U.S. patent application number 16/343217 was filed with the patent office on 2019-08-15 for hot-band annealing equipment, hot-band annealing method and descaling method for si-containing hot rolled steel sheet.
This patent application is currently assigned to JFE Steel Corporation. The applicant listed for this patent is JFE Steel Corporation. Invention is credited to Yoshimitsu Harada, Yukihiro Matsubara, Takahiro Takatsu, Yuta Tamura, Goki Yamada.
Application Number | 20190249270 16/343217 |
Document ID | / |
Family ID | 62018687 |
Filed Date | 2019-08-15 |
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United States Patent
Application |
20190249270 |
Kind Code |
A1 |
Yamada; Goki ; et
al. |
August 15, 2019 |
HOT-BAND ANNEALING EQUIPMENT, HOT-BAND ANNEALING METHOD AND
DESCALING METHOD FOR SI-CONTAINING HOT ROLLED STEEL SHEET
Abstract
Provided is a hot-band annealing equipment provided with a
heating zone, a soaking zone and a cooling zone for subjecting a
Si-containing hot rolled steel sheet to hot-band annealing, wherein
when the Si-containing hot rolled steel sheet is subjected to
hot-band annealing with the hot-band annealing equipment provided
with a rapid heating device at an upstream side of the heating zone
and/or in an inlet side of the heating zone, the hot rolled steel
sheet is heated by not lower than 50.degree. C. at a heating rate
of not less than 15.degree. C./s by using the rapid heating device
to improve a descaling property, whereby descaling can be performed
only by pickling without requiring mechanical descaling or heating
the steel sheet in the pickling process. Also provided are a
hot-band annealing method and a descaling method using the above
equipment.
Inventors: |
Yamada; Goki; (Chiyoda-ku,
Tokyo, JP) ; Harada; Yoshimitsu; (Chiyoda-ku, Tokyo,
JP) ; Takatsu; Takahiro; (Chiyoda-ku, Tokyo, JP)
; Matsubara; Yukihiro; (Chiyoda-ku, Tokyo, JP) ;
Tamura; Yuta; (Chiyoda-ku, Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE Steel Corporation |
Tokyo |
|
JP |
|
|
Assignee: |
JFE Steel Corporation
Tokyo
JP
|
Family ID: |
62018687 |
Appl. No.: |
16/343217 |
Filed: |
October 11, 2017 |
PCT Filed: |
October 11, 2017 |
PCT NO: |
PCT/JP2017/036766 |
371 Date: |
April 18, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 1/34 20130101; C22C
38/00 20130101; C21D 8/12 20130101; C21D 9/46 20130101; C21D 1/42
20130101; C23G 1/08 20130101; C21D 1/40 20130101; C21D 8/1277
20130101; C22C 38/02 20130101; C21D 1/82 20130101; C21D 9/60
20130101; C21D 9/62 20130101; C21D 8/1261 20130101; Y02P 10/25
20151101; Y02P 10/253 20151101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C22C 38/02 20060101 C22C038/02; C23G 1/08 20060101
C23G001/08; C21D 8/12 20060101 C21D008/12; C21D 1/34 20060101
C21D001/34 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 19, 2016 |
JP |
2016-205194 |
Claims
1-8. (canceled)
9. A hot-band annealing equipment provided with a heating zone, a
soaking zone and a cooling zone for subjecting a Si-containing hot
rolled steel sheet to hot-band annealing, characterized in that a
rapid heating device is disposed at an upper stream side of the
heating zone and/or an inlet side of the heating zone.
10. The hot-band annealing equipment according to claim 9, wherein
the rapid heating device is an induction heating device or an
electric heating device.
11. A hot-band annealing method for subjecting a Si-containing hot
rolled steel sheet to hot-band annealing with the hot-band
annealing equipment according to claim 9, characterized in that a
start temperature for heating the Si-containing hot-rolled steel
sheet in the rapid heating device is in a range of room temperature
to 700.degree. C. and the Si-containing hot rolled steel sheet is
heated by not lower than 50.degree. C. at a heating rate of not
less than 15.degree. C./s by using the rapid heating device.
