U.S. patent application number 12/989201 was filed with the patent office on 2011-02-10 for low-carbon steel slab producing method.
Invention is credited to Satoru Mineta, Masafumi Miyazaki, Hideaki Yamamura.
Application Number | 20110030911 12/989201 |
Document ID | / |
Family ID | 41550419 |
Filed Date | 2011-02-10 |
United States Patent
Application |
20110030911 |
Kind Code |
A1 |
Miyazaki; Masafumi ; et
al. |
February 10, 2011 |
LOW-CARBON STEEL SLAB PRODUCING METHOD
Abstract
A low-carbon steel slab producing method includes: adding Ti to
a molten steel decarbonized to have a carbon concentration of 0.05
mass % or less, and subsequently adding at least one of La and Ce
to adjust a constitution, and producing a smelted molten steel; and
pouring the smelted molten steel into a casting mold via a tundish;
wherein at least one of La and Ce in a total amount of 0.2 to 1.2
times an increased amount of oxygen in the smelted molten steel
during contained in the tundish is added to the smelted molten
steel in the tundish, so as to obtain a steel slab having
inclusions which contain oxides of Ti and at least one of La and Ce
as chief components, and so as to make a composition of each of the
inclusions have a mass ratio of 0.1 to 0.7, in terms of
(La.sub.2O.sub.3+Ce.sub.2O.sub.3)/TiO.sub.n (n=1.about.2).
Inventors: |
Miyazaki; Masafumi; (Tokyo,
JP) ; Yamamura; Hideaki; (Tokyo, JP) ; Mineta;
Satoru; (Tokyo, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
41550419 |
Appl. No.: |
12/989201 |
Filed: |
July 15, 2009 |
PCT Filed: |
July 15, 2009 |
PCT NO: |
PCT/JP2009/062795 |
371 Date: |
October 22, 2010 |
Current U.S.
Class: |
164/57.1 |
Current CPC
Class: |
C21C 7/06 20130101; B22D
1/00 20130101; C22C 38/14 20130101; B22D 11/00 20130101; B22D
11/108 20130101; C22C 38/005 20130101; C21C 7/0006 20130101; B22D
11/11 20130101 |
Class at
Publication: |
164/57.1 |
International
Class: |
B22D 27/00 20060101
B22D027/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 15, 2008 |
JP |
2008-183740 |
Claims
1. A low-carbon steel slab producing method, comprising: adding Ti
to a molten steel decarbonized to have a carbon concentration of
0.05 mass % or less, and subsequently adding at least one of La and
Ce to adjust a composition, and producing a smelted molten steel
used for a low-carbon steel slab, containing, by mass %, more than
0% and equal to or less than 0.05% of carbon, more than 0% and
equal to or less than 0.01% of Si, more than 0% and equal to or
less than 0.5% of Mn, more than 0% and equal to or less than 0.05%
of P, more than 0% and equal to or less than 0.02% of S, more than
0% and equal to or less than 0.01% of Al, more than 0.01% and equal
to or less than 0.4% of Ti, and in combination, 0.001% or more and
0.01% or less of at least one of La and Ce, and 0.004% or more and
0.02% or less of oxygen, and iron as a base component; and pouring
the smelted molten steel into a casting mold via a tundish, wherein
at least one of La and Ce in a total amount of 0.2 to 1.2 times an
increased amount of oxygen in the smelted molten steel during
contained in the tundish is added to the smelted molten steel in
the tundish, so as to obtain a steel slab having inclusions which
contain oxides of Ti and at least one of La and Ce as chief
components, and so as to make a composition of each of the
inclusions have a mass ratio of 0.1 to 0.7, in terms of
(La.sub.2O.sub.3+Ce.sub.2O.sub.3)/TiO.sub.n (n=1.about.2).
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for reliably
producing low-carbon steel slabs used for manufacturing low-carbon
thin steel sheets, which are excellent in workability and
moldability, and which have surfaces on which defects hardly
occur.
[0002] Priority is claimed on Japanese Patent Application No.
2008-183740, filed on Jul. 15, 2008, the content of which is
incorporated herein by reference.
BACKGROUND ART
[0003] Molten steel refined in a converter furnace and/or in a
vacuum processing container contains an excessive amount of
dissolved oxygen. The excessive amount of dissolved oxygen is
generally deoxidized with a strong deoxidizing element having a
strong affinity for oxygen, such as Al. This Al becomes alumina
after conducting such deoxidation, and then, alumina aggregates to
form coarse alumina clusters having diameters of hundreds .mu.m or
more.
[0004] Thin steel sheets are used for, for example, outer panels of
vehicles which are subject to severe processing. For this reason,
the carbon concentration in steel for the thin steel sheet is
reduced to 0.05 mass % or less for improving workability of the
thin steel sheet. The reduced carbon concentration, however, leads
to a high concentration of the dissolved oxygen after refining. As
a result, a large amount of alumina is generated by Al deoxidation,
and then, alumina clusters are generated in large amounts.
[0005] If alumina clusters are generated in large amounts, at the
time of continuous casting operation in which molten steel is
poured from a ladle containing the molten steel to casting molds
via a tundish using immersion nozzles, the alumina clusters may be
deposited on the immersion nozzle. These alumina clusters block the
transfer of the molten steel, and disturb the continuous casting
operation. This phenomenon is called "nozzle clogging".
[0006] Further, alumina clusters cause surface defects at the time
of producing steel sheets, and severely impair qualities of the
thin steel sheets. Therefore, countermeasures are required for
reducing the amount of alumina causing alumina clusters.
