15. - 17. 5. 2013, Brno, Czech Republic, EU

ANALYSIS OF SURFACE DEFECTS OF “DENT” AND “PIN-HOLE” TYPES FORMATION

PROCESS DURING COLD ROLLING

Abdrahman NAIZABEKOV, Vitaliy TALMAZAN, Almas YERZHANOV

Karaganda state industrial university, 101400, Republic avenue 30, Temirtau, Republic of Kazakhstan,

kgiu@mail.ru

Abstract

Intensive development of production demands from sheet-rolling production of release of the thin high-quality

leaf intended for manufacturing of various details and the equipment. Sheets besides optimum structure and

demanded mechanical properties, should possess high quality of a surface.

Improvement of quality of steel products is one of the main objectives of modern rolling production, and also

takes a special place in research-and-production activity of the industrial enterprises.

In the field of mechanical engineering requirements to quality of a surface of a leaf, in this regard providing

faultless production constantly raise, and also research of process of a defectform and a formchange of

defects of a surface remains an actual task.

Timely detection of superficial defects excludes marriage hit on the subsequent operations, and also allows

to define in due time possibility of formation of marriage, its reason and in due time to take measures for

prevention of formation of defect. Researches in this direction is very actual.

Results of researches are given in this work about possibility of improvement of quality of a surface of a cold

rolling strip at the expense of increase of a burnishing of superficial defects of the «dent» and «pin-hole»

types by means of the computer.

Keywords: cold rolling, surface defect, finite-element method, the rolling mill, relative depth of flaw, net

reduction, coefficient of friction, case-hardening, regression a sample piece, a reference grid.

1. INTRODUCTION

Existence of strip surface defects is one of the main reasons for rejection and downgrading of rolled products

in cold rolling shops. Practically surface defects in most cases can arise during metal transportation, in the

course of rolling at violation of technological modes, and also at failure of working elements of rolling

equipment.

2. MAIN TEXT

2.1 Methodological bases

In this regard an actual problem is studying and development of ways and methods for prevention and

elimination of defects on the strip surface. One of ways of studying of surface defects shape changing

mechanism is the use of the mathematical modeling based on a method of finite elements.

On the basis of mathematical modeling of strips cold rolling process with surface defects there is

comparative analysis of defects of “dent” and “pin-hole” types is carried out in this work.

There was process of volume surface defects shape formation of “dent” and “pin-hole” types at cold rolling of

strips at continuous five-stand mill 1700 simulated. For the purpose of calculations accuracy increase and in

accordance with product mix of 1700 mill the small area (50×100mm) of ductile metal strip with thickness of

4.5 mm was simulated. Defects were put on a strip surface in the form of deepening.

15. - 17. 5. 2013, Brno, Czech Republic, EU

Material of work rolls was accepted as continuous, incompressible with the diameter of 600mm. strip material

– 08kp steel. On the strip there were six surface defects of cylindrical and conic shapes (fig.1) with various

initial relative depth δ0/h0 simulated.

а) b)

а) – defect with the cylindrical shape of “dent” type; b) – defect with the conical shape of “pin-hole” type; h0 – strip thickness, mm; δ0 – defect depth, mm.

Fig. 1 Cross-sectional profile of the strip with surface defects

2.2 Experimental part

These works [1] testify that practically there are surface defects with relative depth of δ0/h0 = 0.37. For the

purpose of research area expansion it was decided to increase the limit relative depth of surface defects

approximately by 13%. Thus, relative initial depths of defects varied within 10-50% from the strip thickness.

Simulation was spent at constant speed of rolling. During researches widening of strips with defects was not

considered. For the purpose of calculations time reduction there was optimum coordinate grid (100 thousand

elements) set on the strip. Researches were spent at minimum (f =0.03) and medium (f =0.07) friction factors

of cold rolling. Results of surface defects shape change with various initial relative depth δ0/h0, rolled with

different total reduction εΣ and at different friction factors f are given in tables 1 and 2.

Table 1 Surface defects of “dent” type shape changing

Initial relative depth of defect δ0/h0

Initial shape of the defect

Fiction factor

f = 0.03 f = 0.07

1 2 3 4

0.1

εΣ = 19.18%

Defect is rolled

εΣ = 35.11%

Defect is rolled Defect is rolled

0.3

εΣ = 19,18%

εΣ = 35.11%

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Continuation of Table 1

1 2 3 4

εΣ = 45.56%

0.5

εΣ = 19.18%

εΣ = 35.11%

εΣ = 45.56%

Table 2 Surface defects of “pin-hole” type shape changing

Initial relative depth of defect δ0/h0

Initial shape of the defect

Fiction factor

f = 0.03 f = 0.07

1 2 3 4

0.1

εΣ = 19.18%

Defect is rolled

εΣ = 35.11%

Defect is rolled Defect is rolled

0.23

εΣ = 19.18%

εΣ = 35.11%

εΣ = 45.56%

Defect is rolled Defect is rolled

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Continuation of Table 2

1 2 3 4

0.44

εΣ = 19.18%

εΣ = 35.11%

εΣ = 45.56%

During the simulation there was stressed state of inner defects internal parts. In the place of defects there were points at planes and in lattice points, as shown at Fig. 2.

a) b)

а) – defect with the cylindrical shape of “dent” type; b) – defect with the conical shape of “pin-hole” type;

Fig. 2 Scheme of stress tracking in defects

2.3 Results and discussion

Then there was tracking of stressed state in given points (figure 3).

а) б)

а) – defect with the cylindrical shape of “dent” type; b) – defect with the conical shape of “pin-hole” type

Fig. 3 Stressed state in points at planes and in lattice points of defects

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The analysis of the received data on surface defect of “dent” type shows that points located on the plane of

cross-section (point 3 at Fig.2а), are exposed to the greatest stress in comparison with points in lattice points

of planes and on the bottom surface (points 2 and 1 accordingly at Fig.2а). Quite big difference of stress in

points 1 and 3 (Fig.3а), equal to 171MPa is explained by presence of double concentrator of stress that is

caused by the geometrical structure of cylindrical shape defects.

After the consideration of results on surface defect of “pin-hole” type points located on the plane of cross-

section (point 2 at Fig.2b) are exposed to the greatest stress in comparison with points in lattice points of

planes (point 1 at Fig.2b). quite big difference of stress in points 1 and 2 (Fig.3b), equal to 123MPa is

explained by presence of unique concentrator of stress that is caused by the geometrical structure of conical

shape defects.

According to simulation correlation-regression analysis produced models reflecting the character of

technological parameters influence on rolling of surface defects (1 – defect of “dent” type, 2 – defect of “pin-

hole” type):

35,085,0002,0 feh

d, (1)

27,04,0003,0 feh

d, (2)

Achieved results are evidencing the positive role of external friction and relative reduction in the process of

superficial defects relative depth elimination in the process of rolling. External friction forces are making the

stress active, thereby increase defect shape changing. With the increase of friction forces the process of

defect bottom part “buckling” amplifies.

CONCLUSION

By means of finite elements method there was conducted mathematical simulation of strip cold rolling

process with surface defects of “dent” type and “pin-hole”. Stressed state in the zone of defect depending on

the form of cross-section profile for surface defect was analyzed. Models of defect relative depth

dependence on parameters of rolling were received.

LITERATURE

[1] Zenchenko F.I., Chernov P.P., Mazur V.L.. Prevention of strip breakages at cold rolling of strips. / Rolling

production. 1990. № 3, p. 35-42.