Temperature-jump tensile tests to induce optimized TRIP/TWIP effect in a metastable austenitic stainless steel

In the present work,plastic deformation mechanisms were initially tailored by adjusting the deformation temperature in the range of 0 to 200℃ in AISI 304L austenitic stainless steel,aiming to optimize the strength-ductility synergy.It was shown that the combined twinning-induced plasticity(TWIP)/transformation-induced plasticity(TRIP)effects and a wider strain range for the TRIP effect up to higher strains by adjusting the deformation temperature are good strategies to improve the strength-ductility synergy of this metastable stainless steel.In this regard,by consideration of the observed temperature-dependency of plastic deformation,the controlled sequence of TWIP and TRIP effects for archiving superior strength-ductility trade-off was intended by the pre-designed temperature jump tensile tests.Accordingly,the optimum tensile toughness of 846 MJ/m^(3) and total elongation to 133% were obtained by this strategy via exploiting the advantages of the TWIP effect at 100℃ and the TRIP effect at 25℃ at the later stages of the straining.Consequently,a deformation-temperature-transformation(DTT)diagram was developed for this metastable alloy.Moreover,based on work-hardening analysis,it was found that the main phenomenon constraining further improvement in the ductility and strengthening was the yielding of the deformation-induced α′-martensite.

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.

Some Aspects of High Manganese Twinning-Induced Plasticity (TWIP) Steel, A Review

High manganese twinning-induced plasticity （TWIP） steel is a new kind of structural material and possesses both high strength and superior plasticity and can meet the weight-lightening requirement for manufacturing vehicle body. The excellent formability of the TWIP steel comes from the extraordinary strain hardening effect during plastic deformation. The reduction of specific weight by aluminum alloying and strain hardening effect can lead to an effective weight reduction of the steel components, and provide a better choice for materials in vehicle body design. The TWIP effect in high Mn steels is generally associated with the successive work- hardening generated by twins and influenced by some factors, such as Mn content, AI addition revealed by stacking fault energy （SFE）, grain size, deformation temperature and strain rate. The present review introduces some aspects of the TWIP steels relating to their physical metallurgy, influencing factors associated with their deformation mechanisms, and a prospect for the future investigation is also described. Moreover, as a potential candidate for replacing Ni-Cr austenitic stainless steel, researches on the oxidation behavior and corrosion resistance of Fe-Mn-AI-C system steels are also reviewed.

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.

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.

Microstructure evolution of ultra-high-strength twinning-induced plasticity(TWIP)steel under dynamic loading

To prepare ultra-high-yield strength twinning-induced plasticity(TWIP)steel and reveal its work hardening mechanism at different strain rates from the microcosmic range,the microstructure evolution mechanism of Fe–20Mn–0.6C TWIP steel was investigated at strain rates of 10^(-4)–10^(3)s^(-1)using a high-speed tensile testing machine and a transmission electron microscope.The results show that the strain rate and deformation had a significant effect on the twin morphology of TWIP steels.At a strain rate of 10^(2)s^(-1),secondary deformation twins were developed,which intersected with the initial deformation twins and increased the resistance of dislocation movement,as well as the plasticity.TWIP steel at a strain rate of 10^(2)s^(-1)had a higher twin formation speed than that at 10^(0)s^(-1).At the same amount of deformation,the twin boundary fraction was higher and increased linearly at a strain rate of 10^(2)s^(-1),while the rule of twin growth at 10^(0)s^(-1)was conformed to S-curve change of DoseResp model.

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.

Investigation on the Formation Mechanism of Ordered Carbide (FeMn)_3AlC in the Al Added Twinning-Induced Plasticity Steels

The spinodal composition zone in Al added Fe-Mn-Al-C twinning-induced plasticity(TWIP) steels can be determined by contents of Al and C and aging temperature together, based on the thermodynamic analysis. Precipitation of ordered(FeMn)_3AlC carbide by the mechanism of spinodal decomposition occurs in the C-rich and Al-rich zone with low aging temperature. Increase of aging temperature shrinks spinodal composition zone to the high Al and C contents. As a result, the precipitation of(FeMn)_3AlC carbide alters from spinodal decomposition to classical nucleation-growth manner gradually. Further calculation indicates that the diffusion of Al can play a key role in determining the growth rate of(FeMn)_3AlC carbide at high aging temperature.

