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JP6226163B2 - High-tensile steel plate with excellent low-temperature toughness in heat affected zone and its manufacturing method - Google Patents

High-tensile steel plate with excellent low-temperature toughness in heat affected zone and its manufacturing method Download PDF

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JP6226163B2
JP6226163B2 JP2016556564A JP2016556564A JP6226163B2 JP 6226163 B2 JP6226163 B2 JP 6226163B2 JP 2016556564 A JP2016556564 A JP 2016556564A JP 2016556564 A JP2016556564 A JP 2016556564A JP 6226163 B2 JP6226163 B2 JP 6226163B2
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JPWO2016068094A1 (en
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克行 一宮
克行 一宮
長谷 和邦
和邦 長谷
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    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

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Description

本発明は、船舶や海洋構造物、圧力容器、ペンストックなどの鋼構造物に用いられる高張力鋼板とその製造方法に関し、特に、板厚が35〜100mm、降伏応力YSが460MPa以上で、母材の強度・靭性特性に優れるだけでなく、多層盛溶接部における低温靭性にも優れる厚肉の高張力鋼板とその製造方法に関するものである。  The present invention relates to a high-strength steel plate used for steel structures such as ships, offshore structures, pressure vessels, and penstocks, and a method for producing the same, and in particular, a plate thickness of 35 to 100 mm, a yield stress YS of 460 MPa or more, The present invention relates to a thick high-strength steel sheet that not only excels in strength and toughness characteristics of the material, but also has excellent low-temperature toughness in a multi-layer weld zone, and a method for producing the same.

船舶や海洋構造物、圧力容器、ペンストックなどの鋼構造物に用いられる鋼板(厚鋼板)は、溶接接合して所望の形状・寸法の構造物として仕上げられる。そのため、これらの鋼板には、鋼構造物としての安全性を確保する観点から、母材の強度が高く、靭性に優れていることは勿論のこと、溶接部(溶接金属や熱影響部)の靭性にも優れていることが要求される。  Steel plates (thick steel plates) used in steel structures such as ships, offshore structures, pressure vessels, and penstock are welded and finished as structures of desired shapes and dimensions. Therefore, from the viewpoint of ensuring safety as a steel structure, these steel plates have not only high strength of the base metal and excellent toughness, but also of welded parts (welded metal and heat affected zone). It is required to have excellent toughness.

鋼板の靭性を評価する基準としては、従来、主にシャルピー衝撃試験による吸収エネルギーが用いられてきたが、近年では、より信頼性の高い亀裂開口変位試験(CTOD試験;Crack Tip Opening Displacement Test)が用いられることが多くなってきている。この試験は、靭性評価部に疲労予亀裂を発生させた試験片を3点曲げし、破壊直前の亀裂の口開き量(塑性変形量)を測定して脆性破壊の発生抵抗を評価しようとするものである。なお、以降、上記「CTOD試験」で得られた評価結果を「CTOD値」または「CTOD特性」ともいう。  Conventionally, the absorbed energy by Charpy impact test has been used as the standard for evaluating the toughness of steel sheets, but in recent years, the more reliable crack opening displacement test (CTOD test; Crack Tip Opening Displacement Test) It is increasingly used. This test is intended to evaluate the resistance to brittle fracture by bending a specimen with fatigue precracking in the toughness evaluation section at three points and measuring the amount of crack opening (plastic deformation) just before fracture. Is. Hereinafter, the evaluation result obtained in the “CTOD test” is also referred to as “CTOD value” or “CTOD characteristic”.

ところで、上記CTOD試験では疲労予亀裂を用いるので、極めて微小な領域が靭性の評価部となるため、局所的な脆化域が存在すると、シャルピー衝撃試験で良好な靭性が得られても、低いCTOD値を示すことがある。上記局所脆化域は、板厚が厚い鋼板などに多層盛溶接を施した際、複雑な熱履歴を受ける熱影響部(HAZ:Heat Affected Zone)で発生し易く、特に、ボンド部(溶接金属と母材の境界)や、熱影響部の中で上記ボンド部に近く、多層盛溶接時に2相域に再加熱される部分(1サイクル目の溶接で粗粒となり、後続の溶接による入熱でフェライトとオーステナイトの2相域に再加熱される領域、以降、「2相域再加熱部」ともいう)が局所脆化域となり易い。これは、ボンド部近傍の熱影響部は、融点直下の高温に曝されるため、オーステナイト粒が粗大化し、続く冷却時により靭性の低い上部ベイナイト組織が生成し易いことから、ボンド部近傍の熱影響部の靭性が低下する。また、熱影響部のボンド部近傍の2相域再加熱部では、ウッドマンステッテン組織や島状マルテンサイトMAなどの脆化組織が生成し易いため、靭性はさらに低下するからである。  By the way, since the fatigue precrack is used in the CTOD test, a very small region serves as a toughness evaluation part. Therefore, if a local embrittlement region exists, even if good toughness is obtained in the Charpy impact test, it is low. May indicate CTOD value. The above-mentioned local embrittlement zone is likely to occur in a heat affected zone (HAZ: Heat Affected Zone) that undergoes a complex thermal history when multi-layer welding is performed on a thick steel plate or the like. The boundary between the base metal and the heat affected zone is close to the above bond, and is reheated to the two-phase region during multi-layer welding (coarse grains are formed in the first cycle welding, and heat input by subsequent welding) The region reheated to the two-phase region of ferrite and austenite, hereinafter also referred to as “two-phase region reheating portion”) is likely to be a local embrittlement region. This is because the heat-affected zone in the vicinity of the bond part is exposed to a high temperature just below the melting point, and the austenite grains become coarse, and an upper bainite structure with lower toughness is likely to be formed during subsequent cooling. The toughness of the affected area is reduced. Moreover, in the two-phase region reheated part near the bond part of the heat-affected part, brittle structures such as a Woodman Stetten structure and island martensite MA are likely to be generated, so that the toughness is further reduced.

そこで、上記熱影響部の靭性低下を抑制するため、従来から、鋼中にTiNを微細分散させて、オーステナイト粒の粗大化を抑制したり、フェライト変態核として利用したりする技術が実用化されている。しかし、熱影響部のボンド部近傍では、TiNが溶解する温度域にまで加熱されることがあるため、上記TiNの析出効果が消失してしまうという問題がある。  Therefore, in order to suppress the toughness reduction of the heat affected zone, conventionally, a technique for finely dispersing TiN in steel to suppress coarsening of austenite grains or to use as a ferrite transformation nucleus has been put into practical use. ing. However, in the vicinity of the bond part of the heat affected zone, there is a problem that the TiN precipitation effect disappears because it may be heated to a temperature range where TiN dissolves.

その他の技術としては、例えば、特許文献1や特許文献2には、希土類元素(REM:Rare Earth Metal)をTiと共に複合添加し、鋼中に微細粒子を分散させることによって、オーステナイトの粒成長を抑制し、溶接部の靭性を向上させる技術が開示されている。
また、Tiの酸化物を鋼中に微細分散させる技術や、BNのフェライト核生成能と酸化物分散を組み合わせる技術、さらには、CaやREMを添加して硫化物の形態を制御することにより、靭性を高める技術も提案されている。しかし、これらの技術は、比較的低強度で、合金元素量の少ない鋼板を対象としており、合金元素の添加量が多く、熱影響部の組織がフェライトを含まない組織となる高張力鋼板には適用することができない。
As other techniques, for example, in Patent Document 1 and Patent Document 2, a rare earth element (REM: Rare Earth Metal) is added together with Ti, and fine particles are dispersed in steel, thereby austenite grain growth. Techniques for suppressing and improving the toughness of the weld are disclosed.
In addition, the technology of finely dispersing Ti oxide in steel, the technology of combining BN ferrite nucleation ability and oxide dispersion, and further adding Ca and REM to control the form of sulfide, Techniques for increasing toughness have also been proposed. However, these technologies are intended for steel sheets with relatively low strength and a small amount of alloying elements. For high-tensile steel sheets where the amount of alloying elements is large and the structure of the heat affected zone does not contain ferrite. It cannot be applied.

