JPH08224603A - Manufacture of austenitic stainless steel thin slab - Google Patents

Abstract

PURPOSE: To manufacture a cold rolled plate free from scale flaw, defective gloss on a surface, and roping from the thin slab of a continuously cast austenitic stainless steel. CONSTITUTION: The surface smoothing rolling of the thin slab S of an austenitic stainless steel which is continuously cast by a pair of cooling drums 1 and 1, is performed by a preceding stage rolling mill 6 at a rolling temperature of 1150 deg.C or above and 1300 deg.C or below and whose draft is 5% or over. Next, recrystallization rolling whose rolling temperature us 900 deg.C or above and less than 1150 deg.C and whose draft us 10% or over, is performed by a post-stage rolling mill 7. After the rolling, annealing, pickling, and cold rolling are performed under a normal condition to manufacture a cold rolled plate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for casting austenitic stainless molten steel into a thin slab and then hot rolling it to produce a thin slab for cold-rolled steel sheet having good surface properties.

[0002]

2. Description of the Related Art FIG. 1 shows an outline of a twin-drum type continuous casting apparatus as an example of an apparatus for continuously casting thin cast pieces. This device includes a pair of cooling drums 1 and 1 each having a built-in cooling mechanism.
The pair of side dams 2 and 2 pressed against both end surfaces of the cooling drums 1 and 1 are the main constituent members. The molten metal M supplied to the molten metal pool 3 formed by the cooling drums 1, 1 and the side dams 2, 2 is solidified on the peripheral surface of the cooling drums 1, 1 rotating in the arrow direction, A gap of 1 results in a thin cast piece S having a plate thickness of 1 to 10 mm. The thin slab S is sent out downward and driven to rotate, and the pinch rolls 4, 4
Is sent to the winder 5 and wound up. A rolling mill or a cooling device may be installed between the pinch rolls 4 and 4 and the winder 5.

A thin cast piece manufactured by a continuous casting apparatus using such a cooling drum is likely to be cracked due to shrinkage stress when solidified on the peripheral surface of the cooling drum. In order to prevent this, it is often the case that a large number of depressions are provided on the peripheral surface of the cooling drum or shot blasting is performed to roughen the surface to an average surface roughness of about 40 to 120 μm. On the surface of a thin cast piece manufactured using such a cooling drum, irregularities having a height of about 30 to 100 μm are formed. Due to these irregularities, there has been a problem that the surface gloss of the cold rolled steel sheet after cold rolling deteriorates, and scale remains in the recesses to cause scale flaws on the surface of the cold rolled steel sheet. Further, the continuously cast thin slab is annealed as necessary as in the case of the conventional hot-rolled sheet, and subsequently pickled and cold-rolled into a cold-rolled steel sheet, which is a cold-rolled sheet produced from the thin slab. There is a problem that roping occurs because the steel sheet has a larger grain size than the cold-rolled steel sheet manufactured from the hot-rolled sheet.

Conventionally, as a method for reducing the grain size of a hot rolled steel sheet, in the hot rolling of an austenitic stainless steel slab, the hot rolling temperature is about 1000 to 1.
A method for obtaining a recrystallized structure by setting the rolling temperature to 200 ° C. and the rolling reduction to about 15 to 55% is disclosed in, for example, JP-A-58-341.
No. 39 publication. However, when rolling under such conditions, the occurrence of roping could be solved, but the problems of poor surface gloss and scale defects could not be solved.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to produce a cold rolled steel sheet from a continuously cast austenitic stainless steel thin slab, which is free from surface gloss defects, scale defects and roping.

[0006]

According to the present invention, austenitic stainless molten steel is cast into a thin slab, and then the thin slab is cast in a temperature range of 1150 ° C. to 1300 ° C. and a rolling reduction of 5% or more. The surface smoothing rolling is performed. In addition, after the surface smoothing rolling,
In the temperature range of ℃ or more and less than 1150 ℃, and the rolling reduction is 10
% Or more recrystallization rolling is performed. After the surface smoothing rolling or the recrystallization rolling, annealing, pickling, cold rolling and the like are performed under the conditions for producing a normal cold rolled steel sheet to produce a cold rolled steel sheet. The rolling reduction in the present invention is defined by the following equation (1).

