BAKE HARDENABILITY OF COLD-ROLLED TI-BEARING EXTRA LOW-CARBON STEEL SHEETS DUE TO PRECIPITATION CONTROL

Bake hardenability (BH) of cold rolled titanium-bearing extra low carbon steel sheets has been investigated by examining the precipitation behavior of sulfides and titanium-compounds in hot bands with varying sulfur and manganese content. In high sulfur steels (0.0050mas%) without manganese, Ti4C2S2 mainly precipitated in hot bands, while through the addition of manganese MnS was promoted to precipitate before the Ti4C2S2 precipitation and consequently titanium-compounds mainly consist of TiN and TiC in hot bands. On the other hand, in low sulfur steels (0.0015mass%) TiN and TiC mainly precipitated independenly of manganese addition. As the solubility temperature of Ti4C2S2 is higher than that of TiC, in an usual continuous annealing temperature range, Ti4C2S2 does not dissolve and consequently carbon in solution does not exist while carbon is dissolved from TiC during annealing, which provides the bake hardenability for annealed sheets. Through the precipitation control the BH of 30MPa is achieved for titanium bearing extra low carbon steel sheets with an elongation of greater than 50% and a r-value of higher than 2.0 after annealing.