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Laser-induced breakdown spectroscopy applications in the steel industry: Rapid analysis of segregation and decarburization
Spectrochimica Acta Part B 63 (2008) 1122–1129
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s a b
Laser-induced breakdown spectroscopy applications in the steel industry: Rapid analysis of segregation and decarburization☆ Fabienne Boué-Bigne ⁎ Corus Research, Development and Technology, Swinden Technology Centre, Moorgate Road, Rotherham, S60 3AR, South Yorkshire, UK
a r t i c l e
i n f o
Article history: Received 30 November 2007 Accepted 13 August 2008 Available online 27 August 2008 Keywords: Laser-induced breakdown spectroscopy (LIBS) Segregation Decarburization Mapping Steel
a b s t r a c t Rapid chemical analysis is increasingly a prerequisite in the steel making industry, either to check that a steel product complies with customers' specifications, or to investigate the presence of defects that may lead to mechanical property failure of the product. Methods conventionally used for assessment, such as the monitoring of decarburization and segregation, performed by chemical etching of a polished surface followed by optical observation, tend to be relatively fast, simple and applicable to large sample areas; however, the information obtained is limited to the spatial extent of the defect. Other techniques, such as electron probe microscopy and scanning electron microscopy — energy dispersive X-ray, can be used for providing detailed chemical composition at the micro-scale, for a better understanding of the mechanisms involved; however, their use is limited to analyzing comparatively very small sample areas (typically a few mm2). The ability to rapidly generate chemical concentration maps at the micro-scale is one of the many positive attributes of laser-induced breakdown spectroscopy (LIBS) that makes it a useful tool for the steel industry as a laboratory or near-the-line analysis facility. Parameters that influence the detailed mapping of large sample areas were determined and optimized. LIBS scanning measurements were performed on samples displaying segregation and decarburization. A 60 × 60 mm2 area, with a step size of 50 μm, was measured in 35 min on segregation samples, and a 4 × 1 mm2 area with a step size of 20 μm in 2 min on a decarburization sample. The resulting quantified elemental maps correlated very well with data from the methods used conventionally. In the two examples above, the application of LIBS as a micro-analysis technique proved to bring very valuable information that was not accessible previously with other techniques on such large areas in such a short time. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The making of steel involves a series of complex processes where many physical and chemical events take place. Hot metal from the blast furnace is taken to desulphurization. It is then transferred to the basic oxygen steelmaking plant where the carbon content is reduced and alloying elements are added. During secondary steel making, the chemical composition of the steel is taken to within specification by further addition of alloying elements. Once the steel is at the right temperature, its chemical composition adjusted and non-metallic inclusions have been removed as far as possible, it is transferred to the
☆ This paper was presented at the Euro Mediterranean Symposium on Laser Induced Breakdown Spectroscopy (EMSLIBS 2007) held in Paris (France), 11–13 September 2007, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference. ⁎ Tel.: +44 1709 825 225; fax: +44 1709 825 337. E-mail address: [email protected]. 0584-8547/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2008.08.014
tundish for casting, and subsequently rolled. Various finishing processes can be applied to improve or change the surface quality. In order to improve the control of the various stages of the steelmaking process, fast on-line monitoring techniques are required to provide relevant data on a timescale that permits remedy action. Also, more fundamental understanding can be developed by having access to fast analytical techniques that provide thorough data on representative amounts of sample. Laser-induced breakdown spectroscopy (LIBS) is an ideal technique for a variety of on-line and/or fast elemental measurements. The three main advantages that LIBS offers, namely, speed, on-line adaptability and portability, have been exploited by many research groups to address different aspects of process control in the steel industry. Several papers in the literature report the development of on-line applications for the analysis of hot and molten metal [1–6] and for the monitoring of dirt on moving steel strip during rolling [7]. Another type of application exploits the fact that LIBS analysis can be conducted without sample preparation for the rapid bulk analysis of solid steel samples through the surface scale layer [8], or for the direct analysis of solid slag samples [9,10]. Finally, LIBS is also exploited for
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Fig. 1. Influence of the laser focal point in respect with the sample surface on the plasma brightness and on the diameter of the ablated craters.
the spatial resolution that the laser beam offers as a sampling and excitation source. Quantitative mapping with a spatial resolution as small as 3 μm was reported for elemental mapping in different materials [11]. The development of LIBS scanning measurements for the characterization of steel cleanness led to the generation of microscale elemental maps by either using a line-focused laser beam [12], or by using fast repetition rate lasers, pulsing at 1 kHz [13–15]. The analyses of segregation and decarburization in steel samples are the applications considered in this paper. Detailed monitoring of segregation and decarburization would help controlling and improving the quality of the final steel product. In order to characterize these two phenomena with detailed and representative data, high spatial resolution is required as well as speed of measurement in order to make micro-scale mapping possible on large enough areas to provide information showing the complete extent of the defects. Characterization methods already exist for their evaluation, consisting of the optical observation of an etched surface, but they lack precision and consistency. Data in this paper demonstrate the adequacy of LIBS fast scanning measurements. Such LIBS measurements provide thorough and detailed data that will lead to a better monitoring and control of phenomena such as segregation and decarburization.
2. Experimental 2.1. Instrument For this type of analysis, a LIBS instrument offering high spectral resolution is required in order to resolve the lines of interest from the multitude of interfering Fe lines. High spatial resolution is also necessary to generate elemental maps at the micro-scale. The instrument used was manufactured by the Fraunhofer Institute of Laser Technology. It is designed to generate very fast scans with a laser pulsing at 1 kHz, with the associated fast electronics on the detection system. The laser is a custom-made photo-diode pumped Nd:YLF laser, manufactured by EdgeWave (Germany), with a wavelength emission at 1047 nm, a pulse width of 6 ns and a repetition rate of 1 kHz. The laser beam was guided over 38 mm diameter mirrors, expanded to a factor of four and focused through a three lens optical component (f = 60 mm). The plasma light was transmitted directly to the spectrometer via a spherical concave mirror (50 mm diameter, 200 mm radius of curvature, reflective aluminum and protective MgF2 coatings). The laser is coupled to a Paschen–Runge type Optical Emission Spectrometer, manufactured by OBLF (Germany), with a
Fig. 2. Influence of the sample surface roughness on the C and Mn emission signal. The C intensity map (left) displayed additional features related to the surface topography, whereas the Mn map (right) was not influenced by it.
