Method for manufacturing steel sheet

A method for manufacturing a steel sheet, comprising:

• a hot-rolling process in which a steel material is hot-rolled using a finishing mill so as to obtain a hot-rolled steel sheet having an edge wave shape with a wave height periodically changing in a rolling direction;
• and
• a cooling process in which the hot-rolled steel sheet is cooled in a cooling section provided on a sheet-threading path,
• wherein the hot-rolling process includes:
• a target steepness-setting process in which a target steepness of the edge wave shape is set based on first correlation data indicating a correlation between the steepness of the edge wave shape of the hot-rolled steel sheet and a temperature standard deviation Y of temperatures measured across a length of the hot-rolled steel sheet;
• and
• a shape-controlling process in which operation parameters of the finishing mill are controlled so as to match the steepness of the edge wave shape with the target steepness,
• wherein the cooling process includes:
• a target ratio-setting process in which a heat transfer coefficient ratio X1 is set as a target ratio Xt based on second correlation data indicating a correlation between a top and bottom heat transfer coefficient ratio X and the temperature standard deviation Y;
• and
• a cooling control process in which at least one of an amount of heat dissipated from a top surface by cooling and an amount of heat dissipated from a bottom surface by cooling of the hot-rolled steel sheet in the cooling section is controlled so that heat transfer coefficient ratio X matches the target ratio Xt,
• wherein the method further comprising:
• a temperature-measuring process in which a temperature of the hot-rolled steel sheet is measured in chronological order on a downstream side of the cooling section;
• a changing speed-measuring process in which a changing speed of the hot-rolled steel sheet in a vertical direction is measured in chronological order at a same place as a temperature measurement place of the hot-rolled steel sheet on the downstream side of the cooling section;
• a control direction-determining process in which, when an upward side of the vertical direction of the hot-rolled steel sheet is set as positive, in an area with a positive changing speed, in a case in which a temperature of the hot-rolled steel sheet is lower than an average temperature in a range of one or more cycles of a wave shape of the hot-rolled steel sheet, at least one of a direction in which the amount of heat dissipated from the top surface by cooling decreases and a direction in which the amount of heat dissipated from the bottom surface by cooling increases is determined as a control direction, in a case in which the temperature of the hot-rolled steel sheet is higher than the average temperature, at least one of a direction in which the amount of heat dissipated from the top surface by cooling increases and a direction in which the amount of heat dissipated from the bottom surface by cooling decreases is determined as the control direction,
• in an area with a negative changing speed, in a case in which the temperature of the hot-rolled steel sheet is lower than the average temperature, at least one of a direction in which the amount of heat dissipated from the top surface by cooling increases and a direction in which the amount of heat dissipated from the bottom surface by cooling decreases is determined as the control direction, and, in a case in which the temperature of the hot-rolled steel sheet is higher than the average temperature, at least one of a direction in which the amount of heat dissipated from the top surface by cooling decreases and a direction in which the amount of heat dissipated from the bottom surface by cooling increases is determined as the control direction;
• and
• an amount of heat dissipated by cooling-adjusting process in which at least one of the amount of heat dissipated from the top surface by cooling and the amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet in the cooling section is adjusted based on the control direction determined in the control direction-determining process,
• wherein:
• heat transfer coefficient X is a ratio of heat transfer coefficients of top and bottom surfaces of the hot-rolled steel sheet,
• temperature standard deviation Y is the temperature standard deviation during or after cooling of the hot-rolled steel sheet under conditions in which steepness and sheet-threading speed of the hot-rolled steel sheet are constant values, and
• heat transfer coefficient X1 is a top and bottom heat transfer coefficient ratio at which the temperature standard deviation Y becomes a minimum value Ymin.