U.S. patent number 3,805,571 [Application Number 05/370,756] was granted by the patent office on 1974-04-23 for apparatus for continuous treatment of low-carbon cold-rolled steel sheet having excellent cold working properties.
This patent grant is currently assigned to Nippon Steel Corporation. Invention is credited to Mitsunobu Abe, Hisashi Gondo, Ryoseki Katsutani, Tsuyoshi Kawano, Bunichiro Kawasaki, Masao Morimoto, Teruhiko Nishimura, Yoshio Saito, Masahiko Shiraishi, Kenichiro Suemune, Yoshifumi Tadashige, Kenzo Toda, Takao Tsukamura, Norimasa Uehara, Kurayoshi Watanabe.
United States Patent |
3,805,571 |
Toda , et al. |
April 23, 1974 |
APPARATUS FOR CONTINUOUS TREATMENT OF LOW-CARBON COLD-ROLLED STEEL
SHEET HAVING EXCELLENT COLD WORKING PROPERTIES
Abstract
An apparatus for continuous treatment of cold rolled steel
sheet. The apparatus has a continuous annealing furnace through
which the sheet is continuously passed and which is composed of, in
succession, a heating chamber for heating the cold rolled steel
sheet to a temperature range of 700.degree. to 900.degree.C within
2 minutes, a soaking chamber for keeping the sheet in the above
temperature range for two minutes or less, a primary cooling
chamber for rapidly cooling the sheet from the above temperature
range to an overageing at a rate of 5.degree. to 30.degree.C/sec.,
an overageing chamber for keeping the sheet between 300.degree. to
400.degree.C for up to eight minutes, and a secondary cooling
chamber for cooling the sheet from the above overageing temperature
to below 50.degree.C within 2 minutes. Adjacent the output end of
said furnace is apparatus for successively skin pass rolling,
levelling and recoiling the sheet from the annealing furnace. The
skin pass rolling apparatus has apparatus for replacing the working
rolls.
Inventors: |
Toda; Kenzo (Kisarazushi,
Chibaken, JA), Gondo; Hisashi (Kisarazushi, Chibaken,
JA), Kawasaki; Bunichiro (Kisarazushi, Chibaken,
JA), Abe; Mitsunobu (Kisarazushi, Chibaken,
JA), Katsutani; Ryoseki (Kisarazushi, Chibaken,
JA), Kawano; Tsuyoshi (Kimitsugun, Chibahen,
JA), Uehara; Norimasa (Kimitsugun, Chibahen,
JA), Saito; Yoshio (Kimitsugun, Chibaken,
JA), Suemune; Kenichiro (Yahataku, Kitakyushi,
Fukuokaken, JA), Shiraishi; Masahiko (Yahataku,
Kitakyushi, Fukuokaken, JA), Tadashige; Yoshifumi
(Yahataku, Kitakyushi, Fukuokaken, JA), Morimoto;
Masao (Kimitsgun, Chibaken, JA), Tsukamura; Takao
(Nakanoku, Tokyo, JA), Watanabe; Kurayoshi (Nakanoku,
Tokyo, JA), Nishimura; Teruhiko (Kawasakishi,
Kanagawaken, JA) |
Assignee: |
Nippon Steel Corporation
(Tokyo, JA)
|
Family
ID: |
27548614 |
Appl.
No.: |
05/370,756 |
Filed: |
June 18, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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102671 |
Dec 30, 1970 |
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Foreign Application Priority Data
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Dec 30, 1969 [JA] |
|
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44-105435 |
Feb 26, 1970 [JA] |
|
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45-15891 |
Oct 9, 1970 [JA] |
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45-88988 |
Dec 1, 1970 [JA] |
|
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45-105458 |
Dec 1, 1970 [JA] |
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45-105457 |
Dec 1, 1970 [JA] |
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45-106063 |
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Current U.S.
Class: |
72/202; 266/111;
266/108 |
Current CPC
Class: |
C21D
8/0452 (20130101); B21B 31/10 (20130101); C21D
9/56 (20130101); C21D 8/0426 (20130101); C21D
8/0473 (20130101); C21D 9/52 (20130101); C21D
8/0436 (20130101); B21B 39/08 (20130101); B21B
2001/228 (20130101); C21D 9/563 (20130101); B21B
2015/0071 (20130101); C21D 9/573 (20130101); C21D
8/0442 (20130101) |
Current International
Class: |
C21D
9/56 (20060101); C21D 8/04 (20060101); C21D
9/52 (20060101); B21B 31/00 (20060101); B21B
31/10 (20060101); C21D 9/573 (20060101); B21B
39/02 (20060101); B21B 39/08 (20060101); B21B
1/22 (20060101); B21B 15/00 (20060101); B21b
045/02 () |
Field of
Search: |
;72/200,201,202,238,239,342,364 ;148/12 ;266/2.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Dewsnap, R. F. An Investigation of the Radio Annealing of Sheet
Gauge Mild Steel, Special Report No. 79, The Iron and Steel
Institute, London, 1963, pp. 112-120..
|
Primary Examiner: Lanham; Charles W.
