Segregation and structure incontinuously cast high carbon steel

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Authors Sung, Pil Kyung, 1961-

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Segregation and structure in continuously cast high carbon steel

Sung, Pil Kyung, M.S.

The University of Arizona, 1989

U'M'I 300 N. Zccb Rd. Ann Arbor, MI 48106

SEGREGATION AND STRUCTURE IN CONTINUOUSLY CAST

HIGH CARBON STEEL

by

Pil Kyung Sung

A Thesis Submitted to the Faculty of the

DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING

In Partial Fulfillment of the Requirements For the Degree of

MASTER OF SCIENCE

In the Graduate College

THE UNIVERSITY OF ARIZONA

19 8 9

2

STATEMENT BY AUTHOR

This thesis has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his or her judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.

This thesis has been approved on the date shown below:

APPROVAL BY THESIS DIRECTOR

David R. Poirier Professor of Materials Science And Eng

Date

3

ACKNOWLEDGMENTS

The author would like to express his great appreciation to Professor David R. Poirier for his continual guidance and encouragement. Special thanks also go to Professor Louis J. Demer, Professor William G. Davenport, and other faculty for their kindly teaching.

The author is grateful to North Star Steel - Texas for providing financial support and specimens of this research. In particular, Mr. Bhaskar Yalamanchili and Dr. Gordon Geiger are appreciated.

The author also wishes to thank Dr. Kook S. Yeum for his advice, especially for computer programming, Mr. Richard Hastings for EDX analyses, Mr. Tom Teska in Planetary Science for microprobe analyses, and Mr. Jin-Goo Park, Mr. Byeong-Soo Bae, and all of my colleagues at the University of Arizona for their generous help during the course of this work.

Finally, the author expresses his gratitude to his parents for their love.

4

TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS 6

LIST OF TABLES 8

ABSTRACT 9

1. INTRODUCTION 10

2. BACKGROUND ON SEGREGATION IN CONTINUOUSLY CAST STEEL 12

3. EXPERIMENTAL PROCEDURES 17

3. l Specimens 17

3.1.1 Wire-Rod Specimens 17

3.1.2 Billet Specimens 17

3.2 Analyses of Wire-Rod Specimens 20

3.2.1 Structure and Segregation in Wire-Rod Specimens 20

3.2.2 Microhardness Measurement 21

3.3 Analyses of Billet Specimens 21

3.3.1 Macrostructure of Billet Specimens .. 21

3.3.2 Dendrite Arm Spacing in Billet Specimens 21

3.3.3 Macrosegregation in Billet Specimens 22

3.3.4 Microsegregation in Billet Specimens 25

TABLE OF CONTENTS - Continued

5

Page

4. RESULTS AND DISCUSSION 27

4.1 Analyses of Wire-Rod Specimens 27

4.1.1 Structure in the Wire-Rod Specimens 27

4.1.2 Microsegregation in Central Region of Wire-Rod Specimen 27

4.1.3 Microhardness Measurements in Wire-Rod Specimen 3 5

4.2 Analyses of Billet Specimens 35

4.2.1 Macrostructure of Billet Specimens .. 35

4.2.2 Secondary Dendrite Arm Spacings in Billet Specimen 37

4.2.3 Macrosegregation in Central Region of Billet Specimens 4 3

4.2.4 Microsegregation in Billet Specimens 49

5. CONCLUSIONS 59

APPENDIX A: Operating Conditions of SEMQ and Method of Microprobe Analysis 62

REFERENCES 68

6

LIST OF ILLUSTRATIONS

Figure Page

1. Diagram of continuous casting process and rod-making process 13

2. Unit operations for producing wire rod 18

3. Five paths on a transection of a billet for measurements of secondary dendrite arm spacing ... 23

4. Changes of carbon and manganese percentages on the number of spots selected for analyses. Each spot is from an area of 100 urn diameter 24

5. Schematics of center segregation revealed by Stead's reagent in samples taken from Coil 210348 28

6. Microstructure of central region in Sample 1 from Coil 210348, Stead's Reagent etch, 200X: (a) Location 1; (b) Location 2 29

7. Microstructure of central region in Sample 2 from Coil 210348, Stead's Reagent etch, 200X: (a) Location 1; (b) Location 2 30

8. Microstructure of central region in Sample 3 and 4 from Coil 210348, Stead's Reagent etch, 200X: (a) Sample 3; (b) Sample 4 31

9. Microstructure of central region in Sample 5 and 6 from Coil 210348, Stead's Reagent etch, 20OX: (a) Sample 5; (b) Sample 6 32

10. Microstructure of central region in Sample 7 from Coil 210348, Stead's Reagent etch, 200X: (a) Location 1; (b) Location 2 33

11. Microsegregation of C and Mn in central region of Coil 210348: (a) carbon; (b) manganese 34

12. Center porosity in longitudinal sections of billets: (a) before reheating; (b) after reheating 36

7

LIST OF ILLUSTRATIONS - Continued

Figure Page

13. Macrostructures of longitudinal sections revealed by sulfur prints: (a) before reheating; (b) after reheating 38

14. Dendritic structures of billet (before reheating), 50% HC1 etch : (a) 1 cm from the center; (b) 3 cm from the center; (c) 4 cm from the center (Path c in Fig. 12) 39

15. Secondary dendrite arm spacing in billet material (before reheating). Distances are measured along the paths and from the center, x 40

