Effect of Ultra-high Magnesium on SKS51 Liquid Steel Cleanliness and Microstructure

Abstract

This study emphasizes that ultra-high Mg ([%Mg]>0.03) content is very difficult to be achieved under conventional smelting conditions. The content of Mg in steel has significant influence on the cleanness of molten steel and microstructure. As the content of Mg increases, the content of O and S in steel decreases significantly, with O content as low as 0.0002% and S content as low as 0.0008%. Almost all inclusions in ultra-high Mg steel are magnesium-bearing oxide, sulfide, even carbide. As the content of Mg increases, the number of inclusions in the steel increases and the size decreases. But if too much Mg is added into the steel, the size and number of inclusions will rapid increase. The as-cast secondary dendrite spacing of steel decreased obviously with the increase of Mg content in steel. As the content of Mg increases, the as-cast microstructure changes from lamellar pearlite to granular pearlite. The phase diagram of SKS51 steel was calculated by Thermal-calc software. The calculated results showed that MgC₂ was precipitated in the austenite before the austenite was converted into perlite. MgC₂ may become the nuclear core, leading to perlite transformation. Suspected MgC₂ was found in spheroidized annealing Fe₃C core.

1. Introduction

Mg has good metallurgical properties.^{1,2,3,4,5,6,7,8)} The trace Mg in steel is not only a good deoxidizer and desulfurizer, but also has many metallurgical functions such as modifying and refining inclusions,^3,4,5) refining and spheroidizing carbides.^6,7,8) It is worth mentioning that the current research on Mg metallurgy is limited to the study of trace magnesium on steel metallurgy. The research on the effect of ultra-high Mg on steel metallurgy is still in blank field. The study was largely limited to two factors. One is the physical and chemical properties of Mg itself. The boiling point of Mg is very low and it has high steam pressure. Therefore, it is very volatile at the steelmaking temperature. The other is that it is difficult for current smelting equipment to make Mg stay in the liquid steel for a long time and form effective steam pressure above the liquid steel.

In this study, argon was injected into the furnace to increase the static pressure above the molten steel to increase the Mg yield in the steel. In the study, the static pressure of each furnace was the same, and different amounts of Mg were added to the molten steel, so that the content of Mg in each heat varied in gradient. The capability of purifying liquid steel, refining dendrite and changing microstructure of Mg was tested by using steel with different Mg content to reveal the metallurgy of ultra-high Mg.

2. Experiments

The main steps of the experiment are as follows:

(1) put about 20 kg of industrial pure iron and non-oxidizable alloy into the crucible made of magnesium sand;

(2) put Al into 1# built-in feeding bin; put C, Si and Mn into 2# built-in feeding bin; put the Ni-Mg alloy into 3# built-in feeding bin;

(3) vacuum up to within 10 Pa, heating up to 1600°C, and then keep constant temperature;

(4) after the molten steel melts, argon is flushed into the furnace and pressurized to predetermined pressure.

(5) add Al to the steel from 1# built-in feeding bin for pre-deoxygenation and keep constant temperature about 3 min;

(6) C, Si and Mn were added from 2# built-in feeding bin, with a constant temperature for about 5 min.

(7) add Ni–Mg alloy from 3# built-in feeding bin and keep constant temperature about 5 min.

(8) casting under predetermined pressure;

(9) power off after casting;

(10) decompression after 30 minutes.

After smelting, slice from the same part of the ingot. Spectral analysis and chemical analysis were performed by sampling from slice to detect the steel components. Two 10*10*10 mm samples were taken from each slice. One sample was tested for inclusions, and the other carries on the casting structure analysis. Samples for inclusions detection were polished by 240, 400, 600, 800, 1000, 1200, 1500, 2000 mesh sandpaper, and polished by polishing cloth and DNW1.5 polishing paste after completion. The number and size of inclusions were detected by metallographic microscope. The composition and morphology of the inclusions were detected by scanning electron microscope after metallographic examination. The samples for tissue analysis were prepared according to the above sample preparation process for inclusions detection. After being polished, the samples were corroded with 2% alcohol nitrate solution. Then the microstructure of dendrite and cast structure was observed respectively by metallographic microscope and scanning electron microscope.

3. Results and Discussion

The experiment has four heats which number as: 1#, 2#, 3#, 4#. The smelting pressure of 1–4# is the same, both of which are 3 MPa. The industrial pure iron composition used in the experiment is shown in Table 1. The test results of the composition of each heat steel are shown in Table 2.

Table 1. Industrial pure iron composition (mass fraction, %).

