JP5644234B2 - Manufacturing method of structural members with excellent punching fatigue characteristics - Google Patents
Manufacturing method of structural members with excellent punching fatigue characteristics Download PDFInfo
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Description
本発明は、自動車のホイールディスクやフレームなどの打抜き加工を受ける構造部材、特に、引張強度TSが700MPa以上の熱延鋼板を用いた打抜き疲労特性に優れた構造部材の製造方法に関する。 The present invention relates to a method for producing a structural member excellent in punching fatigue characteristics using a hot-rolled steel sheet having a tensile strength TS of 700 MPa or more, such as a structural member subjected to punching processing such as a wheel disk or a frame of an automobile.
自動車のホイールディスクやフレームなどの構造部材には、意匠性の付与、軽量化および他の部材との接合などを目的に、打抜き加工を受けた熱延鋼板が用いられている。 For structural members such as wheel disks and frames of automobiles, hot-rolled steel sheets subjected to punching are used for the purpose of imparting design properties, reducing the weight, and joining with other members.
近年、自動車車体の軽量化の要請がますます強くなっているが、それに伴いこのようなホイールディスクやフレームに対しても、高強度熱延鋼板を適用し、薄肉化を図る検討が活発に行われている。最近では、板厚3mm前後の540MPa級のTSを有する高強度熱延鋼板を用いたホイールディスクが実用化されている。さらなる高強度化も検討されているが、こうした打抜き加工を受けた自動車の構造部材では、長時間にわたって繰返し応力が負荷されるため打抜き加工部から疲労破壊が起きやすいので、1×107サイクル後に疲労破壊の起こらない最大応力で定義された打抜き疲労限が高いことも必要である。 In recent years, there has been an increasing demand for weight reduction of automobile bodies. Along with this, studies to reduce the thickness by applying high-strength hot-rolled steel sheets to such wheel discs and frames have been actively conducted. It has been broken. Recently, a wheel disc using a high-strength hot-rolled steel sheet having a 540 MPa class TS with a thickness of about 3 mm has been put into practical use. Higher strength is also being investigated, but structural members of automobiles that have undergone such punching are subject to repeated stress over a long period of time, so fatigue failure tends to occur from the punched part, so after 1 × 10 7 cycles It is also necessary that the punching fatigue limit defined by the maximum stress at which fatigue failure does not occur is high.
そこで、特許文献1には、鋼の組成や組織を最適化し、これまで540MPa以上のTSを有する熱延鋼板では高々200MPa程度しか得られなかった打抜き疲労限を、600〜800MPaのTSでも230〜250MPaが得られるように高めた熱延鋼板が提案されている。 Therefore, in Patent Document 1, the punching fatigue limit, which has been obtained only about 200 MPa at the most with hot-rolled steel sheets having a TS of 540 MPa or more so far, is optimized for the TS of 600 to 800 MPa. A hot-rolled steel sheet that has been increased to obtain 250 MPa has been proposed.
しかしながら、特許文献1に記載の熱延鋼板では、高々250MPa程度の打抜き疲労限しか得られず、ホイールディスクやフレームなどの構造部材をさらに軽量化する上で疲労破壊の不安を払拭することができない。 However, the hot-rolled steel sheet described in Patent Document 1 can only obtain a punching fatigue limit of about 250 MPa at most, and cannot eliminate the fear of fatigue failure in further reducing the weight of structural members such as wheel disks and frames. .
本発明は、TSが700MPa以上の高強度熱延鋼板を用いて、275MPa以上の打抜き疲労限が得られる打抜き疲労特性に優れた構造部材の製造方法を提供することを目的とする。 An object of the present invention is to provide a method for producing a structural member excellent in punching fatigue characteristics that can provide a punching fatigue limit of 275 MPa or more using a high-strength hot-rolled steel sheet having a TS of 700 MPa or more.
本発明者等は、上記の目的を達成すべく鋭意検討を行った結果、次の知見を得た。すなわち、フェライト相中に少量のマルテンサイト相を含むミクロ組織を有し、TSが1000MPa以下の熱延鋼板を用い、打抜き加工時のクリアランスを15%以下にすれば275MPa以上の打抜き疲労限が得られる。 As a result of intensive studies to achieve the above object, the present inventors have obtained the following knowledge. That is, if a hot rolled steel sheet with a microstructure containing a small amount of martensite phase in the ferrite phase and TS of 1000 MPa or less is used and the clearance during punching is 15% or less, a punching fatigue limit of 275 MPa or more can be obtained. It is done.
