JP5097047B2 - Steel for hot forging and method for producing the same - Google Patents
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本発明は、熱間鍛造用鋼材およびその製造方法に関し、詳しくは、１０００℃を超える温度に加熱して熱間鍛造を行うような中・高炭素系の熱間鍛造用鋼材とその製造方法、なかでも、自動車、産業機械および建設機械など高い転動疲労特性が要求される機械部品の素材として好適な熱間鍛造用鋼材とその製造方法に関する。 The present invention relates to a steel material for hot forging and a method for producing the same, and more specifically, a medium / high carbon steel material for hot forging which is heated to a temperature exceeding 1000 ° C. and subjected to hot forging, and a method for producing the same. In particular, the present invention relates to a steel material for hot forging suitable as a material for machine parts that require high rolling fatigue characteristics such as automobiles, industrial machines, and construction machines, and a method for manufacturing the same.
従来、自動車、産業機械および建設機械などの機械部品は、中・高炭素系の構造用鋼材を素材としてこれに熱間鍛造を繰り返して製造されているが、この様な熱間鍛造のための加熱温度は１０００℃を超え、時には１１００℃以上の高温となるため、オーステナイト粒（以下、オーステナイトを「γ」ともいう。）が粗大化する。 Conventionally, machine parts such as automobiles, industrial machines, and construction machines have been manufactured by repeatedly performing hot forging on medium- and high-carbon structural steel materials. Since the heating temperature exceeds 1000 ° C. and sometimes becomes a high temperature of 1100 ° C. or higher, austenite grains (hereinafter, austenite is also referred to as “γ”) are coarsened.
γ粒の粗大化に対しては、これを抑止するために、しばしばピン止め粒子として、窒化物が利用される。しかしながら、例えば、ハブに代表されるような高い転動疲労特性が求められる機械部品の場合、素材としてＴｉ、Ｎなどを添加した鋼材を用いると、これらの窒化物である粗大なＴｉＮなど硬質の介在物および析出物が疲労破壊の起点となるため、転動疲労特性が低下してしまう。 In order to suppress the coarsening of γ grains, nitrides are often used as pinning particles. However, for example, in the case of a machine part that is required to have high rolling fatigue characteristics such as a hub, when a steel material to which Ti, N, or the like is added is used as a material, a hard material such as coarse TiN that is a nitride of these is used. Inclusions and precipitates are the starting points for fatigue failure, so the rolling fatigue characteristics are degraded.
したがって、上記のハブに代表されるような高い転動疲労特性が要求される機械部品の素材には、特に、粗大な窒化物系の介在物および析出物を形成させないために、Ｔｉ、Ｎなどの含有量を低減させた鋼材が用いられるので、γ粒の粗大化傾向が強く、熱間鍛造のための加熱時のγ粒度は、ＪＩＳ Ｇ ０５５１（２００５）に記載の「鋼−結晶粒度の顕微鏡試験方法」に準じる粒度番号で負の値となるほどに粗大化する場合がある。 Therefore, in order to prevent the formation of coarse nitride-based inclusions and precipitates in the material of mechanical parts that require high rolling fatigue characteristics such as the hub described above, Ti, N, etc. Since a steel material with a reduced content is used, the tendency of coarsening of γ grains is strong, and the γ grain size at the time of heating for hot forging is described in JIS G 0551 (2005). In some cases, the particle size number according to the “microscopic test method” becomes coarser as it becomes a negative value.
このようなγ粒の粗大化は、素材そのものの機械特性の低下、あるいは熱間鍛造後に行われる冷間加工時の変形能の低下などの問題を引き起こす場合があり、硬質の介在物および析出物は極力低減させつつ、γ粒の粗大化抑制効果を有する鋼が求められている。 Such coarsening of γ grains may cause problems such as deterioration of mechanical properties of the raw material itself or deterioration of deformability during cold working performed after hot forging, and hard inclusions and precipitates. Therefore, there is a demand for steel having an effect of suppressing the coarsening of γ grains while reducing as much as possible.
このため、例えば、特許文献１〜３に、高温γ域加熱時のγ粒粗大化抑制効果を有した鋼を得る技術が開示されている。 For this reason, for example, Patent Documents 1 to 3 disclose techniques for obtaining steel having an effect of suppressing γ grain coarsening during high-temperature γ region heating.
具体的には、特許文献１に、特定の化学組成を有する鋼材を１１５０℃〜Ａ3点の温度に加熱した後熱間圧延することを特徴とする再加熱時のオーステナイト結晶粒の粗大化を抑制した「含Ｂ肌焼鋼の製造法」が開示されている。 Specifically, in Patent Document 1, a steel material having a specific chemical composition is heated to a temperature of 1150 ° C. to A 3 points and then hot-rolled, and then austenite crystal grains are coarsened during reheating. Suppressed “Method for Producing B-Skin Hardened Steel” is disclosed.
特許文献２に、特定量の成分元素を含む熱間圧延鋼線材または冷間伸線された鋼線材において、線材の中心軸に平行な縦断面の被顕面積１５０ｍｍ2内に存在する硫化物または硫化物を主体とする複合化合物のうち、被顕面積内に観察される断面積が６０μｍ2以上のものが６０個以下、かつ断面積が１０〜２０μｍ2のものが５００個以上である「冷間鍛造性の優れた鋼線材」が開示されている。 In Patent Document 2, in a hot-rolled steel wire or a cold-drawn steel wire containing a specific amount of component elements, sulfide existing in an exposed area of 150 mm 2 in a longitudinal section parallel to the central axis of the wire or Among the composite compounds mainly composed of sulfide, 60 or less having a cross-sectional area of 60 μm 2 or more observed within the exposed area and 500 or more having a cross-sectional area of 10 to 20 μm 2 A steel wire rod having excellent forgeability is disclosed.
特許文献３に、特定量の成分元素を含む鋼を製造する際、Ａｌ、Ｔｉで脱酸しＡｌ、Ｔｉの酸化物を晶出核としてＭｎＳを微細分散する「浸炭熱処理時に結晶粒が粗大化しない肌焼鋼の製造方法」が開示されている。 Patent Document 3 states that when steel containing a specific amount of component elements is produced, MnS is finely dispersed by deoxidizing with Al and Ti and using Al and Ti oxides as crystallization nuclei. “A method for producing a case-hardened steel” is disclosed.
前記の特許文献１で開示された技術は、ＴｉとＮの含有量を規定して、ＮをＴｉで固定してＢの焼入れ性向上効果を発現させるとともに、ＴｉＮによりオーステナイト粒をピン止めしてγ粒が粗大化することを抑制するものである。しかしながら、ＴｉＮは硬質な窒化物であるため、このような硬質粒子をピン止め粒子として析出させると、既に述べたように、比較的粗大なＴｉＮの介在物および析出物が疲労破壊の起点となり、このため、ハブに代表されるような高い転動疲労特性が要求される部品の素材には、転動疲労特性が低下してしまうことがある。 The technique disclosed in Patent Document 1 defines the contents of Ti and N, fixes N with Ti, exhibits the effect of improving the hardenability of B, and pins austenite grains with TiN. This suppresses the coarsening of the γ grains. However, since TiN is a hard nitride, when such hard particles are precipitated as pinning particles, relatively coarse inclusions and precipitates of TiN become the starting point of fatigue failure, as described above. For this reason, the rolling fatigue characteristics may be deteriorated in a material of a part that requires high rolling fatigue characteristics such as a hub.
特許文献２で開示された技術は、鋼中の硫化物形態を規定して鋼の変形能を高めることを図るものである。このため、硫化物は冷間鍛造性改善のために用いられるだけであって、オーステナイト粒の粗大化を抑制するという観点からのＭｎＳの形態制御については全く考慮されていない。 The technique disclosed in Patent Document 2 is intended to enhance the deformability of steel by defining the sulfide form in the steel. For this reason, the sulfide is only used for improving the cold forgeability, and no consideration is given to the morphology control of MnS from the viewpoint of suppressing the coarsening of the austenite grains.
