Controlled cooling method of L450MB pipeline steel

The invention provides a controlled cooling method of a L450MB pipeline steel. A sectional type controlled cooling mode combined of fast cooling and air cooling are employed, after finish the rolling, steel plate is rapidly cooled by a large cooling rate of 30-40 DEG C/s to the ferrite phase transition temperature scope, air cooling is carried out for 4-10s, steel plate is rapidly cooled by a small cooling rate of 5-10 DEG C/s to the target final cooling temperature. The air cooling of steel plate in critical temperature range of the ferrite transformation is in favor of fully precipitation and the growth of the ferrite crystal grain, the size and distribution of ferrite crystal grain tend to uniformization and equiaxial, pearlitic pellet gets smaller and tends to dispersion and distribution, thereby an ideal tissue state and good comprehensive mechanical property can be obtained. Plastic strain ratio R and the strain hardening index n are increased, the cold-forming performance is improved, the yield ratio of the L450 MB steel plate is reduced, the shape of the steel plate after controlled cooling is improved, and the original type rolling yield of the steel plate is increased.