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EP 1 599 299 B1 |
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EUROPEAN PATENT SPECIFICATION |
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Mention of the grant of the patent: |
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04.07.2007 Bulletin 2007/27 |
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Date of filing: 13.02.2004 |
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International Patent Classification (IPC):
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International application number: |
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PCT/EP2004/001502 |
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International publication number: |
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WO 2004/073900 (02.09.2004 Gazette 2004/36) |
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A METHOD FOR PROCESSING A STEEL PRODUCT, AND PRODUCT PRODUCED USING SAID METHOD
VERFAHREN ZUR VERARBEITUNG EINES STAHLPRODUKTES UND PRODUKT, DAS UNTER BENUTZUNG DES
VERFAHRENS HERGESTELLT WIRD
PROCEDE DE TRAITEMENT D'UN PRODUIT EN ACIER, ET PRODUIT OBTENU PAR CE TRAITEMENT
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Designated Contracting States: |
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AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HU IE IT LI LU MC NL PT RO SE SI SK TR |
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Priority: |
24.02.2003 EP 03075546
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Date of publication of application: |
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30.11.2005 Bulletin 2005/48 |
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Proprietor: Corus Technology BV |
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1970 CA IJmuiden (NL) |
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Inventor: |
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- VAN DER WINDEN, Menno, Rutger
NL-2311 BR Leiden (NL)
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Representative: Kruit, Jan |
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Corus Technology BV
PO Box 10000 1970 CA IJmuiden 1970 CA IJmuiden (NL) |
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References cited: :
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- PATENT ABSTRACTS OF JAPAN vol. 004, no. 088 (M-017), 24 June 1980 (1980-06-24) & JP
55 045507 A (NIPPON STEEL CORP), 31 March 1980 (1980-03-31)
- PATENT ABSTRACTS OF JAPAN vol. 010, no. 214 (C-362), 25 July 1986 (1986-07-25) & JP
61 052317 A (KOBE STEEL LTD), 15 March 1986 (1986-03-15)
- PATENT ABSTRACTS OF JAPAN vol. 009, no. 172 (M-397), 17 July 1985 (1985-07-17) & JP
60 044104 A (NIPPON KOKAN KK), 9 March 1985 (1985-03-09)
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Note: Within nine months from the publication of the mention of the grant of the European
patent, any person may give notice to the European Patent Office of opposition to
the European patent
granted. Notice of opposition shall be filed in a written reasoned statement. It shall
not be deemed to
have been filed until the opposition fee has been paid. (Art. 99(1) European Patent
Convention).
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[0001] The invention relates to a method for processing a steel product, in which the steel
product is passed between a set of rotating rolls of a rolling mill stand. This rolling
mill stand may be part of a rolling mill device consisting of one or more rolling
mill stands.
[0002] Rolling is a very standard operation for imparting desired dimensions and properties
to metal in general and steel in particular. Apart from obtaining the desired final
geometry of the steel product, rolling also results in an improvement to the structure
as a result of the metallurgical processes taking place during and after the rolling.
[0003] However, the conventional rolling, which for wide products is usually considered
to be a plane strain compression process, results in a considerable change in thickness,
which in some cases is undesirable or impossible. For example, in heavy construction
it is necessary to have steel plate with a thickness of 60 to 150 mm for, inter alia,
the production of off-shore platforms or bridges. Since cast steel slabs currently
have a maximum thickness of less than 400 mm, the change in thickness caused by the
rolling to 150 mm would only amount to approximately 60%. Each pass through a conventional
rolling mill stand usually results in a change in thickness of 10 to 30%.
[0004] The casting of slabs sometimes results in the formation of porosity in the slab,
a characteristic which is inherent to the casting process. This porosity is closed
up by the pressure applied as a result of the slabs being rolled a sufficient number
of times. However, if it is necessary to form a plate with a very high thickness,
the rolling only closes up the pores in the outermost layers of the slab, and not
those in the core of the material. However, the pores in the core of the material
are highly disadvantageous for the mechanical properties of the material, in particular
for the toughness properties of the plate. Also, grain refinement only occurs in the
outermost layers of the plate. To close up the pores by the application of pressure
and to achieve grain refinement even in the core of the plate, the degree of rolling
through the thick slab therefore has to be high, whereas the combination of starting
thickness of the slab and final thickness of the steel product do often not allow
a large thickness reduction.
[0005] It is possible to introduce a large equivalent strain into a product without imposing
a large thickness reduction under laboratory conditions using small samples with the
Equal Channel Angular Extrusion (ECAE) method in which extreme shear strains are applied
without changing the specimen's dimension. In ECAE a billet is extruded through a
die with two channels of equal cross-section that meet at an angle. Under ideal circumstances
the billet is sheared on crossing the plane of intersection of the channels by an
amount determined by the angle between the two channels. Since the cross section does
not change during the process, it can be repeated thereby accumulating strain. However,
this laboratory technique cannot be used for industrial production of steel products
because of the very high process forces required, and the impossibility to up-scale
this process for flat products of conventional dimensions.
[0006] In
US 4,086,105 a method of producing fine-grain sheet or plate, in particular fine-grain plate with
a thickness of more than 50 mm, of austenitic stainless steels by hot rolling billets
or slabs in a number of passes is proposed. In this method, the mean velocity of the
reduction in thickness is chosen in dependence upon the ferrite content of the steel.
[0007] The method is particularly intended to prevent the formation of coarse grains in
austenitic stainless steel plates having a thickness of more than 50 mm.
[0008] In
JP 55045507 a method is proposed wherein a shearing deformation is imparted to a metal cast piece
by means of rolling between an upper and a lower roll having a different peripheral
speed.
[0009] It is an object of the invention to provide a method for introducing a large equivalent
strain into the steel product without imposing an equivalent reduction in thickness
of the product.
[0010] It is also an object of the invention to provide a method for processing a steel
product which allows the properties of the product produced thereby to be improved.
[0011] Yet another object of the invention is to provide a method for processing a steel
product which results in grain refinement in the product which is thereby produced.
[0012] Yet another object of the invention is to provide a method for processing continuously
cast steel by means of which the properties of the slab or strip are improved.
[0013] It is another object of the invention to provide a method for processing a continuously
cast steel slab or strip with which it is possible to close up pores in the cast material.
[0014] It is also an object of the invention to provide a steel product with improved mechanical
properties which is produced with the aid of this method.
