Phase Transformation and Texture Evolution During Cold Rolling and alpha '-M Reversion in High Manganese TRIP SteelEI北大核心CSCD

To meet the requirement of environment, economy and safety, advanced high strength steels including dual phased (DP), complex phased (CP), transformation- induced plasticity (TRIP) and twinning-induced plasticity (TWIP) steels are widely used for automotive steel. Among them, high manganese TWIP and TRIP steels are particularly appealing due to their outstanding tensile strength and elongation. In contrast to high manganese TWIP steel, high manganese TRIP steel exhibits higher strength and work hardening rate due to strain induced martensitic transformation. The enhanced mechanical properties of high manganese TRIP steel are determined by both the stability of the retained austenite (gamma) and the initial microstructure. Strain induced martensitic transformation and subsequent reversion from deformed martensite to gamma during annealing is often applied as one of the most effective methods for microstructure improvement. Microstructure and texture characteristics of high manganese TRIP steel during cold rolling together with the reversion of deformed bcc martensite (alpha'-M) at high temperature were investigated. It is shown that the gamma was almost completely transformed into alpha'-M at medium cold rolling reduction. And a higher reduction after alpha'-M saturation resulted in dominantly the deformation of alpha'-M, hence thin laths paralleled to the rolling direction (RD) were obtained. The main components in alpha'-M were {113}< 110 >, {554}< 225 > and rotated cube ({001}< 110 >) textures at medium cold rolling reduction, which are the typical phase transformation textures. The {113}<110> texture rotated toward a more stable orientation {223}< 110 > and led to a strong cold rolling texture (< 110 >//RD) with increasing reduction. The reversion of martensite and recrystallization of gamma proceeded at temperature ranging from 650 degrees C to 850 degrees C. The reversion of alpha'-M proceeded in a diffusional mechanism, accompanying with the redistribution of Mn and Al between gamma and alpha'-M. Deformed alpha'-M was merged by the adjacent gamma, and columnar gamma grains with a large amount of subgrains were obtained. The texture of reverted gamma was approximately the same as that of the deformed gamma, this phenomenon called texture inheritance was formed by the direct growth of gamma. Subsequently, recrystallization of gamma grains occurred by sub-grain coalescence and the columnar g grains were instead by equiaxed gamma grains.     

On the transitions of deformation modes of fully austenitic steels at room temperature

The present study was undertaken to provide a more comprehensive understanding of the deformation modes of advanced fully austenitic steels exhibiting an enhanced combination of strength and ductility. For this purpose, a new plasticity, called microband induced plasticity (MBIP), was introduced. In addition, the origin of its superb combination of strength and ductility over the well-known transformation induced plasticity (TRIP) and twin induced plasticity (TWIP) was elucidated. With the aids of previously developed models, we focused on predicting the transitions among TRIP, TWIP, and MBIP, primarily in terms of the stacking fault energy. The analysis revealed that the TRIP-TWIP transition can be reasonably predicted by the energy balance for FCC austenite — HCP ɛ martensite transformation. The TWIP-MBIP transition can be addressed by the critical stress for mechanical twinning, which causes the infinite divergence of the Shockley partials. Lastly, the TWIP-MBIP transition model was validated by comparing it with the experimental data.     

相变/孪晶诱发塑性的β钛合金加工硬化行为研究

研究了具有相变诱发的塑性变形/孪晶诱发的塑性变形(trunsformationinducedplasticity/twinninginduced plasticity,TWIP/TRIP)效应的Ti-12Mo-5Zr钛合金的加工硬化行为,并利用Hollomon理论分析了该合金的加工硬化指数n变化趋势。根据加工硬化率的演变趋势,该合金的加工硬化过程可分为3个不同阶段:Stage I(ε≈0~0.02),弹-塑性变形过渡阶段;Stage II(ε≈0.02~0.15),加工硬化率θ和硬化指数n逐渐增大阶段;Stage III(ε≈0.15~0.34),加工硬化率θ逐渐减小直到颈缩阶段,该阶段硬化指数n达到稳定值0.34。结合金相显微镜(OM)、透射电镜(TEM)等手段详细研究了该合金在这3个不同加工硬化阶段的显微结构演化过程,探讨了每个阶段的变形机理及其硬化行为。     

