JP4433844B2 - Method for producing high strength steel with excellent fire resistance and toughness of heat affected zone - Google Patents

Method for producing high strength steel with excellent fire resistance and toughness of heat affected zone Download PDF

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JP4433844B2
JP4433844B2 JP2004083324A JP2004083324A JP4433844B2 JP 4433844 B2 JP4433844 B2 JP 4433844B2 JP 2004083324 A JP2004083324 A JP 2004083324A JP 2004083324 A JP2004083324 A JP 2004083324A JP 4433844 B2 JP4433844 B2 JP 4433844B2
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克行 一宮
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本発明は、海洋構造物やラインパイプ、圧力容器などに用いられる降伏強さが414MPa以上で多層溶接が施される高張力鋼に関し、特に、耐火性および溶接継手部の靭性に優れる高張力鋼の製造方法にするものである。   The present invention relates to a high-strength steel having a yield strength of 414 MPa or more, which is used for offshore structures, line pipes, pressure vessels, etc., and is particularly excellent in fire resistance and weld joint toughness. This is a manufacturing method.

構造用鋼材は、通常、常温で十分な強度を有するように設計、製造されている。しかし、鋼の強度は、一般的に温度の上昇に伴い低下し、特に、従来の構造用鋼材は、500℃以上の高温になると強度低下が大きいことが知られている。そのため、火災等で高温状態になることが懸念される構造物、特に、人間が居住する建築物では、高温状態でも構造物が倒壊しないよう、鋼材にある程度以上の耐火性(高温強度)を持たせることが必要である。耐火性を付与する手段としては、例えば、特許文献1には、Mo,V等を添加し、析出強化により高温強度を高める技術が開示されている。   Structural steel is usually designed and manufactured to have sufficient strength at room temperature. However, the strength of steel generally decreases with an increase in temperature. In particular, it is known that a conventional structural steel material has a large decrease in strength at a high temperature of 500 ° C. or higher. For this reason, in structures that are likely to become hot due to fire, etc., especially in buildings where humans live, steel materials have a certain level of fire resistance (high temperature strength) so that the structure will not collapse even in high temperatures. It is necessary to make it. As means for imparting fire resistance, for example, Patent Document 1 discloses a technique for increasing high-temperature strength by precipitation strengthening by adding Mo, V, or the like.

また、海洋構造物等に用いられる鋼は、溶接接合により所望の形状の構造物に仕上げられる。そのため、これらの鋼には、構造物の安全性の観点から、母材自体の靭性はもちろん溶接継手の溶接部(溶接金属や熱影響部)の靭性にも優れることが要求される。   Moreover, steel used for offshore structures and the like is finished into a desired shape structure by welding. Therefore, these steels are required to have excellent toughness of the welded joint (welded metal and heat affected zone) of the welded joint as well as the toughness of the base metal itself from the viewpoint of the safety of the structure.

ところで、板厚が厚い鋼の溶接は、通常、多層溶接により施工されるが、このような溶接では、熱影響部は複雑な熱履歴を受けるため、局所脆化域が発生し易く、特にボンド部(溶接金属と母材との境界)や2相域再熱部(1サイクル目で粗粒となり、2サイクル目でαとγの2相域に加熱される領域)の靭性の劣化が問題となる。というのは、ボンド部は、溶融点直下の高温に曝されるため、オーステナイト粒が最も粗大化し、引き続く冷却により、脆弱な上部ベイナイト組織に変態し易いからである。また、ボンド部には、ウッドマンステッテン組織や島状マルテンサイトといった脆化組織も生成するため、靭性はさらに劣化する傾向にある。   By the way, welding of steel with a large plate thickness is usually performed by multi-layer welding, but in such welding, the heat-affected zone is subjected to a complicated thermal history, so that a local embrittlement zone is likely to occur, Degradation of the toughness of the weld zone (boundary between the weld metal and base metal) and the two-phase reheat zone (coarse grain in the first cycle and heated to the two-phase zone of α and γ in the second cycle) It becomes. This is because the bond portion is exposed to a high temperature just below the melting point, so that the austenite grains are most coarsened and are easily transformed into a fragile upper bainite structure by subsequent cooling. Further, since a brittle structure such as a woodman stetten structure or island martensite is also generated in the bond portion, the toughness tends to be further deteriorated.

この問題に対する対策としては、例えば、鋼中にTiNを微細分散させ、オーステナイトの粗大化を抑制したり、フェライト変態の核として利用したりする技術が実用化されている。さらに、特許文献2や特許文献3には、希土類元素(REM)をTiと複合添加して鋼中に微細粒子を分散させることにより、オーステナイトの粒成長を防止し、溶接部の靭性を向上する技術が開示されている。その他に、Tiの酸化物を分散させる技術やBNのフェライト核生成能を酸化物の分散と組み合わせる技術、さらには、CaやREMを添加することにより硫化物の形態を制御し、高靭性を得る技術も提案されている。   As a countermeasure against this problem, for example, a technique of finely dispersing TiN in steel to suppress coarsening of austenite or using it as a core of ferrite transformation has been put into practical use. Further, in Patent Document 2 and Patent Document 3, a rare earth element (REM) is added in combination with Ti to disperse fine particles in the steel, thereby preventing austenite grain growth and improving the toughness of the weld. Technology is disclosed. In addition, Ti oxide dispersion technology, BN ferrite nucleation ability combined with oxide dispersion technology, and addition of Ca and REM control the form of sulfides to obtain high toughness Technology has also been proposed.

