JP3802810B2 - Steel for welded structures having no dependence on heat input of HAZ toughness and method for manufacturing - Google Patents

Description

【００９０】
【表２】

【００９１】
【表３】

【００９２】
鋼１〜２２は本発明の例を示す。表３から明らかなように、これらの発明鋼における旧γ粒径は、小入熱〜超大入熱の広い入熱条件の範囲において、すべて、２００μｍ以下である。鋼２０−２と２１−２は、化学成分が、それぞれ、鋼２０と２１の化学成分とほとんど同様のものであるが、脱酸条件を変化させたものであり、Ｍｇ量が多少異なるものである。鋼２０−２の場合はλ１が、鋼２１−２の場合はλ２が、本発明で規定する範囲の外であるが、これらの場合においても、鋼２０−２では、粒径がほとんど変化していないことが認められ、また、鋼２１−２では、入熱条件６０．０ｋＪ／ｍｍで粒径が２００μｍ以下になっていることがわかる。また、これら発明鋼のシャルピー吸収エネルギーは、すべて、１０ｋｇｆ・ｍを超えていて、上記発明鋼が高靭性の鋼であることを示している。
【００９３】
しかも、小入熱と超大入熱の場合におけるシャルピー吸収エネルギーの差は、最大でも、およそ４ｋｇｆ・ｍ以下と小さく、幅広い入熱条件の範囲においても、ＨＡＺ靱性に大きな変化はない。
【００９４】
なお、表３には、上記シャルピー吸収エネルギーの差にマイナスが付されているものがあるが、これは、旧γ粒径が大きくなったにもかかわらず靱性が向上したことを示している。そして、このことは、本発明においてはＭｇ主体の含有酸化物の粒内変態能が極めて大きいことに起因している。
【００９５】
それに対し、鋼２３〜３５は、本発明の方法から逸脱して製造された比較鋼を示す。すなわち、比較鋼２３、２４、２５、２６、２７、２９、３０、３３、３４および３５は、基本成分あるいは選択元素の内、いずれかの元素が、本発明で規定する組成範囲を越えて添加されている例である。
【００９６】
上記比較鋼においては、本発明において重要な要件である酸化物の平均粒子間隔は、大部分が、本発明で規定する要件を満たしているが、靱性劣化の要因となる元素が過剰に添加されていることにより、小入熱条件および超大入熱条件での溶接時、ＨＡＺ靱性の劣化が助長されたものである。
【００９７】
比較鋼２８と比較鋼３１は、それぞれ、ＡｌとＴｉが、本発明で規定するＡｌとＴｉの下限値より小さい場合の鋼であり、入熱量が大きくなるに従って、旧γ粒径は粗大化しており、比較鋼２８と比較鋼３１は、ともに、靱性値が低いものである。
【００９８】
また、比較鋼３２は、Ｍｇが無添加のものであり、小入熱条件の下では、靱性は良好であるが、超大入熱条件の下では、靭性の劣化が大きく、結局、シャルピー吸収エネルギーの差は、１０．３ｋｇｆ・ｍと大きい。
【００９９】
上記比較鋼においては、いずれも、ＨＡＺ靱性は低いレベルにあり、そのうえ、入熱量が大きい場合には、さらに、ＨＡＺ靱性が低下している。
【０１００】
なお、比較鋼３３と比較鋼３４においては、微細な酸化物が数多く存在していることにより、他に比べて十分に、旧γ粒径が小さくなっているにもかかわらず、靱性が大きく劣化している。
【０１０１】
この理由は、過剰のＭｇあるいはＯが添加されたことに起因して、主として、５μｍ以上の粗大な粒子が生成され、脆性破壊が促進されたためである。
【０１０２】
比較鋼３６と比較鋼３７は、化学成分が、それぞれ、発明鋼１と発明鋼２の化学成分と同じであるが、最終のＭｇを所定量添加する際、溶鋼中の溶存酸素量が５０ｐｐｍを越えていた鋼である。
【０１０３】
結局、比較鋼３６と比較鋼３７は、鋼中において超微細酸化物の生成が少ないことから、旧γ粒径の粗大化と、顕著な靱性劣化が起きているものである。
【０１０４】
産業上の利用可能性
本発明の化学成分および製造方法によれば、Ｔｉ添加後にＭｇを適切に所定量添加するか、あるいは、ＴｉとＭｇを同時に添加した後に、Ｍｇを適切に所定量添加することにより、入熱条件にかかわらず、ＨＡＺにおける旧γ粒の成長を抑制することができる。
【０１０５】
本発明においては、この抑制効果により、ＨＡＺ靱性を、入熱条件の幅広い範囲で向上させることが可能である。
【０１０６】
その結果、海洋構造物、天然ガス・原油輸送用ラインパイプ、建築、造船、橋梁、建設機械などの幅広い技術分野において、溶接構造物の脆性破壊に対する安全性が、大幅に向上する。
【０１０７】
従って、本発明は、各種産業技術の発展に寄与するところが極めて大きいものである。
【図面の簡単な説明】
【図１】 溶接入熱量を変化させた場合のＨＡＺにおける旧γ粒径を示す図である。
【図２】 超微細なＭｇ酸化物を核にした複合粒子の形態を模式的に示す図である。[0001]
  Technical field
  The present invention relates to steel for welded structures used for offshore structures, natural gas / crude oil transportation line pipes, buildings, shipbuilding, bridges, construction machinery, and the like, and a method for producing the same. More specifically, the present invention requires the toughness of the welded portion, and even if welding is performed under a wide range of heat input conditions where the heat input during welding is from 0.5 kJ / mm to over 150 kJ / mm, the weld heat affected zone ( Hereinafter, the prior austenite grain size in “HAZ”) is small, and the toughness of the weld heat-affected zone (hereinafter referred to as “HAZ toughness”) relates to steel for welded structures that is excellent regardless of heat input conditions.
