JP4953172B2 - Method to refine ferrite structure by laser irradiation - Google Patents

Method to refine ferrite structure by laser irradiation Download PDF

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Publication number
JP4953172B2
JP4953172B2 JP2001058489A JP2001058489A JP4953172B2 JP 4953172 B2 JP4953172 B2 JP 4953172B2 JP 2001058489 A JP2001058489 A JP 2001058489A JP 2001058489 A JP2001058489 A JP 2001058489A JP 4953172 B2 JP4953172 B2 JP 4953172B2
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laser irradiation
laser
grain size
irradiation
refining
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JP2002256335A (en
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一政 西尾
雅宏 豊貞
静雄 迎
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Kitakyushu Foundation for Advancement of Industry Science and Technology
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Kitakyushu Foundation for Advancement of Industry Science and Technology
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Description

【0001】
【発明の属する技術分野】
本発明は、鉄基合金の組織を微細化する方法および装置に関する。詳しくは、溶接止端部等疲労亀裂を生じ易い部位や局部的に強度を高くする必要がある部位の金属組織を微細化するための方法および装置に関する。
【0002】
【従来の技術】
従来、加工熱処理制御プロセス(TMCP:Thermo Mechanical Control Process)等による鉄基合金組織の微細化技術が知られている。また、近年、大歪熱間加工と磁場中熱処理を組み合わせたメゾスコピック組織制御技術によって極微細複相組織を得る手段の実用化が推進されている。このような、金属組織の微細化によって鉄基合金わけても低炭素鋼は、高抗張力化、高靱性化の点で大きく進歩した。
【0003】
【発明が解決しようとする課題】
しかしながら、上記優れた特性をもつ低炭素鋼等の鉄基合金も、複数の材料を溶接によって接合すると、溶接継手部の引張強度及び疲労強度が低下する場合があり、低炭素鋼等の鉄基合金の高抗張力、高靱性等の優れた特性を活かし切れない問題がある。また、局部的に強度を高めたい必要性に対して必ずしも有効な手段がなかった。
【0004】
溶接継手部の金属組織を微細化する手段として、たとえば特開2000―301376号公報に開示されている溶接ビードの熱処理方法がある。この先行技術は、溶接トーチと、予熱レーザおよび後加熱レーザを同一平面内に整列させ、2つの部品の継手部内で一緒に移動させて予熱・溶接・後加熱を併せ行うようにしたものである。この構成によって、予熱レーザによる予熱スポットおよび後加熱レーザによる後加熱スポットと、溶接スポットとの温度差を制御し、以て、溶着される溶接ビードの微細組織を決定せんとするものである。
【0005】
しかしながら、上記先行技術によって到達し得る結晶粒径は、数十μm程度であり、本発明が指向している数μmオーダーの微細組織を得ることは不可能である。本発明は、TMCP等によって得られた微細な組織を有する高抗張力鋼を溶接によって接合する場合も、溶接継手部の組織を母材の組織と同等以上に微細化し、溶接継手部を含む金属材料に機械的特性わけても疲労強度むらのない構造体を得ることができる金属組織の微細化方法および装置を提供することを目的とする。
