JP2018090883A - High temperature carburization steel, production method thereof, and carburized component - Google Patents
High temperature carburization steel, production method thereof, and carburized component Download PDFInfo
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 57
- 239000010959 steel Substances 0.000 title claims abstract description 57
- 238000004519 manufacturing process Methods 0.000 title claims description 11
- 229910001563 bainite Inorganic materials 0.000 claims abstract description 46
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 36
- 239000012535 impurity Substances 0.000 claims abstract description 8
- 229910052698 phosphorus Inorganic materials 0.000 claims abstract description 7
- 238000011282 treatment Methods 0.000 claims abstract description 6
- 229910001562 pearlite Inorganic materials 0.000 claims description 83
- 238000005255 carburizing Methods 0.000 claims description 76
- 238000010438 heat treatment Methods 0.000 claims description 30
- 230000007423 decrease Effects 0.000 claims description 8
- 238000010583 slow cooling Methods 0.000 claims description 5
- 230000002159 abnormal effect Effects 0.000 abstract description 8
- 229910052799 carbon Inorganic materials 0.000 abstract description 2
- 235000019362 perlite Nutrition 0.000 abstract 4
- 239000010451 perlite Substances 0.000 abstract 3
- 239000013078 crystal Substances 0.000 description 31
- 229910001566 austenite Inorganic materials 0.000 description 18
- 238000000034 method Methods 0.000 description 16
- 230000008569 process Effects 0.000 description 13
- 238000001816 cooling Methods 0.000 description 12
- 239000000463 material Substances 0.000 description 12
- 230000000052 comparative effect Effects 0.000 description 11
- 238000010606 normalization Methods 0.000 description 11
- 239000002245 particle Substances 0.000 description 11
- 230000003287 optical effect Effects 0.000 description 7
- 238000004088 simulation Methods 0.000 description 7
- 230000000694 effects Effects 0.000 description 5
- OXNIZHLAWKMVMX-UHFFFAOYSA-N picric acid Chemical compound OC1=C([N+]([O-])=O)C=C([N+]([O-])=O)C=C1[N+]([O-])=O OXNIZHLAWKMVMX-UHFFFAOYSA-N 0.000 description 4
- 230000001629 suppression Effects 0.000 description 4
- 238000007796 conventional method Methods 0.000 description 3
- 230000015572 biosynthetic process Effects 0.000 description 2
- 230000006866 deterioration Effects 0.000 description 2
- 239000000203 mixture Substances 0.000 description 2
- 229910052757 nitrogen Inorganic materials 0.000 description 2
- 239000011347 resin Substances 0.000 description 2
- 229920005989 resin Polymers 0.000 description 2
- 230000009466 transformation Effects 0.000 description 2
- 230000000007 visual effect Effects 0.000 description 2
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 description 1
- 230000009471 action Effects 0.000 description 1
- 239000007864 aqueous solution Substances 0.000 description 1
- 238000005452 bending Methods 0.000 description 1
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- 238000005520 cutting process Methods 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 230000006698 induction Effects 0.000 description 1
- 239000004615 ingredient Substances 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 238000002844 melting Methods 0.000 description 1
- 230000008018 melting Effects 0.000 description 1
- 230000008520 organization Effects 0.000 description 1
- 238000005498 polishing Methods 0.000 description 1
- 239000000243 solution Substances 0.000 description 1
- 239000002436 steel type Substances 0.000 description 1
- 238000005728 strengthening Methods 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
Abstract
Description
本発明は高温浸炭用鋼、その製造方法および浸炭部品に関する。 The present invention relates to a high-temperature carburizing steel, a manufacturing method thereof, and a carburized component.
高温での浸炭処理において発生する結晶粒の粗大化を防止するため、従来、いくつかの提案がなされている。
例えばＡｌＮ、ＮｂＣに代表されるピン止め粒子量を増やし、さらに微細に分散させて数を増やすことで、ピン止め力によって粗大化を防止する方法が挙げられる。具体的には、ピン止め元素を増量し、高温熱処理により固溶化させた後に、焼ならし温度程度の低温で熱処理をして微細分散させると粗大化温度が上昇する。これに関連する従来法として、例えば特許文献１，２に記載の方法が挙げられる。
また、例えば、浸炭前のミクロ組織も粗大化温度に影響することが知られており、フェライト＋パーライト組織では浸炭加熱途中のオーステナイト変態した時点で結晶粒が揃っており、また微細粒を抑制できるため粗大化しにくいこと、ベイナイトが多く存在する粗大化温度が低下することが知られている。さらにフェライトのサイズを制御して粗大化を抑制できる。これに関連する従来法として、例えば特許文献３，４に記載の方法が挙げられる。
Several proposals have heretofore been made in order to prevent coarsening of crystal grains generated in carburizing treatment at a high temperature.
For example, there is a method of increasing the amount of pinning particles typified by AlN and NbC and further increasing the number by finely dispersing the particles to prevent coarsening by the pinning force. Specifically, the amount of pinning elements is increased, and after solidifying by high-temperature heat treatment, the coarsening temperature rises when heat-treated at a low temperature of the normalizing temperature and finely dispersed. Examples of conventional methods related to this include the methods described in Patent Documents 1 and 2.
In addition, for example, it is known that the microstructure before carburizing also affects the coarsening temperature. In the ferrite + pearlite structure, crystal grains are aligned at the time of austenite transformation during carburizing heating, and fine grains can be suppressed. Therefore, it is known that the coarsening is difficult and the coarsening temperature at which a large amount of bainite is present is lowered. Furthermore, coarsening can be suppressed by controlling the size of the ferrite. Examples of conventional methods related to this include the methods described in Patent Documents 3 and 4, for example.
しかしながら、従来のピン止め粒子量を増やす方法では、前熱処理温度を高温化する必要がある。実操業では前熱処理の高温化によりピン止め粒子の残存や粗大化が依然として課題となっている。また、ピン止め粒子の過度な微細化は浸炭熱処理中にピン止め粒子が固溶化し、かえって粗大化を促進する。また、浸炭加熱中の結晶粒を微細化させてしまい、かえって異常粒成長の原因となる。このように、従来、高温浸炭時の結晶粒粗大化を安定的に抑制できていなかった。
また、従来の浸炭前の組織制御では、単純にフェライト＋パーライト組織に着目していたが、これでは高温浸炭時の粗大化を抑制できなかった。また、フェライトサイズの制御は焼入れ性、製造性を犠牲にした低Ｍｎまたは実操業の条件を著しく制約してでしか達成しえなかったため、汎用性が低かった。また、このような制約を設けて尚、高温浸炭時の結晶粒粗大化を安定的に抑制できていなかった。
However, in the conventional method of increasing the amount of pinning particles, it is necessary to increase the pre-heat treatment temperature. In actual operation, the remaining and coarsening of pinned particles remains a problem due to the high temperature of the pre-heat treatment. Moreover, excessive refinement of the pinning particles solidifies the pinning particles during the carburizing heat treatment, which promotes coarsening. In addition, the crystal grains during carburizing heating are made finer, which causes abnormal grain growth. Thus, conventionally, the coarsening of crystal grains during high-temperature carburization has not been stably suppressed.
Moreover, in the conventional structure control before carburizing, the ferrite + pearlite structure was simply focused on, but this could not suppress the coarsening during high-temperature carburizing. In addition, the control of the ferrite size could be achieved only by remarkably restricting the conditions of low Mn or actual operation at the expense of hardenability and manufacturability, so the versatility was low. In addition, with such restrictions, the coarsening of crystal grains during high-temperature carburization has not been stably suppressed.
