Abstract
The behavior of MnS, a significant non-metallic inclusion in steel with low sulfur content, is poorly understood, in particular, how the MnS inclusion becomes a favorable secondary phase. To clarify how the sulfur content in DH36 structural steel affects the behaviors of MnS, and the steel properties, samples containing 20–380 ppm sulfur were melted, and the ingots were rolled in a semi-industrial single-roll mill. According to experiments using optical microscopy (OM), scanning electron microscopy (SEM), an automatic inclusion analysis system, and transmission electron microscopy (TEM), the morphology of MnS inclusions changed from thread-shaped to spherical- or spindle-shaped with decreasing sulfur content, and their size and number also decreased. The steel with 20 ppm S contained much more nanoscale MnS, whose pinning effect created a fine grain size in the steel. The reason is that the temperatures of maximum nucleation rate and fastest precipitation of MnS in this sample (900 °C and 920 °C, respectively) are much lower than the soaking temperature. Compared to samples with 70–380 ppm sulfur, the sample with 20 ppm sulfur content demonstrated the highest yield strength, tensile strength, and impact energy. Finally, the industrially produced DH36 samples were analyzed, and the steel with the lower S content had a higher stability and yield strength, in agreement with our laboratory results.
Highlights
DH36 is a high-strength structural steel widely used in the shipbuilding industry, especially naval warship structural applications
We comprehensively examined the effect of sulfur content (20–380 ppm) in
Laboratory experiments were carried out to clarify the effect of S content on the MnS inclusions
Summary
DH36 is a high-strength structural steel widely used in the shipbuilding industry, especially naval warship structural applications. Since the 1980s, it has demonstrated superior mechanical properties through controlled rolling and controlled cooling, which are often called the thermomechanical controlled processes (TMCP) [1]. The obtained final microstructure and mechanical properties are closely connected with the chemical composition of the steel. The content of sulfur (S), which is commonly considered to harm the steel properties, is increasingly being restricted in ship steel. The decreased S content has a direct effect on MnS precipitation in the steel. MnS is a common type of deformable non-metallic inclusion in steel. These inclusions elongate during hot rolling, Metals 2018, 8, 945; doi:10.3390/met8110945 www.mdpi.com/journal/metals
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