The strength and impact behaviour of intercritically annealed C-Mn-Al-Nb steels

An investigation has been made into the influence of intercritical annealing on the tensile and impact properties of C-Mn-Al-Nb steels. Three levels of manganese (1.5%, 1%, and 0.56%) were examined in an 0.1% C steel austenitised at 920°C followed by intercritical annealing at 730°C for various times. The cooling rate from 920°C to 730°C and from 730°C to room temperature was 7°C/minute. Intercritical annealing enabled Mn to diffuse to the a/a and/ or a/y boundaries and into the newly formed y causing the transformation temperature to be lowered, so refining the grain boundary carbides on cooling to room temperature. For the 1% Mn and 0.5 6% Mn steel, intercritical annealing resulted in improved impact toughness , compared to a normalising treatment due to this refinement of the carbides. The impact transition temperature (ITT) was lowered by as much as 35°C with no change in strength. Strength was little influenced by this heat treatment because grain size remained constant. The improvement in impact toughness was greater the longer the holding time at 730°C but was significant even after 15 minutes. Improvements occurred both on cooling from 920°C to the intercritical annealing (730°C) and on heating from room temperature to 730°C; the latter heat treatment being the more beneficial. For the 1.5% Mn, intercritical annealing for long times introduced a high pearlite volume fraction and or small martensite colonies at the grain boundaries causing a deterioration in impact toughness compared to the normalised state. Reducing the cooling rate to 0.8°C/minutes in the higher Mn (1.5%) steel again resulted in improved impact behaviour with no change in strength on intercritical annealing for long times. This is due to the slower cooling rate preventing martensite from forming, so allowing the carbide refinement on intercritical annealing to dominate the impact behaviour. Improvements occurred both on heating to and cooling down to the intercritical annealing temperature and were as much as a 60°C lowering of the ITT. Reducing the C level to 0.067% C in the high Mn (1.5%) steel also gave rise to an improvement in impact behaviour with no significant change in strength even at the higher cooling rate. This is again believed to be due to the reduced C level preventing martensite from forming. Raising the intercritical annealing temperature to 760°C in the 1% Mn steel produced similar improvements in impact behaviour to those obtained for the heating up cycle but only small improvements took place on cooling down. This may be related to the higher intercritical annealing temperature delaying the y/a transformation so reducing the amount of Mn able to diffuse along the a/a boundaries. Finally, the effect of grain size on the impact and tensile properties of plain carbon manganese steel after intercritical annealing at 730°C was examined. Two grain sizes were studied (18(lm, and 37|lm) . At the finer grain size the impact transition temperature (ITT) was reduced by as much as 15 °C with no change in strength. The improvement in ITT was greater the longer the holding time at 730°C. Improvements occurred both on cooling from the austenitising temperature (920°C) to intercritical annealing (730°C) and on heating from room temperature to 730°C, but the latter heat treatment was more beneficial. For the coarser grain size, there was no significant improvement in impact behaviour with intercritical annealing treatments. This is possibly due to a reduction in the Mn segregation along the boundaries caused by it's lower grain boundary surface area/ volume ratio. The magnitude of the improvement in impact behaviour on intercritical annealing may also be related to the range of carbide thickness over which the improvements occur. Steels which on normalising have a carbide thickness in the range 0.55 (lm to 0.25 Jim are the ones which are likely to show the greatest improvements. Thus this treatment is best suited to thick plate