JP3724142B2 - Method for producing coarse grain-resistant case-hardened steel - Google Patents

Description

【００５９】
【表２】

【００６０】
次いで、これらの鋼を表３〜４に記載の条件で加熱、分塊圧延、冷却して１８０ｍｍ角の鋼片とし、更に、表３〜４に記載の条件で再加熱、熱間圧延、冷却して直径５０ｍｍの棒鋼を製造した。ここで、１８０ｍｍ角の鋼片と直径５０ｍｍの棒鋼の製造において、５００℃を下回る温度域の冷却は放冷とした。
【００６１】
なお、本発明例の鋼である鋼８、鋼９、鋼１５及び鋼２２については３トン鋼塊を分割してから上記の鋼片及び棒鋼の製造に供した。
【００６２】
【表３】

【００６３】
【表４】

【００６４】
上記のようにして得た鋼８〜１５、鋼１７〜２１、鋼２３及び鋼２５〜３４の直径５０ｍｍの棒鋼及び一部の鋼（鋼１３、鋼１４、鋼１７、鋼１８及び鋼２０）の１８０ｍｍ角の鋼片を、通常の方法によって７３０℃で球状化焼鈍した。この後、１８０ｍｍ角の鋼片については（Ｔ／４）の部位（Ｔ＝１８０ｍｍ）から、直径５０ｍｍの棒鋼については（Ｒ／２）の部位（Ｒ＝２５ｍｍ）から、それぞれ直径１０ｍｍ×長さ２０ｍｍの円筒状の試験片を切り出し、５００トン高速プレス機を用いて通常の方法で常温（室温）での据え込み試験を行った。ここで、据え込み率（圧縮率）は７０％、７５％、８０％及び８５％の４条件とし、各条件について試験数は２０で行った。据え込み試験後、目視で割れ発生の有無を調査して冷間加工性を評価した。
【００６５】
表３〜４に、上記の冷間加工性評価結果を併せて示す。なお、各条件での２０の試験片のいずれかに割れを認めた場合、割れが生じた最も低い据え込み率を冷間加工性として記載した。表３〜４で冷間加工性が「＞８５」とあるのは、据え込み率８５％の据え込み試験で、２０の試験片のいずれにも割れが生じなかったことを示す。一方、冷間加工性が「≦７０」とあるのは、据え込み率７０％の据え込み試験で、割れが生じたことを示す。
【００６６】
次いで、上記の８５％の据え込み率で据え込んだ試験片を用いて、８５０〜１０５０℃まで２５℃刻みに各温度で６時間加熱して浸炭処理時の加熱をシミュレートした。
【００６７】
加熱処理後は油冷し、光学顕微鏡（倍率は１００倍）でランダムに１０視野観察して粗粒化特性を調査した。なお、粗粒化の判定基準はJIS G 0551に準じた。すなわち、視野間において３以上異なった粒度番号の視野が存在する場合に「混粒」として、異常粒成長が生じたと見なした。一方、「粗粒」と「細粒」の判定はＪＩＳ基準よりも厳しくして、結晶粒度番号６未満の場合に「粗粒鋼」として結晶粒が粗大化したと判定した。
【００６８】
表３〜４に、上記の粗粒化特性の調査結果を併せて示す。なお、粗粒化発生を認めた場合、粗粒化が生じた最も低い加熱温度を「粗粒化開始温度」として記載した。表３〜４で粗粒化開始温度が「＞１０５０」とあるのは、１０５０℃×６時間の加熱条件では粗粒化が生じなかったことを示す。一方、粗粒化開始温度が「≦８５０」とあるのは、８５０℃×６時間の加熱で既に粗粒化が生じていたことを示す。
【００６９】
表３〜４によれば、本発明で規定する化学組成を有する鋼を、本発明で規定する条件で熱間加工及び冷却した場合には、８５％の据え込み率で冷間加工しても割れを生じず、これを６時間オーステナイト化処理した場合の粗粒化開始温度は９７５℃以上と高く、耐粗粒化特性に優れていることが明らかである。
【００７０】
これに対して、成分のいずれかが本発明で規定する含有量の範囲から外れた比較例の鋼のなかで、鋼２５〜２８及び鋼３１〜３４については、本発明で規定する条件で熱間加工及び冷却しても粗粒化開始温度が９５０℃以下と低く耐粗粒化特性に劣る。上記の鋼のうち、鋼２６の場合には冷間加工性も低いため、冷間での所望形状への成形加工時に割れを生じ易くなる。したがって、熱間での粗成形に替えて冷間加工による精密な成形加工を行い、機械加工を省略してコストダウンを図ろうとする産業界の要請に応えられない場合が生ずる。
【００７１】
一方、比較例の鋼のなかでも鋼２９、鋼３０については、６時間オーステナイト化処理した場合の粗粒化開始温度は１０５０℃を超え、耐粗粒化特性には優れているが、冷間加工性が低い。このため、前記の鋼２６の場合と同様に冷間での所望形状への成形加工時に割れを生じ易くなる。したがって、熱間での粗成形に替えて冷間加工による精密な加工を行い、機械加工を省略してコストダウンを図ろうとする産業界の要請に応えられない場合が生ずる。
【００７２】
（実施例２）
前記の（実施例１）で述べた鋼８及び鋼１５の３トン鋼塊から分割した鋼塊を用いて、１０００〜１２００℃まで１００℃刻みで各温度に加熱した後、通常の方法で直径１００ｍｍの鋼片に熱間鍛造した。なお、いずれの場合も熱間鍛造の仕上げ温度は９５０℃とした。又、鍛造終了後は５００℃までの温度域を１０℃／分の冷却速度で冷却し、５００℃を下回る温度域の冷却は放冷とした。
【００７３】
次いで、上記の直径１００ｍｍの各鋼片を９５０〜１１００℃まで５０℃刻みで各温度に再加熱した後、通常の方法で直径５０ｍｍの丸棒に熱間鍛造した。なお、熱間鍛造の仕上げ温度は８１５℃及び８８５℃の２条件とした。又、鍛造終了後は５００℃までの温度域を２５℃／分の冷却速度で冷却し、５００℃を下回る温度域の冷却は放冷とした。
【００７４】
上記のようにして得た直径５０ｍｍの丸棒を、通常の方法によって７３０℃で球状化焼鈍した後、（Ｒ／２）の部位（Ｒ＝２５ｍｍ）から直径１０ｍｍ×長さ２０ｍｍの円筒状の試験片を切り出し、５００トン高速プレス機を用いて通常の方法で据え込み率８５％の常温（室温）据え込み加工を行った。
【００７５】
次いで、前記の据え込み加工した試験片を８５０〜１０５０℃まで２５℃刻みに各温度で６時間加熱して浸炭処理時の加熱をシミュレートした。
【００７６】
加熱処理後は油冷し、光学顕微鏡（倍率は１００倍）でランダムに１０視野観察して粗粒化特性を調査した。なお、粗粒化の判定基準は前記の（実施例１）の場合と同じである。すなわち、JIS G 0551に準じて、視野間において３以上異なった粒度番号の視野が存在する場合に「混粒」として、異常粒成長が生じたと見なした。一方、「粗粒」と「細粒」の判定はＪＩＳ基準よりも厳しくして、結晶粒度番号６未満の場合に「粗粒鋼」として結晶粒が粗大化したと判定した。
【００７７】
表５〜６に、上記の粗粒化特性の調査結果を示す。
【００７８】
【表５】

