JP2018012892A - Vacuum carbonitridation method - Google Patents
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Abstract
Description
本発明は、減圧浸炭浸窒処理方法に関する。 The present invention relates to a reduced pressure carburizing and nitriding treatment method.
鋼部品の表面を浸炭処理により改質した浸炭鋼部品が広く用いられている。近年の鋼部品を用いた製品の軽量化及び小型化の要求から、１つの鋼部品に付与される面圧や、回転部品として用いられる場合の回転数が増加する傾向にあり、従来以上に鋼部品に求められる耐久性が高くなってきている。 Carburized steel parts in which the surfaces of steel parts are modified by carburizing treatment are widely used. Due to recent demands for weight reduction and downsizing of products using steel parts, the surface pressure applied to one steel part and the number of rotations when used as a rotating part tend to increase. The durability required for parts is increasing.
このような耐久性の高い浸炭鋼部品としては、Ｎｉ、Ｍｏ、Ｃｒ等が添加された合金鋼が母材として用いられる場合がある。しかし、これらの合金鋼を浸炭用の母材として用いることは、添加する合金元素が高価なことによるコスト増の問題と、母材強度の必要以上の向上によって加工性が悪化するという問題が生じる場合がある。 As such a highly carburized steel part, alloy steel to which Ni, Mo, Cr or the like is added may be used as a base material. However, the use of these alloy steels as a base material for carburizing raises the problem of cost increase due to the expensive alloying elements to be added and the problem that workability deteriorates due to an unnecessary increase in the strength of the base material. There is a case.
一方、母材となる鋼への合金元素の添加を少なく抑えつつ鋼部品表面強度を高める方法として、侵入型元素であるＣ、Ｎを鋼部品の表面に強制的に固溶させ、転位の歪を形成させる浸炭浸窒処理がある。従来の浸炭浸窒処理は、９００℃以上の処理温度でガス浸炭処理を施した後に、鋼材の温度を８５０℃程度まで降温した条件でガス浸窒処理を行うというものである。この方法では、処理時間が比較的長く、従来の浸窒を行わない浸炭処理に比べてコストが大きく増加することを避けることができない。浸炭浸窒処理の時間短縮を目的とした技術としては、たとえば浸炭時に減圧化された雰囲気内に浸炭ガスをパルス状に導入する減圧浸炭を採用して浸炭処理の時間短縮を図ったり、特許文献１に記載のものが提案されている。 On the other hand, as a method for increasing the surface strength of steel parts while suppressing the addition of alloying elements to the base steel, the interstitial elements C and N are forcibly dissolved on the surface of the steel parts, and the distortion of dislocations There is carburizing and nitriding treatment to form. In the conventional carburizing and nitriding treatment, after the gas carburizing treatment is performed at a treatment temperature of 900 ° C. or higher, the gas nitriding treatment is performed under the condition that the temperature of the steel material is lowered to about 850 ° C. In this method, the treatment time is relatively long, and it is unavoidable that the cost is greatly increased as compared with the conventional carburizing treatment without nitriding. As a technique for shortening the time of carburizing and nitriding treatment, for example, reducing the time of carburizing treatment by adopting reduced pressure carburizing that introduces carburizing gas in a pulsed manner into the atmosphere reduced at the time of carburizing, patent document 1 has been proposed.
浸炭時に減圧する浸炭処理方法の採用は、浸炭処理の時間の短縮に寄与するが、当然のごとく、浸炭処理後に別工程として行う浸窒処理の時間短縮には寄与しない。また、特許文献１は、Ｃ含有率を中炭素鋼の範囲にすると共にＣｒ含有率を低くした鋼を用いることにより、浸炭窒化処理時間の短縮を図るというものであるが、浸炭窒化処理の処理温度条件の最適化については殆ど検討されていない。一方、浸炭処理後の浸窒処理の温度を浸炭処理温度に近づければ、浸炭処理終了から浸窒処理開始までの間の時間を短縮するとともに浸窒処理をより高温で行うことで浸窒処理自体の処理時間も短縮することが可能と考えられる。しかしながら、単純に、浸窒処理温度を浸炭処理温度に近い温度、具体的には９００℃以上の温度とした場合には、浸窒処理時に雰囲気ガスとして用いるアンモニアガスの分解が進みやすくなり、鋼中に狙いとする濃度の窒素を侵入させることができないという問題がある。 Adoption of a carburizing method that reduces the pressure during carburizing contributes to shortening the time of carburizing treatment, but as a matter of course, does not contribute to shortening the time of carburizing treatment performed as a separate process after carburizing treatment. Patent Document 1 is intended to shorten the carbonitriding time by using steel having a C content in the range of medium carbon steel and a low Cr content. There has been little study on optimization of temperature conditions. On the other hand, if the temperature of the nitrocarburizing treatment after the carburizing treatment is brought close to the carburizing treatment temperature, the time from the end of the carburizing treatment to the start of the nitriding treatment is shortened, and the nitriding treatment is performed at a higher temperature to perform the nitriding treatment. It is considered possible to shorten the processing time of itself. However, simply, when the nitriding temperature is set to a temperature close to the carburizing temperature, specifically 900 ° C. or more, the decomposition of ammonia gas used as the atmospheric gas during the nitriding treatment is likely to proceed. There is a problem that the target concentration of nitrogen cannot enter.
