拉伸试验速率对低碳钢力学性能的影响

采用正交试验方法设计拉伸试验方案,对低碳钢坯、低碳热轧及冷轧钢板进行了金属常温拉伸试验,考察试验速率对力学性能的影响.试验结果表明,拉伸试验速率作为拉伸试验过程中最重要的可控制条件,会对拉伸试验结果如抗拉强度、非比例延伸应力、断后延伸率、断面收缩率等产生影响,通过控制拉伸试验速率,可以达到减少测量结果的不确定度分量,提高检验结果真实程度和稳定性的目的.     

板料的基本成形性与模拟成形性的相关性研究

本文分析了单向拉伸、双向等拉、平面应变、纯剪四种基本状态的试验和锥杯、扩孔、拉深、杯突四种模拟试验,取得了一系列基本材料参数和模拟指标。根据这些试验数据,用解析法建立了基本成形性和模拟成形性之间的数学关系式,用数理统计法,探讨了这两类性能之间的线性相关性,为进一步开发基本试验、简化和精化模拟试验打下了基础。     

退火温度对5182车用铝合金冷轧板的组织及性能的影响

对5182铝合金冷轧板材进行了不同温度退火处理及退火后的单向拉伸试验、杯突试验以及微观组织表征.结果表明:随着退火温度的升高,屈服强度和抗拉强度逐渐降低,在300～325℃范围内,分别由原来的240MPa、379MPa快速降低到114MPa、258MPa;杯突值由原来的5.77 mm提高到了8.75 mm;同时,应变硬化指数也大幅度提高,当退火温度达到325℃,并保温30min,冷轧板材已经发生了完全再结晶.     

拉伸速率控制模式及拉伸速率对冷轧薄板强度测试结果的影响

按照GB/T 228.1—2010中推荐的不同拉伸速率控制模式及拉伸速率对冷轧镀锌钢板进行了拉伸对比试验,分析了应变速率、横梁位移速率及应力速率对冷轧薄板规定非比例延伸强度R_p0.2、下屈服强度R以及抗拉强度R_tn测试结果的影响。结果表明:采用应变速率进行控制得到的拉伸试验数据相对其他两种速率控制模式更为稳定,可以最大限度地降低拉伸测试结果的不确定度;采用0.000 25 s^-1的应变速率控制模式所得试验结果的稳定性与采用40～50 MPa·s^-1的应力速率控制模式所得结果较为接近;在不同拉伸速率控制模式下,提高应变、横梁位移和应力速率均会使屈服强度R_p0.2和R_el增加,而抗拉强度R_tn基本上处于稳定状态。     

冷轧态Al-Li-Cu-Mg-Zr合金的超塑变形工艺

研究了未经预处理的冷轧态 Al-Li 合金超塑变形工艺,根据三段拉伸设想,按正交试验设计方案,进行了超塑拉伸试验。结果表明:对于冷轧态 Al-Li 合金,采用三段拉伸工艺可以获得良好的超塑性,其延伸率达到550%以上,超过一段和两段拉伸。     

301奥氏体不锈钢薄板拉伸强度与冷轧硬度之间的关系

为对冷轧不锈钢薄板的产品硬度控制提供指导,尝试用一个新的方法来取代试轧,既达到控制冷轧板硬度的目的,又能降低成本、提高效率。对0.99mm厚的经过退火的301奥氏体不锈钢薄板进行冷轧减薄,并进行室温拉伸试验,测量其维氏硬度。通过观察金相和利用X射线衍射,验证了应变诱导马氏体相变是导致301奥氏体不锈钢冷轧和拉伸时产生加工硬化的主要原因。试验结果表明,冷轧和拉伸有着相似的加工硬化趋势,综合拉伸与轧制试验数据,确定了拉伸强度与冷轧硬度之间的关系,实现了通过拉伸强度来得到对应应变下的冷轧硬度,具有很好的预见性。冷轧可以提高301不锈钢的强度和硬度,显著改善其力学性能。     

焊缝对拼焊板成形性能的影响

采用激光焊接技术对冷轧薄钢板进行拼焊,通过对拼焊板进行拉伸试验和模拟成形性能试验,研究分析了焊缝对拉伸性能参数和成形性能参数的影响,比较了拼焊板与母材性能的差异,分析了强度比和厚度比对塑性变形的影响及应变分布。     

冷轧态Al—Li—Cu—Mg—Zr合金的超塑变形工艺

研究了未经预处理的冷轧态Ａｌ－Ｌｉ合金超塑变形工艺，根据三段拉伸设想，按正交试验设计方案，进行了超拉伸试验，结果表明，对于冷轧态Ａｌ－Ｌｉ合金，采用三段拉伸工艺可以获得良好的超塑性，其延伸率达到５５０％以上，超过一段和两段拉伸。     

