IMMERSION MEMBER FOR HOT DIP GALVANIZING BATH AND METHOD FOR PREPARING THE SAME

Technical Field:

The present invention relates to an immersion member for hot dip galvanizing bath and a method for preparing the same.

More particularly, the present invention relates to an improvement in an improvement in corrosion-resistant and abrasion-resistant properties of a roll to be immersed in a hot dip galvanizing bath, such as a sink roll or a support roll, or a constituent member thereof and also to stabilization of the quality of such a member. The present invention realizes an improvement in the corrosion resistance and abrasion resistance of a sink roll, a support roll or a bearing part or the like thereof in a hot dip galvanizing bath containing up to 5.5% by weight of Al as a bath content and stabilization of the quality thereof.

Background Art:

Heretofore, a roll, a bearing part or the like to be used in a hot dip galvanizing bath has been produced by using a special stainless steel as its base material and flame-spraying a Co-based self-fusing alloy and a carbide cermet on said base material. However, the

material thus produced is poor in corrosion-resistant and abrasion-resistant properties and inferior in stability of quality, and it tends to often cause abnormal corrosion or peeling of the flame-sprayed layer during use. Accordingly, the conventional immersion member is defective in that its life is short, and especially in case of a sink roll, this defect is serious because the working conditions are severe so that it is eagerly desired to overcome this defect. Moreover, the use of this sink roll results in a further disadvantage of formation of flaws on a product obtained by hot dip galvanizing process.

Disclosure of Invention:

It is a general object of the present invention to eliminate the above-described defects of an immersion member to be used in a hot dip galvanizing bath.

It is a specific object of the present invention to provide an immersion member of the above kind which is superior in corrosion-resistant, abrasion-resistant and peeling-resistant properties and has a stable quality under severe conditions.

It is another specific object to provide a method for preparing an immersion member of the above kind in effective manner.

1. In accordance with the invention as defined in the claim, there is provided an immersion member for a hot dip galvanizing bath, which has a flame-sprayed

surface layer comprising 1.0 to 1.5% by weight of C, 2.0 to 4.0% by weight of B, 2.0 to 4.0% by weight of Si, 1.0 to 6.0% by weight of Fe, 10.0 to 16.0% by weight of , 5.0 to 21.0% by weight of Cr and 10.0 to 15.0% by weight of Ni, with the balance being Co.

2. In accordance with the invention as defined in the claim, there is provided an immersion member for a hot dip galvanizing bath according to claim 1, in which the flame-sprayed surface layer comprising 1.0 to 1.5% by weight of C, 2.0 to 4.0% by weight of B, 2.0 to 4.0% by weight of Si, 1.0 to 6.0% by weight of Fe, 10.0 to 16.0% by weight of W, 5.0 to 21.0% by weight of Cr and 10.0 to 15.0% by weight of Ni, with the balance being Co, is formed, and then, the flame-sprayed surface layer is heated at a temperature-elevating rate of 10 to 100°C/hr, is maintained at a pre-heating temperature of 300 to 600°C for at least 0.5 hour and is subjected to a fusing treatment by heating at a temperature of at least 1000°C for up to 30 minutes once or twice, the temperature is dropped, a soaking treatment is carried out at a temperature of 500 to 800°C for at least 1 hour and a cooling treatment is then carried out at a temperature-dropping rate of 10 to 50°C/hr.

3. In accordance with the invention as defined in the claim, there is provided a sink roll for a hot dip galvanizing bath, which has, on the surface thereof a groove having a shape characterized by a groove depth of 0.5 to 5.0 mm, a groove width of 5.0 to 10.0 mm, a

groove bottom radius of at least 5.0 mm and a groove shoulder radius of at least 3.0 mm, and has a flame-sprayed surface layer comprising 1.0 to 1.5% by weight of C, 2.0 to 4.0% by weight of ^' B, 2.0 to 4.0% by weight of Si, 1.0 to 6.0% by weight of Fe, 10.0 to 16.0% by weight of W, 5.0 to 21.0% by weight of Cr and 10.0 to 15.0% by weight of Ni, with the balance being Co.

