edge surface cracks1

Materials Science and Engineering A 402 (2005) 177–187

Analysis and prevention of side cracking phenomenon occurringduring hot rolling of thick low-carbon steel plates

Byoungchul Hwang, Han Sang Lee, Yang Gon Kim, Sunghak Lee∗

Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea

Received in revised form 5 April 2005; accepted 18 April 2005

Abstract

In this study, a microstructural investigation was conducted on the side cracking phenomenon occurring in thick low-carbon steel platesduring hot rolling. Particular emphasis was placed on the role of iron oxides and ferrite–pearlite band structure in the side cracking. Detailedmicrostructural analyses of the cracked region showed that the ferrite–pearlite band structure in the side region was considerably slanted tothe surface, while that in the central region was parallel to the surface. Small cracks were often observed when the iron oxide layer formedby inhomogeneous oxidization along the slanted ferrite–pearlite band structure in the side region within 30 mm from the plate edge wasintruded into the interior after hot rolling. In addition to these small cracks, a few large cracks were also found when some protrusions formedo ment of thef iron oxidef©

K

1

6ssmtpapcttorp

pro-hismilluringced

riespe-m issonse tocon-

henme

ite tors areargehenr then the

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n the plate surface were decarburized, folded and then intruded into the interior. Based on these findings, the parallel arrangeerrite–pearlite band structure in the side region by the appropriate control of the hot rolling process and the minimization of theormation by the minimized time exposed to high temperatures were suggested in order to prevent or minimize the side cracking.

2005 Elsevier B.V. All rights reserved.

eywords:Side cracking; Low-carbon steel plate; Hot rolling; Iron oxide layer; Pearlite band structure

. Introduction

When classifying rolled steel products, those thicker thanmm are generally called “plates”, which are then clas-ified in many types, depending on strength, condition ofide cutting, alloy composition, surface treatment, processingethod, application and so on[1]. Depending on the condi-

ion of side cutting, there are two kinds of plates, i.e., mill edgelate (no-trimming plate) in which only the top and tail partsre cut, but not the side parts and slit edge plate (trimminglate) whose side parts are cut by gas or a machine. The sideracking often occurring on the surface of the side parts alonghe rolling direction after hot rolling significantly decreaseshe productivity of plates. Presently, when side cracks occurn the plate surface, side trimming is applied, or the crackedegion is removed by whole-wide cutting. The side crackingroblem is also alleviated by scarfing slabs before hot rolling.

∗ Corresponding author. Tel.: +82 54 279 2140; fax: +82 54 279 2399.E-mail address:[email protected] (S. Lee).

However, since the scarfing treatment itself affects theductivity, it is better to be omitted. In some mill plants, tside cracking phenomenon is not only minimized, butedge plates are mass produced by applying manufacttechnology of trimming free plates (TFP), leading to reduproduction costs.

It is also known that the side cracking considerably vawith the kind of slabs. For instance, low-quality slabs exrience serious cracking problems, whereas the problemuch less frequent in the high-quality slabs. The reafor the side cracking can be largely divided into one dumaterials themselves or the other case from processingditions. With respect to materials, cracks can initiate wmicrovoids formed along grain boundaries due to voluchange at the time of phase transformation from austenferrite during hot rolling are coalesced[2,3]. Side or cornecracks can also occur when microcracks formed on slabcorroded by oxidization during reheating and are grown lduring hot rolling. In terms of processing conditions, wreheating or hot rolling conditions are not appropriate fosteel plates, cracking problems arise. Previous studies o

921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2005.04.045

178 B. Hwang et al. / Materials Science and Engineering A 402 (2005) 177–187

Table 1Chemical composition and dimensions of the slabs and steel plates used in this study

Hot rollingprocess

SlabT(mm)

SlabW(mm)

Chemical composition (wt.%) PlateT(mm)

PlateW(mm)

PlateL(mm)

C Mn Si P S Al V Ti Cu

A 248 1580 0.15 1.40 0.26 0.019 0.001 0.04 0.003 0.001 0.029 18.94 2645 16816B 250 1580 0.15 1.37 0.29 0.012 0.001 0.05 0.001 0.002 0.013 17.50 2692 20383

side cracking phenomenon which significantly reduces theproductivity of steel plates are scarce. Particularly, the rea-son for the cracking of 1–2% frequency in high-quality slabsis poorly understood. Therefore, the formation mechanismof side cracks and the need for presenting ways to preventthem require imminent attention in view of the productivityof plates, labor cost, energy reduction and quality improve-ment.

