JP6143355B2 - Hot-rolled steel sheet with excellent drawability and surface hardness after carburizing heat treatment - Google Patents

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

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JP6143355B2
JP6143355B2 JP2013219468A JP2013219468A JP6143355B2 JP 6143355 B2 JP6143355 B2 JP 6143355B2 JP 2013219468 A JP2013219468 A JP 2013219468A JP 2013219468 A JP2013219468 A JP 2013219468A JP 6143355 B2 JP6143355 B2 JP 6143355B2
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梶原 桂
桂 梶原
土田 武広
武広 土田
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Kobe Steel Ltd
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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、平均r値が1.2以上、圧延方向のr 値(rL) が1.3以上、圧延方向に対して45゜方向のr値(rD )が、0.9以上、圧延方向と直角方向のr値(rC)が1.2以上であり、鋼板1/2板厚における板面の{111}、{100}、および{110}の各X線反射面ランダム強度比が、それぞれ2.0以上、1.0以上、および0.2以上であるものとすることで、加工性が向上するとされている。   The hot-rolled steel sheet disclosed in Patent Document 2 has an average r value of 1.2 or more, an r value (rL) in the rolling direction of 1.3 or more, and an r value (rD) in the 45 ° direction with respect to the rolling direction. 0.9 or more, and the r value (rC) in the direction perpendicular to the rolling direction is 1.2 or more, and each X of {111}, {100}, and {110} of the plate surface in the steel plate 1/2 plate thickness It is said that the workability is improved when the line reflection surface random intensity ratio is 2.0 or more, 1.0 or more, and 0.2 or more, respectively.

特許文献3に開示された熱延鋼板は、体積率で40〜95%のベイナイト相を含有して残部がフェライト相からなる組織を有し、上記フェライトの平均結晶粒径が1.2μm以上で4μm未満であって、かつ下記の(a)と(b)の少なくとも一方の特徴を有するものとすることで、 延性およびバーリング加工性が向上するとされている。
(a) 鋼板の表面から板厚方向に1/8の厚さの位置におけるフェライトの平均粒径(ds)と板厚中心におけるフェライトの平均粒径(dc)との比率(ds/dc)が0.3〜0.7であること。
(b) 鋼板の表面から板厚方向に1/8の厚さの位置における{110}<111>、{110}<001>および{211}<111>の極密度の和が集合組織をもたないものの5倍以上であり、かつ、それぞれが1.5倍以上であること。
The hot-rolled steel sheet disclosed in Patent Document 3 has a structure containing a bainite phase of 40 to 95% by volume and the balance being a ferrite phase, and the average crystal grain size of the ferrite is 1.2 μm or more. By being less than 4 μm and having at least one of the following characteristics (a) and (b), ductility and burring workability are improved.
(A) The ratio (ds / dc) of the average particle diameter (ds) of ferrite at the position of 1/8 thickness from the surface of the steel sheet to the average thickness (dc) of ferrite at the center of the sheet thickness is 0.3 to 0.7.
(B) The sum of the polar densities of {110} <111>, {110} <001>, and {211} <111> at a position of 1/8 thickness from the surface of the steel plate also has a texture. It should be 5 times or more of that which is not good, and each must be 1.5 times or more.

特許文献4に開示された熱延鋼板は、フェライト相の平均粒径が5μm以下であり、さらにフェライト相が面積率で50%以上存在するミクロ組織を有し、かつ鋼板の1/4板厚における板面の(113)[1−10]〜(223)[1−10]方位における平均のODF解析強度fが4以上であるものとすることで、剛性が向上するとされている。   The hot-rolled steel sheet disclosed in Patent Document 4 has a microstructure in which the average grain size of the ferrite phase is 5 μm or less, and further the ferrite phase is present in an area ratio of 50% or more, and the ¼ thickness of the steel sheet When the average ODF analysis strength f in the (113) [1-10] to (223) [1-10] orientations of the plate surface is 4 or more, the rigidity is improved.

