JPH07204707A - Manufacture of steel sheet from thin cast billet - Google Patents

Abstract

PURPOSE:To manufacture a steel sheet having excellent mechanical property by making crystal grain of a continuously cast thin billet to be fine polygonal ferritic grains. CONSTITUTION:Molten steel M of carbon steel containing >=0.1wt.% C is injected in a space formed with a couple of water cooled casting rolls 1 which are horizontally installed and driven to be rotated mutually in the reverse directions and a couple of side dams 2 which are abutted on both end parts of roll 1. A solidified shell G is generated when the melted steel M is brought into contact with the circumference of rolls 1 to be water cooled. The shell G is synchronously moved with the rolls 1 and integrated on the nearest contacting point with rolls 1 to be a thin billet S having thickness of <=10mm. The thin billet is hot rolled in a range of temperature Ar3 point to Ar1 point with draft of >=20%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel sheet from a thin cast slab, and more particularly to a method for producing a steel sheet having mechanical properties required for a conventional hot rolled steel sheet.

[0002]

2. Description of the Related Art In recent years, in order to reduce the manufacturing cost and save energy, a technique has been developed for omitting or simplifying the hot rolling step by continuously casting a thin cast piece having a thickness close to the product thickness. There is. However, in the production from a thin slab with a thickness close to the product thickness, a large rolling process like the hot rolling process that is performed in the conventional process cannot be added, so the structure of the steel plate is less than that of the steel plate manufactured in the conventional process. As a result, the crystal grains become coarse and sufficient mechanical properties cannot be obtained.

Further, the crystal grains of the slab cast into a thin slab and rapidly cooled and solidified do not become fine polygonal ferrite grains like the crystal grains of the steel sheet after hot rolling, and are coarse. It becomes acicular ferrite grains. When such a thin cast piece is cold rolled and annealed as it is, the ferrite grains have a mixed grain structure, and the mechanical properties such as ductility are inferior to those of the steel sheet produced by the conventional method.

In order to solve this, it is necessary to change the metallographic structure of the slab before cold rolling to fine polygonal ferrite. As a method thereof, for example, JP-A-3-03
An in-line reverse transformation reheating treatment method according to Japanese Patent No. 8941 is known. This method, reheating treatment from a temperature region lower than the temperature at which the transformation from austenite to ferrite ends, by cooling after reverse transformation to austenite, as in the case of performing hot rolling of the slab The crystal grains are made into fine ferrite. However, according to this method, it is necessary to newly provide a heat treatment apparatus, and since this heat treatment apparatus is installed in-line, a long or powerful one is required, and a large amount of energy is required for heating. And For this reason,
The method using the reheating treatment has a problem of increased cost.

[0005]

SUMMARY OF THE INVENTION According to the present invention, coarse crystal grains produced by rapid solidification are finely divided into fine polygonal ferrite grains while achieving a reduction in equipment cost and energy cost by eliminating the need for reheating a thin cast piece. Another object of the present invention is to provide a method for producing a steel sheet using a thin cast piece having excellent mechanical properties.

[0006]

In order to achieve the above object, a method for manufacturing a steel sheet by a thin cast piece of the present invention is a carbon steel having a weight ratio of C ≧ 0.01% and a mold wall surface is synchronized with the cast piece. and by casting to a thickness of 10mm or less of the thin cast strip by continuous casting machine to be moved, below followed by Ar ₃ point to the casting of the thin cast strip, a
It is characterized in that hot rolling with a rolling reduction of 20% or more is performed in a temperature range of ₁ point or more.

[0007]

[Action] Figure 1 shows a temperature history of hot rolling of the thin cast strip, the cast thin slab Ar ₃ point or less, Ar ₁
In the temperature range above the point, the reduction rate of plate thickness, that is, the reduction rate is 20
Hot rolling is performed under the rolling conditions of not less than%. By this hot rolling, strain energy is accumulated in the austenite crystal grains, dislocations are generated in the crystal grains, and sub-crystal grains are generated. Transformation occurs from within the austenite grains with these sub-crystal grains as nuclei, and the coarse crystal grains during casting become fine polygonal ferrite grains. In this case, the mechanism of transformation by accumulating strain energy in the crystal grains is completely different from that by the above-mentioned reverse transformation heat treatment, so that the crystal grains can be made fine without performing reheating treatment. .

