Materials Engineering, Vol. 16, 2009, No. 3

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DEFECTS SIMULATION OF ROLLING STRIP

Rudolf Mišičko1, Tibor Kvačkaj

2, Martin Vlado

2, Lucia Gulová

2, Miloslav Lupták

2,

Jana Bidulská2

Received 26th May 2009; accepted in revised form 8th June 2009

Abstract The defects in the continuous casting slabs can be developed or kept down in principle by rolling

technology, especially depend to sort, size and distribution of primary defects, as well as used of rolling

parameters. Scope of the article is on observation behavior artificial surface and undersurface defects (scores)

without filler (surface defects) and filling by oxides and casting powder (subsurface defects).

First phase of hot rolling process have been done by software simulation DEFORM 3D setting to the

limited condition for samples with surface defects. Samples of material with low-carbon steel of sizes h x b x l

have been chosen and the surface defects shape „U” and „V” of scores have been injected artificially by

software. The process of rolling have been simulated on the deformation temperatures 1200°C and 900°C,

whereas on the both of this deformation temperatures have been applied amount of deformation 10 and 50 %.

With respect to the process of computer simulation, it is not possible to truthful real oxidation condition

(physical – chemical process during heat of metal), in the second phase of our investigation have been observed

influence of oxides and casting powders inside the scores for a defect behavior in plastic deformation process

(hot and cold rolling process) in laboratory condition.

The basic material was STN steel class 11 375, cladding material was steel on the bases C-Mn-Nb-V.

Scores have been filled by scales to get from the heating temperatures (1100°C a 1250°C), varied types of casting

powders, if you like mixture of scale and casting powders in the rate 1:4.

The joint of the basic and cladding material have been done by peripheral welded joint. Experiment

results from both phases are pointed on the evolution of original typology defects in rolling process. Keywords: Continuous casting semi-finished products, Rolling, Steel strips, Surfaces and undersurface defects

1. Introduction

The problem of surface quality of flat rolling products belongs to the basic priority of industrial process. Each imperfection of the material can cause defect or degreasing quality of final product. Possible source of surface defects for hot or cold rolling strips can be some places in manufacturing process as:

- production and casting of steel,

- heating of material,

- hot slabs rolling,

- cold strip rolling.

Between the most problematic surface defects, if you like initiators of surface defects, which are created during process of manufacturing and steel casting include: inclusions, blowholes,

scars, lines of oxides aluminium, holes, longi-tudinal, transverse and the edge cracks [1-4]. The condition of heating slabs in pusher furnace, hot rolling process, next cold strip rolling process, represents further creation and evolution of defects. During technological operation can be to create the next specific surface defects: shell surface, thermal cracks, hangnails, slivers, scales, squeeze of rolls, scratches and another [5-7]. The accumulation of defects in material during the technological operation caused decreases in mechanical properties as well as fractographic analysis revealed another degradation mechanism in material [8-11].

The aims of contribution:

1. With help of computer simulation is to observe and to analyze behavior of artificial surface

1 R. Mišičko, doc. Ing. CSc. – Department of Materials Science, Faculty of Metallurgy, Technical University of Košice, Park Komenského 11, 043 85 Košice, Slovak Republic. 2 T. Kvačkaj, prof. Ing. CSc., M. Vlado, doc. Ing. CSc., L. Gulová, Ing., M. Lupták, Ing. PhD., J. Bidulská, Ing. PhD. – Department of Metal Forming, Faculty of Metallurgy, Technical University of Košice, Vysokoškolská 4, 042 00 Košice, Slovak Republic. *Corresponding author, e-mail address: jana.bidulska@tuke.sk

Materials Engineering, Vol. 16, 2009, No. 3

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defects inside flat steel sample during simulation of hot rolling process by selected parameters.

2. Studying and investigation of used filler scores (oxides, casting powders) in hot rolling labora-tory process on the subject of closed surfaces.