16. Macrosegregation of carbon along the centerline of billet material before and after reheating 44

17. Macrosegregation of manganese along the centerline of billet material before and after reheating .... 45

18. Relation between C/C0 and Mn/Mn0 for macrosegregation data (Fig. 16,17) 46

19. Relations between 0/Co and Mn/Mn0, P/P0, S/S0 48

20. Microsegregation of C and Mn in billet material before reheating: (a) carbon; (b) manganese 50

21. Relation between C/Cc and Mn/Mn„ for microsegregation data (Fig. 20) 51

22. Microsegregation of Mn in billet materials: (a) before reheating; (b) after reheating 55

23. Comparison of Mn microsegregation in billet materials between before and after reheating: (a) center; (b) 2 cm from the center; (c) 4 cm from the center 57

A-l. X-ray intensity curve 64

A-2. Calibration curve for carbon analyses using SEMQ 65

8

LIST OF TABLES

Table Page

1. Heat compositions 19

A-l. Operating conditions for SEMQ analyses 63

A-2. Characteristic wavelengths 67

9

ABSTRACT

After hot rolling, the presence of segregation in the

center of wire rod can lead to a nonuniform transformation,

resulting in bands of martensite in the microstructure. This

is considered to be a defect, called center-martensite,

because it can cause cracks and breaks during wire drawing.

To identify the mechanism for the formation of center-

martensite in wire rod, the structure, macrosegregation and

microsegregation in unworked billets were characterized.

Based on measurements of secondary dendrite arm spacings,

cooling rates during solidification were estimated. It

appears that the macrosegregation of carbon and manganese in

the billets manifests itself as the microsegregation in wire

rod, which is an agent in forming the center-martensite.

Thus, electromagnetic stirring (EMS) is proposed as a means

to reduce the macrosegregation in the billet and, thereby,

reduce the occurrence of center-martensite in wire rod. This

recommendation assumes that axial segregation in continuously

cast billets is, in fact, reduced by EMS.

10

CHAPTER 1

INTRODUCTION

The proportion of world steel produced via the

continuous casting route has increased, because the process

has numerous advantages. The most important advantage is an

improved yield; in addition, thermal efficiency is improved

and, with good control of the process, high quality steel

can be made. In the United States of America, the percentage

of steel continuously cast was less than 10 percent in 1975

(1). A sharp increase in the use of continuous casting has

since taken place, and the ratio reached 60 percent in

September, 1988 (2).

The idea of continuous casting was conceived by Bessemer

about 1860, but the principle failed because of the

inadequacy of engineering materials of his day (3). It was

in the 1950's that continuous casting of steel began to be

used in industry. Since then the use of the process has

grown very rapidly, and of course today continuous casting

is one of the most important unit operations in the steel

industry.

There are problems associated with the continuous

casting process. One of the most serious problems is

11

• centerline segregation, especially for high carbon steel that

is subsequently hot rolled and drawn into rod or wire

products. After hot rolling, the presence of segregation in

the center of the rod leads to a nonuniform transformation.

This segregation may induce the formation of martensite in

the wire-rod product and cause cracks and breaks during

subsequent cold working. Because the martensite usually

appears in the center of the wire rod, it is deemed "center-

martensite".

The major goal of this research is to identify and to

suggest ways in which center-martensite may be minimized.

In this thesis, the structure, macrosegregation, and

microsegregation in wire-rod products and continuously cast

billets, which were produced by North Star Steel - Texas

(NSST)1, were investigated.

1 North Star Steel - Texas, Inc., P.O. Box 2390, Beaumont, Texas 77704.

12

CHAPTER 2

BACKGROUND ON SEGREGATION IN CONTINUOUSLY CAST STEEL

Figure 1 shows a schematic diagram of a conventional

continuous casting process. Solidification starts in the

mold, wherein a solidified shell forms. The shell has an

appropriate thickness to withstand the ferrostatic pressure

in the liquid core. Solidification is completed through a

water-sprayed cooling zone below the mold.

The structure of continuously cast steel consists of

three zones, as in a statically cast ingot:

i) a chill zone adjacent to the mold surface where the

structure consists of fine equiaxed crystals;

ii) a columnar zone which has a directional dendritic

structure; and

iii) an equiaxed zone consisting of randomly oriented

dendritic grains in the center of the cast.

Segregation results from solute rejection at the

interface in the dendritic mushy zone during solidification.

This leads to "microsegregation", and the length scale

associated with the segregation is the dendritic arm spacing

(approx. 200 jLtm) . The cause of "macrosegregation" in a

casting is the movement of liquid or solid within the mushy

Ladle

Tundish

Mold

Reheating Furnace

Hot Working

Mill

Cold Working

Mill Cutting Billets Device

Final Product (Coil)

^ig. 1. Diagram of continuous casting process and rod-making process.

14

zone (4) . In a columnar dendritic region, the solid is fixed

in place so it is the convection of the interdendritic liquid

that results in macrosegregation. Some macrosegregation can

be attributed to the settling or floating of solid when that

solid is free to move, as in the early stages of equiaxial

solidification.

One of the manifestations of macrosegregation phenomena

is "V-segregation". V-segregation occurs if (i) equiaxed

dendrite crystals are free to grow, while the molten steel

pool remains large, (ii) columnar crystals grow to the

vicinity of the central axis and form bridges, and (iii) the

bridges are not ruptured, and thereby function as effective

filters that collect settling equiaxed dendrite crystals (5) .

Most of the previous reports have agreed that the

general level of segregation in continuously cast billets is

less than that typically found in statically cast steel

ingots, along both the longitudinal and transverse directions

(5) . However, the segregation along the centerline in

continuously cast billets is more severe than that in

statically cast steel ingots.

