JP3550996B2 - Method for producing austenitic stainless steel sheet with excellent surface properties - Google Patents

Description

【００２８】
本発明例（鋼板Ｎｏ．１〜Ｎｏ．８、Ｎｏ．１４ ）には、雲状模様の発生はなく、良好な表面性状を有する冷延板である。これに対し、本発明の範囲を外れる比較例（鋼板Ｎｏ．９〜Ｎｏ．１３ ）では、雲状模様の発生が見られた。
【００２９】
【発明の効果】
本発明によれば、オーステナイト系ステンレス鋼の冷延焼鈍酸洗板に発生する雲状模様を安定してかつ短時間の酸洗で防止することが可能となり、表面性状に優れたオーステナイト系ステンレス冷延鋼板が安価に提供できるという産業上格段の効果を奏する。
【図面の簡単な説明】
【図１】雲状模様の発生におよぼす熱延板の巻取温度ＣＴと、弗酸濃度と硝酸濃度の比、αとの関係を示すグラフである。
【図２】溶解量とαの関係を示すグラフである。
【図３】溶解量とαの関係を示すグラフである。
【図４】雲状模様の発生におよぼす酸洗時間と弗酸濃度の関係を示すグラフである。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for manufacturing an austenitic stainless steel sheet, and more particularly to improving the surface properties of a steel sheet.
[0002]
[Prior art]
A cold rolled stainless steel sheet is obtained by hot rolling a slab into a hot rolled sheet, annealing the hot rolled sheet for recrystallization, and pickling (hereinafter, HAP) for removing scale generated by the hot rolled sheet annealing. ), Cold rolling, finish annealing, and pickling (hereinafter, also referred to as CAP).
[0003]
The gloss and surface properties of a steel sheet that has been cold-rolled as an end product and that has been annealed and pickled (hereinafter, also referred to as a cold-rolled annealed pickled sheet) are mainly affected by cold rolling conditions and CAP conditions. However, surface defects that cannot be smoothed by cold rolling after HAP may remain, which deteriorates gloss and surface properties of the final product. Particularly, in austenitic stainless steel, since the surface removal amount during HAP is as small as 2 to 3 μm, defects generated in a steel sheet annealed after hot rolling (hereinafter, also referred to as a hot-rolled annealed sheet) tend to remain.
[0004]
Japanese Patent Publication No. 3-60920 discloses a method of improving the descaling method of a hot-rolled austenitic stainless steel sheet to produce a cold-rolled austenitic stainless steel sheet having excellent surface gloss without rough surface. .
In the technique described in Japanese Patent Publication No. 3-60920, a hot rolled sheet of austenitic stainless steel is subjected to mechanical preliminary descaling, and then 100 to 400 g of nitric acid and 75 to 400 g of hydrofluoric acid per liter of nitric hydrofluoric acid aqueous solution. This is a method for producing an austenitic stainless steel sheet which is descaled using an aqueous nitric hydrofluoric acid solution to form a general corrosion state without groove corrosion and without etch pits, followed by cold rolling and bright annealing. This method is characterized in that pickling is performed in a high-concentration nitric hydrofluoric acid aqueous solution to increase the amount of surface abrasion to smooth the steel sheet surface after HAP.
[0005]
[Problems to be solved by the invention]
One of the problematic surface defects of an austenitic stainless steel sheet is a cloud-like pattern. This cloud-like pattern is basically attributable to the occurrence of regions having a difference in grain boundary erosion and the formation of intragranular pits on the steel sheet surface after HAP. Such a difference depends on the difference of the erosion depth and the erosion width. In an extreme case, a region having almost no grain boundary erosion or intragranular pits may occur.
[0006]
If a region having uneven surface properties is present in the form of a cloud over several centimeters to several meters on the surface of the steel sheet after HAP, it is difficult to uniformize (smooth) by cold rolling, and even after CAP. It remains as a pattern and deteriorates the surface properties of the final product.
