JP2681393B2 - Method for producing austenitic stainless steel strip with good surface properties and excellent ductility - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing an austenitic stainless steel strip having excellent surface properties and excellent ductility.

[Conventional technology]

Conventional austenitic stainless steel cold-rolled steel strip or steel sheet (hereinafter sometimes simply referred to as cold-rolled steel strip)
Was usually cast from molten steel into a slab with a thickness of 100 to 200 mm, and then hot rolled into a hot rolled steel strip, which was usually manufactured by combining cold rolling and annealing.
On the other hand, a thin plate casting method has been proposed in which a thin plate having a plate thickness equivalent to that of a conventional hot-rolled steel strip is directly cast from molten steel mainly for the purpose of reducing manufacturing costs by saving steps and energy. Regarding the production of a cold-rolled steel strip or a steel plate using such a thin continuous cast material as a raw material, see, for example, JP-A-62-19.
There are proposals described in Japanese Patent No. 7247 and Japanese Patent Publication No. 63-27407.

[Problems to be solved by the invention]

Cold-rolled steel strip made of thin continuous cast material can omit the hot rolling process, eliminating the need for hot rolling equipment.
Not only does it require a large amount of equipment costs, but there is also the possibility of significant reductions in manufacturing costs, such as the need for various energy required for hot rolling. However, on the other hand, in terms of material properties, ductility may not be sufficient compared to the case of manufacturing through conventional continuous casting-hot rolling, and in terms of surface quality, surface texture after cold rolling may be rough. The present inventors have found a new problem that the occurrence of the above may occur.

[Findings of the inventors]

The present inventors systematically investigated the influence of manufacturing conditions on the surface quality and ductility of the cold-rolled steel strip of austenitic stainless steel made from a thin continuous cast material, and obtained the following findings.

(1) Using a continuous cast thin sheet as a material, subjecting the cast thin steel strip to cold rolling / annealing, or subjecting the cast thin steel strip to solution treatment and then cold rolling / annealing When a cold rolled strip of austenitic stainless steel is obtained, coarse grains during casting are brought in even after the subsequent solution treatment, so after cold rolling, before cold rolling. Surface roughness, that is, surface roughness, occurs corresponding to the coarse crystal grains.

On the other hand, in cold-rolled steel made from slabs manufactured by the conventional continuous casting method, high-pressure reduction is applied during hot rolling, so coarse grains during casting are destroyed and subsequent hot rolling occurs. Particle size 20 to 4 by plate annealing (usually 1100 to 1150 ℃)
It becomes about 0 μm. Therefore, even if hot-rolled and cold-rolled, no problem in surface quality occurs.

(2) In austenitic stainless steel cold-rolled steel strips made from thin continuous cast material, the amount and distribution of δ-ferrite during casting has a great influence on the material properties, especially the pressure. This means that the as-cast thin-plate continuous cast material has segregation of components mainly due to δ-ferrite, and even if δ-ferrite disappears even after subsequent annealing and cold rolling-annealing, the segregation of components remains. However, the ductility is reduced.

On the other hand, for slabs manufactured by the conventional continuous casting method,
Although δ-ferrite and component segregation due to δ-ferrite exist, high temperature and long time (usually: 115
Δ-ferrite and component segregation are eliminated by slab heating at 0 to 1250 ° C x 200 min) and hot rolling itself.

From the above, in order to secure the surface quality and ductility equivalent to those of the cold-rolled strip made from the conventional continuous cast slab in the cold-rolled steel strip made from the thin continuous cast material, the It is necessary to take some measures to reduce the crystal grain size and at the same time accelerate the disappearance of δ-ferrite.

[Means to solve the problem]

The present invention has been made on the basis of the findings as described above, and its gist is to continuously melt molten austenitic stainless steel in a twin roll type continuous casting machine, and to rotate the circumference of each roll. The solidified shells of the steel formed on the surface are crimped in the narrow space of twin rolls under a crimping load of 40 kgf or less per 1 mm of plate width, and an austenitic stainless steel strip with a thickness of 0.2 to 5.0 mm Is continuously cast, and the cast thin steel strip is expressed by the following formula: r> 8.51 × 10 ^-3 T + 4.79 ・ ・ ・ where T: direct rolling temperature (℃), r: rolling rate (%) It is characterized in that it is annealed-cold-rolled-annealed after being directly rolled under the conditions of satisfying temperature and reduction.

