Free Access
Issue
Metall. Res. Technol.
Volume 117, Number 1, 2020
Article Number 107
Number of page(s) 8
DOI https://doi.org/10.1051/metal/2019070
Published online 27 January 2020

© EDP Sciences, 2020

1 Introduction

The sulfur-bearing stainless steel has excellent properties such as high corrosion resistance and free-cutting performance making it widely applicable in diverse fields [1,2]. This form of the steel contains a large number of sulfides which play a role in separating metal continuity and lubrication of cutting tools [3]. Therefore, good cutting performance ensures that it can greatly reduce tool wear. However, the lack of good deformation ability of MnS makes it easily deformed along the rolling direction during the rolling process. This leads to cracking of the rolled steel and deterioration of the transverse properties of steel [4,5]. Therefore, it is important to effectively control the sulfide shape. Currently, the addition of various elements into the steel and changing of the strip sulfide into spherical or ellipsoidal shape is done to improve the yield of rolled steel by reducing the anisotropy of steel and the effect of sulfide on the transverse properties of steel [68].

As an element in the same main group as S, Te has a strong affinity for Mn as S and has a good modification effect on sulfide [9,10]. Zheng et al. [11] studied the inhibition of Te on the precipitation of MnS and their findings showed that Te significantly inhibited the precipitation of MnS and reduced the segregation of S. In addition, with this increase in Te content, the distribution of MnS in the steel matrix was more uniform. Yaguchi and Onodera [12] analyzed the effect of Te on the machinability of steel and showed that the addition of Te spheroidized MnS inclusions and improved its cutting performance. Ueda and Morita [13] used the CALPHAD model to calculate the phase equilibrium in iron-containing 1 mass %Mn, 0.3 mass %S and 0.3 mass %Te at different temperatures. The results showed that the MnTe formed by Te atom and Mn atom was a pure phase, and did not react with other substances in molten steel. The findings also showed that MnTe readily reacted with MnS to form composite inclusions. However, limited research has been conducted on the effect of Te transformation from solid solution to precipitation on the size and morphology of sulfide, and on the anti-deformation ability of sulfide during rolling. In addition, due to the lack of thermodynamic data, the mechanism of MnTe precipitation in steel is unclear.

Therefore, in this work, the effect of different Te content on the morphology and deformation resistance of sulfide in Y1Cr13 free-cutting stainless steel during rolling was studied. The formation mechanism of MnTe was also analyzed.

2 Experimental details

The composition of the Y1Cr13 high-sulfur free-cutting stainless steel is listed in Table 1. The experiment was performed in a vertical tube type molybdenum-silicon resistance furnace, and the heating and insulation process was done under the protection of argon. When the temperature in the furnace reached 1600 °C, 99.999% pure Te powder was added to the molten steel. Since Te has a boiling point of only 1390 °C, in order to prevent it from evaporating during the addition process, in this experiment, it was put into molten steel by means of iron sheet wrapping. In industrial production, in order to prevent production accidents caused by Te vapor, it is necessary to adopt a wire feeding method, insert Te-containing core wires into the bottom of the ladle, and cover the surface of the molten steel with refining slag, which can greatly reduce the diffusion of Te vapor and improve the yield of Te. When the steel ingot was completely cooled, a 10 × 10 × 10 mm square and a φ7 × 10 mm cylindrical specimen were cut in the center of each ingot for subsequent analysis and simulated rolling. Resistance furnace smelting temperature and Gleeble-3500 simulated rolling mill temperature settings are shown in Figure 1.

After the samples were cleaned with absolute ethanol, the experimental equipments such as automatic polishing machine, OM(Optical Microscope), SEM-EDS(Scanning Electron Microscope-Energy Dispersive Spectrometer), three-dimensional constant temperature etching electrolyzer, MH-5L digital microhardness tester were used in combination with professional image analysis software Image-Pro Plus and thermodynamic calculation software Factsage for comprehensive analysis of samples. There were 9 groups in this experiment and the Te content in each sample detected by ICP-AES are shown in Table 2. In these groups, sample No. 1 was the contrast sample with no Te added.