2. Experiment

The experiment corresponds with the first aim of contribution have been done by simulation program DEFORM 3D for hot rolling process. Sample of material for mathematical simulation with low-carbon steel of sizes h x b x l have been chosen. The surface defects shape „V” and „U” of scores have been injected artificially by software. The location, shape and sizes in point defects are introduced on Fig. 1, 2 and Table 1. In Table 1 are introduced conditions of experiment.

With computer simulation we are not able to affect conditions of real oxidation, therefore in the next phase of research have been done hot rolling laboratory experiment on rolling mill DUO 210 (the experiment with second aim of contribution). The basic material was STN steel class 11 375, cladding material was steel on basis C-Mn-Nb-V. The shape and sizes of samples are introduced on Fig. 3.

The „V“ scores have been filled by scales to get from the heating temperature 1100°C and 1250°C; casting powders „A“, „B“; mixture of scales from temperature 1100°C and casting powder „C“ (ratio 1:4); mixture of scales from temperature 1100°C and casting powder „B“ (ratio 1:4). The joint of the basic and cladding material have been done by peripheral welded joint. The initial size of sample after gladding operation was 20 x 35 x 60 mm (h x b x l).

The condition of laboratory rolling:

- temperature of heating 1200°C; time of heating 30 min,

- thickness of sample after hot rolling 3 mm,

- pickling in 12 % HCl, time 20 min,

- thickness of sample after cold rolling 0.5 up to 1.2 mm, i.e. relative amount of deformation is 60 % up to 83 %.

Fig. 1. Design and location of scores type „V” and

„U“ before rolling

Fig. 2. Parameters of artificial score defects type

„U” and „V“

Fig. 3. Shape and dimension of specimens for

laboratory rolling

04

03

02

01

R

l b

h

l b

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Tab. 1

Sizes of defects - scores „U” and „V” before and

after rolling for given temperatures and amount

of deformation

TS-transverse score, LS-longitudinal score

3. Results and discussion

The results of rolling simulation in software DEFORM 3D close to the deformation temperature 1200°C a 900°C and amount of deformation 10% applied in both temperatures had showed, that selected temperature deformation conditions in relation to the sizes of artificial defects do not change their sizes - input and output of the simulation are identical. The amount of deformation is primary factor from chosen condition of simulation, because deformation temperatures representing initial and final hot rolling condition are not separately express as behavior of watched defects.

While increasing relative amount of deformation to the level 50% and using temperature of deformation 1200°C had occurred except changes of sizes artificial surface defects to their evolution, i.e. creation defects to the type of overlap and scratch. The evolution of original defects during rolling process and their final view after rolling process can be seen in detail within graphics documentation for temperature 1200°C (similar run have been recorded for deformation temperature 900°C). The scores of the shape „V” No. 01 (Fig. 4) and „U” No. 02 (Fig. 5) was orientated transverse on the sample and have been

rolled. After rolling the scores of the shape „V” and „U” it remained only as edge corrugation.

In score of the shape „V“ No. 03 (Fig. 6), „U“ No. 04 (Fig. 7) orientated on sample longitudinal it was going to creation of overlap in back part, which had gone between the rolls as second, only front part have been rolled. The final feature of the score shape „V“ (No. 03) orientated on sample longitudinal after rolling process have been seeing as scratch for whole length, however in score of the shape „U“ (No. 04) with orientation the same as before have been seeing only in corners as scratch.

Fig. 4. Defect of score type „V“ (No. 01)

transverse directed. State after hot rolling for

deformation temperature 1200 °C and deformation

50%.

Fig. 5. Defect of score type „U“ (No. 02)

transverse directed. State after hot rolling for

deformation temperature1200 °C and deformation

50 %.

No.

of

defect

Change of

defects sizes

after rolling

Sizes

before

rolling

[mm]

Sizes after rolling [mm]

Tε [°C]

1200,

900

ε

[%]

10

Tε [°C]

1200,

900

ε

[%]

50

01

V- TS

h01 3 2,2 0

b01 5 5,9 13

l01 100 100 100

02

U-TS

R02 3 h = 1,9 0

b02 6 6,4 15

l02 100 100 100

03

V-LS

h03 3 2,6 0,5

b03 5 5 1 až 2

l03 20 21,7 37

04

U-LS

R04 3 h = 2,6 0

b04 6 5,9 6,2

l04 20 21,9 37,7

okra jová vln itosť edge corrugation

okrajová vlnitosť edge corrugation

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Fig. 6. Defect of score type „V“ (No. 03)

longitudinal directed. State after hot rolling for

deformation temperature 1200 °C and deformation

50 %.

