JP3662335B2

JP3662335B2 - Welded steel sheet for composite forming

Info

Publication number: JP3662335B2
Application number: JP10314796A
Authority: JP
Inventors: 秀則白沢; 道治中屋
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 1996-03-28
Filing date: 1996-03-28
Publication date: 2005-06-22
Anticipated expiration: 2016-03-28
Also published as: JPH09262683A

Description

【０００１】
【発明が属する技術分野】
メンバ，ブラケットなどの自動車を始めとする輸送機器、足場板などの建築用部材などに使用される溶接鋼板であって、プレス成形の際に張り出し、絞り、伸びフランジ、曲げなどの成形加工が複合的に行われる複合成形に適した溶接鋼板に関する。
【０００２】
【従来の技術】
自動車の燃費向上を目的とした車両の軽量化および自動車製造コスト低減の一つの策として、ドア，ピラーなどは１枚の同一鋼板から成形するのではなく、例えば路面に近い、腐食され易い方には耐食性に優れためっき鋼板を使用し、一方路面側に比して腐食され難い車両上部側は冷延鋼板を使用することができるように、特性の異なる鋼板を突き合わせてレーザ溶接した溶接鋼板をプレス成形用素板として用いて所定の部品を製造する技術が開発されている。このような技術は、例えば文献「Tyssenn Technische Berichte,Heft1/92,p.97 」に紹介されている。また、このような鋼板の組み合わせは、同一組成のめっき鋼板と冷延鋼板とに限らず、化学組成及び強度の異なる鋼板の組み合わせについても行われており、例えば「塑性と加工、Vol.34(1993),p917 」、「溶接学会論文集、Vol.10(1992),p196 」に開示されている通り、国内でも実用化されている。
【０００３】
もっとも、これら技術は、接合部に厳しい絞り成形が施されるドアインナパネルなどでは、板厚がいずれも１．４mm未満の極薄鋼板同士の接合についてのものが多く、また材料のＣ量がいずれも０．０５％以下の極軟鋼板同士の接合がほとんどである。また、板厚および板厚差がより大きい鋼板同士の接合や、引張強度が３９０Ｎ／mm²以上の鋼板と軟鋼板とを接合したピラーなども報告されているが、継手部の成形性が十分ではなく、部品形状や成形条件が制約されているのが現状である。
【０００４】
【発明が解決しようとする課題】
最近、車両の軽量化は上記パネル部品の他、メンバ、アーム類などの比較的高強度で板厚の厚い部位を含む部品に対しても検討されるようになっており、種々の加工用鋼板が開発されているものの、上述の通り、これら部品の素板として苛酷なプレス成形に耐え得る溶接鋼板が適用された例はない。
【０００５】
その理由は、パネル部品に使用される軟質な鋼板に比べて、メンバ、アーム類の高強度の部位に使用される高強度（硬質）鋼板はＣ，Ｍｎなどの合金元素含有量が高いために、溶接過程で双方の鋼板端部を単に当接させて溶接するに過ぎないレーザ溶接では、溶接接合部の硬化が著しく、プレス成形段階での溶接鋼板の成形性が大きく劣化するためである。また、レーザ溶接を行うには、鋼板の突き合わせ端面に当接精度が要求されるが、パネル部品に比べて元来板厚が厚い鋼板の場合、鋼板シャー切断による前加工では、十分な端面精度が得られないことも考えられる。
【０００６】
