JP6791179B2 - Non-microalloyed steel and its manufacturing method - Google Patents

Non-microalloyed steel and its manufacturing method Download PDF

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JP6791179B2
JP6791179B2 JP2018035698A JP2018035698A JP6791179B2 JP 6791179 B2 JP6791179 B2 JP 6791179B2 JP 2018035698 A JP2018035698 A JP 2018035698A JP 2018035698 A JP2018035698 A JP 2018035698A JP 6791179 B2 JP6791179 B2 JP 6791179B2
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佳祐 安藤
佳祐 安藤
岩本 隆
岩本  隆
西村 公宏
公宏 西村
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JFE Steel Corp
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Description

本発明は、熱間圧延ままの非調質材であっても調質材と同等以上の引張強度−靭性バランスを有する非調質鋼、特に自動車および各種産業機械で使用される機械構造用部品として好適な非調質鋼に関するものである。 INDUSTRIAL APPLICABILITY According to the present invention, non-tempered steel having a tensile strength-toughness balance equal to or higher than that of the tempered material even if it is a non-heat-rolled material, particularly mechanical structural parts used in automobiles and various industrial machines It relates to a non-tamed steel suitable for the above.

自動車をはじめとして、輸送機械や建設機械に用いられる構造部品には、機械構造用炭素鋼や機械構造用合金鋼を焼入れ焼戻した調質鋼だけでなく、焼入れ焼戻しによらず鋼の化学成分や組織の調整によって強度を確保した非調質鋼が用いられている。 Structural parts used in transportation machinery and construction machinery, including automobiles, include not only tempered steel obtained by quenching and tempering carbon steel for machine structure and alloy steel for machine structure, but also the chemical composition of steel regardless of quenching and tempering. Non-quenched steel whose strength is ensured by adjusting the structure is used.

このような用途に用いられる非調質鋼は、V やNbを添加したフェライト−パーライト二層組織が一般的であり、調質鋼に比べると、引張強度を同程度にした場合には、降伏強度、絞り値および衝撃値が低く、一方降伏強度を同程度とした場合には、引張強度すなわち硬度が過度に上昇し、切削性が低下することが指摘されていた。 Non-tempered steels used for such applications generally have a ferrite-pearlite two-layer structure with V and Nb added, and yield when the tensile strength is similar to that of tempered steel. It has been pointed out that when the strength, drawing value and impact value are low, while the yield strength is set to the same level, the tensile strength, that is, the hardness, increases excessively and the machinability decreases.

上記の背景の下、特許文献1および特許文献2には、高強度で高降伏比、かつ高靱性な非調質鋼を得るために、フェライト、ベイニティックフェライト、疑似マルテンサイトを有する組織をそなえた鋼材を、冷間加工後、600℃ 以下で時効処理し、CuおよびTi-Nb系炭化物を析出させる技術が開示されている。 Against the above background, Patent Documents 1 and 2 include structures having ferrite, bainitic ferrite, and pseudo-martensite in order to obtain high-strength, high yield ratio, and high toughness non-tempered steel. A technique is disclosed in which a prepared steel material is cold-worked and then aged at 600 ° C. or lower to precipitate Cu and Ti-Nb-based carbides.

しかしながら、実際の製造において、上記したような複数の組織の比率を厳格に制御することは極めて難しい。さらに、多量のCu添加による析出強化を利用する場合には高温割れ防止のために高価なNiを多量に添加する必要があるため、大量消費される構造部品としては適当ではないという問題もあった。 However, in actual manufacturing, it is extremely difficult to strictly control the ratio of a plurality of tissues as described above. Further, when the precipitation strengthening by adding a large amount of Cu is used, it is necessary to add a large amount of expensive Ni to prevent high temperature cracking, so that there is a problem that it is not suitable as a structural part to be consumed in large quantities. ..

また、特許文献3には、強度、靭性に加え、被削性にも優れたベイナイト型非調質鋼を得るため、MnSよりも切削加工時の潤滑効果の大きい、Ti炭硫化物を適切なサイズ、清浄度にコントロールさせる技術が開示されている。しかしながら、本発明者らの調査によれば、TiおよびCの添加量のバランスや熱間加工時の加熱および冷却の条件次第では、分散粒子によるピン止め力が低下し、オーステナイト粒が粗大化する結果、靭性が低下する場合のあることが知見された。 Further, in Patent Document 3, in order to obtain bainite-type non-tempered steel having excellent machinability in addition to strength and toughness, Ti carbon sulfide having a larger lubricating effect during cutting than MnS is appropriate. The technology to control the size and cleanliness is disclosed. However, according to the investigation by the present inventors, the pinning force of the dispersed particles decreases and the austenite particles become coarse depending on the balance of the addition amounts of Ti and C and the heating and cooling conditions during hot working. As a result, it was found that the toughness may decrease.

特開2001−123224号公報Japanese Unexamined Patent Publication No. 2001-123224 特開2001−131680号公報Japanese Unexamined Patent Publication No. 2001-131680 特許第3489655号公報Japanese Patent No. 3489655

本発明は、上述した従来技術の問題を解決し、強度−靭性バランスに優れる機械構造用部品を安定して作製するための素材に適した、非調質鋼およびその製造方法を提供することを目的とする。 The present invention provides a non-microalloyed steel and a method for producing the same, which solves the above-mentioned problems of the prior art and is suitable as a material for stably producing mechanical structural parts having an excellent strength-toughness balance. The purpose.