特許文献３で開示された技術は、Ｃの含有量が０．１０〜０．３０％の肌焼鋼において、鋼の脱酸時に生成する微細な酸化物、鋼鋳造後の冷却時に晶析出する酸化物をＭｎＳの晶析出核として利用するため、Ａｌ、Ｔｉを添加し、ＭｎＳに加えて、Ａｌ、Ｔｉなどの窒化物によるオーステナイト結晶粒の粗大化防止効果を得るものであるため、ＴｉＮなど比較的粗大な硬質の介在物および析出物が疲労破壊の起点となり、このため、ハブに代表されるような高い転動疲労特性が要求される機械部品の素材として用いた場合には、転動疲労特性が低下してしまうことがある。しかも、１０００℃以下という浸炭温度ではγ粒の粗大化抑制効果を有していても、１０００℃を超えるような高い温度に加熱して熱間鍛造を行う場合には、十分なγ粒粗大化抑制効果を確保できるものではない。 The technique disclosed in Patent Document 3 is a case of a case-hardened steel having a C content of 0.10 to 0.30%. Fine oxides generated during deoxidation of steel, and crystal precipitation during cooling after steel casting. In order to use the oxide as a crystal precipitation nucleus of MnS, Al and Ti are added, and in addition to MnS, the effect of preventing the coarsening of austenite crystal grains by nitrides such as Al and Ti is obtained. Relatively coarse hard inclusions and precipitates are the starting point for fatigue failure. For this reason, when used as a material for machine parts that require high rolling fatigue characteristics such as hubs, rolling Fatigue properties may be reduced. Moreover, even if the carburizing temperature is 1000 ° C. or less, even if it has an effect of suppressing the coarsening of γ grains, when performing hot forging by heating to a high temperature exceeding 1000 ° C., sufficient γ grain coarsening The suppression effect cannot be ensured.
本発明は、上記現状に鑑みてなされたもので、優れたγ粒の粗大化抑制効果を有する中・高炭素系の熱間鍛造用鋼材とその製造方法、なかでも酸化物系、窒化物系などの介在物や析出物となりうるＴｉの含有量が０．００５質量％以下およびＮｂが０．００５％以下で、かつ窒化物を形成するＮの含有量が０．０１０質量％未満であっても優れたγ粒の粗大化抑制効果を有し、自動車、産業機械および建設機械など高い転動疲労特性が要求される機械部品の素材として好適な、Ｃの含有量が０．３０〜０．６５質量％である中・高中炭素系の熱間鍛造用鋼材とその製造方法を提供することを目的とする。 The present invention has been made in view of the above situation, and has an excellent effect of suppressing the coarsening of γ grains, and a steel material for hot forging of medium and high carbon type, and a manufacturing method thereof, in particular, an oxide type and a nitride type The content of Ti that can be inclusions and precipitates is 0.005% by mass or less, Nb is 0.005% or less, and the content of N that forms nitrides is less than 0.010% by mass. Has an excellent effect of suppressing the coarsening of γ grains, and is suitable as a material for machine parts that require high rolling fatigue characteristics such as automobiles, industrial machines, and construction machines. An object is to provide a medium / high / medium carbon steel for hot forging of 65% by mass and a method for producing the same.
前述したように、炭化物、窒化物など硬質の粒子をピン止め粒子として用いた場合には、転動疲労特性が低下することを避けられない。このため、ハブに代表されるような高い転動疲労特性が要求される機械部品に対して、炭化物、窒化物など硬質粒子のピン止め効果を適用することはできない。 As described above, when hard particles such as carbides and nitrides are used as the pinning particles, it is inevitable that the rolling fatigue characteristics deteriorate. For this reason, the pinning effect of hard particles such as carbides and nitrides cannot be applied to machine parts that require high rolling fatigue characteristics such as hubs.
そこで、本発明者らは、炭化物、窒化物などよりも軟質であるＭｎＳを、１０００℃を超えるような高い温度でのピン止め粒子として用いることについて種々検討した。具体的には、質量％で、０．３０〜０．６５％のＣを含有する中・高中炭素系の鋼材を用いて熱間加工を模擬した試験を実施し、ＭｎＳの形態と分布がγ粒の粗大化に及ぼす影響および加工熱処理の条件がＭｎＳの形態と分布に及ぼす影響について詳細に検討した。 Therefore, the present inventors have made various studies on the use of MnS, which is softer than carbides and nitrides, as pinning particles at a high temperature exceeding 1000 ° C. Specifically, a test simulating hot working was performed using a medium / high / medium carbon steel material containing 0.30 to 0.65% C by mass, and the morphology and distribution of MnS was γ. The influence on grain coarsening and the effect of thermomechanical treatment on the morphology and distribution of MnS were examined in detail.
その結果、下記（ａ）〜（ｅ）の知見を得た。 As a result, the following findings (a) to (e) were obtained.
（ａ）熱間鍛造のための加熱を１０００℃を超えるような高い温度で行った場合に、γ粒の粗大化を抑制するためには、被鍛造材である鋼材中のＭｎＳの数密度を高めることが有効である。 (A) When heating for hot forging is performed at a high temperature exceeding 1000 ° C., in order to suppress the coarsening of γ grains, the number density of MnS in the steel material to be forged is set to It is effective to increase.
（ｂ）鋼材中のＭｎＳの数密度を高めるには、鋼材中のＭｎＳの形態を細く制御し、熱間鍛造のための加熱の際に分断させることが有効である。 (B) In order to increase the number density of MnS in the steel material, it is effective to finely control the form of MnS in the steel material and to divide it during heating for hot forging.
（ｃ）熱間鍛造のための加熱を１０００℃を超えるような高い温度で行った場合に、ＭｎＳを分断させるためには、質量％で、０．００５％以上０．０３０％未満のＳを含む中・高炭素系の素材鋼を、熱間圧延などの方法で熱間加工して、熱間加工方向に平行な面におけるＭｎＳの形態と分布が、
〈１〉アスペクト比、つまり、長さと厚さとの比である「長さ／厚さ」が１０以下のＭｎＳの面積が全ＭｎＳ面積に対して９０％以上であること、
〈２〉厚さが２μｍ以下であるＭｎＳの面積が全ＭｎＳ面積に対して８０％以上であること、
という２つの条件を満たすように制御する必要がある。
(C) When heating for hot forging is performed at a high temperature exceeding 1000 ° C., in order to cut MnS, S of 0.005% or more and less than 0.030% is contained in mass%. The medium and high carbon material steel containing is hot-worked by a method such as hot rolling, and the form and distribution of MnS in the plane parallel to the hot-working direction is
<1> The aspect ratio, that is, the area of MnS having a length / thickness ratio of 10 or less, which is a ratio of length to thickness, is 90% or more with respect to the total MnS area;
<2> The area of MnS having a thickness of 2 μm or less is 80% or more with respect to the total MnS area;
It is necessary to control so as to satisfy these two conditions.
これは、ＭｎＳが、その厚さに関して、上記（ｃ）に記載した条件〈２〉を満たしていなければ、所定の部品形状に熱間鍛造するに際して、１０００℃を超える温度に加熱しても、表面張力によるＭｎＳのさらなる分断が生じないからであり、また、ＭｎＳが、そのアスペクト比に関して、上記（ｃ）に記載した条件〈１〉を満たしていなければ、所定の部品形状への熱間鍛造のための１０００℃を超える温度での加熱によって、表面張力によるＭｎＳのさらなる分断が生じたとしても、ＭｎＳの数密度の顕著な増加が生じることがないからである。 As long as MnS does not satisfy the condition <2> described in (c) above with respect to its thickness, when hot forging into a predetermined part shape, even when heated to a temperature exceeding 1000 ° C., This is because there is no further fragmentation of MnS due to surface tension, and hot forging into a predetermined part shape if MnS does not satisfy the condition <1> described in (c) above with respect to its aspect ratio. This is because the heating at a temperature exceeding 1000 ° C. does not cause a significant increase in the number density of MnS even if further separation of MnS due to surface tension occurs.
（ｄ）質量％で、０．００５％以上０．０３０％未満のＳを含む鋼材において、熱間加工方向に平行な面におけるＭｎＳの形態と分布を上記（ｃ）に記載したものとするためには、例えば、熱間圧延などの熱間加工に供する素材鋼を、１０００〜１２００℃に加熱して９００℃以上の温度域で累積減面率９０％以上の熱間加工を行い、熱間加工終了後９００℃以上の温度域に少なくとも３ｍｉｎ以上保持すればよい。 (D) In a steel material containing 0.005% or more and less than 0.030% S in mass%, the form and distribution of MnS in a plane parallel to the hot working direction are described in (c) above. For example, the material steel to be subjected to hot working such as hot rolling is heated to 1000 to 1200 ° C. and subjected to hot working with a cumulative area reduction rate of 90% or more in a temperature range of 900 ° C. or higher. What is necessary is just to hold | maintain at least 3 minutes or more in the temperature range of 900 degreeC or more after completion | finish of a process.
