JP4038303B2

JP4038303B2 - Manufacturing method of formed body using thin steel plate

Info

Publication number: JP4038303B2
Application number: JP12741699A
Authority: JP
Inventors: 力岡本; 裕一谷口; 俊太郎須藤
Original assignee: Nippon Steel Corp; Toyota Motor Corp
Current assignee: Nippon Steel Corp; Toyota Motor Corp
Priority date: 1999-05-07
Filing date: 1999-05-07
Publication date: 2008-01-23
Anticipated expiration: 2019-05-07
Also published as: JP2000319755A

Description

【０００１】
【発明の属する技術分野】
本発明は、自動車の構造用部品など、構造上の強度、特に変形時の強度及び又は剛性が必要とされる箇所に適用されるに好適な、プレス等による加工成形後に所定温度域で強度上昇熱処理がなされる成形体の素材である高強度薄鋼板を用いた成形体の製造方法に関するものである。
【０００２】
【従来の技術】
高い成形性と優れた衝撃吸収特性を確保するために、加工成形前は比較的軟質の高強度鋼でプレス成形等の加工成形がしやすく、プレス成形等の成形加工後に低温で短時間熱処理（以下、後熱処理）を行うことで強度を上昇させ部材や部品の変形強度を高める技術がある。例えば、特開平１０−３１０８２４号公報は低Ｃ鋼にＭｏ、Ｃｒ、Ｔｉ、Ｎｂ、Ｖ、Ｂを複合添加し強度上昇を狙った鋼である。しかし、炭化物を利用したこれらの鋼の強度上昇量は母材強度４９０ＭＰａ以下の鋼において１００ＭＰａ程度であり、最終強度として５９０ＭＰａ以上の強度が必要な部材への適用は困難である。
【０００３】
一方でＣｕの時効析出を利用することでより高い強度上昇が得られることが知られている。例えば、特公平３−６９９７９号公報は、それぞれ低Ｃ鋼、極低Ｃ鋼にＣｕを１〜２％添加しこのＣｕを固溶状態、すなわち軟化状態でプレス成形することで高加工性とし、成形後の熱処理により強度を高める技術である。更に、特開平３−７２０３４号公報は高い成形性と高い強度に加え、高い防錆性を得るためにＣｕ析出強化を熱延めっき鋼板で利用するための鋼板製造技術である。しかしながら、これらの技術はＣｕ自体の析出現象を変化させるものではないため、Ｃｕによる析出強化量は添加するＣｕ量と、熱処理温度、熱処理時間でほぼ決まってしまい、大きな強度上昇を得るためには、高温長時間の後熱処理が必要となり生産性の低減、めっき層の性能・性状劣化等の弊害が生じる。
【０００４】
【発明が解決しようとする課題】
本発明の課題は高い成形性と部品あるいは成形体としての高い強度また更には極めて高い防錆性を成形体の生産性を落とすことなく達成することにある。すなわち、プレス成形前の低強度、高延性を生かした高成形性と低温・短時間の成形後の熱処理による強度、および各種亜鉛めっき鋼板並みの高耐食性を兼ね揃えた鋼板を用いた成形体の製造方法にかかわる。
【０００５】
【課題を解決するための手段】
上記の目的を達成すべく種々実験、検討を重ねた結果、Ｃｕとの相分離傾向がＦｅよりも高いＣｒ、Ｍｏを添加することによってＣｕの析出は促進され、低温・短時間の後熱処理条件による強化性が向上することを見出した。図１はこの効果を説明するものであり、Ｃｕ添加量が１．８質量％でＣｒ＋Ｍｏ量を変化させた鋼板の６００℃×４０ｓｅｃ処理による強度上昇量を記したものである。これより、この発明をなすに至ったのである。その要旨は、下記の通りである。