[0007]

In the present invention, the reason for limiting the rolling temperature and the rolling reduction in the hot rolling of the austenitic stainless steel thin slab will be described. An example of a device for carrying out the present invention is shown in FIG. In the figure, a thin slab S cast by a pair of cooling drums 1, 1 is fed by pinch rolls 4, 4 in the direction of the arrow to a pre-rolling mill 6 for smoothing the surface of the thin slab and the crystal structure of the slab. After being hot-rolled by the post-rolling mill 7 for recrystallizing, the material is taken up by the winder 5. A looper is provided between the supporting rolls 8, 8 provided at the rear of the former rolling mill 6 by a dancer roll 10 having a weight 9 suspended therein, and a looper is provided between the former rolling mill 6 and the latter rolling mill 7. Tension is applied to the slab S.

FIG. 2 shows a SUS304 austenitic stainless steel thin slab having a plate thickness of 4.0 mm, which was cast using the twin-drum type continuous casting apparatus shown in FIG. It shows the relationship between the rolling temperature and reduction in the former rolling mill 6 and the surface roughness (Rmax) of the pickled hot-rolled steel sheet when smoothed and rolled into a hot-rolled steel sheet.

From FIG. 2, the surface roughness (Rma
In order to set x) to 20 μm or less where surface gloss and scale flaws are not a problem in a cold rolled steel sheet, the rolling temperature should be in the temperature range of 1150 ° C. or higher and 1300 ° C. or lower, and the rolling reduction should be 5% or higher. is necessary. Even if the rolling reduction is increased to more than 15%, the improvement of the surface smoothing is extremely small, so that the upper limit of the rolling reduction is preferably 15% or less.

FIG. 3 shows that, similarly to the above, the twin drum type continuous casting apparatus shown in FIG. 4 was used to cast thin SUS304 stainless steel slabs, and following casting, the rolling temperature was 1150 ° C. or higher using the pre-rolling mill 6. Rolling in the post-rolling mill 7 when the surface smoothing rolling with a reduction ratio of 5% is performed in the temperature range of 1300 ° C. or less, and subsequently the re-rolling rolling is performed using the post-rolling mill 7 to obtain a hot rolled steel sheet. The relationship between the temperature and the rolling reduction and the average grain size of the hot rolled steel sheet after the annealing treatment is shown. From FIG. 3, in order to set the average crystal grain size of the hot rolled steel sheet to 50 μm or less at which roping that occurs in the cold rolled steel sheet does not pose a problem, the rolling temperature is set to a temperature range of 900 ° C. or higher and lower than 1150 ° C., and the rolling reduction is Is required to be 10% or more. Even if the rolling reduction exceeds 35%, the improvement of the crystal grain refinement is extremely small. Therefore, the upper limit of the rolling reduction is preferably 35% or less.

According to the present invention, continuously cast austenitic stainless steel thin slabs are cast at a rolling temperature of 1 after casting.
5% reduction in the temperature range of 150 ° C to 1300 ° C
When the above surface smoothing rolling is performed, the surface of the slab is likely to be plastically deformed because of its high temperature. Therefore, the unevenness on the surface of the thin cast piece caused by the unevenness around the cooling drum is easily smoothed. At this time, if the rolling temperature is less than 1150 ° C., it becomes difficult to plastically deform the unevenness, and the required reduction rate rapidly increases, so that the unevenness on the surface of the thin cast piece cannot be sufficiently smoothed. As a result, defective surface gloss occurs in the cold-rolled sheet, and scale remains in the recesses, causing surface defects. Further, when the rolling temperature exceeds 1300 ° C., a phenomenon of seizure between the thin slab and the rolling roll occurs, and streak-like defects occur on both sides.

