Abstract
The current work explores the strain hardening and stretches formability behaviour of the developed Triple Phase (TP) steel. Double quenched TP steel strips posse three distinguished stages of strain hardening on tensile forming. 1st stage represents the highest n-value reflecting resistance to homogeneous deformation, where steel can be safely stretched. 2nd and 3rd stage reveals lower n-values, where localized thinning exist. On Erichsen testing, the relationship between punch forming force and punch stroke exhibits two forming regions. The 1st region is delineated by a straight line showing an ultra-high strain-hardening rate, which represents a reversible elastic stretch forming. The 2nd forming region continues to a higher Erichsen punch stroke than that of the 1st region and presents the permanent plastic stretch forming behaviour. It is found that bainite and martensite clusters created, by double quenching, in TP-steel exaggerated the elastic stretch forming limit 10 times higher than the as-hot rolled condition. 7 min. holding time of strips in the salt bath is considered the most effective for the creation of a useful volume fraction of the bainite phase. However, 21 min. holding time in salt bath grows martensite laths through the bainite aggregates, affecting negatively on stretch formability.
Highlights
Triple phase (TP) steels have a combination of favourable properties such as high tensile strength, high strain hardening rate at early stages of plastic deformation combined with a reasonable ductility
The 2nd forming region continues to a higher Erichsen punch stroke than that of the 1st region, which would encourage the automotive manufacturer to use the current Triple Phase (TP)-steel in press-formed complicated-shape-parts, such as seat frames or B-pillars, to cope with the growing demand for the automobiles weight reductions [6]
Double quenched TP-steel strips show three stages of strain hardening on tensile forming. 1st stage reveals the highest nvalue reflecting resistance to homogeneous deformation, where steel can be safely stretched. 2nd and 3rd stages possess lower n-values, where localized thinning would exist
Summary
Triple phase (TP) steels have a combination of favourable properties such as high tensile strength, high strain hardening rate at early stages of plastic deformation combined with a reasonable ductility. These properties are related to the special microstructure of the TP steel in which, the soft ferritic network provides a reasonable ductility; while bainite and martensite phases play the load-bearing role. High initial strain hardening rate results in a high n-value, which provides better resistance to local thinning under the forming conditions of drawing and stretching [2, 3] Properties such as continuous yielding behaviour, uniform plastic deformation and reasonable elongation are important features of TP- steels.
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