本発明は、このような知見に基づきなされたもので、組織全体に占めるフェライト相の面積率が90%以上であり、かつマルテンサイト相を含むミクロ組織を有し、TSが700〜1000MPaの熱延鋼板を用い、打抜き加工部をクリアランス15%以下で打抜き加工する打抜き疲労特性に優れた構造部材の製造方法を提供する。 The present invention has been made on the basis of such knowledge, and the area ratio of the ferrite phase in the entire structure is 90% or more, has a microstructure including a martensite phase, and has a TS of 700 to 1000 MPa. Provided is a method for producing a structural member having excellent punching fatigue characteristics by using a rolled steel sheet and punching a punched portion with a clearance of 15% or less.
本発明の構造部材の製造方法では、熱延鋼板としては、質量%で、C:0.05〜0.14%、Si:0.005〜2.0%、Mn:0.5〜2.0%、P:0.03%以下、S:0.01%以下、Al:0.005〜0.1%、N:0.01%以下、Ti:0.05〜0.2%を含有し、残部がFeおよび不可避的不純物からなる組成を有する熱延鋼板を用いることができる。 In the method for producing a structural member of the present invention, as a hot-rolled steel sheet, in mass%, C: 0.05 to 0.14%, Si: 0.005 to 2.0%, Mn: 0.5 to 2.0%, P: 0.03% or less, S: 0.01 It is possible to use a hot-rolled steel sheet containing not more than%, Al: 0.005 to 0.1%, N: 0.01% or less, and Ti: 0.05 to 0.2%, with the balance being composed of Fe and inevitable impurities.
この熱延鋼板には、さらに、Cr:0.01〜0.5%、Mo:0.005〜0.5%、Nb:0.005〜0.1%、V:0.005〜0.5%の中から選択された少なくとも1種やCa:0.0005〜0.03%、REM:0.0005〜0.03%の中から選択された少なくとも1種が、個別にあるいは同時に含有されることが好ましい。 The hot-rolled steel sheet further includes at least one selected from Cr: 0.01 to 0.5%, Mo: 0.005 to 0.5%, Nb: 0.005 to 0.1%, V: 0.005 to 0.5%, and Ca: 0.0005 to It is preferable that at least one selected from 0.03% and REM: 0.0005 to 0.03% is contained individually or simultaneously.
また、こうした熱延鋼板としては、上記の組成を有する鋼スラブを、例えば、(Ar3変態点＋20)℃以上の仕上温度で熱間圧延後、100℃/s以上の平均冷却速度で600〜750℃の冷却停止温度まで一次冷却し、0.5〜8s間空冷し、100℃/s以上の平均冷却速度で二次冷却後、350〜500℃の巻取温度で巻取って製造した熱延鋼板を用いることが好ましい。 In addition, as such a hot-rolled steel sheet, a steel slab having the above composition is, for example, hot rolled at a finishing temperature of (Ar 3 transformation point +20) ° C. or higher, and an average cooling rate of 100 ° C./s or higher is 600 to Hot-rolled steel sheet produced by primary cooling to a cooling stop temperature of 750 ° C, air cooling for 0.5-8s, secondary cooling at an average cooling rate of 100 ° C / s or more, and winding at a coiling temperature of 350-500 ° C Is preferably used.
本発明により、TS が700MPa以上の高強度熱延鋼板を用いて、275MPa以上の打抜き疲労限が得られる打抜き疲労特性に優れた構造部材を製造できるようになった。本発明の方法により製造した自動車のホイールディスクやフレームは打抜き疲労特性に極めて優れているので、そのさらなる軽量化を推進できるものと考える。 According to the present invention, it is possible to produce a structural member excellent in punching fatigue characteristics that can provide a punching fatigue limit of 275 MPa or more using a high-strength hot-rolled steel sheet having a TS of 700 MPa or more. Since the wheel discs and frames of automobiles manufactured by the method of the present invention are extremely excellent in punching fatigue characteristics, it is considered that further weight reduction can be promoted.
