequency induction quenching process treating, a new kind of seamless steel tube with TRIP effect was investigated and produced on the basis of the chemical compositions of Q345plate with silicon addition. The main work of this paper was summarized as follows.(1) DP steel with the phase ratio of6:4between ferrite and martensite was produced from cold-rolled Q345plate with the initial microstructure of ferrite and pearlite after intercritical annealing treating. The process parameters that impact the microstructure and mechanical properties were studied, including soaking time, soaking temperature, rapid cooling rate, rapid cooling start temperature, overaging temperature, overaging time and heating rate. The optimized continuous annealing process was obtained by the analysis of the influencing factors.(2) Dual-phase steels with different thicknesses were obtained by the optimized heat treating process. Nano-indentation method was to be selected for the hardness measuring of ferrite and martensite, and the property differences between the two phases were analyzed by SEM, TEM and nano-indentation hardness. Besides, the hardness and normal tensile property tests, flangeability, deep-drawing, forming limit diagram and springback property were also investigated. The results validated that the assumption of developing cold-rolled Q345plate as DP steel with an excellent performance was feasible.(3) The bake hardening (BH) behavior of the new type DP steel that obtained by continuous annealing cold-rolled Q345steel was studied. Prestrain value, baking temperature and baking time were systematically analyzed. The results showed that the new type DP steel exhibited an excellent bake hardening property.(4) Hot rolled and cold-drawn Q345seamless steel tube with the initial microstructure of ferrite and pearlite were heat treated by the intermediate-frequency induction quenching process, and the dual-phase steels with ferrite and martensite were obtained. Ultrafine-grained dual-phase steel tube was also developed by cyclic heat treatment. The feasibility of the process was validated by the analyzed results of microstructure, mechanical properties, T and cross type hydroforming simulation.(5) The cold-drawn seamless steel tube was experimentally produced on the basis of chemical composition of Q345steel with silicon addition, the tube was heat treated by two-stage salt-bath process and a new type steel tube with TRIP effect was obtained. Referred to the heat treatment process mentioned above, TRIP type seamless steel tube was developed by the use of intermediate-frequency induction quenching process. Microstructure and mechanical properties of the TRIP type tube were tested and the mechanism of deformation induced phase transformation was studied by the analysis of the change in microstructure before and after cold-rolled forming treatment. The experiment further validated the feasibility of new steel tube grades that could be online developed by intermediate-frequency induction quenching process.DP steel sheet and DP seamless tube were developed by Q345plate and Q345steel tube, TRIP type seamless steel tube was also developed by silicon addition on the basis of chemical composition of Q345steel. With excellent formability and bake hardening ability, the new type DP steel sheet was suitable for the stamping of complex parts. With advanced mechanical properties, the new type DP and TRIP seamless steel tube could be applied to hydroforming and cold-rolled forming production.