（ｅ）上記（ｄ）の１０００〜１２００℃に加熱されて９００℃以上の温度域で累積減面率９０％以上という高い減面率での熱間加工を受けたＭｎＳは、厚さが小さくなるだけでなく、さらに上記の熱間加工終了後９００℃以上の温度域で３ｍｉｎ以上保持されることによって、細くくびれた部分から表面張力によって分断されて長さが短くなるので、結果として、アスペクト比が小さくなる。 (E) MnS heated to 1000 to 1200 ° C. in the above (d) and subjected to hot working at a high area reduction rate of 90% or more in a temperature range of 900 ° C. or more has a small thickness. In addition, after the above hot working is completed, the length is shortened by being separated from the narrowed portion by the surface tension by being held at a temperature range of 900 ° C. or higher for 3 minutes or more. The ratio becomes smaller.
本発明は、上記の知見に基づいて完成されたものであり、その要旨は、下記（１）に示す熱間鍛造用鋼材および（２）に示す熱間鍛造用鋼材の製造方法にある。 The present invention has been completed based on the above findings, and the gist of the present invention resides in a steel material for hot forging shown in (1) below and a method for manufacturing a steel material for hot forging shown in (2).
（１）質量％で、Ｃ：０．３０〜０．６５％、Ｓｉ：０．０５％以上０．５０％未満、Ｍｎ：０．２０〜１．５０％、Ｐ：０．０２５％以下、Ｓ：０．００５％以上０．０３０％未満、Ａｌ：０．０４０％以下およびＮ：０．０１０％未満を含有し、残部はＦｅおよび不純物からなり、不純物中のＴｉが０．００５％以下およびＮｂが０．００５％以下であり、熱間加工方向に平行な面におけるＭｎＳの形態と分布が下記の条件〈１〉および〈２〉を満たすことを特徴とする熱間鍛造用鋼材。
〈１〉アスペクト比が１０以下のＭｎＳの面積が全ＭｎＳ面積に対して９０％以上であること、
〈２〉厚さが２μｍ以下であるＭｎＳの面積が全ＭｎＳ面積に対して８０％以上であること。
(1) By mass%, C: 0.30 to 0.65%, Si: 0.05% or more and less than 0.50%, Mn: 0.20 to 1.50%, P: 0.025% or less, S: 0.005% or more and less than 0.030%, Al: 0.040% or less and N: less than 0.010%, the balance is made of Fe and impurities, and Ti in the impurities is 0.005% or less And Nb is 0.005% or less, and the form and distribution of MnS in a plane parallel to the hot working direction satisfy the following conditions <1> and <2>.
<1> The area of MnS having an aspect ratio of 10 or less is 90% or more with respect to the total MnS area;
<2> The area of MnS having a thickness of 2 μm or less is 80% or more with respect to the total MnS area.
（２）質量％で、Ｃ：０．３０〜０．６５％、Ｓｉ：０．０５％以上０．５０％未満、Ｍｎ：０．２０〜１．５０％、Ｐ：０．０２５％以下、Ｓ：０．００５％以上０．０３０％未満、Ａｌ：０．０４０％以下およびＮ：０．０１０％未満を含有し、残部はＦｅおよび不純物からなり、不純物中のＴｉが０．００５％以下およびＮｂが０．００５％以下である素材鋼を、１０００〜１２００℃に加熱して９００℃以上の温度域で累積減面率９０％以上の熱間加工を行い、熱間加工終了後９００℃以上の温度域で３ｍｉｎ以上保持することを特徴とする上記（１）に記載の熱間鍛造用鋼材の製造方法。 (2) By mass%, C: 0.30 to 0.65%, Si: 0.05% or more and less than 0.50%, Mn: 0.20 to 1.50%, P: 0.025% or less, S: 0.005% or more and less than 0.030%, Al: 0.040% or less and N: less than 0.010%, the balance is made of Fe and impurities, and Ti in the impurities is 0.005% or less And Nb is 0.005% or less, the raw steel is heated to 1000 to 1200 ° C., and hot working is performed at a cumulative area reduction rate of 90% or more in a temperature range of 900 ° C. or more. The method for producing a steel material for hot forging as described in (1) above, wherein the temperature is maintained for 3 minutes or more in the above temperature range.
なお、「熱間加工方向に平行な面」とは、例えば、熱間加工が「熱間圧延」であれば「圧延方向の面」を、また、熱間加工が「熱間鍛造」であれば「鍛錬軸に平行な面」を指す。 Note that the “surface parallel to the hot working direction” is, for example, “the surface in the rolling direction” if the hot working is “hot rolling”, and the hot working is “hot forging”. For example, "plane parallel to the training axis".
ＭｎＳの「アスペクト比」とは、ＭｎＳの「長さ／厚さ」を指す。なお、本発明でいうＭｎＳの「厚さ」とは、介在物最大長方向に平行な２本の直線で介在物を挟んだときの２直線間の距離、すなわち最大厚さを指す。 “Aspect ratio” of MnS refers to “length / thickness” of MnS. The “thickness” of MnS in the present invention refers to the distance between two straight lines when the inclusion is sandwiched between two straight lines parallel to the maximum length of inclusion, that is, the maximum thickness.
「全ＭｎＳ面積」とは、ＪＩＳ Ｇ ０５５５（２００３）に記載の「鋼の非金属介在物の顕微鏡試験方法」に準じて、上記の観察面を倍率４００倍で６４視野、合計で１．４４ｍｍ2相当の面積を観察した際のＭｎＳ面積の合計を指す。 “Total MnS area” means that according to “Microscopic test method for non-metallic inclusions in steel” described in JIS G 0555 (2003), the observation surface is 64 fields at a magnification of 400 times, a total of 1.44 mm. 2 indicates the total MnS area when an equivalent area is observed.
「素材鋼」とは、例えば、鋼塊、連続鋳造材、あるいはそれらを分塊圧延したビレットなど、熱間加工に供されるものを指す。 “Raw steel” refers to, for example, a steel ingot, a continuous cast material, or a billet obtained by subjecting them to a hot work, such as a billet obtained by split rolling.
また、上記の「温度」とは対象となる素材鋼やそれを熱間加工したものの「表面温度」を指す。 The above “temperature” refers to the “surface temperature” of the target material steel or hot-worked steel.
以下、上記 （１）の熱間鍛造用鋼材に係る発明および（２）に示す熱間鍛造用鋼材の製造方法に係る発明を、それぞれ、「本発明（１）」および「本発明（２）」という。また、総称して「本発明」という。 Hereinafter, the invention related to the hot forging steel material of (1) and the invention related to the manufacturing method of the steel material for hot forging shown in (2) are referred to as “present invention (1)” and “present invention (2)”, respectively. " Also, collectively referred to as “the present invention”.
本発明の熱間鍛造用鋼材は、Ｔｉ、ＮｂおよびＮの含有量が、それぞれ、質量％で、０．００５％以下、０．００５％以下および０．０１０％未満、より望ましくは０．００３％以下、０．００３％以下および０．００７％以下であるにも拘わらず、熱間鍛造するために、１０００℃を超えるような温度、特に、１１００℃以上の高い温度に加熱しても、γ粒の粗大化が抑制されるので、自動車、産業機械および建設機械など高い転動疲労特性が要求される機械部品の素材として好適である。なお、この熱間鍛造用鋼材は本発明の方法によって製造することができる。 In the steel for hot forging of the present invention, the contents of Ti, Nb, and N are 0.005% or less, 0.005% or less, and less than 0.010%, more desirably 0.003, respectively, in mass%. %, 0.003% or less and 0.007% or less in order to perform hot forging, even if heated to a temperature exceeding 1000 ° C., particularly a high temperature of 1100 ° C. or higher, Since coarsening of γ grains is suppressed, it is suitable as a material for machine parts that require high rolling fatigue characteristics such as automobiles, industrial machines, and construction machines. This hot forging steel can be produced by the method of the present invention.
以下、本発明の各要件について詳しく説明する。なお、化学成分の含有量の「％」は「質量％」を意味する。 Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of the chemical component means “mass%”.
（Ａ）化学組成：
Ｃ：０．３０〜０．６５％
Ｃは、鋼の強度を確保するのに有効であり、また、自動車、産業機械および建設機械などの機械部品においては、耐摩耗性を確保するのにも非常に有効である。これらの効果を得るためには、Ｃは０．３０％以上の含有量とする必要がある。しかしながら、Ｃの含有量が多くなり、特に、０．６５％を超えると、硬さが高くなりすぎて冷間加工性や被削性が劣化する。したがって、Ｃの含有量を、０．３０〜０．６５％とした。なお、Ｃ含有量の望ましい下限は０．３８％であり、また、望ましい上限は０．６０％である。
(A) Chemical composition:
C: 0.30 to 0.65%
C is effective for ensuring the strength of steel, and is very effective for ensuring wear resistance in machine parts such as automobiles, industrial machines, and construction machines. In order to acquire these effects, it is necessary to make C content 0.30% or more. However, if the content of C increases, and particularly exceeds 0.65%, the hardness becomes too high and cold workability and machinability deteriorate. Therefore, the content of C is set to 0.30 to 0.65%. The desirable lower limit of the C content is 0.38%, and the desirable upper limit is 0.60%.