[0002]
  Background art
  From the viewpoint of preventing brittle fracture of a welded structure such as an offshore structure, many studies have been reported on suppressing the occurrence of brittle fracture occurring from a welded portion, that is, research on improving HAZ toughness in a steel sheet to be used. In recent years, from the viewpoint of improving welding work efficiency, super large heat input welding (20 to 150 kJ / mm), in which welding heat input is further increased, has been carried out from the conventional high heat input welding (approximately 20 kJ / mm or less). If you have increased.
  The difference in the effect of large heat input welding and super large heat input welding on the steel sheet is due to the difference in residence time at a high temperature of 1400 ° C. or higher.
[0003]
  That is, in ultra-high heat input welding, since the residence time is extremely long, the region in which the crystal grain size is significantly coarsened in HAZ is widened, and the toughness is significantly reduced.
[0004]
  In general, measures for preventing the coarsening of crystal grains in the HAZ of a steel sheet are described in, for example, TiN described in JP-A-55-26164 and JP-A-52-17314. “By weight%, C: 0.01-0.2%, Si: 0.002-1.5%, Mn: 0.5-2.5%, Ti and / or Zr: 0.002-0. 1%, Ca and / or Mg: 0.004% or less, Ce and / or La: 0.001 to 0.1%, Al: 0.005 to 0.1%, N: 0.002 to 0.015 % Of old austenite grains (hereinafter referred to as “old γ grains”) due to inclusion particles finely dispersed in the steel, such as ZrN in “structural steel for high heat input welding”. , The particle size is called "old γ particle size") There are known measures to use the effect (pinning effect).
[0005]
  However, such a nitride does not melt at the time of welding with a low heat input or a medium heat input and exhibits a pinning effect by pinning the old γ grains and contributes to the refinement of crystal grains. At the time of welding with high heat input or super high heat input, which has a very long residence time at a high temperature, the welding heat easily dissolves in the steel and disappears.
[0006]
  On the other hand, in recent years, a technique using an oxide generated in molten steel has been disclosed for the purpose of further improving the HAZ toughness. For example, JP-A-59-190313 discloses a method for producing a steel material having excellent weldability, characterized by deoxidizing molten steel with Ti or a Ti alloy and then adding Al, Mg, or the like. ing. This manufacturing method uses the effect of causing the Ti oxide to act as a transformation nucleus of ferrite and increasing the ferrite fraction, and is different from the conventional method using the pinning effect due to precipitates such as nitrides. This is a technique for improving the HAZ toughness.
[0007]
  Thereafter, in this technical field, an invention for increasing the number of oxides as intragranular transformation nuclei in JP-A-61-79745, JP-A-5-43977, JP-A-6-37364, etc. Various inventions are disclosed.
[0008]
  In particular, as described in Japanese Patent Application Laid-Open No. 59-190313, the gist of these inventions is that “the formation of ferrite nuclei during the γ → α transformation, ie, the Ti-containing oxidation that can be used to refine the ferrite structure Is to finely disperse the material uniformly, and does not secure the pinning effect by the nitride as described above, but promotes the ferrite transformation during the γ → α transformation that occurs in the cooling process, The formation of a coarse embrittled structure is suppressed, and the refinement of the structure is achieved.
[0009]
  These toughness improving methods are all used by dispersing relatively large oxides of about 1 μm as transformation nuclei in a coarse structure in order to promote ferrite transformation within the grains.
[0010]
  However, in recent years, from the viewpoint of increasing the size and weight of welded structures, the demand for higher-strength high-strength steel is increasing, and in the composition of high-strength steel, the amount of alloying elements tends to increase. is there. In that case, due to the increase in hardenability with HAZ, conventional measures for improving HAZ toughness using ferrite transformation are becoming ineffective.
[0011]
  From the above viewpoint, in order to drastically improve the HAZ toughness, a pinning effect on the old γ grains can be expected even under a wide range of heat input conditions. Similarly, development of a technique that can be finely dispersed in steel is desired. Moreover, in that case, if it is possible to impart a transformation capacity higher than that of conventional ferrite transformation nuclei, it is considered that the HAZ toughness will be dramatically improved with respect to the steel material properties used in this technical field. It is done.