また、2相域再加熱部は、多層盛溶接時における2相域への再加熱で、オーステナイトに逆変態した領域に炭素が濃化し、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が生成されるため、靭性が低下するという問題がある。そこで、鋼成分を低C、低Si化して島状マルテンサイトの生成を抑制して靭性を向上し、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(例えば、特許文献3、4参照)。これらの技術は、時効処理によってCuを析出させて強度を高めるものであるが、多量のCuを添加するため、熱間加工性が低下し、生産性を阻害するという問題がある。  Also, the two-phase region reheating part is a reheating to the two-phase region during multi-layer welding, in which carbon is concentrated in a region reversely transformed into austenite, and a brittle bainite structure including island martensite is formed during cooling. Since it is produced, there is a problem that toughness is lowered. Therefore, a technique is disclosed in which the steel component is made low C, low Si to suppress the formation of island martensite to improve toughness, and further, by adding Cu, the base material strength is increased (for example, Patent Documents 3 and 4) These techniques are intended to increase the strength by precipitating Cu by aging treatment. However, since a large amount of Cu is added, there is a problem that hot workability is lowered and productivity is hindered.

ところで、近年、船舶や海洋構造物、圧力容器、ペンストックなどの鋼構造物は、大型化する傾向にあり、それに伴い、鋼構造物に使用される鋼板の厚肉化と高強度化が進められている。鋼構造物に用いられる鋼板には、主として板厚が35〜100mm程度の厚肉鋼板が用いられているが、降伏応力YSが420MPa級やそれ以上の強度を得るためには、合金元素の添加量が多い方が有利である。しかし、合金元素の多量の添加は、ボンド部や2相域再加熱部の靭性確保を困難にすることは上述したとおりである。  By the way, in recent years, steel structures such as ships, offshore structures, pressure vessels, and penstocks have tended to increase in size, and accordingly, steel sheets used for steel structures have been made thicker and stronger. It has been. For steel plates used in steel structures, thick steel plates with a plate thickness of about 35 to 100 mm are mainly used. In order to obtain a yield stress YS of 420 MPa or higher, addition of alloying elements is required. Larger amounts are advantageous. However, as described above, the addition of a large amount of the alloy element makes it difficult to ensure the toughness of the bond portion and the two-phase region reheated portion.

この問題に対して、特許文献5には、熱影響部の2相域再加熱部の島状マルテンサイトMAを低減するために、MnとCrの添加量を適正化する技術が開示されている。しかし、この技術は、入熱量が500kJ/cmを超える大入熱溶接用鋼材に関する技術であり、入熱量が100kJ/cm以下の小〜中入熱で多層盛溶接する鋼板を対象とするものではない。また、特許文献5の技術は、Nbを添加していないため、制御圧延技術を適用することができず、母材の靭性が十分ではないという問題がある。  With respect to this problem, Patent Document 5 discloses a technique for optimizing the addition amounts of Mn and Cr in order to reduce the island-like martensite MA in the two-phase region reheating portion of the heat affected zone. . However, this technology is a technology related to a steel material for large heat input welding with a heat input amount exceeding 500 kJ / cm, and is not intended for a steel plate that is multilayer-welded with small to medium heat inputs with a heat input amount of 100 kJ / cm or less. Absent. Moreover, since the technique of patent document 5 does not add Nb, there is a problem that the control rolling technique cannot be applied and the toughness of the base material is not sufficient.

また、特許文献6には、所定の成分組成の下で炭素当量Ceqを適正化することによって、合金元素の多い鋼成分であっても、420MPa以上の降伏応力と良好な低温靭性、特にCTOD特性を両立させる技術が提案されている。この技術によって、降伏応力が420MPa以上で、小〜中入熱による多層盛溶接した熱影響部のCTOD特性に優れる鋼構造物に用いて好適な高張力鋼材を製造することが可能となった。  Further, Patent Document 6 describes that by optimizing the carbon equivalent Ceq under a predetermined component composition, a yield stress of 420 MPa or more and good low-temperature toughness, particularly CTOD characteristics, even for a steel component with a large amount of alloy elements. A technique for achieving both has been proposed. This technique makes it possible to produce a high-tensile steel material suitable for use in a steel structure having a yield stress of 420 MPa or more and excellent in the CTOD characteristics of a heat-affected zone multilayer welded by small to medium heat input.

特公平03−053367号公報Japanese Patent Publication No. 03-053367 特開昭60−184663号公報JP 60-184663 A 特開平05−186823号公報JP 05-186823 A 特開2001−335884号公報Japanese Patent Laid-Open No. 2001-335484 特許第5365146号公報Japanese Patent No. 5365146 特開2012−184500号公報JP 2012-184500 A

上述したように、近年、鋼構造物は、重厚長大化する傾向にあり、それに伴い、船舶や海洋構造物の分野においては、高強度(高降伏応力)で板厚が厚く、かつ、溶接熱影響部の低温靭性に優れる鋼板、特に、降伏応力が460MPa以上、板厚が35〜100mmで、多層盛溶接した溶接熱影響部が優れたCTOD特性を有する鋼板に対する要望が高まっている。  As described above, in recent years, steel structures tend to be heavy and long, and accordingly, in the field of ships and marine structures, high strength (high yield stress), thick plate thickness, and welding heat There is a growing demand for steel plates that are excellent in low temperature toughness of the affected zone, particularly steel plates that have a yield stress of 460 MPa or more, a plate thickness of 35 to 100 mm, and a CTOD characteristic in which a multilayer heat-welded weld heat affected zone is excellent.

上記の要望に対しては、前述した特許文献6に開示の技術によって、合金元素の多い鋼成分系であっても420MPa以上の降伏応力と良好な低温靭性(CTOD特性)を実現するための方途は拓かれた。しかし、板厚が50mm超の鋼板においては、板厚が50mm以下の鋼板の場合と同様の強度特性を得るまでには至っていない。すなわち、特許文献6に記載の技術では、板厚が50mm以下の鋼板では降伏応力が500MPa以上の強度が得られるが、例えば、板厚が70mmの鋼板では、降伏応力が高々462MPa程度のものしか得られない。また、高強度化を狙って、合金元素を単に多量に添加するだけでは、CTOD特性が低下してしまう。  In response to the above-described demand, the technique disclosed in Patent Document 6 described above is a method for realizing a yield stress of 420 MPa or more and good low temperature toughness (CTOD characteristics) even in a steel component system having many alloy elements. Was cultivated. However, steel sheets having a thickness of more than 50 mm have not yet achieved the same strength characteristics as those of steel sheets having a thickness of 50 mm or less. That is, in the technique described in Patent Document 6, a steel sheet having a thickness of 50 mm or less can provide a strength with a yield stress of 500 MPa or more. For example, a steel sheet with a thickness of 70 mm has a yield stress of about 462 MPa at most. I cannot get it. Further, simply adding a large amount of the alloy element with the aim of increasing the strength will deteriorate the CTOD characteristics.

本発明は、従来技術が抱える上記問題点に鑑みてなされたものであり、その目的は、板厚が35〜100mm、降伏応力が460MPa以上で、入熱量が100kJ/cm以下で多層盛溶接した溶接熱影響部におけるCTOD特性に優れる高張力鋼板を提供するとともに、その有利な製造方法を提案することにある。  The present invention has been made in view of the above-described problems of the prior art, and the purpose thereof is multilayer overlay welding with a plate thickness of 35 to 100 mm, a yield stress of 460 MPa or more, and a heat input of 100 kJ / cm or less. An object of the present invention is to provide a high-tensile steel sheet having excellent CTOD characteristics in the heat affected zone and to propose an advantageous manufacturing method thereof.

