C-N共渗层中残留奥氏体层深分布曲线的测定

一、前言近年来对钢中残留奥氏体的作用进行了广泛的研究。对于不同零件和使用条件,控制合理的残留奥氏体含量对性能是有利的。本文针对20CrMnTi钢,C-N共渗后采用不同的热处理工艺,得到不同的残留奥氏体量,用X射线衍射测量残留奥氏体量。并且绘出层深与残留奥氏体分布曲线。试验为研究材料性能与残留奥氏体的关系和对20CrMnTi齿轮钢,以C-N共渗制定热处理工艺,控制残余奥氏体量提供了依据。     

Nb对中锰TRIP钢临界退火过程中组织演变的影响

采用临界退火热处理工艺,利用场发射扫描电镜(FE-SEM)观察含铌和不含铌的两种热轧中锰TRIP钢在不同退火制度下的碳化物演变行为及铌对中锰TRIP钢微观组织、残留奥氏体体积分数与稳定性的影响。结果表明:试验钢经临界退火处理后获得超细晶铁素体与残留奥氏体复相组织。随着退火温度的提高,残留奥氏体体积分数出现先升高后降低的趋势;随着退火时间的延长,碳化物逐渐溶解,残留奥氏体体积分数逐渐增加,达到平衡后保持不变。Nb元素的加入可细化奥氏体晶粒,延缓碳化物溶解,推迟奥氏体转变,增加膜状奥氏体,提高奥氏体稳定性。     

深冷处理对GCr15钢冲击性能的影响

对GCr15钢进行了不同工艺的热处理和深冷处理,检测了硬度、残留奥氏体、冲击吸收功并分析了冲击试样的断口。结果表明,淬火组织、残留奥氏体含量及残留奥氏体转变所产生的应力和显微裂纹是影响GCr15钢冲击性能的主要因素。     

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。     

I&Q&P工艺预先Mn配分处理低碳硅锰钢的组织与性能

通过IQP处理工艺,采用SEM、XRD和拉伸试验手段,研究了低碳硅锰钢C-Si-Mn中Mn元素的配分行为及其配分工艺参数对IQP处理试验钢组织与性能的影响。结果表明:经IQP工艺处理,试验钢室温组织为板条状马氏体和残留奥氏体。随配分时间延长或配分温度的升高,试验钢的抗拉强度总体呈降低趋势,残留奥氏体含量均先增加后减小,伸长率与残留奥氏体量的变化趋势基本一致。800℃等温30 min,试验钢中残留奥氏体含量及伸长率最大,分别为10.02%,24.25%,该参数下的综合性能最佳。     

残留/逆转变奥氏体对改善高强度不锈钢-196 ℃超低温冲击性能的影响

采用力学性能测试、SEM和XRD等手段研究了淬火+低温回火处理的0Cr16Ni6高强度不锈钢和过时效处理的00Cr11Ni11MoTi马氏体时效不锈钢,并分析了残留/逆转变奥氏体对试验钢超低温缺口抗拉强度和冲击性能的影响。结果表明,在两种试验钢室温强韧性相近的情况下,0Cr16Ni6钢在超低温下(-196 ℃)的缺口抗拉强度和冲击性能显著优于00Cr11Ni11MoTi钢。根据冲击试样远离断口和断口附近马氏体/奥氏体衍射峰的相对强度分别定量计算的残留/逆转变奥氏体含量,发现在裂纹形成和扩展过程中0Cr16Ni6钢有接近90%的残留奥氏体通过应变诱发相变生成马氏体,显著改善了超低温韧性;而过时效00Cr11Ni11MoTi钢形成的逆转变奥氏体具有较高的稳定性,难以发生应变诱发马氏体相变,改善超低温韧性作用程度有限。     

奥氏体化温度对汽车用QP钢组织性能的影响

利用扫描电镜及透射电镜、X射线衍射仪和拉伸试验机对采用不同的奥氏体化温度处理后QP钢微观组织和力学性能进行观察及测试分析,探讨了奥氏体化温度对QP钢组织与力学性能的影响。研究结果表明:奥氏体化温度对QP钢最终的组织性能有决定性影响。部分奥氏体化时,QP钢的最终组织为马氏体+残留奥氏体+铁素体;完全奥氏体化时,QP钢的最终组织为马氏体+残留奥氏体。随奥氏体化温度提高,铁素体数量减少,马氏体数量增多,QP钢的强度增加,塑性下降。拉伸过程中,QP钢中发生了残留奥氏体向马氏体转变。     

