无碳化物贝氏体耐磨钢板组织与性能的研究

研究了无碳化物贝氏体耐磨钢板组织、力学性能及焊接性能。结果表明,在低碳贝氏体钢基础上,通过加入一定量的硅元素,利用其在贝氏体组织转变过程中抑制碳化物析出作用,得到由非等轴铁素体加马氏体和残余奥氏体(M-A)岛或由板条状铁素体及其板条间残余奥氏体(Ar)膜组成的无碳化物贝氏体组织,以此得到既具有高强度、高硬度,又具有较高的低温冲击韧性,同时具有较好的焊接性能。     

准贝氏体高强耐磨钢的开发和工艺研究

研究在典型贝氏体钢的成分基础上加入阻止碳化物析出的元素S i,开发出以贝氏体铁素体(BF)和残余奥氏体(AR)组成的准贝氏体组织的高强耐磨钢,在适当的工艺下钢板可获得最佳的综合性能,具有良好的强韧性、耐磨性和焊接性。     

无碳化物贝氏体在车轮上应用的展望

无碳化物贝氏体是新近研究和发展起来的一种新型组织结构，是高强度超级钢的一种理想的基体组织，它是含碳过饱和的贝氏体铁素体与富碳的残余奥氏体的复合组织，钢中没有碳化物。适量含硅钢等温处理都可以得到无碳化物贝氏体。由无碳化物贝氏体组织构成的钢具有高强、高韧和耐磨等特性，加工性能也好，因此，它具有广阔的应用前景。火车车轮是重要的行走部件，受载复杂，股役条件恶劣，现有材料再提高综合力学性能指标则比较困难，而无碳化物贝氏体带来了新的曙光，将是21世纪最有发展前途的钢铁材料之一。     

70Si3MnCrMo钢中贝氏体及其回火稳定性

 对一种新型70Si3MnCrMo钢进行了等温和连续冷却贝氏体相变热处理。利用拉伸和冲击试验研究试验钢的力学行为,利用XRD、SEM和TEM等方法对试验钢进行了相组成分析和微观组织形貌观察。研究结果表明,试验钢经等温贝氏体相变,其最佳综合力学性能出现在200 ℃回火,强塑积为26.4 GPa·%。经连续冷却贝氏体相变,其最佳综合力学性能出现在300 ℃回火,强塑积达到28.6 GPa·%。回火温度较低的情况下,热处理后的组织为由贝氏体铁素体和残余奥氏体组成的无碳化物贝氏体组织,这种无碳化物贝氏体由超细贝氏体铁素体板条而获得超高强度,由一定量的高碳残余奥氏体来保证较高的塑性和韧性。试验钢经连续冷却贝氏体相变,其贝氏体铁素体板条中出现了超细亚单元,并且残余奥氏体呈薄膜状和小块状两种形态分布于贝氏体铁素体板条之间,这两种形态残余奥氏体的稳定性不同。拉伸试样在变形过程中残余奥氏体持续发生TRIP效应,直至全部残余奥氏体都发生转变生成应变诱发马氏体,从而使钢得到更好的强、塑性配合,表现出十分优异的综合性能。     

快速热处理超细贝氏体钢构筑梯度结构研究

心部高韧性、表面高硬度的材料是抗冲击载荷轴承、齿轮等关键零部件的重要要求。以超细贝氏体为对象，进行局部快速热处理，得到局部高硬度、心部高韧性的梯度结构，研究快速热处理过程中超细贝氏体组织的相变行为，以及梯度组织和性能变化规律。结果表明，超细贝氏体钢在加热时，残余奥氏体分解为铁素体和碳化物，贝氏体铁素体粗化，在快速热处理中存在相变滞后。超细贝氏体钢经过局部快速热处理后形成一个组织和性能梯度过渡的结构。硬化区组织由淬火得到的马氏体和残余奥氏体组成，随着深度增加，组织中的马氏体含量减少，初始超细贝氏体的回火产物逐渐增加。硬化区的硬度高达63HRC,随着深度增加逐渐降低至38HRC,硬度相对基体下降约6HRC。     