Ｓｉ：０．０５％以上０．５０％未満
Ｓｉは、脱酸効果を有する元素であり、また、フェライトの固溶強化にも有効な元素であるので、０．０５％以上含有させる。しかしながら、Ｓｉの含有量が０．５０％以上になると熱間加工性を損ねるので、その含有量の上限は、フェライトの固溶強化に必要かつ十分な０．５０％未満である。したがって、Ｓｉの含有量を、０．０５％以上０．５０％未満とした。なお、Ｓｉ含有量の望ましい下限は０．１５％であり、また、望ましい上限は０．４０％である。
Si: 0.05% or more and less than 0.50% Si is an element having a deoxidizing effect, and is also an element effective for solid solution strengthening of ferrite, so 0.05% or more is contained. However, since the hot workability is impaired when the Si content is 0.50% or more, the upper limit of the content is less than 0.50% necessary and sufficient for solid solution strengthening of ferrite. Therefore, the Si content is set to 0.05% or more and less than 0.50%. The desirable lower limit of the Si content is 0.15%, and the desirable upper limit is 0.40%.
Ｍｎ：０．２０〜１．５０％
Ｍｎは、鋼中でＭｎＳを形成するために必須の元素であり、０．２０％以上含有させる。しかしながら、Ｍｎの含有量が過剰になるとＭｎＳ量をいたずらに増やして転動疲労特性の低下を招き、特に、１．５０％を超えると転動疲労特性の低下が著しくなる。したがって、Ｍｎの含有量を０．２０〜１．５０％とした。なお、Ｍｎ含有量の望ましい下限は０．４０％であり、また、望ましい上限は１．００％である。
Mn: 0.20 to 1.50%
Mn is an essential element for forming MnS in steel, and is contained by 0.20% or more. However, if the Mn content is excessive, the amount of MnS is increased unnecessarily, leading to a decrease in rolling fatigue characteristics. In particular, if it exceeds 1.50%, the rolling fatigue characteristics are significantly decreased. Therefore, the Mn content is set to 0.20 to 1.50%. The desirable lower limit of the Mn content is 0.40%, and the desirable upper limit is 1.00%.
Ｐ：０．０２５％以下
Ｐは、鋼中に不純物として含有される元素であり、粒界に偏析して粒界脆化割れを助長し、特に、その含有量が０．０２５％を超えると、粒界脆化割れが生じやすくなる。したがって、Ｐの含有量を０．０２５％以下とした。なお、Ｐには強度向上作用があるので、この効果を得たい場合には、Ｐ含有量の下限を０．００５％とすることが望ましい。
P: 0.025% or less P is an element contained as an impurity in steel and segregates at the grain boundary to promote grain boundary embrittlement cracking. In particular, when the content exceeds 0.025%. , Grain boundary embrittlement cracking tends to occur. Therefore, the content of P is set to 0.025% or less. In addition, since P has a strength improvement effect, when it is desired to obtain this effect, it is desirable that the lower limit of the P content is 0.005%.
Ｓ：０．００５％以上０．０３０％未満
Ｓは、被鍛造材である鋼材中のＭｎＳの数密度を高めてγ粒の粗大化を抑制するために必須の元素であり、このような効果を得るためには、Ｓの含有量は０．００５％以上とする必要がある。しかしながら、Ｓの含有量が多すぎると、たとえ後述する（Ｃ）項の「１０００〜１２００℃に加熱して９００℃以上の温度域で累積減面率９０％以上の熱間加工を行い、熱間加工終了後９００℃以上の温度域で３ｍｉｎ以上保持する」方法で製造しても、ＭｎＳのアスペクト比は小さくならず、後述する（Ｂ）項のＭｎＳ形態が得られないばかりか、粗大な介在物を形成して転動疲労特性を低下させ、特に、０．０３０％以上のＳ含有量で転動疲労特性の低下が著しくなる。したがって、Ｓの含有量を０．００５％以上０．０３０％未満とした。なお、Ｓ含有量の望ましい上限は０．０１５％である。
S: 0.005% or more and less than 0.030% S is an essential element for increasing the number density of MnS in a steel material to be forged and suppressing the coarsening of γ grains, such an effect. In order to obtain the above, the S content needs to be 0.005% or more. However, if the content of S is too large, even if it is heated to 1000 to 1200 ° C. in the later-described (C), hot working is performed with a cumulative area reduction of 90% or more in a temperature range of 900 ° C. or higher, Even if it is manufactured by the method of “holding for 3 min or more in a temperature range of 900 ° C. or more after the end of inter-working”, the aspect ratio of MnS is not reduced, and not only the MnS form of the item (B) described later is obtained but also coarse Inclusions are formed to lower the rolling fatigue characteristics. In particular, when the S content is 0.030% or more, the rolling fatigue characteristics are significantly reduced. Therefore, the S content is set to be 0.005% or more and less than 0.030%. A desirable upper limit of the S content is 0.015%.
Ａｌ：０．０４０％以下
Ａｌは、脱酸効果を有する元素であるので添加してもよいが、過剰に含有すれば介在物を形成して転動疲労特性の低下を招き、特に、その含有量が０．０４０％を超えると、転動疲労特性の低下が著しくなる。したがって、Ａｌの含有量を０．０４０％以下とした。なお、Ａｌの含有量は０．０３５％以下とすることが望ましい。
Al: 0.040% or less Al is an element having a deoxidizing effect, and may be added. However, if excessively contained, inclusions are formed to cause deterioration of rolling fatigue characteristics. When the amount exceeds 0.040%, the rolling fatigue characteristics are remarkably deteriorated. Therefore, the Al content is set to 0.040% or less. The Al content is preferably 0.035% or less.
Ｎ：０．０１０％未満
Ｎは、固溶強化に寄与する元素であるので添加してもよいが、過剰に含有すればＡｌ、Ｔｉ、Ｎｂなどと窒化物を形成し、特に、その含有量が０．０１０％以上になると、粗大な窒化物を形成して転動疲労寿命を低下させる。したがって、Ｎの含有量を０．０１０％未満とした。転動疲労寿命の低下を確実に抑制するためには、Ｎの含有量を０．００７％以下とすることが望ましい。
N: Less than 0.010% N is an element that contributes to solid solution strengthening, so it may be added, but if it is contained excessively, it forms nitrides with Al, Ti, Nb, etc., and particularly its content If it becomes 0.010% or more, coarse nitrides are formed and the rolling fatigue life is lowered. Therefore, the N content is less than 0.010%. In order to reliably suppress the decrease in the rolling fatigue life, it is desirable that the N content is 0.007% or less.
本発明においては、さらに、不純物中のＴｉおよびＮｂを次のとおり規定する。 In the present invention, Ti and Nb in the impurities are further defined as follows.
Ｔｉ：０．００５％以下
Ｔｉは、過剰に含有すれば粗大な窒化物を形成して転動疲労寿命の低下を招く。しかしながら、不純物中に０．００５％以下の量で含まれている場合には、転動疲労特性を阻害することがない。したがって、不純物中のＴｉの含有量を０．００５％以下とした。なお、不純物中のＴｉの含有量の上限は０．００３％とすることが望ましい。
Ti: 0.005% or less If Ti is excessively contained, coarse nitrides are formed and the rolling fatigue life is reduced. However, when the impurities are contained in an amount of 0.005% or less, the rolling fatigue characteristics are not hindered. Therefore, the content of Ti in the impurities is set to 0.005% or less. Note that the upper limit of the content of Ti in the impurities is preferably 0.003%.
Ｎｂ：０．００５％以下
Ｎｂは、Ｔｉほどではないが過剰に含有すれば粗大な窒化物を形成して転動疲労寿命の低下を招くおそれがある。しかしながら、不純物中に０．００５％以下の量で含まれている場合には、転動疲労特性を阻害することがない。したがって、不純物中のＮｂの含有量を０．００５％以下とした。なお、不純物中のＮｂの含有量の上限は０．００３％とすることが望ましい。
Nb: 0.005% or less Nb, if not contained as much as Ti, may form coarse nitrides and lead to a decrease in rolling fatigue life. However, when the impurities are contained in an amount of 0.005% or less, the rolling fatigue characteristics are not hindered. Therefore, the Nb content in the impurities is set to 0.005% or less. Note that the upper limit of the Nb content in the impurities is preferably 0.003%.