【0006】
【課題を解決するための手段】
上記課題を解決するための請求項1に記載の発明は、疲労による損傷を受けた低炭素鋼材料の該疲労損傷部にレーザビームを照射して溶融・凝固せしめた後、該溶融・凝固部およびその幾何学的形状急変部近傍に、A変態点以上材料表面が溶融しない温度域にレーザ照射による急速加熱を行い直ちに急速冷却を施すことを2回〜20回繰り返すレーザ照射によるフェライト組織の微細化方法である。
【0007】
請求項2に記載の発明は、低炭素鋼材料に、A変態点以上材料表面が溶融しない温度域にレーザ照射による急速加熱を行い直ちに急速冷却を施すことを2回〜20回繰り返す時のレーザ照射が、レーザビームの焦点を材料表面から所定寸法だけ離隔せしめた焦点外し距離の存在下になされるものである請求項1に記載のレーザ照射によるフェライト組織の微細化方法である。
【0008】
【作用】
本発明は叙上の構成になるから、鉄基合金の溶接接合部等所望の部位の結晶組織を数μmオーダーまで微細化することができ、近年の著しく高抗張力化された鉄基合金の特性を十分に活かすことができる。また、所望の部位の金属組織を微細化し強化することができる。
【0009】
【発明の実施の形態】
以下、本発明をその好ましい実施形態に則して説明する。
【0010】
本発明においては、低炭素鋼といった鉄基合金における所望の部位、たとえば溶接継手部の溶接金属および母材熱影響部に対し、2回乃至20回の、レーザ照射による急速加熱および急速冷却を施すことがポイントの1つである。図1に、厚さ:6mmの軟鋼板(重量で、C:0.1%、Si:0.15%、Mn:0.52%、P:0.02%、S:0.02%、残部:Feおよび不可避的不純物、結晶粒径:23μm)について、溶接継手部にレーザ照射を行ったときの、レーザ照射回数と、フェライト結晶粒径dの関係を示す。このとき用いたレーザはYAGレーザ加工機であり、レーザ出力P:1.5KW、照射速度v:0.5m/分、焦点外し距離Fd:40mm、シールドガス:Arガス(流量:30l/分)の条件でレーザ照射を行ったものである。
【0011】
図1から明らかなように、溶接金属(W.M.)下面(溶融境界部)から0mm、0.1mm、0.2mmの位置におけるフェライト粒径dは、レーザ照射回数の増加に対応して小さくなっている。レーザ照射を1回行うと、結晶粒径は約6μmと初期の結晶粒径の約1/4の大きさとなっている。さらに、レーザ照射を繰り返すと、レーザ照射回数の増加に対応して結晶粒径は次第に小さくなる。5回〜6回以上になるとほぼ4μmと一定の大きさに収束する。この最終的な粒径は、鋼板の化学的組成、レーザ出力、レーザトーチの移動速度、焦点外し距離Fdによって異なるが、何れの条件においても、レーザ照射を最大で20回まで繰り返すことによって、ほぼ一定の粒径の微細粒に到達する。
【0012】
低炭素鋼といった鉄鋼材料は、γ→α変態によってフェライト粒が微細化する。従来の焼ならし処理はこの変態を利用するものであるが、加熱速度および冷却速度が低いために、到達し得る粒径が20μm〜30μmと結晶粒の微細化には限界があった。発明者らは、加熱速度および冷却速度をきわめて高くとれるレーザビームを用いれば結晶粒の微細化を格段に進め得るものと考え、YAGレーザ加工機を用いて鉄鋼材料表面にレーザ照射を施して結晶粒の微細化を試みた。
【0013】
その結果、母材のフェライト粒径:23μmであったものが、1回のレーザ照射による急速加熱、急速冷却によって、6μmとなった。レーザ照射を繰り返すことによってフェライト粒はさらに微細化して行き、5回以上のレーザ照射で4μmとなった。また、溶接部における結晶粒は、母材に比較するとさらに大きくなっているが、これも2回のレーザ照射によって4μm以下に微細化された。
【0014】
