JP5057299B2

JP5057299B2 - Steel plate for strength gradient member and method for producing the same

Info

Publication number: JP5057299B2
Application number: JP2001027524A
Authority: JP
Inventors: 治郎平本; 隆明比良; 真次郎金子; 章男登坂; 坂田　　敬
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2001-02-02
Filing date: 2001-02-02
Publication date: 2012-10-24
Anticipated expiration: 2021-02-02
Also published as: JP2002235147A

Description

【０００１】
【発明の属する技術分野】
本発明は、強度傾斜部材用鋼板とその製造方法に関する。強度傾斜部材とは、部材内で部分的に強度の異なる部材であって、該部材用鋼板（素材）をプレス成形後に塗装後焼付け処理等の熱処理（時効処理ともいう）を施して製造されるものを意味する。この強度傾斜部材は、現状の主用途が自動車の分野にあるが、家電、建築等の分野に適用することも可能である。また、ここで強度傾斜部材は、強度が徐々に変化するか否かにかかわらず、部材内で部分的に強度の異なる部材を意味する。
【０００２】
【従来の技術】
地球環境保護の観点から自動車車体の軽量化が要求されている。その一方で、自動車衝突時の安全確保のために車体の耐衝突強度を高めることも要請されている。この車体の軽量化と耐衝突強度向上とを十分なレベルで両立させることは難しく、例えば耐衝突強度を向上させようとして衝突部材の肉厚を厚くしあるいは補強材を用いると車体重量が増加するというジレンマに陥る。
【０００３】
そのため、キャビンに近い部分は変形しにくい高強度材および／または厚肉材で形成され、キャビンから遠い（衝突物体に近い）部分は長さ方向に蛇腹状に座屈して衝突のエネルギーを吸収しやすい低強度材および／または薄肉材で形成された部材が使用され、衝突のエネルギーを効率よく吸収する部材が必要となっている。かかる部材の如く一つのプレス成形部材において部分部分で属性を違えたものを傾斜部材と称し、違えた属性が強度であるものを強度傾斜部材と称する。
【０００４】
一方、プレス成形の分野ではプレスの効率化やプレス金型の保有数削減が指向されており、これに適合させた強度傾斜部材製作法として、複数の鋼板を予めレーザ等で接合して一体化した部材をプレス成形するテーラードブランク法や、プレス成形品の強度所望部位を該部位に高密度エネルギー源（レーザ等）を照射することにより焼入れ硬化部となす方法（特開平４−72010 号公報）などが知られている。
【０００５】
【発明が解決しようとする課題】
前記従来の強度傾斜部材は、その製作過程で接合や焼入れにレーザ等を使用する必要があるため、設備費が高くまた生産性が悪いという問題があった。本発明の目的は、この問題を解決しうる手段、すなわち強度傾斜部材をレーザ等による接合あるいは焼入れを要さずに生産性良く製造できる強度傾斜部材用鋼板とその製造方法を提供することにある。
【０００６】
【課題を解決するための手段】
本発明は、プレス成形後熱処理された部材内に強度が他部よりも高い高強度部を有する強度傾斜部材の前記プレス成形前素材として用いる鋼板であって、下記の特性を有する高歪時効硬化鋼板の前記高強度部となす箇所にレベリング加工にて予歪５％以上を付与してなることを特徴とする強度傾斜部材用鋼板（本発明鋼板）である。
記
歪５％付与後に170℃×20分の熱処理を施した場合の引張強度が、歪を付与せず同じ熱処理を施した場合の引張強度に比べて40MPa 以上上昇する特性
