JPH08300107A - Production of austenitic stainless steel thin strip excellent in surface quality - Google Patents

Abstract

PURPOSE: To provide an austenitic stainless steel thin strip excellent in surface quality by cold-rolling without developing relative velocity difference between a cast slab and the inner wall of a mold and without coil-grinding the stainless steel thin cast slab which is cast, by a so called synchronize type continuous casting process and has the thickness approximate to the product thickness. CONSTITUTION: The austenitic stainless steel thin cast strip is cast with a continuous caster, in which the cast strip is moved in synchronism with the wall surface of cooling drums having many dispersed dimples, and hot-rolled at >=15% draft. By this method, the dimples transferred to the surface of the thin cast strip are rolled and eliminated and the roughness of surface is made to <=20μm Rt and obtd. hot-rolled thin strip is coiled. Successively, the hot-rolled thin strip is cold-rolled and bright-annealed without coil-grinding to produce the cold-rolled product.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stainless steel strip slab 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 inner wall surface of the mold. The present invention relates to a method for producing an austenitic stainless steel sheet having excellent gloss by hot rolling and cold rolling.

[0002]

2. Description of the Related Art Synchronous continuous casting processes include twin roll method, twin belt method, as described in, for example, a paper featured in "Iron and Steel"'85 -A197 to A256.
It is a synchronous continuous casting process such as a single roll method where there is no relative speed difference between the slab and the mold wall. The twin-roll continuous casting method, which is one of these synchronous continuous casting processes, is a continuous casting method that is composed of a pair of parallel or inclined cooling drums of the same or different diameter and side dams that seal both end faces of the cooling drums. Molten steel is poured into the mold, solidified shells are generated on the circumferential surfaces of both cooling drums, and the solidified shells are united near the closest positions (so-called "kissing points") of both rotating cooling drums, This is a continuous casting method in which a thin strip-shaped slab is formed.

For example, a thin strip slab cast by a twin roll type continuous casting method has a thickness of several mm (usually about 2 to 5 mm), and is cold-rolled without the conventional hot rolling. Thin sheet products can be manufactured. For this reason, a conventional manufacturing method (slab casting-hot rolling process) in which a slab for hot rolling having a thickness of more than 100 mm is cast by continuous casting using a vibration mold or the like, and this is hot rolled and then cold rolled Compared to
The production efficiency and cost will be significantly improved.

However, a product obtained by cold-rolling a thin strip-shaped slab cast by a twin roll type continuous casting method using a cooling drum having dimples formed thereon is not annealed or pickled. (2B product) has uneven gloss, and bright annealing finish (BA product) causes surface quality problems such as poor gloss (only when surface grinding is omitted in the coil grinding process). Typical uneven gloss in 2B products is
(1) Coarse and fine grain parts of the cast structure become micro-grooves during pickling, uneven gloss that occurs as a dense and dense mesh pattern after cold rolling, and (2) after final annealing corresponding to residual δ ferrite unevenness of the cast structure. (3) For BA products, cold rolling occurs when the surface of the thin strip slab is not smoothed by the coil grinding process before cold rolling. Later, there are defects such as poor gloss due to dimple depressions remaining on the surface of the cold-rolled sheet.

As a countermeasure against these problems, first of all, in order to solve the problem (1), the inventors of the present invention have disclosed in Japanese Patent Laid-Open No. 3-66460 that microgrooves generated during pickling have uneven luster after cold rolling. Proposed a method of randomizing coarse and fine grains in the grain structure by using a cooling drum with a dimple pattern in which dimples with a size of 0.1 to 1.0 mm are randomly arranged in the coarse and fine grains of the cast structure. did.

To solve the problem (2), the inventors of the present invention have disclosed in Japanese Unexamined Patent Publication No. 4-158957 that the interval between all the dimples provided on the wall surface of the cooling drum is 0.35.
By setting the thickness to be equal to or less than mm, the uneven cooling corresponding to the dimple unevenness is relaxed and the solidified structure is made uniform, and δ
A method was proposed in which the sensitivity to uneven coagulation tissue was reduced by adjusting -Fe _cat. (%) To 5 to 9%.

However, the problem of (3) can be dealt with by surface grinding in the coil grinding process, but since the coil grinding process before cold rolling is required, it is new from the viewpoint of production efficiency and cost. Necessary measures were taken.

[0008]

DISCLOSURE OF THE INVENTION The present invention is an austenitic system excellent in surface quality without a coil grinding step in a so-called synchronous continuous casting process such as a twin roll method using a cooling drum having dimples formed thereon. It is an object to provide a method for manufacturing a stainless steel thin plate.

[0009]

That is, the gist of the present invention is to provide a continuous casting machine in which a slab moves in synchronization with the wall surface of a cooling drum in which a large number of dimples are scattered.
By casting a thin cast piece of austenitic stainless steel and then hot rolling at a rolling rate of 15% or more, the dimples transferred to the surface of the thin cast piece are rolled down and eliminated, and the surface roughness is reduced. Rmax is set to 20 μm or less, then the obtained band-shaped hot-rolled thin plate is rolled up, the band-shaped hot-rolled thin plate is further descaled, and subsequently cold-rolled and bright annealed without surface grinding to manufacture a cold-rolled product. A method for producing an austenitic stainless steel sheet having an excellent surface characterized by the above.

