JPH10310844A - Non-tempered high-strength angle iron with excellent hot-dip galvanizing crack resistance - Google Patents

Abstract

(57) [Problem] To provide a non-heat treated high-strength angle iron having excellent hot-dip galvanizing crack resistance by controlling the structure of a base material. A non-heat treated high-strength angle iron excellent in hot-dip galvanizing crack resistance of a base material characterized in that a main structure is a mixed structure of ferrite and bainite.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-refined high-strength angle iron having excellent hot-dip galvanizing resistance, which is used in steel towers and is hot-dip galvanized for rust prevention.

[0002]

2. Description of the Related Art In recent years, high-strength steel materials for the purpose of reducing the weight of steel materials used have been actively used in various fields. Such a tendency has also appeared in steel materials for power transmission towers, and steel materials having a tensile strength of 590 MPa class are currently used. In addition, large power transmission towers are often constructed in the mountains, and further higher tension is required to reduce costs in transporting materials. Steel tower steel is hot-dip galvanized after construction to make it maintenance-free. Angle iron for steel towers (for example, equilateral equiangular angle steel) can be used as a steel tower without welding, so plating crack susceptibility of the base material is considered important,
In the case of high-strength angle iron of 690 MPa or more, plating cracks occur at the time of plating from a drilled portion for bolt joining, which is a great hindrance to increasing strength.

[0003] With respect to high-strength steel to be hot-dipped, Japanese Patent Application Laid-Open No. 58-84959 and Japanese Patent Application Laid-Open No.
Techniques such as No. 1316 have been proposed, but all of them relate to steel materials that prevent cracks occurring at welded portions, and there is little knowledge about high-strength steels that prevent cracks from being formed in bolted holes. It is the current situation.

[0004]

SUMMARY OF THE INVENTION The present invention is to fundamentally solve the above-mentioned problems, and is a non-refined high-strength mountain shape having excellent hot-dip galvanizing crack resistance by controlling the structure of the base material. It is intended to provide steel.

[0005]

DISCLOSURE OF THE INVENTION The present invention has been made to achieve this object, and (1) a base material having a main structure comprising a mixed structure of ferrite and bainite; Non-tempered high-strength angle iron with excellent galvanizing cracking characteristics. (2) The non-heat treated high-strength angle iron according to (1), which satisfies Ceq: 0.35 to 0.65.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention obtained a tensile test method shown in FIG. 1 from a hot-rolled angle iron which was obtained by changing the amounts of C and Mn and further changing the amounts of Ti, Nb and V added. And the relationship between the room temperature tensile strength and the elongation during the plating bath tension was examined from the viewpoint of the structure. FIG. 2 shows the results of the study. Less than,
A structure mainly composed of ferrite and bainite is abbreviated as "ferrite-bainite structure", and a structure mainly composed of ferrite and pearlite structure is abbreviated as "ferrite-pearlite structure".

From FIG. 2, it has been found that the room-temperature strength and the elongation at the time of tension in the plating bath greatly differ depending on the structure, and the elongation is remarkably improved in the ferrite-bainite structure compared to the ferrite-pearlite structure at the same strength.

As a result, it has been found that an elongation of 15% or more in tensile in a bath, which is a measure for preventing cracks in the base material during hot-dip galvanizing, can be secured even at high strength. The present invention is effective when the main structure is a mixed structure of ferrite and bainite, but the fraction of bainite is 15% to
80% is desirable. Further, in consideration of use as a structural steel, a component range of Ceq: 0.35 to 0.65 or less is desirable. In addition, if some kind of welding is required, such as attachment of a jig, Pcm: 0.18 to 0.35
And The preferred ranges of the chemical components to be added are as follows.

C: 0.08 to 0.20% (hereinafter,% indicates% by weight) C is an element essential for increasing the strength. If it is less than 0.08%, it is difficult to obtain high strength, and if it exceeds 0.20%, the toughness of the steel is significantly deteriorated.
It is better to limit it to 20% or less.

Si: 0.6% or less Si is related to the appearance after plating, and if it exceeds 0.6%, plating burn tends to occur. Therefore, 0.
It is better to limit it to 6% or less.

Mn: 1.2 to 2.5% Mn is an essential element in view of strength and toughness.
If it is less than 2.5%, it is difficult to form bainite even in combination with other alloying elements. If it exceeds 2.5%, hardenability is increased, coarse bainite is formed, and toughness is significantly deteriorated. % And not more than 2.5%.

P: unavoidable impurity level P segregates at the grain boundaries and degrades toughness. However, since it is sufficiently reduced by the current refining technology, the upper limit is not limited.
Lower is more desirable.

S: Inevitable impurity level S is mainly present in the form of inclusions in steel and causes deterioration of the material due to embrittlement. However, since the current refining technology is sufficiently reduced, the upper limit is limited. No, but lower is better.

