高强钢热成形局部硬化成形实验与温度场模拟

发布时间：2018-10-21 13:58

【摘要】：一直以来汽车工业始终面临着安全、节能、环保等问题。高强钢的使用成为汽车行业的一个重大转折点。目前汽车行业的发展强调轻质高强,这不仅意味着某些零部件需要减厚,而且还需要保证零部件符合碰撞吸能要求,最大化保护乘客安全。高强钢冲压零件在汽车中的应用比例越来越大,市场对高强钢热成形局部硬化的需求越来越迫切,研究者对这方面的研究主要涉及在成形模具内部设置不同冷却形式来控制板料成形过程中的冷却速度。板料与模具型面接触,在成形和保压阶段进行热量的传递,实现淬火,因此我们可以依据高强钢的连续冷却转变曲线,通过控制模具的温度,来间接控制板料的冷却速度,实现局部硬化。本文采用冷轧B1500HS钢板作为原始冲压材料,成形模具材料选用H13热作模具钢,设计了一套局部硬化成形模具,通过热成形实验及有限元模拟进行分析验证。本文主要研究内容如下:(1)通过对热成形局部硬化实验模具的不断优化,确定局部硬化成形实验模型,依据现有实验条件完成模具装配及实验设备的选择,记录实验过程中模具温度值并绘制曲线,结合高强钢连续冷却转变曲线分析得出结论:通过加热模具降低板料冷却速度能够降低板料的马氏体转变程度,获得韧性较好的贝氏体组织;通过空气冷却方式可以控制模具升温速率,获得强度较高的马氏体组织;通过模具预制间隙方法可以减小冷却区模具温度和加热区模具温度的相互影响,提高板料局部强度的可控性。(2)通过不同实验条件得到冲压件,在特定位置处取样进行力学性能及微观组织测试,对比分析强度、硬度及组织成分含量,得出结论:局部硬化成形板料在冷却区及过渡区得到大部分马氏体组织,冷却区材料断裂方式为脆性断裂,断口齐平,抗拉强度达1400Mpa,硬度达460HV;随着加热温度的升高,加热区板料马氏体含量逐渐减少,贝氏体与铁素体含量逐渐增多,材料以韧性断裂方式断裂,断口呈杯锥状,抗拉强度由1185.98Mpa减少至620.44Mpa,硬度由397HV降低至210HV,减少47%,成形件U型底部与侧壁性能均布,局部硬化成形板料冷却区、过渡区、加热区力学性能及微观组织含量均呈梯度性变化。(3)针对加热区,在ABAQUS中建立二维平面U型冲压模型,分析不同保压时间、模具加热温度对热冲压成形板料的影响,得到模具温度场、板料温度场及应力场;针对冷却区,在Fluent中建立三维U型冲压模型,分析空气流动速度对热成形板料的影响,得到模具及板料的温度场,以高强钢连续冷却转变曲线为分析依据,得到以下结论:热冲压过程中最佳保压时间为10s,加热模具可以降低板料的马氏体转变程度,模具内通入的空气流速高于一定流速时即可使板料得到完全马氏体组织,最佳空气流动速度为6m/s。
[Abstract]:The automobile industry has always been faced with safety, energy-saving, environmental protection and other issues. The use of high-strength steel has become a major turning point in the automotive industry. At present, the development of automobile industry emphasizes light weight and high strength, which not only means that some parts need to be thickened, but also need to ensure that the parts meet the requirements of collision energy absorption and maximize the safety of passengers. The proportion of high strength steel stamping parts used in automobile is increasing, and the demand for local hardening of hot forming of high strength steel is becoming more and more urgent. The research involves setting different cooling forms inside the forming die to control the cooling rate in sheet metal forming process. The sheet metal is in contact with the die surface, and the heat transfer is carried out during the forming and holding stage, so we can indirectly control the cooling rate of the sheet metal according to the continuous cooling transformation curve of the high strength steel and by controlling the temperature of the die. Local hardening is realized. In this paper, the cold-rolled B1500HS steel sheet is used as the original stamping material and the forming die material is H13 hot working die steel. A set of local hardening forming die is designed and verified by hot forming experiment and finite element simulation. The main contents of this paper are as follows: (1) through the continuous optimization of the hot forming local hardening experimental die, the experimental model of the local hardening forming is determined, and the assembly of the die and the selection of the experimental equipment are completed according to the existing experimental conditions. The temperature of the die was recorded and the curve was drawn. The conclusion was drawn by analyzing the continuous cooling transformation curve of high strength steel that the martensite transformation degree of the sheet metal could be reduced by heating the mould to reduce the cooling rate of the sheet metal. The bainite structure with good toughness can be obtained and the martensite structure with high strength can be obtained by controlling the heating rate of the mould by air cooling. The influence of die temperature in cooling zone and mold temperature in heating zone can be reduced by the method of mould prefabrication clearance, and the controllability of local strength of sheet metal can be improved. (2) stamping parts can be obtained by different experimental conditions. The mechanical properties and microstructure were measured by sampling at a specific position, and the strength, hardness and content of microstructure were compared and analyzed. It was concluded that most martensite structures were obtained in the cooling zone and transition zone. The fracture mode of the cooling zone is brittle fracture, the fracture surface is even, the tensile strength is up to 1 400 Mpaand the hardness is up to 460 HV.The content of martensite in the heating zone decreases gradually with the increase of heating temperature, and the content of bainite and ferrite increases gradually. The fracture surface is cup cone, tensile strength is reduced from 1185.98Mpa to 620.44MPA, hardness is reduced from 397HV to 210HVH, hardness is reduced by 47mm, the properties of U-shaped bottom and side wall are distributed, local hardening forming sheet metal cooling zone, transition zone, and so on, the material is broken in ductile fracture mode, and the tensile strength is reduced from 1185.98Mpa to 620.44 MPA, and the hardness is reduced from 397HV to 210 HVV. The mechanical properties and microstructure content of the heating zone are gradient. (3) for the heating zone, a two-dimensional plane U-shaped stamping model is established in ABAQUS, and the effects of different holding time and die heating temperature on the hot stamping sheet metal are analyzed. The mold temperature field, sheet metal temperature field and stress field are obtained. For the cooling zone, a three-dimensional U-shaped stamping model is established in Fluent to analyze the effect of air flow velocity on the hot forming sheet, and the temperature field of the die and sheet metal is obtained. On the basis of continuous cooling transformation curve of high strength steel, the following conclusions are obtained: the best holding time is 10 s during hot stamping, and the degree of martensite transformation can be reduced by heating die. When the air velocity in the mould is higher than a certain velocity, the perfect martensite structure can be obtained, and the optimum air flow velocity is 6 m / s.
【学位授予单位】：吉林大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG306

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