JPH082484B2 - Austenitic stainless steel strip-shaped slab with excellent surface quality, thin plate manufacturing method, and strip-shaped slab - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a slab having a thickness close to the product thickness obtained 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. , Its production and cold rolling of the slab to produce an austenitic stainless steel sheet.

[Conventional technology]

The conventional method of producing a stainless steel thin plate using the continuous casting method is to make the thickness of 100 mm while vibrating the mold in the casting direction.
Casting to the above slab, performing surface maintenance of the obtained slab, after heating to 1000 ℃ or more in a heating furnace, hot rolling is performed by a hot strip mill consisting of a rough rolling machine and a finishing rolling mill train. A hot strip with a thickness of several mm,
Further, after annealing as necessary, descaling and cold rolling were performed for final annealing.

In such a conventional process, in order to hot-roll a slab with a thickness of 100 mm or more, a long hot strip mill is required, and a problem of using a large amount of energy for heating and rolling the slab was there.

In response to this problem, 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 in recent years. For example, there is no relative velocity difference between the cast slab and the inner wall of the mold, such as the twin roll method, the twin belt method, etc., as introduced in the paper featuring "Iron and Steel"'85 -A197 to '85 -A256. Synchronous continuous casting processes are known.

When manufacturing stainless steel sheet products through these synchronous continuous casting processes, the value of cast piece thickness / sheet product thickness is small, so to obtain a high quality sheet product surface, the surface texture of the cast piece should be stable. And maintaining a high standard is an important issue.

That is, unlike the case of the slab manufactured by the conventional continuous casting equipment, these thin plate slab continuous casting processes can obtain a high-quality thin plate surface even if the degree of rolling in the subsequent steps is reduced. It was developed with the goal of producing thin strip slabs. for that reason,
If the thin strip-shaped slab has surface cracks or the like, it becomes a product surface defect, which significantly impairs the commercial value, and the object cannot be achieved.

Therefore, in order to prevent the occurrence of defects such as cracks on the surface of the strip-shaped cast piece, casting is performed by forming a plurality of depressions (dimples) having a predetermined size and depth on the peripheral surface of the cooling drum. (Japanese Patent Application Nos. 62-240479, 62-240481 and 63-202962).

[Problems to be Solved by the Invention]

Although surface defects such as cracks are prevented by the above method, when a casting is performed by providing a depression on the peripheral surface of the cooling drum, a quenching portion and a slow cooling portion are generated on the cast piece surface due to the air gap generated by the depression, As a result, in these parts δ
The amount of remaining ferrite differs, and so-called uneven solidification structure occurs on the surface of the cast piece. This solidification structure unevenness becomes apparent as uneven gloss on the surface of the thin plate product after cold rolling.

As shown in FIG. 1, this uneven solidification structure is caused by the large amount of δ-ferrite in the slow-cooled portion and the small amount in the quenched portion. After cold rolling a slab having such uneven solidification structure, when the final annealing is performed, the growth of recrystallized grains is inhibited in a portion where a large amount of δ ferrite is present and a fine grain structure is obtained, and recrystallization occurs in a portion where a small amount of δ ferrite is present. Grains grow to have a relatively coarse structure, and the size of crystal grains varies. This is,
It appears as uneven gloss on the product surface.

The present invention casts a strip-shaped slab by a continuous casting machine in which the slab moves in synchronization with a mold wall surface provided with random depressions (dimples) having a size, shape, and arrangement, and cold-rolling it. In the method of manufacturing stainless steel sheet products, control the unevenness of solidification structure of the slab caused by the large or small amount of δ ferrite remaining in the thin strip slab, which is caused by the dimples formed on the wall surface of the mold to prevent the slab from cracking. By doing so, it is intended to prevent uneven gloss of the thin plate product caused by uneven solidification structure.

[Means for solving the problem]

The above purpose is 3 (Cr + 1.5Si + Mo) -2.8 (Ni + 0.5Mn
+ 0.5Cu) -84 (C + N) -19.8 defined by -19.8
_{The caL.} (%) is set to 5 to 9%, and molten steel with a high δ ferrite component composition, which tends to form a δ solidification structure even in the surface layer of the slab with a fast cooling rate, has a diameter of 0.1 to 1.2 mm and a depth of 50 to 100 μm. A large number of dimples with circular or oval openings within the range are scattered, and the cooling drums are arranged so that the distance between all adjacent dimple edges is 0.35 mm or less. This is achieved by cooling and continuously casting the slab surface portion to a δ solidification structure with no solidification structure unevenness.

