Microstructures and Mechanical Properties of Bearing Steels Modified for Preparing Nanostructured Bainite

Mo containing high-C-Cr bearing steel was modified with Si (0.8–1.5 wt.%) and 0.8Si–1.0Al to prepare nanostructured bainite by low-temperature isothermal heat treatment. The modified steels were isothermal held at 220 to 240 °C after partial austenitization in an intercritical gamma+carbide region, and the resultant microstructure and mechanical properties were studied. Carbide-free nanostructured bainite with plate thickness below 100 nm and film retained austenite, as well as a small amount of undissolved carbide particles, was obtained in the modified steels except in 0.8Si steel, in which carbides precipitated in bainitic ferrite. As Si content increased, the mean thickness of bainitic ferrite plates modestly decreased, whereas the fraction of retained austenite markedly increased. The thickness of bainitic ferrite plate and the fraction of retained austenite in Si-Al-modified steel were smaller than those in Si-modified steels. The hardness and elongation of the Si-Al-modified steel were lower than those of Si-modified steels. The yield strength of Si-Al-modified steel was superior to that of Si-modified steels. Mid-level ultimate tensile strength and impact toughness were achieved in Si-Al-modified steel. For bearing applications, Si-modified steels could provide higher hardness and toughness but lower dimensional stability. Meanwhile, Si-Al-modified steel could offer higher dimensional stability but lower hardness and toughness.     

Ti微合金化对耐火建筑钢组织和性能的影响

采用低Mo及Ti的复合微合金化,设计3种试验钢配以合理的控轧控冷工艺,成功开发出低成本460 MPa级耐火钢。力学性能测试及显微组织分析结果表明,控轧控冷后水冷,试验钢板获得耐火钢的理想组织:粒状贝氏体和M/A岛。随Ti含量的增加,3种试验钢的平均晶粒尺寸递减。3种试验钢的室温屈服强度都大于460 MPa,600 ℃保温3 h的高温屈服强度都大于307 MPa,具有良好的高温力学性能。在相变强化、析出强化、细晶强化及位错强化的共同作用下,不同Ti含量的试验钢获得了良好的高温力学性能。0.07%Ti含量试验钢的YS值(600 ℃屈服强度/室温屈服强度)为0.68,完全满足耐火钢的使用标准。     

Mo对09CuPCrNi耐候钢连续冷却转变的影响

测定了09CuPCrNi耐候钢的连续冷却转变动力学图（CCT曲线）,并对不同冷速下的组织进行了观察。与传统09CuPCrNi耐候钢的CCT曲线相比,加入0．33％（质量分数,下同）的Mo后,贝氏体转变区与铁素体转变区部分分离;加入0．41％的Mo后,贝氏体转变区与铁素体转变区完全分离,同时珠光体转变区与铁素体转变区也完全分离。     

Microstructural Effects on Fracture Behavior of Ferritic and Martensitic Structural Steels

The effect of microstructure on fracture behavior of 1Cr-0.5Mo and 9Cr-1Mo structural steels was evaluated. 1Cr-0.5Mo steel is used in steam pipes and superheater tubes of power stations. Its microstructure is typically comprised of bainite in a pre-eutectoid ferrite matrix with an average grain size of 10 μm. 9Cr-1Mo steel was developed for applications in steam power stations and as a candidate structural material for first-wall and blanket components of future fusion reactors. Its microstructure consisted of a fully martensitic structure with a prior austenite grain size of 25 μm. The fracture properties were measured using instrumented impact testing at temperatures between ?196 and 300 °C. The total impact fracture energy, the crack initiation and propagation energy, the dynamic yield strength, the brittleness temperature, and the cleavage fracture stress were measured. The bainitic-ferritic alloy steel exhibited much higher resistance to ductile fracture at high test temperatures, while its resistance to brittle fracture at low test temperatures was reduced compared to that of the fully martensitic alloy steel. The results were discussed in terms of the chemical composition and microstructure of the two steel types.     

Evolution of Microstructures of a Low Carbon Bainitic Steel Held at High Service Temperature

Low carbon bainitic steel derives the high strength mainly from high density of dislocations rather than carbon and alloy element content, so it tends to evolve into equilibrium microstructure with low density of dislocations under thermal disturbance. In the present investigation, granular bainite and lath-like bainitic ferrite were produced respectively in Mo-free low-carbon steels by changing cooling rate. It has been found that granular bainite possesses a lower strength at room temperature than bainitic ferrite, but it exhibits a slower decrease of strength with temperature increasing. Dislocation density in both granular bainite and bainitic ferrite decreases via recovery and recrystallization at high temperature.However, when reheating of bainite is carried out at temperature below 600 °C, a long time will be needed for incubation of recrystallization, during which the hardness of bainite maintains stable. The property makes bainite, especially granular bainite, become a potential microstructure for matrix of high strength fire-resistant steel.     

