JP4347999B2 - Induction hardening steel and induction hardening parts with excellent torsional fatigue properties - Google Patents

Description

【００５７】
【表２】

【００５８】
【表３】

【００５９】
（実施例２）
次に、表１に示す鋼水準Ｂ、Ｍ、Ｓ、Ｘについて、圧延仕上げ温度８５０〜９８０℃および７００〜８４０℃の二つの条件で圧延した。前者の条件が本発明例、後者の条件が比較例の圧延条件である。これらの材料について、実施例１と同様の評価を行った。さらに、圧延材について圧延方向に平行な断面のフェライトバンドの評点を求めた。
【００６０】
調査結果を表４に示す。比較例３５〜３８では、フェライトバンドの評点が本発明規定の範囲を上回っている。そして、本発明例の捩り疲労強度は比較例に比べて顕著に優れている。
【００６１】
【表４】

【００６２】
【発明の効果】
本発明の捩り疲労特性に優れた高周波焼入れ用鋼ならびに高周波焼入れ部品を用いれば、高周波焼入れ部品の製造に際して、優れた捩り疲労特性を有する製品を得ることができる。本発明鋼と本発明部品を用いることによって、高周波焼入れすることにより製造されるＣＶＪ部品等の各種シャフト類の捩り疲労強度の向上が可能になる。以上のように、本発明による産業上の効果は極めて顕著なるものがある。
【図面の簡単な説明】
【図１】高周波焼入れ前のフェライトバンド組織が高周波焼入れ後に及ぼす影響を示す図である。
【図２】縞状組織の程度を数量的に表示する金属組織の写真（倍率：２８倍）である。
【図３】捩り疲労試験における時間強度とＭｎＳのアスペクト比の関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to steel for induction hardening, and more specifically, excellent torsional fatigue characteristics suitable as a material for various shafts such as drive shafts and outer rings manufactured by induction hardening at a frequency of 5 to 40 kHz. It relates to induction hardening steel. Use of the newly developed steel is effective for preventing cracking during induction hardening. The forming process of the parts to which the present steel is applied is a process of directly performing cold forging without annealing, a process of annealing before or during cold forging, a process including a cutting process, Alternatively, the process of forming the part mainly by cutting, the process of forming the part by cutting including a partial annealing process, and the process including rolling process in any of these, or warm forging in any of these processes These are combined processes. In addition, since the component made into object by this invention is manufactured by cold processing, such as cutting and cold forging, it is also noted about cold workability.
[0002]
[Prior art]
Various shafts manufactured by the induction hardening process have a strong tendency to increase in strength in accordance with the recent increase in output of automobile engines or compliance with environmental regulations. The main required characteristic of these parts is torsional fatigue characteristics.
[0003]
The present invention is intended for products that are induction-quenched at a frequency of 5 to 40 kHz. Products that are induction-quenched at a frequency of around 100 kHz, which is the mainstream in the past, have a hardened layer depth (for example, the hardened layer depth is about one-fourth of the radius) and are primarily responsible for ensuring wear resistance. Therefore, ensuring the hardness of the outermost surface was an important issue. On the other hand, the technique of induction hardening at a frequency of 5 to 40 kHz, which is an object of the present invention, has recently been attracting attention because it can increase the depth of the hardened layer.
[0004]
In JP-A-3-177537, C: 0.38 to 0.45%, Si: 0.35% or less, Mn: 0.3 to 1.0%, B: 0.0005 to 0.0035% , Ti: 0.01 to 0.05, Al: 0.01 to 0.06%, N: 0.01% or less, ferrite crystal grain size number: 6 or more, microstructure: ferrite and pearlite, hardness HRB 80 to 90 The steel material for direct cutting and induction hardening which has the decarburization depth prescribed | regulated by JIS0558: DM-T0.2mm or less is shown. In the present invention, in order to further improve the machinability, S: 0.005 to 0.30%, Ca: 0.0002 to 0.005%, Pb: 0.005 to 0.30%, Te: 0.00. It is said that a free-cutting component such as 005 to 0.10% can be added as necessary. The invention steel is characterized in that steel B is applied and the decarburization depth is defined. This publication describes the characteristics when induction-quenched at a frequency of 100 kHz, but does not describe the characteristics when induction-quenched at a frequency of 5 to 40 kHz, which is an object of the present invention. In addition, the gazette does not mention the torsional fatigue strength characteristics focused on in the present invention at all. In the steel presented as an example, S is added around 0.02, and it is considered that a large amount of stretched MnS is present, and the occurrence of uneven hardness in the induction-hardened part is considered to be a problem. Strength properties are not considered sufficient.
