Influence of Nb(C, N) Precipitation in Austenite on the Mechanical Properties of High Strength Steel

Effect of Nb content on the mechanical properties of high strength steel has been studied for hot strips rolled in the conventional (regular) or controlled process and also for the normalized one.On the conventional rolling, Nb(C, N) precipitated in ferrite, but not in austenite. The yield strength of the steels increased with Nb content.On the controlled rolling where Nb(C, N) was intended to precipitate in austenite on the basis of the kinetics, many particles of Nb(C, N) precipitated during rolling in austenite containing more than 0.10 wt%Nb. The yield strength did not change with Nb contents higher than 0.05 wt%.Because of the low precipitation rate, Nb(C, N) did not precipitate in austenite of steels containing less than 0.05 wt%Nb by any rolling condition.The toughness of the steels subjected to controlled rolling and normalization was improved with increasing Nb content, without any alternation of the yield strength. This has been attributed to the reduction of dissolved nitrogen, which has an extremely unfavorable effect on the toughness, as a result of the formation of noncoherent Nb(C, N) in austenite.Based on the above results, a new type of high strength steel with superior tougness has been developed by somewhat higher Nb addition and by the specially controlled rolling in which Nb(C, N) was intended to precipitate in austenite.