The development of X100, 13mm thick, on coil at ArcelorMittal is on schedule. Different steel compositions grouped in different categories A, C, D with various alloying contents and carbon equivalents have been tested by means of appropriate laboratory simulations. From these simulations, two different concepts have been developed, able to fulfil X100 strength properties in combination with excellent low temperature toughness. These two concepts are suited for low or high coiling temperatures (Type C and D respectively). The influence of the different TMCP (Thermomechanical controlled processing) parameters on the strength-toughness properties is shown and explained by the various microstructures. The low coiling temperature concept is based on a fine granular bainite microstructure avoiding the presence of M/A constituent and providing an excellent combination of strength and toughness. The high coiling temperature concept exhibits a quasi-polygonal ferrite microstructure with M/A as second phase. The M/A constituent seem to strengthen the steel without impairing the toughness while the quasi-polygonal ferritic matrix retains excellent toughness. A first assessment of the weldability has been done through the simulation and analysis of the heat affected zone for different cooling times between 800 and 50°C. The concept with a lower carbon equivalent (Type D) gives excellent weldability results.

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