High-Strength and High-Plasticity TWIP Steel for Modern Vehicle

In this paper new high-strength and high-plasticity twinning induced plasticity （TWlP） steel for modern automobile body was investigated. Some basic experimental results were given. The results indicate the TWlP steel has excellent properties. It exhibits high ultimate tensile strength （600,--1100 MPa） and extremely large elongation of 60% to 90%.In the future it would be capable of satisfying the requirements of new generation of vehicle.

Effects of annealing temperature on the microstructure and properties of the 25Mn-3Si-3Al TWIP steel

Microstructures and mechanical properties of the 25Mn twinning induced plasticity （TWIP） steel at different annealing temperatures were investigated. The results indicated that when the annealing temperature was 1000℃, the 25Mn steel showed excellent comprehensive mechanical properties, the tensile strength was about 640 MPa, the yield strength was higher than 255 MPa, and the elongation was above 82%. The microstructure was analyzed by optical microscopy （OM）, X-ray diffraction （XRD）, and transmission electron microscopy （TEM）. Before deformation the microstructure was composed of austenitic matrix and annealing twins at room temperature; at the same time, a significant amount of annealing twins and stacking faults were observed by TEM. Mechanical twins played a dominant role in deformation and as a result the mechanical properties were found to be excellent.

Efect of high temperature and high strain rate on the dynamic mechanical properties of Fe-30Mn-3Si-4Al TWIP steel

The dynamic mechanical properties of Fe-30Mn-3Si-4A1 twinning induced plasticity （TWIP） steel were studied by the split-Hopkinson pressure bar （SHPB） at temperatures of 298-1073 K and strain rates of 700, 2500, and 5000 s-1. The TWIP steel indicates strain rate hardening effect between 700 and 2500 s-1, but it shows strain rate softening effect between 2500 and 5000 s-1. In addition, the strain rate softening effect enhances with an increase in deformation temperature. After deformation, the microstructures were studied by optical microscopy （OM）. It is shown that the deformation bands become more convergence, a part of which become interwoven with an increase in strain rate, and the dynamic recovery and recrystallization are enhanced with an increase in both temperature and strain rate.

In-situ observation on the deformation behaviors of Fe-Mn-C TWIP steel

The microstructure and crack behaviour of twinning induced plasticity （TWIP） steel during tensile deformation was investigated with in-situ scanning electron microscopy （SEM）. The results show that there are two modes of plastic deformation during tensile test in the Fe-Mn-C TWIP steel： dislocation gliding and deformation twins. During the process of tensile deformation, secondary deformed twins are found. Inclusions have played a role in the course of ductile fracture, and microcracks initiate from inclusions and twin-twin intersections.

Effect of annealing temperatures on dynamic impact mechanical properties of high manganese TWIP steel

Results presented in this paper contribute to the investigation of the effect of annealing temperatures ranging from 800 to 1000 ℃ on the dynamic impact mechanical properties of high manganese TWIP steel. The properties were measured by using an instrumented Charpy impact tester. The impact absorbing energy initially increases with the annealing temperature, climaxes at 900 ℃, and then decreases. The dynamic crack extension resistance curve indicates that the crack initiation force decreases with the annealing temperature, and the steel annealed at 900 ℃ has the best ability against crack expansion

Deformation behavior of Fe-Mn-C TWIP steel within the temperature range of hot rolling

The hot-deformation behavior of Fe-Mn-C twinning induced plasticity （TWIP） steel was investigated by conducting hot compression tests within a recommended hot rolling temperature range at various strain rates. Flow resistance curves during hot-deformation were obtained, and strain rate sensitivities and activation energies for plastic deformation were calculated using the power law. It is found that the addition of Al and Si clearly increases the peak stresses for the present alloys, especially at 950℃. But Mn has a minor effect on the stress-strain curves and activation energy when its content varies from 15 mass% to 22 mass% for the present alloys.

INFLUENCE OF COLD ROLLING REDUCTION ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF TWIP STEEL

The influence of cold rolling reduction on microstructure and mechanical properties of the TWIP （ttwinning induced plasticity） steel was investigated. The results＇ indicated that the steel had better comprehensive mechanical properties when cold rolling reduction was about 65.0% and the annealing temperature was 1000℃. The tensile strength of the steel is about 640MPa and the yield strength is higher than 255MPa, while the elongation is＇ above 82%. The microstructure is composed of austenitic matrix and annealing twins at room temperature, at the same time, a significant amount of annealing twins and stacking faults＇ are observed by transmission electron microscopy （TEM）. Mechanical twins play a dominant role during deformation, and result in exceUent mechanical properties.

