Economical and industrial considerations have introduced TRIP steels for ultra-high strength light gauge automotive parts. These steels are expected to increase fuel efficiency and decrease emissions. Resistance to rolling, climbing and acceleration are all related to the vehicle weight and thus the fuel efficiency is directly related to weight. Reduced fuel consumption also contributes to lower CO{sub}2 emission which is vital to both US and European car manufacturers. The requirements from the 1997 Kyoto conference are an 8% reduction in CO{sub}2 emissions for European car manufacturers before 2012 [1]. For US manufacturers there is also a growing concern over emissions as congressional efforts consider increasing the fuel efficiency requirements. Furthermore, individual states such as California have adopted more stringent requirements on vehicle fuel efficiency and emissions than those set by the federal CAFE (Corporate Average Fuel Economy) standards. Lighter materials will also contribute to improve the vehicle performance and agility especially in sports cars. Secondly due to safety issues materials will have to be tough, i.e., be capable of absorbing large amounts o energy during a crash event. Finally, these light weight and tough materials will have to be manufactured into automotive parts and structures in a way that maintains affordability and competitiveness with respect to inherently lighter weight materials based on Al and Mg or plastics. The combined requirements of lightness, toughness and reasonable cost are addressed by steels that exhibit the Transformation Induced Plasticity (TRIP) effect. TRIP steels are superior to other steels in terms of combined elongation and strength [2,3,4]. The fabrication of TRIP sheets involves continuous casting, hot-rolling and cold rolling into sheets followed by a unique annealing cycle [5]. To expand the market for thin gauge TRIP steels by introducing them into automotive body parts that are exposed to the environment, it is necessary to be able to ensure a good surface quality and the ability to coat the steel surfaces. Protection against corrosion is essential in thin gauge automotive parts. Unfortunately, during hot-rolling, annealing and casting the high Si content results in a surface oxide layer that (i) is strong and thus difficult to remove by pickling and (ii) is not amenable to Zn-coating since liquid Zn alloys do not readily wet oxides, which results in uncoated regimes, called bare spots. There is thus a problem with compatibility with existing galvanizing lines. Several processing strategies offer potential solutions (or partial solutions) to improve the coatability: (i) the application of so called pre-plated metal layers [6,7,8,9], (ii) intentional internal selective oxidation of the problematic elements by controlling the atmosphere and temperature [2] or (iii) the substitution of Si by Al offers a possibility to maintain the TRIP properties (prevent carbides and retain γ-austenite) but form less problematic oxides during the thermo-mechanical steps. Of the three choices, the last 2 appears most attractive from the point of view of avoiding additional processing steps. It should be noted however, that high aluminum alloying will likely pose challenges to steelmaking and continuous casting in terms of Al loss and erosion of refractory vessels. Mahieu et al [10] have shown that, while Si based TRIP steels tend to form Mn{sub}2SiO{sub}4 which deteriorates the coatability, Al additions form a spinel phase, FeAl{sub}2O{sub}4 and Mn-oxides. The latter were were found to have little effect on the wettability of the galvanizing alloy.
展开▼
