JP4646871B2 - Hot-rolled steel sheet with excellent stretch flangeability - Google Patents
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本発明は、自動車や各種産業機械、構造物において、強度と加工性、特に伸びフランジ性が必要な部品の材料として用いられる、強度クラスが９８０ＭＰａ以上の熱延鋼板に関する。 The present invention relates to a hot-rolled steel sheet having a strength class of 980 MPa or more, which is used as a material for parts that require strength and workability, particularly stretch flangeability, in automobiles, various industrial machines, and structures.
強度クラスが９８０ＭＰａ以上の熱延鋼板において、強度と良好な伸びフランジ性を両立させるには組織の均一性を高めることが有効であり、かつその組織としてベイナイトを主体とする組織が有効であることが知られている。
伸びフランジ性改善に関する従来技術として、特許文献１には、Ｎｂ，Ｔｉの添加とプロセス最適化により板厚方向の組織の均一性を高めつつ、ベイナイト主体の組織にすることで強度−伸びフランジ性を両立させることが記載され、特許文献２には、Ｎｂ，Ｔｉのいずれかを添加しつつ、平均粒径３．０μｍの微細ベイナイトを主体とする組織にすることで、強度−伸びフランジ性を両立することが記載されている。しかし、９８０ＭＰａ以上の強度を確保するためには高価なＮｂの添加が必要となっている。
In a hot-rolled steel sheet having a strength class of 980 MPa or more, it is effective to increase the uniformity of the structure in order to achieve both strength and good stretch flangeability, and a structure mainly composed of bainite is effective as the structure. It has been known.
As a conventional technique for improving stretch flangeability, Patent Document 1 describes strength-stretch flangeability by increasing the uniformity of the structure in the thickness direction by adding Nb and Ti and optimizing the process, and by making the structure mainly bainite. In
一方、高価な焼き入れ性改善元素であるＮｂ等を添加せずにベイナイト組織を得る技術として、特許文献３には、Ｃ−Ｓｉ−Ｍｎ鋼を熱間圧延終了後、２秒以内に冷却を開始し、１５０℃／ｓ超の冷却速度で冷却し、４５０〜６５０℃で冷却を停止することにより、強度−伸びフランジ性を両立させることが記載されている。しかし、冷却速度を１５０℃／ｓ超に高めるためには特殊な設備が必要であり、かつこのような非常に高速の冷却速度では停止温度の制御が困難で、停止温度のばらつきが出ることが懸念される。 On the other hand, as a technique for obtaining a bainite structure without adding Nb or the like, which is an expensive hardenability improving element, Patent Document 3 describes that cooling is performed within 2 seconds after hot rolling of C-Si-Mn steel. It is described that both strength and stretch flangeability can be achieved by starting, cooling at a cooling rate exceeding 150 ° C./s, and stopping cooling at 450 to 650 ° C. However, special equipment is required to increase the cooling rate to over 150 ° C./s, and it is difficult to control the stop temperature at such a very high cooling rate, and the stop temperature may vary. Concerned.
本発明は、従来技術の上記問題点に鑑みてなされたもので、比較的単純な成分系及びプロセスにより、強度クラスが９８０ＭＰａ以上の熱延鋼板で良好な伸びフランジ性を確保することを目的とする。 The present invention has been made in view of the above problems of the prior art, and aims to ensure good stretch flangeability with a hot-rolled steel sheet having a strength class of 980 MPa or more by a relatively simple component system and process. To do.
本発明者らが、ベイナイト組織を得るため種々検討した結果、Ｂの添加と、Ｂの添加による焼き入れ性改善効果を発現するためにＴｉ，Ｎの添加バランスの制御が必要であることが分かった。
続いて、伸びフランジ性に及ぼすベイナイト下部組織の影響について検討した結果、Ｂを活用してフルベイナイトが得られている状況においては、ベイナイトのラス間に形成されるマルテンサイト及び残留オーステナイト（略してＭＡ）が、穴広げ試験時の亀裂の発生起点となり、このＭＡを減少させることで伸びフランジ性が改善可能であることが分かった。ＭＡの形成には従来焼き入れ性確保に有効として添加されていたＮｂ，Ｍｏなどの４Ａ〜６Ａ属の元素が寄与しており、これらの元素をできる限り低減することで伸びフランジ性が改善し、９８０ＭＰａ以上の熱延鋼板で良好な伸びフランジ性を確保することができた。
As a result of various studies conducted by the present inventors to obtain a bainite structure, it was found that the addition balance of Ti and N needs to be controlled in order to exhibit the effect of improving the hardenability by adding B. It was.
