JP6068314B2 - Hot-rolled steel sheet with excellent cold workability and surface hardness after carburizing heat treatment - Google Patents

Hot-rolled steel sheet with excellent cold workability and surface hardness after carburizing heat treatment Download PDF

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JP6068314B2
JP6068314B2 JP2013219467A JP2013219467A JP6068314B2 JP 6068314 B2 JP6068314 B2 JP 6068314B2 JP 2013219467 A JP2013219467 A JP 2013219467A JP 2013219467 A JP2013219467 A JP 2013219467A JP 6068314 B2 JP6068314 B2 JP 6068314B2
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梶原 桂
桂 梶原
土田 武広
武広 土田
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株式会社神戸製鋼所
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    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226Hot rolling
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    • C21D9/00Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/60Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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    • C23COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23CCOATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/20Carburising
    • C23C8/22Carburising of ferrous surfaces
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    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/002Bainite
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    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00Microstructure comprising significant phases
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    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/009Pearlite

Description

本発明は、熱処理前の加工中は良好な冷間加工性を示しつつ、浸炭熱処理後は、所定の表面硬さ、および表面から深いところでも所望の硬さを示す熱延鋼板に関し、詳しくは、各種の構造用部品として用いられる鋼材のうち、特に耐摩耗性や耐疲労特性を改善するため、浸炭焼入れまたは浸炭窒化焼入れ処理による表面硬質化処理の行なわれる部品、例えば自動車などの各部に用いられるクラッチ、ダンパー、歯車(ギア)等を製造するための素材として有用な熱延鋼板に関するものである。なお、以下の説明では、クラッチ類に適用する場合を代表的に取り上げて説明を進めるが、本発明はもとより、その製造に限定されるものではなく、その優れた浸炭焼入性や浸炭窒化焼入性を活用し、芯部の高靭性を維持しつつ表層部を硬質化して、高い表面硬さと優れた衝撃特性の求められる部品を製造するための素材として有効に活用される。   The present invention relates to a hot-rolled steel sheet that exhibits a good cold workability during processing before heat treatment, and exhibits a predetermined surface hardness and a desired hardness even deep from the surface after carburizing heat treatment. Of steel materials used as various structural parts, especially for parts subjected to surface hardening by carburizing or carbonitriding to improve wear resistance and fatigue resistance, such as automobiles The present invention relates to a hot-rolled steel sheet that is useful as a material for manufacturing a clutch, a damper, a gear (gear), and the like. In the following description, a case where the present invention is applied to clutches will be taken up as a representative, and the description will proceed. However, the present invention is not limited to the manufacture of the present invention, and its excellent carburizing hardenability and carbonitriding are also described. It is effectively utilized as a material for manufacturing parts that require high surface hardness and excellent impact properties by making the surface layer hard while maintaining the high toughness of the core by utilizing the permeability.

近年、環境保護の観点から、自動車の燃費向上を目的として、自動車用の各種部品、例えばギアなどのトランスミッション部品やケース等に用いられる鋼材の軽量化、すなわち高強度化に対する要求が益々高まっている。このような軽量化・高強度化の要請に応えるために、一般に用いられる鋼材としては、棒鋼を熱間鍛造した鋼材(熱間鍛造材)が用いられてきた(例えば、特許文献1参照)。また、部品製造工程におけるCOの排出量削減のため、これまで熱間鍛造によって加工されていたギヤなどの部品の冷間鍛造化に関する要求も高まっている。 In recent years, from the viewpoint of environmental protection, for the purpose of improving the fuel efficiency of automobiles, there are increasing demands for weight reduction of steel materials used for various parts for automobiles, for example, transmission parts such as gears and cases, that is, higher strength. . In order to meet such demands for weight reduction and high strength, steel materials obtained by hot forging steel bars (hot forging materials) have been used as steel materials that are generally used (see, for example, Patent Document 1). In addition, in order to reduce CO 2 emissions in the component manufacturing process, there is an increasing demand for cold forging of components such as gears that have been processed by hot forging.

ところで、冷間加工(冷間鍛造)は、熱間加工や温間加工に比較して生産性が高く、しかも寸法精度および鋼材の歩留まりがともに良好な利点がある。しかし、このような冷間加工によって部品を製造する場合に問題となるのは、冷間加工された部品の強度を期待される所定値以上に確保するためには、必然的に強度、すなわち変形抵抗の高い鋼材を用いる必要があることである。ところが、使用する鋼材の変形抵抗が高いものほど冷間加工用金型の寿命短縮を招く難点がある。   By the way, cold working (cold forging) has the advantages of higher productivity than hot working and warm working, and good dimensional accuracy and yield of steel materials. However, the problem in manufacturing parts by such cold working is that in order to ensure the strength of the cold-worked parts to be higher than the expected value, the strength, ie deformation, is inevitably required. It is necessary to use a steel material with high resistance. However, the higher the deformation resistance of the steel material to be used, there is a difficulty in reducing the life of the cold working mold.

上記背景より、トランスミッション部品の分野では、従来からの棒鋼の鍛造品(熱間鍛造、冷間鍛造等)から、部品の軽量化や低コスト化を狙いとして鋼板を用いた部品製造に切り替える検討も進んでいる。中でも、ギアやダンパー、クラッチ等で表面に面圧がかかる部品では、耐摩耗性や耐疲労特性を付与するために、鋼板を部品加工した後に浸炭熱処理することにより、表面硬さを高くすることが行われている。これらの部品製造用の鋼板としては、従来は一般的な軟鋼(SPHCなど)が用いられていたが、さらなる高強度化、高硬度化が求められている。   From the above background, in the field of transmission parts, it is also considered to switch from conventional steel bar forgings (hot forging, cold forging, etc.) to parts manufacturing using steel plates with the aim of reducing the weight and cost of parts. Progressing. Above all, in parts where surface pressure is applied by gears, dampers, clutches, etc., the surface hardness should be increased by carburizing heat treatment after the steel plate is processed to provide wear resistance and fatigue resistance. Has been done. Conventionally, a general mild steel (SPHC or the like) has been used as a steel plate for manufacturing these parts, but higher strength and higher hardness are required.

鋼板を所定形状に冷間加工(プレス成形など)した後、浸炭熱処理を行うことで、所定の強度、表面硬さが確保された高強度部品が製造される。浸炭表面の硬度を上げるためには、C量を中心とした主要成分や添加元素の量を増加させることが考えられるが、そうすると熱処理前の冷間加工性が低下する。したがって、冷間加工性の確保と浸炭熱処理後の表面硬さの向上を両立させる解決策が望まれていた。   After the steel sheet is cold-worked into a predetermined shape (press forming or the like), carburizing heat treatment is performed to manufacture a high-strength component with a predetermined strength and surface hardness. In order to increase the hardness of the carburized surface, it is conceivable to increase the amount of main components and additive elements centering on the amount of C. However, if this is done, the cold workability before heat treatment is reduced. Therefore, there has been a demand for a solution that ensures both cold workability and improved surface hardness after carburizing heat treatment.

上述したように、本発明は熱延鋼板を対象とするが、熱延鋼板に関する従来技術として例えば下記特許文献2〜6が挙げられる。   As described above, the present invention is directed to hot-rolled steel sheets, but the following Patent Documents 2 to 6 are given as conventional techniques related to hot-rolled steel sheets.

特許文献2に開示された熱延鋼板は、面積割合で金属組織の70%以上がフェライト相で、その平均結晶粒径が50μm以下、アスペクト比が3以下であり、さらにフェライト粒界の70%以上が大角粒界からなり、大角粒界で形成されたフェライト相の最大径が30μm 以下であり、かつ最小径が5nm以上の析出物の面積割合が金属組織の2% 以下で、フェライト相と析出物とを除く残部相のなかで面積割合が最大である第二相の平均結晶粒径が50μ m 以下であり、最も近い第二相間にフェライト相の大角粒界が存在するものとすることで、強度と伸びフランジ性のバランスが向上するとされている。   The hot-rolled steel sheet disclosed in Patent Document 2 has an area ratio of 70% or more of the metal structure in the ferrite phase, an average crystal grain size of 50 μm or less, an aspect ratio of 3 or less, and 70% of the ferrite grain boundary. The above is composed of large-angle grain boundaries, the maximum diameter of the ferrite phase formed at the large-angle grain boundaries is 30 μm or less, and the area ratio of precipitates with a minimum diameter of 5 nm or more is 2% or less of the metal structure. The average crystal grain size of the second phase having the largest area ratio among the remaining phases excluding precipitates is 50 μm or less, and there is a large-angle grain boundary of the ferrite phase between the nearest second phases. It is said that the balance between strength and stretch flangeability is improved.

特許文献3に開示された熱延鋼板は、フェライト平均粒径が1〜10μm、フェライト粒径の標準偏差が3.0μm以下、介在物の形状比が2.0以下であるものとすることで、伸びフランジ性が向上するとされている。   The hot-rolled steel sheet disclosed in Patent Document 3 has an average ferrite grain size of 1 to 10 μm, a standard deviation of ferrite grain size of 3.0 μm or less, and an inclusion shape ratio of 2.0 or less. It is said that stretch flangeability is improved.

特許文献4に開示された熱延鋼板は、組織が、フェライト相分率が50%以上、残ベイナイトのフェライト・ベイナイト組織であって、板厚tの1/8t〜3/8tの範囲でのMnミクロ偏析が、0.10≧σ/Mnを満たす範囲にあるものとすることで、伸びフランジ性が向上するとされている。   The hot-rolled steel sheet disclosed in Patent Literature 4 has a ferrite phase fraction of 50% or more and a residual bainite ferrite-bainite structure, in a range of 1 / 8t to 3 / 8t of the sheet thickness t. It is said that stretch flangeability is improved when the Mn microsegregation is in a range satisfying 0.10 ≧ σ / Mn.

特許文献5に開示された熱延鋼板は、組織が、フェライト相の面積率が20%以上、焼戻しマルテンサイト相の面積率が10〜60%、マルテンサイト相の面積率が0〜10%、残留オーステナイト相の体積率が3〜15%であるものとすることで、伸びおよび伸びフランジ性が向上するとされている。   The hot-rolled steel sheet disclosed in Patent Document 5 has a structure in which the area ratio of the ferrite phase is 20% or more, the area ratio of the tempered martensite phase is 10 to 60%, the area ratio of the martensite phase is 0 to 10%, By setting the volume ratio of the retained austenite phase to 3 to 15%, elongation and stretch flangeability are improved.

