9th ECCC European Continuous Casting Conference – ECCC 2017

TWIP (Twining Induced Plasticity) steel is very promising AHSS (Advanced High Strength Steel) grade owing to its superior toughness and ductility. Recently it has attracted the interest of the automotive and steelmaking industries, as the need for reducing weight to provide better fuel efficiency is of paramount importance with the gradual depletion of fuel resources. But these steels are difficult to continuous cast and cracking can occur at the slab surface. Therefore it becomes very important to gain an understanding of the cause of this cracking, in order to prevent their occurrence. In order to gain a better understanding of the cracking propensity in high Al TWIP steel slabs(1~1.5%Al), conventional hot tensile tests were performed to simulate the continuous casting process on a variety of TWIP steels in order to determine the influence of such factors as chemical composition, cooling rate and thermal cycle on hot ductility. Using a cooling rate of 60K/min after heating to 1250oC, ductility was generally <40% RA(Reduction of Area) indicating that with these high Al TWIP steels it will be difficult to avoid transverse cracking. The 1.5%Al containing steels had worse ductility than the low Al containing steels(0.02%Al) because of the presence of large amounts of AlN precipitated at the austenite grain boundaries. Higher strength Nb/V high Al containing TWIP steels were also examined although ductility was likely to be worse than the simpler microalloying free TWIP steels. Increasing the cooling rate from 60 to 180K/min after melting caused the ductility to further deteriorate and high N levels produced only a small reduction in the ductility, probably because ductility is so poor. Increasing the S level from 0.003 to 0.023% caused the ductility to deteriorate in TWIP steels free of microalloy. The worse ductility in the higher S steels was found to be not caused by a simple increase in the sulphide volume fraction but more a consequence of the change from coarse hexagonal plate AlN, which are mainly within the matrix and so have little influence on the hot ductility, to very long dendritic rod precipitates, which are situated at the dendritic or close to the austenite grain boundaries. This dendritic precipitation was rarely observed in the low S steel. The MnS inclusions appeared to act as nucleation sites for the precipitation of AlN. The influence of P in the range ~0.01 to 0.07% with high Ti and N additions on the hot ductility of 1.5%Al, boron treated TWIP steels has been examined. P even at the 0.02% level has a small detrimental influence on the hot ductility and ductility decreases progressively as the P content is increased. Low melting point Fe(Mn) phosphide phases were found at the austenite grain boundaries accounting for this deterioration in ductility. As it is difficult to cast these steels, without cracks forming, P levels should be as low as possible, preferably ~0.01%. The ductility of Nb containing high Al, TWIP steels was very poor in the as-cast condition. Adding B and Ti still gave rise to extremely poor ductility when a cooling rate of 60K/min was used but reducing it to 12K/min caused the ductility to improve so that RA values were now close to the 35~40% RA value required to avoid transverse cracking. Both 0.04%Ti and 0.002%B are required to ensure good hot ductility in high Al,TWIP steels. Sufficient Ti is needed to remove all the N as TiN so preventing AlN precipitating as films over the austenite grain surfaces. B is also needed as it can segregate to the boundaries and strengthen them. A SIMS technique confirmed that B had indeed segregated to the boundaries. The slower cooling rate 10~15K/min compared to 60K/min will result in the optimum segregation of B as well as coarsening the TiN precipitates so they are no longer effective in reducing the ductility. Following all these recommendations, i.e. a low S level, slow secondary cooling rate, a Ti level above the stoichiometric for TiN and a boron addition of 0.002%, transverse cracking was avoided commercially in these very difficult to cast high strength TWIP steels