JP2003226990A - Ferritic stainless steel sheet and manufacturing method thereof - Google Patents

Abstract

(57) [Problem] To provide a method for producing a ferritic stainless steel sheet having excellent surface properties. When pickling a ferritic stainless steel hot-rolled steel sheet having a Cu content of 0.03% or less by mass%, after pickling with a shot blast and a grinding brush, the pickling accelerator contains a volume%. In an aqueous sulfuric acid solution having a sulfuric acid concentration of 200 g / l to 400 g / l and an aqueous solution temperature of 80 ° C. to 100 ° C.
Dip for 0 second or more and 600 seconds or less for pickling, and then roll reduction of 7
A method for producing a ferritic stainless steel sheet, which is subjected to cold rolling of 0% or more and 95% so that the major axis / minor axis is 2 or more and the falling defect area ratio is 0.1% or less.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a ferritic stainless steel sheet having excellent surface properties and a method for producing the same.

[0002]

2. Description of the Related Art In the method of producing stainless steel, hot rolling is followed by hot-rolled sheet annealing to obtain a recrystallized structure, and pickling to remove scale. At this time, it is a general method to perform mechanical descaling such as shot blasting and then immerse in an acid solution such as sulfuric acid, hydrochloric acid or nitric hydrofluoric acid.

An important characteristic of stainless steel is surface gloss, and various methods are adopted in each manufacturing process to improve the surface gloss. Since the surface gloss is significantly improved especially in cold rolling, it is extremely important to control the surface properties of the material before cold rolling. For example, if the hot-rolled and annealed sheet is pickled, and if there are large irregularities due to shot blast marks or scale residue, it is difficult to smooth the surface by cold rolling. Therefore,
The surface properties after pickling are extremely important, and it is necessary to smooth the surface under mechanical descaling conditions and pickling conditions.

As described above, sulfuric acid is often used in the pickling solution of ferritic stainless steel, but in this case, the grain boundaries are eroded, which collapses during cold rolling, resulting in mesh defects and gold. There is a problem that surface defects called dust flaws occur. The grain boundary erosion grooves generated during sulfuric acid pickling and the resulting network defects and poor gloss due to gold dust flaws are characteristic of ferritic stainless steel.
Shot blast marks can be prevented by controlling the shot blast projection density, but it is difficult to prevent grain boundary erosion grooves during sulfuric acid pickling, and it is difficult to reduce the stripping speed and surface grinding after pickling. Etc. were required. When the grain boundary erosion grooves cause collapse defects and gold dust flaws, the surface gloss is deteriorated, and corrosion is also deteriorated due to rusting from these points. It is considered that the cause of grain boundary erosion during sulfuric acid pickling is that elements that are easily dissolved in a sulfuric acid solution (such as P) segregate at the grain boundaries, but this is effective when pickling with sulfuric acid. There has been no way to prevent it.

On the other hand, gold dust flaws may also occur due to the unevenness of shot blast marks.
In Japanese Patent No. 43768, in addition to the increase in shot blast projection energy, a pickling accelerator is added to a pickling tank to improve pickling ability, and an increase in surface roughness due to an increase in shot blast projection energy is prevented. It is disclosed that by increasing the pickling rate, it is possible to increase the pickling rate without increasing surface defects called “glitter”. However, in this method, although the pickling accelerator is added to the pickling aqueous solution, the shot blasting projection density is increased and the line passing speed is increased, so that there is variation in the shot blasting. However, there is a problem that a large shot blast mark partially remains and the surface roughness of the steel sheet becomes uneven, and a grain boundary erosion groove, which is a defect smaller than the shot blast mark, cannot be prevented. As a result, it is impossible to prevent a cold rolling product from suffering a collapse defect or a gold dust flaw.