【００７９】
【表６】

【００８０】
表５〜６から、本発明で規定する化学組成を有する鋼の場合であっても、本発明の規定を外れた条件で熱間加工した場合には、粗粒化開始温度が９５０℃以下と低く、耐粗粒化特性に劣ることが明らかである。
【００８１】
（実施例３）
前記の（実施例１）で述べた鋼９の３トン鋼塊から分割した鋼塊を用いて、１２００℃に加熱した後、通常の方法で直径１００ｍｍの鋼片に熱間鍛造した。なお、いずれの場合も熱間鍛造の仕上げ温度は９５０℃とした。又、鍛造終了後は５００℃までの温度域を２℃／分あるいは１５℃／分の冷却速度で冷却し、５００℃を下回る温度域の冷却は放冷とした。
【００８２】
次いで、上記の直径１００ｍｍの各鋼片を１１００℃に再加熱した後、通常の方法で直径５０ｍｍの丸棒に熱間鍛造した。なお、熱間鍛造の仕上げ温度は９００℃とした。又、鍛造終了後は５００℃までの温度域を３℃／分あるいは２５℃／分の冷却速度で冷却し、５００℃を下回る温度域の冷却は放冷とした。
【００８３】
上記のようにして得た直径５０ｍｍの丸棒を、通常の方法によって７３０℃で球状化焼鈍した後、（Ｒ／２）の部位（Ｒ＝２５ｍｍ）から直径１０ｍｍ×長さ２０ｍｍの円筒状の試験片を切り出し、５００トン高速プレス機を用いて通常の方法で据え込み率８５％の常温（室温）据え込み加工を行った。
【００８４】
次いで、前記の据え込み加工した試験片を８５０〜１０５０℃まで２５℃刻みに各温度で６時間加熱して浸炭処理時の加熱をシミュレートした。
【００８５】
加熱処理後は油冷し、光学顕微鏡（倍率は１００倍）でランダムに１０視野観察して粗粒化特性を調査した。なお、粗粒化の判定基準は前記の（実施例１）及び（実施例２）の場合と同じである。
【００８６】
表７に、上記の粗粒化特性の調査結果を示す。
【００８７】
【表７】

