JPH0757891B2 - Method for producing Cr-based stainless steel sheet BA product having excellent surface properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is a Cr alloy that eliminates the occurrence of gold dust defects.
The present invention relates to a method for manufacturing a BA-based stainless steel plate product.

[Conventional technology]

Conventionally, bright annealing products (BA products) of Cr-based stainless steel sheets are manufactured through the steps of annealing a hot rolled sheet, descaling, cold rolling, bright annealing in a non-oxidizing atmosphere, and skin pass rolling. It had been. Annealing of the hot-rolled sheet is performed in a box-type furnace in a state of being wound on a coil (BOX annealing), and the applicant has unrolled the hot-rolled steel strip from the coil.
According to the method described in JP-A-58-56734, etc., annealing is performed by continuously passing through the furnace (continuous annealing). Further, the annealing of the hot rolled sheet is sometimes omitted by changing the hot rolling conditions. For cold rolling, a single cold rolling method that is performed without intermediate annealing to a product sheet thickness and a double cold rolling method that performs intermediate annealing at an intermediate sheet thickness are performed.

A BA-based Cr-based stainless steel sheet produced in this manner often has a protective coating on its surface, and when it is peeled off, minute defects that are shimmering depending on how the light hits can be seen. This defect is usually called a gold dust flaw, and it is caused by the fact that a minute portion of the surface layer of stainless steel is pressure-fitted in a cover-like shape and is turned up.

As a measure against gold dust flaws, there is a method of reducing the unevenness before cold rolling, which causes minute fogging. To this end, a method of controlling heating and lubrication during hot rolling, a method of preventing intergranular corrosion that occurs during annealing and pickling of a hot rolled sheet, and a method of reducing irregularities caused by mechanical descale (Japanese Patent Publication No. 49-16698) After shot blasting, a method of dissolving and removing the surface layer by 5-40 μm by pickling treatment (JP-A-54-
72728), and the method of removing grain boundary corrosion and surface irregularities by belt grinding or the like. Other methods of controlling the surface hardness during shot blasting with Vickers hardness of 180-350 (Japanese Patent Publication No. 60-56768) and peeling off the surface layer by generating heat streak during cold rolling (Japanese Patent Laid-Open No. 61-4970).
There is 5).

[Problems to be Solved by the Invention]

The current situation is that the gold dust flaws of Cr-based stainless steel sheet BA products cannot be completely prevented even by the above conventional measures. And, compared with 2B products manufactured by annealing pickling after cold rolling, the BA products of Cr-based stainless steel sheets have a small surface roughness after processing and have good abrasiveness, so deep drawing It is often used for various processes such as swelling and overhanging.

An object of the present invention is to produce a Cr-based stainless steel sheet BA product which has almost no gold dust and is excellent in workability.

[Means and actions for solving the problems]

The present invention, by cold rolling a hot rolled sheet of Cr-based stainless steel, after intermediate annealing, by acid treatment with a single acid selected from nitric hydrofluoric acid, sulfuric acid, sulfuric nitric acid or a combination of the acid treatment It is a method for producing a Cr-based stainless steel sheet BA product having excellent surface properties, which comprises subjecting a surface phase to 1 μm or more to ablation, followed by cold rolling and bright annealing.

In the present invention, the Cr-based stainless steel is a martensitic stainless steel such as SUS410 and a ferritic stainless steel such as SUS430. The hot-rolled sheet is a hot-rolled steel strip or steel sheet, which is annealed or descaled without annealing and then cold-rolled. The cold rolling can be performed by a double cold rolling method with intermediate annealing, reverse rolling by a multi-stage rolling machine such as Sendzimir rolling, or tandem rolling by a 4Hi rolling machine. After the intermediate annealing, acid treatment is applied to the surface layer of the stainless steel plate to 1 μm.
The above is uniformly ablated. As a pretreatment for the acid treatment, a salt treatment of immersing in a molten salt may be performed. After cold rolling to the product sheet thickness, bright annealing is performed in a non-oxidizing atmosphere, and skin pass rolling is performed to obtain a BA product.

In the present invention, the ablation of the stainless steel sheet surface layer by the acid treatment after the intermediate annealing may be performed by immersing in a single acid selected from hydrofluoric acid, sulfuric acid and sulfuric nitric acid, or by spraying an acid. Alternatively, a combination treatment such as a sulfuric acid treatment followed by a nitric hydrofluoric acid treatment may be performed. By such an acid treatment, the surface of the stainless steel plate is ablated so as not to cause local deep dents such as intergranular corrosion and etch pits.

In order to improve the workability of ferritic stainless steel sheets and suppress the occurrence of ridging, the cold rolling method has been conventionally performed twice. However, in the conventional two-time cold rolling method for a ferritic stainless steel sheet, a salt treatment is performed after intermediate annealing, and then electrolysis (nitric acid electrolysis) is performed in a nitric acid aqueous solution to descale. This is based on the idea of improving the surface gloss of the stainless steel plate after descaling to improve the surface gloss after the subsequent cold rolling. In addition, the intermediate annealing may be performed by bright annealing.

