JP2006291252A - Material steel plate with excellent fatigue characteristic for quenched-and-tempered steel tube, and steel tube - Google Patents
Material steel plate with excellent fatigue characteristic for quenched-and-tempered steel tube, and steel tube Download PDFInfo
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本発明は、焼入れおよび焼戻しの処理を施して使用される高強度鋼管に用いるための素材鋼板、およびその素材鋼板を用いて製造される焼入れ焼戻し鋼管に関するものである。 The present invention relates to a raw steel plate for use in a high-strength steel pipe that is used after being subjected to quenching and tempering treatments, and a quenched and tempered steel pipe manufactured using the raw steel plate.
自動車をはじめ、各種機械構造物においては、高強度と疲労特性が要求される部材に焼入れ焼戻しの処理を施した「鋼管」を使用することが多々ある。
一般に鋼材の疲労特性を向上させるためには、表面を硬化したり平滑化したりすることが有効であるとされる。
特許文献１には、窒化処理により表面を硬化させて疲労特性を向上させる技術が開示されている。引用文献２には、鋼管内面を研削することにより平滑化するとともに脱炭層を除去して鋼管の疲労特性を向上させる技術が開示されている。
Generally, in order to improve the fatigue characteristics of steel materials, it is considered effective to harden or smooth the surface.
Patent Document 1 discloses a technique for improving fatigue characteristics by hardening a surface by nitriding treatment. Cited Document 2 discloses a technique for improving the fatigue characteristics of a steel pipe by smoothing the inner surface of the steel pipe by grinding and removing the decarburized layer.
昨今、機械構造物の各種部材には小型軽量化の要求が高まっている。鋼管で構成される高強度部材についても例外ではない。
鋼管部材を軽量化するには、肉厚の低減が最も有効である。しかし、薄肉化は強度や疲労特性の面で不利となる。特に、鋼管は曲げ加工などにより所望の形状に加工される場合が多いが、曲げの外側では肉厚が薄くなり、耐久性の点で厳しい状況となる。
Recently, there is an increasing demand for reducing the size and weight of various members of mechanical structures. High strength members made of steel pipe are no exception.
In order to reduce the weight of the steel pipe member, it is most effective to reduce the wall thickness. However, thinning is disadvantageous in terms of strength and fatigue characteristics. In particular, the steel pipe is often processed into a desired shape by bending or the like, but the thickness is reduced outside the bending, and the situation is severe in terms of durability.
このように、高強度鋼管の耐久性を維持しながら薄肉化を図ることは必ずしも容易ではない。この問題の解決手段として、特殊元素の添加により鋼材自体の強度・疲労特性レベルを向上させる方法が考えられる。しかし、多くの機械構造物においてそのような素材コストの増加を招く手法は許容されない。ショットピーニングによる圧縮残留応力の付与によって耐久性を向上させる方法が採られることもある。しかし、ショットピーニングは鋼管の外面側には付与できるが、内面側に付与することは困難であるため、圧縮残留応力の不足する内面側から破損することがある。また、特許文献２のように内面を研削することも考えられる。しかし、これは工程の増加につながり、現状の鋼管製造プロセスをそのまま適用できるものではなく、工程増および歩留り低下によるコストアップも避けられない。 Thus, it is not always easy to reduce the thickness while maintaining the durability of the high-strength steel pipe. As a means for solving this problem, a method of improving the strength and fatigue property level of the steel material itself by adding a special element is conceivable. However, such a method that causes an increase in material cost is not allowed in many mechanical structures. A method of improving durability by applying compressive residual stress by shot peening may be employed. However, although shot peening can be applied to the outer surface side of the steel pipe, it is difficult to apply the shot peening to the inner surface side. Moreover, grinding an inner surface like patent document 2 is also considered. However, this leads to an increase in processes, and the current steel pipe manufacturing process cannot be applied as it is, and an increase in costs due to an increase in processes and a decrease in yield is unavoidable.
本発明は、このような現状に鑑み、素材コストおよび製造コストを従来材と同等に抑えながら、鋼管の耐久性、特に疲労特性を向上させる技術の提供を目的とする。 In view of such a current situation, the present invention aims to provide a technique for improving the durability of a steel pipe, in particular, fatigue characteristics, while suppressing the raw material cost and the manufacturing cost to be equal to those of a conventional material.
発明者らは、高強度鋼管の疲労特性に及ぼす種々の要因について詳細に見直しを行った。その結果、鋼管の中間製品である「素材鋼板」の段階において、表面の脱炭層の形成が十分に抑止されていれば、鋼管製造後における疲労特性が顕著に向上することを見出した。また、そのような素材鋼板は、熱延条件や焼鈍条件を工夫することで製造可能であることが確認された。 The inventors have made a detailed review of various factors affecting the fatigue properties of high-strength steel pipes. As a result, it has been found that the fatigue characteristics after steel pipe manufacture is significantly improved if the formation of the surface decarburized layer is sufficiently suppressed at the stage of the “raw steel plate” which is an intermediate product of the steel pipe. Moreover, it was confirmed that such a raw material steel plate can be manufactured by devising hot rolling conditions and annealing conditions.