[0006]

SUMMARY OF THE INVENTION In order to solve the problems of the known art, the object of the present invention is to add a pickling accelerator to a sulfuric acid pickling solution to increase the amount of ablation of the entire ferritic stainless steel sheet. By suppressing the difference between the amount of ablation between the grain boundary and the grain,
The purpose of this study is to reduce the grain boundary erosion grooves of the pickled sheet and to solve the collapse defects and gold dust defects that occur during cold rolling.

[0007]

In order to solve the above-mentioned problems, the present inventors have added a pickling accelerator when pickling ferritic stainless steel with sulfuric acid, and have revealed grain boundary erosion grooves and A detailed study was conducted on mechanical descale by shot blasting and a grinding brush and cold rolling conditions as acid concentration, temperature and pickling time for suppressing collapse defects on the surface of cold-rolled sheet, and as preliminary descaling treatment for pickling. It was

The present invention provides (1) a ferritic stainless steel sheet having a major axis / minor axis of 2 or more and a collapse defect area ratio of 0.1% or less. (2) The chemical composition of ferritic stainless steel is mass%
The Cu content in Cu is 005% or more and 0.03% or less, and the ferri stainless steel sheet according to (1) above is characterized. (3) When pickling a hot rolled ferritic stainless steel sheet, the pickling accelerator is 0.01 to 1.0% by volume, the sulfuric acid concentration is 200 g / l or more and 400 g / l or less, and the temperature of the sulfuric acid aqueous solution is Manufacture of a ferritic stainless steel sheet characterized by being immersed in an aqueous sulfuric acid solution at 80 ° C. or higher and 100 ° C. or lower for 60 seconds or longer and 600 seconds or shorter, pickled, and then subjected to cold rolling at a rolling reduction of 70% or more and 95% or less. Method. (4) The chemical composition of ferritic stainless steel is mass%
2. The method for producing a ferritic stainless steel sheet according to (3) above, wherein Cu is 0.005% or more and 0.03% or less. (5) The method for producing a ferritic stainless steel sheet according to the above (3) or (4), characterized in that preliminary descaling is performed by shot blasting and a grinding brush as a pretreatment for pickling.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limitation of the present invention will be described below.

As a result of detailed investigation of the collapse defect of the cold-rolled sheet, it was found that the defect having the ratio of the major axis to the minor axis of the collapse defect of 2 or more markedly hinders the surface gloss and becomes a starting point of the gold dust defect. . Since the crystal grain boundary of the pickled plate collapses during cold rolling, the collapse defect has a major axis of the collapsed defect in the rolling direction and a minor axis of the defect in the width direction.

The method of measuring the falling defect area ratio is as follows:
The surface area of a stainless steel plate is magnified 100 times with an optical microscope having a surface area of 1 mm 2, and the total area of collapse defects having a major axis / minor axis of 2 or more is divided by 1 mm 2 and multiplied by 100 to obtain a collapse defect area ratio. did. This was measured at 10 arbitrary points and the average value was adopted.

FIG. 1 shows the relationship between the area ratio of collapse defects and the gold dust rank. Here, the gold dust rank refers to whether the surface glitters when a vinyl tape is attached to the surface of the product plate and peeled off, A, B
If it is a rank, there is no problem in surface quality, and if it is a C or D rank, the surface quality is impaired and the corrosion resistance is also deteriorated. Further, the symbols in the figure are changed for each major axis / minor axis ratio of the collapse defect. □ and ◇ with a ratio of major axis / minor axis of 1.5 or less
For the mark, the collapse defect area ratio is 0.2% or less, and the gold dust rank is A or B, which is a good result, but for the Δ and ○ marks with the major axis / minor axis ratio of 2 or more, the collapse defect area rate is 0. When the content is 1% or less, the gold dust rank is A or B, which is a good result. From this, it can be seen that gold dust is not generated and the surface quality is excellent when the surface defects in which the major axis / minor axis of the collapse defect is 2 or more are 0.1% or less.

