[invention Title]
Apparatus for controlling slab width and method for the same
[Technical Field]
The present disclosure relates, in general, to an apparatus and method for controlling a width of a multi-stand rolling mill and, more particularly, to an apparatus and method for controlling a width of a multi-stand rolling mill, capable of rapidly and precisely controlling a width of a steel sheet in response to a change in temperature or in a width of a steel sheet having been subjected to a rolling process.
[Background Art]
In general, molten steel is formed as a slab in a continuous casting process and a rolling process, and then is formed as a hot-rolled steel sheet while being continuously rolled. In a mini mill, the width of the slab should be within a predetermined range before the slab is rolled by a finishing rolling machine. To this end, the width of the slab is controlled to a target width while being rolled by the rolling machine. However, the width of the slab may be varied due to several factors in the continuous casting process. A slab which varies in width in the continuous casting process may undergo a greater change in width due to other rolling

factors including deviations in temperature, while being subjected to the rolling process again. Since the deviation in width of the slab which has gone through the rolling process is beyond a range of width deviation which is controllable in the finishing rolling maehine, it is essential to control the width of the slab before it is rolled through the finishing rolling machine.
Conventionally, the rolling process is performed while tension exerted by a plurality of rolling machines is maintained to be constant. However, this is not suitable to variably adjust the width of the slab, since the rolling process of the mini mill sees a considerable change in temperatures and the width is greatly influenced by various factors.
[Related Art Document]
Korean Patent Laid-Open Publication No. 10-2001-0061656 (2001.07.07)
[Technical Problem]
An exemplary embodiment in the present disclosure may provide an apparatus and method for controlling a width of a multi-stand rolling mill, capable of rapidly and precisely controlling a width of a steel sheet in response to a change in temperature or in a width of a steel sheet having been subjected to a rolling process.

[Technical Solution!
According to an exemplary embodiment in the present disclosure, a width controlling apparatus for a multi-stand rolling mill may be used to control a width of a steel sheet rolled by the multi-stand rolling mill by adjusting tension of a plurality of rolling stands included in the multi-stand rolling mill, and may include a measuring unit measuring the steel sheet passing through the plurality of rolling stands, thus generating at least one of a measured temperature value, a measured stress value and a measured width value corresponding to a surface temperature, a stress and a width of the steel sheet; and a tension control unit calculating a re.ference. stress value corresponding to each of the plurality of rolling stands using target width variables that are set for the plurality of rolling stands, respectively, and calculating a final target stress value by compensating for the reference stress value using any one of the measured temperature value, the measured stress value and the measured width value, thus adjusting the tension of the rolling stand depending on the final target stress value.
The measuring unit may include a temperature sensor unit measuring the surface temperature of the steel sheet passing through the plurality of rolling stands, thus generating the measured temperature value; a stress measuring unit measuring the stress of the steel sheet passing through the plurality of rolling stands, thus generating the measured stress value; and a width measuring

unit measuring the width of the steel sheet passing through the plurality of rolling stands, thus generating the measured width value.
The tension control unit may calculate the reference stress value using the following equation,
where Awi designates a target width variable at an ith rolling stand, Oiw designates a reference stress value for the ith rolling stand, and Kl, K2 and K3 designate respective coefficients.
The tension control unit may calculate an error compensating value to compensate for an error between the measured width value and the target width value through a feedback control, and then may reset the reference stress value using the error compensating value.
The. tension control unit may calculate a temperature compensation stress value using the following equation,
where T designates the measured -temperature value, oiT
designates the temperature compensation stress value to compensate for a change in width of the steel sheet that varies depending on the measured temperature value, and K4, K5 and K6 designate respective coefficients, and then may compensat-e for the reference stress value based on the temperature compensation stress value. The tension control unit may calculate a ratio of stress to

the plurality of rolling stands using the measured-stress value, and may calculate a distribution compensation stress value required for the plurality of rolling stands to maintain a preset stress ratio.
The tension control unit may calculate a stress change amount required for each of the plurality of rolling stands using the following equation,
where Ci designates a preset stress ratio for the ith rolling stand, Oi designates a measured stress value at the ith rolling stand, and Aoi designates a stress change amount for the ith rolling stand, and then may derive the distribution compensation stress value using the stress change amount.
The tension control unit may calculate the final target stress value using the following equation.

mr^S^fmf I*^Pl?%ftrfft
or
where Oi* designates a final target stress value for the ith rolling stand, OiW designates a reference stress value for the ith rolling stand, OiT designates a temperature compensation stress value compensating for a change in the width of the steel sheet that varies depending on the measured temperature value, oiC designates a distribution compensation stress value distributed on the ith rolling stand, and K designates a coefficient.

