JP4424485B2 - Steel with excellent cold cracking resistance and steel for high-strength structures - Google Patents
Steel with excellent cold cracking resistance and steel for high-strength structures Download PDFInfo
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本発明は建築構造物、機械構造物、海洋構造物、圧力容器、タンク等の高強度鋼構造物に適用される溶接時の耐低温割れ性にすぐれた鋼材に関する。 The present invention relates to a steel material having excellent cold cracking resistance at the time of welding applied to a high-strength steel structure such as a building structure, a mechanical structure, an offshore structure, a pressure vessel, or a tank.
近年、溶接鋼構造物が大型化する傾向が顕著となっている。大型化すれば重量低減の要望が強くなり、こうした大型溶接鋼構造物に使用される鋼材に対し、より高強度のものを使用する傾向はますます大きくなってくる。 In recent years, the tendency for welded steel structures to increase in size has become prominent. The demand for weight reduction increases as the size increases, and the tendency to use higher strength steel materials for such large welded steel structures is increasing.
鋼の溶接施工時の重要な課題の一つに低温割れの防止がある。低温割れは溶接後、溶接部の温度が常温付近に低下してから発生する割れで、溶接される鋼材の強度が高くなるほど発生しやすくなる。この低温割れは、溶接部分での（ａ）マルテンサイトなど割れの起点になる硬化組織の生成、（ｂ）溶接時に浸入した水素（拡散性水素）および（ｃ）拘束度（残留応力）などに強く影響を受けるとされている。 One of the important issues when welding steel is prevention of cold cracking. The low temperature crack is a crack that occurs after the temperature of the welded portion is lowered to around room temperature after welding, and is more likely to occur as the strength of the steel material to be welded increases. This low-temperature cracking is caused by (a) generation of a hardened structure such as martensite at the welded portion, (b) hydrogen (diffusible hydrogen) permeated during welding, and (c) restraint degree (residual stress). It is said to be strongly affected.
鋼材の強度を高くするためには、一般に焼入れ焼戻しのような熱処理を施すが、この熱処理による強度向上効果をより大きくする目的で、種々の合金元素が添加される。高強度鋼板には、より多くの量の合金元素が添加されているので、溶接後の急冷で焼きが入りやすく、硬化組織を容易に生成し硬度を上昇させる。拡散性水素に対しては、溶接材料にできるだけ低水素のものを使用するなどにより低減できるが、多少の浸入は避けがたい。また拘束度は、溶接形状の工夫などで抑制できるが、高強度になるほど増大する傾向にある。 In order to increase the strength of the steel material, heat treatment such as quenching and tempering is generally performed, and various alloying elements are added for the purpose of increasing the strength improvement effect by this heat treatment. Since a higher amount of alloying element is added to the high-strength steel plate, it is easy to be quenched by rapid cooling after welding, and a hardened structure is easily generated to increase the hardness. Diffusible hydrogen can be reduced by using as low a hydrogen as possible for the welding material, but some penetration is unavoidable. In addition, the degree of restraint can be suppressed by contrivance of the welding shape, but tends to increase as the strength increases.
このような低温割れの対策として、溶接施工前に鋼材の予熱がおこなわれる。予熱することにより、溶接直後の高温からの冷却速度が低下して硬度の上昇が抑止され、浸入水素の拡散による逸散が進行し、熱変形量を減少させて拘束力を低減する、等の効果があると考えられる。低温割れの発生を確実に防止するには、溶接する鋼材の予熱が有効であり、予熱は鋼材の強度が高くなれば必須作業となってくる。 As a countermeasure against such cold cracking, the steel material is preheated before welding. By preheating, the cooling rate from the high temperature immediately after welding decreases and the increase in hardness is suppressed, the dissipation due to diffusion of intrusion hydrogen proceeds, the amount of thermal deformation is reduced, the binding force is reduced, etc. It is considered effective. In order to reliably prevent the occurrence of cold cracking, preheating of the steel material to be welded is effective, and preheating becomes an essential work if the strength of the steel material is increased.
低温割れ防止のための予熱は、強度が６００〜８００ＭＰａの高強度鋼の母材ではその組成にもよるが５０〜１５０℃程度必要である。しかしながら、高温の予熱は溶接施工コストが嵩むばかりか工期が長期化するし、とくに予熱温度が１２５℃を超える場合には、現場の作業環境が著しく劣悪となるので作業者への負担も急激に増加する。 Preheating for preventing low temperature cracking requires about 50 to 150 ° C. depending on the composition of a high strength steel base material having a strength of 600 to 800 MPa. However, high-temperature preheating not only increases welding construction cost but also increases the work period. Especially, when the preheating temperature exceeds 125 ° C, the work environment at the site is remarkably deteriorated and the burden on the worker is drastically increased. To increase.
