Effect of Thickness Reduction in BIW Weight Reduction: Transition from 600DU to 800DU on Splitting Behavior

Öz

Material selection for creating a rigid and high-energy absorption lightweight BIW is critical in terms of fuel economy and emissions. In this context, material forming limits and mechanical behavior during stamping processes are the most important factors in the creation of lightweight BIW’s. Therefore, enhancing strength and reducing material thickness stand out as the primary ways to comply with industry standards and reduce the body weight. In weight reduction efforts with dual-phase steels, thinning may lead to splitting due to the increased martensitic island structure in regions with the highest Mn and C content in dual-phase materials. In hot- and cold-rolled dual-phase 800DU material with a thickness of 1mm, splitting may occur during forming simulation due to the martensitic structure to distribute inhomogeneously, surpassing the critical threshold of the forming simulation curve when compared to cold-rolled 600DU material with a thickness of 1.2mm. This study addresses the thinning-induced splitting issue encountered in weight reduction applications, determining the optimal choice between different rolling types under a die produced for 600DU material with a thickness of 1.2mm, using both hot and cold-rolled materials. In line with the purpose of the study, trials were conducted with two different 800DU materials under forming dies. Formability simulations and mechanical tests were performed, and analyses were carried out in terms of microstructure, thinning, and edge cracking performance. The characterization of the materials was conducted using a scanning electron microscope (SEM) equipped with an EDX apparatus. Following the characterization, tensile tests were performed in three different rolling directions to determine the mechanical properties of the materials. To evaluate the resistance of the materials to edge cracking, Hole Expansion (HE) tests were conducted. As a result of the improvements and analyses made by replacing the 600DU material with a thickness of 1.2mm with cold-rolled 800DU material with a thickness of 1.0mm, the proposed application appears feasible for the part geometry under consideration.

Anahtar Kelimeler

• BIW(Body-in-White)
• Weight Reduction
• Dual Phases Steels
• Necking
• Edge Crack
• Hole Expansion (HE)

BIW Ağırlık Azaltmada Kalınlık Azaltımının Etkisi: 600DU'dan 800DU'ya Geçişin Yırtılma Davranışına Etkisi

Öz

Rijit ve yüksek enerji emilimi sağlayan hafif Beyaz Gövde (BIW) yaratmak için malzeme seçimi, yakıt ekonomisi ve emisyonlar açısından kritik öneme sahiptir. Bu bağlamda, malzeme şekillendirme sınırları ve presleme işlemi sırasında mekanik davranış, hafif BIW oluşturmadaki en önemli faktördür. Bu nedenle, dayanımı yükseltmek ve malzeme kalınlığını azaltmak, endüstri standartlarına uyum sağlamak ve gövde ağırlığını azaltmanın birincil yolu olarak öne çıkmaktadır. Çift fazlı çeliklerde ağırlık azaltımı çalışmalarında incelme, çift fazlı çeliklerin içerdiği Mn ve C nedeniyle artan martensit adacık yapısının en fazla olduğu bölgeden yırtığa neden olabilmektedir. Sıcak ve soğuk haddelenmiş çift fazlı 800DU kalınlığı 1 mm olan malzemelerde, soğuk haddelenmiş 600DU kalınlığı 1,2 mm olan malzeme için yapılmış metal şekillendirme kalıbında martensit yapının homojen dağılamaması sebebiyle şekillendirme simülasyon eğrisinin kritik eşiğini aşarak yırtılma meydana gelebilir. Bu çalışmada, ağırlık azaltımı uygulamalarında karşılaşılan incelme kaynaklı yırtık problemi için sıcak ve soğuk haddelenmiş malzemeler ve 600DU, kalınlığı 1,2 mm olan sac için imal edilmiş kalıp altında farklı hadde türleri arasından optimum seçim belirlenmiştir. Çalışmanın amacı doğrultusunda, iki farklı 800DU sınıfı malzeme ile şekillendirme kalıbı altında deneme yapılmış; formabilite simülasyonları, mekanik testler gerçekleştirilmiş ve mikro yapı, incelme, kenar çatlama performansı açısından analiz edilmiştir. Malzemelerin karakterizasyonu, EDX aparatına sahip bir taramalı elektron mikroskobu (SEM) kullanılarak gerçekleştirilmiştir. Karakterizasyonun ardından, malzemelerin mekanik özelliklerini belirlemek amacıyla üç farklı haddeleme yönünde çekme testleri gerçekleştirilmiştir. Malzemelerin kenar çatlağına olan dirençlerini değerlendirmek amacıyla Delik Genişleme (HE) testleri uygulanmıştır. 600DU kalınlığı 1,2 mm olan malzemenin yerine 800DU soğuk haddelenmiş, kalınlığı 1,0 mm olan malzeme değişikliği yapılan iyileştirmeler ve analizler sonucunda uygulamanın yapıldığı parça geometrisinde mümkün gözükmektedir.

Anahtar Kelimeler

• BIW(Beyaz Gövde)
• Ağırlık Azaltımı
• Çift Fazlı Çelikler
• İncelme
• Kenar Çatlağı
• Delik Genişleme (DG)

Kaynakça

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