TECHNICAL FIELD
[0001] The present invention concerns high strength hot-rolled steel and a method for manufacturing
such hot-rolled steel.
BACKGROUND OF THE INVENTION
[0002] Martensitic flat steel products have for many years been manufactured using a method
comprising the steps of heating a steel slab to an austenitizing temperature, hot-rolling,
re-heating, quenching and tempering, or alternatively, heating a steel slab to an
austenitizing temperature, hot-rolling, direct quenching and tempering.
[0003] For example, European patent no.
EP 2,576,848 discloses a method for producing a hot-rolled steel from a steel, whose composition
as percentages by weight is C 0.075 - 0.12%, Si 0.1 - 0.8%, Mn 0.8 - 1.7%, Al 0.015
- 0.08%, P less than 0.012%, S less than 0.005%, Cr 0.2 - 1.3%, Mo 0.15 - 0.80%, Ti
0.01 - 0.05%, B 0.0005 - 0.003%, V 0.02 - 0.10%, Nb less than 0.3%, Ni less than 1%,
Cu less than 0.5%, the remainder being iron and unavoidable impurities. The patent
describes direct quenched martensitic sheet-like steels which are temper annealed.
The hot-rolled steel is exceptionally temper-resistant after the direct quenching
process, wherein by tempering, high-strength (i.e. Rp
0.2 of at least 890MPa) combined with good impact toughness (Charpy V (-20°C) =37J) and
flangeability, as well as good weldability are achieved.
[0004] European patent application np.
EP 2,949,775 A1 discloses an ultra-high durability steel plate having a low yield ratio, comprising
the chemical elements in mass percentages of: C: 0.18-0.34%, Si: 0.10-0.40%, Mn: 0.50-1.40%,
Cr: 0.20-0.70%, Mo: 0.30-0.90%, Nb: 0-0.06%, Ni: 0.50-2.40%, V: 0-0.06%, Ti: 0.002-0.04%,
Al: 0.01-0.08%, B: 0.0006-0.0020%, N ¤0.0060%, O ¤0.0040%, Ca: 0-0.0045%, and the
balance of Fe and other unavoidable impurities. A process of manufacturing the steel
plate is also disclosed, wherein the heating temperature is 1080-1250 °C; the quenching
temperature is 860-940 °C; and the tempering temperature is 150-350 °C.
[0005] Japanese patent application no.
JP 2006206942A provides a method of manufacturing a high-tensile steel excellent in hydrogen embrittlement
resistance, which is less likely to cause hydrogen embrittlement such as delayed fracture,
weld delayed cracking, and sulphide corrosion cracking. The steel is composed of,
in mass%, C: 0.02 to 0.25%, Si: 0.01 to 0.8%, Mn: 0.5 to 2.0%, Al: 0.005 to 0.1%,
N: 0.0005 to 0.008%, P: 0.03% or less, S : 0.03% or less, optionally containing one
or two or more of Cu, Ni, Cr, Mo, Nb, V, Ti, B, Ca, REM, Mg, and the balance being
Fe and unavoidable impurities.
[0006] Such flat steel products may be used for applications, such as wear or structural
applications, in which the steel must exhibit high strength in combination with sufficient
hardness, bendability and impact toughness both in the as-produced steel products
and in the HAZ (heat affected zone) area of welded steel products.
SUMMARY OF THE INVENTION
[0007] An object of the invention is to provide improved hot-rolled steel.
[0008] This object is achieved by hot-rolled steel having the features recited in claim
1,
[0009] By adding the alloying elements in the recited amounts, a combination of good base
material toughness and strength properties can be achieved and any fracture occurring
during a tensile test of a weld will occur as far as possible from the fusion line.
[0010] Carbon is needed to achieve high base material strength and the other elements listed
above promote the strength of the weld so as to avoid softened zones in a welded seam
which would "catch" the fracture. Manganese, molybdenum and vanadium also promote
the strength of quenched and tempered steel.
[0011] From a toughness point of view, it is important to have a carbon content that is
as low as possible. The amount of each element in the embodiments a) to d) provides
a good combination of toughness and high strength.
[0012] Hot-rolled steel having the chemical composition given above and manufactured using
the method described herein exhibits high strength (i.e. a yield strength (Rp
0.2) of at least 1100 MPa) along and transverse to a rolling direction and a tensile
strength of at least 1120 MPa along and transverse to a rolling direction, good bendability
(i.e. a minimum bending radius of 5.0 x thickness along and/or transverse to a rolling
direction, preferably 4.0 x thickness along a rolling direction or more preferably
3.5 x thickness along a rolling direction and an impact toughness of at least 34 J/cm
2 and more, preferably an impact toughness of at least 50 J/cm
2 when a Charpy V notched specimen having a thickness of 5-10 mm is measured at -40°C
longitudinally to the rolling direction, and good ductility (i.e. an A%-elongation
of at least 8% along and transverse to the rolling direction, preferably of at least
10% or most preferably at least 12%). Mechanical properties are defined according
to the testing instructions of standard ISO 10025-6.
[0013] Preferably, this combination of properties is achieved both in the as-produced hot-rolled
quenched and tempered steel products and in the HAZ (heat affected zone) area of welded
hot-rolled steel products (which are welded using a filler material that is designed
for steels having a yield strength of at least 1100 MPa, preferably of at least 960
MPa, more preferably at least 900 MPa, most preferably of at least 890 MPa, such as
X90 or preferably X96).
[0014] The prior art includes hot-rolled steel sheets having a yield strength (Rp
0.2) of at least 1100 MPa, although those prior art hot-rolled steel sheets do not have
such good weldability or such good mechanical properties when welded.
[0015] The expression hot-rolled steel as used herein means a steel that is hot-rolled to
be sheet-like, such as a hot-rolled heavy plate or preferably hot-rolled strip steel.
The thickness of the hot-rolled strip steel may be 2 - 15 mm, preferably 2.5 - 10
mm. The thickness of the hot-rolled heavy plate may be 4 - 50 mm, preferably 5 - 25
mm. According to an embodiment of the invention the hot-rolled steel comprises 0.4-1.7
mass-% Cr, preferably 1.0 -1.7 mass-% Cr.
[0016] According to an embodiment of the invention the chemical composition contains both
Ni and Cu, and the amount of Ni ≥ 0.33 x the amount of Cu, preferably the amount of
Ni ≥ 0.5 x the amount of Cu so as to maintain high surface quality of the steel in
hot-rolling. Furthermore, the alloying costs of the hot-rolled steel can be kept as
low as possible while achieving the advantageous properties of the hot-rolled steel
according to the present invention (since Nickel is an expensive alloying element).
Nickel prevents Copper from melting under the scale that may be formed on the outer
surfaces of the steel when it is annealed before hot-rolling and which comprises iron
oxides and thereby prevents Copper from going into the grain boundaries, which can
weaken the grain boundaries. Weakened grain boundaries can promote surface cracking
and defects during the hot-rolling process.
[0017] According to an embodiment of the invention the chemical composition contains both
Ni and Cu in a total amount of at least 0.5 mass-%, or at least 1.0 mass-%, or at
least 1.2 mass-%.
[0018] According to an embodiment of the invention the hot-rolled steel has a tensile strength
of at least 1120 MPa, or at least 1130 MPa, or at least 1200 MPa, and/or maximum 1250
MPa or maximum 1300 MPa, or maximum up to 1450 MPa along and/or transverse to the
rolling direction.
[0019] According to an embodiment of the invention the hot-rolled steel iis used for metal
active gas (MAG) welded with reinforcement, as defined in claim 13.
[0020] The t8/5 time is the time it takes for a weld seam and an adjacent heat-affected
zone (HAZ) to cool from 800°C to 500°C. The expression "weld seam" means the total
weld area (WM and HAZ). A t8/5 time lower than 5 seconds may adversely affect the
toughness properties of the steel. A t8/5 time greater than 20 seconds may adversely
affect the strength of the steel. The MAG-welded transversal tensile test specimen's
fracture does not occur in the weld metal or fusion line and the fracture is moved
≥ 1 mm or ≥ 2 mm or ≥ 3 mm from the fusion line with or without reinforcement when
using welding consumables having a tensile strength of 1100 MPa, preferably 960 MPa,
more preferably 900 MPa, most preferably 890 MPa, and a t8/5 time of 8 - 12 seconds,
preferably 6 - 18 seconds, more preferably 5-20 seconds..
[0021] According to the embodiment defined in claim 7 the hot-rolled steel has an elongation
of at least 7%, preferably at least 8%, more preferably at least 9% when a tensile
test is carried out across a weld seam of a welded hot-rolled steel product where
the weld is longitudinal to the rolling direction. The hot-rolled steel is welded
using welding consumables having a tensile strength of 1100 MPa, preferably 960 MPa,
more preferably 900 MPa, most preferably 890 MPa, and a t8/5 time of 8 - 12 seconds,
preferably 6 - 18 seconds, more preferably 5-20 seconds..
