JP2000129393A - Martensitic/bainitic non-heat treated steel excellent in machinability, and its manufacture - Google Patents
Martensitic/bainitic non-heat treated steel excellent in machinability, and its manufactureInfo
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- JP2000129393A JP2000129393A JP10306723A JP30672398A JP2000129393A JP 2000129393 A JP2000129393 A JP 2000129393A JP 10306723 A JP10306723 A JP 10306723A JP 30672398 A JP30672398 A JP 30672398A JP 2000129393 A JP2000129393 A JP 2000129393A
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Abstract
Description
【０００１】[0001]
【発明の属する技術分野】本発明は、主たる組織がマル
テンサイトとベイナイトの混合組織からなる被削性に優
れたマルテンサイト・ベイナイト型非調質鋼材およびそ
の製造法に関する。更に詳しくは、熱間での加工後に焼
入れ焼戻しの所謂「調質処理」を施さなくとも、機械構
造部品の素材として好適な、高い強度、大きな降伏比及
び優れた靭性を有するマルテンサイト・ベイナイト型の
被削性に優れた非調質鋼材とその製造方法に関する。BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a martensite-bainite type non-heat-treated steel material excellent in machinability, the main structure of which is a mixed structure of martensite and bainite, and a method for producing the same. More specifically, a martensitic bainite type having high strength, a large yield ratio and excellent toughness suitable as a material for mechanical structural parts without performing so-called "tempering treatment" of quenching and tempering after hot working. And a method for producing the same.
【０００２】[0002]
【従来の技術】機械構造部品、なかでも自動車、産業機
械、土木建設機械などのエンジン部品としてのクランク
シャフトやコンロッド、あるいはフォ−フリクトの爪な
どは、従来、機械構造用の炭素鋼（Ｓ４５Ｃ、Ｓ５０Ｃ
など）や合金鋼（ＳＣＭ４４０など）を用いて、熱間加
工で所定の形状に粗加工し、次いで、切削加工によって
所望の形状に仕上げた後、焼入れ焼戻しの調質処理を施
して所望の形状と性能を確保していた。2. Description of the Related Art Conventionally, crankshafts and connecting rods as engine parts such as automobile parts, industrial machines, and civil engineering construction machines, and forefoot claws, etc., have been conventionally used as carbon steel (S45C, S50C
) Or alloy steel (such as SCM440), hot-worked to roughing into a predetermined shape, and then finished to a desired shape by cutting, followed by tempering treatment such as quenching and tempering to obtain the desired shape. And ensured performance.
【０００３】しかし、前記の調質処理を行うには多大の
熱エネルギーを要するので製造コストが嵩む。そのた
め、省エネルギー及びコスト低減の観点から熱間加工の
ままで、熱間加工後に調質処理を施した場合と同等程度
の特性を確保できる非調質鋼の開発が行われ、この非調
質鋼を母材として各種の機械構造部品が製造されてき
た。[0003] However, the above-mentioned refining process requires a large amount of heat energy, which increases the production cost. Therefore, from the viewpoint of energy saving and cost reduction, development of a non-heat treated steel capable of securing the same properties as in the case of performing a tempering treatment after hot working while maintaining hot working has been performed. Various types of mechanical structural parts have been manufactured using as a base material.
【０００４】特開平４−１７６８４２号公報には、ベイ
ナイトあるいはベイナイト・フェライトの組織を有する
ベイナイト型の「熱間鍛造用非調質鋼」が開示されてい
る。しかし、この公報で提案された非調質鋼を母材とす
る熱間鍛造部品の場合、その実施例の記載からも明らか
なように、高々８７ｋｇｆ／ｍｍ2 （８５３ＭＰａ）の
引張強度と高々６５ｋｇｆ／ｍｍ2 （６３７ＭＰａ）の
降伏強度しか得られない。したがって、更に高い強度が
要求される部品に対しては適用し難いものである。Japanese Patent Application Laid-Open No. 4-176842 discloses a bainite-type "non-heat treated steel for hot forging" having a bainite or bainite / ferrite structure. However, in the case of a hot forged part using a non-heat treated steel as a base material proposed in this publication, as apparent from the description of the examples, a tensile strength of at most 87 kgf / mm 2 (853 MPa) and a tensile strength of at most 65 kgf. / Mm 2 (637 MPa). Therefore, it is difficult to apply to parts requiring higher strength.
【０００５】特開平４−２１０４４９号公報には、組織
が主としてフェライト及びベイナイトで一部パーライト
が共存する「高靭性熱間鍛造用非調質鋼」が開示されて
いる。しかし、この公報で提案された非調質鋼を母材と
する熱間鍛造部品の場合も、その実施例の図１から明ら
かなように、高々７５ｋｇｆ／ｍｍ2 （７３５ＭＰａ）
の引張強度と高々５５ｋｇｆ／ｍｍ2 （５３９ＭＰａ）
の降伏強度しか得られない。したがって、更に高い強度
が要求される部品に対しては適用し難いものである。[0005] Japanese Patent Application Laid-Open No. Hei 4-210449 discloses a "non-heat-treated steel for high toughness hot forging" in which the structure is mainly ferrite and bainite and some pearlite coexists. However, in the case of a hot forged part using a non-heat treated steel as a base material proposed in this publication, as is apparent from FIG. 1 of the embodiment, at most 75 kgf / mm 2 (735 MPa).
Tensile strength and at most 55 kgf / mm 2 (539 MPa)
Only the yield strength can be obtained. Therefore, it is difficult to apply to parts requiring higher strength.
【０００６】このため、調質処理を行わずとも各種の機
械構造部品に高い強度、例えば７００ＭＰａ以上の降伏
強度と１０００ＭＰａ以上の引張強度を確保でき、しか
も良好な靭性と大きな降伏比を確保させる技術の開発が
熱望されている。For this reason, it is possible to ensure high strength, for example, a yield strength of 700 MPa or more and a tensile strength of 1000 MPa or more, for various mechanical structural parts without performing a refining treatment, and to secure good toughness and a large yield ratio. The development of is eager.
【０００７】又、近年、機械構造部品の高強度化に伴っ
て、熱間加工後に所望の形状に成形するための切削加工
のコストが嵩むという問題が生じている。このため、切
削加工を容易にし、低コスト化を図るために被削性に優
れた非調質鋼に対する要求がますます大きくなってい
る。Further, in recent years, with the increase in strength of mechanical structural parts, there has been a problem that the cost of cutting for forming into a desired shape after hot working increases. For this reason, there is an increasing demand for non-heat-treated steel having excellent machinability in order to facilitate cutting and reduce costs.
【０００８】従来、被削性を高めるために、鋼にＰｂ、
Ｔｅ、Ｂｉ、Ｃａ及びＳなどの快削元素を単独あるいは
複合添加することが行われてきた。しかし、ＪＩＳ規格
鋼である機械構造用鋼や、前記した特開平４−１７６８
４２号公報や特開平４−２１０４４９号公報に記載され
ているような鋼に、単に上記の快削元素を添加しただけ
の場合には、所望の機械的性質、なかでも耐疲労特性を
確保できないことが多い。Conventionally, in order to enhance machinability, Pb,
Free-cutting elements such as Te, Bi, Ca and S have been used alone or in combination. However, JIS standard steel for machine structural use and the above-mentioned Japanese Patent Application Laid-Open No. H4-1768.
In the case where the above-mentioned free-cutting elements are simply added to steel as described in Japanese Patent Publication No. 42 or JP-A-4-210449, desired mechanical properties, especially fatigue resistance properties, cannot be secured. Often.
【０００９】鉄と鋼（ｖｏｌ．５７（１９７１年）Ｓ４
８４）には、脱酸調整快削鋼にＴｉを添加すれば被削性
が高まる場合のあることが報告されている。しかし、Ｔ
ｉの多量の添加はＴｉＮが多量に生成することもあって
工具摩耗を増大させ、被削性の点からは好ましくないこ
とも述べられている。例えば、Ｃ：０．４５％、Ｓｉ：
０．２９％、Ｍｎ：０．７８％、Ｐ：０．０１７％、
Ｓ：０．０４１％、Ａｌ：０．００６％、Ｎ：０．００
８７％、Ｔｉ：０．２２８％、Ｏ：０．００４％及びＣ
ａ：０．００１％を含有する鋼では却ってドリル寿命が
低下して被削性が劣っている。このように、鋼に単にＴ
ｉを添加するだけでは被削性は向上するものではない。Iron and steel (vol. 57 (1971) S4)
84) reports that the addition of Ti to deoxidized adjusted free-cutting steel may enhance machinability. But T
It is also described that the addition of a large amount of i increases tool wear due to generation of a large amount of TiN, and is not preferable from the viewpoint of machinability. For example, C: 0.45%, Si:
0.29%, Mn: 0.78%, P: 0.017%,
S: 0.041%, Al: 0.006%, N: 0.00
87%, Ti: 0.228%, O: 0.004% and C
a: In the steel containing 0.001%, the drill life is rather shortened and the machinability is inferior. Thus, simply adding T to the steel
Just adding i does not improve machinability.
【００１０】又、硫黄快削鋼の硫化物形態制御の目的で
Ｚｒが添加されることがあるが、例えば、鉄と鋼（ｖｏ
ｌ．６２（１９７６年）ｐ．８８５）に記されているよ
うに、Ｚｒは被削性に対してはほとんど影響を及ぼさな
い。つまり、鋼に単にＺｒを添加するだけでは被削性は
向上するものではない。In some cases, Zr is added for the purpose of controlling the sulfide form of the sulfur free-cutting steel.
l. 62 (1976) p. 885), Zr has little effect on machinability. That is, the machinability is not improved simply by adding Zr to steel.
【００１１】[0011]
【発明が解決しようとする課題】本発明の目的は、各種
機械構造部品の素材として好適な、降伏強度が７００Ｍ
Ｐａ以上、引張強度が１０００ＭＰａ以上で、０．６５
以上の降伏比（降伏強度／引張強度）と６０Ｊ／ｃｍ2
以上の２ｍｍＵノッチシャルピー衝撃値を確保すること
ができ、しかも被削性に優れた非調質鋼材を提供するこ
とにある。SUMMARY OF THE INVENTION An object of the present invention is to provide a material having a yield strength of 700M which is suitable as a material for various mechanical structural parts.
Pa or more, tensile strength is 1000MPa or more, 0.65
The above yield ratio (yield strength / tensile strength) and 60 J / cm 2
An object of the present invention is to provide a non-heat treated steel material which can secure the above-mentioned 2 mm U notch Charpy impact value and is excellent in machinability.
【００１２】[0012]
【課題を解決するための手段】本発明の要旨は、下記
（１）に示すマルテンサイト・ベイナイト型非調質鋼材
及び（２）に示すその製造方法にある。The gist of the present invention lies in a martensite-bainite type non-heat-treated steel material shown in (1) below and a method for producing the same in (2).