In 1972, Mori et al. (5) reported that considerable

segregation was present along the central axis of

15

continuously cast billets of carbon steel. Since then there

have been many studies on the centerline segregation,

including the works of Schmidt and Fredriksson (6) and the

extensive review of Moore (7) . It has been shown that

increasing superheat, decreasing section size, increasing

carbon content, and casting speed extend the columnar zone

in continuously cast steel, resulting in increased centerline

segregation and central porosity (7) . Conventional thinking

is that centerline segregation and porosity result when the

columnar dendrites form a bridge across the centerline and

prevent feeding by blocking the flow of liquid.

To reduce macrosegregation, electromagnetic stirring

(EMS) has been introduced in continuous casting. EMS

improves the quality of continuously cast steel by effecting

equiaxial solidification so that bridging and centerline

segregation associated with columnar dendrites are avoided

(7-13). Nevertheless, stirring should be controlled

carefully, because with intense stirring a narrow band of

negative segregation, i.e. the so-called white band, results

(11, 12).

In this thesis, microsegregation, as well as

macrosegregation in continuously cast carbon steel, are

investigated. A new method of selecting the size of the area

16

to be analyzed for reporting macrosegregation is presented.

This is especially important when one wishes to relate

center-martensite in a wire rod to the centerline segregation

in the unworked billet.

17

CHAPTER 3

EXPERIMENTAL PROCEDURES

3.1 Specimens

Figure 2 shows the unit operations in producing wire

rod. Specimens, for this study, were taken after casting

(before reheating), after reheating (before hot working) and

from the wire rod. The product is called wire rod because

the rod, which is produced in the form of a coil, is used to

form wire.

3.1.1 Wire-Rod Specimens

Wire-rod specimens were provided by North Star Steel -

Texas (NSST). The specimens were selected from coils, which

were known to contain center-martensite. The specimens were

from Heat 23326-1, Coil 210348; the composition of this heat

is in Table 1. The rod product was produced by hot working

from a continuously cast billet (13 cm x 13 cm) to a wire rod

with a diameter of 6 mm.

3.1.2 Billet Specimens

Two billet specimens from Heat 18703-2 were provided by

NSST; the composition of this heat is in Table 1. One billet

specimen was taken before reheating, and the other was taken

18

CASTER

I Billet Specimen

[Before Reheating] (13 cm x 13 cm)

I REHEATING FURNACE (1305 C, 1 52 Min.)

I Billet Specimen

[After Reheating] (1 3 cm x 13 cm)

j HOT WORKING

(Reduction 400:1)

I Rod Product Specimen

(6 mm diameter)

Fig. 2. Unit operation for producing wire rod.

19

Table 1

Heat compositions (wt. percent)

Heat C Mn P S si Cu Ni Cr MO

23326-1 0. 62 0 .79 0.017 0.010 0.20 0.13 0.07

CO o o 0 .014

18703-2 0. 65 0 .61 0.015 0.015 0.17 0.15 0.06 0. 10 0 .010

20

after reheating. Reheating was at 13 05°C (the soak

temperature) for 152 minutes (the total reheating time). The

dimensions of the billet specimens were 13 cm x 13 cm x 32.5

cm.

3.2 Analyses of Wire-Rod Specimens

3.2.1 Structure and segregation in Wire-Hod Specimens

Seven samples from a wire rod were taken every 10 m from

a coil, and longitudinal specimens, with surfaces containing

the center of rod, were prepared. During grinding, the

widths of the longitudinal specimens were carefully measured

under an optical binocular, and when the widths were equal

to the rod diameters, grinding was ceased. In this way, it

was assured that the planes for analyses contained the

centers of the rods. After completing the usual surface

preparation and etching with Stead's reagent1, the structure

was revealed.

The central region of a wire rod was analyzed for

microsegregation across the center-martensite using a

Scanning Electron Microprobe Quantometer (SEMQ - Applied

Research Laboratories) . This is a dedicated electron beam

microprobe analyzer located in the Department of Planetary

1 Stead's reagent: 2.5 g CuCl2, 10 g MgCl2, 5 ml concentrated HC1, 250 ml alcohol.

21

Sciences at the University of Arizona. Important operating

conditions of the SEMQ and other details pertaining to the

microprobe analyses, especially for carbon, are explained in

Appendix A.

3.2.2 Microhardness Measurement

To confirm the appearance of martensite in the center

of the wire-rod specimens, microhardness was measured using

the Durimet Microhardness Tester (Ernst Leitz Wetzlar),

located in the Department of Materials Science and

Engineering at the University of Arizona.

3.3 Analyses of Billet specimens

3.3.1 Macrostructure of Billet Specimens

The billet specimens were cut lengthwise into two equal

pieces. The center-longitudinal surface was ground to a

finish of 200 grit. To reveal the structures of the

longitudinal sections, sulfur prints were taken.

3.3.2 Dendrite Arm Spacing in Billet Specimens

From the remaining portions of the cut billets, smaller

samples for optical microscopy and metallography were

removed. Secondary dendrite arm spacings were measured by

22

averaging more than 30 arm spacings. The dendritic

structures were revealed by deep etching in a 1:1 solution

of concentrated HCl and water (75°C), after polishing with

0.05 ftm alumina. Measurements of secondary dendrite arm

spacings were taken along five different paths from the

center toward the periphery of the billet (Figure 3). The

secondary dendrite arm spacings were measured at l, 2, 3 and

4 cm from the center along each of the five different paths.