[0015] In the context of this invention, steel should be considered to comprise all ferrous
alloys for example ultra-low carbon steels, low-carbon steels, medium to high carbon
steels, electrical steels, and stainless steels. A steel product in the context of
this invention comprises ingots, slabs, blooms, billets, bar, rod, strip and profiled
sections.
[0016] One or more of these objects are achieved by a method for processing a continuously
cast steel product, in which the steel is passed between a set of rotating rolls of
a rolling mill stand in order to roll the steel product, wherein the rolls of the
rolling mill stand have different peripheral velocities such that one roll is a faster
moving roll and the other roll is a slower moving roll, wherein the peripheral velocity
of the faster moving roll is at least 5% and at most 100% higher that that of the
slower moving roll, wherein the thickness of the steel product is reduced by at most
15% for each pass, and wherein that the rolling takes place at a maximum temperature
of 1350°C.
[0017] As a result of the rolls being provided with a different peripheral velocity, shearing
occurs in the steel product and has been found to occur throughout the entire thickness
of the product. It has been found that this requires a velocity difference of at least
5%. The shearing leads to pores in the continuously cast material being closed up
to a considerable extent. This does not require a major change in thickness, but rather
a change in thickness of at most 15% can suffice. Preferably this thickness reduction
is at most 8% and more preferable at most 5%. This is particularly advantageous in
the processing of those steel products where the dimensions of the steel product at
the start of the process do not allow a singificant reduction in the thickness direction,
because the thickness is substantially retained.
[0018] In addition, it is important that the rolling according to the invention can result
in a grain refinement which occurs throughout the entire thickness of the rolled material,
which is advantageous for the mechanical properties of the slab or strip. Inter alia,
the strength of the material increases. The beneficial effects of smaller grain sizes
are commonly known.
[0019] The rolling is preferably carried out at an elevated temperature. However, the maximum
temperature is limited to 1350°C because the formation of low melting oxides on the
surface of the steel product to be produced has to be avoided. The elevated temperature
makes the rolling run more smoothly.
[0020] It is also expected that the processing according to the invention will result in
a rolled sheet with less lateral spread.
[0021] The peripheral velocity of the faster moving roll is preferably at most 50% higher
and more preferably at most 20% higher than that of the slower moving roll. If there
is a high difference in velocity, there is a considerable risk of slipping between
the rolls and the steel product, which would result in uneven shearing.
[0022] According to an advantageous embodiment, the rolling mill is designed in such a manner
that the rolls have different diameters. This makes it possible to obtain the desired
difference in peripheral velocity.
[0023] According to another advantageous embodiment, the rolls have a different rotational
speed. This too makes it possible to obtain the desired difference in rotational speed.
[0024] It is also possible for these latter two measures to be combined, i.e. rolls with
different diameters and different rotational speeds in order to obtain the desired
difference in peripheral velocity of the rolls.
[0025] According to an advantageous embodiment of the method, the steel product is introduced
between the rolls at an angle of between 5 and 45° with respect to the perpendicular
to the plane through the center axes of the rolls. Introducing the steel product between
the rolls at an angle makes it easier for the rolls to grip the steel product, with
the result that the change in thickness can be kept as low as possible. Experiments
have also shown that after rolling the steel product has an improved straightness
if it is introduced at an angle between the rolls. The steel product is preferably
fed in at an angle of between 10 and 25°, and more preferably at angle of between
15 and 25°, since with such an angle the steel product comes out of the rolling mill
with a good level of straightness. It should be noted that the latter effect is also
dependent on the reduction in the size of the steel product, the type of steel product
and the alloy and the temperature.
[0026] For this purpose, after the rolling has been carried out for the first time, the
processing operating is preferably repeated one or more times. For example, sufficiently
good grain refinement is obtained by carrying out the processing operating according
to the invention three times. However, the number of times that the processing operation
has to be carried out depends on the thickness of the steel product, the difference
in peripheral velocity of the rolls and the desired grain refinement. It is desirable
for the steel product to be introduced between the rolls at an angle of between 5
and 45°, preferably between 10 and 25° and more preferably between 15 and 25° during
each processing operation.
[0027] If the processing operation according to the invention is repeated a number of times,
according to an advantageous embodiment the steel product can be passed through the
rolling mill stand in opposite directions for each pass. The steel product then changes
direction after each rolling operation and is always passed through the same rolling
mill stand. In this case, the rolls have to rotate in opposite directions for each
pass. In this case too, it is desirable for the steel product in each case to be introduced
at an angle between the rolls.
[0028] According to another advantageous embodiment, the steel product is successively passed
through two or more rolling mill stands. This method is suitable primarily for strip
material, which in this way can undergo the desired processing operation very quickly.
[0029] According to a preferred embodiment of the invention the rolling is carried out on
a steel product of which at least a skin layer has a substantially austenitic structure,
and preferably on a steel product having a substantially austenitic structure throughout.
Typical minimum temperatures range from 900 °C for an ultra low carbon steel to 800-870
°C for a low carbon steel (depending on the chemical composition of course) to about
723 °C for a steel with 0.8 %C. In all cases the maximum temperature is 1350 °C. In
case of rolling an austenitic stainless steel, the rolling always takes place on an
austenitic structure.
[0030] According to a second preferred embodiment the rolling is carried out on a steel
product of which at least a skin layer has a substantially austenitic-ferritic two-phase
structure, and preferably on a steel product having a substantially austenitic-ferritic
two-phase structure throughout. Typical temperatures range for a low carbon steel
from 723 °C ending at 800-870 °C. The temperature range decreases with increasing
carbon contents to reduce to an eutectoid point of about 723 °C for a steel with 0.8
%C.
[0031] According to a third preferred embodiment the rolling is carried out on a steel product
of which at least a skin layer has a substantially ferritic structure, and preferably
on a steel product having a substantially ferritic structure throughout. For a low
carbon steel with a carbon content higher than 0.02% the maximum temperature is about
723 °C, whereas for steels with lower carbon contents such as ultra low carbon steels
the maximum temperature is about 850 °C. It should be noted here that these temperature
boundaries for the ferritic, ferritic-austenitic and austenitic region depend on the
composition of the steel and on the thermomechanical history of the steel. The phase
transformation is not instantaneous once a critical temperature is exceeded and therefore
a transforming steel may have a skin layer of a different phase compared to the centre
layer of the steel product.
[0032] According to a further advantageous embodiment of the invention the rolling is performed
at temperatures between 0 °C and 720°C. This comprises not only the cold rolling of
the ferritic steel product, but also the advantageous rolling of steel with a martensitic
structure or the austenitic stainless steel structure.
