metals Article
The Effect of Tellurium on the Formation of MnTe-MnS Composite Inclusions in Non-Quenched and Tempered Steel Ping Shen 1 1
2
*
ID
, Qiankun Yang 1 , Dong Zhang 1 , Shufeng Yang 2 and Jianxun Fu 1, *
State Key Laboratory of Advanced Special Steel, Shanghai Key Laboratory of Advanced Ferrometallurgy, School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China;
[email protected] (P.S.);
[email protected] (Q.Y.);
[email protected] (D.Z.) School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China;
[email protected] Correspondence:
[email protected]; Tel.: +86-137-0183-7760
Received: 24 July 2018; Accepted: 11 August 2018; Published: 14 August 2018
Abstract: Te is a seldom-used alloying element, which is mainly used in free-machining steel. The Te content in this kind of steel is relatively low, and only a small amount of MnTe is generated. In previous studies, the effect of Te on the formation of inclusions was not fully studied. In order to clarify the mechanism, different amounts of tellurium were added into 38MnVS6 steel to form a certain amount of MnTe. The MnTe wrapped the MnS, forming a composite inclusion. With the increase of Te content in the steel, the diameter of inclusion increased, while the aspect ratio of inclusion varied little. The aspect ratio of most inclusions was in the range of 1~3. Besides, the MnS in the MnTe-MnS composite inclusions gradually changed from one big particle to several small particles. The solidification of the steel, as well as the precipitation of MnS and MnTe, was the main factor that affecting the formation of the composite inclusion. The mechanism of the process was systematically expressed in the current study. Keywords: tellurium; MnTe; MnS; Te/S; 38MnVS6
1. Introduction In some resulfurized steels, Te has been used as an additive to improve the machinability of steel [1–3]. A small amount of Te can significantly reduce the aspect ratio of MnS inclusion, leading to the globularization of MnS inclusion, which remarkably improves the machinability of the steel, and prevents the anisotropy of the steel property [4–6]. Usually, Te is the key alloying element for the production of ultra-free machining steel. The solubility of Te in steel is very small [7,8], even at 833 ◦ C and the solubility of Te in α-Fe is only 1.5 at % [9]. At room temperature, the solubility can be nearly ignored. Te is mainly used to modify the MnS inclusion. According to the Te content added into the steel, Te may dissolve into MnS inclusions, or exist in the form of MnTe. Te has many similarities to S in steel, and no reaction occurs between telluride and sulfide. When the Te/S value in the MnS exceeds 0.07 [8], MnTe starts to generate, and this will envelop the MnS. There is a eutectic point at 810 ◦ C for the MnTe-MnS binary phase. Above this temperature, a liquid phase can form. Below 810 ◦ C, the solubility of MnS and MnTe in each opposite phase gradually decreases, and it finally reaches a very low value at room temperature. During the solidification of the steel, MnTe and MnS may precipitate and form MnTe-MnS binary inclusions. With the increase of Te content in the steel, the structures of the MnTe-MnS inclusions vary. In most Te-containing free machining steels, the Te content was very low [10,11]. Only a thin MnTe layer was generated on the surface of the MnS inclusion. The formation of MnTe in the high Metals 2018, 8, 639; doi:10.3390/met8080639
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Te-containing steel was seldom studied. It was different from that of low Te-containing steel. Besides, the lack of thermodynamic data also limited the study of the formation mechanism of the MnTe-MnS binary composite inclusion, which was really important to determine how Te made the MnS inclusion spheroidization. In the current study, different amounts of Te were added into a non-quenched and tempered 38MnVS6 steel, and the effect of Te on the formation of the composite inclusion was studied. In addition, the formation mechanisms in both low and high Te-containing steel were proposed. 2. Materials and Methods The composition of the 38MnVS6 steel is listed in Table 1. In each experiment, approximately 500 g of the steel sample was used. The steel sample was put in a corundum crucible and placed in the constant temperature zone of a vertical tube furnace. Ar gas was purged to the reaction tube during the entire experiment. The sample was heated to 1000 ◦ C at a heating rate of 8 ◦ C/min, then changed to 4 ◦ C/min to 1550 ◦ C. The purity of Te powder was 99.999%. Te powder was wrapped in iron foil. After the complete melting of the steel at 1550 ◦ C, Te powder was added into the molten steel, followed by stirring with a quartz tube. The Te content in the steel varied with the amount of Te that was added into the steel. The temperature was held at 1550 ◦ C for 30min. After cooling, the Te content in the steel as measured by ICP (inductively coupled plasma) is shown in Table 2. Table 1. The composition of 38MnVS6 steel (wt %). Composition C Content
0.383
Si
Mn
P
S
Cr
V
Al
Ti
Nb
Ni
0.574
1.399
0.01
0.051
0.179
0.099
0.018
0.017
0.016
0.123
Table 2. Te content in each steel sample (ppm). No.
1
2
3
4
5
6
7
8
Content
130
230
320
400
680
970
1400
2400
A steel sample with 10 mm in height and 8 mm in width was machined from the center of the steel ingot. Figure 1 shows the position of the steel sample that was taken from the ingot. The steel sample was polished and cleaned in ethanol by ultrasonic cleaner, and prepared for the optical microscope (Carl Zeiss, Oberkochen, Germany) and SEM-EDS (Scanning Electron Microscope-Energy Dispersive Spectrometer, Phenom, Eindhoven, Netherlands) analysis. Images captured with the optical microscope at a magnification of 200× were used to analyze the morphology and distribution of inclusions for each sample. Then, 20 optical images at a magnification of 100× were captured for statistics, including diameter, area fraction, and aspect ratio of the inclusions by using Image Pro Plus 6.0 software. After that, SEM-EDS was used to analyze the structure and composition of the inclusions. Four elements, Mn, S, Fe, and Te were selected to measure the content. In order to further clarify the morphology of the inclusions, a non-aqueous electrolyte method was employed to prepare the inclusions embedded in the steel for the SEM-EDS analysis.