[0005]
JP-A-5-179400 discloses C: 0.38 to 0.45%, Si: 0.35% or less, Mn: more than 1.0% to 1.5%, and B: 0.0005. Direct cutting with a fine grain structure of 0.0035%, Ti: 0.01 to 0.05, Al: 0.01 to 0.06%, N: 0.01% or less, ferrite crystal grain size number: 6 or more Induction hardening steel is shown. In claim 4 of the present invention, Pb: 0.01-0.20%, S: 0.005-0.30%, Bi: 0.01-0.10%, Te: 0.0005-0.10 %, Ca: 0.0003 to 0.0050%, or one or more. The invention steel material is a steel material in which the amount of Mn is increased with respect to JP-A-3-177537. The publication describes torsional strength but does not describe torsional fatigue strength. The steel presented as an example is added at around S: 0.02, and a large amount of stretched MnS is present, and the occurrence of uneven hardness in the induction-hardened part is considered to be a problem, and the torsional fatigue strength Etc. are not considered sufficient.
[0006]
Japanese Patent Laid-Open No. 11-1749 discloses that in a longitudinal section passing through the axis of a linear or rod-shaped rolled material, it is parallel to the axis and is 1/4 · D (D is the diameter of the rolled material). Bending characterized in that there are no more than 20 composite inclusions having a diameter of 10 μm or more, made of oxide and sulfide, existing in a test area of 100 mm ² including a distant imaginary line as a center line A steel for induction hardening with excellent fatigue strength and rolling fatigue strength is shown. In the present invention, S: 0.1% or less, B: 0.01% or less, Ca: 0.0005 to 0.01%, Te: 0.1% or less, Zr: 0.1% or less as selective elements It can be contained. The aim of the addition of Ca, Te and Zr of the present invention is to improve the anisotropy by spheroidizing MnS and to improve the machinability without deteriorating toughness and bending fatigue properties. The object of the present invention is to provide a steel for induction hardening that can exhibit excellent bending fatigue characteristics and rolling fatigue characteristics by induction hardening, and is a coarse composite having a diameter of 10 μm or more made of oxide and sulfide. The feature is that inclusions are limited within the above range. However, in the invention, no mention is made regarding torsional fatigue characteristics. Bending fatigue is a phenomenon in which a crack is generated and propagated in a cross section perpendicular to the axial direction due to tensile stress at or near the surface, leading to fracture. In contrast, torsional fatigue, which is taken up in the present invention, causes a crack to occur in a plane parallel to the axial direction due to shear stress on the surface or in the vicinity of the surface, and then propagates on a plane that forms 45 degrees with the axial direction. It is a phenomenon. That is, the torsional fatigue failure and the bending fatigue failure differ in all of the working stress that causes the failure, the cross-section where the crack occurs, and the failure mode. Rolling fatigue is a phenomenon in which cracks are generated and propagated from the surface of the contact area or directly under the surface in the repeated contact phenomenon of rolling elements. In rolling fatigue and torsional fatigue, the stress state, crack initiation and propagation are The mechanism is completely different. From the above, the description regarding bending fatigue characteristics and rolling fatigue characteristics in Japanese Patent Application Laid-Open No. 11-1749 does not give any suggestion regarding torsional fatigue strength taken up in the present invention.
[0007]
[Problems to be solved by the invention]
The disclosed steels as described above are considered to have insufficient torsional fatigue properties due to the presence of elongated MnS and the inappropriate ferrite structure. The present invention solves such problems and provides a steel for induction hardening and an induction-quenched component having excellent torsional fatigue characteristics.
[0008]
[Means for Solving the Problems]
The present inventor has solved the above problems using the following means.