ところで、メンバ、アーム類などの部品の高強度薄肉化に伴う軽量化は、部品にかかる負荷様式によって材料選択の指針が異なる。静的引張り強度で評価した場合、板厚を１０％減少させるには強度を１０％増せばよい。しかし、曲げ、疲労、衝撃などが加わる負荷環境下では、静的引張り強度と異なって、強度と板厚が１対１の関係でなくなり、一般には板厚を１０％減少させる場合、強度は約１５％増大させるように材料の選択が検討される。ところが、このような材料強度の増大は成形性を大きく劣化させるので、成形性が一段と優れた材料の開発が必要になり、またプレス成形で破断しやすい部位は部品形状をよりシンプルにするなどの設計変更が必要となり、部品のわずかな薄肉化でも膨大な実験や試行作業が必要になる。
【０００７】
従って、アーム類などの足回り部品のように、軽量化による燃費向上効果が大きく、薄肉化の要求が強い部品に対しては、部品の強度や成形性の面で、高強度が要求される所定の部位に対してより厚肉のある、あるいはより高強度の材料を使用し、強度の要求されない部位に対しては薄肉の材料が使用できれば好都合である。
【０００８】
本発明は、かかる要求に対してなされたもので、自動車のメンバ、アーム類など比較的高強度で強度差があり、板厚の厚い部位を含む部品の一体成形に適した、複合成形用溶接鋼板を提供することを課題とする。
【０００９】
【課題を解決するための手段】
本発明の複合成形用溶接鋼板は、板厚が異なり、かつ引張強さの差が１２０Ｎ／ mm ² 以上で化学組成が異なる鋼板の端部を突き合わせて溶接する際に加熱溶融状態で突き合わせ方向に加圧して接合した溶接鋼板であって、溶接接合部の最高硬さが３７０Ｈｖ以下、一方の鋼板の溶接熱影響部の平均硬さと他方の同平均硬さの差が９０Ｈｖ以下、一方の鋼板の板厚をｔ₁，他方の同板厚をｔ₂としたとき｜ln（ｔ₁／ｔ₂）｜≦０．６であることを特徴とするものである。
【００１０】
本発明の溶接鋼板によれば、双方の材料を単に加熱溶融して冷却するレーザ溶接とは異なり、溶接の際に所定の溶接条件の下、加熱溶融状態で突き合わせ方向に加圧して接合するため、板厚が異なり、かつ引張強さの差が１２０Ｎ／ mm ² 以上で化学組成が異なる、比較的Ｃ，Ｍｎ量の多い高強度の鋼板同士でも、溶融部の酸化物が外部に押し出されて溶接接合部が清浄になり、また溶接過程で双方の鋼板端部が高温下で熱間押し付け加工が付与されるため、溶接接合部はその後の冷却で微細組織となって靱性、延性が向上し、溶接接合部の硬さを低く保つことができ、前記溶接接合部の清浄化と相まって良好な成形性が得られ、複合成形に対し、良好な成形加工性を有する素板を提供することができる。
【００１１】
以下、溶接部の限定条件について詳細に説明する。溶接鋼板を成形加工する場合、突き合わせ溶接接合部の変形能をいかに高く保つかが重要である。一般に金属材料は硬くなるに従って成形性が劣化するが、溶接接合部は局部的に急熱急冷されるために著しく硬化し、変形に対して割れ易くなるが、溶接時の高温加熱段階で、本発明のように熱間加工が付与される突き合わせ溶接では、溶接条件を適正に選択すればかなり良好な成形性が期待できる。もっとも、自動車部品などの過酷な成形に対しては溶接接合部は母材に近い成形性を有することが望ましい。本発明は板厚が異なり、かつ引張強さの差が１２０Ｎ／ mm ² 以上で化学組成が異なる鋼板を突き合わせ溶接した場合でも母材に近い成形性を付与するために必要な条件を規定するものである。
【００１２】
溶接接合部の最高硬さが３７０Ｈｖ以下（Ｈｖ：ビッカース硬さ）
部品の設計上、高強度が必要な部位には高合金の高強度鋼板を使用し、一方強度を必要としないが複雑な形状に成形する部位には極めて低合金の軟鋼板を使用することが好適であるが、溶接が適正になされても硬化し易い材料では溶接部が著しく硬化し、後述の実施例から明らかな通り、突き合わせ溶接接合部の最高硬さが３７０Ｈｖを超えると、十分な成形性を確保することができないようになる。このため、溶接接合部の最高硬さの上限を３７０Ｈｖ以下とし、好ましくは２８０Ｈｖ以下である。溶接接合部の最高硬さが２８０Ｈｖ以下の溶接鋼板では母材と同等の張り出し成形性が得られるからである。尚、最高硬さの下限は特に規定されないが、実用材料では事実上約９０Ｈｖ程度になるものと考えられる。
