JPH08257703A - Method for continuous casting of high strength steel - Google Patents

Abstract

PURPOSE: To enable a continuous casting of a high strength steel in a small lot by adding a prescribed content of Cu into molten steel at the upstream side of brake magnetic field and simultaneously, adding powder containing Ni to contain the prescribed value or more of Ni at a prescribed point just below the surface layer of a cast slab. CONSTITUTION: A copper wire 3 fed into the molten steel 4 in the condition of impressing the static magnetic field 8 with electric magnets 7 is mixed in a pool of the molten steel 4 at the upper side of the static magnetic field 8 and the alloy component is uniformized, and the length of a transition part which changes from the initial component to the following component, is drastically shortened. The powder 12 supplied into a meniscus of the molten steel 4 is invaded into the interface between a solidified shell 6 and the surface in a mold 2, and the Ni in the powder 12 is invaded so as to be contained in the solidified shell 6. The Ni exists only in the surface of the cast slab, and since the surface flaw developed at a hot-rolling is prevented, at the time of adding >=1wt.% Cu into the molten steel 4, the copper-containing steel even in the small lot can continuously be cast in a good yield by containing >=0.01wt.% Ni at the point of 10mm just below the surface layer of the steel slab.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for casting a high-strength steel by adding an alloy component to molten steel in a continuous casting mold, and more particularly to a method in which an alloy component is added to the molten steel in the mold at one stage of continuous casting. The present invention relates to a method of casting a high-strength steel having a composition different from that of the cast steel of the molten steel itself from a part of the molten steel injected into the steel.

[0002]

BACKGROUND OF THE INVENTION The use of steel materials has increased over time, and as a result, the types of steel materials have increased.
Therefore, in order to meet many demands of customers, steel materials of different materials are manufactured little by little. This is the production of so-called high-mix low-lot materials. In the production of a wide variety of small lot materials, if the characteristics required in the subsequent manufacturing process such as rolling cannot be created separately, it is necessary to manufacture as a different steel type even if there is a slight difference in alloy composition. is there. In that case, in industrial production, refining must be carried out in a large-capacity converter, for example, 300 tons, so 300 tons of steel must be tapped even if the originally required amount is at most about 10 tons. This is a great waste, and when stocking the remaining 290 tons as a slab until it becomes necessary, the inventory will increase, and there will be a problem that storage space and interest rates will be required. Further, in the case of remelting as scrap, there arises a problem that extra cost is required for scraping. These problems will eventually raise the price of steel products. Therefore, reducing the manufacturing cost of a wide variety of small lot materials has been a longstanding problem in the steel industry.

As a proposal of a casting technique for a wide variety of small lot materials, for example, there is a wire addition method presented to Takashi Ito: 72th and 73rd Nishiyama Memorial Technology Course, p143-p172, Japan Iron and Steel Institute. The purpose here is S of A1 killed steel.
ol. Although the Al content is controlled, it can be utilized for casting of various kinds of small lot materials by adding alloy components that change other materials to molten steel when expanded and interpreted. However, this method results in uncertain and non-uniform alloy components. That is, in the connecting part (transition part: component transition part) of the preceding cast product without adding alloy component and the following cast product with adding alloy component, molten steel with alloy component added is mixed with molten steel without alloy component attached. , The alloy composition is uncertain and non-uniform. The transition portion is long and its components are uncertain and non-uniform, so that it cannot be used, resulting in poor yield.

[0004]

On the other hand, it is known that a steel sheet containing copper exhibits various unique properties. For example, copper is added to IF steel in which solid solution carbon and solid solution nitrogen are substantially eliminated by adding Ti and Nb to ultra low carbon steel.
%, And when precipitation treatment is performed after cold rolling annealing, copper does not form carbonitride and is precipitation strengthened alone, and the tensile strength is 590 N / mm ² which is industrially unattainable with solid solution strength. It is a steel sheet that extends to the grade and has excellent formability.
Although the demand for this steel plate is strong, it is used only in a small amount because the number of parts used is extremely small. Therefore, a copper-added steel plate, that is, a high-strength steel is a representative of small lot materials. However, since it is necessary to store as scrap for a long time after being tapped in a large-capacity converter, and because there are many problems in scrapping copper-containing steel as scrap, there is a question of whether or not it can be manufactured. It was a type of steel that steel manufacturers dislike manufacturing.

An object of the present invention is to overcome the problem of a wide variety of small lot materials which copper-containing steel has. Specifically, the first purpose is to facilitate small amount casting of copper-added high-strength steel, and the second purpose is to cast with high yield.

[0006]

SUMMARY OF THE INVENTION The present invention applies a braking magnetic field in a direction intersecting the casting withdrawing direction to a lower part of a mold or a lower part of the mold, and the molten steel above the braking field is applied. Cu is added in an amount of 0.1% or more by weight, and at the same time, Ni is added to the powder so that the content of Ni is 0.01% or more by weight at a point 10 mm immediately below the surface layer of the cast slab.

