자동차용 냉연 강판의 TWB (맞춤식 재단 용접강판) 특성에 관한 연구

Abstract

There is a rising interest in environmental issues over the world. Therefore, Automobile-related manufacturers have made all efforts to cut the emission of greenhouse gas. As a part of the effort, they are trying to reduce the weight of vehicle. There are some methods to reduce the weight of automobile. One is to replace heavy steel with light metal such as aluminum alloys or magnesium alloys etc. The other is to use high strength steels in order to reduce the thickness of the steel plate as thin as possible. However, both of them have some disadvantages. The price of the aluminum alloys or magnesium alloys is quiet expensive and high strength steels have a poor formability and high spring-back. To overcome these disadvantages, hot-stamping technology has been developed. The hot-stamping technique is the forming work to manufacture the high strength parts by cooling rapidly after the press forming in heating at over Austenite transformation temperature. By this method, the strength of steel sheet is increased about three times of original one through die quenching process. The hot stamping technique is expected to be able to ensure the required property at parts of car body, the safety and the light weight. Boron steel is the one of the materials for hot stamping. Boron steels have a very small amount of boron to improve hardenability of the steel. This boron exists as a solid solution in austenite, and it plays important role to improve hardenability. The purpose of this study is to show the micro-structure and to investigate mechanical properties according to the heat-treatment conditions. The water quenching from the verious temperature was conducted under the different-elapsed time as a heat-treatment. Therefore, these property values can be a practical data to use boron steels for hot stamping. Furthermore, the micro-structure and machanical properties of the SPOT welded specimen according to the existence of coating and the counterpart materials(SPRC 340, SPRC 590) is investigated in order to see welding characteristic of boron steel at the specific welding condition. The hot stamping technique is the forming work to manufacture the high strength parts by cooling rapidly after the press forming in heating at over Austenite transformation temperature. The hot stamping technique is expected to be able to confront each other effectively for the required property at parts of car body, the safety and the light weight. And boron steel which contains very small amount of boron is the one of the materials for hot stamping. The optimal process has been conducted on the reduce the cost and cycle time for a serial production. the conventional study about hot stamping is mostly studied on the metallurgical and mechanical properties by boron additive in the room temperature and high temperature. The purpose of this study is to show the micro-structure and to investigate mechanical properties(Tensile strength, Vickers hardness) according to the heat treatment conditions. The die-quenching from the various temperatures was conducted under the different-elapsed time as a heat-treatment. Therefore, these property values can be a practical data to use boron steels for hot stamping. Furthermore, the micro-structure and mechanical properties of the laser welded specimen according to the existence of coating and the welding counterpart material’s thickness is investigated in order to see welding characteristic of boron steel at the specific welding condition. These property values obtained from various methods were applied as the basic data for the forming condition and working. Also, the obtained material properties of boron steel were used as the basic data for the impact analysis. The die-quenching from the various temperatures was conducted under the different elapsed time as a heat-treatment. Laser welded boron steel with quenching has 1454 MPa, max tensile strength and 6 %, elongation. It has 96 % tensile strength of a base metal, 1522 MPa. Therefore, these property values can be a practical data to use boron steels for hot stamping.