Assistant Examiner: Combs; E. M.
Attorney, Agent or Firm: Wenderoth, Lind & Ponack
Parent Case Text
This application is a division of application Ser. No. 102,671,
filed Dec. 30, 1970.
Claims
What is claimed is:
1. An apparatus for continuous treatment of cold rolled steel sheet
comprising a continuous annealing furnace through which the sheet
is continuously passed and which is composed of, in succession, a
heating chamber for heating the cold rolled steel sheet to a
temperature range of 700.degree.C to 900.degree.C within 2 minutes,
a soaking chamber for keeping the sheet in the above temperature
range for two minutes or less, a primary cooling chamber for
rapidly cooling the sheet from the above temperature range to an
overageing temperature at a rate of 5.degree. to 30.degree.C/sec.,
an overageing chamber for keeping the sheet between 300.degree. to
400.degree.C for up to 8 minutes, and a secondary cooling chamber
for cooling the sheet from the above overageing temperature to
below 50.degree.C within 2 minutes, and means adjacent the output
end of said furnace for successively skin pass rolling, levelling
and recoiling the sheet from the annealing furnace.
2. An apparatus according to claim 1 in which the primary cooling
chamber has a jet cooling system for rapid cooling of the sheet
from a temperature range of 700.degree. to 900.degree.C to a
temperature range of 300.degree. to 450.degree.C at a rate of less
than 20.degree.C/sec.
3. An apparatus according to claim 1 in which the secondary cooling
chamber has gas jet means therein for jetting water cooled gas for
cooling the sheet from the overageing temperature to 100.degree.C
and has further gas jet means for further cooling the sheet below
100.degree.C, and gas supply means coupled to said further gas jet
means and including gas refrigerating means for regrigerating the
gas supplied to said further gas jet means.
4. An apparatus according to claim 1 in which the overageing
chamber has a hearth roll therein over which the steel sheet runs
which has a roll diameter which satisfies the following
formula:
d/R .ltoreq. 2.02 .times. 10.sup.-.sup.3 - 1.05 .times.
10.sup.-.sup.3 log (t + 1)
in which d is the thickness in millimeters of the steel sheet, R is
the bending radius in millimeters and t is the time in minutes
during which stress is given to the sheet, and the secondary
cooling chamber has in series gas jet means for blowing gas cooled
by cooling water and gas jet means for blowing gas cooled by
refrigeration.
5. An apparatus according to claim 1 in which the skin pass rolling
means is a skin pass rolling mill having a pair of work rolls and a
pair of back-up rolls backing up said work rolls, and driving means
coupled to one of said pair of rolls, and bridle rolls between said
skin pass rolling mill and said levelling means for continuous
feeding of the steel sheet from the skin pass rolling mill to said
levelling means.
6. An apparatus according to claim 1 in which said skin pass
rolling means comprises a skin pass rolling mill having a housing
and working rolls housed in said housing, a support frame for
supporting a replacement working roll, a truck arranged on one side
of the roll mill on which said support frame is positioned, a means
on said support frame for placing the replacement working roll in
coaxial alignment with a used roll in the rolling mill, a means on
said truck for pushing the replacement roll toward the housing to
thereby push the used roll out of the housing, another truck
arranged on the opposite side of the rolling mill from said
firstmentioned truck, and a further support frame on said further
truck for receiving and holding the used roll when it is pushed
thereonto from the housing.
Description
BACKGROUND OF INVENTION
1. Field to which the Invention belongs
The present invention relates to an apparatus for producing
low-carbon cold steel sheet having cold workability, particularly
press forming quality with use of a continuous annealing
system.
The term "a low-carbon steel sheet" used hereinafter designates a
steel sheet which is produced by the process of cold rolling and
annealing and is for such applications as pressed automobile body
parts, rather than an insufficiently annealed steel sheet which is
for zinc-plating or tin-plating not subjected to drawing.
2. Prior art
Thin steel sheet for cold working, particularly press forming for
use in automobile parts must have among its properties good
ductility as well as good drawability and stretchability. To obtain
these properties, it is required that the grain size be large
enough to prevent the condition known as surface orange peel due to
press forming, that the content of dissolved carbon and nitrogen be
sufficiently low, that the yield point be low and the elongation be
large. To meet these requirements, most of the thin cold rolled
steel sheets for such applications have been conventionally
produced by box annealing.
However, box annealing requires a longer treatment time (normally
more than 60 hours) and thus is undesirable from the standpoint of
efficiency. A method for reducing the treatment time recently has
been proposed as disclosed in Japanese Patent Publication Sho
43-5995. This method still requires more than 30 hours for the
treatment and thus is completely non-competitive with continuous
annealing.
