Note: Descriptions are shown in the official language in which they were submitted.
CA 03135141 2021-09-27
WO 2020/201437 PCT/EP2020/059423
1
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 a good
balance of high hardness and excellent mechanical properties such as impact
strength
and formability/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 is
achieved
due to the high hardness provided by the wear resistant steels.
Wear resistant steels can also function as structural steels for making
construction
components if the wear resistant steels have sufficient mechanical properties
such as
formability, weldability and fatigue resistance that comply with national
standards. The
advantage of using wear resistant steels in the structural part for
construction purposes is
that less welding is needed and the weight can be lowered.
Such high hardness in a 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
CA 03135141 2021-09-27
WO 2020/201437 PCT/EP2020/059423
2
the desired hardness, the resulting martensite reaction causes significant
internal residual
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. 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. To overcome these drawbacks, a tempering
step
after quench hardening can be introduced to improve mechanical properties.
This
however increases the processing efforts and costs.
0N102392186 and 0N103820717 relate to RHQ steel plates having relatively low
carbon
content (0.25-0.30 wt. % in CN102392186; 0.22-0.29 wt. % in CN103820717) 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.
SUMMARY OF INVENTION
The present invention extends the utilization of the cost-effective
thermomechanically
controlled processing (TMCP) in conjunction with direct quenching (DQ) and
possibly also
tempering to produce a high-hardness steel strip product exhibiting excellent
formability/bendability and impact strength values.
CA 03135141 2021-09-27
WO 2020/201437 PCT/EP2020/059423
3
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
formability/bendability
and impact strength values. 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. %):
0.14 ¨ 0.35, preferably 0.17 ¨ 0.31, more preferably 0.20 ¨ 0.28
Si 0 ¨ 0.5, preferably 0.01 ¨ 0.50, more preferably 0.03 ¨ 0.25
Mn 0.05 ¨ 0.40, preferably 0.05 ¨ 0.30
Al 0 ¨ 0.1, preferably 0 ¨ 0.08
Cu 0.1 ¨0.4, preferably 0.10 ¨ 0.35
Ni 0.2 ¨ 0.9, preferably 0.3 ¨ 0.8, more preferably 0.3 ¨ 0.7
Cr 0.2 ¨ 0.9, preferably 0.3 ¨ 0.8, more preferably 0.3 ¨ 0.7
MO 0 ¨ 0.2, preferably 0 ¨ 0.1
Nb 0 ¨ 0.005
Ti 0 ¨ 0.035
V 0 ¨ 0.05
0.0005 ¨ 0.0050, preferably 0.0008 ¨ 0.0040
P 0 ¨ 0.025, preferably 0 ¨ 0.020
0 ¨ 0.008, preferably 0 ¨ 0.005
0 ¨ 0.01, preferably 0 ¨ 0.005
Ca 0 ¨ 0.01, preferably 0 ¨ 0.005, more preferably 0 ¨ 0.003
remainder Fe and inevitable impurities.
The steel product has a low content of Mn, which is important for improving
impact
toughness and bendability.
The levels of Cr and Ni are set to improve hardenability. The level of Ni is
further set to
improve impact toughness and formability.
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
CA 03135141 2021-09-27
WO 2020/201437 PCT/EP2020/059423
4
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, in terms of weight percentages (wt.
%):
C 0.14 ¨ 0.35, preferably 0.17 ¨ 0.31, more preferably 0.20 ¨
0.28
Si 0 ¨ 0.5, preferably 0.01 ¨ 0.50, more preferably 0.03 ¨ 0.25
Mn 0.05 ¨ 0.40, preferably 0.05 ¨ 0.30
Al 0 ¨ 0.1, preferably 0 ¨ 0.08
Cu 0.1 ¨0.4, preferably 0.10 ¨ 0.35
Ni 0.2 ¨ 0.9, preferably 0.3 ¨ 0.8, more preferably 0.3 ¨ 0.7
Cr 0.2 ¨ 0.9, preferably 0.3 ¨ 0.8, more preferably 0.3 ¨ 0.7
Mo 0 ¨ 0.2, preferably 0 ¨ 0.1
Nb 0 ¨ 0.005
Ti 0 ¨ 0.035
/ 0 ¨ 0.05
B 0.0005 ¨ 0.0050, preferably 0.0008 ¨ 0.0040
P 0 ¨ 0.025, preferably 0 ¨ 0.020
S 0 ¨ 0.008, preferably 0 ¨ 0.005
N 0 ¨ 0.01, preferably 0 ¨ 0.005
Ca 0 ¨ 0.01, preferably 0 ¨ 0.005, more preferably 0 ¨ 0.003
remainder Fe and inevitable impurities;
- heating the steel slab to the austenitizing temperature of 1150 - 1300
C;
- hot-rolling to the desired thickness at a temperature in the range of Ar3 to
1250 C,
wherein the finish rolling temperature is in the range of 800 C to 960 C,
preferably 870 C ¨ 940 C, more preferably 880 C ¨ 930 C; and
- direct quenching the hot-rolled steel strip product 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.
