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。     

热处理工艺对9Cr钢组织与力学性能的影响

利用热膨胀试验研究了9Cr钢随冷却速度变化的相变行为,设定奥氏体化温度分别为860和1000℃,利用 OM、SEM、TEM、XRD和室温拉伸对比研究不同热处理温度下9Cr钢的显微组织及力学性能.研究表明：随着冷却速度增加,9 Cr 钢发生铁素体/珠光体相变、贝氏体相变和马氏体相变,其中马氏体相变临界冷速为1.6℃/s;860℃热处理后9Cr钢的显微组织为板条贝氏体/马氏体和少量等轴铁素体,并有4％的残余奥氏体;奥氏体化温度升至1000℃后,奥氏体晶粒尺寸增加,9Cr 钢中铁素体几乎消失,板条特征更加明显,力学性能与860℃热处理后基本相同,均达到 HL级抽油杆钢的要求,说明9Cr钢具有较宽的工艺窗口.     

中碳Cr-Mo-V钢连续冷却时的组织转变规律

采用Formastor-FⅡ全自动相变仪实现不同冷却速度,利用金相显微镜、扫描电子显微镜和透射电子显微镜,研究了45CrMoV钢在不同冷却速度下的组织转变规律以及回火温度对组织的影响。结果表明,随着冷却速度的变慢,45CrMoV钢的组织由马氏体变为马氏体、先共析铁素体、下贝氏体和粒状贝氏体的混合物。冷却速度进一步变慢,先共析铁素体数量增多,下贝氏体和粒状贝氏体总量减少,材料硬度不断下降;45CrMoV钢中的粒状贝氏体为岛状、颗粒状,也有不规则形状,下贝氏体铁素体板条比低碳钢和超低碳钢中的板条更宽,分布更分散,板条形态不规则;随着回火温度的升高,45CrMoV钢中的渗碳体由细针状变为细条状,最后长大为椭球状,材料强度下降,韧性上升。     

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。     

等温温度对超细贝氏体钢组织演变规律的影响

通过热膨胀试验研究实验钢的等温转变动力学,采用盐浴等温淬火工艺制备超细贝氏体组织,利用扫描电镜、透射电镜和X射线衍射仪定量分析工艺参数对微观组织结构的影响.结果显示:实验钢室温组织由大量超细板条状贝氏体铁素体和板条间分布的薄膜状奥氏体的复相组织构成,210℃等温淬火得到的贝氏体板条间距细化到约60 nm,硬度约为HBW610;实验钢的最终组织特征取决于发生贝氏体转变的等温温度和等温时间,等温温度越低时贝氏体转变完成需要的等温时间越长.      

高碳贝氏体钢的低温转变组织与性能

摘要：采用光学与扫描电子显微镜、X射线衍射等手段研究了不同等温温度（300、250、200℃）对于高碳（质量分数0.79%）贝氏体钢低温转变样品的相含量、组织尺寸和力学性能的变化规律。结果表明，随贝氏体等温温度的降低，贝氏体最终转变量更高，贝氏体铁素体板条和薄膜状残余奥氏体宽度、块状残余奥氏体尺寸减小，抗拉强度升高，塑韧性降低。300℃的贝氏体抗拉强度为1525MPa，贝氏体铁素体宽度是116nm，而200℃的贝氏体铁素体板条尺寸达到62nm，抗拉强度达到1 928MPa。研究发现，在未充分转变的贝氏体样品中，尺寸大于4.7μm的块状残余奥氏体在冷却过程中易发生马氏体相变，而小于该尺寸的残余奥氏体比较稳定，可以保留到最终组织中。     

70kg级低碳贝氏体钢相变规律研究

利用MMS-200热模拟试验机和光学显微镜研究了70kg级低碳贝氏体钢板在不同终轧温度和冷却速度下的相变规律。结果表明,随冷却速度的增大,钢中依次出现多边形铁素体、珠光体、针状铁素体、粒状贝氏体、下贝氏体和马氏体组织,奥氏体向铁素体相变温度Ar3降低,晶粒细化。随着终轧温度的降低,铁素体诱导相变明显增加,铁素体晶粒细化。     

显微组织对磨料磨损性能的影响

本文研究了马氏体,贝氏体、残余奥氏体及马氏体—贝氏体混合组织在二体、三体磨料磨损下的磨损特性,对材料磨损表面及磨屑形貌进行了详细分析。结果表明,材料的磨料磨损主要由显微切削和显微裂纹两个因素控制,材料的耐磨性由其强度与韧性的配合所决定。马氏体组织的加工硬化速率及碳含量强烈影响其耐磨性,先转变少量下贝氏体对其耐磨性有益。下贝氏体组织在高应力磨料磨损下比相同硬度,或稍高硬度的回火马氏体组织具有较高的耐磨性,但在低应力磨损下表现为不利的作用。粗大的贝氏铁素体组织在两种磨损下都对耐磨性不利。马氏体—贝氏体混合组织比马氏体组织,贝氏体组织具有更高的耐磨性。残余奥氏体在二体高应力磨损下起非常有益的作用,在三体低应力磨损下则起不利的作用。     