[0012]
  As a method for introducing an oxide, there is a method in which a deoxidizing element such as Ti is added alone in the steel melting step. However, in many cases, aggregates and aggregates of oxides are generated during holding of molten steel, resulting in the formation of coarse oxides. On the contrary, the cleanliness of the steel is impaired and the toughness is reduced. Therefore, in order to miniaturize these oxides, various ideas such as a composite deoxidation method have been made as described in the previous example.
[0013]
  However, the conventionally known methods cannot disperse fine oxides in the steel that have such an effect that the coarsening of crystal grains can be completely prevented when the welding heat input is large.
[0014]
  Disclosure of the invention
  The present invention improves the conventional composite deoxidation method, disperses oxides and / or nitrides more finely and uniformly than before, and further imparts ferrite transformation ability to the finely dispersed particles. An object of the present invention is to provide a steel for a welded structure that is excellent in HAZ toughness in welding under any heat input condition including super large heat input.
[0015]
  The gist of the present invention is as follows.
[0016]
  (1) In mass%,
    C: 0.01-0.2%
    Si: 0.02 to 0.5%,
    Mn: 0.3-2%,
    P: 0.03% or less,
    S: 0.0001 to 0.03%,
    Al: 0.0005 to 0.05%,
    Ti: 0.003 to 0.05%,
    Mg: 0.0001 to 0.01%
    O: 0.0001 to 0.008% is contained, the balance is made of iron and inevitable impurities,
  Mg with a particle size of 0.2-5 μmSubjectWith the oxide as a core, one of sulfide and nitride alone or a combination of both particles is dispersed in steel with an average particle spacing of 30 to 100 μm,And / or
  Mg having a particle size of 0.005 to less than 0.2 μmSubjectWith oxide as a core, one of sulfide and nitride alone or a composite of both is dispersed in steel with an average particle spacing of 30 μm or less.
A welded structure steel having no dependence on heat input of HAZ toughness.
[0017]
  (2) In mass%,
    Cu: 0.05 to 1.5%,
    Ni: 0.05 to 5%,
    Cr: 0.02 to 1.5%,
    Mo: 0.02 to 1.5%,
    V: 0.01-0.1%
    Nb: 0.0001 to 0.2%,
    Zr: 0.0001 to 0.05%,
    Ta: 0.0001 to 0.05%
    B: contains one or more of 0.0003 to 0.005%,
The steel for welded structures having no dependency on heat input of the HAZ toughness as described in (1) above.
[0018]
  (3) In mass%,
    Ca: 0.0005 to 0.005%,
    REM: For welded structures having no dependence on heat input of HAZ toughness according to (1) or (2) above, containing one or more of 0.0005 to 0.005% steel.
[0019]
  (4) The heat input dependence of the HAZ toughness according to (1), (2) or (3) above, wherein the prior austenite grain size of the HAZ structure is 10 to 200 μm regardless of welding heat input. There is no steel for welded structures.
[0020]
  (5) MadesteelIn the stage, after adding Si and Mn and performing weak deoxidation treatment, Ti: 0.003 to 0.05 mass%,MgAre added sequentially or simultaneously to bring the dissolved oxygen amount to 50 ppm or less, and Mg is added to the final content.0.0001-0.01% by mass(1), (2), (3) or (4), wherein the HAZ toughness has no dependency on heat input, and is a method for producing steel for welded structures.
[0021]
  (6) MadesteelIn the stage, after adding Si and Mn and performing weak deoxidation treatment, Ti: 0.003 to 0.05 mass%,Al, Ca and Mg are added sequentially or simultaneously to reduce the dissolved oxygen content to 50 ppm or less, and Mg is added to the final content.0.0001-0.01% by mass(1), (2), (3) or (4), wherein the HAZ toughness has no dependency on heat input, and is a method for producing a steel for welded structures. .
[0022]
  BEST MODE FOR CARRYING OUT THE INVENTION
  Mg has been conventionally known as an element that increases the cleanliness of steel and improves the HAZ toughness as a strong deoxidizer and desulfurizer.
[0023]
  As a technique for improving the HAZ toughness by controlling oxide dispersion, Japanese Patent Application Laid-Open No. 59-190313 discloses a technique for adding Mg after adding Ti.
[0024]
  However, the purpose of the technology is to promote the increase of Ti oxides as intragranular transformation nuclei by adding Mg, as cited above. It does not achieve finer grain size.
[0025]
  Therefore, the present inventors pay attention to the action of Mg as a strong deoxidizer and utilize the property that aggregation and coarsening are less likely to occur than Al, and in the Ti-added steel, the addition of the deoxidizer in the steelmaking process The idea was that if the order and amount were controlled, there was room to expect fine dispersion of the oxide.
[0026]
  Hereinafter, the present invention will be described in detail.
[0027]
  The present inventors systematically investigated the state of oxides when Mg was added to molten steel that was weakly deoxidized by adding Ti.
