Free Access
Issue
Metall. Res. Technol.
Volume 117, Number 2, 2020
Article Number 208
Number of page(s) 9
DOI https://doi.org/10.1051/metal/2020021
Published online 24 April 2020

© EDP Sciences, 2020

1 Introduction

Nowadays, the non-quenched and tempered steel has been widely applied to manufacturing fields, particularly in the form of the machining parts. So a good machinability which is mainly affected by the distribution, composition and quantity of the inclusions in steel is quite vital [13]. However, the machinability of non-quenched and tempered steel improved is mainly to modify the inclusions by adding elements to liquid steel during the refining process [4,5].

Currently, many studies have been conducted to explore the effects of the inclusions on the machinability of the non-quenched and tempered steel [614]. Studies have shown that the sulfide in the steel mainly exists in the form of MnS [15], one of most harmful inclusions. Besides, due to the lower high-temperature hardness than steel matrix, MnS is fiercely deformed to destroy the continuity of steel matrix during the hot rolling, which acts as a stress concentrator in facilitating chip breaking [16,17]. Qi et al. [18] discussed the effect of cooling rate on the precipitation and growth of MnS based on experimental and thermodynamic analyses, and the results showed that with increasing the cooling rate, the time for inclusion growth became shorter and the inclusion size became smaller. Chagas et al. [19] studied the effect of MnS inclusions on the machinability of free-cutting steel by using finite-element numerical simulation method. The results showed that the size of MnS affected the stress-strain in the cutting process. Xia et al. [20] studied the effect of Mn/S mass ratio on the service life of a tool, cutting force and surface roughness of the machining steel, and found that a Mn/S ratio of 3.33 was more advantageous to form MnS lubrication zone in the process of machining.

Presently, a control method of sulfide inclusions is generally to add appropriate modification elements to the liquid steel for obtaining more globular or spindle sulfides. The inclusions can be transformed into calcium aluminates with Ca treatment to reduce the pouring times of steel and to cause the blockage of nozzle. The addition of Zr can produce a large number of fine manganese precipitates and zirconium inclusions to reduce the cleanliness of steel. Rare earth elements have a strong inhibitory effect on oxygen and sulfur, but the mechanism of MnS modification by them has not been clearly elucidated until now and all elements in steel easily react with refractories. Shen et al. [21] studied the Mg–Ca treatment for the re-sulfurized steels, with a result showing that the MnS and Al2O3 inclusions were modified to increase their fatigue life and machinability. Pang et al. [22] have investigated the characteristics of inclusion formation during AOD/LF refining and discussed the change mechanisms of MgO cores and MgO-spinel precipitation, showing that the pure MgO particles were easier to be formed in molten steel under low Al contents.

To study the effect of the inclusions modification on the machinability of the non-quenched and tempered steel by Mg treatment, the industrial tests and machining experiments were carried out in this work. The morphologies and compositions of the inclusions were respectively observed and analyzed by an optical microscope (OM) and a scanning electron microscope (SEM) after the tests. The machining experiments were carried out. Finally, the cutting mechanism of evolution of the inclusions effected by Mg addition was discussed.

2 Experimental methods

2.1 Industrial tests

The production processes of the non-quenched and tempered steel in steel mill were electric arc furnace (EAF), ladle furnace (LF), vacuum degassing furnace (VD) and continuous casting (CC). The industrial tests were divided into two groups (Ca treatment and Mg–Ca treatment). The first one was conducted with Ca treatment, as the contrast group. The other as the experimental group was conducted with Mg–Ca treatment (Mg–Ca cored-wire). In the feeding process of Mg–Ca cored-wire, after the VD, the activity of oxygen was controlled at 8–10 ppm and the molten steel temperature was controlled at 1566–1590 °C. Then the Mg–Ca cored-wire was added to the liquid steel. At the same time, a bottom argon-blowing was supplied when the Mg–Ca core fed. The main chemical composition of Mg–Ca cored-wire was 15%Ca, 10%Mg, and 40%Si (Fig. 1).

