Study on the Segregation and Precipitation Behavior of Boron in Low-Carbon Steels

Abstract

The subject of the grain boundary segregation of solute has attracted considerable attention in recent years, since it is widely recognized that such solute segregations not only control many of the mechanical, chemical and electronic properties of technologically important materials but also induce grain boundary structural transformations. Boron is among the elements which, as small additions, can change the properties of materials considerably. The behavior of segregation and precipitation of boron (B) in low-carbon high-strength 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 due to the phenomenon of non-equilibrium segregation. The precipitation behavior of Fe23(C,B)6 borocarbides at austenite grain boundary was accelerated during cooling because of a significantly increased B concentration segregated at austenite grain boundary by a non-equilibrium segregation mechanism. This borocarbide precipitation along the grain boundaries was not observed after deformation because B concentration along grain boundaries does not exceeds the optimum value by grain refinement. The phase transformation behavior of high-strength boron steel was investigated considering the segregation and precipitation behavior of B. The effects of cooling rate, austenitizing temperature and austenite deformation on the transformation behavior of B-bearing steel were compared to those B-free steel by using dilatometry, microstructural observations and analysis of B distribution. The effects of these variables on hardenability were discussed in terms of non-equilibrium segregation mechanism and precipitation behavior of B. 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. The retardation of austenite-to-ferrite transformation by B addition depends strongly on cooling rate

this is mainly due to the phenomenon of non-equilibrium grain boundary segregation of B. The hardenability effect of B-bearing steel was decreased at higher austenitizing temperature due to the precipitation of borocarbide along austenite grain boundaries. Analysis of B distribution by second ion mass spectroscopy confirmed that the grain boundary segregation of B occurred at low austenitizing temperature of 900 °C, whereas B precipitates were observed along austenite grain boundaries at high austenitizing temperature of 1200 °C. It is thought that a significant increase in B concentration at austenite grain boundaries due to grain coarsening and a non-equilibrium segregation mechanism leads to the B precipitation. In contrast, solute B segregated at austenite grain boundaries during cooling after deformation became more stable because grain refinement does not cause B concentration at grain boundaries to exceed the critical point

thus the effect of B on hardenability could be maximized under controlled cooling after hot deformation. Therefore, the austenite grain size and non-equilibrium segregation behavior of B are the most important variables that determine the magnitude of the hardenability effect of B in steels.