临界淬火对超厚水电钢冲击韧性的影响

通过设计对比实验,研究了临界淬火工艺对超厚水电站用钢板的组织和性能的影响.试验结果发现,临界淬火较低的加热温度得到了细小的奥氏体晶粒和一定量的未溶铁素体,临界淬火后试验钢近表面得到了未溶铁素体+板条马氏体组织,厚度t/4处得到了未溶铁素体+板条贝氏体+粒状贝氏体组织,厚度t/2处得到未溶铁素体+先共析铁素体+粒状贝氏体组织.回火后钢板近表面组织转变为未溶铁素体+回火索氏体组织,厚度t/4和t/2处回火后得到了铁素体+回火贝氏体组织.临界淬火工艺保留了部分未溶铁素体,使碳扩散至奥氏体中,提高了奥氏体的稳定性,淬火后以残余奥氏体存在,提高了超厚水电钢的冲击韧性;厚度t/2处的-20℃的低温冲击韧性达到了156 J,满足了使用要求.     

低于Ms温度变形对等温贝氏体相变动力学和组织的影响

摘要：变形和等温热处理是高强贝氏体钢主要生产工艺,已有研究表明低于马氏体相变起始温度（Ms）的等温热处理可以促进贝氏体相变动力学,低温奥氏体预变形也可以加速贝氏体相变。研究了低于Ms温度变形对后续等温贝氏体相变动力学和组织的影响,结果表明,并未出现预想的加速相变叠加效应,反而,变形温度低于Ms温度时,贝氏体相变动力学减弱,等温贝氏体相变孕育期延长。低于Ms温度等温相变时,贝氏体铁素体与母相奥氏体位向关系接近K-S关系,变形试样虽然获得了一部分先马氏体,且能提高贝氏体形核率,但并非所有的胚核都能发生长大,变形改变母相奥氏体取向,使贝氏体原本的位向关系遭受破坏,导致有效形核率降低。     

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.     

Ms以下温度等温淬火对中碳贝氏体钢相变和性能影响

摘要：设计了马氏体起始相变温度（Ms）以上和以下2个不同温度等温淬火试验，结合热膨胀仪、扫描电镜显微组织、X光衍射和拉伸试验等试验手段，研究了对比于Ms以上温度等温淬火试验，Ms以下等温淬火对中碳贝氏体钢相变、组织和性能的影响。结果表明，贝氏体相变可以发生在Ms温度以下，且其相变动力学被明显促进。相比于Ms以上温度等温淬火，Ms温度以下等温淬火虽然可以加速相变动力学，但导致强度和伸长率下降，因此降低了最终的力学性能。这主要是因为Ms温度以下等温淬火试样组织内部出现了大量的回火无热马氏体（AM）和少量的贝氏体和残余奥氏体（RA）。因此，Ms温度以下等温淬火热处理后的组织性能未必优于Ms温度以上等温处理后组织性能，这主要取决于具体的成分和工艺。     

A Study of the Influence of Thermomechanical Controlled Processing on the Microstructure of Bainite in High Strength Plate Steel

Steels with compositions that are hot rolled and cooled to exhibit high strength and good toughness often require a bainitic microstructure. This is especially true for plate steels for linepipe applications where strengths in excess of 690 MPa (100 ksi) are needed in thicknesses between approximately 6 and 30 mm. To ensure adequate strength and toughness, the steels should have adequate hardenability (C. E. >0.50 and Pcm >0.20), and are thermomechanically controlled processed, i.e., controlled rolled, followed by interrupted direct quenching to below the Bs temperature of the pancaked austenite. Bainite formed in this way can be defined as a polyphase mixture comprised a matrix phase of bainitic ferrite plus a higher carbon second phase or micro-constituent which can be martensite, retained austenite, or cementite, depending on circumstances. This second feature is predominately martensite in IDQ steels. Unlike pearlite, where the ferrite and cementite form cooperatively at the same moving interface, the bainitic ferrite and MA form in sequence with falling temperature below the Bs temperature or with increasing isothermal holding time. Several studies have found that the mechanical properties may vary strongly for different types of bainite, i.e., different forms of bainitic ferrite and/or MA. Thermomechanical controlled processing (TMCP) has been shown to be an important way to control the microstructure and mechanical properties in low carbon, high strength steel. This is especially true in the case of bainite formation, where the complexity of the austenite-bainite transformation makes its control through disciplined processing especially important. In this study, a low carbon, high manganese steel containing niobium was investigated to better understand the effects of austenite conditioning and cooling rates on the bainitic phase transformation, i.e., the formation of bainitic ferrite plus MA. Specimens were compared after transformation from recrystallized, equiaxed austenite to deformed, pancaked austenite, which were followed by seven different cooling rates ranging between 0.5 K/s (0.5 °C/s) and 40 K/s (40 °C/s). The CCT curves showed that the transformation behaviors and temperatures varied with starting austenite microstructure and cooling rate, resulting in different final microstructures. The EBSD results and the thermodynamics and kinetics analyses show that in low carbon bainite, the nucleation rate is the key factor that affects the bainitic ferrite morphology, size, and orientation. However, the growth of bainite is also quite important since the bainitic ferrite laths apparently can coalesce or coarsen into larger units with slower cooling rates or longer isothermal holding time, causing a deterioration in toughness. This paper reviews the formation of bainite in this steel and describes and rationalizes the final microstructures observed, both in terms of not only formation but also for the expected influence on mechanical properties.     