The present inventors have applied the technique described in Japanese Patent Publication No. 3-60920 to make the surface of the steel sheet after HAP a smooth surface free of intergranular erosion and intragranular pits. Although we tried to eliminate the pattern in a laboratory, there was a case where it was effective to perform pickling for 60 seconds or more for a long time, but a comparison of about 30 seconds, which is the pickling time for efficient treatment in HAP. In pickling for a short period of time, the effect varies for each hot-rolled sheet, preventing the formation of a cloud pattern and failing to stably obtain a uniform steel sheet surface.
[0007]
Therefore, an object of the present invention is to provide a method for producing an austenitic stainless steel cold-rolled steel sheet having excellent surface properties with high efficiency, which prevents the above-described cold-rolled annealed pickled plate from forming a cloud pattern. .
[0008]
[Means for Solving the Problems]
The present inventors have earnestly studied the cause of erosion unevenness occurring on the steel sheet surface during HAP, and as a result, have newly found that the erosion unevenness is affected by the scale attached to the steel sheet surface before HAP.
If the coil winding temperature (hereinafter, also referred to as CT) after the completion of hot rolling is high, oxidation proceeds further even in a state where the coil is wound, and the scale thickness of the steel sheet surface increases. For this reason, internal oxidation easily proceeds in the next step of hot-rolled sheet annealing, and a Cr-free layer is deeply formed in the grain boundary oxidized portion and the intragranular pit-shaped oxidized portion. When pickling the hot-rolled annealed sheet in the surface state as described above, if the pickling time is long and the amount of dissolution is sufficient, the Cr removal layer is completely dissolved, and although there are grain boundary erosion and intragranular pits, The nonuniformity of the degree of erosion depending on the location at each position of the steel plate is small and is not recognized as a pattern with nonuniform color tone as a whole. However, if the pickling time is short and the amount of dissolution is insufficient, the Cr-free layer is not completely dissolved, and the erosion depth becomes non-uniform and is recognized as a pattern.
[0009]
On the other hand, when CT after hot rolling is low, oxidation progresses little in the state of being wound up by the coil, and thus the scale thickness of the formed steel sheet surface is relatively small. However, after finishing hot rolling, since the steel sheet is cooled to lower the CT, the scale thickness tends to be partially nonuniform at each position of the steel sheet.
In particular, the scale at the front and rear end portions of the steel sheet is not uniform, and when such a hot-rolled steel sheet is annealed, a region where almost no grain boundary oxidation or intra-grain pit-like oxidation occurs is generated. The region where grain boundary oxidation or intragranular pit-like oxidation is present is not as high as in the case of high CT, but a Cr-free layer is formed, while the region without grain boundary oxidation or intragranular pit-like oxidation is a Cr-free region. Almost no layers are formed.
[0010]
When the hot-rolled annealed sheet having the above-mentioned surface state is pickled, the surface of the steel sheet after pickling is divided into a portion having intragranular pits and grain boundary erosion and a portion having almost no erosion, resulting in uneven erosion and a pattern.
The present inventors further studied a method of pickling a hot-rolled annealed sheet based on the above findings. As a result, they came to the conclusion that the pickling conditions should be adjusted according to the CT conditions of the hot-rolled steel sheet in order to prevent the cloud pattern.
[0011]
In other words, since the surface state of the hot-rolled steel sheet before pickling differs depending on the winding temperature CT, the pickling solution is a nitric hydrofluoric acid aqueous solution with a high hydrofluoric acid concentration. Under the pickling conditions that allow rapid dissolution, on the other hand, when the CT is low, the concentration of hydrofluoric acid is adjusted according to the CT of the hot-rolled steel sheet, under the pickling conditions that make the dissolution rate of the dechromized layer and the dissolution rate of the base metal part as equal as possible. And the concentration of nitric acid and α are adjusted.
[0012]
The experimental results that are the basis of this idea are shown in FIGS. The experimental results in FIGS. 2 and 3 are the results after immersion in an aqueous nitric hydrofluoric acid solution of 100 g / l hydrofluoric acid (liquid temperature: 60 ° C.) for 40 sec.