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Fig. 3. Influence of the positioning of a large sample in relation to the laser beam focal point. The Mn intensity map (left) displayed a strong baseline gradient. Its normalization to the intensity of a Fe line brought the intensity baseline back flat (right).
0.75 m Rowland circle. Forty five photomultiplier tubes enable the detection of thirty one elements simultaneously. Step size as small as 5 μm is available on the scanning stage in x and y directions. The instrument can be operated in air or Ar atmospheres. 2.2. Samples and standards The samples used in this study were cast products obtained from continuous casting of high carbon steel. Bloom samples were used for the segregation measurements, and billet samples for the decarburization. Two sets of certified reference materials were selected to calibrate the emission signals so that they covered the range of Mn and C concentrations encountered in both applications. Set 1 comprised CRM 059-2, 401-2, 402-2 and 434-1 for the analysis of segregation, and set 2, CRM 059-2, 401-2, 451-1 and 455-1 for the analysis of decarburization. Calibration curves were measured after each sample measurement to follow and correct the drift of the instrument's sensitivity. All standards and samples were polished to a finish of 1 μm for the decarburization maps, as the same sample surface was previously
analyzed by electron probe microscopy (EPMA), which requires such a fine finish. Standards and samples for the segregation mapping were prepared to a rougher finish sufficient for LIBS analysis. Automated polishing was used in order to maintain a flat large sample surface, and a silicon carbide 180 grit size paper provided a sufficiently smooth finish. No carbon contamination occurred as inferred by the absence of high intensity carbon/silicon emission signals across the whole sample. 2.3. Measurement conditions All scanning was performed with single-shot measurements on the standards and on the samples, with a laser energy of 1.7 mJ, in an air environment. Standards and samples were placed in a sample holder so that their position coincided with the focus point of the laser beam, yielding the smallest possible ablation craters. The diameters of the resulting craters were approximately 13 μm. Depending on the step size chosen, the consecutive ablation craters were adjacent or spaced. The step size was selected as a compromise between three parameters: the amount of spatial resolution required (related to how
Fig. 4. Quantified Mn map (left) and quantified C map (right) from LIBS measurements; both displaying the presence of elemental segregation.
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Fig. 5. Comparison of the sulphur print (left) and the Mn LIBS map (right), generated on the same surface. Both display segregation zones, as well as dendrites.
fine the features monitored were), the size of the total area measured (enabling generation of more representative data if the features were heterogeneously distributed), and the duration of the overall measurement. C and Mn were measured at 193 nm and 293 nm respectively, without background correction. 2.4. Data handling Large scanning measurements performed with micro-scale spatial resolution generated very large data files that only a custom-made graphical user interface could handle efficiently. For example, the 60 × 60 mm2 measurements on the bloom samples yielded files with 0.72 million points per channel; given that 45 detectors were simultaneously used, the 30 min measurements yielded 32.4 million point data files. A graphical user interface was specifically written to open and manipulate the LIBS data file. 3. Results and discussion 3.1. Parameters influencing signal stability
spatial resolution was obtained. The small depth of field of our system meant that the precision of the position of the samples and the flatness of samples had to be better than at least 50 μm. To achieve this, precision micro-machined sample holders were manufactured to ensure that the positioning of the samples and standards was reproducible each time a new specimen was inserted into the sample chamber. 3.1.2. Sample surface finish Some elements proved to be more sensitive than others to the samples' surface finish. Fig. 2 shows two intensity maps acquired simultaneously on a sample prepared by finishing with a 80 grit size alumina paper. The resulting surface finish was relatively rough with a Sa roughness factor (the arithmetic mean of deviation from the surface of all points measured on an area) of 2.5 μm and a difference between minimum trough and maximum peak of 14 μm. The rough finish of the surface did not influence the Mn map, whereas the C map displayed additional features related to surface topography. For this reason, a finer surface preparation was necessary to achieve a quality C mapping. Automated polishing (Struers Tegra automated polishing
3.1.1. Sample positioning In order to keep a good spatial resolution, and a stable signal, the samples had to be positioned so that they remain in the plane of the laser focal point while performing large area scanning. The influence of the focal point position with respect to the sample's surface on the plasma brightness and on the craters formed was observed (Fig. 1). The brightest plasma was obtained when the laser beam interacted with the sample surface at the focal point. The positioning of the focal point below the sample surface did not relate to stronger emission signal; additionally, the resulting larger craters meant that less good
Table 1 Averaged concentration values found in the numbered zones in Fig. 6 Bloom sample
%C
%Mn
Tundish bulk analysis Average bulk concentration (%) — zone 1 Average bulk concentration (%) — zone 2 Average concentration in segregation (%) — zone 3 Average concentration in segregation (%) — zone 4 Average concentration in segregation (%) — zone 5
0.73 0.76 0.88 1.12 1.06 1.02
0.51 0.48 0.52 0.73 0.69 0.67
Fig. 6. Schematic of areas selected for the calculation of averaged concentration values in segregated and non-segregated zones.
