JPH1190523A - Manufacture of cold rolled austenitic stainless steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate irregularity in glass on the surface of a steel sheet and to make the glossiness uniform by executing finish rolling after executing descaling by jetting high-pressure water having a specific collision pressure after rough rolling. SOLUTION: An austenitic stainless steel slab is made into a sheet bar by hot rough rolling. After rough rolling and before finish rolling, descaling is executed by jetting the high-pressure water on the surface of the sheet bar. The descaling is executed by spraying the high-pressure water having the collision pressure of >=25 kgf/cm<2> . Next, the sheet bar descaled with the high- pressure water is made into a hot rolled sheet by executing finish rolling. The surface scale of the hot rolled sheet which is manufactured executing descaling with the high-pressure water has two-layer structure whose inner layer is a corundum-type oxide and outer layer is a spinel-type oxide. This reason is that the temp. of the steel sheet at the time of finish-rolling and coiling becomes comparatively low temp. by the high-pressure water descaling.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled austenitic stainless steel sheet, and more particularly to the prevention of surface pattern and uneven gloss.

[0002]

2. Description of the Related Art Austenitic stainless steel represented by SUS 304 has excellent heat resistance, corrosion resistance and workability, and is widely used for various purposes. Normally, an austenitic stainless steel sheet is formed into a hot-rolled sheet by hot rolling, and then subjected to a hot-rolled sheet annealing / pickling process to be cold-rolled, finish-annealed, and pickled to form a cold-rolled sheet of a predetermined material.

[0003] In austenitic stainless steel, if a hot-rolled sheet is annealed in a combustion atmosphere, which is the most common annealing atmosphere, and then pickled, uneven gloss may occur and a pattern may be formed. Such a pattern becomes remarkable after cold rolling, finish annealing, and pickling, and becomes a problem when used for roofs, panels, and the like where design is important. Recently, for the purpose of improving productivity, cold rolling by a tandem rolling mill using large-diameter rolls has also been applied in the field of austenitic stainless steel sheets, compared to the case where conventional small-diameter rolls are used. There is a problem that gloss unevenness becomes remarkable.

[0004] Such a pattern and gloss unevenness are caused by surface defects formed on a hot-rolled sheet remaining locally without disappearing even by subsequent cold rolling. In particular, it is considered that the austenitic stainless steel has a smaller amount of ground iron dissolved during pickling than the ferritic stainless steel, and surface defects tend to remain. Examples of the surface defects include grain boundary erosion grooves, pit-like erosion in grains, and bite flaws during hot rolling.

Several methods have been proposed for eliminating these surface defects. For example, Japanese Patent Application Laid-Open No. 60-248889 discloses that a hot-rolled sheet is prepared by adding 100 to 400 g / l of nitric acid to 75 to 400 g / l of hydrofluoric acid.
A method for producing an austenitic stainless steel sheet, which is descaled using an aqueous solution containing a mixture of the above and then cold-rolled and annealed. Japanese Patent Application Laid-Open No. 7-48628 discloses that a hot-rolled sheet is mechanically descaled in advance, and then subjected to an oxygen content of 1 to 15% and a temperature of 950 to 1300 ° C. for 1 to 240 seconds. There has been proposed a method for producing an austenitic stainless steel sheet which is subjected to continuous annealing for holding and recrystallizing, followed by pickling, cold rolling, and then bright annealing.

Japanese Unexamined Patent Application Publication No. 60-177135 discloses that a hot-rolled sheet is annealed in an inert gas, a reducing gas, or a vacuum for a short time, and then rapidly cooled to cause grain boundary erosion of the hot-rolled sheet. A method of suppression is disclosed.

[0007]

SUMMARY OF THE INVENTION However, Japanese Patent Application Laid-Open
The method described in JP-A 60-248889 strengthens the acid dissolving power during pickling and dissolves the ground iron until grain boundary erosion grooves do not occur, so that surface defects such as grain boundary erosion grooves are eliminated, There is a problem that it takes a long time for the pickling treatment, a large amount of the surface of the steel sheet is dissolved, consumption of the pickling solution increases, and pickling sludge increases.

In the method described in Japanese Patent Application Laid-Open No. 7-48628, mechanical descaling is performed before annealing to reduce grain boundary erosion grooves, and the overall gloss is improved. In the case of non-uniformity, it is difficult to perform uniform descaling, and non-uniformity of the underlying structure cannot be completely eliminated. Therefore, there remains a problem that gloss unevenness of the cold rolled sheet is not eliminated.