Several patents have been published regarding continuous annealing
of cold rolled steel sheet for tin-plating. Among these patents,
U.S. Pat. No. 2,832,711 is noteworthy. Although this patent
discloses relatively soft thin steel sheet treated by a continuous
annealing, it discloses or suggests nothing with respect to
low-carbon steel sheet having excellent workability which is for
pressed autobody parts. Commercial production of such steel sheet
has never been carried out.
Reasons why low-carbon cold rolled steel sheet for applications
such as pressed automobile body parts has never been produced by
means of a continuous annealing are as follows.
It has been generally believed that low yield point, large
elongation and Lank-Ford value (r-value) which can be obtained by
box annealing can not be obtained unless the speed of the sheet
pass is greatly reduced to provide an overaging time for effecting
full precipitation of dissolved carbon, or unless the sheet, after
continuous annealing for recrystallization is coiled and given
enough aging time while the sheet is in a coiled state. For the
former case, a treatment furnace for the overaging must be
extremely long and thus is not practical for commercial production.
In the latter case, the advantages of a continuous treatment as a
whole can not be obtained although the annealing is effected
continuously.
Further, from the standpoint of equipment, in order to effect
recrystallization annealing - overaging - temper rolling - leveling
- and recoiling in a continuous manner, it is necessary to rapidly
cool the sheet after the overaging to near room temperatures for
the subsequent temper rolling, and it is also necessary to perform
very quickly the replacement of rolls of the temper rolling mill
without interruption of the continuous treatment of the sheet.
No appropriate means has been provided for meeting the above
requirements.
GIST OF THE INVENTION
Therefore, one of the objects of the present invention is to
provide an apparatus for producing a low-carbon steel sheet having
excellent cold workability equal to or better than those obtainable
by box annealing by utilization of continuous annealing which has
conventionally been believed to be impossible to use in commercial
production.
Another object of the present invention is to provide a temper
rolling apparatus for continuously and efficiently tempering
overaged steel sheet.
Other objects of the present invention will be understood from the
following descriptions and the attached figures.
The present invention is applicable to ordinary low-carbon steel,
but the most desirable results are obtained by steel compositions
as specified hereinafter. Therefore, the following descriptions
will be in connection with the specified steel compositions.
Features of the present invention lie in that a low-carbon steel
containing less than 0.25 percent preferably less than 0.20
percent, by weight of manganese, and having the following relation
among manganese, sulphur and oxygen:
0 .ltoreq. [Mn%] - (Atomic weight of Mn/Atomic weight of O) .times.
[O%]
- (Atomic weight of Mn/Atomic weight of S) .times. [S%] .ltoreq.
0.15
is used as a starting material, and subjected to an ordinary hot
rolling, coiled at high temperatures above 600.degree.C, then
coiled in an ordinary way, and the thus obtained steel sheet is
subjected to recrystallization and overaging treatment in a
continuous process.
Further features of the present invention lie in improvements in
continuous annealing of a steel strip which has been cold rolled in
an ordinary way, which improvements comprise passing the steel
strip through a heating chamber which can heat the strip to a
temperature between 700.degree. to 900.degree.C within 2 minutes, a
soaking furnace which can keep the strip within the temperature
range for 2 minutes, a first cooling chamber which can cool the
strip from the above temperature range to an overaging temperature
between 300.degree. and 450.degree.C at a rate of 5.degree. to
30.degree.C/sec. (preferably less than 20.degree.C/second), an
overaging chamber which can maintain the strip at a temperature
between 300.degree. to 450.degree.C, preferably 300.degree. to
400.degree.C for up to eight, but preferably up to five minutes and
a continuous annealing furnace having a secondary rapid cooling
chamber which can cool the strip from the above overaging
temperature to a temperature below 50.degree.C preferably to room
temperatures within 2 minutes, and then subjecting the strip to
temper rolling and leveling and lastly recoiling the strip. Between
the continuous annealing furnace and the temper rolling apparatus,
an apparatus for a holding of the strip, such as a loop-car, may be
provided, and in the overaging zone it is desirable that a hearth
roll be used having a sufficiently large diameter that bending
stress is not placed on the strip during its passage
therethrough.
DETAILED DESCRIPTION OF INVENTION
The present invention will be described in detail with reference to
the attached figures:
FIG. 1 is a graph showing the relation between the proportions of
the constituents in the steel sheet and the yield point.
FIG. 2 is a graph showing the relation between the proportions of
the constituents in the steel sheet and the elongation.
FIG. 3 is a graph showing the relation between the proportions of
the constituents in the steel sheet and the average Lank-Ford
value.
FIG. 4 is a schematic view of an embodiment of the continuous
annealing apparatus according to the present invention.