[0022] The present invention also concerns a method for manufacturing hot-rolled steel according
to any of the embodiments of the invention, having the features recited in claim 8.
[0023] The method comprises the following steps carried out in the following order:
- heating a steel slab having a chemical composition as recited in claim 1 to an austenitizing
temperature of 1000 - 1350 °C, preferably 1200 - 1350°C,
- hot-rolling such that a finishing rolling temperature is 760 - 1050°C, preferably
760 - 960°C,
- quenching to 300 °C or less, preferably 150°C or less, and
temper annealing at a temperature of 500 - 650 °C, if the tempering time is 1 hour
or more, or temper annealing at a temperature of 500 - 750 °C, if the tempering time
is less than 1 hour after said quenching step, whereby the microstructure of the hot-rolled
steel before the temper annealing step contains at least 90% martensite, when said
microstructure is examined in ¼ thickness and the content of ferrite and pearlite
before the temper annealing step must be in total less than 10%
[0024] Quenching results in at least 90% martensite in the microstructure, preferably 95%
martensite, and more preferably 99% martensite when the microstructure is examined
in ¼ thickness.
[0025] It is beneficial to use such relatively high austenitizing temperatures because in
strip rolling the final thickness is small and steel tends to cool down during rolling.
By using higher heating temperatures, steel is warmer during strip rolling and rolling
forces are smaller. Austenite grain refinement is also easier then. Higher austenitization
temperatures also promote more uniform grain structure before rolling.
[0026] If very high temperatures (of more than 1350°C) are used there is a risk that large
grain size will be obtained. Furthermore, steel may oxidize aggressively and there
may be a yield loss due to high scaling. Additionally, production costs will be increased.
[0027] The quenching step is preferably a direct quenching step, which is for example conducted
a maximum of 15 seconds after the last hot-rolling pass. The cooling rate during quenching
is typically 30 - 150 °C/s.
[0028] For maximizing total elongation in a direction transverse to the rolling direction,
the method comprises the step of temper annealing at a temperature of 500 - 650 °C,
more preferably 550 - 650 °C, whereby the tempering time is 1 hour or more, or temper
annealing at a temperature of 500 - 750 °C, more preferably 550 - 750 °C, if the tempering
time is less than 1 hour. Tempering time is the holding time after the steel has reached
the tempering temperature. The temper annealing improves the impact toughness and
elongation of the hot-rolled steel while maintaining its strength. When maximum total
elongation is not required, the temper annealing step is carried out at a temperature
of 150 - 499°C, more preferably 180 - 250°C with any tempering time used. The microstructure
of the hot-rolled steel before the temper annealing step contains at least 90% martensite,
preferably at least 95% martensite and more preferably at least 99% martensite when
said microstructure is examined in ¼ thickness.
[0029] It should be noted that the temper annealing step may be conducted immediately after
the quenching. Alternatively, one or more additional method steps may be carried out
between the quenching step and the temper annealing step. For example, the quenched
steel may be subjected to an acid pickling step and/or coiling and/or straightening.
[0030] The mechanical properties of the hot-rolled steel as produced and when welded are
good because of the chemical composition of the steel and because the material is
tempered at a relatively high temperature of at least 500°C, preferably of at least
550°C and more preferably of at least 580°C. If the tempering time is relatively short,
i.e. less than 1 hour, (for example when induction tempering is used), the tempering
temperature can be higher, for example 50°C higher or more. The maximum tempering
temperature is 750°C.
[0031] According to an embodiment of the invention the temper annealing is preferably carried
out in a furnace other than a bell-type furnace, i.e. the temper annealing step is
preferably not carried out in a bell-type furnace but any other suitable type of furnace.
A bell-type furnace is a batch furnace that consists of an insulated chamber with
a steel shell and a heating system. Bell furnaces have removable covers called
bells, which are lowered over the load and hearth using a crane. An inner bell is placed
over the hearth and sealed to supply a protective atmosphere. An outer bell is lowered
to provide the heat supply. If temper annealing is carried out in a bell-type furnace,
the steel may typically be subjected to a temperature of 450-600°C for a long period
of time since the temperature inside the insulated chamber rises and falls slowly,
which may cause brittleness in some steels since atomic segregations may form at grain
boundaries, which can weaken the steels and make them very fragile at room temperature.
[0032] According to an embodiment of the invention the method comprises the step of strip
rolling the hot-rolled steel. The hot-rolled steel comprises a maximum of 0.005 mass-%
Niobium and < 0.15 mass-% Carbon when the hot-rolled steel is strip rolled.
[0033] The hot-rolled steel comprises a minimum of 0.005 or 0.04 or 0.02 mass-% Niobium
when the hot-rolled steel is not strip rolled. More than 0.06 mass-% Niobium has no
effect or only a minor effect on the strength properties of the hot-rolled steel.
[0034] When direct quenched, the strip rolling as a process produces a more elongated austenite
grain structure (flattened) compared to plate rolling, where the time for recrystallization
is longer and recrystallization is easier. By using Niobium, the flattening ratio
can be increased. To achieve the same flattening ratio as for strip rolling, the plate
steel is often alloyed with Niobium. Flattening of austenite increases the strength
and impact toughness of the steel.
[0035] When steel is reheated and quenched after hot-rolling, Niobium is needed to get high
strength and impact strength. The minimum amount of Niobium required is then >0.005
mass-%, preferably >0.02 mass-%.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] The present invention will hereinafter be further explained by means of non-limiting
examples with reference to the appended figures where;
Figure 1 shows a flow chart showing the steps of a method according to an embodiment
of the invention, including the alternative to temper anneal at at temperature of
150-499°C falling outside the embodiment.
Figure 2 shows hardness profiles for material having a thickness of 8 mm over the
weld tested from face side (i.e. the side at which welding took place) and the root
side (i.e. the side opposite to the side at which welding took place), and
Figure 3 shows hardness profiles for material having a thickness of 4 mm over the
weld tested from face side and the root side.
It should be noted that all features disclosed with reference to a hot-rolled steel
according to the present invention also apply to a method according to the present
invention, and vice versa.
DETAILED DESCRIPTION OF EMBODIMENTS
[0037] Figure 1 shows the steps of a method for manufacturing hot-rolled steel according
to any of the embodiments of the invention having a chemical composition containing
(in mass-%):
- C 0.10 - 0.2, preferably 0.10 - 0.18, more preferably 0.12 - 0.18,
- Si 0 - 0.7, preferably 0.03 - 0.50, more preferably 0.10 - 0.30,
- Mn 1.3 - 2.2, preferably 1.4 - 1.8, more preferably 1.4 - 1.7,
- Nb 0 - 0.06, preferably 0 - 0.04, more preferably 0 - 0.005,
- Ti 0 - 0.15, preferably 0 - 0.05 more preferably 0.005 - 0.02,
- V more than 0.03 and ≤ 0.25, preferably more than 0.10 and ≤ 0.20,
- Al 0.01 - 0.15, preferably 0.015 - 0.08,
- B 0.0005 - 0.010, preferably 0.0005 - 0.005, more preferably 0.001 - 0.003,
- Cr 0. 1 - 1.7, preferably 0.4 - 1.7 or 0.6 - 1.5, or more than 1.0 mass-%,
- Mo 0.15 - 0.8, preferably 0.2 - 0.5,
- Cu 0 - 1.5, preferably 0.1-1.0,
- Ni 0.3 - 2.5, preferably 0.7 - 1.7,
- P 0 - 0.015, preferably 0 - 0.009,
- S 0 - 0.008, preferably 0 - 0.004,
- Zr 0 - 0.2, preferably 0 - 0.01
- Ca 0 - 0.004, preferably 0.001 - 0.003,
- preferably N 0-0.01 mass-%, more preferably ≤ 0.006 mass-%.
- balance Fe and unavoidable impurities.
whereby:
- a) when 0.10 < C < 0.11 then Mn ≥ 1.6 and V > 0.14 and Mo ≥ 0.5 (in mass-%),
- b) when 0.11 < C < 0.125 then Mn ≥ 1.45 and V ≥ 0.13 and Mo ≥ 0.35 (in mass-%),
- c) when 0.125 < C < 0.15, then Mn ≥ 1.35 and V ≥ 0.12 and Mo ≥ 0.20 (in mass-%),
- d) when C ≥ 0.15 and V > 0.11, then Mn ≥ 1.3 and Mo ≥ 0.15 (in mass-%) or
when C ≥ 0.15 and V 0.03 - 0.11, then Mn > 1.3 and Mo > 0.15 and Nb > 0.02 and Cr+Cu+Ni
>1.4 (in mass-%).
[0038] The method comprises the step of heating a steel slab having the chemical composition
described above to an austenitizing temperature of 1000-1350 °C.
[0039] The thickness of the steel slab is, for example, 210 mm and it is preferably heated
to an austenitizing temperature of 1200 - 1350°C, where it is kept until it is of
adequately even warmth and the alloying elements have adequately dissolved into the
matrix. Typically, this takes several hours. If the austenitizing temperature is below
1200 °C, there can be a danger that not all of the alloying elements will dissolve
into the austenite, i.e. the austenite is not made homogenous and during the tempering,
the precipitation hardening may remain at a low level. On the other hand, if the austenitizing
temperature is higher than 1350 °C, this will result in an exceptionally large grain
size of the austenite and increased oxidation of the slab surface. Annealing time
in reheating is typically varied in the range of 2 - 4 hours, but, depending on the
selected furnace technology and the thickness of the slab, it can also be longer than
4 hours or shorter than 2 hours.
[0040] After the heating step, hot-rolling is conducted, which may typically comprise a
roughing step and a subsequent finish rolling step. The temperature of hot-rolling
at the last pass is 760 - 1050°C. Preferably, the finishing rolling temperature at
the last pass of the hot-rolling is 760 - 960°C. The end temperature of hot-rolling
is preferably above 830 °C or more preferably at least 850°C so that rolling forces
remain reasonable, and at the most 940 °C, and more preferably 920° at the most, wherein
i.a. excellent surface quality is assured.
[0041] After hot-rolling, or strip rolling, the steel is quenched, i.e. cooled at an accelerated
cooling rate, typically of 30 - 150 °C/s, using one step cooling for example, preferably
at a maximum cooling rate of 120°C/s, in a suitable quenching medium, such as water
or oil, to a temperature of 300 °C or less, or preferably 150°C or less, i.e. any
temperature between room/ambient temperature and 300°C. If it is a strip product it
is coiled at that temperature, i.e. at a coiling temperature of 300°C or less. Preferably,
the quenching is direct quenching conducted a maximum of 15 seconds after the last
hot-rolling pass.
[0042] This quenching gives the steel its exceptionally good mechanical properties including
good impact toughness combined with good bendability. Preferably, the end temperature
of quenching is a maximum of 150°C, because, in this case, after quenching, a steel
product with good flatness is achieved.
[0043] The quenched steel is subsequently subjected to temper annealing at a temperature
of 500 - 650 °C if the tempering time is 1 hour or more, or temper annealing at a
temperature of 500 - 750 °C if the tempering time is less than 1 hour. If the tempering
temperature is 400 - 750°C then the temper annealing is typically carried out in a
furnace other than a bell-type furnace so as to avoid the risk of adversely affecting
the strength and toughness properties of the steel. However, if the tempering temperature
is 150 - 250°C then the tempering annealing may also be carried out in a bell-type
furnace without adversely affecting the strength and toughness properties of the steel
and to minimize production costs. Tempering at annealing temperatures of 250 - 400°C
is not recommended due to low temperature tempering embrittlement if good toughness
properties are required. Typically, higher temperatures promote good total elongation
values, and lower tempering temperatures promote higher strength properties.
[0044] A suitable tempering treatment is defined by the formula P = T
∗(20+logt), where the temperature T is in °K and the time is in hours. The Larsen Miller
parameter, P, is between 15 - 19.5, and preferably 16 - 18.
[0045] The temper annealing step may be carried out on quenched steel, such as steel sheet
cut from a coil, or on a steel sheet that is continuously unwound from a coil, or
a heavy plate. In the case of a strip product, the temper annealing step may alternatively
be carried out on a whole coil, for example in a bell type furnace.
[0046] The microstructure of the hot-rolled steel before the temper annealing step contains
at least 90% martensite, preferably at least 95% martensite and more preferably at
least 99% martensite when the microstructure is examined in ¼ thickness. The majority
of the microstructure will be martensite although it may contain some bainite. The
content of ferrite and pearlite before the temper annealing step must be in total
less than 10%, preferably less than 5%.
[0047] Manganese content as a percentage by weight is 1.1 - 2.2 mass-% in order to assure
good hardenability in the weld metal and HAZ of welded hot-rolled steel. Manganese
also promotes hardenability of the base material during the quenching step. The expression
"weld metal" is intended to mean the part of the weld seam that consists mainly of
filler material.
[0048] The maximum Manganese content should be set according the equation so as to prevent
excessive segregations and ensure good impact strength:
maximum Manganese content (in mass-%) = 2.7- 5
∗Carbon content (in mass-%).
[0049] Molybdenum precipitates in temper annealing, which decreases the lowering of strength
caused by tempering treatment and thus helps in achieving high strength. Additionally,
Molybdenum is used
inter alia to prevent the brittleness of steel by slowing infiltration of
inter alia Phosphorus into the grain boundaries during temper annealing. Molybdenum also efficiently
increases the hardenability of the base material and ensures good strength properties
of welded seams of welded hot-rolled steel.
[0050] It has been found that Niobium may decrease the bendability of a hot-rolled steel
if it is present in a large amount. The use of Niobium as an alloying element is however
of advantage in achieving adequate strength and impact toughness in hot-rolled steel.
Niobium promotes smaller grain size in steel, which results in better properties of
the steel. Niobium may be needed, especially in the case of heavy plate, to enable
smaller amounts of other alloying elements that promote good strength and toughness
properties to be used. In the case of a direct quenched strip product, the steel can
be also made without using Niobium. Niobium is therefore an optional alloying element
in the hot-rolled steel according to the present invention, the content of which should
be limited to 0.06 mass-%, preferably to 0.04 mass-%, and more preferably to 0.005
mass-%, wherein the best possible bendability properties for the hot-rolled steel
are assured.
[0051] Titanium is an optional alloying element in the hot-rolled steel according to the
present invention, which may be required for binding Nitrogen in the steel, and so
that Boron functions efficiently as an improver of hardenability and does not form
Boron nitrides. Titanium is used, because it works more reliably with quenched steel
than Aluminium. The Titanium content is 0 - 0.15 mass-%, preferably 0 - 0.05 mass-%
and more preferably 0.005 - 0.02 mass-%. Titanium nitrides exhibit grain growth in
the heat affected zone of a weld and improve the toughness properties of a welded
seam. On the other hand, at contents higher than 0.02 mass-%, the amount of relatively
large-sized Titanium nitrides, TiN may increase, which is detrimental in terms of
the impact toughness and bending properties of the hot-rolled steel. The Ti/N ratio
of the hot-rolled steel is preferably in the range 3 - 4. However, larger Titanium
contents up to 0.15 mass-% may be used to increase strength in the as-tempered condition.
[0052] Vanadium content in the hot-rolled steel according to the present invention must
be more than 0.1 mass-% and ≤ 0.25 mass-%, preferably more than 0.10 and ≤ 0.20 mass-%,
or at least 0.11 mass-% Vanadium, or at least 0.12 mass-% Vanadium, or at least 0.13
mass-% Vanadium, or at least 0.14 mass-% Vanadium in order to assure high strength.
It has however been found that a too high amount of Vanadium is detrimental to the
impact toughness of the quenched and tempered steel. The amount of Vanadium should
not therefore exceed 0.25 mass-%. Vanadium has a strong precipitation strengthening
effect after tempering and is needed to achieve high strength both in base metal and
in the HAZ.
[0053] Aluminium is used to condense steel, i.e. to bind oxygen from the steel. The Aluminium
content is 0.01 - 0.15 mass-%, preferably 0.015 - 0.08 mass-% to prevent excessive
formation of aluminium oxides.
[0054] Boron is an effective alloying element that promotes hardenability of the steel in
quenching. It is an essential alloying element in this invention since it promotes
strength and hardness properties in the weld metal and heat-affected zone (HAZ). During
welding, Boron moves from the base material to the weld metal, thereby increasing
the hardness of the weld metal. This ensures that fracture does not occur in the weld
metal or fusion line. Fracture can be moved as far as possible away from the fusion
line towards the base material in high static loads. The Boron content is 0.0005 -
0.010 mass-%, preferably 0.0005 - 0.005 mass-% and more preferably 0.001 - 0.003 mass-%.
A Boron content of at least 0.0005 mass-% promotes hardenability of the base material
and of the HAZ, ensuring good strength properties. On the other hand, a Boron content
of more than 0.005 mass-% is worthless as regards the hardenability of the base material
and the HAZ. When the Boron content is more than 0.001 mass-%, it ensures matching
strength properties of the weld and fracture location as previously described. A Boron
content of more than 0.010 mass-% can be detrimental for mechanical properties of
the steel.
[0055] The Chromium content of hot-rolled steel according to the present invention is 0.
1 - 1.7 mass-%, preferably 0.4 - 1.7 or 0.6 - 1.5 mass-%, or more than 1.0 mass-%
in order to achieve high strength and good hardenability both in the hot-rolled steel
as produced and in the HAZ of a welded hot-rolled steel product. Chromium also promotes
tempering resistance.
[0056] According to an embodiment of the invention the chemical composition of hot-rolled
steel according to the present invention contains both Ni and Cu in a total amount
of at least 0.5 mass-%, or at least 1.0 mass-%, or at least 1.2 mass-%. Copper is
an optional alloying element. It can be used in an amount up to 1.5 mass-%, preferably
0.1-1.0 mass-% in order to increase the strength or improve the weather resistance
of the hot-rolled steel.