【０００６】
（１）質量％にて
Ｃ：０．０５％以下、
Ｓｉ：１．５％以下、
Ｍｎ：０．０１〜１．５％、
Ｐ：０．１以下、
Ｓ：０．０１以下、
Ａｌ：０．００５〜０．１％、
Ｎ：０．０１％以下、
Ｃｕ：１．２〜２．５％、
Ｎｉ：０．２５〜１．５％
およびＣｒ、Ｍｏの２種を合計量で０．０３〜２．５％含有し、残部が鉄および不可避的不純物からなる鋼成分を有する薄鋼板をプレス成形後、処理時間ｔ（ｓｅｃ）、処理温度Ｔ（℃）が、（B）の式を満たし、かつｔ≦１００の条件で熱処理を行うことにより、プレス成形時よりも引張強度を１９０ＭＰａ以上上昇させることを特徴とする薄鋼板を用いた成形体の製造方法。
ｅｘｐ（−（Ｔ−790)／55）＜ｔ＜ｅｘｐ（−（Ｔ−860)／55）（B）
（２）鋼成分にさらに、
Ｔｉ：０．０１〜０．１５％、
Ｎｂ：０．０１〜０．１５％、
Ｖ：０．０１〜０．１５％
のうち１種または２種以上を含有させた薄鋼板を用いる請求項１記載の薄鋼板を用いた成形体の製造方法。
（３）鋼成分にさらに、
Ｂ：０．０００３〜０．００２％
を含有させた薄鋼板を用いる請求項１または２記載の薄鋼板を用いた成形体の製造方法。
（４）表面処理を施した薄鋼板を用いる請求項１又は請求項２又は請求項３に記載の薄鋼板を用いた成形体の製造方法。
（５）表面処理が亜鉛めっきであることを特徴とする請求項４に記載の薄鋼板を用いた成形体の製造方法。
【０００７】
【発明の実施の形態】
本発明はＣｕを１．０〜２．５％添加した鋼に、Ｃｕとの相分離傾向がＦｅよりも高いＣｒ、Ｍｏを添加し、後熱処理中のＣｕの析出を促進させ、低温・短時間の後熱処理条件による強化性を向上させたものである。本発明は以上のような骨子に基づくが、以下に本発明の個々の構成要件について詳細に説明する。
【０００８】
まず、本発明の成分の限定理由について述べる。
Ｃは、鋼の加工性に影響を及ぼす元素であり、含有量が多くなると、加工性は劣化する。従って、０．０５％以下とする。また、添加量が増加すると結晶粒の微細化が進み粒界面積が増大することによりＣｕの粒内固溶量が低減するため０．０２％以下が更に望ましい。
【０００９】
Ｓｉは、固溶強化を通して強度と延性を向上させるに好ましい元素である。しかし、１．５％を越えると加工性は劣化するので、１．５％を上限とする。
【００１０】
Ｍｎは、０．０１％未満では、鋼中不純物であるＳがＭｎＳとして十分に固定されず熱延時に割れを生じる。さらに、ＭｎはＳｉと同様に固溶強化により強度と延性を向上させる好ましい元素である。しかし、１．５％を越えると加工性が劣化する場合があるため１．５％を上限とする。
【００１１】
Ｐは鋼板の強度を上げる元素として必要な強度レベルに応じて添加する。しかし、０．１％を越えると鋼板の２次加工脆性の問題が発生してくるので上限を０．１％とする。
【００１２】
Ｓは鋼中に存在しない方が好ましい元素であり、特に加工性を高めるためには低い方が望ましく上限を０．０１％とする。
【００１３】
Ａｌは、ＡｌＮとしてＮをトラップし固溶Ｎを減少させることが好ましいので０．００５％以上添加する。ただし添加量が増加すると加工性が劣化するため０．１％以下を上限とする。
【００１４】
Ｎは、加工性を確保するためには少ない方が良い。０．０１％を越えると加工性が劣化してくるので、０．０１％を上限とする。
【００１５】