【００１３】
一方の鋼板の溶接熱影響部の平均硬さと他方のそれとの差が９０Ｈｖ以下
突き合わせ溶接部に厳しい曲げ加工あるいはバーリング加工のように材料が局部的に大きく変形する場合には、前記溶接接合部の最高硬さの限定のみでは成形性が不十分である。すなわち、両材料の熱影響部の平均硬さの差が９０Ｈｖを超えると、溶接部の最高硬さが低くても、局所的に材料の軟化部や硬化部が生じやすくなり、前者では材料の変形が軟化部に集中してくびれが生じやすく、後者では材料変形が硬化部近傍に集中してくびれが生じやすくなり、溶接鋼板の成形性が著しく劣化する。特に強度や化学組成が大幅に異なる鋼板の突き合わせ溶接ではこの問題が生じやすく、両材料の熱影響部の平均硬さの差は７０Ｈｖ以下にすることが好ましい。尚、この差の下限値は特に規定しないが、必要部位に高強度の材料を使用するため、引張強さの差が１２０Ｎ／ mm ² 以上となる強度レベルの異なる鋼板同士を接合する場合には双方の硬さの差は事実上１３Ｈｖ以上になろう。
【００１４】
一方の鋼板の板厚をｔ₁，他方の同板厚をｔ₂としたとき、
｜ln（ｔ₁ ／ｔ₂) ｜≦０．６
フラッシュバット溶接、ＤＣ（直流）バット溶接など溶接加熱時に突き合わせ方向に押し付ける熱間加工が付与される溶接では、溶接接合部の品質は突き合わせの初期荷重、本溶接時のアプセット荷重、溶接電流、アプセット代などによって大きく影響を受けるが、材料面でも板厚差による影響が大きい。双方の材料の板厚比の｜ln（ｔ₁／ｔ₂）｜が０．６を超えると、双方の材料の板厚が大きく異なるため、溶接時の溶融端部の材料流れが不均一となって酸化物が溶接接合部に残留しやすくなる他、溶接後の溶接ビードのトリム加工が困難になり、表面が美麗な溶接鋼板が得られ難くなる。このため、｜ln（ｔ₁／ｔ₂）｜の上限を０．６とする。尚、下限は特に限定されないが、メンバ、アーム部品などの板厚が大きく異なる材料同士を接合する場合では、前記値は０．１以上になるであろう。
【００１５】
本発明の溶接鋼板は、熱延鋼板、冷延鋼板、めっき鋼板など鋼板全般を対象とするものであり、鋼板の板厚は特に限定されないが、一般の溶接機容量、溶接材の形状精度などを考慮すると１．４〜６mm程度が望ましい。また、溶接に当たり鋼板端部へ油付けを行ってもよく、また溶接後に焼純などの熱処理を適宜併用してもよい。
【００１６】
【実施例】
表１及び表２に示す、板厚、化学組成などが異なる種々の鋼板を下記(1) の条件によりＤＣバット溶接し、得られた溶接鋼板に対し、下記(2) および(3) の要領により張り出し成形性、バーリング加工性を調査した。その調査結果を表３に示す。
【００１７】
(1) 溶接条件
・溶接機：ＤＣバット溶接
・突き合わせ荷重：スクイズ（初期）荷重４〜１５ton 、アプセット荷重５〜１６ton
・溶接電流：２次電流３０〜２３０ｋＡ
・本通電時間：１０〜３０サイクル
・アプセット代：９〜１５mm
・トリマ開始時間：溶接後４sec
・試験片幅：約２００mm
【００１８】
(2) 張り出し成形性調査
直径５０mm、パンチ肩半径１６mmの平底円筒パンチにより成形し、材料破断時点での成形高さ（最大成形高さＨmax ：mm）により、成形性を評価した。
【００１９】
(3) バーリング加工性調査
溶接線上に２０mm直径の穴をパンチにて打ち抜き、頂角６０度の円錐パンチで穴拡げ加工（バーリング加工）を実施した。打ち抜き破断面の溶接接合部のくびれ状況を観察し、以下の５段階で評価した。評点５：溶接部からくびれるとは限らず、母材と同等の加工性を示す。評点４：くびれは溶接接合部から生じるが、母材部との伸びの差は小さく、加工が可能である。評点３：くびれは溶接接合部から生じ、母材部との伸びの差は中程度であるが、加工は可能である。評点２：くびれが溶接接合部から生じ、母材部との伸びの差は大きく、加工不可能である。評点１：直ちに溶接部で破断し、加工が不可能である。
【００２０】
【表１】

