JP5126857B2 - Manufacturing method of case-hardened steel pipe with excellent workability - Google Patents

Manufacturing method of case-hardened steel pipe with excellent workability Download PDF

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JP5126857B2
JP5126857B2 JP2009509185A JP2009509185A JP5126857B2 JP 5126857 B2 JP5126857 B2 JP 5126857B2 JP 2009509185 A JP2009509185 A JP 2009509185A JP 2009509185 A JP2009509185 A JP 2009509185A JP 5126857 B2 JP5126857 B2 JP 5126857B2
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steel pipe
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annealing
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研一 別府
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新日鐵住金株式会社
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Description

本発明は、浸炭焼入れ後の破壊荷重の高い高強度高靱性の肌焼鋼管(肌焼鋼製の鋼管)とその製造方法に関する。本発明は特に、加工性に優れた肌焼鋼管とその製造方法に関する。   The present invention relates to a case-hardened steel pipe having a high fracture load after carburizing and quenching (steel pipe made of case-hardened steel) and a method for producing the same. The present invention particularly relates to a case-hardened steel pipe excellent in workability and a manufacturing method thereof.
従来から、自動車用や産業機械用などの各種機械構造部品、特にシャフト、CVJ(等速ジョイント)、CVT(無断変速機連続可変トランスミッション)、および歯車を代表とする表面硬化部品の製造には肌焼鋼が使用されている。素材の肌焼鋼を、熱間鍛造や冷間鍛造、更には機械加工により所望の部品形状に成形加工する。作製された部品は、その耐摩耗性および疲労強度を向上させる目的で、表面に浸炭処理や浸炭窒化処理などの表面硬化処理を施してから使用に供される。   Conventionally, it has been difficult to manufacture various machine structural parts for automobiles and industrial machines, especially surface hardened parts such as shafts, CVJ (constant velocity joint), CVT (continuously variable transmission continuously variable transmission), and gears. Hardened steel is used. The raw case-hardened steel is formed into a desired part shape by hot forging, cold forging, or machining. For the purpose of improving the wear resistance and fatigue strength, the manufactured part is subjected to surface hardening treatment such as carburizing treatment or carbonitriding treatment before use.
こうした機械構造部品に要求される特性はますます高度化しつつある。すなわち、従来からの要求特性である浸炭焼入れ後の高い表面硬度と曲げ疲労強度に加えて、より優れた耐摩耗性や転動疲労特性、更には衝撃的な荷重負荷に対する一段と高い耐衝撃破壊特性および靱性を具備していることが望まれるようになっている。   The properties required for such mechanical structural parts are becoming increasingly sophisticated. In other words, in addition to the conventionally required characteristics of high surface hardness and bending fatigue strength after carburizing and quenching, more excellent wear resistance and rolling fatigue characteristics, and even higher impact fracture resistance characteristics against shock load And it has become desirable to have toughness.
肌焼鋼は、浸炭焼入れ処理時の結晶粒の異常成長による衝撃破壊強度の低下、疲労特性の低下、寸法精度の低下等が問題となる場合がある。特に、浸炭処理の合理化の観点から、浸炭時間を短縮するために990〜1090℃の温度域でいわゆる高温浸炭を行うと、粗大粒が発生して、必要な疲労特性、転動疲労特性等が得られないという問題が生じる。   Case-hardened steel may have problems such as a decrease in impact fracture strength due to abnormal growth of crystal grains during carburizing and quenching, a decrease in fatigue characteristics, a decrease in dimensional accuracy, and the like. In particular, from the viewpoint of rationalizing the carburizing treatment, when so-called high-temperature carburizing is performed in a temperature range of 990 to 1090 ° C. in order to shorten the carburizing time, coarse grains are generated, and necessary fatigue characteristics, rolling fatigue characteristics, etc. The problem that it cannot be obtained arises.
特開2005−240175公報(特許文献1)には、鋼成分やTi系の析出物を制御することによって、肌焼鋼の高温浸炭における粗大粒の発生を抑制することが提案されている。   Japanese Patent Application Laid-Open No. 2005-240175 (Patent Document 1) proposes to suppress the generation of coarse grains in high-temperature carburizing of case-hardened steel by controlling steel components and Ti-based precipitates.
高温浸炭によらずに、比較的温和な浸炭焼入れ条件によって、浸炭層に高い強度と耐摩耗性が確保できる鋼材も検討されている。特開平9−53150号公報(特許文献2)には、浸炭層表面に切欠きが存在する場合にも十分に高い耐衝撃破壊荷重を示す高強度高靱性の肌焼鋼と、それを用いて、加工性に優れると共に浸炭焼入れ後に優れた耐衝撃破壊強度を示す高強度高靱性の肌焼鋼管を安定して製造する方法が開示されている。   Steel materials that can ensure high strength and wear resistance in the carburized layer under the relatively mild carburizing and quenching conditions without using high-temperature carburizing have been studied. Japanese Patent Application Laid-Open No. 9-53150 (Patent Document 2) discloses a high-strength, high-toughness case-hardened steel exhibiting a sufficiently high impact fracture load even when a notch is present on the surface of the carburized layer, and using the same Also disclosed is a method for stably producing a high-strength, high-toughness case-hardened steel pipe that has excellent workability and exhibits excellent impact fracture strength after carburizing and quenching.
特許文献2によれば、従来技術の問題の原因の一つが不完全焼き入れ組織の生成にあり、この不完全焼入れ組織の生成の最大の原因が、浸炭材を焼入れする時に起こるオーステナイト粒界上での炭化物の析出にある。そこで、上記の炭化物析出を防止するためにBを添加し、Bの効果が十分に発現されるように、Nを極力抑制する成分設計が採用されている。   According to Patent Document 2, one of the causes of the problems of the prior art is the generation of an incompletely quenched structure, and the greatest cause of the generation of the incompletely quenched structure is on the austenite grain boundary that occurs when the carburized material is quenched. In the precipitation of carbides. Therefore, a component design is adopted in which B is added to prevent the above carbide precipitation and N is suppressed as much as possible so that the effect of B is sufficiently expressed.
しかし、特許文献2に開示されている高強度高靱性の肌焼鋼管は、特に肌焼鋼の継目無鋼管として優れた特性を有するが、比較的硬度が高いため、需要家段階における鍛造加工等の加工性に問題が生じることがある。   However, the high-strength, high-toughness case-hardened steel pipe disclosed in Patent Document 2 has excellent characteristics, particularly as a seamless steel pipe for case-hardened steel, but because of its relatively high hardness, forging at the customer stage, etc. There may be a problem with the workability.
なお、特許文献2には、肌焼鋼管の製造方法に関して、(i)熱間製管で得られた鋼管に冷間加工を経て応力除去焼鈍を行う方法[実施例3]と、(ii)熱間製管で得られた鋼管に1次焼鈍を施してから、冷間加工を経て応力除去焼鈍(2次焼鈍)を行う方法[実施例4および5]とが記載されている。
特開2005−240175公報 特開平9−53150号公報
In addition, in patent document 2, regarding the manufacturing method of the case-hardened steel pipe, (i) a method of performing stress relief annealing on the steel pipe obtained by hot pipe making through cold working [Example 3], and (ii) It describes a method [Examples 4 and 5] in which a steel pipe obtained by hot pipe making is subjected to primary annealing and then subjected to stress processing annealing (secondary annealing) through cold working.