4. In accordance with the invention as defined in the claim; there is provided a sink roll for a hot dip galvanizing bath, which has on the surface thereof a double-cross groove having a shape characterized by a groove depth of 0.5 to 5.0 mm, a groove width of 5.0 to 10.0 mm, a groove bottom radius of at least 5.0 mm, a groove shoulder radius of at least 3.0 mm and an angle of inclination of 0.3 to 5.0° to the circumferential axis of the groove, and has a flame-sprayed surface layer comprising 1.0 to 1.5% by weight of C, 2.0 to 4.0% by weight of B, 2.0 to 4.0% by weight of Si, 1.0 to 6.0% by weight of Fe, 10.0 to 16.0% by weight of W, 5.0 to 21.0 by weight of Cr and 10.0 to 15.0% by weight of Ni, with the balance being Co.

5. In accordance with the invention as defined in the claim, there is provided a sink roll for a hot dip galvanizing bath according to claim 3 or claim 4, in which the groove pitch is 20.0 - 60.0 mm.

6. In accordance with the invention as defined in the claim, there is provided a method for preparing

a sink roll for a hot dip galvanizing bath according to claim 3, said method comprising forming a flame-sprayed surface layer comprising 1.0 to 1.5% by weight of C,

2.0 to 4.0% by weight of B, 2.0 to 4.0% by weight of

Si, 1.0 to 6.0% by weight of Fe, 10.0 to 16.0% by weight of , 5.0 to 21.0% by weight of Cr and 10.0 to 15.0% by weight of Ni, with the balance being Co, on the surface of a roll having on the surface thereof a groove having a shape characterized by a groove depth of 0.5 to 5.0 mm, a groove width of 5.0 to 10.0 mm, a groove bottom radius of at least 5.0 mm and a groove shoulder radius of at least 3.0 mm, heating the flame-sprayed surface layer at a temperature-elevating rate of 10 to 100°C/hr, maintaining the flame-sprayed surface layer at a pre-heating temperature of 300 to 600°C for at least

0.5 hour, subjecting the flame-sprayed surface layer to a fusing treatment by heating at a temperature of at least 1000°C for up to 30 minutes once or twice, dropping the temperature, carrying out a soaking treatment at a temperature of 500 to 800°C for at least 1 hour and carrying out a cooling treatment at a temperature-dropping rate of 10 to 50°C/hr.

7. In accordance with the invention as defined in the claim, there is provided a method for preparing a sink roll for a hot dip galvanizing bath according to claim 4, said method comprising forming a flame-sprayed surface layer comprising 1.0 to 1.5% by weight of C, 2.0 to 4.0% by weight of B, 2.0 to 4.0% by weight of

Si, 1.0 to 6.0% by weight of Fe, 10.0 to 16.0% by weight of , 5.0 to 21.0% by weight of Cr and 10.0 to 15.0% by weight of Ni, with the balance being Co, on the surface of a roll having on the surface thereof a double-cross groove having a shape characterized by a groove depth of 0.5 to 5.0 mm, a groove width of 5.0 to 10.0 mm, a groove bottom radius of at least 5.0 mm, a groove shoulder radius of at least 3.0 mm and an angle of inclination of 0.3 to 5.0° to the circumferential axis of the groove, heating the flame-sprayed surface layer at a temperature-elevating rate of 10 to 100°C/hr, maintaining the flame-sprayed surface layer at a pre-heating temperature of 300 to 600°C for at least 0.5 hour, subjecting the flame-sprayed surface layer to a fusing treatment by heating at a temperature of at least 1000°C for up to 30 minutes once or twice, dropping the temperature, carrying out a soaking treatment at a temperature of 500 to 800°C for at least 1 hour and carrying out a cooling treatment at a temperature-dropping rate of 10 to 50°C/hr.

Now the descriptions will be given to the reasons for limitations of the composition of the flame-sprayed surface layer in the present invention as defined in claims 1 - 7. In order to complete the present invention, a testing apparatus as shown in Fig. 1 was used to perform tests on samples having a flame-sprayed surface layer of various compositions.

