JP2012177167A - Steel sheet for soft nitriding treatment, and its manufacturing method - Google Patents

Steel sheet for soft nitriding treatment, and its manufacturing method Download PDF

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JP2012177167A
JP2012177167A JP2011041027A JP2011041027A JP2012177167A JP 2012177167 A JP2012177167 A JP 2012177167A JP 2011041027 A JP2011041027 A JP 2011041027A JP 2011041027 A JP2011041027 A JP 2011041027A JP 2012177167 A JP2012177167 A JP 2012177167A
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Takashi Kobayashi
崇 小林
Nobuyuki Nakamura
展之 中村
Yoshimasa Funakawa
義正 船川
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Jfe Steel Corp
Jfeスチール株式会社
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PROBLEM TO BE SOLVED: To provide a steel sheet for soft nitriding treatment excellent in moldability and fatigue resistance.SOLUTION: The steel sheet for soft nitriding treatment has a composition containing, by mass%, C: 0.05% or more and 0.10% or less, Si: 0.5% or less, Mn: 0.7% or more and 1.5% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.01% or more and 0.06% or less, Cr: 0.5% or more and 1.5% or less, V: 0.03% or more and 0.30% or less, and N: 0.005% or less while a ratio between a solid solution V amount and the V content (solid solution V amount/V content) is 0.50 or more, and the balance Fe with inevitable impurities, and has a composite structure including ferrite and pearlite.

Description

本発明は、自動車の変速機部品等、疲労強度や耐摩耗性が要求される機械構造用部品に好適な軟窒化処理用鋼板に係り、特に軟窒化処理前の成形性に優れ、且つ、軟窒化処理後の耐疲労特性に優れた軟窒化処理用鋼板およびその製造方法に関する。   The present invention relates to a steel sheet for nitrocarburizing treatment that is suitable for machine structural parts that require fatigue strength and wear resistance, such as transmission parts for automobiles, and is particularly excellent in formability before soft nitriding treatment and is soft. The present invention relates to a steel sheet for soft nitriding that has excellent fatigue resistance after nitriding and a method for producing the same.
自動車用の変速機部品等、長時間継続して応力負荷された状態で使用される機械構造用部品には、疲労強度や耐摩耗性が要求される。そのため、これらの機械構造用部品は通常、鋼素材を所望の部品形状に加工したのち、表面硬化熱処理を施すことにより製造される。表面硬化熱処理を施すと、鋼表面が硬化するとともに鋼表層部に圧縮残留応力が導入されるため、疲労強度および耐摩耗性が向上する。   Fatigue strength and wear resistance are required for mechanical structural parts that are used in a state in which stress is continuously applied for a long time, such as transmission parts for automobiles. Therefore, these mechanical structural parts are usually manufactured by processing a steel material into a desired part shape and then subjecting it to a surface hardening heat treatment. When the surface hardening heat treatment is performed, the steel surface is hardened and compressive residual stress is introduced into the steel surface layer portion, so that fatigue strength and wear resistance are improved.
上記表面硬化熱処理の代表的なものとしては、浸炭処理と窒化処理が挙げられる。浸炭処理は、鋼をA3変態点以上の温度に加熱し、鋼の表層部に炭素を拡散・浸透(浸炭)させる処理であり、通常、高温状態にある浸炭後の鋼をそのまま焼入れすることにより、鋼の表面硬化を図っている。この浸炭処理では、A3変態点以上の高温域で鋼表層部に炭素を拡散・浸透させるため、炭素が鋼表面から比較的深い位置まで拡散・浸透する結果、大きな表面硬化層深さが得られる。 Typical examples of the surface hardening heat treatment include carburizing treatment and nitriding treatment. Carburization, the steel is heated to a temperature above A 3 transformation point, a process of diffusion and osmosis (carburized) carbon in the surface layer of the steel, usually, be directly quenching the steel after carburizing in a high temperature state Thus, the surface hardening of the steel is achieved. This carburization process, for diffusing and spreading the carbon steel surface layer portion in A 3 high temperature range of lower than the transformation point, a result of carbon is diffused and penetrated to a relatively deep position from the steel surface, resulting a large surface hardened layer depth It is done.
しかしながら、上記表面硬化熱処理として浸炭処理を採用した場合、焼入れ時の変態歪や熱歪に起因する部品形状精度の低下が避けられない。また、浸炭後に焼入れしたままの状態では、鋼の靭性が著しく低下する。そのため、浸炭処理を施して部品を製造する場合、焼入れ後に、部品形状の矯正や靭性回復を目的とした焼戻し(例えばプレステンパー処理)を施すことが必須となり、製造工程数が多くなるため、製造コスト面で極めて不利となる。   However, when carburizing treatment is adopted as the surface hardening heat treatment, a decrease in part shape accuracy due to transformation strain and thermal strain during quenching is inevitable. Moreover, in the state of being quenched after carburizing, the toughness of the steel is significantly reduced. Therefore, when manufacturing parts by carburizing, it is essential to perform tempering (for example, press tempering) for the purpose of correcting the shape of parts and restoring toughness after quenching, which increases the number of manufacturing processes. This is extremely disadvantageous in terms of cost.
一方、窒化処理は、鋼をA1変態点以下の温度に加熱し、鋼表層部に窒素を拡散・浸透(窒化)させる処理であり、浸炭処理のように焼入れすることなく鋼の表面硬化を図るものである。すなわち、窒化処理は処理温度が比較的低温であるうえ、鋼の相変態を伴わないため、窒化処理を施して部品を製造すれば、部品の形状精度を良好に保つことができる。但し、アンモニアガスを用いるガス窒化の場合、窒化に要する時間が約25〜150時間と著しく長いため、大量生産を前提とする自動車部品等には適さない。 On the other hand, nitriding treatment, the steel is heated to a temperature below the A 1 transformation point, a process of diffusion and osmosis (nitride) of nitrogen into the steel surface layer portion, the surface hardening of steel without quenching as carburizing It is intended. In other words, since the nitriding treatment has a relatively low processing temperature and does not involve a phase transformation of steel, if the nitriding treatment is performed to manufacture a part, the shape accuracy of the part can be kept good. However, in the case of gas nitriding using ammonia gas, the time required for nitriding is as long as about 25 to 150 hours, so that it is not suitable for automobile parts and the like on the assumption of mass production.
ガス窒化に見られる上記問題を有利に解決するものとして、近年、普及しつつあるのが軟窒化処理である。軟窒化処理は、浸炭性雰囲気を利用することによって窒化反応を迅速に進行させる窒化処理であり、被処理物は550〜600℃の処理雰囲気中に数時間保持され、鉄炭化物の生成をなかだちとして、鋼表面から鋼中に向けて窒素が拡散導入される。この軟窒化処理によると、得られる鋼表面硬度は従来の窒化処理(ガス窒化)よりも低くなるものの、窒化処理時間の大幅な短縮が可能となる。   In order to advantageously solve the above-mentioned problems found in gas nitriding, in recent years, soft nitriding is becoming popular. Soft nitriding treatment is a nitriding treatment in which a nitriding reaction proceeds rapidly by using a carburizing atmosphere, and the object to be treated is held in a treatment atmosphere at 550 to 600 ° C. for several hours, and the formation of iron carbide is facilitated. Nitrogen is diffused and introduced from the steel surface into the steel. According to this soft nitriding treatment, the steel surface hardness obtained is lower than that of the conventional nitriding treatment (gas nitriding), but the nitriding treatment time can be greatly shortened.
軟窒化処理は、塩浴中で処理する方法とガス中で処理する方法とに大きく分類される。塩浴中で処理する方法(塩浴軟窒化処理)では、シアン系の浴が用いられるため、環境汚染防止対策が必須となる。一方、ガス中で処理する方法(ガス軟窒化処理)では、アンモニアを主成分とする混合ガスを用いるため、環境汚染の原因となる排出物が少ない。以上の理由により、軟窒化処理のうち、特にガス中で処理するガス軟窒化処理の普及率が高まりつつある。   Soft nitriding treatment is roughly classified into a method of treating in a salt bath and a method of treating in a gas. In the method of treatment in a salt bath (salt bath soft nitriding treatment), since a cyan bath is used, measures to prevent environmental pollution are essential. On the other hand, in the method of treatment in gas (gas soft nitriding treatment), since a mixed gas containing ammonia as a main component is used, there are few emissions that cause environmental pollution. For the above reasons, among the soft nitriding treatments, the spread rate of the gas soft nitriding treatment that is processed in a gas is increasing.