本発明は上記のような課題を解決することを目的とする。
すなわち、本発明の目的は、高温での浸炭処理を施しても異常粒成長が抑制される高温浸炭用鋼およびその製造方法を提供することである。また、その高温浸炭用鋼を用いた浸炭部品を提供することである。
An object of the present invention is to solve the above problems.
That is, an object of the present invention is to provide a steel for high-temperature carburizing, in which abnormal grain growth is suppressed even when carburizing at a high temperature is performed, and a manufacturing method thereof. Moreover, it is providing the carburized component using the steel for high temperature carburizing.
本発明者は上記課題を解決するため鋭意検討し、本発明を完成させた。
本発明は以下の（１）〜（６）である。
（１）Ｃ：０．１〜０．３質量％、Ｓｉ：０．０５〜１．０質量％、Ｍｎ：０．３〜２．０質量％、Ｐ：０．０３質量％以下、Ｓ：０．０３質量％以下、Ｃｒ：０．３〜１．５質量％、Ａｌ：０．０２〜０．０５質量％、残部がＦｅおよび不可避的不純物からなり、
フェライト面積率が４０％以上であり、
フェライトで区切られたパーライト単位を個別パーライトと定義したとき、下記式（Ｉ）を満たす個別パーライトの面積を合計して求めた面積率が３０％以上であり、かつ、個別パーライトの円相当径が２５０μｍ未満である、高温浸炭用鋼。
式（Ｉ）： Ｓ1≧（３×１０8）／Ｖ1 2
Ｓ1は各個別パーライトの面積（μｍ2）を意味する。
Ｖ1は３×（Ａｌ含有率（ｐｐｍ）×Ｎ含有率（ｐｐｍ）−３０００）1/2とする。
Ｖ1は、Ｎｂ含有率が３０ｐｐｍ超の場合、３×（Ａｌ含有率（ｐｐｍ）×Ｎ含有率（ｐｐｍ）−３０００）1/2＋（Ｎｂ含有率（ｐｐｍ）−３０）とし、Ｎｂ含有率が３０ｐｐｍ以下の場合、３×（Ａｌ含有率（ｐｐｍ）×Ｎ含有率（ｐｐｍ）−３０００）1/2とする。
（２）Ｃ：０．１〜０．３質量％、Ｓｉ：０．０５〜１．０質量％、Ｍｎ：０．３〜２．０質量％、Ｐ：０．０３質量％以下、Ｓ：０．０３質量％以下、Ｃｒ：０．３〜１．５質量％、Ａｌ：０．０２〜０．０５質量％、残部がＦｅおよび不可避的不純物からなり、
フェライト面積率が４０〜７０％、ベイナイト面積率が１５％以下であり、
フェライトで区切られたパーライト＋ベイナイト単位を個別パーライト＋ベイナイトと定義したとき、下記式（ＩＩ）を満たす個別パーライト＋ベイナイトの面積を合計して求めた面積率が３０％以上であり、かつ、個別パーライト＋ベイナイトの円相当径が２５０μｍ未満である、高温浸炭用鋼。
式（ＩＩ）： Ｓ2≧（３×１０8）／Ｖ2 2
Ｓ2は各個別パーライト＋ベイナイトの面積（μｍ2）を意味する。
Ｖ2は、Ｎｂ含有率が３０ｐｐｍ超の場合、３×（Ａｌ含有率（ｐｐｍ）×Ｎ含有率（ｐｐｍ）−３０００）1/2＋（Ｎｂ含有率（ｐｐｍ）−３０）とし、Ｎｂ含有率が３０ｐｐｍ以下の場合、３×（Ａｌ含有率（ｐｐｍ）×Ｎ含有率（ｐｐｍ）−３０００）1/2とする。
（３）さらにＮｂ：０．１質量％以下、Ｍｏ：０．５質量％以下のいずれか１種または２種を含むことを特徴とする、上記（１）または（２）に記載の高温浸炭用鋼。
（４）浸炭前熱処理として８５０〜１０００℃で焼ならしをし、その後の降温過程において、
８００℃から６８０℃までの保持時間を９０分以内とし、
８００℃から７３０℃までの保持時間を３０分以上とし、
７３０℃から６８０℃までを１０℃／分以下の降温速度で徐冷し、
上記（１）〜（３）のいずれかに記載の高温浸炭用鋼が得られる、高温浸炭用鋼の製造方法。
（５）浸炭前熱処理として８５０〜１０００℃で焼ならしをし、その後の降温過程において、
８００℃から６８０℃までの保持時間を９０分以内とし、
８００℃から７３０℃までを２℃／分以下の降温速度で徐冷し、
７３０℃から６８０℃までを１０℃／分以下の降温速度で徐冷し、
上記（１）〜（３）のいずれかに記載の高温浸炭用鋼が得られる、高温浸炭用鋼の製造方法。
（６）加熱途中の７３０〜８３０℃までの間において５分以上かけて徐加熱するか、または５分以上保持することを特徴とする浸炭処理を上記（１）〜（３）のいずれかに記載の高温浸炭用鋼に施して浸炭部品を得る、浸炭部品の製造方法。
The inventor has intensively studied to solve the above-mentioned problems, and has completed the present invention.
The present invention includes the following (1) to (6).
(1) C: 0.1 to 0.3 mass%, Si: 0.05 to 1.0 mass%, Mn: 0.3 to 2.0 mass%, P: 0.03 mass% or less, S: 0.03% by mass or less, Cr: 0.3 to 1.5% by mass, Al: 0.02 to 0.05% by mass, the balance consisting of Fe and inevitable impurities,
The ferrite area ratio is 40% or more,
When the pearlite unit delimited by ferrite is defined as individual pearlite, the area ratio obtained by summing the areas of the individual pearlite satisfying the following formula (I) is 30% or more, and the circle equivalent diameter of the individual pearlite is Steel for high-temperature carburization that is less than 250 μm.
Formula (I): S 1 ≧ (3 × 10 8 ) / V 1 2
S 1 means the area (μm 2 ) of each individual pearlite.
V 1 is 3 × (Al content (ppm) × N content (ppm) −3000) 1/2 .
When the Nb content is more than 30 ppm, V 1 is 3 × (Al content (ppm) × N content (ppm) −3000) 1/2 + (Nb content (ppm) −30). When the rate is 30 ppm or less, 3 × (Al content (ppm) × N content (ppm) −3000) 1/2 is set.
(2) C: 0.1 to 0.3 mass%, Si: 0.05 to 1.0 mass%, Mn: 0.3 to 2.0 mass%, P: 0.03 mass% or less, S: 0.03% by mass or less, Cr: 0.3 to 1.5% by mass, Al: 0.02 to 0.05% by mass, the balance consisting of Fe and inevitable impurities,
The ferrite area ratio is 40 to 70%, the bainite area ratio is 15% or less,
When the unit of pearlite + bainite separated by ferrite is defined as individual pearlite + bainite, the area ratio obtained by summing the areas of individual pearlite + bainite satisfying the following formula (II) is 30% or more, and individual Steel for high-temperature carburization, in which the equivalent circle diameter of pearlite + bainite is less than 250 μm.
Formula (II): S 2 ≧ (3 × 10 8 ) / V 2 2
S 2 means the area of each individual pearlite + bainite (μm 2 ).
V 2 is 3 × (Al content (ppm) × N content (ppm) −3000) 1/2 + (Nb content (ppm) −30) when Nb content exceeds 30 ppm, When the rate is 30 ppm or less, 3 × (Al content (ppm) × N content (ppm) −3000) 1/2 is set.
(3) The high-temperature carburizing according to (1) or (2) above, further comprising any one or two of Nb: 0.1% by mass or less and Mo: 0.5% by mass or less. Steel.