本発明は、かかる背景に鑑みてなされたものであり、浸炭処理と浸窒処理を連続して行う場合の処理時間を大幅に短縮可能な減圧浸炭浸窒処理方法を提供しようとするものである。 The present invention has been made in view of such a background, and an object of the present invention is to provide a reduced pressure carburizing and nitriding treatment method capable of greatly shortening the processing time when the carburizing treatment and the nitriding treatment are continuously performed. .
本発明の一態様は、０．２０〜２．００質量％のＣｒを含有する鋼材に対して、減圧浸炭処理と減圧浸窒処理とを連続して行う減圧浸炭浸窒処理方法であって、
上記減圧浸炭処理は、９００〜１０５０℃の処理温度にて行い、
上記減圧浸窒処理は、処理温度Ｔ（℃）及び雰囲気圧力Ｐ（ｐａ）が以下の式（１）及び式（２）を満足し、かつ、上記減圧浸窒処理の処理温度を上記減圧浸炭処理の処理温度に対して±１０℃の範囲内の温度とする条件で行うことを特徴とする鋼材の減圧浸炭浸窒処理方法にある。
式（１）：Ｔ≧９００（℃）
式（２）：ａ１×Ｔ＋ｂ１≦Ｐ≦７００００（ｐａ）
（ただし、ａ１＝−３３４×［Ｃｒ］＋１１１７、ｂ１＝３００１５０×［Ｃｒ］−９８０３１６、［Ｃｒ］は鋼材におけるＣｒ含有率（質量％））
One aspect of the present invention is a reduced-pressure carburizing / nitriding method for continuously performing reduced-pressure carburizing treatment and reduced-pressure nitriding treatment on a steel material containing 0.20 to 2.00% by mass of Cr,
The vacuum carburizing process is performed at a processing temperature of 900 to 1050 ° C.
In the reduced pressure nitriding treatment, the treatment temperature T (° C.) and the atmospheric pressure P (pa) satisfy the following expressions (1) and (2), and the treatment temperature of the reduced pressure nitriding treatment is set to the reduced pressure carburizing. The present invention resides in a reduced pressure carburizing and nitriding treatment method for a steel material, which is performed under a condition of a temperature within a range of ± 10 ° C. with respect to the treatment temperature of the treatment.
Formula (1): T ≧ 900 (° C.)
Formula (2): a1 × T + b1 ≦ P ≦ 70000 (pa)
(Where a1 = −334 × [Cr] +1117, b1 = 300150 × [Cr] −980316, [Cr] is the Cr content (% by mass) in the steel material)
上記減圧浸炭浸窒処理方法は、減圧浸窒処理時において被処理材中の含有元素の中で特にＮの鋼中への侵入に大きな影響を及ぼすＣｒの含有率の値と最適な減圧浸窒処理条件との関係を明確にすることにより、減圧浸炭処理と減圧浸窒処理との総合的な処理時間を従来よりも大幅に短縮するものである。 The above-mentioned reduced pressure carburizing and nitriding treatment method includes the value of Cr content which has a great influence on the penetration of N into steel among the elements contained in the material to be treated during the reduced pressure nitriding treatment and the optimum reduced pressure nitriding treatment. By clarifying the relationship with the processing conditions, the total processing time of the reduced pressure carburizing treatment and the reduced pressure nitrocarburizing treatment is significantly shortened compared to the conventional case.
すなわち、上記減圧浸炭処理は浸炭段階での処理時間短縮に寄与する減圧浸炭処理を採用し、これまでと同様に処理温度を上記特定の高温範囲内とし、かつ、上記減圧浸窒処理は上記式（１）及び式（２）を具備する条件で実施する。そして、特に式（２）にあるように、被処理材のＣｒ含有率に応じて被処理材毎に最適な雰囲気圧力Ｐの条件を決定する。これにより、単に処理温度を高めに設定するだけではなく、従来よりも高い温度であっても確実に減圧浸窒処理を進めることができる雰囲気圧力条件を導き出し、被処理材のＣｒ含有率に応じて減圧浸窒処理条件を最適化することができる。そして、この最適化によって、減圧浸窒処理の処理温度と、９００℃以上の処理が不可欠である減圧浸炭処理の処理温度との差を極力小さくすることができ、±１０℃の範囲内という実質的に同じ温度域内とすることも可能となり、浸炭処理温度から浸窒処理温度への降温時間の短縮と浸窒処理温度の高温化による浸窒時間の短縮の効果によって、減圧浸炭処理と減圧浸窒処理とを連続して行う場合の総合的な処理時間を従来よりも大幅に短縮することができる。 That is, the reduced-pressure carburizing process employs a reduced-pressure carburizing process that contributes to shortening the processing time in the carburizing stage, the processing temperature is set within the specific high temperature range as before, and the reduced-pressure carburizing process is performed by the above formula. It implements on the conditions which comprise (1) and Formula (2). And especially in Formula (2), the conditions of the optimal atmospheric pressure P are determined for every to-be-processed material according to Cr content rate of a to-be-processed material. This not only sets the treatment temperature higher, but also derives the atmospheric pressure condition that allows the reduced pressure nitriding treatment to proceed reliably even at a higher temperature than before, depending on the Cr content of the material to be treated. Thus, the pressure nitriding treatment conditions can be optimized. And by this optimization, the difference between the treatment temperature of the reduced pressure nitriding treatment and the treatment temperature of the reduced pressure carburizing treatment in which treatment of 900 ° C. or more is indispensable can be reduced as much as possible, and is substantially within the range of ± 10 ° C. In the same temperature range, reduced carburization and reduced pressure carburization can be achieved by reducing the cooling time from the carburizing temperature to the nitriding temperature and shortening the nitriding time by increasing the nitriding temperature. The total processing time in the case where the nitrogen treatment is continuously performed can be significantly reduced as compared with the conventional case.
