M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 1 3 5 5 – 1 3 5 8

Short communication

Study of aging behavior of CSP hot bands for cold sheets

Guang Xu⁎, Yi Yang, Xinqiang ZhangKey Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology,Wuhan, 430081, ChinaMail box 131, School of Materials and Metallurgy, Wuhan University of Science and Technology, Wuhan, 430081, China

A R T I C L E D A T A

⁎ Corresponding author. Mail box 131, SchoolChina. Tel.: +86 027 63212211; fax: +86 027 86

E-mail addresses: xuguang@wust.edu.cn, ra

1044-5803/$ – see front matter © 2007 Elsevidoi:10.1016/j.matchar.2007.09.007

A B S T R A C T

Article history:Received 6 September 2007Accepted 19 September 2007

In the past, hot bands for the rolling of cold sheets were produced by conventional hot stripmills. More and more hot strips are now produced by Compact Strip Production (CSP) lines.The aging behavior of CSP hot bands was studied by means of age-annealing experimentsperformed to develop suitable rolling schedules. The aging tests revealed that a largernumber of fine precipitates are present in specimens rolled at a lower coiling temperature.The results of these tests showed that the hot bands rolled at higher finishing and lowercoiling temperatures were preferred for producing cold sheets with good drawingproperties.

© 2007 Elsevier Inc. All rights reserved.

Keywords:CSP hot bandsAging behaviorCold sheetAnnealing

1. Introduction

Hot bands for cold sheets are usually produced with conven-tional hot strip mills. The resulting cold sheets generallypossess properties good for subsequent drawing purposes. Inorder to save energy and improve the productivity of steels,more andmore thin slab casting and rolling (TSCR) productionlines have been built in the past decade. Compact stripproduction (CSP) is one of the main TSCR processes and isbeingwidely adopted for steel production [1]. Over 10 CSP lineshave been put into production in China in the past 10 years.Some CSP hot strips are used to produce the drawable coldsheets in cold tandemmills. However, production experienceshave shown that the drawing properties of cold sheets rolledwith CSP bands are not as good as those of cold sheets rolledfrom conventional hot strips. Therefore, in recent years anumber of studies have been made on how to produceacceptable hot bands for drawable cold sheet rolling. Theaging behavior of these alloy steels has been studied by

of Materials and Metallur560679.ndom88@163.com (G. Xu

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several research groups in order to establish better under-standing of the rolling process and the drawability [1–9].However, the overall understanding of these factors remainsunclear. In this paper, the aging behavior of CSP hot bandswasinvestigated with the objective of defining suitable productiontechnology for the reliable production of cold sheets with gooddrawing properties.

2. Experimental Work

The steel used for this program was from the CSP productionline of Lianyuan Iron and Steel Company (LY Steel), China.Chemical compositions of two different hot bands, and thefinishing and coiling temperatures are given in Table 1. Thetest specimens were machined to a dimension of 30×20 mm.

Experimental equipmentused include a SG-7.5–10 salt-bathfurnace, a HR-150A hardness tester and a transmissionelectron microscope (TEM). The heat treatments were carried

gy, Wuhan University of Science and Technology, Wuhan, 430081,

).

.

Table 1 – Chemical composition and rolling parameters of the DC03 experimental steels

Steelgrade

Coilnumber

Concentration (wt.%) Technology parameters

C Si Mn Al P S Ni Cr Cu Finishing temperature Coil temperature

DC03 6270212500 0.02 0.03 0.2 0.0287 0.0131 0.0031 0.0201 0.0301 0.0501 910 °C 646 °CDC03 6380006200 0.04 0.01 0.21 0.0407 0.0031 0.0031 0.0101 0.0201 0.0401 899 °C 568 °C

Table 2 – Aging conditions used for the Sample 6200 [Coil6380006200] hot bands

Aging temperature, °C Aging times, h

550 0.5, 1, 1.5, 2, 2.5600 0.5, 1, 1.5, 2, 2.5650 0.5, 1, 1.5, 2, 2.5700 0.5, 1, 1.5, 2, 2.5750 0.5, 1, 1.5, 2, 2.5

Table 3 – Aging conditions used for the Sample 6200 andSample 2500 hot bands

Coilnumber

Sample Aging temperature,°C

Aging times,h

6270212500 2500 650 0, 0.5, 1, 2, 3, 46380006200 6200 650 0, 0.5, 1, 2, 3, 4

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out in the salt-bath furnace; the aging treatment conditions areshown in Tables 2 and 3. The heat treatments shown in Table 2were used only for Coil 6380006200, referred to hereafter asSample 6200. The 650 °C aging treatments shown in Table 3were used for both coils, referred to hereafter as Sample 6200and Sample 2500. The hardnesses of the samples weredetermined, and the precipitates and the change in dislocationdensity as a result of the aging treatments were characterizedby TEM. Finally, the drawing properties of annealed sheetswere measured on a Zwick materials testing machine.

Fig. 1 –Changes in hardness of Sample 6200 as a function ofaging time at various temperatures.