Ｃｕは、時効析出により鋼を著しく高強度化するが一方、Ｃｕ固溶状態にあっては鋼の延性をそれほど損なわない。時効析出による強度上昇能はＣｕ添加量によるが１．２％未満ではいかにＣｒ、Ｍｏを添加したとしても析出量が少なく、長時間を要するためほとんど強度上昇は起こらない。一方、２．５％超では強度上昇能はいかにＣｒ、Ｍｏを添加したとしてもほとんど飽和しており、これを上限とする。また、Ｃｕの添加は鋼のリサイクル性を損なうため１．８％以下が望ましい。
【００１６】
ＮｉはＣｕ添加鋼において熱延中に生じ易い表面欠陥（通称Ｃｕヘゲ）を抑制するために有効である。このため、Ｎｉは０．２５％以上の添加が必要である。しかし、添加量が大きいとその効果が飽和する上、Ｎｉが高価なため経済性が著しく損なわれるため上限を１．５０％とする。Ｎｉのこのような表面欠陥抑制効果はＣｕ添加量に応じて発揮されるためＮｉ添加量はＮｉ／Ｃｕに応じてこの比を０．２５〜０．６とすることが望ましい。
【００１７】
Ｃｒ、Ｍｏは本発明にあって極めて重要な元素である。前記の図１（Ｃｕ＝１．８％含有）に示すようにＣｕ添加薄鋼板に更にＣｒ＋Ｍｏを０．０３％以上含有せしめることによりプレス成形後の熱処理（例えば６００℃×４０ｓ加熱）によってプレス成形前に比べ著しい引張強度の上昇（ΔＴＳ）が得られることを本発明者らは見出した。
【００１８】
この原因は明確ではないがＣｒ、ＭｏがＦｅよりもＣｕとの相分離傾向が高いので、Ｃｕ鋼にＣｒ＋Ｍｏを添加するとＣｕの短時間析出を促進するものと本発明者らは考えている。Ｃｒ、Ｍｏの１種または２種の合計量として０．０３％未満ではその効果は得られず、一方、添加量が増大すると結晶粒の微細化を促進し、Ｃｕの粒内固溶量が低減するため上限を２．５％とする。
【００１９】
Ｔｉ、Ｎｂ、Ｖは時効性改善、延性向上のためにＴｉ：０．０１〜０．１５％、Ｎｂ：０．０１〜０．１５％、Ｖ：０．０１〜０．１５％の一種または２種以上を添加する。Ｔｉ：０．０１％以上、Ｎｂ：０．０１％以上、Ｖ：０．０１％以上の少なくとも１種を添加するとこれらの炭窒化物が生成されＣとＮが固定され、得られる鋼板は非時効性の鋼板となり、延性の向上が得られる。また、これらの炭窒化物により鋼の強度を上げることができるため強度調整にも利用できる。Ｔｉ、Ｎｂ、Ｖの上限を０．１５％とするのは０．１５％超では効果が飽和しコストアップとなるためである。
【００２０】
Ｂは２次加工脆性を抑制する作用があり、この効果を発揮するためには下限を０．０００３％とする。しかしながら、０．００２％を越えると効果は飽和するため０．００２％を上限とする。
【００２１】
次に本発明の薄鋼板の製造方法について説明する。製造方法は一般に行われている熱延鋼板、冷延鋼板、めっき鋼板の製造方法で構わない。特に熱間圧延でフェライト粒にひずみが過度に加わり加工性が低下するのを防ぐには熱間圧延をＡｒ３以上で行う。また巻き取り時のＣｕ析出が後熱処理による強度上昇量を著しく小さくすることがあるので、巻き取り温度については５００℃以下とすることが望ましい。
【００２２】
加工成形前の鋼板の強度レベルを特定するものでなく、３７０ＭＰａ級、３９０ＭＰａ級、４４０ＭＰａ級、４９０ＭＰａ級、５９０ＭＰａ級、あるいは、６９０ＭＰａ級等で比較的軟質の薄鋼板でも前記成分を有する場合にはプレス成形等の加工成形がしやすく、プレス成形等の成形加工後の鋼板の熱処理強度上昇能に優れている。
【００２３】
本発明の鋼板とは、熱延鋼板、冷延鋼板、溶融亜鉛めっき鋼板、合金化溶融亜鉛めっき鋼板、電気亜鉛めっき鋼板のいずれでもかまわず、発明の効果を享受出来る。さらに、加工性の向上や、加工後の外観のために調質圧延を施した鋼板（ダル仕上げ鋼板、ブライト仕上げ鋼板、表面に特定形状のパターンを転写された鋼板等）、表面に防錆油、潤滑油などの油膜層を有する鋼板など、通常に薄鋼板に用いられる各種の表面処理を施したいずれの鋼板においても、本発明の成分範囲の鋼板であれば本発明の効果を十分に享受することができる。
