JP2003129169A - Slab superior in toughness and mechanical property and steel material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a slab superior in toughness and mechanical properties, and provide a steel material obtained by heating and rolling the slab. SOLUTION: The slab includes, by mass%, 0.005-0.30% C, 1.0% or less Si, 2.0% or less Mn, 0.005-0.1% Al, and one or more of 0.005-0.5% Hf, 0.005-0.5% Zr, and 0.005-0.5% Y, and has 100/mm<2> or more of precipitates with sizes of 5 μm or less. The steel material is obtained by heating the slab, and then hot- working it with a total reduction ratio of 3 or less.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a slab and a steel material having excellent mechanical properties such as toughness, strength, and ductility. Regarding

[0002]

2. Description of the Related Art Regarding the miniaturization of solidification structure after casting,
Conventionally, various methods have been proposed, for example, Japanese Patent Application Laid-Open No. 2000-2000.
JP-A-301306 relates to a slab excellent in quality and processing characteristics and a steel material obtained from the slab, in which 100 inclusions / c of inclusions having a lattice matching degree with δ ferrite of 6% or less are included.
It is characterized by containing m ² or more.

Specifically, SiO ² , MnO, MgO, etc. are added to molten steel to form an inclusion having a size of 10 μm or less as 100
It has been proposed that particles / cm ² or more are dispersed to form solidification nuclei, and the pinning effect suppresses the growth of crystal grains after solidification to form a fine and uniform solidification structure.

Japanese Unexamined Patent Publication (Kokai) No. 11-92860 relates to the refinement of the structure by a special alloying element such as Hf, and the abnormal growth of crystal grains in the carburizing treatment of carbon steel for machine structure is described as Nb, T.
It is proposed to prevent this by a precipitate composed of i, Hf, etc.

Japanese Unexamined Patent Publication (Kokai) No. 8-85850 describes a high Cr ferritic heat resistant steel for a high temperature and high pressure boiler, in which creep strength is improved by adding Y.

Japanese Unexamined Patent Publication (Kokai) No. 6-279928 proposes a technique for a high-strength rail, in which the number and distribution of MnS are controlled by adding Zr to promote the pearlite transformation from within the austenite grains to form a fine structure. There is.

[0007]

As described above, the prior art in which a special element is added for the purpose of refining the structure is proposed for refining the solidification structure after casting or the transformation structure from the austenite region. ing.

[0008] However, in the prior art relating to the refinement of the solidification structure, it is not easy to verify the distribution state of inclusions, and it is not easy to confirm that the obtained slab has the desired effect. .

Further, there is no case where the effect of refining the structure of special alloy elements such as Hf and Y has been confirmed in the solidified structure.

Therefore, the present invention uses a special alloying element such as Hf and Y, and does not require complicated structure verification and can easily obtain a fine cast solidified structure excellent in toughness and workability. We propose a steel material using the slab.

[0011]

The present inventors have found that Hf, Z
r and Y were added to the molten steel, and the behavior in the solidification process was examined in detail.

As a result, these elements are precipitated in the liquid phase or in the solid phase as oxides, sulfides or composite precipitates to make the solidification structure itself fine and the secondary structure after transformation fine. It has been found that there is a correlation between the volume fraction of these precipitates having a certain size or more and the grain size of the solidification structure in casting.

The present invention has been made by further studies based on these findings, that is, the present invention is as follows. % By mass, C: 0.005 to 0.30%, Si:
1.0% or less, Mn: 2.0% or less, Al: 0.005
~ 0.1%, and Hf: 0.005-0.5%, Z
r: 0.005-0.5%, Y: 0.005-0.5%
A slab having excellent toughness and mechanical properties, characterized by containing one or more of

2. % By mass, C: 0.005-0.3
0%, Si: 1.0% or less, Mn: 2.0% or less, A
1: 0.005 to 0.1%, and Hf: 0.005
0.5%, Zr: 0.005-0.5%, Y: 0.00
5% to 0.5% of one or more kinds, and 5
A slab having excellent toughness and mechanical properties, characterized by having 100 / mm ² or more of precipitates having a size of μm or less.