このため、溶接施工前に行う予熱を低くしても低温割れの発生しない高強度鋼が種々提案されている。たとえば、特許文献１には低Ｃ−Ｎｂ−Ｍｏ−Ｔｉ−Ｂ鋼を用い、圧延直後その温度から焼入れをおこなって炭素当量を低くして強度を高くし、溶接熱影響部の硬度上昇を抑制し、予熱温度を低くする鋼板の発明が開示されている。この場合、板厚が２５ｍｍで引張強さが７８０ＭＰａの鋼が例示されているが、より板厚が増した場合、強度の維持や予熱不要が可能かどうか明らかでない。 For this reason, various high-strength steels that do not cause cold cracking even if the preheating performed before welding is reduced have been proposed. For example, in Patent Document 1, low C—Nb—Mo—Ti—B steel is used, and immediately after rolling, quenching is performed from that temperature to lower the carbon equivalent to increase the strength, thereby suppressing an increase in the hardness of the weld heat affected zone. And the invention of the steel plate which makes a preheating temperature low is indicated. In this case, steel with a plate thickness of 25 mm and a tensile strength of 780 MPa is exemplified, but when the plate thickness is further increased, it is not clear whether the strength can be maintained or preheating is not necessary.
また、特許文献２や特許文献３にも熱間圧延と圧延直後急冷による高強度鋼板の製造方法の発明が開示されている。これらの圧延条件とその直後の急冷条件との組合せにより高強度の鋼板を得る方法は、金属結晶粒径を微細にして、より少ない合金元素量で強度を上げしかも靱性を向上させる効果がある。これらの発明による鋼板では、合金元素量の低減は溶接熱影響部の硬度上昇を抑制するので予熱温度を低下できるが、予熱なしとするまでには至っていない。 Patent Documents 2 and 3 also disclose inventions for manufacturing a high-strength steel sheet by hot rolling and rapid cooling immediately after rolling. The method of obtaining a high-strength steel sheet by combining these rolling conditions and the quenching conditions immediately after that has the effect of increasing the strength and improving the toughness with a smaller amount of alloy elements by reducing the metal crystal grain size. In the steel sheets according to these inventions, the reduction in the amount of alloying elements suppresses the increase in hardness of the weld heat affected zone, so that the preheating temperature can be lowered, but it has not reached the point where there is no preheating.
この低温割れを防止する溶接前の鋼材の必要予熱温度は、たとえば非特許文献１等に示されているように、鋼材の化学組成から求まるＰＣＭ値から推測できる。特許文献４には化学組成を適正化してこの値を０．２５％以下にするとともに、熱間圧延の加熱および圧延条件を適正化し、溶接施工時の予熱温度が１００℃以下であっても溶接割れを発生することのない鋼板の製造技術が開示されている。 The necessary preheating temperature of the steel material before welding to prevent this low temperature cracking can be estimated from the PCM value obtained from the chemical composition of the steel material as shown in Non-Patent Document 1, for example. In Patent Document 4, the chemical composition is optimized to reduce this value to 0.25% or less, and the heating and rolling conditions for hot rolling are optimized so that welding is performed even when the preheating temperature during welding is 100 ° C. or less. A technique for manufacturing a steel sheet that does not cause cracking is disclosed.
溶接時の低温割れ発生の確実な防止には、予熱をおこなう必要があり、ことに鋼材の強度が高い場合にはこれは必須になる。しかしながら、この予熱作業は被溶接物が大きくなってくると、加熱のための手配作業や昇温時間などに多くの工数および時間が費やされ、溶接施工能率を著しく低下させる。このため、この予熱温度をより低くできる鋼材が種々開発され、高強度鋼とはいっても相対的には強度が低く、厚さも大きくない範囲では、予熱温度が５０℃以下または予熱を要しない予熱フリーの鋼材が得られている。しかしながら、少し強度が高くなり、厚さが増してくると、予熱なしでは必ずしも溶接の低温割れを十分に抑止できているとはいえない状態である。 In order to surely prevent the occurrence of cold cracking during welding, it is necessary to preheat, especially when the strength of the steel material is high. However, in this preheating work, when the work to be welded becomes large, a lot of man-hours and time are spent for the preparation work for heating and the heating time, and the welding work efficiency is remarkably lowered. For this reason, various steel materials that can lower the preheating temperature have been developed, and preheating temperature is 50 ° C. or less or preheating that does not require preheating as long as high strength steel is relatively low in strength and thickness is not large. Free steel is obtained. However, if the strength is slightly increased and the thickness is increased, it cannot be said that cold cracking of the welding is not sufficiently suppressed without preheating.