According to another exemplary embodiment in the present disclosure, a width controlling method for a multi-stand rolling mill, used to control a width of a steel sheet rolled by the multi-stand rolling mill by adjusting tension of a plurality of rolling stands included in the multi-stand rolling mill may include a feed-forward control step of calculating a reference stress value of the steel sheet corresponding to each of the plurality of rolling stands, using a target width variable set in each of the plurality of rolling stands; a temperature compensating step of measuring a surface temperature of the steel sheet passing through each of the rolling stands, and calculating a temperature compensation stress value to compensate for a change in width of the steel sheet that varies depending on the surface temperature of the steel sheet; a feedback control step of generating a measured width value by measuring the width of the steel sheet that has been rolled in the multi-stand rolling mill, calculating an error compensating value to compensate for an error between the measured width value and a target width value through a feedback control, and resetting the reference stress value using the error compensating value; a stress distribution step of generating a measured stress value by measuring the stress of the steel sheet located between the plurality of rolling stands, and calculating a distribution compensation stress value for the plurality of rolling stands using the measured stress value so that the plurality of rolling stands maintains a preset stress ratio; and a tension control step of calculating a final

target stress value using the reference stress value, the temperature compensation stress value and the distribution compensation stress ^^alue, and adjusting a magnitude of the tension applied by the rolling stand to the steel sheet depending on the final target stress value.
[Advantageous Effects]
According to exemplary embodiments in the present disclosure, the apparatus and method for controlling the width of the multi-stand rolling mill are capable of precisely and rapidly controlling the width of the steel sheet in response to a disturbance such as a change in temperature or width of the steel sheet.
[Brief Description Of Drawings]
FIG. 1 is a block diagram illustrating a width controlling apparatus of a multi-stand rolling mill according to an exemplary embodiment in the present disclosure;
FIGS. 2A and 2B are. graphs illustrating a stress value of a steel sheet in the multi-stand rolling mill according to the exemplary embodiment in the present disclosure; and
FIG. 3 is a flowchart illustrating a width controlling method of a multi-stand rolling mill according to an exemplary embodiment in the present disclosure.

[Detailed Description]
Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. Exemplary embodiments in the present invention will be provided to make the present invention be more completely understood by those skilled in the art. In the following description, if it is decided that the detailed description of known function or configuration related to the invention makes the subject matter of the invention unclear, the detailed description is omitted. Like reference numerals designate like elements throughout the specification.
In addition, it will be understood that, when an element is referred to as being connected to another element, they may be 'directly connected' to each other, and besides be 'indirectly connected' to each other with an intervening element disposed therebetween. Further, the meaning of 'including an element' does not exclude the possibility of further including another element unless otherwise stated.
FIG. 1 is a block diagram illustrating a width controlling "apparatus of a multi-stand rolling mill according to an exemplary embodiment in the present disclosure.
Referring to FIG.' 1, the width controlling apparatus of the multi-stand rolling mill according to the exemplary embodiment in the present disclosure may include temperature sensor units (tl to tn) , stress measuring units (si to sn) , a width measuring unit w, and a tension control unit 20.

Hereinafter, referring to FIG. 1, the width controlling apparatus of the multi-stand rolling mill according to the exemplary embodiment in the present disclosure will be described.
The temperature sensor units tl to tn may measure the surface temperature of the steel sheet b passing through a plurality of rolling stands-11 to ln+1, thus generating a measured temperature value. As shown in FIG. 1, the temperature sensor units tl to tn may be provided between the plurality of rolling stands 11 to ln+1, and may measure the surface temperature of the steel sheet b passing between the plurality of rolling stands 11 to ln+1, thus generating the measured temperature value. Here, each of the temperature sensors units tl to tn may use any member as long as it may measure the temperature of the steel sheet b. The measured temperature value measured by the temperature sensor units tl to tn may be input into the tension control unit 20.
The stress measuring units si to sn may measure the tension of the steel sheet b passing through the plurality of rolling stands 11 to ln+1, thus generating a measured stress value. As shown in FIG. 1, the stress measuring units si to sn may be provided between
the plurality of rolling stands 11 to ln+1, and may measure the
I
stress of the steel sheet b passing between the plurality of rolling stands 11 to ln+1, thus generating the measured stress value. Here, each of the stress measuring units si to sn may use any member as long as it may measure the stress of the steel sheet b. The measured stress value measured by the stress measuring units si to,sn may

