Method for producing hot rolled steel sheet using induction heating and apparatus therefor

A method and hot rolling apparatus for producing a hot rolled steel sheet include a roughing rolling mill to roughing roll a heated slab into a sheet bar, at least one solenoid-type induction heater arranged for re-heating the sheet bar over its entire width, and a finish rolling mill arranged to finish roll the re-heated sheet bar into a hot rolled steel sheet having a predetermined thickness. An initial temperature before rolling is set to be low, and re-heating of the sheet bar is carried out around the middle of the hot rolling apparatus. The thermal energy required for rolling is thereby reduced as a whole without damaging the quality of the resulting hot rolled steel sheet. A descaling apparatus is provided prior to the finish rolling mill to remove oxide scales on surfaces of the sheet bar, whereby the resulting hot rolled steel sheet is free of scale flaws and exhibits superior surface properties.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention broadly relates to hot rolling methods and facilities 
for metallic sheets, and more particularly, to a method and apparatus for 
producing a hot rolled steel sheet. 
2. Description of the Related Art 
In a process of hot rolling a sheet, such as a process for hot rolling a 
steel sheet, the rolled sheet should be rolled at a temperature as low as 
possible but above a required level. In general, when the temperature is 
higher, more energy is lost per unit time period, and the temperature more 
rapidly decreases. Accordingly, in view of efficient utilization of 
thermal energy, hot rolling should preferably be carried out at a 
temperature as low as possible but still capable of securing product 
quality. 
As described above, a decrease in the temperature of the rolled sheet 
during the rolling process can be a cause of some problems. In a rolling 
process including major steps of roughing rolling and finish rolling, the 
rolled sheet should be at a temperature which is higher than a 
predetermined level after the finish rolling step. Further, since 
deformation resistance should be restricted so as not to exceed 
limitations concerning performance of a finish rolling mill, the 
temperature should be controlled so as not to be lower than a 
predetermined level before the sheet is sent to the finish rolling mill. 
Hitherto, due to such requirements, an initial temperature was determined 
taking into account any decrease in the temperature during the roughing 
rolling step. 
In methods disclosed in Japanese Unexamined Patent Publications No. 
59-92114 and No, 62-214804, edge portions of sheets, which particularly 
readily grow cold, are re-heated by transverse-type induction heating. 
Front and rear end portions of sheets also readily grow cold, and Japanese 
Unexamined Patent Publications No. 1-321009 and No. 4-3371, disclose 
methods in which the front and rear end portions of a sheet are heated 
over an entire width by an edge-heating apparatus used in the methods 
disclosed in Japanese Unexamined Patent Publications No. 59-92114 and No. 
62-214804, while moving the apparatus in a sheet-width direction when the 
front and rear end portions pass over it. 
As a method in which heating is performed over the entire sheet-width, 
Japanese Unexamined Patent Publication No. 51-122649 discloses a method in 
which a transverse-type induction heater is arranged, to preheat a steel 
sheet for a subsequent process, as close as possible to an apparatus for 
the subsequent process. 
However, although the initial temperature can be lowered to some extent, a 
drastic resolution has not been achieved yet by the above-described 
methods, disclosed in Japanese Unexamined Patent Publications No. 
59-92114, No. 62-214804, No. 1-321009, and No. 4-33715, which compensate 
for the temperature decrease heavily occurring in the edge portions of 
sheets or at the front and rear ends thereof in order that the initial 
temperature before rolling should be set low. 
In such circumstances, a method has been considered in which a heater is 
intermediately arranged, the initial temperature is aggressively lowered 
to reduce a thermal energy loss in an early stage of rolling, and rolling 
is carried out while being accompanied by re-heating performed at an 
appropriate position. 
Induction heating can be considered as an easily-practiced technique for 
intermediate heating. However, transverse-type induction heating which is 
described in Japanese Unexamined Patent Publication No. 51-122649 has some 
problems such as a complicated apparatus due to a necessity of providing a 
means for controlling a coil gap, and excessive heating of edge portions 
of sheets. 
As described in Japanese Unexamined Patent Publication No. 51-122649, the 
re-heating apparatus is usually arranged as close as possible to the 
apparatus of the subsequent step. According to such an arrangement, 
however, since a surface temperature of a sheet is high, thermal energy 
added by induction heating is readily lost in a case where the subsequent 
step, such as descaling or rolling, can be a cause of cooling from the 
sheet surfaces. 
In a process for producing a hot rolled steel sheet, since the sheet: is 
heated and rolled in a high temperature range from 800 to 1300.degree. C., 
oxide scales are generated on surfaces of the sheet. If such scales are 
left on the surfaces, the scales are pressed during rolling so that they 
are included in the surface portion of the sheet, and the resulting hot 
rolled steel sheet will have scale flaws. 
As is generally known, scale flaws are classified into two types described 
below. 
(1) Inclusion Scales 
Inclusion scales are generated as follows: 
Scales which have not been completely removed in a descaling process 
preceding a finish rolling mill are pressed into the surface portion of 
the sheet during a finish rolling process. 
(2) Particulate Scales 
Particulate scales are generated as follows: 
Secondary scales which have been generated after the descaling process 
preceding the finish rolling mill are pressed into the surface portion of 
the sheet during the finish rolling process. 
In order to prevent the generation of inclusion scales, a surface 
temperature of a sheet before descaling should be set at a high value. The 
higher the surface temperature of the sheet is, the greater the amount of 
generated scales becomes and the larger the internal stress of the scales 
becomes, since the temperature difference between before and after the 
descaling process becomes large, and a thermal stress generated on 
interfaces between the scales and the sheet also becomes large. 
