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.     

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.     

Development of laboratory scale thin strip caster and investigation of direct casting of AISI 304 stainless steel

Abstract

An unequal diameter (1 : 3), two roll thin strip casting machine has been designed and fabricated for investigation of the direct casting of thin strip on a laboratory scale. The system consists of a preheatable shallow tundish with online heating facility, water cooled rotating rolls (chill and auxiliary) for solidification of the liquid metal, and a stripper assembly. The machine has a variable speed. Roll gap setting and roll pressure adjustment are two important features of the machine. It is also possible to vary the placement angle of the auxiliary roll with respect to the chill roll. A heat transfer model was developed, based on experimental casting results. Experiments were conducted using AISI 304 grade stainless steel. Up to 100 kg of steel was cast without interruption into strips of widths 100 and 200 mm and thickness varying between 1 and 2 mm. Some of the process parameters affecting the quality of the strip were identified.     

Study of microstructure of low-temperature plasma-nitrided AISI 304 stainless steel

The microstructure of the low-temperature plasma-nitrided layer on AISI 304 austenitic stainless steel was studied by transmission electron microscopy (TEM). The results show that the surface of the layer consists of a supersaturated solid solution (γ′_N) based on the γ′-Fe₄N phase whose electron diffraction pattern (EDP) has a strong diffuse scattering effect resulting from supersaturating nitrogen (above 20 at. pct) and 〈110〉 streaks arising from matrix elastic strain due to the formation of paired or clustered Cr-N. The latter is due to the N above the 20 at. pct γ′-Fe₄N-phase value and leads to a lattice parameter that is greater than that of the γ′-Fe₄N phase. The subsurface of the layer is composed of a supersaturated solid solution based on γ-austenite, which is an expanded austenite, γ _N. Its morphology shows the basketweave or “tweedlike” contrast consisting of so-called stacking fault precipitates having twin relationships with the matrix whose EDP shows diffuse scattering streaks with certain directions. The ε martensite transformation was observed in the subsurface of the layer. The increase in stacking faults compared with the original stainless steel and formation of ε martensite in the subsurface of the layer indicate that nitrogen lowers the stacking fault energy of austenite.     

Mechanical properties of type 304 stainless steel sheet produced by twin-roll strip casting process

Tensile properties and formability of type 304 stainless steel sheets which were cast by a twin-roll type strip caster and thereafter cold-rolled are examined and compared with those of conventionally processed sheets. The results exhibit that the strip casting processed sheets have mechanical properties almost equal to the conventional ones. Furthermore, it is found that the degree of plastic anisotropy caused by crystallographic anisotropy in the present sheets is weaker than that in the conventional ones.     

Texture enhancement by inoculation during casting of ferritic stainless steel strip

Melt/substrate contacting experiments, designed to approximate conditions encountered during strip casting, were carried out to produce as-cast ferritic stainless steel strip. The results show that inoculation of the melt to produce TiN particles, together with casting onto a smooth substrate, results in the optimum conditions for nucleation and subsequent growth of an exceedingly high volume fraction of ferrite grains with 〈001〉 oriented within a few degrees of the normal direction (ND) of the strip surface. It is argued that, during casting, TiN particles either nucleate or deposit onto the substrate with 〈001〉 parallel to the ND, and since these particles exhibit crystallographic features similar to δ-ferrite, subsequent epitaxial growth inherits the initial particle orientation. Such oriented nucleation of ferrite from a smooth substrate results in the optimum heat-transfer conditions for further growth of dendrites with 〈001〉 perpendicular to the substrate, thus producing the intense through-thickness 〈001〉//ND fiber texture in the as-cast strip. The potential for producing grain-oriented silicon iron by direct strip casting is outlined.     

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\     

薄带连铸结晶辊表面处理形貌研究进展

介绍了薄带连铸结晶辊表面处理形貌的作用及研究进展.结晶辊表面形貌主要通过控制辊面的传热和凝固壳的形核长大两个方面来影响薄带连铸的凝固过程.根据形貌特征,结晶辊表面处理形貌主要可以归为规则沟槽纹理形貌、不规则的表面毛化形貌和复合处理形貌等三类.对三类不同形貌的特点及在薄带连铸工艺中对传热、凝固形核和表面质量的影响进行了分析总结.纹理形貌可以改善薄带连铸的凝固和传热特性,表面毛化形貌能减少裂纹、鳄鱼皮等缺陷的形成,复合处理的表面形貌能够进一步提高铸带的表面质量.     

304不锈钢带板形控制的有限元分析

 为了掌握十二辊轧机中间辊横移量对带钢板形的影响规律,运用ANSYS/LS-DYNA显示动力学有限元软件,对十二辊轧机轧制不同宽度的304奥氏体不锈钢带进行中间辊横移控制的数值模拟。研究了当中间辊沿操作侧进行－30～－50 mm横向移动及带钢宽度变化时,工作辊有载辊缝和带钢应变曲线的变化情况。结果表明,动态调节中间辊横移量使对中长度[L]占来料宽度的55%～60%,可以获得平直度高的板形。     

Ferromagnetic properties of deformation-induced martensite transformation in AISI 304 stainless steel

Ferromagnetic properties of plastically deformed AISI 304ss have been studied using magnetic hysteresis and Barkhausen emissions methods. The present study has been concentrated on low volume fraction of martensite, i.e., below 58 pct, as compared to the available literature for a higher percentage of martensite. In measured materials, the coercivity increased with deformation and had a tendency to go toward saturation value. A linear increase in remanence with the deformation was observed. A large number of small amplitude of Barkhausen emissions were found at low percentage of martensite, indicating that magnetization rotation took place within a small region. However, large amplitude Barkhausen emissions were observed with the increase of deformations. Angular variation of Barkhausen emissions indicated the formation of rolling texture within the materials. A model has been proposed to explain the results. At the initial stage, small martensite clusters are formed, which grow with the deformation, and the intracluster exchange interaction becomes predominant. With the increase of deformation, martensite volume fraction increases. In this process, existing clusters grow and new clusters are formed. As a result, martensite clusters come closer and intercluster exchange interaction becomes important.     

