Acicular Ferrite Amount
As described in Section II, microstructures have been analyzed for all samples after heat treatment in the HT-LSCM. The results are displayed in Figure 4. For each sample, a mean value obtained by analyzing three different positions on the sample is given. In all cases, the microstructure is very homogeneous over the whole sample area. By means of HT-LSCM, the formation of acicular ferrite was observable in situ, as shown for sample A1-1 in Figure 5. It is clearly observed that a needle nucleated on an active nonmetallic inclusion. The needle grew with the ongoing cooling process until the impingement with another transformation product.
Steels A3 and A1 provided the highest potential for acicular ferrite, showing a percentage of greater than 70 pct in the final microstructure (see Figures 6 and 7). Also in steel C1, a significant potential for acicular ferrite was observed. Samples A2 and B1 show only limited potential for acicular ferrite formation under the defined conditions. In the case of A2 also, variations in the heat-treatment and cooling parameters do not improve the results (see Figure 6). One of the major reasons for the low AF content in A2 is the very low matrix content of manganese. Details regarding this aspect will be given in a later subsection.
In the current study, steels without titanium were inactive for acicular ferrite. For example, in steel C1 with 0.03 wt pct titanium, an acicular ferrite fraction of 44 pct resp. 59 pct depending on the cooling rate was detected. Without titanium addition, no acicular ferrite formed regardless of the cooling rate, due to a lack of Ti-rich inclusions that act as active nucleation sites (see Figure 8).
Steel B1 exhibited a lower potential for forming acicular ferrite than steels A and C did; nevertheless, still 8 pct acicular ferrite grains are found. Figure 9 clearly displays that acicular ferrite grains formed in steel B were significantly shorter than the grains formed in steels A or C.
Steel D provided no potential for forming acicular ferrite due to the steel’s composition. The high carbon content of the pearlitic steel suppressed the formation of acicular ferrite completely; consequently, the capability of inclusions in steel D to act as nuclei for acicular ferrite could not be studied.
Active Inclusion Types in Steels A, B, and C
Figure 10(a) summarizes the results of manual and automated SEM/EDS analyses for representative examples of steels A, B, and C. For each case, all detected inclusion types are displayed with their percentages (in black) in relation to the absolute number of detected inclusions. In addition, their percentages (in red) also within the total number of active inclusions are given. The ratio between these two values helps to evaluate inclusions’ acicular ferrite nucleation potential; the higher the ratio of active percentage to total percentage, the higher the inclusion type’s potential.
Considering the absolute number of detected inclusions, a variation between 200 and 300 inclusions per mm2 between the analyzed samples is found. This deviation is seen as quite homogeneous and comparable since samples have not been deformed before SEM/EDS analyses. Nevertheless, sample A1-1 shows the highest number of detected inclusions per mm2, followed by sample A2-1. Inclusion numbers in samples B1 and C1-1 are very similar around 200 per mm2. The lower inclusion number in these samples would correspond to a lower content of AF compared with A1-1, for example. However, the inclusion number in A2-1 is close to A1-1, but the AF content is much lower. Of course, an adequate minimum number of possible nucleation sites is necessary. However, the absolute inclusion number can be very high—if there are not enough active inclusions present, AF formation is not promoted.
Figure 10(b) summarizes the detected inclusion size ranges for every inclusion type in a representative sample for each steel grade. The mean Equivalent Circle Diameter (ECD) of most inclusions (marked with the small square in each box) is below 2 µm, and except Al2O3 inclusions in B-1 for every type, 75 pct of all the detected inclusions are smaller than 2.5 µm ECD (defined through the upper line of each box). The upper size limit for 99 pct of inclusions is marked with a cross for each case. In the literature, several authors[33,34,35,36] described a boost in nucleation potential with the increasing inclusion size, which is in good accordance with earlier findings of Ricks et al.[37,38] However, it should be noted that in many cases also, a lower and an upper critical sizes for active inclusions are defined. The lower critical value is the minimum size beneath which particles are too small to act as active nuclei. This value was determined to be 0.3 to 0.5 µm in various studies.[34,39,40] Ricks et al.,[37,38] Lee et al. and Mu et al.[33,34,35,36] investigated inclusions smaller than 1.5 µm and defined 1 µm as the upper critical value. Above 1 µm, the probability curve flattens out. Huang et al.[34,39,40] who considered inclusions up to 7 µm, observed 1.5 µm as the upper critical value but described a sharp decrease in the nucleation probability above 4 µm, resulting in inert behavior of inclusions larger than 6.5 µm. Similar results were gained by Song et al. and Wang et al. who found inclusions with sizes ranging from 1 to 3 µm and 1 to 2 µm, respectively, as the most appropriate nuclei. Inclusions below and above these ranges were less active. The inclusion sizes observed in the present study were all in a very similar range and in good agreement to the values defined as appropriate in the literature. For these reasons, the influence of inclusion size was not further studied in this study. Instead, the current study was focused on inclusion composition and inclusion morphology. In the following, detailed results for samples A1-1, B-1, and C1-1 are discussed.
