JPS61249603A - Production of thin austenitic stainless steel sheet having excellent surface characteristic - Google Patents

Abstract

PURPOSE:To reduce stages by cold rolling with large-diameter rolls and to obtain a product having good surface characteristic by pickling a hot-rolled steel strip in an aq. soln. consisting of a specific compsn. and cold-rolling the strip with work rolls conforming to the specific equation by using a rolling mill lubricant having a specific saponification value and viscosity. CONSTITUTION:A billet contg. <=0.06wt% C is hot-rolled and is quickly cooled. The hot rolled strip is coiled at <=650 deg.C and is subjected to mechanical descal ing. The strip is then pickled with an aq. soln. consisting of 60-200g/l NHO3 and 15-150g/l HF. The strip is cold-rolled with a rolling mill having the work rolls of the roll diameter and roll surface roughness conforming to the equation I and the equation II by using the rolling mill lubricant meeting either or both conditions of >=30 saponification value and >=30 centistoke viscosity at 50 deg.C, then the strip is subjected to final annealing and pickling. In the 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 <=2.5mu value), Ra(D): roll surface roughness at the diameter D.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for manufacturing thin stainless steel sheets, and particularly relates to a method for manufacturing thin stainless steel sheet products with good surface quality through a manufacturing process that requires fewer steps and is highly productive. It is something.

(Conventional technology) In the production of thin stainless steel sheets, the commercial value of stainless steel sheets is determined not only by corrosion resistance, but also by appearance, gloss, polishability, and even defects called white streaks and gold dust for BA products. Technical challenges exist. Conventionally, these problems have been solved by mechanical descaling and pickling after hot-rolled sheet annealing, and after grinding the entire surface of the coil to remove various defects, a Sendzimir mill consisting of small-diameter, multi-stage rolls is used to process a large number of passes. It has been manufactured by cold rolling. In order to produce thin stainless steel sheets with beautiful surfaces, annealing is required.
The method consisting of pickling, surface grinding, and cold rolling with small diameter rolls has become established as a complete technology, and 2B products, 2B products, and BA products as specified by JIS have been manufactured. Regarding the manufacturing technology of these products, see Sawatani et al. “Steel Manufacturing Research” 4292 (
Ploo (1977).

Of course, there is a great need for process omissions, and the omission of annealing and surface grinding are also being considered.

For example, regarding the omission of hot-rolled sheet annealing of thin austenitic stainless steel sheets, typified by 18Cr-8Ni, much knowledge is already known regarding hot rolling methods and cooling during winding. (Japanese Patent Publication No. 57-38654, Japanese Patent Publication No. 59-46287) In order to prevent intergranular corrosion in the pickling process, it is important to suppress carbide precipitation during winding. First of all, it is necessary that carbide precipitation does not occur. In addition, when hot-rolled sheet annealing is omitted, pickling does not produce a Cr-free layer, as stated in Japanese Patent Publication No. 58-56013. Pickling with HCl is said to be preferable because the washing speed is low and intergranular corrosion occurs.

(Problems to be Solved by the Invention) However, all of the conventional techniques are based on the premise of cold rolling with small diameter rolls, and cannot necessarily be said to be a sufficient technique. In particular, in addition to omitting annealing and surface grinding, large-diameter roll cold rolling is used instead of conventional small-diameter roll cold rolling to solve the problem of creating 2D, 2B, and BA coated surfaces of stainless steel thin sheets. There is no research;
This is a completely new issue.

(Means for Solving the Problems) The present invention is directed to hot-rolled plate annealing and coil grinding processes (hereinafter referred to as CG process), which have traditionally been essential in the production of thin plates of austenitic stainless steel, typically 18Cr-8Ni. omitted), and further utilizes a large-diameter roll cold rolling method, for example, a tandem cold rolling mill for ordinary steel, to achieve JIS standard 2.
It provides a method for manufacturing D, 2B products and BA products.

°The technical problems to be solved by the present invention are: (1) To pickle a coil without hot-rolled plate annealing, and to make a pickled plate that can be made into 2D, 2B, and BA products by omitting the CG process. (2) Cold rolling technology that uses large diameter rolls instead of small diameter rolls to produce 2D, 2B, and BA products at high speed. These will be explained in detail below.