[0028]
  As a result, after deoxidizing the molten steel with Si and Mn, when Ti and Mg are added in the order of Ti and Mg, or Ti addition and Mg addition are performed at the same time, and the Mg is again in an equilibrium state. It has been found that an oxide having two types of particle sizes is formed as the oxide particle size.
[0029]
  In the present invention, in the first stage Mg deoxidation, it is confirmed in the present invention that the same tendency as described above also occurs when Al or Ca is added simultaneously or in the preceding stage.
[0030]
  That is, one is an Mg-containing oxide having a particle size of 0.2 to 5.0 μm, and the other is an ultrafine MgO or Mg-containing oxide having a particle size of 0.005 to 0.1 μm. The generation of such an oxide is presumed to be based on the following reason.The Mg-containing oxide is an “Mg-containing oxide mainly composed of Mg” to be described later, and is hereinafter referred to as “Mg-based oxide”.
[0031]
  First, by addition of Ti or simultaneous addition of Ti and a small amount of Mg, an oxide of μm size mainly composed of Ti or Ti is once generated. Next, when Mg having strong deoxidizing power is further added in this state, the oxide already formed is reduced by Mg, and finally, Mg-containing oxidation mainly composed of Mg having a μm size. object(Mg-based oxide)Is generated.
[0032]
  At this time, despite the fact that dissolved oxygen is low, Mg has a stronger deoxidizing power than Ti, so that a new fine oxide of sub-μm size of Mg alone is also generated at the same time.
[0033]
  As a result, it is possible to achieve an increase in the number of particles and a reduction in size that could not be achieved by the conventional addition method.
[0034]
  In general, with respect to oxides having a size of μm, as the number of oxides of 5 μm or more increases, it tends to become a starting point of destruction. Therefore, when Mg is added, as described in JP-A-9-157787, The Mg addition amount is limited to about 30 to 50 ppm.
[0035]
  However, according to the present invention, such a problem is avoided, and Mg can be added up to 100 ppm.
[0036]
  On the other hand, in Ti deoxidation or Ti + small amount Mg deoxidation, since it is deoxidation by a weak deoxidation element or a small amount of strong deoxidation element, dissolved oxygen still remains in the molten steel, and at that time, again Mg In addition to the above-mentioned μm or sub-μm oxide, the oxidation reaction of the remaining dissolved oxygen and Mg gradually proceeds to further generate an ultrafine oxide. The reason why ultra-fine oxides are generated is that the amount of dissolved oxygen is reduced and the distribution of dissolved oxygen in the molten steel is made uniform, so that oxide clustering is suppressed. It is estimated to be.
[0037]
  As described above, the oxide generated in steel becomes sulfide and nitride nucleation sites during casting, or during the subsequent cooling process or reheating-hot process.
[0038]
  And as a result of investigating the state of the oxide in steel at a magnification of 1000 to 100,000 times using an electron microscope, the existence state of the oxide in steel can be arranged as 1) and 2) below. . Regarding the state of oxides, it is desirable to observe at least 10 fields of view at a specific magnification (for example, about 100,000 times in the case of ultrafine oxides) and measure the average particle spacing and the like.
[0039]
  1) Mg having a particle size of 0.2 to 5 μmSubjectParticles in which one of sulfide and nitride alone or in combination of both with oxide as a nucleus are combined and contained in steel with an average particle spacing of 30 to 100 μm.
[0040]
  2) Mg having a particle size of 0.005 to less than 0.2 μmSubjectParticles in which one of sulfide and nitride alone or in combination of both with oxide as a core are precipitated are contained in steel with an average particle spacing of 30 μm or less.
[0041]
  The present invention relates to a steel material having excellent HAZ toughness achieved by the presence of the oxides of the above 1) and / or 2), and toughness change in HAZ, which has been largely dependent on heat input in the past, as much as possible. It provides innovative technology that can be suppressed.
[0042]
  Below, the improvement of HAZ toughness is further demonstrated.
[0043]
  As known so far, the intragranular transformation is promoted more when the number of oxides is larger and when there is precipitation of sulfides and nitrides on the oxides. As shown in the above 1), the number is increased by 10 times or more compared to the conventional one, and the composite precipitation is confirmed to be 100% sulfide or nitride as long as it is confirmed. Since it is precipitated, Mg of the present inventionSubjectIn the oxide, the intragranular transformation ability becomes extremely large.
[0044]
  Next, the most important refinement of the prior γ grain size in the present invention will be described with reference to FIG.
  FIG. 1 shows the old γ grain size in HAZ with 0.10C-1.0Mn steel as the base component and the amount of heat input on the horizontal axis for each heat input condition [1 kJ / mm, 10 kJ / mm, 50 kJ / mm. , 100 kJ / mm, 150 kJ / mm].