3. % By mass, C: 0.005-0.3
0%, Si: 1.0% or less, Mn: 2.0% or less, A
1: 0.005 to 0.1%, and Hf: 0.005
0.5%, Zr: 0.005-0.5%, Y: 0.00
A slab containing 5 to 0.5% of one kind or two or more kinds,
Steel material with excellent toughness and mechanical properties characterized by hot working after heating.

4. % By mass, C: 0.005-0.3
0%, Si: 1.0% or less, Mn: 2.0% or less, A
1: 0.005 to 0.1%, and Hf: 0.005
0.5%, Zr: 0.005-0.5%, Y: 0.00
5% to 0.5% of one or more kinds, and 5
A steel material excellent in toughness and mechanical properties, characterized in that a slab having 100 μm / mm ² or more of precipitates having a size of μm or less is heated and then hot-worked.

5. The steel material excellent in toughness and mechanical properties described in 3 or 4, wherein hot working after heating the slab is rolling at a total reduction ratio of 3 or less.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The slab of the present invention is made of Hf, Zr, Y.
Is added to molten steel, a precipitate containing one or more of them is precipitated in a liquid phase or a solid phase, the solidification structure itself is made fine, and the secondary structure after transformation is refined, toughness, and It is a slab with excellent mechanical properties.

In the present invention, the precipitate is 5 μm or less,
100 pieces / mm ² or more. In order to secure such a precipitation state, one or more of Hf, Zr, and Y are contained in 0.005 to 0.5% (0.005% or more, 0.5% or more), respectively.
The following) is preferably added.

FIG. 1 shows the influence of the amount of precipitates (the number of precipitates) on the average equiaxed crystal diameter in the cast solidification structure of the molten steel containing Hf, Zr and Y, respectively. The average equiaxed grain size is 1 m near the center of as-cast steel with a mass of 150 kg.
It was measured in m ² .

The average equiaxed crystal diameter is the average value of the size of equiaxed crystals contained in 10 fields of view observed by an optical microscope, and the amount of precipitates (the number of precipitates) is in 10 fields of view observed by a scanning electron microscope. The average value of those included in.

As a result, in each of the steels added with Hf, Zr and Y, when the average particle number of the amount of precipitates (the number of precipitates) is 100 or more, the average equiaxed crystal diameter is usually about 2 mm, It is 1 mm or less and has a fine structure.

FIG. 2 shows the effect of the average equiaxed grain size on the mechanical properties of the steel used in FIG. As the average equiaxed crystal diameter decreases, the tensile properties and Charpy impact properties improve.

The slab of the present invention has a mass% of C: 0.005 to 0.30% and Si: in order to secure toughness and mechanical properties as a practical steel such as JIS standard welded structural steel.
1.0% or less, Mn: 2.0% or less, Al: 0.005
.About.0.1%, S: 0.01% or less, N: 0.005% or less is preferable.

C C is 0.005% or more in order to obtain the strength as a practical steel, and if 0.30% or more is added, the toughness deteriorates.
It is set to 0.005 to 0.30%.

Si 2 Si is added for deoxidation. However, if added in excess of 1.0%, ductility and toughness deteriorate, so the content is made 1.0% or less.

Mn Mn is added to secure the strength, but if it is added in excess of 2.0%, ductility and toughness decrease, so it is made 2.0% or less.

Al Al is deoxidized, so 0.005% or more is added. on the other hand,
If added in excess of 0.1%, AlN becomes excessive and the toughness deteriorates.

In the present invention, S S is an impurity element and is harmful to toughness.
In order to prevent an increase in the value of 0.01% or less, it is desirable to set it to 0.01% or less.

N N is an impurity in the present invention, and solid solution N lowers the toughness, so it is desirable to set it to 0.005% or less.

Further, from the viewpoint of weldability, Cu, N
The addition of an appropriate amount of one or more of i, Cr, Mo, V, Ti, Nb and B does not impair the effects of the present invention.

