JP4466352B2 - Hot rolled steel sheet suitable for warm forming and manufacturing method thereof - Google Patents
Hot rolled steel sheet suitable for warm forming and manufacturing method thereof Download PDFInfo
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本発明は、主として室温での加工が困難な自動車用超高強度部材等の使途に好適な、温間成形に適した熱延鋼板およびその製造方法に関するものである。 The present invention relates to a hot-rolled steel sheet suitable for warm forming, which is suitable for the use of an ultra-high-strength member for automobiles that is difficult to process at room temperature, and a method for manufacturing the hot-rolled steel sheet.
近年、地球環境保全という観点から、自動車の燃費改善が要求されている。また、車両衝突時に乗員を保護する観点からは、自動車車体の安全性向上も要求されている。このため、燃費改善と安全性向上の両方を満足するべく自動車車体の軽量化と強化の双方を図る検討が積極的に進められている。自動車車体の軽量化と強化を同時に満足させるには、部品素材を高強度化かつ薄肉化することが効果的であり、最近では引張強さ590MPa以上の高張力薄鋼板が自動車構造部品に使用され始めている。 In recent years, there has been a demand for improving fuel efficiency of automobiles from the viewpoint of global environmental conservation. Further, from the viewpoint of protecting occupants in the event of a vehicle collision, it is also required to improve the safety of the automobile body. For this reason, in order to satisfy both the improvement in fuel efficiency and the improvement in safety, studies to reduce and strengthen the automobile body are being actively promoted. In order to satisfy the weight reduction and strengthening of automobile bodies at the same time, it is effective to increase the strength and thickness of component materials. Recently, high-tensile steel sheets with a tensile strength of 590 MPa or more have been used for automotive structural parts. I'm starting.
しかしながら、引張強さ590MPa以上の高張力薄鋼板は、従来の室温でのプレス加工による成形方法では成形荷重が大きいため、プレス機への負荷が大きいほか、成形時の割れ・しわの問題や、スプリングバックが大きく所定形状への成形が困難であるといった問題がある。 However, the high-tensile steel sheet with a tensile strength of 590 MPa or more has a large forming load in the conventional forming method by press working at room temperature, so the load on the press machine is large, as well as cracking and wrinkling problems during forming, There is a problem that the spring back is large and it is difficult to mold into a predetermined shape.
これらの問題に対して、例えば特許文献１には、成形時には軟質であり成形後の熱処理により高強度化する、後熱処理型の熱延鋼板の製造方法が開示されている。この製造方法では、Cu:0.8〜2.0%を含有させた鋼板を仕上圧延後、10℃/s以上の冷却速度で500℃以下まで冷却し巻き取ることによって、熱延鋼板中に固溶状態で存在させておいたCuを加工後熱処理時に析出させ最大19kgf/mm2の引張強さの上昇を得ている。 To deal with these problems, for example, Patent Document 1 discloses a method for producing a post-heat treatment type hot-rolled steel sheet that is soft at the time of forming and is strengthened by heat treatment after forming. In this manufacturing method, a steel sheet containing Cu: 0.8 to 2.0% is finish-rolled, and then cooled to 500 ° C. or less at a cooling rate of 10 ° C./s or more, and wound in a solid solution state in the hot-rolled steel sheet. The existing Cu was deposited during the heat treatment after processing, and the maximum tensile strength increase of 19 kgf / mm 2 was obtained.
また、高強度鋼板の成形性を改善する方法として、温間成形方法が提案されている。例えば、特許文献２には、C:0.03〜0.2%、Si：0.5%以下、Mn：1〜3%、P：0.1%以下、S：0.1%以下、Cr：0.01〜1%、Al：0.01〜0.1%、N：0.02%以下を含有させた鋼板を、マルテンサイト相の体積率が10%以上80%以下であり、かつマルテンサイト相の平均径が8μm以下となる組織とすることで、室温における引張強さに対する450℃における引張強さの比が0.6以下である高張力鋼板が開示されている。また、特許文献２では、温間で成形した際に、強度を上昇させる要因となっているマルテンサイト相自体の変形抵抗が低下するため、他の方法で強化した鋼に比べて温間での引張強さの低下が大きいとしている。 Further, a warm forming method has been proposed as a method for improving the formability of a high-strength steel plate. For example, in Patent Document 2, C: 0.03 to 0.2%, Si: 0.5% or less, Mn: 1 to 3%, P: 0.1% or less, S: 0.1% or less, Cr: 0.01 to 1%, Al: 0.01 By making the steel sheet containing ~ 0.1%, N: 0.02% or less, the volume ratio of the martensite phase is 10% or more and 80% or less, and the average diameter of the martensite phase is 8 μm or less, A high-tensile steel sheet is disclosed in which the ratio of the tensile strength at 450 ° C. to the tensile strength at room temperature is 0.6 or less. Moreover, in patent document 2, since the deformation resistance of the martensite phase itself which becomes a factor which raises intensity | strength falls at the time of shape | molding warm, compared with the steel strengthened by the other method, it is warm. The drop in tensile strength is said to be large.
上記以外に成形性を改善すると同時に部材を高強度化する方法として、近年、熱間成形（ホットプレス）が注目されている。この方法は、鋼板をAc3変態点以上の高温に加熱した状態で成形することにより成形性の問題を解決し、成形後の冷却速度を制御しマルテンサイトを主体とする低温変態相とすることにより980MPaを超える高強度を得ようとするものである。例えば、特許文献３にはC:0.18〜0.25%、Si:0.15〜0.35%、Mn:1.15〜1.40%、Cr:0.15〜0.25%、Ti:0.01〜0.03%を含み残部がFeおよび不可避的不純物から成る薄鋼板を熱間成形する車輛用衝突補強材の製造方法および車輛用衝突補強材が提案されている。また、特許文献３では、主として熱間成形条件を制御することで引張強さ1500MPa程度が得られ、スプリングバックの回避にも成功している。
しかしながら特許文献１に記載された技術では、成形時の強度と成形-熱処理後の強度の差がたかだか200MPaであり、780MPa以上の高強度鋼板を得ようとする場合、室温での成形荷重が大きくなってしまうといった問題点がある。また、Cu含有を必須としており、Cu添加鋼の熱間割れ回避のためのNi添加によるコストアップが避けられないといった問題点がある。さらには、Cu添加は鋼のリサイクルの観点からも好ましくない。 However, in the technique described in Patent Document 1, the difference between the strength at the time of forming and the strength after forming and heat treatment is at most 200 MPa, and when trying to obtain a high strength steel plate of 780 MPa or more, the forming load at room temperature is large. There is a problem of becoming. Moreover, Cu content is essential, and there is a problem that the cost increase due to Ni addition for avoiding hot cracking of Cu-added steel cannot be avoided. Furthermore, Cu addition is not preferable from the viewpoint of recycling steel.
