The Effect of Adding V and Nb Microalloy Elements on the Bake Hardening Properties of ULC Steel before and after Annealing

Abstract

Bake hardening (BH) is a vital part of special steel production. Studies in this field have focused on steels under homogeneous yielding, but until now, none have been conducted on the phenomena that occur for steels under heterogeneous yielding. In the current study, the effect of adding Nb and V alloying elements on the strength of ultra-low carbon (ULC) steel after bake hardening was investigated. The effects of pre-strain, grain size, and recrystallization annealing temperature were analyzed, as well as the effect of Nb and V on the yield stress caused by the bake hardening process. For this purpose, five types of alloys with different V and Nb contents were melted, cast in an induction furnace, and subjected to hot hammering and hot rolling. Then, cold rolling was applied to the samples by ~80%. To eliminate the effects of cold working, tensile samples were subjected to recrystallization annealing at 750 and 800 °C for 30 min, and the samples were quickly quenched in a mixture of a NaCl solution and ice. The annealed samples were subjected to a pre-tensile strain in the range of 2–12% and then aged in a silicone oil bath at 180 °C for 30 min. Then they were subjected to a tensile test. The obtained results showed that with the increase of the pre-strain and the annealing temperature, the values of baking hardness increased. The presence of V in the composition of steel reduced the annealing temperature.

The Effect of Niobium Addition and Pre-Annealing on the Tensile Properties of 52CrMoV4 Spring Steel

“…This is the way in which the dislocation locking mechanism works in conjunction with the development of tiny phases to produce materials with improved strength characteristics [6,21]. Similar to previous research, it was discovered that the production of Nb and C particles following heat treatment results in a higher Nb ratio material with a finer grain size [5,[7][8][9][10][11]. The mechanical properties of the steel are greatly affected by the size of the prior austenite grain [22].…”

“…It is understood that when the Nb ratio increases, pre-annealing is effective in reducing the grain size. When the relevant literature was examined, it was seen that the grain size decreased with the effect of Nb [5][6][7][8][9][10][11].…”

“…Ghanaei et al studied the effect of adding different amounts of vanadium and niobium microelements to different types of steel after bake hardening. It was found that the highest tensile strength value and finest grain size were achieved in the sample with 0.031 wt.% Nb [10]. According to Andrade et al, Nb has the greatest solute retardation effect on static recovery and recrystallization.…”