Next, the steel material of the present invention will be described. The steel material of the present invention, after heating the above-mentioned slab, is hot-rolled to be processed into a thick plate, thin plate, shaped steel, etc., and is excellent in toughness and mechanical properties due to a slight reduction of total reduction ratio of 3 or less. To be

[0033]

Example A molten steel having the composition shown in Table 1 was cast, and the structure, precipitates and toughness, and mechanical properties of the cast material were investigated. The casting temperature and cooling conditions of each test material are the same,
The cast material was 150 × 200 × 500 (mm).

Table 2 shows the average equiaxed crystal diameter, the number of precipitates and the tensile properties near the center of the cast material. In the table, the average equiaxed crystal diameter is the average value of equiaxed crystal diameters observed in 10 fields of view by an optical microscope, and the number of precipitate particles is the average value of the number of particles contained in 10 fields of view observed by a scanning electron microscope. And The tensile test was performed using a No. 4 test piece in accordance with JIS standard Z2241.

As a result, the invention steels A to F had a large number of precipitate particles and a fine crystal structure with an average equiaxed crystal diameter of 1 mm or less was obtained, while the comparative steels G and H had an average equiaxed crystal diameter of 2 mm. As described above, the tensile properties were also inferior to the steel of the present invention.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

EFFECTS OF THE INVENTION According to the present invention, a cast solidification structure from molten steel is refined without requiring a complicated manufacturing process, so that a slab and a steel material having excellent toughness and mechanical properties can be obtained. It is extremely useful in industry.

[Brief description of drawings]

FIG. 1 is a diagram showing the effect of the amount of precipitates (the number of precipitates) on the average equiaxed crystal diameter in the cast solidification structure of molten steel containing Hf, Zr, and Y, respectively.

FIG. 2 is a diagram showing the effect of the average equiaxed crystal diameter on the mechanical properties of the cast solidification structure from molten steel containing Hf, Zr, and Y, respectively.

Continued front page    (72) Inventor Mikio Suzuki             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. (72) Inventor Chiaki Ouchi             Aoba, Aoba-ku, Sendai City, Miyagi Prefecture             Shi) Inside the Graduate School of Engineering, Tohoku University

Claims (5)

[Claims] 1. C: 0.005 to 0.30 in mass%
%, Si: 1.0% or less, Mn: 2.0% or less, Al:
0.005 to 0.1%, and Hf: 0.005 to 0.
5%, Zr: 0.005-0.5%, Y: 0.005-
A slab having excellent toughness and mechanical properties, which is characterized by containing 0.5% of one kind or two or more kinds. 2. C: 0.005 to 0.30 in mass%
%, Si: 1.0% or less, Mn: 2.0% or less, Al:
0.005 to 0.1%, and Hf: 0.005 to 0.
5%, Zr: 0.005-0.5%, Y: 0.005-
Contains 0.5% of one kind or two kinds or more, and 5 μm
A slab excellent in toughness and mechanical properties, characterized by having 100 or more of the following precipitates / mm ² . 3. C: 0.005 to 0.30 in mass%.
%, Si: 1.0% or less, Mn: 2.0% or less, Al:
0.005 to 0.1%, and Hf: 0.005 to 0.
5%, Zr: 0.005-0.5%, Y: 0.005-
A steel material excellent in toughness and mechanical properties, characterized by heating and then hot working a slab containing 0.5% of one kind or two or more kinds. 4. C: 0.005 to 0.30 in mass%.
%, Si: 1.0% or less, Mn: 2.0% or less, Al:
0.005 to 0.1%, and Hf: 0.005 to 0.
5%, Zr: 0.005-0.5%, Y: 0.005-
Contains 0.5% of one kind or two kinds or more, and 5 μm
A slab having the following deposits of 100 pieces / mm ² or more,
Steel material with excellent toughness and mechanical properties characterized by hot working after heating. 5. The hot working after heating the slab is a rolling working with a total reduction ratio of 3 or less.
Steel materials with excellent toughness and mechanical properties described.