発明者らは、上記課題を解決するため、鋼の成分組成が溶接部に及ぼす影響に着目して鋭意検討を重ねた。その結果、以下のことを知見した。
i)CTOD特性は、鋼板全厚の試験片で評価されるため、中心偏析部が破壊の起点となる。従って、溶接熱影響部のCTOD特性を向上するためには、中心偏析し易い元素、具体的にはC,Mn,P,NiおよびNbの含有量を適正範囲に制御し、中心偏析部の硬さの上昇を抑制することが重要である。
ii)溶接熱影響部の靭性を向上させるためには、鋼中にTi/Nを均一微細に分散析出させて、熱影響部の溶接ボンド部近傍でのオーステナイト粒の粗大化を抑制することが有効である。
iii)CTOD値と強度とはトレードオフの関係にあるので、単にCeqを上昇させて高強度化を図ると、CTOD特性が低下してしまうが、低C−低P−高Ni系の成分系とすることで、強度−CTOD特性のバランスを改善することができる。
iv)さらに、溶接熱影響部の靭性をより向上させるためには、硫化物の形態制御を目的として添加しているCaの化合物(CaS)を晶出させて、溶接熱影響部の靭性向上に利用するのが有効である。
In order to solve the above-mentioned problems, the inventors have conducted intensive studies focusing on the influence of the composition of steel on the weld. As a result, the following was found.
i) Since the CTOD characteristic is evaluated with a test piece having a full thickness of the steel sheet, the central segregation portion is the starting point of the fracture. Therefore, in order to improve the CTOD characteristics of the weld heat affected zone, the content of elements that easily undergo center segregation, specifically, the contents of C, Mn, P, Ni, and Nb are controlled within an appropriate range, and the hardness of the center segregation zone is increased. It is important to suppress the increase in height.
ii) In order to improve the toughness of the weld heat affected zone, Ti / N is uniformly and finely dispersed and precipitated in the steel to suppress the austenite grain coarsening in the vicinity of the weld bond portion of the heat affected zone. It is valid.
iii) Since the CTOD value and the strength are in a trade-off relationship, simply increasing Ceq to increase the strength decreases the CTOD characteristics, but the low C-low P-high Ni component system By doing so, the balance of strength-CTOD characteristics can be improved.
iv) Further, in order to further improve the toughness of the weld heat affected zone, the Ca compound (CaS) added for the purpose of controlling the form of sulfide is crystallized to improve the toughness of the weld heat affected zone. It is effective to use.

上記知見に基いて開発した本発明は、C:0.010〜0.050mass%、Si:0.01〜0.50mass%、Mn:1.80〜3.50mass%、P:0.012mass%以下、S:0.0035mass%以下、sol.Al:0.010〜0.060mass%、Ni:0.1〜2.0mass%、Cr:1.0〜3.0mass%、Nb:0.005〜0.040mass%、Ti:0.005〜0.025mass%、N:0.0020〜0.0050mass%を含有し、さらに、Crを35Cr+8Mn≧63かつ7Cr+18Mn≦63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成分組成を有する高張力鋼板である。  The present invention developed based on the above knowledge is C: 0.010 to 0.050 mass%, Si: 0.01 to 0.50 mass%, Mn: 1.80 to 3.50 mass%, P: 0.012 mass%. Hereinafter, S: 0.0035 mass% or less, sol. Al: 0.010-0.060 mass%, Ni: 0.1-2.0 mass%, Cr: 1.0-3.0 mass%, Nb: 0.005-0.040 mass%, Ti: 0.005 0.025 mass%, N: 0.0020 to 0.0050 mass%, and further contains Cr satisfying 35Cr + 8Mn ≧ 63 and 7Cr + 18Mn ≦ 63, with the balance being composed of Fe and inevitable impurities It is a tension steel plate.

本発明の上記高張力鋼板は、上記成分組成に加えてさらに、Cu:1.0mass%未満、Mo:0.05〜0.50mass%、V:0.005〜0.05mass%、B:0.0005〜0.0030mass%、Ca:0.0005〜0.0050mass%およびMg:0.0002〜0.0030mass%の中から選ばれる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 high-strength steel sheet of the present invention further includes Cu: less than 1.0 mass%, Mo: 0.05 to 0.50 mass%, V: 0.005 to 0.05 mass%, and B: 0. It is characterized by containing one or more selected from the group consisting of .0005 to 0.0030 mass%, Ca: 0.0005 to 0.0050 mass%, and Mg: 0.0002 to 0.0030 mass%.

また、本発明の高張力鋼板は、Oの含有量が0.0030mass%以下であり、かつ、上記Ca,SおよびOが下記(1)式;
0<{Ca−(0.18+130×Ca)×O}/1.25/S<1 …(1)
ただし、上記式中の各元素記号は、それぞれの元素の含有量(mass%)である。
を満たして含有することを特徴とする。
In the high-tensile steel sheet of the present invention, the O content is 0.0030 mass% or less, and the Ca, S, and O are represented by the following formula (1):
0 <{Ca− (0.18 + 130 × Ca) × O} /1.25/S <1 (1)
However, each element symbol in the above formula is the content (mass%) of each element.
It is characterized by containing.

また、本発明は、上記のいずれかに記載の成分組成を有する鋼素材を1030〜1200℃に加熱した後、950℃以上の温度域における累積圧下率を30%以上、950℃未満の温度域における累積圧下率を30〜70%とする熱間圧延し、その後、600℃以下まで冷却速度1.0℃/s以上で加速冷却することを特徴とする高張力鋼板の製造方法を提案する。  Moreover, this invention heats the steel raw material which has a component composition in any one of said to 1030-1200 degreeC, Then, 30% or more of the cumulative reduction in the temperature range of 950 degreeC or more is a temperature range less than 950 degreeC. A method for producing a high-strength steel sheet is proposed in which hot rolling is performed at a cumulative rolling reduction of 30 to 70% and accelerated cooling is performed at a cooling rate of 1.0 ° C./s or higher to 600 ° C. or lower.

また、本発明の上記高張力鋼板の製造方法は、600℃以下まで加速冷却した後、さらに、450〜650℃の温度で焼戻処理を施すことを特徴とする。  Moreover, the manufacturing method of the said high-strength steel plate of this invention is further characterized by performing a tempering process at the temperature of 450-650 degreeC after accelerated cooling to 600 degrees C or less.

本発明によれば、船舶や海洋構造物などの大型鋼構造物に用いて好適な、板厚が35〜100mm、降伏応力が460MPa以上でも、入熱が100kJ/cm以下で多層盛溶接した熱影響部の低温靭性、特に−60℃以下の極低温でのCTOD特性に優れる高張力鋼板を安定して製造し、提供することができる。  According to the present invention, heat which is multilayer-welded with a heat input of 100 kJ / cm or less, suitable for large steel structures such as ships and marine structures, even when the plate thickness is 35 to 100 mm and the yield stress is 460 MPa or more. It is possible to stably produce and provide a high-tensile steel sheet that is excellent in low-temperature toughness of the affected part, in particular, CTOD characteristics at an extremely low temperature of −60 ° C. or less.

まず、本発明の高張力鋼板が有すべき成分組成について説明する。
C:0.010〜0.050mass%
Cは、高張力鋼板としての母材強度の確保に必要な元素である。Cが0.010mass%未満では焼入性が低下するため、目標の強度(YS≧460MPa)を確保するためには、CuやNi,Cr,Moなどの焼入性向上元素を多量に添加することが必要となり、原料コストの上昇や溶接性の低下を招く。一方、Cが0.050mass%を超えると、溶接部の靭性が低下する。よって、Cは0.010〜0.050mass%の範囲とする。好ましくは0.015〜0.050mass%の範囲である。
First, the component composition that the high-tensile steel sheet of the present invention should have will be described.
C: 0.010-0.050 mass%
C is an element necessary for ensuring the strength of the base material as a high-tensile steel plate. When C is less than 0.010 mass%, the hardenability deteriorates. Therefore, in order to ensure the target strength (YS ≧ 460 MPa), a large amount of a hardenability improving element such as Cu, Ni, Cr, Mo is added. This increases the cost of raw materials and decreases weldability. On the other hand, when C exceeds 0.050 mass%, the toughness of the welded portion decreases. Therefore, C is set to a range of 0.010 to 0.050 mass%. Preferably it is the range of 0.015-0.050 mass%.

Si:0.01〜0.50mass%
Siは、脱酸材として、また、母材の強度を高めるために添加する元素であり、0.01mass%以上添加する必要がある。しかし、0.50mass%を超える多量の添加は、溶接性の低下と溶接部の靭性低下を招く。よってSiは0.01〜0.50mass%の範囲とする。好ましくは0.01〜0.35mass%の範囲である。
Si: 0.01 to 0.50 mass%
Si is an element added as a deoxidizing material and for increasing the strength of the base material, and it is necessary to add 0.01 mass% or more. However, a large amount of addition exceeding 0.50 mass% causes a decrease in weldability and a decrease in toughness of the welded portion. Therefore, Si is set to a range of 0.01 to 0.50 mass%. Preferably it is the range of 0.01-0.35 mass%.

Mn:1.8〜3.5mass%
Mnは、母材および溶接部の強度を確保するため、1.8mass%以上添加する必要がある。しかし、3.5mass%を超える添加は、溶接性を低下させるだけでなく、焼入性が過剰となって母材および溶接部の靭性を低下させる。よって、Mnは1.8〜3.5mass%の範囲とする。好ましくは1.8〜3.3mass%の範囲である。
Mn: 1.8 to 3.5 mass%
Mn needs to be added in an amount of 1.8 mass% or more in order to ensure the strength of the base material and the weld. However, addition exceeding 3.5 mass% not only lowers weldability but also causes excessive hardenability and lowers the toughness of the base metal and the weld. Therefore, Mn is in the range of 1.8 to 3.5 mass%. Preferably it is the range of 1.8-3.3 mass%.

