超高强度钢气压管成型技术的研究与开发

In order to meet the needs of auto industry for reduction of the weight of car bodys, the oil consumption and exhaust gas, the high temperature gas tube forming technology was proposed for manufacturing of thin wall tube parts of ultrahigh strength steel. In this technology, the ultrahigh strength steel tube billets are heated in the forming dies to the temperature above the austenite phase transform point, then the billets are bulged to the desired forms by internal high pressure gas. After forming, the tube parts are quenched rapidly in the dies to increase the strength of the tube material up to above 1200MPa. In this quenching process, the quench hardening effect of the ultrahigh strength steel is utilized. Using this technology, the problems, such as large forming force, large springback and low dimentional accuracy, met in the room temperature forming of the ultrahigh strength steel are solved. In this proposed project, experimental research and numerical simulation will be conducted to investigate the technological parameters and the formability of the ultrahigh strength steel, the efficiency and the feasibility of different heating and cooling methods will be evaluated, and a simulating device for high temperature gas tube forming will be manufactured. The innovative features of the proposed technology are: 1) the tube billets are heated in the die cavity under a condition of vaccum or protecting atmosphere, such that the problems of oxidization is avoided; 2) pulsed pressure gas is used in the forming process to reduce the friction between the tube billets and the dies, as the result the forming force is decreased; 3) gas jet cooling or liquid nitrogen cooling method is used to quench the forming parts which will greatly increased the cooling efficiency and lower the die cost. Another feature of the project is the finite element analysis of the forming process considering the thermal, mechanical and phase transform effects, which jointly determines the final dimention accuracy and mechanical properties of the ultrahigh strength steel tube parts.

为满足汽车工业对减轻车身重量、减少能耗和排放的需求，本项目提出了高温气压管成型技术，用于制造车身用超高强度钢薄壁空心管件。针对超高强度钢常温成型力大、回弹大、零件尺寸精度低等问题，本项目采用高温成型的方法，将超高强度钢在成型模具内加热到奥氏体区（850～950℃），利用高压气体胀形，然后再利用超高强度钢淬火硬化效应，对模具内已成型的管件进行淬火处理，以获得强度在1200MPa以上的薄壁空心管件。本项目采用实验和数值模拟的方法研究超该项技术的工艺问题和材料的成型性能，评估各种加热和冷却方式的效率和可行性，建立模拟实验研究装置。提出真空加热或保护气氛加热方法解决高温成型的氧化问题，提出脉动气压成型方法减小管壁与模具的摩擦，降低成形力，提出喷吹氮气或液氮的方法进行淬火处理，提高冷却效率，降低模具制造成本。本项目还开展热、力和相变耦合的有限元分析，研究超高强度钢热成型的变形和组织性能演变规率。

本课题主要完成四个方面的研究工作。.1. 针对课题，制定了总体研究方案。主要特点：（1）开展有限元模拟研究，分析了气压薄壁管热胀形的应力应变分布以及壁厚变化，建立了气压管成形过程有限元模型；（2）开发了一种由成形水冷模具、电阻加热、管端密封、液压轴向进给机构、高压气体供给控制等组成的实验装置；（3）水冷模具可以对高强钢管进行淬火冷却，提高强度。钢管采用电阻加热，最高加热温度为1000℃。（4）开发了单级增压泵、高压气体压力调节等技术，完成了40MPa级高压气体增压系统设计集成。.2.薄壁管气压热成形数值模拟。采用ANSYS13.0有限元分析软件，对气压热成形过程中胀形模具的受力情况进行了数值模拟，结果表明，在恒定合模力和最大气压力作用下，模具应力应变最大的位置出现在型腔过渡圆角处；利用LS-DYNA对薄壁管高温气压热成形过程展开结构场分析，研究了接触和摩擦处理等问题，给出了几何模型、自适应网格划分、边界条件处理等在数值模拟的方法与实例，分析了不同温度、气压、以及加载路径对成形管坯壁厚分布的影响规律，结果表明，增大进气速率和轴向力可以有效抑制壁厚过度减薄，使壁厚趋于均匀。.3. 研发出薄壁管高气压热成形试验装备。结果表明：模具水冷通道、轴向进给控制、升温时间、气体压力和成形速率等是薄壁管气压热成形的关键技术，最终获得了现有试验条件下管坯贴模性最佳工艺。采用STEP7和WinCC编程软件，完成了试验机的新型高气压与适时成形速度之间的耦合实际值，结果表明：气压与适时成形速度控制模型，解决了原系统的气体供给速度较慢，影响成形温度和贴膜性等关键技术问题。.4. 薄壁管热成形中试实验研究。验证了不同工艺参数对管坯几何形状、壁厚分布和成形性能的影响规律。针对碳钢、304不锈钢的实验结果表明：在一定的温度与气压范围内，温度高，气压大，管坯成型性能最佳；成形温度由800℃增至950℃时，壁厚由成形前1.33mm减薄至成型后1.03mm，得到了贴模性较好的管坯工件。从选定的钢得出的结果来看，不锈钢和碳钢的测试结果与我们初始选钢阶段预测是一致的。值得注意的是，不锈钢304具有较高的变形能力，通过快速冷却，得到了组织均匀性和较好的机械特性。.目前，针对本课研究，已经培养硕士研究生3人，在研博士研究生1人，撰写涉及本课题相关研究工作论文7篇，研究生课题论文4篇，申请发明专利4项。