温度对ZK60镁合金细晶板材成形性能的影响

研究了温度对ZK60镁合金细晶板材成形性能的影响。通过单向拉伸试验研究了ZK60细晶板材在25～300℃下的拉伸成形性和力学性能各向异性,进而通过杯突试验和热拉深试验研究了温度对板材成形性能的影响。研究结果表明:随着温度的升高,板材的各向异性逐渐降低,在300℃下的断裂伸长率在341%以上。ZK60细晶板材的成形性能随着温度的升高显著提高,在本实验条件下,当温度为250℃时获得最大的Erichsen值(IE=18. 7 mm)和最大的极限拉深比(LDR=1. 8),ZK60镁合金细晶板材的最佳杯突成形温度范围为200～250℃,最佳热拉深成形温度范围为250～300℃。     

影响杯突试验数据的因素分析

在GBS-60型半自动数显杯突机上对不同批次、不同厚度的油桶板进行了检验,找出了影响杯突值准确性的主要因素——试样厚度、压边力与冲杯速度。试验结果表明,杯突值随着试样厚度的增加而增大;而压边力与杯突值成反比关系;最佳的冲杯速度控制在10mm／min,可保证试验的准确性。     

Medium carbon steel deep drawing: A study on the evolution of mechanical properties,texture and simulations,from cold rolling to the end product

Medium carbon steels are mostly used for simple applications; nevertheless new applications have been developed for which good sheet formability is required. This class of steels has an inherent low formability. A medium carbon hot rolled SAE 1050 steel has been selected for this study. It has been cold rolled with reductions in the 7–80% range. Samples have been used to assess the cold work hardening curve. For samples with a 50 and 80% thickness reduction, an annealing heat treatment has been performed to obtain recrystallization. The material has been characterized in the “as received”, cold rolled and annealed conditions, using several methods: optical microscopy, X-ray diffraction (texture), Vickers hardness and tensile testing. The 50% cold rolled and recrystallized material has been further studied in terms of sheet metal formability and texture evolution during the actual stamping of a steel toecap that has been used to validate the finite element simulations.     

Comparison between visioplasticity measurements and finite element computations for tensile tests on cold rolled perforated ferritic chromium steel P92

Finite element computations for tensile tests on cold rolled perforated sheet metals are compared to experimental tests with simultaneous visioplasticity measurements to determine requirements for the use of detailed FEM models. First, force–displacement curves and in the secondly the local strains have been compared. The obtained results show that the perforation geometry does not need to be depicted exactly in the FEM-model, an approximation with simple geometries like cylinders deliver reasonable results. Furthermore, the computed tensile test on perforated sheet metals with 2D-FEM models is more acceptable than wide-meshed 3D-models.     

20MnSi热轧带肋钢筋冷弯脆断和无屈服点原因分析

分析和研究了20MnSi热轧带助钢筋在常规力学性能检验中出现冷弯脆断和无屈服点的异常现象。结果表明，钢中存在一定量异常粒状贝氏体组织是20MnSi热轧带助钢筋产生冷弯脆断或在拉伸检验时出现无屈服点的主要原因。     

钛-钢复合钢材力学性能及本构模型研究

为系统研究钛-钢复合钢材的力学性能,对2 mm~12 mm厚TA2+Q235B钛-钢复合钢材进行了系列试验研究,包括拉伸、剪切、粘结、弯曲、冲击韧性、硬度等试验,其中拉伸试件设计考虑了复合比的影响。基于试验结果,得到了该类钛-钢复合钢材的基本力学性能指标,并重点对其单调拉伸荷载下的力学性能进行了分析研究。试验结果表明:钛-钢复合钢材的应力-应变曲线特征及典型力学性能指标与复合比的大小直接相关;随着复合比的增大,屈服平台逐渐消失,弹性模量逐渐减小,屈服强度和断后伸长率逐渐升高,但抗拉强度的变化并不明显,这与钛TA2和Q235B低碳钢本身的力学性能有关。基于拉伸试验数据和有限元数值计算结果,提出了钛-钢复合钢材的力学指标计算方法,建立了其本构模型。此外,剪切和粘结试验得到的复合界面强度尽管较低,但对拉伸力学性能影响十分有限;同时,该类复合钢材的受弯和冲击性能良好,硬度结果呈现两侧高、中间界面层低的情况。研究结果可为钛-钢复合钢材在结构工程领域的研究和应用提供基础参考和材料本构模型,并有利于促进其工程应用。     

Design optimisation of biaxial tensile test specimen using finite element analysis

