Papers by Author: Han Dong

Abstract: nfluences of various heating processes on the amount of retained austenite after an intercritical annealing were investigated experimentally in a Mn-alloyed TRIP steel. The heating with a higher rate or an interruption at lower temperature can lead to the more rapid formation of austenite during intercritical annealing and hence more retained austenite in final microstructure. Such an enhanced austenitization kinetics is attributed to the rapid dissolution of fine carbides having previously precipitated out at low temperature during the heating, which is also confirmed by the numerical simulations.

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: Grain growth kinetics in an AISI 347 stainless steel with Nb content up to 0.7%wt was studied during the isothermal holding in the temperature range of 1100-1270°C for various periods. Abnormal grain growth was observed even in the presence of a large amount of precipitates. The kinetics of normal grain growth was tracked by metallographic measurements and fitted by the classical modeling, which led to two important parameters of activation energy Q and growth exponent n derived. Both of them are larger than the usual values for grain growth in the Nb-microalloyed steels due to the much larger content of Nb in the present steel.

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: Hot-stamping molding for ultra-high-strength steel have some similarities with traditional cold-stamping molding in the aspects of molding process and die design. But due to the effect of temperature variation of blank, hot-stamping have some differences in ultra-high-strength products design, material selection and forming process design. Some special forming defects, such as local thinning, cracking and wrinkling, could appear in hot-stamping process due to these differences. In order to obtain uniform phase structure and get high-quality products, it is very important to be able to predict and control the blank temperature and the consistence of blank cooling rate. The thermo-mechanical characteristics of hot-stamping are studied with the material of ADVANCE1500 (22SiMnTiB). Based on the results of simulations and experiments, conclusion are drawn that the complexity of the product and the blank which contacts with die asynchronously causes the uneven distribution of the blank temperature. This is the key factor that leads to the poor mobility of the blank material and local thinning, cracking, wrinkling and other defects in forming process. Proper clearance between punch and die can reduce the probability of defects which could contribute to the improvement of hot-stamping process.

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.