JPH08267178A - Method for reducing center segregation in cast slab by fine oxide - Google Patents

Abstract

PURPOSE: To provide a steel dispersing fine oxides controlled in the composition into the steel, restraining the development of harmful MnS in the center segregation part and excellent in a hydrogen induced cracking resistant characteristic and sulfric stress corrosion cracking resistant characteristic. CONSTITUTION: The oxide satisfying 0.5-5μm grain diameter and >=20 mass% Ti concn. is dispersed in the position of a cast slab in the range from the center position in the thickness direction to the surface direction corresponding to the center segregation part of the cast slab at 5-50 pieces/mm<2> to reduce the development of MnS.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention is a low carbon steel having a limited S content, which is mainly used for steel materials such as thick plates or pipes. Regarding reduction methods.

[0002]

2. Description of the Related Art Material properties required for high-grade steels for marine structures, ships, line pipes, etc. are becoming more and more severe. In particular, drastic improvements in hydrogen-induced cracking resistance, sulfide stress corrosion cracking resistance, and low-temperature toughness at welds are desired.
At the end of solidification in continuous casting, solute elements are concentrated, and this structure after solidification forms a brittle hardened structure, and MnS sulfide that is the starting point of cracking is generated.

To satisfy the hydrogen-induced cracking resistance and the sulfide stress corrosion cracking resistance, it is necessary to reduce S in molten steel. It is effective to control the slag composition in order to efficiently remove S, which is disclosed in JP-A-63-
It is disclosed in Japanese Patent Publication No. 19320. However, even if S in molten steel is sufficiently reduced, in the center segregated portion of the slab,
Concentration of S and Mn is remarkable and MnS which is a harmful inclusion is easily formed.

[0004]

The present invention is intended to solve such conventional problems, and suppresses the generation of harmful MnS generated at the center of the slab when the low S steel is melted. That is, the object is to provide a method for reducing the segregation of the slab center with fine oxides.

[0005]

In order to achieve the above object, the present invention has a particle size of 0.5 to 5 μm and a Ti concentration of 20 ma.
An oxide satisfying ss% or more is added to the cast piece portion within a range of 10 mm in the surface layer direction from the center position in the thickness direction of the cast piece, which is a portion corresponding to the center segregated portion of the cast piece, in an amount of 5 to 50 per 1 mm ^2.
It is a method for reducing segregation of the center of a cast slab using fine oxides, which is characterized by reducing the generation of MnS by dispersing the individual pieces.

The present invention will be described in detail below. MnS
In order to reduce the generation of S, it is necessary to perform a low S treatment, and a low S steel with S ≦ 0.010 mass% that can be easily achieved by a normal desulfurization treatment industrially is targeted. In this steel, when a small oxide of about several μm is dispersed at the slab position corresponding to the central segregation part and MnS is crystallized on the oxide, MnS is finely dispersed together with the oxide, resulting in harmful coarse oxidation. It is possible to suppress the generation of an object, for example, MnS having a size of 100 μm or more. The particle size distribution of oxides is shown in FIG. 1. The number of oxides having a particle size of 0.5 to 5 μm occupies 95% of the whole, and by considering only oxides of this size, Reaction control is sufficient. Therefore, the particle size of the oxide is limited to 0.5 to 5 μm.

The center segregation portion of the slab varies depending on the molten steel flow, bulging, roll arrangement, etc. at the end of solidification, but within a range of 10 mm vertically from the center position in the thickness direction of the slab to the thickness direction of the slab. The target slab site is controlled. In this case, the distribution and composition of the oxide are important. In order to finely disperse the oxide in the steel, alumina is generated in the case of normal Al deoxidation, and this oxide is likely to cluster and easily float, so that an appropriate deoxidation method cannot be performed.
Therefore, it is easy to finely disperse the oxide by using a method of reducing the Al concentration which is a strong deoxidizing element and generating an oxide in the process of solidification of steel, and it is effective to use Ti instead of Al. is there.

It was also found that this Ti oxide serves as a precipitation site for MnS, and is particularly strongly controlled by the Ti concentration in the oxide. The result is shown in FIG. The composition of 25 to 35 oxides in each steel material was analyzed, and the average Ti concentration in the oxide was determined. The Ti concentration and the ratio of oxides in which MnS was crystallized (number of oxides in which MnS was crystallized / number of oxides) are shown. When the Ti concentration in the oxide is low, M
The proportion of oxides in which nS precipitates is small, and the proportion of oxides in which MnS crystallizes increases as the Ti concentration increases. Especially when the Ti concentration in the oxides is 40 mass% or more, the proportion is 80% or more. Become. FIG. 3 shows the relationship between MnS of 50 μm or more in the central segregated portion and the average Ti concentration in the oxide, which is harmful to the material. The large MnS in this case is a sulfide that is crystallized independently without crystallizing into an oxide. MnS is 0.
When crystallized in a fine oxide of 5 to 5 μm, MnS is dispersed and crystallization of a large single MnS which is a harmful inclusion is almost eliminated.

There is an appropriate range for the number of oxides of this composition. That is, when the number of oxides is less than 5 / mm ² , there are insufficient sites for reacting with and absorbing S. If the number of oxides is more than 50 / mm ² , the number of oxides increases and the number of oxides becomes 100, which is harmful to the material and becomes the starting point of cracking.
The frequency of generation of oxides having a size of μm or more increases. Therefore, the number of oxides is limited to 5 to 50 / mm ² .

In the present invention, the carbon concentration of the steel material is C≤0.20 mass from the viewpoint of toughness and weldability.
% Low carbon steel.

[0011]

EXAMPLES Examples of the present invention will be shown below. Table 1 shows the generation state of MnS in the central portion that governs the material properties and the generation state of coarse oxides of 100 μm or more in steel materials in which the composition and number of oxides are changed. Table 2 shows the composition range of the steel material of this example.

[0012]

[Table 1]

[0013]

[Table 2]

According to the present invention ,, MnS formation was not observed in the central segregation portion, and 10
Clean steel was produced with few large inclusions of 0 μm or more.

[0015]

EFFECTS OF THE INVENTION By dispersing a fine oxide having a controlled composition of Ti ₂ O ₃ in steel, central segregation is reduced and a steel material in which MnS, which is a harmful inclusion, is not produced is stably produced. did it.

[Brief description of drawings]

FIG. 1 is a diagram showing a particle size distribution of oxides in steel.

FIG. 2 is a diagram showing a relationship between an oxide composition and a precipitation rate of MnS on an oxide.

FIG. 3 is a diagram showing a relationship between an oxide composition and the number of individual MnS having a size of 50 μm or more.

Claims (1)

[Claims] 1. A particle size of 0.5 to 5 μm and a Ti concentration of 20.
An oxide satisfying mass% or more is added to a cast piece within a range of 10 mm from the center position in the thickness direction of the cast piece, which is a portion corresponding to the center segregation portion of the cast piece, to the surface layer direction in an amount of 5 to 5 per 1 mm ^2.
A method for reducing segregation of the center of a cast slab by a fine oxide, characterized in that the generation of MnS is reduced by dispersing 0 pieces.