2017 Volume 57 Issue 2 Pages 388-393
Steel is one of the most recyclable materials, and thus it is recycled repeatedly. When steel is recovered from end-of-life products, some materials containing tramp elements besides carbon steel are mixed into the scrap. Therefore, we must prevent contamination by tramp elements during repeated recycling. Meanwhile, there is little knowledge on tramp element contents in carbon steel. This study aims to quantify tramp element contents in carbon steel produced in Japan. The tramp element contents of more than 500 samples of carbon steel were analyzed. The specimens were classified by steel product forms because the tolerance for tramp element contents differs by form. The average contents and confidence intervals for populations were calculated and then multiplied by the annual carbon steel production in Japan. The amounts of Ni and Mo impurities associated with carbon steel produced annually accounted for more than 15% of the annual consumption of these metals in Japan. In addition, the consumptions for plating were considered. The results were confirmed by a comparison with the contents in carbon steel produced from steel scrap generated in Japan after non-ferrous metals were separated from the scrap, which was performed in Vietnam. It is hypothesized that the tramp element contents in the Vietnamese steel are equal to the weighted average contents of carbon steels consumed in Japan. The comparison for Cu and Sn showed good agreement. We conclude that tramp element contents in carbon steel obtained in this study are representative values, even though the number of samples was limited.
Steel is one of the most recyclable materials, and thus it is recycled repeatedly. In the past three decades, end-of-life recycling rates have fluctuated over than 80% in Japan.1) In addition, even if disused steel was not be recovered at its end-of-life and remained within the technosphere for a while, it can be recovered later when its price increases.1) When steel scrap is recovered from end-of-life products, some materials other than carbon steel are mixed into the scrap. In steelmaking processes, some impurity elements, known as tramp elements, cannot be extracted from molten steel. Tramp elements in other materials that are present in the scrap can remain in recycled steel products. Through repeated recycling, contamination by tramp elements is possible.2,3,4) The average length of time from when steel is consumed as a finished product to when steel is recovered as steel scrap is estimated to be around 20 years.5,6) However, at least in reinforced bar steel, the contents of tramp elements have been observed for over 25 years and have been found to not change during this period.7)
Sufficient knowledge of tramp element contents in carbon steel has not been gathered yet. A very limited number of studies have measured tramp element contents in steel other than reinforced steel bars.8,9) We summarize previous results on tramp element contents in carbon steel in Table 1. Although steel bars, in particular reinforced steel bars, exhibit the most tolerance to tramp element contents, we have not observed a time-series change in the tramp element contents of any other steel forms. Contamination with tramp elements by repeated recycling should be affected by the tramp element contents in every product form of steel. Therefore, to understand the current situation on tramp element contents and prevent contamination with tramp elements by repeated recycling, we must recognize tramp element contents in all forms of steel. This study aims to quantify the total amounts of tramp element associated with carbon steel produced in Japan. Here, we use the term ‘carbon steel’, but focus on so-called ordinary steel used in Japanese statistics, because carbon steel and alloy steel are not explicitly distinguished in Japanese statistics and ordinary steel composes a large part of carbon steel.