Subsequently, as a result of examining the influence of the bainite substructure on the stretch flangeability, in the situation where full bainite is obtained using B, martensite and residual austenite (abbreviated for short) formed between bainite laths. MA) became the starting point of cracking during the hole expansion test, and it was found that stretch flangeability can be improved by reducing this MA. The elements of the 4A-6A group such as Nb and Mo, which have been added as effective for securing the hardenability, contribute to the formation of MA. Stretch flangeability is improved by reducing these elements as much as possible. It was possible to secure good stretch flangeability with a hot-rolled steel sheet of 980 MPa or more.
本発明に係る伸びフランジ性に優れた熱延鋼板は、質量％で、Ｃ：０．０３％以上、０．２０％以下、Ｓｉ：０．９％以上、２．０％以下、Ｍｎ：１．０％以上、３．０％以下を含み、Ｐ：０．０５％以下、Ｓ：０．０５％以下であり、Ｖ：０．０１０％以下、Ｚｒ：０．０１０％以下、Ｎｂ：０．００５％以下、Ｍｏ：０．０１０％以下、Ｔａ：０．０１０％以下、Ｈｆ：０．０１０％以下、Ｗ：０．０１０％以下、かつこれら７元素の合計が０．０２％以下であり、Ｎ：０．０１０％以下で、かつＴｉの質量％を［Ｔｉ］とし、Ｎの質量％を［Ｎ］としたとき、［Ｔｉ］−３．４［Ｎ］が−０．００３％以上、０．０３％以下であり、Ｂ：０．０００３％以上、０．００５０％以下を含み、残部Ｆｅ及び不可避不純物からなり、主要組織がベイナイトでその分率が９０％以上、マルテンサイト及び残留オーステナイトからなる第２相分率が３％以下であることを特徴とする。 The hot-rolled steel sheet excellent in stretch flangeability according to the present invention is, in mass%, C: 0.03% to 0.20%, Si: 0.9% to 2.0%, Mn: 1 0.0% or more and 3.0% or less, P: 0.05% or less, S: 0.05% or less, V: 0.010% or less, Zr: 0.010% or less, Nb: 0 0.005% or less, Mo: 0.010% or less, Ta: 0.010% or less, Hf: 0.010% or less, W: 0.010% or less, and the total of these seven elements is 0.02% or less. Yes, N: 0.010% or less, and when Ti mass% is [Ti] and N mass% is [N], [Ti] -3.4 [N] is -0.003%. Or more, 0.03% or less, B: 0.0003% or more and 0.0050% or less, comprising the balance Fe and inevitable impurities, the main structure Bainite in that amount of 90% or more, and wherein the second phase fraction consisting of martensite and residual austenite is 3% or less.
上記高強度熱延鋼板は、必要に応じて、さらにＡｌ：０．００１％以上、１．０％以下、Ｃａ：０．０００１％以上、０．１％以下、Ｍｇ：０．０００１％以上、０．１％以下、Ｃｅ：０．０００１％以上、０．１％以下の１種又は２種以上を含み、又は／及びＣｒ：０．０１％以上、５．０％以下、Ｃｕ：０．０１％以上、５．０％以下、Ｎｉ：０．０１％以上、５．０％以下の１種又は２種以上を含むことができる。 The high-strength hot-rolled steel sheet, if necessary, is further Al: 0.001% or more, 1.0% or less, Ca: 0.0001% or more, 0.1% or less, Mg: 0.0001% or more, 0.1% or less, Ce: 0.0001% or more, including 0.1% or less, or 2 or more types, and / or Cr: 0.01% or more, 5.0% or less, Cu: 0.0. One or more of 01% or more and 5.0% or less, Ni: 0.01% or more and 5.0% or less can be included.
本発明によれば、比較的単純な成分系及びプロセスにより、９８０ＭＰａ以上の熱延鋼板で良好な伸びフランジ性を確保することができる。
本発明に係る熱延鋼板は、自動車や各種産業機械、構造物において、強度と加工性、特に伸びフランジ性が必要な部品の材料として好適に用いることができる。
According to the present invention, good stretch flangeability can be secured with a hot-rolled steel sheet of 980 MPa or more by a relatively simple component system and process.
The hot-rolled steel sheet according to the present invention can be suitably used as a material for parts that require strength and workability, particularly stretch flangeability, in automobiles, various industrial machines, and structures.