また、本発明鋼板では、前記プレス成形前素材が、第１成分（Ｃ：0.01〜0.12％、Si：2.0 ％以下、Mn：0.01〜3.0 ％、Ｐ：0.2 ％以下、Al：0.001 〜0.1 ％、Ｎ：0.003 〜0.02％）を全て含有し、あるいはさらに、第２成分（Ti：0.001 〜0.1 ％、Nb：0.001 〜0.1 ％）のうちの１種または２種、および／または、第３成分（Ni：0.1 〜1.5 ％、Cr：0.1 〜1.5 ％、Mo：0.1 〜1.5 ％）のうちの１種または２種以上を含有し、平均結晶粒径８μm 以下のフェライトを主相とする組織を有し、さらに、固溶Ｎ量＝0.003〜0.01％であり、フェライト結晶粒界面から±５nmの範囲内に存在する平均固溶Ｎ濃度Ngb とフェライト結晶粒内に存在する平均固溶Ｎ濃度Ngとの比 Ngb/Ng ＝100 〜10000 である高張力熱延鋼板であることが好ましい。
【０００７】
また、本発明は、プレス成形後時効処理された部材内に強度が他部よりも高い高強度部を有する強度傾斜部材の前記プレス成形前素材として用いる鋼板の製造方法であって、前記の特性を有する高歪時効硬化鋼板に前記高強度部となす箇所を定め、該箇所に対し予歪５％以上になるレベリング加工を施すことを特徴とする強度傾斜部材用鋼板の製造方法（本発明方法）である。
本発明方法では、前記プレス成形前素材が、第１成分（Ｃ：0.01〜0.12％、Si：2.0 ％以下、Mn：0.01〜3.0 ％、Ｐ：0.2 ％以下、Al：0.001 〜0.1 ％、Ｎ：0.003 〜0.02％）を全て含有し、あるいはさらに、第２成分（Ti：0.001 〜0.1 ％、Nb：0.001 〜0.1 ％）のうちの１種または２種、および／または、第３成分（Ni：0.1 〜1.5 ％、Cr：0.1 〜1.5 ％、Mo：0.1 〜1.5 ％）のうちの１種または２種以上を含有し、平均結晶粒径８μm 以下のフェライトを主相とする組織を有し、さらに、固溶Ｎ量＝0.003〜0.01％であり、フェライト結晶粒界面から±５nmの範囲内に存在する平均固溶Ｎ濃度Ngb とフェライト結晶粒内に存在する平均固溶Ｎ濃度Ngとの比 Ngb/Ng ＝100 〜10000 である高張力熱延鋼板であることが好ましい。
【０００８】
【発明の実施の形態】
本発明は、歪５％付与後に比較的低温の熱処理である400 ℃以下の熱処理を施すことにより、歪を付与せず同じ熱処理を施したものに比して引張強度（引張強さ又はＴＳとも称する。）が40MPa 以上上昇する特性を有する高歪時効硬化鋼板の特徴を有利に引き出したものであり、この高歪時効硬化鋼板に部分的に予歪５％以上を与えてなる本発明鋼板にプレス成形→熱処理を施すと、前記予歪を与えた部分のＴＳが他の部分に比べて顕著に上昇する。
【０００９】
それゆえ、予め実験等で求めておいた前記予歪の歪量と前記プレス成形→熱処理後のＴＳの上昇量との関係に基づいて、前記プレス成形→熱処理後のＴＳの目標値に対応する歪量の値ε_Aを決定し、歪量ε_Aの予歪を高強度部としたい箇所に与えておくだけで、プレス成形→熱処理後に前記高強度部としたい箇所のＴＳが前記目標値通りとなった強度傾斜部材を得ることができる。
【００１０】
すなわち、本発明鋼板によれば、これをそのままプレス成形→400 ℃以下の熱処理するだけで、レーザ等の高価な接合あるいは照射加熱設備を必要とせず安価にかつ生産性良く強度傾斜部材を製造することができる。
通常の鋼板を使用しても予歪による加工硬化によりプレス成形後に部分的に材料が硬くなった部材を製造することが可能であるが、本発明によれば、高歪時効硬化鋼板を使用することにより、通常鋼板を単に加工硬化させた場合よりも異部分間の強度差が格段に大きい強度傾斜部材を製造することができる。
【００１１】
