Effect of Microstructure on Stretch-flangeability of Ferrite-bainite Dual Phase Steel

The relationship between microstructure and the stretch-flangeability of F+B dual phase steels had been studied by observing the microstructure of F+B dual phase steels with different hole expansion ratio.When the microstructure consists of polygonal ferrite and granularbainite,the DP steel exhibited excellent stretch-flangeability. Polygonal ferrite is beneficial to the hole expansion ratio,while lower bainite and lath bainite play negative roles. The M-A islands in bainite have a great effect on the stretch-flangeability. As the size of M-A islands decrease,the hole expansion ratio improves. When the width of M-A islands is 0.7 μm,the hole expansion ratio of F+B steel reaches up to 120%.