すなわち、上記目的は、質量％で、Ｃ：０.１〜０.４％、Ｓi：０.５％以下、Ｍn：０.１〜１.５％、Ｐ：０.０２％以下、Ｓ：０.０１％以下、Ｃr：０.１〜２％、Ｔi：０.０１〜０.１％、Ｂ：０.０００３〜０.０１％、Ｎi：０〜２％、Ｍo：０〜２％以下、Ｖ：０〜０.５％、Ｎb：０〜０.５％、Ｃa：０〜０.０２％、残部がＦeおよび不可避的不純物の組成を有し、板厚中心部と表面から２０μｍ位置のＣ断面硬さの差Δｋが３０ＨＶ以下である、疲労特性の良好な焼入れ焼戻し鋼管用素材鋼板によって達成される。 That is, the above purpose is mass%, C: 0.1 to 0.4%, Si: 0.5% or less, Mn: 0.1 to 1.5%, P: 0.02% or less, S: 0.01% or less, Cr: 0.1-2%, Ti: 0.01-0.1%, B: 0.0003-0.01%, Ni: 0-2%, Mo: 0-2% Hereinafter, V: 0 to 0.5%, Nb: 0 to 0.5%, Ca: 0 to 0.02%, the balance having the composition of Fe and inevitable impurities, 20 μm from the center of the plate thickness and the surface This is achieved by a material steel plate for quenching and tempering steel pipe having good fatigue characteristics, in which the difference Δk in the C cross-sectional hardness at the position is 30 HV or less.
そのような鋼板は、加熱抽出温度１１５０〜１３００℃、巻取温度５７０℃以下での熱間圧延、あるいはさらにＡc1点未満の温度での焼鈍を施すことにより製造される。熱間圧延後には脱スケール工程を入れることができる。また、熱延鋼板に対して、冷間圧延とＡc1点未満の温度での焼鈍を施すこともできる。 Such a steel sheet is manufactured by performing hot rolling at a heating extraction temperature of 1150 to 1300 ° C. and a coiling temperature of 570 ° C. or lower, or annealing at a temperature of less than Ac 1 point. A descaling step can be included after hot rolling. In addition, the hot-rolled steel sheet can be subjected to cold rolling and annealing at a temperature less than Ac 1 point.
なお、Ｎi、Ｍo、Ｖ、Ｎb、Ｃaは任意添加元素であり、その下限０％は通常の製鋼工場での分析手法で測定限界以下である場合を意味する。「Ｃ断面」は、圧延方向に垂直な断面である。板厚中心部と表面から２０μｍ位置のＣ断面硬さは、例えばマイクロビッカース硬度計によって測定できる。Δｋは（板厚中心部の硬さ）−（表面から２０μｍ位置の硬さ）であるが、これは負の値になっても構わない。「鋼板」には「鋼帯」が含まれる。 Ni, Mo, V, Nb, and Ca are arbitrarily added elements, and the lower limit of 0% means that they are below the measurement limit by an analysis method in a normal steel factory. “C section” is a section perpendicular to the rolling direction. The C section hardness at a position of 20 μm from the center of the plate thickness and the surface can be measured by, for example, a micro Vickers hardness meter. Δk is (the hardness at the center of the plate thickness) − (the hardness at the position of 20 μm from the surface), but this may be a negative value. “Steel plate” includes “steel strip”.
また、前記のような素材鋼板を溶接造管したのち必要に応じて成形加工を施し、最終的に焼入れおよび焼き戻しされた鋼管であって、鋼管の肉厚中心部と内表面から２０μｍ位置のＣ断面硬さの差Δｋ1が７０ＨＶ以下である焼入れ焼戻し鋼管が提供される。
この場合の「Ｃ断面」は、鋼管の長手方向に垂直な断面である。Δｋ1は（肉厚中心部の硬さ）−（内表面から２０μｍ位置の硬さ）であるが、これは負の値になっても構わない。
Further, the steel sheet as described above is welded and formed, and then subjected to forming as necessary. Finally, the steel pipe is quenched and tempered, and is 20 μm from the thickness center portion and the inner surface of the steel pipe. A quenched and tempered steel pipe having a difference in cross-sectional hardness Δk1 of 70 HV or less is provided.
The “C cross section” in this case is a cross section perpendicular to the longitudinal direction of the steel pipe. Δk1 is (the hardness at the center of the thickness) − (the hardness at the position of 20 μm from the inner surface), but this may be a negative value.
本発明に従えば、従来材と同等の安価な鋼を用いて、各種機械構造部材に用いる高強度鋼管の疲労特性を向上させることが可能となった。特に、中間製品である素材鋼板の段階で表層部の硬度低下を抑制しておく手法を採用し、これが鋼管の疲労特性向上効果をもたらすので、鋼管製造段階において疲労特性を改善するために特段の手段を要しない。すなわち、鋼管製造工程への負荷が軽減される。また、研削等の機械的除去手段を採用する必要がなく、それによる歩留り低下の心配がない。本発明は、高強度鋼管の薄肉化、ひいては自動車をはじめとする機械構造物の軽量化に寄与し得るものである。 According to the present invention, it is possible to improve the fatigue characteristics of high-strength steel pipes used for various mechanical structural members by using inexpensive steel equivalent to conventional materials. In particular, a method of suppressing the hardness reduction of the surface layer part at the stage of the intermediate steel plate is adopted, and this brings about the effect of improving the fatigue characteristics of the steel pipe. No means are required. That is, the load on the steel pipe manufacturing process is reduced. Further, there is no need to employ mechanical removal means such as grinding, and there is no concern about yield reduction. The present invention can contribute to reducing the thickness of high-strength steel pipes and, in turn, to reducing the weight of mechanical structures such as automobiles.
