JP2898199B2 - Manufacturing method of continuous cast slab - Google Patents

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 continuous cast slab having a different composition of a surface layer and an inner layer directly from molten steel.

[0002]

2. Description of the Related Art JP-A-63-108947 discloses that
A DC magnetic field is applied across the thickness of the slab at a certain distance below the molten metal meniscus in the continuous casting mold in the casting direction, and different types of melting are applied to the pool above and below the DC magnetic field zone. A method and an apparatus for continuously casting a multilayer slab in which a surface layer and an inner layer are formed from metals of different types by performing solidification and drawing while supplying a metal are described. As a result, it became possible to obtain a multi-layered slab in which the metal of the upper pool was separated into the surface layer and the metal of the lower pool was separated into the inner layer and solidified.

[0003]

However, this method of producing a multilayer cast slab is complicated because it is necessary to separately prepare two types of molten steel in a steel smelting stage, that is, a refining stage. If it is carried out during integrated steelmaking, production may be hindered. In addition, the same publication discloses that, when molten steel of one type is injected into a mold, the composition of molten steel in one or both pools is adjusted with a wire of an alloy element and adjusted to different components from each other. Although it is described that it is possible to produce a layered slab, the problem remains that the addition of wire itself complicates the operation.

[0004]

According to the present invention, a continuous casting powder containing a predetermined solute element is used, and the thickness of a slab is set at a position below a molten steel meniscus in a continuous casting mold by a predetermined distance in a casting direction. Multi-layer casting is performed by injecting molten steel of a constant composition while applying a DC magnetic field band of almost uniform strength across the slab width direction across the slab and forming upper and lower pools divided by the DC magnetic field band. Regarding the continuous casting method of pieces,
Add the continuous casting powder to the upper pool , and
A continuous cast slab characterized by producing a multilayer cast slab in which the surface layer concentration of the solute element is higher than that of the inner layer by continuously casting while immersing the lower end of the molten steel injection nozzle into the solute element and mixing the solute element. It is a manufacturing method.

[0005]

According to the present invention, a mold flux is used as a mold lubricant and a surface protective agent, which is constantly used in a continuous casting operation. A DC magnetic field band applied across the thickness of the slab at a position below the meniscus by a certain distance in the casting direction while the powder melts on the inner meniscus and the solute element diffuses and mixes from the meniscus into the molten steel in the upper pool. Thus, continuous casting is performed while suppressing mixing of the solute element mixed in the upper pool into the lower pool, thereby easily producing a slab having a predetermined amount of the solute element added to the surface layer.

A molten steel having a different composition is provided at each position of an upper pool and a lower pool in a continuous cast strand pool which is divided by a magnetic field zone in which a DC magnetic field extends across the slab in the thickness direction across the slab. , The molten steel stays in the portion where the DC magnetic field having a certain magnetic flux density or more acts, and in the other upper and lower pools, a region where each molten steel component is uniform is formed by the flow of the molten steel. If continuous casting is performed while maintaining this state, a multilayer slab in which the surface layer and the inner layer are formed from the compositions of the respective molten steels can be manufactured. That is, as shown in FIG. 3A, when a DC magnetic field band 9 having a magnetic flux density equal to or more than a certain value is applied uniformly over the width of the slab, the upper pool 3 and the lower pool 4 in the region where the DC magnetic field band 9 stays. Although the components of the molten steel injected into the respective pools are maintained, the two types of molten steel diffuse into each other in the stagnation region 5 interposed therebetween.
Mixing forms a concentration gradient, and the component concentration distribution in the casting direction is as shown in FIG. When continuous casting is performed while maintaining such a structure, the cross section of the cast slab has a structure in which a boundary layer 8 having a concentration gradient exists between the surface layer 6 and the inner layer 7 as shown in FIG. Becomes In general, since the thickness of the boundary layer 8 is sufficiently smaller than the thickness of the surface layer 6, a multilayer slab in which the surface layer 6 and the inner layer 7 are apparently separated can be cast.