3.3.3 Macrosegregation in Billet Specimens

The longitudinal variation of composition (C and Mn) in

the central area of the billets was analyzed by using the

previously mentioned SEMQ, with the operating conditions

given in Appendix A. At each point, the average composition

of two adjacent areas, of approximately 250 jum diameter, was

determined. Analyses for every 5 mm along the centerline

were applied to 3 0 points.

To obtain a proper size of beam area for measuring

macrosegregation, the changes of carbon and manganese

percentages with respect to the electron beam diameter were

measured (Figure 4) . Two areas from the billet before

reheating were selected for these measurements: one with

relatively high percentages of carbon and manganese, and the

other with low percentages. For these trials, the beam

diameter was 100 ̂im, and adjacent point-by-point analyses for

23

(Transection of Billet)

M

d 1 r

1 30mm

Fig. 3. Five paths on a transection of a billet for measurements of secondary dendrite arm spacings.

24

Number

Number

Fig. 4. Changes of carbon and manganese percentages on the number of spots selected for analyses. Each spot is from an area of 100 /m diameter.

25

every 100 fim were taken. The horizontal axis of Figure 4 is

the number of points (N) analyzed from the center point, and

the vertical axis is the averaged percentage of N points.

Figure 4 shows that the compositions of carbon and manganese

are constant for six or more points for path 2-2 and

approximately constant for ten or more points along path 3-

2. Because the electron beam of the SEMQ, used in these

analyses, has a maximum beam diameter of 250 jLtm, two adjacent

points of 250 fxm diameter were used in this macrosegregation

analyses. In terms of area, this is equivalent to 12.5 times

the number of points in Figure 4. This area was, therefore,

considered suitable for reporting local macrosegregation

compositions. The area is not too small, so as to confuse

macrosegregation with microseareaation. and not too large,

so as to confuse a local measure of macrosegregation with

some average measure from a sampling area that is too large.

3.3.4 Microsegregation in Billet Specimens

Microsegregation in billet specimens was analyzed using

the SEMQ with the operating conditions given in Appendix A.

Point-by-point analyses, for every 10 /im, along paths in

different locations (the center, 2 cm from the center, and

4 cm from the center), were taken. At each point, the

electron beam was focused to a diameter of approximately 2

fi m.

26

Microsegregation of manganese was also analyzed using

Energy Dispersive X-ray Spectroscopy (EDX)2 located in the

Arizona Materials Laboratories of the University of Arizona.

Analyses were carried out with a "take-off" angle of 40° and

an acceleration voltage of 15 kV. Quantitative results were

obtained for all analyses using ZAF technique3 with the

software code (Tracor Northern Standardless Quantitative

Analysis program: a method which compares the unknown

spectrum with stored standard spectra to obtain the K

ratios).

2 Tracor Northern 5502 Microanalysis System attached to a JEOL JSM-840A Scanning Electron Microscope.

3 Atomic number correction (Z), absorption correction (A) , and fluorescence correction (F).

27

CHAPTER 4

RESULTS AND DISCUSSION

4.1 analyses of Wire-Rod Specimens

4.1.1 Structure in the Wire-Rod Specimens

Figure 5 shows careful sketches of the visual appearance

of the etched longitudinal surfaces of the seven specimens

in the center of the wire rod. The center segregation does

not appear continuously along the length of the coil. This

suggests that the center segregation in the billet, from

which this rod was produced, was periodic. This is expected

because billets with largely columnar structures typically

contain V-segregates and visually apparent porosity along the

centerline (6-8).

Figures 6-10 show microstructures at various locations

along the centers of the wire rod. The banding structure is

evident, and there are white streaks in the central areas.

This banding structure is thought to be caused by the rolling

of the center segregation in the original billet.

4.1.2 Microsegregation in Central Region of Wire-Rod Specimen

As shown in Figure 11, microsegregations of carbon and

manganese were analyzed across the white streak in Location

1 of Sample 1 (Figure 6 a). Figure 11 indicates that the

28

Fig. 5. Schematics of center segregation revealed by Stead's reagent in samples taken from Coil 210348.

29

(a)

, '. I .

. I: II I

(b)

Fig. 6. Microstructure of central region in Sample 1 from Coil 210348, Stead's Reagent etch, 200X: (a) Location 1; (b) Location 2.

(a)

(b)

Fig. 7. Microstructure of central region in Sample 2 from Coil 210348, Stead's Reagent etch, 200X: (a) Location 1; (b) Location 2.

30

----,.

,; I \ ;·

I :

I • ' '( j

. ! ~

t, ·ll ' I . I I

! I .

--

31

(a)

(b)

Fig. 8. Microstructure of central region in Samples 3 and 4 from Coil 210348, Stead's Reagent etch, 200X: (a) Sample 3; (b) Sample 4.

32

I i .. _:

··I . ' !

(a)

(b)

Fig. 9. Microstructure of central region in Samples 5 and 6 from Coi l 21 0348, Stead's Reagent etch, 200X: (a) Sample 5; (b) Sample 6.

33

(a)

(b)

Fig. 10. Microstructure of central region in Sample 7 from Coil 210348, Stead's Reagent etch , 200X: (a) Location 1; (b) Location 2.

34

~ • 2.0

1.5

~ 1.0

u

0.5

0.0 0 20 40 60 80 100

Distance, urn

(a)

2.0

1.5

..-~

1.0 c :;

0.5

0.0 0 20 40 60 80 100

Distance, urn

(b)

Fig. 11. Microsegregation of c and Mn in central region of Coil 210348: (a) carbon; (b) manganese.

