Study And Application On Soft Reduction Operation Models For Bloom Castings Of Special Steels

The macrosegregation,porosity,and shrinkage cavity are popular internal quality defects in continuous casting,especially for high carbon and alloy steels.These defects will have harmful impact on steel performance and the production yield,and thus frequently become the root of some waste products.The countermeasures against the internal quality defects have been well developed in production,in which,for example,there are lower superheat temperature casting,electromagnetic stirring,soft reduction,and intensive cooling in the region of strand final solidification.Among these means,the soft reduction(SR)implemented in the crater end could compensate final volume shrinkage of the strand,and then suppresses the fluid flow of the solute enriched liquid so as to eliminate or reduce these defects.The soft reduction operation models for the effective application of continuous cast bloom have been comprehensively studied in the dissertation.In order to exert the metallurgical effects of soft reduction in continuous casting bloom,first of all,the bloom liquid core profile and solid fraction distribution during solidification should been acquired so as to determine reduction zone and reduction amount.Based on heat transfer and solidification model in the bloom casting process,the quantitative calculation of center solid fraction and liquid core profile have been carried out.The effects of casting speed,steel grade,superheat degree,specific water flowrate and strand sectional dimension on liquid core profile and center solid fraction distribution were analyzed.The results show that the liquid core thickness increases and the solid fraction increasement decreases with increase of the casting speed,carbon and alloy contents,superheat degree and sectional dimension,and decrease of specific water flowrate.For a given steel grade and strand section,the casting speed has the greatest impact,while the superheat degree has the minimal effect on the liquid core thickness and the center solid fraction.Based on overall mass balance in the casting direction of the bloom strand,the necessary reduction amount model was mathematically derived for bloom continuous production.The effects of steel grade,casting speed,superheat degree,specific water flowrate and strand sectional dimension on necessary reduction amount were analyzed.It is found that the necessary reduction amount under each reduction roll should increase upon approaching the crater end due to the increase final solidification.The results also indicate that the strand thickness has the greatest impact,while the superheat degree has the minimal effect on the necessary reduction amount,and all the influence factors of increasing liquid core thickness can increase the necessary reduction amount.The effects of steel grade,casting speed,superheat degree,specific water flowrate and strand sectional dimension on reduction efficiency for flat roll were analyzed on the basis of the previous reduction efficiency model for flat roll.Based on the thermal-mechanical coupling model established,a unified regression reduction efficiency model which is suitable for soft reduction of flat roll and convex roll was presented.The effects of convex roll shape,contact ratio between the roll and strand,and the process parameters on reduction efficiency for convex roll were analyzed.It is shown that the reduction efficiency increases with increase of the casting speed,carbon and alloy contents,superheat degree and sectional dimension,and decrease of specific water flowrate.In other words,all the influence factors of increasing liquid core thickness can increase reduction efficiency.It is also found that under the same conditions the reduction efficiency for convex roll reduction is greater than for flat roll reduction,and the greater the contact ratio,the greater the reduction efficiency,but the reduction efficiency change is rather small when the contact ratio is more than 7.Based on the thermal-mechanical coupling model,the reduction force,stress and strain in the soft reduction process for flat roll and convex roll were analyzed.The results show that under the same conditions the strand reduction force for convex roll is less than for flat roll,and the greater the contact ratio,the smaller the reduction force and the larger the ranger of the tensile stress and tensile strain and the larger tensile strain.It is also found that under the same conditions the greater the center solid fraction,the greater the reduction force exerted by the flat roll or the convex roll.Based on the necessary reduction amount model and reduction efficiency model,the reduction amount model suitable for bloom flat and convex roll was presented.The effects of the process parameters on the required reduction amount were analyzed.It is shown that the reduction amount increases with increase of the casting speed,carbon and alloy contents,superheat degree and sectional dimension,and decrease of specific water flowrate.In other words,the effects of process factors on the reduction amount are consistent with that on necessary reduction amount.It is also found that under the same conditions the reduction amount for convex roll reduction is greater than for flat roll reduction,and the greater the contact ratio,the greater the reduction amount,but the effect of the contact ratio on the reduction amount is rather limited when the contact ratio is more than 7.The soft reduction model was applied on bloom casters with good metallurgical results.The internal quality defects of the bloom casting,which includes macrosegregation,porosity,shrinkage cavity and V-shape segregation,and the quality of the rolled products were improved significantly.The qualified ratio of the rolled rod of steel GCr15 is increased from 85.4%to 98.8%.The microstructure of cord steel wire rod meets the requirement of the cord production with a proper phase combination of sorbite,pearlite and ferrite.