Advanced high-strength steel

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1 1 Technical Article Bridging the Gap: MXRF Technique Rapidly Maps Centerline Segregation Continuously cast slabs with high alloy content are susceptible to centerline segregation defect. Etching techniques are routinely utilized for visual assessment of segregation. This paper introduces a new quantitative technique based on micro x-ray fluorescence analysis that allows rapid generation of chemistry maps from large slab samples. Authors Joydeep Sengupta (left) primary researcher (primary processes), ArcelorMittal Global R&D Hamilton, Hamilton, Ont., Canada joydeep.sengupta@arcelormittal.com Jackie Leung (right) senior researcher, ArcelorMittal Global R&D Hamilton, Hamilton, Ont., Canada jackie.leung@arcelormittal.com Advanced high-strength steel (AHSS) and ultrahigh-strength steel (UHSS) slabs produced from ArcelorMittal casters worldwide have high alloy contents manganese up to 5%, silicon up to 2% and aluminum up to 2% for achieving desired mechanical properties for automotive customers. These specialty steel grade groups include dual-phase (DP) and transformation-induced plasticity (TRIP) steel grades. High-strength, low-alloy (HSLA) steel grades slated for the line pipe industry also have high manganese content (up to 1%) and relatively higher sulfur (>50 ppm) and phosphorus (>100 ppm) contents compared to other steel grades. During continuous casting of these specialty steel grades, alloying elements often segregate along the centerline of slabs (Fig. 1). If a continuous channel of solute-enriched liquid is formed near the final solidification point due to suboptimal casting conditions, a centerline with segregated elements remains frozen inside the slab. In the absence of adequate strand containment, strand bulging often encourages movement of liquid within this solute-enriched continuous channel and aggravates the problem of centerline segregation. Casting Figure 1 parameters such as superheat, spray cooling practice and casting speed determine the severity and extent of centerline segregation in these slabs. Fig. 2 shows the range of centerline segregation severity typically found in AHSS slabs. In addition to the centerline segregation defect, internal midway cracks (Fig. 1) may also appear if excessively large bulging strains are created at the continuous caster near the solidifying front. Equipment maintenance-related parameters, such as caster roll misalignment, roll wear, and plugged and/or leaky spray nozzles also have a detrimental effect on internal quality of slabs. Review of published literature indicates that slab centerline segregation can lead to the formation of martensitic banded structure in hotand cold-rolled sheet products, 1 3 which may have some impact on their final mechanical properties. 4 6 The detrimental impact of severe continuous martensitic banding on mechanical properties of DP steel sheets has been reported. 7,8 Hence, to protect the interest of the customers, minimization and control of centerline segregation is very important to all steelmakers that produce AHSS slabs. Kenny Witherspoon IXRF Systems Inc., Austin, Texas, USA kennyw@ixrfsystems.com Location of centerline segregation defect and internal cracks inside a steel slab.

2 Figure 2 2 Range of centerline segregation severity observed on advanced high-strength steel (AHSS) slab samples as revealed after chemical etching. Toward this goal, over the past several decades, a multitude of qualitative and quantitative techniques have been developed by casting process engineers to evaluate the internal quality of continuously cast products, especially centerline segregation. Etching methods have used sulfur or Baumann printing, 9 ammonium persulfate, 9 copper ammonium chloride, 9 HCl, 9 Nital 10 and Picral. 10 Studies have been conducted to study the effect of process parameters on centerline segregation using qualitative methods, such as sulfur printing, 11,12 ammonium persulfate etching 13 and HCl etching. 14 These chemical reagents reveal the cast structure that is created during primary solidification. Centerline segregation and internal cracks can be clearly delineated within this cast structure typical examples are shown in Fig. 2. Attempts have been made to quantify centerline segregation using predictive segregation models, microprobe analysis, 18 optical emission spectrometry pulse discrimination analysis (OES-PDA), 19 elemental mapping 20 and image analysis. 