JP4963918B2 - Method for annealing low carbon steel containing Cr - Google Patents
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- JP4963918B2 JP4963918B2 JP2006267023A JP2006267023A JP4963918B2 JP 4963918 B2 JP4963918 B2 JP 4963918B2 JP 2006267023 A JP2006267023 A JP 2006267023A JP 2006267023 A JP2006267023 A JP 2006267023A JP 4963918 B2 JP4963918 B2 JP 4963918B2
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Description
本発明は、製造工程中に冷間鍛造や冷間引抜きなどのＡｃ1変態点以下の温度にて塑性変形による加工に用いる機械構造用の低炭素鋼の冷間加工性に優れる焼鈍方法に関する。 The present invention relates to an annealing method that is excellent in cold workability of low-carbon steel for machine structure used for processing by plastic deformation at a temperature below the Ac 1 transformation point such as cold forging and cold drawing during the manufacturing process.
自動車その他の各種の装置の機材の製造、例えば、歯車などの複雑な形状をした部品の製造は、ＪＩＳ−ＳＣＭ４２０、ＪＩＳ−ＳＣｒ４２０などのＣｒやＭｏを含有する低炭素鋼を熱間鍛造、冷間鍛造にて歯車の概略の形状を形成し、切削などにより最終形状に仕上げした後、浸炭を施して製造している。また、直動軸においても、概略の形状に引抜きをし、切削によって最終形状としていた。 In the manufacture of equipment for automobiles and other various devices, for example, the manufacture of parts with complicated shapes such as gears, low-carbon steels containing Cr and Mo such as JIS-SCM420 and JIS-SCr420 are hot forged and cooled. A rough shape of the gear is formed by inter-forging, and after finishing to the final shape by cutting or the like, it is manufactured by carburizing. In addition, the linear motion shaft was also drawn out into a rough shape and cut into a final shape.
しかし、近年では、冷間加工技術の発達に伴い、冷間による複雑な多段鍛造を行うことによる切削仕上げ工程の省略、大型部品に対して熱間鍛造によっていた工程の冷間鍛造化、複雑形状への引抜き加工などが行われている。そのため冷間鍛造、引抜きなど主に塑性変形を伴う加工により製造する部品、例えばＣＶＴプーリー、ギア、シャフト、直動軸などの複雑形状の部品においては、鋼材の冷間加工性に対する要求は一層強くなってきている。 However, in recent years, with the development of cold working technology, the cutting finishing process by performing complicated multi-stage forging by cold is omitted, cold forging of the process that was hot forging for large parts, complicated shape The drawing process is performed. For this reason, in parts manufactured mainly by plastic deformation, such as cold forging and drawing, such as CVT pulleys, gears, shafts, linear motion shafts, etc., there is a stronger demand for cold workability of steel materials. It has become to.
ところで、このような鋼材の冷間加工性を向上するためには、焼鈍により硬さを下げる必要がある。従来は焼鈍により硬さを低減する方法として、２回焼鈍を行う方法や、冷却を極端に遅くする方法や、冷却時Ａｃ1変態点近傍で保持する方法や、１度の焼鈍中に数回の加熱冷却を行う方法などがとられていた。しかし、これらには、それぞれ生産性の悪化や焼鈍の長時間化や鋼種によるＡｃ1変態点の変化やパターンの複雑化などの問題が存在している。 By the way, in order to improve the cold workability of such a steel material, it is necessary to reduce the hardness by annealing. Conventionally, as a method of reducing hardness by annealing, a method of performing annealing twice, a method of extremely slowing cooling, a method of holding near the Ac 1 transformation point during cooling, and several times during one annealing The method of performing heating and cooling of was taken. However, these have problems such as deterioration in productivity, prolonged annealing, changes in Ac 1 transformation point due to steel types, and complicated patterns.
従来、鋼に含まれる炭化物を球状化すると、鋼の硬度が減少するので、塑性変形が容易となる。ところで、炭素を０．３質量％より多く含有する中炭素鋼および高炭素鋼では、球状化焼鈍法が知られている。しかし、低炭素鋼では、含有炭素量が少なく、中炭素鋼および高炭素鋼の方法では炭化物の球状化を行うことは困難である。ところで、このような低炭素鋼においてもＡ1変態点より低い温度に保持することで炭化物を球状化し、鋼の硬度を低下する方法が開発されている（例えば、特許文献１参照。）。 Conventionally, when carbides contained in steel are spheroidized, the hardness of the steel is reduced, so that plastic deformation is facilitated. By the way, the spheroidizing annealing method is known for medium carbon steel and high carbon steel containing more than 0.3% by mass of carbon. However, low carbon steel has a small carbon content, and it is difficult to spheroidize carbides by the methods of medium carbon steel and high carbon steel. By the way, even in such a low carbon steel, a method has been developed in which carbides are spheroidized by maintaining a temperature lower than the A 1 transformation point to reduce the hardness of the steel (for example, see Patent Document 1).