本発明は、含有する合金成分を制御することにより、このような予熱をおこなわなくても、溶接時に低温割れが発生しない耐低温割れ性にすぐれた鋼材および高強度構造物用鋼材を提供するものである。
The present invention provides a steel material excellent in cold cracking resistance and a steel material for high-strength structures that does not generate cold cracking at the time of welding without controlling such an alloy component by controlling the alloy components contained therein. It is.
本発明の要旨は次のとおりである。
Gist of the present invention is Ru der following.
（１）質量％にてＣ：０．０１〜０．１５％、Ｓｉ：０．０２〜０．８％、Ｍｎ：０．６〜３．０％、Ｐ：０．０３％以下、Ｓ：０．０３％以下、ｓｏｌ．Ａｌ：０．００４〜０．０８％、Ｎ：０．０１５％以下、Ｃｏ：０．１〜１０．０％、Ｔｉ：０．００３〜０．３％で、さらにＣｕ：０．１〜２．５％、Ｎｉ：０．１〜１０．０％、Ｃｒ：０．１〜１．５％、Ｍｏ：０．１〜１．５％、Ｖ：０．００１〜０．２％、Ｎｂ：０．００１〜０．０８％およびＢ：０．０００２〜０．００５％のうちの１種以上を含有し残部が鉄および不純物であることを特徴とする溶接の耐低温割れ性にすぐれた鋼材。
( 1 ) By mass%: C: 0.01 to 0.15%, Si: 0.02 to 0.8%, Mn: 0.6 to 3.0%, P: 0.03% or less, S: 0.03% or less, sol. Al: 0.004 to 0.08%, N: 0.015% or less, Co: 0.1 to 10.0% , Ti: 0.003 to 0.3% , and Cu: 0.1 to 2 0.5%, Ni: 0.1 to 10.0%, Cr: 0.1 to 1.5%, Mo: 0.1 to 1.5%, V: 0.001 to 0.2%, Nb: from 0.001 to 0.08% Contact and B: 0.0002 to 0.005% the balance contains one or more of the is excellent in low-temperature cracking of the weld, characterized in that iron and impurities Steel material.
（２）質量％にてＣ：０．０１〜０．１５％、Ｓｉ：０．０２〜０．８％、Ｍｎ：０．６〜３．０％、Ｐ：０．０３％以下、Ｓ：０．０３％以下、ｓｏｌ．Ａｌ：０．００４〜０．０８％、Ｎ：０．０１５％以下、Ｃｏ：０．１〜１０．０％で、さらにＣａ：０．０００２〜０．００５％およびＭｇ：０．０００２〜０．００５％の1種または２種を含有し残部が鉄および不純物であることを特徴とする予熱なし溶接の耐低温割れ性にすぐれた高強度鋼構造物用鋼材。
(2) In mass% C: 0.01 to 0.15%, Si: 0.02 to 0.8%, Mn: 0.6 to 3.0%, P: 0.03% or less, S: 0.03% or less, sol. Al: 0.004 to 0.08%, N: 0.015% or less, Co: 0.1 to 10.0%, Ca: 0.0002 to 0.005%, and Mg: 0.0002 to 0 A steel material for high-strength steel structures having excellent low temperature cracking resistance of preheating-free welding, characterized in that it contains one or two of 0.005% and the balance is iron and impurities .
