Effects of Mn and Al on the Intragranular Acicular Ferrite Formation in Rare Earth Treated C–Mn Steel

Mingming Song; Bo Song; Zhanbing Yang; Shenghua Zhang; Chunlin Hu

doi:10.1515/htmp-2016-0003

Article Open Access

Effects of Mn and Al on the Intragranular Acicular Ferrite Formation in Rare Earth Treated C–Mn Steel

Mingming Song , Bo Song , Zhanbing Yang , Shenghua Zhang and Chunlin Hu

Published/Copyright: September 15, 2016

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From the journal High Temperature Materials and Processes Volume 36 Issue 7

Abstract

The influence of Al, Mn and rare earth (RE) on microstructure of C–Mn steel was investigated. The capacities of different RE inclusions to induce intragranular acicular ferrite (AF) formation were compared. Result shows that RE treatment could make C–Mn steel from large amounts of intragranular AF. Al killed is detrimental to the formation of intragranular AF in RE-treated C–Mn steel. An upper bainite structure would replace the AF when Al content increased to 0.027 mass %. The optimal Mn content to form AF is about 0.75–1.31 mass %. The effective RE inclusion which could induce AF nucleation is La₂O₂S. When patches of MnS are attached on the surface of La₂O₂S inclusion, AF nucleation capacity of RE-containing inclusion could enlarge obviously. The existence of manganese-depleted zone and low lattice misfit would be the main reason of La-containing inclusion inducing AF nucleation in C–Mn steel.

Keywords: microstructure; rare earth; intragranular acicular ferrite; inclusion; heterogeneous nucleation

Introduction

“Oxide metallurgy technology”, which was put forward by Takamura and Mizoguchi [1, 2], is an effective way of steelmaking to refine austenite grain using tiny dispersed intragranular ferrite (IGF), which includes intragranular polygonal ferrite and intragranular acicular ferrite (AF). The AF laths have a chaotic crystallographic orientation, which could result in a retardation of the propagation path for a cleavage crack in steel. Therefore, the steel toughness increases with an increasing amount of AF. Until now, oxide metallurgy technology has been successfully used in high-strength and toughness steel, non-tempered steel and low-carbon steel [3, 4, 5]. A number of mechanisms have been proposed to explain the function of nonmetallic inclusions on the nucleation of AF: (1) a reduction of the interfacial energy for simple heterogeneous nucleation on the surface of inclusion [6, 7]; (2) an epitaxial nucleation on the inclusion external surface, inclusion lattice has a good coherency with ferrite [8, 9, 10]; (3) the increasing stress around inclusion which resulted from the difference in thermal expansion coefficients of the inclusion and matrix metal [11]; (4) solute elements depletion in the matrix near inclusion, such as Mn [12, 13]. Unfortunately, so far there is still strong controversy over the above mechanism for intragranular AF nucleation at inclusion.

As is well known that small amounts of rare earth (RE) added into steel can play the role of deoxidant, desulfurizer and inclusion modifier, forming high melting point compounds, such as RE_xO_y and RE_xS_y. Thewlis [14] and Wen et al. [15, 16, 17] found that a lot of AF is formed and the performance of steel improved after some Ce is added into C–Mn steel. For the mechanism for intragranular AF nucleation, they speculated that low misfit of Ce-containing inclusions with α-Fe phase, such as Ce₂O₃, CeS and Ce₂O₂S, maybe the main reason for intragranular AF nucleation. But they did not indicate which type of RE inclusions could play the role of inducing AF nucleation during the process of austenite to ferrite. Moreover, elements Mn and Al have great effect on inclusion composition and steel microstructure, which has been attracting a great deal of attention in oxide metallurgy [18]. However, in RE-treated C–Mn steel, there is little study about the effect of Mn and Al on the intragranular AF formation.

The aim of this work is to explore which type of RE inclusions could play the role of inducing AF nucleation during the transformation of austenite to ferrite. Meanwhile, the effect of elements Mn and Al on the inclusion and the mirostructure evolution in RE-treated C–Mn steel has been analyzed.

