JP4507708B2 - Low yield ratio high strength high toughness steel sheet manufacturing method - Google Patents
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- 229910000831 Steel Inorganic materials 0.000 title claims description 52
- 239000010959 steel Substances 0.000 title claims description 52
- 238000004519 manufacturing process Methods 0.000 title claims description 35
- 238000001816 cooling Methods 0.000 claims description 64
- 229910000734 martensite Inorganic materials 0.000 claims description 57
- 229910001563 bainite Inorganic materials 0.000 claims description 33
- 229910000859 α-Fe Inorganic materials 0.000 claims description 29
- 238000005096 rolling process Methods 0.000 claims description 22
- 238000000034 method Methods 0.000 claims description 11
- 239000002184 metal Substances 0.000 claims description 7
- 229910052751 metal Inorganic materials 0.000 claims description 7
- 230000008569 process Effects 0.000 claims description 6
- 229910052748 manganese Inorganic materials 0.000 claims description 5
- 239000012535 impurity Substances 0.000 claims description 4
- 229910052750 molybdenum Inorganic materials 0.000 claims description 4
- 229910052759 nickel Inorganic materials 0.000 claims description 4
- 229910052796 boron Inorganic materials 0.000 claims description 3
- 229910052804 chromium Inorganic materials 0.000 claims description 3
- 229910052758 niobium Inorganic materials 0.000 claims description 3
- 229910052791 calcium Inorganic materials 0.000 claims description 2
- 229910052720 vanadium Inorganic materials 0.000 claims description 2
- 238000003303 reheating Methods 0.000 description 36
- 229910001566 austenite Inorganic materials 0.000 description 21
- 238000010438 heat treatment Methods 0.000 description 19
- 230000009466 transformation Effects 0.000 description 18
- 230000000694 effects Effects 0.000 description 12
- 238000005098 hot rolling Methods 0.000 description 6
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- 239000010953 base metal Substances 0.000 description 3
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- 238000010521 absorption reaction Methods 0.000 description 1
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- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 1
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Description
本発明は、建築、海洋構造物、ラインパイプ、造船、土木、建設機械等の分野での使用に好適な、低降伏比高強度高靱性鋼板の製造方法に関するものである。 The present invention relates to a method for producing a low yield ratio, high strength, high toughness steel sheet suitable for use in the fields of architecture, offshore structures, line pipes, shipbuilding, civil engineering, construction machinery and the like.
近年、溶接構造用鋼材においては、高強度、高靱性に加え、耐震性の観点から低降伏比化も要求されている。一般に、鋼材の金属組織を、フェライトの様な軟質相の中に、ベイナイトやマルテンサイトなどの硬質相が適度に分散した組織にすることで、鋼材の低降伏比化が可能であることが知られている。 In recent years, steel materials for welded structures are required to have a low yield ratio from the viewpoint of earthquake resistance in addition to high strength and high toughness. In general, it is known that the yield ratio of steel can be reduced by making the microstructure of steel a structure in which a hard phase such as bainite or martensite is appropriately dispersed in a soft phase such as ferrite. It has been.
上記のような軟質相の中に硬質相が適度に分散した組織を得る製造方法として、焼入れ（Ｑ）と焼戻し（Ｔ）の中間に、フェライトとオーステナイトの２相域からの焼き入れ（Ｑ’）を施す熱処理方法が知られている（例えば、特許文献１参照。）。この熱処理方法では、Ｑ’温度を適当に選択することにより、低降伏比化が達成可能であるが、熱処理工程数が増加するため、生産性の低下、製造コストの増加を招く。 As a production method for obtaining a structure in which a hard phase is appropriately dispersed in the soft phase as described above, quenching from a two-phase region of ferrite and austenite (Q ′) between quenching (Q) and tempering (T). ) Is known (see, for example, Patent Document 1). In this heat treatment method, a low yield ratio can be achieved by appropriately selecting the Q 'temperature, but the number of heat treatment steps increases, resulting in a decrease in productivity and an increase in manufacturing cost.
製造工程が増加することがない方法として、Ａｒ３温度以上で圧延終了後、鋼材の温度をフェライトが生成するＡｒ３変態点以下になるまで加速冷却の開始を遅らせる方法が開示されている（例えば、特許文献２参照。）。しかし、圧延終了から加速冷却開始までの温度域を放冷程度の冷却速度で冷却する必要があるため、生産性が極端に低下する。
このように従来の技術では、生産性の低下、製造コストの増加を招くことなく、低降伏比高強度高靱性鋼板を製造することは困難である。 As described above, it is difficult to manufacture a low yield ratio, high strength, high toughness steel sheet without causing a decrease in productivity and an increase in manufacturing cost.
したがって本発明の目的は、このような従来技術の課題を解決し、高製造効率、低コストで製造できる、低降伏比高強度高靱性鋼板の製造方法を提供することにある。 Accordingly, it is an object of the present invention to provide a method for producing a low-yield-ratio high-strength, high-toughness steel sheet that can solve the problems of the prior art and can be produced with high production efficiency and low cost.
このような課題を解決するための本発明の特徴は以下の通りである。
（１）、質量％で、Ｃ：０．０３〜０．１％、Ｓｉ：０．０１〜０.５％、Ｍｎ：１．２〜２．５％、Ａｌ：０．０８％以下を含有し、残部がＦｅ及び不可避不純物からなる鋼を、Ａｒ３温度以上の圧延終了温度で熱間圧延した後、５℃／ｓ以上の冷却速度で４５０〜６５０℃まで加速冷却を行い、その後直ちに０．５℃／ｓ以上の昇温速度で７５０℃以下まで再加熱を行う鋼板の製造方法であり、該鋼板の金属組織がフェライトとベイナイトと島状マルテンサイトとの合計の面積分率が９７％以上であり、島状マルテンサイトの面積分率が３〜２０％であることを特徴とする低降伏比高強度高靱性鋼板の製造方法。
（２）、さらに、質量％で、Ｔｉ：０．０４％以下、Ｎｂ：０．０７％以下、Ｖ：０．１％以下の中から選ばれるいずれか１種のみを含有することを特徴とする（１）に記載の低降伏比高強度高靱性鋼板の製造方法。
（３）、さらに、質量％で、Ｍｏ：０．４％以下、Ｃｕ：０．５％以下、Ｎｉ：０．５％以下、Ｃｒ：０．５％以下、Ｂ：０．００５％以下、Ｃａ：０．０００５〜０．００３０の中から選ばれる１種又は２種以上を含有することを特徴とする（１）または（２）に記載の低降伏比高強度高靱性鋼板の製造方法。
The features of the present invention for solving such problems are as follows.