The method described in Japanese Patent Application Laid-Open No. 60-177135 suppresses grain boundary erosion during annealing. However, if the scale generated during hot rolling is not uniform, the method after cold rolling may be used. There is a problem that gloss unevenness occurs. Means for Solving the Problems The present invention advantageously solves the above-mentioned problem, has no unevenness in gloss on the surface of the steel sheet, and has a uniform gloss, a method for manufacturing a cold-rolled austenitic stainless steel sheet having excellent surface gloss, and a base plate of the cold-rolled sheet. An object of the present invention is to propose a method for producing a hot-rolled austenitic stainless steel sheet.

[0010]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above object, and as a result, it has been found that the gloss unevenness on the surface of a cold-rolled austenitic stainless steel sheet depends on the type of surface scale formed during hot rolling. It was found that one of the causes was the difference in the oxidation form during annealing of the hot-rolled sheet due to the difference in thickness, mixing ratio, etc.

Next, the results of experiments performed by the present inventors will be described. The present inventors have studied in detail the surface of an austenitic stainless steel sheet after hot rolling, after hot-rolled sheet annealing, after hot-rolled sheet pickling, after cold rolling, and after finish annealing and pickling. As a result, the gloss unevenness of the cold rolled sheet corresponds to the gloss unevenness occurring after the hot rolled sheet annealing and the hot rolled sheet pickling. In addition, such gloss unevenness cannot be confirmed from the surface appearance after hot rolling, but when a portion corresponding to the gloss unevenness is observed with an analytical instrument using X-ray diffraction, the composition and thickness of the oxide are found in the portion. However, after annealing and pickling, it is divided into a low gloss area and a high gloss area, resulting in uneven gloss. I found something.

FIG. 1 is an SEM photograph showing the structure of the steel sheet surface after hot-rolled sheet annealing and pickling. (A) is a portion mainly composed of corundum type oxide ((Fe, Cr) ₂ O ₃ ),
(B) shows the composition of the spinel oxide ((Fe, Cr)
₃ O ₄ ) This is the surface structure after annealing and pickling of the part mainly containing FeO. The difference in scale composition greatly affects the surface gloss after pickling.

Further, the present inventors have considered that such a difference in the composition of the hot-rolled scale on the same surface is caused by the non-uniformity of the ground iron structure at the time of hot-rolling. Therefore, the present inventors further studied a method for eliminating non-uniformity of the hot-rolled scale and the hot-rolled ground iron structure. As a result, after rough rolling,
By performing descaling by impact pressure 25 kgf / cm ² or more high-pressure water before the finish rolling, uneven surface scale and base iron structure of the hot rolled sheet is eliminated, the oxidation of the surface proceeds uniformly, uneven gloss Has disappeared. It was also found that such descaling also reduced local intra-granular pit erosion after pickling.

The present invention has been made based on the above findings. That is, the present invention relates to a method for producing an austenitic stainless steel hot-rolled steel sheet, in which an austenitic stainless steel slab is subjected to hot rough rolling and finish rolling to obtain a hot-rolled sheet. Is a method for producing a hot-rolled austenitic stainless steel sheet for cold rolling, wherein descaling is performed by injecting high-pressure water of 25 kgf / cm ² or more, followed by finish rolling.

[0015] Further, the present invention provides a cold rolled steel sheet obtained by subjecting an austenitic stainless steel slab to hot rough rolling and finish rolling to form a hot rolled sheet, subjecting the hot rolled sheet to an annealing and pickling treatment, and then performing cold rolling. In the method for producing a rolled austenitic stainless steel sheet, after the hot rough rolling, before the finish rolling, the impact pressure is 25 kgf / cm ² or more high-pressure water is injected to perform descaling, and then finish rolling is performed. Is a method of manufacturing a cold-rolled austenitic stainless steel sheet having an excellent surface gloss having a difference in surface gloss of 30 or less, and further performing descaling on the hot-rolled sheet, followed by annealing and pickling. preferable.

In the present invention, it is preferable that the flow rate of the high-pressure water injected in the descaling is set to 0.002 l / cm ² or more.

[0017]

DETAILED DESCRIPTION OF THE INVENTION In the present invention, an austenitic stainless steel slab is formed into a sheet bar by hot rough rolling. The slab heating and rough rolling conditions for hot rolling may be generally known conditions, and need not be particularly specified. For example, the slab heating condition is preferably set to a normal slab heating temperature of 1050 to 1300 ° C.