特許文献２に記載された技術では、温間成形時に低強度とすることは可能であるが、温間成形後の強度も低下してしまうという問題点がある。 With the technique described in Patent Document 2, it is possible to reduce the strength at the time of warm forming, but there is a problem in that the strength after warm forming also decreases.
特許文献３に記載された技術では、A3点以上の高温に加熱する必要があるため、スケールが多量に発生し、成形後にショットブラスト等の手入れが必要となるほか、成形部材の組織がマルテンサイト単相組織となるため、延性に乏しく、適用可能部材が限られるといった問題がある。 In the technique described in Patent Document 3, since it is necessary to heat to a high temperature of A3 or higher, a large amount of scale is generated, and after molding, maintenance such as shot blasting is required, and the structure of the molded member is martensite. Since it becomes a single phase structure, there is a problem that ductility is poor and applicable members are limited.
以上のように、プレス成形時には軟質であり、プレス成形後の部材強度が高強度となる鋼板を得る手法としては、従来から１）Cuの後熱処理析出、２）熱間成形（ホットプレス）等の方法が提案されてきたが、前者１）はCu添加によるコストアップ、Cu添加によるスラブの熱間割れ回避のためのNi添加によるコストアップ、および鋼のリサイクルの観点から好ましくなく、後者２）は成形後の表面手入れによるコストアップ、および鋼組織がマルテンサイト単相組織となるために部材延性に乏しいといった問題点があり、いずれもユーザーのニーズを満足するものではなかった。 As described above, as a technique for obtaining a steel sheet that is soft at the time of press forming and has a high strength after press forming, conventionally, 1) Cu post-heat treatment precipitation, 2) hot forming (hot pressing), etc. However, the former 1) is not preferable from the viewpoints of cost increase due to Cu addition, cost increase due to Ni addition to avoid hot cracking of slab due to Cu addition, and steel recycling. The latter 2) Has problems such as cost increase due to surface care after forming and poor steel ductility due to the steel structure becoming a martensite single phase structure, and none of them satisfied user needs.
以上より、本発明は上記問題点を解決するためになされたもので、温間成形時には低強度であり、温間成形後の部材強度が高強度となる温間成形に適した熱延鋼板およびその製造方法を提供することを目的とする。 As described above, the present invention has been made to solve the above problems, and has a low strength at the time of warm forming, and a hot rolled steel sheet suitable for warm forming in which the strength of the member after warm forming is high and It aims at providing the manufacturing method.
本発明者らは、上記の課題を解決すべく、鋭意研究した。その結果、熱間成形（ホットプレス）に比べ比較的低温でスケール生成の少ない300℃以上A1点以下での加工（温間成形）によって成形強度を低下させ、さらに温間成形中にTi、Nbの合金炭化物を析出させることによって温間成形部材強度を高強度化することができると考えるに至り、そして、従来、熱延鋼板中に析出させ、鋼板を高強度化するために添加されていたTi、Nb等の炭化物生成元素を、熱延鋼板中に析出させず、逆に積極的に固溶させることにより、温間成形時には低強度であり、温間成形後の部材強度が高強度となる薄鋼板が得られることを見いだした。さらに、従来、Ti、Nbは強炭化物生成元素であるため熱延鋼板中に固溶させることが困難とされてきたが、熱間圧延時のスラブ加熱温度、仕上げ圧延温度、仕上げ圧延後の冷却速度、巻取り温度等をト−タルで規定することにより、熱延鋼板中にTi、Nbを固溶させうることが可能となることをも見出した。 The present inventors have intensively studied to solve the above problems. As a result, hot-forming to reduce the molding strength by machining (warm molding) under the following relatively low temperature 300 ° C. or higher A 1 point less scale formation than in (hot pressing), in addition warm forming Ti, It has been thought that the strength of warm-formed members can be increased by precipitating Nb alloy carbide, and it has been conventionally added to increase the strength of steel sheets by precipitation in hot-rolled steel sheets. In addition, by virtue of aggressive solid solution instead of precipitating carbide forming elements such as Ti and Nb in the hot-rolled steel sheet, the strength is low during warm forming, and the strength of the member after warm forming is high. It was found that a thin steel sheet can be obtained. In addition, Ti and Nb have been considered to be difficult to dissolve in hot-rolled steel sheets because Ti and Nb are strong carbide-forming elements, but slab heating temperature during hot rolling, finish rolling temperature, and cooling after finish rolling. It has also been found that Ti and Nb can be dissolved in the hot-rolled steel sheet by prescribing the speed, coiling temperature and the like in total.
本発明は、以上の知見に基づきなされたもので、その要旨は以下のとおりである。 The present invention has been made based on the above findings, and the gist thereof is as follows.
［１］mass%で、C：0.020〜0.20%、Si：1.5%以下、Mn ：0.50〜3.0%、P： 0.10%以下、S ：0.01%以下、Al：0.01〜0.5%、N：0.005%以下を含み、かつ、Ti、Nbの一種又は二種を合計で0.10〜0.50%含有し、残部がFeおよび不可避的不純物からなり、さらに前記Tiおよび前記Nbの合計含有量の60%以上が固溶状態であること特徴とする温間成形に適した熱延鋼板。 [1] In mass%, C: 0.020 to 0.20%, Si: 1.5% or less, Mn: 0.50 to 3.0%, P: 0.10% or less, S: 0.01% or less, Al: 0.01 to 0.5%, N: 0.005% And a total of one or two of Ti and Nb in a range of 0.10 to 0.50%, the balance is made of Fe and inevitable impurities, and more than 60% of the total content of Ti and Nb is solid. A hot-rolled steel sheet suitable for warm forming characterized by being in a molten state.
［２］上記［１］において、さらに、mass％で、V：0.01〜1.0%、Mo：0.01〜1.0%、Cr：0.01〜1.0%の一種または二種以上を含有することを特徴とする温間成形に適した熱延鋼板。 [2] In the above [1], the temperature further includes at least one of mass%, V: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cr: 0.01 to 1.0%. Hot rolled steel sheet suitable for hot forming.
［３］上記［１］または［２］において、マルテンサイト相を体積率で20％以下、残部はベイナイトを主体とする組織からなることを特徴とする温間成形に適した熱延鋼板。 [3] A hot-rolled steel sheet suitable for warm forming, characterized in that, in [1] or [2] above, the martensite phase has a volume ratio of 20% or less, and the balance is composed mainly of bainite.
［４］上記［１］または［２］に記載の組成を有するスラブを、鋳造後、再加熱することなく若しくは1200℃以上に再加熱した後、熱間圧延し、該熱間圧延では粗圧延を経ることなく若しくは粗圧延した後、仕上げ圧延温度：900℃以上で仕上げ圧延を行い、次いで、平均冷却速度: 50℃/s以上で600℃以下まで急冷し、次いで、巻取り温度：520℃以下で巻取ることを特徴とする温間成形に適した熱延鋼板の製造方法。 [4] The slab having the composition described in [1] or [2] is hot-rolled after casting without being reheated or reheated to 1200 ° C. or higher, and in the hot rolling, rough rolling is performed. Or after rough rolling, finish rolling is performed at a finish rolling temperature of 900 ° C. or higher, then rapidly cooled to an average cooling rate of 50 ° C./s to 600 ° C. or lower, and then a winding temperature of 520 ° C. A method for producing a hot-rolled steel sheet suitable for warm forming, characterized by winding in the following.