(4) Normalizing at 850 to 1000 ° C. as a heat treatment before carburizing,
The holding time from 800 ° C. to 680 ° C. is within 90 minutes,
The holding time from 800 ° C to 730 ° C is 30 minutes or more,
Slowly cool from 730 ° C to 680 ° C at a rate of temperature decrease of 10 ° C / min or less,
The manufacturing method of the steel for high temperature carburizing in which the steel for high temperature carburizing in any one of said (1)-(3) is obtained.
(5) Normalizing at 850 to 1000 ° C. as a heat treatment before carburizing,
The holding time from 800 ° C. to 680 ° C. is within 90 minutes,
Slow cooling from 800 ° C to 730 ° C at a rate of temperature decrease of 2 ° C / min or less,
Slowly cool from 730 ° C to 680 ° C at a rate of temperature decrease of 10 ° C / min or less,
The manufacturing method of the steel for high temperature carburizing in which the steel for high temperature carburizing in any one of said (1)-(3) is obtained.
(6) The carburizing treatment characterized by heating gradually over 5 minutes or more in the middle of heating to 730 to 830 ° C. or holding for 5 minutes or more in any of the above (1) to (3) A method for producing a carburized part, which is applied to the steel for high-temperature carburizing described above to obtain a carburized part.
本発明によれば、高温での浸炭処理を施しても異常粒成長が抑制される高温浸炭用鋼およびその製造方法を提供することができる。また、その高温浸炭用鋼を用いた浸炭部品を提供することができる。 ADVANTAGE OF THE INVENTION According to this invention, the steel for high temperature carburizing in which abnormal grain growth is suppressed even if it performs the carburizing process at high temperature, and its manufacturing method can be provided. Moreover, the carburized component using the steel for high temperature carburizing can be provided.
＜本発明の高温浸炭用鋼＞
本発明の高温浸炭用鋼について説明する。本発明の高温浸炭用鋼は、次のような２つの態様を含む。
<High-temperature carburizing steel of the present invention>
The steel for high-temperature carburizing of the present invention will be described. The high-temperature carburizing steel of the present invention includes the following two aspects.
本発明の高温浸炭用鋼の第１の態様は、Ｃ：０．１〜０．３質量％、Ｓｉ：０．０５〜１．０質量％、Ｍｎ：０．３〜２．０質量％、Ｐ：０．０３質量％以下、Ｓ：０．０３質量％以下、Ｃｒ：０．３〜１．５質量％、Ａｌ：０．０２〜０．０５質量％、残部がＦｅおよび不可避的不純物からなり、フェライト面積率が４０％以上であり、フェライトで区切られたパーライト単位を個別パーライトと定義したとき、下記式（Ｉ）を満たす個別パーライトの合計の面積率が３０％以上であり、かつ、個別パーライトの円相当径が２５０μｍ未満である、高温浸炭用鋼である。
式（Ｉ）： Ｓ1≧（３×１０8）／Ｖ1 2
ここで、Ｓ1は各個別パーライトの面積（μｍ2）を意味する。
また、Ｖ1は、Ｎｂ含有率が３０ｐｐｍ超の場合、３×（Ａｌ含有率（ｐｐｍ）×Ｎ含有率（ｐｐｍ）−３０００）1/2＋（Ｎｂ含有率（ｐｐｍ）−３０）とし、Ｎｂ含有率が３０ｐｐｍ以下の場合、３×（Ａｌ含有率（ｐｐｍ）×Ｎ含有率（ｐｐｍ）−３０００）1/2とする。
このような高温浸炭用鋼を、以下では「本発明の高温浸炭用鋼１」または「態様１」ともいう。
1st aspect of the steel for high temperature carburizing of this invention is C: 0.1-0.3 mass%, Si: 0.05-1.0 mass%, Mn: 0.3-2.0 mass%, P: 0.03% by mass or less, S: 0.03% by mass or less, Cr: 0.3-1.5% by mass, Al: 0.02-0.05% by mass, the balance from Fe and inevitable impurities When the ferrite area ratio is 40% or more, and the pearlite unit delimited by ferrite is defined as individual pearlite, the total area ratio of the individual pearlite satisfying the following formula (I) is 30% or more, and This is a steel for high-temperature carburizing, in which the circle-equivalent diameter of individual pearlite is less than 250 μm.
Formula (I): S 1 ≧ (3 × 10 8 ) / V 1 2
Here, S 1 means the area (μm 2 ) of each individual pearlite.
V 1 is 3 × (Al content (ppm) × N content (ppm) −3000) 1/2 + (Nb content (ppm) −30) when the Nb content exceeds 30 ppm. When the Nb content is 30 ppm or less, 3 × (Al content (ppm) × N content (ppm) −3000) 1/2 is used.
Hereinafter, such high-temperature carburizing steel is also referred to as “high-temperature carburizing steel 1 of the present invention” or “mode 1”.
本発明の高温浸炭用鋼の第２の態様は、Ｃ：０．１〜０．３質量％、Ｓｉ：０．０５〜１．０質量％、Ｍｎ：０．３〜２．０質量％、Ｐ：０．０３質量％以下、Ｓ：０．０３質量％以下、Ｃｒ：０．３〜１．５質量％、Ａｌ：０．０２〜０．０５質量％、残部がＦｅおよび不可避的不純物からなり、フェライト面積率が４０〜７０％、ベイナイト面積率が１５％以下であり、フェライトで区切られたパーライト＋ベイナイト単位を個別パーライト＋ベイナイトと定義したとき、下記式（ＩＩ）を満たす個別パーライト＋ベイナイトの合計の面積率が３０％以上であり、かつ、個別パーライト＋ベイナイトの円相当径が２５０μｍ未満である、高温浸炭用鋼である。
式（ＩＩ）： Ｓ2≧（３×１０8）／Ｖ2 2
ここで、Ｓ2は各個別パーライト＋ベイナイトの面積（μｍ2）を意味する。
また、Ｖ2は、Ｎｂ含有率が３０ｐｐｍ超の場合、３×（Ａｌ含有率（ｐｐｍ）×Ｎ含有率（ｐｐｍ）−３０００）1/2＋（Ｎｂ含有率（ｐｐｍ）−３０）とし、Ｎｂ含有率が３０ｐｐｍ以下の場合、３×（Ａｌ含有率（ｐｐｍ）×Ｎ含有率（ｐｐｍ）−３０００）1/2とする。
このような高温浸炭用鋼を、以下では「本発明の高温浸炭用鋼２」または「態様２」ともいう。
The second aspect of the high-temperature carburizing steel of the present invention is C: 0.1-0.3% by mass, Si: 0.05-1.0% by mass, Mn: 0.3-2.0% by mass, P: 0.03% by mass or less, S: 0.03% by mass or less, Cr: 0.3-1.5% by mass, Al: 0.02-0.05% by mass, the balance from Fe and inevitable impurities When the ferrite area ratio is 40 to 70%, the bainite area ratio is 15% or less, and the pearlite + bainite unit separated by ferrite is defined as individual pearlite + bainite, individual pearlite satisfying the following formula (II) + It is a steel for high-temperature carburization, in which the total area ratio of bainite is 30% or more and the equivalent pearlite + bainite equivalent circle diameter is less than 250 μm.
Formula (II): S 2 ≧ (3 × 10 8 ) / V 2 2
Here, S 2 means the area of each individual pearlite + bainite (μm 2 ).
V 2 is 3 × (Al content (ppm) × N content (ppm) −3000) 1/2 + (Nb content (ppm) −30) when the Nb content exceeds 30 ppm. When the Nb content is 30 ppm or less, 3 × (Al content (ppm) × N content (ppm) −3000) 1/2 is used.