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Figure The position steel sample in the ingot. Figure1.1. 1.The Theposition position of of the the Figure of the steel steel sample sampleininthe theingot. ingot.
3. 3. Results Results 3. Results 3.1. Optical and SEM-EDS Analysis Optical and SEM-EDSAnalysis Analysis 3.1.3.1. Optical and SEM-EDS Figure 2 shows the typical morphology of in with Te Figure 2 shows the typical morphology of inclusions inclusions in steels steels with different different Te contents contents Figure 2 shows the typical morphology of inclusions in steels with different Te contents captured captured with an optical microscope. The inclusions were mainly circles or ellipse shapes. captured with an optical microscope. The inclusions were mainly circles or ellipse shapes. In In the the with an optical microscope. The inclusions were mainly circles or ellipse shapes. In the steel with steel with low Te contents, inclusions were distributed along grain boundaries and showed steel with low Te contents, inclusions were distributed along grain boundaries and showed aa low Te contents, inclusions were distributed along grain boundaries and showed a chain-like pattern. chain-like chain-like pattern. pattern. With With the the increase increase of of Te Te content, content, the the inclusions inclusions gradually gradually distributed distributed separately, separately, With the increase of Te content, the inclusions gradually distributed separately, the size of the inclusion the size of the inclusion increased, and the number of chain-like inclusions the size of the inclusion increased, and the number of chain-like inclusions decreased. decreased. In In larger larger increased, and thewere number ofcomposition chain-like inclusions decreased. Indifferent larger inclusions, there wereand some inclusions, there some differences shown with colors, the light grey inclusions, there were some composition differences shown with different colors, the light grey and composition differences shown with different colors, the light grey and the dark grey. the the dark dark grey. grey.
(a) (a)
(b) (b)
(c) (c)
(d) (d)
(e) (e)
(f) (f)
(g) (g)
(h) (h)
Figure Figure 2. 2. Morphology Morphology of of inclusions inclusions in in steels steels with with different different Te Te contents. contents. (a) (a) 130 130 ppm; ppm; (b) (b) 230 230 ppm; ppm; (c) (c) Figure 2. Morphology of inclusions in970 steels with different Te(h)contents. (a) 130 ppm; (b) 230 ppm; 320 ppm; (d) 400 ppm; (e) 680 ppm; (f) ppm; (g) 1400 ppm; 2400 ppm. 320 ppm; (d) 400 ppm; (e) 680 ppm; (f) 970 ppm; (g) 1400 ppm; (h) 2400 ppm. (c) 320 ppm; (d) 400 ppm; (e) 680 ppm; (f) 970 ppm; (g) 1400 ppm; (h) 2400 ppm.
The The steel steel samples samples were were then then analyzed analyzed by by SEM-EDS. SEM-EDS. Figure Figure 33 shows shows the the SEM SEM image image of of typical typical inclusions in steels. The color differences were much clearer. The dark areas acting as the cores the The steel samples were then analyzed by SEM-EDS. Figure 3 shows the SEM image of inclusions in steels. The color differences were much clearer. The dark areas acting as the cores of oftypical the composite inclusion were mainly MnS, while the white areas enveloping the core were MnTe. The inclusions ininclusion steels. The color differences werethe much The dark areas acting the cores composite were mainly MnS, while whiteclearer. areas enveloping the core wereasMnTe. The of some shown in 3. was aa small of in thecomposition composite of inclusion were areas mainly while the white areas the core were MnTe. composition of some selected selected areas are areMnS, shown in Table Table 3. There There was enveloping small solubility solubility of Te Te in MnS, MnS, and S in MnTe, that was 5.0% and 2.3% respectively. When the Te content in the steel was 130 and S in MnTe,of that wasselected 5.0% and 2.3%are respectively. When3.the Te content in thesolubility steel wasof130 ppm, The composition some areas shown in Table There was a small Teppm, in MnS, only trace MnTe could be With of Te content, more MnTe was generated. only MnTe could be observed. observed. With the the increase increase was130 generated. and S intrace MnTe, that was 5.0% and 2.3% respectively. Whenof theTeTecontent, content more in theMnTe steel was ppm, only When the Te content was 2400 ppm, in some inclusions, MnTe occupied the main part of When thecould Te content was 2400 ppm, in some of inclusions, MnTe thegenerated. main partWhen of the thethe trace MnTe be observed. With the increase Te content, moreoccupied MnTe was inclusion. Certainly, there were still many inclusions that were mainly composed of MnS. Certainly, there many inclusions that were mainly MnS. Te inclusion. content was 2400 ppm, inwere somestill inclusions, MnTe occupied the main composed part of theof inclusion. Certainly, there were still many inclusions that were mainly composed of MnS.
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(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Figure 3. SEM (Scanning Electron Microscope, Phenom, Eindhoven, Netherlands) image of typical Figure 3. SEM (Scanning Electron Microscope, Phenom, Eindhoven, Netherlands) image of typical inclusion in steels with different Te contents. (a) 130 ppm; (b) 230 ppm; (c) 320 ppm; (d) 400 ppm; (e) inclusion in steels with different Te contents. (a) 130 ppm; (b) 230 ppm; (c) 320 ppm; (d) 400 ppm; 680 ppm; (f) 970 ppm; (g) 1400 ppm; (h) 2400 ppm. ① MnS; ② MnS; ③ MnTe; ④ MnS; ⑤ 1 MnS; 2 MnS; 3 MnTe; 4 MnS; 5 MnTe; 6 (e) 680 ppm; (f) 970 ppm; (g) 1400 ppm; (h) 2400 ppm. MnTe; ⑥ MnTe; ⑦ MnS 7 MnS MnTe; Table 3. Composition of different points (wt %). Table 3. Composition of different points (wt %).
