Correlation of microstructure and thermal fatigue property of three work rolls

This is a study of the thermal fatigue property in three centrifugally cast work rolls, i.e., a nickel-grain cast-iron roll (Ni-grain roll), a high-chromium cast-iron roll (Hi-Cr roll), and a high-speed steel roll (HSS roll). The thermal fatigue mechanism was investigated with a focus on the roll microstructure and the increase in tensile stress which led the specimen to fracture when it reached the tensile strength. The thermal fatigue test results indicated that the thermal fatigue property was best in the HSS roll, followed by the Hi-Cr roll and the Ni-grain roll, respectively, and that the thermal fatigue life of each roll decreased with the increase of the mean temperature or of the temperature range of the thermal fatigue cycle. The results were then interpreted based on the amount of primary carbides and the cyclic softening phenomenon associated with the exposed time to elevated temperatures. The coarse primary carbides on the specimen surface acted as fatigue crack initiation sites, as they cleaved at a low stress level to form cracks. The HSS roll, having the highest tensile strength and the smallest amount of primary carbides, thus showed better thermal fatigue property than the other rolls. For the improvement of the thermal fatigue property of the rolls, this study suggests a homogeneous distribution of primary carbides by reducing the carbide segregation formed along the solidification cell boundary and by optimizing of the roll-casting process.     

Effects of alloying elements on microstructure,hardness, and fracture toughness of centrifugally cast high-speed steel rolls

A study was made of the effects of alloying elements on the microstructure, hardness, and fracture toughness of centrifugally cast high-speed steel (HSS) rolls. Particular emphasis was placed on the role of hard carbides located along solidification cell boundaries and the type of the tempered martensitic matrix. Microstructural observation, X-ray diffraction analysis, hardness and fracture-toughness measurements, and fractographic observations were conducted on the rolls. The constitution and morphology of carbides observed within the intercellular boundaries varied depending upon the predominant alloying elements that comprised them. These massive carbide formations strongly influenced the bulk material hardness and fracture toughness due to their high hardness. The effects of alloying elements were analyzed on the basis of the liquidus-surface diagram which and indicated that the proper contents of the carbon equivalent (CE), tungsten equivalent, and vanadium were 1.9 to 2.0, 10 to 11, and 5 to 6 pct, respectively. The roll material, containing a small amount of intercellular carbides and lath-type tempered martensitic matrix, had excellent fracture toughness, since carbides were well spaced. Therefore, it was suggested that the optimization of alloying elements was required to achieve the homogeneous distribution of carbides.     

Correlation of microstructure and fracture toughness in three high-speed steel rolls

The objective of this study is to clarify the fracture characteristics of high-speed steel (HSS) rolls in terms of microstructural factors such as matrix phase and primary carbide particles. Three HSS rolls with different chromium contents were fabricated by centrifugal casting, and the effect of the chromium addition was investigated through microstructural analysis, fracture-mechanism study, and toughness measurement. The hard and brittle primary carbides, as well as the eutectic carbides (ledeburites), were segregated in the intercellular regions and dominated overall properties. Observation of the fracture process revealed that these primary carbides cleaved first to form microcracks at low stress-intensity factor levels and that the microcracks then readily propagated along the intercellular networks. The addition of chromium to a certain level yielded microstructural modification, including the homogeneous distribution of primary carbides, thereby leading to enhancement of fracture toughness of the HSS rolls.     

Effects of carbon and chromium additions on the wear resistance and surface roughness of cast high-speed steel rolls

The effects of carbon and chromium additions on the wear resistance and surface roughness of five high-speed steel (HSS) rolls manufactured by a centrifugal casting method were investigated in this study. High-temperature wear tests were conducted on these rolls to experimentally simulate the wear process during hot rolling. The wear-test results revealed that the wear resistance increased with increasing carbon content because of the increased total carbide fraction. In the HSS roll containing a smaller amount of chromium, the wear resistance was improved because it contained a number of very hard MC carbides. However, it showed a very rough roll surface because of the preferential removal of the matrix and the sticking of the rolled material to the roll surface during the wear process. Rolls containing a higher chromium content had very low surface roughness, because of the formation of a larger amount of M₇C₃ carbides of slightly lower hardness and homogeneous wear of the matrix and carbides. In order to improve wear resistance with consideration of surface roughness, the increase in the chromium content, i.e., the increase in the amount of M₇C₃ carbides, which can result in a low surface roughness, was suggested.     

