JPH0825001A - Production of thin cast slab having good scale peelability - Google Patents

Abstract

PURPOSE:To provide the production method of thin cast slab which is easy to remove scale before hot rolling in order to produce the hot rolled product having no surface flaw in producing hot rolled product from thin cast slab. CONSTITUTION:In a belt continuous casting method, by using the belt coating material having 20% or more content of one or >=2 kinds among metallic Al, Ti, Zr, La, Ce. Al are concentrated in the surface layer of cast slab so as to increase a melting point of the low melting point fayalite oxide, which is a cause for hard scale peelabilty of a Si containing steel and a thin cast slab having good scale peelabilty is obtained, accordingly a hot rolled product having no surface flaw is produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for producing a hot rolled product from a thin slab having a thickness of 100 mm or less obtained by continuous casting from molten steel, and particularly to Si and further Cu, Cr, M.
The present invention relates to a method for producing a hot-rolled steel sheet having no scale flaw from a slab containing an element that inhibits scale releasability such as o and Ni.

[0002]

2. Description of the Related Art In recent years, a continuous casting method for directly producing thin slabs having a thickness of several tens of mm from molten steel has attracted attention. When a hot rolled product is manufactured from the thin cast piece obtained by this method, the heating step and the rough rolling step can be omitted or simplified, so that simplification of equipment and energy saving effect can be expected.

In order to produce a hot-rolled product having no scale flaws on the surface from such a thin slab, it is necessary to sufficiently remove the scale produced during casting or reheating before finish rolling. Because the thickness is thin, the normal thickness is 10
It is required to have a good scale releasability of the slab, because there is no method for reducing the scale easily such as descaling of a 0 mm slab to facilitate scale peeling. The thin slab produced by the belt-type continuous casting method is produced without the oscillation mark such as the slab produced by the fixed mold type continuous casting machine because the mold and the slab move synchronously and are cast. Since the scale does not bite into the slab, the slab containing no Si or containing a small amount of Si can be relatively easily removed by a descaling method in which high-pressure water is sprayed.

However, in the Si-containing steel, Si causes selective oxidation at the interface between the scale and the base steel or at the grain boundary to form low melting point oxide firelite (Fe ₂ SiO ₄ ), which firmly adheres to the base steel. It becomes difficult to remove the scale because the scale that is difficult to peel off is formed. Cu, N
When elements such as i, Mo, and Cr are contained, these alloy elements are enriched as metal particles in the firelite, and the scale and base iron are complicated, and removal becomes more difficult.

As a technique for improving the peelability of the primary scale of such Si-containing steel, Japanese Patent Publication No. 57-52926 is available.
Publication No. JP-A No. 1993-96, the surface of the cast slab is coated with metallic Al
Has been disclosed, in which Al ₂ O ₃ coating is formed in the reheating process to make it difficult to oxidize, and firelite and Al ₂ O ₃ are reacted to raise the melting point. Further, Japanese Patent Publication No. 56-42649 discloses a method of heat-treating the surface layer of a cast slab to make the structure fine and uniform, prevent the local progress of oxidation, and suppress the formation of subscale in which Si is concentrated. Is disclosed. However, these techniques cause an increase in extra steps and increase costs, and cannot be an effective means for the scale produced during casting, which is a problem when rolling without reheating after casting. .

[0006]

As described above, no means has been found for producing a hot-rolled steel sheet having no scale flaw from a slab containing Si at low cost. Based on these circumstances, the present invention provides a slab with good scale releasability at a low cost when producing a hot-rolled product from a thin slab containing Si, and obtains a hot-rolled product without surface defects. Is an issue.

[0007]

The present invention provides the following means in order to solve the above-mentioned problems. In the twin-belt continuous casting method, metallic Al, Ti, Z
A method for producing a thin slab having good scale releasability, which comprises casting using a belt coated with a coating agent containing 20% by weight or more of one or more of r, Ce and La.

[0008]

In order to clarify the cause of poor scale releasability of Si-containing steel, the inventors of the present invention heated a steel sheet containing 0.5% by weight of Si to 1200 ° C., held it for 2 minutes, and then sprayed it with high-pressure water. The scale was removed, and the cross section of the part where the scale was not peeled off was examined. As a result, it was found that the interface between the base metal and the scale was intricately complicated as shown in FIG. 1, and low-melting-point oxide firelite was generated in that portion as is conventionally said. Since it has a low melting point, it is selectively generated at the crystal grain boundaries. Therefore, if the melting point is increased, it is possible to suppress the selective generation at the crystal grain boundaries.
It is considered to be effective in improving the scale releasability of the i-containing steel. Therefore, as a result of various studies on a method of increasing the melting point of firelight, Al ₂ O ₃ , TiO, Ca
It has been found that if oxides of elements having a stronger deoxidizing power than Si, such as O and MgO, are reacted with firelite to form oxides with a high melting point, selective generation at grain boundaries does not occur.

