Note: Descriptions are shown in the official language in which they were submitted.
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HIGH-HARDNESS STEEL PRODUCT AND METHOD OF MANUFACTURING THE
SAME
FIELD OF INVENTION
The present invention relates to a high-hardness steel strip product
exhibiting excellent
resistance to climatic corrosion, a good balance of high hardness and
excellent
mechanical properties such as impact strength and bendability. The present
invention
further relates to a method of manufacturing the high-hardness steel strip
product.
BACKGROUND
High hardness has a direct effect on wear resistance of a steel product, the
higher
hardness the better wear resistance. By high hardness it is meant that the
Brinell
hardness is at least 450 HBW and especially in the range of 500 HBW to 650
HBW.
Wear resistant steels are also known as abrasion resistant steels. They are
used in
applications in which high resistance against abrasive and shock wear is
required. Such
applications can be found in e.g. mining and earth moving industry, and waste
transportation. Wear resistant steels are used for instance in gravel truck's
bodies and
excavator buckets, whereby longer service time of the vehicle components are
achieved
due to the high hardness provided by the wear resistant steels. The benefits
of wear
resistant steels are even more crucial when the paint layer on a machine's
outer surface is
frequently exposed to mechanical stresses such as impacts which can cause
scratch to
paint layers.
Such high hardness in steel product is typically obtained by martensitic
microstructure
produced by quench hardening steel alloy having high content of carbon (0.41-
0.50 wt. %)
after austenitization in the furnace. In this process steel plates are first
hot-rolled, slowly
cooled to room temperature from the hot-rolling heat, reheated to
austenitization
temperature, equalized and finally quench hardened. This process is
hereinafter referred
to as the reheating and quenching (RHQ) process. Examples of steels produced
in this
way are wear resistant steels disclosed in CN102199737 or some commercial wear
resistant steels. Due to the relatively high content of carbon, which is
required to achieve
the desired hardness, the resulting martensite reaction causes significant
internal residual
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stresses to the steel. This is because the higher the carbon content the
higher the lattice
distortion. Therefore, this type of steel is very brittle and can even crack
during the quench
hardening. To overcome these drawbacks related to brittleness, a tempering
step after
quench hardening is usually introduced, which however increases the processing
efforts
and costs.
Due to the high carbon content these steels have deteriorated impact strength,
poor
formability or bendability, and low resistance to stress corrosion cracking
(SCC). Stress
corrosion cracking is the cracking induced from the combined influence of
tensile stress
and a corrosive environment. Usually, stress corrosion cracking starts as a
pitting
corrosion with hard-to-detect fine cracks penetrating into the material while
most of the
material surface appears intact. Stress corrosion cracking is classified as a
catastrophic
form of corrosion, as the detection of such fine cracks can be very difficult
and the
damage not easily predicted. There is a need of better approaches to decrease
the
carbon content without compromising the hardness or any of the other
mechanical
properties, such as impact strength, formability/bendability or resistance to
stress
corrosion cracking.
CN102392186 and CN103820717 relate to RHQ steel plates having relatively low
carbon
content (0.25-0.30 wt. % in 0N102392186; 0.22-0.29 wt. % in 0N103820717) and
also
relatively low manganese content. A tempering step after quench hardening is
required for
making such RHQ steel plates, which inevitably increases the processing
efforts and
costs.
EP2695960 relates to an abrasion-resistant steel product exhibiting excellent
resistance to
stress corrosion cracking, which steel sheet can be made in a process where
direct
quenching (DQ) may be performed immediately after hot rolling, without the
reheating
treatment after hot rolling as in the RHQ process. The steel sheet of
EP2695960 has a
relatively low carbon content (0.20-0.30 wt. %) and a relatively high
manganese content
(0.40-1.20 wt. %). In order to increase the resistance to stress corrosion
cracking, the
base phase or main phase of the microstructure of the steel product of
EP2695960 must
be made of tempered martensite. On the other hand, the area fraction of
untempered
.. martensite is restricted to 10% or less because the resistance to stress
corrosion cracking
is reduced in the presence of untempered martensite. In balancing abrasion
resistance
and resistance to stress corrosion cracking, the steel product of EP2695960
has a surface
hardness of 520 HBW or less.
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The present invention extends the utilization of the cost-effective
thermomechanically
controlled processing (TMCP) in conjunction with direct quenching (DQ) to
produce a
high-hardness steel strip product exhibiting improved resistance to climatic
corrosion,
guaranteed impact strength values and excellent formability/bendability.
SUMMARY OF INVENTION
In view of the state of art, the object of the present invention is to solve
the problem of
providing a high-hardness steel strip product exhibiting excellent resistance
to climatic
corrosion, guaranteed impact strength values and excellent
formability/bendability. The
problem is solved by the combination of specific alloy designs with cost-
efficient TMCP
procedures which produces a metallographic microstructure comprising mainly
martensite.