また、冷間加工性に優れた低炭素のＣｒ−Ｍｏ鋼の熱処理方法が開発されている（例えば、特許文献２参照。）。 Further, a heat treatment method for low-carbon Cr—Mo steel having excellent cold workability has been developed (for example, see Patent Document 2).
一方、出願人は、被削性および冷間加工性に優れた０．８〜１．２質量％Ｃの高炭素クロム軸受鋼を開発している（例えば、特許文献３参照。）。しかし、この方法は高炭素鋼に係るものであり、低炭素鋼においては適用することはできなかった。 On the other hand, the applicant has developed a 0.8 to 1.2 mass% C high carbon chromium bearing steel excellent in machinability and cold workability (see, for example, Patent Document 3). However, this method is related to high carbon steel and could not be applied to low carbon steel.
本発明が解決しようとする課題は、０．１〜０．３％の低炭素および０．５〜２．５％のＣｒを含有する歯車などに用いる低炭素鋼において、通常の焼鈍組織よりも球状化炭化物間の間隔が広く、硬さの低い組織とした、高い冷間加工性を有する低炭素鋼を製造するための焼鈍方法を提供することである。 The problem to be solved by the present invention is a low carbon steel used for gears containing 0.1 to 0.3% of low carbon and 0.5 to 2.5% of Cr, rather than a normal annealing structure. An object of the present invention is to provide an annealing method for producing a low carbon steel having a high cold workability and having a structure having a wide interval between spheroidized carbides and a low hardness.
発明者らは以下の知見を見出した。低炭素鋼を焼鈍する際、この低炭素鋼はＡ1変態点〜Ａ3変態点の温度域においては、フェライト・オーステナイトの２相組織となる。しかし、実際は、この２相組織には若干の炭化物が残存している。そこで、この低炭素鋼をＡｃ1変態点（７２０〜７６０℃）付近を１０℃／ｈｒ以下の加熱速度で徐熱する、あるいはＡｃ1変態点（７２０〜７６０℃）付近で等温保持する。この徐熱あるいは等温保持によりパーライトの分断される間隔が大きくなり、この結果、球状炭化物の核となる炭化物数を減少させることができる。その後７６０〜７９０℃まで加熱保持した後、次いで７４０〜７２０℃の温度範囲に空冷以下の速度で冷却し、さらに該温度範囲から１０℃／ｈｒ以下の速度でＡｒ1点終了以下の温度まで徐冷することにより、通常の焼鈍組織よりも球状化炭化物間の間隔が広く、硬さの低い組織を得ることができる。これらの知見から本発明の手段を得たものである。 The inventors have found the following findings. When the low carbon steel is annealed, the low carbon steel has a two-phase structure of ferrite and austenite in the temperature range from the A 1 transformation point to the A 3 transformation point. However, in practice, some carbide remains in this two-phase structure. Therefore, this low carbon steel is gradually heated near the Ac 1 transformation point (720 to 760 ° C.) at a heating rate of 10 ° C./hr or less, or is kept isothermal near the Ac 1 transformation point (720 to 760 ° C.). The interval at which the pearlite is divided is increased by this slow heating or isothermal holding, and as a result, the number of carbides serving as nuclei of the spherical carbides can be reduced. Thereafter, the mixture is heated to 760 to 790 ° C., then cooled to a temperature range of 740 to 720 ° C. at a rate of air cooling or less, and further gradually decreased from the temperature range to a temperature of Ar 1 point or less at a rate of 10 ° C./hr or less. By cooling, it is possible to obtain a structure having a wider interval between spheroidized carbides and a lower hardness than a normal annealed structure. The means of the present invention are obtained from these findings.
すなわち、本発明の手段は、質量％で、０．１〜０．３％のＣおよび０．５〜２．５％のＣｒを含有する低炭素鋼の焼鈍に際し、加熱中のＡｃ1変態点（７２０〜７６０℃）の近傍の昇温速度を遅くするものである。 That is, the means of the present invention is the Ac 1 transformation point during heating when annealing a low carbon steel containing 0.1 to 0.3% C and 0.5 to 2.5% Cr in mass%. The rate of temperature rise in the vicinity of (720 to 760 ° C.) is slowed down.
