JP2004300551A - High-strength medium carbon steel - Google Patents

Abstract

An object of the present invention is to provide a medium carbon steel which suppresses the addition of expensive alloy elements and has excellent pitting resistance and impact strength. In a mass%, C: 0.40 to 0.80%, Si: 0.40 -1.50%, Mn: 0.30-2.00%, P: 0.030% or less, S: 0.030% or less, Ti: 0.02-0.30%, B: 0.0005- High-strength medium-carbon steel containing 0.0050% and N: 0.015% or less, with the balance being Fe and unavoidable impurities. This steel significantly improves the impact strength by adding B, and steel by adding Ti. Fine TiC is dispersed therein, and the addition of Si provides excellent tempering softening resistance.
[Selection diagram] None

Description

【００２７】
【実施例】
表１に示す化学組成の鋼を１００ｋｇ真空溶解炉で溶製した後、１２００℃の熱間鍛造によって直径３２ｍｍの棒鋼を製造した。次いで、前記各素材に対して９００℃以上の温度に加熱した後空冷して焼きならし処理を施した後、各種試験用の試験片を作製した。各試験片に対して、硬化層比率（表面から５００ＨＶとなる点までの距離／部品半径）が０．２５以上となるように高周波焼入れ処理を施した後、１５０℃で焼戻し、各試験を行った。それぞれの特性評価は、耐ピッチング性については、図１に示すローラーピッチング試験片を用いたローラーピッチング試験、衝撃強度については、図２に示す角１０ｍｍ×長さ５５ｍｍの試験片に２ｍｍＵノッチを設けた試験片を用いたシャルピー衝撃試験にて行った。
【００２８】
衝撃強度は、シャルピー衝撃試験を行い、その際の吸収エネルギーをノッチ底の断面積８０ｍｍ^２で除した値をシャルピー衝撃値として評価した。
【００２９】
ここで、ローラーピッチング試験の原理を図３に示す。すなわち、ローラーピッチング試験片１（小ローラー：材質は表１の鋼種Ａ〜Ｎ）と相手材２（大ローラー：ＪＩＳ ＳＣＭ４２０相当）とを３４０ｋｇｆ／ｍｍ^２すなわち、３３３４ＭＰａの面圧下で高速回転させ、ピッチングが発生するまでの寿命である回転数を求めて耐ピッチング性を評価した。なお、この場合、ローラーピッチング試験片１（小ローラー）と相手材２（大ローラー）との周速の差、すなわち滑り率は４０％である。そして、各条件についてそれぞれ５本のローラーピッチング試験片１を対象として試験を行った後、ワイブルプロットを行い、５０％破損確率寿命（Ｂ_５０寿命）を求めて耐ピッチング性を評価した。
【００３０】
また、ローラーピッチング試験片１を用いて表面硬化層断面の表面から８０μｍ位置における硬さを３００℃の高温条件下にて測定し焼戻軟化抵抗性の評価とした。以上の試験結果を表２に示す。
【００３１】
【表２】

