JP3895002B2 - Non-tempered high-tensile steel with excellent resistance to hot-dip galvanizing cracking - Google Patents
Non-tempered high-tensile steel with excellent resistance to hot-dip galvanizing cracking Download PDFInfo
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- JP3895002B2 JP3895002B2 JP12089597A JP12089597A JP3895002B2 JP 3895002 B2 JP3895002 B2 JP 3895002B2 JP 12089597 A JP12089597 A JP 12089597A JP 12089597 A JP12089597 A JP 12089597A JP 3895002 B2 JP3895002 B2 JP 3895002B2
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Description
【0001】
【発明の属する技術分野】
本発明は、鉄塔、橋梁、建築物などの防錆のために、溶融亜鉛メッキを施される耐溶融亜鉛メッキ割れ特性に優れた高張力鋼に関する。
【0002】
【従来の技術】
近年、使用鋼材の重量低減を目的とした高強度鋼材が様々な分野で積極的に使用されるようになってきた。送電用鉄塔向け鋼材にもこのような傾向が現れてきており、現在引張強さが590MPa級の鋼材が用いられている。また、大型送電鉄塔は、山中に建設されることが多く、資材の運搬におけるコスト低減のため更なる高張力化が求められている。鉄塔用鋼材は建設された後にメンテナンスフリーとするため、溶融亜鉛メッキが施される。鉄塔用の形鋼(例えば等辺等厚山形鋼)は溶接施工をすることなく鉄塔とすることが可能であるため、母材のメッキ割れ感受性が重要視されるが、690MPa以上の高強度形鋼ではメッキ時に形鋼のボルト接合のための穴開け部からのメッキ割れが生じ高強度化の大きな妨げとなっている。
【0003】
溶融メッキされる高強度鋼に関しては、従来より特開昭58−84959号公報、特開昭59−11316号公報等の技術が提案されてきたが、いずれも溶接部において発生する割れを防止する鋼材に関するものであり、ボルト穴加工部からの割れを防止する高強度鋼に関しての知見は少ないのが現状である。
【0004】
【発明が解決しようとする課題】
本発明は、上記問題点を根本的に解決するためのものであり、母材の組織制御により耐溶融亜鉛メッキ割れ特性に優れた非調質型高張力鋼を提供することを目的とするものである。
【0005】
【課題を解決するための手段】
本発明は、この目的を達成するためになされたもので、重量%で、C:0.08〜0.20%と、Si:0.6%以下と、Mn:1.2〜2.5%とを含有し、更に、Cu:2.0%以下、Ni:2.0%以下、Cr:1.0%以下、Mo:1.0%以下、Ti:0.005〜0.20%、Nb:0.20%以下、Ca:0.004%以下から選択された一種又は二種以上を含み、残部Fe及び不可避的不純物からなり、主たる組織がフェライトとベイナイトの混合組織からなり、ベイナイトの分率が15%乃至80%(但し、ベイナイト分率15 % は除く)であることを特徴とする母材部の耐溶融亜鉛メッキ割れ特性に優れた引張り強さが690MPa以上の非調質高張力鋼である。
【0006】
【発明の実施の形態】
本発明者らは、C、Mn量を変化させ、さらにTi、Nb、V添加量を変化させた鋼を熱間圧延した鋼材より、図1に示す引張試験法を採取し、常温引張強度とメッキ浴引張時の伸びとの関係を組織の観点から検討した。図2に検討結果を示す。以下、主たる組織がフェライトとベイナイトで構成されているものを、 “フェライト−ベイナイト組織”と略称し、主たる組織がフェライトとパーライト組織で構成されているものを、“フェライト−パーライト組織”と略称する。
【0007】
図2から、常温強度とメッキ浴引張時の伸びは組織により大きく異なり、フェライト−パーライト組織に比べフェライト−ベイナイト組織では同一強度で比較した場合、伸びが著しく向上することを見出した。
【0008】
その結果、溶融亜鉛メッキ時に母材部での割れが防止される目安となる浴中引張での伸び15%以上が高強度においても確保できることが判明した。
本発明は主たる組織がフェライトとベイナイトの混合組織であれば効果が認められるがベイナイトの分率は15%〜80%が望ましい。また構造用鋼としての使用を考慮した場合、Ceq:0.35〜0.65、Pcm:0.18〜0.35で、添加する化学組成は以下の成分範囲が望ましい。
【0009】
C:0.08〜0.20%(以下%は重量%を示す)
Cは強度を高めるのに必須の元素である。0.08%未満では高強度を得るのが困難で、0.20%を越えると鋼の靱性が著しく劣化するため、0.08%以上、0.20%以下に限定するのがよい。
【0010】
Si:0.6%以下
Siはメッキ後の外観状況と関係しており、0.6%を越えるとメッキ焼けが発生しやすくなる。よって、0.6%以下に限定するのがよい。
【0011】
Mn:1.2〜2.5%
Mnは強度、靱性の面から必須の元素であるが、1.2%未満では他の合金元素と組合せてもベイナイトを形成させることが難しく、2.5%を越えると焼き入れ性が高くなり粗いベイナイトが生成し、靱性が著しく劣化するため、Mn:1.2%以上2.5%以下に限定するのがよい。
【0012】
P:不可避不純物レベル
Pは粒界に偏析し、靱性を劣化するが、現状の精錬技術で十分に低減されているため、上限値は限定しないが、低いほど望ましい。
【0013】
S:不可避不純物レベル