Hereinafter, such high-temperature carburizing steel is also referred to as “high-temperature carburizing steel 2 of the present invention” or “mode 2”.
以下において単に「本発明の高温浸炭用鋼」と記した場合、本発明の高温浸炭用鋼１および本発明の高温浸炭用鋼２のいずれをも意味しているものとする。 Hereinafter, when simply described as “the high-temperature carburizing steel of the present invention”, it means both the high-temperature carburizing steel 1 of the present invention and the high-temperature carburizing steel 2 of the present invention.
このような本発明の高温浸炭用鋼は、サイズが限定されたパーライトまたはパーライト＋ベイナイトがフェライトで囲まれているため（すなわち、上限サイズが限定された、個別パーライトまたは個別パーライト＋ベイナイトが一定比率以上で存在しているため）、浸炭加熱過程においてパーライトがオーステナイトに変態した後、フェライトは残存し一定以上の粒度にはならず、フェライト近傍のオーステナイトが揃って成長し、浸炭加熱温度（例えば８３０℃）に到達時点の前には、微細オーステナイト結晶粒が消失し、結晶粒が揃うので、異常粒成長は抑制される。 In such a high-temperature carburizing steel of the present invention, pearlite or pearlite + bainite having a limited size is surrounded by ferrite (that is, a specific ratio of individual pearlite or individual pearlite + bainite having a limited upper limit size). Therefore, after the pearlite is transformed into austenite during the carburizing heating process, the ferrite remains and does not have a particle size of a certain level or more, and austenite near the ferrite grows together, and the carburizing heating temperature (for example, 830). Prior to reaching the time point (° C.), the fine austenite crystal grains disappear and the crystal grains are aligned, so that abnormal grain growth is suppressed.
＜成分＞
本発明の高温浸炭用鋼の成分について説明する。
<Ingredients>
The components of the high-temperature carburizing steel of the present invention will be described.
Ｃは鉄鋼の強化に欠かせない元素であり、芯部強度確保、しかし過度の添加は加工性を低下させる。したがって、本発明の高温浸炭用鋼におけるＣ含有率は０．１〜０．３質量％であり、０．１５〜０．２５質量％であることが好ましい。 C is an element indispensable for strengthening steel, and the core strength is ensured, but excessive addition reduces workability. Therefore, the C content in the high-temperature carburizing steel of the present invention is 0.1 to 0.3% by mass, and preferably 0.15 to 0.25% by mass.
Ｓｉは焼入れ性高め、脱酸元素として有効であるが、過度の添加は加工性の悪化を招く。したがって、本発明の高温浸炭用鋼におけるＳｉ含有率は０．０５〜１．０質量％であり、０．１〜０．５質量％であることが好ましい。 Si enhances hardenability and is effective as a deoxidizing element, but excessive addition causes deterioration of workability. Therefore, the Si content in the high-temperature carburizing steel of the present invention is 0.05 to 1.0% by mass, and preferably 0.1 to 0.5% by mass.
Ｍｎは焼入れ性を向上させ強度を増すが、過度の添加は加工性の悪化を招く。したがって、本発明の高温浸炭用鋼におけるＭｎ含有率は０．３〜２．０質量％であり、０．５〜１．５質量％であることが好ましい。 Mn improves hardenability and increases strength, but excessive addition causes deterioration of workability. Therefore, the Mn content in the high-temperature carburizing steel of the present invention is 0.3 to 2.0 mass%, and preferably 0.5 to 1.5 mass%.
Ｐは粒界脆化の促進するため抑制することが望ましいが、過度の抑制は工程の延長によりコストの増大の要因となる。したがって、本発明の高温浸炭用鋼におけるＰ含有率は０．０３質量％以下である。 P is desirably suppressed to promote grain boundary embrittlement, but excessive suppression causes an increase in cost due to the extension of the process. Therefore, the P content in the high-temperature carburizing steel of the present invention is 0.03% by mass or less.
ＳはＭｎと結合しＭｎＳを形成し得るが、ＭｎＳの量が増大すると疲労強度低下するため抑制することが望ましい。一方、過度の抑制は工程の廷長によりコストの増大の要因となる。したがって、本発明の高温浸炭用鋼におけるＳ含有率は０．０３質量％以下である。 S can be bonded to Mn to form MnS, but it is desirable to suppress because the fatigue strength decreases as the amount of MnS increases. On the other hand, excessive suppression causes an increase in cost due to the process head. Accordingly, the S content in the high-temperature carburizing steel of the present invention is 0.03% by mass or less.
Ｃｒは焼入れ性を向上させる重要元素であり、その効果は０．３質量％以上で顕著である。しかし過度の添加により被削性を悪化させるため、１．５質量％以下とする。したがって、本発明の高温浸炭用鋼におけるＣｒ含有率は０．３〜１．５質量％である。 Cr is an important element for improving hardenability, and the effect is remarkable at 0.3% by mass or more. However, in order to deteriorate machinability by excessive addition, it is made 1.5 mass% or less. Therefore, the Cr content in the high-temperature carburizing steel of the present invention is 0.3 to 1.5 mass%.
Ａｌは脱酸作用があり、また焼ならし工程でＮと結合し、微細なＡｌＮを多量に形成し、結晶粒をピンニングさせることで結晶粒の粗大化を抑制する重要元素である。しかし、過剰の添加は途中工程でＡｌＮを残存させ、周りのＡｌおよびＮを引き寄せることで巨大化し、焼ならし工程でのＡｌＮ形成をかえって妨げるため結晶粒粗大化抑制効果が低下する。また曲げ疲労強度が低下する。したがって、本発明の高温浸炭用鋼におけるＡｌ含有率は０．０２〜０．０５質量％である。 Al has a deoxidizing action, and is an important element that combines with N in the normalizing process to form a large amount of fine AlN and pin the crystal grains to suppress coarsening of the crystal grains. However, excessive addition causes AlN to remain in the middle of the process, enlarging the surrounding Al and N by drawing them around, and prevents the formation of AlN in the normalizing process. Also, bending fatigue strength decreases. Therefore, the Al content in the high-temperature carburizing steel of the present invention is 0.02 to 0.05 mass%.
ＮはＡｌと結合し、ＡｌＮを形成する重要元素である。ＡｌＮ量はＡｌとＮの添加量の積に関係する。 N is an important element that combines with Al to form AlN. The amount of AlN is related to the product of the added amounts of Al and N.
Ｎｂは焼ならし工程で微細なＮｂＣを形成し、ＡｌＮの結晶粒粗大化抑制を補助するが、過度の添加は鋼材のコストを上げ、また被削性を低下させる。そのため微量で効果が発揮される０．１質量％以下ならば添加してもよい。 Nb forms fine NbC in the normalizing process and assists the suppression of AlN crystal grain coarsening, but excessive addition increases the cost of the steel material and reduces the machinability. Therefore, it may be added as long as it is 0.1% by mass or less so that the effect is exhibited in a small amount.
Ｍｏは焼入れ性および耐摩耗性を向上させるが高価である。また、多量添加によってベイナイト組織が現れる。したがって、本発明の高温浸炭用鋼におけるＭｏ含有率は０．５質量％以下であること好ましく、０．３質量％以下であることがより好ましい。 Mo improves hardenability and wear resistance, but is expensive. Moreover, a bainite structure appears by adding a large amount. Therefore, the Mo content in the high-temperature carburizing steel of the present invention is preferably 0.5% by mass or less, and more preferably 0.3% by mass or less.
