Effects Of Bath Annealing Process On Microstructure And Mechanical Properties Of Cold-Rolling Steel Sheet SPCC

The SPCC cold-rolled steel sheet is widely used in household appliancy,automobile,door manufacturing industry,also as a hot-dip galvanizing substrate.In our country,SPCC accounts for a larger proportion of the cold-rolled products and the market demand is larger. However,when taking the CSP hot plates as raw material to produce the cold rolling plates,the products have higher yield strength,lower elongation ratio and poorer stamping performance. The most current studies focus on reducing the yield strength of CSP hot-rolled plates. This paper discusses the effects of different annealing process on microstructure and properties of cold-rolling sheet SPCC through simulating annealing under laboratory conditions. The results show that:The average ferrite grain size of CSP plates SPCC produced by LIANGANG Group is12μm,and the yield strength of SPCC is less than 300MPa,so it is suitable as the base material to produce cold-rolled plates. The ZJ330B produced by ZHUJIANG Group have the better plastic,However,after cold-rolling and annealing processes,the plate,s yield strength is higher,so the ZJ330B steel is suitable to use as hot-rolled plates. The SPCC cold-rolled plates have serious work-hardening situation and higher strength. The recrystallization temperature of SPCC is 560℃, and the main influencing factors of recrystallization temperature are cold rolling reduction rate and heating rate.After double steps annealing for 5h between 680℃to 700℃,the ferrite grain size of SPCC is 35μm,but the pie-forming degree is not very high;annealing with 750℃for 5h,the ferrite grain coarsens and no free cementite precipitats. Along with the annealing temperature's elevation, the intensity and the tensile ratio drop gradually,but elongation ratio reaches the maximum value after 680℃and 700℃annealing. When the holding time is longer,the effects through a single extended holding time to reduce the yield strength is decreasing. The properties fully meet the requirements of deep stamping(DDQ) steel after annealing with 680℃and 700℃for 5h.The ReL,IE and rm values of the SPCC plate are more sensitive to heating rate,but the change of elongation ratio is smaller. Annealing with the heating rate IV and V,the SPCC has small equated grains and lower nm,rm value,so it can only be used as a CQ level sheet steel. After the same annealing conditions,the SPCC sheets with 75% reduction rate have the maximum pie-forming degree. At this point,the sheets have excellent tensile properties. The microstructure and properties difference is very small after Single and double steps annealing,so the 400℃transition temperature and 550℃heat preservation platform can be canceled.The results also show that the chain distribution of cementite mainly is from the balling transition of flake cementite in pearlite;when SPCC sheets are annealed below the critical temperature, the cementite's precipitation is not serious,not affecting the stamping performance.