【００３２】
表２により、比較例Ｊ（ＪＩＳ Ｓ５３Ｃ）に対して本発明の鋼が耐ピッチング性、衝撃強度のいずれも優れる鋼であることが分かる。また、ＳｉをＪＩＳ中炭素鋼レベル（０．１５〜０．３５％）に対して０．４０〜１．５０％に増量添加することによって高温硬さの上昇すなわち焼戻軟化抵抗性が向上したことが分かる。さらに、比較例Ｌ、Ｍ、Ｎの結果から耐ピッチング性、衝撃強度をともに大幅に向上させるには、Ｓｉの０．４０〜１．５０％の添加、望ましくは０．５５〜１．５０％の添加及びＴｉとＢの添加が必須であることが分かる。
【００３３】
【発明の効果】
以上説明したように、本発明は、Ｂ添加により衝撃強度を大幅に向上させ、かつＴｉ添加により鋼中に微細なＴｉＣを分散させ、さらにＳｉ添加により焼戻軟化抵抗性に優れることが特徴である。それにより、高価な合金元素の添加を抑えた耐ピッチング性、衝撃強度に優れる中炭素鋼が得られ、この発明鋼に高周波焼入れ処理を施すことにより機械構造部品である歯車などの構造用鋼として優れた効果を発揮する。
【図面の簡単な説明】
【図１】ローラーピッチング試験片の概略を示す側面図である。
【図２】シャルピー衝撃試験片の概略を示す側面図である。
【図３】ローラーピッチング試験の原理を示す斜視図である。
【符号の説明】
１ ローラーピッチング試験片
２ 相手材[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a part made of medium carbon steel for a machine structure, for example, a steel part for an automobile that requires high strength such as a gear.
[0002]
[Prior art]
In the past, medium-strength steel such as gears, which are mechanical structural parts, which had been subjected to induction hardening or the like on medium carbon steel represented by JIS S45C and S53C, was used. Is increasingly demanding. Therefore, alloy elements such as Ni, Cr, and Mo are added to increase the strength. For example, high strength is achieved by adding 0.30 to 2.00% of Mo and 0.010 to 0.100% of Nb (for example, see Patent Document 1).
[0003]
Further, it has been found that by increasing the amount of Si added compared to the JIS level (0.15 to 0.35%), the tempering softening resistance is improved, and thereby the pitting resistance is improved (Patent Document 1). 2). Temper softening resistance refers to the difficulty in lowering the hardness that occurs when reheating is performed at or above the tempering temperature during induction quenching and tempering.
[0004]
Furthermore, a medium carbon steel in which toughness is improved by adding Ti and B is disclosed (see Patent Document 3), but the amount of Si added is below the JIS level.
[0005]
[Patent Document 1]
JP-A-3-47948 [Patent Document 2]
JP-A-7-258793 [Patent Document 3]
JP 2000-63979 A
[Problems to be solved by the invention]
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a medium-carbon steel which suppresses the addition of expensive alloy elements and has excellent pitting resistance and impact strength. It is.
[0007]
[Means for Solving the Problems]
In order to improve pitting resistance, in the present invention, fine TiC is dispersed in steel by adding Ti, thereby strengthening the dispersion of the steel matrix, and further, the Si content is set to 0.40 to 1.50%. The tempering softening resistance is thereby improved.
[0008]
In order to improve impact strength, B is added to enhance toughness by strengthening grain boundaries.
[0009]
In other words, the means of the present invention for solving the above-mentioned problems are, in the invention of claim 1, C: 0.40 to 0.80%, Si: 0.40 to 1.50%, Mn : 0.30 to 2.00%, P: 0.030% or less, S: 0.030% or less, Ti: 0.02 to 0.30%, B: 0.0005 to 0.0050%, N: 0.015% or less, the balance consists of Fe and unavoidable impurities, the impact strength is greatly improved by adding B, and fine TiC is dispersed in steel by adding Ti, and tempering is softened by adding Si. It is a high-strength medium-carbon steel characterized by excellent resistance.
[0010]
According to the second aspect of the present invention, in addition to the alloy elements of the first aspect, Cr: 0.15 to 3.00%, Mo: 0.03 to 1.50%, and Ni: 0.03% by mass. ３3.50% or less, one or more of which contain Fe and unavoidable impurities, greatly improve the impact strength by adding B, and disperse fine TiC in steel by adding Ti, Further, it is a high-strength medium carbon steel characterized by having excellent temper softening resistance by adding Si.
[0011]
According to the third aspect of the present invention, in addition to the alloy element of the first aspect, one of Nb: 0.02 to 0.15% or less and V: 0.02 to 0.15% by mass%. It contains the above, and the balance consists of Fe and unavoidable impurities. The addition of B greatly improves the impact strength, the addition of Ti disperses fine TiC in the steel, and the addition of Si provides excellent temper softening resistance. It is a high-strength medium-carbon steel characterized by the following.
[0012]
According to a fourth aspect of the present invention, in addition to the alloy element of the second aspect, one of Nb: 0.02 to 0.15% or less and V: 0.02 to 0.15% by mass%. It contains the above, and the balance consists of Fe and unavoidable impurities. The addition of B greatly improves the impact strength, the addition of Ti disperses fine TiC in the steel, and the addition of Si provides excellent temper softening resistance. It is a high-strength medium-carbon steel characterized by the following.
[0013]
The reasons for limiting the alloying elements will be described below. In addition,% is shown by mass%.
[0014]
C: 0.40 to 0.80%
C is an element necessary for securing the core strength as a component for a mechanical structure, but if it is less than 0.40%, the hardness at the time of quenching becomes low, and if it exceeds 0.80%, it becomes hard at the time of quenching. Hardness is saturated and workability is reduced. Therefore, the content is set to 0.40 to 0.80%.
[0015]
Si: 0.40 to 1.50%
Si is an element having an important role in the present invention, and is added to improve tempering softening resistance, which is very effective in improving pitting resistance. To obtain this effect, the content needs to be 0.40% or more, but if it is too large, the toughness is impaired, so the upper limit is made 1.50%. To exert the above effects most, it is desirable to add 0.55 to 1.50%.
[0016]
Mn: 0.30-2.00%
Mn is added because it is an element that improves quenching properties and tempering softening resistance, and requires 0.30% or more to obtain the effect. However, if added in excess, the effect is saturated and the machinability and cold-workability deteriorate, so the upper limit is made 2.00%.
[0017]
P: 0.030% or less Since P segregates at austenite grain boundaries and lowers toughness such as impact strength and bending strength, the upper limit of the content is made 0.030% or less.
[0018]
S: 0.030% or less S impairs cold workability and forms non-metallic inclusions of MnS in steel to impair the toughness in the lateral direction, so the upper limit is made 0.030% or less. .
[0019]
Ti: 0.02 to 0.30%
Ti is an element that precipitates finely in the steel as TiC, thereby dispersing and strengthening the steel matrix, and delaying the generation and propagation of cracks that lead to fatigue fracture and pitting. 0.02% or more. However, if added in excess of 0.30%, machinability deteriorates, so the upper limit is made 0.30%.
[0020]
N: 0.0150% or less N is an element effective to combine with Al, Ti, Nb, V and the like to form nitrides and carbonitrides and to prevent crystal grains from being coarsened during induction hardening. However, when added in a large amount, a large hard primary nitride is formed with Ti, Nb, and V, thereby lowering the rolling fatigue properties, bending fatigue properties, and machinability. Therefore, the upper limit is set to 0.0150%.
[0021]
B: 0.0005 to 0.0050%
B is an element that significantly improves the hardenability of steel by adding a trace amount thereof, and significantly improves the strength and toughness after induction hardening and tempering by segregating at grain boundaries and suppressing grain boundary fracture. If it is less than 0.0005%, the effect is not sufficient, and if it exceeds 0.0050%, the hot workability is reduced. Therefore, the upper limit is made 0.0050%.
[0022]
Cr: 0.15 to 3.00%, Mo: 0.03 to 1.50%, Ni: 0.03 to 3.50%
Cr is an element that contributes to improvement of hardenability and also contributes to increase tempering softening resistance and improve pitting resistance. To obtain such an effect, Cr is made 0.15% or more. . However, even if it is added in excess of 3.00%, the effect of improving the pitting resistance is saturated and the machinability is rather deteriorated. Mo contributes to the improvement of the hardenability and also improves the toughness of the steel. Therefore, Mo can be optionally added to secure the hardenability and the toughness. If the content is less than 0.03%, such an effect cannot be obtained. If the content is more than 1.50%, it becomes difficult to soften and anneal the material, and the machinability and cold forgeability are deteriorated. The upper limit is set to 1.50% because the economy is impaired. Ni imparts a predetermined hardenability to the steel and improves the toughness of the steel. Ni can be arbitrarily added in order to secure the hardenability and the toughness. If the content is less than 0.03%, such an effect cannot be obtained. Even if the content exceeds 3.50%, the effect is saturated and the economy is impaired, so the upper limit is 3.50%. One or more of these may be added as needed.
[0023]
Nb: 0.02 to 0.15%, V: 0.02 to 0.15% or less Nb and V both form fine carbides or carbonitrides, and are used for the refinement of crystal grains during induction hardening. Since it is an element that contributes and is effective in increasing the toughness of the mechanical structural member, one or two of these elements are appropriately added as necessary to refine the austenite crystal grains. The effect cannot be obtained by addition. Also, if the added amount is too large, the fine effect is saturated and the mechanical properties may be rather deteriorated. Therefore, the upper limit is set to 0.15%.
[0024]
BEST MODE FOR CARRYING OUT THE INVENTION
The steel according to the embodiment of the present invention is shown in Table 1 and the following tables and examples. In Table 1, steel type A, steel type H, and steel type I are embodiments of the first aspect of the present invention. Steel type B and steel type C are embodiments of the invention of claim 2. Steel type D and steel type F are embodiments of the third aspect of the present invention. Steel type E and steel type G are an embodiment of the invention of claim 4. Steel types A to C are embodiments of the invention in which the amount of Si is 0.40 to 0.55%, and steel types D to I are embodiments of the invention in which the amount of Si is 0.55 to 1.50%. It is a form.
[0025]
The comparative steels are steel type J with JIS S53C, steel type K with JIS S53C with Ti and B added, steel type L with JIS S53C with B added, steel type M with JIS S53C with Ti added, steel type N is obtained by adding Si and B to JIS S53C.
[0026]
[Table 1]

