F45MnVS非调质钢动态再结晶模型与晶粒尺寸数值模拟

在变形温度950～1050℃、应变速率0.01～5 s-1下对F45MnVS非调质钢进行不同变形量(5％～56％)的单道次压缩试验,研究了变形温度、应变速率和变形量对该钢变形行为和晶粒尺寸的影响;基于试验数据建立动态再结晶临界应变模型和平均晶粒尺寸模型,嵌入Deform软件中模拟了试验钢的动态再结晶平均晶粒尺寸.结果表明:随着变形量或应变速率的增大,或者变形温度的降低,试验钢的平均晶粒尺寸减小;较高应变速率下加工软化导致的应力下降不明显,动态再结晶程度较小,较低应变速率下则相反;模拟得到的再结晶平均晶粒尺寸与试验结果较吻合,且平均晶粒尺寸随变形温度、应变速率和变形量的变化规律与试验结果相符.     

高温变形对含Ti无间隙原子钢铁素体晶粒直径的影响

在形变温度1233K，形变速率3．3×10-3s-1的试验条件下，采用拉伸变形试验方法研究了含Ti无间隙原子钢（IF钢）高温变形对铁素体晶粒尺寸的影响。讨论了高温变形奥氏体组织状态（动态回复和动态再结晶）、等温保持时间和冷却速度对铁素体晶粒大小的影响。     

组织特征对980MPa级先进超高强钢成形性能和拉伸行为的影响

以CP980钢、DP980钢、QP980钢为研究对象，采用单轴拉伸试验并结合数字图像相关技术，研究了组织特征对980 MPa级先进超高强钢单轴拉伸行为以及全局、局部成形性的影响。结果表明：QP980钢由马氏体、铁素体、残余奥氏体组织组成，在均匀变形阶段奥氏体相变产生的相变诱导塑性效应使该钢具有最优的全局成形性，但是新生马氏体相与其他相的硬度差较大，导致其局部成形性最差并形成准解理断裂；DP980钢由铁素体和马氏体组织组成，其强化机制以马氏体硬相强化和铁素体位错强化为主，全局成形性居中，同时因铁素体和马氏体之间具有一定的协调变形能力，其局部成形性较好，断裂形式主要为韧性断裂；CP980钢为铁素体、贝氏体、马氏体的多相组织，各相硬度差小，协调变形能力较强，局部成形性最好，断裂形式为韧性断裂。     

车用双相高强钢的动态力学性能及本构模型的对比

通过不同应变速率(0.001～500 s^-1)下的室温拉伸试验,研究了车用HC340/590DP、HC700/980DP双相高强钢的动态力学性能;分别采用Johnson-Cook模型、Swift-Hockett/Sherby模型,以及将Swift-Hockett/Sherby模型引入到Ludwik模型中的修正模型对2种钢的流动应力-应变曲线进行拟合,对比分析3种本构模型的拟合结果。结果表明：随着应变速率的增加,2种钢均表现出增强增塑现象;Johnson-Cook模型、Swift-Hockett/Sherby模型和修正Ludwik模型的拟合度平均值分别为0.950,0.999,0.997;修正Ludwik模型既具有各应变速率间应力耦合的特点,又保持了高拟合精度,可以准确描述车用双相高强钢的动态流变行为。     

汽车用高强度钢板的动态变形行为

对DP590钢板和CR340LA钢板在应变速率为0.003s-1(准静态)和20~700s-1(动态)下进行了室温拉伸试验,研究了试验钢板的动态拉伸变形行为、应变速率敏感性和动态断裂行为。结果表明:两种试验钢板的动态真应力-真应变曲线均无屈服平台,屈服后真应力随真应变的增加先快速增大后缓慢增大;应变速率对屈服强度的影响略高于对抗拉强度的影响,并且DP590钢板的应变速率敏感性和硬化指数均高于CR340LA钢板的;两种试验钢板的均匀伸长率均随应变速率的增加而降低;随应变速率的增加,DP590钢板中的位错密度增加,当应变速率不小于200s-1时出现位错胞;DP590钢板在准静态拉伸时发生明显颈缩,而动态拉伸时未发生颈缩,且随应变速率的增加,拉伸断口上的C形韧窝数量减少,等轴状韧窝数量增加。     

淬火加热速率对980MPa冷轧双相钢显微组织和力学性能的影响

用扫描电镜、透射电镜、拉伸试验机等研究了淬火加热速率对980 MPa双相钢显微组织和力学性能的影响。结果表明：在5～150℃·s-1加热速率范围内加热后淬火时，随加热速率的增大，铁素体晶粒有细化的趋势，马氏体尺寸更加均匀，抗拉强度增加到1 300MPa后基本保持不变，其屈服强度和伸长率先增加后减小，当加热速率为100℃...     

