JPH0366466A - Method for casting steel - Google Patents

Abstract

PURPOSE:To form fine equi-axed crystal and to improve the strength and toughness by solidifying molten steel while stirring in a heated mold. CONSTITUTION:At the time of casting the steel, the molten steel is stirred in the range of 5 - 1000 turns/min number of revolutions at the temp. range from <= +50 deg.C to the liquidus temp. to the solidus temp. and higher, and solidi fied by using the mold heated at <= -20 deg.C and >= -150 deg.C to the solidus temp. By this method, as the fine equi-axed crystal is developed, by using the steel ingot, the steel having excellent strength and toughness can be manufactured with direct rolling.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a method for casting steel.

[Prior Art] It is known that it is effective to make the metal structure fine in order to satisfy the basic properties of steel such as strength and toughness.

In order to reduce energy costs and improve productivity, a direct rolling method has been attempted in recent years, in which steel is rolled as it is without cooling after casting. Such techniques include, for example, JP-A-59-208018 and JP-A-6L-141.
There is a technique disclosed in the i072 publication.

[Problem to be solved by the invention] However, when such casting is normally performed, the solidified structure becomes coarse and difficult to recrystallize during subsequent rolling, and as a result, the finally obtained metal structure is also relatively coarse. The drawback is that it is difficult to obtain a product with good strength and toughness because it becomes a mixed grain state of coarse grains and fine grains due to partial recrystallization.

[Means for Solving the Problems] The present invention eliminates the drawbacks of the conventional methods as described above and provides a method for casting hanging steel, and its gist is as follows:
When casting steel, molten steel is rotated at least 5 revolutions per minute in the temperature range from below liquidus temperature +50℃ to above solidus temperature! 0
Stir at a rotation speed of 0.00 rpm or less, and the solidus temperature is -20
This is a steel casting method characterized by producing a fine solidified structure by solidifying using a mold heated to a solidus temperature of −150° C. or lower.

The present invention will be explained in detail below.

In the solidified structure, the columnar crystal part is particularly coarse and extremely difficult to recrystallize, so when direct rolling is performed, the metal structure after transformation becomes coarse or a mixed structure of fine and coarse structures, which deteriorates the material quality. . However, the present inventors have discovered a new fact that makes it possible to overcome the above-mentioned drawbacks, and based on this discovery, a new steel casting method has been developed.

If fine isometric islands can be generated over almost the entire surface of the slab in the solidified state, the solidified structure will easily re-crystallize during subsequent hot rolling, making it possible to manufacture thick steel plates with excellent material properties. However, in conventional casting methods, the cooling capacity of the mold is high, and a strong solidified shell is formed on the mold wall surface in the early stage of solidification, inhibiting the release of crystals, resulting in the formation of coarse columnar crystals. The present invention heats the mold and lowers the cooling capacity of the mold to prevent the formation of a strong solidified shell on the mold wall, and also promotes the release of crystals from the mold wall by stirring the molten steel and promotes the release of crystals. The free crystals are precipitated and solidified, making it possible to obtain fine isometric islands over almost the entire surface of the slab.

The reasons for limiting the manufacturing method of the present invention will be explained below.

The reason why the upper limit of the stirring temperature is set to below the liquidus temperature + 50°C is because if the molten steel temperature is above the liquidus temperature + 50°C, the generated free crystals will re-dissolve, inhibiting the formation of fine equicyclic islands. It is from. Further, the lower limit temperature is set to be higher than the solidus temperature because stirring is impossible below the solidus temperature.

Regarding the limitation of rotation speed, if it is less than 5 revolutions per minute, the stirring of the molten steel will be too weak and free crystals will not be released from the mold sufficiently, and if it is more than 1000 revolutions, the stirring of the molten steel will be too strong and smooth casting will not be possible. This is because it is not possible. Possible methods for stirring the molten steel include mechanical stirring using a rotor, electromagnetic stirring, etc., but are not particularly limited. When using a rotor, the material of the rotor may be oxides or nitrides of boron or silicon, or metals coated or sprayed with these heat-resistant substances, but there are no particular limitations. . The shape may be any shape as long as it promotes stirring of the molten steel, and it is effective to add projections, depressions, or blades to the surface. Furthermore, it is also effective to cool the rotor in order to use the rotor as a solidification shell generation site.

As for the heating conditions for the mold, if it is heated above the solidus temperature of -20°C, the cooling ability of the mold will become too small, and the number of solidified shells generated on the casting wall will decrease, and a sufficient number of isocycle islands will be released by stirring. As a result, the islands become coarser.

In addition, when heating below the solidus temperature of -150°C, the cooling ability of the mold is too large and a strong solidified shell is generated, making it difficult for crystals to separate from the mold even if there is a flow of molten steel due to stirring. , coarse columnar crystals are formed.

Therefore, solidus temperature -20℃ or lower solidus temperature -150℃
The above range was set.

[Example] Using the steel types shown in Table 1, casting was performed by applying the methods and methods of the present invention shown in Table 2. As a result, the solidified structure has the isocycle size and the isocycle ratio as shown in Table 2, and it is clear that the present invention has increased the number of fine isocycle islands, and thus the present invention is effective.

[Effects of the Invention] The present invention generates fine isometric islands by solidifying molten steel while stirring it in a heated mold, so if this steel ingot is used, steel with excellent strength and toughness can be directly delivered I+: rolled. can be manufactured.

Claims (1)

[Claims] 1. When casting steel, molten steel is stirred at a rotation speed of 5 revolutions per minute or more and 1000 revolutions per minute or less in a temperature range from liquidus temperature + 50 ° C or lower to solidus temperature or higher, and solidus temperature -
A method for casting steel, characterized in that a fine solidified structure is produced by solidifying using a mold heated to a solidus temperature of 20° C. or lower and a solidus temperature of −150° C. or higher.