Metallurgist, Vol. 57, Nos. 5–6, September, 2013 (Russian Original Nos. 5–6, May–June, 2013)
STUDY OF THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF STEEL CAST USING PLASMA HEATING IN A CBCM TUNDISH
E. Kh. Isakaev,1 A. S. Tyuftyaev,1 G. A. Filippov,2 and D. I. Yusupov2
UDC 669.18(075.8)
The effect of technology for steel plasma heating in a CBCM tundish developed and assimilated for the first time within Russia on macrostructure and mechanical properties of a CCB is considered. It is shown that the technology provides a reduction in the degree of steel overheating above the liquidus temperature during casting. There is an increase in macrostructure uniformity, average level of slab metal ductility indices, and impact strength from the start to end of melting, and also a reduction in property scatter over both slab width and thickness. Keywords: steel plasma heating, continuous casting, macrostructure, mechanical properties, impact strength.
Continuous casting of steel is a most effective resource and energy saving technology for the concluding stage of steel smelting production. One of the main tasks of improving this technology is an increase in continuously cast billet (CCB) quality. A tundish, being the next vessel in the path of steel to a mold, is intended for receiving metal from a steel pouring ladle (SL) and distributing it over a mold, and also for providing the possibility of pouring steel continuously, i.e., “melt to melt.” However, there is additional cooling of steel, which predetermines a requirement for additional steel heating in a tundish with respect to liquidus temperature [1]. This metal heating promotes an increase in billet crack sensitivity, development of a columnar ingot structure, and also such macrostructural defects as axial liquation and central porosity [2]. In addition, an extremely high temperature for pouring metal may lead to breakthrough of the solidifying CCB skin with respect to cracks. With increased metal overheating in a tundish, it is necessary to reduce the CCB extraction rate [1, 3]. Using plasma metal heating in a tundish makes it possible to reduce the required metal temperature in the SL and to stabilize its change in the course of pouring, which has an additional effect on CCB macrostructure uniformity and quality [4, 5]. In view of this, in many overseas metallurgical plants the technology of plasma heating metal in a tundish is used extensively. However, within Russia this technology has not been used until recently [6]. Specialists of the Bardin Central Research Institute of Ferrous Metallurgy (TsNIIchermet) together with the Joint Institute of High Temperatures, Russian Academy of Sciences (OIVT RAN), the AGNI-K company, and Magnitogorsk Metallurgical Combine (MMK) have implemented measures for creating industrial test equipment and fitting [7] of a steel plasma heating device (SPHD) as applied to MMK conditions. In accordance with a planned agreement of MMK, a tundish was prepared intended for pouring experimental melts.
1 2
Joint Institute of High Temperatures, Russian Academy of Sciences (OIVT RAN), Moscow, Russia. Bardin Central Research Institute of Ferrous Metallurgy (TsNIIchermet), Moscow, Russia; e-mail:
[email protected].
Translated from Metallurg, No. 5, pp. 69–74, May, 2013. Original article submitted January 17, 2013.
0026-0894/13/0506-0427 ©2013 Springer Science+Business Media New York
427
TABLE 1. CCB Macrostructure with the Use of Plasma Heating in Tundish Structure defect (scale) Melt
Tliq, °C
ΔT, °C
1
1515
+30
axial chemical inhomogeneity (ACI)
axial cracks (AC)
cracks perpendicular to wide lab arms (CP)
clustered cracks (CC)
localized inhomogeneity (LI)
14-4
1.5
2
0
1
1.5
2
15-4
1.5
2
0
1
1.5
1
14-1*
1.5
1.5
0
1
1
0.5
*
1.5
1.5
1
1
1
0.5
14-3*
1.5
2
0.5
1
2
1
14-4*
1.5
1.5
0.5
1
1
1
*
1.5
1.5
1
1
1.5
1.5
14-6*
1.5
1.5
0
1
1.5
1
14-7*
1.5
2
0
0.5
1.5
0
14-8*
1.5
2
0
0
0.5
0
14-9
1.5
2
0
1
2
0
+26
15-1
1.5
2
0.5
0.5
0.5
0
+ 23
15-4*
1.5
2
0
0.5
1
0
+ 24
13-4*
1.5
1.5
0
0.5
1
1
+16 1515
+13
4 *
1514 1516
14-2
14-5 +11
3
Stream (template)
axial porosity (AP)
+22
2
*
Pouring with the use of plasma heating in tundish.