P:0.012mass%以下
Pは、鋼中に不可避的に混入してくる不純物元素であり、母材および溶接部の靭性を低下させる有害元素でもある。特に0.012mass%を超えると、CTOD特性が著しく低下するため、本発明ではPの上限を0.012mass%に制限する。好ましくは0.008mass%以下である。
P: 0.012 mass% or less P is an impurity element that is inevitably mixed in steel, and is also a harmful element that lowers the toughness of the base material and the weld. In particular, if it exceeds 0.012 mass%, the CTOD characteristics are remarkably deteriorated. Therefore, in the present invention, the upper limit of P is limited to 0.012 mass%. Preferably it is 0.008 mass% or less.

S:0.0035mass%以下
Sは、鋼中に不可避的に混入してくる不純物元素であり、0.0035mass%超え含有すると、母材および溶接部の靭性を低下させる。よって、Sの上限は0.0035mass%とする。好ましくは0.0030mass%以下である。
S: 0.0035 mass% or less S is an impurity element inevitably mixed in steel, and when contained in excess of 0.0035 mass%, the toughness of the base material and the welded portion is lowered. Therefore, the upper limit of S is set to 0.0035 mass%. Preferably it is 0.0030 mass% or less.

Cr:1.0〜3.0mass%
Crは、鋼の焼入性を向上して母材の強度や靭性を確保するのに有用な元素である。また、フェライト安定化元素であり、Mnによるオーステナイトの過度の安定化を防止し、島状マルテンサイトの生成を抑制する効果を有する。このような効果を得るためには、1.0mass%以上の添加を必要とする。しかし、Crを過剰に添加すると、熱影響部の硬さが上昇し、靭性が低下するので、上限は3.0mass%とする。好ましくは1.0〜2.8mass%の範囲である。
Cr: 1.0-3.0 mass%
Cr is an element useful for improving the hardenability of steel and ensuring the strength and toughness of the base material. Moreover, it is a ferrite stabilizing element and has the effect of preventing the excessive stabilization of austenite by Mn and suppressing the formation of island martensite. In order to acquire such an effect, addition of 1.0 mass% or more is required. However, if Cr is added excessively, the hardness of the heat-affected zone increases and the toughness decreases, so the upper limit is made 3.0 mass%. Preferably it is the range of 1.0-2.8 mass%.

sol.Al:0.010〜0.060mass%
Alは、溶鋼を脱酸するために添加される元素であり、sol.Alで0.010mass%以上含有させる必要がある。一方、0.060mass%を超えて添加すると、母材および溶接部の靭性を低下させるとともに、溶接による希釈によって溶接金属に混入し、靭性を低下させるので、上限は0.060mass%とする。好ましくは、sol.Alで0.017〜0.055mass%の範囲である。
sol. Al: 0.010-0.060 mass%
Al is an element added to deoxidize molten steel. It is necessary to contain 0.010 mass% or more with Al. On the other hand, if added in excess of 0.060 mass%, the toughness of the base metal and the welded portion is reduced and mixed with the weld metal by dilution by welding to reduce the toughness, so the upper limit is made 0.060 mass%. Preferably, sol. The range of Al is 0.017 to 0.055 mass%.

Ni:0.1〜2.0mass%
Niは、鋼の強度と靭性の向上に有効である他、溶接部のCTOD特性の向上にも有効な元素である。これらの効果を得るためには、0.1mass%以上の添加が必要である。しかし、Niは高価な元素であること、過度の添加は、鋳造時にスラブ表面疵の発生を招くことから、上限を2.0mass%とする。好ましくは0.1〜1.8mass%の範囲である。
Ni: 0.1 to 2.0 mass%
Ni is an element effective not only for improving the strength and toughness of steel but also for improving the CTOD characteristics of the weld. In order to obtain these effects, it is necessary to add 0.1 mass% or more. However, since Ni is an expensive element and excessive addition causes generation of slab surface flaws during casting, the upper limit is set to 2.0 mass%. Preferably it is the range of 0.1-1.8 mass%.

Nb:0.005〜0.040mass%
Nbは、オーステナイトの低温域における未再結晶域の形成に寄与する元素であり、この温度域で熱間圧延することにより、母材の組織微細化および高靭性化を図ることができる。また、焼入れ性の向上や焼戻し軟化抵抗にも効果があり、母材強度の向上に有効な元素でもある。上記の効果を得るためには、0.005mass%以上含有させる必要がある。しかし、0.040mass%を超えて添加すると、溶接熱影響部の靭性が低下するため、上限を0.040mass%とする。好ましくは0.007〜0.035mass%の範囲である。
Nb: 0.005-0.040 mass%
Nb is an element that contributes to the formation of a non-recrystallized region in the low temperature region of austenite. By hot rolling in this temperature region, the structure of the base material can be refined and the toughness can be increased. In addition, it is effective in improving hardenability and tempering softening resistance, and is also an effective element for improving the base material strength. In order to acquire said effect, it is necessary to contain 0.005 mass% or more. However, if added over 0.040 mass%, the toughness of the weld heat affected zone decreases, so the upper limit is made 0.040 mass%. Preferably it is the range of 0.007-0.035 mass%.

Ti:0.005〜0.025mass%
Tiは、溶鋼が凝固する際、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し、溶接部の靭性向上に寄与する。しかし、0.005mass%未満の含有では、その効果が小さく、一方、0.025mass%を超えて含有すると、TiNが粗大化して母材や溶接部の靭性改善効果が得られない。よって、Tiは0.005〜0.025mass%の範囲とする。好ましくは0.006〜0.023mass%の範囲である。
Ti: 0.005-0.025 mass%
Ti precipitates as TiN when the molten steel is solidified, and suppresses austenite coarsening in the welded portion, thereby contributing to improvement in the toughness of the welded portion. However, when the content is less than 0.005 mass%, the effect is small. On the other hand, when the content exceeds 0.025 mass%, TiN is coarsened and the toughness improving effect of the base material and the welded portion cannot be obtained. Therefore, Ti is set to a range of 0.005 to 0.025 mass%. Preferably it is the range of 0.006-0.023 mass%.

N:0.0020〜0.0050mass%
Nは、TiやAlと結合して析出物を形成することによって結晶粒を微細化し、母材の靭性を向上させる効果がある。また、溶接熱影響部の組織の粗大化を抑制するTiNを形成するために必要な元素でもある。これらの効果を発現させるためには、Nを0.0020mass%以上含有させる。しかし、0.0050mass%を超えて添加すると、固溶Nの増加により、母材や溶接部の靭性が著しく低下したり、TiNb複合析出物の生成に伴う固溶Nbの減少によって強度低下をまねいたりすることから、上限を0.0050mass%とする。好ましくは0.0020〜0.0047mass%の範囲である。
N: 0.0020 to 0.0050 mass%
N has an effect of refining crystal grains by combining with Ti or Al to form precipitates and improving the toughness of the base material. It is also an element necessary for forming TiN that suppresses the coarsening of the structure of the heat affected zone. In order to express these effects, N is contained in an amount of 0.0020 mass% or more. However, if added over 0.0050 mass%, the toughness of the base metal and the welded portion is remarkably lowered due to an increase in the solid solution N, and the strength is lowered due to the decrease in the solid solution Nb accompanying the formation of TiNb composite precipitates. Therefore, the upper limit is set to 0.0050 mass%. Preferably it is the range of 0.0020-0.0047 mass%.

35Cr+8Mn≧63かつ7Cr+18Mn≦63
Crは、フェライト安定化元素であり、Mnによる過度のオーステナイト安定化を緩和し、溶接熱影響部における島状マルテンサイトMAの生成を抑制する効果がある。すなわち、Mn単独添加では焼入れ性を高めるだけであるが、フェライト安定化元素であるCrを添加することによって、Mnによる過度のオーステナイト安定化が緩和され、島状マルテンサイトの生成が抑制される。上記効果を得るためには、CrをMnとの関係において、35Cr+8Mn≧63かつ7Cr+18Mn≦63を満たすよう添加することが必要である。なお、好ましくは35Cr+8Mnは65以上、7Cr+18Mnは62以下である。
35Cr + 8Mn ≧ 63 and 7Cr + 18Mn ≦ 63
Cr is a ferrite stabilizing element, and has an effect of relaxing excessive austenite stabilization by Mn and suppressing generation of island martensite MA in the weld heat affected zone. That is, the addition of Mn alone only enhances the hardenability, but the addition of Cr, which is a ferrite stabilizing element, alleviates excessive austenite stabilization by Mn and suppresses the formation of island martensite. In order to obtain the above effect, it is necessary to add Cr so as to satisfy 35Cr + 8Mn ≧ 63 and 7Cr + 18Mn ≦ 63 in relation to Mn. In addition, Preferably 35Cr + 8Mn is 65 or more, and 7Cr + 18Mn is 62 or less.