Pure titanium oxides or titanium oxides with small amounts of dissolved manganese were detected in the sample. Pure TiOx was not found as acicular ferrite nuclei in this steel. (Ti,Mn)Ox was identified as highly active in steel A1; despite (Ti,Mn)Ox representing only a small fraction of the total inclusion number, 10 pct of the active inclusions were of this type.
TiOx was very often found in combination with MnS forming (Ti,Mn)OxSy particles. This inclusion type was found to be composed of a TiOx-containing core and a MnS layer. Due to the interacting volume during EDS analysis and the resulting overlap of signals from TiOx and MnS, no definitive statement about dissolved manganese in the TiOx could be made. (Ti,Mn)OxSy had proven to be an effective nucleation site for acicular ferrite; 7 pct of the active inclusions in steel A1 were of this composition.
Due to the reaction with the used alumina crucible, Al2O3 was frequently included in the nonmetallic inclusions. High amounts of (Ti,Al)Ox and (Ti,Mn,Al)Ox inclusions were found in the sample A1. Manual SEM/EDS analyses revealed these inclusions were composed of homogeneous oxidic phases or multiphase inclusions consisting of Al2O3, TiOx, and (Ti,Mn)Ox. The manual analyses of active inclusions revealed that (Ti,Al)Ox particles accounted for 10 pct of the acicular ferrite nuclei. (Ti,Mn,Al)Ox was also confirmed as an active type, but with a low potential. Although (Ti,Mn,Al)Ox was the most common inclusion type in steel A1, only 14 pct of the acicular ferrite nuclei were of this type.
Occasionally, (Ti,Al)Ox and (Ti,Mn,Al)Ox inclusions had a MnS layer. The formed (Ti,Mn,Al)OxSy inclusions were highly active and represented 38 pct of the active particles in this grade.
MnS was found to nucleate heterogeneously on preexisting oxidic phases in many cases, but a significant amount of pure MnS particles were also detected. MnS was asserted to be an effective inclusion type because 7 pct of the analyzed acicular ferrite nuclei were of pure MnS.
In addition, TiN was found, which was determined to be inert for acicular ferrite nucleation in steel A1.
(Ti,Mn,Al)Ox inclusions were identified in sample B1 by automated SEM/EDS. As previously explained, based on the automated SEM/EDS results, this type of inclusions could not be distinguished between homogenous and multiphase TiOx-(Mn)Al2O3 particles. However, this inclusion type was not found as nuclei for acicular ferrite in steel B; however, it was active in steel A1.
MnS was detected as a stable inclusion phase in steel B. Despite the small number, MnS represented 25 pct of the active inclusions in steel B. Like in steel A1, (Ti,Mn)OxSy inclusions were formed by heterogeneous nucleation, but in contrast to steel A1, this type was ineffective in steel B.
The largest fraction of all inclusions and, also, the largest fraction of active inclusions (75 pct) was that of (Ti,Mn,Al)OxSy. Therefore, this inclusion type was identified as highly active.
As in steel A1, high alumina inclusions were found in steel B1 owing to the crucible material. These inclusions were found to be inert in the present case, in accordance with most reports in the literature.[5,9,19,40,43,44,45]
Considerable amounts of (Ti,Mn,Al,Si)Ox particles were detected in steel C1. However, this inclusion type was identified as inactive for acicular ferrite, contrasting with the literature reports.[12,13,42]
Small amounts of pure MnS particles were found in steel C1. Most MnS particles were composed of a heterogeneously nucleated layer on oxidic inclusions. Nevertheless, 15 pct of the active inclusions in steel C1 were pure MnS. Hence, MnS was confirmed as highly active in steel C1.
No pure TiOx was contained in this steel. However, small amounts of (Ti,Mn)OxSy inclusions were encountered, which consisted of a titanium-containing oxidic core and a MnS layer. As explained before, it was not possible to distinguish between TiOx and (Ti,Mn)Ox cores. 10 pct of the active inclusions in steel C1 were of the (Ti,Mn)OxSy type.
