Materials Science and Engineering A 425 (2006) 201–204

Effect of complex electromagnetic stirring on inner quality ofhigh carbon steel bloom

Jianchao Li a,b,∗, Baofeng Wang b, Yonglin Ma b, Jianzhong Cui a

a Key Laboratory of National Education Ministry for Electromagnetic Processing of Materials, Northeastern University, Shenyang 110004, Chinab School of Material Science and Engineering, Inner Mongolia University of Science and Technology, Baotou 014010, China

Received 4 August 2005; received in revised form 3 March 2006; accepted 20 March 2006

Abstract

To obtain a better internal quality of high carbon steel bloom, complex electromagnetic stirring combined M-EMS with F-EMS was designed.In order to find a proper location for positioning the F-EMS stirrer, the solid shell thickness profile of a continuous casting bloom was computed.It was found that the F-EMS stirrer should be placed 11.39 m below the meniscus. Based on the finding, an industrial plant trial was conducted onthe operating bloom casting machine at Baotou Iron and Steel (Group) Corp. The results show that the problem of inner porosity and compositionsegregation can be solved with the complex electromagnetic stirring technique. The ratio and index of central segregation and porosity can bert©

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educed significantly. Therefore, good casting technique, optimum mould design and final electromagnetic stirring are all important for improvinghe inner quality of high carbon steel.

2006 Elsevier B.V. All rights reserved.

eywords: Continuous casting; Complex electromagnetic stirring; Solidification; Bloom; High carbon steel

. Introduction

Use of electromagnetic fields to control flows and particleovements has become one of the most promising methods inany engineering applications. It is becoming widespread in

ndustry. Electromagnetic stirring of steel in the continuous cast-ng bloom and slab is a well established technique for improv-ng the quality of cast products. Today’s technology employsow-frequency rotating magnetic field generated by multi-phasenduction coils to stir the liquid steel in the mould (M-EMS),eneath the mould in the secondary cooling zone (S-EMS) andn the final solidification zone (F-EMS). The benefits of equiaxedtructure with respect to the reduction of centerline segregationnd overall improvements in high carbon bloom due to EMS areow widely recognized by industry.

Marked improvements in bloom quality have been achievedt Baotou Iron and Steel (Group) Corp. as a result of the imple-entation of M-EMS. But in the production process of high

arbon steel, internal defects such as center segregation andorosity are produced in the final solidification phase. To obtain

a better internal quality of high carbon steel bloom, complexelectromagnetic stirring combined M-EMS and F-EMS wasdesigned. Although there is a great deal of theoretical work [1–5]to date on the stirring induced in a column of liquid metal bya rotary electromagnetic field, its relevance to industrial elec-tromagnetic stirring remains limited. It is, therefore, importantto obtain a better understanding of the combined effects of M-EMS and F-EMS on the solidification structure and centerlineand V-segregations. These issues have been studied from a prac-tical perspective by combing M-EMS with F-EMS in order toimprove quality of bloom for rail and cable wire applications.The results of this study are presented in this paper.

2. Optimal positioning of complex electromagneticstirring

In complex electromagnetic stirring, the M-EMS is locatedoutside the mould, as usual. So the positioning of the F-EMSis critical. Proper position of the F-EMS can greatly improvethe effect of stirring on the quality of bloom, so it is important

∗ Corresponding author. Tel.: +86 472 2207531; fax: +86 472 2207530.E-mail address: jchli2000@126.com (J. Li).

to compute and analyze the temperature profile on the bloomduring continuous casting. The process parameters are showedin Table 1.

921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.

oi:10.1016/j.msea.2006.03.061

202 J. Li et al. / Materials Science and Engineering A 425 (2006) 201–204

Table 1Material parameters for the computation of electromagnetic stirring

Parameter Values Units

Bloom dimensions 380 × 280 mm × mmMould length 700 mmCasting speed 0.70 m/minArch radius 12000 mmSegment 0 600 mmSegment 1 2200 mmSegment 2 7200 mmWater temperature 20 ◦CCasting temperature 1492 ◦CWater flux in Segment 0 0.786 l/m2 sWater flux in Segment 1 0.379 l/m2 sWater flux in Segment 2 0.079 l/m2 s

In order to mathematically model the continuous casting pro-cess, it was assumed that the slice of the bloom being consideredmoves with the strand at a casting speed which was governedby bloom size and steel production. With the high aspect ratioof blooms, and the low thermal conductivity of steel relativeto typical casting speeds, heat conduction down the bloom issmall. Thus, a two dimensional model across the bloom, basedon transient heat conduction with latent heat evolution, is capa-ble of accurate predictions.

The following assumptions are made for the computing pro-cess:

(1) The effect of mould shake on the heat conduction isneglected.

(2) The boundary condition of upper surface is considered asthe thermal insulation.

(3) Heat conduction in the casting direction is omitted.(4) The latent heat of steel is 298 kJ/kg and is applied on nodal

points according to equivalent specific heat method [6].

The details of model and the treatment of different boundaryconditions are referred to the literature [6].