1) 熱延鋼板のミクロ組織とクリアランスと打抜き疲労限
図1に、本発明に適用できるTSが780MPa級の熱延鋼板と従来のTSが540MPa級の熱延鋼板(板厚はいずれも3.1mm)について求めた打抜き加工時のクリアランスと打抜き疲労限との関係を示す。なお、クリアランスは鋼板の板厚に対する割合で表しており、打抜きポンチと打抜きダイの間隔(片側):CL(mm)と板厚:t(mm)から、CL/t×100(%)で算出される。ここで、本発明に適用できるTSが780MPa級の熱延鋼板は、C:0.07%、Si:1.5%、Mn:1.8%、P:0.007%、S:0.001%、Al:0.044%、N:0.00019%、Ti:0.05%、Cr:0.3%を含有し、残部がFeおよび不可避的不純物からなる組成を有し、組織全体に占めるフェライト相の面積率が94%で、マルテンサイト相の面積率が6%のミクロ組織を有する熱延鋼板であり、従来のTSが540MPa級の熱延鋼板は、C:0.14%、Si:0.03%、Mn:1.1%、P:0.02%、S:0.007%、Al:0.045%、N:0.0020%、Nb:0.025%を含有し、残部がFeおよび不可避的不純物からなる組成を有し、フェライト相とパーライト相からなるミクロ組織を有する熱延鋼板である。また、上記のフェライト相、マルテンサイト相の面積率は、走査型電子顕微鏡(SEM)用試験片を採取し、圧延方向に平行な板厚断面を研磨後、ナイタール腐食し、板厚中央部を倍率1000倍でSEM写真を10視野で撮影し、フェライト相、マルテンサイト相を画像処理により抽出し、画像解析処理によりフェライト相、マルテンサイト相の面積および観察視野の面積を測定して、(各相の面積)/(観察視野の面積)×100(%)より算出した。打抜き疲労限は、図2に示すような中央に10mmφの打抜き加工部を有する試験片を用い、平面曲げを繰り返し(応力比R:-1、周波数:25Hz)、上記打抜き加工部に疲労破壊が起こることなく1×107サイクル可能な最大応力で評価した。
1) Microstructure, clearance, and punching fatigue limit of hot-rolled steel sheet Fig. 1 shows a hot rolled steel sheet with a TS of 780MPa and a conventional TS of 540MPa that can be applied to the present invention. ) Shows the relationship between the clearance during punching and the fatigue limit for punching. The clearance is expressed as a percentage of the sheet thickness of the steel sheet, and is calculated as CL / t x 100 (%) from the distance between the punching punch and the punching die (one side): CL (mm) and the sheet thickness: t (mm). Is done. Here, TS is 780 MPa class hot rolled steel sheet applicable to the present invention, C: 0.07%, Si: 1.5%, Mn: 1.8%, P: 0.007%, S: 0.001%, Al: 0.044%, N: It contains 0.00019%, Ti: 0.05%, Cr: 0.3%, the balance is composed of Fe and inevitable impurities, the area ratio of ferrite phase occupying the whole structure is 94%, the area ratio of martensite phase Is a hot rolled steel sheet having a microstructure of 6%, and the conventional TS 540 MPa grade hot rolled steel sheet is C: 0.14%, Si: 0.03%, Mn: 1.1%, P: 0.02%, S: 0.007% , Al: 0.045%, N: 0.0020%, Nb: 0.025%, and the balance is a composition composed of Fe and inevitable impurities, and has a microstructure composed of a ferrite phase and a pearlite phase. In addition, the area ratio of the ferrite phase and martensite phase was obtained by taking a specimen for a scanning electron microscope (SEM), polishing the plate thickness cross section parallel to the rolling direction, then corroding the nital, and centering the plate thickness. SEM photographs were taken at 10 magnifications at a magnification of 1000, ferrite phase and martensite phase were extracted by image processing, the area of ferrite phase and martensite phase and the area of observation visual field were measured by image analysis processing, (each Phase area) / (observation field area) × 100 (%). For the punching fatigue limit, a test piece having a 10 mmφ punched part at the center as shown in Fig. 2 is used, and plane bending is repeated (stress ratio R: -1, frequency: 25 Hz), and fatigue failure occurs in the punched part. Evaluation was made at the maximum stress possible for 1 × 10 7 cycles without happening.
図1に示すように、本発明に適用できるTSが780MPa級の熱延鋼板を用いると、打抜き加工時のクリアランスを15%以下にすれば275MPa以上の打抜き疲労限が得られることがわかる。一方、従来のTSが540MPa級の熱延鋼板では、打抜き加工時のクリアランスを低下すれば打抜き疲労限が向上する傾向は認められるが、高々270MPaの打抜き疲労限しか得られない。 As shown in FIG. 1, it is understood that when a hot rolled steel sheet having a TS of 780 MPa class applicable to the present invention is used, a punching fatigue limit of 275 MPa or more can be obtained if the clearance during punching is 15% or less. On the other hand, in conventional hot-rolled steel sheets with a TS of 540 MPa, a tendency to improve the punching fatigue limit can be recognized if the clearance during punching is reduced, but only a punching fatigue limit of 270 MPa at most can be obtained.