本発明者らは、上記課題を解決するため、熱間加工後の強度並びに靭性に及ぼす、成分および組織の影響について鋭意検討を行った。その結果、以下のa)〜c)の事項を見出すに到った。
a)熱間加工後の鋼組織を面積率で70%以上のベイナイトにすることにより、優れた強度−靭性バランスを得ることができる。
b)靭性の向上には、上記a)のベイナイト組織化に加え、該組織中の固溶C量を減少させることが効果的である。そのため、炭化物形成により、固溶C量を減少せしめるTiを、鋼材中のC含有量に応じた、所定量以上で添加する必要がある。
c) 熱間加工の加熱時に未固溶となるTi炭化物量を増やせば、該析出物によるピンニング効果が発揮され、非調質ままで微細なベイナイト組織を得ることができる。一方で、TiとCを過剰に添加すると、オーステナイト粒が顕著に微細化し、焼入れ性が低下するため、上記a)で述べた所望のベイナイト組織を得ることが困難となる。また、未固溶のTi炭化物量が少ない場合、オーステナイト粒が粗大化するため、微細ベイナイト組織に比べて靭性が劣位になる。そのため、安定的に優れた強度−靭性バランスを得るためには、CとTiの含有量を厳密に制御する必要がある。
In order to solve the above problems, the present inventors have diligently studied the effects of components and structures on the strength and toughness after hot working. As a result, we have found the following items a) to c).
a) An excellent strength-toughness balance can be obtained by making the steel structure after hot working into bainite with an area ratio of 70% or more.
b) In order to improve toughness, it is effective to reduce the amount of solid solution C in the tissue in addition to the bainite organization described in a) above. Therefore, it is necessary to add Ti, which reduces the amount of solid solution C by forming carbides, in an amount of a predetermined amount or more according to the C content in the steel material.
c) By increasing the amount of Ti carbides that become unsolidified when heated in hot working, the pinning effect of the precipitates is exhibited, and a fine bainite structure can be obtained without tempering. On the other hand, if Ti and C are added in excess, the austenite grains become significantly finer and the hardenability is lowered, so that it becomes difficult to obtain the desired bainite structure described in a) above. Further, when the amount of undissolved Ti carbide is small, the austenite grains become coarse, so that the toughness is inferior to that of the fine bainite structure. Therefore, in order to obtain a stable and excellent strength-toughness balance, it is necessary to strictly control the contents of C and Ti.

本発明は、上記の知見に基づいてなされたものであり、その要旨構成は以下のとおりである。
1.質量%で、
C:0.10〜0.25%、
Si:0.10〜1.00%、
Mn:0.30〜3.00%、
P:0.050%以下、
S:0.005〜0.150%、
Cr:0.10〜2.50%、
Al:0.010〜0.080%、
Ti:0.25〜0.60%および
N:0.0070%以下
を、次式(1)で定義されるT1が3.0以下、かつ次式(2)で定義されるT2が1620〜1750となる範囲で含有し、残部はFeおよび不可避不純物の成分組成を有し、ベイナイト相の面積率が70%以上の組織を有する非調質鋼。
T1=(C /12)/(Ti/48) ・・・・(1)
T2=−7430/[Log(C×Ti)−3.23] ・・・・(2)
ここで、各元素記号は該元素の含有量(質量%)を示す。
The present invention has been made based on the above findings, and its gist structure is as follows.
1. 1. By mass%
C: 0.10 to 0.25%,
Si: 0.10 to 1.00%,
Mn: 0.30 to 3.00%,
P: 0.050% or less,
S: 0.005 to 0.150%,
Cr: 0.10 to 2.50%,
Al: 0.010 to 0.080%,
Ti: 0.25 to 0.60% and N: 0.0070% or less are contained in the range where T1 defined by the following formula (1) is 3.0 or less and T2 defined by the following formula (2) is 1620 to 1750. The balance is a non-tamed steel having a composition of Fe and unavoidable impurities, and a structure in which the area ratio of the bainite phase is 70% or more.
T1 = (C / 12) / (Ti / 48) ... (1)
T2 = -7430 / [Log (C x Ti) -3.23] ... (2)
Here, each element symbol indicates the content (mass%) of the element.

2.前記成分組成は、さらに、質量%で
Mo:1.0%以下、
Nb:0.3%以下、
V:0.3%以下、
W:0.3%以下および
B:0.0100%以下
のうちから選んだ1種以上を含む前記1に記載の非調質鋼。
2. 2. The component composition is further increased by mass%.
Mo: 1.0% or less,
Nb: 0.3% or less,
V: 0.3% or less,
The non-microalloyed steel according to 1 above, which comprises at least one selected from W: 0.3% or less and B: 0.0100% or less.

3.前記成分組成は、さらに、質量%で
Cu:1.0%以下および
Ni:1.0%以下
のうちから選んだ1種以上を含む前記1または2に記載の非調質鋼。
3. 3. The component composition is further increased by mass%.
Cu: 1.0% or less and
Ni: The non-tampered steel according to 1 or 2 above, which contains at least one selected from 1.0% or less.

4.質量%で、
C:0.10〜0.25%、
Si:0.10〜1.00%、
Mn:0.30〜3.00%、
P:0.050%以下、
S:0.005〜0.150%、
Cr:0.10〜2.50%、
Al:0.010〜0.080%、
Ti:0.25〜0.60%および
N:0.0070%以下
を、次式(1)で定義されるT1が3.0以下、かつ次式(2)で定義されるT2が1620〜1750となる範囲で含有し、残部はFeおよび不可避不純物の成分組成を有する鋼材に、加熱温度が1300℃以下および仕上温度が850℃以上1300℃以下の熱間加工を施し、次いで700〜550℃の温度域を0.5℃/s以上で冷却する非調質鋼の製造方法。
T1=(C /12)/(Ti/48) ・・・・(1)
T2=−7430/[Log(C×Ti)−3.23] ・・・・(2)
ここで、各元素記号は該元素の含有量(質量%)を示す。
4. By mass%
C: 0.10 to 0.25%,
Si: 0.10 to 1.00%,
Mn: 0.30 to 3.00%,
P: 0.050% or less,
S: 0.005 to 0.150%,
Cr: 0.10 to 2.50%,
Al: 0.010 to 0.080%,
Ti: 0.25 to 0.60% and N: 0.0070% or less are contained in the range where T1 defined by the following formula (1) is 3.0 or less and T2 defined by the following formula (2) is 1620 to 1750. The rest is a steel material with a composition of Fe and unavoidable impurities, which is hot-worked with a heating temperature of 1300 ° C or less and a finishing temperature of 850 ° C or more and 1300 ° C or less, and then in the temperature range of 700 to 550 ° C at 0.5 ° C / s. A method for producing non-tempered steel to be cooled.
T1 = (C / 12) / (Ti / 48) ... (1)
T2 = -7430 / [Log (C x Ti) -3.23] ... (2)
Here, each element symbol indicates the content (mass%) of the element.