[Work]

Figure 2 shows Cr in the equilibrium diagram of the Fe-Cr-Ni ternary system.
_eq. + Ni _eq. ≈ 30% Cross-sectional state diagram of the equivalent part (Transact
Ion of JWRI Vol.14, No.1,1985, p125). Cr _eq. And Ni _{eq. Are} as follows and calculated from the components.

Cr _eq. = Cr (%) + 1.5Si (%) + Mo (%) + Nb (%) Ni _eq. = Ni (%) + 0.5Mn (%) + 0.5Cu (%) + 30 {C (%) + N ( %)} Complete γ solidification (Zone I) in the region where Cr _eq. Is small, but as Cr _eq. Increases, primary γ → δ + γ solidification (Zone II), primary δ → δ + γ solidification (Zone III), It is expected that the coagulation morphology will change to complete delta coagulation (Zone IV).

As a result of experiments with many component systems, 3 (Cr + 1.5Si + Mo)
-2.8 (Ni + 0.5Mn + 0.5Cu) -84 (C + N) -19.8 defined as δ-Fe _caL. (%) _Within the range of -2% or less and 5% or more, complete γ solidification structure (zone I). ) And primary crystals δ → δ + γ solidification structure (zone III) and complete δ solidification structure (zone IV).
The metallographic photographs of FIGS. 3 (a), (b), (c), and (d) show the cross-sectional _textures of cast _slabs cast with a component system in which δ-Fe _caL. (%) _Is changed. As is clear from the figure, δ-Fe
_{Those with caL.} (%) of -2.3% (zone I) have a complete γ solidification structure, and no solidification structure unevenness due to the large or small amount of δ ferrite is observed. When δ-Fe _caL. (%) is 2.3%, it is a primary crystal γ → δ + γ solidification structure (zone II), which is apparent in the residual state of δ ferrite in the slow-cooled part and the quenched part due to depressions (dimples). Differences are seen and the unevenness of the coagulation structure is remarkable. When δ-Fe _caL. (%) Is 6.3%, it is a primary crystal δ → δ + γ solidification structure (zone III). It is a δ solidification structure at a depth of about 150 μm from the surface layer, and a δ + γ solidification structure at a depth higher than that. Is observed. The δ solidification structure in the surface layer is uniform regardless of the slow-quenched portion, and no solidification structure unevenness is observed. Further, when δ-Fe _caL. (%) Is 8.8%, a complete δ solidified structure (zone IV) is obtained up to the center of the plate thickness.

Thus, the selection of the composition of components in the Fe-Cr-Ni system has a great influence on the solidification structure of the slab, δ-Fe _caL.
By controlling the (%) to -2% or less (complete γ solidification) and 5% or more (primary crystal δ → δ + γ solidification, complete δ solidification), it is clear that a slab with relatively few solidification microstructures can be obtained. became. However, δ-Fe _caL. (%) _Is -2
%, It is necessary to increase Ni _eq. Elements such as C, N and Ni, C has a risk of impairing corrosion resistance, and N is disadvantageous for softening. In addition, since the addition of Ni is restricted from the viewpoint of cost, it is not desirable as a practical steel of 18-8 series. Furthermore, on the high δ ferrite component side, there is a concern that deterioration such as age cracking characteristics that occurs during press working of thin plate products may occur, so the upper limit was set to 9%, and a suitable δ-Fe
It is judged that the _caL. (%) range is preferably 5 to 9%.

Next, the cooling drum of the present invention has a large number of circular or oval depressions formed on its surface. When the initially solidified shell is formed on the surface of the cooling drum, the depressions form independent air gaps that are not continuous with each other.
Therefore, the air gap causes a part of the solidified shell that is slowly cooled and a part that is rapidly cooled in contact with the cooling drum surface, and the amount of δ ferrite in the rapidly cooled part decreases compared to the slowly cooled part. Thus, uneven solidification structure occurs due to the large or small amount of δ ferrite. In order to reduce the unevenness of the solidification structure caused by such uneven cooling of the initial solidified shell, the size and depth of the depression formed in the cooling drum should be optimized while considering the relationship with the crack, and It was solved by narrowing the space between.

