| Solute segregation in dendrite boundary during steel solidification and the thermal/mechanical states of solidifying shell in continuous casting mold have significant effects on slab surface and subsurface quality. So it has great significance to study the laws of solute microsegregation in mushy zone and its influence on steel solidification under mold cooling condition as well as the thermal/mechanical change law of solidifying shell in the mold and the effects of process parameters of continuous casting on it for determinating the reasonable process parameters of continuous casting, improving slab quality, and realizing the high efficiency continuous casting. In the present study, taking the metallurgical behaviors of shell solidification in continuous casting mold as the contents, a microsegregation model of solute elements in mushy zone with8/y transformation and a themo-mechenical coupling finite element model of shell solidification in slab continuous casting mold were estabilished by numerical simulation method, which the distribution characteristics of C, Si, Mn, P, S in the mushy zone and the influence of their segregation on crack susceptibility at solidification front, and the dynamic distribution laws of air gap, mold flux, heat flux, shell temperature field and stress field in mold were investigated to enrich the metallurgical theory of continuous casting mold and offer the theoretical basis and guidance for practical producation. The main research contents and results obtained in this paper are as follows:(1) Based on the regular hexagon transverse cross section of dendrite shape proposed by Ueshima, the microsegregation model was established by finite volume method. Simulation results show that:①C would not segregate severely in dendrite boundary during steel solidification. Changing C content in molten steel can affect the steel solidification mode significantly in the mold and make the segregation behaviors of solute elements in mushy zone change. P and S are the easy segregation elements. Increasing P and S content in molten steel would also make the solidification mode transform and reduse C solubility of8-Fe and peritectic point C content. The absolute segregation amount of S in solidification terminal stage in dendrite boundary shows a good linear relation with initial S content in molten steel. However, P just when its initial content is lower than0.030%, the absolute segregation amount shows a good linear relation with its initial content, and when it exceeds the value, the increasing speed of absolute segregation amount slows down.②The critical fracture strain (εc) of solidification front shows a wholely descending trend with C content increase in molten steel, and when the C content falls into the range of peretictic steel, εc appears abrupt fall. The influence of P and S segregation on εc is related to C content. When C content is lower than0.30%, increasing P and S initial content makes εc decrease rapidly, and when the content exceeds the value, the influence weakens. Increasing C content in molten steel can improve the critical fracture stress of solidification front. Increasing P and S content in molten steel can also improve the critical fracture stress at fs=1.0slightly, whereas it may change the solid phase fraction and phase composition in mushy zone significantly, and the tensile strength of solidification front would decrese at similar temperature. It is harmful to continuous casting.(2) Based on microsegregation model, one2D transient themo-mechenical coupling finite element model which has considered the dynamic distribution of air gap and mold flux in shell-mold interface in slab continuous casting mold during peritectic steel solidification in a domestic steel work were developed. The results show that:①Under the common condition of casting process, air gap forms at shell corner firstly at height of160mm bellow steel bath level, and mainly concentrates in the region of0-20mm nearby the shell corner. To the slab wide face corner, the air gap thickness increases with slab moving down in the mold, and increases suddenly at the height of280mm up the mold exit. The evolution of narrow face air gap at corner is just in the region from160to450mm bellow steel bath level. The thickness of mold flux at shell wide face corner is thinner than narrow face, and generates a peak at the height of200mm up the mold exit. The distribution trend of mold flux in the interface gap both increases firstly and then decreases from shell corner to the midst at shell wide face and narrow face, and the main form of mold flux in flux channel is solid flux.②Shell off-corner in slab mold is the centration region of hot spots appearance during peritectic steel casting. Under the common condition of casting process, the maximum surface temperature differences and shell thickness differences of wide face and narrow face off-comer reach to120℃,1.5mm and61℃,0.9mm to wide face and narrow face midst respectively, and the regions become weak link of shell coming up surface and subsurface defects. The stress of shell surface is tensile stress, and the solidification front stress is compressive stress. Both of them increase with slab moving down in the mold, and changes significantly around shell.③With other process conditions keeping invariable, increasing casting speed can reduce the air gap thickness wholely at shell corner, make the air gap mutative region at lower part of mold wide face move upward along the direction of mold height, and cause the mold flux thickness at shell wide face and narrow face corner increase and decrease respectively, as well as lowers the extent of shell surface temperature recovery of off-corner region at mold exit. With other process conditions keeping invariable, increasing mold narrow face taper slightly decreases the thickness of air gap and mold flux at shell narrow face corner, whereas intensifies the growth of air gap at shell wide face corner, and causes the surface temperature of shell wide face off-comer at mold exit appears obvious fluctuation. With other process conditions keeping invariable, intensifying the cooling intensity of mold slightly increases the air gap thickness at shell corner and decreases the shell surface temperature wholely. With other process conditions keeping invariable, improving superheat of molten steel can intensify the growth of air gap at shell corner and cause the distribution of mold flux thickness appear opposite trend with that of air gap. With each10℃increase of the superheat, the shell surface temperature will increase8～12℃wholely, and the shell off-corner surface temperature tends to more uniform at mold exit. With other process conditions keeping invariable, improving solidus of mold flux causes the location where mold flux in shell-mold interface solidified completely move upward, and makes the thickness of mold flux and the air gap at shell corner decrease and increase respectively. The lower solidus of mold flux is, the more uniform distribution of shell surface temperature along the circumferential at mold exit, and the more favorable for peritectic steel slab continuous casting.(3) In terms of the uniformity of heat transfer, the behaviors of heat transfer for peritectic steel continuous casting using different copper combination with three type mold taper structures were investigated based on the solidification characters of peritectic steel in mold. The results show that applying the copper combination of wide face copper with wedge-shape taper structure at corner and narrow face ideal taper structure with curved hot face and wedge-shape structure taper at corner to cast peritectic steel can almost eliminate the nonuniform distribution of mold flux and air gap in the narrow face interface, compress the distribution acreage of air gap and nonuniform mold flux at shell wide face corner effectively, and make both heat flux and shell temperature in wide face and narrow face uniform. which is favorable for peritectic steel continuous casting. |
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