上記減圧浸炭浸窒処理方法は、上述したごとく、０．２０〜２．００質量％のＣｒ（クロム）を含有する鋼材に対して行うものである。ここで、Ｃｒ含有率の下限を０．２０％とするのは、鋼の強度（内部硬さ）を確保するためである。Ｃｒ含有率の上限を２．００質量％とするのは、Ｃｒ含有率が高すぎると侵入させたＮが固溶せずにＣｒと結合して粗大なＣｒＮが生成されやすくなり、粗大なＣｒＮによる疲労強度低下が懸念されると共に本来狙っているＮ固溶による表面硬度向上効果が得られにくくなるためである。 As described above, the reduced-pressure carburizing and nitriding treatment method is performed on a steel material containing 0.20 to 2.00% by mass of Cr (chromium). Here, the reason why the lower limit of the Cr content is 0.20% is to ensure the strength (internal hardness) of the steel. The upper limit of the Cr content is set to 2.00% by mass. If the Cr content is too high, the intruded N does not form a solid solution but is bonded to Cr and easily forms coarse CrN. This is because there is a concern about a decrease in fatigue strength due to N, and it is difficult to obtain the effect of improving surface hardness due to the originally aimed N solid solution.
また、上記減圧浸炭処理は、９００〜１０５０℃の処理温度（鋼材の温度）にて行う。減圧浸炭処理の下限温度を９００℃とするのは、９００℃未満では処理時間の点で不利となるからである。減圧浸炭処理の上限温度を１０５０℃とするのは、温度が高すぎると結晶粒が粗大化しやすくなるためである。なお、上記減圧浸炭処理は、具体的には、ガスによる減圧浸炭処理であり、上記処理温度に維持した処理炉内に浸炭用ガスをパルス状に導入しながら行う。すなわち、減圧浸炭処理時の雰囲気圧力は、大気圧よりも低い圧力に減圧した状態で行う。具体的には、パルス状に導入した浸炭雰囲気の最大時圧力は、５０〜３０００ｐａの範囲とすることが好ましい。浸炭用ガスとしては、たとえば、アセチレン、プロパン等を用いることができる。 Moreover, the said reduced pressure carburizing process is performed at the process temperature (temperature of steel materials) of 900-1050 degreeC. The reason why the lower limit temperature of the reduced pressure carburizing process is set to 900 ° C. is that if it is less than 900 ° C., it is disadvantageous in terms of processing time. The reason why the upper limit temperature of the reduced pressure carburizing treatment is set to 1050 ° C. is that the crystal grains are likely to be coarsened if the temperature is too high. The reduced-pressure carburizing process is specifically a reduced-pressure carburizing process using gas, and is performed while introducing a carburizing gas in a pulsed manner into a processing furnace maintained at the processing temperature. That is, the atmospheric pressure during the reduced pressure carburizing process is performed in a state where the pressure is reduced to a pressure lower than the atmospheric pressure. Specifically, it is preferable that the maximum pressure of the carburizing atmosphere introduced in a pulse shape is in the range of 50 to 3000 pa. As the carburizing gas, for example, acetylene, propane or the like can be used.
また、上記減圧浸窒処理は、処理温度（鋼材の温度）Ｔ（℃）及び雰囲気圧力Ｐ（ｐａ）が上記の式（１）及び式（２）を満足する条件で行う。
式（１）は、Ｔ≧９００（℃）であり、減圧浸窒処理時の処理温度Ｔの下限値を示すものである。浸炭処理温度との温度差を小さくして浸炭処理後の降温に必要な時間を短縮し、かつより高い温度で減圧浸窒処理を行って処理時間短縮を確実に行うために、減圧浸窒処理温度の下限値を従来の８５０℃程度に比べて高く定めたものである。
The reduced-pressure nitriding treatment is performed under the condition that the treatment temperature (temperature of the steel material) T (° C.) and the atmospheric pressure P (pa) satisfy the above formulas (1) and (2).
Formula (1) is T ≧ 900 (° C.), and represents the lower limit value of the processing temperature T during the reduced pressure nitriding process. In order to reduce the temperature difference from the carburizing temperature to reduce the time required for temperature reduction after carburizing, and to reduce the processing time by performing reduced pressure nitriding at a higher temperature, the reduced pressure nitriding treatment The lower limit value of the temperature is set higher than the conventional value of about 850 ° C.
