JPH04191386A - Descaling method for steel strip - Google Patents

Abstract

PURPOSE:To improve descaling by penetrating a soln. of salts consisting of >=one kind among Al, Fe, Mn, and Zn, and halogen elements into scale and heating a ferritic stainless steel strip at the time of descaling this steel strip. CONSTITUTION:The ferritic stainless steel strip S subjected to batch annealing after hot rolling is transported to a soln. applying and penetrating device 21. While the steel strip S is immersed into the salt soln. 23 consisting of >=1 kinds among the Al, Fe, Mn, and Zn, and the halogen elements, the steel strip is pressurized between a pair of upper and lower press rolls 24A, 24B. The salt soln. 23 is penetrated into the surface oxide scale 50 of the steel strip formed by annealing until the soln. arrives at the base iron part of the steel strip S. Cracks 52 are generated in the scale 50 by the pressurization at this time and the salt soln. 23 penetrates down to the dull seam parts 51 of the base of the steel strip S. This steel strip S is then sent to a heating section and is heat treated at the temp. exceeding the m.p. of the salt. The steel strip is sent to a scale removing device after cooling.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for descaling ferritic stainless steel strip, and in particular to a steel strip with improved descaling efficiency on the surface of the steel strip after batch annealing. Regarding descaling method.

[Conventional technology]

Conventionally, stainless steel strips are easier to work harden than common steel, and are annealed after hot rolling to reduce the load during cold rolling.

In particular, ferritic stainless steel strips require a long time of annealing, unlike austenitic stainless steel strips, so batch annealing is performed.

Therefore, in the case of a ferritic stainless steel strip, annealing is performed for 60 to 80 hours in an atmosphere of 24% H and 96% N using a batch-type bell-type annealing furnace. At this time,
A thickness of 5μ generated on the surface of the stainless steel strip during hot rolling.
The oxidized scale of approximately 100 m in size is further oxidized by a trace amount of H 20 present in the reducing atmosphere gas, and grows into a dense oxide scale layer with a thickness of approximately 10 to 15 μm. This oxide scale is extremely difficult to remove compared to the oxide scale that forms on the surface of ordinary steel strips, so mechanical descaling such as shot blasting and pickling using multiple pickling tanks are required. Line (Anneal
ing and Pickling Line)
is used to remove oxide scale.

Furthermore, in the descaling process of the stainless steel strip, when performing the cleaning, a high concentration of sulfuric acid, nitric-fluoric acid (a mixed acid of nitric acid and hydrofluoric acid), nitric acid, etc. with a concentration of about 20 wt% is used. Strong acids were required, and long-term treatment was requested.

For this reason, the descaling process becomes a rate-limiting step, and descaling takes time, making it impossible to improve productivity.

In addition, in the shot plast process, shot particles form unevenness on the surface of the steel strip, increasing the surface roughness of the steel strip. There was also the problem that the skin became rough and the surface roughness worsened.

If there are any irregularities on the surface of the steel strip, there is a problem in that in the next step of cold rolling, the convexities on the surface of the steel strip will fall down, causing overlapping and causing sparkling scratches.

Therefore, in order to solve this problem, Japanese Patent Application Laid-Open No. 54-93620 and Japanese Patent Application Laid-open No. 55-4732 are proposed as conventional examples to reduce the descaling process which is the main cause of the problem.
No. 8 is known.

The conventional example described in JP-A-54-93620 is
When box-shaped annealing a cold-rolled steel sheet coil, a chemical mainly consisting of at least one of an alkali metal compound, an alkaline earth metal compound, and an oxygen compound of boric acid is attached to the steel sheet coil before the annealing. The descaling process is reduced by annealing the oxide scale and causing the oxide scale to react with the compound, thereby making it easier to remove the oxide scale.

On the other hand, in the conventional example described in JP-A No. 55-47318, after immersing or coating a Cr-based stainless steel material containing 10% or more of Cr in an aqueous solution of iron chloride,
By annealing the r-based stainless steel material, the oxide scale formed during the heat treatment of the Cr-based stainless steel can be easily removed in the subsequent pickling process, thereby reducing the descaling process.

[Invention or problem to be solved]

However, in the conventional example described in JP-A No. 54-93620, the alkali or alkaline earth metal compound applied to the cold-rolled steel coil has a high melting point for boat fishing, so it is particularly difficult to use ferrite. Annealing temperature of stainless steel strip (
600 to 900°C), the compound exists as a stable solid on the surface of the steel strip, making it difficult for the reaction with the oxide scale to proceed. Further, if the compound is attached to the surface of the steel strip, the reaction efficiency with the oxidized scale is very poor. For this reason, there was a problem in that it was not possible to sufficiently reduce descaling.