JP-A-2005-240175 JP-A-9-53150
本発明は、加工性に優れ、具体的には、硬度がHRB(ロックウエルBスケール硬度)で72〜80の肌焼鋼管であって、成形加工後に、比較的温和な条件での浸炭焼入れを施して最終製品にしたときに、浸炭層が高い強度および耐摩耗性と十分に優れた耐衝撃破壊特性とを示す肌焼鋼管とその製造方法を提供する。   The present invention is excellent in workability, specifically, is a case-hardened steel pipe having a hardness of 72 to 80 in HRB (Rockwell B scale hardness), and is carburized and quenched under relatively mild conditions after forming. The present invention provides a case-hardened steel pipe and a method for producing the case-hardened steel pipe, in which the carburized layer exhibits high strength and wear resistance and sufficiently excellent impact fracture resistance when it is made into a final product.
本発明は、以下の知見に基づく。
(1)特許文献2に記載された熱間製管後の熱処理方法では、冷間加工後に焼鈍を施しても、硬度はHRBで85以上となることが多く、硬質で客先での成形加工は容易ではない。
The present invention is based on the following findings.
(1) In the heat treatment method after hot pipe making described in Patent Document 2, even if annealing is performed after cold working, the hardness is often 85 or more in HRB, and it is hard and is molded by the customer. Is not easy.
(2)特に特許文献2に記載されている、上記(ii)の冷間加工前に1次焼鈍を行う方法では、この時の焼鈍温度が700℃程度であると、冷間加工後に特許文献2に記載の条件で2次焼鈍を施しても、鋼を軟化させることは困難である。この場合、2次焼鈍温度が730℃程度ならばベイナイト組織となる。   (2) In the method of performing the primary annealing before the cold working of (ii) described in Patent Document 2 in particular, if the annealing temperature at this time is about 700 ° C., the Patent Document after the cold working Even if the secondary annealing is performed under the conditions described in 2, it is difficult to soften the steel. In this case, if the secondary annealing temperature is about 730 ° C., a bainite structure is obtained.
(3)上記(2)で、冷間加工後の2次焼鈍において熱処理温度を930℃程度まで高めて徐冷すると、HRB=75前後に鋼を軟化できる。しかし、この温度条件では、Ac点以上の領域での相変態によって冷間加工の影響が解消されてしまうため、冷間加工度と2次焼鈍での熱処理条件とを組み合わせて硬度をHRB=72〜80の範囲で自由に制御することができなくなる。また、冷間加工後の高温加熱のため、寸法精度が低下し、表面脱炭が生じることがある。鋼組織はフェライト+パーライト組織となり、粗粒化しやすい。(3) In the above (2), when the heat treatment temperature is raised to about 930 ° C. in the secondary annealing after the cold working and the steel is gradually cooled, the steel can be softened around HRB = 75. However, under this temperature condition, the effect of cold working is eliminated by the phase transformation in the region of Ac 3 or more points. Therefore, the hardness is set to HRB = by combining the cold working degree and the heat treatment condition in the secondary annealing. It becomes impossible to control freely in the range of 72-80. In addition, due to high temperature heating after cold working, dimensional accuracy may be reduced and surface decarburization may occur. The steel structure becomes a ferrite + pearlite structure and is easy to coarsen.
(4)特許文献2は、冷間加工前の1次焼鈍の温度条件として870℃を具体的に示している。この熱処理条件では、Ac点より高い温度に一旦加熱されることから、1次焼鈍後の組織はフェライト+パーライト組織となる。しかし、1次焼鈍温度が低いため、冷間加工後の2次焼鈍で十分に長い時間をかけて徐冷を行わないと、鋼の軟化を期待できない。(4) Patent Document 2 specifically shows 870 ° C. as a temperature condition of primary annealing before cold working. Under this heat treatment condition, since the material is once heated to a temperature higher than the Ac 3 point, the structure after the primary annealing becomes a ferrite + pearlite structure. However, since the primary annealing temperature is low, the softening of the steel cannot be expected unless the annealing is sufficiently long in the secondary annealing after the cold working.
(5)特許文献2が開示する方法とは異なり、冷間加工前の熱処理として880℃以上の温度での焼準(冷却速度は70℃/分以下)を行い、好ましくは断面積減少率20〜50%の冷間加工後に、700〜820℃で焼鈍を行うと、パーライトの一部が球状化する。以下、本明細書においては、特段の断わりがない限り、「パーライトの球状化」とは、パーライト中のセメンタイトの球状化を意味する。それにより、所望の硬度低減が達成できると同時に、焼準後の冷却速度と2次焼鈍の温度とを調整することによって、好ましくは、さらに冷間加工における断面積減少率の調整も加えることによって、硬度の調整が可能となる。   (5) Unlike the method disclosed in Patent Document 2, normalization is performed at a temperature of 880 ° C. or more (cooling rate is 70 ° C./min or less) as the heat treatment before cold working, and preferably the cross-sectional area reduction rate is 20 When annealing is performed at 700 to 820 ° C. after ˜50% cold working, part of the pearlite is spheroidized. Hereinafter, unless otherwise specified, “perlite spheroidization” means spheroidization of cementite in pearlite. Thereby, the desired hardness reduction can be achieved, and at the same time, by adjusting the cooling rate after normalization and the temperature of the secondary annealing, preferably by further adjusting the reduction rate of the cross-sectional area in cold working The hardness can be adjusted.
すなわち、本発明では、浸炭焼入れを可能にする鋼組成を有する、熱間製管により作製された鋼管に、まず焼準(焼ならし、normalizing)を施してから、冷間加工を行い、その後に応力除去焼鈍を行う。この焼鈍により、焼準で形成されたフェライト+パーライト組織における少なくとも一部のパーライトの球状化(すなわち、パーライト中のセメンタイトの球状化)が起こり、鋼が軟質化して、加工性に優れた肌焼鋼管が製造される。   That is, in the present invention, a steel pipe having a steel composition that enables carburizing and quenching is manufactured by first normalizing the steel pipe manufactured by hot pipe making, and then performing cold working. Stress relief annealing is performed. This annealing causes spheroidization of at least a part of pearlite in the ferrite + pearlite structure formed by normalization (that is, spheroidization of cementite in pearlite), softening the steel, and skin hardening with excellent workability. Steel pipe is manufactured.
また、本発明では、焼準で生じたフェライト+パーライト組織に対して、後工程の冷間加工での加工度とその後の焼鈍での熱処理条件を調整することによって、焼鈍中に球状化するパーライトの割合を変化させることができ、こうして鋼材硬度の微調整が可能になる。   Further, in the present invention, pearlite that is spheroidized during annealing by adjusting the degree of processing in the cold working of the subsequent process and the heat treatment conditions in the subsequent annealing for the ferrite + pearlite structure generated in the normalization. Thus, the steel material hardness can be finely adjusted.