FIG. 5 is a view showing the third cooling section in detail.
FIG. 6 is a schematic flow-sheet showing the production line
subsequent to the continuous annealing furnace.
FIGS. 7 to 11 are illustrative views showing the manner of carrying
out rapid replacement of skin-pass mill rolls.
In the present invention low-carbon steel with a low manganese
content is used as starting material.
Generally, a thin steel sheet used for press forming contains about
0.3 percent manganese. Excessive manganese makes it difficult to
obtain steel sheet which can be used for press forming by
continuous annealing, and in the case of low-carbon rimmed steel
sheet, it is necessary to minimize the manganese content in order
to obtain good surface qualities of the steel sheet.
The present inventors have found that an appropriate amount of MnS
and MnO which is assured by a low manganese content is useful in
steel for a continuous annealing treatment. Namely in the present
invention, manganese has only to be present in an amount necessary
for satisfactorily fixing the sulphur and oxygen which are harmful
impurities, and thus manganese has only to be present in a
stoichiometric amount or a little more to combine with sulphur and
oxygen. That is, the manganese content should be less than 0.25
percent by weight, preferably less than 0.20 percent, and
manganese, sulphur and oxygen must be present in amounts which will
produce the relationship 0 .ltoreq. K .ltoreq. 0.15 in the
following equation.
K = [Mn%] - (Atomic weight of Mn/Atomic weight of O) .times. [O%] -
(Atomic weight of Mn/Atomic weight of S)
.times. [S%] = [Mn%] - (55/16) .times. [O%] - (55/32) .times.
[S%]
When the relationship 0 .ltoreq. K .ltoreq. 0.15 is produced in a
conventional steel sheet having about 0.3 percent manganese,
excessive MnS and MnO are formed and they are harmful.
One of the features of the present invention, therefore, is that
the relationship among manganese, sulphur and oxygen is in a
specific range, and it is also another feature of the present
invention that the hot rolled steel sheet is coiled at high
temperatures so as to convert harmful sulfur and oxygen contents
into harmless precipitates. The sulphur and oxygen contents cause
red-hot embrittlement during the hot rolling and also suppress
grain growth during recrystallization annealing. Sulphur
inclusions, mainly FeS, which occur during annealing produce many
crystal nuclei which prevent grain growth, and the sulfur
inclusions, mainly FeS, themselves hinder grain growth. Oxygen also
has a similar harmful effect on the grain growth. Therefore by
fixing such harmful sulphur and oxygen into harmless forms with
manganese, it is possible to reduce the distribution density of
sulphur and oxygen so as to provide conditions permitting
satisfactory rapid recrystallization.
According to the present invention, the harmful sulphur and oxygen
contents are converted into harmless MnS and MnO by the high
temperature coiling after hot rolling. As a result it is possible
to provide conditions under which the rapid recrystallization is
assured. This high temperature coiling also can permit grain growth
through the self-annealing action of the hot rolled coil so that
the strain energy after the cold rolling can be reduced and the
grains can grow in a short time during recrystallization annealing.
For the above purposes the coiling temperature should be more than
600.degree.C. However, if the coiling temperature is too high, the
local temperature differences in the hot rolled steel sheet will be
extremely large and adjustment thereof will be difficult. Thus the
upper limit of the coiling temperature should be about
800.degree.C, and a desirable range for the coiling temperature is
675.degree. - 800.degree.C.
Conversion of the impurities into harmless form and the grain
growth while coiled in the hot state by the high temperature
coiling as described above can assure a short time
recrystallization during the recrystallization annealing and
provide steel materials which are very suitable for a continuous
annealing treatment with a heat cycle of rapid heating and cooling.
Further, continuous annealing can eliminate sticking of the coiled
steel strip which occurs when low-manganese steel strip is box
annealed. Thus the continuous annealing can improve the production
efficiency as compared with box annealing.
As mentioned above, cold rolled steel sheet having excellent
properties can be produced by a process of continuous annealing
using a high temperature coiling of hot rolled steel sheet. One
illustrative embodiment of a continuous annealing and temper
rolling apparatus is shown in FIG. 4.
In FIG. 4, the continuous annealing furnace A is composed of the
following chambers connected in series:
a. a heating chamber 4 for gas heating with a radiant tube
system
b. a soaking chamber 5 with electrical resistance elements for
electrical heating
c. a primary cooling chamber 6 with a jet cooler system in which
water-cooled air is blown into the chamber to effect a forced
cooling
d. an overaging chamber means 7 with electrical heating, and
e. a secondary cooling chamber 9 comprising a secondary cooling
section 10 with a jet cooler system of the water-cooled gas type
and a third cooling section 11 with a jet cooler system in which
the chamber gas cooled by a refrigerator is blown against the sheet
steel to effect a forced cooling.
