U.S. patent application number 11/884502 was filed with the patent office on 2010-06-24 for extremely low carbon steel plate excellent in surface characteristics, workability, and formability and a method of producing extremely low carbon cast slab.
Invention is credited to Wataru Ohashi, Katsuhiro Sasai.
Application Number | 20100158746 11/884502 |
Document ID | / |
Family ID | 42266402 |
Filed Date | 2010-06-24 |
United States Patent
Application |
20100158746 |
Kind Code |
A1 |
Sasai; Katsuhiro ; et
al. |
June 24, 2010 |
Extremely Low Carbon Steel Plate Excellent in Surface
Characteristics, Workability, and Formability and a Method of
Producing Extremely Low Carbon Cast Slab
Abstract
A method of producing an extremely low carbon steel cast slab
characterized by casting molten steel obtained by reducing the
carbon concentration of the molten steel to 0.005 mass % or less,
then adding Cu, Nb, and B to the molten steel, furthermore, and
adjusting the concentration of dissolved oxygen in the molten steel
to 0.01 mass % to 0.06 mass % and extremely low carbon steel plate
comprised of steel containing C: 0.005 mass % or less, acid soluble
Al: 0.005 mass % or less, and further Cu, Nb, and B, characterized
in that the steel has fine oxides of a diameter of 0.5 .mu.m to 30
.mu.m dispersed in it in an amount of 1000 particles/cm.sup.2 to
1,000,000 particles/cm.sup.2.
Inventors: |
Sasai; Katsuhiro; ( Chiba,
JP) ; Ohashi; Wataru; (Chiba, JP) |
Correspondence
Address: |
KENYON & KENYON LLP
ONE BROADWAY
NEW YORK
NY
10004
US
|
Family ID: |
42266402 |
Appl. No.: |
11/884502 |
Filed: |
February 16, 2006 |
PCT Filed: |
February 16, 2006 |
PCT NO: |
PCT/JP06/03201 |
371 Date: |
August 15, 2007 |
Current U.S.
Class: |
420/92 ; 164/47;
164/499; 164/61; 420/89 |
Current CPC
Class: |
C21D 8/0236 20130101;
C22C 38/004 20130101; C22C 38/08 20130101; B22D 11/115 20130101;
C21D 8/0226 20130101; C22C 38/16 20130101; B22D 11/001 20130101;
C22C 38/14 20130101; C22C 38/02 20130101; C22C 38/12 20130101; B22D
7/00 20130101; C22C 38/04 20130101 |
Class at
Publication: |
420/92 ; 164/47;
164/61; 164/499; 420/89 |
International
Class: |
C22C 38/08 20060101
C22C038/08; B22D 25/00 20060101 B22D025/00; B22D 27/15 20060101
B22D027/15; B22D 27/02 20060101 B22D027/02; C22C 38/16 20060101
C22C038/16 |
Claims
1. A method of producing an extremely low carbon steel cast slab
characterized by casting molten steel obtained by reducing the
carbon concentration of the molten steel to 0.005 mass % or less,
then adding Cu, Nb, and B to the molten steel, furthermore, and
adjusting the concentration of dissolved oxygen in the molten steel
to 0.01 mass % to 0.06 mass %.
2. A method of producing an extremely low carbon steel cast slab
characterized by casting molten steel obtained by reducing the
carbon concentration of the molten steel to 0.005 mass % or less,
then adding Cu, Nb, and B to the molten steel to make the molten
steel include Cu by 0.01 to 3.0 mass % and Nb and B by respectively
-0.021.ltoreq.Nb-(93/12).times.C.ltoreq.0.1
-0.0023.ltoreq.B-(11/14).times.N.ltoreq.0.0045 and further
adjusting the concentration of dissolved oxygen in the molten steel
to 0.01 mass % to 0.06 mass %.
3. A method of producing an extremely low carbon steel cast slab
characterized by casting molten steel obtained by reducing the
carbon concentration of the molten steel to 0.005 mass % or less,
adding Cu, Ni, Nb, and B to the molten steel to make the molten
steel include Cu by 0.01 to 3.0 mass %, Ni by 0.5.times.Cu
concentration or less, and Nb and B by respectively
-0.02.ltoreq.Nb-(93/12).times.C.ltoreq.0.1
-0.0023.ltoreq.B-(11/14).times.N.ltoreq.0045 and further adjusting
the concentration of dissolved oxygen in the molten steel to 0.01
mass % to 0.06 mass %.
4. A method of producing an extremely low carbon steel cast slab
according to claim 1 characterized by decarburizing said molten
steel by vacuum degassing.
5. A method of producing an extremely low carbon steel cast slab
according to claim 1 characterized by casting said molten steel by
casting it while electromagnetically stirring it.
6. A method of producing an extremely low carbon steel cast slab
according to claim 5 characterized by casting said molten steel by
casting it while electromagnetically stirring it to make the molten
steel at the meniscus position swirl by an average flow rate of 40
cm/s to 100 cm/s.
