Effect of the Ti/N ratio on the hardenability and mechanical properties of a quenched-and-tempered C-Mn-B steel

Abstract

Ten experimental 0.18 pct C-1.2 pct Mn- 0.002 pct B steels with various Ti/N ratios were evaluated in this study. The hardenability of these steels was first determined using Jominy tests. Slab sections were then rolled to produce 12.5-mm-thick plates, and subsequently quenched and tempered for mechanical property evaluation. The volume fraction of coarse (greater than 1 µm) TiN particles was measured in all steels using quantitative metallographic techniques. Scanning transmission electron microscopy was used to investigate fine precipitates, and scanning electron microscopy was used to examine the fracture surface of Charpy specimens. The results show that a complete boron (B) hardenability effect is obtained with Ti/N ratios ≥2.9, a value slightly below the stoichiometric Ti/N ratio of 3.4. Any excess Ti, above that which combines with N, provides an additional increase in hardenability on quenching (effect of Ti in solution) and an increase in strength on tempering (Ti (C,N) precipitation). Steels with a higher (Ti)(N) product develop a higher volume fraction of coarse TiN particles during solidification. These coarse TiN particles result in reduced toughness levels of the heat-treated plates evaluated in the present study.