本発明の高温浸炭用鋼は、上記のように、Ｃ、Ｓｉ、Ｍｎ、Ｐ、Ｓ、ＣｒおよびＡｌを特定範囲の含有率で含有する。また、Ｎｂおよび／またはＭｏを、上記のような特定範囲の含有率でさらに含有してもよい。そして、残部はＦｅおよび不可避的不純物である。 As described above, the high-temperature carburizing steel of the present invention contains C, Si, Mn, P, S, Cr, and Al in a specific range. Moreover, you may further contain Nb and / or Mo with the content rate of the specific range as mentioned above. The balance is Fe and inevitable impurities.
＜組織＞
本発明の高温浸炭用鋼１（態様１）について光学顕微鏡を用いて１００倍で観察すると、フェライト面積率が４０％以上である。
また、光学顕微鏡を用いて１００倍で観察するとフェライトで区切られたパーライト単位が観察され、これを「個別パーライト」と定義する。別の言い方をすれば、フェライトで囲まれたパーライトの部分を、１つの「個別パーライト」とする。そして、この個別パーライトの面積をＳ1（μｍ2）とすると、前述の式（Ｉ）を満たす個別パーライトが多く存在する。そして、それらの式（Ｉ）を満たす個別パーライトの面積を合計し、面積率（視野内に占める面積率）を計測すると、３０％以上となる。さらに、個別パーライトの円相当径は２５０μｍ未満である。
<Organization>
When the steel for high-temperature carburization 1 (embodiment 1) of the present invention is observed at 100 times using an optical microscope, the ferrite area ratio is 40% or more.
Further, when observed with an optical microscope at a magnification of 100, pearlite units separated by ferrite are observed, and this is defined as “individual pearlite”. In other words, a part of pearlite surrounded by ferrite is defined as one “individual pearlite”. When the area of this individual pearlite is S 1 (μm 2 ), there are many individual pearlites that satisfy the above-mentioned formula (I). And when the area of the individual pearlite which satisfy | fills those formula (I) is totaled and an area rate (area ratio occupied in a visual field) is measured, it will be 30% or more. Furthermore, the circle-equivalent diameter of the individual pearlite is less than 250 μm.
本発明の高温浸炭用鋼２（態様２）について光学顕微鏡を用いて１００倍で観察すると、フェライト面積率が４０〜７０％以上、ベイナイト面積率が１５％以下である。
また、光学顕微鏡を用いて１００倍で観察するとフェライトで区切られたパーライト＋ベイナイト単位が観察され、これを「個別パーライト＋ベイナイト」と定義する。別の言い方をすれば、フェライトで囲まれたパーライト＋ベイナイトの部分（パーライトとベイナイトとの集合体の部分）を、１つの「個別パーライト＋ベイナイト」とする。そして、この個別パーライト＋ベイナイトの面積をＳ2（μｍ2）とすると、前述の式（ＩＩ）を満たす個別パーライト＋ベイナイトが多く存在する。そして、それらの式（ＩＩ）を満たす個別パーライト＋ベイナイトの面積を合計し、面積率（視野内に占める面積率）を計測すると、３０％以上となる。さらに、個別パーライト＋ベイナイトの円相当径は２５０μｍ未満である。
When the steel for high-temperature carburizing 2 of the present invention (Aspect 2) is observed 100 times using an optical microscope, the ferrite area ratio is 40 to 70% or more and the bainite area ratio is 15% or less.
Further, when observed with an optical microscope at a magnification of 100, a pearlite + bainite unit separated by ferrite is observed, which is defined as “individual pearlite + bainite”. In other words, the part of pearlite + bainite surrounded by ferrite (the part of the aggregate of pearlite and bainite) is defined as one “individual pearlite + bainite”. When the area of this individual pearlite + bainite is S 2 (μm 2 ), there are many individual pearlites + bainite satisfying the above-mentioned formula (II). Then, when the areas of individual pearlite + bainite satisfying the formula (II) are totaled and the area ratio (area ratio occupied in the field of view) is measured, it becomes 30% or more. Furthermore, the circle equivalent diameter of individual pearlite + bainite is less than 250 μm.
浸炭前の組織構成でフェライトで分断された個別のパーライト（またはパーライト＋ベイナイト）のサイズが次工程の浸炭加熱途中にオーステナイト変態した時のオーステナイト結晶粒径を決める。微細オーステナイト結晶粒はその周りのオーステナイト結晶粒に食われ周りの結晶粒粗大の要因となる。そのため粗大パーライトは粗大オーステナイトの原因となり異常粒成長を引き起こす。粗大パーライトはその面積を円相当径で示したときに２５０μｍ未満であり、２００μｍ未満であることが好ましい。 The size of individual pearlite (or pearlite + bainite) divided by ferrite in the structure before carburization determines the austenite crystal grain size when austenite transformation occurs during carburizing heating in the next step. The fine austenite crystal grains are eaten by the surrounding austenite crystal grains and cause the surrounding crystal grains to be coarse. Therefore, coarse pearlite causes coarse austenite and causes abnormal grain growth. Coarse pearlite is less than 250 μm, and preferably less than 200 μm, when the area is indicated by the equivalent circle diameter.
―方、微細パーライトからは微細オーステナイト結晶粒が形成するため、隣の粗大オーステナイトの異常粒成長を助長する。よって、微細パーライトは、一定量以下に制限する必要がある。この時のパーライトの許容サイズは焼ならし時のピン止め粒子ＡｌＮおよびＮｂＣの量が関係し、ピン止め粒子量を前述のＶ1またはＶ2のパラメータで示したときに、光学顕微鏡による観察の結果、個別パーライトの面積（Ｓ1）または個別パーライト＋ベイナイトの面積（Ｓ2）が、（３×１０8）／Ｖ1 2以上または（３×１０8）／Ｖ2 2以上のサイズであり、それらのパーライトサイズ（Ｓ1）または許容パーライト＋ベイナイトサイズ（Ｓ2）が面積率で３０％以上であると異常粒成長の抑制をする。
フェライトはパーライトを分断し、パーライトサイズを小さくすることで、浸炭加熱途中でのオーステナイト結晶粒径が粗大になるのを防ぐため、面積率にして４０％以上であることが望ましい。
ベイナイトは微細オーステナイト結晶粒の原因となるため抑制することが望ましいが、過度の抑制は鋼種・工程の制限を招く。少量ならベイナイト由来の微細オーステナイトはすぐさま周りのパーライト由来の粗大オーステナイト結晶粒により消滅するため、面積率にして１５％程度のベイナイトは許容される。好ましくは０％である。
-On the other hand, since fine austenite crystal grains are formed from fine pearlite, it promotes abnormal grain growth of adjacent coarse austenite. Therefore, it is necessary to limit the fine pearlite to a certain amount or less. The allowable size of pearlite at this time is related to the amount of pinning particles AlN and NbC during normalization. When the amount of pinning particles is indicated by the parameter V 1 or V 2 described above, observation with an optical microscope is possible. As a result, the area of individual pearlite (S 1 ) or the area of individual pearlite + bainite (S 2 ) is a size of (3 × 10 8 ) / V 1 2 or more or (3 × 10 8 ) / V 2 2 or more. When the pearlite size (S 1 ) or the permissible pearlite + bainite size (S 2 ) is 30% or more in terms of area ratio, abnormal grain growth is suppressed.
Ferrite divides pearlite and reduces the pearlite size to prevent the austenite crystal grain size from becoming coarse during carburizing heating, so that the area ratio is preferably 40% or more.
Since bainite causes fine austenite crystal grains, it is desirable to suppress bainite, but excessive suppression causes a restriction on the steel type and process. If the amount is small, the fine austenite derived from bainite is immediately disappeared by the coarse austenite crystal grains derived from the surrounding pearlite, so that an area ratio of about 15% bainite is allowed. Preferably it is 0%.