[0027]
【Example】
After 100 kg of steel having the chemical composition shown in Table 1 was melted in a vacuum melting furnace, a steel bar having a diameter of 32 mm was manufactured by hot forging at 1200 ° C. Next, each material was heated to a temperature of 900 ° C. or higher, air-cooled, and subjected to normalizing treatment, and then test specimens for various tests were prepared. Each test piece was subjected to induction hardening treatment so that the ratio of the hardened layer (the distance from the surface to the point at which 500 HV was reached / the part radius) was 0.25 or more, then tempered at 150 ° C., and each test was performed. Was. For each property evaluation, for the pitting resistance, a roller pitting test using the roller pitting test piece shown in FIG. 1 was used, and for the impact strength, a 2 mm U notch was provided on a 10 mm square × 55 mm long test piece shown in FIG. The test was performed in a Charpy impact test using the obtained test piece.
[0028]
For the impact strength, a Charpy impact test was performed, and the value obtained by dividing the absorbed energy at that time by the sectional area of the notch bottom of 80 mm ² was evaluated as the Charpy impact value.
[0029]
Here, the principle of the roller pitching test is shown in FIG. That is, the roller pitching test piece 1 (small roller: materials are steel types A to N in Table 1) and the mating material 2 (large roller: JIS SCM420 equivalent) are rotated at a high speed under a surface pressure of 340 kgf / mm ^2, that is, 3334 MPa, The number of revolutions, which is the life until the occurrence of pitting, was determined, and the pitting resistance was evaluated. In this case, the difference in the peripheral speed between the roller pitching test piece 1 (small roller) and the mating material 2 (large roller), that is, the slip ratio is 40%. Then, after performing a test for each of the five roller pitching test pieces 1 under each condition, Weibull plotting was performed, and a 50% failure probability life ( _B50 life) was obtained to evaluate pitting resistance.
[0030]
Further, the hardness at a position of 80 μm from the surface of the cross section of the surface hardened layer was measured under a high temperature condition of 300 ° C. using the roller pitting test piece 1 to evaluate the tempering softening resistance. Table 2 shows the test results.
[0031]
[Table 2]

