JP4284258B2 - Steel sheet with low yield ratio and excellent toughness and welded joint toughness and its manufacturing method - Google Patents
Steel sheet with low yield ratio and excellent toughness and welded joint toughness and its manufacturing method Download PDFInfo
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本発明は、低降伏比で靭性および溶接継手部靭性に優れた鋼板に関するものであり、特に低温条件に曝される用途に適用した場合でも脆性破壊を起こし難い低降伏比特性を有すると共に靭性に優れ、更には溶接熱影響部の靭性も改善された鋼板とその製法に関するものである。 The present invention relates to a steel sheet having a low yield ratio and excellent toughness and welded joint toughness, and has a low yield ratio characteristic that hardly causes brittle fracture even when applied to applications exposed to low temperature conditions, and toughness. The present invention relates to a steel plate that is excellent and has improved the toughness of the heat affected zone, and a method for producing the same.
現在、種々の分野で低温靭性に優れた鋼板が求められている。例えば、液化石油ガス(LPG)や液化アンモニウムを貯蔵する低温用タンクなどでは、ガスを液化状態に保つため−60℃程度の低温で使用されるので、この様な低温条件下でも優れた靭性を保つことが求められる。 Currently, steel sheets having excellent low temperature toughness are required in various fields. For example, low temperature tanks that store liquefied petroleum gas (LPG) or liquefied ammonium are used at a low temperature of about −60 ° C. in order to keep the gas in a liquefied state. It is required to keep.
特に低温タンク材においては、脆性破壊防止の観点から、母材靭性を高めるため結晶粒を極力微細化し得るような圧延法を採用することが多く、結晶粒の微細化を進めると降伏比が上昇するという問題が生じてくる。即ち低温タンクでは、安全性を確保するため低降伏比化も重要な課題となってくるが、現状では、母材靭性の向上と低降伏比の両立は容易でない。 In particular, for low temperature tank materials, from the viewpoint of preventing brittle fracture, rolling methods that can make crystal grains as fine as possible are often used to increase the base material toughness, and the yield ratio increases as crystal grains become finer Problem arises. That is, in a low temperature tank, lowering the yield ratio is an important issue in order to ensure safety, but at present, it is not easy to improve the base material toughness and the lower yield ratio.
こうした状況の中で例えば特許文献1には、鋼組織中に所定量のマルテンサイトまたはマルテンサイトとオーステナイトの混合相を存在させると共に、当該相のサイズを極力小さくすることによって降伏点を示さない鋼に改質し、溶接性と低温靭性が良好で且つ低降伏比を示す高張力鋼が開示されている。 In such a situation, for example, Patent Document 1 discloses a steel that does not exhibit a yield point by making a predetermined amount of martensite or a mixed phase of martensite and austenite in the steel structure and reducing the size of the phase as much as possible. A high-strength steel is disclosed that has improved weldability and low temperature toughness and exhibits a low yield ratio.
また特許文献2には、上記特許文献1の更なる改善技術として、鋼組織中に所定量のマルテンサイトまたはマルテンサイトとオーステナイト混合相を存在させ、当該相のサイズを極力小さくすると共にアスペクト比を特定することにより、低降伏比で溶接性と低温靭性に優れた高張力鋼を得る技術が開示されている。 In Patent Document 2, as a further improvement technique of Patent Document 1, a predetermined amount of martensite or a mixed phase of martensite and austenite is present in the steel structure, and the aspect ratio is reduced while minimizing the size of the phase. By specifying, a technique for obtaining a high-strength steel having a low yield ratio and excellent weldability and low-temperature toughness is disclosed.
しかしこれら低降伏比を有する鋼も、母材自体の靭性や脆性破壊防止の観点、あるいはその製造方法の簡便性などからすると、更なる改善の余地を残している。 However, steels having these low yield ratios still have room for further improvement in terms of the toughness of the base metal itself, the prevention of brittle fracture, or the simplicity of its manufacturing method.
また、低温用鋼を用いて貯蔵用タンクなどを構築する際には、溶接継手部が溶接熱によるミクロ組織変化によって靭性劣化を起こし易いため、溶接継手部の低温靭性に優れた鋼板に対する要望も強い。 In addition, when constructing storage tanks using low-temperature steel, welded joints tend to deteriorate toughness due to microstructural changes due to welding heat, so there is also a demand for steel sheets with excellent low-temperature toughness in welded joints. strong.
そこで例えば特許文献3には、鋼板の化学成分を調整することによって溶接継手部の低温靭性を改善することが提案されている。この特許文献では、溶接継手部に島状マルテンサイトが生成すると、この島状マルテンサイトはC(炭素)が濃縮した硬質相であって破壊の起点になり易いことに注目し、該島状マルテンサイトを低減するため鋼材(母材)のC量を低減している。しかし最近では、化学成分の調整による低温靭性改善効果を超えて更なる低温靭性の向上が求められている。例えば、溶接時に1400℃程度の高温に加熱される領域(溶接熱影響部、以下、溶接継手部全体を含めてHAZ部ということがある)などでは、結晶粒が粗大化して低温靭性が劣化してしまうため、更なる低温靭性の改善が求められる。 Thus, for example, Patent Document 3 proposes improving the low-temperature toughness of the welded joint by adjusting the chemical composition of the steel sheet. In this patent document, it is noted that when island martensite is generated in a welded joint, this island martensite is a hard phase enriched in C (carbon) and is likely to be a starting point of fracture. In order to reduce the site, the amount of C in the steel (base material) is reduced. Recently, however, further improvement in low temperature toughness has been demanded beyond the effect of improving low temperature toughness by adjusting chemical components. For example, in a region heated to a high temperature of about 1400 ° C. during welding (hereinafter referred to as a “haz heat-affected zone, which may be referred to as a HAZ zone including the entire weld joint)”, the crystal grains become coarse and low-temperature toughness deteriorates. Therefore, further improvement in low temperature toughness is required.
そこで特許文献4には、TiNなどの微細析出物を分散させることによって、HAZ部での結晶粒の粗大化を抑制する方法が提案されている。しかし大入熱溶接を行う場合は、高温(1400℃以上)になる領域(例えば、大入熱溶接時の溶接線近傍など)があり、この様な領域ではTiNなどの析出物が固溶してしまうため、結晶粒の粗大化抑制効果を期待することができない。また二相温度域の加熱部ではフェライトとオーステナイトが平衡状態となり、このフェライトからCがオーステナイトへ移行してオーステナイトにCが濃縮されるため、結果的に島状マルテンサイトが生成する(この様な現象は、二相域脆化と称される)が、上記微細析出物は二相域脆化の防止効果も乏しい。 Therefore, Patent Document 4 proposes a method of suppressing the coarsening of crystal grains in the HAZ part by dispersing fine precipitates such as TiN. However, when high heat input welding is performed, there are regions (for example, the vicinity of the weld line at the time of high heat input welding) that become high temperature (1400 ° C. or higher). In such regions, precipitates such as TiN are dissolved. Therefore, the effect of suppressing the coarsening of crystal grains cannot be expected. Also, in the heating part in the two-phase temperature range, ferrite and austenite are in an equilibrium state, and since C shifts from this ferrite to austenite and C is concentrated in austenite, island martensite is formed as a result (such as this The phenomenon is referred to as two-phase region embrittlement), but the fine precipitates are also poor in preventing two-phase region embrittlement.
また本出願人は、耐アンモニア応力腐食割れ性が良好で且つHAZ靭性に優れた低温用鋼板として、先に特許文献5に記載の技術を提案している。この発明は、C含量を0.06%以下に抑えることで耐アンモニア応力腐食割れ性を高めると共に、特定量のTiとNを含有させることによってHAZ靭性を改善したもので、特に金属組織をフェライト主体とし、フェライト粒界にパーライトが層状に分散した層状パーライト組織とすることによって、耐アンモニア応力腐食割れ性とHAZ靭性という2つの要求特性を高めている。 The present applicant has previously proposed the technique described in Patent Document 5 as a low-temperature steel plate having good ammonia stress corrosion cracking resistance and excellent HAZ toughness. The present invention improves ammonia stress corrosion cracking resistance by suppressing the C content to 0.06% or less, and improves HAZ toughness by containing specific amounts of Ti and N. By forming a layered pearlite structure in which pearlite is dispersed in layers at the ferrite grain boundaries, the two required properties of ammonia stress corrosion cracking resistance and HAZ toughness are enhanced.
