Cu-containing TRIP steels

摘要

For environmental reasons, electric furnaces are more and more used for the steel production. This implies the recycling of scrap containing high levels of tramp elements like Cu and Ni. In order to valorise this scrap, the CRM proposes to turn the tramp elements to advantage, in particular for the production of TRIP steels. Indeed, copper, like silicon, seems to prevent the formation of carbides during bainitic transformation. The replacement of Si by Cu could allow the production of TRIP steels without the numerous drawbacks of Si (at high temperature, formation of Fe_2SiO_4 which melts down in the heating furnaces, surface defects on the hot strip, galvanisation problems...). Moreover, Cu has other advantages: it improves the corrosion resistance of steel and it generates a steel hardened by precipitation, solid solution or by ferrite grain size refinement. The effect of Cu associated with Ni has been extensively studied on steels of various chemical composition. These steels have been hot rolled in laboratory. After pickling, the hot strips have been cold rolled and annealed with a cycle which has been optimised for the production of TRIP steels. Very interesting mechanical properties have been obtained with the Cu and Ni-containing steel: a tensile strength up to 1000 MPa with a total elongation of 20%. The microstructure of the Cu and Ni-containing steel is a very fine and complex one composed of three phases: bainite, ferrite and retained austenite.