In a first aspect, the present invention provides a hot-rolled steel strip
product comprising
a composition consisting of, in terms of weight percentages (wt. %):
0.17¨ 0.38, preferably 0.21 ¨0.35, more preferably 0.22 ¨0.28
Si 0 ¨ 0.5, preferably 0.01 ¨ 0.5, more preferably 0.03 ¨ 0.25
Mn 0.1 ¨0.4, preferably 0.15 ¨ 0.3
Al 0.015 ¨ 0.15
Cu 0.1 ¨ 0.6, preferably 0.1 ¨ 0.5, more preferably 0.1 ¨ 0.35
Ni 0 ¨ 0.8, preferably 0.2 ¨ 0.8
Cr 0.1 ¨1, preferably 0.3 ¨ 1, more preferably 0.35 ¨ 1,
even more preferably 0.35 ¨ 0.8
Mo 0.01 ¨ 0.3, preferably 0.03 ¨ 0.3, more preferably 0.05 ¨ 0.3
Nb 0 ¨ 0.005
Ti 0 ¨ 0.05, preferably 0 ¨ 0.035, more preferably 0 ¨ 0.02
V 0 ¨ 0.2, preferably 0 ¨ 0.06
0.0005 ¨ 0.005, preferably 0.0008 ¨ 0.005
0 ¨ 0.025, preferably 0.001 ¨ 0.025, more preferably 0.001 ¨ 0.012
0 ¨ 0.008, preferably 0 ¨ 0.005
0 ¨ 0.01, preferably 0 ¨ 0.005, more preferably 0 ¨ 0.004
Ca 0 ¨ 0.01, preferably 0 ¨ 0.005, more preferably 0.0008 ¨ 0.003
remainder Fe and inevitable impurities.
Preferably, the aforementioned composition comprises, in terms of weight
percentages
(wt. %):
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Ti 0 ¨ 0.005
0 ¨ 0.003
Preferably, the aforementioned composition comprises, in terms of weight
percentages
(Wt. %):
Ti > 0.005 and 0.05
> 0.003 and 0.01
Preferably, [Ni] > [Cu]/3, and more preferably [Ni] > [Cu]/2, wherein
[Ni] is the amount of Ni in the composition,
[Cu] is the amount of Cu in the composition.
The steel product is alloyed with the essential alloying elements Si, Cu, Ni
and Cr, which
provides good resistance against climatic corrosion and increases durability
of a paint
layer.
The steel product has a low content of Mn, which is important for improving
impact
toughness and bendability.
The Ca/S ratio is adjusted such that CaS cannot form thereby improving impact
toughness and bendability. The Ca/S ratio is preferably in the range of 1 ¨ 2,
more
preferably 1.1 ¨1.7, and even more preferably 1.2- 1.6.
The level of Nb should be restricted to the lowest possible to increase
formability or
bendability of the steel product. Elements such as Nb may be present as
residual contents
that are not purposefully added.
The difference between residual contents and unavoidable impurities is that
residual
contents are controlled quantities of alloying elements, which are not
considered to be
impurities. A residual content as normally controlled by an industrial process
does not
have an essential effect upon the alloy.
In a second aspect, the present invention provides a method for manufacturing
hot-rolled
steel strip product comprising the following steps of
- providing a steel slab consisting of the chemical composition as
mentioned
previously in the Summary and according to any one of the claims 1 to 5;
- heating the steel slab to the austenitizing temperature of 1200 - 1350
C;
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- hot-rolling to the desired thickness at a temperature in the range of Ar3
to 1300 C,
wherein the finish rolling temperature is in the range of 800 C to 960 C,
preferably 870 C ¨ 930 C, more preferably 885 C ¨ 930 C; and
- direct quenching the hot-rolled steel strip product to a cooling end and
coiling
5 temperature of 450 C or less, preferably 250 C or less, more preferably
150 C
or less, and even more preferably 100 C or less.
Optionally, a step of temper annealing is performed on the direct quenched and
coiled
strip product at a temperature in the range of 150 C - 250 C. However, the
step of
temper annealing is not required according to the present invention.
The steel product is a steel strip having a thickness of 10 mm or less,
preferably 8 mm or
less, and more preferably 7 mm or less.
The obtained steel product has a microstructure comprising, in terms of volume
percentages (vol. %), at least 90 vol. % martensite, preferably at least 95
vol. %
martensite, and more preferably at least 98 vol. % martensite, measured from
1/4 thickness
of the steel strip product. The martensitic structure may be untempered,
autotempered
and/or tempered. Preferably, the martensitic structure is not tempered. More
preferably,
the aforementioned microstructure comprises more than 10 vol. % untempered
martensite.
Preferably, the microstructure comprises 0 ¨ 1 vol. % residual austenite, and
more
preferably 0 ¨ 0.5 vol. % residual austenite. Typically, the microstructure
also comprises
bainite, ferrite and/or pearlite.
The obtained steel product has a prior austenite grain size of 50 pm or less,
preferably 30
pm or less, more preferably 20 pm or less, measured from 1/4 thickness of the
steel strip
product.