＜本発明の高温浸炭用鋼の製造方法＞
以上のような本発明の高温浸炭用鋼を得るためには、浸炭前熱処理として８５０〜１０００℃で焼ならしをし、その後の降温過程において、８００℃から６８０℃までの保持時間を９０分以内（好ましくは６０分以内）とし、８００℃から７３０℃までの保持時間を３０分以上とし、７３０℃から６８０℃までを１０℃／分以下の降温速度で徐冷する。
または、浸炭前熱処理として８５０〜１０００℃で焼ならしをし、その後の降温過程において、８００℃から６８０℃までの保持時間を９０分以内（好ましくは６０分以内）とし、８００℃から７３０℃までを２℃／分以下の降温速度で徐冷し、７３０℃から６８０℃までを１０℃／分以下の降温速度で徐冷する。
これにより焼ならし温度でオーステナイト結晶粒が揃った状態から冷却時にはその揃ったオーステナイト結晶粒の粒界にフェライトが生成・成長し、一方、粒内フェライトの生成を防ぐことができるため、その後生成するパーライトはフェライトに分断されることなく、個別パーライトサイズが揃った組織となる。
<The manufacturing method of the steel for high-temperature carburizing of this invention>
In order to obtain the steel for high-temperature carburizing of the present invention as described above, normalizing is performed at 850 to 1000 ° C. as a heat treatment before carburizing, and the holding time from 800 ° C. to 680 ° C. is 90 minutes in the subsequent temperature lowering process. (Preferably within 60 minutes), the holding time from 800 ° C. to 730 ° C. is set to 30 minutes or more, and the temperature is gradually cooled from 730 ° C. to 680 ° C. at a temperature decreasing rate of 10 ° C./min or less.
Alternatively, normalizing is performed at 850 to 1000 ° C. as a heat treatment before carburizing, and the holding time from 800 ° C. to 680 ° C. is within 90 minutes (preferably within 60 minutes), and 800 ° C. to 730 ° C. Is gradually cooled at a temperature lowering rate of 2 ° C./min or less, and is gradually cooled from 730 ° C. to 680 ° C. at a temperature lowering rate of 10 ° C./min or less.
As a result, ferrite forms and grows at the grain boundary of the aligned austenite grains at the normalizing temperature from the state in which the austenite grains are aligned, and on the other hand, it can prevent the formation of intragranular ferrite, so it is generated afterwards. The pearlite to be made has a structure with individual pearlite sizes without being divided into ferrite.
上記のような本発明の高温浸炭用鋼を浸炭して浸炭部品を製造することができる。
ここで本発明の高温浸炭用鋼を浸炭するときの加熱途中の７３０〜８３０℃までの間において５分以上かけて徐加熱するか、または５分以上保持すると、結晶粒粗大化防止効果がさらに向上する。
Carburized parts can be manufactured by carburizing the high-temperature carburizing steel of the present invention as described above.
Here, when the steel for high-temperature carburizing of the present invention is carburized at a temperature of 730 to 830 ° C. during the heating, gradually heating for 5 minutes or more, or holding for 5 minutes or more, the effect of preventing grain coarsening is further increased. improves.
真空高周波誘導溶解炉を用いて、第１表に示す組成を備える１１種類の円柱状のインゴット（断面直径：１００ｍｍ、５０ｋｇ）を作製した。そして、インゴットを１２５０℃で２時間加熱した後に、断面直径が３０ｍｍとなるまで鍛造し、得られた棒材を９５０℃で焼きならした。
その後、小型炉に移して各種熱処理および制御冷却を実施した。熱処理および冷却条件を第２表および第３表に示す。
Using a vacuum high-frequency induction melting furnace, 11 types of cylindrical ingots (cross-sectional diameter: 100 mm, 50 kg) having the compositions shown in Table 1 were produced. And after heating the ingot at 1250 degreeC for 2 hours, it forged until the cross-sectional diameter became 30 mm, and obtained bar material was normalized at 950 degreeC.
Then, it moved to the small furnace and implemented various heat processing and controlled cooling. The heat treatment and cooling conditions are shown in Tables 2 and 3.
次に、各種熱処理および制御冷却を実施した棒材から、断面直径８ｍｍ、長さ１２ｍｍの円柱状試料を切り出し、浸炭を模擬した処理を施した。具体的には、円柱状試料の側面であって、長さ方向の中心部に、加工フォーマスタに繋がる熱電対を接合し、１０５０℃まで９０℃／ｍｉｎで加熱し、その温度にて１時間保持した後、Ｈｅガスで冷却して、結晶粒粗大化判定用の試料とした。ここで、加熱温度を１０５０℃の他、１１００℃とし、その他を同様とした処理も行って、別の結晶粒粗大化判定用の試料も得た。さらに、加熱温度を１０５０℃または１１００℃とし、９０℃／ｍｉｎでの加熱の途中である８００℃において１０分間保持し、その後、再度、９０℃／ｍｉｎで加熱する徐加熱状態を模擬する処理も行って、さらに別の結晶粒粗大化判定用の試料も得た。すなわち、４種類の結晶粒粗大化判定用の試料も得た。
次に、各試料を長さ方向に切断し、切断面を研磨できるように樹脂に埋め込み、研磨した後、研磨面をピクリン酸（ピクリン酸１０ｇを水５００ｍｌに加えた水溶液）を用いて腐食して、研磨面に結晶粒を現出させた。そして、試料の長さ方向の中心から３ｍｍ以内かつ半径方向の中心から１．５〜２．５ｍｍの位置（１／２Ｒ）において、ＪＩＳＧ０５５１に従った光学顕微鏡（１００倍）を用いた粒度測定を行った。そして、５視野の全てにおいて、粒度が５番以下のオーステナイトが存在しない場合に、粗大化を抑制できたと判断した。第２表では、粗大化を抑制できたものを「○」、できなかったものを「×」と記した。
Next, a columnar sample having a cross-sectional diameter of 8 mm and a length of 12 mm was cut out from the bar material subjected to various heat treatments and controlled cooling, and subjected to a process simulating carburization. Specifically, a thermocouple connected to the processing formaster is joined to the side surface of the cylindrical sample at the center in the length direction, heated to 1050 ° C. at 90 ° C./min, and at that temperature for 1 hour After being held, the sample was cooled with He gas to obtain a sample for crystal grain coarsening determination. Here, the heating temperature was set to 1100 ° C. in addition to 1050 ° C., and the other treatments were also performed to obtain another sample for determining the coarsening of crystal grains. Further, there is a process for simulating a gradually heating state in which the heating temperature is set to 1050 ° C. or 1100 ° C., held at 800 ° C. during heating at 90 ° C./min for 10 minutes, and then heated again at 90 ° C./min. Then, another sample for determining the coarsening of the crystal grains was obtained. That is, four types of crystal grain coarsening determination samples were also obtained.
Next, each sample is cut in the length direction, embedded in a resin so that the cut surface can be polished, polished, and then the polished surface is corroded with picric acid (an aqueous solution in which 10 g of picric acid is added to 500 ml of water). Thus, crystal grains appeared on the polished surface. Then, particle size measurement using an optical microscope (100 times) according to JISG0551 is performed at a position (1 / 2R) within 3 mm from the center in the length direction of the sample and 1.5 to 2.5 mm from the center in the radial direction. went. And in all five visual fields, when the austenite whose particle size was 5 or less did not exist, it was judged that coarsening could be suppressed. In Table 2, “○” indicates that the coarsening could be suppressed, and “X” indicates that the coarsening could not be achieved.