一方、2相域再熱部、即ち最初の溶接時に溶融点直下の高温に曝され、続く重ね溶接時の再加熱によりフェライトとオーステナイトの2相域となる領域は、ボンド部以上に脆化する傾向がある。これは、2パス目以降の再加熱により、オーステナイト領域に炭素が濃化し、これが冷却中に、島状マルテンサイトを含む脆弱なベイナイト組織を生成し、靭性を劣化させるからである。この対策として、特許文献4には、低C、低Si化することにより島状マルテンサイトの生成を抑制し、さらにCuを添加することにより母材強度を高める技術が開示されている。
特開平5-311324号公報 特公平03-053367号公報 特開平60-184663号公報 特開平05-186823号公報
On the other hand, the two-phase region reheat zone, that is, the region that is exposed to a high temperature just below the melting point during the first welding and becomes a two-phase region of ferrite and austenite by reheating during the subsequent lap welding becomes more brittle than the bond portion. Tend. This is because carbon is concentrated in the austenite region by reheating after the second pass, and this generates a fragile bainite structure including island-like martensite during cooling, and deteriorates toughness. As a countermeasure, Patent Document 4 discloses a technique for suppressing generation of island martensite by reducing C and Si and further increasing the strength of the base material by adding Cu.
JP-A-5-311324 Japanese Patent Publication No. 03-053367 Japanese Patent Laid-Open No. 60-184663 JP 05-186823 A

一般に、耐火性の改善に有効な析出強化は、鋼の靭性を著しく低下させることが知られている。そのため、海洋構造物等に用いられる鋼のように母材および溶接部に対して厳しい低温靭性が要求される鋼に対して、耐火性の向上を図るために析出強化を用いた場合には、靭性を大きく劣化させることになる。そのため、従来、耐火性と母材および溶接部の低温靭性とを共に兼ね備えた鋼は存在していなかった。   In general, it is known that precipitation strengthening effective in improving fire resistance significantly reduces the toughness of steel. Therefore, for steels that require severe low temperature toughness for the base metal and welds, such as steel used for offshore structures, etc., when precipitation strengthening is used to improve fire resistance, Toughness will be greatly degraded. Therefore, conventionally, there has been no steel having both fire resistance and low temperature toughness of the base material and the welded portion.

本発明の目的は、従来技術が抱える上記問題点を解決し、母材の強度や靭性および溶接熱影響部の靭性に優れると共に、従来鋼では考慮されていなかった耐火性にも優れる高張力鋼を有利に製造する方法を提案することにある。   The purpose of the present invention is to solve the above-mentioned problems of the prior art, and is excellent in the strength and toughness of the base metal and the toughness of the heat affected zone of the base metal, and also in the fire resistance not considered in the conventional steel It is in proposing the method of manufacturing advantageously.

発明者らは、高張力鋼の溶接熱影響部の靭性を向上させつつ母材の高温強度をも向上することができる方法について鋭意検討を行った。その結果、TiNのピンニング効果に加えてさらに、硫化物の形態制御のために添加しているCaの添加量を適正範囲にすることにより、溶接熱影響部の組織を微細にし、耐火性を向上させるために添加するMo等の元素の悪影響を緩和でき、ひいては、母材の耐火性と溶接熱影響部の靭性との両立が図られることを見出した。さらに、母材の強度靭性に及ぼす圧延条件の影響について検討したところ、圧延温度、冷却速度および冷却停止温度を制御すれば、母材の強度靭性に優れた高張力鋼を製造できることを見出した。   The inventors diligently studied a method capable of improving the high temperature strength of the base material while improving the toughness of the weld heat affected zone of the high strength steel. As a result, in addition to the pinning effect of TiN, the amount of Ca added to control the morphology of the sulfide is made within an appropriate range, so that the structure of the heat affected zone is refined and the fire resistance is improved. It has been found that the adverse effects of elements such as Mo added to the effect can be mitigated, and that both the fire resistance of the base metal and the toughness of the weld heat affected zone can be achieved. Furthermore, when the influence of the rolling conditions on the strength toughness of the base metal was examined, it was found that a high-tensile steel excellent in the strength toughness of the base material can be produced by controlling the rolling temperature, cooling rate and cooling stop temperature.