The aspect ratio of a prior austenite grain structure is one of the factors
affecting a steel
product's impact toughness and bendability. In order to improve impact
toughness, the
prior austenite grain structure should have an aspect ratio of at least 1.5,
preferably at
least 2, and more preferably at least 3. In order to improve bendability, the
prior austenite
grain structure should have an aspect ratio of 7 or less, preferably 5 or
less, and more
preferably 1.5 or less. The obtained steel product according to the present
invention has a
prior austenite grain structure with an aspect ratio in the range of 1.5 ¨ 7,
preferably 1.5 ¨
5, and more preferably 2 ¨ 5, which ensures that a good balance of excellent
impact
toughness and excellent bendability can be achieved.
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The obtained steel product has a good balance of hardness and other mechanical
properties such as improved resistance to climatic corrosion and excellent
impact strength.
The steel product has at least one of the following mechanical properties:
a Brinell hardness in the range of 420 ¨ 580 HBW, preferably 450 ¨ 550 HBW,
and more
preferably 470 ¨ 530 HBW;
a corrosion index (ASTM G101-04) 5, preferably 5.5, and more preferably 6;
a Charpy-V impact toughness of at least 34 J/cm2 at a temperature of -20 C or
-40 C.
The steel product exhibits excellent bendability or formability. The steel
product has a
.. minimum bending radius of 3.4 t or less in a measurement direction
longitudinal to the
rolling direction wherein the bending axis is longitudinal to rolling
direction; a minimum
bending radius of 2.7 t or less in a measurement direction transversal to the
rolling
direction wherein the bending axis is transversal to rolling direction; and
wherein t is the
thickness of the steel strip product.
The steel product has a good balance of high hardness and excellent mechanical
properties such as impact strength and formability/bendability. Consequently,
the steel
product exhibits excellent resistance to climatic corrosion.
BRIEF DESCRIPTION OF DRAWINGS
Figure 1 illustrates the microstructures.
DETAILED DESCRIPTION OF THE INVENTION
The term "steel" is defined as an iron alloy containing carbon (C).
The term climatic corrosion (a.k.a. atmospheric corrosion) refers to outdoor
corrosion
caused by local environmental conditions. Environmental conditions are formed
from
weather phenomena like rain and sunshine. They are also affected by different
impurities
in the air like chlorides from sea water and sulfur compounds coming from
volcanic activity
and industry or mining.
The term "Brinell hardness (HBVV)" is a designation of hardness of steel. The
Brinell
hardness test is performed by pressing a 10 mm spherical tungsten carbide ball
against a
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clean prepared surface using a 3000 kilogram force, producing an impression,
measured
and given a special numerical value.
The term "corrosion index (ASTM G101-04)" refers to the American Society for
Testing
and Materials (ASTM) standard G101 which is currently the only available guide
to
quantify the atmospheric corrosion resistance of weathering steels as a
function of their
composition.
The term "accelerated continuous cooling (ACC)" refers to a process of
accelerated
cooling at a cooling rate down to a temperature without interruption.
The term "ultimate tensile strength (UTS, Rm)" refers to the limit, at which
the steel
fractures under tension, thus the maximum tensile stress.
The term "yield strength (YS, Rp02)" refers to 0.2 % offset yield strength
defined as the
amount of stress that will result in a plastic strain of 0.2 %.
The term "total elongation (TEL)" refers to the percentage by which the
material can be
stretched before it breaks; a rough indicator of formability, usually
expressed as a
percentage over a fixed gauge length of the measuring extensometer. Two common
gauge lengths are 50 mm (A50) and 80 mm (Aso).
The term "minimum bending radius (RO" is used to refer to the minimum radius
of bending
that can be applied to a test sheet without occurrence of cracks.
The term "bendability" refers to the ratio of Ri and the sheet thickness (t).
The alloying content of steel together with the processing parameters
determines the
microstructure which in turn determines the mechanical properties of the
steel.
Alloy design is one of the first issues to be considered when developing a
steel product
with targeted mechanical properties. Next the chemical composition according
to the
present invention is described in more details, wherein % of each component
refers to
weight percentage.
Carbon C is used in the range of 0.17 % to 0.38 %.
C alloying increases strength of steel by solid solution strengthening, and
hence C
content determines the strength level. C is used in the range of 0.17 % to
0.38%
depending on targeted hardness. If the carbon content is less than 0.17%, it
is difficult
to achieve a Brinell hardness of more than 420 HBW. However, C has detrimental
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effects on weldability, impact toughness, formability or bendability, and
resistance to
stress corrosion cracking. Therefore, C content is set to not more than 0.38
%.
Preferably, C is used in the range of 0.21 % to 0.35 %, and more preferably
0.22 % to
0.28 %.
Silicon Si is used in an amount of 0.5 % or less.
Si is added to the composition to facilitate formation of a protective oxide
layer under
corrosive climate conditions, which provides good resistance against climatic
corrosion
and increases the durability of a paint layer that is easily damaged or
removed from
machines surfaces due to wear. Si is effective as a deoxidizing or killing
agent that can
remove oxygen from the melt during a steelmaking process. Si alloying enhances
strength
by solid solution strengthening, and enhances hardness by increasing austenite
hardenability. Also the presence of Si can stabilize residual austenite.