[0045]
  For the measurement of the prior γ particle size, in the case of an actual joint, after extracting a part of HAZ by cutting or the like, polishing treatment is performed, and the microstructure obtained by Nital corrosion is further measured using an optical microscope. Photographs were taken (at least 5) at a magnification of 50 to 200 times, and cutting was performed. The old γ particle size of 1 to 50 kJ / mm in FIG. 1 is a value obtained by this method.
[0046]
  In the case of ultra-high heat input, grain boundary ferrite is generated along the old γ grain boundary, so it is calculated as old γ grain including the grain boundary ferrite, or reproducible heat with the same heat input equivalent amount. It is usual to measure the prior γ particle size from the microstructure obtained by heating to a predetermined condition using a cycle tester and then quenching. The old γ particle size in the case of 100 kJ / mm and 150 kJ / mm in FIG. 1 is a value obtained from a microstructure formed using the latter reproduction thermal cycle tester.
[0047]
  Here, measurement examples of Al deoxidized steel, Ti-added Al deoxidized steel, and Mg deoxidized steel are shown, and the dependence of the old γ particle size on heat input is completely different depending on the presence or absence of Mg oxide in 2) above. You can see that
[0048]
  That is, with the exception of Mg deoxidized steel, the old γ grain size is clearly increased as the heat input increases.
[0049]
  On the other hand, when the presence state of the oxides 1) and 2) is realized, or when the presence state of the oxide 2) is realized, the heat input greatly changes. In the case of the Mg deoxidized steel, the change in the prior γ grain size is found to be extremely small.
  In particular, the presence state of the oxide of 2) is a dominant factor for the refinement of the prior γ grain size.
[0050]
  However, if the heat input amount is up to about 60 kJ / mm, the refinement of the old γ grain size is achieved even in the presence state (single) of the oxide of 1) above.
[0051]
  In addition, the pinning force is effective even in the presence state of the above-mentioned 1) single oxide, although its effectiveness is small, but when coexisting with the presence state of the oxide of 2) above, the refinement of the old γ grains is reduced. Remarkably promoted.
[0052]
  As a result of observing the steel sheet with refined old γ grains with an electron microscope, it was found that the face-centered cubic structure of MgO or Mg of 0.1 μm or less and the spinel type structure MIIMIII₂O_Four (MII: Mg, Ca, Fe, Mn, etc., MIII: Al, Ti, Cr, Mn, V, etc.) particles, or Mg schematically shown in FIG.SubjectIt was revealed that there are a large number of oxide-sulfide and / or nitride [TiN etc.] composite particles.
[0053]
  In electron microscope observation, MgSubjectExamination of the crystallographic orientation relationship between the contained oxide-sulfide or nitride particles revealed that they all had a completely parallel orientation relationship.
[0054]
  This indicates that the ultrafine Mg oxide acts as a preferential precipitation site for sulfides and nitrides. In other words, it is considered that the number of nitrides effective for crystal grain pinning is increased due to the presence of many preferential precipitation sites.
[0055]
  In other words, when the heat input is small, it is considered that these composite particles are acting as particles for pinning, and furthermore, when the residence time at high temperatures such as during super-high heat input welding is long, Although dissolution of nitride particles occurs, in the present invention, a large number of MgO or MgSubjectEven if the contained oxide is present, even if the nitride particles are dissolved, the fine oxide particles still present are considered to act as pinning particles at a high temperature.
[0056]
  Therefore, in the present invention, it is possible to achieve growth suppression of old γ grains in HAZ that could never be obtained with conventional steel.
[0057]
  That is, one of the features of the present invention is that, in addition to the remarkable improvement in intragranular transformation ability, unlike conventional steel in which crystal grains are pinned using a nitride such as TiN, oxidation of MgO or the like. Fine precipitates are introduced into the steel to create nitride precipitation nuclei, thereby increasing the number of nitrides, and in small heat input welding where nitrides work effectively, these The presence of the composite particles is that old γ grains having a HAZ of 10 to 200 μm or less can be obtained.
[0058]
  Furthermore, another feature of the present invention is that the nitride is dissolved, and in the high heat input to super high heat input welding, where the effect of improving toughness has not been obtained at all, the effect of suppressing the grain growth by the oxide alone. In HAZ, the old γ grain size is hardly changed.
[0059]
  As described above, the Mg addition method in the present invention is as follows. First, Si and Mn are added. Thereafter, Ti is added to adjust the amount of oxygen in the molten steel, and then a small amount of Mg is gradually added. Either Ti and a small amount of Mg are added at the same time, and then Mg is finally added again.
[0060]
  The optimum amount of Mg depends on the amount of oxygen present in the molten steel after the addition of Ti. However, according to experiments, the oxygen concentration at that time depends on the amount of Ti addition and the time until the addition of Mg. In this case, the addition amount of Ti and the addition amount of Mg may be controlled within an appropriate range.
[0061]
  The appropriate amount of dissolved oxygen at the final Mg addition is about 0.1 to 50 ppm. The minimum of 0.1 ppm is the minimum amount of dissolved oxygen that can form fine Mg oxide. On the other hand, if the amount of dissolved oxygen exceeds 50 ppm, a coarse Mg oxide is formed, and the pinning force becomes weak.