低碳锰钢强韧化机制的研究

在实验室对低碳锰钢进行了控轧控冷试验.利用光学显微镜和透射电子显微镜等测试手段,对试验结果进行了研究.结果表明,具有铁素体和贝氏体复相组织的低碳锰钢具有较高的强度和良好的韧性.贝氏体相变强化对低碳锰钢屈服强度的贡献可达30%,贝氏体铁素体板条的细化和铁素体亚晶的存在可以降低碳锰钢的脆性转变温度.具有铁素体和贝氏体复相组织的低碳锰钢除了固溶强化之外,主要强化机制为细晶强化和贝氏体相变强化.     

Effects of Nb on the Microstructure and Mechanical Properties of Water-Quenched F<Subscript>GBA</Subscript>/B<Subscript>G</Subscript> Steels

In order to reduce the alloying cost, Mn-series low carbon water-quenched grain boundary allotriomorphic ferrite (F_GBA)/granular bainite (B_G) steels have been developed. The effect of 0.06 wt.% Nb on microstructure and mechanical properties of F_GBA/B_G steel was investigated. The result showed that the addition of 0.06 wt.% Nb improved the hardenability of the F_GBA/B_G steel, refined the grain size of F_GBA, promoted the granular bainitic transformation, and refined the granular bainite including its bainitic ferrite and martensite/austenite (M/A) constituents. With the addition of 0.06 wt.% Nb, the yield strength increased from 560 to 741 MPa, and the impact energy increased from 93 to 151 J, respectively, for 30-mm thickness steel plates. It is supposed that the addition of 0.06 wt.% Nb could improve the mechanical properties of the F_GBA/B_G steel by refining the microstructure and increasing the amount of strengthening phases.     

Microstructure,Mechanical Properties,and Unlubricated Sliding Wear Behavior of Air-Cooled MnSiCrB Cast Steels

Two medium carbon low-alloy MnSiCrB cast steels containing different Si contents (0.5 and 1.5 wt.%) were designed, and the effects of Si contents on the microstructure, mechanical properties, and unlubricated sliding wear behavior of the cast steels after air-cooling from 850 °C and subsequent tempering at 220 °C was studied. The results show that the microstructure of the cast steel containing 0.5 wt.% Si consists of granular bainite and lower bainite/martensite multi-phase. In the cast steel containing 1.5 wt.% Si, granular bainite was not observed. The microstructure consists of carbide-free bainite/martensite multi-phase. Excellent hardenability can be obtained at both low and high Si levels. The cast steel containing 0.5 wt.% Si exhibits excellent combination of strength, ductility, and impact toughness superior to the cast steel containing 1.5 wt.% Si. Also, the wear-resistance of the former steel is better than that of the latter in the unlubricated sliding wear condition. The air-cooled MnSiCrB cast steel containing low Si levels, with excellent mechanical properties and wear-resistance, is a potential high-performance and low-cost wear-resistant cast steel for unlubricated sliding wear condition.     

The Effects of Alloying Elements on the Continuous Cooling Transformation Behavior of 2¼Cr-1Mo Steels

The continuous cooling transformation (CCT) behavior of eight 2¼Cr-1Mo steels from a statistically designed matrix was determined. These steels contained two levels of carbon (0.07 and 0.16 wt.%), manganese (0.35 and 0.85%), chromium (1.5 and 2.8%), and molybdenum (0.3 and 1.25%). Each steel was tested in a quenching dilatometer at five to six cooling rates between 725 and 1.2 °C/min. For each CCT sample, the change in length, microstructure, and macrohardness were determined. The ferrite content was also measured for samples cooled near the ferrite nose. Pearson correlation and multiple regression analyses were performed for various CCT diagram parameters. The correlation analysis showed that carbon and chromium contents significantly affected the critical temperatures and the bainite and martensite transformation temperatures. Increasing carbon content significantly increased the hardness for the bainite and martensite range of cooling rates, but hardness at slower cooling rates was unaffected by alloying elements. Regression equations were obtained for the critical temperatures and the ferrite nose cooling rate.     