Japanese Unexamined Patent Publication No. 6-269840 discloses a method in 
which surfaces of a sheet are heated using gas burners at a position just 
preceding a descaling apparatus. 
On the other hand, in order to prevent the generation of particulate 
scales, the surface temperature of the sheet after descaling should be 
restricted to inhibit the generation of secondary scales. 
The surface temperature of the sheet before descaling should preferably be 
as high as possible to prevent the generation of inclusion scales, while 
it should preferably be as low as possible to prevent the generation of 
particulate scales. Accordingly, there is an optimum temperature range in 
which neither type of scales are generated, and the temperature of the 
sheet before descaling should be controlled so that it falls within the 
optimum range in which neither type of scales are generated. 
In a method disclosed in Japanese Unexamined Patent Publication No. 
6-269840, temperature control is substantially impossible since surfaces 
of a sheet are heated using gas burners. Since a temperature of the sheet 
may be too low in some cases while it may be too high in other cases, it 
is difficult to prevent the generation of both types of scales. 
Further, the use of gas burners is also accompanied by problems such as 
those described below. 
(1) The productivity is lowered because a time period for preparation such 
as preheating is required for ignition and extinction of the gas burners. 
(2) The working environment readily deteriorates due to generation of 
combustion gases. 
SUMMARY OF THE INVENTION 
Aiming to solve the above-described problems, an object of the present 
invention is to provide a method and hot rolling apparatus for producing a 
hot rolled steel sheet, in which thermal energy required for rolling can 
be reduced as a whole without damaging the quality of the hot rolled steel 
sheet. 
Another object of the present invention is to provide a method and hot 
rolling apparatus for producing a hot rolled steel sheet having excellent 
surface properties without scale flaws. 
In order to solve the above-described problems and to achieve the above 
objects, the present invention provides a method and hot rolling apparatus 
for producing a hot rolled steel sheet. The apparatus comprises: 
(a) a roughing rolling mill to roughing-roll a heated slab into a sheet 
bar; 
(b) at least one solenoid-type induction heater to re-heat the sheet bar; 
and 
(c) a finish rolling mill to finish-roll the re-heated sheet bar. 
The apparatus further comprises an apparatus for adjusting a heating 
position such that a surface temperature of the sheet bar before the 
finish rolling mill is lower than a temperature in a thicknesswise center 
of the sheet bar. 
According to the above method and apparatus, the thermal energy required 
for rolling can be reduced as a whole without damaging the quality of the 
hot rolled steel sheets. 
The present invention provides another method and hot rolling apparatus for 
producing a hot rolled steel sheet, wherein the apparatus comprises: 
(a) a roughing rolling mill to roughing-roll a slab having a predetermined 
temperature into a sheet bar; 
(b) at least one solenoid-type induction heater to re-heat the sheet bar 
over an entire width of the sheet bar; 
(c) a descaling apparatus for descaling oxide scales on surfaces of the 
sheet bar; and 
(d) a finish rolling mill to finish-roll the sheet bar, 
wherein the solenoid-type induction heater and the descaling apparatus are 
arranged between the roughing rolling mill and the finish rolling mill in 
an order of the roughing rolling mill, the solenoid-type induction heater, 
the descaling apparatus, and the finish rolling mill. 
The above-described apparatus further comprises apparatus for controlling a 
surface temperature of the sheet bar on an inlet side of the descaling 
apparatus within a range of from about 1000.degree. C. to about 
1020.degree. C. 
According to the above method and apparatus, hot rolled steel sheets having 
excellent surface properties without scale flaws can be produced.

DETAILED DESCRIPTION 
Description of the First Embodiment 
The inventors conducted investigations concerning a method and a hot 
rolling apparatus for producing a hot rolled steel sheet, in which an 
initial temperature before rolling can be set at a low value, a re-heating 
apparatus is arranged in the middle of the hot rolling apparatus, and 
thermal energy required for rolling can be reduced as a whole without 
damaging the quality of the hot rolled steel sheet. As a result, they have 
found that the following techniques are is effective for reducing the 
thermal energy required for rolling as a whole without damaging the 
quality of the hot rolled steel sheet: 
employing at least one solenoid-type induction heater as the re-heating 
apparatus arranged in the middle of the hot rolling apparatus; and 
arranging and operating the heater in a manner by which a thermodiffusion 
time period can be set and controlled such that heat added to a sheet bar 
is sufficiently diffused in the thickness direction of the sheet bar and 
is not readily last from surfaces in a subsequent step, and such that a 
surface temperature of the sheet bar is lower than a temperature in the 
thicknesswise center of the sheet bar. 
Further, the present inventors have obtained the following findings: 
the above-described thermodiffusion time period can be determined according 
to a relational equation based on properties and a thickness of a sheet 
bar; 
the decrease in temperature in edge portions can be compensated for by 
heating the side edge portions of the sheet bar using an edge heater 
arranged in the hot rolling apparatus, and a uniform quality over the 
sheet bar can be thereby achieved; 
defect generation due to an excessive increase in a surface temperature by 
heating can be prevented by arranging a leveler preceding the 
solenoid-type induction heater; and 
if the position of the heater is limited to a region preceding the finish 
rolling mill, a high heating efficiency can be achieved by setting an 
excitation frequency of the heater within a specific range with respect to 
a specific range of sheet bar thickness. 
