U.S. patent application number 10/546192 was filed with the patent office on 2007-03-08 for method for processing a steel product produced using said method.
This patent application is currently assigned to CORUS TECHNOLOGY B.V.. Invention is credited to Menno Rutger Van Der Winden.
Application Number | 20070051439 10/546192 |
Document ID | / |
Family ID | 32731582 |
Filed Date | 2007-03-08 |
United States Patent
Application |
20070051439 |
Kind Code |
A1 |
Van Der Winden; Menno
Rutger |
March 8, 2007 |
Method for processing a steel product produced using said
method
Abstract
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 in order to roll the steel
product. According to the invention, 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, and
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,
and the thickness of the steel product is reduced by at most 15%
per pass, and the rolling takes place at a maximum temperature of
1350.degree. C. The invention also relates to a steel product
produced using the method, and to the use of this steel
product.
Inventors: |
Van Der Winden; Menno Rutger;
(Leiden, NL) |
Correspondence
Address: |
STEVENS DAVIS MILLER & MOSHER, LLP
1615 L STREET, NW
SUITE 850
WASHINGTON
DC
20036
US
|
Assignee: |
CORUS TECHNOLOGY B.V.
P.O. BOX 10000
IJMUIDEN
NL
NL-1970 CA
|
Family ID: |
32731582 |
Appl. No.: |
10/546192 |
Filed: |
February 13, 2004 |
PCT Filed: |
February 13, 2004 |
PCT NO: |
PCT/EP04/01502 |
371 Date: |
October 3, 2006 |
Current U.S.
Class: |
148/546 ;
148/610; 148/624; 148/648 |
Current CPC
Class: |
B21B 3/02 20130101; B21B
1/026 20130101; B21B 2267/065 20130101; C21D 8/0236 20130101; C21D
8/00 20130101; B21B 2001/383 20130101; B21B 2001/225 20130101; B21B
2275/05 20130101; C21D 8/0226 20130101 |
Class at
Publication: |
148/546 ;
148/610; 148/624; 148/648 |
International
Class: |
C21D 8/00 20060101
C21D008/00; C21D 7/02 20060101 C21D007/02 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 24, 2003 |
EP |
03075546.6 |
Claims
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, 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, 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, the thickness of the steel product is reduced by at
most 15% per pass, and the rolling takes place at a maximum
temperature of 1350.degree. C.
2. The method as claimed in claim 1, wherein the thickness of the
steel product is reduced by at most 8% each pass,
3. The method as claimed in claim 1 wherein the peripheral velocity
of the faster moving roll is at most 50% higher than that of the
slower moving roll.
4. The method as claimed in claim 1, wherein the rolling mill is
designed in such a manner that the rolls have different
diameters.
5. The method as claimed in claim 1, wherein the rolls have
different rotational speeds.
6. The method as claimed in claim 1, wherein the steel product is
introduced between the rolls at an angle of between 5 and
45.degree. with respect to the perpendicular to the plane through
the center axes of the rolls.
7. The method as claimed in claim 1, wherein 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, wherein the steel product is
passed through the rolling mill stand in opposite directions for
each pass.
9. The method as claimed in claim 7, wherein the steel product is
successively passed through two or more rolling mill stands.
10. The method as claimed in claim 1, wherein the passing of the
steel product between the set of rotating rolls having different
peripheral velocities 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 claim 1, wherein the rolling is
carried out on a steel product of which at least a skin layer has a
substantially austenitic structure.
12. The method as claimed in claim 1, wherein the rolling is
carried out on a steel product of which at least a skin layer has a
substantially austenitic-ferritic two-phase structure.
13. The method as claimed in claim 1, wherein the rolling is
carried out on a steel product of which at least a skin layer has a
substantially ferritic structure.
14. The method as claimed in claim 1, wherein the rolling is
carried out while the temperature of the steel product is higher
than 0.degree. C. and lower than 720.degree. C.
15. The method according to claim 14, wherein 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
wherein between casting the strand and accelerated cooling or
coiling or cooling, or after cooling, the steel product is
subjected to the method of claim 1.