As a result of observing the surface of the stainless steel plate in each step in the conventional two-time cold rolling method in detail, the present inventors have found that the uneven projections on the surface of the stainless steel plate before cold rolling are spread by rolling, I found out that the edges of the sword spread and became a tiny cover, which became a gold dust flaw. The minute fog generated in the first cold rolling remains almost as it is after the intermediate annealing and the acid treatment in the next step, and remains as it is in the intermediate bright annealing. In the method of performing the acid treatment after the intermediate annealing, nitric acid electrolysis is performed in the conventional acid treatment, so that the surface layer of the stainless steel plate is only abraded by about 0.2 μm. Therefore, most of the minute caps that cause gold dust flaws remain almost as they are. In the second cold rolling, since the material is softened, this fogging further spreads on the surface of the stainless steel plate and a part is peeled off, but many remain as a fine fogging.

According to the present invention, the fine rust is ablated by the acid treatment after the intermediate annealing, so that the gold dust flaw does not occur. As shown in FIG. 1, when the amount of weld cutting of the stainless steel sheet by the acid treatment after the intermediate annealing is 1 μm or more, the gold dust flaws of the BA product are extremely reduced. In Fig. 1, the gold dust defect rating A is 25 cm when the vinyl tape is applied and then peeled off.
It is 10 or less per ² , B is 11 to 100, C is
101 to 1,000 and D is 1,000 or more. As a manufacturing standard, A and B pass, and C and D fail. What carried out nitric acid electrolysis in the conventional method was CD.

In the present invention, as the acid for the acid treatment performed after the intermediate annealing, hydrofluoric acid is preferable for the Cr-based stainless steel sheet containing no Mo or Cu. In a Cr-based stainless steel sheet containing no carbon or nitride but a carbonitride forming element such as Al, Ti, Nb, sulfuric acid can be used in addition to nitric hydrofluoric acid. When the sulfuric acid treatment is performed, the amount of stainless steel to be ablated is large, and in the Cr-based stainless steel containing the carbonitride-forming element, intergranular corrosion does not occur or is slight. Since Cr-based stainless steel sheets containing Mo and Cu have good corrosion resistance, sulfuric acid nitric acid with a large amount of fusing is suitable. In other words, as the acid treatment after the intermediate annealing, it is necessary to melt and shave the surface layer as uniformly as 1 μm or more in order to remove minute fog on the surface smoothed by the first cold rolling. . Depending on the type of acid, deep intergranular corrosion and etch pits occur, so by the second cold rolling,
It becomes a minute cover that is the cause of gold dust flaws.

As a characteristic of the acid, nitric hydrofluoric acid has a larger amount of ablation than nitric acid electrolysis, but when a Cr deficient layer is present at the grain boundary, intergranular corrosion occurs. For Cr-based stainless steel that does not contain Mo or Cu and does not contain carbonitride-forming elements such as Al, Ti, Nb, the intermediate annealing temperature is 900
If the temperature is lower than ℃, nitric hydrofluoric acid can be used because Cr deficient layer hardly occurs. Target materials are SUS410L, SUS4
29, SUS430, SUS403, SUS410, SUS410S, SUS420J1, SUS420J2,
Examples include SUS429J1 and SUS440A that does not contain Mo. In addition, Al-containing SUS405 and Al-containing 16% Cr steel can also use nitric hydrofluoric acid because the annealing temperature is 900 ° C or lower. Nitric acid has a larger amount of ablation than hydrofluoric acid, but causes grain boundary corrosion due to P grain boundary segregation. The grain boundary segregation of P is remarkably generated by heating at around 800 ° C., so that it cannot be applied unless the annealing temperature is far from this. However, Mo
Not containing Cu or Cu, Al, Ti, those containing carbonitride forming elements such as Nb, due to the increase in the A _C1 transformation point and the recrystallization temperature,
It can be applied because the annealing temperature is as high as 900 ℃ or higher.
Target steel types include 10 to 20% Cr steel containing 1% or less of Ti and SUS430LX. Further, sulfuric acid is more easily dissolved in a material having a high Cr, so that it is used for 20% or more of Cr steel. On the other hand, in the case of not containing Mo or Cu but containing Ti or Nb, since C is fixed, even if the annealing temperature exceeds 900 ° C., a Cr-deficient layer hardly occurs. Acids are also applicable. Sulfuric acid nitric acid has a larger amount of fusing than sulfuric acid,
Material with good corrosion resistance containing Cu can also be ablated. Further, even if the Cr-deficient layer or the grain boundary segregation of P does not occur, intergranular corrosion does not occur and the entire surface is dissolved. Applicable steel types are SUS434, SUS436L, SUS444, SUS44J1, SUSXM27, Mo
SUS440A containing 19% Cr steel containing Nb and Cu. In addition, it is also applicable to SUS430, which is a representative steel type of ferritic stainless steel.