The reasons for limitations of the respective

compositions are as described below. C:

Carbon forms a carbide dispersed in the matrix of the flame-sprayed surface layer and is effective in improving the corrosion resistance and abrasion resistanc in a hot dip galvanizing bath. If the content of C is lower than 1.0% by weight, a carbide of Cr is mainly formed and the above-mentioned effect is not sufficient. If the content of C exceeds 1.5% by weight, the flame-sprayed surface layer is brittle and cracking or peeling is often caused. Accordingly, the content of C is limited to 1.0 to 1.5% by weight. B and Si:

Boron and silicon are components indispensable for imparting a self-fusing property, and they form a boride and a suicide dispersed in the matrix of the flame-sprayed surface layer and are effective in improving the corrosion resistance and abrasion resistance in a hot dip galvanizing bath. If the contents of B and Si are lower than 2.0% by weight, formation of the boride and suicide is insufficient, and if the contents of B and Si exceed 4.0% by weight, the flame-sprayed surface layer is brittle and cracking or peeling is often caused. Accordingly, the contents of B and Si are limited to 2.0 to 4.0% by weight. Fe:

Iron exerts an effect of stabilizing the matrix texture of the flame-sprayed surface layer. If the

content of Fe is lower than 1.0% by weight, the above-mentioned effect cannot be obtained since the content of C in the matrix is reduced because of formation of a carbide and a boride. If the content of Fe exceeds 6.0% by weight, the amount of the formed carbide increases, and the flame-sprayed surface layer is brittle and cracking or peeling is often caused. Accordingly, the content of Fe is limited to 1.0 to 6.0% by weight. W:

Tungsten forms a boride and a carbide in the flame-sprayed surface layer and exerts an effect of improving the corrosion resistance and abrasion resistance in a hot dip galvanizing bath. If the content of is lower than 10.0% by weight, the effect of improving the corrosion resistance by formation of the boride is reduced. If the content of exceeds 16.0% by weight, heat cracks are liable to be formed and cracking or peeling is caused during the use, though the effect of improving the corrosion resistance and abrasion resistance by formation of the boride and carbide is attained. Accordingly, the content of is limited to 10.0 to 16.0% by weight. Cr:

Chromium is an element forming the matrix and exerts an effect of reinforcing the matrix. If the content of Cr is lower than 5.0% by weight, the content of Cr in the matrix is reduced by formation of a carbide and the above-mentioned effect is not attained. If the

content of Cr exceeds 21.0% by weight, the corrosion resistance in a hot dip galvanizing bath is reduced. Accordingly, the content of Cr is limited to 5.0 to 21.0% by weight, preferably 17.0 to 21.0% by weight. Ni:

Nickel is an element forming the matrix and is effective in increasing the toughness of the matrix and improving the workability in the deposition treatment. If the content of Ni is lower than 10.0% by weight, the resistance to heat cracking on receipt of a thermal shock is insufficient. If the content of Ni exceeds 15.0% by weight, the corrosion resistance in a hot dip galvanizing bath is reduced, though the workability in the flame-spraying treatment is improved. Accordingly, the content of Ni is limited to 10.0 to 15.0% by weight.

According to the invention as defined in claim 2, there is provided an immersion member for a hot dip galvanizing bath according to claim 1, in which a flame-sprayed surface layer comprising 1.0 to 1.5% by weight of C, 2.0 to 4.0% by weight of B, 2.0 to 4.0% by weight of Si, 1.0 to 6.0% by weight of Fe, 10.0 to 16.0% by weight of , 5.0 to 21.0% by weight of Cr and 10.0 to 15.0% by weight of Ni, with the balance being Co, is formed, and then, the flame-sprayed surface layer is heated at a temperature-elevating rate of 10 to 100°C/hr, is maintained at a pre-heating temperature of 300 to 600°C for at least 0.5 hour and is subjected to a fusing treatment by heating at a temperature of

at least 1000°C once or twice, the temperature is dropped, a soaking treatment is carried out at a temperature of 500 to 800°C for at least 1 hour and a cooling treatment is then carried out at a temperature-dropping rate of 10 to 50°C/hr.