5.前記成分組成は、さらに、質量%で
Mo:1.0%以下、
Nb:0.3%以下、
V:0.3%以下、
W:0.3%以下および
B:0.0100%以下
のうちから選んだ1種以上を含む前記4に記載の非調質鋼の製造方法。
5. The component composition is further increased by mass%.
Mo: 1.0% or less,
Nb: 0.3% or less,
V: 0.3% or less,
The method for producing a non-microalloyed steel according to 4 above, which comprises at least one selected from W: 0.3% or less and B: 0.0100% or less.

6.前記成分組成は、さらに、質量%で
Cu:1.0%以下および
Ni:1.0%以下
のうちから選んだ1種以上を含む前記4または5に記載の非調質鋼の製造方法。
6. The component composition is further increased by mass%.
Cu: 1.0% or less and
Ni: The method for producing a non-microalloyed steel according to 4 or 5 above, which comprises one or more selected from 1.0% or less.

本発明によれば、強度−靭性バランスに優れた機械構造用部品を、安定的に作製するための素材に適した、非調質鋼およびその製造方法を提供することができる。すなわち、輸送機械や建設機械に用いられる各種構造部品を、本発明鋼を用いて作製した場合に、非調質ままで高強度および靭性に優れた部品を量産することが可能になる。 According to the present invention, it is possible to provide non-microalloyed steel and a method for producing the same, which are suitable as a material for stably producing mechanical structural parts having an excellent strength-toughness balance. That is, when various structural parts used in transportation machines and construction machines are manufactured using the steel of the present invention, it becomes possible to mass-produce parts having high strength and excellent toughness without being tempered.

回転曲げ疲労試験片の形状を示す図である。It is a figure which shows the shape of the rotary bending fatigue test piece. 強度−靭性バランスを発明例と比較例とで比較するためのグラフである。It is a graph for comparing the strength-toughness balance between the invention example and the comparative example.

まず、本発明において、鋼の成分組成を上記の範囲に限定した理由について説明する。なお、成分に関する「%」表示は、特に断らない限り質量%を意味するものとする。
C:0.10〜0.25%
Cは、Tiと共に熱間加工時の加熱温度で安定に存在するTi炭化物を形成するため、組織の微細化に有効な元素である。また、ベイナイト相の生成ならびに強度向上にも寄与するため、0.10%以上の添加を必要とする。一方、C含有量が0.25%を超えると、固溶C量の増加によりベイナイト相の靭性が低下するため、C量は0.25%以下とする。好ましくは0.14%以上0.22%以下の範囲である。
First, in the present invention, the reason why the composition of steel is limited to the above range will be described. In addition, unless otherwise specified, the "%" indication regarding the component shall mean mass%.
C: 0.10 to 0.25%
C is an element effective for microstructure refinement because it forms Ti carbides that are stably present at the heating temperature during hot working together with Ti. In addition, since it contributes to the formation of the bainite phase and the improvement of strength, it is necessary to add 0.10% or more. On the other hand, if the C content exceeds 0.25%, the toughness of the bainite phase decreases due to the increase in the solid solution C content, so the C content is set to 0.25% or less. It is preferably in the range of 0.14% or more and 0.22% or less.

Si:0.10〜1.00%
Siは、脱酸だけでなく、ベイナイト相の生成に有効なために、0.10%以上で添加する。一方、Si 量が1.00%を超えると、フェライト相およびベイナイト相に固溶し、その固溶硬化により、靭性および機械加工性を劣化させるため、Si量は1.00%以下とする。好ましくは0.10%以上0.50%以下の範囲である。
Si: 0.10 to 1.00%
Si is added at 0.10% or more because it is effective not only for deoxidation but also for the formation of bainite phase. On the other hand, if the amount of Si exceeds 1.00%, it dissolves in the ferrite phase and the bainite phase, and the solid solution hardening deteriorates the toughness and machinability. Therefore, the amount of Si is set to 1.00% or less. It is preferably in the range of 0.10% or more and 0.50% or less.

Mn:0.30〜3.00%
Mnは、ベイナイト相の生成ならびに強度向上に有効なために添加する。しかしながら、Mn量が0.30%未満では、ベイナイト相の生成量が少なくなり、靭性確保が困難となる。したがって、Mn量は0.30%以上で添加する。一方、3.00%を超えると焼入れ性が顕著に向上し、靭性に悪影響を及ぼす島状マルテンサイト等の硬質層が形成されるため、Mn量は3.00%以下とする。好ましくは0.80%以上2.00%以下の範囲である。
Mn: 0.30 to 3.00%
Mn is added because it is effective in forming the bainite phase and improving the strength. However, if the amount of Mn is less than 0.30%, the amount of bainite phase formed is small, and it becomes difficult to secure toughness. Therefore, the amount of Mn should be 0.30% or more. On the other hand, if it exceeds 3.00%, hardenability is remarkably improved and a hard layer such as island-shaped martensite that adversely affects toughness is formed, so the amount of Mn should be 3.00% or less. It is preferably in the range of 0.80% or more and 2.00% or less.

P:0.050%以下
Pは、結晶粒界に偏析し、靭性および疲労強度を低下させるため、低いほど望ましい。具体的には、0.050%を超えると、上記弊害が現れるため、P量は0.050%以下とした。一方、0.003%未満に低減するには多大な製造コストを要することから、コストの観点からは、0.003%を下限とすることが好ましい。
P: 0.050% or less P is preferably as low as it segregates at the grain boundaries and reduces toughness and fatigue strength. Specifically, if it exceeds 0.050%, the above-mentioned adverse effects appear, so the amount of P was set to 0.050% or less. On the other hand, since a large manufacturing cost is required to reduce the content to less than 0.003%, it is preferable to set 0.003% as the lower limit from the viewpoint of cost.

S:0.005%〜0.150%
Sは、Mnと硫化物を形成し、被削性を向上させる作用を有する。そのためには、0.005%以上で含有させる。一方、過剰な添加は、部品の疲労強度および靭性を低下させるため、上限を0.150%とした。好ましくは0.010%以上0.060%以下の範囲である。
S: 0.005% to 0.150%
S has an action of forming a sulfide with Mn and improving machinability. For that purpose, the content should be 0.005% or more. On the other hand, excessive addition reduces the fatigue strength and toughness of the parts, so the upper limit was set to 0.150%. It is preferably in the range of 0.010% or more and 0.060% or less.

