热冲压成形工艺参数及冷却系统的研究
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摘要
超高强度钢在汽车上的应用，不但实现了车体轻量化，减少了燃油的排放，同时其超高的强度也提高了车体的弯曲刚性和扭转刚性，保证了整车的安全性，因此超高强度钢在汽车制造领域的应用越来越广泛。但是随着材料强度的提高，其冲压性能下降，常规的冷冲压工艺很难实现其成形。热冲压成形是一项专门用于成形超高强度钢板的新技术，热冲压成形技术的原理是将常温下强度为500MPa—600MPa的高强度硼合金钢板加热到奥氏体化温度以上，保温若干分钟，使其完全奥氏体化，然后送入内部带有冷却水道的热冲压成形模具内成形，保压的同时钢板被模具表面冷却淬火，把高温的奥氏体转变成板条马氏体，发生相变强化，强度大幅提高，可以达到1500MPa左右。
本文首先以厚度为2mm的22MnB5硼钢为研究对象，采用金相显微镜、激光共焦扫描显微镜、扫描电子显微镜、拉伸试验机和显微硬度仪等设备研究了超高强度钢热冲压成形中奥氏体化加热温度和保温时间对材料晶粒大小、显微组织、平均显微硬度和抗拉强度的影响。试验结果表明，对于厚度为2mm的22MnB5钢板，最佳的奥氏体化加热参数为900℃—950℃、5min。
将有限元模拟和试验研究相结合，以热冲压成形模具中的冷却系统为研究对象，分析了冷却水道几何参数对工件冷却效果的影响。研究结果认为，在满足模具强度、制造成本和加工可行性等要求的情况下，采用大水道直径、小水道侧壁间距和小水道顶部距工件距离的几何参数，可以使水道中水流流速更加均匀，冷却效率更高，冷却效果更好。冷却水道直径越大、水道侧壁间距越小及水道顶部距工件距离越小，马氏体转变越完全，马氏体板条束越细小，材料的显微硬度和抗拉强度等力学性能越好。
The applications of ultra-high strength steel for automotive, not only lightened the carbody and reduced the fuel emissions, but also improved the bending rigidity and torsionalrigidity to ensure the safety of the whole vehicle. So that, ultra-high strength steel was usedmore widely in automobile manufacturing field. But it is hard to use traditional coldstamping process to form this type of steel, due to the high strength and its bad stampingproperty. Hot stamping is a new technology, especially used for forming ultra-high strenghsteel. The principle of hot stamping is that the boron alloy steel plate which tensile strengthis500MPa-600MPa at room temperature is heated to austenitizing temperature for a fewminutes, making sure that the microstructure is austenitized compeletly. And then send theplate into hot stamping tools with cooling channels inside, which can form the plate, at thesame time, the steel plate is quenched by the surface of the die. The high temperatureaustenite changed into lath martensite, which caused the strength greatly enhanced. Thestrength can reach to1500MPa or so.
First of all, ultra high strength steel22MnB5with the thickness of2mm adopted forautomobile was selected for this study with the aid of metallographic microscope, Laserconfocal scanning microscope, SEM, tensile test enginery and microhardness tester, whichare aiming for analyzing the effect of the heating parameters of austenization on the materialgrain size, microstructures, average microhardness and tensile strength of ultra high strengthsteel. The tested results exhibited that the optimal heating parameters of austenization for22MnB5steel with the thickness of2mm were the temperature between900℃and950℃and the heating time of5min.
Through finite element modeling and experimental research, we used the cooling system of the hot stamping tools for this study and studied the effect of the parameters of thecooling pipes on the workpiece’s cooling effect. The results concluded that when we couldsatisfy the requirements such as the die strength, manufacturing costs and processingfeasibility, the using of the cooling channel geometry parameters of the large diameter, smallsidewall spacing and small distance between the cooling pipe and the workpiece could makethe flow rate of cooling water in the pipe more uniform, cooling efficiency higher andcooling effect better. When we used the larger diameter pipe, smaller sidewall spacing andsmaller distance between the cooling pipe and the workpiece, the martensitic transformationwas more complete, the martensite lath was much smaller, and the mechanical propertiessuch as micro hardness and tensile strength were much better.
本文首先以厚度为2mm的22MnB5硼钢为研究对象，采用金相显微镜、激光共焦扫描显微镜、扫描电子显微镜、拉伸试验机和显微硬度仪等设备研究了超高强度钢热冲压成形中奥氏体化加热温度和保温时间对材料晶粒大小、显微组织、平均显微硬度和抗拉强度的影响。试验结果表明，对于厚度为2mm的22MnB5钢板，最佳的奥氏体化加热参数为900℃—950℃、5min。
将有限元模拟和试验研究相结合，以热冲压成形模具中的冷却系统为研究对象，分析了冷却水道几何参数对工件冷却效果的影响。研究结果认为，在满足模具强度、制造成本和加工可行性等要求的情况下，采用大水道直径、小水道侧壁间距和小水道顶部距工件距离的几何参数，可以使水道中水流流速更加均匀，冷却效率更高，冷却效果更好。冷却水道直径越大、水道侧壁间距越小及水道顶部距工件距离越小，马氏体转变越完全，马氏体板条束越细小，材料的显微硬度和抗拉强度等力学性能越好。
The applications of ultra-high strength steel for automotive, not only lightened the carbody and reduced the fuel emissions, but also improved the bending rigidity and torsionalrigidity to ensure the safety of the whole vehicle. So that, ultra-high strength steel was usedmore widely in automobile manufacturing field. But it is hard to use traditional coldstamping process to form this type of steel, due to the high strength and its bad stampingproperty. Hot stamping is a new technology, especially used for forming ultra-high strenghsteel. The principle of hot stamping is that the boron alloy steel plate which tensile strengthis500MPa-600MPa at room temperature is heated to austenitizing temperature for a fewminutes, making sure that the microstructure is austenitized compeletly. And then send theplate into hot stamping tools with cooling channels inside, which can form the plate, at thesame time, the steel plate is quenched by the surface of the die. The high temperatureaustenite changed into lath martensite, which caused the strength greatly enhanced. Thestrength can reach to1500MPa or so.