まず、本発明に係る熱延鋼板の組成及び組織限定理由について説明する。
・Ｃ：０．０３％以上、０．２０％以下
Ｃは強度を確保するために有効で、フェライト形成を抑制し、ベイナイト形成を促進する元素である。しかし、０．２０％を越えると強度が高くなりすぎて伸びフランジ性が劣化し、一方、０．０３％未満ではベイナイト分率が低下して必要な強度及び伸びフランジ性が得られない。好ましくは、０．０５％以上、０．１５％以下、さらに好ましくは０．０７％以上、０．１２％以下である。
・Ｓｉ：０．９％以上、３．０％以下
Ｓｉは固溶強化により、伸びフランジ性の低下を抑えつつ強化に寄与する元素である。しかし、３．０％を越えると強度が高くなり過ぎ、伸びフランジ性が劣化し、一方、０．９％未満であると十分な強度が得られない。好ましくは１．０％以上、２．５％以下、さらに好ましくは１．１％以上、２．０％以下である。
First, the composition of the hot-rolled steel sheet according to the present invention and the reason for limiting the structure will be described.
C: 0.03% or more and 0.20% or less C is an element effective for securing strength, suppresses ferrite formation, and promotes bainite formation. However, if it exceeds 0.20%, the strength becomes too high and the stretch flangeability deteriorates. On the other hand, if it is less than 0.03%, the bainite fraction decreases and the required strength and stretch flangeability cannot be obtained. Preferably, they are 0.05% or more and 0.15% or less, More preferably, they are 0.07% or more and 0.12% or less.
Si: 0.9% or more and 3.0% or less Si is an element that contributes to strengthening while suppressing a decrease in stretch flangeability by solid solution strengthening. However, if it exceeds 3.0%, the strength becomes too high and the stretch flangeability deteriorates. On the other hand, if it is less than 0.9%, sufficient strength cannot be obtained. Preferably they are 1.0% or more and 2.5% or less, More preferably, they are 1.1% or more and 2.0% or less.
・Ｍｎ：１．０％以上、３．０％以下
Ｍｎはフェライトの形成を抑制し、ベイナイトの形成を促進して強度向上及び伸びフランジ性向上に有効な元素である。しかし、３．０％を越えるとベイナイトの形成が抑制されてＭＡが形成され、伸びフランジ性が劣化し、一方、１．０％未満であるとフェライトが形成されて強度が得られず、伸びフランジ性も劣化する。好ましくは１．２％以上、２．７％以下、さらに好ましくは１．５％以上、２．４％以下である。
・Ｐ：０．０５％以下
Ｐは粒界に偏析し、粒界強度を低下させて粒界破壊を促進するため、伸びフランジ性が劣化する。従って、０．０５％以下に制限し、好ましくは０．０３％以下、さらに好ましくは０．０２％以下に制限する。
・Ｓ：０．０５％以下、
ＳはＭｎＳなどの硫化物を形成し、この硫化物が穴広げ試験時の破壊の起点となるため、伸びフランジ性が劣化する。従って、０．０５％以下に制限し、好ましくは０．０３％以下、さらに好ましくは０．０２％以下に制限する。
Mn: 1.0% or more, 3.0% or less Mn is an element effective in suppressing the formation of ferrite and promoting the formation of bainite to improve strength and stretch flangeability. However, if it exceeds 3.0%, the formation of bainite is suppressed and MA is formed, and the stretch flangeability deteriorates. On the other hand, if it is less than 1.0%, ferrite is formed and the strength cannot be obtained, and the elongation is not achieved. Flangeability also deteriorates. Preferably they are 1.2% or more and 2.7% or less, More preferably, they are 1.5% or more and 2.4% or less.
-P: 0.05% or less P segregates at the grain boundary, lowers the grain boundary strength and promotes grain boundary fracture, so the stretch flangeability deteriorates. Therefore, it is limited to 0.05% or less, preferably 0.03% or less, and more preferably 0.02% or less.
S: 0.05% or less,
S forms a sulfide such as MnS, and this sulfide serves as a starting point of fracture during the hole expansion test, so that the stretch flangeability deteriorates. Therefore, it is limited to 0.05% or less, preferably 0.03% or less, and more preferably 0.02% or less.