In the oxygen converter workshop (OCW) in CBCM-4 of MMK, industrial testing was carried out successfully for SPHD equipment. This made it possible to determine the operating capacity of equipment and SPHD fittings, and the possibility of heating steel in a tundish [7]. An algorithm was worked out from the results of testing the device for computer control of metal heating and maintenance of the required steel overheating temperature in the tundish during pouring. Testing of plasma heating was carried out in accordance with the program developed for performing experimental melts of SPHD in a tundish of CBCM-4 of the OCW of MMK. The program included carrying out two series of test melts using plasma heating in a tundish. At the start of the experiment, a continuous temperature measuring system was checked for metal in the tundish, and a curve was composed for the change in temperature during pouring without using plasma heating for the first melt in a series. Then plasma heating was switched on for the tundish with a power of 0.2 MW (10% of nominal) for 5 min starting from the 11th minute of pouring a melt. Plasmatrons were switched off for 15 min and pouring was carried out without heating. Then the heating was switched on again for 10 min, at the end of which pouring was completed without heating. During the whole melt pouring process, there was continuous metal temperature measurement in the tundish and curves were composed for the changes. In the last two melts, pouring was carried out by a similar stage using heating and without it, but with SPHD power of 0.4 and 0.6 MW, respectively. A stagewise increase in heating power during testing made it possible to evaluate the plasmatron component wear, to check the plasmatron supply system, and to correct regime parameters, making it possible to stabilize the arc and optimize heat input to a melt. The second series of test melts was carried out without prior metal heating. Melt cooling in the SL and tundish during pouring was compensated by dosed heating in the tundish. Evaluation of the macrostructure of experimental melts was carried out according to GOST 10243–75. Results of studying steel macrostructure for selected transverse templates are provided in Table 1. As seen from the data in Table 1, there 428
TABLE 2. Test Melt Chemical Composition Element content, %
Melt number
C
Si
Mn
S
P
Cr
Ni
Cu
Al
Ti
B
1
0.16
0.23
0.53
0.0056
0.0074
0.04
0.025
0.037
0.035
0.0018
0.0003
2
0.17
0.24
0.56
0.0075
0.0158
0.05
0.032
0.038
0.042
0.0011
0.0003
Fig. 1. Typical slab steel macrostructure with overheating with respect to liquidus temperature on entry into mold: a) 11°C; b) 30°C.
is a tendency for an increase in the number of NMI defects with an increase in the degree of steel overheating with respect to the liquidus temperature during pouring. In fact, the macrostructure of metal, poured with maximum overheating (30°C) has increased axial chemical inhomogeneity (ACI) and local inhomogeneity (LI). In order to study the macrostructure and mechanical properties through the section of slabs, they were poured from melts of similar chemical composition (No. 1 and No. 2), and templates were selected with overheating above the liquidus temperature from 30 to 11°C. The chemical composition of test melts is presented in Table 2. Specimens were prepared from templates (see Table 1) 14-4 (30°C overheating), 14-1 (22°C overheating), 14-8 (13°C overheating), and 14-6 (11°C overheating) for studying macrostructure by the Bauman method and after deep etching, and also for mechanical tests in tension and impact bending. Templates for studying macrostructure were cut in a transverse section of a slab and longitudinally (with a pitch of about 100 mm). It was established by studies that the distribution of sulfide inclusions through the cross section of a slab occurs more uniformly with lower steel overheating with respect to liquidus temperature during pouring. In steel with overheating during pouring of 22°C, in contrast to steel with overheating of 11°C, there is clearly expressed concentration of sulfur over the slab horizontal axis. A photograph is presented in Fig. 1 of steel specimen macrostructure cast with a different overheating with respect to liquidus temperature (ΔT). In templates of steel cast with ΔT = 30°C (see Fig. 1b), there are significantly more defects, particularly cracks, than in steel with the minimum ΔT = 11°C (see Fig. 1a). A perceptible difference is revealed with respect to number of cracks in steel (Fig 2) cast with a different overheating with respect to liquidus temperature. The specific overall length of cracks in templates with ΔT = 30 and 13°C in a slab transverse section is 2.5 and 1.6 mm/dm2, respectively. In a longitudinal cross 429
Fig. 2. Crack in specimen surface of slab axial area.
