JP4956907B2 - High-strength, high-deformability thick steel plate with excellent ductile crack initiation characteristics and its manufacturing method - Google Patents
High-strength, high-deformability thick steel plate with excellent ductile crack initiation characteristics and its manufacturing method Download PDFInfo
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Description
本発明は、建築物や海洋構造物、造船、橋梁、ラインパイプなどに用いて好適な構造用鋼板に関し、特に地震多発帯などで用いられる鋼板に要求される耐延性き裂発生特性に優れる、引張強度が５５０ＭＰａ以上の高強度高変形能厚鋼板とその製造方法に関するものである。 The present invention relates to a structural steel plate suitable for use in buildings, marine structures, shipbuilding, bridges, line pipes, etc., and is particularly excellent in ductile crack initiation characteristics required for steel plates used in earthquake-prone bands, The present invention relates to a high-strength, high-deformability thick steel plate having a tensile strength of 550 MPa or more and a method for producing the same.
近年、建築物や海洋構造物、造船、橋梁、ラインパイプなどの分野で用いられている鋼材は、安全性の向上や、操業効率の向上(例えば、パイプラインでの輸送ガスの高圧化)、使用鋼材の削減によるトータルコスト低減等を目的として、高強度化が進められている。また、上記鋼材が使用される地域は、自然環境が過酷な地域へと拡大しているため、例えば、地震多発地帯などで使用される構造物用鋼材などには、従来の要求特性とは異なる、優れた塑性変形能や耐延性破壊特性が求められるようになってきている。 In recent years, steel materials used in the fields of buildings and offshore structures, shipbuilding, bridges, line pipes, etc. have improved safety and operational efficiency (for example, increased pressure of transport gas in pipelines), Higher strength is being promoted for the purpose of reducing the total cost by reducing the amount of steel used. In addition, since the area where the steel materials are used has expanded to areas where the natural environment is severe, for example, structural steel materials used in earthquake-prone areas etc. are different from the conventional required characteristics. Therefore, excellent plastic deformability and ductile fracture resistance have been demanded.
このような状況から、特許文献１〜３には、降伏応力と引張強度の比である降伏比を低下させることにより塑性変形能を向上させた鋼材が、また、特許文献４，５には、同じく降伏比を低下させることにより優れた耐座屈性を有する高変形能鋼材が開示されている。しかし、たとえ低降伏比で変形能に優れた鋼材であっても、欠陥部などの応力集中部から延性き裂が発生し、進展する場合には、その塑性変形能が発揮される前に、長距離き裂伝播、すなわち、不安定延性破壊を生じてしまう虞がある。
From such a situation, in
そこで、不安定延性破壊を防止することを目的として、高強度高変形能鋼材の開発が行われている。例えば、特許文献６には、金属組織を細粒フェライト主体の組織とすることによって吸収エネルギーを高めた高張力鋼の製造方法が、また、特許文献７には、金属組織をフェライトとマルテンサイトの２相混合組織とすることによって、不安定延性破壊の停止性能を高めた高強度鋼管が開示されている。
上記特許文献６および７に記載された技術は、不安定延性破壊における耐延性き裂伝播特性を向上させた鋼材に関するものである。しかし、例えば、高圧で操業されるガスパイプラインでは、一旦、き裂が発生すると、き裂の伝播停止が困難になる場合があり、その結果、局所的な破壊でも重大な被害をもたらすことが危惧される。そのため、ガスパイプラインに用いられる鋼材は、延性き裂伝播が防止されるだけでなく、たとえ損傷が生じてもき裂発生に至らない、もしくは最小規模のリークに止めることができるものであることが望ましい。
The technologies described in
上記課題を解決するには、鋼材自体に内在する欠陥や外的要因によって受ける損傷あるいは腐食による減肉部等からのき裂の発生を抑制することが必要である。しかし、上記特性を満たすような、延性き裂発生抵抗が大きい高強度高変形能厚鋼板は、今のところ存在していないのが実情である。 In order to solve the above-described problems, it is necessary to suppress the occurrence of cracks from a reduced thickness portion or the like due to damage or corrosion caused by defects or external factors inherent in the steel material itself. However, there is currently no high-strength, high-deformability thick steel plate that satisfies the above characteristics and has high ductile crack initiation resistance.
