贝氏体形态对细晶铁素体/贝氏体双相钢室温力学性能的影响

基于过冷奥氏体动态相变的思想,通过两道次压缩变形结合控制冷却的热模拟轧制工艺,获得不同贝氏体含量及形态的细晶铁素体贝氏体双相钢。通过显微组织观察及力学性能测试,考察了第二相贝氏体特征对双相钢室温拉伸变形行为的影响。研究结果表明,形变后快速冷却可获得无碳板条状贝氏体,较慢的冷速或在贝氏体转变区保温处理可获得粒状贝氏体。贝氏体体积分数大于20%左右的细晶铁素体/贝氏体双相钢具有低的屈服强度,高的抗拉强度,高的伸长率,低屈强比以及连续屈服特性。屈服强度既与铁素体晶粒尺寸相关,也与贝氏体形态和数量相关。板条贝氏体引起的屈服强度提高大于粒状贝氏体,粒状贝氏体具有比板条贝氏体更好的塑性。     

低碳贝氏体钢的组织类型及其对性能的影响

低碳贝氏体钢受控冷工艺的影响会得到不同类型的组织，在较慢速冷却时，在奥氏体中先形成针状铁素体，残余奥氏体会被包裹在铁素体之中，形成粒状贝氏体团。工业轧制试验表明．不同控制冷却工艺可得到两类组织，一类出现黑珠组织(富碳马氏体组织)．具有该组织的钢轧态冲击韧性低。另外一类为细化的板条贝氏体组织，具有该组织的钢轧态强度高，冲击韧性好，但伸长率不足。通过回火处理，存在黑珠组织钢的冲击韧性能得到提高，超细化板条贝氏体组织钢的伸长率也能得到改善，但后者屈服强度会比前者高100MPa左右。     

粒状贝氏体组织对低碳钢力学性能的影响

 在化学成分相同的条件下,分别研究了粒状贝氏体、铁素体 珠光体对低碳合金钢强度、塑性、冲击韧性的影响。结果表明：低碳粒状贝氏体钢比铁素体 珠光体钢具有更高的强度和冲击韧性,而且粒状贝氏体组织对钢的强化方式不仅仅是细晶强化和位错强化,弥散强化也是强化方式之一。     

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。     

卷取温度对热轧酸洗低碳铁素体/贝氏体双相钢力学性能的影响

为了生产出力学性能理想的热轧酸洗低碳铁素体/贝氏体双相钢,针对不同卷取温度对C–Mn系低碳铁素体/贝氏体双相钢力学性能的影响进行研究。结果表明:对于含碳量较低的铁素体/贝氏体双相钢,在贝氏体区采取不同的卷取温度,将获得不同形貌的贝氏体组织,同时影响组织的晶粒度级别,进而对屈服强度、抗拉强度和延伸率等力学性能产生重要影响。通过合理控制卷取温度,可以生产出高强度、高延伸率、低屈强比的优质铁素体/贝氏体双相钢。     

铬对C-Si-Mn-Nb铁素体贝氏体双相钢相变规律的影响

通过比较C-Si-Mn-Nb和C-Si-Mn-Cr-Nb两种实验钢热变形后的CCT曲线,分析了合金元素Cr对铁素体贝氏体双相钢连续冷却过程相变规律和组织演变的影响.结果表明,Cr可轻微抑制实验钢的铁索体转变,细化铁素体晶粒,但对铁素体显微硬度没有影响.在铁素体转变量很少或未转变的情况下,Cr可增强亚稳奥氏体的稳定性和淬透性,细化贝氏体产物中的铁素体板条和MA岛,提高贝氏体的显微硬度.     

