Research On Microstructure And Properties Of Medium Manganese Steel With High Strength And High Ductility

With the development of economy and progress in exploration of marine resource and energy storage,and the demand for low cost steels with high strength and high ductility is increasing.In the present,lots of expensive Ni and Mo are added in steels in order to increasing the strength and ductility of steels,and the applications are limited because of the high cost.In this paper,two steels with medium manganese low nickel and low manganese high nickel were studied under the rolling and heat treatment processing,and the main research contents and results are showed as following:(1)The critical temperatures and continuous cooling transformation behaviors for steels were researched under simulation analysis,and CCT curves were established.The results showed critical temperatures Ac1 and Ac3 are 630℃,740℃ and 650℃,800℃ for medium manganese and low manganese steels,respectively;martensite was obtained at the cooling rate of 0.2～30 ℃/s for medium manganese steel;Granular bainite was obtained at the cooling rate of 0.2～2 ℃/s,and granular bainite plus lath bainite was obtained at the cooling rate of 2～10℃/s,and matensite was obtained at the cooling rate of 10～30℃/s for low manganese steel.(2)The effect of deformation and heat treatment processing on microstructure was researched through simulation analysis.The results showed that the size of martensite lath decreased and hardness value increased with the increasing of strain for medium manganese steel,and the amount of granular bainite increased and the amount of lath bainite decreased and hardness decreased with the increasing strain for low manganese steel;Hardness value increased with the increasing of cooling rate for both steels;Refined quenching microstructure was obtained at the quenching temperature of 725 ℃～800 ℃ and 800 ℃～850℃ for high and low manganese steels,respectively;The hardness value was higher at tempering temperature of 600℃,and the hardness value increased with the increasing of tempering time for both steels.(3)The effect of quenching temperature and tempering time on microstructure and properties was researched through quenching and tempering experiments.The results showed that yield strength,tensile strength and impact energy decreased with the increasing quenching temperature for medium manganese steel;Yield strength and tensile strength increased,then decreased,and the impact energy was higher than 200J at-40 ℃ with the increasing of quenching temperature for low manganese steel;Good quenching temperature were at 750～800℃ and 800～850℃ for medium manganese and low manganese steels,respectively;Yield strength,tensile strength and impact energy at-40 ℃ were 878MPa,820MPa and 196J for medium manganese steel,and better performance was obtained for medium manganese steel compared with low manganese steel;The strength decreased and the impact energy increased with the increasing of tempering time.(4)The effect of online quenching and offline tempering processing on microstructure and properties of experiment steels was researched through direct tempering experiment.The results showed that yield strength,tensile strength and impact power at-40 ℃ was 836MPa,897MPa,252J and 740MPa,782MPa,219J for medium manganese steel and low manganese steel,respectively;Better performance was obtained for medium manganese steel compared with low manganese steel.