本発明は、1面において、質量%で、C:0.1〜0.25%、Si:0.2〜0.4%、Mn:0.3〜0.9%、P:0.02%以下、S:0.001〜0.15%、Cr:0.5〜0.9%、Mo:0.15〜1%、Al:0.01〜0.1%、B:0.0005〜0.009%、N:0.006%未満を含み、残部が本質的にFeから成る鋼組成を有する鋼から管を作製し、得られた鋼管に対して、880〜980℃の温度に保持した後に880〜400℃の温度範囲を70℃/分以下の冷却速度で冷却することによって焼準を施し、焼準された鋼管に冷間加工を行い、冷間加工された鋼管に700〜820℃の温度で焼鈍を行うことを特徴とする、肌焼鋼管の製造方法である。   In one surface, the present invention is in mass%, C: 0.1-0.25%, Si: 0.2-0.4%, Mn: 0.3-0.9%, P: 0.02 %: S: 0.001-0.15%, Cr: 0.5-0.9%, Mo: 0.15-1%, Al: 0.01-0.1%, B: 0.0005 -0.009%, N: less than 0.006%, the tube is made from steel having a steel composition consisting essentially of Fe, the temperature of the resulting steel tube being 880-980 ° C After holding, normalization is performed by cooling the temperature range of 880 to 400 ° C. at a cooling rate of 70 ° C./min or less, and cold-working is performed on the normalized steel pipe, and 700 to 700 to the cold-worked steel pipe It is a manufacturing method of a case hardening steel pipe characterized by performing annealing at the temperature of 820 ° C.
別の面からは、本発明は、質量%で、C:0.1〜0.25%、Si:0.2〜0.4%、Mn:0.3〜0.9%、P:0.02%以下、S:0.001〜0.15%、Cr:0.5〜0.9%、Mo:0.15〜1%、Al:0.01〜0.1%、B:0.0005〜0.009%、N:0.006%未満を含み、残部が本質的にFeから成る鋼組成を有し、かつ鋼組織がフェライト+パーライト+球状化セメンタイトの混合組織またはフェライト+球状化セメンタイトの混合組織であることを特徴とする、冷間仕上げ肌焼鋼管である。   From another aspect, the present invention provides, in mass%, C: 0.1 to 0.25%, Si: 0.2 to 0.4%, Mn: 0.3 to 0.9%, P: 0 0.02% or less, S: 0.001 to 0.15%, Cr: 0.5 to 0.9%, Mo: 0.15 to 1%, Al: 0.01 to 0.1%, B: 0 .0005 to 0.0009%, N: less than 0.006%, the balance is essentially composed of Fe, and the steel structure is a mixed structure of ferrite + pearlite + spheroidized cementite or ferrite + spherical A cold-finished case-hardened steel pipe characterized by having a mixed structure of cementitious cementite.
前記鋼組成は、下記(1)および(2)から選ばれた1種または2種以上の元素をさらに含有することができる:
(1)質量%で、Ni:0.3〜4.0%
(2)質量%で、Ti:0.01〜0.3%、Nb:0.01〜0.3%、V:0.01〜0.3%、Zr:0.01〜0.3%から選ばれた1種または2種以上。
The steel composition may further contain one or more elements selected from the following (1) and (2):
(1) By mass%, Ni: 0.3-4.0%
(2) By mass%, Ti: 0.01-0.3%, Nb: 0.01-0.3%, V: 0.01-0.3%, Zr: 0.01-0.3% 1 type or 2 types or more selected from
前記鋼組成において、B含有量は、B:0.0005〜0.003%であることが好ましい。
「肌焼鋼」および「肌焼鋼管」とは、製品(例、上述した機械構造部品)の所定形状に加工してから、最終的に浸炭焼入れを施して、表面層(浸炭層)を高硬度化してから用いられる鋼および鋼管である。言うまでもないが、前述した硬度は、肌焼鋼の硬度、すなわち、部品形状への成型加工前の(当然に、浸炭焼入れ前の)硬度である。所定の部品形状への成形加工と浸炭焼入れは、通常は客先で(需要家側で)行われる。
In the steel composition, the B content is preferably B: 0.0005 to 0.003%.
“Skin-hardened steel” and “Skin-hardened steel pipe” are processed into a predetermined shape of a product (for example, the above-mentioned machine structural parts) and finally carburized and hardened to increase the surface layer (carburized layer). Steel and steel pipe used after being hardened. Needless to say, the above-described hardness is the hardness of the case-hardened steel, that is, the hardness before molding into a part shape (naturally before carburizing and quenching). Molding to a predetermined part shape and carburizing and quenching are usually performed at the customer (on the customer side).
「残部が本質的にFeからなる」とは、残部が不可避的不純物を含有しうることを意味する。   “The balance consists essentially of Fe” means that the balance can contain inevitable impurities.
実施例で得られた本発明に係る肌焼鋼管の顕微鏡組織写真である。It is a microscope picture of the case hardening steel pipe concerning the present invention obtained in the example.
本発明において肌焼鋼管の鋼組成を前記の如くに限定した理由を、その作用と共に説明する。本明細書において、鋼組成を表す「%」は「質量%」である。
C:
Cは、鋼の硬度・強度を確保する基本的成分である。浸炭焼入れ部品の使用中に変形しないだけの強度を鋼に確保するにはHv250以上の硬度を必要とする。この必要硬度の確保のためにC含有量を0.1%以上とする。一方、0.25%を超えてCを含有させると、鋼の芯部靱性が劣化する。従って、C含有量は0.1〜0.25%であり、好ましくは0.12〜0.20%である。
The reason why the steel composition of the case-hardened steel pipe is limited as described above in the present invention will be described together with its action. In this specification, “%” representing the steel composition is “mass%”.
C:
C is a basic component that ensures the hardness and strength of steel. Hardness of Hv250 or higher is required to secure the steel with a strength that does not cause deformation during use of the carburized and quenched parts. In order to ensure the required hardness, the C content is set to 0.1% or more. On the other hand, when C is contained exceeding 0.25%, the core toughness of steel deteriorates. Therefore, the C content is 0.1 to 0.25%, preferably 0.12 to 0.20%.
Si:
浸炭層の焼入れによって高い耐衝撃破壊特性を実現するために、Siの焼入性向上効果を積極的に利用する。Si含有量が0.2%未満であると、所望する高い浸炭層の焼入性を確保できない。一方、0.4%を超えてSiを含有させると、浸炭時の粒界近傍でのSiの酸化による粒界の脆弱化が顕著となる。そこで、Si含有量を0.2〜0.4%とする。
Si:
In order to realize high impact fracture resistance by quenching the carburized layer, the effect of improving the hardenability of Si is actively used. If the Si content is less than 0.2%, the desired high carburized layer hardenability cannot be ensured. On the other hand, when Si is contained exceeding 0.4%, the grain boundary becomes brittle due to oxidation of Si in the vicinity of the grain boundary during carburizing. Therefore, the Si content is set to 0.2 to 0.4%.