次に、各種熱処理および制御冷却を実施した棒材から、組織観察用の試料を採取した。そして、棒材の長さ方向に切断し、切断面を研磨できるように樹脂に埋め込み、研磨した後、研磨面をピクラール（ピクリン酸２５ｇをアルコール１５０ｍｌに加えた溶液）を用いて腐食して、研磨面においてフェライトとパーライト＋ベイナイトとを識別できるようにした。そして、試料の半径方向の中心から５〜１０ｍｍの位置を光学顕微鏡（１００倍）を用いて５視野観察し、市販の画像処理ソフトを用いて、フェライト、パーライト、ベイナイトの各々の面積と、フェライトで区切られたパーライト＋ベイナイトの面積を測定した。 Next, a sample for observing the structure was collected from the bar material subjected to various heat treatments and controlled cooling. And after cutting in the length direction of the bar, embedding in the resin so that the cut surface can be polished and polishing, the polished surface is corroded using Picral (a solution of 25 g of picric acid in 150 ml of alcohol), It was made possible to distinguish between ferrite and pearlite + bainite on the polished surface. Then, a 5-10 mm position from the center in the radial direction of the sample was observed using an optical microscope (100 times), and five areas were observed using a commercially available image processing software, and each area of ferrite, pearlite, and bainite, and ferrite The area of pearlite + bainite separated by.
＜焼ならし後の保持温度とその後の冷却の影響＞
実施例１〜６は、焼ならし後に３分間、大気中にて冷却して８００℃程度に冷却した後、７５０℃、７８０℃または７３０℃に調整した炉内に３０分保持し、その後、５℃／ｍｉｎにて冷却した材料である。これらの場合、１０５０℃および１１００℃のいずれで処理した場合であっても、結晶粒の粗大化は確認されなかった。また、個別パーライトの中で式（Ｉ）を満たすものの面積率は３０％以上であった。
<Effect of holding temperature after normalization and subsequent cooling>
Examples 1 to 6 were cooled in the atmosphere for 3 minutes after normalizing and cooled to about 800 ° C., then held in a furnace adjusted to 750 ° C., 780 ° C. or 730 ° C. for 30 minutes, It is a material cooled at 5 ° C./min. In these cases, coarsening of the crystal grains was not confirmed regardless of whether the treatment was performed at 1050 ° C. or 1100 ° C. Moreover, the area ratio of what satisfy | fills Formula (I) in individual pearlite was 30% or more.
＜比較例：焼ならし後の冷却工程省略＞
一方、比較例１、４は７５０℃で３０分保持後、大気中にて冷却した材料、比較例２、５は焼ならし後、大気中にて冷却した材料であり、いずれも浸炭模擬後の結晶粒に粗大化が確認された。また、パーライト＋ベイナイトのサイズが微細となるため、個別パーライトの中で式（Ｉ）を満たすものの面積率は３０％未満であった。このことから、７５０℃保持と徐冷を省くと、組織が微細となるため、次工程の浸炭時に結晶粒が粗大になることが分かる。
<Comparative example: Omission of cooling process after normalization>
On the other hand, Comparative Examples 1 and 4 are materials cooled in the air after being held at 750 ° C. for 30 minutes, and Comparative Examples 2 and 5 are materials cooled in the air after normalization, both after carburizing simulation The coarsening of the crystal grains was confirmed. Moreover, since the size of pearlite + bainite became fine, the area ratio of the individual pearlite satisfying the formula (I) was less than 30%. From this, it can be understood that when the holding at 750 ° C. and the slow cooling are omitted, the structure becomes fine, so that the crystal grains become coarse at the time of carburizing in the next step.
＜比較例：焼ならし後の保持温度の影響＞
一方、比較例９〜１４は、焼ならし以後の保持温度が７３０℃未満であった材料である。いずれも浸炭模擬後に結晶粒粗大化が確認された。また、パーライトサイズが微細であり、個別パーライトの中で式（Ｉ）を満たすものの面積率は３０％未満であった。
<Comparative example: Influence of holding temperature after normalization>
On the other hand, Comparative Examples 9 to 14 are materials whose holding temperature after normalization was less than 730 ° C. In both cases, grain coarsening was confirmed after carburizing simulation. Further, the pearlite size was fine, and the area ratio of the individual pearlite satisfying the formula (I) was less than 30%.
＜焼ならし後の冷却制御の影響＞
実施例７〜１０は焼ならし後の８００℃から７３０℃までは１℃／ｍｉｎまたは２℃／ｍｉｎで徐冷し、７３０℃以下は冷却速度を５℃／ｍｉｎに速くした材料である。浸炭模擬後の結晶粒の粗大化は一部与条件を除いて確認されなかった。また、個別パーライトの中で式（Ｉ）を満たすものの面積率は３０％以上であった。
<Influence of cooling control after normalization>
In Examples 7 to 10, the material was annealed from 800 ° C. to 730 ° C. after normalizing at 1 ° C./min or 2 ° C./min, and below 730 ° C., the cooling rate was increased to 5 ° C./min. Grain coarsening after simulated carburization was not confirmed except for some given conditions. Moreover, the area ratio of what satisfy | fills Formula (I) in individual pearlite was 30% or more.
一方、比較例１５、１６は、焼ならし後の８００℃から７３０℃までは５℃／ｍｉｎで徐冷し、７３０℃以下についても冷却速度を５℃／ｍｉｎに速くした材料である。この場合、浸炭模擬後の結晶粒の粗大化が確認された。また、個別パーライトの中で式（Ｉ）を満たすものの面積率は３０％未満であった。これより、８００℃から７３０℃まで徐冷することで、次工程の浸炭時における結晶粒粗大化を抑制できることが分かる。 On the other hand, Comparative Examples 15 and 16 are materials that are gradually cooled from 800 ° C. to 730 ° C. after normalization at 5 ° C./min, and the cooling rate is increased to 5 ° C./min even at 730 ° C. or lower. In this case, coarsening of the crystal grains after the carburization simulation was confirmed. Moreover, the area ratio of what satisfy | fills Formula (I) in individual pearlite was less than 30%. From this, it turns out that crystal grain coarsening at the time of carburizing of the next process can be controlled by gradually cooling from 800 ° C to 730 ° C.
＜比較例：焼ならし後に遅い冷却＞
比較例３、６は、焼ならし後に炉の電源を落として炉内で冷却した材料である。８００℃から６８０℃に下がるまでには１２０分かかっていた。これらの材料はいずれも浸炭模擬後に結晶粒の粗大化が確認された。また、焼ならし後の組織はフェライトとパーライトがそれぞれバンド状に存在し、円相当径で２５０μｍ超の粗大なパーライトとなっていた。このことから８００℃から６８０℃までをただ徐冷すればよいのでなく、９０分以内に冷却することで粗大パーライトを抑制できると考えられる。特に６０分以内で冷却することで円相当径が２００μｍ未満となっており、１１００℃でも浸炭模擬時に結晶粒の粗大化が抑制できている。
<Comparative example: Slow cooling after normalization>
Comparative Examples 3 and 6 are materials that were cooled in the furnace by turning off the power of the furnace after normalization. It took 120 minutes to drop from 800 ° C to 680 ° C. All of these materials were confirmed to be coarsened after simulated carburization. Moreover, the structure after normalization had ferrite and pearlite in a band shape, respectively, and was a coarse pearlite having an equivalent circle diameter of more than 250 μm. From this, it is considered that not only slow cooling from 800 ° C. to 680 ° C. is required, but coarse pearlite can be suppressed by cooling within 90 minutes. In particular, by cooling within 60 minutes, the equivalent circle diameter is less than 200 μm, and coarsening of crystal grains can be suppressed even at 1100 ° C. during carburizing simulation.