35

maxima were located approximately at the midpoint of the

white streak. In the streak the composition is 1.25 %C and

1 %Mn/ whereas the average composition in this area is

approximately 0.85 %C and 0.75 %Mn.

4.1.3 Microhardness Measurements in Wire-Rod Specimen

That the white constituent (i.e., white streak) is

center-martensite was confirmed by microhardness

measurements. The average Vickers hardness number (load :

15g) of the white constituent was 421 and compared well to

the average hardness of martensite (VHN 414), obtained by

austenizing and quenching a piece from the same wire-rod

specimen. The average Vickers hardness of the dark

constituent (i.e., pearlite) was VHN 149. For further study

on the cause of the appearance of martensite in the wire rod,

billet specimens were analyzed as described in the following

section.

4.2 analyses of Billet Specimens

4.2.1 Macrostructure of Billet Specimens

Figure 12 shows the visual appearance of the

longitudinal sections after grinding. Center porosity is

obvious. Porosity is also important, because the porosity

may induce microcracks in subsequently hot or cold rolled

(a) <b)

3 cm

Fig. 12. Center porosity in longitudinal sections of billet (a) before reheating; (b) after reheating.

37

products. In this research, porosity in the billets was not

investigated in detail. However, the formation of porosity

is an important solidification aspect and should be studied.

Figure 13 shows the macrostructures revealed by sulfur

prints. The darker indications in the center of the billets

correspond to sulfide segregation. The central equiaxed zone

is approximately 2 cm in width, with the remainder of the

cross-section occupied by columnar grains.

4.2.2 secondary Dendrite Arm Spacings in Billet Specimen

Figure 14 shows the dendritic structures at three

distances (1, 3, and 4 cm) from the center of the billet.

As shown in Figure 15, the secondary dendrite arm spacings

were measured along five different paths radiating from the

center. Secondary dendrite arm spacing increases with

increasing distance from the surface of the billet, because

the secondary dendrite arm spacing depends upon the cooling

rate during solidification.

The relationship between secondary dendrite arm spacing

and cooling rate (14) is

d2 = a en = b e"n (1)

where

d2 = secondary dendrite arm spacing, jtim

$ = local solidification time, s

38

e .2 1

e o

o w

(a) (b)

3 cm

Fig. 13. Macrostructures of longitudinal sections revealed by sulfur prints: (a) before reheating; (b) after reheating.

39

(a)

(b)

(c)

Fig. 14. Dendritic structures of billet (before reheating), 50% HCl etch: (a) 1 ern from the center; (b) 3 ern from the center; (c) 4 ern from the center (Path c in Fig. 12).

40

240

_ 220 H E s

#»

<zi < Q 200 ->» L. ea •o n o u O) W 180 H

Path

-o- a b

-o- c d e

1ci0

2

Distance, cm

•

d ' i

130mm

(Transection of Billet)

Fig. 15. Secondary dendrite arm spacing in billet material (before reheating). Distances are measured along the paths and from the center, x.

41

c = average cooling rate during solidification, K s"1

and a, b, n = constants.

The exponent n is in the range of 1/3 to 1/2 for secondary

dendrite arm spacing and is generally determined empirically

(14) .

For Fe-C-Cr alloys (for %C: 0.77 to 0.82 and %Cr: 1.81

to 2.61), Okamoto et al. (15) assumed that n = 1/3 in

Equation (1) and determined "a" for various compositions.

For approximately 2 %Cr, "a" varies with carbon content as

a = -15.2 (%C) + 85 (2)

Equation (2) applies to steels that solidify to austenite( 7 ) .

In terms of the local solidification time, 6, we can

use

Tl - Ts C = (3)

9

where

Tl = liquidus temperature, and

TS = nonequilibrium solidus temperature.

For Fe-C-Cr alloys that solidify to 7 , Ts =* 1160°C, and

Tl = a0 + ax (%C) + a2 (%C)2 (4)

where a0 = 1531, aa = -62.793, and a2 = -6.1167 (16).

42

For 0.65 %C (the billet composition in this research),

Equations (2) and (4) give a = 75.12 and TL = 1488 °C,

respectively. Then, the secondary dendrite arm spacing for

0.65 %C steel is

d2 = 75.12 c"1/3 (5)

or

d2 = 10.89 e1/3 (6)

Jacobi et al. (17) measured secondary dendrite arm

spacings for steel with various contents of carbon and

manganese. In doing a regression analysis on the data, they

did not assume a value for n, as did Okamoto et al. (15).

They obtained:

d2 = 15.S 0°-" (7)

or

d2 = 109.2 £-°-" (8)

Equations (7) and (8) are for 0.59 %C - 1.10 %Mn steel, which

is close to the composition of the billet in this research.

Using the results of Jacobi et al. (17) (Equations (7)

and (8)), the cooling rate at 1 cm from the center is in the

range of 0.167 to 0.204 K s"1. At 4 cm from the center and

toward the midface (Path c, d and e in Figure 15) , the

cooling rates are approximately 0.347 to 0.402 K s"1.

It is found that the secondary dendrite arm spacings

43

differed between paths that would have cooled at equal rates

if uniform cooling had been employed (e.g., Paths e and c).

It can be deduced that the cooling condition on the exterior

surfaces of the billet was not uniform during solidification.

4.2.3 Macrosegregation in Central Region of Billet Specimens

Figures 16 and 17 show the macrosegregation, in the

center of the billets, of carbon and manganese, respectively.

