JPH0447011B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an annealing pretreatment method that is applied to a cold rolled steel strip before continuous annealing and descaling in the manufacturing process of stainless steel cold rolled steel strip, and in particular to a method for suppressing and removing oxide scale during annealing. The present invention relates to an annealing pretreatment method for improving scale properties and improving the surface properties of a steel strip after descaling. As is well known, in the manufacturing process of cold-rolled stainless steel strips, annealing is usually performed after cold rolling, mainly to remove strain caused by cold rolling and impart good mechanical properties. Such annealing after cold rolling is usually carried out by continuous annealing, and in terms of the annealing atmosphere, it is roughly divided into bright annealing, which is carried out in a reducing atmosphere, and annealing, which is carried out in an oxidizing atmosphere. be done. In particular, when annealing is performed in the latter oxidizing atmosphere, oxide scale is formed on the surface of the steel strip during annealing, so it is necessary to perform descaling treatment during annealing. Therefore, when annealing is carried out in an oxidizing atmosphere, the process is generally carried out using a so-called continuous annealing/descaling line that has annealing equipment in the first half and descaling equipment in the latter half. In addition, there are cases in which the rolling oil adhering to the surface of the steel strip is washed and removed with a degreaser before continuous annealing and descaling, and cases in which the rolling oil is burned and removed in a direct-fired annealing furnace without degreasing. However, in the former case, the thickness of the oxide scale generated during annealing tends to be thinner. By the way, in continuous annealing and descaling lines for cold rolled stainless steel strips, excellent surface properties such as surface gloss after descaling are required.
Furthermore, it is required to be inexpensive in terms of cost, that is, to be able to perform descaling efficiently. In order to meet these demands, it is necessary to suppress the generation of oxide scale during annealing as much as possible, and to efficiently remove the oxide scale film generated during annealing without damaging the surface of the steel. is necessary. Conventionally, methods for descaling such continuous annealing/descaling lines for cold-rolled stainless steel strips include immersion in inorganic acids such as sulfuric acid, nitric acid, and nitric-fluoric acid (a mixed acid of nitric acid and hydrofluoric acid), or Treatment methods that combined electrolysis were often adopted, but
As a result of various research and development efforts,
A combination of the so-called molten alkali salt immersion method (also known as the salt method), in which the material is immersed in a molten alkali salt heated to 400 to 500°C containing NaOH as the main component, and electrolytic treatment with a neutral salt aqueous solution such as _NaSO4 . This method has been put into practical use, and the descaling method itself has achieved some success. On the other hand, controlling the atmosphere during annealing can be considered to suppress the formation of oxide scale during annealing, but in the case of a direct-fired furnace that uses light oil, LPG, etc. as fuel,
The reality is that oxide scale production is not controlled by atmosphere control because it is extremely difficult to control the atmosphere. Therefore, as a method for controlling the formation of oxide scale during annealing without controlling the atmosphere in the annealing furnace, a method described in JP-A-56-133478 has been proposed. In this proposed method, a black film is generated on the surface of the stainless steel strip by electrolysis in an electrolytic solution containing carbon black as the main component before annealing, and the film is annealed during the annealing process to form a black film on the steel strip. In addition to suppressing surface oxidation,
It is said that it is possible to improve descaling performance by promoting the formation of defects in the scale layer to improve the oxide scale. However, when this proposed method is applied to actual operation, it is necessary to include not only an immersion tank but also electrolytic equipment as annealing pretreatment equipment, which poses a problem in that the equipment costs are unavoidably high. This invention was made against the background of the above-mentioned circumstances, and it suppresses the formation of oxide scale during annealing in a continuous annealing/descaling line in the production process of stainless steel cold rolled steel strips, thereby facilitating descaling. It is an object of the present invention to provide an annealing pretreatment method that can also improve the surface quality after descaling and does not cause a significant increase in equipment costs. The inventor of the present application has conducted various experiments and studies to achieve the above object, and in particular, immersed a cold rolled stainless steel strip before annealing in various alkaline aqueous solutions and annealed it in an oxidizing atmosphere. When a steel strip before annealing is immersed in an aqueous solution of a specific alkaline substance in a specific concentration range, the formation of oxide scale is suppressed to a considerable extent and descaling becomes easy.
In addition, the inventors discovered that the surface properties of the steel strip after descaling also become good, leading to the present invention. That is, the annealing pretreatment method of the present invention is a method for producing cold rolled stainless steel strip in which continuous annealing and descaling treatment is performed after cold rolling, in which cold rolling is performed after cold rolling and before continuous annealing and descaling treatment. The surface of the rolled steel strip is degreased, and then the pH is 9 to 13.