Mn:
Mnも、浸炭層の焼入性を高め、高い耐衝撃破壊特性を実現するために添加する。Mn含有量を0.3%未満に低減すると、浸炭層の焼入性が低下して、所望する高い耐衝撃破壊特性を確保することができない。浸炭時の粒界近傍でのMnの酸化による粒界の脆弱化は、Mn含有量が0.9%を超えても、実用上問題がないことが判明した。しかし、0.9%を超えてMnを含有させると、打抜き加工性と砥石研削性の劣化が顕著となる。打ち抜き加工性や砥石研削性といった特性は、CVJ等の効率的な加工には特に重要である。従って、Mn含有量を0.3〜0.9%とする。
Mn:
Mn is also added to improve the hardenability of the carburized layer and realize high impact fracture resistance. If the Mn content is reduced to less than 0.3%, the hardenability of the carburized layer is lowered, and the desired high impact fracture resistance cannot be ensured. It has been found that the weakening of the grain boundary due to oxidation of Mn in the vicinity of the grain boundary during carburizing has no practical problem even when the Mn content exceeds 0.9%. However, when Mn is contained exceeding 0.9%, the deterioration of the punching workability and the grindstone grindability becomes remarkable. Properties such as punchability and grindability are particularly important for efficient machining such as CVJ. Therefore, the Mn content is set to 0.3 to 0.9%.
P:
Pは、浸炭焼入れ時にオーステナイト粒界上にセメンタイトが析出することによる粒界の脆弱化を著しく促進するので、肌焼鋼においては極めて有害な不純物元素である。従って、P含有量は極力低減することが好ましい。ただ、Pの低減は原料や精錬工程でのコスト増大を伴うので、目標性能とコストとのバランスから許容値が設計される。本発明では、後述するBの効果を勘案して、許容できるP含有量の上限値を、0.02%とする。
P:
P is an extremely harmful impurity element in case-hardened steel because it significantly promotes the weakening of grain boundaries due to precipitation of cementite on austenite grain boundaries during carburizing and quenching. Therefore, it is preferable to reduce the P content as much as possible. However, since the reduction of P is accompanied by an increase in costs in the raw materials and the refining process, an allowable value is designed based on a balance between target performance and cost. In the present invention, the upper limit of the allowable P content is set to 0.02% in consideration of the effect of B described later.
S:
Sは、鋼の靱性劣化を招く一方で、機械加工性(被削性、打抜き性)を改善するという点では積極添加が望まれる成分でもある。S含有量が0.001%未満であると機械加工性改善効果が顕著とならず、0.15%を超えてSを含有させると、鋼の靱性劣化が著しくなる。そこで、S含有量を0.001〜0.15%とする。機械加工性をあまり要求されなくて済むような使われ方の場合には、S含有量は低めに抑えることが得策である。
S:
S causes a deterioration in the toughness of the steel, but is also a component that should be positively added in terms of improving the machinability (machinability and punchability). When the S content is less than 0.001%, the machinability improving effect is not remarkable, and when the S content exceeds 0.15%, the toughness of the steel is significantly deteriorated. Therefore, the S content is set to 0.001 to 0.15%. In the case of usage that requires less machinability, it is a good idea to keep the S content low.
Cr:
Crは、鋼素地(表面の浸炭層を除いた部分の鋼)の焼入性確保や、浸炭層に必要な炭素濃度を短時間で達成するために欠かせない成分であり、そのためには0.5%以上のCr含有量が必要である。しかし、Crは同時に、浸炭焼入れ時にオーステナイト粒界上にセメンタイトが析出することによる粒界の脆弱化を著しく促進するので、その含有量を0.9%以下に制限する。ただし、Cr含有量を0.9%以下に制限すると、鋼の焼入性、とりわけC量の高い浸炭層分の焼入性が不十分となる。そのため、本発明では、粒界の脆弱化を招くことのないB、Mo、Niの添加によって、焼入性を補う。このように、Cr含有量は0.5〜0.9%とするが、好ましくは0.5〜0.65%に調整する。
Cr:
Cr is an indispensable component for ensuring the hardenability of the steel substrate (the portion of the steel excluding the carburized layer on the surface) and achieving the carbon concentration necessary for the carburized layer in a short time. A Cr content of 0.5% or more is required. However, at the same time, Cr significantly promotes the weakening of the grain boundary due to the precipitation of cementite on the austenite grain boundary during carburizing and quenching, so the content is limited to 0.9% or less. However, if the Cr content is limited to 0.9% or less, the hardenability of the steel, particularly the hardenability of the carburized layer with a high C content, becomes insufficient. Therefore, in this invention, hardenability is supplemented by addition of B, Mo, and Ni which does not cause the grain boundary to become brittle. Thus, the Cr content is set to 0.5 to 0.9%, but is preferably adjusted to 0.5 to 0.65%.
Mo:
Moは、鋼素地および浸炭層の強度および靱性の向上、並びに浸炭層に必要な炭素濃度を短時間で達成するために必須の成分である。Moの焼入性向上効果は鋼素地のC含有量に殆ど影響を受けないため、高炭素になった浸炭層においても焼入性向上効果は安定して発揮される。
Mo:
Mo is an essential component for improving the strength and toughness of the steel substrate and the carburized layer and achieving the carbon concentration required for the carburized layer in a short time. Since the effect of improving the hardenability of Mo is hardly affected by the C content of the steel substrate, the effect of improving the hardenability is stably exhibited even in a carburized layer having a high carbon content.
上記のように、本発明では、浸炭に伴う粒界の脆弱化を抑制するためにCr含有量を低減し、Bの添加により焼入性の補充を図る。そのような鋼では、高炭素になっても焼入性が著しく低下するので、Moによる浸炭層の焼入性補償は非常に重要である。Mo含有量が0.15%未満では、十分な焼入性補償ができないだけでなく、短時間の浸炭処理で浸入するCの量も低下する。上記効果を付与する観点からはMo含有量は多い方が好ましいが、1%までの添加で十分な効果が得られ、これを超える量のMoの添加は経済的に得策ではない。従って、Mo含有量は0.15〜1%とし、好ましくは0.2〜0.7%、より好ましくは0.2〜0.6%である。   As described above, in the present invention, the Cr content is reduced in order to suppress the weakening of grain boundaries accompanying carburization, and the hardenability is replenished by adding B. In such steel, the hardenability is remarkably lowered even when the carbon content becomes high. Therefore, the hardenability compensation of the carburized layer by Mo is very important. If the Mo content is less than 0.15%, not only sufficient hardenability compensation can be performed, but also the amount of C that is infiltrated by a short time carburizing treatment is reduced. From the viewpoint of imparting the above effects, it is preferable that the Mo content is large, but a sufficient effect can be obtained by adding up to 1%, and addition of Mo in an amount exceeding this is not economically advantageous. Therefore, the Mo content is 0.15 to 1%, preferably 0.2 to 0.7%, more preferably 0.2 to 0.6%.