[0032]
Table 2 shows that the steel of the present invention is a steel excellent in both pitting resistance and impact strength compared to Comparative Example J (JIS S53C). In addition, by increasing the amount of Si to 0.40 to 1.50% with respect to the carbon steel level (0.15 to 0.35%) in JIS, the increase in high-temperature hardness, that is, the resistance to temper softening, was improved. You can see that. Further, from the results of Comparative Examples L, M and N, in order to greatly improve both the pitting resistance and the impact strength, 0.40 to 1.50% of Si is added, preferably 0.55 to 1.50%. And the addition of Ti and B are indispensable.
[0033]
【The invention's effect】
As described above, the present invention is characterized by significantly improving impact strength by adding B, dispersing fine TiC in steel by adding Ti, and being excellent in temper softening resistance by adding Si. is there. As a result, a medium carbon steel with excellent pitting resistance and impact strength, in which the addition of expensive alloying elements is suppressed, can be obtained. Demonstrates excellent effects.
[Brief description of the drawings]
FIG. 1 is a side view schematically showing a roller pitching test piece.
FIG. 2 is a side view schematically showing a Charpy impact test piece.
FIG. 3 is a perspective view showing the principle of a roller pitching test.
[Explanation of symbols]
1 Roller pitching test piece 2 Counterpart material

Claims (4)

In mass%, C: 0.40 to 0.80%, Si: 0.40 to 1.50%, Mn: 0.30 to 2.00%, P: 0.030% or less, S: 0.030 %, Ti: 0.02 to 0.30%, B: 0.0005 to 0.0050%, N: 0.015% or less, the balance being Fe and unavoidable impurities. A high-strength, medium-carbon steel characterized by significantly improving Ti, dispersing fine TiC in the steel by adding Ti, and having excellent tempering softening resistance by adding Si. In addition to the alloy elements according to claim 1, in mass%, Cr: 0.15 to 3.00%, Mo: 0.03 to 1.50%, Ni: 0.03 to 3.50% or less , Containing at least one element, with the balance being Fe and unavoidable impurities, the impact strength is greatly improved by adding B, and fine TiC is dispersed in the steel by adding Ti, and the tempering softening resistance is added by adding Si. High-strength medium-carbon steel characterized by excellent properties. In addition to the alloying elements according to claim 1, one or more of Nb: 0.02 to 0.15% or less and V: 0.02 to 0.15% by mass% are contained, and the balance is Fe. And high-strength steel, characterized by significantly improving impact strength by adding B, dispersing fine TiC in steel by adding Ti, and being excellent in temper softening resistance by adding Si. Carbon steel. In addition to the alloy elements according to claim 2, one or more of Nb: 0.02 to 0.15% or less and V: 0.02 to 0.15% by mass% are contained, and the balance is Fe. And high-strength steel, characterized by significantly improving impact strength by adding B, dispersing fine TiC in steel by adding Ti, and being excellent in temper softening resistance by adding Si. Carbon steel.

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