本発明の高張力鋼板は、上記成分組成に加えてさらに、Cu,Mo,V,B,CaおよびMgのうちから選ばれる1種または2種以上を下記の範囲で含有することができる。
Cu:1.0mass%未満
Cuは、母材の強度を高めるのに有効な元素であり、上記効果を得るためには0.1mass%以上添加するのが好ましい。しかし、1.0mass%以上の添加は、熱間加工性を低下するため、1.0mass%未満とすることが好ましい。より好ましくは、0.7mass%以下である。
The high-tensile steel sheet of the present invention can further contain one or more selected from Cu, Mo, V, B, Ca, and Mg in the following range in addition to the above component composition.
Cu: Less than 1.0 mass% Cu is an element effective for increasing the strength of the base material, and it is preferable to add 0.1 mass% or more in order to obtain the above effect. However, since addition of 1.0 mass% or more reduces hot workability, it is preferable to make it less than 1.0 mass%. More preferably, it is 0.7 mass% or less.

Mo:0.05〜0.50mass%
Moは、母材を高強度化するのに有効な元素であり、特に高張力鋼板での強度向上効果が大きい。上記効果を得るためには0.05mass%以上添加するのが好ましい。しかし、過剰の添加は靭性に悪影響を及ぼすため、上限は0.50mass%とするのが好ましい。より好ましくは0.10〜0.45mass%の範囲である。
Mo: 0.05-0.50 mass%
Mo is an element effective for increasing the strength of the base material, and has a particularly large strength improvement effect in a high-tensile steel plate. In order to acquire the said effect, adding 0.05 mass% or more is preferable. However, excessive addition adversely affects toughness, so the upper limit is preferably 0.50 mass%. More preferably, it is the range of 0.10-0.45 mass%.

V:0.005〜0.05mass%
Vは、母材の強度と靭性の向上に有効な元素であるので、0.005mass%以上添加することができる。しかし、0.05mass%を超えると、靭性の低下を招くため、添加する場合は0.005〜0.05mass%の範囲とするのが好ましい。より好ましくは0.005〜0.045mass%の範囲である。
V: 0.005-0.05 mass%
V is an element effective for improving the strength and toughness of the base material, and therefore can be added in an amount of 0.005 mass% or more. However, if it exceeds 0.05 mass%, the toughness is reduced, so when added, it is preferably in the range of 0.005 to 0.05 mass%. More preferably, it is the range of 0.005-0.045 mass%.

B:0.0005〜0.0030mass%未満
Bは、オーステナイト域から冷却される際にオーステナイト粒界に偏析し、焼き入れ性を向上することで強度を高める効果がある。この効果を得るためには、0.0005mass%以上の添加するのが好ましい。しかし、過剰の添加は、溶接熱影響部に、島状マルテンサイト(MA)を多量に含むベイナイト組織を生成し、靭性を低下させる。よって、Bは0.0005〜0.0030mass%の範囲で添加するのが好ましい。より好ましくは0.0006〜0.0020mass%の範囲である。
B: 0.0005 to less than 0.0030 mass% B is segregated at the austenite grain boundary when cooled from the austenite region, and has the effect of increasing the strength by improving the hardenability. In order to acquire this effect, it is preferable to add 0.0005 mass% or more. However, excessive addition generates a bainite structure containing a large amount of island martensite (MA) in the weld heat-affected zone, and lowers toughness. Therefore, it is preferable to add B in the range of 0.0005 to 0.0030 mass%. More preferably, it is the range of 0.0006-0.0020 mass%.

Ca:0.0005〜0.0050mass%
Caは、Sと結合してCaSを形成し、Sを固定することによって硫化物の形態を制御し、靭性を向上する元素である。この効果を得るためには、0.0005mass%以上添加するのが好ましい。しかし、0.0050mass%を超えて添加しても、その効果は飽和する。よって、Caは0.0005〜0.0050mass%の範囲で添加するのが好ましい。より好ましくは0.0007〜0.0035mass%の範囲である。
Ca: 0.0005 to 0.0050 mass%
Ca combines with S to form CaS, and by fixing S, the form of sulfide is controlled and the toughness is improved. In order to acquire this effect, it is preferable to add 0.0005 mass% or more. However, even if added over 0.0050 mass%, the effect is saturated. Therefore, Ca is preferably added in the range of 0.0005 to 0.0050 mass%. More preferably, it is the range of 0.0007-0.0035 mass%.

Mg:0.0002〜0.0030mass%
Mgは、Caと同様にSを固定することによって靭性を向上する元素である。この効果を得るためには、0.0002mass%以上添加するのが好ましい。しかし、0.0030mass%を超えて添加しても、その効果は飽和する。よって、Mgは0.0002〜0.0030mass%の範囲で添加するのが好ましい。より好ましくは0.0003〜0.0020mass%の範囲である。
Mg: 0.0002 to 0.0030 mass%
Mg is an element that improves toughness by fixing S in the same manner as Ca. In order to acquire this effect, it is preferable to add 0.0002 mass% or more. However, even if added over 0.0030 mass%, the effect is saturated. Therefore, it is preferable to add Mg in the range of 0.0002 to 0.0030 mass%. More preferably, it is the range of 0.0003-0.0020 mass%.

O:0.0030mass%以下
Oは、母材の靭性を低下する元素であり、特に0.0030mass%を超えて含有すると、上記の悪影響が顕著となるので、上限は0.0030mass%とするのが好ましい。より好ましくは0.0025mass%以下である。
O: 0.0030 mass% or less O is an element that lowers the toughness of the base material. In particular, when the content exceeds 0.0030 mass%, the above-described adverse effect becomes significant, so the upper limit is made 0.0030 mass%. Is preferred. More preferably, it is 0.0025 mass% or less.

本発明の高張力鋼板は、上記に説明したCa,SおよびOが下記(1)式;
0<{Ca−(0.18+130×Ca)×O}/1.25/S<1 …(1)
ただし、上記式中の各元素記号は、それぞれの元素の含有量(mass%)である。
を満たして含有することが好ましい。
上記(1)式の中辺である({Ca−(0.18+130×Ca)×O}/1.25/Sが、<1)は、硫化物形態制御に有効なCaとSの原子濃度の比を示す指標値(ACR(Atomic Concentration Ratio)とも称される)であり、硫化物の形態を推定することができる。
In the high-tensile steel plate of the present invention, the above-described Ca, S and O are represented by the following formula (1);
0 <{Ca− (0.18 + 130 × Ca) × O} /1.25/S <1 (1)
However, each element symbol in the above formula is the content (mass%) of each element.
It is preferable to contain and satisfy.
The middle side of the above formula (1) ({Ca− (0.18 + 130 × Ca) × O} /1.25/S <1) is an effective atomic concentration of Ca and S for sulfide form control. This is an index value (also referred to as ACR (Atomic Concentration Ratio)) indicating the ratio of the sulfide, and the form of sulfide can be estimated.

Caの硫化物であるCaSは、酸化物に比べて低温で晶出するため、均一に微細分散させるのに有利である。そこで、CaSを晶出させるとともに、CaS晶出後も固溶Sを確保するようにすれば、晶出したCaSの表面上にMnSが析出して、高温でも溶解し難い複合硫化物を形成する。さらに、上記MnSの周囲には、Mnの希薄帯が形成されるので、フェライト変態がより促進される。  Since CaS, which is a sulfide of Ca, crystallizes at a lower temperature than oxides, it is advantageous for uniform fine dispersion. Therefore, if CaS is crystallized and solid solution S is ensured even after CaS crystallization, MnS precipitates on the surface of the crystallized CaS to form a composite sulfide that is difficult to dissolve even at high temperatures. . Further, since a Mn dilute band is formed around the MnS, the ferrite transformation is further promoted.