式（２）は、減圧浸窒処理の雰囲気圧力Ｐと処理温度Ｔを、ａ１×Ｔ＋ｂ１≦Ｐ≦７００００の関係を満たす条件にするというものである。ここで、ａ１＝−３３４×［Ｃｒ］＋１１１７、ｂ１＝３００１５０×［Ｃｒ］−９８０３１６であり、ａ１とｂ１とはいずれも［Ｃｒ］つまり鋼材におけるＣｒ含有率（質量％）に基づく係数である。式（２）並びに式(２)に含まれるａ１及びｂ１のＰ及びＴとＣｒ含有率との関係は、多数の実験に基づいて導かれたものである。そして、上述したごとくＣｒ含有率が０．２０〜２．００質量％の鋼材を処理する場合には、式（１）及び式（２）を具備する処理温度Ｔ及び圧力Ｐの条件を選択することにより、減圧浸窒処理を確実かつ効率的に行うことができる。 Formula (2) is to make the atmospheric pressure P and the processing temperature T of the reduced-pressure nitriding process satisfy the condition of a1 × T + b1 ≦ P ≦ 70000. Here, a1 = −334 × [Cr] +1117, b1 = 300150 × [Cr] −980316, and both a1 and b1 are coefficients based on [Cr], that is, the Cr content (mass%) in the steel material. . The relationship between P and T in a1 and b1 and Cr content contained in Formula (2) and Formula (2) has been derived based on numerous experiments. And as above-mentioned, when processing the steel materials whose Cr content rate is 0.20-2.00 mass%, the conditions of the process temperature T and the pressure P which comprise Formula (1) and Formula (2) are selected. As a result, the reduced-pressure nitriding treatment can be performed reliably and efficiently.
また、上記減圧浸窒処理は、浸窒用ガスとしてアンモニア（ＮＨ3）を用い、アンモニアを主体とする窒化用ガスを処理炉内に導入すると共に余剰雰囲気ガスを排出しながら、処理温度と圧力を制御して行う。ここで、減圧浸窒処理は、被処理材である鋼材表面にてＮＨ3が、ＮＨ3→［Ｎ］＋３／２Ｈ2の反応により分解し、窒素［Ｎ］が鋼中に侵入して浸窒が行われる。一方、アンモニアが鋼材表面に到達する前にＮ2とＨ2に分解してしまった場合には、減圧浸窒処理を進めることが難しくなる。特に減圧浸窒処理時の処理温度を単純に９００℃以上の高い温度に設定すると、処理炉中において比較的容易にアンモニアが分解するため減圧浸窒処理が進みにくくなる。 The reduced-pressure nitriding treatment uses ammonia (NH 3 ) as a nitriding gas, introduces a nitriding gas mainly composed of ammonia into the processing furnace, and discharges excess atmosphere gas, while treating the processing temperature and pressure. To control. Here, the reduced-pressure nitriding treatment is performed by the reaction of NH 3 on the surface of the steel material to be treated by the reaction NH 3 → [N] + 3 / 2H 2 , and nitrogen [N] penetrates into the steel and soaks. Nitrogen is performed. On the other hand, when ammonia has decomposed into N 2 and H 2 before reaching the steel surface, it is difficult to proceed with the reduced pressure nitriding treatment. In particular, if the processing temperature during the low-pressure nitriding treatment is simply set to a high temperature of 900 ° C. or higher, the ammonia is decomposed relatively easily in the processing furnace, so that the low-pressure nitriding treatment is difficult to proceed.
これに対し、式（２）は、処理炉内の処理温度Ｔ及び鋼材のＣｒ含有率に応じて、減圧浸窒処理時の雰囲気圧力の範囲を導くものである。すなわち、９００℃以上の温度に加熱した場合、アンモニアガスの一部について分解が進むことは避けられないものの、式（２）の条件の範囲内であれば、９００℃以上の処理温度であっても、各鋼材の表面にＮを侵入型元素として固溶させるのに必要な量のアンモニアガスを確保でき、効率よく減圧浸窒処理を進めることができることを本発明者らが見出したのである。 On the other hand, Formula (2) leads the range of the atmospheric pressure at the time of reduced pressure nitriding according to the processing temperature T in the processing furnace and the Cr content of the steel material. That is, when heated to a temperature of 900 ° C. or higher, it is inevitable that a part of the ammonia gas will be decomposed. However, if it is within the range of the condition of formula (2), the processing temperature is 900 ° C. or higher. In addition, the present inventors have found that an amount of ammonia gas necessary for solid-dissolving N as an interstitial element can be secured on the surface of each steel material, and the reduced pressure nitriding treatment can be carried out efficiently.