In the present invention, after hot rough rolling and before finish rolling, descaling is performed by injecting high-pressure water onto the sheet bar surface. For descaling, the collision pressure is 25 kgf / cm ² or more,
Preferably, a high-pressure water spray having a flow density of 0.002 l / cm ² or more is used. Incidentally, impact pressure p (MPa) from the discharge pressure of the nozzle P (MPa), discharge rate Q (l / sec), the distance between steel sheet surface and nozzle H (cm), determined by p = 5.64PQ / H ² Can be

If the descaling by injection of high-pressure water is performed before the rough rolling, surface defects such as biting occur during the rough rolling. On the other hand, if the descaling is performed after the finish rolling, it is difficult to uniformly remove the surface structure of the ground iron. In particular, uniform peeling is difficult at the leading end of the coil and the trailing end of the coil. The pressure of the high-pressure water to be injected is
It requires a collision pressure of 25 kgf / cm ² or more. Impact pressure is 25kgf /
If it is less than cm ^2, it is not possible to remove the surface scale and the heterogeneous structure of the surface layer of the ground iron. Further, if the flow density is less than 0.002 l / cm ^2, it is not possible to remove the surface scale and the non-uniform structure of the ground iron surface layer, and it takes a lot of time to remove the scale.
It causes productivity to be impaired.

The sheet bar descaled with high-pressure water is then subjected to finish rolling to form a hot-rolled sheet. The finish rolling condition does not need to be particularly limited, and may be a generally known condition capable of rolling to a predetermined size. As described above, the surface scale of the hot-rolled sheet manufactured by performing descaling with high-pressure water under the conditions described above has a corundum type oxide ((F
e, Cr) ₂ O ₃ ), the outer layer is a spinel oxide ((Fe, Cr)
₃ O ₄ ). This is because the steel sheet temperature during finish rolling and winding becomes relatively low due to high-pressure water descaling. The scale of the hot-rolled sheet of the present invention has a uniform thickness and a small thickness over the entire surface of the sheet, as compared to that of the hot-rolled sheet not subjected to high-pressure water descaling. Note that the thickness of the spinel-type oxide in the outer layer may vary due to the effect of cooling conditions. In addition, in this oxide, oxygen and water vapor are easily diffused in an annealing atmosphere, and pit-like erosion may be observed in grains.

In order to reduce the occurrence of gloss unevenness,
In the present invention, descaling of the hot-rolled sheet is preferably performed before annealing of the hot-rolled sheet. Annealing is performed by descaling the hot-rolled sheet to remove the spinel-type oxide in the outer scale layer of the hot-rolled sheet. This descaling before annealing of the hot-rolled sheet only needs to remove at least the outer layer scale uniformly, and does not require strong descaling. Of course, it may be a descaling that removes all scales. As the descaling method, mechanical descaling such as repeated bending, pulling, shot blasting, brush grinding, high-pressure water injection, and the like are preferable. Further, descaling by pickling may be used.

Next, the hot-rolled sheet is subjected to hot-rolled sheet annealing and pickling, and is then cold-rolled into a cold-rolled sheet. Tandem rolling using a large-diameter roll can be applied to cold rolling. Needless to say, conventional rolling using a small-diameter roll can be applied. The cold-rolled sheet is further subjected to finish annealing and pickling to form a product sheet. Further, temper rolling may be performed as necessary.

The temperature and time for hot-rolled sheet annealing and the temperature and time for finish annealing are not particularly limited. In particular, the conditions of the finish annealing may be appropriately determined according to the use and characteristics of the product sheet. Further, it is not necessary to limit the conditions of hot-rolled sheet pickling and finish pickling. What is necessary is just to combine salt processing, neutral salt electrolysis, sulfuric acid pickling, and nitric hydrofluoric acid pickling generally used generally.

The conditions of the cold rolling or the temper rolling affect the overall gloss but do not affect the gloss unevenness, so that there is no particular limitation. The steel composition in the present invention is preferably in the range of all ordinary austenitic stainless steel compositions, and is not particularly limited.

[0025]

EXAMPLE An austenitic stainless steel slab having the composition shown in Table 1 was heated to 1250 ° C., rough-rolled into a sheet bar, descaled with high-pressure water under the conditions shown in Table 2, and then subjected to finish rolling. A 4.0 mm thick hot rolled sheet was used. In addition, as a conventional example, experiments were performed when descaling with high-pressure water was not performed at all, and as a comparative example, experiments were performed with descaling using high-pressure water before rough rolling.

