Fatigue properties of transformation-induced plasticity and dual-phase steels for auto-body lightweight: Experiment,modeling and application

The substitution of conventional high strength steels (HSS) with advanced high strength steels (AHSS), e.g., low-alloy multiphase transformation-induced plasticity steel (TRIP steel) or dual-phase steel (DP steel), for body lightweight brings about increased stress of notched components. Thus the fatigue properties of TRIP and DP steels and the fatigue life of notched lightweight design are important considerations for reasonable material selection during the design stage of auto-body. For the mentioned issue, cyclic strain-controlled fatigue properties of TRIP and DP steels with equivalent grade and lightweight result were investigated experimentally. Different cyclic behaviors of TRIP and DP steels were observed due to different interior microstructures. The cyclic stress behavior of TRIP steel is characterized by cyclic hardening followed by stable at lower strain amplitudes, and softening at higher strain amplitudes; however cyclic softening followed by stable occurs consistently for DP steel throughout entire strain amplitude range of test. TRIP steel possesses enhanced fatigue life and cyclic stress at the same strain amplitude than DP steel. Furthermore, local strain-life models of two steels were developed by linear regression of experimental data, to predict and compare the fatigue life of notched body structures made of them by finite element method. The simulation result illustrates that TRIP steel can provide more beneficial potential than DP steel for the lightweight design of notched body structures from the viewpoint of fatigue resistance.     

On Optimizing Batch Annealing for the Production of IF Steels

Batch annealing characterized by a long heating cycle and low heating temperature is widely used to produce sheet steels including interstitial-free (IF) steels. Optimizing batch-annealing conditions would be helpful in cutting lead time and saving energy while keeping the necessary deep drawability; however, little research has been done on it. The effects of batch-annealing parameters on the deep drawability of a Ti-stabilized IF steel and Nb+Ti-stabilized IF steel have been investigated. In general, a slight improvement in r_mvalue was found with the increase of annealing temperature. Because of the higher recrystallization temperatures of Nb+Ti-IF steel, it showed higher sensitivity to annealing temperature than Ti-IF steel. In addition, greater dependency of mechanical properties and deep drawability on soaking time was observed when annealing temperature was lower. Moreover, a heat transfer analysis showed that the annealing time could be reduced by 40% if the temperature decreases from 730°C to 650°C. The industrial production has verified that the analytical results are in good agreement with the recorded annealing temperature curve. Meanwhile, it has been shown that Ti-IF steels annealed at 650°C have very close deep drawability and other tensile properties compared with these annealed at 730°C.     

A Study on the Wear Behaviour of Dual Phase Steels

Dual phase （DP） steels containing four different amounts of martensite ranging from 43 vol. pct to 81 vol. pct have been developed from 0.2 wt pct carbon steel by intercritical heat treatment at a fixed temperature of 780℃ with varying holding times followed by water quenching. Dry sliding wear tests have been conducted on DP steels using a pin- on-disk machine under different normal loads of 61.3, 68.5, 75.7 and 82.6 N and at a constant sliding speed of 1.20 m/s. At these loads, the mechanism of wear is primarily delamination, which has been confirmed by SEM micrographs of subsurface and wear debris of samples. Wear properties have been found to improve with the increase in martensite volume fraction in dual phase steels.     

TEXTURE DEVELOPMENT IN EXTRA LOW CARBON (ELC) AND INTERSTITIAL FREE (IF) STEELS DURING WARM ROLLING

The warm rolling and subsequent annealing textures of an extra low carbon (ELC) and two interstitial free (IF) steels have been investigated using both single pass and multipass rolling in the ferritic temperature range. A total amount of 80% deformation by rolling was given in each case. It was observed that texture development in ELC and IF steels during warm rolling displays striking differences. In general, ELC steels acquire only a weak texture after finish rolling in the upper ferritic range, for both single and multipass rolling. Single pass warm rolling in the lower ferritic range, however, produces very sharp γ fiber in this alloy, as an exception. Recrystallization anneal after warm rolling, in general, has been found to decrease the overall texture. By contrast, much sharper overall texture (and γ fiber) develops in the IF steels during warm rolling, and it generally sharpens after subsequent recrystallization anneal. This is true for both single pass and multipass rolling. The best texture results (highest intensities of γ fibers) are invariably obtained when most of the rolling deformation is given in the ferritic regime. Thus, between the two grades, the IF steels are definitely far better than the ELC steels for the purpose of developing sharp γ fibers by warm rolling process. Out of the two IF steels, the Ti-containing steel appears to develop a perceptibly sharper γ fiber after warm rolling and annealing as compared to the Nb-containing one.     