[0007]

When a braking magnetic field is applied to the molten steel in a direction intersecting with the slab drawing direction, the molten steel is injected into the mold at the braking magnetic field and flows downward (in the slab drawing direction).
An electromagnetic force that tries to return upward is applied, that is, a force that blocks the downward flow is applied, so that at the position where the braking magnetic field is reached, the molten steel flow bends in the direction toward the inner surface of the mold and then rises along the inner surface of the mold. And That is, with respect to the braking magnetic field, the molten steel on the upper side (immersion nozzle side) is restrained from flowing downward, and as a result, a circulating flow is formed on the upper side (hereinafter, this region is referred to as a blocking pool). And The alloy component is supplied to the shut-off pool, which is efficiently dispersed in the molten steel in the pool by the circulating flow. That is, the braking magnetic field separates the blocking pool from the molten steel preceding the blocking pool (blocks or suppresses the molten steel flow), and stirring of the supplied alloy components is realized in the blocking pool. , Thereby, the diffusion of the alloy component supplied to the preceding molten steel to which the alloy component is not supplied is small, and moreover, after the alloy component is started to be supplied to the desired component ratio of the alloy component of the molten steel in the mold. Convergence is fast, and therefore, the length of the above-mentioned transition portion (component transition portion) in the slab drawing direction is significantly reduced. That is, the yield of the different steel type casting is significantly improved.

On the other hand, the powder enters the interface between the solidified shell and the inner surface of the mold from above the meniscus of the molten steel in the mold, and from this, Ni in the powder is contained therein together with the formation of the solidified shell. So as to infiltrate. Therefore, Ni added from the powder exists only in the surface layer of the cast slab. The reason why Ni is required for the surface layer in this way is to prevent the generation of surface defects, which are so-called Cu, generated by hot-rolling the cast slab, that is, Cu. In the case of the present invention using the powder method, the amount required therefor is 10 below the surface layer.
It is 0.01% or more at the mm point. The upper limit is not particularly specified, but the upper limit may be determined depending on the amount of Cu, and from the viewpoint of economy, it may be approximately the same as the weight ratio of Cu.

Next, a more specific description will be given. FIG. 1 is a vertical cross-sectional view of a mold 2 for carrying out the present invention in one mode, taken along a plane perpendicular to a wide surface. At the bottom of the mold 2 is an electromagnetic blade.
Equipped with an electromagnet 7 for g. The electromagnet 7 for the electromagnetic brake is configured to cross a molten steel 4 in the continuous casting mold 2 with a magnetic field in one direction which produces a magnetic flux having a uniform density in the width direction of the wide surface and orthogonal to the slab drawing direction 10. Then, it can be applied to the molten steel 4 in the mold. Reference numeral 8 is a static magnetic field (static magnetic field) at this time. The electromagnet 7 of the electromagnetic brake is the mold 2
You may arrange | position below. However, it is above the lowest point f where the flow of molten steel injected into the mold 2 substantially extends.

The molten steel flow injected from the tundish into the mold 2 through the nozzle 11 reaches point f when the static magnetic field 8 is not applied. However, when the static magnetic field 8 is formed, the Lorentz force that blocks the molten steel injection flow passing through the static magnetic field 8 acts from the static magnetic field 8, and the penetration depth of the injection flow is approximately c / d (dashed line). Up to the level. With the static magnetic field 8 applied, the copper wire 3 is fed to the molten steel. The copper wire 3 that has entered the molten steel is uniformly mixed in the molten steel pool above the static magnetic field 8 by the injected molten steel flow. Molten steel (a slab) having a solidified shell on the surface of the molten steel pool is extruded downward from this pool at a constant speed. That is, the molten steel flow that is about to flow down is stopped by the electromagnetic brake of the static magnetic field 8 and therefore turns its direction to the inner surface of the mold, forming a small pool for mixing in the mold, and mixing within that range. As a result, the time for the concentration to become uniform is shortened, and the flow of the slab extruded downward from this pool is less likely to form convection due to plug flow. Does not cause new mixing. Therefore, the length of the seam portion (transition portion) where the first component changes to the second component becomes the shortest.

On the other hand, the powder 12 supplied to the molten steel meniscus moves in the direction from the contact points a and b between the molten steel meniscus and the inner surface of the mold, that is, enters the interface between the solidified shell and the inner surface of the mold 2, and from there. The Ni in the powder penetrates so as to be contained therein as the solidified shell forms. Therefore, the Ni added from the powder exists only in the surface layer of the slab. As described above, Ni is required for the surface layer because the surface defects generated after hot rolling the slab, so-called Cu.
This is to prevent the generation of baldness due to. The amount required for that is, in the case of the present invention using the powder method,
It is 0.01% or more at a point 10 mm immediately below the surface layer. Although the upper limit is not particularly specified, the upper limit may be determined depending on the amount of Cu, and from the viewpoint of economy, it may be approximately the same as the weight ratio of Cu. As described above, the alloy component Cu is sufficiently mixed in the pool having a constant volume above the static magnetic field 8 by the electromagnetic braking action of the static magnetic field 8. Therefore, a predetermined time has elapsed since the addition of the alloy was started, and the steady state was achieved. The component concentration in the slab obtained when the slab is in a stable state is stable, and is constant in the slab transverse section and the slab length direction. Therefore, by implementing the present invention, the length of the component change region (transition portion) caused by the addition of the alloy component is significantly reduced, and the concentration of the alloy component becomes uniform at any portion in the cast slab. Manufacturing yield is greatly improved.