上記知見に基き開発された本発明は、C:0.03〜0.10mass%、Si:0.05〜0.30mass%、Mn:0.6〜1.7mass%、P:0.015mass%以下、S:0.0005〜0.0050mass%、Al:0.005〜0.06mass%、Cr:0.05〜1.0mass%、Mo:0.2〜0.5mass%、Nb:0.005〜0.05mass%、Ti:0.005〜0.02mass%、N:0.0030〜0.0065mass%、Ca:0.0005〜0.0030mass%を含有し、かつCa,OおよびSの含有量が、下記関係式;
<(Ca−(0.18+130×Ca)×O)/(1.25×S)<1
を満たして含有し、残部がFeおよび不可避的不純物からなる鋼素材を1050〜1250℃に加熱後、(Ar+100℃)以上の温度域における累積圧下率を50%以上とし、仕上げ温度を(Ar−100℃)〜(Ar+100℃)とする熱間圧延をした後、冷却速度2〜20℃/secで600℃以下まで冷却することを特徴とする耐火性および溶接継手部靭性に優れる高張力鋼の製造方法である。
The present invention developed based on the above knowledge is C: 0.03-0.10 mass%, Si: 0.05-0.30 mass%, Mn: 0.6-1.7 mass%, P: 0.015 mass%. Hereinafter, S: 0.0005-0.0050 mass% , Al: 0.005-0.06 mass% , Cr: 0.05-1.0 mass% , Mo: 0.2-0.5 mass% , Nb: 0.005 to 0.05 mass%, Ti: 0.005 to 0.02 mass%, N: 0.0030 to 0.0065 mass%, Ca: 0.0005 to 0.0030 mass%, and Ca, O and S content is the following relational expression;
0 <(Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) <1
The meet and containing, after heating the steel material and the balance being Fe and unavoidable impurities in 1,050 to 1250 ° C., and (Ar 3 + 100 ℃) over a cumulative rolling reduction in the temperature range 50% or more, a finishing temperature Fire rolling and weld joint toughness characterized by cooling to 600 ° C. or less at a cooling rate of 2 to 20 ° C./sec after hot rolling with (Ar 3 −100 ° C.) to (Ar 3 + 100 ° C.) It is a manufacturing method of high-tensile steel excellent in.

本発明の鋼素材は、上記成分組成に加えてさらに、V:0.01〜0.06mass%、Cu:0.1〜1.0mass%およびNi:0.02〜1.5mass%のうちから選ばれる1種または2種以上を含有することを特徴とする。   In addition to the above component composition, the steel material of the present invention further includes one or more selected from V: 0.01 to 0.06 mass%, Cu: 0.1 to 1.0 mass%, and Ni: 0.02 to 1.5 mass%. It is characterized by containing.

また、本発明は、上記冷却後の高張力鋼を、さらに700℃以下の温度で焼戻し処理することを特徴とする。   Further, the present invention is characterized in that the high-tensile steel after cooling is further tempered at a temperature of 700 ° C. or lower.

本発明によれば、母材の常温強度や高温(600℃)強度ならびに靭性が良好で、しかも溶接熱影響部の靭性にも優れる、降伏強さが414MPa以上、引張強さが517MPa以上(API規格2W−60)の高張力鋼を安価に製造することができる。その結果、建築物や海洋構造物等の大型化や安全性向上、施工能率の向上に大いに寄与する。   According to the present invention, the base metal has good strength at normal temperature and high temperature (600 ° C.) and toughness, and also has excellent toughness in the heat affected zone, yield strength is 414 MPa or more, and tensile strength is 517 MPa or more (API Standard 2W-60) high strength steel can be manufactured at low cost. As a result, it greatly contributes to increasing the size, safety and construction efficiency of buildings and offshore structures.

次に、本発明において、各成分の組成範囲を限定した理由について説明する。
C:0.03〜0.10mass%
Cは、構造用鋼としての必要な強度を得るためには、0.03mass%以上含有させる必要がある。一方、多過ぎると、溶接部の靭性低下を招くので、上限を0.10mass%とする必要がある。好ましくは、0.04〜0.09mass%である。
Next, the reason why the composition range of each component is limited in the present invention will be described.
C: 0.03 ~ 0.10mass%
C is required to be contained in an amount of 0.03 mass% or more in order to obtain the necessary strength as structural steel. On the other hand, if the amount is too large, the toughness of the welded portion is reduced, so the upper limit needs to be 0.10 mass%. Preferably, it is 0.04-0.09 mass%.

Si:0.05〜0.30mass%
Siは、脱酸成分として0.05mass%以上添加する必要があり、一方、0.30mass%を超えると、多層盛溶接部の靭性を劣化させるため0.30mass%以下に制限する必要がある。
Si: 0.05-0.30mass%
Si needs to be added in an amount of 0.05 mass% or more as a deoxidizing component. On the other hand, if it exceeds 0.30 mass%, the toughness of the multi-layer welded portion is deteriorated, so it is necessary to limit it to 0.30 mass% or less.

Mn:0.6〜1.7mass%
Mnは、母材の強度を確保するために0.6mass%以上添加する必要がある。一方、1.7mass%を超えると、溶接部の靭性を著しく劣化させるため、1.7mass%以下とする必要がある。好ましくは、0.8〜1.6mass%である。
Mn: 0.6-1.7mass%
Mn needs to be added by 0.6 mass% or more in order to ensure the strength of the base material. On the other hand, if it exceeds 1.7 mass%, the toughness of the welded portion is remarkably deteriorated, so it is necessary to make it 1.7 mass% or less. Preferably, it is 0.8-1.6 mass%.