[Action]

According to the method of the present invention, when a cold-rolled steel strip of austenitic stainless steel made of a thin continuous cast material is manufactured, an appropriate work strain is applied before annealing before cold rolling. The crystal grain size of the latter step becomes small, and at the same time, the disappearance of δ-ferrite can be promoted. This solves the above-mentioned problems of surface quality and ductility deterioration of cold-rolled steel strips made from thin continuous cast material at the same time.

[Detailed Description of the Invention]

FIG. 1 shows a main part of a twin-roll continuous caster to which the present invention is applied and a state during casting. As shown in FIG. 1, the molten austenitic stainless steel 1 in the tundish is formed on the circumferential surface of the internal water-cooled twin rolls 3, 3'rotating in opposite directions from the opening of the tundish. The twin rolls 3, 3'are continuously poured into the pool 4
Rapidly solidifies on the circumferential surface to form thin solidified shells 5, 5 ', which are twin rolls 3, 3'as the rolls rotate.
The continuous steel strip 6 is manufactured by being pressure-bonded and rolled in the narrow gap portion. The crimping load at this time is shown as a load applied to the load cells 8 and 8 ′ attached to the roll bearings (roll chock) 7 and 7 ′. This crimping load increases as solidification proceeds at a low roll speed, and decreases when the roll speed increases.

Austenitic stainless molten steel is continuously poured into the twin roll type continuous casting machine, and the solidified shells of the steel formed on the circumferential surfaces of the rolls are pressure-bonded in the narrow gap portion of the twin rolls to cast. At that time, when the pressure load exceeds 40 kgf per 1 mm of plate width, surface defects such as vertical cracks and lateral cracks frequently occur. Therefore, the crimping load should be 40 kgf or less per 1 mm of plate width. At this time, the thickness of the cast thin steel strip is 0.2 mm or more when the thickness is thin because the sheet shape is "wakame", and when it is thick, it is 5.0 mm or less because breakouts occur frequently. It is good to say

In the present invention, before or after being wound as a coil in-line on the cast thin steel strip cast in this way, it is directly rolled at a temperature and a rolling reduction satisfying the above formula, and then annealed-cold rolled-annealed. Give. As a result, it is possible to produce an austenitic stainless steel strip having excellent surface properties and excellent ductility, which prevents surface roughening.

 This is shown below as a representative test example.

Austenitic stainless molten steel having the chemical composition shown in Table 1 was used in the twin roll type continuous casting machine described above for crimping load 7.5k.
After casting at gf / mm, winding the cast thin steel strip as a coil, directly rolling at various temperatures and reductions, then annealing-cold rolling (rolling rate 60%)-annealing .

FIG. 2 shows the presence or absence of surface roughness at that time, arranged by the rolling reduction and temperature of the direct rolling. As is clear from Fig. 2, the continuously annealed cast thin steel strip is directly annealed by directly rolling it at a temperature and reduction ratio that satisfy the condition of r> 8.51 × 10 ^-3 T + 4.79. -It can be seen that the occurrence of surface roughening after cold rolling can be prevented.

As for ductility, as shown in the examples below, the introduction of processing strain by direct rolling makes it possible to make the grain size uniform by eliminating the disappearance of δ-ferrite during subsequent annealing and to eliminate the segregation of components due to δ-ferrite. As a result, the ductility is improved and the ductility is almost the same as that of the cold-rolled steel strip made from the slab manufactured by the conventional continuous casting method.

[Example 1] Molten austenitic stainless steels (steel Nos. 1 to 4 of the present invention) whose chemical components are shown in Table 2 were continuously poured into the twin roll type continuous casting machine described in this section, The solidified shells of the steel formed on the circumferences of the rolls are crimped in the narrow gap portion of the twin rolls under the crimping load shown in Table 2, and a thin steel strip having a thickness of 0.6 to 2.3 mm Was continuously cast and directly rolled at 1200 ° C at various reduction ratios before being wound in-line as a coil. Then, a surface quality investigation and a tensile test were carried out on the material which was annealed at 1150 ° C for 1 minute, annealed at a rolling rate of 60%, and subjected to finish annealing at 11050 ° C for 1 minute.

In addition, as a comparative example, the same cast thin steel strip as described above was not directly rolled, was subjected to a solution treatment at 1150 ° C for 1 minute of soaking, and was cold-rolled at a rolling rate of 60%, A surface quality inspection and a tensile test were carried out on the product that had been subjected to finish annealing for 1 minute soaking at 1050 ° C.

