Effect of Mg Treatment on Inclusions of X65 Pipeline Steel

Three heats of X65 pipeline steels are produced in a factory, two of which were treated with magnesium. The characteristics of inclusions in those steels were analyzed by SEM-EDS and ASPEX. And the influence of magnesium treatment on inclusions of X65 pipeline steel are studied in this paper. The results show that: (1) The oxide inclusions in non-magnesium treated steel are mainly Al2O3 inclusions, while the oxide inclusions are mainly MgO·Al2O3 in the steel with magnesium-treated. (2) The proportion of the small size (1-5 μm) inclusions in magnesium-treated steel is relatively high, and that of the large size (> 15-μm) inclusions is obviously reduced. (3) The larger size inclusions are mainly Al2O3 and the inclusions near the low melting zone. The inclusions close to MgO·Al2O3 spinel composition have smaller size, indicating that the magnesium treatment could make the size of inclusions less and disperse the inclusions. (4) Magnesium-treated steel contained a large number of fine MgO·Al2O3 inclusions, which could provide nucleation core for the precipitation of sulfide, thus reducing the precipitation of sulfide at a grain boundary. And the number of pure MnS decrease after magnesium treatment.


Introduction
Hydrogen induced cracking (HIC) is one of the main failure modes of pipeline steel. In recent years, oil and gas pipeline have developed to the large diameter and high pressure direction, which require pipeline steel to keep higher strength and toughness [1]. However, the higher the strength of steel, the greater the sensitivity of hydrogen-induced cracks [2]. Inclusions in steel are usually regarded as hydrogen traps. When the pipeline steel undergoes a rotting reaction in an acidic environment, the resulting hydrogen enters the steel ification of inclusions in pipeline steel was calcium treatment [7,8], so that, the MnS inclusions in the steel are modified into spherical CaS inclusions and CaO-Al 2 O 3 -CaS complex inclusions, and the B type inclusions are modified into low melting point of calcium aluminate inclusions, which are easy to grow up and float. However, to some extent, calcium treatment could improve the adverse effect of MnS, but the low-melting calcium aluminate inclusions would be formed after calcium treatment. And if the low-melting calcium aluminate inclusions don't eliminate completely in time, they will form a cluster CaO-Al 2 O 3 inclusions in rolled metal, which seriously affect the anti-HIC performance of the product [3,[9][10][11]. It is obvious that calcium treatment has some deficiencies in modification of inclusions in pipeline steel.
Magnesium treatment could make the inclusions small and dispersed [12][13][14], and MnS inclusions also would be modified to complex sulfide by magnesium treatment [15][16][17]. In view of the advantages of magnesium treatment, this paper carries out the magnesium treatment on X65 pipeline steel, and the effect of Mg on inclusions are analyzed in pipeline steel. The results could provide a reference for the application of magnesium treatment of pipeline steel.

Experimental
Three heats of industrial trials are performed in a 200t ladle and recorded as No.1, No.2 and No.3 respectively. The production route is "BOF(basic oxygen furnace) → LF(ladle furnace) → RH → magnesium treatment → soft blowing by Ar gas → CC (continuous casting)". In particular, magnesium treatment was not applied to heat No. 1 but the other process is identical. The details of the process practices could be described as follows.
During tapping of BOF, synthetic slag and ferroalloys with aluminum are added into the stream of liquid steel. After that, the melt is stirred by argon to made the composition and temperature homogenized. And then the melt is transported to LF refining station, in which high basicity slag is used and aluminum is added to decrease the oxygen and sulfur of the steel. After 40 min, the melt is delivered to RH refining station, at this station the liquid steel is treated at the pressure of 67 Pa for 20 min, and then 120 m and 220 m magnesium wire are inserted into the ladle by a wire-feeding mechanism for Heat 2 and 3 respectively. After magnesium treatment, the molten steel is gently stirred for 10 min to promote the inclusions floating upward. And then the ladle is transferred to the continuous caster.
The steel samples are taken from casting billet. The contents of C and S of the samples are analyzed with infrared analysis. The concentrations of Si, Mn, P, Mg, Nb, V, Ti, Als (dissolved aluminum) and O (dissolved oxygen) are determined using ICP-OES (inductively coupled plasma-atomic emission spectroscopy). The N content is analyzed using thermal conductance. The results are shown in Table 1. All the samples are cross-sectioned and polished, and then morphology and composition of inclusions are analyzed by SEM(Scanning Electron Microscopy), EDS (Energy Dispersive Spectrometer) and ASPEX (Aspex scanning electron microscopes).

