Actual multiphase steels guarantee tensile strengths in different grades between 500 and 1000 MPa. Specifications and standards permit a wide field of chemical compositions. On one hand, carbon amounts higher than 0.12% are not seldom for grades exceeding 600 MPa tensile strength. However, frequently encountered difficulties during body manufacturing, due to limited formability, like enhanced crack formation at trim edges, increased springback, or deteriorated fracture behavior of weld joints are typically related to unfavorable microstructures. On the other hand, a strict limitation of the carbon content to below 0.1 wt % across all strength classes up to 1000 MPa allows avoiding the mentioned manufacturing difficulties. Carbon reduction as well as grain refinement and precipitation hardening by means of Nb-microalloying and solid solution hardening through further addition of manganese, silicon, chromium, or molybdenum are the major key factors on metallurgical site. A more homogenous combination of ferrite and martensite with certain amounts of bainite is adjusted by heat treatment to defuse the critical interfaces to increase forming properties measured by bending angle and hole expansion.

Abstract. In the scope of the optimization of multi phase steels, e.g. for the automotive industry, control of the microstructure is essential to tailor the mechanical properties. In this study, two cold rolled steels varying in carbon content were annealed and cooled under different laboratory conditions. The microstructure is investigated using optical and electron microscopy and EBSD. The results are correlated to the mechanical properties obtained from tensile, hole expansion and bending test. It is found that tensile strength and elongation are mainly dependent on martensite volume fraction, while yield strength is less affected by chemical composition or annealing treatment. In contrast, hole expansion capacity and maximum bending angle are significantly improved by the homogenization of the microstructure which is independent of strength and elongation. The microstructure homogeneity is expressed by analyzing the Lorenz curves derived from the kernel average misorientation from EBSD measurements.