Abstract
The effect of isothermal heat treatments (1 hour at 200, 400, 600 and 800°C) on mechanical properties of thermo-mechanically rolled S700MC steel has been investigated by extensive mechanical characterizations. Treatments at 600°C increase yield and tensile strength and decrease impact energy. Below 600°C the steel retains its bainitic structure. Precipitation kinetics simulations indicate that this secondary hardening effect arises from the nucleation of fine (Nb,Ti)C particles, indicating that the bainitic structure is unstable above 600°C due to its high supersaturation with respect to C, Nb and Ti. These results can help to optimize the operating practices for post-weld heat treatments.
Highlights
In the last years the industrial demand for high strength steels (HSS) with increasing mechanical performance to reduce the overall weight of the components, has stimulated the steel producers to develop and commercialize many steel grades capable of reaching a strength up to 1 GPa and even more [1]
The behavior of S700MC to welding is generally good due to its relatively low carbon equivalent [7,8,9,10,11], this study provides additional information on joining of this high strength steel, especially regarding the optimization of the operating practices for post-weld heat treatments when they are required to reduce hardness peaks in the heat affected zone caused by the joint geometry or by a not adequate selection of the welding conditions
Total elongation appears not affected by the treatments and the observed fluctuations can be considered within the scatter of the experimental determinations (Figure 3b)
Summary
In the last years the industrial demand for high strength steels (HSS) with increasing mechanical performance to reduce the overall weight of the components, has stimulated the steel producers to develop and commercialize many steel grades capable of reaching a strength up to 1 GPa and even more [1]. Among the most performing high strength grades, S700MC is largely employed since its high tensile properties are associated to a good toughness. In some cases additional thermal treatments are employed to correct or improve the mechanical properties of as-rolled strips, especially of large thickness, in-line with the hot rolling mill [4]. The behavior of S700MC to welding is generally good due to its relatively low carbon equivalent [7,8,9,10,11], this study provides additional information on joining of this high strength steel, especially regarding the optimization of the operating practices for post-weld heat treatments when they are required to reduce hardness peaks in the heat affected zone caused by the joint geometry or by a not adequate selection of the welding conditions
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