Furthermore, a slab with a thickness of 200 mm was manufactured from the austenitic stainless steel melt having the chemical composition described in Table 2 as the conventional example by the conventional continuous casting method.
After hot rolling at 0 ° C to form a hot rolled steel strip with a thickness of 2.0 mm, 115
A hot-rolled sheet was annealed at 0 ° C for 1 minute, cold-rolled at a rolling rate of 60%, and finish-annealed at 1050 ° C for 1 minute to perform a tensile test.

Table 3 shows the direct rolling reduction and the presence or absence of surface roughness after finish annealing. It can be seen that when the direct rolling temperature is 1200 ° C., the surface roughness does not occur if the reduction ratio exceeds the lower limit of 15% of r (reduction ratio) determined by the above formula.

Further, FIG. 3 shows the average elongations of the examples of the present invention, comparative examples and conventional examples. From the results shown in Fig. 3, direct rolling promotes the disappearance of δ-ferrite to make the grain size uniform and eliminates the segregation of the components, which is several% higher than the case without direct rolling (comparative example marked with ○). It can be seen that the elongation is improved and that it is almost the same as the conventional example.

[Example 2] Austenitic stainless molten steel (steel Nos. 5 to 8 of the present invention example) shown in Table 2 above was continuously poured into a twin roll type continuous casting machine in the same manner as in Example 1, and the rolls were rolled. The solidified shells of the steel formed on the respective circumferences of the above are crimped by the crimping load shown in Table 2 in the narrow gap portion of the twin rolls, and the thickness is 1.9 to 2.1.
mm thin steel strip was continuously cast. Before they are wound in-line as a coil, they are directly rolled at 500 ° C with various reduction ratios, annealed at 1150 ° C for 1 minute, and cold rolled at a rolling ratio of 60%. A surface quality inspection and a print tension test were carried out on the product that had been subjected to finish annealing for 1 minute soaking at 1050 ° C.

As a comparative example, the cast thin steel strips of Nos. 5 to 8 were subjected to solid solution treatment at 1150 ° C. for 1 minute without direct rolling,
After cold rolling at a rolling ratio of 0%, finish annealing was performed at 1050 ° C for 1 minute, and surface quality investigation and printing test were carried out.

Furthermore, a slab with a thickness of 200 mm was manufactured from conventional austenitic stainless steel molten steel having the chemical composition shown in Table 2 as a conventional example by the conventional continuous casting method, and this was hot-rolled at an extraction temperature of 1200 ° C After making a 2.0 mm hot rolled steel strip, hot-rolled sheet is annealed at 1150 ° C for 1 minute, cold-rolled at a rolling rate of 60%, and finish-annealed at 1050 ° C for 1 minute. A tensile test was carried out on the material subjected to the test.

Table 4 shows the direct rolling reduction and the presence or absence of surface roughness after finish annealing. As can be seen from the results in Table 4, when the direct rolling temperature is 500 ° C., the surface roughness does not occur if the rolling reduction exceeds the lower limit of 9% of r determined by the above formula.

Further, FIG. 4 shows the average elongations of the present invention example, the comparative example and the conventional example. As is clear from the results shown in Fig. 4, direct rolling promotes the disappearance of δ-ferrite to make the grain size uniform and eliminates the segregation of the components. It can be seen from the comparative example) that the elongation is improved by several percentage points and that the elongation is almost the same as the conventional example.

[Example 3] Austenitic stainless molten steel (Steel Nos. 9 to 12 of the present invention example) shown in Table 2 was continuously poured into a twin roll type continuous casting machine in the same manner as in Example 1, The solidified shells of the steel formed on each circumference of the rolls are crimped by the crimping load shown in Table 2 in the narrow gap part of the twin rolls, and the thickness is 2.0 to 4.0.
mm thin steel strips were continuously cast, and these were directly rolled at 25 ° C with various reduction ratios, then annealed at 1150 ° C for 1 minute and cold rolled at 60% rolling ratio. The surface quality and tensile test of the rolled material, which was subjected to finish annealing for 1 minute soaking at 1050 ° C, were carried out.