(1) By mass%, C: 0.03 to 0.1%, Si: 0.01 to 0.5%, Mn: 1.2 to 2.5%, Al: 0.08% or less Then, the steel consisting of Fe and unavoidable impurities in the balance is hot-rolled at a rolling end temperature of Ar3 temperature or higher, accelerated cooling to 450-650 ° C at a cooling rate of 5 ° C / s, and A method for producing a steel sheet that is reheated to 750 ° C. or less at a temperature rising rate of 5 ° C./s or more, and the metal structure of the steel sheet has a total area fraction of 97% or more of ferrite, bainite, and island martensite , and the method for producing a low yield ratio high-strength and high toughness steel plate area fraction of the island martensite, wherein 3-20% der Rukoto.
(2) Furthermore, it is characterized by containing only one selected from Ti: 0.04% or less, Nb: 0.07% or less, and V: 0.1% or less in mass%. The manufacturing method of the low yield ratio high strength high toughness steel sheet according to (1) .
(3) Further, in mass%, Mo: 0.4% or less , Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, B: 0.005% or less Ca: One or more selected from 0.0005 to 0.0030, or a method for producing a low yield ratio high strength high toughness steel sheet according to (1) or (2) .
本発明によれば、低降伏比高強度高靱性鋼板を、高能率、低コストで製造することができる。このため建築、海洋構造物、ラインパイプ、造船、土木、建設機械等の溶接構造物に使用する鋼板を、安価で大量に安定して製造することができ、生産性および経済性を著しく高めることができる。 According to the present invention, a low yield ratio high strength high toughness steel sheet can be manufactured with high efficiency and low cost. For this reason, steel sheets used for welding structures such as architecture, offshore structures, line pipes, shipbuilding, civil engineering, construction machinery, etc. can be manufactured stably in a large amount at a low price, and the productivity and economy are significantly increased. Can do.
本発明者らは前記課題を解決するために、鋼板の製造方法、特に制御圧延後の加速冷却とその後の再加熱という製造プロセスについて鋭意検討した結果、Ｍｎ、Ｍｏ等の焼入性向上元素を添加した鋼を用い、加速冷却過程でベイナイト変態途中すなわち未変態オーステナイトが存在する温度領域で冷却を停止し、その後ベイナイト変態終了温度（以下、Ｂｆ点と記載する。）以上から再加熱を行うことにより、鋼板の金属組織が、フェライト、ベイナイトの混合相中に、フェライト、ベイナイトより硬質相である島状マルテンサイト（以下ＭＡと記載する。）が均一に生成した３相組織となり、低降伏比化が可能であるという知見を得た。ＭＡは、たとえば３％ナイタール溶液(nitral:硝酸アルコール溶液)でエッチング後、電解エッチングして観察すると、容易に識別可能である。図１は走査型電子顕微鏡（ＳＥＭ）で鋼板のミクロ組織を観察した場合の写真であるが、ＭＡは白く浮き立った部分として観測され、フェライト、ベイナイトの混合組織にＭＡが均一に生成している様子が確認できる。 In order to solve the above-mentioned problems, the present inventors have intensively studied a manufacturing process of a steel sheet, particularly a manufacturing process of accelerated cooling after controlled rolling and subsequent reheating. As a result, a hardenability-enhancing element such as Mn and Mo is added. Using the added steel, cooling is stopped during the bainite transformation in the accelerated cooling process, that is, in a temperature region where untransformed austenite exists, and then reheating is performed from the bainite transformation finish temperature (hereinafter referred to as Bf point) or higher. Thus, the metal structure of the steel sheet becomes a three-phase structure in which island-like martensite (hereinafter referred to as MA), which is a harder phase than ferrite and bainite, is uniformly formed in the mixed phase of ferrite and bainite, and has a low yield ratio. We obtained the knowledge that it is possible. MA can be easily identified by, for example, etching with a 3% nital solution (nitral: nitric alcohol solution) and then observing it by electrolytic etching. FIG. 1 is a photograph when the microstructure of a steel sheet is observed with a scanning electron microscope (SEM). MA is observed as a white floating portion, and MA is uniformly generated in a mixed structure of ferrite and bainite. The state can be confirmed.
本発明は上記の知見により得られたもので、圧延後の加速冷却、再加熱によって生成したベイナイト、フェライト相と、再加熱後の空冷中に生じる硬質相であるＭＡが均一に生成した３相組織を有する低降伏比高強度高靱性鋼板に関するものである。 The present invention was obtained from the above knowledge, and the three phases in which MA, which is a hard phase generated during air cooling after reheating and bainite and ferrite phases generated by accelerated cooling after rolling and reheating, were uniformly formed. The present invention relates to a low yield ratio high strength high toughness steel sheet having a structure.
以下、本発明の高強度鋼板について詳しく説明する。まず、本発明の高強度鋼板の組織について説明する。 Hereinafter, the high-strength steel sheet of the present invention will be described in detail. First, the structure of the high-strength steel sheet of the present invention will be described.