Fig. 7. Defect of score type „U“ (No. 04)

longitudinal directed.

State after hot rolling for deformation,

temperature 1200 °C and deformation 50 %.

The fundamental knowledge consequential from laboratory experiment followed to the second aim of contribution (2.) for scores with filler by

scales can summarize: - influence varied types of scales to the

character of changes in the free surfaces are not approved oneself,

- the scales are made decarburization on the basic material to depth approximately 0.1 mm,

- on the boundary surfaces as basic – cladding material are created oxidizing envelopes, or narrow strips of petite oxides (Fig. 8),

- on the sample after hot rolling and pickling have not be observed any defects,

- the dark flecks of irregular shape had occurred on the surfaces of sample after cold rolling, mainly in the places of primary scales.

Fig. 8. Oxidizing envelopes, or narrow strips of

petite oxides

For scores with filler by casting powders: - casting powder „B“ causes carbonization of

basic material in surroundings scores up to depth 0.2 mm (Fig. 9),

- casting powder „A“ (with lower volume of carbon than „B“) causes carbonization of basic material in surroundings scores up to depth 0.1 mm,

- in the carbonization areas are created cracks during hot rolling (Fig. 10),

- the others expressions have been similar than in the case of scores with filler of scales,

- on the samples after hot rolling and pickling on the cladding surface appeared flecks, than have not been pickling,

- the ratio of dark flecks occurrence on the cladding surface are increased after cold rolling, the casting powders are activated intersection inside undersurface defects on the surface also for materials with increased strength.

Fig. 9. Carbonization of basic material in

surroundings scores

pohľad zhora - vznik ryhy from above view – creation of score

p ohľad spredu počas valcovan ia front view - during of rolling

pohľad spredu - bez vzniku preložky poh ľad zozadu - vzn ik p reložk y

front view – without creation of overlap

from behind view – creation of overlap

pohľad zhora – vznik ryhy v rohoch defektu from above view –

creation of score in the defect corners

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Fig. 10. Cracks during hot rolling

For scores with filler by mixture of casting

powders and scales:

- the fundamental structural expressions are the same as previous assignments,

- mixture of casting powder and scales do not form in own surroundings carbonization of material,

- mixture of casting powder and scales during heating to the temperature 1200 °C forms conglomeration consisting with light oxides and dark fundament composed by melting of casting powder (Fig. 11), as well as eutectics films of melting powder (Fig. 12),

- casting powder „B“ is more aggressive then casting powder „C“, what is express as precipitation oxidizing envelopes in fundamental material very close to the cladding metal sheet,

- on the samples after cold rolling have been observed isolated occurrence of smaller dark flecks and drop out slivers.

Fig. 11. Conglomeration

Fig. 12. Eutectics films

4. Conclusions

- by simulation of the hot rolling process in software DEFORM 3D had occurred during 50 % deformation applied on temperature 1200 °C and 900 °C to evolution of artificial surface defects of given typology,

- the original defects become evident on the output (after finishing of simulation) as overlaps, scratches, corrugation,

- by laboratory experiment on the rolling mill DUO 210 have been proved, that inside defects having oxidative filler are caused creation of dark iron flecks and slivers after cold rolling on the samples,

- the defects type dark steel flecks and slivers are occurred on the surface sample after thickness reduction to value 1.2 mm as a result of cold rolling process, what is introducing value 0.25 mm for depth of undersurface defect,

- the present of casting powders in inner sample defects support creation of dark steel flecks and slivers after cold and hot rolling on the samples.

Acknowledgements

Published results were acquired using the

project VEGA No. 1/4136/07.

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