3. Results and Discussion

3.1. Effect of Aging Treatment at Different Temperatures

Aging experiments were carried out for specimens of Sample6200 under the conditions given inTable 2. The hardness resultsfor these specimensare given onFig. 1, where it can be seen thatthe hardness increased after aging for half an hour at all agingtemperatures. The carbon content of the steel is more than0.02wt.%,which exceeds themaximumsolubility of carbon inαphase. It is difficult to stabilize the solute carbon effectively,which makes aging–overaging phenomena unavoidable. Theinitial hardness increase is caused by an increase in thenumberof carbide and nitride precipitates in the steel. After the initialvery rapid increase, thehardness showed little further changeattemperatures of 550 °C, 600 °C and 650 °C, suggesting littlechange in the number density and size of the precipitates overthis temperature range, for times up to 2.5 h. When the agingtemperature was increased to 700 °C and 750 °C, the samplehardness decreased sharply with aging time after the peakvalue. This may be attributed to the coarsening of the fineprecipitates, which reduced the effectiveness of the precipitatesin impeding dislocation movement.

3.2. The Effect of Aging on Different Samples

Samples 2500 and 6200 were aged at 650 °C for times up to fourhours, Table 3. The sample hardnesses after the aging treat-ments were measured and are shown on Fig. 2.

Fig. 2 –Hardness of Samples 6200 and 2500 as a function ofaging time at 650 °C.

Fig. 3 –TEMmicrographs of Sample 6200. (a) Prior to aging, (b) aged at 650 °C for 2 h [15 kX), and (c) aged at 650 °C for 2 h (150 kX).

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The finishing and coiling temperatures were 899 °C and568 °C, respectively, for Sample 6200 and 890 °C and 646 °C,respectively, for Sample 2500. It can be seen from Fig. 2 thatthe hardness values reached a peak for both samples afteraging at 650 °C for 0.5 h. The reason for this rapid increase inhardness is the same as that discussed previously. Thehardness of both samples decreased abruptly after reachingtheir peak values because of the coarsening of the fine carbideand nitride precipitates. The hardness of Sample 6200 isslightly greater than that of Sample 2500, reflecting the greateramount of carbon in the former sample. Also, the coilingtemperature of Sample 6200 was lower, resulting in morecarbon being stabilized in the solid solution.

3.3. TEM and Analysis

The morphology of precipitates and dislocation densities inSample 6200 were characterized by TEM, Fig. 3. Fig. 3(a) shows

Fig. 4 –Annealing schedule used for comparative drawabilitytests.

the structure prior to aging, and Fig. 3(b) and (c) show themicrostructure after aging at 650 °C for two hours. Comparisonof Fig. 3(a) and (b) indicate little change in dislocation densityresults from the aging treatment. This suggests that thecontribution to strengthening by the dislocation substructureis not much affected by the aging. However, the numbers offine precipitates before and after aging differ significantly, asillustrated by comparing Fig. 3(a) and (c). This indicates thatthe hardness increase of this specimen after 650 °C–2 h agingis the result of the presence of fine precipitates as compared tothe specimen without aging treatment.

3.4. Annealing Tests

In order to determine the drawing properties of cold sheetsrolled from experimental hot bands, annealing tests wereperformed on Samples 6200 and 2500. First, the hot bandswere rolled to cold sheets with a reduction of 73%. Then thecold-rolled sheets were annealed according to the annealingschedule shown in Fig. 4. The mechanical properties of theannealed sheets were measured on a Zwick materials testingmachine; the results are given in Table 4. Refer also to thefinishing and coiling temperatures used for these samples,Table 1. The finishing temperatures were nearly the same for

Table 4 – Mechanical properties of the cold sheetsdetermined by a Zwick materials testing machine

Coilnumber

0.2% YS,MPa

Tensilestrength, MPa

Elongation,%

Work-hardeningindex, n

Sample6200

214.3 254.2 31.5 0.25

Sample2500

227.8 299.7 28.7 0.21

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the two samples, but the coiling temperature of Sample 6200was considerably lower.

The tensile test results, Table 4, clearly indicate that Sample6200, processed with a lower coiling temperature has betterdrawing properties than that of Sample 2500 processed at ahigher coiling temperature. The higher the finishing temper-ature and the lower the coiling temperature in a certain range,themore carbon andnitrogen can be retained in solid solution.These elements precipitate out of solid solution duringannealing in the form of fine carbide and nitride precipitates,which are useful for producing ellipsoidal-shaped grains withgood drawing properties. This is in agreement with the agingtreatment results discussed previously.

4. Conclusions

By using a relatively lower coiling temperature for DC03 steels,it is possible to retain greater amounts of alloying elements,notably carbon and nitrogen, in solid solution. Upon subse-quent aging (at 650–680 °C) these elements precipitate ascarbides and nitrides, hardening the matrix and assisting inthe formation of ellipsoidal-shaped grains. Ultimately, thisstructure improves the drawing properties of the cold sheets.

Aging at 650 °C for short times has little effect on thedislocation substructure. The bulk of the hardening observedafter aging is due to changes in the precipitate density and sizecompared to the structure prior to aging.

Acknowledgments

The authorswould like to express their thanks to Lianyuan Ironand Steel Company (LY Steel) for providing financial support tothis research project.

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