Papers by Author: Wen Quan Cao

Abstract: The advantages and disadvantages of the conventional hot stamping steel and the hot tamping technology were shortly reviewed. It was proposed that new type steel with relative low austenization temperature and air hardening ability should be designed and produced to overcome the surface oxidation and avoid the complexity of cooling system of the stamping dies. In this study, it was shown that the 3rd generation sheet steel with medium-Mn composition could be austenized and pressed at 700-750°C and air cooled into automobile parts with strength no less than 1.5GPa and ductility larger than 12%. This new steel and corresponding forming technology were called warm stamping technology, which overcomes the disadvantages of the conventional hot stamping steel. The fabrication trials of the automobile parts in the automobile industries were demonstrated with high strength and relative good ductility. It was concluded that the 3rd generation sheet steel and the warm stamping technology could be applied to replace the conventional hot stamping steel and conventional hot stamping technology in near future.

Abstract: The austenisation and quenching behaviors of a medium-Mn steel with a chemical composition of C 0.14% and Mn 5.0% were presented in this paper. The heating and holding temperature varied from 500^OC to 950^OC was applied to find the austenization temperature. The holding time varied from 1-5minutes was used to choose the suitable austenisation time. Air cooling were applied to check the hardening capacity of the studied steel. It was found that the tensile strength is lower than 900Mpa and total elongation varied from 20-30% when the austenisation temperature is lower than 700^OC. However, the tensile strength could be high up to 1600MPa and the total elongation about 12% when the austenisation temperature is higher than 700^OC with air cooling. Based on the low austenisation temperature and the air hardening capacity, it was proposed that the studied medium-Mn steel could be used as the hot stamping steel in the car industry.

Abstract: The mechanical stability of the retained austenite in the cold-rolled medium-Mn steel was studied. Tensile tests were carried out to measure the mechanical properties of the annealed steel. Scanning electron microscopy was applied to characterize the microstructure evolution during the tensile process; X-ray diffraction analysis was used to determine the residual austenite content in the deformed steel. It was found that the volume fraction of retained austenite gradually decreases with strain .The value of the stability coefficient of retained austenite k was small in the test steel, which indicated high mechanical stability of retained austenite. Due to TRIP effect, the high mechanical stability of the retained austenite strongly delays the onset of necking, which resulted in good comprehensive mechanical properties with ultrahigh strength and plasticity.

Abstract: The behavior of 60% cold-rolled medium-manganese steel (0.1C5Mn) during intercritical annealing, has been examined using various techniques. Microstructural observations showed a slight coarsening of the subgrain/grain structure during intercritical annealing, without any apparent change in the misorientation distribution. In addition, the formation of ultrafine austenite grains took place mainly at high-angle boundaries and rarely at low angle boundaries, suggesting a heterogeneous austenite nucleation process in this steel. The results indicated that the annealing behavior of cold rolled medium manganese steels is controlled by the extensive recovery of the ferrite phase and formation of austenite phase with an austenite volume fraction of ~20%. It was proposed that the segregation of manganese and carbon to high-angle boundaries promoted austenite nucleation and growth, as such segregation decreases the Gibbs energy of austenite.

Abstract: The yield strength and impact energy properties for martensitic steel fabricated by vacuum induction melting, is investigated. It is found that the addition of Ti can improve the yield strength property of the martensitic steel after reheat quenching process, which can be attributed to increase in precipitation hardening from formation of TiC precipitates in the martensitic matrix and a superfine sized (~8μm) grains in the martensitic structure. Moreover, the yield strength can be further enhanced by Mo addition, which can be ascribed to a large amount of freshly nano-sized (1-10nm) precipitates in the final martensitic structure. The experimental and theoretical results on the contribution of TiC precipitates to hardening of the martensitic steel are in excellent agreement. In addition, the impact toughness also has been improved along with yield strength followed by the heat treatment, which can be attributed to the grain refinement and high ratio of high-angle grain boundaries after Mo addition.

Abstract: In this study a C-Mn High Strength Low Alloy steel (HSLAs) was processed by quenching and austenite reverted transformation during annealing (ART-annealing), which results in an ultrafine grained duplex microstructure characterized by scanning electron microscopy equipped with electron back scattered diffraction, transmission electron microscopy and x-rays diffraction (SEM/EBSD, TEM and XRD). Microstructural observation revealed that the full hard martensitic microstucture gradually transformed into ultrafine grained duplex structure with austenite volume fraction up to 30% at specific annealing conditions. Mechanical properties of this processed steel measured by uniaxial tensile testing demonstrated that an excellent combination of strength (Rm~1GPa) and total elongation (A5~40%) at 30% metastable austenite condition in studied C-Mn-HSLAs. This substantially improved strength and ductility were attributed to the strain induced phase transformation of retained austenite dispersed throughout the ultrafine grained microstructure. At last it is proposed that ART-annealing is a promising way to produce high strength and high ductility steel products.

Abstract: In this study Quenching and Partitioning (Q&P) as proposed by Speer was applied to improve the ductility of C-Mn high strength Low Alloy steel (HSLAs). Microstructural observations revealed a multiphase microstructure including first martensite, fresh martensite and retained austenite in the Q&P processed steel. During tensile process, the austenite volume fraction gradually decreased with strain increasing, suggesting the phase transformation induced plasticity for the Q&P processed steel. Ultrahigh strength about 1300-1800MPa and tensile elongation about 20% were obtained after Q&P processing at specific conditions, which is significant higher than that of ~10% of conventional martensitic steel. The the product of tensile strength to total elongation increased from 25 to 35GPa% with increasing carbon content in studied steel. This improved mechanical properties were related to the ductility contribution from TRIP effects of the retained austenite and strength contribution from the hard martensitic matrix. At last it was turned out that the Q&P process is a promising way to produce ultrahigh strength steel with relative high ductility under tailored heat treatment conditions for different micro-alloyed carbon steel.