JPH05293601A - Manufacture of cold rolled austenitic stainless steel having excellent surface quality - Google Patents

Abstract

PURPOSE:To provide a manufacturing method for a cold rolled sheet having no unevenness of luster on its surface by controlling the solidified structure of a casting strip relating to the manufacturing method for a cold rolled austenitic stainless steel sheet using a continuous casting machine in which a casting strip and the wall face of a mold synchronously move. CONSTITUTION:When casting a thin belt-like casting strip using a continuous casting machine in which the casting strip and the wall face of the mold synchronously move, 8 ferrite on the surface layer of the casting strip is lost by slowly cooling the casting strip appearing from the mold in a high temperature zone and the cold rolling is carried out after the casting strip is descaled and ground while leaving the zone.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention cools a stainless steel strip slab having a thickness close to the product thickness cast by a so-called synchronous continuous casting process in which there is no relative velocity difference between the slab and the mold wall. The present invention relates to a method for producing a thin plate product by hot rolling.

[0002]

2. Description of the Related Art A stainless steel hot coil manufactured by a conventional hot rolling process has a thin oxide scale during hot rolling as compared with ordinary steel, so that the lubrication effect of the hot rolling is not sufficient, and a rolling roll is used. The seizure phenomenon of (3) is apt to occur, and scale biting flaws in which the scale is bitten into the base material in layers are often generated. In particular, this tendency is remarkable in stainless steel containing high Cr or Si, Al and the like and having excellent oxidation resistance.

In the production of cold-rolled sheet, it is necessary to completely remove the scale from the viewpoint of the surface quality of the product, and it is necessary to remove the above-mentioned scale biting flaw and oxide scale before cold rolling. As a typical conventional method, mechanical descaling such as shot blasting is performed to cause cracks and peeling of the scale, and then final descaling is performed with a pickling solution such as hydrofluoric acid, sulfuric acid and hydrochloric acid, and then the shape is corrected. In order to do so, cold rolling is generally performed, and then, heavy grinding (up to 100 μm) by belt grinding is performed to remove scale biting flaws.

In the stainless steel descaling process,
Since the finishing descaling is performed by pickling, there are various problems in terms of waste liquid treatment and deterioration of working environment. In order to solve these problems, a method of descaling only by a mechanical method has been studied for hot-rolled material and its annealed material. That is, a method of crushing the scale on the surface of the steel strip by rolling while supplying the slurry-like polishing agent as rolling oil, and then crushing and removing the crushed scale with a brazilian roll (JP-A-57-137023) and After repeatedly applying repeated bending to perform scale braking, a method of grinding with a nylon bristle braid roll containing abrasive grains (Japanese Patent Laid-Open No. 54-122656) and the like.

However, these methods are not intended to positively remove the surface flaws of the steel strip because the grinding function is intended for removing scales, and the scale-engaging flaws need to be removed by belt grinding in a subsequent step. was there. In addition, it is necessary to correct the shape by cold rolling in order to uniformly remove surface flaws by belt grinding in the plate width direction. Generally, cold rolling mills are used to perform the original gauge down (thickening) work. In addition to that, the shape correction work is additionally performed, and the cold rolling efficiency is reduced.

In such a treatment method, it is necessary to pass three lines of a pickling line, a rolling line and a grinding line, and there is a problem that the productivity is extremely poor and the cost is high. In recent years, research on a process for producing a slab having a thickness equal to or close to that of a hot strip by continuous casting has been advanced. For example, "Iron and Steel"'85
In the synchronous continuous casting process in which there is no relative speed difference between the slab and the inner wall of the mold, such as the twin roll method and the twin belt method, which are introduced in the article featured in A197 to '85 -A256, Since hot rolling is not required to obtain a hot coil having a thickness of 10 mm, there is no biting of the layered scale as described above. Therefore, heavy grinding, which was necessary in the conventional hot rolling process, is unnecessary or can be significantly reduced.

However, in this process, in the steel grade in which the ferrite phase remains in the austenite phase (SUS304 as a representative steel grade), the air gap generated between the cast slab and the cooling drum causes a quenching part and a slow cooling part on the surface of the slab. As a result, the residual amounts of ferrite in these portions differ, and uneven solidification structure occurs on the surface of the slab. When this structure unevenness is subjected to annealing and pickling after cold rolling, gloss unevenness occurs and the surface quality of the product is significantly impaired. This is a surface defect peculiar to this process material that is not found in the current hot-rolled process material.