P:0.015mass%以下
Pは、0.015mass%を超えると、溶接部の靭性を劣化させるため、0.015mass%以下に制限する。好ましくは、0.012mass%以下である。
P: 0.015 mass% or less When P exceeds 0.015 mass%, the toughness of the welded portion is deteriorated, so that it is limited to 0.015 mass% or less. Preferably, it is 0.012 mass% or less.

S:0.0005〜0.0050mass%
Sは、0.0050mass%を超えて含有すると、母材および溶接部の靭性を劣化させるため、0.0050mass%以下とする。好ましくは、0.0040mass%以下である。一方、Sが0.0005mass%未満では、HAZの組織微細化に有効な硫化物の量を確保できないので、Sは0.0005mass%以上とする。
S: 0.0005-0.0050 mass%
If S is contained in excess of 0.0050 mass%, the toughness of the base metal and the welded portion is deteriorated, so it is 0.0050 mass% or less. Preferably, it is 0.0040 mass% or less. On the other hand, if S is less than 0.0005 mass%, the amount of sulfide effective for refining the HAZ structure cannot be secured, so S is set to 0.0005 mass% or more.

Al:0.005〜0.06mass%
Alは、溶鋼を脱酸するために、0.005mass%以上含有させる必要があり、一方、0.06mass%を超えて含有すると、母材の靭性を低下させるとともに、溶接時の希釈により溶接金属部に混入して靭性を劣化させるため、0.06mass%以下に制限する必要がある。
Al: 0.005-0.06mass%
In order to deoxidize molten steel, Al must be contained in an amount of 0.005 mass% or more. On the other hand, if it exceeds 0.06 mass%, the toughness of the base metal is reduced and the weld metal part is diluted by dilution during welding. In order to deteriorate toughness by mixing, it is necessary to limit to 0.06 mass% or less.

Cr:0.05〜1.0mass%
Crは、鋼の常温および高温での強度上昇に有効に作用するが、その効果を得るためには0.05mass%以上添加する必要がある。一方、1.0mass%を超える添加は、靭性を劣化させる。よって、Crは、0.05〜1.0mass%の範囲で添加する。
Cr: 0.05-1.0mass%
Cr effectively works to increase the strength of steel at room temperature and high temperature. To obtain this effect, it is necessary to add 0.05 mass% or more. On the other hand, addition exceeding 1.0 mass% deteriorates toughness. Therefore, Cr is added in the range of 0.05 to 1.0 mass%.

Mo:0.2〜0.5mass%
Moは、焼入性を向上すると共に、析出強化等により鋼の強度を上昇させる有効な元素であり、特に、中・高温強度の上昇に対して有効である。一方、大量の添加は、コスト上昇になる上、溶接性の劣化を招くため、0.2〜0.5mass%の範囲に制限する。
Mo: 0.2-0.5mass%
Mo is an effective element for improving the hardenability and increasing the strength of the steel by precipitation strengthening and the like, and is particularly effective for increasing the medium / high temperature strength. On the other hand, addition of a large amount increases the cost and causes deterioration of weldability, so is limited to a range of 0.2 to 0.5 mass%.

Nb:0.005〜0.05mass%
Nbは、鋼の常温および高温強度の向上に有効であるが、その効果を得るには0.005mass%以上添加する必要がある。一方、0.05mass%を超える添加は、溶接部の靭性を劣化させる。よって、Nbは0.005〜0.05mass%の範囲で添加する。
Nb: 0.005-0.05mass%
Nb is effective in improving the normal temperature and high temperature strength of steel, but 0.005 mass% or more needs to be added to obtain the effect. On the other hand, addition exceeding 0.05 mass% deteriorates the toughness of the welded portion. Therefore, Nb is added in the range of 0.005 to 0.05 mass%.

Ti:0.005〜0.02mass%
Tiは、凝固時にTiNとなって析出し、溶接部におけるオーステナイトの粗大化を抑制すると共に、フェライト変態核となってフェライト粒を微細化し高靭性化に寄与する。0.005mass%未満ではその効果が少なく、一方、0.02mass%を超えると、TiN粒子の粗大化によって期待した効果が得られなくなる。
Ti: 0.005-0.02mass%
Ti precipitates as TiN during solidification and suppresses austenite coarsening in the weld zone, and also serves as a ferrite transformation nucleus to refine the ferrite grains and contribute to higher toughness. If it is less than 0.005 mass%, the effect is small. On the other hand, if it exceeds 0.02 mass%, the expected effect cannot be obtained due to the coarsening of TiN particles.