[0007] As a countermeasure for reducing the amount of alumina,
Patent Document 1 discloses a method for removing alumina by adding
flux for absorbing inclusions into a molten steel surface. Further,
as another countermeasure for reducing alumina, Patent Document 2
discloses a method for adsorbing and removing alumina by adding CaO
flux into molten steel. With these methods, however, it is
extremely difficult to sufficiently remove a large amount of
alumina generated in low-carbon molten steel.
[0008] Meanwhile, as a method for suppressing generation of alumina
(instead of removing alumina), there is a method for removing
dissolved oxygen after a decarburizing process, by deoxidizing
elements other than Al. For example, Patent Document 3 discloses a
method for smelting molten steel used for thin steel sheets, and in
this method, Mg is used for deoxidation. However, Mg vapor pressure
is high and the yield ratio to molten steel is significantly low.
For this reason, in a case that only Mg is used for deoxidizing
molten steel with a high concentration of dissolved oxygen such as
low-carbon steels, a large amount of Mg is required. Therefore, in
view of manufacturing cost, it is not considered that the above
method is practical.
[0009] Considering the above problems regarding deoxidation of
molten steel using Al, Patent Document 4 discloses a method of
using Ti, and La and/or Ce in combinations as deoxidizing elements.
According to this method, inclusions contained in deoxidized molten
steel become compound inclusions of Ti oxide, and La oxide and/or
Ce oxide. Since these compound inclusions finely disperse in the
molten steel rather than aggregating one another, the
above-mentioned coarse alumina cluster will not be generated, that
is, neither nozzle clogging nor surface defects on the steel sheet
occur.
[Related Art Documents]
[Patent Documents]
[0010] [Patent Document 1] Japanese Unexamined Patent Application,
First Publication No. H05-104219
[0011] [Patent Document 2] Japanese Unexamined Patent Application,
First Publication No. S63-149057
[0012] [Patent Document 3] Japanese Unexamined Patent Application,
First Publication No. H05-302112
[0013] [Patent Document 4] PCT publication No. WO 03/002771 A1
DISCLOSURE OF THE INVENTION
Problems that the Invention is to Solve
[0014] However, even in the method disclosed in Patent Document 4,
molten steel may be subject to oxidation by ambient oxygen or slag
in a tundish at the time of pouring the molten steel from a ladle
containing the molten steel to the tundish.
[0015] More specifically, in the case that Ti, and La and/or Ce are
used as deoxidizing elements for oxidizing molten steel, Ti in the
molten steel is preferentially oxidized and then, the content rate
of Ti oxide in the inclusions will increase. As a result,
composition of the inclusion changes from the above-described
composition in which aggregation hardly occurs, to a composition in
which aggregation frequently occurs, thereby causing nozzle
clogging or surface defects on the steel sheet.
[0016] An object of the present invention is to provide a
low-carbon steel slab producing method that can prevent nozzle
clogging and surface defects on a steel sheet which are caused by
aggregated inclusions, by using Ti, and La and/or Ce as deoxidizing
elements for molten steel, controlling composition change of the
inclusions in the molten steel due to oxidation of the molten steel
in a tundish, and preventing inclusions from aggregating.
Means for Solving the Problems
[0017] In order to solve the above-described problems, the present
invention employs the following.
(1) A low-carbon steel slab producing method according to an
invention including: adding Ti to a molten steel decarbonized to
have a carbon concentration of 0.05 mass % or less, and
subsequently adding at least one of La and Ce to adjust a
composition, and producing a smelted molten steel used for a
low-carbon steel slab containing, by mass %, more than 0% and equal
to or less than 0.05% of carbon, more than 0% and equal to or less
than 0.01% of Si, more than 0% and equal to or less than 0.5% of
Mn, more than 0% and equal to or less than 0.05% of P, more than 0%
and equal to or less than 0.02% of S, more than 0% and equal to or
less than 0.01% of Al, more than 0.01% and equal to or less than
0.4% of Ti, and in combination, 0.001% or more and 0.01% or less of
at least one of La and Ce, and 0.004% or more and 0.02% or less of
oxygen, and iron as a base component; and pouring the smelted
molten steel into a casting mold via a tundish, wherein at least
one of La and Ce in a total amount of 0.2 to 1.2 times an increased
amount of oxygen in the smelted molten steel during contained in
the tundish is added to the smelted molten steel in the tundish, so
as to obtain a steel slab having inclusions which contain oxides of
Ti and at least one of La and Ce as chief components, and so as to
make a composition of each of the inclusions have a mass ratio of
0.1 to 0.7, in terms of (La.sub.2O.sub.3+Ce.sub.2O.sub.3)/TiO.sub.n
(n=1.about.2).
EFFECTS OF THE INVENTION
[0018] According to the present invention in (1), the composition
of inclusions in molten steel to be subject to oxidation in a
tundish can be controlled within an appropriate range. Therefore,
it is possible to produce low-carbon steel slabs excellent in
workability and moldability while reliably preventing nozzle
clogging and product surface defects.
BRIEF DESCRIPTION OF A DRAWING
[0019] FIG. 1 is a flowchart illustrating processes for producing
low-carbon steel according to an embodiment of the present
invention.
EMBODIMENT OF THE INVENTION
[0020] Hereinafter, an embodiment of the present invention will be
described in detail.
[0021] Firstly, the composition range of deoxidized molten steel
and the composition range of inclusions contained in the deoxidized
molten steel according to the embodiment of the present invention
will be explained together with the reasons therefor.