Effects of Alloying Elements on Microstructure,Hardness, Wear Resistance,and Surface Roughness of Centrifugally Cast High-Speed Steel Rolls

A study was made of the effects of carbon, tungsten, molybdenum, and vanadium on the wear resistance and surface roughness of five high-speed steel (HSS) rolls manufactured by the centrifugal casting method. High-temperature wear tests were conducted on these rolls to experimentally simulate the wear process during hot rolling. The HSS rolls contained a large amount (up to 25 vol pct) of carbides, such as MC, M₂C, and M₇C₃ carbides formed in the tempered martensite matrix. The matrix consisted mainly of tempered lath martensite when the carbon content in the matrix was small, and contained a considerable amount of tempered plate martensite when the carbon content increased. The high-temperature wear test results indicated that the wear resistance and surface roughness of the rolls were enhanced when the amount of hard MC carbides formed inside solidification cells increased and their distribution was homogeneous. The best wear resistance and surface roughness were obtained from a roll in which a large amount of MC carbides were homogeneously distributed in the tempered lath martensite matrix. The appropriate contents of the carbon equivalent, tungsten equivalent, and vanadium were 2.0 to 2.3, 9 to 10, and 5 to 6 pct, respectively.     

轧辊材料疲劳裂纹扩展理论计算与分析

辊面剥落是轧辊失效的主要形式之一。依据疲劳裂纹扩展理论,计算了轧辊材料裂纹扩展过程中裂纹扩展速率与应力强度因子幅的关系即da/d N—ΔK,分析了各因素对裂纹扩展速率的影响以及不同轧辊的裂纹扩展特征。结果表明,弹性模量、应力比、残余应力、晶粒度、断裂塑性、工作应力、断裂强度等对轧辊裂纹扩展速率的影响越来越不明显,弹性模量影响最大,而屈强比几乎没有影响;轧辊材料较高的弹性模量、较低的残余应力、较粗大的晶粒、较高的断裂塑性可以有效抑制轧辊裂纹的疲劳扩展;轧辊工作层比心部、支承辊比工作辊、锻钢辊比铸铁辊具有更高的耐裂纹扩展断裂能力。结果有助于分析轧辊失效机理并采取有效措施,防止轧辊剥落。     

Composition, microstructure, hardness, and wear properties of high-speed steel rolls

The effects of alloying elements on the microstructural factors, hardness, and wear properties of four high-speed steel (HSS) rolls fabricated by centrifugal casting were investigated. A hot-rolling simulation test was carried out using a high-temperature wear tester capable of controlling speed, load, and temperature. The test results revealed that the HSS roll containing a larger amount of vanadium showed the best wear resistance because it contained a number of hard MC-type carbides. However, it showed a very rough roll surface because of cracking along cell boundaries, the preferential removal of the matrix, and the sticking of the rolled material onto the roll surface during the wear process, thereby leading to an increase in the friction coefficient and rolling force. In order to improve wear resistance with consideration to surface roughness, it is suggested that a reduction in the vanadium content, an increase in solid-solution hardening by adding alloying elements, an increase in secondary hardening by precipitation of fine carbides in the matrix, and formation of refined prior austenite grains by preaustenitization treatment be employed to strengthen the matrix, which can hold hard carbides in it.     

Temperature and thermal stresses in work rolls during the hot rolling of strip

In this paper the solution of the unsteady, three-dimensional heat transfer in work rolls was derived by using Bessel functions and the δ-function and expressed by an infinite series. In this manner other problems of heat transfer in solid cylinders and hollow cylinders can also be calculated. The temperature profile of the work roll due to the distribution of the temperature in the axial direction can cause unregular pressure distribution between work roll and backup roll and can influence the quality of strip. Therefore, the thermal crowns must be considered at various moments during rolling. The thermal crown can be determined by using the FE-Method or others. The reason for the fire-cracking in the roll surface can be determined as the thermal shock load in the contact zone, in which the sharp compressive stresses lead to local plastic deformation. The local plastic deformations are followed by residual deformations. Therefore, residual tensile stresses occur in the cooling zone. On each revolution, the surface undergoes plastic strain in compression and in tension. The result is thermal fatigue. The fine network of cracks in the roll surface can result in a sharp stress concentration which is dangerous for the rolls loaded with a bending moment. The maximum tensile stresses due to the temperature distribution occur in the roll core, which usually do not lead to damages of the rolls.     

首钢3500 mm中板轧机辊型研究与应用

采用影响函数法建立轧辊的弹性变形解析模型,针对首钢3 500 mm中板轧机在轧制过程中,支承辊端部出现剥落或掉肉现象,根据首钢中板厂的轧制工艺,分析了不同板宽、不同轧辊辊型、不同轧辊端部倒角以及直倒角参数对辊间压力分布的影响规律.研究结果表明,在轧制能力范围内,轧件越宽,轧辊采用凹辊型可以使辊身部的辊间压力分布更为均匀;倒角的选择和改变直倒角的参数都会相应地改变端部的应力集中现象.研究结果为该轧机的辊型的改造和配置以及提高轧辊寿命提供了理论依据.     