First of all, ultra high strength steel22MnB5with the thickness of2mm adopted forautomobile was selected for this study with the aid of metallographic microscope, Laserconfocal scanning microscope, SEM, tensile test enginery and microhardness tester, whichare aiming for analyzing the effect of the heating parameters of austenization on the materialgrain size, microstructures, average microhardness and tensile strength of ultra high strengthsteel. The tested results exhibited that the optimal heating parameters of austenization for22MnB5steel with the thickness of2mm were the temperature between900℃and950℃and the heating time of5min.
Through finite element modeling and experimental research, we used the cooling system of the hot stamping tools for this study and studied the effect of the parameters of thecooling pipes on the workpiece’s cooling effect. The results concluded that when we couldsatisfy the requirements such as the die strength, manufacturing costs and processingfeasibility, the using of the cooling channel geometry parameters of the large diameter, smallsidewall spacing and small distance between the cooling pipe and the workpiece could makethe flow rate of cooling water in the pipe more uniform, cooling efficiency higher andcooling effect better. When we used the larger diameter pipe, smaller sidewall spacing andsmaller distance between the cooling pipe and the workpiece, the martensitic transformationwas more complete, the martensite lath was much smaller, and the mechanical propertiessuch as micro hardness and tensile strength were much better.
引文
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[2] P.AKERSTROM，B.WIKMAN， M.MATERIAL. Material parameter estimation forboron steel from simultaneous cooling and compression experiments[J]. Modelling andSimul. Mater. Sci. Eng.，2005，13：1291-1308.
[3] D.KUHN. Hot forming makes higher strength[J]. Modern Metal Processing，2006，11：24-25.
[4]唐靖林，曾大本.面向汽车轻量化材料加工技术的现状及发展[J].金属加工，2009(11)：11-13.
[5]马鸣图.先进汽车用钢[M].北京：化学工业出版社，2007：18-130.
[6]江海涛，唐荻，米振莉.汽车用先进高强度钢的开发及应用进展[J].钢铁研究学报，2007，19(18)：1-6.
[7] HEINZ STEINBEISS， HYUNWOO SO. Method for optimizing the cooling design ofhot stamping tools[J]. Production Process，2007，1：149-155.
[8] M.NADERI， M.KETABCHI. Analysis of microstructure and mechanical properties ofdifferent high strengh carbon steels after hot stamping[J]. Journal of materialsProcessing Technology，2011，211：1117-1125.
[9] B.ABDULHAY，B.BOUROUGA，C.DESSAIN. Experimental and theoretical study ofthermal aspects of the hot stamping process[J]. Applied Thermal Engineering，2011，31(5)：674-685.
[10] T.TR STER，W.ROSTEK. The International Conference New Development in SheetMetal Forming Technology，2004[C]. Germany： Stuttgart，2004.
[11] M.NADERI，M.KETABCHI，M.ABBASI. Analysis of microstructure and mechanicalproperties of different high strength carbon steels after hot stamping[J]. Journal ofMaterials Processing Technology，2011，211(6)：1117-1125.
[12] M.MERKLEIN， J.LECHLER， M.GEIGER. Characterization of the Flow Propertiesof the Quenchenable Ultra High Strength Steel22MnB5[J]. CIRPAnnals-Manufacturing Technology，2006，55(1)：229-232.
[13] QIANG WU， JINKE GONG. Research on laser welding of vehicle body[J].Optics&laser Techonogy，2008，40：420-426.
[14]谷诤巍，单忠德，徐虹.汽车高强度钢板冲压件热成形技术研究[J].模具工业，2009，35(4)：27-29.
[15]谷诤巍，姜超，单忠德.超高强度钢板冲压件热成形工艺[J].汽车工艺与材料，2009，4：15-17.
[16] JUNYING MIN， JIANPING LIN， JIAYUE LI. Investigation on hot forming limits ofhigh strength steel22MnB5[J]. Computational Materials Science，2010，49(2)：326-332.
[17] KOLLECK R， VEIT TEST R. Investigation on induction heating for hot stamping ofboron alloyed steels[J]. CIRP Annals-Manufacturing Technology，2009，58：275-8.
[18] LIU HS， XING ZW， BaO J. Investigation of the hot-stamping process for advancedhigh-strength steel sheet by numerical simulation[J]. J Mater Eng Perform，2010，19：325–34.
[19] P.F.BARIANI, S.BRUSCHI, A.GHIOTTI. Testing formability in the hot stamping ofHSS[J]. CIRP Annals-Manufacturing Technology,2008,57(1):265-268.
[20]徐虹，孟佳，谷诤巍.22MnB5钢热冲压高温防氧化涂层[J].汽车技术，2011，(12)：48-51.
[21]庄奕玲，张齐健.汽车涂层耐老化性能的研究[J].汽车技术，2004，(7)：26-30.
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