21 Currently, the etching technique is the most widely utilized method for controlling slab internal quality control at continuous casters and for investigating failures reported by customers. However, this qualitative technique has two inherent disadvantages as discussed in the following section. Qualitative Evaluation of Segregation: The Technical Gaps Qualitative Evaluations Cannot Be Used for Technical Benchmarking For slab internal quality assessment (IQA), samples are periodically obtained either directly from the continuously cast strand (on-line) or from cold slabs (off-line). Etched cast macrostructures are then obtained from these samples at IQA laboratories, which are usually located at the proximity of the continuous casting equipment. After visual inspection of these macrostructures, an internal quality rating is assigned and recorded in the corporate quality database. The internal quality rating system is usually adapted from the Mannesmann Demag (MD) standards that were established several decades ago. This adaptation is necessary because the MD ratings were based on sulfur prints a widely used method for steels with %S >50 ppm that now fail to reveal

3 3 Technical Article Table 1 Comparison Between Centerline Segregation Ratings Assigned by IQA Laboratories Located at Plant A and Plant B to the Same Five Samples Obtained From Five Different Slabs Slab origin Plant A Plant B Slab ID Normalized centerline rating IQA lab at Plant A IQA lab at Plant B A A A A A B B the cast macrostructure for AHSS grades, as these modern steels have extremely low sulfur content (<20 ppm). Etching methods and equipment vary from caster to caster, and hence each steelmaker has developed its own IQA standards independently. These standards cannot be compared and lead to confusion while performing any benchmarking study. To demonstrate this problem, a study was recently conducted at ArcelorMittal Global R&D to compare the centerline segregation rating systems between two sister plants: Plant A and Plant B. The same samples obtained from five slabs cast at Plant A and two slabs cast at Plant B (i.e., one sample from each slab) were processed at the IQA laboratories at the two plants per the standard operating procedures developed independently by them. It is evident from Table 1 and Fig. 3 that the IQA ratings assigned by the two labs are not comparable. Two factors are primarily responsible for this discrepancy: Etching techniques are different the etchant used by Plant A is more reactive than the acidic etchant used by Plant B. Hence, macrographs produced at Plant A have better resolution and led to harsher internal quality ratings. Digital cameras used for recording the cast macrographs have different specifications Plant A s digital camera captures more megapixels per unit area, generating photographs that have better contrast and resolution. Thus, artifacts are accurately recorded, which artificially enhances the segregation severity. Thus, it can be concluded that site-specific qualitative rating systems are not comparable and can easily confuse the customer if slabs are being purchased from different steelmakers. Furthermore, pursuing internal or external benchmarking studies on centerline segregation can be challenging due to this incompatibility between the IQA rating systems. Qualitative Evaluations Cannot Be Used for Statistical Data Analysis It is a well-known fact that casting speed and superheat have the greatest impact on centerline segregation severity. Casting speed controls the location of the final solidification point (crater end) Figure 3 (a) (b) Full-width transverse sample from same slab (B-1 see Table 1) and same location etched and photographed at two IQA laboratories at: Plant A (a) and Plant B (b). The sample received a harsher rating at Plant A (3 vs. 2 at Plant B) highlighting the ambiguity of etching-based rating systems.

4 4 Figure 4 inside the caster higher casting speeds encourage enhanced movement of alloying elements near the crater end and create segregated structure. Superheat determines the amount of heat entering the mold and evolution of solidification structure higher superheats encourage columnar crystal structures that have increased susceptibility for alloy segregation. To characterize the impact of speed and superheat on centerline segregation at ArcelorMittal Dofasco Inc. s No. 1 continuous caster, 40 AHSS slab samples (with varying degrees of manganese content) were obtained from slabs cast at various casting speed and superheat combinations. Care was taken to keep the speed and width constant (i.e., to achieve steady state casting) for the slabs selected for internal quality evaluation. The samples were etched and rated as per established standard procedures for cupric ammonium chloride-based etching method developed by ArcelorMittal Global R&D Hamilton (AM Global R&D). 