Furthermore, in the conventional example described in JP-A No. 55-47318, the Cr stainless steel material is annealed after being immersed or coated in an aqueous iron chloride solution. It is very difficult to apply the solution evenly over the entire length of the coil on the surface of the ferritic stainless steel strip. As a result, there was an issue of not being able to fully descale.

Therefore, in order to solve these problems, the present invention aims to improve descaling, thereby improving the surface quality of the steel strip after descaling, and furthermore, it is possible to produce ferritic stainless steel strip with good surface quality. The purpose is to provide a descaling method that improves performance.

[Failure to solve the problem]

Therefore, in order to solve the above problems by achieving such an object, the present invention provides a method for descaling a steel strip for removing oxidized scale on the surface of a ferritic stainless steel strip after batch annealing. A solution of a salt consisting of at least one of Al, Fe, Mn, and Zn elements and a halogen element is infiltrated into the oxide scale formed on the surface of the steel strip, and then heated, followed by descaling. It is.

[Function] According to the steel strip descaling method according to the present invention, after batch annealing of a ferritic stainless steel strip, A-
j? , Fe, Mn, and Zn elements and a halogen element are applied to the ferritic stainless steel strip coil, for example, when unwinding the steel strip during the hot rolling and annealing. It becomes possible to penetrate into all the oxide scale layers formed on the surface, all the way to the bare iron part of the steel strip, by capillary action.

Next, before descaling the steel strip coated with the solution, the solution is heated to a temperature higher than the melting point of the salt contained in the solution (300
By heat-treating the ferritic stainless steel strip at a temperature of ~500'C or higher), the efficiency of the reaction between the oxide scale and the salt melt improves, and descaling becomes easier.

That is, as a result of the heat treatment, the water in the solution that has penetrated into the oxide scale layer first evaporates at 100°C, and the solution becomes a solid salt.

Since the melting point of the solid is very low compared to the melting point of alkaline earth metals, etc., the solid has a melting point of 200 to 300
It becomes molten at a very low temperature of around °C and diffuses densely into the oxide scale. At this time, since the melting temperature of the salt is low, it is economical to save the energy required for heating after the salt solution permeates. Next, the steel strip
Upon heating to ˜500° C., a solid-liquid reaction occurs between the molten solid and the oxidized scale.

Due to this solid-liquid reaction, a very strong oxide scale with a spinel structure is generated on the surface of the ferritic stainless steel strip during the hot rolling process and the batch annealing process.
For example, Cr2O3, Fe3O4, FeCr2O4, etc. change into halides of Cr and Fe which are amorphous and have very low mechanical strength. Therefore, the oxide scale can be easily removed from the surface of the steel strip even if shot blasting and pickling treatment with nitric-fluoric acid, which are indispensable in the descaling process of stainless steel strips, are omitted.

As a result, it is possible to prevent the quality deterioration of the steel strip surface caused by shot blasting and pickling treatment with nitric-fluoric acid, and the surface roughness, which is most important for stainless steel strips, has been significantly improved. We can provide steel strips. Furthermore, since descaling efficiency can be improved, productivity can be improved, and the working environment can also be improved.

Furthermore, the solution can be applied and permeated onto the steel strip by using an existing AP line, and furthermore, the steel strip can be heat-treated in an open flame atmosphere on the AP line, which is economical. Here, since the solution is permeated to the base iron portion of the steel strip, the solid matter in the molten state is densely diffused into the oxide scale layer of the steel strip during the heat treatment.

Therefore, it is physically impossible for the oxidizing components (0□, H2O, Co□) in the atmospheric gas used during the heat treatment to penetrate into the base steel of the steel strip, and new oxidation occurs. It never scales or grows.

〔Example〕

Next, embodiments of the present invention will be described according to the drawings.

FIG. 1 is a block diagram of a descaling line according to an embodiment of the present invention, and FIG. 2 is a block diagram of a salt solution application/infiltration device installed in the middle of this line.

At the starting point of the line shown in FIG. 1, a hot-rolled and batch-annealed ferritic stainless steel strip (hereinafter referred to as stainless steel strip) S is wound around a payoff reel 1. This wound stainless steel strip S is placed at the starting point of the existing AP line, unwound at this starting point, and cut at the leading end or trailing end by the entry shear 2. At this time,
In the welder 3, the tip of the stainless steel strip in the subsequent stage is welded to the rear end of the stainless steel strip in the previous stage, and continuous descaling is performed.