12. A hot-band annealing method for subjecting a Si-containing hot
rolled steel sheet to hot-band annealing with the hot-band
annealing equipment according to claim 10, characterized in that a
start temperature for heating the Si-containing hot-rolled steel
sheet in the rapid heating device is in a range of room temperature
to 700.degree. C. and the Si-containing hot rolled steel sheet is
heated by not lower than 50.degree. C. at a heating rate of not
less than 15.degree. C./s by using the rapid heating device.
13. A descaling method, characterized in that the Si-containing hot
rolled steel sheet subjected to the hot-band annealing by the
method according to claim 11 is pickled without conducting
mechanical descaling.
14. A descaling method, characterized in that the Si-containing hot
rolled steel sheet subjected to the hot-band annealing by the
method according to claim 12 is pickled without conducting
mechanical descaling.
15. A descaling method, characterized in that the Si-containing hot
rolled steel sheet subjected to the hot-band annealing by the
method according to claim 11 is pickled after being subjected to
mechanical descaling.
16. A descaling method, characterized in that the Si-containing hot
rolled steel sheet subjected to the hot-band annealing by the
method according to claim 12 is pickled after being subjected to
mechanical descaling.
17. The descaling method according to claim 13, wherein the
Si-containing hot rolled steel sheet contains not less than 1.0
mass % of Si.
18. The descaling method according to claim 14, wherein the
Si-containing hot rolled steel sheet contains not less than 1.0
mass % of Si.
19. The descaling method according to claim 15, wherein the
Si-containing hot rolled steel sheet contains not less than 1.0
mass % of Si.
20. The descaling method according to claim 16, wherein the
Si-containing hot rolled steel sheet contains not less than 1.0
mass % of Si.
21. The descaling method according to claim 13, wherein the
Si-containing hot rolled steel sheet is a raw material for an
electrical steel sheet.
22. The descaling method according to claim 14, wherein the
Si-containing hot rolled steel sheet is a raw material for an
electrical steel sheet.
23. The descaling method according to claim 15, wherein the
Si-containing hot rolled steel sheet is a raw material for an
electrical steel sheet.
24. The descaling method according to claim 16, wherein the
Si-containing hot rolled steel sheet is a raw material for an
electrical steel sheet.
25. The descaling method according to claim 17, wherein the
Si-containing hot rolled steel sheet is a raw material for an
electrical steel sheet.
26. The descaling method according to claim 18, wherein the
Si-containing hot rolled steel sheet is a raw material for an
electrical steel sheet.
27. The descaling method according to claim 19, wherein the
Si-containing hot rolled steel sheet is a raw material for an
electrical steel sheet.
28. The descaling method according to claim 20, wherein the
Si-containing hot rolled steel sheet is a raw material for an
electrical steel sheet.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is the U.S. National Phase application of
PCT/JP2017/036766, filed Oct. 11, 2017, which claims priority to
Japanese Patent application No. 2016-205194, filed Oct. 19, 2016,
the disclosures of each of these applications being incorporated
herein by reference in their entireties for all purposes.
FIELD OF THE INVENTION
[0002] This invention relates to a hot-band annealing equipment, a
hot-band annealing method and a descaling method, and more
particularly to an equipment and a method for performing a hot-band
annealing of a Si-containing hot rolled steel sheet containing a
large amount of Si as a raw material for a grain-oriented
electrical steel sheet, a non-oriented electrical steel sheet, a
high-strength cold rolled steel sheet or the like, and a descaling
method of the Si-containing hot rolled steel sheet after the
hot-band annealing.
BACKGROUND OF THE INVENTION
[0003] Electrical steel sheets mainly used as an iron core material
of electric equipment are roughly divided into a non-oriented
electrical steel sheet and a grain-oriented electrical steel sheet.