Year | Country | Cr | Cu | Ni | Sn | Description on samples | Ref. |
---|---|---|---|---|---|---|---|
1954 | U.S. | 0.52*1 | 0.17 | 0.107 | 0.035 | Cold scrap shop only*3 | 12) |
1962 | U.S. | 0.71 | 0.151 | 0.085 | 0.017 | Cold scrap shop only*3 | 12) |
1977 | U.S. | 0.115 | 0.162 | 0.086 | 0.011 | Cold scrap shop only*3 | 12) |
1987 | Japan | 0.14 | 0.28 | 0.08 | 0.03 | EAF steel bar*4 | 7) |
1989 | Japan | 0.159 | 0.327 | – | 0.024 | EAF steel bar*4 | 7) |
1991 | Japan | 0.169 | 0.325 | – | 0.024 | EAF steel bar*4 | 7) |
1996*2 | Japan | 0.158 | 0.297 | – | 0.031 | EAF steel scrap (n=50) | 8) |
1997 | Japan | 0.179 | 0.289 | – | 0.018 | EAF steel bar*4 | 7) |
2001 | Japan | 0.174 | 0.288 | – | 0.019 | EAF steel bar*4 | 7) |
2004 | Japan | 0.192 | 0.318 | – | 0.021 | EAF steel bar*4 | 7) |
2007 | Japan | 0.185 | 0.281 | – | 0.020 | EAF steel bar*4 | 7) |
2011 | Japan | 0.175 | 0.278 | 0.072 | 0.017 | EAF steel bar*5 (n=99) | 9) |
2011 | China | 0.093 | 0.034 | 0.048 | 0.012 | EAF steel bar (n=16) | 9) |
2013 | Japan | 0.201 | 0.284 | – | 0.019 | EAF steel bar*4 | 7) |
2011–2013 | Switzerland | – | 0.37 | – | 0.019 | EAF steel | 13) |
Substance flows of tramp elements associated with carbon steel were classified into four types. The first and second flows are the intentional use of those elements as coatings on surfaces, f1, and as alloying additives, f2, respectively. For instance, the former includes the consumption of tin that is intentionally coated on steel surfaces as tin plate. The third flow is tramp elements alloyed in carbon steel which composes a large part of carbon steel scrap, f3. The fourth flow is tramp elements contained in materials beside carbon steel, such as alloy steel and non-ferrous materials, which composes some part of steel scrap, f4.9,14,15) The mass balance between raw materials and products in steel mills is shown in Fig. 1. The four flows defined above are indicated as f1, f2, f3, and f4 in this figure. We distinguished between the flows of tramp elements associated with steel products, fprod, by their origins. Tramp elements associated with steel products are explicitly expressed as unintentionally mixed ones, f3+f4, and intentionally added ones, f1 and f2. In raw materials, metals composed of tramp elements for plating, f1, and alloying, f2, pig iron, fpig, and steel scrap were considered. In addition, the steel scrap was divided into carbon steel, fscr, and unintentionally mixed materials other than carbon steel, fmix. If yield loss during production is ignored, the mass balance between the raw materials and the products holds. Here, pig iron, which seems not to contain any tramp elements, is neglected in the mass balances of tramp elements.
Mass balances of carbon steel and tramp elements associated with carbon steel in steel mills.
The flows of f3 and f4 could be quantified by observing raw materials, produced steel, or both. Steel scrap, which is a major component of raw materials, is a mixture of several different materials in a solid state. Much effort is necessary to quantify the average composition of the steel scrap by analyzing the element contents of each material after liberating them from the mixture. Steel products are already composed of the averaged tramp elements of raw materials, including several different materials. Therefore, we observed the tramp element contents in steel products.
We analyzed five elements: Cu, Sn, Cr, Ni, and Mo. Chromium can be distributed into slag phase because the standard Gibbs energy of formation for chromium oxide is smaller than that for iron oxide in the range of smelting temperatures. Therefore, chromium is not included in tramp elements in theory. However, in practice, some chromium remains in molten steel. As shown in Table 1, in many cases, steel bars produced in electric arc furnaces (EAFs) contain more than 0.1% chromium on average. Moreover, high contents of chromium cause difficulties during the deep drawing process.10,11) In addition, we may consider the Cr content in steelmaking slag for the mass balance in steel mills. However, it is difficult to obtain representative chromium contents in steelmaking slag generated by carbon steel production. Therefore, we did not take substance flow to slag into account. In this study, chromium was regarded in the same way as other tramp elements.
The amounts of tramp elements in steel products were calculated by the average contents of the elements multiplied by the amounts of production. The average contents and production were distinguished by steel product form because tolerances for tramp elements differ according to steel form.16) The amounts of production relied on statistics.17) The analytical methods used in this study, which are explained later in detail, will include f2 and not f1 in the results of the elemental analysis. Therefore, the consumption for coatings was estimated by the production of coated steel and the amounts of coating metals derived from specifications described in catalogues from steel producers. Alloying additives exist as alloys in steel, which cannot be distinguished from unintentionally mixed ones, as represented by f3 and f4, respectively. The amounts of alloying additives were assumed to be zero, because almost no steel forms in carbon steel require alloying additives.