In the present study, microstructural analysis was con-ducted on the side cracking phenomenon occurring duringhot rolling of thick low-carbon steel plates to elucidate thecrack formation mechanism. Based on the analysis results,correlation between microstructural factors and hot rollingconditions was investigated to present ways to prevent or min-imize the occurrence of side cracks. The kind of slabs and hotrolling conditions were varied, and the resulting microstruc-tures were investigated in order to discuss the effects ofmicrostructural factors on the side cracking.

2. Experimental

The slabs used in this study were 250 mm thick, and man-ufactured at Dillinger Hutte, Dillingen, Germany, by contin-uous casting, and the chemical compositions and dimensionso ares sig-n r, iti s int influ-e enitegfi m atw y. Inh lt-i epthb Thes usedi as toe sidec arts,

and both sides of four parts out of the eight were scarfed.This was to eliminate any possible defects that might not bevisible from the surface but present inside the slabs. For con-venience, those scarfed are referred to as specimens ‘1–4’following the alphabet of ‘A’ or ‘B’ indicating the process,while those non-scarfed are referred to as specimens ‘5–8’.

Observation of the scarfed slab surface revealed excel-lent surface condition without visible defects. To observeany possible microcracks present inside the slabs, the cor-ner and center regions of the scarfed slabs were cut, and theinterior was observed by an optical microscope. The eightslab specimens in each process of A and B were cut againinto two parts, and the parts were put under hot rolling testtwice in each process of A and B. The rolling method usedin this study was a conventional rolling composed of bothrough rolling and finish rolling[7], and the detailed condi-tions of reheating and hot rolling of the A and B processes arelisted inTables 2 and 3. Comparison of the preheating condi-tions of the A and B processes showed that the holding timein a furnace in the B process (3 h and 55 min) was slightlyshorter than in the A process, since the furnace temperaturewas kept relatively high and the preheating furnace temper-ature abruptly increased to 1020◦C. The holding time in theA process was longer (4 h and 27 min), as the slab temper-ature increased relatively slowly and the preheating furnacetemperature was 621◦C. In the both A and B processes, ther loadi ss ata nishr nsid-e andm

tioni cals edi ttomo ther mallc epth.M ckedp nning

TR

Hp

o.2Heatemp

A 122B –

f the slabs and plates for the processes ‘A’ and ‘B’hown inTable 1. Generally, the carbon content has noificant influence on the hot ductility of steels. Howeve

s well established that susceptibility to cracking occurhe carbon range from 0.10 to 0.16% because of thence of the peritectic reaction in producing large austrains, together with concomitant poor ductility[4–6]. Thenal thickness of the plates was decided to be 15–20 mhich the side cracking was reported to occur frequentlot-rolled strips or thin plates (<16 mm), side cracks resu

ng from slab cracks are less prevalent as the crack decomes shallower with increasing rolling reduction.ize and chemical composition of the slabs and platesn both processes do not show much difference. Soxamine the effects of scarfing before hot rolling on theracking, the two slabs were equally divided into eight p

able 2eheating conditions

ot rollingrocess

Preheating furnacetemperature (◦C)

Heating furnace no.1temperature (◦C)

Heating furnace ntemperature (◦C)

621 944 12111020 1150 1173

olling temperature was about the same, but the rollingn the B process was higher than that in the A procell passes. Since the rolling reduction during the final fiolling (passes 10, 11 and 12) of the B process was corably higher than that of the A process, finer grain sizeore excellent mechanical properties were expected.A sketch of the side cracking spots and the orienta

s presented inFig. 1(a), and a photograph showing typiide cracks are shown inFig. 1(b). Side cracks are observn both of the plate sides and in both the top and bof the plate surface, and all the cracks are parallel toolling direction. Most of them are fine thread-shaped sracks, but some are large cracks of sub-millimeters in dicrostructures of the center and side regions of the cralates were observed by an optical microscope and a sca

ting furnace no.3erature (◦C)