[0057] According to an embodiment of the invention the chemical composition contains both
Ni and Cu, and the amount of Ni ≥ 0.33 x the amount of Cu, preferably the amount of
Ni ≥ 0.5 x the amount of Cu. Cr+Cu+Ni is between 0.4 - 5.7, preferably between 1.4
- 3.5 and more preferably between 2 - 3.
[0058] Nickel is an essential alloying element in the hot-rolled steel according to the
present invention and it improves the toughness of the heat-affected zones and the
weld metal of welded seams and it also improves the surface quality of the hot-rolled
steel containing Copper but may, under some circumstances, slightly decrease the impact
toughness of the tempered steel.
[0059] Phosphorus weakens the impact toughness of quenched and tempered steel and the Phosphorus
content should therefore be limited to a maximum of 0.015 mass-%, preferably to a
maximum of 0 - 0.009 mass-%.
[0060] Sulphur content is limited to a maximum of 0.008 mass-%, preferably to a maximum
of 0.004 mass-%, to assure good impact toughness and formability in the hot-rolled
steel according to the present invention.
[0061] Zirconium is an optional alloying element that may replace Niobium if needed. The
Zirconium content can be between 0 - 0.2 mass-%, preferably 0 - 0.01 mass-%.
[0062] Calcium is an optional alloying element that may be used to modify the morphology
of inclusions in the steel. The Calcium content can be between 0 - 0.004 mass-%. If
the amount of Calcium exceeds 0.004 mass-% the inclusions in the steel may be too
large, which may adversely affect the physical properties of the steel.
[0063] The hot-rolled steel has a tensile strength of at least 1120 MPa and up to 1450 MPa.
[0064] According to an embodiment of the invention the hot-rolled steel has an A%-elongation
of at least 8% (i.e. permanent elongation of length, expressed in percent of length)
or even of at least 10% or at least 12% along and transverse to the rolling direction.
The hot-rolled steel has such an elongation in its as-produced condition. The hot-rolled
steel also has an elongation of at least 7%, preferably at least 8%, more preferably
at least 9% when a tensile test is carried out across a weld seam of a welded hot-rolled
steel product where the weld is longitudinal to the rolling direction.
[0065] According to an embodiment of the invention the hot-rolled steel an impact toughness
of at least 34 J/cm
2 and more preferably an impact toughness of at least 50 J/cm
2 when a Charpy V notched specimen having a thickness of 5-10 mm is measured at -20°C
and more preferably at -40°C longitudinally and transverse to the rolling direction.
The hot-rolled steel has such an impact strength in its as-produced condition.
[0066] The mechanical properties of the hot-rolled steel cited in this document were determined
in accordance with the testing instructions of standard ISO 10025-6:2004.
[0067] According to an embodiment of the invention the hot-rolled steel is metal active
gas (MAG) welded with our without reinforcement, using a V- or Y-groove welding method,
whereby a first pass is welded from a bottom or top side, preferably from a bottom
side, and other passes from a top side, using welding consumables having a tensile
strength of 1100 MPa, preferably 960 MPa, more preferably 900 MPa, most preferably
890 MPa, and a t8/5 time of 8 - 12 seconds, preferably 6 - 18 seconds, more preferably
5-20 seconds, which may be determined by welding the hot-rolled steel and measuring
the time it takes for the weld seam and the adjacent heat-affected zone (HAZ) to cool
from 800°C to 500°C.
[0068] According to an embodiment of the invention the hot-rolled steel has a minimum bending
radius of 5.0 x thickness or more preferably a minimum bending radius of 4.0 x thickness
or more preferably a minimum bending radius of 3.5 x thickness longitudinally and
transverse to the rolling direction. With a plate thickness of 7 mm or more the steel
has a minimum bending radius of 5.0 x thickness or preferably a minimum bending radius
of 4.0 x thickness or more preferably a minimum bending radius of 3.5 x thickness
longitudinally to the rolling direction, and a minimum bending radius of 5.0 x thickness
transverse to the rolling direction.
[0069] The hot-rolled steel according to the present invention is suitable for applications,
such as wear or structural applications, in which the steel must exhibit high strength
in combination with sufficient hardness, bendability and impact toughness both in
the as-produced products and in the HAZ (heat affected zone) of welded hot-rolled
steel products. For example, the hot-rolled steel according to the present invention
may be used to produce any component for construction, mining, material-handling,
earth-moving, pile driving, snow-plowing, landscaping or rock drilling equipment.
The hot-rolled steel may for example be used to produce a lifting boom for an excavator
or crane.
TEST RESULTS
[0070] Tests were conducted using the steels having the chemical compositions presented
in Table 1 below. The amount of each element is given in mass-%, the balance being
Fe and unavoidable impurities other than Nitrogen. It should be noted that Nitrogen
may also be considered to be an unavoidable impurity. The amount of Nitrogen is however
given in Table 1 along with the intentionally added alloying elements. The amount
of Nitrogen is in the range of 0-0.01 mass-%.
[0071] It should be noted that the compositions labelled "INV" in Table 1 are steels that
have the chemical composition and the physical properties of steel according to the
present invention and which have been manufactured using a method according to the
present invention. Comparative examples that do not have the chemical composition
or the physical properties of steel according to the present invention, or which have
not been manufactured using a method according to the present invention are labelled
"REF" in Table 1.
Table 1: Chemical compositions
Comp. |
C |
Si |
Mn |
P |
S |
Al |
Nb |
V |
Cu |
Cr |
Ni |
N |
Mo |
Ti |
Ca |
B |
INV/ REF |
A1 |
0.13 |
0.20 |
1.5 |
0.01 |
0.001 |
0.05 |
0.00 |
0.15 |
0.4 |
1.3 |
1.0 |
0.004 |
0.4 |
0.01 |
0.002 |
0.001 |
INV |
B |
0.14 |
0.18 |
1.5 |
0.01 |
0.001 |
0.05 |
0.00 |
0.15 |
0.5 |
1.3 |
2.0 |
0.004 |
0.1 |
0.01 |
0.002 |
0.001 |
REF |
C |
0.17 |
0.19 |
1.5 |
0.01 |
0.001 |
0.05 |
0.04 |
0.05 |
0.5 |
0.7 |
1.0 |
0.004 |
0.4 |
0.01 |
0.002 |
0.001 |
INV |
A2 |
0.13 |
0.20 |
1.5 |
0.01 |
0.002 |
0.05 |
0.00 |
0.15 |
0.5 |
1.4 |
1.0 |
0.004 |
0.4 |
0.01 |
0.003 |
0.002 |
INV |
A3 |
0.13 |
0.18 |
1.5 |
0.01 |
0.002 |
0.06 |
0.00 |
0.15 |
0.4 |
1.3 |
1.0 |
0.004 |
0.4 |
0.01 |
0.003 |
0.001 |
INV |
D |
0.17 |
0.30 |
1.5 |
0.01 |
0.001 |
0.05 |
0.00 |
0.04 |
0.4 |
0.7 |
1.0 |
0.003 |
0.4 |
0.02 |
0.001 |
0.000 |
REF |
E |
0.13 |
0.19 |
1.5 |
0.01 |
0.001 |
0.05 |
0.00 |
0.04 |
0.4 |
0.7 |
1.5 |
0.003 |
0.4 |
0.02 |
0.001 |
0.000 |
REF |
F |
0.17 |
0.19 |
1.5 |
0.01 |
0.001 |
0.06 |
0.00 |
0.04 |
0.4 |
0.7 |
1.0 |
0.003 |
0.4 |
0.02 |
0.001 |
0.001 |
REF |
G |
0.18 |
0.03 |
1.5 |
0.01 |
0.001 |
0.05 |
0.00 |
0.04 |
0.4 |
1.0 |
1.0 |
0.003 |
0.4 |
0.02 |
0.001 |
0.001 |
REF |
[0072] Steels having the chemical compositions presented in Table 1 were hot-rolled to an
end thickness of 4 mm, 6 mm and 8 mm. Hot-rolling was performed in a hot strip rolling
line and hot-rolled strips were directly quenched after rolling before coiling. Depending
on the type of tempering furnace used, the tempering was carried out before or after
a cut-to-length process. If the tempering was performed in bell type furnace (tempering
code "C" in Table 2 below), then the cut-to-length processing for quenched strips
was carried out after tempering. In the case of sheet tempering (tempering code "S"
in Table 2), the cut-to-length processing was carried out before the tempering annealing.
Depending on the tempering method, the holding time during tempering varied between
15 - 720 minutes.
[0073] More specific manufacturing parameters are presented in Table 2.