硅和铬对中碳钢贝氏体显微组织的影响

为了获得具有良好强度一韧性平衡弹簧钢的重要信息,检测了Si和Cr含量对中碳钢贝氏体显微组织的影响。将4种实际的中碳钢JIS-S55C、SUP9、SUP7和SUP12在1000℃奥氏体化后,在温度介于300oC和500℃之间进行等温转变,借助扫描电子显微镜以及透射电子显微镜观察显微组织。在没有Si和Cr的$55C钢贝氏体转变早期,形成碳化物,而在SUP7和SUPl2钢中,碳化物的析出受Si含量的增加而受到抑制。在贝氏体转变中期,由于残余奥氏体中的碳浓度增加,导致残余奥氏体的分数随Si和Cr的增加而增加。事实上,添加硅可促进游离碳化物贝氏体铁素体,并且通过碳的富集,导致残余奥氏体的数量较大。     

准贝氏体组织及新型系列准贝氏体钢

新型钢中的准贝氏体由贝氏体铁素体和分布其上的残余奥氏体膜组成,处于贝氏体转变的初期阶段。新型准贝氏体钢具有优异的综合力学性能和简单易行的工艺性能,应用前景广阔。     

奥氏体—贝氏体球墨铸铁组织与耐磨性研究

本文研究了采用等温淬火的方法，获得奥氏体－贝氏体球墨铸铁组织的过程，并讨论了其组织形态与硬度，耐磨性之间的关系，研究表明，其等温淬火组织为无碳化物贝氏体加残余奥氏体及石墨、硬度高，耐磨性好。     

镍系无碳化物贝氏体无缝钢管的残余奥氏体

研究开发了一种适用于高强高韧抗腐蚀无缝钢管的镍系无碳化物贝氏体钢,其微观组织由无碳化物贝氏体、马氏体和片状残余奥氏体组成.片状残余奥氏体的存在使无碳化物贝氏体无缝钢管在保持高强度的同时,具有良好的韧性.为了满足多种强韧性配合的应用,使用Gleeble热模拟、X射线衍射仪、金相等手段对不同热处理制度下得到的片状残余奥氏体含量进行了研究,得到了不同热处理制度下残余奥氏体形态和含量的变化规律,为制造高强高韧抗腐蚀石油专用无缝管材提供了重要的试验依据.     

高碳铬钼钢贝氏体碳化物形貌及其形成机理

 取9Cr2Mo钢和GCr15钢,奥氏体化后进行中温等温转变,采用金相显微镜和QUANTA-400扫描电子显微镜研究贝氏体碳化物的形貌及其形成机理。结果表明,9Cr2Mo钢和GCr15钢的上贝氏体组织呈羽毛状,上贝氏体碳化物呈长短不一的薄片状或短棒状,分布在铁素体亚片条或亚单元之间,其排列与贝氏体铁素体片条轴向大体上平行分布;下贝氏体组织呈竹叶状或针状,下贝氏体碳化物呈细片状或纤维状等形状,分布在铁素体片中间,大多数与片条的主轴方向交角排列,但角度不等。钢中贝氏体碳化物在γ/α相界面上形核,向奥氏体中和铁素体亚单元间长大,是碳原子沿着相界面扩散的过程。     

The effect of Mn and Si on the morphology and kinetics of the bainite transformation in Fe-C-Ti alloys

The overall kinetics of the isothermal transformation of austenite to ferrite and bainite at intermediate temperatures and the microstructure of the transformation products in Fe-C-Ti, Fe-C-Ti-Si, Fe-C-Ti-Mn, and Fe-C-Ti-Mn-Si alloys were investigated with dilatometry, quantitative metallography, and TEM. The rate of transformation of austenite is retarded and the precipitation of carbide is inhibited in Ti-containing alloys by additions of Si, Mn, or Si + Mn. In addition to bainite and degenerate ferrite plates, a large amount of granular structure consisting of martensite and retained austenite (so-called granular bainite) is observed in these alloys if the isothermal reaction time is not long enough to complete austenite decomposition. It is suggested that the inhibiting effect of Si on carbide precipitation, the segregation of carbideforming elements to prior γ grain boundaries and interphase interfaces, and the enhancing effect of Si on the segregation of these elements lead to the results obtained. The results also support the view that so-called granular bainite is a mixture of transformation products produced by the partial decomposition of austenite. Former Graduate Student, Southwest Jiaotong University This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Metals Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

