U.S. patent application number 17/601234 was filed with the patent office on 2022-06-09 for high-hardness steel product and method of manufacturing the same.
The applicant listed for this patent is SSAB TECHNOLOGY AB. Invention is credited to Magnus Gladh, Mikko Hemmila, Magnus Larsson, Tommi Liimatainen, Pasi Suikkanen, Esa Virolainen.
Application Number | 20220177997 17/601234 |
Document ID | / |
Family ID | 1000006210763 |
Filed Date | 2022-06-09 |
United States Patent
Application |
20220177997 |
Kind Code |
A1 |
Larsson; Magnus ; et
al. |
June 9, 2022 |
High-Hardness Steel Product and Method of Manufacturing the
Same
Abstract
Described is a hot-rolled steel strip product including a
composition consisting of, in terms of weight percentages, 0.14% to
0.35% C, 0% to 0.5% Si, 0.05% to 0.40% Mn, 0.1% or less Al, 0.1% to
0.4% Cu, 0.2% to 0.9% Ni, 0.2% to 0.9% Cr, 0.2% or less Mo, 0.005%
or less Nb, 0.035% or less Ti, 0.05% or less V, 0.0005% to 0.0050%
B, 0.025% or less P, 0.008% or less S, 0.01% or less N, 0.01% or
less Ca, and the remainder being Fe and inevitable impurities,
wherein the steel product has a Brinell hardness in the range of
420 to 580 HBW.
Inventors: |
Larsson; Magnus; (Borlange,
SE) ; Gladh; Magnus; (Borlange, SE) ; Hemmila;
Mikko; (Vihanti, FI) ; Liimatainen; Tommi;
(Raahe, FI) ; Virolainen; Esa; (Oulu, FI) ;
Suikkanen; Pasi; (Oulu, FI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SSAB TECHNOLOGY AB |
Sockholm |
|
SE |
|
|
Family ID: |
1000006210763 |
Appl. No.: |
17/601234 |
Filed: |
April 2, 2020 |
PCT Filed: |
April 2, 2020 |
PCT NO: |
PCT/EP2020/059423 |
371 Date: |
October 4, 2021 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 9/52 20130101; C22C
38/42 20130101; C21D 6/008 20130101; C22C 38/06 20130101; C22C
38/54 20130101; C22C 38/04 20130101; C21D 8/0205 20130101; C22C
38/002 20130101; C21D 6/004 20130101; C22C 38/50 20130101; C22C
38/02 20130101; C21D 8/0226 20130101; C22C 38/46 20130101; C21D
8/0263 20130101; C21D 6/005 20130101; C22C 38/44 20130101; C22C
38/48 20130101; C22C 38/001 20130101 |
International
Class: |
C21D 9/52 20060101
C21D009/52; C21D 8/02 20060101 C21D008/02; C21D 6/00 20060101
C21D006/00; C22C 38/54 20060101 C22C038/54; C22C 38/50 20060101
C22C038/50; C22C 38/48 20060101 C22C038/48; C22C 38/46 20060101
C22C038/46; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 5, 2019 |
EP |
19167552.9 |
Jul 11, 2019 |
EP |
19185759.8 |
Claims
1. A hot-rolled steel strip product comprising a composition
consisting of, in terms of weight percentages (wt. %): C 0.14-0.35,
Si 0-0.5, Mn 0.05-0.40, Al 0-0.1, Cu 0.1-0.4, Ni 0.2-0.9, Cr
0.2-0.9, Mo 0-0.2, Nb 0-0.005 Ti 0-0.035 V 0-0.05 B 0.0005-0.0050,
P 0-0.025, S 0-0.008, N 0-0.01, Ca 0-0.01, remainder Fe and
inevitable impurities, wherein the steel product has a Brinell
hardness in the range of 420-580 HBW.
2. The steel product according to claim 1, wherein the steel
product has a Brinell hardness in the range of 450-550 HBW.
3. The steel product according to claim 1, wherein the steel
product has a Charpy-V impact toughness of at least 50 J/cm.sup.2
at a temperature of -40.degree. C.
4. The steel product according to claim 1, wherein the steel
product has a minimum bending radius of 3.2 t or less in a
measurement direction longitudinal to the rolling direction; a
minimum bending radius of 2.5 t or less in a measurement direction
transversal to the rolling direction; and wherein t is the
thickness of the steel strip product.