Homogenizationtemperature (◦C)

Extractiontemperature (◦C

Holding time infurnace (min)

3 1153 1149 2671170 1109 235

B. Hwang et al. / Materials Science and Engineering A 402 (2005) 177–187 179

Table 3Detailed hot rolling conditions

Rolling strand of process 1 2 3 4 5 6 7 8 9 10 11 12 13

Thickness (mm) A 223 195 177 158 138 116 94 70 48 33 26 21 17B 231 211 190 168 146 122 100 82 65 49 34 25 17

Rolling reduction (%) A 10.8 12.6 9.2 10.7 12.7 15.9 19.0 25.5 31.4 31.3 21.2 19.2 19.0B 7.6 8.7 10.0 11.6 13.1 16.4 18.0 18.0 20.7 24.6 30.6 26.5 32.0

Temperature (◦C) A 1156 1151 1144 1141 1138 1134 1123 1118 1111 1096 1068 1040 1003B 1126 1125 1116 1116 1115 1112 1101 1064 1065 1065 1058 1041 1005

Rolling force (tonnes) A 1364 1417 2038 2091 2206 2420 2764 3352 3644 3690 2932 2671 2524B 1496 1585 3121 3227 3318 3410 3494 3593 3911 4510 4446 3348 2794

Initial slab thickness: 250 mm.

electron microscope (SEM, model; JSM-6330F, JEOL). Thecracked and non-cracked regions were cut perpendicular tothe rolling direction, and their microstructures were observed.Inclusions of the cracked region were analyzed by energydispersion spectroscopy (EDS). The sectioned area of thecracked region and that of the non-cracked region were com-

F(

paratively examined to investigate the correlation betweenmicrostructure and side cracking.

3. Results

3.1. Microstructure of slabs and plates

Low-magnification optical micrographs of the corner andcenter regions of a slab are presented inFig. 2(a) and (b),showing that the slab is composed of typical solidified den-drite structures. The dark region and the light region arealternating. The former is the C–Mn-rich region, while thelatter is the C–Mn-depleted region. Cracks are hardly foundnear the surface or interior of the slab. Ervasti and Stahlberg[8] conducted a hot rolling simulation test using finite elementmethods after introducing a large, 20 mm deep V-type verti-cal crack in slabs of 220 mm in thickness. They reported thatall the cracks were eventually disappeared from the surfaceafter the cracks were changed to a Y pattern, despite minordifferences in the final closure depending on the rolling con-ditions. Thus, it can be expected that a small number of smallcracks in the slabs used in this study would disappear afterhot rolling, and that they are not related to cracks in the sideregion of plates.

andt on-c , allt andp rains

ig. 1. (a) Sketch illustrating side cracks and orientations of the planes, andb) a photograph showing typical side cracks formed on the plate surface.

a insii plates thant avierr ni rbur-i to bes area

truc-t n the

Fig. 3(a–f) are optical micrographs of the surfacehe interior of the center and side regions of the nracked plate, respectively. Irrespective of the locationhe microstructures are typically composed of ferriteearlite. Near the plate surface, a number of equiaxed gre observed (Fig. 3a–c), and the number of elongated gra

ncreases as it gets deeper into the interior (Fig. 3d–f). Thiss because dynamic recrystallization proceeds on theurface as the stored energy of the surface is higherhat of the interior due to the lower temperature and heolling load on the surface[7,9,10]. Since the surface regios exposed to high temperatures after hot rolling, decazation readily occurs, and the amount of pearlite tendsmaller than that of the interior. Impurities such as MnSlso observed, but their amount is negligible.

Though the arrangement of the ferrite–pearlite band sure is parallel to the surface both on the surface and i


Fig. 2. Optical micrographs of the (a) corner and (b) center regions of the slab used in the A hot rolling process.

Fig. 3. Optical micrographs of (a) and (d) the center region, (b) and (e) the region of 50 mm far from the side, and (c) and (f) the side region, showing the bandstructure of ferrite and pearlite. The upper and lower parts of the figure indicate the surface and interior regions, respectively. Note the slope difference of theband structure.


