Improvement of formability of continuous organic-coated coils by controlling ageing behaviour of low carbon steel substrates

The objective of this project was to define low C steels and process conditions to manufacture continuous 'coil-coated' coils, with low susceptibility to strain ageing and uniform mechanical properties (MP), exploring the possibility of using hot coils rolled both in the austenitic and in the ferritic fields, with different chemical compositions. Industrial trials were performed to investigate the behaviour of ferritic (ELC-F) and austenitic hot rolled coils (ELC-AUS) in terms of MP after organic coil-coating (OCC) process. The results show differences in terms of ageing resistance between the two hot-rolling procedures in austenitic and ferritic fields: the ELC-F coils exhibit a yield point elongation after OCC of around 1-2 %, whereas the ELC-AUS coils show a yield point elongation >10%. Due to the use of high coiling temperatures, around 720°C in hot rolling mill, a study on scale morphology was performed: there was no significant difference in frequency and type of scale defects between the ELC-F and the ELC-AUS hot-rolled coils. The laboratory simulations of the industrial process confirmed the good results of the industrial trials regarding the ELC-F steel and screened other steels with P and B additions. The conclusions of the project were that the aimed coils can be produced with a standard ELC-grade, an optimised ferrite rolling strategy (FRT≈770°C and CT≈715°C) combined with an optimal soaking temperature in the galvanising line (≈750°C) and a conventional total SP elongation of 2-2.2 %. Boron is a very interesting element for this steel grade. If the ratio B/N is not too far from 1, the product aimed at could be obtained not only by means of ferritic rolling but also by means of conventional austenitic rolling.