［５］上記［１］または［２］に記載の組成を有するスラブを、鋳造後、再加熱することなく若しくは1200℃以上に再加熱した後、熱間圧延し、該熱間圧延では粗圧延を経ることなく若しくは粗圧延した後、仕上げ圧延温度：900℃以上で仕上げ圧延を行い、次いで、平均冷却速度: 50℃/s以上で600〜350℃まで急冷し、次いで、巻取り温度：520〜350℃で巻取ることを特徴とする温間成形に適した熱延鋼板の製造方法。 [5] The slab having the composition described in [1] or [2] is hot-rolled after casting without being reheated or reheated to 1200 ° C. or more, and in the hot rolling, rough rolling is performed. Or after rough rolling, finish rolling is performed at a finish rolling temperature of 900 ° C. or more, then rapidly cooled to 600 to 350 ° C. at an average cooling rate of 50 ° C./s or more, and then a winding temperature of 520 A method for producing a hot-rolled steel sheet suitable for warm forming, characterized by winding at ~ 350 ° C.
なお、本明細書において、鋼の成分を示す%は、すべてmass%である。また、本発明において、「温間成形」とは、300℃以上A1点以下の温度で部材形状に成形加工することである。また、本発明において、鋼板とは、鋼板、鋼帯を含むものとする。 In addition, in this specification,% which shows the component of steel is all mass%. In the present invention, “warm forming” means forming into a member shape at a temperature of 300 ° C. or higher and A1 point or lower. Moreover, in this invention, a steel plate shall include a steel plate and a steel strip.
本発明によれば、温間成形時には低強度であり、温間成形後の部材強度が高強度となる温間成形に適した熱延鋼板が得られる。そして、熱間成形（ホットプレス）に比べ比較的低温でスケール生成の少ない300℃以上A1点以下での温間成形を行うことにより、成形時の強度を成形後の部材強度の0.60以下とすることができ、室温での加工が困難な自動車用超高強度部材等を容易に得ることができる。さらに、本発明の熱延鋼板は、成形後の組織が延性の乏しいマルテンサイト単相組織とならないため、部材延性が要求される広範囲な部材にも適用可能である。 According to the present invention, it is possible to obtain a hot-rolled steel sheet that has low strength during warm forming and is suitable for warm forming in which the strength of the member after warm forming is high. By performing the warm molding in the following hot-forming at a relatively low temperature 300 ° C. or higher A 1 point less scale formation than in (hot press), 0.60 parts strength after molding strength at the time of molding or less and Therefore, it is possible to easily obtain an ultra-high strength member for automobiles that is difficult to process at room temperature. Furthermore, the hot-rolled steel sheet of the present invention is applicable to a wide range of members that require member ductility because the structure after forming does not become a martensite single-phase structure with poor ductility.
本発明の熱延鋼板は、下記に示す成分に規定し、TiおよびNbの合計含有量の60%以上を固溶状態と規定したことを特徴とする。そして、マルテンサイト相を体積率で20％以下、残部はベイナイトを主体とする組織とすることを第二の特徴とする。これらは本発明において最も重要な要件であり、上記のように成分、固溶状態、さらには組織を最適化することにより、温間成形に適した熱延鋼板を得ることができる。また、上記熱延鋼板は、鋳造後、再加熱することなく若しくは1200℃以上に再加熱した後、熱間圧延し、該熱間圧延では粗圧延を経ることなく若しくは粗圧延した後、仕上げ圧延温度：900℃以上で仕上げ圧延を行い、次いで、平均冷却速度50℃/s以上で600℃以下まで急冷し、次いで、巻取り温度：520℃以下で巻取ることにより製造が可能となる。 The hot-rolled steel sheet of the present invention is characterized in that it is defined as the following components, and 60% or more of the total content of Ti and Nb is defined as a solid solution state. The second feature is that the martensite phase has a volume ratio of 20% or less and the balance is mainly bainite. These are the most important requirements in the present invention, and a hot-rolled steel sheet suitable for warm forming can be obtained by optimizing the components, the solid solution state, and the structure as described above. Further, the hot-rolled steel sheet is subjected to hot rolling after casting or after reheating to 1200 ° C. or higher without being reheated, and after hot rolling without rough rolling or rough rolling, and then finish rolling. Production is possible by performing finish rolling at a temperature of 900 ° C. or higher, then rapidly cooling to 600 ° C. or lower at an average cooling rate of 50 ° C./s or higher, and then winding at a winding temperature of 520 ° C. or lower.
以下、本発明を詳細に説明する。 Hereinafter, the present invention will be described in detail.
