Causes And Control Of Corner Transverse Cracks Defects In Baosteel Micro-alloyed Steel Continuous Casting Slabs

Since Baosteel started production in September 1985,a modern steel plant has been built.The No.3 continuous casting machine was built in 2004 for the production of thick plates.This paper is to solve the problems encountered in the production of Baosteel's No.3 continuous casting machine.The main focus is on the products of micro-alloyed slabs.In order to reduce the corner crack problem which is easy to occur in this kind of slab,and to reduce the occurrence of edge cracking of thick slab continuous casting billet Rate,a series of summary and research were made.The main research contents are as follows:(1)Combing and study the transverse cracks in the corners of micro alloyed steel slabs and their influencing factors,including MD vibration marks,austenite grain size,pro-eutectoid ferrite,micro alloy element precipitates,etc.The effect of micro alloy elements such as niobium,titanium and vanadium on the high temperature plasticity of billets were analyzed.(2)On the austenite grain boundary,chain-like micro-alloyed carbonitrides,reticular ferrite films formed by pro-eutectoid ferrite,and coarse or uneven austenite grains promote cracking of continuous casting billets produce.(3)In order to further study the angular transverse crack defects,OM,SEM,TEM,EDS and other experimental methods were adopted to pass continuous casting of austenitic ferrite film,austenite grains,MD vibration marks,N/V/Ti precipitates,etc.The comprehensive analysis of the transverse crack at the corner of the billet confirms the root cause of the formation of the transverse crack at the corner of the continuous casting billet.(4)On-site production confirmed that under the traditional cooling process,the microstructure of the continuous casting billet and the distribution of micro-alloying elements under the traditional cooling conditions are basically the same as those of the current continuous casting billet,but the structure of the continuous casting billet produced by the SMC process cooling condition is small.Two-phase dispersion distribution,especially no pro-eutectoid ferrite film at the grain boundary.Under the continuous casting slab SMC process,the low plastic zone of the product slab is significantly better than that under the traditional process.After hot tensile test,the product has good plasticity in the range of 700?900 ?.(5)The coarse austenite grains,the large degree of mixed crystals,and the grain ferrite structure and chain precipitates of the original austenite grain boundaries cause stress concentration in the continuous casting billet under bending and straightening.The original austenite grain size of the outer arc corner of JT5Q6BP1 steel continuous casting billet is about 1.8 times of the austenite grain size of JT5P58P6 steel.There is a large amount of membranous ferrite structure and crystal in the surface depth of 4?6mm.The second phase particles precipitated in the chain,and the ferrite mixed crystal is relatively high,and the mixed crystal index is 3.5.(6)By analyzing the inner and outer arc angle transverse cracks of JT5Q6BP1 and JT5P58P6 billets produced by Baosteel 3#caster in actual production,the conclusion that the outer arc is more likely to crack is obtained.For this reason,according to the optimized organization control principle,the defect rate of the slab edge crack caused by the corner crack of the continuous casting billet is steadily decreased.The incidence of slab edge cracking has decreased from the previous 1.2%to the current 0.13%.