7. Extremely low carbon steel plate having excellent surface
characteristics, workability, and formability comprised of steel
containing C: 0.005 mass % or less, acid soluble Al: 0.005 mass %
or less, and further Cu, Nb, and B, characterized in that the steel
has fine oxides of a diameter of 0.5 .mu.m to 30 .mu.m dispersed in
it in an amount of 1000 particles/cm.sup.2 to 1,000,000
particles/cm.sup.2.
8. Extremely low carbon steel plate having excellent surface
characteristics, workability, and formability comprised of steel
containing C: 0.005 mass % or less, acid soluble Al: 0.005 mass %
or less, Cu: 0.01 to 3.0 mass %, and further Nb and B of
respectively -0.02.ltoreq.Nb-(93/12).times.C.ltoreq.0.1
-0.0023.ltoreq.B-(11/14).times.C.ltoreq.0.0045 characterized in
that the steel has fine oxides of a diameter of 0.5 .mu.m to 30
.mu.m dispersed in it in an amount of 1000 particles/cm.sup.2 to
1,000,000 particles/cm.sup.2.
9. Extremely low carbon steel plate having excellent surface
characteristics, workability, and formability comprised of steel
containing C: 0.005 mass % or less, acid soluble Al: 0.005 mass %
or less, Cu: 0.01 to 3.0 mass %, Ni: 0.5.times.Cu mass % or less,
and further Nb and B of respectively
-0.02.ltoreq.Nb-(93/12).times.C.ltoreq.0.1
-0.0023.ltoreq.B-(11/14).times.N.ltoreq.0.0045 characterized in
that the steel has fine oxides of a diameter of 0.5 .mu.m to 30
.mu.m dispersed in it in an amount of 1000 particles/cm.sup.2 to
1,000,000 particles/cm.sup.2.
10. Extremely low carbon steel plate having excellent surface
characteristics, workability, and formability comprised of steel
containing C: 0.005 mass % or less, acid soluble Al: 0.005 mass %
or less, and further Cu, Nb, and B, characterized in that at least
40% of the number of oxides present in the steel include at least
Si, Mn, and Fe.
11. Extremely low carbon steel plate having excellent surface
characteristics, workability, and formability comprised of steel
containing C: 0.005 mass % or less, acid soluble Al: 0.005 mass %
or less, Cu: 0.01 to 3.0 mass %, and further Nb and B of
respectively -0.02.ltoreq.Nb-(93/12).times.C.ltoreq.0.1
-0.0023.ltoreq.B-(11/14).times.N.ltoreq.0.0045 characterized in
that at least 40% of the number of oxides present in the steel
include at least Si, Mn, and Fe.
12. Extremely low carbon steel plate having excellent surface
characteristics, workability, and formability comprised of steel
containing C: 0.005 mass % or less, acid soluble Al: 0.005 mass %
or less, Cu: 0.01 to 3.0 mass %, Ni: 0.5.times.Cu mass % or less,
and further Nb and B of respectively
-0.02.ltoreq.Nb-(93/12).times.C.ltoreq.0.1
-0.0023.ltoreq.B-(11/14).times.C.ltoreq.0.0045 characterized in
that at least 40% of the number of oxides present in the steel
include at least Si, Mn, and Fe.
13. An extremely low carbon steel plate having excellent surface
characteristics, workability, and formability according to claim 10
characterized in that the contents of the at least Si oxides, Mn
oxides, and Fe oxides which at least 40% of the number of oxides
present in the steel include is 20 mass % or more.
Description
TECHNICAL FIELD
[0001] The present invention relates to extremely low carbon steel
plate excellent in surface characteristics, workability, and
formability and a method of producing extremely low carbon cast
slab.
BACKGROUND ART
[0002] Molten steel refined in a converter or vacuum treatment
vessel contains a large amount of dissolved oxygen. This excess
oxygen is generally removed by Al which is a strong deoxidizing
element with a strong affinity with oxygen. However, Al forms
alumina-based inclusions by deoxidation. These aggregate and form
coarse alumina clusters.
[0003] The alumina clusters become the cause of the formation of
surface defects during steel plate production and greatly
deteriorate the quality of the thin-gauge steel plate. In
particular, in extremely low carbon molten steel, which is a
material for thin-gauge steel plate having a low carbon
concentration and high concentration of dissolved oxygen after
refining, the amount of the alumina clusters is extremely large and
the rate of formation of surface defects is extremely high, so
measures for reducing the alumina-based inclusions have become
major issues.
[0004] As opposed to this, in the past, the method of adding flux
for adsorption of inclusions to the molten steel surface to remove
alumina-based inclusions such as disclosed in Japanese Patent
Publication (A) No. 5-104219 and the method of utilizing the
injection flow disclosed in Japanese Patent Publication (A) No.
63-149057 to add a CaO flux in the molten steel and remove
alumina-based inclusions by adsorption have been proposed and
worked.
[0005] On the other hand, as a method not removing the
alumina-based inclusions, but not forming them, Japanese Patent
Publication (A) No. 5-302112 discloses the method of producing
molten steel for thin-gauge steel plate not being deoxidized much
at all by Al by deoxidizing the molten steel by Mg.
DISCLOSURE OF THE INVENTION
[0006] However, with the method of removing alumina-based
inclusions as described in Japanese Patent Publication (A) No.