そこで、上記の課題を解決するための本発明の手段は、請求項１の発明では、質量％で、０．１〜０．３％のＣおよび０．５〜２．５％のＣｒを含有する低炭素鋼に対し、７００〜７３０℃の温度範囲に加熱した後、該温度範囲から７４０〜７６０℃まで１０℃／ｈｒ以下の加熱速度で徐熱し、さらに７６０〜７００℃まで加熱し保持した後、７４０〜７２０℃の温度範囲に空冷以下の速度で冷却し、さらに該温度範囲からＡｒ1変態点終了以下の温度域まで１０℃／ｈｒ以下で徐冷した後、空冷以下の速度で冷却することを特徴とする低炭素鋼の焼鈍方法である。本発明の手段とすることにより冷間加工性を良好とすることができる。 Therefore, the means of the present invention for solving the above-mentioned problem is that, in the invention of claim 1, the mass contains 0.1 to 0.3% of C and 0.5 to 2.5% of Cr. The low-carbon steel is heated to a temperature range of 700 to 730 ° C., then gradually heated from the temperature range to 740 to 760 ° C. at a heating rate of 10 ° C./hr or less, and further heated to and maintained at 760 to 700 ° C. After that, it is cooled to a temperature range of 740 to 720 ° C. at a rate below air cooling, and further gradually cooled at a rate of 10 ° C./hr or less from the temperature range to a temperature range below the end of the Ar 1 transformation point, and then cooled at a rate below air cooling. It is the annealing method of the low carbon steel characterized by doing. By employing the means of the present invention, the cold workability can be improved.
請求項２の発明では、質量％で、０．１〜０．３％のＣおよび０．５〜２．５％のＣｒを含有する低炭素鋼に対し、７２０〜７６０℃まで加熱した後、４時間以上等温保持し、さらに７６０〜７９０℃まで加熱して保持した後、さらに７４０〜７２０℃の温度範囲に空冷以下の速度で冷却し、さらに該温度からＡｒ1変態点終了以下の温度域まで１０℃／ｈｒ以下で徐冷した後、空冷以下の速度で冷却することを特徴とする低炭素鋼の焼鈍方法である。上記と同様に、本発明の手段とすることにより冷間加工性を良好とすることができる。 In invention of Claim 2, after heating to 720-760 degreeC with respect to the low carbon steel containing 0.1 to 0.3% of C and 0.5 to 2.5% of Cr by mass%, Hold isothermal for 4 hours or more, further heat and hold to 760-790 ° C, then cool to a temperature range of 740-720 ° C at a rate below air cooling, and further from this temperature to the temperature range below the Ar 1 transformation point end This is an annealing method for low carbon steel, characterized by slowly cooling to 10 ° C./hr or less until cooling to air cooling or less. Similarly to the above, the cold workability can be improved by using the means of the present invention.
上記手段の各上下限の限定理由を以下に説明する。
請求項１の手段で、炭化物の間隔を大きくし硬さを低減するためには、鋼材の成分によって決まるＡｃ1点付近を可能な限りゆっくり徐熱することが必要であるため、徐熱開始をＡｃ1直下の７００〜７３０℃とし、徐熱終了をＡｃ1直上の７４０〜７６０℃とした。加熱速度は可能な限り遅くすることが望ましいが、生産性と硬さ低減の効果から１０℃／ｈｒ以下の速度とした。
The reasons for limiting the upper and lower limits of the above means will be described below.
In order to increase the carbide interval and reduce the hardness with the means of claim 1, it is necessary to gradually heat the vicinity of the Ac 1 point determined by the components of the steel material as slowly as possible. The temperature was 700 to 730 ° C. immediately below Ac 1 , and the end of the slow heating was 740 to 760 ° C. immediately above Ac 1 . Although it is desirable to make the heating rate as slow as possible, the heating rate is set to 10 ° C./hr or less from the viewpoint of productivity and hardness reduction.
請求項２の手段で、炭化物の間隔を大きくし硬さを低減するためには、鋼材の成分によって決まるＡｃ1点直上で保持するのが最も効果的であるため、７２０〜７６０℃で保持することとした。保持時間は可能な限り長く取ることが望ましいが、生産性と硬さ低減の効果から４時間以上とした。 In order to increase the interval between carbides and reduce the hardness by means of claim 2, it is most effective to hold just above the Ac 1 point determined by the components of the steel material, so hold at 720 to 760 ° C. It was decided. Although it is desirable to keep the holding time as long as possible, the holding time is set to 4 hours or more from the viewpoint of productivity and the effect of reducing hardness.