[0033] It is possible for the method to be preceded or followed by a rolling operation which
is carried out using a rolling mill in which the rolls have substantially identical
péripheral velocities. In this way, by way of example, an accurately desired thickness
or smoothness can be imparted to the product.
[0034] According to another advantageous embodiment, a steel product is produced according
to a method comprising the steps of:
- continuous casting of a steel strand;
- optionally heating and/or temperature homogenising the steel strand between a casting
machine and a rolling device;
- optionally rolling the steel product in one or more rolling mill stands of the rolling
device with rolls having substantially identical peripheral velocities;
- optionally accelerated cooling after the last rolling step;
- optionally cutting the steel product into slabs or coils before or after rolling;
- optionally coiling the steel product
- cooling the steel product
[0035] The most commonly used method to produce steel slabs is by continuous casting of
a steel strand and cutting it into steel slabs with a thickness of between 200 and
400 mm. After casting, these slabs are usually allowed to cool down to ambient temperatures
before being introduced in the furnaces of a hot strip mill. In some cases the slabs
can be introduced into the furnace while it is still warm or hot from casting (respectively
so-called "hot-charging" or "direct-charging").
[0036] The thickness of the continuously cast strand is preferably below 150 mm, more preferably
below 100 mm and even more preferably below 80 mm for thin slab casting.
[0037] The cast strand may be cut after casting by means of a cutting device. The thus obtained
slabs may be stored for later processing and allowed to cool down or they may be processed
immediately. In the former case the slabs may require reheating prior to rolling,
in the latter case the slabs may require to be homogenised in temperature. After finish
rolling the rolled product may be cooled using accelerated cooling and optionally
coiled. After the final processing step the steel product cools or is cooled to ambient
temperatures. In case the cast strand is not cut into slabs, but processed immediately
by continuous, endless or semi-endless rolling, the rolled product will be cut in
a later stage of the rolling process e.g. before the optional coiler. It will be obvious
that the rolling according to the invention may take place anywhere between the casting
step and the final cooling step, or even thereafter.
[0038] Prior to coiling the steel product may be subjected to accelerated cooling. After
the final processing step the steel product cools or is cooled to ambient temperatures.
[0039] According to another embodiment of the invention the thickness of the continuously
cast strand is preferably below 20 mm, more preferably below 10 mm and even more preferably
below 5 mm.
[0040] The cast strand having a cast microstructure may be cut after casting by means of
a cutting device. The thus obtained slabs may be stored for later processing and allowed
to cool down or they may be processed immediately. In the former case the slabs may
require reheating prior to rolling, or they may be used as final product. In the latter
case the slabs may require to be homogenised in temperature. One drawback of the strip-cast
steel products is that the end product still largely has the cast microstructure,
since the strip has scarcely been rolled. Consequently, the mechanical properties
of the end products are relatively poor, and consequently the use of the end products
is limited and do not meet the standards of the products obtained through the conventional
thick slab or even the more recent thin slab route. During the rolling process according
to the invention the microstructure is transformed from a casting structure to a wrought
microstructure without substantial reduction in thickness thereby improving the final
properties of the steel product significantly. After finish rolling the rolled product
may be cooled using accelerated cooling and optionally coiled. After the final processing
step the steel product cools or is cooled to ambient temperatures. In case the cast
strand is not cut into slabs, but processed immediately by continuous, endless or
semi-endless rolling, the rolled product will be cut in a later stage of the rolling
process e.g. before the optional coiler. After finish rolling the rolled product may
be cooled using accelerated cooling. After the final processing step the steel product
cools or is cooled to ambient temperatures. Again, it will be obvious that the rolling
according to the invention may take place anywhere between the casting step and the
final cooling step, or even thereafter.
[0041] A further advantage is obtained if the steel product to be processed according to
the previous two embodiments is a stainless steel.
[0042] In the context of this invention, stainless steel comprises both ferritic, austenitic-ferritic
duplex steels and austenitic stainless steels. These steels are commonly applied in
application where the corrosion resistance of unalloyed or low-alloy steel is inadequate.
The combination of corrosion resistance, high strength and good ductility usually
associated with the duplex stainless steels results in applications where the formability
of ferritic and austenitic stainless steels is inadequate. Typical examples of a ferritic
stainless steels according to EN 10088 (1995) are X2CrNi12- 1.4003 (410) X6Cr14 -
1.4016 (430), and of austenitic stainless steels are X5CrNiMo17-12-2 1.4401 (316)
X5CrNi18-10 - 1.4301 (304). These steels are typically used as general-purpose stainless
steels in plate, strip, semi-, bar, rod and applied as construction steels for buildings,
pipelines, kitchenware, components in pumps and valves etc.
[0043] The thickness of the slab or strip is preferably reduced by at most 15% for each
pass, and preferably by at most 8% and more preferably by at most 5% for each pass.
Since the shearing and therefore the grain refinement are brought about by the difference
in peripheral velocity between the rolls, the reduction in thickness of the material
is not required to obtain grain refinement. The reduction in thickness is required
primarily in order to enable the rolls to grip the material. This only requires a
slight change in thickness, which is advantageous in the case of thin continuously
cast steel slab, strip cast material and strip material. The smaller the reduction,
the thicker the slab or strip remains after each pass. The possible applications of
continuously cast slabs and strip material increase as a result. With the aid of the
method according to the invention, better mechanical properties can be imparted to
the steel product, without the need for a substantial reduction in thickness. Since
the method according to the invention can be used to impart better properties to an
already relatively thin steel product, it is to be expected that thicker continuously
cast plate and strip material, now with better mechanical properties, will also find
industrial applications.
[0044] In the production of high strength steel strip microalloyed with one or more of the
elements Nb, V, Ti or B(these steelgrades are usually called HSLA-steels (high strength,
low alloy)), in a hot strip mill acording to the well-known principles of thermomechanical
rolling it is a problem to produce strip with a higher thickness. The continuously
cast slabs that are used to start the rolling process with usually have a fixed thickness
of between 200 and 350 mm, for example 225 mm. The rolling mills also usually are
divided in a roughing section where the slab is rolled down in a number of passes,
for example 5 passes, to a chosen thickness of, for example, 36 mm. This so-called
transfer bar thickness is usually a fixed thickness within a given hot strip mill
and the deviations from this fixed value are minimal. Deviations from this value by
increasing its value usually results in rolling forces or torques in the finishing
mill which exceed operational limits, thereby causing risks to the rolling mill or
resulting in unacceptable changes in the shape and profile of the product. Decreasing
the thickness of the transfer bar usually results in rolling forces or torques in
the roughing mill which exceed operational limits. However, the fixed value of the
transfer bar also causes a problem because it results in different values of reduction
for a thick strip of for example 18 mm and a thin strip of for example 4 mm. In the
first case the total reduction in the finishing mill is 50%, in the second case it
is 89%. This has large repercussions on the development of the microstructure of the
steel during and after hot-rolling because the thermomechanical conditions are quite
different which results in different recrystallisation of the deformed austenite and
different precipitation kinetics of micro-alloying elements. Consequently also the
phase transformation during cooling after rolling is affected. In an advantageous
embodiment of the invention the degree of deformation of the steel product can be
increased without the need to increase the transfer bar thickness, or the degree of
deformation can be kept unchanged while the final thickness of the steel product is
increased.