[0022] The present inventors experimentally evaluated aggregating
action of inclusions, by using, as deoxidizers to be added to
molten steels, Al, Ti, La and Ce in appropriate combinations
thereof. Analysis was made on inclusions in the molten steel, by
cooling samples of the molten steel and studying the inclusions in
the steel using SEM-EDX.
[0023] As a result, it was confirmed that Al.sub.2O.sub.3
inclusions, TiO.sub.n inclusions (n=1.about.2, the same applies to
hereinafter), Al.sub.2O.sub.3--La.sub.2O.sub.3--Ce.sub.2O.sub.3
compound inclusions, Al.sub.2O.sub.3--La.sub.2O.sub.3 compound
inclusions, and Al.sub.2O.sub.3--Ce.sub.2O.sub.3 compound
inclusions were aggregated with relative ease. It was further
confirmed that, on the contrary,
TiO.sub.n--La.sub.2O.sub.3--Ce.sub.2O.sub.3 compound inclusions,
TiO.sub.n--La.sub.2O.sub.3 compound inclusions, and
TiO.sub.n--Ce.sub.2O.sub.3 compound inclusions were not aggregated
but dispersed in the molten steel as fine inclusions in spherical
shapes or in spindle shapes.
[0024] A reason of the above phenomenon may be suggested that
TiO.sub.n--La.sub.2O.sub.3--Ce.sub.2O.sub.3,
TiO.sub.n--La.sub.2O.sub.3, and TiO.sub.n--Ce.sub.2O.sub.3 have
smaller interface energy between inclusions and molten steel than
that of Al.sub.2O.sub.3, TiO.sub.n,
Al.sub.2O.sub.3--La.sub.2O.sub.3--Ce.sub.2O.sub.3,
Al.sub.2O.sub.3--La.sub.2O.sub.3, and
Al.sub.2O.sub.3--Ce.sub.2O.sub.3. That is, if the interface energy
is small, inclusions may be reliably presented in molten steel, and
aggregation of the inclusions may be suppressed.
[0025] Further, it was confirmed from experiments that aggregation
of the inclusions depended on the mass ratio of
La.sub.2O.sub.3+Ce.sub.2O.sub.3 and TiO.sub.n. More specifically,
for suppressing aggregation of the inclusions in molten steel, if
the value regarding the mass ratio of
La.sub.2O.sub.3+Ce.sub.2O.sub.3 and TiO.sub.n contained in the
inclusions obtained from the equation
(La.sub.2O.sub.3+Ce.sub.2O.sub.3)/TiO.sub.n (hereinafter, this
value may be described as "modification index") is 0.1 or more, the
interface energy between the inclusions and the molten steel
decreases. That is, aggregation of the inclusions can be
suppressed. It should be noted that the modification index is
preferably 0.15 or more, and more preferably, 0.2 or more.
[0026] Meanwhile, if the modification index exceeds 0.7, the
melting point of the inclusions will decrease and the inclusions
will enter a liquid state in molten steel. Therefore, inclusions
rather frequently aggregate and form coarse inclusions. For this
reason, the modification index should be 0.7 or less. The
modification index is preferably 0.6 or less, and more preferably
0.5 or less.
[0027] In the case of carrying out pre-deoxidation with Al (as
described later), inclusions may contain not only Ti, and La and/or
Ce, but also Al. As a result of studying this fact, it was
confirmed from experiments that if the amount of Al oxides in the
inclusions does not reach 25 mass %, the effect of suppressing
aggregation of the inclusions was not disturbed.
[0028] Accordingly, in the present invention, with respect to each
of the inclusions contained in deoxidized molten steel, oxidation
products of Ti, and La and/or Ce are generated as chief
components.
[0029] In the case of not carrying out pre-deoxidation with Al, the
total amount of oxides of Ti, and La and/or Ce in each inclusion
reaches almost 100 mass %. However, even if the pre-deoxidation
with Al is carried out and Al oxides are contained in the
inclusions, it is still possible to regard oxidation products of
Ti, and La and/or Ce as chief components.
[0030] Here, as a criterion regarding the chief components, a state
in which inclusions contain 75 mass % or more of oxidation products
of Ti, and La and/or Ce in total may be proposed. In this state, as
same to the case that the total amount of the oxidation products of
Ti, and La and/or Ce does not reach about 100 mass %, the
aggregation of the inclusions may be suppressed.
[0031] Since all of Ti, La, and Ce are deoxidizing elements, oxygen
concentration in molten steel is decreased by adding these
elements. Upon decreasing the oxygen concentration, the surface
tension of the molten steel increases. If the surface tension of
the molten steel increases too much, even if the modification index
of the inclusion is controlled to be fallen within the
above-described range, it is impossible to sufficiently reduce the
interface energy between the molten steel and the inclusions. As a
result, the inclusions aggregate and form coarse inclusions.
[0032] Meanwhile, if the oxygen concentration in the molten steel
increases too much, a large amount of inclusions are generated due
to deoxidation. Then, the collision probability of the inclusions
increases, thereby promoting aggregations.
[0033] Therefore, it was discovered that the oxygen concentration
has an appropriate range defined by the upper limit and the lower
limit for sufficiently preventing inclusions from coarsening, and
in order for the oxygen concentration to fall within the
appropriate range, there is an appropriate range for the amount of
deoxidizing elements. More specifically, as a result of
experimentally studying, it was discovered that the aggregation of
the inclusions may be sufficiently suppressed if the oxygen
concentration of the molten steel lies in a range of 0.004 mass %
or more and 0.02 mass % or less.