22 Fig. 4 shows the variation of internal quality of slab samples with casting speed and superheat. Referring to this figure, it can be concluded that: (a) in general, samples cast at lower superheats had better internal quality and the risk of centerline segregation increased with casting speed, and (b) large variability in internal quality was observed at any given casting speed. This indirectly confirms that other factors, such as roll and spray nozzle conditions, can also influence the extent of centerline segregation. The figure also simultaneously highlights the limitation of using qualitative data to support any caster improvement projects that aim to minimize centerline segregation. Using the qualitative data presented in the figure, it is impossible to establish impact of casting process and equipment life on centerline segregation using any statistical multi-variable regression analysis. Qualitative data cannot also be used to validate any numerical models that are capable of predicting segregation severity indices. Clearly, from the standpoint of segregation-sensitive AHSS slabs, etching-based qualitative procedures pose two technical challenges for casting engineers: The method is subjective, and hence it is difficult to establish a unified detection procedure so that interests of all customers can be protected. This is especially true if these slabs are produced by a global company (such as ArcelorMittal) that operates multiple casters around the world. Variation of AHSS internal quality with casting speed and superheat at ArcelorMittal Dofasco Inc. s No. 1 Continuous Caster. The method does not allow quantitative or statistical analysis, and hence it is not possible to identify cost-effective and quick-to-implement solution strategies that are required at the casters to minimize segregation defects in these slabs. To overcome these two technical gaps, quantitative procedures for detecting centerline segregation are required. These methods allow measurement of alloy chemistry variation along the segregated slab centerline. Quantitative Evaluation of Segregation: Bridging the Gap Table 2 presents a concise summary of a benchmark study conducted by AM Global R&D and compares the procedures that are either currently in use or can be utilized to obtain chemistry data for quantitative assessment of centerline segregation. The most widely used quantitative technique is the OES method. Portable handheld units have been developed and commercial units are available. Handheld units have been successfully used to measure steel chemistry variation along the slab centerline for full-width transverse slab samples. OES analysis can provide accurate analysis of lighter elements, such as C, S, P and Al, without the use of vacuum around the area of interest. However, handheld units need frequent calibration and any loss of contact between the spark and sample surface can result in measurement

5 5 Technical Article Table 2 Quantitative Methods for Slab Centerline Segregation Analysis Technique Features Pro Con Wet chemistry Handheld optical emission spectrometry (OES) analyzers Automated OES systems Automatic rating by image processing of etched macrographs Chemistry analysis done after drilling holes along centerline Etching is required to reveal centerline on slab sample Spot size is ~5 mm dia. Chemistry analysis from OES sparks along centerline Etching is required to reveal centerline on slab sample Spot size is smaller (~3 mm dia.) OES sparks spacing on full-width slab sample is ~10 mm Equipped with X-Y stage for controlled OES sparking that leads to better accuracy Etching is required to reveal centerline on slab sample OES sparks spacing can be as low as ~1 mm on full-width sample Uses sulfur prints that are scanned at ~10 dpi grayscale Automatic image analysis software provides a numerical rating Accurate full chemistry analysis can be obtained including carbon and sulfur Large full-width samples can be analyzed Accurate full chemistry analysis can be obtained including carbon and sulfur Sparking instead of drilling provides faster feedback (days vs. weeks) Better accuracy and faster feedback compared to handheld OES (<1 day) Chemistry plots have finer resolution Post-processing software for data analysis Quantitative rating system with fast feedback for full-width transverse samples Feedback can be slow up to 3 weeks Spot size is very large and chemistry plots have coarse resolution Frequent calibration of device is required Handheld system introduces errors and lowers accuracy Frequent calibration of device is required Accuracy is dependent on contact between sparks and sample surface No chemistry feedback Low accuracy Low resolution scans trigger false positives SEM energy dispersive x-ray spectroscopy (EDXS) SEM is used for EDXS analysis Spot size is small: µm EDXS is well-established as an academic or R&D tool Semi-accurate chemistry analysis (see Fig. 5) No etching is required SEM is required Only small mounted samples are analyzed Chemistry maps from large areas cannot be obtained Micro x-ray fluorescence (MXRF) systems Spot size is 40 µm 1 mm Scan time is 45 minutes to 5 hours for ~50 x 100 mm area Accurate chemistry