In the spraying of a self-fusing alloy, the fusing treatment is an important step determining the function of the flame-sprayed layer, and this treatment is usually accomplished by gas flame heating, heating in a furnace and high-frequency heating. However, in case of the roll for hot dip galvanizing bath, the gas flame heating is mainly adopted from the viewpoint of equipment, because the roll for a hot dip galvanizing bath has a diameter of 200 to 1000 mm and a length of 1200 to 2100 mm and has a hollow structure or solid structure. An oxygen-acetylene or oxygen-propane flame is used for the gas flame heating. Since the self-fusing alloy flame-sprayed layer has a porosity of 20 to 25% and the roll has large diameter and length, a large quantity of heat is necessary, and when a thick flame-sprayed layer having a thickness of 1 mm or more is formed, insufficient diffusion is often caused in the boundary between the base and the flame-sprayed layer. We have made research with a view to solving the problem of insufficient diffusion in the boundary between the base and the flame-sprayed layer in case of formation of a thick flame-sprayed layer having a thickness of 1 mm or more while taking it into consideration that

the self-fusing alloy flame-sprayed layer has 20 to 25% of pores and the heat conductivity is reduced by the presence of these pores, and we have found a method in which the fusing treatment is carried out by using an oxygen-propylene flame having a low maximum flame temperature and the flame-sprayed layer is stabilized without degradation of the surface layer portion of the base. An example of the heat cycle in the fusing treatment of this method is illustrated in Fig. 2. Fig. 2(a) shows an example in which the fusing treatment is carried out once and Fig. 2(b) shows an example in which the fusing treatment is carried out twice. Fig. 3 shows the surface texture of a sink roll for a hot dip galvanizing bath, which is obtained by carrying out the fusing treatment according to the heat cycle shown in Fig. 2(a) (Fig. 3(a)), the surface texture of a sink roll for a hot dip galvanizing bath, which is obtained by carrying out the fusing treatment according to the heat cycle shown in Fig. 2(b) (Fig. 3(b)), and the surface texture of a conventional sink roll for a hot dip galvanizing bath, which is obtained by carrying out the conventional fusing treatment using an oxygen-acetylene flame ( Fig. 3(c) ) .

As is apparent from the foregoing description, the present invention is characterized in that before the fusing treatment, the entire roll is maintained at 300 to 600°C where diffusion of alloy components of the base is not caused and the heat loss in the fusing

treatment is reduced, whereby a difference in the temperature between the surface portion of the flame-sprayed layer and the boundary portion close to the base is reduced and the quality of the flame-sprayed layer is uniformalized.

In case of a roll having large diameter and length, such as a sink roll, the heat loss is readily brought about and the difference in the texture is caused because of the difference in the temperature between the upper portion of the treated layer and the boundary portion close to the base. In order to avoid this disadvantage, it is preferred that the fusing treatment be carried out twice. According to this embodiment, in order to solve the problem of insufficient diffusion in the boundary between the flame-sprayed layer and the base in formation of a thick flame-sprayed layer, the first fusing treatment is carried out at a temperature lower by 20 to 30 ^C C than the appropriate treatment temperature to melt only the surface layer portion, and after the surface layer portion is brought into a state of a solid solution and the heat conductivity becomes substantially equal to the level of austenitic stainless steel, the second fusing treatment is carried out at an appropriate treatment temperature, whereby insufficient diffusion and unevenness of the grain size in the boundary portion close to the base and the difference in the texture between the surface portion and the boundary portion close to the base are eliminated.

When sink roll for a hot dip galvanizing bath, which are obtained by forming a flame-sprayed layer having a thickness of 2.5 mm and then, carrying out the machining operation so that the thickness of the flame-sprayed layer is at least 1 mm, are compared with respect to the surface texture, it is seen that, as is apparent from Fig. 3, the porosity is low in the roll obtained by carrying out the fusing treatment according to the present invention (Fig. 3(a)) and in the roll obtained by carrying out the fusing treatment twice, where the quantity of heat is large, the porosity is further reduced and the boride grows in the form of rods (Fig. 3(b)). As compared thereto in the roll obtained by carrying out the fusing treatment by using an oxygen-acetylene flame, the porosity is high, the growth of the boride is small, the quantity of heat is small and the quantity of heat used for diffusion in the boundary portion close to the base is further reduced (Fig. 3(c)).

In order to finely divide coarse crystal grains in the boundary diffusion layer portion of the flame-sprayed layer, the temperature in the fusing treatment after the second fusing treatment can be set at a lower level.