[0034] Basically, in the present invention, Ti is added and
subsequently, one or more of La and Ce is added. Thus, Ti is mostly
worked as an element for deoxidation, and one or more of La and Ce
is mostly worked as elements for modifying the composition of the
inclusions. Therefore, Ti may be considered as a chief element for
deoxidation. That is, in order to fall the value of the oxygen
concentration in the molten steel within the above-mentioned range
of 0.004 mass % or more and 0.02 mass % or less, the Ti amount in
the steel should be fallen within the range of 0.01 mass % or more
and 0.4 mass % or less, considering deoxidation equilibria.
[0035] Furthermore, in order to fall the modification index of the
inclusions within the above-mentioned appropriate range, the total
amount of La and Ce in the steel should fall within the range of
0.001 mass % or more and 0.01 mass % or less, which is lower than
the amount of Ti in the steel.
[0036] Next, the reason of the limitation regarding compositions in
the present invention will be explained below.
[C], [Si], [Mn], [P]
[0037] Elements of C, Si, Mn, and P improve the strength and the
hardness of steel sheets. Therefore, in order to improve the
workability and the moldability of product sheets, the upper limits
of these elements are respectively set to 0.05 mass %, 0.01 mass %,
0.5 mass %, or 0.05 mass %. Meanwhile, the lower limits of them are
set to more than 0 mass %.
[S]
[0038] An element S becomes sulfide such like MnS, and is expanded
by rolling process. The expanded sulfide becomes a starting point
of fracture at the time of processing the product sheet, and thus
deteriorates the workability. The practical upper limit is set to
0.02 mass %. Since the lower amount is preferable, the lower limit
includes 0 mass %.
[Al]
[0039] An element Al, which is a strong deoxidizing element, is
added to adjust the amount of [oxygen] in molten steel. However, if
the Al is added excessively, a large amount of alumina will be
generated in the molten steel to form alumina cluster. Then, this
alumina cluster may cause nozzle clogging at the time of casting
operation and generate surface defects on the product sheet. The
practical upper limit at the time of carrying out pre-deoxidation
with Al is set to 0.01 mass %. Since Al is not added in the case of
not carrying out the pre-deoxidation, the lower limit includes 0
mass %.
[Ti], [La], [Ce], [O]
[0040] The limitations of the ranges regarding elements of Ti, La,
Ce, and O, and the reasons thereof are explained above.
[0041] Next, a molten steel deoxidation process, a composition
change of the inclusion due to oxidation, and a method for
controlling the modification will be explained below.
[0042] In order to improve workability and moldability of the
products, molten steel in which the amount of elements other than
Fe are adjusted to: C: 0.05 mass % or less, Si: 0.01 mass % or
less, Mn: 0.5 mass % or less, P: 0.05 mass % or less, S: 0.02 mass
% or less, is decarbonized in a converter furnace and/or a vacuum
processing container.
[0043] The dissolved oxygen contained in the molten steel is
usually deoxidized by, mainly, adding Al. As a result, a large
amount of alumina is generated, and alumina aggregates to form
coarse alumina clusters having a diameter of hundreds .mu.m or
more. Then, alumina clusters may cause nozzle clogging or surface
defects on the steel sheet at the time of a continuous casting
operation.
[0044] Then, in the present invention, dissolved oxygen after
decarburization is deoxidized by, mainly, deoxidizers other than Al
so as to prevent generation of alumina clusters in large amounts.
More specifically, molten steel is refined in a steel furnace such
as a converter furnace or an electric furnace, and is subject to a
vacuum degassing and the like, thereby reducing the carbon
concentration in the molten steel to 0.05 mass % or less. To this
molten steel, Ti+La, Ti+Ce, or Ti+La+Ce are added, and before a
tundish stage, compound inclusions of Ti oxide, and La oxide and/or
Ce oxide are generated in the molten steel.
[0045] If the deoxidation is carried out only with Ti, a large
amount of Ti is required. Thus, for adjusting the amount of the
dissolved oxygen before adding Ti, pre-deoxidation with a small
amount of Al may also be carried out. In this case, 1-10 minute(s)
should be allowed after the small amount of Al is added, for
floating alumina.
[0046] Then, for carrying out continuous casting operation, molten
steel contained in a ladle is poured from the ladle into casting
molds via a tundish, using immersion nozzles. At this time,
generally, in order to prevent the molten steel in the tundish from
being exposed to air and oxidized in the tundish, the atmosphere in
the tundish may be changed to an inert gas such as Ar, and a molten
steel surface may be sealed by a molten flux.
[0047] However, industrially, it is difficult and substantially
impossible to completely change the atmosphere in the tundish to
oxygen-free atmosphere. Further, molten steel may be oxidized by
slag mixed into the molten steel from the ladle. Therefore, the
oxidation of the molten steel during contained in the tundish
inevitably occurs to some extent.
[0048] In particular, when the casting speed decreases, for example
at the time of replacing the ladle, flow volume of the molten steel
via a tundish decreases. Therefore, the residence time of the
molten steel during contained in the tundish is increased, that is,
the molten steel is exposed to the atmosphere and slag for a long
time. Therefore, the oxidation is likely to occur. Hereinafter,
oxidation of molten steel during contained in a tundish by the
atmosphere or slag is described as "reoxidation".