さらに、請求項１または請求項２の手段において、加熱する最高温度は低すぎると溶け残った炭化物の数が多くなり、また高すぎると炭化物が溶けすぎてしまうので、適切な炭化物量とするため７６０〜７９０℃とした。さらに溶け残った炭化物を成長させパーライト組織の発生を抑えるためＡｒ１点付近を徐冷却する必要があるので、徐冷却開始温度を７４０〜７２０℃とし終了温度をＡｒ１以下とした。冷却速度は可能な限り遅くすることが望ましいが、生産性と硬さ低減の効果から１０℃／ｈｒ以下の速度とした。 Furthermore, in the means of claim 1 or claim 2, if the maximum temperature to be heated is too low, the number of undissolved carbides increases, and if it is too high, the carbides are excessively dissolved. The temperature was 760 to 790 ° C. Furthermore, since it is necessary to gradually cool the vicinity of the Ar 1 point in order to grow the undissolved carbide and suppress the generation of the pearlite structure, the slow cooling start temperature is set to 740 to 720 ° C., and the end temperature is set to Ar 1 or less. Although it is desirable to make the cooling rate as slow as possible, the cooling rate is set to 10 ° C./hr or less from the viewpoint of productivity and hardness reduction.
本発明の手段における低炭素鋼のＣ量は、少なすぎた場合、あるいは多すぎた場合のどちらにおいても、本発明の熱処理で硬さを低減することが困難であるため、質量％で、０．１〜０．３％とした。 The C content of the low carbon steel in the means of the present invention is 0% by mass% because it is difficult to reduce the hardness by the heat treatment of the present invention, both when it is too small and when it is too much. 0.1 to 0.3%.
さらに、Ｃｒ量は、最高温度時に適量の炭化物を残留させるために必要であるため、質量％で、０．５〜２．５％とした。 Furthermore, the Cr amount is 0.5% to 2.5% in mass% because it is necessary to leave an appropriate amount of carbide at the maximum temperature.
以上に説明したように、本発明の加熱工程からなる焼鈍方法は、従来の２回焼鈍を行う方法や、冷却を極端に遅くする方法や、冷却時Ａｃ1変態点近傍で保持する方法や、１度の焼鈍中に数回の加熱冷却を行う方法などの方法をとることなく、ただ１回の焼鈍を行うことで、硬度を低減した低炭素鋼を得ることができ、この低炭素鋼は優れた冷間加工性のものである。 As explained above, the annealing method comprising the heating step of the present invention is a conventional method of performing annealing twice, a method of extremely slowing cooling, a method of holding near the Ac 1 transformation point during cooling, Without taking a method such as heating and cooling several times during one annealing, a low carbon steel with reduced hardness can be obtained by performing only one annealing. It has excellent cold workability.
本発明を実施するための最良の形態を図を参照して説明する。１００ｋｇ真空溶解炉にて、表１に示す化学成分を有するＡ〜Ｄ鋼を溶解して得た鋼材から、熱間鍛造によりφ３２ｍｍ×Ｌ１００ｍｍ丸棒を製作した。ここで、Ａ鋼、Ｄ鋼はＭｏを有意に含有するＳＣＭ系の鋼であるが、Ｂ鋼、Ｃ鋼はＭｏを不可避不純物として含有するものである。 The best mode for carrying out the present invention will be described with reference to the drawings. A φ32 mm × L100 mm round bar was manufactured by hot forging from a steel material obtained by melting A to D steels having chemical components shown in Table 1 in a 100 kg vacuum melting furnace. Here, steel A and steel D are SCM steels that contain Mo significantly, while steel B and steel C contain Mo as an inevitable impurity.
その後、請求項１の発明の実施例を本発明１とし、請求項２の発明の実施例を本発明２とし、従来方法の焼鈍例として、通常の焼鈍を例１とし、冷却速度の遅いパターンをを例２とし、冷却中一定温度を取るパターンを例３とし、通常の焼鈍を２回行うものを例４とし、図１に示す焼鈍パターンとし、これらの焼鈍パターンによる焼鈍を焼鈍炉にて行い、さらに表１のＡ鋼について炭化物状態を調査してその結果を図２に示し、さらに表１のＡ〜Ｄ鋼の試験片を図１に示す焼鈍パターンで焼鈍し、それぞれの焼鈍後の試験片の硬さを図３に示す。 Thereafter, the embodiment of the invention of claim 1 is set as the present invention 1, the embodiment of the invention of claim 2 is set as the present invention 2, and as an example of the annealing of the conventional method, the normal annealing is set as the example 1, and the slow cooling pattern Is set to Example 2, a pattern in which a constant temperature is taken during cooling is set to Example 3, an example in which normal annealing is performed twice is set to Example 4, and the annealing pattern shown in FIG. 1 is used, and annealing by these annealing patterns is performed in an annealing furnace. Further, the carbide state of the steel A in Table 1 was investigated, and the results are shown in FIG. 2, and the test pieces of the steels A to D in Table 1 were annealed with the annealing pattern shown in FIG. The hardness of the test piece is shown in FIG.
