Investigation Of Multi-level Microstructure On Mechanical Property Of High Strength Steels

Refining multi-level microstructure is the important development direction, which improves the strength, toughness and fatigue property of metallic material.Many researchs on discipline between each-level microstructure and strength, toughness of martensite steels are studied. For martensite steels,the effective grain size wuich may dominate strength and toughness has not fully understood.Moreover,study on the coupling relationship between multi-level microstructure and toughness has not reported.In this paper, the multi-level martensite microstructure is obtained by using the heatreatment of different quenching temperature, different cooling rate in 20CrNi2 Mo steels.The each-level microstructure size is quantitative measured by using OM(optical microscope),SEM(scanning electron microscope),EBSD(electron backscattered scattering detection) and TEM(Transmission electron microscope). Under those conditions, the relationship between multi-level microstructure and mechanical property of martensite is studied and the effective grain size on lath martensite strength, toughness is discussed.Moreover, relative relation model between multi-level microstructure and toughness is established.The microstructure in quenched and tempered 20CrNi2 Mo steels was lath martensite.Prior austensite grain size,martensite packet size and martensite block width increased with quenching temperature.With the quenching temperature increasing,the Rm,Rp0.2, Z and hardness decreased.With the cooling rate increasing,the prior austensite grain size was not affected,others microstructure was refined.The Rm,Rp0.2,Z,HRC and J1 C of A process increased with cooling rate contrary to A and Aku.With the cooling rate increasing, for B process, Rm and hardness first and decreased afterwards, however elongation first drop after increased.There was a Hall-Petch relationship between the Rm, Rp0.2 and-1/2 of pachet, block and lath of 20CrNi2 Mo steels. The block width can be regarded as the microstructural unit controlling the strength in lath martensite steels.A Hall-Petch relationship existed between-1/2 of martensite lath and Aku, J1 C under different quenching temperature. The lath width can be regarded as themicrostructural unit controlling the toughness in lath martensite steels. A Hall-Petch relationship existed between-1/2 of martensite lath and J1 C under different cooling rate. The lath width can be regarded as the microstructural unit controlling the J1 C in lath martensite steels.The relation model of multi-level microstructure and toughness in martensite steels was preliminary established, namely, Di=di/nδ. By the calculation,the prior austensite grain size was1/3 of n ?, and dr/n ?(A4) ﹥ 1.The pocket size, block width and lath width less than n ?.Moreover,the lath width was much less than 2?. With the quenching temperature increasing, J1 increased, J2 and J3 decreased. With the cooling increasing, J1 and J2 increased, J3 decreased.