JPH04314844A - Steel material excellent in toughness at low temperature and its production - Google Patents

Abstract

PURPOSE:To produce a steel material where fine oxides are uniformly dispersed and which has superior toughness at low temp. CONSTITUTION:In a continuous casting method for a laminated steel material, the steel material is produced by adding 0.005-0.05wt.% Ti to molten steel to be a surface layer and also adding 0.005-0.05wt.%, in total, of one or >=2 elements among Zr, Ca, Hf, Ce, and Y to a molten steel to be an inner layer. Particularly, it is possible to feed and add deoxidation elements by means of a wire. The resulting steel material is characterized by having the above composition and also having a structure where the maximum grain size of oxides is regulated to <=50/mum and also (the number of the oxides in the surface layer part)/(the number of the oxides in the part at 1/2 thickness) is regulated to >60%.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material with excellent low-temperature toughness obtained by uniformly dispersing fine oxides, and a method for continuously casting the steel material as a composite steel material.

0002

[Prior Art] Until now, a direct current magnetic flux in the direction across the thickness of the slab is applied to the continuous casting mold over the entire width, and a static magnetic field band formed in the vertical direction of the mold by the direct current magnetic flux is used as the boundary. JP-A-63-108947 discloses a continuous casting method for a composite metal material in which metals having different compositions are supplied above and below.

On the other hand, a method for improving the low-temperature toughness of steel by dispersing fine oxides in the steel was disclosed in Japanese Patent Laid-Open No. 61-23.
It is disclosed in Japanese Patent No. 8940. That is, a small amount of T
By adding i as a deoxidizing element and finely dispersing the secondary deoxidizing product, this becomes a transformation nucleus, and fine ferrite is generated from within the austenite grains to refine the structure, especially in the weld heat affected zone. It has been described that the toughness of steel is improved. In addition, as a method disclosed by the present inventors in JP-A-1-228643, Zr, Ti, and Ce are added to molten steel.
By adding a deoxidizing element such as , Y or Hf and cooling and solidifying it, MnS, which is one of the transformation nuclei, is deoxidized to a product (ZrO2, Ti2 O3, Ce2 O3, etc.)
There is a method of making fine precipitation on top. It is recognized that these methods yield finer structures as the number of oxides increases. Therefore, the objective of any of the above methods is to increase the number of oxides and uniformly disperse them.

0004

[Problems to be Solved by the Invention] The present invention focuses on refining the structure of steel and thereby improving its low-temperature toughness, and aims to reduce the oxides (deoxidized The object of the present invention is to provide a steel material in which a larger number of materials are dispersed than before, and a method for manufacturing the same.

[0005]

[Means for Solving the Problems] The gist of the present invention is as follows: (1) A steel comprising a surface layer and an inner layer having different components, the surface layer containing 0.005% to 0.05% Ti by weight. The inner layer contains 0.005% to 0.05% of the total of one or more of Zr, Ca, Hf, Ce, and Y.
%, the maximum grain size of the oxides contained is 50 μm or less, and the ratio of (number of oxides in the 1/2 thick part) to (number of oxides in the surface layer) is 60%. This steel material is characterized by being larger than (2
) Direct current magnetic flux in the direction across the thickness of the slab is applied to the continuous casting mold over the entire width, and metals with different compositions are placed above and below the static magnetic field zone formed by the direct current magnetic flux in the vertical direction of the mold. In a manufacturing method using continuous casting of supplied composite steel materials, Mn
, after deoxidizing with Si, Ti is 0.005% to 0 by weight%.
．． After deoxidizing the molten steel to be injected under the static magnetic field zone, which will become the inner layer, with Mn and Si, Zr, C
Production of a steel material with excellent low-temperature toughness characterized by adding one or more of a, Hf, Ce, and Y in a total amount of 0.005% to 0.05%. Obtained by method. It is also possible to add and supply these deoxidizing elements, ie, Ti, Zr, Ca, Hf, Ce, or Y wire.

[0006]

[Function] The present invention will be explained in detail below along with its function. The inventors of the present invention have conducted detailed research to solve the problems in the conventional technology, and have discovered that there are large differences in the dispersion state of oxides in the thickness direction of the slab depending on the type of deoxidizing element. .

That is, in the case of Ti-deoxidized steel, the number of oxides decreases from the surface layer to the inside of the slab. this is,
Ti is a weak deoxidizing element, the dissolved oxygen concentration of molten steel after deoxidation is high, and the deoxidation products crystallize due to segregation during solidification.
This is because the next product is the main component.

[0008] The reason for this is that the cooling rate near the surface layer is high and segregation is significant, so the number of oxides increases.
In the /2 thick part, the cooling rate is low and segregation is slight, so the number is smaller than in the vicinity of the surface layer. On the other hand, Zr, Ca, Hf
, Ce, Y, etc. are strong deoxidizing elements, and deoxidizing products are mainly primary products produced immediately after deoxidizing.

Therefore, the influence of the cooling rate on the number of oxides is smaller than in the case of Ti deoxidation, the distribution in the thickness direction of the slab becomes more uniform, and the number of oxides in the 1/2 thick part of the slab becomes sufficient. Can be secured. However, since it aggregates and floats before solidification starts, the number of oxides near the surface layer is smaller than in Ti deoxidation.

[0010] Using each of the molten steels prepared by the two types of deoxidizing methods described above, molten steel deoxidized with Ti in the surface layer and molten steel deoxidized with Zr, Ca, Hf, Ce, Y, etc. in the inner layer, In the present invention, the steel material of the present invention is manufactured by forming a static magnetic field zone in the mold and continuously casting while preventing the mixing of two types of molten steel, as explained in the section of the above-mentioned means.