2.3. Sampling Steels in JapanSteel products were randomly sampled and analyzed to determine the average tramp element contents of carbon steel produced in Japan. The tramp element contents preferred to be obtained in this study are the average of every form of carbon steel produced in Japan. If specimens were supplied by a steel mill, the obtained data could be biased because the raw materials consumed and the conditions of operation at each mill may have local conditions, which lead to unique tramp element contents in products. However, it is not easy to collect specimens from a large number of steel mills. Therefore, for the sake of easy implementation, we collected specimens from scrap yards where steel produced by different mills is randomly gathered. Ideally, industrial scrap specimens were preferred in this study because such specimens were produced recently. However, in reality, although industrial scrap and obsolete scrap were mixed in the specimens, they were not completely distinguishable. As a result, it was not feasible to specify the times at which the obtained steel specimens were produced. In Japan, no significant changes in elemental content, except for that of Cr, were found over time from 1987 to 2013, at least for reinforced steel bars.7,10) Therefore, we treated the specimens without identifying the production date.
Because we sampled from scrap yards, it was not clear what steelmaking processes were employed and what types of raw materials were used for the obtained steel specimens. Contents of tramp elements in steel products may differ according to the fraction of obsolete scrap in raw materials. In general, basic oxygen furnace (BOF) steel, for which pig iron is the dominant raw material, and EAF steel, for which steel scrap is the dominant raw material, should have quite different contents.9) Statistics on production and consumption cannot be distinguished by steelmaking processes either. Therefore, in this study, steelmaking processes were not distinguished.
The numbers of samples for each steel product form were determined to reduce uncertainty in the total amounts of tramp elements associated with steel production. Larger numbers of samples were achieved for steel forms that have larger production or higher contents of tramp elements (Table 2). With regard to steel plate and sheet, it was not easy to distinguish forms of sampled steel according to the classification in statistics. The number of samples for steel plate and sheet was counted in total. Then, forms of steel plate and sheet could not be distinguished in the analysis. A sufficient number of specimens for the form of sheet pile steel could not be sampled at scrap yards. Some of reasons were that sheet pile is rarely to end up its life due to its (semi-)eternal usage such as shore protection, and that sheet pile is rarely to be recovered even after its end of life. Sheet pile was eliminated from forms of steel for analysis, by which sensitivity caused was evaluated in discussions. Sampling was conducted four times; the first and the second times were conducted in August 201118) and August 2015 in the Chubu region, respectively, and the third and the fourth times were in July 2016 and August 2016 in the Kanto region, respectively. The chemical composition was analyzed by a handheld x-ray fluorescence analyzer (Olympus, DELTA Premium) after gliding the surface of specimens in scrap yards. Optical emission spectrochemical analysis (Thermo Fisher Scientific, ARL™ 4460 Optical Emission Spectrometer) was conducted only for bars, wires, and rods sampled in the second and third samplings.
Steel product forms | Production [1000 t] | Number of samples | Tramp element contents | |
---|---|---|---|---|
Rail | 786 | 7 | Middle | |
Sheet pile | 803 | 2 | Middle | |
Section | 6270 | 148 | High | |
Bar | 8403 | 216 | High | |
Wire rod | 2261 | 14 | Low | |
Plate and sheet | Heavy and medium plate | 10485 | 65 | Low |
Hot rolled sheet | 17639 | Low | ||
Cold rolled sheet | 6141 | Low | ||
Electric sheet | 1689 | Low | ||
Tin plate | 901 | Low | ||
Galvanized sheet | 533 | Low | ||
Other surface treated steel | 12324 | Low | ||
Pipe | 3617 | 57 | Low |
The contents of tramp elements in steel classified by product forms were analyzed. Some specimens obtained at scrap yards were revealed to be alloy steels, such as structural low-alloy steels, including SMn420, SCr415, SCM415, etc. These alloy steels were not counted in the number of specimens. The numbers shown in Table 2 are the numbers of carbon steel specimens that were valid for evaluation. The results of elemental analysis corresponding to steel product form are shown in Table 3. This study employed a method of random sampling. Therefore, the results have confidence intervals for the average of the population, which is all carbon steel products produced in Japan. In Table 3, 90% confidence intervals are also shown. By using the average shown in Table 3 and the annual steel production shown in Table 2, the amounts of tramp elements unintentionally mixed into carbon steel produced in Japan, f3 and f4, were estimated. Here, confidential intervals for sheet pile were not calculated because of a limited number of samples. An uncertainty caused by sheet pile was evaluated on the assumption of having the same confidential intervals to those of bars. By taking it into account, the range of the confident intervals may increase at most 0.3% for each element.