17. The method for producing a steel product according to claim 16,
wherein the thickness of the cast strand is below 150 mm.
18. The method for producing a steel product according to claim 16,
wherein the thickness of the cast strand is below 20 mm.
19. The method according to claim 16, wherein the steel product
that is produced is a stainless steel product.
20. The method for producing a steel product according to claim 16,
wherein the rolling is carried out on a steel product having a
substantially austenitic structure, the steel is acceleratedly
cooled thereafter, and the steel product essentially comprises
ferrite, bainite and/or martensite.
21. The method for producing a steel product according to claim 16,
wherein the average grainsize of the steel product is smaller than
5 .mu.m.
22. The method according to claim 1, wherein the steel product is
subjected to a heat treatment before or after the rolling step.
23. The method as claimed in claim 1, 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.
25. A steel product produced according to the method of claim 1
having a thickness of between 10 and 300 mm.
26. A steel product produced according to the method of claim 1,
wherein the steel product is a steel billet.
27. A steel section, produced using a billet according to claim
26.
28. A steel product produced according to the method of claim 1,
wherein the starting point is a steel ingot.
29. A steel plate, strip or billet produced by continuous casting,
using the method as claimed in claim 1.
30. A steel strip produced according to the method of claim 16.
31. A clad steel product produced according to claim 23 for use in
a member of the group consisting of pipes, chemical plants, power
plants, vessels, and pressure vessels.
32. A steel strip produced according to claim 16, wherein the steel
is a HSLA-steel comprising at least one of the elements niobium,
titanium, vanadium or boron.
33. The method as claimed in claim 1, wherein the thickness of the
steel product is reduced by at most 5% each pass.
34. The method as claimed in claim 1, wherein the peripheral
velocity of the faster moving roll is at most 20% higher than that
of the slower moving roll.
35. The method as claimed in claim 1, wherein the steel product is
introduced between the rolls at an angle of between 10 and
25.degree. with respect to the perpendicular to the plane through
the center axes of the rolls.
36. The method as claimed in claim 1, wherein the steel product is
introduced between the rolls at an angle of between 15 and
25.degree. with respect to the perpendicular to the plane through
the center axes of the rolls.
37. The method as claimed in claim 1, wherein the rolling is
carried out on a steel product having a substantially austenitic
structure throughout.
38. The method as claimed in claim 1, wherein the rolling is
carried out on a steel product having a substantially
austenitic-ferritic two-phase structure throughout.
39. The method as claimed in claim 1, wherein the rolling is
carried out on a steel product having a substantially ferritic
structure throughout.
40. The method for producing a steel product according to claim 16,
wherein the thickness of the cast strand is below 100 mm.
41. The method for producing a steel product according to claim 16,
wherein the thickness of the cast strand is below 80 mm.
42. The method for producing a steel product according to claim 16,
wherein the thickness of the cast strand is below 10 mm.
43. The method for producing a steel product according to claim 16,
wherein the thickness of the cast strand is below 5 mm.
44. The method for producing a steel product according to claim 16,
wherein the rolling is carried out on a steel product having a
substantially austenitic structure, the steel is acceleratedly
cooled thereafter, the steel product essentially comprises ferrite,
bainite and/or martensite, and the ferrite content after cooling is
at least 60%.
45. The method for producing a steel product according to claim 16,
wherein the rolling is carried out on a steel product having a
substantially austenitic structure, the steel is acceleratedly
cooled thereafter, the steel product essentially comprises ferrite,
bainite and/or martensite, and the ferrite content after cooling is
70%.
46. The method for producing a steel product according to claim 16,
wherein the rolling is carried out on a steel product having a
substantially austenitic structure, the steel is acceleratedly
cooled thereafter, the steel product essentially comprises ferrite,
bainite and/or martensite, and the ferrite content after cooling is
more than 80%.
47. The method for producing a steel product according to claim 16,
wherein the average grainsize of the steel product is smaller than
2 .mu.m.
48. The method for producing a steel product according to claim 16,
wherein the average grainsize of the steel product is smaller than
1 .mu.m.