Optionally, a step of temper annealing is performed on the direct quenched
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.
CA 03135141 2021-09-27
WO 2020/201437 PCT/EP2020/059423
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
5 and/or tempered. Typically, the microstructure also comprises retained
austenite, bainite,
ferrite and/or cementite.
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.
The steel product has a good balance of high hardness and excellent mechanical
properties such as impact strength and formability/bendability.
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,
more
preferably 460 ¨ 530 HBW, and even more preferably 470 ¨ 530 HBW;
a Charpy-V impact toughness of at least 50 J/cm2 at a temperature of -40 C.
The steel product exhibits excellent bendability or formability. The steel
product has a
minimum bending radius of 3.2 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 2.5 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.
CA 03135141 2021-09-27
WO 2020/201437 PCT/EP2020/059423
6
DETAILED DESCRIPTION OF THE INVENTION
The term "steel" is defined as an iron alloy containing carbon (C).
The term "Brinell hardness (HBVV)" is a designation of hardness of steel. The
Brinell
hardness test is performed by pressing a spherical tungsten carbide ball with
a diameter
of 10 mm against a clean prepared surface of a metal sheet using a 3000
kilogram force,
producing an impression, measured and given a special numerical value. A
spherical
tungsten carbide ball with a diameter of 5 mm and a load of 750 kilogram force
are
applied to test samples with thinner gauges, e.g. 3 mm in thickness.
The term "gauge" refers generally to a measure of the thickness of a metal
sheet.
.. The term "ultimate tensile strength (UTS, Rip)" refers to the limit, at
which the steel
fractures under tension, thus the maximum tensile stress.
The term "yield strength (YS, Rp0.2)" 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 (A80).
The term "minimum bending radius (R,)" 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 R, and the sheet thickness (t).
The term
"bendability" can also be used interchangeably with "formability" in the
context of the
current description.
The term "heat-affected zone (HAZ)" refers to a non-melted area of a metal
material that
has experienced changes in its material properties as a result of exposure to
high
.. temperatures. The alterations in material properties are usually a result
of welding or high-
heat cutting procedures. The HAZ is identified as the area between the weld or
cut and
the base metal material. These areas can vary in size and severity depending
on the
properties of the materials involved, the intensity and concentration of heat,
and the
process employed.
CA 03135141 2021-09-27
WO 2020/201437 PCT/EP2020/059423
7
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 0.14 % to 0.35 %.
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 0.14 % to 0.35 %
depending on
targeted hardness. If the carbon content is less than 0.14%, it is difficult
to achieve a
Brinell hardness of more than 420 HBW. C is also an austenite stabilizing
element.
However, C has detrimental effects on weldability, impact toughness,
formability or
bendability, and resistance to stress corrosion cracking. Therefore, C content
is set to not
more than 0.35 %.
.. Preferably, C is used in the range of 0.17 % to 0.31 %, and more preferably
0.20 % 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 retained 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 the
Si level is excessively high.
Preferably, Si is used in the range of 0.01 % to 0.50 %, and more preferably
0.03 % to
0.25 %.
Manganese Mn is used in the range of 0.05 % to 0.40 %.