しかしながら、上記特許文献2〜5に開示された熱延鋼板は、冷間加工性に優れるものの、浸炭熱処理後の表面硬さについては何ら言及がなく、その改善効果は不明である。   However, although the hot-rolled steel sheets disclosed in Patent Documents 2 to 5 are excellent in cold workability, there is no mention of the surface hardness after carburizing heat treatment, and the improvement effect is unknown.

一方、特許文献6に開示された熱延鋼板(浸炭鋼帯)は、板厚方向表層部の50μm深さまでの平均硬さが170HV以上で、且つ、金属組織がフェライト+パーライトであり、表面炭素濃度CS(質量%)と鋼中平均炭素濃度CM(質量%)の差ΔC=CS−CMが0.1質量%以上であるものとすることで、打ち抜き時の「だれ」を軽減するとともに、打ち抜き後の浸炭処理を省略できるとされている。   On the other hand, the hot-rolled steel sheet (carburized steel strip) disclosed in Patent Document 6 has an average hardness of 170 HV or more up to a depth of 50 μm in the thickness direction surface layer portion, and has a metal structure of ferrite + pearlite, surface carbon The difference ΔC = CS-CM between the concentration CS (mass%) and the average carbon concentration CM (mass%) in the steel is 0.1 mass% or more, thereby reducing the “dripping” at the time of punching, It is said that carburizing after punching can be omitted.

しかしながら、上記特許文献6に開示された熱延鋼板(浸炭鋼帯)は、浸炭熱処理後の表面硬さに優れるものの、冷間加工性については何ら言及がなく、その改善効果は不明である。   However, although the hot-rolled steel sheet (carburized steel strip) disclosed in Patent Document 6 is excellent in surface hardness after carburizing heat treatment, there is no mention of cold workability and its improvement effect is unknown.

上記のように、冷間加工性と浸炭熱処理後の表面硬さを兼備する熱延鋼板については、これまでほとんど検討がなされていなかった。   As mentioned above, the hot-rolled steel sheet having both the cold workability and the surface hardness after the carburizing heat treatment has been hardly studied so far.

特許第3094856号公報Japanese Patent No. 3094856 特許第3821036号公報Japanese Patent No. 3821036 特許第4276504号公報Japanese Patent No. 4276504 特許第4644075号公報Japanese Patent No. 4644075 特開2011−168861号公報JP 2011-168861 A 特開2010−222663号公報JP 2010-222663 A

そこで、本発明は、冷間加工性と浸炭熱処理後の表面硬さを兼備する熱延鋼板を提供することを目的とする。   Therefore, an object of the present invention is to provide a hot-rolled steel sheet that has both cold workability and surface hardness after carburizing heat treatment.

請求項1に記載の発明は、
板厚が2〜10mmであり、
成分組成が、
質量%で(以下、化学成分について同じ。)、
C :0.05〜0.30%、
Mn:0.3〜3.0%、
Al:0.015〜0.1%、
N :0.003〜0.030%を含み、
残部は鉄および不可避的不純物からなり、
組織が、
面積率で、
フェライト:10〜50%、
パーライト:15〜50%、
残部:ベイナイトからなり、
前記フェライトおよびパーライトを含む全ての相の結晶粒(以下、「全結晶粒」という。)について、
アスペクト比(長軸/短軸)が3以下の結晶粒の個数が前記全結晶粒の個数の60%以上であるとともに、前記全結晶粒の平均結晶粒径が3〜50μmの範囲である
ことを特徴とする冷間加工性と浸炭熱処理後の表面硬さに優れる熱延鋼板である。
The invention described in claim 1
The plate thickness is 2 to 10 mm,
Ingredient composition
% By mass (hereinafter the same for chemical components)
C: 0.05 to 0.30%
Mn: 0.3-3.0%
Al: 0.015-0.1%
N: 0.003 to 0.030% included,
The balance consists of iron and inevitable impurities,
Organization
In area ratio,
Ferrite: 10-50%,
Perlite: 15-50%,
The rest: bainite
About crystal grains of all phases containing the ferrite and pearlite (hereinafter referred to as “all crystal grains”),
The number of crystal grains having an aspect ratio (major axis / minor axis) of 3 or less is 60% or more of the number of all crystal grains, and the average crystal grain size of all the crystal grains is in the range of 3 to 50 μm. Is a hot-rolled steel sheet having excellent cold workability and surface hardness after carburizing heat treatment.

請求項2に記載の発明は、
成分組成が、さらに、
Cr:3.0%以下(0%を含まない)、
Mo:1.0%以下(0%を含まない)、
Ni:3.0%以下(0%を含まない)よりなる群から選択される少なくとも1種
を含むものである請求項1に記載の熱延鋼板である。
The invention described in claim 2
Ingredient composition further
Cr: 3.0% or less (excluding 0%),
Mo: 1.0% or less (excluding 0%),
The hot-rolled steel sheet according to claim 1, comprising at least one selected from the group consisting of Ni: 3.0% or less (excluding 0%).

請求項3に記載の発明は、
前記不可避的不純物のうち、Si:0.5%以下、P:0.030%以下、S:0.035%以下である請求項1または2に記載の熱延鋼板である。
The invention according to claim 3
3. The hot rolled steel sheet according to claim 1, wherein among the inevitable impurities, Si is 0.5% or less, P is 0.030% or less, and S is 0.035% or less.

請求項4に記載の発明は、
成分組成が、さらに、
Cu:2.0%以下(0%を含まない)、および/または、
Co:5%以下(0%を含まない)
を含むものである請求項1〜3のいずれか1項に記載の熱延鋼板である。
The invention according to claim 4
Ingredient composition further
Cu: 2.0% or less (excluding 0%), and / or
Co: 5% or less (excluding 0%)
It is a hot-rolled steel plate of any one of Claims 1-3.

請求項5に記載の発明は、
成分組成が、さらに、
V:0.5%以下(0%を含まない)、
Ti:0.1%以下(0%を含まない)、
Nb:0.1%以下(0%を含まない)よりなる群から選ばれる少なくとも1種
を含むものである請求項1〜4のいずれか1項に記載の熱延鋼板である。
The invention described in claim 5
Ingredient composition further
V: 0.5% or less (excluding 0%),
Ti: 0.1% or less (excluding 0%),
5. The hot-rolled steel sheet according to claim 1, comprising at least one selected from the group consisting of Nb: 0.1% or less (not including 0%).

請求項6に記載の発明は、
成分組成が、さらに、
Ca:0.08%以下(0%を含まない)、および/または、
Zr:0.08%以下(0%を含まない)
を含むものである請求項1〜5のいずれか1項に記載の熱延鋼板である。
The invention described in claim 6
Ingredient composition further
Ca: 0.08% or less (excluding 0%), and / or
Zr: 0.08% or less (excluding 0%)
It is a hot-rolled steel plate of any one of Claims 1-5.

請求項7に記載の発明は、
成分組成が、さらに、
Sb:0.02%以下(0%を含まない)
を含むものである請求項1〜6のいずれか1項に記載の熱延鋼板である。
The invention described in claim 7
Ingredient composition further
Sb: 0.02% or less (excluding 0%)
It is a hot-rolled steel plate of any one of Claims 1-6.

請求項8に記載の発明は、
成分組成が、さらに、
REM:0.05%以下(0%を含まない)、
Mg:0.02%以下(0%を含まない)、
Li:0.02%以下(0%を含まない)、
Pb:0.5%以下(0%を含まない)、
Bi:0.5%以下(0%を含まない)よりなる群から選ばれる少なくとも1種
を含むものである請求項1〜7のいずれか1項に記載の熱延鋼板である。
The invention according to claim 8 provides:
Ingredient composition further
REM: 0.05% or less (excluding 0%),
Mg: 0.02% or less (excluding 0%),
Li: 0.02% or less (excluding 0%),
Pb: 0.5% or less (excluding 0%),
Bi: The hot-rolled steel sheet according to any one of claims 1 to 7, which contains at least one selected from the group consisting of 0.5% or less (not including 0%).

本発明によれば、フェライト+パーライト主体の組織において、結晶粒を等軸化かつ微細化することで、冷間加工性を確保しつつ、浸炭熱処理後に所定の表面硬さが得られる熱延鋼板を提供できるようになった。   According to the present invention, in a structure mainly composed of ferrite and pearlite, a hot rolled steel sheet that has a predetermined surface hardness after carburizing heat treatment while ensuring cold workability by equiaxing and refining crystal grains. Can now be provided.

以下、本発明に係る熱延鋼板(以下、「本発明鋼板」、あるいは、単に「鋼板」ともいう。)について、さらに詳細に説明する。本発明鋼板は、上記特許文献1に記載された熱間鍛造材(高強度高靭性肌焼き用鋼)と成分組成が重複するが、組織をフェライト+パーライト主体組織とするとともに、結晶粒を等軸化かつ微細化する点で異なっている。   Hereinafter, the hot-rolled steel sheet according to the present invention (hereinafter also referred to as “the steel sheet of the present invention” or simply “the steel sheet”) will be described in more detail. The steel sheet of the present invention overlaps with the hot forging material (high strength and high toughness case hardening steel) described in Patent Document 1 above, but the structure is ferrite + pearlite main structure and the crystal grains are the same. It differs in that it is made axial and finer.

〔本発明鋼板の板厚:2〜10mm〕
まず、本発明鋼板は、板厚が2〜10mmのものを対象とする。板厚が2mm未満では、構造体としての剛性が確保できなくなる。一方、板厚が10mmを超えると、本発明で規定する組織形態を達成することが難しく、所望の効果が得られなくなる。好ましい板厚は3〜9mm、さらに好ましい板厚は4〜7mmである。
[Thickness of the steel plate of the present invention: 2 to 10 mm]
First, the steel sheet of the present invention has a thickness of 2 to 10 mm. If the plate thickness is less than 2 mm, rigidity as a structure cannot be secured. On the other hand, if the plate thickness exceeds 10 mm, it is difficult to achieve the tissue form defined in the present invention, and the desired effect cannot be obtained. A preferable plate thickness is 3 to 9 mm, and a more preferable plate thickness is 4 to 7 mm.

次に、本発明鋼板を構成する成分組成について説明する。以下、化学成分の単位はすべて質量%である。   Next, the component composition which comprises this invention steel plate is demonstrated. Hereinafter, all the units of chemical components are mass%.