However, silicon
content of higher than 0.5 % may unnecessarily increase carbon equivalent (CE)
value
thereby weakening the weldability. Furthermore, surface quality may be
deteriorated if Si
is present in excess.
As previously mentioned, Si is an important alloying element for providing
sufficient
hardness and good resistance to climatic corrosion, and for increasing
durability of a paint
layer. Preferably, Si is used in the range of 0.01 % to 0.5 %, and more
preferably 0.03 %
to 0.25 %.
Manganese Mn is used in the range of 0.1 % to 0.4%.
Mn alloying lowers martensite start temperature (Ms) and martensite finish
temperature
(Mf), which can suppress autotempering of martensite during quenching. Reduced
autotempering of martensite leads to higher internal stresses that enhance the
risk for
quench-induced cracking or distortion of shape. Although a lower degree of
autotempered
martensitic microstructures is beneficial to higher hardness, its negative
effects on impact
strength should not be underestimated.
Mn alloying also enhances strength by solid solution strengthening, and
enhances
hardness by increasing austenite hardenability. However, if the Mn content is
too high,
hardenability of the steel will increase at the expense of impact toughness.
Excessive Mn
alloying may also lead to C-Mn segregation and formation of MnS, which could
induce
formation of initiation sites for pitting corrosion and stress corrosion
cracking.
Thus, Mn is used in an amount of at least 0.1 % to ensure hardenability, but
not more than
0.4 % to avoid the harmful effects as described above and to ensure excellent
mechanical
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properties such as impact strength and bendability. Preferably, a low level of
Mn is used in
the range of 0.15% to 0.3%.
Aluminum Al is used in the range of 0.015% to 0.15%.
Al is effective as a deoxidizing or killing agent that can remove oxygen from
the melt
during a steelmaking process. Al also removes N by forming stable AIN
particles and
provides grain refinement, which is beneficial to high toughness, especially
at low
temperatures. Also Al stabilizes residual austenite. However, an excess of Al
may
increase non-metallic inclusions thereby deteriorating cleanliness.
Copper Cu is used in the range of 0.1 % to 0.6 %.
Cu is added to the composition to facilitate formation of a protective oxide
layer under
corrosive climate conditions, which provides good resistance against climatic
corrosion
and increases the durability of a paint layer that is easily damaged or
removed from
machines surfaces due to wear. Cu may promote formation of low carbon bainitic
structures, cause solid solution strengthening and contribute to precipitation
strengthening.
Cu may also have beneficial effects of inhibiting stress corrosion cracking.
When added in
excessive amounts, Cu deteriorates field weldability and the heat affected
zone (HAZ)
toughness. Therefore, the upper limit of Cu is set to 0.6%.
As previously mentioned, Cu is an important alloying element for providing
sufficient
hardness and good resistance to climatic corrosion, and for increasing
durability of a paint
layer. Preferably, Cu is used in the range of 0.1 % to 0.5 %, and more
preferably 0.1 % to
0.35 %.
Nickel Ni is used in in an amount of 0.8 % or less.
Ni is used to avoid quench induced cracking and also to improve low
temperature
toughness. Ni is an alloying element that improves austenite hardenability
thereby
increasing strength with no or marginal loss of impact toughness and/or HAZ
toughness.
Ni also improves surface quality thereby preventing pitting corrosion, i.e.
initiation site for
stress corrosion cracking. Ni is added to the composition to facilitate
formation of a
protective oxide layer under corrosive climate conditions, which provides good
resistance
against climatic corrosion and increases the durability of a paint layer that
is easily
damaged or removed from machines surfaces due to wear. However, nickel
contents of
above 0.8 % would increase alloying costs too much without significant
technical
improvement. An excess of Ni may produce high viscosity iron oxide scales
which
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deteriorate surface quality of the steel product. Higher Ni contents also have
negative
impacts on weldability due to increased CE value and cracking sensitivity
coefficient.
As previously mentioned, Ni is an important alloying element for providing
sufficient
hardness and good resistance to climatic corrosion with no or marginal loss of
impact
5 toughness, and for increasing durability of a paint layer. Ni is
preferably used in the range
of 0.2% to 0.8%.
Chromium Cr is used in the range of 0.1 % to 1 %.
Cr is added to the composition to facilitate formation of a protective oxide
layer under
corrosive climate conditions, which provides good resistance against climatic
corrosion
10 and increases the durability of a paint layer that is easily damaged or
removed from
machines surfaces due to wear. Cr alloying provides better resistance against
pitting
corrosion thereby preventing stress corrosion cracking at an early stage. As
mid-strength
carbide forming element Cr increases the strength of both the base steel and
weld with
marginal expense of impact toughness. Cr alloying also enhances strength and
hardness
by increasing austenite hardenability. However, if Cr is used in an amount
above 1 % the
HAZ toughness as well as field weldability may be adversely affected.
As previously mentioned, Cr is an important alloying element for providing
sufficient
hardness and good resistance to climatic corrosion with no or marginal loss of
impact
toughness, and for increasing durability of a paint layer. Preferably, Cr is
used in the
range of 0.3 % to 1 %, more preferably 0.35 % to 1 %, and even more preferably
0.35 %
to 0.8 %.
