A Three Dimensional Model to Characterize the Centerline Segregation in CC Slabs

Authors


Abstract

In this investigation the formation of centerline segregation and shrinkage holes in the final stages of solidification during the continuous casting process of slabs is described. Therefore experimental and theoretical investigations of the melt behavior during the solidification are carried out. Based on these results, a model is presented to explain the formation of V-shaped segregation and core porosities caused by the restricted flow of melt through the mushy zone in slabs. The theoretical explanation is manifested by various observation, calculations and analyses by electron microprobe mappings. The main results are that V-segregation forms in quasi periodical frequency in longitudinal direction within continuous cast slabs. The areas of formation seem to be independent, like little billets lying side by side in one slab. Consequential a three dimensional model about the behavior of the liquid melt in the final stages of solidification is designed.

1 Introduction

During the solidification of CC (Continuous Casting) slabs a homogenous and controlled solidification is significantly to achieve during the production. In order to produce on maximum cost efficiency, plant operators have to continually improve their steel products and reduce potential discrepancies. Macrosegregations are a major reason for quality limitation of steel semis.[1] For sustainable improvement of the inner quality of slabs, it is decisive to investigate the formations of shrink holes and macrosegregations. Plant operators[2] suppose that crater end is not a clear line, but without having a precise description and explanation of the procedures at high fraction solid values. Based on these research activities, improvement measures can be carried out, which will help the producer and plant designer to minimize these negative effects for the production of slabs during the continuous casting process.

In general, the phenomenon of segregation occurs during the solidification of metallic alloys. Thereby different alloying elements segregate because of separation processes at the solidification front due to different solubility of the substances which are contained in both steel phases, in the solid and liquid state.[3-5] These separation processes are divided in positive (local enrichment) and negative (local depletion) segregation.[6]

Segregation is classified in micro- and macrosegregation,[7] microsegregation in the scale of less than 1000 µm in the dendritic solidification, macrosegregation in the scale of millimeters by moving microsegregated melt to a sink in the structure. Different resources distinguish the scale of segregation types even in three forms, with an intermediate size, the so-called semi-macrosegregation.[8, 9] Semi-macrosegregations usually develop in the spaces of equiaxed crystals. The formation of centerline segregations is known for blooms and billets[2, 10-14] but for thick slabs general investigations of V-segregations are not conducted. Figure 1 shows the general understanding for the formation of V-shaped segregation for small formats[10, 15] because of the two-dimensional heat removal.

Figure 1.

a) As cast structure of a 105 mm sq. billet with V- shaped segregation after[15] and b) model concept of the formation of inhomogeneities during the solidification of billets after.[16]

In contrast, in the middle part of slabs heat extraction is one-dimensional to the loose and fix sides. In this case the common understanding for the formation of centerline segregations and porosities during the casting of thick slabs is because of bulging. Thereby enriched melt is sucked and squeezed out from the solidification front and flows to the centerline because of negative pressure by shrinkage.[16, 17] However, new continuous casting facilities usually have small-sized supporting rolls and therefore the possibilities for severe bulging effects are considerably minimized. Due to the smaller roll pitches the movement of the solidified strand has been nearly overcome. But despite the innovations, plant operators still find inhomogeneities in the centerline area of slabs. For this reason a new model needs to be designed to describe the formation process of centerline segregation for thick slabs.

2 Experimental Procedure and Calculations

To investigate the behavior of the liquid melt in the mushy zone in the final stages of solidification, selected samples of slabs in longitudinal direction were taken at Salzgitter Flachstahl GmbH. In total, four strand samples of different heats with a particular length of 900 mm were taken from 350 mm × 1700 mm strands. In order to enable good comparability between them, just samples of the steel grade S355 were considered (see Table 1). The material to be inspected was produced at the continuous caster No. 4 at the Salzgitter Flachstahl GmbH company in Germany and every heat was cast at superheat between 13 and 18 K, because investigations revealed a different center microstructure with barely center segregation at lower superheat. It is well known that the superheat has a major impact of the center microstructure which has already been quantified for blooms and billets.[18] The caster No. 4 was built by SMS Siemag AG in 2010 with a metallurgical length of 34.42 m.[19] The characteristic values of this caster are summarized in Table 2. From every sample, the parts B, D and F (see Figure 2) were taken for further investigations whereby the pieces where located in the middle of the slab and on the left and right at ¼ of the width. To avoid an influence on the microstructure because of the mechanical preparation, on each side of the sample 35 mm was grinded. For better handling the samples were cut into three parts.

