JPH06207245A - Moltilayer steel sheet excellent in fatigue characteristic and its production - Google Patents
Moltilayer steel sheet excellent in fatigue characteristic and its productionInfo
- Publication number
- JPH06207245A JPH06207245A JP5001365A JP136593A JPH06207245A JP H06207245 A JPH06207245 A JP H06207245A JP 5001365 A JP5001365 A JP 5001365A JP 136593 A JP136593 A JP 136593A JP H06207245 A JPH06207245 A JP H06207245A
- Authority
- JP
- Japan
- Prior art keywords
- fatigue
- steel sheet
- rolling
- steel
- surface layer
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
Landscapes
- Heat Treatment Of Steel (AREA)
Abstract
Description
【０００１】[0001]
【産業上の利用分野】本発明は、溶接構造物の疲労強度
を向上させるために、表層部に溶接熱サイクルによる変
態膨張を生じせしめ圧縮残留応力を発生させ、疲労亀裂
発生特性を向上させ、かつ疲労亀裂伝播時に亀裂の分岐
を多数発生させて、板厚方向の疲労亀裂伝播抵抗を向上
させた鋼板とその製造方法に関するものである。BACKGROUND OF THE INVENTION The present invention, in order to improve the fatigue strength of a welded structure, causes transformation expansion due to welding heat cycle in the surface layer to generate compressive residual stress and improve fatigue crack initiation characteristics, Further, the present invention relates to a steel sheet in which a large number of crack branches are generated during fatigue crack propagation to improve fatigue crack propagation resistance in the sheet thickness direction, and a manufacturing method thereof.
【０００２】[0002]
【従来の技術】構造物の軽量化、大容量化の要求に応
え、構造用鋼板の高強度化が急速に進んでいる。しかし
ながら、繰り返し荷重を受ける構造物では、降伏強度の
みならず疲労強度を考慮しなければならず、高強度化の
ニーズに応えることができない場合があり、疲労強度の
向上が切望されている。特に、溶接構造物では溶接止端
部から疲労亀裂の発生する場合が多く、鋼材の強度を向
上させても疲労強度は殆ど向上しない。2. Description of the Related Art In response to demands for weight reduction and large capacity of structures, structural steel sheets have been rapidly increased in strength. However, in a structure subjected to repeated loading, not only the yield strength but also the fatigue strength must be considered, and there are cases where it is not possible to meet the needs for higher strength, and there is a strong demand for improvement in fatigue strength. In particular, in a welded structure, a fatigue crack often occurs from the weld toe, and even if the strength of the steel material is improved, the fatigue strength is hardly improved.
【０００３】溶接構造物の疲労強度は、主として溶接部
の止端部形状によって支配されることが知られており、
溶接部の止端部処理等の疲労強度向上策が適用されるこ
とがある。しかし、止端部処理は、構造物の建造工数を
増大させるばかりでなく、溶接部位によっては止端部処
理が実施できない場合も多く、鋼材面から疲労強度向上
が切望されている。It is known that the fatigue strength of a welded structure is mainly governed by the shape of the toe of the weld,
Measures for improving the fatigue strength such as processing the toe of the welded portion may be applied. However, the toe treatment does not only increase the man-hours for constructing a structure, but also often does not allow the toe treatment to be performed depending on the welded portion, and it is eagerly desired to improve fatigue strength from the viewpoint of steel materials.
【０００４】溶接継手部の疲労破壊は一般に溶接止端部
から発生するため、溶接止端部での疲労亀裂発生の抑制
と、止端部で発生した疲労亀裂の板厚方向への伝播を遅
延させることが有効である。溶接継手部の疲労破壊は一
般に応力集中の大きな溶接止端部から発生するため、発
生特性は溶接止端部形状に大きく影響されることが知ら
れている。また、疲労亀裂伝播を遅延させるためには、
疲労亀裂伝播面に垂直方向に亀裂を分岐させることが有
効であることが、Ｐｒｏｃｅｅｄｉｎｇｓ ｏｆ ａｎ
ｉｎｔｅｒｎａｔｉｏｎａｌ ｃｏｎｆｅｒｅｎｃｅ
ｓｐｏｎｓｏｒｅｄ ｂｙ Ｍｅｔａｌｓ Ｓｏｃｉｅ
ｔｙ（２１−２３，Ｏｃｔｏｂｅｒ，１９８１，Ｌｏｎ
ｄｏｎ）のＰ．７９〜に記載されている。また同様な方
法として日本造船学会論文集Vol.１６９，pp．２５７〜
２６６では微小セパレーションによる疲労亀裂伝播速度
向上効果を示しており、セパレーション指数が大きい程
微小セパレーションも発生しやすいとの報告がなされて
いる。しかしながら、通常のセパレーション指数として
用いられているＳＩmax はシャルピー試験片破面から求
めるものであり、ＳＩmax と疲労伝播速度の関係を調査
した結果、ＳＩmax と疲労伝播特性の間には直接関係が
認められないことがあった。Fatigue fracture of a welded joint generally occurs from the weld toe, so that the occurrence of fatigue cracks at the weld toe is suppressed and the propagation of fatigue cracks at the toe in the plate thickness direction is delayed. It is effective to let It is known that the fatigue fracture of the welded joint generally occurs from the weld toe where stress concentration is large, and therefore the generation characteristics are greatly affected by the weld toe shape. Also, to delay fatigue crack propagation,
It is effective to branch the crack in the direction perpendicular to the fatigue crack propagation surface.
international conference
sponsored by Metals Society
ty (21-23, October, 1981, Lon
don) P.M. 79-. As a similar method, the Shipbuilding Society of Japan Vol.169, pp. 257-
266 shows the effect of improving the fatigue crack propagation rate by the minute separation, and it is reported that the larger the separation index, the more easily the minute separation occurs. However, SImax, which is used as a normal separation index, is obtained from the fracture surface of a Charpy test piece, and as a result of investigating the relationship between SImax and fatigue propagation velocity, a direct relationship was found between SImax and fatigue propagation characteristics. There was nothing.
