Influence of intercritical annealing on strength and impact behaviour of niobium containing steels

Abstract

The influence of inter critical annealing at 730°C on the impact properties and strength of C–Mn–Al–Nb steels has been examined. For low Mn (0·56%), Nb steels, intercritical annealing resulted in improved impact performance and the impact transition temperature (ITT) was reduced by as much as 35 K with no change in strength. The improvement in impact performance is considered to be due to Mn segregating to the α/γ boundaries leading to refinement of the grain boundary carbides. This refinement increased with holding time at 730°C in accordance with an increased grain boundary segregation of Mn. Strength was not influenced because grain size remained unchanged on intercritical annealing. The improvement in impact behaviour was greater the longer the holding time at 730°C but was significant even after 15 min. Improvements occurred both on cooling from the austenitising temperature (9·20°C) to 730°C and on heating from room temperature to 730°C, the latter heat treatment being the more beneficial. For higher Mn (1·4%), Nb steels, improvements in impact performance resulting from intercritical annealing depended on cooling rate. Again, the Mn build-up in the y increases with time of intercritical annealing. Owing to the initial overall higher Mn level and finer grain size, the steels were susceptible to martensite formation if the cooling rate was too high. At a cooling rate of 40 K min - 1, improvements in impact behaviour occurred only after short intercritical annealing times (30 min) when only a small amount of martensite had formed. Long times caused a serious deterioration in impact behaviour due to the presence of high volume fractions of martensite. Slow cooling (1 K min⁻¹), however, ensured ferrite–pearlite structures and significant improvements in impact behaviour (20–60 K reductions in ITT) were noted on intercritical annealing with no change in strength. The short holding times required to achieve an improvement in impact behaviour in these fine grained steels are encouraging for the possible commercial exploitation of this heat treatment.

MST/1382