高锰TRIP/TWIP效应共生钢高速变形过程中的组织演变及变形行为

对18Mn-3Al-3Si和21Mn-3Al-3Si高锰TRIP/TWIP效应共生钢动态变形过程中的变形行为,应变硬化速率、真应力和应变硬化指数随真应变的变化,以及应变硬化和基体软化间的相互作用等进行了研究,采用OM,SEM,TEM和XRD等方法对变形前后的组织进行了分析.结果表明,高应变速率下,TRIP/TWIP效应共生钢应变诱发相变途径为γ→ε→α;高速变形对滑移的抑制、奥氏体向马氏体的相变和形变孪晶对奥氏体晶粒的细化是应变硬化的主要因素;造成基体软化的原因是绝热温升效应、ε→γ的逆相变和孪晶的动态再结晶.     

Work hardening induced by martensite during transformation-induced plasticity in plain carbon steel

Transformation-induced plasticity (TRIP) steels are becoming increasingly exploited for industrial applications because they show high strength and high uniform elongation (ductility). Despite this interest, the relative contributions of the various strengthening and straining mechanisms are often poorly understood. In this study, neutron diffraction is employed to quantify the contribution of different mechanisms to ductility and work hardening for a 0.25 wt.% C steel. Differences in stress–strain response at different temperatures are related to the extent of the transformation of metastable austenite into martensite during deformation. At room temperature (RT) the transformation of austenite occurs gradually with straining, while at ?50 °C the transformation occurs almost from the onset of loading. The associated transformation strain is reduced, comprising nearly half the total strain, lowering the apparent elastic modulus and explaining the relatively low work hardening compared to RT straining. By contrast, deformation at RT after pre-straining at ?50 °C results in larger work hardening than for solely RT straining due to the higher martensite levels introduced at ?50 °C. This is due to composite load transfer to the strong constituent from the soft matrix. The extent of the transformation is quantified as a function of strain at both temperatures as well as its effect on the work hardening and elongation.     

亚稳β型TRIP/TWIP钛合金研究进展

综述了当前国内外学者关于具有相变诱发塑性/孪晶诱发塑性(transformation induced plasticity/twinning induced plasticity,TWIP/TRIP)效应的亚稳β型钛合金的研究进展。介绍了该类钛合金的设计方法。统计了近几年开发的TWIP/TRIP钛合金家族成员及其力学性能,归纳了合金在塑性变形过程中的变形产物及其形成机制,指出了该类钛合金在发展过程中遇到的一些问题。     

304奥氏体不锈钢应变诱发马氏体的研究

用X射线衍射(XRD)、磁针法、力学性能和显微分析研究了商用热轧态和冷轧态304奥氏体不锈钢在不同变形方式下应变诱发马氏体的行为。结果表明:304钢热轧态由于存在多量碳化物和组织不均匀性,其奥氏体稳定性低,拉伸应变诱发马氏体量达40%,冷轧(固溶)态组织均匀、奥氏体稳定性高,拉伸应变诱发马氏体量仅9%;304冷轧板材扩孔、杯突成形时其切向和径向的二向拉应力有助于γ→α’转变,诱发马氏体量(30%～35%)多于单向拉应力的拉伸诱发马氏体量(8%～10%)。对于冷轧304不锈钢,在20%以上拉伸工程应变的驱动下才能较明显地诱发马氏体。304明显的强化效果(△σ达320～400 MPa)来自应变硬化和马氏体相变强化两方面:冷轧304钢的强化主要来自应变硬化本身;热轧304钢的强化不光有应变硬化的贡献,还有应变诱发的多量马氏体的重要贡献,而且后者是主要的。     

N对TWIP钢马氏体相变及力学性能的影响

以传统TWIP钢为对比,测试了含N TWIP钢的力学性能,并利用XRD进行物相分析和TEM进行做观结构表征.结果表明,在由fcc或hcp结构向bcc结构马氏体进行相变时,晶体结构中的最大间隙由0.1047 nm降低至0.0725 nm.间隙原子N的存在显著增大bcc结构的晶格畸变能,提高α马氏体切变的阻力,因而强烈抑制α马氏体相变,导致组织中hcp结构ε相含量大幅度增加,提高了TWIP钢的强度,但也降低了钢的塑性.另外,奥氏体平均和区域层错几率的计算及微观组织分析结果表明,形变增加层错的数量,而马氏体相变消耗层错,从而减少层错数量.     