この発明は、低降伏比化を重視すると同時に、低温雰囲気下でも優れた耐応力腐食割れ性を示すと共に、大入熱溶接を行った場合でもHAZ靭性を良好に保つ点で優れた低温用鋼と言える。しかし需要者の要求は止まるところを知らず、特にHAZ部の低温靭性については更なる改善を求めている。
本発明は上記の様な事情に着目してなされたものであって、その目的は、母材そのものとして優れた靭性を有し、低降伏比で脆性破壊を起こし難く、更に、HAZ靭性においても前述した様な先行技術を凌駕する特性を有する鋼板とその製法を提供することにある。 The present invention has been made paying attention to the circumstances as described above, and its purpose is to have excellent toughness as a base material itself, hardly cause brittle fracture at a low yield ratio, and also in HAZ toughness. It is an object of the present invention to provide a steel plate having characteristics superior to those of the prior art as described above and a method for manufacturing the steel plate.
上記課題を解決することのできた本発明に係る低降伏比で靭性および溶接継手部靭性に優れた鋼板とは、
C :0.02〜0.10%、(質量%を意味する、以下、化学成分の場合は同じ)、
N :0.002〜0.010%、
Nb:0.003〜0.025%、
Ti:0.005〜0.025%、
を含む鋼材からなる鋼板であって、該鋼板の縦断面に現われるミクロ組織は、フェライトとパーライト若しくはこれらとベイナイトからなり、フェライトは下記の2種類を含み、
1)板厚方向の厚みが5〜10μmで、アスペクト比が2以上である伸長フェライト が15面積%以上、
2)円相当径が5μm以下でアスペクト比が1.5以下である等軸状フェライトが1 0〜50面積%、
且つ、横断面内に、Ti窒化物とNb炭窒化物との複合析出物が100個/mm2以上分散しているところに特徴を有している。
The steel sheet excellent in toughness and weld joint toughness at a low yield ratio according to the present invention, which was able to solve the above problems,
C: 0.02 to 0.10% (meaning mass%, hereinafter the same for chemical components),
N: 0.002 to 0.010%,
Nb: 0.003 to 0.025%,
Ti: 0.005 to 0.025%,
The microstructure that appears in the longitudinal section of the steel sheet is composed of ferrite and pearlite or these and bainite, and the ferrite includes the following two types:
1) Elongated ferrite having a thickness in the thickness direction of 5 to 10 μm and an aspect ratio of 2 or more is 15 area% or more,
2) 10-50 area% of equiaxed ferrite having an equivalent circle diameter of 5 μm or less and an aspect ratio of 1.5 or less,
In addition, it is characterized in that composite precipitates of Ti nitride and Nb carbonitride are dispersed at 100 pieces / mm 2 or more in the cross section.
本発明に係る上記鋼材の中でも、鋼板内にTi窒化物と共にNb炭窒化物として複合析出しているNb量が0.003%以上であるものは、HAZ靭性において一段と優れた性能を発揮する。 Among the above steel materials according to the present invention, those in which the amount of Nb compound-deposited as Nb carbonitride together with Ti nitride in the steel sheet is 0.003% or more exhibits further excellent performance in HAZ toughness.
また本発明に係る上記鋼材は、基本成分としてSi:0.7%以下、Mn:0.5〜1.8%、Al:0.1%以下を満たすものが好ましい。この鋼材には、低温靭性の更なる改善を期して、B:0.0003〜0.005%および/またはNi:0.01〜0.5%を含有させることができ、あるいは鋼板の一層の特性向上を期して、更に他の元素として、Zr:0.0003〜0.05%、Ca:0.0005〜0.005%、Mg:0.0005〜0.005%、REM:0.0005〜0.01%よりなる群から選択される少なくとも1種、あるいは更に、Cu:0.01〜0.5%、Cr:0.05〜0.5%、Mo:0.01〜0.5%、V:0.005〜0.1%よりなる群から選択される少なくとも1種の元素を、要求特性に応じて含有させてもよい。 The steel material according to the present invention preferably satisfies Si: 0.7% or less, Mn: 0.5 to 1.8%, and Al: 0.1% or less as basic components. This steel material can contain B: 0.0003 to 0.005% and / or Ni: 0.01 to 0.5% for further improvement of low temperature toughness, In order to improve the characteristics, other elements include Zr: 0.0003 to 0.05%, Ca: 0.0005 to 0.005%, Mg: 0.0005 to 0.005%, REM: 0.0005. Or at least one selected from the group consisting of 0.01%, or Cu: 0.01 to 0.5%, Cr: 0.05 to 0.5%, Mo: 0.01 to 0.5 %, V: At least one element selected from the group consisting of 0.005 to 0.1% may be contained according to the required characteristics.
また本発明の製法は、上記特性を備えた鋼板をより確実に得ることのできる方法として位置付けられる発明であり、前記化学成分の要件を満たす鋼材を熱間圧延する際に、800℃以上の温度域の圧下率を40%以上、800〜750℃の温度域の圧下率を20〜50%、750℃以下の温度域の圧下率を30%以上に制御し、得られる鋼板の縦断面に現われるミクロ組織の主体をフェライトとパーライト、又はこれらとベイナイトとし、且つフェライトを上記1),2)の2種類の混合組織とし、且つ横断面内に、Ti窒化物とNb炭窒化物との複合析出物を100個/mm2以上分散させるところに特徴を有している。 Further, the production method of the present invention is an invention that is positioned as a method that can more reliably obtain a steel plate having the above characteristics, and when hot-rolling a steel material that satisfies the requirements of the chemical components, a temperature of 800 ° C. or higher. The rolling reduction in the region is controlled to 40% or more, the rolling reduction in the temperature region of 800 to 750 ° C. is controlled to 20 to 50%, and the rolling reduction in the temperature region of 750 ° C. or less is controlled to 30% or more. The microstructure is mainly composed of ferrite and pearlite, or these and bainite, and ferrite is a mixed structure of the above two types 1) and 2), and composite precipitation of Ti nitride and Nb carbonitride in the cross section. It is characterized in that 100 or more pieces / mm 2 are dispersed.
上記製法を実施するに当たっては、熱間圧延の後に1〜50℃/secの速度で加速冷却を行なうことが好ましく、更に該加速冷却の後、500℃以上Ac1変態点以下の温度で焼戻しを行えば、鋼板の性能を一段と高めることができるので好ましい。 In carrying out the above production method, it is preferable to perform accelerated cooling at a rate of 1 to 50 ° C./sec after hot rolling. Further, after the accelerated cooling, tempering is performed at a temperature of 500 ° C. or more and an Ac 1 transformation point or less. This is preferable because the performance of the steel sheet can be further improved.
本発明の鋼板は、鋼中のC,N,Nb,Ti量を特定すると共に、ミクロ組織の主体をフェライトとパーライト、又はこれらとベイナイトとし、且つフェライトを比較的長尺のものと短尺のものからなる2種類の混合組織に制御することによって、母材靭性と低降伏比特性を与え、更には、鋼材中にTi窒化物とNb炭窒化物との複合析出物を微細分散状態で析出させることによってHAZ靭性を改善することができ、低降伏比で母材靭性およびHAZ靭性の全てに優れた鋼板を提供できる。 The steel sheet of the present invention specifies the amount of C, N, Nb, and Ti in the steel, and the microstructure is mainly composed of ferrite and pearlite, or these and bainite, and the ferrite is relatively long and short. The base metal toughness and the low yield ratio characteristic are given by controlling to two kinds of mixed structures consisting of, and further, a composite precipitate of Ti nitride and Nb carbonitride is precipitated in the steel material in a finely dispersed state. Accordingly, the HAZ toughness can be improved, and a steel sheet excellent in all of the base metal toughness and the HAZ toughness can be provided with a low yield ratio.
本発明者らは上述した様な課題の解決を期して鋭意研究を重ねた結果、鋼材中の炭素含量を低レベルに抑えることを前提として適量のN,Nb,Tiを含有させ、ミクロ組織を、フェライトとパーライト若しくはこれらとベイナイトからなる適正な組織に制御すると共に、該フェライトを、上記1)、2)からなる特定寸法サイズの混合組織とし、且つ鋼板内にTi窒化物とNb炭窒化物との複合析出物を分散析出させてやれば、低降伏比で高レベルの母材靭性とHAZ靭性を確保できることを突き止め、上記本発明を完成した。 As a result of intensive studies aimed at solving the problems as described above, the present inventors have included an appropriate amount of N, Nb, Ti on the premise of suppressing the carbon content in the steel material to a low level, In addition, the ferrite is controlled to have an appropriate structure made of ferrite and pearlite or these and bainite, and the ferrite is made to have a mixed structure having a specific size size of 1) and 2) above, and Ti nitride and Nb carbonitride in the steel sheet. The present invention has been completed by ascertaining that a high level of base metal toughness and HAZ toughness can be secured with a low yield ratio if the composite precipitate is dispersed and precipitated.
