US-12618132-B2 - Method for manufacturing hot-dip galvanized steel sheet

Abstract

When a hot-dip galvanizing treatment is performed on a steel sheet containing Si in an amount of 0.2 mass % or more by using a continuous hot-dip galvanizing apparatus including an annealing furnace in which a heating zone, a soaking zone, and a cooling zone are arranged in this order, a snout adjacent to the cooling zone, and hot-dip galvanizing equipment, a humidified nitrogen-hydrogen gas mixture containing moisture in such a manner that a certain expression is satisfied is supplied into a region on the downstream side of the soaking zone, gas nozzles are arranged over the entire perimeter of an inner wall of the snout, nitrogen gas or a nitrogen-hydrogen gas mixture is supplied through the gas nozzles downward along the inner wall, and the dew point in the snout is controlled to be −50° C. to −35° C.

Inventors

• Gentaro TAKEDA
• Hideyuki Takahashi
• Mai AOYAMA
• Maiko Watanabe
• Tatsuya Ehashi

Assignees

• JFE STEEL CORPORATION

Dates

Publication Date

20260505

Application Date

20220609

Priority Date

20210714

Claims (2)

1. 1 . A method for manufacturing a hot-dip galvanized steel sheet, the method comprising performing a hot-dip galvanizing treatment on a steel sheet containing Si in an amount of 0.2 mass % or more by using a continuous hot-dip galvanizing apparatus including an annealing furnace in which a heating zone, a soaking zone, and a cooling zone are arranged in this order, a snout adjacent to the cooling zone, and hot-dip galvanizing equipment, wherein a humidified nitrogen-hydrogen gas mixture containing moisture in such a manner that expression (1) below is satisfied is supplied into a region on the downstream side of the soaking zone, wherein gas nozzles are arranged over the entire perimeter of an inner wall of the snout, wherein nitrogen gas or a nitrogen-hydrogen gas mixture is supplied through the gas nozzles downward along the inner wall, wherein at least two exhaust ports are disposed in an upper part of the snout to discharge the gas that is supplied through the gas nozzles, and wherein the dew point in the snout is controlled to be −50° C. to −35° C.: 1 ⁢ 58 < M / X < 178 , ( 1 ) where M denotes the amount of moisture contained in the humidified gas that is supplied into the soaking zone and X denotes a parameter regarding an influence on of a surface area of the steel sheet wherein M and X are calculated by the following equations below: M= 0.08074× Vh× 10 7.5Th/(Th+237.3) X= 0.2× w×S+ 0.4935 Vh: flow rate of the humidified gas that is supplied into the soaking zone (Nm3/hr) Th: dew point of the humidified gas that is supplied into the soaking zone (° C.) w: width of the steel sheet (m) S: sheet passing speed (m/s).
2. 2 . The method for manufacturing a hot-dip galvanized steel sheet according to claim 1 , wherein 70 volume % or more of the amount of the gas that is supplied through the gas nozzles is discharged through the exhaust ports in the upper part of the snout.

Description

CROSS REFERENCE TO RELATED APPLICATIONS This is the U.S. National Phase application of PCT/JP2022/023212, filed Jun. 9, 2022, which claims priority to Japanese Patent Application No. 2021-116033, filed Jul. 14, 2021, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes. FIELD OF THE INVENTION The present invention relates to a method for manufacturing a hot-dip galvanized steel sheet by using a continuous hot-dip galvanizing apparatus including an annealing furnace in which a heating zone, a soaking zone, and a cooling zone are arranged in this order, a snout adjacent to the cooling zone, and hot-dip galvanizing equipment. BACKGROUND OF THE INVENTION Nowadays, in the fields of, for example, automobiles, home electric appliances, and building materials, there is an increasing demand for a high-strength steel sheet (high-tensile strength steel material) which can be used for, for example, reducing the weight of structures. As examples of a high-tensile strength steel material, it is known that a steel sheet having good stretch flangeability can be obtained by adding Si to steel, and a steel sheet having good ductility can be obtained by adding Si, Al, and Mn to steel so that retained y tends to be formed. However, in the case where a hot-dip galvanized steel sheet is manufactured by using a high-strength steel sheet containing Si or Mn in a large amount (in particular, 0.2 mass % or more) as a base material, since Si or Mn in steel, which is an easily oxidizable element, is selectively oxidized even in a reducing atmosphere or a non-oxidizing atmosphere, which is used generally, Si or Mn is concentrated on the surface of the steel sheet to form oxides. Since such oxides cause a deterioration in wettability with molten zinc when a coating treatment is performed, bare spots occur. Therefore, there is a sharp deterioration in wettability due to an increase in the concentration of Si or Mn in steel, which results in frequent bare spot occurrence. In addition, even in the case where a bare spot does not occur, there is a problem of a deterioration in coating adhesiveness. Moreover, in the case where a hot-dip galvannealed steel sheet is manufactured, when Si or Mn in steel is selectively oxidized and concentrated on the surface of the steel sheet, since alloying is markedly delayed in an alloying process, which is performed after a hot-dip galvanizing process, there is also a problem of a marked deterioration in productivity. In response to such problems, Patent Literature 1 discloses a technique for inhibiting Si from being concentrated on the surface of a steel sheet by performing annealing to promote internal oxidation of Si in a continuous annealing and hot-dip coating method utilizing an annealing furnace having an anterior part of a heating zone, a posterior part of the heating zone, a heat-retaining zone, and a cooling zone in this order and a hot-dip coating bath, in which heating or heat-retaining is performed on the steel sheet at least in a steel sheet temperature range of 300° C. or higher by using an indirect heating method, in which the furnace atmosphere in each of the zones contains hydrogen in an amount of 1 volume % to 10 volume % and a balance of nitrogen and incidental impurities, in which the maximum end-point temperature of the steel sheet in the anterior part of the heating zone is 550° C. or higher and 750° C. or lower, in which the dew point of the anterior part of the heating zone is lower than-25° C., in which the dew point of the posterior part of the heating zone and the heat-retaining zone is −30° C. or higher and 0° C. or lower, and in which the dew point of the cooling zone is lower than −25° C. In addition, Patent Literature 1 also states that a humidified gas mixture of nitrogen and hydrogen is supplied into the posterior part of the heating zone and/or the heat-retaining zone. Patent Literature 2 discloses a technique for inhibiting Si from being concentrated on the surface of a steel sheet by measuring the dew point of the furnace gas of a reducing furnace and changing the supply and exhaust positions of the furnace gas in accordance with the measurement results so that the dew point of the furnace gas is higher than −30° C. and 0° C. or lower. Patent Literature 2 states that, although any one of a DFF (direct-fired furnace), a NOF (nonoxidizing furnace), and a radiant tube-type furnace may be used as a heating furnace, it is preferable that a radiant tube-type furnace be used because this markedly realizes the effects according to aspects of the invention. Patent Literature 3 discloses a method for achieving good slidability as a result of achieving uniform coating weight by controlling the dew point of an atmosphere gas in a snout to be within a predetermined range (preferably, −50° C. or lower) in accordance with the chemical composition of steel (the contents of Si and Al). Patent Literature 4 discloses a method fo