Experimental procedure

Sample steels were obtained from a high-temperature vertical resistance furnace, which was equipped with an Al₂O₃-coated PtRh30-PtRh6 thermocouple and an FP93 automatic temperature controller with a temperature accuracy of ±2 K. About 300 g high-purity electrolytic iron was melted in Al₂O₃ crucible with an inner diameter of Ø 45 mm. After the high-purity electrolytic iron melted and thermal insulated for 5 min at 1,873 K, high-purity raw materials were added into the melt to adjust the contents of C, Si, Mn, P, S, Al and La, sequentially. Then, the crucible was held at 1,873 K for 5 min again to finish the reaction and to uniform the chemical composition. After that, the power was turned off. When the temperature dropped to 1,373 K at an average cooling rate of 15 K/min, the crucible was taken out from the furnace quickly and quenched into water. Throughout the experiment course, pure argon was fed into the furnace from the bottom at a flow of 1 L/min to avoid oxidation.

Table 1:

The composition of each tested steel/mass %.

Sample	C	Si	Mn	Ni	P	S	O	Al	La	Fe
AR1	0.18	0.22	1.31	–	0.018	0.025	0.0072	<0.003	–	Bal.
AR2	0.18	0.25	1.31	–	0.018	0.025	0.0065	<0.003	0.020	Bal.
AR3	0.18	0.26	1.32	–	0.015	0.022	0.0045	0.027	0.022	Bal.
MN1	0.17	0.24	0.47	–	0.017	0.027	0.0065	<0.003	0.021	Bal.
MN2	0.18	0.25	0.75	–	0.019	0.028	0.0061	<0.003	0.022	Bal.
MN3	0.18	0.25	2.83	–	0.018	0.021	0.0068	<0.003	0.022	Bal.
MN4	0.18	0.26	1.31	–	0.017	<0.001	0.0067	<0.003	0.021	Bal.
MN5	0.18	0.25	–	1.31	0.016	0.026	0.0065	<0.003	0.022	Bal.

To reduce the impact of uncertainty, different amount of high-purity indispensable materials were added into different melts. Table 1 shows the chemical composition in prepared samples. The La content was determined by inductively coupled plasma mass spectrometry; the Si, Mn, P and Al contents were measured by inductively coupled plasma atomic emission spectroscopy; and the C, S and total oxygen contents were obtained by infrared spectroscopy.

Samples were sectioned under the same process in the middle of the cooled ingots. They were mechanically ground and polished to prepare metallographic samples and then were etched for 5–10 s in 4 vol. % nital. A scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS) and an optical microscope were used to examine the changes of inclusions and microstructures in the steels.

The thermodynamic stabilities of various nonmetallic inclusion phases at present were analyzed by the calculation of phase diagram method and the actual calculation was fulfilled by computer software FactSage 6.4.

Results and discussion

Comparison of microstructures

Influence of Al on the microstructure of RE-treated steel

Figure 1 shows the microstructure variation in the steel quenched from 1,373 K with different Al content. The microstructure is mainly martensite (M) and has small amount of bainite in the RE-free steel AR1 with low content of Al, as shown in Figure 1(a). Significant changes are seen in the microstructure of steel AR2 after 0.020 mass % La is added, the main microstructures are intragranular AF, some ferrite side plate (FSP) and a small quantity of pearlite, as shown in Figure 1(b). When the Al content is up to 0.027 mass %, after 0.022 mass % La is added, the microstructure of steel AR3 is bundles of parallel ferrite laths, as shown in Figure 1(c), which is quite different from that of the first two samples. It is obviously that Al has a great influence on the microstructure for the treatment of La. The volume fraction of AF decreases pronouncedly as the Al content increases. Higher Al content is detrimental to AF formation by current experiment.

Figure 1:

The microstructure in steels with different content of Al (a) AR1, (b) AR2 and (c) AR3.