First, the present inventors focused on the scale characteristics of the hot-rolled plate annealed material, and investigated including mechanical descaling methods, and used a pickling solution consisting of G03 and Go at a higher concentration and higher temperature than usual. By doing so, it was possible to obtain a pickled board without intergranular corrosion and with small surface irregularities after pickling. Barefoot,
If intergranular corrosion occurs or the surface after pickling has large irregularities or roughness, it will not be possible to obtain a satisfactory surface even after the cold piezoelectric process described below due to overlapping in the cold pressing plate. C
This is because the G process will not be omitted.

Immediately after hot rolling, water cooling is performed to prevent carbide precipitation.
Winding at 50° C. or lower is important for preventing carbide precipitation. Of course, the lower the amount of C in the steel, the better (C0,060%
The following are desirable.

The scale of a hot-rolled sheet that is rapidly cooled after hot rolling and wound up at 650° C. or lower is thinner than that of a hot-rolled sheet that is subsequently annealed. For this scale, conventional mechanical descaling such as shot blasting is effective, but wet mechanical descaling using iron sand particles mixed in high-pressure water is effective, and is particularly effective in reducing surface roughness after pickling. It turned out to be. A slurry containing 50 to 70% iron sand particles was added to 80 to 300 kg of 7 cm2 of high pressure water at a rate of 0.
When mixed at a ratio of 2 to 3 times and sprayed, the scale is mechanically removed by the abrasive effect of iron sand particles, and then the concentration and temperature are selected and a pickling solution consisting of 9HNO and 9HNO is used in an extremely short period of time. It was found that descaling was completed in .

Figure 1 shows 18Cr-8Ni material (C≦0.0
After mixing and spraying a 55% slurry of iron sand grains in high-pressure water (nokg/sub 2) at a ratio of 0.33 to the amount of high-pressure water at a winding temperature of 590°C), various concentrations were prepared. The amount of solvent after 20 seconds of Ssolley pickling with HNO and pickling solutions at various temperatures is shown. The descaling limit corresponds to a solvent volume of 1.5μ. The results show that descaling occurs in a very short time in a pickling solution consisting of high concentrations of HNO at high temperatures.

Furthermore, no intergranular corrosion is observed on the surface after descaling.

In the present invention, the content of C in the austenitic stainless steel slab is
The amount was set to 0.060% by weight or less, but if it exceeds this value, Cr carbides will precipitate at the grain boundaries even if the coiling temperature is 650°C or less after hot rolling, and HNO3 and nitric acid will form during pickling. This is because intergranular corrosion occurs due to the pickling solution consisting of. Further, even if the steel is rapidly cooled after hot rolling, intergranular corrosion occurs if the coiling temperature exceeds 650°C. Regarding the pickling solution composition consisting of HNO3 and pickling liquid, as shown in Figure 1, the concentration of HNO is 6097 ~
20011/l, ■Concentration 15117-13~1501
It was set as 1/J. Each lower limit is determined by the descaling effect and is the concentration at which high-speed descaling becomes impossible. The higher the concentration, the higher the descaling distance, but the upper limit is because the concentration effect is saturated. The temperature is related to the concentration of HNO, and 10 to 80
℃ is preferable.

As a result of measuring the surface roughness of the pickled board after pickling in this way, the maximum roughness (Rmax) was 20 to 10μ.
A center line average roughness (Ra) of 2.1 to 1.5 μ was obtained. A normal annealed plate that was shot-blasted and then pickled with a pickling solution consisting of HNO and sardine had a thickness of 25μ and 3μ, respectively. In this way, C was set to 0.060- or less, the coiling temperature was set to 650°C or less, hot-rolled plate annealing was omitted, and high-pressure water and iron sand grains were mixed and sprayed to produce high-temperature, high-concentration HNO.
By pickling with a pickling solution consisting of . If there is no intergranular corrosion and the surface of the pickled plate is smooth as described above, no overburden will occur in the cold rolling process even if the post-process CG is omitted. Thus, it has been found that if the centerline average roughness Ra is 2.5 μ or less, preferably 1.5 to 2.1 μ, the material has sufficient performance as a cold rolling material even if the CG process is omitted.

Next, rolling of stainless steel using a cold rolling mill with large diameter rolls will be described.