本発明では、フェライト相、ベイナイト相に硬質相であるＭＡが均一に生成した組織とすることで、低降伏比化を達成している。本発明における、ＭＡ生成のメカニズムは以下の通りである。スラブを加熱後、オーステナイト領域で圧延を終了し、その後Ａｒ３変態温度以上で加速冷却を開始する。加速冷却をベイナイト変態途中すなわち未変態オーステナイトが存在する温度域で終了し、その後Ｂｆ点以上で再加熱を行い、その後冷却するという製造プロセスである。その組織の変化は次の通りである。加速冷却終了時のミクロ組織はベイナイトと未変態オーステナイトであり、Ｂｆ点以上で再加熱を行うことで未変態オーステナイトからのフェライト変態が生じるが、フェライトはＣ固溶量が少ないためＣが未変態オーステナイトへ排出される。そのため、再加熱時のフェライト変態の進行に伴い、未変態オーステナイト中のＣ量が増加する。このとき、焼き入れ性を高め、オーステナイト安定化元素である、Ｍｎ、Ｃｕ、Ｎｉ等が一定以上含有されていると、再加熱終了時でもＣが濃縮した未変態オーステナイトが残存し、再加熱後の冷却でＭＡへと変態し、最終的にベイナイト、フェライト、ＭＡの３相組織となる。本発明では、加速冷却後、未変態オーステナイトが存在する温度域から再加熱を行うことが重要であり、再加熱開始温度がＢｆ点以下となるとベイナイト変態が完了し未変態オーステナイトが存在しなくなるため、再加熱開始はＢｆ点以上とする必要がある。また、再加熱後の冷却については、ＭＡの変態に影響を与えないため特に規定しないが、基本的に空冷とすることが好ましい。本発明では、ベイナイト変態途中で加速冷却を停止し、その後連続的に再加熱を行うことで、製造効率を低下させることなく硬質相であるＭＡを生成させることができ、硬質相を含んだ複合組織である３相組織とすることで低降伏比が達成できる。３相組織中のＭＡの割合は、ＭＡの面積分率（圧延方向や板幅方向等の鋼板の任意の断面におけるＭＡの面積の割合）で、３〜２０％とすることが望ましい。ＭＡの面積分率が３％未満では低降伏比化を達成するには不十分な場合があり、また２０％を超えると母材靱性を劣化させる場合がある。また、低降伏比化および母材靭性の観点から、ＭＡの面積分率は５〜１５％とすることが特に望ましい。なお、ＭＡの面積分率は、例えばＳＥＭ観察により得られたミクロ組織を画像処理することによってＭＡの占める面積率を求めることで得ることができる。また、ＭＡが粗大であると破壊の起点となり母材靭性を劣化させるため、ＭＡの平均粒径は、１０μｍ以下であることが望ましい。なお、ＭＡの平均粒径は、ＳＥＭ観察により得られたミクロ組織を画像処理し、個々のＭＡと同じ面積の円の直径を個々のＭＡについて求め、それらの直径の平均値として求めることができる。 In the present invention, a low yield ratio is achieved by forming a structure in which MA, which is a hard phase, is uniformly formed in the ferrite phase and the bainite phase. The mechanism of MA generation in the present invention is as follows. After heating the slab, rolling is finished in the austenite region, and then accelerated cooling is started at the Ar3 transformation temperature or higher. This is a manufacturing process in which accelerated cooling is terminated in the middle of bainite transformation, that is, in a temperature range where untransformed austenite exists, and then reheated at the Bf point or higher and then cooled. The changes in the organization are as follows. Microstructures at the end of accelerated cooling are bainite and untransformed austenite, and reheating at the Bf point or higher causes ferrite transformation from untransformed austenite. However, since ferrite has a small amount of C solid solution, C is untransformed. Discharged into austenite. Therefore, the amount of C in untransformed austenite increases with the progress of ferrite transformation during reheating. At this time, if the hardenability is increased and austenite stabilizing elements such as Mn, Cu, Ni, etc. are contained in a certain amount or more, untransformed austenite in which C is concentrated remains even after reheating, and after reheating It is transformed into MA by cooling, and finally becomes a three-phase structure of bainite, ferrite, and MA. In the present invention, after accelerated cooling, it is important to perform reheating from a temperature range where untransformed austenite exists, and when the reheating start temperature is below the Bf point, bainite transformation is completed and untransformed austenite does not exist. The reheating start needs to be higher than the Bf point. In addition, the cooling after reheating is not particularly specified because it does not affect the transformation of MA, but basically it is preferably air cooling. In the present invention, accelerated cooling is stopped in the middle of bainite transformation, and then reheating is performed continuously, so that MA that is a hard phase can be generated without lowering the production efficiency, and a composite containing a hard phase. A low yield ratio can be achieved by using a three-phase structure. The proportion of MA in the three-phase structure is preferably an area fraction of MA (ratio of the area of MA in an arbitrary cross section of the steel sheet in the rolling direction, the sheet width direction, etc.) and 3 to 20%. If the area fraction of MA is less than 3%, it may be insufficient to achieve a low yield ratio, and if it exceeds 20%, the base material toughness may be deteriorated. Further, from the viewpoint of lowering the yield ratio and the base material toughness, the area fraction of MA is particularly preferably 5 to 15%. Note that the area fraction of MA can be obtained, for example, by determining the area ratio occupied by MA by image processing the microstructure obtained by SEM observation. Further, if the MA is coarse, it becomes a starting point of fracture and deteriorates the toughness of the base material. Therefore, the average particle size of the MA is preferably 10 μm or less. The average particle diameter of MA can be obtained as an average value of the diameters obtained by subjecting the microstructure obtained by SEM observation to image processing, obtaining the diameter of a circle having the same area as each MA, and obtaining the diameter of each MA. .
なお、金属組織が、実質的にフェライトとベイナイトと島状マルテンサイトとの３相組織からなるとは、本発明の作用効果を無くさない限り、フェライト、ベイナイトおよびＭＡ以外の組織を含有するものが、本発明の範囲に含まれることを意味する。 Note that the metal structure substantially consists of a three-phase structure of ferrite, bainite, and island martensite, as long as the effects of the present invention are not lost, those containing structures other than ferrite, bainite, and MA, It is meant to be included in the scope of the present invention.