本発明で用いる高歪時効硬化鋼板は、歪５％付与後に400 ℃以下の熱処理を施した場合のＴＳが、歪を付与せず同じ熱処理を施した場合に比べて40MPa 以上上昇する特性を有する鋼板でなければならない。このＴＳ上昇量（ΔＴＳ＝（歪５％付与後に熱処理を施した場合のＴＳ）−（歪を付与せず同じ熱処理を施した場合のＴＳ））が40MPa に達しない鋼板では、部材内での強度の差が小さく、部材内で部分的に強度を変化させるメリットを享受することができない。
【００１２】
ここで、熱処理温度は400 ℃以下とする。400 ℃を超える高温処理では、処理を行うための設備費が高くなる。また、熱処理として一般的に行われているプレス形成後の塗装焼き付け処理を採用することが、設備費を小さくする上で、非常に有利である。このため、プレス形成後の塗装焼き付け相当処理、すなわち170 ℃×２０分処理する熱処理により40MPa 以上引張り強度が上昇する高歪時効硬化鋼板が好ましい。
【００１３】
高歪時効硬化鋼板としては、例えば特開2000−297350号公報に開示される鋼板、すなわち、第１成分（Ｃ：0.01〜0.12％、Si：2.0 ％以下、Mn：0.01〜3.0 ％、Ｐ：0.2 ％以下、Al：0.001 〜0.1 ％、Ｎ：0.003 〜0.02％）を全て含有し、あるいはさらに、第２成分（Ti：0.001 〜0.1 ％、Nb：0.001 〜0.1 ％）のうちの１種または２種、および／または、第３成分（Ni：0.1 〜1.5 ％、Cr：0.1 〜1.5 ％、Mo：0.1 〜1.5 ％）のうちの１種または２種以上を含有し、平均結晶粒径８μm 以下のフェライトを主相とする組織を有し、さらに、固溶Ｎ量＝0.003 〜0.01％であり、フェライト結晶粒界面から±５nmの範囲内に存在する平均固溶Ｎ濃度Ngb とフェライト結晶粒内に存在する平均固溶Ｎ濃度Ngとの比 Ngb/Ng ＝100 〜10000 である高張力熱延鋼板が好適である。
【００１４】
本発明では、高歪時効硬化鋼板に付与する予歪の歪量は５％以上とする必要がある。この歪量が５％に満たないと、高歪時効硬化鋼板を用いてもプレス成形→熱処理後の部材内強度差が過小となるおそれがあり、強度傾斜部材用鋼板として不十分なものとなる。
本発明において、予歪の歪量は次のように定義される。すなわち、予歪導入用の加工装置（例：レベラ、圧延機など）で加工する前の鋼板について引張試験を行って真応力（σ）−真歪（ε）曲線：σ＝Ｆ（ε）を求め、次に同鋼板を同加工装置で加工したものについて引張試験を行って降伏点強度（ＹＰ）を求め、次に前記真応力−真歪曲線σ＝Ｆ（ε）を用いてσ＝ＹＰに対応する真歪εの値ε_YPを求め、これを当該加工装置で導入された予歪の歪量とする（図１参照）。加工装置の操作量（加工操作量）とＹＰとの関係は別途実験により定めうるから、この関係と前記真応力−真歪曲線とを用いて、加工操作量を予歪の目標歪値に対応した値に設定することができる。
【００１５】
本発明では、前記高歪時効硬化鋼板への予歪付与方法はレベリング加工とすることが好ましい。高歪時効硬化鋼板に予歪を与える方法としては、レベリング加工の他に圧延加工が考えられるが、圧延加工は１対のロールで板を挟んで圧下する加工であるため鋼板の被加工部分（高強度部としたい箇所）の板厚（肉厚）減少を伴い、剛性の面で不利となって好ましくない。これに対してレベリング加工は複数のロールを上下互い違いに配置したレベラ２を用いた曲げ曲げもどし主体の加工であるから鋼板１の被加工部分の板厚減少をほとんど生じることがなく、予歪導入方法として最適である（図２参照）。
【００１６】
そして本発明方法によれば、レベラの加工操作量（矯正量、鋼板通し部位、鋼板通し回数など）を設定するだけで、任意の鋼板長さ部位に予歪を任意の歪量で与えることができるから、強度傾斜部材にテーパ状、ステップ状等の様々な強度分布を容易に付与することができる。
【００１７】
【実施例】
