JPH08257704A - Method for continuously casting high strength steel containing copper - Google Patents

Abstract

PURPOSE: To enable the continuous casting of a high strength steel in a small lot in the case of need, by adding a specific content of Cu into molten steel at the upstream side of brake magnetic field and simultaneously adding powder containing Ni to contain the specific 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 electomagnet 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 contained into the solidified shell 6. The Ni prevents the surface flaw developed in a hot-rolling, and at the time of adding >=1% Cu into the molten steel 4, Ni is made to be contained of >=0.01% at the point of 10mm just below the surface layer of the cast slab and the copper-containing high strength steel can continuously be cast in a good yield even in the small lot by executing the casting containing no Cu in the other strands.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is to continuously cast a plurality of slabs of the same component in parallel by a continuous casting machine comprising a plurality of strands, and at the same time, to provide high strength containing a different component of copper in one strand. A method of casting 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 this case, industrial production requires refining in a large capacity, for example, 300 ton converter,
Even if the amount originally required is at most about 10 tons, 300
Tons must be tapped. 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, the production 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, Takashi Ito: No. 72.7
There is a wire addition method presented in the 3rd Nishiyama Memorial Technology Course, p143-172, Japan Iron and Steel Institute. The purpose here is Sol. The Al content is controlled, but if this is expandedly interpreted, it is possible to use it for the production of a large variety of small lot materials by adding alloy components that change other materials into the molten steel.

However, in this method, it takes time from immediately after the addition of the alloy component until the additive component in the cast reaches a desired value, and the cast in that period (transition part: component transition part) is the additive component. The concentration (component ratio) of is indeterminate and non-uniform. That is, it means that the transition portion is long and its components are not uniform, so that it cannot be used, and the yield is poor.

On the other hand, it is known that a steel sheet containing copper shows various peculiar characteristics. For example, ultra low carbon steel with Ti
When approximately 1.5% of copper is added to IF steel from which solid solution carbon and solid solution nitrogen are substantially eliminated by adding Nb and Nb and precipitation treatment is performed after cold rolling annealing, copper is carbonitride. The steel sheet has precipitation strength of 590 N / mm ² class which is industrially unattainable by solid solution strengthening without forming a steel sheet and has excellent formability. Demand for this steel sheet is strong, but it is used only in a small amount because the number of parts used is extremely small. Therefore, it is a representative of small lot materials, and from a steel manufacturer's perspective, it needs to be stored as a slab for a long time after being tapped in a large-capacity converter, and because copper-containing steel is scrapped. Since there are many problems in steelmaking, it was a steel grade that manufacturing is disliked in iron and steel manufacturers to the extent that the possibility of manufacturing is questioned.

[0007]

SUMMARY OF THE INVENTION The first object of the present invention is to increase the degree of freedom of casting of copper-containing high-strength steel, which is a typical example of small lot materials, and to improve the yield of casting. The third purpose is to reduce the cost required for casting and managing the copper-containing high-strength steel.

[0008]

According to the present invention, while pouring molten steel of the same kind into each strand of a continuous casting machine composed of a plurality of strands, one strand is cast at the bottom of the mold or below the mold. A braking magnetic field in a direction intersecting with the slab withdrawing direction is applied to the piece, Cu is added to the molten steel above the braking magnetic field in a weight ratio of 0.1% or more, and at the same time, the powder contains Ni. Directly below the surface layer of cast slab 10
At the point mm, Ni is 0.01% or more by weight, and in the other strands, the molten steel is cast by a usual method.

[0009]

In one strand, when the supply of the alloy component Cu is started and ended, the above-mentioned transition portion (component transition portion) can be formed into a slab.

Here, if there is a braking magnetic field while supplying the alloying component Cu, the molten steel flow injected from the nozzle at the braking magnetic field and trying to flow downward (in the slab drawing direction) will be returned upward. Electromagnetic force is applied, that is, a force that blocks the flow is applied, which causes the molten steel flow to bend toward the inner surface of the mold at the position where the braking magnetic field is reached,
Then, it tries to rise along the inner surface of the mold. That is,
At the boundary of the braking magnetic field, the molten steel on the upper side (immersion nozzle side) is suppressed from flowing downward, and as a result, tries to form a circulation flow on the upper side (hereinafter, this area is referred to as a blocking pool). . 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).
In the shut-off pool, the stirring of the supplied alloy components is realized, and as a result, the diffusion of the alloy components subsequently supplied to the preceding molten steel that does not supply the alloy components is small, and the alloy components After starting the supply, the molten steel in the mold quickly converges to the desired composition ratio of the alloy component, and therefore the length of the transition portion (component transition portion) in the slab drawing direction is significantly reduced. That is, the casting yield of precipitation-strengthened high-strength steel in a large variety of small lots with one strand is significantly improved. In the other strand, continuous casting of only the molten steel that enters the mold is performed, which is a conventional large-scale continuous casting of the same type of ingot, and the casting yield is as high as the conventional one.