Table 1. Main chemical composition of the investigated steel grade S355
Chemical elementConcentration: [wt%]
C0.16–0.18
Si0.15–0.25
Mn1.50–1.60
P<0.020
S<0.005
Table 2. Characteristic values of the caster No.4 at Salzgitter Flachstahl GmbH based on ref.[20]
Plant parameters
Caster itemNo. 4
Year of construction2010, by SMS Siemag
Number of strands1
Dimensions1200 mm
 2600 mm × 250 mm/350 mm
Capacity100 000 t/a
Radius11.5 m
Bending points7
Metallurgical length34.42 m
Content of the tundish34 t
Segments15 (0–14)
StraighteningSegment 8
Soft reductionSegment 9–14
Figure 2.

Plan of sampling from the slab.

Afterwards the longitudinal samples with the dimensions 350 mm × 10 mm × 300 mm (height × thickness × length) were hot deep etched by the SMS Siemag AG in Hilchenbach for an optical evaluation of internal quality. The deep etching occurs with hydrochloric acid at 70°C and unveils the structure of the samples. By this method segregations and porosities can be identified. Both, etching and qualitative evaluation is standardized for all investigations by SMS Siemag AG.[20]

For a quantification of the segregation results electron microprobe mappings and line scans were conducted at the Central Facility for Electron Microscopy at RWTH Aachen University.

To verify the effect of bulging, a bulging calculation with the numerical solidification program SlabSol© has been carried out at IEHK. These calculations are based on the bulging formula, which depends on specific plant parameters, especially roll pitch and casting speed.[21]

3 Experimental Results

The optical segregation analyses of the etched samples were conducted after a standardized reference row.[20]

The external areas (sample B and F) at ¼ of the width had a smaller segregation index than the samples at the middle of slab width (sample D). In addition to an uneven secondary cooling zone over the width, independent areas in the slab might be reasonable.

Analogous to the conduct of billets,[22] 92% of all occurring V-shaped segregations were more pronounced on the loose side. Moreover the metallurgical center is shifted about 3–4 mm to the fix side. The amount of V-shaped segregations of all samples differs between 0.33 and 1.67 per 100 mm of a sample in longitudinal direction. Thereby the medial distance between the segregations varies between 10 and 200 mm with a median value of 50 mm, so that no correlation with the plant parameters is noticeable, which is an indication for the limited influence of bulging.

Generally, three different categories of centerline areas in the longitudinal sections were identified. The first one does not exhibit any visible defects at the centerline. The second category indicates selective inhomogeneities around the centerline. Distinct V-shaped segregations and a region of porosities are characteristic for the third division.

For quantifying the various classifications among the segregation pattern electron microprobe line scans are carried out. These line scans confirm the previous implemented categories of the optical analyzes. Figure 3 shows exemplary the results of manganese for a V-shaped segregation from sample B, where the V-shape is closed in casting direction. Centerline segregation has a negative basic level because of the dominant suction flow caused by the volume contraction in the final solidification zone.[11] The negative pressure sucks melt from higher regions of the strand through this area. The peaks of high manganese concentration are caused by the formation of V-segregations, where the micro segregated remaining melt flows through preferred channels in mainly solidified areas.[23, 24]

Figure 3.

Macro-EPMA mappings for the manganese distribution from sample B. The concentration ratio is the relation between the actual concentration c and the mean concentration c0.