Microstructure and Texture Evolution of Fe-33Mn-3Si-3Al TWIP Steel on Strain

The microstructure and texture evolution of Fe-33Mn-3Si-3Al twinning induced plasticity(TWIP) steel were studied by the scanning electron microscope(SEM) and X-ray diffraction(XRD) at room temperature. After quasi-static tensile, the texture evolution of different strain was observed. It was shown that the Goss and Brass components increased within the strain range of less than 0.6. Whereas, the main components were decreased when the strain levels were greater than 0.6. This behavior was attributed to the low stacking fault energy(SFE) and was related to the strain energy of this high manganese steel. At high strain levels, the high strain energy may contribute to the Brass components transition to the A(rot-Brass) components.

Reveal Hydrogen Behavior at Grain Boundaries in Fe-22Mn-0.6C TWIP Steel via In Situ Micropillar Compression Test

In this study,the effect of hydrogen on dislocation and twinning behavior along various grain boundaries in a high-manganese twinning-induced plasticity steel was investigated using an in situ micropillar compression test.The compressive stress in both elastic and plastic regimes was increased with the presence of hydrogen.Further investigation by transmission electron backscatter diffraction and scanning transmission electron microscope demonstrated that hydrogen promoted both dislocation multiplication and twin formation,which resulted in higher stress concentration at twin-twin and twin-grain boundary intersections.

表面喷丸及退火处理对Fe-30Mn-3Al-3Si TWIP钢组织与性能的影响

为实现强韧化,对Fe-30Mn-3Al-3Si TWIP钢进行了表面喷丸及退火热处理。采用金相显微镜、X射线衍射仪和透射电子显微镜等手段分析了喷丸以及喷丸结合退火处理对TWIP钢显微组织结构的影响,并利用显微维氏硬度计和万能材料试验机对原始样品、喷丸以及喷丸结合退火处理样品的硬度分布、力学性能及加工硬化行为进行了研究。分析结果表明,喷丸可在TWIP钢表面引起强烈的塑性变形,并形成包含60~100 nm大小的位错胞块的强烈形变层和包含微纳尺度形变孪晶结构的过渡层。该纳米-微米尺度的晶粒尺寸梯度结构可以将TWIP钢表面强烈形变层的硬度显著提升至366 HV0.1,并在残余压应力联合作用下使试样的屈服强度提升至485 MPa,均远高于初始样品的对应值。但受喷丸引起的表面粗糙和强烈形变层纳米晶塑性低的影响,其断后延伸率仅为41.8%。退火处理后,喷丸样品中的形变组织发生回复与再结晶,其中强烈形变层位置的再结晶晶粒尺寸为1~3μm,硬度也降低至215 HV0.1。所形成的微米-微米尺度晶粒尺寸梯度结构试样的屈服强度降低至261 MPa,但延伸率增加至65.3%。晶粒尺寸梯度结构有助于增加TWIP钢屈服与抗拉强度,但不利于延伸率的提升。

TWIP钢表面Ni-P化学镀层组织结构及性能研究

目的在孪生诱发塑性钢(TWIP)表面制备Ni-P镀层,提高TWIP钢的耐腐蚀性能。方法通过化学镀工艺在TWIP钢表面制备了高磷Ni-P镀层,并进行不同温度的热处理,利用扫描电镜及能谱仪、X射线衍射仪及原子力显微镜等探究了热处理温度和时间对Ni-P镀层形貌和组织结构的影响。通过电化学方法研究了TWIP钢表面镀层的耐蚀性能。结果随着热处理温度的升高,镍磷原子发生迁移,胞状组织边界模糊,粗糙度降低,其中600℃热处理镀层表面最为致密平整。随着热处理温度的升高,镀层组织结构演变过程如下:非晶态(普通镀层)→非晶态部分晶化,Ni12P5、Ni5P2等亚稳态镍磷化合物析出(300℃)→非晶态完全晶化、稳态Ni3P相长大(400℃)→晶粒长大(500℃、600℃)。Ni-P镀层能够明显提升TWIP钢的耐腐蚀性能。随着热处理温度的升高,镀层的耐蚀性能先降低后升高。600℃热处理1h镀层的腐蚀电流密度为0.25μA/cm^(2),与普通Ni-P镀层相比降低了80.9%,与TWIP钢基体相比降低了99.6%。600℃热处理镀层光滑致密无缺陷的表面促进了保护性氧化膜的产生,使镀层的耐蚀性能提高。结论合适的热处理工艺提高了Ni-P镀层的致密性和保护能力,光滑致密无缺陷的镀层能够为TWIP钢提供良好的防护。