通常の焼ならし処理の場合の鉄鋼材料の加熱速度および冷却速度は、何れも0.1℃/sであるのに対し、鋼板の表面にレーザ照射を行ったときの加熱速度は約300℃/s以上(500℃〜800℃における平均加熱速度)通常約800℃/s程度であり、冷却速度は約10℃/s〜400℃/s(800℃〜500℃における平均冷却速度)である。このように、レーザ照射によるときの加熱速度および冷却速度は、焼きならし処理における加熱速度および冷却速度の100倍―8000倍である。このことが、α→γ変態によって生じるγ(オーステナイト)粒の成長を抑制し、引き続く冷却過程におけるγ→α変態によるα粒の成長も抑制することになる。また、加熱速度がきわめて高いために、1回のレーザ照射では材料全体の組織がγ(オーステナイト)化されることはなく、材料全体をγ(オーステナイト)化するにはレーザ照射の繰り返しが必要となるが、レーザ照射回数が増大するにつれて微細フェライト粒界の総長さが増大するために、次のレーザ照射によるγ(オーステナイト)化のための核生成サイトが増加する。そして、γ粒も微細化され、その後の冷却過程におけるα粒の核生成サイトも増加する。その結果、フェライト核は成長できずに微細粒になるのである。
【0015】
次に、焦点外し距離について説明する。図2に、レーザビーム焦点外し距離(レーザビームの焦点と照射対象表面間の距離)Fd(mm)と、微細化結晶領域の深さの関係を示す。図3に、レーザビームの焦点外し距離Fd(mm)と、微細化結晶領域の幅の関係を示す。図2および図3何れも鋼板の厚さ6mmのときのものである。図2および図3から明らかなように、材料表面が溶融しない条件下で、レーザビームの焦点外し距離Fdが47mmのときに最大の深さおよび幅が得られている。このように、レーザビームの焦点外し距離Fdは、結晶粒の微細化プロセスの制御における重要な操作パラメータである。
【0016】
本発明のレーザ照射による金属組織の微細化方法は、鉄鋼材料の疲労による亀裂等の損傷に対して、その損傷箇所をレーザビーム照射によって溶融・凝固させ、その部分に適用することによって、機械的特性を回復させることができる。
【0017】
次に、本発明のレーザ照射による金属組織の微細化方法を実施するときに用いる装置について、説明する。図4に、本発明のレーザ照射による金属組織の微細化装置の概略を示す。図4において、1はレーザ発振器、2は光ケーブル、3はレーザトーチ、である。4はレーザビームであって、Fdが焦点外し距離である。5は鋼板であり、レーザ照射対象である。6はレーザトーチ操作ロボット制御機構である。このレーザトーチ操作ロボット制御機構6によって、ロボットの進行方向および進行速度が制御される。7は焦点外し距離Fd制御機構であって、鋼板5の表面位置(高さ)やレーザトーチ位置を操作パラメータとして、焦点外し距離Fdを制御する。レーザ発振器1から出たレーザビーム4は、光ケーブル2によって導出され、レーザトーチ3により鋼板5の表面に照射される。レーザ照射は、鋼板の結晶粒径が所望の大きさに微細化されるまで繰り返される。鋼板5が疲労を受けていない場合は、レーザ照射部が溶融しない条件下にレーザ照射を行う。疲労に起因する亀裂等の損傷を受けている場合は、その領域をレーザビーム照射によって溶融・凝固させた後、再溶融しない条件下にレーザ照射を繰り返して表面部の結晶を微細化させる。
【0018】
【実施例】
重量で、C:0.1%、Si:0.15%、Mn:0.52%、P:0.02%、S:0.02%、残部:Feおよび不可避的不純物からなる低炭素鋼の厚さ:3mm、4mm、5mmおよび6mmの鋼板を、50mm×50mmに切断し、YAGレーザ加工機を用いて、出力P:1.5KW、照射速度v:0.5m/分、焦点外し距離Fd:40mm、シールドガス:Arガス(流量:30l/分)の条件の下で試験片中央部に長さ:100mmのレーザ照射を繰り返し行った。
このレーザ照射によって溶融・凝固した領域は、鋼板表面から深さ:0.2mmであった。冷却速度を一定するために、試験片が室温まで降温した後に次のレーザ照射を行った。レーザ照射回数は、最大で10回までとした。
【0019】
厚さ:3mm、4mm、5mmおよび6mmの鋼板について、レーザ照射部の熱サイクルの実測を行った。測定位置における最高加熱温度は、930℃〜980℃であった。500℃〜800℃における平均昇温速度は800℃/sで、板厚によっては殆ど変化しなかった。冷却速度は、板厚が3mmのときに120℃/sであり、板厚が大きくなるに従って冷却速度が高くなり、板厚が6mmの場合は440℃/sであった。何れにしても、レーザ照射による加熱・冷却挙動は急速加熱、急速冷却であることが分かった。
【0020】
板厚:6mmの鋼板について、レーザ照射前の組織ならびにレーザ照射回数1回および10回のときの結晶組織を図5に示す。レーザ照射前のフェライト粒径は23μmであって、レーザ照射回数が1回の場合は、フェライト粒径はレーザ照射前の粒径に比し小さくなっているけれども、結晶粒の大きさにばらつきが見られる。また、元々パーライトであった部分が焼入れ組織に変化している。照射回数が10回の場合は、結晶粒はさらに細かくなり、レーザ照射1回の場合に見られた焼き入れ組織は殆ど見られない。