そこで、本発明の目的は、地震多発地帯などで使用される鋼材に要求される耐延性き裂発生特性に優れた高強度高変形能厚鋼板を提供すると共に、その有利な製造方法を提案することにある。 Accordingly, an object of the present invention is to provide a high-strength, high-deformability thick steel plate having excellent ductile crack generation characteristics required for steel materials used in earthquake-prone areas and the like, and to propose an advantageous production method thereof. There is.
発明者らは、上記課題の解決に向け、外的要因による損傷が存在する場合を想定して、切欠きを有する高強度鋼板の延性破壊挙動について鋭意研究を重ねた。その結果、軟質相であるフェライトと硬質相であるベイナイトの２相組織からなるミクロ組織と、それら両相の機械的特性を最適化することにより、変形能を低下させることなく切欠きからの延性き裂発生を抑制することができる事実を見出し、本発明を完成するに至った。 In order to solve the above-mentioned problems, the inventors have conducted intensive research on the ductile fracture behavior of a high-strength steel sheet having a notch, assuming that damage due to external factors exists. As a result, by optimizing the microstructure of the two-phase structure of ferrite, which is a soft phase, and bainite, which is a hard phase, and by optimizing the mechanical properties of both phases, ductility from notches is reduced without reducing deformability. The present inventors have found the fact that crack generation can be suppressed and have completed the present invention.
すなわち、本発明は、Ｃ：０．０３〜０．１ｍａｓｓ％、Ｓｉ：０．２９〜１ｍａｓｓ％、Ｍｎ：１．５１〜２ｍａｓｓ％、Ｓ：０．００２ｍａｓｓ％以下、Ａｌ：０．０１〜０．０７ｍａｓｓ％を含有し、残部がＦｅおよび不可避的不純物からなり、金属組織がフェライト相とベイナイト相の２相組織からなる鋼板であって、前記ベイナイト相の相分率が１０〜５０％、前記ベイナイト相のビッカース硬さとフェライト相のビッカース硬さの比（ＨｖＢ／ＨｖＦ）が１．６以上、引張強度が５５０ＭＰａ以上で、降伏比が０．８以下であることを特徴とする耐延性き裂発生特性に優れる高強度高変形能厚鋼板である。 That is, the present invention includes C: 0.03 to 0.1 mass%, Si: 0.29 to 1 mass%, Mn: 1.51 to 2 mass%, S: 0.002 mass% or less, Al: 0.01 to 0 0.07 mass%, the balance being Fe and inevitable impurities, and the metal structure being a two-phase structure of a ferrite phase and a bainite phase, the phase fraction of the bainite phase being 10 to 50%, The ratio of the Vickers hardness of the bainite phase to the Vickers hardness of the ferrite phase (Hv B / Hv F ) is 1.6 or more, the tensile strength is 550 MPa or more, and the yield ratio is 0.8 or less. It is a high-strength, high-deformability thick steel plate with excellent crack initiation characteristics.
本発明の厚鋼板は、上記成分組成に加えてさらに、Ｃｕ：０．０１〜０．５ｍａｓｓ％、Ｎｉ：０．０５〜０．５ｍａｓｓ％、Ｃｒ：０．０１〜０．５ｍａｓｓ％およびＭｏ：０．０１〜０．５ｍａｓｓ％のうちから選ばれる１種または２種以上を含有することを特徴とする。 In addition to the above component composition, the thick steel plate of the present invention further includes Cu: 0.01 to 0.5 mass%, Ni: 0.05 to 0.5 mass%, Cr: 0.01 to 0.5 mass%, and Mo: It contains one or two or more selected from 0.01 to 0.5 mass%.
また、本発明の厚鋼板は、上記成分組成に加えてさらに、Ｎｂ：０．００５〜０．１ｍａｓｓ％、Ｖ：０．００５〜０．１ｍａｓｓ％、Ｔｉ：０．００５〜０．１ｍａｓｓ％のうちから選ばれる１種または２種以上を含有することを特徴とする。 In addition to the above component composition, the thick steel plate of the present invention further includes Nb: 0.005 to 0.1 mass%, V: 0.005 to 0.1 mass%, Ti: 0.005 to 0.1 mass%. It contains one or two or more selected from among them.
