핫스탬핑용 보론 강판의 Water Quenching 및 Spot 용접 특성 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 various 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 mechanical 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 results obtained from this study are given as follows. (1) Martensite transformation temperature is about 800 ~ 840 ℃. (2) The specimens that water-quenched within 15 sec from 870 ℃ and within 20 sec from 900 ℃ show more than 1450 MPa tensile strength and Hv 400. (3) Tensile shear load of the spot welded specimen is unrelated with coating and the mechanical properties of counterpart materials influenced the tensile shear load when the boron steel's mechanical property is about the same. (4) Vickers hardness of spot welded zone shows certain trend of Hv 380 regardless of the hardness of water-quenched boron steel. (5) Martensite structure was observed at spot welded zone, heat affected zone and water-quenched boron steel. Ferrite however, was seen between the boron steel and heat affected zone. This results indicate same trend with the result of hardness.