・Ｖ：０．０１０％以下
・Ｚｒ：０．０１０％以下
・Ｎｂ：０．００５％以下
・Ｍｏ：０．０１０％以下
・Ｔａ：０．０１０％以下
・Ｈｆ：０．０１０％以下
・Ｗ：０．０１０％以下、かつこれら７元素の合計が０．０２％以下
これらの元素は添加することでフェライトの形成を強く抑制し、ベイナイト形成を促進する作用がある。しかし、ベイナイトのラス間に微細なＭＡが形成され、これが穴広げ試験時の破壊の起点となり、伸びフランジ性を低下させる。そのため、良好な伸びフランジ性を得るうえでは、添加しない方がよい。添加する場合でも、Ｖ，Ｚｒ，Ｍｏ，Ｔａ，Ｈｆ，Ｗは個別にそれぞれ０．０１０％以下、Ｎｂは０．００５％以下、合計で０．０２％以下に抑える。好ましくは、Ｖ，Ｚｒ，Ｍｏ，Ｔａ，Ｈｆ，Ｗは個別にそれぞれ０．００５％未満、Ｎｂは０．００３％未満に抑え、合計で０．０１５％以下、さらに好ましくは合計０．０１０％以下に抑える。
V: 0.010% or less Zr: 0.010% or less Nb: 0.005% or less Mo: 0.010% or less Ta: 0.010% or less Hf: 0.010% or less W : 0.010% or less, and the total of these 7 elements is 0.02% or less By adding these elements, the formation of ferrite is strongly suppressed, and the action of promoting bainite formation is promoted. However, fine MA is formed between the laths of bainite, and this becomes a starting point of fracture during the hole expansion test, and the stretch flangeability is lowered. Therefore, in order to obtain good stretch flangeability, it is better not to add. Even when added, V, Zr, Mo, Ta, Hf, and W are individually controlled to 0.010% or less and Nb is 0.005% or less, and the total is suppressed to 0.02% or less. Preferably, V, Zr, Mo, Ta, Hf, and W are individually controlled to less than 0.005% and Nb to less than 0.003%, and the total is 0.015% or less, more preferably 0.010%. Keep it below.
Ｂ：０．０００３％以上、０．００５０％以下
Ｂはフェライト変態を強く抑制する元素であり、ベイナイト主体の組織を形成して伸びフランジ性を改善するうえで非常に有効な元素である。しかし、０．００５０％を越えるとフェライト変態抑制効果が得られなくなり、一方、０．０００３％未満であると、焼き入れ性改善効果が得られない。好ましくは０．０００５％以上、０．００４０％以下、さらに好ましくは０．０００５％以上、０．００３％以下である。
・Ｎ：０．０１０％以下
Ｎは不可避的に存在する元素である。固溶Ｎが存在すると、Ｂと結合してＢＮを形成し、Ｂ添加による焼き入れ性改善効果（フェライト形成抑制効果）を阻害するので、０．０１０％以下に低減する。好ましくは０．００８％以下、さらに好ましくは０．００６％以下である。
B: 0.0003% or more and 0.0050% or less B is an element that strongly suppresses ferrite transformation, and is an extremely effective element for improving stretch flangeability by forming a bainite-based structure. However, if it exceeds 0.0050%, the effect of suppressing ferrite transformation cannot be obtained, whereas if it is less than 0.0003%, the effect of improving hardenability cannot be obtained. Preferably they are 0.0005% or more and 0.0040% or less, More preferably, they are 0.0005% or more and 0.003% or less.
N: 0.010% or less N is an unavoidable element. When solid solution N exists, it combines with B to form BN and inhibits the hardenability improving effect (ferrite formation suppressing effect) by adding B, so it is reduced to 0.010% or less. Preferably it is 0.008% or less, More preferably, it is 0.006% or less.
