Study On Refinement Of Prior Austenite Grain Of Slab Subsurface For Microalloyed Steel Casting

The research，production and application of high strength micro-alloyed steel hasbecome an important symbol for measuring the level of industrial development for thenational steel industry. However, the transverse cracking on the slab surface is a majorproblem encountered during the continuous casting of microalloyed steel. In thetraditional continuous casting process, making slab surface temperature avoid the rangeof the third brittle zone during bending or straightening operation is not enough toprevent cracking. Because the slab corner is affected by the two-dimensional heattransfer from both narrow and wide sides, it is inevitable for the corner temperature toreach the third brittle temperature range. According the formation mechanism oftransverse cracking，the prior austenite grain size is an important factor affecting thesurface transverse cracking; and refinement of the prior austenite grain size will be aeffective measures to reduce the cracking susceptibility.In this paper, accoring to the above idear of refining the prior austenite grain in themicro-alloyed slab surface layer which from the Pansteel and Baosteel, the process ofinitial solidification and thermal cycle was simulated by confocal scanning lasermicroscope (CSLM). The behavior of austenite grain growth (for low carbonmicroalloyed steel) during the initial solidification, austenite grain refinementmechanism during double phase transformation process and method of assessing slabtransverse cracking susceptibility were investigated. Based on the laboratory researchresults, the slab arc continuous caster fron Pansteel was used for the casting of lowcarbon microalloyed steel to analye the effect of caster parameter and technologicalparameter control on the refinement of prior austenite grains. The main work is listed inthe followings:①Based on the investigation of behavior of austenite grain growth, it shows thatthe starting temperature for austenite grain growth (Tγ) can be measured through in situobservation using CLSM. For low carbon micro-alloyed steel (CP≤0.18%), Tγis mainlyrelated to the CPunder the certain cooling rate, and shows the exponential functionrelation to the later; the pinning effect of the precipitates on the grain boundaries can bereflected by the amount of titanium precipitates PTiwhich is related to the solubilityproduct of TiN. The revision factor K, used for describing the effect of micro-alloyed elements on austenite grain size, has a quadratic function relation with PTi. Both the Tγand K were used to revise the C.Bernhard model. The results demonstrated that thecalculated value by the revision model closely corresponds to the measured values; andthe average relative error of the Bernhard’s model can be reduced by5%after revising.②Austenite grain refinement mechanism was analyzed via simulating doublephase transformation process. The introduction explains the effect of “γ/α”ratio anddistribution and reheating control on the grian refinement, to solve the inefficiency ofcurrent literatures. For the experiment steel, when the cooling rate, cooling temperature,reheating temperature and reheating rate are5℃/s,640℃,1100℃,3℃/s, respectively.The coarse prior austenite grain(1.2mm) can be refined as fine grains(0.47mm) witheven distribution, the mixed grains index is only0.5.③The results of continuous casting test show that there are three transformationregions for microstructure evolution at different distance to the slab subsurface: strongcooling transformation region near the slab surface, double phase transformation regionin which double phase transformation occurs due to the thermal cycles, and thesubsequent mild cooling transformation region. The key to reduce slab crackingsusceptibility is to obtain widely-distributed double phase transformation region in slabsubsurface. Under control cooling pattern, not only is film-like ferrite eliminated, in thewidely-distributed DPT region, but homogeneous and fine microstructure is obtained,and the cracking susceptibility is decreased with a crack index of0.4. The refinement ofaustenite grain is suitable to the thermal physical property of steel (Ar3and Ac3temprature), secondary cooling segment parameters of slab caster and slab sections. Inthe future, if the secondary cooling segment parameters can be improved accdoding toheat transfer principle inorder to solve the contradiction between the cooling andreheating, the cracking susceptibility will be reduced via double phase transformation.④Features of high-temperature microstructure (HTM) can be demonstrated by theparameters of as-cast microstructure (ACM): Ferrite grain size (DF), ferrite mixed grainindex (E), film-like ferrite precipitation fraction (fMF). The three parameters essentiallyreflect the austenitinte grain size and distribution in HTM. The transverse crackingsusceptibility can be reflected by the DF, fMFand E in the ACM, and shows the positivecorrelation with the parameters. Based on the stiffness ansalysis of salb surface,Equivalent diameter of austenite grain was put forward (DγP). DγPis a correction to theprior austenite grain diameter during the calculation of stiffness; and the effect of film-like territe and precipitates microalloyed elements on the austenite grain boundaryare included in the DγP. The DΓp, used to assessing the transverse cracking susceptibility,can be calculated by the parameters of as-cast microstructure. It makes up for theinadequacy of the hot tensile test for determination of cracking susceptibility.