【００１３】（１）重量％で、Ｃ：０．１５〜０．３５
％、Ｓｉ：０．１〜１．０％、Ｍｎ：１．５〜３．０
％、Ｐ：０．１０％以下、Ｓ：０．００２〜０．１０
％、Ｃｕ：０．０１〜０．５％、Ｎｉ：０．２％以下、
Ｃｒ：０．５〜１．５％、Ｍｏ：０．２％以下、Ｖ：
０．５０％以下、Ｎｂ：０．０５％以下、Ｔｉ：１．０
％以下、Ｚｒ：１．０％以下で、且つ、Ｔｉ（％）＋Ｚ
ｒ（％）：０．０４〜１．０％、Ｂ：０．０００５〜
０．０１０％、Ｎ：０．００８％以下、Ａｌ：０．１０
％以下、Ｐｂ：０．３０％以下、Ｔｅ：０．１０％以
下、Ｃａ：０．０１０％以下を含み、下記式で表され
るｆｎ１の値が０％以上、下記式で表されるｆｎ２の
値が０％を超え、残部はＦｅ及び不可避不純物の化学組
成で、鋼中のＴｉ炭硫化物及びＺｒ炭硫化物の最大直径
が１０μｍ以下で、且つ、その量の和が清浄度で０．０
５％以上で、且つ、全組織中に占めるマルテンサイトの
割合が面積率で２０〜９５％で、マルテンサイト以外の
組織中に占めるベイナイトの割合が面積率で７０％以上
であることを特徴とする被削性に優れたマルテンサイト
・ベイナイト型非調質鋼材。(1) In weight%, C: 0.15 to 0.35
%, Si: 0.1 to 1.0%, Mn: 1.5 to 3.0
%, P: 0.10% or less, S: 0.002 to 0.10
%, Cu: 0.01 to 0.5%, Ni: 0.2% or less,
Cr: 0.5 to 1.5%, Mo: 0.2% or less, V:
0.50% or less, Nb: 0.05% or less, Ti: 1.0
% Or less, Zr: 1.0% or less, and Ti (%) + Z
r (%): 0.04 to 1.0%, B: 0.0005 to
0.010%, N: 0.008% or less, Al: 0.10
%, Pb: 0.30% or less, Te: 0.10% or less, Ca: 0.010% or less, and the value of fn1 represented by the following formula is 0% or more, and fn2 represented by the following formula: Exceeds 0%, the balance is the chemical composition of Fe and unavoidable impurities, the maximum diameter of Ti carbosulfide and Zr carbosulfide in steel is 10 μm or less, and the sum of the amounts is 0% in cleanliness. .0
5% or more, and the ratio of martensite in the entire structure is 20 to 95% in area ratio, and the ratio of bainite in the structure other than martensite is 70% or more in area ratio. Martensitic bainite type non-heat treated steel with excellent machinability.
【００１４】 ｆｎ１＝Ｔｉ（％）＋０．５Ｚｒ（％）−３．４Ｎ（％）・・・・ ｆｎ２＝Ｔｉ（％）＋Ｚｒ（％）−１．２Ｓ（％）・・・・」である。Fn1 = Ti (%) + 0.5Zr (%)-3.4N (%)... Fn2 = Ti (%) + Zr (%)-1.2S (%). .
【００１５】（２）重量％で、Ｃ：０．１５〜０．３５
％、Ｓｉ：０．１〜１．０％、Ｍｎ：１．５〜３．０
％、Ｐ：０．１０％以下、Ｓ：０．００２〜０．１０
％、Ｃｕ：０．０１〜０．５％、Ｎｉ：０．２％以下、
Ｃｒ：０．５〜１．５％、Ｍｏ：０．２％以下、Ｖ：
０．５０％以下、Ｎｂ：０．０５％以下、Ｔｉ：１．０
％以下、Ｚｒ：１．０％以下で、且つ、Ｔｉ（％）＋Ｚ
ｒ（％）：０．０４〜１．０％、Ｂ：０．０００５〜
０．０１０％、Ｎ：０．００８％以下、Ａｌ：０．１０
％以下、Ｐｂ：０．３０％以下、Ｔｅ：０．１０％以
下、Ｃａ：０．０１０％以下を含み、前記式で表され
るｆｎ１の値が０％以上、前記式で表されるｆｎ２が
０％を超え、残部はＦｅ及び不可避不純物の化学組成
で、鋼中のＴｉ炭硫化物及びＺｒ炭硫化物の最大直径が
１０μｍ以下で、且つ、その量の和が清浄度で０．０５
％以上である鋼を、１１００℃以上の温度に加熱して熱
間加工し、熱間加工を９００℃以上の温度で終了した後
３０〜３００℃／分の冷却速度で冷却する被削性に優れ
たマルテンサイト・ベイナイト型非調質鋼材の製造方
法。(2) In weight%, C: 0.15 to 0.35
%, Si: 0.1 to 1.0%, Mn: 1.5 to 3.0
%, P: 0.10% or less, S: 0.002 to 0.10
%, Cu: 0.01 to 0.5%, Ni: 0.2% or less,
Cr: 0.5 to 1.5%, Mo: 0.2% or less, V:
0.50% or less, Nb: 0.05% or less, Ti: 1.0
% Or less, Zr: 1.0% or less, and Ti (%) + Z
r (%): 0.04 to 1.0%, B: 0.0005 to
0.010%, N: 0.008% or less, Al: 0.10
%, Pb: 0.30% or less, Te: 0.10% or less, Ca: 0.010% or less, the value of fn1 represented by the above formula is 0% or more, and fn2 represented by the above formula Exceeds 0%, the balance is the chemical composition of Fe and inevitable impurities, the maximum diameter of Ti carbosulfide and Zr carbosulfide in steel is 10 μm or less, and the sum of the amounts is 0.05% in cleanliness.
% Of steel is heated to a temperature of 1100 ° C. or more, hot worked, and after hot working is completed at a temperature of 900 ° C. or more, it is cooled at a cooling rate of 30 to 300 ° C./min. Excellent martensite bainite type non-heat treated steel manufacturing method.
【００１６】なお、本発明でいう「Ｔｉ炭硫化物」には
単なるＴｉ硫化物を、又、「Ｚｒ炭硫化物」には単なる
Ｚｒ硫化物をそれぞれ含むものとする。又、「（Ｔｉ及
びＺｒの炭硫化物の）最大直径」とは「個々のＴｉ及び
Ｚｒの炭硫化物における最も長い径」のことを指す。Ｔ
ｉ炭硫化物の清浄度やＺｒ炭硫化物の清浄度は、光学顕
微鏡の倍率を４００倍として、JIS G 0555に規定された
「鋼の非金属介在物の顕微鏡試験方法」によって６０視
野測定した値をいう。In the present invention, "Ti carbosulfide" includes a simple Ti sulfide, and "Zr carbosulfide" includes a simple Zr sulfide. Also, the "maximum diameter (of Ti and Zr carbosulfides)" refers to "the longest diameter of individual Ti and Zr carbosulfides." T
i The cleanliness of carbosulfides and Zr carbosulfides were measured in 60 fields of view using the “microscope test method for nonmetallic inclusions in steel” specified in JIS G 0555, with the magnification of an optical microscope set to 400 times. Value.
【００１７】「組織」は鋼材の中心部における組織を指
す。"Structure" refers to the structure at the center of the steel material.
【００１８】鋼の加熱温度は鋼表面における温度をい
い、熱間加工の終了温度も熱間加工した鋼材の表面にお
ける温度をいう。冷却速度とは、鋼材の表面における９
００〜４００℃の平均冷却速度を指す。The heating temperature of steel refers to the temperature at the steel surface, and the end temperature of hot working also refers to the temperature at the surface of the hot-worked steel material. The cooling rate is defined as 9 at the surface of the steel material.
Refers to the average cooling rate of 00-400 ° C.
【００１９】マルテンサイト以外の組織部分において、
ベイナイト以外の組織はフェライト、パーライトやオー
ステナイトが変態せずに残った所謂「残留オーステナイ
ト」などである。In the organizational parts other than martensite,
Structures other than bainite include so-called “retained austenite” in which ferrite, pearlite, and austenite remain without being transformed.
【００２０】本発明者らは、調質処理を行うことなく各
種の機械構造部品に、高い強度、良好な靭性と大きな降
伏比を確保させるために、熱間加工した後のミクロ組織
及びその母材鋼となる鋼の化学組成について種々検討し
た。その結果、下記の知見を得た。The present inventors have developed a microstructure after hot working and its mother material in order to ensure high strength, good toughness and a large yield ratio in various mechanical structural parts without performing a tempering treatment. Various studies were made on the chemical composition of the steel used as the material steel. As a result, the following findings were obtained.
【００２１】（ａ）面積率で一定以上のマルテンサイト
を含む組織であれば、熱間鍛造を初めとする熱間加工の
ままでも高い強度が得られる。(A) As long as the structure contains martensite of a certain area ratio or more, high strength can be obtained even with hot working such as hot forging.
【００２２】（ｂ）上記（ａ）のマルテンサイトを含む
組織において、マルテンサイト以外の組織中にベイナイ
トの占める割合が面積率で７０％以上であれば、高い強
度と良好な靭性とを兼備できる。(B) In the structure containing martensite described in (a) above, if the proportion of bainite in the structure other than martensite is 70% or more in terms of area ratio, both high strength and good toughness can be obtained. .
【００２３】（ｃ）適正量のＭｎとＣｕとを複合添加し
た鋼を非調質の機械構造部品の母材鋼に用いれば、高い
降伏強度が得られ、降伏比も大きくなる。更に、靭性も
良好になる。(C) If a steel to which a proper amount of Mn and Cu are added in combination is used as a base steel for non-heat-treated mechanical structural parts, a high yield strength is obtained and a yield ratio is increased. Further, the toughness is improved.
【００２４】（ｄ）鋼に適正量のＴｉやＺｒを添加し、
鋼中の介在物制御として硫化物をＴｉ炭硫化物やＺｒ炭
硫化物に変え、更にＴｉ炭硫化物やＺｒ炭硫化物を鋼材
に微細に分散させれば、鋼材の被削性が飛躍的に向上す
る。(D) adding an appropriate amount of Ti or Zr to the steel;
If the sulfide is changed to Ti carbosulfide or Zr carbosulfide to control inclusions in the steel, and the Ti carbosulfide or Zr carbosulfide is finely dispersed in the steel, the machinability of the steel is dramatically improved. To improve.
【００２５】そこで、更に検討を続けた結果、下記の事
項を見いだした。Then, as a result of further study, the following matters were found.
【００２６】（ｅ）Ｓとのバランスを考慮して鋼にＴｉ
とＺｒのいずれかを積極的に添加すると、鋼中にＴｉ炭
硫化物あるいはＺｒ炭硫化物が形成され、Ｔｉ及びＺｒ
を添加すると、鋼中にはＴｉ炭硫化物とＺｒ炭硫化物と
が形成される。(E) Considering the balance with S, Ti
If either of Zr and Zr is positively added, Ti carbosulfide or Zr carbosulfide is formed in the steel, and Ti and Zr
When Ti is added, Ti carbosulfide and Zr carbosulfide are formed in the steel.
【００２７】（ｆ）鋼中に上記したＴｉ炭硫化物やＺｒ
炭硫化物が生成すると、ＭｎＳの生成量が減少する。(F) The above-mentioned Ti carbosulfide or Zr in steel
When the carbosulfide is formed, the amount of MnS generated decreases.
【００２８】（ｇ）鋼中のＳ含有量が同じ場合には、Ｔ
ｉ炭硫化物やＺｒ炭硫化物はＭｎＳよりも大きな被削性
改善効果を有する。これは、Ｔｉ炭硫化物やＺｒ炭硫化
物の融点がＭｎＳのそれよりも低いため、切削加工時に
工具のすくい面での潤滑作用が大きくなることに基づ
く。(G) When the S content in steel is the same, T
i carbosulfide and Zr carbosulfide have a greater machinability improvement effect than MnS. This is based on the fact that the melting point of Ti carbosulfide or Zr carbosulfide is lower than that of MnS, so that the lubricating action on the rake face of the tool during cutting is increased.