【００２１】
【表２】

【００２２】
【表３】

【００２３】
表３のデータを用いて、溶接接合部の最高硬さと張り出し成形性（溶接鋼板のＨmax ／母材鋼板のＨmax ）との関係を整理した。その結果を図１に示す。尚、母材鋼板のＨmax は突き合わせた母材鋼板の内のＨmax の低い材料の値を使用した。また、両母材鋼板の熱影響部平均硬さの差とバーリング加工時のくびれ評点との関係を整理した結果を図２に示す。
【００２４】
図１より、溶接接合部の最高硬さが２８０Ｈｖまでは母材鋼板と同等の成形性が得られ、３７０Ｈｖを超える領域では溶接接合板の張り出し成形性が大きく劣化することが分かる。一方、図２より、プレス成形部品用の素板として、くびれ評価３以上のバーリング加工性を付与するためには、両母材鋼板の熱影響部平均硬さの差ΔＨｖを９０Ｈｖ以下にすべきことが分かる。もっとも、図１及び図２において溶接接合部最高硬さ及び熱影響部平均硬さの差が発明範囲内にありながら、成形性が著しく低いもの（試料No. １８）があるが、これは板厚の組み合わせが不適切であるため、溶接接合部に酸化物が混入したものである。
【００２５】
【発明の効果】
以上説明した通り、本発明によれば、溶接の際に所定の溶接条件の下、加熱溶融状態で突き合わせ方向に加圧して接合するため、板厚が異なり、かつ引張強さの差が１２０Ｎ／ mm ² 以上で化学組成が異なる、比較的Ｃ，Ｍｎ量の多い高強度の鋼板同士を用いても、溶接接合部が清浄になり、また靱性、延性が向上するため、成形性、清浄度等の品質を高く維持でき、複合成形に対して良好な加工性を備える。
【図面の簡単な説明】
【図１】実施例における溶接接合部の最高硬さと張り出し成形性との関係を示すグラフ図である。
【図２】実施例における母材鋼板の熱影響部平均硬さの差とバーリング加工時のくびれ評点との関係を示すグラフ図である。[0001]
[Technical field to which the invention belongs]
It is a welded steel sheet used for transportation equipment such as automobiles such as members and brackets, and building materials such as scaffolding boards, and it is combined with forming processes such as overhanging, drawing, stretching flange, bending, etc. during press forming. The present invention relates to a welded steel plate suitable for composite forming performed in a conventional manner.
[0002]
[Prior art]
As a measure to reduce vehicle weight and reduce vehicle manufacturing costs for the purpose of improving automobile fuel efficiency, doors, pillars, etc. are not molded from the same sheet of steel, but are close to the road surface and easily corroded. Uses a plated steel plate with excellent corrosion resistance, and on the other hand, a welded steel plate that has been welded by laser welding with steel plates of different characteristics so that the upper part of the vehicle, which is less likely to be corroded, can be a cold-rolled steel plate. A technique for manufacturing a predetermined part by using it as a base plate for press molding has been developed. Such a technique is introduced, for example, in the document “Tyssenn Technische Berichte, Heft1 / 92, p. 97”. Further, such a combination of steel plates is not limited to a plated steel plate and a cold rolled steel plate having the same composition, but also a combination of steel plates having different chemical compositions and strengths. For example, “Plasticity and processing, Vol. 34 ( 1993), p917 "," Journal of the Japan Welding Society, Vol.10 (1992), p196 ".
[0003]
Of course, these technologies, such as door inner panels where severe drawing is applied to the joints, are often used for joining ultra-thin steel plates with a thickness of less than 1.4 mm, and the amount of C in the material is also high. In either case, the joining of ultra-soft steel sheets of 0.05% or less is almost all. In addition, it has been reported that steel plates with larger plate thicknesses and plate thickness differences are joined together, and pillars in which a steel plate having a tensile strength of 390 N / mm ² or more and a mild steel plate are joined, but the formability of the joint is sufficient. Instead, the current state is that the part shape and molding conditions are limited.
[0004]
[Problems to be solved by the invention]
Recently, in addition to the panel parts described above, the weight reduction of vehicles has been studied for parts including relatively high strength and thick parts such as members and arms. However, as described above, there is no example in which a welded steel plate that can withstand severe press forming is applied as a base plate of these parts.
[0005]
The reason is that the high strength (hard) steel plate used for the high strength parts of the members and arms has a higher content of alloy elements such as C and Mn than the soft steel plate used for the panel parts. This is because, in laser welding, in which the end portions of both steel plates are simply brought into contact with each other in the welding process and welding is performed, the welded joints are extremely hardened and the formability of the welded steel plates in the press forming stage is greatly deteriorated. In addition, in order to perform laser welding, contact accuracy is required for the butt end surfaces of the steel plates. However, in the case of a steel plate that is originally thicker than the panel parts, sufficient end surface accuracy is obtained by pre-processing by steel plate shear cutting. May not be obtained.
[0006]
By the way, the weight reduction associated with the high strength and thinning of parts such as members and arms, the material selection guidelines differ depending on the load mode applied to the parts. When evaluated by static tensile strength, the strength may be increased by 10% in order to reduce the plate thickness by 10%. However, in a load environment where bending, fatigue, impact, etc. are applied, unlike the static tensile strength, the strength and the plate thickness are not in a one-to-one relationship. Generally, when the plate thickness is reduced by 10%, the strength is about Material selection is considered to increase by 15%. However, such an increase in material strength greatly deteriorates moldability, so it is necessary to develop a material with better moldability, and parts that are easily broken by press molding can be made simpler. A design change is required, and even a slight thinning of the parts requires an enormous amount of experimentation and trial work.