耐衝突性評価試験では、図３に示す寸法形状を有する板状材料（強度傾斜部材用鋼板に相当）をプレス成形により図４に示す寸法形状を有するハット型成形部材（強度傾斜部材に相当）となし、これに熱処理として170 ℃×20分を施したものを試験片としその部位Ａ側から重さ270kg の重りを時速20km/hで衝突させ、ハット型成形部材の潰れ量60mmまでの吸収エネルギーにより耐衝突性が評価される。
【００１８】
表１に示す特徴を有する板厚1.4mm の熱延鋼板▲１▼〜▲４▼を用いて表２に示す方法で図４のハット型成形部材の成形素材となすべき図３の板状材料を作製して上記の耐衝突特性評価試験を行った。その結果を表２に示す。なお、表１においてΔＴＳは以下のような形で求めた。
ΔＴＳ＝（歪５％付与後に170 ℃×20分熱処理を施した場合のＴＳ）
−（歪を付与せず170 ℃×20分熱処理を施した場合のＴＳ）
【００１９】
【表１】

【００２０】
【表２】

【００２１】
実施例１、２はΔＴＳが40MPa 以上になる鋼板（高歪時効硬化鋼板）▲１▼、▲２▼の部位Ａ相当部分に予歪を与えず部位Ｂ相当部分にレベラで予歪５％を与えたものでこの場合の板状材料が本発明鋼板に該当するもの、比較例１はΔＴＳが40MPa 未満になる鋼板▲３▼の部位Ａ相当部分に予歪を与えず部位Ｂ相当部分にレベラで予歪５％を与えたもの、比較例２は部位Ａ側に鋼板▲１▼、部位Ｂ側に高強度鋼板▲４▼を充当して両者をテーラードブランク法で接合したもの、比較例３は鋼板▲１▼の部位Ａ相当部分にも部位Ｂ相当部分にも予歪を与えなかったものである。
【００２２】
実施例１、２では、比較例２に匹敵する耐衝突性が得られており、本発明鋼板を強度傾斜部材の素材に用いることで、高価な接合装置等によらずとも生産性良く強度傾斜部材を製造できることがわかる。
【００２３】
【発明の効果】
本発明によれば、特別な装置を必要とせずに生産性良く強度傾斜部材を製造することができるという効果を奏する。
【図面の簡単な説明】
【図１】予歪の歪量の定義説明図である。
【図２】部分的に施すレベリング加工の説明図である。
【図３】ハット型成形部材用の板状材料を示す平面図である。
【図４】ハット型成形部材を示す立体図である。
【符号の説明】
１鋼板
２レベラ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel plate for a strength gradient member and a method for producing the same. A strength-graded member is a member having partially different strength within the member, and is manufactured by subjecting the steel sheet (material) for the member to heat treatment (also referred to as aging treatment) such as post-coating baking after press forming. Means things. This strength gradient member has the current main application in the field of automobiles, but can also be applied to fields such as home appliances and architecture. Moreover, an intensity | strength inclination member means the member from which intensity | strength differs partially within a member irrespective of whether an intensity | strength changes gradually here.