analysis MXRF technique is wellestablished for macrosegregation analysis Software suite for spectral data analysis and mapping line and area scans Not established currently under testing at ArcelorMittal Global R&D errors. Automated portable OES units have also been developed, providing improved accuracy and finer chemistry plot resolutions, but use of these units for macrosegregation analysis has not been reported in published literature. Automated rating systems developed using image analyses of photographs of etched macrographs are quantitative but do not provide any chemistry information. Furthermore, etching artifacts (e.g., dark spots, blotches, etc., that are ignored by the human eye) on photographs introduce errors in the segregation ratings; and hence, the technology has not gained any popularity. Energy dispersive x-ray spectroscopy (EDXS) technique using scanning electron microscopes (SEM) is routinely used for obtaining elemental composition by industrial and academic researchers. In this technique, the electron beam of the SEM impinges on the steel sample and excites atomic electrons causing the production of characteristic x-rays. These characteristic x-rays are at energies specific to elements in the sample. The EDXS detector collects these x-rays as a signal and produces a spectrum that reveals the elements near the segregated centerline. This technique provides semi-quantitative analysis on small steel samples, which have to be mounted and polished so that they can be placed inside an SEM. To obtain chemistry data from large slab samples, regularly spaced small samples are cut and analyzed, and then a chemistry contour map for the entire area is created by manipulating the EDXS chemistry data. Obviously, this procedure for a large area of interest (e.g., 100 x 200 mm) is extremely labor-intensive and impractical. Steel samples also can be excited by x-rays. X-rays for use in the SEM are generated by passing a current through a filament, which causes electrons to be emitted. The electrons are then accelerated toward a target or anode, which in turn produces characteristic

6 6 Figure 5 x-ray fluorescence from the anode material. These x-rays then pass through a small aperture to focus the x-ray beam onto a sample. When x-rays are used as the source of excitation, the method is then called energy dispersive x-ray fluorescence (ED-XRF). Generally, SEM/EDXS analysis takes place at an excitation energy of 20 kev. This method can detect most elements that are present in amounts above 0.1 wt.%. However, in terms of detectability, electron excitation is best suited for light elements with x-ray energies below 2 kev, while XRF can detect the heavier elements at lower concentrations than can be seen with SEM/EDXS. In fact, for energies above 2 kev, XRF can detect elements below 100 ppm. XRF can also resolve peaks of heavier elements at higher energies than EDXS can. This is demonstrated in Fig. 5, which shows the comparison between EDXS and XRF analysis of the same metal sample. As the overlay shows, at 2 kev the EDXS shows less detection as the energy is higher, while XRF is able to resolve the peaks with higher energy excitation. Also, XRF is able to identify both Kα and Kβ peaks, which aids in identification of multiple elements that are in the same area of interest. Volz et al. 23 and Pickering et al. 24 have successfully used the XRF technique to map solute distribution in metals to generate composition maps over large distances. Volz et al. used a large-scale fixed facility at the Los Alamos National Laboratory to generate composition maps for studying solidification phenomena in Ni-based superalloys and uranium-niobium alloys. The XRF system can accommodate samples up to 700 x 700 x 250 mm. This system uses a rhodium tube with maximum power of 35 kv and 100 μa, and a liquid nitrogen cooled, lithium-drifted silicon detector. The smallest spot size is approximately 0.4 mm. Pickering et al. used standard off-the-shelf components to build an automated XRF apparatus for generating composition maps for Cr, Mn, Ni, Mo and Si to analyze macrosegregation phenomenon in 200-ton steel ingots. The apparatus used a rhodium source and a 10-mm silicon drift detector. The XRF unit is mounted on a robotic gantry system with laser-guided movement control. The authors reported that their automated XRF apparatus was capable of accurately and reproducibly detecting macrosegregation present in the steel samples. AM Global R&D explored the application of the XRF technique 22 for obtaining composition maps for quantifying manganese segregation along the centerline of continuously cast slabs. A prototype assembled unit was used to obtain these measurements. Comparison between energy dispersive x-ray spectroscopy (EDXS) and x-ray fluorescence (XRF) analysis of the same metal sample EDXS spectrum is shown in red (or shaded) and the XRF spectrum is indicated by the black line. To calibrate the XRF measurements, two standard specimens (ASMI210B and ASMI199B) were used to convert the manganese count rates measured by the XRF system to manganese wt.