Cooling Rate Dependence of Boron Distribution in Low Carbon Steel

Abstract

The behavior of boron (B) segregation to austenite grain boundaries in low carbon steel was studied using particle tracking autoradiography (PTA) and secondary ion mass spectroscopy (SIMS). An effective time method was used to compare the cooling rate (CR) dependence of this segregation during continuous cooling and its time dependence during isothermal holding. Comparison of these segregation behaviors has confirmed that the CR dependence of B segregation agrees well with its time dependence and is mainly a result of the phenomenon of nonequilibrium segregation. Based on the CR dependence of B segregation, the continuous cooling transformation behavior of B-bearing steel as compared with B-free steel was also investigated using dilatometry and microstructural observations. The addition of a small amount of B to low carbon steel retarded significantly the austenite-to-ferrite transformation and finally expanded the range of cooling programs that result in the formation of bainitic microstructures. Analysis of the B distribution has confirmed that this retardation effect of B on ferrite transformation is attributed to the CR dependence of B segregation to austenite grain boundaries during cooling after austenitization.

Effect of phosphorous and boron addition on microstructural evolution and Charpy impact properties of high-phosphorous-containing plain carbon steels

“…This is because the non-equilibrium segregation on grain boundaries, which works for preventing the formation and movement of grains, decreases with increasing grain size [14,25,31]. Thus, the 650 1C-treated steels, whose grains are larger than those of the 550 1C-treated steels (Table 2), do not clearly show the effect of B addition on grain refinement.…”

“…The slight increment in strength by the P addition might be associated with the solid solution hardening [39]. The large increment in strength by the B addition is related with the grain refinement and the increased amount of bainite and degenerated pearlite [13][14][15]. In the 650 1C-treated steels, the strength is slightly increased by the solid solution hardening effect due to the P addition.…”

“…B is known to improve the grain boundary cohesion, as it is segregated on grain boundaries [14,15,25,31,40]. The 2PB-5 steel has finer and more homogeneous microstructures than the 2P-5 steel ( Fig.…”

“…B is segregated on grain boundaries in a form of non-equilibrium segregation. Many researchers [14,30,31,45,46] investigated the segregation of B at various quenching temperatures, holding temperatures, and cooling rates, and reported that the critical times of B were quite short in the range of 10 $ 100 s. According to Song et al [12], the critical time of B was calculated to be 364 s and 17 s at the isothermal temperatures of 650 1C and 550 1C, respectively. Thus, B is sufficiently segregated on grain boundaries during the phase transformation from austenite, and beneficially influences the formation of low-temperature transformation phases and the grain refinement effect.…”

“…B is known to enhance the grain boundary cohesion energy as it is readily distributed on grain boundaries [8][9][10], which can decrease the intergranular segregation of P and consequently the ductilebrittle transition temperature [11][12][13]. Furthermore, the addition of B stabilizes austenites, and helps to form fine grains, thereby simultaneously improving the strength and toughness [13][14][15]. Thus, it is expected that the addition of B can control the reduction in toughness due to grain boundary segregation of P.…”