【摘要】：采用(超)高強度鋼板沖壓件制造車身零件不僅可以減輕車身重量,降低油耗,而且可以確保、提高車的安全性,是同時實現(xiàn)車體輕量化和提高碰撞安全性的最佳途徑。但(超)高強鋼板在常溫下沖壓易開裂、回彈嚴(yán)重。尤其是當(dāng)強度達到1500MPa時,常規(guī)的冷沖壓成形工藝幾乎無法成形。因此,如何實現(xiàn)高強度鋼板的高精度沖壓成形就成為一項緊迫需要解決的技術(shù)難題。超高強度硼鋼板的熱沖壓新工藝被認(rèn)為是解決上述難題的有效方法。其工藝原理是：硼鋼板加熱,使其充分奧氏體化(900～950℃)后,迅速轉(zhuǎn)移到壓力機上,利用配有冷卻系統(tǒng)的模具成形后,在模具中冷卻淬火獲得馬氏體組織,淬火后鋼板的抗拉強度達到1500MPa左右甚至更高。本文主要是圍繞超高強硼鋼熱沖壓成形工藝的工程科學(xué)問題和技術(shù)難題開展研究。通過Gleeble-3500熱模擬試驗機上進行熱膨脹試驗。研究了加熱速率、加熱溫度和保溫時間對硼鋼高溫成形性能的影響規(guī)律,研究表明：在保證生產(chǎn)節(jié)拍的前提下,經(jīng)過快速加熱曲線加熱的試件高溫成形性能最好；加熱溫度對硼鋼高溫下成形的性能有比較明顯的影響；保溫時間則對硼鋼高溫下的峰值應(yīng)力影響顯著,對極限應(yīng)變影響不大�；趭W氏體形核長大理論,考慮加熱速率的影響,建立非等溫條件下硼鋼HC1500HS統(tǒng)一奧氏體相變動力學(xué)模型。應(yīng)用Matlab進化算法工具箱對模型中的材料常數(shù)進行求解,獲得的統(tǒng)一相變動力學(xué)模型能夠準(zhǔn)確的預(yù)測硼鋼奧氏體在非等溫加熱過程中的熱膨脹曲線及奧氏體體積百分?jǐn)?shù)。基于響應(yīng)面法建立了熱沖壓零件的抗拉強度、屈服強度和延伸率的響應(yīng)面模型,考慮的工藝參數(shù)有加熱溫度(800-1000℃)、保溫時間(60-540 s)、成形溫度(560-800℃)以及模具溫度(20-220℃)。在獲得響應(yīng)面模型的基礎(chǔ)上研究各個工藝參數(shù)對熱沖壓零件機械性能的影響規(guī)律,并采用NSGA-Ⅱ多目標(biāo)進化算法對工藝參數(shù)進行了優(yōu)化,獲得了具有良好綜合力學(xué)性能的熱沖壓零件。優(yōu)化結(jié)果將為熱沖壓工藝參數(shù)選擇提供實驗依據(jù)和理論指導(dǎo)。設(shè)計加工了一套圓臺裝置用于測定硼鋼淬火過程中板料和模具在熱沖壓保壓淬火過程中的溫度曲線。計算得到模具與硼鋼之間的界面換熱系數(shù)。并研究了壓力和氧化皮厚度對界面換熱系數(shù)的影響規(guī)律。該研究為熱沖壓冷模具淬火過程中板料與模具溫度的計算提供理論依據(jù),為準(zhǔn)確計算熱沖壓過程中的相變提供了數(shù)據(jù)基礎(chǔ)。在Gleeble-3500熱模擬實驗機上進行了硼鋼HC1500HS的熱膨脹實驗,研究了硼鋼HC1500HS連續(xù)冷卻中冷卻速度和變形對過冷奧氏體相變過程的影響,確定了該鋼組織轉(zhuǎn)變的臨界冷卻速度,制定了硼鋼HC1500HS的動態(tài)奧氏體連續(xù)冷卻轉(zhuǎn)變曲線(DCCT曲線)�；诤辖馃崃W(xué)理論,建立了硼鋼HC1500HS的非等溫條件下鐵素體和貝氏體的相變動力學(xué)模型,獲得的模型能夠很好的預(yù)測硼鋼在不同冷卻速度和變形程度下的轉(zhuǎn)變產(chǎn)物。建立了車門防撞梁熱沖壓及冷模具淬火過程的有限元模型,進行熱沖壓過程中零件微觀組織的有限元模擬,得到了車門防撞梁熱沖壓過程中的板料溫度、微觀組織及維氏硬度的分布特征,并通過實驗驗證熱力相變耦合的有限元模型的數(shù)值計算有效性,實現(xiàn)了熱沖壓零件微觀組織演化過程有限元預(yù)測,以控制熱沖壓零件的機械性能。利用有限元模型研究了成形溫度,模具溫度,保壓壓力和保壓時間對防撞梁熱沖壓零件的微觀組織的影響規(guī)律。
[Abstract]:Using (ultra) high strength steel sheet to make body parts can not only reduce body weight and fuel consumption, but also ensure and improve vehicle safety. It is the best way to realize lightweight and improve collision safety at the same time. The conventional cold stamping process is almost impossible to form. Therefore, how to achieve high-precision stamping of high-strength steel sheet becomes an urgent technical problem to be solved. After austenitizing (900-950 C), it is transferred to the press quickly. After forming with a cooling system, the martensite structure is obtained by cooling and quenching in the die. The tensile strength of the quenched steel sheet is about 1500MPa or even higher. This paper mainly focuses on the engineering scientific problems and technical difficulties of hot stamping forming process of ultra-high strength boron steel. The effects of heating rate, heating temperature and holding time on the high temperature formability of boron steel were studied by the thermal expansion test on Gleeble-3500 thermal simulator. Based on the austenite nucleation and growth theory and considering the effect of heating rate, a unified austenite transformation kinetics model of HC1500HS for boron steel under