【０００５】また、セパレーションによる疲労亀裂伝播
速度遅延効果はΔＫ値の低いレベルで有効なことが、前
述の日本造船学会論文集Vol.１６９，pp．２５７〜２６
６に記載されているが、セパレーションは結晶方位の異
なる集合組織間の塑性異方性により発生するものであ
り、ΔＫ値の低い領域では塑性域が小さいためセパレー
ションの発生が困難となる。そこで、低ΔＫ値レベルで
も疲労亀裂の伝播を遅延させるようなマイクロクラック
を発生させるのに必要な組織制御に関する技術の開発が
望まれている。Further, the effect of retarding the fatigue crack propagation rate by separation is effective at a low ΔK value, as described in the above-mentioned Proceedings of the Japan Shipbuilding Society Vol.169, pp. 257-26
As described in No. 6, separation occurs due to plastic anisotropy between textures having different crystal orientations, and it is difficult to generate separation in a region where the ΔK value is low because the plastic region is small. Therefore, it is desired to develop a technique relating to the structure control necessary to generate microcracks that delay the propagation of fatigue cracks even at a low ΔK value level.
【０００６】[0006]
【発明が解決しようとする課題】本発明は、疲労強度を
向上させるために、疲労亀裂発生部である溶接止端部に
圧縮残留応力を発生せしめ疲労亀裂発生までの寿命を増
大させるとともに、疲労亀裂先端に低ΔＫ領域でもマイ
クロクラックを多数発生させる組織制御を行い疲労伝播
寿命をも増大させる鋼板とその製造技術を提供すること
を課題とするものである。SUMMARY OF THE INVENTION In order to improve the fatigue strength of the present invention, a compressive residual stress is generated at the weld toe, which is the fatigue crack occurrence portion, to increase the life until fatigue crack initiation, and to reduce fatigue. It is an object of the present invention to provide a steel sheet and a manufacturing technique for the steel sheet, in which the microstructure is controlled to generate a large number of microcracks even in the low ΔK region at the crack tip and the fatigue propagation life is also increased.
【０００７】[0007]
【課題を解決するための手段】本発明の要旨は、次の通
りである。 （１）鋼板の表裏面からそれぞれ０．２mm以上板厚２５
％以下の領域が３％以上のＮｉ量を含有する化学成分を
有し、鋼板の表裏面からそれぞれ少なくとも板厚の５％
以上の範囲にわたって、隣接する結晶粒どうしで結晶方
位の等しい粒から構成されるコロニーのアスペクト比
（長軸径／短軸径の比）が４以上でかつその短軸径が５
μｍ以下の組織からなることを特徴とする疲労特性の優
れた複層鋼板。The gist of the present invention is as follows. (1) 0.2 mm or more from the front and back sides of the steel plate 25
% Or less has a chemical composition containing 3% or more of Ni, and at least 5% of the plate thickness from the front and back surfaces of the steel plate, respectively.
In the above range, the aspect ratio (ratio of major axis diameter / minor axis diameter) of a colony composed of grains having the same crystal orientation between adjacent crystal grains is 4 or more and the minor axis diameter is 5 or less.
A multi-layer steel sheet having excellent fatigue properties, which is characterized by having a structure of μm or less.
【０００８】（２）鋼板厚みをｔ、圧延前の鋳片厚みを
ｔ0 とした時に、鋼板の表裏面からそれぞれ０．２×ｔ
0 ／ｔ（mm）以上０．２５×ｔ0 ／ｔ（mm）以下の領域
が３％以上のＮｉ量を含有する化学成分を有する鋳片
を、圧延中途中水冷時の板厚をｔ1 とした時、表層から
少なくとも板厚方向に０．０５×ｔ1 (mm)以上の領域を
２℃/sec以上の冷速で急冷して、その後、当該表層部が
Ａｒ3 以下の温度から圧延を開始もしくは再開し、Ａｃ
1 ℃からＡｃ3 ℃の範囲で圧延を終了し、その後Ａｃ3
点以上に復熱させないことを特徴とする前記１の組織を
有する鋼板の製造方法。(2) When the thickness of the steel sheet is t and the thickness of the slab before rolling is t 0 , 0.2 × t from the front and back surfaces of the steel sheet, respectively.
A slab having a chemical composition in which a region of 0 / t (mm) or more and 0.25 × t 0 / t (mm) or less has a Ni content of 3% or more has a plate thickness of t 1 during water cooling during rolling. At that time, at least a region of 0.05 × t 1 (mm) or more in the plate thickness direction from the surface layer is rapidly cooled at a cooling rate of 2 ° C./sec or more, and then the surface layer portion is rolled from a temperature of Ar 3 or less. Start or restart, Ac
Rolling is completed within the range of 1 ℃ to Ac 3 ℃, then Ac 3
A method for producing a steel sheet having the above-mentioned structure 1, characterized in that the steel sheet is not reheated to a point or more.