まず、本発明における鋼の化学成分（組成）の限定理由は以下の通りである。
C：0.020〜0.20%
Cは鋼板の強度増加や温間成形中の炭化物生成の観点から、本発明において最も重要な元素の一つであり、本発明では温間成形後に目的とする強度を確保するために0.020%以上の含有を必要とする。一方、0.20%を越える含有は、熱延板へのTi、Nbを含む炭窒化物の固溶化を困難にし、さらには溶接性を著しく劣化させる。以上より、Cは0.020％以上0.20%以下、好ましくは、0.040以上0.15％以下とする。
Si:1.5％以下
Siは鋼の延性を顕著に低下させることなく、鋼板を高強度化させることができる有用な強化元素であり、鋼板の強度レベルに応じて添加してもよく、このような効果を得るには0.01%以上の添加するのが好ましい。しかしながら、特に、高い表面美麗性や耐食性を要求される自動車用鋼板の場合、1.5%を越えるSiの含有は、表面性状、化成処理性等に悪影響を与えるうえ、これらの悪影響を抑制するために鋼板表面の酸洗処理を長時間にわたり行わなければならない等、大きなコストアップを招く。以上より、Siは1.5%以下とする。より優れた表面美麗性、耐食性が求められる用途では好ましくは0.5%以下、より好ましくは0.25%以下である。
Mn：0.50〜3.0%以下
Mnは、オーステナイトを安定化し、フェライト変態を遅延させる元素であり、熱間圧延時にフェライト変態と同時に起こるTiおよびNbの炭化物の析出を安定して抑制する。このような効果を得るためには0.50%以上の添加が必要である。一方、3.0%を越えるMnの含有は上記した効果が飽和するだけでなく、温間成形時の強度が増大する。以上より、Mnは0.50％以上3.0%以下、好ましくは1.0％以上2.5％以下とする。
Ｐ：0.10％以下
Ｐは鋼を強化する作用があり、鋼板の強度レベルに応じて添加してもよく、このような効果を得るには0.005%以上添加するのが好ましい。一方、Ｐ含有量が0.10％を超えると、溶接性が劣化する。以上より、Ｐは0.10％以下である。また、より優れた溶接性が要求される場合には、Ｐは0.05％以下が好ましい。
Ｓ：0.01％以下
Ｓは、鋼板中では介在物として存在し、溶接性の劣化をもたらすだけでなく、Sを含む粗大介在物は自動車衝突時に鋼板の破壊の起点となり、衝突の衝撃を十分に吸収することなく鋼板が破断する恐れがある。そのため、自動車用構造部材としてはできるだけ低減するのが好ましく、0.01％以下であればこれらの悪影響が無視できることから、本発明ではＳは0.01％以下とする。また、より優れた溶接性や衝撃吸収特性を要求される場合には、Ｓは0.005％以下が好ましい。
Al：0.01〜0.5%
Alは鋼の脱酸元素として添加され、鋼の清浄度を向上させるのに有用な元素であり、鋼の組織微細化のためにも添加が望ましい元素である。また適性範囲のAlを添加したアルミキルド鋼のほうが、Alを添加しない従来のリムド鋼に比して、機械的性質が優れている。以上の理由により、下限は0.01%とする。また、Siと同様に強度-延性バランスを向上させる効果も有するが、一方で、Al含有量が多くなると表面性状の悪化につながるため上限は0.5%とする。以上より、Alは0.01％以上0.5％以下とする。
N：0.005%以下
NはTi、NbとTiN、NbNを形成し、熱延鋼板中の固溶Ti、Nbを減少させるため、本発明においてはできるだけ少ないほうが好ましい。よって、Nは0.005%以下、好ましくは0.003%以下とする。
TiおよびNbの一種又は二種を合計で0.10〜0.50％
TiおよびNbは、本発明においてもっとも重要な元素の一つであり、温間成形中に極微細炭化物として析出することにより成形後の部材の強度を高める。TiおよびNbの合計含有量が0.10%未満では、いかなる方法で製造したとしても、温間成形後に所望の強度上昇を得ることができない。一方、0.50%を超えて添加しても温間成形時の強度上昇効果が飽和し、逆に加工性の劣化をもたらす。以上より、TiおよびNbの合計含有量を0.10％以上0.50％以下とする。より好ましくは、0.15％以上0.40％以下である。
TiおよびNbの合計含有量の60%以上が固溶状態
さらに、本発明では、熱延鋼板中にTiおよびNbの合計含有量の60%以上を固溶させることとする。このようにTiおよびNbを熱延鋼板中に析出させず、積極的に固溶させることにより、温間成形時に所望の強度上昇を得ることができる。TiおよびNbの固溶量が合計含有量の60%未満では、含有量に見合う強度上昇を得ることができない。よって、TiおよびNbの固溶量は合計含有量の60%以上とする。より好ましくは70%以上である。なお、本発明において、固溶Ti、Nb量は、10%AA系電解法（参照：高山ら：鉄と鋼, 82(1996),147.）により抽出した残さの化学分析により得られた析出Ti、Nb量を、鋼中Ti、Nb量から差し引いて求めるものとする。
First, the reasons for limiting the chemical composition (composition) of steel in the present invention are as follows.
C: 0.020-0.20%
C is one of the most important elements in the present invention from the viewpoint of increasing the strength of the steel sheet and generating carbides during warm forming.In the present invention, 0.020% or more is required in order to ensure the desired strength after warm forming. It is necessary to contain. On the other hand, if the content exceeds 0.20%, it becomes difficult to solidify the carbonitride containing Ti and Nb into the hot-rolled sheet, and the weldability is remarkably deteriorated. From the above, C is 0.020% or more and 0.20% or less, preferably 0.040 or more and 0.15% or less.
Si: 1.5% or less
Si is a useful strengthening element that can increase the strength of a steel sheet without significantly lowering the ductility of the steel, and may be added according to the strength level of the steel sheet. It is preferable to add 0.01% or more. However, especially in the case of automotive steel sheets that require high surface aesthetics and corrosion resistance, the content of Si exceeding 1.5% adversely affects the surface properties, chemical conversion properties, etc., in order to suppress these adverse effects. The steel plate surface must be pickled for a long time, resulting in a large cost increase. From the above, Si is 1.5% or less. In applications where superior surface aesthetics and corrosion resistance are required, it is preferably at most 0.5%, more preferably at most 0.25%.
Mn: 0.50 to 3.0% or less
Mn is an element that stabilizes austenite and delays ferrite transformation, and stably suppresses precipitation of Ti and Nb carbides that occur simultaneously with ferrite transformation during hot rolling. In order to obtain such an effect, addition of 0.50% or more is necessary. On the other hand, the content of Mn exceeding 3.0% not only saturates the above effect, but also increases the strength during warm forming. Accordingly, Mn is set to 0.50% to 3.0%, preferably 1.0% to 2.5%.
P: 0.10% or less P has an effect of strengthening steel, and may be added according to the strength level of the steel sheet. To obtain such an effect, 0.005% or more is preferably added. On the other hand, if the P content exceeds 0.10%, the weldability deteriorates. From the above, P is 0.10% or less. In addition, when more excellent weldability is required, P is preferably 0.05% or less.
S: 0.01% or less S is present as an inclusion in the steel sheet, not only causing deterioration of weldability, but also a coarse inclusion containing S serves as a starting point for the destruction of the steel sheet at the time of automobile collision, and the impact of the collision is sufficient. There is a risk that the steel sheet will break without absorption. Therefore, it is preferable to reduce as much as possible as a structural member for automobiles, and if it is 0.01% or less, these adverse effects can be ignored. Therefore, in the present invention, S is set to 0.01% or less. Further, when more excellent weldability and impact absorption characteristics are required, S is preferably 0.005% or less.
Al: 0.01-0.5%
Al is added as a deoxidizing element for steel, is an element useful for improving the cleanliness of steel, and is also an element that is desirable for addition for refining the structure of steel. In addition, aluminum killed steel to which Al in the appropriate range is added has better mechanical properties than conventional rimmed steel to which Al is not added. For the above reasons, the lower limit is set to 0.01%. In addition, it has the effect of improving the strength-ductility balance like Si, but on the other hand, an increase in Al content leads to deterioration of surface properties, so the upper limit is made 0.5%. From the above, Al is made 0.01% to 0.5%.
N: 0.005% or less
N forms Ti, Nb and TiN, NbN, and reduces the solid solution Ti and Nb in the hot-rolled steel sheet. Therefore, it is preferable that N be as small as possible in the present invention. Therefore, N is 0.005% or less, preferably 0.003% or less.
0.10 to 0.50% in total of one or two of Ti and Nb
Ti and Nb are one of the most important elements in the present invention, and precipitate as ultrafine carbides during warm forming to increase the strength of the member after forming. If the total content of Ti and Nb is less than 0.10%, a desired increase in strength cannot be obtained after warm forming, regardless of the production method. On the other hand, even if added over 0.50%, the effect of increasing the strength during warm forming is saturated, and conversely, the workability is deteriorated. From the above, the total content of Ti and Nb is made 0.10% to 0.50%. More preferably, it is 0.15% or more and 0.40% or less.
