The effect of microstructure on the sheared edge quality and hole expansion ratio of hot-rolled 700 MPa steel

Abstract

Abstract. The effects of microstructure on the cutting and hole expansion properties of three thermomechanically rolled steels have been investigated. The yield strength of the studied 3 mm thick strip steels was approximately 700 MPa. Detailed microstructural studies using laser scanning confocal microscopy (LCSM), FESEM and FESEM-EBSD revealed that the three investigated materials consist of 1) single-phase polygonal ferrite, 2) polygonal ferrite with precipitates and 3) granular bainite. The quality of mechanically sheared edges were evaluated using visual inspection and LSCM, while hole expansion properties were characterised according to the methods described in ISO 16630. Roughness values (R a and R z ) of the sheet edge with different cutting clearances varied between 12 μm to 21 μm and 133 μm to 225 μm, respectively. Mean hole expansion ratios varied from 28.4% to 40.5%. It was shown that granular bainite produced the finest cutting edge, but the hole expansion ratio remained at the same level as in the steel comprising single-phase ferrite. This indicates that a single-phase ferritic matrix enhances hole expansion properties even with low quality edges. A brief discussion of the microstructural features controlling the cutting quality and hole expansion properties is given.

Design and Development of Complex Phase Steels with Improved Combination of Strength and Stretch-Flangeability

“…Thus, the formation of granular bainite should be avoided in order to achieve better HER. Two recent studies [34,35] reported that large precipitates such as micrometric TiN or cementite can act as the primary source of void nucleation in complex phase steels, which deteriorates HER. Large TiN are also known to deteriorate the toughness [16].…”

Strain evolution during hole expansion testing of 800 MPa tensile strength hot-rolled steels

“…In addition to punch shape and hole edge condition, microstructure of the material is also known to have effects on the hole expansion. For example, high hardness difference between ferrite and martensite phases [6] and secondary constituents, such as inclusions and cementite [7], have been reported to decrease HER results.…”