一方、自動車の変速機部品をはじめとする機械構造用部品は、従来、鋳造や鍛造により得られた中間品に機械加工を施し、所望の形状に加工・接合して製造されるのが一般的であったが、近年、素材として鋼板(薄鋼板)が積極的に用いられるようになり、鋼板(薄鋼板)にプレス加工等を施し、所望の形状に成形して製造するようになっている。これにより、従来よりも製造工程が短縮され、製造コストの大幅な削減が可能となっている。このような背景から、自動車の変速機部品等、機械構造用部品の素材に好適な、成形性に優れた軟窒化処理用鋼板の要望が高まり、現在までに様々な技術が提案されている。   On the other hand, machine structural parts such as automobile transmission parts are conventionally manufactured by machining intermediate products obtained by casting or forging, and processing and joining them into desired shapes. However, in recent years, steel plates (thin steel plates) have been actively used as raw materials, and the steel plates (thin steel plates) are subjected to press working, etc., and formed into a desired shape and manufactured. . Thereby, a manufacturing process is shortened compared with the past, and the manufacturing cost can be significantly reduced. Against this background, there has been a growing demand for steel sheets for nitrocarburizing treatment with excellent formability, which are suitable for materials for machine structural parts such as automobile transmission parts, and various techniques have been proposed to date.
例えば、特許文献1および特許文献2には、重量比でC:0.01〜0.08%未満、Si:0.005〜1.00%、Mn:0.010〜3.00%、P:0.001〜0.150%、N:0.0002〜0.0100%、Cr:0.15超〜5.00%、Al:0.060超〜2.00%を含有し、さらに、Ti:0.010%以上および4C[%]未満、V:0.010〜1.00%の1種または2種を含有する組成の鋼を、熱間圧延後500℃以上で巻き取るか、その後50%以上の圧下率で冷間圧延を施し、再結晶焼鈍を行う、成形性に優れた窒化用鋼板の製造方法、および、上記した組成を有する成形性に優れた窒化用鋼板が開示されている。また、係る技術によると、成形性に悪影響を及ぼすC含有量を0.08%未満に抑制するとともに、Cr、Al等を窒化促進元素として含有することにより、成形性および窒化性に優れた窒化用鋼板となるとされている。   For example, in Patent Document 1 and Patent Document 2, C: 0.01 to less than 0.08%, Si: 0.005 to 1.00%, Mn: 0.010 to 3.00%, P: 0.001 to 0.150%, N: 0.0002 to 0.0100% by weight ratio , Cr: more than 0.15 to 5.00%, Al: more than 0.060 to 2.00%, Ti: more than 0.010% and less than 4C [%], V: 0.010 to 1.00% The steel is rolled up at 500 ° C. or higher after hot rolling, and then cold rolled at a reduction rate of 50% or higher, and recrystallization annealing is performed. A steel sheet for nitriding having the above composition and excellent formability is disclosed. In addition, according to the technology, the steel content for nitriding is excellent in formability and nitriding properties by suppressing the C content that adversely affects formability to less than 0.08% and containing Cr, Al, etc. as nitriding promoting elements. It is supposed to be.
また、特許文献3には、質量%で、C:0.03%以上0.10%未満、Si:0.005〜0.10%、Mn:0.1〜1.0%、Cr:0.20〜2.00%を含有し、不純物として、S:0.01%以下、P:0.020%以下、sol.Al:0.10%以下、N:0.01%以下であり、残部が実質的にFeからなる組成とし、JIS G 0552で規定されるフェライト結晶粒度を粒度番号で5以上12以下とする軟窒化処理用鋼が提案されている。そして、係る技術によると、Ti、V等の高価な元素を添加しないため安価な鋼板が得られるとともに、鋼の結晶粒径を微細化することによりプレス加工性に優れた鋼板が得られるとされている。   Patent Document 3 contains, in mass%, C: 0.03% or more and less than 0.10%, Si: 0.005 to 0.10%, Mn: 0.1 to 1.0%, Cr: 0.20 to 2.00%, and as impurities, S: 0.01% or less, P: 0.020% or less, sol.Al: 0.10% or less, N: 0.01% or less, with the balance being substantially composed of Fe, and the ferrite grain size specified by JIS G 0552 as the grain size number A steel for nitrocarburizing treatment of 5 to 12 is proposed. And according to such technology, it is said that an inexpensive steel sheet is obtained because expensive elements such as Ti and V are not added, and a steel sheet excellent in press workability is obtained by refining the crystal grain size of steel. ing.
また、特許文献4には、質量%で、C:0.01%超、0.09%以下、Si:0.005 〜0.5 %、Mn:0.01〜3.0 %、Al:0.005 〜2.0 %、Cr:0.50〜4.0 %、P:0.10%以下、S:0.01%以下およびN:0.010 %以下、或いは更にV:0.01〜1.0 %、Ti:0.01〜1.0 %およびNb:0.01〜1.0 %のうちから選んだ1種または2種以上を含有する組成とし、単位体積当たりの粒界面積Sv を80mm-1以上、1300mm-1以下とする窒化処理用薄鋼板が提案されている。そして、係る技術によると、Cr,Al,V,Ti,Nbといった窒化物形成元素を鋼板の成形性を阻害しない範囲で含有させたうえで、単位体積当たりの粒界面積を所定の範囲に制御することにより、窒化処理後に高い表面硬さと十分な硬化深さの両者が併せて得られるとされている。 Further, Patent Document 4 includes mass%, C: more than 0.01%, 0.09% or less, Si: 0.005 to 0.5%, Mn: 0.01 to 3.0%, Al: 0.005 to 2.0%, Cr: 0.50 to 4.0%, P: 0.10% or less, S: 0.01% or less and N: 0.010% or less, or V: 0.01-1.0%, Ti: 0.01-1.0% and Nb: 0.01-1.0% There has been proposed a thin steel sheet for nitriding with a composition containing the above and having a grain interface area Sv per unit volume of 80 mm -1 or more and 1300 mm -1 or less. And according to such a technique, after containing nitride forming elements such as Cr, Al, V, Ti, and Nb within a range that does not impair the formability of the steel sheet, the grain boundary area per unit volume is controlled within a predetermined range. By doing so, it is said that both high surface hardness and sufficient curing depth can be obtained after nitriding.
また、特許文献5には、C:0.01〜0.10mass%、Si:0.1mass%以下、Mn:0.1〜l.0mass%、P:0.05mass%以下、S:0.01mass%以下、Al:0.01〜0.06mass%、Cr:0.05〜0.50mass%、V:0.01〜0.30mass%、N:0.01mass%以下を含み、残部がFeおよび不可避的不純物からなる軟窒化用鋼板が提案されている。そして、係る技術によると、窒化促進元素としてCr:0.05〜0.50mass%およびV:0.01〜0.30mass%を含有することにより軟窒化処理による表面硬化特性が向上し、多量の合金元素を添加することなく、軟窒化処理前の成形性に優れ、軟窒化処理による表面硬化特性にも優れる軟窒化処理鋼板を安価に製造することができるとされている。   In Patent Document 5, C: 0.01 to 0.10 mass%, Si: 0.1 mass% or less, Mn: 0.1 to 1.0 mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, Al: 0.01 to There has been proposed a steel sheet for soft nitriding including 0.06 mass%, Cr: 0.05 to 0.50 mass%, V: 0.01 to 0.30 mass%, N: 0.01 mass% or less, and the balance being Fe and inevitable impurities. And according to the technique concerned, the surface hardening characteristic by a soft nitriding process improves by containing Cr: 0.05-0.50mass% and V: 0.01-0.30mass% as a nitriding acceleration | stimulation element, and adding a lot of alloy elements However, it is said that a nitrocarburized steel sheet having excellent formability before nitrocarburizing treatment and excellent surface hardening characteristics by nitrocarburizing treatment can be produced at low cost.
また、特許文献6には、C:0.04〜0.08mass%、Si:0.1mass%以下、Mn:0.05〜0.6mass%、P:0.03mass%以下、S:0.01mass%以下、Al:0.1mass%以下、Cr:0.6〜1.2mass%、V:0.002〜0.01mass%未満およびN:0.01mass%以下を含有し、残部がFeおよび不可避的不純物からなる軟窒化処理用鋼板が提案されている。そして、係る技術によると、極微量のV(0.002〜0.01mass%未満)を含有することにより、軟窒化処理によって高硬度であり且つポーラス層の形成が少ない窒化層を形成することができるため、加工性に優れるとともに耐摩耗性にも優れる軟窒化処理用鋼板が得られるとされている。   In Patent Document 6, C: 0.04 to 0.08 mass%, Si: 0.1 mass% or less, Mn: 0.05 to 0.6 mass%, P: 0.03 mass% or less, S: 0.01 mass% or less, Al: 0.1 mass% Hereinafter, a steel sheet for soft nitriding treatment containing Cr: 0.6 to 1.2 mass%, V: less than 0.002 to 0.01 mass% and N: 0.01 mass% or less, with the balance being Fe and inevitable impurities has been proposed. And according to such a technique, by containing a very small amount of V (less than 0.002 to 0.01 mass%), it is possible to form a nitrided layer with high hardness and less porous layer formation by soft nitriding, It is said that a steel sheet for soft nitriding that is excellent in workability and wear resistance is obtained.
特開平9−25513号公報Japanese Patent Laid-Open No. 9-25513 特開平9−25543号公報Japanese Patent Laid-Open No. 9-25543 特開2003−105489号公報Japanese Patent Laid-Open No. 2003-1054889 特開2003−277887号公報JP 2003-277877 A 特開2005−171331号公報JP 2005-171331 A 特開2008−280598号公報JP 2008-280598 A
しかしながら、特許文献1および特許文献2で提案された技術では、窒化促進元素として多量のAlを含有するため、Al介在物に起因する内部欠陥および表面欠陥の発生が懸念される。また、精錬時にAl系スラグが多く生成するため、溶製コストの高騰を招くという問題も見られる。   However, since the techniques proposed in Patent Document 1 and Patent Document 2 contain a large amount of Al as a nitriding promoting element, there is a concern about the occurrence of internal defects and surface defects due to Al inclusions. In addition, since a large amount of Al-based slag is produced during refining, there is also a problem that the melting cost increases.
