Microstructural characteristics of prematurely failed cold-strip mill work-rolls: Some observations on spalling susceptibility

A comprehensive metallurgical investigation was carried out on samples of prematurely failed cold-strip mill work-rolls used in an integrated steel plant to study the influence of microstructural characteristics on failure propensity and roll life. The samples pertained to 3 wt.% Cr-base forged steel work-rolls, which exhibited variations in roll life despite operation under similar mill environments. Optical and SEM revealed that while a uniform dispersion of fine globular carbides was conducive to higher roll life, carbides of angular and/or elongated morphologies acted as stress-raisers, induced microcracking of the tempered martensite matrix, and accentuated eventual spalling. From the standpoint of nonmetallic inclusions, higher life rolls were cleaner. Low/poor life rolls showed higher incidences of MnS and angular TiN inclusions, which often provided sites for the precipitation of undesirable elongated carbides. Although microprobe analysis indicated that carbides in these rolls were essentially M₇C₃, quantitative metallography revealed that, apart from morphology, roll performance was profoundly influenced by carbide content and count. Interestingly, while higher life rolls were characterized by carbide contents of >4.5 vol.% and counts of >200,000 number/mm², rolls exhibiting low and poor lives showed significantly lower values of these features. X-ray diffractometry of spalled roll specimens indicated that while higher life rolls contained minimal retained austenite, rolls exhibiting lower lives inevitably contained >10% retained austenite. The deleterious effect of excessive retained austenite on the spalling susceptibility of cold-strip mill work-rolls was attributed to its possible transformation to martensite under imposed rolling stresses.     

热轧板带轧机高速钢复合轧辊断裂失效分析

针对热轧板带工作辊在轧制过程中发生的断辊事故,对其失效原因进行了分析。通过对轧辊断口宏观形貌,材质化学成分、显微组织、工作层残余奥氏体含量及断口形貌的分析,认为轧辊断裂属于应力断裂.而造成轧辊断裂的主要原因是工作层残余奥氏体含量过高,轧辊在使用过程中残余奥氏体发生马氏体相变,巨大的内应力导致裂纹的产生;同时轧辊工作层显微组织中网状碳化物,芯部C、Mn含量不足以及铸造孔洞缺陷加速了裂纹扩展而造成的。为此,对轧辊的制造工艺、使用及维护提出了相应的措施,确保了轧辊的正常使用。     

低合金高硼高速钢轧辊的铸态组织

设计了一种用廉价的硼元素部分（或全部）取代普通高速钢轧辊中价格昂贵的合金元素的轧辊。借助光学显微镜（OM）、X射线衍射（XRD）、扫描电镜（SEM）和透射电镜（TEM）等方法对高硼高速钢轧辊铸态组织进行研究。结果表明：高速钢轧辊铸态组织主要由马氏体基体和少量的残余奥氏体以及M23（B,C）6,M3（B0.7C0.3）和（W,Mo）2（B,C）碳硼化合物组成,基体显微硬度达到820～950 HV,轧辊的铸态硬度大于64 HRC。     

Microstructural features of prematurely failed hot-strip mill work rolls: Some studies in spalling propensity

Work rolls made of indefinite chill double-poured (ICDP) iron are commonly used in the finishing trains of hot-strip mills (HSMs). In actual service, spalling, apart from other surface degeneration modes, constitutes a major mechanism of premature roll failures. Although spalling can be a culmination of roll material quality and/or mill abuse, the microstructure of a broken roll can often unveil intrinsic inadequacies in roll material quality that possibly accentuate failure. This is particularly relevant in circumstances when rolls, despite operation under similar mill environment, exhibit variations in roll life. The paper provides an insight into the microstructural characteristics of spalled ICDP HSM work rolls, which underwent failure under similar mill operating environment in an integrated steel plant under the Steel Authority of India Limited. Microstructural features influencing ICDP roll quality, viz. characteristics of graphite, carbides, martensite, etc., have been extensively studied through optical microscopy, quantitative image analysis (QIA), and electron-probe microanalysis (EPMA). These are discussed in the context of spalling propensity and roll life.     