Rolling stainless steel with large-diameter rolls of 150 mφ or more is aimed at improving productivity by rolling at high speed, but the challenge in this case is to ensure that defects such as cover υ do not occur even at high speeds. It is.

The present inventors have found that when cold rolling stainless steel, if the stainless steel is rolled using the above-mentioned large-diameter roll, the unevenness existing on the surface of the material before cold rolling is less likely to cover up during cold rolling. When cold-rolled using a conventional Sendzimir cold-rolling mill with a roll diameter of 100 mm or less, the surface of the material tends to become uneven, whereas cold-rolling with large-diameter rolls like the one described above This is because the surface shear deformation is small and the surface irregularities do not overlap, but instead mainly undergo compressive deformation and gradually become shallower. From this result, 2B with small diameter roll cold rolling,
The meaning of the CG tool, which has been considered indispensable when creating a BA coating surface, and the fineness of the abrasive material used in CG is required, but if the advantages of the large diameter roll cold rolling described above can be utilized, During cold rolling, the surface unevenness becomes shallower without being covered, and finally disappears. Thus, it has been found that by utilizing large-diameter roll cold rolling in the cold rolling of stainless steel, 2B and BA products with excellent surfaces can be obtained even if the CG process is omitted.

Covering during cold rolling depends on the relative deformation behavior between the roll surface and the cold rolled material surface, but the larger the diameter roll, the smaller the surface roughness of the material, and the smaller the roll surface roughness, the better the lubrication. It has been found that the better the condition, the smaller the shear deformation stress on the surface, and the less likely it is to cover. As a result of a detailed investigation of these relationships, it was found that the following three conditions are necessary to prevent the unevenness existing on the pickled plate, which is a cold rolled material, from being covered during large diameter roll cold rolling.

(1) Assuming good lubrication conditions, (2)
The centerline average roughness Ra(t)) of the surface of the pickled plate of the material is as small as possible, Ra(t)≦2.5μ, and the centerline average roughness RaQ)) of the roll surface is as small as possible. It is preferable to roll, and in relation to R & (c)
Use a roll that satisfies a(t)/Ra■)≧1.5, and (3) the relative relationship between the roll diameter■) (however, D≧150■φ) and the material thickness (1) is t/D. ≦Apply 30% or more cold rolling that satisfies the above.

First, good lubrication conditions will be explained.

The overburden phenomenon in large-diameter roll cold rolling is also deeply related to the lubrication state within the roll bite during cold rolling. In order to prevent the unevenness on the surface of the pickled plate from being compressively deformed and causing cover, it is necessary to strengthen the oil film strength of the rolling oil in the roll bite, and for this purpose, the saponification value (SV) of the rolling oil must be increased. 30 or higher or temperature 50℃
The rolling oil viscosity at 30 centistokes (cst
) or more. That is, if the above-mentioned values are not met, the pressure resistance of the oil film is insufficient and fogging occurs. In this case, it is optimal to use a rolling oil that satisfies the above two conditions.

However, it is necessary to secure an appropriate amount of lubricating oil in the roll bit, and the suitable amount of oil per square meter of the plate surface on the entry side of the cold rolling mill is 0.01 to 1 gram. If the oil amount is less than 0.01 g, shear deformation will increase due to insufficient oil amount, and there will be a lot of cover. If it exceeds 1 g, there will be excessive lubrication and the neutral point will shift, causing unstable rolling phenomena such as chattering and slipping. At the same time, it is not good for Kabusashi. Adjustment of this lubricating oil amount is based on the rolling oil concentration, rolling oil supply amount,
Controlled by rolling speed, etc.゛Next, talking about the pickled plate surface roughness and the roll surface roughness, if the plate surface roughness exceeds 2.5μ, overburden tends to occur during the cold rolling process, which impairs the surface quality.
It is necessary to suppress it to 2.5μ or less.

In addition, the finer the roll surface roughness R, 0))), the less likely it will be covered, so it is necessary to satisfy the following formula % formula % (1). For the same purpose, with respect to the thickness t of the pickled board, the cold rolling roll diameter D (however, D≧150■φ) is set to satisfy the following formula % formula % (2), and the cold rolling roll diameter is set to the material It is necessary to increase the thickness of the plate as much as possible to increase the effect of suppressing surface irregularities from forming during cold rolling.

