MICROSTRUCTURE AND MECHANICAL-PROPERTIES OF BAINITE CONTAINING MARTENSITE AND RETAINED AUSTENITE IN LOW-CARBON HSLA STEELS

The microstructure of a new low carbon HSLA steel with a high tensile strength and a high toughness in the as-hot forged condition for automotive parts was observed in detail in an 0.12% C-2% Mn-1% Cr-0.25% Mo-0.12% Nb-0.021% Ti-0.0018% B steel. Lath-like microstructure observed at the cooling rate ranging from 250 to 6-degrees-C/s mainly was revealed to consist of bainitic-ferrite with retained austenite and auto-tempered martensite. Granular microstructure observed at the cooling rate ranging from 0.17 to 0.04-degrees-C/s was characterized to be martensite-austenite constituent islands in featureless matrix. This microstructure is considered to be formed the following process. The neighboring laths of bainitic ferrite coalesced and the length of untransformed austenite shortened and consequently its shape became to resemble islands during slow cooling. Toughness of granular microstructure was lowered by twinned martensite in islands and no boundaries in packets compared with lath-like microstructure. Toughness of the low carbon HSLA steel with as-transformed martensite and bainite microstructure is equivalent or superior to that of conventional medium carbon steels.