〔本発明鋼板の成分組成〕
<C :0.05〜0.30%>
Cは、最終的に得られる浸炭(もしくは浸炭窒化)焼入れ部品としての芯部強度を確保するうえで欠くことのできない元素であり、0.05%未満では十分な強度が得られなくなる。しかし、過剰に含有させると靭性が劣化するほか、被削性や冷間鍛造性が低下して加工性を損なうので0.30%を上限とする。Cの好ましい含有量は0.08〜0.25%の範囲である。
[Component composition of the steel sheet of the present invention]
<C: 0.05 to 0.30%>
C is an element indispensable for securing the core strength of the carburized (or carbonitrided) quenching part finally obtained. If it is less than 0.05%, sufficient strength cannot be obtained. However, if it is contained excessively, the toughness is deteriorated, and the machinability and cold forgeability are deteriorated and the workability is impaired, so 0.30% is made the upper limit. The preferable content of C is in the range of 0.08 to 0.25%.

<Mn:0.3〜3.0%>
Mnは、溶鋼の脱酸に有効な元素であり、その効果を有効に発揮させるには0.3%以上含有させなければならないが、過度に含有させると、冷間加工性や被削性に悪影響を与えるとともに、結晶粒界への偏析量の増大によって粒界強度を低下させ、ひいては衝撃特性に悪影響を及ぼすようになるので、3.0%以下に抑えなければならない。Mnの好ましい含有量は0.5〜2.0%の範囲である。
<Mn: 0.3 to 3.0%>
Mn is an element effective for deoxidation of molten steel, and in order to exert its effect effectively, it must be contained in an amount of 0.3% or more. However, if it is excessively contained, cold workability and machinability are reduced. In addition to having an adverse effect and increasing the amount of segregation to the crystal grain boundary, it lowers the grain boundary strength and thus adversely affects the impact characteristics, so it must be suppressed to 3.0% or less. A preferable content of Mn is in the range of 0.5 to 2.0%.

<Al:0.015〜0.1%>
Alは鋼材の脱酸材として鋼中に含まれてくる元素であり、鋼中のNと結合してAlNを生成し、結晶粒の粗大化を防止する作用を有している。こうした効果を有効に発揮させるには0.015%以上含有させなければならないが、その効果は0.1%程度で飽和し、それを超えると酸素と結合して非金属系介在物となり、衝撃特性等に悪影響を及ぼすようになるので、0.1%を上限と定めた。 好ましくは0.08%以下であり、さらに好ましくは0.06%以下、特に好ましくは0.04%以下である。
<Al: 0.015-0.1%>
Al is an element contained in steel as a deoxidizing material for steel, and has an action of binding to N in steel to produce AlN and preventing coarsening of crystal grains. In order to exert such an effect effectively, it must be contained at 0.015% or more, but the effect is saturated at about 0.1%, and beyond that, it combines with oxygen to form a non-metallic inclusion, Since it adversely affects the characteristics, etc., the upper limit was set to 0.1%. Preferably it is 0.08% or less, More preferably, it is 0.06% or less, Most preferably, it is 0.04% or less.

<N :0.003〜0.030%>
Nは鋼中でAl,V,Ti,Nb等と結合して窒化物を生成し、結晶粒の粗大化を抑制する作用を有しており、その効果は0.003%以上含有させることによって有効に発揮される。好ましくは、0.005%以上である。しかし、それらの効果は約0.030%で飽和し、それ以上に含有させると窒化物が介在物となって物性に悪影響を及ぼすようになるので、それ以上の添加は避けなければならない。好ましくは0.02%以下であり、さらに好ましくは0.015%以下である。
<N: 0.003-0.030%>
N combines with Al, V, Ti, Nb, etc. in steel to produce nitrides, and has the effect of suppressing the coarsening of crystal grains. The effect is by containing 0.003% or more. Effectively demonstrated. Preferably, it is 0.005% or more. However, these effects saturate at about 0.030%, and if it is contained more than that, nitrides become inclusions and adversely affect the physical properties, so addition beyond this must be avoided. Preferably it is 0.02% or less, More preferably, it is 0.015% or less.

本発明鋼板は上記成分を基本的に含有し、残部が鉄および不可避的不純物であるが、不可避的に混入してくるSi,PおよびSは、下記の理由からそれぞれできるだけ少なく抑えることが望ましい。   The steel sheet of the present invention basically contains the above components, and the balance is iron and unavoidable impurities, but it is desirable to keep Si, P and S mixed unavoidably as small as possible for the following reasons.

<Si:0.5%以下>
Siは、強化元素あるいは脱酸性元素として有効に作用する反面、粒界酸化を助長して曲げ疲労特性を劣化させるとともに冷間鍛造性にも悪影響を及ぼす。したがってこうした障害をなくすにはその含有量を0.5%以下に抑えなければならず、特に高レベルの曲げ疲労特性が求められるときは、その含有量を0.1%以下に抑えることが望まれる。こうした観点から、Siのより好ましい含有量は0.02〜0.1%の範囲である。
<Si: 0.5% or less>
Si effectively acts as a strengthening element or a deacidifying element, but promotes grain boundary oxidation to deteriorate bending fatigue properties and adversely affects cold forgeability. Therefore, in order to eliminate such obstacles, the content must be suppressed to 0.5% or less, and particularly when a high level of bending fatigue characteristics is required, it is desirable to suppress the content to 0.1% or less. It is. From such a viewpoint, the more preferable content of Si is in the range of 0.02 to 0.1%.

<P:0.030%以下>
Pは結晶粒界に偏析して靭性を低下させるので、その上限は0.030%と定めた。Pのより好ましい含有量は0.020%以下、さらに好ましくは0.010%以下である。
<P: 0.030% or less>
P segregates at the grain boundaries and lowers the toughness, so the upper limit was set to 0.030%. The more preferable content of P is 0.020% or less, and further preferably 0.010% or less.

<S:0.035%以下>
SはMnSを生成し、被削性の向上に寄与するが、本発明を歯車等に適用する場合は、縦目の衝撃特性だけでなく横目の衝撃特性も重要であり、横目の衝撃特性向上には異方性の低減が必要となり、そのためにはS含有量を0.035%以下に抑えなければならない。Sのより好ましい含有量は0.025%以下、さらに好ましくは0.020%以下である。
<S: 0.035% or less>
S generates MnS and contributes to the improvement of machinability. However, when the present invention is applied to gears and the like, not only the impact characteristics of the vertical eyes but also the impact characteristics of the horizontal eyes are important, and the impact characteristics of the horizontal eyes are improved. In order to reduce the anisotropy, the S content must be suppressed to 0.035% or less. The more preferable content of S is 0.025% or less, and further preferably 0.020% or less.

また本発明鋼板には、上記の基本成分に加えて、本発明の作用を損なわない範囲で、以下の許容成分を含有させることができる。   Moreover, in addition to said basic component, this invention steel plate can be made to contain the following allowable components in the range which does not impair the effect | action of this invention.

<Cr:3.0%以下(0%を含まない)、
Mo:1.0%以下(0%を含まない)、
Ni:3.0%以下(0%を含まない)よりなる群から選択される少なくとも1種>
これらの元素は、焼入性を高めあるいは焼入れ組織を微細化する作用を有する点で有用元素であり、特にCrは優れた焼入性向上効果を有しており、またMoは不完全焼入れ組織の低減と焼入性の向上、さらには粒界強度の向上に有効に作用し、さらにNiは焼入れ後の組織を微細化して耐衝撃性の向上に寄与する。こうした効果は、好ましくはCr:0.2%程度以上、Mo:0.08%程度以上、Ni:0.2%程度以上のうち1少なくとも1種を含有させることによって有効に発揮されるが、Cr量が3.0%を超えるとCrが炭化物を生成して粒界偏析を起こし、粒界強度を低下させて靭性に悪影響を及ぼし、Moの上記効果は約1.0%で飽和し、またNiの上記効果も3.0%で飽和するので、それ以上の添加は経済的に全く無駄である。
<Cr: 3.0% or less (excluding 0%),
Mo: 1.0% or less (excluding 0%),
Ni: at least one selected from the group consisting of 3.0% or less (not including 0%)>
These elements are useful elements in that they have a function of enhancing hardenability or refining the hardened structure, particularly Cr has an excellent effect of improving hardenability, and Mo is an incompletely hardened structure. This effectively acts to improve the hardenability and the grain boundary strength, and Ni contributes to the improvement of impact resistance by refining the structure after quenching. Such an effect is preferably exhibited by including at least one of Cr: about 0.2% or more, Mo: about 0.08% or more, Ni: about 0.2% or more, If the Cr content exceeds 3.0%, Cr generates carbides and segregates at the grain boundaries, lowers the grain boundary strength, adversely affects toughness, and the above effect of Mo is saturated at about 1.0%. Further, the above effect of Ni is saturated at 3.0%, so addition beyond that is economically useless.

<Cu:2.0%以下(0%を含まない)、および/または、
Co:5%以下(0%を含まない)>
Cuは耐食性の向上に有効に作用する元素であり、その効果は好ましくは0.3%以上含有させることによって有効に発揮されるが、その効果は2.0%で飽和するのでそれ以上の含有は無駄である。なおCuを単独で含有させると、鋼材の熱間加工性が悪くなる傾向があるので、こうした弊害を回避するには、熱間加工性向上効果を有するNiを前記含有量の範囲で併用することが望ましい。
<Cu: 2.0% or less (not including 0%), and / or
Co: 5% or less (excluding 0%)>
Cu is an element that effectively acts to improve corrosion resistance, and the effect is preferably exerted by inclusion of 0.3% or more, but the effect is saturated at 2.0%, so it is contained more than that. Is useless. If Cu is contained alone, the hot workability of the steel material tends to deteriorate. Therefore, in order to avoid such adverse effects, Ni having the effect of improving the hot workability should be used in the above content range. Is desirable.

またCuとCoは、いずれも鋼材をひずみ時効させ、硬化させる作用があり、加工後強度を向上させるのに有効な元素である。このような作用を有効に発揮させるためには、これらの元素は、それぞれ0.1%以上、さらには0.3%以上含有させることが好ましい。しかし、Coの含有量が過剰であると、鋼材をひずみ時効および硬化させる効果、さらに、加工後強度を向上させる効果が飽和し、また、割れを促進させるおそれがあるため、Coの含有量は5%以下、さらには4%以下、特に3%以下とすることが推奨される。   Cu and Co are both effective in strain-aging and hardening the steel and are effective in improving the strength after processing. In order to effectively exhibit such an action, these elements are preferably contained in an amount of 0.1% or more, and more preferably 0.3% or more. However, if the Co content is excessive, the effects of strain aging and hardening of the steel material, and the effect of improving the strength after processing are saturated, and there is a possibility of promoting cracking, so the Co content is It is recommended to set it to 5% or less, further 4% or less, especially 3% or less.

