Development of Ultra High Strength Steels for Reduced Carbon Emissions in Automotive Vehicles.

The need for reducing the carbon footprint of automotive vehicles has driven companies to develop high strength components that would allow less material to be used. This project focuses on the safety critical chassis, which also requires steels with high elongation, fatigue and stretch-flangeability. Pure ferrite steels which have been strengthened by nano-sized Interphase Precipitation (IP) are the only material that can meet all of these requirements. Modern IP steels are heavily reliant on the use of Mo to prevent coarsening of the precipitates and to maintain strength. Unfortunately Mo is a difficult element to use economically due to unstable market prices and sources, along with recycling problems as Mo is banned in the food industry. Therefore the key objective of this project was to minimise or remove entirely Mo from these development IP steels. Two strategies were carried out to meet this outcome, the first was altering the thermodynamic processing and Mn content to maximise IP formation and minimise the grain size. The second strategy was to use Nb, V, C and N to produce complex IP that would have the same positive effect of adding Mo. Increasing the Mn content reduced the grain size, which is as expected, however it also un-expectantly changed the conditions where IP was favourable and reduced the particle density, resulting in no increase in strength. The cooling rate after hot-rolling was changed from 50 to 100 °C/s which increased the sub-grain structure, resulting in a decrease in the elongation. The gain in precipitation strengthening was mitigated by the loss in solution strengthening resulting in no change to strength. Finally the coiling temperature was changed between 630 and 600 °C to find the ideal value for these experimental steels. Coiling at 630 °C resulted in significantly more precipitates, but the Hall-Petch strengthening gain at 600 °C more than compensated for this resulting in the 600 °C steels being consistently stronger. The EDS and EELS carried out on carbon-replica samples confirms for the first time the existence of (Nb, V, Mo)(C, N) and (Ti, Nb, V)C precipitates along with variable compositions of this nature. It was found that adding Mo to (Nb, V)(C, N) resulted in particle growth and with increasing Mo content a severe reduction in the particle density occurred, which is contradictory to established literature on Mo. It was found through dilatometer analysis that Mo greatly reduces the α/γ phase boundary speed, with the effect being greater on V micro-alloyed steels compared with Ti. This effect resulted in the phase boundary being pinned long enough for extensive coarsening of the precipitates to occur. The addition of N to the steel always resulted in refinement of the particle size for both hot rolled and forged steels. Additionally for the hot-rolled samples N caused a significant increase in the particle density by improving the efficiency of micro-alloying elements leaving the solid solution and entering precipitates. The use of Ti to form (Ti, Mo)C or (Ti, Nb, V)C always resulted in a high particle density distribution and a very fine (3 nm) average size, with the size range being smaller than the other samples. It has been found that Cr does not enter the precipitates but does decrease the grain size. The effect of Cr on the particle density is yet to be fully established, but it appeared to have a minimal effect on precipitation. Finally VC and (V, Mo)C were studied using atom-probe topography in partnership with The University of Oxford. They were able to study the morphology of IP in significant detail and also obtained the composition of the grain boundary. This confirmed the composition of the precipitates and demonstrated that Mn was segregated to the ferrite grain boundaries.