Molybdenum Mo is used in the range of 0.01 % to 0.3 %.
Mo alloying improves impact strength, low-temperature toughness and tempering
resistance. The presence of Mo enhances strength and hardness by increasing
austenite
hardenability. Mo can be added to the composition to provide hardenability in
place of Mn.
In the case of B alloying, Mo is usually required to ensure the effectiveness
of B. However,
Mo is not an economically acceptable alloying element. If Mo is used in an
amount of
above 0.3 % toughness may be deteriorated thereby increasing the risk of
brittleness. An
excessive amount of Mo may also reduce the effect of B. Furthermore, the
inventors have
noticed that Mo alloying retards recrystallization of austenite thereby
increasing the aspect
ratio of a prior austenite grain structure. Therefore, the level of Mo content
should be
carefully controlled to prevent excessive elongation of the prior austenite
grains which
may deteriorate bendability of the steel product.
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Preferably, Mo is used in the range of 0.03 % to 0.3 %, and more preferably
0.05 % to
0.3 %.
Niobium Nb is used in an amount of 0.005 % or less.
Nb forms carbides NbC and carbonitrides Nb(C,N). Nb is considered to be the
major grain
.. refining element. Nb contributes to strengthening and toughening of steels.
Yet, Nb
addition should be limited to 0.005 % since an excess of Nb deteriorates
bendability, in
particular when direct quenching is applied and/or when Mo is present in the
composition.
Furthermore, Nb can be harmful for HAZ toughness since Nb may promote the
formation
of coarse upper bainite structure by forming relatively unstable TiNbN or
TiNb(C,N)
precipitates. The level of Nb should be restricted to the lowest possible to
increase
formability or bendability of the steel product.
Titanium Ti is used in an amount of 0.05 % or less.
TiC precipitates are able to deeply trap a significant amount of hydrogen H,
which
decreases the H diffusivity in the materials and removes some of the
detrimental H from
the microstructure to prevent stress corrosion cracking. Ti is also added to
bind free N that
is harmful to toughness by forming stable TiN that together with NbC can
efficiently
prevent austenite grain growth in the reheating stage at high temperatures.
TiN
precipitates can further prevent grain coarsening in the HAZ during welding
thereby
improving toughness. TiN formation suppresses BN precipitation, thereby
leaving B free to
make its contribution to hardenability. For this purpose, the ratio of Ti/N is
at least 3.4.
However, if Ti content is too high, coarsening of TiN and precipitation
hardening due to
TiC develop and the low-temperature toughness may be deteriorated. Therefore,
it is
necessary to restrict titanium so that it is less than 0.05%.
Preferably, Ti is used in an amount of 0.035 % or less, and more preferably
0.02 % or less.
If the steel product has a low nitrogen content of 0.003 % or less, it is
unnecessary to add
Ti to ensure the boron hardenability effect, and the Ti content can be as low
as 0.005 % or
less. If the nitrogen content is more than 0.003 % but no more than 0.01%, the
Ti content
can be more than 0.005 % but no more than 0.05%.
Vanadium V is used in an amount of 0.2 % or less.
V has substantially the same but smaller effects as Nb. V403 precipitates are
able to
deeply trap a significant amount of hydrogen H, which decreases the H
diffusivity in the
materials and removes some of the detrimental H from the microstructure to
prevent HIC.
V is a strong carbide and nitride former, but V(C,N) can also form and its
solubility in
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austenite is higher than that of Nb or Ti. Thus, V alloying has potential for
dispersion and
precipitation strengthening, because large quantities of V are dissolved and
available for
precipitation in ferrite. However, an addition of more than 0.2 % V has
negative effects on
weldability and hardenability.
Preferably, V is used in an amount of 0.06 % or less.
Boron B is used in the range of 0.0005 % to 0.005 %.
B is a well-established microalloying element to increase hardenability. The
most effective
B alloying would preferably require the presence of Ti in an amount of at
least 3.42 N to
prevent formation of BN. In the presence of an amount of 0.003 % or less
nitrogen, the Ti
content can be lowered to 0.005 % or less, which is beneficial to low-
temperature
toughness. Hardenability deteriorates if the B content exceeds 0.005 %.
Preferably, B is used in the range of 0.0008 % to 0.005 %.
Calcium Ca is used in an amount of 0.01 % or less.
Ca addition during a steelmaking process is for refining, deoxidation,
desulphurization,
and control of shape, size and distribution of oxide and sulphide inclusions.
Ca is usually
added to improve subsequent coating. However, an excessive amount of Ca should
be
avoided to achieve clean steel thereby preventing the formation of calcium
sulfide (CaS)
or calcium oxide (CaO) or mixture of these (Ca0S) that may deteriorate the
mechanical
properties such as bendability and SCC resistance.
Preferably, Ca is used in an amount of 0.005 % or less, and more preferably
0.0008 % to
0.003 % to ensure excellent mechanical properties such as impact strength and
bendability.
