Correlation of microstructure and thermal-fatigue properties of centrifugally cast high-speed steel rolls

This is a study of thermal-fatigue properties in centrifugally cast high-speed steel (HSS) rolls. The thermal-fatigue mechanism was investigated, with special focus on the roll microstructure and the increase in tensile stress which led the specimens to fracture when it reached the tensile strength. The thermal-fatigue test results indicated that the thermal-fatigue life decreased with increasing maximum temperature of the thermal-fatigue cycle. The results were then interpreted based on the amount of carbides and the cyclic-softening phenomenon associated with the exposed time to elevated temperatures. The coarse intercellular carbides on the specimen surface acted as fatigue-crack initiation sites as they cleaved at a low stress level to form cracks. The roll having the lowest matrix hardness and the smallest amount of intercellular carbides, thus, showed better thermal fatigue properties than the other rolls. For the improvement of the thermal-fatigue properties of the rolls, this study suggests a homogeneous distribution of carbides by reducing the carbide segregation formed along the solidification cell boundary and by optimizing the roll compositions.     

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.     

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.     

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。     

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.     

New Method for Evaluating Thermal Wear of Rolls in Rolling Process

A new method was developed by a thermal wear machine to evaluate the thermal wear of roils in steel rolling process. The steel strip and rolls were simulated by upper and lower heating disks. The upper heating disk could he kept at a temperature of over 900 ℃ by induction heating. The pressure between the disks as high as 323.2 MPa could be achieved and the slipping rate could be 12. 7 %. The thermal wear of high speed steel （HSS） roll material, the wear rate of the HSS roll, and the SEM morphology of a worn HSS roll surface were investigated. This method was useful and could be employed to simulate friction and wear between strip and roll during the strip rolling process.     

Effect of rare earth elements on the thermal cracking resistance of high speed steel rolls

The effect of rare earth elements on the thermal cracking resistance of high speed steel (HSS) rolls was investigated. Laser rapid heating was used for thermal fatigue experiments. Thermal cracks and microstructure were observed using metalloscopy and scanning electron microscopy. The results showed that thermal cracks initiated from the interface between the matrix and eutectic carbides (including M6C and M7C3 type carbides),and propagated along the interface between the two phases. MC type carbides enriched with vanadium could prevent the propagation of thermal cracks. The presence of rare earth elements decreased the quantity of big eutectic carbides,and proportionally increased spherical and rod-shaped MC type carbide content. HSS0 (0.00% RE) had approximately three times the thermal cracking density of HSS3 (0.12 wt.% RE). Rare earth elements were shown to significantly improve the microstructure and thermal cracking resistance of HSS rolls.     

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.     

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.     

Resistance to High-Temperature Oxidation and Wear of Various Ferrous Alloys Used in Rolling Mills

Various materials are commonly used to manufacture work rolls for hot rolling mills, such as ICDP (Indefinite Chill Double Pour) cast irons, high-chromium white cast irons, and high speed steels (HSS). Various chemical compositions and microstructures are studied in order to optimize the in-use behavior of those grades of rolls. In this paper, six grades of ferrous alloys (an ICDP cast iron; an ICDP cast iron enriched in vanadium, niobium, and molybdenum; a HSS; a graphitic HSS; a high-chromium white cast iron (Hi-Cr); and a niobium-molybdenum-doped high-chromium white cast iron) were investigated. High-temperature oxidation tests with gravimetric means at 575 °C in water vapor atmosphere and sliding wear tests were carried out. The oxidation kinetics was followed during oxidation test. The microstructure was observed by optical and scanning electron microscopies. The oxides formed on the surface of the samples were analyzed by XRD and EDS. The thickness of the oxide scales and the mass gain were measured after oxidation test. The results showed that the behavior of all the grades differed. The oxide scale of HSS and HSS-G grades was fine and their friction coefficient was low. The weight gain after oxidation test of HSS was high. Hi-Cr and M-Hi-Cr grades presented highly porous oxide layer and an important increase of the friction coefficient during wear test. ICDP and M-ICDP had intermediate behavior.     

喷射成形含铌M3型高速钢的组织和耐磨性

采用喷射成形技术制备了M3型高速钢和以Nb替代V的M3型高速钢.利用扫描电镜、X射线衍射、差示扫描量热仪和金相显微镜研究了Nb对M3型高速钢组织的影响.喷射成形能有效消除宏观偏析,细化组织.以Nb代V,提高了MC型碳化物开始析出温度,大量MC相先于共晶反应析出,呈独立的近球形分布于晶界,同时其尺寸减小.由于消耗大量C,抑制了共晶反应,M₂C片层数量减少且厚度变薄,其在热变形过程中更易于分解,进一步增加了组织均匀性.低温低载荷时含铌的M3型高速钢抗磨损性能显著优于M3高速钢,温度升高到500℃时磨损机制逐渐以氧化磨损为主,两合金的抗磨损性能差距减小,主要原因是大量呈弥散球形分布的含铌MC型碳化物能有效提高高速钢的磨粒磨损抗性,而其对抗氧化性能并无明显作用.      

