Microalloyed Low-Carbon Multiphase Steels

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 spring-back, 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.