[0002]
[Prior art]
From the viewpoint of protecting the global environment, there is a demand for lighter automobile bodies. On the other hand, it is also required to increase the collision resistance of the vehicle body in order to ensure safety during a car crash. It is difficult to achieve a sufficient level of weight reduction of the vehicle body and collision resistance strength. For example, if the thickness of the collision member is increased or a reinforcing material is used to improve the collision resistance strength, the weight of the vehicle body increases. It falls into the dilemma.
[0003]
Therefore, the part close to the cabin is made of a high-strength material and / or thick material that is difficult to deform, and the part far from the cabin (close to the collision object) buckles like a bellows in the length direction to absorb the energy of the collision. A member formed of an easy-to-use low-strength material and / or a thin-walled material is used, and a member that efficiently absorbs collision energy is required. A member having a different attribute in a part of one press-formed member such as this member is referred to as an inclined member, and a member having a different attribute is strength is referred to as a strength inclined member.
[0004]
On the other hand, in the field of press forming, the aim is to increase the efficiency of presses and reduce the number of press dies, and as a method of manufacturing strength gradient members adapted to this, a plurality of steel plates are joined together in advance with a laser or the like. A tailored blank method in which a molded member is press-molded, and a method in which a desired strength portion of a press-formed product is formed into a quenching and hardening portion by irradiating the portion with a high-density energy source (laser or the like) (JP-A-4-72010) Etc. are known.
[0005]
[Problems to be solved by the invention]
The conventional strength gradient member has a problem that the equipment cost is high and the productivity is low because it is necessary to use a laser or the like for joining and quenching in the manufacturing process. An object of the present invention is to provide means for solving this problem, that is, a steel plate for a strength gradient member that can be manufactured with high productivity without requiring joining or quenching of the strength gradient member by a laser or the like, and a method for producing the same. .
[0006]
[Means for Solving the Problems]
The present invention is a steel plate used as the pre-press forming material of a strength gradient member having a high strength portion whose strength is higher than other portions in a member heat-treated after press forming, and has a high strain age hardening having the following characteristics: A steel plate for strength gradient members (steel plate of the present invention) obtained by applying a pre-strain of 5% or more to a portion formed by the high strength portion of the steel plate by leveling.
The tensile strength when heat treatment at 170 ° C for 20 minutes is applied after 5% strain is given, and the tensile strength increases by 40 MPa or more compared to the tensile strength when the same heat treatment is performed without applying strain. The material before press molding is the first component (C: 0.01 to 0.12%, Si: 2.0% or less, Mn: 0.01 to 3.0%, P: 0.2% or less, Al: 0.001 to 0.1%, N: 0.003 to 0.02). %)), Or further, one or two of the second components (Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1%), and / or the third component (Ni: 0.1 to 1.5) %, Cr: 0.1 to 1.5%, Mo: 0.1 to 1.5%), and has a structure whose main phase is ferrite with an average crystal grain size of 8 μm or less, The amount of dissolved N is 0.003 to 0.01%, and the average dissolved N concentration Ngb is within ± 5 nm from the ferrite crystal grain interface and is present in the ferrite crystal grain. It is preferably a high-tensile hot-rolled steel sheet having a ratio Ngb / Ng = 100 to 10,000 with respect to the average solid solution N concentration Ng.
[0007]
Also, the present invention provides a steel sheet production method of use as the press molding before the material of the intensity gradient members strength press forming after the aging treated in member has a high strength portion than the other portion, of the A method for producing a steel plate for a strength gradient member, characterized in that a location to be the high strength portion is defined in a high strain age-hardened steel plate having characteristics, and a leveling process is applied to the location to a pre-strain of 5% or more (the present invention) Method).
