JPH08165523A - Production of austenitic stainless steel thin cast slab excellent in cold-rolled surface quality - Google Patents

Abstract

PURPOSE: To obtain a thin steel sheet excellent in the surface quality without uneven luster by applying hot-rolling having a specific rolling ratio to a cast slab which is cast by a synchronous type continuous casting process using cooling rolls having dimples and specifies the componental composition. CONSTITUTION: The austenitic stainless steel having component composition satisfying the inequality: Ni equivalence <=0.63×Cr equivalence -1.29 to the Cr equivalence defined with Cr+Mo+1.5Si and the Ni equivalence defined with Ni+30 (C+N)+0.5 (Mn+Cu) by wt.% in all elements, is cast with a continuous caster, such as twin rolls, synchronously moving the cast slab with the wall surface of the cooling rolls and randomly dispersing many dimples by a shot blasting method. Successively, the dimples transferred onto the cast slab is rolled and disappeared by the hot-rolling having >=10% reduction ratio and also, the thin sheet forming equi-axed crystal grain by recrystallizing the surface layer is coiled. The uneven luster caused by the remaining δ-ferrite is not at all developed by the limitation of the components in the cast slab.

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 surface quality without uneven gloss by 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 in which there is no relative speed difference between the slab and the inner wall surface of the mold. 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 the 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 with, the production efficiency and the cost are significantly advantageous.

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, without hot rolling, has various glosses on the surface. The unevenness occurs. Typical uneven glossiness is (1) unevenness of coarseness and fineness in the cast structure during pickling, resulting in unevenness of the mesh pattern after cold rolling, and (2) δ-ferrite in the cast structure. Coarse and fine grain structure after final annealing corresponding to residual unevenness, uneven glossiness occurs as color tone unevenness, and (3) unevenness of dimple itself promotes non-uniformity of oil film thickness in roll bite during cold rolling. There is uneven gloss, etc.

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
We proposed a method of reducing the sensitivity to uneven coagulation by adjusting δ-Fe _cal. (%) to 5 to 9%.

In the above invention, the method for processing the predetermined dimples is limited to the expensive photo-etching method,
There is a problem that an inexpensive processing method such as shot blasting cannot be used, and in addition to these problems (3), these inventions cannot be perfect measures. In particular,
When the coil grinding step before cold rolling was omitted, the surface quality of the bright annealed finished product (BA product) was a problem, and new measures were required.

[0008]

DISCLOSURE OF THE INVENTION The present invention provides an austenitic system having excellent surface quality in which uneven gloss is improved by a so-called synchronous continuous casting process such as a twin roll method using a cooling drum having dimples formed thereon. An object of the present invention is to provide a method for manufacturing a stainless steel thin plate.

[0009]

That is, the gist of the present invention resides in that a continuous roll such as a twin roll in which a slab moves in synchronization with the wall surface of a cooling drum in which a large number of random dimples are scattered by a shot blast method. Austenitic stainless steel having a composition such that the Cr equivalent defined by Cr + Mo + 1.5Si and the Ni equivalent defined by Ni + 30 (C + N) +0.5 (Mn + Cu) satisfy the following formula (1) in weight% by a casting machine. Steel is cast, and subsequently, the dimples transferred to the slab are rolled and extinguished by hot rolling with a rolling ratio of 10% or more, and the surface layer is recrystallized to wind a thin plate slab with equiaxed grains. A method for producing an austenitic stainless steel thin slab having excellent cold rolled surface quality characterized by the above.

Ni equivalent ≦ 0.63 × Cr equivalent −1.29 (1)

[0011]

In the continuous casting method such as the twin roll method, the purpose of randomly distributing the dimples on the cooling drum is to directly cool the portion of the dimple that is still low in rigidity (dimple center) by the air gap directly to the cooling drum. The thermal stress at the time of enclosing a highly rigid part (dimple edge part) that is sufficiently cooled by contacting is reduced by dimple dispersion, and cracks due to shrinkage of the solidified shell occur across multiple decreased rigidity parts. It is to prevent doing.

