Materials Science and Engineering A 421 (2006) 307–316 Inconsistent flow stress in low carbon boron steels during finishing Kevin Banks ∗ , Waldo Stumpf, Alison Tuling Department of Materials Science and Metallurgical Engineering, University of Pretoria, Pretoria 0002, South Africa Received 22 October 2005; accepted 27 January 2006 Abstract The influence of deformation parameters and composition on flow stress behaviour of boron-containing steels is described. High boron steels displayed a consistent flow stress during finishing due to the boron concentration at austenite grain boundaries having achieved steady state followed by the precipitation of BN. Higher flow stresses than those found in boron-free steel were due to solute boron at grain boundaries, which delayed the onset and the rate of dynamic recrystallisation (DRX) and consequently enhanced the solute drag effect. In low B steels, inconsistent flow stress behaviour at finishing temperatures above the BN solubility temperature was attributed to deformation at or near the maximum non-equilibrium grain boundary segregation (NGS) of boron, which occurred rapidly. The flow stress in low B steels was consistent when DRX occurred during roughing and early finishing and was promoted by weak NGS and a finer grain size when the reheat temperature was decreased. Flow stress consistency was also promoted by avoiding a NGS peak by deforming at temperatures below the BN solubility temperature. A high driving force for AlN precipitation in austenite increased flow stress inconsistency through the protection of solute boron for NGS and increased solute drag. © 2006 Elsevier B.V. All rights reserved. Keywords: Boron; Flow stress; Non-equilibrium grain boundary segregation; Hot rolling 1. Introduction During the hot strip rolling of thin (<2.2 mm) low C–low B (8–20 ppm) steels, mill instability due to interstand mass flow variations, is sometimes experienced in the final stages of finish- ing (950 ◦ C), which results in geometrical inconsistencies. No instability was observed when thin, high B (30–50 ppm) low N steel was rolled under normal conditions. Instability can arise from (1) fluctuating flow properties of the material itself or (2) attempts by the mill to conform to incorrect setup flow stress data or (3) dynamic transformation from austenite-to-ferrite in the roll gap in cases where the temperature may drop below the Ar 3 . Since ferrite has a lower flow stress than austenite when both phases co-exist at a given temperature [1], sudden drops in rolling load may be experienced during finishing. Dumetrescu [2] reported that the high rolling forces experienced in thin, low C–low B–low N steels in the last finishing passes below 900 ◦ C were attributed to the formation of carboborides. Boron segregation to austenite grain boundaries retards their mobility and hence, the recrystallisation kinetics during hot working [3]. Watanabe et al. [4] suggested that when austen- ∗ Corresponding author. Tel.: +27 12 420 4552; fax: +27 12 362 5304. E-mail address: kevin.banks@up.ac.za (K. Banks). ite grain boundaries move slowly enough (i.e. comparable to the diffusion rate of B at low austenite temperatures), solute B atoms are “swept up” leading to an increased flow stress. In recent work [5], high steady state flow stresses and high peak strains were found in low C–low B–high N steel over the entire hot working temperature range if compared to low C–high N steel. This was ascribed to solute drag by the B on the austenite grain boundaries, leading to retarded dynamic recrystallisation. Two types of boron segregation can occur at austenite grain boundaries in rapidly cooled or deformed steel [6,15]: Firstly, equilibrium grain boundary segregation (EGS) arises after isothermal holding for longer times. Its severity decreases with increasing holding temperature and is independent of the prior cooling rate. Segregation increases progressively, then attains a stable value. Secondly, non-equilibrium segregation or NGS develops during rapid cooling from high temperatures and leads to the formation of a temporary boron-depleted zone next to the segregated regions. This is ascribed to the migra- tion of boron–vacancy complexes to sinks at the grain bound- aries, where the B atoms are deposited after annihilation of the vacancies. The NGS of boron is obtained entirely from the B-depleted zones, whilst the boron distribution remains homo- geneous solely in regions far from the grain boundaries [6]. NGS is very sensitive to the cooling rate, as well as pre-deformation, and the degree of segregation is many times higher than that 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.01.073