【０００９】本発明において、対象とする構造用鋼は、
溶接部での変態膨張を生じせしめるために表層部の鋳片
に含有させるＮｉ以外の元素に関しては、例えば前記し
た特公昭５８−１４８４９号公報に記載され、次記する
ように、通常の溶接構造用鋼が所要の材質を得るため
に、従来から当業分野での活用で確認されている作用・
効果の関係を基に定めている添加元素の種類と量を同様
に使用して同等の作用と効果が得られる。従って、これ
等を含む鋼を本発明は対象鋼とするものである。これ等
の各成分元素とその添加理由と量を以下に示す。Ｃは、
鋼の強度を向上する有効な成分として添加するものであ
るが、０．２０％を超える過剰な含有量では、２相域圧
延時の変形抵抗を増して圧延を困難にするばかりか、溶
接部に島状マルテンサイトを析出し、鋼の靭性を著しく
劣化させるので、０．２０％以下に規制する。Ｓｉは溶
鋼の脱酸元素として必要であり、また強度増加元素とし
て有用であるが、１．０％を超えて過剰に添加すると、
鋼の加工性を低下させ、溶接部の靭性を劣化させる。ま
た、０．０１％未満では脱酸効果が不十分なため、添加
量を０．０１〜１．０％に規制する。In the present invention, the target structural steel is
Elements other than Ni contained in the slab of the surface layer portion for causing transformation expansion in the welded portion are described, for example, in Japanese Patent Publication No. 58-14849 mentioned above, and as described below, a normal welded structure. In order to obtain the required material for use steel, the action that has been confirmed in the past in the field of application
The same action and effect can be obtained by similarly using the types and amounts of the additional elements defined based on the effect relationship. Therefore, the steel containing these is the subject steel of the present invention. Each of these component elements and the reason and amount of addition are shown below. C is
Although it is added as an effective component to improve the strength of steel, an excessive content exceeding 0.20% not only increases the deformation resistance during two-phase rolling, making rolling difficult, but it also causes a weld zone. Since island-like martensite is precipitated in the steel and markedly deteriorates the toughness of steel, the content is restricted to 0.20% or less. Si is necessary as a deoxidizing element for molten steel and is also useful as a strength increasing element, but if added in excess of 1.0%,
It reduces the workability of steel and deteriorates the toughness of the weld. Further, if less than 0.01%, the deoxidizing effect is insufficient, so the addition amount is regulated to 0.01 to 1.0%.
【００１０】Ｍｎも脱酸成分元素として必要であり、
０．３％未満では鋼の清浄度を低下し、加工性を害す
る。また鋼材の強度を向上する成分として０．３％以上
の添加が必要である。しかし、Ｍｎは変態温度を下げる
ので、過剰の添加により２相域圧延温度が下がりすぎ、
変形抵抗の上昇をきたすので、２．０％を上限とする。
ＡｌおよびＮは、Ａｌ窒化物による鋼の微細化の他、圧
延過程での固溶、析出により、鋼の結晶方位の整合およ
び再結晶に有効な働きをさせるために添加する。しか
し、添加量が少ない時にはその効果がなく、過剰の場合
には鋼の靭性を劣化させるので、Ａｌ：０．００１〜
０．２０％、Ｎ：０．０２０％以下に限定する。以上
が、本発明が対象とする鋼の基本成分である。Mn is also necessary as a deoxidizing component element,
If it is less than 0.3%, the cleanliness of the steel is lowered and the workability is impaired. Further, it is necessary to add 0.3% or more as a component for improving the strength of the steel material. However, since Mn lowers the transformation temperature, the excessive addition of Mn lowers the two-phase region rolling temperature too much.
Since it causes an increase in deformation resistance, the upper limit is 2.0%.
Al and N are added in order to make the steel effective by matching the crystal orientation of the steel and recrystallization by solid solution and precipitation in the rolling process in addition to the refinement of the steel by Al nitride. However, when the addition amount is small, the effect is not exerted, and when it is excessive, the toughness of the steel is deteriorated.
0.20%, N: 0.020% or less. The above is the basic composition of the steel targeted by the present invention.
【００１１】これらの基本成分に加え、表層部には、溶
接施工時に熱影響を受けて変態する際にマルテンサイト
変態して膨張し、止端部で通常生じる引張残留応力を低
減させ、むしろ圧縮残留応力を発生させるようにＮｉ量
を３％以上添加するものである。しかしながら、Ｎｉ量
の添加はコスト上昇を引き起こし、かつ疲労亀裂伝播を
遅延させるために導入する集合組織を発達させるために
は、Ａｒ3 点温度が高い方が望ましいので疲労亀裂発生
に効果のない板厚内部でのＮｉ量含有は不必要である。
そこで、Ｎｉ含有層の厚みを５mm以下と規制している。In addition to these basic components, the surface layer portion undergoes martensitic transformation and expands when transformed under the influence of heat during welding, reducing the tensile residual stress that normally occurs at the toe portion, and rather compressing it. The amount of Ni is 3% or more so as to generate residual stress. However, the addition of the amount of Ni causes a cost increase, and in order to develop the texture introduced to delay the fatigue crack propagation, it is desirable that the Ar 3 point temperature is higher, so that a plate that is not effective for fatigue crack initiation It is not necessary to include the Ni content in the thick interior.
Therefore, the thickness of the Ni-containing layer is regulated to 5 mm or less.
