JPH0569101A - Production of steel material - Google Patents

Abstract

PURPOSE:To provide the above method to produce the steel material with which fine oxides in Al less killed steel can be uniformly dispersed in said steel material. CONSTITUTION:In the cast of casting the poor deoxidized steel having <=0.005% Al with continuous casting, by adding and dipping an iron wire containing 0.0005-0.005wt.% iron oxide in terms of oxygen per unit amount of molten cast steel into a mold from mold meniscus to the deep part so that the melting reaction position of the added wire becomes >=30% and <70% of the total thickness of a cast slab to the solidified thickness in the cast slab, many fine oxides of <=10mum are uniformly dispersed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a low carbon steel material which hardly uses an Al alloy as a deoxidizing material, and the main application of this steel material is a thick plate or UO pipe.

[0002]

2. Description of the Related Art In recent years, the demands on the material characteristics of marine structures, ships, line pipes, etc. have become more severe, and in particular, a drastic improvement in the low temperature toughness at the welded portion has been desired.
On the other hand, by finely dispersing minute oxides (oxide particle size ≤ 10 μm) containing Ti as a main component in the steel material, a large number of austenite particles are formed in the austenite grains in the cooling process after welding. Of minute ferrite (hereinafter referred to as intragranular ferrite) is generated, and excellent toughness is obtained by substantially refining the crystal grains in the heat-affected zone of welding. Thus, the method of melting this steel material is disclosed in JP-A-60-70.
No. 15 publication.

However, in the manufacturing method disclosed in Japanese Patent Laid-Open No. 60-7015, it was found that the number of minute oxides in the continuously cast slab greatly changed in the thickness direction of the slab. That is, the number of oxides is large in the vicinity of the surface layer of the slab where the cooling rate is high, but the number decreases at the center of the slab where the cooling rate is low. In order to make the material of the steel material more uniform, it is necessary to suppress the decrease in the number of minute oxides that serve as nuclei for intragranular ferrite in the thickness direction of the cast piece.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for reducing a minute decrease in the number of oxides in a steel material in the thickness direction during continuous casting.

[0005]

[Means for Solving the Problems] An investigation and analysis of the formation mechanism of minute oxides have revealed that the cooling rate and the dissolved oxygen content are governed. That is, if the cooling rate is high, the driving force for crystallizing the oxide inside the cast piece is large, and the solidification time is short, so the frequency of aggregation and coalescence due to the collision of the crystallized oxides decreases. The number of oxides also increases. Also, if the dissolved oxygen content is high, the absolute amount of oxide formation increases.

The increase in the cooling rate inside the slab due to the increase in the amount of secondary cooling water in the continuous casting equipment is due to the heat transfer within the solidified slab, and the effect of the thickness of the slab is significant to obtain efficient cooling. However, excessive cooling causes quality defects such as surface cracks. Further, if the amount of oxygen is increased in the molten steel stage before casting, not only the decreasing central portion of the cast slab but also the total number of oxides increases, and the cleanliness of molten steel deteriorates. Therefore, in order to more efficiently increase the number of oxides in the slab that is decreasing, iron oxide, which is an oxidant, is directly added into the molten steel and reacted to increase the number of oxides. Specifically, an iron wire containing iron oxide is supplied to the molten steel in the mold, the iron wire is melted inside the molten steel, the iron oxide reacts with the molten steel, and the number of oxides inside is increased. By adding the oxide to the inside, the cooling rate is also increased, and the aggregate growth of the oxide can be suppressed by increasing the number of crystallized oxides and shortening the solidification time.

The melting position of the iron wire containing iron oxide is desired to be a position where the number decreases remarkably in the thickness direction of the cast piece, and the iron wire is added and immersed from the mold meniscus to a deep portion.
Specifically, the slab is melted at a position where the solidified thickness is 30% or more of the total slab thickness, and the cooling rate inside the slab is increased to increase the number of oxides inside the slab. However, if the melting position is too deep, the uncoagulated portion is reduced and unmelting of the wire occurs. Therefore, the melting position is limited. From the results of the experiments so far, it was found that the melting position at which the unmelting of the wire did not occur was such that the cast solidification thickness was less than 70% of the total cast thickness.

If the amount of oxide added is too small, the amount of oxide produced is insufficient and the effect is small. If it is too large, undissolved oxide of several tens of μm or more remains, resulting in a non-uniform steel material. Therefore, specifically, the added oxygen amount is 0.0005 to
It is limited to 0.005 wt%.