即ち本発明では第1の特徴点として、鋼板の縦断面に現われるミクロ組織を上記の様に制御することで、必要最小限の強度を確保しつつ降伏比を大幅に低減することが可能となる。そして、該ミクロ組織をフェライトとパーライト若しくはこれらとベイナイトとし、且つ該フェライト組織が、相対的に厚肉で伸長したフェライトと、微細で等軸状のフェライトとの混在状態で存在するものは、適度の母材靭性と低降伏比を有するものとなる。 That is, in the present invention, as a first feature point, by controlling the microstructure appearing in the longitudinal section of the steel sheet as described above, it becomes possible to significantly reduce the yield ratio while ensuring the necessary minimum strength. . And when the microstructure is ferrite and pearlite or these and bainite, and the ferrite structure exists in a mixed state of relatively thick and elongated ferrite and fine and equiaxed ferrite, It has a base material toughness and a low yield ratio.
上記において、フェライトを前記1),2)の混合組織と定めたのは、同様に適度の母材靭性を確保しつつ低降伏比化を実現するためである。ちなみに、板厚方向の厚みが5〜10μmで、アスペクト比が2以上である伸長フェライトの占める比率が15面積%未満では、伸長フェライト量の不足により鋼板の引張特性が不足気味となり、例えば400MPa以上といったレベルの強度を確保し難くなる。また、円相当径が5μm以下でアスペクト比が1.5以下である等軸状フェライトが10面積%未満では、母材靭性が不十分となり、逆に50面積%を超えて過度に多くなると、降伏点が上昇すると共に降伏比も高くなり、低降伏比化の目的が果たせなくなる。 The reason why the ferrite is defined as the mixed structure of 1) and 2) in the above is to realize a low yield ratio while ensuring an appropriate base metal toughness. By the way, if the ratio of the elongated ferrite having a thickness in the thickness direction of 5 to 10 μm and the aspect ratio of 2 or more is less than 15% by area, the tensile properties of the steel sheet are insufficient due to the insufficient amount of the elongated ferrite, for example, 400 MPa or more. It becomes difficult to secure such a level of strength. Further, if the equiaxed ferrite having an equivalent circle diameter of 5 μm or less and an aspect ratio of 1.5 or less is less than 10 area%, the base material toughness becomes insufficient, and conversely, if it exceeds 50 area% and excessively increases, As the yield point rises, the yield ratio increases, and the purpose of lowering the yield ratio cannot be achieved.
適度の母材靭性を確保しつつ過度の降伏点の上昇を抑えて低降伏比化を増進する上でより好ましい上記伸長フェライトの含有比率は20面積%以上、より好ましい上記等軸状フェライトの含有比率は15面積%以上である。 More preferably, the content ratio of the stretched ferrite is 20 area% or more, more preferably the content of the equiaxed ferrite, in order to suppress the increase of the excessive yield point while ensuring an appropriate base material toughness and promote the reduction of the yield ratio. The ratio is 15 area% or more.
なお本発明において、“伸長フェライト”を『板厚方向の厚みが5〜10μmで、アスペクト比が2以上であるフェライト』、“等軸状フェライト”を『円相当径が5μm以下でアスペクト比が1.5以下であるフェライト』と定めたのは、圧延方向に引き伸ばされた伸長フェライトと、短尺で球形に近いフェライトの存在比率を定量化するための基準を明確にするためであり、上記1),2)の規定範囲を外れる寸法・サイズのものは、各々の面積率には含めないものとする。 In the present invention, “elongated ferrite” is “ferrite having a thickness in the thickness direction of 5 to 10 μm and an aspect ratio of 2 or more”, and “equiaxial ferrite” is “equivalent axis diameter of 5 μm or less and the aspect ratio is The reason for defining “ferrite of 1.5 or less” is to clarify the standard for quantifying the abundance ratio of the elongated ferrite stretched in the rolling direction and the short, nearly spherical ferrite. ) And dimensions outside the specified range in 2) shall not be included in each area ratio.
また本発明では、上記フェライト組織に関する要求特性を満たす鋼板については、その前提として、当該鋼材中に適量のNbを含有させることが不可欠の要件となる。ちなみにNbは、熱延時におけるフェライト再結晶粒を長尺化し、アスペクト比の大きい伸長フェライトの存在率を高める作用を有している。 Moreover, in this invention, about the steel plate which satisfy | fills the required characteristic regarding the said ferrite structure, it becomes indispensable requirements to contain an appropriate quantity of Nb in the said steel materials as the premise. Incidentally, Nb has the effect of elongating ferrite recrystallized grains during hot rolling and increasing the abundance of elongated ferrite having a large aspect ratio.
そして、前記1),2)で規定する伸長フェライトと等軸状フェライトの面積率の要件を満たす金属組織を得るには、鋼中のNb含量を0.003%以上とすべきである。しかしNb含量が多過ぎると、該フェライト再結晶粒の長尺化作用が過度に強く現われて伸長フェライトの比率が過大となり、等軸状フェライトの比率が不足気味となって強度不足になるので、0.025%以下に抑えるべきである。Nbのより好ましい含有率は0.005%以上、0.022%以下であり、Nb含量をこの範囲に設定して前記1),2)で規定する伸長フェライトと等軸状フェライトの面積率の範囲に制御してやれば、例えば400MPaレベルの強度を確保しつつ、例えば85%前後の低い降伏比を得ることが可能となる。 And in order to obtain the metal structure which satisfies the requirements for the area ratio of the elongated ferrite and the equiaxed ferrite defined in 1) and 2), the Nb content in the steel should be 0.003% or more. However, if the Nb content is too much, the lengthening effect of the ferrite recrystallized grains appears excessively strong, the ratio of the elongated ferrite becomes excessive, the ratio of the equiaxed ferrite becomes insufficient and the strength becomes insufficient. It should be kept below 0.025%. A more preferable content of Nb is 0.005% or more and 0.022% or less. The Nb content is set within this range, and the area ratio of the elongated ferrite and the equiaxed ferrite defined in the above 1) and 2) If it is controlled within the range, it is possible to obtain a low yield ratio of, for example, about 85% while securing a strength of, for example, 400 MPa.
また本発明における第2の特徴点は、鋼中の上記Nb含量に加えて、C,N,Ti含量を適正に制御し、Nbに由来するNb炭窒化物とTi窒化物とからなる複合析出物を鋼材内に微細析出させることで、溶接継手部やHAZ部の低温靭性を向上させるところにある。 The second feature of the present invention is that, in addition to the Nb content in the steel, the C, N, and Ti contents are appropriately controlled, and a composite precipitation comprising Nb carbonitride and Ti nitride derived from Nb. By precipitating the material finely in the steel material, the low temperature toughness of the welded joint part and the HAZ part is improved.
従って本発明では、上記Nb含量に加えて、まずC(炭素)含量を0.02〜0.10%の範囲内に制御する。その理由は、鋼材として必要な構造強度を確保するには0.02%以上のC含量を確保することが必要であり、これ未満では強度不足になるからである。より好ましいC含量は0.03%以上である。一方、本発明は、前述した如く例えば−60℃以下といった低温条件に曝される用途に適用されことがあることを想定しており、こうした低温条件での使用にも耐える母材靭性を確保すると共に、HAZ靭性の劣化を防止するには、C含量を0.10%以下に抑えるべきであり、より好ましくは0.08%以下に抑えるのがよい。 Therefore, in the present invention, in addition to the Nb content, first, the C (carbon) content is controlled within the range of 0.02 to 0.10%. The reason is that it is necessary to secure a C content of 0.02% or more in order to ensure the structural strength necessary as a steel material, and if it is less than this, the strength is insufficient. A more preferable C content is 0.03% or more. On the other hand, it is assumed that the present invention may be applied to applications exposed to low temperature conditions such as −60 ° C. or less as described above, and the toughness of the base material that can withstand use under such low temperature conditions is ensured. At the same time, in order to prevent the degradation of the HAZ toughness, the C content should be suppressed to 0.10% or less, and more preferably 0.08% or less.
次にN(窒素)含量は、0.002〜0.010%の範囲に制御すべきである。即ちNは、Nb炭窒化物の生成源になると共に、後述する如くTiと結合してTi窒化物の生成源となり、HAZ部の靭性を高める作用を有しており、0.002%未満では上記窒化物としての析出量が不十分となって、意図するレベルの靭性改善効果が得られない。しかしN含量が過度に多くなると、非金属介在物量の増大により却って靭性を劣化させるので、0.010%以下に抑えるべきである。より好ましいN含量は0.003%以上、0.008%以下である。 Next, the N (nitrogen) content should be controlled in the range of 0.002 to 0.010%. In other words, N is a source of Nb carbonitride and, as will be described later, combines with Ti to become a source of Ti nitride, and has the effect of increasing the toughness of the HAZ part. The precipitation amount as the nitride becomes insufficient, and the intended toughness improving effect cannot be obtained. However, if the N content is excessively large, the toughness is deteriorated due to an increase in the amount of non-metallic inclusions, so it should be suppressed to 0.010% or less. A more preferable N content is 0.003% or more and 0.008% or less.
