1988 Volume 74 Issue 1 Pages 167-174
In the production of 80 kgf/mm2 grade steel plate with high toughtness by direct-quenching and tempering (DQ-T) process, optimum amounts of B in steel plate is indispensable. Because, if the plate does not contain that, the tensile strength of plate is not necessarily higher than that of reheat-quenched and tempered plate. And, optimum amounts of B are changed by parameters of DQ-T process such as slab reheat, and finish-rolling temperatures and plate chemistry.
In this paper, an investigation has been made on the effect of B behavior and change in γ-grains during the period between finish-rolling and quenching on the plate tensile strength. In particular, existing state and distribution of B were observed by using modified autoradiography (modified FTE technique) which can clearly show B-distribution at the γ-grain boundary and γ-grain matrix.
Boron-distribution and γ-grain structure dynamically change depending on the time from finish of rolling to quenching, and this results in change in the hardenability of plate. Enough segregation of solute B at recrystallized γ-grain boundaries increases the hardenability of plate. Whereas the formation of fine precipitate of B conpounds within recrystallized γ-grain and at grain boundaries lowers the hardenability of plate.
The model showing relationships between γ-grain structure, B-distribution and hardenability of plates are schematically represented on the basis of experimental results.
