Effect of YAG Laser Cutting on Stretch-flangeability of Ultra High Strength TRIP sheet steels

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  • 超高強度TRIP鋼の伸びフランジ性に及ぼすYAGレーザ切断の影響
  • チョウコウキョウド TRIP コウ ノ ノビ フランジセイ ニ オヨボス YAG レーザ セツダン ノ エイキョウ

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The transformation-induced plasticity (TRIP) of retained austenite is very useful to enhance the press formability of ultra high strength sheet steels. The TRIP steels with different matrix structure and different retained austenite morphology have been developed for weight reduction and shock safety up to now. The conventional TRIP-aided dual-phase steel or "TRIP-aided polygonal ferrite (PF) steel" composing of polygonal ferrite matrix and blocky retained austenite plus bainite second phase possessd an excellent stretch-formability and deep drawability. We have recently developed new type of TRIP steel or "TRIP-aided annealed martensitic (AM) steel" composing of bainitic ferrite matrix by annealed martensite matrix, and "TRIP-aided bainitic ferrite (BF) steel" composing of bainitic ferrite matrix and interlath retained austenite films. It is supposed that carbon addition is effective to rise the tensile strength of TRIP steel. So, in the present study the effect of YAG laser cutting on the stretch-flangeability of 0.1-0.6C-1.5Si-1.5Mn TRIP steels was investigated. The strength-stretch-flangeability balance (TS×λ) of holes obtained by either laser cutting and hole-punching decreased with increasing carbon content in PF, AM and BF steels. When the carbon content of 0.3 mass% or higher, the λ value in the case of laser cutting decreased to a level comparable to that in the case of hole-punching in PF steels. On the other hand, we demonstrated that YAG laser cutting contributes to the improvement of the stretch-flangeability of 1200 MPa class BF steels with 0.4 mass%.

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