N:0.0030〜0.0065mass%
Nは、TiNを必要量確保するために必要であり、十分なTiN量を得るためには0.0030mass%以上含有することが必要である。一方、0.0065mass%を超えると、溶接時の加熱によってTiNが溶解する温度領域における固溶N量が増加し、靭性を著しく低下させるため、0.0065mass%以下に制限する。
N: 0.0030-0.0065mass%
N is necessary for securing a necessary amount of TiN, and in order to obtain a sufficient amount of TiN, it is necessary to contain 0.0030 mass% or more. On the other hand, if it exceeds 0.0065 mass%, the amount of solid solution N in the temperature region where TiN dissolves by heating during welding increases, and the toughness is significantly reduced.

Ca:0.0005〜0.0030mass%
Caは、Sを固定して靭性を改善する元素である。このような効果を発揮させるためには、少なくとも0.0005mass%含有することが必要である。しかし、0.0030mass%を超えて含有してもその効果が飽和するため、Caの添加量は、0.0005〜0.0030mass%の範囲とする。
Ca: 0.0005 to 0.0030 mass%
Ca is an element that fixes S and improves toughness. In order to exhibit such an effect, it is necessary to contain at least 0.0005 mass%. However, since the effect is saturated even if it contains exceeding 0.0030 mass%, the addition amount of Ca shall be the range of 0.0005-0.0030 mass%.

<(Ca−(0.18+130×Ca)×O)/(1.25×S)<1
Ca,OおよびSは、高温でも溶解しないフェライト変態生成核を微細分散させるためには、次式;
<(Ca−(0.18+130×Ca)×O)/(1.25×S)<1
ここに、Ca,O,S:各元素の含有量(mass%)
の関係を満足するよう含有する必要がある。上記式における、(Ca−(0.18+130×Ca)×O)/(1.25×S)は、硫化物形態制御に有効なCa濃度とSとの原子濃度の比を示す値であり、硫化物の形態を推定することができる(拝田他、「鉄と鋼」、日本鉄鋼協会、第66年(1980)、第3号、p.354〜362)。この式を満たした場合には、硫化物の析出物は、CaS上にMnSが析出した複合硫化物の形態となる。一方、(Ca−(0.18+130×Ca)×O)/(1.25×S)≦0の場合には、CaSが晶出しないため、Sは、MnS単独の形態で析出する。このMnSは、鋼板圧延時に伸長されて、母材の靭性の低下を引き起こすとともに、本発明の主眼である溶接熱影響部でのα変態生成核の微細分散が達成されない。逆に、1≦(Ca−(0.18+130×Ca)×O)/(1.25×S)の場合には、Sが完全にCaによって固定され、フェライト生成核として働くMnSがCaS上に析出しないため、複合硫化物がα生成核として十分な機能を発揮しない。よって、(Ca−(0.18+130×Ca)×O)/(1.25×S)は、0超え1未満の範囲とする。好ましくは0.2〜0.8の範囲である。
0 <(Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) <1
In order to finely disperse the ferrite transformation nuclei that do not dissolve even at high temperatures, Ca, O, and S have the following formula:
0 <(Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) <1
Here, Ca, O, S: content of each element (mass%)
It is necessary to contain so as to satisfy the relationship. In the above formula, (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) is a value indicating the ratio of the atomic concentration between Ca concentration and S effective for sulfide form control, The form of sulfide can be estimated (Hirida et al., “Iron and Steel”, Japan Iron and Steel Institute, 66 (1980), No. 3, p. 354-362). When this equation is satisfied, the sulfide precipitate is in the form of a composite sulfide in which MnS is deposited on CaS. On the other hand, when (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) ≦ 0, since CaS does not crystallize, S precipitates in the form of MnS alone. This MnS is elongated at the time of rolling the steel sheet to cause a decrease in the toughness of the base material, and the fine dispersion of α transformation generation nuclei in the weld heat affected zone, which is the main point of the present invention, is not achieved. On the contrary, when 1 ≦ (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S), S is completely fixed by Ca, and MnS acting as ferrite nuclei is formed on CaS. Since it does not precipitate, the composite sulfide does not exhibit a sufficient function as an α-forming nucleus. Therefore, (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) is in the range of more than 0 and less than 1. Preferably it is the range of 0.2-0.8.

本発明では、上記の必須成分に加えてさらに、強度および靭性を高めるために、V,CuおよびNiから選ばれる1種または2種以上を含有させることができる。
V:0.01〜0.06mass%
Vは、微量の添加であっても高温強度の上昇に有効であるが、その効果を得るには0.01mass%以上の添加が好ましい。一方、過剰の添加は、溶接部の靭性を劣化させるので、0.06mass%以下とすることが好ましい。
In the present invention, in addition to the essential components described above, one or more selected from V, Cu and Ni can be contained in order to further increase the strength and toughness.
V: 0.01-0.06mass%
V is effective for increasing the high temperature strength even if added in a small amount, but in order to obtain the effect, addition of 0.01 mass% or more is preferable. On the other hand, since excessive addition deteriorates the toughness of the welded portion, it is preferably made 0.06 mass% or less.

