高强钢板材冲裁力实验研究

目的解决不同高强钢板材在冲裁时所需冲裁力不同的计算问题。方法搭建了高强钢板材冲孔实验平台,结合车身上广泛应用的DP、QP钢系列以及MS系列超高强钢板,设计了相应的冲孔试样,通过冲孔实验,获得了不同超高强钢板材所需的冲裁力,并结合超高强钢板的冲裁力经验计算公式,得到适用于计算不同高强钢材料冲裁力的经验系数,最后将经验公式进行修正,获得了适用于不同高强钢板材冲裁力与抗拉强度的修正关系式。结果选用DP980及1700MS高强钢进行冲孔实验可知,两者实验冲裁力与通过修正公式计算所得冲裁力误差分别为2%和2.2%。结论验证了所得修正关系式的可行性,可为高强钢零件冲切压力机吨位的选取和模具结构设计提供参考。     

拉深成形对于Q&P980高强钢氢致延迟断裂影响的实验研究

目的从整车厂应用的角度出发,基于拉深工艺,研究QP980高强钢的氢致延迟断裂敏感性。方法选取1.6 mm板厚的QP980高强钢,进行拉深系数为0.56和0.63的两种冲杯实验,以电化学充氢结合摄像头定时拍摄的方法,并结合ABAQUS软件计算杯口应力集中处的应力和应变。结果对于1.6 mm板厚的QP980高强钢,在拉深试样杯口边缘应力集中处,应力大致在900～1000 MPa范围,而应变大于等于0.32,则必然发生延迟开裂;若应变小于等于0.23,则延迟断裂敏感性较低。结论应力和应变同时影响高强钢的氢致延迟断裂敏感性,即对QP 980(1. 6 mm厚度)零件,当拉深边缘应变小于0.23,则该位置氢致延迟断裂可能性低;若应力集中处残余应力达到900 MPa以上,应变达到0.3以上,则该位置氢致延迟断裂敏感性高。此结论对工程应用判断拉深零件氢致延迟断裂有一定指导意义,     

超高强钢 QP980 液压成形 B 柱仿真及试验研究

目的研究超高强钢QP980液压成形汽车B柱的可成形性。方法基于Autoform有限元模拟软件,仿真对比了宝钢第三代超高强钢QP980及当前广泛应用的DP980超高强钢液压成形B柱的可成形性,通过试验试制了QP980液压成形B柱,并与仿真结果进行对比。结果在相同工艺条件下,QP980具有较高安全裕度,DP980具有开裂风险,采用2种材料模拟壁厚减薄率及回弹趋势一致,DP980壁厚减薄率大于QP980,QP980回弹大于DP980;QP980液压成形B柱模拟及试验对比显示,壁厚减薄率和回弹变化趋势一致,试验壁厚减薄率大于模拟,样件实际回弹小于模拟,QP980液压成形B柱实测最大壁厚减薄率7.6%,一端施加约束,另一端回弹约6 mm。结论超高强钢QP980液压成形B柱成形性良好,满足零件性能要求。     

薄板和厚板材料中不同裂纹形状的断裂分析

本文导出了与破损时线弹性应力强度因子弹性名义破损应力和两种材料参数有关的两参量断裂准则。断裂准则以前用于拉伸裁荷条件下簿板和厚板试样表面开裂和穿透裂纹的数据分析。本文的断裂准则则应用于中心裂纹拉伸试样、紧凑试样和缺口弯曲断裂试样,这些试样用钢、钛或鋁合金制成,在室温下进行试验。断裂数据包括了宽范围的裂纹长度、试样宽度和厚度。所分析的材料具有一个宽范围的拉伸性能。用此准则计算的破损应力与试验破损应力相当吻合(±10％)。同时还发現,此准则与不同试样类型(如中心裂纹拉伸试样和紧凑试样)所取得数据相关;对于同样材料、厚度和试验温度来说为±10％。     