Next, the reasons for limitation in the present invention will be described. C contained in carbon steel is the most important element that determines the structural morphology in the transformation from austenite to ferrite, and its lower limit is 0.01% in order to stably cause the transformation treatment in the present invention. The above is necessary. The upper limit is 0.1 to prevent deterioration of weldability.
5% is preferable. The plate thickness of the thin cast piece was set to 10 mm or less because the structure becomes coarse when the thickness exceeds 10 mm and the crystal grains are not sufficiently refined by the transformation treatment.

In the hot rolling of the thin slab, the larger the reduction rate, the larger the number of sub-crystal grains formed, and the higher the growth rate of the sub-crystal grains, so that the transformation easily occurs. When the rolling reduction is less than 20%, the strain energy in the austenite crystal grains is not sufficiently accumulated, so that the subcrystal grains are not sufficiently generated. Therefore, in order to sufficiently generate the sub-crystal grains, it is necessary to set the reduction rate of hot rolling to 20% or more. If the rolling reduction exceeds 80%, the improvement in the effect is small and the cost increases, so the rolling reduction is 8%.
It is preferably 0% or less.

When the temperature of the thin slab at the time of starting hot rolling next exceeds the Ar ₃ point, the strain energy is not sufficiently accumulated in the crystal grains, so that the sub-crystal grains are not sufficiently generated. . Further, when the temperature of the thin cast piece at the start of rolling falls below the Ar ₁ point, ferrite transformation from the austenite grain boundary occurs preferentially, so that sub-crystal grains are not sufficiently generated in the ferrite grain. Therefore, in order to sufficiently generate the sub-crystal grains in the hot rolling of the thin cast slab, it is necessary to start and end the hot rolling in the temperature range of Ar ₃ point to Ar ₁ point.

The method for manufacturing the thin strip-shaped cast piece according to the present invention will be described below with reference to FIG. FIG. 2 is a diagram showing an example of a continuous casting machine in which the mold wall surface moves in synchronization with the slab.
The apparatus shown in FIG. 2 includes a pair of water-cooled casting rolls 1, 1 for casting a thin cast slab S, and a pair of side dams 2, 2 for preventing leakage of molten steel M injected between the water-cooled casting rolls 1, 1. (The opposite side is not shown), a rolling mill 3 for hot rolling the cast thin-walled slab S, and a winder 4 for winding the hot-rolled thin-walled slab S into a coil. .

The present invention will be described in detail. The pair of water-cooled casting rolls 1 and 1 are horizontally installed, and are rotationally driven in the arrow direction by a driving device (not shown). The water-cooled casting rolls 1, 1 have a peripheral surface formed of, for example, copper or a copper alloy, and have a water-cooling mechanism built therein. A pair of side weirs 2 and 2 made of a refractory material for sealing the sides are pressed against both ends of the water-cooled casting rolls 1 and 1, and the pair of water-cooled casting rolls 1 and 1 and the side weirs 2 and 2 are used. Molten steel M from the tundish 5 to the space formed
Is injected. Molten steel M is a rotating water-cooled casting roll 1,1
When it is contacted with the peripheral surface of the and cooled, it forms solidified shells G, G. The solidified shells G and G thus produced are water-cooled casting rolls 1 and 1.
And the water-cooled casting rolls 1 and 1 are integrated at the closest contact point to form a thin cast piece S. The thin cast slab S is gripped by the pinch rolls 6 and 6, and by the time it reaches the rolling mill 3, the temperature thereof is cooled to a region below the Ar ₃ point and supplied to the rolling mill 3.

The temperature of the thin-walled slab S when leaving the water-cooled casting roll 1 depends on the casting speed, that is, the rotation speed of the water-cooled casting roll 1, the contact length between the molten steel M and the water-cooled casting roll 1, the heat flux to the water-cooled casting roll. Generally, the thinner the thickness of the thin cast piece S, the higher the temperature, although it depends on the above. However, the thinner the thickness of the thin slab S is, the faster the temperature decreases after exiting the water-cooled casting roll 1. Therefore, the temperature of the thin slab S when reaching the rolling mill 3 varies depending on the thickness. Instead, it becomes almost constant.
Here, if it is desired to shorten the machine length regardless of the fact that the apparatus may be slightly complicated, a cooling facility such as water cooling is installed in front of the rolling mill 3 to force the temperature of the thin-walled cast slab S to ₃ Ar or less. You may cool to.