【００２９】（ｈ）Ｔｉ炭硫化物やＺｒ炭硫化物の効果
を充分発揮させるためには、Ｎ含有量を低く制限するこ
とが重要である。これは、Ｎ含有量が多いとＴｉＮやＺ
ｒＮとしてＴｉやＺｒが固定されてしまい、Ｔｉ炭硫化
物やＺｒ炭硫化物の生成が抑制されてしまうためであ
る。(H) In order to sufficiently exert the effects of Ti carbosulfide and Zr carbosulfide, it is important to limit the N content to a low level. This is because when the N content is large, TiN or Z
This is because Ti or Zr is fixed as rN, and the generation of Ti carbosulfide or Zr carbosulfide is suppressed.
【００３０】（ｉ）製鋼時に生成したＴｉ炭硫化物やＺ
ｒ炭硫化物は、通常の熱間加工のための加熱温度では基
地に固溶しないし、凝集もしない。したがって、オース
テナイト領域において所謂「ピン止め作用」が発揮され
るので、オーステナイト粒の粗大化防止に有効である。(I) Ti carbosulfide or Z produced during steelmaking
The r carbosulfide does not form a solid solution in the matrix at the normal heating temperature for hot working and does not agglomerate. Therefore, a so-called "pinning action" is exhibited in the austenite region, which is effective in preventing austenite grains from becoming coarse.
【００３１】（ｊ）Ｔｉ炭硫化物やＺｒ炭硫化物によっ
て被削性を高めるとともに大きな強度、特に、大きな疲
労強度を確保するためには、Ｔｉ炭硫化物やＺｒ炭硫化
物のサイズと、その清浄度で表される量（以下、単に
「清浄度」という）を適正化しておくことが重要であ
る。(J) In order to enhance machinability by Ti carbosulfide or Zr carbosulfide and to secure large strength, especially large fatigue strength, the size of Ti carbosulfide or Zr carbosulfide and It is important to optimize the amount represented by the cleanliness (hereinafter simply referred to as “cleanliness”).
【００３２】本発明は上記の知見に基づいて完成された
ものである。The present invention has been completed based on the above findings.
【００３３】[0033]
【発明の実施の形態】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「重
量％」を意味する。DESCRIPTION OF THE PREFERRED EMBODIMENTS Each requirement of the present invention will be described in detail below. In addition, “%” of the content of the chemical component means “% by weight”.
【００３４】（Ａ）鋼の化学組成 Ｃ：Ｃは、ＳとともにＴｉやＺｒと結合してＴｉ炭硫化
物やＺｒ炭硫化物を形成し、被削性を高める作用を有す
る。Ｃは、強度を確保するのにも有効な元素である。し
かし、その含有量が０．１５％未満では所望の１０００
ＭＰａ以上の引張強度が得られない。一方、０．３５％
を超えて含有すると、靭性が低下して６０Ｊ／ｃｍ2 以
上の２ｍｍＵノッチシャルピー衝撃値が得られ難くなる
し、被削性が劣化するようになって切削コストが嵩んで
しまう。したがって、Ｃの含有量を０．１５〜０．３５
％とした。(A) Chemical composition of steel C: C combines with S and Ti or Zr to form Ti carbosulfide or Zr carbosulfide, and has an effect of enhancing machinability. C is an element that is also effective for securing strength. However, if the content is less than 0.15%, the desired 1000
A tensile strength higher than MPa cannot be obtained. On the other hand, 0.35%
If the content exceeds 2 , the toughness is reduced and it is difficult to obtain a 2 mm U notch Charpy impact value of 60 J / cm 2 or more, and the machinability is deteriorated and the cutting cost increases. Therefore, the content of C is set to 0.15 to 0.35.
%.
【００３５】Ｓｉ：Ｓｉは、脱酸を促進するとともに、
静的強度と疲労強度を高める作用がある。前記の効果を
充分発揮させるためには、Ｓｉの含有量を０．１％以上
とすることが必要である。一方、Ｓｉを１．０％を超え
て含有させても前記の効果は飽和し、コストが嵩むばか
りである。したがって、Ｓｉの含有量を０．１〜１．０
％とした。なお、Ｔｉ炭硫化物やＺｒ炭硫化物のサイズ
と清浄度を所定の値とするためには、ＴｉやＺｒの酸化
物が過剰に生成することを防ぐことが必要であるので、
Ｓｉの含有量は０．２０％以上とすることが好ましく、
特に、Ａｌを添加しない場合のＳｉ含有量は０．４０％
以上とすることが好ましい。Si: Si promotes deoxidation, and
It has the effect of increasing static strength and fatigue strength. In order to sufficiently exhibit the above effects, the content of Si needs to be 0.1% or more. On the other hand, even if Si is contained in excess of 1.0%, the above effects are saturated and the cost is increased. Therefore, the content of Si is set to 0.1 to 1.0.
%. In order to set the size and cleanliness of Ti carbosulfide or Zr carbosulfide to a predetermined value, it is necessary to prevent an excessive generation of Ti or Zr oxide.
The content of Si is preferably set to 0.20% or more,
Particularly, when Al is not added, the Si content is 0.40%.
It is preferable to make the above.
【００３６】Ｍｎ：Ｍｎは、脱酸作用や強度を高める作
用がある。更に、Ｃｕと複合添加すると降伏強度、降伏
比及び靭性を高める作用も有する。こうした効果を充分
発揮させるためには、１．５％以上の含有量を必要とす
る。しかし、Ｍｎを３．０％を超えて含有させるとその
効果は飽和してコストが嵩むだけでなく、むしろ焼入れ
性が高くなりすぎてマルテンサイトの単相組織となり易
く、降伏比が低下してしまう。更に、被削性が低下する
ので切削コストが嵩んでしまう。したがって、Ｍｎの含
有量を１．５〜３．０％とした。Mn: Mn has a deoxidizing effect and an effect of increasing strength. Further, when added in combination with Cu, it also has the effect of increasing the yield strength, yield ratio and toughness. In order to sufficiently exhibit such effects, a content of 1.5% or more is required. However, when Mn is contained in excess of 3.0%, the effect is not only saturated and the cost is increased, but also the hardenability becomes too high to easily form a martensite single phase structure, and the yield ratio decreases. I will. Furthermore, since the machinability is reduced, the cutting cost is increased. Therefore, the content of Mn is set to 1.5 to 3.0%.
【００３７】Ｐ：Ｐは、鋼中に不純物として含有される
ものであり、必須成分として添加しなくても良い。添加
すれば降伏強度を高める作用がある。この効果を確実に
得るには、Ｐは０．００５％以上の含有量とすることが
好ましい。しかし、その含有量が０．１０％を超えると
靭性の著しい低下を招く。したがって、Ｐの含有量を
０．１０％以下とした。P: P is contained as an impurity in steel, and need not be added as an essential component. If added, it has the effect of increasing the yield strength. In order to ensure this effect, it is preferable that the content of P be 0.005% or more. However, when the content exceeds 0.10%, the toughness is significantly reduced. Therefore, the content of P is set to 0.10% or less.
【００３８】Ｓ：Ｓは、ＣとともにＴｉやＺｒと結合し
てＴｉ炭硫化物やＺｒ炭硫化物を形成し、被削性を高め
る作用を有する。しかし、その含有量が０．００２％未
満では所望の効果が得られない。S: S combines with C with Ti and Zr to form Ti carbosulfide and Zr carbosulfide, and has an effect of improving machinability. However, if the content is less than 0.002%, the desired effect cannot be obtained.
【００３９】従来、快削鋼にＳを添加する目的は、Ｍｎ
Ｓを形成させて被削性を改善させることにあった。しか
し、本発明者らの検討によると、上記のＭｎＳの被削性
向上作用は、切削時の切り屑と工具表面との潤滑性を高
める機能に基づくことが判明した。しかもＭｎＳは巨大
化し、鋼材本体の地疵を大きくし、欠陥となる場合があ
る。本発明におけるＳの被削性改善作用は、適正量のＣ
とＴｉ、Ｚｒとの複合添加によってＴｉやＺｒの炭硫化
物を形成させることで初めて得られる。このためには、
上記したように０．００２％以上のＳの含有量が必要で
ある。一方、Ｓを０．１０％を超えて含有させても被削
性に与える効果に変化はないが、鋼中に粗大なＭｎＳが
再び生じるようになって、地疵等の問題が生じる場合が
ある。更に、熱間での加工性が劣化して熱間加工が困難
になることもある。したがって、Ｓの含有量を０．００
２〜０．１０％とした。Conventionally, the purpose of adding S to free-cutting steel is to add Mn
The purpose is to improve the machinability by forming S. However, according to the study of the present inventors, it has been found that the above-described action of improving the machinability of MnS is based on a function of enhancing lubricity between chips and the tool surface during cutting. In addition, MnS increases in size, increases the ground flaw of the steel material main body, and sometimes becomes a defect. In the present invention, the machinability improving action of S is based on an appropriate amount of C
Can be obtained for the first time by forming a carbosulfide of Ti or Zr by complex addition of Ti and Zr. To do this,
As described above, an S content of 0.002% or more is required. On the other hand, although the effect on machinability is not changed even when S is contained in excess of 0.10%, coarse MnS is again generated in the steel, and a problem such as ground flaw may occur. is there. Furthermore, hot workability may deteriorate and hot work may become difficult. Therefore, the content of S is 0.00
2 to 0.10%.
【００４０】Ｃｕ：Ｃｕは、Ｍｎと複合添加すると降伏
強度、降伏比及び靭性を高める作用を有する。しかしな
がら、その含有量が０．０１％未満では添加効果に乏し
い。一方、Ｍｎと複合添加した場合にはＣｕを０．５％
を超えて含有させても前記の効果は飽和して経済性が損
なわれるだけでなく、靭性の著しい低下をもたらす。し
たがって、Ｃｕの含有量を０．０１〜０．５％とした。Cu: Cu, when added in combination with Mn, has the effect of increasing yield strength, yield ratio and toughness. However, if the content is less than 0.01%, the effect of addition is poor. On the other hand, when combined with Mn, Cu is added by 0.5%.
In addition, the above effect is saturated and the economic efficiency is impaired, and also the toughness is remarkably reduced. Therefore, the content of Cu is set to 0.01 to 0.5%.
【００４１】Ｎｉ：Ｎｉは添加しなくても良い。添加す
れば靭性を高める作用がある。この効果を確実に得るに
は、Ｎｉは０．０５％以上の含有量とすることが好まし
い。しかし、Ｎｉを０．２％を超えて含有させても前記
の効果は飽和して経済性を損なうし、被削性が低下す
る。したがって、Ｎｉの含有量を０．２％以下とした。Ni: Ni may not be added. Addition has the effect of increasing toughness. To ensure this effect, the content of Ni is preferably set to 0.05% or more. However, even if Ni is contained in an amount exceeding 0.2%, the above-mentioned effect is saturated, impairing economic efficiency and reducing machinability. Therefore, the content of Ni is set to 0.2% or less.
【００４２】Ｃｒ：Ｃｒは、強度を高める作用を有す
る。この効果を確実に得るには、Ｃｒは０．５％以上の
含有量とする必要がある。しかし、１．５％を超えて含
有させても前記の効果は飽和しコストが嵩むばかりであ
る。したがって、Ｃｒの含有量を０．５〜１．５％とし
た。Cr: Cr has the effect of increasing the strength. To ensure this effect, the content of Cr must be 0.5% or more. However, even if the content exceeds 1.5%, the above effect is saturated and the cost is increased. Therefore, the content of Cr is set to 0.5 to 1.5%.
