Regular Article
Characteristics of MgAl2O4-TiN Complex Inclusion Precipitation and Growth during Solidification of GCr15SiMn in ESR Process
2015 Volume 55 Issue 8 Pages 1693-1698
Details
2015 Volume 55 Issue 8 Pages 1693-1698
MgAl2O4-TiN complex inclusion found in GCr15SiMn ESR ingot with MgAl2O4 as the core and TiN as the periphery is of great harm to steel. The characteristics of this complex inclusion precipitation and growth during solidification of metal molten pool have been theoretically investigated with the help of thermodynamics and kinetics. The results demonstrate that during solidification of metal molten pool, TiN will precipitate on the existed core MgAl2O4 due to the microsegregation of solutes Ti and N. The effect of high cooling rate on the size of complex inclusion with the core MgAl2O4 from the consumable electrode is not significant, particularly for larger size of the core, while with the core MgAl2O4 formed during solidification, its size is greatly affected by the cooling rate, which will possibly provide a useful method of distinguishing the source of the core MgAl2O4 under high cooling rate. At the same cooling rate, the growing extent of periphery TiN decreases with the increasing size of the core MgAl2O4. Meanwhile, decreasing C content in steel within the upper and lower limit will decrease the percentage of this complex inclusion.
GCr15SiMn is one of the most important special steel, with higher demand for inclusion. During ESR (Electroslag Remelting) process, MgAl2O4-TiN complex inclusion is usually found in this steel’s ESR ingot, which will limit its application, such as high speed train and large-scale equipment.
Many studies have reported the precipitation and growth of oxide and TiN during solidification of steel, providing some understanding on the distribution and composition of oxide,1,2,3) and the control of TiN inclusion in steel.4,5) For the complex inclusion with oxide as the core and TiN as the periphery, Wang6) proposed that TiN will precipitate on the core Al2O3 due to decreasing temperature and microsegregation of solutes, Li7) found that the smaller the size of Al2O3, the larger the extent of TiN growth for IF steel. However, at present the behavior of MgAl2O4-TiN complex inclusion precipitation and growth during solidification of GCr15SiMn in ESR process has not been reported.
In this paper, based on models proposed by Ma,8) Liu9) and Zhang,10) the precipitation and growth of MgAl2O4-TiN complex inclusion during solidification of GCr15SiMn in ESR process have been theoretically investigated with the help of thermodynamics and kinetics, which will provide some ideas on controlling this complex inclusion in ESR ingot.
For MgAl2O4 inclusion in steel, some are from the consumable electrode, which has not been absorbed by ESR slag, and some MgAl2O4 inclusions will precipitate during decreasing temperature of metal molten pool. Meanwhile, during solidification of steel, solutes Ti and N will enrich at the solidification front interface due to different solubility in liquid and solid phases, when the actual concentration product exceeds the equilibrium value in liquid steel, TiN will precipitate and growth with MgAl2O4 as the core due to the low disregistry between these two kinds inclusions.11)
2.1. Solutes MicrosegregationTable 1 is the average chemical composition of researched GCr15SiMn ESR ingot, the contents of Ti and N are 23 ppm and 74 ppm, respectively.
| C | Si | Mn | Cr | Alt | Als | P | S | Ca | Mg | Ti | N | O |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| % | % | % | % | % | % | ppm | ppm | ppm | ppm | ppm | ppm | ppm |
| 1.00 | 0.48 | 1.06 | 1.50 | 0.014 | 0.014 | 72 | 11 | 2 | 4 | 23 | 74 | 10 |
The calculation domain of the complex inclusion precipitation and growth corresponds to the half secondary dendrite spacing L, which is divided into Q nodes as shown in Fig. 1.
Schematic diagram for the dividing of calculation domain by nodal elements.
Chemical reaction equation of Ti and N is given as Eq. (1).12,13,14)
| (1) |
Based on previous work,15) at time t, when there is no TiN precipitation, according to the mass conservation of Ti and N, the microsegregation model is given in Eqs. (2), (3) to calculate the TiN precipitated time in steel.
| (2) |
| (3) |
Where
At the beginning of solidification of metal molten pool, the total content T[A] of solute A (Mg, O or Al) in steel is shown in Eq. (4).
| (4) |
Where r0 represents the radius of existed MgAl2O4 in steel at the beginning of solidification, w[A] is the mass percentage of solute A in MgAl2O4. ρMgAl2O4 and ρm respectively represent the densities of MgAl2O4 and steel.
| (5) |
| (6) |
Where Na represents the number of inclusion per unit area, d* is harmonic mean of inclusion size, and di represents apparent particle size of ith inclusion among n inclusions.