【００２４】
特に耐食性を付与する目的では表面処理鋼板としては亜鉛系めっきを用いることが有効である。亜鉛系めっきとしては溶融亜鉛めっき、合金溶融亜鉛めっき、電気亜鉛めっき、電気亜鉛合金（亜鉛−鉄合金、亜鉛−ニッケル合金、亜鉛−クロム合金等）めっき、合金化電気亜鉛めっき等、めっき層に亜鉛を含有するものであればいづれでもかまわない。
【００２５】
ついで、上記本発明成分の鋼板を用いて加工成形、例えば絞り加工などのプレス加工を行う。プレス成形法は、特に規定するものではなく、絞り加工、張り出し加工、曲げ加工、しごき加工、打ち抜き加工等を加えても何等差し支えない。
【００２６】
プレス成形後に成形体の引張強度を１９０ＭＰａ以上上昇可能な薄鋼板からなる成形体のプレス後の熱処理（後熱処理）条件としては処理時間が処理温度に対して短すぎる場合には、十分なＣｕ析出が起こりにくく好ましくない。一方、長すぎる場合には、Ｃｕの過時効析出により強度が確保困難となり好ましくない。特に長時間時効は成形体の生産性を低下させたり、表面処理性能が低下する場合があるので、処理時間ｔ≦１００の条件及び（Ｂ）の条件を満たすものとする。
ｅｘｐ（−（Ｔ−790)／５５）＜ｔ＜ｅｘｐ（−（Ｔ−860)／５５）（Ｂ）
【００２７】
後熱処理温度の好ましい範囲は３５０〜８００℃、更に好ましい範囲は４５０〜７００℃である。３５０℃未満では、処理時間が著しく長くなる場合があり、必ずしも工業的でない。８００℃超では、いかにＣｒ、Ｍｏを添加しても、Ｃｕの析出が十分に起こり難く後熱処理にて引張強度上昇量が小さい場合が有るので好ましくない。４５０〜７００℃では、Ｃｒ、Ｍｏの添加により、Ｃｕの析出を比較的安定的に利用出来易く、例えば数秒〜数分程度の短時間の熱処理時間でも所要の加工後熱処理強度上昇を得ることが出来るので好ましい。
加熱する熱処理方法としては、特に規定するものではなく、部分高周波加熱、通電加熱、温浴熱処理、赤外線加熱、熱風加熱など、強度の必要な所定範囲に加熱する方法であれば、いずれでもかまわない。
【００２８】
【実施例】
次に本発明を実施例に基づいて説明する。
表１に示す成分の鋼を溶製し、常法に従い連続鋳造でスラブとした。符号Ｍ，Ｎ，Ｏ，Ｑ，Ｓが本発明に従った成分の鋼で符号Ｕ、Ｖの鋼はＣｒ、Ｍｏの添加量が、Ｗ、Ｘの鋼はＣｕ添加量が本発明の範囲外である。これらの鋼を加熱炉中で１２００℃まで加熱し、８８０℃の仕上げ温度で熱間圧延を行い、５００℃以下にて巻き取る。これに続いて酸洗後、冷間圧延を行い、８５０℃×６０ｓｅｃの再結晶焼鈍を行い冷延鋼板となした。
【００２９】
得られた鋼板をＪＩＳ５号引張試験片に加工し、処理無しの機械特性値の評価を行った。各試験片のＴＳを表１に示す。さらに、成形加工後熱処理に相当するように、５〜１０％予歪み付与後、ＪＩＳ５号引張試験片に表２に示す条件にて熱処理（後熱処理）を行い、機械特性値の評価を行った。後熱処理後のＴＳおよび各熱処理によるＴＳの上昇（ΔＴＳ）を表２に示す。表２の結果より、各温度において本発明で定めた範囲の時間の熱処理を行うことで１９０ＭＰａ以上の強度上昇が得られており、この範囲外ではこれ以下の強度上昇量となる。さらに、表３においてＵ、Ｖ、Ｗ、Ｘは本発明成分外の鋼板であるが、Ｕ、Ｖの鋼板はＣｒ、Ｍｏの効果が不十分なため本発明の短時間熱処理においては１９０ＭＰａ以上の引張強度上昇を得ることはできない。Ｗ、Ｘの鋼板はＣｕ添加量が少ないためＣｒ、Ｍｏの効果により短時間化は進むものの、ΔＴＳの絶対値として十分な引張強度上昇が得られない。
【００３０】
【表１】