特許文献5に開示された熱延鋼板は、下記式(1)で規定されるΔr1が−0.20以上0.20以下であり、下記式(2)で規定されるΔr2が0.42以下であるものとすることで、絞り成形性が向上するとされている。
Δr1=(r0−2r45+r90)/2 ・・・ (1)
Δr2=rmax−rmin ・・・ (2)
ここで、式中の各記号は以下の値を表す:
r0:板面の圧延方向に対して平行に採取した試験片で測定したr値、
r45:板面の圧延方向に対して45°方向に採取した試験片で測定したr値、
r90:板面の圧延方向に対して90°方向に採取した試験片で測定したr値、
rmax:r0、r45およびr90のうち最大の値、
rmin:r0、r45およびr90のうち最小の値。
In the hot-rolled steel sheet disclosed in Patent Document 5, Δr1 defined by the following formula (1) is −0.20 or more and 0.20 or less, and Δr2 defined by the following formula (2) is 0.42 or less. It is said that drawability is improved by assuming that.
Δr1 = (r0-2r45 + r90) / 2 (1)
Δr2 = rmax−rmin (2)
Where each symbol in the formula represents the following value:
r0: r value measured with a test specimen taken in parallel with the rolling direction of the plate surface,
r45: r value measured with a test piece taken in the direction of 45 ° with respect to the rolling direction of the plate surface,
r90: r value measured with a test specimen taken in the 90 ° direction with respect to the rolling direction of the plate surface,
rmax: the maximum value of r0, r45 and r90,
rmin: The minimum value of r0, r45, and r90.

特許文献6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 6 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.

しかしながら、上記特許文献2〜6に開示された熱延鋼板は、絞り加工性などの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 6 are excellent in cold workability such as drawability, there is no mention of the surface hardness after carburizing heat treatment, and the improvement effect is unknown. is there.

一方、特許文献7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 7 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7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、絞り加工性はもとより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 7 is excellent in surface hardness after carburizing heat treatment, there is no mention of cold workability as well as drawing workability, and its improvement effect Is unknown.

上記のように、絞り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 described above, the hot-rolled steel sheet having both drawing workability and surface hardness after carburizing heat treatment has hardly been studied so far.

特許第3094856号公報Japanese Patent No. 3094856 特許第3742559号公報Japanese Patent No. 3742559 特許第4161935号公報Japanese Patent No. 4161935 特許第4267257号公報Japanese Patent No. 4267257 特開2013−119635号公報JP 2013-119635 A 特許第426504号公報Japanese Patent No. 426504 特開2010−222663号公報JP 2010-222663 A

そこで、本発明は、絞り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 having both drawability 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 :0.003〜0.090%を含み、
残部は鉄および不純物からなり、
深さt/4(t:板厚)の位置に存在する、フェライトおよびパーライトを含む全ての結晶粒(以下、「全結晶粒」という。)に関し、
板面方位が(123)面から10°以内の結晶粒の面積率が20%以上であり、
圧延方向で、結晶方向が<001>方向から10°以内の結晶粒と、結晶方向が<110>方向から10°以内の結晶粒との合計面積率が25%以下であるとともに、
前記全結晶粒の平均粒径が3〜50μmである
ことを特徴とする絞り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 0.090% included,
The balance consists of iron and impurities,
Regarding all the crystal grains including ferrite and pearlite (hereinafter referred to as “all crystal grains”) existing at a depth t / 4 (t: plate thickness),
The area ratio of crystal grains whose plate plane orientation is within 10 ° from the (123) plane is 20% or more,
In the rolling direction, the total area ratio of the crystal grains whose crystal direction is within 10 ° from the <001> direction and the crystal grains whose crystal direction is within 10 ° from the <110> direction is 25% or less,
An average grain size of all the crystal grains is 3 to 50 μm. A hot rolled steel sheet excellent in drawing workability and surface hardness after carburizing heat treatment.

請求項3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 according to claim 3
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 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、熱延鋼板の集合組織を所定の組織形態に制御することで、絞り加工性が求められる部品においても、加工中の変形能を高めることにより、金型の寿命が延長されるとともに、鋼板に割れが発生しにくく、浸炭熱処理後に得られる部品は所定の表面硬さを確保できる熱延鋼板を提供できるようになった。   According to the present invention, by controlling the texture of the hot-rolled steel sheet to a predetermined structure form in the structure mainly composed of ferrite and pearlite, by improving the deformability during processing even in a part that requires drawing workability. In addition to extending the life of the mold, the steel sheet is less prone to cracking, and the parts obtained after the carburizing heat treatment can provide a hot-rolled steel sheet that can ensure a predetermined surface hardness.

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したうえで、熱延鋼板の集合組織を所定の組織形態に制御するとともに、結晶粒を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, high-toughness case hardening steel) described in Patent Document 1 above. Are different from each other in that the texture is controlled to a predetermined structure and crystal grains are refined.

