Studying the Effect of Tramp Elements in Scrap on

Studying the Effect of Tramp Elements in Scrap on Industrial Recycled Steel Processing and Quality Ahmed Ramadan1, A. Y. Shash2, I. S. El-Mahallawi2, S. Dieter3 and Taha Mattar4 1)

Beshay Steel, Industrial Zone, Sadaat City, Egypt Faculty of Engineering Cairo University, Giza, Egypt 3) Institute of Ferrous Metallurgy, RWTH-Aachen University, Aachen, Germany4) Centeral Metallurgical Research and Development Institute, Helwan, Egypt 2)

Abstract Increasing the steel demand world-wide made steel scrap a critical resource in ferrous industries. In Electric steelmaking industry, the used metal scrap reach 100% of the metallic charge, which results in rising the amount of residual elements such as Cu, Pb and Sn in the produced steel. These elements don’t intend to be added and don’t remove by sequence treatments. It is found also that the behaviour of copper and tin in steel during the reheating step in hot working is the main reason for the loss of hot ductility and the appearance of hot shortness during hot rolling processes, as the liquid copper-rich phase settles on the grain boundaries as result of selective oxidation of iron and formation of Cu rich phase. This phase migrates to word grain boundaries and leading to surface cracking. These cracks are not eliminated during subsequent processing and cause surface defects in the final product. In some instances, complete cracking of the slab or billet occurs as a result of hot shortness. Hot shortness and the resulting increased tendency for cracking are commonly associated with the presence of copperenriched phases at the scale–metal interface. Accordingly, the effect of tramp elements in the scrap used for recycled steel production has been intensively studied in this work. From the obtained results and measurments, it was found that the solubility of tramp elements will decrease as temperature decreases, where the tramp elements (Cu, Pb, Sn) will diffuse toward the grain boundaries and form intermetallic compounds or reach phases which have low melting points causing reduction in ductility and failure during the bending test. The results showed also that the samples with high percentage of tramp elements (Cu, Pb, Sn) in the billet, that have been rolled and subjected to tempcore process, do not show a decrease in elongation or failure in bending test. This may be attributed to the fact that tempcore process will hinder the occurrence of the harmful effect of the tramp elements on the elongation or failure at bending test where the quenching will form super saturated solid solution of tramp inside the ferrite as well as the self tempering will form a uniform distribution of small particles of tramp elements through the matrix. Keywords: Recycled Steel, Hot Shortness, Crack Susceptibility, Tempcore Process, Tramp Elements, Residual Melt.