Steel product forms | Sample mean [%] | 90% confident interval for the population mean [±%] | ||||||||
---|---|---|---|---|---|---|---|---|---|---|
Cu | Sn | Cr | Ni | Mo | Cu | Sn | Cr | Ni | Mo | |
Rail | 0.165 | 0.000 | 0.215 | 0.063 | 0.021 | 0.041 | 0.000 | 0.066 | 0.020 | 0.007 |
Sheet pile | 0.248 | 0.000 | 0.023 | 0.000 | 0.000 | – | – | – | – | – |
Section | 0.176 | 0.003 | 0.109 | 0.055 | 0.016 | 0.019 | 0.001 | 0.013 | 0.006 | 0.002 |
Bar | 0.269 | 0.015 | 0.175 | 0.078 | 0.014 | 0.013 | 0.002 | 0.010 | 0.005 | 0.001 |
Wire rod | 0.016 | 0.000 | 0.011 | 0.000 | 0.002 | 0.013 | 0.000 | 0.010 | 0.000 | 0.002 |
Plate and sheet | 0.039 | 0.001 | 0.048 | 0.014 | 0.003 | 0.017 | 0.001 | 0.020 | 0.006 | 0.001 |
Pipe | 0.013 | 0.000 | 0.010 | 0.010 | 0.001 | 0.007 | 0.000 | 0.002 | 0.017 | 0.000 |
The elemental analysis of the specimens could not take the weights for plating, f1, into account. In addition to the results of the elemental analysis, the weights were estimated as follows. Coatings with the metals analyzed in this study included tin plate, nickel coatings, and chromate conversion coatings. Sufficient information to estimate weight of chromium for chromate conversion coatings was not available. Therefore, tin plate and nickel coatings were taken into consideration. In tin plate, the tin for the coating was assumed to be the average of four coating weights specified in the producers’ catalogues:19,20) 2.8 g/m2, 5.6 g/m2, 8.4 g/m2, and 11.2 g/m2. The thickness of the steel plate was 0.18–0.50 mm for single-rolled plating and 0.15–0.36 mm for double-rolled plating.19,20) The average thickness of steel plating was assumed to be the average of the upper and lower bounds, that is, 0.3 mm. With these assumptions, the weight of Sn on 1 ton of steel as tin plate was estimated to be 2.95 kg. Steel consumption for tin plate is available in statistics. Steel consumption for nickel coatings is not distinguished in statistics and is included in the category of other coating plate. Therefore, the steel consumption as other coating plate for two specific end use sectors, electric machinery and automobiles, was regarded as the steel consumption for nickel-coated steel because the major usages of nickel coatings are batteries and fuel tanks in automobiles.21,22) The nickel for coatings was assumed to be the average of four coating weights specified in the producers’ catalogue:21) 8.9 g/m2, 17.8 g/m2, 16.7 g/m2, and 35.6 g/m2. The thickness of the steel plate was 0.15–0.80 mm.21) The average thickness of steel plate was assumed to be the average of the upper and lower bounds. With these assumptions, the weight of Ni in 1 ton of steel with nickel coating was estimated to be 5.26 kg.
