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.     

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.     

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.     

304不锈钢水槽应力腐蚀开裂分析

针对304不锈钢水槽底面特别是出水口处发生的锈蚀现象,通过直读光谱仪、金相显微镜、图像分析仪、扫描电镜及能谱仪,对304不锈钢锈蚀水槽的化学成分、显微组织、裂纹形貌及腐蚀产物的成分进行了检测分析.结果表明,304不锈钢水槽的化学成分、显微组织都符合JIS G 4305标准,水槽锈蚀的主要原因为冲压油未及时清洗,在马氏体、拉应力和氯离子的共同作用下,发生晶间应力腐蚀所致.     

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.     

Hydrogen assisted cracking of type 304 stainless steel

This paper reports a study of hydrogen assisted cracking in type 304 stainless steel. It shows that the most detrimental effect in increasing the susceptibility of the material to hydrogen cracking is the formation of martensite upon deformation. This is particularly damaging if the martensite is localized at the grain boundaries. With martensite present intergranular impurities such as phosphorus play a secondary role. As martensite becomes more difficult to form, the importance of impurities increases.     

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\     

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.     

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。     

Analysis and prevention of cracking phenomenon occurring during cold forging of two AISI 1010 steel pulleys

This study is concerned with the effects of microstructural parameters on the cracking phenomenon occurring during cold forging of two AISI 1010 steels that were fabricated by converter steel making and electric furnace steel making, respectively. This allowed a comparison between microstructures that contained a small or large amount of nitrogen. Detailed microstructural analyses of the cracked region showed that a number of adiabatic shear bands, along which cracks initiated and propagated, were formed in the top interior part of the cold-forged pulley. Dynamic torsional tests were conducted using a torsional Kolsky bar in order to investigate the dynamic deformation behavior during cold forging, and then the test data were compared via microstructures, mechanical properties, adiabatic shear banding, and fracture mode. From the dynamic shear stress-strain curves, the steel containing a considerable amount of nitrogen showed a smaller shear strain of 0.2 at the maximum shear stress point, after which the shear stress decreased rapidly prior to fracture, whereas the other steel containing a smaller amount of nitrogen showed relatively homogeneous shear deformation. This dynamic torsional behavior correlated well with the cracking and adiabatic shear banding behavior, together with the yield-point phenomenon occurring in the steel containing more nitrogen. Because the cracking occurring during cold forging was associated with the adiabatic shear banding and the yield-point phenomenon, the minimization of nitrogen and the fast cooling rate after hot rolling were suggested to prevent the cracking.     

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.     

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.     

Stress-corrosion cracking of sensitized type 304 stainless steel in thiosulfate solutions

The stress corrosion cracking of a sensitized Type 304 stainless steel has been studied at room temperature using controlled potentials and two concentrations of sodium thiosulfate. In both constant extension rate and constant load tests, the crack velocities attain extremely high values, up to 8 μm s^-1. Scratching electrode experiments conducted at various pH values on simulated grain boundary material show that both the crack initiation frequency and crack velocity are closely related to the repassivation rate of the grain boundary material as expected on a dissolution-controlled mechanism; however, the maximum crack velocity at any potential is consistently about two orders of magnitude higher than that predicted from the electrochemical data. Frequent grain boundary separation ahead of the crack tip is thought to occur, but retarded repassivation of the grain boundary material is a necessary feature of the cracking. Effects of strain-generated martensite are discussed.     

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。     

304不锈钢精密钢带表面“炉灰”缺陷分析

 在304不锈钢精密带光亮退火生产环节，钢带表面易出现金属粉的现象，导致产品表面质量存在不稳定性。针对304不锈钢精密钢带表面出现的“炉灰”缺陷，利用扫描电子显微镜观察其形貌，并用能谱仪对缺陷处局部成分进行了点扫描和面扫描检测。结果表明，304精密钢带“炉灰”缺陷微观形貌为白色微颗粒。结合能谱微区分析结果和相关研究分析，推测“炉灰”缺陷主要是由于304不锈钢基体组织中硼元素质量分数过高，钢带在光亮退火过程中硼原子易与保护气氛中分解的活性较高的氮原子结合，生成氮化硼析出而形成的。通过控制不锈钢基体中硼元素质量分数不大于0.001 5%，从而达到有效降低304不锈钢精密钢带“炉灰”缺陷的目的。     

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.