On the other hand, in the mold in which the alloy component Cu is added as described above, the powder enters the interface between the solidified shell and the mold inner surface from above the meniscus of the molten steel in the mold along the mold inner surface, and from there. The Ni in the powder penetrates so as to be contained therein as the solidified shell forms. 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 is
This is to prevent the generation of surface defects, which are so-called Cu, generated by hot rolling of the slab. In the case of the present invention using a method using powder, the amount necessary for that purpose is 0.01% or more at a point 10 mm immediately below the surface layer. 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 shows a one-strand mold 2 embodying the invention in one aspect,
It is a longitudinal cross-sectional view cut by a plane perpendicular to the wide surface. An electromagnet 7 for an electromagnetic brake is installed at the bottom of the mold 2.
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 may be arranged below the mold 2. However,
The lowest point f at which the flow of molten steel injected into the mold 2 substantially extends
Above the point.

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. 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.

Although the method of adding the alloy component Cu is not particularly limited, 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).

[0017]

EXAMPLES Steel A having the composition (composition ratio: weight%) shown in Table 1
300 tons were obtained by ordinary refining, and in a continuous casting machine having two strands, first, 100 tons of steel A was first cast using a normal powder, and Cu was reduced to 1.2% from the middle. So that the copper wire 3 is fed into the molten steel so that copper-added steel (N
o. 1 + No. 2 + No. 3) was cast into 50 tons. In the other strand, Steel A was cast in the usual way, i.e. without addition of copper, 150 tons. In this casting method, copper-added steel (No. 1 + No. 2 + N
o. For 3), no braking magnetic field 8 and N to powder
No addition of i (No. 1), application of braking magnetic field 8 but no addition of Ni to powder (No. 2), and application of braking magnetic field 8 and addition of Ni to powder (No. 3) The above three conditions were carried out during casting. That is, casting was performed as shown in Table 2. When Ni was included (No. 3), the amount of Ni in 10 mm below the surface layer was 0.3% by weight. The shape of the slab in the width direction of the mold is 230 mm in thickness and 12 in width.
It was 50 mm. The electromagnet 7 for the electromagnetic brake was installed at a position of 500 to 700 mm from the meniscus in the mold.

[0018]

[Table 1]

[0019]

[Table 2]

No. in which the braking magnetic field 8 was not applied No. 1 in which the braking magnetic field 8 was applied. Regarding No. 2, after starting the feeding of the copper wire 3 for the addition of Cu, the length of the slab until the Cu content in the slab becomes 0.2%, and after the Cu became 1.2% Table 3 shows the variation in Cu.

[0021]

[Table 3]

As can be seen from Table 3, according to the present invention, the length of the slab until the amount of Cu in the slab reaches a predetermined value is greatly shortened, and the variation of the Cu component ratio thereafter is also small.

Next, N containing no Ni in the surface layer
o. 2 and No. 2 containing it. Extraction temperature: 1200 ° C, finish rolling temperature: 900 ° C, winding temperature:
Hot rolling was performed at 700 ° C. The hot rolled steel strip was visually inspected for surface flaws. The results of the inspection are shown in Table 4.

[0024]

[Table 4]

As shown in Table 4, steel No. containing Ni in the surface layer was used. No. 3 was good with no surface flaws, but Ni was used as the surface layer.
Steel No. In No. 2, the baldness due to Cu occurred.

[0026]

As described above, according to the method of the present invention, it is possible to produce a copper-containing high-strength steel, which has a strong demand but is used only in an extremely small amount, with high yield, very easily, and only when necessary. Therefore, it is possible to manufacture a large-capacity converter that does not have a large amount of stock and does not make it scrap. Therefore, the cost of the copper-containing high strength steel can be reduced.

[Brief description of drawings]

FIG. 1 is a longitudinal cross-sectional view of a long piece of a mold for casting copper-added high-strength steel in a multi-strand continuous casting machine that embodies the present invention in one aspect.

[Explanation of symbols]

 2: Mold 3: Copper wire 4: Molten steel in the mold 6: Solidified shell 7: Electromagnet for electromagnetic braking 8: Braking magnetic field 10: Slab drawing direction 11: Immersion nozzle 12: Powder

Claims (1)

[Claims] 1. While pouring molten steel of the same kind into each strand of a continuous casting machine consisting of a plurality of strands, in one strand, the cast strip withdrawal direction is set to the bottom of the mold or the cast below the mold. A braking magnetic field in an intersecting direction is applied, and Cu is added to the molten steel above the braking magnetic field in a weight ratio of 0.1% or more, and at the same time, Ni is added to the powder at a point 10 mm immediately below the surface layer of the cast slab. A continuous casting method for high-strength steel containing copper, characterized in that the weight ratio of Ni is 0.01% or more and the molten steel is cast in the other strands by a usual method.