The stainless steel strip S passes through the entrance looper 4 and is coated with solution.
It is transported to the infiltration device 21. This solution application/penetration device 2
1, as shown in FIG.
The stainless steel strip S guided by 5A is immersed in a salt solution (hereinafter simply referred to as "salt solution") 23 containing at least one of AA, Fe, Mn, and Zn elements and a halogen element in a tank 22. Then, the stainless steel strip S is pressed between a pair of upper and lower pressure rolls 24A and 8.

FIG. 3 is an enlarged view of the properties of the surface layer of the stainless steel strip S after pressurization. As shown in FIG. Penetrate into oxide scale 50.

Here, cracks 52 are formed in the surface oxide scale 5o due to pressure.
occurs, and the salt solution 23 penetrates through the cracks 52 into the rounded portions 51 of the base of the stainless steel strip S due to capillary action.

Thereafter, the stainless steel strip S is sent to the heating section 6 constituting the annealing furnace 5 in the AP line via the exit guide roll 25B, and is heated to a temperature of 300 to 500, which exceeds the melting point of salt, in a combustion gas atmosphere.
Heat treated at temperatures above °C. Here, the water content of the salt solution 23 that has permeated into the oxide scale 50 is
Evaporates at °C to salt or solid form. When the heating temperature further increases, the solids melt and diffuse densely into the oxide scale 50, and a solid-liquid reaction proceeds with the oxide scale 50, producing reaction products with weak mechanical strength (chloride ) occurs. Next, the stainless steel strip S is sent to the cooling section 7 and cooled to a predetermined temperature.

After that, the stainless steel strip S is removed by the oxide scale removing device 31.
sent to. As shown in FIG. 4, this device has two sets of grinding brushes 32 arranged in series and facing each other, upper and lower, in order to remove reaction products 33 on the surface of the stainless steel strip S. The reaction product 33 is removed from the stainless steel strip S by the grinding force of the grinding brush 32 generated by rotating the grinding brush 32 opposite to the advancing direction of the stainless steel strip S. The reaction product 33 is washed away by a water spray nozzle 34 and discharged from a discharge pipe 35 installed at the lower end of the removal device 31 . In this manner, the reaction product 33 can be easily removed from the stainless steel strip S by the grinding force of the grinding flange 32. For this reason, there is no need for shot blasting.

Next, for the purpose of surface treatment to obtain a stainless steel strip with a beautiful metallic luster and whiteness on the surface, the stainless steel strip S is transported to pickling [41] and treated with sulfuric acid, hydrochloric acid, etc. other than nitric-hydrofluoric acid. A trace amount of chloride (reaction product) remaining on the surface of the steel strip S is pickled with acid, and then the steel strip S is transported to a nitric acid tank 42 where it is subjected to finishing pickling and passivation treatment. At this time, when the stainless steel strip S is pickled using the pickling layer 41, the surface of the base iron portion of the stainless steel strip S is not eroded because nitrofluoric acid is not used.

Next, the stainless steel strip S is cleaned by a cleaning device I2, a dryer I
3, passes through the output looper 14, is cut into a predetermined size by the dividing shear 15, and is wound onto a tension reel I6.

As described above, in the above embodiments, since the salt solution is applied to the stainless steel strip before it is unwound after being batch annealed, the salt solution can be sufficiently applied to the entire surface of the oxide scale on the front and back surfaces. As a result, descaling efficiency can be greatly improved.

Next, specific examples of the present invention will be described.

Using the AP line shown in Figure 1, the stainless steel strip (
SUS430; plate thickness 4.0vun, TFI radiation 15001
nffl) was descaled using the solutions listed in Table 1. In addition, the descaling status was investigated on the outlet side of each facility shown in Table 2 and expressed as an area ratio. The results are shown in Table 2.

After hot rolling, the batch annealed stainless steel strip was conveyed to the solution coating/penetration device 21, and a salt solution having the components shown in Table 1 was applied to the surface of the steel strip after hatch annealing, and then conveyed to the heating section 6. (Steel strips A to E). The temperature of the heating section 6 was set as shown in Table 1 in order to melt the salt and promote the solid-liquid reaction with the oxidized scale of the stainless steel strip.

Table 1 Next, the steel strip leaving the heating section 6 is conveyed to the cooling section 7.
After that, it reaches the oxide scale removing device 31, and after passing the grinding brush 32 through two buses, it is transported to the pickling tank 4I and H
It was pickled with 2SO4 (15%).

Thereafter, the steel strip is transported to a nitric acid bath 42, where it is subjected to passivation treatment, passed through a cleaning device 12, a dryer 13, passed through an exit looper 14, and then cut to a predetermined size by a dividing shear 15, and then placed on a tension reel. 16 to obtain steel strips A-E.

