Processing and microstructure of direct-quenched and tempered ultra-high strength steels

Thesis event information

Date and time of the thesis defence

Place of the thesis defence

Linnanmaa, auditorium IT115

Topic of the dissertation

Processing and microstructure of direct-quenched and tempered ultra-high strength steels

Doctoral candidate

Master of Science (Eng.) Ari Saastamoinen

Faculty and unit

University of Oulu Graduate School, Faculty of Technology, Materials and Mechanical Engineering Research Group

Subject of study

Material science and physical metallurgy


Professor Ronald Schnitzer, Montanuniversität Leoben

Second opponent

Doctor Per Hansson, SSAB Special Steels


Professor Jukka Kömi, University of Oulu

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Reduced carbon footprint by optimizing heat treatment of modern ultra-high strength steels

In the doctoral thesis, processing route of direct-quenched and tempered steels was investigated. Results of the thesis lead to improved mechanical properties as well as more enhanced productivity during processing. Both these factors allow decrease in CO2 emissions during both manufacturing and full life cycle.

Total production of steel is predicted to be increased by 50 % by 2050 from current 1600 million tons. Development of modern ultra-high strength steels will allow more environmental friendly steel footprint during both manufacturing and life cycle. Enormous volume of steel production will lead to huge global impact even with marginal gains.

Thesis consisted of four original publications, where the effect of chemical composition, hot rolling parameters and post-quenching heat treatment paramaters on microstructure and properties were investigated. Results of the thesis show that by using modern direct-quenching procedure combined with tempering treatment, productivity and final properties of ultra-high strength steels can be enhanced compared to traditional re-austenitized and tempered steels. Enhanced productivity was obtained by using direct-quenching instead of reheating and one single tempering treatment for whole steel coil. Chemical composition was tailored for this demanding heat treatment process and it retained it's strength despite long heat treatment at 570 °C.

Traditionally re-austenitized and quenched steel has possessed low toughness and additional tempering treatments have been necessary to promote toughness of these high-strength products. This doctoral thesis showed that thermomechanically processed and direct-quenched low-carbon 960 MPa grade steel has high toughess already at as-quenched condition and further heat treatment is not necessary required. In the thesis, conventional re-austenitizing process route lead to higher grain size and lower toughness compared to direct-quenched steel despite additional heat treatments. However, the total level of yield strength, elongation and bendability of the steel was improved with optimal tempering provess.

Results of the thesis provide new information about processing of modern ultra-high strength steels and helps in the design of new commercial steel grades.
Last updated: 1.3.2023