Hot ductility of TWIP steels

Abstract

The hot ductility of twin induced plasticity (TWIP), 0·6 wt-C steels containing 18–22 wt-Mn with N levels in the range 0·005–0·023 wt- and the Al additions either low (<0·05 wt-) or high (1·5 wt-) has been examined. Little change in ductility occurred in the temperature range 1100–650°C as the structure was always fully austenitic. Ductility was generally poor (<40), reduction of area values, the best ductility at the higher Al level being given by the steel with the lowest N and S levels. Because the steel is fully austenitic, the ductility is solely dependent on that for unrecrystallised austenite. Therefore, to avoid transverse cracking the volume of second phase particles should be kept to a minimum, i.e. the N should be low to reduce the amount of AlN that can be precipitated out and the S level should be as low as possible to limit the amount of MnS inclusions. Metallographic and TEM studies were carried out and the poor ductility was found to be due to extensive precipitation of AlN at the austenite grain boundaries. Increasing the cooling rate from the melting point to the test temperature from 60 to 180°C min−1 or introducing an undercooling step both led to even worse ductility.

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

“…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.…”

“…Previous work [3,5,7] has concentrated on the simpler high Al, TWIP steels giving a room temperature yield strength of 800MPa. However, if a higher strength level is required ~1000MPa, then Nb needs to be added [8].…”

“…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].…”

Mechanical properties and deformation behaviour of Fe-(20/27)Mn-4Al-0.3C austenitic steels

“…20Mn in present work should be ascribed to the larger amount of AlN inclusions and less addition of Mn solute [16]. The AlN impurity is mainly produced by the higher N content in steel making, and prefers to distribute along the rolling direction during the hot rolling process.…”

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

“…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.…”