Process for manufacturing compact coils of ultra-fine grained, martensite-free steel bars

A process for manufacturing compact coils of ultra-fine grained, martensite-free steel bars having a microstructure with an actual grain size equal to or higher than 9 according to standard ASTM E112-13, and wherein a difference of hardness (HV) measured between surface and core of the steel bar is less or equal than 40 HV, the process comprising the following stages:

• a) rolling a steel billet with an initial surface temperature of 850-1200° C. by means of a roughing rolling mill producing a steel bar;
• wherein the steel billet is a billet of low or medium carbon steel consisting of, in weight percentage, carbon in a range of 0.20-0.25%, silicon in a range of 0.20-0.70%, manganese in a range of 0.80-1.30%, possible vanadium in a range of 0.020-0.050%, the remaining being iron and unavoidable or possible impurities;
• b) performing at least one first cooling stage so that the steel bar has a surface temperature higher than the martensite start temperature (Ms), and performing at least one first equalization stage in air to minimize a difference of temperature between core and surface of the steel bar until reaching the surface temperature in a range of 850-920° C.;
• c) rolling the steel bar by means of at least one intermediate rolling mill;
• d) performing at least one second cooling stage always maintaining the surface temperature higher than the martensite start temperature (Ms), and performing at least one second equalization stage in air to minimize the difference of temperature between core and surface of the steel bar until reaching the surface temperature in a range of 700-900° C.;
• e) rolling the steel bar by means of a finishing rolling mill in a non-recrystallization temperature range, maintaining a whole cross-section of the steel bar within said non-recrystallization temperature range, and with a total reduction between 25 and 50% with respect to a cross-section of the steel bar at an entry of the finishing rolling mill, in order to obtain an ultra-fine-grained austenitic matrix;
• f) winding the steel bar in a compact coil, by means at least one spooling device, at a winding temperature in a range of 500-800° C. so that the ultra-fine-grained austenitic matrix transforms in a mixture of ferrite and pearlite.