[0007]
Therefore, high-strength parts are required in terms of the strength and formability of parts, such as underarm parts such as arms, that have a significant effect on improving fuel economy due to weight reduction and a strong demand for thinning. It would be advantageous if a thicker or higher strength material could be used for a given site and a thinner material could be used for sites where strength is not required.
[0008]
The present invention has been made in response to such a demand, and is a composite molding weld suitable for integral molding of parts including parts having a relatively large strength, such as automobile members and arms, which have a relatively high strength and a large thickness. It is an object to provide a steel plate.
[0009]
[Means for Solving the Problems]
The welded steel sheet for composite forming according to the present invention is different in thickness and has a difference in tensile strength of 120 N / mm ² or more and has a different chemical composition. A welded steel plate joined by pressurization, wherein the maximum hardness of the welded joint is 370 Hv or less, the difference between the average hardness of the weld heat affected zone of one steel plate and the same average hardness of the other is 90 Hv or less, When the plate thickness is t ₁ and the other plate thickness is t ₂ , | ln (t ₁ / t ₂ ) | ≦ 0.6.
[0010]
According to the welded steel sheet of the present invention, unlike laser welding in which both materials are simply heated and melted and cooled, pressure is applied in the butt direction in the heated and melted state under welding conditions during welding. , different thickness, and the difference between the tensile strength of the chemical composition is different 120 N / mm ² or more, relatively C, even steel plates of more high strength of the Mn content, oxides of the molten portion is pushed out to the outside Since the welded joint becomes clean and both steel plate ends are hot pressed at high temperatures during the welding process, the welded joint becomes a microstructure by subsequent cooling, improving toughness and ductility. It is possible to keep the hardness of the welded joint low, provide a good moldability in combination with the cleaning of the welded joint, and provide a base plate having a good moldability for composite molding. it can.
[0011]
Hereinafter, the limited condition of a welded part will be described in detail. When forming a welded steel sheet, it is important to keep the deformability of the butt weld joint high. In general, as metal materials become harder, formability deteriorates, but the welded joint is locally hardened and rapidly cooled, making it extremely hard to break against deformation. In the butt welding to which hot working is applied as in the invention, a considerably good formability can be expected if the welding conditions are appropriately selected. However, it is desirable that the welded joint has a formability close to that of the base material for severe forming such as automobile parts. What the invention the different thickness, and that the difference between the tensile strength defines the conditions necessary to impart formability is close to the base material even when butt welding a steel plate chemical compositions are different 120 N / mm ² or more It is.
[0012]
Maximum hardness of welded joint is 370 Hv or less (Hv: Vickers hardness)
When designing parts, use high-strength steel sheets with high alloy for parts that require high strength, while using mild steel sheets with extremely low alloy for parts that do not require strength but are formed into complex shapes. Although it is preferable, if the material is hard to be cured even if it is properly welded, the welded portion is remarkably hardened. As will be apparent from the examples described later, when the maximum hardness of the butt welded joint exceeds 370 Hv, sufficient molding is achieved. It becomes impossible to secure sex. For this reason, the upper limit of the maximum hardness of the welded joint is 370 Hv or less, preferably 280 Hv or less. This is because a welded steel sheet having a maximum hardness of the weld joint of 280 Hv or less can provide an overhang formability equivalent to that of the base material. In addition, although the minimum of the maximum hardness is not particularly defined, it is considered to be practically about 90 Hv in practical materials.
[0013]