One of the most restricting aspects of the biaxial tensile test for sheet metal is the design of the cruciform specimen. Although specimens of the cruciform type have been investigated quite extensively previously, no standard geometry for the cruciform specimen exists. Using a specifically designed pantograph apparatus for operation in a standard tensile testing machine, various cruciform specimens machined from low-carbon cold rolled steel sheet were analysed experimentally. Finite element modelling of the specimens was conducted in parallel to the experimental test programme to establish optimum specimen geometry. Through a process of optimisation, a standard cruciform specimen was designed which can be used to accurately predict the mechanical behaviour of the mild steel when formed in multiple directions simultaneously. This paper describes the optimisation process and the results obtained from both the experimental testing and numerical modelling.     

The microstructure and mechanical properties of prolonged and lower temperature aged Fe–Ni–Mn–Mo–Ti–Cr maraging steel

In this study, the microstructure and mechanical properties of Fe–Ni–Mn–Mo–Ti–Cr maraging steel at low temperature and prolonged aging condition were investigated. Optical and scanning electron microscopy examinations, tensile and hardness tests were conducted to study the microstructure, aging behavior and mechanical properties of the cold‐rolled steel. The results showed that aging of cold rolled Fe–Ni–Mn–Mo–Ti–Cr maraging steel resulted in the formation of Mo rich and Ti rich Lave phase precipitates. Existence of many dislocation cores due to cold rolling and subsequently, low temperature aging caused to formation of uniform distribution of very fine precipitates. The presence of these precipitates increased the yield and ultimate tensile strengths but couldn't improve the uniform tensile ductility. This alloy showed ultra‐high fracture stress of about 1950 MPa with a negligible tensile elongation (about 2 %) at the peak aged condition. The fractographic studies indicated this alloy shows semi‐brittle fracture in the subsequent aging treatment.     

Fatigue behaviour of clinched joints in a steel sheet

Clinch joining has been used in sheet metal work owing to its simplicity and because it facilitates the joining dissimilar metal sheets. In this study, monotonic and fatigue tests were conducted using coach‐peel and cross‐tension type specimens to evaluate the fatigue strength of clinch joints in a cold‐rolled mild steel sheet. The monotonic experimental results reveal that the coach‐peel specimen exhibits the lowest monotonic strength among the three specimen configurations. The coach‐peel and cross‐tension specimen geometries exhibit very low fatigue ratios, compared with the tensile‐shear specimen. The maximum von Mises and principal stresses at the fatigue endurance limit are much higher than the engineering tensile strengths of the steel sheet used to determine the three specimen geometries. Compared with the effective stress and maximum principal stress, the Smith–Watson–Topper fatigue parameter can be used for an appropriate prediction of the current experimental fatigue life. With regard to the coach‐peel specimen geometry, all samples exhibit pull‐out failure mode in the fatigue testing range. However, for the cross‐tension specimen geometry, mixed (pull‐out and interface) and interface failure modes occurred, depending on the number of cycles to failure.     

Investigation of Non‐Linear Springback for High Strength Steel Sheets by ESPI

Abstract: The springback of the sheet metals after large deformations during deep drawing is not a strongly linear process with a constant Young’s modulus, but the stress–strain behaviour during the unloading phases shows considerably non‐linear and inelastic effects. Unloading of two types of steel sheets for cold forming, a cold‐rolled high‐strength microalloyed steel and a low‐carbon steel sheet, has been analysed using the method of electronic speckle pattern interferometry (ESPI). The specimens were investigated by uniaxial tension tests, and the influences of different testing parameters upon springback were analysed. The experimental measurements showed that the stress–strain curve during unloading is non‐linear, the influence of the prestrain path upon unloading is minor, and the secant moduli of unloading curves decrease with increasing prestrain. When the prestrain value becomes high enough, a saturated value for the secant modulus is approached. An empirical relation was found to describe the changes in the unloading modulus in accordance with the prestrain value.     

Enhanced tensile properties of 316L steel via grain refinement and low-strain rolling

Excellent strength–ductility synergy of metallic materials is significant for their industrial applications. This study presents a fine-grained 316L stainless sheet (average grain size of ～5?µm) with a good combination of strength and ductility achieved via low-strain cold rolling (rolling strain of 30%). The fabricated steel sheet exhibits maximum yield strength and ultimate tensile strength values of 1045 and 1080?MPa, respectively, with a uniform elongation of 7%. Experimental results confirm that the high density of dislocations, strain-induced martensitic phase, and deformation twins together contribute to the high strength of the rolled stainless steel. Moreover, its good ductility is attributable to the strain-induced martensitic transformation and deformation twins.