During solidification of node m shown in Fig. 1, the sum of solute A content TS,I,[A] in MgAl2O4 existed in solid phase and the remaining amount of solute A content TS,[A] in solid phase from node 1 to node m are shown as Eqs. (7), (8).
| (7) |
| (8) |
Where ri,MgAl2O4,S
represents the radius of MgAl2O4 in node i of solid phase, and
The sum of solute A content TL,I,[A] contained in MgAl2O4 presented in the left liquid steel and the remaining amount of solute A content TL,[A] in the left liquid steel from node m+1 to node Q are given in Eqs. (9), (10).
| (9) |
| (10) |
Where
According to the mass conservation of solute A, Eq. (11) can be obtained.
| (11) |
Based on above Eqs. (4), (7), (8), (9), (10), (11) and approximately mathematical solution, the size of MgAl2O4 in liquid steel at different solid fractions fst can be calculated.
Assume that during solidification of node p, TiN begins to precipitate on the core MgAl2O4 in liquid steel, the calculation domain becomes node p to node Q, as shown in Fig. 2. In Fig. 2, small circles represent MgAl2O4, and the small squares outside the circles represent TiN.
Schematic diagram for the growing calculation of complex inclusion.
With similar method mentioned above, the contents of Ti and N presented in complex inclusion, liquid and solid phases will be obtained, as shown below.
Taking solute Ti for example, total content T[Ti] of solute Ti in liquid steel corresponding to the time of the beginning of TiN precipitation is given in Eq. (12).
| (12) |
Where
During solidification of node m in Fig. 2, the sum of solute Ti content TS,I,[Ti] in MgAl2O4-TiN complex inclusion existed in solid phase and the remaining amount of solute Ti content TS,[Ti] in solid phase from node p to node m are given in Eqs. (13), (14).
| (13) |
| (14) |
Where ρTiN represents the density of TiN. w[Ti] is the mass percentage of Ti in TiN, and ri,MgAl2O4-TiN,S is the radius of MgAl2O4-TiN complex inclusion in node i of solid phase.
The sum of solute Ti content TL,I,[Ti] contained in MgAl2O4-TiN complex inclusion presented in the left liquid steel and the remaining amount of solute Ti content TL,[Ti] in the left liquid steel are shown as Eqs. (15), (16).
| (15) |
| (16) |
Where rt,MgAl2O4-TiN,L is the radius of this complex inclusion in the left liquid steel at time t.
After TiN precipitation, solutes Ti and N in the left liquid steel will reach equilibrium state as shown in Eq. (17). So the coupled growth of periphery TiN will be calculated according to the mass conservation of solutes Ti and N.
| (17) |
Where f[Ti] and f[N] are the activity coefficients of Ti and N in steel, respectively, which can be calculated by using the data in Ref. 17) through the model of Wagner.
The equilibrium partition coefficients of solutes Mg, Al, O, Ti and N between liquid and solid phases and their diffusion coefficients in γ-Fe phase are shown in Table 2.18) In Table 2, R is the gas constant.
| Solutes |
| DS (cm2 * s−1) |
|---|---|---|
| Mg | 0.02 | 0.055 * exp
|
| Al | 0.6 | 5.9 * exp
|
| O | 0.03 | 5.75 * exp
|
| N | 0.48 | 0.91 * exp
|
| Ti | 0.33 | 0.15 * exp
|
The temperature T at liquid-solid interface is given as Eq. (18).8)
| (18) |
Where T0, TL and TS respectively represent the melting point of pure iron (1811 K), liquidus and solidus temperatures of steel studied.
The secondary dendrite arm spacing L can be calculated by Eq. (19).19)
| (19) |
Where Rc is the cooling rate of steel and
The densities of steel, TiN and MgAl2O4 inclusion (ρm, ρTiN and ρMgAl2O4) are about 7000 kg/m3, 5430 kg/m3 and 3580 kg/m3,17) respectively.
The effect of cooling rate on the time of TiN precipitation in steel is shown in Fig. 3, which demonstrates that there is nearly no influence of cooling rate on TiN precipitated time, which means that, from edge area to center area of ESR ingot, TiN will precipitate on the core MgAl2O4 at the same solid fraction during solidification. Under present condition, when the solid fraction reaches to about 0.965, TiN will precipitate in steel.
Influence of cooling rate on the time of TiN precipitation.
Figure 4 illustrates the growth of MgAl2O4-TiN complex inclusion with different initial size 0 μm, 3 μm and 5 μm of the core MgAl2O4 under different cooling rates. It is obvious that the size of complex inclusion, with the core MgAl2O4 formed during solidification of ingot, is greatly affected by the cooling rate. On the other hand, for the core MgAl2O4 from the consumable electrode, its size is influenced by a low cooling rate, while hardly affected by the high cooling rate, particularly in the case of larger size of the core MgAl2O4 and higher cooling rate.
Growth of MgAl2O4-TiN complex inclusion with different initial size of the core under different cooling rates.
Meanwhile, with different initial size of the core MgAl2O4, the final size of complex inclusion is different under high cooling rate, which will possibly provide a useful method of distinguishing the core MgAl2O4 whether are from the consumable electrode. For instance, for the part of ESR ingot corresponding to the cooling rate of 5 K/s, the core of complex inclusion with size above approximately 5 μm is considered from the electrode.
