Development of ultra high strength steels for reduced carbon emissions in automotive vehicles

• Main Author: Chamisa, Alfonce
• Other Authors: Rainforth, W. M.
• Published: University of Sheffield 2014
• Subjects: 620
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.605502

Automotive steels with enhanced strength and ductility beyond the current bounds can be engineered through microstructural strategies that take into account the benefits brought about by nanoprecipitates formed during the transformation from austenite to ferrite. Three multiphase steel compositions were initially studied. A patented Ti-Al-Mo steel composition was selected as the baseline for comparison with the other two steels. It is claimed that this steel has exceptional mechanical properties. The Ti and V microalloyed steels were selected to check whether interphase precipitates (IP), which can yield a high degree of precipitation strengthening, could be produced. Results showed that Ti-Al-Mo had superior microstructure and properties as compared to the other two. The microstructure was composed of ferrite, martensite, bainite and retained austenite. Unlike the other two steels, IP was also observed and the UTS of 780MPa and uniform elongation of 21% previously reported by other authors were also confirmed. The V microalloyed steel composition was selected for the next part of the project since it would be commercially viable to produce for Tata Steel. The time/temperature/transformation behaviours of the V microalloyed steel were extensively studied. The microstructures developed were analysed and high precipitate number densities averaging 394 particles/m2 were recorded in the sample transformed at 700oC for 1200s. A high uniform elongation of 30.8% and the highest UTS of 627MPa were also reported on the same sample. The UTS value was attributed to the high precipitate number density which made an overall contribution to the yield strength of 270MPa. However, further studies need to be carried out, since the properties were not optimised and were inferior to some of the steels in current use for automotive applications. Questions were asked as to why IP was not observed. The low austenising temperature of 950oC was cited as the possible reason. Thermodynamic calculations using Thermo-Cal software had predicted that the optimum should have been 1050oC. As a result, 950oC was believed to be inadequate to effectively dissolve the carbides present to allow effective formation of interphase precipitates during the temperature hold in the α + γ temperature region. The high N content was cited as another possible reason, but this was not conclusive and shown in itself not to be true by work in the next stage of the project. It has since been established that Mo retards precipitate growth in both Nb and Ti alloyed steels. However, nothing has been reported on the effects of Mo on V microalloyed steels. As a result, the next stage of the project studied the effects of Mo on V microalloyed steels. Predominantly ferritic steels with Nb-V-Mo microalloying additions were produced and coiled at different temperatures. Samples microalloyed with Ti-Mo, Ti, V-Mo, V, Nb-Mo and Nb were also produced for comparison purposes. IP was observed in most of the Nb-V-Mo steels. IP with average interparticle distances of 8 ± 2nm and row spacing of 22 ± 3nm were observed in sample 10-630Nb+VMo. High YS of 925MPa, UTS of 1023MPa and total elongation of 16.8% were recorded for this sample. The exceptional mechanical properties were attributed to high number densities of fine IP averaging 1766 particles/m2. 82% of the precipitates had average sizes below 7nm and these made a contribution to YS of approximately 546MPa. It was then concluded that Mo additions were likely to have influenced the formation of fine precipitates that strengthened the ferritic steels. Hence Mo is likely to influence the high nucleation rate and slow precipitate growth in the same way that it influences Ti and Nb microalloyed steels. Since one of the steels studied at this stage had high N additions, it was also confirmed that the precipitate number densities in the previous V microalloyed steels batch had nothing to do with the N content; instead, it all had to do with the low austenising temperature which failed to put the carbides into solution.