Element Mn S Fe Te
Element 1Mn S 56.53 Fe 28.64 Te 11.35 3.47
1 2 3 56.53 55.87 323.00 2 28.64 29.14 2.33 55.87 23.00 11.35 13.00 29.14 9.62 2.33 3.47 5.37 61.67 9.62 13.00 5.37
61.67
4 59.35 4 30.38 59.35 5.97 30.38 4.29 5.97 4.29
5 6 26.15 525.45 2.52 2.18 26.15 5.51 2.52 6.93 65.82 5.51 65.43 65.82
7 56.41 6 28.51 25.45 8.23 2.18 6.85 6.93 65.43
7 56.41 28.51 8.23 6.85
Except for the MnTe-MnS composite inclusions, there were also a few oxide inclusions. It ought to be in the original inclusions in the raw material. In the current study, this was not the main topic Except for the MnTe-MnS composite inclusions, there were also a few oxide inclusions. It ought and was not discussed in detail. to be in the original inclusions in the raw material. In the current study, this was not the main topic and was not discussed in detail. 3.2. Statistics of Inclusions In eachofsample, 20 optical images at the magnification of 100× were captured. The images were 3.2. Statistics Inclusions processed with an image analyzer (Image Pro Plus 6.0) to obtain the equivalent diameter and the In each sample, 20 optical images at the magnification of 100× were captured. The images were aspect ratio of the inclusions. The diameter of most inclusions was larger than 1 μm. Thus, in order processed with image analyzer Pro Plus 6.0) to image obtainorthe diameter and the to prevent thean statistical error due(Image to the definition of the theequivalent defects that were formed aspect ratio the inclusions. The diameter of most inclusions was1 larger than µm. Thus, Figure in order4 to during theofpolishing, the inclusion with a diameter smaller than μm was not1considered. prevent the statistical error due to the definition of the image or the defects that were formed during shows the number percentage of inclusions in different diameter ranges. With the increase of Te thecontent polishing, the inclusion with a diameter smaller than 1 µm was not considered. Figure 4 shows in the steel, the number of inclusions with small diameters decreased, while the number thewith number of inclusions different diameterdiameter ranges. of With of Te largepercentage diameters increased. The in average equivalent the the totalincrease inclusions in content each in sample the steel, theshown number of inclusions small diameters decreased, while theincreased number with with the large was in Figure 5. The with average equivalent diameter of inclusions diameters increased. The average equivalent diameter of the total inclusions in each sample was increase of Te content. Besides, the area fraction of inclusions also increased with the increase of Te content, as shown in Figure 6. equivalent diameter of inclusions increased with the increase of Te shown in Figure 5. The average solubility of Tefraction in solid of steel was limited. A small amount of increase Te causedofthe Te content.The Besides, the area inclusions also increased with the Te saturation content, asofshown in the steel; thus, most Te existed in the form of telluride. The increase of Te content caused the in Figure 6. increase of total volume of telluride; the area fraction inclusions was increased. The solubility of Te in percentage solid steel was limited. thus A small amount of Teofcaused the saturation of Te in telluride precipitated around the sulfide or adhered to the of sulfide instead of distributing theThe steel; thus, most Te existed in the form of telluride. The increase Te content caused the increase in the steel, which caused an increase of equivalent of separately total volume percentage of telluride; thus the area fraction of diameter. inclusions was increased. The telluride precipitated around the sulfide or adhered to the sulfide instead of distributing separately in the steel, which caused an increase of equivalent diameter.
Number percentage percentage of inclusion (%) (%) Number Number percentageofofinclusion inclusion (%)
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50 50 50
130 ppm Te 130 ppm Te 130 ppm Te 230 ppm Te 230 ppm Te 230 ppm Te 320 ppm Te 320 ppm Te 320 ppm Te 400 ppm Te 400 ppm Te 400 680ppm ppmTe Te 680 ppm Te 680 ppm Te 970 ppm Te 970 ppm Te 970 ppm TeTe 1400 ppm 1400 ppm Te 1400 ppm Te 2400 ppm Te 2400 2400ppm ppmTe Te
40 40 40 30 30 30 20 20 20 10 10 10 0 00
1~2.5 1~2.5 1~2.5
2.5~5 2.5~5 2.5~5
5~10 5~10 5~10
10~15 10~15 10~15
Diameter ofinclusion inclusion (μm) Diameter Diameterof of inclusion(μm) (μm)
>15 >15 >15
Average equivalent equivalent diameter (μm) (μm) Average Average equivalentdiameter diameter (μm)
Figure 4. Numberpercentage percentageofofinclusions inclusions in in different different diameter ranges. Figure diameter ranges. Figure 4.4.Number Number ranges. Figure4. Numberpercentage percentageof ofinclusions inclusionsin indifferent differentdiameter diameter ranges.
10 10 10 8 88 6 66 4 44 2 22 0 00 0 00
500 500 500
1000 1000 1000
1500 1500 1500
Te content(ppm) (ppm) Te Tecontent content (ppm)
2000 2000 2000
2500 2500 2500
Figure 5. Average equivalent diameters. Figure 5. Averageequivalent equivalentdiameters. diameters. Figure Figure5. 5.Average equivalent diameters.
Area fraction fraction of inclusion (%) (%) Area Area fractionofofinclusion inclusion (%)
2.0 2.0 2.0 1.5 1.5 1.5 1.0 1.0 1.0 0.5 0.5 0.5 0.0 0.0 0.0 0 00
500 500 500
1000 1000 1000
1500 1500 1500
Te content(ppm) (ppm) Te Tecontent content (ppm)
2000 2000 2000
Figure 6. The area fractions of inclusions. Figure Figure6. 6.The Thearea areafractions fractions inclusions. Figure 6. The area fractionsofof ofinclusions. inclusions.