[0062]
  In addition, as for the Mg material and the addition method used when adding Mg, as a result of trying a method of adding metal Mg by wrapping it in Fe foil, a method of adding using Mg alloy, etc., the former In this case, it was found that the oxidation reaction was intense and the yield was lowered. Therefore, when melting at normal atmospheric pressure, it is preferable to add Mg alloy having a relatively heavy specific gravity.
[0063]
  Hereinafter, the reasons for limiting the component composition of the present invention will be described.
[0064]
  C is a basic element that improves the strength of the base metal of steel. In order to secure the improvement effect, addition of 0.01% or more is necessary. However, if over 0.2% is added excessively, the weldability and toughness of the steel material are reduced, so the upper limit is 0.2%. And
[0065]
  Si is an element necessary as a deoxidizing element in steelmaking, and it is necessary to add 0.02% or more to the steel. However, if added over 0.5%, the HAZ toughness is reduced, so that the content of Si is 0.001%. The upper limit is 5%.
[0066]
  Mn is an element necessary for ensuring the strength and toughness of the base metal. However, if added over 2%, the HAZ toughness is significantly inhibited. Since securing becomes difficult, the addition amount range is set to 0.3 to 2%.
[0067]
  P is an element that affects the toughness of steel. If contained over 0.03%, not only the base metal but also the toughness of HAZ is significantly inhibited, so the upper limit is made 0.03%. .
[0068]
  When S is contained in an amount exceeding 0.03%, coarse sulfides are generated and toughness is inhibited. However, when the content is less than 0.0001%, MnS and the like effective for the formation of intragranular ferrite are produced. Therefore, the amount of sulfide is significantly reduced, so 0.0001 to 0.03% is made the range of the addition amount.
[0069]
  Al is usually added as a deoxidizer, but in the present invention, if added over 0.05%, the effect of adding Mg is inhibited, so 0.05% is made the upper limit. Also, stable, MIIMIII₂O_Four Since 0.0005% is necessary at least to generate, 0.0005% is set as the lower limit.
[0070]
  Ti is an element that exerts an effect on grain refinement as a deoxidizer and further as a nitride-forming element, but a large amount of addition causes a significant decrease in toughness due to the formation of carbides. The upper limit needs to be 0.05%. And in order to acquire a predetermined effect, since addition of 0.003% or more is required, the range of the addition amount shall be 0.003-0.05%.
[0071]
  Mg is a main alloy element in the present invention, and is mainly added as a deoxidizing agent. However, when it is added over 0.01%, a coarse oxide is easily generated, and the base material and the HAZ This results in a decrease in toughness. However, addition of less than 0.0001% makes it impossible to sufficiently expect the formation of oxides necessary for intragranular transformation and pinning particles. Therefore, the range of the addition amount is set to 0.0001 to 0.010%.
[0072]
  O (oxygen) is MgSubjectIt is an essential element for generating an oxide. If the amount of oxygen finally remaining in the steel is less than 0.0001%, the number of oxides is not sufficient, so 0.0001% is made the lower limit. On the other hand, when the content exceeds 0.008%, coarse oxides increase and the base material toughness and the HAZ toughness are lowered, so the upper limit is made 0.008%.
[0073]
  In the present invention, one or more elements selected from Cu, Ni, Cr, Mo, V, Nb, Zr, Ta, and B may be added as elements for improving strength and toughness. it can.
[0074]
  Cu is an element effective for increasing the strength without reducing toughness, but if it is less than 0.05%, it is not effective, and if it exceeds 1.5%, cracking is likely to occur during heating of the slab or welding. . Therefore, the content range is set to 0.05 to 1.5%.
[0075]
  Ni is an element effective for improving toughness and strength, and in order to obtain the effect, addition of 0.05% or more is necessary, but if added over 5%, weldability is reduced. The upper limit is 5%.
[0076]
  Addition of 0.02% or more is effective to improve the strength of steel by precipitation strengthening, but Cr increases the hardenability and produces a bainite structure when added in excess of 1.5%. And toughness is reduced. Therefore, the upper limit is made 1.5%.
[0077]
  Mo is an element that improves hardenability and at the same time forms carbonitride to improve strength. To obtain the effect, addition of 0.02% or more is required, but 1.5% Addition exceeding a large amount brings about a significant decrease in toughness as well as an unnecessary strengthening, so the content range is 0.02 to 1.5%.
[0078]
  V is an element that forms carbides and nitrides and is effective in improving the strength. However, the addition of less than 0.01% has no effect, and the addition of more than 0.1% reduces the toughness. Therefore, the content range is set to 0.01 to 0.1%.
[0079]
  Nb is an element that forms carbides and nitrides and is effective in improving the strength. However, if it is added less than 0.0001%, it is not effective, and if it exceeds 0.2%, the toughness is reduced. The content range is 0.0001 to 0.2%.