Effect of reduction in area per pass on strain distribution and microstructure during caliber rolling in twinning-induced plasticity steel

The effects of reduction in area(RA)per pass during caliber rolling on microstructure and strain distribution of twinninginduced plasticity steel have been investigated to find solutions to make a more homogeneous material along the radial direction for wire rod applications.The steel wires subjected to an average RA per pass of 2.5%(skin pass caliber rolling)and 10.0%(conventional caliber rolling)were analyzed.The skin pass caliber-rolled wire was characterized as a duplex fiber texture of major<111>and minor<100>.and the textures were almost same at/between center and surface area,which is totally different from those of conventional caliber rolling and wire drawing.The skin pass caliber rolling led to more homogeneous microstructure and mechanical properties along the radial direction in comparison with the conventional caliber rolling and wire drawing due to the more homogeneous Hall-Petch hardening,dislocation hardening,and texture behavior with area,resulting in higher formability.

高强度高塑性TWIP钢的开发研究

通过调整成分 ,研究了一种新型的高锰钢成分对组织结构和 TWIP效应以及对强度、塑性的影响。结果表明 ,该钢在变形后基体中存在大量细小的形变孪晶 ,室温下可具有相变诱导塑性和孪晶诱导塑性的 TWIP效应 ,因而具有高的强度 (10 0 0 MPa以上 )和极高的伸长率 (6 0 %～ 90 % )。该钢种是很有前途的高强度、高塑性钢种 。

汽车用TWIP钢的力学性能与微观组织

采用热轧、冷轧及退火处理等工艺,对成分为25Mn-3Si-3Al的TWIP钢进行了试制,研究了钢板的力学性能、微观组织及其断裂机制,并采用X射线测定了钢板的晶体学织构.实验结果表明钢板拉伸时发生典型的延性断裂;拉伸前的组织为伴有大量退火孪晶的奥氏体;在拉伸过程中退火孪晶转变成形变孪晶,使产品的强度和塑性提高;退火过程中形成的织构组分有利于塑性变形.

Effect of drawing speed on microstructure distribution and drawability in twinning-induced plasticity steel during wire drawing

The effect of drawing speed on temperature rise and microstructure distribution in twinning-induced plasticity(TWIP)steel during wire drawing has been investigated to improve drawability for wire rod applications.Although wire drawing process is performed at room temperature,heat is generated due to the plastic deformation and friction at the wire-die interface.The steel wires subjected to the low drawing speed(LD)of 0.5 m/min and the high drawing speed(HD)of 5.0 m/min were analyzed using the numerical simulation and electron backscatter diffraction techniques.Interestingly,the specimens subjected to the HD had a higher drawability by about 18%compared to the LD,which is totally different from the general behavior of plain carbon pearlitic steels.The LD wire had uniform temperature distribution along the radial direction during wire drawing.In contrast,the HD wire had a temperature gradient along the radial direction due to the higher frictional effect at surface:the minimum temperature of 58℃ at center area and the maximum temperature of 143 C at surface area.The higher stacking fault energy of HD wire at the surface area due to the high temperature rise retarded twinning rate,resulting in the prevention of fast exhaustion in ductility in comparison with the LD wires since the earlier depletion of twins at surface area is known as the main reason for the fracture of TWIP steel during wire drawing.Consequently,HD process delayed the fracture strain of wire and increased the uniformity of microstructure and mechanical properties along the radial direction.

热处理工艺对TWIP钢组织性能的影响

对3种不同成分的TWIP钢(Fe-25Mn-4Si-2Al,Fe-30Mn-4Si-2Al,Fe-30Mn-3Si-3Al)进行热轧后的热处理实验,结果表明,增加固溶处理的保温时间,可提高TWIP钢的延伸率,而强度的变化并不明显.从晶界的角度分析了性能变化的原因:增加固溶处理的保温时间,有利于生成退火孪晶,不但增加退火孪晶界的面积,也有利于增加其他特殊晶界(CSL晶界)的数量.CSL晶界对材料的抗晶间腐蚀、断裂和蠕变强度的提高均有有益的作用,因此,通过控制TWIP钢的热处理工艺来提高原始组织中特殊晶界的含量,可以提高TWIP钢的强韧性.

汽车用TWIP钢的探索研究

采用金相组织观察与X射线衍射等方法对5种不同Mn含量的TWIP钢拉伸前后的组织进行了研究。结果表明,淬火态Fe-15Mn-3Si-3Al钢存在条状分布的马氏体;Fe-25Mn-3Si-3Al、Fe-30Mn-3Si-3Al、Fe-33Mn-3Si-3Al钢在淬火与退火两种热处理工艺下均能获得淬火态奥氏体和退火孪晶组织,淬火态的奥氏体晶粒更细小;母相奥氏体影响形变孪晶形貌与分布,均匀分布的细小孪晶导致优异的力学性能,随着Mn含量的增高,母相奥氏体尺寸逐步增加,形变孪晶层片逐渐增厚且分布不均匀;Fe-25Mn-3Si-3Al钢具有优异的力学性能。

Cr含量对高锰奥氏体TWIP钢高温氧化行为的影响

通过恒温氧化增重实验和扫描电镜观察及XRD分析,研究了Cr含量对高锰奥氏体孪晶诱发塑性(TWIP)钢在700℃氧化8 h的高温氧化动力学过程及氧化产物的影响.结果表明:随着Cr含量的提高,高锰奥氏体TWIP钢的抗氧化能力增强;当Cr的质量分数为1.13%时,经过在700℃下8 h的高温氧化,样品单位面积的氧化增重为30μg/mm2;而当Cr的质量分数增加到3.95%时,单位面积的氧化增重降低至3μg/mm2.随着Cr含量的增加,高锰奥氏体TWIP钢氧化层的致密度提高,氧化产物的构成也具有明显的差异.

