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Stretch-flangeability of High Manganese TWIP Steels

Stretch-flangeability of High Manganese TWIP Steels
Chen, Lei
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It is well known that second generation AHSS (Advanced High Strength Steel) TWIP (TWinning Induced Plasticity) steels achieve high strengths (800-1200MPa) and large uniform elongations (45%-60%) in uni-axial tension tests. In sheet deformation operations involving cut edge stretching, however, the deformation properties of TWIP steels are poor. It is usually assumed that it is a combination of (1) an increased tensile strength and (2) a multi-phase microstructure with phases of different hardness, which is the main cause of poor stretch-flangeability. High Mn TWIP steel, however, is a single phase austenitic steel and the reasons for its low HER are therefore not directly obvious. The present contribution is an in depth study of the edge formability of High Manganese fully austenitic TWIP steel. Two types of Hole Expansion (HE) test were carried out: the flat bottom punch HE test and the ISO/TS 16630 conical punch HE test. The punch force, and the sample strain and temperature were continuously monitored, by means of in situ Infrared (IR) thermo-graphy and in situ optical strain analysis. A fully ferritic, Ti stabilized interstitial-free (IF) steel was used as ?Dvery\ formable reference single phase materials. Although TWIP steel showed a much larger uniform elongation in uniaxial tensile tests, the hole expansion properties were less than for the Ti IF steel in both types of HE tests. It was found that the normal anisotropy (r-value), and the strain rate sensitivity (m-value) were critical material parameters in hole expansion. The r-value determines the strain paths of the hole edge during a HE test. A large r-value leads to a better HER. A high and positive m-value reduces the rate of diffuse necking, delays fracture and leads to a better HE performance. In the flat punch HE test, the edge of the hole deformed in the deep drawing mode with a constant strain ratio which corresponded to a constant r-value. The strain path was similar to that of the tensile test throughout the test. The thickness strain was essentially the same across the flat part of the HE specimen, with no radial dependence. In the conical punch HE test, the hole edge of the samples deformed in a combination of drawing and bending deformation. This lead to a complicate strain path for the hole edge during the conical punch HE test. The deep drawing type deformation strain path of the hole edge of a TWIP steel HE sample with a carefully prepared, drilled hole began with a very small strain ratio, i.e. a large r-value. The strain ratio increased gradually to a large value corresponding to an r-value (0.4), which is much lower than the corresponding r-value recorded during the tensile test (0.8). Unlike the flat punch HE process, the thickness gradient of sample from the hole edge to the edge of blank was distinct. The hole edge strain path change caused by the bending process during a conical punch HE test resulted in slightly higher HER values. IR thermo-graphy revealed considerable thermal effects in the case of the TWIP steel in both the flat bottom punch and the conical punch HE tests. Whereas the HE test was essentially isothermal in the case of the IF steel, high temperatures were generated at the edge of the TWIP steel HE sample
a large thermal gradient was formed during the test, with the temperature rising to almost 90 ▲C at the root of the edge crack. The higher temperatures developed at the hole edge are therefore a cause for an earlier failure by the local softening the TWIP steel. The edge characteristics of specimen before and after edge cracking were studied by Electron Microscopy. The fracto-graphy of the edge cracks revealed that in the case of the IF steel the edge of a hole thinned evenly and the cracking was preceded by a considerable amount of diffuse necking. Although the fracto-graphy of the TWIP steel revealed ductile features, the TWIP sample failed in the HE test by a catastrophic crack. The hole edge of a TWIP steel HE specimen cracked without obvious prior diffuse strain localization. A high density of fractured AlN inclusions were found in the Al-added TWIP steels after HE testing. These voids were nucleated and expanded in the vicinity of the cracked AlN particles. The voids formed at fractured AlN inclusion are the most likely initial crack nucleation sites.
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