When the weights for plating were added, the substance flows of tramp elements associated with carbon steel production in Japan were estimated to be 5.45×104 ton, 4.65×103 t, 4.22×104 t, 1.74×104 t, and 3.80×103 t for Cu, Sn, Cr, Ni, and Mo, respectively (Table 4). The amounts used in Sn and Ni coatings are also indicated in Table 4. Here, the amounts of chromium could be underestimated because two flows were not considered: the chromium that may be distributed to slag and the chromium used in coatings. When these substance flows are compared with the annual metal consumption, the flow of Cu was 4% of electrolytic copper production, which was 1.15 million tons. The flow of Sn was 17% of the total of production and import of electrolytic tin, which was 28 thousand tons, more than a half of which was intentionally added for coatings. The flow of Cr was 9% of the total imported chromium ore and ferro-chromium, which was 458 thousand tons. The flow of Ni was 16% of the total of production of ferro-nickel and metal nickel, which was 111 thousand tons. The flow of Mo was 19% of imported calcined molybdenum ore, which was 19 thousand tons. It was found that a relatively large fraction of nickel consumption was dissipated into carbon steel as a non-functional alloy. This result has not previously been revealed by the substance flow of nickel.23)
Element | Total amounts [1000 t] | Coating weights on carbon steel [1000 t] | 90% confidence interval [1000 t] | Fraction of 90% confidence intervals to the total amounts | Annual consumption24) [1000 t] | Fraction of the total amounts to the annual consumption (Numbers in parentheses are fractions for the amounts used in coatings) |
---|---|---|---|---|---|---|
Cu | 54.6 | – | ±11.3 | ±21% | Electrolytic copper 1516 | 3.6% |
Sn | 4.65 | 2.7 | ±0.758 | ±16% | Electrolytic tin 1.1 Imported electrolytic tin 27.1 | 16.5% (9.4%) |
Cr | 42.2 | –* | ±8.98 | ±21% | Imported Cr ore 16 Imported ferro-chrome 443 | 9.2% |
Ni | 17.4 | 0.8 | ±4.56 | ±26% | Metal nickel 42 Ferro-nickel 69 | 15.7% (0.8%) |
Mo | 3.80 | – | ±0.892 | ±24% | Imported calcined Mo ore 19.6 | 19.4% |
When confidence intervals are taken into consideration, the substance flows of tramp elements are estimated to be 5.45±1.13×104 ton, 4.65±0.76×103 t, 4.22±0.90×104 t, 1.74±0.46×104 t, and 3.80±0.89×103 t for Cu, Sn, Cr, Ni, and Mo, respectively (Table 4). Even in the 90% confidence intervals, the intervals are approximately a quarter of the average. More than a half of confident intervals were derived from the results for steel plate and sheet of which the steel production counts for around 70% to the total steel production. One of reasons could be that several steel product forms that involve steel plate and sheet were not distinguished in our study because of the difficulty in identifying these forms. Another reason could be that the number of specimens for plate and sheet was relatively small because of their low contents of tramp elements. Although the confidence intervals derived from that were relatively large, the number of specimens for that was not insufficient because the fraction of the confidence intervals derived from that is smaller than the share of its production except for that of Cu. Even in the case of Cu, the fraction of the confidence interval is almost the same as the share of its production. For future studies, we can note that larger number of samples for steel plate and sheet and making a distinction among different forms of plate and sheet lead to less uncertain results.
3.3. Representativeness of the Composition Obtained in this StudyAs a larger number of specimens are sampled, the results would become more representative. In general, it is very hard to confirm the representativeness of the results. In contrast, in this study, specimens of steel products were obtained from the electric furnace where scrap generated in Japan was consumed after separating non-ferrous metals from the scrap. The steel products produced from Japanese scrap seem to have the average chemical composition among steel consumed in Japan. It is hypothesized that the tramp element contents in recycled steel after the scrap is pre-treated are equal to the weighted average contents of carbon steels consumed in Japan.
The furnace that consumes scrap generated in Japan after the separation of non-ferrous metals was found near Hanoi city, Vietnam. One hundred and twenty-one samples produced at mutually different heat in the furnace during September 2015 were sampled. The raw materials for the steel products were dominated by Japanese obsolete steel scrap. They rarely used domestically generated scrap or pig iron. In reality, it was possible that the consumed scrap was imported not only from Japan but also from the U.S. and Canada. According to the national statistics on steel scrap imported into Vietnam, the dominant exporter of steel scrap was Japan. Around 60% of imported steel scrap was come from Japan in the year 2015, followed by Hong Kong 19% and the U.S. 11%.25) Therefore, the imported scrap was assumed to be scrap generated in Japan. In the pre-treatment of the imported steel, non-ferrous materials, such as copper wires, connectors, aluminum parts, and print circular boards, were separated from the steel scrap by human hands. However, alloy steel and stainless steel were not separated because they do not have any market where alloy steel and stainless steel could be sold at higher price than carbon steel scrap. Given the pre-treatment, the Japanese average for Cu and Sn contents are comparable with those contents in the steel produced by the Vietnamese mill. On the other hand, the Japanese average contents of Cr, Ni, and Mo, most of which are contained in alloy steel and stainless steel, are supposed to be lower than those contents in the steel produced by the Vietnamese mill. The Vietnamese specimens were analyzed by optical emission spectrochemical analysis and compared with the Japanese average contents of tramp elements.