【００４３】Ｍｏ：Ｍｏは添加しなくても良い。添加す
ればＮｉと同様に靭性を向上させる作用がある。この効
果を確実に得るには、Ｍｏは０．０５％以上の含有量と
することが好ましい。しかし、０．２％を超えて含有さ
せても前記の効果は飽和し、コストが嵩むばかりであ
る。したがって、Ｍｏの含有量を０．２％以下とした。Mo: Mo may not be added. If added, it has the effect of improving the toughness, similarly to Ni. To ensure this effect, it is preferable that the content of Mo be 0.05% or more. However, even if the content exceeds 0.2%, the above effect is saturated and the cost is increased. Therefore, the content of Mo is set to 0.2% or less.
【００４４】Ｖ：Ｖは添加しなくてもよい。添加すれば
強度を高める作用がある。この効果を確実に得るには、
Ｖは０．０５％以上の含有量とすることが好ましい。し
かし、０．５０％を超えて含有させても前記の効果は飽
和し、経済性を損なうばかりである。したがって、Ｖの
含有量を０．５０％以下とした。V: V may not be added. Addition has the effect of increasing the strength. To ensure this effect,
V is preferably set to a content of 0.05% or more. However, even if the content exceeds 0.50%, the above effect is saturated, and the economic efficiency is only lost. Therefore, the content of V is set to 0.50% or less.
【００４５】Ｎｂ：Ｎｂは添加しなくてもよい。添加す
れば強度を高める作用がある。この効果を確実に得るに
は、Ｎｂは０．０１％以上の含有量とすることが好まし
い。しかし、０．０５％を超えて含有させても前記の効
果は飽和し、コストが嵩むばかりである。したがって、
Ｎｂの含有量を０．０５％以下とした。Nb: Nb may not be added. Addition has the effect of increasing the strength. In order to surely obtain this effect, the content of Nb is preferably set to 0.01% or more. However, if the content exceeds 0.05%, the above effect is saturated and the cost is increased. Therefore,
The content of Nb was set to 0.05% or less.
【００４６】Ｔｉ、Ｚｒ：Ｔｉ、Ｚｒは本発明において
重要な元素であって、それぞれＣ及びＳと結合してＴｉ
炭硫化物やＺｒ炭硫化物を形成し、被削性を高める作用
を有する。上記の効果は、ＴｉとＺｒの含有量に関し、
Ｔｉ（％）＋Ｚｒ（％）の値が０．０４％以上の場合に
確実に得られる。しかし、Ｔｉ（％）＋Ｚｒ（％）の値
で１．０％を超えるＴｉとＺｒを含有させても被削性向
上効果は飽和するのでコストが嵩んでしまう。なお、Ｔ
ｉ（％）＋Ｚｒ（％）の値が０．０４〜１．０％であり
さえすれば良いので、必ずしもＴｉとＺｒを複合して含
有させる必要はない。Ｚｒを添加しない、つまり、Ｔｉ
を単独添加する場合に、Ｔｉを１．０％を超えて含有さ
せるとＴｉ炭硫化物による被削性向上効果が飽和してコ
ストが嵩むばかりか、Ｔｉ炭硫化物が粗大化して却って
靭性の低下を招いてしまう。逆に、Ｔｉを添加しない、
つまりＺｒを単独で添加する場合に、Ｚｒを１．０％を
超えて含有させるとＺｒ炭硫化物による被削性向上効果
が飽和してコストが嵩むばかりか、Ｚｒ炭硫化物が粗大
化して却って靭性の低下を招いてしまう。したがって、
ＴｉとＺｒの含有量をいずれも１．０％以下で、且つ、
Ｔｉ（％）＋Ｚｒ（％）の値を０．０４〜１．０％とし
た。なお、良好な被削性と靭性を安定して得るために
は、ＴｉとＺｒの含有量の上限はそれぞれ０．８％とす
ることが好ましい。Ti, Zr: Ti and Zr are important elements in the present invention.
It forms carbosulfides and Zr carbosulfides and has the effect of enhancing machinability. The above effects are related to the contents of Ti and Zr,
It is reliably obtained when the value of Ti (%) + Zr (%) is 0.04% or more. However, even if the content of Ti and Zr exceeds 1.0% in the value of Ti (%) + Zr (%), the machinability improving effect is saturated and the cost increases. Note that T
Since the value of i (%) + Zr (%) only needs to be 0.04 to 1.0%, it is not always necessary to include Ti and Zr in a composite manner. No Zr is added, that is, Ti
In the case where Ti is added alone, if the content of Ti exceeds 1.0%, the effect of improving the machinability by Ti carbosulfide is saturated and not only the cost increases, but also the Ti carbosulfide becomes coarse and the toughness increases. It causes a decline. Conversely, no Ti is added,
That is, when Zr is added alone, if Zr is contained in excess of 1.0%, the effect of improving the machinability by Zr carbosulfide is saturated and not only costs are increased, but also Zr carbosulfide becomes coarse. On the contrary, the toughness is reduced. Therefore,
The content of each of Ti and Zr is 1.0% or less, and
The value of Ti (%) + Zr (%) was set to 0.04 to 1.0%. In order to stably obtain good machinability and toughness, the upper limits of the contents of Ti and Zr are each preferably 0.8%.
【００４７】Ｂ：Ｂは、鋼の焼入れ性を高めるととも
に、靭性を向上させる作用がある。しかし、その含有量
が０．０００５％未満では添加効果に乏しい。一方、
０．０１０％を超えて含有させてもその効果は飽和する
ばかりか、熱間加工性の低下を招くようになる。したが
って、Ｂの含有量を０．０００５〜０．０１０％とし
た。B: B has the effect of improving the hardenability of steel and improving the toughness. However, if the content is less than 0.0005%, the effect of addition is poor. on the other hand,
If the content exceeds 0.010%, the effect is not only saturated, but also causes a reduction in hot workability. Therefore, the content of B is set to 0.0005 to 0.010%.
【００４８】Ｎ：本発明においてはＮの含有量を低く制
御することが極めて重要である。すなわち、ＮはＴｉや
Ｚｒとの親和力が大きいために容易にＴｉやＺｒと結合
してＴｉＮやＺｒＮを生成し、ＴｉやＺｒを固定してし
まうので、Ｎを多量に含有する場合には前記したＴｉ炭
硫化物やＺｒ炭硫化物の被削性向上効果が充分に発揮で
きないこととなる。特に、ＴｉやＺｒの含有量が低めの
場合には、Ｎ含有量の影響が顕著となる。更に、粗大な
ＴｉＮやＺｒＮは靭性を低下させてしまう。したがっ
て、Ｎ含有量を０．００８％以下とした。なお、Ｔｉ炭
硫化物やＺｒ炭硫化物の効果を高めるために、Ｎ含有量
の上限は０．００６％とすることが好ましい。N: In the present invention, it is extremely important to control the N content to a low level. That is, since N has a large affinity for Ti and Zr, it easily binds to Ti and Zr to form TiN and ZrN, and fixes Ti and Zr. Thus, the effect of improving the machinability of the obtained Ti and Zr carbosulfides cannot be sufficiently exhibited. In particular, when the content of Ti or Zr is relatively low, the influence of the N content becomes significant. Further, coarse TiN or ZrN lowers toughness. Therefore, the N content is set to 0.008% or less. In order to enhance the effects of Ti carbosulfide and Zr carbosulfide, the upper limit of the N content is preferably set to 0.006%.
【００４９】Ａｌ：Ａｌは添加しなくてもよい。添加す
れば鋼を脱酸する作用を有する。更に、結晶粒を微細化
し、強度及び靭性を高める作用を有する。こうした効果
を確実に得るには、Ａｌは０．０１％以上の含有量とす
ることが好ましい。しかし、０．１０％を超えて含有さ
せると被削性や熱間加工性の低下を招く。したがって、
Ａｌの含有量を０．１０％以下とした。ここで、Ａｌ含
有量とは所謂「ｓｏｌ．Ａｌ（酸可溶性Ａｌ）量」のこ
とをいう。なお、Ｔｉ炭硫化物やＺｒ炭硫化物のサイズ
と清浄度を所定の値とするためには、ＴｉやＺｒの酸化
物が過剰に生成することを防ぐことが必要であるので、
０．０１％以上のＡｌを含有させることとするのが良
い。Al: Al may not be added. Addition has the effect of deoxidizing steel. Further, it has the effect of refining crystal grains and increasing strength and toughness. In order to surely obtain such an effect, the content of Al is preferably set to 0.01% or more. However, when the content exceeds 0.10%, machinability and hot workability are reduced. Therefore,
The content of Al was set to 0.10% or less. Here, the Al content refers to a so-called “sol. Al (acid-soluble Al) amount”. In order to set the size and cleanliness of Ti carbosulfide or Zr carbosulfide to a predetermined value, it is necessary to prevent an excessive generation of Ti or Zr oxide.
It is preferable to contain 0.01% or more of Al.
【００５０】Ｐｂ：Ｐｂは添加しなくても良い。添加す
れば被削性を大きく高める作用を有する。この効果を確
実に得るには、Ｐｂは０．０５％以上の含有量とするこ
とが好ましい。しかし、その含有量が０．３０％を超え
ると疲労強度が著しく低下して耐疲労特性の劣化を招
く。したがって、Ｐｂの含有量を０．３０％以下とし
た。Pb: Pb may not be added. When added, it has the effect of greatly improving machinability. In order to surely obtain this effect, the content of Pb is preferably set to 0.05% or more. However, when the content exceeds 0.30%, the fatigue strength is remarkably reduced, and the fatigue resistance is deteriorated. Therefore, the content of Pb is set to 0.30% or less.
【００５１】Ｔｅ：Ｔｅは添加しなくてもよい。添加す
れば被削性を一層高める作用を有する。この効果を確実
に得るには、Ｔｅは０．０１％以上の含有量とすること
が好ましい。しかし、その含有量が０．１０％を超える
と熱間加工性が著しく低下し、生産性を損なう。したが
って、Ｔｅの含有量を０．１０％以下とした。Te: Te need not be added. When added, it has the effect of further enhancing machinability. To ensure this effect, the content of Te is preferably set to 0.01% or more. However, if the content exceeds 0.10%, hot workability is significantly reduced, and productivity is impaired. Therefore, the content of Te is set to 0.10% or less.
【００５２】Ｃａ：Ｃａも添加しなくてもよい。添加す
ればＰｂやＴｅと同様に被削性を一層高める作用を有す
る。この効果を確実に得るには、Ｃａは０．００１％以
上の含有量とすることが好ましい。しかし、その含有量
が０．０１０％を超えると熱間加工性が低下して生産性
を損なうようになる。したがって、Ｃａの含有量を０．
０１０％以下とした。Ca: Ca may not be added. When added, it has the effect of further improving machinability, similarly to Pb and Te. In order to surely obtain this effect, the content of Ca is preferably set to 0.001% or more. However, when the content exceeds 0.010%, the hot workability is reduced and productivity is impaired. Therefore, the content of Ca is set to 0.1.
010% or less.
【００５３】ｆｎ１：前述の式で表されるｆｎ１の値
が０％以上の場合に前記したＢの焼入れ性向上効果が確
保でき、所望の高強度と良好な靭性が得られる。したが
って、本発明では式で表されるｆｎ１の値を０％以上
と規定する。このｆｎ１の値の上限は特に規定されるも
のではなく、１．０であっても良い。Fn1: When the value of fn1 represented by the above formula is 0% or more, the effect of improving the hardenability of B can be secured, and the desired high strength and good toughness can be obtained. Therefore, in the present invention, the value of fn1 represented by the equation is defined as 0% or more. The upper limit of the value of fn1 is not particularly defined, and may be 1.0.