C	Si	Mn	Cr	Ni	Al	O	P	S
0.0018	0.006	0.022	0.024	0.018	0.010	0.0066	0.0061	0.0042

Table 2. Chemical compositions of SKS51 steel (mass fraction, %).

Steels	C	Si	Mn	Cr	Ni	Al	Mg	O	P	S
1#	0.82	0.20	0.40	0.32	1.59	0.043	0.0006	0.0033	0.0072	0.0042
2#	0.82	0.19	0.40	0.34	1.59	0.054	0.0460	0.0012	0.0076	0.0031
3#	0.82	0.20	0.39	0.32	1.61	0.047	0.0720	0.0005	0.0068	0.0013
4#	0.82	0.20	0.40	0.33	1.59	0.051	0.2000	0.0002	0.0073	0.0008

It can be seen from Table 2 that Mg has a strong ability to remove O and S from steel. Before the Mg deoxidizes, a certain amount of Al is first added for deoxidation and Mg is more active than Al, so when Mg is added into the liquid steel, it will modify alumina inclusions in addition to its own reaction with oxygen. In order to better reflect the effect of Mg on inclusions, the related chemical reactions before and after Mg addition were calculated thermodynamically. Some thermodynamic data related to calculations are shown in Table 3.

Table 3. Thermodynamic data of related chemical reactions.

Reaction equation	ΔGθ, J·mol−1	lgK	Reference
2[Al]+3[O]=Al2O3(s)	−1225000+393.80T	−20.57+64000/T	11
[Mg]+[O]=MgO(s)	−89960−82.00T	4.28+4700/T	11
4(Al2O3)+3[Mg]=3(MgO·Al2O3)+2[Al]		34.37−46950/T	12
(MgO·Al2O3)+3[Mg]=4MgO(s)+2[Al]		33.09−50880/T	12

In the experiment, Al was first added for deoxygenation   

$$[\text{Al}]+[\text{O}]={\text{Al}}_2{\text{O}}_{3(\text{S})}$$(1)

It can be obtained from Table 3 and chemical reaction equilibrium   

$$[\text{\%O}]={\left({\displaystyle \frac{a_{{\text{Al}}_2{\text{O}}_3}}{{10}^{64\mathrm{\hspace{0.17em} }000/T-20.57}\cdot {[\text{\%Al}]}^2\cdot f_{\text{Al}}^2\cdot f_{\text{O}}^3}}\right)}^{{\displaystyle \frac{1}{3}}}$$(2)

a_Al2O3 can be considered as 1 relative to the pure solid standard state. The steel standard composition used in thermodynamic calculation are C: 0.82, Si: 0.2, Mn: 0.4, Cr: 0.35, Ni: 1.6, Al: 0.045, O: 0.001, P: 0.006, S: 0.002. According to the first-order Wanger activity formula and Table 4, f_O = 0.34, f_Al = 1.17 can be calculated.

Table 4. Interaction coefficients used in thermodynamic calculation at 1873 K.

$e_i^j$	C	Si	Mn	Cr	Ni	Mg	Al	O	P	S
Mg	−0.24	−0.09	–	0.05	−0.031	–	−0.12	−460	–	−1.38
Al	0.091	0.0056	0.035	0.0096	−0.0173	−0.13	0.043	−0.98	–	0.03
C	0.14	0.078	−0.12	−0.024	0.012	−0.07	0.043	−0.34	–	0.046
O	−0.45	−0.131	−0.021	−0.033	0.006	−300	−1.17	−0.20	0.07	−0.133
S	0.11	0.063	−0.026	−0.011	0	−1.82	0.035	−0.27	0.029	−0.028
Mn	−0.07	0	0	0.0039	–	–	0.027	−0.083	–	−0.048

The deoxygenation reaction of Mg is generally considered as two parts. One part is in the floating process of the Mg bubbles, Mg bubbles react with oxygen. The other part is the reaction of dissolved Mg react with dissolved oxygen after Mg dissolved into liquid steel. The two kinds of Mg deoxygenation reaction are as follows:   

$${\text{Mg}}_{(\text{g})}+[\text{O}]={\text{MgO}}_{(\text{S})}$$(3)

  

$$[\text{Mg}]+[\text{O}]={\text{MgO}}_{(\text{S})}$$(4)

Since Mg dissolves rapidly in liquid steel, Eq. (4) is considered as the main reaction. Therefore, this reaction is only considered to calculate and analyze the Mg deoxygenation reaction. The deoxygenated product MgO is solid pure MgO, so its activity can be considered as 1 relative to the pure solid standard state, then there is:   

$$[\text{\%O}]=\frac{{10}^{-\hspace{0.17em}\frac{4\mathrm{\hspace{0.17em} }700}{T}\hspace{0.17em}-4.28}}{f_{\text{O}}{f}_{\text{Mg}}[\text{\%Mg}]}$$(5)

According to the first-order Wanger activity coefficient formula and Table 4, f_Mg = 0.5 can be calculated. Substitute relevant data into Eqs. (2) and (5), and Fig. 1 can be obtained.