一般に、高強度熱延鋼板では、TSが増加しても打抜き疲労限はほとんど増加しないことが知られている。しかし、本発明に適用可能な780MPa級の熱延鋼板では、図1に示すように、打抜き加工時のクリアランスが15%の場合でも、従来の540MPa級の熱延鋼板と比較し10%程度の打抜き疲労限の向上が認められる。これは、本発明に適用可能な熱延鋼板では、打抜き疲労限に対して組成とミクロ組織の最適化が図られているためと考えられる。また、本発明に適用可能な780MPa級の熱延鋼板では、打抜き加工時のクリアランスを15%以下に制御することで、従来のTSが540MPa級の熱延鋼板を遥かに凌ぐ打抜き疲労限が得られることが新たに明らかになったが、これは、打抜き加工時のクリアランスが15%以下の領域で、本発明に適用可能な熱延鋼板のミクロ組織がより一層有効に打抜き疲労限に寄与しているためと推察される。 In general, it is known that in a high-strength hot-rolled steel sheet, the punching fatigue limit hardly increases even if TS increases. However, in the hot rolled steel sheet of 780 MPa class applicable to the present invention, as shown in FIG. 1, even when the clearance during punching is 15%, it is about 10% compared with the conventional hot rolled steel sheet of 540 MPa class. An improvement in the punching fatigue limit is observed. This is considered to be because in the hot-rolled steel sheet applicable to the present invention, the composition and the microstructure are optimized with respect to the punching fatigue limit. In addition, the 780 MPa class hot-rolled steel sheet applicable to the present invention has a punching fatigue limit that far exceeds that of conventional 540 MPa class hot-rolled steel sheets by controlling the clearance during punching to 15% or less. However, this is because the microstructure of the hot-rolled steel sheet applicable to the present invention contributes to the punching fatigue limit more effectively in the region where the clearance during punching is 15% or less. It is guessed that this is because.
本発明者らは、上記知見に基づき、種々検討した結果、フェライト相の面積率が90%以上でかつマルテンサイト相を含むミクロ組織を有する引張強度700MPa以上の高強度鋼板を、クリアランス15％以下で打抜き加工することで、上記したような打抜き疲労限275MPa以上の優れた打抜き疲労特性を確保できることを見出した。なお、マルテンサイト相の面積率は3%以上とすることが好ましい。また、板厚としては、2〜8mm程度が好ましい。 As a result of various studies based on the above findings, the present inventors have found that a high strength steel sheet having a tensile strength of 700 MPa or more and having a microstructure containing a martensite phase with a ferrite phase area ratio of 90% or more, clearance of 15% or less. It was found that the punching fatigue characteristics with a punching fatigue limit of 275 MPa or more as described above can be ensured by punching with. The area ratio of the martensite phase is preferably 3% or more. The plate thickness is preferably about 2 to 8 mm.
また、本発明に適用可能な熱延鋼板では、TSが1000MPaを超えると、打抜き疲労限が低下するとともに、加工性も劣化するので、TSは1000MPa以下とする必要がある。ここで、鋼板のミクロ組織には、フェライト相とマルテンサイト相以外に、ベイナイト相や残留オーステナイト相、パーライト相などを含むことができる。さらに、フェライト相には、TiやNbの炭化物を析出させることもできる。 Further, in the hot-rolled steel sheet applicable to the present invention, when TS exceeds 1000 MPa, the punching fatigue limit is lowered and workability is also deteriorated. Therefore, TS needs to be 1000 MPa or less. Here, the microstructure of the steel sheet can include a bainite phase, a retained austenite phase, a pearlite phase, and the like in addition to the ferrite phase and the martensite phase. Further, Ti and Nb carbides can be precipitated in the ferrite phase.