Characteristics of the Transformation of Retained Austenite in Tempered Austempered Ductile Iron

Controlling the amount of retained austenite is a concern in austempered ductile iron formation. Retained austenite has a strong influence on austempered ductile iron properties, such as hardness and wear resistance. In this research, the characteristics of the transformation of retained austenite were investigated as a function of the number of tempering cycles. The hardness of the austempered ductile iron samples was measured, and the specific amount of retained austenite was analyzed by x-ray diffraction (XRD). Wear tests were conducted on a ball-on-flat sliding fixture. The tempering process was found to have no effect on the hardness of the austempered ductile iron samples. This may be due to retained austenite being partially converted into brittle quenched martensite during the tempering process. However, tougher tempered martensite was also formed from existing martensite. The two effects seemed to offset each other, and no significant differences occurred in overall hardness. XRD analysis showed that under the same austempering temperature and holding time, the amount of retained austenite decreased with additional tempering cycles. Also, with the same holding time and tempering cycles, less retained austenite was contained in the matrix at higher austempering temperatures. This was due to more high carbon content austenite and needle-like ferrite being present in the austempered ductile iron matrix. In addition, tempered austempered ductile iron exhibited significantly higher wear resistance as compared to traditionally treated ductile iron.     

Q&P钢残留奥氏体含量的热动力学计算

基于CALPHAD方法建立了Q&P钢的配分扩散模型,并建立了一套特定成分在特定QP工艺下的组织转变计算任务流,通过计算QP钢一次淬火过程的马氏体/残留奥氏体含量和配分过程中残留奥氏体的碳富集量,并结合Thermo-Calc软件内置的基于吉布斯自由能的马氏体相变本构模型,预测稳定保留至室温的残留奥氏体含量。利用该模型计算文献钢种(Fe-0.2C-1.28Mn-0.37Si-0.0018B, wt%)的室温残留奥氏体含量,结果显示计算马氏体转变温度比试验数据高60 ℃,计算室温残留奥氏体含量为4.41%,与试验数据基本吻合,从而验证了该计算模型的半定量性。利用该模型进一步计算分析了碳、锰元素含量和热处理制度对AQT980和AQT1180钢一次残留奥氏体含量的影响规律,计算结果显示碳、锰元素含量的增加可使钢中相变点(A₃、Ms、Mf)温度下降;在固定淬火温度下,钢中的碳含量和锰含量增加可使一次残留奥氏体含量大幅增加;当碳、锰元素含量一定时,一次淬火温度的上升会使一次残奥含量大幅增加。     

Effect of quenching temperature on structure and properties of centrifugal casting high speed steel roll

The critical points and time-temperature-transformation (TTT) curves of the isothermal transformation diagrams for a high-speed steel casting on a horizontal centrifugal casting machine had been determined experimentally in the study. The effects of quenching temperature on the microstructures and properties of centrifugal casting high speed steel (HSS) roll has been investigated using scanning electron microscopy (SEM), light optical microscopy (LOM) and X-ray diffraction (XRD) as well as using tensile, impact, and hardness tests. The results show that the HSS roll has excellent hardenability and its matrix structure can be transformed into the martensite after being quenched in the sodium silicate solution. The retained austenite in the quenching structure increases and the hardness decreases when the quenching temperature exceeds 1,040℃. The tensile strength and impact toughness of HSS roll increase once the quenching temperature is raised from 980℃ to 1,040℃. However, the tensile strength and impact toughness have no significant change when the quenching temperature exceeds 1,040℃. The HSS roll quenched at 1,040℃ exhibits excellent comprehensive mechanical properties.     