Based on the above findings, the present inventors conceived a method and 
hot rolling apparatus for producing a hot rolled steel sheet in which the 
thermal energy required for rolling can be reduced as a whole without 
damaging the quality of the hot rolled steel sheet, and have accomplished 
a preferred embodiment, wherein at least one solenoid-type induction 
heater is employed as the re-heating apparatus arranged in the middle of 
the hot rolling apparatus; the thermodiffusion time period in the 
thickness direction, which allows the heat added to the sheet bar to be 
sufficiently diffused in the thickness direction so that the heat is not 
readily lost from the surfaces in the subsequent step, and which achieves 
a surface temperature of the sheet bar lower than the temperature in the 
thicknesswise center of the sheet bar, is determined in accordance with 
the properties and the thickness of the sheet bar; and the heater is 
arranged and operated in accordance with the time period. The inventors 
developed a further preferred embodiment based on the above, in which at 
least one edge heater for heating the side edge portions of the sheet bar 
is arranged in the above hot rolling apparatus; a leveler is arranged at a 
position preceding the solenoid-type induction heater; and the excitation 
frequency of the heater is set within a range with respect to a specific 
thickness of the sheet bar in the case where the heater is arranged at a 
position preceding the finish rolling mill. 
In summary, according to a preferred embodiment, the apparatus and the 
manufacturing conditions are specified within the ranges described below, 
and there are provided a method and hot rolling apparatus in which the 
thermal energy required for rolling can be reduced as a whole without 
damaging the quality of the hot rolled steel sheet. 
A first preferred embodiment of the invention is described below with 
reference to FIG. 1. In a conventional hot rolling technique, a slab or 
steel ingot 1 is roughing rolled by a roughing rolling mill 2, while being 
maintained at a high temperature directly after solidification or by 
re-heating, into a sheet bar 3 having an intermediate thickness. 
After this, the sheet bar is carried by table rollers 7, subjected to 
surface scale removal by a descaling apparatus 5 or the like, and is 
finish rolled by a finish rolling mill 6 into a steel sheet having a final 
thickness. Subsequently, an appropriate cooling step by a cooling 
apparatus or the like which is not shown in FIG. 1, a step of coiling the 
sheet into a coil, and other known steps are carried out. 
According to the first embodiment, in such an apparatus, at least one 
solenoid-type induction heater 4 is provided as a re-heating apparatus 
between the roughing rolling mill 2 and the finish rolling mill 6, and the 
at least one heater 4 is situated such that the temporal distance 8 
(thermodiffusion time period) from the end of the heating step to the 
descaling step or the subsequent finish rolling step is longer than a 
predetermined time period, the thermal energy is thereby sufficiently 
diffused into the inside of the sheet bar 3 in which only the surface 
temperature is high due to the skin effect, and thus the surface 
temperature becomes lower than the temperature in the thicknesswise center 
of the sheet. 
The above is based on the following principle: In maintaining the material 
temperature at above a certain value by adding a certain thermal energy to 
a material, thermal radiation can be further restricted and the 
high-temperature state can be maintained for a longer time period by a 
procedure in which the energy is divided into two, and the divided 
energies are added with a temporal interval, as compared with a procedure 
in which the thermal energy is added once at the beginning. In order to 
apply this principle to hot rolling for steel sheets, the re-heating 
apparatus requires mechanical simplicity, ease of installation, and 
superior heating efficiency. From this point of view, to achieve these 
characteristics, at lest one solenoid-type induction heater 4 is employed. 
More specifically, for a hot rolling apparatus, an electrical heating unit 
achieving a higher power (high electric power) is preferred in view of a 
limitation concerning the time period (the position) for heating, but 
electric-resistance-type heating cannot be employed in view of the 
negative influence of sparks on the surfaces of the steel sheet, and 
therefore the heating apparatus is limited to an induction heating 
apparatus. The induction heating apparatus can be classified into a 
solenoid type and a transverse type. The transverse type, however, 
exhibits irregular heating ability depending on the heated portions, is 
defective in uniformly heating, and requires that the positional, 
relationship between a coil and a bar (steel sheet) should be maintained 
at an optimum level. Accordingly, the solenoid type heating apparatus is 
employed for the induction heater in the preferred embodiment, since the 
power (electric power) can be applied almost uniformly in the sheet-width 
direction and problems caused by biased heating are reduced as compared 
with the transverse type heater in the case where a sheet bar with a 
thickness of a few tens of millimeters is heated, and the heater has a 
simple structure so that the sheet bar can be heated simply by passing 
through the heater. 
Further, the thermodiffusion time period after heating is set longer than a 
certain time period since the surface temperature inevitability becomes 
high in the case of solenoid-type heating, and therefore, sufficient 
thermal diffusion in the sheet-thickness direction and an appropriate 
temperature distribution in the sheet-thickness direction are necessary 
for preventing easy loss of the applied heat from the surfaces in the 
subsequent step. In the preferred embodiment, the heater is situated and 
operated such that the time period from the re-heating step to the 
subsequent step can be adjusted and controlled. 
The thermodiffusion time period is determined such that the thermal energy 
applied by the solenoid-type induction heater at a constant level remains 
sufficiently high after finish rolling. 
More specifically, the thermodiffusion time period is determined such that 
the difference according to the subtraction of the steel central 
temperature from the steel surface temperature has at least a minus value, 
and preferably, about -10 .degree. C. or below. 
Further, in the case where the finish rolling rate is varied, the 
re-heating apparatus 4 may be arranged to include a device or unit for 
moving the apparatus 4, such as rails in the longitudinal direction of the 
apparatus and on which the re-heating apparatus 4 is movably mounted, to 
adjust a heating position, or the time period for thermal diffusion may be 
adjusted in a manner in which a plurality of re-heating apparatuses 4 are 
arranged (see FIG. 8) and the effective heating position is adjusted by 
selecting at least one turned-on heating apparatus 4 out of the plurality 
of heating apparatuses 4. 
