Effect of direct quenched and tempering temperature on the mechanical properties and microstructure of high strength steel

Abstract

The effect of direct quenched (DQ) and tempering temperature on the microstructure and mechanical properties of high strength steel were studied by means of SEM, EBSD, TEM and mechanical properties test. The results showed that in DQ state, the tensile strength could reach 1420 Mpa, the yield strength could be 1050 Mpa, and the elongation was about 9.0%, impact energy at −20 °C was 59 J. High density entangled dislocation was distributed inside the lath and at the boundary. A small amount of Nb and Ti carbide was precipitated at the dislocation and lath boundary, and a small amount (about 2.15%) of residual austenite distributed between the lath. With the tempering temperature rising from 500 °C to 720 °C, the tensile strength of the experimental steel decreased from 1220 MPa to 840 MPa, the yield strength decreased from 1190 MPa to 780 MPa, the elongation increased from 10% to 13%, and the impact energy at −20 °C increased from 84 J to 153 J. When the tempering temperature rised from 500 °C to 640 °C, the structure was mainly composed of lath martensite and a large number of dislocations were still distributed inside the lath. The size of carbides precipitated inside and on the boundary of the lath was about 20–30 nm. When tempered at 680 °C, the structure was mainly composed of martensite and a small amount of polygonal ferrite. There were still a large number of entangled dislocations inside and on the boundary of martensite. The carbide precipitate at the matrix boundary and dislocation line was obviously coarsening (70–80 nm). When tempered at 720 °C, the microstructure was mainly polygonal ferrite, the dislocation density in the matrix significantly decreased, and the carbide precipitated at the matrix boundary and dislocation line significantly coarsened (about 100 nm). With the tempering temperature rising from 500 °C to 720 °C, the proportion of small-angle grain boundary was gradually decreased from 88.64% to 70.50%.