Effect of Process Parameters on Microstructures and Mechanical Properties of a Nano/ultrafine Grained Low Carbon Steel Produced by Martensite Treatment Using Plane Strain Compression

Abstract

In this work, the martensite treatment consisting of cold deformation by plane strain compression and subsequent annealing was used for producing the nano/ultrafine grained structure in a low carbon steel. The equivalent strain was varied from 0.1 to 2, while the annealing process was carried out in the temperature range of 400-600°C for 0-180 min. The microstructural evolution and mechanical properties of the as-deformed and annealed specimens were investigated. The results showed that in the as-deformed specimens, increase in strain intensified the volume fraction of the martensite cell blocks and consequently the strength. Fully equiaxed nano/ultrafine grained ferrite was developed from the martensite cell blocks during the annealing at warm temperature around 500°C for sufficient time lengths. It was concluded that the final multi-phased microstructure composed of ultrafine ferrite grains, block-tempered martensite, and fine cementite precipitates was responsible for the obtained superior mechanical properties.

Effect of Niobium on the Formation of Nano/Ultrafine Grain Structure in a Low Carbon Steel by Thermomechanical Treatment

“…Cold rolling of such ultrafine microstructure enhanced the dislocation density and the beginning martensite fragmentation. As reported by Ghasemali et al 10) , more than 65% reduction in thickness was required for martensite fragmentation and Hosseini et al 14) proved that increasing cold deformation to 85% reduction in thickness led to achieving finer ferrite grains. Figure 3 reveals SEM image of 85% cold rolled martensite in Nb .04 sample, which was composed of fully lamellar dislocation cells (LDCs).…”

“…Figure 5 reveals SEM microstructure of 85% cold rolled samples after warm temperature annealing at 550 ºC for 300 s. Annealing of deformed Nb .00 sample in such circumstances led to the complete formation of ferrite with mean grain size of 87 nm, but there were some grains bigger than others (Figure 5(a)). This phenomenon had been reported by Hoseini et al 14) and it is called "abnormal grain growth". The reason for this occurrence is minimizing the role of cementite carbide in the prevention of grain boundaries migration at higher temperatures.…”

“…Figure 2(b) shows the color metallography image of Nb .04 as quenched microstructure. Martensite appeared as blue and bluish brown using the mentioned etching solution agent, while pearlite and ferrite seemed orange and yellow 14) . Hence, there was up to 85% martensite phase in as quenched steels.…”

improvement of tensile properties of low-carbon steels via short-time intercritical annealing

“…Some research has been conducted on the development of thermomechanical processes for the production of ultrafine grain low carbon steels. Martensite treatment including cold deformation by plane strain compression followed by annealing has been used to produce nano/ultrafine grain low carbon steel [8]. It was reported that 91% cold rolling of dual phase starting microstructure including ferrite and martensite, and subsequent annealing below the eutectoid temperature, resulted in the production of ultrafine grain structure in low carbon steel [9].…”