【００３１】
【表２】

【００３２】
【表３】

【００３３】
【００３４】
【００３５】
【本発明の効果】
本発明によれば加工成形前は強度レベルが３７０ＭＰａ級、３９０ＭＰａ級、４４０ＭＰａ級、４９０ＭＰａ級、５９０ＭＰａ級、あるいは、６９０ＭＰａ級等で比較的軟質の薄鋼板でプレス成形等の加工成形がしやすく、そして、プレス成形等の成形加工後には比較的低温での短時間熱処理を行うことで、引張強さが上昇し部品あるいは成形体の変形強度を高めることを可能とすることができる。
【図面の簡単な説明】
【図１】Ｃｕ添加薄鋼板のＣｒ＋Ｍｏ含有量と加工成形後熱処理による引張強度上昇（ΔＴＳ）との関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention is suitable for application to a location where structural strength, in particular, strength and / or rigidity at the time of deformation is required, such as a structural component of an automobile. The present invention relates to a method for manufacturing a formed body using a high-strength thin steel sheet that is a material of the formed body to be heat-treated.
[0002]
[Prior art]
In order to ensure high formability and excellent shock absorption properties, it is easy to perform press molding and other processing with relatively soft high-strength steel before processing, and heat treatment at a low temperature for a short time after press molding or other processing ( Hereinafter, there is a technique for increasing the strength of the members and parts by increasing the strength by performing post-heat treatment. For example, Japanese Patent Application Laid-Open No. 10-310824 is a steel that aims to increase strength by adding a combination of Mo, Cr, Ti, Nb, V, and B to low C steel. However, the increase in strength of these steels using carbide is about 100 MPa in steel with a base material strength of 490 MPa or less, and it is difficult to apply to members that require a strength of 590 MPa or more as the final strength.
[0003]
On the other hand, it is known that higher strength increase can be obtained by utilizing aging precipitation of Cu. For example, Japanese Examined Patent Publication No. 3-69979 discloses high workability by adding 1 to 2% of Cu to low C steel and extremely low C steel, respectively, and press forming this Cu in a solid solution state, that is, in a softened state. This is a technique to increase the strength by heat treatment after molding. Furthermore, JP-A-3-72034 is a steel plate manufacturing technique for utilizing Cu precipitation strengthening in hot-rolled steel plates in order to obtain high rust prevention properties in addition to high formability and high strength. However, since these techniques do not change the precipitation phenomenon of Cu itself, the amount of precipitation strengthening by Cu is almost determined by the amount of Cu to be added, the heat treatment temperature, and the heat treatment time. In addition, post-heat treatment at a high temperature for a long time is required, resulting in problems such as a reduction in productivity and deterioration of performance and properties of the plating layer.
[0004]
[Problems to be solved by the invention]
An object of the present invention is to achieve high moldability, high strength as a part or a molded body, and extremely high rust prevention without reducing the productivity of the molded body. In other words, a compact using a steel plate that combines low strength before press forming, high formability utilizing high ductility, strength by heat treatment after forming at low temperature and short time, and high corrosion resistance comparable to various galvanized steel plates. Involved in the manufacturing method.
[0005]
[Means for Solving the Problems]
As a result of various experiments and studies to achieve the above-mentioned purpose, the precipitation of Cu is promoted by adding Cr and Mo, which have a higher phase separation tendency from Cu than that of Fe. It has been found that the strengthening property by is improved. FIG. 1 illustrates this effect, and shows the amount of increase in strength due to the 600 ° C. × 40 sec treatment of a steel sheet in which the amount of Cu added is 1.8% by mass and the amount of Cr + Mo is changed. Thus, the present invention has been made. The summary is as follows.
[0006]
(1) By mass% C: 0.05% or less,
Si: 1.5% or less,
Mn: 0.01 to 1.5%,
P: 0.1 or less,
S: 0.01 or less,
Al: 0.005 to 0.1%,
N: 0.01% or less,
Cu: 1.2-2.5%,
Ni: 0.25 to 1.5%
And a total amount of 0.03 to 2.5% of Cr and Mo, with the balance being a steel sheet having a steel component consisting of iron and unavoidable impurities, press forming, treatment time t (sec), treatment A thin steel sheet characterized in that the tensile strength is increased by 190 MPa or more as compared to the time of press forming by performing a heat treatment under a condition where the temperature T (° C.) satisfies the formula (B) and t ≦ 100 is used. Manufacturing method of a molded object.
exp (-(T-790) / 55) <t <exp (-(T-860) / 55) (B)
(2) In addition to steel components,
Ti: 0.01 to 0.15%,
Nb: 0.01 to 0.15%,
V: 0.01 to 0.15%
The manufacturing method of the molded object using the thin steel plate of Claim 1 using the thin steel plate which contained 1 type, or 2 or more types.
(3) In addition to steel components,
B: 0.0003 to 0.002%
The manufacturing method of the compact | molding | casting using the thin steel plate of Claim 1 or 2 using the thin steel plate containing this.
(4) The manufacturing method of the molded object using the thin steel plate of Claim 1 or Claim 2 or Claim 3 using the thin steel plate which performed the surface treatment.
(5) The method for producing a formed body using the thin steel sheet according to claim 4, wherein the surface treatment is galvanization.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
The present invention adds Cr and Mo, which have a higher phase separation tendency than Cu to steel added with 1.0 to 2.5% of Cu, and promotes precipitation of Cu during post-heat treatment. The strengthening property by the post-heat treatment condition for the time is improved. Although the present invention is based on the above-mentioned outline, the individual components of the present invention will be described in detail below.