5. The steel product according to claim 1, wherein the steel
product has a microstructure consisting of, in terms of volume
percentages (vol. %), martensite in an amount of at least 90 vol.
%; and remainder being retained austenite, bainite, ferrite,
pearlite and/or cementite.
6. The steel product according to claim 1, wherein the steel
product has a prior austenite grain size of 50 .mu.m or less.
7. The steel product according to claim 1, wherein the steel
product has a prior austenite grain structure with an aspect ratio
in the range of 1.5-7.
8. The steel product according to claim 1, wherein the steel strip
product has a thickness of 10 mm or less.
9. A method for manufacturing the steel comprising the following
steps of providing a steel slab consisting of the chemical
composition according to claim 1; heating the steel slab to the
austenitizing temperature of 1150-1300.degree. C.; hot-rolling to
the desired thickness at a temperature in the range of Ar.sub.3 to
1250.degree. C., wherein the finish rolling temperature is in the
range of 800.degree. C. to 960.degree. C.; direct quenching the
hot-rolled steel strip product to a cooling end and coiling
temperature of 450.degree. C. or less; and optionally, temper
annealing at a temperature in the range of 150.degree.
C.-250.degree. C.
Description
FIELD OF INVENTION
[0001] 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
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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 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.
[0006] CN102392186 and CN103820717 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.
[0007] 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
[0008] 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.
[0009] 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.
[0010] 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. %):
TABLE-US-00001 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 V 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
[0011] remainder Fe and inevitable impurities.
[0012] The steel product has a low content of Mn, which is
important for improving impact toughness and bendability.
[0013] The levels of Cr and Ni are set to improve hardenability.
The level of Ni is further set to improve impact toughness and
formability.
[0014] 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.
[0015] 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.
[0016] In a second aspect, the present invention provides a method
for manufacturing hot-rolled steel strip product comprising the
following steps of [0017] providing a steel slab consisting of, in
terms of weight percentages (wt. %):
TABLE-US-00002 [0017] 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 V 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
[0018] remainder Fe and inevitable impurities; [0019] heating the
steel slab to the austenitizing temperature of 1150-1300.degree.
C.; [0020] hot-rolling to the desired thickness at a temperature in
the range of Ar.sub.3 to 1250.degree. C., wherein the finish
rolling temperature is in the range of 800.degree. C. to
960.degree. C., preferably 870.degree. C.-940.degree. C., more
preferably 880.degree. C.-930.degree. C.; and [0021] direct
quenching the hot-rolled steel strip product to a cooling end and
coiling temperature of 450.degree. C. or less, preferably
250.degree. C. or less, more preferably 150.degree. C. or less, and
even more preferably 100.degree. C. or less.
[0022] Optionally, a step of temper annealing is performed on the
direct quenched product at a temperature in the range of
150.degree. C.-250.degree. C. However, the step of temper annealing
is not required according to the present invention.
[0023] The steel product is a steel strip having a thickness of 10
mm or less, preferably 8 mm or less.
[0024] 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. Typically, the
microstructure also comprises retained austenite, bainite, ferrite
and/or cementite.
[0025] The obtained steel product has a prior austenite grain size
of 50 .mu.m or less, preferably 30 .mu.m or less, more preferably
20 .mu.m or less, measured from 1/4 thickness of the steel strip
product.
[0026] 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.
[0027] The steel product has a good balance of high hardness and
excellent mechanical properties such as impact strength and
formability/bendability.
[0028] 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/cm.sup.2 at a temperature of -40.degree. C.
[0029] 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.
DETAILED DESCRIPTION OF THE INVENTION
[0030] The term "steel" is defined as an iron alloy containing
carbon (C).
[0031] The term "Brinell hardness (HBW)" 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.
[0032] The term "gauge" refers generally to a measure of the
thickness of a metal sheet.
[0033] The term "ultimate tensile strength (UTS, R.sub.m)" refers
to the limit, at which the steel fractures under tension, thus the
maximum tensile stress.
[0034] The term "yield strength (YS, R.sub.p0.2)" refers to 0.2%
offset yield strength defined as the amount of stress that will
result in a plastic strain of 0.2%.
[0035] 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).
[0036] The term "minimum bending radius (R.sub.i)" is used to refer
to the minimum radius of bending that can be applied to a test
sheet without occurrence of cracks.