C Si Mn Cr Nb钢双相组织性能的柔性控制

 根据C Si Mn Cr Nb试验钢的双道次变形和分段冷却热模拟试验结果,进行了试验钢控轧控冷试验,分析了工艺参数对试验钢组织和性能的影响,获得了具有不同力学性能的铁素体+马氏体或铁素体+贝氏体双相组织。结果表明,试验钢两段轧制分段冷却后550 ℃卷取获得铁素体+马氏体双相组织,屈服强度415 MPa,抗拉强度710 MPa,伸长率23.0％,屈强比0.59。500 ℃卷取得到铁素体加粒状贝氏体双相组织,与550 ℃卷取相比,屈服强度升高35 MPa,抗拉强度降低45 MPa,伸长率略微降低。     

粒状贝氏体和粒状组织强韧化机制的研究

M-A岛以双相强化方式可有效地提高粒状组织的强度；而对粒状贝氏体起主要作用的是碳的固溶强化和板条亚晶强化，这是造成粒状贝氏体强度高于拉状组织的主要原因。粒状组织的韧性来源于由铁素体断裂强度所决定的裂纹形成功，但粒状贝氏体的韧性主要部分是裂纹扩展功，铁素体板条亚矗厦机械稳定性较高的残案奥氏体对此起主要韧化作用。     

显微组织对X80管线钢力学性能的影响

利用光学显微镜研究了20 mm厚度X80管线钢显微组织类型对强韧性的影响。结果表明:显微组织为针状铁素体和少量的粒状贝氏体时,钢的屈服强度达到512MPa;针状铁素体的晶粒细化、粒状贝氏体和板条贝氏体可以使X80管线钢具有更高的屈服强度,达到600MPa。细小而均匀的粒状贝氏体可以获得良好的冲击韧性。掺杂在一起的细小的针状铁素体、准多边形铁素体、粒状贝氏体促使裂纹扩展路径曲折,改善冲击韧性。     

低碳贝氏体钢的回火转变及内耗研究

针对低碳贝氏体钢,采用扫描电镜(SEM)、透射电镜(TEM)、X射线衍射和内耗手段分析了600℃回火过程中的显微组织变化,测试了回火后的屈服强度。结果表明,试验钢轧制态的显微组织是板条贝氏体与粒状贝氏体的混合组织,有少量Mo_2C和NbC析出。随着回火时间延长,贝氏体板条的宽度不断增大,板条内部形成较多不同取向的胞状结构,有些板条逐步演化为多边形状,粒状贝氏体不断吞噬板条贝氏体,使显微组织中粒状贝氏体增多且粗化;Mo_2C和NbC析出物进一步增多,贝氏体铁素体中的位错密度不断降低;内耗-温度谱中Snoek峰值不断降低,表面处的SKK峰值先降低后升高,心部的SKK峰值先降低后变化不大;屈服强度呈现先提高后降低的趋势。     

低碳贝氏体钢连续冷却相变规律研究

利用Gleeble1500热/力模拟实验机,研究了抗拉强度为700 MPa级的低碳贝氏体钢的相变规律,分析了不同冷却速率对钢组织及性能的影响。结果表明：轧后试样在1℃/s～30℃/s较宽的冷却速度范围内均可得到贝氏体组织,并随着冷却速度的增加,显微组织以粒状贝氏体为主转变为以性能优良的板条贝氏体和马氏体为主,在不降低韧性指标的前提下,提高了强度,为700 MPa级的低碳贝氏体钢工艺制度的制订提供了依据。     

