JPH11269539A - Manufacture of austenitic stainless steel sheet excellent in descaling property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a descaling technique capable of always completing descaling independently of fluctuations in pickling conditions by controlling annealing conditions in the continuous annealing and pickling stage of an austenitic stainless steel sheet. SOLUTION: At the time of continuously annealing an austenitic stainless steel sheet, the thickness of the scale formed at the time of annealing is estimated on the basis of annealing conditions and the annealing conditions are modified so that a scale thickness capable of facilitating scale removal at pickling is obtained. At the time of this modification, it is preferable, for example, that the scale thickness capable of facilitating scale removal at pickling is set at <=0.157 μm or >=0.25 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing an austenitic stainless steel sheet, and more particularly to a method for producing austenitic stainless steel sheet in a continuous annealing pickling process by controlling annealing conditions in accordance with the thickness of oxide scale generated during annealing. The present invention relates to a method for producing an austenitic stainless steel sheet with improved descaling properties.

[0002]

2. Description of the Related Art Conventionally, as a method for descaling steel strip in a continuous annealing pickling facility, for example, Japanese Unexamined Patent Publication No.
No. 569 discloses a method of replenishing an acid by analyzing the acid concentration or replenishing the acid according to the consumption of the acid. Also, Japanese Patent Application Laid-Open No. 54-069527 discloses a method of maintaining an appropriate pickling ability by periodically measuring the amount of dissolved metal. Japanese Patent Application Laid-Open No. Hei 03-28386 discloses a method of increasing the pickling capacity by controlling the ratio of the total iron concentration in the pickling solution to the concentration of divalent and trivalent iron ions. Discloses a method for controlling pH.

[0003]

In the method of maintaining the acid condition at the time of pickling and descaling the steel strip to the optimum condition as in each of the above conventional examples, it is necessary to measure or predict the acid concentration. Acid must be replenished based on the results. However, since the measurement and prediction of the acid concentration are performed in a batch process, the control is not performed in real time, and the effective acid concentration necessarily decreases and the dissolved metal ion concentration increases until the next measurement. Therefore, the acid capacity decreases. Therefore, under the condition immediately before acid replenishment, descaling failure may occur.

The present invention has been made in order to solve the problems of the conventional descaling technique, and controls the annealing conditions in the continuous annealing and pickling process of an austenitic stainless steel sheet to thereby achieve pickling. It is an object of the present invention to provide a descaling technique that can always complete descaling regardless of changes in conditions.

[0005]

Means for Solving the Problems In order to achieve the above object, the present invention according to claim 1 provides a method for continuously annealing an austenitic stainless steel sheet, the thickness of a scale formed during annealing based on annealing conditions. And annealing conditions are corrected so that the scale thickness is easily removed by pickling.

According to a second aspect of the present invention, in the method for manufacturing an austenitic stainless steel sheet according to the first aspect of the present invention, in correcting the annealing conditions of the continuous annealing, scale is easily removed by pickling. The scale thickness is set to 0.15 μm or less or 0.25 μm or more.

According to a third aspect of the present invention, in the method for manufacturing an austenitic stainless steel sheet according to the first or second aspect, the annealing conditions include a soaking temperature and a soaking time of an annealing furnace. Is used.

Here, the soaking temperature is the temperature of the steel strip on the exit side of the annealing furnace (maximum temperature reached), and the soaking time is the annealing time after the temperature of the steel strip reaches the recrystallization start temperature (soaking temperature). It is time to leave the furnace.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The inventor of the present invention has conducted intensive studies on the improvement of descaling properties in the annealing pickling process for producing an austenitic stainless steel sheet. The inventors have found that the descalability of the oxide scale sometimes formed depends only on the thickness of the oxide scale, and have accomplished the present invention. That is, in the present invention, the soaking plate temperature and the soaking time in the annealing step are controlled so that the thickness of the oxide scale is always maintained in the appropriate descaling region. Thus, even if the pickling conditions gradually change within the range shown in Table 1, for example, and gradually approach the lower limit, the desalting of the austenitic stainless steel sheet can always be completed.

The pickling tank is composed of three tanks, the first tank being neutral salt electrolysis, the second tank being mixed acid (hydrofluoric acid and nitric acid), and the third tank being nitric acid electrolysis.

[0011]

[Table 1]

The embodiment will be described below. First,
The present inventor has made it possible to indirectly estimate the thickness of the oxide scale from the annealing conditions from the relationship between the annealing conditions and the oxide scale thickness obtained as a result of many experiments. The details are described below.