The pickling accelerator has a function of promoting generation of hydrogen during a dissolution reaction of steel and sulfuric acid during sulfuric acid pickling, and promotes abrading of steel. The occurrence of intergranular erosion grooves during pickling with sulfuric acid is due to the difference in the ablation between the grain boundaries and within the grains. I found that it would disappear. As the pickling accelerator, an organic compound having a carboxyl group and sulfur such as mercaptoacetic acid is effective, sulfur is separated in a sulfuric acid solution, and a hydrogen generation reaction, that is, a steel is generated by adhering to the surface of the steel sheet during pickling. Promotes dissolution.
FIG. 2 shows the relationship between the pickling accelerator concentration and the occurrence of grain boundary erosion grooves in the pickled plate. Here, as the grain boundary erosion groove rank, the smallest level was set as A, and the following most severe level E was evaluated. When the rank is A or B, the cold rolled sheet is at a level at which no collapse defect or gold dust flaw occurs. From this, the concentration of the pickling accelerator needs to be 0.01% by volume. Further, the addition of a large amount makes the manufacturing cost increase and a large amount of a metal dissolution product called sludge occurs, which makes it difficult to control the operation of the pickling line. Desirably, 0.05 to 0.25% is good.

The sulfuric acid solution needs to have a concentration of 200 g / l or more and a temperature of 80 ° C. or more because it has a low concentration even if a pickling accelerator is added, and scale remains at low temperatures and grain boundary erosion grooves occur. . However, if the concentration is too high and the temperature is too high, the cost of the pickling solution is significantly increased and the pickling tank is deteriorated. Therefore, the concentration is 400 g / l or less and the temperature is 100 ° C. or less. Desirably, the concentration is 250 to 300 g / l, the temperature is 8
5 to 90 ° C is preferable.

FIG. 3 shows the relationship between the pickling time and the occurrence of grain boundary erosion grooves on the pickled plate. Therefore, the pickling time needs to be 60 seconds or more. This is because even if pickling accelerator is added in a short time,
This is because the grain boundary erosion groove appears at the initial stage of pickling, but in pickling for 60 seconds or more, the ablation in the grain progresses and the unevenness with the grain boundary disappears. However, excessively long time immersion causes quality problems such as deterioration of pickling efficiency and rough surface.
It was set to 600 seconds or less. Desirably, the pickling time of 100 seconds to 250 seconds is good. Further, according to the main pickling conditions, the base material is sufficiently abraded, so that no shot blast mark or grinding brush mark remains, and no scale remains naturally. Note that FIG.
In the above experiment, the pickling time was 100 seconds.

Conventionally, the sulfuric acid pickling time is 30 to 50.
A short pickling time of about a second was used. It has been found that the cause of generation of defects due to intergranular erosion grooves in the conventional method described in, for example, JP-A-9-143768 is mainly due to insufficient pickling time.

The cold rolling reduction rate is determined in order to obtain a predetermined sheet thickness, but the collapse defect disappears as the reduction rate increases because it is crushed as the surface of the steel sheet is smoothed by the roll. The reduction ratio required for this purpose is 70% or more. However, excessive high-pressure rolling leads to a reduction in rolling ability, so the content was made 95% or less. Preferably 7
A reduction rate of 5 to 85% is preferable.

Cu may be added to the ferritic stainless steel in order to obtain predetermined characteristics, but even if it is not added, a small amount (about 0.03 to 0.05%) of Cu is contained. I will end up. It is mainly mixed as a component contained in scrap used as a raw material for melting stainless steel. This small amount of Cu deteriorates the pickling property during the sulfuric acid pickling, resulting in a decrease in the amount of fusing. When the amount of ablation is small, the difference in ablation between the grain boundary and the grain is large, so that grain boundary erosion grooves are likely to occur. In addition, scale residue is likely to occur and scale defects are likely to occur in the product. Further, Cu in the steel is once dissolved during the sulfuric acid pickling, but becomes Cu oxide and adheres to the surface of the steel sheet. When the amount of Cu exceeds 0.03%, the Cu oxide makes the color tone of the steel sheet surface dark, which may impair the surface quality of the pickled sheet. Therefore, the amount of Cu is 0.03
It is necessary to keep the percentage below. However, excessive Cu
Reduction of the amount leads to increase of refining cost, so 0.0
It was set to be 05% or more and 0.03% or less. Further, considering the pickling property and the raw material cost, it is preferably 0.007 to 0.01.
5% is good. Cu content in stainless steel 0.03%
In order to make the following, it suffices to select the scrap raw material, for example, Al that may be used as a deoxidizer.
In order to exclude Cu contained in etc., it is sufficient to make a device such as selecting pure Al. 2 and 3, the experiment was conducted with the Cu content in the stainless steel being 0.03%.