<V:0.5%以下(0%を含まない)、
Ti:0.1%以下(0%を含まない)、
Nb:0.1%以下(0%を含まない)よりなる群から選ばれる少なくとも1種>
これらの元素はCやNと結合して炭化物や窒化物を生成し、結晶粒を微細化して靭性(耐衝撃性)の向上に寄与するが、それぞれ上限値付近でその効果は飽和し、かえって被削性や冷間加工性に悪影響を及ぼすおそれがでてくるので、それぞれ上限値以下に抑えなければならない。これら元素の添加効果を有効に発揮させるための好ましい下限値はV:0.03%程度、Ti:0.005%程度およびNb:0.005%程度である。
<V: 0.5% or less (excluding 0%),
Ti: 0.1% or less (excluding 0%),
Nb: at least one selected from the group consisting of 0.1% or less (not including 0%)>
These elements combine with C and N to form carbides and nitrides, refine the crystal grains and contribute to the improvement of toughness (impact resistance), but the effect is saturated near the upper limit, respectively. Since there is a risk of adversely affecting the machinability and cold workability, each must be suppressed to the upper limit value or less. Preferable lower limit values for effectively exhibiting the effect of addition of these elements are V: about 0.03%, Ti: about 0.005% and Nb: about 0.005%.

< Ca:0.08%以下(0%を含まない)、および/または、
Zr:0.08%以下(0%を含まない)>
Caは、硬質の介在物を柔軟な介在物で包み込み、またZrはMnSを球状化させ、いずれも被削性の向上に寄与するほか、両元素ともMnSの球状化による異方性の低減によって横目の衝撃特性を高める作用を有しているが、それらの効果はそれぞれ0.08%で飽和するので、それぞれ0.08%以下、さらには0.05%以下、特に0.01%以下とすることが推奨される。なおこれらの元素の上記効果を有効に発揮させるための好ましい下限値は、Ca:0.0005%程度(さらには0.001%程度)、Zr:0.002%程度である。
<Ca: 0.08% or less (excluding 0%), and / or
Zr: 0.08% or less (excluding 0%)>
Ca wraps hard inclusions with flexible inclusions, and Zr spheroidizes MnS, both of which contribute to improving machinability, and both elements are reduced by anisotropy due to spheroidization of MnS. Although it has the effect | action which improves the impact characteristic of a horizontal eye, since those effects are saturated at 0.08%, respectively, it is 0.08% or less, Furthermore, 0.05% or less, Especially 0.01% or less It is recommended to do. In addition, the preferable lower limit for effectively exhibiting the above effects of these elements is about Ca: 0.0005% (further about 0.001%) and Zr: about 0.002%.

<Sb:0.02%以下(0%を含まない)>
Sbは、粒界酸化を抑制して曲げ疲労強度を高めるうえで有効な元素であるが、その効果は0.02%で飽和するので、それ以上の添加は経済的に無駄である。該Sbの添加効果を有効に発揮させるための好ましい下限値は0.001%程度である。
<Sb: 0.02% or less (excluding 0%)>
Sb is an element effective for suppressing the grain boundary oxidation and increasing the bending fatigue strength, but since the effect is saturated at 0.02%, the addition of more is economically useless. A preferable lower limit value for effectively exhibiting the effect of adding Sb is about 0.001%.

<REM:0.05%以下(0%を含まない)、
Mg:0.02%以下(0%を含まない)、
Li:0.02%以下(0%を含まない)、
Pb:0.5%以下(0%を含まない)、
Bi:0.5%以下(0%を含まない)よりなる群から選ばれる少なくとも1種>
REMは、Caと同様にMnSなどの硫化化合物系介在物を球状化させ、鋼の変形能を高めるとともに、被削性の向上に寄与する元素である。このような作用を有効に発揮させるためには、REMは、0.0005%以上、さらには0.001%以上含有させることが好ましい。しかし、過剰に含有しても、その効果が飽和し、含有量に見合う効果が期待できないため、0.05%以下、さらには0.03%以下、特に0.01質量%以下が推奨される。
なお、本発明において、REMとは、ランタノイド元素(LaからLnまでの15元素)およびSc(スカンジウム)とY(イットリウム)を含む意味である。これらの元素のなかでも、La、CeおよびYよりなる群から選ばれる少なくとも1種の元素を含有することが好ましく、より好ましくはLaおよび/またはCeを含有するのがよい。
<REM: 0.05% or less (excluding 0%),
Mg: 0.02% or less (excluding 0%),
Li: 0.02% or less (excluding 0%),
Pb: 0.5% or less (excluding 0%),
Bi: at least one selected from the group consisting of 0.5% or less (excluding 0%)>
REM is an element that, like Ca, spheroidizes compound inclusions such as MnS to increase the deformability of steel and contribute to the improvement of machinability. In order to effectively exhibit such an action, REM is preferably contained in an amount of 0.0005% or more, more preferably 0.001% or more. However, even if contained excessively, the effect is saturated, and an effect commensurate with the content cannot be expected, so 0.05% or less, further 0.03% or less, particularly 0.01% by mass or less is recommended. .
In the present invention, REM means a lanthanoid element (15 elements from La to Ln), Sc (scandium) and Y (yttrium). Among these elements, it is preferable to contain at least one element selected from the group consisting of La, Ce and Y, more preferably La and / or Ce.

Mgは、Caと同様にMnSなどの硫化化合物系介在物を球状化させ、鋼の変形能を高めるとともに、被削性の向上に寄与する元素である。このような作用を有効に発揮させるためには、Mgは、0.0002%以上、さらには0.0005%以上含有させることが好ましい。しかし、過剰に含有しても、その効果が飽和し、含有量に見合う効果が期待できないため、0.02%以下、さらには0.015%以下、特に0.01%以下が推奨される。   Mg, like Ca, is an element that spheroidizes sulfide-based inclusions such as MnS to increase the deformability of steel and contribute to the improvement of machinability. In order to effectively exhibit such an action, Mg is preferably contained in an amount of 0.0002% or more, more preferably 0.0005% or more. However, even if contained excessively, the effect is saturated and an effect commensurate with the content cannot be expected, so 0.02% or less, further 0.015% or less, and particularly 0.01% or less is recommended.

Liは、Caと同様にMnSなどの硫化化合物系介在物を球状化させ、鋼の変形能を高めることができ、また、Al系酸化物を低融点化して無害化して被削性の向上に寄与する元素である。このような作用を有効に発揮させるためには、Liは、0.0002%以上、さらには0.0005%以上含有させることが好ましい。しかし、過剰に含有しても、その効果が飽和し、含有量に見合う効果が期待できないため、0.02%以下、さらには0.015%以下、特に0.01%以下が推奨される。   Li can spheroidize sulfide compound inclusions such as MnS and increase the deformability of steel like Ca, and lower the melting point of Al-based oxides to make them harmless and improve machinability. It is a contributing element. In order to effectively exhibit such an action, Li is preferably contained in an amount of 0.0002% or more, and more preferably 0.0005% or more. However, even if contained excessively, the effect is saturated and an effect commensurate with the content cannot be expected, so 0.02% or less, further 0.015% or less, and particularly 0.01% or less is recommended.

Pbは、被削性を向上させるために有効な元素である。このような作用を有効に発揮させるためには、Pbは0.005%以上、さらには0.01%以上含有させることが好ましい。しかし、過剰に含有させると、圧延疵の発生等の製造上の問題を生じるため、0.5%以下、さらには0.4%以下、特に0.3質量%以下が推奨される。   Pb is an effective element for improving machinability. In order to effectively exhibit such an action, Pb is preferably contained in an amount of 0.005% or more, and more preferably 0.01% or more. However, if it is contained excessively, production problems such as the occurrence of rolling defects occur, so 0.5% or less, further 0.4% or less, and particularly 0.3% by mass or less are recommended.

Biは、Pbと同様に、被削性を向上させるために有効な元素である。このような作用を有効に発揮させるためには、Biは0.005%以上、さらには0.01%以上含有させることが好ましい。しかし、過剰に含有させても被削性向上の効果が飽和するため、0.5質量%以下、さらには0.4%以下、特に0.3%以下が推奨される。   Bi is an element effective for improving the machinability like Pb. In order to effectively exhibit such an action, Bi is preferably contained in an amount of 0.005% or more, and more preferably 0.01% or more. However, since the effect of improving the machinability is saturated even if contained excessively, 0.5% by mass or less, further 0.4% or less, particularly 0.3% or less is recommended.

次に、本発明鋼板を特徴づける組織について説明する。   Next, the structure characterizing the steel sheet of the present invention will be described.

〔本発明鋼板の組織〕
上述したとおり、本発明鋼板は、フェライト+パーライトを主要組織とするものであるが、特に、フェライトおよびパーライトを含む全ての相の結晶粒について、等軸化の程度とサイズをそれぞれ特定範囲に制御することを特徴とする。
[Structure of the steel sheet of the present invention]
As described above, the steel sheet of the present invention is mainly composed of ferrite and pearlite, and in particular, the degree of equiaxing and the size of crystal grains of all phases including ferrite and pearlite are controlled within a specific range. It is characterized by doing.

<組織が、フェライト:10〜50%、パーライト:15〜50%、残部:ベイナイトからなる>
相の割合は、鋼板の強度レベルを決める重要な因子である。本発明では、冷間加工性と熱処理後の板厚中心部における母材強度を確保する観点から、引張強度で350〜700MPa程度とする必要がある。引張強度が350MPa未満では、浸炭熱処理後でも、表面硬さは確保されるが、表面から深い部分の硬さ不足、板厚中心部の強度、硬さが不足する。一方、引張強度が700MPaを超えると、熱処理前の冷間加工性が確保できない。
強度レベルに対応して、フェライトが少なすぎる、および/または、パーライトが多すぎると、引張強度が高くなりすぎて、成形ができない。一方、フェライトが多すぎる、および/または、パーライトが少なすぎると、ベース強度が不足し、板厚中心部での強度が不足し、疲労強度が低下する。このため、組織が、面積率で、フェライト:10〜50%、パーライト:15〜50%からなるものとする。なお、残部はベイナイトである。
<Structure is composed of ferrite: 10 to 50%, pearlite: 15 to 50%, balance: bainite>
The proportion of phases is an important factor that determines the strength level of the steel sheet. In the present invention, it is necessary to set the tensile strength to about 350 to 700 MPa from the viewpoint of ensuring the cold workability and the strength of the base material at the center of the plate thickness after the heat treatment. When the tensile strength is less than 350 MPa, the surface hardness is ensured even after the carburizing heat treatment, but the hardness of the portion deep from the surface is insufficient, and the strength and hardness of the center portion of the plate thickness are insufficient. On the other hand, if the tensile strength exceeds 700 MPa, cold workability before heat treatment cannot be ensured.
Corresponding to the strength level, if there is too little ferrite and / or too much pearlite, the tensile strength becomes too high and molding is impossible. On the other hand, when there is too much ferrite and / or too little pearlite, the base strength is insufficient, the strength at the center of the plate thickness is insufficient, and the fatigue strength is reduced. For this reason, a structure shall consist of ferrite: 10-50% and pearlite: 15-50% by an area rate. The balance is bainite.