2) 本発明に適用できる熱延鋼板
本発明に適用できる熱延鋼板としては、例えば、C:0.05〜0.14%、Si:0.005〜2.0%、Mn:0.5〜2.0%、P:0.03%以下、S:0.01%以下、Al:0.005〜0.1%、N:0.01%以下、Ti:0.05〜0.2%を含有し、残部がFeおよび不可避的不純物からなる組成を有する熱延鋼板を用いることができ、あるいはさらに、Cr:0.01〜0.5%、Mo:0.005〜0.5%、Nb:0.005〜0.1%、V:0.005〜0.5%の中から選択された少なくとも1種やCa:0.0005〜0.03%、REM:0.0005〜0.03%の中から選択された少なくとも1種が、個別にあるいは同時に含有される熱延鋼板を用いることができる。また、例えば、上記成分組成を有する鋼スラブを、(Ar3変態点＋20)℃以上の仕上温度で熱間圧延後、100℃/s以上の平均冷却速度で600〜750℃の冷却停止温度まで一次冷却し、0.5〜8s間空冷し、100℃/s以上の平均冷却速度で二次冷却後、350〜500℃の巻取温度で巻取って製造した熱延鋼板を用いることができる。以下に、組成および製造条件の限定理由を説明する。
2) Hot-rolled steel sheet applicable to the present invention As the hot-rolled steel sheet applicable to the present invention, for example, C: 0.05-0.14%, Si: 0.005-2.0%, Mn: 0.5-2.0%, P: 0.03% or less, S: 0.01% or less, Al: 0.005-0.1%, N: 0.01% or less, Ti: 0.05-0.2% can be used, and a hot-rolled steel sheet having a composition consisting of Fe and inevitable impurities can be used, Alternatively, at least one selected from Cr: 0.01 to 0.5%, Mo: 0.005 to 0.5%, Nb: 0.005 to 0.1%, V: 0.005 to 0.5%, Ca: 0.0005 to 0.03%, REM: 0.0005 A hot-rolled steel sheet in which at least one selected from ˜0.03% is contained individually or simultaneously can be used. Also, for example, a steel slab having the above composition is hot-rolled at a finishing temperature of (Ar 3 transformation point +20) ° C. or higher, and then reaches a cooling stop temperature of 600 to 750 ° C. at an average cooling rate of 100 ° C./s or higher. A hot-rolled steel sheet produced by primary cooling, air cooling for 0.5 to 8 s, secondary cooling at an average cooling rate of 100 ° C./s or more, and winding at a winding temperature of 350 to 500 ° C. can be used. The reasons for limiting the composition and manufacturing conditions will be described below.
2-1) 組成
C:0.05〜0.14%
マルテンサイト相を生成させ、必要な強度を確保するのに効果的な元素である。700MPa以上のTSを得るためにはC量を0.05%以上とする必要がある。一方、C量が0.14%を超えると加工性、打抜き疲労限が低下する。従って、C量は0.05〜0.14%とする。
2-1) Composition
C: 0.05-0.14%
It is an element that is effective in generating a martensite phase and ensuring the required strength. In order to obtain a TS of 700 MPa or more, the C content needs to be 0.05% or more. On the other hand, when the C content exceeds 0.14%, the workability and the punching fatigue limit are lowered. Therefore, the C content is 0.05 to 0.14%.
Si:0.005〜2.0%
Siは固溶強化により強度を上昇させるとともに、変態過程で二相分離を促進する元素である。Si量が2.0%を超えると強力なデスケーリング装置を使用しても表面性状の著しい劣化を招く。また、0.005%未満では上記の効果を発現しない。従って、Si量は0.005〜2.0%とする。
Si: 0.005-2.0%
Si is an element that increases strength by solid solution strengthening and promotes two-phase separation during the transformation process. If the Si content exceeds 2.0%, the surface properties will be significantly degraded even if a powerful descaling device is used. In addition, if it is less than 0.005%, the above effects are not exhibited. Therefore, the Si content is 0.005 to 2.0%.
Mn:0.5〜2.0%
Mnは固溶強化およびマルテンサイト相生成に有効な元素である。700MPa以上のTSを得るにはMn量を0.5%以上とする必要がある。一方、Mn量が2.0%を超えると溶接性が低下したり、偏析が著しくなり加工性を低下させる。従って、Mn量は0.5〜2.0%とする。
Mn: 0.5-2.0%
Mn is an effective element for solid solution strengthening and martensite phase formation. In order to obtain a TS of 700 MPa or more, the Mn content needs to be 0.5% or more. On the other hand, if the Mn content exceeds 2.0%, the weldability is deteriorated, and segregation becomes remarkable, thereby reducing the workability. Therefore, the Mn content is 0.5 to 2.0%.