49. The method according to claim 1, wherein the steel product is
subjected to a heat treatment before or after the rolling step
selected from at least one member of the group consisting of a
normalising treatment, a full anneal, a stress relief anneal or a
speroidisation annealing treatment.
50. The method as claimed in claim 23, wherein the covering layer
is a steel with a different composition than that of the steel
product.
51. The method as claimed in claim 23, wherein the covering layer
is a stainless steel.
52. The method as claimed in claim 23, wherein the covering layer
comprises a member of the group consisting of Titanium, Nickel,
Copper, Aluminium and alloys thereof.
53. A steel product produced according to the method of claim 1,
having a thickness between 20 and 160 mm.
54. A steel product produced according to the method of claim 1,
having a thickness between 20 and 60 mm.
55. A steel product produced according to the method of claim 25,
for use in a member of the group consisting of buildings, bridges,
earth moving equipment, pipe line, ship building, and off shore
constructions.
56. An H-section, produced using a billet according to claim
26.
57. A steel product produced according to the method of claim 1,
wherein the starting point is a steel ingot, wherein the steel
product has a core and pores in the core of the product have a
maximum dimension of less than 200 .mu.m.
58. A steel product produced according to the method of claim 1,
wherein the starting point is a steel ingot, wherein the steel
product has a core and pores in the core of the product have a
maximum dimension of less than 100 .mu.m.
59. A steel product produced according to the method of claim 1,
wherein the starting point is a steel ingot, wherein the steel
product has a core and pores in the core of the product have a
maximum dimension of less than 20 .mu.m.
60. A steel product produced according to the method of claim 1,
wherein the starting point is a steel ingot, wherein the steel
product has a core and pores in the core of the product have a
maximum dimension of less than 10 .mu.m.
61. A steel plate, strip or billet produced by continuous casting,
using the method as claimed in claim 1, wherein the steel product
has a core and pores in the core of the plate, strip or billet have
a maximum dimension of less than 200 .mu.m.
62. A steel plate, strip or billet produced by continuous casting,
using the method as claimed in claim 1, wherein the steel product
has a core and pores in the core of the plate, strip or billet have
a maximum dimension of less than 100 .mu.m.
63. A steel plate, strip or billet produced by continuous casting,
using the method as claimed in claim 1, wherein the steel product
has a core and pores in the core of the plate, strip or billet have
a maximum dimension of less than 20 .mu.m.
64. A steel plate, strip or billet produced by continuous casting,
using the method as claimed in claim 1, wherein the steel product
has a core and pores in the core of the plate, strip or billet have
a maximum dimension of less than 10 .mu.m.
65. A steel strip produced according to the method of claim 16, for
use in parts of automobiles, transport equipment, piling,
buildings, construction.
66. A steel strip produced according to claim 16, wherein the steel
is an ultra low carbon steel.
67. A steel strip produced according to claim 16, wherein the steel
is an ultra low carbon steel at least partly stabilized with at
least one of the elements titanium, niobium or boron.
Description
[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] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.degree. C.
[0014] 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.
[0015] 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 kwnown.
[0016] The rolling is preferably carried out at an elevated
temperature. However, the maximum temperature is limited to
1350.degree. 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.
[0017] It is also expected that the processing according to the
invention will result in a rolled sheet with less lateral
spread.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] According to an advantageous embodiment of the method, the
steel product is introduced between the rolls at an angle of
between 5 and 450.degree. 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.degree. and more preferably at angle of between 15 and
25.degree. 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.
[0023] 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.degree. preferably between 10 and 250 and more
preferably between 15 and 25.degree. during each processing
operation.
[0024] 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.
[0025] 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.
[0026] 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.degree. C.
for an ultra low carbon steel to 800-870.degree. C. for a low
carbon steel (depending on the chemical composition of course) to
about 723.degree. C. for a steel with 0.8% C. In all cases the
maximum temperature is 1350 .degree. C. In case of rolling an
austenitic stainless steel, the rolling always takes place on an
austenitic structure.
[0027] 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.degree. C.
ending at 800-870.degree.C. The temperature range decreases with
increasing carbon contents to reduce to an eutectoid point of about
723.degree. C. for a steel with 0.8% C.
