JP2003117640A - Method for continuously casting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for continuously casting a steel with which secondary deoxidized product at the solidified time is positively applied and γgrain in 1/4-1/2 parts of the product thickness can further be made fine and thus a slab having sufficient quality can be supplied as a blank of steel material for large heat input welding. SOLUTION: One or more kinds among REM, REM-containing alloy, Zr and Zr-containing alloy is added into molten steel near the lower part of an immersion nozzle in a mold while pouring the molten steel into the mold from the immersion nozzle. In the case of singly adding REM and/or REM-containing alloy, the REM concentration in the cast product is made to 0.001-0.01%, and in the case of singly adding Zr and/or Zr-containing alloy, the Zr concentration in the cast product is made to 0.001-0.01%, and in the case of compositely adding REM and/or REM-containing alloy and Zr and/or Zr-containing alloy, the total of the REM concentration and the Zr concentration in the cast product is made to 0.001-0.01%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for steel, and more particularly to a continuous casting method for steel capable of effectively refining austenite grains (γ grains) of a slab. In the present invention,% as a unit symbol of chemical component content (concentration)
Means% by mass.

[0002]

2. Description of the Related Art In recent years, with the stricter specifications required for product quality, techniques for positively and effectively utilizing fine particle inclusions in continuous casting have been advanced. Particularly in the case of a material having strict specifications regarding low temperature toughness, it is mentioned as one of the effective methods to make the γ grain fine and to make the fracture path complicated in order to improve the toughness of the weld heat affected zone.

As a determinant factor of γ grain refinement, the heat treatment-rolling process after continuous casting has a great influence, but grain coarsening occurs in a portion exposed to a high temperature of 1400 ° C. or higher, such as a welding heat affected zone. In order to prevent the occurrence of γ, it is an extremely important technique to make the γ grains fine in the slab stage of the solidification-cooling process. In this regard, a number of techniques utilizing secondary deoxidation products generated at the solidification interface during solidification have been proposed, and examples thereof include the following.

In Japanese Examined Patent Publication No. 5-17300, dissolved oxygen = 0.0
20%, Al, Si = 0.005% or less in each molten steel, Ti =
0.01 to 0.10% is added, and Ti oxide (T
Disclosed is a method for producing a steel material having excellent toughness in the heat-affected zone of a welded joint in which iO or Ti ₂ O ₃ ) is present in the range of 0.004 to 0.100%. In Japanese Patent Laid-Open No. 61-238940, Ti ≦
Deoxidation by 0.030%, particles of 0.1-3.0 μm 5 ×
Disclosed is a method for producing a steel having excellent toughness in a weld heat affected zone, which is characterized by containing 10 ⁴ to 1 × 10 ⁶ pieces / mm ³ .

In Japanese Patent Laid-Open No. 4-2713, Mn and Si alloys are charged as a first deoxidizing material into molten steel in a ladle discharged from a converter, and then Ti, Zr and Ca alloys are used as a second deoxidizing material. Dissolving oxygen in the molten steel to 50 ppm or less to melt the steel in this order, and at that time, the weight concentration of the components in the steel is set to a specific range, a deoxidizing method in the steel manufacturing process is disclosed. Japanese Patent Fair 5-2770
In the third publication, C, Si, Mn, P, S, Al, Ti, Zr, N,
It is a component system in which the O component is specified, and when Ti + Zr = 0.005 to 0.022% is satisfied, the particle diameter is 0.05 to 10 μm and the number of particles is 3 × 10 ^5.
It is disclosed that it is possible to produce a high-strength steel having excellent weld heat-affected zone toughness by including a composite oxide of Ti and Zr of up to 1 × 10 ¹⁰ pieces / mm ³ .

[0006]

However, in the slab cast by the above-mentioned conventional technique, the heat input amount after hot rolling.
Product thickness 1 when subjected to high heat input welding of 500 kJ / cm or more
It was difficult to stably satisfy the low temperature toughness of the welded part of / 4 to 1/2 part. Therefore, the present invention positively utilizes the secondary deoxidation product at the time of solidification to make the γ grains in the 1/4 to 1/2 part of the product thickness even finer as a material for large heat input welding steel materials. An object of the present invention is to provide a continuous casting method for steel capable of supplying a slab of sufficient quality.

