| In comparision with EAF process, BOF special steel making has many characteristics such as high efficientcy, large batch production, low residual impurity of material and low cost, etc. However, some quality problems exist in the BOF special steel product due to the rapid process rhythm, lacking of enough refining time, less purity, shortage of professional steel rolling and subsequent equipments. Therefore, research on smelting process to improve BOF special steel quality has a great significance.This paper focus on BOF steelmaking process to produce special steel in Shougang steelworks. It investigated the low oxygen smelting technology of gear steel with BOF process; studied the refining slag, calcium treatment of inclusion and control of wideness harden ability bands by analyzing the statistics of history production data, testing of steel product and theoretical analysis.The main research achievements are as follow:(1) Shougang products and process analysisAccording to the results of industry test and historical statistics data, combining with literature survey, the shortages of shougang product compared with domestic and overseas advanced manufacturer were found. The results show that the BOF slag flowover was up to12-15kg/t; w(FeO) that entered LF was up to3.0%and at the end of LF refining was2.0%; w[T.O]was up to27×10-6of mass concentration; LF refining time was close to30minutes which longer than BOF or CC.There were excessive A and B type inclusions in steel. The inclusions modification was insufficient; most calcium aluminate inclusions located in CA6-CA2area which showed the calcium treatment was deficient. The accuracy on precise composition control was low, qualification rate of hardenability narrow band control was low, it was81.3%if J9defined as36.0±3.00HRC, however, decreased to65.3%if J9defined as36.0±2.00HRC. It means that improve the smelting process and composition control is necessary.(2) Research on smelt Technology of gear steel with BOF processAccording to thermodynamics calculating, the relationship between aluminium and oxygen content in the aluminum deoxidization steel, and the condition of inclusion generation and the condition of inclusion removing were studied by plant historical statistics data analysis. The results showed that the key points were to shorten refining time, to control slag flowover and to enhance slag modification. In order to produce steel with low oxygen w[T.O]<15x10-6, follow conditions must be satisfied: w[Al]s>0.03%, αAl2O3<0.02, w[CaO]/w[SiO2]=3.5-5and w[CaO]/w[Al2O3]=1.5-2.0. Calcium treatment should be made when the temperature of refining steel is above1600℃. The ratio of calcium and aluminum in steel should be controlled in0.017～0.19when acid-soluble aluminum is0.03%. The CaO·Al203and12CaO·7Al2O3with low melting point can be precipitated because Al2O3is liquid state when the calcium is (5-57)×10-6. When the sulphur content is higher than0.01%, the Al2O3inclusion will turn into CA easily, not C12A7.(3) Research on oxide inclusion in aluminum deoxidization steel during refining processElectrolysis method was used to analysis type, distribution and variability regulation of oxide inclusion in refining process of aluminum deoxidization steel, The result indicated, for Shougang steel, total quantity of oxide inclusion in the gear steel was0.0027%equally, A12O3reaches as high as0.00219%, counted69.30%of the oxide inclusion, the content of the common oxide inclusion in steel such as MnO、FeO、CaO are extremely low, less than0.0002%. Oxygen content from oxide inclusion almost took90%in ths steel,80%are from Al2O3and SiO2, so that it is the key to remove stable oxide. Shougang gear steel has massive M3C and M(C,N) precipitate, mainly precipitated during the solidification process, have great advantage to the grain refinement in heat treatment.(4) Research on the influence of non-metallic inclusions to gear steel fatigue lifeThrough the rotating bending fatigue experimental, fatigue breakdown behaviours of gear steel in different metallurgy quality from industrial producing were researched, and the gear steel S-N curve was determined, also fracture break mechanism analyzed and discussed. The result indicated that, to the20CrMnTi steel in hot rolling, hammers and the normalizing condition, rotating bar bending fatigue test showed that the disparity of dates was low, and the metallurgy quality was high especially the uniformity of micro-structure, there was no obvious difference on the rotating bar bending fatigue quality between20CrMnTi steel in the normalizing condition of different oxygen content. This mainly was related to that their fatigue cracking stems from the fatigue cracking mechanism from the superficial substrate. To the sample cracked from the inclusion of the surface layer, the inclusion was Al2O3·(CaO)x and large. Therefore, special attention should be paid to the unusual inclusions.(5) Research on the narrow hardenability band control technologies of gear steelImpact of gear steel hardenability bandwidth factor was identified using mathematical statistics and regression analysis.The value of gear steel hardenability prediction models was established to predict the hardenablility values on-line. The composition adjustment was guided based on Multi-SVM prediction model. Narrow component control precision is w[C]±0.01%、 w[Si]±0.02%、w[Ti]±0.02%、w[Mn]±0.02%、w[Cr]±0.03%、w[Ti]±0.01%, the respective proportions is96.1%、94.7%、90.5%、87.4%、81.9%、90.5%. The data were improved greatly than before; J9±2.0HRC qualification rate of narrow hardenability bandwidth value was93.3%, qualification rate of J9value±3.0was99.3%; Qualification rate of J15value±2.0was89.2%, qualification rate of J9value±3.0was98.4%.Through optimization process, the main technical data were greatly improved. The results were as follows:w[T.O]≤15×106, inclusion rating A+B+C+D<5.0seris, grain size rating>8.5, percent of pass of hardenability bandwidth<6HRC was99.3%and <4HRC was93.3%, macroscopic test of general loosen was less than0.5grade, center loosen was less than1.0grade, no cracks, inclusions, porosity and other defects in the low-fold and the refining time was shorten15.6min than before.In conclusion, this paper gave a detailed study on the converter smelting process to produce gear steel, and realized the object to produce high-quality gear steel by converter process. The statistics of the product qualities were up to international advanced level. |
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