Study On Microstructures And Deep-Drawabilities Of Aluminum Killed Low-Carbon Steel Sheets During Single Stand Reversing Cold Rolling

The aluminum killed low-carbon steel sheets, deoxidized by adding aluminum, are one of high quality carbon structural steel sheets. They are widely used for automobile manufactory, building industry and packaging industry because of their good formability and low strength. However, most scholars paid their attentions to recrystallization behavior of continually cold rolling steel sheets and their annealing processing parameters, but few are concerned with the influence of cold rolling reduction ratios on recrystallization behavior of reversely cold rolling steel sheets and their annealing processing parameters. With the development of modern industry, demands of the steel sheets owned different thickness specifications become increasingly pronounced. So making the most appropriate production parameters for different steel sheets, saving energy-consumption and reducing cost are the hotspots in research field at present.The research objects in this article are low-carbon aluminium killed steel sheets by reversely cold rolling. Influence of metallurgy process parameters and acid soluble aluminum content on microstructure and deep drawability of the steel sheets are studied systematically by using X-ray diffraction (XRD), electron back scattering diffraction (EBSD), scanning electron microscope (SEM), transmission electron microscopy (TEM), optical microscope (OM) and tensile testing machine.The research on deformation law of steel sheets during reversely cold rolling process by copper wire fixed-point notation shows that shear stress and shear deformation don’t increase obviously with increasing reduction ratios. Because steel sheets reciprocate through rolling mill, shear stress direction is also changed correspondingly. Deformation mechanism in surface and core of steel sheets are all compressed during reversely rooling process, so microstructure is homogeneous in thickness direction. The microstructure is composed of pie recrystallization grains along rolling elongation after annealing process. Volume fraction of{111} orientation textures is dominant and good for deep drawability of the steel sheets.The study on microstructure and deep drawability of steel sheets owned different cold rolling reduction ratios shows that reduction ratios have a little influence on deep drawability. Though the rolling plane isn’t changed, the rolling direction has been completely reversed after every reversely rolling pass. So the orientation texures are different from those by continually rolling. Seen from macro-textures measured using XRD, volume fractions of favorable{111}, unfavorable{100} and{110} orientation textures hasn’t yet been changed obviously with reduction ratios. However, reduction ratios have some influences on micro-textures measured using EBSD. The quantity of favorable micro textures increases and unfavorable micro textures decreases as reduction ratios increase. It mainly because steel sheets owned different reduction ratios have different fragmentation degrees of grains in cold rolling process and recrystallization driving forces in annealing process.Study on recrystallization law of steel sheets owned different cold rolling reduction ratios shows that as cold rolling reduction ratio increases from40%to81%, recrystallization temperature of steel sheets decreases from602℃to572℃during isochronal annealing for4h, and recrystallization holding time of steel sheets reduces from117min to5min during isothermal annealing at610℃. However, recrystallization process doesn’t need incubation period during isothermal annealing at700℃, and reduction ratios have less influence on recrystalization. Recrystallization temperatures and holding time of the steel sheets with different reduction ratios during different annealing processes all can be calculated using the experiment results in this paper.The Microstructure of steel sheets owned64%reduction ratios was examined by EBSD technology. The results show that preferentially growing direction of recrystallization grains is rolling direction and equiaxed grains grow into cake-type grains during recrystallization process.{111} orientation grains preferentially nucleate and grow up. Not only that, characteristic distributions of grain orientation in α and γ orientation line haven’t been changed obviously from early to just finishing recrystallization stage. So formation mechanism of texture can be explained by directional nucleation.Number fraction of low misorientation angle boundary (LMAB<15°) of cold rolling steel sheets owned64%reduction ratio reaches up to90.7%, because its microstructure contains a lot of subgrains with LMAB. As annealing tmeperatures increase from490℃to610℃, number fraction of LMAB decreases from88.0%to5.1%after annealing for4h. This is because recrystallization nucleuses owned great misorientation angle boundary (GMAB) grow up by consuming sub-grains owned similar orientation in recrystallization process. Increase of grain size is accomplished by annexing smaller grains in grain growth stage. Because smaller grains own larger curvature and surface area in per unit volume, they have higher energy and are annexed easily by larger grains. So the driving force coming from reducing of grain boundary surface energy is different from deformation storage energy in recrystallization stage.Study on the deep drawability of steel sheets owned68%and75%reduction ratios after annealing with different temperatures shows that annealing temperatures have some influences on r value, yield strength and tensile strength of the steel sheets. The optimal temperatures are680℃-700℃for the steel sheets owned68%-75%reduction ratios. Higher annealing temperatures (700℃) should be selected for the steel sheets owned lower reduction ratios (68%) in order to fully recrystallization. Lower annealing temperatures (680℃) should be selected for the steel sheets owned higher reduction ratios (75%) in order to save resources and reduce the cost.Effect of acid-soluble aluminium content on microstructure and deep drawability of the steel sheets have been researched. The results show that yield strength and tensile strength have already reached to257MPa and336MPa, respectively, for the steel sheets owned more acid-soluble (mass fraction0.1%). Moreover, the plastic strain ratio is0.84shows the steel sheets have almost lost theirs deep drawability. Textures haven’t been changed obviously during metallurgical process. Volume fraction of favorable{111} orientation texture is still very low after anneling. This is mainly because aluminum particles owned low melting point generate in the steel sheets. The aluminum particles are dissolved and precipitated over and over again during metallurgical production process. They appear in the steel sheets in forms of small droplets when temperatures are higher than its melting point. This has severe influences on deep drawability of the steel sheets. However, yield and tensile strength of the steel sheets owned less acid-soluble aluminium (mass fraction0.021%) is190MPa and282MPa, respectively. The plastic strain ratio is1.19. Favorable textures have been increased obviously after annealing process. The deep drawability of the steel sheets is very excellent. Therefore, the mass fraction of acid-soluble should be about0.021%in this paper.