温成形中锰钢的组织性能评价

 热成形零件已在汽车安全件上广泛应用,为了进一步提升零件碰撞安全性、提高表面质量、降低成本,基于中锰钢提出了一种降低加热温度的热成形技术,通过将完全奥氏体化的中锰钢在模具中淬火成马氏体组织获得超高强度力学性能,与22MnB5钢热成形相比,在获得1 500 MPa抗拉强度时,中锰钢温成形的加热温度可降低150 ℃以上,断后伸长率提高30%以上,同时提高零件的表面质量。综述并评价了中锰钢经温成形后的微观组织与力学性能以及冷弯性能、成形性能、电阻点焊等工艺性能,并与22MnB5钢热成形进行了系统地比较,体现出温成形中锰钢节能环保、提高碰撞安全性的技术优势。     

热成形加热速率对22MnB5钢组织和性能的影响

22MnB5是常见的热成形高强钢，广泛应用于汽车车身，可减轻车身质量，提高汽车抗冲击和防温性。随着用户对热成形零件生产效率要求的不断提高，特别是感应力加热，火焰直燃加热等快速加热技术的推广应用，热减形加热速率快速提升到120℃/s。为了考察不同加热速率对22MnB5钢热成形后的组织和性能的影响规律，利用淬火膨胀仪、扫描电子显微镜(SEM)和拉伸试验机，分别对加热冷却过程中的热膨胀曲线、热成形后的微观组织和力学性能进行了测定。结果表明，随加热温度升高，试样冷却组织由铁素体向马氏体、贝氏体转变，且150℃/s加热速率下组织中贝氏体和铁素体含量要高于10℃/s加热速率的试样；且当加热速率从10℃/s提高到150℃/s时，22MnB5钢在950℃仍然能够完全奥氏体化，但是冷却试样的条件屈服强度、抗拉强度、断后伸长率都有所下降，分别降低了334 MPa、56 MPa、0.9%。     

加热温度对热成形中锰钢氢脆敏感性的影响

对0.1C–5Mn中锰钢在不同温度(850、950和1000℃)加热后进行热成形处理,利用电化学预充氢、慢应变速率拉伸及氢渗透实验等研究了加热温度对其氢脆敏感性的影响.结果表明,试验钢在不同温度加热后进行热成形处理,其组织全部为马氏体,同时因自回火而生成一定量的ε-碳化物,且随着加热温度的升高,原奥氏体晶粒尺寸增加,而试验钢的强度和塑性逐渐降低.当加热温度为850℃时获得了较好的强度与塑性配合,强塑积为22 GPa·%.随着加热温度升高,充氢样中的可扩散氢含量明显降低而非可扩散氢含量有所增加,而以相对缺口抗拉强度损失表征的氢脆敏感性指数及有效氢扩散系数呈现先升高后显著降低的变化趋势,当加热温度为1000℃时,氢脆敏感性最低.进一步断口分析表明,试验钢充氢断口起裂区均为沿着原奥氏晶界的沿晶断裂.试验钢的这种氢脆断裂行为主要与热成形中锰钢的强度水平及自回火析出的ε-碳化物有关.与常用的传统热成形钢22MnB5相比,试验钢的氢脆敏感性较高,这主要与其M_s点(马氏体转变开始温度)较低而使得自回火程度较低等有关.     

Ti-V-Mo微合金化22MnB5钢中析出相及其强化作用

使用温成形替代热成形可以避免热成形过程中表面氧化等问题,但热成形常用22MnB5钢在高温回火后出现明显的软化现象.而通过向钢中添加Ti、V、Mo等微合金元素可以在钢中形成细小的析出相以及细化晶粒,起到提高强度的作用,从而可以解决该问题.因此,通过在22MnB5钢中添加Ti、V、Mo微合金元素,利用OM(光学显微镜)、F...     

38MnB5热成形钢高温变形行为及本构方程

利用Gleeble-3500热模拟试验机对38MnB5热成形钢的高温变形行为进行研究, 分别在650~950℃温度区间内, 以0.01、0.1、1和10 s-1的应变速率对其进行等温单向拉伸测试, 并得到相应条件下的真应力-应变曲线.结果表明: 38MnB5热成形钢流变应力随着变形温度的升高而减小, 随着应变速率的增大而增大.当应变速率逐渐增加时, 热变形时发生的动态回复和动态再结晶效果并不显著, 而当温度逐渐升高时, 二者作用逐渐加强.考虑了温度、应变速率和应变的综合复杂影响, 建立38MnB5热成形钢高温下的本构方程.此本构方程通过对流变应力、应变、应变速率等实验数据的回归分析, 得到与变形温度、应变速率和应变相关的材料参数多项式.计算结果与实验结果对比发现, 通过本构方程所获得的计算值与试验值吻合良好.      