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.     

高温渗碳轴承钢旋弯疲劳裂纹萌生与扩展行为

 为了提高航空轴承的服役寿命,借助QBWP-10000X型旋转弯曲疲劳试验机,研究了高温渗碳轴承钢的旋转弯曲疲劳性能和裂纹萌生扩展行为。结果表明,钢的中值疲劳强度达到913.3 MPa。有效渗层中大量M₂₃C₆和少量M₆C碳化物显著提高了试验钢的表面硬度,渗层不同碳浓度导致马氏体先后发生相变而形成408 MPa表面压应力,进而提高了钢的疲劳性能。疲劳裂纹主要萌生在表面缺陷和次表面碳化物,分别占比71.4%和 28.6%。萌生裂纹缺陷特征尺寸及承载应力对应力强度因子和循环次数影响显著,深犁沟形状由于涉及应力集中而直接影响疲劳循环次数,承受相同加载应力碳化物特征尺寸越大,循环次数越低。裂纹萌生后沿渗碳层碳化物边界快速扩展同时向芯部缓慢扩展,最后在试样疲劳源对侧近边缘区域发生准解理和韧性混合断裂。     

Oxidation and Thermal Fatigue Behaviors of Two Type Hot Work Steels During Thermal Cycling

Thermal fatigue test has been carried out on widely used hot work steel 4Cr5MoSiVl and a low alloyed steel 3Cr3MoV in temperature range of 200 to 700 °C. Tempering resistance, as well as high temperature hardness/strength of steel specimens, works as a dominating material parameter on thermal fatigue resistance. During the heating period, high hardness can depress the inelastic deformation. This deformation is the origination of tensile stress, which acts as the driving force of heat checking during the cooling period. The cyclic strain-oxidation interaction can speed up the damage on surface defects, which plays an obvious role in initiation of thermal cracks. On 4Cr5MoSiVl steel specimens, borders between the matrix and inclusions such as titanium compounds, or lager carbides such as primary carbides, are focused by strain and attacked by oxidation, and are main initiating places of cracks. While on 3Cr3MoV steel specimens, larger strain causes plastic deformation concentrating around grain boundaries. Then the following oxidation accelerates this grain boundary damage and creates cracks.     

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.     

热连轧精轧高速钢轧辊重复上机初始辊形预报

针对热连轧精轧高速钢轧辊重复上机时存在残留磨损辊形和残留热辊形影响初始辊形的问题,分析了残留磨损辊形和残留热辊形对板形控制精度的影响及难点,得出轧辊温度场是高速钢轧辊重复上机初始热辊形最佳表征方式的结论,提出高速钢轧辊下机后空冷过程温度场建模思路,并建立轧辊空冷过程温度场计算模型。在此基础上,基于热连轧生产线二级系统框架,研究形成了高速钢轧辊重复上机初始辊形预报技术设计方案,并在首钢京唐公司1 580 mm热连轧生产线应用。应用结果表明,该技术对实现热连轧精轧高速钢轧辊更高效应用和提高板形控制精度有显著效果。     

汽车排气系统用铁素体不锈钢的耐热性研究

利用实验室条件下冶炼的Fe-Cr-Nb-Mo铁素体不锈钢,对其高温强度和热疲劳性能进行了研究。研究结果表明:实验钢的高温抗拉强度和屈服强度优于或基本相当于目前所使用的铁素体不锈钢的性能;实验钢的热疲劳裂纹从V形缺口处萌生,夹杂物为裂纹萌生优先区域;热疲劳裂纹长度和扩展速度随热疲劳上限温度的升高而增大;热疲劳裂纹优先沿晶界扩展,高温氧化是疲劳裂纹扩展的主要因素。     

全机架高速钢轧辊轧制无取向硅钢的应用研究

研究分析了高速钢轧辊的特性、轧辊磨损及高速钢的热膨胀,并通过在无取向硅钢的轧制中使用全机架高速钢轧辊的上机试验,优化轧制工艺,结果表明:与普通轧辊相比,高速钢轧辊轧制无取向硅钢后,有利于延缓边降,利于无取向硅钢的断面控制,从而提升后工序的同板差精度.     

High-speed steel rolls used for cold rolling

During cold rolled production of steel,each change of rolls causes a halt in production and affects the roll’s grinding maintenance and consumption.Consequently,rolls are very critical to the costs of steel production. Besides the rolling accidents,surface quality problems,including inhomogeneous wear and a decrease of the surface roughness of the rolls are other main reasons for outage and a change of the rolls.Therefore,safe rolls,with superior wear resistance and roughness retentivity will be a future trend in the cold rolling steel industry.In this study,the property characteristics and in-service performance of high-speed steel(HSS)cold rolling work rolls at Baosteel are discussed.The results of this study indicate that in-service performance of HSS cold work rolls has an improvement over conventional rolls.Implementation of HSS work rolls will prolong the rolling campaign and improve the rolling stability,thus,the cost of cold rolling production can be better controlled.