（３）質量％にてＣ：０．０１〜０．１５％、Ｓｉ：０．０２〜０．８％、Ｍｎ：０．６〜３．０％、Ｐ：０．０３％以下、Ｓ：０．０３％以下、ｓｏｌ．Ａｌ：０．００４〜０．０８％、Ｎ：０．０１５％以下、Ｃｏ：０．６〜１０．０％で、さらにＣｕ：０．１〜２．５％、Ｎｉ：０．１〜１０．０％、Ｃｒ：０．１〜１．５％、Ｍｏ：０．１〜１．５％、Ｖ：０．００１〜０．２％、Ｎｂ：０．００１〜０．０８％、Ｔｉ：０．００３〜０．３％およびＢ：０．０００２〜０．００５％のうちの１種以上と、Ｃａ：０．０００２〜０．００５％およびＭｇ：０．０００２〜０．００５％の1種または２種を含有し残部が鉄および不純物であることを特徴とする予熱なし溶接の耐低温割れ性にすぐれた高強度鋼構造物用鋼材。
(3) In mass%, C: 0.01 to 0.15%, Si: 0.02 to 0.8%, Mn: 0.6 to 3.0%, P: 0.03% or less, S: 0.03% or less, sol. Al: 0.004 to 0.08%, N: 0.015% or less, Co: 0.6 to 10.0%, Cu: 0.1 to 2.5%, Ni: 0.1 to 10 0.0%, Cr: 0.1-1.5%, Mo: 0.1-1.5%, V: 0.001-0.2%, Nb: 0.001-0.08%, Ti: One of 0.003-0.3% and B: 0.0002-0.005%, Ca: 0.0002-0.005% and Mg: 0.0002-0.005% A steel material for high-strength steel structures having excellent low-temperature cracking resistance for preheating-free welding, characterized by containing seeds or two types, with the balance being iron and impurities .
本発明の鋼材は、溶接作業に先立ってその予熱を施さなくても低温割れを防止できるため、溶接施工の作業能率を大幅に向上させることができる。 Since the steel material of the present invention can prevent cold cracking without performing preheating prior to welding work, the work efficiency of welding work can be greatly improved.
本発明者らは、より強度の高い鋼材を用い、アーク溶接法により溶接構造物を構築する場合に生じやすい、溶接部の低温割れを抑止するための対策について種々調査を行った。低温割れの対策として母材の予熱は極めて効果的であるが、その十分な実施には多大の工数を要し、時には実施困難な場合がある。そこで、予熱なしか、おこなうとしてもできるだけ低温にする方法について検討した結果、鋼の合金元素の一つとしてＣｏを含有させると低温割れ防止に顕著な効果のあることを見出した。 The inventors of the present invention conducted various investigations on measures for suppressing cold cracking of a welded portion, which is likely to occur when a welded structure is constructed by an arc welding method using a steel material having higher strength. Although preheating of the base material is extremely effective as a countermeasure against cold cracking, it takes a lot of man-hours to fully implement it, and sometimes it is difficult to implement. Therefore, as a result of investigating a method for reducing the temperature as much as possible even if it is not preheated, it was found that inclusion of Co as one of the alloy elements of steel has a remarkable effect in preventing low temperature cracking.
Ｃｏは、鋼材の強化を目的に含有させる元素が一般にＭｓ点を低下させるのに対し、Ｍｓ点を上昇させるので強度向上の効果は小さい。また、耐熱鋼などで高温強度を向上させる効果から添加される場合もあるが、通常の溶接して用いられる低合金鋼材では、高価であることもあって、ほとんど顧みられることのない元素である。ところが、溶接母材となる鋼材に含有させることにより、低温割れを抑止でき、予熱をおこなわなくても、予熱したと同等の効果が現れるのである。 Co, which is included for the purpose of strengthening the steel material, generally lowers the Ms point, whereas it raises the Ms point, so the effect of improving the strength is small. In addition, it may be added from the effect of improving the high-temperature strength with heat-resistant steel, etc., but the low-alloy steel material used by ordinary welding is an element that is rarely considered because it is expensive. . However, when it is contained in the steel material as the welding base material, low temperature cracking can be suppressed, and even if preheating is not performed, the same effect as when preheating occurs.
このような効果の現れる理由については必ずしも明らかではないが、Ｍｓ点が上昇することにより溶接の熱影響部に生成されるマルテンサイト組織中の転位密度が低下し、転位密度の低下は硬さを低下させ変形能を向上させることから、先述の溶接部硬さ低下や残留応力低減をもたらして、割れ発生を抑止するのではないかと思われる。さらに転位密度の低下は金属組織中に吸蔵される水素量を低下させるので、拡散性の水素量を減少させていると推測される。 The reason why such an effect appears is not necessarily clear, but as the Ms point increases, the dislocation density in the martensitic structure generated in the heat-affected zone of the weld decreases, and the decrease in dislocation density decreases the hardness. It is thought that it may reduce cracking by reducing the hardness of the weld and reducing the residual stress as described above, because it lowers and improves the deformability. Furthermore, since the reduction in dislocation density reduces the amount of hydrogen occluded in the metal structure, it is presumed that the amount of diffusible hydrogen is reduced.
