【発明の詳細な説明】[Detailed description of the invention]
本発明は歪時効脆化の少ない構造用高張力鋼に
関するものである。
海洋構造物、フイツテング素材、タンク等の構
造用鋼には例えば強度が60キロ以上の高張力鋼が
要望されることが多く、またこれらの構造用鋼に
は板厚が例えば100mmと云うような厚手が要望さ
れる。
ところで、従来の構造用高張力鋼としては例え
ばMo、Ti、Nb、等の強化元素を添加した高張力
鋼があり、強度特性の高いものであるが、前記強
化元素は鋼の価格アツプの一因となること、これ
らの含有量が多くなると鋼の靭性が劣化の傾向に
あること、および前記強化元素を添加した高張力
鋼は板厚の厚みに制限を受ける等の問題がある。
またＶを添加するとともにＮを添加した高張力
鋼があり、これは厚手の高張力鋼として有用であ
るが、該高張力鋼には歪時効により脆化すると云
う欠点があつて、とに歪時効で靭性が劣化し、構
造用鋼としての安全性に問題をきたす。
このような実情に鑑みて、本発明者達は種々研
究検討したところ、ＶとＮを添加した鋼に、Ca
またはMgを添加すると、歪時効後も衝撃吸収エ
ネルギーの低下が殆んどないこと、即ち歪時効に
よる靭性劣化が防止できることを見出し、この知
見に基づき歪時効脆化の少ない構造用高張力鋼を
提供するものである。
そしてその要旨は、
Ｃ：0.12〜0.25％、Si：0.1〜1.0％、Mn：0.5〜
2.0％、Ｐ：0.025％以下、Ｓ：0.01％以下、Al：
0.006〜0.06％、Ｖ：0.05〜0.2％、Cu：0.1〜0.5
％、Ni：0.1〜0.5％、Cr：0.05〜0.4％、Ti：
0.005〜0.03％、Ｎ：0.01〜0.03％に、Ca：0.0005
〜0.0070％、Mg：0.0005〜0.0070％、の少なくと
も１種を含有し、残部鉄および不可避不純物より
なる歪時効脆化の少ない構造用高張力鋼にある。
次に本発明を詳細に述べる。
まず本発明鋼における各成分元素の限定理由を
説明する。Ｃは強度を高めるために有効であるか
ら、0.12％以上含有せしめる。一方含有量があま
り多くなると溶接性を害するので0.25％以下とす
る。
Siは製鋼作業上から0.10％以上必要で、1.0％
を超えると靭性が劣化するので上限を1.0％とす
る。
Mnは強度と靭性に有効な成分で、その効果を
奏するためには0.5％以上必要であるが、一方含
有量が過多になると脆化するので2.0％以下とす
る。
Ｐは靭性を劣化するので、0.025％以下とす
る。Ｓも前記Ｐと同様な理由から0.01％以下とす
る。
Alは脱酸しキルド鋼とするため0.006％以上必
要である。一方AlはＮとの親和力が強いのでこ
の含有量が多くなると後記するVNの生成を阻害
するから0.06％以下とする。
Ｖは本発明鋼で重要な成分で後記するＮと析出
物VNを形成し、析出強化作用をもたせるために
0.05％以上添加する。一方この含有量が0.20％を
超えると析出強化作用が飽和し、これ以上含有さ
せても強度上昇は少ないから上限を0.20％とす
る。
Ｎは前記Ｖとともに本発明鋼で重要な成分で、
析出物VNにより強度を高めるよう従来鋼のＮ含
有量約0.0060％に較べ多い0.010％以上含有させ
る。一方0.03％以上になると製鋼作業で健全な鋼
塊を作ることがむずかしいので上限を0.03％とす
る。
このVNによる析出強化は、他のNb、Tiなどの
析出窒化物と比べて厚手の空冷材のような、緩冷
でも効果を発揮するという特徴がある。
CaとMgはともに歪時効による靭性劣化防止に
極めて有効な成分で、この効果を奏するために
Caは0.0005〜0.0070％、Mgは0.005〜0.0070％含
有される。Ca、Mgとも0.0005％以下では前記の
効果が得られず、0.0070％以上では靭性が劣化す
る。このCaとMgはともに含有させてもよいが、
いずれか一方を含有させてもよい。
第１図にCa含有による歪時効による靭性劣化
防止効果の１例を示す。この図から明らかなよう
に本発明鋼のCa含有鋼Ａ（Ca：40ppm）はCaを
含有しない鋼Ｂにくらべ、歪時効による衝撃吸収
エネルギーvE−20の劣化が非常に少ない。尚、
図示例のベース成分は0.18C―0.45Si―1.45Mn―
0.02Al―0.08V―0.0190Nでｔ＝50mmである。この
効果の理由は現在必ずしも明らかになつていない
がCaの窒化物が形成されてVN析出物以外のＮを
無害化していると考えられる。
Cu、Ni、Cr、Tiは、すべて強度を高める成分
で、各々、効果を発揮するには、少なくとも
Cu0.1％、Ni0.1％、Cr0.05％、Ti0.005％は必要
で、コスト的に不利であると同時に多量添加は、
靭性に問題をきたすので、その上限をCu0.5％、
Ni0.5％、Cr0.4％、Ti0.03％とした。
これらの元素は、同時に添加することによつて
その効果が大きい。
本発明鋼は転炉あるいは電気炉などで溶製さ
れ、分塊圧延あるいは連続鋳造された後、熱間圧
延され圧延まま、あるいはその後焼準される。
本発明鋼はＶとＮをともに多量含有させている
ことからVN析出による強化作用は例えば熱間圧
延後の冷却速度が遅くとも、十分発揮されるので
厚手材の高張力鋼が製造される。
以下実施例を示す。
第１表に実施例の化学成分を示し、第２表に機
械的性質を示す。この実施例においてＡ―１、Ｈ
―１〜Ｈ―９は本発明鋼で、Ａ―５は比較鋼であ
る。
第２表から明らかなように、本発明鋼は強度σ
B（引張り強さ）が60キロ級の高強度であり、歪
時効後も靭性の劣化が少ないことが認められる。
The present invention relates to a high tensile strength steel for structural use with less strain aging embrittlement. High tensile strength steel with a strength of 60 kg or more is often required for structural steels such as marine structures, building materials, tanks, etc., and these structural steels also require plate thicknesses of 100 mm or more. Thick material is required. By the way, conventional high-tensile strength steels for structural use include high-strength