Niobium modification of microcomposite iron/carbon/chromium/manganese steel

A microcomposite steel which is desired to be attained in this investigation, is a composite microstructure consisting of fine packets of heavily dislocated martensite laths surrounded by continuous thin films of interlath retained austenite. Heavily dislocated lath martensite gives ultrahigh strength; a softer, tough austenite phase provides high fracture toughness. This composite microstructure has been achieved in the Fe/Cr/Mn/C system containing more than a total of four percent of alloying elements in previous work.; The present work is concerned with obtaining superior combinations of mechanical properties by adding the microalloy element of niobium to the basic quaternary alloy of Fe/2.0 wt% Cr/1.2 wt% Mn/0.25 wt% C and by applying controlled rolling practice to processing of the experimental alloys to attain fine austenite grains, giving rise to high Charpy V-notch impact toughness. The basis is to have sufficient hardenability after retrenching the chromium content, so that processing will not destroy the microcomposite structure of heavily dislocated martensite laths and thin films of interlath retained austenite after the martensitic phase transformation, following rolling and quenching.; Continuous cooling transformation diagrams of the basic quaternary and the niobium bearing alloys have been determined to predict the appropriate cooling rate required for martensitic transformation by dilatometry method. Niobium effected a considerable increment of martensitic hardenability, which alleviates the otherwise stringent quenching rate arising from the reduced alloy contents and which is needed to obtain the composite microstructure.; Controlled rolling practice has been widely utilized commercially for refining ferrite grain size in ferrite/pearlite steels, by applying heavy deformation below the recrystallization temperature, in order to obtain the pancake structure of austenite. However, in this work, heavy deformation is added just above the recrystallization temperature in order to obtain fine and recrystallized austenite grains which result in fine microcomposite structures. Thus the rolling conditions (amount of deformation, temperature) were optimised in this study. As for the microalloy element, 0.02 wt% of niobium is added to utilize the interactions of microalloyed element precipitation and austenite recrystallization kinetics during hot rolling. Application of controlled rolling practice to Fe/0.25 wt% C/2.0 wt% Cr/1.2 wt% Mn/0.02 wt% Nb alloy resulted in a great enhancement of Charpy impact toughness by about 15 joules than Fe/0.25 wt% C/2.0 wt% Cr/1.2 wt% Mn alloy without addition of niobium.; Plane strain fracture toughness tests were done on both experimental alloys which were treated with optimum process condition. This condition involves finish rolling at 900{dollar}spcirc{dollar}C followed by water quenching and tempering at 250{dollar}spcirc{dollar}C. The Nb bearing alloy showed an excellent plane strain fracture toughness value reaching 155 MPa{dollar}surd{dollar}m, after tempering but even 147 MPa{dollar}surd{dollar}m directly quenched on-line. These good values are due to the composite structure.