60% or more of the total content of Ti and Nb is in a solid solution state Furthermore, in the present invention, 60% or more of the total content of Ti and Nb is dissolved in the hot-rolled steel sheet. Thus, a desired increase in strength can be obtained during warm forming by positively dissolving Ti and Nb in the hot-rolled steel sheet without causing precipitation. If the solid solution content of Ti and Nb is less than 60% of the total content, an increase in strength commensurate with the content cannot be obtained. Therefore, the solid solution amount of Ti and Nb is set to 60% or more of the total content. More preferably, it is 70% or more. In the present invention, the amounts of solid solution Ti and Nb are the precipitates obtained by chemical analysis of the residue extracted by the 10% AA electrolysis method (see: Takayama et al .: Iron and Steel, 82 (1996), 147.). The amount of Ti and Nb shall be subtracted from the amount of Ti and Nb in the steel.
本発明の鋼板は、上記の必須添加元素で目的とする特性が得られるが、所望の特性に応じて以下の元素を含有することができる。
V：0.01〜1.0%、Mo：0.01〜1.0%、Cr：0.01〜1.0%の一種または二種以上
V、Mo、Crはそれぞれ単独で添加しても、温間成形時に添加量に見合う強度上昇を得ることができないが、Ti、Nbと複合で添加することにより、温間成形時の強度上昇量を増加させる効果を有し、このような効果はV、Mo、Crそれぞれを0.01%以上添加したときに顕著となる。これは、V、Mo、CrはTi、Nbに比べ炭化物形成能が弱いため、単独添加では添加量にみあう十分な強度上昇を得ることができないが、Ti、Nbと複合添加することで、Ti、Nbを含む炭化物に複合して析出し、温間成形時の強度上昇量を増加させると考えられる。一方で、V、Mo、Crそれぞれを1.0%を超えて添加すると、コストアップや温間成形時の加工性の劣化をもたらす。以上より、含有する場合、Vは0.01％以上1.0％以下、Moは0.01％以上1.0％以下、Crは0.01％以上1.0％以下とする。
The steel sheet of the present invention can achieve the desired characteristics with the above-mentioned essential additive elements, but can contain the following elements depending on the desired characteristics.
V: 0.01-1.0%, Mo: 0.01-1.0%, Cr: 0.01-1.0%
Even if each of V, Mo, and Cr is added alone, it is not possible to obtain an increase in strength commensurate with the amount added during warm forming, but by adding Ti and Nb in combination, the amount of increase in strength during warm forming can be obtained. This effect becomes significant when each of V, Mo, and Cr is added in an amount of 0.01% or more. This is because V, Mo, Cr has a weak carbide forming ability compared to Ti, Nb, so it is not possible to obtain a sufficient strength increase with the addition amount alone, but by adding Ti, Nb in combination, It is considered that it is combined with carbides containing Ti and Nb and precipitates to increase the amount of strength increase during warm forming. On the other hand, if each of V, Mo, and Cr is added in excess of 1.0%, the cost is increased and workability is deteriorated during warm forming. Therefore, when contained, V is 0.01% to 1.0%, Mo is 0.01% to 1.0%, and Cr is 0.01% to 1.0%.
上記以外の残部はFe及び不可避不純物とする。不可避的不純物としては、例えば、Sb、Sn、Zn、Co等が挙げられ、これらの含有量の許容範囲としては、Sb:0.01％以下、Sn：0.1％以下、Zn：0.01％以下、Co：0.1％以下の範囲である。また、本発明では、本発明の作用効果を害さない微量元素として、Ni、Cu、Mg、Ca、Zr、REMを通常の鋼組成の範囲内で含有してもよい。 The remainder other than the above is Fe and inevitable impurities. Inevitable impurities include, for example, Sb, Sn, Zn, Co, etc. The allowable ranges of these contents are Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less, Co: The range is 0.1% or less. Moreover, in this invention, you may contain Ni, Cu, Mg, Ca, Zr, and REM as a trace element which does not impair the effect of this invention within the range of a normal steel composition.
次に、本発明の温間成形に適した熱延鋼板の組織について説明する。本発明の温間成形に適した熱延鋼板の組織は、マルテンサイト相を体積率で20％以下、残部はベイナイトを主体とする。以下に詳細に説明する。
マルテンサイト体積率：20％以下（ただし、0％の場合も含む）
マルテンサイトが多量に生成すると鋼板強度が高くなりすぎるほか、マルテンサイト相は温間成形時に軟化するため、TiおよびNbの炭化物による強度上昇効果を相殺してしまう。よって、マルテンサイト相の体積率は20％以下が好ましい。より好ましくは15％以下である。なお、上記主旨により、本発明においては、マルテンサイト体積率が0％の場合においても本発明の効果を奏するため、マルテンサイト体積率が0％の場合も含むものとする。
残部：ベイナイトを主体
マルテンサイトを除いた残部組織はベイナイトを主体とする組織とする。ただし、若干量（体積率で20%以下）のフェライト、パーライト、残留オーステナイト等の組織が混在していても構わない。また、本発明でいうベイナイトとは、ラス状フェライトの界面に沿って板状のセメンタイトが析出した所謂上部ベイナイト、およびラス状フェライト内にセメンタイトが微細分散した所謂下部ベイナイトを含むものとする。
Next, the structure of the hot rolled steel sheet suitable for the warm forming of the present invention will be described. The structure of the hot-rolled steel sheet suitable for warm forming according to the present invention is mainly composed of a martensite phase of 20% or less by volume and the balance being bainite. Details will be described below.
Martensite volume ratio: 20% or less (including 0%)
When a large amount of martensite is generated, the steel sheet strength becomes too high, and the martensite phase softens during warm forming, thereby offsetting the strength increase effect due to Ti and Nb carbides. Therefore, the volume ratio of the martensite phase is preferably 20% or less. More preferably, it is 15% or less. In the present invention, the present invention includes the case where the martensite volume ratio is 0% in order to achieve the effects of the present invention even when the martensite volume ratio is 0%.
The remainder: mainly bainite The remainder structure excluding martensite is a structure mainly composed of bainite. However, a slight amount (20% or less by volume) of a structure such as ferrite, pearlite, and retained austenite may be mixed. The bainite as used in the present invention includes so-called upper bainite in which plate-like cementite is precipitated along the interface of lath-like ferrite and so-called lower bainite in which cementite is finely dispersed in lath-like ferrite.
なお、ベイナイトと、セメンタイトの析出の無いマルテンサイトとは走査電子顕微鏡等で容易に区別可能である。 Bainite and martensite without cementite precipitation can be easily distinguished by a scanning electron microscope or the like.
