Double-hit isothermal deformation and multi-pass continuous cooling hot compression tests were carried out to study the recrystallization behavior of a 0.11 (in mass %) Ti-microalloyed complex phase (CP) steel. The influence of different deforming temperatures and holding times on microstructure evolution was investigated. The results showed that a pronounced austenite grain refinement after appropriate recrystallization process has been detected. The grain size decreases continuously from 176μm to 20μm after four-pass compression. It has been verified that non-crystallization temperature (Tnr) of the experimental steel is about 975°C under the deformation conditions. Based on the stress-strain curves, a kinetic method was established to predict the non-recrystallization temperature of the studied steel during nonisothermal continuous hot deformation.
Complex phase steel CP 800, a kind of advanced high strength steel (AHSS), exhibited quite high carbon equivalent (CE) which was a detrimental factor for weldability of steels. Thus the weldability of CP 800 steels containing (in wt%) 0.06C-0.45Si-1.71Mn-0.11Ti was extensively studied. Mechanical properties and impact toughness of butt joint, the welding crack susceptibility of weld and heat-affected-zone (HAZ) for tee joint, Control Thermal Severity (CTS) welded joint, and 60°Y-groove butt joint were inspected after gas shielded arc welding tests. The impact toughness was larger than 27J either at room temperature (RT) or at -20°C, indicating good impact toughness of the weld of the steel. In addition, welding crack susceptibility tests revealed that the weldments were free of surface crack and other imperfection, showed fairly good weldability. In application, the longitudinal control arm of automobile made of this steel exhibited excellent fatigue and durability performance.
Continuous cooling transformation (CCT) diagram of steel 22MnB5 was studied using dilatometer method. The influence of the hot forming process parameters, such as the heating temperature and holding time on the mechanical properties and microstructure of stamped parts was analyzed by tensile tests and the metallographic observation on the parts with various processing parameters. The microstructural evolution obtained from the dilatometer samples reveals that the cooling rates not smaller than 20K/s induced fully martensitic microstructure. As the cooling rate decreasing, more ferrite and pearlite or more bainitic microstructure forms. Tensile tests results show an excellent tensile strength and ductility combination of 22MnB5. The tensile strength and yield strength reach 1500MPa and 1200MPa respectively, with total elongation of around 10%.
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