Steel sheet shape control method and steel sheet shape control apparatus

A steel sheet shape control apparatus which is provided in a continuous hot-dip metal coating apparatus including a wiping nozzle disposed to be opposite to a steel sheet lifted from a coating bath, and which controls a shape in a transverse direction of the steel sheet by applying an electromagnetic force in a through-thickness direction with respect to the steel sheet, the apparatus comprising:

• a plurality of pairs of electromagnets which are disposed along the transverse direction in both sides in the through-thickness direction of the steel sheet above the wiping nozzle;
• a control device which controls the electromagnet;
• rolls which are provided in the coating bath and which includes at least a pair of support rolls between which a steel sheet moves and which contact the steel sheet conveyed to a vertical upper side;
• and
• a plurality of pairs of second sensors which are disposed along the transverse direction in both sides in the through-thickness direction of the steel sheet at the position of the electromagnet, and measure the position in the through-thickness direction of the steel sheet,
• wherein the control device is configured to:
• (A) set a target correction shape in the transverse direction of the steel sheet at a position of the electromagnet to a curved shape by performing a first numerical analysis based on a passing condition of the steel sheet,
• (B) measure the shape in the transverse direction of the steel sheet at a predetermined position between the wiping nozzle and the electromagnet or measures coating amount of the hot-dip metal with respect to the steel sheet at the subsequent stage of the electromagnet position when the steel sheet is conveyed in a state where the electromagnetic force is applied to the steel sheet by the electromagnet so that the shape in the transverse direction of the steel sheet at the position of the electromagnet is the curved shape set in (A),
• (C) calculate the shape in the transverse direction of the steel sheet at the position of the wiping nozzle based on the shape or the coating amount measured in (B),
• (D) repeat (B) and (C) by adjusting the target correction shape to a curved shape having an amount of warp different from the curved shape set in (A) by performing the first numerical analysis when the amount of warp of the shape calculated in (C) is equal to or more than a first upper limit value,
• (E) measure vibration in the through-thickness direction of the steel sheet at the predetermined position when the amount of warp of the shape calculated in (C) is less than the first upper limit value,
• (F) calculate vibration in the through-thickness direction of the steel sheet at the position of the wiping nozzle by performing a second numerical analysis based on the vibration measured in (E), and
• (G) adjust a control gain of the electromagnet by performing the second numerical analysis to make amplitude of the vibration calculated in (F) be less than a second upper limit value when the amplitude is equal to or more than the second upper limit value,
• wherein the control device is configured to,
• when the target correction shape is set in (A),
• (A1) measure the position in the through-thickness direction of the steel sheet at the position of the electromagnet by the second sensor when the steel sheet is conveyed in a state where the electromagnetic force is not applied by the electromagnet,
• (A2) calculate a warp shape in the transverse direction of the steel sheet at the position of the electromagnet in the state where the electromagnetic force is not applied by the electromagnet, based on the position measured in (A1), and
• (A3) set the target correction shape to a curved shape which is symmetrical in the through-thickness direction to the warp shape calculated in (A2), and
• wherein in (A) and (D), the control device is configured to adjust a pushing-in amount of the steel sheet by the pair of support rolls so that the amount of warp of the shape in the transverse direction of the steel sheet at the position of the electromagnet, in the state where the electromagnetic force is applied, is 2.0 mm or more.