In the present invention, the element to be added at this time is 0.1% or more by weight of Cu for the purpose of producing a high strength steel containing copper which is a small lot in the present invention. If the added amount of copper is less than 0.1%, the characteristics as copper-containing steel do not appear.
Although this is conventionally known, it is specified here in order to manufacture copper-containing steel extremely inexpensively. Although the upper limit is not specified, addition of an alloy causes an increase in cost.
% Or less is preferable.

The method of adding the alloy component Cu is not particularly limited, but since Cu is extremely easy to form a wire and the copper wire manufacturing technology is highly developed, the copper wire is fed into molten steel. The method of doing is most preferable. According to this method, the alloy components to be added can be easily dissolved in the molten steel, and can be evenly distributed in the cast slab. When the static magnetic field 8 is applied to start the feeding of the copper wire 3, that is, when the Cu addition to the molten steel in the mold 2 is started, the Cu component ratio of the above-mentioned pool partitioned by the static magnetic field 8 is quickly increased to the target weight. After the copper wire 3 was fed at a high speed to achieve the ratio, and the amount required to reach the target weight ratio was fed, the speed was proportional to the casting speed, that is, the amount of molten steel injected into the mold 2 was commensurate. It is preferable to set the amount to be delivered. This further reduces the length of the component change region (transition part).

[0014]

EXAMPLES Steel A having the composition (composition ratio: weight%) shown in Table 1
300 tons by ordinary refining, and by adding Cu by the copper wire 3 into the molten steel in the continuous casting machine,
First, 100 tons of steel A was first cast using ordinary powder, and 5 parts of steel A-1 were added so that Cu would be 1.6% from the middle.
I cast 0 tons. Next, with the addition of Cu stopped, steel A
50 tons of steel, then a magnetic field was applied to restart the addition of Cu so as to reach 1.6%, and 50 tons of steel A-2 was cast. For the last 50 tons, Ni was contained in the surface layer of the slab. Steel A-3 was cast while adjusting the amount of Ni using a powder containing Ni powder so that the amount supplied to the molten steel would be constant. At this time, the amount of Ni 10 mm below the surface layer was 0.4% by weight. The shape of the slab in the width direction of the mold was 230 mm in thickness and 1250 mm in width. The electromagnet 7 for the electromagnetic brake is 500 to 70 from the meniscus in the mold.
It was installed at a position of 0 mm.

[0015]

[Table 1]

For the steel A-1 to which no magnetic field was applied and the steel A-2 to which a magnetic field was applied, the length of the slab from the addition of Cu until the amount reached 1.6%, and the Cu content was 1 The variation of Cu after reaching 0.6% is as shown in Table 2.

[0017]

[Table 2]

As can be seen from Table 2, the length of the slab until the amount of Cu in the slab reaches a predetermined value can be greatly shortened by the method of the present invention, and the variation thereafter is small. Next, the slabs of Steel A-2 containing no Ni in the surface layer and Steel A-3 containing it were extracted at an extraction temperature of 120.
Hot rolling was performed under the conditions of 0 ° C., finish rolling temperature: 900 ° C., and winding temperature: 700 ° C. The hot rolled steel strip was visually inspected for surface flaws. The results of the inspection are shown in Table 3.

[0019]

[Table 3]

As shown in Table 3, the steel A-3 containing Ni in the surface layer was good with no surface flaws, but the steel A-2 containing no Ni in the surface layer was affected by Cu. There was a baldness.

[0021]

As described above, the use of the method of the present invention makes it possible to manufacture copper-containing steel, which has a strong demand but is used only in an extremely small amount, very easily and only when necessary, and has a large capacity. In a steel mill operating with the converter of No. 1, it is possible to perform casting with a good yield without having a large amount of inventory and without producing it without scrapping it.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view of a long piece of a continuous casting mold that embodies the present invention in one aspect.

[Explanation of symbols]

2: Mold 3: Copper wire 4: Molten steel 6: Solidified shell 7: Electromagnet for electromagnetic brake 8: Static magnetic field (static magnetic field) 9: Immersion nozzle 10: Slab drawing direction 11: Powder on molten steel surface

Claims (1)

[Claims] 1. A braking magnetic field in a direction intersecting with the casting withdrawing direction is applied to a lower part of the mold or a lower part of the mold, and Cu is added to the molten steel above the braking field in a weight ratio of 0. A continuous casting method for high-strength steel in which 1% or more is added, and at the same time Ni is added to the powder so that Ni is 0.01% or more by weight ratio at a point 10 mm immediately below the surface layer of the slab.