ISSN 09670912, Steel in Translation, 2014, Vol. 44, No. 4, pp. 264–267. © Allerton Press, Inc., 2014. Original Russian Text © Yu.F. Ivanov, V.E. Gromov, D.A. Kosinov, S.V. Konovalov, S.A. Barannikova, 2014, published in “Izvestiya VUZ. Chernaya Metallurgiya,” 2014, No. 4, pp. 51–55.
Structure of LowCarbon Steel Sheet after Scale Removal Yu. F. Ivanova, b, V. E. Gromovc, D. A. Kosinovc, S. V. Konovalovc, and S. A. Barannikovad, e, f a
Tomsk Polytechnic University, Tomsk, Russia Institute of HighCurrent Electronics, Siberian Branch, Russian Academy of Sciences, Tomsk, Russia cSiberian State Industrial University, Novokuznetsk, Russia email:
[email protected] d Institute of Strength Physics and Materials Science, Siberian Branch, Russian Academy of Sciences, Tomsk, Russia e Tomsk State University, 36 Lenin Prospekt, Tomsk, Russia f Tomsk State University of Architecture and Building, 2 Solyanaya Ave., Tomsk, Russia b
Received April 23, 2013
Abstract—The structural and phase changes in lowalloy steel during scale removal are analyzed by means of optical and transmission electron microscopy. A band with increased pearlite content is observed; it may be attributed to the liquation of carbon. Keywords: lowcarbon steel, dislocational substructure, phase composition, plastic deformation, scale, dislo cation density DOI: 10.3103/S0967091214040081
Steel sheet is of great economic value. However, in hot rolling and cooling, the surface of the sheet is coated with scale, as a result of oxidation in air. That impairs its performance. In view of the constantly increasing requirements on the quality and mechani cal properties of steel sheet, the evolution of its struc ture and phase composition during manufacture, heat treatment, and scale removal is of great interest [1–3]. In the present work, we analyze the structural and phase changes in lowcarbon steel during scale removal. We investigate 08ps steel, with the following compo sition: 0.05–0.11% C, 0.05–0.17% Si, 0.35–0.65% Mn, up to 0.25% Ni up to 0.1% Cr, up to 0.25% Cu, up to 0.04% S, and up to 0.035% P (wt % in all cases); the balance is iron [4]. Samples of 08ps steel are cut from hotrolled strip. The strip is rolled with initial and final temperatures of 1250 and 860–890°C, respectively, coiled at 670°C, and cooled in air to room temperature. Some of the samples are taken from the strip with scale; for others, the scale is removed by mechanical means (flexure of the strip between rollers, with simultaneous tension) and by subsequent passage through a bath of sulfuricacid solution. The phase composition and defect substructure of the steel samples are investigated by optical and transmission (diffractional) electron microscopy [5, 6]. Thermomechanical treatment creates polycrystal line structure based on α phase (a solid solution with the bcc lattice of iron). The grains of α phase are frag mented; that is, they are separated by smallangle boundaries into regions of predominantly nonequiax
ial form (Fig. 1). The relative area of the grain occu pied by the fragments is about 0.8. The mean transverse dimensions of the fragments are 0.603 ± 0.310 μm; the range is 0.3–1.5 μm (Fig. 2a). The azimuthal component of the fragment’s total disorientation angle, determined from the rela tive broadening of the reflexes on the electron micro diffraction patterns, is Δ = 2.8° [5]. Within the frag ments and within grains with no smallangle bound aries, we see reticular dislocational substructure (0.56 of the grain volume) and chaotic distribution of the dislocations (0.44 of the grain volume). The scalar dislocation density 〈ρ〉, averaged over all types of sub structure, is about 2.4 × 1010 cm–2. Flexural extinction contours are seen on the elec tronmicroscope images of the steel’s structure (Fig. 1). The presence of flexural extinction contours indicates flexure and torsion of the α phase lattice, which, in turn, indicates internal stress fields arising during thermomechanical treatment of the steel [5–8].
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0.5 µm Fig. 1. Electronmicroscope image of steel structure after rolling. The arrows indicate flexural extinction contours.
STRUCTURE OF LOWCARBON STEEL SHEET AFTER SCALE REMOVAL
The morphology of the flexural extinction contours corresponds to the flexure–torsion gradient of the steel’s crystal lattice; the transverse dimension of the contours corresponds to the degree of flexure and tor sion of the lattice and the amplitude of the internal stress fields. In 08ps steel, the contours begin at the boundaries of the grains and fragments. Hence, the stress concentrators responsible for the flexure and tor sion of the steel are the intraphase grain boundaries. Sta tistical analysis of the flexural extinction contours shows that their mean transverse dimension is h ≈ 190 nm. The density of the contours (the number of contours per unit area of the foil image) is η ≈ 1.3 × 105 mm–2. Note that this characteristic indicates the number of stress concentrators in the steel. Tables 1 and 2 present these structural parameters of 08ps steel in general form. Scale removal does not significantly change the steel structure (Fig. 3). The grains are fragmented. Within the fragments, we see reticular dislocational substructure and chaotic distribution of the disloca tions; and the boundaries of the grains and fragments are sources of flexure and torsion of the steel lattice, as indicated by the flexural extinction contours. The dif ference in structure of the steel after scale removal is shown by quantitative analysis. Analysis of Tables 1 and 2 yields the following conclusions. (1) Scale removal significantly reduces (by a factor of two or more) the mean dimensions (Fig. 2b) and increases (by a factor of about 1.3) the azimuthal com ponent of the fragments’ total disorientation angle. (2) Scale removal changes the dislocational sub structure: chaotic distribution of the dislocations is replaced by reticular dislocational substructure, whose content (vol %) more than doubles; the increase in scalar dislocation density is slight. (3) Scale removal significantly (by a factor of about 1.8) increases the density of stress concentrators (the number of flexural extinction contours per unit area of the foil), with slight decrease in transverse contour dimensions. (4) Scale removal increases (by a factor of 2.6) the dislocation density at the fragment boundaries, as fol lows from estimates based on the relation between the density of dislocations concentrated at subbound aries (〈ρ〉), the disorientation angle of the subbound aries (Δ), and the distance between subboundaries (d) [5, 6]: 〈ρ〉 = 2Δ/bd, where b is the Burgers vector of the dislocations. Thus, on the basis of transmission (diffractional) electron microscopy, we may conclude that scale removal from the surface of 08ps steel sheet signifi cantly increases its defect content. Scale removal from steel sheet is accompanied by the appearance of bands with increased (type I) and reduced (type II) etchability on the metallographic sections (Fig. 4). The structural and phase state of the bands is analyzed by studying the corresponding foil samples. STEEL IN TRANSLATION
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(a) W, % 40 20 0 0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 (b) H, µm W, % 30 20 10 0 0.05
0.25
0.45
0.65
0.85 H, µm
Fig. 2. Histogram of the transverse fragment dimensions of steel after rolling (a) and after rolling and scale removal (b).