Fig. 4 shows the relationship between the distance between the edges of adjacent dents on the surface of the cooling drum and the solidification structure. From the figure, when the distance between the edges of the dents becomes 0.35 mm or less, the surface between the edges is in contact. The solidification structure observed in the solidified shell is a normal δ + γ structure in which δ ferrite is uniform, whereas when the distance between the edges of the cavities exceeds 0.35 mm, the normal δ +
A solidified structure with a small amount of δ ferrite (referred to as a γ structure) was observed together with a γ structure. The reason for this is that, as shown in FIG. 5, in the initial stage of solidification, the molten steel first contacts the edges of the depressions and is cooled, so when the gap between adjacent depressions is narrow, the solidification shell between the depressions becomes thicker. In a region with a wide depression interval, this effect does not extend over the entire depression interval, so that a local coagulation delay occurs in the solidification shell in that portion (fifth part).
Figure-b). Since the shell at that portion is thin and its strength becomes small, it is pressed by the static pressure of molten steel and adheres to the surface of the cooling drum (No. 5).
Fig. C), that part is rapidly cooled rapidly, the growth of δ ferrite is suppressed, and δ ferrite is likely to disappear due to the subsequent thermal diffusion, and δ ferrite is less than in other slowly cooled parts, and the slab surface has a large amount. -It is presumed that coagulated tissue unevenness with a small amount is generated (Fig. 5-d).

If the diameter of the opening of the recess is 0.1 mm or less, not only the effect of gentle cooling due to the air gap is small, but also brush processing such as recess processing and dirt is difficult. On the other hand, if the diameter of the opening of the recess exceeds 1.2 mm, the recess itself becomes a starting point for microcracks, which is not preferable.

If the pit depth is less than 50 μm, the effect of slow cooling of the entire cooling drum is not sufficient, and if it exceeds 100 μm, the convex dimples transferred to the slab become high, and processing such as grinding with a coil grinder is required before cold rolling. Therefore, productivity is reduced.

The ratio of the protrusion height of the convex dimples transferred to the cast piece to the recess depth (recess filling rate) is 80 to 100%.

Therefore, the shape of the projections on the surface of the cast slab according to the present invention is a circle or an ellipse having a diameter of 1.0 to 1.2 mm and a height of 50 to 100 μm, and the projection has a minimum distance between adjacent projections of 0.35 mm or less. Many are scattered.

As described above, the present invention controls the molten steel component to be cast on the high δ ferrite side, and by providing the depressions on the cooling drum surface so that the initial solidified shell can be uniformly and slowly cooled, uneven solidification structure It is possible to produce a small number of strip-shaped slabs, thereby preventing uneven gloss of the thin plate product after cold rolling.

〔Example〕

Austenitic stainless steel _{melts of} various δ-Fe _caL. (%) _{Shown in} Table 1 were cast by a twin roll continuous casting machine in which depressions were randomly or uniformly arranged on a cooling drum to obtain strip-shaped cast pieces. After polishing the surface of this slab for about 100 μm, a solidified structure was revealed by nitric acid electrolytic etching, and the unevenness of the structure was observed. Also, descaling the slab,
50-90% cold rolling and 1050-1200 ℃ annealing,
After the salt treatment, the occurrence of uneven gloss on the surface of the thin plate finished by pickling with a mixed acid of nitric acid-hydrofluoric acid was observed. Furthermore, the obtained thin plate was processed into a disk of 80 mmφ and 32 mmφ (drawing ratio = 2.
Cylindrical drawing was performed to 5) and the presence of aging cracks was checked after 48 hours. Table 2 shows the results.

As shown in Table 2, the manufacturing method according to the present invention (No. 1 to
No. 6) showed no unevenness in the solidification structure of the slab and uneven gloss of the thin plate product, which was good. On the other hand, Comparative Example No. 7
The samples obtained from No. 8 and No. 8 are suitable δ-ferrite components, but due to the wide spacing of the dimples, uniform solidification was not performed, and uneven solidification structure and uneven gloss occurred. Further, in the samples obtained in Comparative Examples No. 9 and No. 10, unevenness in solidification structure and uneven gloss occurred because the δ ferrite component was not appropriate. Further, the product obtained in Comparative Example No. 11 had good solidification structure unevenness and gloss unevenness, but δ-Fe _caL. > 9% or more of high δ ferrite component, so that age cracking occurred. did.

