The microstructures and mechanical properties of 17-4 PH stainless steel at each steps of
heat treatment, such as homogenizing, solid solution treatment followed by aging treatment, longterm
aging at 400 °C, and recovery treatment, in order to obtain a better understanding of the
embrittlement phenomena on aging, was investigated. As the homogenizing treatment time
increased, the length of δ-ferrite decreased and elongated shape of δ-ferrite turned to sphere shape
with the decrease of volume fraction. The solution treated specimen mainly consists of lath
martensite with a small fraction of elongated δ-ferrite. The spherical particles existed a little in the
martensite matrix, while no precipitates were found in the δ-ferrite at the solution treated specimen.
As the aging treatment temperature increased, the strength decreased while the toughness increased.
The fcc Cu-rich particles precipitated in the δ-ferrite during the long-term aging at 400 °C after the
solution heat treatment. This precipitation causes the aged hardening after long-term aging
accompanied by decreases in elongation and charpy V-notch energy absorption. The strength and
elongation was restored after recovery treatment and the fcc-Cu precipitated were almost dissolved
into the δ-ferrite matrix.
In this study, a wear test method was proposed to predict the wear life of the CrN layer coated on the surface of the press tools for manufacturing the auto-parts with ultra high strength steel (UHSS) with a tensile strength of 1.5 GPa. The pin-ondisc type wear test was carried out to confirm the feasibility and the reproducibility of the wear amount according to the test conditions such as the normal force, the sliding velocity, and the sliding speed. The test conditions were obtained from the finite element stamping analysis and the wear simulation. With the wear amount from the wear test, a prediction model of the wear depth in the CrN coating layer was proposed according to the test conditions with the design of experiments such as Taguchi method and the response surface method. The derived prediction model was then compared to the result of the Archard wear model, fully describing that the proposed model can effectively predict the wear life of the press tools for the auto-parts with UHSS.
Vehicle weight reduction is a known strategy to improve fuel efficiency and strengthen exhaust gas regulation. The use of high strength steel can satisfy safety, efficiency, emission, manufacturability, durability, and cost requirements. However, springback of high strength steel results in undesired shape defects. Springback is one of the most important issues of high strength steel for many applications in an automobile. Servo press has various capabilities to improve defects, process conditions, and productivity problems when forming a high strength steel. Especially, servo press can be set to function with variable slide motion in a single operation. In this study, the effect of servo press slide motion on 980MPa high strength steel was investigated through U-bending test.
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