Method of producing galvannealed steel sheet

Abstract

The present invention provides a method of producing a galvannealed steel sheet with which plating adhesion is high and a favorable plated appearance can be obtained even when galvannealing is performed on a steel strip comprising 0.2 mass% Si or more and with which reduction of tensile strength can be limited by lowering alloying temperature. In the present invention, dry gas and a mixed gas of humidified gas with dry gas are supplied to the soaking zone of an annealing furnace. By supplying the mixed gas at suitable times from a position in the lower half of the soaking zone while supplying the dry gas at suitable times from near an upper hearth roll of the soaking zone and discharging gas in the furnace at suitable times from a gas discharge port located above the upper hearth roll, the dew point of at least the uppermost area in the soaking zone is controlled to -20 DEG C to 0 DEG C.

Description

technical field [0001] The present invention relates to a method for manufacturing alloyed hot-dip galvanized steel sheets using a continuous hot-dip galvanizing device. The continuous hot-dip galvanizing device has: an annealing furnace in which a heating zone, a soaking zone and a cooling zone are arranged in sequence; and A hot-dip galvanizing facility adjacent to the cooling zone; and an alloying facility adjacent to the hot-dip galvanizing facility. Background technique [0002] In recent years, in the fields of automobiles, home appliances, building materials, etc., the demand for high-strength steel sheets (high-strength steel materials) that contributes to weight reduction of structures is increasing. It can be seen that as high-strength steel materials, for example, a steel sheet having good hole expandability due to the inclusion of Si in the steel, and a steel sheet having excellent toughness due to the easy formation of retained austenite due to the inclusion of ...

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Problems solved by technology

Moreover, in addition to changes in the amount of moisture brought into the furnace due to changes in the outside air temperature and the type of steel plate, the dew point of the mixed gas is also prone to change due to changes in the outside air temperature, and it is difficult to stably control it within the optimum dew point range.

In this way, since the dew point fluctuates greatly, even in the above dew point range and temperature range, surface defects such as missing plating will occur, making it difficult to manufacture stable products.

[0015] In the method described in Patent Document 3, if DFF is used in the heating furnace, oxidation of the surface of the steel sheet will occur. However, since the humidified gas is not actively supplied to the annealing furnace, it is difficult to achieve a high dew point even within the control range. The dew point can be stably controlled in the area of ​​-20~0℃

In addition, assuming that when the dew point rises, the dew point at the upper part of the furnace tends to increase, and when the dew point meter at the lower part of the furnace measures 0 °C, there may be a high dew point atmosphere of +10 °C or higher at the upper part of the furnace. Long-term operation will cause adhesion defects

[0016] In the method described in Patent Document 4, although the surface concentration of Si, Mn, etc. can be suppressed, and the wettability of hot-dip galvanizing can be increased, the alloying reaction of iron and zinc is delayed due to solid solution elements, so In order to form a predetermined degree of alloying, it is necessary to increase the alloying temperature too high, and it is difficult to achieve the coexistence with the mechanical properties of the material

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