[0049] The amount of reoxidation of the molten steel during
contained in the tundish is precisely defined by the difference
between the amount of oxygen contained in molten steel which exists
at a molten steel inlet located in an up stream of the tundish, and
the amount of oxygen contained in molten steel which exists at a
molten steel outlet located in an downstream of the tundish.
However, considering the design of the equipment, it is difficult
to measure the amount of oxygen contained in the molten steel at
the molten steel inlet or molten steel outlet of the tundish.
Therefore, molten steel in the ladle which contains substantially
the same amount of the oxygen to that of upstream of the tundish,
and molten steel in the vicinity of tundish outlet which contains
substantially the same amount of oxygen to that of downstream of
the tundish may be used as practical measuring points and the
measured values at these measuring points may be used for the
evaluation.
[0050] The amount of Ti contained in the molten steel which has Ti
as chief deoxidizing element is larger than the amount of La and/or
Ce. Thus, Ti is preferentially oxidized by the reoxidation of the
molten steel, and Ti oxide is generated substantially in proportion
to the amount of the reoxidation.
[0051] Ti oxide which is newly generated by significant reoxidation
becomes TiO.sub.2. This TiO.sub.2 has a strong aggregation
property, therefore, the TiO.sub.2 and the compound inclusions of
Ti oxide, and La oxide and/or Ce oxide which are already presented
in the molten steel before a ladle stage are aggregated. As a
result, the modification index of the compound inclusions will be
decreased.
[0052] This phenomenon is notable when the casting speed decreases,
for example at the time of replacing the ladle as mentioned above.
For this reason, it was recognized as difficult to reliably prevent
nozzle clogging or surface defects of the steel sheet caused by the
aggregated inclusions, in a long-running casting operation.
[0053] The present inventor, in view of these circumstances,
discovered that the deterioration of the modification index can be
suppressed by adding an appropriate amount of La and/or Ce to a
tundish containing molten steel in which the modification index of
the inclusions has been decreased by the reoxidation occurred in
the tundish, for reducing Ti oxide in the molten steel by La and/or
Ce, and decreasing the amount of TiO.sub.n in the compound
inclusions of Ti oxide, and La oxide and/or Ce oxide. Hereinafter,
the details will be described.
[0054] La and Ce have strong deoxidation ability in comparison with
that of Ti. Therefore, TiO.sub.2 just after being generated by
reoxidation may be reduced only by a small amount of La or Ce.
Here, if TiO.sub.2 is partially reduced to modify fine compound
oxides having a diameter of 0.5 .mu.m-30 .mu.m such as
TiO.sub.2--La.sub.2O.sub.3, TiO.sub.2--Ce.sub.2O.sub.3,
TiO.sub.2--La.sub.2O.sub.3--Ce.sub.2O.sub.3, and the modification
index after the modification falls within the above-mentioned
appropriate range, aggregation of the inclusions generated by the
reoxidation may be prevented. Then, the inclusions may be modified
to compound oxides in spherical shapes or spindle shapes.
[0055] For the deoxidation, one or more of La and Ce should be
added to the molten steel in an amount required for the
modification, in accordance with the amount of TiO.sub.2 generated
by the reoxidation.
[0056] The amount of TiO.sub.2, which is generated by the
reoxidation, is determined based on the increased mass of the
oxygen in the molten steel during contained in the tundish.
Accordingly, using the increased mass of the oxygen in the molten
steel during contained in the tundish as a management index, one or
more of La and Ce may be added to the molten steel in an amount
required for the modification, based on the management index.
[0057] Here, the increased mass of the oxygen in the molten steel
during contained in the tundish may be calculated by multiplying
the amount of molten steel supplied to the tundish (that is, poured
amount of the molten steel to the tundish per unit of time) by the
amount of reoxidation of the molten steel (that is, the oxygen
concentration increased in the tundish per unit of molten steel
amount). The amount of the reoxidation of the molten steel can be
obtained by using zirconia oxygen sensors at the above-mentioned
measuring points for measuring the value of the oxygen in the
molten steel, and calculating the difference between the measured
values upstream of the tundish and downstream of the tundish.
[0058] It should be noted that the increased mass of the oxygen in
the molten steel during contained in the tundish may vary when the
ladle is replaced (that is, for each of charges). Further, even in
the same charge, the increased mass of the oxygen in the molten
steel during contained in the tundish may vary according to the
change of the operating conditions. Therefore, it is preferable to
measure, using the zirconia oxygen sensor and the like, the amount
of oxygen in the molten steel during contained in the tundish for
each of the charges, or every time the operating condition changes
in order to grasp the increased mass of the oxygen in the molten
steel during contained in the tundish.
[0059] In order for the modification index to fall within the
above-described appropriate range (i.e., 0.1 or more and 0.7 or
less) by adding one or more of La and Ce in the tundish so as to
partly reduce TiO.sub.2 generated by the reoxidation for modifying
them to compound oxides such as TiO.sub.2--La.sub.2O.sub.3,
TiO.sub.2--Ce.sub.2O.sub.3, and
TiO.sub.2--La.sub.2O.sub.3--Ce.sub.2O.sub.3, to the molten steel,
it is necessary to add to the molten steel, one or more of La and
Ce in an amount with a mass equal to 0.2 to 1.2 times the increased
mass of the oxygen in the molten steel during contained in the
tundish, based on the calculation using the molecular weight ratio
with respect to before and after of the modification.
[0060] One or more of La and Ce is preferably added in an amount
with a mass equal to 0.3 to 1.1 times the mass of the increased
oxygen, and more preferably, 0.4 to 0.9 times the mass of the
increased oxygen, in order for the modification index to fall
within the above-described range.
