HP295钢板冲压开裂原因的分析

通过试验找出了ＨＰ２９５钢板开裂的主要原因是ＭｎＳ夹杂沿轧向分布，使钢板的横向延性降低，并提级弯开裂是由于试样加工当造成ＭｎＳ夹杂物外露而成为裂纹源所致。     

20MnSi热轧带肋钢筋冷弯断裂及拉伸脆断分析

对２０ＭｎＳｉ热轧带肋钢筋冷弯断裂及拉伸脆断试样进行金相观察、断口扫描及能谱分析，发现钢中碳、锰、硅成份不均、非金属夹杂、连铸坯内裂纹、轧制工艺控制不当等是造成２０ＭｎＳｉ热轧带肋钢筋性能不合的主要原因，并据此提出冶炼、连铸轧制过程中的改进措施。     

钢水凝固过程中MnS夹杂形态的控制

利用金相显微镜，电子显微镜，我们研究了钢水凝固过程中形成的ＭｎＳ夹杂形成及加入合金元素Ａｌ，Ｓｉ，Ｃ，Ｔｉ对其形态的改善作用。低硫钢水在凝固期间，形成铁素体后生成的二次ＭｎＳ夹杂形态划分如下：（ｉ）由包晶反应产生的球状或滴状ＭｎＳ；（ｉｉ）由共晶反应产生的条状ＭｎＳ；（ｉｉｉ）伪共晶反应产生的鱼骨状ＭｎＳ。在富含硫的钢水中，初生相是ＭｎＳ。根据钢水中各元素性质及熔炼气氛，这些初生ＭｎＳ夹杂的形态可     

20钢水平连铸圆坯中间裂纹形成研究

通过对衡阳钢管厂２０钢水平连铸坯的（Ｐ），（Ｓ），Ｍｎ／Ｓ及其对应的中间裂纹统计，得出：（Ｐ），（Ｓ），Ｍｎ／Ｓ对中间裂纹有较大影响，随（Ｐ）的增加，中间裂纹的敏感性增大，随（Ｓ）的增加，敏感性增大；随Ｍｎ／Ｓ增加，敏感性变小。     

粒状贝氏体组织对200MnSi带肋钢筋性能的影响

２０ＭｎＳｉ带肋钢筋进行拉伸和冷弯试验时,有σｓ有明显和脆断现象发生。金相显微组织检查发现,有粒状贝氏体存在,这是导致σｓ不明显和脆断的根源。而粒状贝氏体的形成主要取决于钢的化学成分,尤其与Ｍｎ含量偏高（〉１．６％）及残留元素Ｍｏ含量有着密切联系,也与轧制时的加热温度和冷却速度有关。因此,必须严格控制２０ＭｎＳｉ钢中的残留元素含量和Ｍｎ偏析,严格轧制工艺。     

夹杂物短疲劳裂纹的产生和早期扩展

根据６种主要的试样取向，在不同应力和应力方的所形成和测定的数据，研究了细长ＭｎＳ夹杂物含量高的Ｅｎ７Ａ钢的短疲劳裂纹的产生和早期扩展行为。短裂纹通常产生基基体和夹杂物之间断开的地方，在体和夹杂物之间是连续的，那么，裂纹有时会在夹杂物中形成，在早期阶段，短裂纹是这夹杂物影响扩展的机制蔓延，在低应力之下，通常产生单一的并在很大程度上，决定疲劳寿命的短裂纹，而在高应力状态下则存在多个裂纹的相互作用。     

55SiMnVB连铸坯的凝固组织与质量

分析了５５ＳｉＭｎＶＢ弹簧钢铸坯的凝固组织，测定了树枝晶二次枝晶臂间距，同时对铸坯中的非金属夹杂物了金相形貌的观察与复写，结果表明，５５ＳｉＭｎＶＢ弹簧钢铸坯的凝固组织较疏轻，二次枝晶臂间距较一般合金结构钢小（１００～２５０μｍ）；激冷层较薄（６～８ｍｍ）；中心等轴晶率低（２０％～２６％），铸坯夹杂物含量偏高（０．７１％～１．２８％），降低钢液过热度和提高凝固末端电磁搅拌强度，对减轻偏析，提高中心     

高强度矿用链条钢的开发

通过对２０ＭｎＶ、２５ＭｎＶ高强度矿用链条钢的开发及用户现场跟踪试验，探索出了合理的２０ＭｎＶ、２５ＭｎＶ冶炼、轧制工艺制度，并分析了用户在生产圆环链时产生裂纹的原因。     