The difference between the average hardness of the welded heat affected zone of one steel sheet and that of the other is 90 Hv or less. If the material is greatly deformed locally, such as severe bending or burring, in the butt weld, Formability is insufficient only by limiting the maximum hardness. That is, if the difference in the average hardness of the heat-affected zone between the two materials exceeds 90 Hv, even if the maximum hardness of the welded portion is low, locally softened and hardened portions are likely to occur. Deformation tends to concentrate in the softened part, and constriction tends to occur. In the latter case, material deformation concentrates in the vicinity of the hardened part and constriction tends to occur. In particular, this problem is likely to occur in butt welding of steel plates having significantly different strengths and chemical compositions, and the difference in average hardness of the heat affected zone of both materials is preferably 70 Hv or less. The lower limit of this difference is not specified in particular. However, since a high-strength material is used for a necessary part, when joining steel plates having different strength levels at which the difference in tensile strength is 120 N / mm ² or more. The difference in hardness between the two will effectively be greater than 13 Hv.
[0014]
When the thickness of _one steel plate is t ₁ and the thickness of the other steel plate is t ₂ ,
| Ln (t ₁ / t ₂ ) | ≦ 0.6
In welding where hot working is applied in the butt direction during welding heating, such as flash butt welding and DC (direct current) butt welding, the quality of the welded joint is the initial load of butt, upset load during main welding, welding current, upset Although it is greatly affected by cost, etc., it is also greatly influenced by the difference in sheet thickness in terms of material. If the thickness ratio | ln (t ₁ / t ₂ ) | of both materials exceeds 0.6, the thickness of both materials is greatly different, so that the material flow at the molten end during welding is not uniform. As a result, the oxide tends to remain in the welded joint, and it becomes difficult to trim the weld bead after welding, making it difficult to obtain a welded steel sheet having a beautiful surface. Therefore, the upper limit of | ln (t ₁ / t ₂ ) | is 0.6. Although the lower limit is not particularly limited, the value will be 0.1 or more in the case of joining materials having greatly different plate thicknesses such as members and arm parts.
[0015]
The welded steel sheet of the present invention is intended for all steel sheets such as hot-rolled steel sheet, cold-rolled steel sheet, and plated steel sheet, and the thickness of the steel sheet is not particularly limited, but general welding machine capacity, shape accuracy of the welding material, etc. Considering the above, it is desirable that the thickness is about 1.4 to 6 mm. Further, in welding, oil may be applied to the end of the steel sheet, and heat treatment such as tempering may be used in combination as appropriate after welding.
[0016]
【Example】
Various steel plates with different thicknesses and chemical compositions shown in Tables 1 and 2 were DC-butt welded under the following conditions (1), and the following procedures (2) and (3) were applied to the obtained welded steel plates. Was used to investigate the stretch formability and burring processability. The survey results are shown in Table 3.
[0017]
(1) Welding conditions / welding machine: DC butt welding / butting load: squeeze (initial) load 4-15 ton, upset load 5-16 ton
・ Welding current: Secondary current 30-230 kA
・ Main energization time: 10-30 cycles ・ Upset cost: 9-15 mm
・ Trimmer start time: 4 sec after welding
・ Specimen width: About 200mm
[0018]
(2) Overhang formability investigation Formed by a flat bottom cylindrical punch having a diameter of 50 mm and a punch shoulder radius of 16 mm, and the formability was evaluated by the forming height at the time of material breakage (maximum forming height Hmax: mm).
[0019]
(3) Burring workability investigation A 20 mm diameter hole was punched on the weld line with a punch, and a hole expanding process (burring process) was performed with a conical punch with a vertex angle of 60 degrees. The constriction of the welded joint of the punched fracture surface was observed and evaluated in the following five stages. Rating 5: Not necessarily constricted from the welded part, but shows workability equivalent to that of the base material. Score 4: Constriction occurs from the welded joint, but the difference in elongation from the base metal is small and processing is possible. Score 3: Constriction occurs from the welded joint, and the difference in elongation from the base metal is moderate, but processing is possible. Score 2: Constriction occurs from the welded joint, and the difference in elongation from the base metal is large, so that machining is impossible. Score 1: The weld breaks immediately and cannot be processed.
[0020]
[Table 1]