After cold rolling with a large-diameter roll at a reduction rate of at least 30%, a product with a 2B surface can be obtained by performing the usual final annealing and pickling. To obtain the product, the roughness of the steel plate surface is made shallow by rolling with a large-diameter roll with a reduction wheel of 30% or more of the total reduction amount, and then the roll roughness is After finish rolling is performed by 10% or more of the total reduction using a Sendzimir cold rolling mill with rolling rolls having Ra≦0.3μ, usual final annealing, pickling, and temper rolling are performed. By using such a single layer, the surface roughness can be further improved, the gloss can be improved, and the material properties as a thin plate can be sufficiently satisfied. Furthermore, if bright annealing is performed in place of the final annealing and pickling described above, a BA surface cracked product with the most severe surface conditions can be produced. In this case, it is desirable to make the surface roughness of the small diameter roll very fine.

(Embodiments of the invention) Example (1) SUS 3 as a representative of austenitic stainless steel
04 was dissolved using the usual EF-AOD method, and 150wnC
C cast slab. The main components of these test materials are listed in Table 1.

After that, hot rolling was carried out as usual to a thickness of 3.0 and 2.5 ws, the finishing temperature was raised to a high temperature, and immediately quenched to a thickness of 580 to 5 ws.
It was wound up at 60°C. After that, hot rolled plate annealing was omitted and 110
kg/crr? After performing mechanical descaling by mixing and spraying a slurry of iron sand with a concentration of 70 inches into high aqua regia at a high pressure water ratio of 0.3 to 0.5, the steel plate surface was pickled. Preheat to 70℃ in the bath at the front of the tank, and heat to 80℃.
~1501/l HNO3 and 15~55 E/11 H
Pickling was carried out by spraying a mixture of F at 60-70°C for 15-30 seconds. Descaling properties were also good. However, although intergranular corrosion clearly occurred in the specimen with 0.074% C, no intergranular corrosion was observed in the other specimens. After these pickling processes, the sample material■
The surface of ~■ was coated with 2.6 to 3.4μ of solvent on one side, and the sample material ■ was coated with 4.4μ of solvent. As a result, the average roughness Ra of the sample materials (1) to (2) was 1.5 μ to 2.3 μ, and the surface roughness of the sample material (2) was 4.1 μ, which was abnormal.

Next, these pickled plates were rolled in a 5-stand tandem cold rolling machine with a roll diameter of 450 mm (#) 500 to 1000 m/
Cold rolled in the range of 150 to 500 m/min in a reverse cold rolling mill with a roll diameter of 200 mm and a roll diameter of 1.5, 1.2, and 1.0 m. Roll surface roughness. In a tandem rolling mill, the KRa per stand is 1.
Those changed from 2μ to 0.1μ and all stands Ra =
The experiment was conducted with a constant value of 0.3μ.

In the reverse rolling mill, Ra was kept constant at 0.3μ.

During such rolling, the cold rolling oil used was (1) saponification value (sv) s o , viscosity 25 cst (temperature 50
°C) mineral oil, tallow mixed emulsion oil, (2) viscosity 60
(3) Saponification value (SV) 150, viscosity 60 cst (temperature 50°C).
) regarding three cases of tallow-based emulsion rolling oil.

Spray oil was applied at a temperature of 60° C. and a tIk degree of 5° so that the amount of rolling oil adhering to the entrance side of each rolling mill stand was 0.01 to 197 m”.

For those whose roll roughness during cold rolling is made finer at the final stand, 1.2mm and 1.0m thick thin sheets are subjected to the usual final annealing at 1075-1100°C and then carried out the usual pickling to produce 2B products. This was followed by temper rolling to obtain a 2B product. The surface roughness of these was 0.2 to 0.3μ in terms of Ra.

As a result of the above, the plate thickness is 1.2 m and 1
．． 2D which was made into 0 gourd, final annealed and then pickled.

For 2B products, those with C of 0.060% or less and surface roughness of Ra of 2.5μ or less after pickling are large diameter products.
Roll roughness during cold rolling is at least Raυ)1
．． Excellent surface properties were obtained when the thickness was 2μ or less. However, when the C content was 0.070% and the surface roughness after pickling was large, the gloss of the 2B surface was poor.