[0045] With profiled sections the degree of deformation is essential for the properties
of the final product as well. For example, it is known that steel billets which are
rolled into profiled sections, such as H-sections, often have a part which has undergone
scarcely any rolling, with the result that little or no grain refinement occurs in
this part. Steel billets for sections usually have a gauge between 200 and 400 mm,
for example 230 mm or 310 mm. These are rolled in the slab/bloom/billet stage after
reheating to a temperature of maximal 1350°C. Finish rolling occurs usually at a temperature
where the steel is austenitic and flange thicknesses range from 10 to 150 mm. Non-limitative
examples for typical steel grades used for these sections comprise CMn-steels and
HSLA-steels. The process according to the invention allows a finer grainsize of the
billet because of the larger degree of deformation in the billet, and also allows
a reduction in the pore size of the billet, resulting in better fracture toughness.
[0046] Recently it has become clear from the results of basic research that properties such
as strength, toughness and corrosion resistance can be improved by reducing grain
size. Steels have been developed with a very fine grain size by controlling the structure
of the grain. These steels not only provide higher tensile strengths compared to conventional
steel, but also improved toughness, endurance and corrosion resistance. This technology
has been implemented in the hot strip mill by imposing a very large thickness reduction
at low rolling temperatures, as a result of which the rolling forces and torques increase
to extremely high levels. However, the proposed solution for obtaining ultra fine
ferrite grains relies on grain refinement by ordinary rolling (i.e. plane strain compression)
at low hot rolling temperatures and requires a very powerful rolling mill. Furthermore,
a strong thickness reduction is imposed to the material to attain the required levels
of deformation. In the process according to the invention, a significant grain reduction
can be achieved because of the accumulation of strain in the steel without substantially
reducing the thickness. The average grainsize of the steel product obtained is preferably
smaller than 5 µm, more preferably smaller than 2 µm and even more preferably smaller
than 1 µm.
[0047] According to another embodiment of the invention the properties of complex phase
steels are unexpectedly improved because of the accumulation of strain in the steel
without substantially reducing the thickness. When the steel product is rolled in
the austenitic state and subsequently acceleratedly cooled, the large degree of accumulated
deformation allows the steel to transform to a very fine ferrite grain in combination
with a very finely distributed fine-grained second phase consisting of bainite or
martensite. A small amount of carbides may also be present. The ferrite content of
this steel product is preferably at least 60%, more preferably at least 70% and even
more preferably at least 80%. The average grainsize of the steel product obtained
is preferably smaller than 5 µm, more preferably smaller than 2 µm and even more preferably
smaller than 1 µm.
[0048] In conventional production of steel plates, for example of the carbon-manganese type
or of the HSLA-type, the starting point is a continuously cast slab with a typical
thickness between 200 and 350 mm. These slabs are reheated in a reheating furnace
to a temperature between 1000 and 1350°C. After reheating these slabs are rolled to
a thickness of between 30 to 200 mm, preferably 40 to 150 mm and held at temperature,
for instance by shielding it against cooling. During this holding period at high temperature
grain growth takes place as a result of which the final mechanical properties of the
finished plate may also deteriorate. It is common knowledge that a larger grain size
decreases the ductility properties and the toughness of a steel product. It is also
well known that the yield strength decreases with an increase in grainsize. Consequently,
grain growth during holding should be avoided. Conventionally this is done by accelerated
cooling. However, the use of accelerated cooling has the disadvantage of enlarging
the temperature difference between the centre part of the slab and the surface part
of the slab. This temperature difference adversely affects the homogeneity of the
final microstructure of the slab.
[0049] In many cases the plate receives a heat treatment during the production process.
This may for example be a normalisation treatment wherein the slab is reheated into
the austenite region and allowed to cool down in still air or a tempering anneal or
stress relief anneal which both aim to reduce the level of internal stresses. Another
example of a heat treatment is the speroidisation treatment in which elongated carbides
are transformed into more or less spheroidal particles. These carbides may be iron
carbides (e.g. cementite) or other metal carbides like chromium carbides. This type
of annealing treatment is used often in steels with carbon contents in excess of 0.8%.
Unfortunately, the majority of these heat treatments and particularly the spheroidisation
treatments take a long time and frequently lead to decarburisation of the surface
part of the strip thereby adversely affecting the properties.
[0050] The rolling according to the invention can also be carried out at low temperatures
between 0 and 720 °C. Special benefits from the rolling can be expected when performed
at low temperatures (i.e. cold rolling) because of the resulting breaking up of undesired
particles. As a result of the break up of the particles the final properties of the
steel product are improved. The shearing as a result of the rolling process breaks
up the particles in the steel products, for example metal carbides like cementite
or chromium carbides which may result in an improved toughness. The break up of the
particles also affects the heat treatment response of the steel product. Different
heating and cooling regimes can be employed leading to improved throughput through
the heat treatment stage, e.g. a spheroidisation annealing treatment, or an improved
product.
[0051] It is also possible for the method according to the invention to be preceded or followed
by a heat treatment of the steel product. Examples of these heat treatments are the
well known normalising treatment, stress relief annealing treatment, temper annealing
treatment or spheroidisation annealing treatment.
[0052] In the context of this invention, a steel product also comprises a steel where one
or both steel surfaces which are to be rolled are covered with one or more layers
prior to rolling according to the invention. This combination of a steel product covered
on one or both surfaces with one or more layers of metal is commonly referred to as
cladded plate or strip. In producing clad plate there are three options by which the
covering metal is bonded to the steel substrate: explosive bonding, roll bonding and
weld overlay. One of the important factors affecting the quality of clad plate is
the quality of the adhesion between the substrate and the cladding layer. This is
a particular problem for the clad plate which is produced by roll bonding, because
in conventional rolling the stress state at the interface between the substrate and
the cladding layer, or between cladding layers is compressive only. According to an
advantageous embodiment, a surface of the steel product which is to be rolled is covered
by one or more layers prior to rolling. The covering layer can be a metal, preferably
another steel, e.g. a steel with a different composition or a stainless steel, Titanium,
Nickel, Copper, Aluminium or alloys thereof. This way it is possible, for example,
to produce laminated material, such as what is known as clad material for use in,
for example, pipes and pipe lines, chemical plants, power plants, vessels, pressure
vessels.
