JPH07115129B2 - Continuous casting method for composite steel with excellent internal quality - 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 directly producing a composite steel material by continuous casting and rolling the composite steel material into a multi-layer steel sheet, which prevents the occurrence of internal defects in the continuous casting step. is there.

[0002]

2. Description of the Related Art Already, a direct current magnetic flux in the direction transverse to the thickness of a slab is applied to a continuous casting mold over its entire width, and the static magnetic field band formed by the direct current magnetic flux in the vertical direction of the mold is used as a boundary above and below that. Japanese Patent Application Laid-Open No. 63-108947 discloses a method for continuously casting a composite metal material in which molten metals having different compositions are supplied.

In the method of supplying molten metal having different compositions into the mold as described above, when the pouring amount control balance is disturbed or the like, a region where both molten metals are mixed is formed in the static magnetic field band, and a cast piece is produced. A different layer is formed in the solidified structure near the interface between the inner and outer layers (hereinafter referred to as the transition layer).

[0004] When casting a stainless ordinary steel clad, depending on the casting conditions, a large number of cracks are generated in the transition layer, which significantly deteriorates rolling characteristics, and in some cases, problems such as tearing from the interface during rolling occur.

[0005]

SUMMARY OF THE INVENTION The present invention has been made for the purpose of solving the problems of the prior art as described above and providing a method for producing a composite steel material having excellent internal quality.

[0006]

In order to achieve the above-mentioned object, the present invention provides a method for continuously casting a composite metal material disclosed in Japanese Patent Laid-Open No. 63-108947, wherein stainless steel and ordinary steel are placed in a mold. When manufacturing a composite steel material consisting of, the cooling rate of the interface between the inner and outer layers is set to 10 ° C / min or less in the range of 1200 ° C to 500 ° C, and there is no crack near the interface of the inner and outer layers. It is a continuous casting method for high-quality composite steel.

Further, in the above method, the hydrogen concentration in the ordinary steel is set to 5 ppm or less, or the cooling rate of the interface between the inner and outer layers is set to 10 ° C./min or more at 500 ° C. or less to further prevent cracking in the transition layer. It is possible to suppress it.

[0008]

The operation of the present invention will be described in detail below.

The present inventors have made detailed studies to solve the above-mentioned problems in the prior art, and as a result, have clarified that the cause of cracking in the transition layer is due to the following two points. That is, 1) During cooling, the stress generated due to the difference in heat shrinkage between stainless steel and ordinary steel concentrates in the transition layer.

2) During cooling, diffusion of hydrogen from the inner layer side to the outer layer side occurs, and particularly when austenitic stainless steel is used for the surface layer, hydrogen accumulates at the interface due to the difference in hydrogen diffusion rate and the transition layer is formed. Embrittle.

The stress concentration can be evaluated by the difference in linear expansion coefficient. That is, while carbon steel has 8.8 to 14.4 × 10 ^-6 / ° C, 18Cr8Ni which is a typical stainless steel.
For steel, it is 16.4 × 10 ^-6 / ° C (Steel Manual, 3rd edition;
When using stainless steel for the surface layer, the amount of heat shrinkage is larger than that of the carbon steel in the inner layer, and compressive stress acts on the interface.

Further, it was clarified from the analysis of the solidification structure photograph that the transition layer is a martensite transformation during cooling because the components of the surface layer and the inner layer are mixed. Therefore, stress concentrates on the transition layer during cooling, and if it is brittle like a martensite structure, cracking is likely to occur.

On the basis of the above findings, a rapid concentration of stress can be prevented by reducing the cooling rate after solidification and cracking can be reduced. The condition is to cool at a cooling rate of 10 ° C./min or less. It was confirmed.

Regarding the accumulation of hydrogen at the interface, it has been clarified that it occurs by the following mechanism. It is known that the solubility of hydrogen in α iron is smaller than that in γ iron.

Therefore, when the inner layer is α iron and the surface layer is γ iron,
Hydrogen diffuses from the inner layer to the outer layer in the solid phase.

Also, Kobe Steel Technical Report, Vol.32, No.3, p61
As shown in (1), the diffusion rate of hydrogen in γ-iron is several orders of magnitude lower than that in α-iron, so that hydrogen accumulates at the γ / α interface, and the interface becomes brittle. This phenomenon remarkably occurs, for example, when austenitic stainless steel is used for the surface layer and carbon steel is used for the inner layer.