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machine) was used with silicon carbide paper of 180 grit size, yielding a final Sa roughness factor of 0.15 μm, along with a minimum/ maximum difference of 4.7 μm. The finish obtained was sufficient to not influence the C signal. Also, this sample preparation ensured that the large samples prepared remained as flat as possible. Flatness better than 10 μm variation was achieved on the 70 mm long samples. 3.1.3. Signal normalization For all measurements, the emission intensities were normalized to correct for the laser shot-to-shot variation and for potential remaining variation in the sample's position in relation to the laser beam focal point. Fig. 3 shows the example of a Mn map acquired on an area of 60 × 60 mm2 displaying central segregation features and a strong intensity gradient in the intensity baseline. The normalization of the Mn line at 293 nm to the Fe line at 187 nm brought the baseline back flat, and left the central concentration variation distinct. The black pixels at coordinates (20, 20) show the presence of porosity in the sample. 3.2. Segregation mapping by LIBS Segregation in cast steel product is the heterogeneous distribution of alloying elements in the steel matrix. When molten steel is cast, it starts solidifying from the outer shell. Solute elements in metal are more soluble in the liquid phase than in the solid phase. As the solidification fronts move towards the center of the cast volume, the solute concentrates towards the middle; this is what produces segregation. It is detrimental to the properties of the steel and needs to be monitored to enable a better control. Two conventional methods exist to assess the presence of segregation: ‘sulphur print’ and ‘carbon drillings’. ‘Sulphur print’ is a wet chemical method where the spatial distribution of sulphides can be revealed on large areas (up to 100 s of mm2). The print is produced by applying on the polished surface of the sample a photographic paper previously soaked in dilute sulphuric acid. The acid attacks sulphur and sulphides present in the steel that liberate hydrogen sulphide gas, which in turn stains the photographic paper. The darker the stain on the paper, the higher the sulphur content. By extension, the sulphur print indicates the presence and location of the segregation of the other elements in the steel. The information obtained this way is limited, as no direct information on the elements' concentration variation is available. Another method, called ‘carbon drillings’, involves the drilling of several volumes of the steel, with a spatial resolution of 10 mm, into the metallurgical center spot of the cast sample and at locations half way between the center spot and the sample edge to provide material that is non-segregated. The spatial resolution of this method is poor
and a large number of drillings needs to be taken to obtain reliable results. The possibility to actually produce rapidly elemental concentration maps by LIBS is a very attractive method for the assessment of segregation, on either large or small samples. Elemental maps were generated on a 60 × 60 mm2 area of a bloom sample of high carbon steel. Step sizes of 50 μm and 100 μm were used along the x-axis and the y-axis, respectively. The measurement had a duration of 35 min and provided maps with 720,000 data points for each of the 31 elements simultaneously detected. Quantified maps were obtained for Mn at 293 nm and C at 193 nm by applying the respective calibration curves equation. to the qualitative maps. A very distinct center-line segregation was visible in similar locations on the C and Mn maps (Fig. 4). The C map displayed two main zones of segregation in which the C distribution appears diffused. The Mn map revealed a much more structured distribution, where the same segregated areas appeared as for C. The different aspects of C and Mn segregation were as expected, as C has a much higher diffusion coefficient in steel than Mn, so the same original patch of segregation diffuses in the case of C, in a time for which Mn essentially remains in its original structure [16]. Needle-shape Mn depleted zones, also appeared on the map, corresponding to dendrites formed during solidification of the steel. Also, a multitude of high Mn intensity pixels relating to the presence of Mn-rich non-metallic inclusions were visible when zooming on the Mn map. The concentration maps generated with LIBS compared well with the sulphur print that was previously produced on the same sample surface (Fig. 5). Not only the segregated areas appeared in the same locations, but also, dendritic arms visible on the Mn map corresponded to the ones found on the sulphur print. The nominal C and Mn concentration values for the bloom sample, determined during the steelmaking process by the analysis of the liquid steel in the tundish, were compared with the C and Mn concentrations found in the homogenous and segregated areas of the bloom sample determined from the LIBS scanning, with the graphical user interface allowing the selection of areas of the concentration map and providing the corresponding local concentration values (Table 1). Results from both techniques correlated very well, with segregated areas, as well as dendrites, displaying the same spatial patterns. Such concentration maps provide two types of information on the segregation: its spatial extent and the concentration variation of the actual elements of interest. Averaged concentration values were calculated on selected regions to observe as a first approximation the segregation for C and Mn. Fig. 6 shows the regions observed and the corresponding concentrations were given in Table 1. The average C and Mn concentration found in zone 1 correlated well with the bulk chemical composition of the steel measured in the tundish before
Fig. 7. Different stages of the extraction of C segregation data on the bloom sample: a) cropped quantitative map, b) statistical and image filter map, c) threshold filter map.
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Fig. 8. C (top) and Mn (bottom) concentration distribution (left) and size distribution of segregated areas (right) in bloom sample.
casting. Further exploitation of the data was then performed to provide more detailed information on the segregation. The concentration maps were manipulated by applying a statistical and image analysis filter to remove the micro-scale salt-and-pepper noise of the data; the pixels related to segregation were finally extracted by applying a threshold filter of mean plus two standard deviations calculated on the whole cropped map. The different stages of the extraction are shown on the C map in Fig. 7 where the concentration map was first cropped, filtered, and the segregated zones were finally identified in white on black background after application of the threshold filter. The distribution of C and Mn concentrations in the segregated areas could then be plotted, as well as the distribution of the size of the segregated areas in mm2 (Fig. 8). The total segregated areas for the C could be estimated to 64 mm2 in the 3600 mm2 measured. Such detailed data provided thorough information of the segregation, and in a short time. Work was reported in the literature whereby spark-optical emission spectrometry was used to map segregation on large cast
samples [17]. However, LIBS offers two additional advantages for the mapping of segregation on cast samples. Firstly, the controlled directionality of the laser beam in comparison to the spark scattered distribution means that a much finer spatial resolution is obtained with LIBS. Secondly, although the frequent presence of porosity in cast steel samples causes excitation problem with a spark source, the laserinduced ablation of the surrounding areas of the cavities is not affected due to the nature of the laser excitation source and again due to its high spatial resolution. 3.3. Decarburization mapping by LIBS Decarburization is also a phenomenon that occurs when the steel starts solidifying during casting. The C in the steel, close to the surface, is lost by reaction with the surrounding oxygen and moisture in the air. Pearlite is the crystal structure formed in 0.8% C solid steel. The loss of C near the surface during cooling results in areas of pure ferrite and mixture of ferrite and pearlite (Fig. 9). Decarburization also affects the
Fig. 9. Photograph detail of a steel sample displaying a decarburized layer at its surface, after polishing and etching. a) Pure ferrite zone b) Ferritic grain boundary around pearlitic grain c) Pearlite.
