Influence of B on hot ductility of high Al, TWIP steels

Abstract

The influence of B on the hot ductility of high Al, Ti containing twinning induced plasticity (TWIP) steels has been examined. It was established that provided the B was fully protected by adding sufficient Ti to combine with all the N, then B could segregate to the austenite grain boundaries and improve ductility. This improvement was particularly marked for the temperature range of 700–900°C, the range in which the straightening operation often takes place in continuous casting. Of most importance in the present work has been the detection of B at the boundaries using a secondary ion mass spectrometry technique. The cooling rate from the reheating temperature of 1250°C to the tensile testing temperature range of 700–1200°C was 60 K min−1, but it is likely that slower cooling rates ≤25 K min−1, more in keeping with the secondary cooling rate on continuous casting, will give even better ductility. Ti additions in themselves are beneficial to the hot ductility of these steels as precipitation of AlN at the austenite boundaries is avoided, but only if the cooling rate is sufficiently slow to allow the TiN particles to coarsen. However, to ensure freedom from cracking, an addition of B is also required.

Effect of Ti and B microadditions on the hot ductility behavior of a High-Mn austenitic Fe–23Mn–1.5Al–1.3Si–0.5C TWIP steel

“…Thereby, because of the non-equilibrium segregation of boron at grain boundaries during the steel solidification, formation of the second-phase precipitates is promoted, whose nature may be as compounds of the type M 23 (B,C) 6 , Fe 2 B, M 3 (B,C) [32,57] and even C x B y , localized both inside and at grain boundaries of the austenite. Similar results were obtained in the research work of Kang et al [28] in TWIP steels containing different boron concentrations. They argued the improvement in the hot ductility at the temperature range of 700-800°C is associated with the presence of BN and Fe 23 (B,C) 6 at grain boundaries.…”

“…Actually, few work has been done about the study of the hot ductility behavior of TWIP steels [21][22][23][24][25][26][27][28], taking into account various experimental parameters, where the most significant is the microstructural conditioning before tensile testing. Generally, the results of these research works have shown a decrease of the hot ductility in the temperature range of 700-1000°C due to the AlN precipitation and the high Mn content (i.e.…”

“…Additionally, boron facilities the hot plastic flow due to a solute drag effect by boron atoms on the austenitic grain boundaries and also to a solid solution softening effect. According to Mintz et al [16] and Kang et al [24,28] there is a need to find ways of improving the hot ductility of TWIP steels to make them more amenable to casting, and this is where adding Ti with B might be a solution. The Ti level should be high enough to ensure that the entire N is combined as TiN and that B is in solid solution, able to migrate to the boundaries.…”

“…Their most important transverse cracking. More recently, Kang et al [28] have carried out a detailed secondary ion mass spectrometry (SIMS) characterization in Ti and B containing TWIP steels. The SIMS images at the temperature range of 700-900°C indicate that boron segregates to grain boundaries [37], results which greatly agree with the improvement in hot ductility at the same temperature range.…”

Solidification Structure, Non-metallic Inclusions and Hot Ductility of Continuously Cast High Manganese TWIP Steel Slab

“…7) P. Lan et al have analyzed the solidification microstructure, solidification defects, thermophysical properties and hot ductility of Fe-22Mn-0.7C TWIP steel ingot. [8][9][10][11][12] S. E. Kang et al [13][14][15][16][17][18] have investigated the influences of composition and AlN on the hot ductility of high aluminum TWIP steels, which were produced by 50 kg vacuum induction furnace. I. Mejía, 19) A. E. Salas-Reyes 20) and A. S. Hamada 21) have studied the hot ductility behavior of high-manganese aus-tenitic TWIP steels.…”