Development of low alloy titanium and niobium microalloyed ultrahigh strength steel through electroslag refining

The objective of the investigation is to develop an ultrahigh strength steel (UHSS) with a yield strength in excess of 1500 MPa, with a minimum elongation of 9–10% for the fabrication of rocket motor casing. Attempts have been made to develop a special steel by the electroslag refining (ESR) process. The base alloy was prepared with a chemical composition of 0·28C–1·0Mn–1·0Mo–0·35V–4·2Cr and a trace amount of S and P. This alloy has displayed yield strength of ∼ 1450 MPa in the quenched and tempered condition with an average austenite grain size of ∼65 μm. The mechanical properties were improved further by microalloying with 0·07%Ti and Nb. The yield strengths of Ti and Nb microalloyed steels are 1500 and 1570 MPa respectively, in the as cast quenched and tempered condition. The optical, scanning electron microscope (SEM) and transmission electron microscope (TEM) studies confirmed that the microstructure consists of tempered lath martensite. Transmission electron microscope (TEM) carbon replica and EDS analysis showed that the Ti(C,N) and Nb(C,N) precipitates might have helped to reduce the austenite grain size in the Ti and Nb steel. It is interesting to note that due to the smaller size of Nb (C,N), the grain size obtained in the Nb steel is in the range of 22–27 μm compared to 35–45 μm in the Ti steel. The improvement of yield strength in Ti and Nb steel might be due to smaller austenite grains and tempered lath martensite microstructures.