[0037] The term "bendability" refers to the ratio of R.sub.i and
the sheet thickness (t). The term "bendability" can also be used
interchangeably with "formability" in the context of the current
description.
[0038] 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.
[0039] The alloying content of steel together with the processing
parameters determines the microstructure which in turn determines
the mechanical properties of the steel.
[0040] 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%.
[0041] 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%.
[0042] 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.
[0043] 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.
[0044] 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%.
[0045] Mn alloying lowers martensite start temperature (M.sub.s)
and martensite finish temperature (M.sub.f), which can suppress
autotempering of martensite during quenching. Reduced autotempering
of martensite leads to higher internal stresses that may 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.
[0046] 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.
[0047] 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.
[0048] 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 AlN 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%.
[0049] 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%.
[0050] 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%.
[0051] 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 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.
[0052] 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%.
[0053] 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.
[0054] 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.
[0055] 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 content should be carefully controlled to prevent
excessive elongation of the prior austenite grains which may
deteriorate bendability of the steel product.
[0056] Preferably, Mo is used in the range of 0.1% or less.
Niobium Nb is used in an amount of 0.005% or less.
[0057] 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 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.
[0058] 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.
[0059] V has substantially the same but smaller effects as Nb. V4C3
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 (HIC). 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.
Boron B is used in the range of 0.0005% to 0.0050%.
[0060] 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%.
[0061] 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.
[0062] 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 (CaOS) that may deteriorate the mechanical
properties such as bendability and stress corrosion cracking (SCC)
resistance.
[0063] 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.
[0064] Unavoidable impurities can be phosphor P, sulfur S and
nitrogen N. Their content in terms of weight percentages (wt. %) is
preferably defined as follows:
TABLE-US-00003 P 0-0.025, preferably 0-0.020 S 0-0.008, preferably
0-0.005 N 0-0.01, preferably 0-0.005
[0065] Other inevitable impurities may be hydrogen H, oxygen O and
rare earth metals (REM) or the like. Their contents are limited in
order to ensure excellent mechanical properties, such as impact
toughness.
[0066] 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.
[0067] 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.
D i = 6 .times. exp .function. [ 7.1 .times. ( C + Mn 5.87 + Mo
3.13 + Cr 6.28 + Si 18 + Ni 15 ) ] ##EQU00001##
[0068] 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.
[0069] The first step is to provide a steel slab by means of, for
instance a process of continuous casting, also known as strand
casting.
[0070] In the reheating stage, the steel slab is heated to the
austenitizing temperature of 1150-1300.degree. 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.
[0071] The final steel product has a prior austenite grain size of
50 .mu.m or less, preferably 30 .mu.m or less, more preferably 20
.mu.m or less, measured from 1/4 thickness of the steel strip
product.
[0072] In the hot rolling stage the slab is hot rolled to the
desired thickness at a temperature in the range of Ar.sub.3 to
1250.degree. C., wherein the finish rolling temperature (FRT) is in
the range of 800.degree. C. to 960.degree. C., preferably
870.degree. C.-940.degree. C., more preferably 880.degree.
C.-930.degree. C.
[0073] 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.
[0074] 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.
[0075] The obtained steel strip product has a thickness of 10 mm or
less, preferably 8 mm or less.
[0076] The hot-rolled steel strip product is direct quenched to a
cooling end and coiling temperature of 450.degree. C. or less,
preferably 250.degree. C. or less, more preferably 150.degree. C.
or less, and even more preferably 100.degree. C. or less. The
cooling rate is at least 30.degree. C./s.
[0077] The direct quenched steel strip product is coiled at
temperature of 450.degree. C. or less, preferably 250.degree. C. or
less, more preferably 150.degree. C. or less, and even more
preferably 100.degree. C. or less.
[0078] 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.
[0079] Optionally, an extra step of temper annealing is performed
at a temperature in the range of 150.degree. C.-250.degree. C.
[0080] The steel strip product has a good balance of hardness and
other mechanical properties such as excellent impact strength and
excellent formability/bendability.
[0081] 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.
[0082] The steel strip product with high hardness has a Charpy-V
impact toughness of at least 50 J/cm.sup.2 at a temperature of
-40.degree. C. thereby fulfilling the conventional impact strength
requirements.