The position of the re-heating apparatus 4 is not limited to a place 
between the roughing rolling mill 2 and the finish rolling mill 6, and the 
re-heating apparatus 4 may be situated in the middle of the roughing 
rolling mills. 
Moreover, although the thermodiffusion time period varies depending on the 
properties of a sheet bar and a thickness thereof at the time of induction 
heating, a value of the time period suitable to properties and a thickness 
can be determined according to the following equation (1): 
EQU T=.alpha..times.(.rho.C.sub.p /.lambda.).times.H.sup.2 (1) 
where: 
T represents the time period for thermodiffusion, 
.alpha. represents a coefficient inherent in the hot rolling apparatus, 
.rho. represents a density of the sheet bar, 
C.sub.p represents the specific heat of the sheet bar, 
.lambda. represents the thermal conductivity of the sheet bar, and 
H represents a thickness of the sheet bar. 
In summary, the thermodiffusion time period, which varies according to a 
change in conditions such as properties and a thickness, is determined by 
the above equation (1) and is set by adjusting the position of the 
induction heater 4, and a high heating efficiency can thereby be 
maintained. 
In the above equation (1), the situation of thermal diffusion toward the 
inside of the sheet while the sheet is radiation-cooled after the 
completion of heating is approximately applied to the heat-transfer 
equation under adiabatic conditions, and the time constant for the largest 
attenuation value, namely, the time constant (.rho.C.sub.p 
/.lambda.)(H/2.pi.).sup.2 of the following solution of the Fourier series 
is used: 
##EQU1## 
In the above equation, the time constant is a generic value, and the 
optimum thermodiffusion time periods for various sheet properties and 
thicknesses can be determined by determining the constant .alpha. in 
accordance with each apparatus. 
Practically, .alpha. can be determined by determining an optimum 
thermodiffusion time period for a certain condition. 
Furthermore, as shown in FIG. 1, an edge heater 9 to heat side edge 
portions of a sheet bar 3 is provided in the hot rolling apparatus of the 
preferred embodiment. This edge heater 9 is provided in the vicinity of 
the at least one solenoid-type heater 4 in order to compensate for a 
temperature decrease in the edge portions of the sheet bar 3, and to 
thereby obtain further uniform quality over the materials. The edge heater 
9 can be freely positioned, and may be arranged at a position preceding a 
solenoid-type induction heater 4, as shown in FIG. 1. 
Additionally, as shown in FIG. 1, a leveler 10 is provided at the inlet 
side of the heaters 4 to stably send a sheet bar toward the solenoid-type 
induction heater(s) 4, and preferably, the leveler 10 is arranged such 
that solenoid-type induction heating starts within the above-described 
thermodiffusion time period. An excessive increase in the surface 
temperature during re-heating can be prevented in such a manner, namely, 
by starting re-heating before thermal recovery on the sheet surface whose 
temperature has been reduced by the tools in the leveler 10 or the like. 
When a shape of a sheet is inferior, the sheet bar cannot pass through nor 
be heated by the solenoid-type induction heater(s) 4 because of a gap at 
the opening portion thereof. In this case, the sheet bar 3 is reformed 
prior to being sent to the solenoid-type induction heater(s) 4. The 
surface temperature is, however, inevitably reduced by leveling at the 
leveler 10. The solenoid-type induction heater largely raises the sheet 
surface temperature, but a surface temperature increase during heating can 
be restricted by arranging the leveler 10 prior to the starting point for 
induction heating (as shown in FIG. 1), and preferably, within the 
temporal distance range 8 for the above-described thermodiffusion time 
period. In such a manner, the thermal energy loss by radiation during 
heating can be minimized, and in addition, defect generation due to an 
excessive increase in the surface temperature can be prevented. 
Although edge portions of the sheet bar 3 are also heated, a degree of the 
temperature increase in those portions is the same as that in the center 
portion. According to the preferred embodiment, the temperature decrease 
in the edge portions can be completely compensated for by additionally 
providing an edge heater 9, as described above. 
Further, when the solenoid-type induction heater 4 is arranged at a 
position preceding the finish rolling mill 6, the heating step is 
performed under the conditions that an excitation frequency of the heater 
is set at about 1,000 to about 3,000 Hz. 
More specifically, in the hot rolling apparatus of the preferred 
embodiment, if the position of the induction heater 4 is limited to a 
region preceding the hot finish rolling mill 6 for steel, the sheet bar 
thickness is limited to approximately 10 to approximately 50 mm, and the 
properties are also limited. Accordingly, by setting the coil excitation 
frequency at from about 1,000 to 3,000 Hz, the effects of the preferred 
embodiment on the temperature distribution can be sufficiently exhibited, 
and a high healing efficiency can be achieved. 
The heating efficiency by a solenoid-type induction heater depends on the 
material thickness, especially in the stage prior to hot finish rolling 
for steel. The surface temperature excessively increases with a frequency 
above about 3,000 Hz, and the induction heating efficiency is lowered with 
a frequency below about 1,000 Hz. Accordingly, the lower limit of the 
frequency is about 1,000 Hz and the upper limit of the frequency is about 
3,000 Hz. The frequency may be adjusted according to the material 
thickness, or may be set to a typically used value. 
As described above, according to the preferred embodiment, there can be 
provided a method and hot rolling apparatus for producing a hot rolled 
steel sheet, in which the thermal energy required for rolling can be 
reduced as a whole, without damaging the quality of the resulting rolled 
sheet. 
EXAMPLE 
An example according to a preferred embodiment of the invention is 
described below, referring to FIG. 1. 
The effects of the preferred embodiment in a case where a sheet bar 3 with 
a thickness of 30 mm was finish rolled into a thickness of 25 mm by an 
experimental rolling mill are described below. 