FIG. 2 shows the relationship between the amount of 13% Cr-8.5Ni steel dissolved by pickling and α corresponding to the composition of the Cr-free layer. When α is in the range of 1.25 to 2.5, a high dissolution amount is obtained.
[0013]
FIG. 3 shows the relationship between α and the amount of 18% Cr-8.5% Ni steel corresponding to the base metal dissolved by pickling. When α is in the range of 2.5 to 6.5, a high dissolution amount is obtained. If α is within this range, the difference in the amount of dissolution between the base material portion and the Cr-free layer is about three times. On the other hand, when α is about 1.5 times, this difference becomes 10 times.
Thus, it has been found that by changing α, the pickling conditions can be easily changed to conditions under which the de-Cr layer can be rapidly dissolved, and conditions under which the dissolution rate of the de-Cr layer and the base material can be made as close as possible. Was.
[0014]
The present invention has been made based on the above findings.
That is, the present invention provides a hot-rolled sheet of austenitic stainless steel material by hot rolling, winding the hot-rolled sheet around a coil, further annealing the hot-rolled sheet, and then descaling by pickling. And then cold-rolled to the final thickness, followed by annealing and pickling, in a method for producing an austenitic stainless steel sheet, wherein the hot-rolled sheet has a winding temperature CT (° C.) of 500 to 900 ° C. Further, the pickling solution used in the descaling treatment is a nitric hydrofluoric acid aqueous solution containing 100 to 300 g / l of hydrofluoric acid, and the hydrofluoric acid concentration (g / l) and the nitric acid concentration in the nitric hydrofluoric acid aqueous solution (G / l), α within the range of 1.25 or more and 6.5 or less, and according to the winding temperature CT, the following equations (1) and (2): CT ≦ −75 × α + 1100 (1)
CT ≧ −100 × α + 950 (2)
(Here, CT: winding temperature of hot-rolled sheet (° C.), α: ratio of hydrofluoric acid concentration (g / l) and nitric acid concentration (g / l) in nitric acid aqueous solution (= hydrofluoric acid concentration / (Nitric acid concentration)) is a method for producing an austenitic stainless steel sheet having excellent surface properties, characterized in that descaling is carried out within a range satisfying the following conditions. Is preferably performed.
[0015]
The above-mentioned relationship between α and CT indicates the relationship between CT and CT (A, 1.25, 900), B (1.25, 825), and C (4.5, 500) in FIG. , D (6.5, 500), E (6.5, 612.5), and F (2.67, 900).
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
First, the reasons for limitation in the present invention will be described.
In the present invention, the winding temperature CT of the hot rolled sheet is limited to a temperature range of 500 to 900 ° C.
If the CT exceeds 900 ° C., oxidation proceeds even after being wound on a coil, and the thickness of the scale becomes thicker, so that long-time pickling is required for scale removal, and the productivity is reduced. On the other hand, when the CT is lower than 500 ° C., the deformation resistance of the steel sheet becomes high, and it becomes difficult to wind the coil. For these reasons, the CT of the hot rolled sheet was limited to 500 to 900 ° C.
[0017]
The hot rolled sheet wound around the coil is preferably subjected to hot rolled sheet annealing, and then subjected to preliminary descaling by mechanical means or chemical means.
Pre-descaling by mechanical means is preferably performed by using shot blasting, but mechanical means such as roll bending and light rolling may be used.
The preliminary descaling by chemical means may be pickling with nitric acid or sulfuric acid, or electrolytic treatment using these acids.
[0018]
The hot rolled annealed plate is then subjected to descaling by pickling.
The pickling liquid used in the descaling treatment is a nitric hydrofluoric acid aqueous solution containing hydrofluoric acid in the range of 100 to 300 g / l, and the hydrofluoric acid concentration (g / l) and the nitric acid concentration (g / l) in the nitric hydrofluoric acid aqueous solution are determined. Is within the range of 1.25 or more and 6.5 or less, and according to the winding temperature CT, the following equations (1) and (2): CT ≦ −75 × α + 1100 (1)
CT ≧ −100 × α + 950 (2)
(Here, CT: winding temperature of hot-rolled sheet (° C.), α: ratio of hydrofluoric acid concentration (g / l) and nitric acid concentration (g / l) in nitric acid aqueous solution (= hydrofluoric acid concentration / Nitric acid concentration)).