For comparison, the cast thin steel strips of Steel Nos. 9 to 12 described above were not directly rolled, were subjected to solution treatment at 1150 ° C for 1 minute of soaking, and were cold rolled at a rolling rate of 60%. After that, surface quality inspection and tensile test of the product subjected to finish annealing for 1 minute soaking at 1050 ° C were conducted.

In addition, a slab with a thickness of 200 mm was manufactured from the austenitic stainless steel melt having the chemical composition shown in Table 2 as a conventional example by the conventional continuous casting method.
After hot rolling at 0 ℃ to make a hot rolled steel strip with a thickness of 2.0 mm, 1
A hot-rolled sheet was annealed at 150 ° C for 1 minute, cold-rolled at a rolling rate of 60%, and finish-annealed at 1050 ° C for 1 minute to perform a tensile test.

Table 5 shows the reduction ratio of direct rolling and the presence or absence of surface roughness after finish annealing. As can be seen from Table 5, when the direct rolling temperature is 25 ° C., the surface roughness does not occur if the rolling reduction exceeds the lower limit of 5% of r determined by the above formula.

Further, FIG. 5 shows the average elongations of the examples of the present invention, comparative examples and conventional examples. As is clear from the results in Fig. 5, direct rolling promotes the disappearance of δ-ferrite, which leads to uniform grain size and elimination of component segregation. It can be seen from the comparative example) that the elongation is improved by several percentage points and that the elongation is almost the same as the conventional example.

Example 4 Molten austenitic stainless steel shown in Table 2 (Steel Nos. 1 to 4 of the present invention) was continuously poured into a twin roll type continuous casting machine, and the same procedure as in Example 1 was performed. The solidified shells of the steel formed on each circumference are crimped to the narrow gap of the twin rolls by the crimping load shown in Table 2, and the thickness is 0.6 to 2.3mm.
The thin steel strip of was continuously cast. After coiling these into coils, they were reheated to 1200 ° C and directly rolled at various reduction ratios, then annealed at 1150 ° C for 1 minute to 60%.
A surface quality investigation and a tensile test were carried out for the material which was cold-rolled at a rolling rate of 10 ℃ and subjected to finish annealing at 1050 ° C for 1 minute.

As a comparative example, the cast thin steel strip (steel No. 1 to 4)
Was not subjected to direct rolling, was subjected to a solution treatment at 1150 ° C for 1 minute of soaking, and was cold-rolled at a rolling rate of 60%, then at 1050 ° C.
Then, a surface quality investigation and a print tension test of the product subjected to finish annealing for 1 minute soaking were carried out.

Furthermore, a slab with a thickness of 200 mm was manufactured from the austenitic stainless steel melt having the chemical composition shown in Table 2 as a conventional example by the conventional continuous casting method.
After hot rolling at 0 ℃ to make a hot rolled steel strip with a thickness of 2.0 mm, 1
A hot-rolled sheet was annealed at 150 ° C for 1 minute, cold-rolled at a rolling rate of 60%, and finish-annealed at 1050 ° C for 1 minute to perform a tensile test.

Table 6 shows the direct rolling reduction and the presence or absence of surface roughness after finish annealing. As seen in Table 6, r (reduction ratio) determined by the above formula when the direct rolling temperature is 1200 ° C.
If the rolling reduction exceeds the lower limit of 15%, surface roughness does not occur.

FIG. 6 shows the average elongations of the examples of the present invention, comparative examples and conventional examples. As is clear from the results in Fig. 6, direct rolling promotes the disappearance of δ-ferrite to make the grain size uniform and eliminates the segregation of the components. It can be seen from the example) that the elongation is improved by several percentage points and that the elongation is almost the same as the conventional example.

[Example 5] The molten austenitic stainless steel shown in Table 2 (Steel Nos. 5 to 8 of the present invention) was continuously poured into a twin-roll type continuous casting machine in the same manner as in Example 1, and the rolls were rolled. The solidified shells of the steel formed on the circumference of each of them are crimped to the narrow gaps of the twin rolls by the crimping load shown in Table 2, and thin steel strips with a thickness of 1.9 to 2.1 mm are continuously cast. Then, after being wound in-line as a coil, it was reheated to 500 ° C and directly rolled at various reduction ratios, then annealed at 1150 ° C for 1 minute for annealing,
The surface quality and tensile test were performed on the material that was cold-rolled at a rolling ratio of 10% and subjected to finish annealing at 1050 ° C for 1 minute.