In accordance with the invention as defined in claim 3, there is provided a sink roll for a hot dip galvanizing bath, which has on the surface thereof a groove having a shape characterized by a groove depth of 0.5 to 5.0 mm, a groove width of 5.0 to 10.0 mm, a

groove bottom radius of at least 5.0 mm and a groove shoulder radius of at least 3.0 mm, and has a flame-sprayed surface layer comprising 1.0 to 1.5% by weight of C, 2.0 to 4.0% by weight of B, 2.0 to 4.0% by weight of Si, 1.0 to 6.0% by weight of Fe, 10.0 to 16.0% by weight of W, 5.0 to 21.0% by weight of Cr and 10.0 to 15.0% by weight of Ni, with the balance being Co.

As the groove on the sink roll for a hot dip galvanizing bath may be formed in the shape of a spiral groove formed spirally on the surface of the roll, a symmetric groove formed spirally on the surface of the roll symmetrically with respect to the center line, or the like.

The groove shape has no direct influences on the corrosion resistance and abrasion resistance, but has influences on the quality of a product, that is, a hot dip galvanized steel sheet and on the film characteristics when a flame-sprayed surface layer is formed.

When the groove shape is set as defined in the claims, incorporation of the dross into the strip can be prevented and the function of discharging zinc and the dross can be enhanced.

The reasons for limitations of the groove shape as defined in claim 3 will now be described with reference to the results obtained with respect to various sizes in the actual operation.

Groove Depth

When the strip is kept in contact with the surface of the sink roll, a negative pressure is produced in the groove, and zinc and the dross are discharged by the pumping action. If the groove depth is smaller than 0.5 mm, the sectional area of the groove is too small and the discharging effect becomes insufficient. In contrast, if the groove depth exceeds 5.0 mm, the discharging speed is reduced and incorporation of the dross is often caused. Accordingly, the groove depth is limited to 0.5 to 5.0 mm. Groove Width

The groove width has an effect similar to the effect of the groove depth. If the groove width is smaller than 5.0 mm, the sectional area of the groove is small and the discharging effect is insufficient. In contrast, if the groove width exceeds 10.0 mm, the discharging speed is reduced and the dross is apt to be incorporated into the strip. Accordingly, the groove width is limited to 5.0 to 10.0 mm. Groove Bottom Radius

Since the groove bottom undergoes erosion by the discharged zinc and dross, the rounding of the bottom is indispensable. If the bottom radius is smaller than 5.0 mm, peeling of the flame-sprayed layer is readily caused by concentration of the stress, and furthermore, a groove mark is often formed. Accordingly, the groove bottom radius is limited to at least 5.0 mm, preferably

5.0 to 25.0 mm. Groove Shoulder Radius

Since the groove shoulder undergoes erosion by the discharged zinc and dross, the rounding of the shoulder is indispensable. If the groove shoulder radius is smaller than 3 mm, a groove mark is often formed and peeling of the flame-sprayed layer is apt to be caused by concentration of the stress. Accordingly, the groove shoulder radius is limited to at least 3 mm, preferably 3.0 to 30.0 mm.

It is preferred that the groove pitch be 20 to 60 mm. If the groove pitch is adjusted to 20 to 60 mm, the problems of local erosion and incorporation of the dross can be solved in a good balance, and the quality of the strip product can be improved and the life of the sink roll can be greatly prolonged. Accordingly, in the invention as defined in claim 5, the groove pitch is limited to 20 to 60 mm in the invention as defined in claim 3.

In the invention as defined in claim 4, there is provided a sink roll for a hot dip galvanizing bath, which has on the surface thereof a double-cross groove having a shape characterized by a groove depth of 0.5 to 5.0 mm, a groove width of 5.0 to 10.0 mm, a groove bottom radius of at least 5.0 mm, a groove shoulder radius of at least 3.0 mm and an angle of inclination of 0.3 to 5.0° to the circumferential axis of the groove, and has a flame-sprayed surface layer comprising 1.0 to 1.5%

by weight of C, 2.0 to 4.0% by weight of B, 2.0 to 4.0% by weight of Si, 1.0 to 6.0% by weight of Fe, 10.0 to 16.0% by weight of W, 5.0 to 21% by weight of Cr and 10.0 to 15.0% by weight of Ni, with the balance being Co.