电磁连续铸造法制备高速钢复合轧辊

进行了高速钢复合轧辊的电磁连续铸造实验,在工艺参数匹配得当的情况下,制备出了外观形状规整、振痕轻微,内部组织致密,结合面质量良好的辊坯。分析结果表明,辊坯从外到内依次为激冷凝固层、枝状晶区、界面结合区和辊芯金属区。外层高速钢主要是马氏体+贝氏体+残余奥氏体+合金碳化物组织,其中的共晶碳化物呈迷宫状、短条状和粒状,辊芯为珠光体+沿晶界析出的少量铁素体,晶粒细小。双金属复合界面的结构主要由扩散层、激冷凝固层和柱状晶区组成。     

边界润滑条件下18Cr2Ni4WA钢的摩擦诱发马氏体相变及其耐磨特性的研究

用18Cr2Ni4WA钢,经渗碳、淬火和冷处理,以获得不同残留奥氏体含量的试样,进行了与T10钢金属盘对磨的边界润滑磨损试验研究.结果表明,磨损过程中确实存在摩擦诱发马氏体相变,且碳含量低的残留奥氏体较碳含量高的残留奥氏体更易发生诱发相变;摩擦诱发马氏体对提高材料的耐磨性是有利的,而残留奥氏体对耐磨性的影响却存在一个临界应力σc;当外加应力＞σc时,残留奥氏体对提高材料的耐磨性有利;当外加应力＜σc时,残留奥氏体对提高材料的耐磨性不利,并且当残留奥氏体和外加应力搭配适当时,耐磨性可达最高值.     

Effect of Heat Treatment on Microstructure and Mechanical Properties of Quenching and Partitioning Steel

In order to investigate the effect of microstructural characterization on the mechanical properties and retained austenite stability, a different type of quenching and partitioning steel(I-QP) through intercritical annealing before the quenching and partitioning process was designed, which can realize lamellar intercritical microstructure compared to the conventional quenching and partitioning(QP) process. The morphology of ferrite and martensite/retained austenite is lamellar in the I-QP steel while it is equiaxed after being heat-treated by conventional QP process. The I-QP steel is proved to have better formability and mechanical properties than conventional QP steel, which is due to the highervolume fraction of retained austenite in the I-QP steel and confirmed by electron backscattering diffraction patterns and X-ray diffraction. Furthermore, the stability of retained austenite in I-QP steel is also higher than that in conventional QP steel, which is investigated by tensile tests and differential scanning calorimetry.     

炼钢粉尘回收用热轧工作辊辊套的热处理工艺

制定了炼钢粉尘回收用热轧工作辊辊套的热处理工艺参数并应用于实际生产。检验结果表明,该辊套辊身硬度达52~54 HRC;其金相组织为回火马氏体+未溶碳化物+残余奥氏体;淬硬层深度不低于40mm,各项力学性能指标均达到设计要求。     

On The Enhancement of Wear Resistance of Hardened Carbon Tool Steel （AISI 1095） With Cryogenic Quenching

Many experimental investigations reveal that it is very difficult to have a completely martensitic structure by any hardening process. Some amount of austenite is generally present in the hardened steel. This austenite existing along with martensite is normally referred as the retained austenite. The presence of retained austenite greatly reduces the mechanical properties and such steels do not develop maximum hardness even after cooling at rates higher than the critical cooling rates.Strength can be improved in hardened steels containing retained austenite by a process known as cryogenic quenching.Untransformed austenite is converted into martensite by this treatment. This conversion of retained austenite into martensite results in increased hardness, wear resistance and dimensional stability of steel. Wear can be defined as the progressive loss of materials from the operating surface of a body occurring as a result of relative motion at the surface. Hardness, load,speed, surface roughness, temperature are the major factors which influences wear. Many studies on wear indicate that increasing hardness decreases the wear of a material. With this in mind, to study the surface wear on a surface modified (Cryogenic treated) steel material an attempt has been made in this paper. In this study as a Part -I Hardening was carried out on carbon tool steel (AISI 1095) of different L/D ratio with conventional quenchants like purified water, aqueous solution and Hot mineral oil. As a Part -II hardening was followed by quenching was carried out as said in Part- I and the hardened specimen were quenched in liquid Nitrogen which is at sub zero condition. The specimens were tested for its microstructure, hardness and wear loss. The results were compared and analyzed. The alloying elements increases the content of retained austenite hence the material used was AISI 1095 (Carbon 0.9%, Si 0.2%, Mn0.4% and the rest Iron)     