P:0.03%以下
P量が0.03%を超えると偏析による加工性の低下を招く。従って、P量は0.03%以下とする。
P: 0.03% or less
If the amount of P exceeds 0.03%, workability is deteriorated due to segregation. Therefore, the P content is 0.03% or less.
S:0.01%以下
SはMnやTiと硫化物を形成して加工性を低下させるとともに、高強度化に有効なMnやTi量の低下を招く。従って、S量は0.01%以下とする。より好ましくは0.003%以下である。
S: 0.01% or less
S forms sulfides with Mn and Ti to reduce workability, and causes a decrease in the amount of Mn and Ti effective for increasing the strength. Therefore, the S content is 0.01% or less. More preferably, it is 0.003% or less.
Al:0.005〜0.1%
Alは鋼の脱酸材として重要な元素であり、それにはAl量を0.005%以上とすることが必要である。一方、Al量が0.1%を超えると鋼中に多量の介在物が残存し材質や表面性状の低下を招く。従って、Al量は0.005〜0.1%とする。
Al: 0.005-0.1%
Al is an important element as a deoxidizing material for steel, and it is necessary to make the Al content 0.005% or more. On the other hand, if the Al content exceeds 0.1%, a large amount of inclusions remain in the steel, leading to deterioration of the material and surface properties. Therefore, the Al content is 0.005 to 0.1%.
N:0.01%以下
N量が0.01%を超えると製造工程で多量の窒化物を生成し熱間延性を劣化させる。従って、N量は0.01%以下とする。
N: 0.01% or less
If the amount of N exceeds 0.01%, a large amount of nitride is generated in the manufacturing process and the hot ductility is deteriorated. Therefore, the N content is 0.01% or less.
Ti:0.05〜0.2%
TiはC、Nと結合し微細な炭化物や窒化物を形成し、主にフェライト相に析出して高強度化に寄与する元素である。こうした効果を得るには、0.05%以上の添加が必要である。一方、Ti量が0.2%を超えると加工性の劣化を招く。従って、Ti量は0.05〜0.2%とする。
Ti: 0.05-0.2%
Ti is an element that combines with C and N to form fine carbides and nitrides, and precipitates mainly in the ferrite phase to contribute to high strength. In order to obtain such an effect, addition of 0.05% or more is necessary. On the other hand, when the Ti content exceeds 0.2%, workability is deteriorated. Therefore, the Ti content is 0.05 to 0.2%.
残部はFeおよび不可避的不純物であるが、以下の理由で、さらに、Cr:0.01〜0.5%、Mo:0.005〜0.5%、Nb:0.005〜0.1%、V:0.005〜0.5%の中から選択された少なくとも1種やCa:0.0005〜0.03%、REM:0.0005〜0.03%の中から選択された少なくとも1種が、個別にあるいは同時に含有されることが好ましい。 The balance is Fe and inevitable impurities, but for the following reasons, it is further selected from Cr: 0.01-0.5%, Mo: 0.005-0.5%, Nb: 0.005-0.1%, V: 0.005-0.5% Preferably, at least one selected from Ca: 0.0005 to 0.03% and REM: 0.0005 to 0.03% is contained individually or simultaneously.
Cr:0.01〜0.5%
Crは焼入れ性を向上させマルテンサイト相を得るのに有効な元素である。Cr量が0.01%未満ではその効果が小さく、0.5%を超えると加工性の低下を招く。従って、Cr量は0.01〜0.5%とする。
Cr: 0.01-0.5%
Cr is an effective element for improving the hardenability and obtaining a martensite phase. If the Cr content is less than 0.01%, the effect is small, and if it exceeds 0.5%, the workability is reduced. Therefore, the Cr content is 0.01 to 0.5%.
Mo:0.005〜0.5%
Moは焼入れ性を向上させマルテンサイト相を得るのに有効な元素である。Mo量が0.005%未満ではその効果が小さく、0.5%を超えると加工性の低下を招く。従って、Mo量は0.005〜0.5%とする。
Mo: 0.005-0.5%
Mo is an element effective for improving the hardenability and obtaining a martensite phase. If the amount of Mo is less than 0.005%, the effect is small, and if it exceeds 0.5%, the workability is lowered. Therefore, the Mo amount is set to 0.005 to 0.5%.