2500 2500 2500
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3.3. Analysis Analysis of of the the Three-Dimensional Three-Dimensional Morphology Morphology of 3.3. of Inclusions Inclusions 3.3. Analysis of the Three-Dimensional Morphology of Inclusions The non-aqueous non-aqueous electrolyte the inclusions from steel or The electrolyte method methodisisan anexcellent excellentmethod methodtotoextract extract the inclusions from steel The non-aqueous electrolyte method is an excellent method to extract the inclusions from steel leave inclusions embedded in steel by etching the steel matrix. The MnS and MnTe did not dissolve into or leave inclusions embedded in steel by etching the steel matrix. The MnS and MnTe did not or inclusions in steel etching the steelwere matrix. The in MnS and MnTe didsteel. not theleave solution; thus theembedded inclusions kept the by morphology asmorphology they present the steel. By in using SEM, dissolve into the solution; thus the inclusions kept the as they were present the dissolve into thethe solution; thus the inclusions kept could the as they were present the steel. the using three-dimensional morphology ofmorphology inclusions be observed. Figure 7 shows theininclusions By SEM, three-dimensional of morphology inclusions could be observed. Figure 7 shows By using SEM, the three-dimensional morphology of inclusions could be observed. Figure 7 shows that were embedded in the steel. The white color and the dark color of the inclusions showed similar the inclusions that were embedded in the steel. The white color and the dark color of the inclusions the inclusions were embedded in the steel. white and thebydark color of the inclusions information asthat in Figure 3. The was by color MnTe, where only part of MnS could showed similar information as inMnS Figure 3. enveloped TheThe MnS was enveloped MnTe, where only part be of showed similar information as in Figure 3. The MnS was enveloped by MnTe, where only part observed. The composite inclusions showed spherical or spheroidicity shapes. With the increase of of Te MnS could be observed. The composite inclusions showed spherical or spheroidicity shapes. With MnS could be observed. composite inclusions showed or spheroidicity shapes. With content, more MnSThe particles embedded in MnTe couldspherical be observed. the increase ofsmall Te content, more small MnS particles embedded in MnTe could be observed. the increase of Te content, more small MnS particles embedded in MnTe could be observed.
(a) (a)
(b) (b)
(c) (c)
(d) (d)
(e) (f) (g) (h) (e) (f) (g) (h) Figure 7. Three-dimensional Three-dimensionalmorphology morphology of the composite inclusion in steels with different Te Figure 7. 7. ofof thethe composite inclusion in steels with different Te content Figure Three-dimensional morphology composite inclusion in(f)steels with (g) different Te content (a) 130 ppm; (b) 230 ppm; (c) 320 ppm; (d) 400 ppm; (e) 680 ppm; 970 ppm; 1400 ppm; (a) 130 ppm; (b) 230 ppm; (c) 320 ppm; (d) 400 ppm; (e) 680 ppm; (f) 970 ppm; (g) 1400 ppm; content (a) 130 ppm; (b) 230 ppm; (c) 320 ppm; (d) 400 ppm; (e) 680 ppm; (f) 970 ppm; (g) 1400 ppm; (h) ppm. (h) 2400 2400 ppm. (h) 2400 ppm.
Except for the inclusions discussed above, there was also a kind of inclusion that was quite Except for for the the inclusions inclusions discussed above, above, there there was was also also aa kind kind of of inclusion inclusion that that was was quite quite Except different from the MnTe-MnSdiscussed composite inclusion, not only in the morphology, but also in the different from from the the MnTe-MnS MnTe-MnS composite composite inclusion, inclusion, not not only only in in the the morphology, morphology, but but also also in in the the different composition. As shown in Figure 8, there were some dendritic inclusions in the steels. The EDS composition. As Asshown shownininFigure Figure8,8, there there were were some dendritic dendritic inclusions in in the the steels. steels. The The EDS EDS composition. result indicated that these were MnS inclusions,some and no MnTeinclusions could be detected. This kind of result indicated that these were MnS inclusions, and and no MnTe could could be detected. This kind of inclusion result indicated that these were MnS inclusions, no MnTe be detected. This kind of inclusion mainly existed in the steels with low Te content. With an increase of Te content, the mainly existed in existed the steels low Tewith content. With an increase of Te content, the amount of this inclusion in with the was steels amount ofmainly this kind of inclusion reduced.low Te content. With an increase of Te content, the kind of inclusion was amount of this kind of reduced. inclusion was reduced.
Figure 8. Three-dimensional morphology of the dendritic inclusions. (a) typical dendritic inclusion 1; Figure 8. Three-dimensional morphology the dendritic inclusions. (a) typical dendritic inclusion 1; (b) typical dendritic inclusion 2; (c) typicalof inclusion 3. Figure 8. Three-dimensional morphology ofdendritic the dendritic inclusions. (a) typical dendritic inclusion 1; (b) typical dendritic inclusion 2; (c) typical dendritic inclusion 3. (b) typical dendritic inclusion 2; (c) typical dendritic inclusion 3.