Cr:0.10〜2.50%
Crは、ベイナイト相の生成ならびに強度向上に有効なため添加する。しかしながら、Cr量が0.10%未満の場合、ベイナイト相の生成量が少なくなり、靭性確保が困難となる。したがって、Cr量は0.10%以上とする。一方、2.50%を超えると焼入れ性が顕著に向上し、靭性に悪影響を及ぼす島状マルテンサイト等の硬質層が形成されるため、Cr量は2.50%以下とする。好ましくは、0.50%以上1.50%以下の範囲である。
Cr: 0.10 to 2.50%
Cr is added because it is effective in forming the bainite phase and improving the strength. However, when the amount of Cr is less than 0.10%, the amount of bainite phase formed is small, and it becomes difficult to secure toughness. Therefore, the amount of Cr is set to 0.10% or more. On the other hand, if it exceeds 2.50%, hardenability is remarkably improved and a hard layer such as island-shaped martensite that adversely affects toughness is formed, so the amount of Cr is set to 2.50% or less. It is preferably in the range of 0.50% or more and 1.50% or less.

Al:0.010〜0.080%
Alは、脱酸に必要な元素であるが、0.010%未満ではその効果が十分に得られない。一方、0.080%を超えると、その効果が飽和するとともに連続鋳造時のノズル詰まりの発生や、アルミナクラスター介在物の発現により靭性や疲労強度の低下を招く。従って、Al量は0.010〜0.080%の範囲に限定した。好ましくは0.020〜0.050%の範囲である。
Al: 0.010 to 0.080%
Al is an element necessary for deoxidation, but its effect cannot be sufficiently obtained if it is less than 0.010%. On the other hand, if it exceeds 0.080%, the effect is saturated, nozzle clogging occurs during continuous casting, and alumina cluster inclusions appear, resulting in a decrease in toughness and fatigue strength. Therefore, the amount of Al was limited to the range of 0.010 to 0.080%. It is preferably in the range of 0.020 to 0.050%.

Ti:0.25〜0.60%
Tiは、Cと共に熱間加工時の加熱温度で安定に存在するTi炭化物を形成するため、組織の微細化に有効な元素である。このような効果を発揮させるためには、0.25%以上の添加が必要である。一方、その効果は0.60%で飽和し、かつ0.60%を超える添加はコスト増をまねく。従って、Ti量は0.25〜0.60%の範囲に限定した。好ましくは0.30〜0.50%の範囲である。
Ti: 0.25 to 0.60%
Ti, together with C, forms Ti carbides that exist stably at the heating temperature during hot working, and is therefore an effective element for microstructure miniaturization. In order to exert such an effect, it is necessary to add 0.25% or more. On the other hand, the effect is saturated at 0.60%, and addition exceeding 0.60% leads to cost increase. Therefore, the amount of Ti was limited to the range of 0.25 to 0.60%. It is preferably in the range of 0.30 to 0.50%.

N:0.0070%以下
Nは、Tiと結合してTiNを形成し、オーステナイト結晶粒の微細化に寄与する元素である。しかし、TiNの形成により、熱間加工温度でのオーステナイト粒微細化に寄与するTi炭化物量が減少してしまうだけでなく、過剰に添加すると凝固時の鋼塊に気泡が発生したり、鍛造性の劣化を招くため、上限を0.0070%とする。好ましくは0.0020〜0.0070%の範囲である。
N: 0.0070% or less N is an element that combines with Ti to form TiN and contributes to the refinement of austenite crystal grains. However, the formation of TiN not only reduces the amount of Ti carbides that contribute to the miniaturization of austenite grains at the hot working temperature, but also causes bubbles to be generated in the steel ingot during solidification when added in excess, and forging property. The upper limit is set to 0.0070% because it causes deterioration of. It is preferably in the range of 0.0020 to 0.0070%.

本発明では、各々の元素が単に上記の範囲を満足するだけでは不十分であり、次式(1)で定義されるT1が3.0以下、かつ次式(2)で定義されるT2が1620〜1750となる範囲を満足させることが重要である。
T1=(C /12)/(Ti/48) ・・・・(1)
T2=−7430/[Log(C×Ti)−3.23] ・・・・(2)
ここで、各元素記号は該元素の含有量(質量%)を示す。
In the present invention, it is not sufficient for each element to simply satisfy the above range, and T1 defined by the following formula (1) is 3.0 or less, and T2 defined by the following formula (2) is 1620 to It is important to satisfy the range of 1750.
T1 = (C / 12) / (Ti / 48) ... (1)
T2 = -7430 / [Log (C x Ti) -3.23] ... (2)
Here, each element symbol indicates the content (mass%) of the element.

すなわち、ベイナイト組織からなる非調質鋼の靭性は、固溶C量の増加に伴い低下するため、炭化物形成を通して固溶C量を減少せしめるTiについて、鋼材中のC添加量に応じた、所定量以上を添加する必要がある。ここで、上記したT1はベイナイト組織中の固溶C量を表すパラメータであり、T1が3.0を超えると固溶C量が増加してしまうため、靭性の向上効果が得られなくなる。そのため、T1は3.0以下とする。 That is, since the toughness of the non-tamed steel having a bainite structure decreases as the amount of solid solution C increases, the amount of Ti that reduces the amount of solid solution C through the formation of carbides is determined according to the amount of C added to the steel material. It is necessary to add more than a certain amount. Here, T1 described above is a parameter representing the amount of solid solution C in the bainite structure, and when T1 exceeds 3.0, the amount of solid solution C increases, so that the effect of improving toughness cannot be obtained. Therefore, T1 should be 3.0 or less.