GH5188合金板材高温拉伸变形流动行为与本构方程研究

目的 研究GH5188合金板材高温拉伸变形流动行为,为高温合金板材高温成形工艺的制定和优化提供指导。方法 基于GH5188合金板材高温拉伸试验,分析了变形工艺参数对GH5188合金板材高温拉伸变形时真应力、应变速率敏感性指数和应变硬化指数的影响规律,建立了本构模型对其流动行为进行描述和预测。结果 GH5188合金板材高温拉伸变形流动行为受应变硬化、流动软化和应变速率硬化的共同影响,其变形过程分为弹性变形、加工硬化、稳态流动和断裂4个阶段。随变形温度的升高和应变速率的降低,真应力减小。变形温度、应变速率和真应变对GH5188合金板材的应变速率敏感性指数和应变硬化指数具有显著影响。基于Johnson-Cook和Hensel-Spittel模型,建立了考虑应变硬化效应、流动软化效应和应变速率硬化效应耦合影响的GH5188合金板材高温拉伸变形本构模型(JC-HS模型),采用该模型预测的真应力与试验值的平均相对误差为6.02%。结论 建立的JC-HS模型能够较好地描述和预测GH5188合金板材的高温拉伸流动行为。     

液压成形同步冲孔工艺仿真分析及试验

对液压冲孔过程进行分析,介绍了液压成形同步冲孔的工艺原理,分析了不同类型液压冲孔的特点和应用范围。采用有限元分析方法,建立了液压冲孔仿真有限元模型,并对冲孔过程中应力分布和应变变化情况进行分析。在此基础上,进行液压冲孔工艺试验,把工艺试验结果与有限元分析结果进行比较。结果显示,液压冲孔仿真分析与工艺试验呈现相同的变化规律,仿真分析能够较好预测实际冲孔过程。     

Stellite12钴基合金热循环冲击前后拉伸断裂机理研究

对不同缺口的Stellite12钴基合金试样(700℃/20℃进行不同次数的热循环冲击和未冲击)进行原位拉伸,并结合试验数据的分析以及断口形貌的扫描电镜观察,分析了Stellite12钴基合金热循环冲击前后的拉伸断裂过程和断裂机理。结果发现:热循环冲击后不同半径试样的断裂过程略有不同,热循环冲击后的小圆弧缺口试样在缺口根部产生表面微裂纹,试样边缘及微裂纹两侧产生氧化微孔;原位拉伸时,该试样热冲击过程产生的裂纹先向试样厚度方向扩展,待厚度方向贯通,然后裂纹尖端的基体发生变形、黑相(白相)穿晶开裂、少量沿氧化微孔裂开,试样瞬间发生断裂;而经历热循环冲击后的大圆弧试样表面并未产生明显的裂纹,拉伸加载过程经历大圆弧根部基体变形、黑白相内开裂、边缘氧化微孔张开,试样突然断裂;对于未冲击试样,在加载过程中,试样的断裂过程经历基体变形、黑白相内部开裂,能量聚集到一定程度试样突然断裂。对于未热冲击的三种不同试样其断裂过程基本类似,仅仅是由于小圆弧半径的试样应力集中程度更大,从而使得其断裂应力低于平板以及大圆弧试样。     

超高强QP980钢冲压成形性能

目的研究宝钢新一代先进高强钢QP980与第一代先进高强钢DP980的冲压成形性能。方法采用单向拉伸、光学应变分析和帽型拉弯成形等试验方法,分析两种超高强钢在加工硬化、成形极限、拉弯成形侧壁减薄和回弹等性能特性。结果 QP980的伸长率达到21.6%,与DP980钢相比有更高的强塑积,在变形过程中能够维持较高的瞬时n值,进而提高了材料的成形极限。帽型拉弯试验中,在不同的流入距离条件下,QP980钢的侧壁减薄率均低于DP980钢。在不同压边力条件下,QP980(厚度1.0 mm)材料的回弹量大于DP980(厚度1.2 mm)材料。结论 QP980在梁型件成形过程中具有优良的抗减薄特性,采用QP980钢进行冲压成形时应考虑比同级别DP980钢更大的回弹补偿量,或者采用更大变形量的工艺设计。     

评价金属精密冲裁适应性的新方法

简要分析了金属板材冲裁时应力状态对变形过程的影响,介绍了一种用拉伸、扭转、压缩变形评价金属材料精密冲裁适应性的新方法.它一改传统的以断面光亮带相对高度来评判的方法,利用材料在达到破坏之前所允许的变形总量来评价其对精密冲裁工艺的适应性.与传统方法相比,其具有不必制备试验模具等优点,迅速便捷,可靠性高.     

