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.

The phase transformation behavior of high-strength boron steel was studied considering the segregation and precipitation behavior of boron (B). The effects of cooling rate, austenitizing temperature and austenite deformation on the transformation behavior of B-bearing steel as compared with B-free steel were investigated 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 retardation of austenite-to-ferrite transformation by B addition depends strongly on cooling rate (CR); this is mainly due to the phenomenon of non-equilibrium grain boundary segregation of B. The hardenability effect of B-bearing steel 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 degrees C, whereas B precipitates were observed along austenite grain boundaries at high austenitizing temperature of 1200 degrees C. The significant increase in B concentration at austenite grain boundaries due to grain coarsening and a non-equilibrium segregation mechanism may lead to the B precipitation. In contrast, solute B segregated to austenite grain boundaries during cooling after heavy deformation became more stable because the increase in boundary area by 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 important variables that determine the magnitude of the hardenability effect of B in steel. Crown Copyright (c) 2012 Published by Elsevier B.V. All rights reserved.

The behavior of the non-equilibrium grain boundary segregation of boron in low carbon steel was studied through a particle tracking autoradiography. The behavior of the non-equilibrium grain boundary segregation of boron during continuous cooling was compared with the isothermal kinetics of the non-equilibrium grain boundary segregation of boron at the holding temperature using an effective time method. On the basis of the experiments, the cooling rate dependence of the non-equilibrium segregation of boron was explained using the time dependence of the non-equilibrium segregation of boron in low carbon steel. The experimental observations for the cooling rate dependence of the grain boundary segregation of boron are in good agreement with the time dependence of the grain boundary segregation of boron. The mechanisms of the non-equilibrium segregation of boron during cooling in low carbon steel are also discussed.

Hot ductility of boron containing steel (B steel) with adding Nb (0.03 wt.%) (B-Nb steel) and B-Nb steel with adding Ti (0.0079 wt.%) (B-Nb-Ti steel) was quantified using hot tensile tests. The specimens were solution-treated at 1350 degrees C and cooled at 20 degrees C s(-1) to tensile test temperature (T) in the range of 750 <= T <= 1050 degrees C. After that, they were strained to failure at a strain rate of 2.5 x 10(-3) s(-1). For the B-Nb steel, severe hot ductility loss was observed at 850 <= T <= 950 degrees C, which covered the low temperature in which austenite (gamma) single-phase exists, and the high temperature at which gamma and ferrite (alpha) coexist. Ductility loss in the B-Nb steel was caused by the presence of a network of BN precipitates, rather than by Nb(C, N) precipitates at the gamma grain boundaries. In contrast, hot ductility of the B-Nb-Ti steel was remarkably improved at 850 <= T <= 950 degrees C. In the B-Nb-Ti steel, BN precipitates preferentially on TiN particles, resulting in increased BN precipitation in the gamma grain interior and a decrease in the network of BN precipitates at the gamma grain boundaries. These changes reduce strain localization at the gamma grain boundaries and therefore increase the hot ductility of the steel. (C) 2011 Elsevier B.V. All rights reserved.

The effect of boron (B) precipitation behavior on the hot ductility of B containing steel was investigated. Hot ductility of B containing steel was sensitive to the cooling rate (CR) in the range of 1 to 20 K/s (1 to 20 °C/s), whereas that of B-free steel showed little change with CR. Increased CR causes deepening and widening of the ductility trough in B containing steel. Particle tracking autoradiography (PTA) analysis and transmission electron microscope (TEM) image of the samples show that boron nitride (BN) particles form along prior austenite grain boundaries, and that as CR increases, these particles become smaller and more numerous. This increase in the number of small BN precipitates may promote intergranular fracture, leading to a decrease in hot ductility in the lower austenite temperature region (1173 to 1273 K (900 to 1000 °C)). Furthermore, the formation of filmlike ferrite at ~1123 K (850 °C) causes a decrease in the hot ductility of this steel regardless of B addition and CR.

Hot ductility of Boron (B)-bearing steel has been examined in view of slab corner cracking problem. Addition of B to the low carbon steel reduced its hot ductility under a thermal cycle in which samples were cooled directly to the test temperature before straining. The change in hot ductility of B-bearing steel with deformation temperature showed one trough in the temperature range of 800–1000°C, which covered the lower temperature region of austenite single phase (region (I)), and near the austenite/ferrite transformation temperature (Ae3) (region (II)). An abrupt temperature decrease and reheating before straining heavily deteriorated the hot ductility of B-bearing steel in the region (I). In all steels, the strain concentration in the film-like ferrite primarily reduced hot ductility in region (II) regardless of the addition of B and the thermal cycles before straining. The ductility reduction of B-bearing steel is caused by the distribution and amount of BN precipitation, which is determined by the thermal cycles and the N content. Increase in the N content remarkably reduced hot ductility of B-bearing steel in region (I), where the behavior of BN precipitates controlled hot ductility. The results shows that the improvement of hot ductility in B-bearing steel can be attained by decreasing the N content and by avoiding an abrupt temperature decrease in the secondary cooling stage of the slab after solidification.