[0021]
[Table 2]

[0022]
[Table 3]

[0023]
Using the data in Table 3, the relationship between the maximum hardness of the welded joint and the stretch formability (Hmax of the welded steel sheet / Hmax of the base steel sheet) was organized. The result is shown in FIG. For the Hmax of the base steel plate, the value of the material having a low Hmax among the matched base steel plates was used. Moreover, the result of having arranged the relationship between the difference of the heat affected zone average hardness of both base-material steel plates and the constriction score at the time of burring is shown in FIG.
[0024]
From FIG. 1, it is understood that the formability equivalent to that of the base steel plate is obtained up to the maximum hardness of the welded joint up to 280 Hv, and that the stretchability of the welded joined plate is greatly deteriorated in the region exceeding 370 Hv. On the other hand, as shown in FIG. 2, in order to give a burring workability of a necking evaluation of 3 or more as a base plate for a press-formed part, the difference ΔHv in the heat affected zone average hardness between the two base steel plates should be 90 Hv or less. I understand that. Of course, in FIGS. 1 and 2, there is a difference in the maximum hardness of the welded joint and the average hardness of the heat affected zone within the invention range, but the formability is remarkably low (Sample No. 18). Since the combination of thicknesses is inappropriate, oxide is mixed in the weld joint.
[0025]
【The invention's effect】
As described above, according to the present invention, since welding is performed by pressing in the butt direction in a heat-melted state under predetermined welding conditions during welding, the plate thickness is different and the difference in tensile strength is 120 N / mm chemical composition is different in ^two or more, relatively C, even with steel sheets of high strength of the Mn amount, will weld joint is clean and toughness, to improve ductility, formability, cleanliness, etc. High quality can be maintained, and good processability is provided for composite molding.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the maximum hardness of welded joints and stretch formability in Examples.
FIG. 2 is a graph showing the relationship between the difference in average hardness of the heat affected zone of the base steel sheet and the squeezing score during burring.

Claims

It is a welded steel plate that is welded and joined by pressing in the butt direction in the heat-melted state when welding the ends of steel plates with different plate thicknesses and differences in tensile strength of 120 N / mm ² or more and different chemical compositions. And
The maximum hardness of the welded joint is 370 Hv or less, the difference between the average hardness of the welding heat-affected zone of one steel sheet and the other same average hardness is 90 Hv or less, the thickness of _one steel sheet is t ₁ , and the same thickness of the other steel sheet when was the _{_{t 2 | ln (t 1 /}} t 2) | welding steel composite molding, which is a ≦ 0.6.