【００８８】
表７から、本発明で規定する化学組成を有する鋼の場合であっても、熱間加工後に本発明の規定を外れた条件で冷却した場合には、粗粒化開始温度が８５０℃以下と低く、耐粗粒化特性に劣ることが明らかである。
【００８９】
【発明の効果】
本発明の方法で製造された耐粗粒化肌焼鋼鋼材は、冷間加工による精密な成形加工を行い、次いで浸炭処理しても結晶粒の粗大化や異常粒成長を生ずることがないので、自動車や産業機械用の歯車やシャフト類など浸炭処理が施される部品の母材として利用することができる。本発明の方法で製造された耐粗粒化肌焼鋼鋼材を用いれば、熱間での粗成形後に行われていた歯切りなどの機械加工による整形加工を省略することができるので、自動車や産業機械用の歯車やシャフト類などを低コストで製造することが可能である。 BACKGROUND OF THE INVENTION
[0001]
  The present inventionIs resistantRegarding the manufacturing method of coarse grained case-hardened steel, more specifically, the mother of parts subjected to carburizing treatment such as gears and shafts for automobiles and industrial machineryWith materialCoarse grain-resistant skinSteelThe present invention relates to a method for manufacturing a material.
[Prior art]
[0002]
  Parts to be carburized (hereinafter referred to as carburized parts) such as gears and shafts for automobiles and industrial machines are conventionally roughly formed by hot working such as hot forging, and then machined such as gear cutting. After being subjected to the shaping process, the carburizing process has been applied. As the base steel for the carburized parts, case hardening steels such as chrome steels represented by SCr415 and SCr420, which are JIS standard steels, and chromium molybdenum steels represented by SCM415 and SCM420, have been used.
[0003]
  However, recently, it has been directed to reduce costs by omitting a shaping process by machining such as gear cutting. For this reason, as a method of manufacturing carburized parts, there are many methods of carrying out precise forming processing by cold working (such as cold forging) with excellent forming accuracy instead of hot rough forming, and then carburizing. It's getting on.
[0004]
  However, in the case of carburized parts using conventional JIS standard steel as the base steel and carburized after cold forging as described above, conventional carburized parts (carburized parts subjected to rough forming by hot working) ), Grain coarsening and abnormal grain growth (hereinafter referred to as “coarse graining”) are caused by distortion and material characteristics during quenching. The problem of reduction is likely to occur.
[0005]
  For this reason, as a case hardening steel used as a base material steel for carburized parts, an alternative to the conventional JIS standard steel is disclosed in, for example, Japanese Patent Application Laid-Open No. 60-21359. However, even when the steel proposed in this publication, especially the steel added with Nb is used as the base steel, the ratio of Al (N) / N (%), which is the ratio of the content of Al and N, Since no consideration is given to the value, when carburized parts are manufactured by carburizing after cold forging, coarsening may occur.
[0006]
  Japanese Unexamined Patent Publication No. 63-140031 discloses carburizing that restricts the rolling heating temperature and rolling end temperature of carburizing steel containing specific amounts of Al and N to prevent abnormal growth of crystal grains during carburizing treatment. A steel manufacturing method is disclosed. However, even when steel containing Al and N within the range proposed in this publication is manufactured at the specified rolling heating temperature and rolling end temperature, carburized parts are manufactured by carburizing after cold forging. As a result, coarsening may still occur.
[0007]
  For this reason, precision forming by cold working is performed, and then coarse grain-resistant case-hardening steel that can prevent crystal grain coarsening and abnormal grain growth from occurring even if carburized under normal conditions, and its It has been eager to develop a method for producing case-hardened steel.
[Problems to be solved by the invention]
[0008]
  The present invention has been made in view of the above-mentioned present situation, and performs precise forming processing by cold working, and then prevents grain coarsening and abnormal grain growth even when carburizing treatment is performed. Skin burningSteelIt aims at providing the manufacturing method of.
[0009]
  The gist of the present invention is as follows.(1) andAnd(2)It exists in the manufacturing method of the coarse-grain-hardening case hardening steel material shown.
[0010]
  (1) It becomes a base material of parts manufactured by carburizing after cold working and formingCoarse grain resistanceIt is a manufacturing method of case hardening steel materials, Comprising: In weight%, C: 0.1-0.3%, Si: 0.1-1.0%, Mn: 0.3-2.0%, Al: 0.01 to 0.06%, N: 0.005 to 0.03%, Al (%) / N (%): 1.0 to 2.0, Nb: 0 to 0.07%, V: 0 0.005 to 0.1% and Nb (%) + V (%) ≧ 0.005%, Cu: 0 to 0.3%, Ni: 0 to 0.5%, Cr: 0 to 2.0% , Mo: 0 to 0.5%, W: 0 to 0.5%, Pb: 0 to 0.3%, Te: 0 to 0.08%, Ca: 0 to 0.01%, Bi: 0 to 0 0.3%, S: 0.005 to 0.08%, P: 0.03% or less, with the balance being heated by heating steel having a chemical composition composed of Fe and inevitable impurities to a temperature of 1100 ° C. or higher The above hot working is performed in a temperature range of 850 ° C. or higher. Finishing temperature, then manufacturing method of 耐粗 granulated hardened steels characterized by cooling the temperature range of up to 500 ° C. at a cooling rate of 5 to 500 ° C. / min.
[0011]
  (2) It becomes a base material for parts to be manufactured by carburizing after cold workingCoarse grain resistanceIt is a manufacturing method of case hardening steel materials, Comprising: In weight%, C: 0.1-0.3%, Si: 0.1-1.0%, Mn: 0.3-2.0%, Al: 0.01 to 0.06%, N: 0.005 to 0.03%, Al (%) / N (%): 1.0 to 2.0, Nb: 0 to 0.07%, V: 0 0.005 to 0.1% and Nb (%) + V (%) ≧ 0.005%, Cu: 0 to 0.3%, Ni: 0 to 0.5%, Cr: 0 to 2.0% , Mo: 0 to 0.5%, W: 0 to 0.5%, Pb: 0 to 0.3%, Te: 0 to 0.08%, Ca: 0 to 0.01%, Bi: 0 to 0 0.3%, S: 0.005 to 0.08%, P: 0.03% or less, with the balance being heated by heating steel having a chemical composition composed of Fe and inevitable impurities to a temperature of 1100 ° C. or higher The above hot working is performed in a temperature range of 850 ° C. or higher. After finishing at a temperature, and then cooling the temperature range up to 500 ° C. at a cooling rate of 5 to 500 ° C./min, it is further reheated to a temperature of 1000 ° C. or higher to perform hot working. Finishing at a temperature in a temperature range of 850 ° C. or higher, and then cooling the temperature range up to 500 ° C. at a cooling rate of 5 to 500 ° C./min.
[0012]
  Here, “Al” refers to so-called “acid-soluble Al”. In the following, the above (1),(2)Inventions of (1) and (2) respectively.With lightSay.Also, it may be collectively referred to as “the present invention”.
DETAILED DESCRIPTION OF THE INVENTION
[0013]
  The present inventors melt steel having various chemical compositions in an experimental furnace, produce steel materials by changing hot working conditions and cooling conditions after hot working, spheroidizing annealing, cold working by ordinary methods Processing and carburizing and quenching were performed, and the effects of component elements, hot working conditions and cooling conditions after hot working on the structure after carburizing and quenching were investigated.
[0014]
  As a result, the following(1)~(8)The knowledge of was obtained.
[0015]
  (1)When Al, Nb, and V that generate nitrides and carbonitrides are added alone, a certain degree of coarsening prevention effect is observed. However, simply adding two or more of the above elements in combination does not necessarily increase the effect of preventing coarsening, and may even cause coarsening.
[0016]
  (2)When the contents of Al, Nb and V are controlled to add Al, Nb and V in combination, and the N content is also controlled, a large effect of preventing coarsening can be obtained. If the control of the content of each element described above is not appropriate, the effect of combined addition of Al, Nb, and V does not appear, and the cost only increases.
[0017]
  (3)In order to prevent coarsening during the carburizing process, in addition to the control of the contents of Al, Nb, V and N described above, the ratio of the contents of Al and N (Al (%) / N (%)) Need to be controlled.
[0018]
  (Four)Since the nitrides and carbonitrides of Al, Nb, and V produced in the solidification process after melting are agglomerated and coarsened, the effect of preventing coarsening during the carburizing process is small in that state. However, once the nitride or carbonitride is heated to a high temperature during hot processing and then dissolved in the base austenite, the nitride or carbonitride during hot processing and subsequent cooling Since the product can be finely precipitated, the effect of preventing coarsening during the carburizing process is extremely increased.
[0019]
  (Five)the above(Four)If it is necessary to further hot-work the steel material on which the nitride or carbonitride is finely precipitated by further hot working and subsequent cooling to the desired shape and size, the reheating temperature for hot working should be reduced. It is important to control and prevent the coarsening of nitrides and carbonitrides.
[0020]
  (6)In order to finely precipitate nitrides and carbonitrides during and after hot working, it is important to finish hot working in an appropriate temperature range. If the finishing temperature range is inappropriate, nitrides and carbonitrides are agglomerated and coarsened, and the effect of preventing coarsening becomes extremely small.
[0021]
  (7)In order to prevent coarsening of nitrides and carbonitrides, it is necessary to cool under appropriate cooling conditions after the hot working is completed.
[0022]
  (8)When the component elements, hot working, and subsequent cooling conditions are optimized, nitride and carbonitride are not coarsened even during spheroidizing annealing as a pre-process of cold working. Can be prevented from coarsening.
[0023]
  The present invention has been completed based on the above findings.
[0024]
  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the component content means “% by weight”.
[0025]
  (A) Chemical composition of steel
  C: 0.1 to 0.3%
  C is an element effective for enhancing the hardenability of steel and improving the static strength. However, if its content is less than 0.1%, the effect of addition is poor, while if it exceeds 0.3%, the toughness of the steel decreases. Therefore, the content of C is set to 0.1 to 0.3%.
[0026]
  Si: 0.1 to 1.0%
  Si is an element effective for deoxidation, improving hardenability, and improving static strength by solid solution strengthening. However, if the content is less than 0.1%, the desired effect cannot be obtained. On the other hand, if the content exceeds 1.0%, the carburizability is lowered and the fatigue strength is lowered. Furthermore, it causes a decrease in cold workability. Therefore, the Si content is set to 0.1 to 1.0%.
[0027]
  Mn: 0.3 to 2.0%
  Mn also has a deoxidizing action like Si. It also has the effect of improving hardenability and improving strength and toughness. However, if the content is less than 0.3%, the effect of addition is poor. On the other hand, even if it contains more than 2.0, the effect is saturated and economical efficiency is only impaired. Therefore, the Mn content is set to 0.3 to 2.0%.
[0028]
  Al: 0.01 to 0.06%
  Al combines with N in the steel to form AlN and has the effect of preventing coarsening. In order to exhibit this effect, it is necessary to make Al content 0.01% or more. On the other hand, even if the content exceeds 0.06%, the above-described effect is saturated and the machinability is reduced. Therefore, the Al content is set to 0.01 to 0.06%. As described above, “Al” here means “acid-soluble Al”.
[0029]
  N: 0.005 to 0.03%
  N combines with Al, Nb and V to form nitrides and carbonitrides, and is effective in preventing coarsening. However, if the content is less than 0.005%, the desired effect cannot be obtained. On the other hand, if the content exceeds 0.03%, the deformation resistance of the steel increases and the cold workability decreases. Therefore, the N content is set to 0.005 to 0.03%.
[0030]
  Al (%) / N (%): 1.0 to 2.0
  The ratio of Al and N content (Al (%) / N (%)) is not appropriate even in the range of the content of Al and N described above and the range of the content of Nb and V described later. In some cases, it is impossible to prevent coarsening during carburizing. That is, when the value of Al (%) / N (%) exceeds 2.0, the combined effect of Al, Nb and V for preventing coarsening cannot be obtained. Furthermore, when performing spheroidizing annealing as a pre-process of cold working, since the nitride and carbonitride are extremely coarsened even if the hot working conditions and the subsequent cooling conditions described later are satisfied, carburizing treatment The coarsening of the time cannot be prevented. On the other hand, if Al (%) / N (%) is less than 1.0, the cold workability is deteriorated, and cracking is likely to occur during the forming into a desired shape in the cold. In this case, it is not possible to meet the demands of the industry for reducing the cost by carrying out precise forming processing by cold processing instead of rough hot forming and omitting machining. Therefore, Al (%) / N (%) was set to 1.0 to 2.0.
[0031]
  Nb: 0 to 0.07%
  Nb may not be added. When added, it has a function of forming a nitride or carbonitride to prevent coarsening. If steel and aluminum are controlled in combination with Al and V, an extremely excellent effect of preventing coarsening can be obtained. However0. Even if the content exceeds 07%, the above-described effects are saturated and the economic efficiency is deteriorated, and the deformation resistance is increased and the cold workability is deteriorated. Therefore, the Nb content is set to 0 to 0.07%.
[0032]
  V:0.005~ 0.1%
  VNiIt has a function of preventing coarsening by forming a fluoride or carbonitride. If steel and N and the content of N are combined and added together with Al and Nb, an extremely excellent effect of preventing coarsening can be obtained. HoweverThisTo obtain the effect ofIs VNeeds to be 0.005% or more. On the other hand, if the content exceeds 0.1%, the above effect is saturated and the economic efficiency is impaired, and the deformation resistance is increased and the cold workability is deteriorated. Therefore, the V content is0.005˜0.1%.
[0033]
  Nb (%) + V (%): 0.005% or more
  In order to obtain an extremely excellent coarsening prevention effect by adding Nb and V together with Al to steel with controlled N contentIn addition,The sum of the contents of Nb and V (Nb (%) + V (%)) is 0.005% or more.did.
[0034]
  Cu: 0 to 0.3%