be input into the tension control unit 20.
The width measuring unit w may measure the width of the steel sheet b passing through the plurality of rolling stands 11 to ln+1, thus generating a measured width value. Particularly, the width measuring unit w may measure a final width of the steel sheet b reduced in the multi-stand rolling mill 10, thus generating a final width value. As shown in FIG. 1, the width measuring unit w may be located at a final output end of the multi-stand rolling mill 10, and may measure the final width of the steel sheet b rolled in the multi-stand rolling mill 10. That is, it is possible to generate the final width value by measuring the final width of the steel sheet b, and the measured width value or the final width value may be input into the tension control unit 20. Here, the width measuring unit w may measure the width of the steel sheet b via an image processing unit such as a camera. In addition, any member may be used as the width measuring unit w as long as it may measure the width of the steel sheet b.
Here, the width controlling apparatus of the 'multi-stand rolling mill may include a measuring unit having at least one of the temperature sensor units tl to tn, the stress measuring units si to sn, and the width measuring unit w. That is, the measuring unit may generate at least one of the measured temperature value, the measured stress value and the measured width value corresponding to the surface temperature, the stress and the width of the steel sheet, by measuring the steel sheet passing through the plurality

of rolling stands.
The tension control unit 20 may adjust the tension for each of the rolling stands 11 to ln+1, thus allowing the width of the steel sheet b produced in the multi-stand rolling mill 10 to match a preset target width value. To be more specific, the tension control unit 20 may set a reference stress value corresponding to each of the rolling stands 11 to ln+1, and may compensate for the reference stress value depending on a change in temperature of the steel sheet b and a change in width of the steel sheet b, thus calculating a final target stress value . Subsequently, the tension control unit 20 may adjust the tension for each of the rolling stands 11 to ln+1, thus allowing the measured stress value to follow the final target stress value.
First, the tension control unit 20 may calculate the reference stress value corresponding to each of the plurality of rolling stands using a target width variable set for each of the rolling stands 11 to ln+1. Here, the target width variable may be set to correspond to the target width value for the steel sheet b. To be more specific, the tension control unit 20 may calculate the reference stress value using the following equation,
where AWi designates a target width variable at an ith rolling stand, oiw designates a reference stress value for the ith rolling stand, and Kl, K2 and K3 designate respective coefficients. Thus, the target width variable Aw and the reference-stress value ow may

be represented in the form of an exponential function, as shown in FIG. 2A.
Here, the target width variable AWi may be calculated using
I
the following equation, --^ *" ^". In this equation, Wi designates a width value of the steel sheet b measured or predicted at the ith rolling stand, and Wi* designates a target width value for the steel sheet b at the ith rolling stand. In this case, Wi may be directly measured by each width measuring unit additionally provided between the rolling stands, or may be predicted from the final width value.
For the calculated reference stress value, the tension control unit 20 may perform the feedback control. To be more specific, the final width value corresponding to the final width of the steel sheet b rolled in the multi-stand rolling mill 10 may be fed back, and an error between the final width value and the target width value for the steel sheet may be calculated. Here, if the error is not zero, it is possible to calculate an error compensating value at which the error becomes zero using a feedback control algorithm, and. besides, it is possible to reset the reference stress value using the error compensating value. For example, a stress change amount required to make the error be zero may be calculated as the error compensating value. Here, the calculated error compensating value may be divided to the respective rolling stands through a preset weighting coefficient or predictive function, and then the

reference stress value may be reset by adding or subtracting the divided error compensating value to or from each reference stress value. Alternatively, a change value of the target width variable may be calculated based on the error compensating value. That is, it is possible to reset the target width variable by adding or subtracting the target width variable to or from the change value, and then it is possible to reset the reference stress value corresponding to the reset target width variable. Thus, the tension control unit 20 may perform control such that the final width value of the steel sheet b follows the target width value through the feedback control for the final width value.
Further, the tension control unit 20 may compensate for the
reference stress value, in view of a change in width of the steel
sheet b depending on a temperature. That is, since the width of
the steel sheet b may vary depending on the temperature, the
reference stress value may be compensated for depending on the
temperature of the steel sheet b. To be more specific, the tension
control unit 20 may calculate a temperature compensation stress
value using the following
equation, ^ -™ ^..«- j^-^' ■■^■■Cit-uA t^ g^^^ ^^y compensate for
the reference stress value using the temperature compensation stress value. In this equation, T designates the measured
temperature value, oiT designates the temperature compensation stress value that compensates for a change in width of the steel