[0053] The invention also relates to an improved metal plate or strip which has been produced
by continuous casting, preferably with the aid of the method according to the first
aspect of the invention, in which the pores in the core of the plate or strip have
a maximum dimension of less than 200 µm, preferably less than 100 µm, more preferably
less than 20 µm and even more preferably less than 10 µm. As a result of the continuous
casting, continuously cast plate and strip material always has pores which can be
significantly larger than 200 µm. The standard rolling operations can only close up
these pores in the core to a slight extent or cannot do so at all. The rolling operation
according to the invention makes it possible to provide continuously cast plate and
strip material having pores which are much smaller.
[0054] The invention also relates to an improved metal plate or strip which is produced
by continuous casting, preferably with the aid of the method according to the first
aspect of the invention, in which the metal plate or strip, after recrystallisation,
has a substantially homogenous degree of recrystallisation over its entire thickness.
The fact that the grains have all been subjected to shearing as a result of the rolling
operation according to the invention, including those in the core, means that the
continuously cast plate and strip material will recrystallize over the entire thickness.
[0055] It also relates to a steel product produced according to the invention, in which
the starting point is a steel ingot, and in which steel product the pores in the core
of the product preferably have a maximum dimension of less than 200 µm, more preferably
less than 100 µm, still more preferably less than 20 µm and even more preferably less
than 10 µm as well as to a steel product produced by continuous casting and processed
according to the invention, in which the pores in the core of the plate or strip have
a maximum dimension of less than 200 µm, more preferably less than 100 µm, still more
preferably less than 20 µm and even more preferably less than 10 µm.
[0056] The invention also relates to a steel strip produced according to the invention for
use in for example parts of automobiles, transport equipment, piling, buildings, construction
and to a clad steel product for use in for example pipes, chemical plants, power plants,
vessels, pressure vessels and to a steel strip wherein the steel is a HSLA-steel comprising
at least one of the elements niobium, titanium, vanadium or boron, or wherein the
steel is an ultra low carbon steel, preferably at least partly stabilised, preferably
with at least one of the elements titanium, niobium or boron.
[0057] The invention will be explained with reference to an exemplary embodiment.
[0058] Experiments were carried out using slabs of a Titanium stabilised ultra low carbon
steel, carbon-manganese steels and Niobium microalloyed HSLA-steel.
[0059] The slabs were introduced at different angles varying between 5° and 45°. The temperature
of the slabs when they were introduced into the rolling device was approximately 1000
°C. The two rolls were driven at a speed of 5 revolutions per minute.
[0060] After rolling, the slabs had a certain curvature, which is highly dependent on the
angle of introduction. The straightness of the slab after rolling can to a large extent
be determined by the angle of introduction, in which context the optimum angle of
introduction will be dependent on the degree of reduction of the slab, the type of
material and alloy, and the temperature. For the slabs of steel which have been rolled
in the experiment described above, an optimum introduction angle is approximately
20°.
[0061] A shear angle of 20° was measured in the steel slabs which were rolled in accordance
with the experiment described above. Using this measurement and the reduction in the
size of the slab, it is possible to calculate an equivalent strain in accordance with
the following formula:
[0063] Therefore, in the slabs which have been rolled in accordance with the experiment,
the equivalent strain is
[0064] In the case of rolling with an ordinary rolling mill, shearing does not take place
across the thickness of the plate and the equivalent strain is therefore only
(working on the basis of a uniform strain over the entire thickness of the steel
product).
[0065] Therefore, the rolling using the method according to the invention results in an
equivalent strain which is three to four times higher than with conventional rolling
without any difference in peripheral velocity. A high equivalent strain means less
porosity in the slab, greater recrystalization and therefore greater grain refinement,
and more extensive breaking up of the second-phase particles (constituent particles)
in the slab. These effects are generally known to the person skilled in this field
of engineering if the equivalent strain increases. Therefore, the rolling according
to the invention means that the resulting properties of the material are greatly improved
as a result of the use of the method according to the invention.
1. A method for processing a steel product, in which the steel product is passed between
a set of rotating rolls of a rolling mill stand in order to roll the steel product,
the rolls of the rolling mill stand having different peripheral velocities such that
one roll is a faster moving roll and the other roll is a slower moving roll, characterized in that the peripheral velocity of the faster moving roll is at least 5% higher and at most
100% higher than that of the slower moving roll, in that the thickness of the steel product is reduced by at most 15% per pass, and in that the rolling takes place at a maximum temperature of 1350°C.
2. The method as claimed in claim 1, in which the thickness of the steel product is reduced
by at most 8% each pass, and preferably at most 5% each pass.
3. The method as claimed in claim 1 or 2, in which the peripheral velocity of the faster
moving roll is at most 50% higher and preferably at most 20% higher than that of the
slower moving roll.
4. The method as claimed in one of the preceding claims, in which the rolling mill is
designed in such a manner that the rolls have different diameters.
5. The method as claimed in one of the preceding claims, in which the rolls have different
rotational speeds.
6. The method as claimed in one of the preceding claims, in which the steel product is
introduced between the rolls at an angle of between 5 and 45° with respect to the
perpendicular to the plane through the center axes of the rolls, preferably at an
angle between 10 and 25° and more preferably at a angle of between 15 and 25°.
7. The method as claimed in one of the preceding claims, in which the rolling operation
is repeated one or more times after the rolling has been carried out for the first
time.
8. The method as claimed in claim 7, in which the steel product is passed through the
rolling mill stand in opposite directions for each pass.
9. The method as claimed in claim 7, in which the steel product is successively passed
through two or more rolling mill stands.
10. The method as claimed in one of the preceding claims, in which the rolling operation
as described in one of claims 1 - 9 is preceded or followed by a rolling operation
which is carried out using a rolling mill in which the rolls have substantially identical
peripheral velocities.
11. The method as claimed in any of the claims 1 to 10, in which the rolling is carried
out on a steel product of which at least a skin layer has a substantially austenitic
structure, and preferably on a steel product having a substantially austenitic structure
throughout.