A descaling apparatus 5 was arranged at a position three (3) meters 
preceding a finish rolling mill 6, and finish rolling was carried out at a 
rate of 60 meters per minute. A solenoid-type induction heater 4 is 
arranged at a position preceding the (descaling apparatus 5, and the 
thermodiffusion time period 8 was varied by altering the position of the 
heater 4. 
As a conventional way, an induction heater 4 was is arranged very close to 
the descaling apparatus 5, i.e, at a position one (1) meter preceding the 
descaling apparatus 5, and in this case, the time period from the end of 
heating to descaling was 1 second (conventional example). The following 
conditions were used in the preferred embodiment: the applied energy was 
constant, and the induction heater 4 was distantly placed at positions 
requiring 4 sec. and 9 sec., respectively, for the sheet bar to travel 
from the induction heater 4 to the descaling apparatus. 
As shown in FIG. 2, the surface temperature of the sheet bar just previous 
to the descaling apparatus 5 was high in the conventional example in which 
the thermodiffusion time period 8 was short. 
In each case according to the preferred embodiment of the invention, where 
the thermodiffusion time period was set at 4 sec. or longer, the surface 
temperature became higher than that of the conventional example when 
finish rolling was completed, although the surface temperature just 
previous to the descaling apparatus 5 was lower. Therefore, the thermal 
energy loss during descaling and finish rolling was found to be lowered in 
the present invention. 
For a further study concerning the conditions to achieve the object of the 
preferred embodiment of the invention, the temperature difference between 
the surface and the thicknesswise center of each sheet bar was measured in 
the cases of the thermodiffusion time periods shown in FIG. 2. As a 
result, the temperature after finish rolling can be higher than that of 
the conventional example when, as shown in FIG. 3, the surface temperature 
is lower than the temperature at the thicknesswise center of the sheet bar 
3. 
Further, rolling processes with various steel materials and various 
thicknesses as shown in Table 1 (hereinbelow) were carried out to seek an 
appropriate thermodiffusion time period (which satisfies a preferred 
requirement that the sheet bar 3 has a surface temperature about 
10.degree. C. lower than the thicknesswise center temperature), and to 
determine .alpha. (a coefficient inherent in the hot rolling apparatus). 
As a result, this value was found to be constant. 
In order to evaluate the effect of an edge heater, the temperature 
distribution in the sheet-width direction after finish rolling was 
measured. The results are shown in FIG. 4. 
By arranging an edge heater 9 as shown in FIG. 1, a temperature 
distribution which is uniform in the sheet-width direction could be 
obtained, and a product having properties which are uniform in the 
sheet-width direction could be achieved. Due to this, it: was not 
necessary to set the initial temperature or the re-heating temperature 
high for securing the desired temperature of the side edge portions. 
In the example according to the preferred embodiment, a 3-roller-type 
leveler 10 was arranged at a position preceding the experimental induction 
heater 4. The position of the leveler 10 was then altered to change the 
time period from the end of leveling to the start of re-heating, and the 
surface temperature of the sheet bar 3 just after re-heating was measured 
using a radiation thermometer. As is obvious from the results shown in 
FIG. 6, if the time period from the end of leveling to the start of 
re-heating is set within a range of the thermodiffusion time period 
according to the preferred embodiment, the surface temperature can be 
restricted. When the time period exceeds the desired range for the time 
period from leveling to re-heating according to the preferred embodiment 
shown in FIG. 6, the surface temperature becomes higher. In this example, 
the surface temperature reached 1,250.degree. C., the quality of the sheet 
surface deteriorated, and the amount of heat radiated from the surfaces 
increased. 
FIG. 5 shows the relationship between the excitation frequency of the 
solenoid-type heater(s) 4 and the sheet-surface temperature after finish 
rolling observed in the case where a constant induction-heating electric 
power was applied to the heaters(s) 4 of the hot rolling apparatus of. the 
preferred embodiment. 
In view of the thickness before finish rolling, the heating ability for a 
sheet bar 3 having a thickness of 10 mm was extremely low when the 
frequency was below about 1,000 Hz, and the beating ability for a sheet 
bar having a thickness of 50 mm was lowered when the frequency was above 
about 3,000 Hz. 
As described above, even in an arrangement of the apparatus according to 
the preferred embodiment, there exists a frequency range for effectively 
utilizing the applied electric power to the solenoid-type heater(s) 4 to 
secure the desired temperature for finish rolling, which is from about 
1,000 to about 3,000 Hz. 
TABLE 1 
______________________________________ 
Thickness Thickness 
at Heating after Finish 
Steel Type (mm) Rolling (mm) 
.alpha. 
______________________________________ 
Low Carbon Steel 
30 26 0.040 
Low Carbon Steel 
50 43 0.038 
Low Carbon Steel 
10 7 0.043 
SUS304 30 27 0.041 
SUS410 30 27 0.042 
______________________________________ 
According to the preferred embodiment, at least one solenoid-type induction 
heater 4 as a re-heating apparatus is arranged around the middle of a 
plurality of rolling mills, and the apparatuses are arranged such that a 
time period for diffusion of an added heat to the inside of a sheet bar 
can be secured, or the apparatuses are set up so as to be operated in 
accordance with the required conditions. In this manner, a temperature for 
heating a slab in a furnace prior to rolling can be set low, quality can 
be secured, loads upon a finish rolling mill can be reduced, thermal 
energy loss during roughing rolling can be restricted, and thermal energy 
added by re-heating can be efficiently utilized. 
Description of the Second Embodiment 
FIG. 7 shows another preferred embodiment of a hot rolling apparatus 
according to the present invention. 