Table 2 shows the range of chemical components of the austenitic stainless steel to which the present invention can be applied.

[0014]

[Table 2]

An austenitic stainless steel having a component composition in this range is annealed by varying the soaking plate temperature and soaking time within the range of annealing conditions shown in Table 3.
The relationship between the change in the annealing conditions and the thickness of the oxide scale was determined by measuring the thickness of the formed oxide scale. However, it is generally difficult to directly measure the thickness of the oxide scale. Therefore, in the present invention, the scale thickness is calculated indirectly from the sputtering time up to the Si peak by GDS emission spectroscopy.

[0016]

[Table 3]

FIG. 1 shows the relationship between the annealing conditions described by the soaking plate temperature and the soaking time and the sputtering time. In the present invention, the soaking plate temperature is defined as "temperature of steel strip on the exit side of the annealing furnace (maximum reached plate temperature), ° C, and may be abbreviated as Tss hereinafter). In addition, the soaking time is defined as "the time from when the temperature of the steel strip reaches the recrystallization start temperature (soaking start temperature) to when the steel strip exits the annealing furnace, sec. I do.

FIG. 1 shows that the higher the temperature and the longer the annealing time, the longer the sputtering time up to the Si peak, that is, the thicker the oxide scale layer.

[0019] Analysis result of the GDS, most of oxide scale was found to be present as Cr ₂ 0 _3. Oxidation Behavior of cr ₂ 0 ₃ in accordance with the parabolic law shown in the following equation (1), it is known to form a dense and uniform outer oxide layer.

ΔW = (kp.t) ^1/2 (1) ΔW: increase in oxidation (g / cm ² ) kp: oxidation rate constant (g ² / cm ⁴ · sec) t: time (sec) oxide scale produced is only Cr ₂ 0 _3, when the generation time of a parabolic law holds, the temperature dependence of the oxidation rate constants kp for Cr ₂ 0 ₃ follows the kinetically following equation based on the Arrhenius equation.

Kp = 3.16 × 10 ⁻⁴ exp (−2.06 × 10 ⁴ / T) (2) T: temperature (° C.) Substituting equation (2) into equation (1), Rearranging ^{T = 2.06 × 10 4 /ln(3.16×10 -4} · t / ΔW 2) ... (3) On the other hand, Cr ₂ 0 ₃ of density 5.2g / cm ^3, Cr atomic weight of 52 Assuming that the atomic weight of oxygen is 16, the relationship between the thickness ΔL (μm) of the oxide scale and the increase in oxidation ΔW is as follows: ΔW = 5.2 × 10 ⁻⁴ ΔL × 3 × 16 / (2 × 52 × 16 × 16) (4) By substituting equation (4) into equation (3), T = 2.06 × 10 ⁴ / ln (1.17 × 10 ⁴ · t / ΔL ² ) (5) is obtained.

Further, when the temperature T and the time t in the equation (5) are converted into the soaking plate temperature Tss and the soaking time ts defined above, the following equation (6) is obtained. Tss = 2.06 × 10 ⁴ /ln(2.93×10 ³ · ts / ΔL ² ) −273 (6) By using this equation (6), an arbitrary soaking plate temperature and soaking time can be obtained. It is now possible to determine the oxide scale thickness ΔL generated for. FIG. 2 is a contour diagram showing the relationship between the theoretical scale thickness and the annealing conditions according to equation (6).

The fact that the contour diagrams of FIGS. 1 and 2 are similar indicates that the formation behavior of the oxide scale of the austenitic stainless steel sheet follows the parabolic law. Then, when the thickness of the oxide scale is linearly approximated by the sputtering time up to the Si peak by GDS analysis, equation (7) is obtained. FIG. 3 shows the relationship between the theoretical oxide scale thickness ΔL and the sputtering time tsi up to the Si peak by GDS.

Tsi = 4.42 × 10 · ΔL + 1.02 (7) In the present invention, the oxide scale thickness can be indirectly estimated based on the annealing conditions as described above.

Next, control of annealing conditions based on the thickness of the oxidized scale in descaling according to the present invention will be described. From the oxide scale thickness obtained as described above, the appropriate descaling annealing range (heating plate temperature and heating time) of the austenitic stainless steel sheet can be limited as follows. The conditions are the lower limit of the range of pickling conditions that can be practically used as shown in Table 1 above.