Table 1

The composition of Y1Cr13 steel (wt.%).

thumbnail Fig. 1

Temperature setting. a: resistance furnace; b: Gleeble-3500 simulated rolling mill.

Table 2

Te content in each steel sample (ppm).

3 Results and discussion

3.1 Effect of Te on sulfide morphology in ingot

3.1.1 Statistics of sulfide size

The distribution of sulfide in each ingot at 100x magnification of OM is shown in Figure 2. MnS precipitates mainly at grain boundaries were distributed in chains and clusters. An increase in Te content did not correspond to an increase in the number of sulfides. For a more detailed analysis, 20 metallographic photographs of each sample at 100x magnification were selected, and the average equivalent diameter and proportion of aspect ratio counted using professional image analysis software Image-Pro Plus. The statistical results are shown in Figure 3. Due to the high S content in the steel, the size of the sulfide was relatively large and was limited by the resolution accuracy of the statistical software, so diameters less than 1 µm were not calculated.

Figure 3a shows that at 0 ppm Te content, the average equivalent diameter was 4.4 µm. Addition of Te increased its concentration to 4.7 µm an indication that Te can effectively coarsen sulfide. With the increase of Te content, its effect of inhibiting the precipitation of sulfide began to appear [11], the average equivalent diameter began to decrease, but it was still higher than the before Te addition. This indicated that in Y1Cr13 with high S content, Te has limited inhibition ability on sulfide precipitation, and its coarsening of sulfide dominates. When Te content exceeded 180 ppm, the average equivalent diameter did not decrease but rather increased to 4.6 µm. In addition, a 310 ppm increase in Te content, the increase in diameter reached 5.2 µm. Therefore, it can be infered that Te precipitation occurs at 180 ppm and the solid solution Te in sulfide precipitates as MnTe. The enveloped sulfide leads to an increase in an equivalent diameter, and the precipitated MnTe has a better and improved effect on the sulfide morphology than the solid solution Te in sulfide.

For statistical analysis in this experiment, sulfide with an aspect ratio of 1 to 2 was classified as the type I (spherality) and III (irregular), and the sulfides with an aspect ratio of more than 2 were classified as the type I (strip) [14]. Figure 3b shows that the proportion of type I and III sulfides was 70.2% prior to Te addition. However, with an increase in Te content, the proportion increased, especially when Te content exceeded 180 ppm. This indicated that the main function of Te is to promote the transformation of sulfide from strip to spherical and irregular, rather than simply coarsening it. Moreover the precipitated MnTe was found to be more involved in promoting this transformation.

thumbnail Fig. 2

Distribution of sulfides in ingot with different Te contents-100x (ppm). a: 0; b: 44; c: 94; d: 150; e: 180; f: 200; g: 230; h: 240; i: 310.

thumbnail Fig. 3

Statistical results of sulfide in ingot. a: average equivalent diameter; b: proportion of aspect ratio.

3.1.2 Composition and three-dimensional morphology of sulfide

The morphology and composition analysis of typical sulfide in each ingot was determined by SEM + EDS as shown in Figure 4 and Table 3. Typical sulfide in Y1Cr13 steel was grey or grey-black MnS, but with the increase of Te content some of the strip MnS began to change into spheroid and the overall morphology of MnS became more spherical. When Te content reached 180 ppm, white MnTe began to precipitate at the edge of MnS. This caused the statistical of sulfide size to significantly change when Te content reached 180 ppm as shown in Figure 3. With the continuous increase of Te content, more and more MnTe precipitated and began to gradually wrap MnS, by the time Te content reached 310 ppm MnS was completely wrapped by MnTe.

