Research On High Temperature Diffusion Anealing Process Of GCr15 Bearing Steel Of Continuous Casting Bloom

GCr15 is a typical high-carbon low alloy steel which shows an excellent fatigue and wearing resistance,high stiffness and good stability of the size and shape after a series of heat treatment,and this steel has been used to manufacture the bearing ring and rolling element.Due to the poor working condition of the bearing,a long fatigue life which is determined mainly by the banded structure is required.Banded structure is originated from continuous casting and formed in the rolling process,and it is a product of solid state transformation.Therefore,the key point to insure the good quality of the bearing steel is to reduce the content of the banded structure.For the large size production,it’s hard to improve the quality though cold rolling while a better structure can only be achieved in the continuous casting and heating process.Therefore,this paper is objected on the GCr15 bearing steel with the cross section of the 350mmx470mm from one steel plant,acid etching and chemical analysis have been adopted to study the microstructure and macro segregation of the continuously casting slab.Moreover,the main factor which leads to the internal quality of the slab and the existing problems in the continuous casting process has been analyzed.The production conditions such as heating temperature and the holding time have been set by the chamber electric furnace,aiming at studying the growth behavior of the austenite crystal and analyzing the effect different high temperature diffusion conditions have on the dendritic segregation.At last,the experimental results will be conducted in the research work of industrial test,which provides the basis and guidance for setting a reasonable heating plan in the practical production process.The main conclusions are summarized as follows:(1)Compared with the geometric center of the GCr 15 bearing steel,the position of the solidification center rightly deviates to(-16.23,19.47),which can be improved by the cooling condition in the continuous casting process.(2)A great difference of the cooling condition has been observed in the mixed crystal region,equiaxed region and columnar region,and their cooling speeds are 63.5℃/min,9.641℃/min,6.12℃/min respectively.Severe center segregation and the center porosity have been observed due to the element of C.The technology of reduction combined with proper electromagnetic stirring parameter can improve the slab quality in the industrial production.(3)Different high temperature diffusion annealing processes have been conducted on the sample which shows the rising of the temperature is more efficient for diffusion than the prolonging of the holding time.When the holding time is 6 hours at 1200℃ the dendritic structure will disappear.Meanwhile,if the holding time is extended to 9 hours,a larger austenite grains will form,and the oxidation of the sample becomes serious,and some deeper pits will form in the surface.(4)The size of the austenite grains under different heating temperatures and holding times has been analyzed.The result indicates that the austenite grain size shows a lognormal distribution at the same holding time or heating temperature,and the austenite grain size ofGCr15 bearing steel has been fitted as follow:(?) = 3.87 × 106t0.319 exp(-1.49×105/RT).(5)Under the same heating condition,the grade of the banded structure will gradually decrease with the increasing of the whole rolling reduction,and the grade of the banded structure after annealing is about 1.5 grade higher than the hot rolling state.Based on the original process,the holding time at high temperature will be extended to a certain period,leading to the improvement of the banded structure.When the holding time is extended to 320 min at high temperature,the banded structure will be properly controlled.