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4. Discussion
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4.1. The Effect of Te on the Aspect Ratio of the Inclusion 4.4.Discussion Discussion
The aspect ratio of the inclusion is an important parameter for the steel property. The morphology 4.1. on ofofthe 4.1.The TheEffect Effectofan ofTe Teaspect onthe theAspect Aspect Ratio the Inclusion for the steel properties [1,12,13]. It not only of inclusion with ratio Ratio of 1~3 is Inclusion beneficial The aspect ratio of the inclusion is for the property. increases the machinability, but also prevents the anisotropy of the steel. 9 shows the The number The aspect ratio of the inclusion is an an important important parameter parameter forFigure the steel steel property. The morphology of inclusion with an aspect ratio of 1~3 is beneficial for the steel properties [1,12,13]. ItIt of percentage of inclusions in different adding theproperties steel, the aspect ratio morphology of inclusion with anaspect aspectratio ratio ranges. of 1~3 isAfter beneficial for Te theto steel [1,12,13]. not only increases the machinability, but also prevents the anisotropy of the steel. Figure 9 shows the most inclusions is in the of 1~3; few inclusions a big aspect ratio, and no big differences not only increases therange machinability, but also preventshave the anisotropy of the steel. Figure 9 shows the number percentage of inclusions in different aspect ratio ranges. After adding Te to the steel, the number percentage of inclusions in different aspect ratio ranges. After adding Te to the steel, theratio exist among these samples. Taking all of the inclusions into consideration, the average aspect aspect ratio of most inclusions isisin the range of 1~3; few inclusions have aabig aspect ratio, and no aspect ratio of most inclusions in the range of 1~3; few inclusions have big aspect ratio, and no of of all inclusions are calculated. The effect of Te/S value in different samples on the aspect ratio big differences exist among these samples. Taking all of the inclusions into consideration, the big differences exist among these samples. Taking all of the inclusions into consideration, the the inclusions is shown of in Figure 10. With the increaseeffect of Te content from different 130 ppm to 2400onppm, average averageaspect aspectratio ratio ofall allinclusions inclusionsare arecalculated. calculated.The The effectof ofTe/S Te/Svalue valuein in differentsamples samples on the average aspect ratio varies a little. According to the references, once theTe Te/S value in steelppm exceeds the theaspect aspectratio ratioof ofthe theinclusions inclusionsisisshown shownin inFigure Figure10. 10.With Withthe theincrease increaseof of Tecontent contentfrom from130 130 ppm 0.07, MnTe will bethe generated [8]. When the Te/S value exceedsto0.2, the Te content has little influence to 2400 ppm, average aspect ratio varies a little. According the references, once the Te/S value to 2400 ppm, the average aspect ratio varies a little. According to the references, once the Te/S value on theininaspect ratio of theMnTe MnSwill inclusion. Katoh etWhen al. [14] the aspect inclusions steel 0.07, be [8]. the Te/S 0.2, the content has steelexceeds exceeds 0.07, MnTe will begenerated generated [8]. When themeasured Te/Svalue valueexceeds exceeds 0.2,ratio theTe Teof content has in little aspect the MnS Katoh etetis al. [14] aspect ratio steel with different Te/S afterofofhot and the result shown inthe Figure 10. Theofofsteel littleinfluence influenceon onthe thevalues aspectratio ratio therolling, MnSinclusion. inclusion. Katoh al.also [14]measured measured the aspect ratio inclusions in steel with different Te/S values after hot rolling, and the result is also shown in Figure sample in the current was not rolled under high temperature, however, it is reported that with the inclusions in steel with different Te/S values after hot rolling, and the result is also shown in Figure 10. The steel sample in the current was not rolled under high temperature, however, it is reported presence MnS inclusions more not high onlytemperature, in as-cast steel, but also hot-rolled 10. of TheTe, steel sample in the become current was notglobular, rolled under however, it isin reported that with the presence of MnS inclusions become more globular, not only in as-cast steel, but thatIn with presence ofTe, Te, inclusions become moreare globular, only as-castthe steel, butalso also steel [1]. the the current study, theMnS Te/S values in all steels larger not than 0.2,inwhile aspect ratio is in hot-rolled steel study, the Te/S in all are than 0.2, while the hot-rolled steel[1]. [1].In Inthe thecurrent current study, theaspect Te/Svalues values allsteels steels arelarger larger than while thewas nearlyinthe same, with the variation trend of the ratio in being consistent with the0.2, result that aspect ratio isisnearly the same, with the variation trend of the aspect ratio being consistent with the aspect ratio nearly the same, with the variation trend of the aspect ratio being consistent with the provided Katoh et al. resultby that was provided by Katoh et al.
Numberpercentage percentageofofinclusion inclusion(%) (%) Number
result that was provided by Katoh et al. 100 100
130 ppm Te 130ppm ppmTe Te 230 230ppm ppmTe Te 320 320 ppm Te 400 ppm Te 400 ppm Te 680 ppm Te 680ppm ppmTe Te 970 970 ppm 1400 ppm Te Te 1400 ppm Te 2400 ppm Te 2400 ppm Te
80 80 60 60 40 40 20 20 00
1~3 1~3
3~6 3~6
>6 >6
Aspect Aspectratio ratio
Figure 9.9.Number percentage inclusions ratio ranges. Figure 9. Number percentage ofofof inclusions in different aspect ratio ranges. Figure Number percentage inclusionsin indifferent differentaspect aspect ratio ranges.
18 18 16 16
Katoh et al. Katoh et al. Current study Current study
Aspectratio ratio Aspect
14 14 12 12 10 10 88 66 44 22 00
0.0 0.0
0.5 0.5
1.0 1.0
Te/S Te/S
22
44 66 88
Figure 10. The aspect ratio of inclusions in steels with different Te/S values.
Figure 10. The aspect ratio inclusionsininsteels steels with with different values. Figure 10. The aspect ratio of of inclusions differentTe/S Te/S values.