Table 2: Process parameters
composition |
process |
tempering |
thickness |
furnT |
FRT |
CT |
T |
t |
I NV/REF |
code |
code |
code |
mm |
°C |
°C |
°C |
°C |
min |
A1 |
R02 |
S600 |
8.0 |
1280 |
856 |
50 |
600 |
30 |
INV |
B |
R04 |
S600 |
8.0 |
1280 |
862 |
50 |
600 |
30 |
REF |
c |
R06 |
S560 |
8.0 |
1280 |
878 |
50 |
560 |
30 |
INV |
A2 |
R12 |
S600 |
4.0 |
1278 |
924 |
50 |
600 |
15 |
INV |
A2 |
R13 |
S585 |
8.0 |
1267 |
871 |
50 |
585 |
15 |
INV |
A2 |
R13 |
S600 |
8.0 |
1267 |
871 |
50 |
600 |
15 |
INV |
A3 |
R14 |
S585 |
6.0 |
1279 |
897 |
50 |
585 |
15 |
INV |
A3 |
R14 |
S600 |
6.0 |
1279 |
897 |
50 |
600 |
15 |
INV |
A2 |
R16 |
S585 |
4.0 |
1279 |
917 |
50 |
585 |
15 |
INV |
A2 |
R16 |
S600 |
4.0 |
1279 |
917 |
50 |
600 |
15 |
INV |
D |
B05 |
C420 |
6.0 |
1250 |
919 |
100 |
420 |
720 |
REF |
A1 |
B09 |
C200 |
8.0 |
1250 |
891 |
100 |
200 |
720 |
REF |
E |
B10 |
C600 |
6.0 |
1250 |
876 |
100 |
600 |
240 |
REF |
F |
B12 |
C450 |
6.0 |
1250 |
897 |
100 |
450 |
540 |
REF |
G |
B14 |
C450 |
6.0 |
1250 |
918 |
100 |
450 |
540 |
REF |
D |
B15 |
C600 |
6.0 |
1250 |
917 |
100 |
600 |
240 |
REF |
where: furnT = reheat temperature before hot-rolling
FRT = finishing rolling temperature
CT = coiling temperature
T = tempering temperature
t = tempering time
and the process code indicates the geographical location at which each process was
carried out.
[0074] Test results from mechanical tests and bending tests are presented in Table 3. Steels
according to invention have a mechanical properties as stipulated in claim 1.
[0075] Bending tests were carried out using three-point bending with samples having an area
of 300 x 300 mm
2. Samples were bent to an angle of 90° with one press and all samples were bent into
a Z-shape so that both the upper and lower surfaces of the samples were tested. Mechanical
properties and bendability of the samples were tested both longitudinally with respect
to the rolling direction, and transversely with respect to the rolling direction.
Table 3: Physical properties
composition code |
rolling code |
tempering code |
thickness mm |
test direction T/L |
Rp0.2 MPa |
Rm MPa |
A% % |
T_test °C |
CV J |
CV J/cm2 |
Bendability |
INV/REF |
Ri/t |
A1 |
R02 |
S600 |
8.0 |
T |
1151 |
1188 |
13.6 |
-40 |
33 |
55.0 |
4.5 |
INV |
A1 |
R02 |
S600 |
8.0 |
L |
1170 |
1180 |
15.0 |
-40 |
46 |
76.7 |
2.8 |
INV |
B |
R04 |
S600 |
8.0 |
T |
1096 |
1128 |
14.7 |
-40 |
28 |
46.7 |
4.0 |
REF |
B |
R04 |
S600 |
8.0 |
L |
1108 |
1119 |
15.4 |
-40 |
35 |
58.3 |
2.8 |
REF |
C |
R06 |
S560 |
8.0 |
T |
1158 |
1196 |
13.3 |
-40 |
23 |
38.3 |
3.5 |
INV |
C |
R06 |
S560 |
8.0 |
L |
1171 |
1185 |
14.2 |
-40 |
42 |
70.0 |
2.8 |
INV |
A2 |
R12 |
S600 |
4.0 |
T |
1111 |
1166 |
13.9 |
-40 |
10 |
50.0 |
2.0 |
INV |
A2 |
R12 |
S600 |
4.0 |
L |
1117 |
1170 |
15.1 |
-40 |
15 |
75.0 |
2.3 |
INV |
A2 |
R13 |
S585 |
8.0 |
T |
1170 |
1204 |
14.0 |
-40 |
29 |
48.3 |
2.1 |
INV |
A2 |
R13 |
S585 |
8.0 |
L |
1155 |
1177 |
15.1 |
-40 |
46 |
76.7 |
2.8 |
INV |
A2 |
R13 |
S600 |
8.0 |
T |
1163 |
1196 |
14.0 |
-40 |
28 |
46.7 |
2.1 |
INV |
A2 |
R13 |
S600 |
8.0 |
L |
1151 |
1171 |
15.4 |
-40 |
47 |
78.3 |
2.5 |
INV |
A3 |
R14 |
S585 |
6.0 |
T |
1129 |
1173 |
14.5 |
-40 |
18 |
45.0 |
2.3 |
INV |
A3 |
R14 |
S585 |
6.0 |
L |
1118 |
1152 |
15.6 |
-40 |
30 |
75.0 |
2.7 |
INV |
A3 |
R14 |
S600 |
6.0 |
T |
1128 |
1171 |
14.2 |
-40 |
18 |
45.0 |
2.2 |
INV |
A3 |
R14 |
S600 |
6.0 |
L |
1115 |
1147 |
15.5 |
-40 |
32 |
80.0 |
2.3 |
INV |
A2 |
R16 |
S585 |
4.0 |
T |
1134 |
1178 |
14.6 |
-40 |
16 |
50.0 |
2.0 |
INV |
A2 |
R16 |
S585 |
4.0 |
L |
1134 |
1163 |
15.8 |
-40 |
24 |
75.0 |
2.8 |
INV |
D |
B05 |
C420 |
6.0 |
T |
1182 |
1277 |
7.3 |
-40 |
17 |
42.5 |
3.2 |
REF |
D |
B05 |
C420 |
6.0 |
L |
1125 |
1251 |
9.0 |
-40 |
21 |
52.5 |
3.2 |
REF |
A1 |
B09 |
C200 |
8.0 |
T |
1349 |
1551 |
6.7 |
-40 |
53 |
88.3 |
2.5 |
REF |
A1 |
B09 |
C200 |
8.0 |
L |
1327 |
1519 |
9.0 |
-40 |
79 |
131.7 |
3.4 |
REF |
E |
B10 |
C600 |
6.0 |
T |
1098 |
1137 |
8.9 |
-40 |
27 |
67.5 |
2.2 |
REF |
E |
B10 |
C600 |
6.0 |
L |
1023 |
1108 |
11.8 |
-40 |
35 |
87.5 |
2.3 |
REF |
F |
B12 |
C450 |
6.0 |
T |
1170 |
1257 |
7.9 |
-40 |
20 |
50.0 |
2.8 |
REF |
F |
B12 |
C450 |
6.0 |
L |
1098 |
1229 |
8.9 |
-40 |
29 |
72.5 |
3.3 |
REF |
G |
B14 |
C450 |
6.0 |
T |
1180 |
1265 |
7.0 |
-40 |
19 |
47.5 |
2.7 |
REF |
G |
B14 |
C450 |
6.0 |
L |
1105 |
1234 |
9.9 |
-40 |
28 |
70.0 |
3.2 |
REF |
D |
B15 |
C600 |
6.0 |
T |
1095 |
1143 |
8.7 |
-40 |
19 |
47.5 |
1.8 |
REF |
D |
B15 |
C600 |
6.0 |
L |
1028 |
1132 |
11.3 |
-40 |
30 |
75.0 |
2.0 |
REF |
Welding tests
[0076] Welding tests were carried out using a metal active gas (MAG) welding method and
V- and Y -grooves. The welding consumables used were according to standard ENG 89
5 M21 Mn4Ni2.5CrMo (Commercial grade X96). The first pass was welded from bottom or
top side, preferably from the bottom side, and others passes were welded from the
top side. Welding consumables having a tensile strength of 960 MPa were used and t8/5
was varied between 6 - 18 seconds. Tensile tests across the weld showed that the weld
had a yield strength of 1100 MPa (Rp0.2) and the fracture was located at the base
metal (BM).
[0077] The target was to achieve a combination of strength and toughness properties which
are as good as possible in the weld so that matching tensile properties can be achieved
without losing toughness. In addition, the aim was to obtain a fracture in a static
tensile test over the weld that is located as far as possible from the weld metal
(WM) and fusion line (FL), which enables extremely good elongation to fracture values
for the welded structure. The behaviour of a welded structure is predictable and safe
when the fracture in static loading takes place as far as possible away from the WM
and FL and the elongation to fracture is high. The inventors have found that steels
according to the present invention can fulfill these requirements even if the yield
strength for the base material is more than 1100 MPa. Usually, known steels with such
a high strength have a fracture located over the weld (WM or FL) when a tensile test
is performed, especially when un-matching welding consumables are used (the yield
strength of un-matching welding consumables is typically less than 1100 MPa).