不同工艺下低碳Mn-Si钢的组织与力学性能

通过Gleeble-1500热模拟压缩试验,借助光学显微镜、扫描电镜、X射线衍射及拉伸试验等,研究一种低碳Mn-Si钢在基于热轧动态相变的热轧TRIP钢工艺和基于贝氏体等温处理工艺下的组织与力学性能,比较了通过两种工艺获得的不同复相组织状态对材料的加工硬化能力的影响.结果表明:实验钢在基于动态相变的热轧TRIP钢工艺下获得了以细晶铁素体为基体和贝氏体、残余奥氏体组成的复相组织,而在基于贝氏体等温处理工艺下得到了以板条贝氏体为基体和残余奥氏体组成的复相组织,前者中残余奥氏体含量较高且其碳含量也较高.实验钢具有以板条贝氏体为基体的复相组织时屈服强度和抗拉强度较高;但由于残余奥氏体稳定性较差,实验钢的加工硬化能力较弱,导致其均匀延伸率和总延伸率较小.      

表面高硬度和耐腐蚀性高强贝氏体钢制备新工艺

 铁路运输和工程机械等领域对贝氏体钢的耐磨性和耐腐蚀性具有较高的要求,而表面渗硼、渗铬处理是常见的提高表面硬度和耐腐蚀性的有效方法。在已有的研究工作中,获得贝氏体基体的等温淬火热处理和表面改性处理是分开进行的,工序复杂且生产成本增加。提出制备表面高硬度、高耐腐蚀性中碳高强度贝氏体钢的新工艺,将表面改性处理和贝氏体等温淬火工艺一体化,既可以简化制备工艺,又降低了生产成本、减少了环境污染。采用渗硼/铬-等温淬火一体化新工艺制备表面高硬度、高耐腐蚀性中碳高强贝氏体钢,通过组织观察、硬度测试和腐蚀试验等,对比分析了渗硼-等温淬火和渗铬-等温淬火一体化工艺对中碳高强贝氏体钢组织和性能的影响。结果表明,与仅经过等温淬火工艺处理相比,渗硼/铬-等温淬火一体化工艺处理后,贝氏体钢表面均形成了维氏硬度超过1 500HV的渗层(约为贝氏体基体硬度的3.3倍),且在0.5%NaCl溶液中的腐蚀性能明显提高。暴露在0.5%NaCl溶液中3 h后,渗硼层表现出更好的耐腐蚀性能。两种一体化工艺均可制备表面渗层、基体以贝氏体相为主的新型高强贝氏体钢,在相同处理时间下,与渗铬-等温淬火一体化工艺相比,渗硼-等温淬火一体化工艺处理的渗层生长速率快,且渗硼层与贝氏体钢基体的结合强度更高。     

Bainite morphologies in a 0.2 C–1.5 Mn steel

The isothermal transformation products of austenite over a wide range of temperatures and times in the bainitic range in a 0.2 wt.% C–1.5 wt.% Mn steel have been studied by transmission electron microscopy in order to characterise the bainitic microstructures in low-carbon low-alloy steels. Widmanstätten ferrite has formed with alternate layers of austenite (martensite) as a transition product at 600 and 500°C that has finally transformed on further isothermal transformation to either pearlite (at 600°C) or upper bainite (at 500°C). This type of transformation product was referred to as BI bainite by earlier investigators, but on the basis of the present investigation it is concluded that such ferrite-austenite (martensite) structures are not bainitic as this is not the final transformation product either at 600 or 500°C. Both upper bainite and lath-type lower bainite are formed at 450°C while the transformation product has been only lath-type lower bainite at 400°C.     

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。