The main goal of this paper is to obtain a hot-rolled steels having tensile strength of 98OMPa with good formability such as stretch-flangeability using low temperature transformation microstructure. The currently developed hot-rolled steels having 78OMPa and even to 98OMPa with superior stretch-flangeability were ferrite-based steels. However, it was difficult to sufficiently attain 98OMPa of tensile strength and it had limited on the future development of high strength steels up to 1,18OMPa. Moreover, the currently reported researches on crack initiation and propagation were not obvious after hole expanding test. Therefore,
to achieve a high strength over 98OMPa and a excellent stretch-flangeability, this study was performed using low temperature transformation microstructure and to know the clear fracture behavior, and this study was conducted by analysing the crack generation and propagation path. In order to obtain balanced mechanical properties, a steel of composition Fe-Cr-Ti-Nb-B was developed for hot-rolling. After the rolling and cooling, the steels were respectively manufactured by coiling for 1 hour at 430, 450, and 470 。C. The final microstructure of hot-rolled steels was composed of tempered martensite matrix and granular bainite. The ultimate tensile strength 998MPa, 11% total elongation, and 59% of good hole expansion ratio were achieved when increased from 430 to 470 。C, the fraction of tempered martensite decreased and that of granular bainite oppositely increased. Thus, the hole expansion ratio (HER) increased. Furthermore, as the hardness deviation of tempered martensite and granular bainite decreased, the HER value increased. As the fraction of high angle grain boundary increased, the HER value increased due to the delay of the crack propagation. As the crack propagation path was dispersed on the second fracture surface and significantly deflected, the HER value increased. After the hole expanding test, the generated shape of fracture was mixed with dimple fracture and quasi-cleavage fracture. As the dimple fracture of ductile increased, the HER value increased. The point of main crack generation was observed at the interface between tempered martensite and granular bainite having large hardness deviation and elastic modulus. So, dislocation concentration occurred at the interface of two phases during plastic deformation. And the generated crack was
propagated along the transgrani끄ar. Consequently, 98OMPa grade hot-rolled steels composed of tempered martensite and granular bainite achieved the same level of HER by using martensite matrix steels as compared with ferrite matrix steels. For high strength steels with superior HER, the phase fraction, hardness deviation, and high angle grain boundary were acknowledged as the important factors to increase the stretch-flangeability.