ここで、式（１）及び式（２）から定められる減圧浸窒処理時の雰囲気圧力Ｐと処理温度Ｔとの関係を図１に示す。同図は、横軸に減圧浸窒処理時の処理温度Ｔ（℃）を、縦軸に減圧浸窒処理時の雰囲気圧力Ｐ（ｐａ）をとったものである。同図中、設定しうる最大雰囲気圧力をＰ2、最小雰囲気圧力をＰ1、最高処理温度をＴ2、最低処理温度をＴ1として示す。 Here, FIG. 1 shows the relationship between the atmospheric pressure P and the processing temperature T during the reduced-pressure nitriding treatment determined from the equations (1) and (2). In this figure, the horizontal axis represents the treatment temperature T (° C.) during the reduced pressure nitriding treatment, and the vertical axis represents the atmospheric pressure P (pa) during the reduced pressure nitriding treatment. In the figure, shows P 2 up to atmospheric pressure which can be set, P 1 the minimum ambient pressure, maximum process temperature T 2, the minimum processing temperature of T 1.
同図から知られるように、減圧浸窒処理の最高処理温度Ｔ2は、雰囲気圧力Ｐを最高のＰ2（７００００ｐａ）とした場合に式（２）から導かれる。そして、同図に示されるように、式（１）に相当するＴ＝Ｔ1（９００℃）の境界線と、Ｐ＝Ｐ2（７００００ｐａ）の境界線と、Ｐ＝ａ１×Ｔ＋ｂ１で表される境界線の、３つの境界線に囲まれる三角形の領域が減圧浸窒処理時の雰囲気圧力Ｐと処理温度Ｔとを選択しうる範囲（条件設定可能範囲Ａ）である。この領域の条件であって、先に行う減圧浸炭処理の処理温度と実質的に同じ温度あるいは近い温度を選択すれば、減圧浸炭浸窒処理全体の処理時間を短縮することができる。なお、雰囲気圧力の下限（ａ１×Ｔ＋ｂ１）は、アンモニアガスの分解を上回るアンモニアを確保し、狙いの表面Ｎ濃度を確保可能とするために多くの実験から導出されたものである。また、上限の７００００ｐａは、過剰浸窒による残留オーステナイトの増加や窒化物の析出による強度への悪影響の可能性を低減するために定めてある。 As can be seen from the figure, the maximum treatment temperature T 2 of the reduced pressure nitriding treatment is derived from the equation (2) when the atmospheric pressure P is the highest P 2 (70000 pa). As shown in the figure, the boundary line of T = T 1 (900 ° C.) corresponding to the equation (1), the boundary line of P = P 2 (70000 pa), and P = a1 × T + b1 A triangular area surrounded by three boundary lines is a range (condition setting range A) in which the atmospheric pressure P and the processing temperature T at the time of reduced pressure nitriding can be selected. By selecting a temperature that is substantially the same as or close to the processing temperature of the low-pressure carburizing process that is performed earlier, the processing time of the entire low-pressure carburizing and nitrogening process can be shortened. The lower limit (a1 × T + b1) of the atmospheric pressure has been derived from many experiments in order to secure ammonia exceeding the decomposition of ammonia gas and to ensure the target surface N concentration. Further, the upper limit of 70000 pa is set in order to reduce the possibility of an increase in retained austenite due to excessive nitriding and the adverse effect on strength due to precipitation of nitrides.
また、減圧浸窒処理は、上述したごとく、窒化用ガスとしてのアンモニアを処理炉内に導入すると共に余剰の雰囲気ガスを排出して、雰囲気圧力を維持する。このとき、雰囲気中のアンモニアの分圧を高くするほど減圧浸窒処理にとっては好ましい。アンモニア分圧の上昇には、たとえば、雰囲気ガスの排気量の抑制、アンモニアと窒素の同時導入等の手法をとりながら上記条件設定可能範囲内での高い雰囲気圧力条件を維持することによって実現可能である。また、窒化用ガスとしてのアンモニアの導入流量は多いほど減圧浸窒処理に有利である。 In addition, as described above, the reduced-pressure nitriding treatment introduces ammonia as a nitriding gas into the processing furnace and discharges excess atmospheric gas to maintain the atmospheric pressure. At this time, the higher the partial pressure of ammonia in the atmosphere, the better for the reduced pressure nitriding treatment. Increasing the ammonia partial pressure can be achieved, for example, by maintaining high atmospheric pressure conditions within the above-mentioned range where the conditions can be set while taking measures such as suppressing the exhaust amount of atmospheric gas and introducing ammonia and nitrogen simultaneously. is there. Further, the larger the introduction flow rate of ammonia as the nitriding gas, the more advantageous is the reduced pressure nitriding treatment.
また、上記減圧浸炭処理と減圧浸窒処理とは連続して実施されるが、上記減圧浸炭処理の処理温度と上記減圧浸窒処理の処理温度を実質的に同一温度とすることが好ましい。これにより、鋼材の温度変更時間を余分にとる必要がなく、浸炭処理後すぐに浸窒処理を行うことができるため、その分全体の処理時間をより短縮することができる。ここで、実質的に同一温度とは、全く同じ温度である場合だけでなく、たとえば±１０℃以内の差違であって、温度変更時間を別途とる必要がない場合をも含むものである。 Moreover, although the said reduced pressure carburizing process and a reduced pressure nitriding process are implemented continuously, it is preferable to make the process temperature of the said reduced pressure carburizing process, and the process temperature of the said reduced pressure nitriding process substantially the same temperature. Thereby, it is not necessary to take extra time for changing the temperature of the steel material, and the nitriding treatment can be performed immediately after the carburizing treatment, so that the entire treatment time can be shortened accordingly. Here, “substantially the same temperature” includes not only the case where the temperatures are exactly the same, but also the case where there is a difference within ± 10 ° C., for example, and it is not necessary to separately take a temperature change time.