また、特許文献3で提案された技術では、高価な元素を含まないため安価な軟窒化処理用鋼板が得られるものの、その強度は引張強さで高々420MPa程度であるため、高応力負荷状態で使用される部品への適用は制限される。   In addition, the technique proposed in Patent Document 3 does not contain an expensive element, so that an inexpensive steel sheet for soft nitriding can be obtained. However, the tensile strength is about 420 MPa at the most, so that it is in a high stress load state. Application to the parts used is limited.
また、特許文献4で提案された技術では、500MPaを超える引張強さを有する窒化処理用薄鋼板が得られるものの、窒化処理後の板厚方向の硬度分布についての考慮がなされておらず、実際に窒化処理が施された場合の部品耐久性能が必要十分な水準に達しない場合が多い。   Further, in the technique proposed in Patent Document 4, although a thin steel sheet for nitriding treatment having a tensile strength exceeding 500 MPa is obtained, the hardness distribution in the thickness direction after nitriding treatment is not taken into consideration, and actually In many cases, the durability of the parts does not reach the necessary and sufficient level when the nitriding treatment is applied.
また、特許文献5で提案された技術では、軟窒化処理による表面硬化特性に優れた軟窒化処理用鋼板が得られるものの、その引張強さは390MPaにも満たない。そのため、高い応力が負荷される機械構造用部品への適用は困難であり、汎用性に乏しい。   Further, with the technique proposed in Patent Document 5, although a steel sheet for nitrocarburizing treatment having excellent surface hardening characteristics by nitrocarburizing treatment can be obtained, its tensile strength is less than 390 MPa. Therefore, it is difficult to apply to mechanical structural parts to which high stress is applied, and the versatility is poor.
また、特許文献6で提案された技術では、Cr(0.6〜1.2mass%)とともに極微量のV(0.002〜0.01mass%未満)を含有することにより良質な窒化層を形成し、耐摩耗性に優れた軟窒化処理用鋼板が得られるものの、その強度は引張強さで高々400MPa程度であるため、特許文献3で提案された技術と同様に、高応力負荷状態で使用される部品への適用は制限される。   In addition, the technique proposed in Patent Document 6 contains a very small amount of V (less than 0.002 to 0.01 mass%) together with Cr (0.6 to 1.2 mass%), thereby forming a high-quality nitrided layer and improving wear resistance. Although an excellent steel sheet for nitrocarburizing treatment can be obtained, its strength is at most about 400 MPa in tensile strength, so that it can be applied to parts used under high stress load conditions, similar to the technique proposed in Patent Document 3. Is limited.
更に、鋼板に軟窒化処理を施す場合、通常、鋼板は約550〜600℃の処理温度に加熱され、該処理温度に約1〜5時間保持されるため、軟窒化処理により、鋼板表層部の硬さが著しく上昇する一方、鋼板の板厚内部(非窒化部)の強度は低下することがある。そのため、たとえ軟窒化処理前に所望の強度(引張強さ)を有していても、軟窒化処理により鋼板の板厚内部(非窒化部)の強度が大幅に低下し、軟窒化処理後の最終製品に所望の強度、並びに耐疲労特性を付与することができない場合が想定される。   Further, when the steel sheet is subjected to soft nitriding treatment, the steel sheet is usually heated to a processing temperature of about 550 to 600 ° C. and maintained at the processing temperature for about 1 to 5 hours. While the hardness increases remarkably, the strength inside the plate thickness (non-nitrided portion) of the steel sheet may decrease. Therefore, even if it has the desired strength (tensile strength) before the soft nitriding treatment, the strength inside the plate thickness (non-nitriding portion) of the steel sheet is greatly reduced by the soft nitriding treatment, It is assumed that desired strength and fatigue resistance cannot be imparted to the final product.
以上の理由により、軟窒化処理用鋼板においては、軟窒化処理後であっても鋼板の板厚内部(非窒化部)で所望の強度を有することが、重要な特性の1つとなる。しかしながら、上記した何れの従来技術においても、軟窒化処理前後に見られる板厚内部の強度変化について、何ら検討されていない。   For the above reasons, one of the important characteristics of a steel sheet for nitrocarburizing treatment is that it has a desired strength within the thickness (non-nitriding portion) of the steel sheet even after nitronitriding treatment. However, in any of the above-described conventional techniques, no consideration has been given to a change in strength inside the plate thickness seen before and after soft nitriding.
本発明は、上記した従来技術が抱える問題を有利に解決し、所望の強度(引張強さ:440MPa以上)を有し、且つ、軟窒化処理後の耐疲労特性に優れた軟窒化処理用鋼板およびその製造方法を提供することを目的とする。   The present invention advantageously solves the above-mentioned problems of the prior art, has a desired strength (tensile strength: 440 MPa or more), and has excellent fatigue resistance after nitrocarburizing treatment. And it aims at providing the manufacturing method.
上記課題を解決すべく、本発明者らは、軟窒化処理用鋼板の強度、成形性、並びに、軟窒化処理前後に見られる鋼板の板厚内部(非窒化部)の強度変化に及ぼす各種要因について鋭意検討した。その結果、以下のような知見を得た。
1)鋼板組織を、フェライトおよびパーライトを含む複合組織とすることにより、軟窒化処理後の強度低下が抑制され、強度安定性に優れた鋼板が得られること。
2)鋼板組成に関し、所望量のVを含有させ、該V含有量のうちの過半を固溶Vとすることにより、軟窒化処理を通じて、鋼板の表層部のみならず鋼板の板厚内部(非窒化部)の強度も増加し、耐疲労特性が向上すること。
3)軟窒化処理後に、鋼板の板厚内部(非窒化部)の硬さが、軟窒化処理前の硬さの5%超増加することにより、耐疲労特性が安定して向上すること。
In order to solve the above-mentioned problems, the present inventors have various factors affecting the strength and formability of a steel sheet for nitrocarburizing treatment, and the strength change inside the plate thickness (non-nitrided portion) of the steel sheet before and after the nitronitriding treatment. We studied earnestly. As a result, the following findings were obtained.
1) By making the steel sheet structure a composite structure containing ferrite and pearlite, a decrease in strength after nitrocarburizing treatment is suppressed, and a steel sheet having excellent strength stability is obtained.
2) Concerning the steel sheet composition, a desired amount of V is contained, and the majority of the V content is made into solute V, so that not only the surface layer portion of the steel sheet but also the inside of the steel sheet thickness (non- The strength of the nitrided part is also increased, and the fatigue resistance is improved.
3) After the nitrocarburizing treatment, the hardness inside the plate thickness (non-nitriding portion) of the steel sheet increases by more than 5% of the hardness before the nitrocarburizing treatment, so that the fatigue resistance is stably improved.
本発明は上記の知見に基づき完成されたものであり、その要旨は次のとおりである。
(1)質量%で、
C :0.05%以上0.10%以下、 Si:0.5%以下、
Mn:0.7%以上1.5%以下、 P :0.05%以下、
S :0.01%以下、 Al:0.01%以上0.06%以下、
Cr:0.5%以上1.5%以下、 V :0.03%以上0.30%以下、
N :0.005%以下
を含有し、且つ、固溶V量と前記V含有量との比(固溶V量/V含有量)が0.50超であり、残部がFeおよび不可避的不純物からなる組成と、フェライトおよびパーライトを含む複合組織とを有することを特徴とする、軟窒化処理用鋼板。
The present invention has been completed based on the above findings, and the gist thereof is as follows.
(1) In mass%,
C: 0.05% or more and 0.10% or less, Si: 0.5% or less,
Mn: 0.7% to 1.5%, P: 0.05% or less,
S: 0.01% or less, Al: 0.01% or more and 0.06% or less,
Cr: 0.5% to 1.5%, V: 0.03% to 0.30%,
N: a composition containing 0.005% or less, the ratio of the solute V content to the V content (solid solution V content / V content) is more than 0.50, and the balance is composed of Fe and inevitable impurities A steel sheet for nitrocarburizing treatment, characterized by having a composite structure containing ferrite and pearlite.
(2)(1)において、前記組成に加えてさらに、質量%でNb:0.005%以上0.025%以下を含有することを特徴とする、軟窒化処理用鋼板。 (2) A steel sheet for nitrocarburizing treatment according to (1), further containing Nb: 0.005% to 0.025% by mass% in addition to the above composition.