高铁螺纹道钉用TD16钢的动态再结晶行为

在变形温度为850~1 150℃、应变速率为0.01~10s-1下,利用Gleeble-3500型热模拟试验机对高铁螺纹道钉用TD16钢进行了高温压缩试验,研究了其动态再结晶行为,建立了动态再结晶模型并进行了试验验证。结果表明:TD16钢在试验条件下变形过程中存在动态再结晶行为,温度越低、应变速率越大,动态再结晶越不容易发生,动态再结晶晶粒越细;由动态再结晶模型预测得到的峰值应变/临界应变,ε0.5,晶粒尺寸与试验值的最大相对误差分别为7.08%,11.06%,21.3%,均吻合较好,建立的动态再结晶模型可用于预测TD16钢的动态再结晶行为。     

双相钢拼焊板温拉伸流变应力研究

采用CMT5205微机控制电子万能试验机对B340/590DP双相钢母材及拼焊板进行温拉伸试验,研究母材及拼焊板在变形温度为550～700℃、应变率为0.000 1～0.1 s–1条件下的流变应力行为。采用等应变法计算获得焊接区应力-应变曲线,通过改进后的含软化因子模型分别建立母材和焊接区材料的流变应力模型,并验证流变应力模型的准确性。结果表明,双相钢母材及拼焊板在温拉伸试验中发生了明显的动态回复与动态再结晶,流变应力随变形温度的升高而降低,随应变率的增加而增加;流变应力的预测值与试验值吻合较好,具有较高的可信度。     

热压缩变形参数对海底用X70管线钢再结晶行为的影响

采用热模拟试验机研究了应变速率为1s-1时变形温度、变形量和变形后保温时间对海底用X70管线钢再结晶行为的影响。结果表明:在变形量为25%的条件下,试验钢发生动态再结晶的变形温度为1 0001 020℃,奥氏体晶粒可细化到33μm左右;在变形温度为980℃的条件下,发生动态再结晶的变形量为25%30%,当变形量达到30%时,再结晶基本完成,晶粒尺寸随变形量增大而减小;在变形温度为980℃、变形量为25%的条件下变形完成保温5s后,试验钢开始发生静态再结晶,保温30s后静态再结晶基本完成,此时晶粒尺寸最小,为41μm。     

变形速率对含磷高强无间隙原子钢高温变形行为的影响

采用Gleeble 3800型热模拟试验机测定了含磷高强无间隙原子钢(IF钢)在变形温度为950,850 ℃,单道压缩变形量为50%,变形速率为0.01,0.1,1,10 s-1时的应力应变曲线,对其变形行为进行了分析.结果表明:应变速率为10 s-1,变形量为50%时,应力-应变曲线仅为动态回复型,不因温度的变化而改变类型;当变形温度为950 ℃时,变形速率越高,铁素体晶粒越大;而当变形温度为850℃时,这种差别比较小.说明在变形速率不太高的情况下,变形温度是影响奥氏体或铁素体晶粒尺寸的主要因素.     

Dynamic tensile characteristics of TRIP-type and DP-type steel sheets for an auto-body

This paper is concerned with the dynamic tensile characteristics of transformation-induced plasticity (TRIP)-type and dual phase (DP)-type steel sheets at intermediate strain rates ranging from 0.003 to 200 s⁻¹. The dynamic responses of TRIP600, TRIP800, DP600 and DP800 steel sheets are investigated with the evaluation of stress–strain curves, the strain rate sensitivity, the fracture elongation and the effect of pre-strain. The dynamic responses were acquired from dynamic tensile tests at the intermediate strain rates with a high-speed material testing machine developed. Experiments were carried out with specimens whose dimensions were carefully determined by finite element analyses and experiments to induce uniform deformation in the gauge section at the intermediate strain rates. The tensile tests provide stress–strain curves and the strain rate sensitivity. Experimental results show two important aspects for TRIP-type and DP-type steel sheets quantitatively: The flow stress increases as the strain rate increases and the fracture elongation and the formability of TRIP-type sheets are better than those of DP-type sheets at the intermediate strain rates. The pre-strain effect was also investigated for two types of metals at the intermediate strain rates. TRIP600 and DP600 steel specimens pre-stained by 5% and 10% were elongated at the strain rate of 0.003 s⁻¹ for quasi-static loading, and then tested at strain rates of 0.003, 1, 10 and 100 s⁻¹. The results demonstrate that the mechanical properties of TRIP600 and DP600 steels are noticeably influenced by the pre-strain when the strain rate is over 1 s⁻¹. The ultimate tensile strength as well as the yield stress increases due to the pre-strain.     