また、上記減圧浸炭浸窒処理方法を適用する鋼材としては、Ｃｒ含有率が上記特定の範囲内であれば、他の添加成分の含有率に大きな制限はなく、従来から浸炭処理や浸炭浸窒処理を施されてきたいわゆる肌焼鋼であれば問題なく使用できる。主要な添加成分と含有率の例を以下に示す。 In addition, as a steel material to which the reduced pressure carburizing and nitriding treatment method is applied, as long as the Cr content is within the above specific range, the content of other additive components is not greatly limited, and conventionally, carburizing and carburizing and nitrocarburizing are performed. So-called case-hardened steel that has been treated can be used without problems. Examples of main additive components and contents are shown below.
Ｃ（炭素）は、必須添加成分である。Ｃ含有率は、０．１０〜０．３０質量％程度が好ましい。 C (carbon) is an essential additive component. The C content is preferably about 0.10 to 0.30% by mass.
以下に、Ｃ以外の主要成分の好ましい含有率の例を示す。
Ｓｉ（ケイ素）：０．１０〜１．００質量％、
Ｍｎ（マンガン）：０．３０〜１．５０質量％、
Ｐ（リン）：０．０３５質量％以下、
Ｓ（硫黄）：０．０３５質量％以下、
Ｍｏ（モリブデン）：０〜０．８０質量％以下（任意添加元素）、
Ａｌ（アルミニウム）：０．０２０〜０．０６０質量％、
Ｎ（窒素）：０．００８０〜０．０２５０質量％
Below, the example of the preferable content rate of main components other than C is shown.
Si (silicon): 0.10 to 1.00% by mass,
Mn (manganese): 0.30 to 1.50 mass%,
P (phosphorus): 0.035% by mass or less,
S (sulfur): 0.035 mass% or less,
Mo (molybdenum): 0 to 0.80 mass% or less (optionally added element),
Al (aluminum): 0.020 to 0.060 mass%,
N (nitrogen): 0.0080 to 0.0250 mass%
なお、鋼材には、上記添加元素以外には、Ｆｅ（鉄）及び不可避的不純物が含まれる。 The steel material contains Fe (iron) and inevitable impurities in addition to the additive elements.
本願における減圧浸炭浸窒処理方法の実施例につき、比較例と共に説明する。
まず、表１に示すごとく、Ｃｒ含有率が異なる５グループ（Ｇ１〜Ｇ５）の１５種類の鋼材を試料として準備した。Ｇ１グループの試料１１〜１３は、Ｃｒ含有率が約０．２５％の試料である。Ｇ２グループの試料２１〜２３は、Ｃｒ含有率が約０．６５％の試料である。Ｇ３グループの試料３１〜３３は、Ｃｒ含有率が約１．１０％の試料である。Ｇ４グループの試料４１〜４３は、Ｃｒ含有率が約１．５０％の試料である。Ｇ５グループの試料５１〜５３は、Ｃｒ含有率が約１．９０％の試料である。なお、各グループ毎に３種類の鋼材を準備したが、Ｃｒ以外の成分の含有率の組み合わせは、全てのグループにおいてほぼ同じとなるよう成分調整した。
Examples of the reduced pressure carburizing and nitriding method in the present application will be described together with comparative examples.
First, as shown in Table 1, 15 groups of steel materials (G1 to G5) having different Cr contents were prepared as samples. Samples 11 to 13 of the G1 group are samples having a Cr content of about 0.25%. Samples 21 to 23 of the G2 group are samples having a Cr content of about 0.65%. Samples 31 to 33 of the G3 group are samples having a Cr content of about 1.10%. Samples 41 to 43 of the G4 group are samples having a Cr content of about 1.50%. Samples 51 to 53 of the G5 group are samples having a Cr content of about 1.90%. In addition, although three types of steel materials were prepared for each group, the combination of the content rates of components other than Cr was adjusted so as to be substantially the same in all groups.
上記各試料は、いずれも、表１に示された化学成分組成に調整して得られた鋼塊に対して、鍛伸、焼き鈍し処理を施し、機械加工によりφ２６ｍｍ×４０ｍｍの円柱状の試験片に加工したものである。これらに試料に対して、次のような減圧浸炭浸窒処理を実施した。 In each of the above samples, the steel ingot obtained by adjusting the chemical composition shown in Table 1 is subjected to forging and annealing treatment, and a cylindrical test piece having a diameter of 26 mm × 40 mm by machining. Is processed. The following vacuum carburizing and nitriding treatment was performed on the samples.
