JPH02247049A - Method for producing thin slabs - Google Patents

Abstract

PURPOSE:To produce a cast strip having fine crystallized grains by running the cast strip cast with cooling drums under the condition in contact with the drum surface, rapidly cooling and rolling with hot-rolling at the specific draft. CONSTITUTION:Molten metal in pouring basin part 2 grows as solidified shell on peripheral surface of the cooling drums 1a, 1b by conducting heat through the cooling drums 1a, 1b to be drawn as the cast strip 3. Then, the cast strip 3 is run under the condition of being pushed to one side of the cooling drum 1a with a pushing roll 4 and rapidly cooled in the temp. range of 1,250 - 1,100 deg.C at >=200 deg.C/sec cooling velocity. In this process, coarsening of crystallized grains is restrained. The cast strip 3 is fed into rolling rolls in the hot-rolling mill 5 as it is comparatively fine crystallizing condition, and the hot rolling is executed at <=40% draft according to cooling velocity to the rolling temp. This cast strip has <=about 50mum grain diameter of gamma grain and in the following process, the product without surface defect of roping and uneven glossiness, etc., can be obtd.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a cooling drum-type continuous casting device and a hot rolling roll that rotate thin-walled slabs having fine crystal grains and excellent workability and surface quality. Relating to a method of manufacturing.

[Conventional technology]

In recent years, by directly manufacturing thin slabs in a shape close to the final shape from molten metal such as molten steel by continuous casting, hot rolling, heat treatment, etc. in subsequent processes are reduced or eliminated, and processes and equipment are simplified. Attempts are being made. Known continuous casting methods for such thin-walled cast slabs include twin-drum, single-drum, twin-belt, single-belt, and belt-drum methods.

Among these, the twin-drum method is seen as a promising means for producing slabs with excellent shape characteristics because the shape of the thin slab is almost determined by the drum gap. In this method, the time from when the molten metal starts to solidify in the pool formed between the pair of cooling drums until the solidification is completed is extremely short.

Therefore, the thin slab immediately after being cast has relatively fine crystal grains. However, the thin slab sent out from the drum gap is still in a high temperature state, and there is a large temperature difference between the surface layer of the slab that contacts the drum and the center of the plate thickness, and there may be cases where it contains unsolidified parts inside. be. After the thin slab leaves the drum gap, the heat held in the high-temperature part inside the slab, including the unsolidified portion, is transmitted to the surface layer and regenerated. Coupled with the fact that the thin slab itself is at a high temperature, crystal grains become coarse throughout the thickness of the slab.

This coarsening of crystal grains occurs when the slab produced in this process, which has a certain working rate and omit hot rolling, is made into a final product through the conventional manufacturing process of pickling, cold rolling, and annealing. This may cause deterioration of workability or generation of surface defects. For example, when thin-walled stainless steel slabs manufactured using the twin-drum method are turned into products using conventional manufacturing processes without hot rolling, roughness and uneven gloss called roving occur, which significantly reduces the product value. .

For thin (60 to 150 μm) 81-steel produced by the twin drum method, in order to obtain fine crystals or amorphous, for example, in Japanese Patent Publication No. 19258/1983, a thin cast slab sent out from a drum gap is The thin slab is rapidly cooled by running it in close contact with one of the cooling drums and removing heat through the cooling drum. Also, JP-A-63-68
In Japanese Patent No. 248, a plurality of water-cooled auxiliary rolls are arranged along the conveyance path of the thin slab to prevent breakout and rapidly cool the thin slab. However, in none of these conventional examples, there is no specific mention of coarsening of crystal grains in thin slabs and measures to prevent it. .

[Problem to be solved by the invention]

However, there is a limit to the ability to refine crystal grains only by rapidly cooling a cast thin slab at a temperature of 1350° C. immediately after solidification. For example, when trying to make the crystal grains have a grain size of 50 μm or less, it is necessary to maintain the cooling rate of the thin slab after it leaves the drum gap at 700° C./min or more.

However, at this stage, no practical means have been developed to rapidly cool such high-temperature objects.

For example, in order to rapidly cool a thin slab at such a cooling rate through the circumferential surface of the cooling drum, the heat extraction capacity of the cooling drum must be increased, and the insulation between the circumferential surface of the cooling drum and the thin slab must be increased. It is necessary to press the thin cast slab against the circumferential surface of the cooling drum so that there are no gaps between layers. However, due to thermal contraction or the like, the thin slab tends to locally separate from the cooling drum circumferential surface, making it impossible to form a heat radiation path through which a large heat flux passes. This results in uneven contact,
Temperature unevenness occurs in the slab, and crystal grains become irregular.