[0009]
That is, in mass%,
C: 0.3 to 0.58%,
Si: 0.01 to 1.0%,
Mn: 0.85 to 1.7%,
S: 0.005 to 0.018% ,
B: 0.0005 to 0.005%,
Al: 0.001 to 0.1%,
Zr: 0.0003 to 0.01%
In addition,
Te: 0.0005 to 0.02%,
Ca: 0.0005 to 0.02%,
Zr: 0.0003 to 0.01%
Mg: 0.001 to 0.035%,
Y: 0.001 to 0.1%
Rare earth elements: 0.001 to 0.15%
One or more of these, or
Ti: 0.05% or less, or further
Nb: 0.04% or less,
V: contains one or two of 0.4% or less, or
Mo: 0.3% or less,
Ni: Contains 1 or 2 of 1% or less,
P: 0.025% or less,
Cr: 0.35% or less,
N: less than 0.0070%,
O: Each is limited to 0.0025% or less,
The balance consists of iron and inevitable impurities,
And the microstructure is substantially a ferrite pearlite structure,
The structure area ratio of the ferrite is 1-1.05 × C or less with respect to the carbon content C (%), the ferrite crystal grain size is 25 μm or less, or a cross section parallel to the hot rolling direction. It is a steel for induction hardening excellent in torsional fatigue characteristics characterized by having a ferrite band score of 1 to 5 in the structure.
[0010]
The invention of claim 6 is a component obtained by induction hardening of the steel for induction hardening according to any one of claims 1 to 5 , wherein the aspect ratio of MnS is 10 or less. Induction-hardened parts with excellent fatigue characteristics.
[0011]
By using the steel and parts of the present invention, a product excellent in torsional fatigue characteristics after induction hardening can be obtained.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
In the production of induction-hardened parts, the present inventors have conducted intensive investigations and clarified the following points in order to realize excellent torsional fatigue characteristics after induction hardening.
[0013]
(1) Torsional fatigue failure of induction-hardened members occurs in the following process.
A. A crack is generated on the surface or a plane parallel to the axial direction at the boundary between the hardened layer and the core.
B. The crack propagates in the plane parallel to the axial direction. This is hereinafter referred to as mode III destruction.
C. After mode III fracture, a brittle fracture is caused with a grain boundary crack at a plane of 45 degrees in the axial direction, and a final fracture is caused. This is hereinafter referred to as mode I destruction.
[0014]
(2) Torsional fatigue crack generation and initial propagation occur in a plane parallel to the axial direction. At this time, if extended MnS exists in the axial direction, crack generation and initial propagation occur along the extended MnS. Crack initiation and initial propagation are promoted. FIG. 1 (c) shows the fracture behavior under stress loading, but the extended MnS usually exists along the low carbon martensite part shown in the figure, and the occurrence of cracks shown in the figure. Is promoted by the presence of elongated MnS. For the above reasons, the generation and initial propagation of cracks are suppressed and the torsional fatigue strength is dramatically improved by granulating and refining MnS. Addition of Te, Ca, Zr, Mg, Y and rare earth elements is effective for preventing the formation of elongated MnS, granulating and refining MnS. The addition of a large amount of these elements is inappropriate because it causes formation of nitrides and oxides such as coarse ZrN and inhibits cold workability. Granulation of MnS by adding these elements is also effective for preventing cracks during induction hardening. As described in the prior art, Japanese Patent Application Laid-Open No. 11-1749 discloses that Ca, Te, and MnS are spheroidized to improve anisotropy and to improve machinability. The addition of Zr is described. However, the aim of granulating MnS by adding Ca, Te, Zr is to improve anisotropy and improve machinability without degrading toughness and bending fatigue properties. On the other hand, in the present invention, the improvement in torsional fatigue characteristics is clearly different between the two. In addition, although the invention refers to bending fatigue, it does not refer to any torsional fatigue characteristics, and as described above, in torsional fatigue fracture and bending fatigue fracture, acting stress that causes fracture, Since both the cross-section where the crack occurs and the form of fracture differ greatly, Japanese Patent Application Laid-Open No. 11-1749 does not contain any information that suggests the above technical idea of the present invention.
[0015]
(3) Next, if the ferrite fraction is large and the ferrite grains are coarse, after the induction hardening, the ferrite portion originally becomes low carbon martensite and hardness unevenness occurs. Since ferrite normally exists in a band shape parallel to the axial direction, after induction hardening, a portion having low hardness exists on a plane parallel to the axial direction. FIG. 1 is a diagram schematically showing the influence after induction hardening when a ferrite band exists in the structure before induction hardening. When the ferrite band is prominent, as shown in FIG. 1, after induction hardening, the part of the pearlite structure originally becomes high carbon martensite and the part of the ferrite band originally becomes low carbon martensite, and the hard layer and the soft layer are axial. It will be present in layers along the direction. When torsional stress is applied to such a steel material, the axial direction becomes the direction of maximum shearing stress, so shear cracks are generated and propagated along the soft, low-carbon martensite layer, resulting in low-strength fracture. Invite. For the above reasons, when the ferrite fraction is high and the ferrite grains are coarse, generation of torsional fatigue cracks and initial propagation in a plane parallel to the axial direction after induction hardening are promoted. Therefore, in order to prevent this, it is essential to regulate the ferrite fraction and refine the ferrite grains together with the granulation of MnS.
