| With the rapid growth of steel production in China and the changes ofmacroeconomic environment, competition has become more intense in the steel industry.For today’s steel industry, economic cleaning technology of steel production hasimportant significance to improve product quality and reduce production costs, and ithas been widely concern. However, refining equipment functions are not fully takeninto consideration, and that the combination of refining methods is not quite optimized,these problems have become the bottleneck restricting the modern steel enterprises tobuild the economiccleaning steel production platforms. AFT702and CDQ203producedby Chongqing steel have been selected for research subjects in this paper.Thecleanliness of these steels had been investigated and analyzed under existing converterand refining process conditions, and the results indicate that: due to large numbers ofhigh-melting-point Al2O3inclusions existed in molten steel, which frequently clog thenozzle during casting and easily interrupt production, such inclusions could cause thesteel surface damage in cold-rolling; thus, the steel cannot meet the hot-chargingrequirementsand the compact layout needed for continuous casting and rolling. Manysteel mills had to adopt calcium treatment (Ca treatment) in order to transform thealumina inclusions into low-melting-point calcium aluminates,to a certain extent,calcium treatment can reduce or avoid clogging. However, because of the calcium yieldis very low during processing,its use increases the cost of production at the same time.The reduction or elimination of the calcium treatment can greatly reduce the operatingcosts of production for steel enterprises, which have an annual output of thousands oftons.In this paper, the aim is to work out the most economical system for the refiningprocess and ensure good cleanliness, not happen nozzle clogging or blockage caused bythe inclusions and improved the heat numbers of continuous casting without calciumtreatment, more can not hazard mechanical properties of steel. So the key researchcontent of this paper is how to achieve the spheroidisation of inclusions in steel with thebest denaturation treatment way.Firstly, according to the investigation of the refiningprocess of target steel in existing state, the reasonable control level of steel cleanlinessneed be confirmed. Then, the controlrange of steel and refining slag composition hadbeen calculated by FactSage when the inclusions located in low melting areain steel, On this basis, the effect on the cleanliness of steel grade treated with refining slag orcomposite cored wire had been researched in experimental study, so that we could getthe suitable composition range of refining slag and composite cored wire for adaptingdifferent kinds of steel. Finally, combination of actual production, the industrialapplication had been carried out in Chongqing steel. The main conclusions of this studyare as follows:According to the thermodynamic calculation results by FactSage software, forAFT702, to control the compositions of MnO-SiO2-Al2O3inclusions in soft killed steelinto1520℃liquid region,[Al] content in the liquid steel should be controlled to0.0010%~0.0015%,[O] content is controlled in the range of10~25ppm. To control thecompositions of CaO-SiO2-Al2O3inclusions in strong killed steel into1520℃liquidregion,[Al] content in the liquid steel should be controlled at0.013%or less,[O]content is controlled about2ppm. When the melting points of inclusions are below1520℃, it can avoid clogging during casting process and can be slightly deformed inthe hot rolling process, thereby greatly improving the ductility and toughness of steelproduct. For CDQ203, to control the compositions of CaO-MgO-Al2O3inclusions in theliquid phase region of MgO <10%, Al2O3:45%~60%, CaO:40%~55%, when thebalance [Al] content is0.03%, a[Ca]in the liquid steel is1~10ppm, a[Mg]is0.01~0.3ppm;when the balance [Al] content is0.06%, a[Ca]in the liquid steel is1~30ppm, a[Mg]is0.05~0.9ppm. For the CaO-Al2O3-CaS inclusions can be controlled in the liquid regionbelow1600℃, when a[Ca]in the liquid steel were5ppm,15ppm and30ppm separately,the balance [Al] content should be controlled in the range of0.0010~0.050%,0.0050~0.250%and0.010~0.50%.On the basis of theoretical analysis, to get the relationship between the cleanlinessof liquid steel and slag compositions in this research, twenty two temperatureequilibrium experiments have been carried out on the molten steel of AFT702and thedifferent slag systems in the laboratory, in which include16heats experiments with slagA (CaO/Al2O3:1.0~1.8, SiO2:11~20%mass) and6heats experiments with slag B(CaO/SiO2:1.0~1.2, Al2O3:25~35%mass). Experimental results show that: comparedwith slag B, the slag A has stronger deoxidation and desulfurization capacity. The sizesof CaO-MgO-SiO2-Al2O3inclusions in the steel reaction with slag A are overall smallerthan the same inclusions in the steel reaction with slag B. To generate more less than3μm spherical CaO-MgO-SiO2-Al2O3inclusions and lower T[O] content in steel, itshould be selected in the composition of slag A: CaO/Al2O3ratio is between1.2to1.8, SiO2content of the slag is around11%or20%.Secondly, twelve temperature equilibrium experiments have been carried out onthe molten steel of CDQ203([Al]:0.020%~0.070%) and the different slag systems inthe laboratory, in which include6heats experiments with slag C (CaO/SiO2:9,CaO/Al2O3:1.0, SiO2:5%) and6heats experiments with slag D (CaO/SiO2:6,CaO/Al2O3:1.5,SiO2:9%), so that we can study the interacting relationship betweentop slag and the compositions of liquid steel or inclusions. Experimental results showthat the desulfurization capacity of this two slag is very close, and the average sulfurcontent of the steel is all0.002%, but the deoxidation capacity of slag D is slightlystronger than the slag C. The quantity of lower than5μm inclusions in steel balancewith the slag D is more than slag C, inclusions appear more tiny. Under the slag Dconditions, it is also more conducive to generate CaO-MgO-Al2O3-SiO2systeminclusions. To generate more smaller than5μm spherical liquid CaO-MgO-Al2O3-SiO2inclusions and ensure lower T[O] content in steel, the better components should beselected as the composition of slag D: CaO/Al2O3ratio is about1.5, SiO2content in slagis nearby9%.To analyze the effect of cleanliness of liquid steel with calcium aluminatecomposite cored wire treatment, experiments have also been conducted to make it outthe evolution process of inclusions in steel after60min reaction. Initial content ofcalcium aluminate (90%,80%,70%) in the composite core wire reacted with low alloysteel were changed (the powder yield is assumed to10%and20%).It can be concluded:when the content of calcium aluminate in the core powder is80%to90%(the yield is10%) or the content is90%(the yield is20%), inclusions in the steel is the typicallylow-melting spherical CaO-MnO-SiO2-Al2O3, when the calcium aluminate contentreaches to70%(the yield is10%) or the content reaches to70%to80%(the yield is20%), CaO-MgO-Al2O3or CaO-Al2O3is more easily generated in the steel, Al2O3isgradually reduced.In summary, Chongqing steel were selected for production test of steel AFT702and CDQ203. In this study, the tapping deoxidization system, slag washing and refiningslag system are optimized primarily so that the number of oxide inclusions and harmfulelements are reduced to a very low level, and the multi-furnace continuous casting ofmolten steel can be achieved without calcium treatment under the “basic oxygenfurnace (BOF)'composite deoxidization and slag washing'composition adjustmentby sealed argon bubbling (CAS)/ladle furnace (LF)'continuous casting (CC)” process.There is an important practical significance to solving this problem: not onlycan it improve the cleanliness of carbon steel and low alloy steel, resulting in economicbenefits, but it also has a demonstrated beneficial effect on other production enterprisesof high-grade steel. |
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