Both of these steel sheets usually contain a large amount of Si or
Al increasing specific resistance of steel to decrease an iron
loss. The non-oriented electrical steel sheet is usually produced
by melting a steel adjusted to a predetermined chemical
composition, shaping the steel into a slab by a continuous casting
method, and subjecting the slab to hot rolling or the like,
hot-band annealing if necessary, pickling, cold rolling and finish
annealing for primary recrystallization. The grain-oriented
electrical steel sheet is usually produced by melting a steel
adjusted to a predetermined chemical composition, shaping the steel
into a slab by a continuous casting method or the like, and
subjecting the slab to hot rolling, hot-band annealing if
necessary, pickling, cold rolling, primary recrystallization
annealing that also works as decarburization annealing, application
of an annealing separator and finish annealing for secondary
recrystallization.
[0004] The hot-band annealing is conducted by subjecting the steel
sheet after the hot rolling (hot rolled sheet) to an annealing to
resolve insufficient recrystallization of the hot rolled sheet and
to coarsen or regulate crystal grains before the cold rolling so
that the texture of the steel sheet before the cold rolling can be
improved into favorable one for magnetic properties thereof or
ridging can be prevented.
[0005] When the steel sheet with residual oxidized scale formed on
the surface by the hot rolling or hot-band annealing is subjected
to cold rolling, the oxidized scale is pushed into the steel sheet
surface by rolling rolls to make the unevenness of the steel sheet
surface severe, or the stripped oxidized scale is adhered to the
surface of the roll by deposition and transferred to the steel
sheet surface to cause surface defects, whereby the surface quality
of a final product is substantially damaged. The pickling is a
process for removing the oxidized scale from the surface of the
steel sheet before the cold rolling (descaling process) and is an
inevitable process in the production of not only a hot rolled steel
sheet for an electrical steel sheet but also a steel sheet to be
cold rolled. As the pickling solution is usually used any one of
hydrochloric acid, sulfuric acid, hydrofluoric acid and a mixed
acid of these acids.
[0006] The oxidized scale of the hot rolled steel sheet as a raw
material for the electrical steel sheet or high-strength cold
rolled steel sheet containing a large amount of Si or Al is
composed of outer scales such as FeO, Fe.sub.3O.sub.4,
Fe.sub.2O.sub.3 and the like formed by the diffusion of Fe from the
inside of the steel sheet toward the outside, and subscales
comprised of Si-based oxides such as SiO.sub.2 and
Fe.sub.2SiO.sub.4 and Al.sub.2O.sub.3 formed by the diffusion of
oxygen from the outside of the steel sheet into the inside as shown
in FIG. 1. It is known that the oxidized scale is poor in the
descaling property, and particularly, the subscale containing
Al.sub.2O.sub.3 is extremely poor in the descaling property.
Therefore, the descaling of the Si-containing hot rolled steel
sheet is frequently performed by combining a light draft rolling
with a mechanical descaling such as roller levelling, tension
levelling, shot blasting or the like before the pickling as shown
in FIG. 2(a).
[0007] There are two methods for improving the descaling property
of the Si-containing hot rolled steel sheet, one of which is a
method of suppressing the generation of the oxidized scale itself,
while the other is a method of promoting the stripping of the
oxidized scale.
[0008] As the former method of suppressing the generation of the
oxidized scale, for example, Patent Literature 1 discloses a method
of producing a silicon steel substrate comprising a steelmaking
process, a hot rolling process and a normalizing process using a
normalizing furnace with a non-oxidation heating furnace part
provided with three or more furnace zones, wherein an energy supply
rate in the furnace zone used in the non-oxidation heating furnace
part is adjusted within a range of 15-95% and an excess factor
.alpha. (a rate of an actual combustion air amount to a theoretical
combustion air amount) in the non-oxidation heating furnace part is
adjusted within a range of 0.8.ltoreq..alpha.<1.0 to prevent the
formation of a high-density oxide in the normalizing process. In
this method, however, the temperature distribution of the heating
furnace is made non-uniform by the adjustment of the energy supply
rate or the excess factor .alpha., so that there is a problem that
it is difficult to perform an adequate heating in an actual
operation.