回火时间对高强抗震耐火钢板组织与力学性能的影响

采用OM、SEM、TEM、拉伸试验和冲击试验等,研究了600 ℃回火不同时间对690 MPa级高强抗震耐火钢板的力学性能、微观组织及析出行为的影响。结果表明,不同回火时间对耐火钢板的力学性能和微观组织有重要影响。耐火钢板经过600 ℃回火后强度稍有降低,但伸长率增大,屈强比降低,综合力学性能提高,低温冲击吸收能量随回火时间的延长而降低。最优回火保温时间为15 min,此时试验钢板的屈服强度为976 MPa、硬度为396 HV,-40 ℃冲击吸收能量为164 J,其组织由贝氏体+铁素体+少量马氏体构成,在马氏体和铁素体中均存在位错和细小析出相,析出相为富Nb的Nb、Ti复合碳化物,发挥沉淀强化作用;当保温时间延长至60 min后,析出大量细小Nb、Ti和Mo复合碳化物,但此时的沉淀强化作用不能弥补铁素体造成的强度损失,所以在相同温度回火过程中,随着回火时间的延长,抗拉强度和硬度下降。     

Effect of alloying elements and coiling temperature on the recrystallization behavior and the bainitic transformation in TRIP steels

The effects of alloying elements and coiling temperature on recrystallization behavior and bainitic transformation were investigated based on 0.07C-Mn-Cr-Nb steel with a low carbon equivalent. Based on the ferrite recrystallization behavior, the proper intercritical annealing temperature of all studied steels was suggested to produce TRIP steel with good strength and elongation balance. All steels coiled at 550 °C showed much faster ferrite recrystallization behavior than steels coiled at 700 °C. In addition to the coiling temperature, the effect of increasing carbon content on the ferrite recrystallization was minor at a coiling temperature of 550 °C, but much more prominent at a coiling temperature of 700 °C. The highest Mo added steel showed the best strength and elongation balance, and the highest carbon and Mo added steel showed the highest tensile strength at a coiling temperature of 550 °C. The steel containing a higher amount of elemental Al (0.7 wt.% Al) exhibited much better elongation than the lower Al added steel (0.04 wt.% Al) in TS 780 MPa grade, about 24 % and 19 %, respectively.     

Q350EWR1高耐蚀钢板组织和耐大气腐蚀性能研究

当前,对铁路车辆用钢耐大气腐蚀性能的要求越来越严苛.采用高Cr成分体系和轧后两段冷却工艺,试制了Q350EWR1铁路车辆用高耐蚀钢.研究了其奥氏体连续冷却相变行为,获得了不同冷却速率下其组织演变规律.开展了轧制工艺对Q350EWR1耐蚀钢组织和性能影响的研究,并对其腐蚀性能进行了评价.结果表明,当冷速为0.1℃/s时,...     

Effects of phosphorous addition on mechanical properties and retained austenite stability of 0.15C−1.5Mn−1.5Al TRIP-aided cold rolled steels

The effects of a P addition on the mechanical properties and austenite stability are investigated for 0.15C−1.5Mn−1.5Al TRIP-aided cold-rolled steels containing 0.05 and 0.1 wt.% of P. The strength and retained austenite fraction are increased by an increment of the P content. The strengthening of P-added TRIP-aided steel partially comes from the solid-solution hardening effect of P, and a higher fraction of strain-induced martensite plays an important role as well. The elongation of steel containing 0.1 wt.% P is diminished compared with that containing 0.05 wt.% P. This is attributed to the lower mechanical stability of retained austenite in TRIP-aided steel containing 0.1 wt.% of P, which inhibits persistent work hardening during deformation.     

1.25Cr-0.5Mo钢热处理后的组织与力学性能

利用光学显微镜、扫描电镜和力学试验机研究了3种正火组织的1.25Cr-0.5Mo钢在回火和模拟焊后热处理过程中的组织和力学性能演变。结果表明:1.25Cr-0.5Mo钢粗大的铁素体+珠光体组织经模拟焊后热处理,大量粗大的碳化物沿晶界析出,显著降低钢的冲击性能,铁素体晶内细小弥散碳化物的析出略微改善了钢的强度;贝氏体组织中的贝氏体铁素体板条宽化和碳化物粗化降低了钢的强度,但对冲击性能影响不大;1.25Cr-0.5Mo钢的综合力学性能随着铁素体组织含量的增加而变差,当铁素体组织的含量高于38％时,钢板的力学性能将难以满足要求。     

Cr元素及轧后控冷工艺对高强H型钢组织性能的影响

针对高强H型钢的开发,研究了 Cr元素及轧后控冷工艺对高强H型钢组织性能的影响.利用Jmatpro软件、光学显微镜、ImagePro-Plus等试验方法对含Cr和不含Cr两种试验钢的奥氏体连续冷却转变(CCT)曲线,组织演变、力学性能及强韧化机制进行了研究.结果表明,随着冷速的增加,钢中铁素体和珠光体体积分数逐渐减小,...     