[0009]

[Function] The number distribution of oxides in cast slab according to the conventional method in which the iron wire containing iron oxide is not added to the mold,
The distribution of the number of oxides in a cast piece when an iron wire containing iron oxide is added to the mold is compared using the drawings.
As shown in FIG. 1, the number of oxides in the thickness direction of the cast piece by the conventional method is 70 to 100 pieces / mm ² in the vicinity of the surface layer of the cast piece, but decreases in the inside of the cast piece, and 15 pieces / mm ² in the central portion.
It is a degree. On the other hand, Fig. 2 shows the result when iron wire containing iron oxide was added to the mold and the iron melting point was controlled so that the solidification thickness was about 35% of the total slab thickness and the iron oxide was reacted. Show. In the vicinity of the surface layer of the slab, the number of oxides was 80 to 90 / mm ² , which was almost the same as that of the conventional method, but the decrease in the number of oxides inside the slab was smaller than that of the conventional method, and About 60 pieces / mm ² could be secured.

If the wire melting position is too shallow, for example, if the solidification thickness of the wire melting position is about 15% of the total slab thickness, the number of oxides from the surface layer to 60 mm increases slightly as shown in FIG. No increase in the number of oxides in the central part was observed. If the wire melting position is too deep, for example, the solidification thickness at the wire melting position is about 75% of the total cast piece thickness.
When the target was set, the wire remained unmelted, the effect could not be obtained, and the quality became uneven. Therefore, in the wire melting position, the solidified thickness of the cast piece is 30% or more and less than 70% of the total cast piece thickness within the proper range.

FIG. 4 shows the results of investigating the increase in the number of oxides by changing the amount of oxides added. If the amount of oxygen contained in the oxide is less than 0.0005 wt% with respect to the amount of molten steel cast, no remarkable increase effect is observed. 0.0005-0.0
Although the effect of increasing the number of oxides is recognized at 05 wt%,
If it exceeds 0.005 wt%, undissolved iron oxide remains,
The effect of increasing small oxides diminished, and undissolved iron oxide was present, resulting in a heterogeneous steel material. Therefore, 0.0005 to 0.005 wt% is an appropriate range for the amount of oxide to be added.

As for the effect on the material, as shown in FIG. 5, a wire containing iron oxide was used in an amount of 0.001 equivalent to oxygen.
If the solidification thickness is controlled to about 35% of the total slab thickness at 5 wt% and the cooling rate inside the slab is increased to increase the number of oxides, the area of intragranular ferrite that governs the material You can see that the rate is increasing.

[0013]

[Examples] Molten steel having the components shown in Table 1 was cast by a two-strand continuous casting machine having a thickness of 240 mm, and the iron wire containing iron oxide was added to the mold on the one-strand side by adding the iron oxide content and the melting position. Was changed, and a comparative casting test was performed in which the iron wire was not added in the mold on the 2 strand side. Table 2 shows the results of measuring the number of oxides in the cast piece
Shown in.

[0014]

[Table 1]

[0015]

[Table 2]

[0016]

INDUSTRIAL APPLICABILITY As described above, according to the present invention, iron wires are added into the mold during casting in continuous casting to increase the cooling rate inside the slab to uniformly disperse a large number of fine oxides into the slab. By doing so, it is possible to increase the amount of intragranular ferrite produced and manufacture a steel material having excellent toughness and weldability.

[Brief description of drawings]

FIG. 1 is a diagram showing an oxide distribution in a conventional cast slab.

FIG. 2 is a view showing an oxide distribution in a cast piece by adding an iron wire containing iron oxide (melting position within the range of the present invention).

FIG. 3 is a diagram showing an oxide distribution in a cast slab by adding an iron wire (a melting position outside the range of the present invention).

FIG. 4 is a diagram showing the relationship between the amount of added iron oxide and the increase in the number of oxides in the center of the cast slab.

FIG. 5 is a diagram showing the relationship between the number of oxides and the intragranular ferrite production rate.

Claims (1)

[Claims] 1. Al: 0.005% or less of weakly deoxidized steel is cast in a continuous casting facility, and iron oxide corresponding to an oxygen content of 0.0005 to 0.005 wt% per cast molten steel is cast in a mold. 10 μm or less by adding and immersing the iron wire containing the iron wire from the mold meniscus to the deep part so that the melting reaction position of the addition wire is 30% or more and less than 70% of the total thickness of the slab. A method for producing a steel material, which comprises uniformly dispersing a large number of oxides.