本発明では、この鋼材中の各元素の質量％で表す含有範囲を、次のように限定する。
Ｃ：０．０１〜０．１５％
Ｃは、鋼材の強度を確保するために、０．０１％以上含有させる。一方、過剰なＣは強度を必要以上に上昇させ、また、炭化物の析出量増加および粗大化を招き、靱性を劣化させるので、上限を０．１５％とする。より良好な靱性を得るためには、０．１％以下とするのが望ましく、さらに好ましくは０．０８％までとする。
In the present invention, the content range represented by mass% of each element in the steel material is limited as follows.
C: 0.01 to 0.15%
C is contained in an amount of 0.01% or more in order to ensure the strength of the steel material. On the other hand, excessive C raises the strength more than necessary, and also causes an increase in the amount of precipitation of carbides and coarsening and deteriorates toughness, so the upper limit is made 0.15%. In order to obtain better toughness, the content is desirably 0.1% or less, more preferably 0.08%.
Ｓｉ：０．０２〜０．８％
Ｓｉは脱酸作用を有し、鋼材の強度を向上させる元素である。その効果を得るために０．０２％以上含有させる。しかし、過剰なＳｉ添加は、鋼材の靱性低下の原因となるとともに、溶接時の耐割れ感受性を劣化させるので、その上限は０．８％とする。
Si: 0.02 to 0.8%
Si is an element that has a deoxidizing action and improves the strength of the steel material. In order to acquire the effect, it contains 0.02% or more. However, excessive addition of Si causes a reduction in the toughness of the steel material and deteriorates the resistance to cracking during welding, so the upper limit is made 0.8%.
Ｍｎ：０．６〜３．０％
Ｍｎは、鋼材の強度を向上させる元素である。その効果を得るため、下限を０．６％とする。しかし、多すぎる含有は鋼材およびその溶接熱影響部の靱性低下の原因となり、溶接の耐割れ感受性も劣化させるので、上限は３．０％までとする。靱性および耐割れ感受性をさらに良好にするためには、２．５質量％以下とするのが望ましく、より好ましくは１．７％とまでとするのがよい。
Mn: 0.6 to 3.0%
Mn is an element that improves the strength of the steel material. In order to obtain the effect, the lower limit is made 0.6%. However, if the content is too large, it causes a reduction in the toughness of the steel material and its weld heat-affected zone, and also deteriorates the crack resistance sensitivity of the weld, so the upper limit is made 3.0%. In order to further improve the toughness and cracking resistance, the content is desirably 2.5% by mass or less, more preferably 1.7%.
Ｐ：０．０３％以下
Ｐは原料から混入してくる不純物の一つで、鋼材の靱性を劣化させる。顕著な影響を及ぼさない範囲として０．０３％以下にする必要があるが、少なければ少ないほどよい。望ましくは０．０１５％以下である。
P: 0.03% or less P is one of the impurities mixed from the raw material, and deteriorates the toughness of the steel material. Although it is necessary to make it 0.03% or less as a range which does not exert a remarkable influence, the smaller the better. Desirably, it is 0.015% or less.
Ｓ：０．０３％以下
Ｓは原料から混入してくる不純物の一つで、鋼の熱間加工性を低下させ、鋼材の靱性を劣化させるので、少なければ少ないほどよい。顕著な害を及ぼさない範囲として０．０３質量％以下にする必要があり。望ましくは０．０１５％以下である。
S: 0.03% or less S is one of the impurities mixed from the raw material, and decreases the hot workability of the steel and deteriorates the toughness of the steel material. It should be 0.03% by mass or less as a range that does not cause significant harm. Desirably, it is 0.015% or less.