<アスペクト比(長軸/短軸)が3以下の結晶粒の個数が全結晶粒の個数の60%以上>
結晶粒の形状が等軸粒であることが、伸びフランジ性(穴広げ性)の向上と、熱処理後における表面から深い部分である板厚中心部の硬さの確保の両立に必要である。このため、等軸粒である、アスペクト比(長軸/短軸)が3以下の結晶粒の個数を、全結晶粒の個数の60%以上、好ましくは70%以上、さらに好ましくは80%以上とする。ここに、「全結晶粒」とは、上記フェライトおよびパーライトを含む全ての相の結晶粒を意味するものとする。
<The number of crystal grains with an aspect ratio (major axis / minor axis) of 3 or less is 60% or more of the total number of crystal grains>
It is necessary for the shape of the crystal grains to be equiaxed grains in order to improve both stretch flangeability (hole expandability) and secure the hardness of the center of the plate thickness, which is a deep portion from the surface after heat treatment. Therefore, the number of crystal grains that are equiaxed grains and have an aspect ratio (major axis / minor axis) of 3 or less is 60% or more, preferably 70% or more, more preferably 80% or more of the total number of crystal grains. And Here, “all crystal grains” mean crystal grains of all phases including the ferrite and pearlite.

<全結晶粒の平均結晶粒径が3〜50μmの範囲>
結晶粒が大きくなりすぎると、表面性状が悪化し、表面割れを起こすとともに、穴拡げ性が劣化する。このため、全結晶粒の平均結晶粒径を、50μm以下、好ましくは40μm以下、さらに好ましくは30μm以下とする。一方、下限値については、結晶粒は微細になるほど特性は良くなるが、圧延能力や冷却能力を高くする必要があり、生産性を低下させる。このため、全結晶粒の平均結晶粒径は3μm以上、好ましくは5μm以上、さらに好ましくは7μm以上とする。
<The average crystal grain size of all crystal grains is in the range of 3 to 50 μm>
If the crystal grains become too large, the surface properties deteriorate, causing surface cracks, and the hole expandability deteriorates. For this reason, the average crystal grain size of all crystal grains is 50 μm or less, preferably 40 μm or less, and more preferably 30 μm or less. On the other hand, as the lower limit value, the finer the crystal grains, the better the characteristics, but it is necessary to increase the rolling capacity and cooling capacity, and the productivity is lowered. For this reason, the average crystal grain size of all crystal grains is 3 μm or more, preferably 5 μm or more, and more preferably 7 μm or more.

〔各相の面積率の測定方法〕
上記各相の面積率については、各供試鋼板をt/4(t:板厚)深さまで研削した後、ナイタール腐食し、走査型電子顕微鏡(SEM;倍率1000倍)により5視野撮影し、フェライトおよびパーライトの各比率を点算法で求めることができる。残部はベイナイトとした。
[Measurement method of area ratio of each phase]
About the area ratio of each said phase, after grind | pulverizing each test steel plate to t / 4 (t: board thickness) depth, it corrodes Nital, and image | photographs 5 visual fields with a scanning electron microscope (SEM; magnification 1000 times), Each ratio of ferrite and pearlite can be obtained by a point calculation method. The balance was bainite.

〔結晶粒のアスペクト比の測定方法〕
上記フェライトおよびパーライトを含む全ての相の結晶粒について、最大フェレ径、最小フェレ径を測定し、その比(長軸/短軸)をアスペクト比と定義した。
[Measurement method of crystal grain aspect ratio]
The maximum ferret diameter and the minimum ferret diameter were measured for the crystal grains of all phases including the ferrite and pearlite, and the ratio (major axis / minor axis) was defined as the aspect ratio.

〔平均結晶粒径の測定方法〕
上記走査型電子顕微鏡による撮像を画像解析することにより全結晶粒について個々の重心直径を求め、この重心直径を全結晶粒の個数で算術平均したものを、全結晶粒の平均結晶粒径とした。
[Measurement method of average crystal grain size]
The image of the above scanning electron microscope is used to perform image analysis to determine the individual centroid diameter for all crystal grains. The centroid diameter is arithmetically averaged by the number of all crystal grains as the average crystal grain size of all crystal grains. .

次に、上記本発明鋼板を得るための好ましい製造方法を以下に説明する。   Next, the preferable manufacturing method for obtaining the said steel plate of this invention is demonstrated below.

〔本発明鋼板の好ましい製造方法〕
本発明鋼板は、例えば、上記成分組成を有する原料鋼を溶解、鋳造してスラブとし、スラブまま、または、表面面削したスラブを、加熱、熱間粗圧延、仕上げ圧延の各工程を経て得られた熱延コイル上がり材として製造することができる。その後、表面状態や板厚精度等の必要条件に応じて、さらに、酸洗、スキンパスを施してもよい。
[Preferred production method of the steel sheet of the present invention]
The steel sheet of the present invention is obtained, for example, by melting and casting raw material steel having the above-mentioned composition to form a slab, and by subjecting the slab as it is or surface chamfered to each step of heating, hot rough rolling, and finish rolling. It can be manufactured as a hot rolled coil rising material. Thereafter, pickling and skin pass may be further performed according to necessary conditions such as surface condition and plate thickness accuracy.

[溶鋼の調製]
まず、溶存酸素量と全酸素量を調整した溶鋼に、所定の順番で所定の合金元素を添加することによって、所望の酸化物を生成させることができる。特に本発明では、粗大な酸化物が生成しないように、溶存酸素量を調整した後、全酸素量を調整することが極めて重要である。
[Preparation of molten steel]
First, a desired oxide can be generated by adding a predetermined alloy element in a predetermined order to molten steel in which the dissolved oxygen amount and the total oxygen amount are adjusted. Particularly in the present invention, it is extremely important to adjust the total oxygen amount after adjusting the dissolved oxygen amount so that coarse oxides are not formed.

溶存酸素とは、酸化物を形成しておらず、溶鋼中に存在するフリーな状態の酸素を意味する。全酸素とは、溶鋼に含まれる全ての酸素、すなわち、フリー酸素と酸化物を形成している酸素の総和を意味する。   Dissolved oxygen means oxygen in a free state that does not form an oxide and exists in molten steel. Total oxygen means the sum of all oxygen contained in molten steel, that is, free oxygen and oxygen forming oxides.

まず、溶鋼の溶存酸素量を0.0010〜0.0060%の範囲に調整する。溶鋼の溶存酸素量が0.0010%未満では、溶鋼中の溶存酸素量が不足するため、Al−O系酸化物を所定量確保することができず、所望のサイズ分布が得られない。また、溶存酸素量が不足すると、REMを添加する場合は、REMが硫化物を形成するため、介在物が粗大となり特性を劣化させる原因となる。したがって、上記溶存酸素量は0.0010%以上とする。上記溶存酸素は、好ましくは0.0013%以上、より好ましくは0.0020%以上である。   First, the dissolved oxygen content of the molten steel is adjusted to a range of 0.0010 to 0.0060%. When the amount of dissolved oxygen in the molten steel is less than 0.0010%, the amount of dissolved oxygen in the molten steel is insufficient, so that a predetermined amount of Al—O-based oxide cannot be secured, and a desired size distribution cannot be obtained. In addition, when the amount of dissolved oxygen is insufficient, when REM is added, REM forms sulfides, so that inclusions become coarse and deteriorate characteristics. Therefore, the dissolved oxygen amount is set to 0.0010% or more. The dissolved oxygen is preferably 0.0013% or more, more preferably 0.0020% or more.

一方、上記溶存酸素量が0.0060%を超えると、溶鋼中の酸素量が多くなりすぎるため、溶鋼中の酸素と上記元素の反応が激しくなって溶製作業上好ましくないばかりか、粗大な酸化物を生成して却って特性を劣化させる。したがって、上記溶存酸素量は0.0060%以下に抑えるべきである。上記溶存酸素量は、好ましくは0.0055%以下、より好ましくは0.0053%以下とする。   On the other hand, if the amount of dissolved oxygen exceeds 0.0060%, the amount of oxygen in the molten steel becomes too large, and the reaction between the oxygen in the molten steel and the above elements becomes violent, which is not preferable for melting work, and is coarse. Oxide is produced and the characteristics are deteriorated. Therefore, the amount of dissolved oxygen should be suppressed to 0.0060% or less. The amount of dissolved oxygen is preferably 0.0055% or less, more preferably 0.0053% or less.

ところで、転炉や電気炉で一次精錬された溶鋼中の溶存酸素量は、通常0.010%を超えている。そこで本発明の製法では、溶鋼中の溶存酸素量を何らかの方法で上記範囲に調整する必要がある。   By the way, the amount of dissolved oxygen in molten steel primarily refined in a converter or electric furnace usually exceeds 0.010%. Therefore, in the production method of the present invention, it is necessary to adjust the amount of dissolved oxygen in the molten steel to the above range by some method.

溶鋼中の溶存酸素量を調整する方法としては、例えばRH式脱ガス精錬装置を用いて真空C脱酸する方法や、SiやMn、Alなどの脱酸性元素を添加する方法などが挙げられ、これらの方法を適宜組み合わせて溶存酸素量を調整してもよい。また、RH式脱ガス精錬装置の代わりに、取鍋加熱式精錬装置や簡易式溶鋼処理設備などを用いて溶存酸素量を調整してもよい。この場合、真空C脱酸による溶存酸素量の調整はできないため、溶存酸素量の調整にはSi等の脱酸性元素を添加する方法を採用すればよい。Si等の脱酸性元素を添加する方法を採用するときは、転炉から取鍋へ出鋼する際に脱酸性元素を添加しても構わない。   Examples of the method for adjusting the amount of dissolved oxygen in the molten steel include a method of vacuum C deoxidation using an RH type degassing refining device, a method of adding a deacidifying element such as Si, Mn, and Al. The amount of dissolved oxygen may be adjusted by appropriately combining these methods. Moreover, you may adjust the amount of dissolved oxygen using a ladle heating type refining apparatus, a simple molten steel processing facility, etc. instead of the RH type degassing refining apparatus. In this case, since the amount of dissolved oxygen cannot be adjusted by vacuum C deoxidation, a method of adding a deacidifying element such as Si may be adopted to adjust the amount of dissolved oxygen. When employing a method of adding a deoxidizing element such as Si, the deoxidizing element may be added when steel is removed from the converter to the ladle.