[0010]

In the continuous casting method such as the twin roll method, the purpose of scattering the dimples on the cooling drum is to directly contact the portion (dimple center portion) that is still cooled and has a low rigidity by the air gap. As a result, the thermal stress at the time of surrounding a sufficiently cooled high rigidity part (dimple edge part) is reduced by dimple dispersion, and cracks due to shrinkage of the solidified shell occur across multiple decreased rigidity parts. Is to prevent that.

However, due to the formation of the dimples, the unevenness of the transferred dimples remains on the surface of the cast slab,
The surface is subjected to a cold rolling step in a state that is significantly different from the conventional slab casting-hot rolling process material, and the unevenness left on the surface even after cold rolling causes defective gloss.

The present invention eliminates or reduces the unevenness of the transfer dimples formed on the surface of the thin cast piece by hot rolling. The hot rolling in the present invention is 900-12.
It is carried out at a reduction rate of 15% or more, preferably 30% in the temperature range of 00 ° C. A thin-walled slab cast from a molten steel having a component indicated by the material symbol A in Tables 1 and 2 described in the examples of the present application by a twin roll continuous casting machine was rolled at a temperature of 1044 ° C., and a transfer dimple formed at that time was formed. FIG. 1 shows the relationship between the depth of the irregularities and the rolling reduction in hot rolling. From this figure, the unevenness of the transfer dimples is about 70 to 8 on average.
In the case of 0 μm, it has been found that it is possible to stably maintain the roughness of the surface of the slab at Rmax of 20 μm or less by giving a rolling rate of 15% or more. That is, the present invention can obtain a material having a surface roughness smaller than that of the material obtained by the slab casting-hot rolling process of the prior art, and as shown in FIG. The resulting poor gloss could be significantly improved.

In the present invention, the hot rolling temperature is 9
If the temperature is below 00 ° C, the rolling roll will be scratched, and
If the temperature exceeds 00 ° C, wear of the rolling roll will increase, so 90
A temperature range of 0 to 1200 ° C is suitable. in this way,
According to the present invention, it is possible to prevent cracking of a thin cast slab by the dimples of the cooling drum, and to improve the unevenness of the dimples transferred to the surface of the slab by in-line rolling to solve the gloss defect.

[0014]

EXAMPLES Austenitic stainless steel having the chemical composition (% by weight) shown in Tables 1 and 2 (continued from Table 1) was cast on a twin roll casting machine using a cooling drum having dimples on the surface, and cast plate thickness. About 1030 to about 1030 by a 2Hi rolling mill, which is cast at 3 to 5 mm, and the obtained slab is placed on the rear surface of the casting machine.
One-pass rolling of 5 to 30% was performed in the temperature range of 1100 ° C, and wound at about 850 ° C to produce a strip-shaped hot-rolled thin plate. During the rolling, the rolling reduction force was adjusted during the rolling to change the rolling reduction during casting, and a heat insulating cover was provided between the cooling drum and the rolling mill to raise the temperature before rolling as high as possible. Then, descale pickling, cold rolling, and bright annealing were performed to manufacture a thin plate having a thickness of 0.6 mm, and the glossiness was evaluated. Furthermore, the transfer dimple unevenness of the wound band-shaped hot rolled thin plate after hot rolling was measured with a roughness meter. The results are shown in Table 3.
Shown in

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

[Table 3]

Manufacturing method according to the present invention (No. 1 to No. 5)
The surface roughness of the thin strip slab obtained by
It was less than m, Gs 45 ° ≧ 850 was secured, and a cold rolled product with good surface quality was obtained. On the other hand, comparative examples (No. 6 to N)
o. In the slab obtained by 9), since the rolling is insufficient, the unevenness of dimples remains, and the surface roughness is 20 μm in Rt.
And the resulting cold rolled product had poor gloss.

[0019]

As described above, according to the present invention,
An austenitic stainless steel thin plate having a surface quality equivalent to that of the conventional manufacturing method by the slab casting-hot rolling process can be manufactured at low cost and in a short period of time.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the surface roughness of a transfer dimple after rolling and the rolling reduction.

FIG. 2 is a diagram showing the relationship between the surface roughness of a transfer dimple after rolling and the glossiness.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location B22D 11/12 B22D 11/12 Z C21D 6/00 102 C21D 6/00 102A 8/02 9270-4K 8/02 D 9/52 101 9/52 101 C22C 38/00 302 C22C 38/00 302Z (72) Inventor Yusuke Oikawa 3434 Shimada, Hikari City, Yamaguchi Prefecture Shin Nippon Steel Co., Ltd.

Claims (1)

[Claims] 1. A continuous casting machine in which a slab moves in synchronism with a wall surface of a cooling drum having a large number of dimples scattered therein, a thin slab of austenitic stainless steel is cast, and the slab has a thickness of 15% or more. By hot rolling at a rolling rate, the dimples transferred to the surface of the thin-walled slab are rolled down and eliminated, the surface roughness is set to Rmax of 20 μm or less, and then the obtained strip-shaped hot-rolled thin plate is wound up, Further, the method for producing an austenitic stainless steel sheet having excellent surface quality, which comprises descaling the band-shaped hot rolled sheet, followed by cold rolling / bright annealing without surface grinding to produce a cold rolled product. .