sheet b that varies depending on the measured temperature value, and K4, K5 and K6 correspond to the respective coefficients. Therefore, the tension control unit 20 may compensate for the reference stress value using the temperature compensation stress value, thus coping with the change in width of the steel sheet b depending on the temperature of the steel sheet b. Here, the temperature of the steel sheet b and the temperature compensation stress value corresponding to the temperature of the steel sheet b may be represented in the form of the exponential function, as shown in FIG. 2B.
Moreover, in order to prevent the stress from being concentrated in a specific rolling stand, the stress ratio may be preset in each of the rolling stands 11 to ln+1 included in the multi-stand rolling mill 10. The stress ratio may be represented as the ratio of a stress applied to any one of the rolling stands, among the stress applied to the entire multi-stand rolling mill 10. But, when the rolling process for the steel sheet b proceeds, the stress may be undesirably concentrated in the specific rolling stand, differently from the preset stress ratio. In such a case, it is necessary to distribute the concentrated stress to each rolling stand. Thus, the tension control unit 20 may calculate a distribution compensation stress value required for the plurality of rolling stands 11 to ln+1 to maintain the preset stress ratio, using the measured stress value measured by the stress measuring units si to sn.
To be more specific, the tension control unit 20 may calculate

a stress change amount required for each of the plurality of rolling
stands using the following equation, -■ , and may derive
the distribution compensation stress value using the stress change
amount. Here, Ci designates a stress ratio of the ith rolling stand,
Oi designates a measured stress value at the ith rolling stand, and
Aoi designates a stress change amount for the ith rolling stand.
Here, the distribution compensation stress value may be derived
by applying a proportional-integral control algorithm to the stress
change amount.
The tension control unit 20 may compensate for the reference
stress value using at least one of the measured temperature value,
the measured stress value and the measured width value, and may
calculate the final target stress value by compensating for the
reference stress value. For example, it is possible to calculate
the final target stress value using the following equation,
4 " !& *Si Q„ ii s a ii Ea f? Here
oi* designates a final target stress value for the ith rolling stand, OiW designates a reference stress value for the ith rolling stand, oiT designates a temperature compensation stress value to compensate for a change in width of the steel sheet that varies depending ■ on the measured temperature value, oiC designates a distribution compensation stress value distributed to the ith rolling stand, and K designates a coefficient. As discussed above.

the temperature compensation stress value may be calculated using the measured temperature value, and the distribution compensation stress value may be calculated using the measured stress value. Further, since the reference stress value may be compensated for through the feedback control with the measured width value, the tension control unit 20 may compensate for the reference stress value and then calculate the final target stress value, using at least one of the measured temperature value, the measured stress value and the measured width value.
Thereafter, the tension control unit 20 may control the tension of the rolling stand by adjusting the speed of the rolling stand or using a device such as a lopper. Accordingly, it allows the plurality of rolling stands 11 to ln+1 to follow the final target stress value.
FIG. 3 is a flowchart illustrating a width controlling method of a multi-stand rolling mill according to an exemplary embodiment in the present disclosure.
Referring* to FIG. 3, the width controlling method of the multi-stand rolling mill according to the exemplary embodiment in the present disclosure may include a feed-forward control step SIO, a temperature compensating step S20, a feedback control step S30, a stress distribution step S40, and a tension control step S50.
Hereinafter, the width controlling method of the multi-stand rolling mill according to the exemplary embodiment in the present disclosure will be described with reference to FIG. 3.