[0078] Table 4 shows the welding parameters that were used in the tests and the test results
obtained. Steels according the invention have a fracture that is located at a distance
from the weld (WM and/or FL) when a static load in a tensile test is set over the
weld. It is surprising compared to known steels that such behaviour can be achieved
with or even without reinforcement. Without reinforcement, the achievement of such
behaviour is very innovative. Fracture location is labelled "BM" in Table 4 when the
fracture is in the base material, "HAZ" when it occurs in the Heat Affected Zone,
and "WM" when the fracture occurs in the Weld Metal.
Table 4: Welding results
Processing variables |
Transverse tensile test on weld |
Cha rpy V (5x10x55 mm) |
INV/REF |
Without reinforcement |
With reinforcement |
J /-40°C (mea n of 3 tests) |
composition code |
rolling code |
tempering code |
thickness mm |
t8/5s |
Rp0.2 MPa |
Rm MPa |
A%% |
fracture location |
Rp0.2 MPa |
Rm MPa |
A%% |
fracture location |
WM |
FL |
FL+1 |
FL+3 |
FL+5 |
A1 |
R02 |
S600 |
8 |
12 |
1125 |
1201 |
11,8 |
BM |
1140 |
1194 |
10,8 |
BM |
18 |
≠ |
25 |
37 |
20 |
INV |
B |
R04 |
S600 |
8 |
12 |
1078 |
1130 |
11,2 |
BM |
1080 |
1123 |
11,2 |
BM |
16 |
19 |
31 |
33 |
19 |
REF |
C |
R05 |
S560 |
8 |
6 |
1154 |
1213 |
9,8 |
BM |
1160 |
1204 |
9,6 |
BM |
≠ |
18 |
31 |
18 |
17 |
INV |
C |
R05 |
S560 |
8 |
12 |
1140 |
1212 |
10,0 |
BM |
1143 |
1201 |
10,3 |
BM |
20 |
18 |
24 |
32 |
18 |
INV |
C |
R05 |
S560 |
8 |
18 |
1115 |
1207 |
10,5 |
BM |
1135 |
1198 |
10,4 |
BM |
22 |
19 |
26 |
34 |
15 |
INV |
D |
B05 |
C420 |
6 |
6 |
1128 |
1233 |
4,7 |
HAZ |
1174 |
1275 |
8,4 |
BM |
27 |
27 |
36 |
24 |
25 |
REF |
D |
B05 |
C420 |
6 |
12 |
1081 |
1185 |
4,0 |
WM |
1157 |
1252 |
6,2 |
HAZ |
31 |
33 |
49 |
32 |
28 |
REF |
D |
B05 |
C420 |
6 |
18 |
1067 |
1175 |
5,0 |
WM |
1162 |
1248 |
5,5 |
HAZ |
35 |
43 |
53 |
35 |
30 |
REF |
F |
B12 |
C450 |
6 |
12 |
1101 |
1208 |
5,1 |
WM |
1162 |
1244 |
7,3 |
HAZ |
31 |
30 |
33 |
53 |
28 |
REF |
G |
B14 |
C450 |
6 |
12 |
1106 |
1217 |
5,2 |
WM |
1161 |
1247 |
5,7 |
HAZ |
27 |
29 |
32 |
40 |
26 |
REF |
A2 |
R12 |
S600 |
4 |
12 |
1107 |
1176 |
11,7 |
BM |
1118 |
1190 |
11,6 |
BM |
≠ |
≠ |
≠ |
≠ |
≠ |
INV |
where t8/5 = cooling time from 800 °C to 500 °C in the welded seam
Impact toughness was measured using specimens having a thickness of 5 mm.
[0079] Figures 2 and 3 shows typical hardness profiles over the welded seam tested near
the face side and the root side of the welded samples. It is surprising that steels
according to the present invention can have a very smooth hardness profile over the
weld and that there are no soft zones that could start to neck during a tensile test
and thereby influence the location of the fracture. Normally, steels with a yield
strength of 1100 MPa welded with un-matching welding consumables (X90 and/or X96)
exhibit some softening in the HAZ and especially in the WM. Steel according to the
invention can maintain good hardness in the HAZ but also have good hardness in the
WM due to the diffusion of alloying elements promoting hardening (i.e. Boron). Low
carbon content in the steels according to the invention (i.e. 0.1 - 0.20 mass-% Carbon)
ensures that a weld has high toughness as well as good hardness.
[0080] Further modifications of the invention within the scope of the claims would be apparent
to a skilled person.
1. Hot-rolled steel having a yield strength Rp
0.2 of at least 1100 MPa along and transverse to a rolling direction and a tensile strength
of at least 1120 MPa and up to 1450 MPa along and transverse to a rolling direction,
characterized in that: it has an A%-elongation of at least 8% along and/or transverse to the rolling direction,
an impact toughness of at least 34 J/cm
2 when a Charpy V notched specimen having a thickness of 5-10 mm is measured at -40°C
longitudinally to the rolling direction, a minimum bending radius of 5.0 x thickness
longitudinally and/or transverse to the rolling direction, and a chemical composition
containing in mass-%:
• C 0.10 - 0.2
• Si 0 - 0.7
• Mn 1.3 - 2.2
• Nb 0 - 0.06
• Ti 0 - 0.15
• V more than 0.03 and ≤ 0.25
• Al 0.01 - 0.15
• B 0.0005 - 0.010
• Cr 0.1 - 1.7
• Mo 0.15 - 0.8
• Cu 0 - 1.5
• Ni 0.3 - 2.5
• P 0 - 0.015
• S 0 -0.008
• Zr 0 - 0.2
• Ca 0 - 0.004
• N 0-0.01
• balance Fe and unavoidable impurities,
whereby:
a) when 0.10 < C < 0.11 then Mn ≥ 1.60 and V > 0.14 and Mo ≥ 0.5
b) when 0.11 ≤ C < 0.125 then Mn ≥ 1.45 and V ≥ 0.13 and Mo ≥ 0.35
c) when 0.125 ≤ C < 0.15, then Mn ≥ 1.35 and V ≥ 0.12 and Mo ≥ 0.20, and
d) when C ≥ 0.15 and V > 0.11, then Mn ≥ 1.3 and Mo ≥ 0.15 or
when C ≥ 0.15 and V 0.03 - 0.11, then Mn > 1.3 and Mo > 0.15 and Nb > 0.02 and Cr+Cu+Ni
>1.4.
2. Hot-rolled steel according to claim 1, whereby said chemical composition comprises
0.4-1.7 mass-% Cr, preferably 1.0 -1.7 mass-% Cr.
3. Hot-rolled steel according to claim 1 or 2, whereby said chemical composition contains
both Ni and Cu, in a total amount of at least 0.5 mass-%, preferably at least 1 mass-%.
4. Hot-rolled steel according to any of the preceding claims, whereby the steel has an
A%- elongation of at least 10% or more preferably at least 12% along and/or transverse
to the rolling direction.
5. Hot-rolled steel according to any of the preceding claims, whereby the steel has an
impact toughness of at least 50 J/cm2 when a Charpy V notched specimen having a thickness of 5-10 mm is measured at -40°C
longitudinally to the rolling direction.
6. Hot-rolled steel according to any of the preceding claims, whereby the steel has a
minimum bending radius of 4.0 x thickness longitudinally and/or transverse to the
rolling direction, more preferably a minimum bending radius of 3.5 x thickness longitudinally.
7. Hot-rolled steel according to any of the preceding claims, whereby the steel has an
A%- elongation of at least 7%, preferably of at least 8% or more preferably at least
9% when a tensile test is carried out across a weld seam of a welded hot-rolled steel
product where the weld is longitudinal to the rolling direction, whereby said hot-rolled
steel is welded using metal active gas (MAG) welding with reinforcement, using a V-
or Y-groove welding method, whereby a first pass is welded from a bottom or top side,
preferably from a bottom side, and other passes from a top side, using welding consumables
according to standard ENG 89 5 M21, commercial grade(s) X90 and/or X96, having a tensile
strength of 890 MPa, preferably 960 MPa, more preferably 1100 MPa and a t8/5 of 5
- 20 seconds, preferably 6 - 18 seconds, or 8-12 seconds, whereby the welded steel
has a fracture that is located at a distance of at least 1 mm from the fusion line.
8. Method for manufacturing hot-rolled steel according to claim 1, whereby the method
comprises the following steps:
- heating a steel slab having a chemical composition according to claim 1 to an austenitizing
temperature of 1000 - 1350 °C,
- hot-rolling such that a finishing rolling temperature is 760 - 1050°C,
- quenching to a temperature of 300 °C or less, and temper annealing at a temperature
of 500 - 650 °C if the tempering time is 1 hour or more, or temper annealing at a
temperature of 500 - 750 °C if the tempering time is less than 1 hour after said quenching
step, whereby the microstructure of the hot-rolled steel before the temper annealing
step contains at least 90 area-% martensite when said microstructure is examined in
¼ thickness, and the content of ferrite and pearlite before the temper annealing step
must be in total less than 10 area-%.