【００５４】ｆｎ２：Ｎの含有量が０．００８％以下
で、前述の式で表されるｆｎ２の値が０％を超える場
合に前記したＴｉ炭硫化物とＺｒ炭硫化物の被削性向上
効果が確保できる。ｆｎ２の値が０％以下の場合には、
Ｓ量が過剰となるため、その分ＭｎＳが過剰生成してＴ
ｉ炭硫化物とＺｒ炭硫化物による被削性向上効果が低下
してしまう。したがって、式で表されるｆｎ２に関し
て０％を超える値と規定した。このｆｎ２の値の上限は
特に規定されるものではなく、Ｔｉ（％）＋Ｚｒ（％）
の値が１．０％でＳが０．００２％の場合の値であって
も良い。When the content of fn2: N is 0.008% or less and the value of fn2 represented by the above equation exceeds 0%, the machinability of the Ti and Zr carbosulfides is improved. The effect can be secured. When the value of fn2 is 0% or less,
Since the amount of S is excessive, MnS is excessively generated and T
The effect of improving the machinability by i carbosulfide and Zr carbosulfide is reduced. Therefore, the value of fn2 represented by the formula was defined to be more than 0%. The upper limit of the value of fn2 is not particularly defined, and Ti (%) + Zr (%)
May be 1.0% and S may be 0.002%.
【００５５】（Ｂ）Ｔｉ炭硫化物、Ｚｒ炭硫化物のサイ
ズと量 上記の化学組成を有する非調質鋼材の被削性をＴｉ炭硫
化物やＺｒ炭硫化物によって高めるとともに、所望の強
度と靭性を確保するためには、Ｔｉ炭硫化物やＺｒ炭硫
化物のサイズと清浄度（ＴｉとＺｒを複合添加する場合
にはＴｉ炭硫化物とＺｒ炭硫化物の清浄度の和）で表さ
れる量を適正化しておくことが重要である。(B) Size and amount of Ti carbosulfide and Zr carbosulfide The machinability of the non-heat treated steel having the above chemical composition is enhanced by Ti carbosulfide and Zr carbosulfide, and the desired strength is obtained. To ensure toughness and toughness, the size and cleanliness of Ti and Zr carbosulfides (or the sum of cleanliness of Ti and Zr carbosulfides when Ti and Zr are added in combination) It is important to keep the amount represented.
【００５６】鋼中のＴｉ炭硫化物及びＺｒ炭硫化物の最
大直径が１０μｍを超えると疲労強度や靭性が低下して
しまう。なお、Ｔｉ炭硫化物及びＺｒ炭硫化物の最大直
径はいずれも７μｍ以下とすることが好ましい。Ｔｉ炭
硫化物とＺｒ炭硫化物は、それらの最大直径が小さすぎ
ると被削性向上効果が小さくなってしまう。したがっ
て、Ｔｉ炭硫化物とＺｒ炭硫化物の最大直径の下限値は
０．５μｍ程度とすることが好ましい。If the maximum diameter of Ti carbosulfide and Zr carbosulfide in steel exceeds 10 μm, the fatigue strength and toughness will decrease. Note that the maximum diameter of each of the Ti carbosulfide and the Zr carbosulfide is preferably 7 μm or less. If the maximum diameter of Ti carbosulfide and Zr carbosulfide is too small, the effect of improving machinability is reduced. Therefore, it is preferable that the lower limit value of the maximum diameter of Ti carbosulfide and Zr carbosulfide is about 0.5 μm.
【００５７】最大直径が１０μｍ以下のＴｉ炭硫化物及
びＺｒ炭硫化物の量の和が清浄度で０．０５％未満の場
合には、Ｔｉ炭硫化物及びＺｒ炭硫化物による被削性向
上効果が発揮できない。したがって、Ｔｉ炭硫化物及び
Ｚｒ炭硫化物の最大直径が１０μｍ以下で、且つその量
の和を清浄度で０．０５％以上とした。なお、前記の清
浄度の和は０．０８％以上とすることが好ましい。上記
のＴｉ炭硫化物とＺｒ炭硫化物の清浄度の和の値が大き
すぎると疲労強度が低下する場合があるので、上記の清
浄度の和の上限値は２．０％程度とすることが好まし
い。When the sum of the amounts of Ti carbosulfide and Zr carbosulfide having a maximum diameter of 10 μm or less is less than 0.05% in cleanliness, the machinability is improved by Ti carbosulfide and Zr carbosulfide. No effect. Therefore, the maximum diameter of Ti carbosulfide and Zr carbosulfide was 10 μm or less, and the sum of the amounts was 0.05% or more in terms of cleanliness. It is preferable that the sum of the cleanliness is 0.08% or more. If the sum of the cleanliness of the Ti and Zr carbosulfides is too large, the fatigue strength may decrease. Therefore, the upper limit of the sum of the cleanliness should be about 2.0%. Is preferred.
【００５８】上記したようなＴｉ炭硫化物とＺｒ炭硫化
物の形態は基本的にはＴｉ、Ｚｒ、Ｓ及びＮの含有量で
決定される。しかし、Ｔｉ炭硫化物やＺｒ炭硫化物のサ
イズと清浄度（清浄度の和）を上述の値とするために
は、ＴｉやＺｒの酸化物が過剰に生成することを防ぐこ
とが重要である。このためには、鋼が前記（Ａ）項で述
べた化学組成を有しているだけでは充分でない場合があ
るので、例えば、Ｓｉ及びＡｌで充分脱酸し、最後にＴ
ｉやＺｒを添加する製鋼法を採れば良い。The form of Ti carbosulfide and Zr carbosulfide as described above is basically determined by the contents of Ti, Zr, S and N. However, in order to set the size and cleanliness (sum of cleanliness) of Ti carbosulfides and Zr carbosulfides to the above-mentioned values, it is important to prevent the oxides of Ti and Zr from being excessively generated. is there. In some cases, it is not sufficient for the steel to have the chemical composition described in the above item (A). For example, the steel is sufficiently deoxidized with Si and Al, and finally T
A steelmaking method in which i or Zr is added may be employed.
【００５９】なお、Ｔｉ炭硫化物とＺｒ炭硫化物は、鋼
材から採取した試験片を鏡面研磨し、その研磨面を被検
面として倍率４００倍以上で光学顕微鏡観察すれば、色
と形状から容易に他の介在物と識別できる。すなわち、
前記の条件で光学顕微鏡観察すれば、Ｔｉ炭硫化物及び
Ｚｒ炭硫化物の「色」は極めて薄い灰色で、「形状」は
ＪＩＳのＢ系介在物やＣ系介在物に相当する粒状（球
状）として認められる。Ｔｉ炭硫化物及びＺｒ炭硫化物
の詳細判定は、前記の被検面をＥＤＸ（エネルギー分散
型Ｘ線分析装置）などの分析機能を備えた電子顕微鏡で
観察することによって行うこともできる。Incidentally, Ti carbosulfide and Zr carbosulfide can be obtained from the color and the shape by polishing a test piece taken from a steel material to a mirror surface and observing the polished surface as a test surface with an optical microscope at a magnification of 400 or more. It can be easily distinguished from other inclusions. That is,
Observation under an optical microscope under the above conditions shows that the “color” of Ti carbosulfide and Zr carbosulfide is extremely light gray, and the “shape” is a granular (spherical) equivalent to JIS B-based inclusion or C-based inclusion. ). The detailed determination of Ti carbosulfide and Zr carbosulfide can also be performed by observing the test surface with an electron microscope equipped with an analysis function such as EDX (energy dispersive X-ray analyzer).
【００６０】前記のＴｉ炭硫化物やＺｒ炭硫化物の清浄
度は、既に述べたように、光学顕微鏡の倍率を４００倍
として、JIS G 0555に規定された「鋼の非金属介在物の
顕微鏡試験方法」によって６０視野測定した値をいう。
なお、Ｔｉ炭硫化物やＺｒ炭硫化物の最大直径も、倍率
が４００倍の光学顕微鏡で６０視野観察して調査すれば
良い。As described above, the cleanliness of the Ti carbosulfide and the Zr carbosulfide is determined by setting the magnification of the optical microscope to 400 times, as described in JIS G 0555. It refers to the value measured in 60 visual fields by the "test method".
In addition, the maximum diameter of Ti carbosulfide or Zr carbosulfide may be investigated by observing 60 visual fields with an optical microscope having a magnification of 400 times.
【００６１】（Ｃ）組織 非調質鋼材である熱間加工したままの鋼材の全組織中に
占めるマルテンサイトの割合が面積率で２０％未満の場
合には、所望の１０００ＭＰａ以上の引張強度が安定し
て得られない。一方、マルテンサイトの面積率が９５％
を超えると靭性の著しい低下を招く。(C) Structure When the ratio of martensite in the entire structure of the as-heat-treated non-heat-treated steel material is less than 20% by area, the desired tensile strength of 1000 MPa or more is obtained. It cannot be obtained stably. On the other hand, the martensite area ratio is 95%
If it exceeds, a remarkable decrease in toughness is caused.
【００６２】全組織中のマルテンサイトの面積率が２０
〜９５％の場合であっても、マルテンサイト以外の組織
中に占めるベイナイトの割合が面積率で７０％未満の場
合には、１０００ＭＰａ以上の引張強度と６０Ｊ／ｃｍ
2 以上の２ｍｍＵノッチシャルピー衝撃値とを安定して
同時に確保することが困難となる。したがって、非調質
鋼材の組織を、全組織中のマルテンサイトが面積率で２
０〜９５％で、更に、マルテンサイト以外の組織中にベ
イナイトの占める割合が面積率で７０％以上であるもの
と規定した。The area ratio of martensite in the whole structure is 20
Even in the case of ~ 95%, when the ratio of bainite in the structure other than martensite is less than 70% in area ratio, a tensile strength of 1000 MPa or more and 60 J / cm
It is difficult to stably and simultaneously secure 2 or more 2 mm U notch Charpy impact values. Therefore, the structure of the non-heat treated steel material was changed to 2% martensite in the entire structure.
It was specified that the ratio of bainite in the structure other than martensite was 70% or more in terms of area ratio.
【００６３】なお、全組織中にマルテンサイトが占める
割合は、面積率で４０〜６０％であることが好ましい。The ratio of martensite in the entire structure is preferably 40 to 60% in area ratio.
【００６４】又、全組織中のマルテンサイトの面積率が
２０〜９５％の場合であれば、マルテンサイト以外の組
織においてベイナイトの占める割合が面積率で１００
％、つまり、組織がマルテンサイトとベイナイトとの完
全な混合組織であっても良い。When the area ratio of martensite in the entire structure is 20 to 95%, the ratio of bainite in the structure other than martensite is 100% in area ratio.
%, That is, the structure may be a perfect mixed structure of martensite and bainite.
【００６５】（Ａ）項に記載の化学組成を有する鋼は、
（Ｂ）項に記載したＴｉ炭硫化物、Ｚｒ炭硫化物のサイ
ズと量を確保するために、例えば、Ｓｉ及びＡｌで充分
脱酸し、最後にＴｉやＺｒを添加して溶製され、その
後、熱間での加工（圧延や鍛造）を受け、（Ｃ）項に記
載した組織に調整され、更に、切削加工されて所定形状
の機械構造部品に仕上げられる。The steel having the chemical composition described in (A) is
In order to ensure the size and amount of Ti carbosulfide and Zr carbosulfide described in the section (B), for example, they are sufficiently deoxidized with Si and Al and finally melted by adding Ti or Zr, Thereafter, it is subjected to hot working (rolling or forging), adjusted to the structure described in the section (C), and further cut to finish into a machine structural part having a predetermined shape.