New bainite kinetics of high strength low alloy steel in fast cooling process

Based on Kolmgorov-Johnson-Mehl-Avrami analysis, a new bainite kinetics of high strength low alloy steel in fast cooling process was developed by utilizing different experimental methods.Upper bainite transformation morphological evolutions at a cooling rate of 8.3 K/s were directly observed by laser scan-ning confocal microscopy.This qualitative analysis suggests that bainite packet is more suitable to give a one-dimensional growth model if it is considered as a transformation unit.The nucleation rate of bainite packets in fast cooling process is assumed to give an a priori item.One-dimensional growth model with constant growth rate which is assumed as a function of cooling rate is adopted as well.Thus, the devel-oped new bainite kinetics is simple in expression and contains an adjustable parameter and an empirical pa-rameter.Experimental results show upper bainite and lower bainite transformations in fast cooling proces-ses.Their referential phase volume fractions are calculated by the expanded lever rule on the first deriva-tive dilatometer curves.For the similar transformation mechanisms, upper bainite and lower bainite are considered to give the same kinetics.With considering the Nakamura′s equation, the bainite kinetics is fit-ted with experimental data.Results show that bainite volume fractions and bainite transformation rates can be expressed precisely by the newly developed bainite kinetics.     

Microstructures and Mechanical Properties of X100 Pipeline Steel Strip简

Dynamic CCT （continuous cooling transformation） curves and effects of microstructures and M/A islands of X100 hot rolled strip on mechanical properties were studied by means of a thermal simulator, a scanning electron mi- croscope （SEM）, a transmission electron microscope （TEM）, a digital micro-hardness tester and Image-Pro Plus analysis software. The results show that high contents of C and Mo can make transformation lines of acicular ferrite and bain- ite shift rightward. High Mo tested strip has higher strength and micro-hardness and lower toughness than the other one because lath bainite appears instead of quasi-polygonal ferrite. Tested strip with granular bainite, lath bainite and M/A islands has better mechanical properties and in the microstructures lath bainite content is around 36.50% and M/A islands are fine and disperse in the matrix.     

Morphology and properties of low-carbon bainite

Morphology of low-carbon bainite in commercial-grade high-tensile-strength steels in both isothermal transformation and continuous cooling transformation is lathlike ferrite elongated in the 〈11l〉_b direction. Based on carbide distribution, three types of bainites are classified: Type I, is carbide-free, Type II has fine carbide platelets lying between laths, and Type III has carbides parallel to a specific ferrite plane. At the initial stage of transformation, upper bainitic ferrite forms a subunit elongated in the [−101]f which is nearly parallel to the [lll]_b direction with the cross section a parallelogram shape. Coalescence of the subunit yields the lathlike bainite with the [−101]_f growth direction and the habit plane between (232)_f and (lll)_f. Cementite particles precipitate on the sidewise growth tips of the Type II bainitic ferrite subunit. This results in the cementite platelet aligning parallel to a specific ferrite plane in the laths after coalescence. These morphologies of bainites are the same in various kinds of low-carbon high-strength steels. The lowest brittle-ductile transition temperature and the highest strength were obtained either by Type III bainite or bainite/martensite duplex structure because of the crack path limited by fine unit microstructure. It should also be noted that the tempered duplex structure has higher strength than the tempered martensite in the tempering temperature range between 200 °C and 500 °C. In the case of controlled rolling, the accelerated cooling afterward produces a complex structure comprised of ferrite, cementite, and martensite as well as BI-type bainite. Type I bainite in this structure is refined by controlled rolling and plays a very important role in improving the strength and toughness of low-carbon steels. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials 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.     

冷却工艺对超低碳贝氏体钢强韧性影响的研究

研究了一种含有Cu、Ni、Mo、Nb、B等元素的超低碳贝氏体钢,以搞清楚其在不同的热机械处理 弛豫-析出-控制相变技术 回火工艺(TMCP RPC T)条件下组织和强韧性能的变化规律.实验室研究和工业试制表明,随着工艺制度的不同,钢的显微组织表现为粒状贝氏体和板条贝氏体的比例、形态、尺寸不同;在一定的冷却速度下,轧态钢的屈服强度、抗拉强度和屈强比随终冷温度的降低呈现上升趋势;回火后钢的屈强比较热轧态有所提高.试验条件下,回火温度对Nb析出数量的影响不明显,加热时Nb的固溶程度对该钢的最终组织有明显影响;采用TMCP RPC、TMCP RPC T工艺路线,通过调整工艺参数,能够获得不同性能组合的钢板,实现高性能钢种的柔性化设计.     