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 may enhance
the risk
CA 03135141 2021-09-27
WO 2020/201437 PCT/EP2020/059423
8
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 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 0.05 % to ensure hardenability, but
not more
than 0.40 % to avoid the harmful effects as described above and to ensure
excellent
mechanical properties such as impact strength and bendability. Preferably, a
low level of
Mn is used in the range of 0.05 % to 0.30 % to further improve the
bendability.
Aluminum Al is used in the range of 0.1 % or less.
Al is effective as a deoxidizing or killing agent that can remove oxygen from
the melt
during a steelmaking process. Al removes N by forming stable AIN particles and
provides
grain refinement, which is beneficial to high toughness. Also, Al stabilizes
retained
austenite. However, an excess of Al may increase non-metallic inclusions
thereby
deteriorating cleanliness.
Preferably, Al is used in the range of 0.08 % or less.
Copper Cu is used in the range of 0.1 % to 0.4 %.
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 0.4 %.
Preferably, Cu is used in the range of 0.10 % to 0.35 %.
Nickel Ni is used in the range of 0.2 % to 0.9 %.
Ni is used to avoid quench induced cracking and also to improve toughness and
formability. Ni is an alloying element that improves austenite hardenability
thereby
CA 03135141 2021-09-27
WO 2020/201437 PCT/EP2020/059423
9
increasing strength with no or marginal loss of impact toughness and/or heat-
affected
zone (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 0.9 % would increase alloying costs too much without
significant
technical improvement. An excess of Ni may produce high viscosity iron oxide
scales
which 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.
Ni is preferably used in the range of 0.3 % to 0.8 %, and more preferably 0.3
% to 0.7 %.
Chromium Cr is used in the range of 0.2 % to 0.9 %.
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
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 0.9 %
the heat-affected zone (HAZ) toughness as well as field weldability may be
adversely
affected.
Preferably, Cr is used in the range of 0.3 % to 0.8 %, and more preferably 0.3
% to 0.7 %.
Molybdenum Mo is used in the range of 0.2 % or less.
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 0.2 % 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
CA 03135141 2021-09-27
WO 2020/201437 PCT/EP2020/059423
content should be carefully controlled to prevent excessive elongation of the
prior
austenite grains which may deteriorate bendability of the steel product.
Preferably, Mo is used in the range of 0.1 % or less.
Niobium Nb is used in an amount of 0.005 % or less.
5 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 heat-affected zone (HAZ) toughness since Nb
may
10 .. 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 0.035 % 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 heat-affected zone
(HAZ) during
.. welding thereby improving toughness. TiN formation suppresses BN
precipitation, thereby
leaving B free to make its contribution to hardenability. However, if Ti
content is too high,
coarsening of TiN and precipitation hardening due to TiC develop and toughness
may be
deteriorated. Therefore, it is necessary to restrict Ti so that it does not
exceed 0.035%.
Vanadium V is used in an amount of 0.05 % 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
hydrogen induced cracking (H IC). V is a strong carbide and nitride former,
but V(C,N) can
also form and its solubility in 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 0.05
% V has negative effects on weldability, hardenability and alloying cost.
CA 03135141 2021-09-27
WO 2020/201437 PCT/EP2020/059423
11
Boron B is used in the range of 0.0005 % to 0.0050 %.
B is a well-established microalloying element to increase hardenability. Boron
can be
added to retard phosphorus segregation to grain boundaries thereby reducing
embrittlement during welding in the heat-affected zone (HAZ). Effective B
alloying requires
the presence of Ti to prevent formation of BN. In the presence of B, Ti
content can be
lowered to be less than 0.02%, which is beneficial for toughness. However,
hardenability
deteriorates if the B content exceeds 0.005 %.
Preferably, B is used in the range of 0.0008 % to 0.0040 %.
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 stress corrosion cracking (SCC) resistance.
Preferably, Ca is used in an amount of 0.005 % or less, and more preferably
0.003 % or
less to ensure excellent mechanical properties such as impact strength and
bendability.
Unavoidable impurities can be phosphor P, sulfur S and nitrogen N. Their
content in terms
of weight percentages (wt. %) is preferably defined as follows:
P 0 ¨ 0.025, preferably 0 ¨ 0.020
0 ¨ 0.008, preferably 0 ¨ 0.005
0 ¨ 0.01, preferably 0 ¨ 0.005
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.