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The aspect ratio corresponding to the Te/S value that is smaller than 0.2 is not included in the current thatcorresponding case, most oftothe dissolves intoisthe MnS than inclusion, andincluded few MnTe are Thestudy. aspect In ratio theTe Te/S value that smaller 0.2 is not in the generated. The of of the of MnS inclusions areand different [15]are and will be current study. In mechanism that case, most themodification Te dissolves into the MnS inclusion, few MnTe generated. discussed elsewhere. The mechanism of the modification of MnS inclusions are different [15] and will be discussed elsewhere. Figure 11 11shows showsthe theaverage averageaspect aspectratio ratioofofinclusions inclusions different equivalent diameter ranges. Figure in in different equivalent diameter ranges. Te Te has same effect on inclusion the inclusion of different diameters. the increase Te content the has the the same effect on the of different diameters. WithWith the increase of Te of content in thein steel, steel, big differences be observed. no bigno differences can becan observed. 4.0
Average aspect ratio
3.5 3.0 2.5
130 ppm Te 230 ppm Te 320 ppm Te 400 ppm Te 680 ppm Te 970 ppm Te 1400 ppm Te 2400 ppm Te
2.0 1.5 1.0 0.5 0.0 1~2.5
2.5~5
5~10
10~15
>15
Equivalent diameter (μm) Figure 11. 11. Average aspect ratio ratio of of inclusions inclusions in in different different equivalent equivalent diameter diameterranges. ranges. Figure Average aspect
As mentioned mentioned above, above, the the Te/S Te/S values samples are all larger larger than than 0.2, 0.2, with with most most MnS MnS As values in in these these samples are all inclusions being saturated with Te, and MnTe being generated. In the current conditions, the Te has inclusions being saturated with Te, and MnTe being generated. In the current conditions, the Te has the same same effect effect on on all all inclusions, inclusions, no no matter matter what what the the size size of of the the inclusion inclusion is. is. There There is is no no big big difference difference the of the aspect ratio among these inclusions with different diameters and among these samples with of the aspect ratio among these inclusions with different diameters and among these samples with different Te contents. different Te contents. 4.2. The The Precipitation Precipitation of of the the MnTe-MnS MnTe-MnSBinary BinaryPhase Phasefrom fromSteel Steel 4.2. In the the current current study, study, few few single single MnTe MnTe inclusions inclusions can can be be observed; observed; MnTe MnTe mainly mainly exists exists in in In accompany with with MnS. MnS. MnTe MnTe precipitates precipitates from from the the molten molten steel steel together together with with MnS MnS inclusion inclusion in in the the accompany solidification process. process.InIn low Te content, some inclusions dendritic distributed inclusions solidification thethe steelsteel withwith low Te content, there arethere some are dendritic distributed along grainasboundaries, as shown in Figure indicating thatare these inclusions are along grain boundaries, shown in Figure 8, indicating that8,these inclusions precipitated in the precipitated in the interdendritic regions from the molten steel. With the increase of Te content, the interdendritic regions from the molten steel. With the increase of Te content, the inclusions become inclusions larger, with fewer distributes along grain boundaries. larger, withbecome fewer distributes along grain boundaries. The equilibrium equilibrium solidification solidification of of the the original original 38MnVS6 38MnVS6 sample sample from from 1550 1550 ◦°C to 1350 1350 ◦°C was The C to C was calculated by FactSage 7.0. The result is shown in Figure 12. The molten steel starts to solidify at 1488 calculated by FactSage 7.0. The result is shown in Figure 12. The molten steel starts to solidify at ◦ C, δ-Fe °C, δ-Fe generates first,first, andand then it gradually transforms process 1488 generates then it gradually transformstotoγ-Fe. γ-Fe.With Withthe the solidification solidification process ◦ C.1434 occurring, MnS MnSstarts starts precipitate the molten °C.theWhen the temperature occurring, to to precipitate fromfrom the molten steel atsteel 1434at When temperature decreases decreases 1412 °C, steelcompletely is almost completely solidified. In the temperature 1434◦°C to 1412 ◦ C,tothe steel is the almost solidified. In the temperature range from range 1434 ◦from C to 1412 C, to 1412of°C, theistotal MnS is precipitated. only a smallofamount of MnS precipitates 94.8% the94.8% total of MnS precipitated. After that,After only athat, small amount MnS precipitates from the from steel. the solid steel. solid The FactSage software software does does not not include include the the entire entire Te Tedatabase. database. Although Although the the solidification solidification of of The FactSage 38MnVS6containing containingdifferent differentTeTe contents be calculated when taking Teaccount, into account, the 38MnVS6 contents cancan stillstill be calculated when taking Te into the result result is unreliable; thus, it is not presented or discussed here. is unreliable; thus, it is not presented or discussed here. Figure1313shows shows MnTe-MnS binary diagram a high temperature, the Figure thethe MnTe-MnS binary phasephase diagram [16]. At [16]. a highAttemperature, the MnTe-MnS MnTe-MnS phase can form a liquid the of temperature the eutectic point 810 °C, binary phasebinary can form a liquid solution, the solution, temperature the eutecticofpoint is 810 ◦ C, andisthe mole and the mole ratio of MnS corresponding to the eutectic point is close to 10%. In the current study, ratio of MnS corresponding to the eutectic point is close to 10%. In the current study, the mole ratio the mole ratio of MnS ranges from 32% to 94%, as shown in Figure 13. The solidus temperature and
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Liquid δ-Fe γ-Fe MnS
γ-Fe
liquid 0.15
MnS 0.10 ℃
℃
40
1488
60
0.05 20 1434℃
0 1350
1400
1450
δ-Fe 1500
0.00 1550
Weight percent of precipitate (wt%)
0.20
120
1412
Weight percent of iron phase (wt%)
the liquidus temperature 38MnVS6 are also presented in Figure The higherand Te content, the of MnS ranges from 32% of to 94%, as shown in Figure 13. The solidus13. temperature the liquidus closer to the eutectic point.are Thealso addition of Tein may affect13. theThe solidification of the steel. in temperature of 38MnVS6 presented Figure higher Te content, the However, closer to the order to facilitate the analysis, the affection is not considered, since the Te content is not so high. eutectic point. The addition of Te may affect the solidification of the steel. However, in order to facilitate According to the theaffection result calculated by FactSage, mainly precipitates during the solidification of the analysis, is not considered, sinceMnS the Te content is not so high. According to the result steel. The solubility of Te in solid steel is very limited, assuming thatofMnTe calculated by FactSage, MnS mainly precipitates duringthus the solidification steel. also The precipitates solubility of in Te this temperature range. in solid steel is very limited, thus assuming that MnTe also precipitates in this temperature range.