[0008]
First, the reasons for limiting the components of the present invention will be described.
C is an element that affects the workability of steel, and the workability deteriorates as the content increases. Therefore, it is made 0.05% or less. Further, when the addition amount is increased, the refinement of crystal grains is advanced and the interfacial area of the grains is increased, so that the amount of solid solution of Cu is reduced.
[0009]
Si is a preferable element for improving strength and ductility through solid solution strengthening. However, if it exceeds 1.5%, the workability deteriorates, so 1.5% is made the upper limit.
[0010]
If Mn is less than 0.01%, S which is an impurity in steel is not sufficiently fixed as MnS, and cracks occur during hot rolling. Furthermore, Mn is a preferable element that improves the strength and ductility by solid solution strengthening, like Si. However, if it exceeds 1.5%, the workability may deteriorate, so 1.5% is made the upper limit.
[0011]
P is added according to the strength level required as an element for increasing the strength of the steel sheet. However, if it exceeds 0.1%, there will be a problem of secondary work brittleness of the steel sheet, so the upper limit is made 0.1%.
[0012]
S is preferably an element that does not exist in steel, and in order to improve the workability, the lower one is desirable and the upper limit is set to 0.01%.
[0013]
Al is preferably added in an amount of 0.005% or more because it is preferable to trap N as AlN to reduce the solid solution N. However, since the workability deteriorates when the addition amount increases, the upper limit is made 0.1% or less.
[0014]
N is preferable to be small in order to ensure workability. If it exceeds 0.01%, workability deteriorates, so 0.01% is made the upper limit.
[0015]
Cu remarkably increases the strength of the steel by aging precipitation, while it does not significantly reduce the ductility of the steel in the Cu solid solution state. The strength increasing ability due to aging precipitation depends on the amount of Cu added, but if it is less than 1.2%, no matter how much Cr or Mo is added, the amount of precipitation is small, and it takes a long time, so that the strength hardly increases. On the other hand, if it exceeds 2.5%, the strength increasing ability is almost saturated no matter how Cr and Mo are added, and this is the upper limit. Further, the addition of Cu is desirably 1.8% or less because it impairs the recyclability of steel.
[0016]
Ni is effective for suppressing surface defects (commonly known as Cu heges) that are likely to occur during hot rolling in Cu-added steel. For this reason, Ni needs to be added at 0.25% or more. However, if the addition amount is large, the effect is saturated, and since Ni is expensive, the economy is remarkably impaired, so the upper limit is made 1.50%. Since such a surface defect suppressing effect of Ni is exhibited according to the amount of Cu added, it is desirable that the ratio of Ni added is 0.25 to 0.6 according to Ni / Cu.
[0017]
Cr and Mo are extremely important elements in the present invention. As shown in FIG. 1 (Cu = 1.8% contained), by adding 0.03% or more of Cr + Mo to the Cu-added thin steel sheet, press forming is performed by heat treatment after press forming (for example, heating at 600 ° C. × 40 s). The inventors have found that a significant increase in tensile strength (ΔTS) is obtained compared to before.
[0018]
Although the cause of this is not clear, the present inventors consider that Cr and Mo have a higher phase separation tendency from Cu than Fe, and therefore, when Cr + Mo is added to Cu steel, Cu precipitation is promoted for a short time. If the total amount of one or two of Cr and Mo is less than 0.03%, the effect cannot be obtained. On the other hand, when the added amount is increased, the refinement of crystal grains is promoted, and the in-grain solid solution amount of Cu is increased. In order to reduce, the upper limit is set to 2.5%.
[0019]
Ti, Nb, and V are one of Ti: 0.01 to 0.15%, Nb: 0.01 to 0.15%, and V: 0.01 to 0.15% for improving aging and ductility. Add two or more. When at least one of Ti: 0.01% or more, Nb: 0.01% or more, and V: 0.01% or more is added, these carbonitrides are produced, C and N are fixed, and the resulting steel sheet is non- An aging steel sheet is obtained, and ductility is improved. Moreover, since the strength of steel can be raised by these carbonitrides, it can also be used for strength adjustment. The reason why the upper limit of Ti, Nb, and V is 0.15% is that if it exceeds 0.15%, the effect is saturated and the cost is increased.
[0020]
B has an action of suppressing secondary processing brittleness, and in order to exert this effect, the lower limit is made 0.0003%. However, if it exceeds 0.002%, the effect is saturated, so 0.002% is made the upper limit.
[0021]
Next, the manufacturing method of the thin steel plate of this invention is demonstrated. The manufacturing method may be a commonly used method for manufacturing a hot-rolled steel sheet, a cold-rolled steel sheet, or a plated steel sheet. In particular, hot rolling is performed at Ar3 or higher in order to prevent the ferrite grains from being excessively strained by hot rolling to reduce workability. In addition, since Cu precipitation during winding may significantly reduce the strength increase due to post-heat treatment, the winding temperature is preferably 500 ° C. or lower.
[0022]