次に本発明の温間成形に適した熱延鋼板の製造方法について説明する。 Next, a method for producing a hot-rolled steel sheet suitable for warm forming according to the present invention will be described.
前述の化学成分範囲に調整された溶鋼から、連続鋳造または造塊でスラブを溶製する。鋳造後、再加熱することなく若しくは1200℃以上に再加熱した後、熱間圧延し、該熱間圧延では粗圧延を経ることなく若しくは粗圧延した後、仕上げ圧延温度：900℃以上で仕上げ圧延を行い、次いで、平均冷却速度:50℃/s以上で600℃以下まで急冷し、次いで、巻取り温度：520℃以下で巻取る。 From the molten steel adjusted to the above-mentioned chemical composition range, the slab is melted by continuous casting or ingot forming. After casting, re-heated to 1200 ° C or higher without reheating, then hot rolled, and after hot rolling without rough rolling or rough rolling, finish rolling at a finish rolling temperature of 900 ° C or higher Then, it is rapidly cooled to 600 ° C. or lower at an average cooling rate of 50 ° C./s or higher, and then wound at a winding temperature of 520 ° C. or lower.
使用する鋼スラブは、成分のマクロ偏析を防止すべく連続鋳造法で製造することが好ましいが、造塊法、薄スラブ鋳造法によっても可能である。 The steel slab to be used is preferably produced by a continuous casting method in order to prevent macro segregation of components, but can also be produced by an ingot-making method or a thin slab casting method.
鋳造後、再加熱することなく若しくは1200℃以上に再加熱する。本発明では、スラブを製造したのち、いったん室温まで冷却し、その後再度加熱する従来法に加え、冷却しないで、温片のままで加熱炉に装入する、あるいは保熱をおこなった後に直ちに圧延する、あるいは鋳造後そのまま圧延する直送圧延・直接圧延などの省エネルギープロセスも問題なく適用でき、Ti、Nb等の固溶の観点からも好ましい。 After casting, reheat to 1200 ° C or higher without reheating. In the present invention, after the slab is manufactured, in addition to the conventional method of once cooling to room temperature and then heating again, without cooling, it is charged in a heating furnace as it is, or rolled immediately after heat retention. Alternatively, an energy saving process such as direct feed rolling or direct rolling in which rolling is performed as it is after casting can be applied without any problem, and it is preferable from the viewpoint of solid solution of Ti, Nb and the like.
いったん室温まで冷却し再加熱する場合、スラブ加熱温度は1200℃以上とする。1200℃未満では未固溶の粗大なTiおよびNbを含む炭窒化物および硫化物が多く存在し、温間成形時にTiおよびNbの含有量に見合う強度上昇が得られない。より好ましくは1250℃以上である。 When cooling to room temperature and reheating, the slab heating temperature should be 1200 ° C or higher. Below 1200 ° C, there are many carbonitrides and sulfides containing undissolved coarse Ti and Nb, and the strength increase corresponding to the Ti and Nb contents cannot be obtained during warm forming. More preferably, it is 1250 ° C or higher.
また、冷却しないで、温片のままで加熱炉に装入し再加熱する場合も、スラブ加熱温度は1200℃以上とする。1200℃以上とする理由は、上記冷却後再加熱の場合と同様である。 Moreover, also when not charging but charging with a heating furnace with a warm piece and reheating, slab heating temperature shall be 1200 degreeC or more. The reason why the temperature is 1200 ° C. or higher is the same as in the case of reheating after cooling.
次いで、必要に応じて粗圧延を行った後、仕上げ圧延温度：900℃以上で仕上げ圧延を行う。仕上げ圧延温度が900℃を下回ると、熱間圧延中および熱間圧延直後にTiやNbを含む炭化物が、圧延により導入された転位により歪誘起析出してしまい、温間成形中に含有量に見合う強度上昇が得られなくなってしまう。よって、仕上げ圧延温度は900℃以上とする。圧延中または圧延後の合金炭化物析出抑制の観点から、より好ましくは950℃以上である。 Next, after rough rolling as necessary, finish rolling is performed at a finish rolling temperature of 900 ° C. or higher. When the finish rolling temperature is lower than 900 ° C, carbides containing Ti and Nb during hot rolling and immediately after hot rolling are strain-induced precipitation due to dislocations introduced by rolling, and the content is increased during warm forming. The corresponding increase in strength can no longer be obtained. Therefore, the finish rolling temperature is 900 ° C. or higher. From the viewpoint of suppressing precipitation of alloy carbide during or after rolling, the temperature is more preferably 950 ° C or higher.
次いで、平均冷却速度:50℃/s以上で600℃以下まで急冷する。平均冷却速度が50℃/s未満であると冷却中にフェライトが多量に生成し、同時にTiおよびNbの炭化物も析出してしまうため、温間成形時に所望の強度上昇が得られない。平均冷却速度が50℃/s以上であれば、上記した鋼組成の範囲において、安定してTiおよびNbを固溶させることができる。より好ましくは80℃/s以上、さらに好ましくは100℃/s以上である。急冷停止温度は、600℃以下までとする。600℃を超える場合、急冷停止後巻取りまでの間にTiおよびNbの炭化物が多量に析出してしまうため、温間成形時に所望の強度上昇が得られない。より好ましくは550℃以下である。急冷停止温度の下限はTiおよびNbの固溶の観点からは厳しく限定されないが、350℃以上とすることが好ましい。350℃未満となるとマルテンサイトの体積率が20％を超えて多くなり、鋼板強度が高くなりすぎるほか、マルテンサイトは温間成形時に軟化するため、TiおよびNbの炭化物による強度上昇効果を相殺してしまう。より好ましくは400℃以上である。 Subsequently, it is rapidly cooled to an average cooling rate of 50 ° C./s or more and 600 ° C. or less. If the average cooling rate is less than 50 ° C./s, a large amount of ferrite is generated during cooling, and Ti and Nb carbides are precipitated at the same time, so that a desired strength increase cannot be obtained during warm forming. If the average cooling rate is 50 ° C./s or more, Ti and Nb can be stably dissolved in the above-described range of the steel composition. More preferably, it is 80 ° C./s or more, and further preferably 100 ° C./s or more. The quenching stop temperature shall be up to 600 ° C. When the temperature exceeds 600 ° C., a large amount of carbides of Ti and Nb are precipitated between the quenching stop and the winding, so that a desired strength increase cannot be obtained during warm forming. More preferably, it is 550 ° C or lower. The lower limit of the quenching stop temperature is not strictly limited from the viewpoint of solid solution of Ti and Nb, but is preferably 350 ° C. or higher. When the temperature is lower than 350 ° C, the volume fraction of martensite exceeds 20%, the steel sheet strength becomes too high, and martensite softens during warm forming, offsetting the strength increase effect of Ti and Nb carbides. End up. More preferably, it is 400 ° C. or higher.