However, the non-uniform cooling caused by the formation of the dimples has a great influence on the uniformity of the solidified structure, resulting in unevenness of crystal grains and residual δ-ferrite corresponding to the dimple irregularities. In addition, the unevenness of the transfer dimples remains on the surface layer of the slab, and the internal quality of the surface and immediately below is slab-cast.
The material is subjected to a cold rolling process in a state significantly different from that of the hot rolling process material, and these cause uneven gloss after cold rolling.

In particular, the occurrence of residual δ-ferrite unevenness is a phenomenon peculiar to austenitic stainless steel. The present inventors have made a dimple interval by the shot blast method,
Casting austenitic stainless steel with various components adjusted from a twin roll casting machine using cooling drums with randomly arranged dimples without controlling the size etc., and casting from a temperature of about 1100 ° C. 1-pass rolling of 5 to 35% was performed by a 2Hi rolling mill disposed on the rear surface of the mill, and wound at about 850 ° C. to produce a thin strip slab. Then, annealing / pickling, cold rolling, and final annealing / pickling were performed to manufacture a thin plate having a thickness of 0.6 mm, and uneven gloss was evaluated.

As a result, as shown in FIG.
Cr equivalent defined by Cr + Mo + 1.5Si and Ni +
N defined by 30 (C + N) +0.5 (Mn + Cu)
Regarding the i equivalent, it was confirmed that gloss unevenness due to residual δ ferrite did not occur at all due to the component limitation satisfying Ni equivalent ≦ 0.63 × Cr equivalent−1.29.

With the components satisfying the above relationships, the solidification structure after casting is an ashular δ structure (the ashular δ is the main component and some lacy δ is present).
It is presumed to be F-mode solidification, which is called in weld metal. The inventors of the present invention have found that the uneven glossiness due to δ-ferrite is an ash-like δ that occurs when it is completely solidified in the δ single phase and transformed into γ rather than the vermicular δ that occurs in the eutectic structure of the δ + γ phase during solidification. Found that the unevenness of residual ferrite in the solidified structure was small by observing the solidified structure of many twin roll cast materials, but it was necessary to reduce the dimple spacing from the conventional knowledge. However, it was clarified from this result that the dimple spacing is not always a necessary and sufficient condition. The reason is considered as follows, as shown in FIG.

The photo-etched dimple edge is
Due to the almost right angle, molten steel does not enter the dimple center due to surface tension, and the air gap layer in that part becomes thicker, whereas the dimples formed by shot processing have a gentle edge inclination, so the edge part As a result of the molten steel entering the dimple central portion along the slope of, the air gap layer is thin and the solidification cold speed difference between the dimple central portion and the dimple-dimple portion is reduced. Further, in the shot dimple, small dimples are present in large dimples, and the effect of direct contact with the cooling drum changing from surface to point disperses the solidification cold speed difference in each part of the dimple. It is considered that these effects notably replace the action of narrowing the dimple spacing, which is called photoetching dimples, and that the uneven glossiness of the product is substantially improved only by limiting the specific components.

Next, the non-uniform crystal grains during solidification can be improved by rolling and recrystallization. As shown in FIG. 3, it was clarified that the recrystallization of only the surface layer, which affects the uneven gloss, becomes a recrystallization / sized grain structure by hot rolling at a rolling reduction of 10% or more. The reason why the surface layer is completely recrystallized at a rolling reduction of 10% or more is that the applied shear strain component is larger in the surface layer than in the central layer and the accumulated strain necessary for recrystallization is high, and δ in F mode solidification is high. It is considered that the reduction of the initial grain size due to the pinning effect of ferrite and the quenching effect of the surface layer worked effectively.