  Cu may not be added. Addition improves the hardenability of the steel. In order to reliably obtain this effect, the Cu content is preferably 0.1% or more. However, if it is contained in a large amount, the hot ductility is lowered and the hot workability is lowered. In particular, when the content exceeds 0.3%, the hot workability is significantly deteriorated. Therefore, the Cu content is set to 0 to 0.3%.
[0035]
  Ni: 0 to 0.5%
  Ni need not be added. If added, it has the effect of enhancing the hardenability and toughness of the steel. In order to reliably obtain this effect, the Ni content is preferably 0.05% or more. However, even if the content exceeds 0.5%, the above effects are saturated and the cost is increased. Therefore, the Ni content is set to 0 to 0.5%.
[0036]
  Cr: 0 to 2.0%
  It is not necessary to add Cr. If added, it has the effect of enhancing the hardenability, strength and toughness of the steel. In order to reliably obtain this effect, the Cr content is preferably 0.1% or more. However, even if the content exceeds 2.0%, the above effect is saturated and the economic efficiency is only impaired. Therefore, the content of Cr is set to 0 to 2.0%.
[0037]
  Mo: 0 to 0.5%
  Mo need not be added. If added, it has the effect of enhancing the hardenability, strength and toughness of the steel. In order to reliably obtain this effect, the Mo content is preferably 0.05% or more. However, if the content exceeds 0.5%, the machinability deteriorates. Therefore, the Mo content is set to 0 to 0.5%.
[0038]
  W: 0 to 0.5%
  W does not need to be added. If added, it has the effect of enhancing the hardenability, strength and toughness of the steel. In order to reliably obtain this effect, the W content is preferably 0.05% or more. However, if the content exceeds 0.5%, the machinability deteriorates. Therefore, the W content is set to 0 to 0.5%.
[0039]
  Pb: 0 to 0.3%
  Pb may not be added. However, if added, it has a function of improving machinability. For this reason, when it is desired to finish the inner surface of a part formed by cold working by further precise cutting, it may be added in order to improve the machinability. In this case, the Pb content is preferably 0.005% or more in order to reliably obtain the machinability improving effect. However, if the content exceeds 0.3%, the fatigue characteristics are deteriorated. Therefore, the Pb content is set to 0 to 0.3%.
[0040]
  Te: 0 to 0.08%
  Te may not be added. If added, it has the effect of improving the machinability like Pb. For this reason, when it is desired to finish the inner surface of a part formed by cold working by further precise cutting, it may be added in order to improve the machinability. In this case, in order to reliably obtain the effect of improving machinability, Te is preferably set to a content of 0.01% or more. However, when the content exceeds 0.08%, the hot workability is lowered. Therefore, the Te content is set to 0 to 0.08%.
[0041]
  Ca: 0 to 0.01%
  Ca need not be added. If added, there is an effect of improving the machinability. For this reason, in the case where it is desired to further finish the inner surface of a part formed by cold working like Pb or Te by further precise cutting, it may be added to improve the cutting ability. In this case, in order to reliably obtain the machinability improving effect, the Ca content is preferably 0.001% or more. However, when the content exceeds 0.01%, the hot workability is lowered. Therefore, the content of Ca is set to 0 to 0.01%.
[0042]
  Bi: 0 to 0.3%
  Bi may not be added. If added, it has the effect of improving the machinability. For this reason, when it is desired to further precisely cut and finish the inner surface of a part formed by cold working like Pb, Te and Ca, it may be added to improve the cutting ability. In this case, in order to reliably obtain the machinability improving effect, the Bi content is preferably set to 0.01% or more. However, when the content exceeds 0.3%, the hot workability is lowered. Therefore, the Bi content is set to 0 to 0.3%.
[0043]
  S:0.005~ 0.08%
  SIs offHas the effect of improving machinabilityThe coldInternal surfaces of parts formed by hot workingSpermFinish by dense cuttingGellHas an S content of 0.005% or more.There is a need.However, when the content exceeds 0.08%, the toughness is lowered. Therefore, the S content0.005-0.08%.
[0044]
  P: 0.03% or less
  P segregates at the grain boundaries and reduces toughness. In particular, when the content exceeds 0.03%, the toughness is significantly reduced. Therefore, the P content is set to 0.030% or less.
[0045]
  (B) Hot working and subsequent cooling
  (B-1) Heating
  Nitride and carbonitride of Al, Nb, and V in steel ingots generated in the solidification process after melting (herein, “steel ingot” includes “slab” as in JIS G 0203) Is agglomerated and coarsened, the effect of preventing coarsening at the time of carburizing is small. Therefore, as described above(1)Hot working of the invention and(2)In the previous (primary) hot working of the present invention, the above nitride or carbonitride is once heated to a high temperature to be dissolved in austenite as a base, and is nitrided during hot working and after cooling. It is necessary to finely precipitate materials and carbonitrides to achieve prevention of coarsening during carburizing treatment.
[0046]
  Therefore, it has the chemical composition (A) described above.SteelThe(1)Hot working in the invention of(2)In the previous (primary) hot working in the invention, the above-described nitride or carbonitride is heated to a temperature of 1100 ° C. or more to be dissolved in austenite. The heating temperature is preferably 1150 ° C. or higher, and more preferably 1200 ° C. or higher. There is no restriction | limiting in particular in the upper limit of this heating temperature. However, it is preferable to set the upper limit at about 1350 ° C. in order to suppress cost increase due to decarburization and scale loss, and to further reduce energy costs.
[0047]
  (B-2) Finishing temperature
  In order to precipitate Al, Nb and V dissolved in austenite as fine nitrides and carbonitrides during hot working and subsequent cooling, the finishing temperature of hot working needs to be 850 ° C or higher. is there. When the finishing temperature of hot working is lower than 850 ° C., Al, Nb and V nitrides and carbonitrides partially precipitated during hot working are agglomerated and coarsened, and excellent coarse-resistant grains during carburizing treatment. In some cases, the object of the present invention for obtaining the conversion characteristics cannot be achieved. Therefore,(1)The finishing temperature of hot working in the invention of(2)The finishing temperature of the previous (primary) hot working in the present invention was set to 850 ° C. or higher. In addition, there is no restriction | limiting in particular in the upper limit of the finishing temperature of the said hot working. However, it is preferable to set the upper limit at about 1050 ° C. in order to suppress decarburization.
[0048]
  (B-3) Cooling after hot working