In Figure 16, the macrosegregation ratio for carbon is

defined as C/C0, where C is the weight percent of carbon and

CD is the weight percent of carbon reported for the heat

(Table 1, Heat 18703-2). In Figure 17, the macrosegregation

ratio for manganese, Mn/Mn0, is defined similarly. In terms

of this ratio, the segregation of carbon is more severe than

that of manganese. According to these results, reheating

reduces the macrosegregation of carbon in billets. These

rather surprising results indicate that further study on the

homogenization of macroseareaated carbon during reheating

should be done.

Figure 18 shows the macrosegregation ratio of manganese

plotted against that of carbon. Iwata et al. (9) also

plotted this relationship, using segregation data from an

overall section of billet (115 mm square), and found

relationships between the segregation ratio of carbon (i.e.,

macrosegregation ratio) and the ratios for manganese,

44

o U

Distance, mm 150

-o- Before • After

(Reheating)

Fig. 16. Macrosegregation of carbon along the centerline of billet material before and after reheating.

45

50 100

Distance, mm

-n- Before After

(Reheating)

1 50

50 mm

1 30 mm 325 mm

Fig. 17. Macrosegregation of manganese along the centerline of billet material before and after reheating.

46

2.4-

2.0'

© c s

1 .2 -

y = 0.9315 + 0.2206X R = 0.37

• Before • After

•

•

• \ (Reheating)

• 3 BE"

p - •

1.0 1.2 1.4 1 .6 1 .8 2 .0 2 .2

C/Co

Fig. 18. Relation between C/C0 and Mn/Mn0 for macro-segregation data (Fig. 16, 17).

47

phosphorus, or sulfur, respectively (Figure 19).

As shown in Figures 18 and 19, the results of this

research agree with those of Iwata et al. (9) in that the

slopes are almost equal, although the macrosegregation ratio

for manganese in this research is approximately 0.15 greater

than that of Iwata et al.. The slope of this research is

0.22, and that of Iwata et al. is approximately 0.2.

However, the macrosegregation ratio for carbon is as great

as 2 and is typically 1.5, whereas the maximum observed by

Iwata et al. (9) is 1.45. The difference between the two

sets of data is the large difference between the volume

analyzed for the individual analyses. Iwata et al. (9)

reported analyses taken from chips, machined from paths that

were 200 mm long, 0.1 mm wide and 2.5 mm deep. Thus, their

sample volume was 50 mm3. The results of this research are

based on analyses of two beam areas, with a diameter of 250

fim on the surface. Assuming that the irradiated material is

a sphere with a diameter 1.5 times 250 /xm, then the sample

volume is 0.055 mm3. Therefore, Iwata et al. (9) used a

sample volume that is approximately 900 times greater than

the sample volume used for this work. Also, the

macrosegregation ratios in this research are restricted to

a path along the axis of the billet, whereas Iwata et al. (9)

sampled material across an entire section of a billet.

48

2.6 J(

0 p 2.4 - )( s

~ Mn S/So

2.2

0 .....

2.0 ~ 1-o

c 0

~ 1.8

bO Q)

1-o bO

1.6 )(. )< Q) -

U)

1.4 1---+-----..

1.2 -

1.0 1.2 1.4 Segregation ratio of C

Fig. 19 Relations between C/C0 and Mn/Mn0 , P/P0

, S/So (9).

49

Figure 18 indicates that the data in this research show

a high degree of scatter. Accuracy in the carbon analyses

might be questioned because low atomic number elements, like

carbon, are difficult to analyze with electron beam

microprobes.

4.2.4 Microsegregation in Billet Specimens

Figure 20 shows the microsegregation of carbon and

manganese, at three distances (0, 2, and 4cm) from the center

of the billet (before reheating). These microsegregation

data were obtained by microprobe analysis, described in

detail in Appendix A. At the center of the billet, average

percentages for carbon and manganese are 0.89 and 0.71,

respectively, and are higher than those at the other two

locations (2 and 4 cm from the center). This, of course, is

consistent with the aforementioned observation that there is

positive macrosegregation of both carbon and manganese in the

center of the billet. Using these microsegregation data, the

relationship between the microsegregation ratio of manganese

and that of carbon was plotted as in Figure 21, where C0 and

Mn0 are based on the heat analysis (Table l) . Figure 21

shows the slopes of 0.31 and 0.19 for microsegregation along

the axis of the billet (i.e., 0 cm) and at 2 and 4 cm from

the center, respectively. Examination of Figure 20 shows that

there are corresponding trends of the data for carbon and

50

c o i_ ea U •w C o> u s-at CU

Distance, |um

(a)

500 Distance from center

0 cm

•o- 2 cm

— 4 cm

C s s CJ V u <u &

Distance, pm

(b )

500

Fig. 20 Microsegregation of C and Mn in billet material before reheating: (a) carbon; (b) manganese.

51

2.5

© e

n s

2.0-

1.5'

1.0-

Q 0 cm a 2 cm + 4 cm

(Distance from center)

y = 0.73 + 0.31 X R=0.76

0.5- y = 0.80 +0.19 X R=0.48

o.o i 1 1 1 r 0.8 1.0 1.2 • 1.4 1 . 6 1 . 8

C/Co

Fig. 21 Relation between C/CD and Mn/Mn0 for micro-segregation data (Fig. 20).

52

manganese at 0 cm from the center. This is also evident by

the "R-value" of 0.76 for the same data plotted in Figure 21.