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Fig. 10. Comparison between the photographs of the crystal structure revealed by sample polishing and etching (top) and the C map generated by LIBS measurement on the same sample surface (bottom). Fig. 12. Comparison of the C concentration gradient obtained by LIBS and EPMA, the method conventionally used for micro-scale mapping, on a sample area of 10 × 0.75 mm2. The measurement on the sample took 4 min by LIBS and 2 h by EPMA.
steel properties and needs to be monitored. The conventional method to asses the depth of decarburization involves the polishing and etching of a cut sample in order to reveal its crystal structure. The depth of decarburization is then observed under the optical microscope. This method is subjective as it is down to the operator to estimate where the network of ferritic grain boundaries becomes negligible. LIBS offers the possibility to generate very rapidly quantified C maps, which would provide consistent and reproducible data for decarburization assessment. A billet sample displaying a decarburization surface layer was polished and etched to reveal the extent of decarburization. The same sample surface was subsequently mapped with LIBS. The laser scanning was performed on an area of 4 × 1 mm2, with a step size of 20 μm in the x and y directions. The duration of the measurement was 2 min. A photograph of the etched sample surface and the corresponding LIBS C map were compared in Fig. 10. Both maps correlated perfectly, showing where the ferritic zone and the ferritic grain boundaries displayed very low-to-no C content, and where pearlite structure coincided with C concentrations close to 0.8%. It was observed that the ablation of the sample surface etched or re-polished yielded the same C map. From such a C map, a C concentration curve can be drawn from the C concentrations averaged over the width of the measurement (Fig. 11). The averaging smoothed the large concentration variations between ferritic boundaries and pearlitic grains that the extraction of a single scan line would have displayed, and instead, showed the progressive C concentration
gradient at proximity of the sample surface. The C quantified data obtained by LIBS were validated by comparison with EPMA measurements. A 10 × 0.75 mm2 area of the above sample was first measured by EPMA, and subsequently by LIBS. The EPMA measurement took 2 h, against 4 min for the LIBS measurement. The concentration maps obtained by both techniques were averaged over the measurement area width. Both C gradient curves correlated very well (Fig. 12), validating the quantification of the C by LIBS on such a sample. 4. Conclusions In the two examples above, the application of LIBS as a microanalysis technique proved to bring very valuable information that was not accessible up to now with other techniques on such large areas in such a short time. The speed of analysis and the appropriate spatial resolution that LIBS can offer were, in both cases, the major advantages that made these two applications possible. The possibility to generate fast micro-scale elemental concentration maps on relatively large areas is a very significant step forward to providing representative data of heterogeneous chemical features that occur on a relatively large scale. By generating concentration distribution maps, LIBS gives access to detailed quantified data, whereas current methods
Fig. 11. C concentration gradient (bottom) obtained from the averaging over the width of the area measured by LIBS (top).
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used during production can only provide qualitative data, or quantified information from a non-representative area. In order to successfully generate elemental maps on large areas, fast automated sample surface preparation was necessary to avoid the influence of surface finish on sensitive lines such as C at 193 nm. The adequate design of sample holders ensured that a stable signal was obtained from large area scanning. The systematic normalization of the data to the line of a major element also ensured that quantification could be achieved. LIBS maps generated on samples displaying segregation and decarburization correlated very well when compared with data from conventional micro-analysis techniques. The LIBS data were produced faster and provided thorough and detailed elemental and spatial information. Acknowledgments I would like to thank Corus Construction and Industry Business Unit for supporting this work, as well as Oliver Binns, Ian Gaunt and Martyn Whitwood, at Corus RD&T UK, for providing respectively Talysurf surface roughness measurement, samples preparation support and EPMA maps. Finally, I would like to thank Romain Gautier from the Ecole Nationale Supérieure de Chimie de Rennes for running LIBS measurements on decarburized samples. References [1] C. Lopez-Moreno, S. Palanco, J.J. Laserna, Calibration transfer method for the quantitative analysis of high-temperature materials with stand-off laser-induced breakdown spectroscopy, J. Anal. At. Spectrom. 20 (2005) 1275–1279. [2] A.K. Rai, F.Y. Yueh, J.P. Singh, H. Zhang, High temperature fiber optic laser-induced breakdown spectroscopy sensor for analysis of molten alloy constituents, Rev. Sci. Instrum. 73 (2002) 3589–3599. [3] H.W. Gudenau, K.T. Mavrommatis, L. Ernenputsch, Laser beam induced analysis and optical temperature measurement of hot metal and steel melts, Stahl Eisen 121 (2001) 45–50.