The thin cast slab S supplied to the rolling mill 3 has Ar
Hot rolling is performed at a rolling reduction of 20% or more in a temperature range of ₃ points to ₁ point of Ar. By this hot rolling, strain energy is accumulated in the austenite crystal grains, and thereby sub-crystal grains are generated in the crystal grains. By this hot rolling, austenite transforms into polygonal ferrite with subgrains in the grains as nuclei.

[0015]

EXAMPLES Table 1 shows the chemical composition of the test steels produced, Table 2 shows the hot rolling conditions of the slabs, and Table 3 shows the crystal structure and mechanical properties of the produced steel sheets.

In the examples of the present invention and the comparative examples shown in Tables 2 and 3, the cast pieces cast by using the thin cast continuous casting machine shown in FIG. 2 are successively hot rolled and wound into a coil. This is an example of producing a cold-rolled steel sheet having a plate thickness of 0.7 mm by performing cold rolling and annealing after forming a strip-shaped cast piece having a plate thickness of 4.0 mm. Further, in the conventional example, a slab having a thickness of 130 mm cast by using a continuous casting machine for slabs is hot-rolled into a hot-rolled plate having a thickness of 4.0 mm, and cold rolling and annealing are performed in the same manner as described above. This is an example of manufacturing a cold-rolled steel sheet having a plate thickness of 0.7 mm.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

[Table 3]

No. manufactured by the method of the present invention In 1-4,
Fine polygonal ferrite was formed, showing an excellent strength-ductility balance. Comparative example No. 5 and No.
In No. 6, since the starting temperature of hot rolling exceeds the Ar ₃ point (880 ° C.), No. 7 and No. In _No. 8, since the end temperature of hot rolling is below the Ar ₁ point (727 ° C.),
Coarse acicular ferrite is generated from both grain boundaries,
The yield strength, total elongation, and fatigue strength were low. No. No. 9 has a low yield point and low fatigue strength due to the formation of martensite because the start and end temperatures of hot rolling deviate to a higher degree. In No. 10, since the start and end temperatures of hot rolling were deviated relatively low, martensite was partly generated and the total elongation was low. In addition, No. 11 and N
o. In No. 12, since the rolling reduction of the hot rolling was slightly off, coarse acicular ferrite was generated, and the yield point and fatigue strength were low.

[0021]

As described above, according to the present invention, it is possible to obtain a steel sheet of fine polygonal ferrite grains without the need for reheating a thin cast slab, thus reducing equipment costs and energy costs. While achieving the above, it becomes possible to manufacture a steel sheet having mechanical properties comparable to those of a steel sheet manufactured by a conventional hot rolling method.

[Brief description of drawings]

FIG. 1 is a diagram showing a heat history in hot rolling of a thin cast piece according to the present invention.

FIG. 2 is a diagram showing an example of an apparatus for carrying out the present invention.

[Explanation of symbols]

 1 Water-cooled casting roll 2 Side weir 3 Rolling machine 4 Winder 5 Tundish 6 Pinch roll M Molten steel G Solidified shell S Thin-walled slab

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hiroyuki Nakajima Inventor Hiroyuki Yamaguchi 3434 Shimada, Hikari-shi, Yamaguchi Shin Nippon Steel Co., Ltd. Hikari Steel Works (72) Inventor Satoshi Akamatsu 20-1 Shintomi, Futtsu-shi, Chiba Made in New Japan Iron & Steel Co., Ltd.

Claims (1)

[Claims] 1. A carbon steel having a weight ratio of C ≧ 0.01% is cast into a thin cast piece having a thickness of 10 mm or less by a continuous casting machine in which a mold wall surface moves in synchronization with the cast piece. The method for producing a steel sheet by a thin cast slab, which is characterized by performing hot rolling with a reduction rate of 20% or more in a temperature range of Ar ₃ points or less and Ar ₁ point or more, following the casting of _{No. 1} .