溶鋼の溶存酸素量を0.0010〜0.0060%の範囲に調整した後は溶鋼を攪拌し、溶鋼中の酸化物を浮上分離することによって溶鋼中の全酸素量を0.0010〜0.0070%に調整する。このように本発明では、溶存酸素量が適切に制御された溶鋼を撹拌し、不要な酸化物を除去してから、粗大な酸化物、すなわち、粗大な介在物の生成を防止できる。   After adjusting the dissolved oxygen content of the molten steel to the range of 0.0010 to 0.0060%, the molten steel is stirred, and the oxide in the molten steel is floated and separated, whereby the total oxygen content in the molten steel is 0.0010 to 0.00. Adjust to 0070%. As described above, in the present invention, the molten steel in which the amount of dissolved oxygen is appropriately controlled is stirred to remove unnecessary oxides, and then generation of coarse oxides, that is, coarse inclusions can be prevented.

上記全酸素量が0.0010%未満では、所望の酸化物量不足になるため、介在物の微細なサイズ分布に寄与する酸化物量を確保することができない。したがって、上記全酸素量は0.0010%以上とする。上記全酸素量は、好ましくは0.0015%以上、より好ましくは0.0018%以上である。   If the total oxygen amount is less than 0.0010%, the desired amount of oxide is insufficient, so that the amount of oxide contributing to the fine size distribution of inclusions cannot be ensured. Therefore, the total oxygen amount is set to 0.0010% or more. The total oxygen amount is preferably 0.0015% or more, more preferably 0.0018% or more.

一方、上記全酸素量が0.0070%を超えると、溶鋼中の酸化物量が過剰となり、粗大な酸化物、すなわち、粗大な介在物が生成して特性が劣化する。したがって、上記全酸素量は0.0070%以下に抑えるべきである。上記全酸素量は、好ましくは0.0060%以下、より好ましくは0.0050%以下とする。   On the other hand, if the total oxygen amount exceeds 0.0070%, the amount of oxide in the molten steel becomes excessive, and coarse oxides, that is, coarse inclusions are generated to deteriorate the characteristics. Therefore, the total oxygen amount should be suppressed to 0.0070% or less. The total oxygen amount is preferably 0.0060% or less, more preferably 0.0050% or less.

溶鋼中の全酸素量は、概ね溶鋼の攪拌時間に相関して変化することから、撹拌時間を調整するなどして制御することができる。具体的には、溶鋼を撹拌し、浮上してきた酸化物を除去した後の溶鋼中の全酸素量を適宜測定しながら、溶鋼中の全酸素量を適切に制御する。   Since the total amount of oxygen in the molten steel changes in correlation with the stirring time of the molten steel, it can be controlled by adjusting the stirring time. Specifically, the total amount of oxygen in the molten steel is appropriately controlled while appropriately measuring the total amount of oxygen in the molten steel after stirring the molten steel and removing the floating oxide.

鋼材にREMを添加する場合は、溶鋼中の全酸素量を上記範囲に調整した後に、REMを添加してから鋳造する。全酸素量を調整した溶鋼へ上記の元素を添加することによって所望とする酸化物が得られる。   When adding REM to a steel material, after adjusting the total oxygen amount in molten steel to the said range, it casts, after adding REM. The desired oxide can be obtained by adding the above elements to the molten steel with the total oxygen content adjusted.

溶鋼へ添加するREMの形態は特に限定されず、例えば、REMとして、純Laや純Ce、純Yなど、あるいは純Ca、さらにはFe−Si−La合金、Fe−Si−Ce合金、Fe−Si−Ca合金、Fe−Si−La−Ce合金、Fe−Ca合金、Ni−Ca合金などを添加すればよい。また、溶鋼へミッシュメタルを添加してもよい。ミッシュメタルとは、セリウム族希土類元素の混合物であり、具体的には、Ceを40〜50%程度、Laを20〜40%程度含有している。ただし、ミッシュメタルには不純物としてCaを含むことが多いので、ミッシュメタルがCaを含む場合は、本発明で規定する好適範囲を満足する必要がある。   The form of REM added to the molten steel is not particularly limited. For example, as REM, pure La, pure Ce, pure Y, etc., pure Ca, Fe—Si—La alloy, Fe—Si—Ce alloy, Fe— A Si—Ca alloy, a Fe—Si—La—Ce alloy, a Fe—Ca alloy, a Ni—Ca alloy, or the like may be added. Moreover, you may add misch metal to molten steel. Misch metal is a mixture of cerium group rare earth elements, and specifically contains about 40 to 50% Ce and about 20 to 40% La. However, since misch metal often contains Ca as an impurity, when the misch metal contains Ca, it is necessary to satisfy the preferred range defined in the present invention.

本発明でREMを添加した場合は、粗大な酸化物の除去を促進する目的で、REMを添加した後は、40分を超えない範囲で溶鋼を攪拌することが好ましい。攪拌時間が40分を超えると、微細な酸化物が溶鋼中で凝集・合体するため酸化物が粗大化し、特性が劣化する。したがって、攪拌時間は40分以内とすることが好ましい。攪拌時間は、より好ましくは35分以内であり、さらに好ましくは30分以内である。溶鋼の攪拌時間の下限値は特に限定されないが、攪拌時間が短過ぎると添加元素の濃度が不均一となり、鋼材全体として所望の効果が得られない。したがって、容器サイズに応じた所望の攪拌時間が必要となる。   When REM is added in the present invention, it is preferable to stir the molten steel within a range not exceeding 40 minutes after adding REM for the purpose of promoting the removal of coarse oxides. When the stirring time exceeds 40 minutes, fine oxides aggregate and coalesce in the molten steel, so that the oxides become coarse and the characteristics deteriorate. Therefore, the stirring time is preferably within 40 minutes. The stirring time is more preferably within 35 minutes, and further preferably within 30 minutes. The lower limit of the stirring time of the molten steel is not particularly limited, but if the stirring time is too short, the concentration of the additive element becomes non-uniform, and the desired effect cannot be obtained as a whole steel material. Accordingly, a desired stirring time corresponding to the container size is required.

以上のようにして、成分組成が調整された溶鋼が得られる。得られた溶鋼を用いて鋳造し、鋼片を得る。   As described above, a molten steel having an adjusted composition can be obtained. It casts using the obtained molten steel, and obtains a steel piece.

次に、加熱、仕上げ圧延を含む熱間圧延、熱延後の急冷、急冷停止後の緩冷、緩冷後の急冷、巻取りを行って製造する。   Next, production is performed by heating, hot rolling including finish rolling, rapid cooling after hot rolling, slow cooling after quenching stop, rapid cooling after slow cooling, and winding.

[加熱]
熱間圧延前の加熱は1150〜1300℃で行う。この加熱によりオーステナイト単相とする。これにより固溶元素(V、Nbなどの添加元素を含む)は、オーステナイトに固溶させる。加熱温度が1150℃未満ではオーステナイトに固溶できず、粗大な炭化物が形成されるため疲労特性改善効果が得られない。一方、1300℃を超える加熱温度は操業上困難である。また、添加元素としてTiが含まれる場合、炭化物のうち最も溶体化温度の高いTiを固溶させる点でも、TiCの溶体化温度以上1300℃以下が必要である。加熱温度のより好ましい下限は1200℃である。
[heating]
Heating before hot rolling is performed at 1150 to 1300 ° C. An austenite single phase is obtained by this heating. Thereby, solid solution elements (including additive elements such as V and Nb) are dissolved in austenite. If the heating temperature is less than 1150 ° C., it cannot be dissolved in austenite, and coarse carbides are formed, so that the effect of improving fatigue characteristics cannot be obtained. On the other hand, heating temperatures exceeding 1300 ° C. are difficult to operate. Moreover, when Ti is contained as an additive element, the TiC solution solution temperature or higher and 1300 ° C. or lower are necessary also in terms of solid solution of Ti having the highest solution temperature among carbides. A more preferable lower limit of the heating temperature is 1200 ° C.

[熱間粗圧延]
粗圧延では、本発明で規定する所定形状の等軸粒の存在割合を確保するため、再結晶オーステナイトの組織制御を行う。粗圧延温度は、以後の仕上げ圧延の温度確保も考慮して900〜1100℃とし、粗圧延でのオーステナイト粒の微細化、繰り返し再結晶化させることで、所定形状の等軸粒の存在割合を制御することができる。粗圧延温度は、より好ましくは900〜1000℃である。
[Hot rough rolling]
In rough rolling, the microstructure control of recrystallized austenite is performed in order to ensure the proportion of equiaxed grains having a predetermined shape defined in the present invention. The rough rolling temperature is set to 900 to 1100 ° C. in consideration of securing the temperature of the subsequent finish rolling, and the austenite grains in the rough rolling are refined and repeatedly recrystallized, so that the proportion of equiaxed grains having a predetermined shape is increased. Can be controlled. The rough rolling temperature is more preferably 900 to 1000 ° C.

[熱間仕上げ圧延]
熱間圧延は、仕上げ圧延温度が800℃以上になるように行う。仕上げ圧延温度を低温化しすぎるとフェライト変態が高温で起るようになり、フェライト中の析出炭化物が粗大化するため、一定以上の仕上げ圧延温度が必要である。仕上げ圧延温度は、オーステナイト粒を粗大化してベイナイトの粒径を大きくするため、850℃以上とするのがより好ましい。
[Hot finish rolling]
Hot rolling is performed so that the finish rolling temperature is 800 ° C. or higher. If the finish rolling temperature is too low, ferrite transformation occurs at a high temperature and the precipitated carbides in the ferrite are coarsened, so that a certain finish rolling temperature is required. The finish rolling temperature is more preferably 850 ° C. or higher in order to coarsen austenite grains and increase the grain size of bainite.