[0016]
(4) Next, in addition to optimizing the ferrite structure, it is also effective to suppress the ferrite band itself. As shown in FIG. 2, the grade of the ferrite band is scored in seven stages of 1 to 7 in “Japan Institute of Metals, Vol. 34, No. 9, page 961” published by the Japan Institute of Metals in 1970. That is, the above-mentioned Journal of the Japan Institute of Metals, Vol. 34, No. 9, pp. 957 to 962, “As to the title,“ about the influence of austenite grain size and forging ratio on ferrite stripe structure ”is described, In the left column, lines 7 to 8 on page 961, “Photo. 4 standard photo was created in order to quantitatively display the degree of the striped structure” is described, and “Photo. “4 Classifications of ferritic bands (× 50 × 2/3 × 5/6)” includes reference photos 1 to 7. The score indicates that the smaller the score number is, the lighter the ferrite band is, and the higher the score number is, the more prominent the ferrite band is. In order to improve the torsional fatigue characteristics after induction hardening, the ferrite band score defined in the above-mentioned Journal of the Japan Institute of Metals, Vol. 34, page 961, of the cross-sectional structure parallel to the hot rolling direction is 1-5. It is effective to be.
[0017]
(5) Furthermore, it is effective to regulate the upper limit of the Cr content in order to reduce the hardness unevenness caused by the large ferrite fraction. This is due to the effect of preventing the poor penetration of carbides and reducing the hardness unevenness due to the ferrite fraction.
[0018]
(6) It should be noted that the effect of granulating MnS and the effect of optimizing the ferrite structure on the improvement of torsional fatigue properties are almost the same.
[0019]
(7) Next, in order to suppress the brittle fracture mode I accompanied by intergranular cracking at a 45 degree surface in the axial direction described in the column of the torsional fatigue fracture process "C." Boundary strengthening is effective.
(1) Add B as an essential element. B is due to the effect of expelling the grain boundary segregation P from the grain boundaries.
(2) Reduction of the amount of P and O which are segregation elements at grain boundaries.
(3) Refinement of austenite grain structure by refinement of ferrite structure of previous structure.
(4) In order to further improve the torsional fatigue strength, it is effective to refine the grain boundary carbide by increasing the Si content.
[0020]
(8) Since many of the parts targeted by the present invention are manufactured by cold working such as cutting or cold forging, ensuring cold workability is also an important issue. Addition of Mn and B is effective for suppressing the improvement in hardness at the material stage and improving the induction hardenability. In order to make B effective for hardenability, it is necessary to reduce N. In the present invention, the amount of N is reduced to less than 0.0070%.
[0021]
The present invention has been made based on the above new findings.
[0022]
Hereinafter, the present invention will be described in detail.
[0023]
C is an element effective for imparting the necessary strength to steel after induction hardening, but if it is less than 0.3%, the required strength cannot be ensured, and if it exceeds 0.58%, it becomes hard. Since cold workability is deteriorated, it is necessary to be within the range of 0.3 to 0.58%. A preferable range is 0.4 to 0.56%.
[0024]
Si is an element effective for deoxidation of steel and is an element effective for imparting necessary strength and hardenability to steel and improving temper softening resistance. However, if it is less than 0.01%, the effect is ineffective. It is enough. On the other hand, if it exceeds 1.0%, the hardness is increased and the cold workability is deteriorated. For the above reasons, the content needs to be in the range of 0.01 to 1.0%. The preferred range when the cold workability is emphasized is 0.01 to 0.5%, and the preferred range when the cold workability is particularly important is 0.01 to 0.15%. Further, the preferred range when the torsional fatigue characteristics are emphasized is more than 0.35 to 1.0%, and in the case where high strength is particularly desired, the addition in the range of 0.5 to 1.0% is desirable.
