Study On Martensite Microstructure And Properties Of Medium Carbon Steel After Warm-rolled And Annealing

In this paper, the high strength steel with the ultrafine grain and nano grain wereobtained by low temperature annealing from350C to600C after55%and72%warmrolling at450℃and550℃, by usingmedium carbon steel with lath martensite structure.After microhardness and tensile for rolling sample, the mechanical properties of theexperimental steel after rolling were determined, analyzed the microstructure and fracturemorphology for the steel after rolling and annealing by means of OM, TEM, SEM.The tensile strength of medium carbon steel with martensite structure is1584~1723MPa after warm rolling. The results of annealing experiment show thatnano-grain which size is from31nm to60nm，tensile strength is form1536MPa to1317MPa, strain is from5.3%to7.0%, were obtained by annealing between400℃to500℃; ultrafine grain which size is from182nm to485nm, tensile strength is form1098MPa to1004MPa, strain is from7.9%to9.4%, were obtained by annealing between550℃to600℃after the tested steel warm rolling at450℃in72%reduction. The tensilestrength varied little after annealing in below500℃, and it decreased significantly afterannealing at550℃or above. The recrystallization activation energy is214kJ/mol. Therecrystallization process is relatively slow under550℃and the organizational stability isgood in a certain temperature range in annealing process.With the rolling at550℃, when the reduction is55%, the tensile strength is1463~1206MPa and the strain is5.7%~7.4%after annealing in400~500℃; the tensilestrength is under1002MPa and the strain is above9.5%after annealing in500~600℃.The tensile strength is1565~1324MPa and the strain is5.0%~6.8%after in annealing400~500℃, the tensile strength is under1067MPa and the strain is above8.1%afterannealing in550~600℃, when the reduction is72%. The deformation degree has a greaterinfluence on the performance after annealing, and the tesile strength after annealingincreases as the reduction increasing. Warm rolling temperature almost has no effect onthe performance after annealing.Room temperature tensile tests show that the test steel tensile fracture in variousprocess showing significant stratification fracture between layers are a large number of small pancake-shaped dimple-like organization, the rupture situation is typical of ductilefracture. With the increasing of annealing temperature and time, the width of thedelamination crack decreases. The delamination crack disappear after600℃annealingfor10min and550℃annealingfor120min.