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[4] J. Gruber, J. Heitz, H. Strasser, D. Bauerle, N. Ramaseder, Rapid in-situ analysis of liquid steel by laser-induced breakdown spectroscopy, Spectrochim. Acta Part B 56 (2001) 685–693. [5] C. Aragon, J.A. Aguilera, J. Campos, Determination of carbon content in molten steel using laser-induced breakdown spectroscopy, Appl. Spectrosc. 47 (1993) 606–608. [6] G. Hubmer, R. Kitzberger, K. Morwald, Application of LIBS to the in-line process control of liquid high-alloy steel under pressure, Anal. Bioanal. Chem. 385 (2006) 219–224. [7] D.J.O. Orzi, G.M. Bilmes, Identification and measurement of dirt composition of manufactured steel plates using laser-induced breakdown spectroscopy, Appl. Spectrosc. 58 (2004) 1475–1480. [8] V. Sturm, J. Vrenegor, R. Noll, M. Hemmerlin, Bulk analysis of steel samples with surface scale layers by enhanced laser ablation and LIBS analysis of C, P, S, Al, Cr, Cu, Mn and Mo, J. Anal.At. Spectrom. 19 (2004) 451–456. [9] M. Kraushaar, R. Noll, H.U. Schmitz, Slag analysis with laser-induced breakdown spectrometry, Appl. Spectrosc. 57 (2003) 1282–1287. [10] G. Doujak, R. Mertens, W. Ramb, J. Flock, J. Geyer, S. Lungen, Slag analysis by laserinduced breakdown spectroscopy, Stahl Eisen 121 (2001) 53–58. [11] D. Menut, P. Fichet, J.L. Lacour, A. Rivoallan, P. Mauchien, Micro-laser-induced breakdown spectroscopy technique: a powerful method for performing quantitative surface mapping on conductive and nonconductive samples, Appl. Opt. 42 (2003) 6063–6071. [12] M.P. Mateo, L.M. Cabalín, J.J. Laserna, Automated line-focused laser ablation for mapping of inclusions in stainless steel, Appl. Spectrosc. 57 (2003) 1461–1467. [13] H. Bette, R. Noll, High speed laser-induced breakdown spectrometry for scanning microanalysis, J. Phys. D: Appl. Phys. 37 (2004) 1281–1288. [14] H. Bette, R. Noll, G. Muller, H.W. Jansen, C. Nazikkol, H. Mittelstadt, High-speed scanning laser-induced breakdown spectroscopy at 1000 Hz with single pulse evaluation for the detection of inclusions in steel, J. Laser Appl. 17 (2005) 183–190. [15] F. Boué-Bigne, Analysis of oxide inclusions in steel by fast laser-induced breakdown spectroscopy scanning: an approach to quantification, Appl. Spectrosc. 61 (2007) 333–337. [16] H. Bakker, H.P. Bonzel, C.M. Bruff, M.A. Dayananda, W. Gust, J. Horvath, I. Kaur, G.V. Kidson, A.D. LeClaire, H. Mehrer, G.E. Murch, G. Neumann, N. Stolica, N.A. Stolwijk, Diffusion in solid metals and alloys, Landolt Bornstein New Series, Group 3, vol. 26, Springer Verlag, 1990. [17] H. Presslinger, S. Ilie, P. Reisinger, A. Schiefermuller, A. Pissenberger, E. Parteder, C. Bernhard, Methods for assessment of slab centre segregation as a tool to control slab continuous casting with soft reduction, ISIJ Int 46 (2006) 1845–1851.
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s a b
Laser-induced breakdown spectroscopy applications in the steel industry: Rapid analysis of segregation and decarburization☆ Fabienne Boué-Bigne ⁎ Corus Research, Development and Technology, Swinden Technology Centre, Moorgate Road, Rotherham, S60 3AR, South Yorkshire, UK
a r t i c l e
i n f o
Article history: Received 30 November 2007 Accepted 13 August 2008 Available online 27 August 2008 Keywords: Laser-induced breakdown spectroscopy (LIBS) Segregation Decarburization Mapping Steel
a b s t r a c t Rapid chemical analysis is increasingly a prerequisite in the steel making industry, either to check that a steel product complies with customers' specifications, or to investigate the presence of defects that may lead to mechanical property failure of the product. Methods conventionally used for assessment, such as the monitoring of decarburization and segregation, performed by chemical etching of a polished surface followed by optical observation, tend to be relatively fast, simple and applicable to large sample areas; however, the information obtained is limited to the spatial extent of the defect. Other techniques, such as electron probe microscopy and scanning electron microscopy — energy dispersive X-ray, can be used for providing detailed chemical composition at the micro-scale, for a better understanding of the mechanisms involved; however, their use is limited to analyzing comparatively very small sample areas (typically a few mm2). The ability to rapidly generate chemical concentration maps at the micro-scale is one of the many positive attributes of laser-induced breakdown spectroscopy (LIBS) that makes it a useful tool for the steel industry as a laboratory or near-the-line analysis facility. Parameters that influence the detailed mapping of large sample areas were determined and optimized. LIBS scanning measurements were performed on samples displaying segregation and decarburization. A 60 × 60 mm2 area, with a step size of 50 μm, was measured in 35 min on segregation samples, and a 4 × 1 mm2 area with a step size of 20 μm in 2 min on a decarburization sample. The resulting quantified elemental maps correlated very well with data from the methods used conventionally. In the two examples above, the application of LIBS as a micro-analysis technique proved to bring very valuable information that was not accessible previously with other techniques on such large areas in such a short time. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The making of steel involves a series of complex processes where many physical and chemical events take place. Hot metal from the blast furnace is taken to desulphurization. It is then transferred to the basic oxygen steelmaking plant where the carbon content is reduced and alloying elements are added. During secondary steel making, the chemical composition of the steel is taken to within specification by further addition of alloying elements. Once the steel is at the right temperature, its chemical composition adjusted and non-metallic inclusions have been removed as far as possible, it is transferred to the
☆ This paper was presented at the Euro Mediterranean Symposium on Laser Induced Breakdown Spectroscopy (EMSLIBS 2007) held in Paris (France), 11–13 September 2007, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference. ⁎ Tel.: +44 1709 825 225; fax: +44 1709 825 337. E-mail address: [email protected]. 0584-8547/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2008.08.014
tundish for casting, and subsequently rolled. Various finishing processes can be applied to improve or change the surface quality. In order to improve the control of the various stages of the steelmaking process, fast on-line monitoring techniques are required to provide relevant data on a timescale that permits remedy action. Also, more fundamental understanding can be developed by having access to fast analytical techniques that provide thorough data on representative amounts of sample. Laser-induced breakdown spectroscopy (LIBS) is an ideal technique for a variety of on-line and/or fast elemental measurements. The three main advantages that LIBS offers, namely, speed, on-line adaptability and portability, have been exploited by many research groups to address different aspects of process control in the steel industry. Several papers in the literature report the development of on-line applications for the analysis of hot and molten metal [1–6] and for the monitoring of dirt on moving steel strip during rolling [7]. Another type of application exploits the fact that LIBS analysis can be conducted without sample preparation for the rapid bulk analysis of solid steel samples through the surface scale layer [8], or for the direct analysis of solid slag samples [9,10]. Finally, LIBS is also exploited for
F. Boué-Bigne / Spectrochimica Acta Part B 63 (2008) 1122–1129
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Fig. 1. Influence of the laser focal point in respect with the sample surface on the plasma brightness and on the diameter of the ablated craters.
the spatial resolution that the laser beam offers as a sampling and excitation source. Quantitative mapping with a spatial resolution as small as 3 μm was reported for elemental mapping in different materials [11]. The development of LIBS scanning measurements for the characterization of steel cleanness led to the generation of microscale elemental maps by either using a line-focused laser beam [12], or by using fast repetition rate lasers, pulsing at 1 kHz [13–15]. The analyses of segregation and decarburization in steel samples are the applications considered in this paper. Detailed monitoring of segregation and decarburization would help controlling and improving the quality of the final steel product. In order to characterize these two phenomena with detailed and representative data, high spatial resolution is required as well as speed of measurement in order to make micro-scale mapping possible on large enough areas to provide information showing the complete extent of the defects. Characterization methods already exist for their evaluation, consisting of the optical observation of an etched surface, but they lack precision and consistency. Data in this paper demonstrate the adequacy of LIBS fast scanning measurements. Such LIBS measurements provide thorough and detailed data that will lead to a better monitoring and control of phenomena such as segregation and decarburization.
