Deformation Behavior Of Large Section Continous Casting Steel And Evolution Of Porosity During Heavy Reduction

Since the 21st century,the demand of high quality and large-size steel products largely increases with the rapid development of high-end equipment manufacturing industry in our country.Large section size has thus become one of the major development trends for continuous casting technology.However,with section size of casting steel significantly increased,its internal quality defects,like centerline segregation,porosity,etc.,become much more serious,which severely restrictes the efficient and stable production of high quality continuous casting steel and rolling products.In view of this,the deformation behavior of wide-thick slab and bloom during continuous casting process was investigated with thermal-mechanical coupling method in the present paper,and the evolution of internal porosity during heavy reduction(HR)was quantitatively studied.The main contents and results are as follows:1.A three-dimensional(3D)bulging model for wide-thick slab under unven cooling condition was developed.Influence of uneven cooling,casting speed and roll misalignment on bulging deformation of the wide-thick slab was systematically investigated.The concentrated position of tensile strains along the slab thickness direction,width direction and casting direction caused by bulging and the mechanism of inducing triple-point cracks,mid-way cracks and corner cracks repectively by tensile strains along these three directions were elucidated.Due to the uneven cooling condition,the bulging deformation and the tensile strain on the solidification front of the wide-thick slab significantly increases,which aggravates the risk of internal cracks.With the increase of roll misalignment,the bulging deformation continuously increases,the tensile strains present a linearly increasing trend,and the tensile strains along the thickness direction and casting direction increase more rapidly.2.A 3D thermal-elastic-plastic contraction model for wide-thick slab during the whole continuous casting process was developed.The influence of casting speed,superheat and slab width on the thermal contraction deformation of the wide-thick slab during the whole continuous casting process were quantitatively studied.In consideration of the non-uniform distribution of thermal contraction along the slab width direction,the weighted average calculation method for thermal contraction was proposed:#12 where,c is the weighted average thermal contraction,mm;Sb/Se and Mb/Me are the start/end position of the slab wide surface corresponding to the internal solidified and unsolidified regions;i/j is position number of the slab wide surface corresponding to the internal unsolidified/solidified region;C is thermal contraction of different positions on the slab wide surface,mm;? and y are the weighted factors of position and region.3.A 3D thermal-mechanical coupled model for wide-thick slab during HR at the solidification end was established.The evolution of macro-porosity with size of 1?5 mm and micro-porosity with size of 200?500 ?m during HR was systematically investigated.The theoretical calculation models for macro-porosity closure degree of the wide-thick slab based on equivalent strain and hydrostatic integration were derived respectively,which can be expressed as:?s=-4.90x?eq2+3.35×?eq+1.76×10-3?V=-2141.31×Q4+669.13×Q3-77.62×Q2+6.85×Q-6.50×10-2 where,?s and ?v are the prosity closure degree defined based on variation of the porosity aspect ratio along thickness direction and porosity volume,respectively;?eq and Q are equivalent strain and hydrostatic integration at the corresponding position of porosity,respectively.Investigation on the distribution,morphology and evolution of the micro-porosity during HR indicates that micro-porosity in the wide-thick slab is approximated as ellipsoid and mainly distributed whithin 30 mm from the slab centerline.On the slab transverse section and the midwidth longitudinal section,with the long axis orientation of the micro-porosity deveating from the reduction direction(the slab thickness direction),closure degree of the micro-porosity on the transverse section continuously increases,and closure degree of the micro-porosity on the mid-width longitudinal section firstly decreases continuously and then increases.On the slab mid-thickness longitudinal section,with the long axis orientation of the micro-porosity deveating from the slab width direction,closure degree of the micro-porosity firstly decreases and then continuously increases.4.A 3D thermal-mechacnial coupled model for bloom during HR at the solidification end was developed.The evolution of central porosity under condition of equal/differential roll ration speed between two adjacent withdraw and straightening units was systematically investigated.Afer 10%HR deformation under condition of equal roll rotation speed,the porosity size significantly decreases along the bloom thickness direction with a large deformation degree of?0.46 while the porosity size increases slightly along the casting direction and the width direction with a small deformation degree of<0.1.With reduction position moving away after the solidification end by 2.2 m(twice the distance between two units),the reduction force increases by 15%due to the decrease of the bloom temperature field.Compared with bloom with uniform temperature distribution(1100?)during rough rolling,the closure degree after HR at the solidification end(temperarure difference between the internal and external region of the bloom is?400?)can be increased by 25%.Compared with the required reduction force to implement the same reduction amount by adopting two conventional flat rolls,the required reduction force by adopting one covex roll and both convex rolls can be largely decreased by 41.9%and 56.2%respectively.Differential roll rotation speed under condition of rotation speed ratio between front and reduction rolls Rv>1(the rotation speed of the front rolls is larger than that of the reduction rolls)can significnantly promote the HR process effect on minimizing porosity.The promoting efficiency of differential roll rotation speed on HR process effect significantly decreases when Rv is larger than 1.2,and the optimal rotation speed ratio Rv=1.0?1.2.The theoretical calculation models for porosity closure degree of the bloom were derived,which can be expressed as:?s=-3.27×?eq2+2.94×?eq+4.39×10-3?v=-681.13×Q4+306.43×Q3-50.26×Q2+5.01×Q+1.13×10-2...