＜減圧浸炭浸窒処理＞
上記各試料を処理炉に装入し、各試料の温度を減圧浸炭処理温度まで昇温する。本例では、表２に示すごとく、減圧浸炭処理温度（表２中の「処理温度」）として、９００℃、９３０℃、９５０℃及び９７０℃の４種類の温度を用いた。そして、浸炭ガスとしてアセチレンを処理炉内にパルス状に導入し、導入時の最大雰囲気圧力を１５０ｐａに設定して、９００秒保持し、減圧浸炭処理を行う。
<Low pressure carburizing and nitriding treatment>
Each said sample is inserted into a processing furnace, and the temperature of each sample is raised to the reduced pressure carburizing temperature. In this example, as shown in Table 2, four temperatures of 900 ° C., 930 ° C., 950 ° C., and 970 ° C. were used as the reduced pressure carburizing temperature (“treatment temperature” in Table 2). Then, acetylene as a carburizing gas is introduced into the processing furnace in a pulsed manner, the maximum atmospheric pressure at the time of introduction is set to 150 pa and held for 900 seconds to perform a reduced pressure carburizing process.
次に、この減圧浸炭処理に連続して減圧浸窒処理を行う。減圧浸窒処理の処理温度Ｔ（℃）は、直前の減圧浸炭処理温度と同じ温度（表２中の「処理温度」）に設定する。そして、アンモニアを処理炉内に導入しながら、雰囲気圧力Ｐを表２中の「圧力」の値に設定して、９００秒保持し、その後油焼き入れを行う。本例では、浸窒処理の雰囲気圧力Ｐとして、２００００ｐａ、３００００ｐａ、４００００ｐａ、５００００ｐａ、６００００ｐａ及び７００００ｐａの６種類の圧力を用いた。 Next, a reduced-pressure nitriding process is performed following the reduced-pressure carburizing process. The treatment temperature T (° C.) of the reduced pressure nitriding treatment is set to the same temperature (“treatment temperature” in Table 2) as the immediately preceding reduced pressure carburizing treatment temperature. Then, while introducing ammonia into the processing furnace, the atmospheric pressure P is set to the value of “pressure” in Table 2 and held for 900 seconds, after which oil quenching is performed. In this example, six types of pressures of 20000 pa, 30000 pa, 40000 pa, 50000 pa, 50000 pa, 60000 pa and 70000 pa were used as the atmospheric pressure P for the nitriding treatment.
＜浸窒性評価＞
減圧浸窒処理が効果的になされたか否かについては、上記の減圧浸炭浸窒処理後の各試料表面の窒素濃度を測定することにより評価した。その結果は表２に示した。測定した窒素濃度の結果から、処理後の窒素濃度は、ほぼＣｒ含有率で決定され、同じグループに属する材料間に大きな値の相違がなかったため、表２中の窒素濃度（質量％）の値は、それぞれ、同じグループに属する３種類の試料について測定した３つの値の平均値を示した。窒素濃度の測定は、ＥＰＭＡを用いて行った。表面窒素濃度が０．３０質量％以上の場合には、減圧浸窒処理が十分効果的に行われたと判断することができる。なお、表２中に数値が記載されていない部分は、浸窒反応が正常に進まず、窒素濃度の上昇が確認できなかったことを意味している。
<Nitrogen evaluation>
Whether or not the reduced-pressure nitriding treatment was effectively performed was evaluated by measuring the nitrogen concentration on the surface of each sample after the reduced-pressure carburizing and nitriding treatment. The results are shown in Table 2. From the measured nitrogen concentration results, the nitrogen concentration after the treatment was almost determined by the Cr content, and there was no significant difference between the materials belonging to the same group, so the values of nitrogen concentration (mass%) in Table 2 Represents the average value of three values measured for three types of samples belonging to the same group. The nitrogen concentration was measured using EPMA. When the surface nitrogen concentration is 0.30% by mass or more, it can be determined that the reduced-pressure nitriding treatment has been performed sufficiently effectively. In addition, the part in which the numerical value is not described in Table 2 means that the nitriding reaction did not proceed normally and an increase in the nitrogen concentration could not be confirmed.
表２には、処理温度条件及び式（２）から導かれる最低雰囲気圧力Ｐ1＝ａ１×Ｔ＋ｂ１の計算結果を示す。
表２から知られるように、減圧浸窒処理温度が式（１）を満たす９００℃以上の高温の場合であっても、上述した式（２）を満たすように処理温度Ｔ及び雰囲気圧力Ｐを選択すれば、十分な表面窒素濃度が得られている。一方、雰囲気圧力Ｐが式（２）を満たさず低い場合には、９００℃を超える処理温度では十分な減圧浸窒処理ができないことがわかる。
Table 2 shows the calculation result of the processing temperature condition and the minimum atmospheric pressure P 1 = a1 × T + b1 derived from the equation (2).
As is known from Table 2, even if the reduced pressure nitriding temperature is a high temperature of 900 ° C. or higher that satisfies the formula (1), the processing temperature T and the atmospheric pressure P are set so as to satisfy the above-described formula (2). If selected, a sufficient surface nitrogen concentration is obtained. On the other hand, when the atmospheric pressure P does not satisfy the formula (2) and is low, it can be seen that sufficient reduced pressure nitriding treatment cannot be performed at a treatment temperature exceeding 900 ° C.