Example (2) Hot-rolled, pickled, and large-diameter cold-rolled steel strips with a thickness of 1.5 m and 1.2 m were obtained under the same conditions as in Example (1).

Next, using a Sendzimir cold rolling machine with a normal roll diameter of 55 mm, the roll roughness was made as fine as possible to 0.3 μ or less, and a roll of 200 to 50 mm was rolled using normal rolling oil.
Finish cold rolling is performed in 3 to 6 passes at a speed of 0 m/mln, and some are final annealed, pickled, and temper rolled to produce a 2B surface product. Most of the parts are bright annealed and tempered. It was rolled and made into BA products.

This product has a high C content of 0.070%, has a large surface roughness after pickling, and has a large diameter roll roughness of 1.0 in R1.
Regarding K, which had a thickness of ~0.5μ, excellent brightness could not be obtained with the BA product. However, C is low at 0.060% or less, the surface roughness of the pickled plate is Ra (t) 2.5 μ or less, and the roll roughness during cold rolling with a large diameter roll of 150 mm or more is at least Products that are cold rolled with Ra @ 1.2 μ or less, then combined with cold rolling with small diameter rolls of Ra O, 3 μ or less, and then subjected to final annealing pickling or bright annealing have no cover. 2B and BA products with good gloss could be obtained.

All of these can only be obtained if the requirements mentioned above are met.

(Effect of the invention) Traditionally, cystine-free steel sheets were manufactured by hot-rolled sheet annealing, pickling,
After going through the coil grinding process to make the surface smooth, only a Sendzimir cold rolling machine with small diameter rolls can produce a 2B-? stainless steel surface with excellent gloss. Although nine methods have been developed for manufacturing BA products, the present invention provides a method for manufacturing 2B and BA products with excellent surfaces using a large-diameter roll tandem cold rolling mill that is process-saving and highly productive. The effects of the present invention in terms of high productivity and cost are extremely large.

[Brief explanation of drawings]

Figure 1 shows 18 Cr-8Ni stainless steel (C50,
060%) mechanical descaling material HNOs/
i (F 9 degrees, diagram showing the relationship between pickling temperature and solvent amount (
The numbers in the figure are the amount of solvent on one side (I4) for 20 seconds.

Claims (1)

[Claims] (1) In the manufacturing method of austenitic stainless steel thin plate, typified by 18Cr-8Ni, C is 0% by weight.
．． A steel strip containing 0.060% or less is hot-rolled, and after rolling is rapidly cooled and coiled at 650°C or less, the resulting hot-rolled steel strip is subjected to mechanical descaling, and then 60 to 200 g
The pickled steel strip was pickled in an aqueous solution consisting of HNO_3/l and 15 to 150 g/l HF, and then the obtained pickled steel strip was prepared using a work roll having a roll diameter and roll surface roughness according to the following relational expression. Using a rolling mill, the saponification value is 30 or more and the temperature is 50.
Austenitic stainless steel with excellent surface properties, characterized by being cold rolled using a rolling oil having either or both of the following conditions: a viscosity of 30 centistokes or more at °C, followed by final annealing and pickling. Method for manufacturing thin steel sheets. 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): Board thickness Surface roughness of pickled board at t (having a value of 2.5 μ or less) Ra (D): Roll surface roughness at roll diameter D (2)
In a method for producing a thin austenitic stainless steel plate typically made of 18Cr-8Ni, C is added to 0.06% by weight.
A steel strip containing 0% or less is hot-rolled, and after rolling, it is rapidly cooled and coiled at 650°C or less, and the resulting hot-rolled steel strip is mechanically descaled. and 15 to 150 g/l of HF, and then the obtained pickled steel strip was processed by a rolling mill having a work roll with a roll diameter and roll surface roughness according to the following relational expression. Cold rolling is carried out using a rolling oil having either or both of the following conditions: a saponification value of 30 or more and a viscosity of 30 centistokes or more at a temperature of 50°C, and then a roll diameter of 100 mm or less and a roll surface roughness of Ra0. 3
A method for producing an austenitic stainless thin steel sheet with excellent surface properties, which comprises finishing rolling in a cold rolling mill with work rolls of less than .mu.m, followed by 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): Board thickness Surface roughness of pickled board at t (having a value of 2.5 μ 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.