Influence of B and Ti on hot ductility of high Al and high Al, Nb containing TWIP steels

Abstract

The addition of ∼0·002B and ∼0·04Ti as microalloying additions to improve the poor hot ductility and high risk of cracking on continuous casting of high Al containing twinning induced plasticity (TWIP) steels has been examined. Tensile specimens were either cast in situ or heated to 1250°C before cooling at 60 K min−1 to test temperatures in the range 700–1100°C and strained to failure at 3×10−3 s−1. For tensile specimens reheated to 1250°C, the presence of B with sufficient Ti to combine with all the N improved ductility over the temperature range of 700–950°C, the reduction in area (RA) values being >40. For the higher strength more complex high Al, TWIP steels having Nb present, there was no improvement in ductility with a similar B and Ti addition, when the average cooling rate after melting to the test temperature was 60 K min−1. Reducing the cooling rate to 12 K min−1 resulted in the RA values being close to the minimum required to avoid transverse cracking throughout the temperature range 800–1000°C. Using these additions of B and Ti, transverse cracking was found not to be a problem when continuously casting these high Al containing TWIP steels.

Hot ductility of high Al TWIP steels containing Nb and Nb-V

“…The work has shown that a low S and P content (preferably ~0.005%), by reducing the volume fraction of sulphides [5,7] and in the case of P preventing the low melting point iron phosphide phase forming, gives rise to better ductility [7]. More importantly, it is found that a small addition of B is needed so it can segregate to the boundaries and strengthen them [6]. In order for this to occur, the B must remain in solution and be protected by adding Ti to combine with all the N so that BN is prevented from forming.…”

“…The cooling rate in previous work [3,5,8] after reheating has generally been 60 o C/min, which is higher than the recommended cooling rate of ≤25 o C/min needed to coarsen the TiN particles to obtain the maximum ductility or to give the maximum segregation for B to strengthen the grain boundaries [13]. In this exercise two cooling rates, 12 o C/min min and 60 o C/min have been examined.…”

“…However, if a higher strength level is required ~1000MPa, then Nb needs to be added [8]. Nb additions have, unfortunately, been found to give rise to poor ductility in peritectic C, HSLA steels (~0.1-0.2%C) making it more difficult to avoid transverse cracking [15,16].…”

“…Previous work [8] has chosen a high Ti/N ratio of 6-7:1 so that typically the Ti level would need to be 0.06% in a 0.01%N steel. However, as long as the stoichiometric composition for TiN is attained (Ti/N ratio 3.4:1) all the N can be taken out of solution as TiN, enabling B to segregate un-impedingly to the boundaries.…”

“…Al is particularly favoured because of its ability to delay fracture in deep drawn products [1,2]. However, these high Al, TWIP steels have been found difficult to continuous cast and considerable work has been carried out in defining the optimum composition for reducing the likelihood of transverse cracking occurring during the straightening operation [3][4][5][6][7][8]. The work has shown that a low S and P content (preferably ~0.005%), by reducing the volume fraction of sulphides [5,7] and in the case of P preventing the low melting point iron phosphide phase forming, gives rise to better ductility [7].…”