The effect of microalloying on the sheared edge ductility of ferritic steels

Abstract

Sheared edge ductility plays a critical role in many automotive and structural applications. The effect of microalloying elements on the microstructure, mechanical properties, sheared edge quality and hole expansion properties of six thermomechanically laboratory hot rolled and coiled, approximately 4 mm thick, precipitation hardened ferritic steels have been investigated. The studied V-microalloyed steels had low carbon contents, 0.05 and 0.09 wt.%, and, in some cases, microalloying additions of either Nb or Nb+Ti. Yield strengths were in the range 500 MPa to 750 MPa and hardness values between 176 and 260 HV10 depending on the carbon content and level of precipitation strengthening. Detailed microstructural analysis using laser scanning confocal microscopy (LCSM), FESEM and FESEM-EBSD revealed that all the steels consisted of mainly single-phase ferrite. FESEM analysis revealed the presence of relatively coarse precipitates at grain boundaries and inside some of the ferrite grains, but the number density of visible precipitates varied greatly between grains. The addition of Nb or Nb+Ti reduced the ferrite grain size, especially the size of the largest grains as defined by the 90th percentile in the cumulative grain area distribution (d90%). Mean hole expansion (HE) ratios varied from 39 % to 76 % with the highest values associated with the lowest carbon contents. Despite raising the strength, microalloying with Nb or Nb+Ti had little if any detrimental effect on HE ratios compared to the V-microalloyed variants without Nb or Ti, presumably as a result of the grain refinement. Titanium nitrides acted as void nucleation sites close to the sheared edge. Microalloying with Nb and Nb+Ti led to a decrease in the depth of the shear affected zone (SAZ) and a smaller difference in hardness between the base material and the SAZ.