12. The method as claimed in any of the claims 1 to 10, in which the rolling is carried
out on a steel product of which at least a skin layer has a substantially austenitic-ferritic
two-phase structure, and preferably on a steel product having a substantially austenitic-ferritic
two-phase structure throughout.
13. The method as claimed in any of the claims 1 to 10, in which the rolling is carried
out on a steel product of which at least a skin layer has a substantially ferritic
structure, and preferably on a steel product having a substantially ferritic structure
throughout.
14. The method as claimed in any of the claims 1 to 10, in which the rolling is carried
out while the temperature of the steel product is higher than 0°C and lower than 720°C.
15. The method according to claim 14, in which the rolling is carried out on a steel product
having a substantially martensitic structure.
16. A method for producing a steel product comprising the steps of:
• continuous casting of a steel strand;
• optionally heating and/or temperature homogenising the steel strand between a casting
machine and a rolling device;
• optionally rolling the steel product in one or more rolling mill stands of the rolling
device with rolls having substantially identical peripheral velocities;
• optionally accelerated cooling after the last rolling step;
• optionally cutting the steel product into slabs or coils before or after rolling;
• optionally coiling the steel product
• cooling the steel product
characterised in that between casting the strand and accelerated cooling or coiling or cooling the steel
product is subjected to the method of any one of the claims 1 - 10.
17. The method for producing a steel product according to claim 16 characterised in that the thickness of the cast strand is below 150 mm and preferably below 100 mm, even
more preferably below 80 mm.
18. The method for producing a steel product according to claim 16 characterised in that the thickness of the cast strand is below 20 mm and preferably below 10 mm, even
more preferably below 5 mm.
19. The method according to claim 16 to 18, wherein the steel product that is produced
is a stainless steel product.
20. The method for producing a steel product according to claim 16 - 19 characterised in that the rolling is carried out on a steel product having a substantially austenitic structure,
in that the steel is acceleratedly cooled thereafter, in that the steel product essentially comprises ferrite, bainite and/or martensite, and in that the ferrite content after cooling is preferably at least 60%, more preferably more
than 70% and even more preferably more than 80%.
21. The method for producing a steel product according to claim 16 - 20 wherein the average
grainsize of the steel product is smaller than 5 µm, preferably smaller than 2 µm
and more preferably smaller than 1 µm.
22. The method according to any of the claims 1 - 21 wherein the steel product is subjected
to a heat treatment before or after the rolling step, for example a normalising treatment,
a full anneal, a stress relief anneal or a speroidisation annealing treatment.
23. The method as claimed in any of the claims 1 - 21 wherein a surface of the steel product
which is to be rolled is covered by one or more layers prior to rolling.
24. The method as claimed in claim 23 wherein the covering layer is a metal, preferably
another steel, e.g. a steel with a different composition or a stainless steel, Titanium,
Nickel, Copper, Aluminium or alloys thereof.
25. A steel product produced according to the method of any one of claims 1-10, in which
the starting point is a steel ingot, in which steel product the pores in the core
of the product preferably have a maximum dimension of less than 200µm, more preferably
less than 100 µm, still more preferably less than 20 µm and even more preferably less
than 10 µm.
26. A steel product according to claim 25 further characterized in that the steel product is a steel, plate, strip or billet produced by continuous casting.
1. Verfahren zur Verarbeitung eines Stahlprodukts, bei dem das Stahlprodukt zwischen
einem Satz rotierender Walzen eines Walzstraßengerüsts befördert wird, um das Stahlprodukt
zu walzen, wobei die Walzen des Walzstraßengerüsts unterschiedliche Umfangsgeschwindigkeiten
haben, so dass eine Walze eine schneller laufende Walze und die andere Walze eine
langsamer laufende Walze ist, dadurch gekennzeichnet, dass die Umfangsgeschwindigkeit der schneller laufenden Walze mindestens 5% höher und
höchstens 100% höher als die der langsamer laufenden Walze ist, dass die Dicke des
Stahlprodukts um höchstens 15 % je Durchgang bzw. Stich reduziert wird und dass das
Walzen bei einer Maximaltemperatur von 1350°C erfolgt.
2. Verfahren nach Anspruch 1, bei dem die Dicke des Stahlprodukts um höchstens 8% je
Stich, und bevorzugt um höchstens 5% je Stich reduziert wird.
3. Verfahren nach Anspruch 1 oder 2, bei dem die Umfangsgeschwindigkeit der schneller
laufenden Walze höchstens 50% über und bevorzugt höchstens 20% über der der langsamer
laufenden Walze liegt.
4. Verfahren nach einem der vorhergehenden Ansprüche, bei dem die Walzstraße so gestaltet
ist, dass die Walzen unterschiedliche Durchmesser haben.
5. Verfahren nach einem der vorhergehenden Ansprüche, bei dem die Walzen unterschiedliche
Drehgeschwindigkeiten haben.
6. Verfahren nach einem der vorhergehenden Ansprüche, bei dem das Stahlprodukt zwischen
die Walzen in einem Winkel zwischen 5 und 45° in Bezug auf die Senkrechte zur Ebene
durch die Mittelachsen der Walzen eingeführt wird, bevorzugt in einem Winkel zwischen
10 und 25° und bevorzugter in einem Winkel zwischen 15 und 25°.
7. Verfahren nach einem der vorhergehenden Ansprüche, bei dem der Walzvorgang ein- oder
mehrmals wiederholt wird, nachdem das Walzen das erste Mal ausgeführt wurde.
8. Verfahren nach Anspruch 7, bei dem das Stahlprodukt durch das Walzstraßengerüst in
entgegengesetzte Richtungen für jeden Stich geführt wird.
9. Verfahren nach Anspruch 7, bei dem das Stahlprodukt nacheinander durch zwei oder mehrere
Walzstraßengerüste geführt wird.
10. Verfahren nach einem der vorhergehenden Ansprüche, bei welchem dem in einem der Ansprüche
1 - 9 beschriebenen Walzvorgang ein Walzvorgang vorausgeht oder folgt, der unter Benutzung
einer Walzstraße erfolgt, in der die Walzen im Wesentlichen identische Umfangsgeschwindigkeiten
haben.
11. Verfahren nach einem der Ansprüche 1 bis 10, bei dem das Walzen auf einem Stahlprodukt
erfolgt, von dem wenigstens eine Hautschicht eine im Wesentlichen austenitische Struktur
hat, und bevorzugt auf einem Stahlprodukt, das durchgängig eine im Wesentlichen austenitische
Struktur hat.
