STEEL PRODUCT WITH IMPROVED WEATHERING CHARACTERISTICS IN SALINE ENVIRONMENT

PRODUIT D'ACIER POSSEDANT DES CARACTERISTIQUES DE TENUE AUX INTEMPERIES AMELIOREES DANS UN ENVIRONNEMENT SALIN

English Abstract

The present invention is related to a steel product of the weathering steel type, i.e. a product exhibiting increased corrosion resistance after having been exposed to the outside environment for a given time. According to the invention, the steel product is defined by a composition consisting of (in wt%) : - Carbon between 0.03 and 0.2, - Manganese between 0.5 and 2, - Copper between 0 and 0.5, - Ti between 0 and 0.1, - Cr between 0 and 0.5, - Nickel between 0 and 0.2, - Niobium between 0 and 0.1, - Nitrogen between 0 and 0.01, - Sulphur between 0 and 0.01, Phosphor between 0 and 0.01, - Aluminium higher than 0 and maximum 1.5, - Silicon between 0.25 and 1.5, the balance being Fe and accidental impurities, and wherein the sum of the Al and Si levels is higher than about 0.85wt%.

French Abstract

La présente invention a pour objet un produit d'acier du type acier résistant aux intempéries, c'est-à-dire un produit présentant une résistance à la corrosion accrue après avoir été exposé à l'environnement extérieur pendant un temps donné. Selon l'invention, le produit d'acier est défini par une composition constituée (en % en poids) : - de carbone entre 0,03 et 0,2, - de manganèse entre 0,5 et 2, - de cuivre entre 0 et 0,5, - de Ti entre 0 et 0,1, - de Cr entre 0 et 0,5, - de nickel entre 0 et 0,2, - de niobium entre 0 et 0,1, - d'azote entre 0 et 0,01, - de soufre entre 0 et 0,01, de phosphore entre 0 et 0,01, - d'aluminium, plus de 0 et au maximum de 1,5, - de silicium entre 0,25 et 1,5, le reste étant constitué de Fe et d'impuretés accidentelles, et la somme des niveaux d'Al et de Si étant supérieure à environ 0,85 % en poids.

Claims

8
CLAIMS
1. A steel product consisting of, in wt%:
- Carbon between 0.03 and 0.2,
- Manganese between 0.5 and 2,
- Copper between 0.2 and 0.4,
- Titanium between 0.01 and 0.07,
- Chromium between 0.2 and 0.5,
- Nickel between 0.1 and 0.2,
- Niobium between 0.02 and 0.1,
- Nitrogen between 0 and 0.01,
- Sulphur between 0 and 0.01,
- Phosphorus between 0 and 0.01,
- Aluminium higher than 0 and maximum 1.5,
- Silicon between 0.25 and 1.5,
the balance being Fe and impurities, and wherein the sum of
the Al and Si levels is higher than 0.85wt%.
2. The steel product according to claim 1,
comprising no Sn and no Sb.
3. A steel product according to any one of
claims 1 and 2, said product being a sheet or beam.
4. Use of a product according to any one of
claims 1 to 3 in saline outdoor conditions.

Description

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STEEL PRODUCT WITH IMPROVED WEATHERING CHARACTERISTICS IN
SALINE ENVIRONMENT
Field of the Invention
[0001] The present invention is related to steel
types and products made thereof, which exhibit an increase
in corrosion resistance when exposed to a chlorine-rich
environment. These steels are known as weathering steels.
State of the art
[0002] Weathering steels have been studied and
documented for some time. The corrosion resistance is
caused by a layer of rust that forms on the steel surface
when it is exposed to the outside environment. Traditional
alloying elements which enhance this type of weathering
resistance are Cu, P, Cr and Ni. However, traditional
weathering steels haven proven to be ineffective in an
environment containing Cl ions, i.e. marine or seaside
environments. It was observed that the Cl ions destabilize
some of the stable components that form the protective
oxide layer. Solutions to this problem have been proposed,
mainly in the form of changes in the steel composition.
The best results have been achieved so far by applying
significantly higher levels of Ni. However, given that a
high amount of Ni is needed and taking into account that
the price of Ni is a highly critical parameter in the
commercial exploitation of steels, this solution is not
regarded as ideal, and a need has arisen for an alternative
to the Ni-solution.