1. Introduction: The issue of sustainability should be addressed from a view of life-cycle of materials, in which the life-cycle of manufactured products and their interaction with and impact on the environment are considered simultaneously. To save the world resources "resource consumption" which would mean reducing the use of raw materials since it leads to: increased costs of raw materials, increased energy needs, stockpiles of materials waste, and increased gas (CO2) emissions, the use of recycled steel is expanding all over the world, were the cities of today are the mines of tomorrow. The EAF share of steel production has been increasing at 66% per annum rate since the 1950s according to Yellishetty et. al. [1]. It has been reported by Janke et. al. [2] that on the average 70% of the amount of steel end products is returned to the materials cycle after 20 years of its manufacture, whilst the remaining 30% is lost by rusting of steel. During the last 60 years the world steel production has increased dramatically, according to Yellishetty et. al. [1] the world steel production increased from 187 MT to 1299 MT between 1950 and 2006. It has been shown that since 1950, scrap consumption has been growing at 12% per annum in steel industry, with an estimated doubling in consumption between 2009 and 2019, based on estimated EAF steel production [1]. The major environmental benefits of using recycled steel or scrap is shown in the fact that the production of one ton of steel through the EAF routes consumes only 9- 12.5 Gj/tcs, whereas the BOF steel consumes 28-31 Gj/tcs, this significant saving in energy means in addition to savings in energy costs, minimizing CO2 emissions [1]. Manufactured Steels become at sometime scrap or waste at the end of their life cycle. The amount of waste, or scrap, from fabrication and manufacture varies, depending on individual processes. The scrap resulting from fabrication and manufacturing processes is generally recycled and is called new scrap. Steel organization reports that roughly 100% of steel reinforcement is made from recycled scrap and 25% of steel sections are made from recycled scrap. Scrap steel is almost totally recycled and allowed repeated recycling, Coventry (1999).Scrap recycling technology is called secondary metallurgy in steelmaking. In secondary metallurgy obsolete scrap with home scrap are classified according to quality with respect to chemical analysis, specific weight, non-metal ingredients, etc.. The scrap includes, obsolete scrap, home scrap, shredded scrap, steel turnings, quoted from Janke et. al [2]. Tramp elements, mainly Cu, Sn and Ni content are given as maximum values. These alloying elements are added intentionally during the production of high tensile steel for some rebars, copper bearing steel 0.35% Cu austenitic stainless steel up to 1% Cu Corten steel (0.25-0.55) Cu % weather steel 0.25% Cu known for moderate corrosion resistance are utilized in bridges and steel construction. This type of steel contains Cu, Ni and Cr known in scrap as tramp elements. These residual elements (Cu, Ni, As, Pb, Sn, Sb, Mo, Cr, etc.) are defined as elements that are not added to steel and which cannot be removed by current metallurgical processes. The source of tramp elements is usually one of three: a. parts attached with steel products and made from different metals like discarded electric motors, b. coated steel parts like galvanized steel and c. alloying additions in certain steel grades like Ni, Cr, and Mo. Hiroyuki Katayama et. al. [3] has spotted the effect of accumulation of Ni, Cr, Sn, Cu and Zn resulting from extensive use of recycled steel or scrap. Tramp elements affect steel properties in two different ways: influencing steel mechanical properties or influencing processing quality of steel especially in the continuous caster and during deformation processes.

2. Experimental Procedure 2.1 Bending Tests:

Samples of size 16 mm were taken from reinforcing steel bars of different heats for bending tests, produced by applied conventional rolling product without tempcore process. Also Samples of size 32 mm, were taken from reinforcing steel bars of different heats for bending tests produced by to Tempcore process. 2.2 Tensile Tests:

Tensile tests were then performed using a universal testing machine model 5590-HVL tensile testing machine. The displacement was measured with a strain gauge. 2.3 Microstructure Examination:

To characterize the microstructure specimens were ground and polished and finally etched with a solution of H2O2 16 mm (35%) / HNO3 (65%) / HCl (32%). 3. Results and Discussion Samples of 16 mm, 32 mm size were broken during bending tests having the following chemical composition reported in Table 1. Size

No

C

Si

S

P

Mn

Ni

Cr

Cu

1

0.43

0.26

0.05

0.03

1.33

0.10

0.04

0.42

2

0.4

0.29

0.03

0.03

1.35

0.12

0.14

0.47

3

0.21

0.16

0.04

0.032

0.93

0.142

0.074

0.652

4

0.20

0.162

0.044

0.032

0.92

0.142

0.074

0.56

16 mm

32 mm

Table 1 Broken samples during bending tests

Tensile tests were carried out for small specimens taken from the broken bending samples. The displacement was measured with a strain gauge 5 D and the following results were recorded in Table 2. No. 1 2 3 4

Size

T.S (MPa) 782

Elongation % 11.6

T.S/Y.S

16

Y.S (MPa) 497

16

488

782

10

1.61

32

579

674

15.8

1.16

32

560

653

16

1.16

1.57

Table 2 Tensile test results.

The macroscopic picture was taken at magnification 2X for fracture surface for size 32 mm samples as illustrated in Figure 1. Two Microscopic pictures were taken at Magnifications 200X from the surface, and core of cross section samples, as shown in Figures 2 and 3.a, b and c.

Fig. 1 Macroscopic fracture surface for 32 mm size sample.

a.

b.

c.

Fig. 2 a. Surface 80X, b. Middle 80X and c. Core 80X.

a.

b.

c.

Fig. 3 a. Surface 200X, b. Middle 200X and c. Core 200X.