[0061] One or more of La and Ce may be added by using a pure metal
of one or more of La and Ce, but for example, alloyed metal
including one or more of La and Ce such as mish metal may be used
as well. If the total concentration of the La and Ce in the alloyed
metal is more than 30 mass % or more, the effects of the present
invention will not be lost even if other impurities are mixed in
the molten steel at the time of adding one or more of La and
Ce.
[0062] However, it should be noted that it is important to adjust
the amount of alloyed metal added according to the concentration of
La and/or Ce, so that the amount of La and/or Ce added falls within
an appropriate range. Further, as a method of adding them, the
metal may be directly added to the molten steel, but taking the
loss due to slag into account, it is preferable to continuously
supply the metal in a wire form coated with an iron tube.
[0063] Further, the present invention may also be employed for an
ingot casting operation and a continuous casting operation. As for
the continuous casting operation, the present invention may be
employed not only for a continuous casting operation for producing
normal slabs in the thickness of about 250 mm, but also for a
continuous casting operation which uses a casting machine having
thinner casting molds for producing thin slabs of a thickness of
150 mm or less, and sufficient effects may be derived. Then, nozzle
clogging can be reliably prevented. The steel slabs obtained by the
above-described method may be used for producing steel sheets using
a hot rolling process and/or a cold rolling process.
EXAMPLES
[0064] Hereinbelow, examples regarding the present invention and
comparative examples will be described with reference to a
flowchart in FIG. 1.
Example 1
[0065] 300 tons of molten steel containing 0.0013 mass % of C,
0.004 mass % of Si, 0.25 mass % of Mn, 0.009 mass % of P, and 0.006
mass % of S was produced through refining in a converter furnace
and process in an RH degasser, and was prepared in a ladle (S1 in
FIG. 1). After adding Ti to the molten steel, La and Ce were added
thereto (S3 in FIG. 1). Then, molten steel containing 0.053 mass %
of Ti, 0.0007 mass % of La, 0.0005 mass % of Ce, and 0.0046 mass %
of oxygen was obtained.
[0066] The molten steel in the ladle was taken as a sample for
studying inclusions. Then, it was found that there existed
inclusions in spherical shape or spindle shape having a diameter of
0.5 .mu.m-30 .mu.m. Further, all of the inclusions were oxides
consisting of TiO.sub.2, La.sub.2O.sub.3, and Ce.sub.2O.sub.3, and
the modification indexes of these inclusions fall within a range of
0.16 or more and 0.58 or less.
[0067] From the ladle, the molten steel in the amount of 4.4 tons
per a minute was poured into casting molds via a tundish, using
immersion nozzles. At the time of pouring, the oxygen concentration
of molten steel at a downstream of the tundish (in the vicinity of
tundish outlet) was measured with a zirconia oxygen sensor, and it
was found that the oxygen concentration was 0.0088 mass %, that is,
the increased oxygen concentration in the tundish was 0.0042 mass
%.
[0068] Then, alloyed metal containing 50 mass % of La and 50 mass %
of Ce in a wire form coated with a steel pipe was added into the
tundish in the amount of 40 g/minute, 80 g/minutes, or 200
g/minutes, so that the adding amount of La+Ce to the molten steel
becomes 0.22 times, 0.43 times, or 1.08 times the increased mass of
the oxygen contained in the molten steel in the tundish (that is, a
value obtained by multiplying 4.4 tons/minute which is the amount
of molten steel poured into the tundish in a unit time, by 0.0042
mass % which is the concentration of increased oxygen in the
tundish in a unit amount of the molten steel) (S4 in FIG. 1).
[0069] Employing a continuous casting method, this molten steel was
cast at a casting speed of 1.4 m/min for producing slabs having a
thickness of 250 mm and a width of 1800 mm. At the time of casting,
clogging was not occurred in the immersion nozzle.
[0070] The casted slabs were cut to 8500 mm in length, as a coil
unit. Analysis was made on inclusions in an area up to 20 mm in
depth from a surface of the slab. As a result, it was found that in
any of slabs to which alloyed metal in the amount of 40 g, 80 g, or
200 g per minute was added, there existed oxide inclusions
consisting of TiO.sub.2, La.sub.2O.sub.3, and Ce.sub.2O.sub.3 in
spherical shape or spindle shape each having a diameter of 0.5
.mu.m-.mu.m. The modification indexes of these inclusions fell
within a range of 0.15 or more and 0.55 or less.
[0071] The slabs thus obtained were hot rolled and subsequently
cold rolled, in a usual manner. Then, coils of cold-rolled steel
sheets each having a thickness of 0.7 mm and a width of 1800 mm
were obtained. Qualities of the steel sheet surfaces were visually
observed in an inspection line after the cold rolling, for
evaluating the number of occurrences of surface defects per coil.
As a result, it was found that no surface defect was generated.
Example 2
[0072] 300 tons of molten steels respectively containing 0.0013
mass % of C, 0.004 mass % of Si, 0.25 mass % of Mn, 0.009 mass % of
P, 0.006 mass % of S were produced through refining in a converter
furnace and process in an RH degasser, and were respectively
prepared in a first ladle and a second ladle (S1 in FIG. 1). Then,
to each of the ladles containing the molten steel, 100 kg of Al for
pre-deoxidation was added and refluxed for three minutes, thereby
obtaining molten steel containing 0.002 mass % of Al and 0.012 mass
% of oxygen (S2 in FIG. 1).
