Hakcheol Lee

Effects of Processing Parameters on Microstructure and Properties of Ultra High Strength Linepipe Steel

To examine the effect of processing parameters on microstructural evolution and to obtain the excellent combination of strength and toughness, simulation of thermo-mechanical processing was conducted using the Gleeble machine. Trial production was then conducted under the conditions obtained by Gleeble tests. Based on the results of microstructure analysis and mechanical property evaluation, the relationship between microstructural features and mechanical properties was elucidated. The result shows that the volume fraction of constituted phases can be controlled through adjusting the cooling rate and finish cooling temperature in order to get different strength levels. As cooling rate increases, the volume fraction of upper bainite increases, which leads to the increase of strength. The upper shelf energy (USE) increases with increasing volume fraction of acicular ferrite in bainite base because of the small effective acicular ferrite grain size. Ductile-brittle transition temperature (DBTT) decreases with increasing acicular ferrite volume fraction. High reduction in the rough stage has great influence on grain refinement.

Improvement of impact toughness of 5Mn-1Al-0.5Ti steel by intercritical annealing

The present study is aimed at improving the impact toughness of 5Mn-1Al-0.5Ti steel by incorporating ferrite-martensite dual phase microstructure by intercritical annealing. Although (8-12)Mn martensitic steels usually show very low impact toughness due to the occurrence of intergranular fracture, the martensitic structure of the present 5Mn-1Al-0.5Ti steel fails by transgranular cleavage fracture due to higher grain boundary strength than matrix strength incurred by reduced Mn content and segregation of Ti along grain boundaries. Nevertheless, it still shows very poor impact toughness at room temperature due to its coarse grain size. The application of intercritical annealing, i.e., formation of dual phase microstructure, is shown to significantly decrease ductile-to-brittle transition temperature (DBTT), with only a small degradation of tensile properties; however, microstructural examinations show that most of ferrite/martensite interfaces have a character of low angle boundaries and therefore such decrease in DBTT is not necessarily due to the formation of ferrite-martensite dual phase structure, but rather to the refinement of grain size by low temperature annealing.

Improvement of low temperature toughness of ferritic Mn steels by alloy modification

A study has been made on the effects of Mn content and addition of Al on the microstructure and mechanical properties of (4-6)Mn steels, with the aim of developing ferritic Mn steels having good combination of tensile properties and impact toughness. It has been shown that the reduction of Mn content to 6 wt% and lower from 8-12 wt% of conventional steels can effectively prevent the occurrence of intergranular fracture, which has been a major problem of (8-12)Mn steels with poor toughness at cryogenic temperature. While increasing Mn content in the binary (4-6)Mn steels results in an increase in strength and a decrease in impact toughness, the addition of 1Al to (4-6)Mn steels results in improvements in both strength and impact toughness as compared to those of the binary (4-6)Mn steels and the best impact toughness has been obtained in the as-rolled 1Al added 4Mn steel, whose microstructure consists of fine acicular ferrite grains.