フェライトとベイナイトとＭＡとの３相組織に、パーライトなどの異なる金属組織が１種または２種以上混在する場合は、強度が低下するため、フェライト、ベイナイトおよびＭＡ以外の組織の面積分率は少ない程良い。しかし、フェライト、ベイナイトおよびＭＡ以外の組織の面積分率が低い場合は影響が無視できるため、トータルの面積分率で３％以下の他の金属組織を、すなわちパーライトやセメンタイト等を１種または２種以上含有してもよい。また、強度確保の観点からベイナイトの体積分率を１０％以上にする事が望ましい。 When one or more different metal structures such as pearlite are mixed in the three-phase structure of ferrite, bainite, and MA, the strength decreases, so the area fraction of the structure other than ferrite, bainite, and MA is small. Moderately good. However, if the area fraction of the structure other than ferrite, bainite, and MA is low, the influence can be ignored. Therefore, other metal structures of 3% or less in total area fraction, that is, pearlite, cementite, etc. You may contain more than a seed. Moreover, it is desirable to make the volume fraction of bainite 10% or more from a viewpoint of ensuring strength.
本発明の鋼板は以上のように、フェライトと、ベイナイトと、ＭＡとの３相からなる複合組織を有するが、このような組織は以下のような組成の鋼を用いて、以下のような方法で製造することにより得ることができる。 As described above, the steel sheet of the present invention has a composite structure composed of three phases of ferrite, bainite, and MA. Such a structure uses a steel having the following composition, and the following method. Can be obtained.
まず、本発明の高強度鋼板の化学成分について説明する。以下の説明において％で示す単位は全て質量％である。 First, chemical components of the high-strength steel plate of the present invention will be described. In the following description, all units represented by% are mass%.
Ｃ：０.０３％〜０.１％とする。ＣはＭＡ生成に重要な元素であるが、０.０３％未満ではＭＡの生成に不十分であり、また十分な強度が確保できない。０.１％を越える添加ではＨＡＺ靭性を劣化させるため、Ｃ含有量を０.０３％〜０.１％に規定する。さらに好適には０．０３〜０．０８％である
Ｓｉ：０.０１〜０.５％とする。Ｓｉは脱酸のため添加するが、０.０１％未満では脱酸効果が十分でなく、０.５％を超えると靭性や溶接性を劣化させるため、Ｓｉ含有量を０.０１〜０.５％に規定する。さらに好適には０．０１〜０．３％である。
C: Set to 0.03% to 0.1%. C is an important element for MA generation, but if it is less than 0.03%, it is insufficient for generation of MA, and sufficient strength cannot be secured. If the addition exceeds 0.1%, the HAZ toughness is deteriorated, so the C content is specified to be 0.03% to 0.1%. More preferably, it is 0.03 to 0.08% Si: 0.01 to 0.5%. Si is added for deoxidation, but if it is less than 0.01%, the deoxidation effect is not sufficient, and if it exceeds 0.5%, the toughness and weldability are deteriorated, so the Si content is 0.01 to 0.00. Specify 5%. More preferably, it is 0.01 to 0.3%.
Ｍｎ：１．２〜２.５％とする。Ｍｎは強度、靭性向上、更に焼き入れ性を向上しＭＡ生成を促すために添加するが、１．２％未満ではその効果が十分でなく、２.５％を超えると靱性ならびに溶接性が劣化するため、Ｍｎ含有量を１．２〜２.５％に規定する。成分や製造条件によらず、安定してＭＡを生成させるためには、１．５％以上の添加が望ましい。 Mn: 1.2 to 2.5%. Mn is added to improve strength and toughness, further improve hardenability and promote MA formation. However, if it is less than 1.2%, its effect is not sufficient, and if it exceeds 2.5%, toughness and weldability deteriorate. Therefore, the Mn content is specified to be 1.2 to 2.5%. Addition of 1.5% or more is desirable in order to stably produce MA regardless of the components and production conditions.
Ａｌ：０.０８％以下とする。Ａｌは脱酸剤として添加されるが、０.０８％を超えると鋼の清浄度が低下し、靱性が劣化するため、Ａｌ含有量は０.０８％以下に規定する。好ましくは０．０１〜０．０８％とする。 Al: 0.08% or less. Al is added as a deoxidizer, but if it exceeds 0.08%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content is specified to be 0.08% or less. Preferably, the content is 0.01 to 0.08%.
本発明では、鋼板の強度靱性をさらに改善し、且つ焼き入れ性を向上させＭＡの生成を促す目的で、以下に示すＭｏ、Ｔｉ、Ｎｂ、Ｖ、Ｃｕ、Ｎｉ、Ｃｒ、Ｂ、Ｃａの１種又は２種以上を含有してもよい。 In the present invention, for the purpose of further improving the strength toughness of the steel sheet and improving the hardenability and promoting the production of MA, one of Mo, Ti, Nb, V, Cu, Ni, Cr, B, and Ca shown below is used. You may contain a seed or two or more sorts.
Ｍｏ：０.４％以下とする。Ｍｏは焼入性向上元素の１種であり、強度上昇に有効であり、またＭＡ生成を促す。しかし、０．４％を越える添加はＨＡＺ靭性の劣化を招くことから、Ｍｏ含有量を０.４％以下に規定する。さらに、溶接熱影響部靭性の観点からＭｏ含有量を０．１〜０．３％とすることが好ましい。 Mo: Set to 0.4% or less. Mo is one of the hardenability improving elements, is effective for increasing the strength, and promotes the formation of MA. However, addition exceeding 0.4% leads to deterioration of HAZ toughness, so the Mo content is specified to be 0.4% or less. Furthermore, it is preferable to make Mo content into 0.1 to 0.3% from a viewpoint of weld heat affected zone toughness.