上記のようにCaSを微細に分散して晶出させるためには、Caの添加量および添加時の溶鋼中のSやOの含有量を適正範囲に制御する必要があり、ACRの値を上記(1)の範囲に制御することによってのみ、フェライト変態生成核となるCaSを微細に分散させることができる。
上記ACRが0以下では、CaSが晶出せず、SはMnS単独の形態で析出するので、溶接熱影響部のフェライト生成核が得られない。また、単独で析出したMnSは、圧延時に伸長されて母材の靭性低下を引き起こす原因となる。一方、ACRが1以上では、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上に析出しなくなり、フェライト生成核となる複合硫化物の微細分散を実現することができないため、溶接熱影響部の靭性向上効果が得られない。したがって、上記ACRの値が0超え1未満の場合にのみ、CaS上にMnSが析出して複合硫化物を形成し、これがフェライト生成核として有効に機能する。なお、より好ましいACRの値は0.2〜0.8の範囲である。
In order to finely disperse and crystallize CaS as described above, it is necessary to control the addition amount of Ca and the content of S and O in the molten steel at the time of addition to an appropriate range, and the ACR value is Only by controlling to the range of (1), CaS that becomes ferrite transformation nuclei can be finely dispersed.
When the ACR is 0 or less, CaS does not crystallize, and S precipitates in the form of MnS alone, so that no ferrite formation nuclei in the weld heat affected zone can be obtained. Further, MnS precipitated alone is elongated at the time of rolling and causes a decrease in the toughness of the base material. On the other hand, when the ACR is 1 or more, S is completely fixed by Ca, and MnS acting as a ferrite nuclei does not precipitate on CaS, and fine dispersion of the composite sulfide that becomes a ferrite nuclei cannot be realized. The effect of improving the toughness of the weld heat affected zone cannot be obtained. Therefore, only when the ACR value is greater than 0 and less than 1, MnS is deposited on CaS to form a composite sulfide, which effectively functions as a ferrite nuclei. A more preferable ACR value is in the range of 0.2 to 0.8.

なお、本発明の高張力鋼板は、上記成分以外の残部は、Feおよび不可避的不純物である。ただし、上記成分以外であっても、本発明の作用効果を害しない範囲内であれば、含有することを拒むものではない。  In the high-tensile steel sheet of the present invention, the balance other than the above components is Fe and inevitable impurities. However, even if it is other than the said component, if it is in the range which does not injure the effect of this invention, it does not refuse containing.

次に、本発明の高張力鋼板の製造方法について説明する。
本発明の高張力鋼板は、転炉や電気炉、真空溶解炉等で鋼を溶製し、必要に応じて真空脱ガス処理等の二次精錬を施す常法の精錬プロセスで本発明に適合する成分組成に調整した溶鋼を連続鋳造して鋼素材(スラブ)とした後、該スラブを再加熱し、熱間圧延して所望の板厚とし、加速冷却する、あるいは、上記加速冷却後の鋼板に、さらに焼戻し処理を施す工程を経て製造することができ、従来の高張力鋼板の製造方法と違いはない。
Next, the manufacturing method of the high-tensile steel plate of this invention is demonstrated.
The high-strength steel sheet of the present invention conforms to the present invention in a conventional refining process in which steel is melted in a converter, electric furnace, vacuum melting furnace, etc., and secondary refining such as vacuum degassing is performed as necessary. After the molten steel adjusted to the component composition is continuously cast into a steel material (slab), the slab is reheated and hot-rolled to a desired plate thickness, accelerated cooling, or after the accelerated cooling The steel sheet can be manufactured through a process of further tempering, and there is no difference from a conventional manufacturing method of a high-strength steel sheet.

ただし、上記工程において、スラブ再加熱温度および熱間圧延における圧下率、加速冷却の冷却速度、ならびに、その後、必要に応じて施す焼き戻し処理の条件は、以下の要件を満たすことが重要である。
なお、本発明においては、特に記載しない限り、鋼板温度は、板厚中心部の温度とする。上記板厚中心部の温度は、板厚、表面温度および冷却条件などから差分法等を用いた計算により求めることができる。
However, in the above process, it is important that the slab reheating temperature and the rolling reduction in hot rolling, the cooling rate of accelerated cooling, and the conditions of the tempering treatment to be performed as necessary thereafter satisfy the following requirements. .
In the present invention, unless otherwise specified, the steel plate temperature is the temperature at the center of the plate thickness. The temperature at the central portion of the plate thickness can be obtained by calculation using a difference method or the like from the plate thickness, surface temperature, cooling conditions, and the like.

スラブ再加熱温度:1030〜1200℃
熱間圧延前に行う前のスラブ再加熱温度は、スラブ内部に存在する鋳造欠陥を熱間圧延によって確実に圧着させるため、1030℃以上とする必要がある。しかし、1200℃を超える温度に加熱すると、凝固時に析出したTiNが粗大化し、母材や溶接部の靭性が低下するため、加熱温度の上限は1200℃とする。好ましくは1030〜1170℃の範囲である。
Slab reheating temperature: 1030 to 1200 ° C
The slab reheating temperature before the hot rolling needs to be 1030 ° C. or higher in order to surely press-bond the casting defects existing inside the slab by hot rolling. However, when heated to a temperature exceeding 1200 ° C., TiN precipitated during solidification becomes coarse and the toughness of the base material and the welded portion decreases, so the upper limit of the heating temperature is 1200 ° C. Preferably it is the range of 1030-1170 degreeC.

950℃以上の温度域における熱間圧延の累積圧下率:30%以上
熱間圧延時における再結晶を利用してオーステナイト粒を微細化するためには、950℃以上の再結晶温度域における累積圧下率を30%以上とすることが必要である。30%未満では、スラブ再加熱時に生成した異常粗大粒が残存し、母材の靭性に悪影響を及ぼすからである。好ましくは35%以上である。なお、この段階における累積圧下率の上限は、950℃未満の温度域における圧下率を確保できればよく、特に制限はない。
Cumulative rolling reduction of hot rolling in a temperature range of 950 ° C. or higher: 30% or higher Cumulative rolling in a recrystallization temperature range of 950 ° C. or higher in order to refine austenite grains using recrystallization during hot rolling The rate needs to be 30% or more. If it is less than 30%, abnormal coarse grains generated during reheating of the slab remain, which adversely affects the toughness of the base material. Preferably it is 35% or more. The upper limit of the cumulative rolling reduction at this stage is not particularly limited as long as the rolling reduction in a temperature range of less than 950 ° C. can be secured.

950℃未満の温度域における熱間圧延の累積圧下率:30〜70%
950℃未満の温度域で圧延されたオーステナイト粒は、十分に再結晶しないため、圧延後のオーステナイト粒は偏平に変形したままで、内部に変形帯などの欠陥を多量に含んだ内部歪の高い組織となる。上記内部歪は、フェライト変態の駆動力として働き、フェライト変態を促進する。しかし、累積圧下率が30%未満では、歪エネルギーの蓄積が十分でなく、フェライト変態が起こり難くなるため母材の靭性が低下する。一方、累積圧下率が70%を超えると、逆にポリゴナルフェライトの生成が促進されて、高強度と高靭性を両立できなくなる。好ましくは40〜65%の範囲である。
Cumulative rolling reduction of hot rolling in a temperature range below 950 ° C .: 30 to 70%
Since austenite grains rolled in a temperature range below 950 ° C. do not recrystallize sufficiently, the austenite grains after rolling remain flatly deformed and have a high internal strain containing a large amount of defects such as deformation bands inside. Become an organization. The internal strain acts as a driving force for ferrite transformation and promotes ferrite transformation. However, if the cumulative rolling reduction is less than 30%, the strain energy is not sufficiently accumulated, and the ferrite transformation hardly occurs, so that the toughness of the base material is lowered. On the other hand, when the cumulative rolling reduction exceeds 70%, the formation of polygonal ferrite is promoted, and it becomes impossible to achieve both high strength and high toughness. Preferably it is 40 to 65% of range.

なお、熱間圧延の終了温度は650〜850℃の範囲とするのが好ましい。650℃未満では、加工フェライトが残留して靭性が低下し、一方、850℃を超えると、組織が粗粒となり、靭性が低下するからである。より好ましくは680〜820℃の範囲である。  In addition, it is preferable that the completion | finish temperature of hot rolling shall be the range of 650-850 degreeC. When the temperature is lower than 650 ° C., the processed ferrite remains and the toughness is lowered. On the other hand, when the temperature exceeds 850 ° C., the structure becomes coarse and the toughness is lowered. More preferably, it is the range of 680-820 degreeC.