轧制工艺对460 MPa级低屈强比耐火耐候钢组织性能的影响

利用热模拟方法测定低屈强比耐火耐候钢不同速率冷却后的组织。对比轧后弛豫工艺与未弛豫工艺以及终冷温度对试验钢性能的影响,利用光学显微镜、扫描电镜、透射电镜分析不同工艺对钢轧后显微组织的影响。结果表明,随冷却速度的增加,钢板组织由多边形铁素体变为针状铁素体+粒状贝氏体复相组织;由于弛豫处理过程中过冷奥氏体部分转变为多边形铁素体,钢板屈服强度和屈强比均下降;随着终冷温度的降低,钢板的屈服强度和屈强比上升,与钢中针状铁素体的细化与M/A组元的弥散强化有关;轧后直接水冷,并控制终冷温度至500~560 ℃,可获得高强度与低屈强比的良好匹配。     

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 利用光学显微镜、扫描及透射电镜对热轧态下的耐火钢的显微组织进行了观察和研究，结果表明：耐火钢的显微组织为等轴状铁素体、粒状贝氏体和粒状组织的混合产物。同时分析了这种耐火钢的组织与性能之间的关系。     

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  对马钢生产的耐火H型钢MGFR490B和Q345B H型钢进行了对比试验研究,完成了高温拉伸和高温持久两项力学性能测试和对两种钢微观组织的观察研究。结果表明,耐火H型钢MGFR490B的组织为多边形铁素体﹢珠光体﹢少量贝氏体混合多相组织,此种组织在高温下能保持很好的稳定性,并能在高温下析出合金碳化物,这对降低耐火H型钢MGFR490B的室温屈强比和保持高温屈强比小的波动范围有益。耐火H型钢MGFR490B获得了比Q345B H型钢好的高温强度性能,这与耐火钢的混合多相组织及添加合金元素密不可分。耐火H型钢MGFR490B在600 ℃高温下能保持高的强度,并且其持久断裂时间超过3 h,满足耐火钢的性能指标要求。     

690MPa级高强韧低碳微合金建筑结构钢设计及性能

基于JMatPro热力学软件计算并考虑化学元素间相互影响,设计了690 MPa级抗震耐蚀防火功能结构一体化高强建筑用钢,其化学成分(质量分数,%)主要为:Fe-0.08C-0.3Si-1.1Mn-0.12(Nb+V+Ti)-1.6(Cr+Cu+Ni+Mo)-0.002B-0.004N。经实验室冶炼和控轧控冷工艺(TMCP)处理后,采用EPMA、EBSD等多种微观分析和性能测试手段对该低碳微合金钢的微观组织特征、强韧化机理和力学性能、防火性及耐蚀性等进行了表征和分析。结果表明,所设计的低碳微合金钢TMCP状态下的微观组织包含粒状贝氏体、板条贝氏体和贝氏体铁素体;室温下屈服强度达700 MPa,抗拉强度为878 MPa,屈强比为0.80,断后延伸率为20%,并具有良好的低温韧性。低碳微合金钢在600℃保温1~3 h时,均达到耐火性能要求;并对其在海洋环境下的耐蚀性进行了评价,发现粒状贝氏体对耐腐蚀性能具有积极作用。进一步分析表明,低碳微合金钢具有良好的强韧性源于析出强化、细晶强化、位错强化和固溶强化的综合作用;对低温冲击断口截面组织分析表明,裂纹会多次穿过板条贝氏体呈"Z"字型扩展以消耗更多的能量,也是该钢具有良好低温韧性的原因。     

Microstructure-Related Properties of Some Novel Reinforcement Bar Steel

Mechanical properties particularly Charpy impact toughness of two low-carbon [(a) 0.11% phosphorus and (b) 0.009% niobium] thermomechanically treated reinforcing bar steels were investigated. The phosphorus and niobium steels showed tensile to yield strength ratio of 1.25 and 1.19, ductile-brittle transition temperature of 223 K and below 193 K at yield strength levels of 428 and 472 MPa, respectively. The improved toughness of phosphorus steel is attributed to a mixed transformation microstructure comprising low-carbon bainite and fine polygonal ferrite. Lowest ductile-brittle transition temperature was observed in the niobium steel due to overall fineness of microstructure consisting mainly of low-carbon bainite, acicular ferrite, and polygonal ferrite.