In the figures, 8 is a hearth roller for tensioning and
transporting the steel strip 2 up and down as it moves through the
chambers, and 12 represents a special hearth roller in the
overaging chamber 7.
The details of the third cooling section 11 are shown in FIG. 5. In
this embodiment, eight passes or ten passes of the strip through
the secondary cooling chamber 9 are effected in the secondary
cooling section 10, and the remaining two passes are effected in
the third cooling section 11. 23 designates gas jet chambers, 24 a
heat exchanger, 25 a refrigerator, 26 a piping for sending a
high-temperature gas into the heat exchanger 24, 27 a piping for
circulating a refrigerant, 28 a piping for passing forcedly cooled
chamber gas under pressure into the gas jet chambers, 29 a piping
for recovering waste gas used for cooling, 30 a fan for delivering
the refrigerated gas under pressure to respective gas jet chambers,
31 a piping for gas supply.
It has been found very effective for reducing the stress hardening
of steel strip to increase the radius of curvature of the steel
strip at the turning point, for example, by increasing the diameter
of the hearth roller in the overaging zone.
When the diameter of the hearth roller is small, the bending stress
on steel strip is not negligible at the turning point and thus an
increase in elongation due to overaging is suppressed by stress age
hardening under stress, and when such bending stress is large, the
steel strip hardens at the time of bending, rather than before the
overaging treatment.
Since it is desirable that no such stress be given to the steel
strip, the present inventor investigated the ratio of the sheet
thickness to the bending radius which is critical for avoiding the
remarkable hardening of the strip due to the stress hardening. As a
result, the following relation has been established:
d/R = 2.02 .times. 10.sup.-.sup.3 - 1.05 .times. 10.sup.-.sup.3 log
(t + 1)
in which d is the thickness in millimeters of the steel strip, R is
the bending radius in millimeters (the radius of a circular arc
around the turning point of the strip), t is the time in minutes
during which the stress occurs (the time it takes a point on the
strip to travel along the circular arc).
According to the above, if the time during which the stress occurs
in the strip is shortened, the critical value of d/R which will not
cause hardening of the steel strip due to the stress hardening
becoming higher.
For example, if the time during which the bending stress occurs in
the strip is 1 minute, it is necessary to use a hearth roller
having a diameter more than 1,200 times the strip thickness.
The turning point of the steel strip may be formed by several rolls
having smaller diameters instead of a single hearth roll, and
therefore the radius R at the turning point of the steel strip in
the present invention should be the diameter of the hearth roller
or its equivalent.
At the entrance of the continuous annealing furnace A is a strip
surface cleaning apparatus 3, and at its outlet is a loop car 14 to
which the steel strip is lead around a bending roll 13. 15 and 17
are respectively bridle rolls positioned before and after the skin
pass mill 16, and 18 and 20 are respectively bridle rolls
positioned before and after the leveler 19, and 21 is a bridle roll
positioned ahead of the coiling reel 22.
Details of the arrangement of apparatus subsequent to the skin pass
mill 16 are shown in FIG. 6.
In FIG. 6, 15' and 17' are motors for driving the bridle rolls 15
and 17, and by adjusting the rotation speed of the rolls a desired
tension is given to the steel strip at the temper mill 16. 39 are
the work rolls of the temper mill 16, which are driven by motors
39. Instead, back-up rolls 35 and 36 may be driven for the same
purpose. 18'and 20' are motors for driving the bridle rolls 18 and
20, which serve to adjust the tension of the steel strip, and 21'
are motors for driving the bridle rolls 21, which serve to adjust
the coiling tension.
Next, the operation of the present invention will be described.
In FIG. 4, the cold rolled steel strip 2 uncoiled from the coil 1
is surface cleaned by the surface cleaning apparatus 3 and led to
the continuous annealing furnace A. Then the steel strip 2 is first
introduced to the heating chamber 4 where the strip is heated at
temperatures between 700.degree. and 900.degree.C. The heating up
to the above temperature range is effected within 2 minutes. If the
heating rate, however, is too rapid, grains with unfavorable
orientations increase and thus the Lank-Ford value (r value) which
is an index of press formability, particularly drawability, is
lowered, and the number of recrystallization nuclei increase to
make the grain size fine so that the yield point rises.
Then the strip passes through the soaking chamber 5 in which the
strip is kept at the above temperature range of 700.degree. to
900.degree.C for up to 2 minutes. The above time for keeping the
strip at the temperature has a certain relation with the heating
temperature. With a higher temperature only a short time of soaking
or no soaking is required, while with a lower temperature, a
relatively long time of soaking is required. Namely, in the soaking
chamber 5, recrystallization and grain growth of the steel strip
must be effected, and the soaking time is appropriately adjusted
depending on the temperature.
