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;
• and
• a control device which controls the electromagnet,
• wherein the control device,
• (A) sets 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) measures 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) calculates 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) repeats (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) measures 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) calculates 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) adjusts 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.