上記の理由から、本発明（１）に係る熱間鍛造用鋼材の化学組成は、Ｃ、Ｓｉ、Ｍｎ、Ｐ、Ｓ、ＡｌおよびＮを上述した範囲で含有し、残部はＦｅおよび不純物からなり、不純物中のＴｉおよびＮｂを上述した範囲で制限したものとした。 For the above reasons, the chemical composition of the steel for hot forging according to the present invention (1) contains C, Si, Mn, P, S, Al, and N in the above-mentioned range, and the balance consists of Fe and impurities. Further, Ti and Nb in the impurity were limited within the above-described range.
なお、本発明（１）に係る熱間鍛造用鋼材においては、以上に述べたＣからＮまでの元素以外は、本質的に不純物であって、意図的に添加することはない。 In addition, in the steel for hot forging according to the present invention (1), elements other than the elements from C to N described above are essentially impurities and are not intentionally added.
ここで、不純物除去のための製鋼工程でのいたずらなコストアップを避け、また、過剰な含有による熱間割れを防止するという観点から、不純物中のＣｕおよびＮｉの含有量は、それぞれ、Ｃｕ：０．３０％以下の範囲およびＮｉ：０．２０％以下の範囲で許容できる。 Here, from the viewpoints of avoiding a tedious cost increase in the steel making process for removing impurities and preventing hot cracking due to excessive inclusion, the contents of Cu and Ni in the impurities are respectively Cu: It is acceptable in the range of 0.30% or less and Ni: 0.20% or less.
（Ｂ）熱間加工方向に平行な面におけるＭｎＳの形態と分布：
本発明（１）に係る熱間鍛造用鋼材が、前記（Ａ）項に記載の化学組成を有する場合であっても、熱間加工方向に平行な面におけるＭｎＳの形態と分布が、「アスペクト比が１０以下のＭｎＳの面積が全ＭｎＳ面積に対して９０％以上であること」という前記〈１〉の条件、および、「厚さが２μｍ以下であるＭｎＳの面積が全ＭｎＳ面積に対して８０％以上であること」という前記〈２〉の条件の少なくとも一方から外れる場合には、ＭｎＳによるγ粒の粗大化抑制効果が得られないので、所定の機械部品形状に熱間鍛造するに際して、１０００℃を超えるような高い温度に加熱すれば、γ粒が粗大化してしまう。
(B) MnS morphology and distribution in a plane parallel to the hot working direction:
Even when the steel for hot forging according to the present invention (1) has the chemical composition described in the item (A), the form and distribution of MnS in the plane parallel to the hot working direction is “Aspect”. The condition of the above <1> that the area of MnS with a ratio of 10 or less is 90% or more with respect to the total MnS area, and the area of MnS with a thickness of 2 μm or less with respect to the total MnS area When it deviates from at least one of the above conditions <2> that it is `` 80% or more '', since the effect of suppressing the coarsening of γ grains by MnS cannot be obtained, when hot forging into a predetermined machine part shape, If heated to a high temperature exceeding 1000 ° C., the γ grains become coarse.
すなわち、ＭｎＳの形態と分布が上記の条件〈１〉および条件〈２〉の双方を満たしていれば、後述する「（Ｃ）熱間鍛造用鋼材の製造方法」における条件で処理された場合に、容易に表面張力によって特に細くくびれた部分から分断し、しかも、所定の機械部品形状に熱間鍛造するための１０００℃を超える温度での加熱によって、表面張力によるさらなる分断が生じるので、数密度が増加してγ粒の粗大化を抑制する効果を発揮することができる。しかしながら、アスペクト比が１０より大きいＭｎＳは分断してもＭｎＳの数密度が増加せず、また、厚さが２μｍより大きいＭｎＳは分断しないため、やはり数密度が増加せず、このためγ粒の粗大化を抑制する効果が得られない。 In other words, if the form and distribution of MnS satisfy both the above conditions <1> and <2>, when processed under the conditions in “(C) Method for producing steel for hot forging” described later. Because it is easily divided by the surface tension, especially from the narrow and narrow part, and further heating by a temperature exceeding 1000 ° C. for hot forging into a predetermined machine part shape, further division by the surface tension occurs. The effect of suppressing the coarsening of the γ grains can be exhibited. However, even if MnS having an aspect ratio of more than 10 is divided, the number density of MnS does not increase, and MnS having a thickness of more than 2 μm does not break, so the number density does not increase. The effect of suppressing coarsening cannot be obtained.
したがって、本発明（１）に係る熱間鍛造用鋼材においては、ＭｎＳによるγ粒の粗大化抑制効果が得られるようにするために、熱間加工方向に平行な面におけるＭｎＳの形態と分布が、条件〈１〉の「アスペクト比が１０以下のＭｎＳの面積が全ＭｎＳ面積に対して９０％以上であること」および条件〈２〉の「厚さが２μｍ以下であるＭｎＳの面積が全ＭｎＳ面積に対して８０％以上であること」を満たすことと規定した。 Therefore, in the steel for hot forging according to the present invention (1), in order to obtain the effect of suppressing the coarsening of γ grains by MnS, the form and distribution of MnS in the plane parallel to the hot working direction are In condition <1>, “the area of MnS having an aspect ratio of 10 or less is 90% or more with respect to the total MnS area” and in condition <2>, the area of MnS having a thickness of 2 μm or less is the total MnS. It is defined as satisfying that it is 80% or more with respect to the area.
なお、既に述べたように、「熱間加工方向に平行な面」とは、例えば、熱間加工が「熱間圧延」であれば「圧延方向の面」を、また、熱間加工が「熱間鍛造」であれば「鍛錬軸に平行な面」を指す。 As already described, the “surface parallel to the hot working direction” means, for example, “the surface in the rolling direction” if the hot working is “hot rolling”, and the hot working is “ “Hot forging” refers to “a plane parallel to the forging axis”.
また、ＭｎＳの「アスペクト比」とは、ＭｎＳの「長さ／厚さ」を指し、「全ＭｎＳ面積」とは、ＪＩＳ Ｇ ０５５５（２００３）に記載の「鋼の非金属介在物の顕微鏡試験方法」に準じて、上記の観察面を倍率４００倍で６４視野、合計で１．４４ｍｍ2相当の面積を観察した際のＭｎＳ面積の合計を指す。 The “aspect ratio” of MnS refers to “length / thickness” of MnS, and “total MnS area” refers to “microscopic examination of non-metallic inclusions in steel” described in JIS G 0555 (2003). According to “Method”, the above observation surface is a total of MnS areas when observing an area corresponding to 1.44 mm 2 in total with 64 fields of view at a magnification of 400 times.
なお、本発明でいうＭｎＳの「厚さ」とは、介在物最大長方向に平行な２本の直線で介在物を挟んだときの２直線間の距離、すなわち最大厚さを指すことも既に述べたとおりである。 The “thickness” of MnS in the present invention already refers to the distance between two straight lines when the inclusion is sandwiched between two straight lines parallel to the maximum length of inclusion, that is, the maximum thickness. As stated.
（Ｃ）熱間鍛造用鋼材の製造方法：
本発明（１）に係る熱間鍛造用鋼材は、例えば、前記（Ａ）項に記載の化学組成を有する素材鋼を、「１０００〜１２００℃に加熱して９００℃以上の温度域で累積減面率９０％以上の熱間加工を行い、熱間加工終了後９００℃以上の温度域で３ｍｉｎ以上保持する」ことを特徴とする前記本発明（２）によって、製造することができる。
(C) Manufacturing method of steel for hot forging:
The steel for hot forging according to the present invention (1) is, for example, a material steel having the chemical composition described in the item (A) described above, which is “cumulatively reduced in a temperature range of 900 ° C. or higher by heating to 1000 to 1200 ° C. It can be manufactured by the present invention (2), characterized in that hot working with an area ratio of 90% or more is performed, and the hot working is completed for 3 minutes or more in a temperature range of 900 ° C. or more ”.