A slab heated in a furnace or produced by continuous casting and having a 
predetermined temperature is rough rolled by a roughing rolling mill 2 
into a sheet bar 3. While being sent to a finish rolling mill 6 by table 
rollers 7 and when passing through an adjustable-position solenoid-type 
induction heater 4, the sheet bar 3 is induction-heated over its entire 
width, and high pressure water is then jetted from a descaling apparatus 5 
onto surfaces of the sheet bar 3 to remove scales. After this, the sheet 
bar 3 is finish rolled by a finish rolling mill 6 to result in a hot 
rolled steel sheet having a predetermined thickness. 
Further in FIG. 7, a thermometer 11 is provided on the inlet side of the 
heater 4, and a table roller 12 is provided at the inlet side of the 
heater 4 to detect a conveying speed. A thermometer 14 is provided on the 
inlet side of the descaling apparatus 5, and a controller 13 is provided 
to control the heater 4 based on the detected temperature of the sheet bar 
3 and the detected conveying speed. As shown in FIG. 7 (and also in FIG. 
8), a moving means 15 is provided for moving or changing a heating 
position of the solenoid-type induction heater 4. The moving means 15 may 
take the form of longitudinal rails extending in the longitudinal 
direction of the overall apparatus, and on which the solenoid-type 
induction heater or heaters 4 are mounted so as to be movable along the 
rails, as described hereinabove with respect to FIG. 1. The moving means 
15 is only schematically shown in FIG. 7 for illustrative purposes. 
The controller 13 shown in FIG. 7 can also adjust the time period of 
heating, for example, by adjusting the time period that the heater 4 is 
turned on. The controller 13 can also adjust the excitation frequency of 
the solenoid-type induction heater, or a separate means 18 can be provided 
for adjusting the excitation frequency of the solenoid heater. 
In the embodiment of FIG. 8, the heating position is effectively changed by 
selecting one or more of the solenoid-type heating units 4 for heating the 
sheet bar. The selection of one or more of the solenoid-type heating units 
4 is accomplished by means of a selecting unit 16, which can be in the 
form of a switch device, or which can also include additional control 
circuits for the heating units 4. As shown in FIG. 8, a control unit 17 
can be provided, which is connected to each of the heating units 4, to 
adjust a time period for turning on the respective heating units 4. A 
control unit 18 can also be provided, as shown in FIG. 8, to adjust the 
excitation frequency of the respective heating units 4. 
Although not shown in FIG. 8, each of the heating units 4 could also be 
adjustably mounted on, for example, elongated rails (not shown) extending 
along the length of the apparatus, so that the physical position of the 
respective heating units 4 can be varied along the rails, as in FIGS. 1 
and 7. 
First, a method for further securely carrying out descaling, which is 
directed to the prevention of inclusion-scale generation, is now 
described. 
Scales on the surface of the sheet bar are removed by the following three 
forces: 
(1) The impact force from high pressure water jetted by the descaling 
apparatus 5 onto the surfaces of the sheet bar 3. 
(2) The thermal stress which is derived from the difference in the 
coefficient of thermal expansion between steel and scales, and which is 
generated by the temperature decrease on the surface of the sheet bar due 
to the water stream. 
(3) The internal stress which is generated since scale generation is 
accompanied by volume expansion. 
In order to further securely remove scales, the above three forces should 
preferably be enlarged. 
Among them, the impact force can be enlarged by increasing the water 
pressure or flow rate, or by arranging the nozzles of the descaling 
apparatus 5 closer to a sheet bar 3. Increasing the water pressure or flow 
rate, however, requires increasing the pressure and the volume capacity of 
the pump of the descaling apparatus 5. As to the descaling apparatus 5, it 
is difficult to achieve water pressure or flow rate levels higher than the 
existing levels in view of the problems concerning costs, the space for 
installation, or destabilization of the water stream. 
As for a method of arranging the nozzles of the descaling apparatus 5 
closer to a sheet bar 3, when deformation of the sheet bar 3 such as 
upward warping occurs, the sheet bar 3 may meet with (i.e., contact) the 
descaling apparatus to crash or otherwise damage the descaling apparatus 
5. For fear of this, excessive reduction of the distance between the 
nozzle(s) of the descaling apparatus 5 and the sheet bar 3 is regarded as 
risky. 
From the above-described viewpoints, in the hot rolling apparatus according 
to the preferred embodiment of FIG. 7, the scale-exfoliation properties of 
the sheet bar 3 are improved by enlarging the thermal stress and the 
internal stress. 
The sheet bar 3 is heated over its entire width by the solenoid-type 
induction heater 4, and is then subjected to scale removal by jetting 
water streams from the descaling apparatus 5. 
FIGS. 9 and 10 show the results of comparison on the temperature 
distributions in the thickness direction of the sheet bar 3 before and 
after descaling. FIG. 9 shows the results of the case where induction 
heating before descaling was not performed, while FIG., 10 shows the 
results in the case where induction heating before descaling was 
performed. The solid line in FIGS. 9 and 10 represents the temperature 
distribution before descaling, and the broken line represents the 
temperature after descaling. 
By induction-heating the sheet bar 3 just before descaling, the temperature 
difference between before and after descaling becomes large, the thermal 
stress derived from the difference in the coefficients of thermal 
expansion between the steel and scales also becomes large, and therefore, 
the scale-exfoliation properties are improved. 
According to the preferred embodiment of FIG. 7, the internal stress of 
scales can also be enlarged. The higher the temperature is, the more the 
amount of generated scales increases. Since scales exhibit a volume 
expansion of approximately 1.4 times that of steel, the internal stress of 
scales becomes large in proportion to the amount of generated scales, and 
the stress generated on the interface between scales and steel also 
becomes large. As a result, the removal of scales becomes easy. 
