Roles of nanoscale precipitates and metastable austenite in determining strength and toughness of high-strength Nb-bearing steel

The effects of tempering temperature on the microstructure and mechanical properties of high-strength structural steel containing niobium were investigated to examine the roles of nanoscale precipitates and metastable austenite in determining the yield strength and toughness. After hot-rolling and quenching, three experimental steels were tempered at 590, 630, and 670 °C. During tempering, nanoscale Nb(C, N) precipitates were formed with the recovery of quenched martensite. The average diameters of Nb(C, N) precipitates increased from 5.4 to 8.2 nm as the tempering temperature was increased. Notably, reversed austenite with a volume fraction of 9% was formed at tempering temperatures up to 670 °C. The yield strengths of steel containing tempered martensite tempered at 590 and 630 °C were 965 and 831 MPa, and the tensile strengths were 998 and 879 MPa, respectively. However, the steel comprising reversed austenite and tempered martensite tempered at 670 °C showed continuous yielding behavior, affording yield and tensile strengths of 610 and 889 MPa, respectively. The impact energy increased from 105 to 260 J at − 60 °C with increasing tempering temperature. Reversed austenite improves low-temperature toughness by significantly increasing the crack propagation energy.