Shim et al. [19] and Lee and Pan [20] also found that Al could restrain the formation of intragranular AF in Ti-containing C–Mn steel. They thought that Al could modify the Ti-containing inclusions as the deoxidizing capacity of Al is stranger than that of Ti in C–Mn steel, which made the effective intragranular nucleation sites insufficient. It is well known that Al is a kind of strong deoxidizing element. Al could react with oxygen firstly to form Al₂O₃ inclusion after adding into C–Mn steel. At present, when the content of oxygen and Al is about 0.0070 and 0.027 mass %, respectively, almost all of the Al could change into Al₂O₃ inclusion. Though deoxidizing capacity of Al is weaker than that of La, Al₂O₃ could also react with RE very easily. This could make the composition of intragranular nucleus changed as well.

Microstructures in La-treated steels with different Mn content

Figure 2 shows the typical microstructure in prepared steels quenched from 1,373 K with different Mn content after La treatment. It could found that when Mn content is 0.47 mass %, the microstructure of steel MN1 is composed of pearlite (P), grain boundary ferrite (GBF) and IGF, as shown in Figure 2(a). As the Mn content increased to 0.75 and 1.31 mass %, the microstructure in steels MN2 and AR2 was mainly intragranular AF, respectively, as shown in Figures 2(b) and 1(b). When the content of Mn is up to 2.83 mass % in steel MN3, the microstructure is composed of martensite (M) and small amount of bainite, as shown in Figure 2(c). It is clear that the content of Mn has a great effect on microstructure as well. The optimized Mn content for AF formation is about 0.75–1.31 mass % at present study.

Figure 2:

The microstructure in La-treated C–Mn steel with different Mn content (a) MN1, (b) MN2 and (c) MN3.

Nonmetallic inclusion species in steel depend on the composition of steel. There is little composition difference of steel MN1, MN2, MN3 and AR2 except Mn content. It is easy to infer that the nonmetallic inclusion species in these steels is not quite different. However, the hardenability effect of Mn on austenite is strong. It is quite natural to expect that the microstructure difference in La-treated steel is possibly caused by different Mn content in steel. As Mn content increases, coarse GBF, which forms at relatively high temperature, could be suppressed. When the content of Mn was too high, the hardenability of austenite was so good that it made austenite form bainite and martensite directly.

Comparison of inclusions

The composition of inclusions in steels AR1, AR2 and AR3 is quite different. Figure 3 shows the calculated equilibrium weight percent of inclusion phases in steels AR1, AR2 and AR3. The main conclusion in La-free steel AR1 is MnS and little Mn–Si–Al–O inclusion. While after some La is added into steel AR2, the main conclusions changed into MnS + La₂O₂S. When the content of Al is up to 0.027 mass %, after La treated the main inclusions in steel AR3 are turned into MnS + LaAlO₃ + La₂S₃. Figure 4 shows the SEM micrographs and the energy dispersive spectrum (EDX) of typical nonmetallic inclusions found in steels AR1, AR2 and AR3. The Fe peaks in the EDX spectra are believed to originate from the steel matrices. Figure 4(a) shows the MnS in La-free steel AR1. The typical precipitation of MnS on the surface of Mn–Si–Al–O inclusion in steel AR1 is shown in Figure 4(b). The La₂O₂S inclusion with patch of MnS on its surface in steel AR2 treated by 0.020 mass % La was shown in Figure 4(c). The micrographs of LaAlO₃ and La₂S₃ in steel AR3 are shown in Figure 4(d) and 4(e). There are also some MnS adhered on their surface. This is consistent with the previous analysis of Al which made the nucleus composition change.

Figure 3:

Calculated equilibrium weight percent of inclusions in steels (a) AR1, (b) AR2 and (c) AR3.

Figure 4:

The micrographs and compositions of typical nonmetallic inclusions in the steels (a and b) AR1, (c) AR2, (d and e) AR3.

