低合金超高强度钢的微观组织与强化机理

使用光学显微镜、SEM、TEM、XRD观察G50钢淬火和低温回火后的微观组织形貌,结合低温回火前后G50钢的屈服强度和冲击韧性等力学性能的变化,分析其强韧化机理。结果表明,淬火态G50钢微观组织主要为细小的板条状马氏体,使用TEM可以观测到有残留奥氏体薄膜在马氏体板条之间,残留奥氏体量很低。G50钢的超高强度来自马氏体相变和晶粒的细化,低温回火处理后G50钢的微观组织是细小的板条状回火马氏体,在保持强度的同时提高了冲击韧性。     

18Ni马氏体时效钢循环相变细晶工艺研究

研究了循环相变对18Ni(1800 MPa级)马氏体时效钢奥氏体晶粒细化的影响.结果表明,通过加热温度递减的变温循环相变,18Ni马氏体时效钢奥氏体晶粒尺寸可由201 μm的粗大等轴晶细化为约10 μm的均匀细小晶粒;而通过950、920和900℃的等温循环相变,得到细化的晶粒尺寸为12～14μm;通过循环相变显著细化了马氏体时效钢的晶粒尺寸,而温度递减的变温循环相变是该钢细化晶粒的有效措施.     

热处理对超高强度20Mn2Cr汽车钢组织和性能的影响

研究了热处理工艺参数对20Mn2Cr钢显微组织和性能的影响规律,并采用扫描电镜、透射电镜、电子背散射衍射技术等研究了不同奥氏体化温度和回火温度下实验钢中的马氏体组织特征和碳化物析出形貌.结果表明,实验钢经900℃奥氏体化处理时可以保证较小的原奥氏体晶粒尺寸及细小的马氏体板条束宽度;550℃再结晶退火可以进一步细化原奥氏体晶粒尺寸及马氏体板条束宽度;淬火后的回火处理有利于Cr碳化物粒子的析出.通过调整热处理工艺,20Mn2Cr钢可以获得1000～1700 MPa级的系列超高强度,同时可以实现超高强度与高塑性的良好匹配.     

65Mn钢的组织超细化与超塑性

利用盐浴加热循环淬火对６５Ｍｎ钢进行奥氏体晶粒的超细化，并对晶粒细化后的６５Ｍｎ钢进行超塑拉伸试验。结果表明，热轧或正火态的６５Ｍｎ钢经８１０℃循环淬火３次，奥氏体晶粒即可细化至１３级，在温度６６０～７２０℃、变形速率０．５～３．７×１０（－２）ｍｉｎ（－１）范围内呈现超塑性，δ≥２００％，最高达２９４％，σ最低仅３８ＭＰａ，ｍ值为０．３８左右。     

低合金超高强度钢中的相变及组织控制

概述了低合金超高强度钢中所涉及的相变及组织控制方法。马氏体相变是低合金钢获得超高强度的最基本的途径,通过优化热处理或形变热处理工艺获得细化的马氏体板条是保证超高强度的关键。马氏体钢中足够的塑韧性通过适度回火来保障,回火过程中组织控制的关键是避免脆性渗碳体碳化物的析出。对低合金超高强钢起重要作用的贝氏体主要有两种,下贝氏体和无碳化物贝氏体,其中下贝氏体主要与马氏体一起形成复合组织,细化马氏体板条尺寸。无碳化物贝氏体通过得到超细亚结构或超细板条而获得超高强度,同时利用贝氏体转变的不完全性获得稳定的高碳残留奥氏体来保证塑韧性。残留奥氏体在低合金超高强钢韧性改善方面起着重要作用,Q-P(或Q-P-T)钢和TRIP钢中较多的残留奥氏体可赋予低合金超高强钢超乎寻常的高塑韧性。     