单相组织TWIP钢的扩孔性研究

超高强钢的扩孔性能是冲压成形的重要性质.为评价980 MPa TWIP钢的扩孔性能,本文以单相铁素体IF钢和980 MPa双相钢作为参考材料,用扫描电镜观察了3个钢种的微观组织,并对3个钢种进行了拉伸实验和扩孔实验,采用背散射电子衍射(EBSD)技术分析了拉伸后和扩孔实验后TWIP钢的微观组织.实验结果表明:拉伸前TWIP钢呈现类似于IF钢均匀的单相奥氏体组织,而拉伸后TWIP钢呈现类似于DP钢不均匀的硬质变形孪晶奥氏体和软质奥氏体;扩孔后TWIP钢的开裂位置集中在奥氏体和变形孪晶奥氏体界面;虽然TWIP钢显现出更大的均匀伸长率和加工硬化,但扩孔率明显小于IF钢.TWIP钢扩孔率增加源于早期孪晶诱发塑性(TWIP效应)导致的均匀变形.同时,这种变形机制导致组织中的硬质变形孪晶奥氏体,硬质变形孪晶奥氏体与软质奥氏体匹配(类似于双相钢中马氏体铁素体匹配)将恶化局部变形,阻碍扩孔性能进一步提升.     

马氏体对C-Si-Mn冷轧双相钢屈服特性的影响

通过实验室退火与回火,采用力学测定与显微组织分析研究了马氏体体积分数与回火温度对双相钢屈服特性的影响.结果表明:随马氏体体积分数从0%增加到约15%,双相钢的屈服强度显著降低,降低值约为80～100MPa;而马氏体体积分数在15%～35%之间,双相钢的屈服强度维持在一个较低的水平.当马氏体体积分数大于35%,双相钢屈服...     

Springback prediction in sheet metal forming of high strength steels

Both increased weight reduction and improved passive safety have been simultaneously required for components of new vehicle generation. Thus, advanced high strength Dual Phase (DP) steels have been progressively used when making automotive parts. During each sheet metal forming process the high strength steels exhibit distinct springback effect, which is governed by strain recovery of material after load removal. The springback is variably sensitive to materials and process parameters. Considering springback occurred in a formed part is significant for designing tools and dies. In this work, both experiments and Finite Element Analyses (FEA) of a U-shape forming test were performed and compared for investigating the springback effect. Two DP steels (JSC590R and JSC780Y) with different strengths and a mild steel (JSC270C) were taken into account. The planar anisotropic material model according to Hill’s 1948, Barlat’s yield 2000, and Yoshida–Uemori kinematic hardening model were applied in the simulations. Various mechanical testing as hydraulic bulge test, disk compression test, and in particular cyclic test under tension and compression load were carried out in order to determine required materials parameters of the models. Obviously, steel with higher yield and tensile strength definitely showed an increasing in magnitude of both springback and curling. All presented material models restricted ability to predict springback effect of the examined steels, although the Yoshida–Uemori criterion provided more accurate results than other ones. The model is therefore preferred for describing the strain recovery mechanism of high strength steels, while parameter determination plays a decisive role. The cyclic test was verified to successfully describe the kinematic behaviour of material.     

450~780 MPa系列双相钢扩孔性能实验

目的避免双相钢在成形过程中经常出现的翻边或扩孔开裂的问题,提高生产效率。方法以几种强度级别为450~780 MPa的双相钢为实验对象,测试材料的单向拉伸和扩孔性能,并从微观组织上分析影响双相钢强度和扩孔性能的因素。结果 450~780 MPa强度级别双相钢的屈服强度随铁素体晶粒尺寸的减小而增加,抗拉强度随马氏体体积分数的增加呈近似线性增加。双相钢的扩孔性能随着材料强度的提高呈下降的趋势。DP500与DP450和DP600相比,抗拉强度(579 MPa)居中,伸长率最高,但由于其马氏体形态和分布的差异,其扩孔性能反而最低。结论双相钢的扩孔性能会受到马氏体的含量、尺寸、形态和分布的影响,与材料强度和伸长率没有必然的关系,当马氏体呈颗粒状均匀分布时,具有更好的扩孔性能。     

连续镀锌DP600双相钢的组织与性能研究

制备了连续镀锌DP600双相钢,利用光学显微镜、SEM、EBSD技术对其显微组织进行了观察和分析;利用X射线衍射和EBSD技术分别对钢板的宏观织构和微观织构进行了测定;并对其力学性能、n值和r值进行了检测.实验结果表明:试样组织为铁素体+马氏体岛的双相组织,该钢板具有良好的综合力学性能和成形性,达到了DP600级别双相钢的性能要求.     