On the other hand, when cooling a thin slab with a refrigerant, if the refrigerant used is cooling water or the like, the vaporized refrigerant stays near the surface of the thin slab and acts as a heat insulating layer. Therefore, the expected cooling effect cannot be obtained.

Further, a non-boiling refrigerant suitable for cooling the above-mentioned high-temperature object exceeding 1000° C. has not been developed to a practical stage so far.

Therefore, the present invention aims to refine the crystal grains by hot rolling a thin slab that is in a high temperature state immediately after casting, and to produce a product using the conventional pickling-cold-rolling-annealing process. The aim is to produce thin-walled cast slabs, which can sometimes yield products without surface defects or with good mechanical properties.

[Means to solve the problem]

In order to achieve the purpose of the thin-walled slab manufacturing method of the present invention, a thin-walled slab cast by a rotating cooling drum immediately after solidification is pressed and run while being in contact with the cooling drum by a roll. The thin slab is cooled to 125°C at a cooling rate of 200°C/sec or more by heat removal through
The thin-walled casting is rapidly cooled to a temperature range of 0 to 1100°C, and the pressing is performed by rolling the thin-walled casting in the temperature range by a hot-rolling roll placed immediately downstream of the roll, at a cooling rate of 10°C to the rolling temperature. It is characterized by hot rolling at a rolling reduction of ~40%.

[Effect]

In order to ensure a surface quality that does not cause roving or gloss unevenness in austenitic stainless steel sheets manufactured by the conventional pickling-cold-rolling-annealing process, it is necessary to Hot rolling of 50% or more on thin slabs, ■ Two-time cold rolling method in which the first cold rolling is performed before the final cold rolling, and then annealing, ■ Rapid cooling during production of thin slabs, etc. is possible. However, (1) and (2) do not take advantage of the original aim of the twin-drum method, which is to omit or reduce post-processes. In addition, in case (2), as mentioned above, it is not possible to rapidly cool the thin slab at a sufficient cooling rate.
There are also difficulties in stable production.

In contrast, in the present invention, after rapidly cooling the thin slab immediately after casting and solidification to a temperature range of 1250 to 1100°C,
This thin slab is hot rolled. This hot rolling makes the crystal grains finer and prevents them from growing too large.

At this time, the higher the temperature of the thin slab, the more remarkable the effect of grain refinement by hot rolling.

Recrystallization that occurs when thin slabs are deformed at high temperatures includes
There are dynamic recrystallization and static recrystallization. However, when deforming at high strain and high speed as in hot rolling, dynamic recrystallization usually occurs, and grain refinement is mainly carried out by static recrystallization. As shown in FIG. 2, the critical reduction ratio that causes this static recrystallization depends on the initial grain size and rolling temperature of the slab, and increases as the initial grain size increases and as the rolling temperature decreases.

Therefore, in the present invention, by rapidly cooling the thin slab coming out of the drum gap of the cooling drum immediately after the completion of casting and solidification, the crystal grains formed during casting are prevented from becoming finer and coarser due to rapid cooling. do. The cooling rate at this time is 2 to ensure an initial particle size of 300 μm or less.
It is preferable to set the temperature to 00°C/sec or more. In addition, as shown in Fig. 3, when the thin slab is in the temperature range of 1250 to 1100°C, hot rolling is performed at a cooling rate that corresponds to the initial grain size before hot working. Crystal grains can be effectively refined with a small rolling reduction rate of less than %.

The thin-walled slabs hot-rolled in this way have a grain size of 50 μm or less, and when rolled in the subsequent process, they do not produce defects such as roving or uneven gloss, and have excellent surface condition and material quality. It becomes a thin plate product.

〔Example〕

Hereinafter, the features of the present invention will be specifically explained using examples with reference to the drawings.

FIG. 1 is a schematic diagram showing a twin-drum continuous casting equipment used in this example. a pair of cooling drums la,
lb are arranged in parallel so as to rotate in opposite directions. The axial side of the space between the circumferential surfaces of the cooling drums la and lb is partitioned off by a side weir (not shown) to form a pool 2.

The molten metal poured into the molten metal sump 2 is transferred to cooling drums la,
It cools and solidifies by removing heat through the cooling drums la and lb, and grows as a solidified shell on the circumferential surfaces of the cooling drums la and lb. The solidified shells formed on the circumferential surfaces of the respective cooling drums la and lb are integrated at a kinging point P and sent out as a thin slab 3. The thin slab 3 travels while being pressed against one of the cooling drums 1a by a roll 4. During this traveling process, the thin slab 3 is rapidly cooled by heat removal through the cooling drum 1a. The contact state of the thin slab 3 with the circumferential surface of the cooling drum 1a can be improved by blowing gas, liquid, etc. from the surface side of the thin slab 3, or by pressing with a roll for auxiliary pressing.

Although it depends on the thickness of the thin slab 3, the cooling drum la.