Fig. 1.

Equilibrium relationship Al-O as well as Mg-O. (Online version in color.)

It can be seen from Fig. 1 that Mg has stronger deoxygenation capacity than Al. When the Mg content in the steel solution reaches about 0.01%, the oxygen can be removed to about 0.0002%. The content of Mg in steel is very high under pressure smelting. The high Mg content can make the oxygen in the steel to reach the extremely low level. The calculated results agree with the experimental results.

When Mg is added to steel, Al₂O₃ and dissolved Mg react to form MgO·Al₂O₃, and the reaction is as follows:^9,10)   

$$4{\text{Al}}_2{\text{O}}_{3(\text{s})}+3[\text{Mg}]=3\text{MgO}\cdot {\text{Al}}_2{\text{O}}_{3(\text{S})}+2[\text{Al}]$$(6)

  

$$\text{lg}K=\text{lg}\frac{a_{\text{Al}}^2\cdot a_{\text{MgO}\cdot {\text{Al}}_2{\text{O}}_3}^3}{a_{{\text{Al}}_2{\text{O}}_3}^4\cdot a_{\text{Mg}}^3}=34.37-\frac{46\mathrm{\hspace{0.17em} }950}{T}$$(7)

Then MgO·Al₂O₃ and dissolved Mg react to form MgO, and the reaction is as follows:^11,12)   

$$\text{MgO}\cdot {\text{Al}}_2{\text{O}}_{3(\text{S})}+3[\text{Mg}]=4\text{MgO}+2[\text{Al}]$$(8)

  

$$\text{lg}K=\text{lg}\frac{a_{\text{MgO}}^4\cdot a_{\text{Al}}^2}{a_{\text{Mg}}^3\cdot a_{\text{MgO}\cdot {\text{Al}}_2{\text{O}}_3}}=33.09-\frac{50\mathrm{\hspace{0.17em} }880}{T}$$(9)

In Eq. (6), the activity of MgO·Al₂O₃ is 0.47,¹³⁾ while that of Al₂O₃ is 1, because the solubility of MgO in Al₂O₃ can be ignored.¹⁴⁾ In Eq. (8), because Al₂O₃ has very small solubility in MgO, the activity of MgO is 0.99,¹⁴⁾ while the activity of MgO·Al₂O₃ is 0.8¹³⁾ obtained by experiment by k.fujii et al. The activity of [Mg] and [Al] has been calculated. The MgO/MgO·Al₂O₃/Al₂O₃ phase stability diagram of SKS51 steel can be obtained by substituting relevant data into Eqs. (7) and (9) respectively, as shown in Fig. 2.

Fig. 2.

Phase stability diagram of MgO/MgO·Al₂O₃/Al₂O₃. (Online version in color.)

It can be seen from Fig. 2 that when the Al content of steel is 0.045% and the dissolved Mg content in the liquid steel reaches about 0.0034%, the MgO·Al₂O₃ inclusions in the liquid steel can be modified into MgO inclusions. Ultra-high Mg can easily modify Al₂O₃ inclusions into MgO inclusions.

SKS51 steel has a certain amount of Mn. Before Mg is added into liquid steel, Mn can react with S in liquid steel to form MnS inclusions. Mg and S have a stronger affinity. When Mg is added into molten steel, Mg will directly react with S and MnS in the liquid steel will be modified into MgS inclusions.   

$$[\text{Mg}]+[\text{S}]={\text{MgS}}_{(\text{S})}\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\Delta }{G}^{\theta }=-522\mathrm{\hspace{0.17em} }080+201.02T$$(10)

  

$$[\text{Mn}]+[\text{S}]={\text{MnS}}_{(\text{S})}\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\Delta }{G}^{\theta }=-136\mathrm{\hspace{0.17em} }154.67+44.88T$$(11)

It can be obtained from Eqs. (10) and (11):   

$$[\text{Mg}]+{\text{MnS}}_{(\text{S})}=[\text{Mn}]+{\text{MgS}}_{(\text{S})}\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\Delta }{G}^{\theta }=-385\mathrm{\hspace{0.17em} }925.33+156.14T$$(12)

  

$$\text{lg}K=\text{lg}\frac{a_{\text{Mn}}\cdot a_{\text{MgS}}}{a_{\text{Mg}}\cdot a_{\text{MnS}}}=-8.614+\frac{20\mathrm{\hspace{0.17em} }155.767}{T}$$(13)

The activity of MgS and MnS in Eq. (12) is assumed to be 1. According to Eq. (13), the advantage region diagram of MnS/MgS can be obtained, as shown in Fig. 3. It can be seen from the Fig. 3 that when [%Mn]=0.40 in the steel, as long as [%Mg]>1.78×10⁻³, MnS in the liquid steel can be modified into MgS. For ultra-high Mg SKS51 steel, adding Mg under the condition of pressure causes sufficient amount of Mg to be dissolved in the liquid steel, which is enough to modify MnS into MgS.