The appearance of sulfide as shown in Figure 4 is only a two-dimensional observation which cannot reflect the true shape of the sulfide. Therefore, this paper adopted the three-dimensional constant temperature etching to corrode the iron matrix and expose the sulfide so as to observe its three-dimensional appearance using SEM as shown in Figure 5. The changing trend of sulfide in three-dimensional morphology was similar to that shown in Figure 4, as the increase in Te content, MnTe gradually enveloped MnS making its shape more closer to spherical. However, when Te was not precipitated, the two-dimensional morphology of MnS was ellipsoidal, but the three-dimensional morphology was strip, which indicated that solid solution of Te cannot significantly improve the morphology of MnS. When MnTe precipitated, the two-dimensional and three-dimensional morphology of MnS was completely close to each other and were both spherical. Therefore, MnTe precipitation is particularly important in changing of MnS morphology.

thumbnail Fig. 4

Morphologies of typical sulfides in ingot with different Te contents (ppm). a: 0; b: 44; c: 94; d: 150; e: 180; f: 200; g: 230; h: 240; i: 310.

Table 3

EDS results for atom contents shown in Figure 4 (wt.%).

thumbnail Fig. 5

3D morphologies of typical sulfides in ingot with different Te contents (ppm). a: 0; b: 44; c: 94; d: 150; e: 180; f: 200; g: 230; h: 240; i: 310.

3.2 Effect of Te on sulfide in the rolling process

3.2.1 Statistics of sulfide deformation

Gleeble-3500 was used to compress the ingot to simulate the rolling process in order to investigate the deformation of sulfide during the rolling process. Sulfide is compressed and deformed after simulated rolling and some sulfide is crushed. In order to observe the deformation of sulfide more carefully, photographs were taken at 200x magnification and the sulfide morphology in the compressed sample is as shown in Figure 6. After compression, most sulfides were deformed to varied degrees and the strip sulfide was significantly increased. The increase in Te content decreased the deformation of sulfide.

Figure 7 shows the variation of the average aspect ratio of sulfide before and after compression.

The increase in Te content promoted the spheroidization of sulfide and the average aspect ratio of sulfide in ingot decreased with an increase in Te content. Therefore, under the same compression deformation condition, the change trend of the average aspect ratio of sulfide in the compression sample was similar to that in the ingot. However, further analysis of the data revealed that the growth rate of the deformation rate of sulfide markedly changed. When Te was not added, the sulfide deformation rate was 21% and theincrease in Te content decreased the deformation rate. When Te content was 310 ppm, the deformation rate was only 7%. This shows that Te not only changes the morphology of sulfide but also improves the anti-deformation ability of sulfide.

thumbnail Fig. 6

Morphology of sulfides in compressed sample with different Te contents-200x (ppm). a: 0; b: 44; c: 94; d: 150; e: 180; f: 200; g: 230; h: 240; i: 310.

thumbnail Fig. 7

Average aspect ratio before and after compression.

3.2.2 Effect of Te on deformation resistance of sulfide

Due to the low hardness of the sulfide, its resistance to deformation is poor, so it is easily deformed during the rolling process. However, its anti-deformation ability was significantly enhanced after the addition of Te, so it was presumed that Te could improve sulfide’s hardness. MH-5L digital Microhardness tester was used to measure the hardness of the compressed samples. Although a minimum load of 10 kg was selected in this experiment, the results may be inaccurate due to the small size of the precipitated sulfide, but it could still be used for the qualitative analysis of the effect of Te content on the trend of sulfide’s hardness change. For each sample, 40 sulfides were selected for testing, and the average value is shown in Figure 8. The increase in Te content increased sulfide’s hardness, and its anti-deformation ability became stronger and this caused the deformation rate of sulfide to decrease. However, when the Te content reached 180 ppm, that is, when MnTe began to precipitate, the hardness of the sulfide began to decrease slightly, but the deformation rate of the sulfide was still decreasing as shown in Figure 7. This indicated that after the precipitation of MnTe, there were other factors affecting the anti-deformation ability of sulfide.

