Study On Surface Crack Control Mechanism Of ND Steel Billet

ND steel is prone to cracks on the surface of the slab during continuous casting.Therefore,the causes and mechanisms of surface cracking of ND steel in this paper are explored,and proposes improvement measures for casting process.Through the analysis of the on-site continuous casting billet,there are a large number of lateral cracks in the inner arc corner.There is Cu enrichment inside the crack,and most of the cracks have no protective slag composition.It is proved that the secondary cooling zone is the high-incidence zone of ND steel crack.When exploring the high temperature mechanical properties of ND steel,it is found that the third brittle temperature zone of ND steel is 700?800°C and the valley bottom temperature is775°C.Meanwhile,there is also a distinct brittle zone in the high temperature zone,the temperature range is 1150?1250°C,and the valley bottom temperature is 1200°C.The heat transfer model is established to find out the variation of surface temperature of the continuous casting billet during the continuous casting process.It is found that under the original water amount,when the slab enters the straightening zone,the angular temperature is too low.The specific water quantity is reduced from 0.48L·kg^-1 to 0.44L·kg^-1,This can reduce the overlap of the corner temperature and the brittle temperature zone.Maintaining the amount of raw water,the second cold cut of the slab can increase the temperature fluctuation range of the slab in the straightening zone to above the brittle temperature zone.The precipitation and enrichment of Cu on the surface of continuous casting slab were studied by high temperature cooling.The results show that Cu rich phase exists in all samples along grain boundary.Air cooling and with furnace cooling,the initial temperature of 1150°C,have reached the maximum penetration depth and Cu content.Both the decrease of cooling strength and the increase of initial surface temperature aggravate the process of Cu rich phase being replaced by oxide,which leads to the sharp increase of grain boundary embrittlement.Therefore,optimizing the secondary cooling system,reducing the temperature fluctuation range on the surface of the casting slab and accelerating the cooling speed can reduce the precipitation and enrichment of copper on the surface of the casting slab.Figure 36;Table 14;Reference 66.