9. Method according to claim 8, whereby the microstructure of the hot-rolled steel before
said temper annealing step contains at least 95 area-% martensite and more preferably
at least 99 area-% martensite when said microstructure is examined in ¼ thickness.
10. Method according to claim 8 or 9, whereby said quenching step is a direct quenching
step.
11. Method according to any of claims 8-10, whereby the method comprises the step of strip
rolling said hot-rolled steel and said hot-rolled steel comprises a maximum of 0.005
mass-% Niobium and < 0.15 mass-% Carbon.
12. Method according to any of claims 8-10, whereby said hot-rolled steel comprises a
minimum of 0.005 mass-% Niobium, preferably a minimum of 0.02 mass-% Niobium when
the hot-rolled steel is not strip rolled.
13. Use of a hot-rolled steel according to any of claims 1-7 for metal active gas (MAG)
welding with reinforcement, using a V- or Y-groove welding method, whereby a first
pass is welded from a bottom or top side, preferably from a bottom side, and other
passes from a top side, using welding consumables according to standard ENG 89 5 M21,
commercial grade(s) X90 and/or X96, having a tensile strength of 1100 MPa, preferably
960 MPa, more preferably 900 MPa, most preferably 890 MPa, and a t8/5 time of 8 -
12 seconds, preferably 6 - 18 seconds, more preferably 5-20 seconds, wherein the t8/5
time is the time it takes for the total weld area to cool from 800°C to 500°C , whereby
the welded steel has a fracture that is located at a distance of at least 1 mm from
the fusion line.
1. Warmgewalzter Stahl mit einer Streckgrenze Rp
0,2 von mindestens 1100 MPa entlang und quer zu einer Walzrichtung und einer Zugfestigkeit
von mindestens 1120 MPa und bis zu 1450 MPa entlang und quer zu einer Walzrichtung,
dadurch gekennzeichnet, dass: er eine A%-Dehnung von mindestens 8 % entlang und/oder quer zu der Walzrichtung,
eine Kerbschlagzähigkeit von mindestens 34J/cm
2, wenn eine Probe mit einer V-Kerbe nach Charpy mit einer Dicke von 5-10 mm bei -40
°C längs zu der Walzrichtung gemessen wird, einen minimalen Biegeradius von 5,0 x
Dicke längs und/oder quer zu der Walzrichtung und eine chemische Zusammensetzung aufweist,
die in Masse-% enthält:
• C 0,10-0,2
• Si 0-0,7
• Mn 1,3-2,2
• Nb 0-0,06
• Ti 0-0,15
• V mehr als 0,03 und ≤ 0,25
• Al 0,01-0,15
• B 0,0005-0,010
• Cr 0,1-1,7
• Mo 0,15-0,8
• Cu 0-1,5
• Ni 0,3-2,5
• P 0-0,015
• S 0-0,008
• Zr 0-0,2
• Ca 0-0,004
• N 0-0,01
• Rest Fe und unvermeidbare Verunreinigungen,
wobei:
a) wenn 0,10 < C < 0,11 dann Mn ≥ 1,60 und V > 0,14 und Mo ≥ 0,5
b) wenn 0,11 ≤ C < 0,125 dann Mn ≥ 1,45 und V ≥ 0,13 und Mo ≥ 0,35
c) wenn 0,125 ≤ C < 0,15 dann Mn ≥ 1,35 und V ≥ 0,12 und Mo ≥ 0,20 und
d) wenn C ≥ 0,15 und V > 0,11 dann Mn ≥ 1,3 und Mo ≥ 0,15 oder
wenn C ≥ 0,15 und V 0,03-0,11 dann Mn > 1,3 und Mo > 0,15 und Nb > 0,02 und Cr+Cu+Ni
> 1,4.
2. Warmgewalzter Stahl nach Anspruch 1, wobei die chemische Zusammensetzung 0,4-1,7 Masse-%
Cr, vorzugsweise 1,0-1,7 Masse-% Cr umfasst.
3. Warmgewalzter Stahl nach Anspruch 1 oder 2, wobei die chemische Zusammensetzung sowohl
Ni als auch Cu in einer Gesamtmenge von mindestens 0,5 Masse-%, vorzugsweise von mindestens
1 Masse-% enthält.
4. Warmgewalzter Stahl nach einem der vorstehenden Ansprüche, wobei der Stahl eine A%-Dehnung
von mindestens 10 % oder besonders bevorzugt von mindestens 12 % entlang und/oder
quer zu der Walzrichtung aufweist.
5. Warmgewalzter Stahl nach einem der vorstehenden Ansprüche, wobei der Stahl eine Kerbschlagzähigkeit
von mindestens 50J/cm2 aufweist, wenn eine Probe mit einer V-Kerbe nach Charpy mit einer Dicke von 5-10
mm bei -40 °C längs zu der Walzrichtung gemessen wird.
6. Warmgewalzter Stahl nach einem der vorstehenden Ansprüche, wobei der Stahl einen minimalen
Biegeradius von 4,0 x Dicke längs und/oder quer zu der Walzrichtung, besonders bevorzugt
einen minimalen Biegeradius von 3,5 x Dicke längs aufweist.
7. Warmgewalzter Stahl nach einem der vorstehenden Ansprüche, wobei der Stahl eine A%-Dehnung
von mindestens 7 %, vorzugsweise von mindestens 8 % oder besonders bevorzugt von mindestens
9 % aufweist, wenn ein Zugversuch quer über eine Schweißnaht eines geschweißten, warmgewalzten
Stahlprodukts ausgeführt wird, wo die Schweißung längs zu der Walzrichtung verläuft,
wobei der warmgewalzte Stahl durch Anwenden von Metall-Aktivgas-Schweißen (MAG-Schweißen)
mit Verstärker geschweißt wird, durch Anwenden eines Schweißverfahrens mit V- oder
Y-Naht, wobei eine erste Lage ausgehend von einer Unter- oder Oberseite geschweißt
wird, vorzugsweise von einer Unterseite, und weitere Lagen ausgehend von einer Oberseite,
unter Verwendung von Schweißzusätzen gemäß der Norm ENG 89 5 M21, Handelssorte(n)
X90 und/oder X96, mit einer Zugfestigkeit von 890 MPa, vorzugsweise 960 MPa, besonders
bevorzugt 1100 MPa und einer t8/5 von 5-20 Sekunden, vorzugsweise 6-18 Sekunden oder
8-12 Sekunden, wobei der geschweißte Stahl einen Bruch aufweist, der sich in einem
Abstand von mindestens 1 mm von der Schmelzlinie befindet.
8. Verfahren zur Herstellung von warmgewalztem Stahl nach Anspruch 1, wobei das Verfahren
die folgenden Schritte umfasst:
- Erhitzen einer Stahlbramme mit einer chemischen Zusammensetzung nach Anspruch 1
auf eine Austenitisierungstemperatur von 1000-1350 °C,
- Warmwalzen, sodass eine abschließende Walztemperatur 760-1050 °C beträgt,
- Härten auf eine Temperatur von 300 °C oder weniger und Spannungsarmglühen bei einer
Temperatur von 500-650 °C, wenn die Anlasszeit 1 Stunde oder mehr beträgt, oder Spannungsarmglühen
bei einer Temperatur von 500-750 °C, wenn die Anlasszeit weniger als 1 Stunde beträgt,
nach dem Härtungsschritt, wobei die Mikrostruktur des warmgewalzten Stahls vor dem
Schritt des Spannungsarmglühens mindestens 90 Flächen-% Martensit enthält, wenn die
Mikrostruktur in einer ¼-Dicke untersucht wird, und der Gehalt von Ferrit und Pearlit
vor dem Schritt des Spannungsarmglühens insgesamt weniger als 10 Flächen-% betragen
muss.
9. Verfahren nach Anspruch 8, wobei die Mikrostruktur des warmgewalzten Stahls vor dem
Schritt des Spannungsarmglühens mindestens 95 Flächen-% Martensit und besonders bevorzugt
mindestens 99 Flächen-% Martensit enthält, wenn die Mikrostruktur in einer ¼-Dicke
untersucht wird.
10. Verfahren nach Anspruch 8 oder 9, wobei der Härtungsschritt ein Direkthärtungsschritt
ist.
11. Verfahren nach einem der Ansprüche 8-10, wobei das Verfahren den Schritt des Bandwalzens
des warmgewalzten Stahls umfasst und der warmgewalzte Stahl höchstens 0,005 Masse-%
Niob und < 0,15 Masse-% Kohlenstoff umfasst.
12. Verfahren nach einem der Ansprüche 8-10, wobei der warmgewalzte Stahl mindestens 0,005
Masse-% Niob, vorzugsweise mindestens 0,02 Masse-% Niob umfasst, wenn der warmgewalzte
Stahl nicht bandgewalzt wird.