低碳锰(铌)钢控轧控冷实验研究

通过TMCP研究了低碳锰钢和添加微量Nb的低碳锰铌钢的组织和力学性能的影响因素.研究了Nb对实验钢显微组织和力学性能的影响.Nb的添加能够细化铁素体晶粒,促进铁素体转变,对贝氏体形成有一定的抑制作用.相对低碳锰钢来说,铌的加入减少了贝氏体的体积分数,贝氏体强化效果减弱,钢的屈强比升高.通过增加冷速和降低卷取温度,可以使低碳锰铌钢获得一定量的贝氏体,综合性能较佳.低碳锰铌钢的主要强化机制有细晶强化、贝氏体相变强化和析出强化.     

低碳钢淬火等温碳配分中粒状贝氏体组织演变及性能

利用彩色金相、扫描电镜及拉伸实验等方法,研究了IQPB热处理工艺下低碳硅锰钢在450℃时不同等温时间淬火碳配分工艺对其组织及力学性能影响。结果表明,经不同时间等温碳配分工艺处理,实验钢显微组织基本由粒状贝氏体及粒状组织构成。当配分时间在200~600s时,晶界边缘大块状M/A岛数量逐渐减少,但细小颗粒状M/A岛数量逐渐增多并趋于有序化排列,导致抗拉强度升高,伸长率降低。随碳配分时间延长,细小颗粒状M/A岛又趋于弥散化排列,并且当碳配分时间大于1 200s时出现无碳化物板条贝氏体,其贝氏体板条间的薄膜状残余奥氏体更加稳定,同时受弥散排列的细小颗粒状M/A岛影响,伸长率得到提高,抗拉强度减少。     

Low-Temperature Toughening Mechanism in Thermomechanically Processed High-Strength Low-Alloy Steels

High-strength low-alloy (HSLA) steels were fabricated by varying thermomechanical processing conditions such as rolling and cooling conditions in the intercritical region, and the low-temperature toughening mechanism was investigated in terms of microstructure and the associated grain boundary characteristics. The steels acceleratedly cooled to relatively higher temperature had lower tensile strength than those acceleratedly cooled to room temperature due to the increased volume fraction of granular bainite or polygonal ferrite (PF) irrespective of rolling in the intercritical region, while the yield strength was dependent on intercritical rolling, and start and finish cooling temperatures, which affected the formation of PF and low-temperature transformation phases. The steel rolled in the intercritical region and cooled to 673 K (400 °C) provided the best combination of high yield strength and excellent low-temperature toughness because of the presence of fine PF and appropriate mixture of various low-temperature transformation phases such as granular bainite, degenerate upper bainite (DUB), lower bainite (LB), and lath martensite (LM). Despite the high yield strength, the improvement of low-temperature toughness could be explained by the reduction of overall effective grain size based on the electron backscattered diffraction (EBSD) analysis data, leading to the decrease in ductile-to-brittle transition temperature (DBTT).     

两阶段冷却工艺对基于应变设计X70管线钢组织的影响

 铁素体-贝氏体双相组织钢能够通过软硬相协调屈服抵抗大变形,这是基于应变设计管线钢的研究热点。为探究生产工艺对双相组织形态的影响规律,利用Gleeble-3800热模拟试验机,通过压缩试验模拟轧制和冷却,研究了两阶段冷却工艺对基于应变设计X70管线钢形变奥氏体组织转变的影响。结果表明：一阶段缓冷后的待温处理使铁素体形核温度降低,有效提高了铁素体形核率,起到细化晶粒作用;降低二阶段快冷开冷温度可以增加铁素体析出时间,从而增加铁素体的含量;二阶段快冷中,提高冷却速率和降低终冷温度均可细化贝氏体组织的板条间距以及板条间的碳化物,提高了贝氏体显微维氏硬度。