【００１２】本発明における組織の規定理由を次に示
す。疲労亀裂先端でマイクロクラックを発生しやすくす
るためには、集合組織の発達が有用である。このマイク
ロクラックを発生させるために必要な集合組織を得るた
めに、隣接する結晶粒どうしの方位が等しいコロニーの
長径と短径の比（アスペクト比）を４以上と規定した。
さらに、集合組織を集合組織コロニーの短軸径を５μｍ
以下にしたのは、繰り返し荷重下において塑性域の小さ
な低ΔＫ領域でも結晶方位の異なるコロニー間での塑性
異方性から局所変形を生じさせマイクロクラックを容易
に発生させるためである。The reasons for defining the organization in the present invention are as follows. The development of texture is useful for facilitating the generation of microcracks at the tips of fatigue cracks. In order to obtain the texture necessary for generating the microcracks, the ratio of the major axis to the minor axis (aspect ratio) of the colonies in which the adjacent crystal grains have the same orientation is defined as 4 or more.
Furthermore, the short axis diameter of the texture colony is 5 μm.
The reason for this is to cause local deformation due to plastic anisotropy between colonies having different crystal orientations even in a low ΔK region having a small plastic region under repeated loading, and thus easily generate microcracks.
【００１３】[0013]
【作用】発明者らは、疲労亀裂発生寿命を増大させるた
めに溶接止端部での残留応力を圧縮側にすることで疲労
亀裂発生を抑制する方法に着眼し、鋼材の表（裏）層部
にＮｉを含有した成分とすることで、溶接止端部で熱影
響を受けた部分が変態する際に膨張し、圧縮残留応力を
付与させうることを見いだした。Ｎｉ量を変化させて変
態膨張挙動を測定した結果、Ｎｉ量が３％以上になると
顕著な膨張が確認された。The present inventors have focused on a method of suppressing the occurrence of fatigue cracks by increasing the residual stress at the weld toe on the compression side in order to increase the fatigue crack initiation life. It has been found that by using a component containing Ni in the portion, the portion affected by heat in the weld toe portion expands when transformed and can give a compressive residual stress. As a result of measuring the transformation expansion behavior while changing the Ni content, remarkable expansion was confirmed when the Ni content was 3% or more.
【００１４】隅肉溶接や角巻溶接のような鋼板表面に溶
接する際には、鋼板表面で熱影響を受けるので表層部が
十分なＮｉ含有量であれば表層部での膨張が期待でき
る。しかし、突合せ溶接を適用する際には、板厚表層部
から内部にわたって熱影響部が存在するため、疲労亀裂
発生の問題となる表層部において圧縮残留応力を発生さ
せるためには板厚内部には変態膨張を生じさせないこと
が必要である。そこで、疲労亀裂発生の問題となる表層
部のみをＮｉ含有させることとし、その効果を発揮させ
るために必要なＮｉ含有表層部の厚みは溶接部のミクロ
欠陥を考慮して０．２mm以上とし、板厚方向での膨張差
を活用するために板厚の２５％以下の厚みとした。When welding to the surface of a steel sheet such as fillet welding or square winding welding, the surface of the steel sheet is affected by heat, so if the surface layer portion has a sufficient Ni content, expansion at the surface layer portion can be expected. However, when applying butt welding, since there is a heat-affected zone from the surface layer to the inside of the plate thickness, in order to generate compressive residual stress in the surface layer that causes fatigue cracking, It is necessary not to cause transformation expansion. Therefore, it is assumed that only the surface layer portion that causes a problem of fatigue cracking is made to contain Ni, and the thickness of the Ni-containing surface layer portion necessary to exert the effect is 0.2 mm or more in consideration of micro defects in the welded portion, In order to utilize the difference in expansion in the plate thickness direction, the thickness is set to 25% or less of the plate thickness.
【００１５】さらに、従来から注目されているセパレー
ションの発生による疲労強度向上効果に着眼して実験を
種々実施してきた結果、日本造船学会論文集Vol.１６
９，pp．２５７〜２６６に記載されているように、ΔＫ
値の小さい領域でしかセパレーション指数であるＳＩma
x の大きな鋼材でも疲労伝播速度の遅延効果のないこと
を確認した。これは、セパレーションの発生による疲労
強度向上機構が図１に示すように発生した微小セパレー
ションどうしの間を疲労亀裂が進展する時に生じる遅延
効果によっているためである。Further, as a result of conducting various experiments focusing on the fatigue strength improving effect due to the occurrence of separation, which has been noticed in the past, as a result, the Shipbuilding Society of Japan, Vol. 16
9, pp. 257-266, ΔK
SIma which is the separation index only in the small value area
It was confirmed that there is no effect of delaying the fatigue propagation speed even for steel materials with large x. This is because the fatigue strength improving mechanism due to the occurrence of separation is due to the delay effect that occurs when the fatigue crack propagates between the minute separations generated as shown in FIG.
【００１６】そこで、特開平３−４４４４４号公報に記
載されているように、伝播中の脆性亀裂に先だってセメ
ンタイト相からマイクロクラックが発生し亀裂先端の応
力状態を緩和させていることに着眼し、伝播中の疲労亀
裂先端にマイクロクラックを生じせしめる方法について
種々検討を行った。その結果、疲労亀裂先端では、脆性
亀裂先端よりも低いΔＫ値でマイクロクラックを生じさ
せる必要があり、かつ疲労亀裂伝播速度遅延にはマイク
ロクラックの発生頻度も増加させる必要のあることを知
見した。Therefore, as described in Japanese Patent Application Laid-Open No. 3-44444, it has been noted that microcracks are generated from the cementite phase prior to the brittle crack during propagation to relax the stress state at the crack tip, Various studies were carried out on the method of producing microcracks at the tips of fatigue cracks during propagation. As a result, it was found that it is necessary to generate microcracks at a ΔK value lower than that of a brittle crack tip at the fatigue crack tip, and to increase the frequency of occurrence of microcracks to delay the fatigue crack propagation rate.