[0073] Further, to each of the molten steels, 200 kg of Ti was
added and refluxed for one minute, and subsequently, 40 kg of Ce
was added to the first ladle, and 40 kg of La was added to the
second ladle (S3 in FIG. 1). Then, molten steels containing 0.033
mass % of Ti and 0.01 mass % of oxygen, which further contain La or
Ce in the concentration of 0.005 mass % were obtained.
[0074] Each of the molten steels in the ladles was taken as a
sample for studying inclusions. Then, it was found that there
existed inclusions in spherical shape or spindle shape having a
diameter of 0.5 .mu.m-30 .mu.m. Further, all of the inclusions were
oxides including 10 mass % or less of Al.sub.2O.sub.3 and the
balance consisted of TiO.sub.2 and La.sub.2O.sub.3 or
Ce.sub.2O.sub.3. The modification indexes of these inclusions fell
within a range of 0.22 or more and 0.48 or less.
[0075] From the ladle, the molten steel in the amount of 4.4 tons
per a minute was poured into casting molds via a tundish, using
immersion nozzles. At the time of pouring, the oxygen concentration
of molten steel at a downstream of the tundish (in the vicinity of
the tundish outlet) was measured with a zirconia oxygen sensor, and
it was found that the oxygen concentration was 0.02 mass %, that
is, the increased oxygen concentration in the tundish was 0.01 mass
%.
[0076] Then, alloyed metal containing La was added into the tundish
in the amount of 110 g/minute or 485 g/minutes, so that the adding
amount of La to the molten steel in the first ladle becomes 0.25
times or 1.1 times the increased mass of oxygen in the molten steel
during contained in the tundish (that is, a value obtained by
multiplying 4.4 tons/minute which is the amount of molten steel
poured into the tundish in a unit time, by 0.01 mass % which is the
concentration of oxygen increased in the tundish in a unit amount
of the molten steel) (S4 in FIG. 1).
[0077] Further, alloyed metal containing Ce was added into the
tundish in the amount of 220 g/minute, so that the adding amount of
Ce to the molten steel in the second ladle becomes 0.5 times the
amount of the increased mass of oxygen, in the same manner (S4 in
FIG. 1). Employing a continuous casting method, these molten steels
were cast at the casting speed of 1.4 m/min for producing slabs
having a thickness of 250 mm and a width of 1800 mm. At the time of
casting, clogging had not occurred in the immersion nozzle.
[0078] These slabs thus produced were hot rolled and subsequently
cold rolled, in a usual manner. Then, coils of cold-rolled steel
sheets having a thickness of 0.7 mm and a width of 1800 mm were
obtained. Qualities of the steel sheet surfaces were visually
observed in an inspection line after the cold rolling, for
evaluating the number of occurrences of surface defects per coil.
As a result, it was found that no surface defects were
generated.
[0079] Further, analysis was made on inclusions in the cold rolled
steel sheet. As a result, it was found that in any case of adding
La or Ce, there existed oxide inclusions in a spherical shape or a
spindle shape including 10 mass % or less of Al.sub.2O.sub.3 and
the balance consisting of TiO.sub.2, and La.sub.2O.sub.3, or
TiO.sub.2 and Ce.sub.2O.sub.3 in spherical shapes or in spindle
shapes having diameter of 0.5 .mu.m-30 .mu.m. The modification
indexes of these inclusions fell within a range of 0.2 or more and
0.45 or less.
Comparative Example 1
[0080] 300 tons of molten steel containing 0.0013 mass % of C,
0.004 mass % of Si, 0.25 mass % of Mn, 0.009 mass % of P, and 0.006
mass % of S was produced through refinement in a converter furnace
and process in an RH degasser, and was prepared in a ladle. After
adding Ti to the molten steel, La and Ce were added thereto. Then,
molten steel containing 0.037 mass % of Ti, 0.001 mass % of La,
0.0008 mass % of Ce, and 0.008 mass % of oxygen was obtained.
[0081] The molten steel in the ladle was taken as a sample for
studying inclusions. Then, it was found that there existed
inclusions in spherical shape or spindle shape each having a
diameter of 0.5 .mu.m-0.30 .mu.m. Further, all of the inclusions
were oxides consisted of TiO.sub.2, La.sub.2O.sub.3, and
Ce.sub.2O.sub.3, and the modification indexes of these inclusions
fell within a range of 0.12 or more and 0.33 or less.
[0082] From the ladle, the molten steel in the amount of 4.4 tons
per a minute was poured into casting molds via a tundish, using
immersion nozzles. At the time of pouring, the oxygen concentration
of molten steel at a downstream of the tundish (in the vicinity of
tundish outlet) was measured with a zirconia oxygen sensor, and it
was found that the oxygen concentration was 0.0165 mass %, that is,
the increased oxygen concentration in the tundish was 0.0085 mass
%.
[0083] Employing a continuous casting method, this molten steel was
cast at a casting speed of 1.4 m/min for producing slabs having a
thickness of 250 mm and a width of 1800 mm. At the time of casting,
clogging occurred in the immersion nozzle, and thus, casting was
forced to be terminated and 100 tons of the molten steel was
remaining in the ladle.
[0084] The casted slabs were cut to 8500 mm in length, as a coil
unit. Analysis was made on inclusions in an area up to 20 mm in
depth from a surface of the slab. As a result, it was found that
there existed oxide inclusions consisting of TiO.sub.2,
La.sub.2O.sub.3, and Ce.sub.2O.sub.3 in a spherical shape or in a
spindle shape having a diameter of 0.5 .mu.m-30 .mu.m, in a state
of aggregated cluster having more than 150 .mu.m were aggregated.