[0083] The steel strip product exhibits excellent bendability or
formability. The steel product has a minimum bending radius
(R.sub.i) 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 (R.sub.i) 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.
[0084] 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.
[0085] 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).
[0086] The manufacturing conditions for producing the tested steel
strip products are presented in Table 2.
[0087] The mechanical properties of the tested steel strip products
are presented in Table 3.
Microstructure
[0088] 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
[0089] 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.
Charpy-V Impact Toughness
[0090] The impact toughness values at -40.degree. 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/cm.sup.2 at a temperature of -40.degree.
C. if the 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/cm.sup.2 at a
temperature of -40.degree. 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.
Elongation
[0091] 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 impact toughness and bendability.
Bendability
[0092] 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.degree. 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.
[0093] According to the bend tests, each one of the inventive
examples no. 1-3 has a minimum bending radius (R.sub.i) of 2.8 t or
less in a measurement direction longitudinal to the rolling
direction; a minimum bending radius (R.sub.i) 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.sub.i) of 3.7 t and 3.3 t respectively in a measurement
direction longitudinal to the rolling direction, and a minimum
bending radius (R.sub.i) of 3.0 t and 2.7 t respectively in a
measurement direction transversal to the rolling direction (Table
3).
Yield Strength
[0094] 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 (R.sub.p0.2) 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 (R.sub.p0.2) of 1375 MPa and 1397 MPa
respectively, measured in the longitudinal direction (Table 3).
Tensile Strength
[0095] Ultimate tensile strength (R.sub.m) 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 (R.sub.m) 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
(R.sub.m) of 1617 MPa and 1654 MPa respectively, measured in the
longitudinal direction (Table 3).
TABLE-US-00004 TABLE 1 Chemical compositions (wt. %). Steel type C
Si Mn P S N Cr Ni .sup.1A 0.2390 0.1720 0.2000 0.0100 0.0018 0.0028
0.3840 0.4760 .sup.1B 0.2290 0.1790 0.2000 0.0070 0.0006 0.0024
0.3900 0.5100 .sup.1C 0.2500 0.1770 0.2000 0.0070 0.0006 0.0022
0.4000 0.5000 .sup.2D 0.2290 0.1740 1.2000 0.0090 0.0005 0.0023
0.2100 0.0600 .sup.2E 0.2550 0.1770 1.1900 0.0100 0.0007 0.0026
0.2000 0.0500 Steel type Cu Mo Al Nb V Ti B Ca .sup.1A 0.1600
0.0580 0.0550 0.0010 0.0150 0.0020 0.0011 0.0013 .sup.1B 0.1600
0.0500 0.0510 0.0010 0.0100 0.0020 0.0011 0.0008 .sup.1C 0.1500
0.0140 0.0580 0.0010 0.0090 0.0020 0.0011 0.0008 .sup.2D 0.0100
0.0230 0.0390 0.0010 0.0090 0.0100 0.0015 0.0007 .sup.2E 0.0100
0.0340 0.0390 0.0010 0.0090 0.0090 0.0013 0.0007 .sup.1inventive
composition .sup.2comparative composition
TABLE-US-00005 TABLE 2 Manufacturing conditions Steel Strip Hot
rolling Cooling Temper annealing strip Steel thickness Heating RT
FRT Cooling Cooling Annealing Holding no. type (mm) temp. (.degree.
C.) (.degree. C.) (.degree. C.) temp. (.degree. C.) rate (.degree.
C./s) temp. (.degree. C.) time (h) Remarks 1 A 3 1250 1130 890
<100 262 200 8 inventive example 2 B 6 1200 1100 900 <100 127
200 8 inventive example 3 C 6 1210 1090 890 <100 128 -- --
inventive example 4 D 3 1270 1140 905 <100 290 -- -- comparative
example 5 E 6 1230 1090 905 <100 142 -- -- comparative
example
TABLE-US-00006 TABLE 3 Mechanical properties Steel Strip R.sub.p0.2
R.sub.m A80 strip Steel thickness (L) (L) (L) ChV (-40)
(J/cm.sup.2) Bending (R.sub.i/t) no type (mm) (MPa) (MPa) (%) HBW
Longit. Transv. Longit. Transv. Remarks 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 5
E 6 1397 1654 8.0 502 58 50 3.3 2.7 comparative example
* * * * *