As a result of the accumulation of hydrogen at the interface, the interface becomes brittle and cracks easily occur due to stress concentration during cooling. Therefore, as a method of preventing cracking, it is important to prevent the accumulation of hydrogen at the interface, and it is effective to lower the initial hydrogen concentration of the molten carbon steel in the inner layer at the refining stage. The limit component value is 5ppm considering the degree of integration.

Further, when the inner layer is carbon steel, since the α phase is transformed to the γ phase at the A _c3 transformation temperature, the difference in the diffusion rate of hydrogen becomes large and the hydrogen accumulation becomes remarkable. Therefore, by increasing the cooling rate below the A _c3 transformation temperature, the diffusion of hydrogen can be hindered and the amount of hydrogen accumulated can be reduced.

Considering the above-mentioned stress concentration due to shrinkage,
Cooling rate below 500 ° C, which is below the A _c3 transformation temperature
If it is 10 ° C./minute or more, the effect of preventing hydrogen accumulation is increased and cracking can be reduced.

[0020]

【Example】

[0021]

Example 1 A stainless clad steel was cast by using a molten steel having the composition shown in Table 1 by the continuous casting method according to the above-mentioned JP-A-63-108947. The slab is 250 mm thick and 1200 mm wide.

The cooling rate of the slab was changed by changing the secondary cooling spray water amount. The surface temperature of the slab was measured with a radiation thermometer, and the cooling rate of the interface was obtained based on heat transfer calculation.

The vertical cross section of the obtained cast piece in the casting direction was observed, and the number of cracks having a length of 1 mm or more at the interface was measured.
FIG. 1 is a diagram showing the relationship between the cooling rate at the interface and the number of cracks per unit area at that time.

From these results, the condition of the present invention is 1200 ° C.
By setting the cooling rate from 10 to 500 ° C to 10 ° C / min or less, the number of cracks at the interface could be suppressed to a level without problems.

[0025]

Example 2 Six kinds of stainless clad steels were cast by using the molten steel having the composition shown in Table 2 by the continuous casting method according to the above-mentioned JP-A-63-108947. The slab has a thickness of 250 mm × width
It is 1200 mm.

The vertical cross section of the obtained cast piece in the casting direction was observed, and the number of cracks having a length of 1 mm or more at the interface was measured.
On the other hand, the surface temperature of the slab was measured with a radiation thermometer, and the cooling rate of the interface was obtained based on heat transfer calculation. When the temperature reached 500 ° C, the amount of secondary cooling spray water was adjusted to change the cooling rate.

The cooling rate from 1200 ° C. to 500 ° C. is 10 ° C./min or less for all the steel materials. The steel materials A and B were subjected to degassing treatment so that the hydrogen concentration of the molten carbon steel in the inner layer was 5 ppm or less.

For steel materials B to D, the amount of secondary cooling spray water was adjusted so that the cooling rate at 500 ° C. or lower was 10 ° C./minute or more.

As can be seen from Table 2, the number of cracks at the interface of steel materials A to D, which are materials of the present invention, is at a level without any problem, whereas the steel materials E and F, which are comparative materials, have many cracks. did.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[0033]

As is apparent from the above examples, according to the present invention, it is possible to obtain a composite steel material having no cracks at the interface between the surface layer and the inner layer.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the cooling rate and the number of cracks at the interface between the inner and outer layers from 1200 ° C. to 500 ° C.

Claims (3)

[Claims] 1. A continuous flow casting mold is provided with a DC magnetic flux in a direction transverse to the thickness of a slab over its entire width, and a composition is formed above and below a static magnetic field band formed by the DC magnetic flux in the vertical direction of the mold. In the continuous casting method of composite steel materials that supply different molten steels, the cooling rate at the interface between the inner and outer layers is 1200 ℃ to 500 ℃ when manufacturing the composite steel material consisting of stainless steel and ordinary steel.
The continuous casting method of the composite steel material with excellent internal quality, which is characterized in that the temperature is up to 10 ° C / min. 2. The continuous casting method for a composite steel material having excellent internal quality according to claim 1, wherein the hydrogen concentration in the ordinary steel is 5 ppm or less. 3. The cooling rate at the interface between the inner and outer layers is 10 at 500 ° C. or less.
The continuous casting method for a composite steel material having excellent internal quality according to claim 1, wherein the temperature is not less than ° C / min.