On the Microstructural Strengthening and Toughening of Heat-Affected Zone in a Low-Carbon High-Strength Cu-Bearing Steel

In this article, the influence of simulated thermal cycles for the heat-aff ected zone(HAZ) on the microstructural evolution and mechanical properties in a low-carbon high-strength Cu-bearing steel was investigated by microstructural characterization and mechanical tests. The results showed that the microstructure of the coarse-grained heat-aff ected zone(CGHAZ) and the fine-grained heat-aff ected zone(FGHAZ) was mainly comprised of lath martensite, and a mixed microstructure consisting of intercritical ferrite, tempered martensite and retained austenite occurred in the intercritically heat-aff ected zone(ICHAZ) and the subcritically heat-aff ected zone(SCHAZ). Also, 8–11% retained austenite and more or less Cu precipitates were observed in the simulated HAZs except for CGHAZ. Charpy impact test indicated that the optimum toughness was obtained in FGHAZ, which was not only associated with grain refinement, but also correlated with deformation-induced transformation of the retained austenite, variant confi guration as interleaved type and a relatively weak variant selection. The toughness of ICHAZ and SCHAZ exhibited a slight downtrend due to the presence of Cu precipitates. The CGHAZ has the lowest toughness in the simulated HAZs, which was attributed to grain coarsening and heavy variant selection. In addition, the contribution of Cu precipitates to yield strength in simulated HAZs was estimated based on Russell–Brown model. It demonstrated an inverse variation trend to toughness.     

Control of Austenite Characteristics and Ferrite Formation Mechanism by Multiple-Cyclic Annealing in Quenching and Partitioning Steel

Quenching and partitioning (Q&P) treatment is a novel method to produce advanced high strength steel with excellent mechanical properties. In this study, combination of multiple-cyclic annealing and Q&P process was compared with traditional cold-rolled Q&P steel to investigate the microstructural characteristics and austenite retention. The results showed that retained austenite in traditional Q&P sample was principally located in the exterior of austenite transformation products, while those in multiple-cyclic annealing samples were mainly distributed inside the transformation products. With the increase in cyclic annealing number, both of austenite fraction and austenite carbon content increased, attributing to higher initial austenite carbon content and larger number of austenite/neighbored phase interface to act as carbon partitioning channel. In traditional Q&P sample, the deformed ferrite was recrystallized by sub-grain coalescence, while the austenite was newly nucleated and grew up during annealing process. As a comparison, the ferrite in multiple-cycle annealing samples was formed by means of three routes: tempered martensite that completely recovered with retention of interior martensite variant, epitaxial ferrite that formed on basis of tempered martensite, ferrite that newly nucleated and grew up during the final annealing process. Both of lath martensite and twin martensite were formed as initial martensite and then tempered during partitioning process to precipitate ε carbide with C enrichment, Mn enrichment and homogeneous Si distribution. Compared with the traditional cold-rolled Q&P steel, the Q&P specimens after multiple-cyclic annealing show smaller strength and much larger elongation, ascribing to the coarser microstructure and more efficient transformation induced plasticity (TRIP) effect deriving from retained austenite with high carbon content and larger volume fraction. The application of double annealing treatment can optimize the mechanical properties of Q&P steel to show a striking product of strength and elongation as about 29 GPa%, which efficiently exploit the potential of mechanical performance in low carbon steel.     