  After finishing the hot working, it is necessary to cool the temperature range up to 500 ° C. at a cooling rate of 5 to 500 ° C./min. When the cooling rate of this cooling is less than 5 ° C./min, the nitrides and carbonitrides of Al, Nb, and V are agglomerated and coarsened, and an attempt is made to obtain excellent grain resistance characteristics during carburizing treatment. The object of the present invention may not be achieved. In particular, when performing spheroidizing annealing as a pre-process of cold working, even in the case of steel having the chemical composition of item (A) described above, nitrides and carbonitrides are coarsened. It cannot prevent coarsening during carburizing. On the other hand, when the cooling rate of the cooling exceeds 500 ° C./min, a so-called “low temperature transformation structure” such as bainite or martensite may be generated and it may be difficult to cut into a desired size. Therefore,(1)The cooling rate in the temperature range up to 500 ° C. after the hot working finish of the invention of(2)The cooling rate in the temperature range up to 500 ° C. after the previous (primary) hot working finish of the invention was defined as 5 to 500 ° C./min. The upper limit of the cooling rate is preferably about 300 ° C./min, and more preferably about 100 ° C./min.
[0049]
  Cooling in a temperature range below 500 ° C. may be quenched in order to increase productivity. In addition, said cooling rate says the cooling rate of the steel material surface.
[0050]
  By the steps (B-1) to (B-3) above(1)The present invention is configured. In addition,(1)According to the present invention, after the above hot working and cooling, no further hot working (secondary hot working) is applied, and carburizing is performed in a desired shape by cold working and then carburized. The present invention provides a method for producing a coarse-grained case-hardened steel material that is a base material for parts. That is, it has the chemical composition described in the item (A) and is manufactured by the steps (B-1) to (B-3).(1)The coarse grain-resistant case-hardened steel material according to the invention of (B-3) is not subjected to further hot processing after the step (B-3), and is subjected to spheroidizing annealing as necessary. It is formed into a desired part shape by cold working such as cold forging, and further, the inner surface and the like are precisely cut as necessary, and then carburized and quenched. Then, if necessary, the final part is finished by tempering, grinding and polishing at a low temperature.
[0051]
  On the other hand, after the step (B-3), the steel material is reheated and hot-worked (secondary hot-working) for the purpose of further forming a desired size and shape, and then cooled. No(2)It is a manufacturing method of the coarse grain-resistant case hardening steel material which concerns on this invention. Less than,(2)The requirements according to the invention will be further described.
[0052]
  (B-4) Reheating after cooling
  Al, Nb, and V nitrides and carbonitrides, which are produced in the solidification process after melting and are agglomerated and coarsened, are dissolved in austenite and finely formed as nitrides and carbonitrides during hot working and subsequent cooling. After the precipitation, the reheating temperature at the time of hot working (secondary hot working) for obtaining a desired size or shape of the shaped material needs to be 1000 ° C. or higher. If this reheating temperature is lower than 1000 ° C., the nitrides and carbonitrides are coarsened, and the deformation resistance of the steel material increases and the load on the processing machine such as a rolling mill becomes excessive. is there. Therefore,(2)In this invention, the reheating temperature for hot working after cooling (secondary hot working) was set to 1000 ° C. or higher. The reheating temperature is desirably 1100 ° C. or higher. There is no restriction | limiting in particular in the upper limit of this heating temperature. However, in order to suppress an increase in cost due to decarburization and scale loss and to further reduce energy costs, the upper limit is preferably about 1250 ° C., and more preferably 1200 ° C.
[0053]
  (B-5) Hot working finishing temperature after reheating
  The finishing temperature of the hot working after the reheating (secondary hot working) needs to be 850 ° C. or higher. When this temperature is lower than 850 ° C., Al, Nb and V nitrides and carbonitrides partially precipitated during hot working are agglomerated and coarsened, and excellent coarse grain resistance characteristics are obtained during carburizing treatment. In some cases, the object of the present invention is not achieved. Therefore,(2)In this invention, the finishing temperature of the hot working after the reheating (secondary hot working) was set to 850 ° C. or higher. The upper limit of the temperature is not particularly limited. However, it is preferable to set the upper limit at about 1050 ° C. in order to suppress decarburization.
[0054]
  (B-6) Cooling after hot working after reheating
  After finishing the hot working after the reheating (secondary hot working), when the cooling rate in the temperature range up to 500 ° C. is lower than 5 ° C./min, nitrides or charcoal of Al, Nb, V In some cases, the nitride is agglomerated and coarsened, and the object of the present invention, which is intended to obtain excellent coarse grain resistance characteristics during carburizing treatment, may not be achieved. In particular, when performing spheroidizing annealing as a pre-process of cold working, even in the case of steel having the chemical composition of item (A) described above, nitrides and carbonitrides are coarsened. It cannot prevent coarsening during carburizing. On the other hand, when the cooling rate is higher than 500 ° C./min, a so-called “low temperature transformation structure” such as bainite or martensite may occur and it may be difficult to cut into a desired size. Therefore,(2)In this invention, after finishing the hot working after reheating (secondary hot working), the cooling rate in the temperature range up to 500 ° C. was defined as 5 to 500 ° C./min. The upper limit of the cooling rate is preferably about 300 ° C./min, and more preferably about 100 ° C./min.
[0055]
  In addition, you may quench rapidly the cooling of the temperature range below 500 degreeC in order to improve productivity. Here, the cooling rate means the cooling rate of the steel material surface as described above.
[0056]
  By the steps (B-1) to (B-6) above(2)The present invention is configured. It has the chemical composition described in the item (A) and is manufactured by the steps (B-1) to (B-6).(2)The coarse grain-resistant case-hardened steel according to the invention of the present invention is formed into a desired part shape by cold working including cold forging after being subjected to spheroidizing annealing as necessary. Accordingly, the inner surface and the like are precision cut and then carburized and quenched. Then, if necessary, the final part is finished by tempering, grinding and polishing at a low temperature.
【Example】
[0057]
  Example 1
  Steels having chemical compositions shown in Tables 1 and 2 were melted by a usual method using a 3 ton test furnace. In Table 1 and Table 2.Steel 8-15, Steel 17-21 and Steel 23The chemical composition is within the range specified in the present invention.Steel (invention steel)It is. On the other hand, steels 25 to 34 in Table 2 are out of the content range defined in the present invention by any of the components.Steel (Comparative steel)It is.
[0058]
[Table 1]