[0080]
  Zr and Ta are elements that form carbides and nitrides as in Nb and are effective in improving the strength. However, the addition of less than 0.0001% has no effect, and the addition of more than 0.05% On the contrary, since the toughness is reduced, the content range is set to 0.0001 to 0.05%.
[0081]
  B is an element that generally increases the hardenability when dissolved, but lowers the solid solution N as BN and improves the toughness of the weld heat affected zone. Therefore, the effect can be utilized by addition of 0.0003% or more, but excessive addition causes a decrease in toughness, so the upper limit is made 0.005%.
[0082]
  Ca and REM suppress the production | generation of the extended | stretched MnS by producing | generating a sulfide, and improve the characteristic of the plate | board thickness direction of steel materials, especially lamellar resistance. Both Ca and REM cannot obtain this effect when added in an amount of less than 0.0005%, so the lower limit is made 0.0005%. On the contrary, if added over 0.005%, the number of Ca and REM oxides increases, and ultrafine MgSubjectSince the number of oxides decreases, the upper limit is made 0.005%.
[0083]
  The steel containing the above components is subjected to thick plate heating and rolling after melting in the steel making process, through continuous casting and the like. In this case, in the rolling method, the heating / cooling method, and the heat treatment method, there is no problem with respect to the HAZ toughness even if a method conventionally applied in this field is used.
[0084]
  In particular, the smaller the particle size of the base material, the larger the particle size difference from the HAZ particle size. Therefore, the refinement of the old γ particle size in HAZ according to the present invention is not only HAZ toughness but also hardness matching. Even when it is necessary to consider, etc., it is effective.
[0085]
  Example
  Next, examples of the present invention will be described.
[0086]
  A steel ingot having the chemical components shown in Table 1 and Table 2 (continuation of Table 1) is subjected to hot rolling and heat treatment under the conditions shown in Table 3 to form a steel plate, and then the welding heat input to the steel plate is 1.7 kJ. / Mm small heat input, 20 kJ / mm large heat input, and 150 kJ / mm super large heat input welding were performed. Then, the old γ particle size in HAZ was measured by the above-described cutting method, and the heat input dependence of HAZ toughness (test piece sampling position was the coarsest grain region) was evaluated by a Charpy impact test. The results are also shown in Table 3.
[0087]
  In Table 3, ΔvEo is the difference in Charpy absorbed energy between small heat input (1.7 kJ / mm) and super large heat input (150 kJ / mm), that is, [Toughness at small heat input: vEo (J)] − [Toughness at ultra-high heat input: vEo (J)] was calculated, and each absorbed energy is an average value of values measured for three test pieces at 0 ° C.
[0088]
  Further, λ1 and λ2 are average particle spacings of oxides calculated from 10 electron micrographs with λ1 of 1000 times and 10 electron micrographs with λ2 of 100,000 times, respectively.
[0089]
[Table 1]

[0090]
[Table 2]

[0091]
[Table 3]

[0092]
  Steels 1-22 show examples of the present invention. As is apparent from Table 3, the prior γ grain sizes in these inventive steels are all 200 μm or less in a wide range of heat input conditions from small heat input to super large heat input. Steels 20-2 and 21-2 have chemical components almost the same as those of steels 20 and 21, respectively, except that the deoxidation conditions are changed, and the amount of Mg is slightly different. is there. In the case of steel 20-2, λ1 is outside the range specified in the present invention, and in the case of steel 21-2, λ2 is outside the range specified in the present invention. Further, it can be seen that, in the steel 21-2, the particle diameter is 200 μm or less under the heat input condition of 60.0 kJ / mm. In addition, the Charpy absorbed energy of these inventive steels exceeds 10 kgf · m, indicating that the inventive steel is a high toughness steel.
[0093]
  Moreover, the difference in Charpy absorbed energy between the small heat input and the super large heat input is as small as about 4 kgf · m or less at maximum, and there is no significant change in the HAZ toughness even in a wide range of heat input conditions.
[0094]
  In Table 3, there is a difference in the Charpy absorbed energy difference, which indicates that the toughness is improved despite the increase in the prior γ grain size. And this means that in the present invention MgSubjectThis is because the intragranular transformation ability of the contained oxide is extremely large.
[0095]
  On the other hand, steels 23 to 35 represent comparative steels produced by deviating from the method of the present invention. That is, in the comparative steels 23, 24, 25, 26, 27, 29, 30, 33, 34 and 35, any one of the basic components or the selected elements is added beyond the composition range defined in the present invention. This is an example.
[0096]
  In the comparative steel, the average particle spacing of the oxide, which is an important requirement in the present invention, mostly satisfies the requirements specified in the present invention, but an element that causes toughness deterioration is excessively added. Therefore, the deterioration of the HAZ toughness is promoted at the time of welding under the small heat input condition and the super large heat input condition.
[0097]
  The comparative steel 28 and the comparative steel 31 are steels in which Al and Ti are smaller than the lower limits of Al and Ti defined in the present invention, respectively, and as the heat input increases, the old γ grain size becomes coarser. Both the comparative steel 28 and the comparative steel 31 have low toughness values.