本発明では各元素の含有量を以下のように調整した鋼を使用する。なお、合金元素含有量の「％」は「質量％」を意味する。
Ｃは、機械構造用高強度鋼管に望まれる強度とばね特性を確保するために、０.１％以上の含有が必要である。ただし、多量に含有させると靱性低下による脆性破壊が生じやすくなり、造管時の加工性や溶接部の健全性が劣化する。このためＣ含有量は０.１〜０.４％に規定される。
In the present invention, steel whose content of each element is adjusted as follows is used. Note that “%” of the alloy element content means “mass%”.
C needs to be contained in an amount of 0.1% or more in order to ensure the strength and spring characteristics desired for a high-strength steel pipe for machine structures. However, if contained in a large amount, brittle fracture is likely to occur due to a decrease in toughness, and the workability during pipe making and the soundness of the welded portion deteriorate. For this reason, C content is prescribed | regulated to 0.1 to 0.4%.
Ｓiは、製鋼での脱酸に有効な元素であるが、０.５％を超えると熱間圧延時のスラブ加熱においてファイアライトが生成し易くなり、これが熱延でのデスケール性を劣化させ、表面疵や疲労破壊の起点を作りやすい。また、多量のＳi含有は溶接性および靱性を劣化させる。このためＳi含有量は０.５％以下に制限される。 Si is an element effective for deoxidation in steel making. However, if it exceeds 0.5%, it becomes easy to generate firelight in slab heating during hot rolling, which deteriorates descaleability in hot rolling, It is easy to make surface flaws and fatigue fracture starting points. In addition, a large amount of Si contained deteriorates weldability and toughness. For this reason, the Si content is limited to 0.5% or less.
Ｍnは、焼入れ性および強度を確保する上で有効な元素であり、その効果を十分に得るためには０.１％以上含有させる必要である。しかし、過剰に添加すると、炭素当量も高くなり、加工性および溶接部の健全性に悪影響を及ぼす。このためＭn含有量は０.１〜１.５％に規定される。 Mn is an element effective in securing hardenability and strength, and in order to sufficiently obtain the effect, it is necessary to contain 0.1% or more. However, if added in excess, the carbon equivalent also increases, which adversely affects workability and the soundness of the weld. For this reason, Mn content is prescribed | regulated to 0.1-1.5%.
Ｐは、焼入れ時にオーステナイト粒界に偏析して延性、靱性を劣化させる。このためＰ含有量は０.０２％以下に制限される。 P segregates at the austenite grain boundaries during quenching and deteriorates ductility and toughness. For this reason, the P content is limited to 0.02% or less.
Ｓは、鋼中でＭnＳを形成し、これが亀裂の起点となって強度、靱性を低下させる要因になる。このためＳ含有量は０.０１％以下に制限される。 S forms MnS in steel, which becomes a starting point of cracks and becomes a factor of reducing strength and toughness. For this reason, the S content is limited to 0.01% or less.
Ｃrは、Ｍnと同様に焼入れ性の向上に有効であり、少なくとも０.１％以上の含有が必要である。しかし、２％を超えると焼入れ・焼戻し後の組織が未溶解炭化物を多量に含むものとなり、この炭化物が亀裂を助長させる起点となって、靱性や疲労特性の低下を招く。このためＣr含有量は０.１〜２％に規定される。 Cr is effective for improving the hardenability like Mn, and must be contained at least 0.1% or more. However, if it exceeds 2%, the structure after quenching and tempering contains a large amount of undissolved carbide, and this carbide serves as a starting point for promoting cracks, leading to deterioration of toughness and fatigue characteristics. For this reason, Cr content is prescribed | regulated to 0.1 to 2%.
Ｔiは、鋼中のＮをＴiＮとして固定することにより、焼入れ性向上に有効な固溶Ｂの確保に寄与する。また、焼入れ時に旧オーステナイト粒径の粗大化を抑制する。これらの効果を十分に得るには０.０１％以上のＴi含有が必要となる。ただし、０.１％を超えてＴiを添加しても旧オーステナイト粒径の粗大化抑制効果は飽和し、却って疲労破壊の起点となるＴi系介在物が増加する。このためＴi含有量は０.０１〜０.１％に規定される。 Ti contributes to securing solid solution B effective in improving hardenability by fixing N in steel as TiN. Further, the coarsening of the prior austenite grain size is suppressed during quenching. In order to sufficiently obtain these effects, it is necessary to contain 0.01% or more of Ti. However, even if Ti is added in excess of 0.1%, the effect of suppressing the coarsening of the prior austenite grain size is saturated, and on the other hand, Ti-based inclusions that become the starting point of fatigue fracture increase. For this reason, Ti content is prescribed | regulated to 0.01 to 0.1%.