The possibility of measuring manganese sulphide (MnS) precipitates instead of segregations is relatively small because of the low sulfur content in the melt. Previous investigations contribute this assessment, where MnS was hardly found in the centerline of a slab at even higher sulfur contents due to the accelerated solidification speed at the end of the solidification.[25]

By the solidification software SlabSol© the amount of bulging was calculated for the continuous caster No. 4 from Salzgitter Flachstahl GmbH. Figure 4 shows the result of the calculation and points out the neglectable influence of bulging during final solidification of the strand at this caster. The negative basic level of center line segregation supports this result.

Figure 4.

Bulging calculations with SlabSol© calculated with the bulging formula as a function of plant parameters.[10]

4 Model Conception

Based on the previously reported results, a model for the formation of porosities and segregations in the final stages of solidification is constructed. In further investigations of samples in width direction of the same casting machine, two different zones in the centerline area and different catchment areas of porosities and segregations, which are partially overlapped, can be pointed out (see Figure 5a). This circular positioning of the porosities is similar to billets and blooms.[23] In width direction, the feeding channels of the V-shaped segregation are shown through the area of inhomogeneities around the centerline. Near center line, the V-channels coincide. This is the area with pronounced macrosegregation.

Figure 5.

a) Two different zones in the centerline area in width direction with different catchment areas from the porosities and segregations. b) V-Segregations in longitudinal direction.

The presented studies in longitudinal direction detected V-Segregations (see Figure 5b). Directly at the region around the geometric center, the macrosegregation and especially V-shaped segregation occur in the longitudinal sections. The areas with inhomogeneities were measured for the samples and have been identified in an area of ±20–25 mm from centerline, which is marked in Figure 5a. These porosities are caused by the limited feeding with liquid melt in the final solidification zone where volume shrinkage occurs. The connection between porosities and centerline V-shaped segregations in the final stages of the solidification will be discussed.

A description for the interaction between these areas is explainable with partial leading zones within the slab by which formation of independent parts occurs (see Figure 6). Thus a forming of a local parabolic shaped solidification front is reasonable analogous to the appearance in billets,[18] which is schematically drawn in Figure 7. Regarding this idea of longitudinal direction, V-shaped segregation can be formed with a certain radius of influence, like it is reported for blooms and billets, where only one center exists.[15, 22]

Figure 6.

Partial leading zones within the slab, schematically shown.

Figure 7.

Schematically forming of a parabolic shaped solidification front, similar to blooms and billets.

In longitudinal direction at the centerline, positive segregation peaks in the basically negative segregated area were measured by microprobe analysis as shown in Figure 3.

The formation of independent regions inside the slab takes place because otherwise a continuous, plane segregation through the entire length of the slab would be visible.

Thus a flow of the enriched melt through the mushy zone is blocked by bridge formation, similar to the schematic explanation in Figure 1b. But due to the shrinkage of the melt during solidification forms a vacuum. The enriched liquid melt is aspirated to compensate the free volume. The suction flow of the segregated melt through this equiaxed core zone is blocked by low permeability. Feeding channels with enriched melt can form afterwards through the mechanism of forming V-segregations at high fraction solid. In this case the inhomogenities in width direction are the filled or non filled V-cannels formed in longitudinal direction.

In addition, the description about the centerline segregation matches with the category 3 of the above mentioned classification of the segregations. In this category the form of V-shaped segregation is pronounced and not just concentrated on the centerline. All different shapes of centerline segregation are analyzed by electron microprobe investigations at RWTH Aachen University. From these mappings, an asymmetry of the center segregation is visible on the fix side of the strand for categories 1 and 2. For category 3 instead, on the loose and fix side of the slab, segregated areas are visible. This is presented in Figure 3, exemplary for the manganese contribution. Simultaneously, the chemical elements silicon and aluminum were measured for all samples. The segregation distribution for silicon was similar to manganese. As expected, aluminum did not have any tendency to segregate and it was evenly distributed in the slab. These results were similar for all series of measurement.