Mechanical Behavior and Microstructure Evolution during Tensile Deformation of Twinning Induced Plasticity Steel Processed by Warm Forgings

The mechanical behavior and microstructural evolution of an Fe-30Mn-3Al-3Si twinninginduced plasticity(TWIP)steel processed using warm forging was investigated.It is found that steel processed via warm forging improves comprehensive mechanical properties compared to the TWIP steel processed via cold rolling,with a high tensile strength(R_(m))of 793 MPa,a yield strength(R_(P))of 682 MPa,an extremely large R_(P)/R_(m)ratio as high as 0.86 as well as an excellent elongation rate of 46.8%.The microstructure observation demonstrates that steel processed by warm forging consists of large and elongated grains together with fine,equiaxed grains.Complicated micro-defect configurations were also observed within the steel,including dense dislocation networks and a few coarse deformation twins.As the plastic deformation proceeds,the densities of dislocations and deformation twins significantly increase.Moreover,a great number of slip lines could be observed in the elongated grains.These findings reveal that a much more dramatic interaction between microstructural defect and dislocations glide takes place in the forging sample,wherein the fine and equiaxed grains propagated dislocations more rapidly,together with initial defect configurations,are responsible for enhanced strength properties.Meanwhile,larger,elongated grains with more prevalently activated deformation twins result in high plasticity.

Fe-Mn-C-Al系TWIP钢拉伸过程中的加工硬化行为

对退火后Fe-Mn-C-Al系孪生诱发塑性(TWIP)钢进行不同变形量(1.5%,4.0%,39.0%,47.0%,52.0%,62.0%)的室温单向拉伸试验,研究了该钢在拉伸过程中的加工硬化行为及显微组织演变。结果表明:随着变形量增加,TWIP钢奥氏体晶粒内部开始出现形变孪晶,且密度不断提高,变形机制由位错滑移变为孪生和位错共同作用,拉伸断口呈典型韧性断裂特征;试验钢弹性变形阶段短暂,无明显的屈服平台和物理屈服点,塑性变形阶段存在长加工硬化过程;试验钢的加工硬化率先随真应变增加而急剧下降,此时加工硬化机制以位错强化为主,随着真应变继续增加,加工硬化率增加,在应变为0.55时达到峰值,后趋于平缓,加工硬化机制主要为孪晶强化。

TWIP钢低温塑性变形机理的原位电镜研究

本文采用透射电镜中的原位低温拉伸测试技术,进行了-50℃、-120℃及-180℃下孪晶诱导塑性变形(TWIP)钢均匀塑性变形阶段的变形缺陷行为分析,研究了温度变化对于TWIP钢中的位错滑移与孪生行为的影响。研究发现,TWIP钢在-50℃下激活了常规滑移之外的其它滑移系;随着温度的进一步降低,TWIP钢中位错活性未发生明显下降,特别是需要热激活辅助的位错交滑移在-180℃下仍可发生。同时研究结果说明,低温下TWIP钢强塑性的保持由多种变形机制共同协调作用:形变孪生、孪晶与位错相互作用、位错交滑移及其引起的位错相互作用,本质是其孪生行为与位错滑移的活性同时得到了保持。

激光能量密度对TWIP钢表面氧化物清洗效果及机理的影响

针对孪晶诱发塑性(TWIP)钢板表面氧化层难去除的问题,采用近年来兴起的激光清洗方法,通过观察和测量不同能量密度激光清洗TWIP钢表面的形貌以及氧元素含量,研究了激光清洗技术对TWIP钢表面氧化层的作用效果及机理。结果表明:当能量密度高于12.7 J/cm^(2)时,氧化层开始发生热膨胀而从表面剥离,因此确定此值为氧化层的清洗阈值;当能量密度高于38.2 J/cm^(2)时,材料蒸发引起的热烧蚀起主导作用;当能量密度高于50.9 J/cm^(2)时,增大激光能量会造成基体的氧化,因此确定此值为氧化层的损伤阈值。本研究不仅可为激光清洗在TWIP钢表面氧化层的去除方面提供理论指导,同时也可为TWIP钢的高效、清洁化生产加工提供有效方法。

Grain Size Effect on the Martensite Formation in a High-Manganese TWIP Steel by the Rietveld Method

The aim of the present work was to study the effect of austenite grain size （AGS） on the martensite formation in a high-manganese twinning-induced plasticity （TWIP） steel. The results of a quantitative microstructural characterization of the steel by the whole X-ray pattern fitting Rietveld software, materials analysis using diffraction （MAUD）, indicated that the volume fraction of αbcc-martensite increases with increasing AGS. However, the value of the stacking fault probability （Psf） does not show a large variation for samples with different values of AGS under water-quenching conditions.