ｓｏｌ．Ａｌ：０．００４〜０．０８％
Ａｌは、脱酸剤として重要な元素であり、その効果を発揮させるためには、ｓｏｌ．Ａｌの含有量の下限を０．００４％とする。一方、過剰なＡｌは、粗大な介在物の生成原因となるので、その上限は０．０８％とする。
Ｎ：０．０１５％以下
Ｎは不純物として混入してくる元素であり、他の元素の存在や製造条件によっては鋼材の強度や靱性を向上させたり、逆に靱性を劣化さたりすることもある。しかしＴｉを含む場合には、ＴｉＮとなって溶接部の細粒化、高靱化の役目を果たすので、０．００１％以上あることが望ましい。ただし、過剰に含む場合は、低合金鋼においては、悪影響をもたらすことが多いので、その含有による影響が顕著でない範囲として０．０１５％以下であることとする。
sol. Al: 0.004 to 0.08%
Al is an important element as a deoxidizer. In order to exert its effect, sol. The lower limit of the Al content is 0.004%. On the other hand, excessive Al causes the formation of coarse inclusions, so the upper limit is made 0.08%.
N: 0.015% or less N is an element mixed as an impurity. Depending on the presence of other elements and production conditions, the strength and toughness of the steel material may be improved, or the toughness may be deteriorated. . However, when Ti is contained, it becomes TiN and plays the role of making the welded portion finer and tougher, so 0.001% or more is desirable. However, in the case where it is excessively contained, low alloy steel often has an adverse effect, so that the effect of the inclusion is 0.015% or less as a range where the influence is not remarkable.
Ｃｏ：０．１％〜１０．０％
Ｃｏは本発明の鋼板の溶接における耐低温割れ性向上に重要な元素である。このような効果を得るためには、０．１％以上の含有を必要とする。しかしながら、多すぎる含有は改善効果が飽和しむだな含有となるため、多くても１０％までとする。より顕著な効果をもたらす好ましい含有量の下限は１％以上であり、さらに望ましくは２％以上である。
Co: 0.1% to 10.0%
Co is an important element for improving cold cracking resistance in welding of the steel sheet of the present invention. In order to obtain such an effect, the content of 0.1% or more is required. However, if the content is too large, the improvement effect becomes a wasteful content, so the content is at most 10%. The lower limit of the preferable content that brings about a more remarkable effect is 1% or more, and more desirably 2% or more.
Ｃｕ：０．１〜２．５％
Ｃｕは含まなくても良い。Ｃｕは、焼入れ性向上に加えて析出効果によっても強度向上に寄与するので、高強度を必要とする場合には添加する。含有の効果を得るためには、０．１％以上とするのが望ましい。しかし、２．５％を越える含有は靱性が劣化するので、多くても２．５％以下とする。含有させる場合、望ましいのは０．８％以下であり、より望ましくは０．６％以下である。
Cu: 0.1 to 2.5%
Cu may not be included. Since Cu contributes to strength improvement not only by improving hardenability but also by precipitation effects, it is added when high strength is required. In order to acquire the effect of containing, it is desirable to set it as 0.1% or more. However, if the content exceeds 2.5%, the toughness deteriorates, so the content is at most 2.5%. When it is contained, it is preferably 0.8% or less, more preferably 0.6% or less.
Ｎｉ：０．１〜１０．０％
Ｎｉは含まなくてもよい。しかし、Ｎｉは強度を向上させ、靱性を高めるとともに低温割れ性を改善する効果が大きいので、より高靱性とし、割れ性も改善する目的で含有させてもよい。含有させる場合には０．１％以上とし、できれば１．５％以上とすることが望ましい。しかし、多すぎる含有はその量に見合った効果が得られなくなるので、１０．０％以下とするのがよい。
Ni: 0.1 to 10.0%
Ni may not be included. However, since Ni has a great effect of improving strength and increasing toughness and improving low-temperature cracking properties, Ni may be contained for the purpose of achieving higher toughness and improving cracking properties. When contained, it is preferably 0.1% or more, and preferably 1.5% or more. However, if the content is too large, an effect commensurate with the amount cannot be obtained, so the content is preferably 10.0% or less.
Ｃｒ：０．１〜１．５％
Ｃｒは含まなくてもよい。しかし、Ｃｒは焼入れ性向上に有効なので、鋼材をより高強度にする場合に含有させる。その場合、含有の効果を得るためには０．１％以上とすることが望ましい。しかし、１．５％を超える含有は、溶接時の低温割れを生じやすくなるので、１．５％までとする。
Cr: 0.1 to 1.5%
Cr may not be included. However, since Cr is effective for improving hardenability, it is contained when making the steel material stronger. In that case, in order to acquire the effect of containing, it is desirable to set it as 0.1% or more. However, the content exceeding 1.5% tends to cause cold cracking during welding, so the content is limited to 1.5%.