This indicates that the measured microsegregation of carbon

is real. On the other hand, the "R-value" of only 0.48 for

the remaining data in Figure 21 indicates that, perhaps, the

scatter of the data for carbon is too great to make a similar

conclusion. It should be pointed out, however that the

variation of the concentrations of carbon and manganese along

the particular paths in the regions 2 and 4 cm from the

center were not large, and this fact alone could be

responsible for the rather low "R-value".

Microsegregation results from solute redistribution at

the interface in the dendritic mushy zone. To quantify the

solute redistribution during dendritic solidification, Clyne

and Kurz (18) presented a modified form of the Scheil

equation:

C5* = k CD [1 - (1 - 2 a1 k) fs] <k-1>/<1-2a'k> (9)

where

CE* = solute concentration in the solid at the solid/

liquid interface,

k = equilibrium partition ratio,

CD = uniform alloy composition before solidification, and

fs = weight fraction of solid.

53

a' is an adjusted parameter of a , which is the diffusion

Fourier number and represents the extent of diffusion of

solute in the solid; these are defined as

a = D8 — (10) d2

11 1 a' = a [ 1 - exp(- ) ] - exp (- } (11)

a 2 2 a

where

Ds = diffusion coefficient of solute in solid,

tf = local solidification time, and

d = dendrite arm spacing.

There are two extremes:

(a) No diffusion in solid; a = a' =0

Cs* = k C0 [1 - fB] (12)

This is known as the Scheil equation.

(b) Solidification occurs under equilibrium condition

(complete diffusion in solid); a = » and a' = 0.5

Cs* = k CD [1 - (1-k) fE] _1 (13)

This is described by the lever rule.

The diffusion coefficients of carbon and manganese in

austenite are approximately 10~9 and 10~" m2 s"1, respectively,

at 1350"C \19). Under the assumptions that the secondary

54

dendrite arm spacing is the controlling spacing and the local

solidification time is 600 s, the diffusion parameters for

carbon and manganese are

® carbon 9.6, ^'carbon 0.451 r

® mangonaso 9.6 X 10 , 0£ ' mansanono °* ^ manganese*

a1 for carbon is close to 0.5 (i.e., almost complete

diffusion in solid) , and that for manganese is close to 0

(i.e., no diffusion in solid). Based on this argument, it

can be deduced that carbon would exhibit very little

microsegregation, compared to the microsegregation of

manganese. If there is microsegregation of carbon, and

according to Figures 20 and 21 such is the case, then there

must be a thermodynamic effect. That is the concentration

of carbon adjusts itself according to the microsegregation

of manganese in order to achieve a constant activity.

In order to measure the homogenization during reheating,

the microsegregation of manganese before reheating was

compared to that after reheating (Figure 22) . These

microsegregation data were obtained by the EDX. A smooth

curve passing through the 50 data points of an individual

path in Figure 22 was drawn using a computer code based on

a cubic spline interpolation (20). From the smooth curve the

fraction of lengths with a composition less than a

prespecified composition of manganese was determined. This

gave an estimate of the fraction solid associated with a

Before Reheating After Reheating

Distance from center

0 cm

-o- 2 cm

• 4 cm

100 200 300 400 500 0 100 200 300 400 500

Distance, jim

( a )

Distance, jim

( b )

Fig. 22 Microsegregation of Mn in billet materials: (a) before reheating? (b) after reheating.

U1 U1

56

particular concentration of manganese. The relationship

between the fraction solid and manganese composition is

plotted in Figure 23. With no diffusion in the solid and

constant equilibrium partition ratio, then Equation (13)

applies; it can be written as

In (Ca70 = (k-1) Ink - (1-k) In (l-fB) , (14)

which suggests a linear relationship between ln(Ca*/C0) and

ln(l-fa). Here Cc'indicates the average concentration of

manganese for a given path in Figure 22. C0' is used, rather

than C0 (concentration of manganese in the heat), because the

intent describes microsegregation at a given location where

the macro-concentration is C0'.

Figure 23 shows the plots of ln(Cs*/C0) versus -ln(l-fB) .

For the three plots labeled "before reheating", linear curves

were not obtained. This suggests that the equilibrium

partition ratio for manganese varies during solidification.

By comparing the curves labeled "after reheating" to the

curves labeled "before reheating", an assessment of the

extent of homogenization of the microsegregation of manganese

during reheating can be made. For Figures 23a and 23b, it

appears that very little homogenization has taken place,

although the maximum concentration of manganese has been

reduced in both cases. These plots are shown for billet

material from the center, where the secondary dendrite arm

spacings are approximately 220-240 ̂ m (Figure 15). However,

0.10-

after reheating before reheating 0.05-

after reheating before reheating CJ 0.00-

-0.05 -

-In(l-fs)

(a)

0.10-

0.05-

0.00-

-0.05 -

H -o.io 3.0

before reheatin

after reheating

-In(l-fs)

(b)

-In(l-fs)

(c )

Fig. 23 Comparison of Mn microsegregation in billet materials between before and after reheating: (a) center; (b) 2 cm from the center; (c) 4 cm from the center.

58

based on the plots c, d and e in Figure 15, the secondary

dendrite arm spacing is approximately 170 /im, at 4 cm from

the center. Because the secondary dendrite arm spacing is

less for this material, there has been more homogenization

in this sample (Figure 23c) than in the other two.

The extent of homogenization can be explained in terms

of the index of residual microsegregation, S (21). For no

homogenization during reheating, S = 1; for complete

homogenization, S = 0. Furthermore, S varies with the

dimensionless group, D0/dz, where D is the diffusion

coefficient, 0 is the diffusion time and d is the secondary

dendrite arm spacing. Because 6 decreases with d~2, the

dimensionless group for the sample 4 cm from the center is

approximately 1.8 times the dimensionless group for the

samples at the center and 2 cm from the center.

