An Investigation On Smelting Process Optimization For Cr5 Large Steel Ingot

With the increasing use and performance requirements in cold and hot rolling mill,Cr5 backup roll has been developed.Due to its good hardenability,wear resistance,deep hardening layer and good accident resistance and so on,it has very broad application prospects.In order to meet above performance requirements,the appropriate heat treatment process should be carried out for backup roll,and this is closely related to the quality of ingot.So there is a strict requirement for the quality of the steel ingot,i.e.parent material for backup roll.In order to improve the molten steel cleanliness,the morphology,distribution of all kinds of inclusions should be well controlled.In this thesis,the Cr5 ingot(forgings)produced by Ansteel Casting Company was used as experimental material,the types and origins of inclusions in forgings and steel ingot were studied by a series of test and analysis.The number,size and type of in situ and ex situ inclusions were also statistically counted during smelting and casting,and compared with those in steel ingot(forgings).Through adjusting the deoxidization process and optimizing the pouring process,this will lay a solid foundation to improve the cleanliness of large steel ingot.The main research contents and results obtained in this thesis can be summarized as follows.(1)For unqualified Cr5 backup roll forgings,the non-metallic inclusion in forgings was analysized to determine its chemical composition.Based on the analysis result,the main source of the inclusions was determined and thus the problems in original deoxidization process clearified.Some measures were suggested to be taken to reduce inclusions in steel.(2)Since the original deoxidization process mainly adopts aluminum in deoxidization,some new Al2O3 inclusions can be produced during this process.If there are some problem in this process,not only can the effect of deoxidization not improved,but secondary pollution can occur as well in molten steel due to the higher the Al content in molten steel and even more serious secondary oxidation.By discussing the vacuum carbon deoxidization technology,the double vacuum carbon deoxidization route "LVCD+VCD" was finally adopted.After evaluating the quality of Cr5 backup roll produced by double vacuum carbon deoxidization process and comparing it with the original process of steel ingot,the results showed that cleanliness of the molten steel can be effectively improved bu using double vacuum carbon deoxidization.This is attributed to the not formation of Al2O3 inclusions and its salts in steel that the A1 was not used in the deoxidation process.(3)Due to the tundish used in traditional vacuum casting ingot,secondary oxidation phenomenon will inevitably exist in the process of pouring molten steel.So,an alternative scheme of refined package instead of the tundish was used and it can effectively avoid the oxidation of molten steel in the process of casting.By conducting simulation of the ladle flow field with Fluent software,the minimum ladle injection margin can be calculated to avoid the ladle slag and further optimize refined package in direct casting ingot process.A comparative study result shows that the number and size of inclusions in steel have been significantly reduced.(4)In order to solve the feeding problem in large vacuum casting ingot and even the ingot internal shrinkage cavity or inclusions,a method was used to optimize the ingot insulation riser to improve the effect of riser insulation.After simulating the optimization of improving the internal quality of ingot by ProCAST,the results show that it not only can insure the effect of feeding but also can reduce the amount of molten steel to some extent.And the bad heat preservation performance will result in excessive riser speed and increased solidification shrinkage stress.When the surface shrinkage stress reaches a certain value,the protecting slag shell will be tensile cracked and then fell into the bottom of ingot and sedimentary cone large inclusions will be formed.After improvement of heat preservation,it can effectively solve the riser shell phenomenon and riser contraction is much gentle the depth of shrinkage cavity becomes shallow.The large non-metal oxide inclusions at sedimentary cone can be effectively avoided to form.