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[0003] One
solution is described in JP2006118011, in
the form of a steel composition comprising C, Al, Si, P,
Ni, Cu, N, with Sn and/or Sb added in an amount between
0.03 and 0.5wt% and one or more of Ti, Nb, Mo, W, V, Ca or
Mg, with the mass ratio of Ni/Cu smaller than or equal to
0.5. The
weathering capability in saline environment is
mainly obtained by the presence of Sb and/or Sn, which
stabilize the amount of Fe2+ ions when the steel is in a
wet and saline environment, by reacting with Fe3+ ions,
produced by oxidation of Fe2+.
However, the use of Sn
and/or Sb is disadvantageous for various reasons :
= both Sn and Sb have a detrimental effect on hot
ductility as they decrease the solubility of Cu in
austenite at high temperature
= Sb has a detrimental effect on weld toughness
= Sn can not be removed from the steel during refining
= Sb causes a risk of forming Stibine (SbH3) - a toxic and
flammable compound.
Alms of the invention
[0004] The
present invention aims to provide a
weathering steel with a Ni-content in the same order of
magnitude as in traditional weathering steels, and with an
increased corrosion resistance in marine or seaside
conditions, as well as with excellent mechanical
properties.
Summary of the invention
[0005] The
invention is related to a steel product as
disclosed. The invention is in particular related to a
steel product suitable for use in outdoor environments, in
particular in saline conditions. The

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steel product of the invention may be any type of
construction element such as a steel sheet or beam. In
particular, the invention is related to a steel composition
characterized by the following contents (in wt%) :
- Carbon between 0.03 and 0.2,
- Manganese between 0.5 and 2,
- Copper between 0 and 0.5,
- Ti between 0 and 0.1,
- Cr between 0 and 0.5,
- Nickel between 0 and 0.2,
- Niobium between 0 and 0.1,
- Nitrogen between 0 and 0.01,
- Sulphur between 0 and 0.01,
- Phosphor between 0 and 0.01,
- Aluminium higher than 0 and maximum 1.5,
- Silicon between 0.25 and 1.5,
the balance being Fe and accidental impurities, and wherein
the sum of the Al and Si levels is higher than about
0.85wt%.
[0006]
According to a preferred embodiment, the
following narrower ranges are defined for Cu, Ti, Cr, Ni
and Nb (in wt%) :
- Copper between 0.2 and 0.4
- Ti between 0.01 and 0.07,
- Cr between 0.2 and 0.5,
- Nickel between 0.1 and 0.2,
- Niobium between 0.02 and 0.1.
[0007]
According to the preferred embodiment, the
steel product according to the invention comprises no Sn
and no Sb.

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Detailed description of the invention
[0008] The present invention is related to a steel
product of the weathering steel type, i.e. a product
exhibiting increased corrosion resistance after having been
exposed to the outside environment for a given time.
[0009] According to the invention, the steel product
is defined by a composition consisting of (in wt%):
¨ Carbon between 0.03 and 0.2,
¨ Manganese between 0.5 and 2,
¨ Copper between 0 and 0.5,
¨ Ti between 0 and 0.1,
¨ Cr between 0 and 0.5,
¨ Nickel between 0 and 0.2,
¨ Niobium between 0 and 0.1,
¨ Nitrogen between 0 and 0.01,
¨ Sulphur between 0 and 0.01,
¨ Phosphor between 0 and 0.01,
¨ Aluminium higher than 0 and maximum 1.5,
¨ Silicon between 0.25 and 1.5,
the balance being Fe and accidental impurities, and wherein
the sum of the Al and Si levels is higher than 0.85wt%.
The word 'between' as used above is to be understood as
inclusive of the borders of the range in question.
[0010] According to a preferred embodiment, the
amounts of Cu, Ti, Cr, Ni and Nb are as follows (in wt%) :
¨ Copper between 0.2 and 0.4
¨ Ti between 0.01 and 0.07 ,
¨ Cr between 0.2 and 0.5 .
¨ Nickel between 0.1 and 0.2,
¨ Niobium between 0.02 and 0.1,
This embodiment combines good weathering behaviour in
saline conditions with excellent mechanical properties, due