Al:
Alは、鋼の脱酸および結晶粒微細化に有効な成分である。その含有量が0.01%未満では効果が十分でない。一方、0.1%を超えてAlを含有させると、靱性に有害な介在物が増加する。従って、Al含有量は0.01〜0.1%とする。
Al:
Al is an effective component for deoxidation and grain refinement of steel. If the content is less than 0.01%, the effect is not sufficient. On the other hand, when Al is contained exceeding 0.1%, inclusions harmful to toughness increase. Therefore, the Al content is set to 0.01 to 0.1%.
B:
Bは、浸炭材を焼入れする時に生成するオーステナイト粒界上での炭化物(Cr炭化物等)の析出を抑え、これにより浸炭層の不完全焼入れ組織の生成を阻止し、かつ粒界脆化を防止して、浸炭焼入れ材に十分な耐衝撃破壊特性、耐摩耗性、転動疲労特性等を確保するために欠かせない成分である。特に、本発明では浸炭焼入れ時に粒界上に炭化物が析出することにより粒界の脆弱化を著しく促進するというCrの弊害を防止するために、Cr含有量を制限している。Bは、このようにCr含有量を低減した結果として生ずる鋼素地の焼入性低下を補って、鋼芯部の焼入性を確保する作用も分担する。
B:
B suppresses the precipitation of carbides (Cr carbides, etc.) on the austenite grain boundaries generated when quenching the carburized material, thereby preventing the formation of incompletely quenched structures in the carburized layer and preventing grain boundary embrittlement. Thus, it is an indispensable component for securing sufficient impact fracture resistance, wear resistance, rolling fatigue characteristics and the like for the carburized and quenched material. In particular, in the present invention, the Cr content is limited in order to prevent the adverse effect of Cr, which significantly promotes the weakening of the grain boundaries by precipitation of carbides on the grain boundaries during carburizing and quenching. B also contributes to the effect of securing the hardenability of the steel core part by compensating for the hardenability deterioration of the steel base resulting from the reduction of the Cr content.
B含有量が0.0005%未満であると、上記作用による所望の効果が得られない。一方、0.009%を超えてBを含有させると、逆にBによる粒界脆化が起きるようになる。そこで、B含有量は0.0005〜0.009%とする。   If the B content is less than 0.0005%, the desired effect due to the above action cannot be obtained. On the other hand, if B exceeds 0.0009%, grain boundary embrittlement occurs due to B. Therefore, the B content is set to 0.0005 to 0.009%.
本発明では、後述するように、冷間加工前にAc点以上、具体的には880℃以上の温度で熱処理(焼準)を行う。この熱処理は、冷間加工後の焼鈍により硬度を低下させるという目的を達成するために、一旦、Bを固溶させることを想定している。Bが多いと、Bの固溶、従って、焼準での熱処理に時間を要するので、B含有量は前記範囲の中では低めであることが望ましい。具体的には、B含有量は、0.003%以下(すなわち、0.0005〜0.003%の範囲内)が特に望ましい。In the present invention, as will be described later, heat treatment (normalization) is performed at a temperature of Ac 3 points or higher, specifically, 880 ° C. or higher before cold working. This heat treatment assumes that B is once dissolved in order to achieve the purpose of reducing the hardness by annealing after cold working. When the amount of B is large, it takes a long time for the solid solution of B, and therefore the heat treatment in normalization, so the B content is desirably low in the above range. Specifically, the B content is particularly preferably 0.003% or less (that is, within a range of 0.0005 to 0.003%).
N:
特許文献2にも述べられているように、鋼中のN量は、Bの作用を有効にするのに非常に重要である。すなわち、鋼中のN量が0.006%未満の領域にまで低減された場合に初めて、B添加による浸炭材の焼入れ処理時に起きる粒界への炭化物析出を防止する効果が顕著化し、十分な衝撃荷重強度が確保されるばかりか、転動疲労特性も著しく改善される。鋼中のN含有量は少ないほど望ましいが、大気中での工業的生産においては、現在の製鋼技術でN量を0.001%未満にすることは極めて困難である。
N:
As described in Patent Document 2, the amount of N in steel is very important for making the action of B effective. That is, only when the N content in the steel is reduced to a region of less than 0.006%, the effect of preventing carbide precipitation at the grain boundaries that occurs during the quenching treatment of the carburized material due to the addition of B becomes significant and sufficient. Not only the impact load strength is ensured, but also the rolling fatigue characteristics are remarkably improved. The smaller the N content in steel, the better. However, in industrial production in the atmosphere, it is extremely difficult to reduce the N content to less than 0.001% with the current steelmaking technology.
Ni:
本発明の肌焼鋼管では、一般的な自動車の駆動車軸用継手のインナーレースやボールケージ等に使用される場合には、Niや、次に説明するTi、Nb、VまたはZrの添加を行わなくても、強度、靱性等の特性は十分である。しかし、更に条件が苛酷な用途に使用する場合には、これら元素の1種または2種以上を含有させて、強度や靱性の向上を図ることが有効である。
Ni:
In the case-hardened steel pipe of the present invention, when used for an inner race or a ball cage of a general drive axle joint of an automobile, Ni, Ti, Nb, V, or Zr described below is added. Even if not, properties such as strength and toughness are sufficient. However, when used in applications where the conditions are more severe, it is effective to improve the strength and toughness by containing one or more of these elements.
Niは、鋼素地の強度と靱性の向上に有効な成分であり、またMoと協働して浸炭層の強度・靱性の向上にも大いに寄与する。Ni含有量が0.3%未満では、前記効果が不十分である。一方、4.0%を超えてNiを含有させてもその効果が飽和する。従って、Niを添加する場合には、その含有量を0.3〜4.0%とする。   Ni is an effective component for improving the strength and toughness of the steel substrate, and contributes greatly to improving the strength and toughness of the carburized layer in cooperation with Mo. If the Ni content is less than 0.3%, the effect is insufficient. On the other hand, the effect is saturated even if it contains Ni exceeding 4.0%. Therefore, when adding Ni, the content is made 0.3 to 4.0%.
Ti、Nb、VおよびZr:
これらの元素には、鋼の結晶粒を微細化して靱性を向上させる効果がある。従って、苛酷な使用条件が予想される場合には、これらの1種または2種以上を含有させるのが好ましい。これら各成分の含有量がそれぞれ0.01%未満では上記効果が不十分である。一方、それぞれ0.3%を超えて含有させると、逆に鋼の靱性や転動疲労特性の劣化を招く。従って、Ti、Nb、VおよびZrの含有量はそれぞれ0.01〜0.3%とする。
Ti, Nb, V and Zr:
These elements have the effect of improving the toughness by refining the crystal grains of steel. Therefore, when severe use conditions are expected, it is preferable to contain one or more of these. If the content of each of these components is less than 0.01%, the above effects are insufficient. On the other hand, if each content exceeds 0.3%, the toughness and rolling fatigue characteristics of the steel are deteriorated. Therefore, the contents of Ti, Nb, V and Zr are set to 0.01 to 0.3%, respectively.
