Method for cooling steel strip and cooling apparatus

1. A method for cooling a steel strip by mist cooling in a cooling installation of a galvannealing furnace configured to perform galvannealing treatment on a hot-dip galvanized steel strip, the method comprising:

by an adjusted cooling installation provided at an upstream side in a sheet-passing direction of the cooling installation, jetting mist to the steel strip passing through the cooling installation in a manner that an amount of mist jetted to the steel strip passing through the cooling installation is smaller in an edge portion in a width direction of the steel strip than in a center portion;

by a mist suction installation provided at least at a downstream side in the sheet-passing direction of the cooling installation, sucking at least part of mist jetted to the steel strip; and

cooling the steel strip at a sheet-passing speed such that, during a period between start and end of cooling of the steel strip, a temperature of the steel strip is within a film boiling temperature range and a temperature of the edge portion in the width direction of the steel strip is equal to or higher than a temperature of the center portion in at least a range of ⅔ or more from the upstream side in the sheet-passing direction of a total cooling length of the cooling installation.

2. The method for cooling a steel strip according to claim 1 , wherein, with respect to an installation length L [m] of the adjusted cooling installation, a speed of the steel strip is set to be equal to or less than an upper limit speed V max [m/s] calculated using a formula (a) below,

V max =( L ×( T in −β′){circumflex over ( )} m ×( T in −γ′))/(α′× th )  (a),

where T in [° C.] denotes a temperature of the center portion of the steel strip at an entrance of the cooling installation, th [m] denotes a thickness of the steel strip, and α′, β′, γ′, and m are constants set according to a hot-dip galvannealing installation.

3. The method for cooling a steel strip according to claim 2 , wherein the constants are set as follows: α′=1870000, β′=330, γ′=45, m=0.6.

4. A cooling installation by mist cooling of a galvannealing furnace configured to perform galvannealing treatment on a hot-dip galvanized steel strip, the cooling installation comprising:

an adjusted cooling installation provided at an upstream side in a sheet-passing direction of the cooling installation, the adjusted cooling installation being capable of adjusting, in a width direction of the steel strip, an amount of mist jetted to the steel strip passing through the cooling installation;

a mist suction installation provided at least at a downstream side in the sheet-passing direction of the cooling installation, the mist suction installation being configured to suck at least part of mist jetted to the steel strip, and

a control apparatus configured to control the adjusted cooling installation and the mist suction installation,

wherein the adjustment cooling installation includes, in the sheet-passing direction, a plurality of headers each including a plurality of mist jet nozzles arranged along the width direction,

the adjusted cooling installation is provided in a manner that an installation length L [m] of the adjusted cooling installation in the sheet-passing direction of the steel strip satisfies a formula (b) below,

the adjusted cooling installation is adjusted in a manner that an amount of mist jetted from the plurality of mist jet nozzles to the steel strip passing through the cooling installation is smaller in an edge portion in the width direction of the steel strip than in a center portion by stopping cooling of the edge portion or that the edge portion is sprayed with a smaller amount of water than the center portion is,

the control apparatus adjusts the water amount of the header by controlling opening and closing valves of the header including a plurality of mist jet nozzles,

the control apparatus controls the amount of mist sucked by the mist suction installations to adjust damper opening of exhaust blowers based on a pressure value is measured by a pressure gauge provided near a mist suction port, and

a control apparatus control such that, during a period between start and end of cooling of the steel strip, a temperature of the steel strip is within a film boiling temperature range and a temperature of the edge portion in the width direction of the steel strip is equal to or higher than a temperature of the center portion in at least a range of ⅔ or more from the upstream side in the sheet-passing direction of a total cooling length of the cooling installation

L ≥(α× V×th )/(( T in −β){circumflex over ( )} m )×( T in −γ))  (b)

where T in [° C.] denotes a temperature of the center portion of the steel strip at an entrance of the cooling installation, V └m/s┘ denotes a speed of the steel strip, th [m] denotes a thickness of the steel strip, and α, β, γ, and m are constants set according to a hot-dip galvannealing installation.

5. The cooling installation according to claim 4 , wherein α=1700000, β=330, γ=45, m=0.6.

6. The cooling installation according to claim 4 ,

wherein each header is configured in a manner that mist is not jetted to the steel strip in the edge portion in the width direction of the steel strip.

7. The cooling installation according to claim 6 , wherein each header of the adjusted cooling installation is configured in a manner that the number of the nozzles that jet mist to the steel strip in the center portion in the width direction of the steel strip increases from the upstream side toward the downstream side in the sheet-passing direction.

8. The cooling installation according to claim 5 ,

wherein each header is configured in a manner that mist is not jetted to the steel strip in the edge portion in the width direction of the steel strip.

9. The cooling installation according to claim 8 , wherein each header of the adjusted cooling installation is configured in a manner that the number of the nozzles that jet mist to the steel strip in the center portion in the width direction of the steel strip increases from the upstream side toward the downstream side in the sheet-passing direction.