Evolution of non-metallic inclusions during heat treatment

Abstract

Isothermal heat treatment can not only modify steel microstructure, but also non-metallic inclusions. In this work, heat treatment experiments were conducted between 1373 and 1573 K (1100 and 1300 °C) to study the evolution of inclusion composition, morphology, and size distribution. Results showed that during the heat treatment at 1473 and 1573 K (1200 and 1300 °C), two main kinds of inclusions initially in the steel, CaS and MgO–Al2O3–CaO–CaS, gradually transformed to (Ca, Mn)S and MgO–Al2O3–(Ca, Mn)S inclusions, and some MgO–Al2O3–CaO inclusions also transformed to MgO–Al2O3–(Ca, Mn)S. At the lowest temperature studied, 1373 K (1100 °C), little change was observed. No significant changes in number density and area fraction of the measured inclusions were observed, while the average size of inclusions increased after the heat treatment. The extent of transformation of CaS, MgO–Al2O3–CaO–CaS and MgO–Al2O3–CaO inclusions increased with decreasing inclusion size and higher temperature.

Thermodynamic and Kinetic Analyses of Inclusion Transformations in Linepipe Steel during Isothermal Heating at 1473 K (1200 °C)

“…Integrated with the apparent area, the average chemical composition of the inclusions with different sizes could be calculated by Equation (5). [ 28 ] where n represents the inclusion number in a specified size range; and are the exposed apparent area and the chemical composition of a single inclusion ( k ), respectively; and represents the average chemical composition of the inclusions in the specified size range. As shown in Figure 3, the apparent radii of the inclusions mainly ranged from 0 to 5 μm.…”

“…Integrated with the apparent area, the average chemical composition of the inclusions with different sizes could be calculated by Equation ( 5). [28] j avg ¼…”

“…Interfacial solid‐state reactions have been shown to occur between inclusions and steel. In our previous work, [ 28 ] the isothermal heating experiments were conducted at different temperatures [from 1373 to 1573 K (1100 to 1300 °C)] for 3 h to investigate the thermodynamic mechanisms of inclusion transformation in linepipe steels. The initial complex CaS–CaO–MgO–Al 2 O 3 and pure standalone CaS in steels gradually turned into (Ca, Mn)S–MgO–Al 2 O 3 and (Ca, Mn)S, respectively, by reacting with Mn during isothermal heating at relatively high temperatures [1473 and 1573 K (1200 and 1300 °C)], while any inclusion transformation can hardly be perceived at 1373 K (1100 °C).…”

Evolution of Nonmetallic Inclusions in Ti‐Bearing Al‐Killed Steel during Heat Treatment at 1200 °C

“…This trend is quite similar to that in the present study. In contrast, Liu et al [ 35 ] found that the inclusions of MgO–Al 2 O 3 –CaO–CaS system would change into MgO–Al 2 O 3 –(Ca, Mn)S inclusions during heat treatment. The CaO in the inclusions would react with the S and Al in steel to generate CaS and Al 2 O 3 .…”

“…Similar experiments were also carried out by Ren et al, [ 32–34 ] and they proposed that the evolution of MnO–SiO 2 system inclusions was closely related to heating temperature, inclusion size, and holding time as well as the contents of Cr and Si in steel. Liu et al [ 35 ] observed the inclusions in calcium‐treated Al‐killed steel after heat treatment and found that the inclusions of CaS and MgO–Al 2 O 3 –CaO–CaS in steel would transform into (Ca, Mn)S and MgO–Al 2 O 3 –(Ca, Mn)S inclusions, respectively. In addition, the average size of inclusions increased as well.…”