【００６６】（Ｄ）鋼の加熱温度 熱間加工のための鋼の加熱温度が１１００℃未満では、
炭化物などがオーステナイト中に充分固溶せずに焼入れ
性が低下したり、変形抵抗が大きくなって熱間での加工
が困難になる場合がある。したがって、鋼の加熱温度を
１１００℃以上とした。なお、この加熱温度の上限は１
３００℃程度とすることが好ましい。(D) Heating temperature of steel If the heating temperature of steel for hot working is less than 1100 ° C,
In some cases, carbides or the like do not sufficiently dissolve in austenite to reduce hardenability, or deformation resistance increases, and hot working becomes difficult. Therefore, the heating temperature of the steel was set to 1100 ° C. or higher. The upper limit of the heating temperature is 1
Preferably, the temperature is about 300 ° C.
【００６７】（Ｅ）熱間加工終了温度 熱間加工終了温度が９００℃未満では、変形抵抗が大き
くなるばかりか、炭化物や窒化物が凝集粗大化し、その
結果、結晶粒が著しく粗大化して靭性の低下を招く場合
がある。したがって、熱間加工終了温度を９００℃以上
とした。なお、この熱間加工の終了温度は１０５０℃程
度を上限とすることが好ましい。(E) Hot working end temperature When the hot working end temperature is lower than 900 ° C., not only the deformation resistance is increased, but also carbides and nitrides are agglomerated, and as a result, the crystal grains are remarkably coarsened and the toughness is increased. May be reduced. Therefore, the hot working end temperature is set to 900 ° C. or higher. In addition, it is preferable that the end temperature of this hot working has an upper limit of about 1050 ° C.
【００６８】（Ｆ）冷却速度 熱間加工後の冷却速度が３０℃／分未満の場合には、所
望の組織、つまり全組織中のマルテンサイトが面積率で
２０〜９５％で、更に、マルテンサイト以外の組織中に
ベイナイトの占める割合が面積率で７０％以上である組
織を、安定して鋼材に付与することが困難となる。一
方、３００℃／分を超える場合には後述の実施例で示す
ように全組織中のマルテンサイトの面積率が９５％を超
えてしまい靭性の著しい低下を招く。したがって、熱間
加工後の冷却速度を３０〜３００℃／分とした。(F) Cooling Rate When the cooling rate after hot working is less than 30 ° C./min, the desired structure, that is, the martensite in the entire structure has an area ratio of 20 to 95%, It is difficult to stably impart a structure in which bainite occupies 70% or more in the area ratio of the structure other than the site to the steel material. On the other hand, when the temperature exceeds 300 ° C./min, the area ratio of martensite in the entire structure exceeds 95% as shown in the examples described later, resulting in a significant decrease in toughness. Therefore, the cooling rate after hot working was set to 30 to 300 ° C./min.
【００６９】[0069]
【実施例】（実施例１）表１、表２に示す化学組成の鋼
を通常の方法によって試験炉を用いて１５０ｋｇ真空溶
製した。なお、Ｔｉ酸化物及びＺｒ酸化物の生成を防ぐ
ために、ＳｉやＡｌで充分脱酸し種々の元素を添加した
最後にＴｉとＺｒを添加して、Ｔｉ炭硫化物とＺｒ炭硫
化物のサイズと清浄度（清浄度の和）を調整するように
した。EXAMPLES (Example 1) 150 kg of steel having the chemical composition shown in Tables 1 and 2 was vacuum-melted by a conventional method using a test furnace. In order to prevent the formation of Ti oxide and Zr oxide, sufficient deoxidation with Si or Al was performed, and various elements were added. Finally, Ti and Zr were added to reduce the size of Ti carbosulfide and Zr carbosulfide. And the cleanliness (sum of cleanliness) were adjusted.
【００７０】表１における鋼１〜１５は化学組成が本発
明で規定する範囲内にある本発明例の鋼であり、表２に
おける鋼１６〜３０はその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である。Steels 1 to 15 in Table 1 are steels of the examples of the present invention whose chemical compositions are within the range specified by the present invention, and steels 16 to 30 in Table 2 have any of the components specified by the present invention. It is a steel of a comparative example out of the range of the content.
【００７１】[0071]
【表１】 [Table 1]
【００７２】[0072]
【表２】 [Table 2]
【００７３】次いで、通常の方法によって前記鋼番号の
鋼塊を１２００℃に加熱した後、１０００℃の仕上げ温
度で厚さ３５ｍｍ×幅９０ｍｍ幅×長さ１０００ｍｍの
鋼板に熱間鍛造し、その後鋼組成に応じて３０〜４５℃
／分の冷却速度で冷却した。Next, the ingot having the above-mentioned steel number was heated to 1200 ° C. by a usual method, and then hot forged into a steel plate having a thickness of 35 mm × width 90 mm × width 1000 mm at a finishing temperature of 1000 ° C. 30-45 ° C depending on the composition
/ Min cooling rate.
【００７４】こうして得られた鋼板の中心部から、ＪＩ
Ｓ４号引張試験片とＪＩＳ３号シャルピー衝撃試験片
（２ｍｍＵノッチシャルピー衝撃試験片）を切り出し、
常温で試験を行った。鋼板の中心部からは組織観察用の
試験片も切り出し、光学顕微鏡による中心部の組織観察
を行った。なお、全組織中のマルテンサイトの面積率及
びマルテンサイト以外の組織中にベイナイトの占める割
合（面積率）は、いずれも通常の方法で画像処理して判
定した。From the central part of the steel sheet thus obtained, JI
Cut out S4 tensile test specimen and JIS No. 3 Charpy impact test specimen (2 mm U notch Charpy impact test specimen),
The test was performed at room temperature. A test piece for structure observation was also cut from the center of the steel sheet, and the structure of the center was observed with an optical microscope. In addition, the area ratio of martensite in the entire structure and the ratio (area ratio) of bainite in the structure other than martensite were both determined by image processing using a normal method.
【００７５】上記の熱間鍛造した鋼板から、JIS G 0555
の図６に準じて試験片を採取し、鏡面研磨した３００ｍ
ｍ2 の被検面を、倍率が４００倍の光学顕微鏡で６０視
野観察して、Ｔｉ炭硫化物及びＺｒ炭硫化物を他の介在
物と区分しながらその清浄度（清浄度の和）も測定し
た。Ｔｉ炭硫化物及びＺｒ炭硫化物の最大直径も、倍率
が４００倍の光学顕微鏡で６０視野観察して調査した。From the hot forged steel sheet, JIS G 0555
A test piece was sampled according to FIG.
The test surface of m 2 was observed in 60 visual fields with an optical microscope having a magnification of 400 times, and the cleanliness (sum of cleanliness) of Ti carbosulfide and Zr carbosulfide was determined while separating them from other inclusions. It was measured. The maximum diameters of Ti carbosulfide and Zr carbosulfide were also examined by observing 60 visual fields with an optical microscope having a magnification of 400 times.
【００７６】被削性評価のためのドリル穿孔試験も実施
した。すなわち、前記した熱間鍛造した厚さ３５ｍｍで
幅が９０ｍｍの鋼板を用いて、その厚さ方向に貫通孔を
開け、刃先摩損により穿孔不能となったときの貫通孔の
個数を数え、被削性の評価を行った。貫通孔の個数が５
００個に達したものはその時点で穿孔試験を中止した。
穿孔条件は、ＪＩＳ高速度工具鋼ＳＫＨ５１のφ８ｍｍ
ストレ−トシャンクドリルを使用し、水溶性の潤滑剤を
用いて、穴の中心間隔１０ｍｍ、送り０．１５ｍｍ／ｒ
ｅｖ、回転数７４５ｒｐｍの条件で行った。A drill test for evaluating machinability was also performed. That is, using a hot forged steel plate having a thickness of 35 mm and a width of 90 mm, a through hole is opened in the thickness direction, and the number of the through hole when drilling becomes impossible due to abrasion of the cutting edge is counted. The sex was evaluated. 5 through holes
When the number reached 00, the perforation test was stopped at that time.
The drilling condition is φ8mm of JIS high speed tool steel SKH51.
Using a straight shank drill, using a water-soluble lubricant, center distance between holes 10 mm, feed 0.15 mm / r
ev, the number of rotations was 745 rpm.
【００７７】表３に各種試験の結果を示す。なお、「Ｔ
ｉ、Ｚｒ炭硫化物」とした欄において、ＴｉとＺｒとを
複合添加した場合には「最大直径」はいずれか大きい方
の炭硫化物の値であり、清浄度は清浄度の和を意味す
る。Table 3 shows the results of various tests. Note that "T
In the column labeled "i, Zr carbosulfide", when Ti and Zr are added in combination, the "maximum diameter" is the value of the larger one of the carbosulfides, and the cleanliness means the sum of the cleanliness. I do.
【００７８】[0078]
【表３】 [Table 3]
【００７９】表３から、化学組成及び最大直径が１０μ
ｍ以下のＴｉ炭硫化物及びＺｒ炭硫化物の清浄度（清浄
度の和）が本発明で規定する範囲内にある本発明例の鋼
１〜１５を素材とするものについては、組織はいずれも
全組織中のマルテンサイトが面積率で２０〜９５％で、
更に、マルテンサイト以外の組織中にベイナイトの占め
る割合が面積率で７０％以上であり、所望の７００ＭＰ
ａ以上の降伏強度、１０００ＭＰａ以上の引張強度、
０．６５以上の降伏比と６０Ｊ／ｃｍ2 以上の２ｍｍＵ
ノッチシャルピー衝撃値が得られている。From Table 3, it can be seen that the chemical composition and the maximum diameter are
The structure of any of the steels 1 to 15 of the present invention in which the cleanliness (sum of the cleanliness) of Ti carbosulfide and Zr carbosulfide of m or less is within the range specified in the present invention, Also, the martensite in the whole structure has an area ratio of 20 to 95%,
Furthermore, the ratio of bainite in the structure other than martensite is 70% or more in area ratio, and the desired 700MP
a or more yield strength, 1000MPa or more tensile strength,
Yield ratio of 0.65 or more and 2 mmU of 60 J / cm 2 or more
Notch Charpy impact values are obtained.
【００８０】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は、以下に
述べるように、疲労強度、降伏強度、引張強度、降伏
比、２ｍｍＵノッチシャルピー衝撃値の少なくとも１つ
が所望の値に達していないか、被削性が低い。On the other hand, the steel of the comparative example in which any of the components is out of the range of the content specified in the present invention has a fatigue strength, a yield strength, a tensile strength, a yield ratio of 2 mmU as described below. At least one of the notch Charpy impact values does not reach the desired value or the machinability is low.
【００８１】鋼１６〜２７及び鋼３０は、組織はいずれ
も全組織中のマルテンサイトが面積率で２０〜９５％
で、更に、マルテンサイト以外の組織中にベイナイトの
占める割合が面積率で７０％以上であるが、降伏強度、
引張強度、降伏比と２ｍｍＵノッチシャルピー衝撃値の
うち少なくとも１つが所望の値に達していない。Steels 16 to 27 and steel 30 each have a structure in which martensite in the entire structure has an area ratio of 20 to 95%.