Tiは、特にHAZ靭性の向上に不可欠の成分であって、Ti窒化物の生成によりNb炭窒化物との複合析出物を生成させる上で重要な元素であり、0.005%以上、0.025%以下で含有させる。0.005%未満ではTi窒化物の生成量が不足し、Nb炭窒化物との複合析出物の生成量も不十分となって満足のいくHAZ靭性が得られなくなる。しかし多過ぎると、非金属系介在物の増大により却って靭性を劣化させる原因になるので、0.025%以下に抑えるべきである。より好ましいTi含量は0.008%以上、0.022%以下である。 Ti is an indispensable component particularly for improving the HAZ toughness, and is an important element for forming a composite precipitate with Nb carbonitride by the formation of Ti nitride. It is contained at 025% or less. If it is less than 0.005%, the amount of Ti nitride produced is insufficient, and the amount of composite precipitates produced with Nb carbonitride is also insufficient, so that satisfactory HAZ toughness cannot be obtained. However, if it is too much, it causes the deterioration of toughness due to the increase of non-metallic inclusions, so it should be suppressed to 0.025% or less. A more preferable Ti content is 0.008% or more and 0.022% or less.
本発明で必須的に含有させるNbは、前述した如くフェライト再結晶時における伸長フェライトの面積率を増大させるのに不可欠の元素である他、上記C(炭素)およびN(窒素)と結合しNb炭窒化物の生成源となって上記Ti窒化物と共に複合析出し、HAZ靭性の向上に顕著な効果を発揮する。こうした効果を有効に発揮させるには少なくとも0.002%以上含有させねばならず、好ましくは0.005%以上、更に好ましくは0.008%以上含有させるのがよい。但し、こうしたNbの添加効果は約0.025%で飽和し、それ以上に多くなると焼入性過剰となってHAZ部の島状マルテンサイトが増大し、HAZ靭性が却って悪くなる傾向が現れてくるので、多くとも0.025%以下、好ましくは0.022%程度以下に抑えるのがよい。 Nb essential to be contained in the present invention is an element indispensable for increasing the area ratio of the elongated ferrite at the time of ferrite recrystallization as described above, and also combines with C (carbon) and N (nitrogen) to form Nb. It becomes a carbonitride generation source and is precipitated together with the Ti nitride, and exhibits a remarkable effect in improving HAZ toughness. In order to exhibit such an effect effectively, it should be contained at least 0.002%, preferably 0.005% or more, more preferably 0.008% or more. However, the effect of Nb addition is saturated at about 0.025%, and if it exceeds this, the hardenability becomes excessive and island martensite in the HAZ part increases, and the HAZ toughness tends to deteriorate instead. Therefore, it is good to keep it at most 0.025% or less, preferably about 0.022% or less.
本発明鋼板における重要な構成元素は、上記の様に第1にNb、更にC,N,Tiの4元素であるが、鋼材の基本元素としては、Si:0.7%以下、Mn:0.5〜1.8%、Al:0.1%以下を満たすものであることが望ましい。 As described above, the important constituent elements in the steel sheet of the present invention are firstly Nb, and further four elements of C, N, and Ti. As basic elements of the steel material, Si: 0.7% or less, Mn: 0 It is desirable to satisfy 5 to 1.8% and Al: 0.1% or less.
上記Siは、鋼中の含量が多くなり過ぎると島状マルテンサイトを増加させ、HAZ部の低温靱性を劣化させる傾向が強まるので、0.7%以下、好ましくは0.5%以下、更に好ましくは0.3%以下に抑えることが望ましい。なおSiは溶鋼の脱酸に使用されることが多いため、殆どの鋼中には不可避的に混入してくる。また鋼板の強度向上にも有効に作用することから、Siは、0.01%程度以上、好ましくは0.05%程度以上含有させるのがよい。 If the Si content is too high, the island-like martensite is increased and the tendency to deteriorate the low temperature toughness of the HAZ part is increased, so 0.7% or less, preferably 0.5% or less, more preferably Is preferably suppressed to 0.3% or less. Since Si is often used for deoxidation of molten steel, it is inevitably mixed in most steels. Further, since it effectively works to improve the strength of the steel sheet, Si is preferably contained in an amount of about 0.01% or more, preferably about 0.05% or more.
またMnは、焼入性を高めて鋼板の強度を高める作用を有しており、好ましくは0.5%以上、より好ましくは0.7%以上、更に好ましくは1.0%以上含有させることが望ましい。しかしMn量が過剰になると、島状マルテンサイトが増加してHAZ部の低温靱性を劣化させる傾向があるので、好ましくは1.8%以下、より好ましくは1.6%以下に抑えるのがよい。 Mn has the effect of increasing hardenability and increasing the strength of the steel sheet, and is preferably 0.5% or more, more preferably 0.7% or more, and further preferably 1.0% or more. Is desirable. However, if the amount of Mn becomes excessive, island-like martensite tends to increase and the low temperature toughness of the HAZ part tends to deteriorate, so it is preferable to keep it at 1.8% or less, more preferably 1.6% or less. .
Alは脱酸剤として使用されることが多いため、溶鋼の成分調整過程で混入してくる元素であるが、過剰になると酸化物系介在物量の増大によって鋼材の靭性を劣化させるので、0.1%以下、好ましくは0.08%以下、より好ましくは0.06%以下に抑えるのがよい。一方、AlはAlN系析出物を生成して大入熱溶接時のHAZ靭性を向上させる作用も有しているので、AlによるHAZ靭性向上効果を有効に発揮させたい場合は、Alを積極的に例えば0.01%以上、好ましくは0.02%以上含有させることも有効である。 Since Al is often used as a deoxidizer, it is an element mixed in the process of adjusting the components of the molten steel. However, when it is excessive, the toughness of the steel material deteriorates due to an increase in the amount of oxide inclusions. It should be suppressed to 1% or less, preferably 0.08% or less, more preferably 0.06% or less. On the other hand, Al also has an effect of improving the HAZ toughness at the time of high heat input welding by generating AlN-based precipitates. For example, it is effective to contain 0.01% or more, preferably 0.02% or more.
本発明の鋼板は、前記元素以外にも必要に応じて他の元素、例えば、靭性向上元素[B:0.005%以下、Ni:0.5%以下など]、析出物形成元素[Zr:0.05%以下、Ca:0.005%以下、Mg:0.005%以下、およびREM:0.01%以下など]、強度向上元素[Cu:0.5%以下、Cr:0.5%以下、Mo:0.5%以下、V:0.1%以下など]などを含むものであってもよい。 In addition to the above elements, the steel plate of the present invention may contain other elements such as toughness improving elements [B: 0.005% or less, Ni: 0.5% or less, etc.], precipitate forming elements [Zr: 0.05% or less, Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.01% or less], strength improving elements [Cu: 0.5% or less, Cr: 0.5 % Or less, Mo: 0.5% or less, V: 0.1% or less, and the like].
靭性向上元素:B、Niなど
Bは、BNの生成によってHAZ靭性に有害な固溶Nを固定する作用を発揮する。B含量の下限は特に規定しないが、こうした作用を積極的に活かそうとする場合は、0.0003%以上、好ましくは0.0005%以上含有させることが望ましい。但し、B含量が過剰になると大入熱溶接時のHAZ靭性を却って劣化させるので、0.005%以下、好ましくは0.004%以下に抑えるべきである。
Toughness improving elements: B, Ni, etc. B exhibits the action of fixing solute N that is harmful to HAZ toughness by the formation of BN. The lower limit of the B content is not particularly defined, but when such an action is to be actively utilized, 0.0003% or more, preferably 0.0005% or more is desirable. However, if the B content is excessive, the HAZ toughness at the time of high heat input welding is deteriorated, so it should be suppressed to 0.005% or less, preferably 0.004% or less.
Niは靭性を向上させるのに有効である。Ni含量の下限は特に規定しないが、こうした作用を積極的に期待する場合は、0.01%以上、好ましくは0.05%以上含有させることが望ましい。但し、Ni含量が過剰になると鋼板にスケール疵が発生し易くなるので、0.5%以下、好ましくは0.4%以下に抑えるべきである。上記靭性向上元素は、単独で添加してもよく、複数元素を任意の組合せで併用してもよい。 Ni is effective in improving toughness. The lower limit of the Ni content is not particularly specified, but when such an action is positively expected, it is desirable to contain 0.01% or more, preferably 0.05% or more. However, if the Ni content is excessive, scale flaws are likely to occur in the steel sheet, so it should be suppressed to 0.5% or less, preferably 0.4% or less. The toughness improving elements may be added alone or a plurality of elements may be used in any combination.