[0059]
[Table 2]

[0060]
  Next, these steels were heated, split-rolled and cooled under the conditions described in Tables 3 to 4 to obtain 180 mm square steel pieces, and further reheated, hot-rolled and cooled under the conditions described in Tables 3 to 4 Thus, a steel bar having a diameter of 50 mm was manufactured. Here, in the production of a 180 mm square steel piece and a 50 mm diameter steel bar, cooling in a temperature range below 500 ° C. was allowed to cool.
[0061]
  Note that the steel of the present invention example.Steel8, steel 9, steel15 andThe steel 22 was divided into 3 ton steel ingots and then used for the production of the above steel slabs and steel bars.
[0062]
[Table 3]

[0063]
[Table 4]

[0064]
  Steel obtained as above8-15, steel 17-21, steel 23 and steel 25~ 34 diameter 50mm steel bar and someSteel (steelSteel slabs of 13, mm 14, steel 17, steel 18, steel 18 and steel 20) were spheroidized at 730 ° C. by a conventional method. After this, from the part of (T / 4) (T = 180 mm) for the steel piece of 180 mm square, from the part of (R / 2) (R = 25 mm) for the steel bar having a diameter of 50 mm, the diameter is 10 mm × length. A 20 mm cylindrical test piece was cut out and subjected to an upsetting test at room temperature (room temperature) by a normal method using a 500-ton high-speed press. Here, the upsetting rate (compression rate) was set to four conditions of 70%, 75%, 80%, and 85%, and the number of tests was 20 for each condition. After the upsetting test, the cold workability was evaluated by visual inspection for occurrence of cracks.
[0065]
  Tables 3 to 4 also show the results of the cold workability evaluation described above. In addition, when a crack was observed in any of the 20 test pieces under each condition, the lowest upsetting rate at which the crack occurred was described as cold workability. In Tables 3 to 4, the cold workability of “> 85” indicates that no cracks occurred in any of the 20 test pieces in the upsetting test at an upsetting rate of 85%. On the other hand, the cold workability being “≦ 70” indicates that a crack occurred in an upsetting test with an upsetting rate of 70%.
[0066]
  Next, using the test piece installed at the upsetting rate of 85%, the heating at the carburizing process was simulated by heating at 850 to 1050 ° C. in increments of 25 ° C. for 6 hours at each temperature.
[0067]
  After the heat treatment, oil cooling was performed, and 10 visual fields were randomly observed with an optical microscope (magnification: 100 times) to investigate the coarsening characteristics. Note that the criteria for coarsening were in accordance with JIS G 0551. That is, it was considered that abnormal grain growth occurred as “mixed grain” when there were fields with different particle size numbers of 3 or more between the fields. On the other hand, the judgment of “coarse grain” and “fine grain” was made stricter than the JIS standard, and when the grain size number was less than 6, it was judged that the crystal grain was coarsened as “coarse grain steel”.
[0068]
  In Tables 3-4, the investigation result of said coarse grain characteristic is shown collectively. In addition, when coarsening generation | occurrence | production was recognized, the lowest heating temperature which coarsening produced was described as "the coarsening start temperature." The coarsening start temperature “> 1050” in Tables 3 to 4 indicates that no coarsening occurred under heating conditions of 1050 ° C. × 6 hours. On the other hand, the coarsening start temperature “≦ 850” indicates that coarsening has already occurred by heating at 850 ° C. × 6 hours.
[0069]
  According to Tables 3-4, when a steel having the chemical composition defined in the present invention is hot worked and cooled under the conditions defined in the present invention, it is cold worked at an upsetting rate of 85%. No cracking occurs, and when this is austenitized for 6 hours, the coarsening start temperature is as high as 975 ° C. or higher, and it is clear that the coarsening resistance is excellent.
[0070]
  On the other hand, among the steels of comparative examples in which any of the components deviates from the content range specified in the present invention, the steels 25 to 28 and the steels 31 to 34 are heated under the conditions specified in the present invention. Even if it is cold-worked and cooled, the coarsening start temperature is as low as 950 ° C. or less, and the coarsening resistance is poor. Among the above steels, in the case of steel 26, cold workability is also low, so that cracking is likely to occur during forming into a desired shape in the cold. Therefore, there is a case where it is not possible to meet the demands of the industry for reducing the cost by carrying out precise forming by cold working instead of rough hot forming and omitting machining.
[0071]
  On the other hand, among the steels of the comparative examples, the steel 29 and the steel 30 have a coarsening start temperature exceeding 1050 ° C. when the austenitizing treatment is performed for 6 hours, which is excellent in the coarsening resistance property. Low processability. For this reason, it becomes easy to produce a crack at the time of the shaping | molding process to the desired shape in cold similarly to the case of the said steel 26. FIG. Therefore, there is a case in which it is not possible to meet the demands of the industry to reduce costs by performing precision processing by cold processing instead of hot rough forming and omitting machining.
[0072]
  (Example 2)
  Described in (Example 1) aboveSteelUsing steel ingots divided from 3 ton steel ingots of 8 and 15 to 1000 to 1200 ° C. in 100 ° C. increments, each was hot forged into a steel piece having a diameter of 100 mm by a normal method. In either case, the finishing temperature for hot forging was 950 ° C. Moreover, after the forging was completed, the temperature range up to 500 ° C. was cooled at a cooling rate of 10 ° C./min, and the cooling in the temperature range below 500 ° C. was allowed to cool.
[0073]
  Next, each steel piece having a diameter of 100 mm was reheated to each temperature in increments of 50 ° C. from 950 to 1100 ° C., and then hot forged into a round bar having a diameter of 50 mm by an ordinary method. In addition, the finishing temperature of hot forging was made into two conditions, 815 degreeC and 885 degreeC. Further, after the forging was completed, the temperature range up to 500 ° C. was cooled at a cooling rate of 25 ° C./min, and the cooling in the temperature range below 500 ° C. was allowed to cool.
[0074]
  The round bar having a diameter of 50 mm obtained as described above was subjected to spheroidizing annealing at 730 ° C. by a usual method, and then a cylindrical shape having a diameter of 10 mm × length of 20 mm from the portion (R / 2) (R = 25 mm). A test piece was cut out and subjected to normal temperature (room temperature) upsetting with an upsetting rate of 85% by a normal method using a 500-ton high-speed press.
[0075]
  Next, the upsetting processed test piece was heated at 850 to 1050 ° C. in increments of 25 ° C. for 6 hours at each temperature to simulate heating during the carburizing treatment.
[0076]
  After the heat treatment, oil cooling was performed, and 10 visual fields were randomly observed with an optical microscope (magnification: 100 times) to investigate the coarsening characteristics. Note that the criteria for determining the coarsening are the same as in the case of (Example 1). That is, according to JIS G 0551, it was considered that abnormal grain growth occurred as “mixed grain” when there was a field having a grain number different by 3 or more between the fields. On the other hand, the judgment of “coarse grain” and “fine grain” was made stricter than the JIS standard, and when the grain size number was less than 6, it was judged that the crystal grain was coarsened as “coarse grain steel”.
[0077]
  Table 56Shows the results of the investigation of the coarsening characteristics.
[0078]
[Table 5]