Ｂは、微量の添加で焼入れ性を高める効果がある。また、焼入れ・焼戻し後の旧オーステナイト粒界を強化して脆性破壊を抑制し、靱性を向上させる効果を有する。これらの効果を十分に発揮させるためには０.０００３％以上のＢ含有が必要である。ただし、０.０１％を超えるとこれらの効果は飽和する。このためＢ含有量は０.０００３〜０.０１％に規定される。 B has an effect of enhancing the hardenability by adding a small amount. Moreover, it has the effect of strengthening the prior austenite grain boundaries after quenching and tempering to suppress brittle fracture and improve toughness. In order to fully exhibit these effects, 0.0003% or more of B content is necessary. However, if it exceeds 0.01%, these effects are saturated. For this reason, the B content is specified to be 0.0003 to 0.01%.
Ｎiは、焼入れ性および靱性の向上に有効である。これらの効果を十分に得るには０.０１％以上含有させることが望ましい。ただし、２％を超えると上記効果は飽和し、不経済となる。このためＮiを添加する場合は２％以下の範囲で行う必要があり、０.１〜２％の範囲で含有させることが好ましい。 Ni is effective in improving hardenability and toughness. In order to obtain these effects sufficiently, it is desirable to contain 0.01% or more. However, if it exceeds 2%, the above effect is saturated and uneconomical. For this reason, when adding Ni, it is necessary to carry out in the range of 2% or less, and it is preferable to contain in 0.1 to 2% of range.
Ｍoは、焼入れ性と焼戻し軟化抵抗の向上に有効な元素であり、Ｍn、Ｃrの過剰な添加による靱性劣化を抑えるために補助的に添加することができる。Ｍoを添加する場合は０.０５％以上の含有量とすることが効果的である。ただしＭoは高価な元素であり、多量添加は経済性を損なうので、Ｍo添加は２％以下の範囲で行う必要がある。 Mo is an element effective for improving hardenability and temper softening resistance, and can be supplementarily added to suppress toughness deterioration due to excessive addition of Mn and Cr. When adding Mo, it is effective to make the content 0.05% or more. However, Mo is an expensive element, and addition of a large amount impairs economic efficiency. Therefore, addition of Mo needs to be performed within a range of 2% or less.
Ｖは、焼入れ時に結晶粒を微細化する作用があり、靱性向上に有効である。Ｖを添加する場合は０.０５％以上の含有量とすることが好ましい。ただしＶも高価な元素であり、多量添加は経済性を損なうので、Ｖ添加は０.５％以下の範囲で行う必要がある。 V has the effect of refining crystal grains during quenching, and is effective in improving toughness. When V is added, the content is preferably 0.05% or more. However, V is also an expensive element, and addition of a large amount impairs economic efficiency. Therefore, addition of V must be performed within a range of 0.5% or less.
Ｎbは、炭窒化物を形成し、旧オーステナイト結晶粒の粗大化を抑制して靱性を向上させる作用がある。Ｎbを添加する場合は０.０５％以上の含有量とすることが好ましい。ただし、０.５％を超えても上記作用の上昇はあまり期待できない。このためＮb添加は０.５％以下の範囲で行う。 Nb has the effect of forming carbonitrides and suppressing toughening of prior austenite crystal grains to improve toughness. When Nb is added, the content is preferably 0.05% or more. However, even if it exceeds 0.5%, the increase in the above action cannot be expected so much. Therefore, Nb addition is performed within a range of 0.5% or less.
Ｃaは、圧延鋼材中における圧延方向に長く伸びたＭnＳ系介在物を低減することにより、鋼材の異方性を軽減する効果がある。Ｃaを添加する場合は０.００１％以上の含有量とすることが好ましい。しかし、多量に添加するとＣa系介在物が増加して疲労特性に悪影響を及ぼすので、Ｃa含有量は０.０２％以下に制限される。 Ca has the effect of reducing the anisotropy of the steel material by reducing the MnS inclusions elongated in the rolling direction in the rolled steel material. When adding Ca, the content is preferably 0.001% or more. However, when Ca is added in a large amount, Ca-based inclusions increase and adversely affect fatigue properties, so the Ca content is limited to 0.02% or less.
Ｎは、ＢをＢＮの形成によって消費し、前記のＢ添加の効果を引き出す上でマイナスの要因となる。このためＮ含有量はできるだけ低い方が望ましく、最大でも０.０１％以下にすべきである。 N consumes B by the formation of BN, and becomes a negative factor in extracting the effect of the B addition. Therefore, the N content is preferably as low as possible, and should be 0.01% or less at the maximum.
以上の化学組成を有する鋼を用いて素材鋼板を製造し、これを造管して各種機械構造物に用いる鋼管を得る。 A raw steel plate is manufactured using steel having the above chemical composition, and this is piped to obtain steel pipes used for various machine structures.