Springback prediction of TWIP automotive sheets

In an effort to reduce the weight of vehicles, automotive companies are replacing conventional steel parts with light weight alloys and/or with advanced high strength steels (AHSS) such as dual-phase (DP), twinning induced plasticity (TWIP), and transformation induced plasticity (TRIP) steels. The main objective of this work is to experimentally and numerically evaluate the macro-performance of the automotive TWIP sheet in conjunction with springback. In order to characterize the mechanical properties, simple tension and tension-compression tests were performed to determine anisotropic properties, as well as the Bauschinger, transient, and permanent softening behaviors during reverse loading. For numerical simulations, the anisotropic yield function Yld2000-2d was utilized along with the combined isotropic-kinematic hardening law based on the modified Chaboche model. Springback verification was performed for the unconstrained cylindrical bending and 2D draw bending tests.     

含TRIP效应的Fe-18Mn-Si-C热轧TWIP钢的设计与研究

本文采用层错能估算和相图计算的方法,通过增C降Mn的成分优选,设计了Fe-18Mn-0.528Si-0.6C(质量分数,%)实验钢,研究表明,该钢种在室温拉伸变形时会发生γ→ε相变.借助OM,XRD和TEM对热轧实验钢板室温拉伸性能测试前后的组织进行了分析与研究,结果表明:经过1100℃开轧,850℃终轧后空冷的热轧钢板由于孪晶诱发塑性(TWIP)+相变诱发塑性(TRIP,γ→ε)双重效应的作用,实现了抗拉强度超过1 GPa,延伸率大于60%的优良性能,达到了第三代汽车用钢的要求;淬火ε马氏体和应力诱发ε马氏体的存在会导致力学性能下降.     

Kinetics of deformation processes in high-alloyed cast transformation-induced plasticity/twinning-induced plasticity steels determined by acoustic emission and scanning electron microscopy: Influence of austenite stability on deformation mechanisms

The austenite stability and the stacking fault energy of high-alloyed metastable transformation-induced plasticity/twinning-induced plasticity (TRIP/TWIP) steels, both depending on the chemical composition, have a strong influence on the deformation processes and stress/strain-induced martensitic phase transformation. Aiming at a better understanding of the kinetics of TRIP/TWIP-assisted plastic deformation, acoustic emission (AE) measurements were performed during room temperature tensile deformation of high-alloyed cast model steels with different austenite stability. The real-time AE investigations were complemented by detailed scanning electron microscopy investigations of deformed microstructures using electron backscattered diffraction to determine the martensitic phase transformation and electron channelling contrast to visualize dislocations and their arrangements. The quantitative AE analysis revealed different AE patterns at different plastic strains, which were correlated with underlying deformation mechanisms and microstructural transformations.     

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 借助于X射线衍射，研究了C、Mn、Cr和Ni 含量对304奥氏体不锈钢拉伸力学性能和应变诱发 马氏体相变倾向的影响。结果表明：C、Mn、Cr和Ni在允许的成分范围内变化，应变诱发α′马氏体相变倾向差异很大，这导致屈服强度和抗拉强度复杂的变化，尽管应变诱发α′马氏体相变使加工硬化速率提高，相变可以诱发塑性，但相变速率较快，相变倾向较大的钢塑性反而下降，此外，由于室温变形还增大热诱发马氏体相变倾向，从而限制了C、Mn、Cr和Ni下限钢在高精度和低温环境下构件的应用。     

