Effect of direct quenching on microstructure and mechanical properties of medium-carbon Nb-bearing steel

The influence of direct quenching (DQ) on microstructure and mechanical properties of 0.19C-1.7Si-1.0 Mn-0.05Nb steel was studied. The microstructure and mechanical properties of reheat quenched and tempered (RQ&T) steel plate were compared with those of direct quenched and tempered (DQ&T) steel plates which were hot rolled at different finish rolling temperatures (1173 K and 1123 K), i.e., recrystallization-controlled-rolled direct-quenched (RCR&DQ) and controlled-rolled direct-quenched (CR&DQ), respectively. The strengths generally increased in the following order: RQ&T<RCR&T<RCR&DQ&DQ&T<CR&T<CR&DQ&DQ&T. Strength differences between the CR&DQ&DQ&T and RQ&T conditions as high as 14% were observed at the tempered temperature of 573 K. The optical microscopy of the CR&DQ&DQ&T steel showed deformed grains elongated along the rolling direction, while complete equiaxed grains were visible in RQ&T and RCR&DQ&DQ&T steels. Transmission electron microscopy (TEM) and electron backscattering diffraction (EBSD) of the DQ steels showed smaller block width and higher density of dislocations. Inheritance of austenite deformation substructure by the martensite and differences in martensite block width were ruled out as major causes for the strength differences between DQ and RQ steels.