CA 03135141 2021-09-27
WO 2020/201437 PCT/EP2020/059423
12
Furthermore, the equation shows that any single element is not crucial for
martensite
formation and that the absence of one element may be compensated with the
amount of
other alloying elements and processing parameters, such as e.g. cooling rate.
.1-1 4 Cr
= 6 x exp [7.1 x ( fl C + +- 1-
5.6; 3.13 io
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 1150 -
1300 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.
In the hot rolling stage the slab is hot rolled to the desired thickness at a
temperature in
the range of Ar3 to 1250 C, wherein the finish rolling temperature (FRT) is in
the range of
800 C to 960 C, preferably 870 C ¨ 940 C, more preferably 880 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.
CA 03135141 2021-09-27
WO 2020/201437 PCT/EP2020/059423
13
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.
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 microstructure comprises 1 vol. % or less
retained
austenite, and more preferably 0.5 vol. % or less retained austenite.
Typically, the
microstructure also comprises bainite, ferrite, pearlite and/or cementite.
Optionally, an extra step of temper annealing is performed at a temperature in
the range
of 150 C ¨ 250 C.
The steel strip product has a good balance of hardness and other mechanical
properties
such as excellent impact strength and excellent formability/bendability.
The steel strip product has a high Brinell hardness in the range of 420 ¨ 580
HBW,
.. preferably 450 ¨ 550 HBW, more preferably 460 ¨ 530 HBW, and even more
preferably
470 ¨530 HBW.
The steel strip product with high hardness has a Charpy-V impact toughness of
at least 50
J/cm2 at a temperature of -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 (R,) of 3.2 t or less in a measurement direction
longitudinal to the
CA 03135141 2021-09-27
WO 2020/201437 PCT/EP2020/059423
14
rolling direction wherein the bending axis is longitudinal to rolling
direction; a minimum
bending radius (R,) of 2.5 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
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. Steel types A ¨ C are the inventive compositions
according to the
present disclosure. Steel types D and E are comparative compositions which
comprise a
relatively high Mn content of 1.20 wt. % and 1.19 wt. % respectively (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 ¨ 3 all have a main phase of martensite in an
amount of
.. at least 90 vol. %.
Brinell hardness HBW
The Brinell hardness test is performed by pressing a spherical tungsten
carbide ball with a
diameter of 10 mm against a clean prepared surface of the steel strip samples
with a
thickness of 6 mm using a 3000 kilogram force, producing an impression,
measured and
given a special numerical value. For the strip samples with a thickness of 3
mm, a
spherical tungsten carbide ball with a diameter of 5 mm and a load of 750
kilogram force
are applied. 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 ¨ 3 exhibits a Brinell harness in the range of 467 ¨
489 HBW.
The comparative examples no. 4 exhibits a Brinell harness of 485 HBW while the
comparative example no. 5 exhibits a Brinell harness of 502 HBW.
CA 03135141 2021-09-27
WO 2020/201437 PCT/EP2020/059423
Charpy-V impact toughness
The impact toughness values at -40 C are obtained by Charpy V-notch tests
according to
the ISO 148 standard. Each one of the inventive examples no. 1 ¨ 3 has a
Charpy-V
impact toughness in the range of 78 - 118 J/cm2 at a temperature of -40 C if
the
5 measurement direction is longitudinal to the rolling direction. Each one
of the inventive
examples no. 1 ¨ 3 has a Charpy-V impact toughness in the range of 65 - 90
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 ¨ 3 is improved compared
to the
comparative examples no. 4 and 5.
10 Elongation
Elongation was determined according ISO 6892 standard using longitudinal
specimens.
The mean value of total elongation (A80) of the inventive examples no. 1, 2
and 3 is 4.5,
7.6 and 7.7 respectively (Table 3).The comparative examples no. 4 and 5 have
better
elongation values than the inventive examples no. 1 ¨ 3 at the expense of
Charpy-V
15 impact toughness and bendability.
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 ¨ 3 has
a minimum
bending radius (R,) of 2.8 t or less in a measurement direction longitudinal
to the rolling
direction; a minimum bending radius (R,) of 2.0 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 examples no. 4 and 5 exhibit a minimum bending
radius (R,) of
3.7 t and 3.3 t respectively in a measurement direction longitudinal to the
rolling direction,
and a minimum bending radius (R,) of 3.0 t and 2.7 t respectively in a
measurement
direction transversal to the rolling direction (Table 3).