[0008]

DISCLOSURE OF THE INVENTION The present invention is directed to an austenitic stainless steel ribbon having a thickness close to that of a product cast by a so-called synchronous continuous casting process, in which there is no relative speed difference between the slab and the mold wall surface. The solidification structure unevenness of the slab is eliminated in a short time by in-line heat treatment, and preferably the function of removing oxide scale and mild surface flaws generated on the surface is arranged in the casting line, and individual cold rolling is performed. Manufacturing of austenitic stainless steel cold-rolled sheet with excellent surface quality by directly annealing the molten steel into a strip-shaped strip without surface scale and directly going to the cold-rolling step, omitting the annealing / pickling process and surface flaw removal process It is intended to provide a way.

[0009]

According to the present invention, there is provided a slab that emerges from a mold when casting a strip-shaped slab by a continuous casting machine in which the wall surface of the mold moves in synchronization with the slab. The δ-ferrite in the surface layer of the slab is eliminated by slowing the cooling of the slab in the high temperature region, and the δ-ferrite disappearance region of the surface layer is exposed by subjecting the obtained slab to mechanical descaling and grinding, followed by cold rolling. It is achieved by a method for producing an austenitic stainless steel cold-rolled sheet having excellent surface quality.

Cooling of the cast piece discharged from the mold is carried out at 1250 ° C.
When the average cooling rate in the temperature range of ˜1100 ° C. is 5 ° C./sec or less, the δ ferrite in the surface layer of the slab can be easily eliminated. In the present invention, if the depth at which the δ ferrite disappears is set to about 100 μm or more from the surface of the slab and the total depth for mechanical descaling and grinding is set to about 50 μm or less from the surface of the slab, the occurrence of uneven gloss of the cold rolled sheet can be prevented. Is enough for

When the surface roughness due to grinding is set to Rmax of 20 μm or less, it is advantageous from the viewpoint of preventing grinding marks on the cold-rolled sheet.

[0012]

In FIG. 1, in the case of a SUS304 slab (thickness 3 mm) as a typical example, the cooling time from the temperature of about 1250 ° C. to each water cooling start temperature was changed by covering the line after casting with heat retention. The coagulated tissue in the case is shown. When the above temperature range is cooled at about 5 ° C / sec or less, about 10% from the surface layer
A solidified structure in which δ ferrite in the 0 μm depth region was almost disappeared was obtained. This is sufficient to ensure the uniform texture region needed to prevent uneven gloss even with removal of surface scale and mild surface flaws (about 50 μm). The figure shows the amount of residual δ-ferrite under each processing condition.
For example, when cooling from 1250 ° C. to 1100 ° C. and 1000 ° C. at about 5 ° C./sec, there is almost no difference in the amount of residual δ ferrite, and the temperature at which the diffusion disappearance of δ ferrite is recognized is 1100 ° C. or higher. it is conceivable that. Also, 12
Since the temperature range of 50 ° C. or higher is a δ / γ two-phase region, the slow cooling promotes the growth of the δ phase.

Therefore, it is desirable to pass the temperature range of 1250 ° C. to 1100 ° C. at 5 ° C./second or less. As a comparative example in FIG. 2, the same SUS304 slab (thickness 3 m
In case of m), about 20 ℃ /
The solidification structure when the slab cooled in seconds is reheat-treated is shown. In order to obtain a solidified structure equivalent to that of the inline heat treatment shown in FIG. 1, it was necessary to hold at 1200 ° C. for 120 seconds. The reason for this is that once the slab is cooled, the composition of the δ ferrite is considered to change from the high temperature state to the high Cr, low Ni side, and the deviation from the equilibrium composition at the heat treatment temperature due to reheating is large, In a short time, it takes time to approach the equilibrium composition at that temperature, but in in-line heat treatment, the composition of δ-ferrite at high temperature is close to the equilibrium composition at heat treatment temperature, so it is possible to eliminate it in a short time. .