non-isothermal conditions is established. The model is progressed by using MATLAB. The material constants in the model are solved by the algorithm toolbox, and the unified phase transformation kinetic model can accurately predict the thermal expansion curve and austenite volume percentage of boron steel austenite during non-isothermal heating process. The model considers the process parameters such as heating temperature (800-1000 C), holding time (60-540 s), forming temperature (560-800 C) and die temperature (20-220 C). Based on the response surface model, the influence of each process parameter on the mechanical properties of hot stamping parts is studied, and the process parameters are processed by NSGA-II multi-objective evolutionary algorithm. The optimized results will provide experimental basis and theoretical guidance for the selection of hot stamping process parameters. A set of round table device is designed and manufactured to measure the temperature curves of sheet metal and die during hot stamping and pressure-holding quenching of boron steel. The interfacial heat transfer coefficient between boron steel and the influence of pressure and oxide thickness on the interfacial heat transfer coefficient are studied. The study provides a theoretical basis for calculating the temperature of plate and die during quenching of hot stamping cold die, and provides a data basis for accurately calculating the phase transformation in hot stamping process. The thermal expansion test of boron steel HC1500HS was carried out. The effect of cooling rate and deformation on the transformation process of supercooled austenite during continuous cooling of boron steel HC1500HS was studied. The critical cooling rate of microstructure transformation of the steel was determined. The dynamic Austenite Continuous Cooling Transformation Curve (DCCT curve) of boron steel HC1500HS was worked out. The transformation kinetics model of ferrite and bainite of boron steel HC1500HS under non-isothermal condition is established. The obtained model can well predict the transformation products of boron steel at different cooling rates and deformation degrees. Finite element simulation of microstructure is carried out to obtain the distribution characteristics of sheet temperature, microstructure and Vickers hardness in hot stamping process of car door crashproof beam. The numerical calculation validity of coupled thermo-mechanical phase transformation finite element model is verified by experiment, and the finite element prediction of microstructure evolution process of hot stamping parts is realized to control hot stamping parts. The effects of forming temperature, mold temperature, holding pressure and holding time on the microstructure of hot stamping parts of anti-collision beam were studied by finite element model.
【學(xué)位授予單位】：北京科技大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2016
【分類號】：TG306

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本文編號：2225622