变强度22MnB5热成型钢热处理工艺研究

利用Gleeble-3500型热模拟试验机研究了冷却速率和等温温度对22MnB5热成型钢的力学性能和组织转变的影响.结果表明,将22MnB5由室温加热到930℃保温5 min后,以2℃/s冷却到500～550℃范围内进行等温淬火,显微组织为铁素体和珠光体,屈服强度为430～450 MPa,抗拉强度为650～680 MP...     

热冲压成形钢22MnB5焊点断裂模式的研究

研究了不同焊接工艺条件下热冲压成形钢22MnB5焊点断裂的模式,结果表明,22MnB5点焊接头的断裂模式和结合强度受点焊过程中出现的3种不同类型的缺陷模式影响,即:结合线伸入、缩孔和内部飞溅。     

热处理工艺对22MnB5车用超高强度马氏体钢的组织性能影响

本试验使用金相显微镜、拉伸试验仪,配合Jmatpro及Imageproplus等软件对22MnB5超高强度马氏体钢板材在不同温度下的水淬组织以及相关力学性能进行了测试分析。结果表明:22MnB5综合性能在920℃时最优,马氏体转变量在860℃时达到最值,马氏体转变量与22MnB5的力学性能无直接相关性,且水淬性能优于油淬性能。综合考量,1.2mm厚22MnB5钢板材的最佳淬火工艺参数为:920℃保温后水淬至室温。     

超高强度热冲压用钢中H的扩散与氢致滞后开裂行为

采用阴极充H、恒载荷拉伸和电化学H渗透等试验方法,研究了超高强度钢22MnB5Nb的H扩散行为及氢致滞后开裂性能,并与常用热冲压钢22MnB5进行了对比。结果表明,H在22MnB5Nb钢中的扩散系数为3.02×10-7 cm2/s,显著低于22MnB5钢;与22MnB5钢相比,22MnB5Nb钢具有较好的耐氢致滞后开裂性能;这是由于22MnB5Nb钢晶粒较细小,增加了晶界的有效面积,使H陷阱分布更均匀,进而抑制H向裂纹尖端扩展,避免了局部H的富集。     

26MnB5钢的动态再结晶行为

利用Gleeble-1500热模拟压缩试验获得了26MnB5钢在880～1000℃、0.01～10s-1、最大变形55％条件下的真应力-真应变曲线,研究了26MnB5钢在试验条件下的动态再结晶行为.结果表明:26MnB5的真应力-真应变曲线在高温、低应变速率条件下出现明显峰值点特征,意味着样品发生了动态再结晶;26MnB5再结晶程度和奥氏体晶粒均匀度随温度的增加或应变速率的降低而提高,而晶粒平均尺寸则表现出先减小后增大的趋势;利用Johnson-Mehl-Avrami(JMA)方程可以建立26MnB5钢动态再结晶动力学模型,模型预测值与实测值基本吻合.     

一种Si-Mn系超高强度钢板的热冲压成形试验研究

 采用弯曲件热冲压成形试验研究了板料加热温度、保温时间及移送时间等工艺参数对一种Si-Mn系超高强度钢板热成形零件的力学性能及微观组织的影响规律。结果表明,通过控制热成形工艺参数,在所设计的模具上可实现Si-Mn系超高强度钢板热成形零件的有效淬火,在合适的工艺参数下,可获得细小均匀的马氏体组织,从而获得抗拉强度1700MPa以上,伸长率10%以上的性能,达到原始板料抗拉强度的3倍左右,并明显高于传统的Mn-B系超高强度钢板。     

A Numerical and Experimental Investigation into Hot Stamping of Boron Alloyed Heat Treated Steels

Hot stamping of steel sheets using water or nitrogen cooling media was studied on a laboratory scale. Sheets of grade 22MnB5 boron steels in three different thicknesses were investigated and the results of experimental hot stamping tests were considered. Microstructural analysis, linear and surface hardness profiling as well as tensile tests of formed samples were carried out. After hot stamping, mostly fully martensitic microstructures, which yield ultra high strength levels, were produced. It is concluded that die cooling media, i.e., water or nitrogen, have a significant effect on material properties after hot stamping. Using liquid nitrogen as coolant in the punch instead of water increases yield strength by 50 to 65MPa. Moreover, the evolution of the temperature and force during the hot stamping process was simulated by using a coupled thermomechanical FEM program. The results of numerical simulation and experimental results are in good agreement.     

Modeling of Microstructure Evolution in 22MnB5 Steel during Hot Stamping简

Automobile manufacturers have been inereasingl~r adopting hot-stamped parts for use in newly designed ve- hicles to improve crash worthiness and fuel efficiency. However, the simulation of hot stamping is rather complex and challenging, and further research still needs to be done on hot stamping hardening mechanism. The microstruc- ture evolution and hardening mechanisms during hot stamping of 22MnB5 steel were thoroughly investigated, using information provided in the literatures as well as experimental results. New models were developed to predict the grain growth during heating and the flow stress of a manganese boron steel （22MnB5） with high hardenability by the Gleeble simulation experimental results. The deformed austenite decomposition during stamping and quenching was emphatically quantified based on the transformation thermodynamic and kinetic theories, and the relationship of mi- crostructure to properties was analyzed. The results showed that the optimal process to obtain homogeneous and small lath martensite is heating at 900--950 ℃ for 5 min and then auenching at 50 ℃/s with a Dressing time about 8 s.     

