DP 590, 780, 980. Ultra fine ferrite grain will accommodate less strain compared with coarser one, will bimodal distribution helps to improve HER ? The aspect ratio is ferrite is more than 3 times near the hole or ID after HER.
What is hole expansion resistance? - Course grains accommodates more slip since there are more space for dislocations to get packed. At the same time, you also build up the stress concentration by piling up of dislocations. There is a balance between the slip, crack initiation, and slip transmission to the next grain. In bimodal distribution, the chances of encountering larger grain next to small grain and vice versa helps to disperse the stress concentration. In principle the rule of mixtures gives some approximate answer, the distribution of the grains are uniform.
Dear Kuntimaddi,
Thanks for your answer. I am sorry for the delayed response. Hole expansion test (shear edge ductility) is the most commonly used method to evaluate the Advanced High Strength Steel (AHSS) sheet edge cracking resistances.
"Modern methods of manufacture of automobile components such as wheels, suspension parts and structural components using sheet metal, mainly steel and possibly other metals, involve, primarily, shearing, bending and stretch drawing operations.Included with these processes are the bending up (plunging) of flanges (rims) around punched holes, and this can result in rupture of the material.Various test methods are available to establish the suitability of the sheet metal for the forming processes involved. The hole expanding test is one of the best methods for evaluating the suitability of the sheet metal for forming such “flanges” because it closely resembles the process used under production conditions to form such flanges (plunged rims) starting with punched holes."
During the test, crack will initiate mostly at ferrite or at ferrite- martensite interface in dual phase (DP) steels due to plastic-instability. DP steel is one of the AHSS, which is used for weight reduction in cars. I believe that bimodal distribution of Ferrite in DP will have a significant impact on HER for automotive body in white applications.
Will bimodal grain size distribution in DP steel improve the hole expansion ratio (HER) /stretch flangeability? - ResearchGate. Available from: https://www.researchgate.net/post/Will_bimodal_grain_size_distribution_in_DP_steel_improve_the_hole_expansion_ratio_HER_stretch_flangeability [accessed Sep 16, 2016].
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