[0009] As the latter method of promoting stripping of the oxidized
scale is a method of adopting the aforementioned mechanical
descaling such as shot blasting, tension levelling or the like, but
there is a problem that the steel sheet containing a large amount
of Si is easily broken to cause serious operation troubles because
it is a hard material. Moreover, there is a method of increasing
the concentration or temperature of the pickling solution. However,
it has a problem that an over-pickling is caused when a threading
rate is decreased for some reason to badly affect the surface
quality or deteriorate the working environment in the pickling.
[0010] Patent Literature 2 proposes a method for improving the
descaling property by heating the steel sheet before pickling with
an induction heating device so as to have cracking of the oxidized
scale reach to the surface of the base iron and thereafter blowing
a pickling solution into the crack. Patent Literature 3 discloses a
method wherein a scale breaker is disposed before a pickling tank
and a plurality of deflector rolls, jetting nozzles and an
induction heating device are disposed in the pickling tank to cause
cracking in the scale by the scale breaker and thereafter the crack
is opened by bending using the deflector rolls and a pickling
solution is sprayed to the opening and the temperature of the steel
sheet is increased by induction heating to promote the chemical
reaction.
PATENT LITERATURES
[0011] Patent Literature 1: JP-A-2015-511995
[0012] Patent Literature 2: JP-A-S61-079790
[0013] Patent Literature 3: JP-A-H09-078273
SUMMARY OF THE INVENTION
[0014] In the method disclosed in Patent Literature 2, however,
when the steel sheet is heated before the pickling, there is a
problem that the temperature of the pickling solution is increased
to generate harmful acid fume or the steel sheet is excessively
pickled or the durability of the pickling tank is decreased. In the
method disclosed in Patent Literature 3, it is necessary to dispose
a heating equipment, roll, nozzle and so on in the pickling tank,
and there is a problem that the service life of the facility
equipment is shortened or the maintenance thereof is difficult.
[0015] The present invention is made in view of the aforementioned
problems inherent to the conventional techniques, and the object
thereof is to provide a hot-band annealing equipment capable of
improving a descaling property of a Si-containing hot rolled steel
sheet without requiring the mechanical descaling or the heating of
the steel sheet in the pickling process and to propose a hot-band
annealing method and a descaling method using the above
equipment.
[0016] The inventors have focused the heating method from a
viewpoint that it is important to increase the stripping property
of the oxidized scale in order to improve the descaling property
and made various studies to solve the above problems. As a result,
they have found out that since the oxidized scale layer on the
steel sheet surface has a thermal expansion coefficient different
from that of the base iron, thermal stress is caused resulting from
the difference of the thermal expansion amounts between the
oxidized scale and the base iron when heated to the same
temperature as the base iron, and that a temperature difference is
caused between the base iron and the oxidized scale due to the
difference of the thermal conductivity when an induction heating or
an electric heating, which causes heat generation of the steel
sheet itself, is adopted instead of radiation heating, which heats
the steel sheet from the outside thereof, as the method of heating
the steel sheet, and that when the steel sheet is rapidly heated by
using the induction heating or the electric heating, the
temperature difference between the base iron and the steel sheet
becomes larger to generate a remarkably large thermal stress
between the oxidized scale and the base iron and hence cracking is
caused in the oxidized scale formed on the steel sheet surface to
improve the stripping property. Thus, the inventors have examined
the application of the rapid heating in the process of subjecting
the hot rolled steel sheet to hot-band annealing and found out that
it is very effective for the improvement of the decaling property,
and as a result the invention has been accomplished.
[0017] That is, according to an embodiment of the present invention
is a hot-band annealing equipment provided with a heating zone, a
soaking zone and a cooling zone for subjecting a Si-containing hot
rolled steel sheet to a hot-band annealing, characterized in that a
rapid heating device is disposed at an upper stream side of the
heating zone and/or near the inlet side of the heating zone.
[0018] The hot-band annealing equipment according to an embodiment
of the invention is characterized in that the rapid heating device
is an induction heating device or an electric heating device.
[0019] Also, according to an embodiment of the invention is a
hot-band annealing method by subjecting a Si-containing hot rolled
steel sheet to hot-band annealing using the aforementioned hot-band
annealing equipment, characterized in that the Si-containing hot
rolled sheet is heated by not lower than 50.degree. C. at a heating
rate of not less than 15.degree. C./s by using the rapid heating
device.
