Crash Characteristics of Partially Quenched Curved Products by Three-Dimensional Hot Bending and Direct Quench

Recently, improvement of hybrid and electric vehicle technologies, equipped with batteries, continues to solve energy and environmental problems. Lighter weight and crash safety are required in these vehicles body. In order to meet these requirements, three-dimensional hot bending and direct quench (3DQ) technology, which enables to form hollow tubular automotive parts with a tensile strength of 1470 MPa or over, has been developed. In addition, this technology enables to produce partially quenched automotive parts. In this study, the crash characteristics of 3DQ partially quenched products were investigated as the fundamental research of the design for improving the energy absorption. Main results are as follows: (1) for partially quenched straight products in axial crash test, buckling that occurs at nonquenched portion can be controlled; (2) for the nonquenched conventional and overall-quenched curved products, buckling occurs at the bent portion at the initial stage in axial crash tests, and its energy absorption is low; (3) by optimizing partially quench conditions, buckling occurrence can be controlled; and (4) In this study, the largest energy absorption was obtained from the partially quenched curved product, which was 84.6% larger than the energy absorption of the conventional nonquenched bent product in crash test.

Microstructural Observations of the Direct Quenched Offshore Steel

The purpose of this investigation is focused on the direct quench and temper mechanisms of the high strength offshore steel. Microstructural analyses of martensite and retained austenite in the direct quenched steel, simulations of martensitic transformation temperatures, Ms/Mf, and morphologies of bainite and ferrite have been evaluated in the experiment. Additionally, carbide formation after temped at various temperatures and microhardness depth profiles after heat treating are also included in the study. The direct quenched steel is primarily comprised of martensite, bainite and a few retained austenite with Vickers microhardness above 300. Tempered martensite, iron carbide and bainite are widely observed from all tempered specimens.