Fig. 4. Optical micrographs of the non-cracked side region of the steel platesrolled by the (a) A and (b) B processes.

interior of the center region (Fig. 3a, b, d and e), it is slantedon the surface and in the interior of the side region (Fig. 3cand f). This slanted pattern is observed down to about 30 mmdeep from the side. The slope of the ferrite–pearlite bandstructure decreases as it gets deeper from the surface, andbecomes almost parallel to the surface near the 1/2 thickness.Such a microstructural arrangement is observed, irrespectiveof hot rolling conditions. Magnified optical micrographs ofthe non-cracked side region of the plates rolled by the A andB processes are shown inFig. 4(a) and (b). Ferrite grainsof the plate rolled by the B process (Fig. 4b) are finer thanthose of the plate rolled by the A process (Fig. 4a), and moredecarburization is observed on the plate surface rolled by theB process.

3.2. Frequency of side cracking

A total of 32 plates were fabricated by varying the condi-tions of hot rolling and scarfing. The numbers of small and

Table 4Number of small side cracks formed on the plate surface

Specimena Number of small side cracks

Top part Tail part

A1 0, 0 0, 0A2 0, 0 0, 0A3 1, 0 0, 0A4 0, 0 0, 0

A5 0, 3 0, 0A6 0, 1 0, 0A7 1, 0 0, 0A8 0, 4 1, 0

B1 – –B2 1, 0 1, 0B3 1, 1 3, 1 (1)b

B4 5, 1 (1)b 1, 0

B5 1, 0 0, 0B6 0, 1 1, 1B7 0, 1 1, 0B8 1, 1 1, 1

a Specimen numbers of 1–4 and 5–8 indicate the plate specimens rolledfrom non-scarfed and scarfed slabs, respectively.

b Number of large side cracks.

large cracks formed on the side region of the top and tailparts were counted, and are listed inTable 4. When manysmall cracks were aggregated, they were counted as onesmall crack. Most side cracks were microcracks which aretens of millimeters long and 200�m or less deep, but largecracks having over sub-millimeters depth were also observedin some plates. The small and fine cracks were mostly formedwithin 30 mm from the side, and the large cracks were formedbetween 10 and 30 mm from the side.

In the specimens fabricated by the B process, more crackswere formed than in those fabricated by the A process, regard-less of the scarfing treatment. Locations at the plate did notshow much difference, whether they were the top or the tail.The effect of scarfing on cracking was not readily recogniz-able in the A specimens as the number of cracks in them wasvery few. In the case of the B specimens, the number of smallcracks in the scarfed B5–B8 specimens was slightly reducedover the number observed in the non-scarfed B1–B4 speci-mens, and no large cracks were formed in the scarfed B5–B8specimens at all. These findings indicate that the scarfing androlling conditions affect the formation of side cracks.

3.3. Microstructure of small side cracks

Optical micrographs of the cross-sectional area of thecracked region having fine thread-shaped small cracks ares utc reah toSa neart deep

hown inFig. 5(a–d). Unlike typical side regions withoracks (Fig. 4a–d), the surface is rough, and the gray aas infiltrated deeply into the plate interior. AccordingEM micrographs of the non-cracked side regions (Fig. 6and b), the surface seems smooth without cracks, but

he surface, the gray area inhomogeneously intrudes


Fig. 5. Optical micrographs of the cracked region having small side cracks formed in the (a) A8, (b) B3, (c) B4 and (d) B8 specimens.

Fig. 6. (a) and (b) SEM micrographs of the non-cracked side region of the steel plate rolled by the A process. (c) is an EDS spectrum of the iron oxide in (a).


