When a sheet of steel containing elements (metals) liable to oxidize more easily than iron is annealed in a annealing step (a reducing furnace) before galvanization, such elements may concentrate at the surfaces of the steel sheet to cause defective galvanization due to poor affinity with molten zinc later.
Known to cope with the above problem is a method of electroplating a steel sheet (basis sheer) with a ferrous metal before its hot-dip galvanization. This method is not practical, however, because electroplating apparatus has to be installed before hot-dip galvanizing equipment.
On the other hand, known is the oxidization/reduction method of heating, in advance, a steel sheet in an oxidizing gas to form ferrous oxide films on its surfaces and then reducing and hot-dip galvanizing the steel sheet (Patent documents 1 to 9).
For example, Patent document 1 discloses a version of the oxidization/reduction method of forming oxide films 400-10,000 Å thick on the surfaces of a steel sheet in a non-oxidizing furnace and then annealing the steel sheet in a reducing furnace. In the case of this version, however, the non-oxidizing furnace (NOF) is used as an oxidizing furnace and the effect is not stable.
Accordingly, disclosed in Patent documents 1 to 8, etc. are improved versions of the oxidization/reduction method. In these versions, methods of improving alloying characteristics are adopted. Namely, relatively thin oxide films are grown and reduced to form ferrous oxide films on the surfaces of a steel sheet for better alloying characteristics.
Disclosed in Patent document 9, for example, is a version of the oxidization/reduction method to stabilize the effect. In this case too, the concentrations of components of atmospheric gas are controlled to control the thickness of thin oxide films.
In many cases, the excess air ratio of burners and the concentrations of components of atmospheric gas are controlled in order to control the thickness of oxide films.
As for sheets of steel containing silicon, oxide films are grown on their surfaces and reduced to form ferrous layers by using the oxidization/reduction method for better galvanizing characteristics. As for sheets of steel of a high silicon content, iron will not oxidize easily and silicon will concentrate heavily in reduction. Accordingly, it is necessary to form thick oxide films in the step of oxidization of the oxidization/reduction method. This tendency and the necessity of thickening oxide films are salient in the case of steel containing silicon of 1.2 mass percent or more and more salient in the case of steel containing silicon of 1.8 mass percent or more. Besides this tendency and the necessity of thickening oxide films are salient, not only in the case of steel containing silicon but also in the case of steel containing elements which is liable to oxidize more easily than iron.
To increase the thickness of oxide films to be formed by the step of oxidization of the oxidization/reduction method, the line speed may be decreased to increase the time of oxidization in the oxidizing zone, which entails the increase of time of reduction. Thus, silicon concentrates heavily during the step of reduction disturbing the reduction of oxide films into proper ferrous layers.
To cope with the above problem, the reducing power of the reducing furnace is lowered to reduce thin oxide films appropriately. Lowering the reducing power requires the adjustment of concentrations of components of gas in the reducing furnace. The adjustment requires the replacement of reducing gas in the reducing furnace, which takes several tens of minutes. Therefore, lower in the reducing power of the reducing furnace is not practical for a galvanization line to galvanize various kinds of steel sheets.
If the oxidizing furnace is lengthened, the oxidizing time is elongated and thick oxide films can be formed without lowering the line speed. However, the galvanization line galvanizes sheets of steel containing no silicon, too. Besides, the balance of oxidization and reduction of the oxidization/reduction method is determined depending on the kinds of steel Therefore, if the oxidizing furnace is lengthened to oxidize sheets of steel containing silicon, its oxidizing power increases Accordingly, the oxidizing furnace has to be operated so that steel sheets will not oxidize easily, which lengthens the galvanizing line.    Patent document 1: Japanese Patent Laid-open No. 122865/1980    Patent document 2: Japanese Patent Laid-open No. 202360/1992    Patent document 3: Japanese Patent Laid-open No. 202361/1992    Patent document 4: Japanese Patent Laid-open No. 202362/1992    Patent document 5: Japanese Patent Laid-open No. 202363/1992    Patent document 6: Japanese Patent Laid-open No. 254531/1992    Patent document 7: Japanese Patent Laid-open No. 254532/1992    Patent document 8: Japanese Patent Laid-open No. 306561/1994    Patent document 9: Japanese Patent Laid-open No. 34210/1995