5 Discussion

In the previous chapter, the connection between the area of porosities and macrosegregation was analyzed. The results indicate the concept about the quasi periodicity of V-shaped segregation of 10 and 200 mm distance, in longitudinal direction of the slab. Inside the slab, independent parts occur like a number of parallel billets. On the basis of investigations in width and longitudinal direction of the slab, a three-dimensional characterization of the centerline region in continuous casting slabs is necessary. The solidification front in slabs is considered to be uneven. As mentioned previously, a precise description and explanation of the procedures at high fraction solid values is necessary. The correlation between the suction flow and the centerline segregation gives a good explanation about this topic.

For the validation of the model, samples were taken and hot deep etched for an optical macro assessment. Besides EPMA (electron probe micro analysis) line scans were conducted to get quantitative values about the segregation index for manganese, silicon, and aluminum on a millimeter scale. Bulging seems to be suppressed to the greatest extend by narrow roll pitches. The model describes the melt behavior in the final solidification zone, where the shell thickness is high, which is an additional aspect of the low bulging influence. The impact of shell thickness is included in the introduced bulging formula from Lamant.[21] These objective facts strengthen furthermore this model concept as a series of billet- like craters.

At fraction solid values above fs = 0.3, a flow of the melt in the dendritic mushy network is limited. At lower values the solidification front can consume fresh melt without any problems. But for fraction solid values about fs = 0.65, the flow of liquid melt is not further possible and stopped, as shown in the laboratory experiments from Takahashi et al.[26] Figure 8 shows the region where the calculated isothermal curves were plotted. The isothermal curves are numerically calculated. Furthermore the V-shaped segregation was drawn to scale by a parabola with a shape factor of A = 1 to illustrate the course. The length of the V-shaped segregations usually ends about ±15–20 mm away from the centerline. This corresponds to calculated fraction solid values about fs = 0.6–0.7. These values are also found in laboratory investigations of Takahashi et al.[26] as the flow limiting fraction solid. Thereby the centerline segregation ends within the area of inhomogeneities (see Figure 5).

Figure 8.

Plotted isothermal curves with the course of the V-shaped segregation. The isothermal curves are calculated numerically, without taking the development of the microstructure into account.

In the final stages of solidification, a suction flow forms to compensate the shrinkage of the liquid and the already solidified steel. Because of the high fraction solid values, the melt cannot fill the complete shrinkage volume and as a result an area of micro porosities forms. The visible feeding channels, solidified with the enriched melt, are sucked out from the spaces between the dendrites. These routes of the former liquid melt for compensation represent the arms of the V-shaped segregation. The area of the V-shaped segregation and the porosities coincide in the zone of the centerline, as expected from the calculation for fraction solid values in this area.

6 Conclusions

In this study, experimental and theoretical investigation of the macrostructure formation and centerline segregation in thick slabs are presented. The key findings of these research activities can be summarized as follows:

  1. The quasi periodicity of segregations in longitudinal direction within continuous casting slabs is demonstrated.
  2. An independency of the areas within thick slabs, like little billets lying side by side in one slab, is stated out.
  3. The connection between the region of porosities and centerline segregation is shown.
  4. Consequential a three dimensional model about the behavior of the liquid melt in the final stages of solidification is designed.
  5. Sedimentation segregation was identified at the fix side of the continuous casting machine.
  6. Formation of V-shaped segregation and shrinkage holes at fraction solid values about fs = 0.65 were evidenced.

The values for periodicity and spreading of V-segregations are determined for the steel grade S355 casted as 350 mm thick slabs at the continuous caster No. 4 at Salzgitter Flachstahl GmbH. Further investigations about the frequency of V-segregation in thick slabs should expend to different casting machines and steel grades, to work out the dependency from plant parameters and chemical composition of the steel grades.

7 Acknowledgments

The authors gratefully acknowledge the support of SMS Siemag AG, specifically Mrs. Dr. E. Erdem Hornauer, and Mr. Dr. P. Müller from the company Salzgitter Flachstahl GmbH.

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