また、本発明は、上記成分組成を有する鋼スラブを１０００〜１２００℃に加熱し、圧延終了温度をＡｒ３変態点以上とする熱間圧延を施し、その後、（Ａｒ３変態点−３０℃）未満の温度から４００℃以下の温度までを平均冷却速度３０℃／秒以上で冷却することを特徴とする耐延性き裂発生特性に優れる高強度高変形能厚鋼板の製造方法を提案する。 Further, the present invention is a steel slab having the above component composition is heated to 1000 to 1200 ° C., the rolling end temperature subjected to hot rolling to Ar 3 transformation point or higher, then, (Ar 3 transformation point -30 ° C.) The present invention proposes a method for producing a high-strength, high-deformability thick steel plate having excellent ductile cracking characteristics, characterized by cooling from a temperature lower than 400 ° C. to a temperature of 400 ° C. or less at an average cooling rate of 30 ° C./second or more.
本発明によれば、地震などで大きな塑性変形を受けた場合においても、延性き裂を発生することのない高強度高変形能厚鋼板を提供することができる。よって、本発明の鋼板は、建築物や海洋構造物、造船、橋梁、ラインパイプなどに用いて好適である。 According to the present invention, it is possible to provide a high-strength, high-deformability thick steel plate that does not generate a ductile crack even when it undergoes a large plastic deformation due to an earthquake or the like. Therefore, the steel plate of the present invention is suitable for use in buildings, marine structures, shipbuilding, bridges, line pipes and the like.
本発明を開発する契機となった実験について説明する。
フェライト相とベイナイト相の２相組織からなり、成分組成が異なる各種鋼材(厚鋼板)から、図１に示す切欠底半径が0.25mmの環状切欠を有する丸棒試験片を作製し、引張試験を行って延性き裂発生特性を調査すると共に、鋼材のミクロ組織と耐延性き裂発生特性との関係を調査した。なお、耐延性き裂発生特性は、引張試験において、上記丸棒試験片の切欠底からき裂が発生したときの標点間の平均歪を「延性き裂発生歪」として評価した。
An experiment that triggered the development of the present invention will be described.
A round bar test piece with a circular notch with a notch bottom radius of 0.25 mm shown in Fig. 1 is prepared from various steel materials (thick steel plates) with a two-phase structure of ferrite phase and bainite phase. The ductile crack initiation characteristics were investigated and the relationship between the microstructure of the steel and the ductile crack initiation characteristics was investigated. In the tensile test, the average strain between the gauge points when the crack was generated from the notched bottom of the round bar test piece was evaluated as “ductile crack generation strain” in the tensile test.
図２は、Ｃ含有量を変化させてフェライト相とベイナイト相の相分率を変化させた鋼における、降伏比と延性き裂発生歪に及ばすベイナイト相分率の影響を示したものである。図２から、降伏比は、ベイナイト相分率が10〜70％の広い範囲で0.8以下の低い値を示しているが、延性き裂発生歪は、ベイナイト相分率が50％を超えると急激に低下していることがわかる。また、図３は、Ｃ：0.05mass％、Mn：1.8mass％n、Cu：0.13mass％、Ni：0.1mass％およびMo：0.2mass％を含有する鋼を用いて、種々の圧延条件、冷却条件で製造したフェライト＋ベイナイト２相組織鋼における、ベイナイト相の硬さＨvBとフェライト相のビッカース硬さＨvFの比(ＨvB／ＨvF)と、延性き裂発生歪との関係を示したものである。図３から、ＨvB／ＨvFの増加にともない、延性き裂発生歪が増加しており、耐延性き裂発生特性が向上していることがわかる。 FIG. 2 shows the influence of the bainite phase fraction on the yield ratio and ductile crack initiation strain in a steel in which the C content is changed and the phase fraction of the ferrite phase and the bainite phase is changed. . As shown in FIG. 2, the yield ratio shows a low value of 0.8 or less over a wide range of bainite phase fraction of 10 to 70%, but the ductile crack initiation strain increases rapidly when the bainite phase fraction exceeds 50%. It can be seen that it has dropped. FIG. 3 shows various rolling conditions and cooling using steel containing C: 0.05 mass%, Mn: 1.8 mass% n, Cu: 0.13 mass%, Ni: 0.1 mass%, and Mo: 0.2 mass%. Shows the relationship between the ratio of the bainite hardness Hv B to the Vickers hardness Hv F (Hv B / Hv F ) and ductile crack initiation strain in ferrite + bainite dual phase steels manufactured under the above conditions It is a thing. From FIG. 3, it can be seen that the ductile crack initiation strain increases with an increase in Hv B / Hv F , and the ductile crack initiation characteristics are improved.