【0021】
切片法を用いて、ボンド部から各深さ毎のフェライトの大きさを測定した。板厚が6mmの場合の結果を、図6に示す。図6における△、○、●は、ボンド部からの距離(深さ)が0mm、0.1mm及び0.2mmの位置における結晶粒の大きさを示す。レーザ照射前の結晶粒径は、23μmであった。
【0022】
何れの位置においても、レーザ照射回数が増すと、結晶粒径は次第に小さくなっている。ボンド部からの深さが0.2mmの位置において、4回目のレーザ照射で結晶粒径は約4μm程になった。また、△(0mm)のようにボンド部近傍では高温に加熱されるので、深さが0.1mm〜0.2mmの部分に比し、結晶粒は少し大きくなった。
【0023】
同様に、板厚が3mm、4mmおよび5mmの場合の結晶粒の大きさを測定した。図7に、ボンド部からの距離が0.2mmの位置における板厚と結晶粒の大きさを、レーザ照射回数をパラメータとして示す。図7から明らかなように、レーザ照射1回の場合、結晶粒の大きさは板厚間で大きな差違は認められなかった。3回目のレーザ照射では、結晶粒の大きさは板厚:6mm場合に4.3μmであったのに対し、板厚:3mmの場合には5.2μmと大きかった。これは、板厚が小さいほど冷却速度が低くなり、それに伴って結晶粒の微細化効果が小さくなったものと考えられる。しかし、レーザ照射を7回、9回と繰り返すと、結晶粒の大きさに板厚による差違はあまり見られなくなった。
【0024】
レーザ照射を繰り返すと結晶粒は次第に微細化し、母材の結晶粒径:23μmに対して、板厚:6mmの場合、4回目のレーザ照射で約4μm程となった。また、板厚が小さくなると結晶粒の微細化効果は若干低下するが、レーザ照射7回以上になると、約4.3μmの一定値となった。
【0025】
【発明の効果】
本発明によれば、鉄基合金の溶接継手部等所望の局部的領域の結晶組織を数μmオーダーにまで微細化することができて疲労強度を高くすることができるとともに、鉄鋼材料の高抗張力化等特性改良技術の成果を十分に活かすことができる。また、疲労に起因する亀裂等の局部的損傷箇所も、これを再溶融して亀裂をなくすだけでなく、転位密度を減少させて新生化した後、焦点外し距離を制御されるレーザビーム照射によって溶融・凝固せしめた後、組織を微細化し、完全に回復させることができる。さらに、幾何学的形状の急変部の表面部のみの組織を微細化し、以て疲労強度を建設時よりも高くすることができる。
【0026】
請求項2に記載の発明によれば、鉄基合金の組織微細化領域の深さや幅ならびに材料への入熱量を容易に制御できる。
【図面の簡単な説明】
【0027】
【図1】レーザ照射繰り返し回数とフェライト粒径の関係を示すグラフ
【図2】レーザビームの焦点外し距離Fdと結晶微細化領域の深さの関係を示すグラフ
【図3】レーザビームの焦点外し距離Fdと結晶微細化領域の幅の関係を示すグラフ
【図4】本発明のレーザ照射による金属組織の微細化装置を示す略図
【図5】板厚:6mmの鋼板について、レーザ照射繰り返し回数1回および10回のときの結晶組織を示す写真
【図6】本発明の位置実施例におけるレーザ照射繰り返し回数と、溶融境界部からの距離(深さ)毎のフェライト粒径dとの関係を示すグラフ
【図7】鋼板の厚さとフェライト粒径dとの関係を、レーザ照射繰り返し回数をパラメータとして示すグラフ
【符号の説明】
【0028】
1 レーザ発振器
2 光ケーブル
3 レーザトーチ
4 レーザビーム
5 鋼板
6 レーザトーチ操作ロボット制御機構
7 焦点外し距離Fd制御機構
[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for refining the structure of an iron-based alloy. More specifically, the present invention relates to a method and an apparatus for refining a metal structure of a part where a fatigue crack such as a weld toe part is likely to occur or a part where the strength needs to be locally increased.