5-104219 and Japanese Patent Publication (A) No. 63-149057, it is
extremely difficult to reduce the alumina-based inclusions formed
in a large amount in extremely low carbon molten steel, to an
extent not forming surface defects.
[0007] Further, with the Mg deoxidation not forming any
alumina-based inclusions at all such as described in Japanese
Patent Publication (A) No. 5-302112, the vapor pressure of the Mg
is high and the yield to molten steel is extremely low, so
deoxidizing molten steel with a high concentration of dissolved
oxygen such as with extremely low carbon steel by just Mg requires
a large amount of Mg. Considering the production costs, the process
cannot be said to be practical.
[0008] Considering these problems, the present invention has as its
object to provide extremely low carbon steel plate reliably
preventing surface defects and superior in workability and
formability by finely dispersing oxides at the time of
solidification without forming almost any inclusions in the molten
steel and a method of production of the same.
[0009] In order to solve the aforeexplained problems, the present
invention has the following as its gist:
[0010] (1) A method of producing an extremely low carbon steel cast
slab characterized by casting molten steel obtained by reducing the
carbon concentration of the molten steel to 0.005 mass % or less,
then adding Cu, Nb, and B to the molten steel, furthermore, and
adjusting the concentration of dissolved oxygen in the molten steel
to 0.01 mass % to 0.06 mass %.
[0011] (2) A method of producing an extremely low carbon steel cast
slab characterized by casting molten steel obtained by reducing the
carbon concentration of the molten steel to 0.005 mass % or less,
then adding Cu, Nb, and B to the molten steel to make the molten
steel include Cu by 0.01 to 3.0 mass % and Nb and B by
respectively
-0.02.ltoreq.Nb-(93/12).times.C.ltoreq.0.1
-0.0023.ltoreq.B-(11/14).times.N.ltoreq.0.0045
and further adjusting the concentration of dissolved oxygen in the
molten steel to 0.01 mass % to 0.06 mass %.
[0012] (3) A method of producing an extremely low carbon steel cast
slab characterized by casting molten steel obtained by reducing the
carbon concentration of the molten steel to 0.005 mass % or less,
adding Cu, Ni, Nb, and B to the molten steel to make the molten
steel include Cu by 0.01 to 3.0 mass %, Ni by 0.5.times.Cu
concentration or less, and Nb and B by respectively
-0.02.ltoreq.Nb-(93/12).times.C.ltoreq.0.1
-0.0023.ltoreq.B-(11/14).times.N.ltoreq.0.0045
and further adjusting the concentration of dissolved oxygen in the
molten steel to 0.01 mass % to 0.06 mass %.
[0013] (4) A method of producing an extremely low carbon steel cast
slab according to any one of (1) to (3) characterized by
decarburizing said molten steel by vacuum degassing.
[0014] (5) A method of producing an extremely low carbon steel cast
slab according to any one of (1) to (4) characterized by casting
said molten steel by casting it while electromagnetically stirring
it.
[0015] (6) A method of producing an extremely low carbon steel cast
slab according to (5) characterized by casting said molten steel by
casting it while electromagnetically stirring it to make the molten
steel at the meniscus position swirl by an average flow rate of 40
cm/s to 100 cm/s.
[0016] (7) Extremely low carbon steel plate having excellent
surface characteristics, workability, and formability comprised of
steel containing C: 0.005 mass % or less, acid soluble Al: 0.005
mass % or less, and further Cu, Nb, and B, characterized in that
the steel has fine oxides of a diameter of 0.5 .mu.m to 30 .mu.m
dispersed in it in an amount of 1000 particles/cm.sup.2 to
1,000,000 particles/cm.sup.2.
[0017] (8) Extremely low carbon steel plate having excellent
surface characteristics, workability, and formability comprised of
steel containing C: 0.005 mass % or less, acid soluble Al: 0.005
mass % or less, Cu: 0.01 to 3.0 mass %, and further Nb and B of
respectively
-0.02.ltoreq.Nb-(93/12).times.C.ltoreq.0.1
-0.00235.ltoreq.B-(11/14).times.N.ltoreq.0.0045
characterized in that the steel has fine oxides of a diameter of
0.5 .mu.m to 30 .mu.m dispersed in it in an amount of 1000
particles/cm.sup.2 to 1,000,000 particles/cm.sup.2.
[0018] (9) Extremely low carbon steel plate having excellent
surface characteristics, workability, and formability comprised of
steel containing C: 0.005 mass % or less, acid soluble Al: 0.005
mass % or less, Cu: 0.01 to 3.0 mass %, Ni: 0.5.times.Cu mass % or
less, and further Nb and B of respectively
-0.02.ltoreq.Nb-(93/12).times.C.ltoreq.0.1
-0.0023.ltoreq.B-(11/14).times.N.ltoreq.0.0045
characterized in that the steel has fine oxides of a diameter of
0.5 .mu.m to 30 .mu.m dispersed in it in an amount of 1000
particles/cm.sup.2 to 1,000,000 particles/cm.sup.2.