次に本発明の肌焼鋼管の製造条件について工程順に説明する。
製管:
上述した鋼組成を有する鋼(肌焼鋼)から、適当な製管法により、素管となる管を作製する。素管は、熱間製管された継目無鋼管であるのが好ましい。しかし、以下で述べるように、焼準時に一旦Ac以上の温度で熱処理するため、前工程での加工履歴が影響しなくなる。従って、製管法は特に制限されず、例えば、電縫鋼管を素管として使用することも可能である。継目無鋼管の熱間製管についても、特段の制約はないが、上記鋼組成を有する鋼を、例えば鋼塊を熱間鍛造することでビレットの形態にしてから、ビレット→マンネスマン穿孔圧延→マンドレルミルでの延伸圧延→定径圧延によって、継目無鋼管とすればよい。
Next, the manufacturing conditions of the case-hardened steel pipe of the present invention will be described in the order of steps.
Pipe making:
From the steel having the above-described steel composition (skin-hardened steel), a pipe to be a raw pipe is produced by an appropriate pipe making method. The base pipe is preferably a seamless steel pipe that has been hot-made. However, as will be described below, since the heat treatment is performed at a temperature of Ac 3 or higher once during normalization, the processing history in the previous process is not affected. Accordingly, the pipe making method is not particularly limited, and for example, an electric resistance welded steel pipe can be used as a raw pipe. There are no particular restrictions on the seamless pipe making of seamless steel pipes, but the steel having the above steel composition is made into a billet form by hot forging a steel ingot, for example, and then billet → Mannesmann piercing rolling → mandrel What is necessary is just to make a seamless steel pipe by drawing rolling in a mill → constant diameter rolling.
焼準:
上述の方法等より製造された鋼管(素管)に対して、冷間加工前に、特許文献1、2に開示されているような1次焼鈍ではなく、焼準を施す。焼準は、例えば、鋼管を適宜加熱炉に装入して所定温度に保持する熱処理(均熱)と、その後の冷却により行われる。この焼準の目的は、鋼の組織をフェライト+パーライトの混合組織とすることである。鋼組織を一旦フェライト+パーライトとしてから、冷間加工後に特定温度範囲で焼鈍を行うことにより、肌焼鋼管に望ましい特性が発現される。
Normalization:
Prior to cold working, the steel pipe (element pipe) manufactured by the above-described method is subjected to normalization rather than primary annealing as disclosed in Patent Documents 1 and 2. The normalization is performed by, for example, heat treatment (soaking) in which a steel pipe is appropriately charged in a heating furnace and maintained at a predetermined temperature, and then cooling. The purpose of this normalization is to make the steel structure a mixed structure of ferrite and pearlite. Once the steel structure is made ferrite + pearlite and then annealed in a specific temperature range after cold working, desirable characteristics are manifested in the case-hardened steel pipe.
焼準の熱処理温度は880℃以上、980℃以下とする。熱処理温度が980℃を超えると、脱炭が進行する可能性がある。880℃の下限温度は、Bを短時間にオーステナイトに固溶させて組織を均一にするために必要な温度である。Bの固溶により、鋼素地の硬度を低下させることができる。焼準時の熱処理温度が880℃より低いと、Bの十分な固溶が達成できず、長時間の温度保持をしても鋼素地の硬度低下にはつながらない。   The heat treatment temperature for normalization is 880 ° C. or higher and 980 ° C. or lower. If the heat treatment temperature exceeds 980 ° C, decarburization may proceed. The lower limit temperature of 880 ° C. is a temperature necessary to make B dissolve in austenite in a short time to make the structure uniform. Due to the solid solution of B, the hardness of the steel substrate can be reduced. If the heat treatment temperature during normalization is lower than 880 ° C., sufficient solid solution of B cannot be achieved, and even if the temperature is maintained for a long time, the hardness of the steel substrate is not reduced.
均熱時間は、鋼管のすべての部分において前記温度になれば、30秒の短時間でもよいが、特性のバラツキを抑制する観点から1分以上であることが望ましい。均熱時間が30分を超えると、脱炭が進行するおそれがあるので、30分以下が望ましい。   The soaking time may be as short as 30 seconds as long as the temperature reaches all the portions of the steel pipe, but it is preferably 1 minute or longer from the viewpoint of suppressing variation in characteristics. If the soaking time exceeds 30 minutes, decarburization may proceed, so 30 minutes or less is desirable.
熱処理(均熱)後の冷却は、空冷でよいが、熱処理温度から400℃までの範囲(従って、少なくとも800〜400℃の範囲)を70℃/分以下とする。冷却速度がこれより大きくなると、ベイナイトが生成するため、本発明の効果が得られない。冷却速度の下限は、焼準を前提としていることから、空冷程度以上であれば特に限定されないが、処理時間等の経済性を考慮すれば20℃/分以上が望ましい。   The cooling after the heat treatment (soaking) may be air cooling, but the range from the heat treatment temperature to 400 ° C. (therefore, the range of at least 800 to 400 ° C.) is 70 ° C./min or less. If the cooling rate is higher than this, bainite is generated, and the effect of the present invention cannot be obtained. Since the lower limit of the cooling rate is premised on normalization, it is not particularly limited as long as it is about air cooling or higher, but it is preferably 20 ° C./min or higher in consideration of economics such as processing time.
冷間加工:
熱間製管で得られた鋼管に、予め焼準を施した後、冷間加工を行う。冷間加工は、鋼管に所定の寸法、寸法精度を確保するために一般には必要であるが、本発明においては、冷間加工に続く焼鈍による2次熱処理段階においてパーライト中のセメンタイトの球状化(従って、パーライトの球状化)を生じさせる効果がある。
Cold working:
A steel pipe obtained by hot pipe making is preliminarily subjected to cold working. In general, cold working is necessary to secure predetermined dimensions and dimensional accuracy in a steel pipe. In the present invention, spheroidization of cementite in pearlite is performed in the secondary heat treatment stage by annealing following cold working ( Accordingly, there is an effect of generating pearlite spheroidization.
冷間加工の手段としては、冷間引抜、冷間圧延等を採用でき、特に制限されるものではない。冷間加工の加工度は、断面積減少率で20〜50%とすることが望ましく、さらに望ましくは25〜50%である。加工度が20%未満では、次工程においてパーライトの一部を球状化することが困難である。加工度が50%を越えると、冷間加工時に工具と材料間での焼付きが発生しやすくなる上、鋼素地の歪の蓄積が増大して、浸炭熱処理時にオーステナイト結晶粒の異常成長を起こし、焼入れ組織の粗大・混粒化を引き起こす。さらに、冷間加工度が50%を超えると、加工硬化による鋼管の硬度上昇が著しくなり、その後に行われる焼鈍での軟化が難しくなって、鋼管の加工性が劣化する。   As a cold working means, cold drawing, cold rolling or the like can be adopted, and there is no particular limitation. The degree of cold working is preferably 20 to 50%, more preferably 25 to 50%, in terms of the cross-sectional area reduction rate. If the degree of processing is less than 20%, it is difficult to spheroidize part of the pearlite in the next step. If the degree of work exceeds 50%, seizure between the tool and the material tends to occur during cold working, and the accumulation of strain in the steel substrate increases, causing abnormal growth of austenite grains during carburizing heat treatment. Causes the hardened structure to become coarse and mixed. Furthermore, when the cold work degree exceeds 50%, the hardness of the steel pipe is remarkably increased by work hardening, and it becomes difficult to soften by annealing performed thereafter, and the workability of the steel pipe deteriorates.