・［Ｔｉ］−３．４［Ｎ］が−０．００３％以上、０．０３％以下
Ｔｉは不可避的に存在するＮをＴｉＮとして固定し、Ｂ添加による焼き入れ性改善効果を発現するために必要な元素である。一方、過剰に添加すると固溶ＴｉがＭＡの形成に寄与し、伸びフランジ性を劣化させるため、Ｎ量に合わせた添加量に調整する必要がある。［Ｔｉ］−３．４［Ｎ］が０．０３％を越えると、固溶Ｔｉが残存することでＭＡの形成が促進され伸びフランジ性が劣化し、一方、−０．００３％未満ではＴｉＮとならないＮが残存し、そのＮがＢと結合して、Ｂのフェライト形成抑制効果を阻害する。好ましい範囲は−０．００１％以上０．０２％以下、さらに好ましい範囲は０．０％以上、０．０１％以下である。
-[Ti] -3.4 [N] is -0.003% or more and 0.03% or less. Ti fixes unavoidable N as TiN and expresses the effect of improving hardenability by adding B. It is a necessary element. On the other hand, if added excessively, solute Ti contributes to the formation of MA and deteriorates the stretch flangeability. Therefore, it is necessary to adjust the addition amount according to the N amount. If [Ti] -3.4 [N] exceeds 0.03%, solid solution Ti remains, which accelerates the formation of MA and deteriorates stretch flangeability. N which does not become remains and the N binds to B, thereby inhibiting the ferrite formation suppressing effect of B. A preferred range is from -0.001% to 0.02%, and a more preferred range is from 0.0% to 0.01%.
・Ａｌ：０．００１％以上、１．０％以下
・Ｃａ：０．０００１％以上、０．１％以下
・Ｍｇ：０．０００１％以上、０．１％以下
・Ｃｅ：０．０００１％以上、０．１％以下
これらの元素は酸化物系介在物を微細化することで伸びフランジ性を改善する作用があるため、１種又は２種以上を必要に応じて添加する。しかし、上限値を越えると効果が飽和し、下限値未満であると効果が得られない。
-Al: 0.001% or more, 1.0% or less-Ca: 0.0001% or more, 0.1% or less-Mg: 0.0001% or more, 0.1% or less-Ce: 0.0001% or more 0.1% or less Since these elements have the effect of improving stretch flangeability by refining oxide inclusions, one or more elements are added as necessary. However, if the upper limit is exceeded, the effect is saturated, and if it is less than the lower limit, the effect cannot be obtained.
・Ｃｒ：０．０１％以上、５．０％以下
・Ｃｕ：０．０１％以上、５．０％以下
・Ｎｉ：０．０１％以上、５．０％以下
これらの元素はフェライト形成を抑制し、ベイナイト組織形成に寄与するため、１種又は２種以上を必要に応じて添加する。しかし、上限値を越えると効果が飽和し、下限値未満であると効果が得られない。
・ Cr: 0.01% or more, 5.0% or less ・ Cu: 0.01% or more, 5.0% or less ・ Ni: 0.01% or more, 5.0% or less These elements suppress ferrite formation And in order to contribute to bainite structure formation, 1 type (s) or 2 or more types are added as needed. However, if the upper limit is exceeded, the effect is saturated, and if it is less than the lower limit, the effect cannot be obtained.
・ベイナイト分率：９０％以上
・ＭＡ分率：３％以下
ベイナイトは均質な組織であるため、ベイナイト主体組織になると亀裂の発生起点が少なくなり、伸びフランジ性が改善する。ベイナイト分率（面積分率）が９０％未満では組織が不均一になるため伸びフランジ性が劣化する。好ましくは９２％以上、さらに好ましくは９５％以上である。理想的には１００％ベイナイト組織になることである。
硬質なＭＡは穴広げ試験時に亀裂の発生サイトとなり、伸びフランジ性を劣化させるので、ＭＡ分率は低下させることが望ましい。ＭＡ分率（面積分率）が３％以下であれば伸びフランジ性の劣化が抑えられる。好ましくは２％以下、さらに好ましくは１％以下である。
主相であるベイナイト及び第２相のＭＡ以外の組織（主としてフェライト及び少量のパーライト）は少ないことが望ましい。これは、硬質相との界面に歪みが集中し、界面が割れの発生起点になるためである。面積分率は好ましくは５％以下、さらに好ましくは３％以下である。
・ Bainite fraction: 90% or more ・ MA fraction: 3% or less Since bainite is a homogeneous structure, when it becomes a bainite-based structure, the starting point of cracks is reduced and stretch flangeability is improved. If the bainite fraction (area fraction) is less than 90%, the structure becomes non-uniform and the stretch flangeability deteriorates. Preferably it is 92% or more, More preferably, it is 95% or more. Ideally, it should be a 100% bainite structure.
Since hard MA becomes a crack generation site during the hole expansion test and deteriorates stretch flangeability, it is desirable to reduce the MA fraction. If the MA fraction (area fraction) is 3% or less, deterioration of stretch flangeability can be suppressed. Preferably it is 2% or less, More preferably, it is 1% or less.