[0007]

Means for Solving the Problems In order to achieve the above-mentioned object, the present inventors have found that fine inclusions such as secondary deoxidation products are stably contained in a slab thickness of 1/4 to 1/2 part. Or, it is important to make the fine precipitates exist stably, and the method for realizing it was eagerly investigated by experiments. First, RE
In order to confirm the effect of M and Zr addition, a melting experiment using an induction heating furnace in an atmosphere of Ar and a subsequent casting experiment in a mold were performed in the laboratory. In the melting experiment, 30 kg of molten steel was Mg
It was dissolved in an O crucible, and the components and dissolved oxygen amount were adjusted.

As an experimental level, a comparison is made between Experiment 1: when REM or Zr metal is added to molten steel after composition adjustment and casting, and Experiment 2: when REM or Zr metal is blown from the upper part of the mold during casting. It was After casting, the obtained steel ingot cross section was macro-etched with picric acid corrosive liquid, and the γ grain size was measured. As a result, the following findings were obtained.

(1) Experiment 2 is different from Experiment 1 in the thickness direction of the slab
The γ grain size in the 1/4 to 1/2 part is small and the γ grain size distribution in the thickness direction of the cast piece is uniform. (2) In Experiment 2, when REM and Zr metals were added so that the content of each in the slab was 0.001 to 0.01%, a remarkable γ grain refining effect was observed. (3) In Experiment 2, similar effects can be obtained by adding REM and Zr alloys (alloys with Si metal, etc.) instead of elemental metals.

(4) In the case of the combined addition of REM and Zr, a remarkable effect of refining γ grains is recognized when added so that the total content in the cast piece is 0.001 to 0.01%. (5) The γ-grain refinement effect is that the amount of dissolved oxygen in molten steel is 10 to
It is particularly remarkable in the case of 100 ppm. Based on the results of this laboratory experiment, for example, when casting was performed using an actual continuous casting machine as shown in FIG. 1, the method of adding REM and Zr was examined, and the following findings were obtained.

(6) If REM and Zr are added to molten steel near the lower part of the dipping nozzle, the same γ grain refinement effect as in the laboratory experiment can be obtained. (7) As a method of adding REM and Zr to the molten steel near the lower part of the immersion nozzle, as shown in FIG.
A method of blowing the carrier gas (inert gas such as Ar) on the carrier gas through an addition hole 13 provided on the side, or a method of inserting a small-diameter wire coated with a thin iron plate from the addition hole 13 Are preferred.

The present invention has been made on the basis of the above findings, and the summary thereof is as follows. [1] In the continuous casting method of steel, while pouring molten steel into the mold from the immersion nozzle, REM, REM containing alloy, Zr, Zr
If one or more of the contained alloys is added to the molten steel near the lower part of the immersion nozzle in the mold, and if REM and / or REM-containing alloy is added alone, the REM concentration of the cast product is 0.001 to 0.01.
%, And when Zr and / or Zr-containing alloy is added alone, the Zr concentration of the cast product is 0.001 to 0.01%, and REM and / or
Alternatively, in the case of adding REM-containing alloy and Zr and / or Zr-containing alloy composite, a method for continuous casting of steel, characterized in that the total of REM concentration and Zr concentration of the cast product is 0.001 to 0.01%.

[2] The continuous casting method for steel according to [1], characterized in that the molten steel near the lower part of the immersion nozzle is a molten steel containing 10 to 100 ppm of dissolved oxygen.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, for example, in a continuous casting method of steel using a continuous casting machine as shown in FIG. 1, while pouring molten steel 2 from dipping nozzle 5 into mold 6, REM , REM-containing alloy, Zr, and one or more of Zr-containing alloys (referred to as required additives) are added to the molten steel near the lower part of the immersion nozzle in the mold.

The required additives are REM, REM-containing alloy, Zr,
One or more Zr-containing alloys are used in molten steel.
REM (O, S) and / or Zr (O, S) are generated, and the fine dispersion of REM (O, S) increases the nucleation sites of γ grains generated during the solidification / cooling process of steel, and does not aim to refine the γ grains. It is because of REM (O, S) and Zr (O, S) have a larger specific gravity than oxysulfides such as Al, Ti, and Si, and have low floatability after formation, and remain in molten steel to form solidification interfaces. It is most suitable for the purposes of the present invention because it is easy to settle.

