Study On Surface Structure Characteristic And Control Mechanism Of Continuous Casting Slabs Of Micro-alloyed Steels

The addition of of Nb,V and Ti to micro-alloyed steel can ensure that the steel has a good fit of strength,toughness and its welding performance can be greatly improved by the dispersion and precipitation of carbo-nitride precipitates（⁵nm in size）and solid solution of micro-alloyed elements in the case of low carbon equivalent.However,these micro-alloyed steels often have transverse cracks on the surface or corner of the slab during continuous casting production and broken edges or cracks in hot rolling.This phenomenon can only be solved by grinding and cutting corner in continuous casting and rolling cutting edge,which could seriously reduces the yield of the final steel.In this paper,the production process of micro-alloyed steel in a factory in China was fully investigated.Taking structural steel for welding steel HG785 as the research object,the thermodynamics and kinetics of carbonitrides precipitation during cooling process of the slab were analyzed and calculated.On this basis,thermal simulation by Gleeble-3500 and calculation model was used to analyze the influence of different cooling rates on the growth of high-temperature austenite and ductility.The cooling parameters of Surface structure control（SSC process）of HG785 were determined,in particular,the influence of SSC cooling process on microstructure and precipitation distribution.Also,the influence of different thermal cycles on ductility of steel was analyzed by thermal tensile.Based on the above theoretical research results,the following conclusions were mainly drawn:（1）The independent test of each production factor in continuous casting was carried out by using contingency table.The factors of superheat,cooling condition in mould and nozzle penetration depth all have great influence on the corner crack of slab of micro-alloyed steel.（2）Thermodynamics of carbonitrides precipitation of tested steel showed that the dissolved Ti,Nb,C and N in austenite gradually decreased with the decrease of temperature.At high temperature,the dissolved Ti,Nb,C and N were mainly precipitated in the form of TiN.With the decrease of temperature,the occupancy ratio of x in Ti(C_xN_1-x)would increase in range of 0.02⁰.3.The priority nucleation of Ti（C,N）located in the grain boundary,and its maximum precipitation temperature about 1350℃.maximum precipitation temperature of homogeneous nucleation about1200℃.The maximum precipitation temperature of Nb（C,N）grain boundary nucleation about 980℃and homogeneous nucleation about 850℃.（3）The final size of austenite were 1.41,1.08,1.41,1.08,0.48 mm with cooling rate of 1,3,5,7,10℃/s respectively.When the cooling rate less than 3℃/s,its easy to form the coarse austenite grain size（>1 mm）.With the increase of cooling rate,fine Ti（C,N）precipitates out around austenite boundary in chain-like,which can effectively restrict the growth of austente by pinning effect.In hot tensile in brittle zoneⅢ,R.A.is only 29.7%and 23.0%when cooling rate is 1℃/s and 5℃/s respectively,both following with the（Ti,Nb）（C,N）precipitates of 70²00 nm rectangle or irregular shape and Nb（C,N）precipitates of 40¹00nm pinpricks shape.（4）With high cooling rate of 10℃/s,the starting temperature ofγ→αtransformation of HG785 was 573℃in SSC process.Rapid cooled to 600 and preservated by 2 min,microstructure transformation can complete,and precipitates mainly consist of a small number of Ti-riched Ti（C,N）with size above 100 nm.When reheating temperature 1000℃,owing to increase of diffusion capacity of solute elements Ti,Nb,C and N,composite（Ti,Nb）（C,N）began to precipitate uniformly,and precipitate size mainly concentrated in 10⁴0 nm.Then,cooled to 800℃by 1℃/s,the austenite grain size was only about 70μm,and the precipitates were mainly fine（Ti,Nb）（C,N）with size of 5²0 nm,also exist a small number of large size（Ti,Nb）（C,N）precipitates continued to grow with the core of TiN.（5）brittleness zone III in the range of 700⁹00℃exist in tensile test after traditional themal cycle and temperature fluctuations cycle.Pro-eutectoid ferrite films of 2⁵μm were formed around the original austenite grain boundary,and accompanied by large amounts of（Ti,Nb）（C,N）precipitates with 50¹50 nm around austenite grain boundary.In SSC cooling process,since the grain boundary and the inner nucleation were almost simultaneous,ferrite film along austenite grain boundary had not yet been discovered,also there was no aggregation phenomenon of precipitates around grain boundary on account of diffusion limitation of Ti and Nb solution elements.（6）Slab heat transfer model of Micro-alloyed steel with slab section of 230mm×1300 mm,temperature of 1550℃in tundish,and casting velocity of 1 m/min show that the corner slab temperature at the exit of mould was 970℃,average cooling rate was 15.67℃/s,and average cooling rate was 9.51℃/s when temperature above1300℃.In straightening process,the average cooling rate of corner slab was only0.43℃/s,and Nb（C,N）around the austenite grain boundary precipitation constantly formed.When adopted temperature of 1570℃in tundish and casting velocity of 0.8m/min in the model,The temperature at the exit of mould was slightly decreased,while the casting time increased obviously,which promoted the formation of large austenite and carbonitrides at the grain boundary and ferrite film like.（7）By optimizing water amount in mould,when water amount of 1N,1I+O,2I+O,and 3I+O was increased to 282,325,523,487 l/min respectively,4I+O,5I+O,and 6I+O was decreased,and subsequent sections were basically consistent with the original.The simulation results of the corner temperature field met the requirements of SSC cooling process.（8）Measures including:Composition adjustment,avoiding crack sensitive area of carbon content as far as possible,reducing the content of N and Nb in steel and improving the content of Ti.Reducing and stabilization the casting temperature in tundish,giving priority to the use of cycle ladle,stabilization the baking process of heating ladle and tundish were proposed.Optimize nozzle insertion depth and mould slag.Reducing the immersion nozzle insertion depth to 120 mm,adopting high basicity and high crystallization phase premelting type hollow core slag（R¹.3）,and increasing the carbon content（about 6.0%）and CaF₂（about 6⁸%）,that can ensure melting effect of slag in the mould.By process optimization on existing continuous casting with low water amount in mould and secondary cooling,occurrence of transverse cracks in the corner of slab was significantly reduced.