上記において、炭化物の状態は、試験片の断面のＤ／４部（ここで、「Ｄ」は試験片の断面の直径である。）においてミクロ組織を観察して確認した。表面硬さは、試験片の断面のＤ／４部においてロックウェル硬度計を使用して測定した。 In the above, the state of carbide was confirmed by observing the microstructure at D / 4 part of the cross section of the test piece (where “D” is the diameter of the cross section of the test piece). The surface hardness was measured using a Rockwell hardness meter at D / 4 part of the cross section of the test piece.
本発明の実施の形態である低炭素鋼の焼鈍方法において、図１の本発明１および本発明２に示す焼鈍パターンによるものは、図２のミクロ組織の写真の本発明１および本発明２に見られるように、炭化物間の間隔が広い組織が得られている。 In the annealing method of the low carbon steel according to the embodiment of the present invention, the annealing pattern shown in the present invention 1 and the present invention 2 in FIG. As can be seen, a structure with a wide spacing between carbides is obtained.
これに対し、従来方法の焼鈍例である図１の例１の通常の焼鈍パターンによるもの、例２の冷却速度の遅い焼鈍パターンによるもの、例３の冷却中一定温度をとる焼鈍パターンによるものおよび例４の通常の焼鈍を２回行う焼鈍パターンによるものは、図２のミクロ組織の写真の例１、例２、例３および例４に示すように、炭化物間の間隔がそれほど広くない組織となっている。 On the other hand, according to the normal annealing pattern of Example 1 in FIG. 1, which is an example of annealing of the conventional method, according to the annealing pattern with a slow cooling rate in Example 2, according to the annealing pattern having a constant temperature during cooling in Example 3, and According to the annealing pattern in which the normal annealing of Example 4 is performed twice, as shown in Example 1, Example 2, Example 3 and Example 4 of the microstructure photograph of FIG. It has become.
さらに、図３のグラフに示すように、本発明の実施の形態の図１に示す本発明１および本発明２では、硬さ（ＨＲＢ）が低減されていることがわかる。これに対し、従来例である例１、例２、例３および例４では、本発明より硬さが大である。 Further, as shown in the graph of FIG. 3, it can be seen that the hardness (HRB) is reduced in the present invention 1 and the present invention 2 shown in FIG. 1 of the embodiment of the present invention. On the other hand, in Examples 1, 2, 3, and 4, which are conventional examples, the hardness is greater than that of the present invention.
また、表１のＡ鋼の発明２の焼鈍パターンにおける保持時間を２．５時間とした場合の硬さは７４．２ＨＲＢ、本発明１におけるＡｒ1変態点までの冷却速度を１５℃／ｈｒとした場合の硬さは７３．８ＨＲＢ、本発明２の最高加熱温度を７５０℃とした場合の硬さは７４．０、本発明２の最高加熱温度を８００℃とした場合の硬さは７５．１ＨＲＢと従来の熱処理と同程度の硬さ低減効果である。以上から本願発明における０．１〜０．３％のＣおよび０．５〜２．５％のＣｒを含有する低炭素鋼では、本願の焼鈍方法により優れた冷間加工性が得られることがわかる。 Further, the hardness when the holding time in the annealing pattern of the invention A of Table 1 in the invention 2 is 2.5 hours is 74.2 HRB, and the cooling rate to the Ar 1 transformation point in the invention 1 is 15 ° C./hr. The hardness is 73.8 HRB, the hardness when the maximum heating temperature of the present invention 2 is 750 ° C. is 74.0, and the hardness when the maximum heating temperature of the present invention 2 is 800 ° C. is 75. It is the same hardness reduction effect as 1HRB and conventional heat treatment. From the above, in the low carbon steel containing 0.1 to 0.3% C and 0.5 to 2.5% Cr in the present invention, excellent cold workability can be obtained by the annealing method of the present application. Recognize.
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