平整机辊系变形的有限元分析

通过对辊系变形的模拟,分析了轧制条件下对工作辊弯曲变形、支撑辊弯曲变形及接触应力的影响,借助大型有限元分析软件ANSYS的手段,对辊系进行了弹性力学结构分析,得出了在轧制过程中辊系的变形情况及载荷分布等详细直观的分析结果。     

The Transformation of Carbides during Austenization and Its Effect on the Wear Resistance of High Speed Steel Rolls

The transformation of carbides with austenization time of a high speed steel (HSS) roll material, manufactured by a centrifugal casting method, has been studied. The correlation between wear resistance and the type, morphology, volume fraction, and distribution of the carbides has also been investigated. Microstructural observations, X-ray diffraction (XRD) analysis, hardness measurements, and energy dispersive spectroscopy (EDS) have been used to characterize the carbides. The type and volume fraction of carbides were found to change with austenizing time. During austenization, the transformation of the M₃C carbides can be postulated as M₃C + γ-Fe → M₂C, with much finer nodular and rodlike MC carbides also forming through a solid-state transformation. The M₂C carbide decomposes as M₂C + γ-Fe → MC + M₇C₃ + M₆C. The decomposed carbide substantially maintains a platelike shape until the end of decomposition. The most important finding of this study is that austenization results in changes in the type, morphology, volume fraction, and distribution of carbides and that it can be controlled to produced a homogeneous distribution of hard carbides, resulting in an improvement in the wear resistance of HSS rolls. This finding may be of great use for the industrial production of HSS rolls.     

HC可逆冷轧机轧辊失效改进措施的试验研究

以1200HC6辊可逆冷轧机轧辊为研究对象,从轧辊检测、磨削和装配等多方面提出了轧辊缺陷的预防和消除措施.首先,采用磁粉探伤、便携式轧辊表面硬度检测仪、低倍组织分析等先进检测手段,及时发现肉眼难以观测到的轧辊缺陷.其次,通过改进工艺冷却润滑系统和过滤装置,给定轧辊合理的磨削量和换辊周期,采取中间辊、支撑辊两个肩部加工复合倒角等措施,降低轧辊消耗,提高轧辊寿命.改进后,轧辊换辊周期由原来的100 km提高到160 km,轧辊缺陷、轧辊消耗(含中间辊和支撑辊)大幅度降低,轧机小时产量提高11.1%.     

高铬复合辊中铬偏析对剥落的影响

分析了高铬复合辊中铬偏析形成的原因及对剥落的影响。铬偏析影响共晶碳化物的组成及形态,控制组织对裂纹扩展的阻力,对轧辊的使用性能有直接的影响。     

直上钢Q235B板坯高拉速下中间裂纹的形成与调控

 基于凝固数值模拟和板坯应变计算分析了直上钢Q235B连铸中间裂纹的形成机理和影响因素,并根据实际生产条件提出了铸坯高拉速质量控制策略。研究发现,辊缝和对中精度为Q235B铸坯中间裂纹形成的主要因素,两者应变之和占凝固前沿总应变60%以上;而表面回温高于50 ℃/m的铸机前段冷却也有重要影响,其应变占比最高约40%。当辊缝的邻辊正偏差为1.5 mm时,通过强化冷却和控制回温可使铸坯凝固前沿总拉应变整体降低约20%;当辊缝的邻辊正偏差为0.5 mm时,冷却优化对凝固前沿拉应变的影响较小。随着拉速增大,辊缝精度对铸坯中间裂纹的影响愈加显著。当前工况下,将锰硫比提高到25,坯壳可最多承受1.4 m/min时邻辊正偏差 1.5 mm、对中偏差±0.5 mm和回温不超过50 ℃/m带来的附加应变。为抑制1.5 m/min以上高拉速下直上钢Q235B铸坯的中间裂纹,建议将辊缝整体精度控制在±0.5 m且邻辊正偏差控制在0.5 mm以内。     

Design criteria for rolling contact fatigue resistance in back-up rolls

Abstract

This research centres on surface initiated damage on back-up rolls whereby rolling contact fatigue cracks can propagate into the rolls potentially reaching the internal stress fields and leading to catastrophic failure and has sought to establish design criteria for avoiding such failures. The project objectives have been achieved by examining field evidence, determining the loading and tribological conditions at the work roll/back-up roll interface and investigating both theoretically and experimentally the mechanisms involved in rolling contact fatigue in this case. The presented rolling contact, fatigue and fracture mechanics model includes criteria for crack branching either upwards (i.e.relative safety) or downwards (i.e.potentially catastrophic) and the link between these two cases is related, quantitatively, to the properties of the roll material. After linking mechanics to microstructure, the influence of work roll test disc surface roughness on both the surface wear of and the interaction between wear and rolling contact fatigue at the surface of back-up roll test discs has been quantified using the results obtained from experimental simulations carried out on a rolling–sliding testing machine. Finally practical quantitative recommendations are made for the mechanical and microstructural design of bainitic back-up roll materials, back-up roll redressing procedures and the surface roughness of both work rolls and back-up rolls presented to the mill.     