The mixed oxides in this steel, containing titanium, manganese, aluminum, and/or silicon, often showed layers of MnS, forming complex oxysulfides. Although the literature suggests (Ti,Mn,Si)OxSy as active, this inclusion type did not promote the nucleation of acicular ferrite in steel C1. In accordance with other reports, (Ti,Mn,Al,Si)OxSy was found as an operative nuclei type, albeit only occasionally. (Mn,Si)OxSy has been described as inert in the literature, but it was sporadically found as nuclei in this study. These differences between the present observations and the literature may be the result of the variations in the steel compositions.
The SEM/EDS identified a considerable amount of TiN-containing inclusions. There were no, or at least inconsistent, descriptions of these inclusion classes found in the literature. The current results demonstrate that (Ti,Mn,Si)OxSyN inclusions were inactive.
(Ti,Al)Ox, (Ti,Mn,Al)Ox, and (Ti,Mn,Al)OxSy inclusions accounted for only a very small fraction of the total inclusions in steel C1, but these types have been demonstrated to be highly active for acicular ferrite nucleation, as they represent 10, 15, and 20 pct, respectively, of the acicular ferrite nuclei in this steel.
Interaction of Solute Manganese and Manganese Inclusions
Manganese is commonly described as crucial for the formation of acicular ferrite in the literature.[8,46,47,48] In general, the migration of manganese in nonmetallic inclusions and the resulting manganese-depleted zones has been observed as the trigger mechanism for acicular ferrite nucleation. To investigate the effect of manganese fluctuations in the matrix on the acicular ferrite formation, steel A1 was compared with steel A2, which is identical in chemical composition, except that no additional manganese was added to steel A2 in the Tammann-type Furnace experiment. However, as shown in Figure 11, huge amounts of manganese-containing inclusions were detected in the sample. This is explained by the fact that the raw material also already contains 0.07 wt pct Mn as well as a preexisting inclusion population. Automated SEM/EDS analyses of the raw material showed Al2O3, (Al,Mn)Ox, (Si,Mn)Ox, MnS and (Al,Mn)OxSy as the predominant inclusion types. Although the overall inclusion population in the raw material is low compared with the inclusion content after the melting experiment, these preexisting inclusions can influence the final inclusion landscape, for example, through modification to (Ti,Mn,Al)Ox. The formation of new high Mn-containing inclusions in this sample only plays a minor role. Furthermore, it has to be mentioned that the percentage of Mn in these complex (Ti,Mn,Al)Ox inclusions is very low. Inclusion sizes are comparable to A1-1 except the type (Ti,Al)Ox which shows several outliers above 5 µm ECD. However, as already shown in Figure 6, only a low acicular ferrite amount was produced without additional manganese addition in steel A2. With the significantly lower manganese content, the microstructure changed from highly acicular ferritic one to mainly Widmannstätten ferritic type with pearlite and considerable amounts of grain boundary ferrite. A decrease in the cooling rate (CR) increased the amount of pearlite and grain boundary ferrite, but not that of acicular ferrite. Widmannstätten ferrite is a grain boundary nucleated phase; nevertheless, a reduction of the grain boundary surface by increasing the austenite grain size (AGS) also did not enhance the intragranular nucleation.
The characterization of the inclusion landscape demonstrated that the suppression of acicular ferrite in steel A2 was not the result of the absence of manganese-containing inclusions, but the result of lacking soluble manganese. Sample A2 contains inclusion types that were already identified as active in steel A1. Although detailed phase characterization as well as a differentiation between homogeneous and heterogeneous inclusions for sizes <2 µm ECD is difficult with the applied SEM, one interesting difference regarding (Ti,Mn,Al)Ox as the inclusion type has been observed: The Mn-content in (Ti,Mn,Al)Ox inclusions was slightly higher in A1 compared with A2, which could additionally influence the crystal structure and further might contribute to the inclusions’ nucleation potential for AF. However, (Ti,Mn,Al)Ox inclusions detected in A1 and B1 show comparable ratios between Ti, Al, and Mn and should therefore feature the same nucleation potential. This inclusion type is active in A1 and inactive in B1. Further investigations applying other methods (e.g., WDS) are necessary to investigate this effect in more detail.
These results support the theory about the strong interaction between inclusions and steel composition. Steel A1 contained 1.48 wt pct manganese. The migration of manganese in nonmetallic inclusions led to a local zone with significantly lower manganese content in the matrix. Inclusions that were surrounded by manganese-depleted zones became more favorable for ferrite nucleation than grain boundaries where the manganese content remained unchanged. However, if the manganese content in the matrix was already very low, as in steel A2 with 0.07 wt pct, the fluctuations in the manganese content around the inclusions were obviously too small to increase the inclusions’ effectiveness to act as nucleation site.