析出物形成元素:Zr、Ca、Mg、REMなど
Zrは、前記Tiと同様に窒化物を形成し、大入熱溶接時のHAZ靭性の向上に寄与する。Zr含量の下限は特に規定しないが、前記作用を積極的に期待する場合は、0.0003%以上、好ましくは0.0005%以上とすることが推奨される。但し、Zr含量が過剰になると清浄度の低下により靭性劣化を起こす原因になるので、0.05%以下、好ましくは0.005%以下に抑えるべきである。
Precipitate-forming elements: Zr, Ca, Mg, REM, etc. Zr forms nitrides like Ti, and contributes to the improvement of HAZ toughness during high heat input welding. The lower limit of the Zr content is not particularly specified, but when the above action is positively expected, it is recommended that the content be 0.0003% or more, preferably 0.0005% or more. However, if the Zr content is excessive, it causes deterioration of toughness due to a decrease in cleanliness, so it should be suppressed to 0.05% or less, preferably 0.005% or less.
Ca、MgおよびREM(希土類元素)は、酸化物、硫化物、酸硫化物などを形成し、HAZ部における結晶粒の粗大化を抑えるのに有効である。また母材の異方性を軽減する作用も有している。Ca、MgおよびREM含量の下限は特に規定しないが、こうした作用を積極的に活用したい場合は、Caは0.0005%以上(好ましくは0.0010%以上)、Mgは0.0005%以上(好ましくは0.0010%以上)、REMは0.0005%以上(好ましくは0.0010%以上)とするのがよい。但し、これらCa、Mg、REMが多過ぎると、清浄度が低下して物性劣化の原因になるので、Caは0.005%以下(好ましくは0.003%以下)、Mgは0.005%以下(好ましくは0.003%以下)、REMは0.01%以下(好ましくは0.005%以下、特に0.003%以下)に抑えるのがよい。 Ca, Mg, and REM (rare earth elements) form oxides, sulfides, oxysulfides, and the like, and are effective in suppressing coarsening of crystal grains in the HAZ part. It also has the effect of reducing the anisotropy of the base material. The lower limits of the Ca, Mg and REM contents are not particularly specified. However, when such an action is to be actively used, Ca is 0.0005% or more (preferably 0.0010% or more), and Mg is 0.0005% or more ( The REM is preferably 0.0005% or more (preferably 0.0010% or more). However, if these Ca, Mg, and REM are too much, the cleanliness is lowered and physical properties are deteriorated. Therefore, Ca is 0.005% or less (preferably 0.003% or less), and Mg is 0.005%. The following is preferable (preferably 0.003% or less) and the REM is preferably 0.01% or less (preferably 0.005% or less, particularly 0.003% or less).
上記析出物形成元素は、単独で添加してもよく、あるいは任意の組合せで複合添加してもよい。 The precipitate-forming elements may be added alone or in combination with any combination.
強度向上元素:Cu、Cr、Mo、Vなど
Cuは、固溶強化および析出強化による強度向上に有効な元素であり、添加量の下限は特に定めないが、こうした作用を積極的に活用しようとする場合は、例えば、0.01%以上、好ましくは0.05%以上含有させるのがよい。しかし、Cu含量が多過ぎると熱間加工性が劣化し、鋼板が表面割れを起こし易くなるので、0.5%以下、好ましくは0.3%以下に抑えるのがよい。
Strength improving elements: Cu, Cr, Mo, V, etc. Cu is an element effective for improving the strength by solid solution strengthening and precipitation strengthening, and the lower limit of the addition amount is not particularly defined, but it is intended to actively utilize such action. For example, the content may be 0.01% or more, preferably 0.05% or more. However, if the Cu content is too high, the hot workability deteriorates and the steel sheet is liable to cause surface cracking, so it is preferable to keep it to 0.5% or less, preferably 0.3% or less.
CrとMoは、いずれも母材の強度向上に有効に作用する。CrとMoの添加量の下限は特に規定しないが、該作用効果を積極的に期待する場合には、Crは、0.05%以上(好ましくは0.10%以上)、Moは、0.01%以上(好ましくは0.05%以上)とするのがよい。しかし、CrやMoが過剰になると、大入熱溶接時のHAZ靭性を劣化させる傾向があるので、Crは0.5%以下(好ましくは0.3%以下)、Moは0.5%以下(好ましくは0.3%以下)に抑えるべきである。 Both Cr and Mo effectively act to improve the strength of the base material. Although the lower limit of the addition amount of Cr and Mo is not particularly defined, when the effect is positively expected, Cr is 0.05% or more (preferably 0.10% or more), and Mo is 0.8. It should be 01% or more (preferably 0.05% or more). However, if Cr and Mo become excessive, there is a tendency to deteriorate the HAZ toughness during high heat input welding, so Cr is 0.5% or less (preferably 0.3% or less), and Mo is 0.5% or less. (Preferably 0.3% or less) should be suppressed.
Vは析出強化によって強度を高める元素であり、添加量の下限は特に規定しないが、こうした作用効果を積極的に期待する場合は、0.005%以上(好ましくは0.010%以上)含有させるのがよい。しかし、Vが過剰になると、大入熱溶接時のHAZ靭性を劣化させるので、0.1%以下(好ましくは0.05%以下)に抑えるのがよい。これらの強度向上元素は、単独で添加してもよく、複数組合せて添加してもよい。 V is an element that increases the strength by precipitation strengthening, and the lower limit of the addition amount is not particularly specified. However, when such an action effect is positively expected, 0.005% or more (preferably 0.010% or more) is contained. It is good. However, if V is excessive, the HAZ toughness at the time of high heat input welding is deteriorated, so it is preferable to keep it at 0.1% or less (preferably 0.05% or less). These strength improving elements may be added alone or in combination.
本発明に係る鋼材の残部成分は実質的にFeと不可避不純物であり、不可避不純物としては、例えば、PやSなどが挙げられる。これらPやSは、積極的に低減することも有効であり、例えば、Pは0.015%以下に低減するのがよい。Pが多くなると溶接性が劣化するからである。好ましくは0.010%以下に抑えるのがよい。なおPは、0.001%程度であれば殆ど実害は生じない。 The remaining components of the steel material according to the present invention are substantially Fe and unavoidable impurities. Examples of the unavoidable impurities include P and S. It is also effective to actively reduce P and S. For example, P should be reduced to 0.015% or less. This is because weldability deteriorates when P increases. Preferably it is good to restrain to 0.010% or less. If P is about 0.001%, practically no harm occurs.
またSは、0.005%以下に抑えることが望ましい。S量が多くなると硫化物系介在物が増大して鋼板の母材靭性が劣化し易くなるからである。好ましくは0.003%以下とするのがよい。なおSは、0.001%程度以下であれば殆ど実害は生じない。 Further, S is desirably suppressed to 0.005% or less. This is because when the amount of S increases, sulfide inclusions increase and the base material toughness of the steel sheet is likely to deteriorate. Preferably it is 0.003% or less. If S is about 0.001% or less, practically no harm is caused.
本発明で用いる鋼材の好ましい成分組成は上記の通りであるが、本発明ではこうした好ましい成分組成に加えて、前述した如く鋼板の縦断面に現われるミクロ組織が、鋼材の靭性と低降伏比化を両立させる上で極めて重要であり、該ミクロ組織はフェライトとパーライト、もしくはそれらとベイナイトからなり、且つ該フェライト組織は、相対的に薄肉の伸長フェライト(具体的には、板厚方向の厚みが5〜10μmで、アスペクト比が2以上であるもの)が15面積%と、等軸状フェライト(具体的には、円相当径が5μm以下でアスペクト比が1.5以下であるもの)が10〜50面積%からなるものである。 The preferred component composition of the steel material used in the present invention is as described above, but in the present invention, in addition to such a desirable component composition, the microstructure appearing in the longitudinal section of the steel plate as described above reduces the toughness and lower yield ratio of the steel material. It is extremely important to achieve both, and the microstructure is composed of ferrite and pearlite, or they and bainite, and the ferrite structure is a relatively thin elongated ferrite (specifically, the thickness in the thickness direction is 5). 10 μm and an aspect ratio of 2 or more) is 15 area%, and equiaxed ferrite (specifically, an equivalent circle diameter of 5 μm or less and an aspect ratio of 1.5 or less) is 10 to 10%. It consists of 50 area%.