低碳硅锰钢的Q&P热处理工艺

研究了QP工艺对添加硅和锰的低碳钢组织性能的影响。通过X射线衍射仪、电子背散射衍射技术、拉伸试验等对不同QP工艺参数条件下试验钢的组织和性能进行了测试分析。结果表明,经过QP工艺热处理后,试验钢中形成了一定比例的纳米级的残留奥氏体。随着配分温度的升高,试验钢中残留奥氏体含量升高,抗拉强度降低,伸长率增加。配分温度为450℃时,随着配分时间的增加,试验钢中残留奥氏体含量先增加后降低,在配分时间为20 s时达到最大值,但抗拉强度降低,伸长率呈增加趋势。强塑积在450℃配分20 s时最大,与残留奥氏体含量变化一致。     

A_(c1)附近临界区处理对785MPa级船体钢屈强比的影响

通过对一种785 MPa级低合金高强度船体钢Ac1附近临界区热处理试样的性能测试,研究了Ac1附近临界区处理对其性能及屈强比的影响,采用光学和透射电镜观察了试验钢的微观组织,分析了试验钢性能与微观组织结构的相关性。结果表明:Ac1附近临界区热处理改变了试验钢的微观组织结构,热处理后钢板组织为回火马氏体基体上分布细小马氏体/M-A组元、碳化物、少量残留奥氏体、细小弥散的纳米铜析出相;多元复相组织中的转变马氏体及残留奥氏体降低了试验钢的屈服点,提高了钢板的加工硬化率和均匀伸长率,降低了试验钢的屈强比;同时,细化的组织结构保证了材料的塑韧性。     

热处理工艺对高锰含铝TRIP钢残留奥氏体和力学性能的影响

设计了新型低碳、5％锰、含铝TRIP钢,通过扫描电子显微镜(SEM)、X射线衍射仪(XRD)及室温拉伸性能测试研究了不同热处理工艺对试验钢残留奥氏体和力学性能的影响,并借助Therma-Calc热力学软件对试验钢进行了平衡热力学计算.结果表明:1％Al元素的添加,使得试验钢平衡相图中的两相区温度范围扩大并向高温区移动.试验钢在660℃等温5 min后,可以获得15.1％的残留奥氏体,对应的抗拉强度803 MPa,伸长率可以达到24％,即试验钢经过一步简单的短时间两相区退火处理后可以获得数量较多的残留奥氏体和比较理想的力学性能.另外通过在640℃、660℃两个温度进行不同时间的等温退火处理发现,随着两相区等温时间的延长,残留奥氏体的量逐渐增多,最高可以达到20.5％;等温时间越长屈服强度越低,试验钢的加工硬化性能越好.最高强度-塑性匹配出现在640℃等温1h后,抗拉强度720 MPa,伸长率可以达到30％.     

离子研磨对TRIP780钢中残留奥氏体含量和稳定性的影响

研究了经离子研磨对TRIP780钢中残留奥氏体含量与稳定性的影响。首先利用电解抛光技术对TRIP780钢进行样品制备以去除样品表面的应力层,然后利用离子研磨仪对所得样品进行离子研磨,再借助场发射扫描电镜对TRIP780钢中的残留奥氏体进行形貌观察与分析,利用电子背散射衍射技术(EBSD)对离子研磨前后的TRIP钢中残留奥氏体含量进行定性和定量分析。结果表明,与电解抛光相比,离子研磨技术同样可以很好的去除样品表面应力层,但是对于TRIP780钢,经过离子研磨后样品中残留奥氏体的含量大幅度减少,从原来的10.1%骤降至0.02%。从残留奥氏体的菊池花样可以看出,经离子研磨后的残留奥氏体菊池花样明显变差,甚至模糊不清。经离子研磨后的TRIP780钢中残留奥氏体含量明显下降,同时其相结构确实发生了转变,由原来的面心立方结构转变为体心立方结构(即fcc→bcc),由此表明残留奥氏体在受到离子轰击后极其不稳定,易发生相变,这一点在残留奥氏体的准原位试验中得到了进一步的验证。同时离子研磨诱发了晶格的畸变,导致菊池花样清晰度明显下降,花样分辨率降低。     

含铌TRIP钢的显微组织和残留奥氏体稳定性分析

研究了含Nb与不含Nb两种冷轧TRIP钢热处理后的显微组织和力学性能,并用X射线衍射法计算了TRIP钢中残留奥氏体含量及残留奥氏体中的碳含量.试验结果表明,TRIP钢中铁素体体积分数随退火温度的升高逐渐减少,在相同热处理工艺下,与不含Nb试样比较,含Nb试样的残留奥氏体中碳含量较高,强塑积较大.残留奥氏体量大约相同时,含Nb试样残留奥氏体更为稳定,综合力学性能也更好.     