Further, the proportion of bainite in the structure other than martensite is 70% or more in terms of area ratio,
At least one of the tensile strength, the yield ratio and the 2 mm U notch Charpy impact value has not reached the desired value.
【００８２】すなわち、Ｃの含有量が低めに外れた鋼１
６、Ｓｉの含有量が低めに外れた鋼１８、Ｃｒの含有量
が低めに外れた鋼２４及びＮの含有量が高めに外れると
ともにｆｎ１の値が低めに外れた鋼２６は引張強度又
は、降伏強度と引張強度の双方が目標値に未達である。That is, steel 1 having a low C content.
6, steel 18 having a low Si content, steel 24 having a low Cr content, and steel 26 having a high N content and a low fn1 value are tensile strength or Both the yield strength and the tensile strength have not reached the target values.
【００８３】Ｃの含有量が高めに外れた鋼１７、Ｐの含
有量が高めに外れた鋼２０、Ｓの含有量が高めに外れた
鋼２１、Ｃｕの含有量が高めに外れた鋼２３及びＴｉの
含有量が高めに外れた鋼３０は２ｍｍＵノッチシャルピ
ー衝撃値が目標値に達していない。Steel 17 with a higher C content, Steel 20 with a higher P content, Steel 21 with a higher S content, Steel 23 with a higher Cu content The steel 30 in which the content of Ti and Ti was deviated to a high value did not reach the target value of the 2 mm U notch Charpy impact value.
【００８４】Ｍｎの含有量が低めに外れた鋼１９及びＣ
ｕの含有量が低めに外れた鋼２２は降伏比が目標値に達
していない。鋼１９はｆｎ２が負の値であり、「Ｔｉ、
Ｚｒ炭硫化物」欄における清浄度が規定の値を下回るた
め被削性も低い。Steels 19 and C whose Mn content was lower
The yield ratio of the steel 22 having a lower u content is lower than the target value. In steel 19, fn2 is a negative value and “Ti,
Since the cleanliness in the "Zr carbosulfide" column is lower than the specified value, the machinability is also low.
【００８５】Ａｌの含有量が高めに外れた鋼２５は被削
性が低い。Steel 25 having a high Al content has low machinability.
【００８６】ｆｎ２が負の値で、「Ｔｉ、Ｚｒ炭硫化
物」欄における清浄度が規定の値を下回る鋼２７は被削
性が低い。Steel 27 having a negative value of fn2 and a cleanliness lower than the specified value in the column of “Ti, Zr carbosulfide” has low machinability.
【００８７】鋼２８及び鋼２９は本発明で規定する組織
が得られておらず、強度（降伏強度と引張強度）及び２
ｍｍＵノッチシャルピー衝撃値が目標値に達していな
い。すなわち、Ｍｎ、Ｃｒ及びＢの含有量が低めに外れ
るために、組織がマルテンサイトを含まないものとなっ
た鋼２８、及びｆｎ１の値が低めに外れるとともにＭｎ
及びＣｒの含有量が低めに外れるために、やはり組織が
マルテンサイトを含まないものとなった鋼２９は、降伏
強度と引張強度及び２ｍｍＵノッチシャルピー衝撃値が
目標値に達していない。鋼２９はｆｎ２も負の値である
ため被削性も低い。The steels 28 and 29 did not have the microstructure specified in the present invention, and had strength (yield strength and tensile strength) of 2%.
The mmU notch Charpy impact value has not reached the target value. That is, since the contents of Mn, Cr, and B come off relatively low, the steel 28 whose structure does not contain martensite, and the value of fn1 come off relatively low, and
Steel 29 whose structure does not contain martensite due to the lower contents of Cr and Cr, the yield strength, tensile strength and 2 mm U notch Charpy impact value did not reach the target values. Steel 29 also has low machinability because fn2 is also a negative value.
【００８８】（実施例２）表４に示す鋼３１及び鋼３２
を通常の方法によって試験炉を用いて１５０ｋｇ真空溶
製した。鋼３１、鋼３２はいずれも化学組成が本発明で
規定する範囲内にある本発明例の鋼である。なお、Ｔｉ
酸化物及びＺｒ酸化物の生成を防ぐために、ＳｉやＡｌ
で充分脱酸し種々の元素を添加した最後にＴｉとＺｒを
添加して、Ｔｉ炭硫化物とＺｒ炭硫化物のサイズと清浄
度（清浄度の和）を調整するようにした。(Example 2) Steel 31 and steel 32 shown in Table 4
Was vacuum-melted in a usual manner using a test furnace at 150 kg. Steels 31 and 32 are steels of the present invention whose chemical composition is within the range specified in the present invention. Note that Ti
To prevent the formation of oxides and Zr oxides, use Si or Al
Finally, Ti and Zr were added to adjust the size and cleanliness (the sum of cleanliness) of Ti and Zr carbosulfides.
【００８９】[0089]
【表４】 [Table 4]
【００９０】次いで、通常の方法によって前記鋼番号の
鋼塊を１２００℃に加熱した後、１０００℃の仕上げ温
度で厚さ３５ｍｍ×幅９０ｍｍ幅×長さ１０００ｍｍの
鋼板に熱間鍛造した。鍛造後は鋼３１を母材鋼とする鋼
板は４０℃／分及び５００℃／分の冷却速度で、又、鋼
３２を母材鋼とする鋼板は４０℃／分及び１０℃／分の
冷却速度で冷却した。Next, the steel ingot having the above-mentioned steel number was heated to 1200 ° C. by a usual method, and then hot forged into a steel plate having a thickness of 35 mm × width of 90 mm × width of 1000 mm at a finishing temperature of 1000 ° C. After forging, the steel sheet using steel 31 as the base steel is cooled at a cooling rate of 40 ° C./min and 500 ° C./min, and the steel sheet using steel 32 as the base steel is cooled at 40 ° C./min and 10 ° C./min. Cooled at speed.
【００９１】こうして得られた鋼板の中心部から、ＪＩ
Ｓ４号引張試験片とＪＩＳ３号シャルピー衝撃試験片
（２ｍｍＵノッチシャルピー衝撃試験片）を切り出し、
常温で試験を行った。鋼板の中心部からは組織観察用の
試験片も切り出し、光学顕微鏡による中心部の組織観察
を行った。なお、全組織中のマルテンサイトの面積率及
びマルテンサイト以外の組織中にベイナイトの占める割
合（面積率）は、いずれも通常の方法で画像処理して判
定した。From the center of the steel sheet thus obtained, JI
Cut out S4 tensile test specimen and JIS No. 3 Charpy impact test specimen (2 mm U notch Charpy impact test specimen),
The test was performed at room temperature. A test piece for structure observation was also cut out from the center of the steel sheet, and the structure of the center was observed with an optical microscope. The area ratio of martensite in the entire structure and the ratio of bainite in the structure other than martensite (area ratio) were both determined by image processing using an ordinary method.
【００９２】又、上記の熱間鍛造した鋼板から、JIS G
0555の図６に準じて試験片を採取し、鏡面研磨した３０
０ｍｍ2 の被検面を、倍率が４００倍の光学顕微鏡で６
０視野観察して、Ｔｉ炭硫化物及びＺｒ炭硫化物を他の
介在物と区分しながらその清浄度（清浄度の和）も測定
した。Ｔｉ炭硫化物及びＺｒ炭硫化物の最大直径も、倍
率が４００倍の光学顕微鏡で６０視野観察して調査し
た。Further, from the hot forged steel sheet, JIS G
A test piece was sampled according to FIG.
The test surface of 0 mm 2 was examined with an optical microscope having a magnification of 400 × 6.
Observation was performed in zero visual field, and the cleanliness (sum of cleanliness) of Ti and Zr carbosulfides was measured while separating them from other inclusions. The maximum diameters of Ti carbosulfide and Zr carbosulfide were also examined by observing 60 visual fields with an optical microscope having a magnification of 400 times.
【００９３】表５に各試験結果を示す。なお、表で「Ｔ
ｉ、Ｚｒ炭硫化物」とした欄における「最大直径」はＴ
ｉとＺｒのいずれか大きい方の炭硫化物の値であり、清
浄度は清浄度の和を意味する。Table 5 shows the results of each test. In the table, "T
"maximum diameter" in the column of "i, Zr carbosulfide" is T
This is the value of the carbosulfide, which is larger of i and Zr, and the cleanliness means the sum of the cleanliness.
【００９４】[0094]
【表５】 [Table 5]
【００９５】鋼３１及び鋼３２を熱間鍛造後、本発明で
規定する範囲内の冷却速度の４０℃／分で冷却した場合
（試験番号１及び３）には、その組織はいずれも全組織
中のマルテンサイトが面積率で２０〜９５％で、更に、
マルテンサイト以外の組織中にベイナイトの占める割合
が面積率で７０％以上であり、所望の７００ＭＰａ以上
の降伏強度、１０００ＭＰａ以上の引張強度、０．６５
以上の降伏比と６０Ｊ／ｃｍ2 以上の２ｍｍＵノッチシ
ャルピー衝撃値が得られている。After steel 31 and steel 32 were hot forged and cooled at a cooling rate of 40 ° C./min within the range specified in the present invention (test numbers 1 and 3), the structures were all the same. Martensite in the area ratio is 20 to 95%, furthermore,
The proportion of bainite in the structure other than martensite is 70% or more in area ratio, and the desired yield strength of 700 MPa or more, tensile strength of 1000 MPa or more, 0.65
The above yield ratio and a 2 mm U notch Charpy impact value of 60 J / cm 2 or more are obtained.
【００９６】これに対して、本発明鋼であっても熱間鍛
造後の冷却速度が本発明で規定する上限を外れた５００
℃／分の場合（試験番号２）には、全組織中のマルテン
サイトの面積率が９５％を超えるので靭性が著しく低下
し、２ｍｍＵノッチシャルピー衝撃値が目標値に達して
いない。On the other hand, even in the case of the steel of the present invention, the cooling rate after hot forging is out of the upper limit defined by the present invention.
In the case of ° C / min (test number 2), the area ratio of martensite in the whole structure exceeded 95%, so that the toughness was significantly reduced, and the 2 mm U notch Charpy impact value did not reach the target value.
【００９７】又、本発明鋼であっても熱間鍛造後の冷却
速度が本発明で規定する下限を外れた１０℃／分の場合
（試験番号４）には、全組織中のマルテンサイトの面積
率が２０％を下回るので引張強度と降伏強度とが目標値
に達していない。Also, in the case of the steel of the present invention, when the cooling rate after hot forging deviated from the lower limit specified in the present invention by 10 ° C./min (test number 4), the martensite in the entire structure was Since the area ratio is less than 20%, the tensile strength and the yield strength have not reached the target values.
【００９８】[0098]
【発明の効果】本発明による被削性に優れたマルテンサ
イト・ベイナイト型非調質鋼材を用いれば、７００ＭＰ
ａ以上の降伏強度、１０００ＭＰａ以上の引張強度、
０．６５以上の降伏比と６０Ｊ／ｃｍ2 以上の２ｍｍＵ
ノッチシャルピー衝撃値を有する機械構造部品を低コス
トで製造することができる。このマルテンサイト・ベイ
ナイト型非調質鋼材は、本発明の方法によって比較的容
易に得られる。According to the present invention, the use of a non-heat-treated martensite-bainite type steel material excellent in machinability according to the present invention provides 700MPa.
a or more yield strength, 1000MPa or more tensile strength,
Yield ratio of 0.65 or more and 2 mmU of 60 J / cm 2 or more
Machine structural parts having a notch Charpy impact value can be manufactured at low cost. This martensitic bainite type non-heat treated steel material can be obtained relatively easily by the method of the present invention.
