摘要
覆镍深冲钢带用于生产高性能电池外壳，要求覆镍层均匀，与基体结合良好，镍层结构致密，孔隙率低，耐蚀性好，延展性优良，并可经受冲压加工而不出现裂纹、剥离等弊病。性能的改善和提高来源于结构的改造与优化，在制备覆镍深冲钢带后，通过真空热处理在界面形成铁镍过渡层，再经过激光表面热冲击细化表面晶粒，熔覆部分孔隙，提高耐蚀性。
涂层界面结合状况是决定涂层材料性能好坏的关键。采用适当的热处理，镍铁过渡层的形成，使界面具有牢固的冶金结合。扩散过渡层的存在固然重要，但过渡层的厚度需要控制。过渡层过厚，铁露出或过多的露出表面，钢带的耐蚀能力将会下降或严重下降。因此，有效控制过渡层厚度是热处理工艺的关键。用电子探针检测镍铁成分分布曲线，首先假设扩散系数与浓度无关，根据实验结果，采用无限大扩散偶模型，使用扩散方程的误差函数解对实验结果进行拟合，获得铁镍的互扩散系数；然后考虑扩散系数与浓度有关的一般情况，与常规的玻耳兹曼-俣野法相区别，采用最小二乘法对实验数据进行拟合处理，得到元素浓度分布的曲线方程，从而求得与浓度有关的互扩散系数。结合铁镍相图，可以了解界面相结构，X射线相分析结果证明了相图预期结构的存在。
钢带上的镍涂层属阴极性涂层，孔蚀是腐蚀的主要形式。以脉冲激光热冲击覆镍深冲钢带，熔化覆盖部分涂层表面的孔隙，改善涂层的表面结构。激光可以将高度集中的能量，按所需的位置和时间，以预定的量值，准确地投射到材料上。我们以高能量激光束迅速扫描工件，在其表面极薄的一层小区域内快速吸收能量而使温度上升，达到相变温度，利用工件基体的热传导自冷淬火，实现表面相变硬化。应用交流阻抗技术检测，发现激光热冲击处理提高了覆镍深冲钢带的耐蚀性能。与真空热处理比
研力护威公文 甩 色瀑 一 们 常 在鼻 胃 焙构 未 相 要 佳An
较，激光热冲击后基体晶粒基本不受影响，在改善耐蚀性能的
、同时，保证了冲压成型的足够塑性。
覆镍钢带要加工成电池壳的形状必须经过7道工序才能冲
压成型。在冲压成型过程的数值模拟中，材料的应力应变关系，
即本构方程是必需的。分离法分别测定基体镀与未镀情况下的
应力应变曲线，建立简单的模型，利用弹塑性力学，实现涂层
应力应变曲线的分离。本文提出了一种新的薄膜／涂层应力应变
关系的测试方法，即在扫描电镜观察的同时，通过测定涂层／基
体复合体和单一的基体的应力应变关系，由简单的理论模型来
确定涂层的应力应变关系。这种方法新颖，可以原位观察涂层
的变形过程及涂层内裂纹的萌生时刻和裂纹扩展过程的有关信
息，不仅如此，还可以考虑残余应力的影响。
本文从三个方面对覆镍深冲钢带的结构和性能进行研究，
分别得到了铁／镍互扩散系数、定量表征的耐蚀性和镍层的应力
应变曲线。主要的创新点有：涂层应力应变曲线的分离考虑了
残余应力的影响；从X射线能谱线分析的定性方法出发，定量
求解了界面的互扩散系数；激光表面处理和电化学交流阻抗谱
的综合运用，可以确定激光热冲击的最佳工艺参数。
Deeply drawing steel strip with electro-deposited nickel coating is a good material used in high-quality cell shell. The distrubution of electro-deposited nickel coating in the deeply drawing steel strip should be uniform and the coating should firmly adhere to the substrate. The coating's structure should be tight and its porosity ratio should be low. The corrosion-resistance and plasticity of the electro-deposited nickel coating should be good. The transition-layer of Ni and Fe at the interface was grown by vacuum heat-treatment. After that, the surface crystal grain was fined and molten to fill some holes due to the thermal shock of laser beam. In this case, the corrosion-resistance of the coating was improved.
By appropriate heat-treatment method, we could obtain the transition-layer of Ni and Fe which resulted in the interface firmly metallurgy bond. On the other hand, the thickness of the transition-layer shoud be suitable. If the layer is thick, the iron will expose too much on the surface and the corrosion-resistance will be low. The electron-probe was used to measure the concentration curve of iron and nickel. The diffusion coefficient was obtained by fitting method. Firstly, we assume that the diffusion coefficient is independent on the concentration. According to the experimental result, we use the model of infinite diffusion couple and utilize the error solution of diffusion equation to fit the experimental results. The Ni and Fe mutual diffusion coefficients were obtained. Secondly, based on the assumption that the diffusion coefficient is dependent on the concentration, we don't use the commonly-used Boltzman-Matona method but utilize the least square method to fit
the experimental data and get the concentration curve equation. From the concentration curve equation, the concentration-dependent mutual diffusion coefficient was obtained. Combination with the iron and nickel phase graph, we also can know the phase structure of the interface. The results of X-ray diffraction proved the existence of these phases.
Nickel coatings on the steel strip are cathodal coatings, and the hole erode is the main erosion mode. The holes of the exterior coatings were molten down partly by the pulse laser thermal shock. In this case, the quality of corrosion resistance could be improved. When the laser beam with high-energy scans the specimen quickly, the coating's exterior layer absorbs the energy and its temperature can attain to the phase transition temperature. The heat exchange of self-cooling quenching realizes the exterior nickel-coatings phase transition hardening. The alternating current impedance was used to examine the corrosion resistance. It is found that laser thermal shock improved the corrosion-resistant of the deeply drawing steel strip with Ni-coating. The laser thermal shock wouldn't affect the substrate crystal grain and can ensure enough plasticity for punching-molding as the improvement of the corrosion-resistance simultaneously.
There are seven mechanical steps for the deeply drawing steel strip with electro-deposited nickel coating to be a battery cell. It is necessary to know the stress-strain relationship in the numerical simulation of the punching process. The stress-strain curve of electro-deposited nickel coating was determined. In this master thesis, a new method was proposed to determine the stress-strain relationship of the coating. The stress-strain relationship of the substrate /coatings composite and substrate were measured. After that, the stress-strain relationship of the coatings could be
determined by a simple theoretical model. The cracks initiation and propagation could be observed in situ by scanning electron microscope in the deformation of the coating.
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