Ｔｉ：０．０４％以下とする。ＴｉはＴｉＮを形成してスラブ加熱時や溶接熱影響部の粒成長を抑制し、母材や溶接熱影響部の靭性を向上させる効果があるが、０.０４％を超える添加は逆に靭性の劣化を招くため、Ｔｉ含有量は０.００５〜０.０４％に規定する。さらに、溶接熱影響部靭性の観点からＴｉ含有量を０．０２％未満とすることが好ましい。 Ti: 0.04% or less. Ti forms TiN and has the effect of suppressing grain growth at the time of slab heating and welding heat-affected zone and improving the toughness of the base metal and welding heat-affected zone. Therefore, the Ti content is specified to be 0.005 to 0.04%. Furthermore, it is preferable that the Ti content is less than 0.02% from the viewpoint of weld heat affected zone toughness.
Ｎｂ：０.０７％以下とする。Ｎｂは圧延時や焼き入れ時の粒成長を抑制する事によりミクロ組織を微細化し、靭性を向上させる効果がある。しかし、０.０７％を超えると溶接熱影響部の靭性が劣化するため、Ｎｂ含有量は０.００５〜０.０７％に規定する。 Nb: Not more than 0.07%. Nb has the effect of reducing the grain growth during rolling and quenching to refine the microstructure and improve toughness. However, if it exceeds 0.07%, the toughness of the weld heat affected zone deteriorates, so the Nb content is specified to be 0.005 to 0.07%.
Ｖ：０.１％以下とする。Ｖは焼入性向上元素の１種であり、強度上昇に有効であり、またＭＡ生成を促す。しかし、０.１％を超えると溶接熱影響部の靭性が劣化するため、Ｖ含有量は０.００５〜０.１％に規定する。 V: 0.1% or less. V is a kind of hardenability improving element, is effective in increasing the strength, and promotes the formation of MA. However, if it exceeds 0.1%, the toughness of the weld heat-affected zone deteriorates, so the V content is specified to be 0.005 to 0.1%.
Ｃｕ：０.５％以下とする。Ｃｕは靭性の改善と強度の上昇に有効な元素である。その効果を得るためには０．１％以上添加することが好ましいが、多く添加すると溶接性が劣化するため、添加する場合は０.５％を上限とする。 Cu: 0.5% or less. Cu is an element effective for improving toughness and increasing strength. In order to obtain the effect, it is preferable to add 0.1% or more. However, if a large amount is added, weldability deteriorates. Therefore, when added, the upper limit is 0.5%.
Ｎｉ：０.５％以下とする。Ｎｉは靭性の改善と強度の上昇に有効な元素である。その効果を得るためには０．１％以上添加することが好ましいが、多く添加するとコスト的に不利になり、また、溶接熱影響部靱性が劣化するため、添加する場合は０.５％を上限とする。 Ni: 0.5% or less. Ni is an element effective for improving toughness and increasing strength. In order to obtain the effect, it is preferable to add 0.1% or more, but adding a large amount is disadvantageous in terms of cost, and the weld heat affected zone toughness deteriorates. The upper limit.
Ｃｒ：０.５％以下とする。ＣｒはＭｎと同様に低Ｃでも十分な強度を得るために有効な元素である。その効果を得るためには０．１％以上添加することが好ましいが、多く添加すると溶接性が劣化するため、添加する場合は０.５％を上限とする。 Cr: 0.5% or less. Cr, like Mn, is an element effective for obtaining sufficient strength even at low C. In order to obtain the effect, it is preferable to add 0.1% or more. However, if a large amount is added, weldability deteriorates. Therefore, when added, the upper limit is 0.5%.
Ｂ：０.００５％以下とする。Ｂは強度上昇、ＨＡＺ靭性改善に寄与する元素である。その効果を得るためには０．０００５％以上添加することが好ましいが、０.００５％を越えて添加すると溶接性を劣化させるため、添加する場合は０.００５％以下とする。 B: Set to 0.005% or less. B is an element contributing to strength increase and HAZ toughness improvement. In order to obtain the effect, it is preferable to add 0.0005% or more, but if added over 0.005%, the weldability is deteriorated, so when added, the content is made 0.005% or less.
Ｃａ：０．０００５〜０．００３％とする。Ｃａは硫化物系介在物の形態を制御して靭性を改善する。０．０００５％以上でその効果が現れ、０．００３％を超えると効果が飽和し、逆に清浄度を低下させて靭性を劣化させるため、添加する場合には０．０００５〜０．００３％とする。 Ca: 0.0005 to 0.003%. Ca improves the toughness by controlling the form of sulfide inclusions. The effect appears at 0.0005% or more, and when it exceeds 0.003%, the effect is saturated, and conversely, the cleanliness is lowered and the toughness is deteriorated. And
Ｎ：好ましくは０．００７％以下とする。Ｎは不可避的不純物として扱うが、０．００７％を越えると、溶接熱影響部靭性が劣化するため、好ましくは０．００７％以下とする。さらに、Ｔｉ量とＮ量の比であるＴｉ／Ｎを最適化することで、ＴｉＮ粒子により溶接熱影響部のオーステナイトの粗大化を抑制することができ、良好な溶接熱影響部靭性を得ることが出来るため、好ましくはＴｉ／Ｎを２〜８、さらに好ましくは２〜５とする。 N: Preferably it is 0.007% or less. N is treated as an unavoidable impurity, but if over 0.007%, the weld heat affected zone toughness deteriorates, so the content is preferably made 0.007% or less. Furthermore, by optimizing Ti / N, which is the ratio of Ti content and N content, the austenite coarsening of the weld heat affected zone can be suppressed by TiN particles, and good weld heat affected zone toughness can be obtained. Therefore, Ti / N is preferably 2 to 8, and more preferably 2 to 5.
上記以外の残部は実質的にＦeからなる。残部が実質的にＦeからなるとは、本発明の作用効果を無くさない限り、不可避不純物をはじめ、他の微量元素を含有するものが本発明の範囲に含まれ得ることを意味する。例えば、Ｍｇ、ＲＥＭをそれぞれ、０．０２％以下添加しても良い。 The remainder other than the above consists essentially of Fe. The balance substantially consisting of Fe means that an element containing an inevitable impurity and other trace elements can be included in the scope of the present invention unless the effects of the present invention are lost. For example, you may add 0.02% or less of Mg and REM, respectively.
