Evolution of microstructure and texture during casting of AISI 304 stainless steel strip

The solidification behavior of AISI 304 stainless steel strip was studied using a melt/substrate contact apparatus, whereby a copper substrate embedded in a moving paddle is rapidly immersed into a steel melt to produce thin (∼1-mm gage) as-cast coupons. For cases where other casting conditions were kept constant, the effect of substrate topography and melt superheat on the development of microstructure and texture during solidification was studied using electron backscatter diffraction (EBSD) and optical microscopy. It was found that nucleation and growth of grains during solidification were influenced both by substrate topography and melt superheat. A ridged substrate produced a high density of randomly oriented grains at the chill surface with the preferred growth of 〈001〉-oriented grains perpendicular to the substrate wall producing a coarse columnar grain structure exhibiting a strong 〈001〉 fiber texture at the strip center. In contrast, a smooth substrate resulted in a lower nucleation density to produce a very coarse-grained columnar microstructure with moderate and essentially constant 〈001〉 fiber texture throughout the strip thickness. By the manipulation of casting parameters, it is possible to produce strip-cast austenitic stainless steel with a particular microstructure and texture.     

Metallographic observations of stainless steel sheets produced by twin-roll type strip caster

Microstructures and textures of austenitic stainless steel sheets produced by a twin-roll type strip caster are investigated at various stages of casting, cold-rolling and annealing processes. Strip casting is a rapid solidification process and, therefore, the cast strip sheets have fine columnar dendrites and very sharp (100) textures. After cold-rolling and annealing the strip sheets have equiaxial and somewhat finer grains than those produced by the conventional process, and the texture is composed mainly of the (110)[1?2] orientation.     

薄带连铸AISI304不锈钢中铁素体行为

为了确定薄带连铸AISI304不锈钢凝固过程中残留铁素体的生成及转变行为,采用彩色金相、电解侵蚀、电子背散射衍射分析技术及X射线衍射分析等研究手段对双辊薄带连铸AISI304不锈钢凝固组织及残留铁素体特征进行了研究.结果表明AISI304不锈钢薄带的凝固组织由表层胞状晶区、中间柱状晶区和中心等轴晶区三部分组成.薄带表层胞状晶区内残留铁素体呈棒状,柱状晶区的残留铁素体形态为鱼骨状,中心等轴晶区的残留铁素体呈弯曲的树枝状;薄带的表层胞状晶区残留铁素体的质量分数为4.6%～6.6%,柱状晶区内的残留铁素体质量分数为3.6%～3.7%,中心等轴晶区内的残留铁素体质量分数为11.27%～11.34%;残留铁素体沿着厚度方向呈现"W"状分布.      

Analysis of surface microcracks in low-carbon steel produced via twin-roll strip casting

In this study,morphological and microstructural analyses were conducted on net-shaped microcracks appearing on the surface of low-carbon steel manufactured via twin-roll strip casting. The fractograph and microscale distribution of elements in the cracked region were also analyzed. Results revealed that the cracked surfaces were characterized by slight pits,along with inclusions composed of manganese and silicon oxide distributed along both the sides of the cracks. Fractograph analysis revealed that the crack and smooth dendrite surfaces were oxidized. These phenomena indicate that microcracks on the cast strip surface form at the hightemperature stage of the solidification process during twin-roll casting and rolling. Microcracks were present in each region with pits in the cast strip and extended along the dendrite interface because of the combined effects of phasechange stress,thermal stress,mechanical stress,and fractional crystallization during the solidification process.     

高磷耐候钢铸轧薄带的组织和深拉伸性能

采用双辊薄带铸轧机制备出磷含量(质量分数,%)为0.15%的新型高磷耐候钢薄带.对比研究了常规薄带和铸轧薄带冷轧退火后的组织、深拉伸性能,并通过对常规薄带和铸轧薄带中心层织构演变规律的分析探讨了冷轧退火后的铸轧薄带深拉伸性能较好的原因.结果表明,在设定的工艺条件下冷轧退火后,铸轧薄带和常规薄带的组织均为铁素体和珠光体组...     