[熱間仕上げ圧延の入り側温度と出側温度の差]
熱間仕上げ圧延の入り側温度と出側温度の差を150℃以下とする。この温度差が150℃を超える場合は、仕上げ圧延前の温度が高い場合であり、結晶粒(オーステナイト粒)が粗大になるとともに、仕上げ圧延中に生成する再結晶粒も大きくなりやすい。また入側と出側の温度差が大きい場合は、仕上げ圧延中に生成する再結晶組織が不均一になりやすく、アスペクト比が大きい結晶粒が残存しやすい。これらの理由により、アスペクト比が3以下の結晶粒の個数が全結晶粒の個数の60%未満となる。この温度差は、より好ましくは100℃以下である。
[Difference between entry temperature and exit temperature of hot finish rolling]
The difference between the entry side temperature and the exit side temperature of hot finish rolling is set to 150 ° C. or less. When this temperature difference exceeds 150 ° C., the temperature before the finish rolling is high, the crystal grains (austenite grains) become coarse and the recrystallized grains generated during the finish rolling tend to be large. When the temperature difference between the entry side and the exit side is large, the recrystallized structure generated during finish rolling tends to be non-uniform, and crystal grains having a large aspect ratio tend to remain. For these reasons, the number of crystal grains having an aspect ratio of 3 or less is less than 60% of the total number of crystal grains. This temperature difference is more preferably 100 ° C. or less.

[熱延後の急冷]
上記仕上げ圧延終了後、5s以内に20℃/s以上の冷却速度(急冷速度)で急冷し、580℃以上670℃未満の温度(急冷停止温度)で急冷を停止する。フェライト変態の開始温度を低温化することによりフェライト中に形成される析出炭化物を微細化するためである。冷却速度(急冷速度)が20℃/s未満ではパーライト変態が促進され、または、急冷停止温度が580℃未満ではパーライト変態またはベイナイト変態が促進され、冷間加工性が低下する。一方、急冷停止温度が670℃以上になるとフェライト中の析出炭化物が粗大化してしまい、耐疲労特性が確保できない。急冷停止温度は、好ましくは600〜650℃、さらに好ましくは610〜640℃である。
[Rapid cooling after hot rolling]
After finishing the finish rolling, quenching is performed at a cooling rate (quenching rate) of 20 ° C./s or more within 5 s, and the quenching is stopped at a temperature of 580 ° C. or more and less than 670 ° C. (quenching stop temperature). This is because the precipitation carbide formed in the ferrite is refined by lowering the starting temperature of the ferrite transformation. When the cooling rate (quenching rate) is less than 20 ° C./s, pearlite transformation is promoted, or when the quenching stop temperature is less than 580 ° C., pearlite transformation or bainite transformation is promoted, and cold workability is lowered. On the other hand, when the quenching stop temperature is 670 ° C. or higher, the precipitated carbides in the ferrite are coarsened, and fatigue resistance characteristics cannot be ensured. The quenching stop temperature is preferably 600 to 650 ° C, more preferably 610 to 640 ° C.

[急冷停止後の緩冷]
上記急冷停止後は、5℃/s以上20℃/s未満の冷却速度(緩冷速度)で緩冷する。緩冷速度を5℃/s以上とすることで、熱延中における初析フェライトの形成を抑制し、フェライト中の析出炭化物を適度に微細化させること、熱延板での結晶粒組織を制御することにより、最終鋼板における集合組織形態を制御するためである。緩冷速度が5℃/s未満では、初析フェライトの形成量が多くなり、粗大粒が生成するとともに、最終鋼板で粗大粒が生成し、炭化物の不均一状態を生じ、冷間加工性を劣化させる。
[Slow cooling after rapid cooling stop]
After the rapid cooling stop, it is slowly cooled at a cooling rate (slow cooling rate) of 5 ° C./s or more and less than 20 ° C./s. By setting the slow cooling rate to 5 ° C./s or more, the formation of pro-eutectoid ferrite during hot rolling is suppressed, the precipitated carbides in ferrite are appropriately refined, and the grain structure of the hot rolled sheet is controlled. This is to control the texture form in the final steel sheet. When the slow cooling rate is less than 5 ° C./s, the amount of pro-eutectoid ferrite is increased and coarse grains are produced, and coarse grains are produced in the final steel plate, resulting in a non-uniform state of carbides, resulting in cold workability. Deteriorate.

[緩冷後の急冷、巻取り]
上記緩冷後、550℃超650℃以下で巻き取る。巻取り温度が650℃超では、表面酸化スケールが多く形成され、表面性状が劣化し、一方550℃未満では、マルテンサイトが多く形成され、冷間加工性が低下する。
[Rapid cooling after slow cooling, winding]
After the above-described slow cooling, the film is wound at over 550 ° C and below 650 ° C. When the coiling temperature is higher than 650 ° C., many surface oxide scales are formed and the surface properties are deteriorated. On the other hand, when it is less than 550 ° C., many martensites are formed and cold workability is lowered.

以下、本発明を実施例によってさらに詳細に説明するが、下記実施例は本発明を限定する性質のものではなく、前・後記の趣旨に適合し得る範囲で適当に変更して実施することも可能であり、それらはいずれも本発明の技術的範囲に含まれる。   Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

下記表1に示す成分組成の鋼を真空溶解法により溶製し、厚さ120mmのインゴットに鋳造し、これを下記表2に示す条件にて熱間圧延を施し熱延鋼板を作製した。なお、いずれの試験においても、急冷停止後の冷却は10℃/s以下の冷却速度で5〜20s緩冷する条件であった。   Steels having the component compositions shown in Table 1 below were melted by vacuum melting, cast into 120 mm thick ingots, and hot-rolled under the conditions shown in Table 2 to produce hot rolled steel sheets. In all the tests, the cooling after the rapid cooling stop was a condition of slow cooling for 5 to 20 seconds at a cooling rate of 10 ° C./s or less.

真空溶解炉(容量150kg)を用い、表1に示した化学成分を含有する供試鋼を溶製し、150kgのインゴットに鋳造して冷却した。真空溶解炉で供試鋼を溶製するに当っては、Al、REM、Ca以外の元素について成分調整するとともに、C,SiおよびMnから選ばれる少なくとも1種の元素を用いて脱酸して溶鋼の溶存酸素量を調整した。溶存酸素量を調整した溶鋼を1〜10分程度攪拌して溶鋼中の酸化物を浮上分離させることによって溶鋼の全酸素量を調整した。   Using a vacuum melting furnace (capacity 150 kg), a test steel containing chemical components shown in Table 1 was melted, cast into a 150 kg ingot, and cooled. In melting the test steel in a vacuum melting furnace, the components are adjusted for elements other than Al, REM, and Ca, and deoxidized using at least one element selected from C, Si, and Mn. The amount of dissolved oxygen in the molten steel was adjusted. The total amount of oxygen in the molten steel was adjusted by stirring the molten steel in which the amount of dissolved oxygen was adjusted for about 1 to 10 minutes to float and separate oxides in the molten steel.

なお、REMおよびCaを添加する場合は、添加全酸素量を調整した溶鋼に添加することによって成分調整した溶鋼を得た。なお、REMはLaを約25%とCeを約50%含有するミッシュメタルの形態で、CaはNi−Ca合金、またはCa−Si合金、またはFe−Ca圧粉体の形態で、それぞれ添加した。   In addition, when adding REM and Ca, the molten steel which adjusted the component was obtained by adding to the molten steel which adjusted the total amount of added oxygen. REM was added in the form of a misch metal containing about 25% La and about 50% Ce, and Ca was added in the form of Ni-Ca alloy, Ca-Si alloy, or Fe-Ca green compact, respectively. .

そして、得られたインゴットを表2に示す各条件で熱間圧延して所定板厚の熱延上がり板を製造した。   And the obtained ingot was hot-rolled on each condition shown in Table 2, and the hot-rolled board of predetermined thickness was manufactured.

このようにして得られた各熱延上がり板について、上記[発明を実施するための形態]の項で説明した測定方法により、鋼板中の各相の面積率、ならびに、結晶粒のアスペクト比およびその個数等を調査した。   For each hot rolled plate thus obtained, the area ratio of each phase in the steel sheet, the aspect ratio of the crystal grains, and the crystal grain ratio were measured by the measurement method described in the above-mentioned section [Mode for carrying out the invention]. The number etc. were investigated.

また、上記各熱延上がり板について、冷間加工性を評価するため、引張強度および穴広げ率を測定し、引張強度が350〜700MPaの範囲で、かつ穴広げ率が20%以上のものを合格とした。   Moreover, in order to evaluate cold workability about each said hot-rolled board, the tensile strength and the hole expansion rate are measured, and the tensile strength is in the range of 350 to 700 MPa and the hole expansion rate is 20% or more. Passed.

さらに、上記各熱延上がり板について、浸炭熱処理後の表面硬さを評価するため、以下の条件にて浸炭焼き入れ試験を行った。   Furthermore, in order to evaluate the surface hardness after the carburizing heat treatment for each of the hot rolled plates, a carburizing and quenching test was performed under the following conditions.

〔浸炭焼き入れ条件〕
カーボンポテンシャル(CP値)=0.8%のガス雰囲気中で、900℃×2.5h保持後さらに850℃×0.5h保持して浸炭処理を施した後、100℃で油焼き入れをし、その後160℃×2h保持して焼き戻し処理を施した後、空冷した。
[Carburizing and quenching conditions]
In a gas atmosphere of carbon potential (CP value) = 0.8%, after holding at 900 ° C. × 2.5 h and further carburizing by holding at 850 ° C. × 0.5 h, oil quenching was performed at 100 ° C. Then, after holding at 160 ° C. for 2 hours and performing a tempering treatment, it was air-cooled.

<浸炭熱処理後の表面硬さ>
そして、ビッカース硬さ試験機を用いて、荷重:1000g、測定位置:鋼板表面から0.8mm深さの位置を測定回数:5回の条件で、ビッカース硬さ(Hv)を測定し、350Hv以上のものを合格とした。ここで、測定位置を表面から0.8mm深さの位置としたのは、浸炭熱処理後において、表面から深いところでも所望の硬さ(強度)を示すことを必要条件としたことによるものである。
<Surface hardness after carburizing heat treatment>
Then, using a Vickers hardness tester, load: 1000 g, measurement position: position at a depth of 0.8 mm from the surface of the steel sheet, measurement number: 5 times, Vickers hardness (Hv) is measured, 350 Hv or more Was accepted. Here, the measurement position was set at a depth of 0.8 mm from the surface because, after carburizing heat treatment, it was a necessary condition to show a desired hardness (strength) even deep from the surface. .

これらの測定結果を下記表3に示す。   The measurement results are shown in Table 3 below.

