Study On The Cleanliness Of Low Alloy High Strength Steel Q345b By BOF-LF-CC Process

This thesis combining with the production practice of low alloy high strength steelQ345B clean steel by using BOF-LF-CC process of converter plant, they find that slagentrapment and reoxidation can cause the content of inclusion greatly increased inmolten steel during the unsteady casting period, the purity of molten steel becomes worse,then continuous casting billet appeared quality problems. In order to take some effectivemeasures to further improve the purity of molten steel, it is necessary to study theevolution law of impurity, gas and other elements and the control of steel cleanliness insteelmaking process of Q345B steel under the condition of plus lump ore, to ensure thatimprove the Q345B clean steel’s control level.This experiment selects the samples of different process, including LF enter-station,before calcium treatment, after calcium treatment, tundish, mold, casting billet, then bymeans of the oxygen-nitrogen analyzer, image analyzer, big sample electrolysis, spectralanalysis and other methods to study the cleanliness, then we can discuss the evolutionrules, objectively evaluates the level of slab cleanliness, grasp the present controlsituation and put forward some reasonable control measures on the basis of the analysisof nitrogen-oxygen, the residual elements, microstructure and large inclusion.The main conclusions are as follows:（1）In the thickness direction of Q345B slab, the micro-inclusions aggregationoccur in quarter department of arc side, and the maximum of T[O] and nitrogen contentoccur in the same place;（2）The main micro inclusions of LF enter-station and before calcium treatment areAl2O3and MnS, and the main inclusion of after calcium treatment change intoCaO-Al2O3and MnS. Large inclusions in each process of electrolysis of billet steel aremainly SiO2and aluminosilicate salts, we also find a small amount of Na2O, which isinvolved in the crystallizer protection slag in addition. In addition, CaO inclusions areexist alone, which added with lime failed to completely melt by the operating workerswere involved in molten steel.After LF refining and calcium treatment, the average T[O] decreased by11.85×10-6;the average nitrogen content in casting process increased10.45×10-6, the results showthat the plant need to improve the level of protection of casting billet; average T[O] andnitrogen content were31.80×10-6and39.20×10-6; （3）The continuous casting mould fluxesis unsteady and the average alkalinity ofrefining out-station slag samples is4.339, CaO/Al2O3is between2.017~2.310,theaverage is2.146.