The No. 1 sample and No. 2 sample were defined as the contrast group and the experimental group, respectively. The chemical compositions of testing specimens determined by an inductively coupled plasma atomic emission spectrometry (ICP-AES) were shown in Table 1.

thumbnail Fig. 1

Photograph of industrial pilot test with Mg–Ca treatment.

Table 1

The chemical compositions of the testing steels (wt.%).

2.2 Sample analysis

Before SEM-energy disperse spectroscopy (EDS) detection, the surface of each sample with dimensions of 6 × 6 × 8 mm along the rolling direction was polished. The morphologies and chemical compositions of the inclusions were respectively observed and detected with OM and SEM-EDS at different magnifications. Then a non-aqueous electrolytic etching method was used to observe the three dimensional morphologies of the inclusions and their compositions were also detected using SEM-EDS analysis. The number, mean diameter, size distribution and area density of the inclusions were also statistically analyzed by the Image Pro-plus 6.0 software.

2.3 Machining analysis

The machining procedures were described. Before the experiments, each group of samples had four steel bars with 54 mm in diameter and 350 mm in length. The cutting device was a lathe (C61320), with a carbide turning tool (YW2). Cutting experiments were divided into two groups. The first one was the contrast group and the other was the experimental group. Both groups were carried out without cutting fluid. The cutting conditions included feed rate (f) = 0.28 mm/r, cutting depth (ap) = 1.5 mm, and cutting speeds of 130, 180, 210, 260 r/min. At the end of each stage, the chips were collected and the tool wear was observed with a digital microscope. Then the chips produced were counted after the experiments. The roughness of the machining steel surface was recorded by the Profiler SE1200 and the wear degree of the tool after each stage was evaluated.

3 Results and discussion

3.1 Size and morphology of inclusions

As shown in Figures 2a2c, the inclusions in the rolled samples after the Ca treatment were mostly in long-strip shape and their sizes were in range of 100–200 µm, which are detrimental to the mechanical properties when used, caused by stress concentration in the highly dense inclusion direction during the operation of steel. Figures 2d2f show that, when the Mg addition (added) to the molten steel was increased to 8–13 ppm, the quantity and density of the long type inclusions in a certain direction were significantly reduced while some inclusions were presented with spherical or spindle type. The inclusions with sizes of < 100 µm in the rolling direction were greatly reduced, and the spindle and spherical inclusions with size below 20 µm nearly occupied the whole viewing field. It was concluded that Mg addition to steel was beneficial to modifying the morphologies of the inclusions.

Figure 2 also shows that the distribution of the inclusions in the rolled material produced by the Ca treatment was irregular. Most of the inclusions was in long strip, whereas the small number of inclusions was in spindle or spherical shape. After the Mg–Ca refining process, the distribution of inclusions was more uniform, together with more fine spindle-like and spherical inclusions.

Ten metallographic photographs (viewing field: 100X) were selected to count the inclusions by the Image Pro Plus 6.0 software. The average area of the inclusions in the rolled steel produced by the Ca treatment was 30.69 µm2, and the average equivalent diameter of the inclusions was 5.15 µm, and the density of the inclusions was 197/mm2. The average area of the inclusions in the rolled steel produced by the Mg–Ca treatment was 8.54 µm2, the average equivalent diameter was 2.90 µm, and the density was 356/mm2. The inclusions were counted according to different aspect ratios, and the statistical results are shown in Figure 3.

Figure 3 shows that the occupied percentage of the aspect ratio of the inclusions (1–3) produced by the Ca treatment was 3%, whereas the occupied percentage was approximately 80% produced by the Mg–Ca treatment. The slender inclusions could significantly reduce the transverse impact toughness and elongation of the steel, but more short and coarse inclusions are good for the lateral mechanical properties of steel. Besides, the smaller the aspect ratio of the inclusions is, the more favorable the cutting [23].

thumbnail Fig. 2

Optical morphologies of the inclusions.

thumbnail Fig. 3

Statistics for aspect ratios of the inclusions.