こうしたミクロ組織は、例えば供試鋼板の縦断面をバフ研磨した後、ナイタールでエッチングしてから顕微鏡観察し、該顕微鏡写真(倍率:200倍)を画像解析することによってフェライト、パーライト、ベイナイトを識別すると共に、同顕微鏡写真から選択した任意の10領域について上記で定義した伸長フェライトと等軸状フェライトの面積率を求め、平均値として算出する。 For example, after microbuffing the longitudinal section of the test steel sheet, etching with nital and then observing under a microscope, the micrograph (magnification: 200 times) is image-analyzed to identify ferrite, pearlite, and bainite. At the same time, the area ratios of the elongated ferrite and the equiaxed ferrite defined above are obtained for any 10 regions selected from the micrograph, and are calculated as an average value.
また本発明では、前述した如く他の要件として、鋼板断面内に存在するTiとNbの炭窒化物の存在形態がHAZ靭性を高める上で重要であり、鋼板の横断面内にTi窒化物とNb炭窒化物との複合析出物が100個/mm2以上分散しているところに他の特徴を有している。即ち該複合析出物とは、例えばTEM写真によると、図1(図面代用顕微鏡写真)にその一例を示す如く、比較的粗大なTi窒化物の側縁や隅角部にNb炭窒化物が付着した様な状態で析出した複合析出物である。 In the present invention, as described above, as another requirement, the presence of carbonitrides of Ti and Nb existing in the cross section of the steel sheet is important for increasing the HAZ toughness. Another feature is that a composite precipitate with Nb carbonitride is dispersed at 100 pieces / mm 2 or more. That is, for example, according to a TEM photograph, the composite precipitate has Nb carbonitride adhering to the side edges and corners of relatively coarse Ti nitride as shown in FIG. It is a composite precipitate that is deposited in such a state.
そして、後述する如く実施例の中からHAZ靭性に優れた鋼種を選択してTEM/EDS(エネルギー分散型X線元素分析)分析を行ったところ、図1に矢印Aで示した析出物はTi窒化物で、該Ti窒化物からなる析出物の周縁や隅角部に付着した状態で析出しているのはNb炭窒化物であり、この様にTi窒化物とNb炭窒化物が複合析出物として鋼板断面内に100個/mm2以上、より好ましくは200個/mm2以上分散している鋼材は、いずれも優れたHAZ靭性を発揮することが確認された。 Then, as will be described later, a steel type having excellent HAZ toughness was selected from the examples and subjected to TEM / EDS (energy dispersive X-ray elemental analysis) analysis. As a result, the precipitate indicated by arrow A in FIG. It is Nb carbonitride that is deposited in the state of adhering to the periphery and corners of the precipitate made of Ti nitride, and thus Ti nitride and Nb carbonitride are combined. It was confirmed that all steel materials dispersed as 100 / mm 2 or more, more preferably 200 / mm 2 or more in the cross section of the steel sheet exhibit excellent HAZ toughness.
そして該複合析出物の作用について更に追求したところ、Ti窒化物が単独で析出している鋼板に較べて次の様な作用を発揮することが分かった。即ち、溶接時に1400℃程度の高温に加熱されるHAZ領域では、たとえTi窒化物が存在していたとしても、該高温域でTi窒化物は溶融して結晶粒粗大化防止効果が失われるのに対し、Ti窒化物とNb炭窒化物との複合析出物は高温域で溶融し難く、高温となるHAZ領域においても完全には溶融し難いため高い結晶粒粗大化防止効果を発揮すること、また、該複合析出物の一部は高温域で溶融することもあるが、それらは局部的に濃化状態で存在するため溶接熱を受けた後の冷却時に微細に再析出し易く、これがフェライト変態核として作用することで結晶粒微細化効果を発揮することが分かった。 Further investigation of the action of the composite precipitate revealed that the following action was exhibited as compared with a steel sheet in which Ti nitride was precipitated alone. That is, in the HAZ region that is heated to a high temperature of about 1400 ° C. during welding, even if Ti nitride exists, the Ti nitride melts in the high temperature region and the effect of preventing grain coarsening is lost. On the other hand, composite precipitates of Ti nitride and Nb carbonitride are difficult to melt in a high temperature region and exhibit high crystal grain coarsening prevention effects because they are difficult to melt completely even in a high temperature HAZ region. In addition, some of the composite precipitates may melt in a high temperature range, but since they exist locally in a concentrated state, they tend to reprecipitate finely during cooling after receiving welding heat. It was found that the crystal grain refinement effect was exhibited by acting as a transformation nucleus.
なお本発明において上記複合析出物の定量は、供試鋼板の横断面を鏡面研磨し電解エッチングした後にTEM観察し、そのサイズと個数については、倍率15,000倍で撮影した断面写真から無作為に20領域を選択して複合析出物のサイズと個数を測定し、その平均値として求めた。 In the present invention, the amount of the composite precipitate is determined by TEM observation after mirror-polishing the cross section of the test steel sheet and electrolytic etching, and the size and number are randomly determined from a cross-sectional photograph taken at a magnification of 15,000 times. 20 regions were selected, and the size and number of composite precipitates were measured, and the average value was obtained.
尚、上記の様な特性を備えた鋼板の製法は特に制限されないが、好ましい方法としては次の様な方法が挙げられる。 In addition, although the manufacturing method in particular of the steel plate provided with the above characteristics is not restrict | limited, The following methods are mentioned as a preferable method.
まず、前述した様なミクロ組織の要件を満たす他、Ti窒化物とNb炭窒化物との複合析出物を適量分散析出させることにより、低降伏比で靭性およびHAZ靭性に優れた鋼板を製造するには、前述した化学成分の要件を満たす鋼材を熱間圧延する際に、800℃以上の温度域の圧下率を40%以上、800〜750℃の温度域の圧下率を20〜50%、750℃以下の温度域の圧下率を30%以上に制御する方法が採用される。 First, in addition to satisfying the requirements of the microstructure as described above, a steel plate excellent in toughness and HAZ toughness with a low yield ratio is manufactured by dispersing and precipitating a proper amount of composite precipitates of Ti nitride and Nb carbonitride. When hot rolling a steel material that satisfies the above-mentioned chemical component requirements, the rolling reduction in the temperature range of 800 ° C. or higher is 40% or more, the rolling reduction in the temperature range of 800 to 750 ° C. is 20 to 50%, A method is employed in which the rolling reduction in the temperature range of 750 ° C. or lower is controlled to 30% or higher.
即ちこの方法では、固溶Nbによる圧延中の再結晶抑制作用が小さい800℃以上での圧延によってオーステナイト粒が微細化し得るように、800℃以上の温度域での圧下率を40%以上とし、また、部分再結晶域である800〜750℃の温度域の圧下率は20〜50%、未再結晶域である750℃以下の温度域の圧下率は30%以上に夫々制御することによって、オーステナイトの伸長を制御する。即ち、こうした温度と圧下率の条件を適正に制御することで、所定の伸長フェライトと等軸フェライトの混合組成を得るもので、こうした温度と圧下率および鋼中のNb含量が、適正なフェライト混合組織を得る上で重要な要件となってくるのである。 That is, in this method, the reduction rate in the temperature range of 800 ° C. or higher is set to 40% or higher so that the austenite grains can be refined by rolling at 800 ° C. or higher where the recrystallization suppressing action during rolling by the solid solution Nb is small. Further, by controlling the reduction rate in the temperature range of 800 to 750 ° C. which is the partial recrystallization region to 20 to 50% and the reduction rate in the temperature range of 750 ° C. or less which is the non-recrystallization region to 30% or more, Controls the elongation of austenite. That is, by properly controlling the conditions of such temperature and rolling reduction, a predetermined mixed composition of elongated ferrite and equiaxed ferrite can be obtained, and such temperature and rolling reduction and Nb content in the steel can be appropriately mixed with ferrite. It becomes an important requirement for obtaining an organization.
また上記の条件は、前記Ti窒化物とNb炭窒化物との複合析出物によりHAZ靭性の改善された鋼材を得る上でも、好ましい条件となる。すなわち、鋳造後の熱間圧延工程で、800℃以上の温度域での圧下率を40%以上、より好ましくは50%以上とし、且つ、800〜750℃の温度域の圧下率を20〜50%、750℃以下の温度域の圧下率を30%以上に制御すれば、前述した如く成分組成の調整された鋼材を使用することで、上記好適な数の複合析出物も確実に生成させることができるのである。 In addition, the above conditions are preferable for obtaining a steel material having improved HAZ toughness by the composite precipitate of the Ti nitride and Nb carbonitride. That is, in the hot rolling step after casting, the rolling reduction in the temperature range of 800 ° C. or higher is set to 40% or higher, more preferably 50% or higher, and the rolling reduction in the temperature range of 800 to 750 ° C. is set to 20 to 50. %, If the rolling reduction in the temperature range of 750 ° C. or lower is controlled to 30% or more, the above-mentioned suitable number of composite precipitates can be reliably generated by using the steel material with the component composition adjusted as described above. Can do it.