発明者らの詳細な研究によれば、最終的に得られる鋼管部材の疲労特性は、鋼管を製造する前の「素材鋼板」の性状に大きく依存することが明らかになった。すなわち、「素材鋼板」の段階で、表面の脱炭層の形成を十分に抑止しておけば、その素材鋼板に由来する鋼管部材の疲労特性を改善することが可能になるのである。 According to detailed studies by the inventors, it has been clarified that the fatigue characteristics of the steel pipe member finally obtained greatly depend on the properties of the “material steel plate” before the steel pipe is manufactured. That is, if the formation of the surface decarburized layer is sufficiently suppressed at the stage of the “material steel plate”, it becomes possible to improve the fatigue characteristics of the steel pipe member derived from the material steel plate.
具体的には、素材鋼板の板厚中心部と表面から２０μｍ位置のＣ断面硬さの差Δｋが３０ＨＶ以下になるように表層部の脱炭層形成が抑止されていることが必要である。Δｋが３０ＨＶを超える程度に表層部が軟化していれば、その素材鋼板を用いた焼入れ焼き戻し鋼管の疲労特性を安定して改善することが難しくなる。 Specifically, it is necessary that the formation of a decarburized layer in the surface layer portion is suppressed so that the difference Δk in the C cross-sectional hardness at a position 20 μm from the plate thickness center portion and the surface of the raw steel plate is 30 HV or less. If the surface layer portion is softened to such an extent that Δk exceeds 30 HV, it is difficult to stably improve the fatigue characteristics of a quenched and tempered steel pipe using the material steel plate.
Δｋが３０ＨＶ以下になるように脱炭層の生成が抑止された鋼板は、前記化学組成を持つ鋼を用い、熱間圧延あるいは焼鈍を工夫することによって製造できることがわかった。
熱間圧延では加熱抽出温度を１１５０〜１３００℃の範囲とし、かつ、巻取温度を５７０℃以下とすればよい。熱延後の脱スケールは通常、酸洗で十分である。脱スケール後には焼鈍を行うことができる。熱延後〜造管前の段階では基本的に焼鈍は最終焼鈍１回で済ますのが良い。その焼鈍はＡc1点未満の温度で行う。水素雰囲気などの非酸化性雰囲気で行うと脱炭を防ぐことができ、研削等により脱炭層を除去する必要がなくなる。酸化性雰囲気での焼鈍を行った場合、そのままの状態でΔｋを安定して３０ＨＶ以下にすることが難しい。
熱延、脱スケール後には必要に応じて冷間圧延を行うことができる。熱延時にわずかに生じた脱炭層も例えば１５％以上好ましくは３０％以上の冷間圧延を行うと更に薄くなり、疲労特性向上に有利となる。
It has been found that a steel sheet in which the formation of a decarburized layer is suppressed so that Δk is 30 HV or less can be produced by devising hot rolling or annealing using steel having the above chemical composition.
In hot rolling, the heating extraction temperature may be in the range of 1150 to 1300 ° C, and the winding temperature may be 570 ° C or less. For descaling after hot rolling, pickling is usually sufficient. Annealing can be performed after descaling. From the stage after hot rolling to before pipe forming, it is basically good to perform annealing only once. The annealing is performed at a temperature less than Ac 1 point. When performed in a non-oxidizing atmosphere such as a hydrogen atmosphere, decarburization can be prevented, and there is no need to remove the decarburized layer by grinding or the like. When annealing in an oxidizing atmosphere is performed, it is difficult to make Δk stable to 30 HV or less in the same state.
After hot rolling and descaling, cold rolling can be performed as necessary. A decarburized layer slightly generated during hot rolling also becomes thinner when cold rolled, for example, 15% or more, preferably 30% or more, which is advantageous for improving fatigue characteristics.
以上のような素材鋼板は、一般的な手法により溶接造管に供される。溶接法は高周波溶接、通電溶接、ＴＩＧ溶接などが適用できる。造管後、必要に応じて成形加工される。その後、焼入れおよび焼き戻しされ、疲労特性の良好な高強度鋼管が得られる。焼入れおよび焼き戻しの処理では、表面の脱炭ができるだけ進行しないように配慮すべきであるが、素材鋼板の前記Δｋが３０ＨＶ以下になっていれば、通電加熱等の通常の手法で焼入れおよび焼き戻しを行った後に、鋼管の肉厚中心部と内表面から２０μｍ位置のＣ断面硬さの差Δｋ1を７０ＨＶ以下に維持することが可能である。このΔｋ1が７０ＨＶ以下になっていれば、内面の表層部は研削等の機械的手段で除去されていなくても、顕著な疲労特性改善効果が得られるのである。 The material steel plate as described above is used for welded pipe making by a general method. As the welding method, high frequency welding, current welding, TIG welding, or the like can be applied. After pipe making, it is molded as necessary. Thereafter, it is quenched and tempered to obtain a high-strength steel pipe with good fatigue characteristics. In the quenching and tempering treatments, care should be taken so that surface decarburization does not proceed as much as possible. However, if the Δk of the raw steel sheet is 30 HV or less, quenching and quenching can be performed by a normal method such as electric heating. After returning, it is possible to maintain the difference Δk1 in the C section hardness at a position of 20 μm from the thickness center portion of the steel pipe and the inner surface at 70 HV or less. If this Δk1 is 70 HV or less, a remarkable improvement in fatigue characteristics can be obtained even if the surface layer portion on the inner surface is not removed by mechanical means such as grinding.