焼鈍:
冷間加工後の焼鈍は、冷間加工により鋼素地に蓄積された歪を開放して鋼素地を軟化させ、客先の要求する加工性を確保するために一般に行われるが、本発明においては、パーライト中のセメンタイトの少なくとも一部を球状化させる目的もある。この目的のために、冷間加工後の焼鈍温度を700〜820℃の範囲とする。焼鈍温度が700℃未満であるか、或いは820℃を超えると、パーライトの球状化が十分に進行しない。
Annealing:
The annealing after the cold working is generally performed in order to release the strain accumulated in the steel base by the cold working to soften the steel base and ensure the workability required by the customer. There is also an object of spheroidizing at least a part of the cementite in the pearlite. For this purpose, the annealing temperature after cold working is set to a range of 700 to 820 ° C. When the annealing temperature is lower than 700 ° C or higher than 820 ° C, pearlite spheroidization does not proceed sufficiently.
焼鈍によりパーライト(パーライト中のセメンタイト)が全て球状化すると、鋼組織はフェライト+球状化セメンタイトの混合組織となる。一方、パーライトの一部が球状化した場合には、鋼組織はフェライト+パーライト+球状化セメンタイト混合組織となる。本発明に係る肌焼鋼管は、この鋼組織と上記鋼組成とにより特徴づけることができる。   When pearlite (cementite in pearlite) is all spheroidized by annealing, the steel structure becomes a mixed structure of ferrite and spheroidized cementite. On the other hand, when a part of pearlite is spheroidized, the steel structure becomes a mixed structure of ferrite + pearlite + spheroidized cementite. The case-hardened steel pipe according to the present invention can be characterized by this steel structure and the above steel composition.
このようにパーライトの少なくとも一部が球状化することにより、鋼管の硬度が低下する。焼鈍による軟化にこの効果が加わることによって、本発明によれば、硬度がHRBで72〜80という加工性の良好な肌焼鋼管を製造することができる。この硬度は、冷間加工時の加工度と焼鈍条件によりパーライトの球状化の割合を変化させることによって、所望の値に調節することができる。   Thus, the hardness of a steel pipe falls because at least one part of pearlite spheroidizes. By adding this effect to the softening by annealing, according to the present invention, it is possible to manufacture a case-hardened steel pipe having a hardness of 72 to 80 and having a hardness of 72 to 80. This hardness can be adjusted to a desired value by changing the ratio of spheroidization of pearlite depending on the degree of work during cold working and the annealing conditions.
前述したように、肌焼鋼では、通常は需要者において、成形加工および浸炭焼入れ処理が行われ、目的とする部品が製造される。本発明に係る肌焼鋼管から部品を製造する場合、成形加工および浸炭焼入れ条件に何ら制限はないが、比較的温和な浸炭焼入れ条件を採用することが可能であるので、そうすることが好ましい。浸炭焼入れ条件の1例を示すと、920℃×2時間の均熱による浸炭と、その後の870℃からの焼入れである。   As described above, in case-hardened steel, usually, a consumer performs a forming process and a carburizing and quenching process to produce a target part. In the case of producing a part from the case-hardened steel pipe according to the present invention, there is no limitation on the forming process and carburizing and quenching conditions, but it is preferable to do so because relatively mild carburizing and quenching conditions can be adopted. An example of the carburizing and quenching conditions is carburizing by soaking at 920 ° C. for 2 hours, followed by quenching from 870 ° C.
以下の実施例は本発明の例示を目的とし、本発明を何ら制限するものではない。本発明の範囲内において当業者であれば各種の変更修正を加えることができる。   The following examples are intended to illustrate the present invention and do not limit the present invention in any way. Various changes and modifications can be made by those skilled in the art within the scope of the present invention.
真空溶製した溶鋼の鋳込みによって、表1に示す鋼組成の鋼塊(1トン)を得た後、これを丸鋼片に熱間鍛造し、更に穿孔圧延後、マンドレルミルによる延伸圧延、ストレッチ・レデュサーによる定径圧延を行う熱間製管により、外径80mm(直径)、肉厚6.1mmの素管(鋼管)を作製した。   A steel ingot (1 ton) with the steel composition shown in Table 1 was obtained by casting molten steel that was vacuum-melted, then hot forged into round steel pieces, further pierced and rolled, stretched and rolled by a mandrel mill, and stretched. -A raw pipe (steel pipe) having an outer diameter of 80 mm (diameter) and a wall thickness of 6.1 mm was produced by hot pipe making with constant diameter rolling by a reducer.
この鋼管に、表2に示す条件で1次熱処理(焼準)とその後の冷却を行った後、断面積減少率28.4%の冷間引抜きを行い、外径66.2mm(直径)、肉厚5.3mmの寸法の継目無鋼管に仕上げた。この鋼管に、表2に2次熱処理として示す条件で焼鈍を施した。2次熱処理(焼鈍)を終えた鋼管から、試験片を採取し、管断面におけるロックウェルBスケール硬度(HRB)を測定した。その結果を表2に合せて示す。   The steel pipe was subjected to primary heat treatment (normalization) and subsequent cooling under the conditions shown in Table 2, followed by cold drawing with a cross-sectional area reduction rate of 28.4%, an outer diameter of 66.2 mm (diameter), A seamless steel pipe having a wall thickness of 5.3 mm was finished. This steel pipe was annealed under the conditions shown in Table 2 as secondary heat treatment. A test piece was collected from the steel pipe after the secondary heat treatment (annealing), and the Rockwell B scale hardness (HRB) in the pipe cross section was measured. The results are shown in Table 2.
表2のNo.1およびNo.2は、冷間加工前の焼準での熱処理温度がAc点より低い700℃であり、HRB=87以上の硬質の仕上がりとなった。一方、冷間加工前の熱処理温度がAc点を超えるNo.3〜No.7においても、熱処理温度が880℃より低い場合には、No.3を除いて、HRB=82以上の硬度であり、HRBを80以下とする軟質化の目的は達成できなかった。熱処理(均熱)後の冷却速度を10℃/分と遅くしたNo.3は、HRBが77で、軟質化の目的を達成できていたが、冷却過程を含めた熱処理時間が長くなり、連続処理を想定した場合には保温設備が長大となるため、経済的でないことは明らかである。In No. 1 and No. 2 in Table 2, the heat treatment temperature in normalization before cold working was 700 ° C., which was lower than Ac 1 point, and a hard finish of HRB = 87 or more was obtained. On the other hand, in No. 3 to No. 7 where the heat treatment temperature before cold working exceeds Ac 3 points, when the heat treatment temperature is lower than 880 ° C., except for No. 3, the hardness is HRB = 82 or more. The purpose of softening the HRB to 80 or less could not be achieved. No. 3 with a cooling rate as low as 10 ° C./min after heat treatment (soaking) had an HRB of 77 and was able to achieve the purpose of softening, but the heat treatment time including the cooling process became longer and continuous. When processing is assumed, it is obvious that the heat insulation equipment is long and is not economical.
