ISO 21224:2025

International
Standard
ISO 21224
First edition
Evaluation of centreline segregation
2025-12
of continuously cast slabs
Évaluation de la ségrégation de la ligne centrale des brames en
coulée continue
Reference number
© ISO 2025
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and designations . 3
5 Sampling and preparation . 3
5.1 General .3
5.2 Position of extraction, extend of testing and lot size .4
5.3 Preparation of samples .5
5.4 Etching and post-treatment of the samples .6
6 Test methods . 7
6.1 General .7
6.2 Evaluation by comparative charts (Method A) .8
6.3 Evaluation by measuring and counting (Method B) .8
6.3.1 General approach .8
6.3.2 Procedure .9
6.4 Measuring the area of the dark zones by digital image analysis (Method C) .10
7 Test report .11
Annex A (normative) Comparative charts for evaluating the severity of centreline segregation
according to Method A .12
Annex B (informative) Example of a documentation table using Method B.16
Annex C (informative) Procedure for image evaluation according to Method C . 17
Bibliography . 19

iii
Foreword
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Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
Steel pipes produced from hot-rolled semi-finished products (strip or plate) are often used for the transport
of different media (e.g. gas, oil, process- and waste water). Slabs from the continuously casting process are
used as blanks for the production of these precursors. The solidification of the steel results in the formation
of centreline segregation which is inherently caused by the casting process as well as the specific composition
of the steel alloy. Depending on the specific conditions, centreline segregation occurs to varying degrees
of severity and can affect the final material properties as well as the properties for further processing of
[1],[2]
the slab . Several methods for quality monitoring and casting process control of steel grades for the
production of pipes as for instance the “Mannesmann rating system” or the “dot technique” have been
[1],[2],[3]
developed in the past , however, none of these methods have the status of a generally accessible
standard or directive. In 2018, the industry standard SEP 1611 (Stahl-Eisen-Prüfblatt of German steel
[4]
association Steel Institute VDEh) was published in order to establish a first, both supplier and costumer
[5]
independent document on evaluation methods . Methods for macrostructure examination rely on the fact
that dark zones become visible in the centreline area of a slab after macroetching has been performed.
This document includes various test methods that have been established on a practical level and that reflect
the current state-of-the-art for the evaluation of centreline segregation using macroetching techniques
followed by visual inspection. For this purpose, the sampling, the sample preparation, the test method, the
evaluation of the results as well as the required documentation are specified in this document.
The severity of centreline segregation can be evaluated in the following ways, enabling the choice between
different options, addressing the wide range of available laboratory equipment at different testing centres:
— evaluation by comparative charts;
— evaluation by measuring and counting of the dark zones using circular templates;
— area measurement of the dark zones by digital image analysis.
In addition to macroetching and visual inspection, a couple of further methods are available in order to
analyse the interior quality of steel slabs using more sophisticated techniques such as electron probe
microanalysis (EPMA), X-ray fluorescence (XRF), optical spark emission spectroscopy (OES) or ultrasonic
[1], [3]
testing (UT) . However, these will not be covered in this document, since an unambiguous correlation
between the various methods has not been verified in general.

v
International Standard ISO 21224:2025(en)
Evaluation of centreline segregation of continuously cast
slabs
1 Scope
This document specifies a procedure for the evaluation of centreline segregation of continuously cast slabs,
including sampling, sample preparation and evaluation method.
This document is applicable for low carbon steel slabs for pipe production manufactured by continuous
casting. These steels have a typical chemical composition as follows (mass percentages):
— C ≤ 0,12 %
— Mn ≤ 2,0 %
— Si ≤ 0,5 %
— Ti + Nb + V ≤ 0,2 %
The common dimensions of these slabs are in the following ranges:
— Slab width: 800 mm to 2 600 mm
— Slab thickness: 150 mm to 450 mm
The evaluation methods covered by this document are based on macroscopic etching of slab samples
followed by visual inspection by means of human eye or camera systems.
The procedures for sampling described in 5.1 to 5.3 of this document are applicable to steels slabs in general
(e. g. other steel grades or dimensions). However, the etching procedure (see 5.4) in combination with the
evaluation methods described in Clause 6 have been developed especially for the application on pipeline
steels as given above. Thus, they are not applicable to other steel grades, product types or dimensions
without a prior validation. Any extension or modification beyond the scope of this document necessitates a
separate agreement between producer and customer.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 4969, Steel — Etching method for macroscopic examination
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/