The strip coming out of the soaking chamber 5 is immediately led to
the primary cooling chamber 6 where the strip is rapidly cooled to
the overaging temperature. The cooling may be done for example with
a jet cooling system. If the cooling rate is slow, the subsequent
overaging treatment does not attain its full effects. Namely it is
advantageous for attaining satisfactory precipitation of carbon
during the overaging to maintain the carbon in a supersaturated
solid solution before the overaging, and for this purpose, it is
preferable to cool the strip at a cooling rate of 5.degree. to
30.degree.C per second, more preferably less than 20.degree.C per
second.
The strip conditioned as above passes through the overaging chamber
7 for up to 8, but preferably up to 5, minutes where the strip is
heated to 300.degree. - 450.degree.C, and the carbon in the strip
fully precipitates and is fixed as carbide to achieve sufficient
press formability and make the strip sufficiently non-aging.
Above 450.degree.C the properties of the steel strip, such as
elongation and yield point, deteriorate although the overaging is
effected. While below 300.degree.C, a greater length of the chamber
is required and thus an increased cost of equipment results.
More than five minutes of overaging will need a greater length of
the chamber, thus inhibiting commercial production, but less than
five minutes of overaging is enough for desired results if the
temperature is within the above range.
The relation between the temperature and the time is similar to
that for soaking, i.e., with a higher temperature a shorter-time
overaging is desirable while with a lower temperature a longer-time
overaging is desirable.
The strip thus overaged is cooled below 50.degree.C for up to 2
minutes in the secondary cooling chamber 9. This cooling has an
important effect on the continuous system of production. Namely if
the strip can be cooled rapidly to near the room temperature, it is
possible to skin pass the strip immediately, thus greatly improving
the production efficiency.
According to the present invention, the strip is cooled to near
100.degree.C from the overaging temperature in the secondary
cooling section 10, and then in the third cooling section 11 the
strip is rapidly cooled to the room temperature using gas
recirculated to the chamber after being cooled by a refrigerator.
The cooling rate becomes slower at the lower temperature. However,
according to the present invention the strip can easily be cooled
to about 40.degree.C in a short time.
In FIG. 5, the gas passes through the pipe 26 to the heat exchanger
24 where the gas is rapidly cooled by heat-exchange with the
refrigerant coming from the refrigerator 25 through the circulation
pipe 27, and is delivered to respective gas jet chamber 23 by the
fan 30. The gas jet chambers 23 are arranged along the strip path
so that the cooled gas is directed uniformly onto the strip surface
to rapidly cool the strip down to the room temperature. The gas is
recovered through the gas recovery pipe 29 and recirculated.
The strip coming out of the cooling chamber 9 is stored under
tension in the loop-car 14 after passing over the bending roll 13
and then is led to the skin pass mill 16.
Subsequently, the strip is subjected to temper rolling. In the
conventional practice, a continuous system for skin pass rolling
the steel strip continuously from the overaging treatment has not
been used. This is due to the fact that batch type annealing and
overaging treatments are necessary to obtain sufficiently good
mechanical properties in the steel strip and also due to the fact
that changing the rolls of the skin pass mill takes a long time,
and this, in a continuous system, would necessitate stopping
operation of the system so that, it has been believed, irregular
quality and deterioration of the mechanical properties of the steel
strip would result. Another reason for the above is that no
appropriate method has been available for cooling after the
overaging in a continuous annealing treatment.
According to the present invention, firstly a specific material is
provided to overcome the first problem, and a steel strip "storing"
portion 14 is provided between the overaging zone and the skin pass
mill to overcome the second problem. Secondly, a rapid replacement
system of the cassette type, as described in detail later, is
provided to carry out rapid roll replacement; and thirdly the
stepwise cooling as described above is used to eliminate the third
problem.
Another feature of the present invention is that the skin pass mill
16 and the leveler 19 are arranged on the same line with each
other.
Still another feature of the present invention is that the bridle
rolls 17 and 18 are provided between the skin pass mill 16 and
leveler 19 to provide two tension systems: the skin-pass tension
section and the leveler tension system.
Referring to FIG. 6, in the skin-pass tension system, the rotation
speed of the bridle rolls 15 and 17, namely the load on the driving
motors 15' and 17' is adjusted so as to give 2kg/mm.sup.2 of back
tension, for example, and to give 3kg/mm.sup.2 of front tension,
for example, to the skin pass mill 16, to effect skin pass rolling
by rotating the work rolls. In the leveler tension system, the
relative speed of the bridle rolls 18 and 20 is adjusted so as to
give, for example, 10kg/mm.sup.2 of front and back tension to the
leveler.
In this way, according to the present invention, the skin pass
rolling and the tension leveling, which have been conventionally
used for improving the shape and qualities of the annealed steel
strip, are connected in the same line, and on the basis of the fact
that the tension in the skin pass rolling produces delicate effects
on the steel strip, the tension system in the skin pass mill and
that in the tension leveler are separated to permit tension
adjustment in each of the systems individually.
