Towards relieving center segregation in twin-roll cast Al-Mg-Si-Cu strips by controlling the thermal-mechanical process

Serious center segregation greatly limits the application of twin-roll casting(TRC)technology for produc-ing 6xxx alloy strips.Herein,Al-0.9Mg-0.6Si-0.2Cu-0.1Fe(wt.%,6061)strips with different thicknesses were fabricated by TRC,and we found that the center segregation was well relieved with the thick-ness increased from 3 mm to 4 mm.To reveal the mechanisms of mitigation of center segregation in the 4 mm strip,various techniques including solidification simulation,crystallographic calculation,elec-tron backscatter diffraction(EBSD),and electron probe micro-analyzer(EPMA)were utilized.The re-sults disclosed that the Fe-containing phase in the 3 mm strip wasπ-AlFeMgSi,while the counterpart in the 4 mm strip wasα-AlFeSi.Theα-AlFeSi could serve as nucleation substrates for Mg_(2)Si and Q-AlCuMgSi phases,thus promoting the uniform distribution of elements and preventing the accumulation of phases in the center region.Three matching planes between theα-AlFeSi and Q/Mg_(2)Si were exam-ined as:(1120)_(α-AlFeSi)//(0001)_(Q),(0001)_(α-AlFeSi)//(110)_(Mg2Si),and(1120)_(α-AlFeSi)//(110)_(Mg2Si).Meanwhile,the smaller roll separating force during the TRC process in the 4 mm strip could weaken the force-induced liquid flow behavior in the semi-solid region,which is the other reason for the alleviation of center seg-regation.Owing to the elimination of the center segregation,a more excellent fracture elongation was achieved in the as-homogenized 4 mm strip(∼29%)compared with the counterpart of the 3 mm strip(∼20%).This work may provide a strategy to eliminate the center segregation,thus further promoting the application of TRC process and producing high-performance Al alloy strips efficiently.

Influencing Factors on Center Segregation in Continuously Cast Pipeline Steel Slabs

The factors which influence center segregation of continuously cast slabs are obtained through analyzing the database of macrostructrue. It indicates that center segregation becomes severer with the increasing content of carbon, phosphorus and sulfur; Center segregation will be reduced obviously if the content of manganese is higher than 1.5% and the ratio of Mn/S is higher than 300; High degree of superheat , high casting speed and increasing width of slabs will increase the degree of center segregation. Since center segregation below class B has little effect on the property of steel, hence, in order to low the ratio of center segregation of class B-1.0 down to 10%, several control strategies are presented as follows: the contents of carbon, phosphorus and sulfur must be focalized in lower range of steel grade demanded, but manganese upper operating range control. The referenced contents of the elements in molten steel are required like this: [C]【0.07%,[P]【0.01%,[S]【0.005%,[Mn]】1.5%,[Mn]/[S]】300; The degree of superheat should be lower than 24℃ and the casting speed should be defined to 1.0-1.1 m·min-1. At the same time, proper secondary cooling system of water distribution should be developed and the precision of continuous caster should be also improved.

Strength and ductility enhancement of twin-roll cast Al-Zn-Mg-Cu alloys with high solidification intervals through a synergistic segregation-controlling strategy

Center segregation and banded intergranular segregation(B-IGS)should be well controlled to improve the mechanical properties of twin-roll cast(TRC)aluminum alloys,especially for alloys with high solid-ification intervals.In the present work,a synergistic grain refinement strategy was designed using an Al-5Ti-B grain refiner and Ti,Zr,and Sc microalloying elements to simultaneously control center seg-regation and B-IGS in TRC Al-Zn-Mg-Cu alloys.As the grain size decreased,center segregation defects were eliminated and transformed into dispersed B-IGS defects;simultaneously,the width and length of the B-IGS were also reduced.Moreover,the macro-distributions of the alloying elements along the thickness direction became more homogeneous due to a weak shear-induced dilation effect.The well-controlled multiscale segregation improved the uniformity of the alloy macrostructure,accelerated the redissolution of the crystalline phase,dispersed the aggregated residual phase,and refined grains under the T6 state.Hence,the strength and ductility of the alloys under the T6 state were simultaneously improved,and the hardness distribution along the thickness direction became more homogenous.Furthermore,the underlying mechanisms of segregation evolution and strength and ductility enhancements were elucidated.This work provides a novel strategy to effectively control multiscale segregation and produce high-performance aluminum alloys with high solidification intervals by TRC.