[0025]
Mn is an element effective for improving induction hardenability. In order to obtain a sufficient hardened layer depth for obtaining the torsional fatigue characteristics, the effect is insufficient at less than 0.85%. On the other hand, if it exceeds 1.7%, the hardness is significantly increased and the cold workability is deteriorated, so it is necessary to set the content within the range of 0.85% to 1.7%. The preferred range is 0.85 to 1.4%.
[0026]
S forms MnS in the steel and is added for the purpose of improving machinability. However, if it is less than 0.005%, its effect is insufficient. On the other hand, if it exceeds 0.018% , the torsional fatigue characteristics are deteriorated. For these reasons, the S content needs to be in the range of 0.005 to 0.018% . In addition, when MnS is elongated, torsional fatigue characteristics are deteriorated. Therefore, one or more of Te, Ca, Zr, Mg, and rare earth elements are contained as essential elements in order to finely disperse MnS. It is necessary to let
[0027]
B is added for the following three points. (1) In steel bar / wire rolling, boron iron carbide is generated in the cooling process after rolling, thereby increasing the growth rate of ferrite and promoting softening during rolling. (2) Gives hardenability to steel during induction hardening. (3) By improving the grain boundary strength of the induction-hardened material, the fatigue strength and impact strength as a machine part are improved. When the amount is less than 0.0005%, the above effect is insufficient. When the amount exceeds 0.005%, the effect is saturated, so the content needs to be within the range of 0.0005 to 0.005%. There is. The preferred range is 0.001 to 0.003%.
[0028]
Al is useful as a deoxidizer and is useful for securing solid solution B by fixing solid solution N present in steel as AlN. However, if the amount of Al is too large, Al ₂ O ₃ will be generated excessively, increasing internal defects and degrading the cold workability. Therefore, in the present invention, it was 0.001 to 0.1%. In addition, when Ti is not added and has an action of fixing solute N, Al is preferably 0.04 to 0.1%.
[0029]
Next, in this invention, 1 type (s) or 2 or more types are contained as an essential element among Te, Ca, Zr, Y, Mg, and rare earth elements. Each of these elements generates an oxide, and this oxide serves as a nucleus for forming MnS, and MnS is compositionally modified such as (Mn, Ca) S or (Mn, Mg) S. This improves the stretchability of these sulfides during hot rolling and finely disperses the granular MnS, thereby improving the torsional fatigue characteristics after induction hardening. Such effects are Te: less than 0.0005%, Ca: less than 0.0005%, Zr: less than 0.0003%, Mg: less than 0.001%, Y: less than 0.001%, rare earth element: 0 Addition of less than 0.001% is insufficient. On the other hand, Te: more than 0.02%, Ca: more than 0.02%, Zr: more than 0.01%, Mg: more than 0.035%, Y: more than 0.1%, rare earth elements: more than 0.15% The above effects are saturated, and excessive addition of these produces rather coarse oxides such as CaO and MgO and their clusters, and hard precipitates such as ZrN, which are cold workable. It causes deterioration. For these reasons, these contents are set to Te: 0.0005 to 0.02%, Ca: 0.0005 to 0.02%, Zr: 0.0003 to 0.01%, Mg: 0.001 to 0 0.035%, Y: 0.001 to 0.1%, and rare earth elements: 0.001 to 0.15%. In addition, the rare earth element as used in the field of this invention refers to the element of atomic number 57-71.
[0030]
Since P is an element that increases deformation resistance during cold forging and deteriorates toughness, cold workability deteriorates. Further, the fatigue strength of the final product is deteriorated by embrittlement of the grain boundaries of the parts after induction hardening and tempering. Therefore, it is necessary to limit the content to 0.025% or less. The preferred range is 0.015% or less.
[0031]
Cr is dissolved in cementite to stabilize the cementite. For this reason, cementite is poorly melted during short-time heating by induction hardening, causing unevenness in hardness and deteriorating torsional fatigue characteristics. This behavior becomes remarkable especially when it exceeds 0.35%. For the above reasons, the content needs to be limited to 0.35% or less. The preferred range is 0.15% or less.