次に、熱間加工の加熱時に未固溶となるTi炭化物量を増やせば、該析出物によるピンニング効果が発揮され、非調質ままで微細なベイナイト組織を得ることができる。ここで、上記したT2は、微細化に寄与するTi炭化物の必要量を表すパラメータであり、T2が1620以下の場合、Ti炭化物量が不足し、ピンニング力が低下するため、オーステナイト粒が粗大化し、靭性が低下してしまう。一方、T2が1750を超えると、オーステナイト粒が顕著に微細化し、焼入れ性が低下するため、面積率で70%以上のベイナイト組織を得ることが困難となる。そのため、T2は1620〜1750とする。 Next, if the amount of Ti carbides that become unsolidified during heating in hot working is increased, the pinning effect of the precipitates is exhibited, and a fine bainite structure can be obtained without being tempered. Here, T2 described above is a parameter representing the required amount of Ti carbides that contribute to miniaturization. When T2 is 1620 or less, the amount of Ti carbides is insufficient and the pinning force is reduced, so that the austenite grains become coarse. , The toughness is reduced. On the other hand, when T2 exceeds 1750, the austenite grains become significantly finer and the hardenability is lowered, so that it becomes difficult to obtain a bainite structure having an area ratio of 70% or more. Therefore, T2 is set to 1620 to 1750.

本発明における鋼中成分は、上記成分を含み、残部はFeおよび不可避不純物である。さらに、本発明の作用効果を損なわない範囲にて、他の特性付与等を目的として、以下の選択成分を添加することが出来る。 The components in steel in the present invention contain the above components, and the balance is Fe and unavoidable impurities. Further, the following selective components can be added for the purpose of imparting other characteristics, etc., as long as the effects of the present invention are not impaired.

Mo:1.0%以下
Moは、焼入れ性および靭性を向上させるため、添加することができる。添加する場合は、0.05%以上とすることが好ましい。一方、その効果は1.0%を超えると飽和し、かつ1.0%を超えて添加するとコスト増になるため、上限を1.0%とすることが好ましい。より好ましくは、0.10〜0.50%の範囲である。
Mo: 1.0% or less
Mo can be added to improve hardenability and toughness. When added, it is preferably 0.05% or more. On the other hand, the effect is saturated when it exceeds 1.0%, and the cost increases when it is added in excess of 1.0%. Therefore, it is preferable to set the upper limit to 1.0%. More preferably, it is in the range of 0.10 to 0.50%.

Nb:0.3%以下
Nbは、結晶粒を微細化し、粒界を強化して靭性および疲労強度の向上に寄与する。そのためには、0.05%以上で添加することが好ましい。一方、その効果は0.3%を超えると飽和し、かつコスト増をまねくため、上限を0.3%とすることが好ましい。より好ましくは、0.10〜0.20%の範囲である。
Nb: 0.3% or less
Nb refines the crystal grains and strengthens the grain boundaries, contributing to the improvement of toughness and fatigue strength. For that purpose, it is preferable to add at 0.05% or more. On the other hand, if the effect exceeds 0.3%, it is saturated and causes an increase in cost. Therefore, it is preferable to set the upper limit to 0.3%. More preferably, it is in the range of 0.10 to 0.20%.

V:0.3%以下
Vは、Nbと同じく炭窒化物形成元素であり、結晶粒を微細化し、粒界を強化して靭性および疲労強度の向上に寄与する。そのためには、0.05%以上で添加することが好ましい。一方、その効果は0.3%を超えると飽和し、かつコスト増をまねくため、上限を0.3%とすることが好ましい。より好ましくは、0.10〜0.20%の範囲である。
V: 0.3% or less V is a carbonitride-forming element like Nb, which contributes to the improvement of toughness and fatigue strength by refining crystal grains and strengthening grain boundaries. For that purpose, it is preferable to add at 0.05% or more. On the other hand, if the effect exceeds 0.3%, it is saturated and causes an increase in cost. Therefore, it is preferable to set the upper limit to 0.3%. More preferably, it is in the range of 0.10 to 0.20%.

W: 0.3%以下
Wは、Tiと共に析出物を形成して靭性および疲労強度の向上に寄与する。そのためには、0.05%以上で添加することが好ましい。一方、その効果は0.3%を超えると飽和し、かつコスト増をまねくため、上限を0.3%とすることが好ましい。より好ましくは、0.10〜0.20%の範囲である。
W: 0.3% or less W forms a precipitate together with Ti and contributes to the improvement of toughness and fatigue strength. For that purpose, it is preferable to add at 0.05% or more. On the other hand, if the effect exceeds 0.3%, it is saturated and causes an increase in cost. Therefore, it is preferable to set the upper limit to 0.3%. More preferably, it is in the range of 0.10 to 0.20%.

B:0.0100%以下
Bは、微量の添加により焼入れ性を確保するのに有効な元素であり、そのためには0.0005%以上で添加することが好ましい。一方、0.0100%を超えて添加しても、上記の効果が飽和する。よって、上限は0.0100%とすることが好ましい。より好ましくは、0.0010〜0.0040%の範囲である。
B: 0.0100% or less B is an element effective for ensuring hardenability by adding a small amount, and for that purpose, it is preferable to add 0.0005% or more. On the other hand, even if it is added in excess of 0.0100%, the above effect is saturated. Therefore, the upper limit is preferably 0.0100%. More preferably, it is in the range of 0.0010 to 0.0040%.

Cu:1.0%以下
Cuは、焼入性の向上に寄与するとともに、靱性の向上に有用な元素である。これらの効果を得るためには、Cuは0.01%以上とすることが好ましい。一方、Cu含有量が1.0%を超えると、圧延材の表面肌が荒れてしまい、疵として残存する懸念がある。そこで、上限は1.0%とすることが好ましい。より好ましくは、0.10〜0.50%の範囲である。
Cu: 1.0% or less
Cu is an element that contributes to the improvement of hardenability and is useful for the improvement of toughness. In order to obtain these effects, Cu is preferably 0.01% or more. On the other hand, if the Cu content exceeds 1.0%, the surface surface of the rolled material becomes rough, and there is a concern that it may remain as a flaw. Therefore, the upper limit is preferably 1.0%. More preferably, it is in the range of 0.10 to 0.50%.

Ni:1.0%以下
Niは、焼入性の向上に寄与するとともに、靱性の向上に有用な元素である。これらの効果を得るためには、Niは0.01%以上とすることが好ましい。一方、1.0%を超えて含有されても、上記の効果が飽和する。よって、上限は1.0%とすることが好ましい。より好ましくは、0.10〜0.50%の範囲である。
Ni: 1.0% or less
Ni is an element that contributes to the improvement of hardenability and is useful for improving toughness. In order to obtain these effects, Ni is preferably 0.01% or more. On the other hand, even if it is contained in excess of 1.0%, the above effect is saturated. Therefore, the upper limit is preferably 1.0%. More preferably, it is in the range of 0.10 to 0.50%.

