【正文】 2011 Elsevier . All rights reserved.doi:(2007) represented an ef?cient methodology which made it possible to optimize the geometrical design of the cooling ducts for a given set of boundary conditions and parameters. The objective was to quench the hot part effectively and at a constant rate and to provide a cooling rate of at least 27 K/s while marten site was formed. The method was successfully applied for a test tool. Kolleck et al. (2009) developed a twostep inductive heating system as an effective concept to decrease energy consumption of conventional heating methods. Tremendous reduction of heating time and consequently lower investment costs as well as reduced ?oor space for the heating device were some results of using thenew presented technology. Bariani et al. (2008) presented an innovative experimental procedure based on Nakazima test for evaluating the formability limits of high strength steels during hot stamping. The procedure generated formability data suitable for an FE modeling of the hot stamping process. They provided the formability data in the formof binations of strains that caused the onset of necking and fracture for given temperatures and average strain rates in the metastable austenitic phase. Xing et al. (2009) set up a material model under hot stamping condition of quenchable steel, based on the experimental data of mechanical and physical properties. They also simulated the whole hot stamping process by ABAQUS.M. Naderi et al. / Journal of Materials Processing Technology 211 (2011) 1117–1125The main objective of the present research is focused on investigation into microstructure and mechanical properties of different nonboron alloyed steel grades after being hot stamped. This objective was followed by selection of four different highstrength uncoated carbon steel sheets. Microstructural evaluation, lateral and surface hardness measurements and tensile tests after hot stamping were performed and considered.2. Materials and methods. Chemical positionThe investigated materials were different nonboron alloyed Fig. 1. An overview of the hot stamping process sequence (Altan, 2007). steel grades with carbon contents between and wt%.The chemical analyses as well as carbon equivalent values of theinvestigated steels are given in Table 1. Carbon equivalent (Ceq) of investigated steels was calculated according to the equation presented by Patchett (2003) for carbon steels. SteelsA, B, C and D show an increasing trend in their carbon equivalent value in order.As seen in Table 1, carbon equivalent ranges from in steelA to in steelD. Accordingly, all the investigated steels are listed in the low carbon steel grades.. CC。