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。     

亚稳态β钛合金的成分设计、变形机制与力学性能研究进展

本文介绍了当前孪晶塑性/相变塑性(TWIP/TRIP)钛合金的研究现状和设计方法,统计了TWIP/TRIP钛合金中发生{332}<113>、{112}<111>双孪晶钛合金β相的晶粒尺寸、屈服强度和加工硬化率。讨论了Bo-Md图在多组元钛合金设计上的应用和局限性,特别是析出不同二次相(α相与ω相)对基体β相稳定性的影响。从基体β相稳定性、析出相、β相晶粒尺寸和晶体学取向三方面归纳了亚稳态β钛合金TWIP/TRIP变形机制的影响因素,并对其中存在的一些问题和不足进行了分析,简要总结了双孪晶机制对钛合金力学性能的影响。通过综述最新的研究进展及相关问题,对未来高强韧钛合金的发展提出新见解。     

Large deformation behavior of twin-induced plasticity steels under high-pressure torsion

A high-manganese twinning-induced plasticity (TWIP) steel is processed by high-pressure torsion (HPT) for up to 1 turn under 6 GPa pressure. The HPT-processed TWIP steels exhibit a homogeneous microstructure with a peak hardness of Hv 550. Deformation twinning is developed significantly in the early stage of the shear deformation, but is exhausted soon after 1/2 turn. The strength of the HPT-processed TWIP steel significantly increased due to the accumulation of dislocations, but elongation dramatically decreased due to a lack of dislocations available for plastic deformations. An analysis of the evolution of strength by imposed large strain under high pressure suggests that strain hardening due to dislocation and twinning is exhausted in the early stages of the HPT process. Further strategy for enhancing both strength and ductility is proposed.     

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.     

EFFECTS OF TEST TEMPERATURE AND STRAIN RATE ON THE MECHANICAL PROPERTIES IN AN INTERCRITICALLY HEAT-TREATED BAINITE-TRANSFORMED STEEL

1.IntroductionThe demand for high strength steels with excellent ductility has increased in the automotive indus-try in order to improve manufacturing and safety and to reduce weight. High strength transforma-tion-induced plasticity (TRIP) steel sheets have received increased attention, as they have both high strength and ductility due to the martensitic transformation of retained austenite during plastic defor-mation.Transformation-induced plasticity was the phenomenon first found in steel …     

The effects of N on the microstructures and tensile properties of Fe–15Mn–0.6C–2Cr–xN twinning-induced plasticity steels

Three Fe–15Mn–0.6C–2Cr–xN (wt.%) twinning-induced plasticity (TWIP) steels with N concentrations of 0.02%, 0.09% and 0.21% were fabricated using a pressure-induction furnace. The variations in tensile properties and deformed microstructure as a function of N concentration were investigated. The yield and tensile strengths of the steels increased without a loss of total elongation as the concentration of N increased. The strain hardening rates (SHR) of the TWIP steels with 0.02% and 0.09% N decreased gradually as the true strain increased until failure. The SHR of the TWIP steel with 0.21% N decreased sharply until a strain of approximately 0.07 and increased with further strain, exhibiting the highest value at a strain of over 0.15. The addition of N delayed the kinetics of mechanical twinning, particularly secondary twinning. The TWIP steel with 0.02% N had many intersections between primary and secondary twins, at which α′ martensite formed. The TWIP steel with 0.09% N possessed few intersections because primary twins obstructed the growth of secondary twins. The TWIP steel with 0.21% N had primary twins with few secondary twins and intersections. The addition of N increased the critical strain for triggering serrations on the tensile stress–strain curves, indicating a reduction in dynamic strain aging (DSA). The TWIP steel with 0.21% N exhibited the highest SHR at large strains, despite the reduced twinning and DSA, because of both the thinning of the mechanical twins and the hardening of the twin boundaries as the N concentration increased.     

ON THE TENSILE MECHANICAL PROPERTY OF Si-Mn TRIP STEELS AT HIGH STRAIN RATE

二．互ntYO*llCtloliAs kn OWOWn,the beh yyiors of the strength and ductility of material under dynamic con－dltlon are obviously differs尬from those under static condition．几ensure the reft劝ility ofstructure under dynamic or impact loading It Is necessar     

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.