すなわち、熱間加工方向に平行な面において、前記（Ｂ）項で述べたアスペクト比が１０以下のＭｎＳの面積が全ＭｎＳ面積に対して９０％以上であり（条件〈１〉）、かつ厚さが２μｍ以下であるＭｎＳの面積が全ＭｎＳ面積に対して８０％以上である（条件〈２〉）ＭｎＳの形態と分布を有する鋼材を得るためには、
〔１〕素材鋼を、１０００〜１２００℃に加熱して９００℃以上の温度域で累積減面率９０％以上の熱間加工を行い、熱間加工終了後９００℃以上の温度域で３ｍｉｎ以上保持する、
という処理を施すのが効果的である。
That is, on the surface parallel to the hot working direction, the area of MnS having an aspect ratio of 10 or less described in the above item (B) is 90% or more with respect to the total MnS area (condition <1>), and the thickness In order to obtain a steel material having an MnS form and distribution, the area of MnS having a length of 2 μm or less is 80% or more based on the total MnS area (condition <2>).
[1] The raw steel is heated to 1000 to 1200 ° C. and subjected to hot working with a cumulative area reduction of 90% or more in a temperature range of 900 ° C. or higher, and after completion of the hot working, 3 minutes or more in a temperature range of 900 ° C. or higher. Hold,
It is effective to apply the process.
なお、既に述べたように、上記〔１〕の処理でいう「素材鋼」とは、例えば、鋼塊、連続鋳造材、あるいはそれらを分塊圧延したビレットなど、熱間加工に供されるものを指す。 In addition, as already stated, the “material steel” referred to in the above-mentioned process [1] is, for example, a steel ingot, a continuous cast material, or a billet obtained by subjecting them to hot rolling, etc. Point to.
また、上記の「温度」が、対象となる素材鋼やそれを熱間加工したものの「表面温度」を指すことも既に述べたとおりである。 In addition, as described above, the “temperature” refers to the “surface temperature” of the target material steel or hot-worked steel.
上記〔１〕の処理を施せば、ＭｎＳは、表面張力によって、特に、細くくびれた部分から分断されるので、アスペクト比が小さいＭｎＳが増加する。そして、所定の機械部品形状に熱間鍛造するに際して、１０００℃を超える温度、特に、１１００℃以上という高い温度に加熱すれば、上記〔１〕の処理によって分断されたＭｎＳが、表面張力によってさらに細くくびれた部分から分断されるので、ＭｎＳの数密度が極めて高くなって、優れたγ粒の粗大化抑制効果が得られる。 When the treatment [1] is performed, MnS is divided by a surface tension, in particular, from a narrow and narrow portion, so that MnS having a small aspect ratio increases. Then, when hot forging into a predetermined mechanical part shape, if heated to a temperature exceeding 1000 ° C., particularly 1100 ° C. or higher, MnS separated by the process of [1] above is further increased by surface tension. Since it is divided from the narrowly constricted portion, the number density of MnS becomes extremely high, and an excellent effect of suppressing the coarsening of γ grains is obtained.
素材鋼を加熱する温度が１２００℃より高い場合には、ＭｎＳとマトリックスであるオーステナイトとの変形抵抗の差が大きくなり、マトリックスに比べてＭｎＳが変形しにくいので、ＭｎＳの厚さを小さくすることが困難である。また、加熱温度が１０００℃より低い場合には、ＭｎＳ自体の変形抵抗が高くなってＭｎＳが変形しにくいため、ＭｎＳの厚さを効果的に小さくすることが困難である。 When the temperature at which the material steel is heated is higher than 1200 ° C., the difference in deformation resistance between MnS and the austenite matrix is increased, and MnS is less likely to deform than the matrix, so the thickness of MnS should be reduced. Is difficult. In addition, when the heating temperature is lower than 1000 ° C., the deformation resistance of MnS itself is high and MnS is difficult to deform, so it is difficult to effectively reduce the thickness of MnS.
熱間加工の温度が９００℃より低い場合にも、ＭｎＳの変形抵抗が高くなってＭｎＳが変形しにくいため、ＭｎＳの厚さを効果的に小さくすることが困難である。 Even when the hot working temperature is lower than 900 ° C., it is difficult to effectively reduce the thickness of MnS because the deformation resistance of MnS becomes high and MnS hardly deforms.
なお、熱間加工温度の上限は、加熱温度の上限である１２００℃であることが望ましい。 The upper limit of the hot working temperature is desirably 1200 ° C., which is the upper limit of the heating temperature.
また、９００℃以上の温度域での累積減面率が９０％に満たない場合には、ＭｎＳが充分に変形されないため、所望のＭｎＳ厚さを得ることができない。 In addition, when the cumulative area reduction rate in the temperature range of 900 ° C. or higher is less than 90%, the desired MnS thickness cannot be obtained because MnS is not sufficiently deformed.
なお、上記〔１〕の熱間加工において、１回の加熱だけでは、９００℃以上の温度域での累積減面率が９０％以上なるような熱間加工が行えない場合には、１０００〜１２００℃の温度域に再加熱して、９００℃以上の温度域での累積減面率が９０％以上となるようにすればよい。 In addition, in the hot working of [1] above, in the case where hot working cannot be performed such that the cumulative area reduction in a temperature range of 900 ° C. or higher is 90% or more by only one heating, 1000 to What is necessary is just to reheat to the temperature range of 1200 degreeC, and to make it the cumulative area reduction rate in the temperature range of 900 degreeC or more become 90% or more.
９００℃以上の累積減面率を大きくしすぎると、加工時間、加工コストをいたずらに高めることになるため、９００℃以上の温度域での累積減面率は、９９％以下とすることが望ましい。 If the cumulative area reduction rate of 900 ° C. or higher is excessively increased, the processing time and the processing cost are unnecessarily increased. Therefore, the cumulative area reduction rate in the temperature range of 900 ° C. or higher is preferably 99% or less. .
一方、前記の熱間加工後に９００℃以上の温度域で保持される時間が３ｍｉｎに満たない場合には、ＭｎＳが分断するのに十分な時間がないために、アスペクト比が大きなＭｎＳが残存してしまう。 On the other hand, if the time that is maintained in the temperature range of 900 ° C. or higher after the hot working is less than 3 min, there is not enough time for MnS to divide, so that MnS having a large aspect ratio remains. End up.
上記の、９００℃以上の温度域での３ｍｉｎ以上という保持時間は、前記の熱間加工後に連続的に冷却する過程で確保してもよいし、補熱や加熱をして確保してもよい。また、補熱や加熱と冷却を繰り返して確保してもよく、「９００℃以上の温度域に保持される時間が３ｍｉｎ以上」になりさえすればよい。 The holding time of 3 min or more in the temperature range of 900 ° C. or higher may be ensured in the process of continuous cooling after the hot working, or may be ensured by supplementary heating or heating. . Further, supplementary heating, heating and cooling may be ensured repeatedly as long as “the time maintained in the temperature range of 900 ° C. or higher is 3 min or longer”.
ただし、保持する時間が長すぎると、コストをいたずらに高めることになるため、保持する時間の上限は、３０ｍｉｎとするのが望ましい。 However, if the holding time is too long, the cost is unnecessarily increased, so the upper limit of the holding time is preferably 30 min.
なお、９００℃以上の温度域で累積減面率９０％以上の熱間加工を施された場合であっても、熱間加工終了後９００℃以上の温度域で保持される時間が３ｍｉｎに満たずに一旦９００℃未満に温度低下し、その後９００℃以上に再加熱されて、９００℃以上に３ｍｉｎ以上保持されたものでは、アスペクト比が１０以上であるＭｎＳは顕著な分断を生じない。このことから、９００℃以上の温度域で累積減面率９０％以上の熱間加工を行い、熱間加工終了後９００℃以上の温度域で３ｍｉｎ以上保持した場合の表面張力によるＭｎＳの分断は、上記の熱間加工直後に存在する加工歪を駆動力として起こっているものと推察される。 Even when hot working with a cumulative surface reduction rate of 90% or more is performed in a temperature range of 900 ° C. or higher, the time that is maintained in the temperature range of 900 ° C. or higher after hot working is less than 3 min. If the temperature is once lowered to less than 900 ° C., then reheated to 900 ° C. or higher and held at 900 ° C. or higher for 3 minutes or longer, MnS having an aspect ratio of 10 or higher does not cause significant division. From this, the MnS fragmentation due to the surface tension when performing hot working with a cumulative area reduction rate of 90% or more in a temperature range of 900 ° C. or higher and holding for 3 minutes or more in the temperature range of 900 ° C. or higher after the hot working is completed is It is presumed that the processing strain that exists immediately after the above hot processing occurs as a driving force.
以下、実施例により本発明をさらに詳しく説明する。 Hereinafter, the present invention will be described in more detail with reference to examples.
表１に示す化学組成を有する鋼Ａ〜Ｄを真空溶解炉によって溶解し、直径２３０ｍｍの円筒状インゴットを作製した。 Steels A to D having the chemical composition shown in Table 1 were melted in a vacuum melting furnace to produce a cylindrical ingot having a diameter of 230 mm.