[0020] The hot-band annealing method according to an embodiment of
the invention is characterized in that a start temperature for
heating the Si-containing hot rolled sheet by using the rapid
heating device is in a range of room temperature to 700.degree.
C.
[0021] The invention according to an embodiment is a descaling
method characterized by subjecting the Si-containing hot rolled
steel sheet after the hot-band annealing in the aforementioned
method to pickling without conducting a mechanical descaling.
[0022] The invention according to an embodiment is a descaling
method characterized by subjecting the Si-containing hot rolled
steel sheet after the hot-band annealing in the aforementioned
method to pickling after a mechanical descaling.
[0023] The Si-containing hot rolled steel sheet to be targeted in
the descaling method according to an embodiment of the invention is
characterized by containing not less than 1.0 mass % of Si.
[0024] The Si-containing hot rolled steel sheet to be targeted in
the descaling method according to an embodiment of the invention is
characterized by being a raw material for an electrical steel
sheet.
[0025] According to an embodiment of the invention, the rapid
heating device is disposed in the equipment for subjecting a
Si-containing hot rolled steel sheet to hot-band annealing to
conduct a rapid heating at not less than a predetermined heating
rate and by not less than a predetermined temperature rising
amount, whereby cracking is introduced in the oxidized scale to
improve the stripping property, so that it becomes possible to
remove the oxidized scale from the steel sheet surface only by
pickling without conducting a mechanical descaling.
[0026] According to an embodiment of the present invention,
therefore, it is possible not only to increase the pickling
efficiency of the Si-containing hot rolled steel sheet and largely
simplify the pickling process but also to stably produce a product
having an excellent surface quality.
[0027] In an embodiment of the invention, the heating device used
in the rapid heating of the steel sheet acts as a part of the
heating zone in the hot-band annealing equipment and contributes to
the improvement of the thermal energy efficiency.
BRIEF DESCRIPTION OF DRAWINGS
[0028] FIGS. 1(a) and 1(b) are views illustrating a cross-sectional
structure of an oxidized scale formed on a surface of a
Si-containing hot rolled steel sheet.
[0029] FIGS. 2(a) and 2(b) are a view showing an equipment line of
an embodiment of the present invention and a view showing an
equipment line of the conventional technique for comparison,
respectively.
[0030] FIG. 3 is a view showing a weight loss of a Si-containing
hot rolled steel sheet subjected to a mechanical descaling or
pickling after hot-band annealing for comparison.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0031] The inventors have focused on an influence of the difference
among the heating methods of the steel sheet, i.e., the difference
between a radiation heating for slowly heating the steel sheet from
the outside and an induction heating or a direct electric heating
for rapidly heating the steel sheet from the inside upon the
descaling property, from a viewpoint that it is important to
increase the stripping property of the oxidized scale in order to
improve the descaling property of the Si-containing hot rolled
sheet as previously mentioned.
[0032] It is due to the fact that the oxidized scale formed on the
surface of the hot rolled steel sheet is composed of plural layers
having different chemical compositions as shown in FIG. 1, but each
of these layers has a thermal expansion coefficient largely
different from that of the base iron, so that a thermal stress is
caused resulting from the difference of the thermal expansion
amounts between the scale and the base iron, even when the scale is
heated to the same temperature as the base iron.
[0033] In the radiation heating for heating the steel sheet from
the outside, since the oxidized scale is first heated and then the
steel sheet is heated, a large temperature difference is not caused
therebetween. On the other hand, when the steel sheet is heated by
the induction heating or the like, heat generation is caused in the
steel sheet itself to produce a temperature difference between the
oxidized scale and the base iron. Moreover, since the rapid heating
can be conducted in the induction heating or the like, the
temperature difference between the oxidized scale and the base iron
is further increased as compared to the slow heating by thermal
conduction such as the radiation heating. As a result, a large
thermal stress resulting from the difference of the thermal
expansion is caused between the oxidized scale and the base iron to
generate a great number of fine cleavages (cracks) in the oxidized
scale, whereby it is expected to promote the stripping of the
oxidized scale and improve the descaling property.