[0098]
  The comparative steel 32 has no additive of Mg and has good toughness under a small heat input condition, but the toughness is greatly deteriorated under a super large heat input condition, and eventually Charpy absorbed energy. The difference is as large as 10.3 kgf · m.
[0099]
  In all the comparative steels, the HAZ toughness is at a low level, and further, when the heat input is large, the HAZ toughness is further lowered.
[0100]
  Note that, in the comparative steel 33 and the comparative steel 34, due to the presence of many fine oxides, the toughness is greatly deteriorated despite the fact that the old γ grain size is sufficiently small compared to the other. is doing.
[0101]
  The reason for this is that, due to the addition of excess Mg or O, coarse particles of 5 μm or more are mainly generated, and brittle fracture is promoted.
[0102]
  The comparative steel 36 and the comparative steel 37 have the same chemical composition as that of the inventive steel 1 and the inventive steel 2, respectively, but when a predetermined amount of final Mg is added, the dissolved oxygen content in the molten steel is 50 ppm. It was steel that had been exceeded.
[0103]
  As a result, the comparative steel 36 and the comparative steel 37 are those in which the generation of ultrafine oxides is small in the steel, and thus the coarsening of the old γ grain size and the remarkable toughness deterioration occur.
[0104]
  Industrial applicability
  According to the chemical component and the production method of the present invention, the heat input condition can be obtained by adding a predetermined amount of Mg after adding Ti, or by adding a predetermined amount of Mg after simultaneously adding Ti and Mg. Regardless of this, the growth of old γ grains in the HAZ can be suppressed.
[0105]
  In the present invention, this suppression effect can improve the HAZ toughness over a wide range of heat input conditions.
[0106]
  As a result, the safety against brittle fracture of welded structures is greatly improved in a wide range of technical fields such as offshore structures, natural gas / crude oil line pipes, construction, shipbuilding, bridges, and construction machinery.
[0107]
  Therefore, the present invention greatly contributes to the development of various industrial technologies.
[Brief description of the drawings]
FIG. 1 is a diagram showing an old γ grain size in HAZ when the welding heat input is changed.
FIG. 2 is a diagram schematically showing the form of a composite particle having an ultrafine Mg oxide as a nucleus.

Claims (6)

% By mass
C: 0.01-0.2%
Si: 0.02 to 0.5%,
Mn: 0.3-2%,
P: 0.03% or less,
S: 0.0001 to 0.03%,
Al: 0.0005 to 0.05%,
Ti: 0.003 to 0.05%,
Mg: 0.0001 to 0.01%
O 2: contains 0.0001 to 0.008%, the balance consists of iron and inevitable impurities,
Particles with an average particle spacing of 30 to 100 μm are formed by depositing particles containing 0.2 to 5 μm of Mg- based oxide as a nucleus and one of sulfide and nitride alone or a combination of both. Dispersed in and / or
Particle size and the oxide of Mg mainly below 0.005~0.2μm the nucleus, alone is one of the sulfides and nitrides, or both was precipitated by the composite particles, the average particle spacing 30μm or less Dispersed in steel,
A steel for welded structures having no dependence on heat input of HAZ toughness. In mass%,
Cu: 0.05 to 1.5%,
Ni: 0.05 to 5%,
Cr: 0.02 to 1.5%,
Mo: 0.02 to 1.5%,
V: 0.01-0.1%
Nb: 0.0001 to 0.2%,
Zr: 0.0001 to 0.05%,
Ta: 0.0001 to 0.05%,
B: 0.0003-0.005% 1 kind or no heat input dependency of HAZ toughness according to claim 1, characterized by containing two or more welded structure steel of. In mass%,
Ca: 0.0005 to 0.005%,
REM: 0.0005 to 0.005% 1 kind or characterized by containing two or more claims 1 or 2 is not heat input dependency of HAZ toughness described welded structure steel of. The steel for welded structures having no dependence on heat input of HAZ toughness according to claim 1 , wherein the prior austenite grain size of the HAZ structure is 10 to 200 μm irrespective of welding heat input. In steelmaking stage, Si, after addition to weak deoxidation of Mn, Ti: and 0.003 to 0.05 wt%, the Mg, the amount of dissolved oxygen and 50ppm or less by adding sequentially or simultaneously Furthermore, Mg is added so that it may become 0.0001-0.01 mass% in final content, and it casts, The HAZ toughness of Claim 1, 2, 3 or 4 dependence on heat input Method for manufacturing steel for welded structures. In steelmaking stage, Si, after addition to weak deoxidation of Mn, Ti: 0.003 to 0.05 mass% and, Al, Ca, Mg, and dissolved oxygen was added sequentially or simultaneously The HAZ toughness according to claim 1, 2, 3 or 4, wherein Mg is added so that the final content is 0.0001 to 0.01% by mass , and casting is performed. Of manufacturing steel for welded structures that does not depend on heat input.

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