次に、本発明における非調質鋼の鋼組織を前記の範囲に限定した理由を説明する。
ベイナイト相:組織全体に対する面積率で70%以上
本発明では、ベイナイト相を組織全体に対する面積率で70%以上とすることが重要である。本発明では、熱間加工温度域におけるオーステナイト粒のピンニングに、当該温度域で未固溶となるTi炭化物を活用する。該析出物は非整合で存在することから、弾性歪みエネルギー(整合歪み)が小さく、析出物起因の靭性低下リスクを最小限に抑制することが可能となる。一方で、熱間加工後の冷却過程において、微細なTi炭化物が析出すると、靭性向上の観点からは不利となる。この点、ベイナイト変態過程は、フェライト−パーライト変態過程に比べ、母相中にTi炭化物が生成し難い。したがって、本発明の非調質鋼の鋼組織はベイナイト相を主体とする。具体的には、ベイナイト相を組織全体に対する面積率で70%以上とする。好ましくは80%以上、より好ましくは90%以上である。また100%であってもよい。なお、ベイナイト相以外の組織としては、フェライト相、パーライト相またはマルテンサイト相等が考えられるが、これらの組織は少ないほど好ましいのは言うまでもない。
Next, the reason why the steel structure of the non-tempered steel in the present invention is limited to the above range will be described.
Bainite phase: 70% or more in area ratio to the entire structure In the present invention, it is important that the bainite phase has an area ratio of 70% or more to the entire structure. In the present invention, Ti carbides that are undissolved in the hot working temperature range are used for pinning the austenite grains. Since the precipitates exist in a non-matching manner, the elastic strain energy (matching strain) is small, and the risk of toughness reduction due to the precipitates can be minimized. On the other hand, if fine Ti carbides are precipitated in the cooling process after hot working, it is disadvantageous from the viewpoint of improving toughness. In this respect, the bainite transformation process is less likely to generate Ti carbides in the matrix than the ferrite-pearlite transformation process. Therefore, the steel structure of the non-treated steel of the present invention is mainly composed of the bainite phase. Specifically, the bainite phase is set to 70% or more in area ratio with respect to the entire structure. It is preferably 80% or more, more preferably 90% or more. It may also be 100%. As the structure other than the bainite phase, a ferrite phase, a pearlite phase, a martensite phase and the like can be considered, but it goes without saying that the smaller the number of these structures, the more preferable.

ここに、各相の面積率は、次のようにして求めることができる。すなわち、得られた非調質鋼から試験片を採取し、圧延方向に平行な垂直断面(L断面)について、研磨後ナイタールで腐食し、光学顕微鏡および走査型電子顕微鏡(SEM)を用い、L断面組織観察(400倍の光学顕微鏡組織観察)により相の種類を同定し、各相の面積率を求める。なお、島状マルテンサイトは光学顕微鏡では判定が困難であることから、SEMを用いて観察する。島状マルテンサイトは SEMにより、白く浮き立った部分として観察されるため、少なくとも5視野のミクロ組織写真を画像処理することによって、それらの面積率の平均値から算出することができる。 Here, the area ratio of each phase can be obtained as follows. That is, a test piece was taken from the obtained non-tamed steel, and the vertical cross section (L cross section) parallel to the rolling direction was corroded with Nital after polishing, and L was used using an optical microscope and a scanning electron microscope (SEM). The type of phase is identified by cross-sectional structure observation (400x optical microscope structure observation), and the area ratio of each phase is determined. Since it is difficult to determine island-shaped martensite with an optical microscope, observe it using an SEM. Since the island-shaped martensite is observed by SEM as a white and raised part, it can be calculated from the average value of the area ratios of microstructure photographs of at least 5 fields of view by image processing.

次に、本発明の製造条件について説明する。
すなわち、上記した成分組成を有する鋼材を加熱後、仕上温度が850℃以上1300℃以下の熱間加工を施し、次いで700〜550℃の温度域を0.5℃/s以上で冷却することによって、非調質鋼を製造する。以下、製造工程毎の要件について説明する。
[鋼材加熱温度:1300℃以下]
熱間加工に先立つ鋼材加熱を、1300℃超えとした場合、組織の微細化に寄与するTi炭化物量の確保が困難となるため、該加熱温度は1300℃以下とする。一方、下限については、次工程の熱間加工における仕上温度を確保するために、900℃以上とすることが望ましい。なお、加熱に供する鋼材は、鋼片であっても、熱間圧延や熱間鍛造を経た鋼片や棒鋼であってもよい。
Next, the production conditions of the present invention will be described.
That is, after heating the steel material having the above-mentioned composition, hot working is performed at a finishing temperature of 850 ° C or higher and 1300 ° C or lower, and then the temperature range of 700 to 550 ° C is cooled at 0.5 ° C / s or higher. Manufacture tempered steel. Hereinafter, the requirements for each manufacturing process will be described.
[Steel heating temperature: 1300 ° C or less]
If the heating of the steel material prior to hot working exceeds 1300 ° C, it becomes difficult to secure the amount of Ti carbides that contribute to the miniaturization of the structure, so the heating temperature is set to 1300 ° C or less. On the other hand, the lower limit is preferably 900 ° C. or higher in order to secure the finishing temperature in the hot working of the next process. The steel material to be heated may be a piece of steel, or a piece of steel or steel bar that has undergone hot rolling or hot forging.

[熱間加工における仕上温度:850℃以上1300℃以下]
この仕上げ温度が850℃未満ではフェライト組織が生成するため、母相を面積率で70%以上のベイナイト組織とするためには不利である。また、圧延荷重が高くなって、圧延材の真円度も悪化する。このため、仕上温度を850℃以上とする。また、仕上温度を1300℃超えとした場合、組織微細化に寄与するTi炭化物量の確保が困難となるため、仕上温度の上限は、1300℃とする。
[Finishing temperature in hot working: 850 ° C or higher and 1300 ° C or lower]
If the finishing temperature is less than 850 ° C., a ferrite structure is formed, which is disadvantageous for forming the matrix phase into a bainite structure having an area ratio of 70% or more. In addition, the rolling load increases, and the roundness of the rolled material also deteriorates. Therefore, the finishing temperature is set to 850 ° C or higher. If the finishing temperature exceeds 1300 ° C, it becomes difficult to secure the amount of Ti carbides that contribute to microstructure miniaturization. Therefore, the upper limit of the finishing temperature is set to 1300 ° C.

