JPH04294848A - Method for controlling oxide - Google Patents

Abstract

PURPOSE:To offer a continuous casting method for uniformly dispersing fine oxides of an Al-less weakly deoxidized steel material. CONSTITUTION:In the case of casting the weakly deoxidized steel having <=0.005% Al with a continuous casting device by adding and dipping an iron wire from a mold minicus to a deep part in a mold so that melting position comes to >=30% and <70% of the whole cast slab thickness to increase cooling speed in the inner part of the cast slab, the fine oxides having <=10mum, which are ordinarily reduced at center part in the thickness direction of the case slab, are uniformly dispersed in the cast slab.

Description

[Detailed description of the invention]

0001

[Industrial Field of Application] The present invention is a low carbon steel that uses almost no Al alloy as a deoxidizer, and is mainly used in thick plates or UO pipes.

0002

BACKGROUND OF THE INVENTION In recent years, requirements for material properties of marine structures, ships, line pipes, etc. have become more severe, and in particular, drastic improvements in low-temperature toughness of welded parts have been desired.

[0003] On the other hand, by uniformly dispersing minute oxides (oxide grain size ≦ 10μ) containing Ti as a main component in the steel material, austenite grains are formed using these as nuclei during the cooling process after welding. Many minute ferrites (hereinafter referred to as intragranular ferrite) are generated within the weld heat affected zone, and excellent toughness is obtained by effectively refining the crystal grains in the weld heat affected zone. The melting method for this steel material is JP-A-60-701.
As shown in 5.

However, it has been found that in the production method of JP-A-60-7015, the number of minute oxides in continuously cast slabs varies greatly in the thickness direction of the slab.

The number of oxides is large near the surface of the slab where the cooling rate is high, but the number decreases at the center of the slab where the cooling rate is slow. In order to make the material quality of the steel material more uniform, it is necessary to control the decrease in the number of minute oxides that serve as ferrite nuclei within the grains in the thickness direction of the slab.

[0006]

SUMMARY OF THE INVENTION The present invention is a method for reducing the decrease in the number of minute oxides in a steel material in the thickness direction.

[0007]

[Means for solving the problem] When we investigate and analyze the mechanism of the decrease in the number of minute oxides, we find that the cooling rate of the continuously cast slab changes in the slab thickness direction, and this cooling rate changes the number of oxides. was found to be in control.

That is, the cooling rate of the inside of the slab is lower than that of the surface layer of the slab, and the time for solidification is longer. Inside the slab, where the cooling rate is low, the driving force for crystallizing oxides is small and the solidification time is long, so the frequency of agglomeration and coalescence due to collisions between crystallized oxides increases.

Due to the above two reasons, the number of oxides inside the slab is small. Therefore, in order to increase the number of oxides inside the slab, it is necessary to increase the internal cooling rate.

[0010] The increase in the cooling rate inside the slab due to an increase in the amount of secondary cooling water in continuous casting equipment is rate-limited by heat transfer within the solidified slab, and is largely influenced by the thickness of the slab, making it difficult to obtain efficient cooling. is difficult, and excessive cooling can cause quality defects such as surface cracks.

[0011] Therefore, in order to increase the cooling rate inside the slab more efficiently, a coolant is directly added to the inside of the molten steel.

Specifically, an iron wire is supplied to the molten steel in the mold, and the cooling rate inside the mold is increased by melting the iron wire inside the molten steel. This increase in the internal cooling rate increases the number of crystallized oxides and shortens the solidification time, thereby suppressing the agglomeration and growth of oxides.

[0013] The melting position of the iron wire is preferably a position where the number decreases significantly in the thickness direction of the slab, and the iron wire is added and immersed from the mold miniscis to the deep part. Specifically, the melting occurs at a position where the solidified thickness of the slab becomes 30% or more of the total thickness of the slab, 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 unsolidified portion will be small and the wire will not be melted, so the melting position is limited. From the experimental results to date, it has been found that the melting position is such that the solidified thickness of the slab is less than 70% of the total thickness of the slab so that the wire does not remain unmelted.

[0015]

[Operation] The distribution of the number of oxides in the slab according to the conventional method in which no iron wire is added to the mold and the distribution of the number of oxides in the slab when iron wire is added to the mold are compared using a diagram.

According to the conventional method, the number of oxides in the thickness direction of the slab is 70 to 100 near the surface layer of the slab, as shown in FIG.
pieces/mm2, but it decreases inside the slab and in the center,
The number is about 15 pieces/mm2.

On the other hand, adding iron wire into the mold,
Figure 2 shows the results when the wire melting position was controlled so that the solidified thickness was approximately 35% of the total slab thickness to improve the cooling rate inside the slab.

[0018] Near the surface layer of the slab, 80 to 90 pieces/mm2
Although it is almost the same as the conventional method, the reduction inside the slab is smaller than the conventional method, and in the center, it is 50 pieces/mm.
We were able to secure around 2.

If the wire melting position is too shallow, for example,
When the wire melting position is set so that the solidified thickness is approximately 15% of the total slab thickness, no increase in the number of wires at the center is observed as shown in FIG.

If the wire melting position is too deep, although the solidification thickness of the wire melting position is targeted to be about 75% of the total slab thickness, the wire remains unmelted and not only the effect cannot be obtained, but also the quality On the top, the steel material was uneven.

Regarding the effect on the material, as shown in Figure 4, the solidified thickness of the wire melting position is about 35% of the total slab thickness.
It can be seen that when the number of oxides is increased by controlling the cooling rate inside the slab to increase the number of oxides, the area ratio of intragranular ferrite, which controls the material quality, increases.

[0022]

【Example】

[0023]

[Table 1]

[0024] The molten steel having the components shown in Table 1 was heated to a thickness of 240 mm.
A comparative casting test was conducted in which iron wire was added by changing the wire melting position in the mold on the 1st strand side, and no iron wire was added in the mold on the 2nd strand side. did. Table 2 shows the results of measuring the number of oxides in the slab.

[0025]

[Table 2]

[0026]

[Effect of the invention] By adding iron wire to the mold during continuous casting and increasing the cooling rate inside the slab, a large number of minute oxides are uniformly dispersed to the inside of the slab, thereby reducing intragranular ferrite. It has become possible to increase the yield and produce steel materials with excellent toughness and welding toughness.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the oxide distribution within a conventional slab.

FIG. 2 is a diagram showing the oxide distribution in a slab (melting position within the range of the present invention) due to the addition of iron wire.

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

FIG. 4 is a diagram showing the relationship between the number of oxides and the intragranular ferrite production rate.

Claims (1)

[Claims] Claim 1: When casting weakly deoxidized steel with Al: 0.005% or less using continuous casting equipment, an iron wire is placed in the mold, and the melting position of the added wire is such that the solidified slab thickness is equal to the total slab thickness. A method for manufacturing a steel material in which the cooling rate inside the slab is increased by adding and immersing it deep from the mold miniscus so that the concentration is 30% or more and less than 70%, thereby uniformly dispersing a large number of minute oxides of 10 μm or less.