The Ca/S ratio is adjusted such that CaS cannot form thereby improving impact
toughness and bendability. The inventors have noticed that, in general, during
the
steelmaking process the optimal Ca/S ratio is in the range of 1 ¨2, preferably
1.1 ¨1.7,
and more preferably 1.2 - 1.6 for clean steel.
Unavoidable impurities can be phosphor P, sulfur S, nitrogen N. Their content
in terms of
weight percentages (wt. %) is preferably defined as follows:
0 ¨ 0.025, preferably 0.001 ¨ 0.025, more preferably 0.001 ¨ 0.012
5 0 ¨ 0.008, preferably 0 ¨ 0.005, more preferably 0 ¨ 0.002
0 ¨ 0.01, preferably 0 ¨ 0.005, more preferably 0 ¨ 0.004
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Other inevitable impurities may be hydrogen H, oxygen 0 and rare earth metals
(REM) or
the like. Their contents are limited in order to ensure excellent mechanical
properties,
such as impact toughness.
Austenite to martensite transformation in steels depends largely on the
following factors:
chemical composition and some processing parameters, mainly reheating
temperature,
cooling rate and cooling temperature. With regard to chemical composition,
some alloying
elements have a greater impact than others while others have a negligible
impact.
Equations describing austenite hardenability may be used to assess the impact
of
different alloying elements on martensite formation during cooling. One such
equation is
presented below. From this equation we can see that carbon has the biggest
impact, Mn,
Mo and Cr have an intermediate impact while Si and Ni have a lesser impact.
Furthermore,
the equation shows that any single element is not crucial for martensite
formation and thd
the absence of one element may be compensated with the amount of other
alloying
elements and processing parameters, such as e.g. cooling rate.
cr = 2 1
D =6xe lx C+ +-
5.87 ib i , j
The steel product with the targeted mechanical properties is produced in a
process that
determines a specific microstructure which in turn dictates the mechanical
properties of
the steel product.
.. The first step is to provide a steel slab by means of, for instance a
process of continuous
casting, also known as strand casting.
In the reheating stage, the steel slab is heated to the austenitizing
temperature of 1200-
1350 C, and thereafter subjected to a temperature equalizing step that may
take 30 to
150 minutes. The reheating and equalizing steps are important for controlling
the
.. austenite grain growth. An increase in the heating temperature can cause
dissolution and
coarsening of alloy precipitates, which may result in abnormal grain growth.
The final steel product has a prior austenite grain size of 50 pm or less,
preferably 30 pm
or less, more preferably 20 pm or less, measured from 1/4 thickness of the
steel strip
product.
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14
In the hot rolling stage the slab is hot rolled to the desired thickness at a
temperature in
the range of Ar3 to 1300 C, wherein the finish rolling temperature (FRT) is in
the range of
800 C to 960 C, preferably 870 C ¨ 930 C, more preferably 885 C ¨ 930 C.
The aspect ratio of a prior austenite grain structure is one of the factors
affecting a steel
product's impact toughness and bendability. In order to improve impact
toughness, the
prior austenite grain structure should have an aspect ratio of at least 1.5,
preferably at
least 2, and more preferably at least 3. In order to improve bendability, the
prior austenite
grain structure should have an aspect ratio of 7 or less, preferably 5 or
less, and more
preferably 1.5 or less. A desired aspect ratio of prior austenite grains can
be achieved by
adjusting a number of parameters such as finish rolling temperature,
strain/deformation,
strain rate, and/or alloying with the elements such as Mo that retard
recrystallization of
austenite.
The obtained steel product according to the present invention has a prior
austenite grain
structure with an aspect ratio in the range of 1.5 ¨ 7, preferably 1.5 ¨ 5,
and more
preferably 2 ¨ 5, which ensures that a good balance of excellent impact
toughness and
excellent bendability can be achieved.
The obtained steel strip product has a thickness of 10 mm or less, preferably
8 mm or less,
more preferably 7 mm or less.
The hot-rolled steel strip product is direct quenched to a cooling end and
coiling
temperature of 450 C or less, preferably 250 C or less, more preferably 150
C or less,
and even more preferably 100 C or less. The cooling rate is at least 30 C/s.
The direct quenched steel strip product is coiled at temperature of 450 C or
less,
preferably 250 C or less, more preferably 150 C or less, and even more
preferably
100 C or less.
The obtained steel strip product has a microstructure comprising, in terms of
volume
percentages (vol. %), at least 90 vol. % martensite, preferably at least 95
vol. %
martensite, and more preferably at least 98 vol. % martensite, measured from
1/4 thickness
of the steel strip product. The martensitic structure may be untempered,
autotempered
and/or tempered. Preferably, the martensitic structure is not tempered. More
preferably,
the aforementioned microstructure comprises more than 10 vol. % untempered
martensite.
Preferably, the microstructure comprises 0 ¨ 1 vol. % residual austenite, and
more
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preferably 0 ¨ 0.5 vol. % residual austenite. Typically, the microstructure
also comprises
bainite, ferrite and/or pearlite.
Optionally, an extra step of temper annealing is performed at a temperature in
the range
of 150 C - 250 C.