% concentrations. Measurements of manganese wt.% from six different AHSS slab samples at locations away from the centerline (far-field manganese wt.%) were then acquired and compared to OES chemistry measurements obtained from steel samples taken from tundish. The manganese wt.% data determined by XRF matched closely with the tundish data for all the six samples. Additionally, the 1σ error associated with the XRF wt.% measurements from 20 locations was found to be less than 5%. The unit was able to successfully generate composition maps for 20 x 20 mm area within 5 hours using a ~1.0-mm spot size. Based on the above investigation, AM Global R&D recently collaborated with IXRF Systems to develop and fabricate a laboratory-scale micro-x-ray fluorescence (MXRF) unit, which is capable of scanning large steel samples so that quantitative chemistry maps near slab centerline segregation can be generated rapidly. The following section introduces this new MXRF unit currently in operation at AM Global R&D. This unit has now been commercialized by IXRF Systems. New MXRF Unit at ArcelorMittal Global R&D Fig. 6 shows the front-view of the integrated MXRF bench top device. The device consists of a chamber that contains the x-ray excitation source, XRF detector and sample scanning stage. The device is also equipped with an XRF software suite that is capable of spectrum processing and acquisition, mapping line and area scans, and digital imaging. The device can

7 7 Technical Article scan slab samples as large as 200 x 100 x 20 mm thick, as shown in the close-up view of the scanning stage (Fig. 7). The key technical specifications of the unit are shown in Table 3. The device has several unique fail-safe features to ensure safe operation: Two cameras and a laser sensor inside the chamber prevent the sample from hitting the source and the detector. XRF software suite provides easy user interface for x-y-z control and ensures safe stage movement. Two switches on the chamber door automatically close the x-ray source shutter when the door is open. An emergency x-ray shutoff button located in the front control panel, in case of any stage malfunctions. A lockout key disables all operations when removed from its slot, in case of emergency. Figure 6 Micro-x-ray fluorescence chamber (a) and human-machine interface interface (b). Figure 7 (a) (b) The MXRF unit was calibrated for Mn, Cr and Al detection using three standards that are routinely utilized for calibrating the OES unit in ArcelorMittal Dofasco s Steelmaking Chemical Analysis Laboratory i.e., BAS 403/2, BSC 68B and MBH 12X LA2. After calibration, OES (spot size = 4 mm) and MXRF (spot size = 40 mm) measurements were obtained from three different slab samples and the maximum difference between the two measurements was within 10%. A detailed gage capability study was conducted on spot spectral analysis using the BAS 403/2 standard and the Mn wt.% data was found to be repeatable (1σ error within 0.03 wt.%). To demonstrate the capability of the MXRF device for generating compositional maps along the centerline segregation area, the same 70 mm x 10 mm area on a typical AHSS slab sample (with aim Mn wt.% of ~2.0%) was scanned at three different acquisition times by varying the MXRF scan parameters (refer to Table 4). The compositional maps are shown in Fig. 8. After the area scans were obtained, the XRF software suite readily provided the compositional statistics associated with each generated compositional map see Table 4. Referring to the figure and the table, it can be concluded that the MXRF device is able to produce Close-up view of the sample scanning stage showing a large steel sample ready for XRF scanning. Table 3 Key Technical Specifications of the MXRF Device Developed by IXRF Systems Excitation source Detector Measurement media Sample types Element range Sample size Sample stage Software suite 50 W 50 kv 1 ma polycapillary x-ray excitation Spot size: Min 40 µm; Max up to 5 mm Target material: rhodium 80 mm 2 silicon drift detector (140 ev resolution) Air and vacuum Solid, liquid, particle and powder Sodium to uranium Max: 200 mm x 100 mm by 20 mm thick 320 mm x 320 mm motorized X-Y-Z Spectra processing and acquisition, mapping line and area scans, and digital imaging