steels to which reinforcing elements such as Mo, Ti, and Nb are added, and these have high strength properties, but the above-mentioned reinforcing elements contribute to the increase in the price of steel. There are problems such as the fact that the toughness of the steel tends to deteriorate as the content of these elements increases, and that high-strength steels to which the above-mentioned strengthening elements are added are limited in plate thickness. There is also high-strength steel that has both V and N added, and this is useful as a thick high-strength steel, but this high-strength steel has the disadvantage of becoming brittle due to strain aging. The toughness deteriorates due to aging, which poses a safety problem as a structural steel. In view of these circumstances, the present inventors conducted various research studies and found that Ca
We also discovered that adding Mg causes almost no drop in shock absorption energy even after strain aging, which means that deterioration of toughness due to strain aging can be prevented. Based on this knowledge, we have developed high-strength steel for structural use with less strain-aging embrittlement. This is what we provide. And the gist is: C: 0.12~0.25%, Si: 0.1~1.0%, Mn: 0.5~
2.0%, P: 0.025% or less, S: 0.01% or less, Al:
0.006-0.06%, V: 0.05-0.2%, Cu: 0.1-0.5
%, Ni: 0.1~0.5%, Cr: 0.05~0.4%, Ti:
0.005-0.03%, N: 0.01-0.03%, Ca: 0.0005
-0.0070%, Mg: 0.0005-0.0070%, and the balance is iron and unavoidable impurities, making it a structural high-strength steel with little strain aging embrittlement. Next, the present invention will be described in detail. First, the reason for limiting each component element in the steel of the present invention will be explained. Since C is effective for increasing strength, it is contained in an amount of 0.12% or more. On the other hand, if the content is too high, weldability will be impaired, so the content should be 0.25% or less. Si is required at least 0.10% for steelmaking work, and 1.0%
If the content exceeds 1.0%, the toughness deteriorates, so the upper limit is set at 1.0%. Mn is an effective component for strength and toughness, and in order to achieve this effect it is necessary to have a content of 0.5% or more, but on the other hand, too much content causes embrittlement, so the content should be 2.0% or less. Since P deteriorates toughness, it should be kept at 0.025% or less. S is also set to 0.01% or less for the same reason as P above. Al is required at 0.006% or more to deoxidize and make killed steel. On the other hand, since Al has a strong affinity for N, if its content increases, it inhibits the formation of VN, which will be described later, so it is set to 0.06% or less. V is an important component in the steel of the present invention, and is used to form precipitates VN with N, which will be described later, to provide precipitation strengthening effect.