The present inventor has found that the descalability of austenitic stainless steel can be classified into the following three types based on the properties of the formed scale and the scale thickness. (1) The low-temperature short-time type annealing produces a glossy, yellowish, temper-colored scale. The scale thickness is 0.15 μm or less, and most of the scale is dissolved by the first neutral salt electrolytic pickling. As a result, the contact area between the surface of the iron base and the mixed acid increases in the subsequent mixed acid immersion process, and complete descaling is completed even though the dechromized layer has not developed much.

(2) A high-temperature long-time type annealing produces a green scale with no gloss. Although the scale thickness is 0.25 μm or more, it is hardly removed by neutral salt electrolysis and remains covered by a thick scale, but Cr having an affinity for oxygen larger than Fe is preferentially selectively oxidized and diffused outward. The dechromized layer is deeply developed. Therefore, the permeability of the acid in the subsequent mixed acid immersion is high, the dissolution of the scale is promoted, and the descaling is completed.

(3) The intermediate type annealing described above produces a reddish semi-gloss scale. The scale thickness is about 0.20 μm, and after the neutral salt electrolytic pickling, a part where the scale remains thick and a part where the ground iron is visible are mixed. Even after the subsequent immersion in the mixed acid, the scale of the portion that remains thick by the neutral salt electrolysis remains as it is, resulting in poor descaling. This is because the internal dechromed layer in the scale remaining portion is not so deep and does not lead to an increase in the permeability of the acid in the mixed acid immersion.

To summarize the above, in order to prevent poor descaling, it is sufficient to control the annealing conditions so that the descaling does not fall within the range of intermediate type annealing (scale thickness ΔL is about 0.20 μm). I can say.

Based on the above findings, in the present invention,
Scale thickness 0.15μm or less or 0.25μm
The annealing conditions are modified so as to be as described above. This makes it possible to always complete the descaling without depending on the fluctuation of the pickling conditions inevitably occurring in the actual operation, in other words, even when approaching the lower limit of the range of the pickling conditions. (Examples) Examples of the present invention will be described below.

Table 4 shows the chemical components of three types of test materials A, B and C made of austenitic stainless steel.

[0032]

[Table 4]

With respect to each of these test materials, various annealing conditions as shown in Table 5 (soaking plate temperature Tss and soaking time ts)
After the annealing, pickling was performed under the pickling conditions shown in Table 6 (set close to the lower limit of the range of the pickling conditions in Table 1), and the descalability was examined.

[0034]

[Table 5]

[0035]

[Table 6]

FIG. 4 shows the soaking plate temperature Tss and the soaking time ts.
Is a plot of the relationship between the annealing conditions represented by and the resulting descaling results. The crosses in FIG. 4 indicate annealing conditions under which descaling was not completed.

It can be seen that the theoretical scale thickness is 0.20 μm even under the pickling conditions under which the acid capacity was the lowest in actual operation.
m by performing annealing so that it does not become
It was confirmed that no descaling failure occurred.

[0038]

As described above, according to the method for producing an austenitic stainless steel sheet having excellent descaling properties according to the present invention, the thickness of the oxide scale formed during continuous annealing of the austenitic stainless steel sheet is reduced. Since the annealing conditions were estimated from the annealing conditions and the annealing conditions were modified so that the scale thickness was easily removed by pickling, defective descaling of the austenitic stainless steel sheet could be completely eliminated.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between annealing conditions and a sputtering time up to a Si peak by GDS.

FIG. 2 is a diagram showing the relationship between annealing conditions and theoretical scale thickness.

FIG. 3 is a diagram showing a relationship between a theoretical scale thickness and a sputtering time up to a Si peak by GDS.

FIG. 4 is a diagram showing a relationship between annealing conditions and actual descaling.

Claims (3)

[Claims] 1. In continuous annealing of an austenitic stainless steel sheet, the thickness of a scale formed during annealing is estimated based on the annealing conditions, and the annealing conditions are modified so that the scale thickness can be easily removed by pickling. A method for producing an austenitic stainless steel sheet excellent in descaling property. 2. The method according to claim 1, wherein, in correcting the annealing conditions of the continuous annealing, a scale thickness at which scale is easily removed by pickling is set to 0.15 μm or less or 0.25 μm or more. Manufacturing method of austenitic stainless steel sheet with excellent scale. 3. The method for producing an austenitic stainless steel sheet having excellent descalability according to claim 1, wherein the soaking conditions include a soaking temperature and a soaking time of an annealing furnace.