Study on cracking cause and rolling process parameters of low carbon steel

Abstract

When Q215 strip steel is used to coil square tube in iron and steel plant,serious cracking is easy to occur at bending point.In order to reduce the cracking rate of the steel pipe,the causes of the corner cracking of the steel pipe were analyzed from two aspects of the microstructure and the inclusions in the steel pipe.The influence mechanism of the microstructure transformation on the cold deformation properties of medium-carbon and low-carbon steel was discussed.The reasonable hot rolling and cooling processes after rolling were formulated.The properties of the inclusions and microstructure in Q215 steel were analyzed by optical microscope and scanning electron microscope.The transformation point,CCT and TTT curves of Q215 steel were calculated by J Mat Pro software.The results show that the main reason for the cracking of the steel pipe is the existence of ter-tiary cementite precipitated along the ferrite grain boundary in the steel,which belongs to hard brittle phase and causes stress concentration at the grain boundary during bending deformation,and the secondary reason for the cracking is the existence of large-size sulfide inclusions more than 100 μm in the steel.By adjusting the hot rolling and cooling process parameters,the amount of tertiary cementite in the steel is significantly reduced,the amount of pearlite is increased,and the cracking rate of the steel pipe is significantly reduced.The research results provide a reference for preventing the processing cracking of Q215 and similar steel grades.

Keywords

• steel making; low-carbon steel; steel pipe; cracking; microstructure; inclusion; cementite