[0002]
[Prior art]
Conventionally, a technique for refining an iron-based alloy structure by a thermomechanical control process (TMCP) is known. In recent years, practical use of means for obtaining an ultrafine multiphase structure has been promoted by a mesoscopic structure control technique combining large strain hot working and heat treatment in a magnetic field. With such refinement of the metal structure, iron-based alloys, especially low-carbon steels, have made great progress in terms of high tensile strength and high toughness.
[0003]
[Problems to be solved by the invention]
However, even when iron-base alloys such as low carbon steel having the above-mentioned excellent characteristics are joined together by welding, the tensile strength and fatigue strength of the welded joint may decrease. There is a problem that it is not possible to make full use of the excellent properties such as high tensile strength and high toughness of the alloy. In addition, there is not always an effective means for the need to increase the strength locally.
[0004]
As means for refining the metal structure of the weld joint, there is a heat treatment method for a weld bead disclosed in, for example, Japanese Patent Application Laid-Open No. 2000-301376. In this prior art, a welding torch, a preheating laser, and a post heating laser are aligned in the same plane, and are moved together in a joint portion of two parts to perform preheating, welding, and post heating together. . With this configuration, the temperature difference between the preheating spot by the preheating laser, the postheating spot by the postheating laser, and the welding spot is controlled, so that the microstructure of the weld bead to be welded is determined.
[0005]
However, the crystal grain size that can be reached by the above-mentioned prior art is about several tens of μm, and it is impossible to obtain a fine structure of the order of several μm, to which the present invention is directed. In the present invention, even when high strength steel having a fine structure obtained by TMCP or the like is joined by welding, the structure of the welded joint portion is made finer than the structure of the base material, and the metal material including the welded joint portion Another object of the present invention is to provide a method and apparatus for refining a metallographic structure that can obtain a structure having no fatigue strength even if it has mechanical characteristics.
[0006]
[Means for Solving the Problems]
The invention according to claim 1 for solving the above-mentioned problem is that after the fatigue damaged portion of the low carbon steel material damaged by fatigue is irradiated with a laser beam to be melted and solidified, the molten / solidified portion is obtained. and near the geometry sudden change portion, of the ferrite structure by laser irradiation repeated twice to 20 times is subjected to immediate rapid cooling is performed rapidly heated by laser irradiation to a temperature range of a 3 transformation point or above the surface of the material does not melt This is a miniaturization method.
[0007]
According to a second aspect of the invention, a low carbon steel material, when repeating it twice to 20 times to perform immediately rapid cooling is performed rapidly heated by laser irradiation to a temperature range of A 3 transformation point or above the surface of the material does not melt 2. The method for refining a ferrite structure by laser irradiation according to claim 1, wherein the laser irradiation is performed in the presence of a defocus distance in which the focal point of the laser beam is separated from the material surface by a predetermined dimension.
[0008]
[Action]
Since the present invention has the above configuration, the crystal structure of a desired part such as a welded joint of an iron-base alloy can be refined to the order of several μm. Can be fully utilized. Moreover, the metal structure of a desired site can be refined and strengthened.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described according to preferred embodiments thereof.
[0010]
In the present invention, rapid heating and rapid cooling by laser irradiation is performed twice to 20 times on a desired portion in an iron-based alloy such as low carbon steel, for example, a weld metal and a base metal heat-affected zone of a weld joint. This is one of the points. FIG. 1 shows a thickness of 6 mm mild steel sheet (by weight, C: 0.1%, Si: 0.15%, Mn: 0.52%, P: 0.02%, S: 0.02%, (Remainder: Fe and inevitable impurities, crystal grain size: 23 μm) The relationship between the number of laser irradiations and the ferrite crystal grain size d when laser welding is performed on the weld joint is shown. The laser used at this time was a YAG laser processing machine, laser output P: 1.5 kW, irradiation speed v: 0.5 m / min, defocus distance Fd: 40 mm, shield gas: Ar gas (flow rate: 30 l / min) The laser irradiation was performed under the following conditions.