＜組成の影響＞
実施例１１、１２と比較例１７、１８はＡｌ量を変えたときの例で、Ａｌ量を減らしても０．０２％以上あって、かつ、式（Ｉ）を満たす個別パーライトの面積率が３０％以上であると、浸炭模擬後の粗大化は確認されないことが分かる。
一方、比較例１９はＡｌを過剰に添加した例で、ピン止め力が弱まるため、式（Ｉ）を満たす個別パーライトの面積率が３０％以上であるものの、浸炭模擬後の結晶粒の粗大化が確認された。
実施例１３、１４および比較例２０、２１はベイナイト（Ｂ）が存在した例であり、ベイナイト（Ｂ）の面積率が１５％以下では浸炭模擬後の結晶粒粗大化は確認されないが、１５％を超えると粗大化が確認された。
<Effect of composition>
Examples 11 and 12 and Comparative Examples 17 and 18 are examples in which the amount of Al is changed. Even when the amount of Al is reduced, the area ratio of individual pearlite satisfying the formula (I) is 0.02% or more. It turns out that the coarsening after carburizing simulation is not confirmed as it is 30% or more.
On the other hand, Comparative Example 19 is an example in which Al is added excessively, and since the pinning force is weakened, the area ratio of individual pearlite satisfying the formula (I) is 30% or more, but the coarsening of crystal grains after carburizing simulation Was confirmed.
Examples 13 and 14 and Comparative Examples 20 and 21 are examples in which bainite (B) was present, and when the area ratio of bainite (B) was 15% or less, no coarsening of crystal grains after carburizing simulation was confirmed, but 15% Exceeding a coarsening was confirmed.
Claims (6)
フェライト面積率が４０％以上であり、
フェライトで区切られたパーライト単位を個別パーライトと定義したとき、下記式（Ｉ）を満たす個別パーライトの面積を合計して求めた面積率が３０％以上であり、かつ、個別パーライトの円相当径が２５０μｍ未満である、高温浸炭用鋼。
式（Ｉ）： Ｓ1≧（３×１０8）／Ｖ1 2
Ｓ1は各個別パーライトの面積（μｍ2）を意味する。
Ｖ1は３×（Ａｌ含有率（ｐｐｍ）×Ｎ含有率（ｐｐｍ）−３０００）1/2とする。
Ｖ1は、Ｎｂ含有率が３０ｐｐｍ超の場合、３×（Ａｌ含有率（ｐｐｍ）×Ｎ含有率（ｐｐｍ）−３０００）1/2＋（Ｎｂ含有率（ｐｐｍ）−３０）とし、Ｎｂ含有率が３０ｐｐｍ以下の場合、３×（Ａｌ含有率（ｐｐｍ）×Ｎ含有率（ｐｐｍ）−３０００）1/2とする。 C: 0.1-0.3 mass%, Si: 0.05-1.0 mass%, Mn: 0.3-2.0 mass%, P: 0.03 mass% or less, S: 0.03 % By mass or less, Cr: 0.3 to 1.5% by mass, Al: 0.02 to 0.05% by mass, the balance consisting of Fe and inevitable impurities,
The ferrite area ratio is 40% or more,
When the pearlite unit delimited by ferrite is defined as individual pearlite, the area ratio obtained by summing the areas of the individual pearlite satisfying the following formula (I) is 30% or more, and the circle equivalent diameter of the individual pearlite is Steel for high-temperature carburization that is less than 250 μm.
Formula (I): S 1 ≧ (3 × 10 8 ) / V 1 2
S 1 means the area (μm 2 ) of each individual pearlite.
V 1 is 3 × (Al content (ppm) × N content (ppm) −3000) 1/2 .
When the Nb content is more than 30 ppm, V 1 is 3 × (Al content (ppm) × N content (ppm) −3000) 1/2 + (Nb content (ppm) −30). When the rate is 30 ppm or less, 3 × (Al content (ppm) × N content (ppm) −3000) 1/2 is set.
フェライト面積率が４０〜７０％、ベイナイト面積率が１５％以下であり、
フェライトで区切られたパーライト＋ベイナイト単位を個別パーライト＋ベイナイトと定義したとき、下記式（ＩＩ）を満たす個別パーライト＋ベイナイトの面積を合計して求めた面積率が３０％以上であり、かつ、個別パーライト＋ベイナイトの円相当径が２５０μｍ未満である、高温浸炭用鋼。
式（ＩＩ）： Ｓ2≧（３×１０8）／Ｖ2 2
Ｓ2は各個別パーライト＋ベイナイトの面積（μｍ2）を意味する。
Ｖ2は、Ｎｂ含有率が３０ｐｐｍ超の場合、３×（Ａｌ含有率（ｐｐｍ）×Ｎ含有率（ｐｐｍ）−３０００）1/2＋（Ｎｂ含有率（ｐｐｍ）−３０）とし、Ｎｂ含有率が３０ｐｐｍ以下の場合、３×（Ａｌ含有率（ｐｐｍ）×Ｎ含有率（ｐｐｍ）−３０００）1/2とする。 C: 0.1-0.3 mass%, Si: 0.05-1.0 mass%, Mn: 0.3-2.0 mass%, P: 0.03 mass% or less, S: 0.03 % By mass or less, Cr: 0.3 to 1.5% by mass, Al: 0.02 to 0.05% by mass, the balance consisting of Fe and inevitable impurities,
The ferrite area ratio is 40 to 70%, the bainite area ratio is 15% or less,
When the unit of pearlite + bainite separated by ferrite is defined as individual pearlite + bainite, the area ratio obtained by summing the areas of individual pearlite + bainite satisfying the following formula (II) is 30% or more, and individual Steel for high-temperature carburization, in which the equivalent circle diameter of pearlite + bainite is less than 250 μm.
Formula (II): S 2 ≧ (3 × 10 8 ) / V 2 2
S 2 means the area of each individual pearlite + bainite (μm 2 ).
V 2 is 3 × (Al content (ppm) × N content (ppm) −3000) 1/2 + (Nb content (ppm) −30) when Nb content exceeds 30 ppm, When the rate is 30 ppm or less, 3 × (Al content (ppm) × N content (ppm) −3000) 1/2 is set.
８００℃から６８０℃までの保持時間を９０分以内とし、
８００℃から７３０℃までの保持時間を３０分以上とし、
７３０℃から６８０℃までを１０℃／分以下の降温速度で徐冷し、
請求項１〜３のいずれかに記載の高温浸炭用鋼が得られる、高温浸炭用鋼の製造方法。 Normalizing at 850 to 1000 ° C. as a pre-carburizing heat treatment,
The holding time from 800 ° C. to 680 ° C. is within 90 minutes,
The holding time from 800 ° C to 730 ° C is 30 minutes or more,
Slowly cool from 730 ° C to 680 ° C at a rate of temperature decrease of 10 ° C / min or less,
The manufacturing method of the steel for high temperature carburizing in which the steel for high temperature carburizing in any one of Claims 1-3 is obtained.
８００℃から６８０℃までの保持時間を９０分以内とし、
８００℃から７３０℃までを２℃／分以下の降温速度で徐冷し、
７３０℃から６８０℃までを１０℃／分以下の降温速度で徐冷し、
請求項１〜３のいずれかに記載の高温浸炭用鋼が得られる、高温浸炭用鋼の製造方法。 Normalizing at 850 to 1000 ° C. as a pre-carburizing heat treatment,
The holding time from 800 ° C. to 680 ° C. is within 90 minutes,
Slow cooling from 800 ° C to 730 ° C at a rate of temperature decrease of 2 ° C / min or less,
Slowly cool from 730 ° C to 680 ° C at a rate of temperature decrease of 10 ° C / min or less,
The manufacturing method of the steel for high temperature carburizing in which the steel for high temperature carburizing in any one of Claims 1-3 is obtained.
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