【００１７】まず、塑性域寸法の小さな低ΔＫ領域でも
マイクロクラックが生成する条件を求めるために、図１
に示すように、テンパーカラー法により現出させた集合
組織コロニーのアスペクト比と疲労試験において微小セ
パレーション発生限界Ｋ値および板厚方向限界破壊応力
の関係を求めた。アスペクト比が４以上では板厚方向限
界破壊応力が低下しマイクロクラックの生成が容易とな
り、塑性域寸法の小さな低ΔＫ領域でもマイクロクラッ
クが生成することを知見した。First, in order to determine the conditions under which microcracks are generated even in the low ΔK region where the size of the plastic region is small, FIG.
As shown in, the relationship between the aspect ratio of the texture colonies developed by the temper color method and the micro-separation occurrence limit K value and the limit fracture stress in the plate thickness direction in the fatigue test was determined. It has been found that when the aspect ratio is 4 or more, the critical fracture stress in the plate thickness direction is lowered, microcracks are easily generated, and microcracks are generated even in a low ΔK region having a small plastic region size.
【００１８】次に、マイクロクラックの発生頻度を増加
させるために、結晶粒径と集合組織を変化させて実験を
行い、テンパーカラー法により現出させた集合組織コロ
ニーの疲労亀裂進展方向の寸法（短軸径）とマイクロク
ラック発生密度の関係を調査した。その結果、図２に示
すようにマイクロクラック発生密度は、集合組織のコロ
ニー寸法に大きく依存した。Next, in order to increase the frequency of occurrence of microcracks, an experiment was conducted by changing the crystal grain size and texture, and the size of the texture colony developed by the temper color method in the fatigue crack growth direction ( The relationship between the minor axis diameter) and the microcrack generation density was investigated. As a result, as shown in FIG. 2, the microcrack generation density largely depended on the colony size of the texture.
【００１９】集合組織コロニーの短軸径と表面亀裂伝播
試験片での公称繰り返し曲げ応力Δσ＝１６０MPa での
破断回数の関係を図３に示す。集合組織コロニーの短軸
径が１０μｍ以上では破断寿命は変わらず、５μｍ以下
では破断寿命が大幅に向上した。集合組織コロニー短軸
径が１０μｍレベルの場合の２倍以上の破断寿命を確保
するためには、集合組織コロニー短軸径が５μｍ以下で
ある必要を知見した。これらの試験片の破面と組織の関
係を詳細に調査した結果、集合組織の短軸径が５μｍ以
下の組織の試験片では、亀裂先端でのマイクロクラック
の密度が高く実質的に亀裂先端での応力が緩和されてい
ることが示された。また、破面の起伏も短軸径が小さく
なるにつれて顕著となっており、クラッククロージャに
よるΔＫeff の低下効果も発揮されていることが判明し
た。FIG. 3 shows the relationship between the minor axis diameter of the texture colonies and the number of fractures at the nominal cyclic bending stress Δσ = 160 MPa in the surface crack propagation test piece. When the minor axis diameter of the textured colony was 10 μm or more, the breaking life was unchanged, and when the minor axis diameter was 5 μm or less, the breaking life was significantly improved. It was found that the texture colony minor axis diameter is required to be 5 μm or less in order to secure a fracture life of at least twice as long as when the texture colony minor axis diameter is at the level of 10 μm. As a result of a detailed investigation of the relationship between the fracture surface and the structure of these test pieces, in the case of the test piece having a structure having a minor axis diameter of 5 μm or less, the density of microcracks at the crack tip was high and the crack tip was substantially It has been shown that the stress of is relaxed. It was also found that the undulations of the fracture surface became more remarkable as the minor axis diameter became smaller, and the effect of reducing ΔKeff by the crack closure was also exerted.
【００２０】したがって、集合組織の短軸径を小さくし
ていくことにより、従来セパレーション発生鋼材の疲労
亀裂伝播遅延メカニズムとは全く異なった、多数のマイ
クロクラックの発生による亀裂先端での応力低下やクラ
ッククロージャによる疲労亀裂伝播遅延メカニズムによ
り疲労特性を大きく向上できることを知見した。Therefore, by decreasing the minor axis diameter of the texture, the stress reduction and cracks at the crack tip due to the generation of a large number of microcracks, which is completely different from the fatigue crack propagation delay mechanism of the conventional separation-generated steel, It was found that the fatigue characteristics can be greatly improved by the fatigue crack propagation delay mechanism by the closure.
【００２１】この組織を達成するためには、例えば、昇
温過程中のフェライトにある必要量の加工を与え、かつ
オーステナイト化への逆変態を防止すれば、加工フェラ
イトに導入された転位は回復、再配列を起こし、フェラ
イト等のマトリックス組織の超細粒化が図られ、さらに
フェライトへ与えた加工により発達させた集合組織はそ
のまま残留させることにより本発明の組織が達成できる
ことを知見した。To achieve this structure, for example, if the ferrite is subjected to a necessary amount of working during the temperature rising process and the reverse transformation to austenite is prevented, the dislocations introduced into the worked ferrite are recovered. It was found that the structure of the present invention can be achieved by causing rearrangement, ultrafine-graining of a matrix structure such as ferrite is achieved, and the texture developed by processing given to ferrite is left as it is.
