Alloyed hot-dip galvanized steel sheet manufacturing method

Abstract

The present invention provides a method for producing a galvannealed steel sheet capable of obtaining a galvanized steel sheet with high coating adhesion even when galvanized steel is applied to a steel strip containing 0.2% by mass or more of Si. The appearance of the coating is relatively good, and the reduction of the tensile strength can be suppressed by lowering the alloying temperature. In the present invention, the mixed gas of the humidified gas and the dry gas and the dry gas are supplied to the soaking zone of the annealing furnace. The mixed gas is timely supplied from the position in the lower half of the soaking zone, on the other hand, the dry gas is timely supplied from the vicinity of the upper hearth roll in the soaking zone, and the gas exhaust is located higher than the upper hearth roll. The outlet timely discharges the gas in the furnace, thereby controlling the dew point of at least the uppermost part in the soaking zone to be -20°C or more and 0°C or less.

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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