0.5 µm Fig. 3. Electronmicroscope image of 08ps steel structure after rolling and subsequent scale removal.
Fig. 4. Surface structure of 08ps steel after scale removal. The arrows indicate sections of the steel chosen for elec tronmicroscopic analysis of the structure and phase com position.
We find that the two types of band are morphologi cally similar but differ significantly in quantitative terms. Analysis of the steel’s quantitative characteris tics inside and outside the bands (Tables 3 and 4) per mits the following conclusions.
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Table 1. Substructural characteristics of the steel after rolling (1) and after rolling and scale removal (2) Dimensions of fragments, μm
State
H
L
1
0.603 ± 0.31 0.28 1.44
>2.5
2
0.31 ± 0.14 0.09 0.92
0.80 ± 0.32 0.32 2.20
〈ρ〉 × 1010, cm–2
Dimensions of contours
Δα, deg
h, nm
η × 105, mm–2
2.4
188.6
1.3
2.8
3.0
196.0
2.3
3.6
Mean/minimum/maximum values.
Table 2. Content of dislocational substructure in the steel after rolling (1) and after rolling and scale removal (2) Substructure, vol % State
Dislocations, vol %
without fragments
with nonequiaxial fragments
chaotic distribution
reticular order
20 25
80 75
56 6
44 94
Type I Type II
Table 3. Quantitative characteristics of the steel substructure in bands with increased (type I) and reduced (type II) etch ability Dimensions of fragments, μm
State
H
L
Type I
0.31 ± 0.14 0.09 0.92
0.76 ± 0.31 0.32 1.52
Type II
0.31 ± 0.14 0.12 0.84
0.83 ± 0.33 0.32 2.20
〈ρ〉 × 1010, cm–2
Dimensions of contours
Δ, deg
h, nm
η × 105, mm–2
3.2
192.0
2.6
2.80
2.8
198.5
1.9
4.43
Mean/minimum/maximum values.
The bands with reduced etchability are character ized by significantly more fragmentation and disorien tation of the fragments (and correspondingly a greater number of dislocations concentrated at the bound aries). The number of stress concentrators is greater in the bands with increased etchability. Table 4. Content of dislocational substructure in bands with increased (type I) and reduced (type II) etchability Substructure, vol % State
Dislocations, vol %
without with nonequi chaotic reticular fragments axial fragments distribution order
Type I
41
59
5
95
Type II
8
92
7
93
As a rule, the formation of band structure on rolling is associated with liquation of the carbon and other alloying elements in the steel [9–12]. In fact, our structural and phase analysis shows that the content of pearlite is 2–3 times higher in the bands with increased etchability than in the bands with reduced etchability. This means that the formation of a steel ingot is accompanied by stratification of the solid solu tion, with the formation of alternating carbonrich and carbondepleted regions. Subsequent thermome chanical treatment of the ingots does not result in their homogenization. Cooling of the sheet from the final rolling temperature is associated with phase transfor mations, in which ferrite and pearlite grains are formed. The pearlite grains are nonuniformly distrib uted in the steel; as our research shows, they are pre dominantly grouped in the bands with increased etch ability. STEEL IN TRANSLATION
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STRUCTURE OF LOWCARBON STEEL SHEET AFTER SCALE REMOVAL
Obviously, regions of steel with higher carbon con tent (higher content of pearlite grains) will be stronger and will undergo less deformation in scale removal. In fact, the relatively low levels of fragmentation and dis orientation of the fragments (and correspondingly the smaller number of dislocations concentrated at the boundaries) in the bands with increased etchability indicates less plastic deformation in those regions. On the other hand, the difference in strength of the ferrite and pearlite grains is associated with the formation of an elastic stress state on account of their incompatible plastic deformation. In electronmicroscopic analysis by the thinfoil method, this corresponds to the higher content of stress concentrators in the bands with increased etchability (Table 3).
3. 4. 5.
6. 7.
CONCLUSIONS Scale removal—by mechanical action (flexure of the strip between rollers, with simultaneous tension) and by subsequent passage through a bath of sulfuric acid solution—qualitatively changes the structure and phase composition of 08ps steel. The deformation of the steel is nonuniform, on account of the liquation of carbon, which is evident in the formation of bands with increased pearlite content.
10.
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Translated by Bernard Gilbert