【００２２】また、廻し溶接部等の鋼板付加物の応力集
中部から発生する疲労破壊に注目し、その成長過程を観
察した結果、疲労寿命の８割以上が疲労亀裂の発生およ
び板厚方向へ２mm程度の深さに成長するのに要している
ことが判明した。このことから、板表層の組織改質が重
要であることを知見し、表層改質部の厚みを変化させて
疲労破断寿命を調査した。その結果図４に示すように板
厚の３０％が組織改質されていれば板厚全体が改質され
ている場合と同等の寿命を示し、工業的に意味のある現
行材の２倍以上の疲労寿命を達成するために必要な表層
改質層の厚みは板厚の５％以上であることを知見した。Also, as a result of observing the growth process of the fatigue fracture generated from the stress-concentrated portion of the steel plate additive such as the turn welded portion, it was found that 80% or more of the fatigue life was in the fatigue crack generation and the sheet thickness direction. It was found that it takes to grow to a depth of about 2 mm. From this, it was found that the structural modification of the surface layer of the plate is important, and the fatigue fracture life was investigated by changing the thickness of the modified surface layer. As a result, as shown in FIG. 4, if 30% of the plate thickness is structurally modified, the life is equivalent to that of the case where the entire plate thickness is modified, which is more than twice that of the current industrially significant material. It was found that the thickness of the surface modified layer required to achieve the fatigue life of is 5% or more of the plate thickness.
【００２３】鋼板表層部に本発明の組織を形成せしめる
ためには、例えば、圧延中に鋼板表面を水冷し、Ａｒ1
点以下とすることで一旦フェライト変態させてしまい、
冷却によっても殆ど温度の低下しない板厚中心部の顕熱
を利用して、表層部のフェライト組織を昇温させながら
さらに圧延を行い表層部に集合組織コロニーの短軸径が
５μｍ以下となる改質組織が形成できることを知見し
た。本発明は以上の知見に基づきなされたものである。In order to form the structure of the present invention on the surface layer of the steel sheet, for example, the surface of the steel sheet is water-cooled during rolling, and Ar 1
If it is below the point, ferrite transformation will occur once,
Utilizing the sensible heat in the center of the plate thickness where the temperature hardly decreases even by cooling, further rolling is performed while raising the ferrite structure in the surface layer, and the minor axis diameter of the texture colony becomes 5 μm or less in the surface layer. It was found that a quality tissue can be formed. The present invention has been made based on the above findings.
【００２４】[0024]
【実施例】実施例の供試鋼の成分を表１に、製造条件お
よび得られた材質を表２に比較例と共に示す。[Examples] Table 1 shows the components of the test steels of Examples, and Table 2 shows the manufacturing conditions and the obtained materials together with Comparative Examples.
【００２５】[0025]
【表１】 [Table 1]
【００２６】[0026]
【表２】 [Table 2]
【００２７】[0027]
【表３】 [Table 3]
【００２８】[0028]
【表４】 [Table 4]
【００２９】本発明例である試験番号１〜９は表層部に
所定のＮｉ含有層を有し、表層部もしくは板厚全体にわ
たりテンパーカラー法で現出させた集合組織のコロニー
の短軸径が５μｍ以下、アスペクト比が４以上の組織を
有しており、その組織の厚みに応じて疲労強度が向上し
ている。比較例である試験番号１０〜１１は表層部に所
定のＮｉ含有層を有しているものの、試験番号１０はＮ
ｉ含有層の厚みが２５％以上あり、当該厚みが２５％以
下である試験番号１と比較して、疲労特性はむしろ低下
している。Test Nos. 1 to 9, which are examples of the present invention, have a predetermined Ni-containing layer in the surface layer portion, and the minor axis diameter of the colony of the texture revealed by the temper color method over the surface layer portion or the entire plate thickness is It has a structure of 5 μm or less and an aspect ratio of 4 or more, and the fatigue strength is improved according to the thickness of the structure. Test numbers 10 to 11 which are comparative examples have a predetermined Ni-containing layer in the surface layer portion, but test number 10 is N.
The i-containing layer has a thickness of 25% or more, and the fatigue property is rather reduced as compared with Test No. 1 in which the thickness is 25% or less.
【００３０】試験番号１１は表層Ｎｉ層の厚みが０．２
mm以下であり、表層Ｎｉ含有層の効果は得られず、本発
明例である試験番号１より疲労特性は良好でない。試験
番号１２は、粗圧延後の冷却およびその後の昇温加工圧
延を実施していないので、所定の表層Ｎｉ含有層は有し
ているものの所定の表層組織が形成されずに疲労特性
は、本発明例である試験番号１より劣化している。試験
番号１３〜１６は、表層Ｎｉ含有層がなく、試験番号１
３，１５は粗圧延後の所定の冷却、圧延を実施してお
り、所定の表層組織は有しているものの本発明例より疲
労特性は低い。また、試験番号１４，１６は通常の鋳片
で、通常の圧延を適用した結果である。Test No. 11 has a surface Ni layer having a thickness of 0.2.