The fine structure and formation mechanism of lower bainite

Isothermal transformation of austenite to lower bainite was studied by optical microscopy and transmission electron microscopy (TEM), in two high-purity Fe-C-4 wt pct Mn-2 wt pct Si alloys containing 0.4 and 0.6 wt pct carbon, in order to elucidate the fine structure and formation mechanism of lower bainite. The present results support a mechanism for lower bainite formation presented previously in which lower bainite sheaves result from the formation of an aggregate of fine ferrite crystals with thin austenite “gaps” between them; carbide precipitation occurs within these austenite gaps. This mechanism accounts for the carbides oriented at an angle to the sheaf axis repeatedly observed in lower bainite but is inconsistent with models based on the precipitation of a high-volume fraction of carbides within highly supersaturated ferrite formed by high-velocity shear. Observations by a number of other researchers are reviewed and shown to include morphological features consistent with the present mechanism. Finally, the orientation relationships typically observed among ferrite, austenite, and carbides in lower bainite are reviewed and shown in most (but not all) cases to be consistent with the view that carbides precipitate in austenite at ferrite-austenite boundaries, also in agreement with the present model. This article is based on a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S Phase Transformations Committee.     

高碳铬轴承钢的成分设计和热处理工艺的研究进展

叙述了高碳铬轴承钢中Mn、Si、Cr、Mo和Al含量及热处理工艺包括马氏体淬火-回火,贝氏体等温淬火、贝氏体-马氏体和马氏体-贝氏体淬火以及纳米贝氏体钢的研究进展。近10年发展的高强度、高塑性和高韧性的纳米贝氏体钢,因其由纳米尺寸的超细贝氏体铁素体板条和板条间富碳的残余奥氏体薄膜组成的特殊组织结构导致其在耐磨和接触疲劳性能方面也具有优越性,该纳米贝氏体轴承钢有良好的应用前景。     

Metallography of bainitic transformation in silicon containing steels

The formation of carbide in lower bainite was studied in two silicon containing carbon steels by transmission electron microscopy and diffraction techniques. Epsilon carbide was identified in the low temperature isothermally transformed bainite structure. The crystallographic relationship between epsilon carbide and bainitic ferrite was found to follow the Jack orientation relationship,viz, (0001)ε l l(011)α, (101l)ε l 1(101)α. The cementite observed in lower bainite was in the shape of small platelets and obeyed the Isaichev orientation relationship with the bainitic ferrite,viz, (010) cl 1(1-11)α, (103) cl 1 (011)α. Direct evidence showing the sequence of carbide formation from aus-tenite in bainite has also been obtained. Based on the observations and all the crystallo-graphical features, it is strongly suggested that in silicon containing steels the bainitic carbide precipitated directly from austenite instead of from ferrite at the austenite/fer-rite interface as has been proposed by Kinsman and Aaronson (Ref. 1). The uniformity of the carbide distribution is thus envisaged to be the outcome of precipitation at the aus-tenite-ferrite interphase boundary. DER-HUNG HUANG, formerly with the Department of Materials Science and Mineral Engineering, University of California     

Ferrite and bainite in alloy steels

The addition of alloying elements even in small concentrations can alter the properties and structure of ferrite and bainite. The various morphologies of ferrite-carbide aggregates are surveyed including alloy pearlite, fibrous carbide eutectoids and precipitation of fine alloy carbides atγ-α interfaces. Modern ideas on the morphology and growth kinetics of ferrite and upper and lower bainite are also summarized. Using this information, an attempt is made to rationalize subcritical transformations of austenite in low alloy steels. Basic factors influencing the strength of alloy ferrites are discussed, leading to an examination of structure-mechanical property relationships in ferrite and bainite. Finally the exploitation of the ferrite and bainite reactions to produce useful alloy steels by direct transformation of austenite is explored.     