SEM was used to observe the effect of MnTe precipitation on the anti-deformation ability of sulfide infive compressed samples with Te content exceeding 180 ppm, as shown in Figure 9. In the simulated rolling process, the samples were heated to 1150 °C and then compressed while MnTe and MnS had eutectic point at 810 °C [15]. Therefore, at the compression temperature, MnTe at the edge of MnS was in a liquid state. At the beginning of the compression, the liquid MnTe first deformed and extruded to both ends of MnS due to pressure. In this process, MnTe absorbed a lot of rolling stress instead of MnS which reduced the deformation of MnS. However, because of the low Te content, MnTe could not completely encapsulate MnS, therefore, MnS was sill deformed to some extent. When Te content was high and MnTe completely encapsulated MnS, the morphology of MnS did not change significantly. Therefore, as shown in Figure 7, when Te content reached 310 ppm, the deformation rate was only 7%.

The sample could not be heat preservation due to the limitation of equipment conditions in the simulated rolling process. However, hot rolling in industrial production is a process in which the temperature drops slowly. Therefore, the deformed MnTe could slowly restore into a spherical or ellipsoid shape according to the minimum free energy with the action of surface tension [16]. Therefore, Te had a better modification effect in industrial production and the deformation and recovery process of MnTe is as shown in Figure 10.

thumbnail Fig. 8

Hardness of MnS.

thumbnail Fig. 9

Morphologies of typical sulfides in each compressed sample with different Te contents (ppm). a: 180; b: 200; c: 230; d: 240; e: 310.

thumbnail Fig. 10

Deformation and recovery process of MnTe.

3.3 Precipitation mechanism of MnTe–MnS in Y1Cr13

3.3.1 Precipitation of MnS

Based on the analysis above, the typical sulfide in Y1Cr13 steel was MnS, the solution and precipitation of Te were all carried out in MnS. Therefore, it was necessary to study the precipitation of MnS. The thermodynamic software used in this paper was FactSage 7.0. The thermodynamic data of Te in this software was unreliable, so the original composition of steel in Table 1 was used for thermodynamic calculation, without considering the influence of Te on the solidification of molten steel and the precipitation of MnS. Although this will cause some errors between the calculation results and the actual, it was still relatively accurate. The MnS precipitation curve drawn using equilib module in FactSage 7.0 is shown in Figure 11.

The liquidus temperature of Y1Cr13 free-cutting stainless steel was 1481 °C. During the solidification process, δ-Fe began to precipitate first. When the temperature decreased to 1448 °C, MnS began to precipitate, whereas a decrease in temperature increased the amount of precipitation and in this process, δ-Fe was transformed into γ-Fe. When the temperature decreased to 1371 °C, the precipitation of MnS was complete, and the amount of precipitation accounted for 0.6 mass % of the total system. The molten steel was completely solidified when the temperature decreased to 1358 °C.

thumbnail Fig. 11

Precipitation curve of MnS.

3.3.2 Precipitation of MnS–MnTe

Although both the solid solution and the precipitation of Te could modify the sulfide in Y1Cr13, the precipitated MnTe obviously had better effect. Therefore, the precipitation mechanism of MnTe in MnS should be further investigated. Figure 12 shows the MnTe–MnS binary phase diagram [17]. When the point determined by the temperature (x-axis) and the molar fraction of MnTe in Te–Mn–S system (y-axis) was above the dotted line in the graph, MnTe will precipitate in the steel. The addition temperature of Te in this experiment was all 1600 °C, the value of x-axis at the intersection of the temperature and the dotted line was the minimum molar fraction that satisfied the conditions for precipitation of MnTe. Convert this value to Te content, which was about 170 ppm. Therefore, in Figure 4, when the Te content reached 180 ppm, the precipitated MnTe could be observed. In industrial production, since the addition temperature of Te in Y1Cr13 is higher than 1600 °C, as long as the content of Te exceeds 170 ppm, MnTe will be precipitated.

Precipitation process of MnTe and MnS in Y1Cr13 is shown in Figure 13. At 1600 °C, Te was added to form Te–Mn–S solution with Mn and S in the steel. As the temperature decreased, the molten steel began to solidify, when the temperature dropped to 1448 °C, MnS began to precipitate in Te–Mn–S solution and the precipitation was completed at 1371 °C. However, since Te inhibits MnS precipitation [11], Te–Mn–S solution still existed in the steel, and the precipitated MnS was surrounded by the solution. A drop in the temperature to 810 °C, MnTe precipitated in Te–Mn–S solution. Since the precipitation of MnTe reduced the Te content in the solution, resulting in a weakening of its inhibitory effect on the precipitation of MnS, MnS precipitated again. The newly precipitated MnS was bonded to the existing MnS, and the MnTe was pushed out and precipitated alone, thus forming a composite inclusion of MnTe wrapped with MnS.

thumbnail Fig. 12

MnTe–MnS binary phase diagram [17].

thumbnail Fig. 13

Precipitation process of MnTe and MnS in Y1Cr13.