TABLE 3. Average Mechanical Property Value over Slab Width and Height Over slab width Overheating ΔT, °C
1/4 of slab width
Over slab thickness center
small radius
axial area
large radius
σu, N/mm2
δ, %
Ψ, %
σu, N/mm2
δ, %
Ψ, %
σu, N/mm2
δ, %
Ψ, %
σu, N/mm2
δ, %
Ψ, %
σu, N/mm2
δ, %
Ψ, %
11
410
34
56
430
31
51
430
32.5
55
407
31.9
50.5
423.5
32.7
55.1
13
419
31
52
430
35
63
429.5
33.6
60
425.5
33
53.5
419
32
57.7
22
419
34
53
367
26
36
415.5
37.2
59
336.5
19
30
428.5
33.4
45.2
30
438
27
50
427
29
46
435.5
31.8
61.3
430.5
22.5
33.9
432
29.6
43
70 60 50 40 30 20
δrel, %
Ψ, %
60 50 2
2
40 30
1
10 5
10
15
20
a
25
1
20 30
35
5
10
15
20
b
25
30 35 ΔT, °C
Fig. 3. Dependence of average slab reduction of area Ψ (a) and relative elongation δrel (b) over the height on overheating temperature ΔT at center and 1/4 width from slab center.
section in templates with ΔT = 30°C the specific overall crack length is 22.7/dm2, and a reduction in ΔT to 13°C assures the absence of cracks. Transverse specimens were cut in two directions for tensile tests: in the center and at a distance of 1/4 over slab width. In each cross section, specimens were cut from the axial area of a slab, with the CCB surface over the small and large radii. Tensile tests were performed in a universal Instron machine at room temperature according to GOST 1497-84 on type III cylindrical specimens. Average mechanical property values are given in Table 3 over the width and thickness of a slab with tensile tests. It is seen from results provided that the overheating temperature has a significant effect on the uniformity of duc430
TABLE 4. Effect of Overheating on the Impact Strength Value (KCU+20, J/cm2) and Uniformity of Its Distribution Over Slab Cross Section Overheating ΔT, °C Selected specimen location 11
13
22
30
1/4 from slab edge, small radius
124
61
151
119
1/4 from slab edge, axial area
69
41
62
29
1/4 from slab edge, large radius
136
139
130
86
Center, small radius
131
158
177
145
Center, axial area
51
52
77
67
Center, large radius
113
38
142
46
160
KCU+20, J/cm2
140 120
1
100 80 60
2
40 20 0
5
10
15
20
25
30 35 ΔT, °C
Fig. 4. Dependence of impact strength KCU+20 on overheating temperature ΔT at 1/4 width of slab from edge from the side of the large radius (1) and axial area (2).