表１に示す鋼を溶製し、スラブを熱間圧延して板厚３.０〜５.０ｍｍの熱延鋼板を得た。熱間圧延後には酸洗を行った。その後、焼鈍、あるいは冷間圧延と焼鈍に供した材料も含め、最終的に以下の工程で板厚３.０ｍｍの鋼板（「素材鋼板」という）を得た。
工程ａ：熱間圧延→脱スケール（酸洗）
工程ｂ：熱間圧延→脱スケール（酸洗）→焼鈍
工程ｃ：熱間圧延→脱スケール（酸洗）→冷間圧延→焼鈍
焼鈍は、温度６７０〜７２０℃、保持時間１５〜２０時間で行い、雰囲気はＨ2：１００％、露点（ＤＰ）：−６０℃、またはＮ2：９７％＋Ｈ2：３％、露点（ＤＰ）：５０℃とした。焼鈍温度は６７０〜７２０℃、保持時間は１５〜２０時間とした。この温度範囲は再結晶温度以上Ａc1点以下に該当する。焼鈍後に酸洗は行っていない。
表２に製造条件を示してある。表２中、ＦＤＴはスラブの加熱抽出温度、ＣＴは巻取温度である。工程については、上記工程ａを「熱延」、工程ｂを「焼鈍」、工程ｃを「冷延焼鈍」と表示してある。
The steel shown in Table 1 was melted, and the slab was hot-rolled to obtain a hot-rolled steel sheet having a thickness of 3.0 to 5.0 mm. After hot rolling, pickling was performed. Thereafter, a steel plate (referred to as “material steel plate”) having a thickness of 3.0 mm was finally obtained through the following steps, including materials subjected to annealing or cold rolling and annealing.
Process a: Hot rolling → descaling (pickling)
Step b: Hot rolling → Descaling (pickling) → Annealing Step c: Hot rolling → Descaling (pickling) → Cold rolling → Annealing The temperature is 670 to 720 ° C. and the holding time is 15 to 20 hours. The atmosphere was H 2 : 100%, dew point (DP): −60 ° C., or N 2 : 97% + H 2 : 3%, dew point (DP): 50 ° C. The annealing temperature was 670 to 720 ° C., and the holding time was 15 to 20 hours. This temperature range corresponds to the recrystallization temperature or more and the Ac 1 point or less. Pickling is not performed after annealing.
Table 2 shows the manufacturing conditions. In Table 2, FDT is the slab heating extraction temperature, and CT is the coiling temperature. Regarding the process, the process a is indicated as “hot rolling”, the process b as “annealing”, and the process c as “cold rolling annealing”.
各素材鋼板について、マイクロビッカース硬度計を用いて、板厚中心部および表面から２０μｍ位置のＣ断面硬さを測定した。それぞれｎ＝１０とし、平均を採った。これらの特定値から「板厚中心部の硬さ（ＨＶ）−表面から２０μｍ位置のＣ断面硬さ（ＨＶ）」を算出し、これをΔｋとした。 About each raw material steel plate, C cross-section hardness of a 20-micrometer position from a plate | board thickness center part and the surface was measured using the micro Vickers hardness meter. Each was set to n = 10 and the average was taken. From these specific values, “the hardness at the center of the plate thickness (HV) —the C section hardness (HV) at a position 20 μm from the surface” was calculated, and this was taken as Δk.
次に、各素材鋼板を用いて焼入れ焼戻し後の疲労特性を調べた。これは、表面脱炭層の有無が疲労特性に及ぼす影響を素材鋼板によってより精度良く比較するための実験であり、この実験で疲労特性に劣ったものは焼入れ焼戻し鋼管においても良好な疲労特性を期待することはできない。逆に、この実験で疲労特性が顕著に改善されたものは、焼入れ焼戻し鋼管においても相対的に疲労特性の顕著な改善が見込まれる。
各素材鋼板から長手方向が圧延方向となるように幅３５×長さ１１０ｍｍの短冊状試験片を作製し、「９００℃×１５分保持→油冷」の焼入れ、および「３００〜４００℃×１時間保持」の焼戻しを行って調質硬さ３７０〜４３０ＨＶにした。その後、ＪＩＳ Ｚ２２７３の両振り平面曲げ疲れ試験に準拠した方法で最大曲げ応力６００Ｎ／ｍｍ2の平面曲げ疲労試験を実施した。なお、１×１０6サイクルを超えた場合はその時点で終了し、疲労寿命＞１×１０6サイクルと評価した。平面曲げ疲労特性は疲労寿命が１×１０5以上を良好（○評価）、それ未満を不良（×評価）とした。
Next, the fatigue characteristics after quenching and tempering were examined using each material steel plate. This is an experiment to compare the influence of the presence or absence of a surface decarburized layer on the fatigue characteristics more accurately by the steel sheet. Those inferior to the fatigue characteristics in this experiment are expected to have good fatigue characteristics even in quenched and tempered steel pipes. I can't do it. On the contrary, if the fatigue characteristics are remarkably improved in this experiment, the fatigue characteristics are expected to be remarkably improved even in the quenched and tempered steel pipe.