In addition, there are still controversial opinions about the effect of pure MnS inclusions on acicular ferrite formation.[10,49,50] Pure MnS particles are often described as inert and only MnS layers on other inclusion types are observed to promote acicular ferrite formation. In contrast, the results of the current study have shown that pure MnS particles were also active. In steels A1, B, and C1 single-phase MnS inclusions were frequently found as nuclei for acicular ferrite (for examples: see Figures 12 and 13).
The Capability of MgO-Modified Inclusions
MgO-based refractories are widely used in industrial processes, but the effect of magnesium on the formation of acicular ferrite has rarely been investigated. Within the current study, the influence of magnesium in steel A was assessed using MgO crucibles that react with the steel melt. MgO-based refractory material for steel A3 was used, which led to the formation of MgOAl2O3 spinel inclusions. For example, Figures 14 and 15 illustrate an oxidic and oxysulfidic, respectively, inclusions with MA-spinel in the core of the particle. Both inclusions acted as nucleation site for several acicular ferrite plates. The inclusion morphology indicated that the reaction with the MgO crucible and the Al2O3 stirring equipment led to the formation of MA-inclusions early in the process. Later, these particles operated as heterogeneous nuclei for other inclusion phases like (Ti,Mn,Al)Ox or MnS which subsequently act as nuclei for acicular ferrite.
Only a small number of (Ti,Al,Mg)Ox and (Ti,Mn,Al,Mg)Ox inclusions were determined by automated SEM/EDS in steel A3; however, the manual analysis showed a high potential of these inclusion types for acicular ferrite. Hence, as already shown in Figure 7, an inclusion modification by MgO did not suppress the acicular ferrite formation.
The Role of Ti-Containing Inclusions
(Ti,Al)Ox was proved to act as nuclei for acicular ferrite in steel A1, as illustrated in Figure 16. (Ti,Al)Ox particles were often found as nuclei in heterogeneous inclusions, so only very less-pure (Ti,Al)Ox particles were found in the steels B and C1. (Ti,Al)Ox, (Ti,Mn,Al)Ox, and (Ti,Mn,Al)OxSy particles were found as major active types in steel C1, accounting for 45 pct of acicular ferrite nuclei, but their numbers was below 2 mm−2 for each class. Hence, a low number of highly active inclusions can produce the same, or even a higher, amount of acicular ferrite than a large number of moderately effective particles.
The addition of titanium, and the subsequent formation of titanium-containing inclusions, changes the nature of the steel from inactive to highly potent for acicular ferrite. In the present study, steels without titanium were completely inactive for acicular ferrite.
Furthermore, a strong interaction of the inclusion landscape and steel composition is noted. While (Ti,Mn,Al)Ox and (Ti,Mn)OxSy inclusions were active in steel A1 and C1, they were inactive in steel B.
Summary of Observed Inclusion Types
The inclusion potentials for acicular ferrite in the investigated steel grades are listed in Table IV. The findings are compared with the state of the art in literature. In the table, active inclusions are marked with a plus (+), inert inclusions with a minus (−), and inclusions that are described as contradictory in the literature with a wavy line (~). In steel A1, the oxidic inclusion types: (Ti,Mn)Ox, (Ti,Al)Ox and (Ti,Mn,Al)Ox; the oxysulfidic types: (Ti,Mn)OxSy and (Ti,Mn,Al)OxSy, as well as MnS and (Ti,Mn)OxSyN were found as active particles. Similar inclusion types were determined as potent in steel C1, in which the oxidic inclusion types: (Ti,Al)Ox and (Ti,Mn,Al)Ox, the oxysulfidic types (Ti,Mn)OxSy and (Ti,Mn,Al)OxSy, as well as MnS were identified as potent nucleation sites. Steel B only showed two types of active inclusions: (Ti,Mn,Al)OxSy and MnS.
It is clearly displayed that oxides containing titanium and manganese, as well as MnS, were very effective for acicular ferrite formation. In contrast, alumina- and TiN-rich inclusions were inert. However, small amounts of aluminum in titanium-manganese-oxides did not lower the inclusion’s potential.
Three samples of steels C2 and C1 were tested after heat treatment applying the method described in Section II. While C1 showed significant percentages of acicular ferrite in the microstructure, Bainite was predominant in all specimens of C2 (see Section III–A). The tensile test data are summarized in Table V. The obtained stress–strain curves are illustrated in Figure 17. A good reproducibility between the samples was observed. For steel C1, tensile strength values are lower compared with C2. However, a remarkable increase of strain values is found in C1. Also the calculated ductility Z shows a remarkable increase for samples C1 compared with C2. Based on the performed tensile test, values for the fracture toughness cannot be obtained. However, the performed tests were the only accomplishable test due to the very small sample size. Increases in strain and ductility are also seen as indicators for improved fracture toughness.