Preferred Orientation of MnS Inclusions in Rolled Steel

The preferred orientation of MnS inclusions in hot-rolled steel was studied by electron diffraction. 
Specimens of resulfurized steel were examined after casting and after hot rolling to true strains of 
0.5, 1.0 and 1.5. After rolling, the orientations of the inclusions tended toward an ideal 〈100〉 {001}. 
Only a moderate correlation between the final inclusion shape and orientation was found.


INTRODUCTION
This paper addresses the question of whether the MnS inclusions in rolled steel have a preferred orientation. Details of the study are reported elsewhere Mardinly. As steel is hot rolled, MnS inclusions deform as well as the steel itself; the relative amount of deformation of the inclusions and the steel varying with temperature. The primary slip system for MnS is <110> 110}. Chao, Thomassen, and Van Vlack. Chao and Van Vlack studied the deformation of single crystal MnS inclusions in low-carbon steel during uniaxial compression and found that inclusions oriented with <100> parallel to the compression axis deformed more than other inclusions of other orientations because the <110> 110} slip systems were more favorably oriented.

EXPERIMENTAL PROCEDURE
An ingot high in MnS content was made by melting 60 lbs of 1119 steel under an argon atmosphere. Additions of FeS (0.25 lb) and electrolytic manganese (0.425 lbs) were made at a superheat of 100C. These additions raised the manganese and sulfur contents to about 3.65% and 0.48% respectively. To obtain large inclusions, the melt was solidified in the crucible, simulating ingot solidification rates.
Three one-inch thick slices from the ingot were hot rolled at about 2000F (1293C) between 12-in. dia. rolls. Care was taken to ensure that the rolling direction was the same in each pass. The three slices were rolled to true thickness strains of 0.5, 1.0 and 1.5, which correspond to reductions of 39%, 63% and 78%. Image analyses indicated that the strains of the inclusions were about two-third as the matrix strain. Figure 1 shows the microstructure after a rolling strain of 1.0. Specimens for electron microscopy were obtained by cutting 1-mm thick slices perpendicular to the rolling direction. These were prepared for the transmission electron microscope by mechanical grinding followed by a final thinning in an argon ion mill. Because the inclusions were elongated in the direction perpendicular to the section, most of them extended through the thickness of the final sample.
Orientations of individual inclusions were determined using selected-area electron diffraction. Selected-area diffraction was chosen only after other methods including electron channeling, Kossel patterns, and electron back scattering from small polished areas were tried and found lacking, probably because of the high dislocation density.
To avoid ambiguity, the sample was tilted until either a (100) or (110) direction was parallel to the beam. To ensure that the result were not biased by the selection of the inclusions to be examined, every inclusion with a sufficiently large electron-transparent area was studied. Many small inclusions could not be studied because they did not have large enough electron-transparent area. The orientation results for the inclusions studied were later checked to see whether there was a systematic orientation dependence on inclusion size. To assess the tendency toward (100) ideal orientations, the cumulative fractions, F, of the ofientatons that lie at an angle, p, or less from' the nearest (100) axis were determined. Figures 5-7 show experimental plots F vs. p as well as the distribution that would be expected for randomly oriented material. The values of p were calculated from the orientation expressed by a "longitude," 0, and a "latitude", t, using the relation cosp cos0 cos.  A more quantitative assessment of data was made by plotting the normalized orientation density (NOD) vs. p. The normalized orientation density was calculated by dividing the fraction, f, of the inclusions oriented so that P ( P ( P2 by the fraction of orientations that would be so oriented in random material. That is: Figures 8a and 8b are plots of NOD vs. p for the RD and RPN after the various reductions. Because of the limited number of inclusions studied, the data were smoothed in constructing these plots, by taking 10 increments of p and plotting the NOD over this increment at the average p in this increment. (i.e., the value of NOD plotted at p 20 represents the normalized density between 15 and 25.) The intensity of the texture can be characterized by a single parameter, the Texture Index (TI), defined as the integral over the range 0 < p < 54.7 of F-F normalized by the maximum possible value of this integral. TI I(F-Fr)dP/I (1-F,)dp.  Great effort was made to randomly select inclusions for study. The only apparent bias of selection was one of inclusion size. The technique for determining the orientation requires that the inclusions be sufficiently large. The smaller inclusions were not studied. There is a possibility that this selection process could bias the data if there is a statistical correlation of orientation with inclusion size.

DISCUSSION
Mangane_se sulfide crystallizes in the NaC1 structure. The predominant slip system is {110}(110). With 110}(110) slip, plane strain characteristic of flat rolling can be achieved most easily when the rolling direction is parallel to (100) and the rolling plane if {001 }.
One should expect that the relative deformation of the inclusions would depend on their orientation. For an isotropic material, the stress state for flat rolling is equivalent to tension in the rolling direction with compression normal to the rolling plane. The relative ease of deformation of any orientation may be characterized by a pseudo-Schmid factor. m'= COS.rdCOS*rd+COSp.COS* (5) where k and are the angles between the rolling direction and the slip direction and slip plane normal respectively, and Z and are the angles between the rolling plane normal and the slip direction and sli plane normal respectively. Figure 10 shows a weak correlation of the aspect ratio, L/W, of the inclusions with m' after a rolling strain of 1.5. For an inclusion that is initially spherical, the L/W ratio is related to the thickness strain, e by L/W exp(2e) so it can be concluded that favorably oriented inclusions tend to deform more than less favorably oriented inclusions. The observed 100}<001> preferred orientation gives the highest possible pseudo-