Dual-phase (DP) steels have been widely applied in automobile industry, and are famous for continuous yielding in their corresponding stress-strain curves. The appearance of yield-point phenomena, or discontinuous yielding, is considered undesirable for DP steels. Herein, we take advantage of the undesirable appearance of yield-point phenomena in DP steels as the variation of heat-treatment parameters, and apply it to water-heater applications, where only yield-strength is regarded as the mechanical criteria. The effect of this heat-treatment on microstructure evolution and phase transformations is first understood by constructing the continuous cooling transformation (CCT) diagram and microstructural map. These results are then utilized to fine tune the thermal parameters of the heat-treatment, such as annealing temperature, holding time, and cooling rates through correlating microstructure and mechanical properties. It is found that cooling rates make the decisive influence, and a detailed microstructure-property relationship is established as a function of cooling rates. The results reveal the steel’s full potential at yield strength when the major strengthening mechanisms are maximized, and demonstrate that it can be achieved by varying the cooling rates. Before applying the microstructure-property relationship to materials design, we studied an important compositional boundary condition due to the processing of continuous casting: the peritectic reaction. Based on thermodynamic modeling and key experimental data, we built equations to predict the vulnerability to casting problems for a wide range of compositions. With the boundary conditions, materials design is conducted via thermodynamic modeling. Instead of decreasing the cooling rate suggested by the microstructure-property relationship, designing strategy for steel compositions is presented with the decreases of austenite stability and optimization of different strengthening mechanisms. Materials characterization confirms the same microstructure constituents, and similar microstructure-property relationship. Tensile tests reveal the two cost-effective steels designed have even higher yield-strength than the base steel, validating the design strategy and the understanding of the microstructure-property relationship.

Creator

• Ren, Qingqiang

DOI

• https://doi.org/10.21985/n2-0w4b-az79

Subject

• Materials Science

Language

• en

Alternate Identifier

• http://dissertations.umi.com/northwestern:15327
• etdadmin_upload_770896

Keyword

• dual-phase steel
• inter-critical annealing
• microstructure-property relationship
• Materials design

Date created

• 2020-01-01

Resource type

• Dissertation

Rights statement

• In Copyright