The inventors of the present invention have found that the gold dust of ferritic stainless steel is not only caused by the grain boundary erosion grooves as described above but also generated by shot blast. The pickling of hot-rolled sheets and hot-rolled / annealed sheets is generally performed by preliminary descaling by mechanical descaling. The most common method is shot blasting, but only shot blasting may cause surface quality deterioration (gold dust) due to shot blasting marks. Therefore, in the present invention, a grinding brush is added to the shot blast as a preliminary descaling process. A grinding brush, for example, has a diameter of 1 to
5 to 10 brushes made of 2mm nylon and silicon dioxide
The bunch that is bound together is mounted on a disk, and when this is rotated, the brush uniformly acts on the surface of the steel plate. After shot blasting, by applying a grinding brush, the convex and concave parts of the unevenness due to shot blasting are scraped, so even if the hit of the shot blast varies a little, the uneven shape is homogenized and the height of the unevenness is reduced. Can be achieved. Therefore, by applying the grinding brush after the shot blasting, the deterioration of the surface quality due to the shot blasting marks, which has been a conventional problem, can be prevented and the descaling property can be improved. Note that, in FIGS. 2 to 4, since the purpose is to investigate only the presence or absence of gold dust due to the grain boundary erosion grooves, a preliminary descaling process in which a grinding brush is added to shot blast is performed.

The effects of the present invention are also effective in the case of immersing in nitric acid, hydrofluoric acid, or a mixed acid thereof, etc. after washing with sulfuric acid. Further, it is effective when the hot-rolled sheet annealing is omitted, and the hot-rolled sheet annealing may be either continuous annealing or batch annealing. In cold rolling, only the reduction rate may be taken into consideration, and the roll diameter, rolling speed, and rolling oil type may be appropriately selected. Furthermore, the surface finish is 2D, 2B,
It can be applied to all surface finishing materials such as BA products.

[0021]

[Examples] Ferritic stainless steel SUS430 having the chemical composition shown in Table 1 was melted and cast, hot-rolled, then hot-rolled sheet annealed, shot-blasted and sulfuric acid pickled. At this time, mercaptoacetic acid was selected as the pickling accelerator added during the sulfuric acid pickling. After that, it was cold-rolled to a thickness of 0.5 mm, further subjected to continuous annealing and temper rolling to obtain a product plate.

[0022]

[Table 1]

Table 1 shows the results of observing the surface texture of the 0.5 mm-thick product plate obtained as described above and evaluating the occurrence of collapse defects.

As is apparent from Table 1, No. 1 manufactured by the method of the present invention. It can be seen that the steels Nos. 1 to 9 have suppressed falling defects and have excellent surface properties as compared with the comparative steels. No. 1 of the present invention example. In No. 9, the grain boundary erosion groove rank and the collapse defect area ratio were good, and the gold dust due to the grain boundary erosion grooves could be prevented, but since the grinding brush was not used as the preliminary descaling treatment, it was caused by shot blasting. The generation of gold dust is observed.