Microstructure difference discussion

By far, a lot of researches have reported that a variety of inclusions have the capability to induce the AF nucleation in steel. Tomita et al. proposed that the TiN–MnS complex precipitate could produce fine IGF in the 490 MPa class high-strength steel for offshore structures [21]. Furuhara et al. reported that the MnS + V(C, N) complex precipitates could act as ferrite nucleation sites in Fe–Mn–C alloys [22]. Nako et al. investigated AF formation potency of Ti–RE metal –Zr (TRZ) complex oxide in the simulated heat-affected zone of low-carbon steel. He also found that the AF formation potency of TRZ complex oxide was higher than Ti and Al oxides [23]. It is easy to find that there is an important relationship between the nucleation of intragranular AF and the composition of inclusion. As yet, there is little research about the effectiveness of different types of RE inclusions to induce the formation of IGF.

Furthermore, the microstructure of steel AR2 quenched from 1,373 K is primarily AF after a certain amount of La is added. The typical AF nucleation in steel AR2 was shown in Figure 5. Energy spectrum showed that the composition of nucleus is mainly the compound of La₂O₂S and MnS. The composition of nucleus is consistent with the calculation of species inclusions in steel AR2. Therefore, it can be inferred that the complex La₂O₂S and MnS inclusion is effective to induce AF nucleation.

Figure 5:

The nucleation of acicular ferrite nucleation in steel AR2.

Steels MN4 and MN5 were prepared to clarify the effects of different types of RE inclusions on IGF formation. Figure 6 shows the calculated equilibrium weight percent of inclusion phases in steels MN4 and MN5. The main conclusion in sulfur lean steel MN4 was La₂O₃ without MnS. While the main conclusions in Mn-free steel MN5 are La₂S₃ + La₂O₂S + FeS.

Figure 6:

Calculated equilibrium weight percent of inclusions in steels (a) MN4 and (b) MN5.

Figure 7 shows the SEM micrographs and the EDX of inclusions found in steels MN4 and MN5. La₂O₃ and Mn–Si–Al–O inclusions in steel MN4 are shown in Figure 7(a). Figure 7(b) and 7(c) is La₂S₃ and La₂O₂S in steel MN5, respectively. There is no MnS formation in steel MN4 and steel MN5, only some FeS formed in steel MN5. The EDX spectra confirm the presence of the calculated inclusions in steels MN4 and MN5.

Figure 7:

The micrographs and compositions of typical nonmetallic inclusions in steels (a) MN4; (b, c and d) MN5.

The microstructure of La-free steel AR1 is martensite as shown in Figure 1(a). Thus, it could estimate that regardless of MnS and silicate is single or compound, which have little ability to induce the nucleation of intragranular AF. Similarly, the microstructure of steel AR3 is bundle of parallel ferrite laths as shown in Figure 1(c), so it could infer that whether MnS, LaAlO₃ and La₂S₃ are single or compound, the capability of AF nucleation for these inclusions is not very high either. Simultaneously, there is little intragranular nucleate AF in the microstructure of steel MN4 either, in which the microstructure is mainly bundles of parallel ferrite laths (BF) and some martensite, as shown in Figure 8(a). It means that single La₂O₃ could not act as effective nucleation site for intragranular AF. While the microstructure in steel MN5 was intragranular AF, pearlite and some GBF as shown in Figure 8(b). Combined with the foregoing discussion that La₂S₃ appeared to be inert to the nucleation of intragranular AF as the analysis for steel AR3 at present, AF should be induced by single La₂O₂S inclusion in steel MN5. In addition, the amount of AF in steel AR2 is visibly more than that in steel MN5, this means that the nucleation capability of single La₂O₂S for intragranular AF is inferior to the complex inclusion of La₂O₂S + MnS owing to the lack of MnS attached, significantly. The inclusion composition and the main microstructure in prepared steels are listed in Table 2.

Figure 8:

The typical microstructure in prepared steels (a) MN4 and (b) MN5.

Table 2:

Capability comparison of inclusion inducing AF nucleation.