冷轧硅脱氧304不锈钢中氧化物夹杂的演变行为

 为实现固态钢基体内夹杂物在冷轧过程中的控制,将硅脱氧304不锈钢热轧板经多道次冷轧至不同的厚度,利用扫描电子显微镜对试样内夹杂物在冷轧过程中的演变行为进行了研究。结果表明,硅脱氧304不锈钢内夹杂物的类型主要为低熔点SiO2-CaO-MnO-Al2O3,其在热轧板内的形貌为大尺寸长条状。冷轧时,这些长度为2.0～23.0 μm的长条状氧化物夹杂发生断裂,形成多个1.0～3.0 μm小尺寸夹杂物。随着冷轧压下量的增加,断裂后形成的夹杂物尺寸逐渐减小。但当夹杂物尺寸降低至约0.5 μm时,夹杂物不再发生断裂。同时,断裂后形成的小尺寸夹杂物之间的距离与夹杂物的初始尺寸无关,由冷轧板的伸长率决定。     

Experimental observations and analysis of creep cavitation in AISI type 304 stainless steel

Large numbers of creep cavities have been measured in AISI type 304 stainless steel using automatic image analysis techniques. Among other things, the data exhibit substantial amounts of scatter, and indicate that the distribution of creep cavities may not be spatially homogeneous, even in the presence of uniform stress and temperature fields. The cavity radii appear to be approximately distributed according to a conditional Weibull distribution. Indirect evidence is presented to support the view that the nucleation of cavities is heavily influenced by grain boundary sliding. Comparisons are drawn with the observed cavity radius distributions and the predictions of the analytical cavity growth model of Chen and Argon and a constrained cavity growth model due to Rice. It is found that the distribution predicted by the constrained growth model is considerably narrower than the experimental distribution, but that the model of Chen and Argon shows reasonable agreement with experiment.     

304不锈钢热轧钢带边部质量控制

不锈钢热轧带钢边部是容易发生质量问题的敏感区域。介绍了304不锈钢在热轧钢带生产过程中易出现的边部鳞折、狭缝、粗糙痕迹等缺陷的主要形貌特征,分析了这些缺陷产生的原因。针对这些边部质量问题,并结合不锈钢生产经验,提出了一系列相应的改进方法和控制措施,对304不锈钢边部质量控制具有借鉴意义。     

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.     

Twinning-induced sluggish evolution of texture during recrystallization in AISI 316L stainless steel after cold rolling

This paper deals with the evolution of texture in AISI 316L austenitic stainless steel during annealing after 95 pct cold rolling. After 95 pct cold rolling, the texture is mainly of the brass type {110}〈112〉, along with a scatter toward the S orientation {123}〈634〉 and Goss orientation {011}〈100〉. Weak evidence of Cu component is observed at this high deformation level. During annealing, recovery is observed before any detectable recrystallization. Recrystallization proceeds through nucleation of subgrain by twinning within the deformed matrix and, later, preferential growth of those to consume the deformed matrix. After recrystallization, the overall texture intensity was weak; however, there are some discernible texture components. There was no existence of the brass component at this stage. Major components are centered on Goss orientation and Cu component {112}〈111〉 as well as the BR component {236}〈385〉. Also, a few orientations come up after recrystallization (i.e., {142}〈2−11〉 and {012}〈221〉). With increase in annealing temperature, the textural evolution shows emergence of weak texture with another new component, {197}〈211〉. The evolution of texture was correlated with the deformation texture through twin chain reaction.     

双辊薄带连铸生产不锈钢技术探讨

不锈钢是双辊薄带连铸工艺重点开发的钢种之一.综述了不锈钢的分类和性能特点,简要介绍了不锈钢的几种生产型式,详细讨论了双辊薄带连铸生产不锈钢的技术要点和难点.     

304不锈钢连铸过程中结晶器渣圈形成机制

 在连铸过程中,结晶器易结渣圈是造成铸坯产生缺陷的主要原因之一。对304不锈钢板坯连铸过程中结晶器保护渣原渣、距开浇60 min时的液渣和渣圈的化学组成、理化性能、结晶矿相以及渣圈形貌结构进行对比分析。结果表明,连铸过程中TiO2和Cr2O3从钢液进入液渣生成高熔点氧化物,使液渣和渣圈的完全熔化温度和黏度显著增大,碱度、转折温度降低。液渣与渣圈的物相以枪晶石和钙铝黄长石为主。高熔点相钙铝黄长石的大量析出以及TiO2和Cr2O3进入液渣使液渣黏度增大是渣圈形成并长大的重要原因。