Non-equilibrium phosphorus grain boundary segregation and its effect on embrittlement in a niobium-stabilized interstitial-free

Grain boundary segregation of phosphorus in an Nb-stabilized and P-strengthened interstitial-free steel was investigated using Auger electron spectroscopy after the steel specimens were quenched from 880 degrees C and aged at 680 degrees C for different times. The segregation initially increased with increasing aging time until reaching a maximum value and then decreased gradually to its equilibrium value with further increasing aging time, demonstrating a non-equilibrium segregation kinetic characteristic with a critical time of around 30 min. The ductile-to-brittle transition temperatures (DBTTs) of the aged specimens were determined through fracture appearance measurements. There was a linear relationship between DBTT and phosphorus boundary concentration (C-p, at%): DBTT(degrees C)=4.39C(p)-64.06, suggesting that a P-strengthened IF steel should avoid slow cooling or staying in the intermediate temperature range so as to avoid apparent phosphorus grain boundary segregation and in turn to avoid embrittling. (C) 2014 Elsevier B.V. All rights reserved.