The hot ductility behaviour of four high-Mn TWIP steels has been studied by hot tensile testing in the temperature range 700–1300 °C at a strain rate of 1 s −1 . For all the studied steels with the single-phase austenitic structure, the hot ductility based on the reduction of area of tensile strained specimens is good. In the temperature range 1000–1200 °C, dynamic recrystallization takes place and consequently the ductility is enhanced. Grain refinement by reheating at the lower reheating temperature (1150 °C) significantly improves the hot ductility through inducing faster dynamic recrystallization. During high temperature deformation, 1.5 wt.% of Al alloying is found to have a beneficial effect on the hot ductility due to strengthening of the steel, whereas the 0.019 wt.% Nb alloying has insignificant effect. With high Al (3 wt.%) and Si (3 wt.%) alloying, the formation of ferrite phase at high temperatures along the austenite grain boundaries has the most deleterious effect on the hot ductility behaviour. Grain boundary sliding was recognized as the failure mechanism in the studied steels.

Hot ductility behaviour of high-Mn TWIP steels

The current study analyses the influence of boron contents (between 29 and 105 ppm) on the hot ductility of boron microalloyed steels. For this purpose, hot tensile tests were carried out at different temperatures (700, 800, 900 and 1000 °C) at a constant true strain rate of 0.001 s −1 . In general, results revealed an improvement of the hot ductility of steels at increasing boron content. At 700, 900 and 1000 °C the ductility is higher than at 800 °C, where boron microalloyed steels exhibit a region of ductility loss (trough region). Likewise, dynamic recrystallization only occurred at 900 and 1000 °C. The fracture surfaces of the tested steels at temperatures giving the high temperature ductility regime show that the fracture mode is a result of ductile failure, whereas it is ductile–brittle failure in the trough region. Results are discussed in terms of dynamic recrystallization and boron segregation towards austenite grain boundaries, which may retard the formation of pro-eutectoid ferrite and increase grain boundary cohesion.

Hot ductility behavior of boron microalloyed steels

This research work studies the influence of microalloying elements (Ti and V) and the solidification route on the hot ductility behavior of high-manganese TWIP steels. Uniaxial hot tensile tests in the temperature range of 700–1100 °C under a constant strain rate of 10 −3  s −1 were carried out. Hot ductility as a function of reduction of area (RA) showed a significant improvement in the V-microalloyed TWIP steel, when compared to a non-microalloyed TWIP steel with a similar composition, in the intermediate temperature range of 800–900 °C. The highest value of 86% RA is attributed to the onset of dynamic recrystallization (DRX) near to the fracture tip. On the other hand, Ti addition to TWIP steel did not exhibit any improvement on the hot ductility, resulting in the worst hot ductility behavior, with a maximum value of 34% RA. The TWIP steels solidified in metallic ingot molds (MM) showed higher peak stress ( σ p ) and ductility values than the sand mold (SM) cast ingots at low and intermediate temperatures, respectively, which is associated with the finer microstructure generated during solidification. Grain boundary sliding was recognized as the failure mechanism associated with second-phase particles precipitated at the grain boundaries, which play the role of nucleation and propagation sites of void-cracks.

Hot ductility behavior of high-Mn austenitic Fe–22Mn–1.5Al–1.5Si–0.45C TWIP steels microalloyed with Ti and V

The current study analyses the influence of boron addition on the hot ductility of a low carbon advanced high strength NiCrVCu steel. For this purpose hot tensile tests were carried out at different temperatures (650, 750, 800, 900 and 1000 °C) at a constant true strain rate of 0.001 s −1 . Experimental results showed a substantial improvement in hot ductility for the low carbon advanced high strength steel when microalloyed with boron compared with that without boron addition. Nevertheless, both steels showed poor ductility when tested at the lowest temperatures (650, 750 and 800 °C), and such behavior is associated to the precipitation of vanadium carbides/nitrides and inclusions, particularly MnS and CuS particles. The fracture mode of the low carbon advanced high strength steel microalloyed with boron seems to be more ductile than the steel without boron addition. Furthermore, the fracture surfaces of specimens tested at temperatures showing the highest ductility (900 and 1000 °C) indicate that the fracture mode is a result of ductile failure, while in the region of poor ductility the fracture mode is of the ductile–brittle type failure. It was shown that precipitates and/or inclusions coupled with voids play a meaningful role on the crack nucleation mechanism which in turn causes a hot ductility loss. Likewise, dynamic recrystallization (DRX) which always results in restoration of ductility only occurs in the range from 900 to 1000 °C. Results are discussed in terms of boron segregation towards austenitic grain boundaries and second phase particles precipitation during plastic deformation and cooling.

Hot ductility behavior of a low carbon advanced high strength steel (AHSS) microalloyed with boron

The influence of small additions of titanium on the hot ductility of C–Mn–Nb–Al steels has been examined. Titanium and nitrogen levels varied in the ranges 0·014–0·045 and 0·004–0·011 wt-%, respectively, so that a wide range of Ti/N ratios could be studied. The tensile specimens were cast and cooled at average cooling rates of 25, 100, and 200 K min-1 to test temperatures in the range 1100–800°C and strained to failure at a strain rate of 2 × 10-3 s-1. It was found that ductility in the titanium containing niobium steels improved with a decrease in the cooling rate, an increase in the size of the titanium containing precipitates, and a decrease in the volume fraction of precipitates. Coarser particles could be obtained by increasing the Ti/N ratio above the stoichiometric ratio for TiN and by testing at higher temperatures. However, ductility was generally poor for these titanium containing steels and it was equally poor when niobium was either present or absent. For steels with ∼0·005 wt-%N ducti...

Influence of titanium and nitrogen on hot ductility of C–Mn–Nb–Al steels

The extent to which the hot tensile test can be used to assess the relationship of steel composition to the incidence of transverse cracking in continuous casting has been examined. The influence of S, Ca, Ti, and Nb on the hot ductility of C–Mn–Al steels was investigated for the temperature range 700–1100°C and the results were compared with available commercial data on the effect of these elements on transverse cracking. Three different heat treatments that were carried out before straining have been examined: (i) reheating hot rolled plate to 1330°C; (ii) in situ casting of tensile samples; and (iii) as for (ii) followed by reheating to 1330°C. After these treatments, the samples were cooled directly to test temperature at a cooling rate of 100 K min−l and strained to failure at a strain rate of 2×10−3 s−l. All heat treatments showed that Nb gave the worst ductility in accordance with its known propensity to favour transverse cracking. Reheating hot rolled plate was found to be the simplest and...

R. Abushosha

Papers

Effectiveness of hot tensile test in simulating straightening in continuous casting