2. Experimental 2.1. Instrument For this type of analysis, a LIBS instrument offering high spectral resolution is required in order to resolve the lines of interest from the multitude of interfering Fe lines. High spatial resolution is also necessary to generate elemental maps at the micro-scale. The instrument used was manufactured by the Fraunhofer Institute of Laser Technology. It is designed to generate very fast scans with a laser pulsing at 1 kHz, with the associated fast electronics on the detection system. The laser is a custom-made photo-diode pumped Nd:YLF laser, manufactured by EdgeWave (Germany), with a wavelength emission at 1047 nm, a pulse width of 6 ns and a repetition rate of 1 kHz. The laser beam was guided over 38 mm diameter mirrors, expanded to a factor of four and focused through a three lens optical component (f = 60 mm). The plasma light was transmitted directly to the spectrometer via a spherical concave mirror (50 mm diameter, 200 mm radius of curvature, reflective aluminum and protective MgF2 coatings). The laser is coupled to a Paschen–Runge type Optical Emission Spectrometer, manufactured by OBLF (Germany), with a
Fig. 2. Influence of the sample surface roughness on the C and Mn emission signal. The C intensity map (left) displayed additional features related to the surface topography, whereas the Mn map (right) was not influenced by it.
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Fig. 3. Influence of the positioning of a large sample in relation to the laser beam focal point. The Mn intensity map (left) displayed a strong baseline gradient. Its normalization to the intensity of a Fe line brought the intensity baseline back flat (right).
0.75 m Rowland circle. Forty five photomultiplier tubes enable the detection of thirty one elements simultaneously. Step size as small as 5 μm is available on the scanning stage in x and y directions. The instrument can be operated in air or Ar atmospheres. 2.2. Samples and standards The samples used in this study were cast products obtained from continuous casting of high carbon steel. Bloom samples were used for the segregation measurements, and billet samples for the decarburization. Two sets of certified reference materials were selected to calibrate the emission signals so that they covered the range of Mn and C concentrations encountered in both applications. Set 1 comprised CRM 059-2, 401-2, 402-2 and 434-1 for the analysis of segregation, and set 2, CRM 059-2, 401-2, 451-1 and 455-1 for the analysis of decarburization. Calibration curves were measured after each sample measurement to follow and correct the drift of the instrument's sensitivity. All standards and samples were polished to a finish of 1 μm for the decarburization maps, as the same sample surface was previously
analyzed by electron probe microscopy (EPMA), which requires such a fine finish. Standards and samples for the segregation mapping were prepared to a rougher finish sufficient for LIBS analysis. Automated polishing was used in order to maintain a flat large sample surface, and a silicon carbide 180 grit size paper provided a sufficiently smooth finish. No carbon contamination occurred as inferred by the absence of high intensity carbon/silicon emission signals across the whole sample. 2.3. Measurement conditions All scanning was performed with single-shot measurements on the standards and on the samples, with a laser energy of 1.7 mJ, in an air environment. Standards and samples were placed in a sample holder so that their position coincided with the focus point of the laser beam, yielding the smallest possible ablation craters. The diameters of the resulting craters were approximately 13 μm. Depending on the step size chosen, the consecutive ablation craters were adjacent or spaced. The step size was selected as a compromise between three parameters: the amount of spatial resolution required (related to how
Fig. 4. Quantified Mn map (left) and quantified C map (right) from LIBS measurements; both displaying the presence of elemental segregation.
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Fig. 5. Comparison of the sulphur print (left) and the Mn LIBS map (right), generated on the same surface. Both display segregation zones, as well as dendrites.
fine the features monitored were), the size of the total area measured (enabling generation of more representative data if the features were heterogeneously distributed), and the duration of the overall measurement. C and Mn were measured at 193 nm and 293 nm respectively, without background correction. 2.4. Data handling Large scanning measurements performed with micro-scale spatial resolution generated very large data files that only a custom-made graphical user interface could handle efficiently. For example, the 60 × 60 mm2 measurements on the bloom samples yielded files with 0.72 million points per channel; given that 45 detectors were simultaneously used, the 30 min measurements yielded 32.4 million point data files. A graphical user interface was specifically written to open and manipulate the LIBS data file. 3. Results and discussion 3.1. Parameters influencing signal stability
spatial resolution was obtained. The small depth of field of our system meant that the precision of the position of the samples and the flatness of samples had to be better than at least 50 μm. To achieve this, precision micro-machined sample holders were manufactured to ensure that the positioning of the samples and standards was reproducible each time a new specimen was inserted into the sample chamber. 3.1.2. Sample surface finish Some elements proved to be more sensitive than others to the samples' surface finish. Fig. 2 shows two intensity maps acquired simultaneously on a sample prepared by finishing with a 80 grit size alumina paper. The resulting surface finish was relatively rough with a Sa roughness factor (the arithmetic mean of deviation from the surface of all points measured on an area) of 2.5 μm and a difference between minimum trough and maximum peak of 14 μm. The rough finish of the surface did not influence the Mn map, whereas the C map displayed additional features related to surface topography. For this reason, a finer surface preparation was necessary to achieve a quality C mapping. Automated polishing (Struers Tegra automated polishing
3.1.1. Sample positioning In order to keep a good spatial resolution, and a stable signal, the samples had to be positioned so that they remain in the plane of the laser focal point while performing large area scanning. The influence of the focal point position with respect to the sample's surface on the plasma brightness and on the craters formed was observed (Fig. 1). The brightest plasma was obtained when the laser beam interacted with the sample surface at the focal point. The positioning of the focal point below the sample surface did not relate to stronger emission signal; additionally, the resulting larger craters meant that less good
Table 1 Averaged concentration values found in the numbered zones in Fig. 6 Bloom sample
%C
%Mn
Tundish bulk analysis Average bulk concentration (%) — zone 1 Average bulk concentration (%) — zone 2 Average concentration in segregation (%) — zone 3 Average concentration in segregation (%) — zone 4 Average concentration in segregation (%) — zone 5
0.73 0.76 0.88 1.12 1.06 1.02
0.51 0.48 0.52 0.73 0.69 0.67
Fig. 6. Schematic of areas selected for the calculation of averaged concentration values in segregated and non-segregated zones.