[0034] The inventors have conducted the following experiments to
confirm the validity of the above idea.
Experiment 1
[0035] A test specimen is taken out from a hot rolled steel sheet
for a non-oriented/grain-oriented electrical steel sheet containing
3.0 mass % of Si (hot rolled sheet) and subjected to a heat
treatment simulating hot-band annealing at 1050.degree. C. for 60
seconds for the hot rolled sheet. The test specimen is rapidly
heating from room temperature (20.degree. C.) up to 30.degree. C.,
50.degree. C., 70.degree. C., 100.degree. C., 400.degree. C. and
700.degree. C. at a heating rate of 50.degree. C./s with a
solenoid-type induction heating device, heated from the each
temperature after the rapid heating to 1050.degree. C. with a
direct type heating furnace (a radiation type heating furnace),
held for 60 seconds, and then cooled at a rate of 25.degree. C./s
in the heat treatment. The atmosphere in the heat treatment is set
to be a nitrogen atmosphere.
[0036] Next, the test specimen after the heat treatment is
subjected to a pickling by immersing in an aqueous solution having
8 mass % of HCl kept at a temperature of 80.degree. C. for 60
seconds, and a stripping condition of an oxidized scale from the
steel sheet surface after the pickling is visually observed to
evaluate the descaling property.
[0037] The descaling property is evaluated as superior (o) when the
surface appearance after the descaling is equal to or superior to
that by the conventional descaling method (mechanical descaling and
pickling) for the Si-containing hot rolled steel sheet, as inferior
(A) when the surface appearance is inferior to that by the
conventional descaling method (mechanical descaling and pickling)
but superior to that by the conventional pickling only, and as bad
(x) when the surface appearance is equal to that by the
conventional pickling only.
[0038] The evaluation results are shown in Table 1. As seen from
these results, when the steel sheet is rapidly heated at a heating
rate of 50.degree. C./s by the induction heating, the descaling
property only by the pickling is improved as compared to the
conventional condition by the combination of the mechanical
descaling and pickling, by setting the temperature rising amount to
not lower than 50.degree. C.
TABLE-US-00001 TABLE 1 Heating temperature of steel sheet (.degree.
C.) 30 50 70 100 400 700 Temperature rising amount 10 30 50 80 380
680 (.degree. C.) from room temperature (20.degree. C.) Evaluation
of descaling x .DELTA. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. property
Experiment 2
[0039] Next, the influence of the heating rate for the steel sheet
on the descaling property is evaluated based on the results of
Experiment 1 by changing the heating rate to five levels of
5.degree. C./s, 10.degree. C./s, 15.degree. C./s, 20.degree. C./s,
and 50.degree. C./s when the temperature rising amount from room
temperature (20.degree. C.) is 50.degree. C., or the heating
temperature is 70.degree. C. (constant) and when the other
conditions and the evaluation standard of the descaling property
are the same as in Experiment 1.
[0040] The results are shown in Table 2. As seen from these
results, when the temperature rising amount by the induction
heating is set to 50.degree. C. (constant), the descaling property
only by the pickling is improved to a level equal to or more than
that by the conventional condition combining mechanical descaling
and pickling by rapidly heating at a heating rate of not less than
15.degree. C./s.
TABLE-US-00002 TABLE 2 Heating temperature of steel sheet (.degree.
C.) 70 70 70 70 70 Temperature rising amount (.degree. C.) 50 50 50
50 50 from room temperature (20.degree. C.) Heating rate (.degree.
C./s) 5 10 15 20 50 Evaluation of descaling property x .DELTA.
.smallcircle. .smallcircle. .smallcircle.
[0041] As seen from the results in Experiment 1 and Experiment 2,
when the rapid heating is conducted by induction heating in the
heating process of the hot-band annealing, an effect of improving
the descaling property to be equal to or more than that when the
mechanical descaling is applied before pickling is obtained by
making the heating rate not less than 15.degree. C./s and making
the temperature rising amount not lower than 50.degree. C.