The point of addition of the required additive is near the lower part of the immersion nozzle. This is represented by a stroke distance along the casting direction starting from the lower end position of the immersion nozzle and is in the range of 0 to 600 mm. By adding in this range, the slab 1/4
Fine REM (O, S) and / or Zr (O, S) are stably fixed on the solidification interface (between dendrite trees) in a thickness equivalent to ~ 1/2 thickness, and a sufficient γ grain refinement effect is obtained. In the method of adding to the molten steel in the ladle, which is generally performed in the past, the required additive reacts with the dissolved oxygen in the molten steel immediately after the addition to form coarse inclusions in the molten steel, and REM (O, S) and /
Or Zr (O, S) is exhausted before it is fixed on the solidification interface. The addition location is preferably in the range of 300 to 600 mm in the vicinity of the lower part of the immersion nozzle.

In order to accurately add the above-mentioned required additives to the molten steel at the above-mentioned addition points, powders of the above-mentioned additives (preferred particle size: 0.5-
2.0 mm), for example, as shown in FIG. 2, through an addition hole 13 provided on the side of the dipping nozzle 5 while being carried on a carrier gas (inert gas such as Ar) flow, or by the above-mentioned powder. It is preferable to insert a wire (suitable diameter: about 5 to 10 mm) coated with a thin iron plate (suitable thickness: about 0.2 to 0.6 mm) from the addition hole 13. Among them, the method of coating the required additives with a wire and inserting
It is more preferable because the required additive can be effectively prevented from being oxidized by the dissolved oxygen before reaching the adding point and the adding yield is improved.

The addition amount of the required additive is the sum of the REM concentration and the Zr concentration of the cast product (however, in the case of adding REM and / or REM-containing alloy alone, Zr concentration = ε (: a minute value less than the analysis error), When Zr and / or Zr-containing alloy is added alone, REM concentration = ε) is set to 0.001 to 0.01%. The sum of REM and Zr concentrations of cast products is 0.00
If the amount added is less than 1%, the amount of REM (O, S) and / or Zr (O, S) produced is insufficient, and the γ-grain refining effect cannot be obtained. If the added amount exceeds 0.01%, not only the solidification interface but also the molten steel will be coarse.
This is because REM (O, S) and / or Zr (O, S) inclusions appear and cluster, leading to deterioration of the internal quality of the slab.

The addition of the required additives is preferably carried out continuously from the viewpoint of making the fixing amount of REM (O, S) and / or Zr (O, S) in the casting direction uniform. Also, REM
From the viewpoint of more stable generation of fine particles of (O, S) and / or Zr (O, S), the molten steel in the mold (the molten steel near the lower part of the immersion nozzle) when the required additives are added has a dissolved oxygen concentration of 10 ~
It is preferably adjusted to 100 ppm.

[0020]

[Example] Si and Mn were fully melted in a converter with a capacity of about 300 tons.
The molten steel pre-deoxidized by RH was subjected to RH treatment and then cast at a casting speed of 0.85 m / min with the continuous casting machine shown in Fig. 1 to obtain a slab thickness.
The present invention was carried out in the following various cases in the operation of a slab having a width of 215 mm and a slab width of 2150 mm. (Example 1) For molten steel injected into the mold after the dissolved oxygen concentration was controlled to 50 ppm by adding Al and Ti in the tundish, the REM concentration of cast slab = 0.
005% equivalent amount of REM metal powder (particle size 0.5-2.0mm)
When continuously supplied near the bottom of the immersion nozzle at a speed of 0.8 kg / min by conveying Ar gas flow (flow rate 5.0 L / min). (Example 2) In Example 1, the REM concentration of the cast slab = 0.005
% When using slab Zr concentration = 0.005% equivalent Zr metal instead of equivalent REM metal. (Embodiment 3) In Embodiment 1, instead of Ar gas flow conveyance, sheath wire feeding (feeding wire (wire diameter 6 mm) wrapping REM metal powder with a 0.2 mm thick iron plate) And when. (Example 4) For molten steel injected into the mold after the dissolved oxygen concentration was controlled to 50 ppm by adding Al and Ti in the tundish, the REM concentration of cast slab was 0.
003% equivalent REM metal powder (particle size 0.5 to 2.0 mm) and slab Zr concentration = 0.002% equivalent Zr-Si alloy powder (particle size 0.5 to 2.0 mm) in Ar gas flow (flow rate) 5.0L / min) When continuously fed near the bottom of the dipping nozzle at a speed of 0.8kg / min. (Example 5) In Example 1, the dissolved oxygen concentration = 50 ppm
When the dissolved oxygen concentration is 5 ppm instead of. (Example 6) In Example 2, the dissolved oxygen concentration = 50 ppm
When the dissolved oxygen concentration is 150 ppm instead of.