As mentioned above, the steel strip, after the continuous
annealing, is immediately subjected to the skin pass and leveler to
improve and correct the properties and shape, and is then coiled
continuously. For this, a rapid replacement of the work rolls in
the skin pass mill is required.
One illustrative example of the rapid replacement means is shown in
FIGS. 7 to 9.
In these figures, the skin pass mill 16 comprises stand housings 32
and 32' back-up rolls 35 and 36, back-up roll chocks 33 and 31,
work rolls 39 and 40 mounted on shafts 41 and 42, and work roll
chocks 37 and 38. 43 is a screw for adjusting the roll gap, 44 is a
pressure cylinder for giving a predetermined upward pressure to the
roll chock 31, 45 is a cylinder for balancing the rolls, pushing
the roll chock 33 upward. 46 is a cylinder for balancing the rolls,
pushing the roll chock 37 upward. At the lower portion of the roll
chock 38 are provided wheels 47 and rails 48 are correspondingly
provided on the roll chock 34. Cylinders 54 for lifting the rail
are provided in the roll chock 31. 53 is a connecting member for
connecting the upper and lower roll chocks 37 and 38 and the
connecting member is provided with a longitudinal oblong opening
for permitting the up-down movement of the chocks.
Now referring to the removing apparatus, 49 and 49' are roll
support frames for a replaced roll, on the bottom of which is
provided rails 50 corresponding to the wheels 47, and 51 and 51'
are trucks for mounting roll support frames 49 and 49', and the
truck 51 is provided with a pusher 52. The truck 51 is arranged on
the working side of the roll mill and the truck 51' is arranged on
the side from which the back-up rolls are driven.
During the operation of the skin pass mill, the roll gap is
adjusted by means of the screw 43 and then a predetermined pressure
is given to the cylinder 44 to push the roll chock 31 up together
with the roll chock 38. Further, the roll chocks 33 and 37 are
pushed up with a predetermined pressure by means of the cylinders
45 and 46 applied to the roll chock 33 to provide a roll gap so
that the work piece can be rolled to a desired thickness.
Now referring to FIG. 8, when the used work rolls 39' and 40' are
replaced, the roll support frame 49 carrying the new work rolls 39
and 40 is mounted on the truck 51 arranged on the roll working
side. Of course, the support frame must be in place so that the
rails 50 of the support frame and the rails 48 of the roll chock 31
are aligned.
Meanwhile, the pressure of the cylinder 46 for the roll balancing
is reduced to lower the upper work roll 39 to support the upper
work roll on the lower work roll so that the upper and lower work
rolls come together, and the rails 48 are lifted up by the
cylinders 54 to push the upper and lower work rolls 39 and 40
upwards by means of the wheels 47 to disengage the lower work roll
from the lower back-up roll to allow the work rolls 39 and 40 to
move along the rails 47.
On the roll driving side of the mill, the roll support frame 49' is
placed on the truck 51' just as on the mill operation side. The
rails 50' are positioned so as to be aligned with the rails 48.
When the above perparatory steps are completed, the lower roll is
pressed by the pusher 52 provided on the truck 51. As the shafts 41
and 42 of the upper and lower work rolls 39' and 40' in the housing
and the shafts 41 and 42 of the new upper and lower work rolls 39
and 40 are aligned, the end surfaces of the shafts contact each
other as the upper and lower work rolls 39 and 40 move, and the
upper and lower work rolls 39' and 40' are pushed out into the roll
support frame 49' which has been positioned on the roll driving
side.
When the new upper and lower work rolls 39 and 40 are in place in
the housing, the used work rolls 39'and 40' are contained in the
support frame 40' and thus ready for movement to the roll repair
yard.
The roll replacement apparatus of the present invention may be
modified. For example as shown in FIG. 10 and FIG. 11, rails 55,
56, 55' and a rail corresponding to rail 56 above and below on the
support frames 49 and 49', and rails 59 and 61 corresponding to the
above rails are provided on the projection 60 provided in the
window of the roll housing and on the projection 58 of the chock 57
for the upper back-up roll. The back-up roll chock 57 is pushed up
by the pressure of the balance cylinders 63 provided on the
projection 62 in the window of the roll housing to allow the wheel
65 of the upper work roll chock 64 to rest on the rail 59, and at
the same time the work roll 40 is brought down by reducing the
pressure of the pressure cylinder 44 to allow the sheel 67 of the
roll check 66 for the lower work roll 40 to rest on the rail 61 in
the housing to push out the roll chocks 64' and 66' mounted
respectively on the upper and lower rails on the roll support frame
49' and to replace the work rolls 39 and 40 in the housing.
The pusher 52 may be replaced for example by a cylinder provided in
the housing, the end of which cylinder engages with the work roll
chock in the roll support frame and pulls the chock into the
housing.