At the feed-forward control step SIO,. a reference stress value of the steel sheet corresponding to each of the plurality of rolling stands may be calculated using a target width variable set in each of the plurality of rolling stands. Here, if the width of the steel sheet fed into the multi-stand rolling mill varies or there is a change in target width value of the steel sheet, it is possible to rapidly reset the reference stress value by correcting the target width variable, thus enabling a rapid response. To be more specific, at the feed-forward control step SIO, the reference stress value may be calculated using the following
W
ition, ~ - ^^ ^ ^ '-'. In this equation, Awi
. 1 ■■■
equat
designates a target width variable at an ith rolling stand, oiw
designates a reference stress value for the ith rolling stand, and
Kl, K2 and K3 designate respective coefficients.
Subsequently, at the temperature compensating step S20, a
surface temperature of the steel sheet passing through each of the
rolling stands may be measured, and a temperature compensation
stress value for compensating a change in width of the steel sheet
that varies depending on the surface temperature of the steel sheet
may be calculated. Since the width of the steel sheet is variable
depending on the surface temperature of the steel sheet, the
temperature compensation stress value may be calculated and then
used to cope with the change in width of the steel sheet depending
on the surface temperature of the steel sheet. To be more specific,

at the temperature compensating step S20, the temperature compensation stress value may be calculated using the following
equation, ^^ ■ ^ i^^ ,a U^J %^i >^ ^ ^^(^ the reference
stress value may be compensated for using the temperature compensation stress value. In this equation, T designates the measured temperature value, oil designates the temperature compensation stress value, and K4, K5 and K6 correspond to the respective coefficients.
At the feedback control step S30, a measured width value may be measured by measuring the width of the steel sheet that has been rolled in the multi-stand rolling mill, and an error compensating value to compensate for an error between the measured width value and a target width value through a feedback control may be calculated, and the reference stress value may be reset using the error compensating value. To be more specific, the error between the final width value and the target width value for the steel sheet may be calculated by feeding back the final width value. Here, it is possible to calculate the error compensating value at which the error becomes zero, and it is possible to reset the reference stress value using the error compensating value. For example, a variation in target width variable or a stress change amount required to make the error be zero may be calculated as the error compensating value.
At the stress distribution step 40, a measured stress value may be generated by measuring the stress of the steel sheet located

between the plurality of rolling stands. Subsequently, it is possible to calculate a distribution compensation stress value so that the plurality of rolling stands maintains a preset stress ratio, using the measured stress value. That is, in order to prevent the stress from being concentrated in a specific rolling stand, the stress ratio may be preset for each of the rolling stands. A stress change amount that should be distributed to maintain the stress ratio may be calculated for each of the plurality of rolling stands. To be more specific, the stress change amount may be calculated
using the following equation, ' , and the distribution compensation stress value may be derived using the stress change
amount. Here, Ci designates a stress ratio of the ith rolling stand, oi designates a measured stress value at the ith rolling stand, and Aoi designates a stress change amount for the ith rolling stand. Subsequently, at the stress distribution step S40, the distribution compensation stress value may be derived by applying the proportional-integral control algorithm to the stress change amount.
At the tension control step S50, a final target stress value may be calculated using the reference stress value, the temperature compensation stress value and the distribution compensation stress value, and it is possible to adjust a magnitude of the tension applied by the rolling stand to the steel sheet depending on the final target stress value. For example, at the tension control step

S50, the final target stress value may be calculated using the


€£^1: *™",
following equation, ^^ -"^ "^ *■■-'■ or
■' "- . Here, Oi* designates a final target
stress value for the ith rolling stand, oiW designates a reference stress value for the ith rolling stand, OiT designates a temperature compensation stress value to compensate for a change in width of the steel, sheet that varies depending on the measured temperature value, OiC designates a distribution compensation stress value distributed to the ith rolling stand, and K designates a coefficient Subsequently, at the tension control step S50, the speed adjustment of the rolling stand or the tension control of the rolling stand using a device such as a looper allows the plurality of rolling stands to follow the final target stress value.
While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims.
[Description of Reference Numbers] 10 : multi-stand rolling mill 20 : tension control unit SIO : FEED-FORWARD S20 : TEMPERATURE COMPENSATING