双辊连铸不锈钢薄带凝固组织特点

 通过金相观察分析了同径双辊薄带连铸机上生产的奥氏体不锈钢薄带的凝固组织,结果表明：铸带凝固组织包括2个柱状晶区和1个等轴晶区,其等轴晶呈近球形或蔷薇形。与传统连铸板坯相比,其柱状晶区一次及二次枝晶的间距较小,等轴晶粒内部为非枝晶结构,其尺寸大约是连铸坯等轴晶的1/10,凝固组织更致密。     

Microstructures of Austenitic Stainless Steel Produced by Twin-Roll Strip Caster

 The microstructures of austenitic stainless steel strip were studied using color metallographic method and electron probe micro analysis (EPMA). In the cast strips, there are three kinds of solidification structures: fine cellular dendrite in the surface layer, equiaxed grains in the center and fine dendrite between them. The solidification mode in the surface layer is the primary austenite AF mode because of extremely high cooling rate, with the retained ferrite located around the primary cellular austenite. In the fine dendrite zone, the solidification mode of molten stainless steel changes to FA mode and the residual ferrite with fish-bone morphology is located at the core of the dendrite. The retained ferrite of equiaxed grains in the center is located in the center of broken primary ferrite dendrite with vermicular morphology.     

Analysis and prevention of cracking during strip casting of AISI 304 stainless steel

In this study, a microstructural investigation was conducted on the cracking phenomenon occurring during strip casting of an AISI 304 stainless steel. Detailed microstructural analyses of the cracked regions showed that most of the cracks were deep, sharp, and parallel to the casting direction. They initiated at the tip of dendrites and propagated along the segregated liquid films between primary dendrites, indicating that they were typical solidification cracks. This cracking phenomenon was closely related to the inhomogeneous solidification of cast strips, represented by depressions, i.e., uneven and somewhat concave areas on the strip surface. The depressions, which were unavoidable in flat rolls due to the presence of a gas gap between the roll and the cast strip, were finely and evenly distributed over the cast strip surface by intentionally providing homogeneous roughness on the roll surface; then, the number and size of cracks were considerably reduced. In addition, the nitrogen gas atmosphere, which retained high solubility in the melt during cooling and good wettability with the roll surface, was successfully used to prevent cracking, because the thickness of the gas gap was minimized.     

Solidification modelling of microstructure in twin-roller thin strip casting of stainless steels

The influence of process parameters on the dendritic microstructure of thin strips cast by the twin-roll method is analyzed in the framework of a one-dimensional solidification model and compared with experimental results. As a relevant characteristic the secondary dendrite arm spacing Λ₂ as a function of the distance x from the roll surface is investigated. The difference between the local dendrite arm spacing near the strip surface and the strip centre, respectively, increases with the strip thickness and only depends on the casting temperature to a small extent. An increase in the strip/roller heat transfer coefficient due to a rising casting velocity or possibly enhanced roll-separating forces leads to a decrease in the dendrite arm spacing. The effect of a sudden decrease in heat transfer during the solidification process, on the Λ₂(x) characteristics, e.g. by a local separation of the solidified shell from the roller surface, is discussed.     

Numerical Simulation of Filling Process During Twin-Roll Strip Casting

The modeling and controlling of flow and solidification of melt metal in the filling process is important for obtaining the optimal pool level and the formation of the solidified metal layer on the surface of twin-rolls during the twin-roll strip casting. The proper delivery system and processing parameters plays a key role to control flow characteristics in the initial filling stage of the twin-roll strip casting process. In this paper, a commercial CFD software was employed to simulate the transient fluid flow, heat transfer, and solidifications behaviors during the pouring stage of twin-roll strip casting process using different delivery systems. A 3D model was set up to solve the coupled set of governing differential equations for mass, momentum, and energy balance. The transient free-surface problem was treated with the volume of fluid approach, a k–? turbulence model was employed to handle the turbulence effect and an enthalpy method was used to predict phase change during solidification. The predicted results showed that a wedge-shaped delivery system might have a beneficial impact on the distribution of molten steel and solidification. The predicted surface profile agreed well with the measured values in water model.     