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to the minimum amounts of the abovenamed elements (Cu, Ti,
Cr, Ni, Nb).
[0011] The
present invention reveals that the
inventive steel compositions favour the formation of fine
5 grained goethite (a-Fe0OH) in the corrosion layer forming
on the surface of a steel product having these
compositions, due to the specific combined Al and Si
content.
With 'fine grained goethite' is meant goethite
with grain sizes from about 8nm to about 15nm. The
formation of such fine grained goethite is beneficial for
the stability of the oxide layer in saline conditions.
[0012] It
is to be noted that the inventive
compositions do not necessarily include an addition of
significant amounts of the traditional elements regarded as
beneficial for weathering resistance in saline conditions,
such as Ni. The inventive compositions do not comprise Sn
and Sb above an accidental impurity level. Preferably, the
steel product of the invention comprises no Sb or Sn.
Elements such as Nb, Ti, Cu, Cr may be added for improving
the mechanical properties of the steel, but are not
required for ensuring the weathering behaviour in saline
conditions.
[0013] The
invention is related to any type of steel
product having the composition according to the invention,
e.g. steel sheets or beams, obtained by any standard steel-
making process, e.g. hot-rolled or cold-rolled sheets or
beams.
Examples
[0014] Table
1 shows the compositions of 3 test
samples Si to S3 having Al and Si contents according to the
invention, as well as 2 comparative samples Cl and C2
having Al and Si contents falling outside of the present

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invention and/or wherein the combined Al and Si content is
less than 0.85wt%. The
samples are outside the subject
matter of the invention because of the presence of
Molybdenum in 31 and the lower levels of Ti, Cu, Ni, Cr and
Nb. They are however illustrative of the influence of the
combined presence of Al and Si on the goethite formation,
and hence on the weathering steel behaviour of products of
the invention.
[0015] The
test samples have been obtained by
standard steel processing methods, involving at least a hot
rolling step, possibly followed by a cold rolling step.
The samples were subjected to a dip-dry test during 60
days, using a 0.05M NaC1 solution, in order to test
weatherability behaviour in saline conditions. As
a
result, oxide layers were formed on the samples. The
composition of these oxide layers was investigated by
Mossbauer spectroscopy, used in particular to measure the
relative area of superparamagnetic goethite in the oxide
layer.
Superparamagnetic goethite has a particle size
between 8 and 15nm, as opposed to antiferromagnetic
goethite, having particle sizes above 15nm.
(0016]
Table 2 clearly shows that the combined Al
and Si contents according to the present invention give
rise to a considerable increase in the relative area of
fine-grained goethite in the oxide layer.

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Sample C Si Al P Mn S N Ti Mo Cu Ni Cr
Si 0.19 0.35 1.2 0.077 1.6 0.006 0.005 0.007 0.025 0.02 0.025 0.025
S2 0.07 1 0.03 0.02 0.5 0.003 0.003 - 0.3 -

S3 0.07 0.25 0.8 0.02 0.5 0.003 0.003 - 0.3 -
Cl 0.07 0.25 0.4 0.02 0.5 0.003 0.003 - 0.3 -
C2 0.03 0 1 0.01 1.8 0.003 0.007 - 0.4 -
Table 1 : example compositions (in wt%)
Sample %
Si 51
S2 40
S3 29
Cl 15
C3 16
Table 2:Relative area of superparamagnetic goethite (in %)