[0019] (10) Extremely low carbon steel plate having excellent
surface characteristics, workability, and formability comprised of
steel containing C: 0.005 mass % or less, acid soluble Al: 0.005
mass % or less, and further Cu, Nb, and B, characterized in that at
least 40% of the number of oxides present in the steel include at
least Si, Mn, and Fe.
[0020] (11) Extremely low carbon steel plate having excellent
surface characteristics, workability, and formability comprised of
steel containing C: 0.005 mass % or less, acid soluble Al: 0.005
mass % or less, Cu: 0.01 to 3.0 mass %, and further Nb and B of
respectively
-0.02.ltoreq.Nb-(93/12).times.C.ltoreq.0.1
-0.0023.ltoreq.B-(11/14).times.N.ltoreq.0.0045
characterized in that at least 40% of the number of oxides present
in the steel include at least Si, Mn, and Fe.
[0021] (12) Extremely low carbon steel plate having excellent
surface characteristics, workability, and formability comprised of
steel containing C: 0.005 mass % or less, acid soluble Al: 0.005
mass % or less, Cu: 0.01 to 3.0 mass %, Ni: 0.5.times.Cu mass % or
less, and further Nb and B of respectively
-0.02.ltoreq.Nb-(93/12).times.C.ltoreq.0.1
-0.0023.ltoreq.B-(11/14).times.N.ltoreq.0.0045
characterized in that at least 40% of the number of oxides present
in the steel include at least Si, Mn, and Fe.
[0022] (13) An extremely low carbon steel plate having excellent
surface characteristics, workability, and formability according to
any one of (10) to (12) characterized in that the contents of the
at least Si oxides, Mn oxides, and Fe oxides which at least 40% of
the number of oxides present in the steel include is 20 mass % or
more.
[0023] According to the present invention, oxides can be made to
finely precipitate in the molten steel at the time of
solidification without causing formation of almost any inclusions,
so it is possible to reliably prevent surface defects and fix the C
and N in the steel plate and possible to control the texture of the
hot rolled steel plate, so it is possible to produce a thin-gauge
steel plate excellent in workability and formability.
BEST MODE FOR CARRYING OUT THE INVENTION
[0024] Below, the present invention will be explained in
detail.
[0025] The method of production of the present invention adds Cu,
Nb, and B to molten steel refined in a converter, electric furnace,
or other steelmaking furnace or treated by vacuum degassing or the
like to reduce the carbon concentration in the molten steel to
0.005 mass % or less and adjusts the concentration of dissolved
oxygen to 0.01 to 0.06 mass %.
[0026] The basic idea of this melting method is to reduce the
carbon concentration to an extent where it does not react with the
oxygen during casting to generate CO gas and leave behind a large
amount of dissolved oxygen without adding almost any Al so as to
prevent the formation of almost all inclusions in the molten steel
and to add Cu, Nb, and B extremely weak in deoxidizing power to fix
the C and N and control the texture control and thereby secure
quality as steel plate for sheet use.
[0027] The molten steel decarburized in the converter or vacuum
treatment vessel contains a large amount of dissolved oxygen. This
dissolved oxygen forms a large amount of alumina-based inclusions
since almost all of it is removed by addition of Al (reaction of
Formula 1):
2Al+3O.dbd.Al.sub.2O.sub.3 [Formula 1]
[0028] The alumina-based inclusions aggregate immediately after
deoxidation to become coarse alumina-based inclusions which become
the cause of formation of surface defects at the time of steel
plate production. However, if not adding any Al in the molten steel
after decarburization or, even if adding it, adding a small amount
and not removing much oxygen at all, a large amount of dissolved
oxygen is contained in the molten steel, but almost no inclusions
are formed and molten steel of an extremely high cleanliness can be
obtained.
[0029] Usually, if casting molten steel in which dissolved oxygen
is contained in a large amount, CO gas is generated during
solidification, a severe bubbling phenomenon occurs, and a large
amount of bubbles are trapped in the cast slab, so not only is the
castability deteriorated, but also the quality of the cast slab
greatly decreases.
[0030] Consequently, the present invention has focused attention on
not adding Al at all or not adding much at all and leaving
dissolved oxygen, but instead greatly lowering the C concentration
so as to suppress formation of CO gas during solidification. As a
result, from experimental studies, it was learned that if the C
concentration is made 0.005 mass % or less, the rate of formation
of CO gas during solidification drops by an extremely large
margin.
[0031] In order to raise the workability in steel plate for
thin-gauge steel plate, it is important to reduce the C
concentration as much as possible and fix the C and N in solid
solution in the steel by the addition of other elements. Usually,
Al, Ti, etc. are used as elements fixing C and N in the steel, but
if adding sufficient amounts of these elements for fixing the C or
N, the molten steel ends up becoming strongly deoxidized.
[0032] In the present invention, it was discovered to add Nb and B
as elements where even if amounts of an extent able to sufficiently
fix N or C are added, the deoxidizing power is so weak that the
molten steel is not deoxidized much at all.
[0033] Nb and B function to increase the workability of the steel
plate by fixing mainly C and mainly N respectively as
precipitates.