Under the same cooling rate of 0.5 K/s, assume that when TiN begins to precipitate in liquid steel, the size of the core MgAl2O4 in steel are respectively 2 μm, 4 μm, 6 μm, 8 μm and 10 μm. The final size of complex inclusion with these cores will respectively reach to 7.2 μm, 7.5 μm, 8.3 μm, 9.6 μm and 11.1 μm, as shown in Fig. 5.
Influence of size of the core MgAl2O4 on the complex inclusion growth.
Based on Fig. 5, it concludes that under the same cooling rate, the growing extent of periphery TiN decreases with increasing size of the core MgAl2O4, which has been already proved reliable through the analysis of complex inclusion in the steel specimens of ESR ingot using SEM-EDS, as shown in Fig. 6. In Fig. 6, the dark area of the complex inclusion is MgAl2O4, while the light area is TiN.
Morphology of part MgAl2O4-TiN complex inclusions.
Figure 7 demonstrates the influence of C content within the upper and lower limit on the growth of MgAl2O4-TiN complex inclusion under the cooling rate of 0.5 K/s, which reflects that with decreasing C content in steel, the growing extent of periphery TiN declines, which means that it is hard to form the complex inclusion.
Effect of C content on the growth of complex inclusion.
Further, the contents of C and corresponding distributions of single MgAl2O4 and MgAl2O4-TiN complex inclusion at different positions in the rolled steel of GCr15SiMn (as shown in Fig. 8) after ESR process have been investigated. In order to reduce the observed error of inclusion using SEM-EDS, it takes the inclusions in specimens 1-1~1-9, 5-1~5-9 in Fig. 8 as the checked result for edge area, the inclusions in specimens 2-1~2-9, 4-1~4-9 as the checked result for 1/2 radius area and that in specimens 3-1~3-9 as the checked result for center area.
Schematic diagram of specimens cut from the rolled steel of GCr15SiMn.
Table 3 shows the get contents of C in steel at different positions in Fig. 8, it is obvious that the center area of the rolled steel presents the negative segregation of C content. Figure 9 shows the amount distribution of single MgAl2O4 and MgAl2O4-TiN complex inclusion in edge, 1/2 radius and center area of rolled steel, it can be calculated that the percentages of MgAl2O4-TiN complex inclusion in these two kinds inclusions are about 22%, 18% and 12% respectively in edge, 1/2 radius and center area, the percentage of center area is minimal. So the conclusion about the effect of C content on the growth of complex inclusion mentioned above is reasonable and reliable.
| Position | 1-1 | 2-1 | 3-1 | 4-1 | 5-1 |
| C content | 1.01% | 1.00% | 0.98% | 1.00% | 1.02% |
| Position | 1-3 | 2-3 | 3-3 | 4-3 | 5-3 |
| C content | 1.01% | 1.01% | 0.99% | 1.01% | 1.01% |
| Position | 1-5 | 2-5 | 3-5 | 4-5 | 5-5 |
| C content | 1.00% | 1.03% | 0.98% | 1.01% | 1.00% |
| Position | 1-7 | 2-7 | 3-7 | 4-7 | 5-7 |
| C content | 1.00% | 1.02% | 0.98% | 1.00% | 1.09% |
| Position | 1-9 | 2-9 | 3-9 | 4-9 | 5-9 |
| C content | 1.01% | 1.01% | 0.98% | 1.01% | 1.02% |
Number distribution of MgAl2O4 and MgAl2O4-TiN complex inclusion in rolled steel.
Furthermore, this conclusion can also be used to explain the size distribution of single TiN inclusion in this rolled metal, as shown in Fig. 10. In general situation, the size of TiN inclusion in center area will be greater than that in 1/2 radius area due to lower cooling rate, but if the effect of C content on size is greater than that of cooling rate, it will present the different case.
Size distribution of single TiN inclusion in GCr15SiMn rolled steel.
Actually, declining C content in steel will increase the liquidus, solidus temperature and narrow the solidification interval, further delay the time of TiN precipitation in steel15) and decrease the growth time.
The precipitation and growth of MgAl2O4-TiN complex inclusion has been theoretically investigated during solidification of GCr15SiMn in ESR process, the results obtained are as follows.
(1) At the later period of solidification of metal molten pool, TiN will precipitate in steel with MgAl2O4 as the core, and the precipitation time is not affected by the cooling rate.
(2) With the core MgAl2O4 formed during solidification, the size of complex inclusion is greatly affected by the cooling rate, while with the core MgAl2O4 from the consumable electrode, its size is hardly affected by high cooling rate, particularly for larger size of the core, which will possibly provide a useful method of distinguishing whether the core MgAl2O4 are from electrode under high cooling rate.
(3) Under the same cooling rate, the larger the size of MgAl2O4, the smaller the growing extent of periphery TiN. Meanwhile, when decreasing C content in steel within the upper and lower limit, the time of TiN precipitated on the core MgAl2O4 will be delayed and the growing extent of periphery TiN will decline, further result in the decreasing percentage of this complex inclusion in steel.
The authors wish to express their thanks to the workers for this experiment and the financial support provided by 863 Project in China-The Key Technology Development of Bearing Steel for Major Equipment (No. 2012AA03A503).
