Efficiency in steel production requires relatively minimal changes in the upstream procedures with product differentiation occurring during the latter stages of processing.
The austenite transformation to various transformation products is probably the single most important factor in determining the final properties of most steels. The role of local segregation is also critical in the transformation and subsequent heat treatment.
The ability to exert greater control over the transformation gives the ability to have greater control of the transformation product and subsequent final properties of the steel.
This applies across all steel types, whether long products, strip or sections.
The control of retained or reverted austenite, which is present in many high-strength steels, remains a key processing issue.
The stability of the austenite, and therefore the likelihood of it transforming during forming, or in service, is controlled by many factors including composition, shape and size.
Small variations in the amount of austenite in the structure can have dramatic effects on the mechanical properties.
For example, in a Super 13% Cr steel, small changes in heat treatment temperature (as small as 5°C) can lead to changes in austenite content of ~10% which can lead to changes in yield strength of up to 300MPa.
In order to give better control of the heat treatment the fundamental reasons controlling the stability of the austenite need to be understood. This project has used alloy design and detailed microstructural analysis to answer this issue.
Key findings to date
A systematic range of compositions was produced, within the specified composition for Super 13% Cr steels, but with systematic changes in the Ni, Mn and Mo contents. This has shown just how small differences in these alloy additions can have a major effect on the amount of reverted austenite and the temperature at which the austenite content peaks.
A detailed analysis of the austenite has shown how the elements strongly partition between the reverted austenite and the tempered martensite. The carbon content in reverted austenite is a linear function of the tempering temperature for the first temper but does not change on the second temper. Ni strongly partitions to the austenite, while Mn also partitions, but to a lesser extent. Additional Ni increases the growth rate of reverted austenite as well as increasing the total quantity, before the reverted austenite becomes too large and transforms on cooling. An increase in Mn also increases the amount of reverted austenite, although to a lesser extent than Ni, but displaces the peak in austenite content to a higher temperature.
The effect of these additions on the shape of the austenite is under investigation. The change in composition also has a marked effect on the tensile properties, but not in a predictable manner.
Understanding this will be key to solving the problem of the extreme sensitivity of tensile properties to the heat treatment conditions.
The research has led to recommendations for adjustments to the steel composition and the heat treatment schedules at Liberty Steel. This will allow the amount and stability of the reverted austenite to be better controlled which will deliver a more consistent product. The changes are straightforward to implement and therefore there are no barriers.
What’s next for this task?
The task is coming to an end. The next step is to work with Liberty Steel to look at whether the recommendations from the research can be implemented.