次に、本発明の高強度鋼板の製造方法について説明する。 Next, the manufacturing method of the high strength steel plate of this invention is demonstrated.
本発明の高強度鋼板は上記の成分組成を有する鋼を用い、スラブ加熱後、Ａｒ３温度以上で熱間圧延を行い、その後５℃/s以上の冷却速度で４５０〜６００℃まで加速冷却を行い、その後Ｂｆ点以上から０.５℃/s以上の昇温速度で７５０℃以下の温度まで再加熱を行うことで、金属組織をフェライトとベイナイトの混合相中に硬質相であるＭＡが均一に生成した３相組織とすることができる。ここで、温度は鋼板の平均温度とする。平均温度は、スラブもしくは鋼板の表面温度より、板厚、熱伝導率等のパラメータを考慮して、計算により求めたものである。以下、各製造条件について詳しく説明する。 The high-strength steel sheet of the present invention uses steel having the above component composition, and after slab heating, hot rolling is performed at an Ar3 temperature or higher, and then accelerated cooling to 450 to 600 ° C at a cooling rate of 5 ° C / s or higher. Then, by reheating from the Bf point to a temperature of 750 ° C. or less at a rate of temperature rise of 0.5 ° C./s or more, the metal structure becomes uniform in the mixed phase of ferrite and bainite. The generated three-phase structure can be obtained. Here, the temperature is the average temperature of the steel sheet. The average temperature is obtained by calculation based on the surface temperature of the slab or steel plate, taking into account parameters such as plate thickness and thermal conductivity. Hereinafter, each manufacturing condition will be described in detail.
スラブ加熱温度は特に規定しないが、加熱温度が１０００℃未満では炭化物の固溶が不十分で必要な強度が得られず、１３００℃を超えると結晶粒が粗大化し母材靭性が劣化するため、加熱温度は１０００〜１３００℃とすることが好ましい。 The slab heating temperature is not particularly specified, but if the heating temperature is less than 1000 ° C, the required strength cannot be obtained because the solid solution of the carbide is insufficient, and if it exceeds 1300 ° C, the crystal grains become coarse and the base material toughness deteriorates. The heating temperature is preferably 1000 to 1300 ° C.
圧延終了温度：Ａｒ３温度以上とする。圧延終了温度がＡｒ３温度以下であると、その後のフェライト変態速度が低下するため、再加熱時の未変態オーステナイトへのＣの濃縮が不十分となりＭＡが生成しない。そのため圧延終了温度をＡｒ３温度以上とする。 Rolling end temperature: Ar3 temperature or higher. If the rolling end temperature is not higher than the Ar3 temperature, the subsequent ferrite transformation rate is lowered, so that the concentration of C into untransformed austenite at the time of reheating becomes insufficient and MA is not generated. Therefore, the rolling end temperature is set to Ar3 temperature or higher.
圧延終了後、直ちに５℃／ｓ以上の冷却速度で冷却する。冷却速度が５℃／ｓ未満では冷却時にパーライトを生成するため、ベイナイトによる強化が得られないため、十分な強度が得られない。よって、圧延終了後の冷却速度を５℃／ｓ以上に規定する。また、冷却開始温度がＡｒ３温度以下となりフェライトが生成すると、再加熱時に微細析出物の分散析出が得られず強度不足を招き、且つＭＡの生成も起こらないため、冷却開始温度をＡｒ３温度以上とする。このときの冷却方法については製造プロセスによって任意の冷却設備を用いることが可能である。本発明では、加速冷却によりベイナイト変態領域まで過冷することにより、その後の再加熱時に温度保持することなくフェライト変態を完了させることが可能である。 Immediately after the end of rolling, it is cooled at a cooling rate of 5 ° C./s or more. When the cooling rate is less than 5 ° C./s, pearlite is generated at the time of cooling, so that strengthening by bainite cannot be obtained, so that sufficient strength cannot be obtained. Therefore, the cooling rate after the end of rolling is specified to be 5 ° C./s or more. In addition, when the cooling start temperature is lower than the Ar3 temperature and ferrite is generated, fine precipitates are not dispersed and precipitated at the time of reheating, resulting in insufficient strength and no MA formation. Therefore, the cooling start temperature is set to be higher than the Ar3 temperature. To do. About the cooling method at this time, it is possible to use arbitrary cooling equipment by a manufacturing process. In the present invention, the ferrite transformation can be completed without maintaining the temperature during the subsequent reheating by supercooling to the bainite transformation region by accelerated cooling.
冷却停止温度：４５０〜６５０℃とする。このプロセスは本発明において、重要な製造条件である。本発明では再加熱後に存在するＣの濃縮した未変態オーステナイトがその後の空冷時にＭＡへと変態する。すなわち、ベイナイト変態途中の未変態オーステナイトが存在する温度域で冷却を停止する必要がある。冷却停止温度が４５０℃未満では、ベイナイト変態が完了するため空冷時にＭＡが生成せず低降伏比化が達成できない。６５０℃を超えると冷却中に析出するパーライトにＣが消費されＭＡが生成しないため、加速冷却停止温度を４５０〜６５０℃に規定する。 Cooling stop temperature: 450 to 650 ° C. This process is an important manufacturing condition in the present invention. In the present invention, C-concentrated untransformed austenite present after reheating is transformed into MA upon subsequent air cooling. That is, it is necessary to stop the cooling in a temperature range where untransformed austenite during the bainite transformation exists. If the cooling stop temperature is less than 450 ° C., the bainite transformation is completed, so MA is not generated during air cooling, and a low yield ratio cannot be achieved. If it exceeds 650 ° C., C is consumed in the pearlite that precipitates during cooling, and MA is not generated, so the accelerated cooling stop temperature is defined as 450 to 650 ° C.