Therefore, it becomes clear that the descaling property equal to or
more than that by the conventional method can be achieved when the
hot rolled sheet is heated so as to satisfy the above condition in
the heating process of the hot-band annealing even when the
mechanical descaling which was conducted before pickling is
omitted.
[0042] The present invention is based on the above new
knowledge.
[0043] The present invention will be explained in detail below.
[0044] The hot rolled sheet targeted by the present invention is
preferable to contain not less than 1.0 mass % of Si. When it is
less than 1.0 mass %, the decrease of the descaling property by the
pickling is not remarkable, so that the mechanical descaling is not
necessary before the pickling. However, an embodiment of the
present invention is obviously applicable to a hot rolled steel
sheet containing less than 1.0 mass % of Si. Moreover, the upper
limit of Si is not particularly defined, but it is about 5.0 mass %
from a viewpoint of ensuring the threading property to a production
line. Preferably, it is within a range of 1.8 to 4.0 mass %.
[0045] As the hot rolled sheet containing Si in the above range are
concretely included a hot rolled steel sheet as a raw material for
a non-oriented electrical steel sheet or a grain-oriented
electrical steel sheet and a hot rolled steel sheet as a raw
material for a high-strength cold rolled steel sheet or a
high-strength surface-treated steel sheet, each of which is
required to have an excellent surface property.
[0046] The hot-band annealing equipment according to the present
invention is preferable to be a continuous annealing furnace. It is
because the rapid heating is impossible in a batch type (box-type)
annealing furnace.
[0047] The heating device used in the rapid heating is preferable
to be an induction heating device or an electric heating device
capable of generating heat from the steel sheet itself and
conducting the rapid heating. Moreover, the induction heating
device is preferable to be a solenoid-type device rather than a
transverse-type device because the steel sheet is uniformly heated
in the widthwise direction by using the solenoid-type device.
[0048] As seen from Experiment 1 and Experiment 2, it is necessary
to conduct the rapid heating of the steel sheet at a heating rate
of not less than 15.degree. C./s and at a temperature rising amount
of not lower than 50.degree. C. in order to improve the descaling
property of the Si-containing hot rolled steel sheet in the
hot-band annealing. Here, the heating rate is preferably not less
than 30.degree. C./s, more preferably not less than 50.degree. C./s
from a viewpoint of increasing the temperature difference between
the base iron and the oxidized scale. Also, the temperature rising
amount in the rapid heating is preferably not lower than 80.degree.
C., more preferably not lower than 100.degree. C. from the same
viewpoint. However, when the end temperature in the rapid heating
exceeds 700.degree. C., a large current is necessary in the heating
or the heating device becomes large scale. When the steel sheet
temperature exceeds a Curie point in the heating by using a
solenoid-type induction heating, the heating efficiency is
violently decreased, so that the upper limit of the rapid heating
(end temperature) is preferable to be about 700.degree. C. More
preferably, it is not higher than 680.degree. C.
[0049] The position of disposing the rapid heating device in the
equipment for the hot-band annealing is preferable to be an
upstream side of the heating zone (just before the heating zone)
and/or in the heating zone. When the rapid heating device is
disposed just before the heating zone, there is caused no problem,
but when it is disposed in the heating zone, it is preferable to be
disposed in a zone having a temperature inside the furnace of not
higher than 700.degree. C. in the inlet side of the heating zone
(the most upstream side), from a viewpoint of preventing thermal
damage of an induction heating coil or a conducting roll. Thus, the
temperature range for disposing the rapid heating device, or the
start temperature for rapidly heating the steel sheet is preferable
to be within a range of room temperature to 700.degree. C. It is
more preferably a range of room temperature to 620.degree. C.,
further preferably a range of room temperature to 600.degree.
C.