[0034] However, with just the compound addition of Nb and B, the
total elongation of the obtained steel plate is greatly improved,
but the Lankford value (referred to as the "r value") becomes a
somewhat low value compared with Al-deoxidized Ti-added extremely
low carbon steel.
[0035] Therefore, when the present inventors examined in detail the
additive elements which easily cause the formation of texture in
the plate surface orientation {111} suited for r value improvement
in the steel plate, whereupon they discovered that Cu addition is
the most effective in the steel plate of the present invention with
the high oxygen concentration.
[0036] Consequently, in the present invention, to raise the
workability of the steel plate, that is, both the total elongation
and the r value, it is necessary to add the three elements of Nb,
B, and Cu in the molten steel.
[0037] In the above way, even if reducing the C concentration to
0.005 mass % or less, if the concentration of dissolved oxygen in
the molten steel is too high, production of the CO gas during
solidification cannot be suppressed. By experimental study, if the
concentration of dissolved oxygen is over 0.06 mass %, even if
lowering the C concentration to 0.005 mass % or less, CO bubbles
end up being trapped in the cast slab, so bubble-based defects are
generated after rolling.
[0038] On the other hand, in order to suppress production of CO
gas, it is possible to remove the excess concentration of dissolved
oxygen by Al, Ti, or the like, but from experimental studies, if
deoxidized to lower than a concentration of dissolved oxygen of
0.01 mass %, alumina, titania, and other inclusions become
excessive and end up remaining in the molten steel without floating
up and being removed.
[0039] Accordingly, the concentration of dissolved oxygen in the
molten steel has to be 0.01 mass % to 0.06 mass %.
[0040] However, when adding Nb, B, and Cu, if the concentration of
dissolved oxygen is in the range of the present invention, Al, Ti,
etc. need not be added at all. Note that the concentration of
dissolved oxygen in the molten steel can be analyzed by an oxygen
sensor using a solid electrolyte, while the concentration of C can
be analyzed by the molten steel sampling method.
[0041] Next, the preferred concentrations in the molten steel of Nb
and B added to the molten steel will be explained. As explained
earlier, Nb and B increase the workability of the steel plate by
fixing mainly C and mainly N respectively as precipitates.
[0042] However, if adding more than required, they are present as
solid solution Nb and solid solution B in the steel and raise the
recrystallization temperature, so unless treated at the annealing
temperature corresponding to this, a hot worked structure easily
results and the ductility easily decreases.
[0043] Therefore, the preferred range of addition of Nb and B to
the molten steel can be suitably expressed if using the middle part
of the following formulas described using the chemical equivalents
of the elements as indicators. Here, the middle part of [Formula 2]
means the amount of free Nb not bonding with C and forming a
carbide, while the middle part of [Formula 3] means the amount of
free N not bonding with N and forming a nitride.
[0044] That is, in the case of Nb, if the value of the middle part
of [Formula 2] is less than -0.02 or over 0.1, or, in the case of
B, if the value of the middle part of [Formula 3] is less than
-0.0023 or over 0.0045, the ductility easily decreases.
[0045] From the above reasons, it is desirable to satisfy the
relationships of
-0.02.ltoreq.Nb-(93/12).times.C.ltoreq.0.1 [Formula 2]
-0.00235.ltoreq.B-(11/14).times.N.ltoreq.0.0045 [Formula 3]
[0046] Further, if the amounts of addition of Nb and B of this
range, the oxygen concentration balanced with Nb and B is 0.01 mass
% or more. Even if adding Nb and B, dissolved oxygen of 0.01 mass %
or more can be secured.
[0047] Further, the preferable concentration of the Cu added to the
molten steel in the molten steel will be explained. Cu has the
effect of promoting the formation of a texture of the {111}
orientation where a high r value is easily obtained in the steel
plate. At the minimum, if adding 0.01 mass % or more, this effect
does not easily appear, so the amount of addition is preferably
made 0.01 mass % or more.
[0048] On the other hand, if the amount of addition of Cu is over
3.0 mass %, the surface properties of the steel plate after hot
rolling easily deteriorate due to Cu-embrittlement, so the upper
limit is preferably made 3.0 mass %.
[0049] Ni has the effect of easing the deterioration of the hot
rolled surface characteristics due to Cu. On a mass base, it is
general to add the equivalent of more than half of the Cu as a
rule. It was discovered in steel plate with a high oxygen
concentration of the present invention, when the concentration of
dissolved oxygen in the molten steel is 0.01 mass % or more,
Cu-embrittlement is inhibited by smoothing the scale and ferrite
boundaries of the hot rolled plate and improving the scale
peelability.
[0050] Because of this, in the present invention, even in a state
where Ni is not added, the surface characteristics of the hot
rolled plate become good and the features of the present invention
can be extracted to the maximum, but when it is necessary, it is
sufficient to add Ni in an amount of less than half of the Cu.
Originally, in steel plate with good surface characteristics of the
hot rolled plate, even if adding Ni to same extent as conventional
Cu steel, only a rise in cost is incurred. The upper limit of Ni is
preferably made less than 1/2 of the Cu concentration.
[0051] The action of the other ingredients in the molten steel will
be alluded to next.
