Development of Ultra High Strength Steel by Electroslag Refining and Thermomechanical Treatment

Abstract

Development of low alloy ultra high strength steel with yield strength in excess of 1600 MPa and elongation of 10 % with good fracture toughness is the primary objective of this investigation. The alloys were prepared through electroslag refining (ESR) followed by thermomechanical treatment (TMT). The base alloy was prepared with 0.23 % C, 1.09 % Mn, 5.7 % Cr, 1.18 % Mo, 0.40 % V, 0.19 % Si, 0.011 % S, 0.015 % P, 0.02 % Al and 0.013 % N. Other alloy was developed by inoculation of about 0.06 % titanium in which other elements are kept intentionally similar with the base alloy. The ESR ingots further underwent for TMT to convert it into plates. The specimens of TMT plates of the base alloy displayed tensile strength, yield strength, % elongation and fracture toughness of 1792, 1580 MPa, 7.6 % and 70 MPa root m, respectively while titanium inoculated alloy showed tensile strength of 1885 MPa, yield strength of 1700 MPa, elongation of 8.4 % and fracture toughness of 84 MPa root m. It was observed that the mechanical properties of the titanium inoculated alloy were substantially improved compared to base alloy. Optical, SEM, TEM and EDAX investigations confirmed that the microstructures of the base alloy and titanium inoculated alloy consisted of predominantly bcc lath martensites. TEM and corresponding EDAX studies revealed that the inter lath martensite spacing in the titanium inoculated alloy was finer than the base alloy and also confirmed a homogeneous precipitate of 20-30 nm titanium carbonitrides in titanium inoculated alloy. The improvement of mechanical properties in titanium alloy may be due to refinement of prior austenite grains, inter lath martensite spacing as well as precipitation hardening.