[0079]
[Table 6]

[0080]
  Table 56Therefore, even in the case of a steel having a chemical composition specified in the present invention, when hot working is performed under conditions that do not comply with the present invention, the coarsening start temperature is as low as 950 ° C. It is clear that the granulation characteristics are inferior.
[0081]
  (Example 3)
  As described in Example 1 aboveOf steel 9Using a steel ingot divided from a 3-ton steel ingot, it was heated to 1200 ° C. and then hot forged into a steel piece having a diameter of 100 mm by a normal method. In either case, the finishing temperature for hot forging was 950 ° C. Further, after the forging was completed, the temperature range up to 500 ° C. was cooled at a cooling rate of 2 ° C./min or 15 ° C./min, and the cooling in the temperature range below 500 ° C. was allowed to cool.
[0082]
  Next, each steel piece with a diameter of 100 mm was reheated to 1100 ° C., and then hot forged into a round bar with a diameter of 50 mm by an ordinary method. The finishing temperature for hot forging was 900 ° C. Further, after the forging was completed, the temperature range up to 500 ° C. was cooled at a cooling rate of 3 ° C./min or 25 ° C./min, and the cooling in the temperature range below 500 ° C. was allowed to cool.
[0083]
  The round bar having a diameter of 50 mm obtained as described above was subjected to spheroidizing annealing at 730 ° C. by a usual method, and then a cylindrical shape having a diameter of 10 mm × length of 20 mm from the portion (R / 2) (R = 25 mm). A test piece was cut out and subjected to normal temperature (room temperature) upsetting with an upsetting rate of 85% by a normal method using a 500-ton high-speed press.
[0084]
  Next, the upsetting processed test piece was heated at 850 to 1050 ° C. in increments of 25 ° C. for 6 hours at each temperature to simulate heating during the carburizing treatment.
[0085]
  After the heat treatment, oil cooling was performed, and 10 visual fields were randomly observed with an optical microscope (magnification: 100 times) to investigate the coarsening characteristics. Note that the criteria for determining the coarsening are the same as those in the above (Example 1) and (Example 2).
[0086]
  table7Shows the results of the investigation of the coarsening characteristics.
[0087]
[Table 7]

[0088]
  table7From, even in the case of steel having a chemical composition defined in the present invention, when cooled under conditions that deviate from the provision of the present invention after hot working, the coarsening start temperature is as low as 850 ° C. or less, It is clear that the coarse grain resistance is inferior.
[0089]
【The invention's effect】
  The present inventionManufactured by the method ofCoarse grained case hardening steelSteelSince it does not cause coarsening of crystal grains or abnormal grain growth even if carburizing is performed after precise forming by cold working, carburizing such as gears and shafts for automobiles and industrial machinery is performed. Parts motherWith materialCan be used. Of the present inventionManufactured by the methodBy using coarse grain-resistant case-hardened steel, gears and shafts for automobiles and industrial machinery can be omitted, since it is possible to eliminate shaping by mechanical processing such as gear cutting that has been performed after hot rough forming. Can be manufactured at low cost.

Claims (2)

A method for producing a coarse grain-resistant case- hardened steel material, which is a base material for parts produced by cold-working and then carburizing, wherein C: 0.1 to 0.3% by weight , Si: 0.1 to 1.0%, Mn: 0.3 to 2.0%, Al: 0.01 to 0.06%, N: 0.005 to 0.03%, Al (%) / N (%): 1.0 to 2.0, Nb: 0 to 0.07%, V: 0.005 to 0.1%, and Nb (%) + V (%) ≧ 0.005%, Cu : 0-0.3%, Ni: 0-0.5%, Cr: 0-2.0%, Mo: 0-0.5%, W: 0-0.5%, Pb: 0-0. 3%, Te: 0 to 0.08%, Ca: 0 to 0.01%, Bi: 0 to 0.3%, S: 0.005 to 0.08%, P: 0.03% or less included The balance is 1% of steel having a chemical composition consisting of Fe and inevitable impurities. Heating to a temperature of 00 ° C. or higher to perform hot working, finishing the hot working at a temperature range of 850 ° C. or higher, and then cooling the temperature range to 500 ° C. to 5 to 500 ° C./min A method for producing a coarse-grained case-hardening steel material, characterized by cooling at a temperature. A method for producing a coarse grain-resistant case- hardened steel material, which is a base material for parts produced by cold-working and then carburizing, wherein C: 0.1 to 0.3% by weight , Si: 0.1 to 1.0%, Mn: 0.3 to 2.0%, Al: 0.01 to 0.06%, N: 0.005 to 0.03%, Al (%) / N (%): 1.0 to 2.0, Nb: 0 to 0.07%, V: 0.005 to 0.1%, and Nb (%) + V (%) ≧ 0.005%, Cu : 0-0.3%, Ni: 0-0.5%, Cr: 0-2.0%, Mo: 0-0.5%, W: 0-0.5%, Pb: 0-0. 3%, Te: 0 to 0.08%, Ca: 0 to 0.01%, Bi: 0 to 0.3%, S: 0.005 to 0.08%, P: 0.03% or less included The balance is 1% of steel having a chemical composition consisting of Fe and inevitable impurities. Heating to a temperature of 00 ° C. or higher to perform hot working, finishing the hot working at a temperature range of 850 ° C. or higher, and then cooling the temperature range to 500 ° C. to 5 to 500 ° C./min After cooling, the steel is further reheated to a temperature of 1000 ° C. or higher to perform hot working, and the hot working is finished at a temperature in the temperature range of 850 ° C. or higher. A method for producing a coarse grain-resistant case-hardened steel material, which is cooled at a cooling rate of 500 ° C / min.