相变诱发塑性的影响因素研究进展

对国外相变诱发塑性影响因素的研究进展做了概述,介绍了TRIP效应及其影响因素。重点概括了化学成分对残留奥氏体量和稳定性的影响,残留奥氏体量和相变转化率及残留奥氏体中碳含量对相变诱发塑性的影响。介绍了影响TRIP效应的其它因素,如变形条件、轧制工艺和热处理制度等。这些论述有助于相变诱发塑性的进一步研究和应用。     

EFFECTS OF CARBON CONTENT AND ROLLING PROCESSING ON RETAINED AUSTENITE FOR HOT-ROLLED TRIP STEELS

The effects of finishing rolling temperature and coiling temperature on retained austenite were studied for hot-rolled transformation induced plasticity (TRIP) steels with different carbon content. The experimental results showed that an appropriate volume fraction of retained austenite from 6% to 11% could be obtained according to the different carbon content less than 0.20% by controlled finishing rolling and coiling for the hot-rolled TRIP steels. It can be concluded that carbon content has a significant effect on the fraction of retained austenite and coiling processing plays stronger role on retaining austenite than fishing rolling processing.     

低碳-硅-锰系TRIP钢的组织演变规律研究

采用彩色金相、SEM、TEM和X射线衍射技术研究了低碳-硅-锰TRJP钢在单向拉伸状态下的组织演变规律.结果表明,TRIP钢变形前的组织为F、B和残余奥氏体,经拉伸变形后部分残余奥氏体在应变作用下转变为孪晶结构的马氏体,提高了钢的强度;TRIP钢的断裂为韧性断裂,位于F晶界处的残余奥氏体发生相变从而松弛了应力,延缓了断裂的产生,使TRIP钢板获得高塑性.     

Effect of strain rate on deformation behavior of TRIP steels

Tensile deformation behavior of Si–Mn TRIP (TRansformation Induced Plasticity) steel with vanadium and without vanadium and the DP (Dual Phase) steel of the same composition were studied in a large range of strain rate (0.001–2000 s^?1) by routine material testing machine, rotation disk bar–bar tensile impact apparatus and high-speed material testing machine of servo-hydraulic type. In situ measurement of the transformation of retained austenite was performed by means of X-ray stress apparatus in order to have detailed knowledge about the transformation of retained austenite at quasi-static tensile. Microstructure of steels before and after tensile were observed by means of optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). It is shown that there is no yield plateau observed on the stress–strain curve at quasi-static condition for TRIP steel containing vanadium because the vanadium carbide suppress the formation of Cottrell atmosphere in matrix. Retained austenite of Si–Mn TRIP steel containing vanadium transforms to martensite at loading stress of 502 MPa (its yielding strength is 486 MPa), while the transformation of retained austenite in matrix of Si–Mn TRIP steel without vanadium happens when its yielding process is finished at quasi-static tensile. It is confirmed that phase transformation of retained austenite in TRIP steel is strain induced phase transformation. It is noted that tensile elongation of TRIP steel at dynamic tensile is always lower than that at quasi-static tensile. That is because gradually strain induced phase transformation of retained austenite in TRIP steel is suppressed by deformation localization at dynamic tensile.     

冷轧双相钢中残余奥氏体稳定性的研究

研究了不同退火温度和变形条件对DP780钢残余奥氏体含量、尺寸、稳定性的影响。结果表明,通过优化连续退火工艺参数,可以获得5%～7%稳定的残余奥氏体。随着变形量的增加,90%的残余奥氏体发生了TRIP效应,不仅提高了DP780钢的强塑性,也改善了其成形性能。     

MECHANICAL PROPERTIES IN AN INTERCRITICALLY HEAT-TREATED BAINITE-TRANSFORMED 2%Si STEEL

A significant amount of austenite can be retained by rapid cooling following intercritical annealing and holding at the bainite transformation range in steel with comparatively low carbon and silicon contents. Retained austenite is blocky and very fine and moderately stabilized due to C enrichment. The elongation and the strength-ductility balance of the steel can be enhanced considerably due to strain-induced martensite transformation and transformation-induced plasticity (TRIP) of retained austenite.     

Dependency of Deformation Behavior of Retained Austenite in TRIP Steels on Microstructural and Chemical Homogeneity

The room-temperature stability of the retained austenite against strain-induced martensitic transformation, its deformation behavior, the response to the bainitic isothermal treatment, the appearance of yield point elongation and other peculiarities of plastic flow, and the mechanical properties of transformation-induced plasticity(TRIP) steel were tailored based on the chemical homogeneity and the relative distribution of the retained austenite, bainite, and ferrite in the microstructure. The presence of ferritic-pearlitic banded structure in the initial microstructure resulted in an inhomogeneous TRIP microstructure, in which the retained austenite and bainite were confined to some bands and it was found to be responsible for the resultant inferior mechanical properties. The appearance of discontinuous yielding for the chemically inhomogeneous material was related to the martensitic transformation of unstable retained austenite at the initial stage of tensile deformation. These results are essential for better understanding of the behavior of advanced high-strength steels and their applications.