5 The steel strip product has a good balance of hardness and other
mechanical properties
such as excellent impact strength, improved resistance to climatic corrosion
and excellent
formability/bendability.
The steel strip product has a high Brinell hardness in the range of 420 ¨ 580
HBW,
preferably 450 ¨ 550 HBW, and more preferably 470 ¨ 530 HBW.
10 .. The steel strip product has a corrosion index (ASTM G101-04) of at least
5, preferably at
least 5.5, and more preferably at least 6, which indicates improved resistance
against
climatic corrosion. The durability of a paint layer is increased and the
repainting interval
can be 1.5 ¨ 2 times longer by using the steel product of the invention.
The corrosion index (ASTM G101-04) is used for estimating long term
atmospheric
15 corrosion of low alloy steels in various environments. The corrosion
index (ASTM G101-
04) equation is formed with a statistical method from long term outdoor
corrosion
exposure tests, which equation is represented as follows.
IASTMG101 = 26.01(%Cu) + 3.88(%Ni) + 1.20(%Cr) + 1.49(%Si) + 17.28(%P) ¨
7.29(%Cu)(%Ni) ¨ 9.10(%Ni)(%P) ¨ 33.39(%CU)2
The steel strip product with high hardness has a Charpy-V impact toughness of
at least 34
J/cm2at a temperature of -20 C or -40 C thereby fulfilling the conventional
impact
strength requirements.
The steel strip product exhibits excellent bendability or formability. The
steel product has a
minimum bending radius of 3.4 t or less in a measurement direction
longitudinal to the
rolling direction wherein the bending axis is longitudinal to rolling
direction; a minimum
bending radius of 2.7 t or less in a measurement direction transversal to the
rolling
direction wherein the bending axis is transversal to rolling direction; and
wherein t is the
thickness of the steel strip product.
The following examples further describe and demonstrate embodiments within the
scope
of the present invention. The examples are given solely for the purpose of
illustration and
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16
are not to be construed as limitations of the present invention, as many
variations thereof
are possible without departing from the scope of the invention.
The chemical compositions used for producing the tested steel strip products
are
presented in Table 1.
The manufacturing conditions for producing the tested steel strip products are
presented
in Table 2.
The mechanical properties of the tested steel strip products are presented in
Table 3.
Microstructure
Microstructure can be characterized from SEM micrographs and the volume
fraction can
be determined using point counting or image analysis method. The
microstructures of the
tested inventive examples no. 1 ¨ 4 all have a main phase of at least 90 vol.
% martensite.
Figure 1 is an SEM image on the RD-ND plane from 1/4 thickness of the steel
strip no. 1,
where the prior austenite grain boundaries are visualized. The prior austenite
grain
structure of the steel strip no. 1 has an aspect ratio of 3.4.
Brinell hardness HBW
The Brinell hardness test is performed by pressing a 10 mm spherical tungsten
carbide
ball against a clean prepared surface using a 3000 kilogram force, producing
an
impression, measured and given a special numerical value. The measurement is
done
perpendicular to the upper surface of the steel sheet at 10 - 15 % depth from
the steel
surface. As shown in Table 3, each one of the inventive examples no. 1 ¨ 4
exhibits a
Brinell harness in the range of 475 ¨ 491 HBW. The comparative example no. 5
exhibits a
Brinell harness of 486 HBW while the comparative example no. 6 exhibits a
Brinell
harness of 469 HBW.
Corrosion index (ASTM G101-04)
The corrosion index (ASTM G101-04) is calculated based on the American Society
for
Testing and Materials (ASTM) standard G101. As shown in Table 3, each one of
the
inventive examples no. 1 ¨4 has a corrosion index (ASTM G101-04) of at least
5.28. On
the other hand, the comparative examples no. 5 and 6 have a much lower
corrosion index
(ASTM G101-04) of 3.4 and 1.04 respectively.
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Charpy-V impact toughness
The impact toughness values at -20 C or -40 C were obtained by Charpy V-
notch tests
according to the ASME (American Society of Mechanical Engineers) Standards.
The
inventive examples no. 1 and 2 have a Charpy-V impact toughness of 63 J/cm2
and 45
J/CM2 respectively at a temperature of -20 C (Table 3). Each one of the
inventive
examples no. 1 ¨4 has a Charpy-V impact toughness in the range of 38 - 120
J/cm2 at a
temperature of -40 C if the measurement direction is longitudinal to the
rolling direction.
Each one of the inventive examples no. 1 ¨ 4 has a Charpy-V impact toughness
in the
range of 58 - 105 J/cm2 at a temperature of -40 C if the measurement
direction is
transversal to the rolling direction. The impact toughness of the inventive
examples no. 1
¨ 4 is improved compared to the comparative example no. 6. The comparative
example
no. 5 has a better Charpy-V impact toughness values than the inventive
examples no. 1
and 2 at the expense of bendability.
Elongation
Elongation was determined according ASTM E8 standard using transverse
specimens of
a produced batch of 2000 ton of plates. The mean value of total elongation
(A50) of the
inventive examples no. 1 and 2 is 11.6 and 11.3 respectively (Table 3), which
is better
than the comparative examples no. 5 and 6 having a mean A50 value of 10.1 and
9.1
respectively. The comparative examples no. 5 and 6 have better A50 values than
the
inventive examples no. 3 and 4 at the expense of Charpy-V impact toughness.
