Papers by Author: Hui Ping Ren

Abstract: Hot compression experiments were carried out on rare earth (RE) added and RE-free Nb-containing steels by using a Gleeble simulator. Stress-strain curves obtained at various temperatures were analyzed to investigate the dynamic recovery and dynamic recrystallization softening behaviours. Morphology, size and number of precipitates in the both steels were examined by means of transmission electron microscopy (TEM). The results showed that, for the experimental Nb-containing steel, the grain size was fined by the RE addtion. In general, dynamic recrystallization cant occur in two steel under 40% deformation rates, and the deformation resistance of RE-containing steel is higher than that of RE-free steel in both the the austenite and ferrite temperatures range.While under the higher deformation rate, the dynamic recovery starting strains of the RE addition steel are higher than that of RE-free steel.It is also shown that the number of precipitate in the RE-containing steel more than that in the RE-free steel, which is due to the RE increasing nucleation rate and promoting Nb carbonitrides precipitation growth in the austenite region. Furthermore, the carbon activity may change by the RE addition, and thereby promote the precipitation strengthening of Nb-microalloyed steel.

Abstract: Rare earth (RE) has purification, denatured mixture and micro alloy function in steel. High Nb containing in steel can expend austenite non-recrystallization region, improve rolling temperature and lower equipment load. Effect of RE on dynamic recrystallization of low carbon high niobium steel is investigated by using thermal simulation experiment in this paper. Experimental results show that dynamic recrystallization does not occur in two test steel below 1000°C. RE elements can increase dynamic recovery beginning strains, refine and disperse M/A organization and change precipitates into small triangle shape.

Abstract: The stacking fault substructure was observed in the quenched martensite of 35CrMo, 2Cr13 and W6Mo5Cr4V2 steels by JEM-2100 transmission electron microscope. It is significant theoretically to discovery the stacking fault substructure and then to study its formation mechanism. The results show that the stacking faults in the martensite of steels are superfine with a few nanometers spacings, which are often concomitant with the high-density dislocations. It is considered that the stacking fault results from the crystal lattice misarrangement during the crystal lattice reconstruction from austenite to martensite in steels. The shear mechanism cannot explain the formation of the stacking fault.

Abstract: The microstructure and the formation mechanism of martensite in W6Mo5Cr4V2 steel was studied by metallographic microscope and JEM-2100 transmission electron microscope after the samples were austenized between the temperatures of Ac₁~Ac_cm and then quenched. The results show that When heating W6Mo5Cr4V2 steel samples between the temperatures of Ac₁~Ac_cm and then quenching, the cryptocrystalline martensite will be obtained. The cryptocrystalline martensite is plate martensite actually. It is considered that the formation cause of the cryptocrystal martensite is extremely inhomogeneous chemical composition in the austenite grains and the difference of martensite starting point (Ms point) of every small area in austenite grains. Besides the high-density dislocation and the fine twin crystal, the substructure of the cryptocrystalline martensite includes the superfine stacking fault. The stacking fault is caused by the stacking misarrangement during the crystal lattice reconstruction of martensite phase transformation. The midrib exists in the cryptocrystal martensite of W6Mo5Cr4V2 steel, which is composed of the fine twin crystal plates. The shear mechanism can not account for the formation of the martensite midrib.