[0052] The Si concentration in the molten steel is preferably 0.005
mass % to 0.03 mass %. If Si concentration is less than 0.005 mass
%, the strength of the steel plate easily becomes insufficient,
while if the Si concentration is over 0.03 mass %, the workability
of the steel plate decreases.
[0053] Further, if the Si concentration is 0.03 mass % or less, the
equilibrium oxygen concentration also becomes more than 0.02 mass
%. By just adjusting the Si concentration, it is possible to secure
a concentration of dissolved oxygen of over 0.02 mass % to 0.06
mass %. Furthermore, by adding elements having deoxidizing power, a
concentration of dissolved oxygen in the molten steel of 0.01 mass
% to 0.06 mass % can be secured.
[0054] When the Mn concentration in the molten steel is less than
0.08 mass %, scab flaws are easily formed at the time of hot
rolling of the slab. Further, if the Mn concentration is over 0.3
mass %, the workability of the steel plate decreases. Because of
this, the Mn concentration in the molten steel is preferably 0.08
mass % to 0.3 mass %.
[0055] Further, Mn has an extremely weak deoxidizing power compared
to Si, because the Mn concentration is 0.3 mass %, the equilibrium
oxygen concentration is in excess of 0.1 mass %, furthermore, by
adding elements having deoxidizing power, a concentration of
dissolved oxygen in the molten steel no less than 0.01 mass % and
no greater than 0.06 mass % can be guaranteed.
[0056] Furthermore, because Mn has an extremely weak deoxidizing
power, if the Mn concentration is 0.3 mass % or less, almost no Mn
oxides are formed under equilibrium conditions, but if adding Mn in
high oxygen molten steel after converter blowing or after vacuum
degassing, the Mn is adding in the form of large clumps of
ferromanganese or manganese ore, so sometimes regions of high Mn
concentration are locally formed in the molten steel. In such
regions, while small in amount, Mn oxides end up being formed.
[0057] In the present invention, it is more preferable not to form
inclusions in the molten steel, so it is more preferable to adjust
the Mn concentration under operating conditions with no addition of
Mn after converter blowing or after vacuum degassing.
[0058] Usually, molten iron contains Mn. Even without the addition
of Mn, by the operating conditions, it is possible to obtain an Mn
concentration of about 0.15 mass %. Consequently, if considering
even the inclusions in addition to the quality, the more preferable
range of Mn concentration is 0.08 to 0.15 mass % where production
is possible without the addition of Mn after converter blowing or
after vacuum degassing.
[0059] In the present invention, to prevent the formation of the
easily aggregating alumina-based inclusions, Al is not added to the
molten steel or almost not added at all. In experimental studies,
if the acid soluble Al concentration of the steel plate exceeds
0.005 mass %, the alumina-based inclusions in the steel plate
increase, so the upper limit was made 0.005 mass %. Since no
addition of Al in the molten steel is preferable, of course the
lower limit of Al concentration includes 0 mass %.
[0060] Here, the acid soluble Al is the amount of Al dissolved in
acid. Usually, this corresponds to the dissolved Al concentration
(concentration of Al not forming Al.sub.2O.sub.3).
[0061] Further, the alumina-based inclusions inevitably entering
from the refractories etc. do not pose a problem. This if because
with a small amount of alumina-based inclusions, the dissolved
oxygen in the molten steel is high, so the boundary energy of the
molten steel and alumina-based inclusions decreases and almost no
texture is formed.
[0062] Furthermore, the Ti in the molten steel fixes the C and N as
TiN or TiC, so is effective in improving the workability, but if
the amount of addition of Ti also becomes greater, for example, if
the Ti concentration becomes more than 0.01 mass %, the equilibrium
oxygen concentration becomes less than 0.01 mass %, so a sufficient
concentration of dissolved oxygen cannot be secured. Consequently,
when adding Ti from the necessity of further raising the
workability, it should be added in the range of 0.01 mass % or
less.
[0063] Recently, continuous casting machines have been equipped
with in-casting mold electromagnetic stirring devices or magnetic
coils. By using these, it was discovered that casting is possible
without allowing CO bubbles to be trapped in the cast slab.
[0064] The present inventors discovered that if securing a molten
steel flow rate at the meniscus in the casting mold at the time of
electromagnetic stirring during solidification of 40 to 100 cm/s,
casting is possible with almost no CO bubbles trapped in the cast
slab even if making the concentration of dissolved oxygen about
0.06 mass %, so this is more preferable.
[0065] Note that if the swirl flow rate of the molten metal by
electromagnetic stirring is less than 40 cm/s, a sufficient
cleansing effect of CO bubbles is difficult to obtain, while if the
swirl flow rate is over 100 cm/s, the CO bubbles are cleansed, but
mold powder at the surface of the molten steel is entrained and
surface defects easily are formed.
[0066] In the present invention, molten steel reduced in C
concentration to 0.05 mass % or so by converter blowing is further
reduced in C concentration to 0.005 mass % by a vacuum degassing
apparatus. The concentration of dissolved oxygen in the molten
steel is controlled to approach 0.01 to 0.06 mass % after the end
of decarburization considering the amount of decarburization.