なお、式（２）については、次のようにして導いた。
すなわち、本発明の実施例も含め多種類の肌焼鋼と呼ばれるＣｒ鋼、Ｃｒ−Ｍｏ鋼について実験及び調査をした結果、浸炭処理後に０．３０％以上の表面窒素濃度を確保できる条件は、ほぼＣｒ含有率によって決定されることを確認し、その条件（表２の値が０．３０％以上となる条件の範囲）を多種類の試験結果から導出した結果が式（２）である。
Formula (2) was derived as follows.
That is, as a result of experiment and investigation on Cr steel called Cr-Mo steel, including various examples of the present invention, and Cr-Mo steel, the condition for ensuring a surface nitrogen concentration of 0.30% or more after carburizing treatment is as follows: Expression (2) is a result obtained by confirming that it is almost determined by the Cr content, and deriving the condition (the range of conditions in which the value in Table 2 is 0.30% or more) from many kinds of test results.
次に、本発明の減圧浸炭浸窒処理方法を用いる場合の処理時間の短縮効果について述べる。
減圧浸窒処理の処理温度を従来の一般的な温度である８５０℃程度に設定した場合と、本発明のように９００℃以上の温度に設定した場合とを比較すると、本発明の方が、浸炭処理温度と浸窒処理温度との差を最低でも５０℃、浸窒処理温度を高めに設定した場合は１００℃以上小さくすることが可能である。その結果、浸炭処理後浸窒処理開始までの間に鋼材の温度を降温する時間が大幅に短縮される。さらに、浸窒処理が高い温度で行われるのでＮの侵入拡散が早く進み、浸窒処理自体の時間短縮が可能である。ただし、処理温度が高いほどアンモニアガスの分解が進むため、得られる表面窒素濃度は下がる傾向となるが、前述した式（２）を満足する条件で処理することにより、鋼表面の窒素濃度を０．３０質量％以上確保することが可能である。したがって、本発明の減圧浸炭浸窒処理方法は、処理時間短縮に非常に有効であることがわかる。
Next, the effect of shortening the processing time when using the reduced pressure carburizing and nitriding method of the present invention will be described.
When comparing the case where the treatment temperature of the reduced pressure nitriding treatment is set to about 850 ° C. which is a conventional general temperature and the case where the temperature is set to 900 ° C. or more as in the present invention, the present invention is more When the difference between the carburizing temperature and the nitriding temperature is set to at least 50 ° C. and the nitriding temperature is set to be higher, it can be reduced by 100 ° C. or more. As a result, the time for lowering the temperature of the steel material between the carburizing process and the start of the nitriding process is greatly shortened. Furthermore, since the nitriding process is performed at a high temperature, the intrusion and diffusion of N progresses rapidly, and the time for the nitriding process itself can be shortened. However, since the decomposition of ammonia gas proceeds as the treatment temperature increases, the resulting surface nitrogen concentration tends to decrease. However, by treating under the condition satisfying the above-described formula (2), the nitrogen concentration on the steel surface is reduced to 0. It is possible to ensure 30% by mass or more. Therefore, it can be seen that the reduced pressure carburizing and nitriding treatment method of the present invention is very effective in reducing the processing time.
Ａ 条件設定可能範囲 A Condition setting range
Claims (1)
上記減圧浸炭処理は、９００〜１０５０℃の処理温度にて行い、
上記減圧浸窒処理は、処理温度Ｔ（℃）及び雰囲気圧力Ｐ（ｐａ）が以下の式（１）及び式（２）を満足し、かつ、上記減圧浸窒処理の処理温度を上記減圧浸炭処理の処理温度に対して±１０℃の範囲内の温度とする条件で行うことを特徴とする鋼材の減圧浸炭浸窒処理方法。
式（１）：Ｔ≧９００（℃）
式（２）：ａ１×Ｔ＋ｂ１≦Ｐ≦７００００（ｐａ）
（ただし、ａ１＝−３３４×［Ｃｒ］＋１１１７、ｂ１＝３００１５０×［Ｃｒ］−９８０３１６、［Ｃｒ］は鋼材におけるＣｒ含有率（質量％）） A reduced-pressure carburizing and nitriding method for continuously performing a reduced-pressure carburizing treatment and a reduced-pressure nitriding treatment on a steel material containing 0.20 to 2.00% by mass of Cr,
The vacuum carburizing process is performed at a processing temperature of 900 to 1050 ° C.
In the reduced pressure nitriding treatment, the treatment temperature T (° C.) and the atmospheric pressure P (pa) satisfy the following expressions (1) and (2), and the treatment temperature of the reduced pressure nitriding treatment is set to the reduced pressure carburizing. A reduced-pressure carburizing and nitriding method for a steel material, which is performed under a condition that the temperature is within a range of ± 10 ° C with respect to the processing temperature of the processing.
Formula (1): T ≧ 900 (° C.)
Formula (2): a1 × T + b1 ≦ P ≦ 70000 (pa)
(Where a1 = −334 × [Cr] +1117, b1 = 300150 × [Cr] −980316, [Cr] is the Cr content (% by mass) in the steel material)
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