Formation of microstructure of HN5MVNb steel after rolling annealed in (α + β) temperature range and fast cooled

The paper presents the effect of the intercritical heat treatment in (α + β) range at temperature 700 oC during 1 to 10 hours on microstructure evolution of fast cooled HN5MVNb steel. The aim of heat treatment was to develop very fine multiphase microstructure in which the hard obstacles of bainite-martensite islands are dispersed randomly or lamellarly within ferrite matrix. That type of microstructure has usually very good tensile and ductile properties. The increase of temperature from 699 oC to 746 oC and from 812 oC to 855 oC was determined by dilatometry. The heating rate of 1 oC/s was used. The CCT diagram of HN5MVNb steel having 52% of new austenite after 700 oC/1hour was established. The decrease of Bs temperature from 550 oC down to 300 oC/260 oC and Ms temperature from 412 oC to 260 oC/210 oC was observed when compared to the steel after full austenitizing.The microsegregation of Ni and Mn between areas of bainite-martensite (BM) islands and ferrite grains was measured on thin foils in the analytical transmission electron microscope Philips CM20 with EDX technique. In both, lamellar and grainy BM islands there were 6.9 - 7% Ni and 1.5 - 1.7%Mn while in ferrite grains only 2.8 - 3.2% Ni and 0.4 - 0.7% Mn. The occurrence of retained austenite was observed in some islands due to high Ni and mn concentration as well as twinned martensite in areas of higher carbon contents. (author)