Direct quenching is a technological innovation implemented at Ruukki/SSAB two decades ago, and new families of structural steels based on direct quenching technology have been developed under the FIMECC LIGHT programme. Ruukki/SSAB already had a very strong position in thin ultra-high-strength steels when the programme started, but the research carried out has promoted Ruukki's/SSAB’s market leadership in this area. Substantial efforts have been made to improve the mechanical properties and usability of the new steels.
The understanding of the limitations of the steel production process and metallurgical understanding have increased during the programme. This enhanced knowledge has enabled the widening of the dimensional ranges of new and existing products, and also enabled the design of new steel grades.
- The factors affecting the toughness of the steel have been revealed
- The main factors affecting the formability of direct quenched strip steels have been identified.
- Weldability and other workshop properties have been evaluated, and new rules and guidelines developed.
- Empirical / statistical analysis tools have been developed to predict and model the mechanical properties of direct quenched steels.
Considerable research work together with the project partners on combining direct quenching and tempering has enabled the production of new thicker tempered plate products with yield strengths of 700 MPa and above. Research work led to fundamental knowledge that allowed the production of direct quenched structural strip steels up to 10 mm thick with yield strengths of 900 and 960 MPa. Basic knowledge has been developed on potential future steels with yield strengths of 1100MPa and above. In addition, the metallurgical research has enabled development of a totally new kind of strip steel with yield strength of 700MPa. This new steel combines excellent toughness, cut-edge quality and formability in a way that is unsurpassed on the global market. Research showed how control of chemical composition and rolling parameters could affect toughness in thick direct quenched plates leading to significant impact toughness improvements that allow good performance even at -60 °C. In other words, direct quenching can now be used for even the most demanding applications.
In addition, information was gathered concerning the design and dimensioning of structures made from direct quenched steels. As a result a manual was published. The manual helps end-users of such steels to obtain the greatest benefits for their products.
The main objective of the project was to build knowledge enabling the development of the next generation of ultra- high-strength low-alloy steels. New ultra-high-strength steels are required for future lightweight structures in, for instance, transport applications. Use of the next generation ultra-high-strength steels will enable weight reduction, reduced fuel consumption, increased payload, and thus lower life cycle costs and environmental impact.
The research on low-alloy steels focused on the application of direct quenching, which lies at the core of SSAB’s strategy for increasing the share of special steels grades in its product portfolio. For SSAAB, the results generated in the FIMECC LIGHT programme serve as a knowledge platform on which future steels will be developed. These steels will not only provide competitive advantage in existing markets, but will open totally new markets for steels. The coming years will show how well enhanced understanding on metallurgy and steel production processes can be turned into profitable business. This includes new processes, microstructures and phenomena.
Jarkko Vimpari, SSAB Europe Oy
SSAB Europe Oy, University Of Oulu, Aalto University