12. Verfahren nach einem der Ansprüche 1 bis 10, bei dem das Walzen auf einem Walzprodukt
erfolgt, von dem wenigstens eine Hautschicht eine im Wesentlichen austenitisch-ferritische
Zweiphasenstruktur hat, und bevorzugt auf einem Stahlprodukt, das durchgängig eine
im Wesentlichen austenitisch-ferritische Zweiphasenstruktur hat.
13. Verfahren nach einem der Ansprüche 1 bis 10, bei dem das Walzen auf einem Stahlprodukt
erfolgt, von dem wenigstens eine Hautschicht eine im Wesentlichen ferritische Struktur
hat, und bevorzugt auf einem Stahlprodukt, das durchgängig eine im Wesentlichen ferritische
Struktur hat.
14. Verfahren nach einem der Ansprüche 1 bis 10, bei dem das Walzen ausgeführt wird, während
die Temperatur des Stahlprodukts höher als 0°C und niedriger als 720°C ist.
15. Verfahren nach Anspruch 14, bei dem das Walzen auf einem Stahlprodukt mit einer im
Wesentlichen martensitischen Struktur erfolgt.
16. Verfahren zur Herstellung eines Stahlprodukts, umfassend die Schritte:
• kontinuierliches Gießen eines Stahlstrangs;
• wahlweises Erhitzen und/oder Temperaturhomogenisieren des Stahlstrangs zwischen
einer Gussmaschine und einer Walzvorrichtung;
• wahlweises Walzen des Stahlprodukts in einem oder mehreren Walzstraßengerüsten der
Walzvorrichtung mit Walzen, die im Wesentlichen gleiche Umfangsgeschwindigkeiten haben;
• wahlweises beschleunigtes Abkühlen nach dem letzten Walzschritt;
• wahlweises Schneiden des Stahlprodukts in Brammen oder Spulen vor oder nach dem
Walzen;
• wahlweises Wickeln des Stahlprodukts;
• Abkühlen des Stahlprodukts,
dadurch gekennzeichnet, dass das Stahlprodukt zwischen dem Gießen des Strangs und dem beschleunigten Abkühlen
oder dem Wickeln oder dem Abkühlen oder nach dem Abkühlen dem Verfahren nach einem
der Ansprüche 1 bis 10 unterzogen wird.
17. Verfahren zur Herstellung eines Stahlprodukts nach Anspruch 16, dadurch gekennzeichnet, dass die Dicke des Gussstrangs unter 150 mm und bevorzugt unter 100 mm, noch bevorzugter
unter 80 mm liegt.
18. Verfahren zur Herstellung eines Stahlprodukts nach Anspruch 16, dadurch gekennzeichnet, dass die Dicke des Stahlstrangs unter 20 mm und bevorzugt unter 10 mm, noch bevorzugter
unter 5 mm liegt.
19. Verfahren nach Anspruch 16 bis 18, wobei das produzierte Stahlprodukt ein rostfreies
Stahlprodukt ist.
20. Verfahren zur Herstellung eines Stahlprodukts nach Anspruch 16 - 19, dadurch gekennzeichnet, dass das Walzen auf einem Stahlprodukt mit einer im Wesentlichen austenitischen Struktur
erfolgt, dass der Stahl anschließend beschleunigt abgekühlt wird, dass das Stahlprodukt
im Wesentlichen Ferrit, Bainit und/oder Martensit enthält, und dass der Ferritgehalt
nach dem Abkühlen bevorzugt mindestens 60% beträgt, bevorzugter mehr als 70% und noch
bevorzugter mehr als 80%.
21. Verfahren zur Herstellung eines Stahlprodukts nach Anspruch 16 - 20, wobei die durchschnittliche
Korngröße des Stahlprodukts kleiner als 5 µm ist, bevorzugt kleiner als 2 µm und bevorzugter
kleiner als 1 µm.
22. Verfahren nach einem der Ansprüche 1 - 21, wobei das Stahlprodukt vor oder nach dem
Walzschritt einer Wärmebehandlung unterzogen wird, beispielsweise einer Normalisierungsglühbehandlung,
einem Hochglühen, einem Entspannungsglühen oder einer Weichglühbehandlung.
23. Verfahren nach einem der Ansprüche 1 - 21, wobei eine zu walzende Oberfläche des Stahlprodukts
von einer oder mehr Schichten vor dem Walzen bedeckt ist.
24. Verfahren nach Anspruch 23, wobei die Deckschicht ein Metall ist, bevorzugt ein weiterer
Stahl, z.B. ein Stahl mit einer unterschiedlichen Zusammensetzung oder ein rostfreier
Stahl, Titan, Nickel, Kupfer, Aluminium oder Legierungen hieraus.
25. Stahlprodukt, das gemäß dem in einem der Ansprüche 1 - 10 angegebenen Verfahren hergestellt wurde, bei dem der Ausgangspunkt ein Stahlbarren ist, wobei bei dem Stahlprodukt die Poren im Kern des
Produkts ein Größtmaß von weniger als 200 µm haben, bevorzugt weniger als 100 µm,
bevorzugter weniger als 20 µm und noch bevorzugter weniger als 10 µm.
26. Stahlprodukt nach Anspruch 25, ferner dadurch gekennzeichnet, dass das Stahlprodukt eine Stahlplatte oder ein Stahlblech, -band oder -knüppel ist, hergestellt
durch kontinuierliches Gießen.
1. Procédé pour traiter un produit en acier, dans lequel le produit en acier est amené
à passer entre un ensemble de cylindres en rotation d'une cage de laminoir afin de
laminer le produit en acier, les cylindres de la cage de laminoir ayant des vitesses
périphériques différentes de telle sorte qu'un premier cylindre est un cylindre à
mouvement plus rapide et que l'autre cylindre est un cylindre à mouvement plus lent,
caractérisé en ce que la vitesse périphérique du cylindre à mouvement plus rapide est supérieure d'au moins
5%, et au maximum supérieure de 100% à celle du cylindre à mouvement plus lent, en ce que l'épaisseur du produit en acier est réduite d'au moins 15% par passe et en ce que le laminage a lieu à une température maximale de 1350°C.
2. Procédé selon la revendication 1, dans lequel l'épaisseur du produit en acier est
réduite tout au plus de 8% à chaque passe, et de préférence plutôt plus de 5% à chaque
passe.
3. Procédé selon la revendication 1 ou 2, dans lequel la vitesse périphérique du cylindre
à mouvement plus rapide est supérieure tout au plus de 50%, et de préférence supérieure
tout au plus de 20% à celle du cylindre à mouvement plus lent.
