JP3895002B2 - Non-tempered high-tensile steel with excellent resistance to hot-dip galvanizing cracking - Google Patents

Description

【００２７】
【表２】

【００２８】
【発明の効果】
本発明によれば、鉄塔、橋梁、建築物などの防錆のため溶融亜鉛メッキが施されてもボルト穴加工部等で割れが発生しない鋼材を提供することができる。
【図面の簡単な説明】
【図１】溶融亜鉛中における母材の引張試験法を示す図。
【図２】常温強度と溶融Ｚｎ中引張時の伸びの関係に及ぼすミクロ組織の影響を示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength steel excellent in hot-dip galvanized cracking resistance that is hot-dip galvanized for rust prevention of steel towers, bridges, buildings, and the like.
[0002]
[Prior art]
In recent years, high-strength steel materials aimed at 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 are currently used. In addition, large transmission towers are often built in the mountains, and higher tension is required to reduce the cost of transporting materials. Steel steel for steel towers is hot dip galvanized to be maintenance-free after construction. Steel towers (for example, equilateral equal thickness steel) can be made into steel towers without welding, so the susceptibility to plating cracks in the base metal is regarded as important. In this case, plating cracks are generated from the perforated portion for joining the bolts of the shape steel during plating, which greatly hinders high strength.
[0003]
As for high strength steel to be hot dip plated, techniques such as Japanese Patent Application Laid-Open No. 58-84959 and Japanese Patent Application Laid-Open No. 59-11316 have been proposed. It is related to steel, and there is little knowledge about high-strength steel that prevents cracks from bolt hole processed parts.
[0004]
[Problems to be solved by the invention]
The present invention is intended to fundamentally solve the above-mentioned problems, and an object thereof is to provide a non-tempered high-tensile steel excellent in hot-dip galvanized cracking resistance by controlling the structure of a base material. It is.
[0005]
[Means for Solving the Problems]
The present invention has been made to achieve this object, and in terms of weight%, C: 0.08 to 0.20%, Si: 0.6% or less, and Mn: 1.2 to 2.5. In addition, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Ti: 0.005 to 0.20% , Nb: 0.20% or less, Ca: wherein one or two or more selected from 0.004% or less, and the balance Fe and unavoidable impurities, Ri Do from primary tissue ferrite and bainite mixed structure, 15% fraction of bainite to 80% (excluding the bainite fraction 15%) non-heat excellent tensile strength resistant galvanized cracking characteristics of the base metal portion, wherein the at least 690MPa to be a High quality steel.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors collected the tensile test method shown in FIG. 1 from steel materials obtained by hot rolling steel with varying amounts of C and Mn, and further varying the amounts of addition of Ti, Nb, and V. The relationship with the elongation at the time of plating bath tension was examined from the viewpoint of the structure. The examination results are shown in FIG. Hereinafter, a structure in which the main structure is composed of ferrite and bainite is abbreviated as “ferrite-bainite structure”, and a structure in which the main structure is composed of ferrite and pearlite structure is abbreviated as “ferrite-pearlite structure”. .
[0007]
From FIG. 2, it was found that the normal temperature strength and the elongation at the time of plating bath tension differ greatly depending on the structure, and the elongation is remarkably improved when compared with the ferrite-pearlite structure at the same strength in the ferrite-bainite structure.
[0008]
As a result, it has been found that an elongation of 15% or more in the bath tension, which is a measure for preventing cracks in the base metal part during hot dip galvanization, can be secured even at high strength.
In the present invention, the effect is recognized if the main structure is a mixed structure of ferrite and bainite, but the fraction of bainite is preferably 15% to 80%. When considering use as structural steel, Ceq: 0.35 to 0.65, Pcm: 0.18 to 0.35, and the chemical composition to be added preferably has the following component ranges.
[0009]
C: 0.08 to 0.20% (hereinafter% indicates% by weight)
C is an essential element for increasing the strength. If it is less than 0.08%, it is difficult to obtain a high strength, and if it exceeds 0.20%, the toughness of the steel is remarkably deteriorated. Therefore, it should be limited to 0.08% or more and 0.20% or less.
[0010]
Si: 0.6% or less Si is related to the appearance after plating. If it exceeds 0.6%, plating burn tends to occur. Therefore, it is good to limit to 0.6% or less.
[0011]
Mn: 1.2 to 2.5%
Mn is an essential element in terms of strength and toughness, but if it is less than 1.2%, it is difficult to form bainite even when combined with other alloy elements, and if it exceeds 2.5%, the hardenability becomes high. Since coarse bainite is generated and the toughness is remarkably deteriorated, Mn is preferably limited to 1.2% or more and 2.5% or less.
[0012]
P: The inevitable impurity level P is segregated at the grain boundary and deteriorates toughness, but is sufficiently reduced by the current refining technique, so the upper limit value is not limited, but it is more desirable as it is lower.
[0013]