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machine) was used with silicon carbide paper of 180 grit size, yielding a final Sa roughness factor of 0.15 μm, along with a minimum/ maximum difference of 4.7 μm. The finish obtained was sufficient to not influence the C signal. Also, this sample preparation ensured that the large samples prepared remained as flat as possible. Flatness better than 10 μm variation was achieved on the 70 mm long samples. 3.1.3. Signal normalization For all measurements, the emission intensities were normalized to correct for the laser shot-to-shot variation and for potential remaining variation in the sample's position in relation to the laser beam focal point. Fig. 3 shows the example of a Mn map acquired on an area of 60 × 60 mm2 displaying central segregation features and a strong intensity gradient in the intensity baseline. The normalization of the Mn line at 293 nm to the Fe line at 187 nm brought the baseline back flat, and left the central concentration variation distinct. The black pixels at coordinates (20, 20) show the presence of porosity in the sample. 3.2. Segregation mapping by LIBS Segregation in cast steel product is the heterogeneous distribution of alloying elements in the steel matrix. When molten steel is cast, it starts solidifying from the outer shell. Solute elements in metal are more soluble in the liquid phase than in the solid phase. As the solidification fronts move towards the center of the cast volume, the solute concentrates towards the middle; this is what produces segregation. It is detrimental to the properties of the steel and needs to be monitored to enable a better control. Two conventional methods exist to assess the presence of segregation: ‘sulphur print’ and ‘carbon drillings’. ‘Sulphur print’ is a wet chemical method where the spatial distribution of sulphides can be revealed on large areas (up to 100 s of mm2). The print is produced by applying on the polished surface of the sample a photographic paper previously soaked in dilute sulphuric acid. The acid attacks sulphur and sulphides present in the steel that liberate hydrogen sulphide gas, which in turn stains the photographic paper. The darker the stain on the paper, the higher the sulphur content. By extension, the sulphur print indicates the presence and location of the segregation of the other elements in the steel. The information obtained this way is limited, as no direct information on the elements' concentration variation is available. Another method, called ‘carbon drillings’, involves the drilling of several volumes of the steel, with a spatial resolution of 10 mm, into the metallurgical center spot of the cast sample and at locations half way between the center spot and the sample edge to provide material that is non-segregated. The spatial resolution of this method is poor
and a large number of drillings needs to be taken to obtain reliable results. The possibility to actually produce rapidly elemental concentration maps by LIBS is a very attractive method for the assessment of segregation, on either large or small samples. Elemental maps were generated on a 60 × 60 mm2 area of a bloom sample of high carbon steel. Step sizes of 50 μm and 100 μm were used along the x-axis and the y-axis, respectively. The measurement had a duration of 35 min and provided maps with 720,000 data points for each of the 31 elements simultaneously detected. Quantified maps were obtained for Mn at 293 nm and C at 193 nm by applying the respective calibration curves equation. to the qualitative maps. A very distinct center-line segregation was visible in similar locations on the C and Mn maps (Fig. 4). The C map displayed two main zones of segregation in which the C distribution appears diffused. The Mn map revealed a much more structured distribution, where the same segregated areas appeared as for C. The different aspects of C and Mn segregation were as expected, as C has a much higher diffusion coefficient in steel than Mn, so the same original patch of segregation diffuses in the case of C, in a time for which Mn essentially remains in its original structure [16]. Needle-shape Mn depleted zones, also appeared on the map, corresponding to dendrites formed during solidification of the steel. Also, a multitude of high Mn intensity pixels relating to the presence of Mn-rich non-metallic inclusions were visible when zooming on the Mn map. The concentration maps generated with LIBS compared well with the sulphur print that was previously produced on the same sample surface (Fig. 5). Not only the segregated areas appeared in the same locations, but also, dendritic arms visible on the Mn map corresponded to the ones found on the sulphur print. The nominal C and Mn concentration values for the bloom sample, determined during the steelmaking process by the analysis of the liquid steel in the tundish, were compared with the C and Mn concentrations found in the homogenous and segregated areas of the bloom sample determined from the LIBS scanning, with the graphical user interface allowing the selection of areas of the concentration map and providing the corresponding local concentration values (Table 1). Results from both techniques correlated very well, with segregated areas, as well as dendrites, displaying the same spatial patterns. Such concentration maps provide two types of information on the segregation: its spatial extent and the concentration variation of the actual elements of interest. Averaged concentration values were calculated on selected regions to observe as a first approximation the segregation for C and Mn. Fig. 6 shows the regions observed and the corresponding concentrations were given in Table 1. The average C and Mn concentration found in zone 1 correlated well with the bulk chemical composition of the steel measured in the tundish before
Fig. 7. Different stages of the extraction of C segregation data on the bloom sample: a) cropped quantitative map, b) statistical and image filter map, c) threshold filter map.