In the preferred embodiment of FIG. 7, a sheet bar 3 is heated just before 
descaling, and the amount of generated scales is thereby increased, to 
securely perform the removal of scales. 
It is also effective to arrange a plurality of solenoid-type induction 
heaters 4 to repeat heating and radiation-cooling of the sheet bar 3 
before descaling. FIG. 8 shows such a hot rolling apparatus in which three 
solenoid-type induction heaters 4 are arranged prior to the descaling 
apparatus 5. 
The temperature of the sheet bar 3 is raised by induction heating, and the 
temperature is lowered due to radiation in the time period from passing 
out of a solenoid-type induction heater 4 to passing into the succeeding 
solenoid-type induction heater 4. During these time periods, fine cracks 
are generated in the scales due to the thermal stress generated on the 
interface between the scales and the sheet bar 3. These cracks increase 
the rate of oxygen diffusion into the scales during the subsequent 
induction-heating period and make the growing rate of the scales fast, and 
the internal stress of the scales thereby becomes large. 
Next, prevention of particulate-scale generation is described. 
The cause of particulate-scale generation is secondary scales generated 
after descaling. In order to restrict the generation of secondary scales, 
the temperature after descaling is lowered. Particulate scales are readily 
generated if the temperature just before the descaling apparatus 5, 
detected by the thermometer 14 on the inlet side of the descaling 
apparatus 5, exceeds about 1020.degree. C. To prevent the 
particulate-scale generation, the surface temperature of the sheet bar 3 
on the inlet side of the descaling apparatus 5 is set at about 
1020.degree. C. or lower. 
Since the surface temperature of a sheet bar 3 before descaling should 
preferably be as high as possible in order to prevent inclusion-scale 
generation, prevention of both particulate-scale generation and 
inclusion-scale generation can be achieved if the solenoid-type induction 
heater 4 is controlled such that the surface temperature of the sheet bar 
3 detected by the thermometer 14 on the inlet side of the descaling 
apparatus 5 falls within the range of from about 1000 to about 
1020.degree. C. 
Although gas burners, electric resistance heaters, and induction heaters 
can be considered for heating a sheet bar, solenoid-type induction heaters 
4 should be employed for the following reasons: 
Although a method using gas burners is proposed in Japanese Unexamined 
Patent Publication No. 6-269840, such a method is accompanied by problems 
such as, those described above in the section entitled Description of the 
Related Art, and is not capable of coping with the difficulties 
encountered in practical use. In particular, the surface temperature of 
the sheet bar 3 detected by the thermometer 14 on the inlet side of the 
descaling apparatus 5 is controlled within a narrow range of from about 
1000 to about 1020.degree. C. in order to prevent both particulate-scale 
generation and inclusion-scale generation. However, such precise 
temperature control is not possible in the case of using gas burners. 
According to an electric-resistance type heating method in which electrodes 
are placed in contact with a sheet bar 3 and an electric current is made 
to flow therethrough, sparks are generated between the electrodes and the 
sheet bar 3, and the surface of the sheet bar 3 may thereby be damaged. 
Further, since the electrodes of an electric-resistance type heater wear 
severely, they must be changed frequently. Additionally, inferiority in a 
controlling response is also a problem. 
In contrast, an induction heater exhibits a superior controlling response, 
and the surface temperature of a sheet bar 3 can be varied at will within 
the range of the heating capacity. Since the sheet: bar 3 can be heated 
without any contact, the surfaces of the sheet bar 3 are free from the 
possibility of being damaged. Further, as compared with other methods, 
induction heating has other marked advantages that it does not cause 
deterioration of the working environment and has the property of ease of 
maintenance. 
Induction heating can be performed in two types of modes, i.e., a 
transverse type in which magnetic flux is generated in parallel to the 
thickness direction of the sheet bar 3, and a solenoid type in which the 
magnetic flux is generated in parallel to the longitudinal direction of 
the sheet bar 3. 
FIG. 11 shows the temperature distributions in the thickness direction of 
sheet bars just after heating by a transverse-type induction heater and 
just after heating by a solenoid-type induction heater, respectively. 
In the transverse type, since eddy current density is substantially uniform 
in the thickness direction, the temperature distribution after induction 
heating reflects the temperature distribution before induction heating, 
namely, the temperature becomes lowest on the surface of the sheet bar and 
highest at the thicknesswise center. In the solenoid type, due to the skin 
effect, the eddy current density becomes highest in the surfaceportion of 
the sheet bar 3 and lowest at the thicknesswise center. As a result, in 
the temperature distribution after induction heating, the highest 
temperature appears on the surface of the sheet bar 3 and the lowest 
temperature appears at the thicknesswise center. 
As is obvious from FIG. 11, the electric power necessary to obtain the same 
surface temperature is smaller when using the solenoid type induction 
heater. 
FIG. 12 shows the temperature distributions in the thickness direction of 
sheet bars just after descaling which was performed after induction 
heating. As shown in FIG. 11, the temperature at the thicknesswise center 
of a sheet bar 3 before descaling is higher in the transverse type than in 
the solenoid type. Accordingly, the temperature at the thickness center of 
the sheet bar 3 after descaling is also higher in the transverse type, 
even if the surface temperature just after descaling is the same, the 
degree of increase in the surface temperature of a sheet bar 3 is higher 
in the transverse type due to subsequent thermal recovery from the 
thickness-central portion. 