Further, the transfer dimples on the surface layer of the cast slab can be eliminated by rolling. FIG. 4 shows the effect of the rolling reduction on the depth of the transfer dimple unevenness. In order to obtain a surface roughness almost equal to that of the slab casting-hot rolling process material, the depth of the transfer dimple unevenness is about 70 to 80 on average.
In the case of μm, a rolling rate of 10% or more is required. In the present invention, not only by the photoetching method or the like, along with the dimple formation of the cooling drum by the shot blasting method that is easy to perform the dimple processing and repair, the solidified structure is made uniform by limiting the components to a specific range, and By improving inhomogeneity of crystal grains and unevenness of transfer dimples by in-line rolling, various uneven glosses can be solved at once.

[0019]

EXAMPLES Austenitic stainless steels shown in Tables 1 and 2 (continued from Table 1) were cast by a twin roll casting machine using a cooling drum having dimples randomly arranged by a shot blasting method. 3-5 mm casting, 1-pass rolling of 5-35% by a 2Hi rolling mill arranged on the rear surface of the casting machine in a temperature range of about 1030 to 1100 ° C, and winding at about 850 ° C, a thin strip slab Was manufactured. 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, annealing / pickling, cold rolling, and final annealing / pickling were performed to manufacture a thin plate having a thickness of 0.6 mm, and uneven gloss was evaluated. Further, the solidification structure of the slab, the recrystallization condition of the surface layer, and the transfer dimple unevenness were measured by a roughness meter. The results are shown in Table 3, Table 4 (continued-1 of Table 3) and Table 5 (continued-2 of Table 3).

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

[Table 4]

[0024]

[Table 5]

The manufacturing method according to the present invention (No. 1 to No.
The unevenness of solidification structure, the recrystallization condition of the surface layer and the surface roughness of the cast product obtained in 12) were good, and the thin plate product was good without unevenness of gloss. On the other hand, Comparative Example No. 13 and No. The product obtained by No. 14 was a proper component, and no gloss unevenness due to residual ferrite did not occur, but gloss unevenness due to mixed grains and dimples occurred due to insufficient rolling. Further, Comparative Example No. 15-N
o. The product obtained from No. 24 was out of the components of the present invention even when the uneven gloss due to mixed grains or dimples was good, and therefore uneven gloss due to residual ferrite occurred, which was overall poor. In addition, Comparative Example No. 17 and N
o. Since the product obtained by No. 20 was a manufacturing method completely outside the scope of the present invention, any uneven gloss was generated,
It was bad.

[0026]

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

[Brief description of drawings]

FIG. 1 is a diagram showing the influence of Cr equivalent and Ni equivalent on gloss unevenness caused by residual ferrite.

FIG. 2 is a schematic diagram showing a photoetch and shot dimple shape and a molten steel intrusion state.

FIG. 3 is a surface layer γ grain trace diagram comparing unskilled and 10% reduced slabs.

FIG. 4 is a diagram showing the relationship between the surface roughness of a transfer dimple by rolling and the rolling reduction.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinichi Teraoka 3434 Shimada, Hikari City, Yamaguchi Prefecture Nippon Steel Works, Ltd.

Claims (1)

[Claims] 1. A continuous casting machine such as a twin roll in which a slab moves in synchronism with the wall surface of a cooling drum in which a large number of random dimples are scattered by a shot blast method, and is defined as Cr + Mo + 1.5Si in% by weight. Austenitic stainless steel having a composition equivalent to Cr equivalent and Ni equivalent defined by Ni + 30 (C + N) +0.5 (Mn + Cu) satisfying the following formula (1) was cast, and subsequently,
A cold-rolled surface characterized in that the dimples transferred to the slab are rolled and extinguished by hot rolling with a rolling ratio of 10% or more, and the surface layer is recrystallized to wind a thin plate slab with equiaxed grains. A method for producing a high quality austenitic stainless steel thin slab. Ni equivalent ≦ 0.63 × Cr equivalent−1.29 (1)