CA 03135141 2021-09-27
WO 2020/201437 PCT/EP2020/059423
16
Yield strength
Yield strength was determined according ISO 6892 standard using longitudinal
specimens. Each one of the inventive examples no. 1 ¨ 3 has a mean value of
yield
strength (R0.2) in the range of 1310 MPa to 1413 MPa measured in the
longitudinal
direction (Table 3). The comparative examples no. 4 and 5 have a mean value of
yield
strength (R0.2) of 1375 MPa and 1397 MPa respectively, measured in the
longitudinal
direction (Table 3).
Tensile strength
Ultimate tensile strength (Rm) was determined according ISO 6892 standard
using
longitudinal specimens. Each one of the inventive examples no. 1 ¨ 3 has a
mean value of
ultimate tensile strength (Rm) in the range of 1511 MPa to 1609 MPa, measured
in the
longitudinal direction (Table 3). The comparative examples no. 4 and 5 have a
mean
value of ultimate tensile strength (Rm) of 1617 MPa and 1654 MPa respectively,
measured
in the longitudinal direction (Table 3).
17
0
Table 1. Chemical compositions (wt. %).
t..)
o
t..)
o
Steel C Si Mn P S N Cr Ni Cu Mo
Al Nb V Ti B Ca
o
1-
type
.6.
(...)
-4
1A 0.2390 0.1720 0.2000 0.0100 0.0018 0.0028 0.3840 0.4760 0.1600 0.0580
0.0550 0.0010 0.0150 0.0020 0.0011 0.0013
113 0.2290 0.1790 0.2000 0.0070 0.0006 0.0024 0.3900 0.5100 0.1600 0.0500
0.0510 0.0010 0.0100 0.0020 0.0011 0.0008
1C 0.2500 0.1770 0.2000 0.0070 0.0006 0.0022 0.4000 0.5000 0.1500 0.0140
0.0580 0.0010 0.0090 0.0020 0.0011 0.0008
2D 0.2290 0.1740 1.2000 0.0090 0.0005 0.0023 0.2100 0.0600 0.0100 0.0230
0.0390 0.0010 0.0090 0.0100 0.0015 0.0007
2E 0.2550 0.1770 1.1900 0.0100 0.0007 0.0026 0.2000 0.0500 0.0100 0.0340
0.0390 0.0010 0.0090 0.0090 0.0013 0.0007
1 inventive composition
P
2
0
w
comparative composition
,
w
,
,
Table 2. Manufacturing conditions
" "
'7
,
Steel Steel Strip Hot rolling Cooling
Temper annealing Remarks "
,
strip no. type thickness
Heating RT FRT Cooling Cooling
(mm )
Annealing Holding
temp. ( C) ( C) ( C)
temp. ( C) rate ( C/s) temp. ( C) time (h)
1 A 3 1250 1130 890 <100 262
200 8 inventive example
2 B 6 1200 1100 900 <100 127
200 8 inventive example 1-d
3 C 6 1210 1090 890 <100 128
- - inventive example n
1-i
4 D 3 1270 1140 905 <100 290
- - comparative example m
1-d
w
E 6 1230 1090 905 <100 142 -
- comparative example
w
o
5
O-
u,
,z
.6.
t..)
Table 3. Mechanical properties
(...)
18
Steel Steel Strip R0.2 (L) Rm (L) A80 (L) HBW ChV (-40) (J/cm2)
Bending (R/t) Remarks
0
strip no type thickness (M Pa) (M Pa) (%)
Longi
(mm) t. Transv.
Longit. Transv.
1 A 3 1413 1609 4.5 488 105 90
2.6 2.0 inventive example
2 B 6 1310 1511 7.6 467 118 85
2.3 1.3 inventive example
3 C 6 1334 1582 7.7 489 78 65
2.8 1.7 inventive example
4 D 3 1375 1617 6.3 485
3.7 3.0 comparative example
E 6 1397 1654 8.0 502 58 50 3.3
2.7 comparative example
,õ
