The Influence of Hot Rolling and Heat Treatments on the Distribution of Boron in Steel
1983 Volume 23 Issue 1 Pages 31-37
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1983 Volume 23 Issue 1 Pages 31-37
The distribution of boron affected by hot rolling and heat treatments has been studied in an 80kg/mm2 grade high tensile strength steel using the technique of B autoradiography which can distinguish B-rich precipitates (BN) from atomic B segregated to grain boundaries.
A much faster deformation velocity due to hot rolling than that of the migration of B atoms in austenite does not allow B atoms to cover all over the deformed boundaries immediately after final rolling and thus diffuse distribution of B along the deformed boundaries results. Since the velocity of the boundary migration due to recrystallization after hot rolling is not so high as the moving velocity of the deformed boundary, the boundaries sweeping the matrix may trap B atoms which have spread diffusively along the boundaries before the boundaries sweep. Consequently B atoms segregate intensively along the austenite grain boundaries after the recrystallization.
In specimens quenched from 1300°C, B atoms are observed to segregate to austenite grain boundaries but direct observations of thin foils prove no B -precipitates on the boundaries. Reheating above Ac3 after quenching from 1300°C, B-rich precipitates (BN) form on the prior austenite grain boundaries. The intensity of fission tracks of those precipitates in low nitrogen (30ppm) steel is much weaker than that in high N steel. At this stage the segregation of B to the boundaries of austenite grains formed by reaustenitization is not observed in high N steel while it occurs in low N steel.
Reheating at 1000°C the high N steel containing an adequate amount of Al after quenching from 1300°C results in the reduction of the intensity and density of fission tracks of B-rich precipitates (BN) and the segregation of B atoms to the austenite grains. Taking the results of previous paper into account, it is considered that heating at 1000°C results in the dissociation of B precipitates (BN) on the prior austenite grain boundaries by the reaction of [Al]+BN=AlN+[B] and the increase of B in solid solution.