[0019]
If α is less than 1.25, the dissolution rate decreases due to the passivating effect of nitric acid (HNO ₃ ). On the other hand, when α exceeds 6.5, the amount of nitric acid (HNO ₃ ) responsible for the cathode reaction becomes insufficient, and the dissolution rate decreases. For this reason, α was limited to the range of 1.25 to 6.5.
Further, in the present invention, α is adjusted according to the winding temperature CT.
When CT is high, α is reduced in the range of 1.25 to 6.5, and when CT is low, α is increased in the range of 1.25 to 6.5. When α is small, the de-Cr layer is rapidly dissolved, while when α is large, the dissolution rate of the de-Cr layer is reduced and the dissolution rate of the base material is increased. Become. As described above, by adjusting α in accordance with CT, erosion unevenness of the hot-rolled annealed plate is suppressed, and the cloud-like pattern of the cold-rolled annealed pickled plate can be suppressed.
[0020]
Based on the above-mentioned idea, in the present invention, α is specifically in the range of 1.25 to 6.5, and according to the winding temperature CT, the following formulas (1) and (2) CT ≦ − 75 × α + 1100 (1)
CT ≧ −100 × α + 950 (2)
(Here, CT: winding temperature of hot-rolled sheet (° C.), α: ratio of hydrofluoric acid concentration (g / l) and nitric acid concentration (g / l) in nitric acid aqueous solution (= hydrofluoric acid concentration / (Nitric acid concentration)) is adjusted to satisfy the range.
[0021]
The range of the present invention is illustrated by the relationship between CT and α, and is shown in FIG.
A (1.25, 900), B (1.25, 825), C (4.5, 500), D (6.5, 500), E (6.5, 612.5) shown in FIG. , F (2.67, 900) are within the range of the present invention within the range connecting the points with straight lines. () Indicates (α, CT).
If the hot-rolled annealed plate is pickled under conditions that satisfy the relationship between CT and α in this range, the surface is uniformly dissolved, there is no erosion unevenness, and cloud-like patterns on the surface of the cold-rolled annealed pickled plate occur. Be suppressed. On the other hand, when the hot-rolled annealed plate is pickled under conditions outside of the relationship between CT and α in this range, the hot-rolled annealed plate is unevenly dissolved, erosion unevenness occurs, and a cloud-like pattern is generated on the cold-rolled annealed pickled plate. .
[0022]
Note that the pickling temperature is preferably 30 to 80 ° C. The temperature is more preferably 55 to 65 ° C. in consideration of the pickling speed and the evaporation of the acid solution.
In order to carry out the high-efficiency production aimed at by the present invention, the pickling time must be within 45 seconds. If the time is exceeded, the processing speed is reduced, and the productivity is reduced.
[0023]
FIG. 4 shows the relationship between the concentration of hydrofluoric acid in the aqueous nitric hydrofluoric acid solution and the pickling time, which affects the occurrence of the cloud pattern. If the concentration of hydrofluoric acid in the aqueous nitric hydrofluoric acid solution is less than 100 g / l, the pickling time for suppressing the formation of a cloud pattern becomes too long, exceeding 45 sec, while the concentration of hydrofluoric acid exceeds 300 g / l. In this case, the effect is saturated and an effect commensurate with the amount of added hydrofluoric acid cannot be expected, which is economically disadvantageous. For this reason, the concentration of hydrofluoric acid in the aqueous nitric hydrofluoric acid solution is set in the range of 100 to 300 g / l.
[0024]
The austenitic stainless steel according to the present invention includes SUS 304, SUS 304L, SUS 316, SUS 316L, and the like.
In the present invention, hot rolling, cold rolling, cold rolled sheet annealing, cold rolled sheet pickling, and the like may be performed under generally known conditions, and are not particularly limited.