以上の結果から、軟質なフェライト相と硬質なベイナイト相からなる２相組織を有する鋼板が変形する場合、軟質なフェライト相が早期に変形を開始するが、硬質なベイナイト相によって引張強度が維持されるため、降伏応力と引張強度の比（降伏応力／引張強度）である降伏比が低下し、変形能が向上する。そこでさらに、成分組成、製造条件を制御し、ベイナイト相の相分率を適正な範囲に制御すると同時に、上記フェライト相とベイナイト相の硬さの比を特定値以上に限定することにより、延性き裂発生抵抗が高い高強度高変形能厚鋼板が得られることがわかった。本発明は、上記知見に基き、完成したものである。 From the above results, when a steel sheet having a two-phase structure consisting of a soft ferrite phase and a hard bainite phase deforms, the soft ferrite phase starts to deform early, but the tensile strength is maintained by the hard bainite phase. Therefore, the yield ratio, which is the ratio of yield stress to tensile strength (yield stress / tensile strength), is reduced, and the deformability is improved. Therefore, by controlling the component composition and production conditions, controlling the phase fraction of the bainite phase to an appropriate range, and simultaneously limiting the hardness ratio of the ferrite phase and the bainite phase to a specific value or more, ductility is improved. It was found that a high-strength, high-deformability thick steel plate with high cracking resistance was obtained. The present invention has been completed based on the above findings.
次に、本発明の鋼板の成分組成を上記範囲に制限する理由について説明する。
Ｃ：0.03〜0.1mass％
Ｃは、ベイナイトの生成を促進し、鋼板の強度を確保するために必要な元素である。0.03mass％未満では、ベイナイト相分率が低く、十分な強度が得られず、一方、0.1mass％を超えて添加すると、逆にべイナイト相分率が高くなりすぎ、延焼き裂発生抵抗が低下する他、溶接性が劣化する。よって、Ｃは0.03〜0.1mass％の範囲とする。
Next, the reason for limiting the component composition of the steel sheet of the present invention to the above range will be described.
C: 0.03-0.1mass%
C is an element necessary for promoting the generation of bainite and ensuring the strength of the steel sheet. If it is less than 0.03 mass%, the bainite phase fraction is low and sufficient strength cannot be obtained. On the other hand, if it is added in excess of 0.1 mass%, the bainite phase fraction becomes too high and the resistance to the occurrence of fire cracking is reduced. In addition to lowering, weldability deteriorates. Therefore, C is set to a range of 0.03 to 0.1 mass%.
Si：0.01〜１mass％
Siは、製鋼工程での脱酸剤として、また、鋼材の強度を高める元素として添加する。しかし、0.01mass％未満では脱酸効果が十分でなく、一方、１mass％を超えると、靭性や溶接性を劣化させる。そのため、Si含有量は0.01〜１mass％の範囲とする。
Si: 0.01-1 mass%
Si is added as a deoxidizer in the steel making process and as an element for increasing the strength of the steel material. However, if it is less than 0.01 mass%, the deoxidation effect is not sufficient, while if it exceeds 1 mass%, the toughness and weldability are deteriorated. Therefore, Si content shall be the range of 0.01-1 mass%.
Mn：0.5〜２mass％
Mnは、強度、靭性を高めるために添加するが、0.5mass％未満ではその効果が十分でなく、２mass％を超えると溶接性が劣化する。よって、Mn含有量は0.5〜２mass％の範囲とする。
Mn: 0.5-2 mass%
Mn is added to increase the strength and toughness, but if it is less than 0.5 mass%, the effect is not sufficient, and if it exceeds 2 mass%, the weldability deteriorates. Therefore, the Mn content is in the range of 0.5 to 2 mass%.