3.2 Analysis of inclusion composition

Figure 4 shows the chemical compositions of the typical inclusions, including single inclusions and oxy-sulfide duplex inclusions, determined by using SEM-EDS. Figures 4a and 4b show the typical sulfides after Ca treatment, mainly in the form of long-strip MnS inclusions. Figures 4c4e show the typical inclusions in spherical shape by the Mg–Ca treatment. Obviously, the inclusion size in a certain direction was smaller than that by the Ca treatment. Meanwhile, with the addition of Mg, the morphologies of the inclusions were transformed from long strip to spherical or spindle shape. The composite inclusions mainly consisted of MnS and MgO · Al2O3, and most MnS inclusions encapsulated the MgO · Al2O3 inclusions.

Then 105–150 nucleation cores for the composite inclusions were detected by SEM-EDS, and the statistics for the composition of the inclusions is shown in Figure 5. The number of inclusions in two groups was counted, respectively. The Y-axis was defined as the ratio of single-type inclusion number to total inclusion number. With Mg addition to liquid steel, based on the reaction of [O] and [Mg], the permeated and fine MgO particles with the size of 0.1–2 µm were generated as precipitation cores for pure MnS, resulting in the formation of composite inclusions. The number of MnS inclusions was significantly reduced, which was beneficial to improving its machinability of steel. The Mg addition increased the number of composite inclusions.

The content of Ca in steel was about 9 ppm after Ca treatment. After the SEM observations, the ratio of MnS was highly up to 91.35% and the remaining inclusions comprised CaO–Al2O3 and Al2O3. Generally speaking, the melting-point of CaO–Al2O3 and Al2O3 inclusion was 2600 and 2054 °C, respectively. The oxides with sizes of 8–12 µm were generated in liquid steel during Ca treatment and Al-deoxidization, then MnS precipitated at 1620 °C during solidification. However, with Mg addition to liquid steel, the earlier precipitates of MgO · Al2O3 and MgO particles with the melting point of 2135 and 2852 °C, respectively, highly permeated with the size of 0.1–2 µm. So the size of CaO · Al2O3 was always larger than MgO and MgO · Al2O3. The effect of the larger size oxide on the formation of MnS composite inclusions was worse than that of the smaller size oxide. It can be seen that the percentage of composite inclusions increased from 9.23% to 28.38% compared with original sample after statistics. At 13 ppm Mg and 9 ppm Ca, the MnS content decreased to 61.5%, mainly due to the reaction of Mg and CaO · Al2O3 to produce smaller-sized Mg–Al2O3 inclusions. This phenomenon is well matched with inclusions in figures (c)–(e), showing the disappearance of single MnS inclusions and the enhancement of composite inclusions.

Figure 4e also compared the typical morphologies of MnS and spinel composite inclusions between Ca and Mg–Ca treatment [11]. It can be seen that sulfide inclusions with spinel cores had a small aspect ratio and tends to be globular, while manganese sulfide had a larger aspect ratio and tends to be longer, indicating that the Mg–Ca treatment are better than Ca treatment.

More small-sized cores were formed after Mg–Ca treatment, improving the machinability of steel products. Thus, the modification mechanism of Mg treatment was summarized that: with the addition of magnesium alloy, Ca in CaO and Al2O3 inclusions is replaced by Mg to generate MgO, Al2O3, and Mn in MnS is replaced by Mg to form (Mg, Mn)S.

thumbnail Fig. 4

EDS results of the core and shell of typical inclusions.

thumbnail Fig. 5

Statistics for the morphologies of the inclusions.

3.3 Tri-dimensional morphologies of inclusions

The non-aqueous electrolytic etching was used to observe the tri-dimensional morphologies of the inclusions. The solvent was the organic alcohol, the sample was the anode, and the stainless steel electrode was the cathode. A direct current power supply (KXN-3050) as electrolytic power supply was used. A low-temperature thermostat and a thermometer were utilized to detect and control the temperature, respectively. The reacted container and its cover were made of glass and plastic, respectively. The electrolytic parameters included that: current density, 37.5–52.5 mA cm−2; electrolytic time, 15–25 min; electrolytic temperature, −10–0 °C. The steel matrix was etched under low current to fully expose the morphologies of the inclusions in steel. And the three dimensional morphologies of the inclusions were completely exposed, as shown in Figure 6. With the addition of Mg alloys to liquid steel, the number of cores for the formation of sulfides in steel was changed.