13. Verwendung eines warmgewalzten Stahls nach einem der Ansprüche 1-7 zum Metall-Aktivgas-Schweißen
(MAG-Schweißen) mit Verstärker, durch Anwenden eines Schweißverfahrens mit V- oder
Y-Naht, wobei eine erste Lage ausgehend von einer Unter- oder Oberseite geschweißt
wird, vorzugsweise von einer Unterseite, und weitere Lagen ausgehend von einer Oberseite,
unter Verwendung von Schweißzusätzen gemäß der Norm ENG 89 5 M21, Handelssorte(n)
X90 und/oder X96, mit einer Zugfestigkeit von 1100 MPa, vorzugsweise 960 MPa, besonders
bevorzugt 900 MPa, am meisten bevorzugt 890 MPa, und einer t8/5 von 8-12 Sekunden,
vorzugsweise 6-18 Sekunden, besonders bevorzugt 5-20 Sekunden, wobei die t8/5-Zeit
jene Zeit ist, die es braucht, damit die gesamte Schweißfläche von 800 °C auf 500
°C abkühlt, wobei der geschweißte Stahl einen Bruch aufweist, der sich in einem Abstand
von mindestens 1 mm von der Schmelzlinie befindet.
1. Acier laminé à chaud présentant une limite d'élasticité Rp
0,2 d'au moins 1100 MPa longitudinalement et transversalement à une direction de laminage
et une résistance à la traction d'au moins 1120 MPa et jusqu'à 1450 MPa longitudinalement
et transversalement à une direction de laminage,
caractérisé en ce que : il présente un allongement à la rupture d'au moins 8 % longitudinalement et/ou transversalement
à la direction de laminage, une résistance au choc d'au moins 34J/cm
2 lorsqu'une éprouvette Charpy entaillée en V présentant une épaisseur de 5 à 10 mm
est mesurée à -40 °C longitudinalement à la direction de laminage, un rayon de courbure
minimum de 5,0 fois l'épaisseur longitudinalement et/ou transversalement à la direction
de laminage, et une composition chimique contenant en % en masse :
• C 0,10 - 0,2
• Si 0 - 0,7
• Mn 1,3 - 2,2
• Nb 0 - 0,06
• Ti 0 - 0,15
• V supérieur à 0,03 et ≤ 0,25
• Al 0,01 - 0,15
• B 0,0005 - 0,010
• Cr 0,1 - 1,7
• Mo 0,15 - 0,8
• Cu 0 - 1,5
• Ni 0,3 - 2,5
• P 0 - 0,015
• S 0 - 0,008
• Zr 0 - 0,2
• Ca 0 - 0,004
• N 0 - 0,01
• le reste en fer et impuretés inévitables,
selon lequel :
a) lorsque 0,10 < C < 0,11 alors Mn ≥ 1,60 et V > 0,14 et Mo ≥ 0,5
b) lorsque 0,11 ≤ C < 0,125 alors Mn ≥ 1,45 et V ≥ 0,13 et Mo ≥ 0,35
c) lorsque 0,125 ≤ C < 0,15, alors Mn ≥ 1,35 et V ≥ 0,12 et Mo ≥ 0,20, et
d) lorsque C ≥ 0,15 et V > 0,11, alors Mn ≥ 1,3 et Mo ≥ 0,15 ou
lorsque C ≥ 0,15 et V 0,03 - 0,11, alors Mn > 1,3 et Mo > 0,15 et Nb > 0,02 et Cr
+ Cu + Ni > 1,4.
2. Acier laminé à chaud selon la revendication 1, selon lequel ladite composition chimique
comprend 0,4 - 1,7 % en masse de Cr, préférentiellement de 1,0 à 1,7 % en masse de
Cr.
3. Acier laminé à chaud selon la revendication 1 ou 2, selon lequel ladite composition
chimique contient à la fois du Ni et du Cu, en une quantité totale d'au moins 0,5
% en masse, préférentiellement au moins 1 % en masse.
4. Acier laminé à chaud selon l'une quelconque des revendications précédentes, selon
lequel l'acier présente un allongement à la rupture d'au moins 10% ou plus préférentiellement
d'au moins 12 % longitudinalement et/ou transversalement à la direction de laminage.
5. Acier laminé à chaud selon l'une quelconque des revendications précédentes, selon
lequel l'acier présente une résistance au choc d'au moins 50 J/cm2 lorsqu'une éprouvette Charpy entaillée en V présentant une épaisseur de 5 à 10 mm
est mesurée à -40 °C longitudinalement à la direction de laminage.
6. Acier laminé à chaud selon l'une quelconque des revendications précédentes, selon
lequel l'acier présentant un rayon de courbure minimum de 4,0 fois l'épaisseur longitudinalement
et/ou transversalement à la direction de laminage, plus préférentiellement un rayon
de courbure minimum de 3,5 fois l'épaisseur longitudinalement.
7. Acier laminé à chaud selon l'une quelconque des revendications précédentes, selon
lequel l'acier présente un allongement à la rupture d'au moins 7 %, préférentiellement
d'au moins 8 % ou plus préférentiellement d'au moins 9 % lorsqu'un essai de traction
est effectué à travers un cordon de soudure d'un produit en acier laminé à chaud soudé
où la soudure est longitudinale par rapport à la direction de laminage, selon lequel
ledit acier laminé à chaud est soudé en utilisant un soudage MAG avec renfort, en
utilisant un procédé de soudage à chanfrein en V ou en Y, selon lequel une première
passe est soudée à partir d'un côté inférieur ou supérieur, préférentiellement à partir
d'un côté inférieur, et d'autres passes à partir d'un côté supérieur, en utilisant
des consommables de soudage conformément à la norme ENG 89 5 M21, aux qualité(s) commerciale(s)
X90 et/ou X96, présentant une résistance à la traction de 890 MPa, préférentiellement
de 960 MPa, plus préférentiellement de 1100 MPa et un t8/5 de 5 à 20 secondes, préférentiellement
de 6 à 18 secondes, ou de 8 à 12 secondes, selon lequel l'acier soudé présente une
rupture située à une distance d'au moins 1 mm de la ligne de fusion.
8. Procédé de fabrication d'acier laminé à chaud selon la revendication 1, selon lequel
le procédé comprenant les étapes consistant à :
- chauffer une brame d'acier présentant une composition chimique selon la revendication
1 à une température d'austénitisation de 1000 à 1350 °C,
- laminer à chaud de sorte qu'une température de laminage de finition soit de 760
à 1050 °C,
- tremper à une température de 300 °C ou moins, et procéder à un recuit de revenu
à une température de 500 à 650 °C si le temps de revenu est de 1 heure ou plus, ou
procéder à un recuit de revenu à une température de 500 à 750 °C si le temps de revenu
est de moins de 1 heure après ladite étape de trempe, selon lequel la microstructure
de l'acier laminé à chaud avant l'étape de recuit de revenu contient au moins 90 %
en surface de martensite lorsque ladite microstructure est examinée sur ¼ d'épaisseur,
et la teneur en ferrite et en perlite avant l'étape de recuit de revenu doit être
au total de moins de 10 % en surface.
9. Procédé selon la revendication 8, selon lequel la microstructure de l'acier laminé
à chaud avant ladite étape de recuit de revenu contient au moins 95 % en surface de
martensite et plus préférentiellement au moins 99 % en surface de martensite lorsque
ladite microstructure est examinée sur ¼ d'épaisseur.
10. Procédé selon la revendication 8 ou 9, selon lequel ladite étape de trempe est une
étape de trempe directe.
11. Procédé selon l'une quelconque des revendications 8 à 10, selon lequel le procédé
comprend l'étape de laminage en bandes dudit acier laminé à chaud et ledit acier laminé
à chaud comprend un maximum de 0,005 % en masse de niobium et < 0,15 % en masse de
carbone.
12. Procédé selon l'une quelconque des revendications 8 à 10, selon lequel ledit acier
laminé à chaud comprend un minimum de 0,005 % en masse de niobium, préférentiellement
un minimum de 0,02 % en masse de niobium lorsque l'acier laminé à chaud n'est pas
laminé en bandes.
13. Utilisation d'un acier laminé à chaud selon l'une quelconque des revendications 1
à 7 pour le soudage MAG avec renfort, en utilisant un procédé de soudage à chanfrein
en V ou en Y, selon lequel une première passe est soudée à partir d'un côté inférieur
ou supérieur, préférentiellement à partir d'un côté inférieur, et d'autres passes
à partir d'un côté supérieur, en utilisant des consommables de soudage conformément
à la norme ENG 89 5 M21, aux qualité(s) commerciale(s) X90 et/ou X96, présentant une
résistance à la traction de 1100 MPa, préférentiellement de 960 MPa, plus préférentiellement
de 900 MPa, le plus préférentiellement de 890 MPa, et un temps t8/5 de 8 à 12 secondes,
préférentiellement de 6 à 18 secondes, plus préférentiellement de 5 à 20 secondes,
dans lequel le temps t8/5 est le temps nécessaire à la surface totale de la soudure
pour refroidir de 800 °C à 500 °C, selon lequel l'acier soudé présente une rupture
située à une distance d'au moins 1 mm de la ligne de fusion.