This method is characterized by applying a KOH (potassium hydroxide) aqueous solution or a Ca(OH) ₂ (calcium hydroxide) aqueous solution within the range of 1 to 1 on the surface of a cold rolled steel strip. The pretreatment method of the present invention will be explained in more detail below. In the annealing pretreatment method of the present invention, rolling oil adhering to the surface of a cold rolled steel strip that has been cold rolled to a predetermined size is first washed and removed using a suitable degreasing agent. In the conventional method for producing cold rolled stainless steel strip, degreasing may or may not be carried out before annealing, but in the case of the present invention, degreasing is carried out in order to improve the application of the alkaline aqueous solution in the next step. Processing is a necessary condition. That is, if the degreasing is insufficient, the alkaline aqueous solution will not sufficiently wet the surface of the steel strip during the next step of applying the alkaline aqueous solution, and as a result, the effect of the present invention by applying the alkaline aqueous solution will not be sufficiently obtained. The degreasing agent used in this degreasing treatment may be an organic degreasing agent such as trichlorethylene or trichloroethane, or an alkaline degreasing agent containing sodium orthosilicate as a main component, but the type thereof is not particularly limited in this invention. After degreasing, the surface of cold-rolled stainless steel strip is
Apply an aqueous solution of KOH or Ca(OH) ₂ . Here, the concentration of the KOH aqueous solution or Ca(OH) ₂ aqueous solution is adjusted so that its pH is in the range of 9 to 13. By applying a specific type of alkaline aqueous solution whose concentration is adjusted to a specific range in this way, if the coated steel strip is annealed in a normal oxidizing atmosphere, the formation of oxide scale during annealing can be suppressed. It is effective. As a specific means for applying the alkaline aqueous solution in this way, any means such as dip coating, spray coating, and coating using a roll coater may be used. The amount of coating is not particularly limited, but it is not necessary to apply a large amount, and it is sufficient that it adheres to the surface of the steel strip when the steel strip is continuously immersed in an aqueous solution and pulled up. Also, KOH aqueous solution or
The range of the PH value of the Ca(OH) ₂ aqueous solution was determined based on detailed experiments by the inventors, as shown in Example 1 below. In other words, in the case of a KOH aqueous solution or a Ca(OH) ₂ aqueous solution with a pH value lower than 9,
Because the effect of suppressing the formation of oxide scale cannot be obtained,
The lower limit of the PH value was set to 9. On the other hand, if the PH value is higher than 13, the amount of oxide scale produced increases, so the upper limit of the PH value was set at 13. Furthermore, Ca
For (OH) ₂ , the PH value of a saturated aqueous solution is approximately 12.7, and the PH value never exceeds 13. As mentioned above, the mechanism by which the formation of oxide scale during subsequent annealing is suppressed by applying a KOH aqueous solution or a Ca(OH) ₂ aqueous solution with a pH value within a specific range has not been completely elucidated, but the following It is assumed that That is, at the beginning of the heating process of annealing, the alkaline aqueous solution and the surface layer of the cold-rolled stainless steel strip react to form a very thin film, and this film reacts with the oxidation reaction on the surface of the steel strip during subsequent heating and soaking. It is thought that this suppresses the Descaling treatment after annealing is performed by immersion in molten alkali salt heated to 400-500℃, electrolytic treatment in a neutral salt aqueous solution such as _NaSO4 , or inorganic acid aqueous solution such as sulfuric acid, nitric acid, nitrofluoric acid, etc. Known methods such as immersion in aqueous solution of these inorganic acids or electrolytic treatment in an aqueous solution of these inorganic acids may be carried out alone or in an appropriate combination, and in any of the descaling methods, if the pre-annealing treatment of the present invention is performed. , descaling becomes easier compared to the case where this is not done. In other words, if the line speed of the continuous annealing/descaling line is descaling rate-limiting, the line speed can be increased by shortening the descaling time and productivity can be improved; if the line speed is annealing rate-limiting, It is possible to reduce the electrolytic power required for descaling, and to reduce the concentration and temperature of the acid solution used. In addition, if the annealing pretreatment of the present invention is applied to ferritic stainless steel, which is often required to have a good surface gloss after descaling, the formation of oxide scale will be suppressed, resulting in descaling. can be made milder than before (for example, by using milder conditions for electrolysis or pickling, or by shortening the descaling time), and therefore performing a milder descaling treatment than before. This makes it possible to reduce the roughness of the surface of the base metal and improve the surface gloss compared to the conventional method. In addition, although the above explanation has been made only for annealing in an oxidizing atmosphere, the pretreatment method of the present invention is not limited to this, and it goes without saying that it can be applied to all types of annealing other than scale-free annealing. be. Example 1 A typical type of ferritic stainless steel
A test piece was taken from a cold-rolled SUS430 steel strip with a thickness of 1.0 mm, and the surface of the test piece was degreased and cleaned using a commercially available alkaline degreaser containing sodium orthosilicate as the main ingredient, and then adjusted to various pH values. The samples were immersed in a KOH aqueous solution or a Ca(OH) ₂ aqueous solution, annealed in a furnace whose atmosphere was adjusted to the LPG combustion gas composition, and the weight increase due to oxide scale formation was measured. For comparison, similar measurements were performed on test pieces that were only degreased. However, the annealing atmosphere is O ₂ 5Vol%,