[0032]
It is desirable to limit N as much as possible for the following two reasons. (1) B is added for the purpose of improving hardenability and strengthening grain boundaries as described above. However, since the effect of these B is manifested only in the state of solid solution B in steel, the amount of N is reduced. Therefore, it is essential to suppress the generation of BN. (2) When N is combined with Al and Ti in the steel, coarse nitrides are formed, causing cold forging cracks and the cold workability is remarkably deteriorated. The above adverse effect is particularly remarkable when the N amount is 0.007% or more. For the above reasons, the content needs to be less than 0.007%. The preferred range is 0.005% or less.
[0033]
O forms oxide inclusions such as Al ₂ O _{3 in} the steel. When a large amount of oxide inclusions are present in the steel, the cold workability deteriorates. This tendency becomes particularly noticeable when the O content exceeds 0.0025%. For the above reasons, the content needs to be limited to 0.0025% or less. The preferred range is 0.002% or less.
[0034]
The above are the basic components of steel targeted by the present invention. In the second claim of the present invention, by adding Ti, N is fixed as TiN by Ti, and N is rendered harmless. . Ti is an element having a deoxidizing action. However, if Ti is added in excess of 0.05%, precipitation hardening due to TiC becomes remarkable, and cold workability is remarkably deteriorated. For this reason, Ti: 0.05% or less was included as necessary.
[0035]
Next, in the third aspect of the present invention, one or two of Nb and V are contained.
[0036]
Nb combines with C and N in steel to form Nb (CN), and is an effective element for increasing the core hardness by refining crystal grains and precipitation hardening. However, if it exceeds 0.04%, the hardness of the material becomes so hard that the cold workability deteriorates and it becomes difficult to form a solution during heating of the steel bar and wire rod. For the above reasons, the content needs to be 0.04% or less. The preferred range is 0.03% or less.
[0037]
V is added for the same effect as Nb. However, if it exceeds 0.4%, the hardness of the material becomes hard and the cold workability deteriorates, and it becomes difficult to form a solution during heating of the steel bar and wire rod. For the above reasons, the content needs to be 0.4% or less. The preferred range is 0.3% or less.
[0038]
Next, the fourth aspect of the present invention contains one or two of Mo and Ni.
[0039]
Mo is an element effective for imparting strength and hardenability to steel and improving the grain boundary strength after induction hardening to increase strength characteristics. However, if added over 0.3%, the hardness increases and cold workability deteriorates. For the above reasons, the content needs to be 0.3% or less.
[0040]
Ni is also an element effective for imparting strength and hardenability to the steel, but if added over 1%, the hardness increases and cold workability deteriorates. For the above reasons, the content needs to be 1% or less.
[0041]
Next, the structure of the present invention will be described.
[0042]
If the ferrite fraction is large and the ferrite grains are coarse, after the induction quenching as described above, the ferrite portion originally becomes low carbon martensite and hardness unevenness occurs. Since ferrite normally exists in a band shape parallel to the axial direction, after induction hardening, a portion having low hardness exists on a plane parallel to the axial direction. For the above reasons, when the ferrite fraction is high and the ferrite grains are coarse, generation of torsional fatigue cracks and initial propagation in a plane parallel to the axial direction after induction hardening are promoted. Therefore, in order to prevent this, it is essential to regulate the ferrite fraction and refine the ferrite grains together with the granulation of MnS. When the ferrite structure area ratio exceeds 1-1.05 × C or the ferrite crystal grain size exceeds 25 μm with respect to the carbon content C (%), there is an adverse effect due to the ferrite structure as described above. Become prominent. For the above reasons, it is necessary that the structure area ratio of ferrite is 1-1.05 × C or less with respect to the carbon content C (%), and the ferrite crystal grain size is 25 μm or less. Here, the structure area ratio of ferrite is expressed as a fraction, that is, the structure area ratio of ferrite when the area of the entire structure is 1. For example, in the case of 0.4% C steel, 1-1.05 × C = 0.58, and the structure area ratio of ferrite is regulated to 0.58 or less (58% or less if expressed in percentage).
[0043]
Next, according to the fifth aspect of the present invention, the score of the ferrite band having a cross-sectional structure parallel to the hot rolling direction is limited to a range of 1 to 5. The score of the ferrite band is a score defined in the Japan Metallurgy Journal Vol. 34, page 961, as described above. The reason for limiting the tissue factor in this way in the present invention will be described below.