Microstructure and Mechanical Properties of Hot-Rolled Fe-Mn-C-Si TWIP Steel

Mechanical properties and microstructural evolution of the hot-rolled Fe-Mn-C-Si TWIP steel were investigated and the deformation mechanism was analyzed.The results showed that the tensile strength and elongation were about 1050 MPa and 60%,respectively.The hot-rolled steel had high specific energy absorption and impact toughness between-120 ℃ and 20 ℃.Some inhomogeneous dislocation zones were observed in the undeformed steel.Lots of deformation twins and twin-dislocation interactions were observed in the deformed steel.TWIP effect was the major deformation mechanism for the excellent mechanical properties.

Microstructure and Mechanical Properties of TWIP Steel Joints

As a new type of high manganese steel, the twinning induced plasticity （TWIP） steels have attracted a growing interest in the automotive industry due to their good performance. Thin plates of TWIP steel were welded by laser beam welding （LBW） and gas tungsten arc welding （GTAW）. The microstructure result shows that GTAW joint has obvious heat-affected zone （HAZ）, while the HAZ of LBW joint is almost invisible. The X-ray diffraction result shows that the phase compositions of both joints are austenitic and no phase transition occurs. Energy disper- sive spectrometry result shows that there is violent evaporation of Mn element in LBW joint, while the proportion of Mn element in GTAW joint is almost unchanged. Tensile tests and micro-hardness measurements were performed to take into account the mechanical properties of joints manufactured by the two different processes. The micro-hard- ness profiles of both joints present a typical saddle distribution, and the hardness of GTAW seam is lower than that of LBW seam. The failure positions of LBW joints are all located in base metal while the GTAW joints are all at the weld toe due to the softening of HAZ. By means of scanning electron microscopy, a typical ductile fracture is observed in LBW joint, while a brittle fracture with quasi-cleavage fracture characteristic is observed in GTAW joint.

Effects of Twin-Dislocation and Twin-Twin Interactions on the Strain Hardening Behavior of TWIP Steels

Tensile tests of Fe-30Mn-5Si-2Al steel were carried out for different strains of 0.05, 0.14, 0.26, and up to the strain-to-failure in order to observe the evolution of microstructure during deformation and investigating the strain hardening behavior. Three-stage strain hardening behavior was observed in this steel during tensile test. In stage I , planar dislocation structure was observed by TEM and regarded as main deformation mechanism, and low strain hardening rate exponent was exhibited. Primary deformation twinning occurred in stage II , and the strain hardening rate exponent increased due to the blockage of dislocations＇ motion by twin boundaries. In stage III , the strain hardening rate exponent had increased to a value higher than 0.5. The obstacle effect of twin boundaries and twin-twin interact＇ion had been observed by TEM, and the interactions between primary and secondary twins were found to cause the additional hardening in addition to the obstacle effect on dislocations＇ motion, which led to the twinning induced plasticity effect in the later stage of deformation.

Effect of Temperature on Microstructure and Deformation Mechanism of Fe-30Mn-3Si-4Al TWIP Steel at Strain Rate of 700 s^(-1)

As twinning-induced plasticity （TWIP） steel is one potential material for shaped charge liner due to the combination of high strength and high plasticity, deformation mechanism at high strain rate and high temperature is required to study. Compression experiments of Fe 30Mn-3Si-4Al TWIP steel were conducted using a Gleeble 1500 thermal simulation machine and a split-Hopkinson pressure bar （SHPB） between 298 and 1073 K at strain rates of 10 3 and 700 s-1 , respectively. Microstructures were observed using optical microscopy （OM） and transmission e- lectron microscopy （TEM）. Results show that flow stress and densities of deformation twins and dislocations de crease with increasing deformation temperature at strain rates of 10 3 and 700 s -1. The stack fault energy （SFE） values （Г） of Fe-30Mn-3Si-4Al TWIP steel at different temperatures were calculated using thermodynamic data. Based on corresponding microstructures, it can he inferred that at 700 s -1 , twinning is the main deformation mecha- nism at 298-573 K for 30 mJ/m2≤Г≤63 mJ/m2 , while dislocation gliding is the main deformation mechanism above 1 073 K for Г≥145 mJ/m2. In addition, with increasing strain rate from 10-3 to 700 s -1 , the SFE range of twinning is enlarged and the SEF value of twinning becomes higher.