双相不锈钢各组相循环变形行为的纳米压痕试验和有限元表征方法研究

对双相不锈钢的奥氏体相和铁素体相，分别开展了不同加载模式（接触载荷和压入位移）和不同加载波形下的单向、循环纳米压痕试验，对比分析了两相的基本力学性能和压痕循环变形行为的演化规律。基于压痕试验结果和修正ABDEL-KARIM-OHNO非线性随动硬化准则的弹塑性本构模型，提出一套双相不锈钢奥氏体相和铁素体相的塑性和循环塑性行为的本构模型参数表征方法。通过对微结构代表性体积单元整体拉伸和循环变形行为进行模拟，并与宏观试验结果对比，验证了参数表征方法的合理性。研究结果表明，铁素体相的强度、硬度和抗棘轮变形的能力均高于奥氏体相，两相之间通过晶界产生一定的交互作用；在接触载荷控制的循环加载条件下，奥氏体相与铁素体相均产生明显的压痕棘轮现象，且载荷水平越高压痕棘轮变形程度越大；所发展的本构模型参数表征方法可为研究多相材料各组相、小体积材料的循环变形行为提供借鉴和参考。     

HAZ Characterization and Mechanical Properties of QP980-DP980 Laser Welded Joints

The QP980-DP980 dissimilar steel joints were fabricated by fiber laser welding. The weld zone(WZ) was fully martensitic structure, and heat-affected zone(HAZ) contained newly-formed martensite and partially tempered martensite(TM) in both steels. The super-critical HAZ of the QP980 side had higher microhardness(～ 549.5 Hv) than that of the WZ due to the finer martensite. A softened zone was present in HAZ of QP980 and DP980, the dropped microhardness of softened zone of the QP980 and DP980 was Δ 21.8 Hv and Δ 40.9 Hv, respectively. Dislocation walls and slip bands were likely formed at the grain boundaries with the increase of strain, leading to the formation of low angle grain boundaries(LAGBs). Dislocation accumulation more easily occurred in the LAGBs than that of the HAGBs, which led to significant dislocation interaction and formation of cracks. The electron back-scattered diffraction(EBSD) results showed the fraction of LAGBs in sub-critical HAZ of DP980 side was the highest under different deformation conditions during tensile testing, resulting in the failure of joints located at the sub-critical HAZ of DP980 side. The QP980-DP980 dissimilar steel joints presented higher elongation(～ 11.21%) and ultimate tensile strength(～ 1011.53 MPa) than that of DP980-DP980 similar steel joints, because during the tensile process of the QP980-DP980 dissimilar steel joint(～ 8.2% and 991.38 MPa), the strain concentration firstly occurred on the excellent QP980 BM. Moreover, Erichsen cupping tests showed that the dissimilar welded joints had the lowest Erichsen value(～ 5.92 mm) and the peak punch force(～ 28.4 k N) due to the presence of large amount of brittle martensite in WZ and inhomogeneous deformation.     

Micromechanical modeling of dual phase steels

Dual phase (DP) steels having a microstructure consisting of a Ferrite matrix, in which particles of Martensite are dispersed, have received a great deal of attention due to their useful combination of high strength, high work hardening rate and ductility, all of which are favorable properties for forming processes. Experimental investigation into the effect of the harder phase volume fraction, morphology and phase distribution on mechanical properties of the dual phase steels is well established and comprehensive in the literature. In the present work, a micromechanical model is developed to capture the mechanical behavior of such materials, adopting the constitutive behavior of the constituents from the literature. Analytical approaches have been used in the past to model the DP steel material behavior, but theoretical treatments are based on the assumption of uniform deformation throughout the constituents, neglecting the local strain gradients. This assumption contradicts experimental observations, reduces the understanding of the mechanics and mechanism of deformation of such materials. Based on the micromechanical modeling of cells, several idealizations are investigated out of which the axisymmetric model is shown to display intrinsic ability to capture the expected material behavior in terms of the trend of the stress–strain curves with increasing volume fraction of the second phase and in terms of the deformation fields of the constituents.     

VC析出相对Fe-Mn-Si-Al相变诱导塑性钢微观组织演变及力学性能的影响

Fe-Mn-Si-Al相变诱导塑性钢因具有较低屈服强度和良好低周疲劳性能,有潜力替代现有抗震用低屈服点钢制造钢阻尼器。对试验用钢进行准静态拉伸和低周疲劳试验,并借助多种组织表征方法研究试验用钢变形前后的微观组织演变,揭示VC析出相及奥氏体晶粒尺寸对其力学性能的影响规律及作用机理。结果表明:奥氏体晶粒粗化可以促进ε马氏体生成交叉状多变体,从而在准静态拉伸过程中,提高试验用钢断后伸长率;而在低周疲劳变形过程中,交叉状多变体削弱ε马氏体相变可逆性,使其疲劳寿命降低。VC析出相有助于提高试验用钢的屈服强度和抗拉强度,但其对ε马氏体生长具有抑制作用,使断后伸长率降低。在低周疲劳变形过程中,VC析出相钉扎ε马氏体/奥氏体两相界面,抑制ε马氏体逆相变,从而使试验用钢的循环加工硬化程度显著提高,低周疲劳寿命降低。     