Nb:0.005〜0.1%、V:0.005〜0.5%
Nb、VはC、Nと結合し微細な炭化物や窒化物を形成し、主にフェライト相に析出して高強度化に寄与する元素である。こうした効果を得るには、両元素とも0.005%以上の添加が必要である。一方、Nb量が0.1%を、また、V量が0.5%を超えると加工性の劣化を招く。従って、Nb量は0.005〜0.1%、V量は0.005〜0.5%とする。
Nb: 0.005-0.1%, V: 0.005-0.5%
Nb and V are elements that combine with C and N to form fine carbides and nitrides, and mainly precipitate in the ferrite phase to contribute to high strength. In order to obtain such an effect, it is necessary to add 0.005% or more for both elements. On the other hand, when the Nb amount exceeds 0.1% and the V amount exceeds 0.5%, workability is deteriorated. Therefore, the Nb amount is 0.005 to 0.1%, and the V amount is 0.005 to 0.5%.
Ca:0.0005〜0.03%、REM:0.0005〜0.03%
CaやREMは介在物の形態制御に有効な元素であり、それぞれ単独で、あるいは共存して加工性の向上に寄与する。こうした効果を得るにはCaやREM量を0.0005%以上とすることが好ましい。一方、CaやREM量が0.03%を超えると鋼中介在物が増加し材質が劣化する。従って、Ca量、REM量は0.0005〜0.03%とすることが好ましい。
Ca: 0.0005-0.03%, REM: 0.0005-0.03%
Ca and REM are effective elements for controlling the morphology of inclusions, and each contributes to the improvement of workability either alone or in combination. In order to obtain such an effect, the Ca or REM content is preferably 0.0005% or more. On the other hand, when the amount of Ca or REM exceeds 0.03%, inclusions in the steel increase and the material deteriorates. Therefore, the Ca content and the REM content are preferably 0.0005 to 0.03%.
2-2) 製造条件
熱間圧延時の仕上温度:(Ar3変態点＋20)℃以上
仕上温度が(Ar3変態点＋20)℃未満では、鋼板表層部に展伸粒や不均一粒が残存し加工性の低下を招きやすい。従って、仕上温度は(Ar3変態点＋20)℃以上とすることが好ましい。
2-2) Manufacturing conditions Finishing temperature during hot rolling: (Ar 3 transformation point + 20) ° C or higher If the finishing temperature is less than (Ar 3 transformation point + 20) ° C, stretched and non-uniform grains remain in the surface layer of the steel sheet. It is easy to cause deterioration of workability. Accordingly, the finishing temperature is preferably (Ar 3 transformation point + 20) ° C. or higher.
なお、Ar3変態点は、例えば、冷却速度10℃/sの加工フォーマスタ実験で熱膨張曲線を求め、その変化点により求めることができる。 The Ar 3 transformation point can be obtained, for example, by obtaining a thermal expansion curve by a processing formaster experiment at a cooling rate of 10 ° C./s and by using the change point.
熱間圧延後の一次冷却:100℃/s以上の平均冷却速度で600〜750℃の冷却停止温度まで冷却
一次冷却の平均冷却速度が100℃/s未満では高温域からフェライト変態が開始され粗大なフェライト組織が形成されやすく700MPa以上のTSを確保することが困難となる。従って、一次冷却の平均冷却速度は100℃/s以上とすることが好ましい。一次冷却の冷却停止温度が750℃超では、粗大なフェライト組織が形成されやすく700MPa以上のTSを確保することが困難となる。一方、600℃未満ではフェライト相以外の相が形成されやすく、フェライト相を面積率で90%以上確保することが困難となる。
Primary cooling after hot rolling: Cooling to 600-750 ° C cooling stop temperature at an average cooling rate of 100 ° C / s or more If the average cooling rate of primary cooling is less than 100 ° C / s, ferrite transformation starts from the high temperature range and is coarse As a result, it is difficult to secure a TS of 700 MPa or more. Therefore, the average cooling rate of primary cooling is preferably 100 ° C./s or more. When the cooling stop temperature of primary cooling exceeds 750 ° C., a coarse ferrite structure is easily formed, and it becomes difficult to secure a TS of 700 MPa or more. On the other hand, when the temperature is lower than 600 ° C., phases other than the ferrite phase are likely to be formed, and it is difficult to ensure the ferrite phase in an area ratio of 90% or more.
