Development of ultra-high strength ‘lean’ maraging steels
thesisposted on 01.03.2017, 02:58 by Sun, Wenwen
The automotive industry is under increasing pressure from legislation on greenhouse gas emissions because cars are seen as a significant contributor to the atmospheric gases that accelerate global warming. Reducing the weight of automobiles is one way to help increase fuel economy and hence reduce quantities of harmful gases released into the atmosphere. Weight reduction in the body-in-white is one area that has received attention because it accounts for ~25% of the total car mass. Using higher strength materials allows less material to be used to support the same forces. Steels have the merit of being economical, strong and exhibit excellent formability and recyclability, thus being the dominant material employed in vehicle construction. In the automotive manufacturing industry, hot stamping has recently come to the fore as a potential method for forming very high strength metals. New processing methods provide an opportunity to develop new steels and the goal of this project is to develop a new, weldable sheet steel with a yield strength approaching 2GPa that could be formed by hot stamping. This steel should contain a low concentration of total alloying elements and a carbon content less than ~0.03wt.%. The approach chosen is based on precipitation hardening of alloy precipitates in a lath martensite matrix. The lath martensite matrix exhibits remarkable thermal stability and can contribute ~1GPa to the yield strength. The objective is for precipitation hardening to contribute the extra 1GPa without significant loss of strength of the matrix to reach a total yield strength of ~2GPa. A genetic algorithm (GA) has been developed and coupled with a computational thermodynamic database to search through composition and temperature space to identify alloy systems and compositions that could potentially meet these design constraints. A new class of steels exploiting the G-phase were developed with yield strengths of 1700MPa. Combined atom probe tomography (APT) and small angle x-ray scattering (SAXS) techniques allowed a detailed characterization of the precipitate state in these new alloys. In-situ SAXS experiments allowed a direct measurement of the nucleation rate of G-phase precipitates and the effective interfacial energy of the precipitate was extracted using classical homogeneous nucleation theory. The precipitate growth kinetics was closely parabolic, as expected for diffusion controlled growth of equiaxed precipitates, and the mass transfer coefficient of the rate controlling species, Ni, was obtained in the lath martensite matrix. Finally, using the theory for strengthening from shearable obstacles, the individual G-phase precipitate strength, as a function of precipitate size, was extracted. The particle strength was observed to depend linearly on size (as is usually assumed) but a deviation at particles radii below 15Å was observed.