S: The inevitable impurity level S exists in steel mainly in the form of inclusions, and causes deterioration of the material due to embrittlement. However, since the current refining technology is sufficiently reduced, the upper limit value is not limited. The lower the better.
[0014]
Cu: 2.0% or less Cu is an element effective for increasing the strength of steel, but Cu cracking tends to occur when added over 2.0%. Therefore, it should be limited to 2.0% or less.
[0015]
Ni: 2.0% or less Ni is an element effective for increasing the strength of steel and improving toughness, but it is preferable to limit it to 2.0% or less in consideration of economy.
[0016]
Cr: 1.0% or less Cr is an element effective for increasing the strength of steel, but if added over 1.0%, the toughness of the steel deteriorates, so it is limited to 1.0% or less. Good.
[0017]
Mo: 1.0% or less Mo is an element effective for increasing the strength of steel, but if added over 1.0%, the toughness of the steel is significantly deteriorated, so it is limited to 1.0% or less. Is good.
[0018]
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 less than 0.005% is added, the effect is not recognized, and if added over 0.20%, precipitates become coarse and the toughness of the steel deteriorates significantly, so the content is limited to 0.005 to 0.20%. It is good.
[0019]
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 if added over 0.20%, steel Both are limited to 0.20% or less, and one or two kinds can be added as necessary.
[0020]
Ca: 0.004% or less Ca is an element capable of remarkably improving the resistance to hot dip galvanizing cracking when added. However, if added over 0.004%, Ca—O—S clusters are generated and the cleanliness of the steel is lowered. Therefore, it is better to limit Ca to 0.004% or less.
[0021]
Al: 0.005 to 0.60%
In the present invention, Al may be added for deoxidation, and in that case, the usual addition amount (0.005 to 0.60%) is used. Treated as an inevitable impurity in Si deoxidation with mold steel.
[0022]
B: The inevitable impurity level B is controlled to 2 ppm or less when welded in order to remarkably improve the hardenability of the steel while remarkably deteriorating the resistance to hot dip galvanizing cracking of the weld. As a general rule, the steel of the present invention is not intended for welding construction, but is about the bolt hole processing, and the upper limit value of B is about 5 ppm. However, the lower the better.
[0023]
【Example】
[Example 1]
After each steel is heated to 1250 ° C. and hot-rolled steel sheet, the structure, strength at room temperature, and drilling of bolts for joining are performed and then immersed in a hot dip galvanizing bath and cracks are generated from the drilled part. Confirmed whether or not. The results are shown in Table 1. Example No. Since 1 to 10 all have a ferrite-bainite structure that satisfies the present invention, they have a TS of 690 MPa or more and no cracks are observed in the drilled portion. In contrast, Comparative Example No. Since all of Nos. 12 to 13 have a ferrite-pearlite structure, Example No. Although TS is low compared with 1-10, the crack has generate | occur | produced in the drilling part.
[0024]
[Example 2]
Each steel was heated to 1270 ° C., and the steel sheet cooled at various cooling rates after hot rolling was examined for the structure, the strength at normal temperature, and the presence or absence of cracks from the punched portion during hot dip galvanization. The results are shown in Table 2. Comparative Example No. Since all of Nos. 14, 17 and 20 are a cooling material and have a ferrite-pearlite structure, cracks associated with plating occur. On the other hand, Example No. 15, 16, 18, 19, 21, and 22 are steel plates in which accelerated cooling is applied to these steels and the structure is ferrite-bainite, and cracks associated with plating are suppressed despite the increase in strength.
[0025]
As described above, in the present invention, the main structure may be a mixed structure of ferrite and bainite, and it is not necessary to stick to the structure and manufacturing conditions of the examples. The effects of the present invention can be achieved if the above-described structure is obtained by alloy design and selection of manufacturing conditions. Moreover, it is not limited to the steel plate of a present Example.
[0026]
[Table 1]

[0027]
[Table 2]

[0028]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, even if hot dip galvanization is given for rust prevention, such as a steel tower, a bridge, and a building, the steel materials which a crack does not generate | occur | produce in a bolt hole processed part etc. can be provided.
[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 influence of the microstructure on the relationship between normal temperature strength and elongation during tension in molten Zn.

Claims (1)

C: 0.08-0.20%, Si: 0.6% or less, Mn: 1.2-2.5%, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Ti: 0.005 to 0.20%, Nb: 0.20% or less, Ca: 0.004% or less wherein one or two or more selected from, and the balance Fe and unavoidable impurities, mainly tissue Ri is Do a mixed structure of ferrite and bainite, the fraction of bainite is 15% to 80% (however, bainite fraction Non-tempered high-tensile steel having a tensile strength of 690 MPa or more and excellent in hot-dip galvanizing cracking resistance of the base material, characterized in that 15 % is excluded) .

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