关于钢材力学性能试验试样断口厚向开裂的若干问题

针对钢铁产品力学性能试验试样(包括拉伸、弯曲、静压、冲击、落锤撕裂和断裂韧度试验试样)断口厚向开裂现象中涉及的试样断口厚向开裂的原因、试样断口分离/分层开裂如何界定,是否认定为欠缺/缺陷,船板静压大断口试验的合理性以及钢材的合乎使用性等若干问题进行了讨论分析,提出了一些新的观点和建议。     

Experiments and modeling of edge fracture for an AHSS sheet

With the emergence of advanced high strength steels (AHSSs) and other light–weight materials, edge fracture has been one of the important issues evading reliable prediction using CAE tools. To study edge fracture behavior of AHSS, a comprehensive hole expansion test (HET) program has been carried out on a DP780 sheet. Specimen with three different edge conditions (milled edge, water jet cut edge and punched edge) are manufactured and tested. Results reveal that the hole expansion ratio (HER) of the present DP780 sheet is around 38 % for milled specimen and water jet cut specimen, and about 14 % for punched specimen. A novel method of a central hole specimen tension is also introduced for edge fracture study, showing a similar trend as found in HET. The paper briefly presents a procedure and the results for a full calibration of the DP780 sheet for plasticity and fracture, where a hybrid testing/simulation method is used to obtain parameters for Hill 48 plasticity model and modified Mohr–Coulomb fracture model. The finite element simulation gives an accurate prediction of HER, as well as the load displacement response and specimen deflection distribution in the hole expansion tests on uncracked material. The correlation between simulation and tests on central hole specimen also turns out to be very good. The paper also presents a very interesting insight of the initiation and propagation of cracks from the hole edge during a hole expansion test by numerical simulation in comparison with testing observation. The number of final cracks are accurately predicted. Other new aspects of the present paper include an improved 3D DIC measurement technique and a simplified analytical solution, from which a rapid estimation of displacement and hoop strain field can be made (see “Appendix 2”).     

Study of curvature and pre-damage effects on the edge stretchability of advanced high strength steel based on a new simulation model

Edge cracking is one of the common failures during the stamping of Advanced High Strength Steel(AHSS). The limit strains of the edge are hard to predict by traditional methods because edge formability is influenced by many factors such as edge quality, process parameters, material properties and so on. Hole expansion tests are conducted with DP780 under three hole diameters and two punching clearances to investigate the effects of curvature and pre-damage caused by punching on edge stretchability. The corresponding simulations are carried out meanwhile with a new model based on Marciniak and Kuczynski (M-K) model and Hill’s localized necking theory to predict the edge limit strain. This new model is proposed for the hole edge with pre-damage and improves the predictability of edge cracking in hole-expansion simulation. The problem that the traditional ideal model fails to predict the hole expansion ratio (HER) of pre-damage edge is also successfully solved by the new model with the use of the Damage Zone and Effective Failure Strain Ratio (EFSR). The comparison between the experiments and simulations reveals that this new model can be used in edge forming failure prediction of AHSS.     

QP和DP钢面内成形性能与边部成形性能对比研究

目的 以宝钢生产的QP980、QP1180、DP980、DP1180 4种典型超高强钢材料为研究对象,进行QP、DP钢种材料面内与边部成形特性对比分析。方法 采用单向拉伸设备以及成形试验机,并结合DIC分析技术,对4种材料的力学性能、面内成形性及边部成形性进行试验研究。结果 与DP钢相比,同等强度级别QP钢的均匀延伸率及加工硬化系数均明显更高。在面内成形应变状态下,同等强度级别QP钢极限成形深度均明显大于DP钢的,但主、次应变大小差异不大。在边部成形应变状态下,同等强度级别QP、DP钢极限成形深度以及主、次应变大小均差异不大。QP、DP钢面内成形最大主应变均明显大于边部成形最大主应变。结论 与同强度级别DP钢相比,QP钢具有更高的均匀延伸率及加工硬化系数。QP钢材料的加工硬化系数高,材料内部协同变形能力强,面内成形性能明显优于DP钢材料的,但两者的边部成形性能差异不大;QP、DP钢材料能承受更大的面内主应变,受边部加工硬化及毛刺的影响,冲裁后,边部应变明显降低,在QP、DP超高强钢零件设计制造过程中,应尽可能避免边部发生较大的变形。     

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.     