Processing effects in spray casting of steel strip

Spray cast strip of AISI 1026 and M2 has been produced by the Osprey^? process under controlled conditions of deposition. Droplet flight distance was varied over the range 325 to 475 mm and strip was spray cast onto either planar or roller substrates of copper and steel. Substrate surface speed was in the range of 0.02 to 1 m/s, which produced strip of 0.025 to 0.0007 m thickness, respectively, with a width of 0.1 m. Surface condition, microstructure, and extent of porosity in the strip were characterized as a function of distance from top and bottom surfaces. The microstructure of the strip is comprised of three regions —a ‘chill zone’ at the bottom surface consisting of fine grains of ferrite and pearlite with numerous pores; a middle region containing equiaxed or columnar grains, Widmanstätten plates, and fine pores; and a top region made up of equiaxed grains comprising Widmanstätten plates and a few pores. Process variables of primary importance with respect to microstructural integrity and surface condition of the strip are substrate velocity, the surface condition of the substrate, flight distance, and the uniformity of droplet flux in the spray cone. Flight distance determines the amount of cooling of the droplets by the atomizing gas and, therefore, the average temperature of the spray incident on the substrate. Microstructure is determined by convective cooling of the spray, and, to a lesser extent, by the substrate velocity and temperature. The processing conditions required to spray cast strip with a homogeneous microstructure and uniform thickness/surface condition have been established.     

双辊薄带连铸0Cr18Ni9不锈钢的直接冷轧研究

为了改善双辊薄带连铸技术得到的0Cr18Ni9不锈钢薄带的表面质量和综合力学性能,对0Cr18Ni9不锈钢薄带进行了直接冷轧的研究.结果表明,不锈钢薄带经直接冷轧后,组织和性能都有明显的改善,各向异性不明显,耐腐蚀性能大大提高,从而论证了0Cr18Ni9不锈钢薄带直接冷轧的可行性.     

A study on inhomogeneous distribution of temper graphite particles in strip-cast Fe-C-Si alloys

This study examined the relationship between solidification structure and graphitization characteristics of white cast iron strips produced by strip casting. Experimental results showed that there was an unusual distribution of temper graphite particles along the through-thickness direction of the graphitized strips in comparison with gravity-cast chill plate. In particular, the graphite-free zones appeared in the vicinity of the strip surface after the completion of graphitization, especially in the strip with low carbon and silicon content. There were abnormally straight interfaces between matrix and eutectic cementite with a strong preferred [001]_c growth direction caused by the effect of directional solidification found in the near-surface regions of the strips. The interfaces did not form a site for the graphite to nucleate and gave rise to the graphite-free zones close to the strip surface. An increase in carbon and silicon content could significantly increase the number of temper graphite particles and shorten the time for the completion of graphitization, but an inhomogeneous distribution feature of graphite particles was still observed in strips with a higher carbon equivalent value (CE). Furthermore, variations in carbon and silicon content resulted in transitions in carbide morphology and composition, which had a tremendous effect on the graphitization characteristics of the cast iron strips.     

Analysis and prevention of microcracking phenomenon occurring during strip casting of an AISI 304 stainless steel

This study was concerned with the effects of microstructural parameters on the microcracking phenomenon occurring during strip casting of an AISI 304 stainless steel. Detailed microstructural analyses of the microcracked regions showed that microcracks were formed mainly along tortoise-shell-shaped depressions and that their number and size were considerably reduced when strip casting was done right after a shot-blasting or pickling treatment of the casting roll surface. This microcracking phenomenon was closely related to the formation of a black oxide layer, which was mainly composed of manganese-rich oxides, on the roll surface. The black oxide layer acted as a barrier of thermal transfer between the rolls and melt, led to an increased gas gap and inhomogeneous solidification of cast strips, and, thus, played a role in forming both tortoise-shell—shaped depressions and microcracks on the strip surface. The installation of brush rolls behind the casting rolls was suggested as a method to prevent microcracks, because the brush rolls could continuously scrape off the black oxide layer affixed on the roll surface during strip casting.     

Effect of Solidification Texture on the Magnetostrictive Behavior of Galfenol

This study investigates the magnetostrictive functionality of crystallographic textures developed by directional solidification of a vacuum-melted galfenol cast button. A polycrystalline Fe_82.4Ga_17.6 alloy was melted using vacuum arc melting and solidified in a water-cooled copper mold. Optical metallography confirmed the development of large columnar grains in the solidification microstructure. Phase constitution and magnetic domain structures of sample were studied by X-ray diffraction (XRD) and magnetic force microscopy (MFM). The results showed that the cast button has a disordered body-centered cubic (bcc) (A2) single-phase structure that consisted of a combination of well-aligned stripe-like and maze-like magnetic domains. To investigate the magnetostriction behavior, a couple of pins were cut along columnar grains as well as in the transverse direction. Magnetostriction was measured in the presence of an externally applied magnetic field. It was found that pins cut along the columnar grains comprised very high magnetostriction, whereas transverse pins had lower magnetostriction against the applied field.     