In continuous casting such as twin rolls in which the slab moves in synchronization with the wall surface of the mold, the solidification cooling speed of the surface layer is several thousand degrees Celsius /
It is on the order of seconds, and the δ ferrite produced is finely dispersed. This makes it possible to shorten the time by the in-line heat treatment of the present invention, and for this reason, δ + γ
As a means for more finely dispersing δ ferrite by rapidly cooling the phase region, it is considered more effective to press the slab on the cooling drum even after the kissing point of the cooling drum for rapid cooling and then perform the in-line heat retention of the present invention. Be done.

Next, as a descaling and surface grinding method, preferably, a slurry polishing agent such as sand iron is projected along with a medium such as high-pressure water or gas, or descaling by shot blasting, followed by elasticity. Surface grinding with a whetstone roll or the like is used. In this surface grinding, the finished surface roughness is Rma x in order to obtain a surface texture that does not cause grinding marks after cold rolling.
20 μm or less is preferable. Further, a grinding depth of 50 μm or less from the surface layer is desirable in order to leave a uniform structure of the surface layer. In the surface grinding used in the present invention, an elastic grindstone roll that easily fits the shape of the plate, for example, a roll formed by laminating disks coated with abrasive grains (lamination type) or a laminating flap wheel is formed into a roll. Things (flap type)
Therefore, it is important that the roll itself is a sufficiently elastic body.

[0016]

[Example] Austenitic stainless steel SUS304
Was cast into a strip-shaped slab (thickness: 2 to 5 mm) by an internal water-cooled twin roll continuous casting machine. 1250 ° C to 1100 ° C due to the heat insulation cover with heating burner installed in the line
After changing the holding time up to, it was cooled and sprayed with high pressure water of sand iron slurry (NID), or shot blasting was performed to descale. After that, surface grinding was performed while changing the finished surface roughness, cold rolling was performed, and final annealing was performed to manufacture a stainless steel thin plate product. Table 1 shows the results of observing the surface properties of the products together with these manufacturing conditions.

According to Table 1, N obtained by the method of the present invention
o. 1-No. In No. 10, no surface defects due to uneven surface gloss or grinding marks were observed. On the other hand, the comparison method No. In Nos. 11 and 12, uneven solidification structure was not sufficiently eliminated, and uneven gloss was generated. No. In Nos. 13 to 16, the occurrence of uneven gloss was not recognized, but since the roughness after surface grinding was large, grinding residue was generated and the product had poor gloss.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

EFFECTS OF THE INVENTION When a strip cast is processed according to the method of the present invention, homogenization of the surface structure of the cast is achieved, the product after cold rolling does not have uneven gloss, and an austenitic system of good surface quality is obtained. A stainless steel cold rolled sheet can be obtained. Furthermore, individual annealing / pickling treatment steps before cold rolling, shape correction rolling steps, and surface flaw removal heavy grinding steps that are performed in the conventional hot rolling process can be omitted, resulting in a significant reduction in manufacturing cost, The manufacturing period can be shortened.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between cooling conditions and ingot solidification structure in in-line heat treatment of thin strip slabs.

FIG. 2 is a graph showing the relationship between the reheat treatment condition of a thin strip slab and the solidification structure of the slab.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Fumio Fumio 3434 Shimada, Hikari City, Yamaguchi Prefecture Shin Nippon Steel Co., Ltd.

Claims (4)

[Claims] 1. When casting a thin strip-shaped cast product by a continuous casting machine in which the mold wall moves in synchronization with the cast product, the cooling of the surface layer of the cast product discharged from the mold is delayed in a high temperature range to obtain the cast product. The austenitic stainless steel with excellent surface quality is characterized in that δ ferrite in the surface layer part disappears and the obtained slab is subjected to descaling and grinding, followed by cold rolling. Manufacturing method of rolled sheet. 2. The slab surface layer portion discharged from the mold is cooled so that the average cooling rate in the temperature range of 1250 ° C. to 1100 ° C. is 5 ° C./second or less. The method described. 3. The depth of the surface layer portion at which the δ ferrite disappears is about 100 μm or more from the surface of the slab, and the total depth of the descaling and grinding is 50 μm or less from the surface of the slab. The method according to 1 or 2. 4. The surface roughness Rmax20 is obtained by the grinding.
4. The method according to any one of claims 1 to 3, characterized in that the thickness is not more than μm.