表１中の鋼Ａおよび鋼Ｂは、化学組成が本発明で規定する範囲内にある鋼である。一方、鋼Ｃおよび鋼Ｄは、Ｓの含有量が本発明で規定する条件から外れた鋼である。 Steel A and steel B in Table 1 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, steel C and steel D are steels in which the content of S deviates from the conditions defined in the present invention.
上記のインゴットまたはこれらのインゴットから熱間鍛造で作製した厚さ１００ｍｍ×幅１００ｍｍの角材のいずれかを素材鋼として用いた。 Either the above ingot or a square member having a thickness of 100 mm and a width of 100 mm produced from these ingots by hot forging was used as the material steel.
まず、インゴットを素材鋼として、１１００〜１２５０℃に加熱して２時間保持した後、熱間鍛造を行い、厚さ１００ｍｍ×幅１００ｍｍ、厚さ３０ｍｍ×幅６０ｍｍおよび厚さ２５ｍｍ×幅２５ｍｍの角材を作製した。 First, using ingot as raw material steel, heated to 1100 to 1250 ° C. and held for 2 hours, then hot forged, square material of thickness 100 mm × width 100 mm, thickness 30 mm × width 60 mm and thickness 25 mm × width 25 mm Was made.
また、上記の厚さ１００ｍｍ×幅１００ｍｍの角材を一旦大気中で放冷して室温まで冷却したものを素材鋼として、１０５０〜１１００℃に加熱した後、厚さ２５ｍｍ×幅２５ｍｍの角材を製造することも行った。 In addition, the square material having a thickness of 100 mm and a width of 100 mm is cooled to room temperature after being allowed to cool to the room temperature. After heating to 1050 to 1100 ° C., a square material having a thickness of 25 mm and a width of 25 mm is manufactured. I also went to.
なお、これらの素材鋼（インゴット、厚さ１００ｍｍ×幅１００ｍｍの角材）から上記角材への熱間鍛造に際しては、鍛造中の温度が９００℃を下回ることがないように、最初の加熱温度と同じ温度に再加熱して鍛造加工を繰り返した。また、熱間鍛造の終了温度は、加熱温度、製造した角材の寸法により異なるものの、全て９００℃を下回らないように制御した。 In addition, in hot forging from these material steels (ingot, square material with a thickness of 100 mm × width of 100 mm) to the above-mentioned square material, the same as the initial heating temperature so that the temperature during forging does not fall below 900 ° C. The forging process was repeated by reheating to temperature. Moreover, although the completion temperature of hot forging changed with heating temperature and the dimension of the manufactured square bar, it controlled so that it might not fall below 900 degreeC.
なお、熱間鍛造終了後は、大気中での放冷または水冷によって室温まで冷却した。 In addition, after completion | finish of hot forging, it cooled to room temperature by standing_to_cool in air | atmosphere or water cooling.
表２に、素材鋼の寸法形状、熱間鍛造の加熱温度、熱間鍛造終了後の寸法形状、９００℃以上の温度域での累積減面率および熱間鍛造終了後の９００℃以上の温度域での保持時間の詳細を示す。 Table 2 shows the dimensional shape of the raw steel, the heating temperature for hot forging, the dimensional shape after completion of hot forging, the cumulative area reduction in the temperature range of 900 ° C. or higher, and the temperature of 900 ° C. or higher after completion of hot forging. Details of the hold time in the area.
なお、表２において９００℃以上の温度域での保持時間が３〜４ｍｉｎの各試験番号が熱間鍛造終了後に大気中で放冷したものである。また、９００℃以上の温度域での保持時間が３〜４ｓの各試験番号が熱間鍛造終了後に水冷したものである。 In Table 2, each test number having a holding time of 3 to 4 minutes in a temperature range of 900 ° C. or higher is left to cool in the air after hot forging. In addition, each test number having a holding time of 3 to 4 s in a temperature range of 900 ° C. or higher is water-cooled after completion of hot forging.
なお、既に述べたように、上記の各温度は、対象となるインゴットやそれを熱間鍛造したものの「表面温度」を指す。 In addition, as already stated, each said temperature points out the "surface temperature" of what became the target ingot and it hot forged.
上記のようにして得た種々の角材について、その厚さおよび幅の中央部から試験片を切り出し、樹脂に埋め込んで熱間鍛造方向に平行な面、つまり、鍛錬軸に平行な面が観察面になるように鏡面研磨した後、ＪＩＳ Ｇ ０５５５（２００３）に記載の「鋼の非金属介在物の顕微鏡試験方法」に準じて、上記の観察面を倍率４００倍で６４視野、合計で１．４４ｍｍ2相当の面積を共焦点顕微鏡を用いて観察し、画像処理によりＭｎＳの形態と分布を求めた。 For the various square bars obtained as described above, a test piece is cut out from the central part of the thickness and width, embedded in the resin, and the plane parallel to the hot forging direction, that is, the plane parallel to the forging axis is the observation plane. In accordance with “Microscopic test method for non-metallic inclusions in steel” described in JIS G 0555 (2003), the observation surface is 64 fields at a magnification of 400 times, for a total of 1. An area corresponding to 44 mm 2 was observed using a confocal microscope, and the morphology and distribution of MnS were determined by image processing.
また、上記の各角材について、所定の機械部品形状への熱間鍛造の際の加熱を模擬して、１２００℃で５ｍｉｎ保持した後空冷し、その厚さおよび幅の中央部から試験片を切り出し、樹脂に埋め込んで熱間鍛造方向に平行な面、つまり、鍛錬軸に平行な面が観察面になるように鏡面研磨した後、ナイタルで腐食し、粒界フェライトを旧γ粒界とみなして、ＪＩＳ Ｇ ０５５１（２００５）に記載の「鋼−結晶粒度の顕微鏡試験方法」に準じて、γ粒度番号を求めた。 In addition, for each of the above square bars, heating at the time of hot forging into a predetermined machine part shape was simulated, held at 1200 ° C. for 5 minutes, then air-cooled, and a test piece was cut out from the center part of the thickness and width After being mirror-polished so that the surface parallel to the hot forging direction, that is, the surface parallel to the forging axis, becomes the observation surface after embedding in resin, it corrodes with nital and regards the grain boundary ferrite as the old γ grain boundary The γ grain size number was determined in accordance with “A steel-crystal grain size microscopic test method” described in JIS G 0551 (2005).
表２に、上記の試験結果を併せて示す。なお、表２においては、「アスペクト比が１０以下のＭｎＳの面積の全ＭｎＳ面積に対する割合」を「アスペクト比が１０以下のＭｎＳの面積割合」と表記し、また、「厚さが２μｍ以下であるＭｎＳの面積の全ＭｎＳ面積に対する割合」を「厚さ２μｍ以下のＭｎＳの面積割合」と表記した。 Table 2 also shows the above test results. In Table 2, “the ratio of the area of MnS having an aspect ratio of 10 or less to the total MnS area” is expressed as “the area ratio of MnS having an aspect ratio of 10 or less”, and “the thickness is 2 μm or less. “The ratio of the area of a certain MnS to the total MnS area” was expressed as “the area ratio of MnS having a thickness of 2 μm or less”.
表２から、ＴｉＮに代えて、ＭｎＳによるオーステナイト粒のピン止め作用を利用してγ粒の粗大化を抑止するためには、化学組成が本発明で規定する範囲内にある鋼を用いて、しかも、ＭｎＳの形態と分布を本発明で規定する範囲に制御しなければならないことが明らかである。 From Table 2, instead of TiN, in order to suppress the coarsening of γ grains using the pinning action of austenite grains by MnS, using a steel whose chemical composition is within the range defined in the present invention, Moreover, it is clear that the morphology and distribution of MnS must be controlled within the range defined by the present invention.
すなわち、鋼の化学組成が本発明で規定する範囲内にあり、しかも、ＭｎＳの形態と分布が本発明で規定する範囲にある試験番号１〜３の場合には、γ粒の粗大化が抑制されている。 That is, in the case of test numbers 1 to 3 in which the chemical composition of steel is within the range specified by the present invention and the form and distribution of MnS are within the range specified by the present invention, the coarsening of γ grains is suppressed. Has been.
しかしながら、鋼の化学組成が本発明で規定する範囲内にある鋼Ｂを素材鋼とするものであっても、ＭｎＳの形態と分布が本発明で規定する範囲から外れた試験番号４〜７の場合、上記試験番号１〜３の場合に比べてγ粒が粗大化している。 However, even if the steel B having the chemical composition within the range specified by the present invention is used as the raw material steel, the MnS morphology and distribution are out of the range specified by the present invention. In this case, the γ grains are coarser than in the case of the test numbers 1 to 3.
