超高強度鋼板熱沖壓成形製程之研究

Abstract

為了達到汽車輕量化的目的，超高強度鋼板之熱沖壓成形製程已陸續應用於汽車件，透過熱沖壓製程可使成品達到高強度之特性，且成形過程中之回彈問題也較先進高強度鋼板小。然而，由於熱沖壓製程成形機制複雜，其製程包含了材料機械、熱學與材料組織間的偶合，板材與模具間又包含了介面熱傳機制與高溫摩擦，且製程亦須透過模具冷卻或模具加熱之方式來控制成形後之強度，其製程和模具設計複雜，不易直接預測其成形之缺陷。因此，針對熱沖壓製程特性進行了解且建立熱沖壓成形之分析與設計技術即為相當重要之課題。本論文主要是針對超高強度鋼板熱沖壓成形製程進行研究，包含熱沖壓成形製程與熱沖壓用鋼和模具材料之整理與歸納、熱沖壓板材與模具介面熱傳特性、熱沖壓板材和模具高溫摩擦特性、熱沖壓成形CAE分析技術建立、熱沖壓模內冷卻系統設計、裁縫式模具淬火熱沖壓成形分析和熱沖壓成形板材熱變形特性。本研究首先針對熱沖壓成形製程進行說明，並針對常見熱沖壓用鋼之材料性質進行蒐集，以及介紹熱沖壓用鋼之模具材料特性。接著，透過介面熱傳實驗和高溫摩擦實驗針對熱沖壓用鋼和模具材料間之介面熱傳係數和高溫摩擦係數進行實驗量測與分析計算，獲得熱沖壓用鋼與模具間之介面熱傳係數和高溫摩擦係數，同時分析影響介面熱傳係數和高溫摩擦係數之主要因子，以及其介面熱傳特性和高溫摩擦特性，建立出可應用於熱沖壓成形CAE模擬分析之介面參數。本研究同時也針對熱沖壓成形CAE模擬分析進行技術建立，包含成形分析技術和模內淬火技術之建立，並利用熱沖壓成形CAE模擬分析針對不同成形工法進行探討。在熱沖壓模具冷卻系統設計之研究方面，則建立熱沖壓模內冷卻系統之CAE分析技術，並利用模擬分析搭配實驗設計法進行平板模具之熱沖壓模內冷卻管路之設計，建立出可應用於熱沖壓模內冷卻初步設計之冷卻性能經驗式。此外，本研究也針對裁縫式模具淬火熱沖壓成形CAE分析進行技術建立，可應用於熱沖壓裁縫式部件之分析與設計。最後，本研究也針對熱沖壓成形可能會產生之熱變形進行探討，並透過模擬分析之方式探討板材於成形過程中產生變形的主要階段。

In order to reach the target of lightweight vehicle, the hot stamping process of ultra high strength steel sheets has been widely used in automotive parts. By the hot stamping process, the product parts can have the ultra high strength, and it also have fewer springback defects in the forming process compared to the parts with advanced high strength steel sheets. However, the forming mechanism of hot stamping process is quite complex, it contains the material mechanical, thermal, and material metallurgy mechanisms in the process. And it has to consider the interface heat transfer mechanism and high temperature friction mechanism between the blank sheet and tools. Moreover, it also needs to design the tool with cooling system or tool with heating system to control the strength of product parts. The manufacture design and tool design are much more complex, and it also difficulty to predict the forming defects. For these reasons, it becomes an important topic about realizing the characteristics of the hot stamping process and establish the technology of forming analysis and tool design for the hot stamping process. This study is focus on the hot stamping process of ultra high strength steel sheets. The research topics include the introduction of hot stamping process and material properties of blank sheets and tools for the hot stamping process, the characteristics of the interface heat transfer coefficient between blank and tools, the characteristics of high temperature friction between blank and tools, the CAE analysis technology of hot stamping process, the tools cooling system design in hot stamping process, the analysis technology of the tailored die quenching hot stamping process, and the thermal deformation of stamping sheets in hot stamping process. In this study, the hot stamping process was firstly introduced and the material properties of blank sheet and die tool were also described. Then, the experimental platforms for measuring the interface heat transfer coefficient and high temperature friction coefficient were also applied to measure and calculate the interface heat transfer coefficient and high temperature friction coefficient. It also conducted the analysis to realize the effects of process parameters on the interface heat transfer coefficient and high temperature friction coefficient. The characteristics of interface heat transfer and high temperature friction coefficient were also studied, and the parameters of interface heat transfer coefficient and high temperature friction coefficient were also established for simulation analysis. The simulation analysis technology for hot stamping process was also established in this study, including the forming analysis and die quenching analysis. The analysis technology was also applied to conduct the formability analysis with different forming methods. In the study of the hot stamping tools cooling system design, the simulation technology was also established. The simulation analysis and the method of design of experiment were conducted to the cooling system design by flat plate tools. Then the empirical equations of cooling ability of hot stamping tools could be established for initial cooling system design. Moreover, the simulation analysis technology for the tailored die quenching hot stamping process was also established in this study. The technology can be applied to the forming analysis and tool design for the tailored die quenching hot stamping process. Finally, the behaviors of thermal deformation were also discussed in this research. By the simulation analysis, the deformation phenomenon of steel sheets was investigated.