Method of continuous annealing/hot-dipping of steel sheet containing silicon and apparatus for continuous annealing/hot-dipping

Abstract

A method of continuous annealing / hot-dipping using a hot-dipping apparatus having an annealing furnace in which a silicon-containing steel sheet is hot-dipped. In this method, the silicon contained in the steel is caused to undergo internal oxidation without undergoing surface oxidation to thereby avoid a decrease in deposit adhesion to the steel and a delay in galvannealing. Also provided is theapparatus for use in this method. The method of continuous annealing / hot-dipping employs an annealing furnace having a former heating zone, latter heating zone, heat retention zone, and cooling zone in this order and a hot-dipping bath. It comprises conducting annealing under the following conditions. In regions where the steel sheet has a temperature of at least 300 DEG C, the steel sheet is heated or kept hot by means of indirect heating. The atmosphere inside the furnace in each zone is an atmosphere consisting of 1-10 vol.% hydrogen and, as the remainder, nitrogen and unavoidable impurities. In the former heating zone, the steel is heated to a maximum temperature of 550-750 DEG C and the atmosphere is regulated so as to have a dew point lower than -25 DEG C. In the subsequent latter heating zone and heat retention zone, the dew point is regulated to from -30 DEG C to 0 DEG C. In the cooling zone, the dew point is regulated to below -25 DEG C.

Description

technical field [0001] The invention relates to a continuous annealing hot-dip coating method and a continuous annealing hot-dip coating device for Si-containing steel sheets. [0002] In addition, the hot-dip plating of the present invention does not specifically specify the type of plating metal, but includes hot-dip plating of zinc, aluminum, tin, and other metals or their alloys. Background technique [0003] In the case of hot-dip plating of metals such as zinc, aluminum, tin, or their alloys on the steel plate, the surface of the steel plate is usually degreased and cleaned, and the steel plate is annealed in an annealing furnace and the surface of the steel plate is activated by hydrogen reduction. After cooling to the specified temperature, the coating is performed by dipping in a hot-dip coating tank. The problem with this method is that when the steel sheet contains easily oxidizable metals such as Si and Mn, these easily oxidizable elements form individual or com...

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Problems solved by technology

The problem with this method is that when the steel sheet contains easily oxidizable metals such as Si and Mn, these easily oxidizable elements form individual or composite oxides on the surface of the steel sheet during annealing, thereby hindering the coating performance and inducing Defects that cannot be plated, or in the case of alloying treatment by reheating after plating, lead to a decrease in the alloying rate

However, the atmosphere conditions in the annealing furnace for internal oxidation of Si and Mn are also conditions for the surface oxidation of the steel sheet substrate to occur in a region where the temperature of the steel sheet is relatively low. It leads to the occurrence of roller defects in the furnace caused by the oxide film on the surface of the steel plate substrate formed in the low temperature zone, which requires a lot of effort in the control of the atmosphere in the industry

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