CETAS Study of element segregation in steel. Ann De Vyt, Chris Xhoffer and Jan Scheers. ArcelorMittal Global R&D Gent (Belgium)

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1 Abstract No. 46 Study of element segregation in steel Ann De Vyt, Chris Xhoffer and Jan Scheers ArcelorMittal Global R&D Gent (Belgium) Modern types of high strength steel typically contain a lot of Mn and C. These elements, among others, are known to segregate strongly during solidification. This enriched area cannot be removed once it is present. After rolling, bands of segregation occur. These bands are not just bands with different compositions but also with different microstructures. As such, the mechanical properties vary within the steel. Therefore it is necessary to study the extent of this segregation analytically. Studying segregation zones requires a combination of sensitivity and lateral resolution. Depending on the techniques used, either one or the other, or both can be achieved. When segregation occurs near the surface and parallel to it, GDOES is a fast and sensitive technique that allows local analysis due to its excellent depth resolution. With Laser Ablation ICP-MS a line scan studying various elements can be acquired across a section of the sample. ICP-MS also gives excellent sensitivity but its lateral resolution may be insufficient. EPMA and TOF-SIMS combine sensitivity and lateral resolution and provide similar results with an advantage for SIMS regarding the elements in low concentration. Figure 1: After Nital etching of HSLA steels, segregation bands are often observed. Their presence indicates a heterogeneous distribution of specific elements at micro scale. TOF-SIMS elemental maps and line scans across segregation bands (dim.: 300x300μm).

2 Abstract No. 46 Various examples will be given showing the advantages of the used techniques, compared to each other.

Study of element segregation in steel Session 12 Micro and nano scale characterization Definition Segregation in materials refers to the enrichment of a material constituent at a free surface or an

3 Study of element segregation in steel Session 12 Micro and nano scale characterization Definition Segregation in materials refers to the enrichment of a material constituent at a free surface or an internal interface of a material In steel it can be an enrichment or a depletion of certain elements in the central line, at grain boundaries or at the surface It influences the mechanical properties of the steel How can we study it? 1

Element segregation Centre line segregation Some elements are

These bands are not just bands with different compositions but

4 Element segregation Centre line segregation Some elements are known to segregate strongly during solidification This enriched area cannot be removed once present. After rolling, bands of segregation occur. These bands are not just bands with different compositions but also with different microstructures. The mechanical properties vary within the steel. Element segregation EPMA linescan 2 3

5 Element segregation EPMA and TOF-SIMS Decarburization 1 mm C Si Mn 4 1 mm 300µm Carbon present at the surface of steel reacts with oxygen containing species forming CO at high temperatures Surface concentration of carbon decreases Can be beneficial or detrimental depending on the application of the steel Need to know 5

6 Decarburization Metallography combined with GDOES depth profiling before and after annealing Surface hardening Various techniques to analyze N EPMA versus hardness 6 7

7 Surface hardening SIMS Intensités N normalisées 1,00E+04 9,00E+03 8,00E+03 7,00E+03 6,00E+03 5,00E+03 4,00E+03 3,00E+03 2,00E+03 1,00E+03 0,00E+00 Top 1/4 Surface hardening LIBS 1/ Distance 3/4 9 8 Bottom

8 Surface hardening GDOES Local enrichments EPMA gives combination of sensitivity and lateral resolution Fe 4 N needles 10 Grain Boundaries rich in N 11

$fracture visible Inclusion analysis$ $of fracture Search for inter laboratory$

9 Inclusion analysis Local enrichments can lead to particles Fracture risks Centre fracture visible Inclusion analysis SEM-EDX particle analysis Performed at ¼ thickness and ½ thickness Potential cause of fracture Search for inter laboratory cross check 12 13

Conclusions To study variations in the local element concentration several requirements are needed for the technique used: Good sensitivity Good lateral resolution Good depth resolution Examples were

10 Conclusions To study variations in the local element concentration several requirements are needed for the technique used: Good sensitivity Good lateral resolution Good depth resolution Examples were shown, proving that many techniques can be used For sensitive surface analysis GDOES seems a good solution SIMS gives a reasonable combination of sensitivity and lateral resolution For optimal resolution EM and EPMA are preferred Inter laboratory tests are very welcome. 14 Chris Xhoffer Cynthia Roegiers Dimitri Monteyne Henrique Duarte Alvarenga LIST TASCON Applied Spectra For the scientific stuff All of you For you attention 30/03/2012 Confidential - ArcelorMittal Global Research and Development Gent 15

University of Pretoria Z Tang (2006) Chapter 8 Studies of acicular ferrite by thin foil TEM

University of Pretoria Z Tang (2006) Chapter 8 Studies of acicular ferrite by thin foil TEM 8.2 Two types of acicular ferrite 8.2.1 Structure with parallel laths There appeared to be two types of acicular ferrite laths that were observed in those alloys cooled with a rapid cooling rate of 47