Effect of residual stresses on fatigue strength of high strength steel sheets with punched holes

The effect of residual stresses on the reverse bending fatigue strength of steel sheets with punched holes was studied for steels with tensile strength grades of 540 MPa and 780 MPa. Tensile and compressive residual stresses were induced around the punched holes. Heat treatment of the specimens with punched holes at 873 K for 1 h decreased the residual stresses around the holes and improved the fatigue strength of the sheets. This result means that the tensile residual stresses induced in the sidewalls of the holes and near the hole edges by punching reduced fatigue strength. The effect of the residual stresses on the fatigue limits of the edges was estimated by the modified Goodman relation using the residual stresses after cyclic loading and the ultimate tensile strength at the fatigue crack initiation sites.     

一种钢板材料动态拉伸本构方程确定以及试件尺度效应研究

采用数值模拟方法,优化设计了Hopkinson拉伸实验中,QP980CR钢的片状试件尺度。对所设计的试件,在MTS(810材料试验机)上进行了准静态拉伸试验,在Hopkinson拉杆装置上进行了不同应变率及不同温度的动态拉伸试验。由准静态及动态拉伸试验结果,确定了试件材料的J-C型动态拉伸本构方程。基于确定的试件动态拉伸本构方程,采用数值模拟方法,研究了Hopkinson拉伸试验中,试件的形状和尺度对测试结果的影响。     

Ductile-to-brittle transition in tensile failure due to shear-affected zone with a stress-concentration source: a comparative study on punched-plate tensile-failure characteristics of precipitation-hardened and dual-phase steels

The effect of shear-affected zone (SAZ), with a stress-concentration source induced by the punching process, on tensile properties was investigated. Tests using honed specimens (which have the same shapes and stress-concentration without any SAZ) and smooth specimens were conducted to compare the effect with that of the punched specimens. Dual-phase steel, which has a high work-hardening ability and low yield strength, and precipitation-hardened steel, which has a low work-hardening ability and high yield strength, were used in the tests. Materials with two tensile strength grades were prepared from both types of steel. Only the precipitation-hardened steel with higher strength grade punched specimen showed a brittle fracture with extremely short fracture-elongation, whereas the other specimens showed a ductile fracture. The fracture surface analysis revealed that cracks initiated in the maximum shear stress plane of the SAZ under tensile loading at first. We call the crack “shear crack.” The steel which showed brittle fracture used in this study easily exhibited plastic-strain localization compared with the other steels. If the shear crack is sharp, then the transition from ductile to brittle failure tends to occur. Furthermore, the strength characteristics of the punched specimen depend on the crack length dependency of the strength resistance and the failure phenomenon of the original material.     

On the buckling of thin tensioned sheets with cracks and slots

The problem of local buckling in tensioned sheets with holes is discussed in relation to its effect on fracture and fatigue characteristics. The results of an experimental investigation designed to determine the tensile buckling stress are presented. The results indicate that for the range of the parameters investigated, the sheet thickness to hole length ratio and a nondimensional hole shape parameter are the dominant geometric variables. It is shown that the results obtained in the investigation described and the results obtained by other investigators can be described by a single formula.

The physical significance of local buckling is discussed and particular emphasis is placed on the importance of accounting for buckling in residual strength analyses of thin cracked sheets. Recommendations are made for the standardization of the buckling stress determination in terms of its relationship to the effect of middle-surface stretching due to the bending. Finally, the influence of the buckling process on plane stress fracture toughness values, K_c, obtained from center cracked sheet specimens is discussed.     

不锈钢拉伸试验条件与断后伸长率测量值的关系

采用单向拉伸试验研究了不同拉伸速度、不同试样规格对不锈钢冷轧薄板断后伸长率测量值的影响。结果表明:不锈钢的断后伸长率测量值随拉伸速度的提高而下降、随试样横截面的增大而提高。说明拉伸试验条件对不锈钢薄板的断后伸长率测量值影响较大,只有在拉伸试验条件一致的情况下,断后伸长率测量值才能作为选材的依据之一。