[0044]
Since induction hardening is rapid heating, if the ferrite in the structure before induction hardening is coarse, the ferrite part has insufficient carbon diffusion after austenitization, and the carbon concentration is lower than the added carbon concentration. After hardening, the hardness at that position becomes small. Here, generally, a striped structure called a ferrite band is recognized in a cross section parallel to the rolling direction of the steel material after hot rolling. When coarse ferrite is continuously present as a ferrite band in a row, hardness unevenness after quenching becomes particularly remarkable, and a soft band corresponding to the original ferrite band is formed in the longitudinal direction. Therefore, when a torsional moment is repeatedly applied to the final part, a fatigue crack is generated by shearing stress along the soft band and breaks with low strength. The above phenomenon becomes particularly prominent when the ferrite band score exceeds 5. For the above reasons, the ferrite band score of the cross-sectional structure parallel to the hot rolling direction was set to 1-5. A preferable range is a range in which the score of the ferrite band having a cross-sectional structure parallel to the hot rolling direction is 1 to 4.
[0045]
Next, the invention of claim 6 is an invention relating to an induction hardened component having excellent torsional fatigue characteristics. It is the component which induction-hardened the steel for induction hardening as described in any one of Claims 1-5 , Comprising: The aspect ratio of MnS restrict | limits to 10 or less. FIG. 3 shows the results of investigating the relationship between the aspect ratio of MnS and the time strength in torsional fatigue for induction-hardened shaft components. When the aspect ratio of MnS exceeds 10, the torsional fatigue characteristics are significantly deteriorated. For the above reasons, the aspect ratio of MnS was limited to 10 or less.
[0046]
In the present invention, the size of the slab, the cooling rate during solidification, the ingot rolling conditions, the bar rolling conditions and the cooling conditions are not particularly limited, and any conditions may be used as long as the requirements of the present invention are satisfied.
[0047]
【Example】
Hereinafter, the effects of the present invention will be described more specifically by way of examples.
[0048]
Example 1
Steel having the composition shown in Table 1 was melted. Here, it is a method for analyzing Zr in steel. After the sample was processed in the same manner as in Appendix 3 of JIS G 1237-1997, the amount of Zr in steel was measured by ICP (inductively coupled plasma) in the same manner as the analysis of Nb in steel. (Emission spectroscopy). However, the sample used for the measurement in the examples of the present invention was 2 g, and the calibration curve in ICP was set to be suitable for a very small amount of Zr. That is, the Zr standard solution was diluted so that the Zr concentration was 1 to 200 ppm to prepare solutions having different Zr concentrations, and the calibration curve was prepared by measuring the Zr amount. In addition, about the common method regarding these ICP, it is based on JISK0116-1995 (general rules of emission spectroscopic analysis method) and JIS Z 8002-1991 (general rules of tolerance of analysis and test).
[0049]
After forming a 162 mm square rolled material, a steel bar having a diameter of 36 to 45 mm was manufactured by hot rolling. For cooling after hot rolling, some materials were air-cooled, and some materials were cooled at a slower cooling rate than air-cooling using a heat insulating cover installed on the cooling floor.
[0050]
The structure of the rolled steel bar was observed to determine the ferrite fraction and the ferrite crystal grain size.
[0051]
Further, the Vickers hardness of the rolled steel bar was measured. Furthermore, an upsetting test piece was prepared from the rolled steel bar, and the cold deformation resistance and the limit upsetting rate were obtained as indicators of cold workability. The cold deformation resistance was represented by the deformation resistance at an equivalent strain of 1.0.
[0052]
Further, static torsional test pieces and torsional fatigue test pieces having a parallel part diameter of 20 mm were collected from the rolled material. The static torsional test piece and the torsional fatigue test piece were subjected to induction hardening under the conditions of a frequency of 8.5 kHz and a maximum heating temperature of 1000 ° C., and then tempered at 170 ° C. for 1 hour. Thereafter, a static torsion test and a torsional fatigue test were performed. The torsional fatigue characteristics were evaluated by time strength at 1 × 10 ⁵ cycles. Moreover, the aspect ratio of MnS was calculated | required using the image analysis apparatus in the cross section of the longitudinal direction of a torsion test piece.
[0053]
These survey results are shown in Tables 2 and 3. The depth of the hardened layer of the induction-hardened material is indicated by the ratio of the depth t to the radius r of HV450.