As a method of using the acid solution, in addition to the above single use,
A combination of sulfuric acid treatment and nitric hydrofluoric acid treatment is also possible. Sulfuric acid treatment alone produces smut and blackens the surface, so it is necessary to remove it with brushing or high-pressure water, but with nitric hydrofluoric acid treatment, the mast can be easily removed. It can be applied to Cr-based stainless steel that does not contain it.

Conditions of the acid treatment, in the case of nitric-hydrofluoric acid, the concentration HNO _3: 10 to 50 g
/, HF: 10 to 20g /, temperature 40 to 60 ° C, 20 for sulfuric acid
0~400g /, 50~90 ℃, in the case of硫硝acid, H ₂ SO _4: 200~4
_{00g /, HNO 3: 5~20g /} , preferably 50~90 ℃.

As described above, by acid treatment after the intermediate annealing, the surface layer is ablated by 1 μm or more as uniformly as possible,
It is possible to eliminate almost all the gold dust flaws of BA products.

〔Example〕

The ferritic stainless steel shown in Table 1 was melted and hot-rolled to a thickness of 3.8 mm. After that, the hot-rolled sheet was annealed and omitted, and mechanically descaled and pickled. Without removing the irregularities on the surface of the pickled plate by belt grinding or the like, the first stage cold rolling was performed to a thickness of 1.2 mm using a tandem rolling mill or a Sendzimir rolling mill. In addition,
For those that did not undergo intermediate annealing, cold rolling was performed directly with a Sendzimir rolling machine to a product thickness of 0.4 mm.

Next, the intermediate annealing was performed in the atmosphere at the heating temperature shown in Table 2, and after the salt treatment at 430 ° C., the acid treatment was performed. The acid treatment was performed by immersing in a single solution of nitric hydrofluoric acid, sulfuric acid, and sulfuric nitric acid, and the conventional acid treatment was performed by nitric acid electrolysis. Table 2 also shows the amount of fusing during this acid treatment. The subsequent second stage cold rolling was finished to a thickness of 0.4 mm using a Sendzimir rolling machine. The final annealing is bright annealing in a non-oxidizing atmosphere,
The heating used the same temperature as the intermediate annealing. Then, temper rolling was performed to obtain a product.

The surface properties and mechanical properties (workability) are important for the properties of BA product boards. Among them, gold dust defects,
γ value and ridging are important. These characteristics are also shown in Table 2.

After intermediate annealing by the two-stage cold rolling method, nitric hydrofluoric acid, sulfuric acid, and sulfuric acid nitric acid were individually treated with an acid, and the present invention example (No.
Compared with the conventional two-stage cold rolling method (No. 18, 20, 22, 24, 25), No. 15, No. 18, 20, 22, 24, 25 has a gold dust flaw rating of A rank, and the surface property is remarkably improved. For the same test material, the γ value and ridging of the two-stage cold rolling method of the present invention example and the conventional one-stage cold rolling method were compared (No. 3 and 19, No. 6 and 21, No. 9). 23, No.15 and 2
6) Then, the two-stage cold rolling method of the present invention is superior in both cases.

The comparative example (No.16, 17) used nitric hydrofluoric acid as the acid solution in the acid treatment after the intermediate annealing, but the HF concentration was low and the amount of fusing did not reach 1 μm. Dust defect has been ranked C. Conventional one-stage cold rolling method (No. 19, 21, 2
In No. 3,26), there was no fusing due to acid treatment, and thus all the glittering flaws were rated C and failed. In the conventional two-stage cold rolling method (No.18, 20, 22, 24, 25), the amount of fusing during acid treatment after intermediate annealing is as small as 0.2 μm, so the product has a gold dust flaw rating. Failed in C to D.

Regarding the gold dust flaw rating, all of the gold dust flaw ratings of the present invention were rank A, but the number of glittering spots in 25 cm ² was 0 or 1-2, which was almost zero.

As described above, after the intermediate annealing, it was confirmed that the gold dust flaws of the BA product were almost completely eliminated by subjecting the surface layer to an even treatment of 1 μm or more as uniformly as possible by acid treatment.

〔The invention's effect〕

According to the method of the present invention, it is possible to almost completely eliminate the gold dust flaw which was a serious surface defect in the BA stainless steel product.

As an application of the method of the present invention, when mechanical properties (workability) are not required so much and gold dust flaws on the surface are important, the same acid as the method of the present invention is applied during the one-stage cold rolling. If the treatment is carried out, the minute fog which is the cause of the gold dust flaw can be removed, and by performing cold rolling and bright annealing subsequently, a Cr-based stainless steel sheet having excellent surface properties can be obtained.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the amount of fusing by acid treatment after intermediate annealing and the gold dust flaw rating of the final BA product.

Claims (1)

[Claims] 1. A hot rolled sheet of Cr-based stainless steel is cold-rolled,
After intermediate annealing, the surface phase is ablated by 1 μm or more by acid treatment with a single acid selected from nitric hydrofluoric acid, sulfuric acid, sulfuric nitric acid or a combination of the acid treatments, and then cold rolling and bright annealing. A method for producing a Cr-based stainless steel sheet BA product having excellent surface properties, characterized by: