JPS61249602A - Production of thin ferritic stainless steel sheet having excellent surface characteristic - Google Patents

Abstract

PURPOSE:To obtain a product having a good surface characteristic with high productivity by subjecting a hot rolled steel strip to pickling and smut treatment respectively in specific solns. then to cold rolling with work rolls conforming to the specific equations by using a rolling mill oil having a specific saponification value and viscosity. CONSTITUTION:The above-mentioned stainless steel is hot rolled and is coiled at 500-750 deg.C and the coiled strip is subjected to mechanical descaling. The strip is pickled in 200-400g/l H2SO4 soln. and is subjected to the smut treatment in 100g/l HNO3 soln. The strip is then subjected to cold rolling with a rolling mill having the work rolls of the roll diameter and roll surface roughness conforming to equation I and equation II by using the rolling mill oil meeting either or both conditions of >=30 saponification value and >=30 centistoke viscosity at 50 deg.C, then to final annealing and pickling. In equations, D: roll diameter mm (having >=150mm value), t: thickness of the pickled strip mm, Ra(t): surface roughness of the pickled strip having the thickness t (having <=2.5 value), Ra(D): surface roughness of the rolls at the diameter D.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention is directed to the production of stainless steel thin plates, and in particular to the production of stainless steel thin plate products with good surface quality through a manufacturing process that is process-saving and highly productive. This is about the method.

(Prior art) In the production of stainless steel thin plates, the surface, in particular, is not only corrosion resistant, but also has excellent appearance, gloss polishability, and even B.
Regarding A-prepared products, there are technical issues that determine their commercial value, including defects such as white streaks and gold dust.

Conventionally, these problems have been solved by mechanical descaling after hot-rolled sheet annealing, pickling, and grinding of the entire surface of the coil to remove various defects. iJ? It has been manufactured by rolling several times for several times.

In order to manufacture thin stainless steel sheets with beautiful surfaces, the method of annealing, pickling, surface grinding, and cold rolling with small diameter rolls has been established as a complete technology, and is based on the JIS standard 2.
B products, 2B products, and BA preparation polishing products have been manufactured. The manufacturing technology for these products is well described in Sawatani et al., "Steel Manufacturing Research-4292 (1977), p. 1001C.

Of course, there is a strong demand for process omissions, and the omission of annealing and surface grinding are being considered, but these are all based on conventional methods and are not necessarily sufficient technologies. . In particular, in addition to omitting annealing and surface grinding, the problem of manufacturing stainless steel 20, 2B, BA prepared polished products by cold rolling with large diameter rolls instead of the conventional cold rolling with small diameter rolls has not been solved in the past. There is no research on this topic, so it can be said to be a completely new issue.

(Problems to be Solved by the Invention) The present invention eliminates the hot-rolled plate annealing and coil grinding processes (hereinafter abbreviated as CG) that have traditionally been essential in the production of thin plates of ferritic stainless steel, typically 17Cr. omitted,
Furthermore, the present invention provides a method for manufacturing 2D and 2B products and BA preparations, which are surface finishes of cold-rolled stainless steel sheets specified by JIS, by utilizing a large-diameter roll cold rolling method, for example, a tandem cold rolling mill for common steel. It is something.

The technical problems to be solved by the present invention are as follows: (1) Pickling a coil without hot-rolled plate annealing, and
Pickling technology for making a pickled board that can be used as 2D, 2B, and BA preparations even if G is omitted; (2) 2D, 2B, and BA preparations are obtained by high-speed cold rolling with large diameter rolls instead of small diameter rolls; Establishment of cold rolling technology for manufacturing. These will be explained in detail below.

(Means for solving the problem) As a method of omitting hot-rolled sheet annealing of ferritic stainless steel, which is typical of 17cri, there are known techniques related to component systems, techniques using hot rolling methods, and techniques using control of coiling temperature. It is being However, regarding the subsequent pickling process, it is important to select the winding temperature and the pickling solution composition, since carbide precipitation during winding cannot be prevented no matter how rapidly the cooling is performed. I found out. If the hot-rolled coil is wound during winding, it will be cooled down in about 2Q minutes or less, and decomposition of the γ phase and precipitation of carbides will proceed. 750℃ above 600℃
Below, in the wound fC17Cr stainless steel, chromium carbide precipitates, and during subsequent pickling, HNO, HF
If a pickling solution consisting of However, it has been found that when an active dissolving type pickling liquid such as H2SO4 is used, grain boundary corrosion does not occur and a smooth pickled plate can be obtained. However, since the active dissolving type pickling solution produces black smut, smut treatment with HNO3 is necessary. Based on these findings, various studies were conducted on the conditions for obtaining a pickled plate with no intergranular corrosion and a smooth surface through mechanical descaling and pickling smut treatment. This is because when intergranular corrosion occurs, burrs due to intergranular corrosion grooves occur on the surface even in the cold rolling process described below. In this regard, the reason why the winding temperature is set to 600°C or higher is that slow cooling below 2°C promotes the decomposition of γ and the precipitation of carbides, but if the winding temperature is lower than 600°C, the
The phase structure r remains undecomposed, or only a small portion of the transformation from r to α+ carbide occurs, resulting in HNO.

This is because very shallow intergranular corrosion occurs during smut treatment. Also, if the winding temperature is higher than 750℃, grain boundary segregation will occur during slow cooling after winding, and grain boundary corrosion will occur during pickling with active dissolved H2SO4. In order to prevent interfacial corrosion, the temperature must be 600°C or higher and 750°C or lower.

It is necessary that intergranular corrosion does not occur and that the surface after pickling has small irregularities. This is because if the surface has large irregularities, it is likely to be covered during cold rolling. The combination of mechanical descaling and H2SO4 pickling is important in this regard.

After winding at a winding temperature that does not cause the above-mentioned p grain boundary corrosion, mechanical descaling was performed and H2SO4 pickling was carried out. Shot blasting has been well known as a method of mechanical descaling, and shot blasting increases the surface roughness after descaling.

Figure 1 shows the effects of the shot-plast method and the mechanical descaling method in which iron sand particles are mixed in high-pressure water. There is a large difference in surface roughness after pickling.

In particular, we focused on the fact that the surface scale becomes thinner when hot-rolled sheet annealing is omitted, and as a result of examining the mechanical descaling method and surface roughness, we found that iron sand particles were mixed as an abrasive in high-pressure water. It has been found that the spraying method is preferable in terms of reducing surface roughness. In that case, the conditions are to mix a slurry of iron sand grains in high pressure water of 80 to 300 Yu/F2 so that the concentration is 50 to 70 Yu, and the ratio to the amount of high pressure water is in the range of 0.3 to 3.0. The best method is to spray it inside.

After mechanical descaling like this, Zoo
30-20 in ~4001/l 90 °C H2SO4 solution
Immerse for 0 seconds. Descaling was completed after 30 seconds of immersion, and the surface roughness smoothed out with the immersion time until about 200 seconds, and RrnlLx was reduced from 20μ to 7A and Ra was 2.
, 5μ to 1.5μ.

By the way, for materials that have been shot blasted, the maximum
is 25-35μ, Ra reaches 3μ, and by a combination of mechanical descaling using high-pressure water and iron sand mixture and H2SO4 pickling, it is about half of the shotplast method, 0≦20μ, Ra≦
A pickled plate with a surface roughness of 2.5μ can be easily obtained. Such a smooth pickled plate is sufficient as a material for cold rolling in the next process even if the CG process is omitted.

In addition, H2SO4 is 200-40011A as mentioned above.
,! - However, if it is less than 2001/, descaling cannot be performed sufficiently, and 400! This is because the concentration effect is saturated and there is no addition effect when the amount exceeds lit. After pickling with H2SO4 solution, ripe-colored smut is generated, so smut treatment is performed using ShiO5 solution. If HNO exceeds 10011t, intergranular corrosion tends to occur, so 100.97 is
3 solutions are used.

Next, the pickled plate 1 is subjected to reverse or continuous rolling in a rolling mill or rolling mill group having large diameter rolls of 150 mm or more.

As a result of investigating the relationship between rolling conditions and steel plate surface properties, the present inventors found that when cold rolling is performed using a conventional Sendzimir cold rolling mill with a roll diameter of less than 100 mφ, the irregularities on the surface of the material tend to overlap. ■In cold rolling using large-diameter rolls with diameters larger than φ, the shear deformation of the plate surface is small and the compressive deformation is large during cold rolling, and the surface irregularities that existed on the plate surface are not covered and are gradually compressed and deformed. I found that it becomes shallower.

It has been found that the following three conditions are necessary to prevent the unevenness that existed on the pickled plate, which is the cold rolled material, from covering up during cold rolling with large diameter rolls: fc 0 (1) good lubrication conditions; Assuming that.

(2) The surface roughness Ra (t) of the pickling board of the material is small, Ra (t) ≦ 2゜5μ, and the roll roughness R
The relationship with a (D) is Ra (t)/Ra (t)≧1
．． Use a roll that satisfies 5.

(3) Cold rolling is carried out in which the relative relationship between the roll diameter (2) and the material plate thickness (1) satisfies t/b≦1150 by at least 3 degrees.

First, the lubrication conditions will be described. The above-mentioned phenomenon of capping is closely related to the lubrication state within the roll bite during cold rolling. If there is insufficient lubrication, even if large-diameter p-ru rolling is performed, shear deformation due to the uneven shape of the steel plate surface will increase, so the effect will be reduced. During rolling, it is necessary to strengthen the oil film strength of the rolling oil in the roll bit in order to prevent the concavo-convex shape from being compressively deformed and overburden. For this purpose, the saponification value (SV) of the rolling oil must be 30 or more, or the rolling oil viscosity t-30 centistokes (c-t) or more at a temperature of 50°C. That is, if the pressure is below the above-mentioned value, the pressure resistance of the oil film will be insufficient and fogging will occur. Of course, a rolling oil that satisfies the above two conditions is optimal. Further, it is necessary to secure an appropriate amount of lubricating oil in the roll bit, and the amount of oil per square meter of the plate surface on the entry side of the cold rolling mill is preferably 0.01 to 1 gram. If it is less than 0.01 g, there will be a lot of shear deformation due to insufficient oil amount, and if it exceeds 1 g, there will be too much lubrication and the center point of the shear will shift, causing unstable rolling factors such as chattering and slipping. It's also not good against simultaneous K Kabusashi. Adjustment of this lubricating oil amount is based on the rolling oil concentration.
Controlled by rolling oil supply amount, rolling speed, etc.

Next, regarding the roughness of the pickled plate and roll surface, if the surface roughness of the pickled plate (Ra(t)) exceeds 2.5 μ, it is easy to cause burrs in the cold rolling process. Since it causes gold dust and white streak defects, it is necessary to suppress it to 2.5μ or less. In addition, the finer the roll surface roughness (RacD)), the less likely it will be covered, so Ra (t) / Ra
Q)) ≧1.5 - (1). For the same purpose, the cold rolling roll diameter D is calculated based on the pickling board thickness t.
(However, D≧150■φ) is set to t/D≦1750, - (2), and the cold rolling roll diameter is made as large as possible relative to the material plate thickness to suppress roughness of surface irregularities during cold rolling. It is necessary to increase the effect.

In this way, the rolling reduction rate is at least 3 by the large diameter roll.
After cold rolling to a depth of 0 mm or more, the product is subjected to usual final annealing and pickling, followed by further temper rolling on the 2B surface to obtain a product with the 2B surface. In order to obtain a 2B surface product with even better gloss, roll the steel sheet with large-diameter rolls at a reduction rate of 30% or more of the total reduction to make the irregularities on the surface of the steel sheet shallower, and then roll it with the commonly used rolls of 100 txmφ or less. After cold rolling by 10% or more of the total reduction using a Sendzimir cold rolling mill using rolling rolls with a diameter and roll roughness Ra≦0.3 μm, usual final annealing, pickling, and temper rolling are performed. Such a process can further improve the surface roughness and improve the gloss, and the surface irregularities no longer overlap.

Furthermore, if bright annealing is performed in place of the final annealing and pickling described above, a BA surface product with the most severe surface conditions can be produced without generating gold dust or white streaks.

(Example) Example (1) As a representative of ferritic stainless steel, 5US430 containing 0.12% of A7-4 was melted by the LD-VAC method to obtain a 250m+ thick slab. After that, as usual,
! It was hot rolled to a thickness of 74.0 and 3.0 cm, and the hot rolling end temperature was about t-880°C, and the coiling temperature was 630 to 680°C. As a comparative material, a low-temperature coiled material of 580°C was added. The cooling rate after winding was ~IC/e up to 400C. After that, hot-rolled plate annealing was omitted, and mechanical descaling was performed by spraying a slurry of 70% iron sand in high-pressure water at 115 kir/c- at a ratio of 0.33 to the amount of high-pressure water. 200-30011/l H2SO4(8
Immersion pickling was performed at a temperature of 0 to 95°C for 30 to 90 seconds.

Next, after brushing, 501 g/l HNO3 (50
The smut was removed by spray pickling at 15°C to 25°C for 15 to 25 seconds.

Cold rolling uses rolls with a large diameter of 480mφ and a uniform surface roughness of t-0.2μ, which is the same as the 4-stand tandem cold rolling mill, where the roll surface roughness is 0.2μ.
The rolling process was carried out using a reverse cold rolling mill with a roll diameter of 200φ.