ANN based prediction model for fatigue crack growth in DP steel

An artificial neural network (ANN)-based model was developed to analyse high-cycle fatigue crack growth rates (d a /d N ) as a function of stress intensity ranges (Δ K ) for dual phase (DP) steel. The training data consisted of d a /d N at Δ K ranges between 5 and 16 MPa √ for DP steel with martensite contents in the range 32 to 76%. The ANN back-propagation model with Gaussian activation function exhibited excellent agreement with the experimental results. The fatigue crack growth rate predictions were made to demonstrate its practical significance in a given real-life situation. Because of the wide range of data points used during training of the model, it will provide a useful predictor for fatigue crack growth in DP steels.     

IF钢硬化模型试验

目的研究适用于IF钢材料的硬化模型。方法通过对3种不同牌号的IF钢材料进行拉伸试验,基于AUTOFORM软件中4种常用的硬化模型分别对试验数据进行拟合,分析不同硬化模型对试验数据曲线整体拟合效果,并基于曲线重合性均方差最小原则,对比了不同硬化模型的拟合误差。结果 Approximation模型综合了Hockett-Sherby饱和模型和Swift非饱和模型,对应力应变数据拟合效果最好,误差分析显示Ludwick模型拟合误差最大,Swift,Hockett-sherby和Approximation模型拟合误差大致相当,Approximation模型拟合误差最小。结论针对IF钢材料,推荐采用Approximation模型拟合硬化曲线,对提升冲压和回弹仿真精度,具有一定的指导作用。     

Characterisation of formability behaviour of multiphase steels by micromechanical modelling

Multiphase steels are new Advanced High Strength Steels (AHSS), which have been developed for the automobile industry for the purpose of reducing of car body weight. These steels offer an excellent combination of high strength and large elongation due to the coexistence of harder and softer phases in their microstructure. The advantageous properties of multiphase steels can be utilised by regulating the type, amount, formation and spatial distribution of the different constituent phases. To describe the influences of the heterogeneous microstructure on the mechanical properties and the complex fracture mechanisms, an approach is presented using Representative Volume Elements (RVEs) on a micro level. Three dimensional RVE simulations were conducted under considerations of metallographic analysis for a Dual Phase (DP) steel and a Transformation-Induced Plasticity (TRIP) steel. The Gurson–Tvergaard–Needleman (GTN) damage model was applied to investigate the local crack initiation in steel sheets during various forming processes. In conclusion, a failure prediction based on microstructure is proposed for the material characterisation in sheet metal forming of multiphase steels.     

Estimation of oxide scale growth and temperature increase of high (9–12%) chromium martensitic steels of superheater tubes

The improved ferritic steels that have very good creep strength such as grades 91 and 92 have been widely used for power plant applications. For example, a chromium–molybdenum–vanadium steel tube (SA213-T91) has been available in the market for two decades. The SA213-T91 material has better high temperature strength and oxidation resistance than those of the more widely used materials such as T11 and T22. In this paper, estimations of the oxide scale growth and temperature increase of 9–12% Cr martensitic steels of superheater tubes are carried out using an incremental procedure, utilizing the finite element simulations and the relation between the Larson Miller Parameter (LMP) and the scale thickness over a period of time. In order to show the superiority of T91 steels over T22 steels, two different steam temperatures are considered in the models for comparisons. The estimations showed as expected that the oxide scale growth and temperature increase of the 9–12% Cr steels were significantly lower compared to those of 2.25% Cr steels.     

Correlating microstructural features with wear resistance of dual phase steel

In order to explore the tribological potential of the dual phase (DP) steel as a wear resistant material, the wear characteristics of the dual phase (DP) steel have been investigated with varying amounts of martensite from 43 to 81 vol pct, developed by varying holding time at the intercritical annealing temperature of 780 °C. Dry sliding wear tests have been conducted on DP steels using a pin-on-disk machine under different normal loads of 61.3, 68.5, 75.7 and 82.6 N and at a constant sliding speed of 1.20 m/s. At these loads, the mechanism of wear is mainly delamination, which has been confirmed by SEM micrographs of the subsurface and wear debris of the samples. Wear and friction properties have been found to be improved with increasing martensite volume fraction in dual phase steels.     

Microstructural influence on the electrochemical corrosion behaviour of dual-phase steels in 3.5% NaCl solution

Galvanostatic corrosion behavior of five different dual-phase (DP) steels with varying morphologies and martensite content has been assessed in comparison to a ferrite–pearlite steel in 3.5% NaCl solution. It has been observed that both the amount of martensite and the morphology of the phase constituents have definite influence on the corrosion behaviour of DP steel. Higher corrosion tendency has been observed with increased amount of martensite and increased refinement of phase constituents.     

Quantification of bake hardening effect in DP600 and TRIP700 steels

Recently developed advanced high-strength steels with multiphase microstructures show interesting bake-hardening (BH) properties. This research work aims to quantify the effect of BH on dual-phase (DP) and transformation induced plasticity (TRIP) steel. Different pre-strains from 0% to 10% with a subsequent BH annealing cycle with temperatures of 60–220 °C for varying BH holding times from 1 to 10,000 min were applied for both materials. Mechanical properties such as yield and tensile strengths, elongation and BH values in dependency of the BH parameters have been determined and related to specific microstructural features in order to characterize the age and strain hardening behavior.     

海洋耐蚀钢的国内外进展

随着陆地资源日趋枯竭,海洋资源的开发与利用被列为2l世纪的重点目标之一。耐海水腐蚀材料的研究和应用是海洋开发的基础和前提,其中海洋耐蚀钢研究应用成为热点。海洋耐蚀钢主要指应用于原油运输船、海洋平台、海底油气管线和大型跨海桥梁等领域的一类低合金高强度钢,在海洋开发和利用领域应用量大、涉及面广。结合国家海洋事业发展"十二五"规划,对海洋耐蚀钢的需求作了简要分析;在总结油船货油舱内部腐蚀环境及机理的基础上,综述了日本、韩国、乌克兰、俄罗斯和中国在油船货油舱用耐蚀钢方面的研究开发现状;比较了美国、日本、法国和中国研发的耐海水耐蚀钢的特点;指出了我国在油船货油舱用耐蚀钢及耐海水腐蚀钢方面存在的问题、差距和对策;提出了我国发展海洋耐蚀钢需要解决的关键技术与核心科学问题。     

热变形对DP600汽车用钢连续冷却转变曲线的影响

利用Gleeble-3800热模拟试验机测定了DP600汽车用钢静态、动态连续冷却转变(CCT)曲线。对两种不同工艺制度下的CCT曲线进行分析比较,结合显微组织分析,研究了热变形对该钢组织变化规律及性能的影响。变形后的CCT曲线上部向上移动,下部向下移动,变形扩大了铁素体区,缩小了贝氏体区,在铁素体和贝氏体转变区域中间存在较宽的奥氏体稳定区域,为制定其热轧生产工艺制度提供依据。     

Effect of copper on annealing characteristics of interstitial free steels

The effect of copper on the annealing behaviour of interstitial free (IF) steel has been investigated using thermoelectric power (TEP) and resistivity measurements. Kinetics of annealing of cold rolled copper-containing IF steel was found to be sluggish when compared with the base steels. TEP measurement revealed that copper has a negative coefficient of TEP in α-iron.     

超快速退火对Nb+Ti-IF钢组织和织构的影响

研究了超快速退火对一种冷轧变形量为94.2%的Nb+Ti-IF钢的微观组织和织构的影响。结果表明:超快速退火(升温速度为300℃/s)提高了钢的再结晶完成温度,整个再结晶退火可在短至0.41 s内完成。与普通退火(升温速度为20℃/s)后钢的再结晶组织相比,超快速退火处理后晶粒平均尺寸由12.98μm细化到10.12μm,晶粒长大速度由～3μm/s提高到～23μm/s,且再结晶晶粒内存在大量缠结位错。在极短时间内,超快速退火再结晶织构仍以均匀、锋锐的{111}//ND有利深冲性能的γ织构为主,且避免了其它α织构的生成,有利于使退火板具有良好的成形性能。