BendabilitY
The bend test consists of subjecting a test piece to plastic deformation by
three-point
bending, with one single stroke, until a specified angle 90 of the bend is
reached after
unloading. The inspection and assessment of the bends is a continuous process
during
the whole test series. This is to be able to decide if the punch radius (R)
should be
increased, maintained or decreased. The limit of bendability (R/t) for a
material can be
identified in a test series if a minimum of 3 m bending length, without any
defects, is
fulfilled with the same punch radius (R) both longitudinally and
transversally. Cracks,
surface necking marks and flat bends (significant necking) are registered as
defects.
According to the bend tests, each one of the inventive examples no. 1 ¨ 4 has
a minimum
bending radius of 3.3 t or less in a measurement direction longitudinal to the
rolling
direction; a minimum bending radius of 2.6 t or less in a measurement
direction
transversal to the rolling direction; and wherein t is the thickness of the
steel strip product
(Table 3). The comparative example no. 5 exhibits lower bendability with a
minimum
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18
bending radius of 3.7 tin a measurement direction longitudinal to the rolling
direction and
a minimum bending radius of 2.2 tin a measurement direction transversal to the
rolling
direction.
Yield strength
Yield strength was determined according ASTM E8 standard using transverse
specimens
of a produced batch of 2000 ton of plates. Each one of the inventive examples
no. 1 ¨ 4
has a mean value of yield strength (Rp02) in the range of 1302 MPa to 1399
MPa,
measured in the longitudinal direction (Table 3). The comparative examples no.
5 and 6
have a mean value of yield strength (Rp02) of 1262 MPa and 1338 MPa
respectively,
measured in the longitudinal direction (Table 3).
Tensile strength
Tensile strength was determined according ASTM E8 standard using transverse
specimens of a produced batch of 2000 ton of plates. Each one of the inventive
examples
no. 1 ¨4 has a mean value of ultimate tensile strength (Rm) in the range of
1509 MPa to
1566 MPa, measured in the longitudinal direction (Table 3). The comparative
examples no.
5 and 6 have a mean value of ultimate tensile strength (Rm) of 1550 MPa and
1552 MPa
respectively, measured in the longitudinal direction (Table 3).
C
Table 1. Chemical compositions (wt. %).
t..)
t..)
o
i-J
Steel type C Si Mn P S Al Cu Ni Cr Mo Nb Ti
V B Ca (ppm) N (ppm) Remarks =
1-
.6.
A 0,251 0,098 0,246 0,008 0,0016 0,094 0,30 0,493 0,718 0,098 0
0,016 0,04 0,0018 23 39 Inventive example t,.)
oo
B 0,23 0,179 0,200 0,007 -0,0006 0,051 0,16 0,51
0,39 0,05 0,001 0,002 0,01 0,0011 8 24 Inventive example
C 0,233 0,179 0,714 0,009 0,0006 0,035 0,009 0,506 0,713 0,067 0
0,017 0,008 0,0017 21 31 Comparative example
D 0,262 0,175 1,19 0,008 0,0002 0,048 0,01 0,035 0,212 0,005 0
0,015 0,008 0,0014 30 21 Comparative example
8
P
.
,
Table 2. Manufacturing conditions
,
Steel Steel type Strip Hot rolling
Cooling rate Coiling temperature Temper annealing Remarks "
,
,
strip no. thickness Heating temperature FRT
( C/s) ( C) Heating
temperature Holding time
, ,
(mm) ( C) ( C)
( C) (h) " ,
1 A 6 1280 895 70 50
- - Inventive example
2 A 6 1280 925 70 50
- - Inventive example
3 B 6 1280 900 - 50
200 8 Inventive example
4 B 3 1280 905 - 50
200 8 Inventive example
C 6 1280 870 55 50
1-d
Comparative example
n -
_
6 D 6 1280 915 55 50
- - Comparative example
IV
t.)
o
t.)
o
O'
vi
o
4=.
t.)
4=.
0
Table 3. Mechanical properties
Steel Steel type Con. HBW Rpo 2 (L) Rm (L) A50 ChV (-20) T ChV (-
40) L ChV (-40) T Bending r/t Remarks
strip no. Index (MPa) (MPa) (J/cm2) (J/cm2)
(Jicr112) longit. transv.
oe
1 A 6.74 487 1399 1566 11,6 63 63
80 3,3 2,0 Inventive example
2 A 6.74 491 1337 1529 11,3 45 38
58 3,0 2,0 Inventive example
3 B 5.28 475 1355 1509 6,9 120
83 2,3 1,3 Inventive example
4 B 5.28 487 1302 1549 8,8 120
105 2,6 2,6 Inventive example
C 3.40 486 1262 1550 9,4 73 68 83
3,7 2,2 Comparative example
6 D 1.04 469 1338 1552 10,0 32 30
42 - Comparative example
L."