4. Procédé selon l'une quelconque des revendications précédentes, dans lequel le laminoir
est conçu de façon que les cylindres aient des diamètres différents.
5. Procédé selon l'une quelconque des revendications précédentes, dans lequel les cylindres
ont des vitesses de rotation différentes.
6. Procédé selon l'une quelconque des revendications précédentes, dans lequel le produit
en acier est introduit entre les cylindres suivant un angle de 5 à 45° par rapport
à la perpendiculaire au plan passant par les axes centraux des cylindres, de préférence
suivant un angle de 10 à 25° et de préférence encore suivant un angle de 15 à 25°.
7. Procédé selon l'une quelconque des revendications précédentes, dans lequel le laminage
est répété une ou plusieurs fois après le laminage initial.
8. Procédé selon la revendication 7, dans lequel le produit en acier est amené à passer
dans la cage de laminoir dans des directions opposées pour chaque passe.
9. Procédé selon la revendication 7, dans lequel le produit en acier est amené à passer
successivement par deux ou plus de deux cages de laminoir.
10. Procédé selon l'une quelconque des revendications précédentes, dans lequel le laminage
décrit dans l'une des revendications 1 à 9 est précédé ou suivi d'un laminage qui
est effectué à l'aide d'un laminoir dans lequel les cylindres ont des vitesses périphériques
sensiblement identiques.
11. Procédé selon l'une quelconque des revendications 1 à 10, dans lequel le laminage
est effectué sur un produit en acier dont au moins une couche superficielle a une
structure sensiblement austénitique, et de préférence sur un produit en acier ayant
partout une structure sensiblement austénitique.
12. Procédé selon l'une quelconque des revendications 1 à 10, dans lequel le laminage
est effectué sur un produit en acier dont au moins une couche superficielle a une
structure à deux phases sensiblement austénitique-ferritique, et de préférence sur
un produit en acier ayant partout une structure à deux phases sensiblement austénique-ferritique.
13. Procédé selon l'une quelconque des revendications 1 à 10, dans lequel le laminage
est effectué sur un produit en acier dont au moins une couche superficielle a une
structure sensiblement ferritique, et de préférence sur un produit en acier ayant
partout une structure sensiblement ferritique.
14. Procédé selon l'une quelconque des revendications 1 à 10, dans lequel le laminage
est effectué tandis que la température du produit en acier est supérieure à 0°C et
inférieure à 720°C.
15. Procédé selon la revendication 14, dans lequel le laminage est effectué sur un produit
en acier à structure sensiblement martensitique.
16. Procédé pour réaliser un produit en acier, comprenant les étapes consistant à :
• couler en continu une barre d'acier ;
• éventuellement chauffer et/ou rendre homogène la température de la barre d'acier
entre une machine de coulée et un dispositif de laminage;
• éventuellement laminer le produit en acier dans une ou plusieurs cages de laminoir
du dispositif de laminage, les cylindres ayant des vitesses périphériques sensiblement
identiques;
• éventuellement réaliser un refroidissement accéléré après la dernière passe de laminage
;
• éventuellement découper le produit en acier en brames ou en bobines avant ou après
le laminage ;
• éventuellement enrouler en bobine le produit en acier ;
• refroidir le produit en acier.
caractérisé en ce que, entre la coulée de la barre et le refroidissement accéléré ou le bobinage ou le
refroidissement ou après le refroidissement, le produit en acier est soumis au procédé
selon l'une quelconque des revendications 1 à 10.
17. Procédé pour réaliser un produit en acier selon la revendication 16, caractérisé en ce que l'épaisseur de la barre coulée est inférieure à 150 mm, de préférence inférieure
à 100 mm, de préférence encore inférieure à 80 mm.
18. Procédé pour réaliser un produit en acier selon la revendication 16, caractérisé en ce que l'épaisseur de la barre coulée est inférieure à 20 mm, de préférence inférieure à
10 mm, de préférence encore inférieure à 5 mm.
19. Procédé selon la revendication 16 ou 18, dans lequel le produit en acier réalisé est
un produit en acier inoxydable.
20. Procédé pour réaliser un produit en acier selon les revendications 16 à 19, caractérisé en ce que le laminage est effectué sur un produit en acier à structure sensiblement austénitique,
en ce que l'acier est ensuite soumis à un refroidissement accéléré, en ce que le produit en acier est sensiblement constitué de ferrite, de bainite et/ou de martensite,
et en ce que la teneur en ferrite après refroidissement est de préférence d'au moins 60%, de préférence
supérieure à 70% et de préférence encore supérieure à 80%.
21. Procédé pour réaliser un produit en acier selon les revendications 16 à 20, dans lequel
la taille moyenne des grains du produit en acier est inférieure à 5 µm, de préférence
inférieure à 2 µm et de préférence encore inférieure à 1 µm.
22. Procédé selon l'une quelconque des revendications 1 à 21, dans lequel le produit en
acier est soumis à un traitement thermique avant ou après le laminage, par exemple
à un traitement de normalisation, un recuit complet, un recuit de détente ou un traitement
de recuit de sphéroïdisation.
23. Procédé selon l'une quelconque des revendications 1 à 21, dans lequel une surface
du produit en acier destiné à être laminé est couverte par une ou plusieurs couches
avant le laminage.
24. Procédé selon la revendication 23, dans lequel la couche de couverture est un métal,
de préférence un autre acier, par exemple un acier à composition différente ou un
acier inoxydable, du titane, du nickel, du cuivre, de l'aluminium ou des alliages
de ceux-ci.
25. Produit en acier réalisé suivant le procédé selon l'une quelconque des revendications
1 à 10, dans lequel on part d'un lingot d'acier, les pores de la partie centrale de
ce produit en acier ayant de préférence une taille maximale inférieure à 200 µm, de
préférence inférieure à 100 µm, de préférence encore inférieure à 20 µm et de préférence
même inférieure à 10 µm.
26. Produit en acier selon la revendication 25, caractérisé en outre en ce que le produit en acier est une tôle, un feuillard ou une billette d'acier réalisé par
coulée continue.
REFERENCES CITED IN THE DESCRIPTION
This list of references cited by the applicant is for the reader's convenience only.
It does not form part of the European patent document. Even though great care has
been taken in compiling the references, errors or omissions cannot be excluded and
the EPO disclaims all liability in this regard.
Patent documents cited in the description
Non-patent literature cited in the description
- R.H. WAGONERJ.L. CHENOTFundamentals of metal formingJohn Wiley & Sons19970000 [0062]