Fig. 3.

Phase stability diagram of MnS/MgS. (Online version in color.)

Table 5 shows the results of inclusion metallographic statistics of 1–4# steel. As shown in Table 5, as the content of Mg in steel increases, the total number of inclusions increases, but the total area of inclusions decreases, and the average size of inclusions also decreases (except 4# steel). Figure 4 shows the distribution of inclusions of 1–4# steel in the phase diagram. The red number in the figure represents the number of inclusions falling on the same coordinate position. Inclusions in 1# steel are mostly Al₂O₃, MnS, MgO or their complex inclusion. For ultra-high Mg steel, all the inclusions were modified into MgO, MgS or their complex inclusion. No Mn and Al elements were found in the ultra-high Mg steel inclusions. It is worth mentioning that due to too much Mg in the steel, the number and size of inclusions in the steel will rapidly increase. Some pure Mg was found in 4# steel (as shown in Fig. 5). This may indicate that Mg in steel has reached the solubility. A large amount of magnesium which cannot be dissolved into the steel remains in the steel, which directly leads to a sharp increase in the number and size of inclusions in the steel (as show in Fig. 5). In Table 5, without considering the case of 4# steel, with the increase of Mg content, the number of inclusions with 0–2.5 μm in the steel increases significantly, and inclusions above 2.5 μm decrease significantly. In order to explain the reason why the number of inclusions increased, size decreased and oxygen content decreased significantly in ultra-high magnesium steel, a evolution mechanism of inclusion size and quantity was put forward (as shown in Fig. 6). It can be seen from Fig. 6 that the inclusions in the liquid steel before magnesium treatment are mainly Al₂O₃ and MnS. When Mg is added into the liquid steel, almost all Al₂O₃ and MnS will be modified into MgO and MgS. The excess dissolved Mg in the molten steel will continue to react with the remaining [O] and [S] in the molten steel to form new MgO and MgS. Due to remaining [O] and [S] is less, the size of new MgO and MgS is smaller than first formed MnS and Al₂O₃. In the model, the size of the inclusions is assumed to remain unchanged after being modified. So the size of MgO and MgS formed by modification are larger than newly formed MgO and MgS. In the process of newly formed MgO and MgS nucleation and growth, part of the newly formed MgO and MgS will nucleate and grow up around MgO and MgS formed by modification. So the size of MgO and MgS formed by modification will become larger. Larger size inclusions are easier to float and remove.¹⁵⁾ So the dissolved O and non-dissolved O in steel are reduced simultaneously, and the large size of inclusions are significantly reduced, leaving only small dispersed MgO inclusions. Similarly, S in steel will reduce, with a significant reduction in large size MgS, leaving only more finely dispersed MgS. Therefore, theoretically, the more Mg in the molten steel before [O] and [S] are consumed, the more favorable it is for the inclusions to grow up and be removed.

Table 5. Inclusion statistics of four 1–4# steel.

Heat	Inclusion diameter distribution/μm				Total number	Total area/μm2	Mean area/μm2	Equivalent circle diameter/μm	Percentage of inclusion area/%
	0–2.5	2.5–5	5–10	10~
1#	402	450	303	150	1305	46340.6	35.51	5.20	0.302
2#	328	1875	101	50	2754	39189.4	14.23	3.84	0.256
3#	928	1527	300	11	2765	30055.6	10.87	3.72	0.196
4#	1602	5551	2250	100	9503	154043.6	16.21	4.54	1.000

Fig. 4.

The distribution of inclusions of 1–4# steel in the phase diagram. (Online version in color.)

Fig. 5.

Sample surface morphology and pure Mg inclusion of 4# steel.

Fig. 6.

Evolution mechanism of inclusion size and quantity in ultra-high magnesium steel.

