Effects by Normalization Treatment on Microstructure and Mechanical Properties of New-type Steel for Low Temperature Service

Analyzed the effects by normalization treatment on microstructure，mechanical properties and fracture mechanism of the new-type TMCP steel for low temperature service.The results show that along with increase of the normalization temperature，both yield strength and tensile strength of the sample as intercritically normalized are going down，whereas those of the sample as fully normalized are going up.And the laws of changes of the sample’s after-fracture elongation and the impact energy absorption from Charpy V notch test are just on the contrary to that of the change of its strength.The over-all mechanical properties of the sample as fully normalized at 920℃are the best among the test samples.Moreover，the acicular ferrite in the microstructure of the TMCP sample as intercritically normalized disappears gradually，while the grain is being refined，and also is getting equiaxed.The microstructure of samples normalized at 860℃and 920℃is composed of equiaxed ferrite and pearlite.As the normalization temperature further rises up to 1 000℃，the grain is getting coarsened，meanwhile Widmanstatten structure develops.The fracture type is changed from the quasi-cleavage fracture of the TMCP sample to ductile fracture of the normalized sample.