Ｓ：0.002mass％以下
Ｓは、不可避不純物として含有されるが、一般的に、鋼中では、硫化物系介在物として存在し、変形時にはボイド発生の起点となるため、延性き裂の発生を防止するためには、その含有量を厳しく規制する必要がある。しかし、0.002mass％以下であれば、材質への悪影響が少ないので、Ｓ含有量の上限は0.002mass％とする。
S: 0.002 mass% or less S is contained as an unavoidable impurity, but in general, it exists as a sulfide inclusion in steel and becomes the starting point of void formation during deformation. In order to prevent it, it is necessary to strictly regulate its content. However, if it is 0.002 mass% or less, there is little adverse effect on the material, so the upper limit of the S content is 0.002 mass%.
Al：0.01〜0.07mass％
Alは、Siと同様、製鋼工程で脱酸剤として添加するが、0.01mass％未満では脱酸効果が十分でない。一方、0.07mass％を超えると、酸化物系介在物が増加して靭性を劣化させる。よって、Al含有量は0.01〜0.07mass％の範囲とする。
Al: 0.01-0.07mass%
Al is added as a deoxidizer in the steel making process, as is Si, but if it is less than 0.01 mass%, the deoxidation effect is not sufficient. On the other hand, when it exceeds 0.07 mass%, oxide inclusions increase and the toughness is deteriorated. Therefore, Al content shall be the range of 0.01-0.07 mass%.
本発明の鋼板は、上記成分組成の他に、以下の元素を選択元素として含有することができる。
Cu：0.01〜0.5mass％、Ni：0.01〜0.5mass％、Cr：0.01〜0.5mass％およびMo：0.01〜0.5mass％のうちから選ばれる１種または２種以上
Cu、Ni、CrおよびMoは、鋼の強度を高めるため添加する選択元素であり、要求強度に応じて、それらのうちから選ばれる１種または２種以上を添加することができる。各元素とも、0.01mass％未満では効果が無く、0.5mass％を超えると溶接性が劣化するので、添加する場合は0.01〜0.5mass％の範囲とするのが好ましい。
The steel sheet of the present invention can contain the following elements as selective elements in addition to the above component composition.
Cu: 0.01 to 0.5 mass%, Ni: 0.01 to 0.5 mass%, Cr: 0.01 to 0.5 mass%, and Mo: 0.01 to 0.5 mass%
Cu, Ni, Cr and Mo are selective elements added to increase the strength of the steel, and one or more selected from them can be added according to the required strength. If each element is less than 0.01 mass%, there is no effect, and if it exceeds 0.5 mass%, the weldability deteriorates. Therefore, when added, it is preferably in the range of 0.01 to 0.5 mass%.
Nb：0.005〜0.1mass％、Ｖ：0.005〜0.1mass％およびTi：0.005〜0.1mass％のうちから選ばれる１種または２種以上
Nb、ＶおよびTiは、靭性及び強度を高めるために添加する選択元素であり、要求強度に応じて、それらのうちから選ばれる１種または２種以上を添加することができる。各元素とも、0.005mass％未満では効果が無く、0.1mass％を超えると溶接部の靭性が劣化するので、添加する場合は0.005〜0.1mass％の範囲とするのが好ましい。
One or more selected from Nb: 0.005 to 0.1 mass%, V: 0.005 to 0.1 mass% and Ti: 0.005 to 0.1 mass%
Nb, V, and Ti are selective elements added to increase toughness and strength, and one or more selected from them can be added according to the required strength. If each element is less than 0.005 mass%, there is no effect, and if it exceeds 0.1 mass%, the toughness of the welded portion deteriorates. Therefore, when added, the content is preferably in the range of 0.005 to 0.1 mass%.
本発明の鋼板は、上記成分以外の残部は、Feおよび不可避的不純物からなる。ただし、上記成分以外の他の成分が、通常、不可避的不純物として認められる以上の量添加されていても、本発明の作用効果を害しない限り、許容される。 In the steel sheet of the present invention, the balance other than the above components is composed of Fe and inevitable impurities. However, it is permissible as long as other components than the above components are added in an amount larger than that normally recognized as an unavoidable impurity as long as the effects of the present invention are not impaired.