Steel	Main inclusion	Mirostructure composition	Capability
AR1	MnS+little Mn–Si–Al–O	B+M	−
AR2	La₂O₂S+MnS	AF	++
AR3	LaAlO₃+La₂S₃	BF/WF	−
MN4	La₂O₃	BF/WF	−
MN5	La₂O₂S	AF+GBF+P	+

Nucleation mechanism discussion

Ferrite nucleation position depends on the interaction of local nucleation driving force and resistance. Epistaxis between ferrite and the inclusion surface could reduce the interfacial energy opposing nucleation. It is easy to deduce that the very small lattice misfit of inclusion with ferrite could facilitate intragranular AF nucleation. The contribution of lattice misfit to heterogeneous nucleation behavior has been emphasized by Turnbull et al. [24] and Bramfitt [8]. They theorized one dimension misfit (a/aaα−Feaα−Fe) and two-dimensional misfit (2-D misfit), respectively. Table 3 shows the misfit of La-containing inclusions with the α-Fe at 1,185 K. It can be seen that a/aaα−Feaα−Fe and the 2-D misfit of La₂O₂S with the α-Fe at 1,185 K are very small, which means La₂O₂S is a very effective inclusion to induce ferrite nucleation.

Meanwhile, when MnS is patched on the surface of La₂O₂S, the capability of inducing intragranular AF nucleation increased obviously. It can be found that Mn in steel plays an important role in promoting intragranular AF nucleated at La₂O₂S inclusion. Shim et al. [25] proposed that there is a manganese depleted zone (MDZ) around some inclusion such as Ti₂O₃. And the MDZ could greatly enhance the nucleation driving force of intragranular AF at Ti-containing inclusion [26, 27, 28, 29]. Although there is no direct measurement of MDZ in this work, it can be speculated that there may be a MDZ formed at the vicinity of La₂O₂S inclusion in steel AR2, which could improve the inducing capability of nucleus. It could conclude that the combined effects of small misfit and MDZ made La-added C–Mn steel formed a lot of AF in this research.

Table 3:

The lattice mismatch between La inclusion and the α-Fe at 1,185 K.

Inclusion	Lattice structure	Lattice parameters a/nm	a/aaα−Feaα−Fe	2-D misfit/%
La₂O₂S	Cubic	0.4062	1.417	0.2
LaAlO₃	Cubic	0.3807	1.328	6.1
La₂O₃	Hexagonal	0.3943	1.416	2.8
La₂S₃	Cubic	0.8616	3.006	0.2
La₃S₄	Cubic	0.8819	3.077	2.6
LaS	Cubic	0.5848	2.040	0.7

Conclusions

The microstructures evolution of La-containing C–Mn steels with different Al and Mn contents have been investigated. SEM with energy dispersive spectrum (EDS) and thermodynamics were used to study the inclusion composition in steels. The main finds of the present work are listed as follows:

Al has a great effect on the microstructure in La-containing C–Mn steel, the FSP structure replaces the AF structure at high Al contents.
The content of microalloying elements Mn has a great effect on microstructure in La-containing C–Mn steel. The optimized Mn content for AF forming is about 0.75–1.31 mass %.
The most effective single La-containing inclusion to induce intragranular AF nucleation is La₂O₂S. When steel containing La₂O₂S inclusion with patches of MnS on the surface shows the highest amounts of AF, the capacity of the La₂O₂S + MnS composite inclusion is better than single La₂O₂S inclusions in La-containing C–Mn steel.
MDZ and small misfit maybe the main reason of La-containing inclusion inducing AF nucleation in La-containing C–Mn steel.

Funding statement: The authors are grateful for the support of the National Natural Science Foundations of China (NSFC) (grant no. 51274269) and the Fundamental Research Funds for the Central Universities (grant nos. FRF-SD-12-010A and FRF-SD-12-002B).

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Received: 2016-1-5

Accepted: 2016-5-12

Published Online: 2016-9-15

Published in Print: 2017-7-26

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Articles in the same Issue

https://doi.org/10.1515/htmp-2016-0003

Keywords for this article

microstructure; rare earth; intragranular acicular ferrite; inclusion; heterogeneous nucleation

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