ちなみに800℃以上の温度域での圧下率が40%未満では、再結晶温度域での圧下率が不足するためオーステナイトの微細化が不十分となり、また750℃以下の温度域での圧下率が30%未満では、未再結晶温度域での圧下率不足によりTiN核へのNb(CN)の析出が不十分となり、いずれの場合も本発明で意図する数の複合析出物を確保し難くなる。 Incidentally, if the rolling reduction in the temperature range of 800 ° C. or higher is less than 40%, the reduction rate in the recrystallization temperature range is insufficient, so that the austenite becomes insufficiently refined, and the rolling reduction in the temperature range of 750 ° C. or less. If it is less than 30%, precipitation of Nb (CN) on TiN nuclei becomes insufficient due to insufficient reduction in the non-recrystallization temperature range, and in any case, it is difficult to secure the number of composite precipitates intended by the present invention. .
なお本発明に係る鋼板の板厚は特に制限されず、様々の厚さの鋼板に適用できるが、本発明の効果がより有効に発揮されるのは、厚さが7mm程度以上(好ましくは10mm以上)の鋼板である。板厚の上限は特に制限されないが、通常は50mm以下(特に30mm以下)程度である。 The thickness of the steel sheet according to the present invention is not particularly limited and can be applied to steel sheets having various thicknesses. However, the thickness of the steel sheet according to the present invention is more effectively about 7 mm (preferably 10 mm). Above). The upper limit of the plate thickness is not particularly limited, but is usually about 50 mm or less (particularly 30 mm or less).
以下、実験例を挙げて本発明をより具体的に説明するが、本発明はもとより下記実験例によって制限を受けるものではなく、前・後記の趣旨に適合し得る範囲で適当に変更を加えて実施することも勿論可能であり、それらはいずれも本発明の技術的範囲に包含される。 なお下記実験例で得られた鋼板の評価は、以下の様にして行った。 Hereinafter, the present invention will be described more specifically with reference to experimental examples.However, the present invention is not limited by the following experimental examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention. In addition, evaluation of the steel plate obtained by the following experiment example was performed as follows.
[断面ミクロ組織]
各供試鋼板を圧延方向と平行方向に切断し、該切断面をバフ研磨した後、ナイタールによりエッチングしてから、光学顕微鏡を用いて断面ミクロ組織を倍率200倍で写真撮影する。そして、該ミクロ写真を画像解析することによって、フェライトとパーライトおよびベイナイトの面積率を求めると共に、フェライト中に占める「厚みが5〜10μmで、アスペクト比が2以上である伸長フェライト」と「円相当径が5μm以下でアスペクト比が1.5以下である等軸状フェライト」の面積率を求める。
[Cross-sectional microstructure]
Each test steel sheet is cut in a direction parallel to the rolling direction, the cut surface is buffed, etched with nital, and then the cross-sectional microstructure is photographed at a magnification of 200 times using an optical microscope. Then, by analyzing the image of the microphotograph, the area ratio of ferrite, pearlite and bainite is obtained, and “elongated ferrite having a thickness of 5 to 10 μm and an aspect ratio of 2 or more” and “equivalent to a circle” in the ferrite The area ratio of “equiaxial ferrite having a diameter of 5 μm or less and an aspect ratio of 1.5 or less” is determined.
[Ti窒化物とNb炭窒化物の複合析出物の確認]
制御圧延によって得た鋼板の深さt/4位置(tは鋼板の厚さ)における圧延方向に平行な断面を切り出して研磨し、この断面を、電解エッチングした後のTEM分析(視野数:20)によって、Ti窒化物、Nb炭窒化物およびそれらの複合析出物の個数を調べた。
[Confirmation of composite precipitates of Ti nitride and Nb carbonitride]
A section parallel to the rolling direction at the depth t / 4 position (t is the thickness of the steel sheet) of the steel sheet obtained by controlled rolling is cut out and polished, and this section is subjected to TEM analysis (number of fields: 20). ), The number of Ti nitrides, Nb carbonitrides and their composite precipitates was examined.
[HAZ靭性試験]
大入熱(8kJ/mm)片面裏当て溶接を行ったときの熱サイクルを模擬したHAZ靭性評価法として、加熱温度:1400℃、800〜500℃の冷却時間(Tc):100秒の熱サイクルで各供試鋼板を熱処理した後、温度−60℃におけるシャルピー吸収エネルギー(Vノッチ)を測定した。なお試験片としては、サイズ10mm×10mm×55mmの棒状で、中央部片面に深さ;2mmのVノッチを形成したものを使用した。
[HAZ toughness test]
As a HAZ toughness evaluation method simulating the thermal cycle when large heat input (8 kJ / mm) single-sided backside welding is performed, heating temperature: 1400 ° C., cooling time of 800-500 ° C. (Tc): thermal cycle of 100 seconds Then, after heat-treating each test steel plate, Charpy absorbed energy (V notch) at a temperature of −60 ° C. was measured. In addition, as a test piece, the size 10mmx10mmx55mm rod shape which formed the V notch of depth; 2mm in the center part single side | surface was used.
実験例1
C:0.05%、Si:0.15%、Mn:1.50%、P:0.009%、S:0.002%、Al:0.030%、Ti:0.014%、N:0.0045%を含有する鋼材(鋼種記号Z;残部はFeおよび不可避不純物)に、下記表1に示す量のNbを添加して溶製し鋳造した鋼材について、表1に示す如く種々の条件で制御圧延することによって所定の板厚の鋼板を得た。
Experimental example 1
C: 0.05%, Si: 0.15%, Mn: 1.50%, P: 0.009%, S: 0.002%, Al: 0.030%, Ti: 0.014%, N : Steel materials containing 0.0045% (steel type symbol Z; balance is Fe and inevitable impurities), and Nb in the amount shown in Table 1 below is added to melt and cast the steel materials as shown in Table 1. A steel sheet having a predetermined thickness was obtained by controlled rolling under conditions.
得られた各鋼板について、断面組織観察によって圧延方向の断面ミクロ組織を観察し、フェライトとパーライトおよびベイナイトの面積率を求めると共に、フェライト組織中に占める『板厚方向厚みが5〜10μmで、アスペクト比が2以上の伸長フェライト』と『円相当径が5μm以下でアスペクト比が1.5以下である等軸状フェライト』の面積率を求めた。 For each steel plate obtained, the cross-sectional microstructure in the rolling direction was observed by observing the cross-sectional structure to obtain the area ratio of ferrite, pearlite, and bainite, and the “thickness in the plate thickness direction was 5 to 10 μm and the aspect ratio occupied in the ferrite structure. The area ratio of “elongated ferrite having a ratio of 2 or more” and “equiaxial ferrite having an equivalent circle diameter of 5 μm or less and an aspect ratio of 1.5 or less” was determined.
また各供試板について、縦断面に現われるTi窒化物とNb炭窒化物の複合析出物の個数を調べた。また各々の供試板について引張試験を行なって引張強さと降伏比を求めると共に、シャルピー試験によって靭性を測定し、且つ熱サイクル試験後のシャルピー試験によってHAZ靭性を測定した。 For each test plate, the number of Ti nitride and Nb carbonitride composite precipitates appearing in the longitudinal section was examined. Each test plate was subjected to a tensile test to determine the tensile strength and yield ratio, toughness was measured by the Charpy test, and HAZ toughness was measured by the Charpy test after the thermal cycle test.
得られた鋼板の評価結果を下記表2に示す。 The evaluation results of the obtained steel sheet are shown in Table 2 below.
表1,2から次の様に解析できる。 Tables 1 and 2 can be analyzed as follows.
