Experimental Investigation of the Effect of the Material Damage Induced in Sheet Metal Forming Process on the Service Performance of 22MnB5 Steel

The use of ultra-high strength steels through sheet metal forming process offers a practical solution to the lightweight design of vehicles.However,sheet metal forming process not only produces desirable changes in material properties but also causes material damage that may adversely influence the service performance of the material formed.Thus,an investigation is conducted to experimentally quantify such influence for a commonly used steel（the 22MnB5 steel） based on the hot and cold forming processes.For each process,a number of samples are used to conduct a uniaxial tensile test to simulate the forming process.After that,some of the samples are trimmed into a standard shape and then uniaxially extended until fracture to simulate the service stage.Finally,a microstructure test is conducted to analyze the microdefects of the remaining samples.Based on the results of the first two tests,the effect of material damage on the service performance of 22MnB5 steel is analyzed.It is found that the material damages of both the hot and cold forming processes cause reductions in the service performance,such as the failure strain,the ultimate stress,the capacity of energy absorption and the ratio of residual strain.The reductions are generally lower and non-linear in the former process but higher and linear in the latter process.Additionally,it is found from the microstructure analysis that the difference in the reductions of the service performance of 22MnB5 by the two forming processes is driven by the difference in the micro damage mechanisms of the two processes.The findings of this research provide a useful reference in terms of the selection of sheet metal forming processes and the determination of forming parameters for 22MnB5.

Microstructure Evolution and Simulation in 22MnB5 Steel during Hot Stamping

Hot stamping components with 1500 MPa ultra-high strength are obtained by press hardening during hot stamping, and the properties depend on the microstructures. It is very important that the microstructure evolution rule is found out during hot stamping process. To characterize the microstructure evolution during hot stamping, a method combining finite element and experiment is carried out. Samples were heated to 950°C and held for 300 second at a induction heating furnace, then taken out from the furnace and stayed in the air at different time (7 s, 11 s, 13 s, 22 s), respectively, finally the specimens were formed and quenched at a die. Microstructural observation as well as surface hardness profiling of formed specimens was performed. And the numerical simulation to predict the austenite transformation into ferrite, pearlite, bainite, and martensite and the volume fraction of each phase during the hot stamping process was made with ABAQUS software. The results show that the ferrite is observed when the specimen stays in the air for 22 s, and the temperature drops to 325°C when the dwell time increases from 7 s to 22 s. The results of numerical simulation and experimental results are in good agreement. So the method finite element can be used to guide the optimization of hot stamping process parameters.

22MnB5超高强钢热冲压成形工艺及试验

考虑材料的热物理性能参数、力学性能与温度的关系,利用ABAQUS软件建立了22MnB5超高强钢热冲压过程的热力耦合有限元模型,选用合适的热冲压工艺参数进行数值分析,得到了坯料热冲压成形的应力应变分布,并以板料初始温度为变量,研究不同初始温度对零件厚度分布、回弹及冷却速率的影响。进行了板料初始温度为900℃的22MnB5超高强钢热冲压试验,零件厚度分布及回弹量与模拟结果基本吻合,各区域淬火组织均为板条状马氏体,由于零件底部的淬火冷却速率较大,马氏体组织更加均匀细小。

22MnB5超高强钢焊接组织与性能

采用钨极氩弧焊方法,实现了22MnB5超高强钢淬火前后同种材料之间的连接,进行了焊接接头拉伸试验和热影响区内显微组织及硬度分布试验,并对焊接前后材料的抗腐蚀性能进行了试验分析.结果表明,淬火处理后22MnB5组织结构从铁素体+珠光体组织转变为马氏体组织,显微硬度与抗拉强度大幅度提高,原始及淬火后材料的焊接接头具有良好的力学性能和组织形貌,焊后焊接接头抗拉强度和硬度略低于原始母材,经淬火处理后的材料焊后抗拉强度达到1 179.59 MPa,略低于淬火材料但比原始母材高很多,淬火热处理材料的腐蚀速率略有上升,而焊接后将大幅度增加腐蚀速率,通过比较淬火焊缝钢腐蚀速率最大.