[0011]
As is clear from FIG. 1, the ferrite particle diameter d at 0 mm, 0.1 mm, and 0.2 mm from the lower surface (melting boundary) of the weld metal (WM) corresponds to the increase in the number of laser irradiations. It is getting smaller. When laser irradiation is performed once, the crystal grain size is about 6 μm, which is about 1/4 of the initial crystal grain size. Furthermore, when laser irradiation is repeated, the crystal grain size gradually decreases in accordance with an increase in the number of laser irradiations. When it is 5 to 6 times or more, it converges to a constant size of about 4 μm. The final grain size varies depending on the chemical composition of the steel sheet, the laser output, the moving speed of the laser torch, and the defocus distance Fd. However, under any condition, the final particle size is almost constant by repeating the laser irradiation up to 20 times. A fine particle having a particle size of is reached.
[0012]
In steel materials such as low carbon steel, ferrite grains are refined by the γ → α transformation. The conventional normalization treatment uses this transformation, but since the heating rate and the cooling rate are low, the reachable grain size is 20 μm to 30 μm and there is a limit to the refinement of crystal grains. The inventors believe that the use of a laser beam that can take a very high heating rate and cooling rate can significantly advance the refinement of crystal grains, and a YAG laser processing machine is used to apply laser irradiation to the steel material surface to produce crystals. Attempts were made to refine the grain.
[0013]
As a result, the ferrite grain size of the base material: 23 μm became 6 μm by rapid heating and rapid cooling by one laser irradiation. By repeating the laser irradiation, the ferrite grains were further refined and became 4 μm after 5 or more laser irradiations. Moreover, although the crystal grain in the welded part is larger than that of the base material, it was also refined to 4 μm or less by two laser irradiations.
[0014]
The heating rate and cooling rate of the steel material in the normal normalizing treatment are both 0.1 ° C./s, whereas the heating rate when laser irradiation is performed on the surface of the steel plate is about 300 ° C. / S or more (average heating rate at 500 ° C. to 800 ° C.) is usually about 800 ° C./s, and the cooling rate is about 10 ° C./s to 400 ° C./s (average cooling rate at 800 ° C. to 500 ° C.). . As described above, the heating rate and the cooling rate at the time of laser irradiation are 100 times to 8000 times the heating rate and the cooling rate in the normalizing process. This suppresses the growth of γ (austenite) grains caused by the α → γ transformation, and also suppresses the growth of α grains due to the γ → α transformation in the subsequent cooling process. In addition, since the heating rate is extremely high, the structure of the entire material is not changed to γ (austenite) by one laser irradiation, and it is necessary to repeat laser irradiation to change the entire material to γ (austenite). However, as the number of times of laser irradiation increases, the total length of the fine ferrite grain boundaries increases, so that the number of nucleation sites for γ (austenite) formation by the next laser irradiation increases. And γ grains are also refined, and nucleation sites of α grains in the subsequent cooling process also increase. As a result, ferrite nuclei cannot grow and become fine grains.
[0015]
Next, the defocus distance will be described. FIG. 2 shows the relationship between the laser beam defocusing distance (the distance between the focal point of the laser beam and the irradiation target surface) Fd (mm) and the depth of the miniaturized crystal region. FIG. 3 shows the relationship between the defocus distance Fd (mm) of the laser beam and the width of the miniaturized crystal region. Both FIG. 2 and FIG. 3 are those when the thickness of the steel plate is 6 mm. As apparent from FIGS. 2 and 3, the maximum depth and width are obtained when the defocus distance Fd of the laser beam is 47 mm under the condition that the material surface does not melt. Thus, the defocus distance Fd of the laser beam is an important operating parameter in the control of the crystal grain refinement process.
[0016]
The method of refining a metallographic structure by laser irradiation according to the present invention is to mechanically apply a damaged portion such as a crack caused by fatigue of a steel material by melting and solidifying the damaged portion by laser beam irradiation and applying it to the portion. Properties can be restored.