Morphology and composition of the inclusion
The compositions and morphologies of inclusions in samples are analyzed by SED-EDS.

Heat
Chemical composition, wt %            seen from this figure, after the magnesium treatment, the proportion of inclusions of the particle size of 1-2 μm increases.

Effect of magnesium treatment on oxide inclusions of steel
When Mg is added to the steel, the following reactions occur [18]:

Size of inclusions
The size of the inclusions in the samples was quantitatively analyzed by ASPEX, and the results are shown in Table 2. It can be seen from the table, in the steel with magnesium treatment, the number of inclusions with small particle size (1-3 μm) increases. Since more than 90% of the inclusions in the steel have a particle size of less than 5 μm. Statistical classification of the inclusions is carried out, and the results are shown in Figure 10. As can be of other substances are calculated using equations (4) and (5), where the interaction coefficients of the elements are shown in Table 3 [5,19,20].
[ ] The equilibrium phase diagram of MgO-MgO·Al 2 O 3 -Al 2 O 3 at 1873 K can be calculated by the Matlab software, as shown in Figure 11. As can be seen from the figure, after magnesium treatment, the content of [Mg] and [Al] in the steel is in the range of MgO·Al 2 O 3 , so inclusions of the steel added magnesium are mainly MgO·Al 2 O 3 , which is consistent with the experimental results. Classify the oxide inclusions in the steel, which is shown in Table 4, and every oxide type include pure oxide and MnS-oxide. In the heat No.1 without Mg, the inclusions in the steel are mainly Al 2 O 3 inclusions, the proportion is 93.85%, and that of MgO-Al 2 O 3 inclusion are only 2.05%. After magnesium treatment, the proportion of MgO-Al 2 O 3 inclusions in the steel are increased to 57.53% and 74.30% respectively, in the heat No.2 and No. 3, and that of Al 2 O 3 inclusion in these heat are reduced to 33.71% and 17.04%, respectively.
Calculate the average composition of the oxide       the area of low melting point ( Figure 13 and Figure  14). The inclusions in these two regions tend to aggregate and grow, and if they do not float to the surface in time, they will stay in the steel. As can be seen from Figure 13, the size of inclusions closed to MgO·Al 2 O 3 spinel is smaller since it is not polymerize in the molten steel and easy to disperse in the steel [12][13][14]. The average composition of the ox-inclusions in the different particle size ranges and label them in the phase diagram, as shown in ic data, listed in Table 1 and Table 3, respectively.
And the calculation results are shown in Figure  16. From the figure, the composition points of the steel treated by magnesium are below the MgO-MgO·Al 2 O 3 -MgS equilibrium line. Therefore, there are no single MgO inclusions in the steel, which is consistent with the previous analysis results. There will be MgS or (Mn·Mg) S inclusions in the steel. The high temperature anti-deforming capability of this kind of sulfide is higher than that of pure MnS, which can reduce the deformation degree during the rolling process, so as to avoid the form of long strips in the final product, and improve the anti-HIC performance of the pipeline steel.
The liquidus temperature (T L ) and solidus temperature (T Sol ) of the steel is calculated by the following formulas (Eq.(10) and Eq.(11)) [21], and the critical precipitation temperature of MnS can be calculated using the Matlab software. Figure 17 is changes in critical precipitation temperature of MnS and solidus temperature (T Sol ) of the steel with Mn content and S content. The composition of the steel also are plotted in the figure. As can be seen from the figure, the critical precipitation temperature of MnS in every heat are lower than the solidus temperature, thus, MnS could not be formed in the experimental steel over the solidus temperature, and the MnS must be precipitated during solidification process because of the segregation.  Figure 15. From the figure, with the increase of Mg content in the steel, the composition of the oxide inclusions is constantly approaching to the spinel. Therefore, the proportion of the small size inclusions in the heat No.3 is the largest (Figure 10 and Table 2).

Effect of magnesium treatment on sulfide inclusions in steel
The formation of sulfides in steel can be expressed by the following formula [5]: K Mn = 0.76 [23], K S = 0.05 [23], C S is the solid concentration of a given solute element at the solid-liquid interface.
The variations of manganese and sulfur concentration with solid fraction in the steels are calculated and showed in Figure 18. The results show that there is MnS precipitation when the solid fraction is approximately 0.95. And the MnS would be precipitated on the oxide during solidification process, which is agree well with the SEM/EDS results (Figure 1, Figure 2 and Figure  3). In the heat No.1, the oxide is mainly Al 2 O 3 , so the MnS mainly precipitated on it. (Figure 1). In the other heat, the oxide is mainly MgO·Al 2 O 3 , therefore, the cores of MnS are principally spinel (Figure 2 and Figure 3).
In conclusion, because of the large amount of smaller MgO·Al 2 O 3 inclusions in the steel with magnesium, which is possible to provide nucleation cores ( Figure 2 and Figure 3) for the precipitation of sulfides, the precipitation of sulfides at grain boundaries will be reduced and they will be dispersed. And the number of pure MnS decrease in the steel with magnesium ( Figure 7, Figure 8 and Figure 9). At the same time, the sulfide with core  (15) based on the Scheil model [22].
( -1) 0 Where T 0 , T SOL , T L , T are pure iron melting point (1809 K), solidus temperature, liquidus temperature and solid-liquid interface temperature, in K. C L is the liquid concentration of a given solute element at the solid-liquid interface, C 0 is the initial liquid concentration, f S is the solid fraction, K is the equilibrium partition coefficient for that element,    sium treatment can form (Mn·Mg)S composite sulfide in the steel, further reduce the deformation degree of sulfide during the rolling process, so as to avoid the form of long strips MnS in the final product and influence on the performance of pipeline steel.
can further improve its capability of high temperature anti-deformation, and then further reduce its deformation degree during the rolling process. Accordingly, magnesium addition to the pipeline steel could improve its properties.

Conclusion
Effect of Mg treatment on inclusions of X65 pipeline steel is studied. By comparing and analyzing the composition and size of inclusions in non-magnesium-treated steel and magnesium-treated steel, the following conclusions are obtained: (1) The oxide inclusions in non-magnesium-treated steel are mainly Al 2 O 3 inclusions (93.85%). The oxide inclusions in magnesium-treated steel are mainly MgO·Al 2 O 3 inclusions. With the increase of magnesium content in the steel (from 0.0013% to 0.0024%), the proportion of MgO·Al 2 O 3 inclusions increase from 57.53% to 74.30%, and the inclusion components are closer to MgO·Al 2 O 3 spinel.
(2) The proportion of the inclusions in the magnesium-treated steel with a small particle size (1-5 μm) is relatively high, besides, the proportion of inclusions in the range of 1-2 μm is higher than that in non-magnesium-treated steel. In the magnesium-treated steel, large particles (> 15 μm) inclusions are obviously reduced (the proportion is 0.21%, and that in the two magnesium-treated steels are 0.10% and 0%, respectively).
(3) Larger size inclusions are mainly Al 2 O 3 inclusions and the inclusions near the low melting point. These inclusions are easy to polymerize and grow up, and if they fail to float to the surface of molten steel, they will remain in the steel. The particle size of the inclusions near the MgO·Al 2 O 3 spinel is small, indicating that the MA spinel is difficult to polymerize in the molten steel, and the magnesium treatment can make inclusions small and dispersed in the steel.
(4) There are a large number of fine MgO·Al 2 O 3 inclusions in the magnesium-treated steel, which can provide the nucleation core for the precipitation of sulfide, thus reduce the precipitation of sulfide at a grain boundary and disperse it. And the number of pure MnS decrease after magnesium treatment. At the same time, the magne-