The Japanese average contents of tramp elements were estimated not by domestic consumption of steel products but by steel entering use, in which the trade of finished products, which is sometimes called indirect trade, was taken into account. The ratio of the indirect trade by end use sector was obtained for the year 2010, which was subdivided into steel product forms using steel consumption classified by the end use sector and the steel form. The ratios for steel forms for the year 2010, as shown in Table 5, were applied to steel consumption in the year 2013 to obtain the steel entering use classified by steel forms.
Steel product forms | Domestic consumption [1000 t] | Indirect net export ratio | Steel entering use [1000 t] | |
---|---|---|---|---|
Rail | 186 | 0.79 | 40 | |
Sheet pile | 674 | 0.65 | 237 | |
Section | 5488 | 0.10 | 4935 | |
Bar | 8249 | 0.07 | 7640 | |
Wire rod | 1665 | 0.94 | 103 | |
Plate and sheet | Heavy and medium plate | 7914 | 0.54 | 3633 |
Hot rolled sheet | 6147 | 0.46 | 3270 | |
Cold rolled sheet | 3066 | 0.52 | 1479 | |
Electric sheet | 544 | 0.84 | 87 | |
Tin plate | 331 | 0.08 | 304 | |
Galvanized sheet | 404 | 0.45 | 220 | |
Other surface treated steel | 8531 | 0.00 | 8501 | |
Pipe | 2908 | 0.19 | 2347 |
The results of the comparison between the Japanese average contents of tramp elements and those of Vietnamese steel products are shown in Fig. 2. The comparison for Cu and Sn showed good agreement. As supposed, the Japanese average contents of Cr, Ni, and Mo are lower than those for Vietnamese steel products. Given the consistency with Vietnamese steel products, we conclude that the tramp element contents in Japanese steel classified by steel forms obtained in this study are representative values, even though the number of samples was limited. The confidence intervals shown in Fig. 2 indicate a smaller range than the results shown in Table 4 because the steel entering use includes a smaller share of steel plate and sheet than that in steel production.
Comparison of tramp element contents between the carbon steel entering use in Japan and the carbon steel produced from pre-treated Japanese scrap. The bottom and top of the box show the range of a 90 percent confidence on the mean which is indicated as the band inside the box.
The average contents of tramp elements classified by steel forms for carbon steel produced in Japan were obtained by randomly sampling more than five hundred specimens of carbon steel in Japan, for which elemental analysis was conducted. The results were consistent with those for Vietnamese steel products, which are assumed to have been produced by the average of scrap generated in Japan with average Cu and Sn contents, and thus we conclude that the tramp element contents obtained in this study are representative values, even though the number of samples was limited. On the basis of these results, substance flows of tramp elements associated with steel production in Japan for the year 2013 were 55 kt, 4.7 kt, 42 kt, 17 kt, and 3.8 kt for Cu, Sn, Cr, Ni, and Mo, respectively. Most of these elements are non-functionally recycled, which accounts for more than 10% of the annual metal consumption in the cases of Ni and Mo.
This work was supported by JSPS KAKENHI Grant Number 15H02860 and the Environment Research and Technology Development Fund (1-1402) of the Ministry of the Environment, Japan. The authors are grateful to Kenichi Nakajima, Takamitsu Ichimura, and Kunio Shimada at Re-tem Corporation, Niina Fujituka, Yuto Takahashi and Yuichiro Sato formerly at The University of Tokyo, Kazuyo Matsubae at Tohoku University, and Trading Company Limited in ThepViet Duc (ThépViệtĐức) Vinh Phuc city and Dahoi Village for sampling specimens.