A wear-resistant steel was hot rolled at the same finish temperature,and subsequently treated with conventional reheat quenching and tempering process(RQ&T) and direct quenching and tempering process(DQ&T),respectively.The effect of direct quenching on the microstructure and mechanical properties was investigated in detail by using optical microscope,transmission electron microscope and scanning electron microscope equipped with electron backscattered diffraction.The results showed that the microstructures of both the RQ and DQ specimens were complex constituents of lath martensite and lower bainite.Compared with the RQ specimen,the lower bainite content in DQ specimen was much higher.Furthermore,the bainite in the DQ specimen extended into and segmented the prior austenite grains,which can decrease martensite packet size.The proportion of high-angle boundary in the DQ specimen was higher than that in the RQ specimen,which may improve the impact toughness.The carbides in DQ&T specimen were much finer and distributed even dispersively because direct quenching can retain substantive defects which may provide more nucleation sites for carbide precipitation in the tempering process.Besides,the mechanical properties of DQ and DQ&T specimens were superior than those subjected to RQ and RQ&T processes,respectively.
Hongyu SONGShunhu ZHANGLiangyun LANCanming LIHaitao LIUDewen ZHAOGuodong WANG
In order to weaken the bias flow in the submerged entry nozzle (SEN) with slide-gate, the rotating magnetic field was imposed. The numerical method was employed to investigate the effect of rotating magnetic field on the flow field in the SEN and the mold under different slide-gate opening ratios. Numerical results showed that when the slide-gate opening ratio is smaller than 100%, the flow field in the SEN and the mold become asymmetry and there is an obvious circulation under the slidegate in the SEN. With increasing exciting current, the divergent angle of liquid steel at the SEN outlet increases, the impact depth of liquid steel in the mold decreases. With increasing slide-gate opening ratio, the impact depth of liquid steel in the mold increases and the required exciting current to weaken the bias flow should increase.
Aluminum nitride(AlN) precipitates and microstructure of 4 wt.%(Si+Al) non-oriented electrical steel were investigated.The 2.0 mm thick cast strips with three different silicon/aluminum(Si/Al) ratios were produced by twin-roll casting process,then the strips were reheated,warm rolled,cold rolled and annealed.The microstructure and AlN precipitates were characterized using optical microscopy,scanning electron microscopy and transmission electron microscopy.The results showed that with the increase of Si/Al ratio,on the one hand,the casting microstructure changed from columnar grains to equiaxed grains,and the uniformity of annealed microstructure was improved;On the other hand,the number of AlN precipitates in cast strips reduced meanwhile the distribution became dispersed.By the reheat treatment,the size and distribution of the AlN precipitates can be changed.Moreover,the grain size of the annealed strips is in the range of 20-50 μm,at the same time,many AlN precipitates were located at grain boundaries.Therefore,controlling the Si/Al ratio is a simple method to obtain desired microstructure.Then AlN precipitates in non-oriented electrical steel prepared by twin-roll casting process hinder markedly the recrystallized grains growth.A compatible reheat treatment can be an approach worth exploring to control the behavior of AlN precipitates.
Ting ZHANGXiaoming ZHANGZhiyuan GUOYuqian WANGChenggang LILiangyun LAN
Electrical steel sheets with 6.5%(mas fraction) Si with good shapes and superior magnetic inductions were successfully produced by a specially designed processing route including ingot casting, hot rolling and warm rolling both with interpass thermal treatment, and final annealing. The sheets were of 0.2 mm and 0.3 mm thick over 140 mm width. A detailed study of the microstructural and textural evolutions from the hot rolling to annealing was carried out by optical microscopy, X-ray diffraction and electron backscattered diffraction. The hot rolled sheet characterized by near-equiaxed grains was dominated by the mixture of <001>//ND fiber(λ-fiber), <110>//RD fiber(α-fiber) and <111>//ND fiber(γ-fiber) textures owing to the partial recrystallization and strain induced boundary migration(SIBM) during the hot rolling interpass thermal treatment. The static recovery and SIBM during the warm rolling interpass thermal treatment result in large and elongated warm rolling grains. The warm rolling texture is dominated by obvious λ, Goss and strong γ-fiber textures. The application of the interpass thermal treatment during hot and warm rolling significantly enhances the impact of SIBM during annealing, which is responsible for the formation of the moderate λ-fiber, some near-λ fiber texture components and the obviously weakened γ-fiber texture in the annealed sheet, leading to a higher magnetic induction compared to the commercially produced 6.5% Si steel by chemical vapor deposition(CVD).
Texture inhomogeneity usually takes place in ferritic stainless steels due to the lack of phase transformation and recrystallization during hot strip rolling,which can deteriorate the formability of final sheets.In order to work out the way of weakening texture inhomogeneity,conventional hot rolling and warm rolling processes have been carried out with an ultra purified ferritic stainless steel.The results showed that the evolution of through-thickness texture is closely dependent on rolling process,especially for the texture in the center layer.For both conventional and warm rolling processes,shear texture components were formed in the surface layers after hot rolling and annealing;sharp α-fiber and weakγ-fiber with the major component at{111}<110> were developed in both cold rolled sheet surfaces,leading to the formation of inhomogeneousγ-fiber dominated by{111}<112>after recrystallization annealing.In the center layer of conventional rolled and annealed bands,strongα-fiber and weakγ-fiber textures were formed;the cold rolled textures were comprised of sharpα-fiber and weakγ-fiber with the major component at{111}<110>,and inhomogeneousγ-fiber dominated by{111}<112> was formed after recrystallization annealing.By contrast,in the centre layer of warm rolled bands,the texture was comprised of weakα-fiber and sharpγ-fiber,andγ-fiber became the only component after annealing.The cold rolled texture displayed a sharpγ-fiber with the major component at{111}<112>and the intensity ofγ-fiber close to that ofα-fiber,resulting in the formation of a nearly homogeneousγ-fiber recrystallization texture in the center layer of the final sheet.
