Edis Ten, Olga Sharaya
(Moscow Institute of Steel and Alloys, Russia
Karaganda State Technical University, KAZAKHSTAN)
Electronic and microscopic investigation of structure
and properties of sheet steel protection coatings
The
quality of surface of low-carbon construction sheet steel produced at
steelmaking plants requires perfection, as the volume of metal-made products
with protection coatings constantly increases.
The
main purpose of protection coatings is to protect products and structures from
corrosion. That is why steelmaking plants rapidly develop production of rolled
metal with anti-corrosion coatings (galvanization, chromium coatings,
enamelling, etc). The share of sheet steel products with coatings is now up to
40% in industrially developed countries. Such production is used in heavy
engineering, car-making, packaging and packaging materials manufacturing.
The
purpose of this work is to investigate the quality of low-carbon steel 08kp,
08ps and the structures of galvanized and aluminum-zinc coatings spread over
the surface 0.5… 0.22 mm thick.
It is known that the quality of metal products
with protection coatings is determined by the absence of surface deficiencies.
Thus, to develop technological solutions, the following tasks had to be
accomplished:
- the
formation of rolled metal surface deficiencies was analyzed and the development
of preventive measures ensured;
- the
zinc and aluminum-zinc coatings on strip steel 08kp, 08ps surfaces were
studied.
The
object of investigation was hot- and cold-rolled sheets of industrial rolled
metal as well as zinc and aluminum-zinc coated samples. The structure of metals
and coatings were studied by way of light microscopy (Carl Zeiss microscopes)
and with the dot-matrix electronic microscope JSM 5910 (made by JEOL).
Investigation results and their discussion
Sheet rolled metal surface deficiencies. Deficiencies are
classified depending on their character and nature and technological
redistribution on which they formed or appeared. There is complexity in identifying
the nature of deficiencies, as deficiencies of one type may be caused by
several factors (for example, steel-smelting or rolling redistribution). At the
same time, similarly manifested deficiencies may have a different nature. In
the course of further technological operations of steel reprocessing, the outer
appearance of deficiencies may change.
In most cases, thin-sheet strip steel
is used for stamping deep-drawing products, manufacturing tin and sheets with
anti-corrosion protection coatings, which virtually excludes any deficiencies
in the surface (base) layer.
We
have determined that main surface deficiencies in cold-rolled stripes and
sheets from low-carbon steels are scabs and rolled-out blowholes classified in
line with GOST 210014-88.
Deficiencies may be divided by default
into internal and surface ones, but in the process of production of end rolled
metal products internal deficiencies may appear/roll out on the stripe surface
and thus form surface deficiencies.
In the course of experiments, rolled
metal surface deficiencies were grouped into three types: low longitudinal
stripes with exfoliations; dark stripes in the form of parallel lines;
interrupted scaly exfoliations with transversal tears. Deficiencies in the form
of lines have basically appeared 100 mm from the edge of a roll and further
1/3-1/4 of the sheet's thickness deep.
By
using a micro-probe and the method of dot analysis and deficient area mapping,
we have managed to determine the structural and concentration heterogeneity:
non-metal phases of various types were recorded in deficient areas (they are
normally called non-metal inclusions). In 08kp steels, there were accumulations
of oxide non-metal inclusions of globular shape which had the following
composition (%): FeO - 91…93; MnO – 1,42…2,5; SiO2 – 0,16…0,98. Cavities
of blowholes (surface and subcutaneous) are partially or fully filled with
dross (ferrum oxides), allowing to carry out micro-probe analysis. Areas of
fragile-destroyed cindery ferrum-magnesium-silicate inclusions were also
observed. For 08kp and 10ps, the presence of raw non-metal inclusions in the
form of fragile-destroyed silicates was recorded in rolled metal, with the
following composition (%):SiO2 – 12…18; MnO – 30…58; FeO – 8…10. Ferrum oxides with 91.6% of FeO and 2.5 % of MnO were distributed between discontinuity flaws and
micro-pore cavities. In areas of strip rolled metal, where surface deficiencies
were not stated, there were no non-metal inclusions or ferrum oxides in the
form of dross.
Therefore, using micro-probe analysis
for researching micro-structures of steel has allowed to identify
characteristic types of rolled metal surface deficiencies and to classify them:
1 – scabs, 2 – rolled-out (oxidized) blowholes, 3 – raw non-metal inclusions
(deoxidization products, cinder). This permitted to figure out the causes of
deficiencies on the rolled metal surface and to work out recommendations for
their reduction, which contributed to obtaining more high quality coatings.