m, 2006). Additionally as the part remains in the die during the cooling stage, spring back is totally waned. According to Naderi et al. (2008) hot stamping is a nonisothermal hightemperature forming process, in which plex ultrahigh strength parts are produced, with the goal of no spring back. In hot stamping process, the blanks are austenitized and subsequently formed and quenched in the die. During quenching, the austenitic microstructure transforms into a martens tic one because of rapid cooling. The marten site evolution during quenching causes an increased tensile strength of up to 1500 MPa, which is veri?ed in different works using tensile tests (Naderi, 2007) and hardness. Corresponding author at: Department of Mining and Metallurgy, AmirKabir University of Technology (Tehran Polytechnic), PO Box 158754413, 424 Hafez Ave.,Tehran, Iran. Tel.: +98 21 64542978。 2011 Elsevier . All rights reserved.1. IntroductionAlong with the development of automobile industry for lightweight car bodies, more and more advanced high strength and ultrahigh strength steels have been used to produce automotive parts. Making use of the mentioned steels not only reduces the weight of automobiles, but also improves crash safety and decreases gas emissions. Difficulties such as large forming forces, low formability and occurrence of spring back during cold stamping of these steels, have made manufacturers to look for new solutions. The possibility to perform stamping operations at elevated temperatures represents a solution to these problems, allowing lower loads on tools and higher accuracy of formed measurements (197。. . 鍛壓技術(shù)，2006.[18] 陳小芳，扶名福，袁志軍. Dynaform數(shù)值模擬技術(shù)在汽車覆蓋件成形中的應(yīng)用. 新技術(shù)新工藝，2004.[17] 黃慶寶，陳玉全，楊大宇，李金宇，付仲偉， 柴邦林. 汽車覆蓋件沖壓數(shù)值模擬分析及參數(shù)優(yōu)選. 現(xiàn)代制造工程，2007.[16] 唐行虎，陳煒，王曉璐， 彭華. 基于數(shù)值模擬的覆蓋件沖壓成形工藝研究. 機(jī)床與液壓，2005.[14] 趙俠，傅建，余玲，萬長(zhǎng)東. 數(shù)值模擬技術(shù)在汽車覆蓋件成形中的應(yīng)用. .[11] 包向軍，蔣宏范，何丹農(nóng)，李從心. 材料參數(shù)對(duì)汽車覆蓋件沖壓成形性能影響的數(shù)值模擬. . . 模具制造，2003.[6] 蔡其祥. ，2002.[7] 梁云榮. 汽車覆蓋件拉伸起皺和開裂現(xiàn)象分析及控制措施. 電加工與模具，2001.[4] 王洪俊，那景新，.鍛壓設(shè)備及制造技術(shù)，2004.[2] 李春光，胡平，郭威. 汽車覆蓋件深拉延有限元仿真過程中的材料塑性與失效模型模具的設(shè)計(jì)與制造將隨著市場(chǎng)的全球化、競(jìng)爭(zhēng)的激烈化、需求的個(gè)性化而體現(xiàn)出信息化、科學(xué)化, CAE 技術(shù)也必將大大提高模具在成本、質(zhì)量和周期方面的競(jìng)爭(zhēng)力而得到越來越廣泛的重視和應(yīng)用。（4）延X軸方向?qū)ΨQ增加值變大，回彈量大致上呈現(xiàn)變小的規(guī)律。（2）由于壓力的逐漸增加，材料的流動(dòng)阻力也隨之增大，繼而使得各部分的塑形變形變大，導(dǎo)致回彈量較小。若制件出現(xiàn)破裂現(xiàn)象，則若有拉延筋則去掉拉延筋，得不到改觀則減少壓邊力。6 結(jié)論與展望 結(jié)論零件在最后所呈現(xiàn)出的回彈其實(shí)是在整個(gè)成形過程的累積的效應(yīng), 所以, 對(duì)影響回彈的主要因素進(jìn)行研究,盡可能準(zhǔn)確的預(yù)算出回彈量, 這對(duì)提高沖壓件的產(chǎn)品質(zhì)量和提高生產(chǎn)效率有著重要的意義。這是因?yàn)榘辶涎貙挿较虻拿娣e越大，板料在分模線外的材料越多，在成形時(shí)被壓邊圈擠壓到的材料面積也就越大，也就是說阻力也就越大，同樣可以使制件的主型面部分的塑形變形有所增加，從而減少了回彈變形。經(jīng)過數(shù)次的計(jì)算，分別得到坯料在不同的尺寸時(shí)的制件回彈數(shù)值。由上表格可得出，其中n值越大則減薄率越小。圖57為板料參數(shù)是DP500時(shí)的制件端部回彈計(jì)算值：圖57 板料參數(shù)是dp500時(shí)的回彈量 圖58為材料參數(shù)是dp600時(shí)的制件端部回彈計(jì)算值：圖58 材料參數(shù)是dp600時(shí)的回彈量 圖59為材料參數(shù)是dp780時(shí)的制件端部回彈計(jì)算值：圖59 材料參數(shù)是dp780時(shí)的回彈量上述三種材料參數(shù)對(duì)照表：材料參數(shù)厚度總伸長(zhǎng)率屈服抗拉r0r45r90 rbarn0n45n90 nbar K e0DP500 mm%, , , , DP600 mm%, , , , DP780 mm%, , , , 通過分別對(duì)材料的厚度、屈服、n值、抗拉、r值、總伸長(zhǎng)率、K值、e0值分別比較，得出回彈呈現(xiàn)減小的規(guī)律，因此，在制件的調(diào)整過程中，應(yīng)當(dāng)采用較為合理的材料參數(shù)。 材料參數(shù)的影響利用第三章和第四章所運(yùn)用的零件有限元數(shù)值仿真模擬的設(shè)定方法和求解過程，探究零件在不同的坯料材料參數(shù)條件下，回彈值的變化規(guī)律。經(jīng)過多次的計(jì)算，分別可得到壓邊力在以下條件下的制件回彈數(shù)值。5 汽車覆蓋件回彈因素影響分析 本章主要是通過dynaform軟件分別研究壓邊力、板料大小、材料本身參數(shù)的變化對(duì)制件成形的回彈量的影