Cu: 2.0% or less Cu is an effective element for increasing the strength of steel.
When added in excess of 0%, Cu cracks are likely to occur. Therefore, it is better to limit it to 2.0% or less.

Ni: 2.0% or less Ni is an element effective for increasing the strength and toughness of steel, but is preferably limited to 2.0% or less in consideration of economy.

Cr: 1.0% or less Cr is an element effective for increasing the strength of steel.
If added in excess of 0%, the toughness of the steel deteriorates.
It is better to limit it to 1.0% or less.

Mo: 1.0% or less Mo is an element effective for increasing the strength of steel.
If added in excess of 0%, the toughness of the steel will be significantly degraded, so it is better to limit it to 1.0% or less.

Ti: 0.005 to 0.20% Ti is effective for refining the structure, and increases the strength of the steel by precipitation strengthening when added in a small amount. If the addition is less than 0.005%, the effect is not recognized. If the addition exceeds 0.20%, the precipitate becomes coarse and the toughness of the steel is remarkably deteriorated. Therefore, the addition is limited to 0.005 to 0.20%. Is good.

Nb: 0.20% or less, V: 0.20% or less Nb and V are effective elements for increasing the strength of steel by precipitation strengthening with a small amount of addition, but exceeding 0.20%. If added, the toughness of the steel is remarkably deteriorated.
% Or less, and one or two of them can be added as needed.

Ca: 0.004% or less Ca is an element that can significantly improve the hot-dip galvanizing crack resistance by adding Ca. However, 0.00
If it is added in excess of 4%, Ca-OS clusters are generated and the cleanliness of the steel decreases. Therefore, the content of Ca is preferably limited to 0.004% or less.

Al: 0.005 to 0.60% Al may be added for deoxidation in the present invention. In this case, the usual addition amount (0.005 to 0.60%) is used.
%). Treated as inevitable impurities in Si deoxidation in section steel and the like.

B: Level of Inevitable Impurities While B significantly improves the hardenability of steel, it significantly deteriorates the hot-dip galvanizing cracking resistance of the welded portion, and is therefore controlled to 2 ppm or less when welded. The steel of the present invention is not targeted for welding work in principle, and has a hole drilling degree, and the upper limit of B is about 5 ppm. However, lower is more desirable.

[0023]

【Example】

[Example 1] After heating each steel to 1250 ° C, a hot rolled 350x350x35 equi-equal thickness angle steel was subjected to microstructure, strength at room temperature, and a connection bolt, and then melted. It was immersed in a galvanizing bath, and it was confirmed whether or not cracks were generated from the drilled portion. Table 1 shows the results.
Example No. Nos. 1 to 10 each have a ferrite-bainite structure satisfying the present invention, and therefore have a 690 MPa
It has the above TS and no cracks are observed in the drilled portion. On the other hand, in Comparative Example No. Examples 12 to 13 each have a ferrite-pearlite structure. Despite having a lower TS than that of Nos. 1 to 10, cracks occur in the drilled portion.

Example 2 Each steel was heated to 1270 ° C.
When 250 × 250 × 25 equilateral equal thickness angle irons were cooled at various cooling rates after hot rolling, the structure, strength at room temperature, and the presence or absence of cracks from the drilled portion during hot-dip galvanizing were examined. . Table 2 shows the results. Comparative Example No. 1
Nos. 4, 17, and 20 are all cooling materials having a ferrite-pearlite structure, so that cracks due to plating have occurred. On the other hand, in Example No. 15, 16, 18, 1
Nos. 9, 21, and 22 are steel sheets obtained by applying accelerated cooling to these steels and having a microstructure of ferrite-bainite, which suppresses cracking due to plating despite high strength.

As described above, in the present invention, the main structure only needs to be a mixed structure of ferrite and bainite, and there is no need to stick to the structure and manufacturing conditions of the examples. The effects of the present invention can be achieved if the above structure is obtained by selecting the alloy design and the manufacturing conditions.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

According to the present invention, it is possible to provide an angle iron that does not crack in a bolt hole processed portion or the like even when hot-dip galvanizing is performed for rust prevention in a steel tower or the like.

[Brief description of the drawings]

FIG. 1 is a diagram showing a tensile test method for a base material in molten zinc.

FIG. 2 is a graph showing the effect of the microstructure on the relationship between room-temperature strength and elongation during tension in molten Zn.

Claims (2)

[Claims] 1. A non-refined high-strength angle iron having excellent hot-dip galvanizing cracking resistance in a base metal part, wherein a main structure is a mixed structure of ferrite and bainite. 2. The non-heat treated high-strength angle iron according to claim 1, wherein Ceq satisfies Ceq: 0.35 to 0.65.