Ｍｏ：０．１〜１．５％
Ｍｏは含まなくてもよい。Ｍｏは、焼入れ性を向上させ強度上昇に有効なので、鋼材をより高強度にする場合に添加する。含有させてその効果を得るには、０．１％以上とするのがよい。しかし、多すぎる含有は低温割れを発生しやすくするので、多くても１．５％までとするのが望ましい。
Mo: 0.1 to 1.5%
Mo may not be included. Mo improves the hardenability and is effective for increasing the strength, so it is added when the steel material is made to have higher strength. In order to obtain the effect when contained, the content is preferably 0.1% or more. However, too much content tends to cause cold cracking, so it is desirable that the content is at most 1.5%.
Ｖ：０．００１〜０．２％
Ｖは含まなくてもよい。Ｖは窒素や炭素と結合して析出硬化により、鋼の強度上昇に有効なので、鋼材をより高強度にしたい場合に０．００１％以上含有させる。しかし、０．２％を超える含有は、低温割れが発生しやすくなるので、多くても０．２％までとする。
V: 0.001 to 0.2%
V may not be included. V is combined with nitrogen and carbon, and is effective for increasing the strength of the steel by precipitation hardening. Therefore, when the steel material is desired to have a higher strength, it is contained in an amount of 0.001% or more. However, if the content exceeds 0.2%, low-temperature cracking is likely to occur, so the content is limited to 0.2% at most.
Ｎｂ：０．００１〜０．０８％
Ｎｂは含まなくてもよい。Ｎｂは、析出硬化を生じ、強度上昇に有効なので、鋼材をより高強度にする場合には０．００１％以上含有させる。しかし、含有量が多すぎると低温割れを発生しやすくなるので、多くても０．０８％以下とする。
Ｔｉ：０．００３〜０．３％
Ｔｉは窒素と結合して、加熱時の粒粗大化防止により溶接部の靱性を上げるとともに、微量のＢがＢＮとなるのを防ぎ、Ｂ添加による焼入れ性を確保する。その効果を得るために、下限は０．００３％とする。一方、多すぎるＴｉは、炭化物の析出を過剰にして、鋼板母材及び溶接熱影響部の靱性を著しく劣化させるので、その上限は０．３％までとする。
Nb: 0.001 to 0.08%
Nb may not be included. Nb causes precipitation hardening and is effective for increasing the strength, so 0.001% or more is contained in order to make the steel material higher in strength. However, if the content is too large, cold cracking is likely to occur, so at most 0.08%.
Ti: 0.003-0.3%
Ti combines with nitrogen to increase the toughness of the welded portion by preventing grain coarsening during heating, prevent a trace amount of B from becoming BN, and ensure hardenability by adding B. In order to obtain the effect, the lower limit is made 0.003%. On the other hand, too much Ti causes excessive precipitation of carbides and significantly deteriorates the toughness of the steel plate base material and the weld heat affected zone, so the upper limit is made 0.3%.
Ｂ：０．０００２〜０．００５％
Ｂは含有させなくてもよい。しかしＢは、微量で焼入れ性を著しく上昇させ、鋼板の高強度化に大きく寄与する。含有させてその効果を得ようとする場合、０．０００２％以上とするの必要がある。ただし過剰のＢ含有は、耐低温割れ性の劣化を生じるので、上限は０．００５％までとする。強度の向上と良好な耐溶接割れ性を確保するためには、０．００５％以下とするのが望ましい。
B: 0.0002 to 0.005%
B may not be contained. However, B significantly increases the hardenability in a small amount, and greatly contributes to increasing the strength of the steel sheet. When it is intended to obtain the effect by being contained, it is necessary to be 0.0002% or more. However, excessive B content causes deterioration of cold cracking resistance, so the upper limit is made 0.005%. In order to ensure improvement in strength and good weld crack resistance, the content is desirably 0.005% or less.
Ｃａ：０．０００２〜０．００５％
Ｃａは含有させなくてもよい。しかし、含有させると組織を微細化して靭性を高める作用がある。この作用を確実に得るには、Ｃａを０．０００２％以上含有させる。ただし、多すぎる含有はＣａＯなどの介在物量が増し靱性が劣化するので、含有させる場合０．０００５％までとするのがよい。
Ca: 0.0002 to 0.005%
Ca may not be contained. However, when contained, it has the effect of increasing the toughness by refining the structure. In order to reliably obtain this action, Ca is contained in an amount of 0.0002% or more. However, if the content is too large, the amount of inclusions such as CaO increases and the toughness deteriorates.