Next, for comparison, a comparative product was produced using the same steel strip (SUS430; plate thickness: 4.0 mm, plate width: 1500 mm) using the following method, which is a conventional descaling method.

After hot rolling, the batch annealed stainless steel strips were conveyed to the heating section 6 without being coated with the solutions shown in Table 1, and then passed through the cooling section 7, where they were subjected to shot blasting and mechanical descaling.

Next, the steel strip is transported to a pickling tank 41 and treated with H2SO4 (1
5%), then further processed in a nitrofluoric acid bath (HF-H
A pickling treatment was performed with NO,; l 5%).

Thereafter, the steel strip is transported to a nitric acid bath 42, where it is subjected to passivation treatment, passed through a cleaning device 12, a dryer 13, passed through an exit looper 14, and then cut into a predetermined size by a dividing shear 15, and then placed on a tension reel. 16 (comparison product).

Table 2 (%) From Table 2, the steel strips (steel strips A to E) that were subjected to the descaling process after applying a solution of the components shown in Table 1 to the steel strips were subjected to shot blasting, mechanical descaling. It was also demonstrated that even without nitric-fluoric acid treatment, the scale could be sufficiently descaled compared to a comparative product that was not coated with the solution.

Next, the surface roughness (Ra: μm) of steel strips A-E and comparative products
was measured. The results are shown in Table 3.

Table 3 From Table 3, steel strips A-E that were not subjected to shot blasting, mechanical descaling, and nitrofluoric acid treatment had a
It was demonstrated that the surface roughness was reduced by 1 to 2 μm. In addition, the reduced surface roughness suppresses the occurrence of glitter scratches on the steel strip surface during the next process of cold rolling and temper rolling, reducing the load during cold rolling and temper rolling. can do. Therefore, it was possible to eliminate deterioration in the quality of the steel strip surface, and in particular, the surface roughness, which is the most important for stainless steel strips, was significantly improved, making it possible to provide a good steel strip with high commercial value.

In this example, a solution using ions shown in Table 1 as cations and anions was applied to the steel strip, or A1. Two or more of Fe, Mn, and Zn elements may be combined to form a cation group, and two or more halogen elements may be combined.
Of course, two or more may be combined to form an anion group.

Further, in this embodiment, SUS 430 is used as the steel strip, but the present invention is not limited to this, and other steel strips can also be used to improve the effect.

In this embodiment, the solution application/penetration device 21 has a structure as shown in FIG. 2, but is not limited to this, and may be of a spray type.

A fountain type, barcode type, etc. may also be used.

In addition, in the oxide scale removal device 31, a grinding brush 32
Mechanical descaling was carried out using a bending roll, a grindstone, a grinding brush that also serves as a bending roll, etc., as long as it does not adversely affect the surface quality of the steel strip.

Further, the pickling tank 41 may use sulfuric acid, hydrochloric acid, nitric acid, or the like, and by combining it with the mechanical descaling described above, descaling can be carried out more efficiently.

〔Effect of the invention〕

As explained above, according to the present invention, the oxide scale formed on the surface of the ferritic stainless steel strip after batch annealing contains at least one of Al, Fe, Mn, and Zn elements and a halogen element. Since descaling is performed after infiltrating a salt solution and then heating, descaling performance can be improved, and the surface quality of the steel strip after descaling can be improved. It is possible to improve production efficiency.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a descaling line according to an embodiment of the present invention, Fig. 2 is a block diagram of the solution coating/penetration device in Fig. 1, and Fig. 3 is a block diagram of the oxidation formed on the surface of the steel strip. FIG. 4 is a cross-sectional view showing a state in which a salt solution permeates into a scale layer, and FIG. 4 is a configuration diagram of the oxide scale removing apparatus in FIG. 1. In the figure, S is a stainless steel strip, 5 is an annealing furnace, 6 is a heating section,
7 is a cooling unit, 21 is a solution application/penetration device, 22 is a tank, 23 is a salt solution, 24 is a pressure roll, 31 is an oxide scale removal device, 32 is a grinding brush, 33 is a reaction product, 4
1 is a pickling tank, 50 is an oxidized scale, 51 is a dull part,
52 indicates a crack.

Claims (1)

[Claims] (1) In a steel strip descaling method for removing oxide scale on the surface of a ferritic stainless steel strip, Al, Fe, Mn, and Zn elements are present in the oxide scale formed on the steel strip surface after batch annealing. 1. A method for descaling a steel strip, which comprises impregnating a solution of a salt comprising at least one of the above and a halogen element, followed by heating, followed by descaling.