If the strength level of the steel plate before work forming is not specified and the above components are included even in a relatively soft thin steel plate such as 370 MPa class, 390 MPa class, 440 MPa class, 490 MPa class, 590 MPa class, or 690 MPa class Work forming such as press forming is easy, and the heat treatment strength increasing ability of the steel sheet after forming such as press forming is excellent.
[0023]
The steel plate of the present invention may be a hot rolled steel plate, a cold rolled steel plate, a hot dip galvanized steel plate, an alloyed hot dip galvanized steel plate, or an electrogalvanized steel plate, and can enjoy the effects of the invention. In addition, steel sheets that have been temper-rolled for improved workability and appearance after processing (dull-finished steel sheets, bright-finished steel sheets, steel sheets with a specific pattern transferred on the surface, etc.), and anti-rust oil on the surface In any steel sheet that has been subjected to various surface treatments usually used for thin steel sheets, such as steel sheets having an oil film layer such as lubricating oil, the effects of the present invention can be fully enjoyed as long as the steel sheet is within the component range of the present invention. can do.
[0024]
In particular, for the purpose of imparting corrosion resistance, it is effective to use zinc-based plating as the surface-treated steel sheet. As zinc-based plating, hot dip galvanizing, alloy hot dip galvanizing, electro galvanizing, electro zinc alloy (zinc-iron alloy, zinc-nickel alloy, zinc-chromium alloy, etc.) plating, alloyed electro galvanizing, etc. Any material containing zinc may be used.
[0025]
Then, press forming such as drawing, for example, drawing, is performed using the steel plate of the present invention component. The press molding method is not particularly defined, and there is no problem even if drawing, overhanging, bending, ironing, punching, or the like is added.
[0026]
When the processing time is too short with respect to the processing temperature as a heat treatment (post heat treatment) condition after the press of the compact made of a thin steel plate capable of increasing the tensile strength of the compact after press molding by 190 MPa or more, sufficient Cu precipitation Is difficult to occur and is not preferable. On the other hand, when the length is too long, it is difficult to ensure the strength due to the overaging precipitation of Cu, which is not preferable. In particular, long-term aging may reduce the productivity of the molded body and may deteriorate the surface treatment performance. Therefore, the condition of the treatment time t ≦ 100 and the condition (B) are assumed.
exp (-(T-790) / 55) <t <exp (-(T-860) / 55) (B)
[0027]
A preferable range of the post heat treatment temperature is 350 to 800 ° C, and a more preferable range is 450 to 700 ° C. If it is less than 350 degreeC, processing time may become remarkably long and it is not necessarily industrial. Above 800 ° C., no matter how much Cr or Mo is added, Cu is not sufficiently precipitated, and there is a case where the increase in tensile strength is small in the post-heat treatment, which is not preferable. At 450 to 700 ° C., the addition of Cr and Mo makes it possible to use Cu deposition relatively stably. For example, the required post-processing heat treatment strength can be increased even in a short heat treatment time of several seconds to several minutes. It is preferable because it is possible.
The heat treatment method for heating is not particularly limited, and any method may be used as long as it is a method of heating to a predetermined range where strength is required, such as partial high-frequency heating, energization heating, warm bath heat treatment, infrared heating, hot air heating, and the like.
[0028]
【Example】
Next, this invention is demonstrated based on an Example.
Steels having the components shown in Table 1 were melted and slabs were obtained by continuous casting according to a conventional method. Steels with the symbols M, N, O, Q, and S according to the present invention, steels with the symbols U and V have Cr and Mo added, and steels with W and X have Cu added outside the scope of the present invention. It is. These steels are heated to 1200 ° C. in a heating furnace, hot-rolled at a finishing temperature of 880 ° C., and wound up at 500 ° C. or less. This was followed by pickling, cold rolling, and recrystallization annealing at 850 ° C. × 60 sec to obtain a cold rolled steel sheet.
[0029]
The obtained steel plate was processed into a JIS No. 5 tensile test piece, and the mechanical property value without treatment was evaluated. Table 1 shows the TS of each test piece. Further, after applying 5 to 10% pre-strain so as to correspond to post-molding heat treatment, heat treatment (post heat treatment) was performed on the JIS No. 5 tensile test piece under the conditions shown in Table 2, and mechanical property values were evaluated. . Table 2 shows the TS after the post heat treatment and the TS increase (ΔTS) by each heat treatment . From the results in Table 2, a strength increase of 190 MPa or more was obtained by performing heat treatment for each time at a temperature within a range defined by the present invention. Outside this range, the strength increase was less than this. Furthermore, Oite Table 3 U, V, W, X but is steel sheet outside the present invention component, U, the steel plate of V Cr, in rapid thermal of the invention for the effect of Mo is insufficient 190MPa The above increase in tensile strength cannot be obtained. Since the W and X steel sheets have a small amount of Cu added, the time is shortened due to the effects of Cr and Mo, but a sufficient increase in tensile strength cannot be obtained as the absolute value of ΔTS.
[0030]
[Table 1]