〔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 :0.003〜0.090%
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0.090%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: 0.003-0.090 % >
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 are saturated at about 0.090% , and if it is contained more than that, nitrides will become inclusions and adversely affect the physical properties, so addition of further amounts must be avoided. Preferably below 0.02% or less, more preferably 0.015% or less.

本発明鋼板は上記成分を基本的に含有し、残部が鉄および不純物であるが、前記不純物のうち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 impurities. Among the impurities, Si, P, and S are desirably suppressed as little 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、鋼中の集合組織の形態をより厳密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 has ferrite + pearlite as a main structure, and is particularly characterized by more strictly controlling the form of texture in steel.

一般に、従前より鋼板の成形性向上のための集合組織制御に関しては、自動車の車体外板に用いられる薄鋼板の深絞り加工性は、材料の塑性異方性(r値(Lankford値):引張り試験における板幅ひずみと板厚ひずみの比)が大きいほど、その加工性が高くなること、さらに再結晶集合組織において板面方位に平行な{111}面を強く発達させ、{100}面方位を弱めることが深絞り性の向上に不可欠であることが実験的にも理論的にも明らかにされている(日本鉄鋼協会:「再結晶・集合組織とその組織制御への応用 再結晶研究の最前線」,1999年3月,p.208参照)。   In general, with regard to texture control for improving the formability of a steel sheet, the deep drawing workability of a thin steel sheet used for a car body outer plate of an automobile is determined by the plastic anisotropy of the material (r value (Rankford value): tensile The greater the ratio between the plate width strain and the plate thickness strain in the test), the higher the workability, and the stronger the {111} plane parallel to the plane orientation in the recrystallized texture, the {100} plane orientation It has been experimentally and theoretically revealed that it is essential to improve the deep drawability (Japan Iron and Steel Institute: “Recrystallization / texture and its application to microstructure control. Frontline ”, March 1999, p. 208).

このため、車体外板用の鋼板では、上記特許文献2〜4に開示されるように、集合組織制御による加工性向上の取り組みは種々行われているものの、浸炭熱処理用鋼板では、このような取り組みは行われていなかった。   For this reason, as disclosed in the above-mentioned Patent Documents 2 to 4 in the steel plate for the vehicle body outer plate, various efforts have been made to improve the workability by texture control. No efforts were made.

〔本発明鋼板の集合組織〕
本発明鋼板は、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関し、結晶粒の方位および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。
[A texture of the steel sheet of the present invention]
The steel sheet of the present invention is characterized by controlling the orientation and size of the crystal grains within a specific range with respect to the crystal grains of all phases including ferrite and pearlite.

<深さt/4(t:板厚)の位置に存在する、フェライトおよびパーライトを含む全ての結晶粒(以下、「全結晶粒」という。)に関し、
板面方位が(123)面から10°以内の結晶粒の面積率が20%以上>
集合組織のでき方は結晶系が同じでも加工法によって異なり、圧延材の場合は圧延面と圧延方向で表現される。つまり、下記に示すように圧延面は{○○○}で、圧延方向は<△△△>で、それぞれ表現される。なお、○や△は整数を示している。これら各方位の表現については、長島晋一編著:「集合組織」(丸善株式会社刊)などに記載されている。
<Regarding all crystal grains including ferrite and pearlite (hereinafter referred to as “all crystal grains”) existing at a position of depth t / 4 (t: plate thickness).
The area ratio of crystal grains whose plate plane orientation is within 10 ° from the (123) plane is 20% or more>
The formation of the texture differs depending on the processing method even if the crystal system is the same, and in the case of a rolled material, it is expressed by the rolling surface and the rolling direction. That is, as shown below, the rolling surface is represented by {xxx} and the rolling direction is represented by <ΔΔΔ>. In addition, (circle) and (triangle | delta) have shown the integer. The expression of each of these directions is described in, for example, edited by Junichi Nagashima: “Aggregation” (published by Maruzen Co., Ltd.).

本発明では、深さt/4の位置に存在する全結晶粒に関し、板面方位が、(123)面から10°以内のフェライト結晶粒の面積率を20%以上に制御することで、浸炭熱処理用鋼板の冷間加工性、絞り加工性を向上させることができる。   In the present invention, with respect to all the crystal grains existing at the position of the depth t / 4, carburization is achieved by controlling the area ratio of ferrite crystal grains whose plate plane orientation is within 10 ° from the (123) plane to 20% or more. The cold workability and drawing workability of the heat treating steel sheet can be improved.

