Abstract
Recent studies showed that, during stamping of high strength steels at industrially relevant production rates, local temperature in the blank may rise up to 200°C – 300°C due to deformation heating. Moreover, die temperature may also rise up to 100°C – 150°C for progressive stamping dies. Based on the common assumption that the blank softens as the temperature increases, thermal softening creates a margin in Forming Limit Diagram (FLD) and therefore the FLD determined at room temperature can safely be used for those cases. In this article, the validity of this assumption on DP590 steel is questioned by high temperature tensile tests (RT - 300°C) at various strain rates (10-3 s-1 – 1 s-1). The results indicated a decrease both in uniform and total elongation in 200°C – 300°C range together with several other symptoms of Dynamic Strain Aging (DSA) at all strain rates. Concurrent with the DSA, the simulated FLD confirms the lower formability at high temperature and strain rates. Thus, it is concluded FLD determined at RT may not be valid for the investigated steels.
Highlights
Dual phase (DP) steels have been continuously used in automotive industry for decades
The tensile tests performed within the scope of this study clearly reveal that the formability of DP590 steels decreases as the temperature approaches to the 200 - 300 oC range, which can be accessed in the industrial practice
The formability decrease is evident from the flow curves of DP590, as the flow stress increases and the ductility decreases with the increasing temperature (Figure 6)
Summary
Dual phase (DP) steels have been continuously used in automotive industry for decades. Forming limit curve (FLC) is the major tool in determination of formability of sheet blanks. In addition to the FLC, flow curve and yield surface are the other major inputs for the simulation sheet metal forming processes.
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