Temperature (℃) Figure12. 12.Equilibrium Equilibriumphase phasediagram diagramduring duringsolidification solidificationof ofsteel. steel. Figure
When content in inthe thesteel steelisislower lowerthan than 970 ppm, a liquid phase containing MnTe-MnS When the the Te Te content 970 ppm, a liquid phase containing MnTe-MnS and ◦ ◦ and a solid MnS phase will generate in the temperature range from 1488 °C to 1412 °C. The solid a solid MnS phase will generate in the temperature range from 1488 C to 1412 C. The solid MnS MnS contains few MnTe precipitates in interdendritic the interdendritic regions, it finally becomes phasephase contains few MnTe and and precipitates in the regions, andand it finally becomes the the dendritic MnS inclusion, as shown in Figure 8. The liquid MnTe-MnS phase keeps a spherical dendritic MnS inclusion, as shown in Figure 8. The liquid MnTe-MnS phase keeps a spherical shape shape for minimum free After energy. the solidification complete solidification steel, the MnS for minimum surface surface free energy. the After complete of the steel,of thethe MnS continuously continuously precipitates from the liquid MnTe-MnS phase until the composition of the liquid phase precipitates from the liquid MnTe-MnS phase until the composition of the liquid phase reaches the reaches the eutectic point. Finally, both MnS and MnTe precipitate from the liquid phase. During eutectic point. Finally, both MnS and MnTe precipitate from the liquid phase. During this solidification this solidification process, the MnTe-MnS inclusion and the dendritic process, the MnTe-MnS composite inclusion composite and the dendritic MnS inclusion forms. MnS inclusion forms.When the Te content in the steel is higher than 1400 ppm, the MnTe-MnS binary phase keeps the ◦C When theand Te there content in solid the steel higher than 1400 ppm, phase1488 keeps liquid phase, is no MnSisphase that is generated inthe the MnTe-MnS temperaturebinary range from ◦ C. the liquid phase, there is no solid MnS the phase thatphase is generated in the temperature from to 1412 After and the solidification of steel, liquid gradually transforms to therange composite 1488 °C to 1412 °C. After the solidification of steel, the liquid phase gradually transforms to the MnTe-MnS inclusion, as stated above. Actually, the solidification of the steel and the precipitation composite MnTe-MnS inclusion, as performed. stated above. the solidification the steel andsmall the of the inclusions are not uniformly In Actually, the high Te-containing steel,ofthere are still precipitation of the inclusions are not uniformly performed. In the high Te-containing steel, there numbers of dendritic MnS inclusions. are still small numbers of dendritic MnS inclusions.
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Figure Figure 13. 13. The The MnTe-MnS MnTe-MnS binary binary phase phase diagram. diagram.
4.3. 4.3. The The Precipitation Precipitation of of MnS MnS and and MnTe MnTefrom fromthe theMnTe-MnS MnTe-MnSBinary BinaryPhase Phase As As shown shown in in Figure Figure 3, 3, there there are are two two kinds kinds of of composite composite inclusion. inclusion. One One contains contains aa big big MnS MnS core, core, while contains many small MnSMnS cores. The later not observed in previous studies, whilethe theother other contains many small cores. The was laterusually was usually not observed in previous since thesince Te/S the value was verywas lowvery in the steel. studies, Te/S value low in the steel. If If MnS MnS precipitates precipitates from from the the steel steel first first and and became became the the nucleation nucleation point point for for MnTe, MnTe, then then MnTe MnTe precipitates from the steel and wraps MnS, a composite inclusion with MnS core and the MnTe precipitates from the steel and wraps MnS, a composite inclusion with MnS core and the MnTe outer outer layer layer will will form. form. However, However, the the composite composite inclusion inclusion with with many many MnS MnS cores cores cannot cannot be be explained explained by by this this mechanism. Accordingto tothe theMnTe-MnS MnTe-MnSbinary binaryphase phase diagram, there a MnTe-MnS solution in mechanism. According diagram, there is aisMnTe-MnS solution in the the at high temperature. After solidification of the steel, liquid solution may exist. steelsteel at high temperature. After the the solidification of the steel, the the liquid solution may stillstill exist. In In further cooling processes, both MnS and MnTe precipitate from MnTe-MnS solution, instead further cooling processes, both MnS and MnTe precipitate from thethe MnTe-MnS solution, instead of of steel. The formationprocesses processesofofthe thecomposite compositeinclusions inclusionsobserved observed in in the the current current study thethe steel. The formation study are are described described as as follows. follows. When low, thethe composition of MnTe-MnS solution is farisfrom eutectic point. When the theTe Tecontent contentis is low, composition of MnTe-MnS solution far the from the eutectic With in temperature, MnS precipitates from the from MnTe-MnS solution first, and gradually point.a decrease With a decrease in temperature, MnS precipitates the MnTe-MnS solution first, and grows. Atgrows. the same time, the time, composition of the MnTe-MnS solutionsolution tends totends the eutectic point. gradually At the same the composition of the MnTe-MnS to the eutectic Once temperature reaches thereaches eutecticthe temperature, MnS and MnTe precipitate simultaneously. point.the Once the temperature eutectic temperature, MnS and MnTe precipitate Since the initial precipitated MnS inclusion already the newly precipitated adheres to the simultaneously. Since the initial precipitated MnSexists, inclusion already exists, the MnS newly precipitated MnS adheres the surface of MnS while is pushed out andleading precipitates alone, leading to surface of MnStocore, while MnTe is core, pushed outMnTe and precipitates alone, to a distinct interface a distinctthe interface between the MnTe MnS phase the MnTe phase.inclusion Finally, awith composite inclusion between MnS phase and the phase.and Finally, a composite the MnS core andwith the the MnS core andisthe MnTe outer is generated. The generation of the composite inclusion from MnTe outer layer generated. The layer generation of the composite inclusion from the MnTe-MnS