Immediately after coming out of the thin slab 3, an unsolidified portion may remain in the center in the width direction. Further, the inside of the normal thin-walled slab 3 is higher in temperature than the surface layer. Therefore,
The thin slab 3 sent out from the kinong point P is
Heat transfer may occur from within. However, in this embodiment, since the thin slab 3 is rapidly cooled from the surface side by overcoming this reheating, the temperature of the thin slab 3 does not rise or remain at a high temperature.

For example, it has the composition S U'3304 and the temperature is 1530℃.
molten stainless steel is poured into the sump 2, and the drum diameter is 12.
When a thin slab 3 with a wall thickness of 2 mm is cast using 00 mm cooling drums la and lb, the temperature of the thin slab 3 immediately after leaving the knosing point P is approximately 1350 °C, and the pressing is performed on the exit side of the roll 4. The temperature at that time was 1200°C. Therefore, coarsening of the crystal grains was suppressed during the pressing process leading to the rolls 4, and the thin slab 3 was sent to the hot rolling mill 5 while remaining in a relatively fine crystalline state.

The hot rolling mill 5 presses the thin slab 3 along the conveyance path using a diameter 5 whose central axis is located approximately 1 m from the roll 4.
A 00 mm reduction roll was used. Using this hot rolling mill 5, the thin slab 3 was hot rolled at a rolling reduction ratio of 10 to 20%. In addition, depending on the width and reduction ratio of the thin slab 3,
It is also possible to use a hot rolling mill in which a reduction roll is supported by a backup roll.

In addition, when the cooling rate of the thin slab 3 from the kissing point P to the roll 4 is set to 500°C/sec or more, the rolling reduction rate by the hot rolling mill 5 is 10 to 20%, and the grain size is γ of 50 μm or less. Grains were obtained.

Further, when the cooling rate was lower than 500°C/sec, it was necessary to increase the rolling reduction rate to 20 to 30%. In any case, unlike conventional hot rolling, it was not necessary to hot-roll the slab at a large reduction ratio exceeding 50%.

The thin slab 3 rolled by the hot rolling mill 5 is then passed through pinch rolls 6 and processed in a normal hot rolling process. Note that, before winding, a suitable refrigerant was ejected from the nozzle 7 to lower the temperature of the thin plate to, for example, 550° C. or lower, and then the thin plate was wound on the winder 8.

When the thin slab 3 with crystal grains refined in the hot rolling mill 5 is rolled to the target thickness in the subsequent process, it has excellent surface properties without generating surface defects such as roving or uneven gloss. A product with motsu was obtained. Furthermore, since it had a fine crystal structure, it had excellent mechanical properties such as strength and toughness.

By the way, in the above explanation, the stainless steel thin slab 3
We take the case of manufacturing as an example. However, the present invention is not limited to this, and can be similarly applied to other types of steel in which crystal grains are a problem. Also,
The present invention is not limited to twin-drum continuous casting, and may be applied to the other types mentioned above.

〔Effect of the invention〕

As explained above, in the present invention, a thin slab cast in a rotating cooling drum immediately after solidification is moved to a roll while being in contact with the circumferential surface of the cooling drum; It is hot-rolled immediately after rolling. Therefore, pressing is carried out during the traveling process to the rolls, and the thin-walled slab is heat removed through a cooling drum to suppress coarsening of crystal grains due to recuperation, and the thin slab is sent to the hot rolling mill as fine crystal grains. . Since the thin slab is hot rolled at a high temperature, static recrystallization can be caused by a slight reduction blade, making it possible to further reduce the crystal grains of the thin slab. In this way, the thin slab with fine grains exhibits excellent workability when rolled in a subsequent process, and does not generate surface defects such as roving or uneven gloss after rolling.

[Brief explanation of drawings]

Fig. 1 shows an outline of the continuous casting equipment adopted in the examples of the present invention, Fig. 2 is a graph explaining that hot rolling immediately after casting is effective in refining grains, and Fig. 3 The figure is a graph specifically showing grain refinement due to hot rolling. 1a, lb: Cooling drum 3: Thin slab 5: Hot rolling mill 7: Nozzle P/kissing point

Claims (1)

[Claims] 1. A thin slab cast in a rotating cooling drum immediately after solidification is run in contact with the cooling drum by a pressing roll, and the thin slab is heated at a temperature of 200°C/second or more by removing heat through the cooling drum. 1250 ~ at a cooling rate of
The thin slab is rapidly cooled to a temperature range of 1100°C, and the thin slab is cooled in the temperature range by a hot rolling roll disposed immediately downstream of the pressing roll, depending on the cooling rate to the rolling temperature.
A method for producing a thin slab, characterized by hot rolling at a rolling reduction of 40%.