The modification indexes of these inclusions fell within a range of
0.05 or more and 0.1 or less.
[0085] The slabs thus obtained were hot rolled and subsequently
cold rolled, in a usual manner. Then, coils of cold-rolled steel
sheets having a thickness of 0.7 mm and a width of 1800 mm were
obtained. Qualities of the steel sheet surfaces were visually
observed in an inspection line after the cold rolling, for
evaluating the number of occurrences of surface defects per coil.
As a result, it was found that 5 surface defects per coil were
generated.
Comparative Example 2
[0086] 300 tons of molten steels respectively containing 0.0013
mass % of C, 0.004 mass % of Si, 0.25 mass % of Mn, 0.009 mass % of
P, and 0.006 mass % of S were produced through refinement in a
converter furnace and process in an RH degasser, and were
respectively prepared in a first ladle and in a second ladle. Then,
to each of the ladles containing the molten steel, 100 kg of Al for
pre-deoxidation was added and refluxed for three minutes, thereby
obtaining molten steel containing 0.002 mass % of Al and 0.013 mass
% of oxygen.
[0087] Further, to each of the molten steels, 200 kg of Ti was
added and refluxed for one minute, and subsequently, 40 kg of Ce
was added to the first ladle, and 40 kg of La was added to the
second ladle. Then, molten steels containing 0.033 mass % of Ti and
0.01 mass % of oxygen, which further contain La or Ce in the
concentration of 0.005 mass % were obtained.
[0088] Each of the molten steels in the ladles was taken as a
sample for studying inclusions. Then, it was found that there
existed inclusions in spherical shapes or spindle shapes having a
diameter of 0.5 .mu.m-30 .mu.m. Further, all of the inclusions were
oxides including 10 mass % or less of Al.sub.2O.sub.3 and the
balance consisting of TiO.sub.2+La.sub.2O.sub.3, or
TiO.sub.2+Ce.sub.2O.sub.3. The modification indexes of these
inclusions fell within a range of 0.22 or more and 0.48 or
less.
[0089] From the ladle, the molten steel in the amount of 4.4 tons
per a minute was poured into casting molds via a tundish, using
immersion nozzles. At the time of pouring, the oxygen concentration
of molten steel at a downstream of the tundish (in the vicinity of
tundish outlet) was measured with a zirconia oxygen sensor, and it
was found that the oxygen concentration was 0.02 mass %, that is,
the increased oxygen concentration in the tundish was 0.01 mass
%.
[0090] Then, alloyed metal containing La was added into the tundish
in the amount of 65 g/minute so that the amount of La added to the
molten steel in the first ladle becomes 0.15 times the increased
mass of oxygen in the molten steel during contained in the tundish
(that is, a value obtained by multiplying 4.4 tons/minute which is
the amount of molten steel poured into the tundish in a unit time,
by 0.01 mass % which is the concentration of oxygen increased in
the tundish in a unit amount of the molten steel). Further, alloyed
metal containing Ce was added into the tundish in the amount of 600
g/minute, so that the adding amount of Ce to the molten steel in
the second ladle becomes 1.36 times the increased mass of oxygen,
in the same manner.
[0091] Employing a continuous casting method, these molten steels
were cast at the casting speed of 1.4 m/min for producing slabs
having a thickness of 250 mm and a width of 1800 mm. At the time of
casting, clogging was occurring in the immersion nozzle, and thus,
casting was forced to be terminated and 50 tons of the molten steel
were remaining in the ladle.
[0092] The slabs thus obtained were hot rolled and then cold rolled
in a usual manner. Then, coils of cold-rolled steel sheets having a
thickness of 0.7 mm and a width of 1800 mm were obtained. Qualities
of the steel sheet surfaces were visually observed in an inspection
line after the cold rolling, for evaluating the number of
occurrences of surface defects per a coil. As a result, it was
found that, as an average of slabs, 5 defects were generated in the
La added coil and 10 defects were generated in the Ce added
coil.
[0093] Further, analysis was made on inclusions in the cold rolled
steel sheet. As a result, it was found that in the La added coil,
there existed oxide inclusions including 10 mass % or less of
Al.sub.2O.sub.3 and the balance consisting of TiO.sub.2 and
La.sub.2O.sub.3 in spherical shapes or spindle shapes having a
diameter of 0.5 .mu.m-30 .mu.m, in a state of aggregated clusters
having a size of 150 .mu.m. These modification indexes of these
inclusions fell within a range of 0.05 or more and 0.1 or less.
[0094] It was also found that in the Ce added coil, there existed
expanded oxide inclusions including 10 mass % or less of
Al.sub.2O.sub.3 and the balance consisting of TiO.sub.2 and
Ce.sub.2O.sub.3, having a diameter of 1000 .mu.m or longer. The
modification indexes of these inclusions fell within a range of
0.75 or more and 1.0 or less.
INDUSTRIAL APPLICABILITY
[0095] From the foregoing, according to the present invention, it
is possible to control the composition of the inclusions in the
molten steel which was reoxidized in the tundish within an
appropriate range. Therefore, nozzle clogging or product surface
defects can be reliably prevented and it is possible to reliably
produce low-carbon thin steel sheets in a long running casting
operation. Therefore, the present invention has excellent
industrial applicability in a steel manufacturing industry.
* * * * *