A strip-shaped test piece having a width of 35 × 110 mm is produced from each material steel plate so that the longitudinal direction is the rolling direction, quenching of “900 ° C. × 15 minutes holding → oil cooling”, and “300 to 400 ° C. × 1” Tempering hardness was set to 370 to 430 HV by tempering "time retention". Thereafter, a plane bending fatigue test with a maximum bending stress of 600 N / mm 2 was carried out by a method based on the double swing plane bending fatigue test of JIS Z2273. Incidentally, if it exceeds 1 × 10 6 cycles is terminated at that point, was evaluated as fatigue life> 1 × 10 6 cycles. As for the plane bending fatigue characteristics, a fatigue life of 1 × 10 5 or more was evaluated as good (◯ evaluation), and less than that was determined as poor (× evaluation).
次に、各素材鋼板を用いて外径２７.２ｍｍの鋼管を製造した。造管はいずれも高周波溶接で行った。得られた鋼管について管の断面形状および溶接部の欠陥を調べ、造管性を評価した。
これらの結果を表２に示す。表２中、平面曲げ疲労評価、造管性評価、総合評価は○が良好、×が不良と判定されるものである。
Next, a steel pipe having an outer diameter of 27.2 mm was manufactured using each material steel plate. Both pipes were formed by high frequency welding. About the obtained steel pipe, the cross-sectional shape of the pipe and the defect of the welded portion were examined, and the pipe forming property was evaluated.
These results are shown in Table 2. In Table 2, plane bending fatigue evaluation, pipe forming evaluation, and comprehensive evaluation are determined to be good for ◯ and poor for x.
表２から判るように、Ｎｏ.１〜１４の本発明例のものは、板厚中心部と表面から２０μｍ位置のＣ断面硬さの差Δｋが３０ＨＶ以下であり、１×１０5サイクル以上の優れた平面曲げ疲労特性、および優れた造管性を呈した。 As can be seen from Table 2, in the examples of the present invention Nos. 1 to 14, the difference Δk in the C section hardness at the position of 20 μm from the center of the plate thickness and the surface is 30 HV or less, and 1 × 10 5 cycles or more. Excellent plane bending fatigue properties and excellent pipe forming properties were exhibited.
これに対し、比較例Ｎｏ.１５はＣ量が低いので強度不足により鋼板の疲労寿命が短かった。Ｎｏ.１６はＣ量が高いので鋼管の断面形状が不良となった。Ｎｏ.１７はＳi量が高く、Ｎｏ.１８はＭn量が高く、Ｎｏ.１９はＣr量が高いので、これらは造管の際、高周波溶接時にそれぞれＳi、Ｍn、Ｃrの酸化物の生成量が多くなり、この酸化物が溶接部に貫入した形で存在する欠陥（ペネトレータ）が生じた。これらは鋼板では良好な疲労特性を示すものの、鋼管では溶接部を起点とした早期折損が生じることがある。 On the other hand, Comparative Example No. 15 had a low amount of C, so the fatigue life of the steel sheet was short due to insufficient strength. No. 16 had a high C content, so the cross-sectional shape of the steel pipe was poor. No. 17 has a high amount of Si, No. 18 has a high amount of Mn, and No. 19 has a high amount of Cr. Therefore, these are amounts of oxides of Si, Mn, and Cr during pipe forming and high frequency welding, respectively. As a result, defects (penetrator) existed in the form of penetration of this oxide into the weld. Although these steel sheets exhibit good fatigue characteristics, steel pipes may suffer early breakage starting from the weld.
Ｎｏ.２０〜２４は熱延での巻取温度が高いために脱炭量が大きく、Δｋが３０ＨＶを超える大きな表面硬さの低下がみられた。このため疲労特性に劣った。またＮｏ.２４、２５は酸化性雰囲気の焼鈍で仕上げたことによりΔｋが３０ＨＶを超え、疲労特性に劣った。 Nos. 20 to 24 had a large decarburization amount due to the high coiling temperature in hot rolling, and a large decrease in surface hardness exceeding ΔHV of 30 HV was observed. For this reason, it was inferior to the fatigue characteristics. Nos. 24 and 25 were finished by annealing in an oxidizing atmosphere, and Δk exceeded 30 HV, resulting in poor fatigue characteristics.
次に、表１の鋼Ｃ、Ｄを用いて、実施例１で示した工程ａ〜ｃにより素材鋼板を製造し、実施例１と同様の手法で高周波造管し、その後「９００℃×１５分保持→油冷」の焼入れ、および「３００〜４００℃×１時間保持」の焼戻しを行って外径２７.２ｍｍの鋼管を製造した。管の外面、内面とも、研削等の機械的除去手段を何も施していない。 Next, the steel plates C and D in Table 1 were used to manufacture a raw steel plate by the steps a to c shown in Example 1, and high-frequency tube forming was performed in the same manner as in Example 1. Thereafter, “900 ° C. × 15 A steel pipe having an outer diameter of 27.2 mm was manufactured by quenching of “minute retention → oil cooling” and tempering of “300 to 400 ° C. × 1 hour”. No mechanical removal means such as grinding is applied to the outer and inner surfaces of the tube.