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Fig. 8. C (top) and Mn (bottom) concentration distribution (left) and size distribution of segregated areas (right) in bloom sample.
casting. Further exploitation of the data was then performed to provide more detailed information on the segregation. The concentration maps were manipulated by applying a statistical and image analysis filter to remove the micro-scale salt-and-pepper noise of the data; the pixels related to segregation were finally extracted by applying a threshold filter of mean plus two standard deviations calculated on the whole cropped map. The different stages of the extraction are shown on the C map in Fig. 7 where the concentration map was first cropped, filtered, and the segregated zones were finally identified in white on black background after application of the threshold filter. The distribution of C and Mn concentrations in the segregated areas could then be plotted, as well as the distribution of the size of the segregated areas in mm2 (Fig. 8). The total segregated areas for the C could be estimated to 64 mm2 in the 3600 mm2 measured. Such detailed data provided thorough information of the segregation, and in a short time. Work was reported in the literature whereby spark-optical emission spectrometry was used to map segregation on large cast
samples [17]. However, LIBS offers two additional advantages for the mapping of segregation on cast samples. Firstly, the controlled directionality of the laser beam in comparison to the spark scattered distribution means that a much finer spatial resolution is obtained with LIBS. Secondly, although the frequent presence of porosity in cast steel samples causes excitation problem with a spark source, the laserinduced ablation of the surrounding areas of the cavities is not affected due to the nature of the laser excitation source and again due to its high spatial resolution. 3.3. Decarburization mapping by LIBS Decarburization is also a phenomenon that occurs when the steel starts solidifying during casting. The C in the steel, close to the surface, is lost by reaction with the surrounding oxygen and moisture in the air. Pearlite is the crystal structure formed in 0.8% C solid steel. The loss of C near the surface during cooling results in areas of pure ferrite and mixture of ferrite and pearlite (Fig. 9). Decarburization also affects the
Fig. 9. Photograph detail of a steel sample displaying a decarburized layer at its surface, after polishing and etching. a) Pure ferrite zone b) Ferritic grain boundary around pearlitic grain c) Pearlite.
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Fig. 10. Comparison between the photographs of the crystal structure revealed by sample polishing and etching (top) and the C map generated by LIBS measurement on the same sample surface (bottom). Fig. 12. Comparison of the C concentration gradient obtained by LIBS and EPMA, the method conventionally used for micro-scale mapping, on a sample area of 10 × 0.75 mm2. The measurement on the sample took 4 min by LIBS and 2 h by EPMA.
steel properties and needs to be monitored. The conventional method to asses the depth of decarburization involves the polishing and etching of a cut sample in order to reveal its crystal structure. The depth of decarburization is then observed under the optical microscope. This method is subjective as it is down to the operator to estimate where the network of ferritic grain boundaries becomes negligible. LIBS offers the possibility to generate very rapidly quantified C maps, which would provide consistent and reproducible data for decarburization assessment. A billet sample displaying a decarburization surface layer was polished and etched to reveal the extent of decarburization. The same sample surface was subsequently mapped with LIBS. The laser scanning was performed on an area of 4 × 1 mm2, with a step size of 20 μm in the x and y directions. The duration of the measurement was 2 min. A photograph of the etched sample surface and the corresponding LIBS C map were compared in Fig. 10. Both maps correlated perfectly, showing where the ferritic zone and the ferritic grain boundaries displayed very low-to-no C content, and where pearlite structure coincided with C concentrations close to 0.8%. It was observed that the ablation of the sample surface etched or re-polished yielded the same C map. From such a C map, a C concentration curve can be drawn from the C concentrations averaged over the width of the measurement (Fig. 11). The averaging smoothed the large concentration variations between ferritic boundaries and pearlitic grains that the extraction of a single scan line would have displayed, and instead, showed the progressive C concentration
gradient at proximity of the sample surface. The C quantified data obtained by LIBS were validated by comparison with EPMA measurements. A 10 × 0.75 mm2 area of the above sample was first measured by EPMA, and subsequently by LIBS. The EPMA measurement took 2 h, against 4 min for the LIBS measurement. The concentration maps obtained by both techniques were averaged over the measurement area width. Both C gradient curves correlated very well (Fig. 12), validating the quantification of the C by LIBS on such a sample. 4. Conclusions In the two examples above, the application of LIBS as a microanalysis technique proved to bring very valuable information that was not accessible up to now with other techniques on such large areas in such a short time. The speed of analysis and the appropriate spatial resolution that LIBS can offer were, in both cases, the major advantages that made these two applications possible. The possibility to generate fast micro-scale elemental concentration maps on relatively large areas is a very significant step forward to providing representative data of heterogeneous chemical features that occur on a relatively large scale. By generating concentration distribution maps, LIBS gives access to detailed quantified data, whereas current methods
Fig. 11. C concentration gradient (bottom) obtained from the averaging over the width of the area measured by LIBS (top).
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used during production can only provide qualitative data, or quantified information from a non-representative area. In order to successfully generate elemental maps on large areas, fast automated sample surface preparation was necessary to avoid the influence of surface finish on sensitive lines such as C at 193 nm. The adequate design of sample holders ensured that a stable signal was obtained from large area scanning. The systematic normalization of the data to the line of a major element also ensured that quantification could be achieved. LIBS maps generated on samples displaying segregation and decarburization correlated very well when compared with data from conventional micro-analysis techniques. The LIBS data were produced faster and provided thorough and detailed elemental and spatial information. Acknowledgments I would like to thank Corus Construction and Industry Business Unit for supporting this work, as well as Oliver Binns, Ian Gaunt and Martyn Whitwood, at Corus RD&T UK, for providing respectively Talysurf surface roughness measurement, samples preparation support and EPMA maps. Finally, I would like to thank Romain Gautier from the Ecole Nationale Supérieure de Chimie de Rennes for running LIBS measurements on decarburized samples. References [1] C. Lopez-Moreno, S. Palanco, J.J. Laserna, Calibration transfer method for the quantitative analysis of high-temperature materials with stand-off laser-induced breakdown spectroscopy, J. Anal. At. Spectrom. 20 (2005) 1275–1279. [2] A.K. Rai, F.Y. Yueh, J.P. Singh, H. Zhang, High temperature fiber optic laser-induced breakdown spectroscopy sensor for analysis of molten alloy constituents, Rev. Sci. Instrum. 73 (2002) 3589–3599. [3] H.W. Gudenau, K.T. Mavrommatis, L. Ernenputsch, Laser beam induced analysis and optical temperature measurement of hot metal and steel melts, Stahl Eisen 121 (2001) 45–50.
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