次に、本発明の鋼板が有する鋼組織および強度特性について説明する。
鋼板組織：フェライト相とベイナイト相からなる２相組織
本発明の鋼板は、軟質のフェライト相と硬質のベイナイト相からなる２相組織鋼であることが必要である。その理由は、引張強度が550MPa以上の強度と高い変形性能を得るためである。ただし、その他の相として、マルテンサイト相やパーライト相、セメンタイト相が存在しても、それらの相分率の合計が５％以内であれば、本発明の作用効果に悪影響を及ぼすことはないので許容される。
Next, the steel structure and strength characteristics of the steel sheet of the present invention will be described.
Steel sheet structure: two-phase structure composed of ferrite phase and bainite phase The steel sheet of the present invention needs to be a two-phase structure steel composed of a soft ferrite phase and a hard bainite phase. The reason is to obtain a tensile strength of 550 MPa or more and high deformation performance. However, even if a martensite phase, a pearlite phase, and a cementite phase are present as other phases, if the sum of the phase fractions is within 5%, the effects of the present invention are not adversely affected. Permissible.
ベイナイト相分率：10〜50％
上記ベイナイト相の相分率は、10〜50％の範囲であることが必要である。上述した図２に示したように、ベイナイト相分率が10％未満では、降伏比が高く、変形能が低下し、50％を超えでは、延性き裂の発生が容易になるため、高変形能かつ延性き裂発生抵抗に優れる鋼板を得るためには、ベイナイト相分率を10〜50％の範囲とする必要があるからである。なお上記ベイナイト相分率とは、全組織に対するベイナイト相の体積分率(％)のことであり、その体積分率は、鋼板断面の面積分率(％)で代えることができる。
Bainite phase fraction: 10-50%
The phase fraction of the bainite phase needs to be in the range of 10 to 50%. As shown in FIG. 2 described above, when the bainite phase fraction is less than 10%, the yield ratio is high and the deformability is reduced. When it exceeds 50%, ductile cracks are easily generated. This is because the bainite phase fraction needs to be in the range of 10 to 50% in order to obtain a steel sheet having high performance and excellent ductile crack initiation resistance. The bainite phase fraction refers to the volume fraction (%) of the bainite phase relative to the entire structure, and the volume fraction can be replaced with the area fraction (%) of the cross section of the steel sheet.
ベイナイト相とフェライト相のビッカース硬さの比(ＨvB／ＨvF)：1.6以上
上述した図３に示したように、ＨvB／ＨvFの増加にともない、延性き裂発生歪が増加し、耐延性き裂発生特性が向上し、特に、ＨvB／ＨvFが1.6以上でその効果が明確となる。よって、ＨvB／ＨvFは1.6以上とする。
Ratio of Vickers hardness of bainite phase and ferrite phase (Hv B / Hv F ): 1.6 or more As shown in FIG. 3 described above, as Hv B / Hv F increases, ductile crack initiation strain increases. The ductile crack initiation characteristics are improved, and in particular, the effect becomes clear when Hv B / Hv F is 1.6 or more. Therefore, Hv B / Hv F is set to 1.6 or more.
降伏比：0.8以下
降伏比は、0.8を超えると、変形能が低下して、欠陥部近傍での歪集中が大きくなり、塑性変形が欠陥を有する断面で局在化して起こり、延性き裂の発生が容易になる。そのため、降伏比は0.8以下とする。
Yield ratio: 0.8 or less If the yield ratio exceeds 0.8, the deformability decreases, the strain concentration near the defect increases, and plastic deformation occurs locally in the cross section having the defect, resulting in ductile cracks. Easy to generate. Therefore, the yield ratio is 0.8 or less.
延性き裂発生歪：３％以上
本発明の鋼板は、上記成分組成と鋼板組織を有することにより、延性き裂発生歪が３％以上という、優れた延性き裂発生抵抗を有する。ここで、上記延性き裂発生歪とは、図１に示した切欠底半径0.25mmの環状切欠を有する丸棒試験片を作製し、種々の量の引張歪を付与後、除荷してから、その試験片の切欠底断面の組織を観察して、延性き裂発生の有無を調査したときの、切欠底からき裂が最初に発生した標点間の平均歪のことである。
Ductile crack initiation strain: 3% or more The steel sheet of the present invention has excellent ductile crack initiation resistance with a ductile crack initiation strain of 3% or more by having the above-described component composition and steel sheet structure. Here, the above-mentioned ductile crack initiation strain refers to the production of a round bar test piece having an annular notch with a notch bottom radius of 0.25 mm shown in FIG. This is the average strain between the gauge marks where the crack first occurred from the notch bottom when the structure of the notch bottom cross section of the test piece was observed to examine whether or not a ductile crack occurred.