合金元素和固溶温度对TRIP/TWIP钢组织性能的影响

对不含Nb和含Nb的两种高锰TRIP/TWIP钢进行了固溶处理,通过室温拉伸实验研究了两种实验钢的力学性能.利用光学显微镜、透射电子显微镜和X射线衍射观察实验钢变形前后的微观组织,并分析了含Nb实验钢中Nb析出物的形态和分布.实验结果表明:随着固溶温度的升高,两种成分的实验钢抗拉强度均下降.固溶温度为900℃时,含Nb实验钢组织仍有少量的热轧带状组织;固溶温度为1 000℃时,与此时的不含Nb实验钢相比,含Nb钢基体内的晶粒细小;在拉伸实验中,两种实验钢均表现出TRIP/TWIP的特性,不含Nb实验钢的TRIP形变强化机制更为突出.

TWIP钢激光和TIG焊接接头的组织和性能

采用激光焊和TIG焊分别对TWIP钢进行焊接,通过显微组织观察、拉伸试验及显微硬度测试对比研究了两种焊接接头的组织和性能。结果表明:激光焊的焊缝中心为细小的等轴晶,靠近熔合线的组织多为柱状晶,几乎看不到热影响区;TIG焊的焊缝组织为取向基本一致的柱状晶且晶粒较大,热影响区为粗大的奥氏体组织;激光焊接头拉伸试样在远离焊缝的母材处断裂,抗拉强度为560 MPa,伸长率为67%,断口为典型的韧性断裂;TIG焊接头断裂在热影响区,抗拉强度为460 MPa,伸长率为48%,断口表现为脆性断裂,塑性变形能力明显低于激光焊接头的;激光焊接头的平均显微硬度高于TIG焊接接头的。

温度及应变速率对TWIP钢拉伸性能的影响

通过拉伸试验研究了温度和应变速率对孪生诱发塑性(TWIP)钢拉伸性能的影响。结果表明:随着温度升高,TWIP钢的强度逐渐下降,断后伸长率逐渐增加,应变硬化指数随真应变增加达到并维持在较高的水平,试验钢的均匀变形能力得到提高,宏观塑性增加;随应变速率增大,试验钢的流变应力升高,断后伸长率下降,抗拉强度基本保持不变,应变硬化率曲线上的平台区长度明显变短,这表明孪生受到抑制,较早达到硬化极值;应变硬化指数峰值随应变速率的增大而减小,TWIP钢的均匀变形能力及宏观塑性下降。

TWIP钢的孪晶及其对Hall-Petch关系的影响

采用背散射电子衍射、透射电子显微镜和拉伸实验等研究了退火温度对冷轧态Fe-25Mn-3Al-3Si TWIP钢微观组织及力学性能的影响,并分析了Hall-Petch关系.结果表明,完全再结晶组织由等轴晶和退火孪晶组成,再结晶晶粒平均尺寸随退火温度的升高单调增大,∑3晶界面积分数随退火温度升高而呈现波动增加,850℃退火1h后∑3晶界面积分数达到44%.拉伸过程中强度与晶粒大小都服从Hall-Petch关系,但孪晶界影响Hall-Petch关系斜率K(ε)的大小.TWIP钢K(ε)-ε关系不同于一般钢材常温下的K(ε)随ε增加单调上升,TWIP钢K(ε)随着ε的增加逐渐增大,然后出现平台,最后下降.

不同热处理工艺对Fe-30Mn-3Si-4Al TWIP钢力学组织性能的影响

采用拉伸性能测试、金相观察、SEM和EDS等方法研究了不同热处理工艺对Fe-30Mn-3Si-4AlTWIP钢微观组织、拉伸力学性能及断口形貌的影响,并采用X射线衍射仪测定材料的物相组成。结果表明,冷却速度越快,TWIP钢的延伸率和强度越高;热处理后其室温组织为含有退火孪晶的单一奥氏体,冷却速度越小,奥氏体晶粒和退火孪晶的尺寸越大。拉伸时发生典型的延性断裂,在拉伸过程中退火孪晶转变成形变孪晶,使材料的塑性提高。