Influence of mechanical factors on solidification structure of thin strips of Fe–6.5wt-%Si alloy by twin roll casting process

Strips (1.6?mm thick) of Fe–6.5wt-%Si alloy were prepared by twin roll casting. The microstructure was found to be mainly columnar and equiaxed grains. The plastic deformation of the strip is inhomogeneous in the thickness direction. The most serious deformation is located at a certain depth from the strip surface. The influence of mechanical factors on the solidification structure of the strip is discussed. As the two solidifying shells contact with each other at the end of solidification, the columnar growth fronts break into fragments, which finally grow into equiaxed grains. At the hot rolling stage, the inhomogeneous deformation of the strip is attributed to the fact that the ductility of the alloy is sensitive to temperature. There is a large temperature gradient in the thickness direction, which can be up to 510°C?mm^??1.     

铁素体不锈钢铸轧薄带凝固组织对冷轧退火带晶粒簇的影响

 研究了Cr17铁素体不锈钢铸轧薄带凝固组织对其冷轧退火带晶粒簇、成形性和皱折特性的影响。利用电子背散射衍射技术对Cr17铁素体不锈钢铸轧薄带及相应的冷轧退火带进行了显微织构分析。结果表明：①Cr17铁素体不锈钢铸轧薄带冷轧退火带的晶粒簇依赖于初始铸轧薄带的凝固组织类型;②柱状晶组织的铸轧薄带具有显著的{001}∥ND晶体取向特征,而等轴晶组织的铸轧薄带晶体取向随机、分散;③等轴晶组织铸轧薄带比柱状晶组织铸轧薄带的冷轧退火带具有更少的{001}<110>晶粒簇和更多的{111}<112>、{111}<110>晶粒簇;④铸轧薄带的等轴晶组织比柱状晶组织有利于提高冷轧退火带的成形和抗皱性能。     

The Impact of Melt-Conditioned Twin-Roll Casting on the Downstream Processing of an AZ31 Magnesium Alloy

Melt conditioning by intensive shear was used prior to twin-roll casting of AZ31 magnesium alloy strip to promote heterogeneous nucleation and to provide a refined and uniform microstructure without severe macrosegregation. The cast strip was then processed by homogenization, hot rolling, and annealing, and its downstream processing was compared with a similar cast strip produced without melt conditioning. Melt conditioning produced strip with accelerated kinetics of recrystallization during homogenization and improved performance in hot rolling and improved tensile properties. An average tensile elongation of ~28 pct was achieved, which is substantially higher than the ~9 pct obtained for the strip produced without melt conditioning which is consistent with reported values (~6 pct to 16 pct). The as-cast, homogenized, and hot-rolled microstructures of the strip were characterized. The kinetics of homogenization and hot-rolling process have been discussed in detail.     

双辊铸轧凝固组织的数值模拟

对双辊板带铸轧的凝固过程中金属的形核和生长行为进行了数值模拟,建立了一套描述双辊板带凝固组织参数的表征体系,从而为双辊铸轧板带凝固组织特征的定量数学描述奠定了理论基础.同时为该领域的试验研究提供了理论基础.     

M2高速钢铸带组织特征及其后续处理

研究了M2高速钢在不同制备条件下的凝固组织特征以及工业铸带中碳化物在高温热处理、热变形作用下的变化,测量了在不同制备条件下高速钢的凝固速度和共晶碳化物网的厚度,采用透射电镜研究了后续高温热处理、热变形对工业铸带中碳化物相的影响,采用定量金相法分析了制备条件和后续处理工艺对铸带组织的影响.研究结果表明,双辊薄带连铸工艺可以细化高速钢凝固组织的枝晶和共晶碳化物网的厚度,改善碳化物的分布,后续高温热处理和热变形可以进一步优化工业铸带中的碳化物组织.建议在工业铸带的后续处理中同时采用高温热处理和热变形工艺以改善铸带组织.