[0011] Thus, by the method of the present invention, it has become possible for the first time to manufacture a steel material in which a sufficient number of oxides are uniformly dispersed continuously from the surface layer using means for continuously casting a composite steel material. This is because, as shown in FIG. 1, in a normal method of manufacturing composite steel materials such as a crimping method, a rapid decrease in the number of Ti oxides in the thickness direction of the surface steel sheet cannot be avoided.

When deoxidizing molten steel, Ti or the like is added after preliminary deoxidation with Mn, Si, etc. Regarding the composition of the deoxidizing element, if the target composition is less than 0.005%, the amount of oxide produced by reaction with dissolved oxygen is effectively small, and if it exceeds 0.05%, coarse oxide clusters will be produced. The proper composition should be set to 0.00 to avoid negative effects such as becoming a starting point for cracks.
The content was set at 5% to 0.05%.

[0013] An effective method for adding a deoxidizing element such as Zr is to fill an iron pipe with the alloy to form a wire, and to feed the alloy into a mold at a constant rate. This wire addition method shortens the time from addition to the start of solidification, especially for Z
It is extremely effective for reducing the floating amount of primary deoxidation products such as r and dispersing many oxides.

[0014] The steel material thus obtained has excellent low-temperature toughness. As represented in Figure 1, its characteristics are that fine oxides are uniformly dispersed, that is, the maximum particle size of the contained oxides is 50 μm or less, and the oxides (in the surface layer) are The ratio of (number of 1/2 thick parts) to (number of parts) is greater than 60%. Note that the surface layer portion here is defined as a position at a depth of 10 mm from the surface of the steel material.

[0015]

EXAMPLE Steel materials having the compositions shown in Table 1 were produced by the continuous casting method described above. During manufacturing, 5000 mm is placed in the range of 450 mm to 700 mm from the meniscus in the mold.
A static magnetic field with Gaussian strength was applied. The mold shape has a thickness of 2
The size was 50 mm x width 1000 mm, and the casting speed was 1 m/min. The pouring amounts of the surface layer molten steel and the inner layer molten steel were adjusted so that the thickness of the surface layer was 25 mm.

[0016] Regarding steel material F, Zr was added using a wire. The wire has a diameter of 5mm and the feeding speed is 3.5m/
It was a minute. Further, as comparison materials, single-layer continuous casting was performed using molten steel that had been deoxidized with Ti for steel material G and deoxidized by Zr for steel material H. 10 from the surface layer in the thickness direction of the obtained slab
The number of oxides with a particle size of 50 μm or less was investigated at each position of 50 and 125 mm using an X-ray microanalyzer.

[0017] In addition, a sample of 12 mm square and 60 mm length for welding reproduction heat cycle treatment was taken from the slab, and the maximum heating temperature was 1350°C, and the cooling time from 800 to 500°C was 161 seconds.
After carrying out a welding simulation thermal cycle treatment with an equivalent heat input of 130 kJ/cm, the toughness was evaluated by a Charpy test.

Table 1 also shows the fracture surface transition temperature vTrs of each sample.

[0019]

[Table 1]

[0020]

[Table 2]

FIG. 1 is a diagram showing the oxide distribution of the present invention material A and comparative materials G and H. From this figure, it was confirmed that material A of the present invention had a sufficient number of oxides than the comparative material in any part in the thickness direction of the slab. In addition, from Table 1, it can be seen that the low temperature toughness of the present invention is also superior to that of the comparative materials, and the uniformity of the number of oxides is also superior.

Table 2 also shows the results when two or more types of deoxidizing elements were added to the inner layer.

[0023]

[Table 3]

[0024]

[Table 4]

From this, it was found that, as in Comparative Material O, when the total amount of deoxidizing elements added exceeds 0.05%, this leads to a decrease in low temperature toughness, which is not preferable. From observation of the fracture surface, it was found that oxide clusters with a particle size exceeding 50 μm were the cause of the decrease in toughness.

[0026]

[Effects of the Invention] According to the present invention, a large number of fine oxides are uniformly dispersed in the steel material, and as a result, it is possible to obtain a steel material with even better low-temperature toughness than that produced by the conventional method. Become.

[Brief explanation of drawings]

FIG. 1 is a chart showing the relationship between the position in the slab thickness direction and the number of oxides for the present invention material and comparative material.

Claims (3)

[Claims] Claim 1: Consisting of a surface layer and an inner layer having different components, the surface layer contains Ti in an amount of 0.005% to 0.05% by weight.
The inner layer contains one or more of Zr, Ca, Hf, Ce, and Y in a total amount of 0.005% to 0.00%.
05%, the maximum particle size of the contained oxide is 50 μm or less, and the ratio of the oxide (number in the 1/2 thick part) to (number in the surface layer) is 60 %
A steel material with excellent low-temperature toughness that is characterized by its larger size. 2. Direct current magnetic flux in the direction across the thickness of the slab is applied to the continuous casting mold over the entire width, and composition is applied above and below the static magnetic field zone formed by the direct current magnetic flux in the vertical direction of the mold. In a manufacturing method using continuous casting of a composite steel material that supplies different metals, Ti is added to the molten steel injected above the static magnetic field zone, which is the surface layer, so that it contains 0.005% to 0.05% by weight, and the inner layer is Addition of one or more of Zr, Ca, Hf, Ce, and Y in a total amount of 0.005% to 0.05% to the molten steel injected below the static magnetic field zone. A method for manufacturing steel materials with excellent low-temperature toughness, characterized by casting. [Claim 3] Deoxidizing elements Ti, Zr, Ca, H
3. The method for manufacturing a steel material with excellent low-temperature toughness according to claim 2, characterized in that f, Ce, or Y is added using a metal wire.