Figure 7 shows the dendritic structure of 1–4# steel. As can be seen from Fig. 7, the dendrites of the steel with Mg are significantly finer than those without Mg. Figure 8 shows the statistical results of secondary dendrite spacing of 1–4# steel. It can be seen from Fig. 8 that with the increase of Mg content in steel, the secondary dendrite spacing is significantly reduced. Figure 9 shows that almost all the inclusions distributed among the secondary dendrites. These inclusions may directly hinder the growth of dendritic crystal. The more inclusions, the smaller the secondary dendrites. The statistical results of dendritic spacing are consistent with those of inclusions. The statistical result of dendritic spacing are consistent with that of inclusions.

Fig. 7.

Dendritic structure of 1–4# steel.

Fig. 8.

Second dendritic spacing of 1–4# steel. (Online version in color.)

Fig. 9.

Inclusions between secondary dendrites of adding Mg steel. (Online version in color.)

Figure 10 shows the pearlitic structure of 1–4# steel. It is easy to find that the pearlite structure of magnesium steel changes from lamellar pearlite of non-magnesium steel to granular pearlite, and the particles of this granular pearlite are very fine. Figure 11 shows 50000 times scanning electron microscopy of 4# steel pearlite. In the Fig. 11, the particle size of pearlite is only about 100 nm. The formation condition of granular pearlite is the formation of a large number of uniformly dispersed Fe₃C nuclei in austenite grains. This can be achieved through heterogeneous nucleation. Figure 12 shows phase and property diagram of ultra-high Mg steel (Mg = 0.03%). It can be seen from the Fig. 12 that MgC₂ is first precipitated from the austenite before austenite transform to perlite. If MgC₂ were numerous and disperse throughout the austenite, then MgC₂ may become the heteromorphic core of Fe₃C. Therefore, the presence of Mg may directly lead to the transformation of the as-cast structure of ultra-high steel from laminated pearlite to granular pearlite. Figure 13 is the schematic diagram of MgC₂ promoting Fe₃C heteromorphic nuclei. MgC₂ was first precipitated from the austenite during the process of ultra-high steel cooling. MgC₂ will become the first formed Fe₃C nucleation core. At the same time of growth of the Fe₃C crystal nucleus, its surrounding parent austenite become carbon-poor areas, creating conditions for the nucleation of ferrite. At the time of ferrite nuclei, it will affect the surrounding parent austenite, making it become carbon-rich area, creating conditions for new Fe₃C nucleation. This is repeated until the austenite transition to granular pearlite is completed. As Mg content in steel is only about 0.2% at most, and the size of Fe₃C is very small, the existence of MgC₂ in pearlitic particles of ultra-high steel has not been found temporarily. However, suspected MgC₂ was found in spheroidized annealing FeC₃ core. Clearly, FeC₃ tends to grow around the MgC₂ core. Therefore, a similar situation may occur in cast steel. If a large amount of dispersed fine MgC₂ is uniformly distributed in cast steel, it is likely that a large number of FeC₃ will grow up around MgC₂ and the lamellar pearlite will change to granular form.

Fig. 10.

Pearlitic structure of 1–4# steel.

Fig. 11.

Pearlitic structure of 4# steel.

Fig. 12.

Phase and property diagram of adding Mg steel (Mg = 0.03%). (Online version in color.)

Fig. 13.

Fe₃C heterogeneous nucleation diagram of ultra-high Mg steel.

Fig. 14.

Suspected MgC₂ spheroidized annealing FeC₃ core.

4. Conclusions

The SKS51 steel in this study was smelted by a pressurized induction furnace. The content of Mg in steel is increased by pressure smelting.

(1) ultra-high Mg can be added into SKS51 steel to reduce the O and S content to extremely low. When Mg content reaches 0.048%, O content and S content are respectively removed to as low as 0.0002% and 0.0008%.

(2) when ultra-high Mg is added to the steel, almost all oxide and sulfide inclusions in the steel become MgO, MgS and their complex inclusion. As the content of Mg increases, the size of inclusions decreases and the number of inclusions increases. However, when the content of Mg reaches a certain level, the precipitation of pure magnesium will lead to the increase of inclusion size and number.

(3) with the increase of Mg content in steel, the secondary dendrite spacing of steel decreased significantly. The reason for the decrease of secondary dendrite spacing may be the pinning effect of magnesium-bearing inclusions among dendrites.

(4) with the increase of Mg content in the steel, the pearlite structure of the steel gradually changes from laminated to granular. In the process of liquid steel cooling, MgC₂ is first precipitated before the austenite of ultra-high Mg steel transformed into pearlite. MgC₂ may be used as the nucleation core of Fe₃C in granular pearlite to transform lamellar pearlite into granular pearlite.

Acknowledgement

The authors acknowledge National Natural Science Foundation of China (No. 51434004, U1435205) for the financial support.

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