なお、熱間加工としては、熱間鍛造および熱間圧延のいずれか一方または両方を適用することができ、本発明の非調質鋼を用いる部材の形状に応じて適宜選択使用すればよい。なお、熱間加工として熱間圧延と熱間鍛造の両方を行う場合は、最終の熱間加工について、上記の加熱温度および仕上温度の条件を適用すればよい。 As the hot working, either one or both of hot forging and hot rolling can be applied, and it may be appropriately selected and used according to the shape of the member using the non-tamed steel of the present invention. When both hot rolling and hot forging are performed as hot working, the above-mentioned heating temperature and finishing temperature conditions may be applied to the final hot working.

[冷却:700〜550℃の温度域を0.5℃/s以上]
熱間加工後に微細析出物が析出して靭性が損なわれないよう、上述したベイナイト組織を得るためには、熱間加工後の冷却速度を規定する必要がある。すなわち、微細析出物の析出温度範囲である700〜550℃の温度域を、微細析出物が得られる限界冷却速度(0.5℃/s)以上で冷却する必要がある。
[Cooling: 0.5 ° C / s or more in the temperature range of 700 to 550 ° C]
In order to obtain the above-mentioned bainite structure so that fine precipitates do not precipitate after hot working and the toughness is not impaired, it is necessary to specify the cooling rate after hot working. That is, it is necessary to cool the temperature range of 700 to 550 ° C., which is the precipitation temperature range of fine precipitates, at a cooling rate (0.5 ° C./s) or higher at which fine precipitates can be obtained.

以下、実施例に従って、本発明の構成および作用効果をより具体的に説明する。なお、本発明は下記の実施例によって制限を受けるものではなく、本発明の趣旨に適合し得る範囲内にて適宜変更することも可能であり、これらは何れも本発明の技術的範囲に含まれる。
表1に示す成分組成の鋼(No.1〜43)を150kg真空溶解炉にて溶製し、連続鋳造して得た鋼材を1250℃に加熱したのち、仕上温度が1050℃の熱間圧延を行い、その後1℃/sで室温まで冷却し50mmφの丸棒鋼とした。得られた丸棒鋼をさらに、種々の温度で加熱後、1150℃にて仕上げ圧下率40%の熱間鍛造を行って30mmφの棒鋼に仕上げたのち、1.2℃/sまたは0.1℃/s(表2参照)で室温まで冷却した。
ここで、表1中に示す鋼No.1〜23は成分組成が本発明を満足する発明鋼であり、鋼No.24〜43は成分組成が本発明を満足しない比較鋼であり、表2中のNo.44〜46は、鍛造後の冷却速度、熱間鍛造前の加熱温度および熱間鍛造の仕上げ温度のいずれかが本発明の規定値から外れた比較例である。
Hereinafter, the constitution and the action and effect of the present invention will be described more specifically according to Examples. The present invention is not limited by the following examples, and can be appropriately modified within a range that can be adapted to the gist of the present invention, all of which are included in the technical scope of the present invention. Is done.
Steels (No. 1 to 43) having the composition shown in Table 1 are melted in a 150 kg vacuum melting furnace, and the steel material obtained by continuous casting is heated to 1250 ° C. and then hot-rolled at a finishing temperature of 1050 ° C. After that, it was cooled to room temperature at 1 ° C./s to obtain a round steel bar having a diameter of 50 mm. The obtained round bar steel is further heated at various temperatures and then hot forged at 1150 ° C. with a finishing reduction rate of 40% to finish it into a steel bar of 30 mmφ, and then 1.2 ° C / s or 0.1 ° C / s (Table). 2) was cooled to room temperature.
Here, steels Nos. 1 to 23 shown in Table 1 are invention steels whose component compositions satisfy the present invention, and steels Nos. 24 to 43 are comparative steels whose component compositions do not satisfy the present invention, and Table 2 Nos. 44 to 46 in the No. 44 to 46 are comparative examples in which any of the cooling rate after forging, the heating temperature before hot forging, and the finishing temperature of hot forging deviates from the specified values of the present invention.

Figure 0006791179
Figure 0006791179

得られた棒鋼に対して、組織観察、引張試験、シャルピー衝撃試験および小野式回転曲げ疲労試験を実施した。以下にそれぞれの調査内容について詳細に説明する。 The obtained steel bars were subjected to microstructure observation, tensile test, Charpy impact test and Ono-type rotary bending fatigue test. The contents of each survey will be described in detail below.

組織観察
組織観察は、熱間鍛造により得られた直径30mmφの棒鋼から試験片を採取し、鍛造方向に平行な垂直断面(L断面)について、研磨後ナイタールで腐食し、光学顕微鏡およびSEMを用い、断面組織観察により相の種類を同定し、各相の面積率を求めた。
Structure observation For structure observation, a test piece is taken from a steel bar with a diameter of 30 mmφ obtained by hot forging, and the vertical cross section (L cross section) parallel to the forging direction is corroded with nital after polishing, and an optical microscope and SEM are used. The type of phase was identified by observing the cross-sectional structure, and the area ratio of each phase was determined.

引張試験
引張試験はJIS4号引張試験片を30mmφの棒鋼中心部から採取し、引張強度を調査した。
Tensile test In the tensile test, JIS No. 4 tensile test pieces were sampled from the center of a steel bar of 30 mmφ and the tensile strength was investigated.

シャルピー衝撃試験
シャルピー衝撃試験はJIS3号試験片を30mmφの棒鋼中心部から採取し、室温での衝撃値を評価した。
Charpy impact test In the Charpy impact test, a JIS No. 3 test piece was sampled from the center of a 30 mmφ steel bar and the impact value at room temperature was evaluated.

