For preparing large-scale nano-grained and ultrafine-grained steel sheets by warm rolling and annealing,the effects of de- forming temperature on both the flow stress and the microstructure evolution of 09MnNiD steel with lath martensitic microstructure were studied by warm-compression test and transmission electron microscopy(TEM)observation.Thereafter,the steel with the lath martensitic structure was multi-pass warm-rolled and then annealed.TEM results indicate that nano-grained and ultrafine-grained steel sheets are formed by warm rolling at 400℃and annealing at 400-600℃.In comparison with the as-warm-rolled specimen,the tensile strength at room temperature changes a little when the rolled samples are annealed below 450℃,and the tensile strength is greatly lowered as the annealing temperature increases to above 550℃.
The hot deformation characteristics of ductile iron are studied in the temperature range of 973 to 1273K and strain rate range of 0.001 to 1 s-1 by using hot compression tests.Processing maps for hot working are developed on the basis of the variations of efficiency of power dissipation with temperature and strain rate.The results reveal that the flow stress of ductile iron is sensitive to strain rate.In the processing map under strain of 0.7,a domain is centered at 1273 K and 1 s-1,and the maximum efficiency is more than 36%.According to the maps,the zone with the temperature range of 1173 to 1273 K and strain rate range of 0.1 to 1 s-1 may be considered as the optimum region for hot working.
A new plastic deformation process for gray cast iron named cylinder covered compression (CCC) was developed. By CCC process, gray cast iron (GCI) specimens, which are embedded in steel cylinders, were hot compressed up to 80% reduction in height without the cracking problem. It was clearly observed that the uniform distribution of directional graphite flakes appeared after more than 45% reduction hot compression. The strength, ductility, and microhardness of GCI after 80% reduction deformation were significantly enhanced: the tensile strength varied from 117 MPa to 249 MPa, while the total elongation varied from 0 to 5.2%, and the microhardness varied from 153 HV to 217 HV. It was shown that the tensile fracture surface presented ductility characters after more than 45% reduction hot deformation.
Warm deformation was investigated for steels containing carbon of 0.45%, 0.79%,and 1.26% respectively with martensite starting structure, using Gleeble 3500 thermomechanical simulator at the temperature ranging from 873 K to 973 K and the strain rate ranging from 0.1 s-1 to 0.001 s-1. The effect of carbon content on the deformation activation energy, Z value and the deformation resistance was analyzed. The results showed that the deformation resistance decreased with increasing carbon content. The peak stress of the steel containing carbon of 1.26% was decreased by 16.2% compared to the steel containing carbon of 0.45% under the same condition. This may be attributed to the weakening effect of solid solution strengthening which outweighs the precipitation strengthening of cementite.
LI Qun WANG Tian-sheng LI Hong-biao GAO Yu-wei LINing JING Tian-fu
In order to investigate the effect of initial microstructure on warm deformation behavior, some specimens of 45 steel were annealed and some quenched. Then the specimens were isothermally compressed on a Gleeble 3500 machine. The deformation temperature range was 550 to 700 ℃ and the strain rate range was 0.001 to 0.1 s-1. An optical microscope (OM) and a transmission electron microscope (TEM) were used to study the microstructures. The results show that the microstructure of annealed specimens is ferrite and pearlite and that of quenched specimens is martensite. The flow stress of quenched specimens is higher than that of annealed ones at 550 ℃ when strain rates are greater than 0.001 s-1. However, at 600 to 700 ℃ and strain rate of 0.001 s-1, the whole flow curves of quenched specimens are below that of annealed ones. Under the rest conditions, the flow stress of quenched specimens is higher at the beginning of compression and then the opposite is true after the strain is greater than a critical value. The microstructure examination proves that the tempering and dynamic recrystallization easily occur in the specimens with martensite during warm compression, which results in the above phenomena.
ZHAO Xin1, YANG Xiao-ling2, JING Tian-fu3 (1. The Sixth Department, Zhengzhou Institute of Aeronautics, Zhengzhou 450015, Henan, China