Fig. 7. (a–c) SEM micrographs of the cracked region, showing fine lined-up iron oxides infiltrated to the interior. (b) and (c) are high-magnification SEMmicrographs of the marked rectangular regions in (a) and (b), respectively, and (d) is an EDS spectrum of fine iron oxides in (c).

into the interior, just like in the cracked side region. EDSanalysis of these gray areas indicates that they are all ironoxides (Fig. 6c). The iron oxides often form either agglom-erates or pores in the interior (Fig. 6a). When the oxides are

brittle, they are smashed after hot rolling, and leave grooveson the surface, which look like microcracks. In some cases,they infiltrate into the interior, and form fine oxides sizedsmaller than 5�m as shown inFig. 7(a–c). These fine oxides

F rge sid rogft

ig. 8. (a–d) Optical micrographs of the surface region containing a lahe marked ‘A’ through ‘C’ areas, respectively, in (a).

e crack in the B3 specimen. (b)–d) are high-magnification optical micraphs o


Fig. 9. (a) and (b) SEM micrographs of the region containing a large sidecrack in the B3 specimen, showing a number of fine iron oxide particles.

are typically aligned in a row, irrespective of grain boundaries(Fig. 7b and c), and EDS analysis identifies them to be Mn–Si-rich iron oxides (Fig. 7d).

3.4. Microstructure of large side cracks

Fig. 8(a–d) are optical micrographs of the cracked regionhaving a large crack in the B3 specimen. Large cracks aremostly formed in about 1 mm depth, and are propagatedparallel to the aligned direction of the ferrite–pearlite struc-ture (Fig. 8a). Ferrite areas with extremely little pearlites areobserved near the crack because of serious decarburizationand the hardness of these areas is slightly lower than thatof other typical ferrite areas with pearlites (Fig. 8b and d).High-magnification SEM micrographs of the large-crackedregion show that the vicinity of the crack is oxidized, andthat many fine spheroidal oxides sized smaller than 1�m arepresent inside (Fig. 9a and b). EDS analysis of these oxidesidentifies them to be iron oxides containing a larger amountof Mn and Si, just as those formed in the small-crackedregion.

4. Discussion

In steel plates, in general, small and large cracks are readilyformed in many regions. These cracks can arise either frommaterial inappropriateness or fabrication process. Cracksarising from the material problem include scab, skin hole,blow hole, sand mark, longitudinal crack, latitudinal crack,star crack, lamination, pipe, lamellar tear and blister becauseof inclusions, non-metallic inclusions, pores and pin holeswhich are present on the surface or interior of slabs, insuffi-cient dehydrization, deoxidization, improper Mn/S ratio anddamaged refractory[1,10–15]. On the other hand, cracks aris-ing from the fabrication process include edge crack, scale andpit, roll mark, scratch, pick up, processing crack, wave andimpurity mark, together with center or edge cracking, warp-ing and wrinkling due to the roll shape[16–19]. The sidecracks in this study are linear cracks shorter than 100 mmoccurring on the plate surface within 30 mm from the side,which can be seen as a type of longitudinal cracks. Mainmicrostructural factors responsible for this side cracking arethe size, shape and arrangement of grains and oxides. Thepresent study is concerned with investigating the formationmechanism of the side cracking by mainly focusing on hotrolling process conditions as well as these microstructuralfactors.

Though both the center and side surface regions of thep rlites egreeo facesT andb duet urala fromt . It isi atesa r thes sloped it getsf ce tot entm ion.W cturei per-a ccurm gionb

hen espec-t ousi s theo giono sider area ndera form

,

lates fabricated in this study have a typical ferrite–peatructure, its size, shape, arrangement direction and df decarburization vary from each other, and the surtructure also differs from the interior structure (Fig. 3a–f).his is because grains are elongated after hot rolling,ecause dynamic recrystallization partially takes place

o high rolling temperatures. Particularly, the microstructrrangement on the surface of the center region different

he surface of the side region requires special attentiondeal that the microstructures of all regions in rolled plre parallel to the surface, but the microstructure neaurface of the side region is slanted to the surface. Theecreases and becomes almost parallel to the surface as

rom the side region to the center region or from the surfahe interior. This difference in microstructural arrangemay affect the initiation of microcracks in the side reghen the surface of the side region where the microstru

s lined up vertically to the surface is exposed to high temtures after hot rolling, oxidization and decarburization oore readily in the surface region than in the center reecause diffusion is activated.

Figs.4(a and b) and5(a–d) show the side regions of ton-cracked specimens and the cracked specimens, r

ively. The oxidization is relatively stable and homogenen the side region of the non-cracked specimens, whereaxidized layers tend to be thick and irregular in the side ref the cracked ones. Since oxides mostly observed in theegion of the cracked specimens are brittle when theyggregated, they are readily fractured or form voids upplied stress, or can be fallen off, when serious, to


Fig. 10. Schematic diagram showing the formation process of small side cracks during hot rolling: (a) formation of ferrite–pearlite band structure,(b) formationof the carbon layer and iron oxide layer on the plate surface and (c) formation of small side cracks in the side regions of the plate under applied exterior loads.

small, thread-shaped cracks on the surface. Also, when therolling load continuously applies after the cracking, oxidesare broken and infiltrated deeply into the interior in a fineparticle shape, thereby leading to the formation of the line-up of Mn–Si-rich iron oxides in the interior (Fig. 7a–d).When coarse oxides or oxide layers are in contact or col-lision against side guide rolls due to the movement of plates,they can work as crack initiation sites. Furthermore, whenfine oxides are dispersed along grain boundaries, cracks aremore readily initiated. The formation process of these smallside cracks is schematically presented inFig. 10(a–c). C–Mn-rich areas and C–Mn-depleted areas exist in the slab beforehot rolling, but the lined-up ferrite–pearlite band structure isdeveloped as C–Mn-rich areas are squeezed along the rollingdirection after hot rolling (Fig. 10a). The ferrite–pearlitebanding is generally related to the microsegregation of alloy-ing elements, and it can be eliminated by annealing for severalhours at a high austenitization temperature[20–23]. Whenthis band structure aligned parallel to the surface is exposedto high temperatures and loads after hot rolling, oxidiza-tion occurs homogeneously throughout the surface overall,the decarburized layer and iron oxide layer are formed inhomogeneous thickness on the plate surface (Fig. 10b). How-ever, when the band structure is slanted or almost verticalto the surface, oxidization proceeds selectively along theband structure, and the iron oxide layer is inhomogeneouslyi r isb load( thes

In comparison with small cracks, large cracks occurring inthe side region are less likely to be formed. In this study, twolarge cracks were found in plates fabricated by the B pro-cess. Cross-sectional observation of the cracks reveals thattheir direction is parallel to the surface, and that they areabout 1 mm deep, unlike in small cracks. Consequently, theformation mechanism for the large crack should be inter-preted differently from the one for the small crack. Thelarge crack is formed almost parallel to the direction of thepearlite band, and the hardness in some areas around thecrack is lower than other areas because of serious decar-burization. The decarburization is a phenomenon in whichoxygen near the plate intrudes into the interior at high tem-peratures, reacts with carbon, and then is removed in CO orCO2. As a result, the carbon concentration inside the platedecreases, and the surface region consists mainly of softferrites as the oxidization reaction is accelerated. This decar-burization phenomenon primarily arises from oxidizing gas(e.g., air, vapor and carbonic acid gas), reducing gas (e.g.,hydrogen and ammonia), molten salts, or iron oxides, andit can simultaneously occur with oxidization when heatedin an oxidizing atmosphere, thereby forming a black ironoxide layer on the surface[24,25]. The decarburization isa process in which the carbon inside steels is removedas the exterior oxygen reacts with the carbon diffused tothe surface. The detailed chemical reaction is as follows[

F

F

nfiltrated deep into the interior. As the iron oxide layerittle, it can easily develop into cracks under an appliedFig. 10c). The final shape of small cracks occurring inide region can be identified in Figs.5(a–d) and6(a–d).

25,26]:

e(C) + O2 → Fe + CO2

e(C) + CO2 → Fe + 2CO


As a result of this decarburization, mechanical proper-ties deteriorate because of insufficient quenching hardness ordecreased carbon concentration in the decarburized region.Deformation or cracking can take place when tensile stressdevelops on the steel surface since the martensite start pointof the decarburized region is higher than that of the non-decarburized region in the interior. Consequently, the decar-burized microstructure near the large crack may have beenpresent near the surface before the crack formation.