高强度22MnB5钢板加热过程中组织演变规律的研究

对高强度22MnB5钢板采用快速电阻加热方法,并与传统炉子加热作比较,研究了钢板在加热过程中的组织演变规律。以炉子加热(～10℃/s)和电阻加热(～100℃/s)两种速率把试样加热到不同温度、保温不同时间后水淬,观察试样的显微组织并测试力学性能。结果表明,22MnB5钢板使用传统炉子加热时其过热度为70℃左右,电阻加热的过热度比炉子加热时增加了约50℃,加热温度达到各自临界转变温度后淬火组织为马氏体和少量铁素体。使用炉子加热到相应温度后保温一段时间,转变的奥氏体量比未保温的多10%左右。钢板电阻加热并淬火后其抗拉强度为1 536.4 MPa,延伸率为6.8%,达到超高强度钢板要求。

加热速率对22MnB5超高强度钢奥氏体化的影响

在Gleeble 3500型热力模拟试验机上对22MnB5超高强度钢在不同加热速率(10~100℃·s^(-1))下进行了奥氏体化,研究了加热速率对奥氏体化温度Ac1,Ac3及奥氏体转变时间的影响,并建立了加热速率与Ac1的关系式。结果表明:该钢的Ac1与Ac3都随着加热速率的增大而升高;当加热速率低于25℃·s^(-1)时,Ac1随加热速率的增大快速升高,当高于25℃·s^(-1)时,Ac1缓慢升高并趋向恒定;随着加热速率的提高,奥氏体化温度区间(Ac3~Ac1)大小呈升高趋势,奥氏体转变时间缩短。

热冲压用超高强钢22MnB5性能测试与分析

为测试邯钢新开发的热冲压用超高强钢22MnB5的力学性能并验证其是否满足热冲压使用需求,选取了国外某知名钢厂热冲压用超高强钢22MnB5进行了对比实验。实验结果证明,邯钢产品的性能指标接近或部分优于对照产品的指标,可以进行热冲压生产。实验中得到的实验数据,可以为该产品后续市场推广提供技术支撑。

NUMERICAL SIMULATION OF DEFORMATION BEHAVIOR OF 22MnB5 BORON STEEL AT ELEVATED TEMPERATURES AND EXPERIMENTAL VERIFICATION

The effects of strain, strain rate and temperature on the mechanical behavior of 22MnB5 boron steel deformed isothermally under uniaxial tension tests and the experimental characterization of 22MnB5 boron steel in the austenitic region have been investigated. Based on the crystal plasticity theory and thermal kinematics, an improved integration model is presented. In this model, the elastic deformation gradient is the integration variable of the governing equation, which contains not only the elastic deformation but also the thermal effects. In the coupled thermo- mechanical process, this model can reveal the evolution of microstructures such as the rotation of a single crystal and the slip systems in each of them. The plastic behavior of the boron steel can be well described by the presented model.

超高强钢热成形奥氏体化加热参数的优化

以汽车用高强钢22MnB5为研究对象,采用金相显微镜、激光共焦扫描显微镜、洛氏硬度仪等分析手段对热成形中加热参数进行研究。结果表明:对于2mm厚的22MnB5板材在热成形过程中,奥氏体化最佳的加热温度为950℃,保温时间为5min。采用优化后的加热工艺参数进行汽车防撞梁热成形试验,试验件的微观组织为均匀的马氏体组织,抗拉强度达到1500MPa以上,显微硬度在450HV以上,完全满足实际要求。

汽车超高强钢防撞梁热成形试验

以汽车用超高强度钢22MnB5为研究对象,采用直接热成形工艺和间接热成形工艺进行汽车防撞梁成形试验。采用金相显微镜、显微硬度仪和万能拉伸试验机等分析测试手段对成形件的组织、力学性能进行了分析。探讨了直接、间接热成形工艺对成形组织及力学性能的影响。结果表明,成形件的微观组织为理想的板条状马氏体,抗拉强度达到1500MPa以上,硬度在450HV以上,完全满足生产要求,实际生产中可以根据情况选择合适的成形工艺。