Microstructure Evolution And Properties Control Of 30CrNi3MoV Low Alloy Ultra High Strength Steel

How to obtain high-strength materials with high toughness has always been the goal that material researchers are constantly exploring and pursuing.In recent years,China's annual output of steel has exceeded one billion tons,but the strength of structural steel is generally 400 MPa~800 MPa.The strength of steel used for large construction machinery and armor protection is high.However,its plastic toughness performance is not satisfactory.In today's advocacy of green industry,there is an urgent need to reduce environmental pollution,save energy and save mineral resources.This requires material researchers to develop high-strength and ultra-high-strength steel to further reduce the weight of metal devices and achieve the goal of energy saving and emission reduction.In order to obtain high strength,good plasticity and toughness,and low manufacturing cost,the quenching-partitioning(Q-P)heat treatment proposed by Speer et al.was performed on the 30CrNi3 MoV steel.The microstructure of 30CrNi3 MoV steel was observed by optical metallography(OM),scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The mechanism for its high strength,plasticity and toughness were revealed.The tensile test was used to study the effect of small deformation on the phase transformation of the austenite in the non-recrystallization zone and its effect on the mechanical properties of the 30CrNi3 MoV steel at room temperature.The phase transition process and mechanical properties of the 30CrNi3 MoV steel prepared by different Q-P process were analyzed by the control variable method.The main research contents of this paper are as follows:(1)The phase transformation behavior of 30CrNi3 MoV steel during continuous cooling was analyzed.The microstructure was analyzed by high-precision linear dilatometer combined with different kinds of microscopic observation equipment.The phase transformation law of 30CrNi3 MoV steel continuously cooled to room temperature at 0.1~30 ?/s was characterized.The continuous cooling transformation(CCT)curve of the 30CrNi3 MoV steel was measured and plotted,and the microhardness of 30CrNi3 MoV steel at different cooling rates was obtained.The effect of small deformation of austenite in non-recrystallization zone on the bainite and martensite transformation of 30CrNi3 MoV steel was studied by using high-precision linear dilatometer capable of applying load.The results show that the critical cooling rate of the martensitic transformation of 30CrNi3 MoV steel is between 0.4 and 0.5 ?/s.The austenite stores high deformation energy after small deformation at 850 ?,providing additional mechanical driving force for subsequent phase transformation.At the same time,the minimum chemical driving force required to complete the bainite or martensite transformation is reduced.Therefore,the Bainite transformation start temperature Bs and the martensite transformation start temperature Ms of the 30CrNi3 MoV steel sample with small deforming in the austenite non-recrystallization zone are improved.Under slow cooling conditions,the small deformation in the non-recrystallized region of the steel leads to the formation of finely dispersed granular bainite and the lower bainite with smaller size.Under rapid cooling conditions,the small deformation in the non-recrystallization zone increases the proportion of lath martensite in the martensite structure,relative to the acicular martensite.Due to the influence of precipitation strengthening caused by the martensite self-tempering process,the microhardness of the phase transformation product of 30CrNi3 MoV steel does not increase uniformly with the increase of cooling rate.(2)The application of quenching-partitioning process on 30CrNi3 MoV steel was studied systematically.The results show that the microstructure of the 30CrNi3 MoV steel after Q-P treatment is martensite and retained austenite,in which martensite is lath-like and needle-like,and the retained austenite is distributed in fine strips in two lath martensite.The austempering temperature not only affects the size of the martensite matrix slab,but also affects the morphology of the retained austenite in the steel.The lower isothermal temperature can refine the size of martensite and diversify the size of the retained austenite.The speed of the partitioning process is accelerated as the partitioning temperature increases at the same partitioning time.The tensile strength of the 30CrNi3 MoV steel increases,and the elongation and strength-ductility balance decreases,with the increase of the partitioning temperature.The tensile strength decreases first and then increases with the increasing partitioning time,and the elongation exhibits the opposite law.The microstructure and mechanical properties of the quenching-partitioning-tempering treated 30CrNi3 MoV steel are different from that of quenching-tempering treated steel.The tensile strength of the steel after quenching-partitioning-tempering treatment is lower than that of quenching-partitioning and quenching-tempering specimens,but the elongation shows the contrary law.