It is desirable that the structure (mainly ferrite and a small amount of pearlite) other than the main phase bainite and the second phase MA is small. This is because strain concentrates on the interface with the hard phase, and the interface becomes the starting point of cracking. The area fraction is preferably 5% or less, more preferably 3% or less.
次に、本発明に係る熱延鋼板の製造方法について説明する。
好ましい製造方法は、図１に示すように、鋼素材を加熱した後、仕上げ圧延を含む熱間圧延、熱延後の急冷、急冷停止、巻き取りである。以下、各工程について説明する。
・熱間圧延前の加熱
熱間圧延前の加熱は、限定的ではないが１０００℃以上、１２５０℃以下で、３０分以上行う。この加熱によりオーステナイト単相とする。
・熱間圧延
熱間圧延は、仕上げ温度が８５０℃以上、１０００℃以下の範囲になるように行う。この範囲はオーステナイト単相域である。仕上げ温度が１０００℃を越えると、オーステナイトが粗大化して焼き入れ性が高まり、マルテンサイトが形成される。
Next, the manufacturing method of the hot rolled steel sheet according to the present invention will be described.
As shown in FIG. 1, the preferred manufacturing method is hot rolling including finish rolling, rapid cooling after hot rolling, rapid cooling stop, and winding after heating the steel material. Hereinafter, each step will be described.
-Heating before hot rolling Although heating is not limited, it is performed at 1000 ° C or higher and 1250 ° C or lower for 30 minutes or longer. An austenite single phase is obtained by this heating.
-Hot rolling Hot rolling is performed so that the finishing temperature is in the range of 850 ° C or higher and 1000 ° C or lower. This range is the austenite single phase region. When the finishing temperature exceeds 1000 ° C., austenite is coarsened, the hardenability is increased, and martensite is formed.
・熱延後の急冷
熱延後の急冷は停止温度（ベイナイト変態が起こる温度域）まで２０℃／ｓ以上で急冷する。フェライト変態が起こる温度域を急速に冷却することで、フェライト形成を抑制する。この冷却速度は速いことが望ましいが、速すぎると制御が困難となるため、好ましくは１５０℃／ｓ未満、さらに好ましくは１２０℃／ｓ未満とする。
・急冷停止、巻き取り
急冷停止温度は３００℃以上、５００℃以下とする。この温度域はベイナイト変態が起こる温度域であり、この温度域に保持することによりベイナイト主体の組織が得られる。停止温度が５００℃を越えると、パーライト変態が発生して、ベイナイト分率が低下し、強度が低下したり、伸びフランジ性が劣化する。停止温度が３００℃未満であると、マルテンサイト変態が発生して、ベイナイト分率が低下し、伸びフランジ性が劣化する。
-Rapid cooling after hot rolling Rapid cooling after hot rolling is performed at a rate of 20 ° C / s or higher until the stop temperature (temperature range where bainite transformation occurs). The formation of ferrite is suppressed by rapidly cooling the temperature range where ferrite transformation occurs. Although it is desirable that this cooling rate is high, control becomes difficult if it is too fast, so it is preferably less than 150 ° C./s, more preferably less than 120 ° C./s.
-Rapid cooling stop and winding The rapid cooling stop temperature should be 300 ° C or higher and 500 ° C or lower. This temperature range is a temperature range where bainite transformation occurs, and a bainite-based structure can be obtained by maintaining in this temperature range. When the stop temperature exceeds 500 ° C., pearlite transformation occurs, the bainite fraction decreases, the strength decreases, and the stretch flangeability deteriorates. When the stop temperature is less than 300 ° C., martensitic transformation occurs, the bainite fraction decreases, and stretch flangeability deteriorates.
表１，２に示す成分の鋳塊を溶製し、熱間圧延により２５ｍｍ厚の板材とし、そこから２５ｍｍ×２００ｍｍ×１２０ｍｍのスラブを切り出し、１１５０℃で３０分加熱した後、仕上げ温度が８８０℃になるように熱間圧延を施し、厚さ３ｍｍに仕上げた後、停止温度４５０℃までを平均冷却速度５０℃／ｓで冷却した。４５０℃まで冷却後、巻き取りを模擬するため、４５０℃×３０分の保持を行った後、炉冷した。 Ingots having the components shown in Tables 1 and 2 are melted to form a plate having a thickness of 25 mm by hot rolling, and a 25 mm × 200 mm × 120 mm slab is cut out therefrom and heated at 1150 ° C. for 30 minutes, and then the finishing temperature is 880. Hot rolling was performed so that the temperature became 3 ° C., and after finishing to a thickness of 3 mm, the cooling was performed at an average cooling rate of 50 ° C./s up to a stop temperature of 450 ° C. After cooling to 450 ° C., in order to simulate winding, holding was performed at 450 ° C. for 30 minutes, followed by furnace cooling.