[0054]
Comparative Example 22 is a characteristic of JIS S40C, Comparative Example 23 is a characteristic of JIS S45C, and Comparative Example 24 is a characteristic of JIS S53C. Comparative Example 25 is a characteristic of boron steel of 0.4C, Comparative Example 26 is 0.45C, and Comparative Example 27 is a characteristic of boron steel of 0.53C. In each of these comparative examples, the aspect ratio of MnS exceeds the range specified in the present invention. And when the example of this invention and a comparative example are compared about the same C amount, the torsional fatigue strength of the example of this invention is remarkably excellent compared with a comparative example.
[0055]
Next, Comparative Examples 28, 29, and 30 are cases where rolling was followed by annealing in a furnace at 650 ° C., and Comparative Examples 28 and 29 were cases where the ferrite fraction exceeded the range specified in the present invention. Comparative Example 30 is a case where the ferrite crystal grain size exceeds the range specified in the present invention, and the torsional fatigue characteristics are inferior to those of the present invention.
[0056]
[Table 1]

[0057]
[Table 2]

[0058]
[Table 3]

[0059]
(Example 2)
Next, the steel levels B, M, S, and X shown in Table 1 were rolled under two conditions of a rolling finishing temperature of 850 to 980 ° C and 700 to 840 ° C. The former condition is the example of the present invention, and the latter condition is the rolling condition of the comparative example. These materials were evaluated in the same manner as in Example 1. Furthermore, the score of the ferrite band of the cross section parallel to the rolling direction was calculated | required about the rolling material.
[0060]
The survey results are shown in Table 4. In Comparative Examples 35 to 38, the score of the ferrite band exceeds the range specified in the present invention. And the torsional fatigue strength of the example of this invention is remarkably excellent compared with the comparative example.
[0061]
[Table 4]

[0062]
【The invention's effect】
When the induction-quenched steel and the induction-quenched parts excellent in torsional fatigue characteristics of the present invention are used, a product having excellent torsional fatigue characteristics can be obtained when producing the induction-quenched parts. By using the steel of the present invention and the parts of the present invention, the torsional fatigue strength of various shafts such as CVJ parts produced by induction hardening can be improved. As described above, the industrial effects of the present invention are extremely remarkable.
[Brief description of the drawings]
FIG. 1 is a diagram showing the influence of a ferrite band structure before induction hardening on after induction hardening.
FIG. 2 is a photograph (magnification: 28 times) of a metal structure that quantitatively displays the degree of striped structure.
FIG. 3 is a diagram showing the relationship between time strength and MnS aspect ratio in a torsional fatigue test.

Claims (6)

% By mass
C: 0.3 to 0.58%,
Si: 0.01 to 1.0%,
Mn: 0.85 to 1.7%,
S: 0.005 to 0.018% ,
B: 0.0005 to 0.005%,
Al: 0.001 to 0.1%
In addition,
Te: 0.0005 to 0.02%,
Ca: 0.0005 to 0.02%,
Zr: 0.0003 to 0.01%
Mg: 0.001 to 0.035%,
Y: 0.001 to 0.1%
Rare earth elements: 0.001 to 0.15%
Containing one or more of them,
P: 0.025% or less,
Cr: 0.35% or less,
N: less than 0.0070%,
O: Each is limited to 0.0025% or less,
The balance consists of iron and inevitable impurities,
And the microstructure is substantially a ferrite pearlite structure,
Induction hardening with excellent torsional fatigue characteristics, wherein the ferrite structure area ratio is 1-1.05 × C or less with respect to the carbon content C (%) and the ferrite crystal grain size is 25 μm or less. Steel. Furthermore, in mass%,
The steel for induction hardening excellent in torsional fatigue characteristics according to claim 1, characterized by containing Ti: 0.05% or less. Furthermore, in mass%,
Nb: 0.04% or less,
V: Steel for induction hardening excellent in torsional fatigue properties according to claim 1 or 2, characterized by containing one or two of 0.4% or less. Furthermore, in mass%,
Mo: 0.3% or less,
The steel for induction hardening excellent in torsional fatigue characteristics according to any one of claims 1 to 3, characterized by containing one or two of Ni: 1% or less. Furthermore, the rating of the ferrite band of the structure of the cross section parallel to the hot rolling direction is 1 to 5, for induction hardening excellent in torsional fatigue characteristics according to any one of claims 1 to 4 steel. An induction-hardened component excellent in torsional fatigue characteristics, wherein the steel for induction hardening according to any one of claims 1 to 5 is induction-quenched, and the aspect ratio of MnS is 10 or less.

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