Under Ca treatment, the sulfide in steel was in long-strip shape. At Mg content of 13 ppm, the morphologies of the inclusions were transformed from long-strip shape to ellipsoidal shape. The spherical and spindle inclusions with inner hard points and outer softness appeared, showing that Mg addition contributed to the formation of spindle inclusions and optimized the distribution of the inclusions.

thumbnail Fig. 6

Tri-dimensional morphologies of the inclusions.

3.4 Evaluation of machinability

The profile roughness (SE1200) was used to measure the surface roughness of the processing samples. During the detection process, the measured probe moved perpendicular to the machined surface to obtain the undulation curve of the surface topography of the workpiece. The curve can obtain surface roughness Ra (arithmetic mean deviation) and Rz (microscopic unevenness ten point height). For each processed sample, the surface roughness was measured for three times and an average value was calculated. The result is shown in Figure 7. The average surface roughness of the Mg–Ca treated steel was smaller than that of Ca treatment, as well as the surface extreme difference. With increasing the cutting speed, the surface roughness of the workpiece tended to be decreased. Therefore, Mg–Ca treatment reduced the surface roughness of the workpiece at a same cutting speed, improving the processing efficiency and thereby increasing the machinability of the steel.

In the cutting test, the wear degree of rake face of the tool under different cutting speeds was recorded, and the shape of the rake face was photographed in Figure 8, In the figure, the upper part of the red line was the built-up edge, and the part below the red line was the wear of the flank face. According to the international standard, the wear band width of the flank face on the basis of 1/2 back knife was measured under different cutting speeds, as shown in Table 2.

Table 2 shows that under the same cutting speed and cutting length, the wear degree of the tool after Mg–Ca was smaller than that after Ca treatment.

With increasing the cutting speed, the tool wear increased. At the same time, the wear degree of the tool between the experimental group and contrast group also increased. When the cutting speed was 260 rev/min, the tool wear of the contrast group and experimental group was 0.164 mm and 0.133 mm respectively, showing that Mg–Ca treatment improved the machinability of steel.

During the cutting process, the chips in curl forms were generated. Generally, the shape of the chips can directly affect the machining process. The chips with improper shape will scratch the machining surface. Generally, the more the chips are, the easier the chip removal, and the better machinability. Under the cutting, C-type chips belong to good forms. Figure 5 shows different types of chips at 260 rev/min. The chips under different cutting conditions in two groups were collected and counted, as shown in Figure 9.

thumbnail Fig. 7

Surface roughness of different samples under different cutting speeds.

thumbnail Fig. 8

Photo of tool wear.

Table 2

Record of tool wear (mm).

thumbnail Fig. 9

Statistics for different chips under different cutting speeds.

3.5 Cutting mechanism of Mg–Ca treatment

The stress state of the first and second deformation zones in the process of metal cutting was analyzed from a microscopic point of view. The shear deformation of the cutting metal in the first deformation zone during the cutting process was achieved by the movement of dislocations.

Shibata et al. [24] considered the misfit effect, inhomogeneity effect, and plastic deformation effect of internal stress as a function of the aspect ratio and elastic moduli ratio, and evaluated the internal stresses in and around an oblate spheroidal inclusion by using Eshelby theory. The internal stresses inside the inclusion and at the boundary of matrix-inclusion were obtained. Effects of the aspect ratio of the inclusions and differences in the elastic moduli ratio of matrix-inclusion were examined. The internal stress in an oblate MnS inclusion in the steel matrix is showed in Figure 10. The k in the figure was consistent with the reciprocal of the aspect ratio proposed in this paper. As the k value increased, the stress in the inclusion increased significantly. After the Mg–Ca treatment, the number of the inclusions with aspect ratio of 1–3 increased, meaning that more inclusions were subjected to greater stresses. When the stress reached a certain critical value, the inclusions acted as a stress concentration source during cutting, beneficial for chip-breaking and obtaining uniform chip shapes, thereby increasing the machinability of the material.

