Study And Application For Prediction Model Of Prior Austenite Grain Size On The Slab Surface Of Peritectic Steel

Peritectic steel has been widely used in construction,automobile,petroleum and other fields because of its good strength,toughness and weldability.With the increase in productivity and hot charging rate,the surface cracks of the slab become the main defects that affect the forward and quality improvement of the continuous casting of peritectic steel.The coarse austenite grain on the surface of slab is the key factor affecting the surface crack.How to refine the austenite grains on the surface of slab in the continuous casting process through the improvement or optimization of process conditions has become a key problem to be solved by domestic and foreign steel mills.In this paper,the initial solidification process of peritectic steel billets with different composition was simulated by high temperature laser confocal microscope.The measurement of initial austenite grain size and T^?temperature was realized through in-situ observation of austenite grain growth behavior.On this basis,the existing prediction model of austenite grain size was modified by establishing the quantitative relationship among the cooling rate,the initial austenite grain size and theM₀^*...Furthermore,the relationship between(the starting temperature for austenite grain growth)and CP(>0.18%)was obtained by in situ observation,and it was introduced into the modified model,which expanded the application scope of the model.Based on the above model,the austenite grain size at different position on the surface of 10B21and SCM440 cold heading steel billet was predicted,and the relationship among the depth of oscillation mark,the depth of depression and the austenite grain size was established.Taking the 1mm austenite grain size as the control standard of the surface structure of the billet,the control requirements of the surface depression and the oscillation mark depth of 10B21 and SCM440 continuous casting billet were put forward.Combined with the control requirements,the current production process conditions were analyzed,optimized and tested in the factory.The results show that the surface quality of the slab was improved obviously after optimization,the depth of oscillation mark and depression were controlled,the austenite grain on the surface of the slab was refined obviously,and the surface crack of the slab was reduced significantly within 1mm.The main research results were summarized as follows:(1)The dendrite spacing of high temperature ferrite at different cooling rates was measured,and the initial austenite grain size was characterized by in-situ observation of austenite grain growth behavior.Based on Anderson's simplified model,the rule ofM₀^*changing with cooling rate was obtained,which was in accordance with exponential function.Furthermore,the relationship between T^?and carbon equivalent(C_P>0.18%)was obtained through in-situ observation of solidification process of different carbon equivalent billets.The existing model has been modified by introducing the above relationship.After the modification,the model can be applied to a wider range of cold speed and steel grades,and was more accurate.(2)There are deep and irregular oscillation marks on the surface of 10B21 cold heading billet,and cracks were found in the deep oscillation marks and depressions.The austenite grain size on the surface of 10B21 cold heading steel billet was predicted by the prediction model,it was found that the austenite grain size increases with the deepening of oscillation mark and depression depth.When the austenite grain size was larger than 1mm,the more cracks begin to appear in the depression and oscillation mark.Taking the 1mm austenite grain size as the control standard of the surface structure of the billet,the control requirement of the surface oscillation mark depth and depression depth of 10B21 continuous casting billet were put forward.According to the control requirements,the current production process parameters of 10B21 cold heading billet were analyzed.The main reasons for the abnormal oscillation mark on the surface of10B21 cold heading billet were the unreasonable setting of mold oscillation and electromagnetic stirring parameters,and the low viscosity of mold powder.The oscillation parameters and electromagnetic stirring parameters of the mold were optimized according to the actual situation of the factory.After optimization,the surface quality of the slab was improved,the surface crack was reduced greatly,and the defect index was reduced from 4.38 to 0.39.The surface oscillation mark of the slab is straight and regular,and the average value of the bending degree d L of the oscillation mark was reduced from 0.99cm to 0.18cm;the depth of the oscillation mark was obviously shallower,and the depth of the oscillation mark at the middle and the edge was reduced by 99.3?m and 69.6?m respectively,and the average depth of the depression was reduced from 2.03mm to 1.72mm;the austenite grains in the depression and oscillation mark were obviously refined,and the size was basically within 1mm.(3)There were deep and irregular oscillation marks on the surface of SCM440cold heading billet,which were accompanied by network cracks and transverse cracks.The prediction model of austenite grain size on the surface of slab was used to predict the austenite grain size at different positions.The control requirement for the depth of the oscillation mark on the surface of SCM440 billet was put forward.Combined with the control requirement,the existing process conditions were analyzed.The main reasons for the abnormal oscillation marks on the surface of SCM440 cold heading billet were too long negative slippage time,too high electromagnetic stirringt value and low viscosity of mold powder.Combined with the operation conditions of the factory,the oscillation parameters and electromagnetic stirring parameters of the mold were adjusted.After optimization,the surface quality of slab was improved obviously.The surface defect index of slab decreased from 4.06 and 0.88 to 0.78 and 0.10 respectively at 1.80m/min and 2.25m/min.The oscillation marks on the surface of the slab were straight and regular.The average value of the oscillation marks on the surface of the slab under the two speeds decreases from 0.82cm and 1.47cm to 0.29cm and 0.47.The austenite grains in the oscillation mark decreased obviously,and no grains with size more than 1mm were found.