According to the present invention, as shown in FIG. 1, when one type of molten steel melted in a refining stage is poured into a mold 1, a solute element such as C is cast by a continuous casting powder 10 into an upper pool 3.
, A multi-layer slab having different compositions of the surface layer and the inner layer can be easily produced without complicating the continuous casting operation. That is, after melting and adjusting the primary components using a converter or an electric furnace or the like, they are injected into the mold 1 from the tundish through the immersion nozzle 2 through a secondary refining process or a vacuum degassing process as required. In order to add a specific concentration of a specific solute element to the molten steel in the upper pool 3 out of the molten steel, a certain amount of the solute element mixed in a continuous cast powder is used. FIG. 2 shows how the solute element is mixed into the molten steel from the meniscus. The continuous casting powder 10 containing the solute element is melted on the mold meniscus, and the molten powder 11 comes into contact with the molten steel at the meniscus and the solute element is removed. The solute element concentration in the molten steel of the upper pool 3 is diffused and mixed into the molten steel to be adjusted to a predetermined value. As shown in FIG. 1, since the upper pool 3 is separated from the lower pool 4 by a DC magnetic field band 9, it hardly affects the solute element concentration in the lower pool 4.

[0008] The multilayer slab thus produced has a higher surface layer concentration with respect to the specific solute than the inner layer, but its production method is as simple as a general continuous casting method. Giving a fluid to the meniscus by the electromagnetic stirring device in the mold is also effective for uniforming the concentration distribution of the solute element and stabilizing the dissolution mixing.

[0009]

EXAMPLE Using a mold having a width of 1.2 m and a thickness of 0.25 m, the casting speed was 1.6 while applying a DC magnetic field having a uniform magnetic flux density in the width direction 0.4 m below the meniscus.
A multilayer slab was cast at m / min. As the molten steel, the ultra-low carbon steel shown in Table 1 was produced by a converter and vacuum degassing equipment, the molten steel was injected from a ladle into a tundish, and further supplied from the tundish into a mold using an immersion nozzle. As the continuous casting powder, a continuous casting powder for forming a surface layer component shown in Table 2 was added into the mold. This continuous casting powder contains a larger amount of C alone than that of a normal composition. The casting operation was performed in exactly the same way as the conventional continuous casting operation. The solidified shell thickness d in this continuous casting machine
It is known that the growth rate of (m) is given by Equation 1, which indicates that the thickness of the surface layer is 10 mm.

[0010]

[Table 1]

[0011]

[Table 2]

[0012]

D = 0.020 × (L / Vc) ^1/2 where Vc is the casting speed (m / min) L is the distance from the meniscus (m)

Examination of the cast slabs indicated that the surface layer contained the components of Table 2 and the inner layer contained the components of Table 1. It is well known that slab surface defects frequently occur in a slab of a single composition comprising the components shown in Table 1. The slab whose surface layer composition is adjusted as shown in Table 2 can drastically reduce surface defects while maintaining the material obtained with the composition of Table 1.
The direct feed ratio from continuous casting to rolling has improved dramatically.

[0014]

According to the present invention, it is possible to easily produce a multilayer cast slab containing a required amount of a predetermined solute element in the surface layer.

[Brief description of the drawings]

FIG. 1 is a view showing a cross section of molten steel in a mold during continuous casting.

FIG. 2 is a view showing a state of contact between molten steel in a mold and continuous casting powder during continuous casting.

FIG. 3 is a view showing molten steel in a mold during continuous casting;
(A) is a diagram showing a cross section of a continuous casting pool, (b) is a diagram showing a solute concentration distribution in a slab length direction, and (c) is a diagram showing a component distribution of a slab cross section.

[Description of Signs] 1 mold 2 immersion nozzle 3 upper pool 4 lower pool 5 dwelling area 6 surface layer 7 inner layer 8 boundary layer 9 DC magnetic field zone 10 continuous casting powder 11 molten powder 12 solute element C solute concentration C _A surface layer solute concentration C _B Inner layer solute concentration Z Casting direction Z ₁ Upper limit of retention area Z ₂ Lower limit of retention area d Slab thickness direction d ₁ Interface position of surface layer / boundary layer d ₂ Interface position of boundary layer / inner layer d ₃ Interface position of inner layer / boundary layer d ₄ Boundary layer / surface interface position d ₅ Slab back

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-3-243245 (JP, A) JP-A-56-68567 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B22D 11/00 B22D 11/10

Claims (1)

(57) [Claims] 1. A with use of a continuous casting powder which contains the predetermined solute elements, sea urchin slab width direction by crossing the thickness of slab at a position a certain distance downward from the molten steel meniscus in the casting direction in continuous casting mold A continuous casting method of a multilayer slab that is performed by injecting molten steel of a constant composition while applying a DC magnetic field band having substantially uniform strength over the entire area and forming upper and lower pools divided by the DC magnetic field band. The continuous casting powder is added to the pool, and the lower end of the nozzle for injecting molten steel is immersed in the stagnation region , and continuous casting is performed while mixing the solute element, whereby the surface layer concentration of the solute element is higher than that of the inner layer. A method for producing a continuous cast slab, comprising producing a slab.