一次冷却後の空冷:0.5〜8s
一次冷却後の空冷時間は所望のミクロ組織を達成するためにきわめて重要である。特に適正なフェライト＋マルテンサイト相の形成のために、一次冷却を行った後、冷却を停止し空冷とする。空冷時間が0.5s未満ではマルテンサイト相の生成量が過剰になりフェライト相の面積率90%以上を確保することが困難となって加工性が低下し、8sを超えるとフェライト相の結晶粒が粗大化し700MPa以上のTSの確保が困難となる。従って、一次冷却後の空冷時間は0.5〜8sとする。
Air cooling after primary cooling: 0.5-8s
The air cooling time after the primary cooling is very important to achieve the desired microstructure. In particular, in order to form an appropriate ferrite + martensite phase, after the primary cooling, the cooling is stopped and air cooling is performed. If the air cooling time is less than 0.5 s, the amount of martensite phase produced becomes excessive, making it difficult to secure an area ratio of 90% or more for the ferrite phase, resulting in a decrease in workability. It becomes coarse and it becomes difficult to secure TS of 700MPa or more. Therefore, the air cooling time after the primary cooling is 0.5 to 8 s.
空冷後の二次冷却:100℃/s以上の平均冷却速度で冷却
空冷後は空冷中に調整されたフェライト相の生成量が変動しないように、平均冷却速度100℃/s以上で巻取温度まで二次冷却する必要がある。
Secondary cooling after air cooling: Cooling at an average cooling rate of 100 ° C / s or higher After air cooling, the coiling temperature is at an average cooling rate of 100 ° C / s or higher so that the amount of ferrite phase adjusted during air cooling does not fluctuate. Secondary cooling is necessary until.
巻取温度:350〜500℃
二次冷却後まで維持されたオーステナイト相をマルテンサイト相に変態させるために、350〜500℃の巻取温度で巻取ることが好ましい。これは500℃超ではパーライト相が部分的に生成して加工性が低下し、350℃未満では変態したマルテンサイト相が自己焼戻しされずマルテンサイト相の強度が高すぎて打抜き時にボイドを生成しやすくなるためである。
Winding temperature: 350-500 ° C
In order to transform the austenite phase maintained until after the secondary cooling into a martensite phase, it is preferable to wind at a coiling temperature of 350 to 500 ° C. This is because when the temperature exceeds 500 ° C, the pearlite phase is partially formed and the workability is lowered. Below 350 ° C, the transformed martensite phase is not self-tempered and the strength of the martensite phase is too high, and voids are generated during punching. This is because it becomes easier.
表1に示す組成とAr3変態点の鋼スラブNo.A〜Fを、1250℃に加熱し、表2に示す仕上温度で熱間圧延して、表2に示す板厚、ミクロ組織、TSの熱延鋼板No.1〜9を作製した。なお、表1のAr3変態点は上記の方法により求めた。また、表2のミクロ組織は上記の方法により求め、TSは、圧延方向に直角方向の沿ってJIS 5号引張試験片を採取し、引張速度10mm/minで引張試験を行って求めた。 Steel slabs Nos. A to F having the composition and Ar 3 transformation point shown in Table 1 were heated to 1250 ° C. and hot-rolled at the finishing temperature shown in Table 2, and the plate thickness, microstructure, TS shown in Table 2 Hot rolled steel sheets No. 1 to 9 were prepared. The Ar 3 transformation point in Table 1 was determined by the above method. The microstructure in Table 2 was determined by the above method, and TS was determined by taking a JIS No. 5 tensile specimen along the direction perpendicular to the rolling direction and conducting a tensile test at a tensile speed of 10 mm / min.
そして、作製した熱延鋼板を酸洗後、上記の方法により、クリアランスを変えて打抜き疲労限を求め、打抜き疲労特性を評価した。 And after pickling the produced hot-rolled steel sheet, the punching fatigue limit was calculated | required by changing clearance by said method, and the punching fatigue characteristic was evaluated.
結果を表3に示す。本発明範囲の条件を満たす熱延鋼板を用い、本発明範囲の条件を満たすクリアランスで打抜き加工を行えば、275MPa以上の打抜き疲労限が得られ、優れた打抜き疲労特性を有する構造部材を製造できることがわかる。特に、熱間圧延後本発明の条件で冷却した熱延鋼板は高Elであり、構造部材を加工する上でより好ましい鋼板といえる。 The results are shown in Table 3. If a hot-rolled steel sheet that satisfies the conditions of the present invention is used and punching is performed with a clearance that satisfies the conditions of the present invention, a punching fatigue limit of 275 MPa or more can be obtained, and a structural member having excellent punching fatigue characteristics can be manufactured. I understand. In particular, a hot-rolled steel sheet cooled under the conditions of the present invention after hot rolling has a high El, and can be said to be a more preferable steel sheet for processing a structural member.
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