Precipitation and strengthening in a low-carbon steel containing scrap impurities

Impurity elements have been introduced into a commercial low-carbon steel to mimic the compositional effect from increased scrap steel use during steel production. In this study, a base steel (BS) and a scrap steel (SS) composition were austenitized, cooled and held isothermally for up to 180 min at 650 °C and 600 °C, which are relevant to industrial coiling temperatures after hot rolling. Vickers hardness and nanoindentation measurements revealed that the SS exhibits higher bulk hardness and higher ferrite-matrix strength in each condition. Scanning Transmission Electron Microscope (STEM) observations of the SS samples showed Cu precipitates that preferentially formed at cementite interfaces, which was associated with Cu segregation at the cementite/ferrite interface, as revealed by atom probe tomography (APT). At 600 °C, Cu precipitates appeared at ferrite grain boundaries in the samples after 5 mins hold, and also became clearly visible in the ferrite matrix in the samples after 60 min and 180 min holds. Meanwhile, at 650 °C, precipitates were barely detected in either the ferrite matrix or ferrite boundaries. It was estimated that after 180 min hold at 600 °C, Cu precipitates contributed approximately 15–21 MPa, while Cu solutes contributed 18 MPa and Ni/Sn solute contributed 19–25 MPa to the ferrite strengthening. These findings illustrate the heterogeneous precipitation behaviours and the strengthening from impurities in commercial low-carbon steels produced with a high scrap content. • Scrap-level impurities added to mimic effects of recycled steel use. • Cu precipitates form preferentially at cementite/ferrite interfaces. • APT reveals Cu segregation at interfaces and depletion in cementite. • Cu, Ni, Sn solutes and Cu precipitates strengthen ferrite matrix. • Provides insights into impurity elements behaviour in recycled steels.