宣钢转炉冶炼20MnSi钢的质量研究

分析了宣钢转炉生产的２０ＭｎＳｉ钢的化学成分不均匀及氧化物夹杂的形成原因。     

低合金高强度钢焊缝金属夹杂物与针状铁素体形核的研究

针对５００～６００ＭＰａ级低合金高强度钢的韧性问题，研究了针对状铁素体的形成的机理，并对夹杂物的类型，成分对针状铁素体形成的影响进行了分析，探讨，指出Ｔｉ，Ｍｎ，Ａｌ，Ｓｉ等的复合氧化物夹杂可有效地促进针状铁素体形核，而ＭｎＯ．ＳｉＯ２夹杂和ＭｎＳ夹杂不能促进针状铁素体形核。     

Development of microstructural banding in low-alloy steel with simulated Mn segregation

The development of microstructural banding in low-alloy steel with Mn segregation has been investigated through the use of artificially segregated steel, interrupted cooling techniques, and optical microscopy. Mn segregation was simulated by hot roll bonding thin sheets of 5140 steel with 0.82 wt pct Mn and modified 5140M with 1.83 wt pct Mn into a plate with 20- and 160-μm-thick segregated layers. Samples were austenitized at 850 °C, continuously cooled at 1 °C/s and 0.1 °C/s, and quenched from progressively lower temperatures to observe the evolution of the microstructure. The segregated band thickness had a striking effect on microstructural development. Samples with 160 μm bands cooled at 1 °C/s had martensite and bainite in high-Mn bands. In contrast, samples with 20 μm bands cooled at the same rate had pearlite in high-Mn bands. The dramatic effect of band thickness on microstructural development was due to growth of a fully pearlitic band at the interface between segregated layers. The formation of interfacial pearlite is discussed relative to redistribution of carbon between adjacent high- and low-Mn bands during cooling.     

Oxide defects in a vacuum investment-cast ni-based turbine blade

Samples from large turbine blades for power generation, investment-cast in vacuum from a nickel-based superalloy, were investigated. Samples were cut from regions near the top of the casting that contained freckle defects. The microstructures of these segregated regions were compared with those from nonsegregated adjacent regions using both optical microscopy and scanning electron microscopy (SEM). The segregated areas revealed a high density of carbides and a network of cracks. Samples were prepared by carefully fracturing along the cracks so as to observe the surfaces of the cracks. Observation by SEM revealed the presence of inclusions identified as films that appeared to have initiated the growth of carbides. Fractures of random areas of the nonsegregated alloy revealed that the films were numerous and widely distributed. In all cases, the films were principally oxides (and/or possibly nitrides) of aluminum and chromium. It was hypothesized that the films had originated by entrainment of the surface film on the liquid metal during the turbulent pouring of the casting. The films could, therefore, be assumed to be double, because the entrainment mechanism is a folding action. It follows that the doubled-over films constitute (1) the observed cracks and (2) the substrates for carbide precipitation. Evidence from other alloy systems is presented to support this conclusion.     

耐候钢板表面微裂纹研究

通过金相观察和扫描电镜分析等,对连铸坯热轧耐候钢板表面微裂纹进行了研究,发现裂纹附近存在Cu和Cr元素,初期裂纹沿晶界形成和延伸。Cu在γ晶界的偏聚以及Cr在基体和氧化铁层问的富集,是导致表面微裂纹的主要原因。生产耐候钢板应合理控制加热温度和时间,加强高压水除鳞。     

Microstructural effects on high-cycle fatigue-crack initiation in A356.2 casting alloy

The effects of various microconstituents on crack initiation and propagation in high-cycle fatigue (HCF) were investigated in an aluminum casting alloy (A356.2). Fatigue cracking was induced in both axial and bending loading conditions at strain/stress ratios of −1, 0.1, and 0.2. The secondary dendrite arm spacing (SDAS) and porosity (maximum size and density distribution) were quantified in the directionally solidified casting alloy. Using scanning electron microscopy, we observed that cracks initiate at near-surface porosity, at oxides, and within the eutectic microconstituents, depending on the SDAS. When the SDAS is greater than ∼ 25 to 28 μm, the fatigue cracks initiate from surface and subsurface porosity. When the SDAS is less than ∼ 25 to 28 μm, the fatigue cracks initiate from the interdendritic eutectic constituents, where the silicon particles are segregated. Fatigue cracks initiated at oxide inclusions whenever they were near the surface, regardless of the SDAS. The fatigue life of a specimen whose crack initiated at a large eutectic constituent was about equal to that when the crack initiated at a pore or oxide of comparable size.     

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.     

Fatigue damage and crack nucleation mechanisms at intermediate strain amplitudes

Polycrystalline copper was fatigued in rotary bending at constant intermediate surface strain amplitudes at 26 Hz under ambient conditions. The specimens were interrupted at various life fractions, their surfaces prepared metallographically and scrutinised to ascertain the types of fatigue damages, namely, short cracks which are confined to individual grains or isolated grain boundary facets, and their role in fatal crack formation. The results show that, at intermediate strain amplitudes, slip band and twin boundary crack damages predominate during early stages of cycling, while grain boundary crack damages remain relatively insignificant even at the stage when fatal cracks have developed. However, depending on the strain amplitude level, the transgranular crack damages may or may not be instrumental in fatal crack formation. At the lower amplitude end of the transition region, fatal cracks are formed by interlinkage of slip band and twin boundary damages. At the higher amplitude end, even though grain boundary damages are negligible initially, they degenerate rapidly on further cycling and eventually evolve into fatal cracks. The present findings show that some 0.05% plastic strain amplitude is required to propagate intergranular cracks. Once the above condition is met, cracks would propagate rapidly along the interface and the crack nucleation mode would change from transgranular to intergranular.     