[0028] 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.degree. C., whereas for steels with lower
carbon contents such as ultra low carbon steels the maximum
temperature is about 850.degree.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.
[0029] According to a further advantageous embodiment of the
invention the rolling is performed at temperatures between
0.degree. C. and 720.degree. 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.
[0030] 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 peripheral velocities.
In this way, by way of example, an accurately desired thickness or
smoothness can be imparted to the product.
[0031] According to another advantageous embodiment, a steel
product is produced according to a method comprising the steps of:
[0032] continuous casting of a steel strand; [0033] optionally
heating and/or temperature homogenising the steel strand between a
casting machine and a rolling device; [0034] optionally rolling the
steel product in one or more rolling mill stands of the rolling
device with rolls having substantially identical peripheral
velocities; [0035] optionally accelerated cooling after the last
rolling step; [0036] optionally cutting the steel product into
slabs or coils before or after rolling; [0037] optionally coiling
the steel product [0038] cooling the steel product
[0039] 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").
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] 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.
[0045] A further advantage is obtained if the steel product to be
processed according to the previous two embodiments is a stainless
steel.
[0046] 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.
[0047] 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.
[0048] In the production of high strength steel strip micro-alloyed
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.
[0049] 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.degree. 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.
[0050] 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 .mu.m, more preferably
smaller than 2 .mu.m and even more preferably smaller than 1
.mu.m.
[0051] 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 .mu.m, more preferably smaller than 2
.mu.m and even more preferably smaller than 1 .mu.m.
[0052] 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.degree. 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.
[0053] 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.
[0054] The rolling according to the invention can also be carried
out at low temperatures between 0 and 720 .degree. 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.
[0055] 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.
[0056] 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.
[0057] 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 .mu.m, preferably less
than 100 .mu.m, more preferably less than 20 .mu.m and even more
preferably less than 10 .mu.m. As a result of the continuous
casting, continuously cast plate and strip material always has
pores which can be significantly larger than 200 .mu.m. 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.
[0058] 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.
[0059] The invention also relates to a steel product produced
according to the invention having a thickness of preferably between
10 and 300 mm, more preferably between 20 and 160 mm, for example
60 mm, for use in for example buildings, bridges, earth moving
equipment, pipe line, ship building, and off shore
constructions.
[0060] The invention also relates to a steel billet produced
according to the invention, for example for use as starting
material for the production of a steel section, for example an
H-section.
[0061] 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 .mu.m, more preferably
less than 100 .mu.m, still more preferably less than 20 .mu.m and
even more preferably less than 10 .mu.m 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 .mu.m, more preferably
less than 100 .mu.m, still more preferably less than 20 .mu.m and
even more preferably less than 10 .mu.m.
[0062] 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.
[0063] The invention will be explained with reference to an
exemplary embodiment.
[0064] Experiments were carried out using slabs of a Titanium
stabilised ultra low carbon steel, carbon-manganese steels and
Niobium microalloyed HSLA-steel.
[0065] The slabs were introduced at different angles varying
between 50.degree. and 45.degree.. The temperature of the slabs
when they were introduced into the rolling device was approximately
1000.degree. C. The two rolls were driven at a speed of 5
revolutions per minute.
[0066] 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.degree..
[0067] A shear angle of 20.degree. 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: eq = 2 3 ( xx 2 + yy 2 ) .
##EQU1##
[0068] This formula is used to make it possible to present the
strain in one dimension and is known from the book "Fundamentals of
metal forming" by R. H. Wagoner and J. L. Chenot, John Wiley &
Sons, 1997.
[0069] Therefore, in the slabs which have been rolled in accordance
with the experiment, the equivalent strain is eq = 2 3 ( ( ln
.function. ( 32.5 30.5 ) ) 2 + ( 1 2 .times. ( tan .times. .times.
20 .times. .degree. ) ) 2 ) .apprxeq. 0.25 . ##EQU2##
[0070] 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 eq = 2 3 ln .function. (
32.5 30.5 ) 2 .apprxeq. 0.07 ##EQU3## (working on the basis of a
uniform strain over the entire thickness of the steel product).
[0071] 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.
* * * * *