GB1004246A - Improvements in or relating to the heat treatment of steel - Google Patents

Abstract

Steel which has been heated to transform it to the austenitic condition, and which contains fine particles dispersed throughout the austenite to minimize the grain size of the latter, is quenched to a temperature between the Ac1 and Ms temperatures, is mechanically worked in that range for such a period that substantially no transformation occurs and is then quenched to form martensite. The latter may then be tempered, and the steel subsequently cold worked. The dispersed particles may be preformed, such as aluminium oxide, which may be added either by blowing into the metal stream of metal between tundish and mould as a suspension in inert gas, or in the case of finer particles, may be dropped through a silica tube into the centre of the stream leaving the tundish. A preferred embodiment is to form the particles directly in the steel by the inclusion of alloying elements capable of forming a stable nitride or carbide with the carbon or nitrogen present. Particularly suitable carbide formers are niobium, titanium and hafnium, but chromium, vanadium and molybdenum may also be used. For the formation of nitrides aluminium and titanium may be used. The process may be further improved by adding to the steel a small quantity of an element, e.g. up to 5% of uranium, which reduces the stacking fault energy of the metastable austenite. Where quenching to martensite would result in unacceptable distortion, the worked austenite may be isothermally transformed to a bainitic or pearlitic structure which may be subsequently cold worked.