Structural effects and band segregate formation during the electromagnetic stirring of strand-cast steel

Chemical and metallographic studies have been made of the “white band” (a zone of negative segregation) formed by the electromagnetic stirring of 127 mm (5 inch) square strand-cast steel billets. Three stirrer locations were studied. At all locations, the white band was found to consist of a zone of negative segregation corresponding to the position of the solid-liquid interface during stirring, followed by a zone of positive segregation related to the cessation of stirring. Metallographic studies showed the curvature of columnar dendrites within the white band and the presence of a transition zone of nondirectional growth between the white band and the equiaxed core. A mechanism for the formation of the segregated zones at the white band is presented, along with theoretical calculations. Agreement between the calculations and experimental results was good. The role of electromagnetic stirring in the formation of the white band and in the associated columnar-equiaxed transition is discussed. During the period of this work the authors were Research Engineers at the United States Steel Company Research Laboratories, Monroeville, PA 15146.     

Correlation of Microstructure and Cracking Phenomenon Occurring during Hot Rolling of Lightweight Steel Plates

An investigation was conducted into the correlation of microstructure and the cracking phenomenon that often occurred in hot-rolled lightweight steel plates. Two kinds of steels were fabricated with varying Mn and Al contents, and their microstructures, tensile properties, and high-temperature transformation behavior were investigated. In the two steels, banded structures containing ferrite grains and κ-carbides were well developed along the rolling direction. Detailed microstructural analyses showed that cracks initiated at film-type κ-carbides continuously formed interfaces between bands, while the band populated with κ-carbides did not play an important role in initiating cracks. Thus, the formation of band structures and film-type interfacial κ-carbides must be minimized to prevent the cracking. The decreased content of hardenability elements, including aluminum, higher finish-rolling temperature, reduced central segregation during the slabmaking process, and decreased material variation during hot rolling, were suggested as practical methods for preventing the cracking.     

Phenomenological Description of the Surface Morphology and Crack Formation of Continuously Cast Peritectic Steel Slabs

The surface quality of continuously cast material is strongly depending on the initial solidification of the steel. Oscillation marks are formed at the very early stages of the strand shell growing process, thus influencing the microstructure and cracking behaviour of the surface and subsurface region. An industrial study of the oscillation mark morphology and the surface structure of peritectic medium carbon steel slabs was performed. The formation of oscillation marks and their effect on the surface quality was examined by metallographic investigations of slab samples. Although constant casting and oscillation conditions were applied, a variation of oscillation mark geometry along the narrow faces of the slabs was measured. A relation between the depth of the oscillation marks and the thickness of a layer of segregated melt situated inside the bottom of the marks was found. Measuring the distribution and length of surface and subsurface segregated cracks in the vicinity of the marks, existing theories of oscillation mark formation could be confirmed. The austenite grain size was found to increase with increasing oscillation mark depth. There was no clear correlation between the austenite and the δ‐ferrite grain size.     

Effect of Rare Earth on Void Band of Diffusion Layer and Properties of Aluminized Steel

 The effects of the addition of rare earth (RE) elements on the void band in the diffusion layer, and the resistances to both oxidation and spalling of aluminized steel were investigated through high temperature oxidation and spalling tests. The results showed that RE had significant effects on the void band in the diffusion layer and the properties of aluminized steel. After diffusion treatment, a considerable number of the voids between the middle layer and transitional layer of pure aluminized coating, aggregated into wavy line shaped void bands parallel to the outer surface. For the RE added aluminized coating, only a few voids aggregated into intermittent block shapes. During high temperature oxidation at 800 ℃ for 200 h, the wavy void band of pure aluminized coating aggregated further into a linear crack parallel to the outer surface, and the internal oxidation occurred within them; the open cracks perpendicular to the surface penetrated through the diffusion layer. For the RE added aluminized coating, only a few voids aggregated into intermittent meniscus shapes. During cyclic spalling tests, the peeling, spallation, and pulverulent cracking occurred along the void band in the diffusion layer of pure aluminized coating, but only a little spallation occurred in the diffusion layer of the RE added aluminized coating, in which cracks perpendicular to the surface were much smaller than those of pure aluminized coating and did not penetrate through the diffusion layer. It is evident that RE addition can restrain the formation and aggregation of voids and subsequently improve the resistances to oxidation and spalling. The mechanism of the RE effect on the void band in the diffusion layer is also discussed.