[0017]
Next, a description will be given of an apparatus used when carrying out the method for refining a metal structure by laser irradiation according to the present invention. FIG. 4 shows an outline of an apparatus for refining a metal structure by laser irradiation according to the present invention. In FIG. 4, 1 is a laser oscillator, 2 is an optical cable, and 3 is a laser torch. Reference numeral 4 denotes a laser beam, and Fd is a defocus distance. Reference numeral 5 denotes a steel plate, which is a laser irradiation target. Reference numeral 6 denotes a laser torch operating robot control mechanism. The laser torch operating robot control mechanism 6 controls the traveling direction and traveling speed of the robot. Defocusing distance Fd control mechanism 7 controls the defocusing distance Fd using the surface position (height) of the steel plate 5 and the laser torch position as operation parameters. The laser beam 4 emitted from the laser oscillator 1 is led out by the optical cable 2 and is irradiated onto the surface of the steel plate 5 by the laser torch 3. Laser irradiation is repeated until the crystal grain size of the steel sheet is refined to a desired size. When the steel plate 5 is not fatigued, laser irradiation is performed under conditions where the laser irradiation portion does not melt. When damage such as a crack due to fatigue is received, the region is melted and solidified by laser beam irradiation, and then laser irradiation is repeated under the condition that re-melting is not performed, thereby refining the surface crystal.
[0018]
【Example】
Low carbon steel consisting of C: 0.1% by weight, Si: 0.15%, Mn: 0.52%, P: 0.02%, S: 0.02%, balance: Fe and inevitable impurities Thickness: 3 mm, 4 mm, 5 mm and 6 mm steel plates were cut into 50 mm × 50 mm, using a YAG laser processing machine, output P: 1.5 kW, irradiation speed v: 0.5 m / min, defocus distance Under the conditions of Fd: 40 mm and shield gas: Ar gas (flow rate: 30 l / min), laser irradiation with a length of 100 mm was repeatedly performed at the center of the test piece.
The region melted and solidified by the laser irradiation was 0.2 mm from the steel plate surface. In order to make the cooling rate constant, the next laser irradiation was performed after the test piece was cooled to room temperature. The number of times of laser irradiation was up to 10 times.
[0019]
For the steel sheets having thicknesses of 3 mm, 4 mm, 5 mm and 6 mm, the thermal cycle of the laser irradiation part was measured. The maximum heating temperature at the measurement position was 930 ° C to 980 ° C. The average rate of temperature increase from 500 ° C. to 800 ° C. was 800 ° C./s, and hardly changed depending on the plate thickness. The cooling rate was 120 ° C./s when the plate thickness was 3 mm, the cooling rate increased as the plate thickness increased, and was 440 ° C./s when the plate thickness was 6 mm. In any case, it was found that the heating and cooling behavior by laser irradiation is rapid heating and rapid cooling.
[0020]
FIG. 5 shows the structure before the laser irradiation and the crystal structure when the number of times of laser irradiation is 1 and 10 for a steel sheet having a thickness of 6 mm. When the ferrite grain size before laser irradiation is 23 μm and the number of times of laser irradiation is one, the ferrite grain size is smaller than the grain size before laser irradiation, but the crystal grain size varies. It can be seen. Moreover, the part which was originally pearlite has changed into the hardened structure. When the number of irradiations is 10, the crystal grains become finer, and the hardened structure seen in the case of one laser irradiation is hardly seen.
[0021]
Using the section method, the size of the ferrite at each depth was measured from the bond portion. The results when the plate thickness is 6 mm are shown in FIG. In FIG. 6, Δ, ○, and ● indicate the size of crystal grains at positions where the distance (depth) from the bond portion is 0 mm, 0.1 mm, and 0.2 mm. The crystal grain size before laser irradiation was 23 μm.
[0022]
At any position, as the number of times of laser irradiation increases, the crystal grain size gradually decreases. At a position where the depth from the bond portion is 0.2 mm, the crystal grain size is about 4 μm by the fourth laser irradiation. Further, since it was heated to a high temperature in the vicinity of the bond portion as Δ (0 mm), the crystal grains were slightly larger than the portion having a depth of 0.1 mm to 0.2 mm.
[0023]
Similarly, the crystal grain size was measured when the plate thickness was 3 mm, 4 mm, and 5 mm. FIG. 7 shows the plate thickness and crystal grain size at a position where the distance from the bond portion is 0.2 mm, using the number of laser irradiations as a parameter. As can be seen from FIG. 7, in the case of one laser irradiation, there was no significant difference in crystal grain size between the plate thicknesses. In the third laser irradiation, the crystal grain size was 4.3 μm when the plate thickness was 6 mm, whereas it was as large as 5.2 μm when the plate thickness was 3 mm. This is considered to be because the cooling rate was lowered as the plate thickness was reduced, and the effect of crystal grain refinement was reduced accordingly. However, when laser irradiation was repeated 7 times and 9 times, there was not much difference in crystal grain size due to the plate thickness.