With respect to Inventive Example No. 3 and Comparative Example No. 9, the situation of uneven solidification structure of cast pieces and uneven gloss of thin plate products will be illustrated. Sixth
FIG. (A) shows a photograph of the surface texture of the solidification structure unevenness of the slab of the invention sample No. 4 and the uneven glossiness of the thin plate product, and no unevenness is observed at all. FIG. 6 (b) shows Comparative Example N
It shows that the solidification structure unevenness of the slab of o.9 shows that the structure after casting is caused by the amount of δ ferrite, and the unevenness of the gloss of the thin plate product is caused by the size of the recrystallized grains. .
In the comparative example, a large amount of δ-ferrite after casting prevents the growth of recrystallized grains of the thin plate product, resulting in a difference in grain size, resulting in uneven gloss. Since there is no difference in the amount of δ-ferrite, uneven gloss does not occur in the thin plate product.

[Effects of the Invention] As described above, according to the present invention, when casting a strip-shaped cast product, uneven texture does not occur on the surface of the cast product, so that the surface of the thin plate product produced through cold rolling thereafter has uneven gloss. It is possible to obtain a stainless steel thin plate that does not generate and has excellent surface quality.

[Brief description of drawings]

Fig. 1 is a photograph of the metallographic structure of the surface of the slab, which is likely to occur when casting with dimples on the peripheral surface of the cooling drum, and which is likely to occur when casting. Fig. 2 shows Fe-Cr- Cr _eq. + N in the equilibrium diagram of the ternary Ni system
FIG. 3 is a sectional state diagram of a portion corresponding to i _eq. ≈30%, and FIGS. 3 (a), (b), (c), and (d) show δ-Fe.
_Fig. 4 is a photograph of a metallographic structure showing a comparison of the cross-sectional structures of slabs obtained by twin roll casting of molten steel of the composition system in which _caL. It is a figure showing the relation of a solidification organization, Drawing 5 is a figure showing typically a mechanism which a solidification organization with little delta ferrite is generated, and Drawing 6 (a) and (b) shows this invention example No. 3 is a photograph of a metallographic structure showing unevenness of the textures of the slab and the product plate of No. 3 and Comparative Example No. 9.

Continuation of the front page (56) Reference JP-A-2-52152 (JP, A) JP-A 64-83342 (JP, A) JP-A 2-133529 (JP, A)

Claims (3)

[Claims] Claims 1. 3 (Cr + 1.5Si + Mo) -2.8 (Ni + 0.5Mn +
0.5Cu) -84 (C + N) -19.8 defined as δ-Fe _caL.
Austenitic stainless steel with 5% to 9% (%) dispersed with a large number of dimples having circular or oval openings with a diameter of 0.1 to 1.2 mm and a depth of 50 to 100 μm. Distance between adjacent dimple edges is 0.35
a continuous casting machine in which the slab moves in synchronism with the wall surface of the cooling drum arranged to be less than 10 mm, is cast into a strip-shaped slab with a thickness of 10 mm or less, and has an excellent surface quality. A method for producing a strip of stainless steel strip. 2. Claims: 3 (Cr + 1.5Si + Mo) -2.8 (Ni + 0.5Mn +
0.5Cu) -84 (C + N) -19.8 defined as δ-Fe _caL.
Austenitic stainless steel with 5% to 9% (%) dispersed with a large number of dimples having circular or oval openings with a diameter of 0.1 to 1.2 mm and a depth of 50 to 100 μm. Distance between adjacent dimple edges is 0.35
With a continuous casting machine in which the slab moves in synchronism with the wall surface of the cooling drum arranged to be less than or equal to mm, the strip is cast into a strip with a thickness of 10 mm or less, and then the strip is subjected to surface descaling. A method for producing an austenitic stainless steel sheet having excellent surface quality, which comprises forming a sheet product by cold rolling and annealing. 3. (Cr + 1.5Si + Mo) -2.8 (Ni + 0.5Mn +)
0.5Cu) -84 (C + N) -19.8 defined as δ-Fe _caL.
(%) Has a component composition in the range of 5 to 9%, and a circular or oval protrusion having a diameter of 0.1 to 1.2 mm and a height of 50 to 100 μm on its surface is 0.35 mm or less. Austenitic stainless steel thin strip slabs with excellent surface quality, characterized in that they are scattered in large numbers at the distance between adjacent protrusions.