Study on Hot Ductility of Slab of 1 180 MPa Grade F/M High Strength Dual Phase Steel for Automobile

The hot ductility of 1180 MPa F/M high strength dual phase steel for automobile had been investigated in the temperature range of 650℃ to 1300℃ using a Gleeble 3500 thermal simulator. The thermoplastic curve was drawn,and the fracture and microstructure of high temperature tensile specimen were observed. The test results show there is a brittle zone in the test temperature and the brittle temperature zone is in range of 675℃ to 910℃. The minimum shrinkage rate reaches 28.76% at 800℃,and it shows good plasticity in the temperature range of melting point to 910℃ with shrinkage rate above 60%. the high temperature plastic zone is narrow and the Ⅲ brittle zone is a typical "U" curve and wide range,indicating high crack sensitivity. Fracture observationI shows that the fractures at both 650℃ and 950℃ have typical dimple which belong to ductile fractures. Fracture at 800℃ is typical brittle mixing along grain boundary and cleavage fracture. The fracture at 650℃ is mainly caused by precipitation of pro-eutectoid ferrite along original austenite grain boundary. The brittle fracture at 800℃ is mainly caused by grain boundary weakening.The high-temperature strength above 1050℃ is low,because the tensile stress exceeds the maximum strength of the steel. In order to avoid slab cracking during straightening section,the slab straightening temperature should be controlled above 950℃ which fall out of the Ⅲ brittle zone(from 675℃ to 910℃).