高張力鋼板模內電阻式局部加熱孔凸緣沖鍛成形研究

The Research of Hole-Flanging Forming on High Tensile Steel Sheet by Applying Local Resistance Heating within Die

沈信維(Sin-Wei Shen)

指導教授 ： 鄭芳松

國立虎尾科技大學/工程學院/機械與電腦輔助工程系碩士班/碩士(2014年)

摘要

現今金屬製品往輕量化並提升強度方面發展，高張力鋼板具備高強度、良好的韌性、熔接性、加工性、疲勞特性及耐候性等優良性質，因此廣泛應用在汽車板金、3C產品機殼及大型液晶電視框架等產品。目前高張力鋼隨著Ni、Cr、Mn等合金元素的添加，其抗拉強度已由340MPa提升到980MPa及更高的強度，導致衝壓加工需在熱作下來成形，因此常有胚料加熱不均、沖孔孔徑斷面品質不佳及孔凸緣成形破裂等問題發生，尤其強度等級越高之鋼板在衝壓製程上更顯的困難與經驗不足。
有鑑於此，本研究將電阻式加熱方法應用在高張力鋼板孔凸緣沖鍛複合成形，研究要點共分為以下五部份：(1)電阻式加熱機台製作；(2)高張力鋼板模內直接局部加熱可行性探討；(3)孔凸緣熱成形實驗；(4)孔凸緣沖鍛成形實驗；(5)使用有限元素軟體DEFORM-3D對孔凸緣沖鍛複合成形模擬分析。實驗分別探討胚料在不同初始孔徑下、不同加工溫度及不同成形速度等條件下，對高張力鋼板之孔凸緣成形性及斷面分析、孔凸緣強度探討、內部組織、硬度進行研究。
實驗結果顯示：(1)高張力鋼板可在模內直接被電阻局部加熱及孔凸緣沖鍛複合成形，其可在30秒內達到850℃的成形溫度；(2)紅銅電極的整體加熱過程較SKD61電極快速平穩，其加熱速率分別為26.6℃/秒及16.6℃/秒；(3)水冷方式對胚料之麻田散鐵生成的效果及硬度較空冷為佳，其冷卻速率分別為136.2℃/秒及12.4℃/秒，造成加工成形區域之硬度值平均在分別為800MPa及300MPa，尤其水冷之胚料正中間位置(Ch5)高達約1200MPa，顯示成形溫度越高、冷卻速度越快，其麻田散鐵生成的效果及散布越均勻；(4)模擬結果顯示初始孔徑大、加工溫度高能使孔凸緣成形力、等效應力值小；(5)實驗結果顯示加工溫度高及初始孔徑小時，最佳孔凸緣長度約6.76mm，最佳孔凸緣引伸率為2.37，此參數條件有利於孔凸緣成形；(6)成功沖鍛成形孔凸緣，並且孔凸緣軸向力強度達1447kgf，另外其外形上可達到近垂直及頂端面平整的孔凸緣。

關鍵字

高張力鋼板 ； 孔凸緣成形 ； 電阻式局部加熱 ； 沖鍛成形 ； 模內直接加熱

並列摘要

Over the past few years, metal manufactures are heading toward strength enhancing and weight lightening. High Tensile Strength Steel (HTSS) has excellent properties of strength, toughness, weldability, workability, anti-fatigue and weather resistance. Therefore, it has been widely applied on automotive sheet metal, chassis of electrical products, frame of large LCD monitor and other products. By adding Ni, Cr, Mn and other alloy elements, the tensile strength of High Tensile Strength Steel has been raised from 340MPa to 980MPa and its strength is improved as well, which results in applying hot forming for stamping process. Nevertheless, it is more difficult and required more experience to process the higher strength of HTSS while sheet stamping. Under the circumstance, many problems would be revealed such as poor billet heating, bad quality of pore cross section after hole punching process, and flange crack.
In this case, this study applied electrical resistance heating method on HTSS punch hole flange forging composite forming. This research could be divided into the following five sections: (1) electrical resistance heating machine assemble; (2) the feasibility of direct heat injection within mold on HTSS; (3) hole flange thermoforming experiments; (4) hole flange forging experiments; (5) using the finite element software DEFORM-3D to analyze holes flange forging complex forming. This research is to study the formability, cross section analysis, strength, internal organization and hardness of HTSS pore flange under the conditions such as different initial pore diameters, processing temperatures, and forming speeds.
Experimental results demonstrated that: (a)HTSS could be heated via the electrical resistance and processed with hole flanging forging and compound forming, and the forming temperature reached 850 ℃ within 30 seconds; (2)compared to SKD61 electrode, the heating process of copper electrode is faster and more smooth, the heating rates were 26.6 ℃ / s and 16.6 ℃ / s; (3) the hardness and properties of martensite billet by applying water-cooled way are better than the air-cooled way did, and the cooling rate was 136.2 ℃ / s and 12.4 ℃ / s separately, which resulted in the average hardness values of forming area were 800MPa and 300MPa, and especially the middle position of water-cooled billet (Ch5) was up to about 1200MPa. That revealed the higher molding temperature, the faster the cooling rate, and the martensite iron-generated effects and spread condition were more evenly; (4) simulation results showed that the larger initial aperture, and higher processing temperatures would lead to smaller hole flange forming force and equivalent stress; (5) the results illustrated that higher processing temperatures and smaller initial aperture would result in ideal hole flange length, which is about 6.76mm, and the most ideal hole flange, which was 2.37. This parameter condition is conducive for hole flanging forming; (6) hole-flanging forging and forming was successful and the hole flange axial force was up to 1447 kgf. Meanwhile, the shape of the hole flange could be nearly perpendicular to the top surface.

並列關鍵字

High Tensile Strength Steel ； Hole Flanging ； Resistance Local Heating ； Press Forging ； Direct Heating Within Die

參考文獻

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