In addition, as a comparative example, the cast thin steel strips (No. 5 to 8) were not directly rolled but were subjected to solution treatment at 1150 ° C for 1 minute of soaking and then cooled at a rolling rate of 60%. After hot rolling, 10
A surface quality inspection and a tensile test were performed on the product that had been subjected to finish annealing at 50 ° C. for 1 minute.

Furthermore, a slab with a thickness of 200 mm was manufactured from conventional austenitic stainless steel molten steel having the chemical composition shown in Table 2 as a conventional example by the conventional continuous casting method, and this was hot-rolled at an extraction temperature of 1200 ° C After making a 2.0 mm hot rolled steel strip, hot-rolled sheet is annealed at 1150 ° C for 1 minute, cold-rolled at a rolling rate of 60%, and finish annealed at 1050 ° C for 1 minute. A tensile test was performed on the applied material.

Table 7 shows the direct rolling reduction and the presence or absence of surface roughness after finish annealing. As can be seen from Table 7, the lower limit of r determined by the above formula when the direct rolling temperature is 500 ° C is 9%.
If the rolling reduction exceeds, the surface roughness does not occur.

FIG. 7 shows the average elongations of the examples of the present invention, comparative examples and conventional examples. As is clear from the results shown in Fig. 7, direct rolling facilitates uniform grain size and elimination of component segregation by promoting the disappearance of δ-ferrite. When direct rolling is not performed (compared with circles) It can be seen from the example) that the elongation is improved by several percentage points and that the elongation is almost the same as the conventional example.

[Effects of the Invention] As described above, according to the method of the present invention, the cast thin steel strip is directly rolled, and thereafter, the cast thin steel strip is directly rolled by annealing, cold rolling and annealing. Without cold rolling
Surface roughness that occurs when cold rolling / annealing is performed after annealing or solution treatment can be eliminated, and ductility equivalent to that of the cold-rolled steel strip obtained by the conventional continuous casting method can be eliminated. A cold-rolled steel strip of austenitic stainless steel having the same can be manufactured. Therefore, the cost can be reduced by omitting the hot rolling step of the conventional manufacturing method, and the austenitic stainless steel having good surface properties and excellent ductility can be provided at a low cost.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view showing a main part of a twin roll machine to which the method of the present invention is applied, and FIG. 2 is a view showing influences of direct rolling temperature and a rolling reduction on occurrence of surface roughness after cold rolling, Figures 3, 4, and 5 show the average elongations of cold-rolled steel strips when they were directly rolled in-line at 1200, 500, and 25 ° C with a reduction ratio satisfying the formula before being wound as a coil. The figures comparing with those of the comparative example and the conventional example, FIGS. 6 and 7 show the case of rewinding to 1200 and 500 ° C. after being wound as a coil and directly rolling at a rolling reduction satisfying the formula. FIG. 5 is a diagram comparing the average elongation of the cold-rolled steel strip with those of Comparative Example and Conventional Example. 1 …… Austenitic stainless steel melt, 2 …… Tundish, 3,3 ′ …… Internal water-cooled twin rolls, 4 …… Bath pool, 5,5 ′ …… Solid shell, 6 …… Thin steel strip, 7 , 7 '... Roll chock, 8,8' ... Load cell.

Claims (3)

(57) [Claims] 1. A molten austenitic stainless steel is continuously poured into a twin roll type continuous casting machine, and solidified shells of the steel formed on the circumferential surfaces of the rolls are plated in a narrow gap portion of the twin rolls. The austenitic stainless steel strip with a thickness of 0.2 mm to 5.0 mm was continuously cast by crimping under a crimping load of 40 kgf or less per 1 mm width, and the cast thin steel strip was rolled at a temperature and rolling satisfying the following formula. Method for producing an austenitic stainless steel strip with good surface properties and excellent ductility, which is characterized by performing direct annealing under cold rolling conditions, followed by annealing-cold rolling-annealing, r> 8.51 × 10 ^-3 T + 4.79 ・ ・ ・ (T: direct rolling temperature (℃), r: rolling rate (%)) 2. The production method according to claim 1, wherein the direct rolling is performed before the cast thin steel strip produced by the twin roll type continuous casting machine is wound into a coil. 3. The production method according to claim 1, wherein the direct rolling is performed by heating a cast thin steel strip produced by a twin roll type continuous casting machine to a coil and then heating the strip at a required temperature.