In the method of the present invention, the material before press molding is the first component (C: 0.01 to 0.12%, Si: 2.0% or less, Mn: 0.01 to 3.0%, P: 0.2% or less, Al: 0.001 to 0.1%, N : 0.003 to 0.02%), or further, one or two of the second components (Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1%), and / or the third component (Ni : 0.1-1.5%, Cr: 0.1-1.5%, Mo: 0.1-1.5%), and has a structure whose main phase is ferrite with an average crystal grain size of 8 μm or less Furthermore, the amount of solid solution N = 0.003 to 0.01%, and the average solid solution N concentration Ngb existing in the range of ± 5 nm from the ferrite crystal grain interface and the average solid solution N concentration Ng existing in the ferrite crystal grain A high-tensile hot-rolled steel sheet having a ratio Ngb / Ng = 100 to 10000 is preferable.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
By applying a heat treatment at 400 ° C. or lower, which is a heat treatment at a relatively low temperature, after applying 5% strain, the present invention has a tensile strength (both tensile strength or TS) compared to that subjected to the same heat treatment without giving strain. Is advantageously extracted from the characteristics of a high strain age-hardened steel sheet having a characteristic of increasing by 40 MPa or more. The steel sheet of the present invention is obtained by partially applying a pre-strain of 5% or more to this high strain age-hardened steel sheet. When press forming → heat treatment is performed, the TS of the pre-strained portion is significantly increased as compared with other portions.
[0009]
Therefore, based on the relationship between the amount of pre-strain obtained in advance by experiments and the amount of increase in TS after press forming → heat treatment, it corresponds to the target value of TS after press forming → heat treatment. By simply determining the strain amount value ε _A and applying the pre-strain of the strain amount ε _A to the location where the high strength portion is desired, the TS at the location where the high strength portion is desired after press molding → heat treatment is the target value. The intensity | strength inclination member which became can be obtained.
[0010]
That is, according to the steel sheet of the present invention, the strength-graded member can be manufactured at low cost and with high productivity without requiring expensive joining such as laser or irradiation heating equipment, by simply performing press forming → 400 ° C. or less as it is. be able to.
Although it is possible to produce a member whose material has become partially hard after press forming by work hardening due to pre-strain even when using a normal steel plate, according to the present invention, a high strain age-hardened steel plate is used. By doing this, it is possible to produce a strength gradient member in which the strength difference between different parts is significantly greater than when the normal steel sheet is simply work-hardened.
[0011]
The high strain age-hardened steel sheet used in the present invention has a characteristic that TS when subjected to heat treatment at 400 ° C. or lower after applying 5% strain is increased by 40 MPa or more as compared with the case where the same heat treatment is performed without applying strain. Must be a steel plate. In steel plates where this TS increase (ΔTS = (TS when heat treatment is applied after applying 5% strain) − (TS when the same heat treatment is applied without applying strain)) does not reach 40 MPa, The difference in strength is small, and the merit of partially changing the strength within the member cannot be enjoyed.
[0012]
Here, the heat treatment temperature is 400 ° C. or less. High-temperature processing exceeding 400 ° C increases the equipment cost for processing. In addition, it is very advantageous to reduce the equipment cost by adopting a coating baking process after press formation, which is generally performed as a heat treatment. For this reason, a high strain age-hardened steel sheet whose tensile strength is increased by 40 MPa or more by a heat treatment in which paint baking is performed after press formation, that is, heat treatment at 170 ° C. for 20 minutes is preferable.
[0013]
As the high strain age-hardened steel sheet, for example, a steel sheet disclosed in JP 2000-297350 A, that is, the first component (C: 0.01 to 0.12%, Si: 2.0% or less, Mn: 0.01 to 3.0%, P: 0.2% or less, Al: 0.001 to 0.1%, N: 0.003 to 0.02%), or one of the second components (Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1%) or 1 type or 2 types or more of 2 types and / or 3rd component (Ni: 0.1-1.5%, Cr: 0.1-1.5%, Mo: 0.1-1.5%), and average grain size of 8 micrometers It has the following structure with the main phase of ferrite, and further, the amount of solid solution N = 0.003 to 0.01%, and the average solid solution N concentration Ngb and ferrite crystal grains present within a range of ± 5 nm from the ferrite crystal grain interface A high-tensile hot-rolled steel sheet having a ratio Ngb / Ng = 100 to 10,000 with respect to the average solid solution N concentration Ng existing therein is suitable.