3.1
transverse section of the slab
section which is usually flame cut across the width of the slab with a pre-defined depth
Note 1 to entry: See Figure 1.
Note 2 to entry: The process of sampling is described in 5.1.
3.2
centre area
region in the middle part of a slab in which the distribution of the alloying elements can be inhomogeneous
as a result of solidification
Note 1 to entry: See Figure 1.
Note 2 to entry: The centre area is normally located in the geometric centre of the slab (i.e., half-slab thickness,
0,5 × t ); but a displacement of the centre area in the direction of the upper or lower wide faces of the slab can occur
due to the specific casting process. The triple points frame the centreline segregation on the narrow faces (short
rectangular faces) of the slab. The solidification starts from the upper, lower and narrow faces of the slab and ends at
the triple points.
Note 3 to entry: The position of the centre area is determined by the etch image of a macroetching according to 5.4.
3.3
etching sample
part of a transverse slab section after dividing the transverse slab section into smaller representative
sections according to a defined scheme
Note 1 to entry: The defined scheme is described according to 5.2 (see Figure 2 and Figure 3).
3.4
test area
mechanically prepared surface of the etching sample for macroetching with subsequent evaluation
Note 1 to entry: See Figure 3.
3.5
assessment area
part of the test area within an interval of ±15 mm from the centre of the centre area
Note 1 to entry: The height of the assessment area h is thus 30 mm. The centre of the centre area is normally at
0,5 × t but can also be shifted in certain cases (see 3.2). In such cases, the position of the assessment area shall be
corrected accordingly.
Note 2 to entry: The width of the assessment area w depends on how the sample is extracted from the transverse slab
section, see 5.3, Figure 3.
3.6
dark zone
area that is visually distinguishable from the surrounding brighter matrix after macroetching a test area
Note 1 to entry: See Figure 4.
Note 2 to entry: If the severity of centreline segregation is evaluated with computerized image analysis, dark zones
are clearly separated in the histogram of the grey scale image of the test area. Dark zones can vary in size and in the
way they are located on the test area.
Note 3 to entry: The severity of the centreline segregation is evaluated on the basis of the dark zones according to
Clause 6.
3.7
classification category
result of the test according to evaluation methods A, B or C, divided into four classes 1 through 4

3.8
size class of the dark zone
description of the severity level of the centreline segregation in test methods B and C based on the size of the
dark zones
Note 1 to entry: This parameter is divided into the classes I through IV and used as a criterion for the determination of
the classification category.
4 Symbols and designations
The symbols used in this document and their corresponding designations are given in Table 1.
Table 1 — Symbols and designations
Symbol Unit Designation
d mm depth of the transverse slab section (Figure 1)
d mm etching sample depth before processing (Figure 2)
d mm etching sample depth after processing (Figure 3)
h mm assessment area height (Figure 3)
t mm slab thickness (Figure 1)
t mm etching sample thickness (Figure 3)
t mm position of centreline segregation; usually 0,5 × t (Figure 1)
c 1
w mm slab width (Figure 1)
w mm etching sample width (Figure 2, 3)
w mm assessment area width (Figure 3)
Z – dark zone (Figure 4)
A mm area of the dark zone (Figure 4)
Z
E mm equivalent diameter, diameter of the circle with the same area as the corresponding
Z
dark zone (Figure 4)
I mm diameter of the incircle of the dark zone (Figure 4)
Z
O mm diameter of the circumscribed circle of the dark zone (Figure 4)
Z
5 Sampling and preparation
5.1 General
The sampling consists of two steps. In the first step, a transverse slab section over the entire slab width w is
flame cut from the end of the slab. The transverse slab section can be cut from the continuously cast strand
or from a slab that has already been cut to length (see Figure 1). The depth d of the transverse slab section
shall be dependent on the sampling and the steps that are necessary to prepare the etching samples, see 5.3.
A depth d between 25 mm and 100 mm of the transverse slab section has proven to be useful.
The second step involves dividing the transverse slab section into etching samples according to a defined
scheme. During the sampling, preparing and evaluating of samples, the original position from where the
samples in the slab or the strand (for example: top and bottom face of the slab, sample position over the
width, etc.) were extracted shall be traced at all times.

Key
A centre area
B triple point
C transverse section of the slab
D slab (or strand)
d depth of the transverse slab section
t slab thickness
t position of centreline segregation
c
w slab width
Figure 1 — Geometry of the transverse slab section and the original slab
5.2 Position of extraction, extend of testing and lot size
The following parameters for the evaluation of centreline segregation shall be negotiable between supplier
and customer:
— the number of the transverse slab sections that are to be extracted as samples from the strand;
— the position in the strand from where the transverse slab sections are to be extracted;
— the scheme according to which the transverse slab section is to be divided into etching samples.
If there are no specific requirements by the customer, one representative transverse slab section shall be
extracted per casting sequence and per strand. As a standard, centreline segregation shall be evaluated in
the centre area of the transverse slab section (1 etching sample per transverse slab section) as depicted in
Figure 2.
Key
A transverse section of the slab
B etching sample
d etching sample depth before processing
t slab thickness
w slab width
w etching sample width
Figure 2 — Geometry of an etching sample and its position in the transverse section of the slab
(example)
5.3 Preparation of samples
The transverse slab section as well as etching samples from the slab can be extracted by hot or cold cutting
(thermal or mechanical). In order to prepare a test area suitable for macroetching, the etching samples shall
1)
be machined mechanically by milling and/or grinding .
If the samples are extracted by hot cutting, 8 mm of the depth of the etching sample d shall be removed by
machining in order to eliminate thermal effects which may have been caused by hot cutting.
The thickness of the etching sample t is normally identical to the slab thickness t . If the preparation of the
2 1
sample requires a thickness t that is smaller than the thickness of the slab section t , the original position
2 1
from where the sample was extracted in the slab shall remain traceable.
The width of the etching sample w shall depend on the technical possibilities (size of the pickling tank,
weight of the samples, etc.) in the respective test laboratory. If the etching samples are extracted from the
transverse slab section by hot cutting, the width of the etching sample w in comparison to the assessment
area w shall be reduced by at least 8 mm per side that was exposed to high temperatures in order to exclude
the possibility that the high temperatures during hot cutting of the sample might affect the test results (see
Figure 3). The width of the assessment area w should be at least 250 mm. If for technical restrictions as
stated above this minimum width cannot be achieved, the etching sample may be divided up into partial
samples with a width of at least 100 mm. These partial samples shall be located directly next to each other
so that the required minimum width w ≥ 250 mm is achieved.
1) A roughness value of R ≤ 4 μm of the surface of the test area a
...