各素材鋼板の段階で、Ｃ断面におけるΔｋを実施例１と同様の手法で求めた。
また、各焼入れ焼戻し後の鋼管について、肉厚中心部と内表面から２０μｍ位置のＣ断面硬さの差Δｋ1を求めた。硬さの測定法およびΔｋ1の算出法は前述のΔｋの場合に準じた。
さらに、各焼入れ焼戻し鋼管について、曲げ疲労試験を実施した。この場合、鋼管外面の最大引張応力発生部位における引張応力が６００Ｎ／ｍｍ2となる条件で行った。前述の平面曲げ疲労試験と同様、１×１０6サイクルを超えた場合はその時点で終了し、疲労寿命＞１×１０6サイクルと評価した。鋼管疲労評価は、鋼管曲げ疲労寿命が１×１０5以上を良好（○評価）、それ未満を不良（×評価）とした。
結果を表３に示す。
At the stage of each material steel plate, Δk in the C cross section was determined in the same manner as in Example 1.
Further, for each steel pipe after quenching and tempering, a difference Δk1 in C cross-sectional hardness at a position of 20 μm from the thickness center portion and the inner surface was obtained. The method for measuring hardness and the method for calculating Δk1 were the same as those for Δk described above.
Furthermore, a bending fatigue test was performed on each quenched and tempered steel pipe. In this case, it was performed under the condition that the tensile stress at the site where the maximum tensile stress was generated on the outer surface of the steel pipe was 600 N / mm 2 . Similar to the above-described plane bending fatigue test, when the cycle exceeded 1 × 10 6 cycles, the test was terminated at that point, and the fatigue life was evaluated as> 1 × 10 6 cycles. In the steel pipe fatigue evaluation, a steel pipe bending fatigue life of 1 × 10 5 or more was evaluated as good (◯ evaluation), and less than that was determined as poor (× evaluation).
The results are shown in Table 3.
表３から判るように、素材鋼板の段階でΔｋが３０以下となるように脱炭層の形成を抑止した本発明例の鋼管は、焼入れ焼戻し後のΔｋ1が７０ＨＶ以下の範囲に収まっており、最終的に脱炭層の形成が非常に少ない鋼管が得られた。その結果、鋼管における曲げ疲労寿命も１×１０5サイクル以上と良好であった。 As can be seen from Table 3, the steel pipe of the example of the present invention in which the formation of the decarburized layer was suppressed so that Δk was 30 or less at the raw steel plate stage had Δk1 after quenching and tempering in the range of 70 HV or less. In particular, a steel pipe with very little formation of decarburized layer was obtained. As a result, the bending fatigue life of the steel pipe was also as good as 1 × 10 5 cycles or more.
これに対し、比較例Ｃ４、Ｃ６、Ｄ４、Ｄ６は素材鋼板製造段階において熱延巻取温度が高く、またＣ５、Ｄ５は素材鋼板製造段階における焼鈍を酸化性雰囲気で行ったため、これらはいずれもΔｋが３０を超え、Δｋ1は７０を超えた。その結果鋼管の曲げ疲労寿命は短かった。 On the other hand, Comparative Examples C4, C6, D4, and D6 have high hot rolling coiling temperatures in the raw steel sheet manufacturing stage, and C5 and D5 are all annealed in an oxidizing atmosphere in the raw steel sheet manufacturing stage. Δk exceeded 30 and Δk1 exceeded 70. As a result, the bending fatigue life of the steel pipe was short.
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Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2009079280A (en) * | 2007-09-27 | 2009-04-16 | Nisshin Steel Co Ltd | Quenched and tempered steel pipe with high fatigue life, and its manufacturing method |
| JP2009249650A (en) * | 2008-04-01 | 2009-10-29 | Nisshin Steel Co Ltd | High fatigue service life quenching/tempering steel tube and manufacturing method therefor |
| CN114032470A (en) * | 2022-01-07 | 2022-02-11 | 北京科技大学 | Carburizing bearing steel and preparation method thereof |
| CN114622136A (en) * | 2022-02-25 | 2022-06-14 | 包头钢铁（集团）有限责任公司 | Continuous casting round tube blank for rare earth-containing railway vehicle hub and preparation method thereof |
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Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2009079280A (en) * | 2007-09-27 | 2009-04-16 | Nisshin Steel Co Ltd | Quenched and tempered steel pipe with high fatigue life, and its manufacturing method |
| JP2009249650A (en) * | 2008-04-01 | 2009-10-29 | Nisshin Steel Co Ltd | High fatigue service life quenching/tempering steel tube and manufacturing method therefor |
| CN114032470A (en) * | 2022-01-07 | 2022-02-11 | 北京科技大学 | Carburizing bearing steel and preparation method thereof |
| CN114622136A (en) * | 2022-02-25 | 2022-06-14 | 包头钢铁（集团）有限责任公司 | Continuous casting round tube blank for rare earth-containing railway vehicle hub and preparation method thereof |
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