According to actual second cooling condition, the positionwhere the solidification rate is 70% or so is suitable for theF-EMS. The position is showed in Fig. 1. The distance below

Fig. 1. Positioning of F-EMS.

the meniscus is 11.39 m, where shell thickness is 76 mm andsolidification is 72%.

3. Plant trial

To assess more accurately the effects of complex EMS on theinner quality of bloom, a plant trial was carried on two strandscast at the same time with a casting speed of 0.70 m/min, super-heat value of 25 ◦C and water fluxes in second cooling zoneare clear in Table 1. Two grades of high carbon steel have beenassessed for the M-EMS and complex EMS. Typical chemicalcomposition of each high carbon steel is showed in Table 2. Themain parameters of complex EMS are listed in Table 3.

Table 2Typical chemical composition of steels (wt.%)

Grade C Mn P S Si Cu Ni Cr Mo

U71Mn 0.71 1.2 0.021 0.020 0.2 0.010 0.07 0.15 0.02082B 0.804 0.74 0.015 0.010 0.2 0.06 0.02 0.14 0.018

s wit

Fig. 2. Macrograph of U71Mn bloom h (a) M-EMS and (b) complex EMS.

J. Li et al. / Materials Science and Engineering A 425 (2006) 201–204 203

Fig. 3. Macrograph of 82B blooms with (a) M-EMS and (b) complex EMS.

Table 3Electromagnetic stirring parameters

Parameter M-EMS F-EMS

Current (A) 600 300Frequency (Hz) 1.8 8Phases 3 3

3.1. Evaluation methods

For solidification structure and centerline segregation eval-uations, a pair of bloom samples was taken from eachstrand cast with M-EMS and with complex EMS. Trans-verse sections were cut for centerline segregation assessment.The solidification structures of the transverse sections of thebloom were revealed by macrostructure and defect erosioninspection methods. The macrostructure were evaluated in thelaboratory.

The centerline carbon segregation was expressed as a ratioof carbon analysis of the centerline C0 to the averaged carbonanalysis of the cast CB. The ratio is referred to as a carbonsegregation coefficient Kc. σ is the standard deviation of themean value of the segregation coefficient. Carbon segregation inthe blooms was characterized by the mean value of the carbonsegregation coefficient Kave

c . This value was obtained on thebasis of all carbon analysis of a sample:

Kc = C0

CB(1)

Kavec =

∑Kc1 + · · · + Kcn

n(2)

where Kavec is the mean value of centerline carbon segregation

coefficients. Kc1 to Kcn are the individual segregation coeffi-c

dsttt

This enabled us to obtain representative analyses and promisereliable results.

3.2. Results and discussions

The macrographs of U71Mn and 82B steel cast with M-EMSand complex EMS are presented in Figs. 2 and 3. As seen, at

Fig. 4. Centerline carbon segregation on transverse section of U71Mn bloomalong centerline of long side with (a) complex EMS and (b) M-EMS.

ients. n is the number of the individual, Kc.The centerline carbon analyses were obtained from the

rilling the bloom centerline of the long side (380 mm), whereamples were obtained by every 10 mm. The carbon analysis ofhe tundish probes was accepted as the average bulk analysis forhe heat. A drill size of a 6.0 mm diameter was used in ordero cover a reasonable portion of the centerline segregation area.

204 J. Li et al. / Materials Science and Engineering A 425 (2006) 201–204

Fig. 5. Centerline carbon segregation on transverse section of 82B bloom alongcenterline of long side with (a) complex EMS and (b) M-EMS.

the bloom center, where the last remaining portion of the liq-uid pool solidifies, a large shrinkage porosity has been formed.In the transverse sections, fine equiaxed structures predomi-nate. They are caused by the M-EMS. The central shrinkageporosity of blooms with complex EMS is very fine and is com-plemented with a rather uniform distribution. The center porosityand shrinkage cavity with complex EMS are reduced.

The typical centerline carbon segregation profiles of bloomswith M-EMS and with complex EMS of the same heats areshown in Figs. 4 and 5. For U71Mn steel, an average centerlinecarbon segregation of Kave

c = 1.03, with σ = 0.037 was attainedwith complex EMS for casting and Kave

c = 1.04 with σ = 0.06with M-EMS. For 82B steel, an average centerline carbon segre-gation of Kave

c = 1.02, with σ = 0.038 was attained with complexEMS for casting and Kave

c = 1.03 with σ = 0.055 with M-EMS.As seen from these data, the differences between average cen-terline carbon segregation of blooms with M-EMS and withcomplex EMS are not so significant. However, the spread bandsof segregation expressed as a standard deviation are reducedsignificantly in the high carbon steel with complex EMS.

4. Conclusions

The following conclusions can be reached in this paper:

(1) Complex EMS including of M-EMS and F-EMS wasdesigned. The proper positioning of F-EMS was determinedto be 11.39 m below the meniscus.

(2) The application of complex EMS improves the centerlinecarbon segregation and porosity of steel bloom.

(3) Good casting technique, optimum mould design and finalelectromagnetic stirring are all important for improving the

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inner quality of high carbon steel.

cknowledgment

The authors are grateful to Baotou Iron and Steel (Group)orp. for its test support.

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