Effect of alloying elements on the microstructure‐property relationship in thermomechanically processed C‐Mn‐Si TRIP steels

The effect of additions of Nb, Al and Mo to Fe‐C‐Mn‐Si TRIP steel on the final microstructure and mechanical properties after simulated thermomechanical processing (TMP) has been studied. The laboratory simulations of discontinuous cooling during TMP were performed using a hot rolling mill. All samples were characterised using optical microscopy and image analysis. The volume fraction of retained austenite was ascertained using a heat tinting technique and X‐ray diffraction measurements. Room temperature mechanical properties were determined by a tensile test. From this a comprehensive understanding of the structural aspect of the bainite transformation in these types of TRIP steels has been developed. The results have shown that the final microstructures of thermomechanically processed TRIP steels comprise ~ 50 % of polygonal ferrite, 7 ‐12 % of retained austenite, non‐carbide bainitic structure and martensite. All steels exhibited a good combination of ultimate tensile strength and total elongation. The microstructure‐property examination revealed the relationship between the composition of TRIP steels and their mechanical properties. It has been shown that the addition of Mo to the C‐Si‐Mn‐Nb TRIP steel increases the ultimate tensile strength up to 1020 MPa. The stability of the retained austenite of the Nb‐Mo steel was degraded, which led to a decrease in the elongation (24 %). The results have demonstrated that the addition of Al to C‐Si‐Mn‐Nb steel leads to a good combination of strength (~ 940 MPa) and elongation (~ 30 %) due to the formation of refined acicular ferrite and granular bainite structure with ~7 ‐ 8 % of stable retained austenite. Furthermore, it has been found that the addition of Al increases the volume fraction of bainitic ferrite laths. The investigations have shown an interesting result that, in the Nb‐Mo‐Al steel, Al has a more pronounced effect on the microstructure in comparison with Mo. It has been found that the bainitic structure of the Nb‐Mo‐Al steel appears to be more granular than in the Nb‐Mo steel. Moreover, the volume fraction of the retained austenite increased (12 %) with decreasing bainitic ferrite content. The results have demonstrated that this steel has the best mechanical properties (1100 MPa and 28 % elongation). It has been concluded that the combined effect of Nb, Mo, and Al addition on the dispersion of the bainite, martensite and retained austenite in the ferrite matrix and the morphology of these phases is different than effect of Nb, Mo and Al, separately.     

Towards an austenite decomposition model for TRIP steels

The current status of developing a fundamental model for describing the overall austenite decomposition kinetics to ferrite and carbide‐free bainite in low carbon TRIP steels alloyed with Mn and Si is reviewed. For ferrite growth, a model is proposed where both interface and carbon diffusion‐controlled ferrite formation are considered in a mixed‐mode approach. The kinetic model is coupled with Thermocalc to obtain necessary thermodynamic information. Spherical geometry with an outer ferrite shell is assumed to capture in a simple way the topological conditions for growth. The mixed‐mode modelling philosophy has been identified to permit a rigorous incorporation of the solute drag effect of substitutional alloying elements, in particular Mn. The Purdy‐Brechet solute drag theory is adopted to characterize the interaction of Mn with the moving austenite‐ferrite interface. The challenges of quantifying the required solute drag parameters are discussed with an emphasis on a potential solute drag interaction of Mn and Si. The model is extended to non‐isothermal processing paths to account for continuous and stepped cooling occurring on the run‐out table of a hot strip mill or on a continuous annealing line. The transformation start temperature during cooling is predicted with a model combining nucleation and early growth which had previously been validated for conventional low carbon steels. The overall model is evaluated by comparing the predictions with experimental data for the ferrite growth kinetics during continuous cooling of a classical TRIP steel with mass contents of 0.19 % C, 1.49 % Mn and 1.95 % Si. Extension of the model to include bainite formation remains a challenge. Both diffusional and displacive model approaches are discussed for the formation of carbide‐free bainite. It is suggested to develop a combined nucleation and growth model which would enable to capture a potential transition from a diffusional to a displacive transformation mode with decreasing temperature.     

Electron microstructure and mechanical properties of silicon and aluminium ductile irons

Samples of unalloyed silicon and aluminium spheroidal graphite cast iron have been studied in the austempered condition. Austempering times of up 3 h at 400°C for Al SG and 1 h at 350°C for Si SG gives a typical ADI microstructure consising of carbide-free banitic ferrite and stable, high carbon enriched, retained austenite. This has an attractive combination of elongation and strength. For longer austempering times transition carbides are precipitated in the bainitic ferrite, η-carbide in the upper bainitic range, i.e.400°C for Al SG and 350°C for Si SG, and ϵ-carbide in the lower bainite range. Increasing amounts of transition carbide reduce the ductility and produce a mixed model of fracture. For longer austempring times _X-carbide is precipitated at the ferrite/austenite boundaries leading to a more brittle fracture mode.