[0014]
In the present invention, the amount of pre-strain applied to the high strain age-hardened steel sheet needs to be 5% or more. If the amount of strain is less than 5%, even if a high strain age-hardened steel sheet is used, the strength difference in the member after press forming → heat treatment may be too small, and it will be insufficient as a steel sheet for strength gradient members. .
In the present invention, the amount of pre-strain is defined as follows. That is, a tensile test is performed on a steel plate before being processed by a processing apparatus for introducing pre-strain (eg, leveler, rolling mill, etc.), and a true stress (σ) -true strain (ε) curve: σ = F (ε) is obtained. Next, a tensile test is performed on the steel sheet processed by the same processing apparatus to obtain a yield point strength (YP), and then using the true stress-true strain curve σ = F (ε), σ = YP The value ε _YP of the true strain ε corresponding to is obtained and used as the strain amount of the pre-strain introduced by the processing apparatus (see FIG. 1). Since the relationship between the machining device operation amount (machining operation amount) and YP can be determined separately by experiment, using this relationship and the true stress-true strain curve, the machining operation amount corresponds to the target strain value of pre-strain. Value can be set.
[0015]
In the present invention, it is preferable that the prestraining method for the high strain age-hardened steel sheet is leveling. In addition to leveling, rolling can be considered as a method for pre-straining a high strain age-hardened steel sheet. However, since the rolling process is a process of rolling a sheet between a pair of rolls, This is accompanied by a decrease in the plate thickness (thickness) of the portion to be a high-strength portion, which is disadvantageous in terms of rigidity. On the other hand, the leveling process is mainly a bending and returning process using the leveler 2 in which a plurality of rolls are arranged in a staggered manner, so that there is almost no reduction in the thickness of the processed part of the steel sheet 1 and pre-strain is introduced. It is the best method (see FIG. 2).
[0016]
According to the method of the present invention, it is possible to give a pre-strain to an arbitrary length portion of the steel plate with an arbitrary amount of strain only by setting a leveler processing operation amount (correction amount, steel plate passing portion, steel plate passing frequency, etc.). Therefore, various strength distributions such as a taper shape and a step shape can be easily imparted to the strength gradient member.
[0017]
【Example】
In the impact resistance evaluation test, a plate-shaped material having the dimensions shown in FIG. 3 (corresponding to a steel sheet for strength gradient members) is press-molded to form a hat-shaped molding member having a dimensions shown in FIG. 4 (corresponding to strength gradient members). The test piece was treated at 170 ° C for 20 minutes as a heat treatment, and a weight of 270 kg was collided from the part A side at a speed of 20 km / h to absorb the hat-shaped molded member up to a collapse amount of 60 mm. Impact resistance is evaluated by energy.
[0018]
The plate-like material of FIG. 3 to be used as the forming material of the hat-shaped molded member of FIG. 4 by the method shown in Table 2 using hot rolled steel plates (1) to (4) having a thickness of 1.4 mm having the characteristics shown in Table 1. The above-mentioned collision resistance evaluation test was conducted. The results are shown in Table 2. In Table 1, ΔTS was determined in the following manner.
ΔTS = (TS when heat treatment is performed at 170 ° C for 20 minutes after 5% strain is applied)
-(TS when heat treatment is performed at 170 ° C for 20 minutes without applying strain)
[0019]
[Table 1]

[0020]
[Table 2]

[0021]
In Examples 1 and 2, a steel plate having a ΔTS of 40 MPa or more (high strain age-hardened steel plate) No pre-strain is applied to the portion A corresponding to the portion A in (1) and (2). In this case, the plate-like material corresponds to the steel plate of the present invention, and in Comparative Example 1, the portion corresponding to the portion A of the portion A of the steel plate (3) where ΔTS is less than 40 MPa is not pre-strained. In Comparative Example 2, a steel plate (1) was applied to the part A side and a high-strength steel plate (4) was applied to the part B side, and both were joined by the tailored blank method, Comparative Example 3 No. No pre-strain was applied to the portion A or the portion B corresponding to the steel plate (1).