表3に示すように、鋼No.1、2、6〜20はいずれも、本発明の成分組成規定の要件を満足する鋼種を用い、推奨の熱間圧延条件で製造した結果、本発明の組織規定の要件を充足する発明鋼であり、引張強度、穴広げ率および浸炭熱処理後の表面硬さはすべて合格基準を満たしており、良好な冷間加工性を確保しつつ、浸炭熱処理後は所定の表面硬さ(強度)を示す熱延鋼板が得られることが確認できた。   As shown in Table 3, steel no. 1, 2, 6-20 are steels that satisfy the requirements of the compositional composition of the present invention, and are manufactured under the recommended hot rolling conditions, and as a result, the invention steel that satisfies the requirements of the structure of the present invention. Yes, the tensile strength, the hole expansion ratio, and the surface hardness after carburizing heat treatment all meet the acceptance criteria, and show a predetermined surface hardness (strength) after carburizing heat treatment while ensuring good cold workability It was confirmed that a hot-rolled steel sheet was obtained.

これに対し、鋼No.3〜5、21〜27は本発明で規定する成分組成および組織の要件のうち少なくともいずれかを満足しない比較鋼であり、引張強度、穴広げ率および浸炭熱処理後の表面硬さのうち少なくともいずれかが合格基準を満たしていない。   On the other hand, Steel No. 3 to 5, 21 to 27 are comparative steels that do not satisfy at least one of the component composition and the structure requirement defined in the present invention, and at least any of the tensile strength, the hole expansion ratio, and the surface hardness after the carburizing heat treatment. Does not meet the acceptance criteria.

例えば、鋼No.3は、成分組成の要件は満たしているものの、熱延前の加熱温度が推奨範囲を外れて低すぎ、パーライトが過剰に形成されるとともに、結晶粒が偏平化し、穴広げ性が劣っている。   For example, steel no. 3, although the requirements of the component composition are satisfied, the heating temperature before hot rolling is too low outside the recommended range, pearlite is excessively formed, the crystal grains are flattened, and the hole expandability is inferior .

また、鋼No.4は、成分組成の要件は満たしているものの、熱延後の板厚が規定範囲を外れて大きすぎ、フェライトが過剰に形成されるとともに、結晶粒が粗大化し、穴広げ性が劣っている。   Steel No. No. 4, although the requirements of the component composition are satisfied, the plate thickness after hot rolling is too large outside the specified range, ferrite is excessively formed, the crystal grains are coarsened, and the hole expandability is inferior .

また、鋼No.5は、成分組成の要件は満たしているものの、仕上圧延における入側温度と出側温度の差が推奨範囲を外れて大きすぎ、結晶粒が偏平化し、穴広げ性が劣っている。   Steel No. No. 5 satisfies the requirements of the component composition, but the difference between the entry side temperature and the exit side temperature in finish rolling is too large outside the recommended range, the crystal grains are flattened, and the hole expandability is inferior.

また、鋼No.21(鋼種q)は、熱延条件は推奨範囲にあるものの、C含有量が低すぎるため、フェライトが過剰に形成され、引張強度が低くなりすぎるとともに浸炭熱処理後の表面硬さが劣っている。   Steel No. Although 21 (steel type q) has a hot rolling condition in the recommended range, since the C content is too low, ferrite is formed excessively, the tensile strength becomes too low and the surface hardness after carburizing heat treatment is inferior. .

一方、鋼No.22(鋼種r)は、熱延条件は推奨範囲にあるものの、C含有量が高すぎるため、パーライトが過剰に形成されるとともに、結晶粒が偏平化し、引張強度が高くなりすぎるとともに浸炭熱処理後の表面硬さが劣っている。   On the other hand, Steel No. 22 (steel grade r) has a hot rolling condition in the recommended range, but because the C content is too high, pearlite is excessively formed, the crystal grains are flattened, the tensile strength becomes too high, and after carburizing heat treatment The surface hardness of is inferior.

また、鋼No.23(鋼種s)は、熱延条件は推奨範囲にあるものの、Mn含有量が低すぎ、結晶粒が偏平化し、浸炭熱処理後の表面硬さが劣っている。   Steel No. Although 23 (steel type s) has a hot rolling condition in the recommended range, the Mn content is too low, the crystal grains are flattened, and the surface hardness after the carburizing heat treatment is inferior.

一方、鋼No.24(鋼種t)は、熱延条件は推奨範囲にあるものの、Mn含有量が高すぎ、フェライトの形成が不足する一方でパーライトが過剰に形成され、穴広げ性が劣っている。   On the other hand, Steel No. Although 24 (steel type t) has a hot rolling condition in the recommended range, the Mn content is too high and ferrite formation is insufficient, while pearlite is excessively formed and the hole expandability is poor.

また、鋼No.25(鋼種u)は、熱延条件は推奨範囲にあるものの、Al含有量が低すぎ、穴広げ性が劣っている。   Steel No. Although 25 (steel type u) has a hot rolling condition in the recommended range, the Al content is too low and the hole expandability is inferior.

一方、鋼No.26(鋼種v)は、熱延条件は推奨範囲にあるものの、Al含有量が高すぎ、やはり穴広げ性が劣っている。   On the other hand, Steel No. No. 26 (steel type v), although the hot rolling conditions are in the recommended range, the Al content is too high and the hole expandability is still inferior.

また、鋼No.27(鋼種w)は、熱延条件は推奨範囲にあるものの、N含有量が高すぎ、穴広げ性が劣っている。   Steel No. No. 27 (steel type w) has a hot rolling condition in the recommended range, but the N content is too high and the hole expandability is poor.

以上より、本発明の適用性が確認できた。   From the above, the applicability of the present invention was confirmed.

Claims (8)

板厚が2〜10mmであり、
成分組成が、
質量%で(以下、化学成分について同じ。)、
C :0.05〜0.30%、
Mn:0.3〜3.0%、
Al:0.015〜0.1%、
N :0.003〜0.030%を含み、
残部は鉄および不可避的不純物からなり、
組織が、
面積率で、
フェライト:36〜50%、
パーライト:29〜50%、
残部:ベイナイトからなり、
前記フェライトおよびパーライトを含む全ての相の結晶粒(以下、「全結晶粒」という。)について、
アスペクト比(長軸/短軸)が3以下の結晶粒の個数が前記全結晶粒の個数の60%以上であるとともに、前記全結晶粒の平均結晶粒径が〜50μmの範囲であり、
浸炭後の表面硬さが350Hv以上である
ことを特徴とする冷間加工性と浸炭熱処理後の表面硬さに優れる熱延鋼板。
The plate thickness is 2 to 10 mm,
Ingredient composition
% By mass (hereinafter the same for chemical components)
C: 0.05 to 0.30%
Mn: 0.3-3.0%
Al: 0.015-0.1%
N: 0.003 to 0.030% included,
The balance consists of iron and inevitable impurities,
Organization
In area ratio,
Ferrite: 36 to 50 percent,
Perlite: 29-50%,
The rest: bainite
About crystal grains of all phases containing the ferrite and pearlite (hereinafter referred to as “all crystal grains”),
The aspect ratio with (major axis / minor axis) is 3 or less of the number of crystal grains 60% or more of the number of the total grains, the Ri range der average crystal grain size of the total crystal grains 5 ~50Myuemu ,
A hot rolled steel sheet excellent in cold workability and surface hardness after carburizing heat treatment, characterized in that the surface hardness after carburizing is 350 Hv or more .
成分組成が、さらに、
Cr:3.0%以下(0%を含まない)、
Mo:1.0%以下(0%を含まない)、
Ni:3.0%以下(0%を含まない)よりなる群から選択される少なくとも1種
を含むものである請求項1に記載の熱延鋼板。
Ingredient composition further
Cr: 3.0% or less (excluding 0%),
Mo: 1.0% or less (excluding 0%),
The hot-rolled steel sheet according to claim 1, which contains at least one selected from the group consisting of Ni: 3.0% or less (not including 0%).
前記不可避的不純物のうち、Si:0.5%以下、P:0.030%以下、S:0.035%以下である請求項1または2に記載の熱延鋼板。   The hot rolled steel sheet according to claim 1 or 2, wherein among the inevitable impurities, Si is 0.5% or less, P: 0.030% or less, and S: 0.035% or less. 成分組成が、さらに、
Cu:2.0%以下(0%を含まない)、および/または、
Co:0.32%以下(0%を含まない)
を含むものである請求項1〜3のいずれか1項に記載の熱延鋼板。
Ingredient composition further
Cu: 2.0% or less (excluding 0%), and / or
Co: 0.32 % or less (excluding 0%)
The hot-rolled steel sheet according to any one of claims 1 to 3.
成分組成が、さらに、
V:0.5%以下(0%を含まない)、
Ti:0.1%以下(0%を含まない)、
Nb:0.1%以下(0%を含まない)よりなる群から選ばれる少なくとも1種
を含むものである請求項1〜4のいずれか1項に記載の熱延鋼板。
Ingredient composition further
V: 0.5% or less (excluding 0%),
Ti: 0.1% or less (excluding 0%),
The hot rolled steel sheet according to any one of claims 1 to 4, which contains at least one selected from the group consisting of Nb: 0.1% or less (not including 0%).
成分組成が、さらに、
Ca:0.08%以下(0%を含まない)、および/または、
Zr:0.08%以下(0%を含まない)
を含むものである請求項1〜5のいずれか1項に記載の熱延鋼板。
Ingredient composition further
Ca: 0.08% or less (excluding 0%), and / or
Zr: 0.08% or less (excluding 0%)
The hot-rolled steel sheet according to any one of claims 1 to 5.
成分組成が、さらに、
Sb:0.02%以下(0%を含まない)
を含むものである請求項1〜6のいずれか1項に記載の熱延鋼板。
Ingredient composition further
Sb: 0.02% or less (excluding 0%)
The hot-rolled steel sheet according to any one of claims 1 to 6.
成分組成が、さらに、
REM:0.05%以下(0%を含まない)、
Mg:0.02%以下(0%を含まない)、
Li:0.02%以下(0%を含まない)、
Pb:0.5%以下(0%を含まない)、
Bi:0.5%以下(0%を含まない)よりなる群から選ばれる少なくとも1種
を含むものである請求項1〜7のいずれか1項に記載の熱延鋼板。
Ingredient composition further
REM: 0.05% or less (excluding 0%),
Mg: 0.02% or less (excluding 0%),
Li: 0.02% or less (excluding 0%),
Pb: 0.5% or less (excluding 0%),
The hot-rolled steel sheet according to any one of claims 1 to 7, which contains Bi: at least one selected from the group consisting of 0.5% or less (not including 0%).
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