CO ₂ 9Vol%, balance N ₂ , dew point +51°C, annealing temperature 850°C, annealing time 5 minutes. The reason why the annealing time is made longer than the usual time is to more clearly demonstrate the effects of the present invention. The measurement results of weight increase due to oxidized scale in Example 1 are shown in FIG. 1 in correspondence with the pH of each aqueous alkaline solution. From FIG. 1, it is clear that the effect of inhibiting the formation of oxide scale is exhibited in the pH range of 9 to 13 for all aqueous solutions. Example 2 Test pieces were taken from cold-rolled steel strips of SUS410, a typical martensitic stainless steel, and SUS304, a typical austenant stainless steel, to a thickness of 0.8 mm. After applying the same degreasing and cleaning treatment as in 1, the pH was adjusted to 10.
Annealing was performed after immersion in a KOH aqueous solution or a Ca(OH) ₂ aqueous solution adjusted to pH 12, and the weight increase due to oxide scale formation was measured. For comparison, similar measurements were made on test pieces that were only degreased. However, the annealing atmosphere was the same as in Example 1, and the annealing temperature was 800℃ for SUS410 steel and 1100℃ for SUS304 steel.
℃, and the annealing time was 5 minutes in both cases. In this example, as in Example 1, the annealing time was slightly longer than usual in order to more clearly demonstrate the effects of the present invention. The measurement results of the weight increase due to oxide scale in Example 2 are shown in Table 1 according to each steel type and each type of alkaline aqueous solution. It is clear from Table 1 that the effects of the present invention are exhibited in both cases of martensitic stainless steel cold rolled steel strips and austenitic stainless steel cold rolled steel strips. Example 3 A typical type of ferritic stainless steel
A test piece was taken from a cold-rolled SUS430 steel strip with a thickness of 1.0 mm, degreased and cleaned in the same manner as in Example 1, and then immersed in a Ca(OH) ₂ aqueous solution adjusted to PH = 12. Prepare something that does not involve soaking,
Annealing was performed at 850° C. for 3 minutes in the same atmosphere as in Example 1. The difficulty of descaling both test pieces after annealing was investigated using two types of descaling methods shown in (A) and (B) below. (A) Nitric acid electrolysis only Conditions: 110 g/l HNO ₃ aqueous solution, temperature 60°C, current density 20A/dm ² with the test piece as the positive pole (B) Salt treatment → nitric acid electrolysis Salt treatment conditions: Molten alkali at 430°C to salt
Soak for 15 seconds. Nitric acid electrolysis conditions: Same conditions as (A) In both descaling methods, the nitric acid electrolysis time was varied in the range of 1 to 8 seconds, and the surface after descaling was observed with a high-magnification optical microscope to ensure that the scale was complete. It was determined whether or not it had been removed. The determination results are shown in Table 2 in correspondence with the nitric acid electrolysis time. It is clear from Table 2 that by performing the annealing pretreatment of the present invention, descaling becomes easy in any descaling treatment method. In addition, among cold-rolled stainless steel strips, ferritic steel strips are required to have particularly excellent surface gloss, so of the test specimens shown in Table 2, scale was completely removed under each descaling condition. The glossiness of the surface after the descaling treatment was measured for the test pieces at the time, that is, the test pieces a, b, c, and d in Table 2. The glossiness is measured by irradiating white light onto the sample surface at an incident angle of 20°C and measuring the intensity of the opposite light.
Specular reflection light intensity in case of standard plate (black glass)
Shown as a ratio when set to 100. The results obtained are shown in Table 3. It is clear that no matter which method was used for descaling, the surface gloss after descaling was better in the test piece that was annealed after performing the annealing pretreatment of the present invention. This is because the pre-annealing treatment of this invention suppresses the formation of oxide scale during annealing, and therefore the subsequent descaling treatment is performed under mild conditions (in this example, the nitric acid electrolysis time is shortened). This is because it became possible. As is clear from the above examples, by applying the annealing pretreatment method of the present invention to the production of stainless steel cold rolled steel strip, oxide scale formation during annealing can be suppressed and descaling treatment efficiency can be improved. This is a significant improvement over the conventional method, and this results in significant effects such as a significant improvement in productivity or a significant reduction in descaling processing costs. Further, according to the annealing pretreatment method of the present invention, due to the good descaling properties as described above, it is possible to improve The effect of improving surface gloss can also be obtained.

【table】

【table】

【table】 [Brief explanation of the drawing]

Figure 1 shows KOH or Ca(OH) ₂ of various pH values on the surface of a SUS430 cold-rolled steel sheet after degreasing as pre-annealing treatment.
FIG. 2 is a diagram showing the oxidation weight gain of a steel plate when an aqueous solution is applied and annealed, compared with a case without pretreatment.

Claims (1)

[Scope of Claims] 1. A method for producing a stainless steel cold rolled steel strip in which continuous annealing and descaling treatment is performed after cold rolling, wherein the cold rolled steel strip is Pre-annealing treatment for cold-rolled stainless steel strip, which is characterized by degreasing the surface and then applying an aqueous solution of KOH or Ca(OH) ₂ adjusted to a pH of 9 to 13 to the surface of the cold-rolled steel strip. Method.