(3)鋼片を加熱し、粗圧延と仕上げ圧延からなる熱間圧延を施し、仕上げ圧延終了後、冷却し、巻取り、熱延鋼板とするにあたり、
前記鋼片を、質量%で、
C :0.05%以上0.10%以下、 Si:0.5%以下、
Mn:0.7%以上1.5%以下、 P :0.05%以下、
S :0.01%以下、 Al:0.01%以上0.06%以下、
Cr:0.5%以上1.5%以下、 V :0.03%以上0.30%以下、
N :0.005%以下
を含有し、残部がFeおよび不可避的不純物からなる組成とし、前記熱間圧延の加熱温度を1100℃以上1300℃以下とし、前記仕上げ圧延の仕上げ温度をAr3変態点以上(Ar3変態点+100℃)以下とし、前記冷却の平均冷却速度を30℃/s以上とし、前記巻取りの巻取り温度を500℃以上600℃以下とすることを特徴とする、軟窒化処理用鋼板の製造方法。
(3) Heating the steel slab, subjecting it to hot rolling consisting of rough rolling and finish rolling, cooling after completion of finish rolling, winding, and hot-rolled steel sheet,
The steel slab is in mass%,
C: 0.05% or more and 0.10% or less, Si: 0.5% or less,
Mn: 0.7% to 1.5%, P: 0.05% or less,
S: 0.01% or less, Al: 0.01% or more and 0.06% or less,
Cr: 0.5% to 1.5%, V: 0.03% to 0.30%,
N: 0.005% or less, with the balance being composed of Fe and inevitable impurities, the heating temperature of the hot rolling is 1100 ° C. or more and 1300 ° C. or less, and the finishing temperature of the finish rolling is Ar 3 transformation point or more ( Ar 3 transformation point + 100 ° C. or lower, average cooling rate of the cooling is 30 ° C./s or higher, and the winding temperature of the winding is 500 ° C. or higher and 600 ° C. or lower. A method of manufacturing a steel sheet.
(4)(3)において、前記組成に加えてさらに、質量%でNb:0.005%以上0.025%以下を含有することを特徴とする、軟窒化処理用鋼板の製造方法。 (4) In (3), in addition to the said composition, Nb: 0.005% or more and 0.025% or less are further contained in the mass%, The manufacturing method of the steel plate for soft nitriding treatment characterized by the above-mentioned.
本発明によれば、所望の強度(引張強さ:440MPa以上)を有し、且つ、軟窒化処理前の成形性および軟窒化処理後の耐疲労特性に優れた軟窒化処理用鋼板が得られる。このような鋼板であれば、自動車の変速機部品等、高応力負荷状態で使用される部品にも使用することができ、製造コストを大幅に削減することが可能となり、産業上格段の効果を奏する。   According to the present invention, a steel sheet for nitrocarburizing treatment having a desired strength (tensile strength: 440 MPa or more) and excellent in formability before nitronitriding treatment and fatigue resistance after nitronitriding treatment is obtained. . Such a steel plate can be used for parts used in high stress loads such as transmission parts of automobiles, and the manufacturing cost can be greatly reduced. Play.
以下、本発明について詳細に説明する。
まず、本発明鋼板の成分組成の限定理由について説明する。なお、以下の成分組成を表す%は、特に断らない限り質量%を意味するものとする。
C :0.05%以上0.10%以下
Cは、固溶強化および第二相の形成を通じて、鋼の高強度化に寄与する元素である。C含有量が0.05%未満である場合、自動車の変速機部品等、高応力負荷状態で使用される部品の素材として要求される鋼板強度を確保することができない。一方、C含有量が0.10%を超えると、鋼板強度が過度に高まり、成形性が低下する。したがって、C含有量は0.05%以上0.10%以下とする。好ましくは0.05%以上0.08%以下である。
Hereinafter, the present invention will be described in detail.
First, the reasons for limiting the component composition of the steel sheet of the present invention will be described. In addition,% showing the following component composition shall mean the mass% unless there is particular notice.
C: 0.05% or more and 0.10% or less
C is an element that contributes to increasing the strength of steel through solid solution strengthening and formation of the second phase. When the C content is less than 0.05%, it is not possible to ensure the strength of the steel sheet required as a material for parts used in high stress load conditions such as automobile transmission parts. On the other hand, when the C content exceeds 0.10%, the steel sheet strength is excessively increased and formability is deteriorated. Therefore, the C content is 0.05% or more and 0.10% or less. Preferably they are 0.05% or more and 0.08% or less.
Si:0.5%以下
Siは、固溶強化元素であり、鋼の高強度化に有効な元素であるとともに、脱酸剤としても作用する。このような効果を得るためには、0.03%以上含有させることが好ましいが、Si含有量が0.5%を超えると、難剥離性スケールが生成して鋼板の表面性状が顕著に悪化する。したがって、Si含有量は0.5%以下とする。好ましくは、0.1%以下である。
Si: 0.5% or less
Si is a solid solution strengthening element, is an element effective for increasing the strength of steel, and also acts as a deoxidizer. In order to obtain such an effect, it is preferable to contain 0.03% or more. However, if the Si content exceeds 0.5%, a hardly peelable scale is generated and the surface properties of the steel sheet are significantly deteriorated. Therefore, the Si content is 0.5% or less. Preferably, it is 0.1% or less.
Mn:0.7%以上1.5%以下
Mnは、固溶強化元素であり、鋼の高強度化に有効な元素である。また、鋼中に不純物として存在するSを析出物として固定し、鋼に対するS起因の悪影響を低減する元素としても作用する。Mn含有量が0.7%未満である場合、所望の鋼板強度を確保することができない。一方、Mn含有量が1.5%を超えると、鋼板強度が過度に高まり、成形性が低下する。したがって、Mn含有量は0.7%以上1.5%以下とする。好ましくは1.0%以上1.5%以下である。更に好ましくは1.2%以上1.5%以下である。
Mn: 0.7% to 1.5%
Mn is a solid solution strengthening element and is an element effective for increasing the strength of steel. It also acts as an element that fixes S present as impurities in steel as precipitates and reduces the adverse effects of S on steel. When the Mn content is less than 0.7%, the desired steel plate strength cannot be ensured. On the other hand, when the Mn content exceeds 1.5%, the steel sheet strength is excessively increased and formability is deteriorated. Therefore, the Mn content is 0.7% or more and 1.5% or less. Preferably they are 1.0% or more and 1.5% or less. More preferably, it is 1.2% or more and 1.5% or less.
P :0.05%以下
Pは、鋼板の成形性や靭性を低下させる元素であり、本発明ではPを極力低減することが好ましい。したがって、P含有量は0.05%以下とする。好ましくは0.03%以下である。
P: 0.05% or less
P is an element that lowers the formability and toughness of the steel sheet. In the present invention, P is preferably reduced as much as possible. Therefore, the P content is 0.05% or less. Preferably it is 0.03% or less.
S :0.01%以下
Sは、Pと同様、鋼板の成形性や靭性を低下させる元素であり、本発明ではSを極力低減することが好ましい。したがって、S含有量は0.01%以下とする。好ましくは0.005%以下である。
S: 0.01% or less
S, like P, is an element that lowers the formability and toughness of the steel sheet. In the present invention, S is preferably reduced as much as possible. Therefore, the S content is 0.01% or less. Preferably it is 0.005% or less.
Al:0.01%以上0.06%以下
Alは、脱酸剤として作用する元素であり、その効果を確実に得るためにAl含有量は0.01%以上とする。一方、Al含有量が0.06%を超えると、脱酸剤としての効果が飽和するうえ、Al系介在物が増加して鋼板の内部欠陥および表面欠陥を招来する。したがって、Al含有量は0.01%以上0.06%以下とする。好ましくは0.02%以上0.05%以下である。
Al: 0.01% or more and 0.06% or less
Al is an element that acts as a deoxidizer, and the Al content is set to 0.01% or more in order to reliably obtain the effect. On the other hand, if the Al content exceeds 0.06%, the effect as a deoxidizer is saturated, and Al inclusions increase to cause internal defects and surface defects in the steel sheet. Therefore, the Al content is 0.01% or more and 0.06% or less. Preferably they are 0.02% or more and 0.05% or less.
Cr:0.5%以上1.5%以下
Crは、軟窒化処理により鋼中に窒化物を形成し、鋼板表層部の硬度を高める効果を有する元素であり、本発明における重要な元素である。このような効果を顕著なものとするためには、Cr含有量を0.5%以上とする必要がある。一方、Cr含有量が1.5%を超えると、軟窒化処理により得られる表面硬化層(窒化層)の脆化が著しくなる。したがって、Cr含有量は0.5%以上1.5%以下とする。好ましくは0.5%以上1.0%以下である。
Cr: 0.5% to 1.5%
Cr is an element that has the effect of forming nitrides in steel by soft nitriding and increasing the hardness of the steel sheet surface layer, and is an important element in the present invention. In order to make such an effect remarkable, the Cr content needs to be 0.5% or more. On the other hand, when the Cr content exceeds 1.5%, the surface hardened layer (nitrided layer) obtained by soft nitriding becomes brittle. Therefore, the Cr content is 0.5% or more and 1.5% or less. Preferably they are 0.5% or more and 1.0% or less.