In the comparative example, a collapse defect due to the occurrence of intergranular corrosion of the pickled plate, scale residue, and gold dust flaws occur. Comparative Example No. 10, 11
The sulfuric acid aqueous solution conditions for pickling are out of the proper range. In No. 12, the pickling time was insufficient, and both the grain boundary erosion groove rank and the collapse defect area ratio were poor, and further scale residue was generated. Comparative Example N
o. In Nos. 13 to 15, the pickling accelerator was not added or the pickling accelerator conditions were inappropriate, both the grain boundary erosion groove rank and the collapse defect area ratio were poor, and gold dust was generated. Comparative Example No. In No. 16, the cold rolling reduction ratio is insufficient and the collapse defect area ratio is poor. Comparative Example N
o. In Nos. 17 and 18, the Cu content is outside the upper limit of the appropriate range, and further, No. No. 18 does not contain the pickling accelerator, both have poor grain boundary erosion groove ranks and collapse defect area ratios, and gold dust is further generated. Comparative Example No. Since No. 19 has a high Cu of 1.2%, the oxide of dissolved Cu adheres to the surface of the steel sheet to blacken the steel sheet surface even if a pickling accelerator is added, and the surface of the steel sheet is roughened. A fall defect occurs. Comparative Example No. Two
No. 0 does not use a grinding brush as a preliminary descaling treatment and further does not use a pickling accelerator,
Both the grain boundary erosion groove rank and the collapse defect area ratio were inferior, and further scale residue was generated and shot blast-induced gold dust was generated.

[0026]

As is apparent from the above description, according to the present invention, it is possible to provide a ferritic stainless steel sheet having excellent surface properties. That is, gold dust is not generated by setting the collapse defect area ratio in which the major axis / minor axis is 2 or more to 0.1% or less. Further, when pickling a ferritic stainless steel hot rolled steel sheet, the generation of grain boundary erosion grooves and gold dust can be prevented by adding a pickling accelerator and optimizing pickling conditions. Furthermore, by setting the Cu content in the stainless steel to 0.03% or less, it is possible to suppress the generation of grain boundary erosion grooves and reduce the scale residue. Furthermore, by using a grinding brush together with shot blast as a preliminary descaling process before pickling, it is possible to prevent the generation of gold dust due to shot blast.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between an area ratio of collapse defects and a gold dust rank.

FIG. 2 is a diagram showing the relationship between the concentration of a pickling accelerator and the occurrence of grain boundary erosion grooves in a pickled plate.

FIG. 3 is a diagram showing the relationship between pickling time and occurrence of grain boundary erosion grooves in a pickled plate.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Shinji Yoshioka             3434 Shimada, Hikari-shi, Nippon Steel Works             Inside the Shoko Ironworks F-term (reference) 4E002 AA07 AD05 AD06 BC05 BD10                       CB03                 4K053 PA05 PA12 QA01 RA15 RA68                       SA06 SA13 SA17 TA02 ZA02

Claims (5)

[Claims] 1. A ferritic stainless steel sheet characterized in that the area ratio of collapse defects having a major axis / minor axis of 2 or more is 0.1% or less. 2. The ferritic stainless steel sheet according to claim 1, wherein the chemical composition of the ferritic stainless steel is, in mass%, Cu of 0.005% or more and 0.03% or less. 3. When pickling a hot rolled ferritic stainless steel, the pickling accelerator is 0.01 to 1.0% by volume,
Sulfuric acid concentration is 200 g / l or more and 400 g / l or less, and the temperature of the sulfuric acid aqueous solution is 80 ° C. or more and 100 ° C. or less.
Immerse for 0 seconds or more and 600 seconds or less, pickle and then reduce the rolling ratio to 7
A method for producing a ferritic stainless steel sheet, which comprises subjecting cold rolling to 0% or more and 95% or less. 4. The production of the ferritic stainless steel sheet according to claim 3, wherein the chemical composition of the ferritic stainless steel is 0.005% or more and 0.03% or less of Cu in mass%. Method. 5. The method for producing a ferritic stainless steel sheet according to claim 3, wherein shot blasting and preliminary descaling with a grinding brush are performed as a pretreatment for pickling.