加速冷却停止後Ｂｆ点以上の温度から０.５℃／ｓ以上の昇温速度で７５０℃以下の温度まで再加熱を行う。このプロセスも本発明において重要な製造条件である。再加熱時の未変態オーステナイトからフェライト変態と、それに伴う未変態オーステナイトへのＣの排出により、再加熱後の空冷時にＣが濃化した未変態オーステナイトがＭＡへと変態する。すなわちＭＡを生成させるためには、加速冷却後直ちに７５０℃以下の温度域まで再加熱する必要がある。昇温速度が０.５℃／ｓ未満では、目的の再加熱温度に達するまでに長時間を要するため製造効率が悪化し、またパーライト変態が生じるためＭＡが生成せず、低降伏比化を達成することができない。再加熱温度が７５０℃を超えるとベイナイトの軟化により十分な強度が得られないため、再加熱温度を７５０℃以下に規定する。本発明では、加速冷却後、未変態オーステナイトが存在する温度域から再加熱を行うことが重要であり、再加熱開始温度がＢｆ点以下となるとベイナイト変態が完了し未変態オーステナイトが存在しなくなるため、再加熱開始はＢｆ点以上とする必要がある。確実にフェライト変態させるためには、冷却停止温度より５０℃以上昇温することが望ましい。再加熱温度において、特に温度保持時間を設定する必要はない。また、再加熱後の冷却過程において冷却速度によらずＭＡは生成するため、再加熱後の冷却は基本的には空冷とすることが好ましい。 After the accelerated cooling is stopped, reheating is performed from a temperature of the Bf point or higher to a temperature of 750 ° C. or lower at a heating rate of 0.5 ° C./s or higher. This process is also an important production condition in the present invention. Due to the ferrite transformation from the untransformed austenite at the time of reheating and the accompanying discharge of C to the untransformed austenite, the untransformed austenite enriched with C during the air cooling after the reheating transforms to MA. That is, in order to generate MA, it is necessary to reheat to a temperature range of 750 ° C. or less immediately after accelerated cooling. If the rate of temperature rise is less than 0.5 ° C./s, it takes a long time to reach the target reheating temperature, so that the production efficiency deteriorates, and since pearlite transformation occurs, MA is not generated, and the yield ratio is lowered. Cannot be achieved. If the reheating temperature exceeds 750 ° C., sufficient strength cannot be obtained due to the softening of bainite, so the reheating temperature is specified to be 750 ° C. or less. In the present invention, after accelerated cooling, it is important to perform reheating from a temperature range where untransformed austenite exists, and when the reheating start temperature is below the Bf point, bainite transformation is completed and untransformed austenite does not exist. The reheating start needs to be higher than the Bf point. In order to reliably transform the ferrite, it is desirable to raise the temperature by 50 ° C. or more from the cooling stop temperature. There is no need to set the temperature holding time at the reheating temperature. Further, since MA is generated in the cooling process after reheating regardless of the cooling rate, it is preferable that the cooling after reheating is basically air cooling.
図１に上記の製造方法を用いて製造した本発明鋼板（０．０５ｍａｓｓ％Ｃ−０．２ｍａｓｓ％Ｓｉ−１．８ｍａｓｓ％Ｍｎ）を走査型電子顕微鏡（ＳＥＭ）で観察した写真を示す。図１によれば、フェライト（Ｆ）、ベイナイト（Ｂ）の混合組織に島状マルテンサイト（ＭＡ）が均一に生成している様子が確認できる。 The photograph which observed this invention steel plate (0.05 mass% C-0.2 mass% Si-1.8 mass% Mn) using the said manufacturing method in FIG. 1 with the scanning electron microscope (SEM) is shown. According to FIG. 1, it can be confirmed that island martensite (MA) is uniformly formed in the mixed structure of ferrite (F) and bainite (B).
加速冷却後の再加熱を行うための設備として、加速冷却を行うための冷却設備の下流側に加熱装置を設置することができる。加熱装置としては、鋼板の急速加熱が可能であるガス燃焼炉や誘導加熱装置を用いる事が好ましい。 As equipment for performing reheating after accelerated cooling, a heating device can be installed downstream of the cooling equipment for performing accelerated cooling. As the heating device, it is preferable to use a gas combustion furnace or induction heating device capable of rapid heating of the steel sheet.
本発明の製造方法を実施するための設備の一例を図２に示す。図２に示すように、圧延ライン１には上流から下流側に向かって熱間圧延機３、加速冷却装置４、誘導加熱装置５、ホットレベラー６が配置されている。誘導加熱装置５あるいは他の熱処理装置を、圧延設備である熱間圧延機３およびそれに引き続く冷却設備である加速冷却装置４と同一ライン上に設置する事によって、圧延、冷却終了後迅速に再加熱処理が行えるので、圧延冷却後の鋼板温度を過度に低下させることなく加熱することができる。 An example of equipment for carrying out the production method of the present invention is shown in FIG. As shown in FIG. 2, a hot rolling mill 3, an acceleration cooling device 4, an induction heating device 5, and a hot leveler 6 are arranged in the rolling line 1 from the upstream side toward the downstream side. By installing the induction heating device 5 or other heat treatment device on the same line as the hot rolling mill 3 that is a rolling facility and the accelerated cooling device 4 that is a subsequent cooling facility, reheating is performed quickly after the end of rolling and cooling. Since it can process, it can heat, without reducing the steel plate temperature after rolling cooling too much.
表１に示す化学成分の鋼（鋼種Ａ〜Ｊ）を連続鋳造法によりスラブとし、これを用いて板厚１８、２６ｍｍの厚鋼板（Ｎｏ.１〜１３）を製造した。 Steel of chemical composition (steel types A to J) shown in Table 1 was made into a slab by a continuous casting method, and thick steel plates (Nos. 1 to 13) having a thickness of 18 and 26 mm were produced using this.