8 8 Figure 8 Three compositional maps for manganese generated by the MXRF device for the same 75 x 10 mm area on an AHSS slab sample at three different acquisition times: Scan A for 1 hour and 40 minutes, Scan B for 2 hours, and Scan C for 3 hours and 15 minutes. Refer to Table 4 for MXRF scan parameters and Mn Ratio statistics. The aim Mn wt.% in the AHSS grade is ~2.0%. Table 4 MXRF Scan Parameters and Mn Ratio Statistics for the Three Compositional Maps Shown in Fig. 8. Note: Mn Ratio = Spot Mn wt.% / Aim Mn wt.% MXRF scan parameters and Mn ratio statistics Scan A Scan B Scan C Map area 75 mm x 10 mm Voltage (kv) 50.0 Current (ma) Intensity (C/s) 260, , ,000 Pixel size (mm) Point dwell time (msec) Resolution (pixels) Scan acquisition time Scan statistics (Mn ratio) 1 hour 40 minutes 2 hours compositional maps rapidly for large areas of interest near the slab centerline. The compositional statistics provide quantitative data for comparing either scans with different MXRF scan parameters or scans from different AHSS samples. Referring to Fig. 8, it can be seen that some accuracy is lost if the acquisition time is reduced from ~3 hours to ~1.5 hours e.g., max Mn ratio reduces from 3.44 to 2.66 and std. deviation reduces from 0.28 to The impact of MXRF scan parameters, such as, point dwell time (acquisition time per pixel) and resolution (number of pixels along the longest edge of the area being scanned), on Mn ratio statistics cannot be ignored. Therefore, to balance the need for rapid map acquisition with accurate Mn ratio statistics, the MXRF scan parameters have to be chosen judiciously. This investigation is currently in progress at AM Global R&D. Once optimal scan parameters have been selected (i.e., standard procedures have been established), the Mn ratio statistics generated by the MXRF device can be used as quantitative metrics for comparing the intensity and variation of centerline segregation on different AHSS slab samples. This represents a significant improvement from the current method of segregation evaluation by using a rating system that is based on visual inspection of etched slab samples. Thus, rapid compositional maps and Mn ratio statistics generated by the MXRF device installed at AM Global R&D bridges the technological gap related to evaluation of slab centerline segregation. The technique will perhaps encourage continuous casters to abandon confusing and labor-intensive qualitative rating systems and implement MXRF-based quantitative 3 hours 15 minutes Average Std. deviation Min Max

9 9 Technical Article metrics that enable objective evaluation and data analysis. Conclusions Continuously cast slabs with high alloy content are susceptible to centerline segregation defect. Etching techniques are routinely utilized for visual assessment of segregation. Quality ratings are assigned to slabs depending on the extent of segregation generated at different processing conditions. Unfortunately, these ratings are confusing and do not allow any quantitative analysis for identifying solution strategies that aim to improve slab quality. Some quantitative techniques are available but these are either labor-intensive or are applicable only to small samples. A new quantitative technique based on micro x-ray fluorescence (MXRF) analysis has been introduced in this paper that allows automated and rapid generation of compositional maps from large slab samples. A prototype MXRF device (currently commercially available from IXRF Systems) installed at ArcelorMittal Global R&D has been successfully tested and is now able to provide quantitative metrics for evaluation of slab centerline segregation. References 1. J.D. Verhoeven, A Review of Microsegregation Induced Banding Phenomena in Steels, Journal of Materials Engineering and Performance, Vol. 9, No. 3, 2000, pp G. Krauss, Solidification, Segregation, and Banding in Carbon and Alloyed Steels, Metallurgical and Materials Transactions B, Vol. 34B, 2003, pp T.F. Majka, D.K. Matlock and G. Krauss, Development of Microstructural Banding in Low-Alloy Steel With Simulated Mn Segregation, Metallurgical and Materials Transactions A, Vol. 33A, 2002, pp R.G. Davies, Influence of Martensite Composition and Content on the Properties of Dual-Phase Steels, Metallurgical and Materials Transactions A, Vol. 9A, 1978, pp A.R. 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Holland, Detection of Macrosegregation in a Large Metallic Specimen Using XRF, Ironmaking and Steelmaking, Vol. 41, No. 7, 2014, pp F Notice Please note that the information provided in this article is provided without warranty of any kind, express or implied, and is not a recommendation of any product, process, technique or material, nor is it a suggestion that any product, process, technique or material should not be used. Neither ArcelorMittal Dofasco Inc. nor any of its affiliates or employees will be liable for any damage suffered as a result of use of any information provided in this article. Use of any information in this article is entirely at the user s risk. This paper was presented at AISTech 2016 The Iron & Steel Technology Conference and Exposition, Pittsburgh, Pa., USA, and published in the Conference Proceedings.