After the surface smoothing rolling, the rolling temperature is 9
The rolling reduction is 10% in the temperature range from 00 ℃ to less than 1150 ℃.
When the above recrystallization rolling is performed, strain energy is accumulated in the crystal grains inside the thin slab, and the strain energy causes the crystal grains to recrystallize and become fine. At this time, if the rolling temperature is 1150 ° C. or higher or lower than 900 ° C., strain energy is hard to be accumulated in the crystal grains even if the rolling reduction is 10% or higher, and thus recrystallization is hard to occur.

By using both the former rolling mill 6 and the latter rolling mill 7 shown in FIG. 4 at the same time, it is possible to simultaneously smooth the irregularities on the surface of the thin slab and refine the crystal grains. . As a result, it is possible to manufacture a cold-rolled steel sheet in which defective surface gloss, scale defects and roping are prevented.

[0014]

[Example] SUS304 austenitic stainless steel molten steel was cast into a thin slab having a plate thickness of 4.0 mm by using the apparatus shown in FIG. 4, and following casting, the sheet was rolled by using the former rolling mill 6 and the latter rolling mill 7. It was hot rolled to a thickness of 2.4 to 3.4 mm. After hot rolling, annealing, pickling and cold rolling to obtain a plate thickness of 0.7 mm
Manufactured cold-rolled steel sheet. Table 1 shows the hot rolling conditions of the present invention example and the comparative example, and the quality of the hot rolled steel sheet and the cold rolled steel sheet.

[0015]

[Table 1]

No. 1 of the present invention example. Nos. 1 to 8 are examples in which surface smoothing rolling was carried out by the former rolling mill 6 and then recrystallization rolling was carried out by the latter rolling mill 7, and cold rolled steel sheets of quality satisfying surface gloss, scale flaws and roping. was gotten. On the other hand, in Comparative Example No. Nos. 9 to 20 are examples of performing surface smoothing rolling and recrystallization rolling, but No. 9 and 10 have low temperature in surface smoothing rolling, and No. 11 and 12 have low rolling reduction. For,
In both cases, a cold-rolled steel sheet having satisfactory surface gloss and scale flaw was not obtained. Further, in No. 13, the temperature in the surface smoothing rolling was too high, so seizure flaws occurred on the drum and the slab. No. 14 and 15 are low in recrystallization rolling, and Nos. 16 and 17 are low rolling reduction in recrystallization rolling.
In No. 20, since the temperature in recrystallization rolling was too high, a cold-rolled steel sheet with satisfactory quality for roping could not be obtained.

[0017]

According to the present invention, a continuously cast austenitic stainless steel thin slab is subjected to surface smoothing rolling after casting to smooth the ruggedness of the surface of the thin slab to cool it. It is possible to prevent the occurrence of surface gloss defects and surface defects such as scale flaws of the rolled sheet. Further, by performing surface smoothing rolling and then recrystallization rolling to refine the crystal grains, it is possible to prevent roping from occurring in the cold rolled sheet.

[Brief description of drawings]

FIG. 1 is a perspective view showing an outline of a twin-drum type continuous casting device.

FIG. 2 is a diagram showing a relationship between a rolling temperature and a rolling reduction in surface smoothing rolling.

FIG. 3 is a diagram showing a relationship between rolling temperature and rolling reduction in recrystallization rolling.

FIG. 4 is a front view showing an outline of an example of an apparatus for carrying out the present invention.

[Explanation of symbols]

 1 ... Cooling drum 2 ... Side weir 3 ... Hot water pool 4 ... Pinch roll 5 ... Winding machine 6 ... Pre-rolling mill 7 ... Post-rolling mill 8 ... Support roll 9 ... Weight 10 ... Dancer roll M ... Molten metal S ... Thin Slab

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location B22D 11/20 B22D 11/20 C

Claims (1)

[Claims] 1. Austenitic stainless molten steel is cast into a thin slab, and then the thin slab is cast at 1150 ° C. or higher and 130
Surface smoothing rolling is performed in a temperature range of 0 ° C. or less and a reduction rate of 5% or more, and then the thin slab is 900 ° C. or more 11
A method for producing an austenitic stainless steel thin cast product, which comprises performing recrystallization rolling in a temperature range of less than 50 ° C and a rolling reduction of 10% or more.