Figure 4 shows the top of billet split during rolling caused by inter granular fracture.

Fig. 4 Top of billet spilt

4. Conclusion 1. Equiaxed grains appear with normal grain boundaries, which means, that the steel was cooled slowly not quenched as there is no appear for the martensite. Both samples contain high percentage of tramp element of Cu (0.41-0.42), which has a maximum solubility at ferrite 0.35%. With higher percentage, copper will precipitate at grain boundaries of ferrite forming £ phase which is a rich phase of copper and causes reduction in ductility and failure during the bending test. 2. Both samples contain the highest Mn% (1.33 - 1.35%). with manganese to sulfer ratio (26-45). This means that the Manganese combines with the total sulfer and form (MnS). The rich copper phase will precipitate at the grain boundaries around MnS increasing the yield strength and the tensile strength and grain embrittlement while on the other hand the elongation, toughness, and bendability will be reduced where the presence of copper precipitates will reduce the adhesion of grain and help to cause fracture. 3. The brittle copper rich phase formed as result of selective oxidation of iron during reheating process. The rich phase will migrate toward grain boundaries and precipitates around sulfied inclusions. This precipitates will weak the cohesions of grain boundaries and causes copper hot shortness. 4. The expected elongation of the same chemical composition free from copper and other tramp is 17% the effect of existing tramp copper reduces the elongation to (10- 11.6)%, as a result of precipitation of copper at grain boundaries which causes grain boundaries embrittlement and grain boudaries decohesion causing reduction in elongation and fracture at final. Also, the dangerous of tramp elements appears more clearly in bending test than reduction in elongation. 5. Slow cooling rate gives big sizes of precipitates and less volume fraction and this is considered more dangerous case where the effect on elongation for size 16 mm although it has low Cu % (0.42%) on the other hand the high cooling rate produced fine precipitates or produced super saturated ferrite of copper as result of tempcore process so fine precipitates formed through the grain and don't have the chance to migrate or segregate around the grain boundaries and this clearly appears at size 32 mm which contains high copper (0.63% Cu). 5. Further Suggestions and Solutions To reduce the effect of tramp elements which cause a series of problems during production and deformation processes it is suggested to take the following actions: a. Make dilution of scrap by addition DRI to scrap during melting at EAF b. Add Ni or Si or both by percentage 1:1 or 1:0.5 of percentage of copper content to minimize the effect of copper by forming solid solution, which has high melting point higher than the temperature of reheating as well as the addition of Ni or Si increase the solubility of Cu in austenite These two suggested proposals will hopefully reduce the effect of tramp elements.

Acknowledgement The authors would like to thank Dr. Yousif Beshay the Manager of Beshay Steel for his generosity for offering the materials and supporting the experimental tests. Also, they would like to thank Beshay Steel for their support in compiling this work.

References [1] M. Yellishetty, G. M. Mudd, P.G. Ranjith, A. Tharumarajah, Environmental Science & Policy, Volume 14, Issue 6, October 2011, p. 650-663. [2] Janke, L., Czaderski, C., Motavalli, M. & Ruth, J. (2005), Mater. Struct., 38, p. 578-592. [3] H. Katayama and Y. Mizukami: Materia Japan 12, 1996, p. 1283-1289. [4] C. Marique., Liège, Belgium; Tramp elements and steel properties: a progress state of the European project on scrap recycling; ECSC contract N_ 7210-CB/205. [5] B. Mintz, S. Yue, and J. J. Jonas, International Materials Reviews, Vol. 36, 1991, No.

5. [6] Flesch, R.; Bleck, W.: steel res. 69, 1998, No. 7, p. 292-299. [7] O. Gross, Radex Rundschau, Heft 4, 1989, p. 154-166. [8] Thome, R.; Dahl, W.: steel res. 66, 1995, No. 2, p. 63-71. [9] B. Mintz and J. M. Arrowsmith: Met. Technol., 1979, No. 6, p. 24. [10] S. A. Attia, (1999) P.hD Thesis, Alexandria National Company of steel at dekhaila (ANSDK).