Since a spiral groove generally adopted at the present has such a structure that zinc and the dross are discharged in one direction, this spiral groove is defective in that incorporation of the dross into the strip is often caused and the strip tends to meander. According to the present invention, by forming a double-cross groove as shown in Figs. 4 and 5, the function of discharging zinc and the dross is improved and meandering of the strip can be prevented.

The reasons for limitations of the basic groove shape factors, that is, the groove depth, groove width, groove bottom radius and groove shoulder radius, are the same as those described with regard to the invention as defined in claim 3. In the invention as defined in claim 4, the groove is limited to a double-cross groove and the angle of inclination to the circumferential axis of the groove is limited to 0.3 to 5.0°.

The reason why the angle of inclination to the circumferential axis of the groove is limited as defined in the claims is as follows. If the angle is smaller than 0.3°, the crossing angle of the cross portion of the groove is too acute and hence, erosion is apt to be caused in the cross portion. In contrast, if the

angle exceeds 5.0°, erosion in the cross portion is controlled, but the flow of the discharged zinc and dross is disturbed in the cross portion and incorporation of the dross is apt to be caused. Moreover, the number of groove streaks decreases and meandering of the strip is often caused.

It is preferred that the groove pitch be 20 to 60 mm. If the groove pitch is adjusted to 20 to 60 mm, the problems of local erosion and incorporation of the dross can be solved in a good balance, and the quality of the strip product can be improved and the life of the sink roll can be greatly prolonged. According to the invention as defined in claim 5, the groove pitch is limited to 20 to 60 mm in the invention as defined in claim 4.

In accordance with the invention as defined in claim 6, there is provided a method for preparing a sink roll for a hot dip galvanizing bath according to claim 3, said method comprising forming a flame-sprayed surface layer comprising 1.0 to 1.5% by weight of C, 2.0 to 4.0% by weight of B, 2.0 to 4.0% by weight of Si, 1.0 to 6.0% by weight of Fe, 10.0 to 16.0% by weight of W, 5.0 to 21.0% by weight of Cr and 10.0 to 15.0% by weight of Ni, with the balance being Co, on the surface of a roll having on the surface thereof a groove having a shape characterized by a groove depth of 0.5 to 5.0 mm, a groove width of 5.0 to 10.0 mm, a groove bottom radius of at least 5.0 mm and a groove shoulder radius of at

least 3.0 mm, heating the flame-sprayed surface layer at a temperature-elevating rate of 10 to 100°C/hr, maintaining the flame-sprayed surface layer at a pre-heating temperature of 300 to 600°C for at least 0.5 hour, subjecting the flame-sprayed surface layer to a fusing treatment by heating at a temperature of at least 1000°C once or twice, dropping the temperature, carrying out a soaking treatment at a temperature of 500 to 800°C for at least 1 hour and carrying out a cooling treatment at a temperature-dropping rate of 10 to 50°C/hr.

In accordance with the invention as defined in claim 7, there is provided a method for preparing a sink roll for a hot dip galvanizing bath according to claim 4, said method comprising forming a flame-sprayed surface layer comprising 1.0 to 1.5% by weight of C, 2.0 to 4.0% by weight of B, 2.0 to 4.0% by weight of Si, 1.0 to 6.0% by weight of Fe, 10.0 to .16.0% by weight of W, 5.0 to 21.0% by weight of Cr and 10.0 to 15.0% by weight of Ni, with the balance being Co, on the surface of a roll having on the surface thereof a double-cross groove having a shape characterized by a groove depth of 0.5 to 5.0 mm, a groove width of 5.0 to 10.0 mm, a groove bottom radius of at least 5.0 mm, a groove shoulder radius of at least 3.0 mm and an angle of inclination of 0.3 to 5.0° to the circumferential axis of the groove, heating the flame-sprayed surface layer at a temperature-elevating rate of 10 to 100°C/hr, maintaining

the flame-sprayed surface layer at a pre-heating temperature of 300 to 600°C for at least 0.5 hour, subjecting the flame-sprayed surface layer to a fusing treatment by heating at a temperature of at least 1000°C once or twice, dropping the temperature, carrying out a soaking treatment at a temperature of 500 to 800°C for at least 1 hour and carrying out a cooling treatment at a temperature-dropping rate of 10 to 50°C/hr.

