Studying Automotive-Steel Microstructures
Despite its history and ubiquitous presence as an infrastructure material, steels remain a complex material system with many persisting research questions. One of the reasons for this is steel’s enormous variability in properties depending on composition and processing.
I am part of a research group, the Center for Iron and Steelmaking Research (CISR), at Carnegie Mellon that is devoted to these research questions. One of my PhD students has just about finished a project investigating the classic metallurgical problem of “microstructure–processing–properties–performance” relationships in steels but with the twist that we care about performance during processing of the steel, not just performance in service.
The steels of interest in this project are new grades that have potential for automotive vehicle structures. Steel remains the material of choice for vehicle structures because of its low cost and wide variability in properties, particularly strength and ductility. This variability gives vehicle designers flexibility in structural design.
There is competition from other materials, particularly aluminum, and the steel industry has responded with new steels and new processing schemes to produce material with even wider ranges of strength and ductility. These new steels for vehicles are higher in the alloying elements manganese and silicon than many other steels. These elements, combined with carefully controlled heat treatments, result in the desirable properties. There are challenges in making these steels in current steel plants because the equipment and the practices have not typically been used to make steels of these high-alloy contents.
There are essentially five steps to making steel: (1) obtain metal from iron ore or scrap, (2) melt and refine, (3) cast into solid form, (4) shape into desired product and (5) heat treat or galvanize. Our focus has been on step 3.
Steels for automotive applications are cast into approximately 9-inch-thick slabs. The slabs are cooled and sometimes shipped long distances for the shaping and treating steps. Typically, as-cast slabs are not of concern because additional processing steps will completely change their microstructures and properties. That’s not the case for these steels – the as-cast state is of critical importance. This is because these steels are brittle in the as-cast state and easily fracture.
We have been performing experiments and simulations to describe these microstructures and test if certain microstructures cause fracture. Because of the alloying elements, these steels have much more complicated microstructures than are typically observed in the as-cast state.
We have found that slower cooling (typically beneficial in preventing cracking) results in formation of thin films of ferritic iron. Cracks propagate through these ferrite films, and the steels easily fracture. This is illustrated in the figure, which is an optical microscope image of a broken steel sample that shows how the crack moved through the microstructure.
The crack traveled right-to-left along the bright, white ferrite films (the gray part of the microstructure is bainite). With faster cooling, there is insufficient time for the ferrite films to form, and fracture is more difficult. Our fundamental results linking steel composition, microstructural development and mechanical properties can be used by steel plants to design cooling strategies that keep slabs from cracking.
The difficulties in designing and controlling processes to efficiently produce steel with the right properties at scale are not limited to new grades. However, doing something new always creates many challenges. The complexity of steel and the pace of alloy development are likely to increase. Research at CISR helps ensure industrial-scale production can keep up.