次に、本発明の鋼板の製造方法について説明する。
スラブ加熱温度：1000〜1200℃
上記に規定された成分組成を有する鋼を通常公知の方法で溶製して鋼スラブとし、その後、熱間圧延に先立って、該スラブを加熱炉に装入し再加熱する。この際のスラブ加熱温度は、1000〜1200℃とする。1000℃未満では十分な強度が得られず、一方、1200℃を超える場合には組織が粗大化して靭性が劣化するためである。
Next, the manufacturing method of the steel plate of this invention is demonstrated.
Slab heating temperature: 1000 ~ 1200 ℃
The steel having the component composition defined above is melted by a generally known method to form a steel slab, and then, prior to hot rolling, the slab is charged into a heating furnace and reheated. The slab heating temperature in this case shall be 1000-1200 degreeC. When the temperature is lower than 1000 ° C., sufficient strength cannot be obtained. On the other hand, when the temperature exceeds 1200 ° C., the structure becomes coarse and toughness deteriorates.
圧延終了温度：Ａr3変態点以上
圧延終了温度は、Ａr3変態点以上とする。圧延終了温度がＡr3変態点未満の場合には、圧延による塑性歪がフェライト相中に残存してフェライトの強度が高くなり、ベイナイト相とフェライト相のビッカース硬さの比が低下するため、延性き裂発生抵抗が低下するからである。圧延終了温度の上限については、特に規定しないが、未再結晶域の圧延により組織を微細にするためには950℃以下であることが好ましい。なお、上記Ａr3変態点は、鋼板が有する成分組成によって変化し、通常、下記式；
Ａr3変態点(℃)＝910−310＊Ｃ−80＊Mn−20＊Cu−15＊Cr−55＊Ni−80＊Mo
ただし、各元素記号は、その成分の含有量(mass％)
で求めることができる。
Rolling end temperature: Ar 3 transformation point or higher The rolling end temperature is higher than the Ar 3 transformation point. When the rolling end temperature is lower than the Ar 3 transformation point, the plastic strain due to rolling remains in the ferrite phase and the strength of the ferrite is increased, and the ratio of the Vickers hardness of the bainite phase to the ferrite phase is decreased. This is because the crack initiation resistance decreases. The upper limit of the rolling end temperature is not particularly defined, but is preferably 950 ° C. or lower in order to make the structure fine by rolling in the non-recrystallized region. The Ar 3 transformation point varies depending on the component composition of the steel sheet and is usually represented by the following formula:
Ar 3 transformation point (° C) = 910-310 * C-80 * Mn-20 * Cu-15 * Cr-55 * Ni-80 * Mo
However, each element symbol is the content of the component (mass%)
Can be obtained.
(Ａr3変態点−30℃)未満〜400℃以下の温度域まで平均冷却速度：30℃／秒以上
熱間圧延終了後の鋼板は、(Ａr3変態点−30℃)未満の温度域から400℃以下の温度域までを、平均冷却速度30℃／秒以上で冷却することが必要である。冷却開始温度が(Ａr3変態点−30℃)以上の場合には、ベイナイト相分率が高くなりすぎ、延性き裂発生抵抗が低下する。また、平均冷却速度が30℃／秒未満の場合や冷却停止温度が400℃を超える場合には、十分な硬さを有するベイナイト相が得られず、ベイナイト相とフェライト相のビッカース硬さの比が低下するため、優れた延性き裂発生抵抗と低い降伏比が得られない。よって、上記冷却条件とする。
(Ar 3 transformation point-30 ° C) Average cooling rate from less than 400 ° C to less than 400 ° C: 30 ° C / second or more The steel sheet after hot rolling is from a temperature range below (Ar 3 transformation point-30 ° C) It is necessary to cool to a temperature range of 400 ° C. or lower at an average cooling rate of 30 ° C./second or higher. When the cooling start temperature is (Ar 3 transformation point −30 ° C.) or higher, the bainite phase fraction becomes too high, and ductile cracking resistance decreases. Also, when the average cooling rate is less than 30 ° C / second or when the cooling stop temperature exceeds 400 ° C, a bainite phase having sufficient hardness cannot be obtained, and the ratio of the Vickers hardness of the bainite phase to the ferrite phase Therefore, excellent ductile crack initiation resistance and a low yield ratio cannot be obtained. Therefore, it is set as the said cooling conditions.