Figure 11 shows the transverse fracture morphology of the chip in the experimental group at the cutting speed of 260 rev/min. Figure 11a indicates the observation position and viewing direction of the lateral fracture in red line. Figure 11b shows an SEM photograph of the transverse fracture area of the chip. A schematic showing the stress streamlines around the inclusions is shown in Figure 11c. Figures 11d11f are the enlarged SEM photographs of the partial regions shown in Figure 11b. It is shown that a large number of inclusions existed on the lateral fracture of the chip, and a large number of micro-cracks were extended through the inclusion or across the inclusion. When many micro-cracks further extended and connected together, a long and deep crack formed, eventually resulting in the chip breaking. The fracture indicated that the inclusion plays important roles in the generation and extension of micro-cracks.

Advantages of Mg–Ca treatment versus Ca treatment are summarized as follows:

  • Mg–Ca treatment makes the distribution of the inclusions in steel more dispersed. Under the shear stress gradient field, the uniform distribution of the inclusions causes periodic effects of stress propagation, causing crack propagation and fragmentation, resulting in less energy required for cutting, thus reducing the cutting resistance force;

  • Mg–Ca treatment generates a larger number of composite inclusions with oxide cores, and these inclusions were not easily deformed during the rolling and also did not affect the transverse mechanical properties of the steel;

  • Mg–Ca treatment generates more globular inclusions, beneficial to withstanding greater internal stresses and facilitating the generation of micro-cracks around the inclusions.

thumbnail Fig. 10

Internal stress in an oblate MnS inclusion in the steel matrix [24].

thumbnail Fig. 11

Micro-morphology of chip fracture.

4 Conclusions

With Mg addition to steel, most of the long-strip inclusions were transformed into spindle and spherical inclusions with smaller sizes, and both the density of inclusions and the percentage of composite inclusions have increased.

Compared with Ca treatment, the inclusions in steel with Mg–Ca treatment were more evenly distributed, making the chips more uniform during the cutting process, thereby improving the cuttinig performance of the steel.

The morphologies of inclusions were related to the compositions, and the more inclusions with Mg–Al spinel, the more inclusions with ellipsoidal shape, which were benifit to improving the machinability of the steel.

Acknowledgements

The work is financially sponsored by two National Natural Science Foundations of China (Granted No. 51874195, 51671124).

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Cite this article as: Han Sun, Liang-ping Wu, Jian-bo Xie, Ke-nan Ai, Zhi-qi Zeng, Ping Shen, Jian-xun Fu, Inclusions modification and improvement of machinability in a non-quenched and tempered steel with Mg treatment, Metall. Res. Technol. 117, 208 (2020)

All Tables

Table 1

The chemical compositions of the testing steels (wt.%).

Table 2

Record of tool wear (mm).

All Figures

thumbnail Fig. 1

Photograph of industrial pilot test with Mg–Ca treatment.

In the text
thumbnail Fig. 2

Optical morphologies of the inclusions.

In the text
thumbnail Fig. 3

Statistics for aspect ratios of the inclusions.

In the text
thumbnail Fig. 4

EDS results of the core and shell of typical inclusions.

In the text
thumbnail Fig. 5

Statistics for the morphologies of the inclusions.

In the text
thumbnail Fig. 6

Tri-dimensional morphologies of the inclusions.

In the text
thumbnail Fig. 7

Surface roughness of different samples under different cutting speeds.

In the text
thumbnail Fig. 8

Photo of tool wear.

In the text
thumbnail Fig. 9

Statistics for different chips under different cutting speeds.

In the text
thumbnail Fig. 10

Internal stress in an oblate MnS inclusion in the steel matrix [24].

In the text
thumbnail Fig. 11

Micro-morphology of chip fracture.

In the text

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