Next, these hot-rolled sheets were annealed (1150
℃ × 30sec), and nitric hydrofluoric acid (100g / l nitric acid + 30g / l hydrofluoric acid)
The temperature was 60 ° C. and the immersion time was 60 seconds. After pickling, the hot-rolled sheet was cold-rolled by a tandem rolling mill having a roll diameter of 250 mm to obtain a cold-rolled sheet having a thickness of 1.0 mm. Some of the hot-rolled sheets were subjected to descaling shown in Table 2 before the hot-rolled sheet annealing.

Next, finish annealing (1100 ° C. ×
30sec), and then neutral salt electrolytic treatment (Na ₂ SO ₄ : 200g /
l, temperature: 80 ° C, electrolytic current: 120 C / dm ² ), and then immersed in nitric hydrofluoric acid (nitric acid 100g / l + hydrofluoric acid 30g / l, temperature: 60 ° C, immersion time: 60sec), Temper rolling at a rolling reduction of 0.3% was performed. About the obtained steel plate, gloss unevenness was evaluated from visual appearance inspection and gloss degree inspection. Table 2 shows the results.

The gloss was measured according to JIS Z 87 for 10 samples.
In accordance with 41, bright and dark areas were measured at 10 points each, and the difference in the maximum gloss was defined as the unevenness in gloss of the steel sheet. Table 2 shows the results.

[0029]

[Table 1]

[0030]

[Table 2]

From Table 2, the descaling by high pressure water
The applied steel sheet of the present invention has a gloss difference as small as 30 or less.
The occurrence of gloss unevenness is prevented. The flow rate is 0.002
l / cm ^Two As described above, the occurrence of gloss unevenness is further suppressed.
In comparison, comparative examples (No. 11 to N
o.15), In the conventional example (No.16), the gloss difference is large,
The occurrence of uneven gloss is observed.

Further, in the steel sheets (No. 5 to No. 10) further descaled before annealing of the hot-rolled sheet, the difference in glossiness is further reduced, and unevenness in gloss is eliminated. If the impact pressure and the time of descaling are out of the scope of the present invention,
Even if gloss unevenness is observed or the gloss unevenness is eliminated, a new surface defect is generated.

[0033]

According to the present invention, a cold-rolled austenitic stainless steel sheet having a surface gloss difference of 30 or less and having excellent surface gloss without pattern unevenness can be manufactured at low cost. Further, the cold-rolled steel sheet according to the present invention is suitable for uses having excellent design properties, and has an effect that the uses can be expanded.
Further, even if tandem cold rolling with a large-diameter roll is performed, a cold-rolled sheet having excellent surface gloss can be obtained, and an improvement in production efficiency can be expected, and a remarkable industrial effect is achieved.

[Brief description of the drawings]

FIG. 1 is a scanning electron micrograph showing the structure of the steel sheet surface after hot-rolled sheet annealing and pickling.

Continuing from the front page (72) Inventor Takumi Uki 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Kawasaki Steel Corp. (72) Inventor Susumu Suto 1 Kawasaki-cho, Chuo-ku Chiba City, Chiba Prefecture Inside the company technology laboratory

Claims (4)

[Claims] An austenitic stainless steel slab is subjected to hot rough rolling and finish rolling to produce a hot-rolled austenitic stainless steel sheet.
Production of cold-rolled austenitic stainless steel rolled sheet characterized by performing high-pressure water having a collision pressure of 25 kgf / cm ² or more before finishing rolling, performing descaling after rough rolling, and finishing rolling. Method. 2. A cold-rolled austenitic stainless steel obtained by subjecting an austenitic stainless steel slab to hot rough rolling and finish rolling to form a hot-rolled sheet, subjecting the hot-rolled sheet to annealing and pickling, and then cold rolling. In the method for producing a steel sheet, after the hot rough rolling, before the finish rolling, the impact pressure is 25 kgf /
The difference in surface gloss is characterized by performing descaling by spraying high-pressure water of ² cm or more, followed by finish rolling.
A method for producing a cold-rolled austenitic stainless steel sheet having excellent surface gloss of 30 or less. 3. The method for producing a cold-rolled austenitic stainless steel sheet according to claim 2, wherein the hot-rolled sheet is further descaled, and then subjected to annealing and pickling. 4. The method according to claim 1, wherein a flow rate of the high-pressure water is 0.002 l / cm ² or more.