[0025]
【Example】
The SUS 304 steel material was hot-rolled into a hot-rolled sheet having a thickness of 4 mm, and was wound around a coil at a winding temperature CT shown in Table 1. Next, these hot-rolled sheets were annealed at an annealing temperature of 1150 ° C. in a HAP line, and then subjected to preliminary descaling by shot blasting, or without the preliminary descaling, the hydrofluoric acid concentration (g / l) shown in Table 1. The hot-rolled annealed plate was pickled in an aqueous nitric hydrofluoric acid solution having a ratio of hydrofluoric acid concentration (g / l) and nitric acid concentration (g / l) of α. Thereafter, these hot-rolled annealed pickled sheets were cold-rolled into 1 mm cold-rolled sheets, then annealed at 1100 ° C., subjected to neutral salt electrolytic pickling, and aqueous nitric hydrofluoric acid pickling, and visually observed as a cloud. The presence or absence of a pattern was determined. The neutral salt electrolytic pickling conditions of the cold rolled annealed plate were as follows: electrolytic solution concentration Na ₂ SO ₄ : 200 g / l, liquid temperature: 80 ° C., electrolytic condition: 75 C / dm ² . The pickling conditions of the aqueous solution of nitric hydrofluoric acid of the cold-rolled annealed sheet were HF: 25 g / l, HNO ₃ : 50 g / l, and the liquid temperature: 55 ° C., immersion for 30 sec.
[0026]
Table 1 shows the results of the determination of the cloud pattern.
[0027]
[Table 1]

[0028]
The examples of the present invention (steel sheets No. 1 to No. 8 and No. 14) are cold rolled sheets having no cloudy pattern and excellent surface properties. On the other hand, in comparative examples (steel sheets No. 9 to No. 13) out of the range of the present invention, generation of a cloud pattern was observed.
[0029]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to prevent the cloud-like pattern which generate | occur | produces in the cold-rolled annealing pickling plate of austenitic stainless steel stably and in a short time pickling, and to obtain the austenitic stainless steel cold-rolled steel excellent in surface property. This has a remarkable industrial effect in that rolled steel sheets can be provided at low cost.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the winding temperature CT of a hot-rolled sheet and the ratio of the concentration of hydrofluoric acid to the concentration of nitric acid and α, which affect the formation of a cloud pattern.
FIG. 2 is a graph showing the relationship between the amount of dissolution and α.
FIG. 3 is a graph showing the relationship between the amount of dissolution and α.
FIG. 4 is a graph showing the relationship between the pickling time and the concentration of hydrofluoric acid on the occurrence of a cloud pattern.

Claims (2)

The austenitic stainless steel material was hot-rolled into a hot-rolled sheet, the hot-rolled sheet was wound around a coil, then the hot-rolled sheet was annealed, and then subjected to descaling by pickling, followed by final treatment. In a method for producing an austenitic stainless steel sheet, which is cold-rolled to a sheet thickness, and then annealed and pickled, the winding temperature CT (° C.) of the hot-rolled sheet is set to a temperature range of 500 to 900 ° C., and the descaling is further performed. The pickling solution used in the treatment is a nitric hydrofluoric acid aqueous solution containing hydrofluoric acid in the range of 100 to 300 g / l, and the hydrofluoric acid concentration (g / l) and the nitric acid concentration (g / l) in the nitric hydrofluoric acid aqueous solution are determined. Is set within the range of 1.25 to 6.5 and within the range satisfying the following formulas (1) and (2) according to the winding temperature CT, and the descaling process is performed. Aus with excellent surface properties characterized by performing Method of manufacturing a night stainless steel plate.
Notation CT ≦ −75 × α + 1100 (1)
CT ≧ −100 × α + 950 (2)
Here, CT: winding temperature of hot-rolled sheet (° C)
α: ratio of hydrofluoric acid concentration (g / l) and nitric acid concentration (g / l) in nitric acid aqueous solution (= hydrofluoric acid concentration / nitric acid concentration) The method for producing an austenitic stainless steel sheet according to claim 1, wherein preliminary descaling is performed before the descaling treatment by pickling.

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