Cu:0.1〜1.0mass%
Cuは、母材強度の上昇に有効であるが、その効果を得るには0.1mass%以上添加することが好ましい。一方、1.0mass%を超えると熱間脆性を生じ、鋼板の表面性状を劣化させるので、上限は1.0mass%とすることが好ましい。
Cu: 0.1-1.0mass%
Cu is effective for increasing the strength of the base material, but 0.1 mass% or more is preferably added in order to obtain the effect. On the other hand, if it exceeds 1.0 mass%, hot brittleness is caused and the surface properties of the steel sheet are deteriorated. Therefore, the upper limit is preferably set to 1.0 mass%.

Ni:0.02〜1.5mass%
Niは、常温強度および低温靭性の向上に有効であるが、その効果を得るには0.02mass%以上の添加することが好ましい。一方、過剰の添加は、コストの増加を招くので上限は1.5mass%とするのが好ましい。
Ni: 0.02 to 1.5 mass%
Ni is effective in improving the normal temperature strength and the low temperature toughness, but it is preferable to add 0.02 mass% or more in order to obtain the effect. On the other hand, excessive addition causes an increase in cost, so the upper limit is preferably 1.5 mass%.

次に、本発明の製造条件について鋭明する。
上記組成の溶鋼を、転炉、電気炉、真空溶解炉等の通常の方法で溶製し、連続鋳造法、造塊-分塊圧延法などの通常の方法でスラブ等の鋼素材とする。この素材を以下の工程により高張力鋼を製造する。
まず、上述した成分組成に調整した鋼素材を、1050〜1250℃の温度範囲に加熱する。1050℃以上に加熱するのは、鋳造欠陥を圧着させるためである。しかし、1250℃を超える温度に加熱すると、TiNが粗大化して溶接部の靭性の劣化を招くため、加熱温度は1250℃以下に規制する必要がある。
Next, the manufacturing conditions of the present invention will be clarified.
Molten steel having the above composition is melted by a usual method such as a converter, electric furnace, vacuum melting furnace or the like, and is made into a steel material such as a slab by a usual method such as a continuous casting method or an ingot-bundling rolling method. This material is used to produce high-strength steel by the following process.
First, the steel material adjusted to the above-described component composition is heated to a temperature range of 1050 to 1250 ° C. The reason for heating to 1050 ° C. or higher is to crimp the casting defect. However, when heated to a temperature exceeding 1250 ° C., TiN becomes coarse and causes the toughness of the weld to deteriorate, so the heating temperature must be regulated to 1250 ° C. or lower.

鋼素材を上記温度に加熱後、(Ar3+100℃)以上の温度域における累積圧下率を50%以上とする熱間圧延を行う必要がある。この温度域で上記圧延を施すことにより、素材中央部に発生した鋳造欠陥を圧着できると共に、オーステナイト粒の再結晶を促進して組織を微細化し、ひいてはフェライト組織の微細化を達成し、優れた靭性を確保することができる。しかし、累積圧下率が50%未満であると、鋳造欠陥が圧着しなかったり、あるいは、加熱時の異常粗大粒が残存したりして、母材の靭性に悪影響を及ぼす。 After the steel material is heated to the above temperature, it is necessary to perform hot rolling in which the cumulative rolling reduction in a temperature range of (Ar 3 + 100 ° C.) or higher is 50% or higher. By performing the above rolling in this temperature range, it is possible to press the casting defects generated in the center of the material, promote the recrystallization of austenite grains, refine the structure, and thereby achieve the refinement of the ferrite structure. Toughness can be ensured. However, if the cumulative rolling reduction is less than 50%, casting defects will not be crimped or abnormal coarse grains will remain during heating, which adversely affects the toughness of the base metal.

上記(Ar3+100℃)以上の温度域における熱間圧延に引き続き、仕上圧延温度を(Ar3−100℃)〜(Ar3+100℃)の範囲とする熱間圧延を行った後、冷却速度2〜20℃/secで600℃以下まで冷却し、その後、空冷する。
仕上圧延温度を、(Ar3−100℃)〜(Ar3+100℃)の範囲とする理由は、(Ar3+100℃)超えの温度で圧延を終了した場合には、粗大なオーステナイト粒が残存するため板厚中央部の靭性が劣化する。一方、(Ar3−100℃)未満の温度で圧延を終了した場合には、鋼板表面のフェライト粒が加工歪を受けて硬化し、表面の靭性が劣化するためである。また、熱間圧延後の冷却速度を2〜20℃/secとする理由は、2℃/sec未満では、フェライト分率が増加するため母材の強度が大きく低下するからであり、一方、20℃/sec超えでは、鋼板表面が著しく硬化し、靭性が低下するためである。さらに、冷却停止温度を600℃以下とする理由は、600℃超えで冷却を停止した場合には、フェライト分率が増加するため、母材の強度が大きく低下するからである。
Subsequent to hot rolling in the temperature range above (Ar 3 + 100 ° C.), after performing hot rolling with the finish rolling temperature in the range of (Ar 3 −100 ° C.) to (Ar 3 + 100 ° C.), the cooling rate Cool to 2 ° C./sec to 600 ° C. or lower, then air cool.
The reason why the finishing rolling temperature is in the range of (Ar 3 −100 ° C.) to (Ar 3 + 100 ° C.) is that when rolling is finished at a temperature exceeding (Ar 3 + 100 ° C.), coarse austenite grains remain. As a result, the toughness at the center of the plate thickness deteriorates. On the other hand, when the rolling is finished at a temperature lower than (Ar 3 -100 ° C.), the ferrite grains on the steel sheet surface are hardened due to processing strain and the surface toughness is deteriorated. Moreover, the reason for setting the cooling rate after hot rolling to 2 to 20 ° C./sec is that the strength of the base material is greatly reduced because the ferrite fraction increases at less than 2 ° C./sec. This is because when the temperature exceeds ° C./sec, the surface of the steel sheet is remarkably hardened and the toughness is lowered. Furthermore, the reason why the cooling stop temperature is set to 600 ° C. or less is that when the cooling is stopped at a temperature exceeding 600 ° C., the ferrite fraction increases and the strength of the base material is greatly reduced.

