The dynamic recrystallization and static recrystallizationin a lowcarbon steel were investigated throughsingle-pass and double-pass experi ments . The results indicate that as the deformation temperature increases andthe strain rate decreases ,the shape of the stress-strain curve is changed fromdynamic recovery shape to dynamicrecrystallization shape . The austenite could not recrystallize within a fewseconds after deformation at temperaturebelow900 ℃. According to the change in microstructure during deformation,the controlled rolling of lowcarbonsteel can be divided into four stages :dynamic recrystallization,dynamic recovery ,strain-induced ferrite transfor-mation ,and rolling in two-phase region. According to the microstructure after deformation,the controlled rollingof low carbon steel can be divided into five regions : non-recrystallized austenite , partly-recrystallized austenite ,fully-recrystallized austenite ,austenite to ferrite transformation,and dual phase .
DU Lin-xiu ZHANG Zhong-ping SHE Guang-fu LIU Xiang-hua WANG Guo-dong
Utilizing Gleeble-1500 thermomechanical si mulator, the influences of hot deformation parameters oncontinuous cooling bainite transformationin Nb-microalloyed lowcarbon steel were investigated . The results indi-cate that bainite starting temperature decreases with raising cooling rate and increases withincreasing deformationtemperature . Deformation has an accelerative effect onthe bainite transformation whenthe speci mens are deformedat 950 ℃. When the deformation temperature increases ,the effect of deformation on bainite starting temperatureis weakened . The amount of bainite is influenced by strain,cooling rate ,and deformation temperature . When thespeci mens are deformed below 900 ℃, equiaxed ferrites are promoted and the bainite transformation is sup-pressed .
YI Hai-long DU Lin-xiu WANG Guo-dong LIU Xiang-hua
The microstructural evolution in a 700 MPa hot rolled high strength steel was analyzed in terms of strengthening mechanisms.The results show that the hot rolled sheet steel has yield strength of 710 MPa with good elongation and toughness.The strength of the developed 700 MPa hot rolled high strength steel is derived from the cumulative contribution of fine grain size,dislocation hardening and precipitation hardening.The fine grain strengthening and precipitation hardening are the dominant factors responsible for such high strength,and the amount of precipitation hardening is two or four times higher than that of conventional microalloyed hot rolled sheet steels reported in the past.Good toughness is due to refinement of ferrite grain size.
YI Hai-long DU Lin-xiu WANG Guo-dong LIU Xiang-hua
Using Gleeble-1500 system, the influence of holding time on bainite transformation in deformed niobium microalloyed steel during continuous cooling was analyzed, and the carbides in upper bainite were also systematically researched. The results show that the occurrence of the static recrystallization decreases the amount of bainite with an increase in the holding time and the emergence of retained austenite (RA) with the longer holding time. Two types of carbides were observed in upper bainite with regard to their precipitation sites. They either existed between the bainite ferrite laths or co-existed with RA. The formation mechanism of two kinds of carbides was analyzed by combining TEM micrographs with the model.
YI Hai-long DU Lin-xiu WANG Guo-dong LIU Xiang-hua
