A steel-strip production method for producing a hot-dip-plated steel strip and a cold-rolled steel strip, the method being executed by a production apparatus including a continuous annealing furnace, a snout connected to the continuous annealing furnace, a contact-type seal plate device, a noncontact-type seal roll device, a hot-dip-plating tank being movable; and a roll configured to turn the path direction of the steel strip after passing through the snout, wherein a hot-dip-plated steel strip production unit configured to produce the hot-dip-plated steel strip by bringing the steel strip continuously annealed in the continuous annealing furnace into the hot-dip-plating tank; and a cold-rolled steel strip production unit configured to produce the cold-rolled steel strip by transferring the steel strip continuously annealed in the continuous annealing furnace without causing the steel strip to pass through the hot-dip-galvanizing tank, are configured to be switchable with one another.

A jet-type direct-fired preheating system, comprising a direct-fired furnace and a preheating furnace. The direct-fired furnace comprises a furnace casing, a direct-fired heating area being provided in the furnace casing. The preheating furnace comprises: a furnace body, wherein the upper portion of the furnace body is connected to the upper portion of the direct-fired furnace by means of a communicating pipe, the bottom of the furnace body is provided with a strip steel inlet, a sealing apparatus and a steering roller, an upper collection chamber of direct-fired waste gas and a secondary combustion chamber of direct-fired combustion waste gas are provided at the upper portion in the furnace body, and a lower collection chamber of the direct-fired waste gas is provided at the lower portion in the furnace body; and a plurality of heat exchange and jet bellows units, provided in the furnace body in the height direction of the furnace body, a threading channel being formed in the middle. Each heat exchange and jet bellows unit comprises: a bellows body, a heat exchange pipe being provided in the bellows body, and a nozzle being provided on the side surface opposite to the threading channel; a secondary waste gas mixing chamber provided between the bellows bodies; and a circulating fan, an inlet pipeline port being formed in the threading channel, and an outlet pipeline port being located in the bellows body. According to the present invention, strip steel can be quickly preheated to 350 or above and quickly heated to 750 or above; and waste heat of the waste gas is fully utilized, such that an over-thick oxide layer can be prevented from being generated on the surface of the strip steel.