According to the present invention, good effects of shape
correction as well as satisfactory skin pass effects can be
attained in the same apparatus without the deterioration of the
steel strip so that soft steel strip for press forming and other
uses in which the mechanical properties and shape are regarded as
important can be obtained without difficulty. In the conventional
process, the skin pass and shape correction are simultaneously
effected by a skin pass rolling mill. On the other hand, in the
present invention, the skin pass rolling mill is assigned only the
task of skin pass rolling, so that the housing or rolls of the skin
pass rolling mill can be made smaller so that the production cost
can be greatly reduced.
A conventional skin pass rolling mill is limited to a large
capacity of 50,000 to 100,000 tons per month and thus is
uneconomical except for a large demand. On the other hand, a small
skin pass rolling mill is very economical because the capacity of
the rolling mill can be increased depending on the demand.
Further according to the present invention, a side trimmer, an
oiler and so on can be provided and thus steps subsequent to skin
pass rolling mill can be arranged in a single line, and thus
advantages such as improved mechanical properties, rationalization
of man power and reduction of semi-finished products can be
obtained.
Properties of steel sheets produced by the process and apparatus
according to the invention will be described with reference to the
attached figures.
FIG. 1 shows the relation between the K value (see first formula
above) and the yield point (YP). Materials were prepared from a
rimmed steel having a chemical composition as follows: C: 0.03 to
0.05%, Mn: 0.14 to 0.31%, S: 0.007 to 0.022%, O: 0.010 to 0.062%
with the balance being substantially Fe. Sheet of this composition
was coiled at a temperature between 700.degree. to 730.degree.C
after hot rolling and then cold rolled to sheet having a final
thickness of 0.8mm. The cold rolled steel strip was annealed in a
continuous annealing system in which the strip was held at
700.degree.C for 1.5 minutes and then to an overaging treatment at
350.degree.C for 5 minutes.
From FIG. 1, it is seen that when K is between 0 and 0.1 a very low
yield point is obtained, and when K is above 0.2 it is
saturated.
FIG. 2 shows elongation (E1) of the same material as in FIG. 1, and
it is seen that an excellent E1% is obtained when K is 0 to
0.15.
FIG. 3 shows r values for the same material as in FIG. 1, and it is
seen that when K is between 0.04 to 0.15, an r value more than 1.7,
which is required for a super deep drawing quality, is obtained,
and overall when K is less than 0 or more than 0.2, a satisfactory
r value is not obtained. Therefore, when K is between 0 to 0.15 the
properties usually required for good press formability are present.
Also from these figures, it is seen that an exceptional lowering of
the yield point and an exceptional increase in elongation and r
value are attained when 0 .ltoreq. K .ltoreq. 0.05.
The steel according to the present invention may be produced in a
convertor or other steel making furnaces, and subjected to ingot
making, cogging, hot rolling, then cold rolling, continuous
annealing (including overaging) and if necessary temper
rolling.
The conditions of the above processing steps may be selected within
a wide range as long as the specified steel composition range and
the specified coiling temperature range of the hot rolled steel
sheet are maintained.
The carbon content of the steel in the present invention may be
similar to ordinary low-carbon cold rolled steel, and if a lower
carbon content is required, this requirement can be easily
satisfied by vacuum degassing the molten steel or decarburizing the
steel during the annealing.
The present invention will be more clearly understood from the
following examples.
EXAMPLE 1
A hot rolled sheet coil was prepared by forming molten steel from a
convertor into an ingot, cogging and hot rolling in an ordinary way
and coiling at temperatures between 700.degree. to 730.degree.C and
the thus obtained hot rolled sheet coil was left to cool to room
temperatures.
This hot rolled sheet was further subjected to ordinary cold
rolling to a final thickness (0.7mm), then to a recrystallization
annealing in which the sheet was maintained at 700.degree.C for 1
minute and cooled and subsequently to an overaging treatment in
which the sheet was maintained at 350.degree.C for 2 minutes in a
continuous annealing process and lastly was subjected to skin pass
rolling with 1 to 1.5 percent reduction.
The steel composition and properties are shown in Table 1. For
comparison the composition and properties of steel sheet having a
large K value are also shown.
As clearly seen from Table 1, the steel of the present invention
has better mechanical properties than the standard steel produced
by a conventional method. ##SPC1##
EXAMPLE 2
The steel according to the invention having a composition as set
forth in Table 2 was produced in a convertor, and after ordinary
ingot making was treated under the conditions set forth in Table 3.
The results also are set forth in Table 3. The cold rolling in this
example was effected in an ordinary way.
It is clear from Table 3 the inventive steel has excellent
properties.
TABLE 2
Steel Compositions %
C Si Mn P S O K value 0.07 0.01 0.22 0.004 0.011 0.051 0.012
##SPC2##
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