During rolling, the cold rolling oils used were (1) mineral oil with saponification value (sv) go and viscosity 25 ast (temperature 50°C), beef tallow mixed emulsion oil, and (2) viscosity 60 cat.
(Temperature: 50°C), saponification value (SV) Adjust the viscosity to 25, mineral oil, beef tallow mixed emulsion oil, (3) saponification value (SV)
150, viscosity 60 cst (temperature 50°C) for three cases of tallow-based emulsion rolling oil, temperature 60°C,
Rolling oil adhesion amount to on the entrance side of each rolling mill stand at a concentration of 5%
, oil to 9/d, and the cold rolling speed was 700 to 800 m/m with a tandem mill and 150 to 300 ntmin with a reverse mill to 1.6 and 1.6 m/m.
It was cold rolled to a thickness of 0. After that, in a regular Sendzimir cold rolling mill with a roll diameter of 55 mm and a roll surface roughness of 0.2μ or less, 0.6
The final annealing and pickling were performed at 800 to 990°C for a short time, and some parts were brightly annealed under the same conditions. - Both were finished by normal temper rolling. These surfaces were finished using a normal rolling plate annealing process and a CG process using only a small-diameter Rollsendzimir cold rolling mill in about 10 to 13 passes.

When compared with the surface of the BA product, no differences were found in terms of gloss, white streaks, gold dust, etc., and they were the same. However, for the coils whose winding temperature was 580°C, slight fogging was observed on the BA surface product. Of course, in terms of material quality, Invention Material 1 showed excellent properties.

Example (2) Under the same conditions as Example (1), a cold-rolled sheet with 9 rolls of hot rolling, pickling, and cold rolling with a large-diameter roll was applied to obtain a cold-rolled sheet with a pitch thickness of 0.8 mm. After final annealing and pickling, 2B
Manufactured surface products. Further temper rolling was applied to produce a 2B surface product. When the winding temperature was other than 580°C, gloss and other surface properties were good.

(Effect of the invention) As described above, intergranular corrosion can be prevented by selecting the coiling temperature and pickling liquid during hot rolling, and by combining an appropriate mechanical descaling method and pickling liquid, The surface roughness of the material is reduced, and the surface roughness is reduced by cold rolling with a roll having a large diameter and a small surface roughness, and further,
If the surface is created by finish cold rolling using rolls with a small diameter and low roll surface roughness, there will be no cover or white streaks even if the hot-rolled plate annealing and CG steps are omitted in the middle process. Since 2D, 2B and BA surface products can be produced, its industrial effects are extremely large.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the mechanical descaling method and the pickling time with respect to the surface roughness RmaX of a hot rolled plate of 17% CR stainless steel after pickling. Pickling time (seconds)

Claims (3)

[Claims] (1) In the manufacturing method of ferritic stainless steel, typically made of 17% Cr, the steel is hot-rolled at 600°C as usual.
After winding at a temperature of 200 to 750°C, the resulting hot rolled steel strip is mechanically descaled and
pickling in g/l H_2SO_4 solution, 100 g/l
Smut treatment is performed in the following HNO_3 solution, and then the pickled steel strip is rolled using a rolling mill having a work roll with a roll diameter and roll surface roughness according to the following relational expression, with a saponification value of 30 or more and a temperature of 50. A thin ferritic stainless steel with excellent surface properties, which is cold-rolled using a rolling oil with either or both of the following conditions: a viscosity of 30 centistokes or more at °C, final annealing, and pickling. Method of manufacturing steel plates. Ra(t)/Ra(D)≧1.5...(1) t/D≦
1/50...(2) However, D: Roll diameter (mm) (having a value of 150 mm or more), t: Thickness of pickled board (mm), Ra (t): Pickling of board thickness t Surface roughness of plate (having a value of 2.5μ or less) Ra (D): Roll surface roughness at roll diameter D. (2) In the manufacturing method of ferritic stainless steel, typically made of 17% Cr, hot rolling is carried out in the usual manner at 600°C.
After winding at a temperature of 200 to 750°C, the resulting hot rolled steel strip is mechanically descaled and
pickling in g/l H_2SO_4 solution, 100 g/l
Smut treatment is performed in the following HNO_3 solution, and then the pickled steel strip is rolled using a rolling mill having a work roll with a roll diameter and roll surface roughness according to the following relational expression, with a saponification value of 30 or more and a temperature of 50. Cold rolling using a rolling oil with either or both of the following conditions: a viscosity of 30 centistokes or more at °C, and then cold rolling using a work roll with a roll diameter of 100 mm or less and a roll surface roughness Ra of 0.3 μ or less A method for producing a ferritic stainless thin steel sheet with excellent surface properties, which comprises finishing rolling with a machine, final annealing, pickling, and temper rolling. Ra(t)/Ra(D)≧1.5...(1) t/D≦
1/50...(2) However, D: Roll diameter (mm) (having a value of 150 mm or more), t: Thickness of pickled board (mm), Ra (t): Pickling of board thickness t Surface roughness of plate (having a value of 2.5μ or less) Ra (D): Roll surface roughness at roll diameter D. (3) The method according to claim 2, wherein bright annealing and temper rolling are performed after finish rolling.