热变形对35CrMo钢淬火马氏体晶体学特征的影响北大核心CSCD

通过热压缩实验获得不同应变下35CrMo钢的淬火马氏体组织。基于电子背散射衍射(EBSD)测试技术研究了热变形对35CrMo钢淬火马氏体晶体学特征的影响,重点分析了不同变形量下奥氏体晶粒尺寸及马氏体变体组合特征的变化。研究结果表明:多轮动态再结晶的出现造成了高温真应力-真应变曲线的多峰变化,且第1轮动态再结晶明显细化了奥氏体晶粒。原始奥氏体的晶粒取向决定了淬火后马氏体变体的类型,且淬火马氏体变体的组合方式均为密排面组合。不同变形量下淬火马氏体变体间的取向差集中在50°~60°范围内,可通过引入大角度晶界来细化晶粒。     

冷却速度对超高强马氏体钢的马氏体相变起始温度和硬度的影响

采用热膨胀法测量了马氏体试验钢在20~70 ℃/s冷却速度下的马氏体相变起始温度(Ms点温度),并研究了其对试验钢硬度的影响。结果表明,冷却速度的提高可增加奥氏体强度、提高奥氏体的热稳定性,进而降低马氏体相变起始温度。同时,更高的冷却速度容易造成晶格畸变,增大位错密度,还会提高过冷度,细化有效晶粒尺寸,使试验钢的硬度提高。     

双相钢中残余奥氏体对性能的影响及其热稳定化

用声发射法研究了一种低合金钢于双相区热处理所得残余奥氏体的马氏体相变。发现残余奥氏体的热稳定化程度和稳定性均与奥氏体的颗粒尺寸有关。颗粒愈小，热稳定化程度愈高，且愈稳定。不存在马氏体核胚的极小奥氏体颗粒不能仅靠过冷来使其转变。形变能诱发试验钢中残余奥氏体转变，且增加钢的塑性。但只有奥氏体颗粒尺寸有合适的分布，其中小部分稳定性很高，才能使马氏体相变随应变增加而逐渐发生并延伸到大的应变，使延伸率明显增加。     

马氏体相变的弹性波促发形核

使用Gleeble-1500型热模拟机，对30CrMnSiMoV超高强度钢在奥氏体状态下预应变。而后分三种工艺淬火：预应变结束后不卸载立即喷雾冷却；预应变结束后卸载，并于960℃保温5min后再喷雾冷却；预应变后不卸载，重新加热至1050℃，并保温5min，发生再结晶后再喷雾冷却。经对其马氏体组织进行分析发现：处于释放状态的弹性应变能会促发马氏体相变均匀形核；而塑性应变对马氏体的生长有限制作用，可间接影响马氏体的相变形核。两者的恰当配合，可显著细化马氏体，使其板条平均宽度接近纳米量级水平（平均宽度为120nm）。在此基础上，提出了一种马氏体的软模弹性波动形核机制。可解释所获得的实验结果。     

双相钢中残余奥氏体对性能的影响及其热稳定化

用声发射法研究了一种低合金钢于双相区热处理所得残余奥氏体的马氏体相变，发现残余奥氏的热稳定化程度和稳定性均与奥氏体的颗粒尺寸有关。颗粒愈小，热稳定化程度愈高，且愈稳定，不存在马氏体核胚的极小奥氏体颗粒不能仅靠过冷来使其转变，形变能诱发试验钢中残余奥氏体转变，且增加钢的塑性，但只有奥氏体颗粒尺寸有合适的分布，其中小部分稳定性很高，才能使马氏体相变随应变增加而逐渐发生并延伸到大的应变，使延伸率明显增加     

Thermodynamic analysis of shape memory phenomena — I. Effect of transformation plasticity on elastic strain energy

The effect of plastic strains accompanying the thermoelastic transformation on shape memory phenomena is evaluated. It is shown that transformation plasticity can be formally included in Eshelby's macroscopic elastic analysis by defining a net transformation strain equal to the difference of the crystallographic transformation strain and the associated plastic strain. A two variant analysis for a thermoelastic martensite transformation is developed which enables calculation of the variant structure along the path of minimum elastic energy. It is shown that plastic strain accompanying the formation of a martensitic plate reduces the elastic energy stored during the forward transformation. The equilibrium variant structure is shown to be determined by both the applied stress and the fraction of martensite. If the plastic strains accompanying the forward and reverse transformations do not cancel, residual elastic stress fields remain after completion of the reverse transformation. The residual elastic strain fields influence subsequent transformation behaviour and provide the driving force for two-way shape memory behaviour.     

Precise measurement of strain accommodation in austenite matrix surrounding martensite in ferrous alloys by electron backscatter diffraction analysis

Local strain distributions in austenite matrix that is deformed to accommodate shape strain associated with formation of martensite were investigated by means of electron backscatter diffraction (EBSD) analysis for various morphologies of lath, lenticular and thin plate martensite in ferrous alloys. By detecting small changes in EBSD patterns through image analysis of the patterns, components of both strain and rotation tensors in austenite matrix adjacent to martensite were measured quantitatively. In the austenite matrix surrounding thin plate martensite, the magnitude of components of strain tensor is nearly as large as those of rotation tensor, implying that shape strain of thin plate martensite is accommodated by elastic deformation of austenite. On the other hand, in the austenite matrices surrounding lenticular and lath martensite, components of strain tensor are found to be much smaller than those of rotation tensor even near the austenite/martensite interface. This indicates that most of the shape strain associated with the formation of lenticular and lath martensite is accommodated by plastic deformation in the austenite matrix. The misorientation axis of austenite adjacent to lenticular and lath martensite coincides well with that predicted from the phenomenological theory of martensite crystallography.     

Transformation Sequence Rule of Martensite Plates and Temperature Memory Effect in Shape Memory Alloys

In thermoelastic martensitic transformation, it is well established that the first martensite plate appearing upon cooling becomes the final one during reverse transformation to austenite upon heating. The results obtained from this work show that the transformation sequence of the martensite appears to be random. Newly formed martensite plates can modify the elastic strain energy level stored in the already existing martensite. Additionally, the elastic strain energy stored in newly formed martensite is not necessarily to be higher than the remaining martensite. The obtained results may assist in understanding phenomena related to partial transformation of shape memory alloys, such as temperature memory effect.     

Cu Zn Al Mn Ni记忆合金马氏体内弹性应变能的研究

用差示扫描量热分析技术研究了三种不同Ｍｎ含量的Ｃｕ Ｚｎ Ａｌ Ｍｎ Ｎｉ记忆合金中马氏体正逆转变的全过程。发现马氏体在共格初变时所产生的弹性应变能之和，约等于逆、正转变的热效应之差。同时，结果还表明，Ｍｎ含量不同，单位质量的马氏体中所产生的弹性应变能亦不相同，且弹性应变能的变化与Ｍｎ含量的变化并非是简单的线性关系。     

Distortion Analysis of Axial Contraction of Carburized-Quenched Helical Gear

An automotive component (steering helical gear) made from low-alloy structural steel SCr420H was gas-carburized and oil-quenched. Axial contraction of total length was measured after such case-hardening process. Using DEFORM-HT Ver 6.1 simulation tool incorporating phase transformation kinetics, the causal factor of negative axial distortion is studied. Analysis of time-dependent displacement, temperature, phase transformation, and stress-strain generation is presented. Total strain and individual strain (e.g., thermal, elastic, plastic, phase transformation, and transformation plasticity strain) are included. Three simulations consisting of case hardening with transformation plasticity (TP), case hardening without TP, and through hardening with TP were conducted to asses the influence of transformation plasticity and martensite as well as retained austenite in contributing the axial contraction of total length. Finally, transformation plasticity has a greater influence than volume fraction of martensite and retained austenite in producing the negative axial distortion.     

Mechanically-induced martensite transformation of NiTiFe shape memory alloy subjected to plane strain compression

Based on the channel die compression, NiTiFe shape memory alloy (SMA) was subjected to plane strain compression. Mechanically-induced martensite transformation, nanocrystalline and amorphous phase can be observed in the case of large plastic strain. Mechanically-induced martensite transformation is obviously different from the conventional stress-induced martensite transformation. The former generally occurs after dislocation slip, whereas the latter arises prior to dislocation slip. The occurrence of B19’ martensite phase contributes to accommodating subsequent plastic deformation of NiTiFe SMA. Mechanically-induced B19’ martensite is partially stabilized due to the existence of local high stress field and consequently it is unable to be reverted to B2 austenite phase during unloading.     

Evolution of lattice strain and phase transformation of β III Ti alloy during room temperature cyclic tension

An in situ high-energy X-ray diffraction cyclic tension test was carried out on a β III Ti alloy to study its micromechanical behavior and the stress-induced phase transformation. Pre-strained material showed a microscopic multi-stage re-loading behavior following the sequence of elastic deformation, stress-induced martensite (SIM) transformation, a second stage of elastic deformation followed by a final stage of SIM transformation. Based on the relationship of internal strains and diffraction intensities between the β phase and the SIM, it is concluded that after a small strain deformation, the austenite is divided into two different sets of grains with different properties. Those that previously experienced phase transformation have a lower critical stress for the SIM transformation due to residual martensite and dislocations, while the rest have a higher trigger stress and only transform to martensite after the stress is back to levels comparable to where transformation was seen in the previous cycle. The different properties within the same austenite grain family cause the multistage re-loading behavior. The reverse phase transformation during unloading was impeded by the combination of increased dislocation density in the austenite and the increased tensile strain in the martensite prior to unloading.     

Microscopic Deformation in Individual Grains in an Advanced High-Strength Steel

In situ synchrotron microbeam x-ray diffraction experiments were carried out to study the microscopic deformation within individual ferrite grains in a martensite/ferrite dual-phase steel (DP980) under incremental tensile loading. The differential aperture x-ray microscopy technique was used to resolve the strain variations as a function of depth up to 100 μm deep from the sample surface. The highly inhomogeneous distributions of the lattice strain, which is associated with the elastic deformation and stresses inside the grains, were determined by means of monochromatic energy diffraction, whereas insights to the plastic deformation were revealed by polychromatic energy diffraction.     

不同应变率下 TRlP钢的拉伸性能

在自制气动式间接杆杆型冲击拉伸试验机上对含1.6Si-1.58Mn-0.195C的TRIP（Transformation-induced Plasticity)钢在不同应变率下的高速冲击拉伸性能进行了研究，并和静态拉伸性能进行了比较。结果显示，随应变速率的提高，材料的抗拉强度显著增加，延伸率降低。由于TRIP钢组织中残余奥氏体在应力应变作用下向马氏体的相变诱发转变显著改善了材料的塑性，因此在高应变率下的延伸率仍较好。     

An in situ high-energy X-ray diffraction study of micromechanical behavior of multiple phases in advanced high-strength steels

The micromechanical behavior of high-strength steels with multiple phases was characterized using the in situ high-energy X-ray diffraction technique. For the materials investigated, the {2 0 0} lattice strains of the constituent phases (ferrite, bainite and martensite) with similar crystal structures were determined by separating their overlapped diffraction peaks and then examining the respective changes in peak positions during deformation. Based on those experimental data, the anisotropic elastic and plastic properties of the steels were simulated using a self-consistent model for predicting the grain-to-grain and phase-to-phase interactions. The constitutive laws for describing the elastic and plastic behavior of each constituent phase were directly obtained by comparing the predicted lattice strain distributions with the measured ones. The transmission electron microscopy observations of the microstructure development verified the partitioning of plastic strains among different phases. The present investigations provide a fundamental understanding of the stress partitioning of soft and hard phases, and the different work-hardening rates of the multiphase steels.