330 : FEEDBACK CONTROL S40 : STRESS DISTRIBUTION S50 : TENSION CONTROL

WE CLAIMS:-
A width controlling apparatus for a multi-stand rolling mill, used to. control a width of a steel sheet rolled by the multi-stand rolling mill by adjusting tension of a plurality of rolling stands included in the multi-stand rolling mill, the width controlling apparatus comprising:
a measuring unit measuring the steel sheet passing through the plurality of rolling stands, thus generating at least one of a measured temperature value, a measured stress value and a measured width value corresponding to a surface temperature, a stress and a width of the steel sheet; and
a tension control unit calculating a reference stress value corresponding to each of the plurality of rolling stands using target width variables that are set for the plurality of rolling stands, respectively, and calculating a final target stress value by compensating for the reference stress value using any one of the measured temperature value, the measured stress value and the measured width value, thus adjusting the tension of the rolling stand depending on the final target stress value.
[Claim 2]
The width controlling apparatus according to claim 1, wherein the measuring unit comprises:

a temperature sensor unit measuring the surface temperature of the steel sheet passing through the plurality of rolling stands, thus generating the measured temperature value;
a stress measuring unit measuring the stress of the steel sheet passing through the plurality of rolling stands, thus generating the measured stress value; and
a width measuring unit measuring the width of the steel sheet passing through the plurality of rolling stands, thus- generating the'measured width value.
[claim 3]
The width controlling apparatus according to claim 1, wherein the tension control unit calculates the reference stress value using the following equation,
where AWi designates a target width variable at an i'^*' rolling stand, 0 i" designates a reference stress value for the i*^"^ rolling stand, and Ki, Ka and K3 designate respective coefficients.
[Claim 4l
The width controlling apparatus according to claim 1, wherein the tension control unit calculates an error compensating value to compensate for an error between the measured width value and the target width value through a feedback control, and then resets the reference stress value using the error compensating value.

[claim 5]
The width controlling apparatus according to claim 1, wherein the tension control unit calculates a temperature compensation stress value using the following equation,
where T designates the measured temperature value, Oi''' designates the temperature compensation stress value to compensate for a change in width of the steel sheet that varies depending on the measured temperature value, and K4, K5 and Ke designate respective coefficients, and then compensates for the reference stress value based on the temperature compensation stress value.
[claim 6]
The width controlling apparatus according to claim 1, wherein the tension control unit calculates a ratio of the stress to the plurality of rolling stands using the measured stress value, and calculates a distribution compensation stress value required for the plurality of rolling stands to maintain a preset stress ratio.
[Claim 7]
The width controlling apparatus according to claim 6, wherein the tension control unit calculates a stress change amount required for each of the plurality of rolling stands using the following equation.

where Ci designates a preset stress ratio for the i*^*^ rolling stand, a± designates a measured stress value at the i*^^ rolling stand, and AOi designates a stress change amount for the i*^*^ rolling stand, and then derives the distribution compensation stress value using the stress change amount.
[claim 8l
The width controlling apparatus according to claim 1, wherein the tension control unit calculates the final target stress value using the following equation.
*^-^ tt-'tsfl 35^1
where oi* designates a final target stress value for the i'^*' rolling stand, oi" designates a reference s'tress value for the i*^*^ rolling stand, Oi'^ designates a temperature compensation stress value compensating for a change in the width of the steel sheet that varies depending on the measured temperature value, oi^ designates a distribution compensation stress value distributed on the i'*^ rolling stand, and K designates a coefficient.
[claim 9]
A width controlling method for a multi-stand rolling mill, used to control a width of a steel sheet rolled by the multi-stand

rolling mill by adjusting tension of a plurality of rolling stands included in the multi-stand rolling mill, the width controlling method comprising:
a feed-forward control step of calculating a reference stress value of the steel sheet corresponding to each of the plurality of rolling stands, using a target width variable set in each of the plurality of rolling stands; .
a temperature compensating step of measuring a surface temperature of the steel sheet passing through each of the rolling stands, and calculating a temperature compensation stress value to compensate for a change in width of the steel sheet that varies depending on the surface temperature of the steel sheet;
a feedback control step of generating a measured width value by measuring the width of the steel sheet that has been rolled in the multi-stand rolling mill, calculating an error compensating value to compensate for an error between the measured width value and a target width value through a feedback control, and resetting the reference stress value using the error compensating value;
a stress distribution step of generating a measured stress value by measuring the stress of the steel sheet located between the plurality of rolling stands, and calculating a distribution compensation stress value for the plurality of rolling stands using the measured stress value so that the plurality of rolling stands maintains a preset stress ratio; and
a tension control step of calculating a final target stress

value using the reference stress value, the temperature compensation stress value and the distribution compensation stress value, and .adjusting a magnitude of the tension applied by the rolling stand to the steel sheet depending on the final target stress value.