Since it is less than or equal to mm, the effect of the surface Ni-containing layer cannot be obtained, and the fatigue properties are not better than those of Test No. 1 which is an example of the present invention. Test No. 12 does not carry out cooling after rough rolling and subsequent temperature rising work rolling, so that although a predetermined surface layer Ni-containing layer is provided, a predetermined surface layer structure is not formed and fatigue characteristics are It is deteriorated from the test number 1 which is an example of the invention. Test numbers 13 to 16 have no surface Ni-containing layer, and test number 1
Nos. 3 and 15 have undergone predetermined cooling and rolling after rough rolling, and have a predetermined surface layer structure, but have lower fatigue properties than the examples of the present invention. Test numbers 14 and 16 are ordinary slabs and are the results of applying ordinary rolling.
【００３１】[0031]
【発明の効果】本発明は上記した手段を用いて上記した
作用を利用したので、溶接止端部に圧縮の残留応力を生
じせしめ疲労亀裂の発生寿命を延命させるとともに、表
層の組織改質により疲労亀裂進展中の破面にマイクロク
ラックを発生せしめ、その結果、廻し溶接継手やＴ継手
のように鋼板表面から疲労亀裂が発生し、板厚方向に伝
播する形態の疲労損傷の軽減、防止を可能とするもの
で、当業分野はもちろん、関連分野にもたらす効果が大
きい。EFFECTS OF THE INVENTION Since the present invention utilizes the above-mentioned action by using the above-mentioned means, the residual stress of compression is generated in the weld toe portion to prolong the life of fatigue crack initiation, and the surface layer is modified by the structure modification. Microcracks are generated on the fracture surface during fatigue crack development, and as a result, fatigue cracks are generated from the steel plate surface, such as turn-welded joints and T-joints, to reduce and prevent fatigue damage in the form of propagating in the thickness direction. It is possible and has a great effect on not only the field of art but also related fields.
【図面の簡単な説明】[Brief description of drawings]
【図１】テンパーカラー法により求めた集合組織のコロ
ニーのアスペクト比（長径／短径）と板厚方向の限界破
壊応力の関係、およびマイクロクラックを発生させるの
に必要な限界ΔＫ値との関係の図表である。FIG. 1 Relationship between the aspect ratio (major axis / minor axis) of the colony of the texture and the critical fracture stress in the plate thickness direction, and the critical ΔK value required to generate microcracks, obtained by the temper color method. Is a chart of.
【図２】テンパーカラー法により現出させた集合組織コ
ロニーの疲労亀裂進展方向の寸法（短軸径）とマイクロ
クラック発生密度の関係の図表である。FIG. 2 is a table showing the relationship between the size (minor axis diameter) in the fatigue crack propagation direction of a texture colony developed by the temper color method and the microcrack generation density.
【図３】テンパーカラー法により現出させた集合組織コ
ロニーの短軸径と表面亀裂伝播試験における公称繰り返
し応力Δσ＝１６０MPa での破断寿命との関係の図表で
ある。FIG. 3 is a chart showing the relationship between the minor axis diameter of textured colonies developed by the temper color method and the fracture life at the nominal cyclic stress Δσ = 160 MPa in the surface crack propagation test.
【図４】板厚に対する表層組織改質部の厚みの比と、表
面亀裂伝播試験における公称繰り返し曲げ応力Δσ＝１
６０MPa での破断寿命との関係の図表である。FIG. 4 is a ratio of the thickness of the surface layer structure modified portion to the plate thickness and the nominal cyclic bending stress Δσ = 1 in the surface crack propagation test.
It is a chart of the relationship with the breaking life at 60 MPa.
───────────────────────────────────────────────────── フロントページの続き (72)発明者 土師 利昭 大分市大字西ノ洲１番地 新日本製鐵株式 会社大分製鐵所内 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshiaki Haji 1 Nishinosu, Oita-shi, Oita-shi Nippon Steel Corporation Oita Works
Claims (2)
板厚の２５％以下の領域が３％以上のＮｉ量を含有する
化学成分を有し、鋼板の表裏面からそれぞれ少なくとも
板厚の５％以上の範囲にわたって、隣接する結晶粒どう
しで結晶方位の等しい粒から構成されるコロニーのアス
ペクト比（長軸径／短軸径の比）が４以上でかつその短
軸径が５μｍ以下の組織からなることを特徴とする疲労
特性の優れた複層鋼板。1. A steel plate having a chemical composition containing a Ni content of 3% or more in a region of 0.2 mm or more and 25% or less of the plate thickness from the front and back faces of the steel plate, and at least 5 of the plate thickness from the front and back faces of the steel plate, respectively. %, A structure in which the aspect ratio (ratio of major axis diameter / minor axis diameter) of a colony composed of grains having the same crystal orientation between adjacent crystal grains is 4 or more and the minor axis diameter is 5 μm or less. A multi-layer steel sheet with excellent fatigue characteristics, which is characterized by comprising
とした時に、鋼板の表裏面からそれぞれ０．２×ｔ0 ／
ｔ(mm)以上０．２５×ｔ0 ／ｔ(mm)以下の領域が３％以
上のＮｉ量を含有する化学成分を有する鋳片を、圧延中
途中水冷時の板厚をｔ1 とした時、表層から少なくとも
板厚方向に０．０５×ｔ1 (mm)以上の領域を２℃/sec以
上の冷速で急冷して、その後、当該表層部がＡｒ3 以下
の温度から圧延を開始もしくは再開し、Ａｃ1 ℃からＡ
ｃ3 ℃の範囲で圧延を終了し、その後Ａｃ3 点以上に復
熱させないことを特徴とする疲労特性の優れた複層鋼板
の製造方法。2. The thickness of the steel plate is t, and the thickness of the slab before rolling is t 0.