600℃以下までの冷却速度:1.0℃/s以上
上記熱間圧延終了後は、冷却速度1.0℃/s以上で、600℃以下の温度まで加速冷却することが重要である。冷却速度が1.0℃/s未満では、強度が低いフェライトの生成を抑制できないため、十分な母材の強度が得られない。好ましくは1.2℃/s以上である。なお、冷却速度の上限については特に制限はないが、母材の靭性を確保する観点から30℃/s以下が好ましい。
また、上記冷却を停止する温度を600℃とする理由は、600℃より高いと、フェライト+パーライトや上部ベイナイトなどの組織の分率が高くなり、高強度と高靭性とが両立しなくなるからである。ただし、加速冷却の停止温度は、島状マルテンサイトなどの靭性に劣る硬質相の生成を抑制する観点から、350℃以上とするのが好ましい。なお、上記加速冷却後に焼戻し処理を施す場合には、上記加速冷却の停止温度の下限は特に制限はない。
Cooling rate to 600 ° C. or lower: 1.0 ° C./s or higher After completion of the hot rolling, it is important to accelerate cooling to a temperature of 600 ° C. or lower at a cooling rate of 1.0 ° C./s or higher. When the cooling rate is less than 1.0 ° C./s, it is not possible to suppress the formation of ferrite having low strength, and thus sufficient strength of the base material cannot be obtained. Preferably it is 1.2 degrees C / s or more. In addition, although there is no restriction | limiting in particular about the upper limit of a cooling rate, 30 degrees C / s or less is preferable from a viewpoint of ensuring the toughness of a base material.
The reason for setting the temperature at which the cooling is stopped to 600 ° C. is that if the temperature is higher than 600 ° C., the fraction of the structure such as ferrite + pearlite and upper bainite becomes high, and high strength and high toughness are not compatible. is there. However, the stop temperature for accelerated cooling is preferably 350 ° C. or higher from the viewpoint of suppressing the generation of a hard phase having poor toughness such as island martensite. In addition, when performing a tempering process after the said accelerated cooling, there is no restriction | limiting in particular in the minimum of the stop temperature of the said accelerated cooling.

焼戻し温度:450〜650℃
上記加速冷却後、焼戻し処理を施す場合には、鋼板の板厚中心温度で450〜650℃の範囲とするのが好ましい。450℃未満では、十分な焼戻しの効果が得られない。一方、650℃を超える温度で焼戻しを行うと、炭窒化物が粗大に析出して靭性が低下したり、強度の低下を招いたりするため、好ましくない。より好ましくは480〜620℃の範囲である。
また、焼戻し処理の加熱方法は、焼戻し時における炭化物の粗大化を抑制する観点から、誘導加熱で行うことが好ましい。
なお、上記焼き戻し処理時間は、10〜300minの範囲するのが好ましい。
Tempering temperature: 450-650 ° C
When the tempering treatment is performed after the accelerated cooling, it is preferable that the steel sheet has a thickness center temperature of 450 to 650 ° C. If it is less than 450 degreeC, the effect of sufficient tempering is not acquired. On the other hand, tempering at a temperature exceeding 650 ° C. is not preferable because carbonitrides are coarsely precipitated and the toughness is lowered or the strength is lowered. More preferably, it is the range of 480-620 degreeC.
Moreover, it is preferable to perform the heating method of a tempering process by induction heating from a viewpoint of suppressing the coarsening of the carbide | carbonized_material at the time of tempering.
The tempering time is preferably in the range of 10 to 300 min.

上記に説明したように、本発明の高張力鋼板は、溶接熱影響部のオーステナイト粒の粗大化を抑制し、さらに、高温でも溶解しないフェライト変態生成核を微細に分散させて、溶接熱影響部の組織を微細化するため、溶接熱影響部おいても高い靭性が得られる。特に、多層盛溶接時の熱サイクルにより2相域に再加熱される領域においては、最初の溶接により溶接熱影響部の組織が微細化されているため、未変態領域の靭性が向上するだけでなく、再変態するオーステナイト粒も微細化することができるので、溶接時の入熱による靭性の低下を大幅に軽減することができる。  As described above, the high-tensile steel sheet of the present invention suppresses the coarsening of austenite grains in the weld heat affected zone, and further finely disperses ferrite transformation nuclei that do not dissolve even at high temperatures, so that the weld heat affected zone In order to refine the structure, high toughness is obtained even in the heat affected zone. In particular, in the region reheated to the two-phase region by the thermal cycle during multi-layer welding, the structure of the weld heat-affected zone is refined by the first welding, so only the toughness of the untransformed region is improved. In addition, since the retransformed austenite grains can also be made finer, a reduction in toughness due to heat input during welding can be greatly reduced.

表1に示した各種成分組成を有する符号A〜Xの鋼を溶製し、連続鋳造法で鋼素材(スラブ)とした後、表2に示した種々の条件で熱間圧延し、加速冷却し、あるいはさらに焼戻し処理を施して、厚さが50〜100mmの厚鋼板No.1〜30を製造した。
次いで、上記厚鋼板を以下の評価試験に供した。
<母材の評価>
・強度特性
厚鋼板の板厚1/2の位置から、鋼板の圧延方向に直角な方向を長さ方向とするJIS4号試験片を採取し、降伏応力YSおよび引張強さTSを測定し、YS≧460MPaおよびTS≧570MPaを満たすものを母材の強度特性が良好と評価した。
・靭性特性
厚鋼板の板厚1/2の位置から、鋼板の圧延方向に直角な方向を長さ方向とするJIS
Z2202に規定されたVノッチ試験片を採取し、シャルピー衝撃試験で−80℃における吸収エネルギーvE−80℃を測定し、vE−80℃≧200Jを満たすものを母材の靭性特性が良好と評価した。
<溶接部の評価>
・靭性特性
厚鋼板から試験片を採取し、K型開先を付与した後、溶接入熱35kJ/cmのサブマージアーク溶接で多層盛溶接を行って溶接継手を作製し、鋼板の板厚1/4位置のストレート側の溶接ボンド部をノッチ位置とするVノッチ試験片を採取し、シャルピー衝撃試験で−80℃における吸収エネルギーvE−80℃を測定した。上記試験は、各条件で3本行い、平均のvE−80℃が150J以上である溶接継手を靭性良好と評価した。
・CTOD特性
上記と同様にして溶接継手を作製し、ストレート側の溶接ボンド部を三点曲げCTOD試験片のノッチ位置とするCTOD試験片を採取し、−60℃におけるCTOD値(δ−60℃)を測定した。上記試験は、各条件で3本行い、CTOD値(δ−60℃)の最小値が0.50mm以上であるものを、溶接継手のCTOD特性が良好と評価した。
After smelting steels of symbols A to X having various component compositions shown in Table 1 and making them steel materials (slabs) by a continuous casting method, they are hot-rolled under various conditions shown in Table 2 and accelerated cooling. Or a further tempering treatment to obtain a thick steel plate No. 50 to 100 mm in thickness. 1-30 were manufactured.
Subsequently, the said thick steel plate was used for the following evaluation tests.
<Evaluation of base material>
・ Strength characteristics JIS No. 4 test piece having a length direction perpendicular to the rolling direction of the steel sheet was taken from the position of 1/2 the thickness of the thick steel sheet, and the yield stress YS and the tensile strength TS were measured. Those satisfying ≧ 460 MPa and TS ≧ 570 MPa were evaluated as having good strength characteristics of the base material.
・ Toughness characteristics JIS where the length direction is the direction perpendicular to the rolling direction of the steel plate from the position of 1/2 the thickness of the thick steel plate
Samples of V-notch specified in Z2202 were collected, the absorbed energy vE- 80 ° C at -80 ° C was measured by Charpy impact test, and those satisfying vE- 80 ° C ≥200J were evaluated as having good toughness characteristics of the base material did.
<Evaluation of weld zone>
・ Toughness characteristics After taking a specimen from a thick steel plate and providing a K-shaped groove, multi-layer welding is performed by submerged arc welding with a welding heat input of 35 kJ / cm to produce a welded joint. A V-notch test piece having a weld bond portion on the straight side at 4 positions as a notch position was collected, and the absorbed energy vE- 80 ° C at -80 ° C was measured by a Charpy impact test. The above test was performed three times under each condition, and a welded joint having an average vE- 80 ° C of 150 J or more was evaluated as having good toughness.
CTOD characteristics A weld joint was prepared in the same manner as described above, and a CTOD test piece having a straight-side weld bond portion as a notch position of a three-point bending CTOD test piece was taken, and a CTOD value at -60 ° C (δ-60 ° C) ) Was measured. The above test was performed three times under each condition, and those having a minimum CTOD value (δ-60 ° C.) of 0.50 mm or more were evaluated as having good CTOD characteristics of the welded joint.