すなわち、試験番号４の場合、アスペクト比が１０以下のＭｎＳの面積の全ＭｎＳ面積に対する割合が８５％と低く、さらに、厚さが２μｍ以下であるＭｎＳの面積の全ＭｎＳ面積に対する割合も７６％と低いので、γ粒度番号は−１．１であって、γ粒が粗大化している。 That is, in the case of test number 4, the ratio of the area of MnS having an aspect ratio of 10 or less to the total MnS area is as low as 85%, and the ratio of the area of MnS having a thickness of 2 μm or less to the total MnS area is also 76%. Therefore, the γ grain size number is −1.1, and the γ grains are coarsened.
なお、上記の試験番号４の場合に、全ＭｎＳ面積に対するアスペクト比が１０以下のＭｎＳの面積割合が小さく、しかも、全ＭｎＳ面積に対する厚さが２μｍ以下であるＭｎＳの面積割合が小さいのは、表２に示したように、熱間鍛造における加熱温度が１２５０℃と高いので、ＭｎＳの厚さを効果的に小さくすることができなかったことに基づく。 In the case of the above test number 4, the area ratio of MnS having an aspect ratio of 10 or less to the total MnS area is small, and the area ratio of MnS having a thickness of 2 μm or less to the total MnS area is small. As shown in Table 2, since the heating temperature in hot forging is as high as 1250 ° C., the thickness of MnS cannot be effectively reduced.
試験番号５の場合、アスペクト比が１０以下のＭｎＳの面積の全ＭｎＳ面積に対する割合は９８％で本発明の規定を満たすもの、厚さが２μｍ以下であるＭｎＳの面積の全ＭｎＳ面積に対する割合が７３％と低い。このため、γ粒度番号は−０．１であって、γ粒が粗大化している。 In the case of test number 5, the ratio of the area of MnS having an aspect ratio of 10 or less to the total MnS area is 98% and satisfies the provisions of the present invention, and the ratio of the area of MnS having a thickness of 2 μm or less to the total MnS area is It is as low as 73%. For this reason, the γ grain size number is −0.1, and the γ grains are coarsened.
なお、上記の試験番号５の場合に、全ＭｎＳ面積に対する厚さが２μｍ以下であるＭｎＳの面積割合が小さいのは、表２に示したように、熱間鍛造における累積減面率が７９％と低いので、ＭｎＳが十分に変形することができなかったことに基づく。 In the case of the above test number 5, the area ratio of MnS having a thickness of 2 μm or less with respect to the total MnS area is small, as shown in Table 2, the cumulative area reduction in hot forging is 79%. This is based on the fact that MnS could not be sufficiently deformed.
試験番号６の場合、厚さが２μｍ以下であるＭｎＳの面積の全ＭｎＳ面積に対する割合は８３％で本発明の規定を満たすものの、アスペクト比が１０以下のＭｎＳの面積の全ＭｎＳ面積に対する割合が６９％と低い。このため、γ粒度番号は−２．０であって、γ粒が粗大化している。 In the case of test number 6, although the ratio of the area of MnS having a thickness of 2 μm or less to the total MnS area is 83% and satisfies the provisions of the present invention, the ratio of the area of MnS having an aspect ratio of 10 or less to the total MnS area is It is as low as 69%. For this reason, the γ grain size number is −2.0, and the γ grains are coarsened.
同様に、試験番号７の場合、厚さが２μｍ以下であるＭｎＳの面積の全ＭｎＳ面積に対する割合は８２％で本発明の規定を満たすものの、アスペクト比が１０以下のＭｎＳの面積の全ＭｎＳ面積に対する割合が７５％と低いため、γ粒度番号は−０．５であって、γ粒が粗大化している。 Similarly, in the case of test number 7, the ratio of the area of MnS having a thickness of 2 μm or less to the total MnS area is 82%, which satisfies the provisions of the present invention, but the total MnS area of MnS having an aspect ratio of 10 or less Therefore, the γ grain size number is −0.5, and the γ grains are coarsened.
なお、上記の試験番号６および７の場合に、全ＭｎＳ面積に対するアスペクト比が１０以下のＭｎＳの面積が小さいのは、表２に示したように、熱間鍛造終了後９００℃以上の温度域での保持時間がそれぞれ、４ｓおよび３ｓと極めて短いので、厚さが小さいＭｎＳの場合であっても、それが分断する十分な時間がないために、アスペクト比の大きなまま残存してしまうことに基づく。 In the case of the test numbers 6 and 7, the area of MnS having an aspect ratio of 10 or less with respect to the total MnS area is small, as shown in Table 2, in the temperature range of 900 ° C. or higher after the completion of hot forging. Since the holding time at 4 s and 3 s are extremely short, respectively, even in the case of MnS having a small thickness, there is not enough time for the separation, so that the aspect ratio remains large. Based.
試験番号８の場合、ＭｎＳの形態と分布が本発明で規定する範囲にあるものの、Ｓの含有量が本発明で規定する条件を下回る鋼Ｃを素材鋼とするものであるため、鋼材中のＭｎＳの数密度が小さく、十分なγ粒の粗大化抑制効果が得られず、γ粒度番号は−２．０であって、γ粒が粗大化している。 In the case of test number 8, although the form and distribution of MnS are in the range specified in the present invention, the steel C having a content of S lower than the conditions specified in the present invention is used as the material steel. The number density of MnS is small, a sufficient effect of suppressing the coarsening of γ grains is not obtained, the γ grain size number is −2.0, and the γ grains are coarsened.
試験番号９の場合、Ｓの含有量が本発明で規定する条件を超えた鋼Ｄを素材鋼とするものであるため、ＭｎＳのアスペクト比が十分小さくならず、アスペクト比が１０以下のＭｎＳの面積の全ＭｎＳ面積に対する割合が８５％と低くなり、γ粒度番号は−１．０であって、γ粒が粗大化している。 In the case of test number 9, since the steel D having a content of S exceeding the conditions specified in the present invention is used as the material steel, the aspect ratio of MnS is not sufficiently small, and the aspect ratio of MnS having an aspect ratio of 10 or less. The ratio of the area to the total MnS area is as low as 85%, the γ particle size number is −1.0, and the γ particles are coarsened.
本発明の熱間鍛造用鋼材は、Ｔｉ、ＮｂおよびＮの含有量が、それぞれ、質量％で、０．００５％以下、０．００５％以下および０．０１０％未満、より望ましくは０．００３％以下、０．００３％以下および０．００７％以下であるにも拘わらず、熱間鍛造するために、１０００℃を超えるような温度、特に、１１００℃以上の高い温度に加熱しても、γ粒の粗大化が抑制されるので、自動車、産業機械および建設機械など高い転動疲労特性が要求される機械部品の素材として好適である。なお、この熱間鍛造用鋼材は本発明の方法によって製造することができる。 In the steel for hot forging of the present invention, the contents of Ti, Nb, and N are 0.005% or less, 0.005% or less, and less than 0.010%, more desirably 0.003, respectively, in mass%. %, 0.003% or less and 0.007% or less in order to perform hot forging, even if heated to a temperature exceeding 1000 ° C., particularly a high temperature of 1100 ° C. or higher, Since coarsening of γ grains is suppressed, it is suitable as a material for machine parts that require high rolling fatigue characteristics such as automobiles, industrial machines, and construction machines. This hot forging steel can be produced by the method of the present invention.
Claims (2)
〈１〉アスペクト比が１０以下のＭｎＳの面積が全ＭｎＳ面積に対して９０％以上であること、
〈２〉厚さが２μｍ以下であるＭｎＳの面積が全ＭｎＳ面積に対して８０％以上であること。 In mass%, C: 0.30 to 0.65%, Si: 0.05% or more and less than 0.50%, Mn: 0.20 to 1.50%, P: 0.025% or less, S: 0 0.005% or more and less than 0.030%, Al: 0.040% or less and N: less than 0.010%, the balance is made of Fe and impurities, Ti in the impurities is 0.005% or less, and Nb is A steel material for hot forging, characterized in that the form and distribution of MnS in a plane parallel to the hot working direction is 0.005% or less and satisfies the following conditions <1> and <2>.
<1> The area of MnS having an aspect ratio of 10 or less is 90% or more with respect to the total MnS area;
<2> The area of MnS having a thickness of 2 μm or less is 80% or more with respect to the total MnS area.
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