Distribution and Morphology of MnS Inclusions in Resulfurized Non-Quenched and Tempered Steel with Zr Addition

Abstract

Distribution and morphology of MnS inclusions in as-cast ingots and as-forged bars of two Zr-bearing resulfurized non-quenched and tempered (NQT) steels have been performed. In the low Zr-bearing steel (0.001 wt%), MnS inclusions, which are teardrop-shaped or rod-like in two-dimensional (2D) morphology and dendritic or skeletal in three-dimensional (3D) morphology, are mainly distributed and segregated at the grain boundaries. While in the high Zr-bearing steel (0.0066 wt%), MnS inclusions are spherical or angular in both 2D and 3D observation and the distribution is more uniform than those in low zirconium steel. The calculated results by Thermo-Calc software show that the content of oxygen is not the direct factor that influences the morphology and distribution of MnS inclusions in medium-sulfur low-oxygen NQT steels. ZrO 2 particles are ideal partcles for generation of spherical type I MnS inclusions owing to their strong nucleation capability and large amounts. However, the Zr and Al contents should be controlled cautiously to avoid generating large-sized agminated complex oxides, which are not easy to float and be removed owing to their high density. Otherwise, the ideal particles ZrO 2 would decrease sharply in number and fail in offering sufficient heterogeneous nuclei for type I MnS inclusions. Besides, high proportion of complex MnS inclusions would decrease the supersaturation when pure MnS inclusions begin to precipitate, suppressing the generation of dendritic type II MnS inclusions.

Effect of MnS Inclusions Distribution on Intragranular Ferrite Formation in Medium Carbon Non-Quenched and Tempered Steel for Large-Sized Crankshaft

“…For example, IPFs only generate on the MnS + V(C, N) particles located inside of the prior austenite, if the MnS + V(C, N) particles are distributed along the boundary of the prior austenite, they would promote GBF formation instead. 16) Furthermore, in our previous study, 17) it was found that there was huge difference between two-dimensional (2D) and three-dimensional (3D) morphologies of type II MnS inclusions, as shown in Fig. 1.…”

“…In the previous paper, 17) it was found the 3D morphologies of MnS inclusions were quite different in two sample steels. Most MnS inclusions in S1 are actually dendritic.…”

“…With the development of the electronic scanning technology, automatic inclusions analysis systems, such as Aspex 20,21) and INCA Explorer, 8,17) are widely applied in steel material research. It can not only obtain abundant size and number characteristics of inclusions within a short time, but the relative position information is also recorded simultaneously, which means this position information can be analyzed to quantify the inclusions distribution characteristics.…”

“…Two ingots were prepared in laboratory and the detailed smelting process was reported in previous paper. 17) Then, the ingots were homogenized at 1 200°C for 2 hours, subsequently forged into bars with diameter of 40 mm and cooled in air to the ambient temperature. A BS2400u non-contact infrared thermometer was applied to measure the cooling rate and the value was 0.3-…”

“…9) However, the first two factors stated above are almost the same in present study. Hence, inclusions in the steel formed by Zr, O, S and Mn elements, 17) which have been proved to be quite effective as heterogeneous nuclei to promote proeutectoid ferrite, 7,8,[11][12][13] are regarded as the main causes of different microstructures in two sample steels.…”