Add 0.05% or more. On the other hand, if the content exceeds 0.20%, the precipitation strengthening effect will be saturated, and even if the content exceeds this, the strength will not increase much, so the upper limit is set at 0.20%. N is an important component in the steel of the present invention along with the above-mentioned V,
In order to increase the strength by precipitated VN, the N content is increased to 0.010% or more compared to the approximately 0.0060% of conventional steel. On the other hand, if the content exceeds 0.03%, it will be difficult to produce a healthy steel ingot during steelmaking operations, so the upper limit is set at 0.03%. Precipitation strengthening by VN is more effective than other precipitated nitrides such as Nb and Ti in that it is effective even when cooled slowly, such as in thick air-cooled materials. Both Ca and Mg are extremely effective components for preventing toughness deterioration due to strain aging, and in order to achieve this effect,
Ca is contained in the range of 0.0005 to 0.0070%, and Mg is contained in the range of 0.005 to 0.0070%. If both Ca and Mg are less than 0.0005%, the above effect cannot be obtained, and if it is more than 0.0070%, the toughness deteriorates. Both Ca and Mg may be included, but
Either one may be included. Figure 1 shows an example of the effect of Ca inclusion on preventing deterioration of toughness due to strain aging. As is clear from this figure, the Ca-containing steel A (Ca: 40 ppm), which is the steel of the present invention, exhibits very little deterioration in impact absorption energy vE- 20 due to strain aging compared to steel B which does not contain Ca. still,
The base components in the illustrated example are 0.18C―0.45Si―1.45Mn―
0.02Al-0.08V-0.0190N and t=50mm. The reason for this effect is not necessarily clear at present, but it is thought that Ca nitride is formed and renders N other than VN precipitates harmless. Cu, Ni, Cr, and Ti are all components that increase strength, and each requires at least
Cu0.1%, Ni0.1%, Cr0.05%, and Ti0.005% are necessary, and at the same time are disadvantageous in terms of cost, adding large amounts is
Since it causes problems with toughness, the upper limit is set to 0.5% Cu.
Ni 0.5%, Cr 0.4%, Ti 0.03%. Adding these elements at the same time increases the effect. The steel of the present invention is melted in a converter or electric furnace, bloomed or continuously cast, and then hot rolled and as rolled, or subsequently normalized. Since the steel of the present invention contains a large amount of both V and N, the strengthening effect due to VN precipitation is sufficiently exerted even if the cooling rate after hot rolling is slow, so that a thick high-strength steel can be produced. Examples are shown below. Table 1 shows the chemical components of the examples, and Table 2 shows the mechanical properties. In this example, A-1, H
-1 to H-9 are the steels of the present invention, and A-5 is the comparative steel. As is clear from Table 2, the steel of the present invention has a strength σ
It has high strength with B (tensile strength) of 60 kg class, and it is recognized that there is little deterioration in toughness even after strain aging.
【表】【table】
【表】【table】
【図面の簡単な説明】[Brief explanation of the drawing]
第１図は本発明鋼のCa含有鋼は歪時効により
靭性劣化がすくないことを示す線図。
FIG. 1 is a diagram showing that the toughness of the Ca-containing steel of the present invention is less likely to deteriorate due to strain aging.