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TEM Investigation on the Dual Precipitation Behavior of Nanometer-sized Carbides and Copper Precipitates in High Strength Copper-Bearing Dual-Phase Steels
Dual-phase steel,Copper-bearing Steel,Interphase precipitation,Dual precipitation,Aging hardening,Transmission electron microscope,
|Publication Year :||2016|
For the sake of reducing the CO2 emission and improving for the efficiency of fuel consumption in the next generation of vehicles, light-weight design for advanced high-strength steels (AHSSs) as well as requirements for durability and safety play an important role in automotive industries in recent years. Dual-phase (DP) steels as one type of AHSSs commercialized by the end of 20th demonstrate special mechanical properties with combination of good strength and formability. Therefore, DP steels have been widely implemented in automotive industries. In addition, with increasing environmental consideration, copper has called lots of attentions as a part of residual elements contained in recycling steel scraps for producing considerable amounts of commercial steels. Therefore, the development of Cu bearing high-strength steels with better mechanical properties is very attractive from both the economic and environmental points of view.
In the thesis, the concept of interphase precipitation of alloyed carbide with copper precipitation is introduced in ferrite and martensite to reduce the strength mismatch and improve the strength and formability in DP steels. The objective of the present work is to clarify the effect of dual precipitation of interphase precipitation and nanometer-sized copper particles in a low carbon Cu-Ti-bearing dual-phase steel and to investigate hardness evolution with the effect of aging treatment at elevated temperature.
Besides the enhancement in hardness of both phases, the dual precipitation effects of interphase-precipitated carbide and copper particle with microstructure evolution are identified with the aid of TEM. With imaging techniques of STEM ADF with EDS mapping methods, the mechanisms for secondary hardening in both phases are elucidated. Furthermore, tensile test is carried out to investigate the overall mechanical properties of the DP steels with or without aging treatment. Surprisingly, not only the yield strength and the tensile strength are enhanced, the ductility is also improved by the treatment of aging. However, owing to the two-step heat treatment of quench-tempering resulting in higher cost of production, it would be more effective and energy-saving if the aging treatment has been finished during the coiling process with the residual temperature of the coil. As a result, a series of simulated coiling process is conducted to compare with the results of aging treatment.
From the present work, the nano-sized interphase-precipitated carbides have been found to serve as effective nucleation site for copper particles and thus shift the peak aging ahead. This research attempts to design a new material with superior mechanical properties with great interest both for industrial application and scientific research.
|Appears in Collections:||材料科學與工程學系|
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