得られた熱延鋼板からサンプルを採取し、組織観察、引張試験、及び穴広げ試験を下記要領で実施した。
・組織観察１（ベイナイト分率）
鋼板中心部のＴＤ面の組織を観察した。サンプルを鏡面に研磨し、３％ナイタールで腐食後、光学顕微鏡で４００倍で５視野（約３０，０００μｍ２／視野）の観察を行い、ポイントカウンティング法（各視野毎に均等なメッシュで１００ポイント）でベイナイト分率を求めた。
・組織観察２（ＭＡ分率）
同じくレペラ試薬で腐食後、光学顕微鏡で１０００倍で５視野（約５，０００μｍ２／視野）の観察を行い、白い領域をＭＡとして画像解析を行い、当該組織の分率を求めた。
なお、組織観察２で観察するＭＡはベイナイトのラス間に存在する微細組織であり、組織観察１で得られるベイナイト分率はこのＭＡを含む。このため、組織観察１で得られるベイナイト分率と組織観察２で得られるＭＡ分率を足すと１００％を越える場合がある。
A sample was taken from the obtained hot-rolled steel sheet, and a structure observation, a tensile test, and a hole expansion test were performed as follows.
・ Structural observation 1 (Bainite fraction)
The structure of the TD surface at the center of the steel plate was observed. After polishing the sample to a mirror surface and corroding with 3% nital, observe 5 fields (approximately 30,000 μm 2 / field) at 400 times with an optical microscope and point counting method (100 points with uniform mesh for each field) ) To obtain the bainite fraction.
・ Tissue observation 2 (MA fraction)
Similarly, after corroding with the repeller reagent, the optical microscope was used to observe five visual fields (approximately 5,000 μm 2 / visual field) at 1000 times, and image analysis was performed using the white area as MA to obtain the fraction of the tissue.
The MA observed in the
・引張試験
引張試験は、サンプルをＪＩＳＺ２２０１記載の５号試験片に加工し、ＪＩＳＺ２２４１記載の引張試験方法に従い、引張強度を測定した。
・穴広げ試験
伸びフランジ性は穴広げ試験で評価した。穴広げ試験は、日本鉄鋼連盟規格ＪＦＳＴ１００１に従って行い、穴広げ率（λ）を測定した。
-Tensile test The tensile test processed the sample into the No. 5 test piece of JISZ2201, and measured the tensile strength according to the tensile test method of JISZ2241.
-Hole expansion test Stretch flangeability was evaluated by a hole expansion test. The hole expansion test was performed according to the Japan Iron and Steel Federation standard JFST1001, and the hole expansion ratio (λ) was measured.
測定結果を表３に示す。表３において、引張強度は９８０ＭＰａ以上を良好、１０００ＭＰａ以上をさらに良好と評価し、穴広げ率は５０％以上を良好、６５％以上をさらに良好と評価した。 Table 3 shows the measurement results. In Table 3, it was evaluated that the tensile strength was 980 MPa or higher and 1000 MPa or higher was better, and the hole expansion ratio was 50% or higher and 65% or higher was further improved.
表３の測定結果を以下簡単に説明する。
Ｎｏ．１，２，４，５，７，１１，１２，２１〜２８，３０，３３は、クレーム記載の組成、ベイナイト分率及びＭＡ分率の各要件を満たし、良好な引張強度及び穴広げ率を示す。
The measurement results in Table 3 will be briefly described below.
No. 1,2,4,5,7,11,12,21-28,30,33 satisfy each requirement of composition, bainite fraction and MA fraction described in claims, and have good tensile strength and hole expansion ratio Show.