形变热处理工艺对低碳微合金管线钢晶粒细化的影响

以管线钢X52为研究对象，在Gleeble1500热模拟机上，进行了奥氏体未再结晶区不同形变速度、形变量和冷却速度对X52的相变行为及显微组织影响的研究。通过光学显微镜、扫描电镜分析可以发现，随变形速率、形变量和冷却速度的增加，晶粒明显变细。同时，低碳钢不同的是在奥氏体未再结晶区扎制时，第二相的析出可以抑制再结晶，并且析出物的存在不仅阻碍位错的运动，而且全造成位错的增殖，因而微合金钢细化昌粒的机理主要有：形变诱导铁素体、铁素体的动态再结晶和第二相的析出抑制晶粒长大使晶粒细化。     

轧态CrCoNi中熵合金动态拉伸本构模型的建立和变形机理研究

CrCoNi中熵合金在准静态拉伸下具有良好的强度和塑性,而其动态拉伸力学行为还有待研究。利用霍普金森拉杆分别对CrCoNi中熵合金试样进行了室温(298 K)和低温(77 K)下不同应变率的动态拉伸力学行为研究,建立了修正的J-C(Johson-Cook)本构模型对其塑性流动行为进行了较好的描述,通过断后样品的微观组织表征揭示了其变形机理。结果表明：室温下CrCoNi中熵合金的强度和塑性随着应变率增大逐渐提高。与准静态拉伸相比,动态拉伸应变率为1 200～5 000 s^-1时,试样的屈服强度增大至560 MPa到1 150 MPa,伸长率增长至60%到90%;低温下强度表现出相似的应变率效应且强度较室温下更高,但韧性有所降低。变形机理结果表明：相比于室温准静态,室温动态拉伸下试样内部孪晶密度更大且交叉孪晶出现、FCC→HCP相变发生、纳米晶形成,三者共同作用促使CrCoNi中熵合金加工硬化提高;相比于室温动态拉伸,低温动态拉伸下试样孪晶密度过大导致孪晶增厚,且纳米晶形成,促使试样加工硬化进一步提高,而孪晶增厚加强了对位错的阻碍致使韧性降低。     

层片状双相钢的显微组织和拉伸力学行为

在V M＜50%条件下,比较发展的层片状双相组织的抗拉强度和最大均匀延伸率高于岛状的,前者的加工硬化指数m 1和m 2低于岛状的并具有较高加工硬化速率和加工硬化能力。拉伸变形时,在较细层片状铁素体和马氏体内,由于不均匀应变分配产生的协调位错,引起较大硬化作用,将导致提高加工硬化速率。     

Numerical modeling and experimental study of thermally induced residual stress in the direct diode laser heat treatment of dual-phase 980 steel

The direct diode laser application has been found useful in the localized heat treatment of metal parts because of its wider beam and more uniform energy distribution with respect to other lasers with Gaussian-like energy distribution. In this study, an uncoupled thermomechanical finite element model is developed to study the temperature field and thermally induced stress evolution in high-strength dual phase (DP) 980 steel during its direct diode laser heat treatment. Thermal analysis results are experimentally validated through thermocouples and then input into a mechanical model as transient temperature loading in order to acquire the thermally induced stresses and strains. The effect of martensite phase transformation on residual stress distribution in heat-treated DP980 steel is considered. An X-ray diffraction technique is used to measure the residual stress distribution at the top surface of the heat-treated coupons of DP980 steel. The numerical results show that compressive stresses are located at the laser–material interaction zone. After heat treatment, tensile stresses are retained at the heat-treated DP980 steel coupons. There is qualitative agreement between the numerically predicted and experimentally measured residual stresses. The effect of the overlapping ratio on the residual stress and hardness of the heat-treated DP980 steel is also experimentally and numerically investigated.     

低碳钢临界奥氏体区的变形行为

在Gleeble-3500型热模拟试验机上对Q235钢进行了单向压缩试验，研究了不同温度下低碳钢的变形特征以及形变诱导铁素体的演变行为和在保温过程中的变化。结果表明：降低变形温度有利于低碳钢的组织细化，但变形温度低于Ar3时，得到混晶组织，并使珠光体成务状分布；随着变形的进行，形变诱导铁素体首先在晶界形核，然后在相界上反复形核；铁素体数量随着应变量的增加而增加，但存在一个极限值；应变量较高时，将会发生铁素体的动态回复和再结晶；形变诱导铁素体在变形后的保温过程中发生了逆相变并伴随着铁素体晶粒的粗化。