V :0.03%以上0.30%以下
Vは、軟窒化処理により鋼中に窒化物を形成し、鋼板表層部の硬度を高める効果を有し、且つ、軟窒化処理を通じて鋼板の板厚内部(非窒化部)の強度を高める効果を有する元素であり、本発明における最も重要な元素である。また、軟窒化処理前の鋼中に析出しているVは、粒子分散強化(析出強化)によって軟窒化処理用鋼板の強度を高める効果も有する。V含有量が0.03%未満である場合、これらの効果を十分に発現することができない。一方、V含有量が0.30%を超えると、軟窒化処理により得られる表面硬化層(窒化層)の脆化が著しくなるうえ、鋼板の強度向上効果が飽和するため経済的にも不利となる。したがって、V含有量は0.03%以上0.30%以下とする。好ましくは0.05%以上0.20%以下である。
V: 0.03% to 0.30%
V has the effect of increasing the hardness of the steel sheet surface layer by forming nitrides in the steel by soft nitriding, and increasing the strength of the steel sheet thickness (non-nitrided part) through soft nitriding. And the most important element in the present invention. Further, V precipitated in the steel before the soft nitriding treatment also has an effect of increasing the strength of the steel sheet for soft nitriding treatment by particle dispersion strengthening (precipitation strengthening). When the V content is less than 0.03%, these effects cannot be sufficiently exhibited. On the other hand, if the V content exceeds 0.30%, the surface hardened layer (nitrided layer) obtained by soft nitriding becomes brittle and the effect of improving the strength of the steel sheet is saturated, which is economically disadvantageous. Therefore, the V content is 0.03% or more and 0.30% or less. Preferably they are 0.05% or more and 0.20% or less.
N :0.005%以下
Nは、鋼板の成形性を低下させる有害な元素である。また、Nは、軟窒化処理前にCr等の窒化促進元素と化合し、有効な窒化促進元素量の低下を招く元素でもある。したがって、本発明ではN含有量を極力低減することが好ましく、0.005%以下とする。好ましくは0.003%以下である。
N: 0.005% or less
N is a harmful element that reduces the formability of the steel sheet. N is also an element that combines with a nitriding promoting element such as Cr before the soft nitriding treatment and causes a decrease in the effective nitriding promoting element amount. Therefore, in the present invention, it is preferable to reduce the N content as much as possible, and it is 0.005% or less. Preferably it is 0.003% or less.
固溶V量とV含有量との比(固溶V量/V含有量):0.50超
鋼板中の固溶Vは、軟窒化処理を通じて鋼板の表層部および板厚内部(非窒化部)の強度を向上させ、軟窒化処理後の耐疲労特性を確保するうえで重要な役割を担う。そこで、本発明では、軟窒化処理用鋼板、すなわち軟窒化処理前の鋼板における固溶V量とV含有量との比を0.50超とする。
Ratio of solid solution V to V content (solid solution V content / V content): more than 0.50 The solid solution V in the steel sheet is the surface layer part of the steel sheet and the inside of the sheet thickness (non-nitriding part) through soft nitriding. It plays an important role in improving strength and ensuring fatigue resistance after soft nitriding. Therefore, in the present invention, the ratio between the solute V content and the V content in the steel sheet for soft nitriding treatment, that is, the steel sheet before soft nitriding treatment, is set to exceed 0.50.
先述のとおり、鋼板に軟窒化処理を施すと、軟窒化処理の熱履歴を経ることにより鋼板の板厚内部(非窒化部)の強度が低下することがあり、軟窒化処理後に所望の耐疲労特性が得られない場合が想定される。そのため、軟窒化処理用鋼板においては、軟窒化処理を施した後の鋼板の板厚内部(非窒化部)が所望の強度を有するような特性を具えていることが重要である。   As described above, when soft nitriding treatment is applied to a steel sheet, the strength inside the thickness of the steel sheet (non-nitriding part) may decrease due to the thermal history of soft nitriding treatment. It is assumed that the characteristics cannot be obtained. For this reason, it is important that the steel sheet for soft nitriding has characteristics such that the thickness inside the non-nitrided portion of the steel sheet after the soft nitriding treatment has a desired strength.
軟窒化処理を施した後の鋼板の板厚内部(非窒化部)の強度を確保する手段としては、軟窒化処理による鋼板の板厚内部(非窒化部)の強度低下分を考慮して軟窒化処理用鋼板の強度を高めに設定する手段も考えられる。しかしながら、鋼板強度を過度に高めると、鋼板の成形性が低下し、軟窒化処理前に所望の部品形状に成形するうえで不利となる。   As a means to ensure the strength of the steel sheet thickness (non-nitriding part) after the soft nitriding treatment, the strength reduction in the steel sheet thickness (non-nitriding part) due to the soft nitriding treatment is taken into account. A means for setting the strength of the steel sheet for nitriding treatment higher is also conceivable. However, when the steel plate strength is excessively increased, the formability of the steel plate is lowered, which is disadvantageous in forming a desired part shape before the soft nitriding treatment.
疲労強度や耐摩耗性が要求される機械構造用部品を、軟窒化処理用鋼板を素材として用いて製造するに際しては、軟窒化処理用鋼板をプレス加工等により所望の部品形状に成形したのち、軟窒化処理を施して最終製品とする。そのため、軟窒化処理用鋼板(軟窒化処理前の鋼板)の強度を必要以上に高めることは、軟窒化処理前の成形性に悪影響を及ぼし、好ましくない。   When manufacturing mechanical structural parts that require fatigue strength and wear resistance using steel sheets for nitrocarburizing treatment as raw materials, after forming the steel sheet for nitronitriding treatment into a desired part shape by pressing or the like, Soft nitriding is performed to make the final product. Therefore, it is not preferable to increase the strength of the steel sheet for nitrocarburizing treatment (steel plate before nitrocarburizing treatment) more than necessary because it adversely affects the formability before nitrocarburizing treatment.
一方、軟窒化処理用鋼板に軟窒化処理を施すことにより、その板厚内部(非窒化部)の強度を軟窒化処理前よりも上昇させることができれば、軟窒化処理前の成形性を低下させることなく、軟窒化処理後の耐疲労特性を向上させることができる。そのため、軟窒化処理前の成形性とともに軟窒化処理後の耐疲労特性が要求される軟窒化処理用鋼板としては、軟窒化処理を通じて鋼板の板厚内部(非窒化部)の強度が上昇する特性を有するのが理想的である。   On the other hand, if the strength inside the plate thickness (non-nitrided part) can be increased by applying soft nitriding treatment to the steel sheet for soft nitriding treatment, the formability before soft nitriding treatment is lowered. Therefore, the fatigue resistance after the soft nitriding treatment can be improved. Therefore, as a steel sheet for nitrocarburizing treatment that requires fatigue resistance after nitrocarburizing treatment as well as formability before nitrocarburizing treatment, the strength of the steel sheet thickness (non-nitrided part) increases through nitrocarburizing treatment. It is ideal to have
そこで、軟窒化処理を通じて、鋼板の板厚内部(非窒化層)の強度を向上させる手段について、本発明者らが検討した結果、軟窒化処理前の鋼板中に所望量の固溶Vを含有させ、軟窒化処理時に固溶Vを炭化物として析出させることが有効であることを知見した。   Therefore, as a result of the present inventors' study of means for improving the strength of the steel sheet thickness (non-nitriding layer) through soft nitriding, a desired amount of solute V is contained in the steel before soft nitriding. It was found that it is effective to precipitate solid solution V as a carbide during soft nitriding.
係る知見に基づき、本発明においては、鋼板中のV含有量を0.03%以上0.30%以下としたうえで、V含有量の過半を固溶Vとすること、すなわち、固溶V量とV含有量との比(固溶V量/V含有量)を0.50超とすることを必須とする。固溶V量とV含有量との比(固溶V量/V含有量)が0.50以下である場合、軟窒化処理に伴う鋼板の板厚内部(非窒化部)の強度上昇効果を十分に発現することができない。なお、軟窒化処理前の鋼中に炭窒化物として析出させ、軟窒化処理前の鋼板強度の確保と軟窒化処理による硬化量の確保を両立させる観点から、固溶V量とV含有量との比(固溶V量/V含有量)の上限値は、0.80とすることが好ましい。   Based on such knowledge, in the present invention, the V content in the steel sheet is set to 0.03% or more and 0.30% or less, and the majority of the V content is set as solute V, that is, the solute V content and the V content. It is essential that the ratio to the amount (solid solution V amount / V content) exceeds 0.50. When the ratio of solute V content to V content (solid solution V content / V content) is 0.50 or less, the effect of increasing the strength inside the steel plate thickness (non-nitrided part) due to soft nitriding is sufficient. It cannot be expressed. From the viewpoint of precipitating as carbonitride in the steel before soft nitriding treatment, and ensuring both the strength of the steel sheet before soft nitriding treatment and the securing of the hardening amount by soft nitriding treatment, The upper limit of the ratio (solid solution V amount / V content) is preferably 0.80.
以上が、本発明における基本組成であるが、基本組成に加えてさらにNbを含有することができる。
Nb:0.005%以上0.025%以下
Nbは、鋼中に炭窒化物として析出し、粒子分散強化(析出強化)によって鋼板の強度を高めるうえで有効な元素であり、必要に応じて含有できる。Nb含有量が0.005%未満である場合、このような効果を十分に発現することができない。一方、Nb含有量が0.025%を超えると、鋼板強度が過度に高まり、成形性が低下する。したがって、Nb含有量は0.005%以上0.025%以下とする。好ましくは0.010%以上0.020%以下である。
The above is the basic composition in the present invention, but it can further contain Nb in addition to the basic composition.
Nb: 0.005% to 0.025%
Nb precipitates as carbonitride in the steel and is an element effective in increasing the strength of the steel sheet by particle dispersion strengthening (precipitation strengthening), and can be contained as necessary. When the Nb content is less than 0.005%, such an effect cannot be sufficiently exhibited. On the other hand, when the Nb content exceeds 0.025%, the steel sheet strength is excessively increased and formability is deteriorated. Therefore, the Nb content is 0.005% or more and 0.025% or less. Preferably they are 0.010% or more and 0.020% or less.
本発明の鋼板において、上記以外の成分は、Feおよび不可避的不純物である。不可避的不純物としては、例えば、質量%で、Cu:0.05%以下、Ni:0.05%以下、Mo:0.05%以下、Co:0.05%以下、Ti:0.005%以下、Zr:0.005%以下、Ca:0.005%以下、Sn:0.005%以下、O:0.005%以下、B:0.0005%以下等が許容できる。   In the steel sheet of the present invention, components other than those described above are Fe and inevitable impurities. As unavoidable impurities, for example, in mass%, Cu: 0.05% or less, Ni: 0.05% or less, Mo: 0.05% or less, Co: 0.05% or less, Ti: 0.005% or less, Zr: 0.005% or less, Ca: 0.005% or less, Sn: 0.005% or less, O: 0.005% or less, B: 0.0005% or less are acceptable.
次に、本発明鋼板の組織の限定理由について説明する。
本発明の鋼板は、フェライトおよびパーライトを含む複合組織を有する。
Next, the reason for limiting the structure of the steel sheet of the present invention will be described.
The steel sheet of the present invention has a composite structure containing ferrite and pearlite.
鋼板組織に占めるフェライトの割合を高めることは、鋼板の成形性を確保するうえで有効であるが、鋼板をフェライト単相組織とすると、鋼板強度が不足し、機械構造用部品の素材としての適用範囲が狭まり、汎用性に乏しくなる。一方、フェライト主体の組織中に第二相を生成させて鋼板強度を確保する場合において、マルテンサイト、ベイナイト等の硬質な低温変態相を第二相とした場合には、軟窒化処理時の熱履歴によって上記低温変態相が軟化してしまい、鋼板の板厚内部(非窒化部)の強度が大幅に低下してしまう。   Increasing the proportion of ferrite in the steel sheet structure is effective in securing the formability of the steel sheet. However, if the steel sheet has a ferrite single-phase structure, the steel sheet strength is insufficient and can be used as a material for machine structural parts. The range is narrowed and the versatility becomes poor. On the other hand, in the case where the second phase is generated in the structure mainly composed of ferrite to ensure the strength of the steel sheet, when the hard low temperature transformation phase such as martensite and bainite is used as the second phase, The low temperature transformation phase is softened by the history, and the strength inside the plate thickness (non-nitrided portion) of the steel sheet is greatly reduced.
そこで、本発明においては、軟窒化処理の熱履歴による鋼板の板厚内部(非窒化部)の強度低下を抑制すべく、鋼板の組織を、フェライトを主相とし、第二相をパーライトとした複合組織とする。なお、本発明においては、鋼板組織中のフェライト分率を80%以上95%以下とし、パーライト分率を5%以上20%以下とすることが好ましい。また、本発明の鋼板は、フェライトとパーライトからなる複合組織とすることが理想的であるが、その他の相(組織)が不可避的に生じてしまう場合であっても、その分率が合計で1%以下であれば許容できる。   Therefore, in the present invention, the structure of the steel sheet has ferrite as the main phase and the second phase as pearlite in order to suppress the strength reduction of the steel sheet thickness (non-nitriding part) due to the thermal history of the soft nitriding treatment. A complex organization is assumed. In the present invention, it is preferable that the ferrite fraction in the steel sheet structure is 80% to 95% and the pearlite fraction is 5% to 20%. The steel sheet of the present invention is ideally a composite structure composed of ferrite and pearlite, but even if other phases (structures) are inevitably generated, the fractions are in total. 1% or less is acceptable.
次に、本発明鋼板の製造方法について説明する。
本発明は、上記した組成を有する鋼片を加熱し、粗圧延と仕上げ圧延からなる熱間圧延を施し、仕上げ圧延終了後、冷却し、巻取り、熱延鋼板とする。この際、加熱温度を1100℃以上1300℃以下とし、仕上げ温度をAr3変態点以上(Ar3変態点+100℃)以下とし、冷却の平均冷却速度を30℃/s以上とし、巻取り温度を500℃以上600℃以下とすることが好ましい。
Next, the manufacturing method of this invention steel plate is demonstrated.
In the present invention, a steel slab having the above composition is heated, subjected to hot rolling consisting of rough rolling and finish rolling, cooled after completion of finish rolling, and wound into a hot rolled steel sheet. At this time, the heating temperature is set to 1100 ° C to 1300 ° C, the finishing temperature is set to Ar 3 transformation point or higher (Ar 3 transformation point + 100 ° C) or lower, the average cooling rate of cooling is set to 30 ° C / s or higher, and the coiling temperature is set. It is preferable that the temperature is 500 ° C. or more and 600 ° C. or less.
本発明において、鋼の溶製方法は特に限定されず、転炉、電気炉等、公知の溶製方法を採用することができる。また、溶製後、偏析等の問題から連続鋳造法により鋼片(スラブ)とするのが好ましいが、造塊−分塊圧延法、薄スラブ連鋳法等、公知の鋳造方法で鋼片としてもよい。更に、必要に応じて、各種予備処理や二次精錬、鋼片の表面手入等を施してもよい。   In the present invention, the method for melting steel is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. In addition, after melting, it is preferable to form a steel slab (slab) by a continuous casting method from the problem of segregation and the like, but as a steel slab by a known casting method such as an ingot-bundling rolling method or a thin slab continuous casting method. Also good. Furthermore, if necessary, various pretreatments, secondary refining, surface treatment of steel pieces, and the like may be performed.
鋼片の加熱温度:1100℃以上1300℃以下
上記の如く得られた鋼片に、粗圧延および仕上げ圧延を施すが、本発明においては、粗圧延前の鋼片中にVを十分に再固溶させる必要がある。鋼片の加熱温度が1100℃未満である場合、V炭窒化物を十分に分解してVを再固溶させることが困難で、Vを含有することにより得られる前記した所望の効果を発現することができないことがある。また、必要な仕上げ温度の確保も困難となる。一方、鋼片の加熱温度が1300℃を超えると、鋼片の加熱に要するエネルギーが増大し、コスト面で不利となる。したがって、粗圧延前の鋼片の加熱温度は1100℃以上1300℃以下とする。好ましくは1150℃以上1250℃以下である。
Heating temperature of steel slab: 1100 ° C or higher and 1300 ° C or lower The steel slab obtained as described above is subjected to rough rolling and finish rolling. In the present invention, V is sufficiently resolidified in the steel slab before rough rolling. It needs to be dissolved. When the heating temperature of the billet is less than 1100 ° C, it is difficult to sufficiently decompose V carbonitride and re-dissolve V, and the desired effect obtained by containing V is exhibited. There are times when you can't. In addition, it is difficult to secure a necessary finishing temperature. On the other hand, if the heating temperature of the steel slab exceeds 1300 ° C., the energy required for heating the steel slab increases, which is disadvantageous in terms of cost. Therefore, the heating temperature of the steel slab before rough rolling is 1100 ° C. or higher and 1300 ° C. or lower. Preferably they are 1150 degreeC or more and 1250 degrees C or less.
粗圧延前の鋼片を加熱するに際しては、鋳造後の鋼片を常温まで冷却してから加熱してもよいし、鋳造後冷却途中の鋼片を追加加熱或いは保熱してもよい。また、鋳造後の鋼片が十分な温度を保持しており、鋼中にVが十分に固溶している場合には、鋼片を加熱することなく直送圧延しても良い。なお、粗圧延条件については特に限定する必要はない。   When heating the steel slab before rough rolling, the steel slab after casting may be heated after cooling to room temperature, or the steel slab during cooling after casting may be additionally heated or kept warm. Further, when the steel slab after casting maintains a sufficient temperature and V is sufficiently dissolved in the steel, direct rolling may be performed without heating the steel slab. The rough rolling conditions are not particularly limited.
仕上げ温度:Ar3変態点以上(Ar3変態点+100℃)以下
仕上げ圧延における仕上げ温度がAr3変態点未満である場合、圧延方向に展伸したフェライト組織、および、未再結晶フェライト組織が形成され、鋼板の成形性が低下する。また、鋼板の機械的特性の面内異方性が強くなり、均質な成形加工が困難となる。一方、仕上げ温度が(Ar3変態点+100℃)を超えると、鋼板の表面性状が悪化する傾向が見られる。したがって、仕上げ温度はAr3変態点以上(Ar3変態点+100℃)以下とする。なお、仕上げ温度とは、仕上げ圧延の最終パス出側での鋼板温度をさす。
Finishing temperature: Ar 3 transformation point or higher (Ar 3 transformation point + 100 ° C) or lower When the finishing temperature in finish rolling is lower than the Ar 3 transformation point, a ferrite structure stretched in the rolling direction and an unrecrystallized ferrite structure are formed. As a result, the formability of the steel sheet is reduced. In addition, the in-plane anisotropy of the mechanical properties of the steel sheet becomes strong, and uniform forming becomes difficult. On the other hand, when the finishing temperature exceeds (Ar 3 transformation point + 100 ° C.), the surface properties of the steel sheet tend to deteriorate. Therefore, the finishing temperature should be not less than the Ar 3 transformation point and not more than (Ar 3 transformation point + 100 ° C.). The finishing temperature refers to the steel plate temperature at the final pass exit side of finish rolling.
上記仕上げ温度を確保すべく、シートバーヒーター、エッジヒーター等の加熱装置を利用して、圧延中の鋼板を追加加熱してもよい。なお、鋼のAr3変態点については、オーステナイト温度域からの冷却過程における熱収縮を測定して熱収縮曲線を作成して求めても、或いは、合金元素の含有量から概算して求めてもよい。 In order to ensure the finishing temperature, a steel plate being rolled may be additionally heated using a heating device such as a sheet bar heater or an edge heater. Note that the Ar 3 transformation point of steel can be obtained by measuring the heat shrinkage in the cooling process from the austenite temperature range and creating a heat shrinkage curve, or by roughly calculating from the alloy element content. Good.
平均冷却速度:30℃/s以上
平均冷却速度の適正化は、鋼板中の固溶Vを確保するうえで重要であり、本発明においては、仕上げ圧延終了後、直ちに(1s以内に)冷却を開始し、仕上げ温度から巻取り温度までの平均冷却速度を30℃/s以上とする。この平均冷却速度が30℃/s未満である場合、冷却過程でVの炭窒化物が析出し、鋼板中に所望量の固溶Vが残存しなくなるおそれがある。また、結晶粒が過度に粗大化して、鋼板の強度や延性が低下する場合がある。したがって、上記平均冷却速度は30℃/s以上とする。好ましくは40℃/s以上である。
Average cooling rate: 30 ° C / s or more Optimization of the average cooling rate is important for securing solid solution V in the steel sheet. In the present invention, cooling is performed immediately (within 1 s) immediately after finishing rolling. Start, and set the average cooling rate from the finishing temperature to the coiling temperature to 30 ° C / s or higher. When this average cooling rate is less than 30 ° C./s, V carbonitride may precipitate during the cooling process, and a desired amount of solute V may not remain in the steel sheet. Further, the crystal grains may be excessively coarsened, and the strength and ductility of the steel sheet may be reduced. Therefore, the average cooling rate is set to 30 ° C./s or more. Preferably it is 40 ° C./s or more.
上記平均冷却速度の上限は特に規定されないが、強水冷に起因する鋼板の形状不良を避けるためには、100℃/s以下とすることが好ましい。なお、鋼板が巻取り温度に達するまで冷却された後は、注水等による強制冷却は特に不要であり、巻取りまで大気中で放冷すればよい。   The upper limit of the average cooling rate is not particularly defined, but is preferably set to 100 ° C./s or less in order to avoid the shape failure of the steel sheet due to strong water cooling. In addition, after the steel plate is cooled until it reaches the winding temperature, forced cooling by water injection or the like is not particularly necessary, and it may be allowed to cool in the atmosphere until winding.
巻取り温度:500℃以上600℃以下
巻取り温度の適正化は、鋼板中の固溶Vを確保するとともに、鋼板を所望の組織とするうえで重要である。巻き取り温度が500℃未満である場合、低温変態相が生成して鋼板が硬質化し、成形性が低下するとともに、軟窒化処理の熱履歴による鋼板の板厚内部(非窒化部)の強度低下が避けられない。一方、巻取り温度が600℃を超えると、巻取り後にV炭窒化物が多量に析出し、鋼板中に所望量の固溶Vが残存しなくなるおそれがある。したがって、巻取り温度は500℃以上600℃以下とする。好ましくは520℃以上580℃以下である。
Winding temperature: 500 ° C. or more and 600 ° C. or less Optimization of the winding temperature is important for securing the solid solution V in the steel sheet and making the steel sheet into a desired structure. When the coiling temperature is less than 500 ° C, a low-temperature transformation phase is generated, the steel sheet becomes hard, the formability decreases, and the strength inside the steel sheet thickness (non-nitrided part) decreases due to the thermal history of nitrocarburizing treatment. Is inevitable. On the other hand, if the winding temperature exceeds 600 ° C., a large amount of V carbonitride precipitates after winding, and there is a possibility that a desired amount of solute V does not remain in the steel sheet. Therefore, the coiling temperature is set to 500 ° C. or more and 600 ° C. or less. Preferably they are 520 degreeC or more and 580 degrees C or less.
上記によって得られた熱延鋼板は、酸洗、ショットピーニング等により酸化スケールを除去したのちに、軟窒化処理用鋼板として使用される。また、形状矯正や表面粗度の調整を目的とした調質圧延を施しても、本発明の効果が損なわれることはない。
なお、本発明の軟窒化処理用鋼板は、ガス軟窒化処理および塩浴軟窒化処理の何れに対しても適用可能である。
The hot-rolled steel sheet obtained as described above is used as a steel sheet for soft nitriding treatment after removing the oxide scale by pickling, shot peening or the like. Moreover, even if the temper rolling for the purpose of shape correction or surface roughness adjustment is performed, the effect of the present invention is not impaired.
The steel sheet for soft nitriding of the present invention can be applied to both gas soft nitriding and salt bath soft nitriding.
表1に示す化学成分を含有する鋼を溶製し、造塊・分塊圧延して鋼片とした。これらの鋼片を加熱したのち、粗圧延および仕上げ圧延を施し、仕上げ圧延終了後、直ちに冷却し、巻取り、板厚:3.2mmの熱延鋼板とした。なお、上記における鋼片の加熱温度、仕上げ温度、仕上げ温度から巻取り温度までの平均冷却速度、巻取り温度は、表2に示すとおりである。   Steel containing the chemical components shown in Table 1 was melted, and ingot-making and ingot-rolling to obtain steel pieces. These steel slabs were heated and then subjected to rough rolling and finish rolling. After finishing rolling, the steel slabs were immediately cooled and wound to form hot-rolled steel sheets having a thickness of 3.2 mm. In addition, the heating temperature of steel slab, the finishing temperature, the average cooling rate from the finishing temperature to the winding temperature, and the winding temperature in the above are as shown in Table 2.
上記により得られた熱延鋼板を酸洗してデスケーリングし、伸長率:0.5%の調質圧延を施した。そして、調質圧延後の鋼板から試験片を採取し、以下の評価に供した。
(i)固溶V量
固溶V量は、調質圧延後鋼板の板幅1/4位置から試験片を採取し、該試験片を電解液中で定電流電解して得た鋼中析出物中のV量を、V含有量から差し引くことにより求めた。
The hot-rolled steel sheet obtained above was pickled and descaled, and subjected to temper rolling with an elongation of 0.5%. And the test piece was extract | collected from the steel plate after temper rolling, and it used for the following evaluation.
(I) Solid solution V amount The solid solution V amount is the precipitation in steel obtained by taking a test piece from 1/4 position of the plate width of the steel sheet after temper rolling, and subjecting the test piece to constant current electrolysis in an electrolytic solution. The amount of V in the product was determined by subtracting from the V content.
(ii)組織観察
調質圧延後鋼板の板幅1/4位置における、圧延方向に平行な板厚断面の試料を採取し、鏡面研磨してナイタールで腐食したのち、板厚1/4位置を、光学顕微鏡あるいは走査型電子顕微鏡で500〜3000倍の適当な倍率にて撮影した。得られた組織写真を用い、画像解析により、組織全体に対するフェライト面積率、パーライト面積率、並びに、その他の組織の種類およびそれらの面積率を求め、それぞれの分率とした。得られた結果を、表3に示す。
(Ii) Microstructure observation After a temper rolling, a sample of the plate thickness cross section parallel to the rolling direction was taken at 1/4 position of the plate width, mirror polished and corroded with nital, then the 1/4 thickness position was Images were taken at an appropriate magnification of 500 to 3000 times with an optical microscope or a scanning electron microscope. Using the obtained structure photograph, the ferrite area ratio, the pearlite area ratio, the other structure types and their area ratios with respect to the entire structure were obtained by image analysis, and were used as the respective fractions. The results obtained are shown in Table 3.
(iii)引張試験
調質圧延後鋼板の板幅1/4位置において、引張試験方向が圧延方向となるように採取したJIS Z 2201(1998)規定の5号試験片(標点距離L:50mm)を用い、JIS Z 2241(1998)の規定に準拠した引張試験を行い、引張強さ(TS)と伸び(El)を測定し、強度・伸びバランス(TS×El)を求めた。なお、本実施例においては、引張強さ(TS):440MPa以上、強度・伸びバランス(TS×El):17GPa・%以上の鋼板を、高強度かつ良好な成形性を有するものと評価した。
(Iii) Tensile test JIS Z 2201 (1998) No. 5 test piece (marking distance L: 50 mm) taken so that the tensile test direction is the rolling direction at the 1/4 width position of the steel sheet after temper rolling. ) Was used to perform a tensile test in accordance with the provisions of JIS Z 2241 (1998), and the tensile strength (TS) and elongation (El) were measured to determine the strength / elongation balance (TS × El). In this example, a steel sheet having a tensile strength (TS): 440 MPa or more and a strength / elongation balance (TS × El): 17 GPa ·% or more was evaluated as having high strength and good formability.
(iv)断面硬さ試験
上記調質圧延後の鋼板から試験片を採取し、JIS Z 2244(2009)に準拠した方法により、板厚1/2位置におけるビッカース硬さ(HVc)を測定した。
<測定方法>
試験力 :0.98N
測定箇所:5箇所
(Iv) Cross Section Hardness Test A specimen was collected from the steel sheet after the temper rolling, and the Vickers hardness (HVc) at a thickness 1/2 position was measured by a method based on JIS Z 2244 (2009).
<Measurement method>
Test force: 0.98N
Measurement location: 5 locations
(v)軟窒化処理試験
上記調質圧延後の鋼板から小片を採取し、以下に示す条件のガス軟窒化処理を施した。
軟窒化雰囲気:アンモニアガスと吸熱型変成ガスの等量比混合ガス
処理温度 :580℃
処理時間 :2.5時間
なお、上記処理温度(580℃)に上記処理時間(2.5時間)保持したのち、小片を油冷した(油温:70℃)。そして、油冷後の小片を、以下の評価に供した。
(V) Soft nitriding test Small pieces were collected from the temper-rolled steel sheet and subjected to gas soft nitriding under the following conditions.
Soft nitriding atmosphere: Equivalent ratio mixed gas of ammonia gas and endothermic metamorphic gas Processing temperature: 580 ° C
Treatment time: 2.5 hours In addition, after holding the said treatment time (2.5 hours) at the said treatment temperature (580 degreeC), the small piece was oil-cooled (oil temperature: 70 degreeC). And the small piece after oil cooling was used for the following evaluation.
油冷後の小片について、JIS G 0563(1993)に準拠して、板表面から深さ0.1mm位置におけるビッカース硬さ(HV0.1)を測定した。また、JIS G 0562(1993)の規定に準拠した実用窒化層深さを測定した。本実施例においては、ビッカース硬さ(HV0.1):500以上であり且つ実用窒化層深さ:0.40mm以上のものを表面硬化特性が良好なものと評価した。
また、鋼板の板厚内部(非窒化部)の硬さを代表して、板厚1/2位置(非窒化部)におけるビッカース硬さ(HVc’)を、上記(iv)と同様の方法により測定した。そして、上記(iv)で求めた軟窒化処理前の板厚1/2位置におけるビッカース硬さ(HVc)と、軟窒化処理後の板厚1/2位置におけるビッカース硬さ(HVc’)から、軟窒化処理による板厚中央部のビッカース硬さの上昇率:(HVc’−HVc)/HVc×100(%)を求めた。本実施例においては、ビッカース硬さの上昇率が5.0%超であるものを、軟窒化処理後の耐疲労特性が良好なもの(○)とし、それ以外を×として評価した。得られた結果を、表4に示す。
For the small pieces after oil cooling, the Vickers hardness (HV0.1) at a depth of 0.1 mm from the plate surface was measured according to JIS G 0563 (1993). Further, the practical nitrided layer depth in accordance with JIS G 0562 (1993) was measured. In this example, Vickers hardness (HV0.1): 500 or more and practical nitrided layer depth: 0.40 mm or more were evaluated as having good surface hardening characteristics.
In addition, the Vickers hardness (HVc ') at the plate thickness 1/2 position (non-nitrided portion) is represented by the same method as in (iv) above, representing the hardness inside the plate thickness (non-nitrided portion) of the steel plate. It was measured. And from the Vickers hardness (HVc) at the plate thickness 1/2 position before the soft nitriding treatment obtained in (iv) above, and the Vickers hardness (HVc ′) at the plate thickness 1/2 position after the soft nitriding treatment, Increase rate of Vickers hardness at the center of the plate thickness by soft nitriding treatment: (HVc′−HVc) / HVc × 100 (%) was obtained. In this example, the case where the increase rate of the Vickers hardness was more than 5.0% was evaluated as good (◯) with good fatigue resistance after soft nitriding treatment, and the others were evaluated as x. Table 4 shows the obtained results.
表4から明らかであるように、本発明例では、強度、成形性、軟窒化処理による表面硬化特性、耐疲労特性の全てにおいて、良好な結果が得られている。一方、鋼組成や組織が本発明の要件を満足しない比較例では、上記何れかの特性において十分な結果が得られていない。   As is clear from Table 4, in the examples of the present invention, good results were obtained in all of the strength, formability, surface hardening characteristics by soft nitriding treatment, and fatigue resistance characteristics. On the other hand, in a comparative example in which the steel composition and structure do not satisfy the requirements of the present invention, sufficient results are not obtained in any of the above characteristics.

Claims (4)

  1. 質量%で、
    C :0.05%以上0.10%以下、 Si:0.5%以下、
    Mn:0.7%以上1.5%以下、 P :0.05%以下、
    S :0.01%以下、 Al:0.01%以上0.06%以下、
    Cr:0.5%以上1.5%以下、 V :0.03%以上0.30%以下、
    N :0.005%以下
    を含有し、且つ、固溶V量と前記V含有量との比(固溶V量/V含有量)が0.50超であり、残部がFeおよび不可避的不純物からなる組成と、フェライトおよびパーライトを含む複合組織とを有することを特徴とする、軟窒化処理用鋼板。
    % By mass
    C: 0.05% or more and 0.10% or less, Si: 0.5% or less,
    Mn: 0.7% to 1.5%, P: 0.05% or less,
    S: 0.01% or less, Al: 0.01% or more and 0.06% or less,
    Cr: 0.5% to 1.5%, V: 0.03% to 0.30%,
    N: a composition containing 0.005% or less, the ratio of the solute V content to the V content (solid solution V content / V content) is more than 0.50, and the balance is composed of Fe and inevitable impurities A steel sheet for nitrocarburizing treatment, characterized by having a composite structure containing ferrite and pearlite.
  2. 前記組成に加えてさらに、質量%でNb:0.005%以上0.025%以下を含有することを特徴とする、請求項1に記載の軟窒化処理用鋼板。   The steel sheet for soft nitriding according to claim 1, further comprising Nb: 0.005% or more and 0.025% or less by mass% in addition to the composition.
  3. 鋼片を加熱し、粗圧延と仕上げ圧延からなる熱間圧延を施し、仕上げ圧延終了後、冷却し、巻取り、熱延鋼板とするにあたり、
    前記鋼片を、質量%で、
    C :0.05%以上0.10%以下、 Si:0.5%以下、
    Mn:0.7%以上1.5%以下、 P :0.05%以下、
    S :0.01%以下、 Al:0.01%以上0.06%以下、
    Cr:0.5%以上1.5%以下、 V :0.03%以上0.30%以下、
    N :0.005%以下
    を含有し、残部がFeおよび不可避的不純物からなる組成とし、前記熱間圧延の加熱温度を1100℃以上1300℃以下とし、前記仕上げ圧延の仕上げ温度をAr3変態点以上(Ar3変態点+100℃)以下とし、前記冷却の平均冷却速度を30℃/s以上とし、前記巻取りの巻取り温度を500℃以上600℃以下とすることを特徴とする、軟窒化処理用鋼板の製造方法。
    When heating the steel slab, subjecting it to hot rolling consisting of rough rolling and finish rolling, after finishing rolling, cooling, winding, and hot rolling steel sheet,
    The steel slab is in mass%,
    C: 0.05% or more and 0.10% or less, Si: 0.5% or less,
    Mn: 0.7% to 1.5%, P: 0.05% or less,
    S: 0.01% or less, Al: 0.01% or more and 0.06% or less,
    Cr: 0.5% to 1.5%, V: 0.03% to 0.30%,
    N: 0.005% or less, with the balance being composed of Fe and inevitable impurities, the heating temperature of the hot rolling is 1100 ° C. or more and 1300 ° C. or less, and the finishing temperature of the finish rolling is Ar 3 transformation point or more ( Ar 3 transformation point + 100 ° C. or lower, average cooling rate of the cooling is 30 ° C./s or higher, and the winding temperature of the winding is 500 ° C. or higher and 600 ° C. or lower. A method of manufacturing a steel sheet.
  4. 前記組成に加えてさらに、質量%でNb:0.005%以上0.025%以下を含有することを特徴とする、請求項3に記載の軟窒化処理用鋼板の製造方法。

    4. The method for producing a steel sheet for nitrocarburizing treatment according to claim 3, further comprising Nb: 0.005% or more and 0.025% or less by mass% in addition to the composition.

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