In order to prevent particulate-scale generation, the surface temperature 
of a sheet bar 3 after descaling is set at a lower value. In view of this 
requirement, the solenoid type induction heater is also regarded as 
advantageous. 
From the above, it is concluded that the solenoid-type induction heater is 
most excellent as a heating apparatus and is preferred in the present 
invention. 
Further, when a material subjected to heating has a size similar to that of 
a sheet bar 3, the frequency of the solenoid-type induction heater 4 is 
preferably set at from about 1000 Hz or more to sufficiently utilize the 
skin effect. 
Since the solenoid-type induction heater 4 may get wet by water streams 
from the descaling apparatus 5, the solenoid-type induction heater 4 may 
have a waterproof structure. More specifically, for example, the 
solenoid-type induction heater 4 may be placed in a case which does not 
have openings other than the openings for receiving and sending a sheet 
bar 3 therethrough, and clean air may be fed by an air-blowing fan from a 
duct connected with the case to maintain the pressure inside the case at a 
positive pressure value (as described in Japanese Unexamined Patent 
Publication No. 6-330158). 
In the preferred embodiment of FIG. 7, since the surface temperature of a 
sheet bar 3 at the inlet side of the descaling apparatus 5 is precisely 
controlled, there are provided a thermometer 11 on the inlet side of the 
heater 4, a thermometer 14 on the inlet side of the descaling apparatus 5, 
a table roller 7 for detection of conveying speed, and a controller 13 for 
controlling the solenoid-type induction heater 4 based on the surface 
temperature of the sheet bar 3 and the conveying speed which are detected 
by the aforementioned detecting devices. 
For controlling the heater 4, the surface temperature of the sheet bar 3 
detected by the thermometer 11 on the inlet side of the heater 4 may be 
sent to the controller 13 in a feed-forward manner, and/or the surface 
temperature of the sheet bar detected by the thermometer 14 on the inlet 
side of the descaling apparatus 5 may be sent to controller 13 in a 
feed-back manner. 
The conveying speed of a sheet bar 3 which is calculated from the rotating 
speed of the table roller 7 for detection of a conveying speed is applied 
to the following equation to calculate the output P of the solenoid-type 
induction heater 4 from the necessary temperature increase .DELTA.T: 
EQU P=C.multidot.W.multidot.H.multidot.V.multidot..DELTA.T 
where: 
W represents the width of the sheet bar, 
H represents the thickness of the sheet bar, 
V represents the conveying speed of the sheet bar, and 
C represents a constant determined on the basis of the specific heat and 
the specific gravity of the sheet bar and the efficiency of the 
solenoid-type induction heater. 
EXAMPLE 
An example according to the preferred embodiment of FIG. 7 is described 
below. 
The effects according to the preferred embodiment of FIG. 7 are described 
in detail with reference to examples. 
Using a hot rolling apparatus according to the preferred embodiment of Fig. 
7, a sheet bar 3 having a thickness of 30 mm was finish rolled into a hot 
rolled steel sheet having a thickness of 1.4 mm. The steel was of a 
low-carbon type. Table 2 (below) shows the relationship between the 
surface property of the hot rolled steel sheet and the surface temperature 
in the widthwise central portion of the sheet bar 3 measured by the 
thermometer 14 on the inlet side of the descaling apparatus 5. 
TABLE 2 
______________________________________ 
Temperature on Inlet Side 
Surface Property 
of Descaling Apparatus 
of Hot rolled Steel Sheet 
______________________________________ 
1050 Having Particulate Scales 
1030 Having Particulate Scales 
1020 Satisfactory 
1010 Satisfactory 
1000 Satisfactory 
990 Having Inclusion Scales 
960 Having Inclusion Scales 
______________________________________ 
Particulate scales were generated on the hot rolled steel sheets in the 
cases where the temperatures measured on the inlet side of the descaling 
apparatus 5 were respectively 1030 and 1050.degree. C., and inclusion 
scales were generated on the hot rolled steel sheets in the cases where at 
the inlet side of the descaling apparatus 5 the temperatures were 
respectively 960 and 990.degree. C. As is obvious from Table 2, both 
inclusion-scale generation and particulate-scale generation can be 
prevented when the temperature on the outlet side of the heater 4 (inlet 
side of descaling apparatus 5 ) is within 1000 to 1020.degree. C. 
In a similar manner, a sheet bar 3 having a thickness of 30 mm was finish 
rolled into a hot rolled steel sheet having a thickness of 1.4 mm. Table 3 
(below) shows the relationship between the surface temperature of the 
sheet bar measured on the inlet side of the heater 4 and the surface 
temperature of the sheet bar 3 measured on the inlet side of the descaling 
apparatus 5. 
TABLE 3 
______________________________________ 
Temperature on Temperature on Inlet Side 
Inlet Side of Heater 4 
of Descaling Apparatus 5 
______________________________________ 
1000 1010 
950 1010 
900 1010 
______________________________________ 
The solenoid-type induction heater of the present invention exhibited a 
superior controlling response, and achieved a temperature of 1010.degree. 
C. on the inlet side of the descaling apparatus 5 when the temperature on 
the inlet side of the heater 4 ranged from 900 to 1000.degree. C. Neither 
inclusion scales nor particulate scales were observed in the hot rolled 
steel sheets in the cases shown in Table 3. 
According to the preferred embodiment of FIG. 7, descaling before finish 
rolling can securely be carried out, and there can be produced a hot 
rolled steel sheet which does not have scale flaws and which exhibits a 
satisfactory surface property. 
The present examples and embodiments are to be considered as illustrative 
and not restrictive and the invention is not to be limited to the details 
given herein, but may be modified in various forms, such as, for example, 
combining features of the various embodiments, within the scope of the 
appended claims.