If a flame-sprayed surface layer is formed on a sink roll having the groove of the present invention by a conventional flame-spraying method and the fusing treatment is then carried out, it sometimes happens that insufficient diffusion is caused in the boundary portion close to the base and a desired mechanical strength cannot be obtained in the surface-treated layer. Especially, the adhesion between the flame-sprayed surface layer and the base is insufficient and hence, peeling of the flame-sprayed surface layer is caused during the use or the like or the flame-sprayed surface layer is cracked, with the result that the commercial value of the sink roll as the product is lost or the life of the sink roll is drastically shortened. Furthermore, bad influences are imposed on the quality of a strip, that is, a galvanized steel sheet, flaws and other defects are formed on the product. This unstableness of the quality is especially conspicuous in a sink roll on which a double-cross groove as specified in claim 4 or claim 7 is formed. Accordingly, development of a preparation

method guaranteeing a stable quality is also an important problem to be solved.

We have completed the methods as defined in claims 6 and 7 as the means for solving this problem. The characteristic feature of these methods resides in the heat treatment conducted after formation of the flame-sprayed surface layer.

At first, heating is carried out at a temperature-elevating rate of 10 to 100°C/hr. If the temperature-elevating rate is higher than 100°C/hr, cracks are apt to be formed on the flame-sprayed surface layer. If the temperature-elevating rate, is lower than 10°C/hr, the method becomes disadvantageous from the economical viewpoint. It is most important that pre-heating should be then carried out at 300 to 600°C for at least 0.5 hour. The object of this treatment is to prevent cracking by the fusing treatment and attain a soaking effect. This treatment is especially important in case of a sink roll having a double-cross groove formed thereon as in the present invention. The pre-heating effect cannot be attained unless pre-heating is carried out at a temperature of 300 to 600°C for at least 0.5 hour.

It is preferred that heating at a temperature of at least 1000°C as the fusing treatment be carried out twice. This twice-heating method is especially recommended in the production of a sink roll having a double-cross groove formed thereon. According to this treatment, a high effect of uniformalizing the quality

in the flame-sprayed layer can be attained, and if desired, the grain size can be reduced. Therefore, a desired mechanical strength can be easily obtained.

Then, the temperature is dropped and the soaking treatment is carried out at a temperature of 500 to 800°C for at least 1 hour. If the treatment time is shorter than 1 hour, the soaking treatment is insufficient and a stable mechanical strength is hardly attained in the flame-sprayed surface layer. Then, the cooling treatment is carried out at a temperature-dropping rate of 10 to 50°C/hr. The reason is that if the temperature-dropping rate exceeds 50°C/hr, there is a risk of cracking.

The effects of the present invention are especially prominent in the production of a sink roll having a double-cross groove formed thereon.

Brief Description of the Drawings:

Fig. 1 is a schematic view illustrating a main portion of an apparatus for testing the corrosion in a hot dip galvanizing bath, which was used in the immersion test of the immersion members for a hot dip galvanizing bath in the examples of the present invention.

Fig. 2(a) and Fig. 2(b) illustrate heat treatment cycles in the invention as defined in claims 2 and 5, in which Fig. 2(a) illustrates the heat cycle in which the fusing treatment is effected once and Fig. 2(b) illustrates the heat cycle in which the fusing treatment is effected twice.

In Fig. 2, ( ) indicates the maintenance at 500 to 550°C for 1 hour, which is conducted before the deposition treatment, (5) represents the deposition treatment conducted at 1220 to 1250°C, Q) indicates the soaking treatment at 600°C for 2 hours, which is conducted before the second deposition treatment, (4) represents the second deposition treatment conducted at 1250 to 1280°C, and (?) indicates the soaking treatment at 600°C for 3 hours, which is conducted after the deposition treatment.