また、本発明では、鋼板の残留応力の低減を目的とし、上記した冷却後、700℃以下の温度で焼戻し処理を施すことが好ましい。この効果を得るためには、焼戻し温度は500℃以上であることが好ましく、一方、700℃を超えると各種炭化物が析出して粗大化し、その後の高温強度を確保することが難しくなる。より好ましい焼戻し温度は、500〜650℃である。   In the present invention, for the purpose of reducing the residual stress of the steel sheet, it is preferable to perform a tempering treatment at a temperature of 700 ° C. or lower after the above cooling. In order to obtain this effect, the tempering temperature is preferably 500 ° C. or higher. On the other hand, when it exceeds 700 ° C., various carbides are precipitated and coarsened, and it is difficult to ensure the high temperature strength thereafter. A more preferable tempering temperature is 500 to 650 ° C.

表1に示す種々の成分組成に調整した鋼を溶製し、鋼スラブとした後、この鋼スラブを、表2に示す条件で熱間圧延を行い板厚40〜75mmの厚鋼板を製造し、その後、焼戻し処理を行った。かくして得られた各厚鋼板について、引張試験及びシャルピー試験を実施した。引張試駿片は、各鋼板の板厚1/4位置から、引張方向が圧延方向に対して垂直方向に平行になるようJIS4号引張試験片(丸棒)採取し、常温および600℃の高温における降伏強さYS、引張強さTSを求めた。また、シャルピー衝撃試験片は、各鋼板の板厚中央部から、長さ方向が圧延方向に対して垂直方向に平行になるようJIS4号衝撃試鹸片を採取し、−40℃における吸収エネルギーを求めた。また、各鋼板の溶接熱サイクル後の靭性を評価するために、幅80mm×長さ80mm×厚み15mmの試験片を採取し、この試験片を1400℃に加熱後、800〜500℃の冷却時間を40sec(サブマージアーク溶接での入熱量約40kJ/cm相当)とする熱サイクルを付与した後、この試験片から上記と同様にしてJIS4号衝撃試験片を採取してシャルピー衝撃試験を実施し、延性脆性破面遷移温度vTrsを測定した。   Steels adjusted to various component compositions shown in Table 1 were melted into steel slabs, and then the steel slabs were hot-rolled under the conditions shown in Table 2 to produce steel plates having a thickness of 40 to 75 mm. Thereafter, a tempering treatment was performed. Each thick steel plate thus obtained was subjected to a tensile test and a Charpy test. Tensile specimens were collected from ¼ position of each steel plate so that the tensile direction was parallel to the direction perpendicular to the rolling direction. The yield strength YS and the tensile strength TS were determined. In addition, Charpy impact test specimens were obtained from JIS No. 4 impact test specimens from the center of the thickness of each steel plate so that the length direction was parallel to the direction perpendicular to the rolling direction, and the absorbed energy at −40 ° C. was measured. Asked. In addition, in order to evaluate the toughness of each steel sheet after the welding heat cycle, a test piece having a width of 80 mm, a length of 80 mm, and a thickness of 15 mm was collected, and after heating the test piece to 1400 ° C., a cooling time of 800 to 500 ° C. After applying a heat cycle of 40 seconds (equivalent to about 40 kJ / cm heat input in submerged arc welding), a JIS No. 4 impact test piece was collected from this test piece in the same manner as described above, and a Charpy impact test was performed. The ductile brittle fracture surface transition temperature vTrs was measured.