Effect of Central Multiphase Microstructure of Thick Plates on Work Hardening and Crack Propagation

The effect of multiphase microstructure in the center segregation zones(CSZ)of high-strength steel thick plates on work hardening characteristics and crack initiation/propagation were studied.Microstructure was correlated with deformation damage behavior by using Taylor factor(TF)gradients,kernel average misorientation(KAM),and geometric necessary dislocation(GND)density.The results show that the segregation leads to a mixed structure of ferrite(soft phase)and martensite/bainite(hard phase)in the center of the thick plate.Compared to the 1/4 thickness(1/4 T)region,grain refinement occurred in the CSZ,with KAM values increasing from 0.24 to 0.49 and a decrease in the proportion of high-angle grain boundaries(>15°)from 77.4 to 51.8%.In the process of deformation,due to the influence of grain refinement and adjacent martensite/bainite,ferrite grains were divided into structural units with different TF values.The higher KAM value and GND density at the interface between soft/hard phases resulted in severe work hardening.In addition,the presence of micron-sized inclusions in the CSZ caused local strain concentration and non-continuous deformation of the matrix,which induced crack initiation.The fracture surface showed a mixed mode of brittle cleavage fracture and ductile fracture.

Numerical Simulation and Experimental Study of F-EMS for Continuously Cast Billet of High Carbon Steel

A final electromagnetic stirring model was developed for billet continuous casting of high carbon steel using the commercial software ANSYS and CFX, and the numerical model was validated by the magnetic flux density measured under a Teslameter CT-3. The magnetic flux density and fluid flow in the liquid pool at the location of final electromagnetic stirring(F-EMS) were calculated by the present numerical model. Meanwhile, the plant trials were carried out to determine the optimum current intensity and frequency of F-EMS for the continuously cast billet of high carbon steel. The numerical results show that, through increasing the current intensity by 100 A, the corresponding increases of magnetic induction intensity, tangential electromagnetic force and flow velocity at the solid/liquid interface in the strand are 0.025 T, 1933 N/m3 and 6.9 cm/s, respectively. Moreover, the industrial trial results showed that for the continuously cast billet of 60 steel, the optimum current intensity and frequency of F-EMS, which is 8.2 m from the meniscus, are respectively 380 A and 6 Hz. With the optimum F-EMS parameters, the significant improvement of center segregation of billet is achieved, and the center carbon segregation index in billet reaches 1.04.

Development and application of a multi-graphite electrode DC plasma tundish heating

The world’s first three-graphite electrode direct current(DC)plasma heating system(2500 kW)was successfully put into production on the 50-t tundish of a two-strand slab caster.The single metallic torch plasma tundish heaters were reviewed.In addition,the induction heating system was also estimated.The three-graphite electrode DC plasma tundish heating(PTH)system does not require any electrode to be fitted to the tundish.Five electrodes can be used to realize uniform and fast heating of the six-strand tundish.Heating with high power can effectively eliminate the steep temperature drop of the molten steel in the tundish during ladle change-over.The system has turned out to be very reliable,simple,and maintenance-free.The heating rate is high within 0.5 to 2.0℃/min.Some heats with ultra-low superheat(2.6-11.0℃)were observed,and the molten steels were successfully cast by the powerful heating capacity and good control performance of the system in the practical production.It can be concluded that continuous casting with superheat of 5-10℃in the tundish was fully realistic with PTH.