Abstract: Flatness of a hot-rolled steel sheet in a coil is improved when the hot-rolled steel sheet is wound with a mandrel in a hot-rolling process to manufacture a coil. The mandrel has a protruding shape with a center portion in an axial direction protruding from both end portions when seen from a lateral side in the axial direction. Regarding a peripheral length difference, which is a difference between a peripheral length of the center portion of the mandrel and a peripheral length at a predetermined distance from the center portion, the ratio of the peripheral length difference to the peripheral length of the center portion is preferably 0.0002 to 0.012. The protruding shape may be a trapezoidal shape or a polynomial function shape.

Abstract: Flatness of a hot-rolled steel sheet in a coil is improved when the hot-rolled steel sheet is wound with a mandrel in a hot-rolling process to manufacture a coil. The mandrel has a protruding shape with a center portion in an axial direction protruding from both end portions when seen from a lateral side in the axial direction. Regarding a peripheral length difference, which is a difference between a peripheral length of the center portion of the mandrel and a peripheral length at a predetermined distance from the center portion, the ratio of the peripheral length difference to the peripheral length of the center portion is preferably 0.0002 to 0.012. The protruding shape may be a trapezoidal shape or a polynomial function shape.

Abstract: The present invention provides a method to control the flatness of a metal sheet or plate by effectively preventing waviness from occurring at the edge portions of the sheet or plate when it is cooled to the room temperature after completing hot rolling, and an apparatus to carry out the method: and relates to a method to control the flatness of a metal sheet or plate by homogenizing the surface temperature distribution of the metal sheet or plate through; measuring the surface temperatures of the metal sheet or plate at the edge portions and the center portion across its width between two rolling stands of a tandem finishing mill, or at the entry to and/or the exit from a reversing finishing mill, or after completing hot rolling, or after hot leveling; controlling the heating temperatures of the edge portions and/or the center portion based on the measured surface temperatures; and then cooling the metal sheet or plate after completing the finishing rolling.

Abstract: A continuous hot rolling method including the steps of: cutting the tail portion of a preceding material in a steel product and the front portion of a succeeding material in the steel product to form butt faces; previously determining the thickness of scale created on the butt faces to determine the proportion of the weight of the scale; calculating the amount of oxygen dissolved in a weld molten zone, determining a deoxidation capacity of a deoxidizer for deoxidizing the above amount of oxygen; allowing the butt faces to abut against each other to form a butt area; applying a laser beam to the butt area to form a weld molten zone; incorporating the deoxidizer having the above deoxidation capacity into the weld molten zone; and hot rolling the molten joined steel product.