符号1,2は、Nbが含まれていないため、TiNとNb(CN)の複合析出物が存在せず、熱サイクルシャルピー試験によって評価されるHAZ靭性が悪い。また、適正な形態のフェライト量が不足しているため引張強さが低く、降伏比も高めである。符号3,4,5,6,9,10,12,13,14,16,17については、Nb含量は規定要件を満たしているものの、圧延条件が適切でないため、適正な形態のフェライトが不足しており、降伏比が高めである。符号18はNb量が規定値を超えているため、TiNとNb(CN)の複合析出物の個数は本発明の規定数に達しているものの、焼入性過剰となってHAZ部の組織が島状マルテンサイトの多い上部ベイナイトとなり、HAZ靭性が劣化している。上記以外のものは、Nb含量が適切で且つ圧延条件も適切であるため、フェライトは適正比率の伸長フェライトと等軸状フェライトを含んでおり、しかもTiNとNb(CN)の複合析出物が十分量生成していて且つ焼入性過剰になることもなく、高い引張強さを有すると共に降伏比も低く、HAZ靭性が良好で且つ優れた熱サイクル後衝撃特性を有している。 Since codes 1 and 2 do not contain Nb, composite precipitates of TiN and Nb (CN) do not exist, and the HAZ toughness evaluated by the thermal cycle Charpy test is poor. Moreover, since the amount of ferrite in an appropriate form is insufficient, the tensile strength is low and the yield ratio is also high. For the codes 3, 4, 5, 6, 9, 10, 12, 13, 14, 16, and 17, the Nb content satisfies the specified requirements, but the rolling conditions are not appropriate, so there is not enough ferrite in the proper form. The yield ratio is high. Reference numeral 18 indicates that the amount of Nb exceeds the specified value. Therefore, the number of composite precipitates of TiN and Nb (CN) reaches the specified number of the present invention, but the hardenability is excessive and the structure of the HAZ part is reduced. The upper bainite is rich in island martensite, and the HAZ toughness is deteriorated. Other than the above, since the Nb content is appropriate and the rolling conditions are also appropriate, the ferrite contains an appropriate ratio of elongated ferrite and equiaxed ferrite, and a composite precipitate of TiN and Nb (CN) is sufficient. It has a high tensile strength, a low yield ratio, good HAZ toughness, and excellent post-thermal cycle impact characteristics.
実験例2
下記表3に示す成分組成の鋼材を溶製・鋳造し、下記表4に示す種々の条件で所定の板厚まで制御圧延した。
Experimental example 2
Steel materials having the composition shown in Table 3 below were melted and cast, and controlled and rolled to a predetermined thickness under various conditions shown in Table 4 below.
得られた鋼板の評価結果を表5に示す。 The evaluation results of the obtained steel sheet are shown in Table 5.
表3〜5から次ぎの様に解析できる。 It can analyze as follows from Tables 3-5.
符号1〜19は、鋼材の化学成分、圧延条件共に規定範囲にあるため、適切な形態のフェライトを有しているため、十分な強度と低降伏比が確保されており、またTiNとNb(CN)の複合析出物も適正個数存在するため高いHAZ靭性を有している。 Since symbols 1 to 19 are in the specified ranges for both the chemical composition of the steel material and the rolling conditions, and have an appropriate form of ferrite, sufficient strength and a low yield ratio are ensured, and TiN and Nb ( CN) composite precipitates also have a high HAZ toughness due to the presence of an appropriate number.
これらに対し符号20〜24は、鋼中のNb含量またはTi含量が不足するためフェライトの形態が規定要件から外れており、特に熱サイクル後衝撃特性が悪い。また符号25〜27は、鋼材の化学成分は適切であるものの圧延条件が適切でないためフェライトの形態が規定要件を満たしておらず、特に母材の降伏比が高い。符号28,29も、鋼材の化学成分は適切であるが圧延条件が適切でないためフェライトの形態が規定要件を満たしておらず、且つTiNとTi(CN)の複合析出物個数も規定値に満たないため降伏比が高く且つHAZ靭性もよくない。 On the other hand, in Nos. 20 to 24, since the Nb content or the Ti content in the steel is insufficient, the form of the ferrite is out of the prescribed requirements, and the impact characteristics after the thermal cycle are particularly bad. Moreover, although the code | symbols 25-27 have the chemical composition of steel materials appropriate, but the rolling conditions are not appropriate, the form of a ferrite does not satisfy a regulation requirement, and especially the yield ratio of a base material is high. Reference numerals 28 and 29 also indicate that the chemical composition of the steel material is appropriate but the rolling conditions are not appropriate, so the form of the ferrite does not meet the specified requirements, and the number of composite precipitates of TiN and Ti (CN) also satisfies the specified value. Therefore, the yield ratio is high and the HAZ toughness is not good.
Claims (8)
N :0.002〜0.010%、
Nb:0.003〜0.025%、
Ti:0.005〜0.025%、
Si:0.7%以下、
Mn:0.5〜1.8%、
Al:0.1%以下、
を含み、残部がFe及び不可避不純物である鋼材からなる鋼板であって、該鋼板の縦断面に現われるミクロ組織は、フェライトとパーライト若しくはこれらとベイナイトからなり、フェライトは下記の2種類を含み、
1)板厚方向の厚みが5〜10μmで、アスペクト比が2以上である伸長フェライト
が15面積%以上、
2)円相当径が5μm以下でアスペクト比が1.5以下である等軸状フェライトが1
0〜50面積%、
且つ、横断面内に、Ti窒化物とNb炭窒化物との複合析出物が100個/mm2以上分散していることを特徴とする、低降伏比で靭性および溶接継手部靭性に優れた鋼板。 C: 0.02 to 0.10% (meaning mass%, hereinafter the same for chemical components)
N: 0.002 to 0.010%,
Nb: 0.003 to 0.025%,
Ti: 0.005 to 0.025%,
Si: 0.7% or less,
Mn: 0.5 to 1.8%
Al: 0.1% or less,
And the balance is a steel plate made of steel with Fe and inevitable impurities, the microstructure appearing in the longitudinal section of the steel plate consists of ferrite and pearlite or these and bainite, the ferrite includes the following two types,
1) Elongated ferrite having a thickness in the thickness direction of 5 to 10 μm and an aspect ratio of 2 or more
Is 15 area% or more,
2) An equiaxed ferrite having an equivalent circle diameter of 5 μm or less and an aspect ratio of 1.5 or less is 1
0-50 area%,
In addition, composite precipitates of Ti nitride and Nb carbonitride are dispersed at 100 pieces / mm 2 or more in the cross section, and the toughness and weld joint toughness are excellent at a low yield ratio. steel sheet.
B:0.0003〜0.005%および/またはNi:0.01〜0.5%
を含むものである請求項1または2に記載の鋼板。 The steel material is B: 0.0003 to 0.005% and / or Ni: 0.01 to 0.5% as other elements.
The steel plate according to claim 1 or 2 , comprising:
Zr:0.0003〜0.05%、
Ca:0.0005〜0.005%、
Mg:0.0005〜0.005%、
REM:0.0005〜0.01%
よりなる群から選択される少なくとも1種の元素を含むものである請求項1〜3のいずれかに記載の鋼板。 The steel material is further Zr: 0.0003-0.05% as another element,
Ca: 0.0005 to 0.005%,
Mg: 0.0005 to 0.005%,
REM: 0.0005 to 0.01%
The steel plate according to any one of claims 1 to 3 , wherein the steel plate contains at least one element selected from the group consisting of:
Cu:0.01〜0.5%、
Cr:0.05〜0.5%、
Mo:0.01〜0.5%、
V:0.005〜0.1%
よりなる群から選択される少なくとも1種の元素を含むものである請求項1〜4のいずれかに記載の鋼板。 The steel material is further Cu: 0.01-0.5% as another element,
Cr: 0.05 to 0.5%,
Mo: 0.01 to 0.5%,
V: 0.005 to 0.1%
The steel sheet according to any one of claims 1 to 4 , comprising at least one element selected from the group consisting of:
1)板厚方向の厚みが5〜10μmで、アスペクト比が2以上である伸長フェライト
が15面積%以上、
2)円相当径が5μm以下でアスペクト比が1.5以下である等軸状フェライトが
10〜50面積%、
鋼板内にTi窒化物とNb炭窒化物との複合析出物を分散析出させることを特徴とする、低降伏比で靭性および溶接継手部靭性に優れた鋼板の製法。 When hot-rolling a steel material that satisfies the chemical component requirements described in any one of claims 1 to 5, the rolling reduction in a temperature range of 800 ° C or higher is 40% or higher, and in the temperature range of 800 to 750 ° C. The reduction ratio is 20 to 50%, the reduction ratio in the temperature range of 750 ° C. or lower is controlled to 30% or more, and the microstructure appearing in the longitudinal section of the obtained steel sheet is composed of ferrite and pearlite or these and bainite. Including the following two types,
1) Elongated ferrite having a thickness in the thickness direction of 5 to 10 μm and an aspect ratio of 2 or more is 15 area% or more,
2) An equiaxed ferrite having an equivalent circle diameter of 5 μm or less and an aspect ratio of 1.5 or less is 10 to 50 area%,
A method for producing a steel sheet having a low yield ratio and excellent toughness and weld joint toughness, wherein a composite precipitate of Ti nitride and Nb carbonitride is dispersed and precipitated in the steel sheet.
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