加熱したスラブを熱間圧延により圧延した後、直ちに水冷型の加速冷却設備を用いて冷却を行い、誘導加熱炉またはガス燃焼炉を用いて再加熱を行った。誘導加熱炉は加速冷却設備と同一ライン上に設置した。各鋼板（Ｎｏ.１〜１３）の製造条件を表２に示す。なお、加熱温度、圧延終了温度、冷却停止（終了）温度および、再加熱温度等の温度は鋼板の平均温度とした。平均温度は、スラブもしくは鋼板の表面温度を測定し、板厚、熱伝導率等のパラメータを考慮して、計算により求めたものである。また、冷却速度は、熱間圧延終了後、冷却停止（終了）温度（２９０〜５７０℃）までの冷却に必要な温度差をその冷却を行うのに要した時間で割った平均冷却速度である。また、再加熱速度（昇温速度）は、冷却後、再加熱温度（５９０〜６７０℃）までの再加熱に必要な温度差を再加熱するのに要した時間で割った平均昇温速度である。 After the heated slab was rolled by hot rolling, it was immediately cooled using a water-cooled accelerated cooling facility and reheated using an induction heating furnace or a gas combustion furnace. The induction furnace was installed on the same line as the accelerated cooling equipment. Table 2 shows the production conditions of each steel plate (No. 1 to 13). The heating temperature, rolling end temperature, cooling stop (end) temperature, reheating temperature, and other temperatures were the average temperature of the steel sheet. The average temperature is obtained by calculation by measuring the surface temperature of the slab or steel plate and taking into account parameters such as plate thickness and thermal conductivity. The cooling rate is an average cooling rate obtained by dividing the temperature difference required for cooling to the cooling stop (end) temperature (290 to 570 ° C.) after the hot rolling is finished by the time required for the cooling. . The reheating rate (temperature increase rate) is the average temperature increase rate divided by the time required to reheat the temperature difference required for reheating up to the reheating temperature (590 to 670 ° C.) after cooling. is there.
以上のようにして製造した鋼板の引張特性を測定した。測定結果を表２に併せて示す。引張特性は、圧延垂直方向の全厚引張試験片を２本採取し、引張試験を行い、引張特性を測定し、その平均値で評価した。引張強度５８０ＭＰａ以上を本発明に必要な強度とし、降伏比８０％以下を本発明に必要な降伏比とした。母材靭性については、圧延垂直方向のフルサイズシャルピーＶノッチ試験片を３本採取し、シャルピー試験を行い、−１０℃での吸収エネルギーを測定し、その平均値を求めた。−１０℃での吸収エネルギーが２００Ｊ以上のものを良好とした。 The tensile properties of the steel sheet produced as described above were measured. The measurement results are also shown in Table 2. Tensile properties were obtained by collecting two full-thickness tensile test specimens in the vertical direction of rolling, performing a tensile test, measuring the tensile properties, and evaluating the average value. The tensile strength of 580 MPa or more was determined as the strength required for the present invention, and the yield ratio of 80% or less was determined as the yield ratio required for the present invention. For base metal toughness, three full-size Charpy V-notch test pieces in the vertical direction of rolling were sampled, Charpy test was performed, the absorbed energy at −10 ° C. was measured, and the average value was obtained. The absorption energy at −10 ° C. was determined to be 200 J or more.
溶接熱影響部（ＨＡＺ）靭性については、再現熱サイクル装置によって入熱４０ｋＪ／ｃｍに相当する熱履歴を加えた試験片を３本採取し、シャルピー試験を行った。そして、−１０℃での吸収エネルギーを測定し、その平均値を求めた。−１０℃でのシャルピー吸収エネルギーが１００Ｊ以上のものを良好とした。 For the weld heat affected zone (HAZ) toughness, three specimens with a thermal history corresponding to a heat input of 40 kJ / cm were collected by a reproducible thermal cycle apparatus and subjected to a Charpy test. And the absorbed energy in -10 degreeC was measured and the average value was calculated | required. Those having Charpy absorbed energy at −10 ° C. of 100 J or more were considered good.
表２において、本発明例であるＮｏ.１〜７はいずれも、化学成分および製造方法が本発明の範囲内であり、引張強度５８０MPa以上の高強度で降伏比８０％以下の低降伏比であり、母材ならびに溶接熱影響部の靭性は良好であった。また、鋼板の組織はフェライト、ベイナイト、島状マルテンサイトの３相組織であり、島状マルテンサイトの面積分率は３〜２０％の範囲内であった。 In Table 2, all of Nos. 1 to 7 as examples of the present invention have chemical components and production methods within the scope of the present invention, and have a high yield strength of 580 MPa or higher and a low yield ratio of 80% or less. The toughness of the base metal and the weld heat affected zone was good. Moreover, the structure of the steel sheet was a three-phase structure of ferrite, bainite, and island martensite, and the area fraction of the island martensite was in the range of 3 to 20%.
Ｎｏ.８〜１０は、化学成分は本発明の範囲内であるが、製造方法が本発明の範囲外であるため、組織がフェライト、ベイナイトの２相組織であり、降伏比が８０％以上と不十分であった。Ｎｏ.１１〜１４は化学成分が本発明の範囲外であるため、降伏比が高く、強度もしくはＨＡＺ靭性が劣っていた。 In Nos. 8 to 10, the chemical components are within the scope of the present invention, but the production method is outside the scope of the present invention, so the structure is a two-phase structure of ferrite and bainite, and the yield ratio is 80% or more. It was insufficient. Nos. 11 to 14 had a high yield ratio and poor strength or HAZ toughness because the chemical components were outside the scope of the present invention.
１ 圧延ライン
２ 鋼板
３ 熱間圧延機
４ 加速冷却装置
５ 誘導加熱装置
６ ホットレベラー
Ｆ フェライト
Ｂ ベイナイト
ＭＡ 島状マルテンサイト
DESCRIPTION OF SYMBOLS 1 Rolling line 2 Steel plate 3 Hot rolling mill 4 Accelerated cooling device 5 Induction heating device 6 Hot leveler F Ferrite B Bainite MA Island martensite
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