さらに、仕上げ圧延後冷却開始までの時間を1.0sec未満とすることが好ましい。1.0sec未満とすることにより、熱間圧延直後のTi、Nbの析出を抑制することができ、より安定にTi、Nbを固溶させることができる。より好ましくは0.8sec未満である。 Furthermore, it is preferable that the time from finish rolling to the start of cooling is less than 1.0 sec. By setting it to less than 1.0 sec, precipitation of Ti and Nb immediately after hot rolling can be suppressed, and Ti and Nb can be dissolved more stably. More preferably, it is less than 0.8 sec.
なお、急冷停止から巻取りまでの平均冷却速度は空冷以上の速度であればよく、厳しく限定されないが、材質安定性の観点から、20℃/s以上100℃/s以下とすることが好ましい。 In addition, the average cooling rate from the rapid cooling stop to the winding may be a rate equal to or higher than air cooling, and is not strictly limited, but is preferably 20 ° C./s or more and 100 ° C./s or less from the viewpoint of material stability.
次いで、巻取り温度：520℃以下で巻取る。仕上げ圧延終了後の巻取り温度の制御は、本発明の目標とする熱延鋼板中のTiおよびNbの合計固溶量を60%以上に制御する上で極めて重要である。巻取り温度が520℃を超えると、巻取り中にTiおよびNbの炭化物が析出してしまい、温間成形時に所望の強度上昇が得られない。より好ましくは500℃以下である。巻取り温度下限は、TiおよびNbの固溶の観点からは厳しく限定はされないが、350℃を下回ると、マルテンサイト相の体積率が20％を超えて多くなり、目的とする組織とならないため、350℃以上とすることが好ましい。より好ましくは400℃以上である。 Next, the coil is wound at a coiling temperature of 520 ° C. or lower. Control of the coiling temperature after finish rolling is extremely important for controlling the total solid solution amount of Ti and Nb in the hot-rolled steel sheet targeted by the present invention to 60% or more. When the winding temperature exceeds 520 ° C., Ti and Nb carbides precipitate during winding, and a desired strength increase cannot be obtained during warm forming. More preferably, it is 500 ° C. or lower. The lower limit of the coiling temperature is not strictly limited from the viewpoint of solid solution of Ti and Nb, but if the temperature is lower than 350 ° C, the volume ratio of the martensite phase exceeds 20%, and the desired structure is not obtained. , 350 ° C. or higher is preferable. More preferably, it is 400 ° C. or higher.
なお、本発明の温間成形に適した熱延鋼板の製造における熱間圧延では、熱間圧延時に圧延荷重を低減するために仕上げ圧延の一部または全部を潤滑圧延としてもよい。潤滑圧延を行うことは、通常強加工される表層付近のTi,Nbを含む炭化物の歪み誘起析出を抑制し、Ti、Nb等を安定して固溶するのに効果的であり、また、鋼板形状の均一化、材質の均一化の観点からも有効である。なお、潤滑圧延の際の摩耗係数は0.10〜0.25の範囲とすることが好ましい。また、相前後するシートバー同士を接合し、連続的に仕上圧延する連続圧延プロセスとすることが好ましい。連続圧延プロセスを適用することは、熱間圧延の操業安定性の観点からも望ましい。 In the hot rolling in the production of a hot rolled steel sheet suitable for warm forming according to the present invention, part or all of the finish rolling may be lubricated rolling in order to reduce the rolling load during hot rolling. Lubricating rolling is effective for suppressing strain-induced precipitation of carbides containing Ti and Nb near the surface layer, which is usually hard-worked, and is effective for stable solid solution of Ti, Nb, etc. This is also effective from the viewpoint of uniform shape and uniform material. In addition, it is preferable to make the abrasion coefficient in the case of lubrication rolling into the range of 0.10-0.25. Moreover, it is preferable to set it as the continuous rolling process which joins the sheet | seat bars which precede and follow, and finish-rolls continuously. The application of the continuous rolling process is also desirable from the viewpoint of the operational stability of hot rolling.
熱間圧延後、形状矯正、表面粗度等の調整のために、伸び率10%以下の調質圧延を施してもよい。
なお、本発明の温間成形に適した熱延鋼板は、温間成形の前に、表面処理を行うこともできる。表面処理としては、亜鉛めっき（合金系を含む）、すずめっき、ほうろう等がある。また、本発明の温間成形に適した熱延鋼板には、亜鉛めっき後、特殊な処理を施して、化成処理性、溶接性、および耐食性等の改善を行ってもよい。
After hot rolling, temper rolling with an elongation of 10% or less may be performed for shape correction, adjustment of surface roughness, and the like.
In addition, the hot-rolled steel sheet suitable for the warm forming of the present invention can be subjected to a surface treatment before the warm forming. Examples of the surface treatment include galvanization (including alloy system), tin plating, enamel and the like. Further, the hot-rolled steel sheet suitable for warm forming according to the present invention may be subjected to special treatment after galvanization to improve chemical conversion property, weldability, corrosion resistance, and the like.
以上から得られた本発明の熱延鋼板を所望部品形状に温間成形するときは300℃以上A1点以下に加熱し、成形する。この温度範囲で成形することで、成形時の強度を成形後の部材強度の0.60以下とすることができ、室温での成形が困難な自動車用超高強度部材等を容易に得ることができる。300℃を下まわる温度で成形しても、成形時の強度低下は少なく、形状凍結性向上などの十分な効果が得られない。なお、TiおよびNbの炭化物析出の観点から、より好ましくは400℃以上である。一方、A1点を越えて加熱すると、スケールの生成が多量になるだけでなく、温間成形中に析出した炭化物が粗大化してしまい、強度上昇を得るどころか、逆に軟化してしまうため、成形温度はA1点以下とすることが必要である。より好ましくは650℃以下である。なお、設備的使用条件等、設計的事項により温間成形時の加熱温度が上記範囲外の場合においても、本発明の熱延鋼板を適用することが可能である。 When the hot-rolled steel sheet of the present invention obtained as described above is warm-formed into a desired part shape, it is heated to 300 ° C. or higher and A1 point or lower and formed. By molding in this temperature range, the strength at the time of molding can be 0.60 or less of the strength of the member after molding, and an ultra-high strength member for automobiles and the like that are difficult to mold at room temperature can be easily obtained. Even if molding is performed at a temperature below 300 ° C., there is little decrease in strength at the time of molding, and sufficient effects such as improvement of shape freezing property cannot be obtained. In view of precipitation of Ti and Nb carbides, the temperature is more preferably 400 ° C. or higher. On the other hand, since when heated above a point A, as well as the generation of scale is large amount, carbides precipitated in the warm forming ends up coarsening, let alone give strength increases, resulting in softening the contrary, The molding temperature must be A 1 point or less. More preferably, it is 650 ° C. or lower. Note that the hot-rolled steel sheet of the present invention can be applied even when the heating temperature during warm forming is out of the above range due to design matters such as equipment usage conditions.