Then, from the front and back surfaces of the steel plate, 0.2 × t 0 /
A slab having a chemical composition in which a region of t (mm) or more and 0.25 × t 0 / t (mm) or less contains 3% or more of Ni was defined as t 1 as a plate thickness during water cooling during rolling. At this time, at least a region of 0.05 × t 1 (mm) or more in the plate thickness direction from the surface layer is rapidly cooled at a cooling rate of 2 ° C./sec or more, and then the surface layer portion starts rolling at a temperature of Ar 3 or less. Or restart, Ac 1 ℃ to A
A method for producing a multi-layer steel sheet having excellent fatigue properties, characterized in that rolling is completed within the range of c 3 ° C, and thereafter reheating is not carried out to the Ac 3 point or higher.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP136593A JP2662489B2 (en) | 1993-01-07 | 1993-01-07 | Multi-layer steel sheet excellent in fatigue characteristics and method for producing the same |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP136593A JP2662489B2 (en) | 1993-01-07 | 1993-01-07 | Multi-layer steel sheet excellent in fatigue characteristics and method for producing the same |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JPH06207245A true JPH06207245A (en) | 1994-07-26 |
| JP2662489B2 JP2662489B2 (en) | 1997-10-15 |
Family
ID=11499477
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP136593A Expired - Fee Related JP2662489B2 (en) | 1993-01-07 | 1993-01-07 | Multi-layer steel sheet excellent in fatigue characteristics and method for producing the same |
Country Status (1)
| Country | Link |
|---|---|
| JP (1) | JP2662489B2 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2005329461A (en) * | 2004-04-21 | 2005-12-02 | Nippon Steel Corp | Welded steel structure having excellent brittle crack propagation stop characteristic, and method for manufacturing the same |
-
1993
- 1993-01-07 JP JP136593A patent/JP2662489B2/en not_active Expired - Fee Related
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2005329461A (en) * | 2004-04-21 | 2005-12-02 | Nippon Steel Corp | Welded steel structure having excellent brittle crack propagation stop characteristic, and method for manufacturing the same |
| JP4505368B2 (en) * | 2004-04-21 | 2010-07-21 | 新日本製鐵株式会社 | Welded steel structure excellent in brittle crack propagation stopping characteristics and method for producing the same |
Also Published As
| Publication number | Publication date |
|---|---|
| JP2662489B2 (en) | 1997-10-15 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| KR0157540B1 (en) | High tensile strength steel having superior fatigue strength and weldability at welds and method for manufacturing the same | |
| JP5729456B2 (en) | Steel material excellent in ductile crack initiation characteristics of weld heat-affected zone and base metal zone and its manufacturing method | |
| JPH01230713A (en) | Production of high-strength and high-toughness steel having excellent stress corrosion cracking resistance | |
| JP3951429B2 (en) | Manufacturing method of high strength steel sheet with small material difference in thickness direction | |
| JP3569314B2 (en) | Steel plate for welded structure excellent in fatigue strength of welded joint and method of manufacturing the same | |
| JPH06207241A (en) | Steel for welding structure excellent in brittle fracture propagating stop performance of joint part and its production | |
| JPH1017980A (en) | Welded steel pipe with low yield ratio, and its production | |
| JP4985086B2 (en) | High tensile thick steel plate with excellent brittle crack propagation stopping characteristics and method for producing the same | |
| JP2662489B2 (en) | Multi-layer steel sheet excellent in fatigue characteristics and method for producing the same | |
| KR20230041060A (en) | Thick steel plate and its manufacturing method | |
| JP4559673B2 (en) | Thick steel plate for welded structure excellent in fatigue strength of welded joint and method for producing the same | |
| JP3462943B2 (en) | Steel sheet having high fatigue strength at welded portion and method for producing the same | |
| JP2613730B2 (en) | Multilayer steel sheet for welded structure having good fatigue properties and method for producing the same | |
| JPH10147845A (en) | Steel plate with high fatigue strength, and its production | |
| JPH05148540A (en) | Steel plate having good fatigue characteristic and production thereof | |
| JPH06271984A (en) | Steel plate excellent in fatigue propagation resistance and arrest property and its production | |
| JPH05271856A (en) | Steel plate excellent in fatigue characteristic and its production | |
| JPH0649587A (en) | Steel plate with high fatigue strength for welded joint and its production | |
| TW201812046A (en) | Thick high-strength steel plate and production process therefor | |
| CN113226614B (en) | Welded structure | |
| JP2005281816A (en) | High strength cold rolled steel sheet having satisfactory formability and excellent projection weldability, and production method therefor | |
| JPH07316654A (en) | Production of thick steel plate of high strength ni steel excellent in toughness at low temperature | |
| JPH0649593A (en) | Steel sheet with high fatigue strength and its production | |
| JP2002248569A (en) | Manufacturing method for steel welding joint for welding structure excellent in fatigue strength | |
| JP2000017378A (en) | Steel sheet pile excellent in underwater weldability |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| A01 | Written decision to grant a patent or to grant a registration (utility model) |
Free format text: JAPANESE INTERMEDIATE CODE: A01 Effective date: 19970513 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20080613 Year of fee payment: 11 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20090613 Year of fee payment: 12 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20090613 Year of fee payment: 12 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20100613 Year of fee payment: 13 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20100613 Year of fee payment: 13 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20110613 Year of fee payment: 14 |
|
| LAPS | Cancellation because of no payment of annual fees |