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New CCT equations have been developed and optimized to simulate the start temperatures of the austenite decomposition process in low-alloyed steels using experimental CCT data published in the literature. Exceptionally, this optimization does not apply the nominal compositions of the steels, but the corresponding soluble compositions of the grain boundaries calculated using IDS software, depending on the reported austenitization treatments of the steels. These compositions, rather than the nominal ones, are expected to control the start of the austenite decomposition, which usually initiates at the grain boundaries. The new optimization treatment takes into account the solute microsegregation and the possible precipitate formation. Using IDS software, the new equations were validated with new experimental CCT data. Agreement was good not only for the austenite decomposition start temperatures, but also for the final phase fractions, indicating fairly reasonable predictions of phase transformation kinetics by the IDS. In addition, IDS simulations were compared with the experimental CCT data of five high-carbon steels, applying both the new equations based on grain boundary soluble compositions as well as the equations based on the nominal compositions. With the same experimental CCT data used in optimization, better agreement was obtained with the new equations, indicating the importance of determining the soluble compositions at the grain boundaries where the austenite decomposition process is likely to begin.

Hollow sections and cold-formed steels have a key role in modern structures and machinery. In addition, to benefit from full potentials of cold-formed steels, it is usually required to weld them to other steel parts of structures. However, data provided by relevant standards, such as Eurocode 3, do not cover newly developed high strength and ultra-high strength grades of this material. Thus, further study is critical to complete available data in literature and standards. Regarding this matter, having a good weldability for cold-formed high and ultra-high strength steels is vital for development of contemporary steel structures. Thus, newly developed steels S700MC and S1100 were selected to be investigated in this study. To do so, bended base metals with different degrees of coldforming were welded to their straight (virgin) counterparts. Next, welded joints were investigated via microstructural analysis, hardness measurement, tensile test, and Charpy impact test to assess the weldabilities of the cold-formed base metals. Results show that the final joints had acceptable characteristics, and the cold-formed base metals showed good weldability. However, bending and pre-straining criteria recommended by the manufacturer must be satisfied to have an acceptable joint after welding. Beyond that criteria, fracture elongation and notch toughness of the welded joints decreased, and some welded samples failed from their cold-formed base metals.

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