表１に示す組成の鋼を溶製し、得られた鋼スラブを表２に示す熱延条件により、板厚3.0ｍｍの熱延鋼板とした。この熱延鋼板に酸洗、伸び率：1.0%の調質圧延を施した後に試験片を採取し、固溶Ti、Nb量の測定、組織観察および引張試験を実施した。各試験方法の詳細は以下の通りである。
固溶Ti、Nb量測定
固溶Ti,Nb量は、10%AA系電解法（参照：(*)高山ら：鉄と鋼, 82(1996),147. ）により抽出した残さの化学分析により得られた析出Ti,Nb量を、鋼中Ti,Nb量から差し引いて求め、鋼中Ti、Nb量に対する固溶量の比：S（＝固溶(Ti+Nb)/鋼中(Ti+Nb)×100）で評価した。
組織観察
得られた熱延鋼板から試験片を採取し、圧延方向に直交する断面（Ｃ断面）について、光学顕微鏡あるいは走査型電子顕微鏡を用いて微視組織を撮像し、組織の種類の同定を行い、画像解析装置を用いてマルテンサイト相の体積率を求めた。
引張試験
得られた熱延鋼板から長軸を圧延方向に直交する方向としたJIS５号引張試験片を採取し、JIS Z 2241の規定に準拠して引張試験を行い、引張強さ（室温TS）を求めた。また，550℃で10分間保持後、同温度で破断するまで上記と同様の引張試験を行い、温間引張強さ（温間TS）を求めた。さらに、550℃で10分間保持後に同温度で2%の引張変形を行ったのち室温まで冷却した試料に対しても上記と同様の引張試験を行い、温間成形後室温引張強さ（温間成形後室温TS）を求め、さらに、これらの温間TS、温間成形後室温TSから温間TS／温間成形後室温TSであるP値を求めた。得られた結果を表２に併せて示す。
Steel having the composition shown in Table 1 was melted, and the obtained steel slab was made into a hot-rolled steel sheet having a thickness of 3.0 mm under the hot-rolling conditions shown in Table 2. The hot-rolled steel sheet was pickled and subjected to temper rolling with an elongation of 1.0%, and then a test piece was collected and subjected to measurement of solid solution Ti and Nb amounts, structure observation, and tensile test. Details of each test method are as follows.
Measurement of solid solution Ti and Nb content The amount of solid solution Ti and Nb was measured using 10% AA electrolysis method (see (*) Takayama et al .: Iron and Steel, 82 (1996), 147. ) The amount of precipitated Ti and Nb obtained by chemical analysis of the residue extracted by subtracting the amount of Ti and Nb in the steel is subtracted from the amount of Ti and Nb in the steel, and the ratio of the solid solution amount to the Ti and Nb amount in the steel: S (= solid solution (Ti + Nb) / in steel (Ti + Nb) × 100).
Microstructure observation A specimen is taken from the obtained hot-rolled steel sheet, and the cross section (C cross section) perpendicular to the rolling direction is imaged with an optical microscope or a scanning electron microscope to identify the type of the structure. The volume ratio of the martensite phase was determined using an image analyzer.
Tensile test A JIS No. 5 tensile test piece with the long axis perpendicular to the rolling direction was taken from the obtained hot-rolled steel sheet, and a tensile test was performed in accordance with the provisions of JIS Z 2241 to determine the tensile strength (room temperature TS). Asked. Further, after holding at 550 ° C. for 10 minutes, a tensile test similar to the above was performed until fracture at the same temperature to determine the warm tensile strength (warm TS). Furthermore, after holding at 550 ° C for 10 minutes and performing 2% tensile deformation at the same temperature and then cooling to room temperature, the same tensile test was performed as above, and after warm forming, room temperature tensile strength (warm After molding, room temperature TS) was determined, and further, the warm TS / the room temperature TS after warm molding was used to determine the P value which was warm TS / room temperature TS after warm molding. The obtained results are also shown in Table 2.
表２より、本発明例は、いずれも、P値が0.60以下であり、温間での成形強度が低く、温間成形後の室温強度が高く（780MPa以上）なっている。なお、実施例No.12はマルテンサイトの体積率が20%を超えているため、温間成形中のマルテンサイトの軟化が析出による強度上昇を相殺し、問題ない範囲ではあるもののP値が若干高くなっている。 From Table 2, all of the examples of the present invention have a P value of 0.60 or less, a low molding strength at the warm temperature, and a high room temperature strength after the warm molding (780 MPa or more). In Example No. 12, since the volume ratio of martensite exceeds 20%, the softening of martensite during warm forming offsets the increase in strength due to precipitation, and although there is no problem, the P value is slightly It is high.
これに対し、本発明の範囲を外れる比較例では、P値が0.60を超えており、温間で成形する利点を十分に生かしきれていないことが分かる。 On the other hand, in the comparative example outside the scope of the present invention, the P value exceeds 0.60, and it can be seen that the advantages of warm molding cannot be fully utilized.
自動車用超高強度部材以外の家電および建築など、室温での加工が困難であり、成形時には低強度で成形が容易で成形後の部材強度は高強度が必要とされる分野でも好適である。 It is also suitable in fields where processing at room temperature is difficult, such as home appliances and buildings other than ultra-high strength members for automobiles, low strength at the time of molding, easy molding, and high strength of the molded member is required.
Claims (5)
該熱間圧延では粗圧延を経ることなく若しくは粗圧延した後、仕上げ圧延温度：900℃以上で仕上げ圧延を行い、
次いで、平均冷却速度: 50℃/s以上で600℃以下まで急冷し、次いで、巻取り温度：520℃以下で巻取ることを特徴とする温間成形に適した熱延鋼板の製造方法。 The slab having the composition according to claim 1 or 2, after casting, without reheating or after reheating to 1200 ° C or higher, hot rolling,
In the hot rolling, after rough rolling or rough rolling, finish rolling is performed at a finish rolling temperature of 900 ° C. or higher,
Next, a method for producing a hot-rolled steel sheet suitable for warm forming, characterized by quenching at an average cooling rate of 50 ° C./s or more to 600 ° C. or less and then winding at a winding temperature of 520 ° C. or less.
該熱間圧延では粗圧延を経ることなく若しくは粗圧延した後、仕上げ圧延温度：900℃以上で仕上げ圧延を行い、
次いで、平均冷却速度: 50℃/s以上で600〜350℃まで急冷し、次いで、巻取り温度：520〜350℃で巻取ることを特徴とする温間成形に適した熱延鋼板の製造方法。 The slab having the composition according to claim 1 or 2, after casting, without reheating or after reheating to 1200 ° C or higher, hot rolling,
In the hot rolling, after rough rolling or rough rolling, finish rolling is performed at a finish rolling temperature of 900 ° C. or higher,
Next, a method for producing a hot-rolled steel sheet suitable for warm forming, characterized in that it is rapidly cooled to 600 to 350 ° C. at an average cooling rate of 50 ° C./s or more and then wound at a winding temperature of 520 to 350 ° C. .
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