For comparison, the following cases were also carried out. (Comparative Example 1) When no metal component is added after Si and Mn deoxidation. (Comparative Example 2) After deoxidizing Si and Mn, the dissolved oxygen was adjusted to 50 ppm, and the REM concentration of the slab was 0.005% when the RH was degassed.
When REM metal powder is added. (Comparative Example 3) After performing Si and Mn deoxidation, the dissolved oxygen was adjusted to 50 ppm, and during RH degassing, the Zr concentration of the slab was 0.005% equivalent to Zi.
When metal powder is added.

Table 1 shows the chemical composition of the cast pieces in each example. Surface layer in the thickness direction of these slabs, 1/4 thickness, 1/2
A sample cross section taken from the thick portion was macro-etched with picric acid corrosive liquid to reveal γ grain boundaries, and the γ grain size was measured by an image analyzer. The results are shown in Table 1. In addition,
Table 1 shows γ of 1/4 thick part of Comparative Example 1 (only Si and Mn deoxidized).
The particle size was set to 1, and the γ particle size index was shown, in which the γ particle size of each part of the other example was expressed as a relative ratio to the γ particle size of the 1/4 thick part of Comparative Example 1.

From the table, in Comparative Examples 2 and 3, the γ grain size in the 1/4 thick portion and the 1/2 thick portion was reduced to about 1/2 of that in Comparative Example 1, while in the Examples. , And it is reduced to about 1/2 or less, 1/4 to 1/2 respectively.
It is clear that the γ grains in the thick portion are refined to a degree close to that of the surface layer portion. In addition, a Charpy test piece conforming to JIS Z 3112 was taken from the 1/4 thick part of the product obtained by rolling the cast piece of each example into a steel plate, and 800-equivalent to large heat input welding with a heat input of 500 kJ / cm. A simulated thermal cycle test was conducted under the condition of a cooling time of 500 ℃ and 200 seconds. The vTrs value of the test piece treated at the peak temperature of 1400 ° C. corresponding to the weld bond is shown in Table 1. According to the present invention, a steel slab that is excellent in low temperature toughness and can be applied to high heat input steel can be obtained. Was shown.

[0024]

[Table 1]

[0025]

EFFECTS OF THE INVENTION According to the present invention, a cast piece which has been difficult in the past
It is possible to realize γ grain refinement in the 1/4 to 1/2 thick part, and it is possible to stably manufacture a steel plate having strict low temperature toughness specifications of the base metal and the weld heat affected zone.

[Brief description of drawings]

FIG. 1 is a schematic view showing an example of a continuous casting machine suitable for carrying out the present invention.

FIG. 2 is a schematic diagram showing an example of a method for adding required additives.

[Explanation of symbols] 1 ladle 2 Molten steel 3 long nozzle 4 tundish 5 immersion nozzle 6 molds 7 solidification shell 8 guide rolls 9 pinch rolls 10 slab 11 Mold powder 12 Molten steel discharge hole 13 Addition hole 14 Required additives 15 High REM and / or Zr concentration range

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Shuji Takeuchi             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Made in Kawasaki             Technical Research Institute of Iron Co., Ltd. F-term (reference) 4E004 MB14 MB20 NC01

Claims (2)

[Claims] 1. In a continuous casting method for steel, one or more of REM, REM-containing alloy, Zr, and Zr-containing alloy is poured into a mold while pouring molten steel into the mold through a dipping nozzle. Add to nearby molten steel and remove REM and / or
When adding REM-containing alloy alone, the REM concentration of the cast product should be 0.
001 to 0.01%, and when Zr and / or Zr-containing alloy is added alone, the Zr concentration of the cast product is 0.001 to 0.01%.
When M and / or REM containing alloy and Zr and / or Zr containing alloy composite addition, REM concentration and Zr of cast product
A method for continuous casting of steel, characterized in that the total of the concentration and the content is 0.001 to 0.01%. 2. The continuous casting method for steel according to claim 1, wherein the molten steel near the lower part of the immersion nozzle is a molten steel containing dissolved oxygen of 10 to 100 ppm.