従来から、結晶粒の板面方位に関し、板面に平行な(111)面方位を強く発達させる一方、(001)面方位を弱めることが深絞り性の向上に有効であることが知られている。冷間圧延工程と焼鈍工程を施す工程ではこのような板面方位の制御が可能であったが、本発明鋼板では、このような板面方位の制御が難しかった。   Conventionally, with regard to the crystal plane orientation of the crystal grains, it has been known that the (111) plane orientation parallel to the plane is strongly developed while the (001) plane orientation is effective in improving the deep drawability. Yes. Such plate surface orientation control was possible in the cold rolling step and the annealing step, but it was difficult to control such plate surface orientation in the steel sheet of the present invention.

そこで本発明では、新たに(123)面の板面方位を有する結晶粒を導入することによって、本発明鋼板での集合組織制御を可能にし、軟質化状態での冷間加工性の向上を実現できるようになった。   Therefore, in the present invention, by newly introducing crystal grains having a (123) plane orientation, it is possible to control the texture of the steel sheet of the present invention and to improve cold workability in a softened state. I can do it now.

上述したように、板面方位として(123)面を有する結晶粒は、軟質化状態での冷間加工性を向上させる作用を有し、このような作用を有効に発揮させるためには、面積率で20%以上が必要である。好ましくは22%以上、さらに好ましくは24%以上、特に好ましくは26%以上である。   As described above, the crystal grains having the (123) plane as the plate surface orientation have an effect of improving the cold workability in the softened state, and in order to effectively exhibit such an effect, the area A rate of 20% or more is required. Preferably it is 22% or more, more preferably 24% or more, particularly preferably 26% or more.

なお、板材では、板厚方向に組織分布を有するため、板厚の1/4の深さ位置を代表位置として組織形態を規定した。また、板面方位が上記理想面方位((123)面)から10°以内の結晶粒はほぼ同等の作用を有すると考えられるので、その範囲の板面方位を有する結晶粒の面積率で規定することとした。   In addition, since the plate material has a tissue distribution in the plate thickness direction, the tissue form was defined with a depth position of 1/4 of the plate thickness as a representative position. In addition, it is considered that crystal grains having a plate plane orientation within 10 ° from the ideal plane orientation ((123) plane) have almost the same action, and thus are defined by the area ratio of crystal grains having a plane plane orientation within that range. It was decided to.

<圧延方向で、結晶方向が<001>方向から10°以内の結晶粒と、結晶方向が<110>方向から10°以内の結晶粒との合計面積率が25%以下>
この合計面積率が大きくなるほど絞り加工性における面内異方性が大きくなるので、25%以下、好ましくは23%以下、さらに好ましくは20%以下に制限する。
<In the rolling direction, the total area ratio of crystal grains whose crystal direction is within 10 ° from the <001> direction and crystal grains whose crystal direction is within 10 ° from the <110> direction is 25% or less>
Since the in-plane anisotropy in drawing workability increases as the total area ratio increases, it is limited to 25% or less, preferably 23% or less, and more preferably 20% or less.

また、上記面内異方性の低減効果により、浸炭熱処理後の焼き入れ時において、マルテンサイト変態は元の結晶粒の結晶方位関係を維持しつつ進行するので、変態ひずみを低減し、部品の寸法精度を向上する効果も得られる。また、別の解釈として、成形加工時に面内異方性が生じることは、部品内でひずみの不均一が生じていることを示しており、それが原因となって、浸炭熱処理、焼入れ後の部品の寸法精度を悪化させていることから、上記の集合組織制御による面内異方性の低減効果により、部品の寸法精度を向上する効果が得られるとも考えられる。   In addition, due to the effect of reducing the in-plane anisotropy, the martensitic transformation proceeds while maintaining the crystal orientation relationship of the original crystal grains during quenching after the carburizing heat treatment, so that the transformation strain is reduced. The effect of improving dimensional accuracy can also be obtained. Further, as another interpretation, the occurrence of in-plane anisotropy during molding processing indicates that non-uniform strain occurs in the part, which is the cause of carburizing heat treatment, after quenching Since the dimensional accuracy of the component is deteriorated, it is considered that the effect of improving the dimensional accuracy of the component can be obtained by the effect of reducing the in-plane anisotropy by the texture control described above.

<前記全結晶粒の平均粒径が3〜50μm>
全結晶粒の平均粒径は、鋼板の加工性(絞り加工性、曲げ加工性、プレス加工性)を向上させるとともに、加工後の表面性状を満足させるため、3〜50μmの範囲であることが必要である。結晶粒が細かくなりすぎると、変形抵抗が高くなりすぎるため、その平均粒径は3μm以上、好ましくは4μm以上、さらに好ましくは5μm以上とする。一方、結晶粒が粗大化しすぎると、靱性、疲労特性などが劣化するとともに、結晶方位を制御しても、曲げ加工性や張出などのプレス成形性が著しく低下し、成形時の割れや肌荒れなどの不良が生じ易いため、その平均粒径は50μm以下、好ましくは45μm以下、さらに好ましくは40μm以下とする。なお、上記と同様、板厚方向で結晶粒のサイズ分布が存在するが、板厚の1/4の深さ位置を代表位置として全結晶粒の平均粒径を規定した。
<Average grain size of all crystal grains is 3 to 50 μm>
The average grain size of all crystal grains is in the range of 3 to 50 μm in order to improve the workability (drawing workability, bending workability, press workability) of the steel sheet and satisfy the surface properties after working. is necessary. If the crystal grains become too fine, the deformation resistance becomes too high, so the average grain size is 3 μm or more, preferably 4 μm or more, more preferably 5 μm or more. On the other hand, if the crystal grains become too coarse, the toughness, fatigue characteristics, etc. deteriorate, and even if the crystal orientation is controlled, the press formability such as bending workability and overhang is remarkably reduced, and cracks and rough surfaces during molding occur. The average particle size is 50 μm or less, preferably 45 μm or less, and more preferably 40 μm or less. Similar to the above, there is a crystal grain size distribution in the plate thickness direction, but the average grain size of all crystal grains was defined with a depth position of 1/4 of the plate thickness as a representative position.

〔結晶粒の板面方位の測定方法〕
結晶粒の板面方位は、SEM−EBSP[Electron Back Scattering Patternと、EBSD(Electron Back Scattering Diffraction)によって、測定・解析される。SEM装置としては、例えば日本電子社製SEM(JEOLJSM5410)、EBSP測定・解析システムとして、例えばEBSP:TSL社製(OIM)を各々用いる。また、結晶粒の大きさにもよるが試料の測定領域は300〜1000μm×300〜1000μmとし、測定ステップ間隔は例えば1〜3μmとする。このようにして同定した各結晶粒の結晶方位より、上記各理想面方位から10°以内の方位のものを集計して合計面積を求め、測定領域の面積で除すことにより、各理想面方位ごとの面積率を求めた。
[Measurement method of crystal plane orientation]
The crystal plane orientation is measured and analyzed by SEM-EBSP [Electron Back Scattering Pattern and EBSD (Electron Back Scattering Diffraction). As the SEM apparatus, for example, SEM (JEOLJSM5410) manufactured by JEOL Ltd. is used, and for example, EBSP: manufactured by TSL (OIM) is used as the EBSP measurement / analysis system. Although it depends on the size of the crystal grains, the measurement area of the sample is 300 to 1000 μm × 300 to 1000 μm, and the measurement step interval is, for example, 1 to 3 μm. From the crystal orientations of the respective crystal grains identified in this way, those with orientations within 10 ° from each of the ideal plane orientations are totaled to obtain a total area, and divided by the area of the measurement region, thereby obtaining each ideal plane orientation. The area ratio for each was determined.

〔結晶粒の圧延方向での結晶方向の測定方法〕
また、結晶粒の圧延方向での結晶方向は、鋼板の圧延方向の断面(側面)についてEBSP測定を行い、解析によって、圧延方向で、結晶方向が<001>方向から10°以内の結晶粒と、結晶方向が<110>方向から10°以内の結晶粒を同定する。測定方法としては、結晶粒の大きさにもよるが、試料の測定領域は、板厚方向の1/4部で 300〜1000μm×300〜1000μmとし、測定ステップ間隔は例えば1〜3μmとする。このようにして同定した各結晶粒の結晶方向より、上記各理想面方向から10°以内の方位のものを集計して合計面積を求め、測定領域の面積で除すことにより、各理想結晶方向ごとの面積率を求めた。
[Method of measuring crystal orientation in the rolling direction of crystal grains]
Further, the crystal direction in the rolling direction of the crystal grains is EBSP measurement with respect to the cross section (side surface) in the rolling direction of the steel sheet, and by analysis, the crystal direction is within the 10 ° direction from the <001> direction. The crystal grains whose crystal direction is within 10 ° from the <110> direction are identified. As a measuring method, although it depends on the size of the crystal grains, the measurement area of the sample is 300 to 1000 μm × 300 to 1000 μm at a quarter part in the plate thickness direction, and the measurement step interval is, for example, 1 to 3 μm. From the crystal directions of each crystal grain thus identified, those with an orientation within 10 ° from each ideal plane direction are totaled to obtain a total area, and divided by the area of the measurement region, thereby obtaining each ideal crystal direction. The area ratio for each was determined.

〔前記全結晶粒の平均粒径の測定方法〕
前記全結晶粒の平均粒径は、上記SEM−EBSPと、その測定条件を用い、所定の測定領域内に観察される各結晶粒の最大直径を各々測定し、それらの平均値を平均粒径として求めた。
[Measuring method of average grain size of all crystal grains]
The average grain size of all the crystal grains is determined by measuring the maximum diameter of each crystal grain observed in a predetermined measurement region using the SEM-EBSP and measurement conditions thereof, and calculating the average value of the average values. As sought.

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の結晶方位を有する結晶粒の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、以後の仕上げ圧延温度の確保も考慮して例えば900〜1100℃とすることができ、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、所定の結晶方位を有する結晶粒の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 structure control of recrystallized austenite is performed in order to ensure the proportion of crystal grains having a predetermined crystal orientation defined in the present invention. The rough rolling temperature can be set to, for example, 900 to 1100 ° C. in consideration of securing the subsequent finish rolling temperature. By refining austenite grains in the rough rolling and recrystallizing repeatedly, a predetermined crystal orientation can be obtained. It is possible to control the ratio of crystal grains. 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.

[熱間仕上げ圧延の最終パスの圧下率]
熱間仕上げ圧延の最終パスの圧下率が高すぎると、本発明の集合組織の形態が得られず、異方性が大きくなる。逆に圧下率が低すぎると、集合組織が発達しない。このため、熱間仕上げ圧延の最終パスの圧下率は10〜18%とし、より好ましくは11〜17%、特に好ましくは12〜16%とする。
[Rolling ratio of the final pass of hot finish rolling]
If the rolling reduction in the final pass of hot finish rolling is too high, the texture structure of the present invention cannot be obtained and the anisotropy increases. Conversely, if the rolling reduction is too low, the texture does not develop. For this reason, the rolling reduction of the final pass of hot finish rolling is 10 to 18%, more preferably 11 to 17%, and particularly preferably 12 to 16%.

[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℃程度以上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おそれがある。
[Rapid cooling after slow cooling, winding]
After the slow cooling, the film is wound at about 550 ° C. or more and 650 ° C. or less. 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 the coiling temperature is less than 550 ° C., many martensites are formed and cold workability may be deteriorated.

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、結晶粒の板面方位、結晶粒の圧延方向での結晶方向、および、全結晶粒の平均粒径を調査した。   For each hot rolled plate thus obtained, by the measurement method described in the above section [Mode for carrying out the invention], the crystal plane direction of the crystal grains, the crystal direction in the rolling direction of the crystal grains, And the average particle diameter of all the crystal grains was investigated.

また、上記各熱延上がり板について、絞り加工性を評価するため、圧延方向に対して0°(圧延方向に平行)、45°、90°(圧延方向に垂直)になるようにJIS5号片を採取し、引張試験を行って、それぞれの角度でのr値(r0、r45、r90)を求めた。そして、以下の式で平均r値およびΔr値を算出した。ここで、Δr値は、r値の面内異方性を評価するための指標である。
平均r値=(r0+2×r45+r90)/4
Δr値=(r0+r90)/2−r45
そして、r0、r45、r90、および、平均r値が、全て0.85以上で、かつΔr値が±0.1以内のものを合格とした。
Moreover, in order to evaluate drawing workability about each said hot-rolling board, it is JIS No. 5 piece so that it may become 0 degree (parallel to a rolling direction), 45 degrees, and 90 degrees (perpendicular to a rolling direction) with respect to a rolling direction. Were collected and subjected to a tensile test to obtain r values (r0, r45, r90) at respective angles. And the average r value and (DELTA) r value were computed with the following formula | equation. Here, the Δr value is an index for evaluating the in-plane anisotropy of the r value.
Average r value = (r0 + 2 × r45 + r90) / 4
Δr value = (r0 + r90) / 2−r45
Then, r0, r45, r90, and the average r value were all 0.85 or more and the Δr value was within ± 0.1.

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、絞り加工性の指標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、良好な絞り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 drawing processability index and the surface hardness after carburizing heat treatment all meet the acceptance criteria, while ensuring good drawing workability, hot rolling showing a predetermined surface hardness (strength) after carburizing heat treatment It was confirmed that a steel plate 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、絞り加工性の指標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 one of the drawing workability index and the surface hardness after 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、粗圧延温度、仕上げ圧延温度、急冷致死温度および巻取り温度のいずれもが推奨範囲を外れて低すぎ、圧延方向での結晶方向が<001>方向と<110>方向の結晶粒が過剰に形成され、絞り加工性、中でもr値の異方性が劣っている。   For example, steel no. Although the requirements for component composition 3 are satisfied, the heating temperature before hot rolling, rough rolling temperature, finish rolling temperature, quenching lethal temperature, and winding temperature are all out of the recommended range and are too low in the rolling direction. The crystal directions of <001> and <110> directions are excessively formed, and the drawing workability, particularly the r value anisotropy, 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、(123)面の板面方位を有する結晶粒が不足するとともに、結晶粒が粗大化し、絞り加工性が劣っている。   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, the crystal grains having the (123) plane plane orientation are insufficient, and the crystal grains are coarsened. The drawing processability 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、仕上げ圧延の最終パスにおける圧下率が推奨範囲を外れて小さすぎ、(123)面の板面方位を有する結晶粒が不足し、圧延方向での結晶方向が<001>方向と<110>方向の結晶粒が過剰に形成され、絞り加工性、中でもr値の異方性が劣っている。   Steel No. 5, although the requirements of the component composition are satisfied, the rolling reduction in the final pass of finish rolling is too small outside the recommended range, the crystal grains having the (123) plane plate orientation are insufficient, and in the rolling direction Crystal grains with <001> direction and <110> direction are formed excessively, and drawing workability, especially r value anisotropy 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、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, the C content is 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、絞り加工性が劣っている。   On the other hand, Steel No. Although 22 (steel type r) has a hot rolling condition in the recommended range, the C content is too high and the drawability 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、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 and the surface hardness after 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、絞り加工性が劣っている。   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 the drawability is inferior.

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、絞り加工性が劣っている。   Steel No. Although 25 (steel type u) has a hot rolling condition in the recommended range, the Al content is too low and the drawability 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、絞り加工性が劣っている。   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 drawability is inferior.

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 :0.003〜0.090%を含み、
残部は鉄および不純物からなり、
深さt/4(t:板厚)の位置に存在する、フェライトおよびパーライトを含む全ての結晶粒(以下、「全結晶粒」という。)に関し、
板面方位が(123)面から10°以内の結晶粒の面積率が20%以上であり、
圧延方向で、結晶方向が<001>方向から10°以内の結晶粒と、結晶方向が<110>方向から10°以内の結晶粒との合計面積率が25%以下であるとともに、
前記全結晶粒の平均粒径が3〜50μmである
ことを特徴とする絞り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 0.090% included,
The balance consists of iron and impurities,
Regarding all the crystal grains including ferrite and pearlite (hereinafter referred to as “all crystal grains”) existing at a depth t / 4 (t: plate thickness),
The area ratio of crystal grains whose plate plane orientation is within 10 ° from the (123) plane is 20% or more,
In the rolling direction, the total area ratio of the crystal grains whose crystal direction is within 10 ° from the <001> direction and the crystal grains whose crystal direction is within 10 ° from the <110> direction is 25% or less,
An average grain size of all the crystal grains is 3 to 50 μm. A hot rolled steel sheet excellent in drawing workability and surface hardness after carburizing heat treatment.
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。 3. The hot rolled steel sheet according to claim 1, wherein among the impurities , Si is 0.5% or less, P is 0.030% or less, and S is 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: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。
Ingredient composition further
Cu: 2.0% or less (excluding 0%), and / or
Co: 5% 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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