[0067] After the end of decarburization in the vacuum degassing
apparatus, Mn and Si are not added or not added as much as
possible, but Cu, Nb, B, Ni, and the like are added. Further, when
it is necessary to finely adjust the concentration of dissolved
oxygen in the molten steel to the target value, simultaneously
small amounts of Al and Ti are added to adjust the ingredients. The
melted steel produced in this way is continuously cast to produce a
cast slab using continuous casting or electromagnetic stirring.
[0068] Next, the steel plate of the present invention will be
explained. Note that the hot rolled steel plate obtained by hot
rolling the cast slab produced by the above method, cold rolled
steel plate obtained by cold rolling, or other steel plate obtained
by working the cast slab is defined as the "steel plate" in the
present invention.
[0069] Therefore, the steel plate of the present invention contains
Cu, Nb, and B. As other elements, for example, it is possible to
include Si, Mn, etc. from the viewpoint of securing the strength
and a trace amount of Ti and acid soluble Al at 0.005 mass % or
less from the viewpoint of securing workability.
[0070] Further, if making the C concentration in the molten steel
extremely low, the dissolved oxygen precipitates during the casting
as Fe oxide-based inclusions. The Fe oxide-based inclusions are not
formed in the molten steel, but precipitate during solidification,
so disperse finely in the cast slab without aggregating
together.
[0071] Note that the "Fe oxide-based inclusions" are not just pure
Fe oxides and also contain oxides of Si oxides, Mn oxides, etc.
combined.
[0072] Therefore, in steel plate of an extremely low carbon steel
like the present invention, at least Si, Mn, and Fe are included as
oxides. In other words, at least one type of oxide of Si, Mn, and
Fe is included. Here, other than the oxides of Si, Mn, and Fe,
various oxides such as oxides of Mg, Ca, and Al may also be
included.
[0073] Further, when evaluating the state of dispersion of
inclusions in the steel plate of the present invention, fine oxides
of a size of 0.5 .mu.m to 30 .mu.m are dispersed in the steel plate
in an amount of 1000 particles/cm.sup.2 to 1,000,000
particles/cm.sup.2. By finely dispersing inclusions in this way,
prevention of surface defects can be attained.
[0074] Note that the size of the fine oxides is made from 0.5 .mu.m
to 30 .mu.m because the size of the inclusions in the steel plate
of the present invention falls in the range of about 0.5 .mu.m to
30 .mu.m. If the inclusions are of a size of 30 .mu.m or so,
surface defects can be sufficiently prevented.
[0075] Further, the state of dispersion of inclusions was made 1000
particles/cm.sup.2 to 1,000,000 particles/cm.sup.2 because if the
inclusions of the steel plate in the present invention are in this
range of particle density, surface defects are not formed.
[0076] Here, the state of dispersion of inclusions was evaluated by
observing the polished surface of the steel plate by an optical
microscope at 100.times. and 1000.times. power and assessing the
distribution of particle size of the inclusions in a unit area. The
particle size of the inclusions, that is, the diameter, was
obtained by measuring the major axis and minor axis and calculating
(major axisxminor axis).sup.0.5.
[0077] Further, if 40% or more of the number of oxides present in
the steel plate contain at least Si, Mn, and Fe, almost all
inclusions will be formed during solidification and the time for
them to aggregate will be short, so they can finely disperse and
surface defects will be difficult to form. This is therefore
preferable.
[0078] Here, "contain at least Si, Mn, and Fe" means at least one
type of Si, Mn, and Fe. This is used in a similar sense later as
well.
[0079] Further, if 40% or more of the number of oxides present in
the steel plate have a content of at least Si oxides, Mn oxides,
and Fe oxides of 20 mass % or more, more preferably 50 mass % or
more, almost all of the oxides will be formed at a timing close to
the end of solidification and the time for them to aggregate will
be extremely short, so the inclusions will finely disperse and
surface defects will be difficult to form. This is therefore more
preferable.
[0080] A steel plate having this kind of dispersed state of oxides
and composition is resistant to the formation of surface
defects.
[0081] From the above results, according to the present invention,
the Fe oxide-based inclusions can be made to precipitate and finely
disperse during solidification without allowing the formation of
almost any inclusions in the molten steel, so the inclusions do not
become the cause of formation of surface defects at the time of
production of the steel plate. Further, the workability is greatly
improved due to the Nb, B, and Cu in the steel plate, so the
quality and material of the steel plate for sheet use can be
greatly improved.
[0082] Steel plate for sheet use is used for automobile external
sheet and other applications where processing is harsh, so
workability must be added. In order to raise the workability of the
steel plate for sheet use, decreasing the C concentration as much
as possible and further fixing the C and N in solid solution in the
steel by the addition of other elements are important.
[0083] The C concentration is made 0.01 mass % or less, preferably
0.005 mass % or less, from the viewpoint of workability. However,
the condition for prevention of the formation of CO bubbles during
solidification is a C concentration of 0.005 mass % or less, so in
the present invention, the C concentration determined from the
condition of workability is sufficiently satisfied. Note that the
lower limit of the C concentration is not particularly limited.