Fig. 3(a), Fig. 3(b) and Fig. 3(c) show micrographs of surface metal textures of sink rolls of Example 2 of the present invention and Comparative Example, in which Fig. 3(a) shows the surface metal texture of a sink roll for a hot dip galvanizing bath, which was obtained by forming a flame-sprayed surface layer having a thickness of 2.5 mm and conducting the fusing treatment according to the heat treatment cycle shown in Fig. 2(a), Fig. 3(b) shows the surface metal texture of a sink roll in a hot dip galvanizing bath, which was obtained by forming a flame-sprayed surface layer having a thickness of 2.5 mm and conducting the fusing treatment twice according to the heat treatment cycle shown in Fig. 2(b), and Fig. 3(c) shows the surface metal texture of a sink roll for a hot dip galvanizing bath, which was obtained by forming a flame-sprayed surface layer having a thickness of 2.5 mm and conducting the conventional fusing treatment using an

oxygen-acetylene flame. In each of the sink rolls shown in Fig. 3(a), 3(b) and 3(c) after the fusing treatment, the machining operation was carried out so that the thickness of the flame-sprayed layer was at least 1 mm. Etching was carried out by using an alkaline solution of red prussiate of potash, and the magnification of each micrograph is 100.

Fig. 4 is a diagram showing an embodiment of the sink roll according to the inventions as defined in claims 3, 4 and 5.

Fig. 5 is a diagram illustrating an example of the shape of the double-cross groove formed on the surface of the sink roll of the present invention.

Fig. 6 is a diagram showing the state of the use of sink rolls for a hot dip galvanizing bath, obtained in Examples 2 and 3, and other immersion members for a hot dip galvanizing bath.

Best Mode for Carrying Out the Invention:

Now, the invention will be explained with reference to the preferred embodiments. Example 1

Sink rolls, sink roll arms, immersion members and test pieces comprising a flame-sprayed surface layer having a composition shown in Table 1 and a thickness of 1 mm were prepared by flame spraying, and samples were formed by conducting the fusing treatment once or twice or without performing any fusing treatment and

were tested.

The fusing treatment conducted once was carried out according to the heat cycle shown in Fig. 2(a), and the fusing treatment conducted twice was carried out according to the heat cycle shown in Fig. 2(b).

The samples were immersed in a molten zinc bath of a corrosion tester shown in Fig. 1, and the decrease of the weight by corrosion was measured. The obtained results are shown in Table 1. The test conditions are as follows: bath temperature: 500 ^C C Al content in bath: 0.27% immersion time: 144 hours

Table 1

Note The thickness of the flame-sprayed layer was 1 mm in each sample.

Example 2

Immersion members for a hot dip galvanizing bath, such as sink rolls, comprising a flame-sprayed surface layer having a composition shown in Table 2 and a thickness of 1.2 mm were formed by flame-spraying, and samples were prepared by conducting the fusing treatment once or twice or without performing any fusing treatment and were subjected to the actual operation test.

The fusing treatment conducted once was carried out according to the heat cycle shown in Fig. 2(a), and the fusing treatment conducted twice was carried out according to the heat cycle shown in Fig. 2(b).

It was found that the corrosion resistance and abrasion resistance were improved in the immersion members for a hot dip galvanizing bath, such as sink rolls, and therefore, the life was greatly prolonged.

The results of the actual operation test of the immersion members for a hot dip galvanizing bath, such as sink rolls, are shown in Table 2.

Table 2

C

Note 1) The life of the sink roll is expressed by the number of days, during which the roll could be used without grinding (the sink rolls of the present invention could be used continuously without grinding of the roll surface if the shaft and bearing were exchanged periodically). 2) The thickness of the flame-sprayed layer was 1.2 mm in each sample.

Example 3

A flame-sprayed surface layer having a composition shown in Table 3 and a thickness of 1 mm was formed on sink rolls for a hot dip galvanizing bath, on which a double-cross groove shown in Fig. 4 and 5 was formed, and also on sink rolls for a hot dip galvanizing bath, on which an ordinary spiral groove was formed. Samples were formed by conducting the fusing treatment once or twice or without performing any fusing treatment and were tested. It was found that neither slipping nor meandering of the strip was caused and the quality of a galvanized steel sheet as the product was improved.

The results of the use test of respective sink rolls in a hot dip galvanizing bath are shown in Table 3.

The standards of evaluation of meandering (walking), the quality of the obtained galvanized steel sheet as the product and the durability are shown in the following tables.

Standard of Evaluation of Meandering (Walking)

Standard of Quality of Product

Standard of Evaluation of Durability

Table 3

Note The thickness of the flame-sprayed layer was 1 mm in each sample.