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

Abstract

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 y1450 MPa in the quenched and tempered condition with an average austenite grain size of y65 mm. 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 mm compared to 35-45 mm 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.

Development of low alloy ultrahigh strength steel

“…18 The increase in strength and toughness of these alloys compared to those of the base alloy may be a result of the grain refinement. 17 It is also reported in the authors' earlier work 19 that the as cast quenched and tempered specimens mostly consist of tempered lath martensites.…”

“…Average lath width in the specimen of as cooled plate estimated from TEM image (Fig. 9) is about 200-300 nm as against 500-600 nm 17,19 for the same alloy in the as cast condition.…”

Ultrahigh Strength Steel: Development of Mechanical Properties Through Controlled Cooling

“…In the previous study, the effects addition of titanium and niobium, and increase of chromium and vanadium contents on the mechanical and microstructural properties were investigated (Maity et al, 2008a(Maity et al, , 2008b. Most of these alloys in as cast tempered condition displayed minimum yield strengths of 1450 MPa with elongation of about 9-12% and impact toughness in many cases was in excess of 300 kJ.m -2 .…”

Development of Ultrahigh Strength Low Alloy Steel through Electroslag Refining Process

“…It is reported in our earlier work that the inclusions in the electrodes are mainly of oxide and sulphide type, which were substantially removed after the ESR process. 13,15) The measured dimension of the oxide type inclusions in the electrode is OG K5 (9.5) d(2.7-3.5 mm) and the sulphide is SS K2 (1.2) L(9-14 mm) of DIN 50602.…”

“…13) Therefore, in some alloys about 0.07 % titanium is added with the basic composition and improvement in mechanical properties was reported. 14,15) In this study for further improvement of mechanical properties especially to increase the toughness values, the basic steel is alloyed with 1-3 % of nickel. In some alloys about 0.07 % titanium was added.…”