4 Conclusions

The typical sulfide in Y1Cr13 free-cutting stainless steel was MnS. Te had the effect of coarsening MnS, especially the precipitated MnTe could significantly improve the morphology of MnS. The precipitated MnTe wrapped MnS and promoted its transformation from strip to spherical shape.

Te effectively improved the hardness of MnS and enhanced its anti-deformation ability. But more importantly, the precipitated MnTe had the ability to absorb rolling stress which was benefit for the reduction of deformation rate of MnS in the rolling process.

The precipitation of MnTe is a key step in the modification of MnS. In Y1Cr13, when Te content exceeded 170 ppm, MnTe precipitated simultaneously with the newly formed MnS from Te–Mn–S solution at 810 °C. The re-precipitated MnS combined with MnS that had been precipitated at 1371 °C, and repelled MnTe. Under the influence of repulsive force, MnTe encapsulated MnS to form MnTe–MnS composite inclusions.

Acknowledgements

This work is supported by the Natural Science Foundation of China (Granted Nos. 51874195, 51671124).

References

  1. T. Akasawa, H. Sakurai, M. Nakamura, T. Tanaka, K. Takano, Effects of free-cutting additives on the machinability of austenitic stainless steels[J], J. Mater. Process. Technol. 143-144(SI), 66–71 (2003) [CrossRef] [Google Scholar]
  2. H.-L. Wu, Y. Huang, Z. Huang, G.-J. Cheng, Experimental research on the abrasive belt grinding turbine blades material 1Cr13 stainless steel[J], Key Eng. Mater. 487(1), 452–456 (2011) [Google Scholar]
  3. K.S. Atwal, A. Reeder, T.J. Pike, Product characteristics and machinability of bloomcast free-cutting steels[J], Revue de metallurgie : Cahiers d’informations techniques 86(6), 531–542 (1989) [CrossRef] [Google Scholar]
  4. G. Domizzi, G. Anteri, J. Ovejero, Influence of sulphur content and inclusion distribution on the hydrogen induced blister cracking in pressure vessel and pipeline steels[J], Corros. Sci. 43(2), 325–339 (2001) [Google Scholar]
  5. K. Kawakami, T. Tsuyoshi, N. Kunihiko, Generation mechanisms of non-metallic inclusions in high-clean liness steel[J], ISIJ. 93(12), 743–752 (2007) [Google Scholar]
  6. W.-H. Yuan, F. Wang, Present research status and prospects on free cutting steel at home and abroad [J], Res. Iron Steel 36(5), 56–62 (2008) [Google Scholar]
  7. Y.-M. Li, F.-X. Zhu, F.-P. Cui, K. Fang, Analysis of forming mechanism of lamination defect of steel plate[J], J. Northeast. Univ.: Nat. Sci. 28(7), 1002–1005 (2007) [Google Scholar]
  8. A. Segal, J.-A. Charles, Influence of particle size on deformation characteristics of manganese sulphide inclusions in steel[J], Met. Technol. 4(1), 177–182 (1977) [CrossRef] [Google Scholar]
  9. S. Ueda, Y. Matsuki, K. Morita, Experimental evaluation of thermodynamic interactions between tellurium and various elements in molten iron[M], in: Applications of process engineering principles in materials processing, energy and environmental technologies, Springer International Publishing, 2017, pp. 485–493 [CrossRef] [Google Scholar]
  10. E. Costa, N. Luiz, M. Silva, et al., Influence of tellurium addition on drilling of microalloyed steel (DIN 38MnS6)[J], Ind. Lubr. Tribol. 63(6), 420–426 (2011) [CrossRef] [Google Scholar]
  11. L. Zheng, A. Malfliet, P. Wollants, et al., Effect of surfactant Te on the formation of MnS inclusions in steel[J], Metall. Mater. Trans. B 48(5), 2447–2458 (2017) [CrossRef] [Google Scholar]
  12. H. Yaguchi, N. Onodera, Effect of tellurium on the machinability of AISI 12L14 + Te steel[J], Trans. ISIJ. 28(12), 1051–1059 (1988) [CrossRef] [Google Scholar]
  13. S. Ueda, K. Morita, Thermodynamics on the composition and morphology control of MnS–MnTe inclusions[R], in: 171th ISIJ Meeting, 2016 [Google Scholar]
  14. K. Oikawa, H. Ohtani, K. Ishida, T. Nishizawa, The control of the morphology of MnS inclusions in steel during solidification[J], ISIJ. Int. 35(4), 402–408 (1995) [CrossRef] [Google Scholar]
  15. P. Shen, Q.K. Yang, D. Zhang, Y.X. Wu, J.X. Fu, Application of tellurium in free-cutting steels[J], J. Iron Steel Res. Int. 25(8), 787–795 (2018) [CrossRef] [Google Scholar]
  16. O. Kazumi, N. Kiyoshi, Y. Osamu, Effects of selenium and tellurium on the surface tension of molten iron and the wettability of alumina by molten iron [J], ISIJ. 66(2), 179–185 (1980) [Google Scholar]
  17. T.Y. Tien, L.H. Van Vlack, R.J. Martin, The system MnTe-MnS: Progress report[R], University of Michigan, New York, USA, 1967 [Google Scholar]