[0024]
When the laser irradiation was repeated, the crystal grains gradually became finer, and when the plate thickness was 6 mm, the crystal grain size of the base material was about 4 μm by the fourth laser irradiation when the plate thickness was 6 mm. Further, the effect of refining crystal grains slightly decreased as the plate thickness decreased, but when the laser irradiation was 7 times or more, the value became a constant value of about 4.3 μm.
[0025]
【Effect of the invention】
According to the present invention, the crystal structure of a desired local region such as a welded joint portion of an iron-based alloy can be refined to the order of several μm, the fatigue strength can be increased, and the high tensile strength of the steel material can be increased. It is possible to make full use of the results of characteristic improvement technology such as conversion. In addition, local damage such as cracks caused by fatigue is not only remelted to eliminate cracks, but also is generated by reducing dislocation density and renewing, and then by laser beam irradiation with controlled defocus distance. After melting and solidifying, the structure can be refined and fully recovered. Furthermore, the structure of only the surface portion of the suddenly changing portion of the geometric shape can be made finer, so that the fatigue strength can be made higher than that during construction.
[0026]
According to the second aspect of the present invention, it is possible to easily control the depth and width of the structure refinement region of the iron-based alloy and the amount of heat input to the material.
[Brief description of the drawings]
[0027]
FIG. 1 is a graph showing the relationship between the number of repetitions of laser irradiation and the ferrite grain size. FIG. 2 is a graph showing the relationship between the defocus distance Fd of the laser beam and the depth of the crystal refinement region. Graph showing the relationship between the distance Fd and the width of the crystal refinement region. FIG. 4 is a schematic diagram showing the apparatus for refining the metal structure by laser irradiation according to the present invention. FIG. 6 is a graph showing the relationship between the number of repetitions of laser irradiation and the ferrite grain size d for each distance (depth) from the melting boundary in the position example of the present invention. Graph [Fig. 7] Graph showing the relationship between steel sheet thickness and ferrite grain size d, with the number of repetitions of laser irradiation as a parameter [Explanation of symbols]
[0028]
DESCRIPTION OF SYMBOLS 1 Laser oscillator 2 Optical cable 3 Laser torch 4 Laser beam 5 Steel plate 6 Laser torch operating robot control mechanism 7 Defocusing distance Fd control mechanism

Claims (2)

疲労による損傷を受けた低炭素鋼材料の該疲労損傷部にレーザビームを照射して溶融・凝固せしめた後、該溶融・凝固部およびその幾何学的形状急変部近傍に、A変態点以上材料表面が溶融しない温度域にレーザ照射による急速加熱を行い直ちに急速冷却を施すことを2回〜20回繰り返すことを特徴とするレーザ照射によるフェライト組織の微細化方法。After to the fatigue damage of the low carbon steel material damaged by fatigue by irradiating a laser beam allowed melting and solidification, the molten-solidified portion and the vicinity thereof geometry sudden change portion, A 3 transformation point or more A method for refining a ferrite structure by laser irradiation, characterized in that rapid heating by laser irradiation in a temperature range where the material surface does not melt and immediate cooling is repeated 2 to 20 times. 低炭素鋼材料に、A変態点以上材料表面が溶融しない温度域にレーザ照射による急速加熱を行い直ちに急速冷却を施すことを2回〜20回繰り返す時のレーザ照射が、レーザビームの焦点を材料表面から所定寸法だけ離隔せしめた焦点外し距離の存在下になされるものである請求項1に記載のレーザ照射によるフェライト組織の微細化方法。 Low carbon steel material, A 3 laser irradiation when the transformation point or above the surface of the material is repeated twice to 20 times is subjected to immediate rapid cooling is performed rapidly heated by laser irradiation to a temperature range which does not melt is a focus of the laser beam 2. The method for refining a ferrite structure by laser irradiation according to claim 1, wherein the method is performed in the presence of a defocus distance separated from the material surface by a predetermined dimension.
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