In the context of processing within the plant, the

present reheating practice reduces microsegregation in the

center of the billet only by a small amount. To increase the

extent of homogenization, a longer reheat time and/or a

higher soak temperature should be used.

59

CHAPTER 5

CONCLUSIONS

In this research, the structure, macrosegregation, and

microsegregation in continuously cast high carbon steel were

investigated in order to identify the agent responsible for

centerline segregation in wire rod and to suggest ways in

which the centerline segregation may be minimized. The major

conclusions of this research are the following:

(1) The centerline segregation did not appear to be

continuous along the length of the wire rod.

(2) The banding structure and center-martensite were

observed in the central area of the wire rod.

(3) Microsegregations of carbon and manganese were found in

the center-martensite of wire rod. The composition of

the center-martensite was 1.25 %C and 1 %Mn.

(4) That the white streak in the central area of the wire

rod is center-martensite was confirmed by microhardness

measurements.

(5) Center porosity and sulfide segregation were obvious

in the central area of the billets.

(6) The central equiaxed zone occupied approximately 15 %

of the total width of the billet, with the remainder

occupied by columnar grains.

(7) Secondary dendrite arm spacing was in the range of 220

to 240 fim at 1 cm from the center, and was 165 to 175

txm at 4 cm from the center and toward the midface.

(8) Cooling rates during solidification were 0.167 - 0.204

K s"1 (at 1 cm from the center) and 0.347 - 0.402 K s"1

(at 4 cm from the center, toward the midface).

(9) By measuring secondary dendrite arm spacings, it was

deduced that the cooling condition on the exterior

surface of the billet was not uniform.

(10) Carbon exhibits more severe macrosegregation than does

manganese along the axis of the billet, and surprisingly

macrosegregation of carbon was reduced after reheating.

Further study on the homogenization of the

macrosegregation of carbon during reheating should be

done.

(11) The relationship between segregation ratios of manganese

and carbon was determined. The results indicate that

the macrosegregation ratio of carbon can be as great as

2.0, whereas the ratio of manganese is less with a

maximum of 1.5.

(12) Microsegregation of carbon was found in the central area

of the billet. Based on the diffusion of carbon, it was

deduced that carbon would exhibit very little

microsegregation. Thus, this microsegregation of carbon

would be explained by thermodynamic effect.

61

(13) It appears that axial macrosegregation in the billet

manifests itself as the centerline segregation in the

wire rod. Thus, macrosegregation in the billet could

be a factor contributing to the appearance of center-

martensite in wire rod. Electromagnetic stirring (EMS)

could be used to reduce the macrosegregation in the

center of the billets as has often been reported.

Further study on the use of EMS in continuously cast

steel should be done. In particular a study, similar

to this, on the structure and segregation in billets

produced with EMS, as well as in resulting wire rod,

should be carried out.

62

APPENDIX A

OPERATING CONDITIONS OF SEMQ AND

METHOD OF MICROPROBE ANALYSIS

The operation conditions for the microprobe analyses are

shown in Table A-l.

First, the X-ray intensity curve was obtained for

wavelengths in the vicinity of a specific element, Figure A-

1. Two positions (X and Y in Fig. A-l) for background were

selected, and these intensities were measured (a and b in

Fig. A-l) for 100 s each. Interpolation of these two

background points was used to get the background intensity

at the wavelength of interest (c in Fig. A-l) . By

subtracting the background intensity from the intensity of

the peak that was measured for 200 s, the X-ray intensity for

the specific wavelength was calculated (d in Fig. A-l) . With

this X-ray intensity, the percentage of each element was

calculated using the ZAF technique1, embodied in the software

code (TASK II).

For carbon analyses, the ZAF technique was not employed.

Instead a calibration curve (Figure A-2) for carbon was used.

The calibration curve was obtained using standard Fe-C alloys

(0.01, 0.30, 0.62, 1.01, and 1.29 wt% C), which were supplied

1 Atomic number correction (Z), absorption correction (A), and fluorescence correction (F).

63

Table A—1

Operating conditions for SEMQ analyses

Acceleration Voltage 10 kV

"Take-off" angle 52.5°

Sample current2 49.2 nA

Analyzing crystal PbSt3 (for C)

LiF (for Mn,Fe)

Analyzer: Hardware TN 880 and TN 1310

(Tracor Northern)

Software TASK II

2 Sample current measured by Faraday cup.

3 PbSt is lead stearate.

c

X

Wave Length

Fig. A-l X-ray intensity curve.

65

2.0 y = • 0.228 + 24.4923x R = 1.00

1.0-

0.5-

0.0 0.07 0.04 0.01 0.03 O.OG 0.02 0.05

Intensity (/beam)

Fig. A-2 Calibration curve for carbon analyses using SEMQ.

66

by the Microbeam Analysis Society4. Samples and standards

were not carbon coated. Intensity is X-ray count rate

normalized to 0.1 nA sample current.

Table A-2 shows the characteristic wavelengths of iron,

manganese and carbon that were used for the analyses. The

reproducibility of the measured intensities of carbon in the

standards was ±10 %. Thus, the reproducibility of the

concentration of carbon, based on the measured intensities

in the samples studied in this research, is ±14.5 %.

The Microbeam Analysis Society, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125.

Table A-2

Characteristic Wavelengths

Fe Ka

Mn Ka

C Ka

1.9374 A

2.1031 A

44.7 A

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68

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