In the specimens fabricated by the B process, more sidecracks take place than in those by the A process. This canbe interpreted by microstructural differences resulting fromdifferent hot rolling conditions. According to the final rollingreduction at the passes 10, 11 and 12 inTable 3, the B pro-cess applies considerably higher rolling reduction than the Aprocess. Under the higher rolling reduction during final finishrolling, the final grain size can decrease because the growthof austenite grains is considerably prevented[7]. Thus, the Bspecimens having more refined grains than the A specimensmay have more excellent properties. This difference in grainsize may be related to the formation of cracks. As the fine-grained B specimens have a larger grain boundary area thanthe A specimens, which works as a good path for oxidizationor decarburization reactions at high temperatures, active oxi-dization or decarburization takes place in the B specimens. Asa result, the B specimens undergo more irregular or seriouso havem

ckso asi ndi-t nt ofi eat-i ing.B sug-g non:( suf-fi aket truc-t e asp lingc izet (3)p ni-m eveni f ofo ssivee rop-e rolls.T tionb cessc imen-t ver,s ter-p thec the

detailed microstructural variations of plates, and in investi-gating the difference in the formation mechanism of oxidesand in the plastic deformation due to temperature distributionin the plate width and thickness. Furthermore, more funda-mental understanding of the cracking mechanism is required[27–31], as the arrangement and size of grains and oxidesgreatly vary with the location of the side region. In order toclarify the microstructure, oxide shape and cracking mecha-nism more in depth in the future, efforts are to be made onfollowing areas: (1) detailed analysis of oxides using trans-mission electron microscopy, (2) simulation test of hot rollingof slabs, (3) microstructural observation of various slabs andplates and (4) evaluation of the cracking frequency accordingto the plate location and shape.

5. Conclusions

Based on microstructural analysis of the side cracking phe-nomenon occurring during hot rolling of thick low-carbonsteel plates, following conclusions were reached.

(1) Observation of the microstructures in the center and sideregions of steel plates showed that the ferrite–pearliteband structure was arranged parallel to the surface inthe center region, whereas it was seriously slanted to the

ctureerior

( erege-e–d or

iontruc-

( mo-eat-neary, thecar-

ring

A

nterf fC

R

titute

gane

xidization on the surface than the A specimens, andore of small and large cracks in the side region.In this study, the formation mechanism of side cra

ccurring during hot rolling in low-carbon steel plates wnvestigated in terms of microstructures and rolling coions. It was found that the size, shape and arrangemeron oxides and other microstructures formed during rehng and hot rolling significantly affected the side crackased on these findings, following solutions can beested to prevent or minimize the side cracking phenome1) minimize the segregation of C and Mn in slabs byciently homogenizing slabs at high temperatures, (2) mhe microstructural arrangement of the ferrite–pearlite sure located in the side region as parallel to the surfacossible through appropriate controlling of the hot rolonditions such as modified materials design to minimhe broad ratio (final plate width to initial slab width),revent or minimize the formation of iron oxides by miizing the exposure time to high temperatures and (4)

n the presence of the oxide layer, prevent the fall-ofxides or the adherence of cracks resulting from excedge works such as mechanical friction or impact by prly adjusting the spacing between plates and side guidehe results of this study interpreting the crack formaehavior in terms of microstructures and fabrication proonditions are expected to be used as important experal data for studies to prevent the side cracking. Howeince this study is mainly concerned with comparative inretation of the qualitative and phenomenal data fromracked and non-cracked regions, it lacks in analyzing

surface in the side region. The slope of the band strudecreased as it got into the center region or the intfrom the side.

2) Small cracks typically occurring in the side region wformed when the brittle iron oxide layer inhomoneously infiltrated into the interior after the ferritpearlite band structure was arranged to be slantevertical to the surface after hot rolling and oxidizatoccurred selectively along the ferrite–pearlite band sture.

3) Large cracks were formed in the case of material inhogeneity or insufficient homogenization during the rehing process. Since the C–Mn-rich region aggregatedthe plate surface was harder than the region nearblarge cracking could occur when the surface was deburized, folded, and then intruded into the interior duhot rolling.

cknowledgments

The work was supported by Technology Innovation Ceor Metals & Materials (TICM) funded by the Ministry oommerce, Industry and Energy (MOCIE) of Korea.

eferences

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[3] Y. Maehara, K. Yasumoto, Y. Ohmoni, High Temp. Tech. 4 (1986)13.

[4] R.J. Gray, A. Perkins, B. Walker, Proceedings of the InternationalConference on Solidification, 1979, p. 300.

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