Sは主に介在物の形態で鋼中に存在し、脆化により材質の劣化を引き起こすが、現状の精錬技術では十分に低減されているため、上限値は限定しないが、低いほど望ましい。
【0014】
Cu:2.0%以下
Cuは鋼の強度を高めるのに有効な元素であるが、2.0%を越えて添加した場合にはCu割れが発生しやすい。よって2.0%以下に限定するのがよい。
【0015】
Ni:2.0%以下
Niは鋼の強度上昇ならびに靱性向上に有効な元素であるが、経済性を考慮し、2.0%以下に限定するのがよい。
【0016】
Cr:1.0%以下
Crは鋼の強度を高めるのに有効な元素であるが、1.0%を越えて添加すると鋼の靱性を劣化させるため、1.0%以下に限定するのがよい。
【0017】
Mo:1.0%以下
Moは鋼の強度を高めるのに有効な元素であるが、1.0%を越えて添加すると鋼の靱性を著しく劣化させるため、1.0%以下に限定するのがよい。
【0018】
Ti:0.005〜0.20%
Tiは組織の微細化に有効であり、また微量の添加で析出強化により鋼の強度を高める。0.005%未満の添加ではその効果は認められず、0.20%を越えて添加すると析出物が粗くなり、鋼の靱性が著しく劣化するため、0.005〜0.20%に限定するのがよい。
【0019】
Nb:0.20%以下、V:0.20%以下
Nb、Vは微量の添加で析出強化により鋼の強度を高めるのに有効な元素であるが、0.20%を越えて添加すると鋼の靱性を著しく劣化するため、いずれも0.20%以下に限定し、1種または2種を必要に応じて添加できる。
【0020】
Ca:0.004%以下
Caは添加することで耐溶融亜鉛メッキ割れ特性を著しく改善することができる元素である。しかし、0.004%を越えて添加すると、Ca−O−Sのクラスターが発生し、鋼の清浄性が低下してしまう。従って、Caを0.004%以下に限定するのがよい。
【0021】
Al:0.005〜0.60%
Alは本発明においては脱酸のために添加する場合もあり、その場合は通常の添加量(0.005〜0.60%)とする。型鋼などでSi脱酸においては不可避不純物として扱う。
【0022】
B:不可避不純物レベル
Bは鋼の焼き入れ性を著しく向上させる一方、溶接部の耐溶融亜鉛メッキ割れ性を著しく劣化させるため、溶接された場合には2ppm以下に管理されている。本発明鋼は原則として溶接施工を対象とせず、ボルト穴加工程度であり、Bの上限値5ppm程度の管理とする。しかし、低いほど望ましい。
【0023】
【実施例】
[実施例1]
各鋼を1250℃に加熱後、熱間圧延した鋼板について、組織、常温での強度、接合用ボルトの穴開け加工を施した後に溶融亜鉛メッキ浴に浸漬し穴開け加工部から割れが発生するかどうかを確認した。その結果を表1に示す。実施例No.1〜10はいずれも本発明を満足するフェライト−ベイナイト組織を有しているため690MPa以上のTSを有しかつ穴開け加工部に割れは全く認められない。これに対し比較例No.12〜13はいずれもフェライト−パーライト組織を有しているため、実施例No.1〜10に比べTSが低いにもかかわらず穴開け加工部に割れが発生している。
【0024】
[実施例2]
各鋼を1270℃に加熱し、熱間圧延後種々の冷却速度で冷却した鋼板について、組織、常温での強度、溶融亜鉛メッキ時の穴開け加工部からの割れの有無をを調べた。その結果を表2に示す。比較例No.14、17、20はいずれも放冷材でフェライトーパーライト組織を有しているため、メッキに伴う割れが発生している。これに対し実施例No.15、16、18、19、21、22はこれらの鋼に加速冷却を適用し、組織をフェライト−ベイナイトとした鋼板であり、高強度化にもかかわらずメッキに伴う割れが抑制されている。
【0025】
以上のように本発明は主たる組織がフェライトとベイナイトの混合組織であればよく、実施例の組織、製造条件にこだわる必要はない。合金設計、製造条件の選定によって上述組織が得られれば本発明の効果は達成できる。また、本実施例の鋼板に限定されるものでもない。
【0026】
【表1】
【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)
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CN101974722A (en) * | 2010-10-29 | 2011-02-16 | 河北钢铁股份有限公司唐山分公司 | Steel plate for manufacturing concrete mixer tank and production method |
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JP4677868B2 (en) * | 2005-09-26 | 2011-04-27 | 大同特殊鋼株式会社 | Steel that can be welded with high strength and high toughness, and a method for producing a member using the same |
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CN101974722A (en) * | 2010-10-29 | 2011-02-16 | 河北钢铁股份有限公司唐山分公司 | Steel plate for manufacturing concrete mixer tank and production method |
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JPH10310846A (en) | 1998-11-24 |
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