[0031]
[Table 2]

[0032]
[Table 3]

[0033]
[0034]
[0035]
[Effect of the present invention]
According to the present invention, the strength level is 370 MPa class, 390 MPa class, 440 MPa class, 490 MPa class, 590 MPa class, or 690 MPa class, etc. Then, by performing heat treatment at a relatively low temperature for a short time after molding such as press molding, it is possible to increase the tensile strength and increase the deformation strength of the part or the molded body.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the Cr + Mo content of a Cu-added thin steel sheet and the tensile strength increase (ΔTS) due to heat treatment after work forming.

Claims

In mass% C: 0.05% or less,
Si: 1.5% or less,
Mn: 0.01 to 1.5%,
P: 0.1 or less,
S: 0.01 or less,
Al: 0.005 to 0.1%,
N: 0.01% or less,
Cu: 1.2-2.5%,
Ni: 0.25 to 1.5%
And a total amount of 0.03 to 2.5% of Cr and Mo, with the balance being a steel sheet having a steel component consisting of iron and unavoidable impurities, press forming, treatment time t (sec), treatment A thin steel sheet characterized in that the tensile strength is increased by 190 MPa or more as compared to the time of press forming by performing a heat treatment under a condition where the temperature T (° C.) satisfies the formula (B) and t ≦ 100 is used. Manufacturing method of a molded object.
exp (-(T-790) / 55) <t <exp (-(T-860) / 55) (B)

In addition to steel components
Ti: 0.01 to 0.15%,
Nb: 0.01 to 0.15%,
V: 0.01 to 0.15%
The manufacturing method of the molded object using the thin steel plate of Claim 1 using the thin steel plate which contained 1 type, or 2 or more types.

In addition to steel components
B: 0.0003 to 0.002%
The manufacturing method of the compact | molding | casting using the thin steel plate of Claim 1 or 2 using the thin steel plate containing this.

The manufacturing method of the molded object using the thin steel plate of Claim 1 or Claim 2 or Claim 3 using the thin steel plate which surface-treated.

The method for producing a formed body using the thin steel sheet according to claim 4, wherein the surface treatment is galvanization.