No.8〜No.18は、冷間加工前の熱処理を880℃または930℃での均熱により行った例である。均熱後の冷却速度が70℃/分を超えたNo.9および13と冷間加工後の焼鈍温度が低すぎるか高すぎたNo.14および16では、HRBが80を超え、十分に軟質化することができなかった。一方、均熱後の冷却速度が70℃/分以下で、冷間加工後の焼鈍温度が700〜820℃の範囲内であった本発明例では、いずれもHRB80以下の軟質化の目的を達成することができた。   No. 8 to No. 18 are examples in which the heat treatment before cold working was performed by soaking at 880 ° C. or 930 ° C. In No. 9 and 13 in which the cooling rate after soaking exceeded 70 ° C./min and No. 14 and 16 in which the annealing temperature after cold working was too low or too high, the HRB exceeded 80 and was sufficiently soft Could not be converted. On the other hand, in the examples of the present invention in which the cooling rate after soaking was 70 ° C./min or less and the annealing temperature after cold working was in the range of 700 to 820 ° C., all achieved the softening purpose of HRB 80 or less. We were able to.
2次熱処理(焼鈍)を終えた鋼管のミクロ組織を観察すると、No.1およびNo.2はベイナイト組織であり、No.3〜No.7はフェライト+パーライト組織であった。No.3では粗粒化の傾向が認められた。   When the microstructure of the steel pipe after the secondary heat treatment (annealing) was observed, No. 1 and No. 2 were bainite structures, and No. 3 to No. 7 were ferrite + pearlite structures. In No. 3, the tendency of coarsening was recognized.
他方、No.8〜No.18については、HRBが80以下の本発明例では、フェライト+パーライト+球状化セメンタイト組織となっていて、パーライト組織のセメンタイトが一部球状化していることが確認された。しかし、No.8〜No.18のうち、硬度がHRB84を超えたものでは、球状化セメンタイトは認められなかった。冷却速度が80℃/分であったNo.9とNo.13では一部ベイナイトが認められた。   On the other hand, with respect to No. 8 to No. 18, in the present invention example with an HRB of 80 or less, it was confirmed that the ferrite + pearlite + spheroidized cementite structure was formed, and the cementite of the pearlite structure was partially spheroidized. It was. However, spheroidized cementite was not observed in No. 8 to No. 18 in which the hardness exceeded HRB84. In No. 9 and No. 13 where the cooling rate was 80 ° C./min, some bainite was recognized.
したがって、鋼管に予め880℃以上で均熱し、70℃/分以下の冷却速度で冷却することにより焼準を施し、冷間加工後に700〜820℃の温度で焼鈍するプロセスによって、パーライト+フェライトの混合組織から、パーライト+フェライト+球状化セメンタイトの混合組織への変化が進み、軟質化の目的が達成できたものと考えられる。   Therefore, the steel pipe is soaked at 880 ° C. or higher in advance, normalized by cooling at a cooling rate of 70 ° C./min or less, and annealed at a temperature of 700 to 820 ° C. after cold working. The transition from a mixed structure to a mixed structure of pearlite + ferrite + spheroidized cementite progressed, and it is considered that the purpose of softening was achieved.
表2の本発明例の鋼管について、穴あけ試験(ポンチ材質:高速度鋼、ポンチ径15.7mm、穴あけ速度:2.5mm/秒)を実施したところ、穴あけ面の凹凸や寸法精度で特段の問題は無かった。また、ボールケージ模擬試験体による衝撃引張試験においても、満足な結果が得られた。さらに、浸炭焼入れ後の特性も良好であった。   When the drilling test (punch material: high-speed steel, punch diameter 15.7 mm, drilling speed: 2.5 mm / sec) was performed on the steel pipes of the present invention examples in Table 2, the irregularities and dimensional accuracy of the drilling surface were exceptional. There was no problem. Satisfactory results were also obtained in an impact tensile test using a ball cage simulated specimen. Furthermore, the characteristics after carburizing and quenching were also good.
図1は、表2のNo.11で得られた鋼管の顕微鏡組織写真を示す。フェライト+パーライト組織中において炭化物(セメンタイト)が球状化していることが分かる。   1 shows a micrograph of the steel pipe obtained in No. 11 of Table 2. It can be seen that carbide (cementite) is spheroidized in the ferrite + pearlite structure.

Claims (3)

  1. 質量%で、C:0.1〜0.25%、Si:0.2〜0.4%、Mn:0.3〜0.9%、P:0.02%以下、S:0.001〜0.15%、Cr:0.5〜0.9%、Mo:0.15〜1%、Al:0.01〜0.1%、B:0.0005〜0.009%、N:0.006%未満を含み、残部がFおよび不可避的不純物から成る鋼組成を有する鋼から管を作製し、得られた鋼管に、880〜980℃の温度に保持した後に880〜400℃の温度範囲を70℃/分以下の冷却速度で冷却することによって焼準を施し、焼準された鋼管に冷間加工を行い、冷間加工された鋼管に700〜820℃の温度で焼鈍を行うことを特徴とする、肌焼鋼管の製造方法。C: 0.1-0.25%, Si: 0.2-0.4%, Mn: 0.3-0.9%, P: 0.02% or less, S: 0.001 -0.15%, Cr: 0.5-0.9%, Mo: 0.15-1%, Al: 0.01-0.1%, B: 0.0005-0.009%, N: It comprises less than 0.006% or the balance to produce a tube from a steel having a steel composition consisting of F e and unavoidable impurities, the resulting steel pipe, the from 880 to 400 ° C. after maintaining the temperature of eight hundred eighty to nine hundred and eighty ° C. Normalization is performed by cooling the temperature range at a cooling rate of 70 ° C./min or less, the cold-worked steel pipe is cold worked, and the cold-worked steel pipe is annealed at a temperature of 700 to 820 ° C. A method for producing a case-hardened steel pipe.
  2. 前記鋼組成が、下記(1)および(2)から選ばれた1種または2種以上の元素をさらに含有する、請求項1に記載の方法。
    (1)質量%で、Ni:0.3〜4.0%
    (2)質量%で、Ti:0.01〜0.3%、Nb:0.01〜0.3%、V:0.01〜0.3%、Zr:0.01〜0.3%から選ばれた1種または2種以上。
    The method according to claim 1, wherein the steel composition further contains one or more elements selected from the following (1) and (2).
    (1) By mass%, Ni: 0.3-4.0%
    (2) By mass%, Ti: 0.01-0.3%, Nb: 0.01-0.3%, V: 0.01-0.3%, Zr: 0.01-0.3% 1 type or 2 types or more selected from
  3. 前記鋼組成において、B:0.0005〜0.003%である請求項1または2に記載の方法。  The method according to claim 1 or 2, wherein in the steel composition, B is 0.0005 to 0.003%.
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