The Influence of Hot Rolling and Heat Treatments on the Distribution of Boron in Steel

Abstract

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 boron autoradiography, which can distinguish boron-rich precipitates (BN) from atomic boron segregated to grain boundaries.
A much faster velocity of moving grain boundaries due to hot rolling than that of the migration of boron atoms in austenite does not allow boron atoms to follow the moving boundaries to segregate and thus only widely-spread distribution of boron along the deformed boundaries results. Since the velocity of the boundary migration due to recrystallization after hot rolling is not so high as that of the boundary deformation, the boundaries sweeping the matrix may trap the boron atoms which have spread widely along the boundaries before the boundaries sweep. Consequently boron atoms segregate intensively along the austenite grain boundaries after the recrystallization.
In specimens quenched from 1300°, boron atoms are observed to segregate to austenite grain boundaries but direct observations of thin foils prove no boron-precipitates on the boundaries. Reheating above Ac₃ after quenching from 1300°C, boron-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 those in high nitrogen steel. At this stage the segregation of boron to the boundaries of austenite grains formed by reaustenitizing is not observed in high nitrogen steel while it is done in low nitrogen steel.
Reheating, to 1000°C, the high nitrogen 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 boron-rich precipitates (BN) and the segregation of boron atoms to the boundaries of austenite grains. Taking the results of previous paper into account, it is considered that heating at 1000°C results in the dissociation of boron precipitates (BN) on the prior austenite grain boundarie₃ by the reaction of Al+BN=AIN+B and thus boron segregates austenite grain boundaries.