Mechanical properties of ultra-high-strength steels at elevated temperatures

ABSTRACT

This paper presents an experimental study on the mechanical properties of ultra-high-strength steels at elevated temperatures. Tensile tests were carried out at 300–600°C on Docol 1200M and Docol 1400M steel samples. The results indicate that as the temperature increases Young’s Modulus, yield strength (YS) and ultimate tensile strength (UTS) display a decrease. YS/UTS ratios at 300°C are lower than those at room temperature, they make peaks at 400 and 500°C for Docol 1400M and Docol 1200M, respectively, and then decrease again beyond those temperatures. While total elongation continuously increases, uniform elongation slightly decreases with increasing temperature. Present carbides in tempered matrix continue to grow and new carbides are observed at the grain boundaries. Considering all roll forming parameters, 300°C seems the most convenient temperature for warm forming. In this sense, the warm roll forming has a potential for forming complex-shaped parts by reconciling strength with formability.     

新型电磁调控轧机弯辊辊型电磁补偿及初始辊缝

 为解决薄带轧制过程中的各类板形问题,以新型电磁调控轧机为研究对象,利用Marc建立三维热-力耦合有限元模型,分析了弯辊和电磁调控轧辊综合作用下弯辊力和轧制力对轧辊辊型状态、板形分布、板坯边部应力、辊间接触应力、承载辊缝形状的影响规律。结果表明,弯辊机制的施加将直接促进电磁调控轧辊的稳定胀形,使电磁调控轧辊胀形凸度得到整体性补偿,并以板形良好为依据,给出新型调控轧机合理的弯辊力施加范围。对比分析了不同弯辊力和轧制力下辊缝函数的变化情况,形成不同的二次、四次凸度,为板形控制及初始辊缝设定提供依据。     

半高速钢冷轧辊材料的锻后淬火工艺

 研究了半高速钢冷轧辊材料的锻热淬火工艺,并与传统热处理工艺获得的力学性能和微观组织进行比较评价,得到如下结论：半高速钢冷轧辊材料锻热淬火（终锻温度大于等于900℃、油冷）+750℃回火后的碳化物尺寸适中,分布均匀弥散,细小的碳化物在最终热处理时容易固溶,对最终热处理组织较为有利;新工艺轧辊的强度、塑性和冲击韧性介于球化退火和调质热处理之间,有着比较均衡的强韧性,而且硬度适中,易于机械加工。因此,新的锻热淬火工艺可以代替传统的锻后热处理工艺,达到节能降耗、提高生产效率的目的。     

冷轧机工作辊非对称弯辊的板形调控理论研究与应用

为减少冷轧带钢的非对称板形缺陷的产生,设计了工作辊非对称弯辊控制系统.应用影响函数法计算辊系变形,同时考虑辊缝中金属横向流动对带钢出口横向张力分布的影响,通过迭代法计算出工作辊两端施加不同弯辊力后的辊间压力分布、出口厚度横向分布以及出口横向张应力分布.理论分析结果表明,工作辊非对称弯辊可以在一定程度上改善辊间压力分布不均,减轻轧辊磨损和减少轧辊掉皮事故的发生,降低带钢边部的非对称板形缺陷.实际应用结果证明,当倾斜调整量小于10%时,应用工作辊非对称弯辊替代倾斜调整,可以获得更好的板形精度.      

金相分析在轧辊强度计算中的应用

利用金相分析对不同石墨形态的球墨铸铁进行了强度对比。根据金相组织，初步判定了轧辊颈部扁头处发现灰斑的轧辊的芯部强度。结合轧机设计资料，对该轧辊在使用中可能出现的瞬时断裂及疲劳损坏进行了使用安全系数评估。在确保安全的前提下，将此辊装机使用，不仅为宝山钢铁股份公司成功挽回了1支轧辊(价值约14万元)，同时也为今后此类问题的解决，乃至对用理论计算方法确定有缺陷轧辊及新材质轧辊的上机安全性提供了依据。