Analysis of Microstructure and Mechanical Properties of Different Hot Stamped B‐bearing Steels

Hot stamping is a technique to produce ultra high strength automobile components. The common material used in hot stamping process is coated and/or uncoated 22MnB5 boron alloyed steel. Ferritic‐pearlitic microstructure in as‐delivered sheets is transformed to fully lath martensitic after hot stamping. In the present research, hot stamping under water or nitrogen cooling media was investigated using different boron alloyed steel grades. Microstructural analyses, linear and surface hardness profiling as well as tensile tests of hot stamped samples were performed. Various microstructures of fully bainitic and/or fully martensitic were produced. The resulting microstructures provided yield strengths of 650–1370 MPa and tensile strengths of 850–2000 MPa. There is an optimum carbon equivalent content for which the highest formability index value, UTS × A₂₅, is achieved. Using a nitrogen cooled punch resulted in higher yield strength without significant changes in ultimate tensile strength. It is concluded that a wide range of B‐bearing steels having an extended carbon equivalent range with an acceptable formability index value can be used by increasing the cooling rate in the die assembly.     

回火温度对淬火后30MnB5热成形钢组织与性能影响

将30MnB5热成形钢进行淬火和回火处理,利用扫描电镜、透射电镜、能谱仪和拉伸性能检测等方法研究了不同回火温度后的显微组织和力学性能变化.经200℃保温2 min回火后热成形钢的综合力学性能最佳,抗拉强度为1774 MPa,总伸长率为8%,强塑积达14 GPa·%以上,该性能满足热成形后作为汽车结构件的使用要求;并且随着回火温度的升高,力学性能呈非单调性变化.200℃低温回火后,主要为板条马氏体和ε碳化物,位错密度略有降低,析出的ε碳化物粒子呈针状分布在马氏体板条内,长度方向大小为100 nm左右,并与位错发生钉扎作用.随着回火温度的升高,板条马氏体发生回复和再结晶,板条边界逐渐模糊,并向等轴状铁素体转变,位错密度显著降低,ε碳化物逐渐向低能态的近球形渗碳体转变并粗化至200 nm左右,对位错的钉扎作用也随之减弱.      

激光拼焊异种超高强钢组织和力学性能

 汽车轻量化后对安全性和碰撞吸能性提出了更高要求,从而促进了高强、吸能材料及其拼接技术的发展。以汽车安保件之一的汽车B柱为研究对象,采用能满足要求的DP980双相钢和22MnB5热冲压成型钢异种材料进行激光拼焊,研究焊接热输入对焊接接头显微组织与力学性能的影响。通过保持激光输出功率不变(1.3 kW)改变焊接速度的方法控制焊接热输入,考察焊接热输入与拼焊接头组织和力学性能之间的关系。利用光学显微镜、扫描电子显微镜、显微硬度测试仪和拉伸试验机研究接头不同亚区的组织和性能。结果表明,当焊接速度为16~26 mm/s时,均获得了完整而无缺陷的熔化区组织;随着焊接速度的提高,不仅焊缝表面凹陷逐步改善,并且焊接热影响区宽度也随之减小。硬度测试表明,接头中存在明显的软化区域,主要分布在DP980侧热影响区的回火区和不完全相变区,而DP980侧热影响区的细晶区、粗晶区、22MnB5侧热影响区以及焊缝金属区的硬度则有所增加,形成了焊接接头的硬化区。拼焊接头在能形成完整接头的条件下抗拉强度保持为576~597 MPa,断裂均发生在22MnB5侧的母材区,断裂时有明显的颈缩现象;接头断后伸长率为11.9%~15.5%,介于DP980母材(11%)和22MnB5(22%)母材的断后伸长率之间;研究还表明,焊接热输入越大,焊接接头相同区域的组织越粗大。     

Determination of Thermal and Mechanical Material Properties of Ultra‐High Strength Steels for Hot Stamping

Direct and indirect hot stamping presently constitutes one of the most innovative forming technologies in the automotive industry through the combination of forming and hardening in one process step or line. Thus, structural components with strength up to 1600 MPa can be accomplished with the quench hardenable ultra‐high strength steel 22MnB5. With respect to the numerical investigation of the feasibility of different parts the knowledge of various thermal and mechanical material characteristics determined under process relevant conditions are required. Within the scope of this paper different experimental methods will be introduced for the determination of material properties according to the typical time‐temperature characteristics of the hot stamping process, as well as the modelling of it as input data for the FE analysis.