Investigation of zinc and
aluminum-zinc coating structures.
Rolled metal samples (150x150 mm) selected from strip
steel 08kp 0.5…0.22 mm thick and covered with zinc and aluminum-zinc coatings
were investigated with the dot-matrix electronic microscope. The coating's
thickness and chemical composition were measured. Picture 1 shows the structure
of a steel sheet with a protection coating.
• 1 • 2 • 3 • 4 • 5 • 6 10
m
Picture 1. –
Structure of a coated steel sheet
From the base of a sheet there are benchmark
points (1,2,3) drawn towards the edge (4,5,6) following which the distribution
of elements was investigated by analyzing energy spectrums of characteristic
X-ray radiation. The quantity X-ray analysis allowed to identify the
composition of phases inside the coatings and to recommend technological
parameters for zinc and aluminum-zinc coatings.
Picture 2 shows the characteristic spectrums of X-ray
radiation obtained from the relevant points (Picture 1). Point 2 corresponds to
an 
phase, whose energy spectrum (Picture 2,a) determines
the composition of the coating and contains 5.1% of zinc and up to 94.8% of
ferrum. The energy spectrum of radiation in point 3 (contains up to 79% of zinc
and up to 21% of ferrum) corresponds to a G phase (picture 2,b). In points 4
and 5, the decoding of spectrums (picture 2,c) allows to identify the chemical
composition of transition phases.
In accordance with the indicated points, one phase
contains up to 91.9% of zinc and up to 8.1% of ferrum and the other up to 93.8%
of zinc and up to 6.2% of ferrum. These concentrations of elements correspond
to 
phases. In point 6 (picture 2,d) only zinc was
identified at the level of 99.9%, corresponding to an 
phase.
Experimental results of composition of intermediary
coating layers are given in Table 1.
Table 1 – Intermediary coating layers
|
Phase |
Type of structure |
Få/Zn composition |
|
1 |
2 |
3 |
|
|
Solid solution of Zn in Fe |
94.8/5.1 |
|
G |
FeZn3; Fe3Zn10;
Fe5Zn21 |
21/79 |
|
|
FeZn7 |
9.07/90.8 |
|
|
FeZn7 + FeZn13 |
8.18/91.7 |
|
|
FeZn13 |
6.1/93.9 |
|
|
Zn |
0.006/99.9 |
Picture 2 – Energy spectrums of
characteristic X-ray radiation:
à) 
phase; b) G phase; c) 
phases; d) 
phase
As is shown above, the more one moves from the coating
surface deep into the stripe (steel base, that is from point 6 to point 1), the
increase in intensity of energy maximums of ferrum and the decrease in energy
maximums of zinc were recorded, which indicated the alteration of zinc and
ferrum quantity in transition from an 
phase to a 
phase.
The structure of the aluminum-zinc coating put onto
the strip steel 08kp 0.5mm thick was also investigated. When scanning the
structure of samples in secondary electrons perpendicular to the coating and
obtaining energy spectrums of characteristic radiation in reflected electrons,
the distribution of elements inside the coating was studied (picture 3).
The investigation of aluminum-zinc coatings was done
in three points from the base of metal to the coating periphery. Thus, point 1
corresponds to the spectrum shown in picture 4,a, point 2 is depicted in
picture 4,b and point 3 in picture 4,c. The analysis of energy spectrums of
characteristic X-ray radiation shows that the intensity of energy maximums of
aluminum grows and the energy maximums of ferrum decrease, as one moves away
from the base of metal.
Picture 3 – Energy spectrum in secondary
electrons
Conclusions:
1. Causes of surface
deficiencies in rolled sheet from steel 08kp and 08ps have been experimentally
identified, which permits to recommend measures to reduce surface deficiencies in
the production of metal products with protection coatings.
2. The micro-structure of zinc and aluminum-zinc
coatings has been studied.
3. The characteristics of each layer of the zinc
coating have been obtained by way of measuring energy spectrums of
characteristic X-ray radiation, allowing to identify phases in the structure of
the zinc coating.
4. Experimental results have
permitted to measure the impact of technological regimes of hot galvanization
on the structure of the coating.
Picture 4 – Energy spectrums of characteristic X-ray radiation