Ｍｇ：０．０００２〜０．００５％以下
Ｍｇは含有させなくてもよい。しかし、ＭｇもＣａと同様に組織を微細化して靭性を高める作用を有する。この作用を発揮させるには、Ｍｇの含有量を０．０００２％以上とすることが望ましい。しかし、０．００５％を超えて含有させると、ＭｇＯなどの介在物量が増大し却って靭性が劣化するので、上限は０．００５％までとするのがよい。
Mg: 0.0002 to 0.005% or less Mg may not be contained. However, Mg, like Ca, has the effect of increasing the toughness by refining the structure. In order to exert this effect, the Mg content is preferably 0.0002% or more. However, if the content exceeds 0.005%, the amount of inclusions such as MgO increases and the toughness deteriorates, so the upper limit is preferably made 0.005%.
表１に示す化学組成を有する鋼を真空溶解法にて溶製して鋳塊とし、鍛造してスラブを作製した。このスラブを通常の工程にて熱間圧延をおこない、厚さ４５ｍｍの鋼板を製造した。得られた鋼板の板厚圧中心部における強度および衝撃特性を表２に示す。 Steel having the chemical composition shown in Table 1 was melted by a vacuum melting method to form an ingot, and forged to produce a slab. This slab was hot-rolled in a normal process to produce a steel plate having a thickness of 45 mm. Table 2 shows the strength and impact characteristics of the obtained steel sheet at the center of the plate thickness pressure.
ＪＩＳ−Ｚ−３１５８にて規定されているｙ形溶接割れ試験方法に基づいて、これらの鋼板から試験板を作製し、溶接をおこなった。表３に溶接条件を示すが、溶接材料は神戸製鋼所（株）製ＬＢ−８０ＵＬを使用した。この溶接材料による溶接金属の化学組成は、一例としてＣ：０．０８％、Ｓｉ：０．５２％、Ｍｎ：１．５％、Ｐ：０．００９％、Ｓ：０．００６％、Ｎｉ：１．９０％、Ｃｒ：０．２８％であるとされている。溶接は室温３０℃、湿度８０％に調整した室内で実施し、試験板の予熱はなしとし、各鋼板に対し３枚の試験板とした。溶接後の試験板は、上記ＪＩＳにて規定される方法に従い表面割れ率および断面割れ率を測定した。結果を表４に示す。 Based on the y-type weld crack test method defined in JIS-Z-3158, test plates were produced from these steel plates and welded. Although welding conditions are shown in Table 3, LB-80UL made from Kobe Steel Ltd. was used for the welding material. The chemical composition of the weld metal by this welding material is, for example, C: 0.08%, Si: 0.52%, Mn: 1.5%, P: 0.009%, S: 0.006%, Ni: 1.90% and Cr: 0.28%. Welding was performed in a room adjusted to a room temperature of 30 ° C. and a humidity of 80%, the test plates were not preheated, and three test plates were used for each steel plate. The test plate after welding was measured for the surface cracking rate and the cross-sectional cracking rate according to the method specified by the above JIS. The results are shown in Table 4.
本発明にて規定する組成範囲である鋼記号Ａ３〜Ａ８の試験結果は、予熱なしによる断面割れ率が試験体１〜３のいずれにおいても０％であるか、悪くとも１０％以下に抑えられている。これに対し鋼記号Ｘ１〜Ｘ４は、Ｃｏを含まない例である（鋼記号Ｘ３および４は、Ｃｏを含まない点以外は、各々Ａ３およびＡ４とほぼ同じ成分含有量である）が、割れが発生している。鋼記号Ｘ５〜Ｘ７はＣｏを含有しているが、Ｃ、Ｓｉ、Ｍｎなどの元素が本発明にて定める適正量を外れており、割れ率が低く抑えられているものの、割れを無くすことはできていない。
The test results of steel symbols A3 to A8, which are the composition ranges specified in the present invention, show that the cross-sectional crack ratio without preheating is 0% in any of the test bodies 1 to 3, or at most 10% or less. ing. On the other hand, steel symbols X1 to X4 are examples that do not contain Co (steel symbols X3 and 4 have almost the same component contents as A3 and A4, respectively, except that they do not contain Co ) , It has occurred. Although steel symbol X5~X7 is contained Co, C, Si, elements such as Mn are outside the proper amount stipulated in this onset bright, although crack rate is kept low, to eliminate cracking Is not done.
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