两相区轧制后低碳中锰钢中残留奥氏体在单向拉伸过程中的演变行为

通过对轧制后低碳中锰钢薄板在不同预拉伸变形量下的组织进行了分析,并研究了单向拉伸过程中残留奥氏体组织演变的规律。试验结果表明,残留奥氏体向马氏体的相变主要发生在局部塑性变形阶段即屈服阶段,屈服阶段结束后,组织中的残留奥氏体基本全部转变为马氏体,试样进入均匀塑性变形阶段。拉伸过程中位错在局部的大量堆积和逐步迁移,在宏观上表现为吕德斯带的产生和移动,导致局部区域组织中的残留奥氏体发生相变,并且这种现象大多是沿着轧制方向进行的,并不会重复发生。     

φ2800 mm冷轧机Cr5平整辊热处理工艺参数的制定

对Cr5材料φ2 800 mm冷轧平整辊热处理工艺参数进行了研究并应用于实际生产,成功制造出二次硬化冷轧用平整辊。检验结果表明,该平整辊辊身硬度达85~88HS,其金相组织为回火马氏体+未溶碳化物+残余奥氏体,淬硬层深度不低于30 mm,各项力学性能指标均达到设计要求。     

The effect of microstructure on hydrogen permeability of high strength steels

Hydrogen diffusivity and trapping have been studied in two advanced high strength steel grades and model samples using electrochemical permeation test. Microstructures of CP1000 and DP1000 steels consist of ferrite, martensite and a small fraction of retained austenite. In addition, bainite is present in CP1000. Model phases with predominance of a particular phase have been prepared by specific heat treatment. DP1000 has shown the lowest diffusivity among all materials, while ferritic model sample has shown the highest. Differences in hydrogen diffusion coefficient values are linked to trapping microstructural characteristics and grain size.     

Q&P工艺对22MnB5钢微观组织及力学性能的影响

利用光学显微镜、拉伸试验机、扫描电镜、XRD和EBSD等手段对22MnB5钢的微观组织及力学性能进行了表征,并重点分析了一步法Q&P工艺处理后的22MnB5钢中残留奥氏体含量及残留奥氏体中碳含量与力学性能的关系。结果表明：采用一步法Q&P工艺,可以获得抗拉强度超过1400 MPa,伸长率超过15%的超高强度22MnB5钢板。随着淬火温度从240 ℃升高至300 ℃,22MnB5钢的组织由马氏体转变为马氏体+残留奥氏体复相组织,试样中的残留奥氏体含量逐渐增加。相同配分温度延长配分时间,残留奥氏体含量呈现先增加后降低趋势。不同热处理工艺下残留奥氏体中的平均碳含量为1.49wt%。采用一步法Q&P热处理工艺可以使残留奥氏体中富集碳,提高残留奥氏体稳定性,强塑积可以达到22.14 GPa·%。     

Influence of interface mobility on the evolution of austenite–martensite grain assemblies during annealing

The quenching and partitioning (Q&P) process is a new heat treatment for the creation of advanced high-strength steels. This treatment consists of an initial partial or full austenitization, followed by a quench to form a controlled amount of martensite and an annealing step to partition carbon atoms from the martensite to the austenite. In this work, the microstructural evolution during annealing of martensite–austenite grain assemblies has been analyzed by means of a modeling approach that considers the influence of martensite–austenite interface migration on the kinetics of carbon partitioning. Carbide precipitation is precluded in the model, and three different assumptions about interface mobility are considered, ranging from a completely immobile interface to the relatively high mobility of an incoherent ferrite–austenite interface. Simulations indicate that different interface mobilities lead to profound differences in the evolution of microstructure that is predicted during annealing.     

MECHANISM OF TEMPER MARTENSITE EMBRITTLEMENT

计算了残余奥氏体分解和马氏体分解的相变总驱动力,渗碳体自马氏体析出和自奥氏体析出的形核驱动力、长大速率和孕育期。计算结果揭示:尽管奥氏体分解的驱动力大于马氏体分解约一个数量级,但渗碳体自马氏体析出的形核驱动力远大于自奥氏体的析出,长大速率约大两个数量级,孕育期比奥氏体析出的约小一个数量级。可见在低温短时回火时,渗碳体先从马氏体内析出;当回火时间较长时,由于奥氏体分解的相变总驱动力较大,以及由于马氏体条内析出渗碳体引起的收缩,促使条间残余奥氏体的分解,形成回火马氏体脆性。因此当不计及杂质(及渗碳体)在原始奥氏体晶界的偏聚对,回火马氏体脆性的机制应为这两个分解先后互相关联的过程。