tility index distribution, i.e., relative elongation and relative reduction of area over a slab cross section, and there is a significantly lower degree of the effect on strength indices. Values are provided in Fig. 3a for relative reduction of area over slab thickness. With a reduction in overheating, not only is there an increase in the average value of relative reduction of area, particularly in the axial area, but also the difference is reduced to a minimum for specimens cut from the direction of small and large radii, and from an axial area. A similar feature is observed for relative elongation (Fig. 3b). Analysis of the experimental data obtained made it possible to reveal the following tendencies. An increase in steel overheating in a tundish from 11 to 30°C leads to a steady fall in ductility indices: relative reduction of area on average by 13% (Fig. 3a), and relative elongation by 15% (see Fig. 3b), and the reduction in ductility with an increase in overheating temperature is more active in the central area of a slab compared with its edges. Impact bending testing was carried out in accordance with GOST 9454–78 in a pendulum impact machine MK-30 at room temperature. Transverse and longitudinal specimens type I (KCU) were cut for testing in two cross sections: at the center and at a distance of one fourth of the slab width. In each of the sections, specimens were cut from the axial area of a slab, and also from the surface over the small and large radii. This makes it possible to evaluate the failure resistance of cast metal for different areas of a slab in two directions, i.e., along ad across dendrites. Average values of specimen impact strength are presented in Table 4, cut from templates with a different steel overheating with respect to liquidus temperature during pouring. 431
180
KCU, J/cm2 b – Small radius B – Large radius
160 140 120
KCU scatter, % 100
100
80
100 80
60
60
40
40 20 0
24
21
20 30
22
13
11
0
2
30
a
22
13 11 Overheating, °C
b
Fig. 5. Effect of degree of overheating on impact strength (a) and relative scatter of slab surface impact strength values over small and large radii (b).
TABLE 5. Effect of Degree of Overheating on Scatter of Mechanical Properties over Slab Cross Section Overheating ΔT
Δσu, %
ΔΨ, %
Δδ, %
11
10
39.6
21.2
13
9.2
29.8
30.3
30
9.5
76.1
53.6
A dependence is presented in Fig. 4 for impact strength KCU+20 on overheating temperature ΔT at a quarter of the width from the edge in the direction of the larger radius and in the axial area. It is seen that there is a tendency of a reduction in the level of impact strength over the whole slab cross section with an increase in metal overheating temperature during pouring. In addition, it has been established that as a reduction in the degree of overheating there is levelling out of impact strength over the thickness of a slab (Fig. 5). Whereas with ΔT = 30°C average values of impact strength for the surface of larger and smaller radii differed by 66 J/cm2, with ΔT = 10°C this differences was 3 J/cm2. Marked levelling out of mechanical properties is also observed over a slab cross section with a reduction in steel overheating above the liquidus temperature during pouring (Table 5). Thus, increased metal overheating with respect to liquidus temperature during pouring facilitates an increase in billet crack sensitivity, development of a columnar ingot structure, and also defects of the macrostructure, such as axial liquation and central porosity. In addition, an extremely high metal pouring temperature may lead to breakdown of a solidifying CCB skin with respect to cracks, and also an increase in the probability of mold wall burn-through. With an increase in metal temperature in a tundish above the optimum value, it is necessary to reduce the level CCB extraction rate, and this reduces CBCM productivity. Use of steel plasma heating in a tundish with continuous casting makes it possible to maintain the temperature at 10 ± 5°C above the liquidus temperature. The optimum temperature regime during pouring provides the best slab structure due to a reduction in central chemical and structural inhomogeneity of a billet, more uniform non-metallic inclusion (NMI) distribution, a reduction in number of cracks, levelling out of mechanical properties over an ingot cross section, and also a marked increase in steel ductility. In addition, removal of NMI is activated as a result of direct heating of slag by plasma in a ladle, and the possibility develops of carrying out alloying in a ladle in an inert atmosphere. Use of heating guarantees exclu432
sion of “cold” melts, and this makes it possible to reduce metal loss as a result of rejection or scrapping, and to increase finished metal output. Since this system excludes a requirement for maintaining a specific level of metal heating in a melting unit, it makes it possible to save of the order of 4 kW·h per ton of metal. Conclusions 1. Industrial testing of equipment has been carried out for the first time in Russia and technology has been developed for plasma heating of steel in a CBCM tundish. A study has been performed of the macrostructure and distribution of sulfide inclusions and mechanical properties for different steel slab cross sections, cast using plasma heating with different metal heating temperature above the liquidus temperature. 2. A favorable effect has been revealed for a reduction in steel heating temperature during pouring on its macrostructure, specific overall crack length, and sulfide inclusion distribution uniformity. It has been established that a reduction in the degree of steel overheating during pouring increases the average level of ductility and impact strength indices, and also leads to a reduction in scatter of these values both over slab width and thickness.
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