回転曲げ疲労特性
直径30mmφの棒鋼から、図1に示す平行部直径6mmの試験片を採取した。得られた試験片に対して、小野式回転曲げ疲労試験機を用い、回転数:3000rpmで実施し、107回を疲労限度として、回転曲げ疲労強度を測定した。
Rotational bending fatigue characteristics A test piece with a parallel portion diameter of 6 mm shown in FIG. 1 was collected from a steel bar having a diameter of 30 mmφ. The obtained test piece, using a Ono-type rotary bending fatigue testing machine, the rotational speed: conducted at 3000 rpm, 10 7 times as fatigue limit was measured rotating bending fatigue strength.

表2および図2に上記調査の結果を示す。本発明例(No.1〜23)は、比較例(No.24〜46)に対し、優れた強度−靭性バランスを有することが認められた。 Table 2 and FIG. 2 show the results of the above survey. It was found that the examples of the present invention (No. 1 to 23) had an excellent strength-toughness balance as compared with the comparative examples (No. 24 to 46).

Figure 0006791179
Figure 0006791179

Claims (6)

質量%で、
C:0.10〜0.25%、
Si:0.10〜1.00%、
Mn:0.30〜3.00%、
P:0.050%以下、
S:0.005〜0.150%、
Cr:0.10〜2.50%、
Al:0.010〜0.080%、
Ti:0.25〜0.60%および
N:0.0070%以下
を、次式(1)で定義されるT1が3.0以下、かつ次式(2)で定義されるT2が1620〜1750となる範囲で含有し、残部はFeおよび不可避不純物の成分組成を有し、ベイナイト相の面積率が70%以上の組織を有する非調質鋼。
T1=(C /12)/(Ti/48) ・・・・(1)
T2=−7430/[Log(C×Ti)−3.23] ・・・・(2)
ここで、各元素記号は該元素の含有量(質量%)を示す。
By mass%
C: 0.10 to 0.25%,
Si: 0.10 to 1.00%,
Mn: 0.30 to 3.00%,
P: 0.050% or less,
S: 0.005 to 0.150%,
Cr: 0.10 to 2.50%,
Al: 0.010 to 0.080%,
Ti: 0.25 to 0.60% and N: 0.0070% or less are contained in the range where T1 defined by the following formula (1) is 3.0 or less and T2 defined by the following formula (2) is 1620 to 1750. The balance is a non-tamed steel having a composition of Fe and unavoidable impurities, and a structure in which the area ratio of the bainite phase is 70% or more.
T1 = (C / 12) / (Ti / 48) ... (1)
T2 = -7430 / [Log (C x Ti) -3.23] ... (2)
Here, each element symbol indicates the content (mass%) of the element.
前記成分組成は、さらに、質量%で
Mo:1.0%以下、
Nb:0.3%以下、
V:0.3%以下、
W:0.3%以下および
B:0.0100%以下
のうちから選んだ1種以上を含む請求項1に記載の非調質鋼。
The component composition is further increased by mass%.
Mo: 1.0% or less,
Nb: 0.3% or less,
V: 0.3% or less,
The non-microalloyed steel according to claim 1, which comprises at least one selected from W: 0.3% or less and B: 0.0100% or less.
前記成分組成は、さらに、質量%で
Cu:1.0%以下および
Ni:1.0%以下
のうちから選んだ1種以上を含む請求項1または2に記載の非調質鋼。
The component composition is further increased by mass%.
Cu: 1.0% or less and
Ni: The non-tampered steel according to claim 1 or 2, which contains at least one selected from 1.0% or less.
質量%で、
C:0.10〜0.25%、
Si:0.10〜1.00%、
Mn:0.30〜3.00%、
P:0.050%以下、
S:0.005〜0.150%、
Cr:0.10〜2.50%、
Al:0.010〜0.080%、
Ti:0.25〜0.60%および
N:0.0070%以下
を、次式(1)で定義されるT1が3.0以下、かつ次式(2)で定義されるT2が1620〜1750となる範囲で含有し、残部はFeおよび不可避不純物の成分組成を有する鋼材に、加熱温度が1300℃以下および仕上温度が850℃以上1300℃以下の熱間加工を施し、次いで700〜550℃の温度域を0.5℃/s以上で冷却する、ベイナイト相の面積率が70%以上の組織を有する非調質鋼の製造方法。
T1=(C /12)/(Ti/48) ・・・・(1)
T2=−7430/[Log(C×Ti)−3.23] ・・・・(2)
ここで、各元素記号は該元素の含有量(質量%)を示す。
By mass%
C: 0.10 to 0.25%,
Si: 0.10 to 1.00%,
Mn: 0.30 to 3.00%,
P: 0.050% or less,
S: 0.005 to 0.150%,
Cr: 0.10 to 2.50%,
Al: 0.010 to 0.080%,
Ti: 0.25 to 0.60% and N: 0.0070% or less are contained in the range where T1 defined by the following formula (1) is 3.0 or less and T2 defined by the following formula (2) is 1620 to 1750. The rest is a steel material with a composition of Fe and unavoidable impurities, which is hot-worked with a heating temperature of 1300 ° C or less and a finishing temperature of 850 ° C or more and 1300 ° C or less, and then in the temperature range of 700 to 550 ° C at 0.5 ° C / s. A method for producing a non-tempered steel having a structure having a bainite phase area ratio of 70% or more, which is cooled as described above.
T1 = (C / 12) / (Ti / 48) ... (1)
T2 = -7430 / [Log (C x Ti) -3.23] ... (2)
Here, each element symbol indicates the content (mass%) of the element.
前記成分組成は、さらに、質量%で
Mo:1.0%以下、
Nb:0.3%以下、
V:0.3%以下、
W:0.3%以下および
B:0.0100%以下
のうちから選んだ1種以上を含む請求項4に記載の非調質鋼の製造方法。
The component composition is further increased by mass%.
Mo: 1.0% or less,
Nb: 0.3% or less,
V: 0.3% or less,
The method for producing non-microalloyed steel according to claim 4, which comprises at least one selected from W: 0.3% or less and B: 0.0100% or less.
前記成分組成は、さらに、質量%で
Cu:1.0%以下および
Ni:1.0%以下
のうちから選んだ1種以上を含む請求項4または5に記載の非調質鋼の製造方法。
The component composition is further increased by mass%.
Cu: 1.0% or less and
Ni: The method for producing non-microalloyed steel according to claim 4 or 5, which comprises at least one selected from 1.0% or less.
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