Cite this article as: Xiangyu Wu, Liang-pin Wu, Jian-bo Xie, Ping Shen, Jian-xun Fu, Modification of sulfide by Te in Y1Cr13 free-cutting stainless steel, Metall. Res. Technol. 117, 107 (2020)

All Tables

Table 1

The composition of Y1Cr13 steel (wt.%).

Table 2

Te content in each steel sample (ppm).

Table 3

EDS results for atom contents shown in Figure 4 (wt.%).

All Figures

thumbnail Fig. 1

Temperature setting. a: resistance furnace; b: Gleeble-3500 simulated rolling mill.

In the text
thumbnail Fig. 2

Distribution of sulfides in ingot with different Te contents-100x (ppm). a: 0; b: 44; c: 94; d: 150; e: 180; f: 200; g: 230; h: 240; i: 310.

In the text
thumbnail Fig. 3

Statistical results of sulfide in ingot. a: average equivalent diameter; b: proportion of aspect ratio.

In the text
thumbnail Fig. 4

Morphologies of typical sulfides in ingot with different Te contents (ppm). a: 0; b: 44; c: 94; d: 150; e: 180; f: 200; g: 230; h: 240; i: 310.

In the text
thumbnail Fig. 5

3D morphologies of typical sulfides in ingot with different Te contents (ppm). a: 0; b: 44; c: 94; d: 150; e: 180; f: 200; g: 230; h: 240; i: 310.

In the text
thumbnail Fig. 6

Morphology of sulfides in compressed sample with different Te contents-200x (ppm). a: 0; b: 44; c: 94; d: 150; e: 180; f: 200; g: 230; h: 240; i: 310.

In the text
thumbnail Fig. 7

Average aspect ratio before and after compression.

In the text
thumbnail Fig. 8

Hardness of MnS.

In the text
thumbnail Fig. 9

Morphologies of typical sulfides in each compressed sample with different Te contents (ppm). a: 180; b: 200; c: 230; d: 240; e: 310.

In the text
thumbnail Fig. 10

Deformation and recovery process of MnTe.

In the text
thumbnail Fig. 11

Precipitation curve of MnS.

In the text
thumbnail Fig. 12

MnTe–MnS binary phase diagram [17].

In the text
thumbnail Fig. 13

Precipitation process of MnTe and MnS in Y1Cr13.

In the text

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