Effect of Annealing on the Microstructure Evolution and Mechanical Properties of Dual Phase Steel

Abstract

The aim of present article was to consider the influence of annealing parameters on evolution of microstructure and mechanical properties of dual phase steel. Dual phase steel was annealed according to the three chosen cycles of annealing: into intercritical region (780°C), into austenite region (920°C) and into austenite region (920°C) by subsequently cooling into intercritical region (780°C) with the hold at the temperature of 495°C. Tensile tests of the heat-treated specimens were carried out. The obtained microstructure consists from three phases: ferritic matrix, austenite and martensite. Nanoindentation experiments were performed with the peak load of 19.62 mN for ferrite grains and 0.981 mN for austenite and martensite grains, using a Berkovich tip as an indenter. The nanohardness for ferrite and martensite was 2.5 ±1 GPa and 7.1 ±1 GPa and for austenite the nanohardness varied from 4.1 to 4.5 GPa.

Development of high throughput assays for establishing process-structure-property linkages in multiphase polycrystalline metals: Application to dual-phase steels

“…We have selected DP steels for this study because of their importance to several advanced technologies, owing to their excellent combination of continuous yielding behavior, high tensile strength, high work-hardening rate, and good ductility [38,41,45,47,51,62,63,66,[79][80][81][82][83][84][85][86][87]. These properties are generally achieved in DP steels through a special heat treatment process called intercritical annealing [36,52,64,65,67,80,84,88] during which the material is heated up to the austenite/ferrite region, held for a certain amount of time, and quenched to room temperature.…”

“…Intercritical annealing results in formation of hard particles of martensite (α') in a soft matrix of ferrite (α) grains. This step is usually followed by additional cold work and heat treatment called bake hardening (BH [50,66,79,82,[85][86][87][88][89][90][91][92][93][94][95][96]; cf. Figure 1) to achieve the desired combination of mechanical properties.…”

New Insights into the Microstructural Changes During the Processing of Dual-Phase Steels from Multiresolution Spherical Indentation Stress–Strain Protocols

“…These processing steps typically include intercritical annealing at 730-830 • C for a few minutes up to an hour, quenching at different cooling rates, cold working to different deformation levels, and aging at 100-250 • C up to few hours. The last two steps are typically referred to as bake hardening [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. During the intercritical annealing treatment, the material is heated up to a temperature where the austenite and the ferrite phases are stable.…”

“…During the intercritical annealing treatment, the material is heated up to a temperature where the austenite and the ferrite phases are stable. During the subsequent quenching to the room temperature, the austenite transforms to the much harder martensite phase [17,[23][24][25][26][27][28][29], which essentially controls the properties of the DP steel. It is evident from the Fe-C phase diagram (see Figure 1) that the different intercritical annealing temperatures will result in different volume fractions of the martensite phase [24].…”

“…Furthermore, the relevant section of the phase diagram (see Figure 1) suggests that intercritical annealing at lower temperatures results in a smaller volume fraction of martensite but with a higher carbon content. As already mentioned, another important component in the processing of DP steels is the bake hardening (BH) [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] step, which includes cold working followed by aging heat treatment. This step is known to impart DP steels with a characteristic property known as continuous yielding, which is generally attributed to the production and pinning of dislocations in the ferrite component, especially in the vicinity of ferrite/martensite interfaces [1,[7][8][9][10][11][12][13][14]16,18,19,22,28,[30][31][32][33][34].…”