solution the MnTe-MnS solution a divorced eutectic.process The whole solidification is shown in Figure is a divorced eutectic. Theiswhole solidification is shown in Figureprocess 14a. 14a. When the Te content is high, the composition of MnTe-MnS solution is close to the eutectic point. When the Tesolidification content is high, theonly composition of MnTe-MnS solution is closeCompared to the eutectic During the initial process, a small amount of fine MnS is generated. with point. the solution, initial solidification onlytoa provide small amount fine for MnS generated. the restDuring MnTe-MnS the MnS coreprocess, is too small enough of surface theisadhesion of Compared with theMnS. rest MnTe-MnS solution, the reaction, MnS corepart is too to providesolution enoughprecipitates surface for newly precipitated In the following eutectic of small the MnTe-MnS the adhesion of in newly precipitated MnS. In the following eutectic partdue of the MnTe-MnS MnS and MnTe the form of divorced eutectic, while another partreaction, precipitates to the eutectic solution precipitates and MnTe in the form of divorced eutectic, partparticles precipitates reaction, that is, MnS MnS and MnTe precipitates alternately, seeming like awhile lot ofanother small MnS are due to thein eutectic reaction, is, MnS and MnTe precipitates alternately, seemingembedded like a lot in of embedded the MnTe phase.that Finally, a composite inclusion with several MnS particles small phase MnS particles are embedded in the MnTe 14b. phase. Finally, a composite inclusion with several MnTe is generated, just as shown in Figure MnSAccording particles embedded in MnTe phase is generated, just asinshown 14b. to be a eutectoid to the MnTe-MnS binary phase diagram given Figurein 13,Figure there ought toprecipitation the MnTe-MnS phase diagram in Figure 13, there to be a point.According The further maybinary be similar to that whichgiven is mentioned above. Thus,ought the structure eutectoid point. The further precipitation maytoo be much. similar to that which is mentioned above. Thus, the of the composite inclusion should not change structure of the composite inclusion should not change too much.
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(a)
(b) Figure 14. 14.Precipitation PrecipitationofofMnTe MnTe and MnS from the liquid phase (a)Telow Te content; high Te Figure and MnS from the liquid phase (a) low content; (b) high(b) Te content. content.
5. Conclusions 5. Conclusions 1. After adding a certain amount of Te in the 38MnVS6 steel, MnTe was generated and wrapped 1. After adding a certain amount of Te inofthe steel,130 MnTe generated wrapped the MnS inclusion. With the increase Te38MnVS6 content from ppmwas to 2400 ppm, and the diameter the MnS inclusion. With the increase of Te content from 130 ppm to 2400 ppm, the diameter of of the inclusion increased. The aspect ratio of inclusions in all steels was nearly the same. the inclusion increased. Theaspect aspectratio ratiorange of inclusions Most inclusions were in the of 1~ 3. in all steels was nearly the same. Most inclusions were in the aspect ratio range of 1~3. 2. The 38MnVS6 steel solidified in the temperature range of 1488~1412 ◦ C, and MnTe and MnS 2. The steelfrom solidified in the temperature range of 1488~1412 °C, liquid and MnTe andInMnS were38MnVS6 precipitated the liquid steel and formed a MnTe-MnS binary phase. the were precipitated from the liquid steel and formed a MnTe-MnS binary liquid phase. In the steel with low Te content, a small amount of dendritic MnS inclusion was first precipitated in steel with low Te content, small amount of of dendritic MnSfew inclusion was firstinclusions precipitated in the interdendritic regions. aWith the increase Te content, dendritic MnS were the interdendritic With the increase of Te content, binary few dendritic MnSa inclusions observed. After theregions. solidification of the steel, the MnTe-MnS phase kept liquid statewere and observed. After the solidification of the steel, the MnTe-MnS binary phase kept a liquid state maintained a spherical shape for the minimum surface free energy. and maintained a spherical shape for the minimum surface free energy. 3. When the temperature fell down from 1412 ◦ C to 810 ◦ C, MnS was precipitated from the 3. When the temperature fell down from 1412 °C to 810 °C, MnS was precipitated from the MnTe-MnS binary liquid phase. Once the temperature reached 810 ◦ C, both MnTe and MnS MnTe-MnS binary liquid phase. Once the temperature reached 810 °C, both MnTe and MnS precipitated from the MnTe-MnS binary liquid phase. In the steel with low Te content, the MnTe precipitated from the MnTe-MnS binary liquid phase. In the steel with low Te content, the and MnS precipitated in the form of the divorced eutectic, forming a composite inclusion with MnTe and MnS precipitated in the form of the divorced eutectic, forming a composite inclusion one MnS core. In the steel with high Te content, both divorced eutectic reactions and eutectic with one MnS core. In the steel with high Te content, both divorced eutectic reactions and reactions occurred, leading to the inclusion containing many small MnS particles. eutectic reactions occurred, leading to the inclusion containing many small MnS particles. J.F.J.F. conceived and designed thethe experiments; Q.Y.Q.Y. andand D.Z.D.Z. performed the Author Author Contributions: Contributions:P.S., P.S.,S.Y. S.Y.and and conceived and designed experiments; performed experiments; P.S. and Q.Y. Q.Y. analyzed the data; P.S. wrote the paper. the experiments; P.S. and analyzed the data; P.S. wrote the paper. Funding: This research was funded by the National Key Research and Development Program of China (No. Funding: This research was funded by the National Key Research and Development Program of China (No. 2018YFB0704400), the National Natural Science Foundation of China (No. 51474142, 51671124), the China 2018YFB0704400), National (No. Natural Science Foundation of China 51474142, 51671124), the China Postdoctoral Sciencethe Foundation 2018M632082), and the Open Project(No. of State Key Laboratory of Advanced Postdoctoral Science Foundation (No. 2018M632082), and the Open Project of State Key Laboratory of Special Steel (No. SKLASS 2017-05). Advanced Special Steel (No. SKLASS 2017-05). Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest.
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