[0022]
In Examples 1 and 2, the impact resistance comparable to that of Comparative Example 2 was obtained, and by using the steel sheet of the present invention as the material of the strength gradient member, the strength gradient was improved with high productivity without depending on an expensive joining device or the like. It can be seen that the member can be manufactured.
[0023]
【Effect of the invention】
According to the present invention, there is an effect that the strength inclined member can be manufactured with high productivity without requiring a special device.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of definition of pre-distortion distortion amount.
FIG. 2 is an explanatory diagram of leveling processing performed partially.
FIG. 3 is a plan view showing a plate-like material for a hat-shaped molded member.
FIG. 4 is a three-dimensional view showing a hat-shaped molded member.
[Explanation of symbols]
1 Steel plate 2 Leveler

Claims

A steel plate used as the pre-press forming material of a strength gradient member having a high strength portion whose strength is higher than other portions in a member heat-treated after press forming, the steel plate being a first component (C: 0.01 to 0.12%) , Si: 2.0% or less, Mn: 0.01 to 3.0%, P: 0.2% or less, Al: 0.001 to 0.1%, N: 0.003 to 0.02%), or further, the second component (Ti: 0.001 to One or two of 0.1%, Nb: 0.001 to 0.1%) and / or the third component (Ni: 0.1 to 1.5%, Cr: 0.1 to 1.5%, Mo: 0.1 to 1.5%) And having a structure with ferrite as the main phase having an average crystal grain size of 8 μm or less, and a solid solution N content of 0.003 to 0.01%, ± 5 nm from the ferrite crystal grain interface High-tensile hot-rolled steel in which the ratio of the average solute N concentration Ngb existing in the range of Ngb to the average solute N concentration Ng existing in the ferrite crystal grains is Ngb / Ng = 100 to 10,000 , And the high strain age hardening the high intensity part and forming strength inclined member for steel sheet characterized by being obtained by applying the above pre-strain 5% by leveling work at a position of the steel sheet further having the following characteristics.
Tensile strength when heat treatment at 170 ° C for 20 minutes after 5% strain is applied increases by more than 40 MPa compared to the tensile strength when the same heat treatment is applied without strain

A method of manufacturing a steel sheet used as the pre-press forming material of a strength gradient member having a high strength portion having a strength higher than that of the other portion in a member heat-treated after press forming, wherein the steel plate is a first component (C: 0.01 to 0.12%, Si: 2.0% or less, Mn: 0.01 to 3.0%, P: 0.2% or less, Al: 0.001 to 0.1%, N: 0.003 to 0.02%), or the second component ( One or two of Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1%) and / or a third component (Ni: 0.1 to 1.5%, Cr: 0.1 to 1.5%, Mo: 0.1 to 1.5) %)), And has a structure whose main phase is ferrite with an average crystal grain size of 8 μm or less, and the amount of solute N is 0.003 to 0.01%. The ratio of the average solid solution N concentration Ngb existing in the range of ± 5 nm from the interface to the average solid solution N concentration Ng present in the ferrite crystal grains is Ngb / Ng = 100 to 10,000. A tensile hot-rolled steel sheet, the strength further defines a portion serving as the high-strength portion in a high strain age hardening steel sheet having the properties described below, it is characterized by performing the leveling process becomes 5% or more pre-strain to relevant section Manufacturing method of steel plate for inclined members.
Tensile strength when heat treatment at 170 ° C for 20 minutes after 5% strain is applied increases by more than 40 MPa compared to the tensile strength when the same heat treatment is applied without strain