一方、Ｎｏ．３はＣ含有量が不足し、フェライトが形成されてベイナイト分率が低く、引張強度及び穴広げ率が劣り、Ｎｏ．６はＣ含有量が過剰で、穴広げ率が劣る。
Ｎｏ．８はＳｉ含有量が過剰で、穴広げ率が劣り、Ｎｏ．９はＳｉ含有量が不足し、引張強度が劣る。
Ｎｏ．１０はＭｎ含有量が不足してフェライトが形成されてベイナイト分率が低く、引張強度及び穴広げ率が劣り、Ｎｏ．１３はＭｎ含有量が過剰で、ベイナイト分率が低下してＭＡ分率が高く、穴広げ率が劣る。
Ｎｏ．１４〜２０はＶ，Ｚｒ，Ｎｂ，Ｍｏ，Ｔａ，Ｈｆ，Ｗの１種又は２種以上の含有量が過剰で、ＭＡ分率が高く、穴広げ率が劣る。
Ｎｏ．２９はＢ含有量が不足し、フェライト抑制効果が不足してベイナイト分率が低く、強度及び穴広げ率が劣り、Ｎｏ．３１はＢ含有量が過剰でフェライト変態を抑制できず、ベイナイト分率が低下し、穴広げ率が劣る。
Ｎｏ．３２は［Ｔｉ］−３．４［Ｎ］が低すぎるため、フェライト変態が抑制できず、ベイナイト分率が低下し、引張強度及び穴広げ率が劣り、Ｎｏ．３４は［Ｔｉ］−３．４［Ｎ］が高すぎるためＭＡ分率が高く、穴広げ率が劣る。
On the other hand, no. No. 3 has insufficient C content, ferrite is formed, the bainite fraction is low, the tensile strength and the hole expansion ratio are inferior, No. 6 has an excessive C content and a poor hole expansion rate.
No. No. 8 has an excessive Si content and a poor hole expansion rate. No. 9 has insufficient Si content and inferior tensile strength.
No. No. 10 has insufficient Mn content, ferrite is formed, the bainite fraction is low, the tensile strength and the hole expansion ratio are inferior, No. 13 has an excessive Mn content, the bainite fraction is lowered, the MA fraction is high, and the hole expansion ratio is inferior.
No. Nos. 14 to 20 have an excessive content of one or more of V, Zr, Nb, Mo, Ta, Hf, and W, a high MA fraction, and a poor hole expansion rate.
No. No. 29 has insufficient B content, ferrite suppression effect is insufficient, bainite fraction is low, strength and hole expansion ratio are inferior, No. 31 has an excessive B content and cannot suppress ferrite transformation, the bainite fraction is lowered, and the hole expansion rate is inferior.
No. No. 32 [Ti] -3.4 [N] is too low, ferrite transformation cannot be suppressed, the bainite fraction is lowered, the tensile strength and the hole expansion ratio are inferior. Since [Ti] -3.4 [N] is too high, the MA fraction is high and the hole expansion rate is inferior.
Ｎｏ．２と、ＭＡが形成されたＮｏ．１４〜２２，３２，３３について、穴広げ率とＭＡ分率をプロットしたのが図２である。図２に示すように、ＭＡ分率が３％以下のとき、５０％以上の穴広げ率が得られている。 No. 2 and No. in which MA was formed. FIG. 2 is a plot of the hole expansion ratio and the MA fraction for 14 to 22, 32, and 33. As shown in FIG. 2, when the MA fraction is 3% or less, a hole expansion ratio of 50% or more is obtained.
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Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH0598353A (en) * | 1991-10-02 | 1993-04-20 | Sumitomo Metal Ind Ltd | Production of hot rolled high tensile strength steel plate excellent in spreadability |
| JPH1180890A (en) * | 1997-09-04 | 1999-03-26 | Kobe Steel Ltd | High strength hot rolled steel plate and its production |
| JPH11100639A (en) * | 1997-09-24 | 1999-04-13 | Nippon Steel Corp | Good workability and high strength hot rolled steel sheet having high dynamic deformation resistance and its production |
| JP2000109951A (en) * | 1998-08-05 | 2000-04-18 | Kawasaki Steel Corp | High strength hot rolled steel sheet excellent in stretch-flanging property and its production |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH0598353A (en) * | 1991-10-02 | 1993-04-20 | Sumitomo Metal Ind Ltd | Production of hot rolled high tensile strength steel plate excellent in spreadability |
| JPH1180890A (en) * | 1997-09-04 | 1999-03-26 | Kobe Steel Ltd | High strength hot rolled steel plate and its production |
| JPH11100639A (en) * | 1997-09-24 | 1999-04-13 | Nippon Steel Corp | Good workability and high strength hot rolled steel sheet having high dynamic deformation resistance and its production |
| JP2000109951A (en) * | 1998-08-05 | 2000-04-18 | Kawasaki Steel Corp | High strength hot rolled steel sheet excellent in stretch-flanging property and its production |
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