[0050] A large number of fine cracks are introduced into the
oxidized scale formed on the surface of the Si-containing hot
rolled steel sheet subjected to the rapid heating after the
hot-band annealing to improve the stripping property of the
oxidized scale, so that the descaling property equal to or more
than that when the mechanical descaling is performed can be
obtained even when the mechanical descaling is not performed before
pickling. In the pickling process of the Si-containing hot rolled
steel sheet, therefore, the mechanical descaling process before the
pickling has been necessary in the conventional method as shown in
FIG. 2(a), while it is possible to omit the mechanical descaling
process by disposing the rapid heating device just before the
heating zone in the hot-band annealing equipment, for example, as
shown in FIG. 2(b). FIG. 2(b) shows an example that the rapid
heating device is disposed just before the heating zone, but it may
be disposed in a low temperature region at the upstream side of the
heating zone. Here, the mechanical descaling may be obviously
performed in order to further improve the pickling property.
Example
[0051] A test specimen having a width of 100 mm and a length of 300
mm is taken out from a hot rolled steel sheet for an electrical
steel sheet containing 3.5 mass % of Si and having a thickness of
2.5 mm and subjected to a heat treatment under the following
conditions A and B simulating hot-band annealing. The atmosphere in
the annealing under each of the conditions A and B is a N.sub.2
atmosphere.
<Hot-Band Annealing Condition>
[0052] Condition A: The steel sheet is heated from room temperature
(20.degree. C.) to 1050.degree. C. at 10.degree. C./s, held at
1050.degree. C. for 40 seconds and then cooled at a rate of
25.degree. C./s in an experimental furnace simulating a direct-type
side burner heating furnace (radiation heating furnace).
[0053] Condition B: The steel sheet is rapidly heated from room
temperature (20.degree. C.) to 700.degree. C. at 60.degree. C./s
with an induction heating device, further heated from 700.degree.
C. to 1050.degree. C. at 20.degree. C./s with the radiation heating
furnace used in the condition A, held at 1050.degree. C. for 40
seconds and then cooled at 25.degree. C./s.
[0054] Next, the test specimen after the hot-band annealing is
divided into two equal parts in the longitudinal direction. One
part is subjected to a mechanical descaling (shot blasting) under
the following condition, and the other part is not subjected to the
mechanical descaling.
<Shot Blasting Condition>
[0055] Kind of blasting material: steel shot grains with a particle
size of 0.35.+-.0.15 mm, a density of 7.5 g/cm.sup.3 and a hardness
of 40-50 Re [0056] Blasting pressure (speed): 12.5 kg/m.sup.2
[0057] Blasting angle: 90.degree. [0058] Blasting
amount.times.time: 1000 (g/s).times.15 (s)
[0059] Thereafter, the test specimen divided into two equal parts
in the longitudinal direction is further divided into two equal
parts in the widthwise direction. One specimen is subjected to
descaling by pickling under the following condition a and the other
is descaled by pickling under the following condition b.
<Pickling Condition>
[0060] Condition a: Immersion in an aqueous solution containing 8
mass % of HCl at 80.degree. C. for 20 seconds
[0061] Condition b: Immersion in an aqueous solution containing 8
mass % of HCl at 80.degree. C. for 40 seconds
[0062] For comparison, when the mechanical descaling is performed
before pickling in the conventional method, the time required for
descaling (pickling time) in the pickling under the above condition
is about 60 seconds.
[0063] FIG. 3 shows a weight loss (g/m.sup.2) of the test specimen
in each stage after the mechanical descaling and after the
pickling, after the hot-band annealing is conducted.
[0064] As seen from these results, the test specimen that is only
subjected to the pickling without the mechanical descaling has the
descaling property equal to that subjected to the mechanical
descaling, by rapidly heating the specimen to 700.degree. C. by the
induction heating in the heating process of the hot-band annealing.
Also, the oxidized scale is removed sufficiently from the surface
in the test specimen rapidly heated to 700.degree. C. by the
induction heating even when the pickling time is shortened from the
usual time of 60 seconds to 40 or 20 seconds. According to the
present invention, therefore, it is possible not only to omit the
mechanical descaling from the descaling process but also to shorten
the pickling time.
[0065] The method according to embodiments of the present invention
provides the effect of improving the descaling property regardless
of the presence or absence of Si content, so that it is applicable
to not only the Si-containing hot rolled steel sheet for an
electrical steel sheet or high-strength steel sheet but also a
general-purpose hot rolled steel sheet containing no Si.
* * * * *