本発明によれば、上記の成分組成を有する鋼スラブに、上記の製造方法を適用することにより、上記金属組織と強度特性を有する鋼板を製造することができるので、地震などで生じる大きな塑性変形を受けても、延性き裂が発生し難い高強度高変形能厚鋼板を提供することができる。 According to the present invention, a steel plate having the above metal structure and strength characteristics can be manufactured by applying the above manufacturing method to a steel slab having the above component composition, so that a large plastic deformation caused by an earthquake or the like. Even if it receives, a high intensity | strength highly deformable thick steel plate with which a ductile crack is hard to generate | occur | produce can be provided.
表１に示した成分組成を有する鋼を常法に従い溶製し、表２に示す製造条件で、板厚15mmの厚鋼板とした。これらの鋼板について、板厚中心部付近のミクロ組織を観察し、10視野の組織写真を得て、これを画像解析してベイナイトの相分率を求めた。また、ベイナイト相とフェライト相の硬さは、マイクロビッカース硬度計により測定した。引張強度と降伏比は、平行部が６mmφ×30mmの丸棒試験片を用いた引張試験により求めた。延性き裂発生歪は、図１に示した切欠底半径0.25mmの環状切欠を有する丸棒試験片を作製し、引張歪の量を種々に変化させて付与してから除荷し、その後、この試験片の切欠底断面の組織を観察して、延性き裂発生の有無を調査し、切欠底からき裂が最初に発生したときの標点間の平均歪を延性き裂発生歪として求めた。 Steel having the composition shown in Table 1 was melted in accordance with a conventional method, and a thick steel plate having a thickness of 15 mm was produced under the production conditions shown in Table 2. For these steel plates, the microstructure near the center of the plate thickness was observed to obtain 10-view structure photographs, which were subjected to image analysis to determine the bainite phase fraction. The hardness of the bainite phase and the ferrite phase was measured with a micro Vickers hardness meter. The tensile strength and yield ratio were determined by a tensile test using a round bar test piece having a parallel part of 6 mmφ × 30 mm. For the ductile crack initiation strain, a round bar test piece having an annular notch with a notch bottom radius of 0.25 mm shown in FIG. By observing the structure of the notch bottom cross section of this specimen, the presence or absence of ductile cracking was investigated, and the average strain between the gauge points when the crack first occurred from the notch bottom was determined as the ductile cracking strain. .
表２に、ベイナイト相分率、引張強度、降伏比、ベイナイト相とフェライト相のビッカース硬さの比および延性き裂発生歪の測定結果を併せて示す。本発明例の鋼板は、いずれも降伏比が0.8以下、延性き裂発生歪が３％以上であり、優れた変形能と高い延性き裂発生抵抗を有している。一方、比較例の鋼板は、成分組成または製造条件が本発明範囲から外れているため、ベイナイトの相分率やベイナイト相とフェライト相のビッカース硬さの比が本発明の規定範囲外のものとなり、その結果、高降伏比または延性き裂発生歪が小さく、変形能または延性き裂発生抵抗が劣っている。 Table 2 also shows the measurement results of bainite phase fraction, tensile strength, yield ratio, ratio of Vickers hardness of bainite phase to ferrite phase, and ductile crack initiation strain. All the steel sheets of the present invention have a yield ratio of 0.8 or less, a ductile crack generation strain of 3% or more, and have excellent deformability and high ductile crack generation resistance. On the other hand, the steel composition of the comparative example is out of the scope of the present invention in terms of the component composition or manufacturing conditions, so the bainite phase fraction and the ratio of Vickers hardness between the bainite phase and the ferrite phase are outside the specified range of the present invention. As a result, the high yield ratio or ductile crack initiation strain is small, and the deformability or ductile crack initiation resistance is poor.
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