EXAMPLES
[0084] Below, examples and comparative examples will be given to
explain the present invention.
Example 1
[0085] 300 t of molten steel with a C concentration of 0.0019 mass
% was produced by refining at a converter and treatment at a rotary
flow type vacuum degassing apparatus.
[0086] To the molten steel, alloys of Cu, Nb, and B were added,
without adding Al, to give 0.011 mass % Si, 0.16 mass % Mn, 0.014
mass % Nb, 0.003 mass % B, 0.07 mass % Cu, 0.0016 mass % N, 0.043
mass % dissolved oxygen, and 0.001 mass % or less acid soluble
Al.
[0087] This molten steel was cast into a slab of a thickness of 250
mm and a width of 1800 mm by continuous casting. The cast slab was
cut into 8500 mm lengths for use as coil units.
[0088] The thus obtained slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
plate of 0.7 mm thickness and a width of 1800 mm. The quality was
visually examined on an inspection line after cold rolling and the
number of surface defects formed per coil was evaluated.
[0089] As a result, no surface defects were formed and no cracking
due to Cu-embrittlement was observed either. Further, the
inclusions in the cold rolled steel plate were examined, whereupon
fine oxides of a diameter of 0.5 .mu.m to 30 .mu.m were dispersed
in the steel plate in an amount of 35,000 particles/cm.sup.2. 70%
of this included Si oxides, Mn oxides, and Fe oxides in a total of
60 mass % or more.
[0090] Further, the obtained cold rolled steel plate was evaluated
for workability. It was high workability steel plate with a total
elongation of 57% and an r value of 2.6
Example 2
[0091] 300 t of molten steel with a C concentration of 0.003 mass %
was produced by refining at a converter and treatment at a rotary
flow type vacuum degassing apparatus.
[0092] To the molten steel, alloys of Cu, Nb, B, and Ni were added,
without adding Al, to give 0.01 mass % Si, 0.15 mass % Mn, 0.035
mass % Nb, 0.005 mass % B, 1.8 mass % Cu, 0.5 mass % Ni, 0.0025
mass % N, 0.004 mass % Ti, 0.015 mass % dissolved oxygen, and 0.001
mass % acid soluble Al.
[0093] This molten steel was cast to a slab of a thickness of 250
mm and a width of 1800 mm using a continuous casting machine with
an in-mold electromagnetic stirring device while
electromagnetically stirring the molten metal by an average flow
rate of 50 cm/s at the meniscus. The cast slab was cut into 8500 mm
lengths for use as coil units.
[0094] The thus obtained slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
plate of 0.7 mm thickness and a width of 1800 mm. The quality of
the slab was visually examined on an inspection line after cold
rolling and the number of surface defects formed per coil was
evaluated.
[0095] As a result, no surface defects and no cracking due to
Cu-embrittlement occurred. Further, the inclusions in the cold
rolled steel plate were examined, whereupon fine oxides of a
diameter of 0.5 .mu.m to 30 .mu.m were dispersed in the steel plate
in an amount of 23,500 particles/cm.sup.2. 50% of this included Si
oxides, Mn oxides, and Fe oxides in a total of 40 mass % or
more.
[0096] Further, the obtained cold rolled steel plate was evaluated
for workability. It was high workability steel plate with a total
elongation of 56% and an r value of 2.7
Comparative Example 1
[0097] Alloys of Ti and Cu were added to molten steel in a ladle
reduced in carbon concentration to 0.0015 mass % by refining at a
converter and treatment at a rotary flow type vacuum degassing
apparatus and the steel was deoxidized by Al to give 0.01 mass %
Si, 0.15 mass % Mn, 0.02 mass % Ti, 0.3 mass % Cu, 0.002 mass % N,
0.04 mass % Al, and 0.0002 mass % concentration of dissolved
oxygen.
[0098] This molten steel was cast into a slab of a thickness of 250
mm and a width of 1800 mm by continuous casting. The cast slab was
cut into 8500 mm lengths for use as coil units.
[0099] The thus obtained slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
plate of 0.7 mm thickness and a width of 1800 mm. The quality of
the slab was visually examined on an inspection line after cold
rolling and the number of surface defects formed per coil was
evaluated.
[0100] As a result, surface defects occurred at a rate of 5
defects/coil by slab average. Cracks due to Cu-embrittlement also
occurred. Further, when the inclusions in the cold rolled steel
plate were examined, fine oxides of a diameter of 0.5 .mu.m to 30
.mu.m were only found in the steel plate at a rate of 200
particles/cm.sup.2. A large number of inclusions in excess of 30
.mu.m were also observed. 95% of the inclusions in the steel plate
were alumina-based inclusions.
[0101] Furthermore, when the workability of the cold rolled steel
plate was evaluated, a high workability steel plate with a total
elongation of 40% and an r value of 1.4 could not be obtained.
INDUSTRIAL APPLICABILITY
[0102] According to the present invention, extremely low carbon
thin-gauge steel plate having excellent surface characteristics,
workability, and formability can be provided, so the present
invention expands the applications of thin-gauge steel plate and
the industrial applicability is great.
* * * * *