As a method of supplying a high melting point oxide such as Al ₂ O ₃ which reacts with firelite to the surface layer of the cast slab,
As mentioned above, a method of depositing metallic Al, Ca, etc. on the surface of the slab by thermal spraying or heat deposition has been proposed, but this method is effective for the scale produced during reheating, but it is produced during casting. Has no effect on scale. As a result of various studies, Al, Ti, Zr, L
Elements a and Ce that are solid-solved in the molten steel should be solid-solved in the surface layer of the slab and concentrated to form Al ₂ O when the surface layer of the slab is oxidized.
Oxides such as ₃ are preferentially generated and react with firelite to form high melting point oxides, and Ca, Mg
It has been found that such elements that are hardly solid-solved in molten steel cannot be utilized by this method.

Next, attention was paid to the belt coating agent as a method of concentrating Al, Ti, etc. on the surface layer of the slab. In the belt type continuous casting method, a refractory material is coated on the surface of a metal belt as a mold for the purpose of belt cooling and gentle cooling for obtaining a uniform solidified shell. If metallic Al or Ti is contained in this belt coating agent, when molten steel comes into contact with the coating agent, Al or Ti
Assuming that the above melted and concentrated in the surface layer of the cast slab, casting was carried out by changing the content of metallic Al in the coating agent, and the peelability of the scale was investigated. As a result, as shown in FIG. 2, it was found that the scale releasability was good when the Al content in the coating agent was 20% by weight or more. Other elements (Ti, Zr, La, C
The same investigation was carried out for e), and if 20 wt% or more was contained in the coating agent as in the case of Al, good scale releasability was obtained. It was also found that when two or more of these elements are contained, the total content should be 20% by weight or more.

The method of applying the coating agent includes a method of applying the coating agent to the belt in advance and a method of applying it, and a method of continuously applying the coating agent to the belt every one rotation of the belt during casting by using a spray gun or the like. The latter method is suitable because the coating agent of the present invention is worn away by contact with the slab.

The present invention has been achieved by applying a new action based on the above findings, and thereby the object of the present invention has been achieved.

[0013]

EXAMPLE A molten steel having the composition shown in Table 1 and a belt coating agent having the composition shown in Table 2 were used to cast at a casting speed of 6 m / min by a belt type continuous casting method as shown in FIG. A thin slab having a piece size of 1200 mm width and a thickness of 50 to 75 mm was cast. The cast slab after casting was subjected to descaling as cast or after reheating, followed by rolling with a hot rolling mill to investigate surface defects.

The numbers 1 to 8 in Table 3 indicate that Al, Ti, etc. are 20
This is an example of the present invention cast using a belt coated with a coating agent containing at least 5% by weight, and a hot coil can be obtained in which the scale can be completely removed and there is no surface flaw and the material is good.
Nos. 9 to 12 are examples in which the content of Al or Ti in the belt coating agent is small, and it can be seen that the scale peelability is poor and the surface defects due to the scale occur in the hot coil.

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

[Table 3]

[0018]

As described above, according to the present invention, a belt coating agent containing 20% by weight or more of metallic Al, Ti or the like is used, so that the surface layer of the slab is made of Al or T.
By concentrating i, etc., and increasing the melting point of the low melting point oxide firelite, which is the cause of difficult scale peelability of Si-containing steel, and providing thin cast pieces with good scale peelability, hot rolling without surface defects The product can be manufactured.

[Brief description of drawings]

FIG. 1 is a schematic view of a cross section of a portion of Si-containing steel having poor scale releasability.

FIG. 2 is a diagram showing a relationship between an Al content in a belt coating agent and scale releasability.

FIG. 3 is a schematic cross-sectional view showing a twin-belt type continuous casting apparatus as an example of a continuous casting apparatus for casting thin cast pieces.

[Explanation of symbols]

 1 Tundish 2 Sliding nozzle 3 Injection nozzle 4 Top pulley 5 Drying device 6 Coating spray device 7 Tension pulley for tension control of metal belt 8 Metal belt 9 Foot roll

Claims (1)

[Claims] 1. In a twin-belt continuous casting method, casting is performed by using a belt coated with a coating agent containing 20% by weight or more of one or more of metallic Al, Ti, Zr, Ce and La. A method for producing a thin slab having good scale releasability, which comprises: