DESIGN OF DUAL PHASE IRON/MANGANESE/CARBON STEEL FOR LOW TEMPERATURE APPLICATION.

摘要

An investigation has been made to improve the impact properties of a dual phase Fe/1.5Mn/.06C steel for potential low temperature application. The research involved establishing the microstructure-property relationships, especially with regard to the morphology of the constituents. Dual phase processing was done in two ways, viz., controlled rolling and intercritical annealing of the as-hot-rolled structure.; Controlled rolling of a Fe/1.5Mn/.06C alloy produced a microstructure which consists mainly of ferrite and upper bainite. The microstructure and mechanical properties of this as-hot-rolled ferritic-bainitic structure were strongly influenced by the processing variables. There was a simultaneous improvement in tensile and impact properties with a decrease in the finish rolling temperature and an increase in the amount of deformation. This was due to refinement of the ferrite grain size.; The as-hot-rolled ferritic-bainitic structure has superior impact properties to the intercritically annealed ferritic-martensitic structure when compared at the same tensile strength level. The superiority of the former is attributed to its favorable morphology in that the relatively coarse second phase is uniformly dispersed within fine ferrite grains. This research also showed that the initial structure of the alloy before intercritical annealing had a strong influence on the impact properties of the ferritic-martensitic structure. The dual phase structure developed from the martensitic-bainitic structure had a higher transition temperature than that developed from the ferritic-bainitic structure, while they showed similar tensile properties. This behavior was due to the difference in the "effective grain size", which was the prior austenite grain size in the former, and the ferrite grain size in the latter. Changing the volume fraction of martensite does not affect the transition temperature of this dual phase steel.; The mechanical properties attained in the as-hot-rolled ferritic-bainitic steel were shown to be quite attractive for low temperature line pipe application. The properties already exceed those specified for Arctic line pipe in the as-hot-rolled state. Moreover, pipe forming causes a large increase in yield strength with only a small increase in transition temperature.