Figure 0006226163
Figure 0006226163

Figure 0006226163
Figure 0006226163

上記測定の結果を表2に併記した。表2から、表1に示した符号A〜Eの鋼は、いずれも本発明の成分組成を満たすものであり、該鋼のスラブを用いて本発明に適合する条件で製造した発明例の鋼板は、いずれも母材および溶接部の強度特性と靭性に優れている。
これに対して、本発明の成分組成を満たす鋼スラブを用いて本発明から外れる条件で製造した比較例の鋼板、あるいは、本発明の成分組成を満たさない鋼のスラブを用いて本発明に適合する条件で製造した比較例の鋼板は、母材および溶接部の強度特性と靭性が上記発明例の鋼板より劣っていることがわかる。
The results of the above measurements are also shown in Table 2. From Table 2, steels of reference signs A to E shown in Table 1 all satisfy the composition of the present invention, and steel plates of inventive examples manufactured under conditions suitable for the present invention using the steel slabs. Are excellent in the strength characteristics and toughness of the base material and the weld.
On the other hand, the steel plate of the comparative example manufactured on the conditions which deviate from this invention using the steel slab which satisfy | fills the component composition of this invention, or the present invention using the slab of the steel which does not satisfy | fill the component composition of this invention It can be seen that the steel plate of the comparative example manufactured under the conditions to be used is inferior in strength characteristics and toughness of the base material and the welded portion to the steel plate of the above-described invention example.

本発明の高張力鋼板は、船舶や海洋構造物、圧力容器、ペンストックなどの鋼構造物の他、建築や橋梁等の土木・建築分野にも適用することができる。

The high-tensile steel plate of the present invention can be applied to civil engineering / architectural fields such as buildings and bridges, as well as steel structures such as ships, offshore structures, pressure vessels, and penstocks.

Claims (5)

C:0.010〜0.050mass%、
Si:0.01〜0.50mass%、
Mn:1.80〜3.50mass%、
P:0.012mass%以下、
S:0.0035mass%以下、
sol.Al:0.010〜0.060mass%、
Ni:0.1〜2.0mass%、
Cr:1.0〜3.0mass%、
Nb:0.005〜0.040mass%、
Ti:0.005〜0.025mass%および
N:0.0020〜0.0050mass%を含有し、さらに、
上記CrおよびMnが、35Cr+8Mn≧63かつ7Cr+18Mn≦63を満たして含有し、かつ、
O:0.0030mass%以下で、
Ca,SおよびOが下記(1)式を満たして含有し、
残部がFeおよび不可避的不純物からなる成分組成を有する高張力鋼板。

0<{Ca−(0.18+130×Ca)×O}/1.25/S<1 …(1)
ただし、上記式中の各元素記号は、それぞれの元素の含有量(mass%)である。
C: 0.010 to 0.050 mass%,
Si: 0.01-0.50 mass%,
Mn: 1.80 to 3.50 mass%,
P: 0.012 mass% or less,
S: 0.0035 mass% or less,
sol. Al: 0.010 to 0.060 mass%,
Ni: 0.1 to 2.0 mass%,
Cr: 1.0-3.0 mass%,
Nb: 0.005 to 0.040 mass%,
Ti: 0.005-0.025 mass% and N: 0.0020-0.0050 mass%,
The Cr and Mn satisfy 35Cr + 8Mn ≧ 63 and 7Cr + 18Mn ≦ 63, and
O: 0.0030 mass% or less,
Ca, S and O satisfy the following formula (1) and are contained:
A high-strength steel sheet having a component composition with the balance of Fe and inevitable impurities.
Record
0 <{Ca− (0.18 + 130 × Ca) × O} /1.25/S <1 (1)
However, each element symbol in the above formula is the content (mass%) of each element.
上記成分組成に加えてさらに、Cu:1.0mass%未満、Mo:0.05〜0.50mass%、V:0.005〜0.05mass%、B:0.0005〜0.0030mass%、Ca:0.0005〜0.0050mass%およびMg:0.0002〜0.0030mass%の中から選ばれる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に記載の高張力鋼板。 In addition to the above component composition, Cu: less than 1.0 mass%, Mo: 0.05 to 0.50 mass%, V: 0.005 to 0.05 mass%, B: 0.0005 to 0.0030 mass%, Ca The high-tensile steel plate according to claim 1, comprising one or more selected from: 0.0005 to 0.0050 mass% and Mg: 0.0002 to 0.0030 mass%. C:0.010〜0.050mass%、
Si:0.01〜0.50mass%、
Mn:1.80〜3.50mass%、
P:0.012mass%以下、
S:0.0035mass%以下、
sol.Al:0.010〜0.060mass%、
Ni:0.1〜2.0mass%、
Cr:1.0〜3.0mass%、
Nb:0.005〜0.040mass%、
Ti:0.005〜0.025mass%および
N:0.0020〜0.0050mass%を含有し、さらに、
上記CrおよびMnが、35Cr+8Mn≧63かつ7Cr+18Mn≦63を満たして含有し、かつ、
O:0.0030mass%以下で、
Ca,SおよびOが下記(1)式を満たして含有し、
残部がFeおよび不可避的不純物からなる成分組成を有する鋼素材を1030〜1200℃に加熱した後、950℃以上の温度域における累積圧下率を30%以上、950℃未満の温度域における累積圧下率を30〜70%とする熱間圧延し、その後、600℃以下まで冷却速度1.0℃/s以上で加速冷却することを特徴とする高張力鋼板の製造方法。

0<{Ca−(0.18+130×Ca)×O}/1.25/S<1 …(1)
ただし、上記式中の各元素記号は、それぞれの元素の含有量(mass%)である。
C: 0.010 to 0.050 mass%,
Si: 0.01-0.50 mass%,
Mn: 1.80 to 3.50 mass%,
P: 0.012 mass% or less,
S: 0.0035 mass% or less,
sol. Al: 0.010 to 0.060 mass%,
Ni: 0.1 to 2.0 mass%,
Cr: 1.0-3.0 mass%,
Nb: 0.005 to 0.040 mass%,
Ti: 0.005-0.025 mass% and
N: 0.0020 to 0.0050 mass%,
The Cr and Mn satisfy 35Cr + 8Mn ≧ 63 and 7Cr + 18Mn ≦ 63, and
O: 0.0030 mass% or less,
Ca, S and O satisfy the following formula (1) and are contained:
After heating the steel material having a composition comprising the balance of Fe and inevitable impurities to 1030 to 1200 ° C., the cumulative reduction rate in the temperature range of 950 ° C. or higher is 30% or more, and the cumulative reduction rate in the temperature range of less than 950 ° C. A method for producing a high-strength steel sheet, characterized in that hot rolling is performed at 30 to 70%, and then accelerated cooling to 600 ° C. or lower at a cooling rate of 1.0 ° C./s or higher.
Record
0 <{Ca− (0.18 + 130 × Ca) × O} /1.25/S <1 (1)
However, each element symbol in the above formula is the content (mass%) of each element.
上記鋼素材は、上記成分組成に加えてさらに、Cu:1.0mass%未満、Mo:0.05〜0.50mass%、V:0.005〜0.05mass%、B:0.0005〜0.0030mass%、Ca:0.0005〜0.0050mass%およびMg:0.0002〜0.0030mass%の中から選ばれる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3に記載の高張力鋼板の製造方法。In addition to the above component composition, the steel material further includes Cu: less than 1.0 mass%, Mo: 0.05 to 0.50 mass%, V: 0.005 to 0.05 mass%, and B: 0.0005 to 0. 4. One or more selected from the group consisting of .0030 mass%, Ca: 0.0005 to 0.0050 mass%, and Mg: 0.0002 to 0.0030 mass%. Manufacturing method of high-tensile steel plate. 600℃以下まで加速冷却した後、さらに、450〜650℃の温度で焼戻処理を施すことを特徴とする請求項3または4に記載の高張力鋼板の製造方法。 5. The method for producing a high-tensile steel sheet according to claim 3 , further comprising performing tempering at a temperature of 450 to 650 ° C. after accelerated cooling to 600 ° C. or lower.
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