───────────────────────────────────────────────────── フロントページの続き (72)発明者 西村 彰二 福岡県北九州市小倉北区許斐町１番地住友 金属工業株式会社小倉製鉄所内 Ｆターム(参考） 4K032 AA01 AA02 AA05 AA08 AA11 AA12 AA14 AA16 AA17 AA19 AA21 AA22 AA23 AA27 AA28 AA29 AA31 AA34 AA35 AA36 AA39 BA01 BA02 CA02 CA03 CC04 CD01 CD02 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Shoji Nishimura, Inventor F-term (reference) 4K032 AA01 AA02 AA05 AA08 AA11 AA12 AA14 AA16 AA17 AA21 AA21 AA22 AA23 AA27 AA28 AA29 AA31 AA34 AA35 AA36 AA39 BA01 BA02 CA02 CA03 CC04 CD01 CD02
Claims (2)
ｉ：０．１〜１．０％、Ｍｎ：１．５〜３．０％、Ｐ：
０．１０％以下、Ｓ：０．００２〜０．１０％、Ｃｕ：
０．０１〜０．５％、Ｎｉ：０．２％以下、Ｃｒ：０．
５〜１．５％、Ｍｏ：０．２％以下、Ｖ：０．５０％以
下、Ｎｂ：０．０５％以下、Ｔｉ：１．０％以下、Ｚ
ｒ：１．０％以下で、且つ、Ｔｉ（％）＋Ｚｒ（％）：
０．０４〜１．０％、Ｂ：０．０００５〜０．０１０
％、Ｎ：０．００８％以下、Ａｌ：０．１０％以下、Ｐ
ｂ：０．３０％以下、Ｔｅ：０．１０％以下、Ｃａ：
０．０１０％以下を含み、下記式で表されるｆｎ１の
値が０％以上、下記式で表されるｆｎ２の値が０％を
超え、残部はＦｅ及び不可避不純物の化学組成で、鋼中
のＴｉ炭硫化物及びＺｒ炭硫化物の最大直径が１０μｍ
以下で、且つ、その量の和が清浄度で０．０５％以上
で、且つ、全組織中に占めるマルテンサイトの割合が面
積率で２０〜９５％で、マルテンサイト以外の組織中に
占めるベイナイトの割合が面積率で７０％以上であるこ
とを特徴とする被削性に優れたマルテンサイト・ベイナ
イト型非調質鋼材。 ｆｎ１＝Ｔｉ（％）＋０．５Ｚｒ（％）−３．４Ｎ（％）・・・・ ｆｎ２＝Ｔｉ（％）＋Ｚｒ（％）−１．２Ｓ（％）・・・・(1) C: 0.15 to 0.35% by weight, S
i: 0.1 to 1.0%, Mn: 1.5 to 3.0%, P:
0.10% or less, S: 0.002 to 0.10%, Cu:
0.01-0.5%, Ni: 0.2% or less, Cr: 0.
5 to 1.5%, Mo: 0.2% or less, V: 0.50% or less, Nb: 0.05% or less, Ti: 1.0% or less, Z
r: 1.0% or less, and Ti (%) + Zr (%):
0.04 to 1.0%, B: 0.0005 to 0.010
%, N: 0.008% or less, Al: 0.10% or less, P
b: 0.30% or less, Te: 0.10% or less, Ca:
0.010% or less, the value of fn1 represented by the following formula is 0% or more, the value of fn2 represented by the following formula exceeds 0%, and the balance is the chemical composition of Fe and unavoidable impurities. Maximum diameter of Ti and Zr carbosulfides of 10 μm
Below, and the sum of the amounts is 0.05% or more in cleanliness, the ratio of martensite in the entire structure is 20 to 95% in area ratio, and bainite occupies in structures other than martensite. Is a martensitic bainite type non-heat treated steel excellent in machinability, characterized in that the ratio of the area is 70% or more in terms of area ratio. fn1 = Ti (%) + 0.5Zr (%)-3.4N (%) ... fn2 = Ti (%) + Zr (%)-1.2S (%) ...
ｉ：０．１〜１．０％、Ｍｎ：１．５〜３．０％、Ｐ：
０．１０％以下、Ｓ：０．００２〜０．１０％、Ｃｕ：
０．０１〜０．５％、Ｎｉ：０．２％以下、Ｃｒ：０．
５〜１．５％、Ｍｏ：０．２％以下、Ｖ：０．５０％以
下、Ｎｂ：０．０５％以下、Ｔｉ：１．０％以下、Ｚ
ｒ：１．０％以下で、且つ、Ｔｉ（％）＋Ｚｒ（％）：
０．０４〜１．０％、Ｂ：０．０００５〜０．０１０
％、Ｎ：０．００８％以下、Ａｌ：０．１０％以下、Ｐ
ｂ：０．３０％以下、Ｔｅ：０．１０％以下、Ｃａ：
０．０１０％以下を含み、前記式で表されるｆｎ１の
値が０％以上、前記式で表されるｆｎ２が０％を超
え、残部はＦｅ及び不可避不純物の化学組成で、鋼中の
Ｔｉ炭硫化物及びＺｒ炭硫化物の最大直径が１０μｍ以
下で、且つ、その量の和が清浄度で０．０５％以上であ
る鋼を、１１００℃以上の温度に加熱して熱間鍛造し、
熱間鍛造を９００℃以上の温度で終了した後３０〜３０
０℃／分の冷却速度で冷却することを特徴とする被削性
に優れたマルテンサイト・ベイナイト型非調質鋼材の製
造方法。2. C: 0.15 to 0.35% by weight, S
i: 0.1 to 1.0%, Mn: 1.5 to 3.0%, P:
0.10% or less, S: 0.002 to 0.10%, Cu:
0.01-0.5%, Ni: 0.2% or less, Cr: 0.
5 to 1.5%, Mo: 0.2% or less, V: 0.50% or less, Nb: 0.05% or less, Ti: 1.0% or less, Z
r: 1.0% or less, and Ti (%) + Zr (%):
0.04 to 1.0%, B: 0.0005 to 0.010
%, N: 0.008% or less, Al: 0.10% or less, P
b: 0.30% or less, Te: 0.10% or less, Ca:
0.010% or less, the value of fn1 represented by the above formula is 0% or more, the value of fn2 represented by the above formula exceeds 0%, and the balance is the chemical composition of Fe and unavoidable impurities. A steel having a maximum diameter of carbosulfide and Zr carbosulfide of 10 μm or less, and the sum of the amounts is 0.05% or more in cleanliness, is heated to a temperature of 1100 ° C. or more and hot forged,
After hot forging is completed at a temperature of 900 ° C or more, 30 to 30
A method for producing a martensitic bainite type non-heat treated steel excellent in machinability, characterized by cooling at a cooling rate of 0 ° C./min.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP30672398A JP3644275B2 (en) | 1998-10-28 | 1998-10-28 | Martensitic bainite-type non-tempered steel material excellent in machinability and manufacturing method thereof |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP30672398A JP3644275B2 (en) | 1998-10-28 | 1998-10-28 | Martensitic bainite-type non-tempered steel material excellent in machinability and manufacturing method thereof |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JP2000129393A true JP2000129393A (en) | 2000-05-09 |
| JP3644275B2 JP3644275B2 (en) | 2005-04-27 |
Family
ID=17960533
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP30672398A Expired - Fee Related JP3644275B2 (en) | 1998-10-28 | 1998-10-28 | Martensitic bainite-type non-tempered steel material excellent in machinability and manufacturing method thereof |
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| Country | Link |
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| JP (1) | JP3644275B2 (en) |
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2009057731A1 (en) | 2007-10-29 | 2009-05-07 | Nippon Steel Corporation | Martensitic non-heat-treated steel for hot forging and non-heat-treated steel hot forgings |
| JP2010174290A (en) * | 2009-01-28 | 2010-08-12 | Jfe Steel Corp | Steel sheet to be die-quenched superior in hot-punchability |
| WO2012067473A3 (en) * | 2010-11-19 | 2012-09-20 | 주식회사 포스코 | High-toughness cold-drawn non-heat-treated wire rod, and method for manufacturing same |
| CN103380217A (en) * | 2011-02-18 | 2013-10-30 | 蒂森克虏伯钢铁欧洲股份公司 | Hot rolled flat steel product produced from a complex phase steel, and method for producing same |
| CN113755752A (en) * | 2021-08-24 | 2021-12-07 | 江苏利淮钢铁有限公司 | 30Mn2CrTiB steel for high-strength and high-toughness engineering machinery wheel body and production method thereof |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| KR101306241B1 (en) * | 2013-01-18 | 2013-09-10 | 주식회사 세아베스틸 | Direct - quenched low carbon micro-alloyed steels having high strength and high impact toughness and method for manufacturing thereof |
Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS62207821A (en) * | 1986-03-10 | 1987-09-12 | Sumitomo Metal Ind Ltd | Production of unnormalized steel for hot forging |
| JPH08277437A (en) * | 1995-04-07 | 1996-10-22 | Kobe Steel Ltd | Production of high strength and high toughness non-heat treated steel for hot forging and forged product thereof |
| JPH10204578A (en) * | 1996-11-25 | 1998-08-04 | Sumitomo Metal Ind Ltd | High strength free cutting non-heat treated steel material |
| JP3468031B2 (en) * | 1997-06-24 | 2003-11-17 | 住友金属工業株式会社 | Martensite-bainite hot forged part and method of manufacturing the same |
-
1998
- 1998-10-28 JP JP30672398A patent/JP3644275B2/en not_active Expired - Fee Related
Patent Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS62207821A (en) * | 1986-03-10 | 1987-09-12 | Sumitomo Metal Ind Ltd | Production of unnormalized steel for hot forging |
| JPH08277437A (en) * | 1995-04-07 | 1996-10-22 | Kobe Steel Ltd | Production of high strength and high toughness non-heat treated steel for hot forging and forged product thereof |
| JPH10204578A (en) * | 1996-11-25 | 1998-08-04 | Sumitomo Metal Ind Ltd | High strength free cutting non-heat treated steel material |
| JP3468031B2 (en) * | 1997-06-24 | 2003-11-17 | 住友金属工業株式会社 | Martensite-bainite hot forged part and method of manufacturing the same |
Cited By (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2009057731A1 (en) | 2007-10-29 | 2009-05-07 | Nippon Steel Corporation | Martensitic non-heat-treated steel for hot forging and non-heat-treated steel hot forgings |
| JP2010174290A (en) * | 2009-01-28 | 2010-08-12 | Jfe Steel Corp | Steel sheet to be die-quenched superior in hot-punchability |
| WO2012067473A3 (en) * | 2010-11-19 | 2012-09-20 | 주식회사 포스코 | High-toughness cold-drawn non-heat-treated wire rod, and method for manufacturing same |
| US9394580B2 (en) | 2010-11-19 | 2016-07-19 | Posco | High-toughness cold-drawn non-heat-treated wire rod, and method for manufacturing same |
| CN103380217A (en) * | 2011-02-18 | 2013-10-30 | 蒂森克虏伯钢铁欧洲股份公司 | Hot rolled flat steel product produced from a complex phase steel, and method for producing same |
| CN113755752A (en) * | 2021-08-24 | 2021-12-07 | 江苏利淮钢铁有限公司 | 30Mn2CrTiB steel for high-strength and high-toughness engineering machinery wheel body and production method thereof |
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| Publication number | Publication date |
|---|---|
| JP3644275B2 (en) | 2005-04-27 |
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