上記測定の結果を、表2中に併記して示した。本発明の条件を満たすNo.1〜6は、いずれも、常温における降伏応力が414MPa以上、引張り強さが517MPa以上(API規格2W−60)、−40℃における衝撃吸収エネルギーvE-40℃が200J以上の特性を有しており、母材の常温強度、靭性が共に優れていることがわかる。また、常温の降伏応力に対する600℃における降伏応力の比は53(56)%以上であり高温強度にも優れている。さらに再現熱サイクルによるHAZ部の靭性も、延性脆性破面遷移温度(vTrs)が−20℃以下と優れた溶接熱影響部靭性を有する鋼材となっている。これに対して、本発明の範囲を外れる比較例(No.7〜30)は、母材の降伏応力、引張り強さおよびvE-40℃、600℃における降伏応力、もしくは、再現HAZのvTrsのいずれか1つ以上が本発明例と比較して劣っており、母材部の強度、靭性や高温強度および溶接熱影響部の靭性が共に優れた鋼となっていない。 The results of the above measurements are shown together in Table 2. Nos. 1 to 6 satisfying the conditions of the present invention all have a yield stress at room temperature of 414 MPa or more, a tensile strength of 517 MPa or more (API standard 2W-60), and an impact absorption energy vE of −40 ° C. at −40 ° C. It has a characteristic of 200J or more, and it can be seen that the base material has excellent room temperature strength and toughness. Moreover, the ratio of the yield stress at 600 ° C. to the yield stress at normal temperature is 53 (56)% or more, which is excellent in high temperature strength. Furthermore, the toughness of the HAZ part by the reproducible thermal cycle is a steel material having excellent weld heat affected zone toughness with a ductile brittle fracture surface transition temperature (vTrs) of -20 ° C or lower. On the other hand, the comparative examples (Nos. 7 to 30) outside the scope of the present invention are the yield stress and tensile strength of the base material and the yield stress at vE- 40 ° C. and 600 ° C., or the reproduction HAZ vTrs. Any one or more are inferior to the examples of the present invention, and the steel is not excellent in the strength, toughness, high temperature strength of the base metal part, and toughness of the weld heat affected zone.

本発明により、耐火性と溶接部の靭性に優れた構造用の高張力鋼の製造が可能となる。そのため、耐火性が十分ではない従来鋼では、火災時の構造物の倒壊を防止するために、耐火被覆等の余分なコストが必要であったが、本発明鋼では、これを省略することができる。   According to the present invention, it is possible to produce a high-strength steel for a structure excellent in fire resistance and weld toughness. Therefore, in conventional steels with insufficient fire resistance, extra costs such as fireproof coating were necessary to prevent the collapse of the structure in the event of a fire. it can.

Claims (3)

C:0.03〜0.10mass%、
Si:0.05〜0.30mass%、
Mn:0.6〜1.7mass%、
P:0.015mass%以下、
S:0.0005〜0.0050mass%、
Al:0.005〜0.06mass%、
Cr:0.05〜1.0mass%
Mo:0.2〜0.5mass%
Nb:0.005〜0.05mass%、
Ti:0.005〜0.02mass%、
N:0.0030〜0.0065mass%、
Ca:0.0005〜0.0030mass%を含有し、かつCa,OおよびSの含有量が、下記関係式;
<(Ca−(0.18+130×Ca)×O)/(1.25×S)<1
を満たして含有し、残部がFeおよび不可避的不純物からなる鋼素材を1050〜1250℃に加熱後、(Ar+100℃)以上の温度域における累積圧下率を50%以上とし、仕上げ温度を(Ar−100℃)〜(Ar+100℃)とする熱間圧延をした後、冷却速度2〜20℃/secで600℃以下まで冷却することを特徴とする耐火性および溶接継手部靭性に優れる高張力鋼の製造方法。
C: 0.03-0.10 mass%,
Si: 0.05-0.30 mass%,
Mn: 0.6 to 1.7 mass%,
P: 0.015 mass% or less,
S: 0.0005 to 0.0050 mass%,
Al: 0.005 to 0.06 mass%,
Cr: 0.05-1.0 mass% ,
Mo: 0.2-0.5 mass%
Nb: 0.005 to 0.05 mass%,
Ti: 0.005-0.02 mass%,
N: 0.0030 to 0.0065 mass%,
Ca: 0.0005 to 0.0030 mass%, and the contents of Ca, O, and S are the following relational expressions:
0 <(Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) <1
The meet and containing, after heating the steel material and the balance being Fe and unavoidable impurities in 1,050 to 1250 ° C., and (Ar 3 + 100 ℃) over a cumulative rolling reduction in the temperature range 50% or more, a finishing temperature Fire rolling and weld joint toughness characterized by cooling to 600 ° C. or less at a cooling rate of 2 to 20 ° C./sec after hot rolling with (Ar 3 −100 ° C.) to (Ar 3 + 100 ° C.) For producing high-strength steel with excellent resistance.
上記成分組成に加えてさらに、V:0.01〜0.06mass%、Cu:0.1〜1.0mass%およびNi:0.02〜1.5mass%のうちから選ばれる1種または2種以上を含有することを特徴とする請求項1に記載の高張力鋼の製造方法。 In addition to the above component composition, V: 0.01 to 0.06 mass%, Cu: 0.1 to 1.0 mass%, and Ni: 0.02 to 1.5 mass%, one or two selected The method for producing high-tensile steel according to claim 1, comprising the above. 上記冷却後の高張力鋼を、さらに700℃以下の温度で焼戻し処理することを特徴とする請求項1または2に記載の高張力鋼の製造方法。 The method for producing high-strength steel according to claim 1 or 2, wherein the high-tensile steel after cooling is further tempered at a temperature of 700 ° C or lower.
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