Research On Heat Treatment Process And Inclusion Of Low Yield Ratio SPCC Steel Plate

SPCC steel sheet has been more and more widely used in the fields of automobile,agricultural machinery,electronic products,instruments and appliances because of the excellent deep drawability,low yield strength,good ductility and smooth surface properties.As the properties of the sheet material seriously affects the process performance of the finished product,the quality of stamping plates provide a guarantee to make excellent products.Therefore,it is of great significance for enterprises to product steel plates with lower yield ratio and excellent mechanical properties.This topic mainly focuses on two kinds of different specifications of the cold and hard steel sheets produced in Taishan Iron and Steel Co.,Ltd.First,the samples were heat treated at different temperatures(Temperature range 440? to 760?)with the same time(4 hours).After that,Metallographic structure and hardness were analyzed by metallographic microscope and Vivtorinox hardness tester respectively in order to find the suitable temperature of heat treatment.Second,the samples were treated with different holding times at 680? and 720?.The effect of holding times on the metallographic structure and hardness was analyzed through the same method above.Then several optimal heat treatment processes were selected from the results of constant temperature and constant time heat treatment to treat the sample plates from plant production.After heat treatment,the properties such as mechanical characterization,tensile fracture,inclusions of the samples were systematically analyzed by using test machine Scanning Electron Microscopy(SEM)and Energy Dispersive Spectrometer(EDS).The results show that the cold rolled plates have the problems of high yield ratio and excessive deformation resistance.It is found that the annealed grain is too small with the grain size range from 9 to 10.The results of constant heat treatment show that the recrystallization temperature of SPCC steel plate with 950 production lines and 1780 production lines is about 520?.The microstructure of the SPCC cold rolled steel sheet presents a fibrous elongated grain at the annealing temperature below 520?,while new grains without distortion began to appear in amorphous structure at 520? and the number of the new grains gradually increases as well as the size with the increase of the annealing temperature.The recrystallization process of the material is basically completed at 680?.The grain size increases with the increase of annealing temperature.After annealing at 680? and 720?,the grain size and hardness of SPCC steel plate changed slowly with the increase of holding time.Generally speaking,the effect of holding time is smaller than that of annealing temperature on the microstructure and properties of SPCC steel.The tensile fracture surface analysis indicated that the macroscopic fracture was ductile fracture.The dimple size and the number of the tensile fracture of the 950 line decreased slowly with the increased temperature.In addition,the dimple size becomes uneven and the depth becomes shallower.After annealing at 680? for 4 hours,tensile fracture dimple size of 1780 production line is relatively uniform and the average size of tensile fracture after annealing at 720? is the same as that of 680?,but the size of dimple is uneven.When the temperature increases to 760?,the dimple size increases and becomes more uneven with the average dimple size of about 12?m.The more uniform and deeper of the dimple is,the better of the material mechanical properties will be achieved.From the point of inclusions analysis,the effect of inclusions on mechanical properties of SPCC cold rolled steel sheet is mainly reflected in the amount,size and distribution of inclusions.The more content and larger size of inclusions can lead to a worse toughness.The inclusion size of the 950 and the 1780 production lines is generally uniform..No larger inclusions were found.The inclusions mainly include chained Al2O3,long striped MnS and non-deformed silicates.Al2O3 and MnS are often present in the matrix with a complex tadpole shape.Compared to 950 production line,1780 production line has less inclusions,so the elongation is higher and the plasticity is relatively better.